Probing the Proton‐Coupled Electron‐Transfer (PCET) Reactivity of a Cross‐Conjugated Cruciform Chromophore by Redox‐State‐Dependent Fluorescence

Abstract

Proton‐coupled electron transfer (PCET) reactions are of great importance in synthetic chemistry and in biology, but the acquisition of kinetic information for these reactions is often difficult. Herein, we report the synthesis of a new PCET reagent, showing redox‐state dependent fluorescence, by merging the concept of cross‐conjugated cruciform chromophores with the strategy of imposing redox activity and Brønsted basicity to aromatic compounds by substitution with guanidino groups. The compound is isolated and characterized in all stable states—reduced, twofold and fourfold protonated and twofold oxidized—and then applied in PCET reactions by using its redox‐state dependent fluorescence signal for kinetic measurements.

1,2,5,6‐Tetrakis(guanidino)‐Naphthalenes: Electron Donors, Fluorescent Probes and Redox‐Active Ligands

“…The quantum yields for protonated 5 were determined to be approximately 2% for (5+ +2H)(PF 6 ) 2 (l ex = 300 nm) and 15 %f or (5+ +4H)Cl 4 (l ex = 280 nm). The drastic increase in the fluorescencei ntensity with the degree of protonation becomes evident from the photos in Figure 9a.T he increasing energetic separation between electronic ground and excited states might be responsible for as uppression/attenuation of thermalr elaxation pathways, explaining the onset of fluorescence upon protonation.C hanges in the fluorescencei ntensity with the degree of protonation (amplification as well as attenuation of fluorescence)h ave already been observed for guanidines that are fluorescent in their unprotonated form, [29,30] but such ad rastic switching effect from fluorescent silent to fluorescent upon protonation has not been reported previously.I nr ecent years, excited-state intermolecular proton transfer (ESPT) has been evidenced for some protonated guanidines. [31,32] However,p rotonated 5 does not display ESPT,g iven that changes in the degree of protonation do not lead to shifts in the energy of the fluorescence signal.…”

1,2,4,5‐Tetrakis(tetramethylguanidino)‐3,6‐diethynyl‐benzenes: Fluorescent Probes, Redox‐Active Ligands and Strong Organic Electron Donors

“…Conversion to 5,6‐dinitro‐4,7‐bis[2‐[tris(1‐methylethyl)silyl]ethynyl]‐2,1,3‐benzothiadiazole is followed by reduction to give 1,2,4,5‐tetra(amino)‐3,6‐bis‐[(triisopropylsilyl)ethynyl]benzene. Reaction with chloro‐N,N,N’,N’‐tetramethyl‐formamidinium‐chloride leads to 2 a (43 % isolated yield) and reaction with 2‐chloro‐1,3‐dimethyl‐4,5‐dihydro‐1 H ‐imidazolium‐chloride leads to 2 b (14 % isolated yield). The low isolated yield of 2 b is due to its relatively high solubility in organic solvents that hampers its isolation by precipitation.…”

“…Figure shows the Lewis structures of the three compounds 2 a , 2 b and 3 studied in this work. The synthesis of 2 a was described in a preliminary work . As detailed in the following, the peculiarities of these three compounds are the redox‐state dependent fluorescence, and the additional reactivity inscribed by the ethynyl groups (especially for compound 3 ).…”

Redox‐Active Guanidines in Proton‐Coupled Electron‐Transfer Reactions: Real Alternatives to Benzoquinones?

“…We recently developed a new class of PCET reagents, namely redox‐active guanidines, that do not have these disadvantages . One example is 1,2,4,5‐tetrakis‐(tetramethylguanidino)benzene ( 1 ), which could be readily oxidized to the dication 1 2+ (Scheme ).…”

“…[28] We recently developed an ew class of PCET reagents, namely redox-active guanidines, that do not have these disadvantages. [30][31][32][33] One example is 1,2,4,5-tetrakis-(tetramethylguanidino)benzene (1), whichc ould be readily oxidized to the dication 1 2 + (Scheme 1). The loss of aromaticity upon oxidation leads to significantly different CÀCb ond distances in the C 6 ring and ad istinct colourc hange from pale yellow forn eutral 1 to intenseg reen for the dication 1 2 + .P roton-coupled electron transfer (PCET) reactionso fo xidized 1,2,4,5-tetrakis(tetramethylguanidino)benzene (1 2 + ,S cheme 1) [34] with some inorganic (thiol to disulfides, phosphines to diphosphines) [31] and organic substrates with relatively low redox-potential (e.g., phenolst ob iphenols, catechols to benzoquinones) were already reported.…”