Back Electron Transfer Process Research Articles

Induction of chirality in the charge transfer complex photochemistry

Electron donor and acceptor undergo association in solution to form a weak molecular complex, namely a charge transfer complex. The formation of the complex is apparent by its new low-energy absorption band. Upon photoexcitation of this charge transfer band, an intracomplex electron transfer is facilitated to form a contact ion radical pair. The theoretical basis for the complex formation and the subsequent photoinduced electron transfer process has been well established already half a century ago. Nevertheless, the charge transfer complex photochemistry is now in a renaissance, due to its synthetic utility of its radical ion pair. By intentionally sidestepped unproductive back electron transfer process, a desired product is effectively obtained with or without external reagent and/or catalyst. In addition, the photoexcitation of the complex does not require any additional photocatalyst and often conceivable with the visible light. Accordingly, various donor/acceptor pairs and synthetic applications have been emerged as novel photoreaction systems in the last two decades. Some have also addressed the stereoselective transformations. This mini review highlights the recent progress of the asymmetric photoreaction, in particular in synthetic applications, based on the photoexcitation of the charge transfer complex, in comparison with photoredox catalysis.

Exploring the impact of structural rigidification of amino-substituted bio-inspired flavylium dyes in DSSCs

Sharing with anthocyanins the 2-phenyl-1-benzopyrylium structural motif, flavylium derivatives are strongly colored bio-inspired dyes that have been explored in dye-sensitized solar cells (DSSCs). Following on the fact that the most efficient flavylium-based dyes for DSSCs require amine electron-donating groups, a diethylamino group and the corresponding rigidified julolidine group were introduced in the benzopyrylium core. This structural variation was combined with another structural parameter – increased planarization of the flavylium ring system – to yield four flavylium derivatives all with a catechol anchoring group. The several pKa values of the new dyes and the UV–vis absorption data at different pH values and upon adsorption to TiO2 (corroborated by TD-DFT calculations) demonstrate a stronger delocalization of the nitrogen lone pair in the julolidine systems when compared to the diethylamino ones, reflecting the stronger electron-donating ability of the former. However, the julolidine-based dyes resulted in a decrease in all DSSC parameters, with efficiencies of 0.6% vs. 2.3% for the diethylamino devices. Discarding eventual increased self-aggregation processes of the more planar julolidine derivatives through studies with a de-aggregating agent (CDCA), and determining comparable dye loadings for all dyes, the presence of increased back-electron transfer processes for the julolidine-based compounds is advanced to explain their lower efficiencies. Rigidification of the flavylium dyes by bridging the benzopyrylium and the phenyl rings is demonstrated by higher fluorescence quantum yields and by electrochemical data and leads to a slight increase in the efficiency of the respective DSSCs. The results contribute to consolidate the potential of flavylium dyes as sensitizers for DSSCs.

Photoinduced charge transfer in push-pull pyrazoline-based chromophores – Relationship between molecular structure and photophysical, photovoltaic properties

The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.

Stability and Performance Enhancement of an Oligo (phenylene vinylene) Photocatalyst via Surface Grafting onto TiO2 for Visible‐Light Indigo Carmine Degradation

AbstractThe chemical immobilization of a dibenzoic acid oligo (phenylene vinylene) (OPV) derivative for TiO2 sensitization was evaluated as a strategy to enhance both the stability and photocatalytic performance of this π‐conjugated moiety when applied to the photodegradation of indigo carmine dye (IC) as model pollutant. The results show that the electron injection from the photoexcited OPV to the TiO2 conduction band remarkably boosts the IC degradation ability under visible light illumination. The use of SiO2 as non‐photoactive support was also investigated to simultaneously determine the role of the inorganic support in the photocatalytic process. The spectroscopic, thermal, and surface characterization validated the successful grafting of the OPV to the inorganic supports. Moreover, superoxide radical and singlet oxygen were established as the active species involved in the dye degradation identifying an additional direct reducing pathway. Interestingly, the use of electron donors proved to be an ineffective approach to increase the system performance, entailing that the OPV regeneration proceeds chiefly via back electron transfer processes. Finally, the reusability tests confirmed that TiO2/OPV material retains up to 86 % of their photocatalytic activity during the first five cycles, demonstrating a critical enhancement in the organic conjugated framework stability.

Self-Assembly and Photochemistry of a Pyrene-Methyl Viologen Supramolecular Fiber System.

This paper reports the self-assembly of a donor-acceptor system into nanoscopic structures and the photo processes taking place within these structures. The donor employed is pyrene linked to two β-cyclodextrin molecules (CD-PY-CD), and adamantane-linked methyl viologen attached to the three arms of mesitylene (Ms-(MV2+-AD)3) is the acceptor. CD-PY-CD and Ms-(MV2+-AD)3 when dissolved in water self-assembled into vesicles, which joined together to give long fibers. The self-assembly was studied using spectroscopic and microscopic techniques. Fluorescence of the pyrene chromophore was quenched within the self-assembled system due to efficient photoinduced electron transfer to methyl viologen. Photoinduced electron transfer within the assembly is confirmed through identification of product radical ions in flash photolysis experiments. Steady-state irradiation of the self-assembled system in an optical bench led to the formation of methyl viologen radical cation, which was stable for a few hours. Longevity of the radical cation was attributed to the fast reaction of pyrene radical cation with adjacent pyrene to give an unstable adduct, which slows down the back electron transfer process.

The effect of the rate of photoinduced electron transfer on the photodecarboxylation efficiency in phthalimide photochemistry

Reactivity in photoinduced electron transfer reactions (PET) has been investigated in a series of molecules possessing different distances between the electron donor (carboxylate or alkoxyphenyl) and the phthalimide as the electron acceptor. The molecules were strategically designed to separate the donor and the acceptor by a rigid adamantane spacer or through a peptide backbone with different number of amino acid residues. PET was investigated by laser flash photolysis (LFP). Although previous reports demonstrated that the quantum yields of the photodecarboxylation reaction (ΦR) depend on the distance between the donor and acceptor moieties, the measured lifetimes of the triplet excited state did not provide information on the rates of PET. Due to reversible PET and back electron transfer processes in compounds 1-5, the measured lifetimes correspond to the sum of the rate constants for the disappearance of the charge transfer species. In the series of peptides, three different PET processes take place and LFP provided rate constant for the reactions occurring subsequent to the intrastrand PET between the carboxylate and the alkoxyphenyl radical cation. The understanding of the factors that affect intramolecular PET processes is important for the rational design of different applications.

Factors determining formation efficiencies of one-electron-reduced species of redox photosensitizers.

Improvement in the photochemical formation efficiency of one-electron-reduced species (OERS) of a photoredox photosensitizer (a redox catalyst) is directly linked to the improvement in efficiencies of the various photocatalytic reactions themselves. We investigated the primary processes of a photochemical reduction of two series [Ru(diimine)3]2+ and [Os(diimine)3]2+ as frequently used redox photosensitizers (PS2+), by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a typical reductant in detail using steady-irradiation and time-resolved spectroscopies. The rate constants of all elementary processes of the photochemical reduction of PS2+ by BIH to give the free PS•+ were obtained or estimated. The most important process for determining the formation efficiency of the free PS•+ was the escape yield from the solvated ion pair [PS•+-BIH•+], which was strongly dependent on both the central metal ion and the ligands. In cases with the same central metal ion, the system with larger -ΔGbet, which is the free energy change in the back-electron transfer from the OERS of PS•+ to BIH•+, tended to lower the escape yield of the free OERS of PS2+. On the other hand, different central metal ions drastically affected the escape yield even in cases with similar -ΔGbet; the escape yield in the case of RuH2+ (-ΔGbet = 1.68 eV) was 5-11 times higher compared to those of OsH2+ (-ΔGbet = 1.60 eV) and OsMe2+ (-ΔGbet = 1.71 eV). The back-electron transfer process from the free PS•+ to the free BIH•+ could not compete against the further reaction of the free BIH•+, which is the deprotonation process giving BI•, in DMA for all examples. The produced BI• gave one electron to PS2+ in the ground state to give another PS•+, quantitatively. Based on these findings and investigations, it is clarified that the photochemical formation efficiency of the free PS•+ should be affected not only by -ΔGbet but also by the heavy-atom effect of the central metal ion, and/or the oxidation power of the excited PS2+, which should determine the distance between the excited PS and BIH at the moment of the electron transfer.

Dye sensitized solar cell (DSC) based on reduced graphene oxide (rGO)-TiO2 nanocomposite photoelectrode and polyaniline (PANI) counter electrode

We report the successful application of reduced graphene oxide–titania (rGO–TiO2) nanocomposite as an efficient photoelectrode and an inexpensive polyaniline (PANI) synthesized by in-situ polymerization on graphite foam as a platinum substitute for tri-iodide reduction for dye‐sensitized solar cell (DSC). The bulk carrier concentration and conductivity of the PANI was measured to be 3.02x1017cm-3 and 4.89x10-1 W-1cm-1 respectively. Subsequently, three DSCs were assembled with rGO–TiO2 nanocomposite photoelectrode and PANI as counter electrode for one and the other two assembled using unmodified TiO2 photoelectrode with PANI and platinum as counter electrodes, respectively. The rGO loading allows more dye to be adsorbed due its large surface area thus improving the light harvesting efficiency (LHE). This improvement in LHE increases the short circuit current density (JSC). The JSC increase is more substantial compared to the reduction in VOC; thus, the increase in the efficiency of the cell with rGO-TiO2 nanocomposite electrode. The short circuit current density for the rGO-TiO2 DSC with PANI counter electrode is 0.45mAcm-2 while that for the unmodified TiO2 DSCs with PANI counter electrode and platinum counter electrode are 0.11mAcm-2 and 0.10 mAcm-2 respectively. This corresponds to 76% increase in the current density and it increases collection rate at the photoelectrode leading to enhanced power conversion efficiency of 0.13% compared with 0.04% and 0.02% for the DSCs assembled with unmodified TiO2 under full sunlight illumination (100 mW/cm2, AM 1.5G) as a result of the better charge collection efficiency of rGO, which reduces the back electron transfer process. This represent 69% enhancement of energy conversion efficiency in the DSC consisting of rGO modified TiO2

Dye sensitized solar cell (DSC) based on reduced graphene oxide (rGO)-TiO2 nanocomposite photoelectrode and polyaniline (PANI) counter electrode

We report the successful application of reduced graphene oxide–titania (rGO–TiO2) nanocomposite as an efficient photoelectrode and an inexpensive polyaniline (PANI) synthesized by in-situ polymerization on graphite foam as a platinum substitute for tri-iodide reduction for dye‐sensitized solar cell (DSC). The bulk carrier concentration and conductivity of the PANI was measured to be 3.02x1017cm-3 and 4.89x10-1 W-1cm-1 respectively. Subsequently, three DSCs were assembled with rGO–TiO2 nanocomposite photoelectrode and PANI as counter electrode for one and the other two assembled using unmodified TiO2 photoelectrode with PANI and platinum as counter electrodes, respectively. The rGO loading allows more dye to be adsorbed due its large surface area thus improving the light harvesting efficiency (LHE). This improvement in LHE increases the short circuit current density (JSC). The JSC increase is more substantial compared to the reduction in VOC; thus, the increase in the efficiency of the cell with rGO-TiO2 nanocomposite electrode. The short circuit current density for the rGO-TiO2 DSC with PANI counter electrode is 0.45mAcm-2 while that for the unmodified TiO2 DSCs with PANI counter electrode and platinum counter electrode are 0.11mAcm-2 and 0.10 mAcm-2 respectively. This corresponds to 76% increase in the current density and it increases collection rate at the photoelectrode leading to enhanced power conversion efficiency of 0.13% compared with 0.04% and 0.02% for the DSCs assembled with unmodified TiO2 under full sunlight illumination (100 mW/cm2, AM 1.5G) as a result of the better charge collection efficiency of rGO, which reduces the back electron transfer process. This represent 69% enhancement of energy conversion efficiency in the DSC consisting of rGO modified TiO2

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

Summary The traditional view of a chemical change is inherently local and classical, and such a change relies on a mix of thermodynamic and kinetic parameters to control reactivity. Often, the thermodynamic stability of chemical bonds necessitates significant energy input for activation. One fundamental question is potentially transformative: can quantum mechanics enable selective bond activation? A possible approach involves strategic input of energy to reaction-specific vibrational levels. Toward this goal, our work describes the coupling of vibrational motions in a terpyridine-molybdenum complex hosting a nonreactive substrate—dinitrogen. Ultrafast coherence spectroscopies revealed a Fermi-resonance coupling mechanism connecting in-plane breathing motion of the light-harvesting terpyridines with the stretching motion of the spatially disparate dinitrogen bridge. Notably, the coupling is significantly enhanced in the photoexcited state. This Fermi resonance indicates an energy conduit that drives the two motions in sync and thereby amplifies vibrational energy exchange. Achieving selective bond activation by bridging vibrations could present a quantum-inspired design principle in synthetic chemistry.

Photo-induced intermolecular electron transfer-effect of acceptor molecular structures

Photo-induced electron transfer versus molecular structure of acceptors is investigated using ultrafast time-resolved transient grating spectroscopy. Typical laser dyes Rhodamine 101 (Rh101) and Rhodamine 6G (Rh6G) in electron donor solvent—aniline are adopted as the objects. The forward electron transfer time constant from aniline to the excited singlet state of two Rhodamine dyes and subsequent back electron transfer from two dyes to aniline are measured. The experimental results denote that Rh6G presents faster electron transfer rates with aniline in both forward electron transfer and back electron transfer processes. With chemical calculation and qualitative analysis, it is found that the flexible molecular geometry of Rh6G leads to stronger electron coupling with donor solvent and further gives rise to larger electron transfer rates.

Short-Range Electron Transfer in Reduced Flavodoxin: Ultrafast Nonequilibrium Dynamics Coupled with Protein Fluctuations.

Short-range electron transfer (ET) in proteins is an ultrafast process on the similar time scales as local protein-solvent fluctuation, and thus the two dynamics are coupled. Here we use semiquinone flavodoxin and systematically characterized the photoinduced redox cycle with 11 mutations of different aromatic electron donors (tryptophan and tyrosine) and local residues to change redox properties. We observed the forward and backward ET dynamics in a few picoseconds, strongly following a stretched behavior resulting from a coupling between local environment relaxations and these ET processes. We further observed the hot vibrational-state formation through charge recombination and the subsequent cooling dynamics also in a few picoseconds. Combined with the ET studies in oxidized flavodoxin, these results coherently reveal the evolution of the ET dynamics from single to stretched exponential behaviors and thus elucidate critical time scales for the coupling. The observed hot vibration-state formation is robust and should be considered in all photoinduced back ET processes in flavoproteins.

Charge transfer dynamics between MPA capped CdTe quantum dots and methyl viologen

Abstract The understanding of charge carrier dynamics in hybrid materials based on colloidal semiconductor nanocrystals (or quantum dots) and organic moieties is fundamental for the design of efficient photonic and photovoltaic devices. In the present work, we investigate the interactions occurring between CdTe quantum dots, capped with a strong capping agent such as 3-mercaptopropionic acid, and a well known electron acceptor such as methylviologen molecule. The nature of the interactions and of exciton dynamics is investigated by stationary and time-resolved spectroscopies. Luminescence data recorded in presence of increasing methylviologen concentrations, indicate that the organic molecule is able to statically interact with the surface sites of CdTe quantum dots; a biphasic interaction behavior is evidenced by determining the apparent association constants. These latter are obtained through the analysis of luminescence data, and values in the range 103–104 are determined. The nature of the interactions is characterized by nanosecond and femtosecond transient absorption spectroscopies, to clarify the dynamics and the conditions able to foster charge mobility. Nanosecond flash photolysis measurements, carried out upon quantum dots excitation, shows the absorption of methylviologen radical cation specie at 605 nm, suggesting the occurrence of electron transfer from CdTe nanocrystals to the organic acceptor; the relatively long decay time of the transient signal (10.3 μs) indicates that back electron transfer processes are negligible. Ultrafast transient absorption measurements confirm the occurrence of an ultrafast electron transfer process; spectral and kinetic analysis of the transient data show that methylviologen radical cation is formed almost instantaneously on the ps-time scale but mainly when the samples are pumped in the energy continuum at 400 nm. This finding suggests that electron mobility from the nanocrystals to the organic units is achieved mainly when the excitonic states possess an excess of energy. The comparison of the kinetic behaviour of the signals at increasing methylviologen concentrations indicates that the electron transfer process competes with the radiative exciton recombination. In addition, the kinetic data suggests that surface trapping and Auger recombination processes might slow down the charge mobility.

Is back-electron transfer process in Betaine-30 coherent?

The possible role of coherent vibrational motion in ultrafast photo-induced electron transfer remains unclear despite considerable experimental and theoretical advances. We revisited this problem by tracking the back-electron transfer (bET) process in Betaine-30 with broadband pump-probe spectroscopy. Dephasing time constant of certain high-frequency vibrations as a function of solvent shows a trend similar to the ET rates. In the purview of Bixon-Jortner model, high-frequency quantum vibrations bridge the reactant-product energy gap by providing activationless vibronic channels. Such interaction reduces the effective coupling significantly and thereby the coherence effects are eliminated due to energy gap fluctuations, making the back-electron transfer incoherent.

Thionine-graphene oxide covalent hybrid and its interaction with light.

Graphene oxide sheets (GO) were covalently functionalized with thionine molecules. The obtained hybrid material, Th-GO, was characterized by means of scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Subsequently, the interaction of light with the free dye molecules and with dye molecules bound to the graphene oxide sheets was probed via UV-Vis spectroscopy, fluorescence spectroscopy and femtosecond pump-probe spectroscopy. The experimental results proved that thionine was successfully grafted onto the GO sheets, however, only one of the two amino groups of thionine was always involved in the amide bond formation. The Th-GO hybrid suspended in N,N-dimethylformamide (DMF) exhibited suppressed fluorescence as compared to the free dye in the same solvent, pointing to an efficient interaction between the photoexcited dye and the graphene sheets. Yet, no electron transfer products were detected by transient absorption measurements, even though there was a shortening of the singlet excited state lifetime of thionine (from the 567 ps for the free dye to the 313 ps for the dye in Th-GO). These results can be rationalized in terms of a fast back electron transfer process or possibly an energy transfer process.

Tuning the donors to control the lifetimes of charge-separated states in triazine-based donor-acceptor systems

Six donor-acceptor systems with styrene based 9-phenyl carbazole, 4,4’-di(hydro, methyl, methoxy or octyloxy) triphenylamine and 4-methylphenyl indoline derivatives as donors, and s-triazine group as the acceptor were synthesized and characterized. The charge-separated states of these donor-acceptor systems were generated through the effective photoinduced electron transfers from excited donor modules to acceptor modules and had lifetimes ranging from 57 ns to 215 ns. These donor-acceptor systems are capable of fast charge separation, but have extremely slow charge recombination. The back electron transfer processes of these donor-acceptor systems occur in the inverted region of the Marcus curve. The driving forces of back electron transfer and the reorganization energies of electron transfer decrease in the order of 9-phenyl carbazole derivative, 4,4’-di(hydro, methyl, methoxy or octyloxy) triphenylamine derivative and 4-methylphenyl indoline derivative. These two factors work together to determine the different lifetimes of charge-separated states in these donor-acceptor systems.

Enhancement of power conversion efficiency of dye sensitized solar cells by modifying mesoporous TiO2 photoanode with Al-doped TiO2 layer

Abstract The optimization of charge transfer at the TiO2/dye/electrolyte interface is crucial to the design of dye-sensitized solar cells. In this paper, the Al doped TiO2 layers were coated on the mesoporous TiO2 films by a chemical bath deposition process, followed by sintering at 500 °C. Its performance as the TiO2/dye/electrolyte interface layer was investigated. The analysis results reveals that the Al doped TiO2 coating can reduce the oxygen vacancy-Ti3+ concentrations in the photoanodes significantly, which is beneficial to suppress the back electron transfer process from TiO2 to electrolyte. Furthermore, the electron mobility of the modified photoanodes was increased. Consequently, the dye sensitized solar cells (DSSC) based on the photoanodes modified by Al-doped TiO2 coating show an improvement on both enhancing the electron transport and suppressing the charge recombination process. The dye sensitized solar cell based on the photoanode modified by 1.0 mol% Al-doped TiO2 layer, shows a power conversion efficiency as high as 7.66%, increasing 9.12% compared with the unmodified reference cell. Finally, our works show a facile and scalable technique to further optimize interfacial charge transfer process in dye sensitized solar cells.

Modulation of Energy Conversion Processes in Carbonaceous Molecular Bearings.

The energetics and photodynamics of carbonaceous molecular bearings with discrete molecular structures were investigated. A series of supramolecular bearings comprising belt-persistent tubular cycloarylene and fullerene molecules accepted photonic stimuli to afford charge-separated species via a photoinduced electron transfer process. The energy conversion processes associated with the photoexcitation, however, differed depending on the molecular structure. A π-lengthened tubular molecule allowed for the emergence of an intermediary triplet excited state at the bearing, which should lead to an energy conversion to thermal energy. On the other hand, low-lying charge-separated species induced by an endohedral lithium ion in fullerene enabled back electron transfer processes to occur without involving triplet excited species. The structure-photodynamics relationship was analyzed in terms of the Marcus theory to reveal a large electronic coupling in this dynamic supramolecular system.

Reduced graphene oxide-titania nanocomposite-modified photoanode for efficient dye-sensitized solar cells

Summary We report the successful application of reduced graphene oxide–titania (rGO–TiO2) nanocomposite as an efficient photoanode for dye-sensitized solar cell (DSSC). The DSSC assembled with the rGO–TiO2-modified photoanode demonstrated an enhanced solar to electrical energy conversion efficiency of 4.74% compared with the photoanode of DSSC composed with unmodified TiO2 (2.19%) under full sunlight illumination (100 mW/cm2, AM 1.5G) as a result of the better charge collection efficiency of rGO, which reduced the back electron transfer process. Influence of the rGO content on the overall efficiency was also investigated, and the optimal rGO content for TiO2 was 0.5 mg. Further, the modification of rGO–TiO2 on the compact layer TiO2 surface led to an increase in efficiency to 5.83%. The superior charge collection and enhanced solar energy conversion efficiency of the rGO–TiO2 nanocomposite makes it to be used as a promising alternative to conventional photoanode-based DSSCs. Copyright © 2015 John Wiley & Sons, Ltd.

Dynamic NMR and Twisted Intramolecular Charge Transfer Excited States

In this paper the results of dynamic NMR studies on ethylmethylamino-tertiary-butyl-phenylborane (EMABPB) with or without light are reported. The NMR data were recorded on a Bruker 400 MHz NMR equipped with our custom-made optical probe and with our custom-made 450 watts (W) monochromatic light sources. The molecular photochemistry including twisted intramolecular charge-transfer-excited-state (TICT) of the EMABPB in several solvents has been investigated. These results indicate that the aminoborane demonstrates multiple configurations in CD3Cl and CD2Cl2 resulting in the shifts of the signals of the alkyl groups on the nitrogen and boron. This indicates that there are some time-dependent changes at constant temperature over the irradiation interval. At ﹣60°C and the presence of light (λ = 265 nm), we observed a large change in the populations of the two sites, and this by itself indicates a modification in the rotation around the boron nitrogen bond in the excited state. By considering the existence of the TICT state, many important energy technologies may be developed with higher efficiency by controlling the back-electron transfer processes.