Tuning interchain and intrachain interactions in polyfluorene copolymers

We study how intermolecular interactions, conformational changes, and polarization effects control the nature of the electronic excitations in conjugated polymer blends. Quantum-chemical calculations and high-pressure photoluminescence experiments are used to assess these effects in polyfluorene-based donor–acceptor systems. The redshift of charge-transfer–like excitations usually observed under pressure is found to arise primarily from the reduced interchain distance; the corresponding shift of bulk excitons is equally sensitive to the interchain spacing and planarization alterations. Intermolecular charge-transfer states are shown to have shorter radiative lifetimes and higher oscillator strengths at high pressure. In contrast, intramolecular bulk excitons build up interchain charge-transfer character and lose oscillator strength and radiative efficiency with increasing pressure.