Macropolyhedral syn-B18H22, the Forgotten Isomer

The chemistry and physics of macropolyhedral B18H22 clusters have attracted significant attention dueto theinteresting photophysical properties of anti-B18H22 (blue emission, laser properties) and relatedpotential applications. We have focused our attention on the forgotten syn-B18H22 isomer, which has receivedvery little attention since its discovery compared to its anti-B18H22 isomer, presumably becausenumerous studies have reported this isomer as nonluminescent. In ourstudy, we show that in crystalline form, syn-B18H22 exhibits blue fluorescence and becomes phosphorescentwhen substituted at various positions on the cluster, associated withpeculiar microstructural-dependent effects. This work is a combinedtheoretical and experimental investigation that includes the synthesis,separation, structural characterization, and first elucidation ofthe photophysical properties of three different monothiol-substitutedcluster isomers, [1-HS-syn-B18H21] 1, [3-HS-syn-B18H21] 3, and [4-HS-syn-B18H21] 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 fullycharacterized by NMR spectroscopy, mass spectrometry, single-crystalX-ray diffraction, IR spectroscopy, and luminescence spectroscopy.This study also presents the first report on the mechanochromic shiftin the luminescence of a borane cluster and generally enriches thearea of rather rare boron-based luminescent materials. In addition,we present the first results proving that they are useful constituentsof carbon-free self-assembled monolayers.

Automated summary

This study focuses on the syn-B18H22 isomer, which has been overlooked compared to its anti-B18H22 isomer. It demonstrates that syn-B18H22 exhibits blue fluorescence and phosphorescence when substituted at different positions on the cluster. The study also presents the first results on the mechanochromic shift in the luminescence of a borane cluster and the potential application of these derivatives in carbon-free self-assembled monolayers.