Spectroscopic studies of meso-tetraphenylporphyrin and other meso-substituted porphyrin derivatives have revealed a dual fluorescence emission process. The first emission is the usual (1)(pi ->pi*) transition. The second alternative emission process is proposed to arise following tautomerism of the inner macrocyclic hydrogen atoms via an atom tunnelling process. This process occurs when exciting at virtually all wavelengths in the Q band region but, is at its most intense when exciting at the Q(x)( 0,0) band and is not observed when exciting at the Soret band maximum, i.e., the emission spectrum of TPP is dependent on the excitation wavelength. (c) 2008 Elsevier B. V. All rights reserved.