Engineering of Charge-Transfer State to Modify Photophysical Properties in N,N′-Diaryl-dihydrodibenzo[a,c]phenazine

Herein, we explored the charge transfer (CT) effect on the excited-state structural and electronic evolution of N,N′-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) featuring various electron-donating groups (EDGs). Upon photoexcitation, DPAC molecules undergo a sequential multistep structural transformation along the Franck–Condon (FC) state, CT-controlled bent state (responsible for E1─420 nm), and twisted state (responsible for E2─600 nm). By introducing different EDGs to the DPAC core, we can fine-tune the balance between the inherent decay of the CT-controlled bent state and its progression into the twisted state by adjusting the CT effect. As the electron-donating ability of the EDG increases, the strong CT effect can produce a "deep" (stable) CT-controlled bent state, in which the transformation of the excited molecular skeleton from bent into twisted was partly suppressed. This research provides valuable insights into organic chromophores' excited-state structural and electronic evolution, revealing how competitive decay pathways are contingent upon the CT effect.