Abstract
In the field of fluorescent dyes, difluoroboron-dipyrromethenes (BODIPY) have a highly respected position. To predict their photophysical properties prior to synthesis and therefore to successfully design molecules specifically for one’s needs, a solid structure–function understanding based on experimental observations is vital. This work delivers a photophysical evaluation of BODIPY and aza-BODIPY derivatives equipped with different electron-withdrawing/-donating substituents. Using combinatorial chemistry, pyrroles substituted with electron-donating/-withdrawing substituents were condensed together in two different manners, thus providing two sets of molecules. The only difference between the two sets is the bridging unit providing a so far lacking comparison between BODIPYs and aza-BODIPYs structural homologues. Replacing the meso-methine bridge with an aza-N bridge results in a red-shifted transition and considerably different, temperature-activated, excited-state relaxation pathways. The effect of electron-donating units on the absorption but not emission for BODIPYs was suppressed compared to aza-BODIPYs. This result could be evident in a substitution pattern-dependent Stokes shift. The outlook of this study is a deeper understanding of the structure–optics relationship of the (aza)-BODIPY-dye class, leading to an improvement in the de novo design of tailor-made molecules for future applications.
Highlights
A detailed understanding of structure−function relationship is the foundation for successfully tuning molecular properties to fit specific applications
BODIPY dyes and their derivatives have become increasingly popular due to their competitive optical properties like large molar absorption coefficients, sharp fluorescent bands, high fluorescent quantum yields, high photostability, and good biocompatibility.[3]. They are used in fluorescent sensors,[4] as fluorescent probes[5−7] in, for instance, microscopy,[1] and in the field of optoelectronics[8] such as donor−acceptor conjugates in solar cells[9] just to name a few applications for this versatile dye class
This aforementioned small change in the structure can lead to a redshift of about 80 nm, leading to some derivatives absorbing in the red or near-infrared regions of the electromagnetic spectrum.[16−19] a general observation is that aza-BODIPY have lower emission quantum yields compared to BODIPY dyes,[15,19] making them less desirable in applications where photons are used as a readout signal
Summary
A detailed understanding of structure−function relationship is the foundation for successfully tuning molecular properties to fit specific applications. For several biological assays or screening experiments as well as in fluorescence microscopy, just to name an example, dyes with near-infrared absorption and emission are preferable due to less interference from autofluorescence and a larger penetration depth.[1,2] BODIPY dyes and their derivatives have become increasingly popular due to their competitive optical properties like large molar absorption coefficients, sharp fluorescent bands, high fluorescent quantum yields, high photostability, and good biocompatibility.[3] They are used in fluorescent sensors,[4] as fluorescent probes[5−7] in, for instance, microscopy,[1] and in the field of optoelectronics[8] such as donor−acceptor conjugates in solar cells[9] just to name a few applications for this versatile dye class. They show a higher potential to undergo singlet-to-triplet intersystem crossing,[20] which in combination with their red-shifted absorption leads to new utilizations like photodynamic therapy.[21]
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