Bis(aryloxo) derivatives of tin(IV) porphyrins: synthesis, spectroscopy and redox activity

Abstract

Spectroscopic, electrochemical and fluorescence properties of five octahedral tin(IV) porphyrins ([(TpTP)Sn(O-m,p-C 6 H 3 (CH 3 ) 2 ) 2 ] (1), [(TpTP)Sn(O-p-C 6 H 4 (CH 3 )) 2 ] (2), [(TpTP)Sn(O-C 6 H 5 ) 2 ](3), [(TpTP)Sn(O-p-C 6 H 4 O-o,p-C 6 H 3 (NO 2 ) 2 ) 2 ] (4) and [(TpTP)Sn(O-p-C 6 H 4 (NO 2 )) 2 ] (5)) bearing electron donating/electron withdrawing aryloxo subunits as the axial ligands are investigated in detail. These complexes have been synthesized by the reaction of 5,10,15,20-tetra(4-methylphenyl)porphyrinato tin(IV) dihydroxide ([(TpTP)Sn(OH) 2 ]) and either 3, 4-dimethylphenol, p-cresol, phenol, 4-(2,4dinitrophenoxy)phenol or p-nitrophenol and were isolated in good-to-moderate yields. Analysis of the spectral data (FAB mass, IR, UV-vis and 1 H NMR) of 1 -5 suggests that the two trans axial aryloxo ligands are strongly bound in a symmetric manner at the tin center in these complexes. Specifically, the general symmetry observed in the 1 H NMR spectra and the ring current effect experienced by the protons present on the axial ligands are revealing in this regard. Each porphyrin ring undergoes two successive, reversible/quasi-reversible, one-electron reductions in CH 2 Cl 2 , 0.1 M TBAP. The bound nitroaromatic axial ligands of complexes 4 and 5 could also be reduced under the same set of experimental conditions. The wavelengths of maximum emission, the singlet energies and the Stokes' shifts observed in the fluorescence spectra of 1 -5 are close to the corresponding parameters of the reference porphyrin, [(TpTP)Sn(OH) 2 ]. On the other hand, fluorescence intensities of complexes 1 -4 are quenched in comparison with that of [(TpTP)Sn(OH) 2 ] in three different solvents. A detailed analysis of the emission and redox potential data indicates that a photoinduced electron transfer from the axial aromatic subunit to singlet state of the basal tin(IV) porphyrin can, in principle, explain the fluorescence quenching observed in these donor-acceptor systems. The spectroscopic and redox features of these tin(IV) porphyrins are compared with those of the analogous phosphorus(V) porphyrins reported by us earlier.

Tin(IV) Porphyrins Containing β‐Substituted Bromines: Synthesis, Conformations, Electrochemistry and Photophysical Evaluation

“…The electronic influences of axial ligands are also evident in the reduction potential data; compared with phenolate compound 4 , compound 5 is easier to reduce by 0.15 V, consistent with the presence of an electron‐withdrawing nitro group. On the other hand, the oxidation waves are ill‐defined and irreversible in nature, and therefore, are difficult to assign, which is similar to the observations made on other reported tin(IV) porphyrins , , …”

Porphyrin‐Based Molecular Rotors as Fluorescent Probes of Nanoscale Environments

“…[27] The most efficient quenching is observed for 1b and 1c for which fluorescence lifetime analysis suggests significant conformational heterogeneity. Taken with the computational studies above, it is proposed that the ease of rotation around the phenolate-NDI link to form high-energy conformations withsome degree of coplanarity may be the important factor in the efficient fluorescence quenching observed in 1a-c.…”

“…[24][25][26] The high charge on the tin centre renders the porphyrin a relatively hard Lewis acid with selectivity for oxo ligands (phenols, carboxylic acids) over softer nitrogenous ligands in solution. [24][25][26][27][28][29][30][31] Importantly for this study, the inductive effect of the high charge on the metal makes tin(IV) porphyrins very stable to ring reduction. [32] In this respect, studies by Reddy and Maiya indicated that ET can occur from a phenolate ligand to the photoexcited porphyrin, the efficiency of which increases with the basicity of the phenolate.…”

“…[32] In this respect, studies by Reddy and Maiya indicated that ET can occur from a phenolate ligand to the photoexcited porphyrin, the efficiency of which increases with the basicity of the phenolate. [27] Accordingly, the fluorescence quantum yield of phenolate-coordinated tin(IV) porphyrins is strongly influenced by the basicity of the phenolate ligand, varying from 0.022 in deaerated CH 2 Cl 2 for the case of the p-nitrophenolate ligation (similar to the quantum yield of several simple tin(IV) porphyrins) to less than 0.001 for the case of p-methylphenolate ligation.…”

Porphyrin-based supermolecules and supramolecular arrays

“…In this respect, combining the highly oxophilic nature of tin(IV)porphyrins with the electronic complementarity that exists between tin(IV)porphyrins and the corresponding free-base and Zn(II) analogues [35] might provide a practical approach to the formation of multiporphyrin assemblies that function as a light harvesting pigment. Tin(IV)porphyrin phenolates are the stable product of the equilibrium-based condensation reaction of substituted phenols with tin(IV)porphyrin dihydroxide in an organic medium (Scheme 4) [36,37]. In fact, the slow rate of condensation allows us to monitor the formation of the bisphenolates and the increase presence of the water byproduct from the reaction (as indicated by the arrow in Scheme 4).…”