**4. Conclusions**

In this work, a series of twenty dyes comprising indane-1,3-dione or 1*H*-cyclopenta[*b*] naphthalene-1,3(2*H*)-dione have been synthesized and examined for their photophysical properties. Introduction of a naphthalene moiety in **EA4** greatly contributed to improve the electron-withdrawing ability of the group. Notably, a red-shift of the intramolecular charge transfer band of ca. 30 nm was observed for all dyes prepared with this acceptor, compared to the parent series comprising **EA1**. A linear and positive solvatochromism was found for all dyes, demonstrating an important charge redistribution upon excitation. A good correlation between the experimental HOMO-LUMO gaps and the theoretical ones could be found. Examination of their electrochemical properties revealed these dyes to be reversibly oxidized whereas an irreversible reduction monoelectronic process was determined for all dyes. By extending the aromaticity of **EA4**, a significant red-shift of the absorption maximum could be obtained for all dyes compared their analogues comprising **EA1**. Future works will consist of further improving the electron accepting ability of **EA4**, which is achievable by functionalizing **EA4** with malononitrile. Based on the present work, the tetracyano derivatives of **EA4** will certainly allow the design of near-infrared dyes that can find applications in research fields such as telecommunications and defense.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/8/1342/s1. 1H and 13C nmR spectra of all chromophores; Solvatochromism analyses in 23 solvents; contour plots of HOMO and LUMO energy levels of all dyes; cyclic voltammograms of all chromophores.

**Author Contributions:** C.P.; G.N. and F.D. directed the work and supervised the overall project. C.P., G.N. and F.D. carried out the synthesis of the chromophores, the chemical characterization of the dyes, the UV-visible absorption measurements and the solvatochromism experiments. T.-T.B. carried out the fluorescence and cyclic voltammetry experiments. S.P. carried out the theoretical calculations. Writing-Original Draft Preparation, C.P., G.N., F.D., T.-T.B. and S.P.; Writing-Review & Editing, C.P., G.N. and F.D.; M.N. and D.G. contributed to review the manuscript. All authors approved the final manuscript.

**Funding:** This research was funded by Aix Marseille University and The Centre National de la Recherche (CNRS) for financial supports. This research was also funded by the Agence Nationale de la Recherche (ANR agency) through the PhD grants of Corentin Pigot (ANR-17-CE08-0010 DUALITY project) and Guillaume Noirbent (ANR-17-CE08-0054 VISICAT project).

**Conflicts of Interest:** The authors declare no conflict of interest.
