Resolving the Vibronic Effect on Dark Processes of Conjugation Extended Diketopyrrolopyrrole with Red/NIR Emitting

The conjugation extended emitters with bright red/NIR emission are of high interest due to potential bioapplications, while they might be plagued from fluorescence quenching associated with the rapid dark decay channels of excited states. In this work, we employed diketopyrrolopyrrole (DPP) and conjugation extended derivatives as a model system and attempted to resolve the excited-state dynamics by using ultrafast spectroscopy. Further vibrational analysis on electronic states (S₀, S₁, T₁, and T₂) allowed us to evaluate vibronic effects on dark processes of the model system, including S₁ state nonradiative decay (<i>k</i>_NR^S) and plausible ISC (<i>k</i>_ISC) channels. By identification of vibrational modes involved in corresponding dark processes, the relationship between excited-state structure relaxation, vibronic coupling, and the rate constant of dark processes was revealed. Our work indicated that the vibronic effect can greatly affect fluorescent emission by promoting dark processes and should be taken into consideration in molecular design of red/NIR emitters.