Manganese-4,5,6,7-tetrahydrobenzimidazole: Dual Emission and the Effect of Proton Transfers

An organic–inorganic cation–anion manganese(II) tetrabromide compound with diprotonated 1-hydroxy-2-(pyridin-2-yl)-4,5,6,7-tetrahydrobenzimidazole, [H[sub.3] L][MnBr[sub.4] ][H[sub.2] O], has been synthesized and investigated. The compound has a few possible pathways for proton transfers, which play an important role in the observed luminescence, optical, and magnetic properties. The proton transfers result in the appearance of two-band luminescence. One band is caused by the Mn(II) d-d transitions. The other band is caused by the transition from the triplet state of organic cation and the d-d transition of manganese(II) coupled through {[H[sub.3] L]}-{[MnBr[sub.4] ]}-{[H[sub.2] O]} vibrations. The optical absorption spectra of [H[sub.3] L][MnBr[sub.4] ][H[sub.2] O] indicate the presence of two direct and one indirect band transitions. The reason for the two-band luminescence and complex optical absorption in [H[sub.3] L][MnBr[sub.4] ][H[sub.2] O] were additionally considered using the DFT calculations.