In this report, a donor-donor-acceptor (D-D-A) type molecule, CTPS, with the donor moiety of carbazole and triphenylamine as well as the acceptor moiety of dibenzothiophene sulfone was strategically designed and synthesized. CTPS exhibited unique dual emission bands, both of which exhibit solvent polarity dependent solvatochromism and hence ratiometic changes of the emission intensity. The two emission bands originated from two distinct charge-transfer states denoted as CT(B) and CT(S), which are ascribed to different D-A combination, as verified by the control D-A moiety, CPS and CTS. Comprehensive steady-state spectra and nano-to-femtosecond time-resolved emission measurement, together with computational approaches, clarified that CT(B) with a through π-conjugated charge transfer character undergoes optically allowed S0→ CT(B) transition, while S0→ CT(S) excitation is forbidden due to the through space nature in the transition. Upon S0→ CT(B) excitation, fast structural and solvent relaxation take place, which competes with the 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 pathways. CT(B) and CT(S) states possess different optimized geometry and do not interconvert during their life span, exhibiting distinctly different population decay times. Owing to slim HOMO/LUMO overlap CT(S) exhibits thermally activated delayed fluorescence (TADF), which was further supported by its efficient OLED performance upon serving as a host. The results gain new insights into the unusual photophysical behavior of a D-D-A type TADF molecule CTPS. CTPS cannot be treated as a single incidental case. Ambipolar molecules comprising of multiple D and A groups frequently encountered bizarre luminescence and hence photophysical behavior, which, similar to that of CTPS, may involve the interplay among various local charge-transfer states.

Electron donor-donor-acceptor triad: Dual charge-transfer emission showing anti-kasha behavior

2019 Spring National Meeting

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