supplementary materials

π-Bridged donor-acceptor-donor (A—D—A) systems are promising candidates for electronic applications such as organic light-emitting diodes (Liu et al., 2001; Grimsdale et al., 1997), as they are expected to have electronic properties similar to those of conventional OPV-type systems but with a red-shifted emission spectrum. Moreover, due to their high degree of conjugation, these A—D—A oligomers are also excellent candidates for organic non-linear optic (NLO) media with a high second-order hyperpolarizability, γ (Chemla, 1987). In an attempt to synthesize the A—D—A system E,E-2,5-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]pyrazine from dimethylpyrazine and the relevant benzaldehyde under standard condensation conditions, (E)-1-(2,4,6-trimethoxyphenyl)pent-1-en-3-one (Fig. 1) was obtained as the major product. In this compound the C=C bond is not disordered, in contrast to what is the case in the 3-methoxy-4-acetoxyphenyl derivative YODGOO (Zhang et al., 2008) and the molecule adopts the anti conformation indicating that there are no energetically beneficial intermolecular contacts favouring the syn conformation as in the unsubstituted DIBNEH (Degen & Bolte, 1999). The title compound displays two weak intramolecular hydrogen bonds involving the methoxy groups in the ortho positions of the phenyl ring, one in a five- and one in a six-membered ring configuration. In contrast, in the 2-hydroxy-5-bromophenyl derivative NORGOR (Zonouzi et al., 2009) a less stable six-membered ring configuration is observed due to the competing strong intermolecular O—H···O hydrogen bond with an adjacent molecule. In the 2-hydroxyphenyl derivative FONKEZ (Wang et al., 2005), the more favourable five-membered ring configuration is also seen. The packing of the title compound is determined in first instance by contacts between the methoxy groups. Two molecules (symmetry-related via an inversion centre) are connected into a dimer involving O2 and C11 of the methoxy groups in the 2- and 4-positions [C11···O2ⁱ, 3.109 (2) Å, 178.33 (12)°, symm. code i = 1 - x, -y, 1 - z] (Fig. 2); note that the C···O—C angle is almost linear. Sheets are then generated through two weak hydrogen bonds involving H5 (Table 1, entry 1) and H31C (entry 2) contacting the oxygen atom of the carbonyl group (O4). These sheets are then interconnected by four additional weak hydrogen bonds (Fig. 3): H21C and O2 are involved in a second dimer formation (entry 3), H11A and H12C contact the oxygen atom of the carbonyl (O4, entries 4 and 5) and H10B of the methylene group next to the carbonyl group generates a CH···π interaction with a nearby phenyl ring (entry 6).