Organic upconversion is a unique process in which low-energy light (usually NIR light) is converted to high-energy light through either the two-photon absorption (TPA) mechanism or the triplet-triplet annihilation (TTA) mechanism. Both TPA upconversion (TPA-UC) and TTA upconversion (TTA-UC) have been actively investigated in recent years due to their intriguing applications in optics, biophotonics, and solar energy utilization. Although they show some similarity (i.e., belonging to the nonlinear two-quantum process and needing focused excitation light), TPA-UC and TTA-UC are very different, such as in mechanism, characteristics involved, molecular design and potential applications. Here, we collectively reviewed these two kinds of upconversion processes and compared their respective characteristics and merits. We also present recent advances made in the areas of TPA- and TTA-UC, the remaining challenges and opportunities, with particular emphasis on molecular engineering of these two active upconversion materials.

a b s t r a c tPollutant washoff from road-deposited sediment (RDS) is an increasing problem associated with the rapid urbanization of China that results in urban non-point source pollution. Here, we analyzed the RDS grain size distribution and its potential impact on heavy metal pollution in urban runoff from impervious surfaces of urban villages, colleges and residences, and main traffic roads in the Haidian District, Beijing, China. RDS with smaller grain size had a higher metal concentration. Specifically, particles with the smallest grain size (<44 m) had the highest metal concentration in most areas (unit: mg/kg): Cd 0.28-1.31, Cr 57.9-154, Cu 68.1-142, Ni 25.8-78.0, Pb 73.1-222 and Zn 264-664. Particles with smaller grain size (<250 m) contributed more than 80% of the total metal loads in RDS washoff, while suspended solids with a grain size <44 m in runoff water accounted for greater than 70% of the metal mass in the total suspended solids (TSS). The heavy metal content in the TSS was 2.21-6.52% of that in the RDS. These findings will facilitate our understanding of the importance of RDS grain size distribution in heavy metal pollution caused by urban storm runoff.

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Two-photon absorption properties of a series of symmetrically substituted stilbenes and asymmetrically substituted stilbene-type derivatives with the same conjugated length have been investigated. The effective twophoton absorption cross sections, d TPA , as large as 62.0610 248 cm 4 s photon 21 for D-p-D molecules and 48.5610 248 cm 4 s photon 21 for D-p-A counterparts have been observed. The effect of these two types of chromophores on the peak position of the linear absorption, one-photon fluorescence as well as two-photon absorptivity is reported. Dipole moment change between the ground and the first excited states (Dm ge ), and the transition dipole moment between the first and second excited states (M ee' ) have also been calculated. It was found that the asymmetrically substituted derivatives possess relatively large Dm ge , whereas the symmetrical counterparts show an increase in M ee' . Although a large two-photon absorption resonance is due to the simultaneously high values of M ee' and Dm ge , correlated to intramolecular charge transfer, the former function is larger. These results obtained have demonstrated that the magnitude and the peak position of two-photon absorption depend not only on the amount but also on the direction of the intramolecular charge transfer.

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[reaction: see text] A highly efficient catalytic borylation process with aryldiazonium ions was developed using a carbene-palladium catalyst formed in situ to give arylpinacolatoborane products. An X-ray structure for the N-heterocyclic carbene-palladium complex, used as the catalyst formed from bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium chloride, was obtained without added base.

The positive external heavy atom effect on upconversion efficiency (Φuc) of DPA (9,10-diphenylanthracene) emitter combined with palladium(II)tetraphenylporphyrins as triplet sensitizer was first found to significantly increase from 12.33% to 35.17% under excitation of low power density at 30 mW·cm–2 (λex = 532 nm). Dynamics data showed that the increasing phosphorescence lifetime (τp) and the phosphorescence quantum yield (Φp) of sensitizer, accompanied with the decreasing nonphosphorescence decay rate constant (k np), have contribution to the triplet–triplet energy transfer (TTT). Interestingly, the external heavy atom effect was found to be more efficient than the internal heavy atom effect on upconversion efficiency, which has provided a simple approach to increase low-powered upconversion efficiency without the difficulties that can arise by the modification of the sensitizer molecular structure in its inner parts. Moreover, this efficient green-to-blue upconversion has been demonstrated to have potential application for the hydrogen-generation from water under excitation of sun energy.