photochemistry

Question 1

describe the differences between photophysics and photochemistry?

Answer 1

chem- light induces chemistry - bond breaking and forming

physics - reversible changes. energy transition or emission of light

Question 2

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

Answer 2

Gs has an infinite lifetime but the ES does not

Question 3

what are the applications of photoinduced electron transfer?

Answer 3

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

Question 4

describe the Acceptor/ Donor complex?

Answer 4

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

Question 5

what does electron transfer depend on?

Answer 5

Solvent
free energy
distance
coupling/structure

Question 6

how does the solvent affect rate of electron transfer?

Answer 6

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

Question 7

what is wrong with libbys model of ET?

Answer 7

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

Question 8

what does marcus model propose?

Answer 8

That a weak electronic interaction required at the crossing point.

Question 9

what the assumptions for marcus` theory?

Answer 9

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

Question 10

what is the definition of solvent reorganisation?

Answer 10

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

Question 11

what are the two regions of marcus bell shaped curved?

Answer 11

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

Question 12

what are the laws of photochemistry?

Answer 12

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)

Question 13

draw a janlonski diagram and define all the terms

Answer 13

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)

Question 14

When doesn’t Kashas rule work

Answer 14

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

Question 15

What is ISC

Answer 15

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

Question 16

What must be involved for efficient ISC?

Answer 16

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

Question 17

How to Efficient ISC

Answer 17

Similar geometry and small energy gap between two states

Question 18

What is Franck condon principle is E vs r coordinate graphs

Answer 18

Light absorption is a vertical process

Question 19

Is the absorbance energies greater or less than the emission energies

Answer 19

Greater than - stoke shifts

Question 20

When is the emission and absorbance spectra Mirror images

Answer 20

When the two states are similar in Geometry

Question 21

Describe the properties of an ideal flurometer

Answer 21

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

Question 22

How does quantum yield and lifetime depend on k

Answer 22

Draw equations

Question 23

How to measure quantum yield

Answer 23

Absolute and relative method

In the latter divide qy of sample by QY of standard

Question 24

What is emission quenching

Answer 24

Any process that reduces the emission intensity

Question 25

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

Answer 25

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

Question 26

Describe and explain the dexter mechanism for emission quenching

Answer 26

Interaction made through Overlap of orbitals

Question 27

Describe some of the properties in resonance energy transwfer

Answer 27

A and D coupled by dipole dipole not | A and D absorb and emit same photon so no actual emission

Question 28

How is the extent of energy transfer determined?

Answer 28

Equation

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

Easier to get population inversion

Question 32

what are the types of Time resolved spectroscopy?

Answer 32

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

Question 33

what does TRIR spec provide

Answer 33

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

Question 34

what is needed in TRIR?

Answer 34

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

Question 35

what are vibrationally hot electronic states?

Answer 35

excited states with vibrational levels greater than zero are populated

Question 36

How are they populated?

Answer 36

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

Question 37

how are hot states measured and what are the observations?

Answer 37

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

Question 38

what is the importance of hot states

Answer 38

Many deactivation pathways include hot states | ISC or photochemistry often occur from an “unrelaxed”, hot statej

Question 39

what is the process of natural photosynthesis

Answer 39

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.

Question 40

How are Hydrogen and oxygen formed in photosynthesis?

Answer 40

Multi electron redox process - 1.23 eV

Question 41

where does photosynthesis take place

Answer 41

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

Question 42

what is The role of chlorophyll

Answer 42

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.

Question 43

what are the two principle components

Answer 43

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

Question 44

describe the process of light reactions?

Answer 44

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

Question 45

what are the key processes in charge separation?

Answer 45

Energy transfer electron transfer proton transfer

Question 46

In plants where do the two photosystems operate together

Answer 46

thylakoid membrane

Question 47

what are the two photo systems called

Answer 47

PSI and PSII | PSII reacts first

Question 48

what is the reaction center of each PS

Answer 48

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

Question 49

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.

Answer 49

True

Question 50

chlorophyll a absorbs light in PSI and PSII at what wavelengths

Answer 50

PSII - 680 nm | PSI - 700 nm

Question 51

what are the key processes in Artificial photsynthesis?

Answer 51

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

Question 52

what are the challenges in artificial photsynthesis?

Answer 52

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

Question 53

for CO2 reduction and H2 production A PHOTOCATALYTIC system can be

Answer 53

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

Question 54

what is a photoelectrical cell?

Answer 54

electrocatyst immobolised on the light-absorbing electrode

Question 55

what are the benefits of a photo electrical cell

Answer 55

* Reduces the required potential * Less corrosion * Less competition with H2 production * Product separation

Question 56

what are the challenges with water splitting

Answer 56

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

Question 57

what are the key features of photoelectrochemical cell

Answer 57

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.

Question 58

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

Answer 58

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.

Question 59

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”).

Answer 59

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.