Conjugation‐Length Dependent Interplay Between Charge Transfer and Locally Excited States in Sequence‐Defined Donor–Acceptor Substituted Oligo(Phenylene Ethynylene)s

Oligo(para-phenylene ethynylene)s (OPEs) serve as well-defined model compounds to understand electronic effects in conjugated macromolecules. Here, we report a sequence-defined series of OPEs capped with a di-tert-butylcarbazole (DtBCz) donor and a triphenyltriazine (Trz) acceptor to investigate how the donor–acceptor distance along a conjugated OPE backbone governs charge-transfer properties. Using an iterative synthetic strategy, the OPE length was extended stepwise, providing a molecularly highly defined platform to investigate length-dependent excited-state evolution. The lowest excited-state character was examined by steady-state absorption and photoluminescence spectroscopy, Lippert-Mataga analysis, time-resolved photoluminescence (TRPL) spectroscopy, transient absorption (TA) spectroscopy, and quantum-chemical calculations. Shorter OPE sequences ($\textbf{PE0}$ and $\textbf{PE1}$) exhibit stronger charge-transfer (CT) dominant behavior, reflected by clear donor–acceptor coherence in transition-density analyses and pronounced solvatochromism. With increasing OPE length ($\textbf{PE2}$ and $\textbf{PE3}$), locally excited (LE) contributions become more prominent, leading to hybridized LE and CT (HLCT) states which is further supported by TA and TRPL spectroscopy. ... mehrThe observed CT-to-HLCT evolution highlights the importance of precise backbone length control for rationally tuning excited-state properties in conjugated macromolecules.