Abstract
The colour of crystals is a function of their atomic structure. In the case of organic crystals, it is the spatial relationships between molecules that determine the colour, so the same molecules in the same arrangement should produce crystals of the same colour, regardless of whether they arise geologically or synthetically. There is a naturally-occurring organic crystal known as karpatite which is prized for its beautiful blue fluorescence under ultra-violet illumination. When grown under laboratory conditions however, the crystals fluoresce with an intense green colour. For 20 years, this difference has been thought to be due to chemical impurities in the laboratory-grown material. Using electron microscopy coupled with fluorescence spectroscopy and X-Ray diffraction, we report here that this disparity is instead due to differences in the structure of the crystals at the nanoscale. The results show that in nature, karpatite has a nanotexture that is not present in the synthetic crystals, which enables different photonic pathways and therefore a blue, rather than green colour whilst undergoing fluorescence.
Highlights
The optical properties of minerals fundamentally arise from their crystal structure and the spatial arrangement of those crystals across many length scales[1,2,3,4,5]
Karpatite is composed of a crystalline arrangement of molecules of coronene (C24H12) a polyaromatic hydrocarbon (PAH)
The unit cell of KP was only determined from single crystal data as recent as 200714 and was in agreement with data obtained from lab grown crystals
Summary
The colour of crystals is a function of their atomic structure. In the case of organic crystals, it is the spatial relationships between molecules that determine the colour, so the same molecules in the same arrangement should produce crystals of the same colour, regardless of whether they arise geologically or synthetically. If this layering in KP is the fundamental cause of the differences in the emission profiles, it would naturally be expected that KP crystals without this nano-texture would emit in the green wavelength region To determine if this was the case and to rule out the role of any potential polyaromatic impurities, some KP was dissolved in toluene, after confirmation of its expected blue emission, and recrystallized by slow evaporation. Sublimation of KP crystals resulted in the deposition of crystals in the cooler region of the tube that exhibited a fluorescence spectroscopy profile similar to KP crystals, but with the addition of peaks in the CN-like green region of the emission spectrum at 463, 482 and 527 nm (Extended Data Fig. 5) This would suggest that the sublimation resulted in a mixture of layered and non-layered morphologies, which was the case, as evidenced by SEM images of the sublimed crystals. As this study represents the first proof that colour change in organic crystals can be a solid-state morphological phenomenon, we believe that our interrogative method can be applied to many other organic crystal systems to potentially uncover exotic charge transfer pathways in semiconductors, field-effect transistors and organic superconductors
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