Synthesis and characterization of heterofluorenes containing four-coordinated group 13 elements: theoretical and experimental analyses and comparison of structures, optical properties and electronic states

Abstract

Herein, we report the syntheses of dibenzoheteroles, namely, heterofluorenes, containing four-coordinated group 13 elements (boron Bf; aluminum Alf; gallium Gaf; indium Inf) and the relationship between their structures and optical properties. The electronic states of the compounds were considered theoretically by the density functional theory (DFT) calculation. In particular, we focused on their emission behaviors and electronic structures in the excited states. Initially, we confirmed that Bf and Gaf showed high stability in water and air, while Alf and Inf were sensitive. The structures of heterofluorenes, involving the heavier elements in the 13^th group, tend to form trigonal planar structures even in the presence of coordination by nitrogen. Next, in their emissions, larger contribution from the triplet excited states was observed in the heterofluorenes with heavier elements. The major emission of Inf at 77 K was attributed to phosphorescence. These phosphorescence properties can be explained by the heavy atom effect. In Gaf and Inf, their excited states were deactivated by vibrational relaxation in their triplet excited states at room temperature. In Bf, Alf and Gaf when adding B(C₆F₅)₃, the emissions oriented from the triplet exciplex were observed. Time-dependent DFT (TD-DFT) calculations revealed that the optimized structure of Bf in the excited S₁ state has a considerably different geometry from those of Alf and Gaf. Finally, we obtained the data that the B–N bond could be cleaved in the excited S₁ of Bf according to the B–N bond length and bond order. As a result, the lower intensity of the emission of Bf was comparable to that of Alf. This bond cleavage could be caused by an increase of the anti-bonding property in the B–N bond in the Franck–Condon (FC) S₁ state and by weak electrostatic interaction between boron and nitrogen atoms. In Alf and Gaf, although the anti-bonding character of the M–N bonds (M = Al or Ga) in the FC S₁ states also increases, the M–N bonds survive because of their stronger electrostatic interaction. The subsequent stronger emission in Alf and Gaf could be observed by suppressing the molecular motion in the excited states.