# A Palette of Efficient and Stable Far-Red and NIR Dye Lasers
## NIR-Emitting Polyarylated Aza-BODIPYs  
To induce a more pronounced shift of the laser emission to longer wavelengths, we focused on aza-BODIPYs bearing a \(\pi\)-extended delocalize framework through peripheral aromatic substitutions at the dipyrrin core (Table 1) [48]. The replacement of the carbon at position 8 by an aza group is a simple but effective way to induce the desired bathochromic shift without enlarging the molecular size. The electronegativity of the nitrogen at such a chromophoric position induces a preferred stabilization of the LUMO state, reducing the energy gap [44]. Such a red shift can be reinforced through peripheral substitutions with aryl groups and electron-rich moieties to extend the \(\pi\)-system and reach emission in the NIR region. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **R** & \(\lambda_{\text{ab}}\) **(nm)** & \(\epsilon_{\text{max}}\)**-\(10^{-4}\)** & \(\lambda_{\text{ff}}\) **(nm)** & \(\Phi\) & \(\lambda_{\text{la}}\)**(nm)** & **\% Eff** \\ \hline \multirow{4}{*}{\(\phi\)} & F & 614 & 3.2 & 639 & 0.32 & 647 & 13 \\  & CN & 604 & 5.5 & 638 & 0.22 & 644 & 13 \\ \hline \multirow{4}{*}{\(\phi\)} & F & 645 & 8.6 & 668 & 0.12 & 681 & 16 \\  & OCOCF\({}_{3}\) & 646 & 5.6 & 671 & 0.47 & 689 & 19 \\  & CN & 652 & 7.3 & 670 & 0.46 & 686 & 12 \\ \hline \multirow{4}{*}{\(\phi\)} & F & 654 & 8.6 & 682 & 0.15 & 690 & 20 \\  & OCOCF\({}_{3}\) & 655 & 6.7 & 687 & 0.44 & 696 & 16 \\  & CN & 654 & 7.3 & 677 & 0.12 & 692 & 16 \\ \hline \multirow{4}{*}{\(\phi\)} & F & 669 & 7.8 & 696 & 0.46 & 708 & 23 \\  & OCOCF\({}_{3}\) & 678 & 7.9 & 708 & 0.49 & 720 & 33 \\ \cline{1-1}  & CN & 667 & 5.6 & 703 & 0.31 & 717 & 29 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Molecular structure of the NIR aza-BODIPYs and their corresponding main photophysical (at diluted solutions of 2 uM) and laser (at the optimal dye concentration ranging from 0.25 mM to 1 mM) data in ethyl acetate: absorption wavelength (\(\lambda_{\text{ab}}\)), molar absorption coefficient (\(\epsilon_{\text{max}}\)), fluorescence wavelength (\(\lambda_{\text{ha}}\)), quantum yield (\(\phi\)), laser wavelength (\(\lambda_{\text{ha}}\)), and efficiency (% Eff). The corresponding O-aza-BODIPY in **NIR1** could not be isolated owing to its chemical instability. In fact, in _F_-aza-BODIPYs, the spectral bands, both absorption and fluorescence, shift progressively to longer wavelengths (Table 1 and Figure 5) as the number of appended unrestricted phenyls increases (from **NIR1**, with two peripheral phenyls, to **NIR2** with four rings), and even further upon functionalization of the peripheral aryls with electron donor groups (like methoxy in **NIR4**). The fluorescence response of the _F_-aza-BODIPYs was lower (around 45%, Table 1) than that recorded for the conformationally restricted BODIPYs (see section with values up to 95% in Figure 3). This was expected in terms of the energy gap law [54]. A lowering of the energy gap enhances the vibrational coupling of the involved electronic states and, hence, the internal conversion probability. Nevertheless, fluorescence efficiencies of around 45% are notable in the NIR region. Such emission depends mainly on the conformational freedom of the aryls at positions 1 and 7 of the chromophoric core. Thus, the fluorescence of **NIR2** and **NIR3** decreased to 12%, whereas the locking of the free motion of such 1,7-rings (sterically hindered mesityl group instead of phenyl) implied a recovery of the fluorescence signal (up to 46% in **NIR4**, Table 1). In this regard, **NIR4** is the most appealing dye among the tested ones owing to its high fluorescence efficiency at longer wavelength [48]. All these dyes showed laser bands tunable from 650 to 700 nm, in good correlation with the evolution of the fluorescence band position depending on molecular structure (Figure 5). The laser efficiencies ranged from 13% to 23%, with **NIR4** again being the dye with the best laser performance in agreement with the photophysical signatures.