Side-to-face ruthenium porphyrin arrays: Photophysical behavior of dimericand pentameric systems

The investigated systems are side-to-face porphyrin arrays made of two type s of molecular components: a porphyrin unit with meso-pyridyl substituents, and one or more ruthenium carbonyl tetraphenylporphyrin units. The two typ es of unit are assembled by axial coordination of the meso-pyridyl groups o f the former onto the metal center of the latter. The number (one or four) of the meso-pyridyl groups on the axial unit determines whether the arrays are dimeric or pentameric. The geometry of the groups (4'-pyridyl or 3-pyri dyl) determine whether the arrays have perpendicular or canted structures. Furthermore, the meso-pyridyl porphyrin can be either free-base or zinc-sub stituted, leading to a total of eight different arrays. All arrays were sho wn to be stable in toluene, even in the dilute solutions (1x10(-5) M) requi red for photophysical experiments. The energy levels of the molecular compo nents are practically unaltered in the arrays, with the excited states of t he ruthenium porphyrin units always higher, both at the singlet and the tri plet level, than those of the free-base or zinc porphyrin units, The photop hysical behavior of the arrays has been studied in detail. The behavior was found to be practically independent of the perpendicular or canted nature of the systems. The arrays exhibit two main supramolecular features, distin ctive with respect to the behavior of the monomeric molecular components (o r suitable models thereof). At the singlet level, the behavior of the ruthe nium unit is normal (100% efficient intersystem crossing to the tripler sla te) but that of the axial unit is strongly perturbed, as indicated by prono unced fluorescence quenching. This effect is attributed to enhanced intersy stem crossing in the free-base or zinc porphyrin unit, owing to the heavy-a tom effect provided by the attached ruthenium units. At the triplet level, efficient energy transfer between the units takes place. When the axial uni t is a free-base porphyrin, the driving force is large, and the process tak es place irreversibly from the ruthenium to the free-base triplet. When the axial unit is a zinc porphyrin, the energy difference between the triplet states is small, and an equilibrium between the two states is established p rior to deactivation. In the free-base systems, triplet energy transfer rat e constants art found to be in the 10(x)-10(9) s(-1) range.