Substitution, environment, and excitation wavelength effects on the optical nonlinearities of some novel cis -/trans-\u3c0-conjugated azobenzenes

The transient nonlinear optical response of a series of highly \u3c0-conjugated 4,4\u2032-diethynylazobenzene derivatives, symmetrically substituted with different electron-rich aromatic moieties (i.e., a N,N\u2032-dihexylaniline, 4-ethynyl-(N,N\u2032,dihexylaniline), and a Zn(II)-porphyrin fragment, respectively) is investigated by means of the Z-scan technique. The systematic study of their nonlinear optical properties, using 4 ns laser excitation, revealed that all molecules possess very large third-order nonlinearity excited at 532 nm, whereas only the aniline derivatives showed nonlinear optical (NLO) response under 1064 nm excitation. In-depth investigation, both in solutions and in thin films, established that the NLO response of these azobenzene (AB) derivatives is totally attributed to their high refractive nonlinearity, presenting negligible nonlinear absorption. These findings strongly suggest that these molecules can be exploited for the development of new materials suitable for photonic/optoelectronic devices, since their strong nonlinear refraction combined with the absence of any nonlinear absorption ensures low losses, very low heating of the organic material, and eventually longer operative lifetimes. \ua9 2014 American Chemical Society

NLO response of photoswitchable azobenzene-based materials

The nonlinear optical (NLO) response of three π-conjugated azobenzene (AB) derivatives was investigated under picosecond laser excitation by means of the Z-scan technique to evaluate the effect of an ethynyl-based conjugated spacer on the NLO properties of ABs. All modules possessed large third-order nonlinearity, but unexpectedly it was the less extended AB derivative that exhibited the largest NLO response. This finding has been confirmed by means of DFT calculations and was attributed to a higher cis/trans ratio of the particular AB derivative in its investigated photoequilibrated state. Furthermore, the influence of the amount of cis isomer on the third-order nonlinear susceptibility [χ(3)] of the less extended AB derivative has been thoroughly investigated. Specifically, modulation of the NLO response has been successfully achieved by tuning the isomeric composition of the investigated photostationary state. These results highlighted the cis-dependent increase of the NLO response to support the general idea that such compounds can be used for multistep switching NLO materials. Flick of a switch: The nonlinear optical (NLO) response of three π-conjugated azobenzene (AB) derivatives is investigated to evaluate the effect of an ethynyl-based conjugated spacer on the NLO properties of ABs. A cis-isomer-dependent increase of the NLO response, together with intrinsic structure-related high hyperpolarizability values, determined these compounds superb candidates for ultrafast switching NLO devices. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The effect of the degree of oxidation on broadband nonlinear absorption and ferromagnetic ordering in graphene oxide

We report on the effect of the degree of oxidation on the broadband non-linear optical response and magnetic behavior of graphene oxide, as well as on a route for obtaining reduced graphene oxide with enhanced optical properties without sacrificing the high dispersibility of the parent graphene oxide. As more sp3 states evolved with the rise in oxidation degree, it turned out that the sp2/sp3 fraction and sp2 clustering are crucial parameters for tuning the broadband non-linear optical absorption over a wide range from ps to ns laser pulses for both visible and infrared laser irradiation. This was clearly confirmed by two different approaches, namely by a synthetic route through the gradual oxidation of graphene oxide from 1 to 3 oxidizing cycles, and reversely by in situ reduction of graphene oxide by UV laser irradiation. Furthermore, as the sp3 states carry localized magnetic moments, ferromagnetic ordering is observed at low temperatures. The magnetization and temperature at which ferromagnetic ordering evolves are found to increase on increasing the oxidation degree. The tuning of non-linear optical and magnetic properties of graphene oxide by oxidation/reduction thus provides an easy way to endow graphene oxide with tunable physical features highly required in both optoelectronics and spintronics applications

Solvent Molding of Organic Morphologies Made of Supramolecular Chiral Polymers

The self-assembly and self-organization behavior of uracil-conjugated enantiopure (R)- or (S)-1,1′-binaphthyl-2,2′-diol (BINOL) and a hydrophobic oligo(p-phenylene ethynylene) (OPE) chromophore exposing 2,6-di(acetylamino)-pyridine termini are reported. Systematic spectroscopic (UV-vis, CD, fluorescence, NMR, and SAXS) and microscopic studies (TEM and AFM) showed that BINOL and OPE compounds undergo triple H-bonding recognition, generating different organic nanostructures in solution. Depending on the solvophobic properties of the liquid media (toluene, CHCl3, CHCl3/CHX, and CHX/THF), spherical, rod-like, fibrous, and helical morphologies were obtained, with the latter being the only nanostructures expressing chirality at the microscopic level. SAXS analysis combined with molecular modeling simulations showed that the helical superstructures are composed of dimeric double-cable tape-like structures that, in turn, are supercoiled at the microscale. This behavior is interpreted as a consequence of an interplay among the degree of association of the H-bonded recognition, the vapor pressure of the solvent, and the solvophobic/solvophilic character of the supramolecular adducts in the different solutions under static and dynamic conditions, namely solvent evaporation conditions at room temperature. (Chemical Equation Presented)

Postoperative radiation and concomitant bolus fluorouracil with or without additional chemotherapy with fluorouracil and high-dose leucovorin in patients with high-risk rectal cancer: A randomized phase III study conducted by the Hellenic Cooperative Oncology Group

Background: Randomized studies have shown that postoperative chemotherapy with or without radiation therapy (RT) improved local control and survival of patients with stages II or III rectal cancer. However, the optimal sequence of treatments and the optimal chemotherapeutic regimen have not been defined. Modulation of fluorouracil (FU) by leucovorin (LV) has yielded a highly significant difference in response rate from that of FU monotherapy, as suggested by an overview of randomized trials in patients with advanced colorectal cancer. However, this difference in response rate did not translate into a survival benefit. Purpose: To evaluate the impact on the disease-free survival (DFS) and overall survival (OS) of patients with stages II or III rectal cancer of postoperative RT and concomitant bolus FU administration alone or with additional chemotherapy using FU and high-dose LV. Patients and methods: From October 1989 until February 1997, 220 patients were randomized postoperatively to receive either one cycle of chemotherapy with FU (600 mg/m(2)/week x 6 followed by a two-week rest) and leucovorin (LV, 500 mg/m(2)/week x 6 as a two-hour infusion) followed by pelvic RT with concomitant FU (400 mg/m(2)) as a rapid intravenous injection during the first three and last three days of RT, and three more cycles of the same chemotherapy with FU and LV (standard, group A, 111 patients) or pelvic RT with concomitant FU only (experimental, group B, 109 patients). Results: As of August 1998, after a median follow-up of 4.9 years, there was no significant difference in either three-year DFS (Group A, 70.3%; group B, 68.2%, P = 0.53) or OS (group A, 77%; group B, 73.3%, P = 0.75). Cox multivariate analysis revealed stage of disease, number of infiltrated nodes, tumor grade, presence of regional implants and perforation to be significant prognostic factors. The incidence of severe side effects was significantly higher in the patients in group A than in those in group B (32.4% vs. 4.6%, P < 0.0001). Conclusions: The incorporation of additional chemotherapy with FU and LV into postoperative concomitant RT and bolus infusion of FU does not offer a greater than or equal to 10% three-year survival benefit over that of concomitant RT and bolus infusion of FU, and significantly increases toxicity in patients with stages II or III rectal cancer