π-Conjugated zwitterions as paradigm of donor-acceptor building blocks in organic-based materials.

The very peculiar characteristics of zwitterions, as well as a clearand unambiguous definition, have been overlooked in past literature. However, these compounds are particularly important in view of the impact they have had in the recent past and will likely continue to have in the future as components of performing functional organic and hybrid materials. In this Account, we primarily aim to define critically important organic concepts of zwitterions regarding both their design and nomenclature. We will particularly focus on a specific kind of zwitterions we define as π-conjugated zwitterions. These types of zwitterions are systems pertaining to the class ofdonor-acceptor (push-pull) molecules. In the ground state, they are preferentially represented in terms of an electron donor moiety bearing a negative net charge, and electron acceptor one bearing a positive net charge connected by a conjugated bridge. As such, they are possibly the most effective example of push-pull structure, possessing relevant features for applications like nonlinear optics, photovoltaics, imaging, and high capacitance dielectrics. In addition, the interaction between these dipolar compounds and the environment is highly specific and can be exploited in the construction of well-organized nanostructures, both in solution and in the solid state. According to the Gold Book of IUPAC for nomenclature, the distinction between zwitterions and the charged molecule called a betaine is subtle. The betaine is a particular class of zwitterion possessing an onium atom not bearing a hydrogen. The two terms are often considered equivalent, thus generating confusion while retrieving literature. In this Account, we define and describe π-conjugated zwitterions systems that are dipolar in the ground state, admitting resonance limiting structures that are neutral and chargeless. For the purpose of this Account and to the benefit of researchers striving to retrieve materials-related zwitterion literature data, we suggest to use the term π-zwitterions instead of the commonly used plain term "zwitterions". We show that this definition enables the clear identification of a class of compounds having unique properties distinct from "dipolar conjugated compounds." We describe the most common donor and acceptor groups in π-zwitterions. In particular, we focus our attention on the special case of the nitrile functionality, which tends to be contiguous to a negative charge. We also address special emphasis to benzenoid components that are substituted by heteroaromatic units in π-zwitterions, because the HOMO-LUMO energetic consequences are specifically involved in these cases. We make reference to the paradigmatic case of π-zwitterions second order nonlinear optical properties. Here, the value of the first hyperpolarizability ß versus the alternation in bond length turns out to be a measure of the balance of the chargeless and the dipolar contribution to the description of the zwitterion ground state. We also report literature data, collected both from our group and others, concerning π-zwitterions containing heteroaromatic and/or nitrile groups, those based on the most performing acceptors so far described, and merocyanines. With particular reference to merocyanines, we show how π-zwitterions can play a fundamental role in the fast growing field of organic photovoltaics. Finally, we present π-zwitterions made up of heteroaromatic groups that open new scenarios in heteroaromatic chemistry.

Exomethylene-3,4-ethylenedioxythiophene (emEDOT): a new versatile building block for functionalized electropolymerized poly(3,4-ethylenedioxythiophenes) (PEDOTs).

A new versatile thiophene derivative exomethylene-3,4-ethylenedioxythiophene (emEDOT) is introduced. The molecule can be straightforwardly prepared in two steps from commercially available derivatives and enables facile further derivatization through both acid catalyzed additions of alcohols and standard thiol-ene click chemistry. The preparation of electrochromic materials and of an electrochemical avidine sensor is shown by the oxidative polymerizations of several functionalized EDOT monomers straightforwardly prepared from emEDOT.

Gray to colorless switching, crosslinked electrochromic polymers with outstanding stability and transmissivity from naphthalenediimmide-functionalized EDOT.

The design, synthesis, polymerization and full electrochemical and spectroelectrochemical characterization of a new naphtalendiimide-functionalized PEDOT cross-linked electrochromic material is reported. The polymer shows exceptionally high redox reversibility, almost complete colorlessness in the bleached state and a gray color in the reduced state.

Regioselective synthesis of 1,2- vs 1,3-squaraines.

Squaraines have been known for many decades as very stable and versatile vis-NIR absorbing dyes. They have found applications for example as sensitizers in organic photovoltaics and photodetectors. The most common squaraine structure is the 1,3-regioisomer. Their 1,2-regioisomers are seldom mentioned and unanimously regarded as side products. A facile direct synthesis of 1,2-squaraines, highlighting the role played by reaction conditions and electronic factors, is described. The first electrochemical characterization of these dyes is also shown.

UV absorbing zwitterionic pyridinium-tetrazolate: exceptional transparency/optical nonlinearity trade-off.

We present relevant results dealing with the transparency/optical nonlinearity trade-off in high-frequency electro-optic applications. The very simple, stable and high optical gap chromophore, the zwitterion 1-methyl-4-(tetrazol-5-ate)pyridinium, represents the best transparency/optical nonlinearity trade-off so far described in the literature. We rationalize this remarkable performance in the framework of the Bond Length Alternation theory by means of a multidisciplinary approach including: single crystal X-ray structure, Electric Field Induced Second-Harmonic Generation, solvatochromism, electrochemistry and thermal analyses.

Marked alkyl- vs alkenyl-substitutent effects on squaraine dye solid-state structure, carrier mobility, and bulk-heterojunction solar cell efficiency.

We report two new squaraine dyes substituted at the pyrrolic rings with n-hexyl (squaraine 1) or n-hexenyl (squaraine 2) chains. Although internal molecular structure variations are minimal, the presence of the terminal double bond results in a much more compact solid-state structure, dramatically affecting charge transport in the thin films; the hole mobility of 2 is approximately 5x that of 1, and the BHJ OPV power conversion efficiency (PCE) of 2 is approximately 2x that of 1. PCEs surpassing 2% for ambient solution-processed devices are demonstrated, the largest so far achieved for squaraine-based organic solar cells.

Photophysics of squaraine dyes: role of charge-transfer in singlet oxygen production and removal.

The unique optical properties of squaraines render these molecules useful for applications that range from xerography to photodynamic therapy. In this regard, squaraines derived from the condensation of nitrogen-based heterocycles and squaric acid have many promising attributes. Key solution-phase photophysical properties of six such squaraines have been characterized in this study. One feature of these molecules is a pronounced absorption band in the region approximately 600-720 nm that has significant spectral overlap with the fluorescence band (i.e., the Stokes shift is small). As such, effects of emission/reabsorption yield unique excitation wavelength dependent phenomena that are manifested in quantum yields of both fluorescence and sensitized singlet oxygen production. Comparatively small squaraine-sensitized yields of singlet oxygen production and, independently, large rate constants for squaraine-induced deactivation of singlet oxygen are consistent with a model in which there is appreciable intra- and intermolecular charge-transfer in the squaraine and squaraine-oxygen encounter complex, respectively. The results reported herein should be useful in the further development of these compounds for a range of oxygen-dependent applications.

Photooxidation and Phototoxicity of pi-extended squaraines.

This paper describes the synthesis of pi-extended squaraines and their photooxidation properties and gives an in-depth characterization of these molecules as photosensitizing agents. Squaraines show a strong absorption in the tissue transparency window (600-800 nm), and upon irradiation, they undergo a photooxidation process, leading to the formation of peroxide and hydroperoxide radicals according to a type I radical chain process. Confocal laser microscopy demonstrates that the designed squaraines efficiently internalize in the cytoplasm and not in the nucleus of the cell. In the dark, they are scarcely cytotoxic, but after irradition, they promote a strong dose-dependent phototoxic effect in four different cancer cells. In HeLa and MCF-7 cells, squaraines 4 and 5, thanks to their hydrocarbon tails, associate to the membranes and induce lipid peroxidation, as indicated by a marked increase of malonyldialdehyde after photodynamic treatment, in agreement with in vitro photooxidation studies. FACS, caspase-3/7 assays and time-lapse microscopy demonstrate that the designed squaraines cause cell death primarily by necrosis.

A high molecular weight donor for electron injection interlayers on metal electrodes.

The molecular donor 9,9'-ethane-1,2-diylidene-bis(N-methyl-9,10-dihydroacridine) (NMA) has been synthesized, and its electronic properties were characterized both in solution using cyclic voltammetry and optical absorption spectroscopy, and at interfaces to metals with photoelectron spectroscopy (PES). The optical energy gap of NMA in solution increases by 0.10 eV when the compound is doubly oxidized. On the basis of quantum-chemical calculations, this ipsochromic effect is rationalized by a change in geometry involving a severe torsion of the two acridinium moieties with respect to the central double bond, thus reducing conjugation upon oxidation. PES is reported for NMA deposited on Au(111), Ag(111), and Cu(111) single crystals. A decrease of the sample work function is observed that becomes larger with increasing molecular coverage and clearly exceeds values that would be expected for metal surface electron "push back" alone, confirming the electron donating nature of NMA. The growth mode of NMA on all three surfaces is almost layer-by-layer (Frank-van der Merwe). For tris(8-hydroxyquinoline)aluminum (Alq(3)) deposited on top of a NMA-modified Au(111) surface, the electron injection barrier (EIB) is reduced by 0.25 eV compared to that on pristine Au(111). Furthermore, the EIB reduction depends linearly on Phi of the donor-modified Au(111) surface, adjustable by NMA precoverage. This enables continuous tuning of the EIB in organic electronic devices, in order to optimize device efficiency and performance.

A squaraine-phthalocyanine ensemble: towards molecular panchromatic sensitizers in solar cells.

A supramolecular phthalocyanine-squaraine ensemble which exhibits a large coverage of the solar spectrum from 850 to 250 nm has been prepared and characterized both photophysically and in bulk heterojunction solar cells.

Second-order nonlinear optical activity of dipolar chromophores based on pyrrole-hydrazono donor moieties.

A series of highly efficient and thermally stable second-order nonlinear optical (NLO) dipolar donor-auxiliary donor-acceptor chromophores have been synthesised in which a hydrazono group and a pyrrole ring act as donor and auxiliary donor components, respectively, in combination with different aromatic and heteroaromatic acceptors. The new dyes have been systematically investigated by NMR spectroscopy, absorption spectroscopy, NLO measurements and thermal stability studies. NLO properties have been studied in detail by electro-optical absorption (EOA) and hyper-Rayleigh scattering (HRS) measurements in 1,4-dioxane and DMSO, respectively. The results originating from the two different methods have been compared and analysed in detail. We found that the NLO properties measured by the EOA and HRS methods correlate with each other and converge to reveal the dye with the acceptor 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran as the most efficient system. The unprecedented combination of a strong donor hydrazono group and the auxiliary donor effects of pi-excessive heteroaromatic rings afforded NLO chromophores with very high values (mu(g)beta0(EOA) up to 2038 x 10(-48) esu and beta(HRS) up to 3980 x 10(-30) esu at 1.5 microm).

Perfluorinated nitrosopyrazolone-based erbium chelates: a new efficient solution processable NIR emitter.

We show the design and synthesis of new perfluorinated nitrosopyrazolone-based ligands and the original method employed for their complexation of erbium ions in the presence of the co-ligand perfluorotriphenylphosphine oxide; the resulting chelate is non-hygroscopic, solution processable and possesses a NIR emission with lifetimes as long as 16 micros.

Efficient squaraine-based solution processable bulk-heterojunction solar cells.

We report the fabrication of organic bulk-heterojunction solar cells based, for the first time, on squaraine/PCBM blends. The most efficient device, solution-processed in air, exhibits J(sc) = 5.70 mA/cm(2), V(oc) = 0.62 V, fill-factor = 0.35, and power conversion efficiency = 1.24%, one of the highest to date for a small molecule solution-processed bulk-heterojunction cell.

Effective two-photon absorption cross section of heteroaromatic quadrupolar dyes: dependence on measurement technique and laser pulse characteristics.

The linear and nonlinear optical properties of the heteroaromatic push-pull-push two-photon absorbing dye N-methyl-2,5-bis[1-(N-methylpyrid-4-yl)ethen-2-yl]-pyrrole ditriflate (PEPEP) are reported. The determination of the two-photon absorption (TPA) cross-section spectrum has been performed with different techniques: femtosecond TPA-white light continuum probe experiments, two-photon-induced fluorescence, and open aperture Z-scan measurements using both nanosecond and femtosecond laser pulses. The measured TPA cross sections and their wavelength dispersion show a marked dependence on the parameters of the laser pulses and on the measurement technique employed. These properties are discussed in terms of the different microscopic mechanisms that can contribute to the multiphoton absorption processes, with different weight depending on the measurement conditions and on the photophysical parameters of the dye.

Assessment of water-soluble pi-extended squaraines as one- and two-photon singlet oxygen photosensitizers: design, synthesis, and characterization.

Singlet oxygen sensitization by organic molecules is a topic of major interest in the development of both efficient photodynamic therapy (PDT) and aerobic oxidations under complete green chemistry conditions. We report on the design, synthesis, biology, and complete spectroscopic characterization (vis-NIR linear and two-photon absorption spectroscopy, singlet oxygen generation efficiencies for both one- and two-photon excitation, electrochemistry, intrinsic dark toxicity, cellular uptake, and subcellular localization) of three classes of innovative singlet oxygen sensitizers pertaining to the family of symmetric squaraine derivatives originating from pi-excessive heterocycles. The main advantage of pi-extended squaraine photosensitizers over the large number of other known photosensitizers is their exceedingly strong two-photon absorption enabling, together with sizable singlet oxygen sensitization capabilities, for their use at the clinical application relevant wavelength of 806 nm. We finally show encouraging results about the dark toxicity and cellular uptake capabilities of water-soluble squaraine photosensitizers, opening the way for clinical small animal PDT trials.

Extended squaraine dyes with large two-photon absorption cross-sections.

Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.

Novel nontoxic mitochondrial probe for confocal fluorescence microscopy.

We propose a 2,5-Bis[1-(4-N-methylpyridinium)ethen-2-yl)]-N-methylpyrrole ditriflate (PEPEP) as a novel nontoxic, nonpotentiometric mitochondrial probe for confocal fluorescence microscopy. PEPEP is a representative chromophore of a large family of heterocyclic fluorescent dyes that show fluorescence emission in aqueous media and great DNA affinity. We check its cytotoxicity and intracellular localization in mammalian and yeast cell cultures. We demonstrate that PEPEP is a very efficient dye for fluorescence confocal microscopy and a valuable alternative to the most frequently used mitochondrial stains.

Enhancement of two-photon absorption cross-section and singlet-oxygen generation in porphyrins upon beta-functionalization with donor-acceptor substituents.

The microwave-enhanced synthesis, comparative singlet oxygen sensitization efficiency, and nonlinear optical characterization of a new beta-functionalized porphyrin and its copper complex are described. We show that the introduction of a donor-acceptor push-pull conjugated fragment in the beta position strongly perturbs the porphyrin electronic structure leading to a remarkable one- and two-photon NIR absorption enhancement.

Strategies for electrooptic film fabrication. Influence of pyrrole-pyridine-based dibranched chromophore architecture on covalent self-assembly, thin-film microstructure, and nonlinear optical response.

The new dibranched, heterocyclic "push-pull" chromophores bis{1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane (1), 1-(pyrid-4-yl)-2-(N-methyl-5-formylpyrrol-2-yl)ethylene (2), {1-(N-methylpyridinium-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}{(1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane (3), N-methyl-2-[1-(N-methylpyrid-4-yl)ethen-2-yl]-5-[pyrid-4-yl]ethen-2-yl]pyrrole iodide (4), bis{1-(N-methyl-4-pyridinio)-2-[2-(N-methylpyrrol-5-yl)]ethane}methane iodide (5), and N-methyl-2,5-[1-(N-methylpyrid-4-yl)ethen-2-yl]pyrrole iodide (6) have been synthesized and characterized. The neutral (1 and 2) and monomethyl salts (3 and 4) undergo chemisorptive reaction with iodobenzyl-functionalized surfaces to afford chromophore monolayers SA-1/SA-2 and SA-3/SA-4, respectively. Molecular structures and other physicochemical properties have been defined by (1)H NMR, optical spectroscopy, and XRD. Thin-film characterization by a variety of techniques (optical spectroscopy, specular X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and angle-dependent polarized second harmonic generation) underscore the importance of the chromophore molecular architecture as well as film growth method on film microstructure and optical/electrooptic response.