Photoinduced Charge Separation Processes Research Articles

Variation of charge transfer kinetics in structurally closely related dyads with rhenium photosensitizers

The kinetics for photoinduced charge separation and thermal charge-recombination processes in three donor–bridge–acceptor molecules with rhenium(I) tricarbonyl diimine photosensitizers were investigated. Time resolved luminescence and transient absorption spectroscopies reveal that in addition to driving force effects, differences in bridge-mediated electronic donor–acceptor coupling among the three dyads play important roles. Notably, it is seen that charge separation depends strongly on whether initial photoexcitation involves promotion of an electron towards or away from a phenothiazine electron donor. Thermal charge recombination rates are found to differ by a factor of 2 between two isomeric dyads due to an electronic coupling effect.

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

Self-Assembled Supramolecular Ferrocene−Fullerene Dyads and Triad: Formation and Photoinduced Electron Transfer

Supramolecular ferrocene−fullerene constructs in which the donor, ferrocene linked to a benzo-18-crown-6 entity (Fc-crown), was self-assembled with the acceptor, fullerene bearing one or two alkyl ammonium ions (NH3+−C60), yielding dyads or a triad, respectively. The newly formed conjugates were characterized by spectroscopic (fluorescence, electospray ionization-mass, and 1H NMR) and electrochemical methods. The adopted crown ether−alkyl ammonium ion binding strategy resulting in stable donor−acceptor conjugates was also supported by the computational studies performed at the DFT B3LYP/3-21G(*) level in addition to the binding constants obtained from fluorescence quenching studies. The experimentally calculated free-energy changes indicated exothermic light-induced charge-separation process. Accordingly, efficient photoinduced charge-separation processes were confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. The rates of charge recom...

Contrasting Photodynamics between C60–Dithiapyrene and C60–Pyrene Dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.

Synthesis, dynamic light scattering, and luminescence properties of copolymers containing iridium(III) bisterpyridine in the side chain

AbstractSupramolecular block‐random copolymers containing [Ir(terpy)2]3+ in the side chain were synthesized via postfunctionalization of a P(S‐b‐ACterpy) block copolymer. Absorbance and emission spectra compared to a model compound show that the polymer backbone has a minor effect on the polymer absorbance but produces a larger shift for the phosphorescence signals to higher wavelength. Dynamic light scattering of the metal complex containing copolymer studied in various solvents showed monomodal aggregation with decreasing aggregate size as the solvent dielectric constant increased. The copolymer precursor P(S‐b‐ACterpy) shows multimodal aggregation in different solvents with the major population consisting of single chains. This difference in behavior between the two polymers is attributed to the electrolytic nature of the complex and the amphiphilicity induced by the charged metal complex. Supramolecular copolymers like these will continue to have interesting self‐organizational properties and may find applications in multicomponent systems for photoinduced charge separation processes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1109–1121, 2007

Through-space communication in a TTF–C60–TTF triad

A 2-pyrazolino[60]fullerene-based triad, possessing two TTF units connected with flexible long linkages, has been prepared. Fluorescence quenching was observed in polar and non-polar solvents, suggesting the existence of a through-space photoinduced charge separation process with rates faster than 3.8 × 109 s−1 and with high efficiency (>0.85). The charge separated states were confirmed by transient absorption spectra in the visible and near-IR regions. The longest lifetime of the charge separated state was found to be 230 ns in CH2Cl2 solvent.

Photoinduced Charge Carrier Generation in a Poly(3-hexylthiophene) and Methanofullerene Bulk Heterojunction Investigated by Time-Resolved Terahertz Spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.

Ultrafast Photoinduced Intramolecular Charge Separation and Recombination Processes in the Oligothiophene-Substituted Benzene Dyads with an Amide Spacer

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.

Molecular Approaches to Solar Energy Conversion with Coordination Compounds Anchored to Semiconductor Surfaces

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.

Photoexcited carriers in organic light emitting materials and blended films observed by surface photovoltage spectroscopy

The electronic structure of the widely-used light emitting materials, 2,5-bis(5-tert-butyl-2-benzoxazolyl) thiophene (BBOT), poly($N$-vinylcarbazole) (PVK) thin films have been characterized using surface photovoltage spectroscopy. The photo-induced charge separation and transfer processes in both blend films of PVK:BBOT and PVK:TPD:BBOT, where TPD is $N,{N}^{\ensuremath{'}}$-diphenyl-$N,{N}^{\ensuremath{'}}$-bis(3-methylphenyl)-[$1,{1}^{\ensuremath{'}}$-biphenyl]-$4,{4}^{\ensuremath{'}}$-diamine have also been investigated. The results of the photo-induced contact potential difference (CPD) change show that BBOT film is an electron-transporting material while PVK film is a hole-transporting one. The photoluminescence and electroluminescence results of the blend films suggest an exciplex interaction between BBOT and PVK or TPD. A positive CPD change due to photo-excitation of the BBOT in PVK:BBOT blend film is attributed to electron trapping at the localized state induced by dispersed BBOT species. In the PVK:TPD:BBOT blend films, a positive CPD change, which starts at the same transition energy as in the former blend film but is significantly enhanced, is observed and explained in terms of charge transfer between the involved energy structures of the blend components. The dependence of the observed effects on the blend composition and ensuing electronic structure is discussed.

Supramolecular control over donor-acceptor photoinduced charge separation.

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.

Primary charge separation in photoinduced multielectron storage systems. A dinuclear ruthenium(ii) species featuring a charge-separated state with a lifetime of 1.3 µs

The primary photoinduced charge separation and recombination processes occurring in a dinuclear Ru(II) complex capable to perform multielectron storage upon light excitation have been studied in different solvents: the charge-separated state obtained in dichloromethane is, to the best of our knowledge, the longest-lived ever reported for a ruthenium polypyridine complex.

Direct observation of charge-transfer dynamics in a conjugated conducting polymer poly(3-octylthiophene)-fullerene composite by time-resolved infrared spectroscopy

Transient infrared-active vibrational (IRAV) modes observed in the \ensuremath{\pi}-conjugated conducting polymers upon photoexcitation are usually used to characterize the dynamics of the photoinduced charge separation and recombination processes. In this paper, the dynamic behavior of photogenerated charge carrier in the poly(3-octylthiophene) (P3OT) conducting polymer doped with fullerene $({\mathrm{C}}_{60})$ has been studied by using step-scan time-resolved Fourier-transform infrared spectroscopy at room temperature in the $2500--900\ensuremath{-}{\mathrm{cm}}^{\ensuremath{-}1}$ (0.31--0.11-eV) frequency region. Upon photoexcitation with a 10-ns laser pulses, it is observed that IRAV modes appear in the similar vibrational frequency regions for pure P3OT and ${\mathrm{C}}_{60}$-doped P3OT; however, the observed IRAV modes is enhanced by 800% in intensity in the presence of small amount of ${\mathrm{C}}_{60}$ (e.g., 10%). Our results also show that the charge separation occurs instantaneously within the laser pulse width (10 ns), whereas the charge recombination processes occur in the microseconds to a few milliseconds time domain. In addition, the observed enhancement in the IRAV modes upon ${\mathrm{C}}_{60}$ doping indicates an enhanced photogeneration efficiency of the charge separation.

Photoinduced Charge Separation and Recombination Processes in Fine Particles of Oligothiophene-C60 Dyad Molecules

Photoinduced charge separation and recombination processes in fine particles of octathiophene-C60 and dodecathiophene-C60 dyad molecules were investigated by the subpicosecond laser photolysis method. Sizes of the fine particle samples were estimated to be 60−140 nm using SEM observation and dynamic light scattering measurements. Charge separated states were generated within 1 ps upon the 150 fs laser excitation in both fine particle samples of the dyad molecules. Generation yields of the charge separated states were about a half of those of the corresponding dyad molecules in polar solvents, indicating the existence of the competitive deactivation pathways of the singlet excited states, such as exciton migration in each fine particle. On the other hand, the charge separated states deactivated via the two-step-decay processes. Direct charge recombination in a dyad molecule and indirect charge recombination after migration of radical cation (hole) and anion (electron) in fine particles were observed as fast and slow decay components, respectively. The faster charge recombination rates than those of previously reported for aniline-fullerene dyad cluster will result from high hole-mobility in the oligothiophene-moieties of the present dyad molecules composing fine particles.

Photoinduced Microsecond-Charge-Separation in Retinyl-C60 Dyad

Intramolecular photoinduced charge separation and recombination processes in a retinyl-C60 dyad molecule (Ret-C60) have been investigated in various solvents by time-resolved absorption and fluorescence techniques. Upon laser excitation of the C60-moiety in nonpolar toluene, the intersystem crossing proceeded from the excited singlet state of the C60-moiety (Ret-1C60*) to the excited triplet state (Ret-3C60*), followed by energy transfer yielding the excited triplet state of the retinyl-moiety (3Ret*-C60) without charge separation. On the other hand, in polar solvents such as N,N-dimethylformamide and benzonitrile, the charge separation occurred from Ret-1C60* at rate on the order of 1010 s-1. The quantum yield was close to unity in these polar solvents. Most parts of the ion pair (Ret•+-C60•-) changed to Ret-3C60* and 3Ret*-C60 by the charge recombination which took place at rate on the order of 109 s-1. However, some parts of the charge-separated state were kept in microsecond time-region: The lifetimes of Ret•+-C60•- were 16 μs and 19 μs in DMF and benzonitrile, respectively, which were as long as those of Ret-3C60* and 3Ret*-C60, suggesting an equilibrium between the charge-separated state and the excited triplet states.

Modulating charge separation and charge recombination dynamics in porphyrin-fullerene linked dyads and triads: Marcus-normal versus inverted region.

Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin-fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor-acceptor arrays with different donor-acceptor separations and diversified donor strength: freebase porphyrin-C60 dyad (H2P-C60), zincporphyrin-C60 dyad (ZnP-C60), ferrocene-zincporphyrin-C60 triad (Fc-ZnP-C60), ferrocene-freebase porphyrin-C60 triad (Fc-H2P-C60), and zincporphyrin-freebase porphyrin-C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60.-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 micros. Determination of CS and CR rate constants, together with the one-electron redox potentials of the donor and acceptor moieties in different solvents, has allowed us to examine the driving force dependence (-DeltaG0ET) of the electron-transfer rate constants (kET). Hereby, the semilogarithmic plots (i.e., log kET versus -DeltaG0ET) lead to the evaluation of the reorganization energy (lambda) and the electronic coupling matrix element (V) in light of the Marcus theory of electron-transfer reactions: lambda = 0.66 eV and V = 3.9 cm(-1) for ZnP-C60 dyad and lambda = 1.09 eV and V = 0.019 cm(-1) for Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 triads. Interestingly, the Marcus plot in Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60 has provided clear evidence for intramolecular CR located in both the normal and inverted regions of the Marcus parabola. The coefficient for the distance dependence of V (damping factor: betaCR = 0.58 A(-1) is deduced which depends primarily on the nature of the bridging molecule.

Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C60 dyad molecules

Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C60 and dodecathiophene-C60 dyad molecules (8T-C60 and 12T-C60, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, 18T*-C60 and 112T*-C60 showed energy transfer to 1C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and 1C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds. In more polar solvents (dielectric constant (∈s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.

Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins.

The dynamics of photoinduced charge separation and charge recombination processes in synthetic DNA hairpins have been investigated by means of femtosecond transient absorption spectroscopy. The driving force and distance dependence of charge-transfer processes involving singlet acceptors and nucleobase donors are consistent with a single-step superexchange mechanism in which the electronic coupling between the donor and acceptor is strongly distance dependent. The dynamics of reversible hole transport between a primary guanine donor and nearby GG or GGG sequences has also been determined and establishes that these sequences are very shallow hole traps.

Solvent Polarity Dependence of Photoinduced Charge Separation in a Tetrathiophene-C60 Dyad Studied by Pico- and Nanosecond Laser Flash Photolysis in the Near-IR Region

Photoinduced charge separation and recombination processes in a tetrathiophene−C60 dyad molecule (4T-C60) in various solvents were investigated by observing transient absorption spectra in the near-IR region. In polar solvents such as tetrahydrofuran and benzonitrile, charge separation occurred at a rate on the order of 1010 s-1, which decreased down to the order of 109 s-1 in moderately polar solvents. In nonpolar toluene, charge separation was not observed. The quantum yields for the charge separations were nearly unity in polar solvents. Charge recombination occurred in 100 ps − 10 ns; the shorter lifetimes occurred in the highly polar solvents. After recombination, the triplet excited state of C60 was generated predominantly. In polar solvents such as benzonitrile, the charge-separated state was also observed in the microsecond time region. To explain the unprecedented long lifetime of the second charge-separation step, an equilibrium between the charge-separated state and the triplet excited state wa...

C60-Containing Multichromophoric Systems for Studying Photoinduced Charge Separation Processes

This article presents an overview of our investigations into the efficacy of the C60 chromophore as an acceptor component in photoinduced charge separation devices. These investigations combine the novel design and synthesis of rigid multichromophoric Ball-&-Chain systems with computational and photoinduced charge separation rate measurements.