ELECTRON DELOCALIZATION AND AROMATICITY IN PORPHYRIN MOLECULE FROM FLUORESCENCE SPECTROSCOPY, RAMAN SPECTROSCOPY, AND SCANNING TUNNELING MICROSCOPY POINT OF VIEWS

• Main Author: Nurhayati
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/52160
• Institution: Institut Teknologi Bandung

Porphyrins are promising compounds in molecular electronics due to their excellent electronic and photonic properties. This is because the porphyrin skeleton has an extended ?-conjugation system that results in a wide range of wavelengths for light absorption. The main conjugation pathways of porphyrin could be described as a contribution of the larger heterocyclic fragment and local aromaticity of two pyrrole rings that accommodate both 18 and 22 ?-electrons. The most conspicuous features of the porphyrin macrocycle are its aromatic character, which has a strong influence on the spectroscopic properties and chemical reactivity. Aromaticity is a concept of central importance in physical organic chemistry. It has been very useful in the rationalization of the structure, stability, and reactivity of many molecules. Unfortunately, despite their crucial importance, the definition or quantification of aromaticity remains a matter of controversy. This lack of a proper definition has led to many different perceptions of aromaticity, and many indices for its quantification have been introduced. The quantification of aromaticity is done based on energetic, structural, and magnetic criteria. The quantification of aromaticity remains difficult because of the lack of physical bases of this concept. In this research, the physical bases of the aromaticity concept can be approached by probing the electron cloud via direct and indirect methods, namely temperature- dependent photoluminescence (TDPL) spectroscopy and scanning tunneling microscopy (STM). The photoluminescence (PL) is observed within the temperature range of 173-543 K. The temperature evolution of Raman spectra revealed no structural transformation or chemical reaction occur in the range of temperature. In addition, the linearity of the vibration mode frequency shift reveals reversible thermal expansion in the range of measurement temperatures. The focus of our research is the meso-phenyl-substituted porphyrins that are tetraphenylporphyrin (H2TPP) and its metalloporphyrin (ZnTPP and CuTPP). H2TPP highly delocalized ?-electrons with an annulene structure and a high PL intensity. The similar value of the spectral overlap integral (J) between the normalized fluorescence and absorption spectra in crystal and solution system show that both systems are similar electronically at high temperature. In addition, the similar value of the (0, 0) transition energy in both systems verify this assumption. The similarity between the two J values confirms that the integrated PL intensity in the crystal system is comparable to that in the isolated system, i.e., in the solution system. In the single-crystal sample, the PL intensity decreases with increasing temperature. The increase of temperature provides kinetic energy for electrons to overcome the confinement energy barrier, making recombination via nonradiative pathways more favorable. Fitting results of the TDPL integrated intensity with a modified Arrhenius equation suggest two confinement energy values. These energy values are similar to the two different confinement regions, with a size of 9.92 and 2.44 Å for H2TPP. These energy values are related to the size of the delocalized electron cloud along the plane and thickness of the porphyrin ring. These values quantitatively express an abstract form of the aromatic ring size of the H2TPP molecule. These results are in good agreement with the topography images of the H2TPP single-molecule and monolayer via the direct probing method using STM. The porphyrin ring orientation relative to the excited crystal face during the TDPL measurements, which determines the information that can be extracted from the TDPL results. This was observed when measuring single crystal metalloporphyrin ZnTPP and CuTPP. In this measurement, the excited crystal field is more dominant in the crystal thickness axis, while the planar ring is almost perpendicular to the plane. The results of ZnTPP single crystal integrated emission intensity fitting proposed two confinement areas, namely 6.54 and 4.17 Å. Meanwhile, the CuTPP single crystal integrated emission intensity fitting suggested two confinement areas, namely 3.92 and 3.91 Å. These values are only related to the size of the electron cloud delocalized along with the thickness of the porphyrin ring. The presence of metal ions at the center of the ring will increase the porphyrin ring thickness. The importance of orientation of the porphyrin ring in the TDPL measurement was also observed in the results of the integrated PL intensity of crushed single-crystal pellet H2TPP, thin films of H2TPP, and thin films of ZnTPP suggest negative activation energies. The random arrangement of H2TPP molecules in those samples induce the stables trapped charge carries can be excited back to the radiative recombination center. That is the origin of the increase of PL intensity at high temperatures.