Topochemical Photopolymerization above the Bulk Melting Temperature in Noncovalent Monolayers on Graphite.

Abstract

While studies of surface-templated topochemical reactions often focus on the central role of monomer ordering, the reactions themselves frequently require Ångström-scale dynamics to form new bonds. Here, it is showed that elevated temperatures increase the topochemical reaction efficiency of 10,12-tricosadiynoic acid (TCDA), a widely utilized commercial diacetylene monomer, assembled on highly oriented graphite (HOPG) substrates. Up to ≈45 °C, Arrhenius temperature dependence is observed for the reaction, with E_a = 5.9 kcal mol^-1 (0.26 eV), consistent with limitations imposed by the propagation step of the reaction, which requires monomer dynamics within the lattice. At higher temperatures, t_0.5 does not continue to decrease. However, the number average degree of polymerization continues to increase, from 97 at 5 °C to 248 at 65 °C, and the number density of polymers formed per incident photon increases by 6-13-fold at elevated temperatures (45-65 °C) in comparison with polymerization at 5 °C. Together, these changes in the on-surface reaction greatly increase molecular sheet integrity, resulting in a 10-fold increase in the efficiency of a covalent mesh-forming reaction that transfers TCDA sheets to soft polydimethylsiloxane.