Donor/Acceptor Effects on the Linear and Nonlinear Optical Properties of Geminal Diethynylethenes (g‐DEEs)

Abstract

A series of geminal diethynylethenes (g-DEEs) with electron-donating and/or electron-accepting (D/A) groups were synthesized via a Pd-catalyzed cross-coupling sequence. The UV/VIS spectra for donor -acceptor (D -A) functionalized g-DEEs 5, 8, and 11 show distinctive absorption trends attributable to intramolecular charge-transfer (ICT). The bond-length-alternation (BLA) index for the cross-conjugated enediyne framework varies slightly with different terminal substituents as determined by density-functional theory (DFT) calculations and single-crystal X-ray analysis. Ultrafast third-order optical nonlinearities for the g-DEEs were measured by the differential optical Kerr effect (DOKE) technique and show that terminal donor -acceptor substitution of g-DEEs enhances molecular second hyperpolarizabilities (g) in comparison to donor or acceptor g-DEEs. A small increase in the twophoton-absorption cross-section (s (2) ) is observed in the series 9 -11 as a result of increased functionalization. The effects of donor/acceptor substitution on electron delocalization along the cross-conjugated enediyne structure are evaluated on the basis of natural-bond-orbital (NBO) analysis. Solid-state structures of the four derivatives 3b, 4b, 7 and 8 were characterized by single-crystal X-ray structural analysis and show an asymmetric unit cell for one derivative, D -A g-DEE 8.1. Introduction. -Conjugated molecules with highly delocalized and polarizable pelectron systems are prospects for third-order nonlinear optical (NLO) materials, and could form the basis for achieving ultrafast all-optical-based computing and telecommunications technology [1] [2]. To find ideal and applicable organic NLO materials, a large number of organic chromophores have been extensively explored, with many efforts focused on linearly conjugated aromatic and/or nonaromatic pconjugated systems [2 -5]. In assessing the suitability of an organic material for NLO purposes, its efficiency/transparency trade-off at a specific wavelength is an important factor to be considered, and maintaining transparency in the visible region of the spectrum is a significant challenge [6]. Many linearly p-conjugated chromophores tend to perform poorly in this respect, and the pursuit of alternative molecular designs has become more and more appealing and important. In particular, the use of cross-

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