1,721,142 research outputs found
UPS spectra of lithium doped carbon nanostructures: quantum-chemical simulations
No full textWe present a quantum chemical study on the effect of lithium doping in hydrogen rich carbon nanostructures. To model the hydrogen rich carbon nanostructure we selected hexa-peri-hexabenzocoronene, a wellknown polycyclic aromatic hydrocarbon whose UPS spectra have been recorded recently. This study shows that lithium interacts strongly with the carbon nanostructure and does not simply transfer charge to the PAH. As a result, the unusual features observed in the UPS spectra of lithium-doped HBC are correctly reproduced by the calculation
Quantum chemical modeling of Infrared and Raman activities in lithium-doped amorphous carbon nanostructures: hexa-peri-hexabenzocoronene as a model for hydrogen-rich carbon materials
No full textThe preferential sites for lithium doping in hydrogen-rich amorphous carbon materials are investigated by considering, as a model, a large polycyclic aromatic hydrocarbon (PAH): hexa-peri-hexabenzocoronene. Quantum-chemical calculations are carried out with B3LYP/6-31G* density functional theory to study the lowest energy configurations of the PAH doped with two lithium atoms. Infrared and Raman activities are computed for the most stable configurations and compared with the spectra of the pristine material to disclose the effect of doping on the vibrational spectra of hydrogen-rich amorphous carbon materials. It is shown that interaction with lithium atoms perturbs appreciably the atomic and electronic structure of the nanometric graphene sheet and leads to new marker bands in the spectr
Modeling p-type charge transport in thienoacene analogs of pentacene
No full textThe charge transport properties of two fused-ring thienoacenes, (a) the syn-isomer of dibenzo-thieno-dithiophene (DBTDT), packing in the solid state with a π–π 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–Levich–Jortner 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
Dimer and cluster approach for the evaluation of electronic couplings governing charge transport: Application to two pentacene polymorphs
No full textHole 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
The double exciton state of conjugated chromophores with strong diradical character: insights from TDDFT calculations
Full text linkA peculiar characteristic of open-shell singlet diradical molecules is the presence of a double exciton state (H,H → L,L) among low lying excited states. Recent high-level quantum-chemical investigations including a static and dynamic electron correlation have demonstrated that this state can become the lowest singlet excited state, a diagnostic fingerprint of the diradical system. Here we investigate the performance of less computationally demanding TDDFT calculations by employing two approaches: the spin-flip TDDFT scheme and TD calculations based on unrestricted broken symmetry antiparallel-spin reference configuration (TDUDFT). The calculations are tested on a number of recently synthesized, large conjugated systems displaying from moderate to large diradical character and showing experimental trace of the double exciton state. We show that spin-flip (SF) TDB3LYP calculations in the collinear approximation generally underestimate the excitation energy of the double exciton state. When the molecule displays a strong diradical character, the unrestricted antiparallel-spin reference configuration of TDUDFT calculations is characterized by strongly localized frontier molecular orbitals. We show that under these conditions the double exciton state is captured by TDUB3LYP calculations since it is described by singly excited configurations and its excitation energy can be accurately predicted. Owing to the improved description of the ground state, also the excitation energy of the single exciton H → L state generally improves at the TDUB3LYP level. With regard to the double exciton state, SF TDB3LYP performs slightly better for small to medium diradical character while a large diradical character (and strong orbital localization) is a prerequisite for the success of TDUB3LYP calculations whose quality otherwise deteriorates
Eclipsed and Twisted Excimers of Pyrene and 2-Azapyrene: How Nitrogen Substitution Impacts Excimer Emission
Full text linkDue to their unique photophysical and electronic properties, pyrene and its analogues have been the subject of extensive research in recent decades. The propensity of pyrene and its derivatives to form excimers has found wide application in various fields. Nitrogen-substituted pyrene derivatives display similar photophysical properties, but for them, excimer emission has not been reported to date. Here, we use time-dependent density functional theory (TD-DFT) calculations to investigate the low-lying exciton states of dimers of pyrene and 2-azapyrene. The excimer equilibrium structures are determined and the contribution of charge transfer (CT) excitations and intermolecular interactions to the exciton states is disclosed using a diabatization procedure. The study reveals that the dimers formed by the two molecules have quite similar exciton-state patterns, in which the relevant CT contributions govern the formation of excimer states, along with the La/Lb state inversion. In contrast with pyrene, the dipole-dipole interactions in 2-azapyrene stabilize the dark eclipsed excimer structure and increase the barrier for conversion into a bright twisted excimer. It is suggested that these differences in the nitrogen-substituted derivative might influence the excimer emission properties
Tuning Low-Lying Excited States and Optical Properties in IndenoFluorene Diradicaloids and Longitudinally Extended Derivatives: A Computational Perspective
No full textIn this work, we have considered the family of indenofluorene (IF) and
its longitudinally elongated variants fluorenofluorene and diindenoanthracene and
investigated their low-lying excited states and optical properties via quantum-chemical
studies at the density functional theory (DFT) level. Singlet ground-state diradicals
exhibit distinct optical properties due to the presence of a low-lying state dominated
by a doubly excited configuration (DE state), often below the lowest allowed singly
excited state (SE state). IFs and their elongated derivatives, with tunable diradical
character and both symmetric and nonsymmetric structures, provide an ideal platform for
exploring DE state energy modulation and spectroscopic behavior. The study shows
that absorption spectra simulated using time-dependent (TD) calculations based on
unrestricted broken-symmetry antiparallel-spin reference configuration (TDUDFT) closely
match the available experimental data. Additionally, it reveals distinct spectral behavior
for symmetric and nonsymmetric derivatives, highlighting the role of lowest-lying weakly
allowed excited states potentially promoting non-radiative deactivation pathways
Effect of the iodine atom position on the phosphorescence of BODIPY derivatives: a combined computational and experimental study
Full text linkA new BODIPY derivative (o-I-BDP) containing an iodine atom in the ortho position of the meso-linked phenyl group was prepared. Photophysical and electrochemical properties of the molecule were compared to previously reported iodo BODIPY derivatives, as well as to the non-iodinated analog. While in the case of derivatives featuring iodine substituents in the BODIPY core, efficient population of the triplet state is accompanied by a substantial positive shift of the reduction potential compared to pristine BODIPY, o-I-BDP displays phosphorescence and simultaneously maintains the electrochemical properties of unsubstituted BODIPYs. A theoretical investigation was settled to analyze results and rationalize the influence of iodine position on electronic and photophysical properties, with the purpose of preparing a fully organic phosphorescent BODIPY derivative. TD-DFT and spin-orbit coupling calculations shed light on the subtle effects played by the introduction of iodine atom in different positions of BODIPY
Unravelling the Role of Structural Factors in the Luminescence Properties of Persulfurated Benzenes
Full text linkRoom temperature phosphorescence rarely occurs from pure organic molecules, especially in the solid-state. A strategy for the design of highly emissive organic phosphors is still hard to predict, thus impeding the development of new functional materials with the desired optical properties. Herein, we analyze a family of alkyl and aryl-substituted persulfurated benzenes, the latter representing a class of organic solid-state triplet emitters able to show very high emission quantum yield at room temperature. In this work, we correlate structural parameters with the photophysical properties observed in different experimental conditions (diluted solution, crystalline and amorphous phase at RT and low temperature). Our results, corroborated by a detailed computational analysis, indicate a close relationship between the luminescence properties and i) the nature of the substituents on the persulfurated core, ii) the adopted conformations in the solid state, both in crystalline and amorphous phases. These factors contribute to characterize the lowest-energy lying excited-state, its deactivation pathways, the phosphorescence lifetime and quantum yield. These findings provide a useful roadmap for the development of highly performing purely organic solid-state emitters based on the persulfurated benzene platform
Origin, Nature and Fate of the Fluorescent State of the Green Fluorescent Protein Chromophore at the CASPT2//CASSCF resolution
No full textAb initio CASPT2//CASSCF relaxation path computations are employed to determine the intrinsic (e.g., in vacuo) mechanism underlying the rise and decay of the luminescence of the anionic form of the green fluorescent protein (GFP) fluorophore. Production and decay of the fluorescent state occur via a two-mode reaction coordinate. Relaxation along the first (totally symmetric) mode leads to production of the fluorescent state that corresponds to a planar species. The second (out-of-plane) mode controls the fluorescent state decay and mainly corresponds to a barrierless twisting of the fluorophore phenyl moiety. While a "space-saving" hula-twist conical intersection decay channel is found to lie only 5 kcal mol(-1) above the fluorescent state, the direct involvement of a hula-twist deformation in the decay is not supported by our data. The above results indicate that the ultrafast fluorescence decay observed for the GFP chromophore in solution is likely to have an intrinsic origin. The possible effects of the GFP protein cavity on the fluorescence lifetime of the investigated chromophore model are discussed
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