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 (PE0 and 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 (PE2 and 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. The observed CT-to-HLCT evolution highlights the importance of precise backbone length control for rationally tuning excited-state properties in conjugated macromolecules.