Optical Projection and Spatial Separation of Spin-Entangled Triplet Pairs from the S₁ (2¹ A_g^–) State of Pi-Conjugated Systems

Highlights

• Triplet pairs can be separated from the S₁ state of linear polyenes

• Separation is achieved by optical stimulation of transitions

• Spatial separation of triplet pairs occurs via delocalized intermediates

• Molecular space and interchromophore coupling are key requirements for separation

Summary

The S₁ (2¹ 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 S₁ wavefunction, however, have remained scarce. Here, studying an archetypal polymer, polydiacetylene, and carotenoids, we experimentally confirm that S₁ (2¹ 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 S₁ 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 2¹ 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 2¹ A_g^– dynamics in singlet fission, photobiology, and molecular quantum technologies.