Multiple photoluminescence of spiro[acridine-fluorene]-based o-carboranyl compounds with potential as a visual sensory material

Two spiro[acridine-9,9 '-fluorene]-based closo-o-carboranyl compounds, namely p-SAC and o-SAC, were prepared and fully characterized. p-SAC exhibited a weak high energy emission trace only in tetrahydrofuran (THF) at 298 K, while the photoluminescence (PL) spectra at 77 K exhibited intense emission in the low energy region. However, o-SAC exhibited an excellent dual-emissive pattern in THF at both 298 and 77 K. The electronic transition in each excited state (S-1) was calculated, which confirmed that the high and low energy emission originated from locally excited (LE) states on the fluorene moieties and intramolecular charge-transfer (ICT) transitions corresponding to o-carboranes, respectively. All these characteristics indicated that ICT-based radiative decay was only favored in the rigid state, where structural fluctuations were restricted. Energy barriers were calculated based on relative energies at various dihedral angles around the o-carborane cages in p-SAC and o-SAC. The rotational motion of the o-carborane cage was less restricted in p-SAC when compared to o-SAC, resulting in suppression of the ICT-based emission when p-SAC was in solution. The PL experiments in the THF/water mixtures indicated that these features were caused by the aggregation-induced emission (AIE) effect. An acetonitrile solution containing relatively high concentrations of o-SAC (ca. 10(-3) M) exhibited a dramatic emission color change from deep red to sky blue when the temperature was increased. The higher temperature caused a natural conversion from a colloidal state (slightly aggregated) to a clear solution. Consequently, the photophysical features of p-SAC and o-SAC demonstrated the application potential of pi-aromatic conjugated o-carboranyl compounds as visual sensory materials.