Force For Photoinduced Electron Transfer Research Articles

Photosensitized activation of diazonium derivatives for C–B bond formation

Aryl diazonium salts are ubiquitous building blocks in chemistry, as they are useful radical precursors in organic synthesis as well as for the functionalization of solid materials. They can be reduced electrochemically or through a photo-induced electron transfer reaction. Here we provide a detailed picture of the ground and excited-state reactivity of a series of 9 rare and earth abundant photosensitizers with 13 aryl diazonium salts, which also included 3 macrocyclic calix[4]arene tetradiazonium salts. Nanosecond transient absorption spectroscopy confirmed the occurrence of excited-state electron transfer and was used to quantify cage-escape yields, i.e. the efficiency with which the formed radicals separate and escape the solvent cage. Cage-escape yields were large; increased when the driving force for photo-induced electron transfer increased and also tracked with the C-N2 + bond cleavage propensity, amongst others. A photo-induced borylation reaction was then investigated with all the photosensitizers and proceeded with yields between 9 and 74%.

Reconciling the Driving Force and the Barrier to Charge Separation in Donor–Nonfullerene Acceptor Films

We investigate the dependence of charge yields on the driving force for photoinduced electron transfer in a series of all-small-molecule, semiconducting films made from indacenodithiophene nonfullerene acceptors (IDT NFAs). In contrast to reports of efficient, barrierless charge separation at near zero driving force for NFA-containing organic photovoltaics, we find that barrierless charge separation occurs only if the driving force is sufficient to overcome the Coulomb potential, ≥300 meV, based on time-resolved microwave conductivity and PL quenching measurements of 5 mmolal sensitized and 700 mmolal blended films comprising IDT NFAs paired with three different donors. This discrepancy with recent literature is caused by a difference in the way that we calculate driving force. Far from being semantic, the driving force calculation is crucial because its value controls the mechanisms needed to explain experimental observations. We provide a candid assessment of the uncertainties for our methods and popular ones used in the literature and emphasize the importance of standardizing methods across this field.

Tuning Photoinduced Electron Transfer in POM‐Bodipy Hybrids by Controlling the Environment: Experiment and Theory

AbstractThe optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron‐dipyrromethene) light‐harvesting units were examined. Evaluated here in this polyanionic donor‐acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge‐transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.

Tuning Photoinduced Electron Transfer in POM-Bodipy Hybrids by Controlling the Environment: Experiment and Theory.

The optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron-dipyrromethene) light-harvesting units were examined. Evaluated here in this polyanionic donor-acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge-transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.

Molecular Dynamics Simulations of Bimolecular Electron Transfer: Testing the Coulomb Term in the Weller Equation.

Reliable estimation of the driving force for photoinduced electron transfer between neutral reactants is of utmost importance for most practical applications of these reactions. The driving force is usually calculated from the Weller equation, which contains a Coulomb term, C, whose magnitude in polar solvents is debated. We have performed umbrella sampling molecular dynamics simulations to determine C from the potentials of mean force between neutral and ionic donor/acceptor pairs of different sizes in solvents of varying polarity. According to the simulations, C in polar solvents is a factor of 2 more negative than typically calculated according to the Weller equation. Use of the π-stack contact distance in the Weller equation instead of the van der Waals radius recovers the correct value of C, but this is mostly fortuitous due to the compensating effects of overestimating the dielectric screening at contact and neglecting both charge dilution and desolvation.

Photoinduced Electron Transfer and Energy Transfer Processes in a Flexible BODIPY-C60 Dyad.

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C60 acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C60 dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C60 dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C60 dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C60 moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C60 dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

Photoinduced Charge Separation in Retinoic Acid on TiO2: Comparison of Three Anchoring Modes

A complete understanding of the interaction between the dye and the semiconductor is essential for the design of a dye-sensitized solar cell (DSC) because the chemical interactions and the electron coupling between semiconductor and dye directly impact electron transfer and the performance of the dye-sensitized photoanode. Three anchoring modes of retinoic acid (RA), which contains a carboxylic acid group adsorbed on TiO2, were studied by DFT calculations. Of the three anchoring modes, the mode involving surface Ti–OH groups has the highest driving force for photoinduced electron transfer. The mixing of the RA’s lowest unoccupied molecular orbital (LUMO) with the conduction band states of TiO2 is also the highest. The calculated results are supported by EPR measurements, which show enhanced light-driven charge separation from increasing the density of the Ti–OH groups on the surface of TiO2. The anchoring modes affect the bond length, degree of conjugation, energies of the highest occupied molecular orbit...

Factors affecting the uncaging efficiency of 500 nm light-activatable BODIPY caging group

Photoremovable protective groups, or caging groups, enable us to regulate the activities of bioactive molecules in living cells upon photoirradiation. Nevertheless, requirement of UV light for activating caging group is a significant limitation due to its cell toxicity and its poor tissue penetration. Our group previously reported a 500 nm light-activatable caging group based on BODIPY scaffold, however, its uncaging efficiency was lower than those of conventional caging groups. Here we show that the uncaging quantum yield (QY) of BODIPY caging group depends upon the driving force of photo-induced electron transfer (PeT). We also found that the uncaging QY increased in less polar solvents. We applied these findings to develop BODIPY-caged capsaicin, which is well localized to low-polarity intracellular compartments, as a tool to stimulate TRPV1 in live cells in response to blue-green light.

G-C3N4/TiO2 Mesocrystals Composite for H2 Evolution under Visible-Light Irradiation and Its Charge Carrier Dynamics.

The photocatalytic performance of graphitic carbon nitride (g-C3N4) has been limited to low efficiency due to fast charge recombination. Here, we constructed g-C3N4 nanosheets/TiO2 mesocrystals metal-free composite (g-C3N4 NS/TMC) to promote the efficiency of charge separation. The photocatalytic H2 evolution experiments indicate that coupling g-C3N4 NS with TMC increases photogenerated charge carriers in g-C3N4 NS/TMC composite due to efficient charge separation. g-C3N4 NS (31 wt %)/TMC shows the highest photocatalytic activity and the corresponding H2 evolution rate is 3.6 μ mol h-1. This value is 20 times larger than that of g-C3N4 NS without any noble metal cocatalyst under visible-light irradiation (λ > 420 nm). The photocatalytic activity of g-C3N4 NS/TMC (3.6 μmol h-1) is 7 times higher than that of g-C3N4 NS/P25 (0.5 μ mol h-1), confirming the importance of strong interface interaction between two-dimensional g-C3N4 NS and plate-shape TMC. Femtosecond time-resolved diffuse reflectance (fs-TDR) was employed to study the fundamental photophysical processes of bulk g-C3N4, g-C3N4 NS, and g-C3N4/TMC composite which are essential to explain the photocatalytic activity. Using fs-TDR, we demonstrate that the photocatalytic activity depends on the increased driving force for photoinduced electron transfer and a higher percentage of photogenerated charges.

A colorimetric and fluorogenic probe for bisulfite using benzopyrylium as the recognition unit.

A coumarin-benzopyrylium (CB) platform has been developed for the colorimetric and fluorogenic detection of bisulfite. The proposed probe utilizes coumarin as the fluorophore and positively charged benzopyrylium as the reaction site. The method employs the nucleophilic addition of bisulfite to the benzopyrylium moiety of CB to inactivate the electron-deficient oxonium ion. The driving force for photo-induced electron transfer is considerably diminished, thereby promoting the emission intensity of the coumarin fluorophore. The fluorescence intensity at 510nm is linear with bisulfite concentration over a range of 0.2-7.5μM with a detection limit of 42nM (3δ). CB shows a rapid response (within 30s) and high selectivity and sensitivity for bisulfite. Preliminary studies show that CB has great potential for bisulfite detection in real samples and in living cells.

Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions.

Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

Dispersive Electron-Transfer Kinetics of Rhodamines on TiO2: Impact of Structure and Driving Force on Single-Molecule Photophysics

The effects of dye structure, driving force of photoinduced electron transfer, and adsorption affinity on the dispersive electron-transfer dynamics of rhodamines on TiO2 films are investigated using single-molecule microscopy. The time-dependent emission (i.e., blinking dynamics) of rhodamine-sensitized TiO2 films are quantified using maximum likelihood estimation (MLE) and quantitative goodness-of-fit tests based on the Kolmogorov–Smirnov (KS) statistic to determine the best fit to the photophysical data. Although the observation of significant p-values (i.e., ranging from 0.16 to 0.53) seems to support the power-law model for the on-time distributions, only a minor subset of the data are actually represented by power laws (i.e., ∼15–20% of events). Instead, the MLE/KS analysis reveals that log-normal distributions, not power laws, most closely represent the entire on-time and off-time distributions for RB, R6G, R123, and 5-ROX on TiO2. Monte Carlo simulations based on the Albery model for dispersive electron transfer (i.e., activation barriers to electron transfer are Gaussian distributed) demonstrate that the log-normal fit parameters (i.e., μon/off and σon/off) are dependent on the average rate constants for injection and recombination as well as the extent of energetic dispersion about the mean activation barrier. The single-molecule results for RB, R6G, R123, and 5-ROX on TiO2 are interpreted in the context of ensemble-averaged spectroscopic and electrochemical characterization and suggest that heterogeneity in electronic coupling and reorganization (i.e., static and dynamic disorder) play a decisive role in the observed dispersive kinetics.

Electronic excited state redox properties for BODIPY dyes predicted from Hammett constants: estimating the driving force of photoinduced electron transfer.

Here we formulate equations based solely on empirical Hammett substituent constants to predict the redox potentials for the electronic excited state of boron-dipyrromethene (BODIPY) dyes. We utilized computational, spectroscopic, and electrochemical techniques toward characterizing the effect of substitution at the positions C2, C6, and C8 of the 1,3,5,7-tetramethyl BODIPY core. Working with a library of 100 BODIPY dyes, we found that highest occupied molecular orbital (HOMO) energies calculated at the B3LYP 6-31g(d) level correlated linearly with the Hammett σm value for substituents at position C8 and with Hammett σp values for substituents at positions C2 and C6. In turn, we observed that LUMO energies correlated linearly with Hammett σp at position C8 and with Hammett σm at positions C2 and C6. Focusing on a subset of 26 dyes for which reduction potentials were either previously available or measured herein and ranged from -1.84 to -0.52 V (a full 1.3 V), we found a linear relationship between redox potentials in acetonitrile and HOMO and lowest unoccupied molecule orbital (LUMO) energies determined via density functional theory (DFT). A linear correlation was thus ultimately established between redox potentials in acetonitrile and Hammett substituent constants. Combining this with equations derived for the linear relationship existing between the zero vibrational energy of the excited BODIPY and Hammett substituent constants enabled us to provide the parameters toward predicting the oxidizing/reducing power of photoexcited 1,3,5,7,-tetramethyl BODIPY dyes in their singlet excited state.

Photoinduced Electron Transfer in Perylene‐TiO2 Nanoassemblies

The photosensitization effect of three perylene dye derivatives on titanium dioxide nanoparticles (TiO2 NPs) has been investigated. The dyes used, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2) and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3) have in common bisanhydride groups that convert into TiO2 binding groups upon hydrolysis. The different substituents on the bay position of the dyes enable tuning of their redox properties to yield significantly different driving forces for photoinduced electron transfer (PeT). Recently developed TiO2 NPs having a small average size and a narrow distribution (4 ± 1 nm) are used in this work to prepare the dye-TiO2 systems under study. Whereas successful sensitization was obtained with 1 and 2 as evidenced by steady-state spectral shifts and transient absorption results, no evidence for the attachment of 3 to TiO2 was observed. The comparison of the rates of PeT (kPeT ) for 1- and 2-TiO2 systems studied in this work with those obtained for previously reported analogous systems, having TiO2 NPs covered by a surfactant layer (Hernandez et al. [2012] J. Phys. Chem. B., 117, 4568-4581), indicates that kPeT for the former systems is slower than that for the later. These results are interpreted in terms of the different energy values of the conduction band edge in each system.

Kinetics of the photoinduced dissociative reduction of the model alkyl peroxides di-tert-butyl peroxide and ascaridole

Rate constants for the reaction between excited singlet state aromatic donors and the dialkyl peroxide, di-tert-butyl peroxide (DTBP), and the bicyclic endoperoxide, ascaridole (ASC), were measured in acetonitrile using fluorescence quenching techniques. The rate constants, measured with 18 different aromatic donors by Stern-Volmer quenching, range from 9.2  109 to 4.4  106 M-1 s-1. Using accurately measured standard reduction potentials for the peroxides, the driving force for photoinduced electron transfer is predicted to be thermodynamically feasible over the entire range of excited donors ranging from 49 to 10 kcal mol-1 for ASC and 38 to 2 kcal mol-1 for DTBP. However, when the photoinduced kinetics are combined with previously measured electron transfer kinetics by homogeneous redox catalysis with ground state radical-anion donors, a smooth parabolic correlation for a dissociative electron transfer mechanism is not observed. Rather, a discontinuity is observed between the photochemical and electrochemical data sets with the rate constants with singlet excited states donors being over two orders of magnitude larger than the ground state kinetics at the same driving force. The discrepancy is examined considering the importance of attractive interaction between fragments in the dissociative photoinduced electron transfer reactions.

Intermolecular and Intracomplex Photoinduced Electron Transfer from Planar and Nonplanar Metalloporphyrins to p-Quinones

The rate constants of intermolecular photoinduced electron transfer from triplet excited states of metalloporphyrins to a series of p-benzoquinone derivatives in benzonitrile were determined to examine the effects of the driving force, the metal, and the conformational distortion of the porphyrin ring on the reorganization energies (λ) of electron transfer by laser flash photolysis. The λ values were evaluated from the determined rate constants on the basis of the Marcus theory of electron transfer. The λ values of planar metalloporphyrins, [Al(TPP)(PhCOO)] and [Zn(TPP)] (TPP(2-)=tetraphenylporphyrin dianion), are approximately the same, but they are 0.27 eV smaller than those of the corresponding nonplanar (saddle-distorted) metalloporphyrins [Al(DPP)(PhCOO)] and [Zn(DPP)] (DPP(2-)=dodecaphenylporphyrin dianion) when they are compared for the same driving force of photoinduced electron transfer. The axial ligand PhCOO(-) of [Al(TPP)](+) and [Al(DPP)](+) was replaced by anthraquinone-2-carboxylate (AqCOO(-)) to afford the electron donor-acceptor complexes [Al(TPP)(AqCOO)] and [Al(DPP)(AqCOO)], respectively. The X-ray crystal structure of [Al(TPP)(AqCOO)] revealed strong coordination of AqCOO(-) to the Al(3+) ion of [Al(TPP)](+) and the existence of π-π interactions between AqCOO(-) and the porphyrin ring. In the case of the saddle-distorted [Al(DPP)(AqCOO)], however, the AqCOO(-) moiety is nearly perpendicular to the porphyrin ring. The photodynamics of intracomplex photoinduced electron transfer from the singlet excited state of [Al(TPP)](+) and [Al(DPP)](+) to the AqCOO(-) moiety were also examined in comparison with the intermolecular photoinduced electron-transfer reactions, and the determined rate constants were evaluated in light of the Marcus theory of electron transfer to reveal that the electron transfer is adiabatic in each case.

Exploiting Conformational Dynamics To Facilitate Formation and Trapping of Electron-Transfer Photoproducts in Metal Complexes

Three new photoinduced electron donor-acceptor (D-A) systems are reported which juxtapose a Ru(II) excited-state donor with a bipyridinium acceptor via a conformationally active asymmetric aryl-substituted bipyridine ligand participating in the bridge between D and A. Across the series of complexes 1-3, steric bulk is sequentially added to tune the inter-ring dihedral angle theta between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (DeltaG(ET)) and back electron transfer (DeltaG(BET)) are reported based on electrochemical measurements of 1-3 as well as Franck-Condon analysis of emission spectra collected for three new donor model complexes 1'-3'. These preserve the substitution patterns on the aryl substituent in their respective D-A complexes but remove the bipyridinium acceptor. Both DeltaG(ET) and DeltaG(BET) are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D-A systems with approximately 100 fs laser pulses at 500 +/- 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of tau(ET) = 29 ps (1), 37 ps (2), and 57 ps (3). That ET remains relatively rapid throughout this series, even as steric bulk significantly increases the inter-ring dihedral angle theta, is attributed to the effects of ligand-based torsional dynamics driven by intraligand electron delocalization in the D*-A excited state manifold prior to ET. The lifetimes of the charge-separated states (tau(BET)) are also reported with tau(BET) = 98 ps (1), 217 ps (2), and 789 ps (3), representing a more than 8-fold increase across the series. This is attributed to reverse conformational dynamics in D(+)-A(-) driven by steric repulsions, which serves to minimize electronic coupling to the ground state. Steric control of ligand geometry and the range over which theta changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states.

Adiabatic Photoinduced Electron Transfer and Back Electron Transfer in a Series of Axially Substituted Silicon Phthalocyanine Triads

A series of new electron acceptor bearing silicon phthalocyanine (SiPc) triads have been synthesized, using the six-coordinated nature of the central silicon atom, by attachment of two electron-acceptor units, fullerene SiPc−(C60)2, trinitrofluorenone SiPc−(TNF)2, and trinitrodicyanomethylenefluorene SiPc−(TNDCF)2. The redox and photophysical properties of SiPc triads in benzonitrile are determined to evaluate the energy of the charge-separated (CS) states and driving force of photoinduced electron transfer in SiPc triads. Photoexcitation of SiPc triads in benzonitrile results in efficient formation of the CS states, which were detected by femtosecond laser flash photolysis measurements. The fate of the CS state whether it decays to the triplet excited-state of the SiPc unit or to the ground-state was examined by nanosecond laser flash photolysis measurements for the longer time scale. The driving force dependence of the rate constants of photoinduced electron transfer and back electron transfer in SiPc triads is analyzed in light of the Marcus theory of adiabatic intramolecular electron transfer.

Interplay between Barrier Width and Height in Electron Tunneling: Photoinduced Electron Transfer in Porphyrin-Based Donor−Bridge−Acceptor Systems

The rate of electron tunneling in molecular donor-bridge-acceptor (D-B-A) systems is determined both by the tunneling barrier width and height, that is, both by the distance between the donor and acceptor as well as by the energy gap between the donor and bridge moieties. These factors are therefore important to control when designing functional electron transfer systems, such as constructs for photovoltaics, artificial photosynthesis, and molecular scale electronics. In this paper we have investigated a set of D-B-A systems in which the distance and the energy difference between the donor and bridge states (DeltaEDB) are systematically varied. Zinc(II) and gold(III) porphyrins were chosen as electron donor and acceptor because of their suitable driving force for photoinduced electron transfer (-0.9 eV in butyronitrile) and well-characterized photophysics. We have previously shown, in accordance with the superexchange mechanism for electron transfer, that the electron transfer rate is proportional to the inverse of DeltaEDB in a series of zinc/gold porphyrin D-B-A systems with bridges of constant edge to edge distance (19.6 A) and varying DeltaEDB (3900-17 600 cm(-1)). Here, we use the same donor and acceptor but the bridge is shortened or extended giving a set of oligo-p-phenyleneethynylene bridges (OPE) with four different edge to edge distances ranging from 12.7 to 33.4 A. These two sets of D-B-A systems-ZnP-RB-AuP+ and ZnP-nB-AuP+-have one bridge in common, and hence, for the first time both the distance and DeltaEDB dependence of electron transfer can be studied simultaneously in a systematic way.

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.