N‐Heterocyclic carbene adducts with main group elements (NHC=E) have aroused great interest and have been widely investigated in coordination chemistry. Among them, N‐heterocyclic carbene adducts with chalcogens (NHC=Ch) have been known for a long time. Their investigations mostly focused on synthesis, coordination chemistry and electrochemistry. Their photophysical properties still remain unexplored. In this work, the photophysical properties of mesoionic carbene adducts with sulfur and selenium have been investigated both in solution and solid state. These compounds showed blue fluorescence in dichloromethane. While in solid state, orange to red room‐temperature phosphorescence can be observed, and dual emission was found in mesoionic thiones. Furthermore, time‐dependent density functional theory (TD‐DFT) calculations were used to obtain insights into the luminescent mechanism.
The light-harvesting complex II of a green alga Bryopsis corticulans (B-LHCII) is
peculiar in that it contains siphonein and siphonaxathin as carotenoid
(Car). Since the S1 state of siphonein and siphonaxathin
lies substantially higher than the Qy state of chlorophyll a (Chl a), the Chl a(Qy)-to-Car(S1) excitation energy transfer is unfeasible.
To understand the photoprotective mechanism of algal photosynthesis,
we investigated the influence of temperature on the excitation dynamics
of B-LHCII in trimeric and aggregated forms. At room
temperature, the aggregated form showed a 10-fold decrease in fluorescence
intensity and lifetime than the trimeric form. Upon lowering the temperature,
the characteristic 680 nm fluorescence (F-680) of B-LHCII in both forms exhibited systematic intensity enhancement and
spectral narrowing; however, only the aggregated form showed a red
emission extending over 690–780 nm (F-RE) with pronounced blueshift,
lifetime prolongation, and intensity boost. The remarkable T-dependence of F-RE is ascribed to the Chl-Chl charge transfer
(CT) species involved directly in the aggregation-induced Chl deactivation.
The CT-quenching mechanism, which is considered to be crucial for B. corticulans photoprotection, draws strong support
from the positive correlation of the Chl deactivation rate with the
CT state population, as revealed by comparing the fluorescence dynamics
of B-LHCII with that of the plant LHCII.
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