Solvent‐Driven Conformational Exchange for Amide‐Linked Bichromophoric BODIPY Derivatives

Abstract

The fluorescence lifetime and quantum yield are seen to depend in an unexpected manner on the nature of the solvent for a pair of tripartite molecules composed of two identical boron dipyrromethene (BODIPY) residues attached to a 1,10-phenanthroline core. A key feature of these molecular architectures concerns the presence of an amide linkage that connects the BODIPY dye to the heterocyclic platform. The secondary amide derivative is more sensitive to environmental change than is the corresponding tertiary amide. In general, increasing solvent polarity, as measured by the static dielectric constant, above a critical threshold tends to reduce fluorescence but certain hydrogen bond accepting solvents exhibit anomolous behaviour. Fluorescence quenching is believed to arise from light-induced charge transfer between the two BODIPY dyes, but thermodynamic arguments alone do not explain the experimental findings. Molecular modelling is used to argue that the conformation changes in strongly polar media in such a way as to facilitate improved rates of light-induced charge transfer. These solvent-induced changes, however, differ remarkably for the two types of amide.

Solvent‐Sensitive Emitting Urea‐Bridged bis‐BODIPYs: Ready Access by a One‐Pot Tandem Staudinger/Aza‐Wittig Ureation

“…Moreover,s uch quenching correlates with ad rastic change in the fluorescenced ecay curvess incet he time-resolved emission profile acquires ab i-exponential character with the contribution of the shorter-lived component increasing with the solventpolarity (Figure 6). [29] The fluorescenceo f the herein synthesized urea-bridged bis-BODIPYs follows as imilar dependence on solventp olarity,s upporting that an ICT process is underway.H owever,t he process has to be triggered through ad issimilar mechanism taking into account key structural differencesw ith respect to the above-mentioned amidebridged bis-BODIPYs such as:1 )the substitution at the, more constrained, ortho-position rather than at the para-position of the meso-phenyl group leads to am ore rigid structure;2 )the urea linker reduces the conformational freedom compared to that induced by the more flexible phenanonthroline linker bearing side amides, and 3) the already mentioned electronic decoupling of the BODIPY units. Photophysical (diluted solutions,2 10 À6 m of cyclohexane)a nd lasing (concentrateds olutionso fe thyl acetate owingt ot he solubility reasons)p roperties of the bridged bis-BODIPYs.…”

“…However,t his behavior was only followed by dyad 9,d erived from the unsubstituted BDP,s ince when dissolvedi n DMF exhibits af luorescence efficiency lower than 5% and a lifetimew ith am ain component (85 %) as short as 100 ps ( Figure 8a nd Table S1). [29] The unusual behaviori nduced by DMF should be related not only to its polarity but also to its electron-donor ability (basicity scale, SB = 0.613), [50] the highest one among the herein selected solvents.T he basicity of DMF could induce specific interactions betweent his hydrogen-bond-acceptor solventa nd the proton of the urea linker so long as the electron lone pair of the latter is mainly located on the amine moiety and less shifted to the oxygen atom.I na greement, the positive charge( see the blue color in Figure 7) is located mainly aroundt he nitrogen atoms of the urea, highlighting this position as the mosts uitable for interaction with electron donor solvents. Thus, the corresponding dyad 9, albeit more stable (the loss of color is less marked and slower), likely undergoes such bleaching process that also contributes to the fluorescenceq uenching.…”

“…In the case of bis-BODIPY 6 ( Figure 1), adibenzofuran spacer induced ad ifferent bite angle between the two BODIPYunits, affecting their fluorescent properties. [29] In thesed erivatives, it was found that an increase in solvent polarity tended to reduce their fluorescence, and this behavior wasa scribed to light-induced charge-transfer between the two BODIPYs, probably facilitated by conformational changes in strongly polar media. [29] In thesed erivatives, it was found that an increase in solvent polarity tended to reduce their fluorescence, and this behavior wasa scribed to light-induced charge-transfer between the two BODIPYs, probably facilitated by conformational changes in strongly polar media.…”

“…[28b] Bis-BODIPY derivatives 7 (Figure 1), recently described by Harriman and co-workers, represent an interesting example of mesophenyls ubstituted-BODIPYs connected by amide linkages to a rigid phenanthroline core. [29] This tendency,h owever, was disrupted in hydrogen-bond-accepting solvents. [29] This tendency,h owever, was disrupted in hydrogen-bond-accepting solvents.…”

Synthesis and Characterization of Some BODIPY‐based Substituted Salicylaldimine Schiff Bases

“…The synthesis of Schiff bases started with dye 2 that was synthesized by condensation of kryptopyrrole ( 1 ) and p ‐nitrobenzoyl chloride followed by complexation with excess BF 3 ·OEt 2 (Scheme ) [8b,9]. Pd‐catalyzed hydrogenation of dye 2 gave the corresponding amino derivative 3 . Then, condensation of dye 3 with different substituted 2‐hydroxy benzaldehydes in dry methanol separately furnished corresponding dark red colored Schiff bases 4a – e (Scheme ).…”