Probing vibrational relaxation dynamics in charge-transfer excited states: Synthesis, physical, and photophysical characterization of cyano-substituted polypyridyl complexes of ruthenium(II)

摘要

Photo-induced charge separation is the physical phenomenon underlying virtually all schemes geared toward the conversion of light into chemical, electrical, and/or mechanical energy. Charge separation is typically effected in a molecular system through charge-transfer excited states, in which photon absorption causes charge redistribution within the chromophore: maintaining, amplifying, or, in the least favorable circumstances, destroying the resulting chemical potential depends on dynamics that occur within the chromophore immediately following the absorptive event. In transition metal complexes a majority of excited state energy is dissipated through non-radiative decay. Despite the prominent role it plays in the deactivation of charge-transfer excited states, there is relatively known about the mechanism of non-radiative decay in charge transfer complexes.;Probing vibrational relaxation in transition metal complexes is not always a straightforward process. Often times information about vibrational relaxation from the initially excited Franck-Condon state to the long lived excited state is inferred from transient electronic absorption spectroscopy. Infrared spectroscopy is more direct way to probe the vibrational state of an electronic excited state. This dissertation investigates the fundamental photophysics of ruthenium polypyridyl complexes, in particular the non-radiative decay between the initial excited Franck-Condon state and the long lived excited state. The complexes studied in this dissertation incorporate cyanide groups as infrared tags in order to use infrared transient absorption spectroscopy, coupled with visible transient absorption spectroscopy, to probe the vibrational relaxation dynamics in ruthenium polypyridyl complexes.;Before ultrafast dynamics can be interpreted it is critical to have a solid understanding of the properties of both the initial Franck-Condon excited state and the long lived excited state. Nanosecond time-resolved spectroscopic techniques which can be used to probe the long lived excited state of transition metal complexes are discussed. These techniques are used to characterize the long lived excited states of a series of cyano-substituted ruthenium(II) bipyridine and terpyridine complexes. A combination of ultrafast infrared and visible absorption spectroscopies are used to probe the excited state dynamics in the series of cyano-substituted ruthenium(II) bipyridine complexes. By combining the two techniques it can be conclusively shown that small amplitude changes in the time resolved electronic absorption spectra on a ∼1-10 ps time scale are due to vibrational relaxation on the lowest energy excited state potential surface. The large energy difference between the cyano substituted bipyridine ligand and the unsubstituted bipyridine ligand allows for selectively localizing the initial excited state on either the cyano-substituted or unsubstituted bipyridine ligands. This potentially allows the dynamics associated with intramolecular vibrational redistribution to be decoupled from the dynamics associated with interligand electron transfer.