Theoretical regulation of ESIPT behavior by varying the π-expansion of proton acceptor for substituted hydroxyl fluorenes

Theoretical investigation of dual hydrogen-bonding interactions and ESIDPT mechanism associated with halogen substituted 2,5-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,4-diol derivatives

CONTEXT

In this work, using the TDDFT method, we mainly focus on exploring the photo-induced excited state dual hydrogen-bonding interactions for halogen element substituted 2,5-bis(4,5-diphenyl-1H-imidazol-2-yl)benzene-1,4-diol (BDIBD) derivatives (i.e., BDIBD-FF, BDIBD-FCl and BDIBD-FBr). Analyses of chemical bond properties, bond lengths and bond angles, infrared (IR) spectral shifts, as well as the simulated core-valence bifurcation (CVB) parameters, we find upon excitation dual hydrogen bonds of BDIBD-FF, BDIBD-FCl, and BDIBD-FBr are strengthened in the S1 state. Variations about photo-induced charge further reveal the excited state intramolecular double proton transfer (ESIDPT) tendency. By constructing potential energy surfaces (PESs), we not only clarify the stepwise ESIDPT mechanism for BDIBD derivatives, but also present the halogen element regulated ESIDPT behaviors.

METHODS

Based on Gaussian 16 program, all BDIBD derivatives were optimized using DFT and TDDFT methods with D3-B3LYP and TZVP theoretical levels. By comparing geometries variations and exploring core-valence bifurcation indexes for predicting hydrogen-bonding strength using Multiwfn 3.8, dual hydrogen-bonding interactions were analyzed. Potential energy surfaces with transition state forms were explored to reveal the ESIDPT mechanism.

π-Conjugation effects on excited-state intermolecular proton-transfer reactions of anthracene-urea derivatives in the presence of acetate anions

This study investigated emissive urea compounds with an anthryl moiety on one side and a substituent group (biphenyl, naphthyl, benzyl, or cyclohexyl) on the other side across from the urea group. This was performed to determine the contribution of π-conjugation on a substituent group to excited-state intermolecular proton-transfer (ESPT) reactions in the presence of acetate anions. Fluorescence lifetime measurements revealed that the rate constant of the ESPT reaction from the normal form to the tautomer form increased with the length of the π-conjugation. Considering that there were a few differences among the wavelengths of the fluorescence maxima for the anthracene-urea derivatives in the presence of acetate anions, we observed that the extension of π-conjugation promoted tautomer formation. This maintained the energy levels of the normal and tautomer forms in the excited state. Furthermore, an anthracene-urea derivative without π-conjugation did not undergo a reverse ESPT reaction, implying that π-conjugation is considerably involved in the reverse ESPT reaction from the tautomer form to the normal form.