# A Palette of Efficient and Stable Far-Red and NIR Dye Lasers
## 1 Introduction  
The design of stable and bright photoactive systems able to emit in the far-red (within 600-700 nm spectral window) and especially in the near-infrared (NIR, beyond 700 nm) spectral regions is an actively sought task in photonics [1; 2; 3; 4; 5; 6]. This kind of long-wavelength emission is an attractive radiation source for telecommunications, as well as for optics and electronics (diodes, filters, logic gates, and optoelectronic devices in general) [7; 8; 9; 10], owing to its low background interference with the surroundings which allows the light to travel long distances. They are also appealing as photosensitizers for solar cells because thegap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) matches the valence and conduction band gaps ofsemiconductors [11; 12]. It is noteworthy that, in the last few years, NIR luminophores were intensively and successfully applied in biotechnology and biomedicine [13; 14; 15; 16]. This kind of NIR radiation allows a deep penetration into tissues (up to 2-5 cm within the biological window, \(\lambda>650\) nm), avoids autofluorescence of the surrounding biomolecules, and has lower toxicity. These features are ideal to apply high-resolution fluorescence imaging (such as super-resolution microscopy) for non-invasive diagnosis, sensing, and therapeutic purposes. In this regard, dye chemistry is a suitable tool to develop NIR absorbers and emitters thanks to the chemical versatility afforded by the highly accessible organic synthetic routes [17; 18; 19; 20]. In particular, cyanines [21; 22], porphyrins [23], and phthalocyanines [24], all of them featuring highly conjugated \(\pi\)-systems, are the most tested organic fluorophores for this aim (Figure 1). However, these fluorophores feature some drawbacks for their optimal performance. They require tedious synthetic and purification procedures. They display low fluorescence efficiency, mainly in the cyanine-based dyes, owing to the non-radiative deactivation channels afforded by their conformationally flexible structures and push-pull character. Moreover, they usually show poor photostability owing to photobleaching processes [25], likely related to the low energy gap between the excited and ground state. Finally, the chemical versatility for postfunctionalization is limited in these molecular skeletons. To overcome such limitations, the design of a new generation of organic dyes endowed with absorption and emission bands within the biological window, easily accessed and derivatized, and displaying bright and long-lasting emissions is a highly demanded issue. In this regard, the renowned boron-dipyrromethene (BODIPY) dyes, which can be considered as the little sister of porphyrins or cyclic cyanines, drew the attention of researchers. Their boron-dipyrrin core is robust and stable (both chemically and photochemically) [26], and their photophysical signatures can be modulated by the substitution pattern [27]. Furthermore, BODIPYs can be easily accessed through many synthetic routes and straightforwardly functionalized to deeply shift their spectral bands toward the red edge of the electromagnetic spectrum [28]. In the literature, several chemical modifications were reported to this aim [29]. The main strategies are the following:  
1. The induction of intramolecular charge transfer (ICT) phenomena in BODIPYs via the grafting of electron donor and acceptor moieties (push-pull dyes) [30; 31]. The emission from the ICT is strongly shifted to longer wavelengths. However, it is usually very weak and very sensitive to the environmental conditions. 2. The extension of the delocalized \(\pi\)-system through aromatic frameworks. To this aim, aromatic rings are annulated at the dipyrrin core, leading to conformationally restricted BODIPYs [32; 33] or directly connected to the core via single bonds [34; 35].