Alkynyl Expanded Donor–Acceptor Calixarenes: Geometry and Second‐Order Nonlinear Optical Properties

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Abstract
A number of wide‐ and narrow‐rimmed functionalized alkynylcalix[4]arenes have been synthesized by Sonogashira coupling. With respect to their optical properties, these donor–acceptor systems are treated as ensembles of covalently linked, electronically independent tolane subchromophores. Linear UV/visible and fluorescence spectroscopic investigations revealed that the charge‐transfer character of the electronic transitions in calixarenes, and also the second‐order nonlinear optical (NLO) properties depend on the electron‐withdrawing nature of the terminal ethynylphenyl substituent (NO2, CF3, H). The nitro derivatives display high values of the quadratic hyperpolarizability β. Not only do the (nonlinear) optical properties of the target compounds depend on the number and relative disposition of the subchromophores, but also on the geometry of the calixarenes. In particular, the opening angle of the calixarene cavity can be determined by the substitution pattern of the calixarene scaffold (wide‐ versus narrow‐rim substitution) and the number of the acetylene functions introduced. Both the NLO properties and the conformational issues are conveniently assessed by using hyper‐Rayleigh scattering (HRS) in solution, and supported by X‐ray crystallography in the solid state.