Tuning hyperfine fields in conjugated polymers for coherent organic spintronics

An appealing avenue for organic spintronics lies in direct coherent control of the spin population by means of pulsed electron spin resonance techniques. Whereas previous work has focused on the electrical detection of coherent spin dynamics, we demonstrate here the equivalence of an all-optical approach, allowing us to explore the influence of materials chemistry on the spin dynamics. We show that deuteration of the conjugated polymer side groups weakens the local hyperfine fields experienced by electron-hole pairs, thereby lowering the threshold for the resonant radiation intensity at which coherent coupling and spin beating occur. The technique is exquisitively sensitive to previously obscured material properties and offers a route to quantifying and tuning hyperfine fields in organic semiconductors.     

Synthesis and electrochemical characterization of fluorene and benzimidazole containing novel conjugated polymers: Effect of alkyl chain length on electrochemical properties

The synthesis and characterization of two donor acceptor type conjugated polymers were investigated. Electrochemical properties were examined by cyclic voltammetry, spectroelectrochemistry and kinetic studies. The increase in the alkyl chain length attached to the fluorene unit was investigated in terms of electrochemical properties. The synthesis was carried out via Stille coupling of 4,7-dibromo-4′-(tert-butyl)spiro[benzo[d]imidazole-2,1′ cyclohexane] and 2,5-bis(tributylstannyl)thiophene with 9,9-dihexyl-9H fluorine (P1) and 9,9-didodecyl-9H fluorene (P2) respectively. Both polymers were neutral state green polymers. They had optical band gaps of 1.55 and 1.47 eV respectively. Increasing the chain length resulted in an increase in solubility and processibility of the polymer. Polymers are multichromic, revealing colors from neutral state green to doped state blue.     

Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.     

Bipolaron recombination in conjugated polymers

By using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term and an external electric field, we investigate the scattering process between a negative and a positive bipolaron in a system composed of two coupled polymer chains. Our results show that the Coulomb interactions do not favor the bipolaron recombination. In the region of weak Coulomb interactions, the two bipolarons recombine into a localized excited state, while in the region of strong Coulomb interactions they can not recombine. Our calculations show that there are mainly four channels for the bipolaron recombination reaction: (1) forming a biexciton, (2) forming an excited negative polaron and a free hole, (3) forming an excited positive polaron and a free electron, (4) forming an exciton, a free electron, and a free hole. The yields for the four channels are also calculated.     

Triplet-polaron quenching in conjugated polymers

We studied the triplet-polaron quenching in a platinum(II) porphyrin- (PtOEP-) doped polyspirobifluorene (PSF-TAD) copolymer. The copolymer contains a hole-transporting phenylenediamine unit (TAD) as a comonomer. Triplet-polaron quenching was probed by the change in PtOEP phosphorescence lifetime under an applied voltage in a unipolar device. The charge-induced reduction of the optically excited lifetime of PtOEP is one-third for the highest applied bias. The charge density can be obtained from current-voltage characteristics in the space-charge-limited (SCL) regime. The obtained hole mobility under SCL conditions is (7 +/- 2) x 10(-5) cm(2)/(V s). This result is in accord with recent mobility measurements of the time-of-flight mobility in our polymer. The triplet-polaron recombination constant was evaluated to be (4 +/- 1) x 10(-13) cm(3)/s, implying a triplet-polaron interaction radius of 2 x 10(-10) m. The results show that triplet-polaron annihilation cannot be neglected in device models for phosphorescent light-emitting diodes.     

Embedded units in conjugated polymers

The problem of embedding of several monomeric units (benzene, thiophene, isothianaphthene) into linear oligoenes or into a polymer built up by the same units is discussed. Using a simple model Hamiltonian, we evaluate the geometry (bond lengths) and electronic structure (energy gaps) of conjugated oligomers containing up to 200 atoms. Special attention is paid to end effects. The quinoid-aromatic transition and the conjugation interruption due to embedded defects is studied in some detail.     

Through-space conjugated polymers based on cyclophanes

Conjugated polymers display unique electronic and optical properties, which favor their use in applications as optoelectronic materials and molecular devices. Despite the recent remarkable progress in the chemistry of conjugated polymers, the synthesis of conjugated polymers containing cyclophane units in the main chain is limited to only a few examples. This Minireview presents recent developments in the synthesis, properties, and applications of through-space conjugated polymers based on cyclophanes.     

Two-photon dispersion in conjugated polymers

The frequency dependence of the dispersive and absorptive parts of a strongly allowed two-photon transition is determined in a series of conjugated polymer solutions. The data analysis yields the energy and width of the two-photon transition, symmetry assignment for the two-photon transition (A_g → A_g), and oscillator strengths for both the one-photon and two-photon transitions.     

Mechanistic study of homogeneous reactions coupled with electrochemical oxidation of catechols

The electrochemical oxidation of catechols was described and has shown that these compounds can be oxidized to related o-benzoquinones. The electrochemically generated o-benzoquinones are quite reactive and can be attacked by a variety of nucleophiles under various mechanistic disciplines such as CE, EC, EC′, ECE, ECEC, ECEC ₂, ECECE, ECECEC, ECECECE and trimerization, in which E represents an electron transfer at the electrode surface, and C represents a homogeneous chemical reaction. The mechanistic pathways and final products are depending on some parameters such as electron withdrawing or donating properties of nucleophile, electrolysis medium (solvent, acidity or pH) and nature of catechol.     

Conformational disorder of conjugated polymers

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.     

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.     

Amplified quenching in metal-organic conjugated polymers

The luminescence from conjugated polyelectrolytes that contain pendant metal complex units is quenched very efficiently by oppositely charged electron acceptors.     

Spin-dependent polaron recombination in conjugated polymers

We simulate the interchain polaron recombination process in conjugated polymer systems using a nonadiabatic molecular dynamics method, which allows for the coupled evolution of the nuclear degrees of freedom and multiconfigurational electronic wavefunctions. Within the method, the appropriate spin symmetry of the electronic wavefunction is taken into account, thus allowing us to distinguish between singlet and triplet excited states. It is found that the incident polarons can form an exciton, form a bound interchain polaron pair, or pass each other, depending on the interchain interaction strength and the strength of an external electric field. Most importantly, we found that the formation of singlet excitons is considerably easier than triplet excitons. This shows that in real organic light emitting devices, the electroluminescence quantum efficiency can exceed the statistical limitation value of 25%, in agreement with experiments.     

Surface-initiated polymerization of conjugated polymers

In this feature article, we highlight the recent developments in the chain growth polymerization mechanism of conjugated polymers. With a particular emphasis on Kumada catalyst-transfer polycondensation, this article focuses on the surface-initiated polymerization of conjugated polymers, along with the opportunities and challenges associated with this technique.     

Electrochemistry of conjugated organic polymers

The electrochemistry of conjugated organic polymers such as polyaniline, polythiophene, and polypyrrole has been reviewed. The physical chemistry and, especially, the electrochemistry of conducting polymers (CP) are largely determined by their electrosynthesis conditions. The effects of such factors as the dopant, solvent, and base electrolyte on the preparation and electrochemical n- and p-doping of CP have been considered. The outstanding features of the electrochemistry of CP have been delineated. The prospects for the use of CP as active cathode and anode materials in chemical cells have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 3, pp. 111–129, May–June, 1994.     

Advances in liquid crystalline conjugated polymers

We review advances in liquid crystalline (LC) conjugated polymers by focusing on (i) ferroelectric and (ii) photoresponsive LC conjugated polymers. In Part 1, LC polyphenylene derivatives were synthesized through substitution of fluorine‐containing chiral LC groups into side chains. Poly(para‐phenylene) [ P1 ] and poly(meta‐phenylene) [ P2 ] derivatives showed chiral smectic C phases responsible for ferroelectricity. They exhibited quick response to electric field, giving switching times of <1 s between two bistable states. The spontaneous polarization (P_S) of P2 remained unchanged even after the electric field became zero, affording the residual polarization (P_R) whose value was the same as that of P_S. This indicates that P2 has a potential memory function based on FLC nature. In Part 2, poly(para‐phenylenevinylene) [ P5 ] and poly(bithienylene‐phenylene) [ P6 ] derivatives were synthesized by introducing dithienylethene moieties into side chains. Drastic quenching of fluorescence occurred when the photoresponsive moiety changed from an open form to a closed one upon an irradiation of ultraviolet light. The quenched fluorescence was recovered through a photoisomerization from the closed form to the open one. Macroscopically aligned P6 film exhibited a linearly polarized fluorescence with significant anisotropy. Reversible quenching and emitting behavior in anisotropic fluorescence was controlled by the photochemical switching. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2463–2485, 2009     

Electronic transport in sigma conjugated polymers

Time resolved transport studies which have now been carried out on representative alkyl and aryl substituted Si and Ge backbone polymers demonstrate that these unipolar systems to transmit holes through the specimen bulk with negligible loss of carriers to deep traps, making these materials attractive candidates for electronic device applications. Transport has been studied by measuring drift mobilities as a function of field, temperature and composition, using the small signal time-of-flight technique. Transport occurs among chain backbone derived states, yet exhibits behavior essentially identical to systems in which transiting carriers are known to undergo thermally activated tunneling transitions among discrete molecular sites. Thus it is suggested that disorder causes the chain backbone to be suborganized into domain like units of varying conjugation length. The sensitivity of transport to large scale thermodynamic processes, to side group substituent and to the effect of specific dopants is demonstrated.     

Properties of polythiophene and related conjugated polymers: a density-functional study

Using a parameter-free, density-functional method that has been developed explicitly for the theoretical treatment of infinite, periodic, isolated, helical polymers we study various polymers related to polythiophene. In particular we discuss how the electronic properties of polythiophene are changed when replacing some of the H atoms by CH3 group, by incorporating vinylene bridges into the backbone, or when replacing some or all the CH units of the backbone by N atoms. We observe the weakest effects for the methyl-substitution and the strongest for the N-incorporation. The latter leads to an overall downward shift of all bands, but in contrast to the case for polyacetylene, the unrelaxed compound with N atoms does not have N lone-pair orbitals as the highest occupied ones. Instead these occur at somewhat deeper energies. When comparing the aromatic and quinoid forms we found for the pure compound as well as for the methyl-containing one that the gap closes when passing from the one to the other form which was not found for any of the other materials of the present study. Moreover, the energy of the HOMO was found to depend stronger on the bond-length alternation than the energy of the LUMO, ultimately giving that polarons will induce two asymmetrically placed gap states with the energetically lower one appearing deeper in the gap than the other one.     

Selective adsorption of metronidazole on conjugated microporous polymers

Conjugated microporous polymers (CMPs) have recently received extensive attention in oil/organic solvent-water separation field as a kind of ideal porous absorbents with tunable porosity, large surface areas, and super-hydrophobicity. However, reports on the application of CMPs in adsorption of hydrophilic contaminants from water are very few. In this work, we studied the adsorption of metronidazole (MNZ), a polar antibiotic, by two kinds of CMPs. The adsorption characteristics of MNZ by the CMPs, including adsorption kinetics, mechanism, and isotherm parameters were calculated. The adsorption kinetics of MNZ was well expressed by the pseudo-second-order model, and the adsorption process was found to be mainly controlled by film diffusion. The adsorption isotherm data agreed well with the Langmuir isotherm model, and the values of free energy E indicated that the adsorption nature of MNZ on the CMPs was physisorption. Increasing dispersion degree of the CMPs in MNZ solution resulted in greater adsorption. This work may provide fundamental guidance for the removal of antibiotics by CMPs.