Intramolecular exciton relaxation and migration dynamics in poly(3-hexylthiophene)

Exciton dynamics are investigated for size selected poly(3-hexylthiophene) samples in dilute chloroform solutions using time-resolved fluorescence. The sizes range from an average of 39 monomers (M _w = 6490 Da) to an average of 168 monomers (M _w = 27860 Da). Both isotropic emission transients, which monitor downhill energy migration, and time-resolved emission depolarization, used to measure orientational migration of the excitons, are reported as a function of emission energy. Downhill energy migration accelerates significantly as the chains become longer. While amplitude of the initial (sub-100 fs) depolarization increases with chain length, the subsequent rate of exciton reorientation is relatively insensitive to chain length for times less than 30 ps and then slows as the chains become longer. The chain length dependence provides additional insight into the connection between spectral diffusion and exciton spatial migration. The results are considered in terms of the distribution of accessible exciton states and how this distribution changes with chain length.