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 • Examples: Photoacidity, excited state proton transfer (ESPT), photosiomerization • Signatures of excited state processes: 9k 6k Counts 3k 0 0 1 2 3 4 Time / ns Fast decay at blue end, Time evolution of Red shifted rise in the red end emission spectra fluorescence spectra
Pulsed excitation Intensity Excited state population Time A = e c l sample Pulsed probe light Map of time evolution Pulsed Pump light of the excited state 30-12-2019 Ultrafast Dynamics 3
Temperature dependence: Arrhenius k = Ae - E a RT Origin: van’t Hoff equation (thermodynamics) Potential Energy profile Molecular Reaction Dynamics Activated Complex Q A Q B e - E 0 RT k = kT Q * h k = 6 10 12 s -1 Fastest reaction: t = 1/ k = 170 10 -15 s -1 Ultrafast Dynamics: Femtochemistry Physical Chemistry, Atkins, 10 th ed. 30-12-2019 Ultrafast Dynamics 4
Photodissociation of ICN Laser Induced Fluorescence (LIF) v Complete photodissociation within 600 fs t = 205 fs ± 30 fs 30-12-2019 Ultrafast Dynamics 5
Snapshots of bond breaking Nobel Lecture, Ahmed Zewail, 1999; Atkins, Physical Chemistry 30-12-2019 Ultrafast Dynamics 6
The mechanism of acid base reactions Matteo Rini, Ben-Zion Magnes, Ehud Pines, Erik T. J. Nibbering Science , 2003 , 301 , 349 – 352 Pyranine Photobase Photoacid Visible pump, Mid IR probe 30-12-2019 Ultrafast Dynamics 7
Dynamics from the time – resolved IR Spectra CH 3 COOH Photobase CH 3 COOH Increasing CH 3 COO - concn. Smaller risetime of CH 3 COOH B. High CH 3 COO - concn C. Low CH 3 COO - concn
The mechanism of acid-base reactions Further reading: Nibbering and coworkers, Science , 2005 , 310 , 83 - 86
Twisted Intramolecular Charge Transfer (TICT) Strong electronic coupling between D and A
Twisted Intramolecular Charge Transfer (TICT)
Twisted Intramolecular Charge Transfer (TICT) Little, if any electronic coupling between D and A
Dimethyl aminobenzonitrile (DMABN) B , 10 3 cm -1 A D f 1) n -hexane, 2) dibutyl ether, 3) diethyl ether, 4) Remarkable polarity dependence: butyl chloride and 5) acetonitrile. Polarity sensor • Non-polar solvents: Blue emission • Intermediate polarity: Dual emission Proc. Indian Natl. Acad. Sci. (Chem. Sci.) 104 (1992) 197 • High polarity: Stokes shifted emission
Polarity and viscosity dependence Charge separation: Large dipole moment – polarity dependence Twisting motion: Viscosity dependence Mixtures of isoviscous polar and non-polar solvents, room temperature E T (30) = Empirical micropolarity parameter C 3 H 7 CN/octane ln k 1 ( s -1 ) Absorption maximum of a betaine dye no. 30 In kcal mol -1 C 4 H 9 OH/ Polarity dependent rate constants hexadecane Effect of hydrogen bonding E T (30) Chem Phys. Lett. 135 (1987) 413
Effect of temperature and H- bonding Isoviscous neat liquids at different temperatures Increase in the rate of TICT at lower temperatures!! Lower temperature Higher polarity Larger stabilization of A* Lower barrier 0 – A[E T (30) – 30] E a = E a E a for alcohols > E a for nitriles H-bonding decreases the electron density on N TICT is hindered alcohols - E a nitriles = 6 kcal mol -1 , energy of a H - bond E a
Other important TICT probes Nile red TNS Nonpolar LE Polar TICT Almost nonemissive TICT state: Stabilized in polar solvents like water Highly fluorescent LE state: Predominant in non polar media Commonly used as protein markers
Applications: Fluorescent sensors Ratiometric fluorescent sensors For saccharide and fluoride Lewis acidity: Boronate > Boronic acid TICT suppressed : 1 -saccharide adduct TICT enhanced : 2 -fluoride adduct Lewis acidity of boronate is quenched by fluoride Bioorg. Med. Chem. Lett. 17 (2007) 2629 – 2633
Structural modification of DMABN Rigidity single emission only LE emission If rings are parallel only TICT emission If rings are perpendicular CH 2 Cl 2 , room temp. - - - - hexane, - —— acetoniotrile J. Lumin. 18/19 (1979) 420 J. Phys. Chem. 97 (1993) 13500
DMABN: Ultrafast dynamics Acetonitrile LE ICT: 3.07 ps J. Chem. Phys. 131 (2009) 031101
PICT: The counterview J. Am. Chem. Soc.. 126 (2004) 031101; 1705
And the debate rages on
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