Excitonic and vibronic spectra of Frenkel excitons in a two-dimensional simple latice

Excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional (2D) lattice with one molecule per unit cell have been studied and their manifestation in the linear absorption is simulated. We use the Green function formalism, the vibronic approach (see Lalov and Zhelyazkov [Phys. Rev. B \textbf{75}, 245435 (2007)]), and the nearest-neighbor approximation to find expressions of the linear absorption lineshape in closed form (in terms of the elliptic integrals) for the following 2D models: (a) vibronic spectra of polyacenes (naphthalene, anthracene, tetracene); (b) vibronic spectra of a simple hexagonal lattice. The two 2D models include both linear and quadratic FE--phonon coupling. Our simulations concern the excitonic density of state (DOS), and also the position and lineshape of vibronic spectra (FE plus one phonon, FE plus two phonons). The positions of many-particle (MP-unbound) FE--phonon states, as well as the impact of the Van Hove singularities on the linear absorption have been established by using typical values of the excitonic and vibrational parameters. In the case of a simple hexagonal lattice the following types of FEs have been considered: (i) non-degenerate FEs whose transition dipole moment is perpendicular to the plane of the lattice, and (ii) degenerate FEs with transition dipole moments parallel to the layer. We found a cumulative impact of the linear and quadratic FE--phonon coupling on the positions of vibronic maxima in the case (ii), and a compensating impact in the case (i).Comment: 13 pages, 12 figure

Theory of Triplet Optical Absorption in Oligoacenes: From Naphthalene to Heptacene

In this paper we present a detailed theory of the triplet states of oligoacenes containing up to seven rings, i.e., starting from naphthalene all the way up to heptacene. In particular, we present results on the optical absorption from the first triplet excited state $1^{3}B_{2u}^{+}$ of these oligomers, computed using the Pariser-Parr-Pople (PPP) model Hamiltonian, and a correlated electron approach employing the configuration-interaction (CI) methodology at various levels. Excitation energies of various triplets states obtained by our calculations are in good agreement with the experimental results, where available. The computed triplet spectra of oligoacenes exhibits rich structure dominated by two absorption peaks of high intensities, which are well separated in energy, and are caused by photons polarized along the conjugation direction. This prediction of ours can be tested in future experiments performed on oriented samples of oligoacenes.Comment: 50 pages, 12 figures (included

Limitations and Advancements in Soft x-ray Spectroscopy

Soft x-ray absorption spectroscopy (XAS) is a widely used method for probing the electronic structure of materials, yet it suffers from many complications related to the reliable measurement of absorption spectra and the proper theoretical modelling of spectral features. Problems due to different experimental aspects, such as sample charging, surface contamination and saturation effects, often introduce artifacts or distortions into a measured absorption spectrum. Even when measured accurately, the interpretation of absorption spectra is complicated by the absence of rigorous theoretical methods that properly account for the effect of the core hole on the remaining electronic structure, leading to inaccurate spectral simulations. The limits of the one electron model of core excitation were explored through a high resolution XAS study of the linear polyacenes. When measured at high resolution, the linear polyacenes exhibited a fine structure that could be assigned to excitation of non-equivalent carbon atoms in the molecules. The energies and intensities of these transitions were extracted and compared to transition intensities calculated from density functional theory, where the multi-electronic effects were approximated using a full core hole and half core hole model. The full core hole model was found to better approximate the trends in the spectra for the smaller molecules, but neither model was able to properly calculate the changes in the absorption spectra of the larger molecules. These results demonstrated the deficiencies of the one electron picture of core hole excitation and highlighted the need for incorporating multi-electronic effects into XAS simulations. To extend the capabilities of XAS, the use of partial fluorescence yields (PFY) was explored. One of the primary experimental limitation of XAS is that absorption spectra are measured by monitoring the total yield of electrons or photons from the sample. In both of these methods, a direct relationship between the measured spectra and the linear attenuation coefficient of the sample is not guaranteed. To overcome this drawback, the partial fluorescence yield measurement technique, employing a silicon drift detector, was used. By measuring partial fluorescence yields (PFY), as opposed to total yields, the effects of background fluorescence could be avoided. The inverse of the partial fluorescence yield was also demonstrated to be an effective way of avoiding saturation effects in some samples. The utility of the inverse partial fluorescence yield (IPFY) method was demonstrated in a study of several iron oxide minerals that were not possible to measure using conventional total yield methods. IPFY was also demonstrated for liquid samples where differences between the Fe PFY and IPFY were noted and attributed to resonant scattering effects in the PFY. In both the iron oxide study and the polyacene study, the experimental limitations related to the measurement of XAS were evaluated by comparison to x-ray Raman spectroscopy (XRS). XRS is a hard x-ray based method that probes core excitation in low-Z elements by measuring the energy loss of the scattered photons. Both XRS and XAS involve the same electronic transitions allowing for a direct comparison of spectra measured using the two methods, but the limitations of XAS related to the short penetration depth of soft x-rays are not encountered in XRS. While XRS measurements do not suffer from saturation or surface effects, the experimental resolution and count rates of this technique are limited. This dissertation demonstrated that, while XAS is an established method, there are many aspects of the technique that require additional development. Emerging calculation methods that can incorporate the interaction between the electron and core hole will improve data interpretation capabilities. New detector systems that can measure partial yields with high resolution and can be placed in specific geometries will continue to improve measurement quality and allow for the development of novel techniques, like IPFY. A growing use of XAS and XRS in concert will also improve the general applicability of core level excitation spectroscopies and help to advance our understanding of the materials around us

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

A method for calculating the rate constants for internal-conversion (k(IC)) and intersystem-crossing (k(ISC)) processes within the adiabatic and Franck-Condon (FC) approximations is proposed. The applicability of the method is demonstrated by calculation of k(IC) and k(ISC) for a set of organic and organometallic compounds with experimentally known spectroscopic properties. The studied molecules were pyrromethene-567 dye, psoralene, hetero[8]circulenes, free-base porphyrin, naphthalene, and larger polyacenes. We also studied fac-Alq(3) and fac-Ir(ppy)(3), which are important molecules in organic light emitting diodes (OLEDs). The excitation energies were calculated at the multi-configuration quasi-degenerate second-order perturbation theory (XMC-QDPT2) level, which is found to yield excitation energies in good agreement with experimental data. Spin-orbit coupling matrix elements, non-adiabatic coupling matrix elements, Huang-Rhys factors, and vibrational energies were calculated at the time-dependent density functional theory (TDDFT) and complete active space self-consistent field (CASSCF) levels. The computed fluorescence quantum yields for the pyrromethene-567 dye, psoralene, hetero[8]circulenes, fac-Alq(3) and fac-Ir(ppy)(3) agree well with experimental data, whereas for the free-base porphyrin, naphthalene, and the polyacenes, the obtained quantum yields significantly differ from the experimental values, because the FC and adiabatic approximations are not accurate for these molecules.Peer reviewe

Computational materials design of optical bandgaps for bulk heterojunction solar cell

Thesis (M.S.)--Boston UniversityFundamental understanding of the structure-property relationship of pi-conjugated poly- mers is critical to predictive materials designs of bulk heterojunction solar cells. In this thesis, the adapted Su-Schrieffer-Heeger Hamiltonian is implemented as the computational tool to systematically explore the opto-electronic properties of nearly 250 different kinds of pi-conjugated systems. New physical insights on the structure-property relationship are extracted and transformed into practical guiding rules in optical bandgap designs. For the most power efficient donor-acceptor copolymer structures, we find that the energy variation of frontier orbitals, in particular the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), can be controlled either independently or collectively, depending on their specific donor or acceptor structures. In particular, we find that having five-membered conjugated carbon rings in the acceptor units is essential to break the electron-hole charge conjugation symmetry, so that the LUMO levels of the copolymer can be reduced dramatically while holding the HOMO energy levels in the donor units constant. On the other hand, by incorporating heteroatoms into the donors units, we can vary the HOMO levels of the copolymers independently. Predicted optical bandgaps of a total of 780 types of these copolymers constructed by using 39 different types of donor and acceptor units are tabulated in this thesis. In addition, the effects of introducing various side groups(-R, -0, -CO, -COO, and thiophene) on the primitive donor and acceptor structures are investigated and their results are discussed in details. Finally, unexpected localized states are found, for the first time, in our calculations for a few special co-polymer structures. These localized states, with electrons localized on one end of the copolymer chain and holes on the other end, contain large dipole moments and therefore may be treated as a new design dimension when these copolymers are placed in polar and non-polar solvent environments

Solid-State Effects on the Optical Excitation of Push-Pull Molecular J-Aggregates by First-Principles Simulations

J-aggregates are a class of low-dimensional molecular crystals which display enhanced interaction with light. These systems show interesting optical properties as an intense and narrow red-shifted absorption peak (J-band) with respect to the spectrum of the corresponding monomer. The need to theoretically investigate optical excitations in J-aggregates is twofold: a thorough first-principles description is still missing and a renewed interest is rising recently in understanding the nature of the J-band, in particular regarding the collective mechanisms involved in its formation. In this work, we investigate the electronic and optical properties of a J-aggregate molecular crystal made of ordered arrangements of organic push-pull chromophores. By using a time dependent density functional theory approach, we assess the role of the molecular packing in the enhancement and red shift of the J-band along with the effects of confinement in the optical absorption, when moving from bulk to low-dimensional crystal structures. We simulate the optical absorption of different configurations (i.e., monomer, dimers, a polymer chain, and a monolayer sheet) extracted from the bulk crystal. By analyzing the induced charge density associated with the J-band, we conclude that it is a longitudinal excitation, delocalized along parallel linear chains and that its overall red shift results from competing coupling mechanisms, some giving red shift and others giving blue shift, which derive from both coupling between transition densities and renormalization of the single-particle energy levels.Comment: This is the published version of the work, distributed under the terms of the ACS AuthorChoice licence https://pubs.acs.org/page/policy/authorchoice_termsofuse.htm

Vibronic Lineshapes of PTCDA Oligomers in Helium Nanodroplets

Oligomers of the organic semiconductor PTCDA are studied by means of helium nanodroplet isolation (HENDI) spectroscopy. In contrast to the monomer absorption spectrum, which exhibits clearly separated, very sharp absorption lines, it is found that the oligomer spectrum consists of three main peaks having an apparent width orders of magnitude larger than the width of the monomer lines. Using a simple theoretical model for the oligomer, in which a Frenkel exciton couples to internal vibrational modes of the monomers, these experimental findings are nicely reproduced. The three peaks present in the oligomer spectrum can already be obtained taking only one effective vibrational mode of the PTCDA molecule into account. The inclusion of more vibrational modes leads to quasi continuous spectra, resembling the broad oligomer spectra