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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.

In contrast to the detailed understanding of inorganic materials, researchers lack a comprehensive view of how the properties of bulk organic materials arise from their individual components. For conjugated polymers to eventually serve as low cost semiconductor layers in electronic devices, researchers need to better understand their functionality. For organics, traditional materials science measurements tend to destroy the species of interest, especially at low concentrations. However, fluorescence continues to be a remarkably flexible, relatively noninvasive tool for probing the properties of individual molecules and allows researchers to carry out a broad range of experiments based on a relatively simple concept. In addition, the sensitivity of single-molecule spectroscopy allows researchers to see the properties of an individual component that would be masked in the bulk phase. In this Account, we examine several photophysical properties of different conjugated polymers using single-molecule spectroscopy. In these experiments, we probed the relationship between the conformation of single conjugated polymer chains and the distance scale and efficiency of energy transfer within the polymer. Recent studies used polarization anisotropy measurements on single polymer chains to study chain folding following spin-casting from solution. This Account summarizes the effects of monomer regioregularity and backbone rigidity, by comparing a regiorandom phenylene vinylene (MEH-PPV) with both a regiorandom and regioregular thiophene (P3HT). Synthesis of novel polymers allowed us to explore the role of different conformation-directing inclusions in a PPV backbone. We showed that these inclusions control the conformation of individual chains and that molecular dynamics can predict these structural effects. In situ solvent vapor annealing studies explored the dynamics of polymer chains as well as the effect of solvent evaporation on the structural equilibrium of the polymer. We observed that a slower rate of solvent evaporation results in a narrow population of highly ordered polymer chains. These highly ordered single chains serve as a model system to probe the effect of conformation on energy transfer following excitation in single MEH-PPV polymer chains in two distinct experiments. In the first, we correlated the anisotropy of the fluorescence emission of individual chains with the anisotropy of their fluorescence excitation. Using this data, we derived a model for energy transfer in a conjugated polymer, simulating chromophores along a chain, coupled via Förster energy transfer. In the second experiment, super-resolution measurements demonstrated the ability of single-molecule spectroscopy to directly visualize energy transfer along a polymer chain embedded in a model device environment. A capacitive device allowed for controlled localization of hole polarons onto the polymer chain. These positive charges subsequently quenched local excitations, providing insight into the range of energy transfer in these single polymer molecules. As researchers continue to characterize conjugated polymer films and develop methods for creating multichain systems, single-molecule techniques will provide a greater understanding of how polymer morphology influences interchain interactions and will lead to a richer description of the electronic properties of bulk conjugated polymer films.

A new phenomenon is presented in which electrogenerated chemiluminescence (ECL) is generated and propagates laterally as self-reinforcing waves as a result of the oxidation of a poly(9,9-dioctylfluorene-co-benzothiadiazole) thin film. In an ordered array of Au electrode posts that act as effective ECL nucleation sites, soliton-like waves were observed to expand from each site and annihilate upon collision with each other. Simulations of the ECL response supported the experimental observations that the ECL waves propagate at a constant speed. A correlated diffusion mechanism involving the correlated motion of ions, injected holes, and solvent molecules is proposed to interpret the experimental data qualitatively. A rapid increase in the diffusion coefficient of these species in the polymer results in a sharp interface between non-oxidized and oxidized polymer phases wherein the electrochemical (EC) oxidation and mass transport of all pertinent species take place. EC oxidation of conjugated polymers of this type has important implications for the understanding of these materials and their modes of operation in EC conjugated polymer devices.

The Horner method was used to synthesize random copolymers of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) that incorporated different backbone-directing monomers. Single-molecule polarization absorption studies of these new polymers demonstrate that defects that preserve the linear backbone of PPV-type polymers assume the highly anisotropic configurations found in defect-free MEH-PPV. Rigid defects that are bent lower the anisotropy of the single chain, and saturated defects that provide rotational freedom for the chain backbone allow for a wide variety of possible configurations. Molecular dynamics simulations of model defect PPV oligomers in solution demonstrate that defect-free and linearly defected oligomers remain extended while the bent and saturated defects tend toward more folded, compact structures.

The regioregularity of a conjugated polymer can greatly affect bulk film morphologies and properties. However, it remains unclear how regioregularity affects the conformation of isolated individual chains where interchain interactions are absent. Here, the effect of the regioregularity on the conformations adopted by regioregular (rr-) and regiorandom (rra-) poly(3-hexylthiophene) (P3HT) chains was studied using single molecule fluorescence excitation polarization spectroscopy. While every rr-P3HT chain within an ensemble was found to fold into a highly ordered conformation, single rra-P3HT chains adopted a wide variety of conformations, ranging from highly ordered to isotropic. This distribution is likely due to variations in the positions of nonhead-to-tail linkages of the side-chains along the backbone of the different polymer chains. Molecular dynamics simulation on atomistic models of rr- and rra-P3HT chains supports the effect of regioregularity on the collapsed conformations. These results demonstr...

We study the electrochemical oxidation waves of conjugated polymers by examining photoluminescence (PL) quenching and electrogenerated chemiluminescence (ECL) of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Different factors such as the anion concentration, the anion size, the electrochemical potential, and the coreactant concentration are investigated for their effects on the behavior of the oxidation waves. Our results show that the wave speeds of PL quenching or ECL can be increased by increasing the anion concentration, decreasing the anion size, or increasing the electrochemical potentials. In addition, the intensity of ECL can be controlled by the coreactant concentration. The transition from pinned to free ECL waves is also studied when an intermediate coreactant concentration is used. These results provide useful insights for understanding the electrochemical oxidation wave phenomenon.

We recently reported the discovery of soliton-like electrogenerated chemiluminescence (ECL) waves from pure conjugated polymer films and gold-nanoparticles-doped polymer films. In addition to a more detailed study of these polymer systems by changing the film thickness and the distribution of local leaks, we also apply the ECL wave phenomenon to polymer blends of conjugated and nonconjugated polymers. Poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) is used as the active material that is oxidized and produces ECL with the presence of a co-reactant, tri-n-propylamine (TPA). Several factors such as film thickness, artificial leaks, and solubility of doped polymers are examined for their effects on the ECL behavior. When polystyrene (PS), less soluble in the electrolyte, is blended with F8BT, dotted ECL signals are observed and transported as waves. When poly(methyl methacrylate) (PMMA), more soluble in the electrolyte, is blended with F8BT, PMMA serves as local scratches and ECL waves are triggered simultaneously from the whole film.

By using single-molecule spectroscopy (SMS) several effects of solvent vapor induced annealing (SVA) were studied directly on single conjugated polymers, e.g.: SVA-induced translocations, folding/unfolding dynamics, and changes in the morphological order. It is shown that single chains can be trapped by spin-coating in a disordered conformation and subsequent SVA leads to an equilibrated, highly ordered conformation.

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.

We have reexamined the fluorescence polarization anisotropy of single polymer chains of the prototypical conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) isolated in a poly(methyl methacrylate) (PMMA) matrix employing improved synthetic samples that contain a much smaller number of tetrahedral chemical defects per chain. The new measurements reveal a much larger fraction of highly anisotropic MEH-PPV chains, with >70% of the chains exhibiting polarization anisotropy values falling in the range of 0.6-0.9. High anisotropy is strong evidence for a rod-shaped conformation. A comparison of the experimental results with coarse grain, beads on a chain simulations reveals that simulations with the usual bead-bead pairwise additive potentials cannot reproduce the observed large fraction of high polarization values. Apparently, this type of potential lacks some yet to be identified molecular feature that is necessary to accurately simulate the experimental results.

We aim to understand the origins of intrinsic charge carrier traps in organic and polymeric semiconductor materials from a physical chemistry perspective. In crystalline organic semiconductors, we point out some of the inadequacies in the description of intrinsic charge traps using language and concepts developed for inorganic semiconductors. In π-conjugated polymeric semiconductors, we suggest the presence of a two-tier electronic energy landscape, a bimodal majority landscape due to two dominant structural motifs and a minority electronic energy landscape from intrinsic charged defects. The bimodal majority electronic energy landscape results from a combination of amorphous domains and microcrystalline or liquid-crystalline domains. The minority tier of the electronic density of states is comprised of deep Coulomb traps embedded in the majority electronic energy landscape. This minority electronic energy landscape may dominate transport properties at low charge carrier densities, such as those expected ...

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.

The hole injection from a carbazole derivative hole transport layer into nanoparticles ( r = 25 ± 15 nm) of the conjugated polymer MEH-PPV was investigated by an indirect single-particle fluorescence-quenching technique. The results suggest that there is a kinetic barrier for hole injection that prevents polymer particles from being charged in the dark. This barrier can be overcome with the assistance of optical excitation of the MEH-PPV nanoparticles, achieving a thermodynamic population of injected holes at positive bias. The amount of injected holes at equilibrium is observed to depend upon the bias in a manner highly consistent with device simulations based on a continuum model. Overall, the results demonstrate that the hole injection into nano domains of conjugated polymers is a complex process depending upon molecular interfacial effects determined by device geometry and electrostatic interactions.

Chalcopyrite copper indium sulfide (CuInS2) and copper indium gallium selenide (Cu(InxGa(1-x))-Se2; CIGS) nanocrystals ranging from approximately 5 to approximately 25 nm in diameter were synthesized by arrested precipitation in solution. The In/Ga ratio in the CIGS nanocrystals could be controlled by varying the In/Ga reactant ratio in the reaction, and the optical properties of the CulnS2 and CIGS nanocrystals correspond to those of the respective bulk materials. Using methods developed to produce uniform, crack-free micrometer-thick films, CulnSe2 nanocrystals were tested in prototype photovoltaic devices. As a proof-of-concept, the nanocrystal-based devices exhibited a reproducible photovoltaic response.

Detailed single molecule spectroscopic results are presented for the recently reported oxidation of single-chain conjugated polymers of MEH-PPV at the HTL/insulator interface of an MIS device. The ...

An investigation of the effect of an applied electric field on the photoluminescence (PL) intensity of single CdSe nanocrystals has revealed a measurable field induced PL modulation for a large fraction of the nanocrystals studied. The field induced intensity modulation characteristics (i.e. modulation sign and depth) were observed to vary from particle to particle, and even for different time periods for the same particle in many cases. Simultaneous intensity and frequency resolved PL measurement show that the PL intensity modulation is in fact due to an electric field effect on the PL quantum yield. The results are consistent with a model in which the energies of surface charge trapping sites are modulated by the applied electric field, causing in turn a modulation of the rates of exciton quenching by these sites. The complex observed field effects can be explained by the superposition of the applied and internal electric fields due to deeply trapped charges on the surface of the nanoparticle.

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

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.

The low energy onset of the optically accessible density of electronic states (OADOS) of the conjugated polymer MEH-PPV has been investigated by low temperature fluorescence single molecule spectroscopy (SMS). The first extensive set of high signal-to-noise SMS data have been acquired for MEH-PPV samples as a function of molecular weight confirming the previously reported bimodal energy gap distribution . These data have been acquired under sample and radiation conditions that inhibit photophysical dynamics that have distorted previous SMS studies. A molecule by molecule vibronic analysis has yielded an experimentally determined spectral density of the S 0 –S 1 energy gaps for the individual chromophores for MEH-PPV. The SMS data has also led to new insights on the energy transfer pathways for conjugated polymers, and on the molecular nature of the “blue” and “red” sites of conjugated polymers. Finally, a combined analysis of the new SMS data with the previously obtained bulk absorption and emission data for MEH-PPV has offered clear evidence that the low energy onset of the OADOS for MEH-PPV, in analogy to the emission spectral density, confirms the existence of a bimodal energy gap distribution .

Herein, we continue our investigation of the single-molecule spectroscopy of the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene-vinylene] (MEH-PPV) at cryogenic temperatures. First, the low temperature microsecond dynamics of single MEH-PPV conjugated polymer molecules are compared to the dynamics at room temperature revealing no detectible temperature dependence. The lack of temperature dependence is consistent with the previous assignment of the dynamics to a mechanism that involves intersystem crossing and triplet-triplet annihilation. Second, the fluorescence spectra of single MEH-PPV molecules at low temperature are studied as a function of excitation wavelength (i.e. 488, 543, and 568 nm). These results exhibit nearly identical fluorescence spectra for different excitation wavelengths. This strongly suggests that electronic energy transfer occurs efficiently to a small number of low-energy sites in the multichromophoric MEH-PPV chains.

The molecular structure, photochemistry, and device physics of conjugated polymers have been investigated by single-molecule spectroscopy (SMS), using the unique ability of this technique to unravel complex spectra and dynamics. Surprisingly efficient and directional electronic energy funneling was observed for conjugated polymer molecules due to highly ordered conformations. Furthermore, recent studies on the SMS of conjugated polymers embedded in electronic devices demonstrate that SMS is a powerful tool for studying the photophysics and charge-transfer processes of conjugated polymers, giving new insights into the complex interactions among excited and charged species that exist in a device environment.

Electron–cation geminate recombination in isooctane has been reinvestigated by femtosecond spectroscopy. The observed recombination kinetics are well-fit by a single exponential decay ( τ = 400 ± 40 fs) and exhibit a significant hydrogen/deuterium kinetic isotope effect. The kinetics are not affected by varying the incident intensity or by exciting the recombining electrons with a high power 800 nm pulse. These observations strongly suggest that the recombination rate is not limited by diffusive motion of the ions to form a contact ion pair, but rather by the electron transfer reaction rate between the ions in a contact ion pair.

Fluorescence spectroscopy was performed on single molecules of two 9,9-dialkylfluorene-benzothiadazole (FxBT) copolymers with a 10-fold difference in average molecular weight. Molecules of both polymers exhibit red-shifted emission indicative of energy migration to low-energy sites (LES) on the polymer chains; however, "red" spectra are much more common for the longer polymer. Since singlet-exciton migration is found to occur on the molecular length scale in both cases, the increased number of red-shifted spectra observed for the longer polymer is evidence that the likelihood of LES formation increases with chain length. This relationship is discussed in terms of three possible causes of low-energy sites: local polymer conformations, chromophores with extended conjugation lengths, and random chemical defects.

Recently published near field scanning optical measurements (NSOM) on the conjugated polymer MEH-PPV exhibited a strong dependence of the photoluminescence intensity on the applied electric fields at the NSOM tip. The observed effect is apparently due to exciton quenching by hole polarons. In the present paper, a model "single carrier" electro-modulated-photoluminescence device is used to further explore the exciton quenching effect of hole polarons in MEH-PPV. Hole polarons, created by charge injection from an ITO electrode, are observed to dramatically quench the photoluminescence intensity of MEH-PPV. The Stern-Volmer quenching efficiency of a hole polaron in conjugated polymer thin films was measured to be 390 nm 3 . This value, and other data presented herein, are consistent with the published NSOM photoluminescence modulation measurements and offer further evidence that hole polarons are efficient photoluminescence quenchers in MEH-PPV.

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.

The factors affecting the operating life of the light-emitting electrochemical cells (LECs) based on films of tris(2,2'-bipyridine)ruthenium(II) both in sandwich (using an ITO anode and a Ga:Sn cathode) and planar (using interdigitated electrode arrays (IDAs)) configurations were investigated. Stability of these devices is greatly improved when they are produced and operated under drybox conditions. The proposed mechanism of the LEC degradation involves formation of a quencher in a small fraction of tris(2,2'-bipyridine)ruthenium(II) film adjacent to the cathode, where light generation occurs, as follows from the observed electroluminescence profile in the LECs constructed on IDAs, showing that the charge injection in such devices is highly asymmetric, favoring hole injection. Bis(2,2'-bipyridine)diaquoruthenium(II) is presumed to be the quencher responsible for the device degradation. A microscopic study of photo- and electroluminescence profiles of planar light-emitting electrochemical cells was shown as a useful approach for studies of charge carrier injection into organic films.

Organic nanoparticles consisting of single conjugated polymer chains were investigated as a function of degree of conjugation by means of single-molecule spectroscopy. The degree of conjugation was synthetically controlled. For highly conjugated chains, singlet excitons are efficiently funneled over nanometer distances to a small number of sites. In contrast, chains with less conjugation and a high number of saturated bonds do not exhibit energy funneling due to a highly disordered conformation.

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.

A method for studying the degradation of organic light emitting devices (OLEDs) in real time is described. Transparent OLEDs allow for the spatial correlation of cathode topographic images with optical images (transmission, photoluminescence, and electroluminescence) of the devices throughout the degradation process. In this study we focused on the evolution of nonemissive, “dark” spots during device operation. We conclude that the electroluminescent dark spots originate as nonconductive regions at the cathode/organic interface and expand or grow as a result of exposure to atmosphere. We propose a mechanism of dark spot growth involving aerobic oxidation of the cathode/organic interfacial region, leading to a highly resistive, carrier blocking interface at the dark spot locations. No initial defects on the cathode surface, which might be responsible for the formation of dark spots, were detected by atomic force microscopy. Structural changes, such as degradation of organic materials and the cathode surfac...

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.

Direct 1-photon excitation of the hydrated electron from the ground state to the conduction band is achieved with 400 nm excitation. This provides a unique method for investigating the femtosecond dynamics and spectroscopy of the conduction band hydrated electron without interference from other states. The assignment of the 400 nm excited state to the conduction band is supported by the observation of >35 A spatial migration of the hydrated electron, as manifested in complete supression of geminate recombination. Similar dynamics and electron scavenging yields are observed for the 400 nm state and that produced by 2-photon 800 nm excitation.

Detailed investigations of the spectral diffusion of sulforhodamine 101 (SR101) in polymer films have been performed over a range of concentrations from typical concentrations for single molecule (SM) spectroscopic studies (10−10 M) to typical concentrations for bulk (10−6 M) experiments. A “single-molecule” ensemble spectrum, obtained by averaging 42 different SM spectra of SR101 in a poly(methyl methacrylate) film, exhibits extreme broadening and a large vibronic peak that is not observed in the more concentrated samples. The distribution of SM spectral means is extremely broad (FWHM=616 cm−1) with molecule-to-molecule variations that last at least several hours. Autocorrelation analysis of 170 SM spectral mean trajectories on the second times scale reveals that the most probable behavior for SR101 molecules is to exhibit negligibly small amounts of spectral diffusion. The observed effects, i.e., the large vibronic peak, the broad spectral distribution, and rare rapid jumps in the spectral mean, are ass...

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...