Does Förster theory predict the rate of electronic energy transfer for a model dyad at low temperature?

The use of the F?rster model to predict the dynamics of resonant electronic energy transfer (RET) in a model donor-acceptor dyad (a terphenyl-bridged perylene diimide (PDI)-terrylene diimide (TDI) dyad molecule) embedded at low temperature in a PMMA matrix is tested against experiment. The relevant ingredients involved in the F?rster rate for RET, namely electronic coupling, spectral overlap, and screening effects, are accounted for in a quantitative manner. Electronic couplings are obtained from time-dependent density functional theory calculations, and the effect of the PMMA environment is included both on the transition densities and on their interaction through the IEFPCM model. We find that the presence of the terphenyl bridge induces a slight delocalization of the PDI and TDI transition densities over the bridge originating in a 56% increase in the coupling and in the breakdown of the dipole-dipole approximation. The spectral overlap is determined on the basis of a detailed simulation of the homogeneously broadened donor emission and acceptor absorption line shapes determined by fitting the single molecule spectra measured at 1.2 K. The corresponding distribution of spectral overlap throughout the ensemble is then estimated by assuming an uncorrelated inhomogeneous line broadening for the donor and acceptor. Combining the calculated electronic couplings and spectral overlaps sampled from Monte Carlo realizations of the energetic disorder, we obtain a mean RET time (approximately 8 ps) and a distribution in reasonable agreement with experiment.     

Photonic superdiffusive motion in resonance line radiation trapping Partial frequency redistribution effects

The relation between the jump length probability distribution function and the spectral line profile in resonance atomic radiation trapping is considered for partial frequency redistribution (PFR) between absorbed and reemitted radiation. The single line opacity distribution function [M. N. Berberan-Santos et al., J. Chem. Phys. 125, 174308 (2006)] is generalized for PFR and used to discuss several possible redistribution mechanisms (pure Doppler broadening; combined natural and Doppler broadening; and combined Doppler, natural, and collisional broadening). It is shown that there are two coexisting scales with a different behavior: the small scale is controlled by the intricate PFR details while the large scale is essentially given by the atom rest frame redistribution asymptotic. The pure Doppler and combined natural, Doppler, and collisional broadening are characterized by both small- and large-scale superdiffusive Levy flight behaviors while the combined natural and Doppler case has an anomalous small-scale behavior but a diffusive large-scale asymptotic. The common practice of assuming complete redistribution in core radiation and frequency coherence in the wings of the spectral distribution is incompatible with the breakdown of superdiffusion in combined natural and Doppler broadening conditions.     

Oxygen effects on the EPR signals from wood charcoals: experimental results and the development of a model

Charcoals prepared from certain tropical woods contain stable paramagnetic centers, and these have been characterized by EPR spectroscopy in the absence and presence of oxygen. The EPR-detectable spin density has been determined, as has been the temperature- and frequency-dependence of the oxygen broadening of the EPR signal, which is orders of magnitude larger than that observed with other materials, such as lithium phthalocyanine. Three Lorentzian components are required to fit the char EPR spectrum in the presence of oxygen, and the oxygen-dependence of the line width, intensity, and resonance position of the three components have been quantified. These results and the properties of porous carbonaceous materials are used to develop a model to explain the effect of oxygen on the char EPR spectral properties. The model is based on oxygen adsorption on the char surface according to a Langmuir isotherm and a dipolar interaction between the paramagnetic adsorbed gas and the charcoal spins. The three EPR components are correlated with the three known classes (sizes) of pores in charcoal, with the largest line broadening attributed to dipolar relaxation of spins in micropores, which have a larger specific surface area and a higher concentration of adsorbed oxygen. An attenuated, but similar, EPR response to oxygen by chars when they are immersed in aqueous solution is attributed to water competition with oxygen for adsorption on the char surface.     

Theoretical and experimental study of the achievable separation power in resistive‐glass atmospheric pressure ion mobility spectrometry

We present a detailed investigation of the performance of our previously reported nanoelectrospray high‐resolution resistive‐glass atmospheric pressure drift tube ion mobility spectrometer constructed with monolithic resistive‐glass desolvation and drift regions. Using experimental spectral data and theoretical pulse width and diffusion variables, we compare theoretical and experimental resolving powers achievable under a variety of field strengths and ion gate pulse widths. The effects of instrumental and operational parameters on the resolution achievable in chromatographic terms are also discussed. Following characterization of the separation power of the instrument, experimental spectral peak width data is fitted by a least‐squares procedure to a pre‐existing semi‐empirical model developed to study contributions to peak width other than initial pulse width and diffusional broadening. The model suggests possible contributions to the final measured peak width from electric field inhomogeneity and minor contributions from instrumental parameters such as anode size, anode‐to‐anode grid distance and drift gas flow rate. The model also reveals an unexpected ion gate width dependence on the final measured peak width that we attribute to non‐ideal performance of the Bradbury‐Nielsen ion gate and limitations in the design of our pulsing‐electronics. Copyright © 2010 John Wiley & Sons, Ltd.     

Using the diphosphanyl radical as a potential spin label: effect of motion on the EPR spectrum of an R1(R2)P--PR1 radical

The EPR spectrum of the novel radical Mes*(CH3)P--PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P--P bond.     

Single molecule photon emission statistics in the slow modulation limit

A framework for calculating photon emission statistics for single chromophores perturbed by slow environmental fluctuations is introduced. When internal chromophore dynamics are significantly faster than time scales for environmental modulation it becomes possible to invoke a type of adiabatic approximation, allowing for straightforward calculation of photon counting moments including explicitly quantum effects. Unlike previous exact treatments, the present methodology involves calculation of dynamics reflecting only the modulation characteristics of the environment and quantum dynamics of an isolated chromophore separately, i.e., the complicated intermingling of chromophore quantum dynamics and the environmental modulation are suppressed via the adiabatic approximation. This leads to significant conceptual and computational simplifications. Within its regime of applicability, the present approximation reproduces exact calculations quantitatively. We demonstrate this accuracy explicitly for the case of a two-level chromophore modulated by a number of different stochastic models.     

Super- and sub-Poissonian photon statistics for single molecule spectroscopy

We investigate the distribution of the number of photons emitted by a single molecule undergoing a spectral diffusion process and interacting with a continuous wave laser field. The spectral diffusion is modeled based on a stochastic approach, in the spirit of the Anderson-Kubo line shape theory. Using a generating function formalism we solve the generalized optical Bloch equations and obtain an exact analytical formula for the line shape and Mandel's Q parameter. The line shape exhibits well-known behaviors, including motional narrowing when the stochastic modulation is fast and power broadening. The Mandel parameter, describing the line shape fluctuations, exhibits a transition from a quantum sub-Poissonian behavior in the fast modulation limit to a classical super-Poissonian behavior found in the slow modulation limit. Our result is applicable for weak and strong laser fields, namely, for arbitrary Rabi frequency. We show how to choose the Rabi frequency in such a way so that the quantum sub-Poissonian nature of the emission process becomes strongest. A lower bound on Q is found and simple limiting behaviors are investigated. A nontrivial behavior is obtained in the intermediate modulation limit, when the time scales for spectral diffusion and the lifetime of the excited state become similar. A comparison is made between our results and previous ones derived, based on the semiclassical generalized Wiener-Khintchine formula.     

Solvent effects on the resonance Raman and hyper-Raman spectra and first hyperpolarizability of N,N-dipropyl-p-nitroaniline

Linear absorption spectra, resonance Raman spectra and excitation profiles, and two-photon-resonant hyper-Rayleigh and hyper-Raman scattering hyperpolarizability profiles are reported for the push-pull chromophore N,N-dipropyl-p-nitroaniline in seven solvents spanning a wide range of polarities. The absorption spectral maximum red shifts by about 2700 cm(-1), and the symmetric -NO2 stretch shifts to lower frequencies by about 11 cm(-1) from hexane to acetonitrile, indicative of significant solvent effects on both the ground and excited electronic states. The intensity patterns in the resonance Raman and hyper-Raman spectra are similar and show only a small solvent dependence except in acetonitrile, where both the Raman and hyper-Raman intensities are considerably reduced. Quantitative modeling of all four spectroscopic observables in all seven solvents reveals that the origin of this effect is an increased solvent-induced homogeneous broadening in acetonitrile. The linear absorption oscillator strength is nearly solvent-independent, and the peak resonant hyperpolarizability, beta(-2omega;omega,omega), varies by only about 15% across the wide range of solvents examined. These results suggest that the resonant two-photon absorption cross sections in this chromophore should exhibit only a weak solvent dependence.     

Laser-induced fluorescence studies of polycyclic aromatic hydrocarbons (PAH) vapors at high temperatures.

In this work, we present the fluorescence spectra of anthracene and pyrene vapors at different elevated temperatures (from 150 to 650 degrees C) excited with the 337 nm line of a nitrogen laser. We describe the high temperature effects on the resulting spectral properties including spectral intensity, spectral bandwidth and spectral shift. We found that the PAH fluorescence spectral bandwidths become very broad as the temperature increases. The broadening is mainly due to thermal vibrational sequence congestion. We also have found that the fluorescence intensity of pyrene vapor increases with increasing temperature, which results from the increase of the pyrene vapor absorption cross section at 337 nm.     

Insights on charge transfer doping and intrinsic phonon line shape of carbon nanotubes by simple polymer adsorption

Doping of individual single-walled carbon nanotubes via noncovalent adsorption of polyethylenimine which converts p-type semiconducting nanotubes into n-type is examined by micro-Raman studies. Distinctively different responses are observed in metallic and in semiconducting nanotubes. Very little or no changes in the radial breathing and the disorder modes are observed upon polymer adsorption on semiconducting carbon nanotubes indicating noncovalent nature of this process. Tangential G-band spectral downshift of up to approximately 10 cm(-)(1) without line broadening is observed for semiconducting tubes suggesting similar magnitude of electron transfer as commonly observed in electrochemical doping with alkali metals. Strong diameter dependence is also observed and can be explained by thermal ionization of charge carriers with activation barrier that scales as the energy gap of the semiconducting nanotubes. In contrast, metallic nanotubes exhibit very different behavior with significant line broadening of the G-band and concurrent enhancement of the disorder mode. In certain cases, initially symmetric Lorentzian line shapes of the G-band features with narrow line widths similar to semiconducting tubes are converted to a broad, asymmetric Breit-Wigner-Fano line shape. Implications on the effects of electron injection and the local chemical environment on the intrinsic line shape of isolated carbon nanotubes are discussed.     

Lineshape of rotational spectrum of CO in (4)He droplets

In a recent experiment the rovibrational spectrum of CO isotopomers in superfluid helium-4 droplets was measured, and a Lorentzian lineshape with a large line width of 0.024 K (half width at half maximum) was observed [von Haeften et al., Phys. Rev. B 73, 054502 (2006)]. In the accompanying theoretical analysis it was concluded that the broadening mechanism may be homogeneous and due to coupling to collective droplet excitations (phonons). Here we generalize the lineshape analysis to account for the statistical distribution of droplet sizes present in nozzle expansion experiments. These calculations suggest an alternative explanation for the spectral broadening, namely, that the coupling to phonons can give rise to an inhomogeneous broadening as a result of averaging isolated rotation-phonon resonances over a broad cluster size distribution. This is seen to result in Lorentzian lineshapes, with a width and peak position that depend weakly on the size distribution, showing oscillatory behavior for the narrower size distributions. These oscillations decrease with droplet size and for large enough droplets ( approximately 10(4)) the line widths saturate at a value equal to the homogeneous line width calculated for the bulk limit.     

Fluorescence excitation and emission spectroscopy on single MEH-PPV chains at low temperature

Fluorescence emission and excitation spectra of single MEH-PPV polymer molecules dispersed in thin PMMA films have been recorded at 1.2 and 20 K. We observe single as well as multichromophore emission in single chain emission spectra, whereby the relative fractions depend on the two different molecular weights (50 and 350 kDa) studied. The molecular weight also affects the distribution of peak emission maxima, which is monomodal (bimodal) for the low (high) molecular weight. The appearance of an additional "red" subpopulation for the high molecular weight sample is attributed to interactions of multiple chromophores from a sufficiently flexible single chain. The comparison of emission spectra appearing in the "blue" as well as "red" subpopulations suggests that these intrachain interactions rather lead to ground-state aggregates than excimers. Independent of the molecular weight, large variations in spectral shape and apparent line width in the emission spectra have been observed. Occasionally, we find very narrow purely electronic zero-phonon lines both in emission and in excitation spectra, with line widths down to the instrumental resolution. In accordance with earlier literature data it is argued that linear electron-phonon coupling should be quite strong for MEH-PPV in PMMA, leading to only a small fraction of chromophores exhibiting zero-phonon lines. In addition, spectral diffusion, which manifests itself by several time-dependent line shifting and broadening phenomena, contributes to the substantial variations of spectral shapes. Excitation experiments with particularly stable chromophores provide an upper limit for the optical line width (approximately 0.1 cm(-1)), which at 1.2 K can actually approach the lifetime-limited homogeneous width. Raising the temperature to 20 K leads to line broadening and typically, to disappearance of zero-phonon lines. The failure to observe zero-phonon lines of chromophores supposedly serving as donors in intramolecular energy transfer is tentatively attributed to fast transfer rates, resulting in strongly broadened lines which escape detection even at 1.2 K.     

Discrimination of PHIP Signals Through their Evolution in Multipulse Sequences

The antiphase character of the PHIP associated signals after a hydrogenation reaction is particularly sensitive to line broadening introduced by magnetic field inhomogeneities and interferences by the presence of resonance lines steaming from a large amount of thermally polarized spins. These obstacles impose a limitation in the detection of reaction products as well as in the experimental setups. A simple way to overcome these impediments consists of acquiring the signal with a train of refocusing pulses instead of a single r.f. pulse. We present here a number of examples where this multipulse acquisition, denominated PhD-PHIP, displays its potentiality in improving the information related to hyperpolarized spins performed in a sample, where the former parahydrogen nuclei are part of a complex J-coupling network.     

Freezing single molecule dynamics on interfaces and in polymers

Heterogeneous line broadening and spectral diffusion of the fluorescence emission spectra of perylene diimide molecules have been investigated by means of time dependent single molecule spectroscopy. The influence of temperature and environment has been studied and reveals strong correlation to spectral diffusion processes. We followed the freezing of the molecular mobility of quasi free molecules on the surface upon temperature lowering and by embedding into a poly(methyl methacrylate) (PMMA) polymer. Thereby changes of optical transition energies as a result of both intramolecular changes of conformation and external induced dynamics by the surrounding polymer matrix could be observed. Simulations of spectral fluctuations within a two-level system (TLS) model showed good agreement with the experimental findings.     

Color Tuning Mechanism of Human Red and Green Visual Pigments

We investigated the molecular mechanism of a rather large red shift of 31 nm in a human red pigment compared with a human green pigment. In this analysis, we paid special attention to the phenomenon of nonadditivity of spectral shifts due to substitution of the key amino acids (OH-bearing amino acids) and the phenomenon of cooperativity by which the spectral shifts due to substitution of the key amino acids in the protein environment of red pigment are about 1.5 times larger than that in the protein environment of green pigment. The analysis was made by using a model of three active sites on which the key amino acids are located and four effective sites by which the effect of the key amino acids is modified. As a result, we found that the interaction between the active sites that occurs through the repolarization of the chromophore induced by the key amino acid is essential for the nonadditivity phenomenon. We also found that the interaction between the active site and the effective site plays a major role in the cooperativity phenomenon. More directly, we say that the highly polarizable property of the chromophore is the origin of the rather large red shift in red pigment. Based on these analyses, we conclude that the interaction between the polarizable chromophore and the protein moiety has the capability of producing a significant spectral shift, at least 1000 cm-1, even by substitution of moderate polar residues of the OH-bearing amino acids.     

Two-dimensional coulomb-induced frequency modulation in Fourier transform ion cyclotron resonance: A mechanism for line broadening at high mass and for large ion populations

Fourier transform ion cyclotron resonance (FTICR), spectra generated for large ion populations exhibit frequency shifts and line broadening, apparently due to Coulomb forces between ions. Although previous two-dimensional (2D) models of Coulomb effects in FTICR accounted for frequency shifts, they did not account for spectral line broadening. In this article, a 2D model is proposed that predicts line broadening due to Coulomb-induced frequency modulation. The model considers the case of two different-mass ions orbiting at their respective cyclotron frequencies around a common guiding center. A mutual modulation of the cyclotron frequency occurs at the difference frequency between ions. If the modulation period is much shorter than the FTICR observation time, then sidebands spaced at intervals approximately equal to the modulation frequency are predicted. However, if the modulation period is similar in duration to the FTICR observation period, the sidebands can no longer be resolved, which results in spectral line broadening. This latter case is a necessary consequence for isotopic peaks in the high mass region around m/z 2000, where deterioration in FTICR performance has been observed. Computer simulations are used to confirm the mass dependence and to demonstrate other features of the model, including a strong dependence of the modulation on ion number. In support of the model, experimental FTICR spectra for large populations of methylnapthalene ions at m/z 141 and 142 exhibit constant frequency sidebands corresponding to multiples of the difference frequency for the two ions extending from nominal values of m/z 136 to 147.     

Single oligomer spectra probe chromophore nanoenvironments of tetrameric fluorescent proteins

When analyzing the emission of a large number of individual chromophores embedded in a matrix, the spread of the observed parameters is a characteristic property for the particular chromophore-matrix system. To quantitatively assess the influence of the matrix on the single molecule emission parameters, it is imperative to have a system with a well-defined chromophore nanoenvironment and the possibility to alter these surroundings in a precisely controlled way. Such a system is available in the form of the visible fluorescent proteins, where the chromophore nanoenvironment is defined by the specific protein sequence. We analyze the influence of the chromophore embedding within this defined protein environment on the distribution of the emission maximum wavelength for a number of variants of the fluorescent protein DsRed, and show that this parameter is characteristic of the chromophore-protein matrix combination and largely independent of experimental conditions. We observe that the chemical changes in the vicinity of the chromophore of different variants do not account for the different distributions of emission maximum positions but that the flexibility of the chromophore surrounding has a dominant role in determining the distribution. We find, surprisingly, that the more rigid the chromophore surrounding, the broader the distribution of observed maximum positions. We hypothesize that, after a thermally induced reorientation in the chromophore surrounding, a more flexible system can easily return to its energetic minimum position by fast reorientation, while in more rigid systems the return to the energetic minimum occurs in a stepwise fashion, leading to the broader distribution observed.     

trans-cis Photoisomerization of a photoactive yellow protein model chromophore in crystalline phase

We have studied the photoinduced trans/cis isomerization of the protonated form of p-hydroxycinnamic thiophenyl ester, a model chromophore of the photoactive yellow protein (PYP), in crystalline phase, by both fluorescence and infrared spectroscopies. The conversion from trans to cis configuration is revealed by a shift of the fluorescence peak and by inspection of the infrared maker bands. The crystal packing apparently stabilizes the cis photoproduct, suggesting different environmental effects from the solvent molecules for this model chromophore in liquid solutions or from the amino acid residues for the PYP chromophore.     

Probing the decay coordinate of the green fluorescent protein: arrest of cis-trans isomerization by the protein significantly narrows the fluorescence spectra

The fluorescence spectra of the wild-type green fluorescence protein (wt-GFP) and the anionic form of p-hydroxybenzylidenedimethylimidazolone (p-HBDI), which models the protein chromophore, were obtained in the 80-300 K temperature range in glycerol/water solvent. The protein spectra have pronounced and well-resolved vibronic structure, at least at lower temperatures. In contrast, the chromophore spectra are very broad and structureless even at the lowest temperatures. Analysis of the spectra shows that the experimentally observed red-shift of the protein spectrum upon heating is apparently caused by quadratic vibronic coupling of the torsional deformation (TD) of the phenyl single bond of the chromophore to the electronic transition. The broad spectra of the chromophore manifest the contribution of different conformations in the glycerol/water solvent. In particular, the lowest-temperature spectrum reflects the distribution over the same TD coordinate in the excited electronic state, which essentially contributes to the asymmetry of the spectrum. Upon heating, motion along this coordinate leads to a configuration from which the radiationless transition takes place. This narrows the distribution along the TD coordinate, causing a more symmetric fluorescence spectrum. We were able to reconstruct the broad, structureless fluorescence spectra of p-HBDI in glycerol/water solutions at various temperatures by convoluting the original wt-GFP spectra with the function describing the distribution of the transition energies of the p-HBDI chromophore. Thus, both the fluorescence broadening and increase in radiationless transition upon removal of the protein chromophore to bulk solvent are consistent with decay by a barrierless TD of the phenyl single bond.