Mikhail N. Uvarov

Out-of-Phase Electron Spin Echo Studies of Light-Induced Charge-Transfer States in P3HT/PCBM Composite

The light-induced charge-transfer (CT) state in the composite of the conductive polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been studied by electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal corresponding to the spin-correlated radical pair P3HT(+)/PCBM(-) has been observed in this composite material. The time-domain ESE shape for different delays between the laser flash and the microwave pulse sequence has been analyzed. In order to explain the evolution of the out-of-phase ESE signal as a function of the delay between the microwave pulses, a model of the CT state is proposed. The hole is assumed to be delocalized on the P3HT chain over several thiophene subunits, while the point-dipole approximation is used to describe the interaction with the electron on PCBM. The distribution of distances between the positive and negative charges in the CT state has been evaluated.

Effects of spin transitions degeneracy in pulsed EPR of the fullerene C70 triplet state produced by continuous light illumination

X-band echo-detected electron paramagnetic resonance (ED EPR) spectra of triplet state of fullerene C(70) generated by continuous light illumination were found to correspond below 30K to a non-equilibrium electron spin polarization. Above 30K spectra are characteristic of Boltzmann equilibrium. Spectra were simulated fairly well with zero-field splitting parameters D=153 MHz and E and distributed within the range of 6-42 MHz. The origin of E distribution is attributed to the Jahn-Teller effect, which in glassy matrix is expected to depend on the local surrounding of a fullerene molecule (a so-called E-strain). In the center of ED EPR spectra a narrow hole was observed. With increase of the microwave pulse turning angle this hole transforms into a single narrow absorptive line. Numerical simulations by density matrix formalism confirm that central hole originates from a simultaneous excitation of both allowed electron spin transitions of the triplet (T(0)↔T(+) and T(0)↔T(-)), because of their degeneracy at this spectral position. Also explanations are given why this hole has not been observed in the previously reported experiments on continuous wave EPR and on ED EPR under laser pulse excitation.

Spin relaxation and structure of light-induced spin-correlated PCBM−/P3HT+ radical pairs

The electron spin echo (ESE) technique is applied to determine the spin relaxation times of long-lived light-induced radicals and short-term spin-correlated radical pairs (SCRPs) formed by the laser flash of a composite consisting of [6,6]-phenyl-C61-butyric acid methyl ether (PCBM) and poly-(3-hexylthiophene) (P3HT) at 80 K. The ESE signal dependences recorded to measure the longitudinal relaxation times of P3HT+/PCBM− SCRPs and the free P3HT+ radical are fitted by the exp(-(t/T1)0.6) dependence with T1 values lying in the microsecond time scale. The difference in the transverse spin relaxation times of the P3HT+/PCBM− radical paira appeared after selective and non-selective echo-detected EPR spectrum excitation is explained by the instantaneous diffusion model. Based on the model, the magnetic interaction energy between the electron spins in P3HT+/PCBM− SCRPs is estimated; E/ħ ∼ 106 s−1.

Spin relaxation of fullerene C70 photoexcited triplet in molecular glasses: Evidence for onset of fast orientational motions of molecules in the matrix near 100 K

Electron spin echo (ESE) was applied to study transversal spin relaxation of photoexcited triplet state of fullerene C(70) molecules in glassy o-terphenyl and cis-/trans-decalin matrices (glass transition temperatures of 243 and 137 K, respectively). The relaxation rate T(2) (-1) was found to increase sharply above 110 K in o-terphenyl and above 100 K in decalin. It is suggested that this increase arises from interaction of (3)C(70) pseudorotation with fast molecular librations in the matrix. Both these types of motion involve atomic vibrations and are uniaxial in their nature, the known literature data on Raman light scattering and others indicate that molecular librations may be thermally activated in glasses just near 100 K. The increase in T(2) (-1) near 100 K is not observed for photoexcited triplet state of fullerene C(60), for which pseudorotation is not uniaxial. As the fullerene molecule has a size much larger than that for glass solvent molecules, it is likely that molecular librations in the matrix are of collective nature.

Anisotropic pseudorotation of the photoexcited triplet state of fullerene C60 in molecular glasses studied by pulse EPR.

Spin-polarized echo-detected electron paramagnetic resonance (EPR) spectra and the transversal relaxation rate T2(-1) of the photoexcited triplet state of fullerene C60 molecules were studied in o-terphenyl, 1-methylnaphthalene, and decalin glassy matrices. The model is composed of a fast (correlation time approximately 10(-12) s) pseudorotation of (3)C60 in a local anisotropic potential created by interaction of the fullerene molecule with the surrounding matrix molecules. In simulations, this potential is assumed to be axially symmetric around some axis of a preferable orientation in a matrix cage. The fitted value of the potential was found to depend on the type of glass and to decrease monotonically with a temperature increase. A sharp increase of the T2(-1) temperature dependence was found near 240 K in glassy o-terphenyl and near 100 K in glassy 1-methylnaphthalene and decalin. This increase probably is related to the influence on the pseudorotation of the onset of large-amplitude vibrational molecular motions (dynamical transition in glass) that are known for glasses from neutron scattering and molecular dynamics studies. The obtained results suggest that molecular and spin dynamics of the triplet fullerene are extremely sensitive to molecular motions in glassy materials.

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ∼100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.

Light-induced charge separation in a P3HT/PC70BM composite as studied by out-of-phase electron spin echo spectroscopy.

A composite material of semiconducting polymer P3HT and fullerene derivative PC70BM was studied by means of electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal was observed under laser irradiation of the composite at low temperature. We assume that during the charge separation process firstly the spin-correlated radical pairs in the singlet-polarized spin state are formed, and then the net polarization of radical pairs arises due to spin evolution. Both types of polarizations contribute to the out-of-phase ESE signal in the case of non-ideal microwave pulses. Analytical calculation of the echo shape for both types of initial polarization revealed that the contribution of the net polarization becomes zero after averaging over the whole EPR spectrum of the radical pair. This behavior was experimentally confirmed; thus the analysis of the out-of-phase ESE signal was simplified. Interspin distance distributions in the charge transfer state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after laser flash TDAF from 300 ns to 3.3 μs at a temperature of 65 K. Due to geminate recombination and diffusion of the radicals from the interface the distribution becomes significantly broader with larger distances prevailing at longer TDAF values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in TDAF.

Higher triplet state of fullerene C70 revealed by electron spin relaxation

Spin-lattice relaxation times T1 of photoexcited triplets (3)C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm(-1). This indicates that the dominant relaxation process of (3)C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm(-1) above the lowest triplet state t1. Chemical modification of C70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in (3)C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy.

Novel Anthrathiophene-Based Small Molecules as Donor Material for Organic Photovoltaics: Synthesis and Light-Induced EPR Study

Abstract A novel anthrathiophene-based compound, 1,4-bis((5-(6,11-dioxoanthra[2,1-b]thiophene-2-yl)thien-2-yl)ethynyl)-2,5-bis(octyloxy)benzene, was synthesized and characterized. The optical absorption spectrum of the synthesized compound in film is strongly red-shifted as compared to the solution spectrum. The energies of frontier orbitals measured by cyclic voltammetry show that this compound can act as electron donor in a composite with the widely used fullerene derivative PCBM. This is confirmed by light-induced electron transfer from it to PCBM evidenced from light-induced EPR spectroscopy. The spectroscopic data suggest that anthrathiophene is a promising platform for synthesis of small-molecular electron donors for organic solar cells.