Structures, Atomic Charges, Emission Properties and DFT studies of Biquinoline Derivatives Induced by Protonation of a Nitrogen Atom

Abstract We prepared a monoprotonated compound series comprising dmphenHCl ([1H]Cl), dmbpyHCl ([2H]Cl), dmbpyHPF6 ([2H]PF6), bqnHCl ([3H]Cl), bqnHPF6 ([3H]PF6), dmbqnHPF6 ([4H]PF6), naphthyridineHPF6 ([5H]PF6) and phenazineHPF6 ([6H]PF6) and elucidated the crystal structures of [1H]Cl, [2H]Cl, [2H]PF6, [4H]PF6, [5H]PF6 and [6H]PF6. Density functional theory (DFT) calculations of the protonated compounds involving 1, [1H]Cl, [1H]PF6, 2, [2H]Cl, [2H]+, 3, [3H]Cl, [3H]+, 4, [4H]+, 5, [5H]+, 6 and [6H]+ were subsequently performed. Further, time-dependent DFT (TD-DFT) analyses were carried out to elucidate the excited singlet structure. Moreover, the atomic charges of these compounds were examined in detail, and the 1H NMR chemical shifts were correlated to atomic charges in the molecule. Notably, the attachment of one proton to a nitrogen atom in these compounds imparted ionic properties, which resulted in strong red-shifted emissions compared to those of their non-protonated counterparts. In particular, the protonated compound [4H]PF6, with two methyl substituents, displayed the strongest emission.