Bending-Type Electron Donor-Donor-Acceptor Triad: Dual Excited-State Charge-Transfer Coupled Structural Relaxation

The triad types of molecules with various combinations of electron donors (D) and acceptors (A) have been widely explored in optoelectronics. However, their photophysical and photochemical properties, which are frequently unconventional, are relatively unexplored. In this study, a donor-donor-acceptor (D-D-A)-type triad, CTPS, consisting of the donor moiety of triphenylamine (D1) and the acceptor moiety of dibenzothiophene sulfone (A) bridging through the second donor carbazole (D2) into a U-shape configuration, was synthesized. CTPS exhibited dual emission bands, both of which reveal solvent-polarity-dependent solvatochromism and unusual excitation-wavelength-dependent ratiometric emission. Comprehensive studies clarified that two emissions originate from two different D-A charge-transfer (CT) states. The lower-energy CT(S) state possesses D1 -> A through-space CT nature with optically forbidden transition, whereas the higher-lying CT(B) state is associated with optically allowed D2 -> A CT through the pi-conjugation transition. Upon S-0 -> CT(B) excitation, the charge transfer creates D2(delta+)A(delta-) dipolar changes and A(delta-)D1 repulsion, leading to structural relaxation of the CT(B) state that competes with fast CT(B) -> CT(S) internal conversion. Therefore, despite the fact that they originate from the same Franck-Condon excited state, both energy-stabilized CT(B) and CT(S) states are populated through two independent channels. The stabilized CT(B) and CT(S) states possess different optimized geometries and do not interconvert during their lifespans, rendering different population decay time constants. The slim highest occupied molecular orbital/lowest unoccupied molecular orbital overlapped D1-A CT(S) state exhibits thermally activated delayed fluorescence (TADF), the character of which was further exploited as a host in organic light-emitting diode. The results gain new insights into the properties of the bending-type D-D-A TADF triads. CTPS should not be a unique case. Bizarre photophysical behavior encountered in molecules comprising multiple D and A groups may involve the interplay among various local CT states, which might have been overlooked.