The new cyrhetrenyl acylhydrazone [(CO)(3)Re(eta(5)-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 thatE-CyAHadopts anE-configuration around the iminic moiety [-N = C(CH3)]. The absorption spectrum ofE-CyAHdisplays two bands at 270 and 380 nm, which are mainly ascribed to pi -> pi* intraligand (IL) and d(pi) -> pi* metal-to-ligand charge transfer (MLCT) transitions, being consistent with DFT/TD-DFT calculations. Upon 365 nm irradiation,E-CyAHphotoisomerizes toZ-CyAH, as evidenced by UV-Vis and(1)H-NMR spectral changes, with a quantum yield value phi(E)(-CyAH ->)(Z)(-CyAH)of 0.30.Z-CyAHundergoes a first-order thermal back-isomerization process, with a relatively short half-life tau(1/2)of 277 min. Consequently,E-CyAHwas 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-CyAHandZ-CyAHefficiently photosensitize the generation of singlet oxygen (O-2((1)Delta(g))) with good yield (phi(Delta) = 0.342).