Tailoring the Molecular Skeleton of Aza‐BODIPYs to Design Photostable Red‐Light‐Emitting Laser Dyes

Abstract

In this article the design and characterization of a set of novel red‐light‐emitting laser aza‐BODIPY dyes is reported. The applied synthetic method allows an exhaustive and versatile functionalization of both the dipyrrin core and the boron bridge. From the analysis of the photophysical and laser signatures, we determine the suitable modifications of the chromophoric backbone necessary to modulate the emission spectral region, efficiency and photostability under a strong irradiation regime. These dyes are endowed with efficient fluorescence and laser emission, and are particularly outstanding in terms of their high photostability, a key parameter to guarantee long‐lasting emission in any (bio)technological application. The herein‐reported results support, for the first time, the viability of aza‐BODIPYs as tunable red laser dyes. In fact, the laser performances of some of the tested aza‐BODIPYs surpass those of commercially available laser dyes in the same spectral region.

Red/NIR Thermally Activated Delayed Fluorescence from Aza‐BODIPYs

“…As it was expected, the new cassettes based on robust building blocks exhibited good photostability since up to 160 GJ mol −1 were required for inducing a 10 % decrease of its LIF emission (Table 2). Energy transfer from the peripheral BODIPYs to the central red‐emitting aza‐BODIPY reduced the rate and extension of the photodegradation processes and significantly enhanced the photostability of the multichromophoric systems with respect to the own monomeric aza‐BODIPY 8 , as well as other monomeric commercial laser dyes with emission in the same spectral region [36] . This enhanced photostability was widespread to all the cassettes herein developed regardless of the acting EET, ICT and even PET mechanism sustained by its molecular design.…”

“…Despite its low fluorescence quantum yield, the new dyes showed an unexpected high laser efficiency (up to 25 %), which was even higher than that exhibited by the monomeric aza‐BODIPY 8 pumped at its maximum absorption wavelength [36] . This apparent mismatch can be rationalized taking into account the EET and ICT character of the emitting states in these cassettes leading to: ( i ) a high Stokes shift (up to 700 cm −1 ), which reduced the extension of re‐absorption/re‐emission processes and, thus, their deleterious effect in the laser action; ( ii ) a very short lifetime (below 0.6 ns), which ameliorated the population inversion and enhanced the stimulated emission probability, counterbalancing the low probability of spontaneous emission; and ( iii ) a high dipole moment (four‐ and six‐fold higher than the isolated donor and acceptor fragments, respectively), allowing the molecular alignment with respect to the polarization of the exciting laser beam to enhance the emission efficiency of the media [52] .…”

“…To overcome the low absorption of 8 at these laser pump wavelengths highly concentrated solutions (up to 5‐fold higher than those required for the cassettes) were needed. As it was mentioned above, this experimental issue had a deleterious effect on the laser action and, consequently, a 3‐fold decrease in the lasing efficiency and photostability of 8 were recorded with respect to the values achieved pumping it at 654 nm [36] …”

“…Therefore, cassette 2 b , which molecular design avoided deleterious effects (PET and internal conversion) on the emission quantum yield, stands out as NIR‐emitter owing to both its more extended delocalized π‐system and its high fluorescence efficiency, which became even 2‐fold higher than that recorded from its isolated aza‐BODIPY precursor 8 (Table 1). [36] …”

“…These effects can be enlarged through the peripherical arylation of the aza‐BODIPY chromophoric core [32–34] . Recently, efficient and long‐lasting fluorescent and laser emission in the target red/NIR spectral region have been recorded from polyarylated aza‐BODIPYs under drastic pumping conditions [36] …”