Dark State of the Thiele Hydrocarbon: Efficient Solvatochromic Emission from a Nonpolar Centrosymmetric Singlet Diradicaloid.

In this work, a comprehensive investigation of the photoinduced processes and mechanisms linked to the luminescence of a novel nonperchlorinated Thiele hydrocarbon (TTH) is presented. Despite the comparable diradical character of TTH (y0 = 0.32-0.44) and the unsubstituted Thiele hydrocarbon (TH) (y0 = 0.30), the polyhalogenated species is inert and photostable, showing an intense deep-red/near-infrared (NIR) fluorescence (photoluminescence quantum yield (PLQY) = 0.84 in toluene) even at room temperature and in the solid state (PLQY = 0.19). TTH displays a large Stokes shift (307 nm in benzonitrile) and solvatochromic behavior, which is unusual for a centrosymmetric, nonpolar, and low-conjugated species. These outstanding emission features are interpreted through quantum-chemical calculations, indicating that its fluorescence arises from the low-lying dark doubly excited zwitterionic state, typically found at low excitation energies in diradicaloids, acquiring dipole moment and intensity by state mixing via twisting around the strongly elongated exocyclic CC bonds of the excited p-quinodimethane (pQDM) core, with a mechanism similar to sudden polarization occurring in olefins. Such a mechanism is derived from ns and fs transient absorption measurements.

Chiral Diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole Dyes with Highly Tuneable Properties in Solution and Thin Films.

Invited for the cover of this issue are the research groups of Lorenzo Di Bari at the University of Pisa and Gianluca Maria Farinola at the University of Bari Aldo Moro. The image depicts three diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole dyes with the same chiral appendage R* but different achiral substituent groups Y showing profoundly different features in their aggregated form. Read the full text of the article at 10.1002/chem.202300291.

Chiral Diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole Dyes with Highly Tuneable Properties in Solution and Thin Films.

We have studied the impact of achiral substituents on the chiral supramolecular architectures of diketopyrrolo[3,4-c]pyrrole-1,2,3-1H-triazole (DPP) dyes. We decorated the same chiral DPP motif with substituent groups on the nitrogen atoms of the lactam moiety: the hydrophobic n-octyl alkyl chain, the hydrophilic tri(ethylene glycol) (TEG) chain and the thermo-cleavable tert-butoxycarbonyl (t-Boc) carbamate group. In spite of having identical conjugated chromophore and chiral appendages, in aggregated form the three dyes displayed profoundly different optical, chiroptical, electrochemical and thermal features. ECD measurements revealed differences in the aggregation modes, which would be inaccessible by most other techniques. We found strong chiroptical features, which would have major implications in the context of chiral organic opto-electronics and in the development of other highly innovative technological applications.

Aggregation Modes of Chiral Diketopyrrolo[3,4-c]pyrrole Dyes in Solution and Thin Films.

The chiroptical features of chiral diketopyrrolo[3,4-c]pyrrole (DPP) derivatives have been only marginally investigated to date. In this regard, we have synthesized ad hoc four chiral DPP dyes, functionalized with enantiopure alkyl groups from natural sources either on the lactam moieties or on the terminal positions of the π-conjugated backbone, to promote an efficient self-assembly into chiral supramolecular structures. For each of them, the aggregation modes has been investigated by absorbance and ECD spectroscopies in conditions of solution aggregation and on thin films, considering the effects of deposition technique (drop casting vs. spin coating) and post-deposition operations (solvent and thermal annealing). The effect of the structure of lateral π-conjugated units attached to the central DPP scaffold, as well as that of the position of the alkyl chiral group, has been assessed. ECD revealed superior capability, compared to absorbance spectroscopy, to provide information on the aggregation modes and to detect the possible co-existence of multiple aggregation pathways.

Infrared Irradiation-Assisted Solvent-Free Pd-Catalyzed (Hetero)aryl-aryl Coupling via C-H Bond Activation.

The increasing attention towards environmentally friendly synthetic protocols has boosted studies directed to the development of green and sustainable methods for direct C-H bond arylation of (hetero)arenes. In this context, here the infrared (IR) irradiation-assisted solvent-free Pd-catalyzed direct C-H bond arylation of (hetero)arenes was achieved. Several heteroaryl-aryl coupling reactions were described, also involving heterocycles commonly used as building blocks for the synthesis of organic semiconductors. The reaction tolerated many functional groups on the aromatic nuclei. The IR-irradiation as the energy source compared favorably with thermal heating and, in combination with solvent-free conditions, provided an important contribution to the development of protocols fitting with the principles of green chemistry.

Solvent-free Reactions for the Synthesis of Indolenine-based Squaraines and Croconaines: Comparison of Thermal Heating, Mechanochemical Milling, and IR Irradiation.

Squaraines and croconaines are organic dyes characterized by intense absorption in the visible or near-infrared spectral regions with applications ranging from biology to material sciences. They are commonly synthesized by condensation reactions of oxocarbonic acids (squaric or croconic acid, respectively) with electron-rich aromatic compounds in high-boiling organic solvents. Here, a simple, cost-effective, and environmentally benign process was developed for the synthesis of indolenine-based squaraines and croconaines under solvent-free conditions. Protocols based on conventional thermal heating, mechanochemical milling, and IR-light activation were compared.

Peripherical thioester functionalization induces J-aggregation in bithiophene-DPP films and nanoparticles.

In this work we demonstrated that the peripherical thioacetylation of a bithiophene-DPP molecule can greatly influence the solid-state properties triggering the formation of NIR emitting J-aggregates in both bithiophene-DPP films and nanoparticles. The morphology and the kinetic and thermal stability of the organic nanoparticles were also investigated.

Synthesis and Computational Study of Semicroconaines and Nonsymmetric Croconaines.

The synthesis of two new dye families of croconic acid derivatives, semicroconaine and nonsymmetric croconaine dyes, is reported for the first time. These compounds show strong absorption in the UV-visible and NIR, respectively. Semicroconaine dyes were obtained by a scalable and efficient condensation of croconic acid with aromatic heterocyclic methylene-active compounds. The subsequent reaction of the semicroconaine dyes with aromatic heterocyclic compounds affords nonsymmetric croconaines. The structure and electronic properties of the synthesized compounds have been investigated by preliminary theoretical calculations at DFT level of approximation.

Solvent-Free Pd-Catalyzed Heteroaryl-Aryl Coupling via C-H Bond Activation for the Synthesis of Extended Heteroaromatic Conjugated Molecules.

Direct arylation of thienopyrrolodione, diketopyrrolopyrroles, benzodithiophene derivatives, and fluorinated heteroarenes with functionalized aryl iodides is proven in solvent-free and non-anhydrous conditions. The reaction is performed in the presence of air and tolerates several functional groups on both the coupling partners, enabling a convenient synthesis of extended heteroaromatic conjugated molecules.

Highly Stable and Red-Emitting Nanovesicles Incorporating Lipophilic Diketopyrrolopyrroles for Cell Imaging.

Diketopyrrolopyrroles (DPPs) have recently attracted much interest as very bright and photostable red-emitting molecules. However, their tendency to form nonfluorescent aggregates in water through the aggregation-caused quenching (ACQ) effect is a major issue that limits their application under the microscope. Herein, two DPP molecules have been incorporated into the membrane of highly stable and water-soluble quatsomes (QS; nanovesicles composed of surfactants and sterols), which allow their nanostructuration in water and, at the same time, limits the ACQ effect. The obtained fluorescent organic nanoparticles showed superior structural homogeneity, along with long-term colloidal and optical stability. A thorough one- (1P) and two-photon (2P) fluorescence characterization revealed the promising photophysical features of these fluorescent nanovesicles, which showed a high 1P and 2P brightness. Finally, the fluorescent QSs were used for the in vitro bioimaging of Saos-2 osteosarcoma cell lines; this demonstrates their potential as nanomaterials for bioimaging applications.

1,5-Diaminonaphtalene is a Highly Performing Electron-Transfer Secondary-Reaction Matrix for Laser Desorption Ionization Mass Spectrometry of Indolenine-Based Croconaines.

Croconaine dyes are appealing molecules synthesized via the condensation of croconic acid and reactive electron-donating aromatic or heterocyclic systems. Here, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) investigation of indolenine-based croconaines is presented for the first time. Archetype proton-transfer matrices, such as 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA), 9-aminoacridine (9AA) as the protonating/deprotonating matrix, and electron-transfer (ET) secondary-reaction matrices, such as 1,5-diaminonapthalene (DAN) and trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2-propenylidene]malononitrile (DCTB), were investigated. DHB, CHCA, and 9AA generate a mix of odd-electron molecular ions and protonated, sodiated, and potassiated adducts. Among the ET matrices, DAN was found to be capable of directing the ionization process toward the exclusive formation of odd-electron molecular ions M+• without fragmentation. MALDI tandem MS provides useful structural characterization of croconaine dyes, thus making identification very straightforward for all investigated compounds. Interestingly the fragmentation of bromo-containing croconaines revealed, for the first time, the gas-phase formation of a bromime cation [Br]+.

Pd-Catalyzed Thiophene-Aryl Coupling Reaction via C-H Bond Activation in Deep Eutectic Solvents.

Direct arylation of 5-octylthieno[3,4-c]pyrrole-4,6-dione with a series of functionalized aryl iodides via C-H bond activation is demonstrated in a deep eutectic solvent made of choline chloride and urea in non-anhydrous conditions and without exclusion of air. This is the first demonstration of a thiophene-aryl coupling via direct arylation in deep eutectic solvents.

A general procedure for the synthesis of alkyl- and arylethynyl-1,2,3-triazole-fused dihydroisoquinolines.

A general procedure for the synthesis of the title compounds has been devised starting from the available 2-halophenylethyl azides, by means of click reactions with trimethylsilylacetylene or 1-trimethylsilyl-1,3-butadiyne followed by a transition metal-catalyzed functionalization of C-H bond. A further extension of this procedure led us to devise the synthesis of more complex 4,4'-bitriazole-fused dihydroisoquinolines.

Photoinduced Interfacial Electron Injection Dynamics in Dye-Sensitized Solar Cells under Photovoltaic Operating Conditions.

We report a pump-probe spectroscopy study of electron injection rates in dye-sensitized solar cell (DSSC) devices. We examine the case of working devices employing an N719 ruthenium sensitizer and an iodide electrolyte. Electron injection is found to occur mainly on a sub-100 fs time scale, followed by a slower component with a lifetime of 26.9 ps, in accordance with previous reports on model samples. The amplitude of this latter component varies with electrolyte composition from 25 to 9%. The appearance of slower components in the electron injection dynamics may be attributed to an aggregated or weakly bound state of the surface-adsorbed N719 sensitizer. Further measurements are reported varying the cell light bias and load conditions, revealing no influence on electron injection dynamics. No other electron injection event is found to occur up to 1 ns. These results show no evidence for a slowdown of electron injection under working conditions compared to model systems for the electrolytes examined in this study.

Dynamics and mechanisms of interfacial photoinduced electron transfer processes of third generation photovoltaics and photocatalysis.

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aimed at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim to achieve a fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here: Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a Ru(II)(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than vibrational relaxation of the vibronic excited state of the donor. Instead, the back ET occurred on the micros time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor beta = 0.16 angstroms(-1) observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with nonequilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface.

Photoinduced interfacial electron transfer and lateral charge transport in molecular donor-acceptor photovoltaic systems.

Nanostructured liquid/solid and solid/solid bulk heterojunctions designed for the conversion of solar energy offer ideal models for the investigation of light-induced ET dynamics at surfaces. Despite significant study of processes leading to charge generation in third-generation solar cells, a conclusive picture of the photophysics of these photovoltaic converters is still missing. More specifically searched is the link between the molecular structure of the interface and the kinetics of surface photoredox reactions. Fundamental scientific issues in this field are addressed by the research project undertaken in the frame of the NCCR MUST endeavor, an outline of which is given here.