**1. Introduction**

During the past decades, push-pull molecules have attracted much attention due to their numerous applications ranging from non-linear optical (NLO) applications [1,2] to organic photovoltaics (OPVs) [3,4], organic field effect transistors (OFETs) [5], organic light-emitting diodes (OLEDs) [6], photorefractive applications [7], colorimetric pH sensors [8], ions detection [9], biosensors [10], gas sensors [11], or photoinitiators of polymerization [12–16]. Typically, push-pull chromophores are based on an electron donor and an electron acceptor connected to each other by mean of a saturated or a conjugated spacer [17]. As the first manifestation of the mutual interaction between the two partners, a broad absorption band corresponding to the intramolecular charge transfer (ICT) interaction can be detected in the visible to the near-infrared region. Position of this absorption band typically depends on the electron-donating ability of the donor and the electron-releasing ability of the acceptor, and a bathochromic shift of this transition is observed upon improvement of the strength of the donor and/or the acceptor. This ICT band can even be detected in the near and far infrared region for electron-acceptors based on poly(nitrofluorenes) [18]. Among electron acceptors, indane-1,3-dione (1*H*-indene-1,3(2*H*)-dione) **EA1** has been extensively studied due to its commercial

availability, its low cost and the possibility to design push-pull dyes by a mean of one of the simplest reaction of Organic Chemistry, namely the Knoevenagel reaction [19–23]. By chemical engineering, its electron-withdrawing ability can be drastically improved by condensation of one or two malononitrile units under basic conditions, furnishing 2-(3-oxo-2,3-dihydro-1*H*-inden-1-ylidene)malononitrile **EA2** [24] and 2,2'-(1*H*-indene-1,3 (2*H*)-diylidene)dimalononitrile **EA3 [25]** (see Figure 1). Based on the strong electron-withdrawing abilities of **EA2** and **EA3**, push-pull dyes with non-fullerene acceptors and small energy gaps have been developed [26,27]. As a possible alternative to improve the electron-accepting ability, extension of the aromaticity of the acceptor can be envisioned. Using this strategy, a red-shift of the ICT band combined with an enhancement of the molar extinction coefficient can be both obtained for these new push-pull derivatives, relative to that obtained with the parent electron acceptor.

**Figure 1.** Electron acceptors **EA1–EA3**.

In this article, an extended version of the well-known indane-1,3-dione, i.e., 1*H*-cyclopenta[*b*] naphthalene-1,3(2*H*)-dione **EA4**, has been used for the design of ten push-pull dyes. It has to be noticed that even if the synthesis of **EA4** is reported in the literature since 2006 [28], only one report mentions the development of anion sensors with **EA4** in 2013 [29] and the first push-pull dyes have been developed in 2017 for photovoltaics applications [30,31]. Considering the scarcity of studies devoted to this electron acceptor, a series of 10 push-pull chromophores **PP1**–**PP10** have been developed with **EA4**. To evidence the contribution of the additional aromatic ring in **EA4** relative to that of **EA1**, 10 dyes **PP11**–**PP20** comprising **EA1** as the electron acceptor have been synthesized for comparison (see Figure 2).

**Figure 2.** Chemical structures of the 20 dyes **PP1–PP20** examined in this study. **PP1–PP10** were made from **EA4**, while **PP11–PP20** were made from **EA1**.

The photophysical properties of the series of 20 dyes as well as their electrochemical properties have been investigated. To support the experimental results, theoretical calculations have been carried out. Finally, the solvatochromic properties have also been examined.

## **2. Materials and Methods**

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. Electron spray ionization (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. 1H and 13C nuclear magnetic resonance (NMR) spectra were determined at room temperature in 5 mm outer diameter (o.d.) tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1H (400 MHz) and 13C (100 MHz). The 1H chemical shifts were referenced to the solvent peak CDCl3 (7.26 ppm) and the 13C chemical shifts were referenced to the solvent peak CDCl3 (77 ppm). UV-visible absorption spectra were recorded on a Varian Cary 50 Scan UV Visible Spectrophotometer, with concentration of 5 <sup>×</sup> 10−<sup>3</sup> M, corresponding to diluted solutions. Fluorescence spectra were recorded using a Jasco FP 6200 spectrometer. The electrochemical properties of the investigated compounds were measured in acetonitrile by cyclic voltammetry, scan rate 100 mV·s<sup>−</sup>1, with tetrabutylammonium tetrafluoroborate (0.1 M) as a supporting electrolyte in a standard one-compartment, three-electrode electrochemical cell under an argon stream using a VSP BioLogic potentiostat. The working, pseudo-reference and counter electrodes were platinum disk (Ø = 1 mm), Ag wire, and Au wire gauze, respectively. Ferrocene was used as an internal standard, and the potentials are referred to the reversible formal potential of this compound. Computational details: All quantum mechanical calculations were computed using Gaussian Package [32]. All geometry optimizations were performed using the density functional theory (DFT) with the global hybrid exchange-correlation functional B3LYP [33] and all minima on the potential energy surface were verified via a calculation of vibrational frequencies, ensuring no imaginary frequencies were present. The Pople double-zeta basis set with a double set of polarization functions on non-hydrogen atoms (6-3111G(d,p))[34,35] was used throughout. This computational approach was chosen in consistency with previous works, as it provides good agreement with experimental data. Excited states were probed using the time dependent density functional theory (TD-DFT) using the same function. All transitions (singlet-singlet) were calculated vertically with respect to the singlet ground state geometry. Solvent effects were taken into account by using the implicit polarizable continuum model (PCM) [36,37]. Dichloromethane (DCM) where chosen in analogy with the experiments. Computed spectra were simulated by convoluting each transition with Gaussians functions-centered on each absorption maximum using a constant full width at half maximum (FWHM) value of 0.2 eV. The assignment of electronic transitions for λmax has been determined with GaussSum 3.0 software [38]. 4-(Dodecyloxy)benzaldehyde **D1 [39]**, 3,4-dibutoxy-benzaldehyde **D2 [40]**, 2,4-dibutoxy-benzaldehyde **D3** [41], 4-(diphenylamino)benzaldehyde **D6 [42]**, 4-(*bis*(4-bromophenyl)-amino)benzaldehyde **D7** [43], and 9-methyl-9*H*-carbazole-3-carbaldehyde **D8 [44],** 3-(4-(dimethylamino)phenyl)acrylaldehyde **D9** [45], and 3,3-*bis*(4-(dimethylamino)phenyl)-acrylaldehyde) **D10** [46] were synthesized, as previously reported, without modifications and in similar yields.

#### *2.1. Synthesis of the Dyes*

## 2.1.1. 2-Butoxy-4-(Diethylamino)Benzaldehyde **D5**

A mixture of 4-(diethylamino)salicylaldehyde (26.5 g, 137 mmol, M = 193.24 g/mol), 1-bromobutane (21.8 g, 159 mmol) and potassium carbonate (18.9 g, 152.2 mmol) were heated in*N,N*-dimethylformamide (DMF) (250 mL) at reflux overnight. The solution was concentrated before addition of water. Extraction of the product was carried out with diethyl ether. The ethereal solution was washed with several portions of water to remove remaining DMF, dried over MgSO4 and concentrated to yield the product as an orange oil (32.1 g, 128.73 mmol, 94% yield). 1H nmR (CDCl3) δ: 0.91 (t, 3*H*, J = 7.4 Hz), 1.14 (t, 6*H*, J = 7.1 Hz), 1.42–1.48 (m, 2*H*), 1.71–1.78 (m, 2*H*), 3.34 (q, 4*H*, J = 7.1 Hz), 3.96 (t, 2*H*, J = 6.3 Hz), 5.95 (d, 1*H*, J = 2.3 Hz), 6.19 (dd, 1*H*, J = 9.0 Hz, J = 2.3 Hz), 7.63 (d, 1*H*, J = 9.0 Hz), 10.1 (s, 1*H*, CHO); 13C nmR (CDCl3) δ: 12.6, 13.8, 19.3, 31.2, 44.7, 67.8, 93.2, 104.2, 114.4, 130.1, 153.8, 163.9, and 187.1; HRMS (ESI MS) m/z: theor: 249.1729 found: 249.1733 ([M]+. detected).
