Conjugated polymers, third-order susceptibility

Polyarylenevinylene (PAV) expressed by the chemical formula of [-Ar-CH=CH-]n, where Ar is an arylene ring, is an attractive n-conjugated polymer family because of the following features (i) by the thermal conversion from polyelectrolyte or organic-solvent-soluble precursors, one can obtain the PAV films which have large third-order susceptibility and excellent optical quality, and (ii) the band gap can be adjusted by suitable selection of the arylene rings. 

Laser lights with short wavelengths are important in optical data processing, and conjugated polymers are expected to be useful materials for their generation. Optical third-harmonic generation measurements have been performed for poly(phenylacetylenes) " The magnitudes of the third-order susceptibility were 7 x 10 esu for poly(phenylacetylene) and in the order of 10 -10" esu for its derivatives . ... 

The shapes of the orbitals for the other molecules of Fig. 2 are shown in Fig. 5. Molecules B and C of Fig. 2 have donors at the both ends. For B and C in Fig. 5, HOMO-1 spreads somewhat into extremities, and electron-hole symmetry does not hold either between HOMO and LUMO nor between HOMO-1 and LUMO-i-1. These shapes are beneficial for %(3). However, the orbital HOMO does not spread into the extremities. This means that transition from HOMO-1 to HOMO does not have so much moment as that of SBA. The orbital shape for molecules B and C is not so good as SBA. With respect to the orbitals of the molecules E and F, both HOMO and HOMO-1 spread, and the electron-hole symmetry does not hold. These are beneficial for Molecules D, E and F, which have amino at both ends and have nitrogen atoms at the X positions of Fig. 1, are expected to have large third order susceptibilities. These results correspond well with the experimental fact that their measured X is large and as good as conjugated polymers. The x values of SBA and SBAC, which is dichloro SBA, are 1.0 X 10 esu and 1.3 x lO" esu. The X( ) of PU-STAD is 1.5 X 10 esu, which is a derivative of molecule F in Fig. 2. 

Interestingly, cr-conjugated polymers such as polysilanes (see Chart 3.3) also exhibit remarkably large third-order susceptibilities... 

The polymer had a A ,ax of 546 nm in the solid state and 462 nm in a chloroform solution, and an of 1 X 1 O. The Amax values indicated that poly(43) was highly conjugated. Conductivity of iodine-doped polymer was 100 S cm and the doped state would be stable for 10-20 years. This high doping stability was explained in terms of free-volume theory by Inganas [42]. The third order susceptibility (3oj co, CO, (o) was measured to be (4.3 1) x 10 esu on an... 

Linearly expanded porphyrin oligomers and polymers with enhanced third-order susceptibilities have also been reported. In solution the conjugation lengths of single-strand chains can be strongly limited by rotation and disorder. The self-assembly of double-strand ladder complexes of porphyrin butadiyne oligomers 21 achieves amplification in the nonlinearity... 

Conjugated polymers and polymeric systems like polydiacetylene, polyacetylene, poly(arylene vinylene), polythiophene, polyaniline, polyimides, and polyesters have been widely studied for their third-order susceptibility. " In addition to pure polymeric systems, investigations of third-order nonU-neatity have also been carried out on small molecules, polymer-doped systems, hybrid inorganic-organic systems, and organometallic complexes. ... 

Conjugated polymers satisfy these requirements and have thus emerged as the most widely studied materials for their susceptability. Some of the examples of conjugated polymers, that have been studied for their third order NLO properties, are polydiacetylenes, poly-p-phenylenevinylenes and polythiophenes. However, CVD has only been used in the case of poly-p-phenylenevinylenes (PPV) [section 3.4], although values have not been reported. An excellent review of third order nonlinear optical properties of PPV in general, can be found in literature. Recently, McElvain et al. ° reported the values of CVD polyazomethines to be... 

Poly(ferrocenylene vinylene) derivatives 68 with values of 3,000-10,000 and polydispersities of ca. 2.2-2.8 (determined by GPG) were synthesized in 1995 in high yields via a titanium-induced McMurry coupling reaction of the corresponding alkylferrocenyl carbaldehyde monomers (Equation (26)). " Gharacterization of these soluble polymers by NMR and IR revealed the presence of trans-Yinylcnc units. The UV-VIS spectra of the polymers are similar to those of the monomers and this indicates a fairly localized electronic structure in the former. The relatively limited electron localization is also reflected in the electrical and optical properties. For example, the values for iodine-doped conductivity a= 10 Scm ) and non-linear third-order optical susceptibility (x = 1-4 x 10 esu) are lower than those of linear conjugated polymers such as poly(l,4-phenylene-vinylene) (a = 2.5x 10 Scm" = 8 X 10 esu). 

The polymers having delocalized r-electron in the main chain have been expected to possess extremely large third-order optical susceptibility.However, such an extended jr-electron conjugation generally rendered the polymers insoluble and infusible as well, which has seriously limited the fabrication of practical NLO devices. Recently, it was reported that the third-order nonlinear optical properties of poly(l,6-heptadiyne)s which were environmentally stable, soluble, and processable. The third-order optical nonlinearities of poly(l,6-heptadiyne)s bearing NLO active chomophores were evaluated for the first time. The third-order nonlinear susceptibility... 

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