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Year 3 - Chemistry > Photochem > Flashcards

Photochem Flashcards

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1
Q

How do you find the allowed transition of symm polyatomics?

A

Find term of gs (totally symm) and es (direct product table)

Then test es terms with μ, which transforms via x,y,z in term table
TDM must be non-zero (so totally symm)

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2
Q

What occurs to BO approx in polyatomics?

A

Two or more vib modes can be excited simultaneously with elec

Means nuclear and elec motion is no longer separable

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3
Q

How can selec rules be determined for polyatomic molecules?

A

Determined by multiplying representations for each mode (elec and vib)

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4
Q

What is a chromophore?

A

Excitation which is a small group of atoms

Occurs when little/no symm overall in the molecule

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5
Q

What is internal conversion?

A

Non-radiative transition between states of same spin multiplicity
e.g. S1 -> S0

From vib g.s of the elec e.s -> vib e.s of elec g.s

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6
Q

What steps are there in internal conversion (IC)?

A

1) Excitation - elec gs to es
2) Vib relaxation - fast, lowers to vib gs of elec es
3) Internal Conversion - vib gs of elec es -> vib es of elec gs
4) Vib relaxation - prevents system going back

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7
Q

What is intersystem crossing?

A

Non-radiative transition from es to close in energy state with diff spin multiplicity
e.g. S -> T

Then vib relaxation

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8
Q

What occurs in fluorescence?

A

Radiative transition to lower energy state of same spin

Followed by VR

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9
Q

What occurs in phosphorescence?

A

Radiative transition from triplet to singlet state
T1 -> S0

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10
Q

What is a Jablonski diagram?

A

Summarises all transitions

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11
Q

How fast are the different radiative transitions?

A

elec excitation - fs

fluorescence - ns

phosphorescence (slow) - ms

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12
Q

How fast are non-radiative transitions?

A

vib relaxation (fast) - ps

internal conversion - fs/ns

intersystem crossing - ns/μs

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13
Q

When does the BO break down?

A

When ΔEel ~ ΔEvib

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14
Q

What does the Franck-Condon principle assume?

A

e- move faster than nulcei

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15
Q

What occurs to excited molecules in gas and liquid phase?

A

Gas - remains at same energy until collision, then loses vib and maybe el energy

Liquid - VR occurs rapidly, exchanges E fast with solvent

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16
Q

What occurs in reverse intersystem crossing?

A

Goes T1 -> S1

opposite to standard and leads to delayed fluorescence

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17
Q

Why is T1 lower in energy than S1?

A

Less repulsion

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18
Q

How does the frequency of fluorescence and phosphorescence compare?

A

Phosphorescence lower frequency and slower

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19
Q

How does fluorescence compare in rate to inter-system crossing?

A

ISC is around same speed of fluorescence

ISC fast as not highly excited vib state so FC overlap integral is large

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20
Q

How does absorption and fluorescence freq compare to 0 <-> 0 transition?

A

Absorption - higher freq

Fluorescence - slower freq

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21
Q

What is the Beer-Lambert Law?

A

It = I0 - Iabs = I0 exp(-αcl) = I0 10-εcl

where:
It,0,abs = intensity of transmitted, initial, and absorbed respectively
α - natural absorption coeff
ε - decadic absorption coeff

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22
Q

What is assumed for the Beer-Lambert Law?

A

Monochromatic light
Path length fixed
Absorption only - no scattering due to dust, bubbles, etc.
Conc fixed - homogeneous

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23
Q

What are the approx of the Beer-Lambert Law?

A

Weak field - absorption doesnt effect population of absorbing state (all molecules in gs)

Absorbers independent

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24
Q

What occurs to the Beer-Lambert Law at low conc?

A

Linear regime abs prop to conc:

It = I0 exp(-αcl) ~ I0 (1-αcl)

Iabs~ I0(αcl) = I0(εcl x ln10)

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25
Q

What is flux?

A

Rate at which energy flows

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26
Q

What is the SI quantity of intensity?

A

Energy flux per unit area per unit time

Not same as intensity of kinetics (photons absorbed per second)

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27
Q

What is the effect of light on quantum states?

A

Light mixes initial and final elec state

kfi = (2π/hbar) ∫ |ψf*H^ψi| 2

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28
Q

What is Fermi’s Golden Rule?

A

Formula describing transition rate from one energy eigenstate to another as a result of weak perturbation

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29
Q

Where is Fermi’s Golden rule derived from?

A

From time-dependent perturbation theory

Don’t learn

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30
Q

How does light interact with charges in a molecule?

A

Electric vector of light E interacts with charges in a molecule
H^' = -Σ qi E.ri

Sum extends over all charges, e- and nuclei, and related to dipole moment:
H^' = - E.μ

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31
Q

How is BO separation used for μ?

A

Dipole moment separated into sum over e- and nuclei

Overlap integral of nuclear-elec states approx 0 as for fixed nuclei the elec states are approx orthogonal

Molec wavefn separated into product of elec and nuclear

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32
Q

How is the electronic wavefn separated?

A

Separated into orbital and spin components

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33
Q

What is the rate of absorption proportional to?

A

kfi α
|χf*χidτN|2
|φf* μe φedτe|2
|<σf|σi>|2

1st term: Franck-Condon factor
2nd term: Elec matrix element, gives orbtial selection rule
3rd term: spin selection rule

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34
Q

What are the equations for Einstein coeffcients B (2-photon) and A (spontaneous emmision)?

A

Bfi = |μfi|2/6ε0hbar2

Afi = 8πhv3|μfi|2/6ε0hbar2c3

When large v (such as UV or Vis) is relevant to emission rates

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35
Q

What does absorption coefficient depend on?

A

Depends on frequency

Integral of absorption = πvNa|μfi|2/3cε0hbar

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36
Q

What is a mirror image spectrum?

A

Elec absorption is approx mirror image of fluorescence spectrum
with the 0<->0 overlapping in the middle

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37
Q

Where are absorption & fluorescence wrt 0<->0 transition?

A

Absorption: transition @ low λ as go to S1 excited state

Fluorescence: transition @ higher λ as S1 gs to es S0

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38
Q

What is required for a mirror image spectrum to occur?

A

ΔE between vib levels similar & transition prob therefore similar

Condensed phase - geometry of es similar to gs

Solvent relaxation occurs faster than fluorescence so all emission occurs from v=0

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39
Q

Why are mirror spectra observed?

A

Frank-Condon principle states nuclei stationary on elec timescale

Vib overlap occurs quickly so all emmision from S1 v=0

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40
Q

When are mirror image spectra not observed?

A

Not observed when:
change in geometry
es may transfer phenol
es may dissociate
several singlet states may appear in absorption (normally only lowest in fluorescence)

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41
Q

What is Kasha’s Rule?

A

Photon emission (fluorescence/phosphorescece) occurs only from lowest excited state of a given multiplicity

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42
Q

What is the bathochromic shift?

A

0 <-> 0 band shows shift towards red (smaller λ) due to quick VR before emmission

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43
Q

Why do phenol and biphenyl not have mirror spectra?

A

Phenol - es transfers proton

Biphenyl - change in geometry, S0 is twisted with low barrier and S1 is planar

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44
Q

Why do quinine and azomethane not exhibit mirror spectra?

A

Quinine - several singlet states in absorption, and only lowest in fluorescence

Azomethane - excited state may dissociate

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45
Q

What is the lifetime equation?

A

τ = 1 / k

where k is rate const

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46
Q

What is the rough lifetime of phosph/fluor?

A

Phosphorescence: τ in ms/s

Fluorescence: τ in ~10 ns

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47
Q

What processes compete to remove S1 state?

A

Fluorescence

Inter-system crossing

Internal conversion

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48
Q

What are the kinetics for removal of S1 state?

A

[S1] = [S1]0 exp(- (kf + kISC + kIC)t)

still first order

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49
Q

What is fluorescence lifetime?

A

τf = 1/(kf + kISC + kIC + …)

i.e. sum of the removal processes

50
Q

What is the equation for quatum yield, φ, in terms of molecules?

A

φ = (# molecules undergoing a process) / (# of photons absorbed) = kfτf = kf/(kf + kISC + kIC + …)

51
Q

How can # of photons be quantised?

A

[S1]0

52
Q

What is a branching ratio?

A

Simple 1st order competition of different processes

53
Q

What is the triplet yield?

A

φT = 1 - φf

54
Q

What is the phosphorescence yield?

A

φph = φT kph/(kph+kisc’) = φT kph τph

where kisc’ is reverse intersystem crossing

55
Q

When can a quantum yield be larger than 1?

A

Chain reaction
or
Produces gas

Quantum yield may depend on conc if competing 2nd order process

56
Q

What is steady-state illumination?

A

S1 in a true steady state
Iabs = (kf + kisc+ …)[S1]SS

[S1]SS = Iabs/(kf + kisc+ …)

57
Q

What is a quencher and how do they work?

A

S1 + Q -> S0 + Q

Quencher dissipates elec energy via vibrations to give heat

Pseudo-first order as quencher in XS and not destroyed

58
Q

What is the fluorescence yield when a quencher present?

A

φf = kf/(kf + kisc + kQ[Q])

Use a straight line plot for ratios of rate const
1/φf = 1 + (kisc/kf) + (kQ[Q]/kf)

59
Q

What is dynamic quenching?

A

Requires encounters and is diffusion controlled

Decreases lifetime (stern-volmer)

60
Q

What is static quenching?

A

Rigid matrix, quenching occurs immediately if quencher in range

φf reduced but not τ

61
Q

How can you measure # of photons absorbed indirectly?

A

Indirect chemical standards used - where the quantum yields known

62
Q

Why can intramolecular energy transfer (transitions beteween elec states without change in E) occur?

A

Small terms in hamiltonian which mix elec states
Treat them as perturbation via Golden Rule

63
Q

What is spin-orbit coupling hamiltonian in the orbital selection rule?

A

H^’ = Σξs^.l^

HSO is Rx,y,z in char tables
Rotations are rarely totally symm

So if ψintial and ψfinal have same symm the elec matrix element = 0

Rate enhanced by heavy atoms as SO coupling depends on it

64
Q

What is El Sayed’s rule?

A

ISC slow unless accompanied by change of e- config
(between S1 and T1)

65
Q

Why does ISC crossing occur in aromatic ketones?

A

Close nπ* and ππ* states
S1 to T2 occurs
(follows by fast IC from T2 -> T1)

66
Q

Why does ISC occur more with heavy atoms?

A

SO coupling depends on Z4
Therefore as mass increases the rate of ISC increases (which will also increase rate of phos)

67
Q

What is e-type delayed fluorescence?

A

Delayed fluorescence, same rate of phosphorescence when does start

Due to heavy atoms and so processes like ISC occur

68
Q

What energy terms are involved for internal conversion?

A

Sum of nuclear KE

Vibration, Rk, must have correct symm to mix the states

69
Q

What energy terms are involved for internal conversion?

A

Sum of nuclear KE

Vibration, Rk, must have correct symm to mix the states

70
Q

How does the FC factor increase with V?

A

@ high they go to classical turning points

71
Q

How does overlap of FC change as the bond length increases in change?

A
72
Q

How does overlap of FC change as the bond length increases in change?

A
73
Q

What is the energy gap law?

A

Rate of energy transfer decreases with increasing energy gap

S1-T1 < S0-T1 < S0-S1

74
Q

What causes the energy gap law?

A

From FC overlap

75
Q

What does the energy gap law favour?

A

Intersystem over internal conversion

76
Q

How does rate of ISC change with deuteration?

A

As deuteration increases then kISC lower τp increases

As they have lower freq and so elec energy gap requires high vib excitation
(poorer overlap - more nodes)

77
Q

What is Kasha’s rule?

A

Fluorescence from lowest excited singlet state

Phosphorescence from lowest triplet state

78
Q

What is a conical intersection?

A

Intersection of two conical surfaces representing individual wavefn

If occurs then v fast IC can occur

79
Q

How does azulene break Kasha’s rule?

A

Fluroescence from S2

Small energy gap between S1 and S0 meaning interconversion, extremely fast IC

80
Q

What is radiative transfer?

A

Fluorescence of D* absorbed by A

Long range

Normal selec rules

D fl must overlap with A ab

81
Q

What occurs in collisional transfer?

A

D* + A -> [D*A] -> [DA*] -> D + A*

Intramolec transfer within collision complex - golden rule with some perturbation

Franck-Condon principle ensures energy gap small - little energy degraded as translation

82
Q

What occurs in resonant energy transfer?

A

Spectra must overlap - energy conservation

83
Q

What occurs in Forster energy transfer?

A

Elec interaction between dipoles

Follows fermi golden rule
Has its own rate const

84
Q

What is the rate of energy transfer?

A

kT = (1/τ)(r06/r6)

where r0 is distance at which ET rate = spont decay rate

85
Q

What is short range energy transfer?

A
86
Q

What occurs wrt spin when energy transfer occurs (between D and A)?

A

Resultant spin of reactants conserved in complex and products

87
Q

What are the wigner spin correlation rules for energy transfer?

A
88
Q

What is triplet sensitisation?

A

Use a T1 state compound easily accesible to make an S0 compound (which is difficult to make into a triplet )

89
Q

What happens to reaction between two triplet state molecules (in ET)?

A

Leads to two singlets (S0 and S1)

This is a type of delayed fluorescence (p-type)

90
Q

What causes p-type delayed fluorescence?

A

Triplet annihilation occurs to give two singlet states

91
Q

What is an excimer?

A

Excited dimer

92
Q

What is an exiplex and main characteristic?

A

Excited complex between different species

Emission at longer λ than monomer fluorescence

93
Q

How does an exiplex form?

A

es bound and gs repulsive

Exciplex emission broad spectrum
Population inversion leads to laser action

94
Q

Does photoexcitation effect acid/base properties?

A

Yes

Phenol S1 more acidic
Aniline S1 less basic
Carboxylic acids less acidic as reduced delocalisation

95
Q

Do excited aromatic states react differently?

A

React with nucleophiles (opposite direction to normal)

96
Q

Why are reactions in excited state different?

A

Nucleophile usually interacts with LUMO of compound

In photo-reaction then nuc interacts with HOMO

97
Q

How does oxn state change in excited species?

A

e.s. easier to oxidise and reduce than gs

98
Q

What is channel 3?

A

Internal Conversion

99
Q

What are the valence isomers of benzene?

A
100
Q

What occurs when you excite benzene?

A

Small amount of isomerisation to valence isomers in the S1 and S2 state

101
Q

What is geminate recombination?

A

Recombination of fragments of a dissociated molecule
Where the fragments are made in same event

102
Q

What is diffusion control?

A

Atoms diffuse and recombine on encounter
In solution the solvent hinders separation

103
Q

What is the yield of escape from geminate recombination on time?

A

Decreases as t increases

Escapes at Ω=0.5

104
Q

What does the yield of escape from geminant recombination dependent on?

A

Wavelength: as decreases the yield of escape increases
(more energy to more likely to escape)

Viscosity: as increases the yield of escape decreases

105
Q

What is Ω, yield of escape with respect to intial distance between fragments?

A

Ω = 1 - a/r0

where a is a const and r0 is intial distance between fragments

106
Q

What occurs during photoionization and dissociative photoionization?

A

Photo: AB + hv -> AB+ + e-

Photo diss: AB + hv -> A + B+ + e-

107
Q

What occurs during autoionization and field ionization?

A

Auto:
AB + hv -> AB* (E>1) -> AB+ + e-

Field:
AB + hv -> AB* (E<1) -> apply field -> AB+ + e-

108
Q

What is the process of double ionization?

A

AB + hv -> AB2+ + 2e- -> A+ + B+

109
Q

What are the selection rules for single photoionization?

A

Any elec state of cation can be produced if can be accesed by removal of one electron from neutral (without further electron rearrangement)

110
Q

Why is there no resonant condition for single photoionization?

A

Energy of the outgoing electron is not quantised (as a free electron)

111
Q

What is done for photoelectron spec?

A

Ionization of a sample of molecules with hv > Ionisation energy

Produces ions with distribution of internal energies and hence a distribution of KEs

112
Q

What is seen on PES?

A

Each progression is from removal of e- from a different orbital

Progression structure represents vib energy levels of each state

113
Q

What is Koopman’s theorem?

A

In closed-shell the first IE of a molecular system is equal to the negative of orbital energy of HOMO

IE + Eion = -ε (orbital energy)

114
Q

What is a large progression on PES?

A

Extensive vib structure

Means removal of e- from this MO which causes a sig change in bonding

115
Q

What is required for photodissociation?

A

Requires excitation directly/indirectly to state above dissociation limit

116
Q

When is vertical excitation favoured?

A

Franck-Condon factors

Strong overlap when higher vib states in excited state

117
Q

What occurs to absorption spectrum at short wavelengths?

A

Becomes continuous as hv crosses dissociation threshold

118
Q

What occurs in predissociation?

A

Molecule excited to bound state - vibrates for a few periods then undergoes curve crossing and dissociates on repulsive PE curve

Then will dissociates

Franck-Condon determines if occurs

119
Q

What is a stationary state?

A

No time dependence

120
Q

What is the main assumption of Franck-Condon principle?

A

Nuclei are stationary on tiome scale of elec motion

Nuc wavefn unchange on es

121
Q

What causes more than non-0 linewidths?

A

Es has a non-zero lifetime

Homogeneous linewidth is a common feature of all molecules

Year 3 - Chemistry (12 decks)

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