Importance of Conformational Change in Excited States for Efficient Thermally Activated Delayed Fluorescence

Abstract

Novel donor–acceptor (D–A) type (monomer) and twin D–A type (dimer) molecules with pretwisted geometrical conformations have been synthesized to investigate the molecular conformation-dependent mechanism of efficient thermally activated delayed fluorescence (TADF). The chemical structure or conformation of TADF molecules, which intrinsically determines the excited-state dynamics of intramolecular charge transfer, is crucial for obtaining an efficient reverse intersystem crossing (reverse ISC, rISC). In this work, the pretwisted D–A type monomer and dimer were investigated by chemical calculations, steady-state spectroscopy, and ultrafast transient absorption (TA) spectroscopy to see what decisive factor is for efficient TADF, conformations, and/or ambient environment (solvent polarity and/or viscosity). Theoretical calculations suggest that there is a smaller energy difference (ΔEst) between the lowest singlet (S1) and triplet (T1) excited states at the optimized lowest excite state (S1) conformation compared with optimized ground-state (S0) conformation, indicating an excited-state conformational change required upon excitation relative to the ground state to have an efficient delayed fluorescence. It is found that the ΔEst of the dimer at S0 geometry is smaller than that of the monomer, but the ΔEst of the dimer at S1 geometry is larger than that of the monomer, which means conformational changes in the electronic excited process have a more significant effect on the monomer and further affect TADF properties. As a result, the monomer shows a shorter lifetime and higher efficiency relative to the dimer via steady-state spectroscopy and viscosity-dependent control experiments. It is found that suppressing conformational change will lead to a lower delayed fluorescence yield, which is in agreement with the calculated results about ΔEst, indicating that an excited-state conformational change is required for both the monomer and dimer to show a higher delayed fluorescence yield. Ultrafast femtosecond (fs) and nanosecond (ns) TA measurements reveal that the two molecules both emit delayed fluorescence after solvation and conformational relaxed processes in low polar toluene and high polar tetrahydrofuran (THF), while the overall photoluminescence quantum yield reduced and lifetime of TADF decreased in more polar THF. The transient spectroscopy measurements also reveal that the radiative process and ISC process of the monomer are faster than the dimer. These results indicate that the monomer is a better TADF candidate molecule compared with the dimer, providing a guidance to understand the relationship between TADF properties and conformational relaxation dynamics, as well as the ambient environment.