Processable organic materials with large, ultrafast thirdorder susceptibilities, x (3) , could enable all-optical signal processing (AOSP) applications through either fabrication of large-area thick films or integration into hybrid organic/ silicon photonic devices. [1][2][3][4] Conjugated polymers, such as polyacetylene, have been shown to exhibit large optical nonlinearities upon application of intense electromagnetic fields.[3] Despite these nonlinearities, many conjugated polymers are inadequate for photonic applications because they lack sufficient processability, or form films with significant crystallinity [5] or inhomogeneous morphologies [6] resulting in poor optical quality. For example, although films of unsubstituted polyacetylene have been found to possess large third-order susceptibilities, [7][8][9] they also exhibit extensive crystallinity, [10] resulting in poor optical quality and large scattering losses.[11] A more processable soluble diester polyacetylene derivative has been reported; [2] however, only thin films have been produced from this material and the bulky side groups can be anticipated to lead to a reduced nonlinear response through a dilution effect. The diester polyacetylene was successfully used in an image-correlation application performed using degenerate four-wave mixing (DFWM) in the visible region with resonant excitation, although the diffraction efficiency was relatively low, due mainly to the limited interaction length. Ring-opening metathesis polymerization (ROMP) provides for the facile synthesis of a great variety of unsaturated polymers.[12] The ROMP method (Scheme 1) utilizes an organometallic alkylidene-based initiator (e.g., rutheniumbased Grubbs initiators [13][14][15] or four-coordinate tungsten-and molybdenum-based Schrock initiators [16][17][18] ) to polymerize strained-ring olefin monomers. [6,19] The ROMP process allows for in situ polymerization of liquid-phase monomers, which allows facile processing of neat films [11] as well as potential integration of the resulting polymers into micro-or nanostructured photonic devices. Moreover, this method is amenable to independent optimization of monomers and initiators to improve processing conditions, optical quality of films, and the third-order susceptibility of the resulting conjugated polymers.Here we report on the synthesis, processing, and linear and nonlinear optical characterization of substituted polyacetylenes obtained using ROMP of mono-substituted cyclooctatetraene (R-COT) monomers. Polymerization and processing conditions have been developed that have led to thick (2-200 mm), large-area (>1 cm  1 cm) films possessing large third-order nonlinearities in the near-infrared (NIR) region including the telecommunications bands. The good optical quality of a 200 mm thick poly(n-butyl-COT) film allowed for DFWM image correlation under off-resonant excitation to be performed with improved diffraction efficiency relative to that obtained using the diester polyacetylene derivative mentioned above; the resulting di...

We present a simple optical arrangement for phase sensitive detection of degenerate four-wave mixing (DFWM) to characterize the real and imaginary parts of Chi((3)) using an imaged 2-D phase grating. Phase sensitive coherent detection of DFWM signal is demonstrated. Phase stabilization of the interferometric arms is obtained passively with the 2-D grating. A processable polyacetylene sample is characterized at a wavelength of 1.5 microm using this technique. The observed nonlinearity is determined to be a fast (<250 fs) effect using a simple test.

Tyrosine–tryptophan (YW) dyads are ubiquitous structural motifs in enzymes and play roles in proton-coupled electron transfer (PCET) and, possibly, protection from oxidative stress. Here, we describe the function of YW dyads in de novo designed 18-mer, β hairpins. In Peptide M, a YW dyad is formed between W14 and Y5. A UV hypochromic effect and an excitonic Cotton signal are observed, in addition to singlet, excited state (W*) and fluorescence emission spectral shifts. In a second Peptide, Peptide MW, a Y5–W13 dyad is formed diagonally across the strand and distorts the backbone. On a picosecond timescale, the W* excited-state decay kinetics are similar in all peptides but are accelerated relative to amino acids in solution. In Peptide MW, the W* spectrum is consistent with increased conformational flexibility. In Peptide M and MW, the electron paramagnetic resonance spectra obtained after UV photolysis are characteristic of tyrosine and tryptophan radicals at 160 K. Notably, at pH 9, the radical photolysis yield is decreased in Peptide M and MW, compared to that in a tyrosine and tryptophan mixture. This protective effect is not observed at pH 11 and is not observed in peptides containing a tryptophan–histidine dyad or tryptophan alone. The YW dyad protective effect is attributed to an increase in the radical recombination rate. This increase in rate can be facilitated by hydrogen-bonding interactions, which lower the barrier for the PCET reaction at pH 9. These results suggest that the YW dyad structural motif promotes radical quenching under conditions of reactive oxygen stress.