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

Chiral and achiral push-pull chromophores have been prepared by cascades of sequential [2+2] cycloadditions of tetra-cyanoethene (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 letraynes 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 H-1 and C-13 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 clue to sterically enforced pi-deconjugation of the acceptor and donor moieties The new multivalent systems feature Intense, bathochromically shifted intramolecular change-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-older optical nonlinearity was measured for one of these TCBDs by degenerate four-wave mixing (DFWM)