Photoinduced vibrational spectra of oligomers and polymers of thiophene derivatives (original) (raw)
Related papers
Picosecond photoinduced dichroism in solutions of thiophene oligomers
Chemical Physics Letters, 1992
The picosecond photoinduced absorption of a series of a-coupled thiophene oligomers in solution have been measured using the Kerr ellipsometry configuration. We present spectral and dynamic studies of the sharp transient absorption peak observed in the transparency region of these oligomers. Its origin and the reasons for its narrow line shape are discussed. The conversion towards the triplet state is observed in terthiophene.
Physical Review B, 1990
We present measurements of the photoexcited states of poly(p-phenylene vinylene), PPV, and a model oligomer, trans, trans-distyrylbenzene, probed by detection of photoluminescence and photoinduced absorption. Photoluminescence is strong in both materials and due to radiative recombination of the singlet exciton. Triplet excitons are also photogenerated, and in the case of the oligomer are seen directly in light-induced ESR measurements. We find a signal with a peak-to-peak width of 12 G at 1580 6 for a probe frequency of 9.14 GHz. We assign this to the "forbidden" bm, =2 transition between the triplet sublevels. From the position of the minimum field H;" for this microwave transition, we determine the zero-field splitting of the triplet sublevels to be 0.07 cm. We find an associated photoinduced absorption at 1.95 eV in the oligomer which we attribute to a triplet-triplet transition of the excited state. For the polymer we find a photoinduced absorption response at 1.45 eV, very similar to that of the oligomer, and we consider that this is similarly due to a triplet exciton, although we have not been able yet to find direct ESR evidence. In the case of the polymer, charged photoexcitations are also seen in photoinduced absorption measurements, characteristic of bipolarons, with induced absorption at 0.6 and 1.6 eV. %'e do not find similar excitations in the oligomer, and we consider that intermolecular charge transport is necessary for the generation of the long-lived charged excitations probed in our experiments.
Journal of Photochemistry and Photobiology A: Chemistry, 2011
Photoinduced energy-and electron-transfer processes of donor-acceptor molecules composed of [1,2,4,5-tetrakis((E)-2-(50-hexyl-2,20-bithiophen-5-yl)vinyl)benzene] (HPBT) with one, two and four entities of perylenediimide (PDI) forming HPBT-PDI n (n = 1, 2 and 4) have been examined in this article by utilizing steady-state absorption and emission, computational, electrochemical and time-resolved transient absorption studies. The HPBT-PDI n molecules are connected through long non-conjugated-bonds that may prevent the direct overlap of HPBT and PDI energy levels. Electrochemical studies suggest the exothermic photoinduced electron transfer processes when HPBT and PDI are selectively excited. Upon excitation the HPBT entity, the steady-state emission and femtosecond transient absorption measurements of HPBT-PDI n revealed an efficient energy transfer from the singlet excited HPBT to PDI with time constants on the order of ∼10 10 s −1. The energy donor-acceptor distance, r = ∼22Å, is calculated from the experimental energy transfer rates using Förster theory and from the MO calculations using ab initio B3LYP/6-311G method. By selective excitation the PDI entity, the electron-transfer processes take place from HPBT to the singlet excited PDI with time constants on the order of ∼10 8 s −1. The slow rates of electron transfer and energy transfer processes indicated that these molecules tend to take conformations with relatively long distance between HPBT and PDI entities.
Synthetic Metals, 1994
Correlated electron spin resonance (ESR) and optical absorption spectroscopy measurements have been carried out on soluble thiophene oligomers (nT, n standing for the number of rings) of ring size n=6 and n=12. Chemical oxidation leads to the formation of singly and doubly charged molecules. On 6T, the paramagnetic radical-cation 6T "+ and diamagnetic dication 6T +÷ are successively generated, whereas the dication (bipolaron) 12T ÷+ of the twice longer molecule 12T is directly produced. Both the radical cation and dication show two optical absorption bands, which we have used to estimate the bandgap of the corresponding species. This latter is shown to be a pertinent parameter for studying the chain-length dependence of the electronic properties of charged oligomers. From a comparison of our results with previously published optical absorption data on thiophene oligomers and polymers, we find that short (n ~< 8) oligomers are better described in terms of molecular orbitals, whereas the one-electron model of conjugated polymers is more appropriate for longer chains (n = 12 and the polymer). The transition between short and long oligomers, which occurs at n = 10+ l thiophene rings, corresponds to a chain long enough to bear two independent polarons.
Spectral and photophysical studies of poly[2,6-(1,5-dioctylnaphthalene)] thiophenes
Journal of Physical Chemistry C, 2007
A complete spectroscopic and photophysical study of three alternating naphthalene-R-thiophene copolymers was undertaken in solution (room and low temperature) and in the solid state (thin films in a Zeonex matrix). The study comprises absorption, emission, and triplet-triplet spectra together with quantitative measurements of quantum yield (fluorescence, intersystem-crossing, internal conversion, and singlet oxygen formation) lifetimes and singlet and triplet energies. The overall data allow the determination of the rate constants for all the decay processes. Comparison between the behavior of analogous 1-naphthyl(oligo)thiophenes and the 2,6-naphthalene(oligo)thiophene copolymers allows several important observations. First, the polymers display higher fluorescence quantum yields and lower S 1 ∼∼fT 1 intersystem-crossing yields than the oligomers. This can be attributed to the presence of the 1,5-dioctyloxynaphthalene groups in the copolymers leading to a more rigid polymer backbone, which decreases radiationless deactivation and increases the radiative efficiency. Second, the singlet and triplet energies are significantly lower in the polymers than with the corresponding oligomers. This implies a lower HOMO-LUMO energy difference in the polymers due to an extended π-delocalization. Third, the singlet-to-triplet (S 1-T 1) energy splitting is higher in the oligomers than with the polymers, even though the former display higher intersystem-crossing yields. It is suggested that this may result from intersystem-crossing in the oligomers involving significant charge-transfer (CT) character (spinorbit coupling is mediated by CT mixing involving the singlet and triplet states in matrix elements of the type 〈 1 Ψ CT |H′| 3 Ψ 1 〉) of the relevant excited states but that is less important with the polymers. We believe that this may be relevant to understanding the nature of CT states in conjugated copolymers.
Excitonic effects in thiophene oligomers
Synthetic Metals, 1997
Excitonic interactions in conjugated chains are studied by analysing the optical and vibrational properties of unsubstituted thiophene oligomers with different supramolecular organization. Single quasi-planar molecules, electronically decoupled in the solid state, have been obtained by inclusion of the thiophene oligomers in perhydrotriphenylene. Resonant Raman scattering shows that the strong spectral changes which occur upon molecular aggregation can be explained only by electronic excited state interactions, in rather good agreement with the prediction of the molecular exciton model.
Dichroism in photoinduced infrared spectra of polyalkylthiophenes
Acta Polymerica
The photoinduced infrared spectra of stretch-oriented polyhexylthiophene and polyoctylthiophene are reported as a function of the polarization both of the pump light and of the probe IR beam. There is no change in the intensity of photoinduced IR absorption due to the polarization of the pump; however, the spectra show parallel dichroism when the probe is polarized. The experimental results are analyzed on the basis of the “effective conjugation coordinate” theory (ECC).
Femtosecond and continuous-wave photoexcitations of thiophene-based conjugated polymers
Nonlinear Optical Properties of Organic Materials VI, 1993
We present an extensive photoinduced absorption study of alkyl substituted poly(2,5-thienylenevinylene)s in both the solid state and in solution. The optical and conformational properties of the samples are monitored with UV-Vis absorption and emission, IR absorption and Raman scattering. Three photoinduced states are detected under steady state condition which display lifetimes of the order of 100 ms in the solid state. The two low-energy bands are assigned to bipolarons, while a third band peaked near the band edge has a different origin. In solution very long-lived photoexcitated states are observed and they are assumed to recombine via a solvent-assisted photo-doping mechanism. Femtosecond transient bleaching decay indicate that fast dynamics is a sensitive probe of the backbone rigidity, the conjugation length and the supermolecular organization of the chain. The experimental data are interpreted in terms of the formation and decay of bound pairs (polarons-excitons) produced by the absorption of above gap photons.
Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes
The Journal of Chemical Physics, 2001
Regioregular poly͑3-͑4Ј-͑1Љ,4Љ,7Љ-trioxaoctyl͒phenyl͒thiophenes͒ ͑PEOPTs͒ exhibit interesting properties for the use in polymer electronics. Exposing thin films of the amorphous, disordered phase ͑orange phase͒ of the ''as prepared'' polymer to chloroform vapor or annealing them by heat treatment results in a redshift of the absorption maximum due to the formation of nanocrystals in an ordered phase ͑blue phase͒. As such, PEOPT thus is a very interesting conjugated polymeric material, which exhibits two different phases with well-defined order/disorder characters on one-and-the-same material. This property opens up the unique possibility to investigate the role of order/disorder on the photoexcited pattern without being obscured by the differences in chemical structure by using different materials with different crystallinity. The fact, that blue phase PEOPT exhibits absorption edges at relatively low energies around 1.8 eV, thereby demonstrating an enhanced spectral absorption range as compared to the orange phase, makes them attractive for use in photodiodes and solar cells as well. The photoinduced charge generation efficiency in both phases of PEOPT is significantly enhanced by the addition of a strong electron acceptor such as fullerene C 60 , as observed by quenching of the luminescence and by photoinduced absorption measurements in the infrared and uv-visible regime. The average number and the lifetime of photoinduced carriers in composites of PEOPT with a methanofullerene ͓6,6͔-phenyl C 61 -butyric acid methyl ester ͑PCBM͒ are found to depend on the crystallinity of PEOPT in thin films, which gives rise to charged photoexcitations delocalized between polymer chains. Stronger bimolecular recombination in composites of the blue phase PEOPT with PCBM is observed as compared to the orange phase PEOPT/PCBM films. The origin of this enhanced recombination is found to be related to the hole mobility of the polymer.