S.C. Graham

OPTICAL-SPECTRA AND EXCITATIONS IN PHENYLENE VINYLENE OLIGOMERS

Abstract Experimental results of optical absorption spectra, photoluminescence spectra due to singlet exciton emission and photoinduced absorption spectra due to triplet-triplet transitions of triplet excitons for several oligomers of poly( p -phenylene vinylene) are reported. From the red-shift of the peaks with increasing chain length, the effective conjugation length for long-chain precursor-route samples of the polymer are estimated to be about 10 to 17 repeat units. Theoretical studies, using the valence-effective Hamiltonian (VEH) technique, of the effects of ring torsions upon the electronic properties are also reported. Comparison between absorption and luminescence spectra in solution and in the solid state shows a blue-shift for the solution spectra; this is modelled in terms of a greater degree of ring torsion for these oligomers in solution.

Spatial extent of the singlet and triplet excitons in transition metal‐containing poly‐ynes

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal‐containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that π conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain‐length dependence of the singlet–singlet, S0→S1, singlet–triplet, S0→T1, and triplet–triplet, T1→Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.

Photophysical properties of solid films of fullerene, C60

The authors report results on photophysical properties of solid films at C60, including optical absorption, photoluminescence, electroabsorption, photoconductivity, and photoinduced absorption. They observe photoconductivity for excitation energies from the UV to the near IR and find that the excitation spectrum follows the optical absorption. The electroabsorption response varies quadratically with electric field, shows oscillations at the band edge, and has an anisotropy of slightly less than three with respect to the light polarization and the direction of the applied electric field. Photoluminescence from excited states shows vibrational fine structure, a strong temperature dependence, and some variation with excitation energy. They see photoinduced absorption attributed to triplet-triplet transitions at 1.7 eV and also a band peaking at 1.3 eV. A further band at 2.33 eV is seen in C60/C70 mixtures (10% C70) and also in degraded C60 samples. They consider that these bands may arise from extrinsic stabilization of photoseparated charges.

The chemical and electronic structure of the interface between aluminum and conjugated polymers or molecules

Abstract The interaction between aluminum and α-ω-diphenyltetradecaheptaenee (DP7), α-sexithienyl (6T), and poly( p -phenylenevinylene) (PPV), respectively have been studied using both X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS). The UPS valence band spectra, are interpreted with the help of quantum chemical calculations based upon Modified Neglect of Diatomic Overlap (MNDO), Valence Effective Hamitonian (VEH) and ab initio Hartree-Fock methods. DP7 is a model molecule for polyacetylene, while 6T is a model molecule (an oligomer) of polythiophene. The results indicate that aluminum reacts strongly with the surfaces of all of the materials studied. The π-electronic structure of each material was strongly modified. Furthermore, aluminum reacts preferentially with the polyene partof DP7, with the vinylene part of PPV, and with the α-carbons of the thiophene nits of 6T.

New Results on Metal-Polymer Interfaces

Abstract New results on studies of the early stages of formation of the aluminum-poly(p−phenylenevinylene) interface are presented.

Photoinduced doping in conjugated polymers

A range of onium salts were blended with conjugated polymers and exposed to visible light. Photoinduced doping occurred with iodonium salts, but not with sulfonium salts. Cyclic voltammetry was used to study the reaction process. This loss of photoreaction holds promise for use in sensors and radiation detectors.

Experimental and theoretical studies of the electronic structure of substituted and unsubstituted poly(para-phenylenevinylene) (PPV)

Abstract The electronic structure of poly(p-phenylenevinylene) and that of its ring-substituted derivatives, poly(2,5-diheptyl-1,4-phenylenevinylene), poly(2,5-dimethoxy-1,4-phenylenevinylene), and poly(2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene), are studied by Ultraviolet Photoelectron Spectroscopy, UPS, and X-ray Photoelectron Spectroscopy, XPS. It is observed by UPS that the π-bands closest to the valence band edge are strongly affected by the presence of the substituents. The influence of the side groups on the experimental spectra is studied theoretically using the Valence Effective Hamiltonian, VEH, model. Calculations are carried out on isolated polymer chains, including full treatment of the aliphatic side groups. Particular attention is paid to the effect of chain torsion angles on the π-band edge. For the diheptyl derivative, the experimental results can be explained on the basis of side-group-induced torsions of the phenylene rings along the backbone, which influence the π-band widths and contribute to differences in both optical absorption threshold and binding energy of the valence band edge. For the alkoxy derivatives, the side groups cause strong modifications in the shape of the upper two occupied π-bands, which results in significant changes in the electronic density of states.

High sensitivity radiation sensing by photoinduced doping in PPV derivatives

We report the properties of MEH-PPV/onium salt blends which we find to be doped by a photoinduced electron-transfer reaction on photoabsorption in the MEH-PPV. The quantum efficiency of this process is determined as a function of the incident photon energy, and compared with the corresponding photocurrent quantum efficiency. This system can be used to measure a cumulative dose of visible or high-energy radiation.

Local density functional calculation of the three-dimensional band structure of poly (phenylene vinylene) (PPV) and comparison with photoemission data

Abstract We have carried out a local-density-functional calculation of the three-dimensional (3D) band structure of the conducting polymer PPV in order to determine the effects of interchain coupling. The major effects are due to coupling of carbons with hydrogens on rings in adjacent chains. Among the effects of this coupling are: splitting by up to 0.4 eV at various points of the Brillouin Zone of the double degeneracy that would exist if the two chains in the unit cell were uncoupled; loss of electron-hole symmetry; the wave vector at the valence band edge not along the chain direction; a band gap smaller by 0.2 eV than that obtained from 1D calculations; and interchain coupling large enough to suggest that polarons are unstable in PPV. The calculated density of states is in reasonably good agreement with that found in photoemission.

Theoretical study of the electronic structure of poly(2,5-dimethoxyparaphenylenevinylene) and its oligomers

Abstract Using the Valence Effective Hamiltonian (VEH) approach, we have studied the electronic structure of poly(2,5-dimethoxyparaphenylenevinylene) and its short oligomers. Calculations are also performed on the charged polymer in order to simulate the existence of bipolarons. Results obtained on the oligomers allow us to understand the evolution of the electronic properties as a function of chain length.