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.     

Electrofluorescence of MEH-PPV and its oligomers: evidence for field-induced fluorescence quenching of single chains

Electrofluorescence (Stark) spectroscopy has been used to measure the trace of the change in polarizability (trDeltaalpha) and the absolute value of the change in dipole moment (|Deltamu|) of the electroluminescent polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrixes. From electrofluorescence, the measured values of trDeltaalpha increase from 500 +/- 60 A(3) in OPPV-5 to 2000 +/- 200 A(3) in MEH-PPV. The good agreement found between these values and those measured by electroabsorption suggests the electronic properties do not differ strongly between absorption and emission, in contrast to earlier predictions. Evidence of electric-field-induced fluorescence quenching of MEH-PPV in dilute solvent glasses was found. When normalized to the square of the applied electric field, the magnitude of quench is comparable to that reported in the literature for thin films of MEH-PPV. In addition, fluorescence quenching was also observed in the oligomers with a magnitude that increases with increasing chain length. By using the values of trDeltaalpha measured by electrofluorescence, a model is developed to qualitatively explain the chain length dependence to the fluorescence quench observed in the oligomers as a function of exciton delocalization along the oligomer backbone. Various explanations for the origin of this quenching behavior and its chain length dependence are considered.     

AFM single molecule experiments at the solid-liquid interface: in situ conformation of adsorbed flexible polyelectrolyte chains

This paper reports the in situ study of conformations of flexible synthetic polymer molecules of protonated poly(2-vinylpyridine) at the solid-liquid interface at different pH of aqueous solutions. The direct conformation study of such thin ( approximately 0.4 nm) single molecules under liquid is performed for the first time. The highly protonated poly(2-vinylpyridine) chains possess a conformation of 2D equilibrated random coil, while at low degree of protonation, molecules are in the conformation of a strongly compressed 3D coil. Molecules are immobile during the few hours of the AFM experiment.     

Structure-dependent photophysics studied in single molecules of polythiophene derivatives.

We report on the photophysical characterization at the single-molecule level of a graft copolymer consisting of a polythiophene backbone and long polystyrene branches. The presence of the branches prevents the polymer chain from forming a collapsed conformational state. The photophysical properties of the resulting solution-like conformation are studied by measuring single-molecule photobleaching dynamics, emission polarization anisotropy and emission spectra. The results are compared with those obtained on the same polythiophene derivative without the branches. It is found that the presence of the branches is a decisive factor in determining the photophysical properties of the polymers on the single-molecule level.     

Photochemistry and photophysics of organomonosilane and oligosilanes: Updating their studies on conformation and intramolecular interactions

This review article deals with recent studies on photochemistry and photophysics of organosilicon monomer to oligomers (here, chain length of Si is mostly less than 10). Arylmonosilanes have been caught attention as the fluorescent and charge-transporting materials. Increasing in silicon chain length, an importance of conformation for silicon single chain in organooligosilane is significant. The conformation affects the spectroscopic, photochemical, and photophysical characteristics. This review is mainly focused on the properties of arylsilanes and α,ω-diaryloligosilanes in which three types of intramolecular interactions, the σ–σ interaction between silicon atoms in a chain unit, the π–π interaction between two aromatic groups, and the σ–π interaction between an aromatic group and a silicon chain unit are expected. As a result, intramolecular excimer/exciplex formation, charge-transfer (CT) complex formation, excitation energy or photoinduced electron transfers were observed by the steady-state and the time-resolved fluorescence (TR-FL) spectroscopy. For the better understanding, the recent conformational studies of single oligosilane chain are also reviewed.     

Excited-state conformation capture by supramolecular chains towards triplet-involved organic emitters

Nowadays,the development of trip let-involved materials becomes a hot research topic in solid-state luminescence fields.However,the mechanism of trip let-involved emission still remains some mysteries to conquer.Here,we proposed a new concept of excited-state confo rmation capture for the const ructio ns of different types of trip let-involved materials.Firstly,excited-state conformation could be trapped by supramolecular chains in crystal and fo rm a new optimum excited-state structure which is different from that in solution or simple rigid environment,leading to bright thermally activated delayed fluorescence(TADF) emission.Based on excited-state conformation capture methodology,next,we obtained roomtemperature phosphorescence(RTP) by introducing Br atoms for the enhancement of intersystem crossing.It could be concluded from experime ntal results that TADF may originate from aggregate effect while RTP may derive from monomers.Finally,heavy-atom free RTP and ultra RTP were achieved by eliminating aggregate effect.This wo rk could not only exte nd the design methodology of triplet-involved materials but also set clear evidences for the mechanism of triplet-involved emissions.     

Interplay between fluorescence and morphology in composite MEH-PPV/PCBM nanoparticles studied at the single particle level

We report the study of the photoluminescence properties of composite conjugated polymer (MEH-PPV)/fullerene (PCBM) nanoparticles as a function of PCBM doping level. The emission properties of individual nanoparticles were studied by Single Particle Spectroscopy (SPS), and distinct changes in vibronic structure with nanoparticle composition were observed. These changes are found to be due to the presence of domains in the nanoparticles with two distinct types of optical signatures, one with molecular and one with aggregate character, for which the abundance and morphology is found to change with PCBM doping levels. Interestingly, highly delocalized structures with a large extent of exciton migration are formed at low PCBM doping levels, while at high PCBM doping levels the exciton collapses into highly localized structures. Furthermore, at very high doping levels phase separation within the MEH-PPV/PCBM nanoparticles is found, even though the reported nanoparticles are only a few tens of nanometers in diameter.     

Competition between compaction of single chains and bundling of multiple chains in giant DNA molecules

It has been established that in a dilute solution individual giant DNA molecules undergo a large discrete transition between an elongated coil state and a folded compact state. On the other hand, in concentrated solutions, DNA molecules assemble into various characteristic states, including multichain aggregate, liquid crystalline, ionic crystal, etc. In this study, we compared single-chain and multiple-chain events by observing individual chains using fluorescence microscopy. We used spermidine, SPD(3+), as a condensing agent for giant DNA. When the concentration of DNA is below 1 microM in base-pair units, individual DNA molecules exhibit a transition from an elongated state to a compact state. When the concentration of DNA is increased to 10 microM, a thick fiberlike assembly of multiple chains appears. AFM measurements of this thick fiber revealed that more than tens of DNA molecules form a bundle structure with parallel ordering of the chains. The transition between single-chain compaction and bundle formation with multiple-chain assemblies was reproduced by a theoretical calculation.     

Carbonization of single polyacrylonitrile chains in coordination nanospaces

It has been over half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed. However, the mechanism of the carbonization reaction remains largely unknown. Structural evolution of PAN during the preoxidation reaction, a stabilization reaction, is one of the most complicated stages because many chemical reactions, including cyclization, dehydration, and cross-linking reactions, simultaneously take place. Here, we report the stabilization reaction of single PAN chains within the one-dimensional nanochannels of metal–organic frameworks (MOFs) to study an effect of interchain interactions on the stabilization process as well as the structure of the resulting ladder polymer (LP). The stabilization reaction of PAN within the MOFs could suppress the rapid generation of heat that initiates the self-catalyzed reaction and inevitably provokes many side-reactions and scission of PAN chains in the bulk state. Consequently, LP prepared within the MOFs had a more extended conjugated backbone than the bulk condition.

Accommodation of polyacrylonitrile in MOFs facilitated and regulated the transformation to ladder polymer in the carbonization process.     

Structure,conformation and dynamics of polymer chains at solid melt interfaces

We have performed molecular dynamics, and lattice Monte Carlo simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics confirmed that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.     

Density and chain conformation profiles of square-well chains confined in a slit by density-functional theory

Density and chain conformation profiles of square-well chains between two parallel walls were studied by using density-functional theory. The free energy of square-well chains is separated into two contributions: the hard-sphere repulsion and the attraction. The Heaviside function is used as the weighting function for both of the two parts. The equation of state of Hu et al. is used to calculate the excess free energy of the repulsive part. The equation of state of statistical associating fluid theory for chain molecules with attractive potentials of variable range [A. Gil-Villegas et al. J. Chem. Phys. 106, 4168 (1997)] is used to calculate the excess free energy of the attractive part. Because the wall is inaccessible to a mass center of a longer chain, there exists a sharp fall in the distribution of end-to-end distance near the wall as the chain length increases. When the average density of the system is not too low, the prediction of this work is in good agreement with computer simulation results for the density profiles and the chain conformation over a wide range of chain length, temperature, and attraction strength of the walls. However, when the average density and the temperature are very low, the prediction deviates to a certain degree from the computer simulation results for molecules with long chain length. A more accurate functional approximation is needed.     

Photochemistry and photophysics of thienocarbazoles

Two methylated thienocarbazoles and two of their synthetic nitro-precursors have been examined by absorption, luminescence, laser flash photolysis and photoacoustic techniques. Their spectroscopic and photophysical characterization involves fluorescence spectra, fluorescence quantum yields and lifetimes, and phosphorescence spectra and phosphorescence lifetimes for all the compounds. Triplet-singlet difference absorption spectra, triplet molar absorption coefficients, triplet lifetimes, intersystem crossing S1 --> T1 and singlet molecular oxygen yields were obtained for the thienocarbazoles. In the case of the thienocarbazoles it was found that the lowest-lying singlet and triplet excited states, S1 and T1, are of pi,pi* origin, whereas for their precursors S1 is n,pi*, and T1 is pi,pi*. In both thienocarbazoles it appears that the thianaphthene ring dictates the S1 --> T1 yield, albeit there is less predominance of that ring in the triplet state of the linear thienocarbazole, which leads to a decrease in the observed phiT value.     

Role of sequence and conformation on the photochemistry and the photophysics of A-T DNA dimers: an experimental and computational approach

The role of base sequence and conformation on the photochemistry and photophysics of thymidylyl (3'-5')-2'-deoxyadenosine sodium salt (TpdA) and 2-deoxyadenylyl (3'-5')-thymidine ammonium salt (dApT) was studied. To this end, nanosecond transient absorption at 266 nm, steady-state irradiation at 254 nm, and quantum chemical calculations were used. The transient absorption spectra show the solvated electron broad band in the visible region for each dinucleotide. In addition, low-intensity absorption bands are observed in the UV region, which are attributed to the deprotonated and protonated neutral radicals of adenine and thymine bases. Photoionization (PI) occurs by one- and two-photon pathways; the latter accounting for approximately 70% of the net PI yield. A diffusion-limited rate constant of 2.0 x 10(10) M(-1) s(-1) was obtained for the reaction of the neutral molecule with the photoejected electron in both sequences. The photodestruction yield, measured from the chromophore loss at 260 nm, decreases in the presence of well-known electron scavengers. This suggests the participation of base radical anions as one of the photodegradation pathways, which is higher in TpdA than in dApT. The intermediacy of a radical ion pair (charge separated state) between the adjacent adenine and thymine bases is proposed in the formation of the [2 + 2] cycloadduct intermediate. The [2 + 2] cycloadduct intermediate is known to be the precursor of the thymine-adenine eight-member ring photoproduct (TA*). Conformational constrains in the radical ion pair are suggested to explain the absence of the TA* photoproduct in dApT. This hypothesis is supported by semiempirical calculations performed on all relevant reactive intermediates proposed to participate in the mechanism of formation of TA*. Altogether, the results show that sequence and conformation profoundly influence the photochemistry and the photophysics of these DNA model systems.     

Spectral identification of specific photophysics of cy5 by means of ensemble and single molecule measurements

The triplet-state characteristics of the Cy5 molecule related to trans-cis isomerization are investigated by means of ensemble and single molecule measurements. Cy5 has been used frequently in the past 10 years in single molecule spectroscopic applications, e.g., as a probe or fluorescence resonance energy transfer acceptor in large biomolecules. However, the unknown spectral properties of the triplet state and the lack of knowledge on the photoisomerization do not allow us to interpret precisely the unexpected single molecule behaviors. This limits the application of Cy5. The laser photolysis experiments demonstrate that the trans triplet state of Cy5 absorbs about 625 nm, the cis ground state absorbs about 690 nm, and the cis triplet state also absorbs about 690 nm. In other words, the T1-Tn absorptions largely overlap the ground-state absorptions for both trans and cis isomers, respectively. Furthermore, the observation of the cis triplet state indicates an important isomerization pathway from the trans-S1 state to the cis-T1 state upon excitation. The detailed spectra presented in this article let us clearly interpret the exact mechanisms responsible for several important and unexpected photophysical behaviors of single Cy5 molecules such as reverse intersystem crossing (RISC), the observation of dim states with a lower emission intensity and slightly red-shifted fluorescence, and unusual energy transfer from donor molecules to dark Cy5 molecules acting as acceptors in single molecule fluorescence resonance energy transfer (FRET) measurements. Spectral results show that the dim state in the single molecule fluorescence intensity time traces originated from cis-Cy5 because of a lower excitation rate, resulting from the red-shifted ground-state absorption of cis-Cy5 compared to that of the trans-Cy5.     

Structure and photophysics of beta-Octafluoro-meso-tetraarylporphyrins

The structure of THF-coordinated [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]zinc, Zn(F(8)TPP).THF, and photophysical studies of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, F(8)TPP, Zn(F(8)TPP), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, F(28)TPP, and [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphinato]zinc, Zn(F(28)TPP), in benzonitrile, are reported. A key point from these studies is that the octafluorinated F(8)TPP and perfluorinated F(28)TPP porphyrins possess similar absorption spectra, but dissimilar X-ray crystal structures and disparate photophysical characteristics. These data cannot be easily accommodated within currently accepted theories which relate macrocycle distortion and optoelectronic properties.     

Influence of alkyl chains length on the conformation and solubilization properties of amphiphilic carboxymethylpullulans

The existence of inter- and/or intramolecular interactions in aqueous solutions of hydrophobically modified polysaccharides induces specific macromolecular conformations at the air–solution interface and in the bulk, where hydrophobic microdomains may be formed. The present work focuses on the interfacial and solubilizing properties of amphiphilic pullulans (CMP₁₂C₈, CMP₇C₁₄, and CMP₄C₁₆) differing in the length and percentage of alkyl chains grafted to the anhydroglucose units. The surface tension studies of these polymers evidence a marked difference between the interfacial properties of CMP₁₂C₈ on one hand and those of CMP₇C₁₄ and CMP₄C₁₆ on the other hand. Pyrene fluorescence spectroscopy and benzophenone solubilization experiments demonstrate a similar partition. Increasing alkyl chains length from eight up to 14 or 16 carbons improves the solubilization properties of nonpolar molecules, even though at the same time, the average number of grafted chains per 100 anhydroglucose units is decreased from 12 down to seven and four for the three compounds, respectively.