Interchain Interactions

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This topic is an important consideration with light emitting polymer and oligomers. Photoluminescence is easily measured by shining light and measuring the emission.

In dilute solutions or inert matrices the photoluminescence quantum yield of a given conjugated polymers can be very large: up to 60-80 %

But in thin films the solvent goes away, the polymer chains get closer so they can interact and the luminescence can be nearly totally quenched.

There are a few situations where going from solution to solid states you can get increased luminescence. Silos for example can cause an aggregation in the solid state that triggers luminescent qualities. The observation of quenching led researcher to explore the cause and remedy for this.

Transition dipole in isolation

A transition dipole is directly related to the probability of a transition occuring from one excited state to the ground, or the ground to the excited state. It is also related to the intensity of the exciting energy.

If the wave function for the S1 state is localized in a different area of space than the ground state then transition dipole will be zero. There must be wave function overlap for the wavefunction equation to be non-zero.

In stilbene the transition dipole for the S0 ->S1 transition and the S1 ->S0 transition is polarized mainly along the long axis (x axis) of the molecule. This is generally the case in pi conjugated systems because as you extend the length of the chain electrons are delocalized over a longer distance, just as in an electrostatic dipole a larger distance of charge separation make for a larger dipole moment. An exception is pentacine whose first optical transition is polarized at 90 degrees from the long axis of the chain. In most other optical polymers such as polyvinylene and polythiophene the optical transition is polarized along the long axis.

Two chain interaction

When two chains come into close proximity they there is the possibility during excitation the electron will jump from one chain to another, leaving a hole and forming a charge transfer exciton. The probability of finding h+ and e- on separate chains in S1 can be obtained from the excited-state wavefunction .

S1 → S0 intensity can go down since the transition is polarized along x.

symmetrical interaction

Consider first the most symmetric configuration which is perfectly cofacial.

From a distance of infinity down to 8 Å there is no significant interaction. There is no significant wavefunction overlap between the units.� �

• excitation is always localized on a SINGLE UNIT�
• PL is not affected��

This is the situation in dilute solution or inert matrices

When the distance (R) drops below 8 Å , as they do in thin films, the wavefunctions of frontier orbitals (homo and lumo) begin to overlap and you get a new splitting of the homo level. Wavefunctions are delocalized over the two units. They become equally spread in the zone of r~5 Å.

exciton band

The HOMO and LUMO may be split to different degrees. In a highly symmetrical situation the transition from the ground state to the lowest excited state (S1) is forbidden. The HOMO (au) and the LUMO (bu) is are both ungerade which means the transition is forbidden (because odd time odd is even, times odd is odd and when this is integrated over all space the result is zero).

If you were able to stack five molecules on top of each other, you could have five exciting states corresponding the mixing of the five states corresponding to the five S1 states, this is known as an exciton band. Even if a higher excited state such as S2 is achieved, Kasha’s Rule dictates that energy will relax down to lower states and the emission can only occur from the lowest excited state. S1 can not transition to S0, it is a “dark state” .The lowest state of that exciton band will be dark and luminescence is quenched.

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