Complications in the Interpretation of F8T2 Spectra in Terms of Morphology

Photophysical properties of conjugated polymers strongly correlate with chain morphology and interactions between chains. Here, we observed two characteristic types of spectra of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) in both single-chain and bulk cases. Despite the similarities in emission spectra, fluorescence dynamics and intensities corresponding to each type of spectrum are radically different in these cases. We hypothesize that the origin of the spectrum with a suppressed 0-0 vibronic band in photoluminescence is chain bending in the single-chain experiments, while the same phenomenon is caused by interchain aggregation in bulk samples. We propose a microscopic interpretation of the single-chain result in terms of bending that we expect to be characteristic of flexible conjugated polymers with five-membered rings. Caution must be exercised in drawing inferences about the meaning of single-chain spectra by analogy with bulk data.