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.