SLIDE 1
31‐07‐2019 Ultrafast Dynamics 1
31‐07‐2019 Ultrafast Dynamics 2
Pulsed excitation
Time Intensity Excited state population sample Pulsed Pump light Pulsed probe light
A = c l Map of time evolution
- f the excited state
How to follow fast processes? Nobel Lecture, Ahmed Zewail, 1999 31 - - PowerPoint PPT Presentation
How to follow fast processes? Nobel Lecture, Ahmed Zewail, 1999 31 - - PowerPoint PPT Presentation
How to follow fast processes? Nobel Lecture, Ahmed Zewail, 1999 31 07 2019 Ultrafast Dynamics 1 Pulsed excitation Intensity Excited state population Time A = c l sample Pulsed probe light Map of time evolution Pulsed Pump light of
SLIDE 2
SLIDE 3
31‐07‐2019 Ultrafast Dynamics 3
Snapshots of bond breaking
Nobel Lecture, Ahmed Zewail, 1999; Atkins, Physical Chemistry
SLIDE 4
Chemistry beyond the Ground State
Electronic arrangement different from that in the ground state HOMO LUMO Singlet excited state, S1 HOMO LUMO Triplet excited state, T1 HOMO LUMO Ground state, So Reactivity in the excited state is not the same as that in ground state
SLIDE 5
Photoacidity
Acids are more acidic in the excited state, bases are more basic Phenols: Delocalozation of electrons on O‐: Stabilization of the phenoxide ion ‐ * transition: Delocalization involves lower energy orbitals Further stabilization of the phenoxide ion Electron donating groups Aromatic carboxylates: Withdraw p electrons from the ring High electron density on O‐: Efficient proton abstraction ‐ * transition: * electrons are easier to withdraw More efficient proton abstraction Electron withdrawing groups
HOMO LUMO
SLIDE 6
An example: 2‐naphthol
pKa = 9.2, pKa* = 2!
Absorption spectra
pH titration curves Fluorescence spectra pH 3: protonated ground state, partially deprotonated excited state Acids are more acidic in the excited state
- J. R. Lakowicz, Principles of fluorescence spectroscopy, 3rd ed. 2006
SLIDE 7
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The mechanism of acid base reactions
Matteo Rini, Ben‐Zion Magnes, Ehud Pines, Erik T. J. Nibbering Science, 2003, 301, 349 – 352 Pyranine Photoacid Visible pump, Mid IR probe Photobase
SLIDE 8
Dynamics from the time – resolved IR Spectra
- B. High CH3COO‐ concn
- C. Low CH3COO‐ concn
Increasing CH3COO‐ concn. Smaller risetime of CH3COOH CH3COOH Photobase CH3COOH
SLIDE 9
The mechanism of acid‐base reactions
Further reading: Nibbering and coworkers, Science, 2005, 310, 83 ‐ 86
SLIDE 10
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The take home message
- There is more to Chemistry than mixing solutions
- Physical Chemistry vs. Unphysical Chemistry
- We need to know theory, we need to understand instruments
- Look wide and never be scared of new challenges