Red-shifted emission spectra of several meso-substituted copper porphyrins in fluid solution

Abstract

Medium dependence of the emission spectra of several meso-substituted copper porphyrins was studied at 300-77 K. Copper porphyrins emit from the lowest excited (porphyrin triplet) states in fluid solution as well as in rigid media. Emission spectra of several copper porphyrins in toluene liquid solution were red-shifted from those in rigid media such as PMMA (poly(methylmethacrylate)) film. The copper porphyrins, which give red-shifted emission in toluene solution, have 2,4b 1 3(a 2ue g)] configuration in the lowest excited states and the amount of the shift depends on meso-substituents ranging up to 1300 cm −1. As the emission spectra in toluene rigid glass at 77 K coincide with those in PMMA film, the observed shift in 2,4[b 1 3(a 2ue g)]-type porphyrins is attributable to distortion of the excited molecules in fluid solution. T(2,4,6-(MeO) 3)PPCu, in which bulky meso-substituents are likely to suppress torsion of the phenyl rings and distortion of the porphyrin plane, was found to show no shift of emission spectra, in spite of the 2,4[b 1 3(a 2ue g)] configuration. In the case of TPrPCu, which has no phenyl group, red shift occurs as a result of the medium being changed to become non-rigid. An out-of-plane distorted structure is proposed. Lifetimes of the emission in toluene solution are remarkably shorter in the copper porphyrins that show a larger emission red-shift from the emission in PMMA film. This relation suggests that the distortion modes are connected with enhancement of the radiationless decay.