Crystal engineering of conjugated oligomers and the spectral signature of π stacking in conjugated oligomers and polymers

H-bonding was used to alter the molecular packing of a bithiazole oligomer. Comparison of the solid-state UV-vis absorption spectra of the H-bonded oligomer with a related non-H-bonded oligomer allowed for the assignment of the optical signature of pi stacking in crystals or ordered aggregates of conjugated oligomers or polymers. The pi-stacking interactions give rise to the fine structure frequently observed in the absorption spectra of regioregular, conjugated polymers, including P3AT's, PABTz's, etc.