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

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

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

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

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

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

Titanium dioxide (TiO₂) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO₂ is well understood. However, the surface structure of nanoparticulate TiO₂, which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defect structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO₂ nanoparticles. We report the synthesis and characterization of ∼4 nm TiO₂ anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO₂ nanoparticles without affecting parameters such as size, shape, core crystallinity, and solubility. The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO₂ defect states on photoinduced electron transfer dynamics

Spectroscopic Analysis of a Biomimetic Model of Tyr_Z Function in PSII

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole–phenol model for the Tyr_Z-D₁His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole–phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad

Modulating Short Wavelength Fluorescence with Long Wavelength Light

Two molecules in which the intensity of shorter-wavelength fluorescence from a strong fluorophore is modulated by longer-wavelength irradiation of an attached merocyanine–spirooxazine reverse photochromic moiety have been synthesized and studied. This unusual fluorescence behavior is the result of quenching of fluorophore fluorescence by the thermally stable, open, zwitterionic form of the spirooxazine, whereas the photogenerated closed, spirocyclic form has no effect on the fluorophore excited state. The population ratio of the closed and open forms of the spirooxazine is controlled by the intensity of the longer-wavelength modulated light. Both square wave and sine wave modulation were investigated. Because the merocyanine–spirooxazine is an unusual reverse photochrome with a thermally stable long-wavelength absorbing form and a short-wavelength absorbing photogenerated isomer with a very short lifetime, this phenomenon does not require irradiation of the molecules with potentially damaging ultraviolet light, and rapid modulation of fluorescence is possible. Molecules demonstrating these properties may be useful in fluorescent probes, as their use can discriminate between probe fluorescence and various types of adventitious “autofluorescence” from other molecules in the system being studied