Conjugation-Length-Driven Aggregation and Excited-State Dynamics in Pristine Oligothiophenes

Pristine oligothiophenes provide an ideal platform to probe intrinsic conjugation-length effects, yet a systematic understanding of their structure–property relationships has remained unexplored. Here, pristine quaterthiophene ( 4T ), quinquethiophene ( 5T ), and sexithiophene ( 6T ) were examined to investigate the influence of incremental thiophene chain extension on molecular structure, vibrational properties, and excited-state dynamics. Single-crystal X-ray diffraction was employed to characterize solid-state organization, while Raman spectroscopy was used to probe conjugation and vibrational coherence. Steady-state and time-resolved photoluminescence measurements were performed in both solution and the solid state, together with temperature-dependent and aggregation studies to assess emission behavior. Complementary theoretical excited-state calculations were carried out to analyze singlet and triplet energy levels. This integrated experimental and theoretical approach provides a systematic framework for examining intrinsic structure–property relationships in unsubstituted oligothiophenes. Our study deliberately avoids peripheral functionalization, enabling a clear structure–property correlation governed solely by backbone extension. Overall, this work provides new mechanistic insight into the relationship between molecular structure, aggregation, and excited-state dynamics in oligothiophenes, highlighting the critical role of conjugation length and packing effects in controlling their emission behavior.