Computational Modeling of Vibronic Transitions and Stokes Shift in Poly(3-hexylthiophene) Aggregates: Insights into Molecular Ordering and Optical Behavior

Conjugated polymers such as poly(3-hexylthiophene) (P3HT) exhibit optical and electronic properties that depend strongly on molecular ordering and aggregation. This study employed Gaussian-based simulations to model the absorption and emission spectra of P3HT under varying temperature (25–125°C), crystallization time (1–8 h), concentration (0.5–2.5 a.u.), and solvent conditions. In solution, P3HT showed a broad π-π* band at 455 nm with a weak 495 nm shoulder, while aggregates exhibited sharp vibronic peaks at 520 nm, 560 nm, and 610 nm. Increasing temperature and concentration produced red-shifts, spectral narrowing, and higher 0–2/0–1 ratios, signifying enhanced π-π stacking and molecular order. Poor-solvent systems yielded stronger aggregation with 0–2/0–1 ratios of 1.2–1.3 compared to 0.8–0.9 in good solvents. Calculated Stokes shifts of 0.35 eV (solution) and 0.477 eV (aggregate) confirm exciton relaxation and interchain coupling, providing quantitative insight into P3HT’s structure-property relationships for organic optoelectronic design.