Photosubstitution reactions of trans-chloro(azine)(1,4,8,11-tetraazacyclotetradecane)ruthenium(II) complexes

Visible range photolyses of trans-[RuCl(cyclam)L](+) (cyclam = 1,4,8,11-tetraazacyclotetradecane; L = pyridine (py), 4-picoline (4-pic), isonicotinamide (isn), or 4-acetylpyridine (4-acpy)) were studied in acidic aqueous solution, and lead to exclusive azine photoaquation. The py and 4-pic complexes have a relatively high, and irradiation wavelength independent, quantum yield (0.020 < Phi < 0.025), while the isn and 4-acpy complexes have lower quantum yields (Phi(isn) approximate to 0.006; Phi(4-acpy) < 0.001). The relative Phi s show patterns consistent with the excited-state tuning model proposed to explain the photochemistry of other related ruthenium(II) ammines. The results indicate that the excited-state precursor of the photochemistry should be a LF* and has a strong contribution from the d(z2) orbital, which should lie lower in energy than the d(x2-y2). Accordingly, considering an approximate C-4v symmetry for the complexes, this LF* should be a (3)E. The results also suggest that the intramolecular hydrogen bonds to the chloride that occur in the ground state may be still operative in the excited state in order to prevent the photoaquation of chloride.