Conformational torsion, intramolecular hydrogen bonding and solvent effects in intersystem crossing of singlet-triplet excited states for heavy-atom-free organic long persistent luminescence

Abstract

In this work, the effects of conformation torsion in the ground and excited states on the electronic structure, intramolecular hydrogen bonding and solvent effects on photophysics of donor-acceptor (D-A) molecules were investigated. We designed two heavy-atom-free molecules with carbazole (Cz) as donor as well as benzophenone (BP) and 3-benzoylpridine (BPy3) as acceptors. It comes out that both of the two molecules BP-Cz and BPy3-Cz have similar conformational torsion trending from planarization to verticalization and then to planarization, when they change from the ground state to the singlet excited state to the triplet excited state. The difference is that BPy3-Cz forms intramolecular hydrogen bond in the ground state, causing the photophsical properties have significant changes compared with BP-Cz. In the ground state, the configuration of BPy3-Cz is restricted by the weak hydrogen bond. The hydrogen bond is broken in the singlet excited state and then re-formed in the triplet excited state constituting a switch with the state change of “formation-breaking-reformation”. In addition, the calculated UV absorption and phosphorescence spectra are approximate while the fluorescence spectrum of BPy3-Cz has a significant red shift which is induced by the lower excited state energy gap. One can note that the solvation spectra are blue-shifted due to the increased viscosity inhibiting the molecular rotation. Besides, the SOC constant (ξ) of two molecules are larger in the more planar triplet excited state rather than singlet excited state, indicating the possibility of LPL. The ξ sum change of BPy3-Cz portends long persistent luminescence. Thus, this work demonstrates that adjustable photochemical properties can be achieved by conformational torsion, which provides a new strategy for the study of molecular system design.