Wavelength-Dependent Control of the CO Release Kinetics of Manganese(I) Tricarbonyl PhotoCORMs with Benzimidazole Coligands

A series of photoactivatable CO-releasing molecules (PhotoCORMs) was prepared from manganese pentacarbonyl bromide and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine ligands (L) bearing different electron-donating and electron-withdrawing groups R = H, 4-CH₃, 4-OCH₃, 4-Cl, 4-NO₂, 2-, 3-, and 4-COOCH₃ on the phenyl substituent to give octahedral manganese(I) complexes of the general formula [MnBr(CO)₃(L)]. Aerated DMSO solutions of the compounds are stable in the dark for 16 h with no CO release. However, the compounds rapidly release CO upon illumination at 412–525 nm, depending on the substitution pattern. Its influence on the photophysical and photochemical properties was systematically explored using UV/Vis spectroscopy and CO release measurements with a commercial gas sensor system. In the nitro-substituted compound, the electronically excited state switched from benzimidazole- to phenyl-centered, leading to a markedly different photochemical behavior of this visible-light activated PhotoCORM.