The importance of nanoscopic ordering on the kinetics of photoinduced charge transfer in aggregated pi-conjugated hydrogen-bonded donor-acceptor systems

Aggregated complexes of diaminotriazine oligo(p-phenylene vinylene) (OPV) units hydrogen bonded to different complementary perylene bisimide (PERY) compounds have been investigated by means of absorption, circular dichroism, photoluminescence, and photoinduced absorption spectroscopy. These studies reveal that in the aggregated state an ultrafast photoinduced charge separation occurs via an intermolecular pathway in the J-type stack of hydrogen-bonded OPV-PERY arrays. The subsequent charge recombination reaction strongly depends on small structural differences within the J-type geometry as revealed by comparison of stacked supramolecular dimers, trimers, and covalently OPV-PERY linked systems. A coupled oscillator model is used to analyze absorption and circular dichroism spectra and to identify intermolecular arrangements that are consistent with the experimental spectra and the charge-transfer kinetics.     

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.     

Supramolecular p-n-heterojunctions by co-self-organization of oligo(p-phenylene vinylene) and perylene bisimide dyes

Comparative studies on hydrogen-bonded versus covalently linked donor-acceptor-donor dye arrays obtained from oligo(p-phenylene vinylene)s (OPVs) as donor and bay-substituted perylene bisimides (PERYs) as acceptor dyes are presented. Both systems form well-ordered J-type aggregates in methylcyclohexane, but only hydrogen-bonded arrays afford hierarchically assembled chiral OPV-PERY dye superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study). In the case of hydrogen-bonded arrays, the stability of the aggregates in solution increases with increasing conjugation length of the OPV unit. The well-defined co-aggregated dyes presented here exhibit photoinduced electron transfer on subpicosecond time scale, and thus, these supramolecular entities might serve as valuable nanoscopic functional units.     

Alternating oligo(p-phenylene vinylene)--perylene bisimide copolymers: synthesis,photophysics, and photovoltaic properties of a new class of donor--acceptor materials

A Suzuki polycondensation reaction has been used to synthesize two copolymers consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene bisimide (PERY) acceptor chromophores. The copolymers differ by the length of the saturated spacer that connects the OPV and PERY units. Photoinduced singlet energy transfer and photoinduced charge separation in these polychromophores have been studied in solution and in the solid state via photoluminescence and femtosecond pump-probe spectroscopy. In both polymers a photoinduced electron transfer occurs within a few picoseconds after excitation of the OPV or the PERY chromophore. The electron transfer from OPV excited state competes with a singlet energy transfer state to the PERY chromophore. The differences in rate constants for the electron- and energy-transfer processes are discussed on the basis of correlated quantum-chemical calculations and in terms of conformational preferences and folding of the two polymers. In solution, the lifetime of the charge-separated state is longer than in the films where geminate recombination is much faster. However, in the films some charges are able to escape from geminate recombination and diffuse away and can be collected at the electrodes when the polymers are incorporated in a photovoltaic device.     

Influence of supramolecular organization on energy transfer properties in chiral oligo(p-phenylene vinylene) porphyrin assemblies

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies. Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However, the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the stack direction reveals a relative lack of organization in these assemblies.     

Control over charge separation in phthalocyanine-anthraquinone conjugates as a function of the aggregation status

We have prepared three isomeric donor-acceptor systems, in which two phthalocyanine (Pc) units have been attached to the 1-,5- (1a), 1-,8- (1b), or 2-,6- (1c) positions of a central anthraquinone (AQ) moiety, leading to packed (1b) or extended (1a and 1c) topologies. The electronic interactions between the donor and the acceptor in the ground state or in the excited states have been studied by different electrochemical and photophysical techniques. Due to the markedly different topologies, we have been able to modify these interactions at the intramolecular level and, by a proper choice of the solvent environment, at the intermolecular level within aggregates. In triad 1b, the ZnPc units are forced to pi-stack cofacially and out of the plane of the AQ ring. Consequently, this molecule shows strong inter-Pc interactions that give rise to intramolecular excitonic coupling but a relatively small electronic communication with the AQ acceptor through the vinyl spacers. On the contrary, the 1-,5- or 2-,6-connections of triads 1a and 1c allow for an efficient pi-conjugation between the active units that extends over the entire planar system. These two molecules tend to aggregate in aromatic solvents by pi-pi stacking, giving rise to J-type oligomers. Photoexcitation of the Pc units of 1a-c results in the formation of the Pc.+-AQ.- charge transfer state. We have demonstrated that the kinetics of these electron transfer reactions is greatly dependent on the aggregation status of the triads.     

Solvent dependence of the charge-transfer properties of a quaterthiophene–anthraquinone dyad

An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.     

Synthesis and optical properties of triphenylene-based dendritic donor perylene diimide acceptor systems

A donor-acceptor charge transfer system based on two discotic mesogens has been synthesized. The donor is either a triphenylene (POG0) or a triphenylene-based conjugated dendron (POG1), while the acceptor is a perylene diimide (PDI) core. The donors are covalently linked to the bay positions of the PDI core through an ether linkage. In chloroform, due to the short donor-acceptor distance and the matching frontier orbital levels, photoinduced charge transfer from either the donor excitation or the acceptor excitation are both thermodynamically and kinetically favored, resulting in efficient quenching of both donor and acceptor fluorescence. In a less polar solvent, hexane, while charge transfer is still the dominant mechanism for decay of the excited electronic state of POG1, photoinduced charge transfer is no longer energetically favorable for POG0 when the acceptor PDI core is excited, making the PDI core of POG0 weakly fluorescent in chloroform but strongly so in hexane. In solid film, POG0 is highly aggregated through both PDI-PDI and triphenylene-triphenylene homotopic stacking. POG1, on the other hand, aggregates through triphenylene dendrons with limited PDI-PDI core stacking, presumably due to the steric hindrance caused by bulky triphenylene moieties which block the access to the PDI core. The efficient photoinduced charge transfer, coupled with the homotopic stacking that forms separated electron-transporting PDI-stacked columns and hole transporting triphenylene-stacked columns, suggests that the reported donor-acceptor systems based on dual-discotic mesogens are potentially new efficient photovoltaic materials.     

Spectral properties of rhodamine 3B adsorbed on the surface of montmorillonites with variable layer charge

Montmorillonite was thermally treated at several temperatures to reduce the charge density of its layer surface. Absorption and fluorescence (steady-state and time-resolved) spectroscopies are now applied to study the adsorption of rhodamine 3B (R3B) laser dye in reduced charge montmorillonites (RCMs) in aqueous suspensions. The decrease in the charge density increases the intermolecular distance between adsorbed R3B molecules, reducing the tendency of the dye to self-associate. H-type and J-type aggregates of R3B in RCMs are spectroscopically characterized, the fluorescent J-aggregates being more extensively formed by decreasing the charge density. Both the reduction in the dye aggregation and the formation of J-type aggregates enhance the fluorescence efficiency of R3B dye adsorbed in montmorillonite particles. Absorption with linearly polarized light reveals that the H-aggregates are more disposed toward the perpendicular of the clay surface than the monomer and J-aggregates species.     

Synthesis and photophysical properties of conjugated polymers with pendant 9,10-anthraquinone units

We have synthesized and investigated the photophysical properties of a series of electron-donor conjugated copolymers with pendant electron-acceptor units. The copolymers consist of diethynyl-1,4-phenylene, fluorene, or phenylene rings alternating with a phenylene unit bearing a pendant 9,10-anthraquinone moiety. The pendant donor-acceptor polymers were designed to have different optical pi-pi* band gaps, while the oxidation potential of the polymer backbone remains approximately constant in the series. The reduction potential of the donor-acceptor polymers is associated with the pendant acceptor units. This leads to the special situation that the electrochemical gap between oxidation and reduction potentials is constant, while the optical band gap decreases, going from PPP, via PPF, to PPE. This design is used to study the effect of the optical gap on the photoinduced electron-transfer reaction that occurs between the main chain electron donor and the pendant acceptor, while the same polymer architecture and energy of the charge separated state are maintained. Fluorescence and photoinduced absorption spectroscopy are used to study the electron transfer following photoexcitation in relation to solvent polarity and in thin solid films. For the fluorene-phenylene alternating copolymer, intramolecular photoinduced electron transfer occurs in the Marcus optimal region.     

Synthesis,optical properties,and aggregation behavior of a triad system based on perylene and oligo(p-phenylene vinylene) units

A donor-acceptor-donor triad molecule with a perylene bisimide derivative as electron acceptor, and an oligo(p-phenylene vinylene) (OPV) derivative as electron donor was synthesized (OPV-PERY-OPV). The structure of the triad was characterized by (1)H and (13)C NMR spectroscopy, size-exclusion chromatography (SEC), and MALDI-TOF spectrometry. Absorbance spectra and CD spectroscopic measurements of the triad molecule indicated the formation of aggregates in solvents such as toluene, chloroform, and tetrachloroethane, whereas it was present in the molecularly dissolved state in THF. The (1)H NMR spectra of the molecule in chloroform had, unexpectedly, four doublet peaks for the perylene protons, instead of the two doublets that is generally seen in N,N'-substituted perylene molecules. To understand the aggregation behavior and the splitting of the signals in the (1)H NMR spectra, a simple model compound was synthesized, in which the OPV units were replaced by phenyl groups (Ph-PERY-Ph). (1)H NMR spectra in CDCl(3) and tetrachloroethane again had four doublet peaks for the perylene protons, whereas in THF the perylene protons gave only a single peak. NOE and COSY spectroscopy were used to assign the peaks to their corresponding perylene protons. UV/Vis and CD spectroscopic measurements indicated that, similar to the OPV-PERY-OPV triad molecule, the model compound Ph-PERY-Ph was also present in the aggregated form in solvents such as toluene, chloroform, and tetrachloroethane, and in the molecularly dissolved state in THF. IR measurements of the model molecule in the first set of solvents indicated carbamate bond (bond;OCObond;NHbond;)-induced intermolecular hydrogen bonding, whereas in THF, the molecule was mostly present in the free form. CPK models with a dimeric structure, in which two perylene molecules are held together by intermolecular hydrogen bonding with the perylene core shifted slightly with respect to one another, could account for the optical properties and the observation of the four different peaks in the (1)H NMR spectra in polar solvent. Temperature-dependent (1)H NMR spectroscopic, UV/Vis, and CD measurements indicated that the transition from the aggregated to the molecularly dissolved state took place at higher temperatures. The electrochemical studies indicated that OPV-PERY-OPV was both p- and n-dopable, whereas Ph-PERY-Ph was only n-dopable. Cyclic voltammetry measurements of Ph-PERY-Ph in THF had two reduction peaks corresponding to the reduction of the perylene core to the monoanion and dianion, respectively. In dichloromethane, however, an additional reduction peak at lower potential was observed. This new reduction peak might arise from the hydrogen-bonded species.     

INTRAMOLECULAR DONOR-ACCEPTOR SYSTEMS: STRUCTURES OF NOVEL LINKED PORPHYRIN- PHENOLPHTHALEIN MOLECULAR SYSTEMS

Abstract— Intramolecular donor-acceptor systems bearing porphyrins covalently linked to phenol-phthalein at various distances and at different orientations have been synthesized. Optical absorption and emission spectra and magnetic resonance studies reveal interaction between the porphyrin unit and lactone/semiquinone forms of phenolphthalein. The charge-transfer quenching of fluoresence in these systems is found to be more pronounced compared to the intermolecular systems. The lifetime studies are interpreted in terms of intramolecular electron transfer.     

Crossover from superexchange to hopping as the mechanism for photoinduced charge transfer in DNA hairpin conjugates

The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.     

Coherent electronic and nuclear dynamics for charge transfer in 1-ethyl-4-(carbomethoxy)pyridinium iodide

Although polaronic interactions and states abound in charge transfer processes and reactions, quantitative and separable determination of electronic and nuclear relaxation is still challenging. The present paper employs the amplitudes, polarizations, and phases of four-wave mixing signals to obtain unique dynamical information on relaxation processes following photoinduced charge transfer between iodide and 1-ethyl-4-(carbomethoxy)pyridinium ions. Pump-probe signal amplitudes reveal the coherent coupling of an underdamped 115 cm(-1) nuclear mode to the charge transfer excitation. Assignments of this recurrence to intramolecular vibrational modes of the acceptor and to modulation of the intermolecular donor-acceptor distance are discussed on the basis of a high-level density functional theory normal-mode analysis and previously observed wave packet dynamics of solvated molecular iodine. Nuclear relaxation of the acceptor induces sub-picosecond decay of the pump-probe polarization anisotropy from an initial value of 0.4 to an asymptotic value of -0.05. Electronic structure calculations suggest that relaxation along the torsional coordinate of the ethyl group is the origin of the anisotropy decay. Electric-field-resolved transient grating (EFR-TG) signal fields are obtained by spectral interferometry with a diffractive optic based interferometer. These measurements show that the signal phase and amplitude possess similar dynamics. Model calculations are used to demonstrate how the EFR-TG signal phase yields unique information on transient material resonances located outside the laser pulse spectrum. This effect can be rationalized in that the real and imaginary parts of the nonlinear polarization are related by the Kramers-Kronig transformation, which allows the dispersive component of the polarization response to exhibit spectral sensitivity over a larger frequency range than that defined by the absorption bandwidth.     

Synthesis and Characterization of Donor-Acceptor-Donor Triads Containing Tetrathiafulvalene and Naphthalene Diimide Units： Towards Regulation of the Intermolecular Charge-Transfer Interaction by Varying the Attached Side Groups

Introduction Extensive studies have been performed on electron donor-acceptor supramolecular systems, which have been used as models to investigate charge-transfer interactions,1 photoinduced electron and energy transfer reactions (for understanding the natural photosynthesis mechanisms).2 In recent years, molecular devices such as molecular shuttles and molecular switches based on electron donor-acceptor supramolecules have been proposed and studied.3 Since the synthesis of tetrathiafulvalene…     

Morphology transition and aggregation-induced emission of an intramolecular charge-transfer compound

An intramolecular charge-transfer (ICT) compound, (TCBD)2OPV3, has been synthesized and fabricated into one-dimensional nanotubes by a reprecipitation method. The observation of SEM and TEM showed the nanotubes were formed from their zero-dimensional precursors of hollow nanospheres. Reconstruction was found to happen during the morphology transition progress. The morphology transition and reconstruction are proposed to be a "curvature strain releasing" process driven by donor-acceptor dipole-dipole interactions. An aggregation-induced emission (AIE) effect was observed for (TCBD)2OPV3. Both aggregates of vesicles and nanotubes were observed to be good red emitters with near-infrared end emission of 750-850 nm, which endows the material with potential applications in the fields of optical devices, biosensors, and biolabels.     

Efficient charge separation in C60-based dyads: triazolino

Triazoline[4,5][60]fullerenes are strong electron acceptors that form with tetrathiafulvalene (TTF), a novel type of donor-acceptor dyad exhibiting efficient improved electron-transfer dynamics. In particular, a rapid photoinduced intramolecular electron transfer, forming a charge-separated state, is followed by a slow charge recombination to generate the fullerene triplet excited state in moderate quantum yields.     

Tetrathiafulvalene derivatives as NLO-phores: synthesis, electrochemistry, Raman spectroscopy, theoretical calculations, and NLO properties of novel TTF-derived donor-pi-acceptor dyads.

Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy.     

Impact of bridging units on the dynamics of photoinduced charge generation and charge recombination in donor-acceptor dyads.

We estimate, at a full quantum-chemical level, the various molecular parameters governing the rate of photoinduced charge generation and charge recombination in model organic structures containing a donor and an acceptor unit in view of the possible use of such systems in organic solar cells. The rate of through-space excitation dissociation, as predicted in the framework of the Marcus-Levich-Jortner theory, is found to be low in comparison to intramolecular decay processes when the donor and acceptor molecules are lying in a head-to-tail arrangement and high when the donor and acceptor molecules are superimposed in a cofacial arrangement. The charge separation rates for side-by-side donor-acceptor dyads are significantly increased by promoting through-bond interactions in covalently linked donor and acceptor units. This has motivated a detailed quantitative analysis of the influence of the nature, size, and conformation of the bridging moiety on the calculated transfer rates.     

Fullerene Derivatives Functionalized with Diethylamino‐Substituted Conjugated Oligomers: Synthesis and Photoinduced Electron Transfer

Diethylamino‐substituted oligophenylenevinylene (OPV) building blocks have been prepared and used for the synthesis of two [60]fullerene–OPV dyads, F‐D1 and F‐D2 , which exhibit different conjugation length of the OPV fragments. The electrochemical properties of these acceptor–donor dyads have been studied by cyclic voltammetry. The first reduction is always assigned to the fullerene moiety and the first oxidation centered on the diethylaniline groups of the OPV rods, thus making these systems suitable candidates for photoinduced electron transfer. Both the OPV and the fullerene‐centered fluorescence bands are quenched in toluene and benzonitrile, which suggests the occurrence of photoinduced electron transfer from the amino‐substituted OPVs to the carbon sphere in the dyads in both solvents. By means of bimolecular quenching experiments, transient absorption spectral fingerprints of the radical cationic species are detected in the visible (670 nm) and near‐IR (1300–1500 nm) regions, along with the much weaker fullerene anion band at λ_max=1030 nm. Definitive evidence for photoinduced electron transfer in F‐D1 and F‐D2 comes from transient absorption measurements. A charge‐separated state is formed within 100 ps and decays in less than 5 ns.