Macropolyhedral syn-B18H22, the "Forgotten" Isomer

Abstract

The chemistry and physics of macropolyhedral B₁₈H₂₂ clusters have attracted significant attention due to the interesting photophysical properties of anti-B₁₈H₂₂ (blue emission, laser properties) and related potential applications. We have focused our attention on the "forgotten" syn-B₁₈H₂₂ isomer, which has received very little attention since its discovery compared to its anti-B₁₈H₂₂ isomer, presumably because numerous studies have reported this isomer as nonluminescent. In our study, we show that in crystalline form, syn-B₁₈H₂₂ 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-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 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.

Figure 1

(A) Schematic of syn-B₁₈H₂₂ with partial numbering on one of the two subclusters. (B, C) Complete numbering systems of syn-B₁₈H₂₂ and anti-B₁₈H₂₂, respectively, in their net-like representation. (D) Electrostatic potential map of syn-B₁₈H₂₂ with selected BH vertices numbered. On the color scale, red shows areas with the highest negative potential, localized mainly in between vertices 1, 3, and 4. The red part in between the vertices 1′, 10, and 10′ represents the negative pole of the molecule. (E) Schematic of the synthesis of the three HS-syn-B₁₈H₂₁ isomers of syn-B₁₈H₂₂.

Figure 2

X-ray determined structures of three synthesized thiol isomers of syn-B₁₈H₂₂.

Figure 3

Plot of energies of various HS-rotamers for all four computationally analyzed HS-syn-B₁₈H₂₁ isomers. Torsion angles were defined as positive in the B(1)-B(2)-B(3)-B(4) direction, between atoms B(2)-B(1)-S(1)-H(S) for isomer 1, B(6)-B(2)-S(2)-H(S) for the experimentally unobtained isomer 2, B(2)-B(3)-S(3)-H(S) for 3, and B(1)-B(4)-S(4)-H(S) for 4.

Figure 4

Experimentally decoupled ¹¹B{¹H} NMR spectra of syn-B₁₈H₂₂ and HS-syn-B₁₈H₂₁ isomers.

Figure 5

Schematic illustration of selected distances and bond angles of syn-B₁₈H₂₂ and its thiol derivatives.

Figure 6

–S(H)···μH-BHB interaction observed in the supramolecular structures of HS-syn-B₁₈H₂₁ isomers/polymorphs.

Figure 7

Normalized emission spectra (plain lines) of as-received syn-B₁₈H₂₂ (black) and recrystallized syn-B₁₈H₂₂ (red) excited at 380 nm in the air atmosphere; normalized excitation spectra (dashed lines) recorded at the maximum of emission (A). Fluorescence decay kinetics of syn-B₁₈H₂₂ in the air atmosphere, excited at 402 nm, recorded at the maximum of emission (B).

Figure 8

Normalized emission spectra (plain lines) of single crystals of 1, 3, and 4 excited at 380 nm in the air atmosphere; normalized excitation spectra (dashed lines) recorded at the maximum of emission (A). Phosphorescence decay kinetics of single crystals of 1, 3, and 4 in the air atmosphere, excited at 380 nm, recorded at the maximum of emission (B).

Figure 9

Normalized emission spectra (plain lines) of single crystals (black) and powder (red) of 4, excited at 380 nm in an air atmosphere; normalized excitation spectra (dashed lines) recorded at the maximum of emission (A). Phosphorescence decay kinetics of single crystals (black) and powder (red) of 4 in an air atmosphere, excited at 380 nm, recorded at the maximum of emission (B).

Figure 10

High-resolution S 2p (A) and B 1s (B) X-ray photoelectron spectra of the SAM formed by vacuum vapor deposition of isomer [4-HS-syn-B₁₈H₂₁] 4 on a Ag substrate. For better visualization, the S 2p spectrum is multiplied by a factor of 4.

Figure 11

All three isomers are oriented with the HS group downward (top row), the respective top view (middle), and the space-filling projections (also top view). The yellow circle represents the position of the sulfur atom. Circles around the space-filling models show the lateral space requirement of the molecules rotating along their B–S axis.