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

Abstract

Chiral 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 CϵC 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 1,1Ј-binaphthalenes. The [AB]-type oligomers exhibit complex conformational equilibria in solution, as revealed by 1 H and 13 C NMR spectroscopy. Therefore, the circular dichroism (CD) spectra of the chiral [AB]-type oligo-

Comparison of CC Triple and Double Bonds as Spacers in Push–Pull Chromophores

“…Note also that we previously determined the third‐order polarizability of pentayne 24 45 (Table 3) to be quite large, proving that triple bond spacers can be very effective in creating a large nonlinearity, in contrast to what we see here for molecules 1 – 5 .…”

“…We see that pentayne 24 45 is a very efficient molecule, with its intrinsic third‐order polarizability almost reaching the largest value we observed ( 10 ) for the oligoenes and the high value of oligoyne 2 . Thus, the triple‐bond spacers are potentially very effective, at least in the case of this molecule, which is more stable than oligoynes 3 – 5 and also has a different and stronger acceptor system.…”

“…The D–A‐substituted tetrayne 5 was prepared from alcohol 19 45 by the “one‐pot” oxidation/Knoevenagel condensation method (Scheme ). Similarly to its precursor, 5 exhibits very low thermal stability: Decomposition was observed at 40 °C during evaporation of the solvent.…”

“…For the synthesis of D–A‐substituted pentayne 6 , compounds 20 46 and 21 ,45 upon deprotection of the TMS group, were subjected to oxidative Hay heterocoupling to afford 22 together with the symmetric hexayne 23 (Scheme ) 45. Pentayne 22 was handled and stored at temperatures below 40 °C because of its low thermal stability.…”

Reactivity of Polyynes: Complex Molecules from Simple Carbon Rods

“…[105] A push-pull tetrayne 206 was transformed into a complex donor-acceptor system 207 via similar one pot procedure. After the next iteration the whole carbon chain of 198 was transformed into a complex π-conjugated push-pull system 205.…”