Effective stimulated emission and excited-state absorption cross-section spectra of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and t,t′-didecycloxy-II-distyrylbenzene

Theoretical Study of Spectroscopic Properties of Dimethoxy-p-Phenylene-Ethynylene Oligomers: Planarization of the Conjugated Backbone

The optical spectra of the dimethoxy-p-phenylene-ethynylene oligomers (up to n = 10) are calculated by DFT and TD-DFT methods. It is found that the conformational rotations around the cylindrical triple-bonded carbon links impact significantly the optical spectrum. The effective conjugation length (ECL) of the oligomer is obtained by extrapolating the HOMO-LUMO gap to infinite chain length with an alternative exponential function. The spectral shift is mainly dependent on the high pi-conjugation segment of oligomers, resulting from the planarization of the backbone. Although the rotational barrier is very low, the calculated results further indicate that rotation about the cylindrical triple bond still interrupts the conjugation of rod-like oligomers to some extent, and leads to an angle-dependent HOMO-LUMO gap. The results are helpful to interpret the conformational-dependent spectroscopic phenomena of p-phenyleneethynylene oligomers and polymers (PPEs) observed in ensemble and single molecule spectroscopy.

Cruciforms as Functional Fluorophores: Response to Protons and Selected Metal Ions

The photophysics of dialkylamino- and/or pyridine-containing functional chromophores, 1,4-distyryl-2,5-bis(ethynylaryl)benzenes (cruciforms) was investigated; their fluorescence quantum yields and emissive lifetimes were determined. Depending upon their substituents, the frontier molecular orbitals (FMOs) of these cruciforms are either congruent, i.e., HOMO and LUMO occupy the same real space, or disjoint, i.e., the HOMO is located on one branch of the cruciform while the LUMO is located on the second one. Donor-acceptor substitution leads to a disjoint FMO pattern, while the parent 1,4-distyryl-2,5-bis(phenylethynyl)benzene shows congruent FMOs. The photophysics of the cruciforms was investigated upon addition of either an excess of trifluoroacetic acid or an excess of selected metal (Mg(2+), Ca(2+), Mn(2+), Zn(2+)) trifluoromethanesulfonate salts. Addition of either metal ions or protons led to analogous but not identical changes in the spectroscopic properties of the investigated cruciforms. The collected data suggest that the metals bind preferentially at the aniline nitrogen and not at the electron-rich arene. The spatially separated FMOs permit the independent manipulation of the HOMO and the LUMO of such cruciforms. If the branches contain metal-complexing moieties, metal binding leads to either a hypsochromic or a bathochromic shift in emission via interaction of the metal cations with either the HOMO or the LUMO.

Rheofluorescence Technique for the Study of Dilute MEH-PPV Solutions in Couette Flow

A novel rheofluorescence technique has been developed that permits the study of fluorescent polymers in a near-uniform shear field. The system has been used to examine the effects of shear flow on dilute solutions of two commercially available samples of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) in toluene and xylene. A detailed description of the instrument is provided, along with data that confirm a small probe molecule, Rhodamine 6G, is not affected by simple shear flow. MEH-PPV solutions were examined over two decades of concentration for rheochromism indicative of changes in segment length, and shear-induced orientation revealed by measurements of the steady state emission anisotropy. It is demonstrated that these dilute samples were not influenced by shear rates in the range 100-1000 s(-1). In contrast, MEH-PPV dispersed in a concentrated polystyrene solution showed evidence of shear-induced orientation and rheochromism. This new technique shows promise for investigating the impact of shear flow on the conformation of conjugated polymers employed in organic optoelectronic devices.

Optical amplification in a first-generation dendritic organic semiconductor

We report a study of a new class of organic semiconductor as an optical gain medium. We demonstrate amplification of violet light by use of stimulated emission in a solution of a first-generation bis-fluorene-cored semiconducting dendrimer. Amplification is also observed in the solid state by means of amplified spontaneous emission in an optically pumped dendrimer planar waveguide. Gains of 36 dB cm(-1) at 420 nm and 26 dB cm(-1) at 390 nm in solution and 350 dB cm(-1) in the solid state are obtained. These results show that semiconducting dendrimers have potential as visible laser and amplifier materials.

Conjugated polymers as molecular materials: how chain conformation and film morphology influence energy transfer and interchain interactions

The electronic structure of conjugated polymers is of current interest because of the wide range of potential applications for such materials in optoelectronic devices. It is increasingly clear that the electronic properties of conjugated polymers depend sensitively on the physical conformation of the polymer chains and the way the chains pack together in films. This article reviews the evidence that interchain electronic species do form in conjugated polymer films, and that their number and chemical nature depend on processing conditions; the chain conformation, degree of interchain contact, and rate of energy transfer can be controlled by factors such as choice of solvent, polymer concentration, thermal annealing, presence of electrically charged side groups, and encapsulation of the polymer chains in mesoporous silica. Taken together, the results reconcile many contradictions in the literature and provide a prescription for the optimization of conjugated polymer film morphology for device applications.