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

Abstract

Although kinetically inert cationic Co(III)TMPyP 5+ (H2TMPyP 4+ = 5,10,15,20-tetrakis(methylpyridinium-4-yl)porphyrin) was considered earlier to be very weakly emissive, both the spectrum and the lifetime of its fluorescence could be determined. Besides, this complex proved to be favorable for outer-sphere photoinduced reduction of the metal center in the presence of triethanolamine (TEOA) as electron donor quenching the triplet excited state of this metalloporphyrin. The corresponding cobalt(II) porphyrin formed in this way was also photoactive; it forwarded an electron to a suitable acceptor (e.g., methylviologen) upon irradiation, regenerating the starting complex. Hence, this system may be a candidate for hydrogen generation from water by utilization of visible light. Metalloporphyrins play important roles in nature, due to their special spectral, coordination and redox features. Their advantageous photoinduced properties can also be exploited in various photocatalytic procedures [1]. Water-soluble derivatives can be utilized in environmentally benign systems not containing organic solvents. Kinetically inert in-plain metalloporphyrins, in which the metal center coplanarly fits into the cavity of the ligand, may offer promising possibilities for realization of photocatalytic systems based on outer-sphere electron transfer [2]. The so-called hyper-porphyrins can be especially interesting in this respect, due to their distorted structure, which may increase the (photo)redox reactivity of these complexes. From water-soluble metalloporphyrins of this type, photoredox reactions of manganese(III) complexes were thoroughly studied [1,3,4], while scarce attention was paid to the corresponding cobalt(III) porphyrins in this respect. Keywords: cobalt(III) porphyrinPhotocatalytic oxidation of the sulfide content of a wastewater to sulfate was studied with Co(III)TMPyP 5+ , Mn(III)TMPyP 5+ , and Fe(III)TMPyP 5+ (H2TMPyP 4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin) [5]. Upon irradiation in the range of the Soretbands, the cobalt(III) complex proved to be the most efficient, but the results have not been interpreted. This metalloporphyrin can also connect to the chain of DNA and oxidatively split it in the presence of suitable electron acceptors [6]. Since cationic manganese(III) porphyrins proved to be efficient photocatalysts in the presence of appropriate electron donors (such as EDTA and TEOA) and methylviologen as electron acceptor [1,3,7], cationic cobalt(III) porphyrins, the other characteristic representatives of water-soluble hyper-porphyrins, are also worth investigating in this respect. Hence, in this work, some photophysical and photochemical properties of Co(III)TMPyP 5+ were studied, also confirming its photocatalytic behavior, which may be utilized in water-splitting by solar radiation.The compounds used for our experiments were of reagent grade. Water purified in a Millipore/Milli-Q system was applied as solvent. Stock solutions of Co(III)TMPyP 5+ were prepared by in situ generation ...

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

“…Nevertheless, the fluorescence lifetimes for these two spin states slightly deviated: 1.36 ns for the square planar and 1.19 ns for the octahedral complex. The fluorescence quantum yield at Soret-band excitation (fl = 2.010 -3 ) is in the same order of magnitude as that observed earlier for Co(III)TMPyP 5+ (9.910 -4 [7]). The higher value may be attributed to the larger ionic radius of Ni(II) (63 vs. 55 pm [18]) causing a smaller distortion (shrinkage) of the porphyrin ring.…”

“…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].…”

“…This type of metalloporphyrins can be formed with the ions of the iron group, such as Fe(III), Co(III), and Ni(II). 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).…”

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

“…According to the generally accepted concept, earlier the highly distorted complexes with Mn(III) or Co(III) metal center were considered not to display appreciable fluorescence at room temperature [4]. However, in recent studies characteristic emission spectra were observed in the case of Co(III)TMPyP 5+ [5] and manganese(III) complexes with 5-[4-(5-carboxy-1-butoxy)-phenyl-10,15,20-tris(1-methylpyridinium-4-yl)porphyrin [6], 5,10,15,20-tetraphenylporphyrin, and 5,10,15,20-tetra(3-hydroxyphenyl)porphyrin [7].…”