Photophysical and photochemical properties of manganese complexes with cationic porphyrin ligands: Effects of alkyl substituents and micellar environment

Abstract

Although Mn(III) porphyrins were considered earlier to be very weakly emissive, the fluorescence displayed by Mn(III) complexes with the cationic TMPyP 2+ ligand (H2TMPyP 4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin) as well as with its other alkyl (such as hexyl and dodecyl) derivatives proved to be strong enough for a comparative study. Elongation of the alkyl substituent increased both the quantum yield and the lifetime of the emission for the Mn(III) porphyrins, while resulted in an opposite effect for the corresponding free bases in homogeneous solutions. The presence of cationic micelles, however, reversed this tendency regarding the emission lifetime of the complexes. These cationic metalloporphyrins were applied in a photocatalytic system involving triethanolamine (TEOA) as a sacrificial electron donor and methylviologen (MV 2+ ) as an acceptor. In the first step of the catalytic process outer-sphere photoinduced reduction of the metal center took place via quenching of the triplet excited state of these metalloporphyrins by TEOA. The corresponding manganese(II) porphyrins formed in this way were also photoactive; they forwarded an electron to MV 2+ upon irradiation, regenerating the starting complex. Elongation of the alkyl substituents increased the quantum yield of the formation of methylviologen radical (MV + ) in this system, but considerably decreased the durability of the photocatalyst. Anionic micelles totally hindered the photoinduced generation of Mn(II) porphyrins, while cationic micellar environment appreciably increased the efficiency of the accumulation of MV + .

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

“…Accordingly, porphyrin complexes of low-spin Mn(III) and Co(III) were considered to be very weakly fluorescent. However, current investigations using a more sensitive equipment indicated that both Mn(III)TMPyP 5+ and Co(III)TMPyP 5+ display a characteristic fluorescence in the 600-750-nm range (Fig 2), which is more than an order of magnitude weaker (fl = 1.110 -4 and 1.410 -3 for the Mn(III) and Co(III) complexes, respectively [16,17]) than that exhibited by the corresponding free base (0.047 [18], 0.030 [19]). The shorter emission lifetimes of these complexes are also in accordance with this tendency caused by both electronic and steric interactions between the metal center and the ligand in these hyper-porphyrins.…”

“…The shorter emission lifetimes of these complexes are also in accordance with this tendency caused by both electronic and steric interactions between the metal center and the ligand in these hyper-porphyrins. [16,17].…”

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

“…Due to this band slightly redshifted compared to that of the free base, Ni(II) porphyrins were categorized as hypso type [4,27]. The emission spectrum displayed by Ni(II)TMPyP 4+ upon Soret-band excitation (Fig 2) is very similar to those of the corresponding cationic Co(III) and Mn(III) metalloporphyrins [7,17]. The fluorescence bands in the 550-800-nm range of the fluorescence spectra of porphyrins, both metalloporphyrins and free bases, can be assigned as S1S0 transitions (the individual bands correspond to the (0, 0), (0,1) and (0,2) transitions with respect to vibrational states -the latter one is generally not perceptible ) [19].…”

“…However, while the photoredox chemistry of iron(III) and cobalt(III) porphyrins was thoroughly studied in the past 2-3 decades [6,7,8,9,10], the corresponding nickel(II) complexes were hardly examined in this respect [11,12]. Since the complexes of iron(III), cobalt(III) as well as manganese(III) with the cationic 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (H2TMPyP 4+ ) proved to be promising in various photocatalytic systems [3,7,8,13,14,15,16,17], in this work some photophysical and photochemical properties of Ni(II)TMPyP 4+ were studied in order to examin the potential applicability of this metalloporphyrin in utilization of solar radiation in the visible range.…”

“…It did not perceptively affect the emission lifetime, but considerably decreased the quantum yield; by about 25% upon addition of 0.05 M TEOA. Similarly to the corresponding manganese(III) and cobalt(III) porphyrins [7,15,17], a photocatalytic system was realized with Ni(II)TMPyP 4+ too in presence of TEOA and MV 2+ as electron donor and acceptor, respectively, in argon-saturated aqueous solutions at room temperature (Fig. 3).…”