Photochemical "Across-a-Ridge" Z-E Isomerization of (Ethenyl-2-d)anthracenes in the Triplet State

Article abstract of DOI:10.1021/j100053a026

Deuterated ethenylanthracenes, 1-, 2-, and 9-(ethenyl-(Z)-2-d)anthracene, and 2-(ethenyl-(E)-2-d)anthracene ((Z)-1-, 2-, and 9-EAD, and (E)-2-EAD, respectively) undergo photochemical Z-E isomerization under thermal activation.The temperature dependence of their isomerization quantum yields and the temperature independence of lifetimes of their T_n <- T₁ absorptions indicate that EADs isomerize adiabatically between the Z and E isomers in the lowest excited triplet state by overcoming an activation barrier (E_a = 19-46 kJ mol^-1) of the perpendicularly twisted geometry.The activation parameters of isomerization, ΔH^<*> (17-42 kJ mol^-1) and ΔS^<*> (-40 to -113 J K^-1 mol^-1), decrease in the order of 2-, 1-, and 9-isomers; however, ΔG^<*> (50-54 kJ mol^-1) is almost constant irrespective of the substitution position of the anthracene nucleus.