On the Inoperativeness of the ESIPT Process in the Emission of 1-Hydroxy-2-acetonaphthone: A Reappraisal

Insights into the Excited-State Processes in 1-Hydroxy-2-acetonaphthone at ADC(2) and CASSCF Levels

1-Hydroxy-2-acetonaphthone (HAN) has been extensively studied both experimentally and computationally to ascertain the existence of the excited-state proton transfer process. However, the process of full photocycle including the nonradiative relaxation pathways is yet to be proposed. Therefore, in the present study, we aim at providing a comprehensive picture of the excited-state processes in HAN including the proton transfer and relaxation processes through electronic structure calculations at second-order algebraic diagrammatic construction (ADC(2)) and complete active space second-order perturbation theory (CASPT2)//complete active space self-consistent field (CASSCF) and dynamics simulations at ADC(2) levels. Our studies show that the proton transfer process in the S₁ state is barrierless and produces a stable keto form, which is in accordance with previous experimental and computational studies. Adiabatic dynamics simulations at the ADC(2) level confirmed the ultrafast process with an average proton transfer time of 43 fs. The resultant keto conformer then undergoes torsional rotation, leading to a conical intersection that mediates the internal conversion process to the ground state. Our dynamics simulation predicted that this deactivation process occurs at a time scale beyond 600 fs of simulation time. We also explored nonradiative relaxation from the enol Franck-Condon region, and this process was found to be improbable from the static point of view at both the ADC(2) and CASPT2 levels of theory due to a high energy barrier along the torsional coordinate.

Structural and Energetic Insights on Two Dye Compounds: 1-Acetyl-2-Naphthol and 2-Acetyl-1-Naphthol

The energy involved in the structural switching of acyl and hydroxyl substituents in the title compounds was evaluated combining experimental and computational studies. Combustion calorimetry and Knudsen effusion techniques were used to determine the enthalpies of formation, in the crystalline state, and of sublimation, respectively. The gas-phase enthalpy of formation of both isomers was derived combining these two experimental data. Concerning the computational study, the G3(MP2)//B3LYP composite method was used to optimize and determine the energy of the isomers in the gaseous state. From a set of hypothetical reactions it has been possible to estimate the gas-phase enthalpy of formation of the title compounds. The good agreement between the experimental and computational gas-phase enthalpies of formation of the 1-acetyl-2-naphthol and 2-acetyl-1-naphthol isomers, provided the confidence for extending the computational study to the 2-acetyl-3-naphthol isomer. The structural rearrangement of the substituents in position 1 and 2 in the naphthalene ring and the energy of the intramolecular hydrogen bond are the factors responsible for the energetic differences exhibited by the isomers. The gas phase tautomeric keto ↔ enol equilibria of the o-acetylnaphthol isomers were analyzed using the Boltzmann’s distribution.

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.

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].

Excited-state intramolecular proton transfer reaction modulated by low-frequency vibrations: an effect of an electron-donating substituent on the dually fluorescent bis-benzoxazole

Excited-state intramolecular proton transfer (ESIPT) reaction has been studied in a molecule showing dual fluorescence, the 2,5-bis(2-benzoxazolyl)-4-methoxyphenol (BBMP), and its isotopomers, where the methoxy, and alternatively, the OH group has been deuterated. Attention is focused on the influence of electron donating OCH(3) substituent on fast excited state reaction. Comparison between the resonance-enhanced multiphoton ionization spectrum and the laser-induced excitation of the primary and phototautomeric emissions has been done. The geometry, electron density distribution, vibrational structure as well as the potential energy profiles in the S(0) and S(1) states of four possible rotameric forms of BBMP were calculated with application of the density functional theory (DFT). It allowed identifying the most probable conformer and assessing the role of low-frequency motions for the ESIPT efficiency.

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: part 2. 2-Hydroxy-1-naphthaldehyde

The S(1)← S(0) fluorescence excitation spectrum of jet-cooled 2-hydroxy-1-naphthaldehyde (2H1N) with origin at 26,668 cm(-1) has been measured. Nine totally symmetric modes and three non-totally symmetric modes have been assigned in the excitation spectrum. Ab initio calculations indicate that 2H1N undergoes a planarity change upon excitation, which may account for the unusual intensity of non totally symmetric vibrational modes in the excitation spectrum. A number of low intensity features were observed on the low energy side of the origin which have been assigned to the 2H1N dimer rather than different ground state confomers of 2H1N. The origin of the S(1)← S(0) electronic transition of the dimer lies at ~26,401 cm(-1); combinations of two low frequency intermolecular modes of the dimer (59 cm(-1) and 17 cm(-1)) were also observed. The occurrence of excited state intramolecular proton transfer (ESIPT) in 2H1N cannot be proven on the basis of this work. A comparison of the (photo)physical properties of 2H1N with 1-hydroxy-2-naphthaldehyde (1H2N) [A. McCarthy and A.A. Ruth, PCCP, 2011, 13, 7485-7499 (Part 1)], however, indicate the plausibility of an ESIPT process in 2H1N. The strength of the intramolecular hydrogen bond (IMHB) in 2H1N was computed as ~10.6 kcal/mol, a value comparable to the IMHB strength of 1H2N. The establishment of a lower limit on the state lifetimes of 2H1N, of ~1.8 ps, indicates that any proposed ESIPT reaction in 2H1N may not proceed barrierlessly. Above an excess energy of ~1000 cm(-1), the intensity of the fluorescence excitation spectrum reduces significantly, indicating the onset of a non-radiative decay mechanism.

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength, aromaticity, and excited-state intramolecular proton transfer reaction

The inequivalence of substitution pair positions of naphthalene ring has been investigated by a theoretical measurement of hydrogen bond strength, aromaticity, and excited state intramolecular proton transfer (ESIPT) reaction as the tools in three substituted naphthalene compounds viz 1-hydroxy-2-naphthaldehyde (HN12), 2-hydroxy-1-naphthaldehyde (HN21), and 2-hydroxy-3-naphthaldehyde (HN23). The difference in intramolecular hydrogen bond (IMHB) strength clearly reflects the inequivalence of substitution pairs where the calculated IMHB strength is found to be greater for HN12 and HN21 than HN23. The H-bonding interactions have been explored by calculation of electron density ρ(r) and Laplacian ∇(2) ρ(r) at the bond critical point using atoms in molecule method and by calculation of interaction between σ* of OH with lone pair of carbonyl oxygen atom using NBO analysis. The ground and excited state potential energy surfaces (PESs) for the proton transfer reaction at HF (6-31G**) and DFT (B3LYP/6-31G**) levels are similar for HN12, HN21 and different for HN23. The computed aromaticity of the two rings of naphthalene moiety at B3LYP/6-31G** method also predicts similarity between HN12 and HN21, but different for HN23.

A DFT-based theoretical study on the photophysics of 4-hydroxyacridine: single-water-mediated excited state proton transfer

Study of intra- and intermolecular hydrogen-bonding interaction and excited state proton transfer reaction has been carried out in 4-hydroxyacridine (4-HA) and its hydrated clusters theoretically. Density functional theory [B3LYP/6-311++G(d,p)] has been exploited to calculate structural parameters and relative energies of different conformers of 4-HA and its hydrates. The substantial impact of solvent reaction field on hydrogen-bond energies, conformational equilibrium, and tautomerization reaction in aqueous medium have been realized by employing Onsager and PCM reaction field methods, and the stability of the conformers of 4-HA is found to be profusely modulated by the electrostatic influence of the solvent. A deeper insight into the nature of H-bonding in 4-HA and its hydrated clusters has been achieved under the provision of natural bond orbital and atoms in molecule analysis. Elucidation of potential energy curves for proton transfer reaction reveals that an intrinsic and two-water-molecule-assisted proton transfer (PT) reaction in 4-HA is hindered by high energy barrier in the S(1) surface, whereas single-water-assisted PT reaction is practically rendered barrierless. At the same time, the appreciably high barrier height of the ground state potential energy curve in all the cases unambiguously rules out the possibility of ground state proton transfer reaction.