Photoinduced Processes in Dyads Made of a Porphyrin Unit and a Ruthenium Complex

A Ru(II) complex ( Ru ) containing as ligands a tridentate 4‘-p-tolyl-2,2‘:6‘,2‘‘-terpyridine (ttpy) and a tridentate 2,6-bis(4‘-phenyl-2‘-quinolyl)pyridine (bpqpy) has been covalently linked to a porphyrin module ( PH 2 ) to obtain a PH 2 − Ru dyad. The corresponding PZn − Ru dyad has then been obtained by metalation of the free base porphyrin with Zn(II) acetate. The photoinduced processes which occur on excitation of the PH 2 − Ru and PZn − Ru dyads, as well as of the PH 2 and PZn porphyrin units and the [Ru(ttpy)(bpqpy)] 2+ model compound Ru, have been investigated in butyronitrile rigid matrix at 77 K and fluid solution at 295 K. In both dyads at low temperature, the lowest singlet excited state of the porphyrin moiety (S 1 ) is quenched by energy transfer to give the triplet metal-to-ligand charge-transfer excited state of the Ru complex ( 3 MLCT) which, in its turn, is quenched by energy transfer to yield the triplet excited state of the porphyrin moiety (T 1 ). At room temperature, a charge-transfer (CT) excited state corresponding to the transfer of an electron from the porphyrin moiety to the Ru-based moiety comes into play. For the PZn − Ru dyad, where the CT state lies below the S 1 excited state of the porphyrin moiety, the deactivation of S 1 ( k ≥ 5 × 10 10 s -1 ) occurs mainly by electron transfer to give the CT level that then deactivates to the T 1 excited state of the porphyrin moiety (100% efficiency; k = 9.3 × 10 9 s -1 ). Since the T 1 level is intrinsically long lived (τ ∼ 210 μs), its deactivation occurs essentially via an activated process through the upper lying CT level ( k = 5.7 × 10 6 s -1 ). The 3 MLCT excited state of the Ru-based moiety directly formed by light absorption appears to decay unperturbed with its intrinsic lifetime ( k = 1.1 × 10 10 s -1 ). In the case of the PH 2 − Ru dyad, the CT level lies slightly above S 1 . As a consequence, only a fraction (ca. 30%) of the S 1 excited states are quenched by electron transfer, the remaining part being quenched by energy transfer to give the 3 MLCT excited state of the Ru-based moiety. Deactivation of the CT state leads to the formation of T 1 ( k = 8.7 × 10 9 s -1 ), whereas the 3 MLCT excited state undergoes unperturbed deactivation ( k = 1.2 × 10 10 s -1 ) directly to the ground state. For the latter dyad, the T 1 excited state is very long lived (280 μs) since deactivation via the upper lying CT level is precluded for energetic reasons.