Photochemistry

Question 1

What is name of the lowest energy - ground state?

Answer 1

Singlet state, So

Question 2

Can you change an electron’s spin by exciting it with light?

Answer 2

No, it can be excited to another singlet state, S1, but its spin cannot change due to selection rules

Question 3

What is this called?

Answer 3

A triplet state, this is T1.

This is because there are different numbers of up and down electrons

Question 4

What does the Jablonski diagram describe?

Answer 4

It is a simplified portrayal of the relative positions of the electronic energy levels and associated vibrational states

States are arranged vertically by energy

Horizontally separation of different states bears no resemblance to nuclear separation

Question 5

What is W12?

Answer 5

W12 is the transition rate from So —> S1

Question 6

What equation gives W12?

Answer 6

W12 = Bp(lambda)n1

Where B = Einstein coefficient of stimulated absorption

p(lambda) = photon density at wavelength (lambda)

n1 = population (concentration) of ground state (So)

Question 7

What order is the transition rate from so —> S1?

Answer 7

First order kinetics

Note: the rate constant for excitation depends on light intensity

Question 8

What is the Franck-Condon principle?

Answer 8

Electronic transitions occur much faster than nuclei can respond ≈ 10^15 s^-
-nuclei are much larger than electrons

Vertical transitions - an electronic transition without a change in nuclear geometry

The excited state energy profile is typically offset vs the ground state

Question 9

What states does vibrational relaxation occur in?

Answer 9

Can occur within S1, T and So

Question 10

How is energy lost through vibrational relaxation?

Answer 10

Non-radiative relaxation of vibrations

i.e. no photon emission, energy is lost as heat to surroundings

Question 11

Why is vibrational relaxation ignored in kinetics?

Answer 11

Because it is very fast

Overall kinetics often determined by slowest process, hence Krel is irrelevant

Krel (ps)&raquo_space; Kic, Kfl, Kisc (ns)

Question 12

What is intersystem crossing?

Answer 12

It’s the transfer between spin states through spin-orbit coupling

i,e, going from S1 to T1

Question 13

What is the rate constant for intersystem crossing?

Answer 13

K(ISC)

Question 14

What occurs after intersystem crossing?

Answer 14

Relaxation to lowest vibrational level will rapidly occur post ISC

However this is usually not shown

Question 15

What does decay result in?

Answer 15

Photon emission - fluorescence + heat

S1 —> So + hv + heat

Question 16

Is fluorescent decay a spin allowed process?

Answer 16

Yes, emits photon. No selection rules are broken

Question 17

What order of magnitude is the speed of fluorescent decay?

Answer 17

≈1x10^9 s-1

Question 18

What is internal conversion?

Answer 18

When the excited electron relaxes to a lower energy state without the emission of a photon

This is called thermal relaxation

Question 19

What’re the kinetics of Decay of the S1 state?

Answer 19

They follow parallel reaction kinetics

The kinetics are governed by the sum of the rate constants

Question 20

What’s the probability of ending up at T1?

Answer 20

Quantum yield (T1) = Kisc / (Kic + Kfl + Kisc)

Question 21

What’s the probability of ending up at So? What does this mean?

Answer 21

Quantum yield (So) = Kfl / (Kic + Kfl + Kisc)

It means a photon is emitted

Question 22

What is the observed half-life of S1?

Answer 22

Since fluorescence follows first order kinetics…

Half-life = ln2 / (Kic + Kisc + Kfl)

Question 23

What i the observed half-life for the fluorescence?

Answer 23

Lifetime = 1 / (Kic + Kisc + Kfl)

Question 24

Why, even if the quantum yield of fluorescence = 1, is the energy yield lower?

Answer 24

Because even if the quantum yield = 1, the energy yield is lower since hv(fl) < hv(exc)

Some energy will be lost as heat due to the very fast vibrational relaxation of the excited electron

Question 25

What is quantum yield equal to?

Answer 25

Quantum yield = number of events( interested in ) / number of photons absorbed

Question 26

How can you find the rate constant for fluorescence?

Answer 26

You can measure both the amount of light emitted and how many excited state molecules that are left after a certain amount of time using pulsed laser spectroscopy

This then results in:

Question 27

How does quenching happen?

Answer 27

When an excited molecule bumps into another molecule, losing its energy and hence not emitting a photon

No pretty colours :( whomp whomp

Question 28

What is the rate of quenching?

Answer 28

Rate of quenching = Kq[Q][S1]

This is a second order rate, as it is bimolecular

Question 29

Why can quenching be treated as a pseudo-first order rate? Wat is the new rate of quenching?

Answer 29

Because [Q] is usually in vast excess of excited state molecules [S1] and so the rate just becomes kq[Q]

Question 30

What is the quantum yield of fluorescent molecules when a quencher is present?

Answer 30

Quantum yield (fl [Q]) = Kfl / (Kic + Kisc + Kfl + Kq[Q])

Question 31

In Stern-Volmer kinetics, what is the effect of quenching on the rate of the system?

Answer 31

Question 32

what does phosphorescence result in?

Answer 32

Emission of a photon

Question 33

From what state does phosphorescence originate from?

Answer 33

T1: triplet state —> decay to So

Question 34

Is phosphorescence spin allowed and how fast is it?

Answer 34

Phosphorescence IS spin forbidden

Its very slow, magnitude of ms to s instead of ns.

Question 35

Does phosphorescence released a high or low relative amount of energy?

Answer 35

Low

Note: hv(ph) < hv(fl) < hv(exc)

Question 36

What’s the benefit with phosphorescence?

Answer 36

Since it lasts for a relatively long period of time it is more likely to collide with other molecules.

This is useful when working with photochemical catalysts

Question 37

What reaction types are there in photochemical reactions?

Answer 37

Dissociation - from S1 to T1

Reaction with M - more likely to be with longer lived T state

Question 38

What is the yield of the (primary) product for a reaction between M and T1?

Answer 38

Question 39

Why would the quantum yield be > 1?

Answer 39

Because a photo product starts a chain reaction