We report the synthesis and physical study of a series of 1,1‐dicyano‐4‐[4‐(diethylamino)phenyl]buta‐1,3‐dienes in which the number and position of additional CN substituents along the 1,1‐dicyanobuta‐1,3‐dienyl fragment is systematically varied. While X‐ray analysis provided unambiguous information about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that introduction of additional cyano groups in the C2‐ and C4‐positions most affected the optical properties of these molecules in solution, in terms of intramolecular charge‐transfer absorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold results in a remarkable bathochromic shift of the ICT absorption maxima, mainly due to the higher planarity in the present series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push‐pull chromophores as third‐order nonlinear‐optical molecular materials.

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.

No abstract

Planar and non-planar push-pull chromophores (D-p-A), consisting of strong electron donors (D) and acceptors (A) connected by a p-conjugated spacer, feature intense intramolecular charge-transfer (CT) interactions [1, 2] and have recently received considerable attention as functional components in molecular electronics and nonlinear optics (NLO).[3] Transannular CT interactions between nonconjugated donors and acceptors in rigid molecular frameworks have also been extensively investigated. Through-space CT interactions in cyclophane derivatives, in which the donor and acceptor moieties are parallel to one another, were established by Staab and Rebafka [4] and Misumi and Otsubo, [5] and further explored by others.[6] When donor and acceptor are separated by appropriately aligned, rigid s-bond frameworks, long-range through-bond CT interactions were also observed in the UV/Vis spectra. [7] Only a few examples of transannular CT chromophores with a non-parallel arrangement, so-called homoconjugated push-pull systems, have been reported, [8][9][10][11] since Nakazawa and Murata published the first example of homoconjugated CT interactions in 9,10-dihydro-9,10-(1,2-tropylio)anthracene tetrafluoroborate in 1977. [8] In general, the reported homoconjugative CT interactions have been mostly restricted to bicyclic [2.2.2] p systems, and in particular triptycene derivatives.2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a popular strong oxidizing and dehydrogenating agent with many uses. [12, 13] We envisaged that thermal [2+2] cycloaddition between donor-activated electron-rich alkynes and the electron-deficient C=C double bonds in DDQ could take place to give new push-pull systems.[2b] Very recently, during the completion of the present study, Trofimov et al. indeed described [2+2] cycloadditions of DDQ with pyrrole-or indole-activated alkynes to give a variety of cycloadducts.[14]Herein, we describe a new class of homoconjugated push-pull chromophores (AE )-1 to (AE )-9 by [2+2] cycloaddition of N,Ndialkylanilino (DAA) and ferrocene (Fc)-substituted alkynes to DDQ. They feature surprisingly strong intramolecular CT interactions and, to the best of our knowledge, are the first homoconjugated push-pull chromophores with promising third-order nonlinear optical (NLO) properties. We also report the synthesis and characterization of spiro compound (AE )-10 by an electrocyclic ring-opening reaction of (AE )-1 followed by a transannular rearrangement.Good to excellent yields (63-94 %) of DAA-substituted chromophores (AE )-1 to (AE )-6 were obtained by [2+2] cycloaddition of DDQ with the acetylenic precursors 11 to 16 in 1,2-dichloroethane at 25 8C (Scheme 1). Analogously, the Fc-substituted chromophores (AE )-7 to (AE )-9 were formed in 62-90 % yield by reaction of DDQ with alkynes 17 to 19, respectively. The chromophoric products are deeply colored solids that are stable at ambient temperature in air. Thermal gravimetric analysis (TGA) did not show any significant weight loss (< 5 %) below 160 8C. However, upon heating...

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.