Graphitic Dots Combining Photophysical Characteristics of Organic Molecular Fluorophores and Inorganic Quantum Dots.

Thanks to their photophysical properties, both organic molecular fluorophores (MFs) and inorganic quantum dots (QDs) are extensively used for bioimaging applications. However, limitations such as photobleaching for the former or blinking, size, and toxicity for the latter still constitute a challenge for numerous applications. We report here that embedding MFs in graphitic carbon dots (GDs) results in fluorophores which entirely tackle this challenge. Characterized by ultranarrow, bright, and excitation-independent emission devoid of blinking and photobleaching, these hybrid-featured nanoparticles also demonstrate their unique photophysical performances at the single-nanoparticle scale, making them appealing candidates for bioimaging applications.

Modulating the Photophysical Properties and Electron Transfer Rates in Diketopyrrolopyrrole-Based Coordination Polymers.

Molecular self-assembly through noncovalent interactions is a particularly efficient approach to fine-tune the optoelectronic and photophysical properties of electroactive materials. In metal-ligand coordination polymers, the final properties of the assemblies are directly related to the nature of the metal-ligand interaction. To probe for such influence on the photophysical properties of electroactive materials, a series of coordination polymers based on a well-known organic dye, diketopyrrolopyrrole, was prepared through coordination of a terpyridine-containing monomer with various metal sources, including iron, cobalt, zinc, and manganese. The resulting supramolecular polymers were characterized through multiple techniques, including UV-vis and fluorescence spectroscopy, time-correlated single-photon counting, and femtosecond transient absorption spectroscopy to reveal the impact of the metal source on the final photophysical properties of coordination polymers. As expected, important variations were found between different coordination polymers in terms of absorption, fluorescence kinetics, and electron transfer rate. While iron and cobalt-containing polymers showed ultrafast electrons transfer rates, assemblies from manganese were shown to be much less efficient, confirming the importance of metal centers. This detailed fundamental study unravels some important relationships between metal-ligand interactions, supramolecular self-assembly, and photophysical properties, ultimately leading to new avenues for the design of functional polymers based on organic dyes.

Characterization and Minimization of Glaser Competitive Homocoupling in Sonogashira Porphyrin-Based Polycondensation.

A porphyrin-containing polymer exhibiting various degrees of Glaser-Hay coupling is reported. Sonogashira polycondensation of zinc(II) (5,15-bis[3,4,5-tri(2-butyloctyl)phenyl]-10,20-bis(ethynyl)porphyrinate) with N, N'-bis( p-iodobenzene)-2,3,5,6-tetrafluorobenzoquinone-1,4-diimine (fQI) is found to be prone to homocoupling, a problem underestimated in the literature. 1H NMR and photophysical analysis are used to assess the ratio of Glaser versus Sonogashira couplings. Optimized conditions to perform Glaser-free Sonogashira polycondensations are provided, and the optimization increases Mn from 9700 to 18 900. Applied to a conjugated polymer, it shows both decreasing homocoupling and 180% enhancement in Mn.

Photophysical and Optical Properties of Semiconducting Polymer Nanoparticles Prepared from Hyaluronic Acid and Polysorbate 80.

A nanoprecipitation procedure was utilized to prepare novel diketopyrrolopyrrole-based semiconducting polymer nanoparticles (SPNs) with hyaluronic acid (HA) and polysorbate 80. The nanoprecipitation led to the formation of spherical nanoparticles with average diameters ranging from 100 to 200 nm, and a careful control over the structure of the parent conjugated polymers was performed to probe the influence of π-conjugation on the final photophysical and thermal stability of the resulting SPNs. Upon generation of a series of novel SPNs, the optical and photophysical properties of the new nanomaterials were probed in solution using various techniques including transmission electron microscopy, dynamic light scattering, small-angle neutron scattering, transient absorption, and UV-vis spectroscopy. A careful comparison was performed between the different SPNs to evaluate their excited-state dynamics and photophysical properties, both before and after nanoprecipitation. Interestingly, although soluble in organic solution, the nanoparticles were found to exhibit aggregative behavior, resulting in SPNs that exhibit excited-state behaviors that are very similar to aggregated polymer solutions. Based on these findings, the formation of HA- and polysorbate 80-based nanoparticles does not influence the photophysical properties of the conjugated polymers, thus opening new opportunities for the design of bioimaging agents and nanomaterials for health-related applications.

Multiply "trapped" 3[trans-Pt(PR3)2(C[triple bond, length as m-dash]CC6H4X)2]* conformers in rigid media.

The complexes (R = Me, Et, Bu; X = H, SMe) exhibit well-defined multi-exponential emissions (2-4 components) in the solid state at 77 and 298 K and in 2MeTHF glasses at 77 K due to multiple frozen conformers exhibiting variable dihedral angles formed by the PtP2C2 and C6H4 planes. The demonstration was made using X-ray crystallography at various temperatures where different sites are present in the samples, and using geometry optimization (DFT computations) where various stable conformers are noted.

Rendering cross-conjugated azophenine derivatives emissive to probe the silent photophysical properties of emeraldine.

An azophenine derivative was synthesized by coupling truxene and azophenine via the copper-free Sonagashira reaction using Pd2(dba)3 and As(PPh)3 as catalysts. The crystal structure of this heavy azophenine model (∼4000) was made and the identity of the structure was confirmed. By introducing truxene groups into this cross-conjugated structure, the deactivating rotations around the NH-C6H4 bonds were slowed down, which rendered this derivative near-IR emissive at 298 K. This species provided then the appropriate spectral and kinetic signatures for knowing where and what to look for in emeraldine, which was called non-emissive. Besides, two other compounds were also synthesized as models for this azophenine derivative for comparison and interpretation purposes.

Metal Dependence on the Bidirectionality and Reversibility of the Singlet Energy Transfer in Artificial Special Pair-Containing Dyads.

The demetalation of a precursor dyad, 3, built upon a zinc(II)-containing artificial special pair and free-base antenna, leads to a new dyad, 4, for singlet energy transfer composed of cofacial free-base porphyrins (acceptor), [Fb]2 bridged by a 1,4-C6H4 group to a free-base antenna (donor), [Fb]. This dyad exhibits the general structure [M]2-C6H4-[Fb], where [M]2 = [Fb]2, and completes a series reported earlier, where [M]2 = [Mg]2 (2) and [Zn]2 (3). The latter dyads exhibit a bidirectional energy-transfer process at 298 K for 2 and at 77 K for 3. Interestingly, a very scarce case of cycling process is observed for the zinc-containing dyad at 298 K. The newly reported compound 4 exhibits a quasi unidirectional process [Fb]*→[Fb]2 (major, kET = 2 × 1011 s-1 at 298 K), where the remaining is [Fb]2*→[Fb] (minor, kET = 8 × 109 s-1 at 298 K), thus completing all possibilities. The results are analyzed in terms of molecular orbital couplings (density functional theory computations), Förster resonance energy transfer parameters, and temperature dependence of the decay traces. This study brings major insights about artificial special pair-containing dyads and clearly contributes to a better understanding of the communication between the two main components of our models and those already described in the literature.

Excited State N-H Tautomer Selectivity in the Singlet Energy Transfer of a Zinc(II)-Porphyrin-Truxene-Corrole Assembly.

An original corrole-containing polyad for S1 energy transfer, in which one zinc(II)-porphyrin donor is linked to two free-base corrole acceptors by a truxene linker, is reported. This polyad exhibits a rapid zinc(II)-porphyrin*→free-base corrole transfer (4.83×1010  s-1 ; 298 K), even faster than the tautomerization in the excited state processes taking advantage of the good electronic communication provided by the truxene bridge. Importantly, the energy transfer process shows approximately 3-fold selectivity for one corrole N-H tautomer over the other even at low temperature (77 K). This selectivity is due to the difference in the J-integral being effective in both the Förster and Dexter mechanisms. The data are rationalized by DFT computations.

Ultrafast Singlet Energy Transfer in Porphyrin Dyads.

A weakly fluorescent Pt-bridged dyad composed of zinc(II) porphyrin (Zn; donor) and free base (Fb; acceptor) has been designed and exhibits an ultrafast singlet energy transfer between porphyrins. The use of larger atoms within the central linker significantly increases the MO coupling between the two chromophores and inherently the electronic communication.

Luminescent 1D- and 2D-Coordination Polymers Using CuX Salts (X = Cl, Br, I) and a Metal-Containing Dithioether Ligand.

The organometallic synthon trans-[p-MeSC6H4C≡C-Pt(PMe3)2-C≡CC6H4SMe] (L1) reacts with CuX (X = Cl, Br, I) in PrCN and PhCN to form 1D- or 2D-coordination polymers (CP) with a very high degree of variability of features. The copper-halide unit can be either the rhomboids Cu2X2 fragments or the step cubane Cu4I4. The CP's may also incorporate a crystallization solvent molecule or not, which may be coordinated to copper or not. Their characterizations were performed by X-ray crystallography, thermal gravimetric analysis (TGA), and IR, absorption, and emission spectra as well as photophysical measurements in the presence and absence of solvent crystallization molecules. The nature of the singlet and triplet excited state was addressed using DFT and TDDFT computations, which turn out to be mainly ππ* with some minor MLCT (Cu4I4 → L1) contributions. The porosity of the materials has been evaluated by BET (N2 at 77 K). The solvent-free 1D CP's are not prone to capture solvent molecules or CO2, but the efficiency for CO2 absorption is best for the 2D CP, which exhibits the presence of clear cavities in the grid structure, after the removal of the crystallization molecules.

Cyclotriveratrylene-Containing Porphyrins.

The C3-symmetric cyclotriveratrylene (CTV) was covalently bonded via click chemistry to 1, 2, 3, and 6 zinc(II) porphyrin units to various host for C60. The binding constants, Ka, were measured from the quenching of the porphyrin fluorescence by C60. These constants vary between 400 and 4000 M(-1) and are considered weak. Computer modeling demonstrated that the zinc(II) porphyrin units, [Zn], exhibit a strong tendency to occupy the CTV cavity, hence blocking the access for C60 to land on this site. Instead, the pincer of the type [Zn]----[Zn] and in one case [Zn]----CTV, were found to be the most probable geometry to promote host-guest associations in these systems.

Metal Linkage Effects on Ultrafast Energy Transfer.

We report the preparation of several new porphyrin homodimers bridged by a platinum(II) ion in which very intense electronic communication through the coordination link occurs. Moreover, the synthesis of a new porphyrin dyad and its photophysical properties are reported. This dyad exhibits the fastest singlet energy transfer ever reported for synthetic systems between a zinc(II) porphyrin and a porphyrin free base. This extremely fast transfer (∼100 femtoseconds) is in the same range as the fastest one measured in natural systems. This feature is due to the platinum(II) linker, which allows for strong MO couplings between the two porphyrin units as experimentally supported by electrochemistry and corroborated by DFT computations.

Ultrafast energy transfer in a Pd(II)-bridged bisporphyrin dyad.

A free base porphyrin/zinc(II)porphyrin dyad bridged by a Pd(II) unit exhibits an ultrafast S1 energy transfer (1.5 × 10(12) s(-1)) due to significant MO couplings between the acceptor and the donor and a non-nil atomic contribution of the Pd-atom in the frontier MOs, hence ensuring π-conjugation.

Unexpected drastic decrease in the excited-state electronic communication between porphyrin chromophores covalently linked by a palladium(II) bridge.

A dyad built up of a zinc(II) porphyrin and the corresponding free base, [Zn-Fb], fused to N-heterocyclic carbene (NHCs) ligands, respectively acting as singlet energy donor and acceptor, and a bridging trans-PdI2 unit, along with the corresponding [Zn-Zn] and [Fb-Fb] dimers were prepared and investigated by absorption and emission spectroscopy and density functional computations. Despite favorable structural and spectroscopic parameters, unexpectedly slow singlet energy transfer rates are measured in comparison with the predicted values by the Förster theory and those observed for other structurally related dyads. This observation is rationalized by the lack of large molecular orbital (MO) overlaps between the frontier MOs of the donor and acceptor, thus preventing a double electron exchange through the trans-PdI2 bridge, and by an electronic shielding induced by the presence of this same linker preventing the two chromophores to fully interact via their transition dipoles.

A drastic substituent effect on the emission properties of quinone diimine models and valuable insight into the excited states of emeraldine.

The (α-NR,α'-NR,N,N'-(C6H4C≡CSiMe3)4)[Q] models ([Q] = -N=C6H4=N-) exhibit upper excited state emissions Sn,Tn → S0 (n >1, R = Boc), similar to emeraldine, vs. a fluorescence S1 → S0 (R = H), driven by a large change in dihedral angles made by the NR-C6H4 and [Q] planes and intramolecular H-bonds.

Slow and fast singlet energy transfers in BODIPY-gallium(III)corrole dyads linked by flexible chains.

Red (no styryl), green (monostyryl), and blue (distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene) corrole dyads have been prepared in high yields using click chemistry, and their photophysical properties are reported. An original and efficient control of the direction of the singlet energy transfers is reported, going either from BODIPY to the gallium-corrole units or from gallium-corroles to BODIPY, depending upon the nature of the substitution on BODIPY. In one case (green), both directions are possible. The mechanism for the energy transfers is interpreted by means of through-space Förster resonance energy transfer (FRET).

Antenna effects in truxene-bridged BODIPY triarylzinc(II)porphyrin dyads: evidence for a dual Dexter-Förster mechanism.

The antenna process from an energy donor (BODIPY; 4',4'-difluoro-1',3',5',7'-tetramethyl-4'-bora-3a',4a'-diaza-s-indacene) in its singlet state to two acceptors (two zinc(II) 5,15-p-tolyl-10-phenylporphyrin) bridged by a central truxene residue (5',5'',10',10'',15',15''-hexabutyltruxene), 5, has been analysed by means of comparison of the energy transfer rates with those of a structurally similar ß-substituted BODIPY-(zinc(II) 5,10,15-p-tolyl-porphyrin), 6, where no conjugation is present between the donor and the two acceptors using the Förster resonance energy transfer (FRET) approximation. It is estimated that the energy transfer in operates mostly via a Dexter mechanism (>99%), and the remaining proceeds via a Förster mechanism (<1%). This information is useful for the design of future molecular devices aimed at harvesting light.

Is the special pair structure a good strategy for the kinetics during the last step of the energy transfer with the nearest antenna? A chemical model approach.

A cofacial bis(Mg(II)porphyrin)-C(6)H(4)-free base ([Mg(2)]-bridge-FB) dyad shows S(1) energy transfer in both directions and much slower rates than similar monoporphyrin systems are observed.