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By creating two triplet excitons from one photo-excited singlet exciton, singlet fission in organic semiconductors has drawn tremendous attention for its potential applications in boosting the efficiency of solar conversion. Here, we show that this carrier-multiplication effect can also be used to dramatically improve the nonlinear optical response in organic materials. We have observed large optical nonlinearity with a magnitude of χ((3)) up to 10(-9) esu in pentacene films, which is further shown to be a result of singlet fission by monitoring the temporal dynamics. The potential application of such efficient nonlinear optical response has been demonstrated with a singlet-fission-induced polarization rotation.

Abstract:We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

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Nitrogen fixation (N 2 fixation) in benthic sediment contributes a certain amount of bioavailable nitrogen (N) to aquatic systems. However, data on sediment N 2 fixation have been poorly documented and the contribution of sediment N 2 fixation to the N budget in lakes has rarely been studied. In this study, N 2 fixation rates in sediments of Lake Taihu were measured periodically using the traditional acetylene reduction assay. Results showed that the highest sediment N 2 fixation rate occurred in winter in Meiliang Bay, with a value of 258 nmol N·kg À1 ·hr À1 , and the lowest rates occurred in all seasons at some sites, with values near zero. East Taihu Bay, Meiliang Bay, the southwestern coastal area, and Zhushan Bay were found to be hotspots for sediment N 2 fixation activity. The annual N 2 fixation rate and volume in sediments of Lake Taihu were 0.083 g N·m À2 ·year À1 and 195 t N, respectively. The N inputs via fixation were much lower than those from river inflow, sediment release, and atmospheric deposition. This study reveals that N 2 fixation in sediments contributes only a minor part to the overall N budget, while it compensates for about 1.8% of sediment N 2 loss via denitrification. The minor contribution of sediment N 2 fixation to the overall N budget in Lake Taihu may be closely linked to sediment characteristics.