Excited-State Proton Transfer in [2,2'-Bipyridyl]-3,3'-diamine

We have reinvestigated the dynamics of excited-state proton transfer in [2,2'-bipyridyl]-3,3'-diamine, or BP(NH₂)₂. Femtosecond fluorescence upconversion spectroscopy for this molecule [Chem. Phys. Lett. 2005, 407, 487] identified two emission bands following photoexcitation, viz., a shorter wavelength band I identified as the emission from the (normal) diamine form and a longer wavelength band II attributed to the doubly proton transferred diimine form. Both bands were found to have low fluorescence quantum yields, and both decayed in about 250 fs. A subsequent computational investigation [ChemPhysChem 2007, 8, 1199] showed that only the formation of singly proton transferred monoimine is energetically feasible and hence would be the origin of band II. It was also suggested that, following the proton transfer, the timescale of the inter-ring twisting in the monoimine formed may correspond to that of the decay of band II. A recent study including excited-state trajectory simulations [New J. Chem. 2020, 44, 8018] showed that only the monoimine is formed and that the timescale of the proton transfer is commensurate with the experimental timescale. Revisiting BP(NH₂)₂ in the present work, we have used trajectory surface hopping simulations to study the proton transfer dynamics and decay rate of the experimental fluorescence signals. We find that the molecule shows both C₂ and C_i types of ground-state minima, while only a C_i form is present on the lowest bright state S₁. Initiating dynamics on S₁ from both ground-state minima, we also find that only single proton transfer takes place, with our proton transfer times in agreement with both experiments and prior simulation studies. Our key findings are about the dynamics after the proton transfer. The nascent monoimine twists to near perpendicularity in about 200-300 fs and also loses oscillator strength for the S₀ → S₁ transition en route. These offer a dynamical explanation of the band II decay timescale seen in the experiments and also agree with the aforementioned computational study.