Photoexcited triplet ESR studies on thiaporphyrins and their protonated derivatives

Excited State Dynamics of Novel Diporphyrins with Dissimilar Cores

The singlet and triplet excited state properties of two diporphyrins, H 2- H and Zn - H , are described. Steady state fluorescence studies indicate that the emission of the H 2- H diporphyrin is dependent on the excitation wavelength and is dominated by the emission of the individual constituent monomers at their respective excitation. Time-resolved studies show two lifetimes ascribable to the normal and thiaporphyrin subunits. However, the emission of the Zn - H diporphyrin is dominated by the thiaporphyrin subunit irrespective of the wavelength of excitation, suggesting an energy transfer from the Zn porphyrin subunit to the thiaporphyrin subunit. Lifetime measurements in toluene show two lifetimes due to open and folded conformations, while in DMF an additional component due to axial ligation is observed. The efficiency of energy transfer is moderately higher in DMF (72%) than in toluene (68%). Triplet ESR studies on the H 2- H dimer reveal a localized triplet with ZFS parameters and ESP pattern the same as for the individual monomers. On the other hand, triplet ESR of the Zn - H diporphyrin reveals triplet characteristics of the thiaporphyrin subunit, indicating an energy transfer in the triplet state.

Ground and Excited State Dynamics of Core-modified Normal and Expanded Porphyrins

Singlet and triplet excited state data on two new core-modified expanded porphyrins and their protonated derivatives are reported. Absorption spectral studies indicate a gradual red shift of the absorption bands upon sulfur substitution into the core. The singlet lifetimes for expanded S 3-sapphyrin and S 4-rubyrin decrease steadily relative to STPPH and S 2 TPP owing to a higher rate of intersystem crossing. Furthermore, the protonated derivatives of S 3-sapphyrin and S 4-rubyrin encapsulate fluoride ions into their cavities with binding constants of 807 and 48 M−1 respectively. However, the triplet lifetime for S 3-sapphyrin is longer (58.80 μs) than for normal thiaporphyrins (STPPH, 37.45 μs; S 2 TPP , 20.96 μs), suggesting that it could be very well suited for photosensitizing triplet oxygen.

Spectroscopic and Electrochemical Studies on Oxaporphyrins and Oxathiaporphyrins

The synthesis and characterization of oxaporphyrins ( OTPP and O 2 TPP ) containing one and two furan rings in place of pyrrole and a mixed oxathiaporphyrin ( OSTPP ) is reported. Spectroscopic and electrochemical studies reveal that oxaporphyrins exhibit optical and emission characteristics closer to the parent H 2 TPP , while their electrochemical behaviour parallels that reported for mono and dithiaporphyrins. However the characteristics of the mixed oxathia derivative resemble that of monothiaporphyrin. Protonation studies reveal weaker resonance interaction between the β-hydrogens and meso-phenyl substituents in sharp contrast to thiaporphyrins. The reduction potentials for OSTPP and O 2 TPP indicate stabilization and destabilization of their LUMOs relative to the parent H 2 TPP in the ground and excited states, respectively.