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

Abstract

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 π-conjugation transition. Upon S0 → CT­(B) excitation, the charge transfer creates D2δ+Aδ− dipolar changes and Aδ−–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.

Face‐to‐Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

“…In toluene at room temperature, all of the four compounds exhibit structureless emissions while additional weak, structured emission bands are observed for PXZ‐CTZ and DPXZ‐CTZ which are tentatively assigned as local excitation (LE) states of the donor or acceptor fragment (Figure 3 a). The co‐existence of multiple bright states is reminiscent of the photophysical properties of triad molecules [3, 5c, 10a] . Unlike their comparable energies for the charge‐transfer absorption, the dominant emission spectra of PXZ‐CTZ , DPXZ‐CTZ and DPXZ‐BO are significantly redshifted by 69–91 nm from that of CTZ .…”

Through‐space charge transfer polymers for solution‐processed organic light‐emitting diodes

“…One is through‐bond charge transfer (TBCT) mediated by conjugated bonds, [ 1 ] which features strong electron coupling between donor and acceptor through covalent bonds, while the other is through‐space charge transfer (TSCT), which occurs through spatial electron interactions and has relatively weaker electron coupling between donor and acceptor. [ 11–25 ]…”

De novo strategy with engineering anti-Kasha/Kasha fluorophores enables reliable ratiometric quantification of biomolecules

“…As an important principle in photochemistry to most fluorophores, Kasha's rule states that fluorophores can only generate one emission band from the lowestenergy excited singlet state (S 1 ), irrespective of the excitation wavelength 26 . In contrast, anti-Kasha dyes remain exceedingly rare [27][28][29][30][31][32] , and most of them emit in the short wavelength region and are thus not suitable for biological applications 33 . Moreover, to the best of our knowledge, the modulation of anti-Kasha emission for ratiometrically detecting bioanalytes has yet to be demonstrated.…”