The
confinement energy of electrons in an aromatic molecule was
studied by indirect and direct methods, namely, temperature-dependent
photoluminescence (TDPL) spectroscopy and scanning tunneling microscopy
(STM). We observed a decrease in the tetraphenylporphyrin
(H2TPP) PL intensity with increasing temperature. The increase
in temperature provides kinetic energy for the electrons to overcome
the confinement energy barrier, making recombination via nonradiative
pathways more favorable. The results of fitting the integrated TDPL
intensity with a modified Arrhenius equation suggest two confinement
energy values. We propose that these energy values are related to
the size of the delocalized electron cloud along the plane and thickness
of the H2TPP ring. These values quantitatively express
an abstract form of the size of the aromatic ring system. These results
are in good agreement with the topography images of single H2TPP molecules and monolayer H2TPP obtained by a direct
probing method using STM. These results are also supported by the
porphyrin ring orientation relative to the excited crystal face during
the TDPL measurements.
Compound separation plays a key role in producing and analyzing chemical compounds. Various methods are offered to obtain high-quality separation results. Liquid chromatography is one of the most common tools used in compound separation across length scales, from larger biomacromolecules to smaller organic compounds. Liquid chromatography also allows ease of modification, the ability to combine compatible mobile and stationary phases, the ability to conduct qualitative and quantitative analyses, and the ability to concentrate samples. Notably, the main feature of a liquid chromatography setup is the stationary phase. The stationary phase directly interacts with the samples via various basic mode of interactions based on affinity, size, and electrostatic interactions. Different interactions between compounds and the stationary phase will eventually result in compound separation. Recent years have witnessed the development of stationary phases to increase binding selectivity, tunability, and reusability. To demonstrate the use of liquid chromatography across length scales of target molecules, this review discusses the recent development of stationary phases for separating macromolecule proteins and small organic compounds, such as small chiral molecules and polycyclic aromatic hydrocarbons (PAHs).
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