Influence of temperature and pressure on shape and shift of impurity optical bands in polymer glasses

The shape, broadening, and shift of optical absorption spectra of molecular impurity centers in polymer glasses are considered in terms of inhomogeneous energy distributions and coupling of electronic transitions to vibrations. Persistent spectral hole burning was applied for frequency-selective probing of zero-phonon lines. The shift and broadening of spectral holes were studied between 5 and 50 K and by applying a hydrostatic He gas pressure up to 200 bar. Broadband absorption spectra were recorded between 5 and 300 K in poly(methyl methacrylate) and polyethylene. In addition to "normal" thermal broadening, due to the first- and second-order electron phonon coupling, several narrowing components were predicted on the basis of frequency dependent hole behavior. Thermal expansion of the matrix and the relaxation of local strains, previously accumulated on cooling below the glass temperature can lead to shrinking of the inhomogeneous width. A Voigt treatment of absorption band shapes reveals that the Gaussian component can indeed suffer remarkable narrowing. Inhomogeneous band shapes and the frequency-dependent thermal and baric line shifts were rationalized with the aid of a pair of two-body Lennard-Jones potentials. The shift of potential well minima is a crucial factor influencing solvent shifts, inhomogeneous band shapes, pressure shift coefficients, and quadratic electron phonon coupling constants.     

Probing the spectral dynamics of single terrylenediimide molecules in low-temperature solids

Fluorescence excitation lines of single terrylenediimide (TDI) molecules were recorded in the matrices polyethylene (PE) and hexadecane (HD) in the temperature range between 1.4 and 13 K. From line width distributions at 2.5 K in both matrices it was concluded that the disorder, theoretically modeled by a distribution of two-level systems (TLSs), is about three times stronger in PE. Temperature-dependent measurements of the line shape of single chromophores showed a reversible broadening and shift of the zero-phonon lines. We attributed this behavior to dephasing caused by pseudolocal phonons and to spectral diffusion caused by fluctuating TLSs of the disordered host. Following these assumptions, the data could be well approximated with the pertinent expressions. The specific environments of the individual molecules are reflected by different behaviors of their `homogeneous' lines.     

Fluorescence-resolved hole burning of squaraine dyes in polymer matrices at low temperature

A new technique that combines nonphotochemical hole burning with multichannel detected fluorescence line narrowing has been used to obtain vibrationally resolved fluorescence spectra of squaraine chromophores in polymer matrices at 1.4 K. At a fixed excitation frequency, the intensities of the zero-phonon lines decay with time due to nonphotochemical hole burning, leaving behind a broader background attributed to emission from molecules excited into phonon sidebands. Subtracting the spectrum of the hole-burned sample from the initial one leaves predominantly a zero-phonon line excited spectrum exhibiting enhanced vibrational structure. Spectra of the same squaraine in polystyrene and polyethylene matrices show differences in the frequencies and intensities of the phonon sidebands, indicating differences in the frequencies and strengths of the matrix modes coupled to the electronic transition of the chromophore. The phonon densities of states inferred through different measurement techniques are compared and related to electronic dephasing rates.     

Single chromophore spectroscopy of MEH-PPV: homing-in on the elementary emissive species in conjugated polymers.

Low-temperature, single-molecule spectroscopy can provide unparalleled access to the primary emissive species of conjugated polymers. We demonstrate this with the example of one of the most commonly studied polymers, poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), MEH-PPV, which is shown to exhibit sharp fluorescence signatures over one hundred times narrower than the ensemble. These unprecedented narrow emission features can be assigned to single chromophores on the polymer chain, which are selectively addressed by the narrow band excitation. As with organic dye systems, the emission from single chromophores is not static with time, but shows a substantial spectral fluctuation. We find that, for single chromophores, this spectral fluctuation always follows a universal Gaussian statistical distribution. High-resolution spectroscopy provides unique insight into low-energy vibrational modes in the polymer emission, which are generally inaccessible with conventional spectroscopic methods such as site-selective fluorescence or Raman spectroscopy. Interchromophoric coupling can also occur owing to the flexible nature of the polymer backbone. This leads to substantial spectral broadening and a loss of resolution in the vibronic progression. We observe reversible switching within one single molecule between narrow and broad emission, which directly correlates with a discrete switching in emission intensity. We conclude that one and the same single molecule can support aggregated and nonaggregated emission, that is, emission from isolated and aggregated chromophores in one single molecule, rather than the tendency for aggregate emission being intrinsic to the molecule.     

Resonant secondary emission and spectral diffusion in isotopically mixed naphthalene crystals

Resonant secondary radiation (zero-phonon lines) flourescence spectra of isotopic 3D aggregates of traps (NH₈) in a host crystal (ND₈), as well as transfer-induced fluorescence at trap concentrations between 0.01 and 10% and at temperatures between 1.6 and 10 K, are presented. Laser-excited (0–1) resonant secondary (zero-phonon) emission is used to resolve close-lying electronic resonances, corresponding to monomers, aggregates and to their host and trap satellite lines, in a quasi-continuum of 3D trap states. Such states are also calculated by numerically diagonalizing a mixed-crystal 3D hamiltonian. Measured and calculated delocalization indices are given for all trap species. In addition, low-temperature spectral diffusion of photoselected trap species is measured by plotting the attenuation of the zero-phonon lines. We give a simple discussion on the modulation of the resonant secondary radiation spectra by concentration, excitation wavelength and temperature. We show that low-temperature exciton diffusion, assited by acoustical phonon creation, occurs above a very low concentration threshold (C > 0.05% for monomers at T = 1.6 K). Thermally activated transfer of the photoselected excitons is also plotted for temperatures up to 10 K above which the zero-phonon lines are quenched.     

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.     

Excitation transfer induced spectral diffusion and the influence of structural spectral diffusion

The theory of vibrational excitation transfer, which causes spectral diffusion and is also influenced by structural spectral diffusion, is developed and applied to systems consisting of vibrational chromophores. Excitation transfer induced spectral diffusion is the time-dependent change in vibrational frequency induced by an excitation on an initially excited molecule jumping to other molecules that have different vibrational frequencies within the inhomogeneously broadened vibrational absorption line. The excitation transfer process is modeled as Fo?rster resonant transfer, which depends on the overlap of the homogeneous spectra of the donating and accepting vibrational chromophores. Because the absorption line is inhomogeneously broadened, two molecules in close proximity can have overlaps of their homogeneous lines that range from substantial to very little. In the absence of structural dynamics, the overlap of the homogeneous lines of the donating and accepting vibrational chromophores would be fixed. However, dynamics of the medium that contains the vibrational chromophores, e.g., a liquid solvent or a surrounding protein, produce spectral diffusion. Spectral diffusion causes the position of a molecule's homogeneous line within the inhomogeneous spectrum to change with time. Therefore, the overlap of donating and accepting molecules' homogeneous lines is time dependent, which must be taken into account in the excitation transfer theory. The excitation transfer problem is solved for inhomogeneous lines with fluctuating homogeneous line frequencies. The method allows the simultaneous treatment of both excitation transfer induced spectral diffusion and structural fluctuation induced spectral diffusion. It is found that the excitation transfer process is enhanced by the stochastic fluctuations in frequencies. It is shown how a measurement of spectral diffusion can be separated into the two types of spectral diffusion, which permits the structural spectral diffusion to be determined in the presence of excitation transfer spectral diffusion. Various approximations and computational methodologies are explored.     

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.     

Photoluminescence spectra of a conjugated polymer: from films and solutions to single molecules

The purpose of this work is to address the issue of applicability of single-molecule spectroscopy (SMS) results for conjugated polymers to "bulk" samples, e.g. conjugated polymer films. Also, some apparent inconsistencies in the literature on SMS regarding the photoluminescence spectral position of conjugated polymers are discussed. We present a series of photoluminescence spectra of thin films of the conjugated polymer poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) with a wide range of varying thickness. The thickness was varied from approximately 20 nm to the value corresponding to well-separated single molecules (SMS sample). The thickness variation resulted in a strong ( approximately 2000 cm(-1)) blue-shift and broadening of the spectrum. The result was reproduced on isolated molecules embedded into a PMMA matrix. This effect cannot be explained by a decrease in energy transfer "freedom" alone. We performed a comprehensive comparison of presented and elsewhere published spectra of MEH-PPV polymer and oligomers in different samples: films, solutions, isolated-molecule coatings and standard SMS samples. The comparison allows that the main reason behind the blue shift is conformational disorder, which is largely dependent on the sample. We also discuss some experimental aspects of SMS, such as representativeness of detected molecules, spectral sensitivity of a setup and temperature. Together with differences in sample preparation method, these issues can explain the existing inconsistencies in the literature.     

Structural luminescence spectra of biogenous porphyrins under selective laser excitation and dependence of this effect on excitation wavelength

Selective laser excitation removes an inhomogeneous width in wideband luminescence spectra of solid solutions of biogenous porphyrins resulting in spectra consisting of narrow zero-phonon lines and phonon sidebands. The effect depends strongly on excitation frequency and disappears gradually upon increasing this frequency within the inhomogeneously broadened pure electronic transition band.     

Fluorescence polarization studies of B-phycoerythrin oriented in polymer film

Polarized steady-state fluorescence and fluorescence excitation spectra as well as time-resolved fluorescence for B-phycoerythrin (B-PE) from red algae, Porphyridium cruentum, embedded in polyvinyl stretched films were measured. The lifetimes of polarized fluorescence were analyzed using exponential components and fractal models. The interactions between various chromophores of the pigment-protein complexes investigated were discussed. The anisotropy of fluorescence excitation spectra differs from the anisotropy of absorption spectra and depends on the wavelength of observation. This shows that differently oriented chromophores take part in various paths of excitation energy transfer (ET) or change their excitation into heat with various efficiencies (or both). Also, analysis of time-resolved fluorescence measured in various spectral regions gives different polarized components of emission. Fractal analysis of lifetimes, done under supposition of the Foerster resonance ET mechanism, suggests different arrangements of energy donors and acceptors for molecules absorbing in different spectral regions. It shows that several fractions of differently oriented "forms" of chromophores exhibiting different spectral properties occur in B-PE complexes. Small changes in the orientation of the chromophores can be followed by modification of the path of excitation energy migration. Based on the results obtained a new reorientational mechanism of the State 1 --> State 2 transition was proposed: Even small conformational modifications of biliproteins, which could be caused in vivo by the change in the conditions of preillumination of bacteria, are able to modify the path of excitation ET. Such a reorientation may be responsible for the change in the partition of biliprotein excitation energy between photosystem II (PSII) and PSI (State 1 --> State 2 transition). The proposed mechanism needs further verification by the investigation of whole bacteria cells.     

Probing intramolecular Förster resonance energy transfer in a naphthaleneimide-peryleneimide-terrylenediimide-based dendrimer by ensemble and single-molecule fluorescence spectroscopy

We report on the ensemble and single-molecule (SM) dynamics of F?rster resonance energy transfer (FRET) in a multichromophoric rigid polyphenylenic dendrimer (triad) with spectrally different rylene chromophores featuring distinct absorption and emission spectra which cover the whole visible spectral range: a terrylenediimide (TDI) core, four perylenemonoimides (PMIs) attached at the scaffold, and eight naphthalenemonoimides (NMIs) at the rim. For FRET from PMI to TDI taking place with an efficiency of 99.5%, single triad molecules optically excited at 490 nm show fluorescence exclusively from the TDI side in the beginning of their emission. On 360-nm excitation, NMI chromophores transfer their excitation energy either directly or in a stepwise fashion to the core TDI, the latter case involving scaffold-substituted PMIs as intermediate acceptors. Indeed, SM experiments on 360-nm excitation evidence highly efficient FRET from NMI chromophores to the TDI core since individual triad molecules show fluorescence exclusively either from TDI or from an intermediate (oxidized) species but never from PMI. Because PMI and TDI are chromophores with high fluorescence quantum yields and high resistance to photobleaching compared to NMI, 360-nm excitation of a single triad molecule leads to bleaching of NMI chromophores with no chance for PMI to be observed. The spatial positioning and the spectral properties of the chosen rylene chromophores make this multichromophoric system an efficient light collector, able to capture light over the whole visible spectral range and to transfer it finally to the core TDI, the latter releasing it as red fluorescence.     

Site-selective fluorescence and hole-burning spectroscopy of MTHF glasses doped with tetracene or pentacene

Site-selective fluorescence spectra of tetracene (TC) doped in a MTHF glass have been recorded as a function of excitation energy. Based upon the true inhomogeneous absorption profile of the TC S₁ ← S₀ origin determined via vibrational ZP emission intensities the true phonon wing has been evaluated employing an iterative numerical spectral convolution procedure. It is found that both phonon spectra and Debye-Waller factor are virtually constant (α = 0.55 ± 0.08) across the inhomogeneous absorption band as are hole-burning yields in MTHF:TC glasses if probed in the zero-phonon line fluorescence excitation mode. It is concluded that the distributions of both the rate constant for NPHB and the strength of electron-phonon coupling are uncorrelated with the electronic site excitation energies.     

Effect of sample preparation and excitation conditions on the single molecule spectroscopy of conjugated polymers

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.     

Spectroscopic properties and ligand field analysis of cis-diazido(1,4,8,11-tetraazacyclotetradecane)chromium(III) azide

The emission and excitation spectra of cis-[Cr(cyclam)(N3)2](N3) (cyclam = 1,4,8,11-tetraazacyclotetradecane) taken at 77 K are reported. The infrared and visible spectra at room temperature are also measured. The vibrational intervals due to the electronic ground state are extracted from the far-infrared and emission spectra. The ten electronic bands due to spin-allowed and spin-forbidden transitions are assigned. Using the observed transitions, a ligand field analysis has been performed to determine the bonding property of azido group in the chromium(III) complex. It is found that azide ligand has weak sigma- and pi-donor properties toward chromium(III) ion. The zero-phonon line in the excitation spectrum splits into two components by 249 cm(-1), and the large 2Eg splitting can be reproduced by the ligand field theory.     

Conformational fluctuations and large fluorescence spectral diffusion in conjugated polymer single chains at low temperatures

The fluorescence of single chains of the conductive polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) was studied by means of single-molecule spectroscopy at 15 K. MEH-PPV was deposited onto a surface from a toluene solution and covered with a polymer cap layer of poly(vinyl alcohol) spin-coated from an aqueous solution for protection against air. Because MEH-PPV is insoluble in water, such sample preparation guarantees that MEH-PPV chains do not mix with the cap polymer. We found that this "host matrix free" environment results in substantially stronger fluorescence spectral diffusion than that observed for conjugated polymer single chains embedded into polymer matrices. The average spectral diffusion range was 500 cm(-1), and the maximum registered value reached 1100 cm(-1), which is approximately 6 times larger than the values reported before. We analyzed spectral diffusion by observation of temporal evolution of the fluorescence intensity, the position of the maximum, and the width of fluorescence spectra. We propose that the transition energy shifts are caused by the differences of the London dispersive forces in slightly different polymer chain conformations. Such conformational changes are possible even at low temperatures because the MEH-PPV single chains in our samples have more freedom for fluctuations than in the usual "in host" arrangement.     

Spectral characteristics of zero-phonon transitions in crystal impurities: A diagrammatic approach

The zero-phonon line shape of a localized transition in a crystal is treated by use of a general interaction between the local transition and the lattice phonons with the harmonic approximation. The theory is carried out to infinite order perturbation by diagrammatic techniques and is thus valid for arbitrarily large phonon coupling. Within our model it is found that the spectral characteristics of the zero phonon line and their temperature dependence are due to resonant Raman-ike phonon scattering processes which cause a decay of the phase coherence of the excitation. The line shape due to this mechanism is found to be lorentzian, and its width increases with temperature, but is zero at 0 K. The line position is also a function of temperature.     

四(4,4',4'',4'''-N,N-二氨基)四苯乙烯光致发光与分子的构象效应

利用还原偶联方法合成出新化合物四(4,4',4',4''-N,N-二氨基)四苯乙烯（ TDETE）。通过测定该化合物在溶液、掺杂聚合物中及晶体粉末的稳态-瞬态荧光光 谱、荧光量子产率和辐射衰变速率常数等。讨论了分子的构象效应等因素对TDETE 光致发光行为的影响。在一定浓度下TDETE溶液存在着三个发光带,分别为全扭曲 构象分子（位于345nm附近的发光I带）、半扭曲构象分子（位于430nm附近的发光 II带）和激基缔合物（530发光III带）的辐射衰变所致。在聚合物（PMMA）中,一 方面由于分子单键的自由旋转扭曲受到遏制,表现为II带的辐射衰变速率常数(kf 值)增大、同时非辐射衰变速率常数knf值减小;另一方面,TDETE分子之间相互作 用得到加强而有利一缔合物形成,结果,使发光II带和III带合二为一出现强而宽 的发射峰,荧光量子产率从溶液中的0.055提高到0.855。此外,在PMM介制裁中观 测到TDETE分子聚集体在626nm处的发光带（IV）,数粉末态中聚集体IV带的强度骤 增,峰值波长红移至650nm。     

Sonoluminescence of Na atom from NaCl solutions doped with ethanol

Sonoluminescence spectra from argon-saturated NaCl solution were measured in the concentration range of 0.5-4 M at the frequency of 138 kHz. The line broadening of sodium atom emission was observed at various acoustic powers in the range from 1.8 to 16.2 W. The sodium D line showed a maximum intensity at a NaCl concentration of 2 M, which corresponded to the maximum production of OH radicals estimated by KI dosimetry. The effects of the addition of a small amount of ethanol on the line width and intensity were closely investigated at various acoustic powers. The sodium line width increases with ethanol concentration and also with power, whereas the line intensity is strongly quenched with increasing ethanol concentration. The results conclusively show that the sodium emission occurs in the gas phase within bubbles. The line broadening is due to interactions with high-pressure argon, and the maximum relative density of gas at bubble collapse was estimated to be 59.5 from the comparison with spectroscopic data. Further line broadening and quenching upon the addition of ethanol arise from collisions with gaseous products obtained from the decomposition of ethanol. The mechanism of sodium excitation is inferred to be as follows. Sodium ions enter bubbles as droplets, and salts are formed because of the high temperature within bubbles. Sodium atoms are generated by the dissociation of salts and then undergo electronic excitation by OH and H radicals.     

Spectroscopy of alkali atoms (Li,Na, K) attached to large helium clusters

Large liquid helium clusters (He_n, n ≈ 10⁴) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm^?1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.