Abhijit Sarkar

Novel polydiacetylenes for optical materials: beyond the conventional polydiacetylenes

Polydiacetylenes (PDAs) are of great interest due to their π-conjugated backbone related properties, such as linear and nonlinear optical properties. As a consequence, design and preparation of PDAs assume enormous importance today. This review deals with one aspect of PDA, that is, design and preparation of the diacetylene monomers and their polymers, PDAs having conjugated side groups. The aim of such an investigation is to produce materials which exhibit large third-order nonlinear optical susceptibilities and which consequently, can be obtained in a form to be customised for optical devices.

A CONVENIENT SYNTHESIS OF AROMATIC-RING-SUBSTITUTED DIACETYLENES

A simple and convenient way for the preparation of symmetrical diarylbutadiynes 5 with improved yields is reported (Scheme 2). The reaction time is drastically reduced using this method compared to previously reported procedures.

Heteroaryl functionalised diacetylenes: preparation and solid-state reactivity

Diacetylenes are known to undergo solid-state topochemical polymerisation to give polydiacetylenes. The reactivity of the monomer is controlled by the arrangement of the molecules in the crystal lattice wherein certain parametric conditions must be met for 1,4-addition to proceed. In the present paper, we investigated the structure–reactivity relationship of a class of diacetylene monomers. Heteroaryl moieties such as thiophene, benzo[b]thiophene and quinoline as one or both directly bound side groups of a diacetylene backbone were used. Thus various symmetrical as well as unsymmetrical diacetylenes were prepared and characterised. The solid-state polymerisation behaviour of these diacetylenes was studied in the light of their single-crystal X-ray structure. It was found that in order to react in the solid state, the diacetylenes must have the required lattice parameters. However, even when the required lattice parameters are met, the diacetylene monomers do not necessarily undergo solid-state 1,4-addition polymerisation, implying the existence of further controlling factors to determine reactivity.

Improved third-order nonlinear optical properties of polydiacetylene derivatives

In order to improve third-order nonlinear optical properties of polydiacetylenes (PDAs), we have been investigating modification of PDA structures. From the point of view of PDA molecular design, three series of monomers for ladder- type PDAs were prepared. Oligoyne monomers having more than five conjugated acetylenes gave polymers which showed broad bands at longer wavelength than their excitonic absorption bands. This implies production of (pi) -conjugated ladder polymer having two PDA backbones linked by acetylenic groups in each repeating unit. From monomers with two butadiynes connected by an alkylene group, ladder-type PDAs were obtained when the carbon number of the alkylene group is more than four. However, monomers with two butadiynes connected by an arylene group gave only single-chain PDAs. From the point of view of PDA morphological engineering, PDA microcrystal water dispersions were applied to evaluate nonlinear optical susceptibilities by z-scan method together with PDA thin films. High-density microcrystal deposited film prepared by layer-by-layer deposition technique showed more than three orders of magnitude enhanced (chi) (3) than the dispersion state. Polycrystalline thin films compose of ladder-type PDAs were also found to have large (chi) (3). The quite large nonlinear refractive index of -41 cm2/GW was attained just near excitonic absorption maximum for one of the polymers.

SOLID STATE POLYMERIZATION OF DIACETYLENES WITH π-CONJUGATING SUBSTITUENTS FOR THIRD-ORDER NONLINEAR OPTICAL PROPERTIES

Topochemical reactivity of some conjugated diacetylenes with πconjugating substituents upon exposure to γ-radiation, UV-visible radiation and upon thermal annealing is described. Few of the diacetylenes undergo solid state polymerization to provide corresponding polydiacetylenes. The reflectance spectra of the resultant polydiacetylenes showed significant backbone-sidegroup electron coupling. The third-order nonlinear susceptibility of two of the polydiacetylenes obtained were evaluated using Makers fringe technique. The third-order nonlinear susceptibility, χ(3) value obtained for the two samples were in the order of 10−11 esu for the fully polymerized polydiacetylene films. These values are comparable to the values reported previously for PDAs with aromatic sidegroups.

Diacetylenes with Formally Conjugated Sidegroups : Precursors to Liquid Crystalline Polymers ?

Recently, there has been hectic research activity in search of organic materials which show promising non-linear optical properties.1–5 Polydiacetylenes (PDAs) attracted special attention as a candidate material for third order non-linear optical properties (χ 3). This interest is due to the fact that PDAs show large third order non-linear optical susceptibility, with a fast response time. Additional supporting and important features of PDAs are that they can be obtained in single crystal form or LB films and it is possible to tune their optical properties through “Molecular and crystal engineering” through a variety of sidegroups.

Preparation of asymmetrically substituted diacetylenes containing heteroaryl sidegroups and their solid state polymerisation

Abstract Preparation of a series of asymmetrically substituted diacetylenes containing thienyl moiety or quinolyl moiety as one of the sidegroups directly bound to the diacetylene backbone and a urethane derivative as the other sidegroup were carried out and their solid state polymerisation was investigated.

Polydiacetylenes Having Quinolyl Sidegroups for Third-order Nonlinear Optical Materials

The solid-state polymerization of diacetylenes (DAs) [1, 2] to polydiacetylenes (PDAs) has long been investigated to obtain materials for photonics. PDA, being a quasi-one-dimensional ð-conjugated system, shows non-linear optical (NLO) properties as well as important chromic properties arising due to conjugated systems [3±5]. However, due to our limited knowledge of the physics and chemistry of DA polymerization and the relationship of polymer structure with the NLO properties, it is not easy to design a perfect DA, which on polymerization should yield PDA with all the expected properties. In our laboratory, we have been preparing DAs with an aim to get good third-order NLO materials [6]. One class of DAs, which we have been investigating, consists of heteroaryl moieties directly bound to the DA stem [7]. Here, we report a series of quinoline-containing novel DAs that topochemically polymerize to give PDAs, 1.

1,8-Bis(2-thienyl)-1,3,5,7-octatetrayne

Although the title compound, C16H6S2, has parallel stacking of the molecules along the c axis, and the intermolecular distances between the C atoms of the triple bonds are within the required distance of 4.0 A, these atoms are not positioned to form a 1,4-addition polymer. The thiophene rings are disordered over two different orientations, with 87:13 relative occupancies. The molecule has \overline{1} symmetry.

Crystal structure of 1-(2-thienyl)penta-1,3-diyne-5-ol

1-(2-thienyl)penta-1,3-diyne-5-ol (TD-OL), C9H6OS, M-r = 162.2 orthorhombic, Pbcn, a = 22.660(2), b = 7.482(1), c = 9.379(1) Angstrom, V = 1590.0(3) Angstrom(3); Z = 8, D-c = 1.355 Mgm(-3), mu = 0.338 mm(-1), F(000) = 672, lambda(MoKalpha) = 0.71069 Angstrom, final R and wR2 are 0.044 and 0.1266 respectively.