Thermochromism and Thermal E,Z Isomerizations in Bianthrones

The standard heat of the thermochromic equilibrium A⇋B in bianthrones (1) was investigated in the temperature range 40-220μC, with special emphasis on steric effects. The data indicate that there is no direct correlation between Δvalues and possible steric interactions in the vicinity of the central double bond. The thermal decay of the colored B species of 2, 2ʹ-bis(trifluoromethyl)bianthrone (3) was studied by laser flash photolysis. The relation between the thermochromic process and the fast thermal E, Z isomerization found in the bianthrone series is discussed. It is concluded that both thermal phenomena have a common transition state.