Distinguishing the Glass, Crystal, and Quasi-Liquid Layer in
                       1-Methylnaphthalene by using Fluorescence Signatures

   Jan Zezula^ a, David Mužík^ a, Johannes Bachler^ b, Thomas Loerting^ b, and Dominik Heger ^a

^a Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech
                                             Republic

^b Institute of Physical Chemistry, University of Innsbruck, Innrain 52c, A-6020 Innsbruck, Austria


Crystalline and vitrified states differ in numerous properties, such as the mechanical behavior,
expansivity, thermal conductivity, luminescence, X-ray and neutron diffraction, and chemical
reactivity. The decision on whether any given compound will crystallize or vitrify when cooled down
from its liquid phase depends on the time allowed for the solidification [1]. The speciation of
polycyclic aromatic hydrocarbons (PAHs) plays a crucial role in their fate in the atmosphere, fresh
and sea water, soil, ice, and also the interstellar space. [2]

Besides other techniques, the fluorescence of polycyclic aromatic hydrocarbons (PAHs) facilitates
distinguishing their speciation. [3] The fluorescence of crystalline 1-methylnaphthalene (MeNp) is
monomer-like, while liquid and glass exhibit excimeric emissions. The temperature dependence of the
fluorescence emission and excitation spectra in the range between 77 K and 295 K provide
comprensive information on the state of 1-methylnaphthalene. The glass, formed by abrupt quenching
in liquid nitrogen or methane, devitrifies at (155 ± 5) K, allowing the liquid to undergo cold
crystallization at around 170 K. In 1-methylnaphthalene crystals, an excimeric emission appears at
approximately 40 K below the melting point, a process we ascribe to the formation of dimers due to
surface premelting; such a quasi-liquid layer exists at the surface well below the freezing point,
remaining uncrystallized. The premelted layer is clearly distinguishable from the bulk glass via
fluorescence spectroscopy, which facilitates state identification. With this knowledge, MeNp may
find use as a simple molecular probe at sub-zero conditions to study the speciation and aggregation
in different environments; alternatively, it may render MeNp applicable as a molecular switch.


1.            Zobrist, B., et al., Do atmospheric aerosols form glasses? Atmospheric Chemistry and
Physics, 2008. 8(17): p. 5221-5244.

2.            Knopf, D.A., P.A. Alpert, and B. Wang, The Role of Organic Aerosol in Atmospheric Ice
Nucleation: A Review. ACS Earth and Space Chemistry, 2018. 2(3): p. 168-202.

3.            Zezula, J., et al., Distinguishing the glass, crystal, and quasi-liquid layer in
1-methylnaphthalene by using fluorescence signatures. Journal of Luminescence, 2023. 261.