Robinson Anandakathir

Large concentration-dependent nonlinear optical responses of starburst diphenylaminofluorenocarbonyl methano[60]fullerene pentads

We demonstrate an approach toward the design of starburst C60-keto-DPAF assembly by applying a starburst macromolecular configuration with C60 as the core center, which is encapsulated by multiple bulky groups leading to the increase of intermolecular separation and aggregation barrier. Molecular compositions of the resulting C60(>DPAF-C9)2 triad and C60(>DPAF-C9)4 pentads were clearly confirmed by MALDI-MS (positive ion) detection of protonated molecular mass ions. Both C60(>DPAF-C9)22 and C60(>DPAF-C9)4 (structural isomers, 3a and 3b) exhibited nonlinear optical transmittance reduction responses in the femtosecond (fs) region with a lower transmittance value for the latter at the high laser power above 80 GW cm−2. This was attributed to the larger fs 2PA cross-section values of 3a and 3b than that of 2 at the same concentration and, apparently, correlated to a higher number of DPAF-C9 subunits in the structure of 3. As the concentration was decreased to 10−4 M, a clear monotonous increase of the σ2 value change (Δσ2) from 13.9, 33.2, to 48.1 and 68.2 × 10−48 cm4 s photon−1 molecule−1 (or 6820 GM for the latter) for the structural variation from the monoadduct 1, bisadduct 2, to tetraadducts 3b and 3a, respectively, was observed. We interpreted the concentration-dependent phenomenon as being due to the high tendency of fullerene-DPAF chromophores to form nanoscale aggregates at concentrations above 10−3 M. We also proposed that starburst structures, as exemplified by C60(>DPAF-C9)4, in a multipolar arrangement resembling encapsulation of C60 by DPAF-C9 pendants, provide a useful means to increase the degree of molecular dispersion and maintain high nonlinear optical efficiency.

Tuning oxygen permeability in azobenzene-containing side-chain liquid crystalline polymers

A series of poly[4-(4-cyanoazobenzene-4′-oxy)alkyl methacrylate]s, side-chain liquid crystalline polymers (azoLCPs) were synthesized with methylene groups as spacers varying from 5 to 12. The thermal properties and phase transition temperatures of the polymers were characterized with differential scanning calorimetry and polarized optical microscopy and a relationship between spacer lengths, glass transition (Tg) and clearing temperatures (Tc) of the polymers was established. The Tg decreased with increasing spacer length, while the Tc exibited an odd–even effect with varying spacer length. X-ray diffraction was used to determine the degree crystallinity above and below Tc. The azoLCPs exhibited smectic ↔ nematic ↔ isotropic phases. Increasing crystallinity correlated linearly with decreasing gas permeability as measured using an oxygen permeation analyzer, which was used to measure the films’ permeability to oxygen (O2) gas. Switching of the azoLCPs from a liquid crystalline to an isotropic state was accomplished by heating the films above their Tc, which resulted in at least a 10-fold increase in the O2 permeability coefficient (PO2). Increasing the methylene spacer length of the azoLCP had little or no effect on gas permeability however it did decrease the Tc, allowing fine control of the temperature at which the switch in PO2 takes place by tuning the mesophase between nematic and isotropic states.

Fabrication of Dye-sensitized Solar Cells and Fluorescence Quenching Study Using Thiophene Based Copolymers

Photovoltaic performance of dye sensitized solar cells fabricated with a commercially available thiophene based copolymer was investigated. Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)], a highly soluble polythiophene, was used as a sensitizer. An open-circuit voltage of 0.64 V and a short-circuit current density of 0.36 mA/cm2 were measured. The incident photon to current conversion efficiency for the polymer was measured. Fluorescence from the other polythiophene, poly(3,3′-didodecyl quarter thiophene) was found to be quenched when blended with phenyl C61 butyric acid methyl ester (PCBM) (1:1 wt ratio), indicating the charge transfer from the conjugated polymer to PCBM.

Stilbene-Based Fluorescent Sensor for Detection of Organophosphorus Warfare Nerve Agents

In this paper, we report the synthesis of stilbene-based fluorophore, 3,4-dihydroxy-4′-aminostilbene (DHAS) for the detection of chemical warfare agents such as organophosphorus nerve gases. DHAS was characterized by various spectroscopic methods and grafted on to electrospun nanofibers. The interaction of DHAS with nerve agents simulant, diethyl chlorophosphate (DCP) was investigated in solution and vapor phase by fluorescence spectroscopy.

Novel fluorescent polymer for trace explosive detection

Conjugated polymer based on thiophene having high reasonably quantum yield has been synthesized. A bulky butoxycarbonylmethyl urethane moiety as a side group is attached to the polymer backbone. The polymer is characterized by 1H NMR, FT-IR, UV-Visible absorption and fluorescence spectroscopy. Thin film fluorescence sensors based on this material were fabricated through a spin-coating technique. The polymer showed rapid fluorescence quenching upon exposure of nitroaromatics and is capable or detecting 2,4-dinitrotolune (DNT) and 2,4,6-trinitrotolune (TNT) vapors at sub parts per billion levels.

Synthesis and characterization of hexadecaaniline-grafted comb-like poly(maleic acid-alt-1 -octadecene)

Highly soluble hexadecaaniline (A16)‐grafted polyolefin derivatives poly(maleic acid‐hexadecaanilinamide‐alt‐1‐octadecene) (PMAO‐A16) in a comb‐like configuration with alternate linear hexadecane and A16 side‐chains were synthesized and characterized. The structure of PMAO‐A16 was substantiated by infrared and UV‐Vis spectra showing high intensity of characteristic absorption peaks corresponding to a high degree of A16 attachments. Covalent grafting of hexadecaanilines onto the polymer backbone of PMAO was confirmed by the detection of a new amide [–(C˭O)–NH–] absorption band appearing at 1661 cm−1 accompanied with the full disappearance of anhydride carbonyl absorptions. Based on the comparison between TGA profiles of PMAO‐A16 and hexadecaaniline, a 12.5% wt loss at 365–600°C was accounted for full elimination of aliphatic side‐chains that matches approximately with the weight percentage of total hexadecane arms (12.7%). The data revealed a nearly quantitative yield of A16 grafting on anhydride subunits leading to complete conversion of PMAO into PMAO‐A16. Furthermore, preliminary 1H‐NMR study of PMAO‐A16 indicated its capability to undergo molecular self‐assembly in DMSO where A16s were dispersed in the solvent phase with hexadecane side‐chains located in a phase‐separated domain.

A Simple Technique for Submicron Scale Patterning of Silver Using Visible Light Interference

A novel, one step and simple methodology for the fabrication of submicron scale silver patterns is demonstrated. The photosensitivity of an organic silver salt has been utilized for this purpose of fabrication. The silver-organometallic compound is converted to metallic silver selectively in the illuminated regions. Surface morphology was studied by scanning electron microscopy (SEM). Energy dispersion spectroscopy (EDS) shows the presence of silver in the developed film. X-ray photoelectron spectroscopy (XPS) confirms the formation of metallic silver. Feature sizes of the order of 200 nm have been achieved using this technique.

Starburst Encapsulation of C60 by Multiple Hindered Two‐Photon Absorptive Diphenylaminodialkylfluorene Arms

The efficiency of nonlinear optical responses is highly influenced by the state of molecular assembly and aggregation. Introduction of bulky alkyl groups on an organic chromophore enhances its molecular dispersion in solution and, thus, simultaneous multiphoton absorptivity. Accordingly, sterically hindered fullerenyl chromophore triads C60(>DPAF‐C9)2 and pentads C60(>DPAF‐C9)4 were designed and synthesized for the use as optical limiting materials. Photoinduced molecular polarization is a crucial parameter for the enhancement of nonlinear optical responses. For this purpose, we synthesized these conjugates by attaching one or several diphenylaminofluorene moieties to methano[60]fullerene via a covalent keto linkage. The motif increases electronic interactions of DPAF‐Cn rings with the C60 cage in close vicinity. Synthesis of C60(>DPAF‐C9)n (n=1, 2, or 4) was carried out by a four‐step reaction procedure starting from 2‐bromofluorene via dialkylation at C9 position of fluorine ring and followed by attachment of a diphenylamino group at C2 position of dialkylated fluorene, acylation of α‐bromoacetyl group at C7 position of diphenylaminodialkylfluorene, and subsequent Bingel cyclopropanation of DPAF‐acyl bromide with C60. All C60‐DPAF‐Cn derivatives were fully characterized by various spectroscopic analyses. Molecular compositions of these conjugates were clearly confirmed by MALDI‐TOF mass spectra. A method of relative proton counting was applied on the samples of complex C60(>DPAF‐C9)2 and C60(>DPAF‐C9)4 derivatives using DABCO as an internal standard for the calibration of proton integration in 1H‐NMR spectroscopic analyses.

Alternative Synthesis of C60‐Diphenylaminofluorene Derivatives for Nonlinear Photonic Applications: Method of Preparation and Characterization

An alternative synthetic route for the preparation of key intermediate synthons 7‐α‐bromoacetyl‐2‐diphenylaminofluorene (α‐BrDPAF‐H) and 7‐α‐bromoacetyl‐9,9‐dialkyl‐2‐diphenylaminofluorene (α‐BrDPAF‐Cn) was demonstrated. The latter reactions involved the first step of dialkylation of 2‐bromofluorene at C9 position of the fluorene moiety, the second step of a diphenylamino group attachment at C2 position of the resulting dialkylfluorene, and the third step of Friedel‐Craft acylation of α‐bromoacetyl group at C7 position of dialkylated diphenylaminofluorene. From the intermediates α‐BrDPAF‐H and α‐BrDPAF‐Cn, a series of C60‐keto‐DPAF nanostructures, such as the fullerene monoadducts C60(>DPAF‐H) and C60(>DPAF‐Cn), where n is 2, 4, or 10, were synthesized in a reasonable yield. Molecular mass ions of the dyads C60(>DPAF‐H), C60(>DPAF‐C2), C60(>DPAF‐C4), and C60(>DPAF‐C10) were clearly detected in positive ion matrix‐assisted laser desorption ionization mass spectrum (MALDI–MS) that confirmed the composition mass of each compound synthesized.

Sensory Response and Two-Photon-Fluorescence Study of Regioregular Polythiophene Nanoparticles

Nanoparticles of a regioregular and soluble polythiophene were fabricated by mini-emulsion technique. The fabricated nanoparticles were characterized by optical spectroscopy and dynamic light scattering. The fluorescence quenching of these fabricated nanoparticles with 2,4-dinitrotolune (DNT) in aqueous and organic solutions was investigated. Significant fluorescence quenching was observed. The Stern-Volmer constants were determined to be higher than that of the bulk polymer in solution, indicating that the nanoparticles provide better sensitivity in DNT sensing. Strong two-photon-induced fluorescence was measured from these nanoparticles.