Remarkable Effect of Odd-Even Spacer in Supramolecular Polymerization and Piezoresponse of Amide-Functionalized Naphthalene Diimides.

This study reports an unprecedented odd-even spacer-length effect on the supramolecular polymerization and piezoelectricity of a series of amide-functionalized naphthalene-diimide (NDI) derivatives. By varying the number of methylene units (n = 1-4) in the linker between the NDI-core and the hydrogen-bonding amide group, we demonstrate that the parity of the spacer dictates the stability, internal-order, gelation, and functionality of the supramolecular polymers. Even-spacer derivatives exhibit significantly higher thermal stability, melting points, and gel-phase elasticity compared to their odd-spacer counterparts. Supramolecular polymerization for all derivatives follows a nucleation-elongation pathway; however, even-spacer monomers display markedly higher cooperativity and also favor more stable elongation. Molecular dynamics (MD) simulations elucidate the structural origin of these differences: even-spacer derivatives adopt a tightly packed helical π-stacking pattern that results in the cancellation of the in-plane dipoles. In contrast, odd-spacer derivatives form linear, offset stacking motifs where individual dipoles align and accumulate, leading to highly polarizable supramolecular polymers. This divergence in the internal dipole orientation translates into a dramatic odd-even effect on the piezoelectric dipolar hysteresis. Odd-spacer derivatives exhibit remarkably high positive piezoelectric coefficients (maximum d33 ∼ 75 pm/V), whereas even-spacer systems show much weaker, and rarely reported negative responses (minimum d33 -15 pm/V).