Thermoreversiblej Photo Valence Isomerization of 9-t-Butylanthracene to its 9,10-Dewar-Isomer

Abstract

Abstract On excitation the first excited singlet state (1La), 9-t-butylanthracene (1) reacts in dilute (10-6-10-3 mole · 1-1) heptane solution with a quantum yield of Φ = 0.016 ± 0.002 to its valence isomer 1-t-butyl-dibenzo-2,5-bicyclo-(2,2,0)-hexadiene (9-t-butyl-9,10-Dewar-anthracene) (2). UV-spectroscopic and 1H-NMR-data of educt and product confirm the proposed isomerization. The reaction is thermoreversible. The first order rate constant of the back reaction was determined in the temperature range of 293 K to 343 K and yielded the activation enthalpy ΔH≠ = 91 ± 3 [kJ · mole-1] and the activation entropie ΔS≠ = - 25 ± 8 [J · mole-1 · K-1] in heptane and 94 ± 3 [kJ · mole-1] and - 17 ± 8 [J · mole-1 · K-1] in ethanole