**Guillaume Noirbent \* and Frédéric Dumur \***

Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France

**\*** Correspondence: guillaume.noirbent@outlook.fr (G.N.); frederic.dumur@univ-amu.fr (F.D.); Tel.: +33-(0)4-9128-2748 (F.D.)

Received: 11 November 2018; Accepted: 29 November 2018; Published: 30 November 2018

**Abstract:** Push–pull dyes absorbing in the visible range have been extensively studied so that a variety of structures have already been synthesized and reported in the literature. Conversely, dyes absorbing in the near and far infrared region are more scarce and this particularity relies on the following points: difficulty of purification, presence of side-reaction during synthesis, low availability of starting materials, and low reaction yields. Over the years, several strategies such as the elongation of the π-conjugated spacer or the improvement of the electron-donating and accepting ability of both donors and acceptors connected via a conjugated or an aliphatic spacer have been examined to red-shift the absorption spectra of well-established visible dyes. However, this strategy is not sufficient, and the shift often remains limited. A promising alternative consists in identifying a molecule further used as an electron-accepting group and already presenting an absorption band in the near infrared region and to capitalize on its absorption to design near and far infrared absorbing dyes. This is the case with poly(nitro)fluorenes that already exhibit such a contribution in the near infrared region. In this review, an overview of the different dyes elaborated with poly(nitro)fluorenes is presented. The different applications where these different dyes have been used are also detailed.

**Keywords:** fluorene; nitrofluorene; Knoevenagel reaction; near infrared absorption; push–pull chromophore; poly(nitro)fluorene
