2008 Eighth Annual Beckman Scholars Symposium
Friday Poster Session - July 28, 2006

We have been studying a set of trans-dicyanochromium(III) complexes of topologically constrained tetraazamacrocycles, namely, trans-[Cr(1,11-C3-cyclam)(CN)2]+ and trans-[Cr(1,4-C2-cyclam)(CN)2]+, to determine the effect of the additional strap on the chemistry and photophysics relative to the centrosymmetric cyclam complex. The 1,11-C3-cyclam and 1,4-C2-cyclam complexes have room-temperature 2Eg excited state lifetimes of 1-3 orders of magnitude lower than the cyclam complex, yet this may not indicate different mechanisms for excited-state deactivation. Temperature-dependent studies show that the room-temperature lifetimes can be explained by variations among the complexes’ activation barriers for thermally activated relaxation. Also, photodeuteration studies of the cyclam complex provide strong evidence for a thermally activated relaxation mechanism of Cr-macrocyclic N bond cleavage. Studies of the analogous [Cr(N4)F2]+ complexes demonstrate similar room-temperature lifetime variations based on the competition between thermally independent nonradiative relaxation and a thermally activated relaxation pathway.