[5,10,15,20-Tetra-kis(4-meth-oxy-phen-yl)porphyrinato]zinc di-chloro-methane disolvate.

In the title compound, [Zn(C48H36N4O4)]·2CH2Cl2, the Zn(II) ion lies on an inversion center and is coordinated in an almost ideal square-planar geometry. The asymmetric unit also contains one di-chloro-methane solvent mol-ecule. The unique meth-oxy-substituted benzene rings form dihedral angles of 59.38 (6) and 66.77 (6)° with the mean plane (r.m.s. deviation of fitted atoms = 0.0282 Å) of the atoms in the porphyrin core. The packing is characterized by close contacts between the Zn(II) ion and two symmetry-related mol-ecules through the O atoms of a meth-oxy-phenyl group [Zn⋯O = 2.694 (2) Å], forming a two-dimensional network parallel to (100).

[5,10,15,20-Tetra-kis(4-tol-yl)porphyrin]zinc(II) dichloro-methane solvate.

In the title complex, [Zn(C(48)H(36)N(4))]·CH(2)Cl(2), the Zn(II) atom lies on an inversion center and the dichloro-methane solvent mol-ecule is disordered around an inversion center. The tolyl substituents are twisted compared to the central aromatic ring system of the porphyrin, similar to what is seen in previously published structures of this molecule [Dastidar & Goldberg (1996 ▶). Acta Cryst. C52, 1976-1980]. The dihedral angles between the mean planes of the tolyl rings and the central ring are 66.98 (6) and 60.40 (6)°.

Tetrapyrrole singlet excited state quenching by carotenoids in an artificial photosynthetic antenna.

Two artificial photosynthetic antenna models consisting of a Si phthalocyanine (Pc) bearing two axially attached carotenoid moieties having either 9 or 10 conjugated double bonds are used to illustrate some of the function of carotenoids in photosynthetic membranes. Both models studied in toluene, methyltetrahydrofuran, and benzonitrile exhibited charge separated states of the type C*+-Pc*- confirming that the quenching of the Pc S1 state is due to photoinduced electron transfer. In hexane, the Pc S1 state of the 10 double bond carotenoid-Pc model was slightly quenched but the C*+-Pc*- transient was not spectroscopically detected. A semiclassical analysis of the data in hexane at temperatures ranging from 180 to 320 K was used to demonstrate that photoinduced electron transfer could occur. The model bearing the 10 double bond carotenoids exhibits biexponential fluorescence decay in toluene and in hexane, which is interpreted in terms of an equilibrium mixture of two isomers comprising s-cis and s-trans conformers of the carotenoid. The shorter fluorescence lifetime is associated with an s-cis carotenoid conformer where the close approach between the donor and acceptor moieties provides through-space electronic coupling in addition to the through-bond component.

Artificial photosynthetic reaction centers: mimicking sequential electron and triplet-energy transfer.

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (P(F)), C-P-P(F) , and the related triad in which the central porphyrin has been metalated to give C-P(Zn)-P(F) have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P(F) moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P(F) undergoes rapid singlet-singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of P(F) yield the initial charge-separated state C-P(.) (+)-P(F) (.) (-). Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C(.) (+)-P-P(F) (.) (-), which has a lifetime of 1.1 mus and is formed with a quantum yield of 0.24. In triad C-P(Zn)-P(F) energy transfer from the P(Zn) excited singlet to the P(F) moiety yields C-P(Zn)-(1)P(F) . A series of electron-transfer reactions analogous to those observed in C-P-P(F) generates C(.) (+)-P(Zn)-P(F) (.) (-), which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C-P(Zn)-P(F) demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low-dielectric environment of artificial biological membranes. In argon-saturated toluene solutions of C-P-P(F) and C-P(Zn)-P(F) , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the P(F) moiety is channeled to the carotenoid chromophore by a triplet energy-transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy-transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria.