Theoretical study for the TADF nature of through-space conjugated [2.2]paracyclophane derivatives and the design for red-light emission

Abstract

The thermally activated delayed fluorescence (TADF) nature of a series of [2.2]paracyclophane derivatives with spatially conjugated structures were studied by DFT/TD-DFT method. Through-space charge transfer, rather than the usual through-bond charge transfer exist between the frontier molecular orbitals of these molecules. The singlet-triplet energy gaps which affect the reverse intersystem crossing (RISC) process were compared among the molecules reported in the experimental study to explain why 2 and 4 are more likely to show TADF emission than 1 and 3. Especially, when discussing the intersystem crossing (ISC) process according to the excited states properties and the spin-orbit coupling matrix elements, the influence of the spatially conjugated structure was also analyzed detailedly. Transition dipole moments of the absorption states were greatly enhanced in the conjugated structure and the ISC between some high-energy states was promoted in consequence, indirectly leading to the promotion of the RISC process which generates S1 excitons. In addition, we designed 5–8 by replacing the benzophenone group with the thianthrene-9,9′,10,10′-tetraoxide group, in order to obtain red-light emission through decreasing the LUMO energy level. The calculation results show that the new molecules have larger radiation rate and smaller internal conversion rate, which indicates that the designed molecules are potential efficient red-light TADF materials.