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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Arterial aneurysms associated with intracranial dural arteriovenous fistulas: epidemiology, natural history, and management. A systematic review.

Arterial aneurysms are uncommon among patients with dural arteriovenous fistulae (DAVFs), and there is limited information available to guide treatment decisions in such cases. We performed a systematic review of the literature, including a case of a DAVF associated with a flow-related intraorbital ophthalmic artery (OA) aneurysm that we have recently managed. The purpose of our study was to clarify epidemiology, natural history, and management of these lesions. A total of 43 published cases of DAVF associated aneurysms were found in 26 studies on the topic. Anterior cranial fossa was the most common location (40%), and ethmoidal branches were the most common arterial feeders (55%). In about 63% of cases, the aneurysm was located on artery unrelated to DAVF supply. Approximately 10% of intracranial DAVFs were associated with aneurysms located in the intraorbital OA. Overall, 70% of lesions were Borden type III, and 50% of patients presented with hemorrhage. In approximately 17% of cases, the source of bleeding was a feeding artery aneurysm. All of the reported intraorbital OA aneurysms associated with DAVFs remained stable during follow-up. DAVF associated aneurysms are fairly rare. Anterior cranial fossa location and direct cortical venous drainage are common among these lesions. The aneurysms are less likely to be located on feeding arteries, and hemorrhagic presentation related to flow-related aneurysm rupture is uncommon.

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.

An Anthracene-Incorporated [8]Cycloparaphenylene Derivative as an Emitter in Photon Upconversion.

The anthracene-incorporated [8]cycloparaphenylene 2 has been synthesized and its optoelectronic properties studied by UV-vis spectroscopy, cyclic voltammetry, and DFT calculations. NMR and computational studies indicate that the new cycloparaphenylene derivative possesses a Cs point group symmetry. The new CPP 2 exhibits peculiar optoelectronic properties: (i) fluorescence emission is blue-shifted with respect to [8]cycloparaphenylene 1, and its quantum yield is higher; (ii) in the presence of an octaethylporphyrin Pd complex, as sensitizer, it undergoes a visible light upconversion. This is the first case in which a cycloparaphenylene derivative is involved as an emitter in low power light frequency conversion.

Solid State Separation and Isolation of Tautomers of Fused-Ring Triazolotriazoles.

Fine control of the tautomeric forms of [1,2,4]triazolo[3,2-c][1,2,4]triazole derivatives in acidic conditions has been achieved by acting on the electronic character of the substituent at position 7 of the heterobicycle and on the counterion. Strong electron releasing or electron withdrawing substituents lead almost exclusively to a single tautomeric form, the 1H-3H or the 2H-3H, respectively. In the case of the phenol substituent, both tautomeric forms are present in comparable amount in solution; the two tautomers can also be selectively precipitated in different crystalline salts using suitable counterions.

Modeling DNA oxidation in water.

A novel set of hole-site energies and electronic coupling parameters to be used, in the framework of the simplest tight-binding approximation, for predicting DNA hole trapping efficiencies and rates of hole transport in oxidized DNA is proposed. The novel parameters, significantly different from those previously reported in the literature, have been inferred from reliable density functional calculations, including both the sugar-phosphate ionic backbone and the effects of the aqueous environment. It is shown that most of the experimental oxidation free energies of DNA tracts and of oligonucleotides available from photoelectron spectroscopy and voltammetric measurements are reproduced with great accuracy, without the need for introducing sequence dependent parameters.

Absorption Band Shapes of a Push-Pull Dye Approaching the Cyanine Limit: A Challenging Case for First Principle Calculations.

The absorption band shapes of a solvent tunable donor-acceptor dye have been theoretically investigated by using Kubo's generating function approach, with minimum energy geometries and normal coordinates computed at the DFT level of theory. The adopted computational procedure allows us to include in the computation of Franck-Condon factors the whole set of normal modes, without any limitation on excitation quanta, allowing for an almost quantitative reproduction of the absorption band shape when the equilibrium geometries of the ground and the excited states are well predicted by electronic computations. Noteworthy, the functionals that yield more accurate band shapes also provide good prediction of the moment variations upon excitation; because the latter quantities are rarely available, theoretical simulation of band shapes could be a powerful tool for choosing the most appropriate computational method for predictive purposes.

Quantum dynamics of electronic transitions with Gauss-Hermite wave packets.

A new methodology based on the superposition of time-dependent Gauss-Hermite wave packets is developed to describe the wave function of a system in which several interacting electronic states are coupled to a bath of harmonic oscillators. The equations of motion for the wave function parameters are obtained by employing the Dirac-Frenkel time-dependent variational principle. The methodology is applied to study the quantum dynamical behaviour of model systems with two interacting electronic states characterized by a relatively large reorganization energy and a range of energy biases. The favourable scaling properties make it a promising tool for the study of the dynamics of chemico-physical processes in molecular systems.

Hole delocalization over adenine tracts in single stranded DNA oligonucleotides.

Adiabatic ionization energies of single stranded DNA oligonucleotides containing adenine tracts of different sizes have been computed at the DFT level and compared with the oxidation potentials determined by differential pulse voltammetry. Geometry optimizations have been performed at the full quantum mechanical level, including the sugar phosphate backbone and solvent effects. The observed progressive lowering of the ionization energy upon increasing the number of consecutive adenines is well predicted, the computed ionization potential shifts being in very good agreement with the experimental outcomes, both by using pure and hybrid functionals. The spin density of the oligonucleotide radical cations is distributed almost over the whole adenine tract, forming delocalized polarons.

Vibronic couplings and coherent electron transfer in bridged systems.

A computational strategy to analyze the dynamics of coherent electron transfer processes in bridged systems, involving three or more electronic states, is presented. The approach is based on partitioning of the Hilbert space of the time independent basis functions in subspaces of increasing dimensionality, which allows us to easily check the convergence of the time dependent wave function. Vibronic couplings are determined by Duschinsky's analysis of the equilibrium position displacements, carried out using the equilibrium geometries and normal modes of the redox partners obtained at the DFT computational level.