D−π–A Compounds with Tunable Intramolecular Charge Transfer Achieved by Incorporation of Butenolide Nitriles as Acceptor Moieties

Chromophores
where a polyenic spacer separates a 4H-pyranylidene
or benzothiazolylidene donor and three different butenolide
nitriles have been synthesized and characterized. The role of 2­(5H)-furanones as acceptor units on the polarization and the
second-order nonlinear (NLO) properties has been studied. Thus, their
incorporation gives rise to moderately polarized structures with NLO
responses that compare favorably to those of related compounds featuring
more efficient electron-withdrawing moieties. Derivatives of the proaromatic
butenolide PhFu show the best nonlinearities. Benzothiazolylidene-containing
chromophores present less alternated structures than their pyranylidene
analogues, and, unlike most merocyanines, the degree of charge transfer
does not decrease on lengthening the π-bridge.