Fluorene-imidazole dyes excited states from first-principles calculations—Topological insights

We report the theoretical investigation of four organic dyes containing imidazole and fluorene moieties as donor and bridge in donor–bridge–acceptor molecular structures. Those target dyes were recently reported as potential agents for building metal-free light-to-electricity conversion devices [J. Org. Chem. (2014) 79, 3159]. Our contribution consists in the establishment of an appropriate computational protocol for obtaining the excited states of these dyes from a reliable level of theory. This benchmark was performed based on the possibilities offered by density functional theory and its time-dependent variant. The outcome of this screening allowed us to compute absorption properties of the target dyes, as well as the emission properties for one of them. Afterward, the electronic transitions computed from this reference method were characterized by a series of topological analysis tools, aimed for a qualitative and quantitative probing of the excited states nature. These tools rely on the formal depiction of the photogenerated hole and particle from density matrices or through the exploitation of the exciton wavefunction. Further linear algebraic operations based on these two types of objects lead to the elaboration of detachment/attachment density matrices and natural transition orbitals respectively, so that the outcome of these operations provides a qualitative depiction of the photoinduced electronic cloud polarization. Finally, quantitative insights were provided by the evaluation of quantum-mechanical metrics related to the charge transfer phenomenon caused by light absorption.