Spectroscopic characterization of novel polycyclic aromatic polymers

A series of novel polyphenylenevinylene (PPV) derivative polymers were studied by absorption and photoluminescence spectroscopies. The effect of the sequential introduction of polycyclic aromatic ring substituents into the delocalized backbone was examined with relation to hypsochromatic and bathochromatic shifting. While the replacement of the phenyl units by naphthyl units results in a substantial hypsochromic shift of both the absorption and emission spectra, their subsequent substitution by anthryl units results in a bathochromic shift. The system is modeled according to, and is found to be consistent with, a previous study of donor-acceptor polyenes of varying length. The electronic structure of the backbone is found to be a balance between that of the high electron affinity polycyclic ring system and the contribution to conjugation across the linking vinyl unit. The model is adapted based on electron affinities of the constituent units, and a clear structure-property relationship for the absorption and emission properties of the system is elucidated. The Stokes shift is examined and is seen to be well-correlated with the vinyl contribution to the electron affinity total (EAtotal). The trends observed in the optical properties of the polymeric system are supported by Raman spectroscopy, whereby the spectral signature of the connecting vinyl bond is seen to soften in a fashion which is correlated with the modeled electron affinity parameters.     

J-aggregation in alpha-sexithiophene submonolayer films on silicon dioxide

We report on the observation of J-aggregates in submonolayer films of alpha-sexithiophene grown on silicon dioxide. Photoluminescence spectroscopy reveals that submonolayers are formed by molecules lying flat on the substrate with a head to tail configuration. Excitation energy dependence of photoluminescence shows a red-shifted absorption with respect to isolated molecules and a negligible Stokes shift between absorption and emission. The pronounced structural order of J-aggregates is reflected in the fwhm of the emission bands. From time-resolved and low-temperature photoluminescence experiments, we infer a quantum yield of the J-aggregate between 0.6 and 1. The demonstration of spontaneous formation of J-aggregates of pi-conjugated systems on amorphous silicon-based substrates can be relevant for the development of organic-inorganic hybrid photonic devices.     

Two photon absorption properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives

Two-photon absorption (TPA) properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives have been investigated. Comparison of the absorption and fluorescence spectra reveals that these compounds show large Stokes shifts, which increase gradually as the conjugation length increases. One-photon absorption and excitation spectra are similar except that the latter exhibit several peaks near lambda(max). It is also found that the one- and two-photon-induced fluorescence excitation spectra are quite similar, which indicates that the one- and two-photon allowed-excited states are the same. The peak TPA cross section values (delta(max)) measured with nanosecond pulses by the two-photon-induced fluorescence method are in the range (50-2620) x 10(-50) cm4 s/photon. The delta(max) value increases as the donor strength and conjugation length increase. A linear relationship is observed between delta(max) and beta, and this delta-beta relationship is found to serve as a useful synthetic strategy for the design of novel TPA dyes with the octupolar structure.     

Photophysical Properties and Electropolymerization of Regioregular Head-to-Tail Oligothiophenes-Functionalized 9,9′-spirobifluorene Derivatives

The photophysical properties of oligothiophenes-functionalized 9,9′-spirobifluorene derivatives in solution and in solid state were investigated in detail. The enhanced red-shift was observed from the absorption and the photoluminescence (PL) spectra of these derivatives in dilute THF solution with the increase of thiophene unit at one branch of 9,9′-spirobifluorene identical with the increased p-electron delocalization and the more effective conjugation length of the whole molecular system. The investigation on the photophysical properties of these compounds further demonstrated that the sp3-hybrid carbon at the spiro-center completely obstructed the correlation between two branches of the 9,9′-spirobifluorene system and made every branch become an independent chromophore. The electrochemical polymerization of 6b as a representative was also investigated.     

结构规整有序的寡聚噻吩修饰9,9'-螺二芴衍生物的光物理性质及其电化学聚合

研究了结构规整有序的寡聚噻吩修饰的9,9'-螺二芴衍生物在溶液中和在固体膜状态下的光物理性质, 发现在四氢呋喃的稀溶液中, 随着有效共轭长度的增加这些化合物的最大吸收峰和发射峰发生了明显的红移. 实验结果表明, 9,9'-螺二芴骨架中的9位螺碳原子明显地阻断了二个共轭片段的共轭效应, 使它们成为了二个独立的共轭链节. 此外, 还进一步研究了这些化合物的电化学聚合性质.     

On the angular dependence of the optical polarization anisotropy in ladder-type polymers

Two ladder-type polymers have been examined using polarized optical absorption and fluorescence spectroscopy in solution and solid state. The fluorescence anisotropy excitation spectra of ladder-type methyl-substituted poly(para-phenylene) and poly(naphthylene-phenylene) follow their absorption profiles, contrary to the typical behavior in other conjugated polymers, which show excitation wavelength dependent anisotropy until the onset of the typical red edge effect. Using Raman microscopy and linear dichroism of aligned stretched polyethylene films containing the ladder polymer, it has been found that the orientation of the absorption transition dipole moment is dependent on the conjugation length. The transition dipole of the shorter conjugation lengths of the distribution is localized approximately 7 degrees off axis, and that of the longer conjugation lengths is delocalized along the chain.     

Spectroscopic studies of polymerized surfactants: 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine

Spectroscopic studies have been performed on aqueous dispersions of the surfactant 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine before and after polymerization with ul-traviolet light. Monomers of this lipid can, under certain conditions, convert from the expected spherical liposomal form to a unique phase consisting of hollow tubules. To determine the molecular conformation of these structures we have used Raman and infrared spectroscopies to probe the structure of the hydrocarbon chains and head groups of the lipids, and used absorption spectroscopy and resonance enhanced Raman scattering of the colored polymer to monitor the length and structure of the diacetylenic polymer backbone. Unusual C? H stretch-ing Raman bands imply that the hydrocarbon chain packing in the monomeric bilayers is different from that observed in other phosphatidylcholines, and that a distrubance in alkyl chain packing occurs on polymerization. Depending on irradiation conditions and the dispersal state of the lipid the polymer chains may be of at least three different colors, from which distinct resonance Raman spectra are obtained. The effective bond conjugation lengths range from quite short in the yellow polymer produced in sonicated vesicles to extremely long in a blue component seen in polymerized tubules.     

Synthesis of 4-(5-Nitrophenylfur-2-yl)-1,2,3-thia- and Selenadiazoles

Arylation of 2-acetylfuran with o-, m-, and p-nitrophenyldiazonium salts under the conditions of the Gomberg-Bachmann reaction has afforded the corresponding 5-(nitrophenyl)-2-acetylfurans. Their carbo-ethoxyhydrazones have undergone the cyclization into stable 4-(5-nitrophenylfur-2-yl)-1,2,3-thiadiazoles under the conditions of the Hurd-Mori reaction. Analogous semicarbazones have afforded the corresponding selenadiazoles upon oxidation with selenium dioxide. The analysis of electronic absorption spectra of the obtained hybrid heterocycles has shown that the conjugation of the phenyl and the furan ring in o-nitrophenyl derivatives is distorted due to steric hindrance, whereas the effect of direct polar conjugation leading to strong bathochromic shift of the absorption band has been observed in the case of the p-nitro derivatives. The position and intensity of the bands in the electronic absorption spectra of the studied compounds are determined by electronic as well as steric factors. The difference in the length of conjugation chain determined by the position of the nitro group in the phenyl fragment also contributes to the observed trend. The introduction of selenadiazole fragment instead of thiadiazole one has caused slight bathochromic shift of the band in the electron absorption spectra.

     

Raman dispersion and intermolecular interactions in unsubstituted thiophene oligomers

We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.     

Hybrid Conjugated Organic Oligomers Consisting of Oligodiacetylene and Thiophene Units: Synthesis and Optical Properties

Novel and highly soluble hybrid conjugated organic oligomers consisting of oligodiacetylene and thiophene units have been synthesized in high purity through iterative and divergent approaches based on a sequence of Sonogashira reactions. The series of thiophene‐containing oligodiacetylenes (ThODAs) and homocoupled ThODAs (HThODAs) show—both in solution and in the solid state—a strong optical absorption, which is progressively red shifted with increasing chain length. The linear correlation of the absorption maximum (λ^A_max) with the inverse of conjugation length (CL=number of double and triple bonds) shows that the effective conjugation length of this system is extended up to at least CL=20. Furthermore, absorption measurements of dropcast thin films display not only a bathochromic shift of the absorption maxima but also a higher wavelength absorption, which is attributed to increased π–π interactions. The wavelength of the maximum fluorescence emission (λ^E_max) also increases with CL, and emission is maximal for oligomers with CL=7–12 (fluorescence quantum yield Φ_F=～0.2). Both longer and shorter oligomers display marginal emission. The calculated Stokes shifts of these planar materials are relatively large (0.4 eV) for all oligomers, and likely due to excitation to the S₂ state, thus suggesting that the presence of enyne moieties dominates the ordering of the lowest excited states. The fluorescence lifetimes (τ_F) are short (τ_F,_max=?1 ns) and closely follow the tendency obtained for the fluorescence quantum yield. The anisotropy lifetimes show a near‐linear increase with CL, in line with highly rigid oligomers.     

Investigation of the Optical Properties of a Novel Class of Quinoline Derivatives and Their Random Laser Properties Using ZnO Nanoparticles

Quinoline Schiff bases display potential applications in optoelectronics and laser fields because of their unique optical properties that arise from extensive delocalization of the electron cloud, and a high order of non-linearity. In this context, a new class of conjugated quinoline-derivative viz. N-(quinolin-3-ylmethylene)anilines were synthesized from 2-hydroxyquinoline-3-carbaldehyde in two good yielding steps. The ability of these imines to accept an electron from a donor is denoted by their electron acceptor number and sites, which is calculated using density functional theory (DFT). The optical properties such as FT-IR, Raman, UV-VIS, and EDS spectra were calculated using TD-DFT, which also provided the energy gap, HOMO-LUMO structure. The optical properties of the synthesized imino quinolines were experimentally studied using photoluminescence and absorption spectroscopy. The properties such as Stokes shift and quantum yield were calculated using experimental data. Furthermore, the compound bearing a methyl group on the aryl ring and ZnO nanoparticles (hydrothermally synthesized) were dissolved in toluene, and optically excited with a 355 nm nanosecond laser, which produced a random laser.     

Effect of donor-acceptor substitution on the nonlinear optical properties of oligo(1,4-phenyleneethynylene)s studied by third harmonic generation spectroscopy

Third-harmonic generation (THG) spectroscopy was performed for oligo(1,4-phenyleneethynylene)s (OPEs) with terminal donor-acceptor (DA) substitution and compared to the results of merely donor substituted OPEs and regular OPE chains with 2,5-dipropoxy benzene rings. Both, extension of the conjugation and push-pull effect enhance the molecular hyperpolarizability gamma, even for the DAOPEs, which exhibit a hypsochromic shift of the long-wavelength absorption for increasing length L of the conjugated chain.     

Spectroscopic properties of poly(9,9‐dioctylfluorene) thin films possessing varied fractions of β‐phase chain segments: enhanced photoluminescence efficiency via conformation structuring

Poly(9,9‐dioctylfluorene) (PFO) is a widely studied blue‐emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well‐defined chain‐extended “β‐phase” conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β‐phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β‐phase fraction and the method by which it was induced. Notably, a PLQE of ～69% is measured for PFO films possessing a ～6% β‐phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ～55% recorded for other β‐phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β‐phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β‐phase fraction. As a specific example, spatial Raman mapping is used to image a mm‐scale β‐phase stripe patterned in a glassy PFO film, with the extracted β‐phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1995–2006     

Effects of packing structure on the optoelectronic and charge transport properties in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)

Spin-coated poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films of different molecular weights (Mn= 9-255 kg/mol), both in the pristine and annealed state, were studied in an effort to elucidate changes in the polymer packing structure and the effects this structure has on the optoelectronic and charge transport properties of these films. A model based on quantum chemical calculations, wide-angle X-ray scattering, atomic force microscopy, Raman spectroscopy, photoluminescence, and electron mobility measurements was developed to describe the restructuring of the polymer film as a function of polymer chain length and annealing. In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the F8 and BT units, and the BT units in neighboring chains are close to one another. Annealing films to sufficiently high transition temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain ("alternating structure"), and the torsion angle between F8 and BT units is reduced. This restructuring, dictated by the strong dipole on the BT unit, subsequently affects the efficiencies of interchain electron transfer and exciton migration. Films exhibiting the alternating structure show significantly lower electron mobilities than those of the pristine high molecular weight films, due to a decrease in the efficiency of interchain electron transport in this structure. In addition, interchain exciton migration to low energy weakly emissive states is also reduced for these alternating structure films, as observed in their photoluminescence spectra and efficiencies.     

Impact of molecular structure of smectogenic chiral esters (3FmX1X2r) on vibrational dynamics as seen by IR and Raman spectroscopy

Smectogenic chiral esters with three-ring rigid core were examined by infrared and Raman spectroscopy. These compounds differ in the type of chiral centre, length of non-chiral chain and substitution of phenyl ring by fluorine atoms. The influence of molecular structure on the type of handedness of helical structure, as well as the shift and split of absorption bands on IR and Raman spectra were determined. The change of the length of non-chiral terminal chain has the most significant influence on the type of helical twist sense. All considered structural elements of tested compounds have the biggest influence on the value of wavelength of vibrational bands related to the carbonyl group located in the rigid core.     

Experimental CC stretching phonon dispersion curves and electron phonon coupling in polyene derivatives

We analyze the infrared and Raman spectra (both experimentally and with the aid of quantum chemical calculations) of a series of polyenals which provide us with the fortunate case of a set of polyene chains with one of the end groups consisting of a C=O group which not only does take part in the conjugation but also pulls electrons from the chain making the whole system highly polar, thus affecting the vibrational transition moments. In the following we show, for the first time, that it is possible to derive experimental phonon dispersion curves and these prove to be different for each chain length. We support our experimental findings with Density Functional Theory quantum chemical calculations which reproduce with sufficient accuracy the IR and Raman spectral pattern and at the same time help in disentangling the assignment of the fine structure observed in the experimental spectra.     

Opto-electronic properties of conjugated polymers

There is growing evidence, both theoretical and experimental, that the primary optical excitations in conjugated polymers are of excitonic nature. They are formed instantaneously upon photoexcitation and migrate incoherently among chain segments differing in length and, concomitantly, in excitation energy. Migration is associated with spectral relaxation manifest in the occurrence of a dynamic Stokes shift. Time-resolved photoluminescence as well as energy transfer studies support this conceptual framework. In systems of the polyphenylenevinylene family the exciton binding energy is estimated to be about 0.4 eV, comparable to that in polydiacetylenes. However, transfer of a charge from an excited chain segment to neighboring chain segments costs less energy. Taking into account that all energy levels are disorder broadened this ensures that a fraction of excitations will find it energetically more favorable to decompose into an electron-hole pair on adjacent chains (off-chain or indirect exciton) that acts as precursor for photoconduction. Exciton breaking can be stimulated by a strong electric field as documented by cw and time resolved spectroscopy. Recent fluorescence as well as pump and probe measurements on a ladder type polymer (LPPP) with 150 fs time resolution will be reported which support the above conceptual framework and document the occurrence of stimulated emission.     

X-ray absorption, photoemission spectroscopy, and Raman scattering analysis of amorphous tantalum oxide with a large extent of oxygen nonstoichiometry

The electronic structure and modification of the local interatomic structure of a reactive sputtered amorphous tantalum oxide (a-TaO(x)) thin film with the variation of oxygen nonstoichiometry, x in a-TaO(x) have been investigated by X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), Raman scattering spectroscopy, and Rutherford back scattering spectroscopy. A parallel chemical shift of Ta4f(7/2) and O1s core levels observed with the variation of x indicates the Fermi level shift by reduction and oxidation in the framework of the rigid band model. Extended X-ray absorption fine structure (EXAFS) suggests both the increase of average coordination number of the first Ta-O shell in polyhedra and a considerable reduction of the average Ta-O bond length with the increase of x. The relative intensity of Raman shift peaks at 670 cm(-1) and 815 cm(-1), corresponding to Ta-O stretching of TaO(6) octahedra and TaO(5) probably with a pyramidal form, respectively, drastically changes between x = 2.47 to 1.86, suggesting the change in the predominant polyhedron from TaO(6) to TaO(5) with a modification in multiplicity of oxygen by the reorganization of the polyhedral network.     

Cationic polythiophene-surfactant self-assembly complexes: phase transitions, optical response, and sensing

The absorption and photoluminescence spectra of the cationic conjugated polyelectrolyte poly[3-(6-trimethylammoniumhexyl)thiophene] (P3TMAHT) were observed to be dramatically altered in the presence of anionic surfactants due to self-assembly through ionic complex formation. Small-angle neutron scattering (SANS), UV/vis, and photoluminescence spectroscopy were used to probe the relationship between the supramolecular complex organization and the photophysical response of P3TMAHT in the presence of industrially important anionic surfactants. Subtle differences in the surfactant mole fraction and chemical structure (e.g., chain length, headgroup charge density, perfluorination) result in marked variations in the range and type of complexes formed, which can be directly correlated to a unique colorimetric and fluorimetric fingerprint. Our results show that P3TMAHT has potential as an optical sensor for anionic surfactants capable of selectively identifying distinct structural subgroups through dual mode detection.     

Molecular sensors using a resonance Raman template

The potential use of resonance Raman spectroscopy as a molecular sensing tool is illustrated using a metalloporphyrin template and pyridine as an analyte. The equilibrium binding constant for the axial binding of pyridine to zinc tetraphenylporphyrin has been measured using resonance Raman spectroscopy. Although no new peaks are observed and the porphyrin peaks do not shift position, the quantification is made possible by the selective resonance enhancement of the template vibrations. The value for log k was determined by resonance Raman to be 3.65 +/- 0.32, which compares well with previously published values estimated using absorption data. Values for log k were determined for a series of related compounds, the picolines, and these also compare favourably with those previously reported.