Deboronation-Induced Ratiometric Emission Variations of Terphenyl-Based Closo-o-Carboranyl Compounds: Applications to Fluoride-Sensing

Abstract

Closo-o-carboranyl compounds bearing the ortho-type perfectly distorted or planar terphenyl rings (closo-DT and closo-PT, respectively) and their nido-derivatives (nido-DT and nido-PT, respectively) were synthesized and fully characterized using multinuclear NMR spectroscopy and elemental analysis. Although the emission spectra of both closo-compounds exhibited intriguing emission patterns in solution at 298 and 77 K, in the film state, closo-DT mainly exhibited a π-π* local excitation (LE)-based emission in the high-energy region, whereas closo-PT produced an intense emission in the low-energy region corresponding to an intramolecular charge transfer (ICT) transition. In particular, the positive solvatochromic effect of closo-PT and theoretical calculation results at the first excited (S1) optimized structure of both closo-compounds strongly suggest that these dual-emissive bands at the high- and low-energy can be assigned to each π-π* LE and ICT transition. Interestingly, both the nido-compounds, nido-DT and nido-PT, exhibited the only LE-based emission in solution at 298 K due to the anionic character of the nido-o-carborane cages, which cannot cause the ICT transitions. The specific emissive features of nido-compounds indicate that the emissive color of closo-PT in solution at 298 K is completely different from that of nido-PT. As a result, the deboronation of closo-PT upon exposure to increasing concentrations of fluoride anion exhibits a dramatic ratiometric color change from orange to deep blue via turn-off of the ICT-based emission. Consequently, the color change response of the luminescence by the alternation of the intrinsic electronic transitions via deboronation as well as the structural feature of terphenyl rings indicates the potential of the developed closo-o-carboranyl compounds that exhibit the intense ICT-based emission, as naked-eye-detectable chemodosimeters for fluoride ion sensing.

Geometric structural insights for enhanced radiative efficiency: Spiro[fluorene–carbazole]‐based ortho‐carboranyl luminophores

“…ortho ‐ closo ‐Carborane containing π‐conjugated organic luminophores has been extensively investigated as a promising material in the fields of optoelectronic 1–23 and sensory devices 24–31 because it exhibits unique photophysical characteristics and high thermal and electrochemical stabilities 3,8,13–15 . In particular, this o ‐carborane cluster demonstrates interesting photoluminescent behavior derived from the intramolecular charge transfer (ICT) transitions of its appended π‐conjugated aromatic group, because of which, it has applications as a 3D‐analogue molecule in various novel cluster materials 6–21,32–39 .…”

Influence of Electronic Environment on the Radiative Efficiency of 9-Phenyl-9H-carbazole-Based ortho-Carboranyl Luminophores

“…Recent research has shown that the ICT-based luminescence characteristics of D‒A dyads are dramatically affected by the electronic environment of the o -carborane cage. For example, the photophysical properties are drastically altered by the deboronation of a closo - o -carborane to give a nido - o -carborane (nest-like structure, where one boron atom is removed from the icosahedron) [ 33 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 ], as the anionic character of the nido -type structure interrupts the ICT transition [ 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 ]. The intrinsic differences between the electronic properties of closo - and nido - o -carboranes have been investigated, providing inspiration as novel molecular scaffolds for chemodosimeter materials.…”

Celebrating Todd Marder: 65th Birthday and His Contributions to Inorganic Chemistry

“…Professor Todd B. Marder has contributed significantly to expand the scope of boron chemistry, and it is an honor to celebrate his achievements, and mark his 65th birthday with a Special Issue of Molecules : “Boron in Catalysis and Materials Chemistry: A Themed Issue in Honor of Professor Todd B. Marder on the Occasion of His 65th Birthday”. It includes excellent review articles on conjugated N^C-organoboron materials, and tri(boryl)-alkanes and -alkenes as versatile reagents in organic synthesis [ 31 , 32 ], and research articles depicting diversity [ 33 , 34 , 35 , 36 , 37 , 38 , 39 ], as noted in the research interests of Professor Marder. We thank all the authors for joining us to congratulate Todd, and for making this issue a successful one through their excellent scientific contributions.…”