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.