What is the third state?

A very important question in French history was: Qu'est-ce que le tiers état?

Yesterday I had some really nice discussions with Irene Burghardt in Paris. We have many common interests concerning quantum dynamics of excited states in condensed phases.

In many chemical processes we are interested in understanding the mechanism whereby one gets from state A to B. This is usually via a transition state C which is often believed to be something like a linear interpolation between A and B. However, it turns out that there are important processes whereby a third state is crucial for understanding the quantum dynamics of getting from A to B.

One example that Seth Olsen and I have worked on is Photoisomerisation of flourescent protein chromophores and methine dyes. This preprint, describes in detail the topology of possible potential energy surfaces for a three state model.

Another example that Irene and her collaborators have been working on is relevant to bulk heterojunction organic solar cells. She gave me a copy of this nice review of their work.

The key process is photoinduced charge transfer from a donor molecule D to an acceptor molecule A:

D A + photon -> D* A -> D+ A-

Electronic structure calculations have shown that at a TFB:F8BT polymer heterojunction there is a higher lying charge transfer state (IS for intermediate state) which couples strongly to the exciton state (XT) which is predominantly TFB*.
XT eventually decays to a charge transfer state CT (the "exciplex") which is predominantly TFB+:F8BT-.

Here are the potential energy surfaces they have constructed. Note that the conical intersection between the CT and XT states is at a much higher energy than the Franck-Condon point. The figure is from a recent PRL.

There is some experimental evidence supporting this claim of the role of a third state.

A few questions I have are:

What is the nature of the IS?
It is a dark state. Is it just, TFB-:F8BT+, whereas CT= TFB+:F8BT-?
Or is this third state a particle-hole state of TFB which has the same parity as the ground state and so is dark.

Can we derive this effective three-state Hamiltonian from a four orbital, four electron model [i.e, just one HOMO and one LUMO on each of TFB and F8BT?
Irene says no, one needs to take into account the delocalised nature of the excited states along the polymer chains.