Innovative Multiphotochromic Molecular Architectures for NO and 1O2 Release – Photochromics

This proposal gathers theoretical and experimental chemists. Original hybrid compounds will be synthesized using technics in organic synthesis and coordination chemistry. The properties of the different states involved in the multiphotochromic systems and the production / activation of 1O2 and NO• will be analyzed by spectroscopic, photophysical, photochemical and electrochemical studies. A major task will be the rationalization of the underlying mechanisms involved. We will use advanced computational methodologies beyond the current state-of-the-art, which will allow static and dynamic investigations of these systems in their environment.
Classical organic and inorganic methods will be used for the preparation of the compounds and several synthetic strategies will be tested. The properties of the new molecules will be evaluated using mainly spectroscopic methods (UV-vis and emission, NMR, EPR) and spectro-electrochemistry will also be very helpful to determine the main characteristics of their oxidized or reduced states. The luminescence properties will be measured and time-resolved spectroscopy will be done to follow transient species. The theoretical study will be carried out using sophisticated electronic structure methods for the static and nonadiabatic dynamic investigations of the compounds.

So far, we have studied the DHP and Ru-NO photochroms separately. As far as DHP are concerned, two distinct tasks have been performed. On the one hand, theoretical studies of the unsubstituted DHP model system have started. We described and characterized the various electronic excited states of this system, and we simulated the UV-vis absorption spectra of the photoisomers. On the other hand, studies combining experiments and theoretical calculations of substituted DHP have allowed us to design and characterize a photoswitchable system in the visible range thanks to the introduction of electro-withdrawing substituent and polypyridine Ru(II) complexes. These studies are still in progress on various DHP in order to improve the photoswitching properties and the production of 1O2. Notably, a DHP derivative with 4 pyridinium groups has been recently studied and proved disappointing regarding its photoswitchability. We have rationalized this behavior will be useful in the future design of new DHP.
Ru–NO complexes are also the subject of two distinct tasks. On the one hand, the prototype [Ru(py)4NOCl]2+ complex is studied theoretically in order to understand better the N?O linkage photoisomerization and NO photorelease mechanisms involved. A study of the initial photodynamics of this system has been carried out and identified three different excited-state relaxation pathways involving intersystem crossings and/or internal conversions. Moreover, Ru–NO complexes are investigated experimentally and, with the support of theoretical calculations, a rich photochemistry of a series of [Ru(R-tpy)(bpy)(NO)]3+ complexes has been revealed in the dimethylsulfoxyde (DMSO) solvent. Notably, the photosubstitution of the NO ligand by a DMSO solvent molecule followed by S?O linkage photoisomerization has been observed and studied as a function of the nature of the R substituent.

The study of DHP and Ru-NO complexes still need to be continued. In particular, the complete photoisomerization mechanism of DHP needs to be unraveled, which requires characterizing all the excited-state relaxation pathways and critical conical intersections involved in the DHP to CPD photoconversion. Depending on the results of the static study, we will possibly consider performing a nonadiabatic dynamics investigation. The study of new DHP with the aim of improving the photoconversion efficiency and capability to produce 1O2 will be pursued. The use of electron-withdrawing groups, annelated and push-pull systems will be investigated. Regarding Ru-NO complexes, the NO photorelease mechanism still need further studies. We would also like to investigate photochromic Ru-NO systems that can operate at room temperature. Environment effects still need to be accounted for. Finally, the main prospect is the conception and study of hybrid DHP-Ru-NO compounds. Both multiphotochromic compounds and systems capable of NO• and 1O2 release will be prepared.

Peer-reviewed articles :

1. M. Jacquet, L. M. Uriarte, F. Lafolet, M. Boggio-Pasqua, M. Sliwa, F. Loiseau, E. Saint-Aman, S. Cobo, G. Royal, All visible light switch based on the dimethyldihydropyrene photochromic core. J. Phys. Chem. Lett. 11, 2020, 2682–2688.
2. R. Sarkar, M.-C. Heitz, G. Royal, M. Boggio-Pasqua, Electronic excited states and UV–Vis absorption spectra of the dihydropyrene/cyclophanediene photochromic couple: a theoretical investigation. J. Phys. Chem. A 124, 2020, 1567–1579
3. N. Marchenko, P. Lacroix, V. Bukhanko, M. Tassé, C. Duhayon, M. Boggio-Pasqua, I. Malfant, Multistep photochemical reactions of polypyridine-based ruthenium nitrosyl complexes in dimethylsulfoxide. Molecules 25, 2020, 2205.
4. E. Lognon, M. Heitz, A. Bakkar, S. Cobo, F. Loiseau, E. Saint-Aman, G. Royal, M. Boggio-Pasqua, Dependency of the dimethyldihydropyrene photochromic properties on the number of pyridinium electron-withdrawing groups. ChemPhysChem, 2020, DOI: 10.1002/cphc.202000304.
5. F. Talotta, M. Boggio-Pasqua, L. González, Early relaxation dynamics in the photoswitchable trans-[RuCl(NO)(py)4]2+. Chem. Eur. J., 2020, DOI: 10.1002/chem.202000507.

Photochromic molecules are increasingly used in the design of innovative functional materials with applications in nanosciences, biology, and photonic devices. These compounds convert photonic energy into chemical energy on an ultrafast (picosecond) timescale to bring about a reversible transformation between two isomers. The change in electronic and molecular structures following electronic excitation results in a change of physical properties, which forms the basis of the applications listed above.
The majority of the investigated photochromic compounds relies on the use of organic molecules such as azobenzenes or dithienylethenes. A photochromic system with remarkable properties, yet not as well known, is the dimethyldihydropyrene (DHP) molecule. Indeed, DHP can be quantitatively and reversibly isomerized into cyclophanediene (CPD) under visible light, but has also the singular property to be able to produce, store and release singlet oxygen (1O2) upon irradiation at low energy under aerobic conditions. 1O2 is of major interest particularly for medical purposes.
Other photochromic compounds based metal complexes have also been discovered. This is the case of ruthenium nitrosyl complexes (Ru-NO), which can be photoisomerized in their isonitrosyl form (Ru-ON). These complexes also have the remarkable property to have the capability to photorelease nitric oxide (NO•), which plays an important role in many physiological mechanisms. It thus appears that DHP and Ru-NO derivatives display remarkable and complementary properties. However, these compounds have not been much exploited yet and their underlying photoswitching mechanism and releasing capability of biologically active species remain poorly understood.
The PHOTOCHROMICS project plans to intimately associate DHP and Ru-NO units in order to design hybrid DHP-Ru-NO multifunctional compounds. The objectives are the design of i) multiphotochromic compunds and ii) bimodal molecular systems capable of producing simultaneously 1O2 and NO• by photoinduced stimuli. Indeed, the design of switchable molecular architectures that can exist in several stable states is a growing field. Similarly, the possibility to produce at the same time the 1O2 and NO• biologically active species, which both display remarkable antibacterial properties, is of tremendous interest in the medical field, as synergistic effects have already been observed.
This proposal gathers theoretical and experimental chemists. Original hybrid compounds will be synthesized using technics in organic synthesis and coordination chemistry. The properties of the different states involved in the multiphotochromic systems and the production / activation of 1O2 and NO• will be analyzed by spectroscopic, photophysical, photochemical and electrochemical studies. A major task will be the rationalization of the underlying mechanisms involved. We will use advanced computational methodologies beyond the current state-of-the-art, which will allow static and dynamic investigations of these systems in their environment.
The main originality of this proposal lies in the remarkable properties and complementarity of the DHP and Ru-NO derivatives, whether in the field of photochromism or in the activation of small molecules of biological interest. To merge these two units in hybrid systems represent a formidable opportunity, which constitutes the heart of this project. The PHOTOCHROMICS proposal is a fundamental research project whose expected results will bring significant progress in the understanding of the involved systems. However, if the end results proved to be full of promise, the designed compounds might be exploited for application purposes in the future.