Structural, thermal, kinetic and spectroscopic characterization of a metastable polymorph of 2-(2'-hydroxyphenyl)benzothiazole.

2-(2'-hydroxyphenyl)benzothiazole (HBT) exhibits remarkable photophysical properties, in particular intense fluorescence, both in solution and in solid state. For this reason, this compound and related ones are intensely studied for a wide range of optoelectronic applications. In addition, benzothiazole derivatives are known to present polymorphism, that is, the characteristic of crystallizing in more than one crystallographic structure. Herein, following a recently published work in which the existence of a metastable polymorph of HBT was reported, we describe the determination of its crystal structure, by means of powder X-ray diffraction. The new polymorph is found to crystallize in the P21 space group, with lattice parameters a = 17.82605 Å, b = 4.96006 Å, c = 6.00379 Å and β = 93.1630°. The experimentally determined lattice parameters are compared to those obtained by modelling the structure by density functional theory, revealing excellent agreement. In addition, the transition of the metastable polymorph to the stable one is studied both kinetically and thermodynamically, by means of non-ambient X-ray diffraction and DSC experiments, respectively. From the kinetic point of view, an activation energy of 66 ± 6 kJ mol-1 is found, while from the thermodynamic one, an enthalpy of conversion of -4.5 ± 1.4 kJ mol-1 is obtained, consistent with the value calculated by DFT, of -5 kJ mol-1. Additionally, the two forms of HBT are characterized spectroscopically, by means of UV-visible, fluorescence and Raman spectroscopy, and the experimental spectra are compared with the computational ones, revealing very good agreement.