Anion binding with urea and thiourea derivatives

• Anion receptors are important in various biological processes and ecology.
• The urea or thiourea are often incorporated into anion receptors and sensors.
• Urea and thiourea receptors selectively bind anions of complementary geometry.
• This critical review contains 386 references.
• Review gives an overview of sensing mechanisms in complex photophysical systems.
• Receptors are classified with respect to the complexity and the number of urea groups.

The importance of anion receptors is a reflection of the ubiquitousness of anions, their functions in biological and industrial processes, as well as their behaviour as pollutants. In the immense field of anion receptor chemistry which has expanded greatly in the past few decades, one of the most prominent binding sites is (thio)urea. The simple synthesis of diverse (thio)urea derivatives, their possibility to form two H-bonds with anions and a promising selectivity with anions of complementary geometry are factors that gave rise to a plethora of studies reporting new organic molecules with different properties and applicability. This review includes the most important examples of (thio)urea receptors from the very beginning, until present. Many authors have used (thio)urea moieties in the computer-aided design of anion sensors, which was then followed by their synthesis, and utilization in the photophysical characterization of host–guest systems, and studies in the transport of ions and ion pairs through membranes. These endeavours led to their successful utilization in the fields of crystal engineering and development of functional materials. Characterization of supramolecular systems and investigations of the complexation thermodynamics has been conducted by use of different analytical and physico-chemical methods. Consequently, it is highly beneficial to summarize all aspects of anion recognition by (thio)ureas within one review article. This critical review containing 386 references classifies (thio)urea derivatives, published from 1990 to 2015, with respect to the complexity of the receptors and the number of urea groups in the molecule.

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