Investigação Teórica da Formação de Exciplexos de Pireno

The photophysical behaviour of a given system can be strongly influenced by the presence of species in high concentrations due to possible excited state interactions, which may lead to the presence of new species in solution - called exciplexes (distinct molecules) or excimers (identical molecules). Since its discovery, species capable of forming exciplexes have been highlighted in important processes such as photodimerization, singlet fission and their use as biosensors. Exciplex formation can be understood based on two types of interaction: excitonic and charge resonances, which suggests a strong multi-referential character for such systems. However, describing the nature of these states is a challenge for medium to large systems due to their high computational cost, requiring the use of single-reference methods such as the time-dependent density functional theory (TD-DFT) and the algebraic diagrammatic construction scheme for the polarization propagator - ADC(2). A related challenge is the basis set choice. This work explores the effect of basis set and level of method on descriptors relevant for exciplex formation such as excitation energy, charge transfer index (CT), exciton average position, natural transition orbitals and the potential energy curves topologies along the ring spacing coordinate. To evaluate the influence of the number of aromatic rings on the exciplex formation, pyrene - naphthalene, pyrene - anthracene, pyrene - pyrene, pyrene - acylpyrene, acylpyrene - acylpyrene were studied. The results suggest that the cc-pVDZ and def2-SV (P) basis set exhibit similar qualitative descriptions in all studied systems. However, significant changes are observed among the tested methods, namely, TD-DFT with several functionals (PBE0, CAM-B3LYP, BHandHLYP and ωB97-XD) and RI-SOS-ADC(2) Regarding structural changes, the increase in the number of aromatic rings led to the increase of stability gain of these systems with the approximation between the fragments, accompanied by the increase of the CT, which is an indication that this parameter is associated with exciplex formation. Meanwhile, the introduction of a carbonyl group led to substantially different descriptions of the RI-SOS-ADC(2) and TD-DFT methods. It was possible to observe a relationship between the presence of possible crossings and differences in the topology of the exciplex potential energy curves with the charge transfer index behaviour along the same coordinates. This relation could be used qualitatively to understand the behaviour of the exciplexes. excited states of these systems. The functional PBE0 consistently disagrees with the description obtained by the other methods in the entire exciplex series studied, suggesting that this is not an appropriate method for these systems. Overall, the BHandHLYP and CAM-B3LYP functionalities show good agreement with each other, as do the TD-DFT / ωB97-XD and RI-SOS-ADC(2) levels. The choice of calculation level has proved to be a crucial step in the study of pyrene exciplexes and should be tested extensively.