Chiral and Achiral Charge‐Transfer Chromophores with a Dendralene‐Type Backbone by Electronically Controlled Cycloaddition/Cycloreversion Cascades

Abstract

AbstractChiral and achiral push‐pull chromophores have been prepared by cascades of sequential [2+2] cycloadditions of tetracyanoethene (TCNE) and tetrathiafulvalene (TTF) to different oligoynes. Thermal [2+2] cycloaddition of TCNE to donor‐substituted alkynes, followed by electrocyclic ring‐opening of the initially formed cyclobutenes, affords donor‐substituted 1,1,4,4‐tetracyanobuta‐1,3‐dienes (TCBDs). Similarly, TTF reacts with electron‐deficient C≡C bonds to give the corresponding buta‐1,3‐diene derivatives, 1,2‐bis(1,3‐dithiol‐2‐ylidene)ethanes. Thus, achiral [AB]‐type oligomers were synthesized from N,N‐dialkylanilino (DAA)‐substituted tetraynes and hexaynes and chiral [AB]‐type oligomers from alkyne‐substituted 1,1′‐binaphthalenes. The [AB]‐type oligomers exhibit complex conformational equilibria in solution, as revealed by 1H and 13C NMR spectroscopy. Therefore, the circular dichroism (CD) spectra of the chiral [AB]‐type oligomers were measured to investigate whether a preferred conformation of the dendralene‐type backbone is induced by the optically active 1,1′‐binaphthalene moiety. Electrochemical studies by cyclic voltammetry (CV) and rotating‐disk voltammetry (RDV) showed large cathodic shifts of the first oxidation potentials for some of the chiral and achiral [AB]‐type oligomers due to sterically enforced π‐deconjugation of the acceptor and donor moieties. The new multivalent systems feature intense, bathochromically shifted intramolecular charge‐transfer (CT) bands in the UV/Vis spectra. Extended, donor‐substituted TCBDs, which are obtained by mono‐addition of TCNE to the hexaynes, exhibit low optical and electrochemical HOMO–LUMO gaps. In addition, a large third‐order optical nonlinearity was measured for one of these TCBDs by degenerate four‐wave mixing (DFWM).