**5. Intramolecular Charge Transfer Complexes in Metal-Based Dyes**

Organic electron donors were not the only ones to be used for the preparation of push–pull chromophores comprising poly(nitro)fluorene acceptors. Ferrocene was also examined as a potential donor and influence of the length as well as the nature of the spacer introduced between the two fragments was studied (See Figure 21) [100].

**Figure 21.** Push–pull chromophores **233**–**250** based on ferrocene.

For the series of metal complexes **233**–**250**, an absorption band ranging from 450 to 900 nm could be detected. These different complexes being designed for Non-Linear Optical (NLO) applications and in order to facilitate the determination of the quadratic hyperpolarizability by the electric field induced second harmonic (EFISH) generation, a soluble version of each complex was synthesized, namely, **235**, **<sup>238</sup>**, **<sup>241</sup>**, **<sup>244</sup>**, **<sup>247</sup>** and **<sup>250</sup>**. Going from **<sup>235</sup>** to **<sup>250</sup>**, the μβ values increased from 100 × <sup>10</sup>−<sup>48</sup> esu for **<sup>235</sup>** to 200 × <sup>10</sup>−<sup>48</sup> esu for **<sup>238</sup>**, 800 × <sup>10</sup>−<sup>48</sup> esu for **<sup>241</sup>** and 2400 × <sup>10</sup>−<sup>48</sup> esu for **<sup>250</sup>**. Evolution of the hyperpolarizability is consistent with the elongation of the π-conjugated spacer in these structures. While examining the optical properties, two ICT could be detected for all compounds. Only a minor variation of the maximum absorption was observed with the spacer (See Table 10).


**Table 10.** Summary of the optical properties of the different dyes recorded in 1,2-dichloroethane.

Finally, in 2001, the same authors investigated the design of A-D-A triads **233**, **251**–**258** where ferrocene or a ruthenium complex was used as the donor (See Figure 22) [101]. In these series, the higher electron donating ability of ferrocene compared to the ruthenium complex was demonstrated, a red-shift of the ICT band being detected for **253** (622 nm in 1,2-dichloroethane) compared to **256** (521 nm in 1,2-dichloroethane). Comparison of the UV-visible spectra of the triads **253**–**255** with the diads **233**, **251**, **252** revealed the position of the ICT bands to remain almost unchanged. Face to these considerations and considering the difficulty to synthesize the triads, it can be concluded that the synthesis of the diads is sufficient and that the connection of the donor to an additional electron acceptor can't improve its electron-donating ability irrespective of the electron-withdrawing ability of the acceptor.

**Figure 22.** Push–pull chromophores based on metallocenes (iron and ruthenium complexes).

#### **6. Conclusions**

In this review, an overview of the different dyes designed with the fluorene scaffold substituted with three or four nitro groups is presented. As detailed in this review, organic dyes have focused the main interest of researchers, but metal-based chromophores have also been investigated. If poly(nitro)fluorene acceptors constitute a unique class of acceptor by their inherent absorption in the near infrared region, the presence of numerous nitro group drastically limits the solubility of the resulting push–pull dyes and numerous efforts have been devoted to overcome this drawback. At present, the scope of applicability of these structures is quite limited, since the compounds have mostly been investigated as sensitizers for hologram recording, and more scarcely for NLO or electrochromic applications. With regards to the recent interest of compounds absorbing in the near infrared region (photopolymerization, dyes for photovoltaics applications, etc.), clearly, the number of applications involving the use of poly(nitro)fluorene structures will greatly expand in the future.

**Author Contributions:** Writing-Original Draft Preparation, G.N. and F.D.; Writing-Review & Editing, G.N. and F.D.

**Funding:** This research was funded by Aix Marseille University and the Centre National de la Recherche Scientifique (CNRS).

**Conflicts of Interest:** The authors declare no conflict of interest.
