Multiphoton-Induced Picosecond Photophysics of Chromium(III)- Polypyridyl Complexes

Abstract

The photophysics of Cr(III)-polypyridyl complexes, Cr(NN)₃ ³⁺, have been the object of many studies in attempts at understanding the behaviour of these systems on absorption of light (Jamieson 1981; Serpone 1983). The usual photochemically relevant states of Cr(NN)₃ ³⁺ (Fig. 1) are: the ground state, ${\!^4{\text{A}}_2}$; the lowest energy spin-allowed quartet state, ${\!^4{\text{T}}_2}$; and the lowest energy spin-forbidden doublet states, ${\!^2{\text{T}}_1}/ {\!^2{\text{E}}}$. Also, shown are the other higher energy quartet and doublet states, and the ligand-centred states (Ohno 1983 1986) depicted as dashed curves? not shown are some 12 additional doublet states. Several studies have been

directed toward the long-lived (τ ~ 0.1 - 1 ms) luminescent ${\!^2{\text{T}}_1}/ {\!^2{\text{E}}}$ states; prompt fluorescence from ${\!^4{\text{T}}_2}$ has not been observed in fluid media at ambient temperatures, implying either that the lifetime of ${\!^4{\text{T}}_2}$ is too short or that this state is not populated. Recent pulsed laser transient absorption spectroscopy experiments have indicated that τ of ${\!^4{\text{T}}_2}$ of Cr(acac)₃, Cr(NCS)₆ ^3-, t-Cr(NH₃)₂(NCS)₄ ^-, t-Cr(en)₂(NCS)₂ ⁺, Cr(en)₃ ³⁺, and Cr(bpy)₃ ³⁺ are a few ps or less (Kirk 1976; Rojas 1986 1987; Pyke 1978; I.eSage 1983 ; Nicollin 1980).