  The S1 (21Ag-) state is an optically dark state of natural and synthetic
pi-conjugated materials that can play a critical role in optoelectronic
processes such as, energy harvesting, photoprotection and singlet fission.
Despite this widespread importance, direct experimental characterisations of
the electronic structure of the S1 (21Ag-) wavefunction have remained scarce
and uncertain, although advanced theory predicts it to have a rich
multi-excitonic character. Here, studying an archetypal polymer,
polydiacetylene, and carotenoids, we experimentally demonstrate that S1 (21Ag-)
is a superposition state with strong contributions from spin-entangled pairs of
triplet excitons (1(TT)). We further show that optical manipulation of the S1
(21Ag-) wavefunction using triplet absorption transitions allows selective
projection of the 1(TT) component into a manifold of spatially separated
triplet-pairs with lifetimes enhanced by up to one order of magnitude and whose
yield is strongly dependent on the level of inter-chromophore coupling. Our
results provide a unified picture of 21Ag-states in pi-conjugated materials and
open new routes to exploit their dynamics in singlet fission, photobiology and
for the generation of entangled (spin-1) particles for molecular quantum
technologies.
