Modulation of 9,12-Substituted o-Carboranes Toward Anti-Kasha Triple-Fluorescence

Achieving triple-fluorescence from a pure organic compound in solution is nearly impossible, as Kasha's rule only allows single fluorescence from the lowest excited state. In this paper, pyrene-substituted o-carboranes, 1a-c, and 2a, were synthesized within 3-5 steps from commercially available chemicals, and their chemical structures have been confirmed by ¹H NMR, ¹³C NMR, and HRMS. The photophysical properties of these compounds have been studied in detail by UV-vis, steady and transient state fluorescence, temperature-dependent fluorescence, and femtosecond transient absorption spectroscopy, as well as TD-DFT calculations. The results show that while 2a exhibits only local emission, the B,B-dipyrenyl-o-carborane, 1a, shows dual-emission from locally excited and intramolecular excimer states. Triple-fluorescence is achieved for 1b and 1c by introducing a small π-substituent at the carborane carbon vertices, which allows the formation of a charge-transfer state that is fluorescent in the near-infrared (NIR) region tailing to 1200 nm. This work provides a new strategy to activate triple-fluorescence from a single-component organic compound, opening a new avenue toward multichannel photonic materials and NIR‑II bioimaging.