The Optical Properties of CdSe Quantum Dot-Bithiophene Complexes

We studied the optical and electrochemical properties of four model oligomers M3, M5, M9 and M17, Fig. 1. We have performed extensive experimental and theoretical analyses of the M series oligomers in order to determine the oligomers that would be the best charge transfer partners for CdSe quantum dots with respect to spectral overlap and band level alignment. The steady-state absorption and fluorescence spectra are shown in Fig. 2 for the M3, M5, M9, and M17 molecules in toluene solutions. Two main peaks observed in the absorption spectra are assigned to S1 and S2, which correspond to the S1 ← S0 and S2 ← S0 transitions, respectively. Both the large Stokes shift and the vibrational broadening of the fluorescence spectra have contributions from conformational changes between the excited and ground states. The optical bandgap of the materials were determined using the tangential extrapolation through the turning point of the low energy side of the absorption spectrum. Cyclic voltammetry on the M3 and M9 oligomers yielded redox potentials that formed the desired typII junction with the valence and conduction band of CdSe QDs. M3 provides a bigger driving force for CT, and a more complementary absorption spectrum for CdSe than does M9. We therefore initially chose M3 as a starting point studying the QD-oligomer interaction. Fig.1. M(2n+1). For M3, n = 1, for M5, n = 2, etc.