Electron Exchange and the Photophysics of Metal−Quinone Complexes. 1. Synthesis and Spectroscopy of Chromium−Quinone Dyads

The synthesis, structural and spectroscopic characterization of monosemiquinone and monocatechol complexes of chromium(III) are described. Compounds of the general form [Cr(N4)Q]n+, where N4 represents a tetradentate or bis-bidentate nitrogenous ligand or ligands and Q represents a reduced form of an orthoquinone, have been prepared by two different routes from CrIII and CrII starting materials. The complex [Cr(tren)(3,6-DTBSQ)](PF6)2, where tren is tris(2-aminoethyl)amine and 3,6-DTBSQ is 3,6-di-tert-butylorthosemiquinone, crystallizes in the monoclinic space group P21/c with a = 11.9560(2) Å, b = 17.0715(4) Å, c = 17.1805(4) Å, β = 90.167(1)°, V = 3506.6(1) Å3, Z = 4, with R = 0.056 and R w = 0.070. Alternating C−C bond distances within the quinoidal ligand confirm its semiquinone character. Variable temperature magnetic susceptibility data collected on solid samples of both [Cr(tren)(3,6-DTBSQ)](PF6)2 and [Cr(tren)(3,6-DTBCat)](PF6) in the range 5−350 K exhibit temperature-independent values of 2.85 ± 0.1 μB and 3.85 ± 0.1 μB, respectively. These data are consistent with a simple CrIII−catechol formulation (S = 3/2) in the case of [Cr(tren)(3,6-DTBCat)](PF6) and strong antiferromagnetic coupling (|J| > 350 cm-1) between the CrIII and the semiquinone radical in [Cr(tren)(3,6-DTBSQ)](PF6)2. The absorption spectrum of the semiquinone complex exhibits a number of sharp, relatively intense transitions in fluid solution. Group theoretical arguments coupled with a qualitative ligand-field analysis including the effects of Heisenberg spin exchange suggest that several of the observed transitions are a consequence of exchange interactions in both the ground- and excited-state manifolds of the compound. The effect of electron exchange on excited-state dynamics has also been probed through static emission as well as time-resolved emission and absorption spectroscopies. It is suggested that the introduction of exchange coupling and subsequent change in the molecule's electronic structure may contribute to an increase of nearly 4 orders of magnitude in the rate of radiative decay (k r), and a factor of ca. 107 in the rate of nonradiative decay (k nr).