Biradical intermediates in the photoisomerization of dibenzodihydropentalenofurans to dibenzosemibullvalenes

Abstract

The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans 3a-d gave the corresponding dibenzosemibullvalenes 2a-d in nearly quantitative yields. The quantum yields of this photoisomerization were found to be in the range 0.17-0.26. Laser flash photolysis studies of the dibenzodihydropentalenofurans 3a-e showed transients, with absorption maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14-30 μs in degassed benzene at 25°C. These transients were readily quenched (trapped) by molecular oxygen, and the Stern-Volmer quenching constants were found to be in the range (2.45-3.17) × 10⁹ M^-1 s^-1. As a representative example, the 1,3-biradical intermediate from 3e was trapped by molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the range (1.09-3.19) × 10⁶ M^-1 s^-1. The decay rates of the transients were found to be temperature dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient from 3a was ~4.5 kcal mol^-1 and the Arrhenius preexponential factor log(A/s^-1) for the decay of the transients was ~7.5. On the basis of our studies, these transients were assigned as the ground-state triplet biradicals, generated by the cleavage of the C-O bond of the starting dibenzodihydropentalenofurans.