Thermochromic Fluorescence from B18H20(NC5H5)2: An Inorganic-Organic Composite Luminescent Compound with an Unusual Molecular Geometry

B18H20(NC5H5)(2) is a rare example of two conjoined boron hydride subclusters of nido and arachno geometrical character. At room temperature, solutions of B18H20(NC5H5)(2) emit a 690 nm fluorescence. In the solid state, this emission is shifted to 620 nm and intensifies due to restriction of the rotation of the pyridine ligands. In addition, there is a thermochromicity to the fluorescence of B18H20(NC5H5)(2). Cooling to 8 K engenders a further shift in the emission wavelength to 585 nm and a twofold increase in intensity. Immobilization in a polystyrene thin-film matrix results in an efficient absorption of pumping excitation energy at 414 nm and a 609 nm photostable fluorescence. Such fluorescence from polystyrene thin films containing B18H20(NC5H5)(2) can also be stimulated by emission from the highly fluorescent borane anti-B18H22 via energy transfer mechanisms. Polystyrene thin-film membranes doped with 1: 1 mixtures of anti-B18H22 and B18H20(NC5H5)(2) thus emit a 609 nm fluorescence and absorb light across more than 300 nm (250-550 nm); this is a significant spectral coverage possibly useful for luminescent solar concentrators. B18H20(NC5H5)(2) is fully structurally characterized using NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analysis, and its ground-state and excited-state photophysics are investigated with UV-vis spectroscopy and quantum-chemistry computational methods.