Chemistry beyond ground state Excited state phenomena Fluorescence - - PowerPoint PPT Presentation
Chemistry beyond ground state Excited state phenomena Fluorescence spectra are mirror images of absorption spectra UNLESS an excited state process occurs Driving force for excited state processes: Change in electron configuration
an excited state process occurs
Red shifted fluorescence spectra Fast decay at blue end, rise in the red end Time evolution of emission spectra
1 2 3 4 3k 6k 9k Time / ns Counts
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Time Intensity Excited state population sample Pulsed Pump light Pulsed probe light
A = e c l Map of time evolution
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Origin: van’t Hoff equation (thermodynamics) Potential Energy profile Activated Complex Molecular Reaction Dynamics
Physical Chemistry, Atkins, 10th ed.
Fastest reaction: k = 6 1012 s-1 t = 1/k = 170 10-15 s-1 Ultrafast Dynamics: Femtochemistry
h Q* QAQB e-E0 RT
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v Laser Induced Fluorescence (LIF) Complete photodissociation within 600 fs t = 205 fs ± 30 fs
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Nobel Lecture, Ahmed Zewail, 1999; Atkins, Physical Chemistry
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Matteo Rini, Ben-Zion Magnes, Ehud Pines, Erik T. J. Nibbering Science, 2003, 301, 349 – 352 Pyranine Photoacid Visible pump, Mid IR probe Photobase
Increasing CH3COO- concn. Smaller risetime of CH3COOH CH3COOH Photobase CH3COOH
Further reading: Nibbering and coworkers, Science, 2005, 310, 83 - 86
Strong electronic coupling between D and A
Little, if any electronic coupling between D and A
Remarkable polarity dependence: Polarity sensor
1) n-hexane, 2) dibutyl ether, 3) diethyl ether, 4) butyl chloride and 5) acetonitrile.
emission
104 (1992) 197
, 103 cm-1 Df
Chem Phys. Lett. 135 (1987) 413
Charge separation: Large dipole moment – polarity dependence Twisting motion: Viscosity dependence ET(30) = Empirical micropolarity parameter Absorption maximum of a betaine dye no. 30 In kcal mol-1 Mixtures of isoviscous polar and non-polar solvents, room temperature
ln k1 (s-1) ET(30) C3H7CN/octane C4H9OH/ hexadecane
Polarity dependent rate constants Effect of hydrogen bonding
Isoviscous neat liquids at different temperatures Increase in the rate of TICT at lower temperatures!! Ea = Ea
0 – A[ET(30) – 30]
Ea for alcohols > Ea for nitriles H-bonding decreases the electron density on N TICT is hindered Ea
alcohols - Ea nitriles = 6 kcal mol-1, energy of a H - bond
Lower temperature Higher polarity Lower barrier Larger stabilization of A*
Nile red TNS Almost nonemissive TICT state: Stabilized in polar solvents like water Highly fluorescent LE state: Predominant in non polar media Commonly used as protein markers
Nonpolar LE Polar TICT
TICT suppressed : 1-saccharide adduct Lewis acidity: Boronate > Boronic acid TICT enhanced : 2-fluoride adduct Lewis acidity of boronate is quenched by fluoride Ratiometric fluorescent sensors For saccharide and fluoride
Rigidity single emission
13500
CH2Cl2, room temp.
If rings are parallel
If rings are perpendicular
LE ICT: 3.07 ps
Acetonitrile