Electronic transitions in noncovalent BODIPY dimers: TD-DFT study.

Theoretical computations have been performed on the absorption spectra of (8-R-TMB) dimers with varying character of substituents at the 8 position (meso) at monomers units (R = NH, OH, CH, H, COH, CF, CN). The obtained results (TD-CAM-B3LYP) show that the first four lower transitions of studied dimers (S → S, i = 1-4) are intrinsically linked with delocalized HOMO and LUMO orbitals of the two monomers, which constitute a dimer. For all the dimers, S → S and S → S transitions are strongly forbidden, whereas S → S and S → S are allowed. There is a good agreement between the TD-CAM-B3LYP theory and the simple model of exciton coupling for two identical chromophores with the planes of two moieties, which are stacked upon each other. Intensities of the allowed transitions depend strongly on the nature of the substituent at the meso position. For the dimers with monomer units bearing electron-donor groups, S → S transitions are more intense compared with S → S ones. As the donor properties of the meso substituents diminish and electron acceptor properties enlarge, the intensity of these transitions becomes lower, whereas that of S → S transitions becomes higher. For the dimers with NH and CN substituents, the almost inverse ratio of intensities of the transitions discussed is observed (for example, f = 0.80 and 0.06 (in the case of NH) and f = 0.09 and 0.72 (CN) for S → S and S → S, respectively). Protonation of the tertiary amine function drastically 'switches off' its electron-donating properties. As a result, the red shift of the bands and redistribution of intensities of the allowed S → S and S → S transitions is predicted for protonated forms of the (8-NH-TMB) dimer. This peculiarity can be of especial importance for the design of dimers with a sensor function.