Non-adiabatic dynamics simulations of the S1 excited-state relaxation of diacetyl phenylenediamine.

The small molecule built around the benzene ring, diacetyl phenylenediamine (DAPA), has attracted much attention due to its synthesis accessibility, large Stokes shift, etc. However, its meta structure m-DAPA does not fluoresce. In a previous investigation, it was found that such a property is due to the fact that it undergoes an energy-reasonable double proton transfer conical intersection during the deactivation of the S1 excited-state, then returns to the ground state by a nonradiative relaxation process eventually. However, our static electronic structure calculations and non-adiabatic dynamics analysis results indicate that only one reasonable non-adiabatic deactivation channel exists: after being excited to the S1 state, m-DAPA undergoes an ultrafast and barrierless ESIPT process and reaches the single-proton-transfer conical intersection. Subsequently, the system either returns to the keto-form S0 state minimum with proton reversion or returns to the single-proton-transfer S0 minimum after undergoing a slight twist of the acetyl group. The dynamics results show that the S1 excited-state lifetime of m-DAPA is 139 fs. In other words, we propose an efficient single-proton-transfer non-adiabatic deactivation channel of m-DAPA that is different from previous work, which can provide important mechanistic information of similar fluorescent materials.