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photochemistry Flashcards

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1
Q

describe the differences between photophysics and photochemistry?

A

chem- light induces chemistry - bond breaking and forming

physics - reversible changes. energy transition or emission of light

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2
Q

what is the difference between the ground state and excited state if they both exhibit the same chemistry?

A

Gs has an infinite lifetime but the ES does not

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3
Q

what are the applications of photoinduced electron transfer?

A

Radiation damage of DNA- determine conductivity
Analytical chemistry - emission sensing
information storage and optical memory - photochromic or photoswitchable optics

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4
Q

describe the Acceptor/ Donor complex?

A

A and D seperated by a linker. A and D can be metal complexes which are seperated by a bridging ligand. Can tune the properties of A and D by varying the bridge. For the bridge to be catalytic ally active it need to posses energetically low lying orbitals to accomodate an electron

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5
Q

what does electron transfer depend on?

A

Solvent
free energy
distance
coupling/structure

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6
Q

how does the solvent affect rate of electron transfer?

A

ET occurs in solvent but since there is not nuclear rearrangement the way the solvent orientates around the nucleus changes.

A- and D+ formed in a polar solvent, they differ form A and D wrt their charge distributions so solvent spheres undergo reorganisation. 𝞴s denotes solvent reorganisation

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7
Q

what is wrong with libbys model of ET?

A

Assumes configurations are in equilibrium at when curve is at the bottom. Huge energy input required which is unrealistic for thermal rxns

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8
Q

what does marcus model propose?

A

That a weak electronic interaction required at the crossing point.

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9
Q

what the assumptions for marcus` theory?

A
  1. First coordination sphere not affected by ET - Nuclear coordinates do not change
  2. Formation of DA from D and A is negligible
  3. Weak interactions
  4. reason for ΔG = > 0 and its orientational polarization. Linear response assumed
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10
Q

what is the definition of solvent reorganisation?

A

the xs energy required for the system to be on the potential surface of the intial state whilst having a coordinate of the final state without undergoing an ET

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11
Q

what are the two regions of marcus bell shaped curved?

A

inverted region - as ΔGet become more negative the rate of electron transfer decreases

Kinetic region (Normal) - as ΔGet becomes less favourable the rate of ET decreases

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12
Q

what are the laws of photochemistry?

A
  1. only absorbed light causes photochemical reactions to occur
  2. One photon will activate one molecule
  3. photochemical reactions take place from lowest energy excited state, regardless of which one was populated first. implies fast decay times (kashas rule)
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13
Q

draw a janlonski diagram and define all the terms

A

Internal crossing - nr relaxtion from states of the same multiplicity
intersystem crossing - nr transition between states of different multiplicity
Florescence - radiative transition from states of same mulitplicity - spin allowed (s-s)
Phosphorescence - radiative transition from states of different mulitplicity - spin allowed (t-s)

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14
Q

When doesn’t Kashas rule work

A

When relaxation to lowest excited state is slow - particularly in transition metals where there is a manifold of low lying excited states of different origin which do no necessarily interact with each other

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15
Q

What is ISC

A

Species evolve into a different electronic state without gaining or losing every

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16
Q

What must be involved for efficient ISC?

A

Change or configuration as states as spin perturbation is forbidden between states with same configuration. Needs single character to come from a different state

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17
Q

How to Efficient ISC

A

Similar geometry and small energy gap between two states

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18
Q

What is Franck condon principle is E vs r coordinate graphs

A

Light absorption is a vertical process

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19
Q

Is the absorbance energies greater or less than the emission energies

A

Greater than - stoke shifts

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20
Q

When is the emission and absorbance spectra Mirror images

A

When the two states are similar in Geometry

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21
Q

Describe the properties of an ideal flurometer

A
  1. Light source is wl independent
  2. monochromator efficiency is wl and polarisation independent
  3. Detector sensitivity is wl independent
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22
Q

How does quantum yield and lifetime depend on k

A

Draw equations

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23
Q

How to measure quantum yield

A

Absolute and relative method

In the latter divide qy of sample by QY of standard

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24
Q

What is emission quenching

A

Any process that reduces the emission intensity

25
Q

Draw the diagram for the Forster mechanism of emission quenching and how does it work

A

Interaction or D* and A though overlap of electric fields in space

26
Q

Describe and explain the dexter mechanism for emission quenching

A

Interaction made through Overlap of orbitals

27
Q

Describe some of the properties in resonance energy transwfer

A

A and D coupled by dipole dipole not

A and D absorb and emit same photon so no actual emission

28
Q

How is the extent of energy transfer determined?

A

Equation

29
Q

What are the advantages of lanthanides that make them good emission tags

A

Sharp intense emission Bands
Energy not affected by environment
Long lived processes as most forbidden
Do not interact with Ligands so little quenching

30
Q

Describes the process in light Harvesting antennas and draw and energy level diagram

A
  1. Antenna absorbs light, excited
  2. ISC to triplet excited state
  3. Energy transfer to lanthanides, populate excited state
  4. Emits and lanthanides goes to ground state
31
Q

Why is 4 level lasers does stimulated emission occur over two excited states

A

Easier to get population inversion

32
Q

what are the types of Time resolved spectroscopy?

A

UV/VIS/NIR probe - electron spectra of intermediates and excited states
IR probe - TRIR - IR spectra of excited states and intermediates
UV/VIS pump and probe - Raman spectroscopy

33
Q

what does TRIR spec provide

A

Bond specific structural information, the weaker the bond the lower the frequency
Excited state dynamics
The shift in vibrational frequency upon excitation related to excited state nature

34
Q

what is needed in TRIR?

A

Good IR reporter - a group with high extinction coefficient and good IR absorption

35
Q

what are vibrationally hot electronic states?

A

excited states with vibrational levels greater than zero are populated

36
Q

How are they populated?

A

FORMATION:
Absorption of light is very fast – if a laser pulse
is ca. ~120 fs – vibrational distribution
Very fast decay (internal conversion for
example)
deposits large amounts of energy in a very
short period of time
Ultrafast ISC – “hot” triplet states are formed

37
Q

how are hot states measured and what are the observations?

A

METHOD: Time-resolved infrared spectroscopy,
OBSERVATIONS: shift of vibrational spectrum with time to higher energy or narrowing of lines; two-exponential decay kinetics

38
Q

what is the importance of hot states

A

Many deactivation pathways include hot states

ISC or photochemistry often occur from an “unrelaxed”, hot statej

39
Q

what is the process of natural photosynthesis

A
  1. Solar light harvesting by pigments (P),
  2. Energy transfer to the reaction center,
  3. Charge separation using Donor (D) and Acceptor (A),
  4. Production of carbohydrates and oxygen.
40
Q

How are Hydrogen and oxygen formed in photosynthesis?

A

Multi electron redox process - 1.23 eV

41
Q

where does photosynthesis take place

A

occurs in CHLOROPLAST
Thylakoid membrane (Thylakoid from “sack”).
Membrane – where light-driven reactions take place.
Stroma – where synthesis of carbohydrates - “food” - takes place.

42
Q

what is The role of chlorophyll

A

Free chlorophyll in its excited state fluoresces, lifetime is several ns.
However, in chloroplasts it does not fluoresce. Instead, its excited state
donates an electron to a primary electron acceptor and initiates electron
transport chain.
e-Transfer from Chl to Acceptor is much faster than several ns.

43
Q

what are the two principle components

A
  1. Light-harvesting antenna system (LH):
    captures light and transfers its energy to the reaction centre.
  2. Reaction centre (RC): lightdriven
    steps of photosynthesis
44
Q

describe the process of light reactions?

A

Light absorption by antenna
2. Energy transfer to the Reaction Centre. In some RC, there is a “special pair” – a sandwich of chlorophylls which acts as “energy absorber”. P680* is formed –in
its so-called excited state, high energy state.
3. Electron transfer from P680* to the primary acceptor

45
Q

what are the key processes in charge separation?

A

Energy transfer
electron transfer
proton transfer

46
Q

In plants where do the two photosystems operate together

A

thylakoid membrane

47
Q

what are the two photo systems called

A

PSI and PSII

PSII reacts first

48
Q

what is the reaction center of each PS

A

chlorophyll a molecule associated with a particular protein, and a primary electron acceptor

49
Q

True or false - The local surrounding (structure of the protein) affects the absorption properties
of chlorophyll a by slightly changing the energies of its ground and excited states.

A

True

50
Q

chlorophyll a absorbs light in PSI and PSII at what wavelengths

A

PSII - 680 nm

PSI - 700 nm

51
Q

what are the key processes in Artificial photsynthesis?

A
  1. solar light harvesting by molecular antennas,
  2. energy transfer to a “reaction center”,
  3. charge separation,
  4. water splitting, or other reaction
52
Q

what are the challenges in artificial photsynthesis?

A

Surviving intense radiation and big temperature variations
Converting lab scale to commercial scale
Photochemical:
Each process requires multiple electrons that must be stored and used at the right time.
The energies of the electrons and holes must be right
Many compounds are coloured, few are photochemically active and even fewer do the right reactions
Materials:
Many potential materials, at present few split water to H
2 and O2 on irradiation with visible light

53
Q

for CO2 reduction and H2 production A PHOTOCATALYTIC system can be

A

homogeneous (everything in solution) or
heterogeneous (molecular catalyst attached to a surface) or
A hybrid material (inorganic/organic/nano/surface…

54
Q

what is a photoelectrical cell?

A

electrocatyst immobolised on the light-absorbing electrode

55
Q

what are the benefits of a photo electrical cell

A
  • Reduces the required potential
  • Less corrosion
  • Less competition with H2 production
  • Product separation
56
Q

what are the challenges with water splitting

A

Cheap robust materials in water
Catalysts for the redox reactions at each electrode
Nanoscale architecture for electron transfer

57
Q

what are the key features of photoelectrochemical cell

A
  1. The potential required to drive the reaction is provided by light rather than by a battery or
    some other applied energy source.
  2. Reduction and oxidation half-reactions occur at the electrodes; the cathode and anode compartments are separated by a membrane.
  3. The half-reactions are connected by electron transfer through an external circuit with the ion flow between cell compartments to maintain charge neutrality.
58
Q

The electrode material or a photoelectrochemical cell should (apart from, obviously, being conductive):

A

be resistant to corrosion (water, light, pH).

(ii) Have high-surface-area (nanoparticle thin films of doped SnO2, mesoporous materials, etc etc)
(iii) Be transparent if light is absorbed by a photosensitiser. If the electrode is also a light absorber, then it has to absorb visible light.

59
Q

Comment on the following observation: The position (energy) of the absorption maximum, and of the
emission maximum, depend on the polarity of the solvent (this effect is called “solvatochromism”).

A

If excited state formed is a polar state (i.e., charge-transfer state), its energy will depend on solvent
polarity. Therefore, the observed solvatochromism of the absorption and emission confirms/is consistent
with the excited state involved being of a charge-transfer character.

Chemistry (22 decks)

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