Paul A. Cahill

C60H2: Synthesis of the Simplest C60 Hydrocarbon Derivative

The reaction of C60 with BH3: tetrahydrofuran in toluene followed by hydrolysis yielded C60H2. This product was separated by high-performance liquid chromatography and characterized as the addition product of H2 to a 6,6-ring fusion (1alb isomer). The 1H nuclear magnetic resonance (NMR) spectrum of the product remained a sharp singlet between –80� and +100�C, which suggests a static structure on the NMR time scale. Hydrolysis of the proposed borane addition product with acetic acid-d1 or D2O yielded C60HD, and its 3JHD coupling constant is consistent with vicinal addition. The observation of a single C60H2 isomer is in complete agreement with earlier calculations that indicated that at most 2 of the 23 possible isomers of C60 would be observable at equilibrium at room temperature. These results suggest that organoborane chemistry may be applied to further functionalization of fullerenes.

Theoretical studies of selected C60H2 and C70H2 isomers

Abstract Semi-empirical MNDO methods predict that 2 of 23 structurally distinct C 60 H 2 isomers and 4 of 143 C 70 H 2 isomers have particularly low heats of formation. These isomers represent either 1,2-addition across a 6:6-ring fusion or 1,4-addition across a 6-ring, with both hydrogens externally bound. Fully optimized ab initio structures are computed for these low-lying isomers at the Hartree—Fock level using 3-21G and 6-31G* basis sets. For C 60 H 2 , all three methods agree on the isomer ordering, and the lowest energy structure is also the only one that has been observed experimentally. The energy separations among the lowest four C 70 H 2 isomers are found to be quite small, but only the ab initio ordering is consistent with experimental results.

LOSS MECHANISMS IN POLYIMIDE WAVEGUIDES

Waveguide losses in thin film polyimides using waveguide loss spectroscopy and photothermal deflection spectroscopy as a function of cure cycle and structure were studied. Fluorinated sidegroups on the polyimide backbone lead to decreases in birefringence and absorption. The primary waveguide loss mechanism is absorption, not scattering. Waveguide losses as low as 0.4 dB/cm at 800 nm have been measured. Losses as low as 0.3 dB/cm at 1300 nm can be inferred from the photothermal deflection spectroscopy.

Measurements of Kleinman-Disallowed Hyperpolarizability in Conjugated Chiral Molecules

We have designed a hyper-Rayleigh scattering scheme to measure six scalar invariants of the squared hyperpolarizability tensor β2. Our theoretical approach expresses the rotational invariants of the irreducible β components as scalars, which eliminates the need for difficult frame transformations. We applied our scheme to several conjugated chiral molecules and found that there are significant Kleinman-disallowed pseudotensor contributions to their hyperpolarizability. These components, along with a large optical rotation and the results of quantum-chemical calculations, indicate a handed nonplanar delocalization of the charge-transfer system in such molecules as predicted by quantum-chemical calculations and are expected to lead to macroscopic second-harmonic generation in axially aligned polymer materials. Pseudotensor contributions to the hyperpolarizability in chiral molecules were found to be as large as the vector contribution in p-nitroaniline. We qualitatively investigated the dispersion in the Kleinman-disallowed components and confirmed that these components are smaller at longer wavelengths.

Semi-empirical calculations of the isomeric C60 dihydrides

Abstract The reduction of C 60 to C 60 H 2 may result in the formation of 23 distinct, energetically non-equivalent isomers, of which three are meso. The heats of formation of these isomers were calculated at spin-restricted and spin-unrestricted semi-empirical levels using PM3 parameters. Our results indicate that at most two of the 23 isomers are likely to be significantly populated at thermal equilibrium at room temperature. The relative energies of the three lowest-energy isomers at the spin-unrestricted level are 0 kcal/mol for substitution across a 6,6-ring fusion, +2.5 kcal/mol for substitution 1,4 across of a 6 ring, and +9.1 kcal/mol for substitution 2,6 across a naphthalene ring subunit. A concise nomenclature for designating substitutional patterns on C 60 and fullerenes in general is proposed.

Crosslinked polyimide electro-optic materials

We report studies of the optical and electro‐optic properties of guest–host polymeric nonlinear optical materials based on aromatic, fluorinated, fully imidized, organic soluble, thermally, and photochemically crosslinkable, guest–host polyimides. We have introduced temperature stable nonlinear optical chromophores into these polyimides and studied optical losses, electric field poling, electro‐optic properties, and orientational stability. We measured electro‐optic coefficients of 5.5 and 12.0 pm/V for ((2,6‐Bis(2‐(3‐(9‐(ethyl)carbazolyl))ethenyl)4H‐pyran‐4‐ylidene)propanedinitrile (4‐(Dicyanomethylene)‐2‐methyl‐6‐(p ‐dimethylaminostyryl)‐4H‐pyran) DCM‐doped guest–host systems at 800 nm using a poling field of 1.3 MV/cm. Poling induced nonlinearities in single‐layer films were in agreement with the oriented gas model, but were lower in three‐layer films due to voltage division across the layers.

Anomalous-dispersion phase-matched second-harmonic generation in a polymer waveguide.

We demonstrate phase-matched second-harmonic generation in a poled polymer waveguide, using anomalous-dispersion phase matching. Phase matching was achieved between lowest-order fundamental and harmonic modes, TM0ω to TM02ω, at a fundamental wavelength of 815 nm over a propagation length of 32 μm. The maximum conversion efficiency was ηexp = 39%/W cm2, in good agreement with theory.

Anomalous-dispersion phase-matched second-harmonic generation.

We have demonstrated anomalous-dispersion phase-matched second-harmonic generation, using electric-field-induced second-harmonic generation in a liquid solution of Foron Brilliant Blue S-R (FBB). This dye has a sharp absorption maximum near 620 nm and a deep absorption minimum near 450 nm. The microscopic hyperpolarizability (beta) of FBB was measured at 1.58 and 0.954 microm and was found to be 163 x 10(-30) and -118 x 10(-30) cm(5)/esu, respectively. The change in sign of beta and the observation that beta(0), a frequency-independent measure of hyperpolarizability, is the same at both frequencies demonstrate the validity of the two-state model of the second-order hyperpolarizability.

Theoretical studies of derivatized buckyballs and buckytubes

Abstract Computational studies of patterns of addition to fullerenes have been extended (1) from C60H2 and C60H4 to C60H6, (2) from C60H2 and C70H2 to models of buckytubes-H2, and (3) from the products of the reaction of:CH2 with C60 to the corresponding products with buckytubes. A paradigm of multiple addition has appeared: as the number of H2 addends increases, 1,4 addition becomes competitive with 1,2-addition. A significant chemical difference between singly-(buckytube) and doubly-(C60) curved carbon is revealed by the energetics of H2 addition to the sidewalls vs. end-caps of a buckytube model. Remarkably, :CH2 insertion into either sidewalls or endcaps of buckytubes yields structures with similar heats of formation.

Ab initio computational study of selected C60H6 isomers

Abstract The relative energies of 18 isomers of C 60 H 6 have been determined through geometry-optimized ab initio calculations. HF/6-31G ∗ computations on the two lowest energy structures determined at the HF/3-21G level indicate that 1,2,4,11,15,30-C 60 H 6 ( 18 , with a C 60 Br 6 -like structure) lies only 0.4 kcal/mol higher in energy than 1,2,3,4,9,10-C 60 H 6 ( 4 ). This remarkably close energy spacing was unexpected because, unlike in 4 in which addition occurs exclusively to 6,6 ring fusions, H 2 addition in 18 occurs both 1,2 to a 6,6 ring fusion and 1,4 across 6-rings so as to place two double bonds into 6,5 ring fusions.