Asymmetrical Diketopyrrolopyrrole Derivatives with Improved Solubility and Balanced Charge Transport Properties.

The diketopyrrolopyrrole (DPP) unit represents one of the building blocks more widely employed in the field of organic electronics; in most of the reported DPP-based small molecules, this unit represents the electron acceptor core symmetrically coupled to donor moieties, and the solubility is guaranteed by functionalizing lactamic nitrogens with long and branched alkyl tails. In this paper, we explored the possibility of modulating the solubility by realizing asymmetric DPP derivatives, where the molecular structure is extended in just one direction. Four novel derivatives have been prepared, characterized by a common dithyenil-DPP fragment and functionalized on one side by a thiophene unit linked to different auxiliary electron acceptor groups. As compared to previously reported symmetric analogs, the novel dyes showed an increased solubility in chloroform and proved to be soluble in THF as well. The novel dyes underwent a thorough optical and electrochemical characterization. Electronic properties were studied at the DFT levels. All the dyes were used as active layers in organic field effect transistors, showing balanced charge transport properties.

Electron Localization and Mobility in Monolayer Fullerene Networks.

The novel 2D quasi-hexagonal phase of covalently bonded fullerene molecules (qHP C60), the so-called graphullerene, has displayed far superior electron mobilities, if compared to the parent van der Waals three-dimensional crystal (vdW C60). Herein, we present a comparative study of the electronic properties of vdW and qHP C60 using state-of-the-art electronic-structure calculations and a full quantum-mechanical treatment of electron transfer. We show that both materials entail polaronic localization of electrons with similar binding energies (≈0.1 eV) and, therefore, they share the same charge transport via polaron hopping. In fact, we quantitatively reproduce the sizable increment of the electron mobility measured for qHP C60 and identify its origin in the increased electronic coupling between C60 units.

The rates of non-adiabatic processes in large molecular systems: Toward an effective full-dimensional quantum mechanical approach.

Two computational approaches for computing the rates of internal conversions in molecular systems where a large set of nuclear degrees of freedom plays a role are discussed and compared. One approach is based on the numerical solution of the time-dependent Schrödinger equation and allows us to include almost the whole set of vibrational coordinates, thanks to the employment of effective procedures for selecting those elements of the Hilbert space which play a significant role in dynamics. The other approach, based on the time-dependent perturbation theory and limited to the use of the harmonic approximation, allows us to include the whole Hilbert space spanned by the vibrational states of the system. The two approaches are applied to the photophysics of azulene, whose anti-Kasha behavior caused by anomalous internal conversion rates is well assessed. The calculated rates for the decays of the first two excited singlet states are in very good agreement with experimental data, indicating the reliability of both methodologies.

Hydroxyl Radical vs. One-Electron Oxidation Reactivities in an Alternating GC Double-Stranded Oligonucleotide: A New Type Electron Hole Stabilization.

We examined the reaction of hydroxyl radicals (HO•) and sulfate radical anions (SO4•-), which is generated by ionizing radiation in aqueous solutions under anoxic conditions, with an alternating GC doubled-stranded oligodeoxynucleotide (ds-ODN), i.e., the palindromic 5'-d(GCGCGC)-3'. In particular, the optical spectra of the intermediate species and associated kinetic data in the range of ns to ms were obtained via pulse radiolysis. Computational studies by means of density functional theory (DFT) for structural and time-dependent DFT for spectroscopic features were performed on 5'-d(GCGC)-3'. Comprehensively, our results suggest the addition of HO• to the G:C pair moiety, affording the [8-HO-G:C]• detectable adduct. The previous reported spectra of one-electron oxidation of a variety of ds-ODN were assigned to [G(-H+):C]• after deprotonation. Regarding 5'-d(GCGCGC)-3' ds-ODN, the spectrum at 800 ns has a completely different spectral shape and kinetic behavior. By means of calculations, we assigned the species to [G:C/C:G]•+, in which the electron hole is predicted to be delocalized on the two stacked base pairs. This transient species was further hydrated to afford the [8-HO-G:C]• detectable adduct. These remarkable findings suggest that the double-stranded alternating GC sequences allow for a new type of electron hole stabilization via delocalization over the whole sequence or part of it.

Charge Localization in Acene Crystals from Ab Initio Electronic Structure.

The performance of Koopmans-compliant hybrid functionals in reproducing the electronic structure of organic crystals is tested for a series of acene crystals. The calculated band gaps are found to be consistent with those achieved with the GW method at a fraction of the computational cost and in excellent accord with the experimental results at room temperature, when including the thermal renormalization. The energetics of excess holes and electrons reveals a struggle between polaronic localization and band-like delocalization. The consequences of these results on the transport properties of acene crystals are discussed.

Sol-Gel Dipping Devices for H2S Visualization.

In this contribution we report the synthesis and full characterization, via a combination of different spectroscopies (e.g., 1H NMR, UV-vis, fluorescence, MALDI), of a new family of fluorescent zinc complexes with extended π-conjugated systems, with the final aim of setting up higher performance H2S sensing devices. Immobilization of the systems into a polymeric matrix for use in a solid-state portable device was also explored. The results provided proof-of-principle that the title complexes could be successfully implemented in a fast, simple and cost-effective H2S sensing device.

Salen, salan and salalen zinc(II) complexes in the interaction with HS-: time-resolved fluorescence applications.

In the current work we investigate the route of interaction of a newly synthesized family of zinc complexes with HS- by a plethora of different spectroscopic techniques. A computational analysis on the time dependent density functional theory (TD-DFT) level explored the overall fluorescence properties of the title complexes and their different fluorescence responses to HS-. Time-resolved fluorescence experiments were also performed and highlight the great potential of the current systems to be implemented as HS- fluorescent sensors.

Is a thin mechanism appropriate for aromatic nitration?

The mechanism of toluene nitration by NO2BF4 in dichloromethane solution is investigated by performing advanced ab initio MD simulations of the reaction trajectories, including at full quantum mechanical level the effects of both the solvent and of the counterion. The time evolution of the encounter complex, as well as that of the associated electronic structure, for different trajectories reveals that a single electron transfer step fastly occurs after reactants are accommodated in a common solvation shell, always preceding the formation of the σ-complex. The present results strongly suggest that the regioselectivity of the reaction is spin-density driven and that a thin mechanism, one based on reaction intermediates and transition states, can be appropriate to describe aromatic nitration.

Duplex DNA Retains the Conformational Features of Single Strands: Perspectives from MD Simulations and Quantum Chemical Computations.

Molecular dynamics simulations and geometry optimizations carried out at the quantum level as well as by quantum mechanical/molecular mechanics methods predict that short, single-stranded DNA oligonucleotides adopt conformations very similar to those observed in crystallographic double-stranded B-DNA, with rise coordinates close to ≈3.3 Å. In agreement with the experimental evidence, the computational results show that DNA single strands rich in adjacent purine nucleobases assume more regular arrangements than poly-thymine. The preliminary results suggest that single-stranded poly-cytosine DNA should also retain a substantial helical order in solution. A comparison of the structures of single and double helices confirms that the B-DNA motif is a favorable arrangement also for single strands. Indeed, the optimal geometry of the complementary single helices is changed to a very small extent in the formation of the duplex.

Electron Transfer Rates in Polar and Non-Polar Environments: a Generalization of Marcus' Theory to Include an Effective Treatment of Tunneling Effects.

A multistep kinetic model in which solvent motion is treated in the framework of Marcus theory and the rates of the elementary electron transfer step are evaluated at full quantum mechanical level is proposed and applied to the calculation of the rates of intramolecular electron transfer reactions in rigidly spaced D-Br-A (D = 1,1'-biphenyl radical anion, Br = androstane) compounds, for five acceptors (A) in three organic solvents with different polarity. The calculated rates agree well with experimental ones, and their temperature dependence is almost quantitatively reproduced.

Paper-Strip-Based Sensors for H2S Detection: A Proof-of-Principle Study.

In this work, the authors explored the interaction of a suite of fluorescent zinc complexes with H2S. The authors provide evidence that HS- binds the zinc center of all the complexes under investigation, allowing them to possibly function as sensors by a 'coordinative-based' approach. Naked-eye color changes occur when treating the systems with HS-, so the fluorescence responses are modulated by the presence of HS-, which has been related to a change in the energy level and coupling of excited states through a computational study. The results show the potential of the systems to function as HS-/H2S colorimetric and fluorescent sensors. Paper-strip-based sensing experiments foresee the potential of using this family of complexes as chemosensors of HS- in more complex biological fluids.

High-Energy-Density Materials: An Amphoteric N-Rich Bis(triazole) and Salts of Its Cationic and Anionic Species.

The synthesis and characterization of the N-rich bis(triazole) compound 1H,4'H-[3,3'-bis(1,2,4-triazole)]-4',5,5'-triamine (C4H7N9) with a N content of 69.6% by weight is reported. The compound exhibits a rich acid-base behavior because it can accept up to two protons, forming a monocation and a dication, and can lose one proton, forming an anion. Measurement of the acid constants has shown that there exist well-defined pH intervals in which each of the four species is predominant in solution, opening the way to their isolation and characterization by single-crystal X-ray analysis as salts with different counterions. Some energetic salts of the monocation or dication containing oxidizing inorganic counterions (dinitramide, perchlorate, and nitrate) were also prepared and characterized in the solid state for their sensitivity. In particular, the neutral compound shows a very remarkable thermal stability in air, with Td = 347 °C, and is insensitive to impact and friction. Salts of the dication with energetic counterions, in particular perchlorate and nitrate, show increased sensitivities and reduced thermal stability. The salt of the monocation with dinitramide as the counterion outperforms other dinitramide salts reported in the literature because of its higher thermal stability (Td = 230 °C in air) and friction insensitiveness.

The Time Scale of Electronic Resonance in Oxidized DNA as Modulated by Solvent Response: An MD/QM-MM Study.

The time needed to establish electronic resonant conditions for charge transfer in oxidized DNA has been evaluated by molecular dynamics simulations followed by QM/MM computations which include counterions and a realistic solvation shell. The solvent response is predicted to take ca. 800-1000 ps to bring two guanine sites into resonance, a range of values in reasonable agreement with the estimate previously obtained by a kinetic model able to correctly reproduce the observed yield ratios of oxidative damage for several sequences of oxidized DNA.

Quantitative Prediction of the Electro-Mechanical Response in Organic Crystals.

Organic semiconductors' inherent flexibility makes them appealing for advanced applications such as wearable electronics, e-skins, or pressure sensors, and can even be used to enhance their intrinsic electronic properties. Unfortunately, these applications for organic materials are currently hindered by the lack of a quantitative understanding of the interplay between their electrical and mechanical properties. In this work, this gap is filled by presenting an accurate methodology able to predict quantitatively the effects of external deformation on the charge transport properties of any organic semiconductors. Three prototypical materials are investigated, showing that the experimental variation of charge carrier mobility with strain is fully reproduced, even in a wide range of deformations applied along different crystal axes. The results indicate that the intrinsic electro-mechanical response of the materials varies by orders of magnitude within the class of organic semiconductors, a difference rationalized observing that the mobility trend is primarily influenced by the transfer integrals' variation, rather than by a modification of the crystal phonons. In light of its robustness, accuracy, and low computational cost, this protocol represents an ideal tool to quantify the electro-mechanical response in new organic compounds, thus establishing a reliable route for a full exploitation of strain engineering in advanced technologies.

Reliable Predictions of Benzophenone Singlet-Triplet Transition Rates: A Second-Order Cumulant Approach.

Fermi golden rule and second-order cumulant expansion of the time-dependent density matrix have been used to compute from first principles the rate of intersystem crossing in benzophenone, using minimum-energy geometries and normal modes of vibrations computed at the TDDFT/CAM-B3LYP level. Both approaches yield reliable values of the S1 decay rate, the latter being almost in quantitative agreement with the results of time-dependent spectroscopic measurements (0.154 ps-1 observed vs 0.25 ps-1 predicted). The Fermi golden rule slightly overestimates the decay rate of S1 state (kd = 0.45 ps-1) but provides better insights into the chemico-physical parameters, which govern the transition from a thermally equilibrated population of S1, showing that the indirect mechanism is much faster than the direct one because of the vanishingly small Franck-Condon weighted density of states at ΔE of transition.

Fluorescent salen-type Zn(II) Complexes As Probes for Detecting Hydrogen Sulfide and Its Anion: Bioimaging Applications.

In this work, we investigate the mode of interaction of a family of fluorescent zinc complexes with HS- and H2S. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS- binds the zinc center of all the complexes under investigation. Treatment with neutral H2S exhibits a markedly different reactivity which indicates selectivity for HS- over H2S of the systems under investigation. Striking color changes, visible to the naked eye, occur when treating the systems with HS- or by an H2S flow. Accordingly, also the fluorescence is modulated by the presence of HS-, with the possible formation of multiple adducts. The results highlight the potential of the devised systems to be implemented as HS-/H2S colorimetric and fluorescent sensors. Bioimaging experiments indicate the potential of using this class of compounds as probes for the detection of H2S in living cells.

Tautomeric and conformational switching in a new versatile N-rich heterocyclic ligand.

A new N-rich triazolo-triazole derivative, 4-methyl-7-(pyrazin-2-yl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole (C8H7N7), bearing a pyrazine residue at 7-position of the triazolo-triazole bicycle, was synthesized, and its acid-base and metal coordination properties were evaluated in solution. The results showed amphoteric behavior and the formation of stable complexes with Cu(ii) and Zn(ii) in pH intervals in which the ligand is neutral or deprotonated. Computational studies were performed in order to evaluate the stability of the different tautomers/conformers of the ligand, and the proton position in the neutral and acidic forms. Single crystal X-ray analysis of the free neutral ligand (2H/s-trans tautomer/conformer), and of its singly protonated (2H-3H/s-trans), doubly protonated (2H-3H-7H/s-trans) and deprotonated forms showed that the influence of the pyrazine ring on the triazolo-triazole system is mainly as electron withdrawing and chelating group, and proton acceptor. Different coordination modes have been evidenced for the neutral and deprotonated ligand. Upon metal coordination, the neutral ligand switches from 2H/s-trans to 3H/s-cis tautomer/conformer forming five-membered chelate rings, while the anionic deprotonated ligand forms six-membered chelate rings in the s-trans conformation. Altogether, five different tautomers/conformers of the ligand were isolated and characterized. In vitro tests confirmed the general antiproliferative activity of triazolo-triazole compounds and the importance of substitution in position 7 for their selectivity.

Coherent Effects in Charge Transport in Molecular Wires: Toward a Unifying Picture of Long-Range Hole Transfer in DNA.

In the framework of a multistep mechanism in which environmental motion triggers comparatively faster elementary electron-transfer steps and stabilizes hole-transfer products, microscopic coherence is crucial for rationalizing the observed yield ratios of oxidative damage to DNA. Interference among probability amplitudes of indistinguishable electron-transfer paths is able to drastically change the final outcome of charge transport, even in DNA oligomers constituted by similar building blocks, and allows for reconciling apparently discordant experimental observations. Properly tailored DNA oligomers appear to be a promising workbench for studying tunneling in the presence of dissipation at the macroscopic level.

The Dynamics of Hole Transfer in DNA.

High-energy radiation and oxidizing agents can ionize DNA. One electron oxidation gives rise to a radical cation whose charge (hole) can migrate through DNA covering several hundreds of Å, eventually leading to irreversible oxidative damage and consequent disease. Understanding the thermodynamic, kinetic and chemical aspects of the hole transport in DNA is important not only for its biological consequences, but also for assessing the properties of DNA in redox sensing or labeling. Furthermore, due to hole migration, DNA could potentially play an important role in nanoelectronics, by acting as both a template and active component. Herein, we review our work on the dynamics of hole transfer in DNA carried out in the last decade. After retrieving the thermodynamic parameters needed to address the dynamics of hole transfer by voltammetric and spectroscopic experiments and quantum chemical computations, we develop a theoretical methodology which allows for a faithful interpretation of the kinetics of the hole transport in DNA and is also capable of taking into account sequence-specific effects.

Transient and Enduring Electronic Resonances Drive Coherent Long Distance Charge Transport in Molecular Wires.

It is shown that the yields of oxidative damage observed in double-stranded DNA oligomers consisting of two guanines separated by adenine-thymine (A:T) n bridges of various lengths are reliably accounted for by a multistep mechanism, in which transient and nontransient electronic resonances induce charge transport and solvent relaxation stabilizes the hole transfer products. The proposed multistep mechanism leads to results in excellent agreement with the observed yield ratios for both the short and the long distance regime; the almost distance independence of yield ratios for longer bridges ( n ≥ 3) is the consequence of the significant energy decrease of the electronic levels of the bridge, which, as the bridge length increases, become quasi-degenerate with those of the acceptor and donor groups (enduring resonance). These results provide significant guidelines for the design of novel DNA sequences to be employed in organic electronics.