photochemistry Flashcards
describe the differences between photophysics and photochemistry?
chem- light induces chemistry - bond breaking and forming
physics - reversible changes. energy transition or emission of light
what is the difference between the ground state and excited state if they both exhibit the same chemistry?
Gs has an infinite lifetime but the ES does not
what are the applications of photoinduced electron transfer?
Radiation damage of DNA- determine conductivity
Analytical chemistry - emission sensing
information storage and optical memory - photochromic or photoswitchable optics
describe the Acceptor/ Donor complex?
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
what does electron transfer depend on?
Solvent
free energy
distance
coupling/structure
how does the solvent affect rate of electron transfer?
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
what is wrong with libbys model of ET?
Assumes configurations are in equilibrium at when curve is at the bottom. Huge energy input required which is unrealistic for thermal rxns
what does marcus model propose?
That a weak electronic interaction required at the crossing point.
what the assumptions for marcus` theory?
- First coordination sphere not affected by ET - Nuclear coordinates do not change
- Formation of DA from D and A is negligible
- Weak interactions
- reason for ΔG = > 0 and its orientational polarization. Linear response assumed
what is the definition of solvent reorganisation?
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
what are the two regions of marcus bell shaped curved?
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
what are the laws of photochemistry?
- only absorbed light causes photochemical reactions to occur
- One photon will activate one molecule
- photochemical reactions take place from lowest energy excited state, regardless of which one was populated first. implies fast decay times (kashas rule)
draw a janlonski diagram and define all the terms
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)
When doesn’t Kashas rule work
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
What is ISC
Species evolve into a different electronic state without gaining or losing every
What must be involved for efficient ISC?
Change or configuration as states as spin perturbation is forbidden between states with same configuration. Needs single character to come from a different state
How to Efficient ISC
Similar geometry and small energy gap between two states
What is Franck condon principle is E vs r coordinate graphs
Light absorption is a vertical process
Is the absorbance energies greater or less than the emission energies
Greater than - stoke shifts
When is the emission and absorbance spectra Mirror images
When the two states are similar in Geometry
Describe the properties of an ideal flurometer
- Light source is wl independent
- monochromator efficiency is wl and polarisation independent
- Detector sensitivity is wl independent
How does quantum yield and lifetime depend on k
Draw equations
How to measure quantum yield
Absolute and relative method
In the latter divide qy of sample by QY of standard
What is emission quenching
Any process that reduces the emission intensity
Draw the diagram for the Forster mechanism of emission quenching and how does it work
Interaction or D* and A though overlap of electric fields in space
Describe and explain the dexter mechanism for emission quenching
Interaction made through Overlap of orbitals
Describe some of the properties in resonance energy transwfer
A and D coupled by dipole dipole not
A and D absorb and emit same photon so no actual emission
How is the extent of energy transfer determined?
Equation
What are the advantages of lanthanides that make them good emission tags
Sharp intense emission Bands
Energy not affected by environment
Long lived processes as most forbidden
Do not interact with Ligands so little quenching
Describes the process in light Harvesting antennas and draw and energy level diagram
- Antenna absorbs light, excited
- ISC to triplet excited state
- Energy transfer to lanthanides, populate excited state
- Emits and lanthanides goes to ground state
Why is 4 level lasers does stimulated emission occur over two excited states
Easier to get population inversion
what are the types of Time resolved spectroscopy?
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
what does TRIR spec provide
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
what is needed in TRIR?
Good IR reporter - a group with high extinction coefficient and good IR absorption
what are vibrationally hot electronic states?
excited states with vibrational levels greater than zero are populated
How are they populated?
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
how are hot states measured and what are the observations?
METHOD: Time-resolved infrared spectroscopy,
OBSERVATIONS: shift of vibrational spectrum with time to higher energy or narrowing of lines; two-exponential decay kinetics
what is the importance of hot states
Many deactivation pathways include hot states
ISC or photochemistry often occur from an “unrelaxed”, hot statej
what is the process of natural photosynthesis
- Solar light harvesting by pigments (P),
- Energy transfer to the reaction center,
- Charge separation using Donor (D) and Acceptor (A),
- Production of carbohydrates and oxygen.
How are Hydrogen and oxygen formed in photosynthesis?
Multi electron redox process - 1.23 eV
where does photosynthesis take place
occurs in CHLOROPLAST
Thylakoid membrane (Thylakoid from “sack”).
Membrane – where light-driven reactions take place.
Stroma – where synthesis of carbohydrates - “food” - takes place.
what is The role of chlorophyll
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.
what are the two principle components
- Light-harvesting antenna system (LH):
captures light and transfers its energy to the reaction centre. - Reaction centre (RC): lightdriven
steps of photosynthesis
describe the process of light reactions?
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
what are the key processes in charge separation?
Energy transfer
electron transfer
proton transfer
In plants where do the two photosystems operate together
thylakoid membrane
what are the two photo systems called
PSI and PSII
PSII reacts first
what is the reaction center of each PS
chlorophyll a molecule associated with a particular protein, and a primary electron acceptor
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.
True
chlorophyll a absorbs light in PSI and PSII at what wavelengths
PSII - 680 nm
PSI - 700 nm
what are the key processes in Artificial photsynthesis?
- solar light harvesting by molecular antennas,
- energy transfer to a “reaction center”,
- charge separation,
- water splitting, or other reaction
what are the challenges in artificial photsynthesis?
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
for CO2 reduction and H2 production A PHOTOCATALYTIC system can be
homogeneous (everything in solution) or
heterogeneous (molecular catalyst attached to a surface) or
A hybrid material (inorganic/organic/nano/surface…
what is a photoelectrical cell?
electrocatyst immobolised on the light-absorbing electrode
what are the benefits of a photo electrical cell
- Reduces the required potential
- Less corrosion
- Less competition with H2 production
- Product separation
what are the challenges with water splitting
Cheap robust materials in water
Catalysts for the redox reactions at each electrode
Nanoscale architecture for electron transfer
what are the key features of photoelectrochemical cell
- The potential required to drive the reaction is provided by light rather than by a battery or
some other applied energy source. - Reduction and oxidation half-reactions occur at the electrodes; the cathode and anode compartments are separated by a membrane.
- The half-reactions are connected by electron transfer through an external circuit with the ion flow between cell compartments to maintain charge neutrality.
The electrode material or a photoelectrochemical cell should (apart from, obviously, being conductive):
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.
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”).
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.
