Connecting Molecular Conformation to Aggregation in P3HT Using Near Edge X-ray Absorption Fine Structure Spectroscopy

Carbon 1s near edge X-ray absorption fine structure (NEXAFS) and UV–vis spectroscopy are used to examine differences between highly aggregated and poorly aggregated forms of the polymer poly(3-hexylthiophene) (P3HT), based on as-cast and annealed regiorandom and regioregular P3HT samples. UV–vis spectra show characteristic signatures of unaggregated P3HT in regiorandom P3HT, and of H-aggregation in regioregular P3HT samples. Distinct spectroscopic differences, including energy shifts, are observed in the NEXAFS spectra of aggregated P3HT relative to the unaggregated forms. These differences are reproduced with transition-potential density functional theory (TP-DFT) calculations which explore aggregation and molecular conformation. Differences in the NEXAFS spectra of P3HT are assigned to thiophene backbone twisting in the unaggregated forms of P3HT, and to various degrees of chain planarization in aggregated forms of P3HT that also correlate to the exciton bandwidth. These results open up the prospect of characterizing conformation and related difficult to assess structural details through NEXAFS spectroscopy and correlative theory and electronic structure analysis.