The Origin of the Emission Properties of π-Conjugated Molecules that have an Acid-responsive Benzimidazole Unit

Abstract

The photophysical properties, especially the emission properties, of π-conjugated molecules with an acid-responsive N-methylbenzimidazole unit have been characterized and compared between T-shaped cross-conjugated and linear conjugated molecules. X-ray crystal structures and optimized structures based on DFT calculations suggest a planar conformation of the 2-thienyl-N-benzimidazole structure even after protonation where intramolecular S–N and NH–S interactions, respectively, are responsible for the stabilization of planar structures. Despite the planarity in the ground state, the T-shaped cross-conjugated molecules exhibit the twisted intramolecular charge-transfer (TICT) emission in response to protonation, whereas the corresponding linear conjugated molecule without a horizontal π-system does not have a charge-transfer state. The difference in the separation of frontier molecular orbitals of the HOMO and LUMO and the energy barrier of the rotation about the thienyl ring predicted by the DFT calculations accounts for the difference in the emission decay process between the linear and cross-conjugated molecules with a protonated benzimidazole unit. An orthogonally allocated π-conjugated system appears to be essential for the emergence of TICT characteristics upon protonation in terms of the separation of molecular orbitals of the HOMOs and LUMOs, as well as the ease of rotation.