Modulation of the Naked-Eye and Fluorescence Color of a Protonated Boron-Doped Thiazolothiazole by Anion-Dependent Hydrogen Bonding

The reaction of a cyclic alkyl(amino)carbene (CAAC)-stabilized thiazaborolo[5,4-d]thiazaborole (TzbTzb) with strong Bronsted acids, such as HCl, HOTf (Tf=O2SCF3) and [H(OEt2)(2)][BAr4F] (Ar-F=3,5-(CF3)(2)C6H3), results in the protonation of both TzbTzb nitrogen atoms. In each case X-ray crystallographic data show coordination of the counteranions (Cl-, OTf-, BAr4F-) or solvent molecules (OEt2) to the doubly protonated fused heterocycle via hydrogen-bonding interactions, the strength of which strongly influences the H-1 NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb-to-CAAC charge transfer character involved in the S-0 -> S-1 transition, ultimately enabling fine-tuning of their absorption and emission spectral features.

Automated summary

The reaction of a cyclic alkyl(amino)carbene (CAAC)-stabilized thiazaborolo[5,4-d]thiazaborole (TzbTzb) with strong Bronsted acids results in the protonation of both TzbTzb nitrogen atoms. The coordination of counteranions or solvent molecules to the doubly protonated fused heterocycle via hydrogen-bonding interactions influences the visible and fluorescence colors of these salts. DFT calculations show that the hydrogen bonding affects the charge transfer character, enabling fine-tuning of absorption and emission spectral features.