Time-resolved resonance Raman spectroscopy of triplet-state metallated and free-base tetraarylporphyrins

Abstract

Resonance Raman spectra of the T₁ excited states of Zn and free-base tetra-4-sulfonatophenylporphyrin (TPPS) have been recorded at room temperature in aqueous solution using two-colour time-resolved methods. The spectra of both sulfonated molecules are very similar to their tetraphenylporphyrin (TPP) analogues, which have been recorded in THF solution using the same pump–probe conditions, but they have higher signal-to-noise ratios because interference from strong solvent bands is reduced. Although two different T₁ spectra of Zn(TPP) have been reported these spectra differ slightly from each other and from the spectrum reported here, which has band positions very close (±6 cm^–1) to those of Zn(TPPS). The high S/N ratios obtainable for the water-soluble porphyrins have allowed reliable polarization data to be recorded for their S₀ and T₁ states. This data set allows a realistic comparison of the changes in bonding associated with excitation of both free-base and Zn tetraarylporphyrins to the T₁ state.

Semi-empirical (AM1) calculations of the bond-length changes associated with excitation of free-base TPP and the simpler model tetra-4-fluorophenylporphyrin (TFP) to their T₁ states follow qualitative predictions based on the orbital coefficients of the HOMO and LUMO involved in the transition. These structural changes accord satisfactorily with the observed changes in frequency on excitation of the vibrational modes of free-base TPP and TPPS. The calculated bond-length changes are strikingly close to those previously calculated for Zn(TFP), where Jahn–Teller effects are expected. The primary effect of the Jahn–Teller distortion in the molecular orbital calculations is to lift the degeneracy of the e_g LUMO pair of the Zn complexes. Once this degeneracy is lifted the calculated geometry changes for both free-base and Zn complexes converge because they arise from a HOMO to LUMO excitation between two orbitals which have similar coefficients in both. The observation that the directions of the shifts of most of the vibrational modes in the T₁ states of the free-base and Zn porphyrins are the same, supports this general conclusion. The only band which displays significantly different behaviour between the metallated and free-base systems is ν₁₁ which is a B_1g mode. This suggests that the Jahn–Teller distortion that lifts degeneracy of the LUMO in the T₁ states of the Zn complexes occurs along a B_1g coordinate.