STEREOCHEMICAL AND ELECTRONIC CONTRIBUTIONS TO E-2 CHROMIUM(III) EXCITED-STATE RELAXATION BEHAVIOR AT 77K

Variations in the radiative rate constants, kr, for the 2E emission of chromium(III) am(m)ine complexes span at least a 78-fold range and are determined by spin-orbit coupling to the nearest quartet excited states. The 77K lifetimes of most of the (N-H) perdeuterated complexes are found to be correlated with the same spin-orbit coupling parameters. The notable exception found to this correlation is the N-H deuterated 1,2-bis (1,4,7-triaza-1-cyclononyl)ethane-chromium(III), Cr([9]aneN3)2Et3+, complex for which the observed lifetime at 77K is 15% of that predicted by correlation with other am(m)ine complexes. This exception demonstrates that some low frequency mode mediated, non-radiative relaxation pathway can contribute to relaxation of the 2E excited state of Cr(III) complexes. The exceptional photophysical behavior of this complex extends to its emission spectra which are broad and matrix sensitive. It is postulated that the 2E excited state of Cr([9]aneN3)2Et3+ is distorted owing to the tendency of the bis-macrocyclic ligand to twist towards a prismatic geometry. It is inferred that variations in the electronic coupling matrix elements account for much of the variation in 77K (2E)Cr(III) lifetimes. © 1990.