Theoretical investigations on the excited state hydrogen bonding dynamics and proton transfer behavior for novel DHDA-23-00 system

Stepwise Excited-State Intramolecular Double Proton Transfer in 1,8-Dihydroxynaphthalene-2,7-dicarbaldehyde

We theoretically investigate the salient features of stepwise excited-state intramolecular double proton transfer (ESIDPT) in 1,8-dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA). Surface trajectory simulations using the TD-B3LYP/6-31G(d) level of theory reveal that the proton transfer primarily happens via S1, wherein about ∼42% of trajectories (40 out of 95) show the single proton transfer alone and another ∼32% (30 out of 95) show double proton transfer. The average time scale for the single proton transfer originating from those ∼42% trajectories is ∼147 fs. In the case of double proton transfer, the average time for the first step, i.e., single proton transfer, is ∼54 fs, and the subsequent step, i.e., double proton transfer, completes in ∼151 fs. All three tautomers, i.e., normal, single, and double proton-transferred tautomers, possess a stable minimum in their first singlet excited state. This state has a ππ* character in the former two tautomers, resulting in a dual fluorescence emission phenomenon upon photoexcitation of DHDA.

ESIPT mechanism of triple emission with hydroxy-oxadiazole compound in DMSO: A theoretical reconsideration

The compound in solvents with triple fluorescence feature of excited state intramolecular proton transfer (ESIPT) has a broad prospect in fluorescent probes, dye sensors and molecular synthesis of photosensitive dyes. An ESIPT molecule hydroxy-bis-2,5-disubstituted-1,3,4-oxadiazoles (compound 1a) emits two fluorescence peaks in dichloromethane (DCM) and three fluorescence peaks in dimethyl sulfoxide (DMSO). [Dyes and Pigments 197 (2022) 109927]. Two longer peaks were attributed to enol and keto emission in both solvents and the shortest third peak in DMSO was just attributed simply. However, there is a significant difference in proton affinity between DCM and DMSO solvents which has influence on the position of emission peaks. Therefore, the correctness of this conclusion needs to be further verified. In this research, density functional theory and time-dependent density functional theory method are used to explore ESIPT process. Optimized structures indicate ESIPT occurs through molecular bridge assisted by DMSO. The calculated fluorescence spectra demonstrate two peaks indeed originated from enol and keto in DCM, while interestingly three peaks are originated from enol, keto and intermediate in DMSO. Infrared spectrum, electrostatic potential and potential energy curves further prove existence of three structures. We reveal the mechanisms that compound 1a molecule occurs ESIPT in DCM solvent and undergoes an ESIPT through assisted by DMSO molecular bridge. Additionally, three fluorescence peaks in DMSO are reattributed. Our work is expected to provide an insight for understanding intra- and intermolecular interactions and synthesis of efficient organic lighting-emitting molecule.

Unveiling the nonadiabatic photoisomerization mechanism of hemicyanines for UV photoprotection

In this work, the nonadiabatic energy relaxation mechanism of hemicyanines for UV photoprotection were investigated by using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) method for the first time. The absorption spectra and potential energy surfaces (PESs) of four hemicyanines with different positions of substituents were presented. The maximum absorption peaks of the four hemicyanines are located in the UVA region. In addition, all these hemicyanine molecules also have light absorption in both the UVB and UVC regions. At the same time, we found that the trans-cis photoisomerization PESs of all these hemicyanines have a significant conical intersection (CI) point between the first excited state and the ground state. Herein, it was first demonstrated that the UV energy absorbed by the hemicyanines could be dissipated nonadiabatically through the CI point by using the trans-cis photoisomerization dynamics mechanism. This work proves that hemicyanines have the possibility to be applied for UV photoabsorbers, and provides important basis for designing new type of hemicyanines for UV photoprotection.