Volker Settels

Understanding Ground- and Excited-State Properties of Perylene Tetracarboxylic Acid Bisimide Crystals by Means of Quantum Chemical Computations

Quantum chemical protocols explaining the crystal structures and the visible light absorption properties of 3,4:9,10-perylene tetracarboxylic acid bisimide (PBI) derivates are proposed. Dispersion-corrected density functional theory has provided an intermolecular potential energy of PBI dimers showing several energetically low-lying minima, which corresponds well with the packing of different PBI dyes in the solid state. While the dispersion interaction is found to be crucial for the binding strength, the minimum structures of the PESs are best explained by electrostatic interactions. Furthermore, a method is introduced, which reproduces the photon energies at the absorption maxima of PBI pigments within 0.1 eV. It is based on time-dependent Hartree-Fock (TD-HF) excitation energies calculated for PBI dimers with the next-neighbor arrangement in the pigment and incorporates crystal packing effects. This success provides clear evidence that the electronically excited states, which determine the color of these pigments, have no significant charge-transfer character. The developed protocols can be applied in a routine manner to understand and to predict the properties of such pigments, which are important materials for organic solar cells and (opto-)electronic devices.

Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates.

Femtosecond time-resolved experiments demonstrate that the photoexcited state of perylene tetracarboxylic acid bisimide (PBI) aggregates in solution decays nonradiatively on a time-scale of 215 fs. High-level electronic structure calculations on dimers point toward the importance of an excited state intermolecular geometry distortion along a reaction coordinate that induces energy shifts and couplings between various electronic states. Time-dependent wave packet calculations incorporating a simple dissipation mechanism indicate that the fast energy quenching results from a doorway state with a charge-transfer character that is only transiently populated. The identified relaxation mechanism corresponds to a possible exciton trap in molecular materials.

Assessment of TD‐DFT‐ and TD‐HF‐based approaches for the prediction of exciton coupling parameters, potential energy curves, and electronic characters of electronically excited aggregates

The reliability of linear response approaches such as time‐dependent Hartree–Fock (TD‐HF) and time‐dependent density functional theory (TD‐DFT) for the prediction of the excited state properties of 3,4;9,10‐tetracarboxylic‐perylene‐bisimide (PBI) aggregates is investigated. A dimer model of PBI is investigated as a function of a torsional motion of the monomers, which was shown before to be an important intermolecular coordinate in these aggregates. The potential energy curves of the ground state and the two energetically lowest neutral excited and charge‐transfer (CT) states were obtained with the spin‐component scaling modification of the approximate coupled‐cluster singles‐and‐doubles (SCS‐CC2) method as a benchmark for dispersion corrected TD‐HF and a range of TD‐DFT approaches. The highly accurate SCS‐CC2 results are used to assess the other, computationally less demanding methods. TD‐HF predicts similar potential energy curves and transition dipole moments as SCS‐CC2, as well as the correct order of neutral and CT states. This supports an exciton trapping mechanism, which was found on the basis of TD‐HF data. However, the investigated TD‐DFT methods provide generally the opposite character for the excited states. As a consequence, these TD‐DFT results have unacceptably large errors for optical properties of these dye aggregates.

Structure–property relationships for 1,7-diphenoxy-perylene bisimides in solution and in the solid state

To elucidate the impact of widely employed solubilizing phenoxy substituents on the structural and functional properties of perylene bisimide (PBI) dyes a series of 1,7-diphenoxy-substituted PBIs was prepared from 1,7-dibromo PBI which exhibit hydrogen, methyl, isopropyl or phenyl substituents at one or both ortho positions of the phenoxy substituents. Despite increasing sterical congestion high yields of 74–88% could be obtained for all twofold aromatic nucleophilic substitution reactions. The structural and optical properties in solution and in the solid state were investigated by 1H NMR, UV-Vis absorption and fluorescence spectroscopy, single crystal X-ray analyses (four structures) as well as quantum chemical and force field calculations. For the latter we used an adapted force field which correctly reflects the rigidity of the PBI core. Our studies show that these dyes prefer to accommodate a slightly twisted molecular structure in solution that is supported by CH⋯O hydrogen bonds between the 1,7-oxygen and the 6,12-hydrogen substituents. Because of the rather shallow potential energy surface, however, the molecules may planarize in the crystalline state under the influence of packing forces as revealed by single crystal X-ray analyses for two derivatives bearing methyl or phenyl substituents at all phenoxy ortho-positions. Such substituents are also suited to enwrap the PBI π-scaffold and to prohibit PBI aggregation in the bulk state giving rise to defined vibronic progressions in the solid state UV-Vis absorption and emission spectra, and appreciable fluorescence quantum yields of up to 37%. In dichloromethane solution all of these 1,7-diphenoxy-substituted PBI dyes exhibit fluorescence quantum yields of 98–100% despite significant differences in the shape of the UV-Vis absorption band. The latter was explained in terms of rigidity because the molecules bearing four ortho-substituents at the phenoxy substituents were shown to prevail in much more fixed conformations compared to their more simple counterparts. Our findings underline that the conformational flexibility of bay-substituents can have an important impact on the functional properties of PBI dyes.

Comparison of the electronic structure of different perylene‐based dye‐aggregates

Aggregates of functionalized polycyclic aromatic molecules like perylene derivatives differ in important optoelectronic properties such as absorption and emission spectra or exciton diffusion lengths. Although those differences are well known, it is not fully understood if they are caused by variations in the geometrical orientation of the molecules within the aggregates, variations in the electronic structures of the dye aggregates or interplay of both. As this knowledge is of interest for the development of materials with optimized functionalities, we investigate this question by comparing the electronic structures of dimer systems of representative perylene‐based chromophores. The study comprises dimers of perylene, 3,4,9,10‐perylene tetracarboxylic acid bisimide (PBI), 3,4,9,10‐perylene tetracarboxylic acid dianhydride (PTCDA), and diindeno perylene (DIP). Potential energy curves (PECs) and characters of those electronic states are investigated which determine the optoelectronic properties. The computations use the spin‐component‐scaled approximate coupled‐cluster second‐order method (SCS‐CC2), which describes electronic states of predominately neutral excited (NE) and charge transfer (CT) character equally well. Our results show that the characters of the excited states change significantly with the intermolecular orientation and often represent significant mixtures of NE and CT characters. However, PECs and electronic structures of the investigated perylene derivatives are almost independent of the substitution patterns of the perylene core indicating that the observed differences in the optoelectronic properties mainly result from the geometrical structure of the dye aggregate. It also hints at the fact that optical properties can be computed from less‐substituted model compounds if a proper aggregate geometry is chosen.

Identification of ultrafast relaxation processes as a major reason for inefficient exciton diffusion in perylene-based organic semiconductors.

The exciton diffusion length (LD) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials. Our model is based on simulations with a hybrid approach which combines high-level ab initio computations for the part of the system directly involved in the described processes with a force field to include environmental effects. The adequacy of the model is shown by detailed comparison with available experimental results. The model indicates that the short exciton diffusion lengths of α-perylene tetracarboxylicdianhydride (PTCDA) are due to ultrafast relaxation processes of the optical excitation via intermolecular motions leading to a state from which further exciton diffusion is hampered. As the efficiency of this mechanism depends strongly on molecular arrangement and environment, the model explains the strong dependence of LD on the morphology of the materials, for example, the differences between α-PTCDA and diindenoperylene. Our findings indicate how relaxation processes can be diminished in perylene-based materials. This model can be generalized to other organic compounds.

Theoretical analysis of the relaxation dynamics in perylene bisimide dimers excited by femtosecond laser pulses.

We present a model for the relaxation dynamics in perylene bisimide dimers, which is based on ab initio electronic structure and quantum dynamics calculations including effects of dissipation. The excited-state dynamics proceeds via a mixing of electronic states of local Frenkel and charge-transfer characters, which becomes effective upon a small distortion of the dimer geometry. In this way, it is possible to explain the fast depopulation of the photoexcited state, which we characterize by femtosecond transient absorption measurements. The combined theoretical and experimental analysis hints at a trapping mechanism, which involves nonadiabatic and dissipative dynamics in an excited-state vibronic manifold and provides an atomistic picture that might prove valuable for future design of photovoltaic materials.

A general ansatz for constructing quasi-diabatic states in electronically excited aggregated systems

We present a general method for analyzing the character of singly excited states in terms of charge transfer (CT) and locally excited (LE) configurations. The analysis is formulated for configuration interaction singles (CIS) singly excited wave functions of aggregate systems. It also approximately works for the second-order approximate coupled cluster singles and doubles and the second-order algebraic-diagrammatic construction methods [CC2 and ADC(2)]. The analysis method not only generates a weight of each character for an excited state, but also allows to define the related quasi-diabatic states and corresponding coupling matrix elements. In the character analysis approach, we divide the target system into domains and use a modified Pipek-Mezey algorithm to localize the canonical MOs on each domain, respectively. The CIS wavefunction is then transformed into the localized basis, which allows us to partition the wavefunction into LE configurations within domains and CT configuration between pairs of different domains. Quasi-diabatic states are then obtained by mixing excited states subject to the condition of maximizing the weight of one single LE or CT configuration (localization in configuration space). Different aims of such a procedure are discussed, either the construction of pure LE and CT states for analysis purposes (by including a large number of excited states) or the construction of effective models for dynamics calculations (by including a restricted number of excited states). Applications are given to LE/CT mixing in π-stacked systems, charge-recombination matrix elements in a hetero-dimer, and excitonic couplings in multi-chromophoric systems.

Influence of a polarizable surrounding on the electronically excited states of aggregated perylene materials.

To tune the efficiency of organic semiconductor devices it is important to understand limiting factors as trapping mechanisms for excitons or charges. An understanding of such mechanisms deserves an accurate description of the involved electronical states in the given environment. In this study, we investigate how a polarizable surrounding influences the relative positions of electronically excited states of dimers of different perylene dyes. Polarization effects are particularly interesting for these systems, because gas phase computations predict that the CT states lie slightly above the corresponding Frenkel states. A polarizable environment may change this energy order because CT states are thought to be more sensitive to a polarizable surrounding than Frenkel states. A first insight we got via a TD‐HF approach in combination with a polarizable continuum model (PCM). These give limited insights because TD‐HF overestimates excitation energies of CT states. However, SCS‐CC2 approaches, which are sufficiently accurate, cannot easily be used in combination with continuum solvent models. Hence, we developed two approaches to combine gas phase SCS‐CC2 results with solvent effects based on TD‐HF computations. Their accuracies were finally checked via ADC(2)//COSMO computations. The results show that for perylene dyes a polarizable surrounding alone does not influence the energetic ordering of CT and Frenkel states. Variations in the energy order of the states only result from nuclear relaxation effects after the excitation process.

Similarities and Differences in the Optical Response of Perylene-Based Hetero-Bichromophores and Their Monomeric Units

The linear and nonlinear optical response of molecular hetero-dimers and their composing perylene units is explored with fluorometry, steady-state and transient absorption, and coherent two-dimensional electronic spectroscopy. Supported by a Förster theory approach and ab initio calculations, we disclose the photoinduced dynamics comprising excitonic coupling, conformational changes, charge transfer, and relaxation dynamics. The influence of the actual orientation of the two chromophore units on these processes is investigated by employing two bichromophores built of the same monomeric units but linked differently.