*3.5. Fluorescence Spectroscopy*

The fluorescent properties of all compounds were investigated in dichloromethane and toluene, in diluted solutions and the results are summarized in the Figure 10 and the Table 5. Interestingly, most of the chromophores were not emissive whatever the solvent is and this behavior is consistent with results classically reported in the literature for indane-1,3-dione derivatives [57–61]. However, it has to be noticed that a series of indane-1,3-dione derivatives was reported as being highly emissive, in a specific context, by use of oligo(phenylene)vinylene as electron donors [62]. Only eleven of these compounds displayed a weak emission in toluene so that the luminescence lifetime as well as the

photoluminescence quantum yield were not determined for these compounds. While examining the emission maxima, the most red-shifted emissions were found for **PP9** and **PP10** displaying the most extended conjugated spacer but also the most extended electron acceptor. As attended, a blue-shifted emission was found for **PP20** (λem = 604 nm) relative to that of **PP10** (λem = 626 nm), directly resulting from its blue-shifted absorption maxima. While comparing the results obtained in dichloromethane and toluene, absorption maxima were found to be blue-shifted by *ca.* 20 nm for all dyes in toluene relative to that determined in dichloromethane. Based on the Taft parameters for both solvents (0.54 for toluene and 0.82 for dichloromethane), it can be concluded that a positive solvatochromism can also be observed in emission. This point is notably confirmed by the Stokes shifts determined in both dichloromethane and toluene, which are almost identical.

**Figure 10.** Fluorescence spectra of studied compounds in dichloromethane (**a**) and toluene (**b**).


**Table 5.** Fluorescence properties of the different compounds in dichloromethane and toluene solutions.

## *3.6. Electrochemical Properties*

The electrochemical properties of all compounds have been investigated by cyclic voltammetry (CV) in dilute solutions, either in acetonitrile or in dichloromethane. The selected voltammograms are shown in the Figure 11 and CV curves of all compounds are given in the Supplementary Materials. The redox potentials of all compounds are summarized in the Table 6 in which redox potentials are given against the half wave oxidation potential of the ferrocene/ferrocenium cation couple.

**Figure 11.** Selected examples of cyclic voltammograms of studied compounds (**PP9**, **PP19**) and Ferrocene (Fc).

**Table 6.** Electrochemical redox potentials of the studied compounds **PP1–PP20**.



**Table 6.** *Cont*.

<sup>1</sup> All potentials are recorded in 0.1 M TBABF4/CH3CN, except for **PP15** and **PP20** for which electrochemistry was carried out in 0.1 M TBAClO4/CH2Cl2. EHOMO (eV) <sup>=</sup> −4.8 − Eox and **ELUMO** (eV) <sup>=</sup> −4.8 − Ered; <sup>2</sup> Optical bandgaps determined in acetonitrile.

As shown in Figure 11, **PP9** and **PP19** differ from each other only in the nature of the accepting moiety. As expected, both compounds have quasi-reversible oxidation processes with identical oxidation potentials (Figure 11, Table 6). Indeed, the oxidation process for the two chromophores is centered onto the electron-donating part and this latter is the same for the two dyes. Conversely, the reduction potential of **PP9** comprising 1*H*-cyclopenta[*b*]naphthalene-1,3(2*H*)-dione (**EA4**) as the electron-accepting moiety is slightly lower than that of **PP19** (comprising **EA1** as the acceptor), leading to narrower electrochemical bandgap. This is in good accordance with the optical bandgap determined by UV-visible absorption spectroscopy, where **PP9** showed a red-shifted ICT band in comparison to **PP19** (See Figures 7 and 8).

The redox potentials of all other compounds are gathered in the Table 6. As can be seen in the same series (**PP1**–**PP10** and **PP11**–**PP20**) where the nature of the acceptor moiety is identical, their reduction potentials changed very slightly while the oxidation potential importantly vary as a function of the donor moiety.

The number and the substitution position of the alkoxy chains on the phenyl ring slightly influence the redox potentials of the targeted molecules (**PP1**–**3** and **PP11**–**13**). However, important variations were determined when the electron-donating ability of the electron donor was increased by the presence of diakylamino groups on the phenyl ring (see **PP4**–**5** and **PP14**–**15**). This phenomenon was even much more pronounced when a double bond was inserted between the donor and the acceptor moiety leading to more conjugated push pull molecules (see **PP9**–**10** & **PP19**–**20**). The presence of two 4-(*N*,*N*-methylamino)phenyl groups such as in **PP10** and **PP20** has only a negligible impact on the electrochemical property. In fact, while the second 4-(*N*,*N*-methylamino)phenyl group could increase the electron donating ability, examination of the mesomeric forms in **PP10** and **PP20** clearly evidences the two groups not to be able to contribute simultaneously to the electronic delocalization, as previously mentioned in the literature [46]. While comparing the dyes at identical electron donating groups, push pull compounds prepared with **EA4** (**PP1**–**PP10**) have lower reduction potentials than their counterparts **PP11**–**PP20** comprising **EA1** as the electron withdrawing groups. This is in perfect accordance with the higher electron accepting capacity of **EA4**.

The redox behaviors of synthesized molecules were then used to estimate their HOMO and LUMO energy levels by using the ferrocene (Fc) ionization potential value (4.8 eV vs. vacuum) as the standard. The correcting factor of 4.8 eV is based on calculations obtained by Pommerehne et al. [63]. It is also

important to note that some other correcting factors have also been used in the literature [64]. The obtained values of EHOMO and ELUMO issued from electrochemical characterizations are summarized in the Table 6 for comparison, the optical bandgaps of all dyes in acetonitrile have been added. The Figure 12 shows a comparative presentation of the frontier orbitals' energy levels experimentally and theoretically obtained. We can see that the experimental findings fit well with the trend predicted by DFT calculation.

**Figure 12.** Comparison of frontier orbitals' energy levels obtained from cyclic voltammetry and DFT calculations.
