Synthesis and Characterization of New Fluorescent Styrene‐Containing Carborane Derivatives: The Singular Quenching Role of a Phenyl Substituent

Abstract

A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1-Ph-1,2-C(2)B(10)H(11), 1-Me-1,2-C(2)B(10)H(11) and 1,2-C(2)B(10)H(12) with one equivalent of 4-vinylbenzyl chloride leads to the formation of compounds 1-3, whereas the reaction of the dilithium salt of 1,2-C(2)B(10)H(12) with two equivalents of 4-vinylbenzyl chloride gives disubstituted compound 4. The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1-4 were analyzed by X-ray diffraction. All compounds, except 1, display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second C(cluster) atom. In general, closo compounds 2-4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the C(cluster) results in an excellent electron-acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge-separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.

Synthesis and properties of a series of carboranyl-BODIPYs

“…BODIPYs 6 and 7 were poorly emissive in DMSO, as previously reported for 8-sulfur substituted BODIPYs [19,32], while in contrast BODIPYs 3 and 5 showed high fluorescence quantum yields, demonstrating that the ortho -carborane clusters do not quench the fluorescence of boron dipyrromethene dyes. This result is in agreement with previous reports showing that closo -carboranes generally enhance the fluorescence of chromophores [33,34]. …”

Electrochemical and spectroelectrochemical studies of C-benzodiazaborolyl-ortho-carboranes

“…There are many reports of unusual low-energy emissions from 1-mono-and 1,2-di-aryl-orthocarboranes which are explained by charge-transfer between the electron-accepting ortho-carborane and the electron-donating aryl groups. 14, 15 Scheme 2. Excited state geometries for ortho-carboranes proposed to explain lowenergy emissions observed experimentally.…”

Photoluminescence in Carborane–Stilbene Triads: A Structural, Spectroscopic, and Computational Study

“…MO calculations reveal that both HOMO and LUMO are located on the stilbene moieties (see Figure S3 in the Supporting Information); this is very different to carborane‐based dyads with smaller chromophores, in which the HOMO is localized in the conjugated unit and the LUMO in the carborane unit, so that low‐lying charge‐transfer (CT) states are generated . In contrast, the LUMO energy of stilbene (−1.55 eV) is clearly lower than that of carborane (−0.25 eV), so that the lowest excited singlet transition (S 0 →S 1 ), which is formed by a HOMO→LUMO excitation, is determined by stilbene alone.…”

“…Bis‐styrene‐substituted m ‐carborane derivatives 3 , 5 , and 7 were synthesized by nucleophilic substitution at the C cluster atoms (C c ) by using a procedure similar to that for o ‐carborane analogue 1 . For compound 3 nucleophilic substitution was performed with 1,7‐ closo ‐C 2 B 10 H 12 , whereas for 5 and 7 we started from their respective mono‐ and diiodinated derivatives 9‐I‐1,7‐ closo ‐C 2 B 10 H 11 and 9,10‐I 2 ‐1,7‐ closo ‐C 2 B 10 H 10 , which were first prepared by electrophilic substitution at the B atoms .…”