Synthesis and diversification of nitrogen and silicon heterocycles used as organic fluorophores

With the interest in photocatalytic transformations constantly increasing, the need of new catalysts is raising overproportionally. Transition metal polypyridyl catalysts have been the dominant class of catalysts for many years. However, with an increasing demand and the awareness for sustainability steadily growing, the demand for precious metal free alternatives was never greater. Acridinium salts have shown photophysical properties similar to those of polypyridyl transition metal catalysts and proven to be versatile and adequate alternatives with similar photophysical properties and nearly identical catalytic potential. Unfortunately, instability towards irradiation often limits their applications and reduces their efficiency. Herein, the scalable synthesis of acridinium salts with an enhanced photostability is reported. Tailormade precursors and an optimized catalyst formation through a 1,5-bifunctional lithium reagent allowed for the preparation of eleven 6- and 7-substituted asymmetric acridinium dyes from commercially available starting materials.
Precise optimization of the substrate synthesis led to the preparation of symmetric acridinium catalysts with an enhanced photostability by formation of a 1,5,5’ trifunctional reagent. A highly efficient tree step procedure synthetic route allowed the preparation of four symmetric photostable acridinium salts.
Further, the enhanced photostability could be demonstrated in Ni-catalyzed esterification, where the catalyst loading of both photocatalyst and Ni-catalyst could be significantly lowered from 10 mol% to more sustainable levels, coming closer to industrial requirements. After gaining this insight, we could demonstrate the versatility of photostable acridinium salts in a 96-wellplate screening setup that demonstrated to possibilities of base variations
1,5-Bifunctional reagents give access to a variety of heteroatom bearing fluorophores. Previously, we could demonstrate the formation of a symmetric silicon rhodamines from a 1,5-bifunctional reagent and an ester. Cryogenic mono-metalation allowed for the access of an unsymmetric 1,5 halogenated silane which after exposure to elemental magnesium could be transformed to its 1,5-bifunctional reagent that gave way to a new class of so far unknown unsymmetric silicon rhodamines. Compared to previous rhodamines, unsymmetric Si-rhodamines show a unique hypsochormic shift in absorption of about 200 nm compared to their symmetric analogous while maintaining similar emission properties.