Hari Singh Nalwa

A Novel Preparation Method of Organic Microcrystals

Organic microcrystals ranging from several tens nm to µm in size of several chromophores were successfully prepared by simply dispersing ethanol solutions of compounds into stirred water, i.e. by a reprecipitation method. The size of microcrystals was found to depend on concentration of ethanol solutions, dispersing conditions, temperature and so on.

RECENT DEVELOPMENTS IN FERROELECTRIC POLYMERS

Abstract For decades, organic polymers were important only as insulators in the electornic industries. Research on electroactive properties was confined mainly to inorganic materials such as barium titanate, lead zirconate titanate, quartz, silicon, cadmium sulfide, and gallium arsenide. In the 198s, organic polymers emerged as a new class of electronic materials and challenged earlier technologies based on inorganic materials. The superiority of organic polymeric materials lies in their versatility and in the possibility of tailoring materials for a particualr end-use. Organic polymers offer several advantages over inorganic materials: They are lighweight, flexible, and tough. They can be obtained in the form of ultrathin films, fiber, and even liquid crystals. They can be easily transformed into the desired configuartion. Their physical properties can be controlled over a wide range by appropriate chemical modifications. Some of them are biocompatible and hence can be utilized in biological processes.

Third-order non-linear optical properties of donor- and acceptor-substituted metallophthalocyanines

Several types of metallophthalocyanines have been synthesized to establish the relationship between molecular structure and third-order optical non-linearity. Third-order non-linear optical susceptibilities χ(3) of phthalocyanine (Pc) containing central metal atoms oxovanadium, copper, nickel, iron and cobalt, and peripheral substitutes were measured by the third-harmonic generation technique. The χ(3)(−3ω; ω, ω, ω) values of VOPc(NH2)4, CuPc(NH2)4, NiPc(NH2)4 and FePc(COOH)4 were found to be 6.3 × 10−12, 2.0 × 10−12, 1.63 × 10−12 and 1.18 × 10−12 e.s.u. respectively at 2.1 μm. Although the donor and acceptor substituents have a dilution effect, χ(3) was still larger than that of the corresponding unsubstituted phthalocyanines, with the exception of metallophthalocyanines with an axial group where the peripheral substituents required for processability or otherwise are not beneficial for optical non-linearity. In addition, the magnitude of χ(3)(−3ω; ω, ω, ω) was found to be influenced by different functional groups substituted in cobalt phthalocyanine. The effect of various peripheral substituents on the magnitude of the third-order non-linear optical susceptibility is discussed.

Third-order optical nonlinearities of tetrakis-n-pentoxy carbonyl metallo-naphthalocyanines

Abstract Third-order nonlinear optical susceptibilities χ (3) of tetrakis- n -pentoxy-carbonyl (COO-C 5 H 11 ) naphthalocyanines containing central metal atom oxovanadium (VO), copper (Cu), zinc (Zn), palladium (Pd), and nickel (Ni) were measured by the third-harmonic generation (THG) technique. The metal atom substitution influences the third-order nonlinear optical susceptibility but the variation is less dramatic; oxovanadium cation was found to be the most effective. The order of χ (3) (−3ω; ω, ω, ω) at 2.1 μm wavelength was VONc > CuNc > ZnNc > NiNc > PdNc. The χ (3) (−3ω; ω, ω, ω) for VONc is 8.6 × 10 −11 esu, more than an order of magnitude larger than those of CuNc, ZnNc, PdNc, and NiNc compounds. A comparative study of optical and non-linear optical properties of processable metallo-naphthalocyanines is presented.

Large Third-order Optical Non-linearities of Spin-cast Thin Films of Novel Metallo-naphthalocyanines

Third-order non-linear optical susceptibilities of silicon, germanium, tin, aluminum, manganese and vanadium naphthalocyanine derivatives measured by third-harmonic generation technique from 1.05 μm to 2.1 μm are reported. Our results show the dependence of χ(3)(–3ω;ω,ω,ω) values on the variation of the central metal atom associated to different metal-ligand charge-transfer interactions. Silicon naphthalocyanines (SiNc) showed the χ(3)(–3ω;ω,ω,ω) values on the order of 10−10 esu at 1.5 μm, this does not appear to be due to a three-photon resonance, instead the increase in χ(3) could be the two-photon resonance lying just above the one-photon state. Degenerate four-wave mixing measurements on thin films of a SiNc derivative yielded the χ(3)(–3ω;ω,ω,ω) value as high as 5.0 × 10−7 esu at 800 nm, enhanced by one-photon resonance near the Q-band peak region. The figure of merit χ(3)/α at 800 nm is quite large corresponding to 4.2 × 10−12 esu cm. Effect of the metal atoms and plausible resonance contribution accounting for large third-order optical non-linearity of naphthalocyanines are discussed.

Third‐order nonlinear optical properties of processable polyazine thin films

The third‐order nonlinear optical susceptibility χ(3)(−3ω;ω,ω,ω) of propylmethylpolyazine thin films as measured by the third harmonic generation is reported. The χ(3)(−3ω;ω,ω,ω) values of propylmethylpolyazine that possess statistically alternating methyl and propyl substituents on the carbon atoms along the conjugated chain were found to be 8.0×10−12 esu off resonance at 1.5 μm fundamental radiation. The interesting third‐order nonlinear properties presumably originate from the unique periodicity of the main π‐electron‐conjugated polyazine backbone.

Third-order nonlinear optical properties of pyridine- and ferrocene-containing polyazines

Abstract Third-order nonlinear optical susceptibilities χ(3) of pyridine and ferrocene-containing polyazines have been measured by the third harmonic generation in the wavelength region 1.0 μm to 1.95 μm. The χ(3)(−3ω; ω, ω, ω) values of 7.75 × 10−12 esu at 1.5 μm for pyridine-containing polyazine and 2.23 × 10−11 esu at 1.8 μm for ferrocene-containing polyazine were observed. Third order optical nonlinearities of polyazines with and without aromatic units in the π-conjugated backbone are compared to develop a structure property relationship.

Fabrication and spectroscopic characterization of organic nanocrystals

Publisher Summary This chapter reviews the preparation, spectroscopic characterization, and linear and nonlinear optical properties of organic molecular and polymeric nano- and microcrystals. Nanostructured materials can be prepared by a variety of techniques, such as sputtering, gas evaporation, chemical vapor deposition, pyrolysis, anodic etching, mechanical attrition, plasma deposition, ultrasonification, thermolysis, photolysis, ion implantation, sol-gel method, electrochemical method, and high-energy milling. Organic nanocrystals ranging from a few tens of nanometers to micrometers in size can be prepared successfully by this solvent-borne technique. The size of the nanocrystals can be controlled by varying experimental conditions, such as solution concentration, temperature, surfactant, and mixing methodology. The possibility of preparing PDAs with controlled molecular weight was qualitatively demonstrated. In the case of microcrystals of 14-8ADA, size-dependent conversion was found and can be explained by the looseness or thermal vibrations of the crystal lattice. Further efforts to increase dispersion concentration will establish nanocrystallization as a new technique for incorporating crystalline compounds into large isotropic media. In addition, microcrystals composed of single chains of conjugated polymers from one end to the other could be useful for making molecular devices as well.

POLYQUINOLINE/BISMALEIMIDE COMPOSITES AS HIGH-TEMPERATURE-RESISTANT MATERIALS

Polyquinoline/bismaleimide composites containing bismaleimide contents between 5 and 50 wt % have been investigated as high-temperature-resistant materials. The effect of bismaleimide loading on thermal, mechanical, and dielectric properties of polyquinoline has been reported. The dielectric constant of composite thin films was found to vary between 2.90 and 3.10 with the loss tangent ranging between 0.004 and 0.007, depending upon the incorporated bismaleimide content. The thermal, mechanical, and dielectric properties of polyquinoline/bismaleimide composite thin films are discussed.

Third-order nonlinear optical properties of polyazine derivatives

Abstract Experimental and theoretical results on third-order nonlinear optical susceptibility χ (3) and molecular hyperpolarizability (γ) of oligomeric and polymeric azines are reported for the first time. Molecular hyperpolarizabilities (γ) of well-defined conjugation length polyazines containing terminal electron-donating amino groups were calculated by employing the ab initio Coupled Perturbed Hartee-Fock with the 6-31G basis set. An azine monomer showed a molecular hyperpolarizability of the order of 14.7×10 −36 esu, which is the largest γ value ever reported for such a short π-electron conjugation length. Molecular hyperpolarizability increases significantly as the length of the π-electron delocalization increases. The γ values for the dimer, trimer, tetramer, and pentamer azine derivatives were 79.0×10 −36 , 228.3×10 −36 , 575.9×10 −36 , and 903.0×10 −36 esu, respectively. Third-order nonlinear optical susceptibilities χ (3) of polyazine derivatives were determined by third-harmonic generation (THG) measurements. The χ (3) (-3ω; ω, ω, ω, ω) values of an azine monomer and a pentamer were 1.39×10 −13 and 15.3×10 −13 esu, respectively. On the other hand, the χ (3) of a polyazine possessing statistically alternating methyl and propyl substituents on the carbon atoms along the chain was found to be 8.0×10 −12 esu at 1.5 μm. The interesting third-order nonlinear properties of polyazines presumably originate from the multiple quantum-well structure of the main π-electron conjugated backbone.