Singlet oxygen generation versus O–O homolysis in phenyl-substituted anthracene endoperoxides investigated by RASPT2, CASPT2, CC2, and TD-DFT methods

Abstract

The electronic excited states corresponding to singlet oxygen generation versus O–O splitting in o-fluorine-phenyl-9-anthracene-9,10-endoperoxide 1 and its 9,10-bisarylanthracene analog 2 have been investigated using theoretical methods. In the case of the smaller endoperoxide 1, the recently developed second-order perturbation theory restricted active space (RASPT2) method has been employed and the results are compared to those from the complete active space (CASPT2), second-order approximated coupled cluster (CC2), and time-dependent density functional theory (TD-DFT) approaches. In addition to the vertical excited states, the photochemical path leading to homolytic O–O dissociation has been computed. This process is governed by a point, where four singlet and four triplet states are almost degenerate and show substantial spin-orbit coupling. The results obtained with RASPT2 indicate that the S1 state is of πoo*σoo* character, corresponding to the O–O homolytic dissociation, while higher excited states Sn (n ≥ 2) correspond to local and charge transfer excitations and should be correlated to the generation of singlet molecular oxygen. A similar photochemical picture is obtained with CASPT2, although two different active spaces are required to describe different parts of the spectrum. The calculations carried out with CC2 as well as the functionals CAM-B3LYP and the B3LYP(32) containing 32 % of exact exchange show good agreement with the RASPT2 energies, but present a strong mixing of πoo*σoo* and πoo* πan* excitations in the lowest S1 state, contradicting the assignment of RASPT2/CASPT2. The use of BP86 is strongly discouraged since it misplaces a large number of charge transfer states below the πoo*σoo* state. The excited states of 2, calculated with B3LYP(32) are very similar to those of 1, leading to the conclusion that both endoperoxides should show a similar photochemistry, that is, the O–O cleavage seems to be partially quenched and singlet oxygen generation is enhanced, in comparison with the parent compound, anthracene-9,10-endoperoxide.