Modeling nonlocal electron-phonon coupling in organic crystals using interpolative maps: The spectroscopy of crystalline pentacene and 7,8,15,16-tetraazaterrylene

Electron-phonon coupling plays a central role in the transport properties and photophysics of organic crystals. Successful models describing charge- and energy-transport in these systems routinely include these effects. Most models for describing photophysics, on the other hand, only incorporate local electron-phonon coupling to intramolecular vibrational modes, while nonlocal electron-phonon coupling is neglected. One might expect nonlocal coupling to have an important effect on the photophysics of organic crystals because it gives rise to large fluctuation in the charge-transfer couplings, and charge-transfer couplings play an important role in the spectroscopy of many organic crystals. Here, we study the effects of nonlocal coupling on the absorption spectrum of crystalline pentacene and 7,8,15,16-tetraazaterrylene. To this end, we develop a new mixed quantum-classical approach for including nonlocal coupling into spectroscopic and transport models for organic crystals. Importantly, our approach does not assume that the nonlocal coupling is linear, in contrast to most modern charge-transport models. We find that the nonlocal coupling broadens the absorption spectrum non-uniformly across the absorption line shape. In pentacene, for example, our model predicts that the lower Davydov component broadens considerably more than the upper Davydov component, explaining the origin of this experimental observation for the first time. By studying a simple dimer model, we are able to attribute this selective broadening to correlations between the fluctuations of the charge-transfer couplings. Overall, our method incorporates nonlocal electron-phonon coupling into spectroscopic and transport models with computational efficiency, generalizability to a wide range of organic crystals, and without any assumption of linearity.

Binding and electronic level alignment of π-conjugated systems on metals

We review the binding and energy level alignment of π-conjugated systems on metals, a field which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

Synthesis and Electrochemical and Photophysical Properties of Azaterrylene Derivatives

A series of terrylene derivatives, such as monoazaterrylene (MATerry), 1,6-diazaterrylene (DiATerry) and pristine terrylene (Terry), were synthesized by changing the number of nitrogen atoms at the bay region (1 and 6 positions of the Terry core). The electrochemical measurements suggested that the first one-electron reduction and oxidation potentials became positively shifted with increasing numbers of nitrogen atoms. This agreed with the energies of the corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states estimated by DFT methods. In contrast, the HOMO-LUMO gaps approximately remained constant. This trend is quite similar to the spectroscopic behaviors observed by absorption and fluorescence spectra. The solvent polarity-dependent spectroscopic trends of DiATerry suggested the intramolecular charge-transfer (ICT) characters. The evaluation of the excited-state dynamics in various solvents indicated the electronic configurational changes of the excited states relative to the ground state via the ICT. This was supported by the Lippert-Mataga plots. Finally, the reversible protonation and deprotonation processes were also observed.

Ultraviolet photoelectron spectroscopy reveals energy-band dispersion for π-stacked 7,8,15,16-tetraazaterrylene thin films in a donor-acceptor bulk heterojunction

7,8,15,16-tetraazaterrylene (TAT) thin films grown on highly oriented pyrolytic graphite (HOPG) substrates were studied extensively with regard to their intrinsic and interfacial electronic properties by means of ultraviolet photoelectron spectroscopy (UPS). Merely weak substrate-adsorbate interaction occurs at the TAT/HOPG interface, with interface energetics being only little affected by the nominal film thickness. Photon energy-dependent UPS performed perpendicular to the molecular planes of TAT multilayer films at room temperature clearly reveals band-like intermolecular dispersion of the TAT highest occupied molecular orbital (HOMO) energy. Based on a comparison with a tight-binding model, a relatively narrow bandwidth of 54 meV is derived, which points to the presence of an intermediate regime between hopping and band-like hole transport. Upon additional deposition of 2,2':5',2″:5″,2″'-quaterthiophene (4T), a 4T:TAT donor-acceptor bulk heterojunction with a considerable HOMO-level offset at the donor-acceptor interface is formed. The 4T:TAT bulk heterojunction likewise exhibits intermolecular dispersion of the TAT HOMO energy, yet with a significant decreased bandwidth.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Disentangling "Bright" and "Dark" Interactions in Ordered Assemblies of Organic Semiconductors

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10-15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how "dark" (ground-state) interactions in organic semiconductors can be distinguished from "bright" (excited-state) exciton coupling typically assessed by spectral measurements alone.

Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials

The transport and photophysical properties of organic molecular aggregates, films, and crystals continue to receive widespread attention, driven mainly by expanding commercial applications involving display and wearable technologies as well as the promise of efficient, large-area solar cells. The main blueprint for understanding how molecular packing impacts photophysical properties was drafted over five decades ago by Michael Kasha. Kasha showed that the Coulombic coupling between two molecules, as determined by the alignment of their transition dipoles, induces energetic shifts in the main absorption spectral peak and changes in the radiative decay rate when compared to uncoupled molecules. In H-aggregates, the transition dipole moments align "side-by-side" leading to a spectral blue-shift and suppressed radiative decay rate, while in J-aggregates, the transition dipole moments align "head-to-tail" leading to a spectral red-shift and an enhanced radiative decay rate. Although many examples of H- and J-aggregates have been discovered, there are also many "unconventional" aggregates, which are not understood within the confines of Kasha's theory. Examples include nanopillars of 7,8,15,16-tetraazaterrylene, as well as several perylene-based dyes, which exhibit so-called H- to J-aggregate transformations. Such aggregates are typically characterized by significant wave function overlap between neighboring molecular orbitals as a result of small (∼4 Å) intermolecular distances, such as those found in rylene π-stacks and oligoacene herringbone lattices. Wave function overlap facilitates charge-transfer which creates an effective short-range exciton coupling that can also induce J- or H-aggregate behavior, depending on the sign. Unlike Coulomb coupling, short-range coupling is extremely sensitive to small (sub-Å) transverse displacements between neighboring chromophores. For perylene chromophores, the sign of the short-range coupling changes several times as two molecules are "slipped" from a "side-by-side" to "head-to-tail" configuration, in marked contrast to the sign of the Coulomb coupling, which changes only once. Such sensitivity allows J- to H-aggregate interconversions over distances several times smaller than those predicted by Kasha's theory. Moreover, since the total coupling drives exciton transport and photophysical properties, interference between the short- and long-range (Coulomb) couplings, as manifest by their relative signs and magnitudes, gives rise to a host of new aggregate types, referred to as HH, HJ, JH, and JJ aggregates, with distinct photophysical properties. An extreme example is the "null" HJ-aggregate in which total destructive interference leads to absorption line shapes practically identical to uncoupled molecules. Moreover, the severely compromised exciton bandwidth effectively shuts down energy transport. Most importantly, the new aggregates types described herein can be exploited for electronic materials design. For example, the enhanced exciton bandwidth and weakly emissive properties of HH-aggregates make them ideal candidates for solar cell absorbers, while the enhanced charge mobility and strong emissive behavior of JJ-aggregates makes them excellent candidates for light-emitting diodes.

Graphene Ink as a Conductive Templating Interlayer for Enhanced Charge Transport of C60-Based Devices

We demonstrate conductive templating interlayers of graphene ink, integrating the electronic and chemical properties of graphene in a solution-based process relevant for scalable manufacturing. Thin films of graphene ink are coated onto ITO, following thermal annealing, to form a percolating network used as interlayer. We employ a benchmark n-type semiconductor, C₆₀, to study the interface of the active layer/interlayer. On bare ITO, C₆₀ molecules form films of homogeneously distributed grains; with a graphene interlayer, a preferential orientation of C₆₀ molecules is observed in the individual graphene plates. This leads to crystal growth favoring enhanced charge transport. We fabricate devices to characterize the electron injection and the effect of graphene on the device performance. We observe a significant increase in the current density with the interlayer. Current densities as high as ∼1 mA/cm² and ∼70 mA/cm² are realized for C₆₀ deposited with the substrate at 25 °C and 150 °C, respectively.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Directional charge separation in isolated organic semiconductor crystalline nanowires

One of the fundamental design paradigms in organic photovoltaic device engineering is based on the idea that charge separation is an extrinsically driven process requiring an interface for exciton fission. This idea has driven an enormous materials science engineering effort focused on construction of domain sizes commensurate with a nominal exciton diffusion length of order 10 nm. Here, we show that polarized optical excitation of isolated pristine crystalline nanowires of a small molecule n-type organic semiconductor, 7,8,15,16-tetraazaterrylene, generates a significant population of charge-separated polaron pairs along the π-stacking direction. Charge separation was signalled by pronounced power-law photoluminescence decay polarized along the same axis. In the transverse direction, we observed exponential decay associated with excitons localized on individual monomers. We propose that this effect derives from an intrinsic directional charge-transfer interaction that can ultimately be programmed by molecular packing geometry.

Photo-induced charge separation in organic semiconductors usually occurs at interfaces between electron donors and acceptors. Here, the authors show using photoluminescence measurements that charge separation is intrinsic and directional in organic crystalline nanowires made of stacked monomers.

Perylene, Oligorylenes, and Aza-Analogs

An in-depth discussion of the properties of perylene is presented. Tuning the properties of perylene by introducing nitrogens is also explored. Finally, we do not discuss the synthesis and properties of oligorylenes functionalized with dicarboxyimide bonds.

Selective Nucleation of Poly(3-hexyl thiophene) Nanofibers on Multilayer Graphene Substrates

We demonstrate that graphene surfaces provide highly selective nucleation of poly(3-hexyl thiophene) (P3HT) nanofibers (NFs) from supersaturated solutions. Solvent conditions are identified that give rise to a wide hysteresis between crystallization and melting centered around room temperature, yielding metastable solutions that are stable against homogeneous nucleation for long periods of time but that allow for heterogeneous nucleation by graphene. Selective growth of P3HT crystals is found for multilayer graphene (MLG) supported on either Si or ITO substrates, with nucleation kinetics that are more rapid for MLG on Si but slower in both cases than for highly oriented pyrolytic graphite (HOPG). Although the NFs grow vertically from the substrate with face-on orientation of P3HT chains, we observe edge-on orientation in dried films, presumably due to capillary forces that cause collapse of the NFs onto the substrate during solvent evaporation.

Intrinsic and extrinsic parameters for controlling the growth of organic single-crystalline nanopillars in photovoltaics

The most efficient architecture for achieving high donor/acceptor interfacial area in organic photovoltaics (OPVs) would employ arrays of vertically interdigitated p- and n- type semiconductor nanopillars (NPs). Such morphology could have an advantage in bulk heterojunction systems; however, precise control of the dimension morphology in a crystalline, interpenetrating architecture has not yet been realized. Here we present a simple, yet facile, crystallization technique for the growth of vertically oriented NPs utilizing a modified thermal evaporation technique that hinges on a fast deposition rate, short substrate-source distance, and ballistic mass transport. A broad range of organic semiconductor materials is beneficial from the technique to generate NP geometries. Moreover, this technique can also be generalized to various substrates, namely, graphene, PEDOT-PSS, ZnO, CuI, MoO3, and MoS2. The advantage of the NP architecture over the conventional thin film counterpart is demonstrated with an increase of power conversion efficiency of 32% in photovoltaics. This technique will advance the knowledge of organic semiconductor crystallization and create opportunities for the fabrication and processing of NPs for applications that include solar cells, charge storage devices, sensors, and vertical transistors.