A Series of Ultra-Efficient Blue Borane Fluorophores

We present the first examples of alkylated derivatives of the macropolyhedral boron hydride, anti-B₁₈H₂₂, which is the gain medium in the first borane laser. This new series of ten highly stable and colorless organic-inorganic hybrid clusters are capable of the conversion of UVA irradiation to blue light with fluorescence quantum yields of unity. This study gives a comprehensive description of their synthesis, isolation, and structural characterization together with a delineation of their photophysical properties using a combined theoretical and experimental approach. Treatment of anti-B₁₈H₂₂ 1 with RI (where R = Me or Et) in the presence of AlCl₃ gives a series of alkylated derivatives, R_x-anti-B₁₈H_22-x (where x = 2 to 6), compounds 2-6, in which the 18-vertex octadecaborane cluster architectures are preserved and yet undergo a linear "polyhedral swelling", depending on the number of cluster alkyl substituents. The use of dichloromethane solvent in the synthetic procedure leads to dichlorination of the borane cluster and increased alkylation to give Me₁₁-anti-B₁₈H₉Cl₂ 11, Me₁₂-anti-B₁₈H₈Cl₂ 12, and Me₁₃-anti-B₁₈H₇Cl₂ 13. All new alkyl derivatives are highly stable, extremely efficient (Φ_F = 0.76-1.0) blue fluorophores (λ_ems = 423-427 nm) and are soluble in a wide range of organic solvents and also a polystyrene matrix. The Et₄-anti-B₁₈H₁₈ derivative 4b crystallizes from pentane solution in two phases with consequent multiabsorption and multiemission photophysical properties. An ultrafast transient UV-vis absorption spectroscopic study of compounds 4a and 4b reveals that an efficient excited-state absorption at the emission wavelength inhibits the laser performance of these otherwise remarkable luminescent molecules. All these new compounds add to the growing portfolio of octadecaborane-based luminescent species, and in an effort to broaden the perspective on their highly emissive photophysical properties, we highlight emerging patterns that successive substitutions have on the molecular size of the 18-vertex borane cluster structure and the distribution of the electron density within.

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

As the first discovered organoboron compound with photochromic property, B(ppy)Mes₂ (ppy=2-phenylpyridine, Mes=mesityl) displays rich photochemistry that constitutes a solid foundation for wide applications in optoelectronic fields. In this work, we investigated the B(ppy)Mes₂ to borirane isomerization mechanisms in the three lowest electronic states (S₀ , S₁ , and T₁ ) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the photoisomerization in the S₁ state is dominant, which is initiated by the cleavage of the B-C_ppy bond. After overcoming a barrier of 0.5 eV, the reaction pathway leads to a conical intersection between the S₁ and S₀ states (S₁ /S₀ )_x , from which the decay path may go back to the reactant B(ppy)Mes₂ via a closed-shell intermediate (Int1-S₀ ) or to the product borirane via a biradical intermediate (Int2-S₀ ). Although triplet states are probably involved in the photoinduced process, the possibility of the photoisomerization in T₁ state is very small owing to the weakly allowed S₁ →T₁ intersystem crossing and the high energy barrier (0.77 eV). In addition, we found the photoisomerization is thermally reversible, which is consistent with the experimental observations.

Photochromic System among Boron Hydrides: The Hawthorne Rearrangement

Photoswitchable molecules have attracted wide interest for many applications in chemistry, physics, and materials science. In this work, we revisit the reversible photochemical and thermal rearrangements of the two B₂₀H₁₈^2- isomers reported by Hawthorne and Pilling in 1966, whose mechanism had not been understood so far. We investigate the rearrangements by means of a joint experimental and computational study with the outcome that B₂₀H₁₈^2- represents the first boron-based photochromic system ever reported. Both photochemical and thermal isomerizations occur through the same intermediate and involve a diamond-square-diamond (DSD) mechanism. Given the absence within boron chemistry of named chemical reactions as opposed to organic chemistry, we propose to label the B₂₀H₁₈^2- photo- and thermal isomerization processes as the Hawthorne rearrangement.