Photophysical properties of the Triplet and the "bound" Triplet-Triplet state in Pentacene Dimers

Covalently linked dimers can serve as ideal prototypes for intramolecular singlet fission where
free triplets are said to be generated from a photoexcited spin singlet. However, controversies
abound as recent experiments claim to have detected the multi-excitonic ¹(TT)₁ state to be
the final photoproduct. Using the semi-empirical Pariser-Parr-Pople model, we report many-
electron calculations of the energies and wavefunctions of the optical singlets, the lowest
triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state
absorptions from these states in a family of phenyl-linked pentacene dimers. We find that
while both the triplet and the triplet-triplet absorb in the visible (500-600 nm), their transient
absorption spectra in the near infrared region are distinctly di erent. We conclude that the
end product of photoexcitation in these particular bipentacenes is the bound triplet-triplet
and not free triplets. We predict additional transient absorptions at even longer wavelengths,
beyond 1500 nm, to the equivalent of the classic 2¹A^-_g in linear polyenes.