Publication Date: 2025
ACS Physical Chemistry Au (26942445)5(6)pp. 628-638
Chalcones present a potentially promising form of natural photoprotection for inclusion in sunscreen formulations. Here, using femtosecond transient electronic absorption spectroscopy and high-level quantum computations, we explore the differing photophysics of two members of the chalcone family: 4,4’-dihydroxychalcone and 4,4’-dihydroxychalcone-α-methoxylate. From experiment, trapped excited-state population in 4,4’-dihydroxychalcone is alleviated by functionalization at the α carbon, affording vast acceleration in nonradiative deactivation. From theory, the ultrashort excited-state lifetime of the α-substituted analog is explained by a barrierless S1/S0conical intersection, providing a route for ultrafast internal conversion, whereas a significant potential energy barrier prohibits the excited system from approaching this conical intersection in the nonsubstituted chalcone. These observations are supported by results from nonadiabatic dynamics simulations. Our investigations elucidate how targeted chemical modifications can perturb potential energy surfaces, resulting in distinct photophysical behaviors. We demonstrate that chalcones’ deactivation mechanisms are sensitive to substitution at the aliphatic bridge connecting the two aromatic rings. © 2025 The Authors. Published by American Chemical Society
Publication Date: 2025
Physical Chemistry Chemical Physics (14639084)
Hund's multiplicity rule asserts that the energy of the lowest excited singlet state (S1) is invariably higher than that of the lowest excited triplet state (T1) in organic compounds, leading to a positive singlet-triplet energy gap (ΔST). This gap often restricts the efficiency of organic light-emitting diodes (OLEDs). Reducing ΔST has the potential to significantly enhance internal quantum efficiency. A promising approach involves minimizing ΔST, facilitating reverse intersystem crossing (RISC) from T1 to S1 without requiring thermal activation. In this study, we present a novel class of organic compounds exhibiting inverted singlet-triplet gaps (IST), where T1 lies above S1, enabling RISC to occur without thermal excitation. A series of heteroaromatic compounds was designed by substituting C-C bonds in aromatic hydrocarbons with symmetrically arranged B-N groups in hexagonal patterns. Employing ab initio computational methods, we examined their electronic properties and assessed their potential for S1-T1 inversion. Frontier molecular orbitals were analyzed to support our findings regarding ΔST. Furthermore, basic structural designs within networks were developed and evaluated. The results reveal that these compounds possess significantly negative ΔST values, validating the existence of a novel category of boron-nitride-based heteroaromatics with inverted singlet-triplet gaps. This breakthrough paves the way for the development of highly efficient OLED materials, promising both enhanced performance and extended longevity. © 2025 The Royal Society of Chemistry.
Publication Date: 2025
Chemical Physics Letters (00092614)860
High-level Density functional theory (DFT) has been employed to investigate the impact of substitutions onto the imidazole (IM) ring, mimicking the effect of the substituted axial histidine on the bonding of the CO and NO ligands with Ferrous-Heme. It has been predicted that the CO and NO bonding to Heme is sensitive to the nature of substitution groups. Also, it has been predicted that in contrast to electron-donating substitutions on the IM ring, the electron-withdrawing groups have mainly a stabilization effect (though slightly) on the bonding of CO and NO to Heme. © 2024 Elsevier B.V.
Publication Date: 2024
The Journal Of Physical Chemistry Letters (19487185)15(29)pp. 7424-7429
Mycosporine glycine (MyG) was produced by the fermentation of a purposely engineered bacterial strain and isolated from this sustainable source. The ultrafast spectroscopy of MyG was then investigated in its native, zwitterionic form (MyGzwitter), via femtosecond transient electronic absorption spectroscopy. Complementary nonadiabatic (NAD) simulations suggest that, upon photoexcitation to the lowest excited singlet state (S1), MyGzwitter undergoes efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast ring-twisting mechanism toward an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. This study illuminates the workings of the archetype mycosporine, providing photoprotection, in the UV-B range, to organisms such as corals, macroalgae, and cyanobacteria. This study also contributes to our growing understanding of the photoprotection mechanisms of life. © 2024 The Authors. Published by American Chemical Society.
Publication Date: 2024
Journal of Chemical Physics (10897690)161(9)
We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S11nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S21ππ*) is an optically bright 1ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S2, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S1/S0 conical intersection, which directs the excited state population to the electronic ground state (S0). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms. © 2024 Author(s).
Publication Date: 2024
RSC Advances (20462069)14(28)pp. 20278-20289
In this study, we investigate photophysical properties of eight inorganic Pt(ii) complexes containing the bzq (benzoquinoline) ligand for OLED applications using high-level density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We explore the radiative and non-radiative relaxation constants (kr, knr), spin-orbit coupling (SOC) matrix elements, and spectral properties. To ensure compatibility between the host and guest compounds, we determine the HOMO and LUMO energy levels, as well as the triplet excitation energies of the selected systems, and evaluate their efficiency for OLED devices. Our findings indicate that all systems, except for 2a and 2b, exhibit a small S1-T1 energetic gap (ΔE ≤ 0.60 eV) and promising SOC matrix elements (25-93 cm−1), leading to a significant intersystem crossing (ISC) process. These complexes also show promising radiative relaxation rates (kr = ∼10−4 s−1) and high phosphorescent quantum yields (Φ > 30%). Thus, our results confirm that six out of the eight selected Pt(ii) complexes are promising candidates for use in the emitting layer (EML) of OLED devices as efficient green emitters. © 2024 The Royal Society of Chemistry.
Publication Date: 2024
Journal of Physical Chemistry A (15205215)128(50)pp. 10851-10860
We present herein our computational exploration of the conformational landscape and photophysical properties of protonated adenosine (AdoH+). Several different protonated isomers and conformers have been considered and their relevant photophysical properties have been addressed. From our ab initio quantum computational results, an S1/S0 conical intersection (CI) has been located for all considered conformers, providing a significant route for the ultrafast deactivation mechanism of the S1 excited state of AdoH+. Our results are also supported by nonadiabatic dynamics (NAD) simulation results indicating the S1 excited state lifetime of 240-300 fs for the two most stable conformers of AdoH+ (i.e., the most stable syn- and anti-N3 protonated tautomers), which is comparable with protonated adenine, reported in the literature. The results confirm the ultrafast deactivation mechanism as well as photostability in nucleosides in protonated form. © 2024 American Chemical Society.
Publication Date: 2023
Journal of Physical Chemistry A (15205215)127(22)pp. 4880-4887
In this work, different levels of quantum computational models such as MP2, ADC(2), CASSCF/CASPT2, and DFT/TD-DFT have been employed to investigate the photophysics and photostability of a mycosporine system, mycosporine glycine (MyG). First of all, a molecular mechanics approach based on the Monte Carlo conformational search has been employed to investigate the possible geometry structures of MyG. Then, comprehensive studies on the electronic excited states and deactivation mechanism have been conducted on the most stable conformer. The first optically bright electronic transition responsible for the UV absorption of MyG has been assigned as the S2 (1ππ*) owing to the large oscillator strength (0.450). The first excited electronic state (S1) has been assigned as an optically dark (1nπ*) state. From the nonadiabatic dynamics simulation model, we propose that the initial population in the S2 (1ππ*) state transfers to the S1 state in under 100 fs, through an S2/S1 conical intersection (CI). The barrierless S1 potential energy curves then drive the excited system to the S1/S0 CI. This latter CI provides a significant route for ultrafast deactivation of the system to the ground state via internal conversion. © 2023 The Authors. Published by American Chemical Society.
Publication Date: 2022
RSC Advances (20462069)12(53)pp. 34217-34225
High level density functional and time-dependent density functional (DFT, TD-DFT) theoretical methods have been employed to investigate the photophysical properties of 5 inorganic compounds resulting from Pt(ii) and ppy (2-phenyl-pyridine) ligands. This study is intended to provide insight into the capability of the selected systems to be used in OLED devices. In addition to an exploration of their ground and excited state geometry and electronic structures, the electronic transitions responsible for their absorption and spectra, as well as other photophysical properties, have been analyzed. To this end, their charge transfer parameters, the triplet exciton generation, phosphorescence quantum yield, and radiative decay rates have been studied. Overall, the results confirm that the selected systems are promising candidates to be used in OLED devices. Moreover, the results of this study assist in understanding the photophysical properties of Pt(ii) complexes with ppy ligands. © 2022 The Royal Society of Chemistry.
Publication Date: 2022
Physical Chemistry Chemical Physics (14639084)24(24)pp. 14898-14908
The quantum chemical computational method and Born-Oppenheimer (BO) dynamics simulation were employed to investigate the non-radiative relaxation mechanism of protonated 9H- and 7H-adenine (AH+). We located three conical intersections (CIs) between the first 1ππ* excite state and the S0 ground state potential energy surfaces for the two most stable protonated isomers of adenine. It was predicted that the barrier-free potential energy profile along the out-of-plane deformation coordinates of the six-member ring plays the most prominent role in the deactivation of the excited AH+ from 1ππ* to the ground state via ultrafast internal conversions. This ring deformation was predicted to provide a common deactivation pathway in protonated DNA/RNA bases, describing their high level of photostability, and corresponding neutral homologues. © 2022 The Royal Society of Chemistry.
Publication Date: 2022
RSC Advances (20462069)12(8)pp. 4703-4713
The effect of distal histidine on ligation of NO to ferrous and ferric-heme, has been investigated with the high-level density functional theoretical (DFT) method. It has been predicted that the distal histidine significantly stabilizes the interaction of NO ferrous-heme (by −2.70 kcal mol−1). Also, water hydrogen bonding is quite effective in strengthening the Fe-NO bond in ferrous heme. In contrast in ferric heme, due to the large distance between the H2O and O(NO) and lack of hydrogen bonding, the distal histidine exhibits only a slight effect on the binding of NO to the ferric analogue. Concerning the bond nature of FeII-NO and FeIII-NO in heme, a QTAIM analysis predicts a partially covalent and ionic bond nature in both systems. This journal is © The Royal Society of Chemistry
Publication Date: 2022
Journal of Solid State Chemistry (1095726X)305
In the present study, molecular geometry, electronic structure, first row transition metal-polyoxometalate interactions, transition metal-H2O bonding, and simulated infrared (IR) spectrum of H2O ligated/unligated transition metal substituted Keggin type polyoxometalates ([PW11O39(MII/MIIOH2)]5-, MII = Cr, Mn, Fe, Co, Ni, Cu and Zn) have been studied through Density Functional Theory (DFT) calculations. From the results, it has been predicted that the interaction of H2O ligand with MII leads to a stable [PW11O39(MII-H2O)]5- complex, without spin crossing during water attachment. The calculations have also revealed that interaction of H2O ligand with [PW11O39(MII)]5- could be discussed in two different classes: when MII is Cr, Mn and Fe, the H2O molecule interacts directly to MII from O side, while for MII = Co, Ni, Cu and Zn, the H-bond interaction between H2O and O atoms around the M, leads to the stabilization of POM-H2O system. Moreover, it has been predicted that the binding properties of H2O strikingly depends on axial 3dz2 orbital of MII. In this regard, it has been exhibited that the affinity of H2O for MII in [PW11O39(MII)]5- with a filled 3dz2 orbital, is strongly reduced as measured by increasing in calculated MII-ligand distance. Also, strong contributions of MII 3d as well as the 2p orbitals of five oxygen donor atoms in polyoxometalate cluster results in the bonding interaction between MII and lacunary Keggin polyoxometalate. The DFT-derived IR spectra showed displacement and splitting in four previously known vibrational bands of Keggin-type polyoxometalate, α-[PW12O40]3-, owing to transition metal substitution and H2O ligation. © 2021 Elsevier Inc.
Publication Date: 2021
Journal of Molecular Liquids (18733166)332
Drug delivery systems can be designed to safely transfer the medicinal compounds into the body and increase the therapeutic index, consequently. Some useful drug delivery systems such as liposomes, hydrogels, nanoparticles, and polymers are widely used. Polymers mainly block copolymers, smart polymers, and a combination of them are regarded in drug delivery systems by improving controlled release, biocompatibility, bioavailability, and increase the solubility of active pharmaceutical ingredients. For the first time in this study, we thoroughly examined the modified PNIPAAm-b-PEG block-copolymer interaction as a smart nano-drug delivery system with curcumin molecule as a drug, using the molecular dynamics simulation method. We used GROMACS as a molecular dynamic software and Amber99SB as an all-atom force field to investigate the polymer's behavior in the presence of the drug in an aqueous solution. We focused on the encapsulation behaviors of a drug, the intermolecular interactions, the structural properties of drug-polymer nano-micelles, and the capacity of drug-loading in the micelle. Our analysis predicts that PNIPAAm-b-PEG polymer phase change temperature is in the range of 300–305 K and also at the concentration of 9% of the polymer, in 310 K, the effective radius of the formed micelle is equal to 4.36 nm, and it is hydrogel like, in good agreement with the relevant experimental data. Further analysis shows the drug solubility in the presence of the polymer increases by about 88%. The study of the interaction energy between drug and polymer predicts the encapsulation process as favorable. © 2021 Elsevier B.V.
Publication Date: 2021
Journal of Molecular Liquids (18733166)344
In the present study, we have employed a high-level density functional theory (DFT) model to investigate both implicit and explicit effects of solvation (i.e., the bulk solvation effect as well as microhydration) in addition to the effect of distal Histidine on the binding of CO to Ferrous and Ferric-Heme. It has been predicted that the distal N-methylimidazole (MI) in addition to microsolvation, as a simple mode of environment, leads to a significant stabilization on the binding of CO to Heme-FeII (by ∼ 4 kcal mol−1). This is while no clear alteration has been predicted for the implicit solvation model. For the Ferric heme, and in contrast to the Ferrous system, strong destabilization from the environment has been predicted for binding CO to the Ferric heme analog. © 2021 Elsevier B.V.
Publication Date: 2021
Physical Chemistry Chemical Physics (14639084)23(14)pp. 8916-8925
Ab initioand surface-hopping nonadiabatic dynamics simulation methods were employed to investigate relaxation mechanisms in protonated thymine (TH+) and cytosine (CH+). A few conical intersections were located between1pp* and S0states for each system with the CASSCF (8,8) theoretical model and relevant contributions to the deactivation mechanism of titled systems were addressed by the determination of potential energy profiles at the CASPT2 (12,10) theoretical level. It was revealed that the relaxation of the1pp* state of the most stable conformer of both systems to the ground state is mostly governed by the accessible S1/S0conical intersection resulting from the barrier-free out-of-plane deformation. Interestingly, it was exhibited that the ring puckering coordinate driven from the C6position of the heterocycle ring in TH+and CH+plays the most prominent role in the deactivation mechanism of considered systems. Ourab initioresults are also supported by excited-state nonadiabatic dynamics simulations based on ADC(2), describing the ultrashort S1lifetime of TH+/CH+by analyzing trajectories leading excited systems to the ground. It was confirmed that the excited-state population mostly relaxes to the groundviathe ring puckering coordinate from the C6moiety. Overall, the theoretical results of this study shed light on the deactivation mechanism of protonated DNA bases. © the Owner Societies 2021.
Publication Date: 2020
RSC Advances (20462069)10(56)pp. 33718-33730
Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and H2O/OH−/O2−ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4−, possesses a high reactivity for oxygen transfer. © The Royal Society of Chemistry 2020.
Publication Date: 2020
Journal of Physical Chemistry A (15205215)124(25)pp. 5089-5097
We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the 1ππ∗ state of enol-enol form (EE+) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S1 lifetime of this species. For the second most stable isomer (EK+), however, a significant barrier predicted in the CASPT2 S1 potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally. © 2020 American Chemical Society.
Publication Date: 2019
International Journal of Quantum Chemistry (1097461X)119(3)
New platinum(II) complex with picolinate (pic) and 2-phenyl naphtothiazole (pntl) ligand as the guest material has been designed and its capability for OLED applications have been examined. Also, we have studied the effects of different substitutions (ie, electron-withdrawing and electron donating groups) on naphtothiazole moiety on optovoltaic characters. We have employed density functional theoretical (B3LYP/DFT) methods to reveal the photophysical and structure properties relationships with the typical host material. The valence MO energies, vertical and adiabatic triplet energy, reorganization energy, and triplet exciton generation fraction (χT) have been extensively studied to exploring high phosphorescence efficiency in OLEDs. It has been predicted that substituted systems are good candidates for OLED applications as well as their parent system. © 2018 Wiley Periodicals, Inc.
Publication Date: 2019
Physical Chemistry Chemical Physics (14639084)21(4)pp. 1750-1760
The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van't Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(3d) orbitals. This trans σ donor effect is characteristic of ligation to iron in hemes in both ferrous and ferric redox forms; however, to date, this has not been observed for ferric heme. © 2019 the Owner Societies.
Publication Date: 2019
Journal of Photochemistry and Photobiology A: Chemistry (18732666)385
The nonradiative deactivation mechanisms of the lowest 1ππ excited states in four protonated isomers of the 7H- and 9H-xanthine, based on the MP2, CC2, ADC(2) and CASSCF theoretical methods have been investigated. It has been predicted that out-of-plane driving coordinates are effectively responsible for photostability of these systems at the Franck-Condon region of the 1ππ* excited state, while the NH or OH stretching coordinates can be suggested to play the deactivation role in the higher energetic range based on the 1πσ* state. The different deactivation mechanisms provide significant supports for photostability of protonated xanthine in the wide range of UV radiation as well as its neutral homologue. Also, from spectroscopy points of view, no significant alteration in the 1ππ*-S0 electronic transition has been predicted to occur following protonation. © 2019 Elsevier B.V.
Publication Date: 2019
Physical Chemistry Chemical Physics (14639084)21(38)pp. 21329-21340
The interaction of a water molecule with ferric heme - iron protoporphyrin ([PP FeIII]+) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-FeIII-H2O]+ complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-FeIII]+ homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH2 bond has been determined for [PP-FeIII-H2O]+ and [PP-FeIII-(H2O)2]+. The corrected binding energy for a single Fe-H2O bond is -12.2 ± 0.6 kcal mol-1, while DFT calculations at the OPBE level yield -11.7 kcal mol-1. The binding energy of the second ligation yielding a six coordinated FeIII atom is decreased with a bond energy of -9 ± 0.9 kcal mol-1, well reproduced by calculations as -7.1 kcal mol-1. However, calculations reveal features of a weaker bond type, such as a rather long Fe-O bond with 2.28 Å for the [PP-FeIII-H2O]+ complex and the absence of a spin change by complexation. Thus despite a strong bond with H2O, the FeIII atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3dz orbital. It is also observed that the binding properties of H2O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H2O for protonated heme [H PP-Fe]+, an intermediate between FeIII and FeII, is strongly reduced compared to that for ferric heme. © 2019 the Owner Societies.
Publication Date: 2018
Journal of Physical Chemistry A (15205215)122(10)pp. 2653-2662
Thiazolo-[4,5-d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S1 PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH2) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S1 excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well. © 2018 American Chemical Society.
Publication Date: 2018
Journal of Porphyrins and Phthalocyanines (10991409)22(8)pp. 646-657
Publication Date: 2018
Journal of Physical Chemistry A (15205215)122(12)pp. 3182-3189
Hydroxyphenyl benzothiazole (HBT), is a well-known organic system based on its special characteristic of the excited state hydrogen transfer (ESHT) following photoexcitation. However, the capability of this system regarding photochromism and photoswitching has not been addressed yet. In this study, we have investigated this issue by the aim of the MP2, CC2, ADC(2), and CASSCF theoretical methods. Also, we have considered several electron withdrawing groups and investigated their effects on the photophysical characteristics and spectroscopic properties of the enol and keto tautomers of the titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characteristics required for photochromism. Also, substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems. Our theoretical results verify that different substitutions alter the UV absorption of HBT systems from 330 to 351 nm and also the corresponding absorption wavelength of the γ-forms of 526-545 nm. © 2018 American Chemical Society.
Publication Date: 2018
Journal of Physical Chemistry A (15205215)122(45)pp. 8849-8857
The photoswitchability of a protonated aromatic compound (2-aminobenzaldehyde, 2ABZH+) in its individual and microhydrated states has been addressed based on the RI-MP2/RI-CC2 theoretical methods. Our calculated results give insight into the ultrafast nonradiative deactivation mechanism of the 2ABZH+, driven by a conical intersection between the S1/S0 potential energy surfaces. Also, it has been predicted that protonation accompanies a significant blue shift effect on the first 1ππ∗ excited state of 2ABZ by 0.87 eV (at least 50 nm). © Copyright 2018 American Chemical Society.
Publication Date: 2017
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy (13861425)182pp. 8-16
The geometry and electronic structures of protonated serotonin have been investigated by the aim of MP2 and CC2 methods. The relative stabilities, transition energies and geometry of sixteen different protonated isomers of serotonin have been presented. It has been predicted that protonation does not exhibit essential alteration on the S1 ← S0 electronic transition energy of serotonin. Instead, more complicated photophysical nature in respect to its neutral analogue is suggested for protonated system owing to radiative and non-radiative deactivation pathways. In addition to hydrogen detachment (HD), hydrogen/proton transfer (H/PT) processes from ammonium to indole ring along the NH+⋯ π hydrogen bond have been predicted as the most important photophysical consequences of SERH+ at S1 excited state. The PT processes is suggested to be responsible for fluorescence of SERH+ while the HD driving coordinate is proposed for elucidation of its nonradiative deactivation mechanism. © 2017
Publication Date: 2017
Chemical Physics Letters (00092614)679pp. 90-96
Microsolvation effect on geometry and transition energies of protonated serotonin has been investigated by MP2 and CC2 quantum chemical methods. Also, conductor-like screening model, implemented recently in the MP2 and ADC(2) methods, was examined to address the bulk water environment's effect on the isomer stability and electronic transition energies of protonated serotonin. It has been predicted that the dipole moment of gas phase isomers plays the main role on the isomer stabilization in water solution and electronic transition shifts. Also, both red- and blue-shift effects have been predicted to take place on electronic transition energies, upon hydration. © 2017
Publication Date: 2016
RSC Advances (20462069)6(39)pp. 33148-33158
The second-order approximate coupled-cluster (RI-CC2) method was employed to investigate photoinduced hydrogen-bond weakening or strengthening in neutral and protonated indole-, 5-hydroxyindole-water clusters. In addition to the protonation effect on the electronic structure of 5-hydroxyindole, the intermolecular H-bond weakening or strengthening of selected systems in the S1 excited state has been investigated. According to our calculated results, it has been predicted that the electronic excitation effect on the hydrogen-bond strength in protonated clusters is essentially more pronounced than neutral analogues. Also, a charge transfer character of the excited state over the chromophore moiety can be suggested for interpreting the excited state dynamics of H-bonds in protonated complexes. Moreover, it has been predicted that protonation is accompanied by a strong red-shift effect (∼1.10 eV) on the S1-S0 transition energy of 5-hydroxyindole. © The Royal Society of Chemistry 2016.
Publication Date: 2016
Journal of Physical Chemistry A (15205215)120(7)pp. 1012-1019
In the present study, the results of comprehensive theoretical exploration on the nonradiative relaxation of three (hydroxyphenyl)imidazole-based organic compounds (abbreviated AHP, HPIP, and HPBI) in the gas phase are presented. Having small structural differences, the selected systems have commonalities in the excited state intramolecular proton transfer (ESIPT) process. The ground and S1 excited state potential energy profiles of titled systems have been determined on the basis of the RI-MP2 and RI-CC2 methods, and the effect of small structural distinctions on their photophysical characters will be extensively addressed. Although, in the presence of solvent, high fluorescence quantum yield is another characteristic of AHP and HPIP, owing to accessible conical intersections between the S1/S0 state potential energy profiles of both systems, nonradiative relaxation can be proposed as the most important feature of these two systems in the gas phase. These conical intersections are responsible for ultrafast deactivation of excited systems via internal conversions to the ground state. The nonradiative deactivation mechanism determined in this work deals with the remarkable photostability of the AHP and HPIP molecules. (Figure Presented). © 2016 American Chemical Society.
Publication Date: 2016
Journal of Chemical Physics (10897690)145(18)
The MP2/CC2 and CASSCF theoretical approaches have been employed to determine the excited state proton transfer and photophysical nature of the four organic compounds, having the main frame of hydroxyphenyl-imidzaopyridine (HPIP). The nitrogen insertion effect, in addition to amine (-NH2) substitution has been investigated extensively by following the transition energies and deactivation pathways of resulted HPIP derivatives. It has been predicted that the excited state intramolecular proton transfer with or without small barrier is the most important feature of these compounds. Also, for all of the considered HPIP derivatives, a conical intersection (CI) between ground and the S1 excited state has been predicted. The strong non-adiabatic coupling in the CI (S1/S0), drives the system back to the ground state in which the proton may either return to the phenoxy unit and thus close the photocycle, or the system can continue the twisting motion that results in formation of a γ-photochromic species. This latter species can be responsible for photochromism of HPIP derivative systems. © 2016 Author(s).
Publication Date: 2016
RSC Advances (20462069)6(85)pp. 82219-82226
In this study, protonation of porphine (H2P) with a range of weak and strong acids was investigated. The dication of H2P was water soluble and therefore UV-vis and 1H NMR studies were performed in water and D2O as well as in dichloromethane and dimethylformamide. In contrast to the dication of other porphyrins, protonated species of H2P were completely decomposed upon evaporation of solvent at room temperature and therefore were studied in solution. Also, high level ab initio calculations were used to predict the structure, frontier molecular orbitals and transition energies of H2P dication. The Soret band of H2P was only slightly shifted to longer or shorter wavelengths in reaction with weak and strong acids, respectively. The results show that the presence of aryl or at least alkyl substituents at the meso positions of porphyrin macrocycle is necessary for the observation of significant red shifts of the Soret band. In the case of the Q(0,0) band, large blue shifts of the band were observed for the dications that correlate with the absence of any π electron-donating group at the meso position. In 1H NMR spectra, signals for both the β and meso protons were shifted downfield, which shows the negligible decrease in the porphine ring current caused by the out-of-plane distortion of the macrocycle. While ab initio calculations show a saddle shaped conformation for [H4P]2+, H4P(CF3COO)2 was found to adopt an unusual wave conformation that clearly differs from that of previously characterized dications of meso-tetra(aryl)- and meso-tetra(alkyl)porphyrins. Also, the calculations on monoprotonated species show that [H3P]+ and H3P(CF3COO) adopt a nearly saddle type and dome shaped conformation, respectively. On the other hand, the energies of absorption bands of H4P(CF3COO)2 calculated at the TD-B3LYP/cc-pVDZ level of theory predicted the red shift of the Soret band and the blue shift of the Q(0,0) bands that are in accord with the results of UV-vis studies. The results revealed the role played by the acid molecules on the blue shift of the Q(0,0) band of the diprotonated species. Furthermore, the calculated changes in the bond lengths and bond angles show that the involvement of in-plane nuclear reorganization (IPNR) in the observed red shifts of the Soret and Q(0,0) bands cannot be excluded. © 2016 The Royal Society of Chemistry.
Publication Date: 2015
RSC Advances (20462069)5(118)pp. 97619-97628
The second order approximate Moller-Plesset (MP2) and coupled cluster (CC2) methods have been employed to investigate the geometry, electronic transition energies and photophysics of the isoindole-pyridine and quinoline-pyrrole complexes. The most stable geometry of both isoindole-pyridine and quinoline-pyrrole complexes has been predicted to be a perpendicular structure. It has also been found that the first electronic transition in both complexes is responsible for UV absorption owing to its 1ππ∗ nature, while a charge transfer 1ππ∗ state governs the nonradiative relaxation processes of both complexes. In this regard, excited state intermolecular hydrogen/proton transfer (ESHT/PT) via the charge transfer electronic states plays the most prominent role in non-radiative deactivation. In the HT/PT reaction coordinate, the minimum potential energy profile of the lowest CT-1ππ∗ state predissociates the local 1ππ∗ state, connecting the latter to a curve crossing with the S0 state. At the region of this curve crossing, the S0 and CT state become degenerate, enabling the 1ππ∗ state to proceed as the predissociative state and finally direct the excited system to the ground state. © 2015 The Royal Society of Chemistry.
Publication Date: 2015
Photochemical and Photobiological Sciences (1474905X)14(12)pp. 2261-2269
The geometry, electronic structures and potential energy profiles of protonated furan and thiophene have been extensively investigated, using the RI-MP2 and RI-CC2 methods. According to RI-CC2 calculated results, the adiabatic S1(1ππ∗)-S0 transition energies of protonated furan and thiophene, have been predicted to be 4.41 eV and 3.70 eV respectively. Thus, protonation is accompanied by a large red shift effect on the first 1ππ∗ transition of the title systems (ΔE > 1.5 eV). The significant spectral-movements, predicted based on the calculated results of this work, indicate an essential effect of protonation on the geometry, electronic structures and optical characters of the five membered heterocyclic systems. In addition, it has been found that excitation of protonated furan and thiophene, with sufficient excess energy above the band origin of S1(1ππ∗)-S0 transition, is accompanied by the S-C or O-C bond breaking. This mechanism is mostly governed by a dissociative 1πσ∗ PE profile in both protonated systems. © The Royal Society of Chemistry and Owner Societies 2015.
Publication Date: 2015
Journal of Physical Chemistry A (15205215)119(25)pp. 6650-6660
The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-naphthol is essentially different from that of its neutral homologue. In protonated α-naphthol, the calculations reveal that 1σπ∗ state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the 1πσ∗ state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-naphthol, mostly by alteration of the 1ππ∗ PE profile, from a bound state in an isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-naphthol and water; with a small barrier; (E< 0.1 eV). © 2015 American Chemical Society.
Publication Date: 2015
RSC Advances (20462069)5(37)pp. 29032-29039
The potential energy (PE) profiles of neutral and protonated phenylalanine, as the simplest aromatic amino acid, at different electronic states have been investigated extensively using RI-MP2 and RI-CC2 methods. The PE profiles have been determined, considering the Cα-Cβ and Cα-C(COOH) bond stretching following proton transfer to the aromatic ring and CO group, respectively, as well as the hydrogen detachment reaction coordinate. The calculated results reveal that a low-barrier proton transfer process from ammonia to the aromatic chromophore, leading the excited system to Cα-Cβ bond cleavage, plays the most prominent role in the deactivation mechanism of excited PheH+ at the origin of the S1-S0 electronic transition. On the contrary, for excited neutral phenylalanine at the band origin of the S1-S0 transition, a large barrier in the S1 profile along the Cα-Cβ bond-stretching hinders the excited system from approaching the dissociative part of PE curve. This barrier may explain the large lifetime of the S1 excited phenylalanine (nanosecond range), while a low barrier in the S1 PE profile of the protonated species along the PT process explains the short-range lifetime of the protonated species (in the picosecond range). © 2015 The Royal Society of Chemistry.
Publication Date: 2015
Photochemical and Photobiological Sciences (1474905X)14(2)pp. 457-464
Excited state hydrogen transfer in hydroquinone- and catechol-ammonia clusters has been extensively investigated by high level ab initio methods. The potential energy profiles of the title systems at different electronic states have been determined at the MP2/CC2 levels of theory. It has been predicted that double hydrogen transfer (DHT) takes place as the main consequence of photoexcited tetra-ammoniated systems. Consequently, the DHT processes lead the excited systems to the 1πσ∗-S0 conical intersections, which is responsible for the ultrafast non-radiative relaxation of UV-excited clusters to their ground states. Moreover, according to our calculated results, the single hydrogen detachment or hydrogen transfer process essentially governs the relaxation dynamics of smaller sized clustered systems (mono- and di-ammoniated). This journal is © The Royal Society of Chemistry and Owner Societies.
Publication Date: 2014
Journal of Chemical Physics (10897690)140(12)
Vibrational and electronic photodissociation spectra of mass-selected protonated benzaldehyde-(water)n clusters, [BZ-(H2O) n]H+ with n ≤ 5, are analyzed by quantum chemical calculations to determine the protonation site in the ground electronic state (S0) and ππ* excited state (S1) as a function of microhydration. IR spectra of [BZ-(H2O) n]H+ with n ≤ 2 are consistent with BZH +-(H2O)n type structures, in which the excess proton is localized on benzaldehyde. IR spectra of clusters with n ≥ 3 are assigned to structures, in which the excess proton is located on the (H 2O)n solvent moiety, BZ-(H2O)nH +. Quantum chemical calculations at the B3LYP, MP2, and ri-CC2 levels support the conclusion of proton transfer from BZH+ to the solvent moiety in the S0 state for hydration sizes larger than the critical value nc = 3. The vibronic spectrum of the S1 ←S 0 transition (ππ*) of the n = 1 cluster is consistent with a cis-BZH+-H2O structure in both electronic states. The large blueshift of the S1 origin by 2106 cm-1 upon hydration with a single H2O ligand indicates that the proton affinity of BZ is substantially increased upon S1 excitation, thus strongly destabilizing the hydrogen bond to the solvent. The adiabatic S1 excitation energy and vibronic structure calculated at the ri-CC2/aug-cc-pVDZ level agrees well with the measured spectrum, supporting the notion of a cis-BZH+-H2O geometry. The doubly hydrated species, cis-BZH+-(H2O)2, does not absorb in the spectral range of 23 000-27 400 cm-1, because of the additional large blueshift of the ππ* transition upon attachment of the second H2O molecule. Calculations predict roughly linear and large incremental blueshifts for the ππ* transition in [BZ-(H2O)n]H+ as a function of n. In the size range n ≥ 3, the calculations predict a proton transfer from the (H 2O)nH+ solvent back to the BZ solute upon electronic ππ* excitation. © 2014 AIP Publishing LLC.
Publication Date: 2014
Physical Chemistry Chemical Physics (14639084)16(23)pp. 11679-11689
The potential energy profiles of neutral and protonated anisole and p-fluoroanisole at different electronic states have been investigated extensively by the RI-MP2 and RI-CC2 methods. The calculations reveal that the relaxation dynamics in protonated anisole and p-fluoroanisole are essentially different from those of the neutral analogues. In neutral anisole/p- fluoroanisole, the 1πσ* state plays a vital relaxation role along the O-CH3 coordinate, yielding the CH3 radical. For both of these molecules, the calculations indicate conical intersections (CIs) between the ground and excited state potential energy (PE) curves, hindered by a small barrier, and providing non-adiabatic gates for radiation-less deactivation to the ground state. Nevertheless, for the protonated cases, besides the prefulvenic deformation of the benzene ring, it has been predicted that the lowest 1(σ,n)π* state along the C-O-C bond angle plays an important role in photochemistry and the relaxation dynamics. The S1, S0 PE profiles of protonated anisole along with the former reaction coordinate (out-of-plane deformation) show a barrierless relaxation pathway, which can be responsible for the ultrafast deactivation of excited systems to the ground state via the low-lying S1/S0 conical intersection. Moreover, the later reaction coordinate in protonated species (C-O-C angle from 120°-180°) is consequently accompanied with the bond cleavage of C-OCH3 at the 1(σ,n)π* state, hindered by a barrier of ∼0.51 eV, and can be responsible for the relaxation of excited systems with significant excess energy (hν ≥ 5 eV). Furthermore, according to the RI-CC2 calculated results, different effects on the S1-S0 electronic transition energy of anisole and p-fluoroanisole upon protonation have been predicted. The first electronic transitions of anisole and p-fluoroanisole shift by ∼0.3 and 1.3 eV to the red respectively due to protonation. © 2014 the Partner Organisations.
Publication Date: 2014
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy (13861425)122pp. 337-342
The second-order approximate coupled-cluster (CC2) method was performed to investigate the excited state hydrogen-bonding properties of Glyoxal (C 2H2O2, Gl) dimers. Since the strengthening and weakening of hydrogen bonds can be investigated by monitoring the vibrational absorption spectra of some hydrogen-bonded groups in different electronic states, the infrared spectra of the hydrogen-bonded GlGl complexes in both of the ground state and the S1 electronically excited state are calculated using the MP2/CC2 methods respectively. We demonstrated that the intermolecular hydrogen bond COâ̄HC between two glyoxal molecules is significantly strengthened in the electronically excited S1 state upon photoexcitation of the hydrogen-bonded GlGl complexes. © 2013 Elsevier B.V. All rights reserved.
Publication Date: 2014
Physical Chemistry Chemical Physics (14639084)16(6)pp. 2417-2424
Excited state reaction coordinates and the consequent energy profiles of a new Schiff base, N-salicilydenemethylfurylamine (SMFA), have been investigated with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. The potential energy profiles of the ground and the lowest excited singlet state are calculated. In contrast to the ground state, the excited state potential energy profile shows a barrier-less dissociation pattern along the O-H stretching coordinate which verifies the proton transfer reaction at the S1 (ππ*) state. The calculations indicate two S1/S0 conical intersections (CIs) which provide non-adiabatic gates for radiation-less decay to the ground state. At the CIs, two barrier-free reaction coordinates direct the excited system to the ground state of enol-type minimum. According to calculation results, a trans-keto type structure obtained from photoexcitation of the enol, can be responsible for the photochromoic effect of SMFA. Furthermore, our results confirm the suggestion that aromatic Schiff bases are potential candidates for optically driven molecular switches. © 2014 the Owner Societies.
Publication Date: 2014
Journal of Chemical Physics (10897690)140(2)
The CC2 (second order approximate coupled cluster method) has been applied to investigate protonation effect on electronic transition energies of 2-pyridone (2PY), 2-pyridone dimer, and micro-solvated 2-pyridone (0-2 water molecules). The PE profiles of protonated 2-pyridone (2PYH) as well as monohydrated 2PYH at the different electronic states have been investigated. The 1πσ state in protonated species (2PYH) is a barrier free and dissociative state along the O-H stretching coordinate. In this reaction coordinate, the lowest lying 1πσ predissociates the bound S1(1ππ) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the 1ππ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the 1πσ state still remains dissociative but the conical intersection between 1πσ and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1-S 0 electronic transition of monomer, dimer, and microhydrated 2-pyridone. © 2014 AIP Publishing LLC.
Publication Date: 2013
Journal of Physical Chemistry A (15205215)117(12)pp. 2499-2507
The first and second electronic excited states (S1 and S 2) of protonated phenanthrene and protonated pyrene, having the ππ* nature, are strongly red-shifted compared to corresponding electronic transitions in neutral homologues. The CC2 calculations identify an out-of-plane deformation as the most important photochemical reaction coordinate in protonated phenanthrene as well as protonated benzene. It was shown that the excited S1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S 1-S0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S0 potential-energy surface. In contrast to the most stable isomer of protonated phenanthrene, the most stable structure of protonated pyrene shows planar structure in both the S1 and S2 excited states, without considerable geometry deformations. © 2013 American Chemical Society.
Publication Date: 2013
Chemical Physics Letters (00092614)555pp. 19-25
The low-lying electronic excited states of protonated phenol and para-Fluorophenol have been investigated extensively by RI-MP2/RI-CC2 methods. Although, protonation of phenol leads to a small red-shift-effect on the S 1-S0 (ππ) electronic transition in respect to its neutral homologue, a large red-shift-effect, on the same electronic transitions of para-substituted phenol has been predicted. The ππ excited state of protonated phenol stays in the UV range (4.34 eV), while its πσ state lies in the VUV region (8.3 eV). The S1 excited-state geometry optimization of protonated phenol predicted unstable S1 state owing to the strong out-of-plane deformation in the benzene ring. © 2012 Elsevier B.V. All rights reserved.
Publication Date: 2013
Journal of Physical Chemistry A (15205215)117(48)pp. 12842-12850
The CC2 (second-order approximate coupled cluster method) has been employed to investigate microhydration effect on electronic properties of protonated phenol (PhH+) According to the CC2 calculation results on electronic excited states of microhydrated PhH+, for the S1 and S2 electronic states, which are of 1ππ* nature and belong to the A′ representation of molecular Cs point group, a significant blue shift effect on the S1 and S2 electronic states, which are of 1ππ* nature and belong to the A′ representation of molecular Cs point group, in comparison to corresponding transitions on bare cation (PhH+), has been predicted. Nevertheless, for the S3-S0 (1A′′, 1σπ*) transition, a large red shift effect has been predicted. Furthermore, it has been found that the lowest 1σπ* state plays a prominent role in the photochemistry of these systems. In the bare protonated phenol, the 1σπ* state is a bound state with a broad potential curve along the OH stretching coordinate, while it is dissociative in microhydrated species. This indicates to a predissociation of the S 1(1ππ*) state by a low-lying 1σπ* state, which leads the excited system to a concerted proton-transfer reaction from protonated chromophore to the solvent. The dissociative 1σπ* state in monohydrated PhH + has small barrier, while increasing the solvent molecules up to three removes the barrier and consequently expedites the proton-transfer reaction dynamics. © 2013 American Chemical Society.
Publication Date: 2013
Polyhedron (02775387)49(1)pp. 36-40
High level ab initio calculations on the saddle distortion of the porphine backbone with dihedral angles in the range 0-30° show the red shift of the Soret and Q bands of the aromatic macrocycle. Also, according to the optimized structure of the planar and saddle shaped porphine, the involvement of in-plane nuclear reorganization in the observed shifts of the bands upon the out-of-plane deformation of the porphyrin core especially to angles larger than 25° cannot be excluded. The distribution of electron density over the porphine frontier orbitals with D2h symmetry has been also studied. © 2012 Elsevier Ltd. All rights reserved.
Publication Date: 2013
Journal of Physical Chemistry A (15205215)117(4)pp. 718-725
Excited state reaction coordinate and the consequent energy profiles of a new Schiff base, N-salicylidene-2-bromoethylamine, have been investigated at the CC2 level of theory. The electron-driven proton transfer and torsional deformation have been identified as the most important photochemical reaction coordinates. In contrast to the ground state, the excited state potential energy profile shows a barrierless dissociation pattern along the O-H stretching coordinate, which verifies the proton transfer reaction along the O-H coordinate at the S1 state. The calculations showed that the PT is electron driven and that the S1 transition has charge transfer character. The keto-type S1 state attained by barrierless proton transfer is found to be unstable via a torsional motion, which provides fast access to a S 1-S0 conical intersection. From the conical intersection, a barrierless reaction path directs the system back to the enol-type minimum of the S0 potential energy surface, thus closing the photocycle. © 2013 American Chemical Society.
Publication Date: 2012
Chemical Physics (03010104)399pp. 224-231
The excitation spectrum of protonated salicylaldehyde has been recorded in the 20,800-22,400 cm -1 region (480-450 nm). The first excited state of protonated salicylaldehyde is a ππ state, largely red shifted as compared to the ππ transition of its neutral analogue. Like protonated benzaldehyde and in contrast to some other protonated aromatic molecules such as benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed, the vibrational bands are well resolved and assigned. This molecule has many low energy isomers and the simulations of the electronic spectrum via ab initio excited state optimizations and Franck-Condon calculations are precise enough to assign the observed electronic spectrum to one of the isomers. © 2011 Elsevier B.V. All rights reserved.
Publication Date: 2011
Chemical Physics Letters (00092614)518pp. 15-20
The low lying singlet and triplet electronic excited states of neutral and protonated Acenaphthylene (C12H8, ACYN) and Acenaphthene (C12H10, ACN) have been investigated extensively by RI-MP2 and RI-CC2 methods. The first and second electronic excited tates (S 1, S2) of protonated ACYN and ACN have ππ* nature and lie in the visible or UV region. Similar to naphthalene, anthracene, and other linear PAHs, the protonation of ACYN and ACN leads to a strong red shift of the electronic transition as compared to the neutral molecule. The calculations indicate a charged transfer character of S1-S 0 transition in protonated ACYN and ACN as well as protonated naphthalene. © 2011 Elsevier B.V. All rights reserved.
Publication Date: 2011
Journal of Luminescence (00222313)131(6)pp. 1229-1233
The interaction between αamylase from Bacillus subtilis and cetyltrimethylammonium bromide (CTAB) has been investigated at various temperature conditions using fluorescence and circular dichroism (CD) spectroscopic methods. Fluorescence data revealed that the fluorescence quenching of αamylase by CTAB is the result of complex formation between CTAB and αamylase. The thermodynamic analysis on the binding interaction data shows that the interactions are strongly exothermic (ΔH°=-17.92 kJ mol-1) accompanied with increase in entropy (ΔS° between 109 to 135 J mol-1 K-1). Thus the binding of CTAB to α-amylase is both enthalpic and entropic driven, which represent the predominate role of both electrostatic and hydrophobic interactions in complex formation process. The values of 2.17×10-3 M-1 and 1.30 have been obtained from associative binding constant (Ka) and stoichiometry of binding number (n), from analysis of fluorescence data, respectively. Circular dichroism spectra showed the substantial conformational changes in secondary structure of αamylase due to binding of CTAB, which represents the complete destruction of both secondary and tertiary structure of α-amylase by CTAB. © 2011 Elsevier B.V. All rights reserved.
Publication Date: 2011
Inorganic Chemistry Communications (13877003)14(11)pp. 1827-1832
Spectral shifts upon core protonation of a series of electron-rich and electron-deficient porphyrins with five and six membered rings at the meso-positions have been studied by UV-vis and 1H NMR spectroscopy. The direction of the shift of the Q(0,0) band of different porphyrins depends on the electron-donating or electron-withdrawing character of the meso-substituent and therefore it may be used to evaluate the relative electron donor ability of the substituents. Also, the shift of the Soret bands was found to be influenced to a lesser extent by the electron donor ability of substituted groups at the porphyrin periphery. Although, large red shifts of the Soret band of H 4T(o-NO2)P2+, H4T(p-SCH 3P)P2+ and H4T(p-NO2P)P2+ indicate a significant out-of-plane deformation of porphyrin core, negligible upfield shifts of the β protons in their 1H NMR spectra show that the practice of using the change in the chemical shift of the β protons as an indicator of structurally induced changes in the porphyrin ring current should be approached with more caution. Interestingly, high level ab initio and DFT calculations demonstrate that in spite of the better electron donating ability of -OCH3 to benzene ring compared to that of -SCH3, meso-(p-SCH3)phenyl group is a clearly better electron donor to the porphyrin core than meso-(p-OCH3)phenyl one. © 2011 Elsevier B.V. All rights reserved.
Publication Date: 2010
Physical Chemistry Chemical Physics (14639084)12(43)pp. 14456-14458
Protonated naphthalene, the smallest protonated polycyclic aromatic hydrocarbon cation, absorbs in the visible, around 500 nm, which corresponds to an unusually large red shift with respect to the neutral naphthalene counterpart. © 2010 the Owner Societies.
Publication Date: 2009
Journal of Physics: Conference Series (17426588)194(2)
The first members of dipole allowed 3Po doubly excited series in helium have been observed in resonant photoexcitation of 1s2s 3Se metastable atoms. A good agreement measured relative photoionization cross sections is achieved when theory includes the radiation damping and, also important, the effects of spin-orbit multiplet splitting on electron angular distribution. © 2009 IOP Publishing Ltd.
Publication Date: 2009
Physical Chemistry Chemical Physics (14639084)11(48)pp. 11479-11486
The photofragmentation spectrum of protonated benzaldehyde has been recorded in the 435-385 nm wavelength range. The first excited state is a ππ* state, strongly red shifted compared to the ππ* state of neutral benzaldehyde. The spectrum presents well resolved vibronic bands in contrast to some other protonated aromatic molecules like benzene or tryptophan in which the excited state dynamics is so fast that no vibrational structure can be observed. The bands can be assigned on the basis of a Franck-Condon analysis using ground and excited state frequencies calculated at the CC2/TZVP level. © 2009 the Owner Societies.
Publication Date: 2009
Physical Review Letters (0031-9007)102(15)
We present spectra of triplet and singlet metastable helium atoms resonantly photoexcited to doubly excited states. The first members of three dipole-allowed Po1,3 series have been observed and their relative photoionization cross sections determined, both in the triplet (from 1s2s Se3) and singlet (from 1s2s Se1) manifolds. The intensity ratios are drastically different with respect to transitions from the ground state. When radiation damping is included the results for the singlets are in agreement with theory, while for triplets spin-orbit interaction must also be taken into account. © 2009 The American Physical Society.
Publication Date: 2009
Journal of the American Chemical Society (0002-7863)131(31)pp. 11091-11097
The excitation spectrum of the protonated benzene dimer has been recorded in the 415-600 nm wavelength range. In contrast to the neutral iso-electronic benzene dimer, its absorption spectrum extends in the visible spectral region. This huge spectral shift has been interpreted with ab initio calculations, which indicate that the first excited states should be charge transfer states. © 2009 American Chemical Society.
Publication Date: 2009
Optics Communications (00304018)282(23)pp. 4552-4555
In this work, temporal evolution of two-photon laser optogalvanic signals of neon has been studied. Optogalvanic signals for four transitions from the metastable 2p53s[3/2]2 state to 2p54d′[3/2]1, 2p54d′[3/2]2, 2p54d′[5/2]3 and 2p54d′[5/2]2 states were recorded over a range of discharge currents (3.4-9 mA). It was found that the shape of the optogalvanic signal was strongly dependent on the discharge current so that its peak shifted to shorter times and its amplitude increased with the discharge current. The decay rates of the 4d states, calculated from the optogalvanic signals, were found to increase linearly with the discharge current in the range of 6.2-9 mA. However, for the range of 3.4-5.4 mA, the decay rates were observed to slightly decrease with the discharge current. © 2009 Elsevier B.V. All rights reserved.
Publication Date: 2008
Optics Communications (00304018)281(22)pp. 5555-5560
Eleven two-photon transitions originating from the 2p53s[3/2]2, 2p53s′[1/2]o, 2p53s[3/2]1, and 2p53s′[1/2]1 states to the 2p54d configuration states have been investigated in the optogalvanic spectrum of neon in the visible region (570-626 nm) for the first time. The two-photon assignments are confirmed by evaluating the temporal evolution, power dependency, and line widths of the optogalvanic signals. The time evolution of the optogalvanic signals for the two-photon transition originating from the metastable 2p53s[3/2]2 state to the 2p54d′[3/2]2 state has also been studied at different discharge currents. © 2008 Elsevier B.V. All rights reserved.
