A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.

Carborane-based host materials were prepared to fabricate deep blue phosphorescence organic light-emitting diodes (PHOLEDs), which constituted three distinctive geometrical structures stemming from the corresponding three different isomeric forms of carboranes, namely, ortho-, meta-, and para-carboranes. These materials consist of two carbazolyl phenyl (CzPh) groups as photoactive units on each side of the carborane carbons to be bis[4-(N-carbazolyl)phenyl]carboranes, o-Cb, m-Cb, and p-Cb. To elaborate on the role of the carboranes, comparative analogous benzene series (o-Bz, m-Bz, and p-Bz) were prepared, and their photophysical properties were compared to show that advantageous photophysical properties were originated from the carborane structures: high triplet energy. Unlike m-Bz and p-Bz, carborane-based m-Cb and p-Cb showed an unconjugated nature between two CzPh units, which is essential for the blue phosphorescent materials. Also, the carborane hosts showed high glass transition temperatures (T(g)) of 132 and 164 °C for m-Cb and p-Cb, respectively. Albeit p-Cb exhibited slightly lower hole mobility when compared to p-Bz, it still lies at the high end hole mobility with a value of 1.1 × 10(-3) cm(2)/(V s) at an electric field of 5 × 10(5) V/cm. Density functional theory (DFT) calculations revealed that triplet wave functions were effectively confined and mostly located at either side of the carbazolyl units for m-Cb and p-Cb. Low-temperature PL spectra indeed provided unequivocal data with higher triplet energy (T(1)) of 3.1 eV for both m-Cb and p-Cb. p-Cb was successfully used as a host in deep blue PHOLEDs to provide a high external quantum efficiency of 15.3% and commission internationale de l'elcairage (CIE) coordinates of (0.15, 0.24).

o-Carborane-based donor-acceptor dyads comprising an o-carboranyl phenyl unit combined with N-carbazole (1) or 4-phenyl-N-carbazole (2) were prepared, and their dyad characters were confirmed by steady-state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o-carboranyl phenyl units; this indicates negligible interaction between the carbazole and o-carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge-transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o-carborane-based dyads were further analyzed by using Lippert-Mataga plots to show that unit charge separation occurred to form a charge-separated species in the excited state, namely, 1⋅2. This excited-state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o-carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o-carborane cage through a phenylene or multi-phenylene spacer revealed that the photoinduced electron-transfer process occurred intramolecularly.

Abstract:The construction of Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), a 0

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