Fast Photoinduced Electron Transfer Research Articles

A Metal-Free Triazacoronene-Based Bimodal VOC Sensor.

Developing suitable sensors for selective and sensitive detection of volatile organic compounds (VOCs) is crucial for monitoring indoor and outdoor air quality. VOCs are very harmful to our health upon inhalation or contact. Bimodal sensor materials with more than one transduction capability (optical and electrical) offer the ability to extract complementary information from the individual analyte, thus improving detection accuracy and performance. The privilege of manipulating the optoelectronic properties of the polycyclic aromatic hydrocarbon-based semiconducting materials offers rapid signal transduction in multimodal sensing applications. A thiophene-functionalized triazacoronene (TTAC) donor-acceptor-donor (D-A-D) type sensor is reported here for VOC sensing. The single-crystal X-ray structure analysis of the TTAC revealed that a distinctive supramolecular polymer architecture was formed because of cooperative π-π and intermolecular D-A interactions and exhibited rapid signal transduction upon exposure to specific VOCs. The TTAC-embedded green luminescent paper-based test strip exhibited an on-off fluorescence response upon nitrobenzene vapor exposure for 120 s. The selective and rapid response is due to the fast photoinduced electron transfer, as is evident from the time-resolved excited-state dynamics and density functional theory studies. The thick-film-based prototype chemiresistive sensor detects harmful VOCs in a custom-made gas sensing system including benzene, toluene, and nitrobenzene. The TTAC sensor rapidly responds (200 s) at relatively low temperatures (180 οC) compared to other reported metal-oxide-based sensors.

Photoinduced Processes in Molecular and Supramolecular Arrays Based on Porphyrins and Acridinium Units

Multichromophoric arrays based on porphyrins are of high interest for their mimicry of naturally occurring photoinduced energy and electron transfer processes leading to energy collection and conversion. [1] Moreover, the rich photophysics and electrochemistry of porphyrins make them suitable components of molecular wires, switches or chemical sensors. [2] In this perspective, it is interesting to pursue the combination of porphyrins with units that can act as multiresponsive components by modifying their chemical and electronic structure upon chemical or redox stimuli. N-acridinium ions have been recently explored as a new class of switches that respond to various orthogonal stimulations (pH or electrochemical/photochemical inputs) that trigger their reversible conversion into the non-aromatic acridane form. [3] Few examples reporting the association of acridinium units and porphyrins indicate that these systems are characterized by fast photoinduced electron transfer reactions between the porphyrin donor and the acridinium acceptor. [4,5]We recently investigated the photophysical properties of molecular and supramolecular arrays based on porphyrins and acridinium units, where the type of porphyrin (free-base or metallated) and/or the form of the switchable component (acridinium or acridane) has been varied (Figure 1). We found that the association of porphyrins and acridinium ions always leads to ultrafast electron transfer processes, and that these reactions can be altered by changing the temperature or by converting the acridinium electron acceptor into an acridane energy donor. Moreover, the combination of two Zn-porphyrins with two acridinium/acridane units results in switchable molecular receptors able to coordinate suitable hosts. The photophysics of the arrays and of the complexes, analyzed by means of steady-state and time-resolved luminescence techniques and ultrafast transient absorption spectroscopy, will be herein presented and discussed.The work is supported by the H2020-MSCA-ITN-2017-765297 project “NOAH” and by the H2020-LC-SC3-2020-RES-RIA-101006839 project “CONDOR”.[1] M. Beyler, L. Flamigni, V. Heitz, J.-P. Sauvage, B. Ventura, Photochem. Photobiol. 2014, 90, 275-286.[2] Beletskaya I, Tyurin VS, Tsivadze AY, Guilard R, Stern C. Chem. Rev. 2009, 109, 1659-1713.[3] H.-P. J. de Rouville, J. Hu, V. Heitz, ChemPlusChem, 2021, 86, 110-129.[4] A. Edo-Osagie, D. Sánchez-Resa, D. Serillon, E. Bandini, C. Gourlaouen, H.-P. Jacquot de Rouville, B. Ventura, V. Heitz, C. R. Chim. 2021, 24, 1-9.[5] H. Kotani, K. Ohkubo, M. J. Crossley, S. Fukuzumi, J. Am. Chem. Soc., 2011, 133, 11092-11095.Figure 1. Structures of the studied arrays and schemes of the occurring photoinduced processes: a) bisacridinium-Zn(II) porphyrin conjugate, b) bisacridinium-diphenylporphyrin conjugate, c) bis(acridinium/acridane)-Zn(II) porphyrin receptors. Figure 1

Investigating Design Rules for Photoinduced Electron Transfer Quenching in Triangulenium Probes.

Fluorescent probes based on photoinduced electron transfer (PET) quenching of long lifetime triangulenium fluorophores have found multiple applications. For such probes a successful design relies on the right balance between the rates of PET quenching and fluorescence. In a series of ADOTA (A) and DAOTA (D) triangulenium fluorophores appended with aniline-like quencher moieties, we have investigated the rates of quenching and its relation to thermodynamic driving force, distance, and conjugation within the quencher moiety. Three different quenchers, a short (1), a long (2), and a long twisted (3), 4-aminophenyl, 4'-aminobiphenyl, and 2,2'-dimethyl-4'-aminobiphenyl, respectively were investigated. Time-resolved fluorescence measurement was used to measure PET rate constantkPETranging from >4x1011s-1to 2x109s-1. Interestingly, PET quenching is equally efficient/fast from1and2, even with increase in distance between the donor and the acceptor. However, when twisting the biphenyl in3, a 20-fold decrease in quenching is found. Even with the decrease inkPETthe quenching in3A/Dis still highly efficient, with nearly 99% quenching. The study show that long lifetime fluorophores can be efficiently switched even by relative slow PET processes and that PET quencher moieties can be removed far from the fluorophore if conjugated linkers are applied.

Supramolecular p/n‐heterojunction of C60‐functionalized bis(merocyanine) quadruple stack: A model system for charge carrier separation and recombination in organic solar cells

AbstractSupramolecular photosystems can afford important insights into elementary processes in molecular materials. Here, a bis(merocyanine)‐C60 conjugate composed of two covalently tethered electron‐donating merocyanine chromophores and two appended fullerene electron acceptor units is investigated. Concentration‐dependent UV/Vis studies in nonpolar solvents revealed a bimolecular association (dimerization) process, resulting in the formation of an H‐coupled merocyanine dye aggregate. The aggregate structure was confirmed by 2D NMR spectroscopy proving the presence of a well‐defined quadruple dye stack surrounded by four loosely connected fullerenes, which mimics the donor–acceptor interface in bulk heterojunction (BHJ) solar cells. Due to the interdigitation of the dyes in the quadruple stack motif, even at low concentrations, fully aggregated species prevail in nonpolar solvents, which enabled us to elucidate the photophysics of this supramolecular p/n‐heterojunction in dilute solution by femtosecond time‐resolved transient absorption spectroscopy. The observed photophysical processes include a favorably fast photoinduced electron transfer from the electron‐donating dye stack to the electron‐accepting fullerene moieties, a slower geminate charge carrier recombination of the generated electron and hole in the supramolecular p/n‐heterojunction as well as the formation of fullerene triplet states. While the geminate recombination of charge carriers is supposed to be less competitive in BHJ photovoltaic materials due to the possibility of fast electron migration within fullerene domains in thin films, intersystem crossing into the triplet manifold appears as a more critical undesired process. These insights from spectroscopy may help to tailor improved photovoltaic materials, thereby demonstrating the value of fundamental studies on elementary photophysical processes in molecular aggregates.KEY POINTS Supramolecular p/n‐heterojunction by self‐assembly in solution Aggregate structure confirmed by NMR spectroscopy Photoinduced electron transfer studied by transient absorption spectroscopy

Femtosecond Upconversion Study of Interfacial Electron Transfer from Photoexcited CsPbBr3 Perovskite Nanocrystal to Rhodamine 6G.

Photoinduced electron transfer (PET) from an excited-state CsPbBr3 nanocrystal (NC) to rhodamine 6G (r6G) is studied in toluene using different fluorescence-based techniques. Because of weak solubility of r6G in toluene, excess r6G molecules adsorb at NC surface which result in a much slower rotational diffusion time scale of r6G in the presence of NCs. Study of intrinsic PET benefits from the soft molecular interactions leading to donor (NC)-acceptor (r6G) complex formation, where solvent diffusion parameters would not play any role in the PET kinetics. Femtosecond transients of NCs are nicely fit to a Poisson expression originally proposed by Tachiya. Conclusive fittings to the temperature dependence quenching data reveal two interesting observations: (1) Even though the average number of surface trap state in a NC does not change with temperature (5-60 °C), the trap-state-induced quenching time scale is accelerated with increase in temperature, pointing toward a more efficient trapping at higher temperature. (ii) In the presence of r6G, a fast (∼150 ps per r6G molecule) interfacial PET time scale is observed, which remains unaffected by temperature (5-60 °C). Our findings demonstrate that even a simple "perovskite NC-electron acceptor" composite like that in the present study can ensure a rapid interfacial charge separation. Such information will help us to realize the actual potential of perovskites NCs in their real applications.

Enhanced dye-sensitized solar cell performance using strontium titanate perovskite integrated photoanodes modified with plasmonic silver nanoparticles

Plasmonic silver nanoparticles loaded strontium titanate nanocomposites (Ag-SrTiO3 NC) with diverse weight percentages (0.5, 1.5, 2.5 and 5 wt%) of Ag nanoparticles (Ag NPs) are synthesized by a facile chemical reduction method. The prepared nanocomposites are characterized using diffuse reflectance spectroscopy, X-ray diffractometry, photoluminescence spectroscopy, scanning electronic microscopy and transmission electron microscopy. The photovoltaic performance of dye-sensitized solar cells (DSSCs) integrated with Ag-SrTiO3 NC photoanodes has been assessed under simulated sun light intensity of 100 mW cm−2. The Ag-SrTiO3 NC photoanode loaded with 2.5 wt% Ag NPs exhibited higher power conversion efficiency of 4.39% with short-circuit photocurrent density of 11.54 mA cm−2, open circuit voltage of 0.77 V and fill factor of 0.49 in DSSC. This enhanced photovoltaic performance can be credited to high dye loading, improvement in visible light harvesting - and fast photo-induced electron transfer caused by the plasmonic Ag NPs.

β-Diketiminate-supported iridium photosensitizers with increased excited-state reducing power

Triazole and NHC-based bis-cyclometalated iridium β-diketiminate complexes emerge as some of the strongest reported visible-light photoreductants. Fast photoinduced electron transfer and photoredox catalysis on organohalide substrates are observed.

Interaction of light with a non-covalent zinc porphyrin-graphene oxide nanohybrid.

The present study explores the influence of graphene oxide (GO) on deactivation pathways of the excited states of zinc 5,10,15,20-tetrakis(4-(hydroxyphenyl))porphyrin (ZnTPPH). The interaction of light with free ZnTPPH molecules and with ZnTPPH molecules adsorbed on graphene oxide sheets was probed via UV-vis spectroscopy, fluorescence spectroscopy, femtosecond pump-probe technique and nanosecond flash photolysis. Formation of the ground-state ZnTPPH-GO complex in solution was monitored by the red-shift of the porphyrin Soret absorption band. It was found that Stern-Volmer fluorescence quenching can be described in terms of two different quenching regimes depending on the GO concentration. In addition, our comprehensive analysis of the steady-state and time-resolved emission experiments led to the conclusion that the observed quenching was entirely attributable to a static mechanism. Laser flash photolysis showed that the triplet lifetime of the ZnTPPH increased in the presence of GO from 174 μs to 292 μs, which is related to the decrease in the rate constant of a radiationless decay mechanism involving rotation of the peripheral hydroxyphenyl rings of the porphyrin. Femtosecond transient absorption spectroscopy demonstrated the presence of a fast photoinduced electron transfer from the singlet excited state of ZnTPPH to the GO sheets, as indicated by the formation of a porphyrin radical cation. Quantum chemical calculations were used to gain deeper insights into the nature of the electronically excited states in the free ZnTPPH as well as in the ZnTPPH-GO complex.

Light-Driven Water Oxidation with the Ir-blue Catalyst and the Ru(bpy)32+/S2O82- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency.

Light-driven water oxidation is achieved with the Ru(bpy)32+/S2O82- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an IrIV,IV2(pyalc)2 μ-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)33+, with a quantum yield ϕ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)33+, leading to the Ir(IV) monomer I1 (k1 ∼ 108 M-1 s-1). The competent Ir-blue catalyst is then obtained from I1 upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)32+/S2O82- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)33+ [with kIr-blue = (3.00 ± 0.02) × 108 M-1 s-1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a IrV,V2 steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of ϕ(O2) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.

Visible-light-responsive photocatalyst with a microsphere structure: preparation and photocatalytic performance of CQDs@BiOCl

In this study, the effects of carbon quantum dot (CQD) doping on the photocatalytic performance of semiconductor BiOCl microspheres were investigated. Highly dispersed CQDs with up-conversion luminescence properties were prepared using the hydrothermal method, and visible-light-responsive CQDs@BiOCl photocatalysts with regular morphology were prepared via CQD doping. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible (UV–Vis) spectroscopy were used to investigate the morphology and light absorption properties of the materials. The degradation rate of rhodamine B (RhB) was 76.1% after 180 min of visible-light irradiation when CQDs@BiOCl were used and only 19.4% when pure BiOCl was used. The photoluminescence (PL), UV–Vis diffuse reflectance spectra (UV–Vis DRS) and electron paramagnetic resonance (EPR) results were analyzed to determine the possible reasons for the increased photocatalytic activity of CQDs@BiOCl microspheres. The results showed CQD doping expanded the visible light absorption range, CQDs exhibited fast photoinduced electron transfer, and CQDs@BiOCl possessed high mesoporosity, which promoted the effective separation of photogenerated electron–hole pairs. In addition, the microsphere structure of CQDs@BiOCl exhibited a larger specific surface area and a more regular morphology than its sheet-like structure. These features increased the number of photocatalytic reaction sites and the surface adsorption of the catalyst. In addition, the electronic conjugated structure of CQDs was demonstrated to function as an effective electron trap. CQD doping effectively inhibited the photogenerated electron–hole pair recombination of the composite photocatalyst, which enhanced the photocatalytic performance of the system.

Selective photoluminescence enhancement of red emitted surface modified poly(p-phenylenediamine) dots: An ultra-sensitive anion photoluminescence sensor for F− in vitro

• An ultra-sensitive anion PL sensor for F − was reported. • The system’s detection limit is 2 × 10 −10 M, lower than most other methods. • This “OFF-ON” detection system shows value in practical applications. • We also show successful F − detection in vitro based on TPPDs. • This work is helpful for the development of other anion PL sensors. The weak interactions between anions and common functional groups make it difficult for the development of anion photoluminescence sensors. In this work, we report an ultra-sensitive anion photoluminescence (PL) sensor for F − based on triethylene glycol monomethyl ether modified poly(p-phenylenediamine) dots (TPPDs). Due to the PET process in excited TPPDs, no obvious PL behavior can be observed in TPPDs. Due to the fast photoinduced electron transfer (PET) process in excited TPPDs, no obvious photoluminescence behavior can be observed: however, the PET process can be cut off in the presence of F − and TPPDs show bright red PL. The detection limit of this system is 2 × 10 −10 M, which is much lower than most other reported detection methods. Compared with traditional “ON-OFF” PL sensors, such an “OFF-ON” PL detection system shows higher value in practical applications. The detection system exhibits superior photostability and high selectivity for F − . We also demonstrate the successful F − detection in vitro based on TPPDs.

Photoconductive Curved-Nanographene/Fullerene Supramolecular Heterojunctions.

This study presents synthesis and characterizations of two novel curved nanographenes that strongly bind with fullerene C60 to form photoconductive heterojunctions. Films of the self-assembled curved nanographene/fullerene complexes, which served as the photoconductive layer, generated a significant photocurrent under light irradiation. Gram-scale quantities of these curved nanographenes (TCR and HCR) as the "crown" sidewalls can be incorporated into a carbon nanoring to form molecular crowns, and the molecular structure of C60 @TCR is determined by single-crystal X-ray diffraction. The UV/Vis absorption and emission spectra, and theoretical studies revealed their unique structural features and photophysical properties. Time-resolved spectroscopic results clearly suggest fast photoinduced electron transfer process in the supramolecular heterojunctions.

Photoconductive Curved‐Nanographene/Fullerene Supramolecular Heterojunctions

AbstractThis study presents synthesis and characterizations of two novel curved nanographenes that strongly bind with fullerene C60 to form photoconductive heterojunctions. Films of the self‐assembled curved nanographene/fullerene complexes, which served as the photoconductive layer, generated a significant photocurrent under light irradiation. Gram‐scale quantities of these curved nanographenes (TCR and HCR) as the “crown” sidewalls can be incorporated into a carbon nanoring to form molecular crowns, and the molecular structure of C60@TCR is determined by single‐crystal X‐ray diffraction. The UV/Vis absorption and emission spectra, and theoretical studies revealed their unique structural features and photophysical properties. Time‐resolved spectroscopic results clearly suggest fast photoinduced electron transfer process in the supramolecular heterojunctions.

Low-cost Schiff bases chromophores as efficient co-sensitizers for MH-13 in dye-sensitized solar cells

Herein, we reported on the synthesis of four Schiff bases (S1–4) based on salicylaldehyde moieties and their applications as metal-free organic chromophores for sensitization and co-sensitization of dye-sensitized solar cells (DSSCs). These sensitizers are comprised of a phenyl ring as a donor scaffold connected with a carboxylic group as acceptor/anchoring unit. To gauge their applicability as photosensitizers for DSSCs the photophysical, electrochemical, theoretical studies and charge transport characteristics were carried out. Interestingly, uses of S2 and S4 dyes as co-sensitizers with well-known MH-13 dye resulted in enhancing its photovoltaic properties from 8.217 to 8.79% and 8.48%, respectively. Furthermore, the photo-sensitizer S3 individually shows better photovoltaic properties compared to S1, S2 and S4, due to the fast photo-induced electron transfer from the phenyl ring to the nitro group. This fast electron transfer resulted from the overlapping between HOMO and LUMO levels on phenyl ring bearing nitro group. The results indicate that these simple and easy prepared Schiff bases moieties may be considered as promising and low-cost co-sensitizers for DSSCs.

Inside Cover: Ultrafast Electron Transfer in Complexes of Doxorubicin with Human Telomeric G‐Quadruplexes and GC Duplexes Probed by Femtosecond Fluorescence Spectroscopy (ChemPhysChem 9/2016)

The Inside Cover picture illustrates how femtosecond fluorescence spectroscopy provides evidence of fast photoinduced electron transfer in complexes of doxorubicin with d(GC) duplexes and human telomeric G-quadruplexes. More information can be found in the Full Paper by P. Changenet-Barret et al. on page 1264 in Issue 9, 2016 (DOI: 10.1002/cphc.201501091).

“Off–On” switchable sensor: A near-infrared fluorescent pH probe and its imaging applications in vivo

Intracellular or extracellular pH is influenced by diverse physiological and pathological processes, so quantitativelymeasuring pHis useful for cellular analysis or diagnosis. In this work, a new near-infrared (NIR) fluorescence turn-on probe CP utilizing a fluorophorespacer- receptor molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced electron-transfer process was developed. Our strategy was to choose tricarbocyanine (Cy), an NIR fluorescent dye with high extinction coefficients, as a fluorophore, and N-(4-pyridylmethyl)ethylenediamine (Py) as a receptor. The pH titration indicated that CP can monitor the minor physiological pH fluctuations with a pKa of ∼6.40, which is valuable for intracellular pH researches and tumor diagnosis. The probe responds linearly and rapidly to minor pH fluctuations within the range of 5.00-7.50 and exhibits strong dependence on pH changes. As expected, the realtime imaging of cellular pH and the detection of pH in situ was achieved successfully in living A549 and HeLa cells by this probe. It is shown that the probe effectively avoids the influence of autofluorescence and native cellular species in biological systems and meanwhile exhibits high sensitivity, good photostability, low cytotoxicity and excellent cell membrane permeability. This fluorescence probe is further examined as tumor microenvironment-activatable probes for intravenous in vivo tumor imaging, which is expected to be of crucial importance for clinical tumor diagnostics.

Heterogeneous subunit structures in the pyranose 2-oxidase homotetramer revealed by theoretical analysis of the rates of photoinduced electron transfer from a tryptophan to the excited flavin

Abstract Pyranose 2-oxidase (P2O) from Trametes multicolor forms a homotetramer in which each of the subunits contains flavin adenine dinucleotide (FAD). The fluorescence of P2O decays with two lifetime components; a slow (358 ps) and a fast (∼90 fs) decay. The lifetime of the fast component is emission-wavelength dependent and is ascribed to fast photoinduced electron transfer (ET) from Trp168 to the excited isoalloxazine (Iso*) in FAD. The donor–acceptor distances were ∼0.7 nm. The extraordinary heterogeneous decays were analyzed with atomic coordinates obtained by a molecular dynamics simulation and the ET rate by Kakitani and Mataga. The emission-wavelength dependent decays in the fast component were elucidated by introducing emission-wavelength dependent standard free energy related to electron affinity of Iso*. Examination of all possible combinations of the four subunits revealed that the slow component was from subunit A and the fast component was from the other three subunits. Agreements between the observed and calculated decays were all excellent. The large difference in the fast and slow fluorescent lifetimes is ascribed to the difference in the standard free energy gap related to electron affinity of Iso*. The dependence of the logarithmic ET rates on the center-to-center distance displayed approximate linear functions (Dutton rule) when the rate was relatively slow and parabolic functions when the rate was ultrafast. The Dutton rule originated from the exponential term of the ET rate, not from the electronic coupling term.

Fluorescence Study of Polyaniline Doped with Organic Acids

Polyaniline (PANI) was synthesized by chemical oxidation polymerization in the presence of ammonium persulfate as oxidant and different concentration of organic acids. In this study, fluorescence characteristics of polyaniline doped with different concentration of organic acids is soluble in DMF in terms of fluorescence quenching are discussed. The emission spectra of all the polymers solutions were noticed to be a mirror image of the excitation spectra. Upon photo excitation of a conjugated polymer, the electrons from the valence band are excited to the conduction band and then migrated along the polymer backbone. In solution, the quencher and the polymer form a weak complex. This static association prevents the transfer of electron on the conjugated chain, or induces ultra fast photo induced electron transfer to electron acceptor, resulting decrease of fluorescent intensity of conjugated polymer. It has been observed that as the concentration of dopant increases the fluorescence intensity decreases. The intensity of the fluorescence peaks seems to increase as the dopant concentration is increased which suggests that the density of states in the dopant induced energy band increases with dopant concentration.

Long-Lived Charge Separation and Applications in Artificial Photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Artificial chloroplast: Au/chloroplast-morph-TiO2 with fast electron transfer and enhanced photocatalytic activity

The unique structures and functional features of chloroplasts in green plants provide a promising blueprint for greatly improving solar energy utilization efficiency. In this paper, a prototype of artificial chloroplast, Au/chloroplast-morph-TiO2 with natural chloroplasts' nanostructure and analogous functional features, is provided. The nano-layered structures of chloroplast template inherited in the chloroplast-morph-TiO2 lead to a large reaction area and fast photo-induced electron transfer; cocatalyst Au nanoparticles which work as reaction centers promote photo-induced charge separation and improve the overall photocatalytic activity in hydrogen production; Nitrogen and phosphorus self-doped from the bio-template increase visible light absorption, similar to the antenna pigment. With this new inorganic artificial photosynthetic system, we achieve effective light utilization, fast photo-induced charge separation, high electron transfer, enhanced photocatalytic activity for dye degradation rate and improved H2 evolution efficiency. This concept provides the inspiration for constructing efficient photocatalysts by imitating the photosynthesis process from both structures and functions.