Deconstructing triplet-triplet annihilation

Tim Schmidt gave a nice chemistry seminar yesterday about recent work from his group aimed at improving the thermodynamic efficiency of photovoltaic cells using upconversion associated with triplet-triplet annihilation.
It has refreshing to hear a talk which focussed on trying to understand the underlying photophysics, rather than just device fabrication and efficiency. He even had some slides with Hamiltonians!

The underlying idea is illustrated in the figure below. Two sensitizer molecule (in this case porphyrins) absorbs "low energy" photons via a singlet state S1 which decays to a triplet T1 via intersystem crossing. These triplets then excite triplets on two neighbouring emitters (in this case rubrene). The two triplets then annihilate on a single rubrene to produce a high energy singlet which can then decay optically. These "high energy" photons are then absorbed by a tandem solar cell.

The photophysics which is not really that well understood concerns the dynamics and mechanism of triplet-triplet annihilation. It is the opposite of singlet fission [featured in this earlier post] which is discussed nicely in a recent review by Smith and Michl.

A fundamental and basic question concerning both singlet fission and triplet-triplet annihilation is
what is the relevant reaction co-ordinate?
e.g., is it a particular molecular vibrational co-ordinate?

A recent article by Zimmerman, Musgrave and Head-Gordon suggests the relevant co-ordinate is the relative motion of the two chromophores, and that there is a breakdown of the Born-Oppenheimer approximation associated with process.