Lecture 12 - Given by Sarah Hudson PDF

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Given by Sarah Hudson...

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CH4003 – physical chemistry Sarah Hudson Photochemistry Many biological and chemical processes can be initiated by the absorption of EMR. Light is capable of ionisation, electron-transfer, dissociation, addition, abstraction and isomerisation. Photochemical reactions are initiated by the absorption of light. They are wavelength sensitive as a molecule can only be excited by absorbing photons with specific wavelengths. An excited molecule can react to give product or can simply lose its energy as heat (no chemical change.) The quantum yield The quantum yield is a measure of the effectiveness of photon absorption on a reaction. The primary quantum yield is the number of specific primary products (a radical, photon-excited molecule or ion) form by the absorption of each photon. The overall quantum yield is the total number of reactant molecules that react as a result of each photon absorbed. HI + hv  H + I H + HI  H2 + I 2I  I2 The primary quantum yield = 2 (one H, one I produced.) While the overall quantum yield = 2 (2 HI molecules react overall as a result of 1 photon.) Many chain reactions are initiated by photochemical reactions and the overall quantum yield of a chain reaction can be very large (104.) The quantum yield of a photochemical reaction depends on the wavelength of light applied. Common photophysical processes Primary absorption, fluorescence, internal conversion, intersystem crossing and phosphorescence are all common photophysical processes. S0 + hv  S1 (primary absorption)

S1  S0 + hv1 (fluorescence with a rate constant of k1) S1  S0 (internal conversion with a rate constant of kic) S1  T1 (intersystem crossing with a rate constant of kisc) T1  S0 (intersystem crossing with a rate constant of k’isc) T1  S0 + hv2 (phosphorescence with a rate constant of kp) These common photophysical processes compete with the formation of photochemical products from the S1 excited state. Therefore, it is important to consider the timescale of the formation and decay of S1 before describing the mechanisms of photochemical reactions. Kinetic schemes Electronic transitions caused by the absorption of UV or visible light occur in 10-16 to 10-15s – this is the formation of the excited state. Excited states decay through fluorescence in 10-12-10-6s so an excited singlet state can initiate very fast reactions that occur in 10-16-10-12s, anything that takes longer will not be initiated due to the excited state having fluoresced back down to the ground state. Intersystem crossing takes between 10-12-10-4s while phosphorescence takes place between 10-6-10-1s. Excited triplet states are very important photochemically as they can undergo several collisions before losing energy radiatively. Quantum Yields

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Singlet state decay in absence of chemical reactions S0 + hv  S1 (vabs = Iabs where Iabs is the rate of absorbance.) S1  So + hv1 (vf = kf[S1]) S1  So (vIC = kIC[S1]) S1  T1 (visc = kisc[S1]) As such, the rate of disappearance of S1 = Iabs – [S1] (kf+kic+kisc) We then apply the steady state approximation (let above equal 0 to remove the intermediate S1) Thus, [S1] = Ia/(kic + kf + kisc) We can then define the observed lifetime of the excited singlet state as to = 1/(kF + kISC + kIC)

φ F=

k F [ S1 ]

k = F k IC [ S 1 ]+ k F [ S 1 ]+k ISC [ S1 ] k IC +k F + k ISC

φ IC =

k IC k IC +k F +k ISC

φ ISC =

k ISC k IC +k F +k ISC

φ F +φ IC + φ ISC =1

The above phis are the quantum yield of fluorescence, internal conversion and intersystem crossing, respectively. Mechanism of triplet decay in the absence of chemical reactions S0 + hv  S1 S1  T1 (visc = kisc[S1]) T1  So (v’isc = k’isc[T1]) T1  S0 + hv (vP = kP[T1])

T1 is an intermediate state so we apply the steady state approximation and we see that [T1] = kisc*[S1] / (kisc + kp) If we sub into this the term for S1 above, we obtain: [T1] = (kisc*Ia) / (kisc + kp)*(kic + kf + kisc) φ P=

¿

rate of phosphorescence emission k P [ T 1 ] = rate of absorption of radiation Ia

k P k ISC '

(k ISC + k P )(k IC +k F + k ISC )

The observed lifetime of the excited triplet state: tp = 1/(kp+kisc) Excited state quenching A quencher molecule is a molecule that opens an additional method of deactivation of S1 or T1 by absorbing the energy difference. φ F,0 =1+τ 0 k Q [Q ] φF

Above is the Stern-Volmer equation, used to compare the quantum yield with a quencher and the quantum yield without a quencher. It can also be used for phosphorescence.

Fluoresence quantum yield in the absence of a quencher

Fluorescence quantum yield in the presence of a quencher Observed lifetime of the excited singlet state We can linearise the formula to enable graphing: 1 1 = +k [Q ] τ F τ0 Q I F,0 =1+τ 0 k Q [Q ] IF

These equations show that the fluorescence intensity or lifetime is directly proportional to fluorescence quantum yield. Application of photochemical reactions Isotope separation – photochemical reactions enable the separation of isotopes from a sample due to isotope species having different masses which result in different vibrational-rotational frequencies. Due to the difference in frequency, photons of different wavelengths are needed. e.g. light)

I35Cl + I37Cl  I35Cl + I37Cl* (only 37Cl atoms are excited by 608nm C6H5Br + I37Cl*  C6H537Cl + IBr

Photosensitatisation – one of reactant molecules may not be activated directly by the presence of a photon but may be activated by accepting energy from another excited molecule. This may be achieved by the absorption of a photon with an appropriate wavelength. Hg + H2  Hg* + H2 (Hg is excited by 254nm light, not H2) Hg* +H2  Hg + 2H*; Hg* + H2  HgH + H* H*  HCO  HCHO +H* 2HCO  HCHO + CO (formyl group formation – aldehyde)...