Damodar Reddy

Fluorescence properties of distorted short-chain basket handle porphyrins

Abstract The singlet excited state properties of a series of basket handle porphyrins and their protonated and Zn2+ derivatives, determined by fluorescence spectroscopy, are described. The introduction of a short chain across the periphery of the porphyrin ring induces distortion in the porphyrin plane. The decreased quantum yields and the ease of oxidation relative to the corresponding planar porphyrin have been attributed to the induced distortion. On protonation, the emission bands experience a blue shift due to a lack of the expected resonance interaction. This is ascribed to the restricted rotation of the porphyrin-phenyl bond imposed by the ortho substituent. The Zn2+ porphyrins show emission from the S1 and S2 excited states with quenching of fluorescence. The decreased energy gap between the S1 and S2 excited states in the distorted Zn2+ porphyrins promotes the S2??S1 internal conversion, thereby decreasing the S2→S0 fluorescence yield.

Non-planarity and its implications on triplet state characteristics of basket handle porphyrins

Abstract The influence of non-planarity in the porphyrin ring on the triplet state characteristics of free base and Zn(II) derivatives of basket handle porphyrins is investigated by triplet ESR. The deviation of the zero-field splitting parameter, D, from planar H2TPP or ZnTPP is dependent on the degree of non-planarity and increases as the non-planarity increases. The electron spin polarization (ESP) pattern for most derivatives remains the same as observed for H2TPP or ZnTPP. However, MSICI8, PSIBr8, ZnMSIICI8 and Znbutyl II show different ESP patterns, suggesting a redirection of the spin sublevel activity in these isomers. The ESP pattern for PSIBr8 is temperature dependent (eeaeaa at −160°C and eeeaaa at −106°C). The calculated structure using molecular mechanics reveals maximum deformation in these isomers which in turn governs the population and decay of the triplet sublevels. The structures indicate a different orientation of bridging phenyl groups in PSIBr8 and MSICl8 with respect to the porphyrin plane.

Brominated short-chain basket-handle porphyrins and their copper(II) derivatives: spectral and electrochemical studies on the effect of β-substitution versus distortion

Pyrrole-brominated short-chain basket-handle porphyrins and their copper(II) derivatives have been synthesized and characterized. These porphyrins are significantly distorted and the β-substitution by bromines further enhances the degree of distortion. Optical absorption and electrochemical sudies reveal considerable shifts in the energies of absorption maxima and electrode potentials. An analysis of these shifts indicates significant changes in the energies of the highest-occupied (HOMO)[a1u(π) and a2u(π)] and lowest-unoccupied molecular orbitals (LUMOs)[eg(π*)] of the porphyrin ring upon β-substitution and distortion. Specifically: (a) both distortion and β-substitution decreases the energy gap between the HOMO and LUMO due to different stabilization/destabilization mechanisms and (b) the magnitude of separation between a1u(π) and a2u(π) is large for β-substitution while it is small for distortion relative to the corresponding planar unsubstituted derivative. The absorption and redox potential shifts show a non-linear behaviour with the number of bromine substituents and this is ascribed to the combined effect of antagonistic inductive interactions and steric hindrance. ESR spectral studies indicate weakening of the Cu–N σ bond without much change in the electronic structure of Cu2+.

Short-chain basket handle porphyrins: singlet and triplet excited state properties

Abstract The excited state properties of the title compounds and their Zn 2+ derivatives, determined by fluorescence and photoexcited tripled ESR methods, are reported. Fluorescence spectral studies indicate that the basket handle porphyrins are less fluorescent than the parent meso-tetraphenyl porphyrin (H 2 TPP). The combined effects of intersystem crossing and internal conversion account for this difference. Estimated excited state potentials suggest that the cross trans linked isomer (PSI) is a better electron donor in the first singlet excited state. Zero-field splitting parameters D and E of the three isomers evaluated from photoexcited triplet ESR show small but significant differences attributed to distortion of the porphyrin plane caused by introduction of a short bridging group. The electron spin polarization (ESP) pattern remains the same as in H 2 TPP, indicating the usual spin—orbit coupling mechanism for intersystem crossing.

Optical and ESR studies on dimerization of metallotetra-crowned phthalocyanines

Abstract The cation and solvent induced dimerization of free base tetra-crowned phthalocyanine (H 2 tCRPc) and its metal derivatives (Ag 2+ and VO 2+ ) have been studied by optical and ESR methods. Dimerization induces a blue shift of Q and Soret bands in the optical spectrum with a reduction in intensity. The exciton formalism accounts for the spectral blue shift upon dimerization. ESR studies reveal the formation of a cofacial dimer with axial symmetry upon the addition of cations. The measured zero-field splitting values in parallel and perpendicular regions suggest that the two paramagnetic centres are separated by 4.9 A in VOtCRPc and 4.8 A in AgtCRPc. In the micellar medium, the dimerization effects are observed only in 0.1 M SDS, while the tetra-crowned phthalocyanines remain aggregated in 0.1 M CTAB and in 5% TX-100.

π-Cation radicals of iron(III) derivatives of deformed porphyrins

π-Cation radicals of iron(III) derivatives of some deformed porphyrins have been characterised using UV/VIS, IR and 1H NMR spectroscopy and by magnetic susceptibility measurements. The deformation in these systems has been induced by a covalent attachment of short bridging chains across the porphyrin periphery. Molecular mechanics simulation clearly reveals the enforced deformation in the porphyrin cores of the free bases inferred from the multiplet structure for the pyrrole protons by 1H NMR spectroscopy. Significant red shifts in Soret (171–340 cm–1) and Q-band (76–354 cm–1) absorption maxima for the iron(III) derivatives relative to [Fe(tpp)Cl](H2tpp = 5,10,15,20-tetraphenylporphyrin) implies the retention of deformation in solution. Collective evidence from IR, UV/VIS and 1H NMR spectroscopy and magnetic susceptibility measurements suggests for the oxidised derivatives a high-spin iron(III) state (S=5//2) antiferromagnetically coupled to a porphyrin radical cation (S=1//2). Magnetic moment values (4.8–5.1 µβ) measured for solids and CD2Cl2 solutions indicate an intramolecular d–π coupling facilitated through an antiferromagnetic exchange. These results substantiate the emerging correlation between the structure of the macrocycle and the metal–ligand magnetic interactions in high-spin iron(III) porphyrin radical-cation complexes.

Dimerization effects on spectroscopic properties of water-soluble porphyrins in aqueous and micellar media

The effects of cation-crown ether induced dimerization on the optical and fluorescence spectra of free base 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (H2tcpp4–) and 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrin (H2tpps4–) and their metal derivatives (Cu2+, Ni2+ and Zn2+) have been studied in aqueous and micellar media. Dimerization induces a red shift of Q-bands in both optical and fluorescence spectra with quenching of fluorescence intensity. However, the Soret band experiences a blue shift in Cu2+ and Ni2+ derivatives, a red shift in Zn2+ derivatives and no shift in the free bases. The magnitude of these shifts depends on the structure of the metalloporphyrin as well as on the nature of the crown ether and cation present in solution. The formation constants for the dimer follow the order Cu2+≈ Ni2+ > H2tcpp4– > Zn2+. In micellar media the dimerization effect is observed only in anionic sodium dodecyl sulphate. In hexadecyltrimethylammonium bromide and Triton X-100, the porphyrins remain monomeric. An analysis of the data suggests that four cation-crown ether complexes are involved per dimer unit. The exciton formalism coupled with π-π interaction accounts for the observed shifts in Cu2+, Ni2+ and free-base porphyrins while a satisfactory interpretation has to take charge-transfer contributions into account for Zn2+ derivatives.

Cyclotriphosphazene linked tetraphenylporphyrine

Condensation of the functionalised cyclophosphazene aldehyde, N 3 P 3 (OC 6 H 5 ) 5 (m-OC 6 H 4 -CHO)(3) with pyrrole affords the tetraphenylporphyrin (4) containing the cyclophosphazene unit. This has been characterized by spectroscopic and analytical methods. Kinetics of metallation (Cu 2+ , Zn 2+ ) of (4) indicate slower rates of insertion relative to normal tetraphenylporphyrins. The esr spectrum indicates an axial symmetry for the Cu 2+ derivative (6).

Optical, NMR and ESR evidence for the oxidation of cobalt porphyrins in the presence of an electron acceptor

We report our studies on the oxidation of various cobalt(II) porphyrins with different substituents in the presence of an electron acceptor, 4,6-dinitrobenzofuroxan (BFO). The oxidised cobalt(III) porphyrin and the reduced anion radical of BFO have been characterised for the first time in solution using optical, 1 H, 19 F NMR and ESR techniques

Short-chain basket handle porphyrins: synthesis and characterisation

Basket handle porphyrins containing short p- or m-phenylenedimethylenedioxy chains covalently linked at the ortho position of phenyl groups of 5,10,15,20-tetraphenylporphyrin have been synthesised. Isomer I of the para derivative (H2LIa) and isomers II and III(H2LIIb and H2LIIIb) of the meta derivative have been characterised by various spectroscopic methods. Electronic and 1H NMR spectral studies indicate significant distortion of the porphyrin skeleton in these derivatives. Protonation of the free-base derivatives results in a small blue shift of the Q bands, attributed to the lack of conjugation between the phenyl groups and the porphyrin plane because of restricted rotation of the porphyrin–phenyl bond. The bridging phenylenedimethylenedioxy group does not interfare with the axial ligation of metal derivatives (Co2+ and Zn2+) of H2LIIb and H2LIIIb. Electrochemical studies indicate easier oxidations and more difficult reductions for the free-base derivatives relative to the corresponding unstrapped derivative, attributed in the former case to the loss of coplanarity due to distortion and in the latter to destabilisation of the anions and dianions due to the lack of solvation. Redox potential data for [CoLIIb] indicate that the metal centred molecular orbital A1(dz2) is situated above the filled A2u orbital of the porphyrin ring.