Optical studies of molecular aggregates: The photophysics of a thienylene vinylene oligomer

The effect of aggregation on the photophysical properties of 5′,5″-bis[2‴-2‴′-thienyl)-1‴- ethenyl]-1,2-bis(2′-thienyl)ethene (4TV) has been studied by absorption and photoluminescence-detected magnetic resonance (PLDMR) spectroscopy. Upon aggregation the molecules align themselves in a herringbone configuration with an estimated angle between the dipoles of 61°. Intermolecular interactions in 4TV dimers cause Davydov splitting of the lowest-energy absorption band. The absorption spectrum of isolated 4TV molecules has a 0-0 transition at 2.52 eV with an associated vibronic progression and the absorption bands of 4TV dimers have transitions at 2.28 and 3.48 eV. The morphology of a blend of 4TV and PMMA can be reversibly converted between aggregated and isolated phases by simple thermal processing or adjustment of the 4TV concentration. The PLDMR spectra of 4TV films show a direct correlation between aggregation and charge photogeneration.