Solvent polarity dependence on excited charge-transfer state properties in donor-acceptor-donor thermally activated delayed-fluorescence molecule

Abstract

The photophysical properties of a donor-acceptor-donor (D-A-D) molecule based on oxadiazole, 2,5-bis(4-(10H-phenoxazin-10-yl)phenyl-1,3,4-oxadiazole (2PXZ-OXD), have been systematically investigated by means of steady state and transient spectroscopy. A significant increase of Stokes shift with increasing solvent polarity indicates that a dipolar charge-transfer state is produced upon excitation. Further quantum chemical calculations show that the dipole moment of singlet excited state is much larger than that of triplet excited state, where singlet excited state is much more dependent on the polarity of solvents. The symmetry-breaking charge transfer states of quadrupole molecule induced by solvent polarity, exhibit large energy loss of singlet state with increasing of polarity. The energy gap is controlled by solvent polarity because of solvation. Furthermore, the emissive oscillator strength is dependent on the polarity, and the non-emissive relaxation resulted from the strong solvation reduces the luminescence properties of emitter.