*3.3. Optical Properties*

All compounds were characterized by UV-visible spectroscopy and their absorption spectra in dichloromethane are provided in the Figure 7. **PP1**–**PP20** are push-pull compounds that possess an electron donating group and an electron accepting group which interact by mean of a π-conjugated system. The position of the charge transfer band will depend of parameters such as the strength of the electron donating/accepting groups, but also of the length of the π-conjugated system. By combining both effects, a decrease in the energy difference between the HOMO and the LUMO can be obtained, resulting in a shift of the absorption spectrum towards longer wavelengths.

While examining the first series **PP1**–**PP10**, absorption maxima ranging from 412 nm for **PP1** to 524 nm for **PP5** were found in dichloromethane for dyes exhibiting a short spacer. Elongation of the π-conjugated system resulted in a significant red-shift of the absorption maximum, shifting from 471 nm for **PP1** to 571 nm for **PP9**. The most red-shifted absorption was found for **PP10**, peaking at 603 nm. While comparing the **PP1**–**PP10** series with **PP11**–**PP20** based on 1,3-indanedione, absorption spectra of **PP11**–**PP20** were found to follow the same order than that of the **PP1**–**PP10** series, but with an absorption blue-shifted by about 30 nm (see Figure 7). Examination of the molar extinction coefficients for the two series also revealed the **PP1**–**PP10** series to exhibit higher molar extinction coefficients than that of the **PP11**–**PP20** series, consistent with an improvement of the molar absorptivity with the oscillator strength and the conjugation extension (see Figure 8) [48].

The experimental absorption spectra recorded in dichloromethane of all compounds are in good accordance with predicted properties obtained by DFT calculation. Notably, a good accordance between the theoretical absorption maxima can be found for all dyes (See Tables 2–4). Second, considering that the main absorption band was theoretically determined for all dyes originating from a HOMO->LUMO transition, this latter can thus be confidently assigned to the intramolecular charge transfer bands for all dyes. A contribution of the HOMO->LUMO transition to the ICT band ranging between 85% (for **PP20**) to 100% for **PP9** and **PP19** could be calculated.

**Figure 7.** UV-visible absorption spectra of **PP1**–**PP10** (**a**) and **PP11**–**PP20** (**b**) in dichloromethane.

(**a**) **Figure 8.** *Cont*.

(**b**)

**Figure 8.** UV-visible absorption spectra of **PP1**–**PP10** (**a**) and **PP11**–**PP20** (**b**) in chloroform.

**Table 3.** Summary of the optical properties of **PP1**–**PP10** in twenty-three solvents and Kamlet and Taft parameters π\*.


<sup>1</sup> Kamlet and Taft parameters; <sup>2</sup> Position of the ICT bands are given in nm.

**Table 4.** Summary of the optical properties of **PP11–PP20** in twenty-three solvents and Kamlet and Taft parameters π\*.



**Table 4.** *Cont*.

<sup>1</sup> Kamlet and Taft parameters; <sup>2</sup> not determined. <sup>3</sup> Position of the ICT bands are given in nm.

#### *3.4. Solvatochromism*

All dyes **PP1**–**PP20** exhibited a good solubility in most of the common organic solvents so that examination of the solvatochromism could be carried out in 23 solvents of different polarities. It has to be noticed that alcohols such as methanol, ethanol, propan-2-ol, butan-1-ol and pentan-1-ol were initially considered as solvents for the solvatochromic study, but the absorption maxima obtained with these solvents were irregular compared to that obtained with the 23 other solvents. This specific behavior can be assigned to the fact that all dyes precipitated in alcohols such as in ethanol which was the solvent of reaction. Even if the absorption spectra could be recorded in alcohols for all dyes, presence of free molecules and aggregates in solution certainly modify the position of the absorption maxima. A summary of the absorption maxima for the twenty dyes are provided in Tables 3 and 4.

As evidenced in the Tables 3 and 4, analysis of the solvatochromism in solvents of different polarities confirmed the presence of an intramolecular charge transfer in all dyes. Intramolecular nature of the charge transfer was demonstrated by performing successive dilutions, intensity of the charge transfer band linearly decreasing with the dye concentrations. Various empirical polarity scales have been developed over the years to interpret the solvent-solute interaction and the Kamlet-Taft's [49], Dimroth-Reichardt's [50], Lippert-Mataga's [51], Catalan's [52], Kawski-Chamma-Viallet's [53], McRae's [54], Suppan's [55], and Bakhshiev's [56] scales can be cited as the most popular ones. Among all scales, the Kamlet-Taft solvent polarity scale proved to be the most adapted one, linear correlations being obtained for all dyes by plotting the absorption maximum vs. The empirical Taft parameters (see linear regression in Supplementary Materials). For all the other polarity scales based on the dielectric constant or the refractive index of solvents, no reasonable correlations could be established. The Kamlet and Taft equation is also a multiparametric equation that can take into account the dipolarity-polarizability (π\*), the hydrogen-donating and accepting ability (α and β) of the solvents, modelizing more precisely the interactions between the solvent and the solute. However, multiple linear regression analyses carried out on the triparametric Kamlet-Taft equation using the three solvent descriptors (α, β, π\*) did not improve the correlation coefficients, as demonstrated in the Table in SI. It can therefore be concluded that the dipolarity-polarizability of the solvent is the primary cause influencing the position of the ICT band.

As evidenced in the Figure 9 and Figures in Supplementary Materials, **PP1**–**PP20** show negative slopes with good linear correlations, indicative of a positive solvatochromism. Excepted for **PP1**, **PP2**, **PP11**, and **PP12** that possess weak electron donors, all dyes displayed strong negative slopes, indicative of a significant charge redistribution upon excitation. The most important solvatochromism was found for **PP9**, **PP10**, **PP19**, and **PP20** that exhibit the longest conjugated spacers.

**Figure 9.** Variation of the positions of the charge transfer band with Kamlet-Taft empirical parameters for **PP1**–**PP10** (**a**) and **PP11**–**PP20** (**b**).
