Internal conversion from the photoexcited state to a correlated singlet triplet-pair state is believed to be the precursor of singlet fission in carotenoids. We present numerical simulations of this process using a π-electron model that fully accounts for electron–electron interactions and electron–nuclear coupling. The time-evolution of the electrons is determined rigorously using the time-dependent density matrix renormalization group method, while the nuclei are evolved via the Ehrenfest equations of motion. We apply this to zeaxanthin, a carotenoid chain with 18 fully conjugated carbon atoms. We show that the internal conversion of the primary photoexcited state, S 2 , to the singlet triplet-pair state occurs adiabatically via an avoided crossing within ∼50 fs with a yield of ∼60%. We further discuss whether this singlet triplet-pair state will undergo exothermic versus endothermic intra- or interchain singlet fission.
We describe our simulations of the excited state dynamics of the carotenoid neurosporene, following its photoexcitation into the "bright" (nominally 1 1 B u + ) state. To account for the experimental and theoretical uncertainty in the relative energetic ordering of the nominal 1 1 B u + and 2 1 A g − states at the Franck−Condon point, we consider two parameter sets. In both cases, there is ultrafast internal conversion from the "bright" state to a "dark" singlet triplet-pair state, i.e., to one member of the "2A g " family of states. For one parameter set, internal conversion from the 1 1 B u + to 2 1 A g − states occurs via the dark, intermediate 1 1 B u − state. In this case, there is a cross over of the 1 1 B u + and 1 1 B u − diabatic energies within 5 fs and an associated avoided crossing of the S 2 and S 3 adiabatic energies. After the adiabatic evolution of the S 2 state from predominately 1 1 B u + character to predominately 1 1 B u − character, there is a slower nonadiabatic transition from S 2 to S 1 , accompanied by an increase in the population of the 2 1 A g − state. For the other parameter set, the 2 1 A g − energy lies higher than the 1 1 B u + energy at the Franck−Condon point. In this case, there is cross over of the 2 1 A g − and 1 1 B u + energies and an avoided crossing of the S 1 and S 2 energies, as the S 1 state evolves adiabatically from being of 1 1 B u + character to 2 1 A g − character. We make a direct connection from our predictions to experimental observables by calculating the time-resolved excited state absorption. For the case of direct 1 1 B u + to 2 1 A g − internal conversion, we show that the dominant transition at ca. 2 eV, being close to but lower in energy than the T 1 to T 1 * transition, can be attributed to the 2 1 A g − component of S 1 . Moreover, we show that it is the charge-transfer exciton component of the 2 1 A g − state that is responsible for this transition (to a higher-lying exciton state), and not its triplet-pair component. These simulations are performed using the adaptive tDMRG method on the extended Hubbard model of π-conjugated electrons. The Ehrenfest equations of motion are used to simulate the coupled nuclei dynamics. We next discuss the microscopic mechanism of "bright" to "dark" state internal conversion and emphasize that this occurs via the exciton components of both states. Finally, we describe a mechanism relying on torsional relaxation whereby the strongly bound intrachain triplet-pairs of the "dark" state may undergo interchain exothermic dissociation.
A family of novel halogen bonding (XB) and hydrogen bonding (HB) heteroditopic [2]rotaxane host systems constructed by active metal template (AMT) methodology, were studied for their ability to cooperatively recognise lithium halide (LiX) ion-pairs. 1 H NMR ion-pair titration experiments in CD 3 CN:CDCl 3 solvent mixtures revealed a notable "switch-on" of halide anion binding in the presence of a cobound lithium cation, with rotaxane hosts demonstrating selectivity for LiBr over LiI. The strength of halide binding was shown to greatly increase with increasing number of halogen bond donors integrated into the interlocked cavity, where an all-XB rotaxane was found to be the most potent host for LiBr. DFT calculations corroborated these findings, determining the mode of LiX ion-pair binding. Notably, ion-pair binding was not observed with the corresponding XB/HB macrocycles alone, highlighting the cooperative, heteroditopic, rotaxane axle-macrocycle component mechanical bond effect as an efficient strategy for ion-pair recognition in general.
In this study, we present the investigation of the curcumin loading and release properties of four different Metal-Organic Frameworks (MOFs) with varying metal centres and organic ligands. Through our research, we have identified HKUST-1 and MIL-100, highly porous copper and iron-containing MOFs, that exhibit specific interactions with curcumin, leading to high encapsulation efficiencies (55%-75%) even at low concentrations as 6ppm. The binding modes of curcumin onto MOFs have been investigated using a combined experimental and computational approach. Furthermore, our drug-releasing studies have revealed slow and prolonged release for over two days, which further indicates the specific interactions of curcumin with HKUST-1 and MIL-100. To the best of our knowledge, this is the first comparative study that investigates the drug delivery properties of curcumin using Copper, Ferrous, and Zinc MOFs. Our findings pave the way for the development of stable, highly interactive MOFs as drug carriers for curcumin, which has the potential to overcome its poor aqueous solubility and rapid metabolism, and enhance its pharmacological activities in medicine.
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