The photochemical trans → cis and thermal cis → trans isomerization pathways of azobenzo-13-crown ether: A computational study on a strained cyclic azobenzene system.

Abstract

The isomerization of azobenzo-13-crown ether can be expected to be hindered due to the polyoxyethylene linkage connecting the 2,2'-positions of azobenzene. The mixed reference spin-flip time-dependent density functional theory results reveal that the planar and rotational minima of the first photo-excited singlet state (S1) of the trans-isomer pass through a barrier (2.5-5.0 kcal/mol) as it goes toward the torsional conical intersection (S0/S1) geometry (<CNNC ≈ 98°), which is responsible for the cis isomer formation. The second excited singlet state (S2) of the trans form has a nearly planar minimum along the N-N stretching mode, which approaches a sloped S2/S1 intersection geometry. This excited state has a rotational minimum (<CNNC ≈ 99°) as well. Both these minima have a characteristic S2-S1 energy gap of 9 kcal/mol and may undergo internal conversion. A comparison of this system with an analogous 2,2'-dimethoxy-substituted azobenzene system reveals less strain in the rotational path of the latter on the S1 surface, indicating the possibility of its better trans → cis yield than the azocrown. The completely planar S2 geometry of the dimethoxy system has easy access to the linear concerted inversion path, which seems to be the reason behind its reported slightly lower π-π*(S2) yield than n-π*(S1). The thermal cis → trans isomerization path of the azobenzo-13-crown passes through a transition state (frequency 453i cm-1), which corresponds to Gibbs free energy of activation value of 26 kcal/mol in the gas-phase and isooctane. Our study also confirms that its trans isomer strongly binds Li+ among the alkali metal ions, and this observation may open up possibilities for practical applications of this azobenzo-crown.