Adventures in Transition Metal Photophysics
State-of-the-art phosphorescent materials are integral to devices such as flat screen displays and modern energy efficient lighting. Such materials efficiently convert electricity to light. The reverse process, the conversion of light into electricity, is perhaps even more technologically desirable. Our group studies complexes of metals such as iron, titanium, ruthenium, chromium, iridium, and rhodium as possible materials to improve these technologies.
MMCT in donor-π-bridge-acceptor complexes
Charge-transfer (CT) excited states where charge is separated over a long distance are interesting from a fundamental standpoint and for applications involving solar energy conversion and photocatalysis. Metal-to-metal CT (MMCT) excited states fall into this class and have been exploited for such applications. In particular, photocatalysis involving MMCT excited states in solid state Ti-O-M architectures involving TiIV acceptors oxo-bridged to donors (M) such as FeII, CrIII, and MnII has recently been investigated. Of additional relevance is that ferrocyanide photosensitizes TiO2, with the photosensitization being ascribed to an FeII to TiIV charge transfer. Furthermore, FeII to TiIV charge transfer has been implicated as being responsible for the color observed in minerals such as blue sapphire (a corundum based gemstone) and blue kyanite (an aluminosilicate based gemstone). Our group is interested in molecular examples of FeII to TiIV charge transfer (Figure 1) and is involved in a systematic investigation of such complexes. These investigations involve collaborations with research groups outside of Furman who are experts in computational chemistry and ultrafast spectroscopy.
Figure 1. Adjusting the electron density at TiIV tunes the low energy absorption in a manner consistent with the assignment of this band as an FeII to TiIV MMCT. The MMCT is solvatochromic, shifting to lower energy with increased solvent polarizability. The reduction potential suggests the transient TiIII species is appropriate for electron injection into TiO2.
Photophysics of complexes with electron deficient alkyne ligands
Many transition metal complexes with alkynyl ligands are investigated for their luminescent properties. Of particular interest is adjusting the electronic properties of the metal/ligands, as this provides for the variation of colors observed in many complexes used in organic light emitting diodes (OLEDs). One common alkynyl ligand used to diminish the electron density at the metal is the pentafluorophenylethynyl ligand (C6F5C≡C–). Our group is interested in even more electron deficient alkyne ligands and has turned to the trifluoropropynyl ligand (CF3C≡C–). Electronically, this ligand behaves much like the cyano ligand. However, emission from a Rh(III) complex with this ligand, trans-[Rh(cyclam)(C2CF3)2]+, shows intense metal centered luminescence (Figure 2). We are presently investigating the relative electronic properties of C6F5C≡C– and CF3C≡C– through spectroscopic, voltammetric, and computational methods.
Figure 2. Emission of a RhIII complex with the trifluoropropynyl ligand. The emission is metal centered 3A2g to 1A1g (D4h) phosphorescence with a quantum yield of 0.11. Upon N-H deuteration, the quantum yield increases to 0.24 (24%).