Modelling the quenching effect of chloroaluminum phthalocyanine and graphene oxide interactions: implications for phototherapeutic applications.

Photodynamic therapy (PDT) and photothermal therapy (PTT) are promising candidates for cancer treatment and their efficiency can be further enhanced by using a combination of both. While chloroaluminum phthalocyanine (AlClPc) has been studied extensively as a photosensitizer in PDT, nanographene oxide (nGO) has shown promise in PTT due to its high absorption of near-infrared radiation. In this work, we investigate the energy transport between AlClPc and nGO for their combined use in phototherapies. We use density functional theory (DFT) and time-dependent DFT to analyze the electronic structure of AlClPc and its interaction with nGO. Based on experimental parameters, we model the system's morphology and implement it in Kinetic Monte Carlo (KMC) simulations to investigate the energy transfer mechanism between the compounds. Our KMC calculations show that the experimentally observed fluorescence quenching requires modeling both the energy transfer from dyes to nGO and a molecular aggregation model. Our results provide insights into the underlying mechanisms responsible for the fluorescence quenching observed in AlClPc/nGO aggregates, which could impact the efficacy of photodynamic therapy.

Structural Study of Diketopyrrolopyrrole Derivative Thin Films: Influence of Deposition Method, Substrate Surface, and Aging.

We report the morphology and microstructure of n-dialkyl side-chain-substituted thiophene DPP end-capped with phenyl groups (Ph-TDPP-Ph) thin films and compare the influence of deposition method and substrate surface using thermally oxidized Si and graphene substrates as well as monolayer graphene surfaces with an underlying self-assembled octadecyltrichlorosilane monolayer, complemented by an aging study of spin-coated films over a 2 weeks aging period. A distinct difference in morphology was observed between spin-coated and vacuum-deposited thin films, which formed a fiber-like morphology and a continuous layer of terraced grains, respectively. After an initial film evolution, all combinations of deposition method and substrate type result in well-ordered thin films with almost identical crystalline phases with slight variations in crystallinity and mosaicity. These findings point toward strong intermolecular forces dominating during growth, and the templating effect observed for other oligomer films formed on graphene is consequently ineffective for this material type. Upon aging of spin-coated films, a noticeable evolution involving two different morphologies and crystalline phases were observed. After several days, the thin film evolved into a more stable crystal phase and a fiber-like morphology. Moreover, slight variation in optical spectra were elucidated on the basis on density functional theory calculations. These results demonstrate that thin-film properties of DPP derivatives can be tailored by manipulating the film formation process.

Organic Electronics from Nature: Computational Investigation of the Electronic and Optical Properties of the Isomers of Bixin and Norbixin Present in the Achiote Seeds.

Organic compounds have been employed in developing new green energy solutions with good cost-efficiency compromise, such as photovoltaics. The light-harvesting process in these applications is a crucial feature that still needs improvements. Here, we studied natural dyes to propose an alternative for enhancing the light-harvesting capability of photovoltaics. We performed density functional theory calculations to investigate the electronic and optical properties of the four natural dyes found in achiote seeds (Bixa orellana L.). Different DFT functionals, and basis sets, were used to calculate the electronic and optical properties of the bixin, norbixin, and their trans-isomers (molecules present in Bixa orellana L.). We observed that the planarity of the molecules and their similar extension for the conjugation pathways provide substantially delocalized wavefunctions of the frontier orbitals and similar values for their energies. Our findings also revealed a strong absorption peak in the blue region and an absorption band over the visible spectrum. These results indicate that Bixa orellana L. molecules can be good candidates for improving light-harvesting in photovoltaics.

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit.

Organic solar cells (OSC) generally contain long-chain π-conjugated polymers as donor materials, but, more recently, small-molecule donors have also attracted considerable attention. The nature of these compounds is of crucial importance concerning the various processes that determine device performance, among which singlet exciton diffusion is one of the most relevant. The efficiency of the diffusion mechanism depends on several aspects, from system morphology to electronic structure properties, which vary importantly with molecular size. In this work, we investigated the effects of conjugation length on the exciton diffusion length through electronic structure calculations and an exciton diffusion model. By applying extrapolation procedures to thiophene and phenylene vinylene oligomer series, we investigate their electronic and optical properties from the small-molecule point of view to the polymeric limit. Several properties are calculated as a function of oligomer size, including transition energies, absorption and emission spectra, reorganization energies, exciton coupling and Förster radii. Finally, an exciton diffusion model is used to estimate diffusion lengths as a function of oligomer size and for the polymeric limit showing agreement with experimental data. Results also show that longer conjugation lengths correlate with longer exciton diffusion lengths in spite of also being associated with shorter exciton lifetimes.

Charge localization and hopping in a topologically engineered graphene nanoribbon.

Graphene nanoribbons (GNRs) are promising quasi-one-dimensional materials with various technological applications. Recently, methods that allowed for the control of GNR's topology have been developed, resulting in connected nanoribbons composed of two distinct armchair GNR families. Here, we employed an extended version of the Su-Schrieffer-Heeger model to study the morphological and electronic properties of these novel GNRs. Results demonstrated that charge injection leads to the formation of polarons that localize strictly in the 9-AGNRs segments of the system. Its mobility is highly impaired by the system's topology. The polaron displaces through hopping between 9-AGNR portions of the system, suggesting this mechanism for charge transport in this material.

Electronic couplings and rates of excited state charge transfer processes at poly(thiophene-co-quinoxaline)-PC71BM interfaces: two- versus multi-state treatments.

Electronic coupling between adjacent molecules is one of the key parameters determining the charge transfer (CT) rates in bulk heterojunction (BHJ) polymer solar cells (PSCs). We calculate theoretically electronic couplings for exciton dissociation (ED) and charge recombination (CR) processes at local poly(thiophene-co-quinoxaline) (TQ)-PC71BM interfaces. We use eigenstate-based coupling schemes, i.e. the generalized Mulliken-Hush (GMH) and fragment charge difference (FCD) schemes, including 2 to multiple (3-11) states. Moreover, we study the effects of functionals, excited state methods, basis sets, surrounding media, and relative placements of TQ and PC71BM on the coupling values. Generally, both schemes provide consistent couplings with the global hybrid functionals, which yield more charge-localized diabatic states and constant coupling values regardless of the number of states, and so the 2-state schemes may be sufficient. The (non-tuned and optimally tuned) long-range corrected (LRC) functionals result in more notable mixing of the local components with the CT states. Employing multiple states reduces the mixing and thus improves the LRC results, although the method still affects the GMH CR couplings. As the FCD scheme is less sensitive, we recommend combining it with the multi-state treatment for polymer-fullerene systems when using the LRC functionals. Finally, we employ the 11-state FCD couplings to calculate the ED and CR rates, which are consistent with the experimental rates of the polymer-fullerene systems. Our results provide more insight into choosing a suitable eigenstate-based coupling scheme for predicting the electronic couplings and CT rates in photoactive systems.

Dynamical exciton decay in organic materials: the role of bimolecular recombination.

Excitons play a critical role in light emission when it comes to organic semiconductors. In high exciton concentration regimes, monomolecular and bimolecular routes for exciton recombination can yield different products affecting significantly the material's optical properties. Here, the dynamical decay of excitons is theoretically investigated using a kinetic Monte Carlo approach that addresses singlet exciton diffusion. Our numerical protocol includes two distinct exciton-exciton interaction channels: exciton annihilation and biexciton cascade emission. Our findings reveal that these channels produce different consequences concerning diffusion and spectroscopic properties, being able to explain diverging experimental observations. Importantly, we estimate critical exciton densities for which bimolecular recombination becomes dominant and investigate its effect on average exciton lifetimes and diffusion lengths.

Exciton Diffusion in Organic Nanofibers: A Monte Carlo Study on the Effects of Temperature and Dimensionality.

Organic nanofibers have found various applications in optoelectronic devices. In such devices, exciton diffusion is a major aspect concerning their efficiency. In the case of singlet excitons, Förster transfer is the mechanism responsible for this process. Temperature and morphology are factors known to influence exciton diffusion but are not explicitly considered in the expressions for the Förster rate. In this work, we employ a Kinetic Monte Carlo (KMC) model to investigate singlet exciton diffusion in para-hexaphenyl (P6P) and α-sexithiophene (6T) nanofibers. Building from previous experimental and theoretical studies that managed to obtain temperature dependent values for Förster radii, exciton average lifetimes and intermolecular distances, our model is able to indicate how these parameters translate into diffusion coefficients and diffusion lengths. Our results indicate that these features strongly depend on the coordination number in the material. Furthermore, we show how all these features influence the emitted light color in systems composed of alternating layers of P6P and 6T. Finally, we present evidence that the distribution of exciton displacements may result in overestimation of diffusion lengths in experimental setups.

Modeling temperature dependent singlet exciton dynamics in multilayered organic nanofibers.

Organic nanofibers have shown potential for application in optoelectronic devices because of the tunability of their optical properties. These properties are influenced by the electronic structure of the molecules that compose the nanofibers and also by the behavior of the excitons generated in the material. Exciton diffusion by means of Förster resonance energy transfer is responsible, for instance, for the change with temperature of colors in the light emitted by systems composed of different types of nanofibers. To study in detail this mechanism, we model temperature dependent singlet exciton dynamics in multilayered organic nanofibers. By simulating absorption and emission spectra, the possible Förster transitions are identified. Then, a kinetic Monte Carlo model is employed in combination with a genetic algorithm to theoretically reproduce time-resolved photoluminescence measurements for several temperatures. This procedure allows for the obtainment of different information regarding exciton diffusion in such a system, including temperature effects on the Förster transfer efficiency and the activation energy of the Förster mechanism. The method is general and may be employed for different systems where exciton diffusion plays a role.

Combined UMC- DFT prediction of electron-hole coupling in unit cells of pentacene crystals.

Pentacene is an organic semiconductor that draws special attention from the scientific community due to the high mobility of its charge carriers. As electron-hole interactions are important aspects in the regard of such property, a computationally inexpensive method to predict the coupling between these quasi-particles is highly desired. In this work, we propose a hybrid methodology of combining Uncoupled Monte Carlo Simulations (UMC) and Density functional Theory (DFT) methodologies to obtain a good compromise between computational feasibility and accuracy. As a first step in considering a Pentacene crystal, we describe its unit cell: the Pentacene Dimer. Because many conformations can be encountered for the dimer and considering the complexity of the system, we make use of UMC in order to find the most probable structures and relative orientations for the Pentacene-Pentacene complex. Following, we carry out electronic structure calculations in the scope of DFT with the goal of describing the electron-hole coupling on the most probable configurations obtained by UMC. The comparison of our results with previously reported data on the literature suggests that the methodology is well suited for describing transfer integrals of organic semiconductors. The observed accuracy together with the smaller computational cost required by our approach allows us to conclude that such methodology might be an important tool towards the description of systems with higher complexity.

Optical and electronic structure description of metal-doped phthalocyanines.

Phthalocyanines represent a crucial class of organic compounds with high technological appeal. By doping the center of these systems with metals, one obtains the so-called metal-phthalocyanines, whose property of being an effective electron donor allows for potentially interesting uses in organic electronics. In this sense, investigating optical and electronic structure changes in the phthalocyanine profiles in the presence of different metals is of fundamental importance for evaluating the appropriateness of the resulting system as far as these uses are concerned. In the present work, we carry out this kind of effort for phthalocyanines doped with different metals, namely, copper, nickel, and magnesium. Density functional theory was applied to obtain the absorption spectra, and electronic and structural properties of the complexes. Our results suggest that depending on the dopant, a different level of change is achieved. Moreover, electrostatic potential energy mapping shows how the charge distribution can be affected by solar radiation. Our contribution is crucial in describing the best possible candidates for use in different organic photovoltaic applications. Graphical Abstract Representation of meta-phthalocyanine systems. All calculations of this work are based on varying metal position along z axis, considering the z-axis has its zero point matching with the center of phthalocyanine cavityconsidering.

A joint theoretical and experimental characterization of two acene-thiophene derivatives.

Acene-thiophenes compounds have been used successfully as active materials in thin-film transistors and photodetectors. This work aims at obtaining an adequate theoretical framework to accurately characterize the optical and electronic properties of two such compounds: NaT2 and NaT3. This is done by comparing the results of simulations with experimental absorption spectra. Basis size effects are investigated as well as the role of intramolecular vibrations in the simulated spectra. It is shown that the most important feature of a DFT calculation is the appropriate selection of long-range corrected functionals, which allows for the accurate description of the first absorption band of these molecules.

Modeling optical properties of polymer-solvent complexes: the chloroform influence on the P3HT and N2200 absorption spectra.

The optical properties of polymer/solvent systems composed by the polymers P3HT and PolyeraActivInk N2200 under the present of chloroform as solvent are experimentally and theoretically investigated using UV-Vis spectroscopy, molecular dynamics (MD), and density functional theory (DFT) calculations. The study is focused on obtaining the theoretical methodologies that properly describes the experimentally obtained absorption spectra of polymer-solvent complexes. In order to investigate the solvent influence, two different approaches are taken into account: the solvation shell method (SSM) and the polarizable continuum model (PCM). Our findings shown that SSM simulations, which combine MD and DFT calculations, are in good agreement with the experimental data. Moreover, it is obtained that simulations in the framework of PCM do not provide a fair description of the real system. Importantly, these results may pave the way for better descriptions of some optoelectronic properties of interest in polymer/solvent systems. Graphical Abstract ᅟ.

Modeling the Emission Spectra of Organic Molecules: A Competition between Franck-Condon and Nuclear Ensemble Methods.

The emission spectra of flexible and rigid organic molecules are theoretically investigated in the framework of the Franck-Condon (FC) and nuclear ensemble (NE) approaches, both of which rely on results from density functional theory but differ in the way vibrational contributions are taken into account. Our findings show that the emission spectra obtained using the NE approach are in better agreement with experiment than the ones produced by FC calculations considering both rigid and flexible molecules. Surprisingly, the description of a suitable balance between the vibronic progression and the emission spectra envelope shows dependency on the initial sampling for the NE calculations which must be judiciously selected. Our results intend to provide guidance for a better theoretical description of light emission properties of organic molecules with applications in organic electronic devices.