A new multicomponent array (PH2-Ru-PAu) has been synthesized, consisting of a free-base arylporphyrin and a gold(III) arylporphyrin, assembled together with a central ruthenium(II)-bis(terpyridine) complex. The photophysical properties of this triad, of the molecular models and of the related dyads Ru-PH2 and Ru-PAu have been determined by steady-stale and time-resolved methods. Excitation of the Ru-PH2 array in the porphyrin subunit results in energy transfer from the porphyrin singlet to the (MLCT)-M-3-excited level of the ruthenium complex (k = 2.4 x 10(9) s(-1)), followed by a very fast (k > 5 x 10(10) s(-1)) energy transfer to the porphyrin-localized triplet state with an efficiency close to unity. The role of the ruthenium in perturbing the spin multiplicity of the excited states is essential to promote the fast energy transfer between the free-base porphyrin singlet and the lowest MLCT excited state of the complex, which is formally a triplet, Excitation of the Ru-PAu dyad in the MLCT manifold of the ruthenium complex yields the porphyrin-localized triplet within our experimental resolution (20 ps) with unit efficiency. No evidence of photoinduced electron transfer emerges from our data and the observed processes are essentially ascribed to energy transfer by a Dexter-type mechanism. In the PH2-Ru-PAu triad excitation in the free-base-porphyrin subunit produces the excited singlet state, which is quenched with a rate k = 5.6 x 10(9) s(-1) The quenching is assigned to energy transfer by a Dexter mechanism to the (MLCT)-M-3 state of the ruthenium complex, which in turn transfers triplet energy very rapidly (k > 5 x 10(10) s(-1)) to the gold porphyrin and to the free-base porphyrin units with an efficiency ratio of four. The overall quantum yield of porphyrin triplets is unity, Direct excitation in the MLCT manifold of the complex causes a similar photoinduced energy transfer to the peripheral porphyrins. Excitation of the gold(III) porphyrin in the triad produces the corresponding triplet, which is unable to promote the thermodynamically allowed triplet energy transfer to the spatially opposite free-base porphyrin, The prevalence of energy-transfer processes with respect to thermodynamically allowed electron transfer in this and related systems is discussed on the basis of the nature of the metal complex and of current theories.
Photoinduced processes in highly coupled multicomponent arrays based on a ruthenium(II)bis(terpyridine) complex and porphyrins
Armaroli N;
1998
Abstract
A new multicomponent array (PH2-Ru-PAu) has been synthesized, consisting of a free-base arylporphyrin and a gold(III) arylporphyrin, assembled together with a central ruthenium(II)-bis(terpyridine) complex. The photophysical properties of this triad, of the molecular models and of the related dyads Ru-PH2 and Ru-PAu have been determined by steady-stale and time-resolved methods. Excitation of the Ru-PH2 array in the porphyrin subunit results in energy transfer from the porphyrin singlet to the (MLCT)-M-3-excited level of the ruthenium complex (k = 2.4 x 10(9) s(-1)), followed by a very fast (k > 5 x 10(10) s(-1)) energy transfer to the porphyrin-localized triplet state with an efficiency close to unity. The role of the ruthenium in perturbing the spin multiplicity of the excited states is essential to promote the fast energy transfer between the free-base porphyrin singlet and the lowest MLCT excited state of the complex, which is formally a triplet, Excitation of the Ru-PAu dyad in the MLCT manifold of the ruthenium complex yields the porphyrin-localized triplet within our experimental resolution (20 ps) with unit efficiency. No evidence of photoinduced electron transfer emerges from our data and the observed processes are essentially ascribed to energy transfer by a Dexter-type mechanism. In the PH2-Ru-PAu triad excitation in the free-base-porphyrin subunit produces the excited singlet state, which is quenched with a rate k = 5.6 x 10(9) s(-1) The quenching is assigned to energy transfer by a Dexter mechanism to the (MLCT)-M-3 state of the ruthenium complex, which in turn transfers triplet energy very rapidly (k > 5 x 10(10) s(-1)) to the gold porphyrin and to the free-base porphyrin units with an efficiency ratio of four. The overall quantum yield of porphyrin triplets is unity, Direct excitation in the MLCT manifold of the complex causes a similar photoinduced energy transfer to the peripheral porphyrins. Excitation of the gold(III) porphyrin in the triad produces the corresponding triplet, which is unable to promote the thermodynamically allowed triplet energy transfer to the spatially opposite free-base porphyrin, The prevalence of energy-transfer processes with respect to thermodynamically allowed electron transfer in this and related systems is discussed on the basis of the nature of the metal complex and of current theories.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
