Impact of low-cost methods in the description of excimer and exciplex formation: pyrene–pyrene and pyrene–naphthalene case studies

The formation of excimers and exciplexes is relevant to a number of photochemical and photophysical processes such as fluorescence quenching and exciton trapping. Theoretical evaluation of the properties and dynamics of such systems often relies on approximate methods to reduce computational time. In this work, we present an assessment of calculations of the excited states of pyrene–pyrene and pyrene–naphthalene pairs with RI-SOS-ADC(2) and the TD-DFT functionals CAM-B3LYP-D3 and ωB97-XD. We find that ADC(2) yields dissociation energies with relatively small errors for the pyrene–pyrene excimer in S1 and a consistent picture of the splitting of monomer La and Lb states into the L− , L+ , L− , and L+ states as the rings approach. TD-DFT, on the other hand, predicts too large dissociation energies for the excimer and is unfit for the description of the pyrene–pyrene dimer. The pyrene–naphthalene exciplex is predicted by RI-SOS-ADC(2) to have a very small dissociation energy, comparable to that of the benzene–benzene excimer.