Ultrafast Vibrational Cooling Inside of a Molecular Container

do Casal M, Pinheiro M, Toldo JM, Barbatti M. Modeling the heating and cooling of a chromophore after photoexcitation

The heating of a chromophore due to internal conversion and its cooling down due to energy dissipation to the solvent are crucial phenomena to characterize molecular photoprocesses. In this work, we simulated the ab initio nonadiabatic dynamics of cytosine, a prototypical chromophore undergoing ultrafast internal conversion, in three solvents—argon matrix, benzene, and water—spanning an extensive range of interactions. We implemented an analytical energy-transfer model to analyze these data and extract heating and cooling times. The model accounts for nonadiabatic effects, and excited- and ground-state energy transfer, and can analyze data from any dataset containing kinetic energy as a function of time. Cytosine heats up in the subpicosecond scale and cools down within 25, 4, and 1.3 ps in argon, benzene, and water, respectively. The time constants reveal that a significant fraction of the benzene and water heating occurs while cytosine is still electronically excited.

An analytical energy-transfer model is implemented to obtain a chromophore's heating and cooling times in a given solvent by using quantities available in nonadiabatic dynamics simulations.

Towards developing novel and sustainable molecular light-to-heat converters

Light-to-heat conversion materials generate great interest due to their widespread applications, notable exemplars being solar energy harvesting and photoprotection. Another more recently identified potential application for such materials is in molecular heaters for agriculture, whose function is to protect crops from extreme cold weather and extend both the growing season and the geographic areas capable of supporting growth, all of which could help reduce food security challenges. To address this demand, a new series of phenolic-based barbituric absorbers of ultraviolet (UV) radiation has been designed and synthesised in a sustainable manner. The photophysics of these molecules has been studied in solution using femtosecond transient electronic and vibrational absorption spectroscopies, allied with computational simulations and their potential toxicity assessed by in silico studies. Following photoexcitation to the lowest singlet excited state, these barbituric absorbers repopulate the electronic ground state with high fidelity on an ultrafast time scale (within a few picoseconds). The energy relaxation pathway includes a twisted intramolecular charge-transfer state as the system evolves out of the Franck–Condon region, internal conversion to the ground electronic state, and subsequent vibrational cooling. These barbituric absorbers display promising light-to-heat conversion capabilities, are predicted to be non-toxic, and demand further study within neighbouring application-based fields.

The synthesis and photophysical properties of phenolic barbiturics are reported. These molecules convert absorbed ultraviolet light to heat with high fidelity and may be suitable for inclusion in foliar sprays to boost crop protection and production.

How the water-soluble hemicarcerand incarcerates guests at room temperature decoded with modular simulations

Molecular dynamics simulations of hemicarcerands and related variants allow the study of constrictive binding and offer insight into the rules of molecular complexation, but are limited because three-dimensional models of hemicarcerands are tedious to build and their atomic charges are complicated to derive. There have been no molecular dynamics simulations of the reported water-soluble hemicarcerand (Octacid4) that explain how Octacid4 encapsulates guests at 298 K and keeps them encapsulated at 298 K in NMR experiments. Herein we report a modular approach to hemicarcerand simulations that simplifies the model building and charge derivation in a manner reminiscent of the approach to protein simulations with truncated amino acids as building blocks. We also report that in aqueous molecular dynamics simulations at 298 K apo Octacid4 adopts two clusters of conformations one of which has an equatorial portal open but the guest-bound Octacid4 adopts one cluster of conformations with all portals closed. These results explain how Octacid4 incarcerates guests at room temperature and suggest that the guest-induced host conformational change that impedes decomplexation is a previously unrecognized conformational characteristic that promotes strong molecular complexation.

Supramolecular Catalysis of m-Xylene Isomerization by Cucurbiturils: Transition State Stabilization, Vibrational Coupling, and Dynamic Binding Equilibrium

The ability of cucurbit[6]uril (CB6) and cucurbit[7]uril (CB7) to catalyze the thermally activated 1,2-methyl shift isomerization pathway of m-xylene in vacuum is investigated using infrequent metadynamics. CB6 is predicted to effectively and selectively catalyze the meta-to-para isomerization through stabilization of the transition state (TS) by van der Waals push (packing coefficient ≈74%), while inhibiting the meta-to-ortho pathway by molding effects of the cavity. Interestingly, despite the snug binding, a very low rate of host-guest vibrational energy transfer is revealed using a novel approach of host-guest partition of the mode-specific anharmonic relaxation rates and ab initio molecular dynamics. The weak vibrational coupling suggests that CB can act as a thermal buffer, possibly shielding encapsulated guests from outside vibrational perturbations such as solvent effects. This dynamic effect could provide an additional boost to the reaction rate by blocking the occurrence of reaction barrier recrossing caused by the friction with surrounding molecules. Finally, mean residence times of xylene into the hosts' cavity were estimated for a range of host-guest complexes, revealing a highly dynamic equilibrium allowing very high guest turnover rates that could minimize catalyst inhibition effects commonly suffered by other supramolecular catalysts.

Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0

Cyanines derived from heptamethines were mainly discussed regarding their functionalization to broaden the solubility in different surroundings exhibiting either hydrophilic or hydrophobic properties and to tailor made the ΔG et photopysical properties with respect to absorption and fluorescence. Electrochemical properties were additionally considered for some selected examples. The cyanines chosen comprised as end groups either indolenine, benzo[e]- or benzo[cd]indolium pattern, which facilitated to shift the absorption between 750-1000 nm. This enabled their use in applications with light sources emitting in the near-infrared (NIR) region selected from high power LEDs or lasers with line-shaped focus. The absorbers considered were discussed regarding their function as sensitizer for applications related to Chemistry 4.0 standards. These were mainly photopolymer coatings, which can be found for applications in the graphic industry or to protect selected substrates. The huge release of heat on demand upon turning ON or OFF the NIR light source enables them for photothermal treatment in processes requesting heat to initiate either chemical (activated reactions) or physical (melting, evaporation) events.

Computational NMR Spectra of o-Benzyne and Stable Guests and Their Hemicarceplexes

The incarceration of o-benzyne and 27 other guest molecules within hemicarcerand 1, as reported experimentally by Warmuth, and Cram and co-workers, has been studied by density functional theory (DFT). The ¹ H NMR chemical shifts, rotational mobility, and conformational preference of the guests within the supramolecular cage were determined, which showed intriguing correlations of the chemical shifts with structural parameters of the host-guest system. Furthermore, based on the computed chemical shifts reassignments of some NMR signals are proposed. This affects, in particular, the putative characterization of the volatile benzyne molecule inside a hemicarcerand, for which our CCSD(T) and KT2 results indicate that the experimentally observed signals are most likely not resulting from an isolated o-benzyne within the supramolecular host. Instead, it is shown that the guest reacted with an aromatic ring of the host, and this adduct is responsible for the experimentally observed signals.