Kearns_Experiment 6

Kearns_Experiment 6 - Experiment 6 Absorption and...

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Experiment 6, Absorption and Fluorescence Spectroscopy of Porphyrins and Metallo-Porphyrins Author: Jeremy Kearns Group 6 Section 1, M 1:25 PM – 4:25 PM Partner: Tyler Smith 2/7/2011 Abstract: This experiment uses a UV-Vis and fluorescence apparatus in order to measure the absorption and fluorescence of porphyrins with different functional groups. The various porphyrins tested were ZnTPP, NiTPP, H 2 TPP, and TPP 2- . After the data was gathered, excitation energy values were calculated using both the wavelength of the absorbance, as well as the Gouterman Model. It is shown that porphyrin molecules can easily transition to their second excited states, but they only are excited to their first excited state in minimal yields. It was also shown that the number of electrons in the d-orbital of a transition metal can affect the energy separation of π and π * electrons in the porphyrin group. I. Introduction Porphyrins constitute a vital type of molecules. Metallation of porphyrins can occur as the ring system deprotonates and a dianionic ligand is formed. The metal ions can act as Lewis acids and accept electron pairs. Porphyrins have a wide range of use both in and out of the human body. Inside the human body, oftentimes with a functional metal group, porphyrins are used in
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Experiment 6, Absorption and Fluorescence Spectroscopy of Porphyrins and Metallo-Porphyrins proteins, which can do such tasks as oxygen transfer and storage (as in hemoglobin and myoglobin), electron transfer (as in cytochrome c and cytochrome oxidase), and energy conversion (as in chlorophyll). Porphyrins are also used in such processes involving: chemical and photochemical means, organic semiconductors, photovoltaic cells, ion sensors, nanoelectronics, and nanophotonics. Some of the key factors that allow the porphyrin to possess such a vast array of applications include the numerous amounts of metals that can bind in the central “pocket” of the porphyrin, as well as the nitrogen-containing π-conjugated rings that can influence the redox properties of the metal center. In order to study the effect of the central metal group, or lack thereof, on the ability of the molecule to transform energy states, physical chemical methods can be utilized. Electrons in a molecule transfer between different energy levels due to the absorption of emission of energy. Two ways to respectively measure the absorption and emission of porphyrins include the use of a UV-Vis and a fluorescence device. When measuring these values, keep in mind that energy transfer is not 100 percent efficient, as a result of this fact, the wavelengths of fluorescence will be higher than those of absorption, as longer wavelengths correlate to lower energies. Electrons in a porphyrin typically show a strong absorbance around 400 nm, which correlates to a transition from the ground state to the second excited state (S 0 to S 2 ). This results in the Soret, or B band. There is also a weak absorbance to the first excited state (S
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Kearns_Experiment 6 - Experiment 6 Absorption and...

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