Effect of Iodination on the Photophysics of the Laser Borane anti-B18H22: Generation of Efficient Photosensitizers of Oxygen

Expanding Luminescence Horizons in Macropolyhedral Heteroboranes

Luminescence is observed in three novel macropolyhedral nineteen- and eighteen-vertex chalcogenaboranes: Se2B17H17 (1), SeB17H19 (3) and SeB18H20 (4). This led us to the recognition that previously published macropolyhedral heteroborane species might also exhibit luminescence. Thus, the known nineteen- and eighteen-vertex dithiaboranes S2B17H17 (2), n-S2B16H16 (5) and i-S2B16H16 (6) were synthesised and also found to exhibit a range of luminescent properties. These macropolyhedral species are very different from the previously unique fluorescent binary borane B18H22 in terms of their structural architectures, by the presence of borane cluster hetero atoms, and, as in the cases of 5 and 6, that their synthetic origins are not derived simply through the modification of B18H22 itself. They consequently greatly expand the possibilities of finding new luminescent inorganic borane species.

Decaborane: From Alfred Stock and Rocket Fuel Projects to Nowadays

The review covers more than a century of decaborane chemistry from the first synthesis by Alfred Stock to the present day. The main attention is paid to the reactions of the substitution of hydrogen atoms by various atoms and groups with the formation of exo-polyhedral boron-halogen, boron-oxygen, boron-sulfur, boron-nitrogen, boron-phosphorus, and boron-carbon bonds. Particular attention is paid to the chemistry of conjucto-borane anti-[B18H22], whose structure is formed by two decaborane moieties with a common edge, the chemistry of which has been intensively developed in the last decade.

Macropolyhedral syn-B18H22, the "Forgotten" Isomer

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.

A Window into the Workings of anti-B18H22 Luminescence-Blue-Fluorescent Isomeric Pair 3,3'-Cl2-B18H20 and 3,4'-Cl2-B18H20 (and Others)

The action of AlCl₃ on room-temperature tetrachloromethane solutions of anti-B₁₈H₂₂ (1) results in a mixture of fluorescent isomers, 3,3'-Cl₂-B₁₈H₂₀ (2) and 3,4'-Cl₂-B₁₈H₂₀ (3), together isolated in a 76% yield. Compounds 2 and 3 are capable of the stable emission of blue light under UV-excitation. In addition, small amounts of other dichlorinated isomers, 4,4'-Cl₂-B₁₈H₂₀ (4), 3,1'-Cl₂-B₁₈H₂₀ (5), and 7,3'-Cl₂-B₁₈H₂₀ (6) were isolated, along with blue-fluorescent monochlorinated derivatives, 3-Cl-B₁₈H₂₁ (7) and 4-Cl-B₁₈H₂₁ (8), and trichlorinated species 3,4,3'-Cl₃-B₁₈H₁₉ (9) and 3,4,4'-Cl₃-B₁₈H₁₉ (10). The molecular structures of these new chlorinated derivatives of octadecaborane are delineated, and the photophysics of some of these species are discussed in the context of the influence that chlorination bears on the luminescence of anti-B₁₈H₂₂. In particular, this study produces important information on the effect that the cluster position of these substitutions has on luminescence quantum yields and excited-state lifetimes.

Metal-Catalyzed and Metal-Free Nucleophilic Substitution of 7-I-B18H21

In this work, two pathways of reactivity are investigated to generate site-specific substitutions at the B7 vertex of the luminescent boron cluster, anti-B₁₈H₂₂. First, a palladium-catalyzed cross-coupling reaction utilizing the precursor 7-I-B₁₈H₂₁ and a series of model nucleophiles was developed, ultimately producing several B-N- and B-O-substituted species. Interestingly, the B-I bond in this cluster can also be substituted in an uncatalyzed fashion, leading to the formation of various B-N, B-O, and B-S products. This work highlights intricate differences corresponding to these two reaction pathways and analyzes the role of solvents and additives on product distributions. As a result of our synthetic studies, seven new B₁₈-based clusters were synthesized, isolated, and characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The photoluminescence properties of two structurally similar ether and thioether products were further investigated, with both exhibiting blue fluorescence in solution at 298 K and long-lived green or yellow phosphorescence at 77 K. Overall, this work shows, for the first time, the ability to perform substitution of a boron-halogen bond with nucleophiles in a B₁₈-based cluster, resulting in the formation of photoluminescent molecules.

Incorporation of the New anti-Octadecaborane Laser Dyes into Thin Polymer Films: A Temperature-Dependent Photoluminescence and Infrared Spectroscopy Study

New anti-octadecaborane(22) laser dyes have been recently introduced. However, their application in solid thin films is limited, despite being very desirable for electronics. Spectroscopic methods, photoluminescence (PL), and infrared reflection-absorption spectroscopy (IRRAS), are here used to reveal structural responses to a temperature change in thin polymer films made of π- and σ-conjugated and non-conjugated polymers and anti-octadecaborane(22) and its tetra-alkylatedderivatives. It has been observed that borane clusters are not firmly fixed within polymer matrices and that their ability for diffusion out of the polymer film is unprecedented, especially at higher temperatures. This ability is related to thermodynamic transitions of polymer macromolecular chains. PL and IRRAS spectra have revealed a clear correlation with β-transition and α-transition of polymers. The influence of structure and molecular weight of a polymer and the concentration and the substitution type of clusters on mobility of borane clusters within the polymer matrix is demonstrated. A solution is proposed that led to an improvement of the temperature stability of films by 45 °C. The well-known spectroscopic methods have proved to be powerful tools for a non-routine description of the temperature behavior of both borane clusters and polymer matrices.

Macropolyhedral Chalcogenaboranes: Insertion of Selenium into the Isomers of B18H22

High yields of novel macropolyhedral selenaboranes are reported. Reactions of the monoanions of the syn- and anti-isomers of B₁₈H₂₂ with powdered selenium in THF variously give new macropolyhedral selenaboranes: 19-vertex [SeB₁₈H₁₉]^- anion 1, 19-vertex [SeB₁₈H₂₁]^- anion 2, 20-vertex [Se₂B₁₈H₁₉]^- anion 3, and 19-vertex [Se₂B₁₇H₁₈]^- anion 4. Single-cluster [hypho-Se₂B₆H₉]^- anion 5 and neutral arachno-Se₂B₇H₉ 6 also result. All of the macropolyhedrals 1, 2, 3, and 4 are characterized by NMR spectroscopy and mass spectrometry, and by single-crystal X-ray diffraction analyses. Anions 1 and 2 each consist of an 11-vertex subcluster joined by a common two-boron edge to a 10-vertex subcluster. Anion 3 consists of an 11-vertex subcluster joined by a common boron atom and an interboron link to an arachno-type 10-vertex subcluster. Unusually, anion 3 incorporates a hexagonal pyramidal intracluster structural motif in its 11-vertex subcluster. Anion 4 entails two arachno-type 10-vertex subclusters joined by a common boron atom, and with an additional intercluster boron-boron link. NMR data for syn-B₁₈H₂₂ and its mono- and dianions 7 and 8 and single-crystal X-ray diffraction results for these anions and also the monoanion 9 of anti-B₁₈H₂₂ are also reported. The oxaborane [μ-(8,9)-O-syn-B₁₈H₂₀]^2- dianion 10 was serendipitously formed during the work and also characterized by a single-crystal X-ray diffraction study. Experimental NMR and structural findings are supported by DFT calculations throughout.

Benchmarking the dynamic luminescent properties and UV stability of B18H22-based materials

The dynamic photoluminescent properties, and potential quenching mechanisms, of anti-B18H22, 4,4’-Br2-anti-B18H20, and 4,4’-I2-anti-B18H20 are investigated in solution and polymer films. UV stability studies of the neat powders show no decomposition...

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.

Theoretical insights on type I/II photoreactions of potential Zn(II) polypyridyl photosensitizers for two-photon photodynamic therapy

Large two-photon absorption cross-sections are vital to photosensitizers (PSs) in TP-PDT, which can be used to develop in-depth treatment for diseased cells and minimize the harm to surrounding cells. Here, we conduct a study about photophysical properties of one Ru(II) polypyridyl complex and two designed Zn(II) polypyridyl complexes by means of DFT and TD-DFT methods. The main results are as follows: firstly, the two-photon absorption spectrum of two designed complexes Zn-OMe and Zn-OCOOCH₃ are all within the phototherapeutic window (550-900 nm). Secondly, large SOC values and small energy gaps ΔE_S-T of these complexes guarantee the efficiency of ISC process. Thirdly, their T₁ energy is greater than that required for generating ¹O₂ (0.98 eV) via Type II photoreaction. In addition, the calculated results of vertical electron affinities (VEA) and vertical ionization potentials (VIP) show that these complexes are able to form superoxide ions O₂^(-) via Type I photoreaction. Specifically, both of two designed Zn-centric complexes have larger TPA cross-sections than that of Ru-centric complex. In a word, we are pleased to report two potential photosensitizers with excellent performance and reasonable price for Type I/II photoreactions. We expect our study will offer some theoretical guidance and help in TP-PDT.

A molecular boron cluster-based chromophore with dual emission

Bromination of the luminescent borane, anti-B18H22, via electrophilic substitution using AlCl3 and Br2, yields the monosubstituted derivative 4-Br-anti-B18H21 as an air-stable crystalline solid. In contrast to the unsubstituted parent compound, 4-Br-anti-B18H21 possesses dual emission upon excitation with UV light and exhibits fluorescence at 410 nm and phosphorescence at 503 nm, with Φtotal = 0.07 in oxygen-free cyclohexane. Increased oxygen content in cyclohexane solution quenches the phosphorescence signal. The fluorescent signal intensity remains unaffected by oxygen, suggesting that this molecule could be used as a ratiometric oxygen probe.

Swollen Polyhedral Volume of the anti-B18H22 Cluster via Extensive Methylation: anti-B18H8Cl2Me12

Methylation of anti-B₁₈H₂₂ (1) affords the first example of alkyl substitution of terminal hydrogen atoms on the fluorescent octadecaborane-22 molecule to give highly methylated 2,2'-Cl₂-1,1',3,3',4,4',7,7',8,8',10,10'-Me₁₂-anti-B₁₈H₈ (2). This extensive chemical substitution leads to a swelling in the polyhedral volume of the 18-vertex boron atomic skeleton of the molecule and an enhancement of the absorption and solubility characteristics of this highly fluorescent molecule. We propose this "swollen polyhedral volume" to be the result of a marked increase in the relative positivity of the "cluster-only total charge" of the boron atomic skeleton caused by the combined electron-withdrawing capacity of the 12 methyl groups. Enhancement in the absorption and solubility properties may be crucial in the design of new borane-based laser materials.

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.

Macropolyhedral Nickelaboranes from the Metal-Assisted Fusion of KB9H14

The reaction of K[arachno-B₉H₁₄] with [NiCl₂(dppe)] produces four new 19-vertex macropolyhedral metallaboranes that result from borane cluster fusion: [9'-(dppe)-9'-Ni-anti-B₁₈H₂₀] (1) and isomeric [11'-(dppe)-11'-Ni-syn-B₁₈H₂₀] (2), together with the chlorine-substituted derivative of 1, [5'-Cl-9'-(dppe)-9'-Ni-anti-B₁₈H₁₉] (3), and the 18-vertex cluster compound [7'-(dppe)-7'-anti-NiB₁₇H₂₁] (4). Two closo 10-vertex single-cluster species, [1-(dppe)-1-closo-NiB₉H₇Cl₂] (5) and [1-(dppe)-1-closo-NiB₉H₇Cl(OH)] (6), were also isolated from the reaction. The production of the metalated syn-octadecaborane isomer 2 from the fusion of two arachno-nonaborate clusters is the first such case to be observed; in all other reported cases fusion has resulted in products with the anti-octadecaboranyl bis-nido configuration.