LOW-TEMPERATURE DOUBLET LUMINESCENCE AND ABSORPTION-SPECTRA OF MULTIDENTATE HEXAAM(M)INE CR(III) COMPLEXES - EFFECTS OF LIGAND-INDUCED DISTORTION FROM O-H SYMMETRY

Luminescence and absorption spectra of the complexes [Cr([18]aneN6)]Br -3, [Cr(sen)]Br-3, [Cr(taptacn)]Br-3, trans-[Cr(tn)(2)(NH3)(2)](ClO4)( 3), cis-[Cr(tn)(2)(NH3)(2)](ClO4)(3) and [Cr([20]aneN6)]Br-3 have been obtained at low temperatures. ([18]aneN6=1,4,7,10,13,16-hexaazacycloo ctadecane, [20]aneN6=1,4,7,11,14,17-hexaazacycloeicosane, sen=4,4',4'' -ethylidynetris-(3-azabutan-1-amine), n=1,4,7-tris(aminopropyl)-1,4,7- triazacyclononane, tn=1,3-diaminopropane). The observed temperature de pendence of luminescence band intensities, together with the compariso n of the luminescence and absorption spectra, has allowed the assignme nt of the (2)E(g) electronic origins, generally found to be split into two orbital components. The energies of the lowest energy doublet com ponent (the origin splitting) were found to be 14 646{29}, 14 729{16}, 14 818{5?}, 15 108{44}, 15 118{-} and 14 680{72} cm(-1) for the above sequence of complexes, respectively. In addition to the differences i n (2)E(g) splitting, considerable variance was found in the proportion in which the luminescence intensity appeared in the electronic origin s as opposed to the vibronic origins. None of the spectra gave evidenc e for progressions. The data reveal the effect of symmetry lowering at the metal centre on both band intensities and doublet splittings. The observed splittings are compared with those calculated from the angul ar overlap model based on X-ray diffraction determinations of the coor dination geometries around the metal centre. Trans-[Cr(tn)(2)(NH3)(2)] (ClO4)(3) is interesting because it shows the usual doublet splitting plus an additional solid state splitting of 5 cm(-1) observed in the e lectronic origin and repeated with varying magnitude in many of the li nes in the spectrum. This is attributed to the presence of two crystal lographically non-equivalent complex sites in the crystal, which we we re unfortunately not able to verify by X-ray diffraction owing to diso rder in the perchlorate anions.