Second‐Order Nonlinear Optical Activity of Dipolar Chromophores Based on Pyrrole‐Hydrazono Donor Moieties

Donor1+donor2→acceptor : The second‐order NLO molecular properties of a class of dipolar chromophores that incorporate the following design elements are investigated: 1) a substituted hydrazono moiety as a strong donor; 2) a pyrrole ring as an auxiliary donor; 3) strong acceptor groups (see figure). Their first hyperpolarisabilities show good promise for use in electro‐optical devices. magnified image A series of highly efficient and thermally stable second‐order nonlinear optical (NLO) dipolar donor–auxiliary donor–acceptor chromophores have been synthesised in which a hydrazono group and a pyrrole ring act as donor and auxiliary donor components, respectively, in combination with different aromatic and heteroaromatic acceptors. The new dyes have been systematically investigated by NMR spectroscopy, absorption spectroscopy, NLO measurements and thermal stability studies. NLO properties have been studied in detail by electro‐optical absorption (EOA) and hyper‐Rayleigh scattering (HRS) measurements in 1,4‐dioxane and DMSO, respectively. The results originating from the two different methods have been compared and analysed in detail. We found that the NLO properties measured by the EOA and HRS methods correlate with each other and converge to reveal the dye with the acceptor 2‐dicyanomethylene‐3‐cyano‐4,5,5‐trimethyl‐2,5‐dihydrofuran as the most efficient system. The unprecedented combination of a strong donor hydrazono group and the auxiliary donor effects of π‐excessive heteroaromatic rings afforded NLO chromophores with very high values ( μ g β 0 (EOA) up to 2038×10 −48 esu and β (HRS) up to 3980×10 −30 esu at 1.5 μm).