Substitution of the laser borane anti-B₁₈H₂₂ with pyridine: a structural and photophysical study of some unusually structured macropolyhedral boron hydrides

Abstract

Reaction of anti-BH1 with pyridine in neutral solvents gives sparingly soluble BH-3',8'-Py3a as the major product (ca. 53%) and BH-6',9'-Py2 (ca. 15%) as the minor product, with small quantities of BH-8'-Py 4 (ca. 1%) also being formed. The three new compounds 2, 3a and 4 are characterized by single-crystal X-ray diffraction analyses and by multinuclear multiple-resonance NMR spectroscopy. Compound 2 is of ten-vertex nido:ten-vertex arachno two-atoms-in-common architecture, long postulated for a species with borons-only cluster constitution, but previously elusive. Compound 3a is of unprecedented ten-vertex nido:eight-vertex arachno two-atoms-in-common architecture. The single-crystal X-ray diffraction analysis for the picoline derivative BH(NCHMe)3b, similarly obtained, is also presented. BHPy 4 is also previously unreported but is of known ten-vertex nido:ten-vertex nido two-atoms-in-common architecture of anti configuration, but now with the pyridine ligand positioned differently to other reported examples of BHL compounds. Factors behind the remarkably low solubility of 3a and 3b are elucidated in terms of electrostatic potential (ESP) calculations, polarity, and van der Waals complementarities. In view of contemporary developing high interest in the fluorescent properties of macropolyhedral boron-containing species, a detailed assessment of the photophysical characteristics of 3a and 4 is also presented. In contrast to the thermochromic fluorescence of 2 (from 620 nm brick-red at room temperature to 585 nm yellow at 8 K, quantum yield 0.15), compound 3a is only weakly phosphorescent in the yellow region (590 nm, quantum yield 0.01), whereas compound 4 exhibits no luminescence. The far more photoactive nature of compound 2 is associated with S excited-state minima structures that differ from each other only by the relative rotational positions of the pyridine substituents on its disubstituted ten-vertex {arachno-BPy}-subcluster. The wavelength and relative intensity of fluorescence from these structures depends on the rotational positions of the pyridine ligands, which in turn are influenced by temperature and/or rotational inhibition in the solid-state.

Insights into the Photoinduced Isomerization Mechanisms of a N,C–Chelate Organoboron Compound: A Theoretical Study

“…[65] Similar to our results, the around 1.5 cm À 1 SOC values between S 1 state and T 1 state were obtained as evidence for the low but non-negligible ISC probability, allowing triplet state population and weak phosphorescence in pyridine(Py)-substituted boron cluster B 16 H 18 Py 2 . [29] Quantum dynamics simulations showed that ultrafast ISC occurs in small aromatic hydrocarbons such as benzene via spin-vibronic effects activated by the prefulvene mode where weakly coupled states (S 1 , T 2 , T 1 states) are near degenerate, leading to efficient crossing. [66][67][68][69] We anticipate that quantum dynamics simulations will provide more insights into the photochemical and photophysical processes of B(ppy)Mes 2.…”

“…CIs are crossings between the potential energy surfaces of the same multiplicity, which act as efficient decay funnels from excited states to ground state . CIs have proven to play crucial roles in boron cluster chemistry and to be basic mechanistic elements for important photophysical and photochemical processes such as intramolecular electron transfer, photoisomerization, Wolff rearrangement, Curtius rearrangement, and aggregation‐induced emission …”

“…[23] CIs are crossings between the potential energy surfaces of the same multiplicity, which act as efficient decay funnels from excited states to ground state. [25][26][27] CIs have proven to play crucial roles in boron cluster chemistry [28][29][30][31] and to be basic mechanistic elements for important photophysical and photochemical processes such as intramolecular electron transfer, [32] photoisomerization, [33] Wolff rearrangement, [34,35] Curtius rearrangement, [36] and aggregationinduced emission. [37,38] Although the above mentioned computational studies have established that the phototransformations of B(ppy)Mes 2 -based compounds can be rationalized by the reaction pathways on the potential energy surface of electronic singlet states, there exist several experimental arguments about the involvement of photoactive triplet states in this class of reactions.…”

Toward Luminescent Iron Complexes: Unravelling the Photophysics by Computing Potential Energy Surfaces

“…Even if, and especially in the case of ultrafast processes and high excess of kinetic energy, other areas of higher energy can be visited, and only dynamic methods would allow to estimate their relevance, MEPs represent the most probable evolution of a chemical system. Indeed, the calculations of MEPs connecting Franck‐Condon regions with excited‐state minima and conical intersections in the framework of the Photoreactive Path Approach (PRPA) has allowed significant breakthrough in the study of photochemical and photophysical processes of organic and inorganic systems . As pointed out by Lindh and co‐workers, “ PRPA is based on mapping the path for the most efficient energy relaxation in the excited state […] Only this procedure guarantees accurate results within the static model” .…”

A Study on the Fluorescence Properties of New Laser Material-n-B18H22 in SDS Aqueous Solution

“…This invention set a key milestone in the field of lasers. Since then, -B 18 H 22 has aroused deep interest of relevant scientists and witnessed increasingly relevant reports [16][17][18].…”