Photophysical Properties of Spirobifluorene-Based o-Carboranyl Compounds Altered by Structurally Rotating the Carborane Cages.

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.

Synthesis and Dual-Emission Feature of Salen-Al/Triarylborane Dyads.

Novel salen-Al/triarylborane dyad complexes were prepared and characterized with their corresponding mononuclear compounds. The UV-vis and photoluminescence experiments for dyads exhibited photoinduced energy transfer from borane to the salen-Al moiety in an intramolecular manner. Theoretical calculation and fluoride titration results further supported these intramolecular energy-transfer features.

Salen-indium/triarylborane triads: synthesis and ratiometric emission-colour changes by fluoride ion binding.

Salen-based indium triads, [{(3-tBu)2-(5-Mes2B)2-salen}In-Me] (1) and [{(3-tBu)2-(5-Mes2Bphenyl)2-salen}In-Me] (2), bearing triarylborane (TAB) units were prepared and fully characterised by NMR spectroscopy and elemental analysis. The major absorption bands of 1 and 2 appeared in the region centred at 347 nm and 374 nm, respectively, and the intense emission spectra were observed in the sky blue (λem = 491 nm for 1) and bluish-green (λem = 498 nm for 2) regions, respectively. The solvatochromism effects in various organic solvents and computational calculation results strongly suggested that these absorption and emission features are mainly attributed to intramolecular charge transfer (ICT) transitions between the salen ligand moieties and the TAB units. Furthermore, UV-vis and photoluminescence (PL) titration experiments by the addition of fluoride anions demonstrated ratiometric quenching patterns in both the absorption and emission spectra, indicating that binding of the fluoride anion to the boron centres interrupts these ICT transitions in each compound. Interestingly, both triads exhibited a gradual red-shifted response in each emission spectrum upon the addition of the fluoride anions, resulting in a dramatic colour-change to yellow. The computational calculation results of the S1 states revealed that these emission-colour change properties arise from the elevation of HOMO levels, which are mainly localised on the TAB moieties, resulting from the fluoride anion binding to the borane centres.