Ultrafast relaxation of charge-transfer excitons in low-bandgap conjugated copolymers

A photophysical study, employing linear optical and femtosecond two-dimensional electronic spectroscopies, of two conjugated copolymers in solution is reported. The study focuses on the effect of backbone conformation on the properties and dynamics of excitons. For the predominantly planar copolymer, poly((4,8-diethylhexyloxyl) benzo([1,2-b:4,5-b′]dithiophene)-2, 6-diyl)-alt-((5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl) (PBDTTPD), the dipole moment of excitons was found to be unchanged upon photoexcitation, and no excitonic relaxation was detected within 1 ps. For poly[N-11′′- henicosanyl-2,7-carbazole-alt-5,5-(4′,7′-di-thienyl-2′. 1′.3′-benzothiadiazole)] (PCDTBT), however, bathochromic shifts of the absorption and fluorescence spectra as a function of solvent polarity indicate a change in dipole moment of approximately 3 Debye upon photoexcitation at the red-edge of the absorption spectrum. Excitation at the PCDTBT absorption maximum, where an ensemble of polymer chains have a predominantly twisted backbone, causes relaxation in the excited state to excitons with stronger charge-transfer character on a timescale of ∼200 fs. This dynamic Stokes shift was not observed when exciting on the PCDTBT absorption red edge, which photo-selects more planar conformations. Ultrafast relaxation in the excited state involving coupled torsional and solvent coordinates increases the charge-transfer character of the exciton. Although there is a concomitant loss of energy, electron-hole correlation (binding) is weakened, which may contribute to the efficiency of organic solar cells based on PCDTBT as an electron donor.