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).