Blue-shifted emission and enhanced quantum efficiency via π-bridge elongation in carbazole–carborane dyads

Abstract

Carbazole-carborane linear dyads and di(carbazole)-carborane V-shaped dyads with phenyleneethynylene-based bridges have been synthesized. The V-shaped dyads display the expected red-shifts in the location of their UV-Vis absorption maxima on bridge-lengthening, but show unusual blue-shifts in charge-transfer (CT) emission on the same π-system lengthening. These blue-shifts can be attributed to the 2n + 3 electron count within the carborane cluster in the excited state. The linear dyads luminesce via a combination of local excited (LE) and CT emission, with a red-shift in LE emission and a blue-shift in CT emission accompanying π-bridge elongation. A quantum efficiency as high as 86% in the solution state is achieved from the hybrid LE/CT emission. Time-dependent density functional theory (TD-DFT) calculations at the excited state of these compounds have clarified the photoluminescence blue-shift and suggested a typical cluster C-C bond elongation in the V-shaped dyads. Calculations on the elongated linear dyads have suggested that the electron density is localized at the phenyleneethynylene-containing bridge.

Solid‐State Thermochromic Luminescence through Twisted Intramolecular Charge Transfer and Excimer Formation of a Carborane−Pyrene Dyad with an Ethynyl Spacer

“…Boron cluster compounds have recently attracted significant attention as a key component in the development of stimuli‐responsive luminescent solid films, which hold great promise as versatile sensors and probes . To obtain clear responses, such materials need to show dramatic luminochromism that is triggered by the target stimuli.…”

Time‐Dependent Emission Enhancement of the Ethynylpyrene‐o‐Carborane Dyad and Its Application as a Luminescent Color Sensor for Evaluating Water Contents in Organic Solvents

“…We have also regarded o ‐carborane as a conjugated “element‐block”, which is defined as a minimum functional unit containing heteroatoms, for constructing functional luminescent materials according to recent works . Because of the electron‐deficient property when bonded at the carbon in the boron cluster, o ‐carborane can work as an electron‐accepting unit, and intense emission from the intramolecular charge transfer (ICT) state can be observed by combination with aryl moieties . Interestingly, despite emission properties of the most of organic luminescent dyes are usually spoiled in the condensed state due to aggregation‐caused quenching (ACQ), the ICT emission can be preserved even in the solid samples of several kinds of aryl‐modified o ‐carborane derivatives .…”

“…[5][6][7][8] Because of the electron-deficient property when bonded at the carbon in the boron cluster, o-carborane can work as an electron-accepting unit, and intense emission from the intramolecular charge transfer (ICT) state can be observedb yc ombination with aryl moieties. [9,10] Interestingly,d espite emission properties of the most of organic luminescent dyes are usually spoiled in the condensed state due to aggregation-caused quenching (ACQ), the ICT emission can be preserved even in the solid samples of severalk inds of aryl-modified o-carborane derivatives. [11][12][13] It is proposed that intermoleculari nteraction could be suppressed owing to the steric hindrance of the sphere shape of the boron cluster.T hus, intense solid-state emissive materials were accomplished with the o-carborane derivatives.…”

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine

“…o -Carborane [ 5 , 6 , 7 , 8 , 9 , 10 ] is an icosahedral cluster composed of 2 carbon and 10 boron atoms, and it has recently attracted attention as a key component for constructing solid-state luminescent materials [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Because of the electron-deficient nature of the boron cluster, intense emission from the intramolecular charge transfer (ICT) state can be observed from the donor–acceptor system with electron-donating units such as triphenylamine and its structural analogues [ 23 , 24 , 25 , 26 ]. In particular, owing to steric hindrance of the sphere shape of the cluster, suppression of ACQ was often induced in the condensed state, followed by intense solid-state emission [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 27 , 28 , 29 ].…”

“…On the basis of these results, we presumed that the introduction of multiple o -carborane units into the strong electron-donating unit might be effective for realizing intense AIE. To examine the validity of this idea, we focused on triphenylamine as an electron-donor [ 23 , 24 , 25 , 26 , 30 , 31 , 32 , 44 , 45 , 46 , 47 ], synthesized modified triphenylamines with various numbers (1–3) of o -carboranes and compared their optical properties.…”