Fluorescence of New o-Carborane Compounds with Different Fluorophores: Can it be Tuned?

• Published in: Chemistry : a European journal 2014-08, Vol.20 (32), p.9940-9951
• Main Authors: Ferrer-Ugalde, Albert, González-Campo, Arántzazu, Viñas, Clara, Rodríguez-Romero, Jesús, Santillan, Rosa, Farfán, Norberto, Sillanpää, Reijo, Sousa-Pedrares, Antonio, Núñez, Rosario, Teixidor, Francesc
• Format: Article
• Language: English
• Subjects: anthracene; Bonding; Carbon; Carborane; carboranes; Chemical compounds; Chemistry; Correlation; Derivatives; fluorene; Fluorescence; photoluminescence; Quenching
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201402396

Two sets of o‐carborane derivatives incorporating fluorene and anthracene fragments as fluorophore groups have been successfully synthesized and characterized, and their photophysical properties studied. The first set, comprising fluorene‐containing carboranes 6–9, was prepared by catalyzed hydrosilylation reactions of ethynylfluorene with appropriate carboranylsilanes. The compound 1‐[(9,9‐dioctyl‐fluorene‐2‐yl)ethynyl]carborane (11) was synthesized by the reaction of 9,9‐dioctyl‐2‐ethynylfluorene and decaborane (B10H14). Furthermore, reactions of the lithium salt of 11 with 1 equivalent of 4‐(chloromethyl)styrene or 9‐(chloromethyl)anthracene yielded compounds 12 and 13. Members of the second set of derivatives, comprising anthracene‐containing carboranes, were synthesized by reactions of monolithium or dilithium salts of 1‐Me‐1,2‐C2B10H11, 1‐Ph‐1,2‐C2B10H11, and 1,2‐C2B10H12 with 1 or 2 equivalents of 9‐(chloromethyl)anthracene, respectively, to produce compounds 14–16. In addition, 2 equivalents of the monolithium salts of 1‐Me‐1,2‐C2B10H11 (Me‐o‐carborane) and 1‐Ph‐1,2‐C2B10H11 (Ph‐o‐carborane) were reacted with 9,10‐bis(chloromethyl)anthracene to produce compounds 17 and 18, respectively. Fluorene derivatives 6–9 exhibit moderate fluorescence quantum yields (32–44 %), whereas 11–13, in which the fluorophore is bonded to the Ccluster (Cc), show very low emission intensity (6 %) or complete fluorescence quenching. The anthracenyl derivatives containing the Me‐o‐carborane moiety exhibit notably high fluorescence emissions, with ϕF=82 and 94 %, whereas their Ph‐o‐carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the CcCc bond length and the fluorescence intensity in CH2Cl2 solution, comparable to that observed for previously reported styrene‐containing carboranes. Thus, our hypothesis is that for systems of this type the fluorescence may be tuned and even predicted by changing the substituent on the adjacent Cc. Tunable fluorescence: In o‐carborane derivatives bonded to different fluorophores, the fluorescence could be tuned by changing the substituent on the adjacent cluster carbon atom (Cc; see scheme). We have found a correlation between the CcCc bond length and the fluorescence intensity; those substituents causing an enlargement of the CcCc distance give rise to very low fluorescence quantum yield or complete quenching of the fluorescence.