D. Yu. Paraschuk

Raman spectroscopy of intermolecular charge transfer complex between a conjugated polymer and an organic acceptor molecule

Intermolecular donor-acceptor charge transfer complex (CTC) formed in the electronic ground state between poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and 2,4,7-trinitrofluorenone (TNF) has been investigated by Raman and optical absorption spectroscopies. Blending of MEH-PPV and TNF results in appearance of the CTC absorption band in the optical gap of the both components and in changes in the characteristic MEH-PPV Raman bands including shifts, change in bandwidth, and intensity. The experimental data are similar in films and solutions indicating the CTC formation in both. We associate the low-frequency shift of the strongest MEH-PPV Raman band at approximately 1580 cm(-1) reaching 5 cm(-1) with partial electron transfer from MEH-PPV to TNF amounting approximately 0.2e(-). We suggest that polymer conjugated segments can form the CTC of variable composition MEH-PPV:TNF=1:X, where X<or=0.5 is per MEH-PPV monomer unit. Our Raman data indicate that MEH-PPV conjugated segments involved in the CTC become more planar; however, their conjugation length seemingly does not change.

Weak intermolecular charge transfer in the ground state of a π-conjugated polymer chain

We show that a π-conjugated polymer chain can demonstrate weak intermolecular charge transfer in the electronic ground state. Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]/2,4,7-trinitrofluorenone (MEH-PPV/TNF) donor-acceptor films have been studied by optical absorption, Raman, infrared spectroscopy, and differential scanning calorimetry. The factors influencing weak intermolecular charge transfer in π-conjugated chains are discussed.

Ground state of π-conjugated polymer chains forming an intermolecular charge-transfer complex as probed by Raman spectroscopy

The intermolecular charge-transfer complex (CTC) between the conjugated polymer MEH-PPV and the low-molecular organic acceptor trinitrofluorenone (TNF) has been studied by Raman and optical absorption spectroscopy. On mixing MEH-PPV with TNF, an absorption band due to the CTC appeared in the optical gap of the polymer, whereas, in the Raman spectra, characteristic bands of the polymer are shifted and their widths and intensities change. The low-frequency shift of the strongest band at 1580 cm−1 in the Raman spectrum of the polymer, assigned to the symmetric stretching vibration of the phenyl group, is shown to be due to electron density transfer from the π-conjugated system of the polymer to the acceptor and is as large as 5 cm−1, which corresponds to a charge transfer on the order of 0.1e−1. Even at a low acceptor concentration (one TNF molecule per 10 monomer units of the polymer), most Raman-active conjugated chains are involved in the CTC. It is suggested that conjugated segments of the polymer can form a CTC of variable composition MEH-PPV: TNF = 1: X, where 0.1 ≤ X ≤ 0.5 (for each monomer unit of the polymer), and one TNF molecule can thereby interact with two conjugated segments of MEH-PPV. The conjugated polymer chains involved in the CTC can become more planar, and their interaction with the local environment can noticeably change; however, their conjugation length, most likely, remains unaltered.

Temperature evolution of electronic and lattice configurations in highly ordered trans-polyacetylene

We have studied the temperature evolution of the optical absorption and Raman scattering spectra of a trans-cis blend of “soluble” polyacetylene in a polyvinylbutyral/butanol mixture. On decreasing temperature a reversible restructuring of the electronic and lattice systems of trans-(CH)x occurs: the electronic energy gap contracts at the high rate of 0.4 meV/K, and the vibrational modes are modified. The experimental data obtained are interpreted in terms of a peculiar interaction of π-conjugated electrons with lattice fluctuations.

Shot-noise-limited radio-frequency lock-in photodetection with a continuous wave mode-locked laser

We have developed quite a simple and original lock-in photodetection system that works at a fixed radio frequency. The system includes a photodetector, a frequency down converter and a standard low-frequency lock-in amplifier. The resonant implementation of the radio-frequency attachment provides high gain and high selectivity, which have enabled us to attain the shot-noise-limited sensitivity to relative optical power change δP/P≃4×10−8 Hz−1/2 at a detection frequency of 6.2 MHz in measurements with a cw mode-locked Nd:yttrium aluminum garnet laser. We have realized the precise position-sensitive photodetection for laser pointing noise measurements and picosecond photoacoustic spectroscopy of solids.

Thermochromism, Raman activity, and electroabsorption in highly ordered trans-and cis-polyacetylene

Abstract We report the results of thermochromism, electroabsorption, and Raman scattering spectroscopy studies for trans-cis blend of ’soluble’ polyacetylene. We have observed that the properties of the trans and cis isomers differ dramatically: the trans isomer reveals considerably higher Raman scattering activity, electroabsorption, thermochromism, and sensitivity of vibrational modes to temperature.

Non-fullerene acceptors for organic solar cells

Solar cells based on organic semiconductor molecules are a promising alternative to conventional silicon photocells owing to their low cost, simple production, and good mechanical properties. Effective organic photocells are based on a heterojunction using an active layer consisting of two different organic semiconductors, one of which is an electron donor, while the other is an acceptor. Progress in organic photovoltaics is related to the development of new donor materials, while fullerene derivatives are commonly used as acceptors. The advantages and disadvantages of fullerene compounds for organic solar cells are discussed in this review, the principles of their operation are briefly considered, and the most successful new non-fullerene acceptors are described. The application of latter acceptors has made it possible to fabricate organic solar cells with an efficiency of about 2–4%.

A quantum-chemistry study of novel copper- and cobalt-complex based redox mediators for dye-sensitized solar cells

Novel redox mediators are necessary in order to increase the efficiency and lifetime of dye-sensitized solar cells. In this work, complex copper and cobalt ions with phenanthroline and bipyridine ligands are studied in the quantum chemical aspect. New complexes with cyano bipyridine derivatives are studied. The redox potentials of the complexes and inner sphere reorganization energies are calculated.

Threshold formation of an intermolecular charge transfer complex of a semiconducting polymer

It has been found that the formation of an intermolecular charge transfer complex in the ground electronic state between the model conjugated polymer (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and the low-molecular-weight organic acceptor (2,4,7-trinitrofluorenone, TNF) occurs stepwise with an increase in the acceptor concentration in the blend as is observed in the optical absorption spectra of solutions. The threshold dependence of the absorption of the charge transfer complex is attributed to the stepwise change in the concentration of the charge transfer complexes, which is not explained by the standard model describing the optical characteristics of intermolecular charge transfer complexes. A kinematic model has been proposed to explain the threshold increase in the concentration of charge transfer complexes: at low acceptor concentrations, the charge transfer complex is formed primarily on the surface of a polymer coil, whereas as the acceptor fraction in the solution increases, TNF molecules penetrate inside the polymer coils, forming the charge transfer complex with the units of the polymer inside the coil.

Anomalously high raman scattering cross section for carbon-carbon vibrations in trans-nanopolyacetylene

Absolute Raman scattering cross section on the stretching carbon-carbon vibrational modes of trans-nanopolyacetylene (NPA) was measured at an excitation wavelength of 514 nm. It is shown that the carbon-carbon bonds in trans-NPA scatter light more efficiently than in the diamond structure by several orders of magnitude. The role of resonant Raman scattering and the model of coherent vibrations of carbon-carbon bonds are discussed.