Geometry and bond-length alternation in nonlinear optical materials. I. Standard parameters in two precursors

2-{3-Cyano-4-[4-(N-formyl­anilino)-trans-1,3-butadien­yl]-5,5-dimethyl-2,5-dihydro­furan-2-yl­idene}propane­dinitrile, C₂₂H₁₈N₄O₂, (I), and 2-{3-cyano-4-[6-(N-formyl­anilino)-trans,trans-1,3,5-hexa­trien­yl]-5,5-dimethyl-2,5-dihydro­furan-2-yl­idene}propane­dinitrile, C₂₄H₂₀N₄O₂, (II), show the alternating single/double-bond behaviour of push–pull chromophores. In the two structures, the planar polyene chains are twisted with respect to the furanyl­idene ring by 18.2 (2) and 12.4 (2)°, respectively. Comparison with structures of related and parent mol­ecules shows subtle but consistent bond-length variations consistent with charge-delocalized structures. Crystal cohesion is provided by various sets of hydrogen bonds, viz. C—H_methyl...N_cyano and bifurcated (C=C—H)₂...O=C in (I), and C—H_{methyl/phenyl}...O and C=C—H...N_cyano in (II).