Magnetic circular dichroism studies. Part XXX. Vibronic intensity in metalloporphyrins

Abstract

We have studied the vibronic structure in the Q(1,0) band of several metalloporphyrins, using explicit relations based on the four orbital model for porphyrins, and find that most of the absorption intensity is due to vibronic coupling of the quasi-forbidden Q or E_u¹ state with the strong B or E_u² state (Soret band). We also attribute significant intensity to Franck–Condon progressions. The Franck–Condon contribution is manifested primarily in the m.c.d. A value of the Q(0,0) band because the other vibronic components consist of positive and negative A terms, and tend to cancel. The theoretical treatment shows that the variation (with configuartion interaction) of the intensity due to active vibrations is uniquely determined by the symmetry of the vibration. This behaviour has been used to assign tentatively the symmetries of the vibrations responsible for bands in the Q or E_u¹ excited state. A comparison of our assignment with that previously obtained by Spiro and Strekas using resonance Raman spectroscopy leads to a general confirmation of their results. In addition, we have been able to define further the accidental degeneracies observed in the Raman spectra and hence to refine the assignment.