Nature of the excited states in large photochromic dimers: A TD-DFT examination

Using a combination of Time-Dependent Density Functional Theory calculations and molecular orbital analysis, the electronic properties of two diarylethene dimers have been investigated in all their possible forms (doubly closed, closed-open and doubly open). The selected ab initio method allows to systematically reproduce the measured spectral features with a remarkable accuracy, and to gain insights into the observed full or partial photochromism. Indeed, in the first dimer, the two DA are mostly independent, whereas in the second, the photochromism is lost due to the π-conjugation. In this second case, the fluorescence spectrum has been simulated with TD-DFT as well.