Abstract

9,9′-Spirobifluorene-based o-carboranyl compounds C1 and C2 were prepared and fully characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The solid-state structure of C1 was also determined by single-crystal X-ray diffractometry. The two carboranyl compounds display major absorption bands that are assigned to π−π* transitions involving their spirobifluorene groups, as well as weak intramolecular charge-transfer (ICT) transitions between the o-carboranes and their spirobifluorene groups. While C1 only exhibited high-energy emissions (λem = ca. 350 nm) in THF at 298 K due to locally excited (LE) states assignable to π−π* transitions involving the spirobifluorene group alone, a remarkable emission in the low-energy region was observed in the rigid state, such as in THF at 77 K or the film state. Furthermore, C2 displays intense dual emissive patterns in both high- and low-energy regions in all states. Electronic transitions that were calculated by time-dependent-DFT (TD-DFT) for each compound based on ground (S0) and first-excited (S1) state optimized structures clearly verify that the low-energy emissions are due to ICT-based radiative decays. Calculated energy barriers that are based on the relative energies associated with changes in the dihedral angle around the o-carborane cages in C1 and C2 clearly reveal that the o-carborane cage in C1 rotates more freely than that in C2. All of the molecular features indicate that ICT-based radiative decay is only available to the rigid state in the absence of structural fluctuations, in particular the free-rotation of the o-carborane cage.

Highlights

  • To date, closo-ortho-carborane (1,2-C2 B10 H12 ) derivatives, which are well-known icosahedral boron-cluster compounds, have been widely studied for their interesting optoelectronic properties, which are based on a variety of organic/organometallic luminophores [1–28]

  • These properties facilitate the formation of various donor-acceptor systems, leading to intriguing intramolecular charge-transfer (ICT) transitions between numerous π-conjugated aromatic groups and o-carborane

  • All of the operations were performed in an inert nitrogen atmosphere while using standard Schlenk

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Summary

Introduction

Closo-ortho-carborane (1,2-C2 B10 H12 ) derivatives, which are well-known icosahedral boron-cluster compounds, have been widely studied for their interesting optoelectronic properties, which are based on a variety of organic/organometallic luminophores [1–28]. These desirable electronic properties are due to the unique characteristics of the o-carborane unit, such as its strongly electron-deficient nature, as well as its highly polarizable σ-aromaticity. These features have become ultimate sources for generating specific cages [29–62] These haveof become ultimate sources for generating specific luminescence luminescence behavior infeatures a variety o-carborane-based compounds.

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