# A Palette of Efficient and Stable Far-Red and NIR Dye Lasers
## NIR-Emitting Polyarylated Aza-BODIPYs
Nevertheless, the most outstanding result was the excellent photostability shown by these dyes (up to 44 GJ/mol, Figure 6). These _F_-aza-BODIPYs showed a noticeable four-fold enhancement with respect to commercial laser dyes emitting in the same spectral region, such as the oxazine dye called Nile Blue [55], while retaining similar laser efficiencies (around 30% in Nile Blue). In other words, these _F_-aza-BODIPYs display efficient and long-lasting laser signals, drastically improving the photostability of commercially available NIR laser dyes. Figure 5: Absorption, fluorescence, and laser (filled) spectra of two representative _F_-aza-BODIPYs (**NIR1** and **NIR4**) in ethyl acetate. Inset: corresponding contour maps (HOMO and LUMO) of the frontier orbitals involved in the electronic transition. Trying to ameliorate both laser parameters, efficiency and photostability, we carried out structural modifications at the boron atom rendering the corresponding \(C\)- and \(O\)-aza-BODIPYs (Table 1). As a common rule in BODIPYs, the replacement of fluorine atoms (\(F\)-BODIPYs) by aloxys (\(O\)-BODIPYs) or alkyls (\(C\)-BODIPYs) improved the photonic response [56; 57; 58]. Thus, we developed the corresponding \(O\)- and \(C\)-aza-BODIPYs, replacing the fluorine atoms at the boron center with electron acceptor trifluoroacetoxy and cyane moieties [48], respectively (Table 1). According to previous results in simpler BODIPYs, both substituents were the most suitable to improve the photonic performance [56; 57]. Such chemical modification has a minor impact on the spectral band positions (overall a slight bathochromic shift) since the boron atom does not take part in the dipyrrin aromatic framework [48]. However, such substitution has a marked impact on the fluorescence efficiency, and it depends on the peripheral substitution pattern of the dipyrrin (Table 1). Thus, in the \(O\)-aza-BODIPYs derived from **NIR2** and **NIR3**, a clear enhancement (four-fold) of the fluorescence response was recorded. The fluorescence response of the \(O\)-aza-BODIPY derived from **NIR4** remained almost the same as that of the corresponding \(F\)-aza-BODIPYs. Moreover, the \(C\)-aza-BODIPYs did not improve the fluorescence response shown by the \(O\)-aza-BODIPYs, and **NIR2** was better than the corresponding \(F\)-aza-BODIPY. Conversely, such improved fluorescence was not reflected in a marked increase of the lasing efficiency. Improved laser efficiencies were only recorded in the derivatives of **NIR2** and **NIR4**. Additionally, the photostability of the \(O\)- and \(C\)-aza-BODIPYs clearly dropped (two- or three-fold depending of the dye, Figure 6) with regard to the parent \(F\)-aza-BODIPYs. Moreover, we observed that the \(C\)-aza-BODIPYs and especially the \(O\)-aza-BODIPYs were chemically unstable, and the solution lost its color with aging time. Likely, these two factors, low chemical and photochemical stability, could explain the existing mismatch between the fluorescence and laser efficiencies. Such low stability was strengthened under strong pumping in the laser resonator cavity.