Theoretical exploration on excited state behaviors for 2-(1H-benzo[d]imidazole-2-yl)-6-(benzo[d]-thiazol-2-yl)-4-bromophenol in nonpolar and polar solvents

Given the potential significance of novel organic fluorophores with potential dual-way proton transfer (PT) reaction channels, in this work we mainly focus on probing into the excited state behaviors for the novel 2-(1H-benzo[d]imidazole-2-yl)-6-(benzo[d]-thiazol-2-yl)-4-bromophenol (BIBTB) fluorophore. Given the two-way PT channels, we firstly check the stable form with hydroxyl points to imidazole and thiazole rings in solvents with different polarities. By twisting dihedral angles (θ1, θ2 and θ3) in the S₀ state, we theoretically verify that the A form (hydroxyl points to thiazole ring) should be the most important part in nonpolar and moderately polar solvents, while the D structure (hydroxyl points toward imidazole ring) becomes more important in polar solvents. Mainly focusing on nonpolar cyclohexane (CYC) and polar acetonitrile (ACE) solvents, we compare the geometrical changes and infrared (IR) vibrational spectral behaviors related to hydrogen bonds. The charge reorganization resulting from frontier molecular orbitals (MOs) and predicted hydrogen bonding energies further reflects the excited state intramolecular proton transfer (ESIPT) tendency. By constructing potential energy curves (PECs) and searching for transition state (TS) forms, we confirm and expound the ultrafast ESIPT mechanism for the BIBTB fluorophore.