Splitting singlets to save the planet

A really nice talk at the I2CAM workshop on organic photovoltaics this week was the one given by Christopher Bardeen, Exciton Fission and Electronic Delocalisation in Organic Semiconductors.

Here, I will just explain some of the beautiful physics of how you can split a singlet exciton into two triplet excitons.

First, the group theory. The product of two triplet states can be written

Presumably, a similar identity holds for l=1 spherical harmonics.

How might this happen in a conjugated organic molecule?

The Figure below is taken from a seminal paper by Tavan and Schulten.

The singlet Ag- state shown above is "dark" and also plays a key role in photosynthetic systems, as discussed here.