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

The new cyrhetrenyl acylhydrazone [(CO)₃Re(η⁵-C₅H₄)-C(O)-NH-N = C(CH₃)-(2-C₄H₂S-5-NO₂)] (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(CH₃)]. 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 → 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 ¹H-NMR spectral changes, with a quantum yield value Φ_{E-CyAH →Z-CyAH} of 0.30. Z-CyAH undergoes a first-order thermal back-isomerization process, with a relatively short half-life τ_1/2 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 (O₂ (¹Δ_g)) with good yield (Φ_Δ = 0.342).