Two-Photon Spectra of Chlorophylls and Carotenoid–Tetrapyrrole Dyads

We present a direct comparison of two-photon spectra of various carotenoid–tetrapyrrole dyads and phthalocyanines (Pc) as well as chlorophylls (Chl) in the spectral range between 950 and 1360 nm, corresponding to one-photon spectra between 475 and 680 nm. For carotenoids (Car) with 8, 9, or 10 conjugated double bonds, the two-photon absorption cross section of states below the optical allowed carotenoid S₂ is at least about 3–10 times higher than that of Pc or chlorophyll <i>a</i> and <i>b</i> at 550/1100 nm. A quantitative comparison of spectra from Pc with and without carotenoids of eight and nine conjugated double bonds confirms energy transfer from optically forbidden carotenoid states to Pc in these dyads. When considering that less than 100% efficient energy transfer reduces the two-photon contribution of the carotenoids in the spectra, the actual Car two-photon cross sections relative to Chl/Pc are even higher than a factor of 3–10. In addition, strong spectroscopic two-photon signatures at energies below the optical allowed carotenoid S₂ state support the presence of additional optical forbidden carotenoid states such as S*, S_<i>x</i>, or, alternatively, contributions from higher vibronic or hot S₁ states dominating two-photon spectra or energy transfer from the carotenoids. The onset of these states is shifted about 1500–3500 cm^–1 to lower energies in comparison to the S₂ states. Our data provides evidence that two-photon excitation of the carotenoid S*, S_<i>x</i>, or hot S₁ states results in energy transfer to tetrapyrroles or chlorophylls similar to that observed with the Car S₁ two-photon excitation

Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator

In order to investigate the possibility of a fullerene acting as an electron and/or singlet energy relay between a donor chromophore and an acceptor, a triad consisting of a fullerene (C₆₀) covalently linked to both a porphyrin energy and electron donor (P) and a β-tetracyanoporphyrin energy and electron acceptor (CyP) was synthesized. Steady state and time-resolved spectroscopic investigations show that the porphyrin first excited singlet state donates singlet excitation and an electron to the fullerene and also donates singlet excitation to the CyP. All three processes differ in rate constant by factors of ≤1.3, and all are much faster than the decay of ¹P–C₆₀–CyP by unichromophoric processes. The fullerene excited state accepts an electron from P and donates singlet excitation energy to CyP. The P^•+–C₆₀^•––CyP charge-separated state transfers an electron to CyP to produce a final P^•+–C₆₀–CyP^•– state. The same state is formed from P–C₆₀–¹CyP. Overall, the final charge-separated state is formed with a quantum yield of 85% in benzonitrile, and has a lifetime of 350 ps. Rate constants for formation and quantum yields of all intermediate states were estimated from results for the triad and several model compounds. Interestingly, the intermediate P^•+–C₆₀^•––CyP charge-separated state has a lifetime of 660 ps. It is longer lived than the final state in spite of stronger coupling of the radical ions. This is ascribed to the fact that recombination lies far into the inverted region of the Marcus rate constant vs thermodynamic driving force relationship

Hole Mobility in Porphyrin- and Porphyrin-Fullerene Electropolymers

Charge transport within films of several new types of electropolymerized porphyrin and porphyrin-fullerene dyad polymers was studied in order to obtain information on the suitability of these organic semiconductors for applications in solar energy conversion, sensor devices, etc. The films, prepared by electropolymerization on a conductive substrate, were immersed in acetonitrile and studied using chronocoulometric and cyclic voltammetric electrochemical methods. The charge diffusion coefficients were found to be dependent upon the electrolytic medium. Electrolyte anion size plays a significant role in determining the rate of migration of charge through the polymers, demonstrating that migration of positive charge is accompanied by migration of negative counterions. Bulkier anions markedly decrease the charge diffusion coefficient. This strong dependence suggests that anion mobility is the rate-limiting process for diffusional charge transport within the porphyrin polymer films and that the largest rates obtained are lower limits to the intrinsic cation mobility. With electrolytes containing the relatively small perchlorate anion, charge diffusion coefficients of the porphyrin polymers were similar to those reported for polyaniline under acidic conditions. The charge diffusion coefficient for a zinc porphyrin polymer was found to decrease 2 orders of magnitude in the presence of pyridine, suggesting that metal-containing porphyrins polymer films may have sensor applications. Cation (hole) mobilities previously reported in the literature for porphyrin-containing polymers with chemical structures quite different from those investigated here were much smaller than those found for the polymers in this study, but further investigation suggests that the differences are due to choice of electrode size and material

All-Photonic Multifunctional Molecular Logic Device

Photochromes are photoswitchable, bistable chromophores which, like transistors, can implement binary logic operations. When several photochromes are combined in one molecule, interactions between them such as energy and electron transfer allow design of simple Boolean logic gates and more complex logic devices with all-photonic inputs and outputs. Selective isomerization of individual photochromes can be achieved using light of different wavelengths, and logic outputs can employ absorption and emission properties at different wavelengths, thus allowing a single molecular species to perform several different functions, even simultaneously. Here, we report a molecule consisting of three linked photochromes that can be configured as AND, XOR, INH, half-adder, half-subtractor, multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate logic devices, all with a common initial state. The system demonstrates the advantages of light-responsive molecules as multifunctional, reconfigurable nanoscale logic devices that represent an approach to true molecular information processing units

Artificial Photosynthetic Reaction Center Exhibiting Acid-Responsive Regulation of Photoinduced Charge Separation

Nonphotochemical quenching (NPQ) is a photoprotective regulatory mechanism employed by many photosynthetic organisms to dynamically modulate energy flow within the photosynthetic apparatus in response to fluctuating light conditions. Activated by decreases in lumen pH produced during periods of high photon flux, NPQ induces rapid thermal dissipation of excess excitation energy. As a result, the rate of charge separation (CS) decreases, thereby limiting the accumulation of potentially deleterious reactive intermediates and byproducts. Herein, a molecular triad that functionally mimics the effects of NPQ associated with an artificial photosynthetic reaction center is described. Steady-state absorption and emission, time-resolved fluorescence, and transient absorption spectroscopies have been used to demonstrate a 1 order of magnitude reduction in the CS quantum yield via reversible protonation of an excited-state-quenching molecular switch moiety. As in the natural system, the populations of unquenched and quenched states and therefore the overall yields of CS were found to be dependent on acid concentration

Marcus Bell-Shaped Electron Transfer Kinetics Observed in an Arrhenius Plot

The Marcus theory of electron transfer predicts a bell-shaped dependence of the reaction rate on the reaction free energy. The top of the “inverted parabola” corresponds to zero activation barrier when the electron-transfer reorganization energy and the reaction free energy add up to zero. Although this point has traditionally been reached by altering the chemical structures of donors and acceptors, the theory suggests that it can also be reached by varying other parameters of the system including temperature. We find here dramatic evidence of this phenomenon from experiments on a fullerene–porphyrin dyad. Following photoinduced electron transfer, the rate of charge recombination shows a bell-shaped dependence on the inverse temperature, first increasing with cooling and then decreasing at still lower temperatures. This non-Arrhenius rate law is a result of a strong, approximately hyperbolic temperature variation of the reorganization energy and the reaction free energy. Our results provide potentially the cleanest confirmation of the Marcus energy gap law so far since no modification of the chemical structure is involved

Photonic Modulation of Electron Transfer with Switchable Phase Inversion

Photochromes may be reversibly photoisomerized between two metastable states and their properties can influence, and be influenced by, other chromophores in the same molecule through energy or electron transfer. In the photochemically active molecular tetrad described here, a porphyrin has been covalently linked to a fullerene electron acceptor, a quinoline-derived dihydroindolizine photochrome, and a dithienylethene photochrome. The porphyrin first excited singlet state undergoes photoinduced electron transfer to the fullerene to generate a charge-separated state. The quantum yield of charge separation is modulated by the two photochromes: one isomer of each quenches the porphyrin excited state, reducing the quantum yield of electron transfer to near zero. Interestingly, when the molecule is illuminated with white light, the quantum yield <i>decreases</i> as the white light intensity is increased, generating an out-of-phase response of the quantum yield to white light. However, when the same experiment is performed in the presence of additional, steady-state UV illumination, a phase inversion occurs. The quantum yield of electron transfer now <i>increases</i> with increasing white light intensity. Such effects illustrate emergent complexity in a relatively simple system and could find applications in molecular logic, photochemical labeling and drug delivery, and photoprotection for artificial photosynthetic molecules. The photochemistry leading to this behavior is discussed

Artificial Photosynthetic Reaction Center with a Coumarin-Based Antenna System

In photosynthesis, sunlight is absorbed mainly by antenna chromophores that transfer singlet excitation energy to reaction centers for conversion to useful electrochemical energy. Antennas may likewise be useful in artificial photosynthetic systems that use sunlight to make fuels or electricity. Here, we report the synthesis and spectroscopic properties of a molecular hexad comprising two porphyrin moieties and four coumarin antenna chromophores, all organized by a central hexaphenylbenzene core. Light absorbed by any of the coumarins is transferred to a porphyrin on the 1–10 ps time scale, depending on the site of initial excitation. The quantum yield of singlet energy transfer is 1.0. The energy transfer rate constants are consistent with transfer by the Förster dipole–dipole mechanism. A pyridyl-bearing fullerene moiety self-assembles to the form of the hexad containing zinc porphyrins to yield an antenna–reaction center complex. In the resulting heptad, energy transfer to the porphyrins is followed by photoinduced electron transfer to the fullerene with a time constant of 3 ps. The resulting P^•+–C₆₀^•– charge-separated state is formed with an overall quantum yield of 1.0 and decays with a time constant of 230 ps in 1,2-difluorobenzene as the solvent

Photochemical Synthesis of a Water Oxidation Catalyst Based on Cobalt Nanostructures

New cobalt-based nanocomposites have been prepared by photoreduction of Co²⁺ salts to generate cobalt nanoparticles deposited on carbon-based materials such as nanocyrstalline diamond and carbon felt. Spontaneous air oxidation converts the metal to Co₂O₃ which has been tested as a water oxidation catalyst. This work demonstrates that the cobalt oxide nanostructures can be deposited on various carbon surfaces and can catalyze the four-electron oxidation of water to oxygen under anodic bias

Catalytic Turnover of [FeFe]-Hydrogenase Based on Single-Molecule Imaging

Hydrogenases catalyze the interconversion of protons and hydrogen according to the reversible reaction: 2H⁺ + 2e^– ⇆ H₂ while using only the earth-abundant metals nickel and/or iron for catalysis. Due to their high activity for proton reduction and the technological significance of the H⁺/H₂ half reaction, it is important to characterize the catalytic activity of [FeFe]-hydrogenases using both biochemical and electrochemical techniques. Following a detailed electrochemical and photoelectrochemical study of an [FeFe]-hydrogenase from Clostridium acetobutylicum (<i>Ca</i>HydA), we now report electrochemical and single-molecule imaging studies carried out on a catalytically active hydrogenase preparation. The enzyme <i>Ca</i>HydA, a homologue (70% identity) of the [FeFe]-hydrogenase from Clostridium pasteurianum, CpI, was adsorbed to a negatively charged, self-assembled monolayer (SAM) for investigation by electrochemical scanning tunneling microscopy (EC-STM) techniques and macroscopic electrochemical measurements. The EC-STM imaging revealed uniform surface coverage with sufficient stability to undergo repeated scanning with a STM tip as well as other electrochemical investigations. Cyclic voltammetry yielded a characteristic cathodic hydrogen production signal when the potential was scanned sufficiently negative. The direct observation of the single enzyme distribution on the Au-SAM surface coupled with macroscopic electrochemical measurements obtained from the same electrode allowed the evaluation of a turnover frequency (TOF) as a function of potential for single [FeFe]-hydrogenase molecules