Macropolyhedral syn -B 18 H 22 , the "Forgotten" Isomer.

The chemistry and physics of macropolyhedral B 18 H 22 clusters have attracted significant attention due to the interesting photophysical properties of anti -B 18 H 22 (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn -B 18 H 22 isomer, which has received very little attention since its discovery compared to its anti -B 18 H 22 isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn -B 18 H 22 exhibits blue fluorescence and becomes phosphorescent when substituted at various positions on the cluster, associated with peculiar microstructural-dependent effects. This work is a combined theoretical and experimental investigation that includes the synthesis, separation, structural characterization, and first elucidation of the photophysical properties of three different monothiol-substituted cluster isomers, [1-HS- syn -B 18 H 21 ] 1 , [3-HS- syn -B 18 H 21 ] 3 , and [4-HS- syn -B 18 H 21 ] 4 , of which isomers 1 and 4 have been proved to exist in two different polymorphic forms. All of these newly substituted macropolyhedral cluster derivatives ( 1 , 3 , and 4 ) have been fully characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, IR spectroscopy, and luminescence spectroscopy. This study also presents the first report on the mechanochromic shift in the luminescence of a borane cluster and generally enriches the area of rather rare boron-based luminescent materials. In addition, we present the first results proving that they are useful constituents of carbon-free self-assembled monolayers.