Trans/cis photoisomerization of 4‐stilbenecarboxylate coordinated to rhodium (III) and iridium(III)

The electronic absorption spectra of trans‐4‐stilbenecarboxylic acid HTSC, its sodium salt NaTSC, [Rh(NH 3 ) 5 TSC] 2+ , and [Ir(NH 3 ) 5 TSC] 2+ dissolved in different solvents, all show a nearly identical long wavelength absorption band at 319 nm. This band is assigned to the first spin‐allowed (S o → S 1 )ππ* transition of TSC − and appears as an isolated intraligand chromophore in the complexes. Whereas HTSC and TSC − emit a fluorescence, this emission is completely quenched in the complexes. Not only HTSC, but also [Rh(NH 3 ) 5 TSC] 2+ , and [Ir(NH 3 ) 5 TSC] 2+ undergo trans/cis photoisomerization of the stilbene moiety upon S o → S 1 intraligand excitation. While the S 1 state of the free ligand is assumed to initiate the isomerization, the results support the first triplet (T 1 ) of the coordinated TSC − as the photoactive excited state. In the complexes, the heavy metals seem to induce S 1 → T 1 intersystem crossing.