Text

theses

The ability of a dendritic network to intercept electrons and extend the lifetime of a short-lived photoinduced charge separated (CS) state was investigated in a homologous family of a pyrene-methylene-viologen dendron family in which the number of viologen subunits grows exponentially from P−C1−G0 to P−C1−G3. The CS state in the parent diad P−C1−G0 with a single acceptor exhibits an extremely short lifetime of τ=0.72 ps. The expansion of the viologen network introduces slower components to the recombination kinetics by allowing the injected electron to migrate further away from the donor. As a result, the fraction of long-lived population increases with 10-fold or greater lifetime extension in the order of P−C1−G3 > P−C1−G2 > P−C1−G1 > P−C1−G0, which can be interpreted in terms of the electron hopping between two viologen sites and long-range electron tunneling across multiple viologen units. Furthermore, the influences of donor-acceptor separation, connectivity, and solvent on electron transfer rates were studied applying different pyrene-viologen donor-acceptor systems. In addition, the vibrational cooling of hot ground state methyl viologen radical cation (MV+•) was studied using femtosecond pump-probe spectroscopy. The photoexcitation of the D1←D0 transition led to very rapid internal conversion within 460 fs, generating a vibrationally excited ground state of MV+• which thermalized on 1.6~16 ps timescale. The initial stage of vibrational excess energy loss corresponds to the intermolecular energy transfer to vibrational modes of the surrounding medium molecules, and the latter stage involves energy dissipation into relatively low-frequency librational and translational modes of the solvent. The effect of solvent and host-guest complexation on cooling dynamics of MV+• was also investigated, and the results suggested that a good match between the density of vibrational states in MV+• and surrounding medium could efficiently accelerate the vibrational cooling.

ETD_8698

Ph.D.

Includes bibliographical references

by Zheng Gong

doi:10.7282/T3J106J2

ETD doctoral

The author owns the copyright to this work.