Temperature and Pressure Dependences of Thermal Cis-to-Trans Isomerization of Azobenzenes Which Evidence an Inversion Mechanism

The mechanism of thermal cis-to-trans isomerization of azobenzene derivatives (rotation versus inversion) is discussed. cis-1 has an azobenzene covalently bridged to an aza crown ether, and rotation of the benzene rings is sterically restricted. The pressure dependence of the isomerization rate of cis-1 provided a ΔV of 2.0 mL mol”1, indicating that the rotational mechanism which required an increase in the polarity at the transition state is not the case for cis-1. By using cis-1 as the standard azobenzene for the inversion mechanism it was demonstrated that the thermal isomerization of most cis azobenzenes occurs via an inversion mechanism, the activation parameters (ΔH and ΔS) including those of cis-1 being subject to a good ΔH-ΔS compensation relationship. The pressure effect also supported the inversion mechanism, the AV* being too small (ca. -0.4 to -0.7 mL mol'1) to consider the rotational mechanism. On the other hand, ΔH-ΔS plots for an azobenzene with push-pull substituents (e.g., 4-dimethylamino-4'-nitroazobenzene) deviated downward from a linear compensation relationship, and a negative, large ΔV (-22.1 mL mol”1) resulted. Hence, the rotational mechanism may be operative for this azobenzene, the dipolar transition state being stabilized by the resonance power of these substituents. The fact that simple azobenzenes have small negative ΔV values and cw-1 has a small positive ΔV was rationalized in terms of void volume; that is, the crown ether ring is expanded in the inversion transition state. The explanation is well compatible with our previous finding that complexes K+ and primary ammonium ions suppress the isomerization rate of cir-l. These results provide conclusive evidence for the inversion mechanism in most azobenzene derivatives and establish a new, unambiguous method to distinguish between the inversion and rotational mechanisms.