Robust S4 ⋠⋠⋠O Supramolecular Synthons: Structures of Radical-Radical Cocrystals [p-XC6 F4 CNSSN]2 [TEMPO] (X=F, Cl, Br, I, CN).

Cocrystallization of the dithiadiazolyl (DTDA) radicals p-XC6 F4 CNSSN (X=F, Cl, Br, I, CN) with TEMPO afforded the 2 : 1 cocrystals [p-XC6 F4 CNSSN]2 [TEMPO] (1-5) whose structures all reflect a common S4 ⋅⋅⋅O supramolecular motif. The nature of this interaction was probed by DFT calculations (M06/aug-cc-pVDZ) on 1 which revealed that the enthalpy of formation of the [C6 F5 CNSSN]2 [TEMPO] supramolecular motif from [C6 F5 CNSSN]2 and TEMPO is substantial (-54.0 kJ mol-1 ). Electronic structure calculations revealed a TEMPO-based doublet S= 1 / 2 configuration as the ground state with limited spin density on the DTDA rings (2.4 %). The corresponding spin quartet state is +78.9 kJ mol-1 higher in energy. An atoms-in-molecules analysis reveals four bond critical points (BCPs) between the TEMPO O and the DTDA S atoms as well as additional BCPs between selected DTDA S atoms and methyl H atoms of the TEMPO molecule. Herein, the structures of 2-5 are considered within the context of a hierarchical view of competing and complementary intermolecular interactions; in particular, the established supramolecular CN⋅⋅⋅S-S synthon is sacrificed in order to form the new S4 ⋅⋅⋅O interaction.

On the Design of Radical-Radical Cocrystals.

Formation of radical-radical cocrystals is an important step towards the design of organic ferrimagnets. We describe a simple approach to generate radical-radical cocrystals through the identification and implementation of well-defined supramolecular synthons which favor cocrystallization over phase separation. In the current paper we implement the structure-directing interactions of the E-E bond (E=S, Se) of dithiadiazolyl (DTDA) and diselenadiazolyl (DSDA) radicals to form close contacts to electronegative groups. This is exemplified through the preparation and structural characterization of three sets of radical cocrystals; the 2:2 cocrystal [PhCNSSN]2 [MBDTA]2 (4) [MBDTA=methyl benzodithiazolyl] and the 2:1 cocrystals [C6 F5 CNEEN]2 [TEMPO] (E=S, 5; E=Se, 6). In 4 the two types of radical are linked via bifurcated inter-dimer δ+ S⋅⋅⋅Nδ- interactions whereas 5 and 6 exhibit a set of five-centre δ+ E⋅⋅⋅Oδ- contacts (E=S, Se).

On the Fine-Tuning of the Excited-State Intramolecular Proton Transfer (ESIPT) Process in 2-(2'-Hydroxybenzofuran)benzazole (HBBX) Dyes.

Herein, a full investigation of the optical properties and first-principles calculations of a large series of original 2-(2'-hydroxybenzofuran)benzazole (HBBX) dyes is described. The electronic substitution on the π-conjugated core of the fluorophores and the nature of the heteroatom (O, S, N) was varied extensively to assess the necessary parameters to trigger a partial frustration of the excited-state intramolecular proton transfer (ESIPT) process, which results in the emission of both tautomers, that is, enol and keto (E* and K*). The optical properties, studied in solution and in the solid state, revealed the appearance of either an intense single K* or a dual E*/K* emission; a feature that is highly dependent on the electronic substitution (donating or accepting), the heteroelement, and the close environment. Subtle modifications of these parameters allowed the establishment of structure-property relationships that were successfully rationalized by first-principles calculations. In particular, the E*/K* emission intensity ratio was shown to be directly related to the free energies of the two emissive tautomers in the excited state.

Step-by-Step Synthesis of Multimodule Assemblies Engineered from BODIPY, DPP, and Triphenylamine Moieties.

In the present account we describe unsymmetrical triads constructed from extended borondipyrromethene (BODIPY) dyes, diketopyrrolopyrrole (DPP) dyes, and electron donor fragments based on triarylamine. The assemblages are such that each module maintains its individual optical and redox properties. The use of phenyl-alkyne-phenyl or phenyl-alkyne-thienyl spacer units is favorable for weak electronic interaction between the modules. The step-by-step linking of each module using palladium-catalyzed cross-coupling reactions provides both mono- and disubstituted derivatives, the latter obtained by passing in particular through a pivotal monosubstituted DPP building block with a reactive bromo substituent. Thus, grafting of a second dye occurs in a controlled manner, providing the target triads in good yields. This protocol allows also the synthesis of key intermediates and dyads, which appear useful for the understanding of the electrochemical and spectroscopic properties. All the dyes exhibit redox and optical properties suitable for cascade energy transfer and photoinduced electron transfer processes in appropriate solvents.

Highly fluorescent and water-soluble diketopyrrolopyrrole dyes for bioconjugation.

The preparation of highly water-soluble and strongly fluorescent diketopyrrolopyrrole (DPP) dyes using an unusual taurine-like sulfonated linker has been achieved. Exchanging a phenyl for a thienyl substituent shifts the emission wavelength to near λ=600 nm. The free carboxylic acid group present in these new derivatives was readily activated and the dyes were subsequently covalently linked to a model protein (bovine serum albumin; BSA). The bioconjugates were characterized by electronic absorption, fluorescence spectroscopy and MALDI-TOF mass spectrometry, thus enabling precise determination of the labeling density (ratio DPP/BSA about 3 to 8). Outstanding values of fluorescence quantum yield (30% to 59%) for these bioconjugates are obtained. The photostability of these DPP dyes is considerably greater than that of fluorescein under the same irradiation conditions. Remarkably low detection limits between 80 and 300 molecules/µm(2) were found for the BSA bioconjugates by fluorescence imaging with a epifluorescence microscope.

Fluorescent molecular rotors based on the BODIPY motif: effect of remote substituents.

The ability of an unconstrained boron dipyrromethene dye to report on changes in local viscosity is improved by appending a single aryl ring at the lower rim of the dipyrrin core. Recovering the symmetry by attaching an identical aryl ring on the opposite side of the lower rim greatly diminishes the sensory activity, as does blocking rotation of the meso-aryl group. On the basis of viscosity- and temperature-dependence studies, together with quantum chemical calculations, it is proposed that a single aryl ring at the 3-position extends the molecular surface area that undergoes structural distortion during internal rotation. The substitution pattern at the lower rim also affects the harmonic frequencies at the bottom of the potential well and at the top of the barrier. These effects can be correlated with the separation of the H1,H7 hydrogen atoms.

α-Fused dithienyl BODIPYs synthesized by oxidative ring closure.

Both symmetrical and unsymmetrical α-fused dithienyl-BODIPY dyes have been prepared by oxidative ring closure induced by anhydrous FeCl3. Extension of the π-system in the fused BODIPY leads to a progressive shift to 579 and 665 nm respectively for the absorption wavelength maxima of the mono- and difused dyes relative to the unfused species (λ(abs) = 502 nm). Linking such dyes to an NIR emitting module provides a panchromatic chromophore with a large absorption cross section in the visible range associated with efficient intramolecular cascade energy transfer.