Spectroscopic Study of the E/Z Photoisomerization of a New Cyrhetrenyl Acylhydrazone: A Potential Photoswitch and Photosensitizer.

The new cyrhetrenyl acylhydrazone [(CO) Re(η -C H )-C(O)-NH-N = C(CH )-(2-C H S-5-NO )] (E-CyAH) has been designed, synthesized and fully characterized to study the effect of having a cyrhetrenyl fragment (sensitizer) covalently bonded to an acylhydrazone moiety (switch), on its photophysical and photochemical properties. The crystal structure reveals that E-CyAH adopts an E-configuration around the iminic moiety [-N = C(CH )]. The absorption spectrum of E-CyAH displays two bands at 270 and 380 nm, which are mainly ascribed to π → π* intraligand (IL) and d  → π* metal-to-ligand charge transfer (MLCT) transitions, being consistent with DFT/TD-DFT calculations. Upon 365 nm irradiation, E-CyAH photoisomerizes to Z-CyAH, as evidenced by UV-Vis and H-NMR spectral changes, with a quantum yield value Φ of 0.30. Z-CyAH undergoes a first-order thermal back-isomerization process, with a relatively short half-life τ of 277 min. Consequently, E-CyAH was quantitatively recovered after 24 h, making it a fully reversible T-type molecular photoswitch. This remarkable behavior allows us to measure the individual photophysical properties for both isomers. In addition, E-CyAH and Z-CyAH efficiently photosensitize the generation of singlet oxygen (O ( Δ )) with good yield (Φ  = 0.342).