Structural effects on the radiationless transition from the T₁ states of tropone derivatives

The T₁ states of nonphosphorescent 2,3-benzotropone and phosphorescent 4,5-benzotropone were studied by a time-resolved EPR method. For 2,3-benzotropone, the observed magnetic parameters and the decay rate (the average decay rate 1.1 × 10⁵ s^-1 at 4.2 K) are very similar to those of the parent tropone. On the other hand, 4,5-benzotropone has different magnetic properties and a much slower radiationless decay rate (<30 s^-1) than tropone. The mechanism of the fast radiationless transition between the T₁ and S₀ states of tropone has been discussed.