Dipolar chromophore structure

The covalent assembly of functional Jt-systems is a general synthetic principle and in some cases they can even be achieved in a multi-component fashion. One of the most impressive examples is the very elegant access to covalently linked donor-fullerene arrangements by 1,3-dipolar cycloadditions with in situ-generated azomethine ylids [59]. However, here only the multi-component de novo synthesis of the chromophore structures will be considered. The major developments have been achieved in condensation-based and cross-coupling strategies. 

Fig. 11.6 Structures and NLO properties of two ferrocene-based dipolar chromophores [51, 52].

Source Used for Determination of TPA Data for a Series of Dipolar Chromophores with the General Structures Taken in Various Solvents Using Different Methods... 

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... 

Semiempirical quantum mechanical calculations of first hyperpolarizabilities, /3, for such dipolar chromophores have proved quite successful in predicting trends in structure-function relationships [7,8]. Marder et al. [33-37], in particular, have been successful in using simple (two-level) calculations to guide synthetic efforts aimed at optimizing molecular hyperpolarizabilities. Recall that within the framework of a two-state calculation, the hyperpolarizability, jS, depends on the transition dipole, /Ag, the difference between the dipole moments of the ground and excited state, /i-ec /Agg and the optical gap, Explicitly,... 

Fohlisch99 reported a remarkable dependence of the electron spectra of quino-cyclopropenes on their structure. As shown in Table 9, the merocyanine-like quino-cyclopropenes show positive solvatochromy when they contain an anthraquinonoid chromophore (198), but negative solvatochromy when they contain a benzoquino-noid system (199). This can be interpreted in terms of a markedly increased participation of dipolar resonance forms in the ground state of the benzoquinonoid 199 compared to the anthraquinonoid 198. From the dipole moment of 198 (9.4 D99 ) the dipolar contribution was estimated to be in the range of 23%. 

When the oxonol backbone is stabilized by the addition of other, usually ring, structures, the new family is given a more descriptive name. In the case of the chromophores of biological vision, one choice would be to describe these molecules as retinienes to reflect their importance in the retina and their conjugated carbon backbone. However, the historical label retinenes has been used to describe a very similar family of monopolar materials with a conjugated carbon backbone that can be considered chromogens but not chromophores. In addition, the dipolar nature of the chromophores is not indicated. 

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

Second order optical nonlinearity can be induced in polymeric systems containing dipolar (donor-acceptor) chromophores. The chromophore can be a molecular species attached to the host chain or it can be incorporated in the polymeric structure itself. In general, a good chromophore has an electron donating group connected to an electron... 

A complex multi-chromophoric system comprises the purple membrane patches from Halobacterium salinarium. These patches are composed of about 3000 bacter-iorhodopsin proteins. The hyperpolarizability of solubilized monomeric bacterio-rhodopsin was measured by HRS and found to be 2100 x 10 esu at 1064 nm. This high value is due to the presence of a chromophore in the protein, the proto-nated Schiff base of retinal. A purple membrane patch can be treated as a two-dimensional crystal of bacteriorhodopsin proteins, and its structure is known in considerable detail. The analysis of the purple membrane tensor was performed by adding the hyperpolarizabilities of the individual proteins in the purple membrane. From (depolarized) HRS measurements on purple membrane suspensions, the structure of the purple membrane patches, and an average membrane size measured by atomic force microscopy, a fi value of 2200 x 10 esu was calculated for bacteriorhodopsin [22]. The organization of the dipolar protonated Schiff base chro-mophores in the membranes was found to be predominantly octopolar. 

Adding a perfluorinated alkylene group can enhance the electron deficiency at this position. The data in Table 3.10 also show an increase of 8 with increasing size of the chromophore. TPA cross section of 120 is less compared to 119. This was concluded from a systematic study [505] and is confirmed by similar structures [514]. Therefore, systematic expansion of the general pattern of 119 is more successful to increase the TPA cross section as shown for some dendrimers based on 119 [517, 518, 520] as the incorporation of dipolar branches in 120 [505]. 

In this Section we discuss optical susceptibilities of some representative clusters. Specifically, we consider one-dimensional arrays of equivalent molecules (periodic boundary conditions are imposed) with the three geometries sketched in Fig. 3, where tlie arrows represents the dipolar pp chromophores. In cluster A all chromophores are oriented in the same direction, perpendicularly to the stack axis. This geometry then corresponds to repulsive Intermolecular Interactions. Clusters B and C instead describe attractive lattices. In cluster B the antiparallel orientation of chromophores leads to a structure with two molecules per unit cell, an inversion center lying in the middle of each pair of molecules. 

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