Syntheses and Photophysical Investigations of Cr(III) Hexadentate Iminopyridine Complexes and Their Tris(Bidentate) Analogues

We report the preparation, photophysical characterization,
and
computed excited state energies for a family of Cr­(III) complexes
based on iminopyridine (impy) Schiff base ligands: compounds 1 and 2 feature hexadentate ligands where tren
(tris-(2-aminoethyl)­amine) caps three impy groups; compounds 3 and 4 are tris­(bidentate) analogues of 1 and 2; compounds 2 and 4 contain methyl ester substituents to alter ligand donation properties
relative to 1 and 3, respectively. Cyclic
voltammograms exhibit multiple reversible ligand-based reductions;
the hexadentate and tris­(bidentate) analogues have almost identical
reduction potentials, and the addition of ester substituents shifts
reduction potentials by +200 mV. The absorption spectra of the hexadentate
complexes show improved absorption of visible light compared to the
tris­(bidentate) analogues. Over periods of several hours to days,
the complexes undergo ligand-substitution-based decomposition in 1
M HCl(aq) and acetonitrile. For freshly prepared sample
solutions in CH3CN, time-resolved emission and transient
absorption measurements for 4 show a doublet excited
state with 17–19 μs lifetime at room temperature, while
no emission or transient absorption signals from the doublet states
are observed for the hexadentate analogue 2 under the
same conditions. The electronic structure contributions to the differences
in observed photophysical properties are compared by extensive computational
analyses (UB3LYP MD-DFT and TD-DFT-NTO). These studies indicate that
the presence of nonligated bridgehead nitrogen atoms for 1 and 2 significantly reduce excited state doublet, quartet,
and sextet energies and change the character of the low lying doublet
states in comparison to species that show population of doublet excited
states.