Spatial confinement alters the ultrafast photoisomerization dynamics of azobenzenes.

Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation of -azobenzene (-Az) and several alkyl-substituted -Az derivatives encapsulated in a water-soluble supramolecular host-guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of the → photoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstituted -Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-state → isomerization pathway for confined molecules that is not observed in solution. Analysis of the transient spectra following ππ* excitation for a series of 4-alkyl and 4,4'-dialkyl substituted -Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distorted and structures. The variation of the dynamics due to crowding in the confined environment provides new evidence to explain the violation of Kasha's rule for nπ* and ππ* excitation of azobenzenes based on competition between in-plane inversion and out-of-plane rotation channels.