Synthesis and Optical Properties of a Series of Push-Pull Dyes Based on Pyrene as the Electron Donor

Fifteen push-pull dyes comprising the tetracyclic polyaromatic pyrene have been designed and synthesized. The optical properties of the fifteen dyes have been examined in twenty-two solvents of different polarities. Surprisingly, contrarily to what is classically observed for push-pull dyes of D-π-A structures, a negative solvatochromism could be found for numerous dyes. The photoluminescence and thermal properties of the dyes were also examined. Theoretical calculations were carried out to support the experimental results.


Synthesis of Dye 1-Dye 15
Two distinct synthetic strategies were developed to access the different structures, depending on the electron acceptors. Except for Dye 5 and Dye 15, the different compounds Dye 1-Dye 4, Dye 6-Dye 14 were obtained by a Knoevenagel reaction in basic conditions, using piperidine as the base. Conversely, for EA5 and EA15, for which anions are highly stable and unreactive in basic conditions, acidic conditions had to be used and acetic anhydride was selected as the appropriate solvent [7,95,96]. Upon reflux of the solutions for two hours for Dye 5 or heating at 90 °C overnight for Dye 15, Dye 5 and Dye 15 could be obtained, with reaction yields ranging from 86% for Dye 5 to 88% for Dye 15 (see Scheme 1). In turn, the fifteen dyes could be obtained in reasonable yields, ranging from 72% for Dye 4 to 88% for Dye 15. Scheme 1. Synthetic routes to obtain Dye 1-Dye 15.

Optical Properties
Pyrene-based dyes are highly polyaromatic structures, so that the determination of a common solvent in which all dyes could be soluble was not possible. Interestingly, almost all dyes were soluble in N,N-dimethylformamide (DMF), except three dyes, i.e., Dye 5, Dye 7 and Dye 15, for which absorption spectra were recorded in dioxane. For these three dyes, dioxane was used as the appropriate solvent for examining their optical properties. In these conditions, the optical properties of almost of the dyes could be compared in DMF. As shown in Figure 3, all dyes showed strong absorption centered in the visible range. Considering that pyrene is a weak electron donor, all dyes showed an intense Scheme 1. Synthetic routes to obtain Dye 1-Dye 15.

Optical Properties
Pyrene-based dyes are highly polyaromatic structures, so that the determination of a common solvent in which all dyes could be soluble was not possible. Interestingly, almost all dyes were soluble in N,N-dimethylformamide (DMF), except three dyes, i.e., Dye 5, Dye 7 and Dye 15, for which absorption spectra were recorded in dioxane. For these three dyes, dioxane was used as the appropriate solvent for examining their optical properties. In these conditions, the optical properties of almost of the dyes could be compared in DMF. As shown in Figure 3, all dyes showed strong absorption centered in the visible range. Considering that pyrene is a weak electron donor, all dyes showed an intense absorption band in the 350-500 nm region corresponding to the intramolec-ular charge transfer (ICT) band. The most red-shifted absorption was found for Dye 7 (λ max = 549 nm), comprising 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene) malononitrile EA7 as the electron acceptor. As shown in Figure 3a, the charge transfer band of Dye 7 is broad and extends from 450 to 700 nm. Following Dye 7, Dye 5, comprising 2,2 -(1H-Indene-1,3(2H)-diylidene)dimalononitrile EA5 as the acceptor, exhibited the second most red-shifted absorption maximum at 549 nm, outperforming all the other dyes (see Figure 3c).
Molecules 2023, 28, x FOR PEER REVIEW 4 of 24 absorption band in the 350-500 nm region corresponding to the intramolecular charge transfer (ICT) band. The most red-shifted absorption was found for Dye 7 (λmax = 549 nm), comprising 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene) malononitrile EA7 as the electron acceptor. As shown in Figure 3a, the charge transfer band of Dye 7 is broad and extends from 450 to 700 nm. Following Dye 7, Dye 5, comprising 2,2′-(1H-Indene-1,3(2H)-diylidene)dimalononitrile EA5 as the acceptor, exhibited the second most red-shifted absorption maximum at 549 nm, outperforming all the other dyes (see Figure  3c).  These results are consistent with previous works reported in the literature evidencing the remarkable electron-withdrawing ability of this group [32,97]. As anticipated, the bluest-shifted absorption was found for Dye 1, comprising dimethyl malonate as the electron acceptor. Indeed, EA1 is the weakest electron acceptor of the series. In this last case, the maximum absorption located at 374 nm could be determined. Moreover, if the absorption of this dye is strongly UV-centered, absorption could, however, be found in the visible range thanks to the long tail extending to 450 nm. The highest molar extinction coefficient of the series was determined for Dye 12, peaking at 44,800 L −1 .M −1 . A slightly lower molar extinction coefficient (ε = 35,950 L −1 .M −1 ) was determined for Dye 13, also bearing a rhodanine-based electron acceptor (see Table 1). Theoretical studies were also carried out to investigate the energy levels as well as the molecular orbital (M.O.) compositions of the different dyes. DFT calculations were performed for all dyes at the wb97xd/6-311g(d,p) level of theory using the Gaussian 09 program to determine the transitions involved in the different absorption peaks. DMF was used as the solvent model with a polarizable continuum model (PCM) [98][99][100][101][102][103][104]. Theoretical UV-visible absorption These results are consistent with previous works reported in the literature evidencing the remarkable electron-withdrawing ability of this group [32,97]. As anticipated, the bluestshifted absorption was found for Dye 1, comprising dimethyl malonate as the electron acceptor. Indeed, EA1 is the weakest electron acceptor of the series. In this last case, the maximum absorption located at 374 nm could be determined. Moreover, if the absorption of this dye is strongly UV-centered, absorption could, however, be found in the visible range thanks to the long tail extending to 450 nm. The highest molar extinction coefficient of the series was determined for Dye 12, peaking at 44,800 L −1 .M −1 . A slightly lower molar extinction coefficient (ε = 35,950 L −1 .M −1 ) was determined for Dye 13, also bearing a rhodanine-based electron acceptor (see Table 1). Theoretical studies were also carried out to investigate the energy levels as well as the molecular orbital (M.O.) compositions of the different dyes. DFT calculations were performed for all dyes at the wb97xd/6-311g(d,p) level of theory using the Gaussian 09 program to determine the transitions involved in the different absorption peaks. DMF was used as the solvent model with a polarizable continuum model (PCM) [98][99][100][101][102][103][104]. Theoretical UV-visible absorption spectra were obtained by TD-DFT calculations and the different spectra are presented in Figure 4.
Optical characteristics are summarized in Table 1. The position of the absorption maxima was determined from the theoretical spectra. As shown in Table 1, for all dyes, the positions of the theoretical absorption maxima were determined in DMF as the solvent. Noticeably, all theoretical absorption maxima were blue-shifted compared to the experimental ones. In fact, the PCM model only allows us to create an electrostatic field corresponding to the dielectric nature of the solvent around the molecule. In no case does this model allow us to take into account specific interactions between the molecule and the solvent. Notably, it does not allow us to take into account a protic effect of a solvent, for instance. In fact, no solvate model in DFT allows us to take into account easily the molecule/solvent interactions. As a consequence of this, a mismatch between the theoretical and the experimental positions of the absorption maxima is found.
Molecules 2023, 28, x FOR PEER REVIEW 6 of 24 and the acceptor. A similar distribution of both the HOMO and LUMO energy levels can be evidenced for Dye 7 and Dye 15, also comprising sterically hindered electron acceptors (see Figure 7). In these cases, torsion angles of 45.65° and 58.64° were determined between the donors and acceptors in Dye 7 and Dye 15, respectively.   As anticipated, the minor variations of the substitution pattern of Dye 12 and Dye 13 did not modify the UV-visible absorption spectra, as experimentally observed. Examination of the contour plots of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) revealed a classical electronic distribution for the two orbitals. As shown in Figure 5, logically, the HOMO orbital is located on the electron donor, whereas the LUMO level is centered on the electron-accepting one.
To obtain a deeper insight into the different transitions involved in the intramolecular charge transfer bands of the different dyes, theoretical calculations were carried out and the different data are summarized in Table 2. Notably, for Dye 5, which comprises the bulkiest electron acceptor, the intramolecular charge transfer band was determined as being an admixture of HOMO => LUMO, HOMO => LUMO + 1 and HOMO − 1 => LUMO transitions. Optimization of the geometry for Dye 5 also revealed this dye to exhibit strong internal torsion with a dihedral angle of 43.05 • due to the steric hindrance generated by the electron acceptor (see Figure 6). Moreover, despite this internal torsion, a HOMO energy level extending over the pyrene moiety and the dicyanomethylene groups of the acceptor could be determined (see Figure 6). Similarly, the LUMO energy level of Dye 5 is mainly located on the electron acceptor but also extends over the pyrene moiety, demonstrating an electronic communication that is maintained between the donor and the acceptor. A similar distribution of both the HOMO and LUMO energy levels can be evidenced for Dye 7 and Dye 15, also comprising sterically hindered electron acceptors (see Figure 7). In these cases, torsion angles of 45.65 • and 58.64 • were determined between the donors and acceptors in Dye 7 and Dye 15, respectively.         For all dyes, the position of the theoretical absorption maximum was close to the position determined for the HOMO => LUMO transition by TD-DFT, since a difference of a few nanometers was found. Therefore, the main transition involved at the absorption maximum corresponds to a HOMO => LUMO transition. Moreover, the exciton binding energy of the different dyes, which is defined as the difference between the electrochemical and optical bandgaps, could not be determined [105,106], the dyes being not sufficiently soluble to determine with accuracy the positions of the HOMO and LUMO energy levels by electrochemistry. Based on the position of the ICT bands, the different electron acceptors could be classified according to the order presented in Scheme 2. Among all electron acceptors, EA7 and EA5 were determined as exhibiting the highest electron-withdrawing abilities of the series of 15 electron acceptors, consistent with the ordering previously reported in the literature [33]. The weakest electron acceptors were determined as being EA1 and EA2 [107,108]. Notably, EA1 is rarely used for the design of push-push dyes due to the weak electronic delocalization that this electron acceptor involves [87,[109][110][111].  For all dyes, the position of the theoretical absorption maximum was close to the position determined for the HOMO => LUMO transition by TD-DFT, since a difference of a few nanometers was found. Therefore, the main transition involved at the absorption maximum corresponds to a HOMO => LUMO transition. Moreover, the exciton binding energy of the different dyes, which is defined as the difference between the electrochemical and optical bandgaps, could not be determined [105,106], the dyes being not sufficiently soluble to determine with accuracy the positions of the HOMO and LUMO energy levels by electrochemistry. Based on the position of the ICT bands, the different electron acceptors could be classified according to the order presented in Scheme 2. Among all electron acceptors, EA7 and EA5 were determined as exhibiting the highest electron-withdrawing abilities of the series of 15 electron acceptors, consistent with the ordering previously reported in the literature [33]. The weakest electron acceptors were determined as being EA1 and EA2 [107,108]. Notably, EA1 is rarely used for the design of push-push dyes due to the weak electronic delocalization that this electron acceptor involves [87,[109][110][111].

Solvatochromism
Despite the polyaromatic nature of the electron donor and the low solubility of the different push-pull dyes in numerous solvents, the solvatochromism of the fifteen dyes could, however, be examined in a wide range of solvents differing by their polarities. A summary of the optical properties of the fifteen dyes, Dye 1-Dye 15, is provided in Table  3. Solvatochromism corresponds to a charge redistribution upon excitation, and, in this

Photoluminescence Properties
The photoluminescence of all dyes was examined in dichloromethane and in DMF and the different results are summarized in Table 1. As shown in Figure 10, major differences could be determined for the different dyes. Noticeably, the most blue-shifted emission was determined for Dye 1, bearing the weakest electron acceptor (λ max = 461 nm), whereas the most-redshifted absorption was found for Dye 14, bearing EA14 as the electron acceptor. As shown in Figure 10, an emission extending until the near-infrared range was found for Dye 14. Following Dye 14, the second most red-shifted absorption was determined for Dye 6, comprising EA6. Interestingly, the largest Stokes shift was determined for Dye 14, being 152 nm. Indeed, for this dye, an absorption located at 507 nm and an emission at 659 nm was found in DMF. If this Stokes shift is important, it remains, however, lower than that reported for 6-pentafluorostyryl-1-dimethylaminopyrene, recently reported in the literature and exhibiting a Stokes shift of 247 nm [93]. This exceptional value can be assigned to the presence of the dimethylamino group on pyrene, improving the electronic delocalization between pyrene and the electron acceptor. For the rest of the molecules, Stokes shifts ranging between 80 and 100 nm could be found, except for Dye 7, for which a Stokes shift of only 14 nm was calculated. Dye 14 thus constitutes an excellent candidate as a fluorescent probe for various applications, in light of the large Stokes shift detected for this dye. It has to be noted that comparison of the Stokes shift obtained in dichloromethane and DMF revealed the Stokes shift to be more important in the more polar solvent [94]. This trend is consistent with the previous works reported in the literature mentioning the higher sensitivity of the emission of pyrene-based dyes to the solvent polarity than the absorption and reporting a red shift of the emission maximum with the solvent polarity, which is also observed in this study.

Thermal Properties
For numerous applications, the thermal stability of dyes is an important parameter, notably for applications such as solar cells [139][140][141][142]. The thermal properties of the different dyes were examined by thermal gravimetry analyses, and a summary of the decomposition temperatures is given in Table 4 and in Figure 11. Noticeably, despite the presence of the same electron-donating group, major differences could be determined concerning the decomposition temperatures of Dye 1-Dye 15. The lowest one was determined for Dye 3, at 176 • C. Conversely, the highest one was found for Dye 15, determined at 400 • C. Only three dyes showed decomposition temperatures lower than 300 • C, evidencing their remarkable thermal stability.
Stokes shift detected for this dye. It has to be noted that comparison of the Stokes shift obtained in dichloromethane and DMF revealed the Stokes shift to be more important in the more polar solvent [94]. This trend is consistent with the previous works reported in the literature mentioning the higher sensitivity of the emission of pyrene-based dyes to the solvent polarity than the absorption and reporting a red shift of the emission maximum with the solvent polarity, which is also observed in this study.

Thermal Properties
For numerous applications, the thermal stability of dyes is an important parameter, notably for applications such as solar cells [139][140][141][142]. The thermal properties of the different dyes were examined by thermal gravimetry analyses, and a summary of the decomposition temperatures is given in Table 4 and in Figure 11. Noticeably, despite the presence of the same electron-donating group, major differences could be determined concerning the decomposition temperatures of Dye 1-Dye 15. The lowest one was determined for Dye 3, at 176 °C. Conversely, the highest one was found for Dye 15, determined at 400 °C. Only three dyes showed decomposition temperatures lower than 300 °C, evidencing their remarkable thermal stability.

General Information
All reagents and solvents were purchased from Aldrich, Alfa Aesar or TCI Europe and used as received, without further purification. Mass spectroscopy was performed by

General Information
All reagents and solvents were purchased from Aldrich, Alfa Aesar or TCI Europe and used as received, without further purification. Mass spectroscopy was performed by the Spectropole of Aix Marseille University. ESI mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis apparatus driven by the Eager 300 software. 1 H and 13 C NMR spectra (More details could be found in Supplementary Materials) were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1 H (400 MHz) and 13 C (100 MHz). The 1 H chemical shifts were referenced to the solvent peak CDCl 3 (7.26 ppm) and the 13 C chemical shifts were referenced to the solvent peak CDCl 3 (77 ppm).
Thermal properties of the different dyes were investigated by using a TA thermal analyzer (TA Instrument Q50) at a heating rate of 20 • C/min under argon flow. The temperature of thermal degradation (T d ) was measured at the point of 5% weight loss. UV-visible absorption spectra were recorded on a Varian Cary 60 UV-vis spectrophotometer with a concentration of 5 × 10 −3 M, corresponding to diluted solutions. Fluorescence spectra were recorded on a Jasco spectrofluorometer FP-8350. Melting points were determined with a Buchi Melting Point M-560 at a scan rate of 10 • C/min by varying the temperature between 80 and 350 • C.

Synthesis of the Dyes
General procedure for the synthesis of all dyes (except Dye 5 and Dye 15): 1-pyrenecarbaldehyde (2 g, 8.68 mmol) and the appropriate electron acceptor (8.68 mmol, 1 eq.) were dissolved in absolute ethanol (50 mL). A few drops of piperidine were added. Immediately, the solution's color changed. The solution was refluxed and monitoring of the reaction progress was carried out by thin layer chromatography (TLC). After cooling, the solution was concentrated under reduced pressure. Addition of pentane precipitated a solid, which was filtered off and dried under vacuum.

Conclusions
To conclude, a series of fifteen dyes based on pyrene as the electron donor have been designed and synthesized. Noticeably, all dyes could be prepared using standard synthetic procedures, enabling us to obtain the different compounds in reasonable yields. Examination of their solvatochromic properties in 22 solvents of different polarities revealed the most soluble dyes to exhibit a positive solvatochromism. On the contrary, for the less soluble dyes, the opposite trend was found, and a negative solvatochromism was detected for these dyes. This unexpected behavior was assigned to the formation of aggregates of different sizes and shapes in solution, the formation of hydrogen bonds between molecules for all dyes possessing electron-withdrawing groups comprising NH or C=O groups affecting in turn the examination of their solvatochromic properties, as the isolated molecules were concomitantly present with aggregates and hydrogen-bonded molecules. All dyes also showed high thermal stability, and a decomposition temperature higher than 300 • C was determined for most of the dyes. Considering that all dyes strongly absorb in the visible range and that pyrene derivatives are excellent photoinitiators of polymerization, future works will consist of examining their photoinitiating abilities during the free radical polymerization of acrylates upon visible light irradiation.  Table S3. Solventindependent correlation coefficient s of the Kamlet-Taft parameters π*; Table S4. Solvent-independent correlation coefficients a and b of the Catalan parameters SdP and SPP; Table S5. Energy levels of the main orbitals for dyes Dye 1-Dye 15; Table S6