Electronic-vibrational coupling drives the relaxation of optically prepared excitations in both carotenoids and semiconductor quantum dots (QDs). As a result of this electronic-vibrational (vibronic) coupling, coherent wavepacket motions are observed during the excitation relaxation processes. The first part of the dissertation describes the relaxation of the optically prepared bright S2 state to dark S1 state via bridging intermediate, Sx. The spectroscopic signature and the vibrational coherences of the intermediate state Sx involved in the nonradiative decay were characterized using broadband multidimensional spectroscopic techniques. Analysis of vibrational coherences shows that Sx undergoes displacements along out-of-plane coordinates as it passes to the S1 state. The second part of the dissertation discusses the nonradiative relaxation in oleate-capped QDs. This process involves excited-state coherent wavepacket motions through a cascade of conical intersections between exciton potential-energy surfaces. Excited state wavepacket motions are observed at frequencies matching the vibrational modes of the organic ligands. These observations indicate that the ligand vibrations are quantum coherently mixed with the core electronic states of the QDs. The third part of the dissertation presents the role vibronic coupling in photoinduced charge transfer from the QD core to a surface ligand electron acceptor molecule, methyl viologen dication (MV2+). The observation of coherent wavepacket motions is consistent with presence of a charge transfer intermediate with a mixed QD-MV character, and this intermediate initiate photoinduced charge transfer from the core of the QD to the surface acceptor molecule. These results raise new opportunities for the engineering light-harvesting properties of materials through the control of electronic-vibrational coupling and quantum coherences.
Read
