Toward Spatiotemporally Controlled Synthesis of Photoresponsive Polymers: Computational Design of Azobenzene-Containing Monomers for Light-Mediated ROMP

Density functional theory calculations have been used to identify the optimum design for a novel, light-responsive ring monomer expected to allow spatial and temporal control of ring-opening metathesis polymerization (ROMP) via light mediated changes in ring strain energy. The monomer design leverages ring-shaped molecules composed of 4,4'-diaminoazobenzene (ABn) closed by alkene-alpha,omega-dioic acid linkers. The atomic geometries, formation enthalpies and ring strain energies of azobenzene (AB)-containing rings with various length linkers have been calculated. The AB(2,2) monomer is identified as having optimal properties for light-mediated ROMP, including high thermodynamic stability, low ring strain energy (RSE) with cis-AB, and high RSE with trans-AB. Time-dependent DFT calculations have been used to explore the photoisomerization mechanism of isolated AB and AB-containing rings, and calculations show that trans-to-cis and cis-to-trans photoisomerization of the optimal AB(2,2) ring molecule can be achieved with monochromatic green and blue light, respectively. The AB(2,2) monomer identified here is expected to allow precise, reversible, spatial and temporal light-mediated control of ROMP through AB photoisomerization, and to have promising potential applications in the fabrication of patterned and/or responsive AB-containing polymer materials.