## 1 Introduction  
Viologen salts [1], that is salts of \(N\)-disubstituted 4,4\({}^{\prime}\)-bipyridinium, represent a textbook example of compounds whose color can be finely tuned by several stimuli. Thus, viologens are known to exhibit electrochromism [2, 3] (color dependence on electricity), thermochromism [4, 5] (color dependence on temperature), photochromism [6, 7] (color dependence on light), sometimes all included in the same compound [8]. In addition to the external physical stimuli, viologen salts' color is also dependent on the chemistry of the counter-anions, substituents on the bipyridinium core, pH and the solvent (chemochromism) [1, 9].  
One interesting aspect of the photochemistry of viologens is the color of the salts and corresponding solutions in low- or nonpolar solvents due to charge-transfer from the donor anion to the bipyridinium unit. As an example, \(N\)-\(N\)-dialkyl-4,4\({}^{\prime}\)-bipyridinium salts, in the solid state, are white with noncoordinating and hard anions such as hexafluorophosphate and 1,1,1-trifluoro-N-((trifluoromethyl)sulfonyl) methanesulfonamide (also known as bistriflimide) [10, 11, 12, 13]; bromide salts are yellow [11], iodide salts are red [14] while iodide salts of \(N\)-\(N^{\prime}\)-dialkyl-2-2\({}^{\prime}\)-6-\({}^{\prime}\)-tetramethyl-4,4\({}^{\prime}\)-bipyridinium are orange [10], to mention but a few examples which nevertheless highlight how relatively minor chemical differences can affect the UV-Vis absorption spectrum. When dissolved in low- or nonpolar solvents the corresponding solutions have a similar color to the solid salt [9].  
The prediction of the absorption spectrum of organic dyes, by quantum chemical protocols, is nowadays quite advanced: covalent systems do not pose significant problems and it is possible to successfully predict the UV-Vis spectrum of organic compounds, using Time-Dependent Density Functional Theory (TD-DFT), with very high accuracy [15; 16; 17]. On the other hand, the application of the same protocols to noncovalent systems is much more challenging [18]: Which is the correct geometry to be considered for a noncovalent compound? Is DFT capable to correctly account for long-range effects and electron density in noncovalently bound systems? Moreover, especially regarding viologen salts in solution, can intermolecular charge transfer (CT) bands be accurately modelled by DFT methods?  
In this work we will test several TD-DFT protocols for the calculation of the absorption spectra of viologen salts in medium and low polar solvents in order to provide a recipe for the prediction of the maximum of the CT absorption. To this end we select here few experimental data, mostly from the author, concerning different viologen salts in different solvents.  
The experimental data of the maximum of the absorption of the lowest energy band in the UV-Vis spectrum in dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)) for four representative examples of viologen salts, have been taken from the literature and listed in Table 1. For octylviolgen iodide, we also have experimental data in acetone. As discussed in Ref. [9] and also consistent with the observations reported by [PERSON] et al. [19; 20], there is no evidence of significant aggregation of bromides and iodides viologens salts in solvents of low and medium polarity, except for the formation of neutral clusters. This is in striking contrast with the aggregation behavior of viologens salts of \"hydrophobic\" anions, like bistrifimide, which show a strong dependence of the spectroscopic properties on the concentration [21; 22]. However, for the systems investigated here it is reasonable to assume the formation of just neutral clusters of 1:2 stoichiometry.