Application of the electronic spectra of porphyrins for analytical purposes: The effects of metal ions and structural distortions

Abstract

We investigated the UV-Vis absorption, singlet-1 and singlet-2 fluorescence, as well as the formation of several metalloporphyrins from equilibrial and kinetic aspects in aqueous solution. Among these complexes were numerous typical out-of-plane and several in-plane metalloporphyrins, and between the two categories, a few border-line cases. On the basis of our results, we have complemented the categorization introduced by Barnes and Dorough for the metalloporphyrins. According to our observations, also in metalloporphyrins, the distortion, i.e., the planarity or nonplanarity of the macrocycle, is basically responsible for the spectral characteristics, while the electronic structure of metal center is a secondary factor, with a considerable importance mainly in the in-plane complexes. The type of complexes can be spectrophotometrically determined on the basis of their UV-Vis absorption and fluorescence spectra. Beside the spectral and photophysical effects of metalation, also those of the structural distortions were studied, which can originate also from metalation, protonation or overcrowded peripheral substitution of the free-base porphyrins, as well as from the axial ligation of metalloporphyrins. Our observation may be useful for different spectrophotometric analytical detection and determination methods, e.g. size-selective metal detection using free-base porphyrins (or other ringed chelate ligands), as well as the determination of Lewis bases as potential axial ligands, using metalloporphyrins.

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

“…The Soret-band of the corresponding cobalt(II) porphyrin (at 429 nm [20]) is also red-shifted compared to that of the free base, however, it is blue-shifted in the relation of the (more distorted) cobalt(III) hyper-porphyrin. This phenomenon can be accounted for the larger radius of the Co 2+ ion, similarly to the case of manganese (II/III) complexes studied earlier [1,14]. The fluorescence bands in the 550-800-nm range of the emission spectra of porphyrins, both free bases and metalloporphyrins, 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) [14].…”

“…For illumination a LED light of 415-465-nm emission with a 440-nm maximum intensity was utilized. Incident light intensity was determined by ferrioxalate actinometry [13].The Co(III) ion of 55 pm ionic radius [8] is small enough to fit into the cavity of the porphyrin ligands, forming unambiguously in-plane complexes [14]. The metalloporphyrins of the significantly larger Co(II) ion (rion = 75 pm) display spectral properties and photochemical behavior deviating from those of the Co(III) complexes.…”

Peculiar photoinduced properties of water-soluble, early lanthanide(III) porphyrins

“…Insertion of these larger metal ions into the porphyrin cavity can be spectrophotometrically followed due to the redshifts of UVVis, intraligand ππ* absorption bands (Fig. 3), similarly to other typical OOP complexes, which display "common" absorption properties [23]. In the absence of the bidentate, O-donor ligand, bisporphyrin (Ln3P2 3-) can form too, which has slightly redshifted and broadened absorption bands, compared to those of the monoporphyrin.…”

“…4). While the lifetimes for mono-and bisporphyrin complexes are almost equal (~2 ns), only the fluoresence quantum yield decreases slightly because the formation of bisporphyrin results only in the deceleration of radiative decay, as a consequence of a special type of aggregation, probably through the peripheral, sulfonato substituents [16,22,24,25] (tail-to-tail or perpendicular head-to-tail dimerization) without strong π-π interactions between the macrocycles [22,23]. If these interactions were stronger, the absorption bands should show much larger redshifts and hyperchromicities, and the fluorescence should be much weaker, nearly disappear.…”

“…If these interactions were stronger, the absorption bands should show much larger redshifts and hyperchromicities, and the fluorescence should be much weaker, nearly disappear. This was manifested in the case of the (parallel) head-to-tail dimer of the protonated porphyrin, (H4TSPP 2-)2 [26], and of the bisporphyrins of mercury(II) ion: (parallel) head-to-tail Hg II 2(TSPP)2 8-and typical head-to-head Hg II 3(TSPP)2 6- [5,23]. The photoinduced reactions of lanthanide(III) mono-and bisporphyrins are also very similar in both the Soret- (Fig.…”