Dipolar donor-acceptor-substituted molecules

For a small molecule with strong donor-acceptor substitution and charge transfer character, the difference dipole moment Ap. is large and the dipolar contribution can be assumed to dominate the two-photon term. 

Dipolar ID donor/acceptor systems are generally characterized by a prevalent single hyperpolarizabihty tensor, namely parallel to the dipolar z axis of the molecule. However, an appropriate donor/acceptor substitution pattern on the bis(salicylaldiminato) framework, resulting in a C2v molecular synunetry, leads to large off-diagonal Pijk tensors components (e.g., 21, D = NEt2 A = Cl,... 

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)...

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. 

Linear organic molecules were firstly considered for the design of TPA efficient molecules. This family can be divided into two groups (Fig. 1) (1) dipolar molecules constituted by a noncentrosymmetric structure, with a linear delocaUzed electrons system substituted at each end by electron donor D and acceptor groups A (push-pull molecules) (2) quadrupolar systems, consisting of symmetric molecules with a n electrons system substituted by donor or acceptor groups. 

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