Access Type

Open Access Dissertation

Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

John F. Endicott

Second Advisor

Claudio N. Verani

Abstract

The 77 K radiative properties (spectra, quantum yields and lifetimes) of ruthenium-polypyridyl complexes are investigated to better understand the effects of electronic mixing on metal-to-ligand-charge-transfer (3MLCT) excited state properties and how metal-centered (3MC) excited states affect the properties of potential ruthenium photosensitizers.The radiative rate of relaxation (kRAD) determines the maximum possible excited state lifetime when all other relaxation pathways are blocked (kn = 0 for all n  RAD). Thus, the excited state will relax only by means of an emission characteristic of the polypyridyl chromophore. kRAD is expected to increase as the excited state energy increases while the value of the non-radiative decay (kNRD) should decrease. Given this relationship in the decay kinetics, the radiative rate constant can be an important factor in determining the lifetimes of high energy photosensitizers. Additionally, the formalisms used in discussions are based on Einsteinian rate constants for atomic fluorescence spectra and not that of phosphorescent donor-acceptor complexes. Other factors should be considered such as difference in spin multiplicity and molecular distortions in vibrational modes that are coupled to electronic transitions (evident from vibronic side band features in emission spectra). Density functional theory (DFT) has indicated that the excited state distortions for these systems are due to electronic mixing of Ru-bpy 3MLCT excited state with the bpy ligand  and * orbitals, or alternatively from the electronic mixing between the 3MLCT and * excited states. The spectroscopic and computational results suggests that a "pure" diabatic 3MLCT excited state is not greatly distorted and that its emission has weak vibronic contributions in the region of bpy-ligand vibrational modes in addition to a very small radiative rate constant. Furthermore, there is no evidence that a "pure" 3MLCT emission has ever been observed. Additionally, some of the observed spectroscopic properties will depend on the differences in excited state molecular geometries based on the reorganizational energy for crossing between the 3MLCT and 3MC states.

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