| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1311667 | Inorganica Chimica Acta | 2007 | 11 Pages |
The synthesis and characterization of the trans-difluorochromium(III) complexes of the constrained macrocyclic ligands 1,4-C2-cyclam = 1,4,8,11-tetraazabicyclo[10.2.2]hexadecane and 1,11-C3-cyclam = 1,4,8,11-tetraazabicyclo[9.3.3]heptadecane is reported. Only trans complexes are formed, and the structures of both trans-[Cr(1,4-C2-cyclam)F2]ClO4 and trans-[Cr(1,11-C3-cyclam)F2]ClO4 are presented. The photochemical and photophysical behavior of the 1,4-C2-cyclam and 1,11-C3-cyclam complexes is compared with that of the corresponding tet a (C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) complex. The constraint imposed by the additional bridging groups of 1,4-C2-cyclam and 1,11-C3-cyclam distorts the complexes away from both octahedral symmetry and centrosymmetry, as evidenced by the bond angles that deviate from ideal 90° and 180° values and by a significant distortion of the Cr(N4) plane. This reduction in symmetry correlates with a monotonic increase in the extinction coefficients in going from the tet a to the 1,11-C3-cyclam, to the 1,4-C2-cyclam complex. These three complexes also exhibit large variations in their aqueous room-temperature excited state behavior; namely, the lifetimes of the 2T1g (Oh) excited states are 30, 60, and 1.0 μs for the tet a, 1,11-C3-cyclam, and 1,4-C2-cyclam complexes, respectively. Studies of the excited-state lifetime of these complexes in acidified H2O/dimethyl sulfoxide over the temperature range between −30 and +95 °C suggest that the 1,4-C2-cyclam complex accesses a temperature-dependent relaxation mechanism at significantly lower temperatures than do the tet a or 1,11-C3-cyclam complexes. The emission behavior of these complexes is also solvent-dependent, consistent with established theories that the degree of splitting of the emitting 2T1g (Oh) state varies with the hydroxylic nature of the solvent.
Graphical abstractThe synthesis and characterization of the trans-difluorochromium(III) complexes of two constrained tetraazamacrocycles, 1,4-C2-cyclam and 1,11-C3-cyclam, is presented. The solvent and temperature dependence of the 2T1g (Oh) excited state behavior is described as well as possible effects of geometric distortion on the photophysics and photochemistry of these complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide
