Excited-State Hydrogen and Dihydrogen Bonding of a Dihydrogen-Bonded Phenol–Borane–Dimethylamine Complex

The excited-state hydrogen and dihydrogen bonding of a dihydrogen-bonded phenol–borane–dimethylamine (BDMA) complex was investigated theoretically by use of time-dependent density functional theory (TDDFT). The coexistence of an intermolecular dihydrogen bond (B–H···H–O) and hydrogen bond (N–H···O) was confirmed by the optimized geometric structure of the dihydrogen-bonded phenol–BDMA complex. The infrared spectra for both the ground and excited states of the dihydrogen-bonded phenol–BDMA complex were also calculated by use of density functional theory (DFT)/TDDFT. As a result, we demonstrated theoretically that the intermolecular dihydrogen bonds B–H···H–O can be significantly strengthened in the excited state. However, the intermolecular hydrogen bond N–H···O in the dihydrogen-bonded phenol–BDMA complex is weakened upon photoexcitation to the S1state. The dynamic changes of the intermolecular dihydrogen and hydrogen bonding are consistent with the calculated bond lengths in different electronic states. The coexistent dihydrogen and hydrogen bonding in the electronic excited states of this dihydrogen-bonded complex was studied theoretically in this work. Furthermore, it was found that the N–H···O hydrogen bond in the cyclic structure of phenol–BDMA complex hindered the dehydrogenation reaction between dihydrogen bonded O–H and B–H1groups, which has been reported for the phenol–borane–trimethylamine complex.