Probing the nonpolar: microwave structure of the azobenzene-water complex

Azobenzene is a widely used molecular photoswitch, yet its gas-phase structure has been the subject of a long-standing controversy. The global minimum, the trans isomer, is planar and centrosymmetric, possessing no dipole moment and making it impossible to study by microwave spectroscopy. To overcome this challenge, we employed a molecular tagging approach to indirectly probe its molecular structure by generating a complex between azobenzene and water. This complex was characterized using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the 2–8 GHz region, granting for an unambiguous determination of its structure. The cluster is stabilized by an O–H···N hydrogen bonding to one of the azo nitrogen atoms, supplemented by secondary C–H···O interactions. A key structural feature is that the azobenzene monomer within the cluster is significantly distorted from planarity, in contrast to the most stable monomer in the gas phase. • The monohydrated cluster of azobenzene was studied by CP-FTMW spectroscopy. • Hydration enabled detection of the non-polar trans -azobenzene monomer. • The structure analysis revealed a water-induced torsion between both phenyl rings. • This subtle torsion in the geometry of azobenzene was characterized experimentally.