Temporal dynamics of photoexcitations in p-conjugated regular copolymers

We investigate relaxation processes in thiophene-based copolymers through site-selective and time-resolved photoluminescence spectroscopies. We observe a large Stokes-shift between the absorption and luminescence peaks that can be explained in terms of spectral relaxation of excitons within an inhomogeneously broadened density of states. The incoherent motion of the photoexcitations toward non radiative centres leads to a non exponential decay of the photoluminescence intensity. The decay rate of the photoluminescence signal strongly depends on temperature except for long times. The interplay among thermally activated jumps, slow transfer to non nearest neighbour sites and molecular recombination explains satisfactorily these findings.