Aryl-vinylene bipyridine and terpyridine ligands and their Ruthenium(II) and Osmium(II) complex systems: Potential optical limiting materials
Description
The goal for this research described herein is the development of a series of transition metal based visible light absorbing chromophores that have excited state lifetimes long enough to actively participate in bimolecular photoreactions and display both two photon excited state absorption and reverse saturable absorption properties. With this goal in mind, the preparation and photophysical characterization of a series of metal-organic Ru(II) and Os(II) complexes with relatively long excited state lifetimes is presented. The approach to extend the excited state lifetimes is to make Ru(II) and Os(II) complexes with ligands having triplet state energies that can be sensitized by long wavelength excitation into metal-to-ligand charge transfer MLCT transitions of the complexes. An additional characteristic of the complexes that is introduced through the introduction of ligands with low energy triplet states is unique excited state absorption in the near infra-red. The combination of long excited state lifetime and unique excited state absorption demonstrates the potential use of complexes as optical limiting materials (protection of materials from pulsed laser sources) Three areas of interest for this project are addressed here. First, is the determination of triplet energies of aryl vinylene terpyridyl and bipyridyl aromatic hydrocarbon ligands using density functional calculations (B3LYP with a 6-31G* basis running on Gaussian 03 or Gaussian 09) and the synthesis of some of the ligands under Wittig or Knoevenagel conditions. Second is the coordination of the ligands on the ruthenium and osmium metal centers and the subsequent photophysical. Third, is the evaluation of the utility of the complexes as optical limiting materials by a variety of characterization methods including evaluation of intersystem crossing efficiencies, determination of two photon absorption coefficients and determination of rate constants for formation of the triplet absorbing states of the optical limiters The project, which was a cooperative effort with three different groups, was to develop and characterize a new class of aryl-vinylene derivatized bipyridine and terpyridine ligands and long wavelength absorbing organometallic complexes for optical limiting purposes. The complexes were made in our group and further investigated in Emory University by Dr. Tim Lian's group using femtosecond transient spectroscopy; their two photon absorption properties were characterized by Z scan methods in Dr. Igor Rubtsov's group at Tulane University Photoredox reactions of this series of Ru(II) and Os(II) complexes in solution were also examined. These metal complex chromophores couple excited states localized on the metal complex (MLCT) with states of triplet spin multiplicity on a covalently linked organic (aromatic hydrocarbon) component. For the bis-bipyridine Ru(II) complexes the pyrenylvinyl 'localized' excited state of the complex reacts via photoinduced electron transfer with a variety of viologen and diquat electron acceptors. The interesting aspect of the electron-transfer process is that though the excited state is ligand-localized the photoredox reaction product is oxidized Ru(III) The determination of triplet yields, examined by energy transfer experiments, gave us an idea of the effectiveness of intersystem crossing to populate the triplet ligand localized state in a series of ruthenium complexes. Preliminary Z scan measurements demonstrate that some of the ruthenium and osmium complexes are good candidates for optical limiting materials by virtue of their relatively large two photon absorption coefficients at wavelengths of particular interest for laser protection. Femtosecond transient spectra demonstrate that population of the triplet ligand localized states is very fast (less than 5ps) A final chapter is included which represents a continuation of the work of Dr. Jin Chen. Four ruthenium diimine complexes have been synthesized as phosphorescent oxygen sensors