Crystal morphology fixed by interplay of π-stacking and hydrogen bonds – the case of 1-hydroxypyrene

The crystal structure of 1-hydroxypyrene fluorophore has been determined for the first time. Despite the simplicity of the molecule itself and the relatively low symmetry of the crystal structure (space group P 2 1 ), the specific molecular arrangement and intermolecular interactions result in a rather conserved crystal morphology, to some extent dependent on the polarity of the solvents. The structure of 1-hydroxypyrene causes an unusual behavior of its single crystals under pressure: they increase in length and bend and then grow thicker irrespective of the solvent's polarity. Quantum mechanical calculations, covering estimation of intermolecular interaction energies and predicting the structural changes at increased pressure, shed light on the mechanism of this phenomenon. The whole crystal structure can be described as a set of ‘molecular springs’. Crystalline 1-hydroxypyrene also represents a relatively rare example of an H-aggregate which is luminescent in the solid state. The fluorescence spectra of 1-hydroxypyrene in the solid state resembles its spectra in water, reflecting a well-defined H-bond network in the crystal structure.