Bis-Rhodamines Bridged with a Diazoketone Linker: Synthesis, Structure, and Photolysis.

Heydar Shojaei,Vladimir N Belov,Stefan W Hell,Mariano L Bossi

doi:10.1021/acs.joc.1c01721

Abstract

Two fluorophores bound with a short photoreactive bridge are fascinating structures and remained unexplored. To investigate the synthesis and photolysis of such dyes, we linked two rhodamine dyes via a diazoketone bridge (−COCN2−) attached to position 5′ or 6′ of the pendant phenyl rings. For that, the mixture of 5′- or 6′-bromo derivatives of the parent dye was prepared, transformed into 1,2-diarylacetylenes, hydrated to 1,2-diarylethanones, and converted to diazoketones Ar1COCN2Ar2. The high performance liquid chromatography (HPLC) separation gave four individual regioisomers of Ar1COCN2Ar2. Photolysis of the model compound—C6H5COCN2C6H5—in aqueous acetonitrile at pH 7.3 and under irradiation with 365 nm light provided diphenylacetic acid amide (Wolff rearrangement). However, under the same conditions, Ar1COCN2Ar2 gave mainly α-diketones Ar1COCOAr2. The migration ability of the very bulky dye residues was low, and the Wolff rearrangement did not occur. We observed only moderate fluorescence increase, which may be explained by the insufficient quenching ability of diazoketone bridge (−COCN2−) and its transformation into another (weaker) quencher, 1,2-diarylethane-1,2-dione.

Highlights

The possibility to modify two fluorophores in the course of one photochemical reaction is intriguing and remained unexplored
If a single photoreactive group efficiently suppresses the emission of the whole compound, and this group can be transformed into a nonquenching state, both fluorophores may be activated in one step (Scheme 1B)
The synthesis of bromorhodamines 6a,b from aminophenol 54 is given in Scheme 2

Summary

■ INTRODUCTION

The possibility to modify two fluorophores (and change the emission parameters of two dye residues) in the course of one photochemical reaction is intriguing and remained unexplored. The expected product of the photolysis (in the absence of amines in the reaction solution) diphenylacetic acid (13)[24] was detected along with deoxybenzoin (10), benzil (14), and traces of diphenylmethane (15) (Scheme 5) These compounds were identified by comparison with commercial reference substances (retention times, UV, and mass spectra). Having in mind the encouraging results obtained with model diazoketone 11, we performed the photolysis of diazoketones 1a−d (12 μM) in aqueous acetonitrile (acetonitrile/water = 80/20; v/v) in the presence of ammonium formate buffer (pH 7.3-7.4) (Scheme 6) Compounds 1a−d have 3−4 times higher absorption at 365 nm (irradiation wavelength) than the parent fluorophore N,N′-bis(2,2,2-trifluoroethyl)rhodamine.[4] The presence of the azibenzil chromophore The Wolff rearrangement is unfavored because the migration ability of the very bulky dye residue is low

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES