Fluorescence - Introduction Most molecules prefer the...

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Introduction Most molecules prefer the lowest electronic ground state. To be elevated to excited states these molecules must absorb light. Once the molecule absorbs this light, it rapidly loses its excess of energy and returns to its lowest energy level. One way a molecule gains energy is through absorption. The reverse process is emission. A type of emission is Fluorescence and can be visualized in the Jablonski diagram below. Figure 1 : General absorption and emission processes 2 Fluorescence competes with nonradiative pathways (decrease of vibrational energy while maintaining same energy state) and phosphorescence decay. Quenching occurs when another component acts to favor one of the nonradiative pathways. Differing types of quenching include but are not limited to static and dynamic quenching. Dynamic quenching occurs where the fluorophore (the analyte) in the excited state interacts with
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other components and energy is transferred nonradiatively. The relationship between the quantum efficiency in the presence of a quencher (Фf) and in the absence of a quencher (Фa) gives rise to the Stern-Volmer equation. 3 Фf/Фa = 1+Kq[Q] Equation 1 This equation describes the decrease in the fluorescence quantum yield as a fuction of quencher concentration [Q]. The fluorescence intensity is directly proportional to the fluorescence quantum yield and thus the constant (Kq) may be determined from intensity measurements as a function of quencher concentration. If all molecules that are excited return to the original level of their ground state (thus all energy is gained and lost equally) the quantum efficiency is said to be maximum. 1 However, because even at normal conditions some molecules in an environment are temporarily at excited states, there are differences in excitation and emission data. This can be seen in the overlap between the two in a fluorescence intensity spectrum. Figure 2 displays these findings. Figure 2: Fluorescence spectra of LSD tartrate 4
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Fluorescence is an important part of today’s chemistry. In this experiment the chemist will learn the basic applications of the spectrometer and will also be able to apply their findings to common experiments in modern day chemistry. Specifically, in this paper the fluorophore quinine will be studied to evaluate its maximum excitation and emission wavelengths. To do this a spectrometer will be utilized to acquire spectra of quinine under different conditions. Fluorescence will also be analyzed by changing the conditions of the instrument. Experimental
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This note was uploaded on 09/22/2008 for the course CHEM 480 taught by Professor Chen during the Spring '08 term at University of Michigan.

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Fluorescence - Introduction Most molecules prefer the...

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