Abstract
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.
Highlights
We focus on linear pi-conjugated systems with C2h symmetry, which allows us to label the electronic states of the system according to how their wavefunctions transform under the point group symmetry operations
The photoinduced absorption (PIA) associated with these states lie between 1.75 to 2 eV, and given that their lifetime is $200 fs, we do not consider them to be of significance for the results presented here.[66]
In monomeric b-carotene, it has been reported that excitation from the ground state with significant excess photon energy can result in a longer-lived PIA band, blue shifted from the S1-Sn transition,[114] and the same effect is evident in several other carotenoids.[115]
Summary
Conjugated polymers and oligomers are ubiquitous in biological systems, with nature deploying these flexible and chemically tunable systems for a wide variety of advanced optoelectronic functions.[1,2,3] For many photosynthetic organisms, they play a vital dual role as both light-harvesting antennae and photoprotective molecules that can remove deleterious excess excitations.[2,4,5] Synthetic molecular materials developed for organic electronics have transformed the transistor and light-emitting diode technology[6,7,8] and are becoming promising components for next-generation photovoltaic (PV) devices with the potential to overcome the Shockley–Queisser limit via singlet fission (SF).
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