PHOTOCHEMISTRY OF COMPLEX IONS .7. TRANS-4-STILBENECARBOXYLATOPENTAAMMINECOBALT(III) ION

The aqueous ion Co(NH3)5(TSC)2+, where TSC denotes trans-4-stilbenecarboxylate, appears in most respects to be a normal member of the carboxylatopentaamminecobalt(III) family. The second ligand field band is masked, however, by a strong absorption at 320 mμ, essentially identical with that for the free TSC molecule. The first singlet-singlet transition of the TSC moiety thus appears to function as an isolated chromophoric group of the complex. Irradiation of the 320-mμ band of the free TSC ligand leads to trans to cis isomerization; however, irradiation of this same band in the complex produces cobalt(II) in a quantum yield of 0.16 and both some free (unisomerized) and some oxidized TSC ligand. Other members of the carboxylatopentaamminecobalt(III) series are nearly inert to this wavelength of irradiation. Free TSC- ion shows a peak fluorescent emission at 400 mμ, which is partially quenched in the presence of Co(NH3)5(H2O)3+ and essentially completely so when the ligand is coordinated. It is concluded that an intramolecular excitation energy transfer occurs, probably to produce a charge transfer triplet excited state of the complex, which then undergoes redox decomposition. The suggested process is one of intersystem crossing from the first singlet excited state of the TSC ligand.