Unveiling the dehydrogenation mechanism of dihydrogen-bonded phenol-borane-dimethylamine complex in the ground and excited states

In this work, the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods were adopted to explore the dehydrogenation mechanism of phenol-borane-dimethylamine (phenol-BDMA) complex in ground (S-0) and excited (S-1) states. The analysis of the geometric parameters and infrared (IR) vibrational spectra indicate that the dihydrogen bond H-1 center dot center dot center dot H-2 is significantly strengthened in S-1 state and the dehydrogenation process may occur along the dihydrogen bond H-1 center dot center dot center dot H-2. Upon photo-excitation, the electronic density of phenol-BDMA complex is redistributed and the redistribution of the electronic density can provide driving force for the dehydrogenation process in S-1 state. The potential energy curves reveal that the dehydrogenation process in S-1 state is easier than that in S-0 state. The charge redistribution in S-1 state is beneficial to the dehydrogenation process. Furthermore, it has been found that the dihydrogen bond can promote the dehydrogenation process, while the hydrogen bond can hinder the dehydrogenation process. In addition, the dehydrogenation mechanisms were described in detail. First, the H-1 and H-2 atoms gradually approach each other, and the O and H-3 atoms are also getting closer to each other. Subsequently, the O-H-1 and B-H-2 bonds break, and the H-1 and H-2 atoms dissociate from the O and B atoms, respectively. Hydrogen molecular is formed by combing the H-1 and H-2 atoms after crossing the energy barrier hopping point. In the meantime, the H-3 atoms dissociate from the N atom and transfer to the O atom.