Lecture 10 - Electronic transitions

Question 1

What is the Frank Condon principle?

Answer 1

• States the approximation that an electronic transition is most likely to occur without changes in the positions of the metallo-center in the molecular entity and its environment.
• Absorption happens faster than the stretching of the molecule

Question 2

What is the basic absorption process?

Answer 2

• An electron is added to the excited state and the molecule relaxes into the excited geometry
• The absorption band shape is caused by transitions to a number of vibronic-electronic states.

Question 3

What are the 2 emission processes?

Answer 3

• Fluorescence - spin state doesn’t change, fast and efficient.
• Phosphorescence - spin state changes, slow and long lived.

Question 4

What happens in fluorescence?

Answer 4

• Due to shift of absorption, when electron it absorbed and tries to go back down, it cant go to bottom vibronic energy level and stays in a higher level than the GS
• Shift is higher wl - lower energy than the absorption process
• Very fast
• Heat loss in the system can cause fluorescence to be less efficient

Question 5

What is the fluorescence quantum yield?

Answer 5

no of photons emitted/no of photons absorbed

• A higher value will give a larger fluorescence

Question 6

What is phosphorescence?

Answer 6

• Involves absorption between two spin states of the same spin which causes the electron to go to a lower energy state and causes spin to change (T1)
• These are forbidden-
• Electron trapped in the lower excited state
• Going back down to GS is slow
• very stable which is why is takes 100s-minutes to go back down
• Occur at low freq than fluo

Question 7

What are the differences between Resonance Raman and fluo? (both dont go back down to 0)

Answer 7

• Difference due to amount of time molecule stays in excited state
• In res Raman, emission is slow as the nuclei relaxes into equilibrium geometry of excited state (scattering happens before nuclei reaches eq)
• Res Raman is faster as the particle goes to a level that isn’t real so falls back very fast.

Question 8

How can fluorescence be avoided in Raman?

Answer 8

• Use pulsed lasers and detect scattering before flu happens.
• Can use higher laser wl, lowers energy so further away from electronic state

Question 9

What is fluorescence used for?

Answer 9

• In bioanalysis to label biomolecules which are uncoloured, makes them coloured to allow detection
• Example is DNA detection , fluorescent dye attached to a DNA strand through a linker and only the dye detected on emission.

Question 10

What are the uses of phosphorescence?

Answer 10

• Clock and watch hands
• Glow in the dark stickers
• Rocks naturally phos
• Sea plankton
• An example is SrAl2O4, which is used in safety signs, must be visible in dark env.