Synthesis and Photophysical Properties of New Chromium(III) Complexes of N-Derivatized 1,4,8,11-Tetraazacyclotetradecane Ligands cis-[Cr(1,8R2cyclam)Cl2]Cl, Where R Is a Pendant Chromophore. Exclusive Formation of the cis Isomer

Several new chromium(III) complexes have been synthesized utilizing derivatives of the macrocyclic ligand cyclam (1,4,8,11-tetraazacyclotetradecane) with various N-substituted chromophores in the 1 and 8 positions (1,8-R2cyclam) where R = CH3, CH2Ph, CH2Nph (Nph = naphthyl), and CH2Anth (Anth = anthracyl). X-ray crystal structures were determined for all four complexes, and these are formed exclusively in the cis configuration with the two tertiary amines in the "hinge" positions (i.e., along the folding axis) of the coordinated ligand. As a result, the cis isomers appear to be inert to isomerization under conditions dramatically more forcing than needed to effect the cis to trans isomerization of the unsubstituted Cr(cyclam)Cl2+ ion. Photophysical studies demonstrated that emission occurs solely from the metal-centered ligand field doublet excited states regardless of whether initial excitation is into the quartet ligand field bands or into the (pi)-(pi)* bands of the pendant chromophore. Thus, excitation of the pendant chromophore results in efficient intramolecular energy transfer to the metal centered ligand field excited states.