Evidence for Delocalization between o-Carborane and Triphenylethylene via an Alkynyl Bridge

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• High solid-state fluorescence quantum yields (Φ) in o-carborane systems are desirable but hindered by aggregation-caused quenching, typically requiring carbon-vertex disubstitution.
• Current strategies focus on modifying carbon vertices, limiting the exploration of other functionalization sites for luminescence enhancement.

Purpose of the Study:

• To challenge the convention of carbon-vertex disubstitution for high solid-state fluorescence in o-carborane systems.
• To demonstrate the potential of B9 boron-vertex monosubstitution combined with alkynyl-bridge conjugation for achieving superior luminescent properties.
• To elucidate the mechanism behind the enhanced luminescence through structural and computational analyses.

Main Methods:

• Synthesis of a novel o-carborane derivative, B9-TriPE-yn-o-CB, via scalable and selective B9 functionalization.
• Characterization using UV-vis absorption and fluorescence emission spectroscopy (solution at 77 K and solid-state).
• Structural analysis via single-crystal X-ray diffraction and theoretical investigation using Density Functional Theory (DFT) calculations.

Main Results:

• The novel compound B9-TriPE-yn-o-CB achieved a high solid-state Φ of 83.5%, significantly outperforming other monosubstituted and most disubstituted analogues.
• Alkynyl-bridge conjugation was found to enhance electronic coupling between the TriPE chromophore and the o-carborane cage.
• Structural analysis revealed suppressed nonradiative decay due to cage C-C and B-B bond contractions, supported by DFT calculations showing efficient 3D-2D electron delocalization.

Conclusions:

• B9 boron-vertex monosubstitution, when appropriately conjugated, can lead to high-performance solid-state emitters, challenging the necessity of carbon-vertex disubstitution.
• The study establishes a 3D (boron cage)-2D (chromophore) conjugation design principle for activating the B9 boron vertex.
• This work opens new avenues for developing advanced boron cluster-based emitters with tailored photophysical properties.