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[Display omitted] • The enhanced hydrogen bonding provides the impetus to promote the ESIPT reaction. • Photo-induced increased electronic densities around O 14 atom plays vital roles in attracting hydroxyl proton. • Atomic electronegativity could be more favorable for the ESIPT behavior of 4M3HF system. In view of the potential applications via regulating excited state behaviors, in this work, excited state dynamics of 4-methoxy-3-hydroxyflavone (4M3HF) with different atomic electronegativities (i.e., 4M3HF-O, 4M3HF-S and 4M3HF-Se) are explored theoretically. Dominating chemical parameters, infrared (IR) spectra, and bond critical point (BCP) analyses reveal hydrogen bond could be enhanced in S 1 state. Via light-induced excitation, by comparing the energy gaps of frontier molecular orbitals (MOs) and charge recombination, we find low atomic electronegativity promotes ESIPT reaction. Insights into potential energy curves (PECs) and transition state (TS) forms, we present low atomic electronegativity facilitates ESIPT behavior for 4M3HF system. [ABSTRACT FROM AUTHOR]

Abstract A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ 2)ρ, we firstly verify the double intramolecular hydrogen bonds (O1 H2⋯N3 and O4 H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S 0 -state and S 1 -state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future. Graphical Abstract Unlabelled Image Highlights • Hydrogen bond of HMPM should be strengthened in the first excited state. • Charge transfer facilitates the ESIPT process for HMPM molecule. • Only the excited state single proton transfer process occurs for HMPM molecule. [ABSTRACT FROM AUTHOR]

In this present work, we theoretically investigate the excited state dynamical mechanism of a novel sensor trans-2-[4′-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine (t-DMASIP-b). Within the framework of density functional theory (DFT) and time dependent DFT (TDDFT) methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, two methanol (MeOH) solvent molecules should be connected with t-DMASIP-b forming t-DMASIP-b-MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O5-H6 ⋯ N7 one should be largely strengthened in the S 1 state, which might facilitate the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared (IR) vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals (MOs) based on the photo-excitation, we do find tendency of ESIPT reaction due to the most charges located around N7 atom in the LUMO orbital. Even though some indications reveal the ESIPT, we find the moderate potential energy barriers in the S 1 -state potential energy curves. The moderate potential energy barrier 12.89 kcal/mol along with O5-H6 ⋯ N7 hydrogen bonding wire indeed hinders the proceeding of ESIPT, so only one fluorescence peak was reported in previous experiment, which has been reasonably explained in our work. As a whole, we deem our work not only successfully explains previous experimental work, but also paves the way for the further applications about t-DMASIP-b sensor in future. [ABSTRACT FROM AUTHOR]