Modeling charge and energy transfer in organic molecular materials

The understanding of nanoscale physics, chemistry and biology still poses unanswered questions such as how the optical and electrical properties of materials evolve from those of individual molecules, and organic semiconductors fall in this class of materials. The main processes occurring in such systems are both charge and energy transfer, responsible for the practical operation of electronic devices. Therefore, an understanding at a fundamental level of the electronic properties of the involved molecules can help the optimization of each process, for a better global performance of the material. My three years PhD activity was developed along two major lines of research: charge and energy transport, both based on the computational investigation of intramolecular properties and intermolecular interactions. Strictly related to energy transport are the optical properties of condensed phase materials and how they evolve from those of isolated molecular components. The charge transport properties were investigated for several organic molecular crystals showing semiconducting behavior, whose experimental crystal structure and charge mobilities are available. As the same interactions that drive the transport of charge play also a role in determining the optical properties and the energy transport in molecular aggregates, in my research activity I investigated such processes as well. In this regard, I took into account a dimer of perylene-bisimide, with the aim of elucidating the role of charge transfer states and their effect on optoelectronic properties. Additionally, to assess the propagation of excited states in a molecular material a kinetic constant is required, similarly to charge transport, but the expression in this case includes the overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor. To this end I also developed a code devoted to the simulation of linear absorption and emission spectra of an isolated molecule, starting from computed quantum mechanical properties

Resonant TERS of a Single-Molecule Kondo System

Single-molecule tip-enhanced Raman spectroscopy (TERS) under ultra-high vacuum (UHV) and cryogenic conditions enables exploration of the relations between the adsorption geometry, electronic state, and vibrational fingerprints of individual molecules. TERS capability of reflecting spin states in open-shell molecular configurations is yet unexplored. Here we use the tip of a scanning probe microscope to lift a perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecule from a metal surface to bring it into an open-shell spin one-half anionic state. We reveal a correlation between the appearance of a Kondo resonance in the differential conductance spectroscopy and concurrent characteristic changes captured by the TERS measurements. Through a detailed investigation of various adsorbed and tip-contacted PTCDA scenarios, we infer that the Raman scattering on the suspended PTCDA is resonant with a higher excited state. Theoretical simulation of the vibrational spectra enables a precise assignment of the individual TERS peaks to high-symmetry Ag modes, including the fingerprints of the observed spin state. These findings highlight the potential of TERS in capturing complex interactions between charge, spin, and photophysical properties in nanoscale molecular systems, and suggest a pathway for designing spin-optical devices using organic molecules

Charge transport parameters for carbon based nanohoops and donor–acceptor derivatives

The effect of donor–acceptor (D–A) moieties on magnitudes such as reorganization energies and electronic couplings in cycloparaphenylene (CPP) carbon based nanohoops (i.e. conjugated organic molecules with cyclic topology) is highlighted via model computations and analysis of the available crystalline structure of N,N-dimethylaza[8]CPP. For the sake of comparison, intra-molecular and inter-molecular charge transport parameters are concomitantly modelled for the recently determined herringbone polymorph of [6]CPP, along with [8]CPP and [12]CPP. The peculiar contribution of low frequency vibrations to intramolecular reorganization energies is also disclosed by computing the Huang–Rhys factors for the investigated [n]CPPs and the N,N-dimethylaza derivative. In contrast with most planar organic semiconductors where the layer in which molecules are herringbone arranged identifies the high-mobility plane, nanohoops disclose inter-layer electronic couplings larger than the intra-layer counterparts. Charge transfer rate constants modelled with three different approaches (Marcus, Marcus–Levich–Jortner and spectral overlap) suggest that D–A nanohoops, owing to orbital localization, may be more efficient for charge transport than [n]CPPs for suitable solid phase arrangements.FN and SC acknowledge financial support from the University of Bologna. AJPJ and JCSG acknowledge the ‘‘Ministerio de Economía y Competitividad’’ of Spain and the ‘‘European Regional Development Fund’’ through the project CTQ2014-55073-P

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position

In this study, a set of 10 positional indolocarbazole (ICz) isomers substituted with dicyanomethylene groups connected via para or meta positions are computationally investigated with the aim of exploring the efficiency of structural isomerism and substitution position in controlling their optical and electronic properties. Unrestricted density functional theory (DFT), a spin-flip time-dependent DFT approach, and the multireference CASSCF/NEVPT2 method have been applied to correlate the diradical character with the energetic trends (i.e., singlet–triplet energy gaps). In addition, the nucleus-independent chemical shift together with ACID plots and Raman intensity calculations were used to strengthen the relationship between the diradical character and (anti)aromaticity. Our study reveals that the substitution pattern and structural isomerism represent a very effective way to tune the diradical properties in ICz-based systems with meta-substituted systems with a V-shaped structure displaying the largest diradical character. Thus, this work contributes to the elucidation of the challenging chemical reactivity and physical properties of diradicaloid systems, guiding experimental chemists to produce new molecules with desirable properties.Funding for open access charge: Univesidad de Málaga/CBUA. The work at the University of Málaga was funded by the MICINN (PID2019-110305GB-I00) and Junta de Andalucía (UMA18-FEDERJA-080, P09FQM-4708, and P18-FR-4559) projects. The authors thankfully acknowledge the computer resources, technical expertise, and assistance provided by the SCBI (Supercomputing and Bioinformatics) centre of the University of Málaga. The work at the University of Alicante was supported by the MICINN (PID2019-106114GB-I00). The work at the University of Bologna was supported by University of Bologna (RFO) funds

Unveiling the collaborative effect at the cucurbit[8]urilMoS2 hybrid interface for electrochemical melatonin determination

Host-guest interactions are of paramount importance in supramolecular chemistry and in a wide range of applications. Particularly well known is the ability of cucurbit[n]urils (CB[n]) to selectively host small molecules. We show that the charge transfer and complexation capabilities of CB[n] are retained on the surface of 2D transition metal dichalcogenides (TMDs), allowing the development of efficient electrochemical sensing platforms. We unveil the mechanisms of host-guest recognition between the MoS2- CB[8] hybrid interface and melatonin (MLT), an important molecular regulator of vital constants in vertebrates. We find that CB[8] on MoS2 organizes the receptor portals perpendicularly to the surface, facilitating MLT complexation. This advantageous adsorption geometry is specific to TMDs and favours MLT electro-oxidation, as opposed to other 2D platforms like graphene, where one receptor portal is closed. This study rationalises the cooperative interaction in 2D hybrid systems to improve the efficiency and selectivity of electrochemical sensing platform

Beyond the 2D Field‐Effect Charge Transport Paradigm in Molecular Thin‐Film Transistors

Organic field-effect transistors (OFETs) are considered almost purely interfacial devices with charge current mainly confined in the first two semiconducting layers in contact with the dielectric with no active role of the film thickness exceeding six to eight monolayers (MLs). By a combined electronic, morphological, structural, and theoretical investigation, it is demonstrated that the charge mobility and source–drain current in 2,20-(2,20-bithiophene-5,50-diyl)bis(5-butyl-5H-thieno[2,3-c]pyrrole-4,6)-dione (NT4N) organic transistors directly correlate with the out-of-plane domain size and crystallite orientation in the vertical direction, well beyond the dielectric interfacial layers. Polycrystalline films with thickness as high as 75 nm (≈30 MLs) and 3D molecular architecture provide the best electrical and optoelectronic OFET characteristics, highlighting that the molecular orientational order in the bulk of the film is the key-enabling factor for optimum device performance. X-ray scattering analysis and multiscale simulations reveal the functional correlation between the thickness-dependent molecular packing, electron mobility, and vertical charge distribution. These results call for a broader view of the fundamental mechanisms that govern field-effect charge transport in OFETs beyond the interfacial 2D paradigm and demonstrate the unexpected role of the out-of-plane domain size and crystallite orientation in polycrystalline films to achieve optimum electronic and optoelectronic properties in organic transistors

Sensitivity of urolithiasis detection using urinary, radiography and ultrasound parameters

Although many information has been published regarding canine urolithiasis, sensitivity of radiography, ultrasound and urinary parameters have been poorly correlated with number, size, and composition of the stones. One hundred and thirteen clinical files of dogs with diagnosis of urolithiasis were retrospectively selected. Information regarding number, appearance, location, size, and composition of the stones were noted after surgical removed. Urolithiasis data was compared with radiographs and ultrasound images and urinalysis parameters. Pure struvite was found in 42.4% and calcium oxalate in 35.6%. Survey radiographs enabled the detection of radiopaque stones and when an ultrasound examination was also performed, the sensitivity was increased. Double contrast radiography enabled identification in 100% of radiolucent stones, and allowed for size measurement and number counts in 76.9% of radiolucent stones. Crystalluria had low sensitivity (31.5%) and specificity (58.8%). Hematuria (96.3%) and leukocyturia (61.1%) were the most common parameters found. We concluded that double contrast radiography can be considered the method of choice for detection of radiolucent stones. Crystalluria is not a good parameter to detect or predict type of stone. Hematuria and leukocyturia, although non-specific findings, can be used as triage for investigation of urolithiasis

Dimer and cluster approach for the evaluation of electronic couplings governing charge transport: Application to two pentacene polymorphs

Hole transport properties are modeled for two polymorphs of pentacene: the single crystal polymorph and the thin film polymorph relevant for organic thin-film transistor applications. Electronic couplings are evaluated in the standard dimer approach but also considering a cluster approach in which the central molecule is surrounded by a large number of molecules quantum-chemically described. The effective electronic couplings suitable for the parametrization of a tight-binding model are derived either from the orthogonalization scheme limited to HOMO orbitals and from the orthogonalization of the full basis of molecular orbitals. The angular dependent mobilities estimated for the two polymorphs using the predicted pattern of couplings display different anisotropy characteristics as suggested from experimental investigations

Modeling p-type charge transport in thienoacene analogs of pentacene

The charge transport properties of two fused-ring thienoacenes, (a) the syn-isomer of dibenzo-thieno-dithiophene (DBTDT), packing in the solid state with a \u3c0\u2013\u3c0 stacking arrangement and also known as bis-benzo-thieno-thiophene (BBTT) and (b) C6-DBTDT, an alkylated derivative, packing in the more conventional herring-bone arrangement, are investigated computationally in the framework of the non-adiabatic hopping mechanism. Charge transfer rate constants are computed within the Marcus\u2013Levich\u2013Jortner formalism including a single effective mode treated quantum mechanically and are injected in a kinetic Monte Carlo scheme to propagate the charge carrier in the crystal. Charge mobilities are computed at room temperature with and without the influence of an electric field and are shown to compare very well with the measured mobilities in single-crystal devices. Both systems show an almost 1D charge transport with C6-DBTDT displaying about a ten times larger mobility value, in agreement with experiment. It is shown that the role of the HOMO-1 orbital is not relevant for BBTT, while it might contribute to a more marked 2D charge transport character for C6-DBTDT

The Low Lying Double-Exciton State of Conjugated Diradicals: Assessment of TDUDFT and Spin-Flip TDDFT Predictions

Conjugated singlet ground state diradicals have received remarkable attention owing to their potential applications in optoelectronic devices. A distinctive character of these systems is the location of the double-exciton state, a low lying excited state dominated by the doubly excited HOMO,HOMOLUMO,LUMO configuration, (where HOMO=highest occupied molecular orbital, LUMO=lowest unoccupied molecular orbital) which may influence optical and other photophysical properties. In this contribution we investigate this specific excited state, for a series of recently synthesized conjugated diradicals, employing time dependent density functional theory (TDDFT) based on the unrestricted parallel spin reference configuration in the spin-flip formulation (SF-TDDFT) and standard TD calculations based on the unrestricted antiparallel spin reference configuration (TDUDFT). The quality of computed results is assessed considering diradical and multiradical descriptors, and the excited state wavefunction compositio