Photophysical and photocatalytic behavior of nickel(II) 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin

Abstract

Kinetically inert cationic Ni(II)TMPyP 4+ (H2TMPyP 4+ = 5,10,15,20-tetrakis(methylpyridinium-4-yl)porphyrin) displayed a characteristic fluorescence (τ = 1.2-1.4 ns, Φ = 2.0×10 -3 ), which was quenched with triethanolamine (TEOA) in a static way. This complex proved to be an efficient photocatalyst in the system containing TEOA and methylviologen (MV 2+ ) as electron donor and acceptor, respectively. Interestingly, however, deviating from the behavior of the analogous Co(III) and Mn(III) complexes in such a system, TEOA did not dinamically quench the triplet excited state of Ni(II)TMPyP 4+ (τ = 6.31 μs), hence no reduction of the metal center occured upon irradition. Instead, in the presence of this electron donor (at 1×10 -3 M) the excited-state lifetime dramatically increased (to τ = 36.6 μs), indicating the formation of a Ni(II)TMPyP 4+ -TEOA associate. This longer-lived triplet was efficiently quenched by MV 2+ (kq = 9.9×10 6 M -1 s -1 ), leading to the formation of MV •+ . The overall quantum yield of this one-step photoassisted electron transfer is considerably high (Φ = 0.011-0.013 at Soret-band irradiation). Hence, this system, combined with a suitable cocatalyst, may be applicable for visible light-driven hydrogen generation from water.

Visible light-driven photophysics and photochemistry of water-soluble metalloporphyrins

“…The Soret-band of the latter species appears at 449 nm, while that of the low-spin complex can be found at 420 nm (very close to the Soret-band of the free-base ligand). Similarly to the corresponding cationic Mn(III) and Co(III) metalloporphyrins, also Ni(II)TMPyP 4+ displays a characteristic fluorescence upon excitation at the Soret-bands [30]. Interestingly, the shape and the position of the emission spectra do not significantly depend on the excitation wavelength, and they well agree with those of the Mn(III)TMPyP 5+ and Co(III)TMPyP 5+ complexes.…”

Photocatalytic Oxygenation by Water-Soluble Metalloporphyrins as a Pathway to Functionalized Polycycles

“…Recently, Co and Ni complexes have also been photophysically investigated by Horváth et al as potential photocatalysts as well as for the functionalization of organic substrates by photocatalytic oxygenation in comparison to Mn(III) porphyrins [38][39][40]. The obtained results well demonstrated how the size of the metal center determines the structure and, thus, the photoinduced behavior of the porphyrin complexes, along with the substituents on the ligand.…”

Formation, Photophysics, Photochemistry and Quantum Chemistry of the Out-of-Plane Metalloporphyrins

“…Nevertheless, in the typical d-type hyperporphyrins, e.g. the low-spin chromium(III), manganese(III), nickel(II) and cobalt(III) porphyrins, the radius of the metal center, and thus, the metal-nitrogen bonds are too short, resulting in the contraction of the coordination cavity, along with the ruffled deformation of the macrocycle [2,[5][6][7].…”

“…triethanolamine, TEOA), these complexes proved to be efficaciousl photocatalysts that transfer electrons between the ground-state reactants through an outersphere mechanism, generating the MV •+ radical cation. This system can be applied for the production of hydrogen from water [2,[5][6][7].…”

“…Overcrowded substitution on the periphery [3][4] or insufficiently short metal-nitrogen bonds due to the shrinkage of the coordination cavity can cause the ruffled or saddled deformation [2,[5][6][7]. If, however, the M-N bonds are significantly longer than half the length of the diagonal N-N distance in the free-base porphyrin, dome deformation can occur.…”