Origin of the Red-Shifted Optical Spectra Recorded for Aza-BODIPY Dyes

Abstract

The optical properties are compared for two boron dipyrromethene (BODIPY) dyes that differ by virtue of the substituent at the meso-site, namely, aza-N versus C-methine atoms. Both compounds are equipped with aryl rings at the 3- and 5-positions of the dipyrrin backbone, which help to extend the degree of π-delocalization. The aza-BODIPY dye absorbs and fluoresces at much lower energy than does the conventional BODIPY dye, with red shifts of about 100 nm being observed in fluid solution, but with comparable fluorescence yield and lifetime. Hydrogen bonding donors, such as alcohols, attach to the aza-N atom and promote nonradiative decay without affecting the properties of the conventional dye. Triplet formation is ineffective in the absence of a spin-orbit coupler. Quantum chemical calculations indicate that the electronegative aza-N atom lowers the energy of the LUMO while having little effect on the corresponding HOMO energy. The resultant decrease in the HOMO-LUMO energy gap is primarily responsible for the red shift. The HOMO-LUMO energy gap is also affected by the dihedral angle subtended by the aryl rings, but this is insensitive to the geometry around the central 6-membered ring. The aza-N atom, by virtue of restricting spatial overlap between the HOMO and LUMO, decreases the energy gap between excited-singlet and -triplet states.

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

“…The sole replacement of the central meso carbon by an aza group entails a pronounced bathochromic shift of the spectral bands. The electronegative character of the nitrogen stabilizes the LUMO state (around 0.58 eV), whose electronic density is concentrated in such key meso position, lowering the energy gap (Figure ) . To push this shift even further towards the red edge of the visible region, the chromophoric backbone has been decorated with aromatic frameworks, bearing also electronic rich moieties (such as electron donor methoxy and electron acceptor trifluoromethyl), at positions 3 and 5, and 1 and 7 respectively.…”

“…However, Harriman et al . suggested that, albeit the replacement of the methine carbon by an aza group affects the S 1 ‐T 1 energy gap, the intersystem crossing is ineffective in aza‐BODIPYs . They pinpoint that the sensitivity of the fluorescence deactivation with the solvent could be related to a specific interaction between hydrogen bonding solvents (i. e., alcohols) and the chromophoric aza group.…”

Elucidating the Relationship between Multiradical Character and Predicted Singlet Fission Activity

“…These molecules share structural similarities between them, such as the two azaBODIPY style structures (13 and 16) and the two TCNQesque structures (17 and 18), and in fact some azaBODIPY and TCNQ derivatives are thought to be SF active experimentally. [62,63] It is clear from the structures that the presence of a tetraradical character amongst predicted SF compounds is distributed across the chemical space and is not related to a specific chromophore e. g. there are other azaBODIPY derivatives within our dataset which do belong to the biradicaloid class. Since these are only a handful of suspicious entries in a set of 201 satisfactory molecules, the evidence for exothermic SF in a tetraradicaloid cannot be considered conclusive but should warrant further investigation, especially since all of them fall within the efficient SF energy range i. e. 0 < E(S 1 ) -2•E(T 1 ) < 0.5 eV.…”

Enhanced in vivo Optical Imaging of the Inflammatory Response to Acute Liver Injury in C57BL/6 Mice Using a Highly Bright Near‐Infrared BODIPY Dye

“…The title compound BD was available from previous studies [17] and purified by column chromatography before use and its purity checked by NMR spectroscopy.Q uantum yields of fluorescence were measured by the comparison method using AzaBODIPY (5,5-difluoro-3,7bis(4-methoxyphenyl)-1,9-5H-4l4,5l4-dipyrrolo[1,2-c:2',1f[1,3,5,2]triazaborinine). [24] Selected data are shown in Table S1.…”