Solvation effect on the excited-state intramolecular proton transfer mechanism of 1-morpholinylmethyl-2-naphthol

Study on ESIPT of salicylaldehyde derivative EQCN in DCM solvent

The fluorescence of salicylaldehyde derivative (EQCN) as an excitation-wavelength-dependent molecule with long-persistent luminescence has been investigated experimentally and theoretically. However, the excited-state intramolecular proton transfer (ESIPT) process mechanism and optical property associated with photochemical process of EQCN molecule in dichloromethane (DCM) solvent has not been discussed in detail. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to investigate ESIPT process of EQCN molecule in DCM solvent. By optimizing the geometry of the EQCN molecule, the hydrogen bond interaction of EQCN molecule of Enol structure in excited state (S₁ state) is strengthened. The calculated absorption peak and fluorescence peak agree well with the experimental values. Based on the optimized geometric structure, the frontier molecular orbital isosurface (FMOs) were drawn, and the redistribution of electron density in DCM solvent was depicted, which intuitively explain the changes in the photophysical properties of EQCN. Through the calculated potential energy curves (PECs) of EQCN in both DCM solvent and ethanol solvent, the ESIPT process of EQCN was found more likely to occur in ethanol solvents.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Low-Lying Excited States of hqxcH and Zn-hqxc Complex: Toward Understanding Intramolecular Proton Transfer Emission

Excited state intramolecular proton transfer (ESIPT) has been a topic of interest due to its potential to lead to multiple emissions. Although many organic molecules showing ESIPT emission are already known, studies on metal complexes showing ESIPT and their related theoretical understandings are very limited. In this study, we focus on [Zn(hqxc)₂(DMSO)₂] (Zn-hqxc: hqxc = 3-hydroxy-2-quinoxalinecarboxylate, DMSO = dimethyl sulfoxide), which shows ESIPT emission in the solid state, even though the hqxcH ligand does not show ESIPT emission. To gain insights into the role of the zinc atom and the emission mechanisms, we examined excited states of free hqxcH and the Zn-hqxc complex using time-dependent density functional theory calculations. From the results, it was shown that the zinc atom triggers a structural change of the hqxcH ligand from the lactam form (3,4-dihydro-3-oxo-2-quinoxalinecarboxylic acid) to the enol form (3-hydroxy-2-quinoxalinecarboxylic acid), where the latter form has several stable excited states. Several stable geometries were found for singlet and triplet excited states, suggesting that emissions for the Zn-hqxc complex can be both phosphorescence and fluorescence caused by the enol-enol, keto-keto, and keto-enol forms of the two hqcx ligands in the complex. We found that the photophysical properties of the Zn-hqxc complex are dominated by the ligand due to the filled d¹⁰ of Zn(II). The presented results suggest that, to design new ESIPT metal complexes, one possible approach is to combine a metal atom showing ligand centered emission and a ligand that has separate ESIPT and coordination sites.

TDDFT study on excited state intramolecular proton transfer mechanism in 2-amino-3-(2'-benzazolyl)-quinolines

The intramolecular proton transfer reaction of the 2-amino-3-(2'-benzoxazolyl)-quinoline (ABO) and 2-amino-3-(2'-benzothiazolyl)-quinoline (ABT) molecules in both S₀ and S₁ states at B3LYP/6-311++G(d,p) level in ethanol solvent have been studied to reveal the deactivation mechanism of the tautomers of the two molecules from the S₁ state to the S₀ state. The results show that the tautomers of ABO and ABT molecules may return to the S₀ state by emitting fluorescence. In addition, the bond lengths, angles and infrared spectra are analyzed to confirm the hydrogen bonds strengthened upon photoexcitation, which can facilitate the proton transfer process. The frontier molecular orbitals (MOs) and natural bond orbital (NBO) are also calculated to indicate the intramolecular charge transfer which can be used to explore the tendency of ESIPT reaction. The potential energy surfaces of the ABO and ABT molecules in the S₀ and S₁ states have been constructed. According to the energy potential barrier of 9.12kcal/mol for ABO molecule and 5.96kcal/mol for ABT molecule, it can be indicated that the proton transfer may occur in the S₁ state.