Polymerization of Physisorbed Molecular Monolayers via Overhanging Alkynyl Chains: Characterization of Polymerization Kinetics and Monolayer Durability

Abstract

Monolayers self-assembled by triphenyleneethynylene (TPE) compounds bearing two terminal alkynyl chains were polymerized by Glaser−Hay (G-H) alkyne coupling at the acetonitrile−HOPG interface. The alkynyl chains extend into the solution due to the monolayer's dense-packed morphology. Reacting substructures that have no morphology-determining roles is a potential strategy for preserving monolayer morphology throughout polymerization. Monolayer G-H reaction kinetics and polymerized monolayer durability were characterized by using mass spectrometry and fluorescence. Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) and time-of-flight (TOF) MS were used to identify TPE-oligomers in the monolayer and to track the monolayer populations of TPEmonomer, -dimer, and -trimer as a function of G-H reaction duration. Comparison of the observed kinetics to a Monte Carlo simulation provided evidence of step-growth polymerization. The durability of polymerized monolayers depended strongly on the length of the alkynyl chains linked by G-H reaction. Polymerized T6y monolayers (O(CH 2 ) 3 C�CH alkynyl chains) desorbed minimally during 16-h immersion in 90 °C o-dichlorobenzene (oDCB), whereas polymerized T8y (O(CH 2 ) 5 C�CH alkynyl chains) and polymerized T11y (O(CH 2 ) 8 C�CH alkynyl chains), desorbed 33 and 60%, respectively, of their TPE units after 4 h in 90 °C oDCB. All the polymerized monolayers are much more durable than unpolymerized monolayers, which desorb quantitatively from HOPG when rinsed with 25 μL of oDCB. Polymerized T6y monolayer is a highly durable anchor that may be adapted to build multilayer structures "permanently" attached to the HOPG surface. The alkynyl chain length dependence may be useful for tuning polymerized TPE monolayer durability for specific applications.