Benzothiazole dibenzoic acid derivative (BTDB) is well-known organic linkers utilized for the syntheses of various metal organic frameworks, and demonstrates interesting photophysical properties upon concentration variations in solution. The presence of two carboxylic acid functional groups at each side of the rod-like molecule, facilitates dimerization and oligomerization equilibria. Interestingly, dimers and oligomers have completely different emission behaviors from the monomer of the same species. At a low range of concentration, 0.1 – 64 μM, dimerization process is dominant, and that the equilibrium constant of dimer formation found to be 18,000 M-1. On the other hand, in the 64 – 1000 μM concentration range, oligomerization takes over, and that it results in the formation of a small linear chain of 8 molecules, or 4 dimers, with a high equilibrium constant of 1.2 × 1013 M-3. Various experimental measurements and theoretical calculations have suggested hydrogen-bond formation is the main driving force for the dimerization and oligomerization in the nano- and micro- molar regime, and that structure rigidity of a species is a key factor in controlling its photophysical properties, such as emission quantum yield and excited state lifetime.
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