Fluorescence of Radical Ions in Liquid Solution: Wurster's Blue as a Case Study

The fluorescence lifetime of the radical cation of N,N,N',N'-tetramethyl-p-phenylenediamine (Wurster's blue) decreases from 260 ps at 82 K to 200 fs at room temperature. Calculations indicate a small barrier between the excited-state minimum (D1 min) and a conical intersection (CI) of the excited and ground state potentials. The intersection is reached within 200 fs upon torsion of one of the C—N bonds