**1. Introduction**

The reciprocal influence of an electron-donating and an electron-accepting group is capable to give rise to structures with fascinating properties. Optically, the first manifestation of this mutual interaction is a color change when the two partners are mixed, resulting in the presence of an additional absorption band which is not detectable in the absorption spectra of the initial partners considered separately. This absorption band corresponds to the charge transfer (CT) interaction where part of the electronic density is transferred from the electron-rich to the electron-deficient partner. Depending on the fact that the two moieties are (or not) connected to each other, this CT transition can be intermolecular or intramolecular. Historically, intermolecular charge transfer complexes have been extensively studied, focused on the metallic conductivity resulting from the grinding of two insulating organic compounds, i.e., tetrathiafulvalene (TTF) and tetracyano-quinodimethane (TCNQ) [1]. Following this pioneering work, the possibility to develop superconductors at low temperatures has driven extensive research efforts, and the emergence of the Bechgaard salts [2]. If these structures are attractive for their electrical properties, the resulting charge transfer complex renders these structures insoluble, drastically limiting the scope of applicability. Conversely, materials exhibiting an intramolecular charge transfer (ICT) i.e.,

meaning that the electron donor is connected to the electron acceptor by mean of a spacer (aliphatic or conjugated) are not necessarily insoluble so that these structures found widespread applications ranging from dye-sensitized solar cells (DSSCs) [3], nonlinear optical applications [4,5], solvatochromic probes [6–8], photochromes [9], single component semiconductors [10–12], electrochromes [13,14], and piezochromes [15]. Typically, these compounds are composed of a π-conjugated system with an electron-donating, and an electron-accepting part connected at both sides of the conjugated spacer [16–19]. If the number of groups used as electron donors cannot be calculated anymore in regards to the diversity of structures, the number of electron acceptors that can be covalently linked to electron donors is more limited, as exemplified by the list of molecules 1–17 presented in the Figure 1. Notably, malononitrile **1** [20], indanedione derivatives **2** [21], (thio)barbituric derivatives **3** [22], Meldrum derivatives **4** [23], pyridinium **5** [24], methyl-containing tricyanofurans **6** [25], substituted tricyanopropenes **7** [26], pyran derivatives **8** and **9** [27,28], 1,1,3-tricyano-2-substituted propenes **10** [29], isoxazolones **11** [30], hydantions and rhodanines **12** [31], pyrazines **13** [32], dicyanoimidazoles **14** [33], benzo[*d*]thiazoliums **15** [34], benzo[*d*]imidazoliums **16** [35], and dicyanovinyl-thiophen-5-ylidenes **17** [36] can be cited as the most common acceptors.

**Figure 1.** Chemical structures of common electron acceptors.

For all these acceptors and irrespective of the electron donors, the intramolecular charge transfer band is centered in the visible range and this latter can be displaced towards the near infrared region if the length of the π-conjugated spacer introduced between the two partners is extended as much as possible. However, the extension of the π-conjugated spacer is often a hard work from a synthetic point of view and alternatives are actively researched [37]. Considering that for numerous applications, an absorption in the near-infrared region is required and face to the fact that all the above-mentioned electron acceptors are not sufficiently electro-deficient to inherently position the charge transfer band in the near infrared (NIR) region without taking recourse to the elongation of the π-conjugated spacer, electron acceptors already displaying an absorption band in the NIR region are actively researched. In this field, and to the best of our knowledge, only (poly)nitrofluorenes such as dinitrofluorene **19** [38], trinitrofluorene **20** and tetranitrofluorene (TNF) **21** exhibit such an absorption band (see Figure 2). It has to be noticed that the absorption band detected in the NIR for **19**–**21** is not detectable in **18**. Based on this unique property, numerous push–pull dyes displaying an absorption peak in the NIR region have been designed. If (polynitro)fluorenone derivatives have been extensively studied for the design of intermolecular charge transfer complexes [39], the presence of the carbonyl function in

fluorenone totally impede this structure to be used for the design of push–pull molecules by connecting the electron withdrawing/releasing groups to the carbon inserted between the two aromatic rings. Fluorenone derivatives will not be discussed in this review (see Figure 3). Indeed, by the presence of the activated CH2 group standing between the two aromatic rings, poly(nitro)fluorene derivatives constitute candidates of choice for the synthesis in one step of push–pull molecules by a Knoevenagel reaction, when opposed to an electron donor comprising an aldehyde function (see Figure 3).

**Figure 2.** UV-visible absorption spectra of **18**–**21** in *N*,*N*-dimethylformamide (DMF).

As an interesting feature, when substituted with electron-withdrawing groups such as nitro groups, the methylene group standing between the two aromatic rings is sufficiently acid so that the Knoevenagel reaction on **20** and **21** can be carried out without any base, in non-toxic and polar solvents such as *N*,*N*-dimethylformamide (DMF). Principles of Green Chemistry can thus be applied to the synthesis of push–pull molecules (see Figure 3). Conversely, the methylene group in **18** and **19** is often not sufficiently activated to allow the Knoevenagel reaction to be carried out without a base and piperidine or pyridine are classically used.

**Figure 3.** Chemical structures of fluorene and fluorenone.

In this review, an overview of the different push–pull structures comprising (poly)nitrofluorenes as electron acceptors is presented. More precisely, four different aspects will be detailed. First, numerous push–push dyes have been obtained by functionalization of the 9-position of the fluorene acceptor. Second, a series of dyes has also been obtained by nucleophilic substitution of secondary amines on cyano-substituted fluorenes and this second approach constitutes the second most widely used synthetic procedure to access to poly(nitro)fluorene-based dyes. Third, the conceptual unimolecular rectifier proposed by Aviram and Ratner has been a source of inspiration for the design of numerous chromophores and a series of dyes has been designed based on this proposed structure. Finally, parallel to organic donors, organometallic donors have also been investigated, and it constitutes the last part of this review. To end, and parallel to the synthesis and the examination of the different photophysical properties of the dyes, the different applications justifying the design of these structures will be detailed.
