DNA methylation and demethylation are the key steps in epigenetics. Emerging studies have demonstrated that these two processes play crucial roles in mammalian development and pathogenesis. Epigenetic modified cytosine and its further oxidative products, including 5-methylcytosine, 5-hydroxymethylcytosine, 5-formylcytosine, and 5carboxylcytosine, are called the "new four bases of DNA". The appearance of such new epigenetic bases can complicate DNA photodamage and repairing mechanism because they could have drastically different excited-state dynamics compared to canonical DNA nucleobases. In this study, excited-state dynamics of three demethylated nucleosides in buffer solution at physiological pH were investigated by femtosecond to microsecond time-resolved spectroscopy. Distinct excited-state dynamics are found in these demethylated nucleosides. For 2′-deoxy-5-formylcytidine (5fdCyd), direct observation of ultrafast intersystem crossing to the triple state with a 69% quantum yield is presented. Meanwhile, the triplet-state energy of 5fdCyd can transfer to the ambient molecular oxygen and generate destructive singlet oxygen. On the other hand, no such observation is seen in 2′-deoxy-5-hydroxymethylcytidine (5hmdCyd) and 2′-deoxy-5-carboxycytidine (5cadCyd), and these two bases show ultrafast internal conversion similar to that in 5-methylcytidine and cytidine. These results indicate that 5fdCyd is an effective internal triplet photosensitizer in DNA, and it could act as a new hot spot in DNA photodamage.

Conventional wisdom posits that spin-triplet energy transfer (TET) is only operative over short distances because Dexter-type electronic coupling for TET rapidly decreases with increasing donor acceptor separation. While coherent mechanisms such as super-exchange can enhance the magnitude of electronic coupling, they are equally attenuated with distance. Here, we report endothermic charge-transfer-mediated TET as an alternative mechanism featuring shallow distance-dependence and experimentally demonstrated it using a linked nanocrystal-polyacene donor acceptor pair. Donor-acceptor electronic coupling is quantitatively controlled through wavefunction leakage out of the core/shell semiconductor nanocrystals, while the charge/energy transfer driving force is conserved. Attenuation of the TET rate as a function of shell thickness clearly follows the trend of hole probability density on nanocrystal surfaces rather than the product of electron and hole densities, consistent with endothermic hole-transfer-mediated TET. The shallow distance-dependence afforded by this mechanism enables efficient TET across distances well beyond the nominal range of Dexter or super-exchange paradigms.

It remains unclear whether the emission center of ligand-encapsulated metal nanoclusters (MNCs) is the surface ligands or the metal core. In this paper, we simultaneously observed metal-centered and ligand-centered emissions in Ag nanoclusters. The contributions of the surface ligands and the metal core were individually investigated to understand the nature of AgNC photoemission. A new ligand synergistic emission effect was observed. The amino correlated nπ* state provides a pivot to bridge the carboxyl correlated ππ* and nπ* states to enhance the charge transfer efficiency between different surface electronic states. Consequently, the photoluminescence quantum yields were significantly improved (∼1 to ∼10%). Transient absorption studies revealed that decreasing the pH could expand the potential energy curve and generate a conical intersection. This would facilitate the charge transfer and relaxation of excited electrons via a radiative pathway, thereby enhancing the emission intensity. These new insights into the photoemission mechanisms of MNCs should stimulate additional experimental and theoretical studies and could benefit the molecular-level design of luminescent MNCs for optoelectronics and other applications.

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Developing a novel tool capable of real-time monitoring and simultaneous quantitation of multiple molecules in mitochondria across the whole brain of freely moving animals is the key bottleneck for understanding the physiological and pathological roles that mitochondria play in the brain events. Here we built a Raman fiber photometry, and created a highly selective non-metallic Raman probe based on the triple-recognition strategies of chemical reaction, charge transfer, and characteristic fingerprint peaks, for tracking and simultaneous quantitation of mitochondrial O 2 *À , Ca 2 + and pH at the same location in six brain regions of free-moving animal upon hypoxia. It was found that mitochondrial O 2 *À , Ca 2 + and pH changed from superficial to deep brain regions upon hypoxia. It was discovered that hypoxia-induced mitochondrial O 2 *À burst was regulated by ASIC1a, leading to mitochondrial Ca 2 + overload and acidification. Furthermore, we found the overload of mitochondrial Ca 2 + was mostly attributed to the influx of extracellular Ca 2 + .