Mechanochemical Tuning of Pyrene Absorption Spectrum Using Force Probes

An Algorithm Predicting the Optimal Mechanical Response of Electronic Energy Difference

The use of mechanical forces at the molecular level has been shown to be an interesting tool for modulating different chemical and physical molecular properties. The so-called covalent mechanochemistry deals with the application of precise mechanical forces that induce specific changes in the structure, stability, reactivity, and other physical properties. The use of this kind of force to modulate photophysical properties and photochemical reactivity has also been studied. Nevertheless, the general problem of mechanical modulation of the energy gap between two electronic states has been addressed only with the development of simple theoretical models. Here, we develop and implement an algorithm providing the Largest energy Gap variation with Minimal mechanical Force (LGMF) that allows the determination of the optimal mechanical forces tuning the electronic energy gap, as well as to identify the maximum mechanical response of a molecular system to the application of any mechanical stimulus. The algorithm has been implemented for diverse molecular systems showing different degrees of flexibility. The phyton code of the algorithm is available in a public repository.

Mechanical Activation of Forbidden Photoreactivity in Oxa-di-π-methane Rearrangement

In this work, we demonstrate that the forbidden oxirane-type photoproduct (the cyclopropyl ketone photoproduct is the allowed one) of the oxa-di-π-methane photorearrangement can be obtained by mechanochemical control of the photoreactions. This control is achieved by the application of simple force pairs rationally chosen. By analyzing in detail the effect of the applied forces on this photoreaction, it comes to light that the mechanical action affects the diverse properties of the oxa-di-π-methane rearrangement, modifying all the steps of the reaction: (i) the initial ground-state conformers’ distribution becomes affected; (ii) the new conformational population makes the triplet excitation process to be changed, responding to the magnitude of the applied force; (iii) the stability of the different intermediates along the triplet pathway also becomes affected, changing the dynamical behavior of the system and the reaction kinetics; and (iv) the intersystem crossing also becomes strongly affected, making the forbidden oxirane-type photoproduct to decay more efficiently to the ground state. All these changes provide a complex scenario where a detailed study of the effect of applied forces is necessary in order to predict its overall effect on the photoreactivity.

The concept of substituent-induced force in the rationale of substituent effect

Controlling the thermochemistry and kinetics of chemical reactions is a central problem in chemistry. Among factors permitting this control, the substituent effect constitutes a remarkable example. Here, we develop a model accounting for the effect of a substituent on the potential energy surface of the substrate (i.e., substituted molecule). We show that substituents affect the substrate by exerting forces on the nuclei. These substituent-induced forces are able to develop a work when the molecule follows a given reaction path. By applying a simple mechanical model, it becomes possible to quantify this work, which corresponds to the energy variation due to the effect of the substituent along a specific pathway. Our model accounts for the Hammett equation as a particular case, providing the first non-empirical scale for the σ and ρ constants, which, in the developed model, are related to the forces exerted by the substituents (σ) and the reaction path length (ρ), giving their product (σ · ρ) the well-known variation on the reaction energy due to the substituent.

Effect of ring rize on photoisomerization properties of stiff stilbene macrocycles

A series of stiff stilbene macrocycles have been studied to investigate the possible impact of the macrocycle ring size on their photodynamic properties. The results show that reducing the ring size counteracts the photoisomerization ability of the macrocycles. However, even the smallest macrocycle studied (stiff stilbene subunits linked by a six carbon chain) showed some degree of isomerization when irradiated. DFT calculations of the energy differences between the E- and Z-isomers show the same trend as the experimental results. Interestingly the DFT study highlights that the energy difference between the E- and Z-isomers of even the largest macrocycle (linked by a twelve carbon chain) is significantly higher than that of the stiff stilbene unit itself. In general, it is indicated that addition of even a flexible chain to the stiff stilbene unit may significantly affect its photochemical properties and increase the photostability of the resulting macrocycle.

Photoreactivity Control Mediated by Molecular Force Probes in Stilbene

We report theoretical and experimental evidence showing that photochemical reactivity of a chromophore can be modified by applying mechanical forces via molecular force probes. This mechanical action permits us to modulate main photochemical properties, such as fluorescence yield, excited-state lifetime, or photoisomerization quantum yield. The effect of molecular force probes can be rationalized in terms of simple mechanochemical models, establishing a qualitative framework for understanding the mechanical control of photoreactivity in stilbenes.

Mechanical Switching of Aromaticity and Homoaromaticity in Molecular Optical Force Sensors for Polymers

The sensing of mechanical stress in polymers is indispensable for investigating the origin and propagation of cracks that lead to material failure and for designing mechanically responsive polymers. Here the unique approaches of using the force-induced switching of aromaticity and homoaromaticity in molecular optical force sensors for the real-time measurement of mechanical forces acting in stretched polymers are suggested. The mechanical switching of aromaticity in Dewar benzene is an irreversible event, whereas the degree of π-orbital overlap in homoaromatic compounds like homotropylium can be adjusted progressively over a wide range of forces. Using computational methods, it is demonstrated that both approaches lead to significant changes in the visible part of the UV/Vis spectra of the force sensors upon application of weak forces (pN-nN). Polymers that incorporate such molecular force sensors therefore change their color well before material failure occurs.