Photophysics of intramolecularly hydrogen-bonded aromatic systems: ab initio exploration of the excited-state deactivation mechanisms of salicylic acid

The investigation of excited state proton transfer mechanism in water-bridged 7-azaindole

Based on the time-dependent density functional theory (TDDFT), the excited-state intermolecular proton transfer (ESIPT) mechanism of water-bridged 7-azaindole has been investigated theoretically. The calculations of primary bond lengths and the IR vibrational spectra between the S0 state and the S1 state that verified the intramolecular hydrogen bond were strengthened. The fact that reproduced experimental absorbance and fluorescence emission spectra well theoretically demonstrate that the TDDFT theory we adopted is reasonable and effective. In addition, intramolecular charge transfer based on the frontier molecular orbitals demonstrated the indication of the ESIPT reaction. The constructed potential energy curves of ground state and the first excited state based on keeping the H2···O3 and H6···N7 distances fixed at a series of values have been used to illustrate the ESIPT process. A relative lower barrier of 5.94 kcal/mol in the S1 state potential energy curve for type II (lower than that of 9.82 kcal/mol in the S1 state for type I) demonstrates that type II ESIPT process occurs firstly in 7Al-2H2O complex.

Features of partial encapsulation of an ESIPT probe 3-hydroxy-2-naphthoic acid (3HNA) in the nano cavities of β- and γ-cyclodextrin: comparative study with sequestered 3HNA in micelles and reverse micelle

Encapsulation of an excited state intramolecular proton transfer probe within the nanocavity of different confined media.

Mechanistic insights into excited state intramolecular proton transfer in isolated and metal chelated supramolecular chemosensors

Mechanistic studies of the excited state intramolecular proton transfer in a series of related and progressively more complex supramolecular chromophores.

A critical approach toward resonance-assistance in the intramolecular hydrogen bond interaction of 3,5-diiodosalicylic acid: a spectroscopic and computational investigation

The photophysics of a prospective drug molecule, 3,5-diiodosalicylic acid (3,5-DISA), having a wide spectrum of biological and medicinal applications, have been investigated using spectroscopic techniques and computational analyses. The remarkably large Stokes' shifts in various solvents from 3,5-DISA has been intertwined with the occurrence of an excited-state intramolecular proton transfer (ESIPT) reaction. Concurrently, the emergence of an intriguing dual emission feature in less interacting solvents is also reported and the spectral response of 3,5-DISA toward the variation of medium acidity/basicity has been exploited to decipher the nature of various species present in different solvents. Our experimental results, unveiling the occurrence of an ESIPT reaction in 3,5-DISA, have been aptly substantiated from computational studies in which the operation of ESIPT has been explored from structural as well as energetics (analysis of potential energy surface (PES)) perspectives. A major focus of the present study is on the evaluation of the intramolecular hydrogen bond (IMHB) interaction in 3,5-DISA, including the application of various methodologies to estimate the IMHB energy and subsequently, an in-depth analysis of the IMHB interaction reveals its partially covalent nature through the application of advanced quantum chemical tools, e.g., the natural bond orbital (NBO) method. In this context, the interplay between the aromaticity of the benzene nucleus and the IMHB energy has been rigorously explored, showing indications for the occurrence of resonance-assisted hydrogen bonding (RAHB) in 3,5-DISA. To this end, the geometric as well as magnetic criteria of aromaticity have been analyzed.

A theoretical exploration on electronically excited states of protonated furan and thiophene

MPE profiles of the ground and S1 excited states of protonated furan along C–O bond-stretching reaction coordinate, computed at the RI-CC2/aug-cc-pVDZ level of theory.

Hydrogen bond effect on the photophysical properties of 2-ureido-4[1H]-pyrimidinone quadruple hydrogen bonded systems

Two different hydrogen bond effects on the photophysical properties of UPy systems in different solutions have been studied.

Excited-state deactivation mechanisms of protonated and neutral phenylalanine: a theoretical study

Minimum energy paths (MEPs) of protonated phenylalanine (PheH⁺) at the electronic ground and S₁ (¹ππ*) excited states along the C_α–C_β bond stretching coordinate, following proton transfer to the aromatic chromophore.

A theoretical exploration of the nonradiative deactivation of hydrogen-bond complexes: isoindole–pyridine and quinoline–pyrrole

CC2 potential energy profiles of the ground and excited states of the isoindole–pyridine complex along the proton transfer reaction coordinate are studied.

Photophysics and photochemistry of cis- and trans-hydroquinone, catechol and their ammonia clusters: a theoretical study

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 (1)πσ*-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).

ESIPT inspired dual fluorescent probe (Z)-3-((4-(4-aminobenzyl) phenyl) amino)-1,3-diphenylprop-2-en-1-one: experimental and DFT based approach to photophysical properties

A fluorescent probe (Z)-3-((4-(4-aminobenzyl) phenyl) amino)-1,3-diphenylprop-2-en-1-one (L) was synthesized and characterized by IR, (1)H NMR, ESI mass, UV-visible and fluorescence spectroscopy and by single crystal X-ray diffraction. The molecule has a stable helical structure due to intermolecular CH π interaction. The thermal stability of L was studied by TG analysis. The electronic structure calculations of L have been carried out using DFT at B3LYP/6-31G (d,p) level. The vibrational frequencies and (1)H NMR spectra were computed at this level and compared with experimental values. Major orbital contributions for the electronic transitions were assigned with the help of time-dependent density functional theory (TD-DFT). The observed electronic absorption spectra of L in different solvents coincide with the computed spectra in keto form. The dual emission and high Stokes shift values support the excited state intramolecular proton transfer (ESIPT) process. The molecular docking has been employed to get information about the interaction of L with DNA [6BNA].

Ab initio study of potential ultrafast internal conversion routes in oxybenzone, caffeic acid, and ferulic acid: implications for sunscreens

Oxybenzone (OB) and ferulic acid (FA) both find use in commercial sunscreens; caffeic acid (CA) differs from FA by virtue of an -OH group in place of a -OCH3 group on the aromatic ring. We report the results of ab initio calculations designed to explore the excited state nonradiative relaxation pathways that provide photostability to these molecules and the photoprotection they offer toward UV-A and UV-B radiation. In the case of OB, internal conversion (IC) is deduced to occur on ultrafast time scales, via a barrierless electron-driven H atom transfer pathway from the S1(1(1)nπ*) state to a conical intersection (CI) with the ground (S0) state potential energy surface (PES). The situation with respect to CA and FA is somewhat less clear-cut, with low energy CIs identified by linking excited states to the S0 state following photoexcitation and subsequent evolution along (i) a ring centered out-of-plane deformation coordinate, (ii) the E/Z isomerism coordinate and, in the case of CA, (iii) an O-H stretch coordinate. Analogy with catechol suggests that the last of these processes (if active) would lead to radical formation (and thus potential phototoxicity), encouraging a suggestion that FA might be superior to CA as a sunscreen ingredient.

Excited state deactivation pathways of neutral/protonated anisole and p-fluoroanisole: a theoretical study

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 (1)πσ* 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.

Photophysical and excited-state intramolecular proton transfer of 2-(1-(3,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenol: DFT analysis

Fluorescent phenenthrimidazole derivatives have been synthesized and characterized by (1)H, (13)C NMR and mass spectral analyses. Synthesized compounds have been. The solvent effect on the absorption and fluorescence bands has been analyzed and supplement by computational studies. Phenenthrimidazole containing hydroxy group shows a single prominent absorption and emission in polar solvents and dual emission in non-polar solvents due to excited state intramolecular proton transfer (ESIPT). The ESIPT of 2-(1-(3,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenol was cross validated by the analysis of optimized geometry parameters, potential energy surface (PES), mulliken's charge distribution on the heavy atoms and molecular orbitals of its tautomers. Nonviability of ground state electron transfer was explained by HOMO-LUMO analysis. The intramolecular hydrogen bonding (IMHB) interaction has been explored by the calculation of electron density ρ(r) and the Laplacian Δ(2)ρ(r) at the bond critical point (BCP) using atoms-in-molecule (AIM) method and by calculation of hyper conjugative interaction between N17 lone pair and σ*(O53⋯H54) using natural bond orbital (NBO) analysis.

Excited-state intramolecular proton transfer: photoswitching in salicylidene methylamine derivatives

The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012, 13, 4287-4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π-electron-donating and π-electron-withdrawing substituents results in an electron-density push-and-pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization.

Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: a theoretical study

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 (1)πσ∗ 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 (1)πσ∗ predissociates the bound S1((1)ππ∗) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the (1)ππ∗ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the (1)πσ∗ state still remains dissociative but the conical intersection between (1)πσ∗ and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1-S0 electronic transition of monomer, dimer, and microhydrated 2-pyridone.

Time-dependent density functional theory assessment of UV absorption of benzoic acid derivatives

Benzoic acid (BA) derivatives of environmental relevance exhibit various photophysical and photochemical characteristics. Here, time-dependent density functional theory (TDDFT) is used to calculate photoexcitations of eight selected BAs and the results are compared with UV spectra determined experimentally. High-level gas-phase EOM-CCSD calculations and experimental aqueous-phase spectra were used as the references for the gas-phase and aqueous-phase TDDFT results, respectively. A cluster-continuum model was used in the aqueous-phase calculations. Among the 15 exchange-correlation (XC) functionals assessed, five functionals, including the meta-GGA hybrid M06-2X, double hybrid B2PLYPD, and range-separated functionals CAM-B3LYP, ωB97XD, and LC-ωPBE, were found to be in excellent agreement with the EOM-CCSD gas-phase calculations. These functionals furnished excitation energies consistent with the pH dependence of the experimental spectra with a standard deviation (STDEV) of ∼0.20 eV. A molecular orbital analysis revealed a πσ* feature of the low-lying transitions of the BAs. The CAM-B3LYP functional showed the best overall performance and therefore shows promise for TDDFT calculations of processes involving photoexcitations of benzoic acid derivatives.

Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.

Exploring single and double proton transfer processes in the gas phase: a high resolution electronic spectroscopy study of 5-fluorosalicylic acid

Two species that possess different absorption and emission properties have been observed in the low resolution fluorescence excitation spectrum of 5-fluorosalicylic acid (FSA) in the gas phase. The two species were identified as monomer and dimer species using high resolution techniques. Studies of these spectra in the presence of an applied electric field, together with ab initio quantum chemistry calculations, show that the monomer is a "closed" form of FSA exhibiting an intramolecular C = O⋅⋅⋅H-O-C hydrogen bond in the ground state. Absorption of light at ∼344 nm transforms this species into the tautomeric form C-O-H⋅⋅⋅O = C via a barrierless proton transfer process. The large charge rearrangement that accompanies this process results in a significantly red-shifted emission spectrum. The (FSA)(2) dimer exhibits two intermolecular C=O⋯H-O-C hydrogen bonds but in this case the double proton transfer leads to a conical intersection with the ground state and rapid nonradiative decay. The onset of this process and the time scale on which it occurs are revealed by a homogeneous broadening of the dimer's high resolution spectrum.