Ultrafast transient absorption spectroscopy reveals new excited-state dynamics following excitation oftrans-azobenzene (t-Az) and several alkyl-substitutedt-Az derivatives encapsulated in a water-soluble supramolecular host-guest complex. Encapsulation increases the excited-state lifetimes and alters the yields of thetrans→cisphotoisomerization reaction compared with solution. Kinetic modeling of the transient spectra for unsubstitutedt-Az following nπ* and ππ* excitation reveals steric trapping of excited-state species, as well as an adiabatic excited-statetrans→cisisomerization 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 substitutedt-Az molecules suggests that additional crowding due to lengthening of the alkyl tails results in deeper trapping of the excited-state species, including distortedtransandcisstructures. 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.
ASJC Scopus subject areas