Short Chain Basket Handle Porphyrins : Ground and Excited State Dynamics

Department of Chemistry, Indian Institute of Technology, Kanpur-208 016

Manuscript received 31 August 1993

The effect of porphyrin ring distortion on the ground and excited state dynamics of short chain basket handle porphyrins has been investigated. The covalent attachment of phenyl ring of meso-5,10,15,20-tetraphenylporphyrin (Hz TPP) by short bridging groups of varying nature and length induces distortion in the porphyrin ring. The optical absorption bands and emission bands exhibit significant red-shifts with reduction in intensity relative to corresponding planar porphyrin due to ring distortion. On protonation both optical and emission bands experience blue-shift due to lack of expected resonance interaction. Metallation does not alter the degree of distortion. The Zn^II derivatives showed emission from both S₁ and S₂ excited state with quenching of fluorescence. Redox potential data suggest easier oxidations and harder reductions both in the ground and first singlet excited states relative to corresponding planar porphyrins. Zero field splitting parameters D and E of basket handle porphyrins evaluated from photoexcited triplet esr show small but significant differences attributed to distortion of the porphyrin plane. The calculated structures reveal the expected distortion in the porphyrin plane caused by short bridging groups and offers an explanation for the unusual spectral and electrochemical shifts observed in basket handle porphyrins relative to planar porphyrins.