Xiaoyang Xu+, Xueyuan Zhao+, Shuang Xu, Xinning Zhang, Qihao Wang, Lin Wu, Xin Li, Junqing Shi, Jiani Ma,* Lei Ji,* and Wei Huang*. Angew. Chem. Int. Ed. 2025, e21735. DOI: https://doi.org/10.1002/anie.202521735

Organic electronic materials are mostly centered around organic aromatic compounds, as their π - conjugated systems can achieve significant orbital overlap, thereby significantly enhancing electronic transition strength. Carbonborane is a 12 vertex cage shaped molecule composed of carbon, hydrogen, and boron with a three-dimensional aromatic structure. Its surface p-orbitals overlap less with the orbitals of organic substituents, making it rarely used to construct luminescent materials. In 2007, Chujo and colleagues reported that carbon substituted ortho carboboranes exhibited strong charge transfer (CT) emission. Since then, a large number of carbon substituted ortho carboboranes with excellent luminescent properties have been reported and applied in fields such as organic light-emitting diodes (OLEDs), circularly polarized luminescence (CPL), and fluorescence sensing. Chujo and colleagues have reported several boron substituted ortho carboboranes (regardless of whether there is a substituent on the carbon atom), but their absorption or emission spectra have shown that there is no interaction between the boron substituent and the carboborane cage. In the past few decades, almost all research on the luminescence of carboranes has focused on carbon vertices. People believe that boron vertices are 'inert' in electron conjugation and cannot effectively participate in charge transfer processes. How to "activate" the silent boron vertices and involve them in the "dance" of optoelectronics has become an unresolved challenge in this field.

Figure 1. (a) Chemical structures of o-carborane and numbering of the key positions discussed in this paper; the cross point of bonds in carborane represents BH vertex; (b) structures of 9,12-substituted o-carborane and 1,2-substituted o-carborane; (c) compounds reported in this paper. The red arrow indicates the allowed or not allowed charge transfer pathway.

Faced with this challenge, the research team proposed a sophisticated "collaborative activation" strategy. They designed and synthesized three novel 9,12-substituted ortho carboborane molecules (1, 2a, 2b). The essence of its design lies in the installation of a strong electron donor at the boron vertex and the introduction of a phenyl group at the carbon vertex. These two phenyl groups are like antennas. When the aryl group is attached to the carbon vertex, it reduces the energy of the orbital, thereby enhancing the electron accepting ability of the C-C antibonding of the carborane and activating charge transfer from the boron end substituent to the carbon end. These two seemingly inconspicuous benzene rings are the keys that unlock the charge transfer from the boron vertex carbazole to the carborane cage. The photophysical research and theoretical calculations of the system indicate that boron vertex functionalized donors can effectively transfer charges to adjacent carborane cages under photoexcitation, and exhibit both localized excitation (LE) and CT emission peaks simultaneously.

In non-polar solvents such as n-hexane, compounds 2a and 2b exhibit rare double emission phenomena. The two emission peaks of compound 2b in toluene solution are located at 345 nanometers in the ultraviolet region and 850 nanometers in the near-infrared region, spanning a huge range of 505 nanometers (with an energy difference of 2.1 electron volts), and neither peak is in the visible light region. It is an invisible "black" dual emission, which has certain guiding significance for constructing black luminescent materials for special functional applications. The potential energy surface (PES) scan reveals a huge energy gap between the LE state and the CT state, which explains why the anti Kasha rule of LE emission is preserved while CT emission appears.

This paper achieves efficient charge transfer for the first time from the vertices of adjacent carboborane boron, and successfully prepares single-molecule dual emissive materials with a huge wavelength difference (up to 505 nanometers). Fundamentally expanding the design paradigm of carbon boron alkyl functional materials, overturning the long-standing carbon centered view of carbon borane luminescence, revealing a new strategy for activating boron vertices to participate in electron conjugation, and opening up new avenues for developing high-performance dual emission materials based on adjacent carbon boranes. The design of multi location and multi-functional collaborative carbon borane new materials for the future has opened up infinite imagination space.

First Authors: Xu Xiaoyang, doctoral candidate, Zhao Xueyuan, master’s student,  Northwestern Polytechnical University

Correspondence Authors: Academician Huang Wei, Prof. Ji Lei, Northwestern Polytechnical University; Prof. Ma Jiani, Shaanxi Normal University

Full Text Link: https://doi.org/10.1002/anie.202521735