exp27_Sep07 - Experiment 27 Chemistry 541/2 Physical...

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1 Experiment 27 Physical Chemistry Laboratory Time-resolved luminescence of pyrene (revised Sep 2007) Introduction In this lab, subnanosecond laser pulses from a nitrogen laser will be used to excite pyrene dissolved in cyclohexane or ethanol. A fast photodetector and a high bandwidth digital oscilloscope will be used to measure the kinetics of the luminescence decays for pyrene solutions at various concentrations. Nonlinear least squares fitting of the transients will be carried out to obtain estimates of the pyrene monomer and excimer fluorescence lifetimes. This lab provides a basic introduction to modern photophysical measurements using a simple laser-based apparatus. In addition, this lab provides basic training in how to use a modern oscilloscope. Chemistry 541/2 Chemistry 541/2 Laser
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2 Background Photophysics encompasses all of the physical (i.e. non-chemical or non-reactive) events that can occur upon excitation of molecules by light. Absorption of visible and/or ultraviolet light generally gives rise to electronic excited states in organic molecules. In this lab, pyrene will be excited by a 337 nm laser pulse, forming a pyrene excited state. 1.00 0.75 0.50 0.25 0.00 Relative Absorbance 380 360 340 320 300 280 260 240 Wavelength / nm UV/VIS absorption spectrum of pyrene in cyclohexane. We label a single pyrene molecule by the symbol M, and we label a pyrene excited state by M*. An asterisk is the traditional photochemical nomenclature for indicating an electronically excited molecule. We can thus represent the excitation of pyrene in the following way, M ⎯→ hv M* (1) On account of their high energy content, electronically excited molecules may be highly reactive. Alternatively, an excited molecule can return to its electronic ground state by radiative or nonradiative decay. These latter processes are examples of photophysical decay channels for excited states. In radiative decay, an excited molecule emits a photon of light with the light energy equal to the energy difference between the initial and final quantum states. Radiative decay between states of the same parity is called fluorescence, while radiative decay between two states of different parity is called phosphorescence. Fluorescence and phosphorescence are two kinds of luminescence. Since the electronic ground state of most organic compounds is a singlet state, fluorescence usually involves transitions between singlet states. Similarly, phosphorescence usually involves a transition from a molecule’s lowest energy triplet state to its ground electronic singlet state. Triplet-singlet transitions are spin-forbidden making phosphorescence a comparatively slow process. In room temperature solution other means of deactivating excited states have higher rate constants and phosphorescence is generally not observed. Fluorescence, on the other hand, is a fully spin-allowed process and proceeds at a fast rate. Typical fluorescence lifetimes for organic compounds are 5-50 ns. Pyrene is a highly fluorescent polyaromatic hydrocarbon.
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exp27_Sep07 - Experiment 27 Chemistry 541/2 Physical...

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