BODIPYs with Chalcogenophenes at Boron: Synthesis and Properties

Abstract

Reported herein are the synthesis and characterization of BODIPYs bearing heterocycles at boron. To synthesize this series, various chalcogenophenes (furan, thiophene, selenophene, and tellurophene) were lithiated and then used as nucleophiles to attack the boron center of a parent F-BODIPY. Compounds in the series were compared with respect to their photophysical and structural properties, and trends were discussed. By virtue of the "heavy atom effect", as the mass of the heterocycle appended to the BODIPY core increases, compounds exhibit a higher singlet oxygen quantum yield. The BODIPY with tellurophene at boron exhibits the highest quantum yield (Φ Δ = 0.68) in the series and reduced emission (Φ f = 0.01).

A series of potent BODIPY photosensitisers featuring tellurophene motifs at boron

“…in THF at room temperature. 6 After five minutes, and in order to promote double-substitution of the fluoro substituents, the requisite F-BODIPYs (Scheme 1, 3 ) were added dropwise over five minutes to the pale-yellow stirring solution of 2-tellurophenyllithium. The resulting solution was stirred for one hour at room temperature.…”

“…Employing this synthetic method, a series of [Te]-BODIPYs that feature various substituents on the dipyrrinato backbone was synthesised (Table 1). The properties of the ethyl-substituted [Te]-BODIPY 2 have been established, 6 and it was found that replacing ethyl chains with octyl chains ( 4a ) had little effect on the system in terms of yields and ease of purification. Similarly, alkyl groups are well tolerated in the meso position of the dipyrrinato core, as demonstrated by the isolation of the methyl and isopropyl examples 4b and 4c , respectively.…”

“…Similarly to the reported [Te]-BODIPYs, 1 and 2 , the determined Φ f values were minimal and no noticeable fluorescence was observed in aqueous conditions. 5,6 Although aggregation-induced emission (AIE) has been observed for BODIPYs featuring aryl groups at the meso -position and thiophenes, or spiro-linked aryl groups, at boron, investigation of AIE properties for a series of BODIPYs appended with tellurophenes at boron would require an alternative synthetic approach given the challenges encountered herein regarding 4j–4l . 25,26,28 The absorbance properties for all compounds were also measured in solutions of 1% DMSO in MeOH and 1% DMSO in PBS to determine the wavelengths for 1 O 2 determination and in cellulo experiments, respectively.…”

“…, F-BODIPYs, 4,4-difluoro-4-bora-3a,4a-diaza- s -indacenes) lends well to PDT, and thus F-BODIPYS are well-documented as PDT agents. 4–9 The design principles aligning with tuning for photosensitisation aim to promote ISC to the 3 PS* state, with the maintenance of modest fluorescence deemed useful for theranostic purposes. 9 A structural modification known to promote ISC involves the incorporation of a heavy-atom near or on the chromophore, 10 with the resulting ISC enhancement originating from greater spin–orbit coupling.…”

“…We recently showcased a new system in which we appended chalcogenophenes (furan, thiophene, selenophene and tellurophene) to the core of a BODIPY via nucleophilic substitution at the boron atom of an F-BODIPY. 6 The synthesis involved using lithiated heterocycles and thereby avoided the need to activate the B–F bond via the use of Lewis acids, or effect prior halogen exchange at boron. 23–27 When comparing the photophysical data of all compounds in the series we found that the tellurophene-appended [Te]-BODIPY 2 (Fig.…”