Optical Projection and Spatial Separation of Spin-Entangled Triplet Pairs from the S-1 (2(1) A(g)(-)) State of Pi-Conjugated Systems

The S-1 (2(1) A(g)(-)) state is an optically dark state of natural and synthetic pi-conjugated materials that plays a role in optoelectronic processes, such as energy harvesting, photoprotection, and singlet fission. Experimental characterizations of the Si wavefunction, however, have remained scarce. Here, studying an archetypal polymer, polydiacetylene, and carotenoids, we experimentally confirm that S-1 (2(1) A(g)(-)) is a superposition state with strong contributions from spin-entangled pairs of triplet excitons ((1)(TT)). We then show that optical manipulation of the Si 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. Our results provide a unified picture of 2(1) A(g)(-) states in pi-conjugated materials and provide a hitherto unexplored pathway to create near-free triplets. More generally, our findings open new routes to exploit 2(1) A(g)(-) dynamics in singlet fission, photobiology, and molecular quantum technologies.