Photophysical Properties of π-Conjugated Donor–Acceptor–Donor Systems: Influence of Acceptor Type and Donor Segment Length

We report a comparative photophysical study of thiophene-based, donor-acceptor-donor (DAD) π-conjugated oligomers incorporating thienopyrroledione (TPD), benzothiadiazole (BTD), and benzoselenadiazole (BSeD) as acceptor units. Using femtosecond and nanosecond transient absorption spectroscopy, along with steady-state optical and electrochemical measurements, we investigate how acceptor identity and conjugation length influence intersystem crossing (ISC), triplet state formation, and fluorescence behavior. The TPD series exhibit high triplet yields (up to 88%) and long triplet lifetimes (up to 185 μs), with increasing fluorescence quantum yield and lifetime as conjugation length increases. The latter trend is attributed to LUMO delocalization along the oligomer backbone, which reduces acceptor influence and suppresses ISC. In contrast, the singlet excited states of the BTD and BSeD oligomers display strong charge-transfer (CT) character, leading to significantly reduced triplet yields.