Octupolar nonlinearities

LeBozec and co-workers have reported nonlinear behavior in a series ofterpyri-dyl and dipicolinic acid complexes, with further studies on these complexes by Maury and co-workers [83, 84]. Their research was on new molecular materials for optoelectronics, with studies based on octupolar nonlinear optical molecules showing that molecular quadratic hyperpolarizability values were strongly influenced by the symmetry of the complexes [85]. Other studies on organic-lanthanide complexes with nonlinear optics have also reported second- and third-harmonic generation behavior with simultaneous multiphoton absorption properties [50]. Such studies have shown the importance of coordination chemistry as a versatile tool in the design of nonlinear materials. 

Beyond the classical approach, one has to consider the concept of octupolar nonlinearity in order to optimize the NLO response. Thus, it is sometimes convenient to decompose the P tensor into irreducible spherical multipolar components [27, 28]. When Kleinman symmetry applies i.e. under off-resonant conditions, the decomposition for p is as follows. 

Theoretical studies on the nonlinear optical properties of octupolar tri-i-triazines have been carried out <06MI808>. 

The primary objective of this section is to show the importance of lanthanides to nonlinear harmonic generation. This work, which is unpublished, evolved from earlier studies that have shown the correlation of nonlinear optical effects in dipolar and octupolar compounds which involve the use of one or more electron-donating groups connected via a conjugated tt-linker to an electron acceptor group. 

The search for efficient NLO-phores includes also the design of octupolar derivatives [40, 70a]. The advantages associated with this alternative class of NLO-phore include an improved nonlinearity-transparency trade-off and more facile noncentrosymmetric arrangement in the solid state due to the absence of dipolar moments. To this end, the 3-D chromophores 135 and 136 with C3 and D2 (approximate T) symmetry, respectively (Scheme 4.39), have been prepared according to classical synthetic routes [73a]. 

Keywords Nonlinear optical materials Ionic octupolar Zwitterionic Ab initio Stilbazolium ... 

In this chapter we report on some novel strategies that have been pursued to obtain efficient second-order nonlinear molecules starting from the well-known phthalocyanines. In principle, these planar centrosymmetric molecules do not present second-order activity and have been extensively studied for third-order applications. In order to induce asymmetry, two main approaches have been followed a) peripheral substitution of the macrocycle with donor and acceptor groups and b) structural modifications of the Pc core to reduce the symmetry, the resulting-noncentrosymmetric compounds (i.e. subphthalocyanines) presenting variable degrees of dipolarity/octupolarity in the nonlinear response. 

In the phthalocyanine field, the octupolar route provides additional degrees of freedom to help in the design of efficient nonlinear molecules. One of the possible methodologies to reach Pc-based octupolar architectures is the arrangement of the Pc cores into D or structures by means of attaching the macrocycles to benzene [58] or to a tetravalent atom such as phosphorus. Thus, for example, aryl trisphthalocyanine phosphonium salt (Figure 5) has been prepared and the second-order NLO response at the molecular level has been measured by HRS [59]. The jShrs values at X = 1.06/a,m(189 x 10 esu) is superior to those available for other related unsymmetrically substituted phthalocyanines with dipolar characteristics. 

In order to go even deeper into the understanding of the nonlinear optical properties of subphthalocyanines in solution, and in particular of its mixed octupolar/dipolar character, HRS and EFISH experimental studies were performed on four structural isomers of trinitro chlorosubphthalocyanine la-d (Figure 12). These studies were reinforced by theoretical calculation at high to very high level [70]. 

A possible means to increase the nonlinearity without sacrificing transparency is to replace the dipolar with an octupolar composition [16], so more recently we have explored trigonally-branched metal alkynyl complexes as potential NLO materials,... 

This work provides a relatively comprehensive review of studies involving ruthenium coordination and organometallic complexes as nonlinear optical (NLO) compounds/materials, including both quadratic (second-order) and cubic (third-order) effects, as well as dipolar and octupolar chromophores. Such complexes can display very large molecular NLO responses, as characterised by hyperpolarizabilities, and bulk effects such as second harmonic generation have also been observed in some instances. The great diversity of ruthenium chemistry provides an unparalleled variety of chromophoric structures, and facile Ru" —> Ru" redox processes can allow reversible and very effective switching of both quadratic and cubic NLO effects... 

Zyss, J.. Dhenaut. C.. Chauvan, T., Ledoux, I. Quadratic nonlinear susceptibihty of octupolar chiral ions. Chem. Phys. Lett. 206, 409-414 (1993)... 

Dhenaut, C., Ledoux, I., Samuel, I.D.W., Zyss, J., Bourgault, M.. Le Bozec, H. Chiral metal complexes with large octupolar optical nonlinearities. Nature 374. 339-342 (1995)... 

Houbrechts, S., Wada, T., Sasabe, H., Morrall, J.P.L., Whittall, LR., McDonagh, A.M., Humphrey, M.G., Persoons, A. Novel organomctals for nonlinear optics octupolar alkynylmetal complexes. Mol. Cryst. Liq. Cryst. Science Tech.. Sect. B Nonlinear Opt. 22, 165-168 (1999)... 

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