Spectral-Luminescent Properties of meso-Tetraarylporphyrins Revisited: The Role of Aryl Type, Substitution Pattern and Macrocycle Core Protonation

Abstract

Both the ground (SO) and the lowest singlet excited states (S1) for a series of 5,10,15,20-tetraarylporphyrins consisting of two symmetrically and four asymmetrically substituted derivatives (A4, AB3, trans-A2B2, cis-A2 B2, A3B and B4, where A=phenyl and B=mesityl) are studied by absorption and fluorescence spectroscopies. The rotational degree of freedom of the aryl rings is found to play a crucial role in the discrimination between the radiative and radiationless decays of the S1 states. This feature is dramatically enhanced upon going from the free base molecules to their monoand diprotonated forms because of the nonplanar macrocycle conformation of the latters. The progressive A to B replacement of the aryl substituents leads to additive spectral changes over the whole series in the free base form. For both mono- and diprotonated species such a gradual additive pattern is broken by a “spectral jump” from the trans to the cis derivative, which is proposed to be a signature of the transition between two macrocycle conformations with different flexibility