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

Abstract

Both the ground (S0) 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-A2B2, 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 mono- and 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.