Abstract
The photophysical properties of closo-ortho-carboranyl-based donor–acceptor dyads are known to be affected by the electronic environment of the carborane cage but the influence of the electronic environment of the donor moiety remains unclear. Herein, four 9-phenyl-9H-carbazole-based closo-ortho-carboranyl compounds (1F, 2P, 3M, and 4T), in which an o-carborane cage was appended at the C3-position of a 9-phenyl-9H-carbazole moiety bearing various functional groups, were synthesized and fully characterized using multinuclear nuclear magnetic resonance spectroscopy and elemental analysis. Furthermore, the solid-state molecular structures of 1F and 4T were determined by X-ray diffraction crystallography. For all the compounds, the lowest-energy absorption band exhibited a tail extending to 350 nm, attributable to the spin-allowed π–π* transition of the 9-phenyl-9H-carbazole moiety and weak intramolecular charge transfer (ICT) between the o-carborane and the carbazole group. These compounds showed intense yellowish emission (λem = ~540 nm) in rigid states (in tetrahydrofuran (THF) at 77 K and in films), whereas considerably weak emission was observed in THF at 298 K. Theoretical calculations on the first excited states (S1) of the compounds suggested that the strong emission bands can be assigned to the ICT transition involving the o-carborane. Furthermore, photoluminescence experiments in THF‒water mixtures demonstrated that aggregation-induced emission was responsible for the emission in rigid states. Intriguingly, the quantum yields and radiative decay constants in the film state were gradually enhanced with the increasing electron-donating ability of the substituent on the 9-phenyl group (‒F for 1F < ‒H for 2P < ‒CH3 for 3M < ‒C(CH3)3 for 4T). These features indicate that the ICT-based radiative decay process in rigid states is affected by the electronic environment of the 9-phenyl-9H-carbazole group. Consequently, the efficient ICT-based radiative decay of o-carboranyl compounds can be achieved by appending the o-carborane cage with electron-rich aromatic systems.
Highlights
The 1 H{11 B} Nuclear magnetic resonance (NMR) spectra of the o-carboranyl carbazole compounds exhibited resonances corresponding to the 9-phenyl-9H-carbazole moiety and terminal phenyl groups in the region of 8.3–7.0 ppm
Three broad singlet peaks were observed between −3 and −12 ppm in the 11 B{1 H} NMR spectra of all the closo-o-carborane compounds, which clearly confirmed the presence of the o-carborane cage
Conclusions we evaluated the impact of electron-donating effects on the intramolecular charge transfer (ICT)-based radiaHerein,tive we evaluated impact of electron-donatingo-carboranyl effects on thecompounds
Summary
Over the past few decades, π-conjugated aromatic compounds bearing an icosahedral closo-ortho-carborane (closo-o-1,2-C2 B10 H12 ) cluster have been extensively investigated [1–5]. Because of their unique photophysical properties and reasonable electrochemical stabilities [1,3,6–8]. Owing to these features, such o-carboranyl compounds have recently been proposed as promising optoelectronic materials for organic light emitting diodes [7,8] and organic thin-film transistors [9,10]. The intriguing photophysical properties of o-carboranyl luminophores originate from the electronic donor-acceptor (D–A) dyad formed by combining a π-conjugated aromatic organic fluorophore (donor) with an o-carborane (acceptor) [4,7–36]. The strong electron-withdrawing ability of the carbon atoms in the o-carborane
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