Metastability and polymorphism in dihydroxybenzenes – implications for thermal energy storage

State-of-the-art calorimetric techniques have been used to explore the effects of molecular isomerism on the phase behaviour of the three dihydroxybenzenes catechol, resorcinol, and hydroquinone. Within the broader remit of the search and rational design of phase-change materials for thermal-energy storage, these data reveal a surprisingly rich (and hitherto unappreciated) behaviour, ranging from an unavoidable propensity to crystallize (hydroquinone) to the emergence of both disordered and ordered metastable phases well below the range of stability of the normal liquid (resorcinol and catechol). Catechol exhibits the most complex thermophysical response, and ab-initio calculations evince a subtle interplay between intramolecular and intermolecular interactions, ultimately leading to the formation of new crystal phases.