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Herein we describe the use of regioregular (rr-) and regiorandom (rra-) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single molecule fluorescence spectra and excitation/emission polarization measurements were compared with a Förster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was observed for both rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the experimental observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small number of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these experimental and simulated results provide the basis for quantitatively describing energy transfer in an important class of conjugated polymers commonly used in a variety of organic electronics applications.

We report a comparative study in which a single-molecule fluorescence resonance energy transfer approach was used to examine how the binding of two families of HIV-1 viral proteins to viral RNA hairpins locally changes the RNA secondary structures. The single-molecule fluorescence resonance energy transfer results indicate that the zinc finger protein (nucleocapsid) locally melts the TAR RNA and RRE-IIB RNA hairpins, whereas arginine-rich motif proteins (Tat and Rev) may strengthen the hairpin structures through specific binding interactions. Competition experiments show that Tat and Rev can effectively inhibit the nucleocapsid-chaperoned annealing of complementary DNA oligonucleotides to the TAR and RRE-IIB RNA hairpins, respectively. The competition binding data presented here suggest that the specific nucleic acid binding interactions of Tat and Rev can effectively compete with the general nucleic acid binding/chaperone functions of the nucleocapsid protein, and thus may in principle help regulate critical events during the HIV life cycle.

Single-particle fluorescence spectroelectrochemistry was used to investigate the electrochemical oxidation of isolated, immobilized particles of the conjugated polymers BEH-PPV and MEH-PPV at an indium tin oxide (ITO) electrode immersed in an electrolyte solution. Two types of particles were investigated: (i) polymer single molecules (SM) and (ii) nanoparticle (NP) aggregates of multiple polymer single molecules. For the BEH-PPV polymer, the observation of nearly identical lowest oxidation potentials for different SM in the ensemble is evidence for effective electrostatic screening by the surrounding electrolyte solution. A combination of Monte Carlo simulations and application of Poisson-Boltzmann solvers were used to model the charging of polymer single molecules and nanoparticles in the electrochemical environment. The results indicate that the penetration of electrolyte anions into the polymer nanoparticles is necessary to produce the observed narrow fluorescence quenching vs oxidation potential curves. Finally, fluorescence-lifetime single-molecule spectroelectrochemical (SMS-EC) data revealed that at low potential an excited state reduction process (i.e., electron transfer from ITO to the polymer) is probably the dominant fluorescence quenching process.

The injection of positive charge carriers (holes) into a single conjugated polymer chain was observed to be light-assisted. This effect may underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply trapped charge layers in polymer light emitting diodes. The charging/discharging dynamics were investigated indirectly by a variety of single molecule electro-optical spectroscopic techniques, including an “image-capture” approach.

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

Extensive new single molecule spectroscopy (SMS) data on the conjugated polymer MEH-PPV at low temperature were obtained. In particular, the combined effects of sample preparation and excitation condition were explored in detail. The data confirm previous observations from this laboratory that (i) the distribution of emission maxima of single MEH-PPV molecules has a bimodal distribution and (ii) the single molecule emission spectrum of MEH-PPV exhibits few time-dependent fluctuations of the emission intensity, band shape, or spectral maxima. These data also help explain the discrepancy among the various published SMS data on this compound and suggest that environmental impurities, long irradiation times, nearby interfaces, and incomplete data sampling may account for some of the discrepancies among the published data.

Fluorescence emission spectra were recorded for isolated polymer chains of the polyfluorene copolymer, F8BT [poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)], at 298 and 20 K for two molecular weights (chain lengths). For long-chain F8BT at 298 K, the observed distribution of single-molecule emission maxima G(Emax) is bimodal, with peaks at approximately 2.35 eV ("blue") and approximately 2.25 eV ("red"). Previously, the red forms have been assigned to polymer chains that possess intrachain contacts, which lowers the local singlet exciton energy. At approximately 20 K, G(Emax) collapses into a single broad distribution centered at approximately 2.3 eV for long-chain F8BT. However, this distribution can be further divided into a high-energy edge that is dominated by the "blue" form, while the remainder of the distribution is composed of the "red" form. Low-molecular-weight F8BT samples emit exclusively from the blue form, and no shift in peak maxima with low temperature was observed. A Franck-Condon analysis reveals a decrease in emitting state displacements between spectra measured at 298 and 20 K, suggesting that temperature-induced structural displacements are responsible for the change in the bimodal emission.

Relatively narrow bandwidth fluorescence spectra were observed from isolated single molecules of the conjugated polymer, MEH-PPV (poly[2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene]) at low temperature. The spectroscopy reveals rich spectral information in energy relaxation pathways and structure of this important electroluminescent material. From room temperature to 20 K the fluorescence of MEH-PPV molecules was found to red-shift by ∼20 nm along with a decreased electron−phonon coupling. The fluorescence spectra give clear evidence for efficient electronic energy funneling to a small number (one in some cases) of low-energy sites, each with an increased effective conjugation length compared to room temperature.

The recently introduced technique of near field scanning optical microscopy (NSOM) with an electrically biased NSOM probe is applied in this paper to the study of exciton/polaron interactions in single crystals of the mixed organic semiconductor, tetracene doped with pentacene. As in the case of the previously investigated conjugated polymers, the injection of hole polarons induces extreme fluorescence quenching for tetracene/ pentacene. However, the spectral selectivity, dynamics, and other features of the tetracene/pentacene data suggest a different mechanism than conjugated polymer for hole polaron interactions. The tetracene/pentacene mechanism is dominated by hole trapping by pentacene.

This paper describes a preliminary experimental study on charge injection in MEH-PPV polymer nanoparticles incorporated in an OLED type device. The nanoparticles have been investigated with single molecule fluorescence spectroscopy recorded simultaneously with charging and discharging of the particles. Structure and dynamics of an anion/MEH-PPV complex, proposed to be responsible for discrete photobleaching in conjugated polymers, have been investigated with this technique. Device physics and kinetics have been elucidated by using the fluorescent conjugated polymer nanoparticles as probes for detecting the presence of hole polarons, and have been related to the electrically induced oxidation and reduction of the anion/MEH-PPV complex.

We recently introduced a new technique for semiconductor nanoparticle research that involves simultaneous single molecule spectroscopy (SMS) and controllable oxidation/reduction in an electronic device. This paper shows that the new technique, denoted by fluorescence-voltage single molecule spectroscopy (F-V/SMS), is analogous to current vs voltage (I-V) measurements for devices and electrochemical cells. F-V/SMS data are reported for single molecules (nanoparticles) of the conjugated polymer MEH-PPV as a function of bias voltage on the device and bias sweep rate to obtain information on both the energetics and kinetics of the charge transfer (oxidation/reduction) process in situ on the nanoscale. The extensive F-V/SMS data presented herein reveal that the dynamics for oxidation/reduction of MEH-PPV nanoparticles are controlled by various factors including filling of deep hole traps in the charge transporting layer of the device, the oxidation/ reduction chemical "state" of the MEH-PPV molecule, and the molecular scale heterogeneity of the device.

The kinetics of the attachment of isooctane solvated electrons to the water pools of reverse micelles (water/ AOT/isooctane) has been studied by ultrafast time-resolved spectroscopy. The electrons generated in isooctane by photoionization (three photons of 400 nm) are observed to rapidly attach to reverse micelles, resulting in hydrated electrons in the water pools of the reverse micelles. The measured electron attachment rates (inferred from attachment yields) show a greater dependence on reverse micelle size than that predicted by a diffusioncontrolled reaction model. Evidence is presented that highly delocalized, excess electron states of the water pool play an important role in the attachment process.

We describe molecule-by-molecule observations, using single molecule fluorescence spectroscopy, of the ordering of the components in a nematic phase formed by a binary mixture of short and long rods. A stiff conjugated polymer, MEH−PPV (poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene]), present at very low concentrations in a binary nematic−nematic phase with the mesogen 5CB (4-n-pentyl-4‘-cyanobiphenyl) is investigated. We observe near-perfect orientational order of the stiff, rodlike conjugated polymers translating through a planar nematic phase. This represents striking experimental evidence for the predicted enhanced alignment of long rods in the nematic phase of a binary mixture of rods.

Ultrafast spectroscopy was used to investigate the excited state dynamics and photochemistry of the hydrated electron in the water pool of a reverse micelle (RM) comprised of water and the surfactant, AOT, dissolved in an isooctane solution. Solvated electrons were generated in the isooctane solvent by photoionization (three photons of 400 nm). The isooctane-solvated electrons were observed to rapidly attach to the RM and relax in the water pool within 1 ps. A second 400 nm excitation pulse was employed to excite the equilibrated hydrated electron in the RM water pool, inducing an electron transfer reaction with the surfactant molecules. The ultrafast data as a function of RM size offer new insights on spatial confinement effects on the chemistry and relaxation dynamics of excess electrons in nanometer-sized water pools.

The electronic spectrum of CdS colloidal quantum dots (QDs) with a radius of 1.0−2.3 nm is studied by low-temperature photoluminescence excitation spectroscopy. CdS QDs are found to exhibit a resonant Stokes shift of ∼20−70 meV, which is ∼4 times larger than similarly sized CdSe QDs. This effect can be reproduced by an effective-mass theoretical calculation, which reveals that the hole ground state (or highest occupied molecular orbital, HOMO) in CdS QDs is a P state and the ground-state exciton is an optically passive state. Compared to CdSe, in CdS, the smaller spin−orbit splitting causes the orbital-symmetry-forbidden dark exciton in a QD and its larger resonant Stokes shift. The band-edge photoluminescence in CdS QDs exhibits a lifetime of ∼200 ns at 10 K, which is consistent with the dark exciton mechanism.

Femtosecond multicolor pump−probe spectroscopy on the prototypical intramolecular electron-transfer compound ([(NH3)5RuNCRu(CN)5]-; RuRu) in aqueous solution is reported for the first time with sufficient time resolution and spectral coverage to monitor the complete spectral dynamics. From the dynamic absorption spectrum, constructed from the pump−probe data obtained over a wide range of probe wavelengths (570−1300 nm), accurate measurement of the electron transfer (ET) time (1/kET) have been obtained by a global analysis of the data. The data have lead to new insights on the role of solvent and solute nuclear motions in the ET process. Additionally, pump−probe experiments with variable wavelength pump pulses have revealed the presence of a pump wavelength-dependent dynamic feature at early time, which has been assigned to stimulated emission from nonequilibrium excited states.

The migration of charge carriers in response to a pulsed electric field was investigated on the 100-nm length scale and microsecond time scale by a recently introduced method employing photoluminescence detection and near field optical excitation through a sharpened optical fiber. A metal coating on the near field probe serves as a ∼100 nm diameter electrode which acts to either collect or repel majority charge carriers, depending on the sign of the applied voltage. The local carrier density is determined by carrier-induced photoluminescence quenching. The transient response is described by a model including carrier drift and diffusion. The role of oxygen in photoinduced carrier generation, exciton quenching, and charge trapping is also investigated.

We investigate the time scale for hydrated electron solvation as a function of the initial configuration of the electron/water system. The experiments employ various 2-pulse and 3-pulse femtosecond pulse sequences that allow for controllable preparation of the various optically excited and precursor states of the equilibrated hydrated electron. We observe that the conduction band electron, which is produced by detrapping of the hydrated electron, has the slowest time scale for electron solvation with an average solvation time constant of 400 fs. In contrast, the solvation dynamics are significantly faster for electrons that are produced in “pre-solvated” environments. These latter examples include the excited p-state of the hydrated electron and the precursor states involved in UV femtosecond multiphoton ionization of water.

Femtosecond spectroscopy measurements are reported on the electron transfer, ET, reactions of the precursor states of the hydrated electron in the multiphoton ionization of water and the single-photon ionization of Fe(CN)64- in aqueous solutions. The ET reaction corresponds to an electron scavenging by various electron acceptors, such as Cd2+. Using the data reported herein, and previously published data on the scavenging kinetics of other electron precursors (e.g., from radiolysis) and optically excited states of the hydrated electron, it was shown that the rate constant of ET varies inversely with the volume of specific form of the hydrated electron. These data strongly support a unified model for the electron-transfer kinetics of many forms of delocalized electrons with localized electron acceptors in which ET rates are assumed to be proportional to the average electron density of the specific hydrated electron excited state or precursor.

Experimental information on the spatial extent of the putative p and conduction band states of the hydrated electron (eeq) has been obtained by monitoring and controlling the electron/hole spatial separation of a photogenerated eeq/hole pair using a femtosecond laser pulse sequence. Optical excitation of eeq with two photons is observed to dramatically alter its spatial distribution and geminate recombination yield with the hole. One-photon excitation, in contrast, has no effect on the spatial distribution. The results strongly confirm theoretical predictions on the size and location of excited states of eeq.

Quantum chemical analyses of poly(p-phenylenevinylene) (PPV)-based polymers with conformational and chemical defects are presented. It is shown that defects leading to tetrahedral kinks in the polymerization direction that were proposed previously to explain the optical anisotropy of polymer films (Hu et al. Nature 2000, 405, 1030) are indeed energetically stable. These defects are shown not to affect the primary electronic absorption band, but they may localize the electron−hole excitation in one segment of the polymer chain.

A detailed investigation of ultrafast photoinduced electron transfer (ET) in a mixed valence compound ((NH3)5RuNCRu(CN)5-; RuRu) in formamide, ethylene glycol, and glycerol was performed using variable wavelength femtosecond pump−probe spectroscopy over a broad range of pump and probe wavelengths. The ET kinetics were monitored by observing recovery of the ground state population. The spectra are highly dynamic, indicating the need for broad spectral coverage to accurately unravel the ET kinetics from excited state and ground state relaxation dynamics. ET is observed to proceed more slowly in slower solvents in a manner consistent with the Hybrid model of solution phase electron transfer. The ET time (1/e) of RuRu in ethylene glycol is 220 fs compared to 100 fs in water. As recently reported for RuRu in water, the ET time is pump wavelength independent. A previously unobserved pump wavelength dependence of the early time dynamics, however, is observed which is assigned to stimulated emission prior to vibr...

A photoinduced effect on the rate of reverse intersystem crossing (isc) in single sulforhodamine 101 (SR101) molecules has been observed using a novel two-color, multipulse, double resonance technique. The observed photoinduced intersystem crossing can be modeled by a four-electronic-state scheme (T1 → TN → S1 → S0) in which one of the excitation colors is in resonance with the S0 → S1 transition and the other is resonant with the T1 → TN transition. Synchronous averaging over many cycles of the two-color pulsed excitation experiment provides transient data that can be analyzed in a straightforward manner that is analogous to a conventional chemical kinetics experiment. Separate single color pulsed experiments were used to evaluate whether single color excitation (543 nm) alone could be responsible for photoinduced reverse isc (as previously reported for other molecules). However, results from the single color, pulsed excitation experiments show no evidence of this effect for two dyes, DiI and SR101.

The photochemistry of poly[2-methoxy, 5-(2′-ethyl-hexyloxy)- p -phenylene-vinylene] (MEH-PPV) has been found to be highly dependent on the presence of O 2 , which increases singlet exciton quenching dramatically. Spectroscopy on isolated single molecules of MEH-PPV in polycarbonate films that exclude O 2 reveals two distinct polymer conformations with fluorescence maxima near 555 and 580 nanometers wavelength, respectively. Time-resolved single-molecule data demonstrate that the 580-nanometer conformation exhibits a “landscape” for intramolecular electronic energy relaxation with a “funnel” that contains a 580-nanometer singlet exciton trap at the bottom. The exciton traps can be converted to exciton quenchers by reaction with O 2 . Conformationally induced, directed-energy transfer is arguably a critical dynamical process that is responsible for many of the distinctive photophysical properties of conjugated polymers.

The concentrated electric field (E-field) in the vicinity of a voltage-biased near-field optical probe is used to modulate the photoluminescence of organic thin films on the ∼100 nm scale. The samples are bilayers comprising a self-organized-thin-film (50−500 nm) layer of zinc-octakis (β-decoxyethyl) porphyrin (ZnODEP) on top of an indium tin oxide (ITO) coated glass electrode. The Al coated-optical-fiber-near-field probe functions simultaneously as a noncontacting moveable electrode and a local source of optical excitation (30−70 nm aperture). When the ITO electrode is charged positive relative to the probe (yielding a field on the order of 1MV/cm-1), the photoluminescence (fluorescence) intensity decreases ∼5% and the probe−sampleprobe−sample distance (under shear-force feedback control) increases ∼7 A. Opposite effects for both signals occur when the ITO is charged negative. The E-field effect on the near-field luminescence properties of thin films of ZnODEP is discussed in terms of the following physi...

In biological systems, organic molecules exert a remarkable level of control over the nucleation and mineral phase of inorganic materials such as calcium carbonate and silica, and over the assembly of crystallites and other nanoscale building blocks into complex structures required for biological function. This ability to direct the assembly of nanoscale components into controlled and sophisticated structures has motivated intense efforts to develop assembly methods that mimic or exploit the recognition capabilities and interactions found in biological systems. Of particular value would be methods that could be applied to materials with interesting electronic or optical properties, but natural evolution has not selected for interactions between biomolecules and such materials. However, peptides with limited selectivity for binding to metal surfaces and metal oxide surfaces have been successfully selected. Here we extend this approach and show that combinatorial phage-display libraries can be used to evolve peptides that bind to a range of semiconductor surfaces with high specificity, depending on the crystallographic orientation and composition of the structurally similar materials we have used. As electronic devices contain structurally related materials in close proximity, such peptides may find use for the controlled placement and assembly of a variety of practically important materials, thus broadening the scope for 'bottom-up' fabrication approaches.

The migration length of singlet electronic energy excitations (excitons) in thin films of perylene bis(phenethylimide) (PPEI) has been measured by an imaging technique that incorporates near-field optical excitation and far-field fluorescence imaging. The observed energy migration lengths fall in the range of e50 to ∼500 nm depending on sample morphologies. Sample morphology was controlled by “solvent annealing” and characterized by fluorescence NSOM and scanning force microscopy. The potential role of sample morphology in singlet energy migration is discussed. Additionally, the results reported herein are compared with a previous determination of singlet energy migration in PPEI.

Near-field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) have been employed to spatially resolve the complex nanoscale morphologies, spectroscopy, and energy-transfer efficiencies of self-assembled multilayered structures composed of alternating layers of α-zirconium phosphate [α-Zr(HPO4)2] (ZrP) and dye-labeled poly(allylamine hydrochloride) (dye-PAH) (where dye = Fluorescein (FL), Rhodamine B (RhB), or Texas Red (TR)). Two types of multilayer films have been investigated, namely, glass/anchor/ZrP/dye-PAH and glass/anchor/ZrP/dye-PAH/ZrP/dye-PAH, which were formed by the sequential layer-by-layer adsorption of the charged polyelectrolyte component layers. High- and low-coverage films were investigated. The glass/anchor/ZrP assemblies were shown to consist of a densely packed “tiled” motif of ZrP sheets which lie flat on the surface and cover more than 95% of the area, with average plate sizes of height = 13 (7) A, width ≈ 150 nm. The dye-labeled polymer layers in glass/anchor/ZrP/d...

The single molecule fluorescence spectroscopy of various isolated single-chromophoric dye molecules and multiple-chromophoric conjugated polymer molecules has been investigated. For each system the transient fluorescence “intensity”, ICW(t) (i.e., detected photons/dwell time), has been recorded with continuous wave (CW) irradiation. ICW(t) has been analyzed to yield an occurrence histogram for the different “intensities”, H(I), and an intensity time-autocorrelation function CI(t). The histograms H(I) for the various examples show highly diverse behavior with one, two, or even three peaks as well as “flat regions”. The different features in the histograms are shown to arise from distinct photophysical processes. From the study of model systems, characteristic features in the intensity histograms and autocorrelation functions are shown to result from photon shot noise, “blinking” due to triplet bottlenecks, spectral diffusion due to environmental fluctuations, and interchromophoric energy transfer. Classifi...

We recently reported the first pump−probe measurements on the hydrated electron with sufficient time resolution (∼35 fs) to directly observe the initial processes in the solvation dynamics of this key prototype for condensed-phase dynamics [Silva, C.; Walhout, P. K.; Yokoyama, K.; Barbara, P. F. Phys. Rev. Lett. 1998, 80, 1086]. An unprecedented relaxation process for the hydrated electron was observed that occurs on the 35−80 fs time scale and exhibits a solvent isotope effect (τ(D2O)/τ(H2O) ∼ 1.4). The new process was assigned to inertial/librational motion of the water surrounding the excess electron. The present paper reports a more extensive study of the ∼35 fs resolved dynamics of the hydrated electron in H2O and D2O at more probe wavelengths and as a function of pump-pulse intensity. The results are in agreement with the preliminary report and support the importance of librational water motion in the relaxation dynamics of the hydrated electron. New high excitation pulse intensity measurements reve...

The effect of solvent vapor annealing on the fluorescence properties and morphology of titanyl phthalocyanine/perylene phenethylimide thin-film molecular semiconductor bilayers (TiOPc/PPEI) is investigated. A combination of atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) is used in conjunction with bulk absorption and fluorescence measurements to correlate the morphological and photophysical properties of these bilayer systems. AFM data show that treatment of the vacuum-deposited amorphous PPEI and TiOPc/PPEI films results in the crystalline transformation of these materials and severely alters the contact between the TiOPc and PPEI layers. AFM data show extended solvent vapor annealing produces void spaces in the TiOPc coverage on the order of several hundred nanometers. Steady-state fluorescence intensity and fluorescence lifetime measurements are used as a measure of charge-transfer quenching efficiencies. Very efficient charge-transfer quenching is observed when amorpho...

Single - molecule fluorescence spectroscopy of a multichromophoric conjugated polymer (molecular weight ∼20,000) revealed surprising single-step photobleaching kinetics and acute jumps in fluorescence intensity. These jumps were shown not to result from spectral diffusion and were attributed to fluctuations in the quantum yield of emission for the molecules. The data indicate efficient intramolecular electronic energy transfer along the polymer chain to a localized fluorescence-quenching polymer defect. The defects are created by reversible photochemistry of the polymer. These findings have implications for the use of conjugated polymers in light-emitting diode displays and sensors.

The distance dependence of electron transfer (ET) rates between donors and acceptors in a DNA environment has been an area of intense research and heated discussion, especially with regard to the recent report of ultrafast ET over a separation of >40 A. This paper is focused on quantitative modeling of the photodynamics and gel electrophoresis/photocleavage experiments of the untethered DNA/metallointercalator system. Simulations are compared to previously published experimental results in order to quantitatively examine the possible involvement of long-range ET and donor/acceptor clustering in the DNA-mediated ET between metallointercalators. Comparison of the simulation results with a broad range of experimental data strongly indicates that a fairly typical distance dependence for ET (β ≈ 1 A-1) coupled to donor/acceptor clustering is able to account for all features in the data and for related photocleavage studies.

Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA. In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (3 x 10(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (approximately 10(10) per second). This rate is independent of the loading of acceptors on the helix, but is sensitive to sequence and pi stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.

We have studied the conformational dynamics associated with the nanoscale DNA bending induced by human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein using single-molecule Förster resonance energy transfer (SM-FRET). To gain molecular-level insights into how the HIV-1 NC locally distorts the structures of duplexed DNA segments, the dynamics, reversibility, and sequence specificity of the DNA bending behavior of NC have been systematically studied. We have performed SM-FRET measurements on a series of duplexed DNA segments with varying sequences, lengths, and local structures in the presence of the wide-type HIV-1 NC and NC mutants lacking either the basic N-terminal domain or the zinc fingers. On the basis of the SM-FRET results, we have proposed a possible mechanism for the NC-induced DNA bending in which both NC's zinc fingers and N-terminal domain are found to play crucial roles. The SM-FRET results reported here add new mechanistic insights into the biological behaviors and functions of HIV-1 NC as a retroviral DNA-architectural protein which may play critical roles in the compaction, nuclear import, and integration of the proviral DNA during the retroviral life cycle.

Conjugated polymers (CPs) are promising materials for use in electronic applications, such as low-cost, easily processed organic photovoltaic (OPV) devices. Improving OPV efficiencies is hindered by a lack of a fundamental understanding of the photophysics in CP-based thin films that is complicated by their heterogeneous nanoscale morphologies. Here, we report on a poly(3-hexylthiophene)-block-poly(tert-butyl acrylate)-block-poly(3-hexylthiophene) rod-coil-rod triblock copolymer. In good solvents, this polymer resembles solutions of P3HT; however, upon the addition of a poor solvent, the two P3HT chains within the triblock copolymer collapse, affording a material with electronic spectra identical to those of a thin film of P3HT. Using this new system as a model for thin films of P3HT, we can attribute the low fluorescence quantum yield of films to the presence of a charge-transfer state, providing fundamental insights into the condensed phase photophysics that will help to guide the development of the next generation of materials for OPVs.

Ultrafast pump - probe measurements have been made on the benzene - bromine atom charge transfer (CT) complex in CCl4 and cyclohexane solutions. Ultrafast optical excitation of the CT band of the complex yields an ion pair, which is comprised of a benzene cation and a bromide anion. The rate of charge recombination between the bromide and the benzene cation in the ion pair has been observed to be much faster than the rate of diffusion apart. The charge recombination rate is accelerated at high benzene concentrations as a result of the formation of a benzene dimer cation - bromide ion pair which undergoes much faster charge recombination than the benzene cation - bromide ion pair.

This paper describes the simultaneous measurement of excitation and emission anisotropy to visualize energy transfer in single chains of the prototypical conjugated polymer MEH-PPV, for samples with70% of the single chains organized into extended, rod-like conformations. The uniformity and high degree of order of the single molecules in these experiments has allowed direct comparison of our experimental data to energy-transfer simulations in model polymer chains. Increases in average anisotropy from 0.62 to 0.74 from excitation to emission and average changes of15° to the in-plane dipole principal orientation axis confirmed that energy was transferred to a relatively small number of sites in these highly ordered chains. This organization persisted even at large molecular weights (M(n) = 850 kDa). Electronic energy transfer in highly anisotropic model chains was simulated using an incoherent Förster-type mechanism to generate modulation depth histograms in good agreement with the observed data, as well as ensemble emission energies consistent with previously reported results. In these ordered model chains, excitons migrated an average of 6 nm before emission. This distance, far larger than the radius for single-step FRET, implies that energy transfer in MEH-PPV is a multistep funneling process.

We use first-principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals form self-trapped molecular polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values., 5 pages, 3 figures

The role of vibrational modes in ultrafast photoinduced intramolecular electron transfer reactions is explored. Femtosecond resolved experiments on two chemical classes will be described, namely, metalmetal intervalence electron transfer in mixed valence compounds and intramolecular charge recombinatiodseparation in organic donor/acceptor compound class, the betaines. The results have been analyzed to reveal the complex interactions of intramolecular and intermolecular modes in electron transfer reactions. I. INTRODUCTION The study of charge transfer processes, and in particular homogeneous electron transfer (ET) in solution, is at the forefront of the study of the molecular details of chemical reactions in liquids. The last decade has brought progress to many central problems in ET research, including the role of the solvent in these reactions, the involvement of vibrational degrees of freedom, the role of nuclear tunneling of the solvent and solute (and related quantum mechanical effects), the pathway and "mechanism" of electronic interactions, including long range ET, and other aspects of biological and heterogeneous ET. Recent reviews of the foundations of ET theories have been given by Newton and Sutinl and Marcus and Sutin.* Theories of ET reactions in solution are formulated in terms of a model in which the transferring electron is localized at a hmr molecular site in the reactant arid at a different acceptor molecular site in theproduct. Figure 1A portrays the usual curves for an ET reaction with no nuclear degrees of freedom other than the solvent coordinate. The coupling of the solvent coordinate to the ET is quantified by h, the solvent reorganization energy. AGO is the reaction free energy (driving force) for the ET. A central expression in ET theozv due to Marcus is as follows (h+AG")2 AG*= 4h where AG* is the activation energy for the ET reaction.

New insights on the molecular level details of the recently reported light-assisted injection of positive charge into single conjugated polymer chains are reported. Extensive new fluorescence-voltage single molecule spectroscopy (FV-SMS) measurements were performed on single chains of the archetypical conjugated polymer MEH-PPV embedded in a capacitor device to complement previous studies of the influence of the bias scan rate and optical excitation intensity. The use of a vacuum microscope allowed for the precise control of the device atmosphere, demonstrating the influence of triplet states in the MEH-PPV on the FV-SMS modulation. For identical device conditions, little variation was observed in the rate and yield of charging from molecule to molecule. Through the use of thicker supporting matrices and insulating polymer "blocking layers", it was determined that good electrical contact between the hole transport layers and the single molecules was necessary for charge injection. The results demonstrate the complexity of charge transfer processes at the interface of organic semiconductors and highlight the ability of single molecule methods to advance the understanding of such processes at the nanoscale.

The double proton transfer reaction of the isolated formic acid dimer has been investigated within the reaction surface Hamiltonian framework, using a newly calculated three‐dimensional ab initio potential energy surface. The symmetric (synchronous) proton movement, the asymmetric (asynchronous) proton movement and the relative motion of two formic acid molecules have been explicitly included in the calculation. The calculation gives a tunneling splitting of 0.004 cm-1, which is considerably smaller than a previous theoretical prediction (0.3 cm-1). An effective tunneling path has been calculated from the lowest vibrational eigenfunction of the reaction surface Hamiltonian, and the path deviates significantly from the minimum energy path on the potential energy surface. The new results are consistent with the conventional understanding of heavy-light-heavy mass combination reactions. The effective reaction path from this calculation reveals evidence of asymmetric proton movement. However, a synchronous double proton transfer is the major mode of reaction. Tunneling splittings for a few excited vibrational levels have also been calculated within the reaction surface Hamiltonian framework. Vibrational excitation of a large amplitude, heavy atom mode dramatically increases the tunneling splitting.

Reverse transcription of the HIV-1 genome involves several nucleic acid rearrangement steps that are catalyzed (chaperoned) by the nucleocapsid protein (NC), including the annealing of the transactivation response region (TAR) RNA of the genome to the complementary sequence (TAR DNA) in minus-strand strong-stop DNA. It has been extremely challenging to obtain unambiguous mechanistic details on the annealing process at the molecular level because of the kinetic involvement of a complex and heterogeneous set of nucleic acid/protein complexes of variable structure and variable composition. Here, we investigate the in vitro annealing mechanism using a multistep single-molecule spectroscopy kinetic method. In this approach, an immobilized hairpin is exposed to a multistep programmed concentration sequence of NC, model complementary targeted-oligonucleotides, and buffer-only solutions. The sequence controllably “drags” single immobilized TAR hairpins among the kinetic stable states of the reaction mechanism; i.e., reactants, intermediates, and products. This single-molecule spectroscopy method directly probes kinetic reversibility and the chaperone (catalytic) role of NC at various stages along the reaction sequence, giving access to previously inaccessible kinetic processes and rate constants. By employing target oligonucleotides for specific TAR regions, we kinetically trap and investigate structural models for putative nucleation complexes for the annealing process. The new results lead to a more complete and detailed understanding of the ability of NC to promote nucleic acid/nucleic acid rearrangement processes. This includes information on the ability of NC to chaperone “reverse annealing” in single-strand transfer and the first observation of partially annealed, conformational substates in the annealing mechanism.

HIV-1 reverse transcription requires several nucleic acid rearrangement steps that are “chaperoned” by the nucleocapsid protein (NC), including minus-strand transfer, in which the DNA transactivation response element (TAR) is annealed to the complementary TAR RNA region of the viral genome. These various rearrangement processes occur in NC bound complexes of specific RNA and DNA structures. A major barrier to the investigation of these processes in vitro has been the diversity and heterogeneity of the observed nucleic acid/protein assemblies, ranging from small complexes of only one or two nucleic acid molecules all the way up to large-scale aggregates comprised of thousands of NC and nucleic acid molecules. Herein, we use a flow chamber approach involving rapid NC/nucleic acid mixing to substantially control aggregation for the NC chaperoned irreversible annealing kinetics of a model TAR DNA hairpin sequence to the complementary TAR RNA hairpin, i.e., to form an extended duplex. By combining the flow chamber approach with a broad array of fluorescence single-molecule spectroscopy (SMS) tools (FRET, molecule counting, and correlation spectroscopy), we have unraveled the complex, heterogeneous kinetics that occur during the course of annealing. The SMS results demonstrate that the TAR hairpin reactant is predominantly a single hairpin coated by multiple NCs with a dynamic secondary structure, involving equilibrium between a “Y” shaped conformation and a closed one. The data further indicate that the nucleation of annealing occurs in an encounter complex that is formed by two hairpins with one or both of the hairpins in the “Y” conformation.

The translational diffusion constant, D, of a polymer solute in a single-domain, nematic liquid crystal solvent (5CB) is measured in directions parallel and perpendicular to the nematic director using a fluorescence two-beam, cross-correlation technique. The solute under investigation is the stiff, conjugated polymer, MEH-PPV. The ratio D parallel/D perpendicular) of diffusion constants (parallel and perpendicular to the director) is observed to be 1.9 +/- 0.3. This is surprisingly small considering that MEH-PPV is known to be both elongated and highly aligned along the liquid crystal director of 5CB. We therefore argue that the structural order parameter of the solvent governs the anisotropy of the diffusion of the solute.

Single molecule fluorescence correlation spectroscopy has been used to investigate the photodynamics of isolated single multichromophoric polymer chains of the conjugated polymers MEH-PPV and F8BT on the microsecond to millisecond time scale. The experimental results (and associated kinetic modeling) demonstrate that (i) triplet exciton pairs undergo efficient triplet-triplet annihilation on the30 micros time scale, (ii) triplet-triplet annihilation is the dominant mechanism for triplet decay at incident excitation powersor =50 W/cm(2), and (iii) singlet excitons are quenched by triplet excitons with an efficiency on the order of (1)/(2). The high efficiency of this latter process ensures that single molecule fluorescence spectroscopy can be effectively used to indirectly monitor triplet exciton population dynamics in conjugated polymers. Finally, correlation spectroscopy of MEH-PPV molecules in a multilayer device environment reveals that triplet excitons are efficiently quenched by hole polarons.

This paper describes a new single molecule spectroscopy approach for the investigation of triplet−triplet and singlet−triplet interactions in conjugated polymers. The technique involves the irradiation of isolated single, mulitchromophoric, conjugated polymer molecules by a repetitive sequence of variable-intensity microsecond time scale excitation pulses. The fluorescence intensity is synchronously time-averaged for thousands of cycles of the pulse sequence to yield a high signal-to-noise fluorescence transient on the microsecond time scale. The transient can be analyzed with kinetic models to obtain quantitative information about the kinetics of triplet−triplet exciton annihilation and the quenching of singlet excitons by triplet excitons in conjugated polymers.

In HIV-1 reverse transcription, the nucleocapsid protein, NC, induces secondary structure fluctuations in specific DNA and RNA hairpins. Time-resolved single-molecule fluorescence resonance energy transfer was used to study NC chaperoned opening of DNA hairpins over a broader range of conditions and in more depth than in previous studies. The experiments reveal a complex mechanism for secondary structure fluctuations with dynamic processes occurring over a wide time range, i.e., approximately 5 to250 ms and with the involvement of long-lived intermediates. The dynamic role of DNA loop regions and NC binding/dissociation events are discussed.

Research under support of this grant has been focused on the understanding of highly delocalized ''conduction-band-like'' excited states of solvated electrons in bulk water, in water trapped in the core of reverse micelles, and in alkane solvents. We have strived in this work to probe conduction-band-like states by a variety of ultrafast spectroscopy techniques. (Most of which were developed under DOE support in a previous funding cycle.) We have recorded the optical spectrum of the hydrated electron for the first time. This was accomplished by applying a photo-detrapping technique that we had developed in a previous funding cycle, but had not yet been applied to characterize the actual spectrum. In the cases of reverse micelles, we have been investigating the potential role of conduction bands in the electron attachment process and the photoinduced detrapping, and have published two papers on this topic. Finally, we have been exploring solvated electrons in isooctane from various perspectives. All of these results strongly support the conclusion that optically accessible, highly delocalized electronic states exist in these various media.