Synthesis Of Sidechain Liquid Crystal Polymers For Nonlinear Optics

Invited PaperSynthesis of sidechain liquid crystal polymers for nonlinear opticsAnselm C. Griffin, Amjad M. Bhatti and Robert S.L. HungDepartments of Chemistry and Polymer Science, University of Southern MississippiBox 5043 Southern Station, Hattiesburg, Mississippi 39406AbstractSide chain liquid crystalline polymers offer unique advantages as a new class oforganic materials with potential for nonlinear optical (nlo) response. Design andsynthesis of a series of nitroaromatic side chain liquid crystalline polyesters wascarried out employing the concept of having the nitroaromatic species serve concomitant-ly as both the nonlinear optical chromophore and as the (only) liquid crystalline moietyin the polymer.IntroductionIn the design and synthesis of nonlinear optical materials, organic liquidcrystalline polymers offer numerous advantages over other material types. Each of thedescriptors- organic, liquid crystalline, and polymeric- brings an extra meritoriousquality to bear on the problem. As organic materials they haye3a potentially largenonlinear response with pi- electron charge- correlated states. The presence of pi-donor and pi- acceptor groups conjugated through an extended molecular framework isconducive to very rapid response times upon pi- electronic excitation. They areresistant to damage by powerful lasers and have large dipole moment changes upon pi-electronic excitation. In addition being organic materials there is a great variety ofmolecular structures possible to optimize No response.As liquid crystalline (mesogenic) materials these side chain polymers are easily andefficiently oriented by electromagnetic fields both with respect to their director anddipolar orientation. They possess long (and short) range positional order throughcooperative molecular orientation even in the quiescent liquid crystalline phase.Electric poling of the liquid crystalline (aligned) phase and subsequent freezing ofthis orientation into a glassy solid state retains molecular and dipolar orientation.As polymers they have advantages such as improved mechanical strength,processability, and the possibility of forming thin films of high optical quality. Inaddition the polymeric state allows the freezing of liquid crystalline orientation bysolidification of a liquid crystalline phase into a glassy phase on cooling.Molecular DesignFor efficient second harmonic generation, aromatic4nitro compounds are routinelyemployed in small molecule nonlinear optical materials. Although not the onlysynthetically accesible pi- acceptor group, nitro seems to be somewhat unique in itsability to enhance second harmonic generation and is the pi- acceptor of choice. As pi-donor nitrogen and oxygen both find use in the design of small molecule nonlinearoptical compounds with nitrogen being the more common. Oxygen is certainly acceptableas a beginning point and is more convenient for incorporation into a liquid crystallinepolymer. (We are currently also pursuing nitrogen as pi -donor in our polymers.)Liquid crystalline compounds are characterized by a large axial ratio which isoften realized by polar, rod -like chemical structures having para- substituted aromatic