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Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (

Two closely related, rigidly linked porphyrin−naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77−295 K. Dyad 2, which has a similar donor−acceptor linkage but whose quinone carbonyl groups are ∼2 A farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin−quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, whic...

Two approaches to eliciting photoelectrochemical reactions from carotenoids are being pursued. One uses LB films of amphipathic carotenoids deposited on semiconducting electrodes which are immersed in electrolytes containing electron donors or acceptors. Photocurrents are observed and their action spectra implicate the excited carotenoid pigment, possibly in an aggregated form, as the photoactive species. An example of the second approach is a molecular dyad consisting of a carotenoid covalently linked to Gjo. Excitation of the carotenoid moiety generates a charge-separated species in high yield. More complex systems include a molecular mad (C-P-Q) containing a porphyrin moiety (P) linked to a quinone (Q) and to a carotenoid (C). Triads of this type have been incorporated unidirectionally into lipid bilayer membranes. Time-resolved fluorescence experiments reveal that excitation of C-P-Q in the membrane leads to electron transfer from 1P to give C-P'+-Q". A subsequent electron transfer from C to P" yields C'+-P-Q'-. In this species the carotenoid pigment acts as a molecular wire to conduct charge across the membrane. In the presence of appropriate cofactors, this system translocates protons across the bilayer lipid membrane and generates proton motive force.

Three dyad molecules, each consisting of a porphyrin (P) linked to a quinone (Q) through a rigid bicyclic bridge, have been prepared, and their photochemistry has been investigated using time-resolved fluorescence and absorption techniques. In all three molecules, photoinduced electron transfer from the porphyrin first excited singlet state to the quinone occurs with rate constants of approximately 10{sup 12} s{sup -1} in solvents ranging in dielectric constant from approximately 2.0 to 25.6 and at temperatures from 77 to 295 K. The transfer rate is also relatively insensitive to thermodynamic driving force changes up to 0.4 eV. This behavior is phenomenologically similar to photosynthetic electron transfer. The rapid rate of photoinduced electron transfer and its lack of dependence on environmental factors suggests that transfer is governed by intramolecular vibrations. Charge recombination of P{sup {center_dot}+} {sup -}Q{center_dot}, on the other hand, is substantially slower than charge separation and sensitive to both driving force and environmental conditions. Thus, by changing conditions, charge recombination rates can be varied over a wide range while photoinduced electron transfer rates are relatively unaffected. This suggests that rigid dyads of this general type may be useful building blocks for more complex molecular devices. 76 refs., 9 figs., 1more » tab.« less

Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn-C60) and free base (P-C60) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+-C60bull;-. In toluene, P-1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1C60 underoes photoinduced electron transfer to give P•+-C60bull;-. The electron transfer rate constant is −2 × 1011 s−1.

Time-resolved fluorescence and subpicosecond transient absorption experiments have been carried out on a triad molecular device (CPQ) comprising a porphyrin (P) covalently linked to an electron-donating carotenoid pigment (C) and a quinone (Q), which acts as an electron acceptor. Rate constants governing the intramolecular electron transfer processes originating from the excited singlet state of the porphyrin have been determined. Excitation of the porphyrin moiety of the triad in benzonitrile solution results in photoinduced electron transfer to give CP .+ Q .− ( k 1 =2.5 × 10 9 s −1 ) followed by rapid non-photochemical electron transfer to yield C .+  PQ .− ( k 3 ≈ 1 × 10 11 s −1 ). The intermediate CP .+ Q .− , also decays by charge recombination to the ground state with a rate constant k 2 of about 6 × 10 11 s −1 . Nanosecond laser flash experiments were used to measure a quantum yield of 0.13 for the long-lived (370 ns) charge-separated species C .+ PQ .− . Excitation of the carotenoid moiety of the triad results in singlet energy transfer to the attached porphyrin with a quantum yield of about 0.07.

A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q[sub A]-Q[sub B]), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-[sup 1]P-Q[sub A]-Q[sub B] singlet state to yield the charge-separated state C-P-P[sup [sm bullet]+]-Q[sub A][sup [sm bullet][minus]]-Q[sub B]. Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state with a quantum yield of 0.83 and a lifetime of 55 [mu]s. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds. 28 refs., 10more » figs., 1 tab.« less

A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

The fluorescence quantum yield of zinc porphyrin (ZnP) covalently linked to 9,10-bis(phenylethynyl)anthracene (AB) is strongly dependent upon the solvent properties. The bichromophoric system ZnP-AB exhibits 'normal' zinc porphyrin fluorescence in solvents that cannot coordinate to the central zinc atom. In contrast, if a Lewis base, such as pyridine, is added to a sufficiently polar solvent, the fluorescence is significantly quenched. Picosecond transient absorption measurements, in conjunction with fluorescence quenching and cyclic voltammetric measurements, suggest that the quenching mechanism is intramolecular electron transfer from ZnP to AB. The charge separated state, ZnP.+-AB(.-), has a lifetime of not more than 220 ps before recombining. If a secondary electron acceptor, iron(III) porphyrin (FeP), is covalently connected to the AB unit, a second electron transfer from AB(.-) to FeP occurs and the charge separated state, ZnP.+-AB-FeP.-, has a lifetime of at least 5 ns. This demonstrates that electron transfer might be sensitively tuned (switched on) by specific solvent effects. (C) 2001 Elsevier Science BN. All rights reserved.

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi -systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a Forster mechanism. In contrast, systems with a rr-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi -conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.

The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(III) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 Angstrom donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S-1 --> T-1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Forster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for election transfer.