**3. Results and Discussion**

## *3.1. Synthesis of the Dyes and Electron Acceptors*

All dyes **PP1**–**PP20** presented in this work have been synthesized by a Knoevenagel reaction involving the aldehydes **D1**–**D10** and the two electron acceptors **EA4** and **EA1**, respectively (See Figure 3). The different reactions were performed in ethanol using piperidine as the catalyst. **PP1**–**PP20** were obtained with reaction yields ranging from 74% yield for **PP4** and **PP11** to 94% for **PP5** (see Table 1). All compounds were obtained as solids and they were characterized by 1H, 13C nmR spectroscopies, and HRMS spectrometry (see Supplementary Materials). It has to be noticed that the synthetic procedure to **EA4** has been greatly improved compared to that reported in the literature [47], enabling to reach a reaction yield of 91%. Indeed, a common method to synthesize 1,3-indanedione derivatives consists in a Claisen condensation of the corresponding diesters with ethyl acetate under basic conditions (generally sodium hydride).

**Figure 3.** Synthetic pathways to **PP1**–**PP20** and the ten aldehydes used in this study.

**Table 1.** Reaction yields obtained for the synthesis of **PP1**–**PP20**.


The final product is obtained after the condensation step by decarboxylation of the intermediate salt under hot acidic conditions (see Figure 4). To access the starting compound, i.e., diethyl naphthalene-2,3-dicarboxylate, two different reaction pathways were examined, by esterification of naphthalene-2,3-dicarboxylic acid in ethanol in the presence of an excess of thionyl chloride, or by the classical esterification conditions consisting in refluxing the acid in ethanol in the presence of a catalytic amount of H2SO4 (see Figure 4). If diethyl naphthalene-2,3-dicarboxylate could be obtained in almost quantitative yields with the two procedures, our attempt to convert the diester obtained by the first procedure were unfruitful to form **EA4**. This is attributable to remaining traces of SOCl2 in the diester, despites the numerous washings in basic conditions. Due to the presence of water traces in ethyl acetate, SOCl2 could react with water to form HCl, neutralizing part of the NaH introduced. Conversely, in the second procedure, H2SO4 can be easily removed from the diester due to its catalytic use, avoiding this drawback. Using the oil obtained by this second procedure, a major improvement was obtained by replacing NaH 60% dispersion in oil by NaH 95% dispersion in oil for the first reaction step.

**Figure 4.** Synthetic route to 1,3-indanedione derivatives by the Claisen condensation and the two possible esterification procedures for naphthalene-2,3-dicarboxylic acid.

By using this more concentrated dispersion, the reaction yield of the crude materials for the two steps (the intermediate salt was not isolated) could be greatly increased. By modifying the decarboxylation time (increased from 1.5 h instead of 20 min), the quantity of NaH (2.5 eq. instead of 1.45 eq.) and the recrystallization solvent [47] (benzene replaced by the less toxic toluene) compared to that reported in the literature, the overall reaction yield after purification could be increased from 65% (literature) up to 91% in our optimized conditions.
