Following light excitation of acridine to higher singlet states and internal conversion to the lowest excited singlet state S1 the molecule can relax via inter system crossing to the first excited triplet state T1 from which by H-transfer from fluorene the radical pair triplet state TRP can be reached. The radical pair reaction product quickly decays back to the S0 educt ground states. The kinetics of this reversible H-transfer reaction can be monitored via the phosphorescence decay of the acridine T1 precursor state. Too fast? Look at a slower picture sequence. View also the reaction scheme.