# A Palette of Efficient and Stable Far-Red and NIR Dye Lasers
## 2 Red-Emitting Benzofuran-Fused BODIPYs  
The laser dyes designed to cover the far-red region of the visible spectrum are based on a BODIFY core bearing fused benzofuran rings with different substitution patterns (fluorine, alkyls, linear alcovays, or additional fused rings) [47]. The corresponding molecular structures and main photophysical and laser data are summarized in Figure 3.  
The fusion of benzofuran rings at chromophoric positions 2 and 3 and their symmetrical positions 5 and 6 induces the expected bathochromic shift. In fact, the theoretically predicted molecular orbitals show that the \(\pi\)-system is extended through the whole molecule (Figure 4). The exception is the 8-aryl  
Figure 2: Molecular structure of the renowned boron-dipyrromethene (BODIPY) chromophore and the chemical modification carried out to push the spectral bands toward the red edge of the visible spectrum and NIR region. group, not taking part in the delocalized \(\pi\)-system owing to its twisted disposition [26]. The presence of electron donor moieties (**RED7**, **RED9**, and **RED11**) and mainly the further fusion of more aromatic groups (**RED8**) induce a more pronounced shift to the red edge, from 575 to 610 nm (Figures 3 and 4). Such spanning of the aromaticity is reflected in a remarkable enhancement of the absorption probability (molar absorption coefficient), being twice that recorded for simpler BODIPYs, and reaching values approaching 200,000 M\({}^{-1}\).cm\({}^{-1}\) (Figures 3 and 4) [47]. The corresponding fluorescence bands are located in a spectral window ranging from 595 to 625 nm. The spectral proximity between the absorption and fluorescence spectra (low Stokes shift) suggests a low geometrical rearrangement upon excitation (Figure 4). The cyclization of the substituents limits the conformational freedom, and these constrained geometries hinder non-radiative relaxation processes [49]. As a result, the recorded fluorescence efficiencies are high, surpassing 80% in most cases (regardless of the applied substitution pattern) and even approaching 100% in some cases (Figure 3). The exceptions to this behavior are due to an increase in the non-radiative deactivation processes induced by the substitution pattern. For instance, in **RED5**, the combination of electron donor benzofuran with electron formaldehyde at the _para_-8-aryl favors charge separation and reduces the emission capability [47]. In **RED8** and **RED11**, the geometrical tensions induced by the additional fused phenyls could promote the bending of the chromophore, thereby distorting the planarity and increasing the internal conversion [47]. The excellent photophysical properties of these dyes led to an outstanding laser performance in the far-red region. Strong laser emissions were recorded spanning the spectral region from 615 nm to  
Figure 3: Molecular structures of the benzofuran-fused BODIPYs and their corresponding main photophysical (at diluted solutions of 2 uM) and laser (at the optimal dye concentration ranging from 0.5 to 1.2 mM) data in ethyl acetate: absorption wavelength (\(\uplambda_{\text{ab}}\), in nm), molar absorption coefficient (\(\epsilon_{\text{max}}\), in 10\({}^{4}\) M\({}^{-1}\).cm\({}^{-1}\)), fluorescence wavelength (\(\uplambda_{\text{B}}\), in nm), quantum yield (\(\phi\)), laser wavelength (\(\uplambda_{\text{la}}\), in nm), and efficiency (% Eff). The laser emission of **RED11** could not be recorded due to lack of solubility at the required high concentration. 660 nm (Figure 4) with efficiencies approaching 50% (see **RED1-4**, **RED6-7**, and **RED9-10** in Figure 3). The lasing properties followed the photophysical trends. The bathochromic shift of the laser wavelength correlated with a shift of the fluorescence band with the substitution pattern (see **RED8** in Figure 3). Moreover, those dyes showing the lowest fluorescence efficiency displayed the poorest laser efficiency (see **RED5** and **RED8** in Figure 3).