Second harmonic generation in ferrocene based hydrogen bonded assemblies

Supramolecular assemblies formed by electron donors and acceptors might possess very large optical nonlinearity, even larger than the sum of the constituents if they form noncentrosymmetric networks. To probe this hypothesis we have functionalized barbituric acid, by condensing it with ferrocene carboxaldehyde to give 1, a hydrogen bond donor–acceptor–donor (HDAD). This molecule complements the hydrogen bond donor acceptor sites of 2,6-diaminopyridine (2) which is a hydrogen bond acceptor–donor–acceptor (HADA). 2,6-Diaminopyridine was further functionalized with acetyl ferrocene to give 3, which forms complexes with 1. Complex formation was studied using nuclear magnetic resonance spectroscopy. The second order nonlinear optical (NLO) response of these hydrogen bonded assemblies has been probed by hyper Rayleigh scattering (HRS) measurements and was found to be maximum when 1 was mixed with five equivalents of 3. A plausible explanation for the formation of 1:5 complex was given on the basis of control experiments with 4, a methylated analog of 1, and the addition of a strong hydrogen bond acceptor solvent, methanol.