A novel double-layer TiO2 nanorod array (NRA) gas sensor for room-temperature detection of NH3 was fabricated by employing etched fluorine-doped tin dioxide (FTO) glass as the in situ growing substrate and the new-type gas-sensing electrode via the facile droplet-coating and hydrothermal methods. Due to the synergistic effect of forces, special double-layer TiO2 NRAs with a cross-linked and bridgelike structure is formed, in which adequate point junctions can be generated to construct self-assembled electron pathways required for gas-sensing tests. Gas-sensing tests indicate that all samples obtained at different growth times have an excellent gas-sensing response to low-concentration NH3 at room temperature. Among them, the TiO2 NRAs obtained at 6 h (S2) exhibit the highest gas-sensing response to 100 ppm NH3 with a value of 102%. In addition, the growth mechanism, the gas reaction mechanism, and the effect of humidity on the gas-sensing performance are also discussed in the present paper.

The photochemistry of Criegee intermediates plays a significant role in atmospheric chemistry, but it is relatively less explored compared with their thermal reactions. Using multireference CASPT2 electronic structure calculations and CASSCF trajectory surface-hopping molecular dynamics, we have revealed a dark-state-involved AA → XA photoisomerization channel of the simple Criegee intermediate (CHOO) that leads to a cyclic dioxirane. The excited molecules on the AA state, which can have either originated from the BA state via BA → AA internal conversion or formed by state-selective electronic excitation, is driven by the out-of-plane motion toward a perpendicular A/XA minimal-energy crossing point (MECI) then radiationless decay to the ground state with an average time constant of ∼138 fs, finally forming dioxirane at ∼254 fs. The dynamics starting from the AA state show that the quantum yield of photoisomerization from the simple Criegee intermediate to dioxirane is 38%. The finding of the AA → XA photoisomerization channel is expected to broaden the reactivity profile and deepen the understanding of the photochemistry of Criegee intermediates.

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A combination of excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) has opened new opportunities to develop color-tunable luminescent materials with high quantum yield. Understanding the emission mechanism of these luminophores is essential for the molecular design and construction of a functional system. Herein, we report QM (MS-CASPT2// TD-DFT, MS-CASPT2//CASSCF) and ONIOM (QM/ MM) studies on the fluorescence quenching and AIE mechanisms of 2-(2-hydroxy-phenyl)-4(3H)-quinazolinone with typical characteristics of AIE and ESIPT as an example. The computational results indicate that in the tetrahydrofuran solution, once being excited to the S 1 state, the molecule tends to undergo an ultrafast, barrierless ESIPT from enol to keto tautomer and then accesses a S 1 /S 0 conical intersection in the vicinity of a CC bond twisted intramolecular charge-transfer (TICT) intermediate, leading to a nonradiative decay from the excited to ground state. Hence, the TICT-induced nonadiabatic transition, which has been further confirmed by the on-the-fly trajectory surface hopping dynamics simulations, accounts for the fluorescence quenching in solution. In contrast, in the solid state, the nonradiative relaxation pathway via the CC bond rotation is suppressed due to environmental hindrance, leaving the ESIPT-induced enol−keto tautomerization as the only excited-decay channel, thus the fluorescence is observably enhanced in the crystal.

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