Lecture 5 Intermolecular electronic energy transfer Photochemistry Lecture 5 Intermolecular electronic energy transfer

Intermolecular Energy Transfer D* + A  D + A* Donor Acceptor E-E transfer – both D* and A* are electronically excited. Often referred to as “quenching” as it removes excess electronic energy of initially excited molecule.

Radiative Transfer D*  D + h h + A  A* Long range Radiative selection rules Overlap of absorption and emission spectra PabsA - probability of absorption of A FD() – spectral distribution of donor emission A() – molar absorption coefficient of acceptor  - path length of absorption

Overlap of absorption spectrum of A and emission spectrum of D

Non-radiative mechanism A + D*  [AD*]  [A*D]  A* + D Formation of collision complex Intramolecular energy transfer within complex – Apply Fermi’s Golden Rule H’ is perturbation due to intermolecular forces (Coulombic, long range – “Forster”) or electronic orbital overlap (exchange, short range – “Dexter”)

Energy Gap Law Collisional energy transfer most efficient when the minimum energy taken up as translation i.e., ED*-ED  EA*-EA This can be thought of arising from Franck Condon principle within collision complex

Long-range energy transfer Interaction between two dipoles A, D at a separation r. Insert H’ into Fermi’s Golden Rule Dependence on transition moments for A and D Thus transfer subject to electric dipole selection rules r D A

Long range energy transfer Overall energy transfer rate must be summed over all possible pairs of initial and final states of D and A* subject to energy conservation - Depends on overlap of absorption spectrum of A and emission spectrum of D

Long Range (Forster) energy transfer There will be a critical distance r0 at which the rate of energy transfer is equal to the rate of decay of fluorescence of D (Typically r0 = 20 – 50 Å) At this point kT = 1/D. At any other distance, Note fD D-1 is equal to the fluorescence rate constant for D.

Efficiency of energy transfer Define wT the rate of energy transfer, ET the efficiency of transfer relative to other processes w0 is the rate of competing processes (fluorescence, ISC etc) wT can be identified with the rate of energy transfer at the critical distance R0 (see above)

Short range energy transfer (Dexter) Exchange interaction; overlap of wavefunctions of A and D L is the sum of the van der Waals radii of donor and acceptor Occurs over separations  collision diameter Typically occurs via exciplex formation (see below)

Spin Correlation Resultant vector spin of collision partners must be conserved in collision complex and subsequently in products D(S1) + A(S0) both spins zero, thus resultant spin SDA=0 - can only form products of same spin D(T1) + A(S0) SD=1, SA=0, thus SDA=1 – must form singlet + triplet products D(T1) + A(T1) SDA = 2, 1, or 0 thus can form e.g., S + S, S + T, or T + T

Quenching by oxygen 3O2 + D(S1)  3{O2;D(S1)}  3O2 + D(T1) S=1 S=1 S=0,1,2 Oxygen (3g-) recognised as strong inducer of intersystem crossing. De-oxygenated solutions used where reaction from S1 state necessary.

Triplet sensitization Use intermolecular energy transfer to prepare molecules in triplet state e.g., benzophenone (T1) + naphthalene (S0) benzophenone (S0) + naphthalene (T1) Important in situations where S1 state undergoes slow ISC or reacts rapidly.

Triplet-triplet annihilation

P-type delayed Fluorescence Delayed fluorescence (after extinction of light source): Kinetic scheme After initial [S1] population lost

P-type delayed fluorescene

Dynamic versus static quenching Dynamic quenching: in solution energy transfer processes depend on D* and A coming into contact by diffusion – very fast processes may be diffusion limited. As quencher concentration increases, fluorescence decays more rapidly. Static quenching – in a rigid system, energy transfer is effectively immediately if a quenching molecule is within a certain distance of D*. Thus the initial fluorescence intensity is lower.

Dynamic vs static quenching- effect on fluorescence decay of increasing quencher concentration Static quenching – no change in lifetime but initial intensity lower Dynamic quenching – fluorescence decays more rapidly as [A]

Exciplex formation M* +M M + M Electronically excited state of the collision complex more strongly bound than ground state Fluorescence leads to ground state monomers M* +M M + M

Excimer formation Exchange interaction stabilizes M*M (cf helium dimer) Emission at longer wavelength than monomer fluorescence Time dependence of excimer fluorescence - builds up and decays on short time scale Exciplexes are mixed complexes of the above type M* + Q  (M*Q)

Pyrene excimer

Excimer Laser Population inversion between exciplex state and unpopulated unbound ground state