Title

Ultrafast Photophysics and Photochemistry Of Hexacoordinated Bromides of Pt(IV), Os(IV), and, Ir(IV) in the Condensed Phase Studied by Femtosecond Pump-Probe Spectroscopy

Date of Award

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexander Tarnovsky

Second Advisor

Michael Ogawa (Committee Member)

Third Advisor

Mikhail Zamkov (Committee Member)

Fourth Advisor

Raymond Larsen (Committee Member)

Abstract

Transition metal complexes are actively integrated into schemes for solar energy conversion, photocatalysis, photovoltaics, and photodynamic therapy. In these applications, the population optically delivered to the Franck-Condon region of the initial excited electronic state often travels over several non-equilibrated excited state surfaces before arriving to a product state with useful functions. Radiationless relaxation between multidimensional potential energy surfaces of polyatomic molecules takes place through avoided crossings, or more typically through real crossings - conical intersections. The understanding of the forces driving electronic relaxation in polyatomic molecules has attracted considerable interest, especially in recent years, when ultrafast laser experiments made it possible to monitor downhill motion of the wave-packet in real-time.

Ultrafast photophysics and photochemistry of hexabromoplatinate, hexabromoosmate , and, hexabromoiridate dianions in the condensed phase (water, methanol, acetonitrile) was investigated in detail using femtosecond broadband time-resolved spectroscopy. Photoexcitation of hexabromoplatinate, and hexabromoosmate dianions at LF or CT bands leads to remarkably fast formation of the coherently vibrating photoproducts, pentabromoplatinate and pentabromoosmate ions in the triplet state on less than 150 fs time scale. Vibrational coherence in these photoproducts can be launched due to coherence population transfer from the parent molecule or when a dissociation fragment passes through symmetry-induced Jahn-Teller conical intersections. Conical intersections in these systems result in ultrafast radiationless transitions to the ground electronic state followed by a ligand substitution. Hexabromoplatinate complex showed excellent photocatalytic properties. Photoexcitation of this complex results in the formation of highly reactive pentacoordinated species, pentabromoplatinate in the triplet state capable of oxidizing methanol and acetonitrile on an ultrafast time scale.

Photoexcitation of hexabromoiridate at one of the lowest CT bands is followed by fast internal conversion from higher lying excited states to the lowest excited state and then to the electronic ground state. No photodissociation is observed in this complex due to fast vibrational energy transfer from the hot ground state to solvent as opposed to the gas phase study where accepting vibrational energy medium was absent.

Femtosecond broadband pump-probe spectroscopy with 100 fs time resolution has been employed along with DFT/TDDFT theoretical calculations. The experimental and theoretical approaches brought together have provided a detailed understanding of the sequence of events that follow photoexcitation and an insight on central metal effects on the photophysical properties of these complexes. Overall, the results of the present investigation reveal fundamental background knowledge of the electronic factors in the photophysical properties of inorganic transition metal complexes.