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PHYS5530M Physics of Biological Systems > Chlorophyll for Efficient Solar Energy Capture > Flashcards

Chlorophyll for Efficient Solar Energy Capture Flashcards

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1
Q

What are the four phases of photosynthesis light reactions?

A

1) antennas and light collection
2) photochemistry (electronic energy converted to chemical energy via photons)
3) electron transfers stabilise chemical energy
4) storage of energy in new chemical compounds

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2
Q

Where is chlorophyll found?

A

-in high concentration in proteins in photosynthetic membranes which are folded to increase surface area

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3
Q

Atomic Bohr Model / Quantum Theory

A
  • single electron states at different energy levels

- only photons with exact energy of transition (e.g. E2-E1) can be absorbed or emitted

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4
Q

Molecular Bohr Model / Quantum Theory

A
  • electrons can interact with multiple nuclei which can rotate and vibrate
  • a single electron state is split into many sub-states so transitions are possible for a range of energies
  • the distribution of electrons across sub-states is governed by thermal equilibrium
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5
Q

Cholorophyll Electron Transitions

Absorption

A
  • at room temperature, most electrons are near the bottom of the ground state
  • most likely terminus after absorption is the centre of the excited state since the density of subtrates is greatest there
  • this leads to broadening of peaks in the chlorophyll specta
  • absorption is ultra-fast (fs)
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6
Q

Cholorophyll Electron Transitions

Fluoresence

A
  • higher vibrational states are unstable and decay to the lowest vibrational state in picoseconds (energy loss as heat)
  • this means that the most likely starting point for emission is the bottom of the exctied state
  • the higher electronic state in the lowest vibrational state is relatively stable, decays in ns
  • the most likely teminus for emission is the centre of the ground state
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7
Q

Cholorophyll Electron Transitions

Stokes Shift

A
  • since ΔEa > ΔEf, the peak of absorption is at higher frequency than the emission peak
  • the differenece in the location of the peaks is called Stokes Shift
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8
Q

Singlet Ground State, S0

A
  • both electrons in ground state
  • one spin up, one spin down
  • spins are paired, anti-parallel
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9
Q

Singlet Excited State, S1

A
  • one electron in ground state
  • one electron in excited state
  • one spin up, one spin down
  • spins are paired, anti-parallel
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10
Q

Triplet State, T1

A
  • with low probability (very slow) the S1 excited state can become a triplet state, T1
  • one electron in ground state, one electron in excited state
  • electron in excited state flips spins so that has the same spin as the ground state electron
  • this is allowed under the Pauli exclusion principle since the electrons have different energies
  • spins unpaired, parallel
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11
Q

Photoluminesence

Definition

A
  • emission of a photon
  • occurs by two mechanisms:
  • -fluorescence
  • -phosphoresence
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12
Q

Jablonski Diagram

Fluorescence

A
  • photon emission from singlet excited state
  • radiative
  • timescale: 10^(-10) - 10^(-7)s
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13
Q

Jablonski Diagram

Phosphoresence

A
  • photon emission form triplet excited state
  • radiative
  • timescale: 10^(-6)-10s
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14
Q

Photosynthesis and Triplet States

A
  • triplet states can be highly reactive
  • this can be damaging to biological systems since they’re so long lived
  • they must be dealt with (or prevented) during photosynthesis
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15
Q

Absorption Spectroscopy

Experimental Set-up

A
  • light source directed towards monochromator
  • monochromatic light emerges at incident intensity, Io
  • incident light passes through sample substance
  • dectector measures intensity of transmitted light, I
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16
Q

Absorption Spectroscopy

Absorbance Equation

A

A = log_10_(Io/I) = εCx
-where A is absorbance, C is the concentration of sample molcules, x is the thickness of the sample and ε is the molar absorption coefficient

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17
Q

Absorption Spectroscopy

Estimating Absorbing Materials Concentration

A

-since A∝C, A can provide a simple estimate for concentration

18
Q

Absorption Spectroscopy

Calculating the Molar Absorption Coefficient

A
  • generally, ε increases with the number of pi binds
  • for known concentration range, can plot A against C
  • will be a linear relationship, A(λ) ∝ ε(λ) for constant concentration
  • calculate ε form gradient
19
Q

Absorption Spectroscopy

Absorption Spectrum

A
  • plot of A against wavelength has important characteristics:
  • -number, position and shape of peaks
  • -wavelength of maximal absorption
  • -width of absorption peaks
  • the strength of a materials’ absorbancy is quanitified as a function of wavelength (or frequency)
20
Q

Estimating Effective Absorption Strength

A

-integrate ε(λ) over the wavelengths of absorption

21
Q

Chlorophylls Absorption

Wavelengths

A
  • absorption over relatively large wavelength range, 400-900nm, requires extensive pi-system of conjugated bonds meaning a fairly large molecule
  • when energy is absorbed it is distributed over the pi-system
  • X and Y transitions for S0->S1 & S0->S2 means up to 4 absorption bands for one molecule
22
Q

Chlorophylls Absorption

Energy Transfer Between Chlorophylls

A
  • the lowest excited singlet state is sufficiently long-lived (τ~5ns) to allow energy transfer to neighbouring chlorophyll molecule
  • theory and experiment show this τ allows energy propagation between assemblies of many chlorophylls over relatively large distances (10-100nm)
23
Q

How stable is the excited state of a particular fluorophore?

A

-to answer this consider how long an electron stays in the excited state before decaying
-the lifetime of the excited state is equal to the reciprocal of the combined decay rates, typically 10ps-10ns
τ = 1 / (kf + kπr)
-where kf is the rate of radiative decay and kπr is the rate of all non-radiative dissipation

24
Q

How bright is a particular fluorophore?

A

-consider reemission of photons vs. non-radiative dissipation
-fluorescence quantum yield is the rate of fluorescence over all other processes
-can be close to unity or <5%
φf = kf / (kf + kπr)
-where kf is the rate of radiative decay and kπr is the rate of all non-radiative dissipation

25
Q

What does amount of fluoresence depend on?

A
  • fluoresence depends on:
  • -absorption i.e. concentration, ε
  • -efficiency (or rate) of the competing decay processes
26
Q

Intensity of Fluoresence

A

If = φf * Ia = kf / (kf + kπr) * Ia
-where Ia is the intensity of absorbed light, φf is the fluoresence quantum yield, kf is the rate of radiative decay and kπr is the rate of all non-radiative dissipation

27
Q

Relative Fluoresence Intensity

A

If ~ Fr(λ) ∝ φf(λ)
If ~ Fr(λ) ∝ A(λ)
-most fluoresence spectrometers measure ‘relative’ fluoresence, Fr(λ), because it can be challenging to detect every photon in/out

28
Q

Steady State Fluoresence Spectroscopy

A
  • fluoresence measured at 90’ to light source so incident light isn’t picked up
  • typically use a collection time of 10-60sec for reasonable signal to noise ratio
  • fluoresence can be measured as a function of excitation wavelength with fixed emission position or function of emission wavelength with fixed excitation position
  • if we excite a particular wavelength (e.g. where absorption is known to be high) then record fluoresence intensity at a range of emission wavelengths (integrate the signal over a defined time)
29
Q

Measuring Fluorescence Quantum Yield

A

-with a special accessory added to the fluoresence spectrometer, absolute fluoresence quantum yield can be measured
-this is done by collecting every excitation vs emitted photon:
φf = kf / (kf + kπr) = no. of photons emitted / no. of photons absorbed

30
Q

Time-Resolved Fluoresence Spectroscopy

A
  • with an alternative instrument configuration we can measure fluoresence intensity as a function of time
  • to do this we need to measure at a ps-ns timescale
  • the measurement system needs a fast and well-defined light source which excites the sample coherently so the chromophore enters the excited state at a defined t=0
  • use a pulsed laser with 50-100ps duration pulse
  • uses timing electronics with a cumulative histogram graphical display
  • need a single-photon-counting detector with high detection quantum efficiency which can be quickly rest ready for another photon, dead time
31
Q

Fluoresence Decay Histogram

A
  • using time-resolved spectroscopy repeat the time measurement many times and count how many photons have arrived after what time
  • sort the photons into a histogram accordint to their arrival times
  • exponential fit of fluoresence decay to obtain fluorescence lifetime i.e. number of molecules in the excited state decreases at a rate proportional to the current number of electrons in the excited state
32
Q

Fluorescence Decay Curve

Equation

A 
N(t) = No * exp(-kt)
-where k is the rate constant
-OR
N(t) = No * exp(-t/τ)
-where τ=1/k is the mean lifetime, the mean time at which the population of the assembly is reduced to 1/e~37% of the initial value
33
Q

Fluorescence Decay Curve

Multi-exponential Fitting

A

F(t) = Σ Ai * exp(-t/τi)
-sum over i=1 to i=n
-e.g. a bi-exponential fit, if there are two decay processes
F(t) = A1exp(-t/τ1) + A2exp(-t/τ2) + z
-where τ1 is the lifetime of component 1, A1 is the amplitude of component 1 and equivalently for τ2 and A2
-and z is the fitting constant (detector background noise?)

34
Q

Fluorescence Excited State Lifetime and Natural Excited State Lifetime

A

-the natural excited state lifetime is the excited state lifetime in the absence of non-radiative dissipation (knr=0), not possible to measure directly
τ0 = 1/kf
-the fluoresence lifetime of the excited state is the resiprocal of the combined decay rate of the excited state:
τf = 1 / (kf + knr) = τo*φf

35
Q

What does relative fluorescence intensity depend on?

A
  • emission wavelength obsrved
  • excitation wavelength
  • samples fluorescence QY
  • sample’s concentration
  • the detector’s quantum efficiency
  • age of excitation lamp
36
Q

Why is fluorescence relavant to studying photosynthesis?

A
  • it helps us to understand the energy capture processes going on as part of ‘light harvesting’:
  • -energy absorbed / energy of excited state
  • -effectiveness of process (non-radiative decay ~wastage)
  • -to quanitfy energy transfer efficiency, indirectly from fluoresence
37
Q

Jablonski Diagram

Intersystem Crossing

A
  • transition between vibrational levels belonging to electronic states of differnent spin multiplicity, e.g. S1 to T1
  • non-radiative
  • timescale: 10^(-10) - 10^(-6)s
38
Q

Jablonski Diagram

Internal Conversion

A
  • an electron in a higher energy singlet state transfers to a lower lying singlet state, it is immediately followed vibrational relaxation to the lowest vibrational level of the new electronic state
  • non-radiative
  • timescale: 10^(-11) - 10^(-9)s
39
Q

Jablonski Diagram

Absoprtion

A
  • transition from lower to higher electronic state, energy of photon converted to internal energy
  • radiative
  • timescale: 10^(-15)s
40
Q

Jablonski Diagram

Vibrational Relaxation

A
  • transition to lower vibrational level within the same electronic state
  • non-radiative
  • timescale: 10^(-12) - 10^(-10)s
41
Q

Jablonski Diagram

Fluorescence

A
  • transition from the lowest vibrational level of one electronic state to a lower electronic state of the same spin multiplicity
  • radiative
  • timescale: 10^(-10) - 10^(-7)s

PHYS5530M Physics of Biological Systems (12 decks)

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