π-Extended Dipyrrins Capable of Highly Fluorogenic Complexation with Metal Ions

Abstract

Synthesis and properties of a new family of π-extended dipyrrins, capable of forming brightly fluorescent complexes with metal ions, are reported. The metal complexes posses tunable spectral bands and exhibit different emission properties depending on the mode of metal coordination.Boron dipyrrins (BODIPY) form a popular group of fluorophores due to their high emission quantum yields, excellent photostability and versatile chemistry.1 While boron complexes of dipyrrins are by far the most recognized, dipyrrins have also long been known to form stable adducts with metal ions.2 , 3 , 4 Metallodipyrrins are typically isolated as homoleptic biscomplexes (ML 2 , L=dipyrrin), which mainly find use as building blocks for construction of various supramolecular assemblies.5 Curiously, unlike BODIPY, homoleptic metallodipyrrins practically do not fluoresce, although there are several important exemptions.6 For example, Holten, Lindsey et al showed that by increasing the size of the meso-aryl group in Zn bis-aryldipyrrins, the non-radiative decay could be diminished, affording considerable gain in emission.6a On the other hand, some recently reported heteroleptic Al 3+ and Sn 2+ mono-dipyrrinates (MLX n ) exhibit bright fluorescence,7 , 8 suggesting that the emissivity of metallodipyrrins could in part be related to the mode of metal coordination. Overall, the interplay between the structure and photophysics of metallodipyrrins is not well understood; and generally metallodipyrrins are considered poorly emissive species.1aHere we report a new family of aromatically π-extended dipyrrin molecules capable of forming bright fluorescent complexes with metal ions and exhibiting a unique fluorescence modulation effect mediated by exciton coupling.9 A recently described approach to π-extended porphyrins, based on 4,7-dihydroisoindole and its derivatives (Scheme 1),10 paved a new way to π-extended oligopyrroles, including 2,2′-alkoxycarbonyl-dibenzo-(BDP) and dinaphtho [2,3] condensation of 2-substituted 4,7-dihydroisoindole 1 with aldehydes,10c followed by the oxidation of dipyrromethanes with DDQ. Similarly, dinaphthodipyrrins can be synthesized from the corresponding pyrrole-esters reported previously (Scheme 1).10aAll the synthesized dipyrrins exhibit broad absorption bands (ε~4-7×10 4 M −1 cm −1 ) ranging from 550-570 nm for BDP's (2a-f) to 660-700 nm for NDP (3). Taken as free-bases, BDP's and NDP's fluoresce very weakly (φ fl~0 .01-0.02 at 22°C); however, upon addition of metal salts (e.g. Zn(OAc) 2 , Ca(OAc) 2 , YCl 3 , LaCl 3 , GdCl 3 ) their solutions instantly become fluorescent. For example, upon addition of Zn(OAc) 2 , purple solutions of BDP's in DMF immediately change color to deep blue with appearance of bright red fluorescence (Fig. 1b). In contrast, metallation of regular dipyrrins typically requires heating and/or presence of bases and leads to non-emissive products.The optical transitions of the new π-extended dipyrrinates span the entire red spectral range (Fig. 1a), resembling in shape the bands of BOD...

Synthesis of Phosphorescent Asymmetrically π-Extended Porphyrins for Two-Photon Applications

“…73,75,82 In the case of sterically more hindered structures 5 and 7, longer times (up to 72 h) were required to complete the condensations. Generally, using acetic acid as the solvent, as opposed to dichloromethane, was found to speed up the reactions by factors of ∼2.…”

Fluorescent and colorimetric ion probes based on conjugated oligopyrroles

“…2) can be reversibly modulated. 11 Free dipyrrins 5 and 6 do not fluoresce. Addition of Zn(OAc) 2 to an acetone solution of dipyrrin 5 resulted in immediate vivid fluorescence enhancement with a quantum yield of 70%.…”

An Extremely Bright Heteroleptic Bis(dipyrrinato)zinc(II) Complex

“…Cheprakov and co‐workers reported that mono(dipyrrinato)zinc(II) complexes displayed higher ϕ F (>0.5) than the corresponding bis(dipyrrinato)zinc(II) complexes (<0.1) 6. However, mono(dipyrrinato)zinc(II) complexes easily decomposed, or they disproportionated into the corresponding bis(dipyrrinato)zinc(II) complexes 6. 7 Thus, the improvement of ϕ F in dipyrrinato zinc(II) complexes leaves room for further investigation.…”