Side-to-Face Ruthenium Porphyrin Arrays: Photophysical Behavior of Dimeric and Pentameric Systems

Abstract

The investigated systems are side-to-face porphyrin arrays made of two types of molecular components: a porphyrin unit with meso-pyridyl substituents, and one or more ruthenium carbonyl tetraphenylporphyrin units. The two types of unit are assembled by axial coordination of the meso-pyridyl groups of 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'-pyridyl) determine whether the arrays have perpendicular or canted structures. Furthermore, the meso-pyridyl porphyrin can be either free-base or zinc-substituted, leading to a total of eight different arrays. All arrays were shown to be stable in toluene, even in the dilute solutions (1x10 À5 m) required for photophysical experiments. The energy levels of the molecular components are practically unaltered in the arrays, with the excited states of the ruthenium porphyrin units always higher, both at the singlet and the triplet level, than those of the free-base or zinc porphyrin units. The photophysical 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, distinctive with respect to the behavior of the monomeric molecular components (or suitable models thereof). At the singlet level, the behavior of the ruthenium unit is normal (100 % efficient intersystem crossing to the triplet state) but that of the axial unit is strongly perturbed, as indicated by pronounced fluorescence quenching. This effect is attributed to enhanced intersystem crossing in the free-base or zinc porphyrin unit, owing to the heavy-atom effect provided by the attached ruthenium units. At the triplet level, efficient energy transfer between the units takes place. When the axial unit is a free-base porphyrin, the driving force is large, and the process takes 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 prior to deactivation. In the free-base systems, triplet energy transfer rate constants are found to be in the 10 8 ± 10 9 s À1 range.

Photophysical Properties of a Rhodium Tetraphenylporphyrin-tin Corrole Dyad. The First Example of a Through Metal–Metal Bond Energy Transfer†

“…clearly. Moreover, this 106-108 sC1 range compares favorably with those reported for face-to-side arrays where a ruthenium@) tetraphenylporphyrin D is bonded to a pyridyltriphenylporphyrin A via a Ru-N coordination (33). In these cases, the rates for triplet ET were in the 10x-109 s-' range.…”

Strong enhancement of the lanthanide-centred luminescence in complexes with 4-alkylated 2,2′;6′,2″-terpyridines †

“…fivefold shorter lifetime of the ligand-centred phosphorescence for all three ligands, a fact we assign to the heavy atom effect, as observed for instance in porphyrin systems. 25, 26 The large spin-orbit coupling of the lanthanide centre facili- = 1 or 2). A likely explanation is that the alkyl substituents in L 1 and L 2 lower the rate of non-radiative processes by hindering the access of solvent molecules.…”

Synthesis, spectroscopic and electrochemical investigations of supramolecular nickel(II)tetraazaporphyrin complexes

“…The metalloporphyrins/ tetraazaporphyrins and ruthenium(II) polypyridyls are versatile and robust chromophores possessing strong visible region absorption and reversible redox behavior. These compounds have potential applications as photosensitizers [15][16][17][18][19][20][21][22][23][24] for developing molecule based materials and supramolecular organic photonic devices. We have recently synthesized several new metallotetraazaporphyrins and have incorporated them into a PVC matrix as ion carrier agents, in order to fabricate different ion selective electrodes [25][26][27].…”