The electrocatalytic activity of transition-metal-based compounds is strongly related to the spin states. However, the underlying relationship connecting spin to catalytic activity remains unclear. Herein, we carried out density functional theory calculations on oxygen reduction reaction (ORR) catalyzed by Fe single-atom supported on C2N (C2N–Fe) to shed light on this relationship. It is found that the change of electronic spin moments of Fe and O2 due to molecular-catalyst adsorption scales with the amount of electron transfer from Fe to O2, which promotes the catalytic activity of C2N–Fe for driving ORR. The nearly linear relationship between the catalytic activity and spin moment variation suggests electronic spin moment as a promising catalytic descriptor for Fe single-atom based catalysts. Following the revealed relationship, the ORR barrier on C2N–Fe was tuned to be as low as 0.10 eV through judicious manipulation of spin states. These findings thus provide important insights into the relationship between catalytic activity and spin, leading to new strategies for designing transition metal single-atom catalysts.

A new type of prototypical biphotochromic switches combining spirooxazine and fulgide has been synthesized and their nonlinear optical (NLO) responses have been theoretically studied based on density functional theory (DFT).Our DFT calculations show that the introduction of fulgide fragment (E and C) into spirooxazine host can largely enhance the static second and third-order NLO responses. Moreover, the photoisomerization of spirooxazine derivatives (SO-E and SO-C) brings forth a pronounced change in the geometry accompanied with the formation of larger π-conjugated system, and thus the corresponding merocyanine derivatives (PMC-E and PMC-C) obtain ~3.5 times' enhancement of static second and third-order NLO responses. Nevertheless, the difference in NLO responses between E-form series (SO-E and PMC-E) and C-form series (SO-C and PMC-C)respectively is not substantial, which means the fulgide fragment in the prototypical biphotochromic system does not considerably differentiate the corresponding optical nonlinearities. To well separate the NLO responses of four states in our prototype, we further propose a structural modification strategy by introducing the furan moiety (2-vinylfuran) into the π-bridge, and the corresponding NLO performances from DFT results finally meet our demands, indicating that such revised biphotochromic system based on our prototype is a fascinating choice for the architecture of multistate NLO switches.

Systematic comparisons among the oxazinone, quinazoline, and difluoroboron series on optical absorption and fluorescence emission properties have been made. Weaker electron donor–acceptor (D–A) pairs in both the oxazinone and quinazoline series bring about the slight red shifts of absorption spectra, whereas they significantly promote the fluorescence intensities of the oxazinone series but bathochromically shift the maximum emission wavelengths of the quinazoline series. Intrinsically, the charge-transfer (CT) modes govern the electron excitation/de-excitation processes in both the oxazinone and quinazoline series, but with respective different CT features, i.e., the interfragment CT mode for the oxazinone series versus the intrafragment CT mode for the quinazoline series. In the difluoroboron series, Oxa-Cl-OCH3-BF2 undergoes a change of transition mode from the local excitation to the CT de-excitation, whereas a large variation of CT compositions can be observed in Qui-Cl-OCH3-BF2. Experimentally, Oxa-Cl-OCH3-BF2 exhibits higher fluorescence quantum yield, favorable thermo- and photostability, stronger fluorescence intensity, and appropriately large Stokes shift, whereas theoretically, Qui-Cl-OCH3-BF2 benefiting from both the difluoroboron and quinazoline moieties is also promising to be a good fluorescent dye. The motif of combining oxazinone/quinazoline and difluoroboron moieties is believed to improve the prototypical architectures of oxazinone and quinazoline dyes.

Polarized charges on dual-reactive centers of C2N-supported single-atom based transition metal ion catalysts promoting HCOOH dehydrogenation.

As one of the promising photovoltaic technologies, high performance metal-free dye-sensitized solar cells (DSSCs) have been explored due to the fact that they can be potentially produced using low-cost materials, their color can be tuned and they exhibit reasonable stability.