**4. Conclusions**

In this paper, a mild synthetic approach to prepare indigo based dyes is reported. Tyrian Purple is used as starting material to be coupled, after *t*-Boc protection, with different stannanes under Stille-Migita conditions. Three new symmetrical D-A-D dyes featuring an extended conjugation have been prepared and spectroscopically characterized and their optical properties compared with the results of theoretical calculations. Furthermore, non-symmetrical indigo-based D-A-π-A dye has been designed, synthesized and fully characterized both spectroscopically and electrochemically, assessing the possibility of using the indigo scaffold to prepare green dyes for DSSC application

**Supplementary Materials:** The following is the Supplementary data to this article: computational details for compounds **5a**, **5b**, **5c**, **5d**, **6d**, and **DF90**; Tauc plots for the CH2Cl2 solutions of compounds **3a–d** and **4a–c** and for for the EtOH solutions of compounds **3a**,**c**,**d** and **4a–c**; copies of the 1H and 13C NMR spectra of compounds **5a, 5b**, **5c** and **DF90.**

**Author Contributions:** D.F. and M.C. carried out the synthesis and characterization of the new compounds; D.F. and L.Z. carried out the spectroscopic and electrochemical studies of the dyes and prepared the corresponding figures; A.S. and C.C. carried out the DFT computational studies. A.M. and G.R. supervised and coordinated the research work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by Fondazione Cassa di Risparmio di Firenze ("ENERGYLAB" project, grant no. 2016.1113).

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

## **References**


**Sample Availability:** Samples of the compounds **5a–c** and **6a–c** available from the authors.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
