Showing papers by "Stephen Barlow published in 2006"

Extended Squaraine Dyes with Large Two-Photon Absorption Cross-Sections

Abstract: Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 μm has been found to be 7.0 × 10-11 esu for one of these chromophores.

Bisdioxaborine Polymethines with Large Third-Order Nonlinearities for All-Optical Signal Processing

Abstract: Organic materials with large third-order nonlinearities in the near-infrared spectral regime are critical in the development of photonic devices to be utilized in all-optical signal processing. We have developed polymethine materials, specifically bisdioxaborine-terminated polymethine dyes, which possess large ultrafast third-order nonlinearities and low nonlinear loss all in the near-infrared spectral regime. An extended bisdioxaborine polymethine anion exhibited the largest value of γ (third-order microscopic nonlinearity) at 1.3 μm (|γ| = 5.7 × 10-32 esu) and showed no characteristics of symmetry breaking, unlike other polymethines of similar lengths. A neat film of this molecule maintained relatively low linear loss in the near-infrared and showed a large third-order macroscopic nonlinearity at 1.3 μm (|χ(3)| = 3.6 × 10-10 esu), with a temporal response of less than 8 ps. Furthermore, the real part of χ(3) was nearly an order-of-magnitude larger than the imaginary component. Consequently, this materia...

Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries.

Abstract: We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4'-bis[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl)amino]styryl}-3,4-di(n-butoxy)thiophene, [1](2+) and [2](2+) respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray structures, together with (1)H NMR coupling constants for [2](2+), indicate geometries in which the bond lengths are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT calculations with closed-shell singlet configurations reproduce the observed structures well. Our results indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1 A between formally single and double bonds) on the basis of a limiting valence-bond representation of the structure can, in fact, show structures with significantly different patterns of bond lengths.

Quantum-chemical investigation of second-order nonlinear optical chromophores: comparison of strong nitrile-based acceptor end groups and role of auxiliary donors and acceptors.

Abstract: We report a detailed quantum-chemical investigation of donor-acceptor substituted dipolar nonlinear optical chromophores incorporating the 4-(dimethylamino)phenyl donor end group and a variety of strong heterocyclic acceptor end groups, including tricyanofurans and tricyanopyrroles. In particular, we study the variation of the molecular second-order polarizability (β) with the acceptor end group and when inserting auxiliary donors (thiophene) and acceptors (thiazole) into the π bridge. Both finite-field calculations (in the context of local contributions) and sum-over-states calculations were carried out in order to probe the relationship between β and the chemical structure of the various chromophores. The trends obtained with these two methods are fully consistent. The large β values (up to 700×10−30esu) as well as the observed tunability of the optical absorption maximum (λmax) make the chromophores investigated here interesting candidates for use in electro-optic applications at telecommunications wav...

Comparative studies of the geometric and electronic properties of 1,1-disubstituted-2,3,4,5-tetraphenylsiloles and 1,1,2,2-tetramethyl-3,4,5,6-tetraphenyl-1,2-disila-3,5-cyclohexadiene

Abstract: We present an experimental and density functional theory comparison of the geometric structure, electronic characteristics and optical properties of 1,1-dimethyl-2,3,4,5-tetraphenylsilole (I), 1,1-(propane-1,3-diyl)-2,3,4,5-tetraphenylsilole (II), 1,1-dimethoxy-2,3,4,5-tetraphenylsilole (III) and 1,1,2,2-tetramethyl-3,4,5,6-tetraphenyl-1,2-disila-3,5-cyclohexadiene (IV). The molecular structures of II–IV have been determined using X-ray crystallography and are compared to the previously reported structure of I. Many characteristics of IV are significantly different from those of I–III, due to the presence of the additional silicon atom. In contrast to I–III, which have planar central rings, IV has a non-planar ring conformation due to the presence of two approximately tetrahedrally coordinated silicon atoms. Cyclic voltammetry suggests that I–III are reduced at similar potentials to that for tris(8-hydroquinolinato)aluminium and that IV is more readily oxidizable than I–III. Additionally, while IV is non-emissive in both solution and the solid state, the three siloles are fluorescent in the solid state. For the four compounds, we have calculated the intramolecular reorganization energies for electron- and hole-transfer reactions, ionization potentials and electron affinities.