Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy

Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S-1(n pi*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S-1(n pi*) and S-2(pi pi*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S-1(n pi*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of similar to 2 kcal mol(-1). This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.