FIU CHM 4130 - Chapter 15 Xiao_Molecular Luminescence Spectrometry_2018 (51 pages)

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Chapter 15 Xiao_Molecular Luminescence Spectrometry_2018



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Molecular Luminescence Spectroscopy Chapter 15 Fluorescence Phosphorescence and Chemiluminescence What happens to the absorbed EM energy determines whether you have Absorbance molecule returns to the ground or lower energy state via a non radia ve transi on such as vibra on collision with other molecules etc These give o the energy absorbed rather than the emission of light Fluorescence Some energy is lost through various processes e g non radia ve transi ons and then light is given o Phosphorescence The molecule transi ons from an excited triplet state to an lower energy singlet state and gives o light Fluorescence Phosphorescence Chemiluminescence Absorption 10 14 1 to 10 15 s 10 8 For UV Vis need to observe Po and P difference which limits detection 10 9s M M heat Theory of Fluorescence and Phosphorescence 10 5 to 10 8 s fluorescence 10 4 to 10 s phosphorescence Excitation of e by absorbance of h Re emission of hv as e goes to ground state Use h 2 for qualitative and quantitative analysis Method Mass detec on limit mole Concentra on detec on limit molar Advantages UV Vis 10 13 to 10 16 10 5 to 10 8 Universal Fluorescence 10 15 to 10 17 10 7 to 10 9 Sensi ve Fluorescence Pauli Exclusion Principle 2 Fluorescence ground state to single state and back Phosphorescence ground state to triplet state and back Fluorescence Phosphorescence 10 5 to 10 8 s 10 4 to 10 s Spins paired No net magnetic field Spins unpaired net magnetic field Rates of Absorption and Emission Absorp on Emission Rate seconds Comments Photo Absorp on 10 14 to 10 15 Fast Fluorescence 10 5 to 10 10 Fast singlet to singlet transi on Phosphorescence 10 4 to 10 Slow triplet to singlet transi on S1 S0 Deactivation Processes a vibrational relaxation solvent collisions emission excitation Stokes shift vibrational relaxation is efficient and goes to lowest vibrational level of electronic state within 10 12s or less significantly shorter life time than electronically excited state fluorescence occurs from lowest vibrational level of electronic excited state but can go to higher vibrational state of ground level b internal conversion crossing of e to lower electronic state S1 to S0 would also happen efficient therefore many compounds don t fluoresce aliphatic especially probable if vibrational levels of two electronic states overlap can lead to predissociation predissociation relaxation to vibrational state of a lower electronic state with enough energy to break a bond dissociation direct excitation absorption to vibrational state with enough energy to break a bond c external conversion deactivation via collision with solvent collisional quenching decrease collision increase fluorescence or phosphorescence decrease temperature and or increase viscosity decrease concentration of quenching Q agent d intersystem crossing spin of electron is reversed change in multiplicity in molecule occurs singlet to triplet enhanced if vibrational levels overlap more common if molecule contains heavy atoms I Br e Phosphorescence Deactivation from an triplet electronic state to the ground state producing a photon Variables a ec ng uorescence Quantum Yield ratio of the number of molecules that luminesce to the total number of excited molecules efficiency determined by the relative rate constants kx of deactivation processes Increase quantum yield by decreasing factors that promote other deactivation processes Fluorescence Quantum Yield F k F k ec kF k ic k isc k pd k d kec external conversion S1 S0 Influenced by the kic internal conversion S1 S0 environment kisc intersystem crossing S1 T1 kpd predissociation kd dissociation Depend on the chemical structure kf fluorescence Types of Transitions in Fluorescence seldom occurs from absorbance less than 250 nm 200 nm 140 kJ mol breaks many bonds fluorescence not seen with typically or n commonly great quantum efficiency Fluorescence most commonly arises from a transition from the lowest vibrational level of the first excited electronic state to one of the vibrational levels of the electronic ground state Concentration Dependence F K Po P P bc 10 Po P0 P Power of the excitation beam absorbed by the system F K Po 2 303 bc F Kc If 2 303 bc A 0 05 then Fluorescence is proportional to concentration Effect of Concentration on Fluorescence or Phosphorescence At low concentrations F 2 3K bcPo deviations at higher concentrations absorbance 0 05 can be attributed to absorbance becoming a significant factor and by self quenching or selfabsorption Fluorescence Structure usually aromatic compounds with low energy transition quantum yield increases with number of rings and degree of condensation fluorescence especially favored for rigid structures fluorescence increase for chelating agent bound to metal Examples of fluorescent compounds H N N H2 C O Zn N 2 Quinoline indole fluorene 8 hydroxyquinoline Methyl Addition Carboxylic Acid Substitution Halogen Substitution Effect of Structural Rigidity Temperature Solvent pH Effects decrease temperature increase fluorescence deactivation increase viscosity increase fluorescence less collisions heavy atoms effect fluorescence is pH dependent for compounds with acidic basic substituents more resonance forms stabilize excited state H H N H H N H H N resonance forms of aniline Effect of Dissolved O2 increase O2 decrease fluorescence oxidize fluorescing species paramagnetic property increase intersystem crossing spin flipping Quenching methods Quenching methods Components of Fluorescence Spectrophotometers Light sources low pressure Hg lamp 254 302 313 nm lines high pressure xenon arc lamp lasers Wavelength selectors lters Monchromators Cells and sample compartments quartz cells light ght compartments to minimize stray light Detectors photomul pliers CCD cameras Emission and Excita on Spectra Instrumentation Basic design components similar to UV Vis spectrofluorometers observe both excitation emission spectra Extra features for phosphorescence sample cell in cooled Dewar flask with liquid nitrogen delay between excitation and emission Fluorometers simple rugged low cost compact source beam split into reference and sample beam reference beam attenuated fluorescence intensity A 1 filter fluorometer Spectrofluorometer both excitation and emission spectra two grating monochromators quantitative analysis Perkin Elmer 204 Total Fluorescence Instrument Use array detector CCD to collect total uorescence spectrum Tradi onal Emission and Excita on Spectra Total Fluorescence Spectrum


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