We report a rare atom-like interaction between excitons in monolayer WS, measured using ultrafast absorption spectroscopy. At increasing excitation density, the exciton resonance energy exhibits a pronounced redshift followed by an anomalous blueshift. Using both material-realistic computation and phenomenological modeling, we attribute this observation to plasma effects and an attraction-repulsion crossover of the exciton-exciton interaction that mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a strong analogy between excitons and atoms with respect to interparticle interaction, which holds promise to pursue the predicted liquid and crystalline phases of excitons in two-dimensional materials.

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A new methylene-linked flavan-3-ol dimer, bis(8-epicatechinyl)methane (1), was isolated from the pericarps of Litchi chinensis Sonn. (Sapindaceae), together with dehydrodiepicatechin A (2), proanthocyanidin A1 (3), proanthocyanidin A2 (4), (-)-epicatechin (5), 8-(2-pyrrolidinone-5-yl)-(-)-epicatechin (6), (-)-epicatechin 8-C-β-D-glucopyranoside (7), naringenin 7-O-(2,6-di-O-α-L-rhamnopyranosyl)-β-D-glucopyranoside (8), and rutin (9). It was the first report of compound 2 as a natural product and compounds 6-8 from this species. Compounds 1, 2, and 6-8 were evaluated for antioxidant activity. The ferric reducing antioxidant powers (FRAP) of compounds 1 and 6 were comparable to that of L-ascorbic acid, and the scavenging activities of compounds 1, 2, 6, and 7 toward 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) radical cations were more potent than those of L-ascorbic acid; compound 8 was weak in FRAP and DPPH assays.