Exciton migration and charge transfer in chemically linked P3HT-TiO2 nanorod composite

Exciton migration and charge transfer in the chemically linked P3HT-TiO2 nanorod composite (P3HT-Si-nr-TiO2) solution were investigated in comparison with pristine P3HT and physically mixed P3HT/LA-nr-TiO2 solutions. The chemically linked P3HT-Si-nr-TiO2 was made by covalently linking in situ polymerized P3HT onto nr-TiO2 using triethoxy-2-thienylsilane as a linker to replace the initial linoleic acid (LA) capping agent on nr-TiO2. The physically mixed P3HT/LA-nr-TiO2 was prepared by adding ex situ synthesized P3HT into the LA-capped nr-TiO2 solution. In the chemically linked sample, charge transfer from P3HT to TiO2 nanorods was found to occur evidenced by photoluminescence (PL) quenching and ultrafast decay dynamics with a timescale of 0.75 ps. However, both the emission spectra and femtosecond dynamics in physically mixed sample overlapped very well with those from pristine P3HT solution, indicating no PL quenching or charge transfer from P3HT to nr-TiO2. In addition, blue shift in absorbance and PL spectra, larger Stokes shift, and structureless PL spectra found in the chemically linked sample indicated that P3HT formed a more coil-like conformation with more twisted torsion disorders than those in pristine P3HT and physically mixed samples. This is consistent with the femtosecond measurement result that torsional relaxation occurred with a longer decay time and higher amplitude. Moreover, intersystem crossings (ISC) from singlet state (S-1) to triplet state (T-1) in P3HT of the three samples were all found to occur in a comparable timescale of similar to 1 ns and showed no dependence on conformational disorders such as torsional defects.