E/Z Molecular Photoswitches Activated by Two-Photon Absorption: Comparison between Different Families

Abstract

Nonlinear optical techniques as two-photon absorption (TPA) have raised relevant interest within the last years due to the capability to excite chromophores with photons of wavelength equal to only half of the corresponding one-photon absorption energy. At the same time, its probability being proportional to the square of the light source intensity, it allows a better spatial control of the light-induced phenomenon. Although a consistent number of experimental studies focus on increasing the TPA cross section, very few of them are devoted to the study of photochemical phenomena induced by TPA. Here, we show a design strategy to find suitable E/Z photoswitches that can be activated by TPA. A theoretical approach is followed to predict the TPA cross sections related to different excited states of various photoswitches’ families, finally concluding that protonated Schiff-bases (retinal)-like photoswitches outperform compared to the others. The donor-acceptor substitution effect is therefore rationalized for the successful TPA activatable photoswitch, in order to maximize its properties, finally also forecasting a possible application in optogenetics. Some experimental measurements are also carried out to support our conclusions.

Taking up the quest for novel molecular solar thermal systems: Pros and cons of storing energy with cubane and cubadiene

“…Due to the diradical nature of electronic ground transition states and the multi-configurational nature of electronic excited states, all calculations were performed at the CASPT2//CASSCF level, that is, geometry optimization by complete active space self-consistent field (CASSCF) theory (Olsen, 2011), followed by energy correction on top of the CASSCF calculation through second-order perturbation theory (CASPT2) (Andersson et al, 1992). This strategy was proven to be successful for the quantitative description of several photoisomerization processes (Marazzi and Garcia-Iriepa, 2019) based on [2 + 2] photocycloaddition (Runčevski, 2014). The 6-31G (d,p) basis set was used including a "double-zeta" approach for the valence orbitals and polarization functions (d,p) to better describe formation and rupture of chemical bonds.…”

“…Much later, several MOST systems were proposed starting from the '80s, including different mechanisms, such as photocyclization of the dihydroazulene/ vinylheptafulvene system (Cacciarini, 2015;Wang, 2017;Mogensen, 2019;Nielsen, 2020), cis/trans photoisomerization of stilbene and azobenzene derivatives (Masutani et al, 2014;Dong, 2018;Hu, 2019;Sun et al, 2019;Wang, Losantos, et al, 2019;Zhang et al, 2022), photoisomerization of bimetallic complexes (Zhang, 2013), and [2 + 2] photocycloaddition of the norbornadiene/ quadricyclane system Jevric et al, 2018). Some systems, although initially discarded for their poor properties, were recently taken back into account, in light of methodological developments concerning mechanochemistry applied to azobenzene (Marazzi and Garcia-Iriepa, 2019;Nucci et al, 2019), electrochemistry (Franz, 2022), and novel synthetic routes to derivatize the norbornadiene/quadricyclane system (Orrego-Hernández et al, 2020). This latter scientific advancement consolidated, in these last years, the norbornadiene/quadricyclane system as the most promising for MOST applications (Bren', 1991;Jorner, 2017;Elholm, 2022), mainly due to the considerably higher E stored compared to the rest of the proposed systems and, at the same time, its relatively low molecular weight, hence maximizing D storage (see the previous section for its definition).…”

Two-photon isomerization properties of donor–acceptor Stenhouse adducts

“…[11][12][13][14] Diverse molecular designs have recently been developed in order to combine the two-photon absorption properties of molecules with other aspects of their photophysics. [15][16][17] The interest in these photo-actuators has been increasing in recent years as reflected by the development of new molecular switches with significant two-photon absorption cross sections, either through electronic transitions of the actuator itself, [18][19][20] or with the use of a molecular antenna. [21][22][23] Although this is an ongoing research area, at present only a few molecular motifs have been characterized with respect to their two-photon isomerization properties.…”

“…The most frequent compound-class in this regard are azo molecules which undergo E/Z reactions after two-photon excitation. 19,20,24 The typical photoconversion yields in these molecules are of the order of 30%, and their two-photon cross sections can reach,in certain cases-a few hundred GM units. 19 Other examples of two-photon reactive molecules include caged systems, 8,13 Schiff bases, and azlactone compounds.…”

Theory, implementation, and disappointing results for two-photon absorption cross sections within the doubly electron-attached equation-of-motion coupled-cluster framework

“…One class of photoswitching behavior is based on the cistrans isomerization of conjugated chromophores, such as molecules with polyene backbone. It commonly occurs in biological systems, e.g., the initial step in vision is cis-totrans photoisomerization of retinal 8 . The switching process in such systems proceeds via open-shell structures with diradical character.…”