Revealing the Real Size of a Porphyrin Molecule with Quantum Confinement Probing via Temperature-Dependent Photoluminescence Spectroscopy.

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.