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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.

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.

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.

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 .

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.

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.

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.

Single-molecule spectroscopy of the prototypical dye, DiI, has been investigated in rigorously de-oxygenated polymer films. The extraordinarily high signal-to-noise data demonstrate that the intrinsic T 1 → S0 intersystem crossing kinetics of DiI are simple-first-order with little temporal or molecule-to-molecule fluctuations. The results verify that O2 collisions are the major factor responsible for fluctuations of the intersystem crossing rate in non-degassed and partially degassed polymer films. In addition to the intrinsic DiI intersystem kinetics, rare fluctuations are observed in the intersystem crossing rate, which are ascribed to residual impurities and/or to low quantum yield photochemical processes of DiI.

Optical excitation of pentacene, doped into a benzoic acid crystal matrix, induces a reversible proton transfer reaction between the host and the guest. This reaction occurs at low temperatures and leads to the creation of defects corresponding to the displacement, from its regular position, of an acid proton of the host matrix. The formation and evolution of these defects is monitored via the electronic S 0 → S 1 transition of the pentacene guest. Here we report and discuss a variety of measurements made with the aim of obtaining information about the first step of the reaction. It is shown that the rate for the first step of this reaction is reduced by a factor of about 10 4 upon deuteration of the host matrix, demonstrating that this step occurs by tunnelling. Other unsuccessful experiments (optical, ESR, magnetic field effects) made to identify the first intermediate of the reaction as well as molecular orbital calculations of potential intermediates are also reported briefly. It is shown that the formation of the pentacene cation, protonated in the centre position, is consistent with all observations. This species is proposed as the most likely first intermediate of the reaction.

We report femtosecond direct pump/probe data on the s-state to p-state transition of the solvated electron in alcohols. The data reveal a p-state lifetime of ≈ 0.5 ps and a large spectral component due to displacement along the ground-state solvent coordinate involving slow diffusive solvent motions and subsequent relaxation. The large displacement is surprising considering the short excited-state lifetime and the long relaxation times of these modes. This effect is apparently the result of a previously unknown p-state to s-state nonadiabatic rearrangement of the solvent coordinate of solvated electrons in alcohols.

The electron transfer and ground-state solvation dynamics of betaine-30 (B-30) in n-butanol at low temperatures are investigated by femtosecond pump-probe spectroscopy. Two separate components of the solvent response to solute excitation are observed. At higher temperatures (292-273 K), spectral evolution corresponding to diffusional solvation is evident. However, low-temperature studies (273-193 K) show the presence of a second process characterized by a ≈ 100 ps decay time and weak temperature dependence demonstrating that specific intermolecular hydrogen-bond dynamics are involved in the solvation of B-30 in protic solvents.

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.

New tunable pump and probe data are reported for the S1 → S0 reverse electron transfer in betaine in solution. The observed dynamics are a dramatic function of probe wavelength due to energy deposition into solute and solvent molecular degrees of freedom from the electron transfer. The data indicate that in nonpolar solvents, this energy is primarily deposited into intramolecular modes. In polar solvents, most of the energy is deposited into intramolecular modes, but some is specifically deposited into polar collective motions of the solvent, i.e. the solvent coordinate. Modern theories of electron transfer that include diffusive motion along the solvent coordinate as a driving mechanism for electron transfer predict this specific energy deposition into the solvent coordinate. In this paper, we extend a model by Walker et al. to predict the spectral dynamics due to solvent coordinate heating caused by the electron transfer and compare with experimental results. We show that the diffusive electron transfer mechanism creates a displaced distribution along the solvent coordinate that is characterized by an absorption spectrum that is red-shifted from the equilibrium ground-state distribution, in agreement with experimental observations. The model is not in quantitative agreement with experiment, however, because it neglects the contributions of vibrational heating of the intramolecular modes to the spectral dynamics. This type of vibrational heating is evident in the observed dynamics in nonpolar solvents.

We have investigated the photodissociation and vibrational relaxation of I−2 in water, ethanol, and ethanol-d1. Internal conversion, geminate recombination, and vibrational relaxation in the top 70% of the ground state potential in water are complete within the time resolution of our apparatus (

Extensive new ultrafast pump-probe measurements are reported for betaine-30 including excitation of ground state molecules at 800 nm (S 0 →S 1 ) and at 400 nm (S 0 →S 3 ). This paper focuses on the variation of betaine transient pump—probe dynamics with probe wavelength. The observed behavior supports the interpretation that the S 1 →S 0 internal conversion is induced by specific energy transfer to the solvent coordinate in polar solvents. In addition, transient spectra resulting from excitation at both 400 and 800 nm are nearly indistinguishable at times longer than ≈ 200 fs. We interpret this as evidence that the S 3 →S 1 internal conversion is much more rapid and less solvent sensitive than the S 1 →S 0 internal conversion (both are formally intramolecular electron transfer). Comparison of the two charge transfer processes offers insight on electron transfer kinetics in solution.

Novel measurements on the microscopic solvation dynamics of coumarin probes in several simple polar solvents and solvent mixtures have been made using the time dependent fluorescence Stokes shift technique. The microscopic solvent relaxation function, C(t), is observed to be poorly modeled by a single exponential decay in many cases. The average experimental solvation times, , for pure solvents and binary solvent mixtures are close to values predicted by dielectric continuum theory, but in many cases, the observed C(t) shape does not agree with that predicted by dielectric continuum theory. The results suggest that molecular motion of solvent molecules near the solute can be responsible for microscopic solvation components of C(t) that are not predicted using bulk dielectric data of the neat solvent and the dielectric continuum theory. In addition to the solvation dynamics results, some potential sources of probe molecule non-ideality are examined, and it is shown that these effects are not significant contributors to experimental error for these C(t) measurements.

The underlying chemical dynamics of the excited state electron transfer of electronically excited 9,9′-bianthryl in polar solvents is explored via a semi-empirical comprehensive theoretical model for the reaction coordinate energy profile and the dynamics along the reaction coordinate. The predictions of the model are in excellent agreement with new femtosecond fluorescence data on bianthryl, which are presented in this paper. The model is comprised of several key elements, including: (i) an Onsager cavity/ semi-empirical treatment for the solvent coordinate; (ii) an electronically adiabatic description of the mixing between the reactant and product zero-order states; (iii) a generalized Langevin equation treatment of the reaction coordinate dynamics where the friction kernel is determined using independent experimental results on solvation dynamics of coumarin probes; and (iv) an empirical solvatochromic/vibronic description for predicting fluorescence and absorption spectra. With a limited amount of parameterization the overall model is able to account in detail for many observables for bianthryl, including the static absorption spectra, the solvent dependence of the static fluorescence spectra, and the time resolved fluorescence spectra. The model supports our previous proposal that the electron transfer kinetics of bianthryl is controlled by polar solvation dynamics.

The static and transient fluorescence spectroscopy of 3-hydroxyflavone, 3HF, has been investigated as a function of deuteration of the 3-hydroxy group and the solvent. Small H/D isotope effects are observed for at least two of the simple rate constants for the excited-state dynamics. The isotope effects are not significantly temperature dependent. The results suggest that the proton transfer process is not a simple barrier crossing reaction.

This paper describes a laser spectrometer that yields picosecond time-gated emission spectra which are characterized by a high signal-to-noise ratio and a broad spectral coverage. The crucial experimental methods are presented, with particular emphasis given to the experimental techniques that are necessary to achieve broad spectral coverage. Examples are given of the application of the apparatus in the investigation of the torsional isomerization of 2-vinylanthracene and the transient solvation of electronically excited coumarin 440 in alcoholic solutions.

Three representative polyacene quinones doped in thin polymer films have been studied by degenerate four-wave mixing to elucidate the dominant factors in the third-order nonlinear optical effects of these compounds. The experiments have been made near or on resonance at 532 nm. The nonlinear effects of these materials are found to be of electronic origin with rapid response times. The third-order nonlinear optical coefficients are not simply correlated with the delocalization length of the compounds as expected from simple theory.

The seeded free jet LIF spectroscopy of the S 1 ← S 0 transition of 9-(3′-methylphenyl)anthracene, 9-(4′-methylphenyl)anthracene, 9-(3′-fluorophenyl)anthracene, and 9-(4′-fluorophenyl)anthracene has been studied. The lowest-frequency S 1 mode for all of these molecules is assigned to torsional motions about the bond that joins the phenyl and anthracene rings. Effective torsional potentials have been determined by fitting the torsional vibrational spacings to a one-dimensional potential of the form V (φ) = 1 2 Σ n V n (1-cos n φ).

Two procedures have been devised for minimizing the distorting effects of relaxation on NMR spectra obtained from continuously reacting samples which exhibit CIDNP. The techniques employ (90- τ ) N or ( P sat - τ - P obs - t obs ) N pulse sequences. These record the chemically driven component of the spectrum which develops a short time after the signal has been saturated by one or more rf pulses. The theory of the method is developed for several models of reaction kinetics and illustrated with experimental determinations of the absolute and relative CIDNP enhancement factors for 1 H and 13 C spectra obtained from two thermally decomposing organic peroxides. It is found that the enhancement factors obtained from relaxation-free CIDNP spectra are in excellent agreement with predictions of the high-field radical pair theory. Errors arising from incomplete saturation are estimated and the compromise between signal/noise and removal of relaxation effects is discussed.

The infrared spectrum of malonaldehyde (MA) isolated in an argon matrix has been studied in the frequency range 4000–270 cm−1 at temperatures between 15 and 30 K. In addition, the ultraviolet spectrum of MA isolated in cyclohexane crystals has been studied in the frequency range 28100–30700 cm−1 at temperatures between 1.2 and 50 K. In contrast to the previously observed gas phase spectrum of MA, no tunneling splitting due to proton motion in a double minimum potential is apparent in the spectrum of matrix isolated MA. Furthermore, the relative intensities of the MA bands are not temperature dependent in contrast to expectations for a tunneling system. These results strongly suggest that matrix/MA interactions localize the proton on the time scale of the infrared and optical experiments.