Equilibrium, Photophysical, Photochemical, and Quantum Chemical Examination of Anionic Mercury(II) Mono- and Bisporphyrins

Abstract

Mercury(II) ion and 5,10,15,20-tetrakis(parasulfonato-phenyl)porphyrin anion can form 1:1, 2:2, and 3:2 (metal ion/porphyrin) out-of-plane (OOP) complexes, from which Hg2P2(8-) has not been identified until now. Identification of this species significantly promoted the confirmation of the composition and the precise elucidation of the equilibrium of Hg3P2(6-). Since the formation of each complex is too fast, their kinetic behavior was studied from the side of dissociation. The rate-determining step in dissociations, as well as in the formation of the 2:2 complex, that is, the dimerization of 1:1 complex, proved to be virtually first-order under these conditions, while the consecutive formations of HgP(4-) and Hg3P2(6-) are second-order reactions. The equilibria can be spectrophotometrically investigated because the Soret- as well as the Q-absorption bands of the free-base ligand are more and more red-shifted in the series of 1:1, 2:2, and 3:2 complexes, and the split of Q-bands disappears as the singlet-1 excited states become degenerate; in the case of bisporphyrins, the bands broaden, especially in the longer-wavelength region of the spectra. The quantum yield and the lifetime of S1-fluorescence from the macrocycle is decreased by the insertion of a mercury(II) ion due to distortion, and in bisporphyrins the luminescence totally ceases because their more complicated structure promotes other ways of energy dissipation. The lifetime of the triplet excited-state is also reduced by metalation. The transient absorption measured upon excitation of Hg3P2(6-) probably originates from Hg2P2(8-) formed by efficient photodissocation during the laser pulse. This photoinduced dissociation is characteristic to out-of-plane complexes, but in metallo-monoporphyrins it needs the energetically higher Soret-excitation; in bisporphyrins, it can take place during irradiation at the longer Q-wavelengths. Investigation of the intramolecular photoredox reactions has proved that for the increased efficiency of the indirect photoinduced LMCT, not the redox potential, but the position of the metal center is responsible. The two orders of magnitude higher photoredux quantum yield for the 3:2 complex, compared to that of the 2:2 species, can be explained by the repulsive effect of the inner mercury(II) ion pushing the other two farther out of the ligand cavity. In bisporphyrins the second excited states are photochemically more reactive than the first ones, while most of the photochemical processes of HgP(4-) originate from the first excited state. According to our quantum chemical calculations, the mercury(II) ion causes the expansion of the porphyrin-cavity; therefore its out-of-plane position is smaller than the value expected based on its ionic radius. In the hitherto unknown 2:2 dimer two 1:1 saucer-shaped monomers are kept together by secondary forces, mostly by pi-pi interaction, but their relative arrangement was not unequivocally determined by the two DFT functionals used. The arrangements with a symmetry axis or plane perpendicul...

Equilibrium, photophysical and photochemical examination of anionic lanthanum(iii) mono- and bisporphyrins: the effects of the out-of-plane structure

“…The calculated geometrical parameters are listed in Table 3, which also involves the data regarding the free base (abbreviated 35 as H2P) and its deprotonated form (P 2-) 44,47 for comparison. The parameters presented are: d(N-N), the distance between diagonally located N atoms, i.e.…”

“…44,47 The fourfold symmetry is inherited 10 by the LaP + complex. As expected, in lanthanum(III) porphyrins the metal ion protrudes from the plane of the ligand the magnitude of which depends on the composition.…”

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

“…6). Also this phenomenon may confirm this special type of aggregation through the peripheral sulfonato substituents because in the case of post-transition metal ions the differences are much larger, mainly in the range of Q-bands [5,6]. The out-of-plane position of the metal center in porphyrin complexes can generally cause two types of photochemical reactions: 1) photoinduced dissociation of the metal ion from the cavity of the ligand (without charge transfers) as a consequence of the lability of the complex; 2) photoredox degradation of the macrocycle initialized by an irreversible LMCT reaction resulting in oxidation of the ligand and reduction of the metal center.…”

“…Besides, the OOP position promotes the formation of bis-or oligoporphyrins, so called sandwich complexes, mainly if the metal center is susceptible to higher coordination numbers ( Fig. 1) [3][4][5][6]. Lanthanide(III) complexes are the model compounds of metallo-oligoporphyrins, in which the π-π interaction depends on the distance between the macrocycles, strongly influenced by the size of the metal center [7][8][9].…”

“…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.…”