Quadrupolar chromophores

The synthesis of quadrupolar chromophores has also been achieved from 2,6-DTT-dicarboxaldehyde 117. Push-push (i.e., bis-donor) compound 118 was prepared via a double Wittig reaction carried out under solid-liquid phase transfer conditions. Pull-pull (i.e., bis-acceptor) compounds 119 were obtained from a symmetrical bis-aldehydes via a double Horner-Emmons-Wittig condensation (Scheme 9) <2002SM17, 1999CC2055>. 

The class of compounds that has been most extensively investigated from the point of view of two-photon absorption is that of so-called quadrupolar chromophores. hi essence, these molecifles are linear conjugated chains with electron donating or withdrawing substituents arranged symmetrically with respect to the center of the molecifle (Fig. 8, classes I-IV). With the inversion center being preserved, the lowest order moment supported by these molecules is the quadrupole moment. 

Fig. 8 Schematics of various linear (one-dimensional) chromophores classified based on the substitution pattern. I-IV quadrupolar molecules V dipolar molecules (D = donor group 7T = 7T-conjugated bridge A = acceptor group)...

Some of the chromophores in this table are not strictly centrosymmetric, but their description as quadrupolar structures is a good approximation, based on the properties that have been observed or calculated. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

TPA chromophore 107 represents, in analogy to its dipolar analog 52, a typical quadrupolar chromophore with both increased linear and nonlinear absorption properties [428]. Thus, the extinction coefficient and 8 are about 1.3 and 1.4 times larger (about /2) in comparison with that of 52. This again shows that incorporation of the electron-deficient triazine results in an increase of 8 for quadrupolar compounds considering both OP and TP quantities. 

Comparison of the energies for TPA and OPA shows a higher energy for the TP excited state compared to the lowest allowed OP excited state (Si) for all chromophores (99-107) examined in this section. Hence, the amplitude of TPA can be described by Eq. (27). There are no dipolar contributions, no matter that solvatochromism occurs for the samples 99-104. The influence of the quadrupolar pattern is reflected in M12. Application of models of the symmetry break are not likely to provide clarification of the relation between TPA properties and the distinct quadrupolar substitution of the chromophores 99-107. 

D-7i-A)3-D chromophores exhibit large TPA, as shown for chromophore 122 [490]. This compound can be considered a propeller-shaped molecule consisting of three quadrupolar arms. This study demonstrates that addition of each individual arm results in a twofold increase of 8 as shown for the TPA of 122 (5 = 5030 GM) in comparison with the quadrupolar chromophore 100c (7) = 1370 GM). The latter comprises one arm of 122. Substitution of one arm by a methyl group results in a twin chromophore exhibiting a 8 of 3130 GM. Thus, one can conclude that TPA depends on the number of arms (na), na > 2, incorporated in both (D-rc- A)na-D and (D-7i)na-D chromophores. The structural pattern of 122 comprises three quadrupolar arms. Thus, Eqs. (59) and (60) cannot be applied to describe TPA of 122 because A/(m 0 due to the quadrupolar pattern of each arm. Therefore, the enhacement of 8 is better explained by the increase of the transition dipole moments Mqi and M 2 because... 

The dynamic coupling between the electric hexadecapole moment of a d—d transition in the metal ion and an induced electric dipole in each ligand is forbidden in a four-coordinate complex containing a tetrahedral chromophore, although the mechanism becomes formally allowed on reduction to D2(i or lower chromophoric symmetry. In tetrahedral complexes the dynamic coupling between a d—d quadrupolar... 

Table 3 Properties of quadrupolar bis(styryl)benzene chromophores...

Figure 14 Structures of some dipolar and quadrupolar group 8 alkynyl chromophores discussed in Section 12.04.1.3.2.(...

Sissa et al have presented theoretical models for describing the electroabsorption (EA) spectra of dipolar, quadrupolar, and octupolar organic chromophores in solution and therefore for addressing molecular information that go well beyond the linear absorption spectra and are complementary to nonlinear spectroscopic techniques. So, for dipolar molecules EA spectra provide the excited state dipole moment and polarizability whereas for more complex quadrupolar and octupolar compounds, they also give information on the dark states. 

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