Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states

Substituent Effects on Excited-State Intramolecular Proton Transfer Reaction of 2-Aryloxazoline Derivatives

Different substituents and benzene ring numbers had significant effects on the fluorescence phenomenon of 2-aryloxazoline derivatives as observed in an experiment. Here, we select five 2-aryloxazoline derivatives with different substituents and benzene ring numbers (2u, 2ad, 2af, 2ai, and 2ah) to analyze the effects on the fluorescence phenomena. For 2ad, 2ah, and 2ai, first, the geometric structures are optimized based on the density functional theory and time-dependent density functional theory methods. The analysis of the obtained bond parameters reveals the variation of hydrogen bond interactions from S₀ to S₁ states. Second, the calculated absorption and emission spectra are consistent with the experimental values, which proves that the theoretical method is feasible. Finally, through the analysis of the infrared vibrational spectrum, reduced density gradient isosurfaces, frontier molecular orbitals, and potential energy curves, the strengthening mechanism of the hydrogen bond interaction and the ability of the excited-state intramolecular proton transfer (ESIPT) reaction to occur are further explained. Since the proton transfer reactions of 2u and 2af occur spontaneously under photoexcitation, they have no stable structures in the S₁ state. In conclusion, due to the different substituents, 2u is more prone to the proton transfer reaction than 2ad. For 2af, 2ai, and 2ah with different benzene ring numbers, the ESIPT reaction is more difficult to occur as the number of benzene rings increases. The ability of the ESIPT reaction to occur follows the order 2af → 2ah → 2ai. For 2-aryloxazoline derivatives with different substituents or different benzene ring numbers, the hydrogen bond strengthening mechanism has been authenticated, which promotes the occurrence of the ESIPT reactions.

The theoretical study of excited-state intramolecular proton transfer of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

The symmetrical structures 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBTD) can take shape two intramolecular hydrogen bonds in chloroform. In order to research the molecular dynamic behavior of BBTD upon photo-induced process, we utilize density functional theory (DFT) and time-dependent density functional theory (TDDFT) to complete theoretical calculation. Through the comparison of bond length, bond angle, IR spectra, and frontier molecular orbitals between ground state (S₀) and first excited state (S₁), it clearly indicates that photoexcitation have slightly influence for intensity of hydrogen bond. For the sake of understanding the mechanism of excited state intramolecular proton transfer (ESIPT) of BBTD in chloroform, potential energy surfaces have been scanned along with the orientation of O₁-H₂ and O₄-H₅ in S₀ and S₁ state, respectively. A intrigued hydrogen bond dynamic phenomenon has been found that ESIPT of BBTD is not a synergetic double proton transfer process, but a stepwise single proton transfer process BBTD→BBTD-S→BBTD-D. Moreover, the proton transfer process of BBTD-S→BBTD-D is easier to occur than that of BBTD→BBTD-S in S₁ state.

10-Hydroxybenzo[h]quinoline: switching between single- and double-well proton transfer through structural modifications

Proton transfer in HBQ and modified compounds was investigated experimentally (steady state absorption and emission spectroscopy, NMR and chemometrics) and theoretically (DFT and TD-DFT M06-2X/TZVP calculations) in ground and excited singlet state.

THEORETICAL INVESTIGATION OF SUBSTITUTION EFFECT ON THE PROTON TRANSFER MECHANISM IN 3-MERCAPTO-PROPENETHIAL

We present detailed theoretical studies of the proton transfer (PT) in the 3-Mercapto-propenethial (MP) and in some of its derivatives. The effect of substitution on the transition state structures corresponding to the PT in R2 or R1(3) positions is studied by using B3LYP/6-311++G** level of theory in gas phase and water solution. The following substituents have been taken into consideration: Cl, F, OH, SH, OCH3, SCH3, CHO, NO2, CCH and OCF3 . For the different derivatives of 3-MP, we have computed geometries and potential energy curves for the intramolecular PT in their ground. Also, the excited-state properties of intramolecular hydrogen bonding in substituted systems have been investigated theoretically using the time-dependent density functional theory method. The π-electron delocalization parameter (Q) and HOMA index as a geometrical indicator of a local aromaticity are investigated. Natural bond orbital (NBO) analysis was also performed for better understanding the nature of intramolecular interactions. The results of analysis by Quantum theory of "Atoms in Molecules" and NBO method fairly support the DFT results. Correlations between the PT reaction barrier versus topological parameters, ν(S–H) and γ(S–H) are also probed. The obtained results showed a strong influence of the R substituent on the PT reaction barrier changes. Numerous correlations between topological, geometrical, thermodynamic properties and energetic parameters were also found.

Computational study of the intramolecular proton transfer reactions of 3-hydroxytropolone (2,7-dihydroxycyclohepta-2,4,6-trien-1-one) and its dimers

The proton transfer reaction and dimerization processes of 3-hydroxytropolone (3-OHTRN) have been investigated using density functional theory (DFT) at the B3LYP/6-31+G** level. The influence of the solvent on the proton transfer reaction of 3-OHTRN was examined using the self-consistent isodensity polarized continuum model (SCI-PCM) with different dielectric constants (ε = 4.9, CHCI₃; ε = 32.63, CH₃OH; ε = 78.39, H₂O). The intramolecular proton transfer reaction occurs more readily in the gas phase than in solution. Results also show that the stability of 3-OHTRN dimers in the gas phase is directly affected by the hydrogen bond length in the dimer structure.

Solvatochromism and preferential solvation of 1,4-dihydroxy-2,3-dimethyl-9,10-anthraquinone by UV-vis absorption and laser-induced fluorescence measurements

Solvation characteristics of 1,4-dihydroxy-2,3-dimethyl-9,10-anthraquinone (1) in pure and binary solvent mixtures have been studied by UV-vis absorption spectroscopy and laser-induced fluorescence techniques. The binary solvent mixtures used as CCl(4) (tetrachloromethane)-DMF (N,N-dimethylformamide), AN (acetonitrile)-DMSO (dimethylsulfoxide), CHCl(3) (chloroform)-DMSO, CHCl(3)-MeOH (methanol), and MeOH-DMSO. The longest wavelength band of 1 has been studied in pure solvents as well as in binary solvent mixtures as a function of the bulk mole fraction. The Vis absorption band maxima show an unusual blue shift with increasing solvent polarity. The emission maxima of 1 show changes with varying the pure solvents and the composition in the case of binary solvent mixtures. Non-ideal solvation characteristics are observed in all binary solvent mixtures. It has been observed that the quantity [nu (12)-(X(1)nu (1)+X(2)nu (2))] serves as a measure of the extent of preferential solvation, where nu and X are the position of band maximum in wavenumbers (cm(-1)) and the bulk mole fraction values, respectively. The preferential solvation parameters local mole fraction (X(2)(L)), solvation index (delta(s2)), and exchange constant (k(12)) are evaluated.