EFFECT OF THE EXCITON COUPLING ON THE OPTICAL AND PHOTOPHYSICAL PROPERTIES OF FACE-TO-FACE PORPHYRIN DIMER AND TRIMER - A TREATMENT INCLUDING THE SOLVENT STABILIZATION EFFECT

A treatment of the exciton coupling interaction including the solvent stabilization effect is given. The method is successfully applied to porphyrin dimer and trimer: the maxima of the Soret bands can be predicted with a high accuracy. The effect of exciton coupling on the photophysical properties of basket handle cofacial, doubly linked porphyrin dimer and trimer is investigated and compared to that in the singly linked dimer. Exciton coupling is very weak for the latter, whose chromophores behave independently from each other: the quantum yields of the singlet decay pathways (fluorescence, intersystem crossing, internal conversion) remain approximately the same as in the monomer. For the face-to-face compounds, the drastic decrease of the fluorescence and triplet yield could not be entirely explained in terms of the unique exciton coupling effect. The existence of a low virtual charge-transfer state lying near above the lowest singlet excited state is postulated and discussed. It allows us (i) to explain the increase of the internal conversion rate constant at the expense of the other radiative and radiationless processes and (ii) to account for the invariance of the singlet and triplet lifetimes of excited dimer and trimer with respect to the monomer.