First observation of the triplet state of fluorobenzene

A triplet–triplet (T–T) absorption spectrum of fluorobenzene was observed for the first time. The broad absorption in the region of 330–550 nm with lifetime of τ=0.7 μs was ascribed to the T–T absorption spectrum by the T–T energy transfer experiment using naphthalene as an acceptor. The spectrum reported previously as the T–T absorption was elucidated to be the spectrum of radical. Quantum yield of the intersystem crossing φISC was determined to be 0.27 by use of the time-resolved thermal lensing (TRTL) experiment.