Acidochromic Turn‐on 2,4‐Diarylpyrano[2, 3‐b]indole Luminophores with Solubilizing Groups for A Broad Range of Polarity

Abstract

Acidochromic turn‐on 2,4‐diarylpyrano[2, 3‐b]indole luminophores with different solubilizing groups are readily synthesized in a two‐step process, consisting of a Pd−Cu‐catalyzed domino insertion‐coupling‐cycloisomerization reaction of alkynoyl ortho‐iodo anilides and terminal arylacetylenes furnishing the chromophore, and a desilylation‐alkylation sequence of the 2‐TMS‐ethylthio functionality for introducing solubilizing moieties, i. e. polar and aliphatic swallowtails as well as propylsulfonate potassium. These title compounds are able to cover a broad spectrum of solubility in nonpolar to polar and even aqueous solvent systems. Indeed, the turn‐on fluorescence can be shown to proceed without precipitation in aqueous solvent systems. In addition, a selected pyrano[2, 3‐b]indole forms clusters of 127 nm diameter at high water concentrations as determined by dynamic light scattering.

Amphipolar, Amphiphilic 2,4-diarylpyrano[2,3-b]indoles as Turn-ON Luminophores in Acidic and Basic Media

“…Based on our previous synthetic protocols [ 61 , 62 ], a novel generation of halochromic 2,4-diarylpyrano[2,3- b ]indoles containing a hydroxyl group has been created using our potassium fluoride mediated deprotection-alkylation one-pot reaction. Starting from secondary amide 1 [ 62 ] and terminal alkyne 2a [ 63 ] , the O -acyl protected 2,4-diarylpyrano[2,3- b ]indole 3a is formed by a Pd- and Cu-catalyzed domino insertion-coupling-cycloisomerization reaction in moderate yields of up to 54% ( Scheme 2 ). Alternatively, 2,4-diarylpyrano[2,3- b ]indole 3b containing a 2-tetrahydropyranyl (THP) group can be synthesized by the coupling of alkyne 2b in 73% yield.…”

“…The solubilizing swallowtail substituent can be introduced by fluoride-induced desilylative cleavage of the (trimethylsilyl)ethyl group of 2,4-diarylpyrano[2,3- b ]indoles 3 [ 62 ]. Interestingly, KF simultaneously acts as a desilylation and deacylation agent and for substrate 3a (PG = Ac) both O - and S -protecting groups are concomitantly cleaved in the presence of an excess of potassium fluoride and the subsequent addition of the swallowtail tosylate 4 selectively leads S -alkylation furnishing hydroxy 2,4-diarylpyrano[2,3- b ]indole 5 in 30% yield in a one-pot fashion ( Scheme 3 ).…”

“…Similarly as 2,4-diarylpyrano[2,3- b ]indoles of the first [ 61 ] and second [ 62 ] generation, 2,4-diarylpyrano[2,3- b ]indole 5 is also non-luminescent in the solid state and solution. Upon protonation with stronger acids, such as trifluoroacetic acid (TFA), solutions of 2,4-diarylpyrano[2,3- b ]indole 5 exhibit yellow luminescence upon excitation with UV-light ( Scheme 4 , Figure 1 left).…”

“…Recently, we have demonstrated that 2,4-diarylpyrano[2,3- b ]indoles are readily accessible from alkynoyl ortho -iodo anilides and terminal arylalkynes through a Pd-Cu-catalyzed domino insertion-coupling-cycloisomerization reaction [ 61 , 62 ]. Investigations of their photophysical properties revealed that halochromic luminescence in solution is induced through protonation in acid media or complexation with quantum yields of up to 15% ( Scheme 1 ).…”

“…Investigations of their photophysical properties revealed that halochromic luminescence in solution is induced through protonation in acid media or complexation with quantum yields of up to 15% ( Scheme 1 ). Based on these observations, the use of 2,4-diarylpyrano[2,3- b ]indoles as turn-ON luminophores and proton-sensitive chemosensors was considered, even expanding the solvent systems and the solubility [ 62 ].…”