Photoluminescence of Enantio-Selectively Formed Chromium(III) Double Salts

Abstract

Chromium complex double salts of the compositions [Cr(en)₃] [Cr(ox)₃] and [Cr(pn)₃][Cr(ox)₃] (en=ethylenediamine, pn=propylenediamine, ox=oxalate), denoted as [en][ox] and [pn][ox], respectively, were prepared from racemic and optically resolved component complexes. (+) or (-) [Cr(en)₃]³⁺ selectively combines with homochiral [Cr(ox)₃]^3- to form a chiral double salt, which has a different structure and a different low-temperature photoluminescence spectrum with respect to the racemic [en][ox] compound. The luminescence at 15 K is due to the ²E_g → ⁴A_2g transition of the oxalate complex. The total luminescence spectrum and the circularly polarized luminescence (CPL) spectrum of chiral [en][ox] are dominated by a progression in a 213 cm⁻¹ mode with S = 0.31. Racemic [pn][ox] also shows a luminescence spectrum with a pronounced vibronic progression.