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

Abstract

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.