Solvent-polarity-dependent conformation and ESIPT behaviors for 2-(benzimidazol-2-yl)-3-hydroxychromone: a novel dynamical mechanism

In view of the potential importance of 3-hydroxychromone derivatives, in this work, we mainly focused on exploring the solvent-polarity-dependent photoinduced behaviors of 2-(benzimidazol-2-yl)-3-hydroxychromone (BI3HC). Distinguished from previous work, we first checked the coexisting three conformations (i.e., BI3HC-A, BI3HC-B, and BI3HC-C) and the presence of a coexistence mechanism between BI3HC-A and BI3HC-C. Most importantly, BI3HC-A is the main component in non-polar solvents, while BI3HC-C is the main one in polar solvents. Through combined analysis of the infrared (IR) vibrational spectra and geometrical variations as well as predicted hydrogen bonding energy via the bond critical point (BCP) between S0 and S1 states, we present a hydrogen bonding strengthening phenomenon that facilitates the excited-state intramolecular proton-transfer (ESIPT) behavior for BI3HC-A and BI3HC-C. To qualitatively investigate photoinduced behaviors based on frontier molecular orbitals (MOs), we found that photoinduced intramolecular charge transfer (ICT) and charge redistribution promoted an ESIPT tendency. By comparing the barriers of the potential energy curves (PECs) between twisting dihedral angles and ESIPT paths, we could rule out mutual transformations in the S1 state and also propose a solvent-polarity-regulated ESIPT behavior for BI3HC-A and BI3HC-C. Furthermore, via searching the transition state (TS) and performing intrinsic reaction coordinate (IRC) simulations, we further checked the ESIPT mechanism. Through gaining insights in spectral variations in solvents, we uncovered the solvent-polarity-dependent spectral behaviors. We sincerely hope this work will not only help clarify the solvent-polarity-regulated dynamical behaviors of BI3HC but also pave the way for further explorations and applications of other 3-hydroxychromone derivatives.