Distinct Photophysics of the Isomers of B₁₈H₂₂ Explained

Abstract

The photophysics of the two isomers of octadecaborane(22), anti- and syn-B18H22, have been studied by UV–vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B18H22 shows fluorescence with a high quantum yield, ΦF = 0.97, and singlet oxygen O2(1Δg) production (ΦΔ ∼ 0.008). Conversely, isomer syn-B18H22 shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S1 minimum for anti-B18H22 and a conical intersection (CI) between its S0 and S1 states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B18H22, and the system therefore has sufficient initial energy on excitation to reach the (S0/S1) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B18H22 may be due to the significant differences in the geometrical rearrangements at their respective conical intersections. Thus, the syn isomer shows one very large, B–B elongation of 1.2 Å from 1.8 Å in the ground state to 3.0 Å at the CI, whereas the anti isomer shows smaller elongations (below 1 Å) in several B–B connectivities at its (S0/S1)CI. The absorbed energy in S1 for the anti-B18H22 is therefore redistributed vibrationally into several regions of the molecule rather than almost completely into a single vibrational mode as in the case for the syn isomer. The consequent prolonged S1 lifetime for the anti isomer allows for relaxation via fluorescence.

A borane laser

“…A third consideration as a further plausible source of losses is the presence of excited-state absorption at the laser emission wavelength (s al 1 40), although this effect cannot be easily proved. Finally, triplet-triplet and ground-state (s al 0 ) absorptions at the laser wavelength can be ruled out, as they were not detected in the previous work 22 . Interestingly, despite all these losses, anti-B 18 H 22 still emits blue laser light with an efficiency of nearly 10%.…”

“…Of all the known binary boranes (compounds consisting only of boron and hydrogen), anti-B 18 H 22 is the only one to have the property of fluorescence. This attribute was first noticed by Hawthorne et al 21 , and yet only recently described comprehensively, including a full consideration of all transitions between ground and excited electronic states of the molecule 22 . In this latter study, anti-B 18 H 22 was revealed to exhibit an intense blue fluorescence at 407 nm ( Fig.…”

“…We have recently become interested in the photophysical properties of the binary borane anti-B 18 H 22 , the centrosymmetric isomer of octadecaborane (22) that has a polyhedral structure resembling a split soccer ball (Fig. 1).…”

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

“…This attribute was first noticed by Hawthorne et al some years ago, [13] and subsequently described comprehensively, including a full consideration of all transitions between ground and excited electronic states of the molecule. [14] In this latter study, anti-B 18 H 22 was revealed to exhibit an intense blue fluorescence at 407 nm with a quantum yield approaching unity (Φ F = 0.97). [14] Most recently anti-B 18 H 22 has been proved to be useful as a blue-emitting laser material of good efficiency and considerable photostability.…”

Borane Polyhedra as Building Blocks for Unknown but Potentially Isolatable New Molecules – Extensions based on Computations of the Known B18H22 Isomers

“…32 From a consideration of the two-dimensional (2D) 'unfolded' projections from Figure 2, it is reasonable to speculate an extension of the B 18 H 22 structural principle by the addition of further pentagonal pyramids, using the two central fused pentagonal pyramids as a starting structure, as represented schematically in Figure 3.…”