Photochemistry of Metalloporphyrins in Polymer Matrices. Laser Photolysis Studies of Chlorochromium(III) Tetraphenylporphyrin in Polystyrene Films in the Temperature Range 77−300 K

Abstract

Nanosecond laser photolysis studies of chloropyridinato-and chloroaquachromium(III) tetraphenylporphyrin, ClCr III TPP(Py) and ClCr III TPP(H 2 O), dissolved in polystyrene films have been carried out in the temperature range 77-300 K. The transient observed for ClCr III TPP(Py) after laser pulsing is solely the excited states (the tripquartet, 4 T 1 , and tripsextet, 6 T 1 , states). The emission from the tripquartet state, 4 T 1, is detected upon laser excitation of ClCr III TPP(Py). No photodissociation of axial pyridine is detected in the temperature range studied. The laser photolysis of ClCr III TPP(H 2 O) in polystyrene films gives two transients; the excited states ( 4 T 1 and 6 T 1 ) of ClCr III TPP(H 2 O) and the five coordinate ClCr III TPP yielded by the photodissociation of axial H 2 O from ClCr III TPP(H 2 O). The yields of the excited states ( 4 T 1 and 6 T 1 ) for both ClCr III TPP(Py) and ClCr III TPP(H 2 O) were found to increase with a decrease in temperature. The lifetimes and the relative yields of the excited states and the relative yields of the photodissociation of the axial ligands were measured in the temperature range 77-300 K. These results lead to the conclusions that (1) the energy gaps between the 4 T 1 and 6 T 1 states are 1.5 and 1.1 kcal mol -1 for ClCr III TPP(Py) and ClCr III TPP(H 2 O), respectively, (2) the full photodissociation of axial H 2 O from ClCr III TPP(H 2 O) occurs from the singquartet state, 4 S 1 , and (3) the photodissociation of the axial ligand is the major pathway for excitation energy dissipation at the 4 S 1 state of six-coordinate chromium(III) porphyrin.

Laser Photolysis of Chromium(lll) Porphyrins in Toluene Solutions. Effects of Oxygen on the Photodissociation Yields of the Axial Ligands

“…The respective values are smaller than those obtained in toluene solutions, indicating that the high viscosity of the polystyrene film suppresses the ligand dissociation process at the 6Tl state. In contrast to the results mentioned above, the laser photolysis of Cr11'TPP(Cl)(H20) at 298 K gives only CrI''TPP(CI) as a transient product (24). The 'TI state could not be detected.…”

“…1) except for small differences around the 435 nm region. Previous study has demonstrated that the transient spectrum detected in polystyrene film is solely ascribed to the 6Tl state of Cr"'TPP(Cl)(Py) (24). Thus, as described previously (14,19), the 34 ns spectrum is explained as a spectrum of the 6TI state overlapping with that of the five coordinate species, CrlllTPP(CI), which is produced by photoejection of Py from Cr"'TPP(Cl)(Py).…”

“…Laser photolysis of CrU'TPP(CI)(L) in polystyrene films was carried out at 298 K to elucidate the effects of solvent rigidity on the photochemical reactions. The transient spectra observed for Cr"'TPP(CI)(L) (L = Py, Pip, 3-CNPy and 1-MeIm) in the polymer films were solely for the 6TI state (24). No photochemical dissociation of L from Cr"' TPP(Cl)(L) could be detected.…”

“…The 0, concentration in toluene solutions was obtained by measuring the 0, pressure by a mercury manometer: the Bunsen coefficient of 0, in toluene is 0.22 at 298 K (23). Polystyrene films dissolving chromium(II1) porphyrin were prepared according to a method previously reported (24). Absorption spectra were recorded on a Hitachi U-3OOO spectrophotometer.…”

Coordination of Aza Heterocycles with Macroheterocyclic Metal Complexes in Amphiprotic Media: I. Reaction of Imidazole with Chromium(II) (Acetate)tetraphenylpophyrinate

“…According to [9,10], in aqueous solutions Cr(III) complexes tend to lose the acido ligand localized in the fifth coordination position to give charged bisaqua complexes whose reaction with imidazole gives rise to bisimidazole derivatives. Conductometric and spectral studies [4,13] showed that in organic solvents (acetonitrile, DMSO) coordination of heterocycles involves no dissociation of acido ligand and formation of charged complexes, whereas analysis of the electronic absorption spectra of chromium(III) (acetate)tetraporphyrinate (AcO)CrTPP in toluene made Tipugina and Lomova [8] to suggest that axial coordination involves formation of ion pairs. Figure 1 illustrates the changes in the electronic absorption spectra, produced by addition of imidazole (Im) into a solution of (AcO)CrTPP in ethanol.…”