Nitroaromatics photophysics and photochemistry: a quantum dynamics study

The main problem addressed in the present project was the study and characterization of the photophysics and photochemistry of the nitroaromatic compounds nitrobenzene, 1-nitronaphthalene, and 2-nitronaphthalene. This task was pursued performing both static ab-initio electronic structure theory computations and quantum dynamics simulations. In particular the CASSCF and CASPT2 methods were employed as electronic structure theory methods, while the dynamics were performed using mainly the vMCG method in its DD-vMCG version. The overall objectives of the work were: characterize the decay paths and related critical points accounting for the photoindunced known phenomena in the systems; identify the timescale for the known ultrafast internal conversion processes and singlet-triplet intersystem crossing processes happening in the molecules; evaluate the importance of the roaming mechanism in the photochemistry of the mentioned compounds. The work is important for society because it leads to a significant increase of the knowledge of the photophysics and photochemistry of the reported molecules, which are key in various important fields: all three molecules are common urban atmospheric pollutants whose undergone photodegradation under solar exposure; nitroaromatic molecules, a family of compound here well represented by the selected systems, are normally employed in the drug delivery sector due to their ability to photorelease nitrogen oxide, which in turn has various fundamental biological effects on the human body; nitroaromatics systems, and in particular nitrobenzene, are used as prototypes for energetic materials.

During the period of the project various works were performed. The main photophysical and photochemical decay paths of the molecules were described, performing ground and excited states geometry optimizations, optimizations of conical intersections and singlet-triplet crossing regions, minimum energy path computations and frequencies calculations. Those were performed using the CASSCF and CASPT2 methods, employing different basis sets and active spaces. In order to study their dynamics with the DD-vMCG method, an interface between the electronic structure program Molcas and the dynamics package Quantics was created. Also the DD-vMCG code present in the Quantics program was further develop so to account for the coupling and consequently population transfer between states of different spin multiplicity, as the singlet and triplet states involved in the photoresponse of the mentioned molecules. Using the so implemented Quantics code, the photophysics and photochemistry of the systems were study running quantum dynamics using the vMCG methods in conjunction with the CASSCF and CASPT2 electronic structure theory methods, using both Cartesian coordinates and normal modes.
The main results obtained so far are: characterization of all main photophysical and photochemical decay paths of nitrobenzene, including the characterization of two non-previously reported reactive singlet-triplet crossing regions leading respectively to nitric oxide and epoxide ring formation, and the evaluation of a possible roaming path mediated by a conical intersection between the S1 and S0 states; characterization of the main critical points and decay paths for 1- and 2-nitronaphthalene, comparing the main similarities and differences among the three molecules; improvement of the DD-vMCG code in the Quantics package so to be able to treat both vibronic and spin-orbit couplings; resuls on the timescale and amount of population transfers between both singlet and triplet states after excitation of the S1 state of the three compounds, based on DD-vMCG dynamics.
The results obtained during the project were disseminated through talks and poster in various congresses and conferences. In particular: 4 four talk were delivered, five posters were presented; a total of eleven scientific meetings were attended. The various results obtained on nitrobenzene were published in the Journal of Chemical Theory and Computation (JCTC 2017, 13, 2777-2788).

One of the main progress beyond the state of the art resulting from the present project is the implementation in the vMCG method included in the Quantics package of a code for treating both vibronic and spin-orbit couplings, so to describe population transfers among states having different spin-multiplicity, as the singlet and triplet states involved in the photoresponse of nitrobenzene, 1-nitronaphthalene, and 2-nitronaphthalene. This code development has a potentially huge impact in the field of photophysics and photochemistry, since at present very few programs for running dynamics simulations can take into account population transfers among state with different spin multiplicity. Also, most of such programs are not based on a full quantum dynamics method. The description of states with different spin multiplicity in the framework of the DD-vMCG methods instead allows a full quantum dynamics treatment of them, and of the related population transfers and dynamics. Singlet-triplet population transfers are important in many different molecules, as the ones here studied. Other examples in which singlet-triplet transfers play an important role are canonical nucleobases and modified canonical nucleobases, in particular the ones used in phototherapy as photosensitizers. The project has had an important impact on the ER career, making improve its scientific knowledges and skills, his supervision abilities, his communication skills, and making expand his scientific network. Also the host organization has gain from the transfer of knowledge acquired from the ER.

Periodic Reporting for period 1 - NAP-QDYS (Nitroaromatics photophysics and photochemistry: a quantum dynamics study)

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