ULTRAFAST ENERGY DISSIPATIONS AND QUANTUM EFFICIENCIES OF CONJUGATED POLYMERS BY SEGMENTAL STRESSES

Quantum efficiencies of conjugated polymers (CPs) were found to increase dramatically by elevation of segmental stresses, therefore, we explore the emission behavior in the time scales comparable to segmental motions to unveil the relationship. We stretched CP (MEH-PPV) to ~300%, resulting photoluminescence (PL) enhancements of 20-60 folds depending on the stress level. By using up-conversion time-resolved confocal PL spectroscopy, we found the absorbed energy quickly dissipates via segmental rotations and vibrations, leading to PL red shifts. The damping via rotations was ~10 times faster than vibrations but stopped after ca. 2 ps, while the latter continued functioning, but abated slowly to diminishment. The segmental stresses can reduce or even switch off the rotation damping but yielded no influences on vibrations. The latter was found in fact heat dissipation, while the rotations, driven by local electrostatics from excitation, can trigger self-trapping and the 2 ps represents the time limit before escape from self-trapping for recombination. This model explains satisfactorily how segmental stresses interacting with CP backbones to result dramatic efficiency enhancements.