RESUMEN
A series of donor-acceptor-substituted alkynes, 2 a-f, was synthesized in which the length of the π-conjugated polyyne spacer between the N,N-diisopropylanilino donor and the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) acceptor was systematically changed. The effect of this structural change on the optoelectronic properties of the molecules and, ultimately, their third-order optical nonlinearity was comprehensively investigated. The branched N,N-diisopropyl groups on the anilino donor moieties combined with the nonplanar geometry of 2 a-f imparted exceptionally high solubility to these chromophores. This important property allowed for performing INADEQUATE NMR measurements without (13) C labeling, which, in turn, resulted in a complete assignment of the carbon skeleton in chromophores 2 a-f and the determination of the (13) C-(13) C coupling constants. This body of data provided unprecedented insight into characteristic (13) C chemical shift patterns in push-pull-substituted polyynes. Electrochemical and UV/Vis spectroscopic studies showed that the HOMO-LUMO energy gap decreases with increasing length of the polyyne spacer, while this effect levels off for spacers with more than four acetylene units. The third-order optical nonlinearity of this series of molecules was determined by measuring the rotational averages of the third-order polarizabilities (γrot ) by degenerate four-wave mixing (DFWM). These latter studies revealed high third-order optical nonlinearities for the new chromophores; most importantly, they provided fundamental insight into the effect of the conjugated spacer length in D-A polyynes, that can be exploited in the future design of suitable charge-transfer chromophores for applications in optoelectronic devices.
RESUMEN
A formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between N,N'-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was explored, providing a new class of π-conjugated donor-acceptor chromophores. These DCNQI adducts exist in the solid state as single diastereoisomers, but as two interconverting diastereoisomers in solution. Solid- and solution-state evidence for intramolecular charge transfer (CT) was obtained; additionally, the DCNQI adducts exhibit positive solvatochromism and significant solution-state third-order polarizabilities.