Photochemistry Flashcards
What is name of the lowest energy - ground state?
Singlet state, So
Can you change an electron’s spin by exciting it with light?
No, it can be excited to another singlet state, S1, but its spin cannot change due to selection rules
What is this called?
A triplet state, this is T1.
This is because there are different numbers of up and down electrons
What does the Jablonski diagram describe?
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
What is W12?
W12 is the transition rate from So —> S1
What equation gives W12?
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)
What order is the transition rate from so —> S1?
First order kinetics
Note: the rate constant for excitation depends on light intensity
What is the Franck-Condon principle?
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
What states does vibrational relaxation occur in?
Can occur within S1, T and So
How is energy lost through vibrational relaxation?
Non-radiative relaxation of vibrations
i.e. no photon emission, energy is lost as heat to surroundings
Why is vibrational relaxation ignored in kinetics?
Because it is very fast
Overall kinetics often determined by slowest process, hence Krel is irrelevant
Krel (ps)»_space; Kic, Kfl, Kisc (ns)
What is intersystem crossing?
It’s the transfer between spin states through spin-orbit coupling
i,e, going from S1 to T1
What is the rate constant for intersystem crossing?
K(ISC)
What occurs after intersystem crossing?
Relaxation to lowest vibrational level will rapidly occur post ISC
However this is usually not shown
What does decay result in?
Photon emission - fluorescence + heat
S1 —> So + hv + heat
Is fluorescent decay a spin allowed process?
Yes, emits photon. No selection rules are broken
What order of magnitude is the speed of fluorescent decay?
≈1x10^9 s-1
What is internal conversion?
When the excited electron relaxes to a lower energy state without the emission of a photon
This is called thermal relaxation
What’re the kinetics of Decay of the S1 state?
They follow parallel reaction kinetics
The kinetics are governed by the sum of the rate constants
What’s the probability of ending up at T1?
Quantum yield (T1) = Kisc / (Kic + Kfl + Kisc)
What’s the probability of ending up at So? What does this mean?
Quantum yield (So) = Kfl / (Kic + Kfl + Kisc)
It means a photon is emitted
What is the observed half-life of S1?
Since fluorescence follows first order kinetics…
Half-life = ln2 / (Kic + Kisc + Kfl)
What i the observed half-life for the fluorescence?
Lifetime = 1 / (Kic + Kisc + Kfl)
Why, even if the quantum yield of fluorescence = 1, is the energy yield lower?
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
What is quantum yield equal to?
Quantum yield = number of events( interested in ) / number of photons absorbed
How can you find the rate constant for fluorescence?
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:
How does quenching happen?
When an excited molecule bumps into another molecule, losing its energy and hence not emitting a photon
No pretty colours :( whomp whomp
What is the rate of quenching?
Rate of quenching = Kq[Q][S1]
This is a second order rate, as it is bimolecular
Why can quenching be treated as a pseudo-first order rate? Wat is the new rate of quenching?
Because [Q] is usually in vast excess of excited state molecules [S1] and so the rate just becomes kq[Q]
What is the quantum yield of fluorescent molecules when a quencher is present?
Quantum yield (fl [Q]) = Kfl / (Kic + Kisc + Kfl + Kq[Q])
In Stern-Volmer kinetics, what is the effect of quenching on the rate of the system?
what does phosphorescence result in?
Emission of a photon
From what state does phosphorescence originate from?
T1: triplet state —> decay to So
Is phosphorescence spin allowed and how fast is it?
Phosphorescence IS spin forbidden
Its very slow, magnitude of ms to s instead of ns.
Does phosphorescence released a high or low relative amount of energy?
Low
Note: hv(ph) < hv(fl) < hv(exc)
What’s the benefit with phosphorescence?
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
What reaction types are there in photochemical reactions?
Dissociation - from S1 to T1
Reaction with M - more likely to be with longer lived T state
What is the yield of the (primary) product for a reaction between M and T1?
Why would the quantum yield be > 1?
Because a photo product starts a chain reaction
