Visible-light NO photolysis of ruthenium nitrosyl complexes with N2O2 ligands bearing pi-extended rings and their photorelease dynamics

NO photorelease and its dynamics for two {RuNO}(6) complexes, Ru(salophen)(NO)Cl (1) and Ru(naphophen)(NO)Cl (2), with salen-type ligands bearing pi-extended systems (salophenH(2) = N,N'-(1,2-phenylene)-bis(salicylideneimine) and naphophenH(2) = N,N'-1,2- phenylene-bis(2-hydroxy-1- naphthylmethyl-eneimine)) were investigated. NO photolysis was performed under white room light and monitored by UV/Vis, EPR, and NMR spectroscopies. NO photolysis was also performed under 459 and 489 nm irradiation for 1 and 2, respectively. The photochemical quantum yields of the NO photolysis (phi(NO)) of both 1 and 2 were determined to be 9% at the irradiation wavelengths. The structural and spectroscopic characteristics of the complexes before and after the photolysis confirmed the conversion of diamagnetic Ru(II)(L)(Cl)-NO+ to paramagnetic S = 1/2 Ru(III)(L)(Cl)-solvent by photons (L = salophen(2-) and naphophen(2)(-)). The photoreleased NO radicals were detected by spin-trapping EPR. DFT and TDDFT calculations found that the photoactive bands are configured as mostly the ligand-to-ligand charge transfer (LLCT) of pi(L) -> pi*(Ru-NO), suggesting that the NO photorelease was initiated by the LLCT. Dynamics of NO photorelease from the complexes in DMSO under 320 nm excitation were investigated by femtosecond (fs) time-resolved mid-IR spectroscopy. The primary photorelease of NO occurred for less than 0.32 ps after the excitation. The rate constants (k(-1)) of the geminate rebinding of NO to the photolyzed 1 and 2 were determined to be (15 ps)(-1) and (13 ps)(-1), respectively. The photochemical quantum yields of NO photolysis (phi(NO), lambda = 320 nm) were estimated to be no higher than 14% for 1 and 11% for 2, based on the analysis of the fs time-resolved IR data. The results of fs time-resolved IR spectroscopy and theoretical calculations provided some insight into the overall kinetic reaction pathway, localized electron pathway or resonance pathway, of the NO photolysis of 1 and 2. Overall, our study found that the investigated {RuNO}(6) complexes, 1 and 2, with planar N2O2 ligands bearing pi-extended rings effectively released NO under visible light.