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OMIECs undergo structural rearrangements to accommodate ions and electronic charge carriers during which percolating networks are formed for efficient electronic charge transport.

Ocean mesoscale and submesoscale features, such as eddies and filaments, play a key role in the foraging ecology of marine predators, by concentrating nutrients and acting as aggregative structures for pelagic organisms. Highly pelagic seabirds may exploit these features to find profitable food patches in a dynamic and complex 3-dimensional spatial environment. Using miniaturized GPS loggers, we investigated whether foraging habitat selection of the Mediterranean storm petrel Hydrobates pelagicus melitensis, one of the smallest (ca. 28 g) seabirds worldwide, was affected by different static and dynamic oceanographic features during the breeding period. Individuals performed long foraging trips (up to 1113 km) in a relatively short time (1 to 2 d), covering large home ranges (up to 34370 km2), particularly during incubation. Different oceanographic features affected the at-sea distribution of storm petrels at different spatio-temporal scales. During incubation, individuals selected areas characterized by shallow waters and strong currents, conditions that may enhance vertical water mixing and increase food availability. During chick-rearing, they foraged closer to the colony, selecting shallow and productive areas, where increasing Lagrangian coherent structures and eddy kinetic energy enhanced foraging probability. These features could play an important role in storm petrels’ foraging habitat selection, especially during chick-rearing, given their need to find predictable food patches in a short timespan. Overall, our results suggest that marine circulation processes are key drivers of the at-sea distribution of this small pelagic surface predator.

Understanding sediment transport dynamics is key to understanding landscape evolution, and has important implications for engineering projects, aquatic ecosystem dynamics, and transmission of water-borne diseases. Multiple elevated temperature infra-red stimulated luminescence (MET-IRSL) has great potential to provide detailed information on the transport of sediments using infra-red light to stimulate the luminescence signal of feldspars. MET-IRSL uses a series of elevated temperature stimulations to access multiple signals with different characteristic rates of signal reduction by light exposure (bleaching), for example, during grain transport. During deposition and storage, trapped charge accumulates, leading to growth of the different IRSL signals, until the grain is again subject to transport. Applied in this manner, MET-IRSL measurements can constrain past sediment burial and exposure histories.MET-IRSL measurements of different grain and clast sizes (e.g. silt, sand, pebbles and cobbles) can provide a range of sediment transport information, providing further constraint to sediment dynamics and system behaviour. Different clast size groups are associated with varied ways to structure the MET-IRSL measurements, e.g. depth bleaching profiles observed within pebbles and cobbles. In this presentation we demonstrate the potential of combining these approaches, and of constructing time-space equivalence models for real world situations, including the site of Allt Dubhaig, Perthshire, Scotland.

The efficiency of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices depends significantly upon absorption of photons and the migration of the photogenerated excited state to the heteroju...

Unlike the Frenkel exciton model, the essential state model (ESM) for aggregates of donor–acceptor (DA) chromophores accounts self-consistently for the effect of the reaction field on the ground an...

The exciton band dispersion in π-stacks of conjugated organic chromophores is a critical factor in determining the photophysical response and transport properties. In such stacks, the exciton band ...

The essential states model (ESM) for donor–acceptor–donor (D–A–D) chromophores is used to explore absorption and photoluminescence (PL) in molecular dimers composed of centrosymmetric and non-centrosymmetric squaraine molecules. The spectral line shapes and shifts relative to the monomer spectrum are due to an interesting interplay between three-center charge distributions responsible for ground- and excited-state (permanent) dipole and quadrupole moments and two-center charge distributions responsible for transition dipole moments. The Davydov splitting is sensitive only to the interactions between the (extended) transition dipoles , whereas the permanent dipole-dipole and quadruple-quadrupole interactions impact the midpoint frequency of the two Davydov components, leading to a generally asymmetric splitting relative to the peak monomer transition frequency. The theory accurately reproduces the steady-state absorption and PL line shapes recently obtained for covalently bound squaraine dimers. The ESM also predicts an extreme type of non-Kasha behavior, where both Davydov components are blue-shifted above the monomer transition frequency.

An extension of the essential states model is developed to investigate the absorption and electronic properties of a highly dipolar merocyanine dye and double and quadruple π-stacks formed upon the...

The shape of the mid-IR absorption spectrum provides valuable information about the “hole” polaron coherence length in doped and undoped conjugated polymer films. In poly(3-hexylthiophene) (P3HT) f...

A highly sensitive short-wave infrared (SWIR, λ > 1000 nm) organic photodiode (OPD) is described based on a well-organized nanocrystalline bulk-heterojunction (BHJ) active layer composed of a dicyanovinyl-functionalized squaraine dye (SQ-H) donor material in combination with PC\(_{61}\)BM. Through thermal annealing, dipolar SQ-H chromophores self-assemble in a nanoscale structure with intermolecular charge transfer mediated coupling, resulting in a redshifted and narrow absorption band at 1040 nm as well as enhanced charge carrier mobility. The optimized OPD exhibits an external quantum efficiency (EQE) of 12.3% and a full-width at half-maximum of only 85 nm (815 cm\(^{-1}\)) at 1050 nm under 0 V, which is the first efficient SWIR OPD based on J-type aggregates. Photoplethysmography application for heart-rate monitoring is successfully demonstrated on flexible substrates without applying reverse bias, indicating the potential of OPDs based on short-range coupled dye aggregates for low-power operating wearable applications.

The Mediterranean islands’ blue economy and, more specifically, the tourism sector, largely regulate Europe’s gross product. Climate change threatens the ecological, societal, and economic sustainability of the islands in many ways, with increasing wildfires making up one of the most critical components of the climate change impacts on tourism. Here, we aim to identify and assess forest fire vulnerability and risk due to climate change for seven Mediterranean islands through the application of the “impact chain” conceptual framework. The backbone of this approach requires the integration of quantitative and qualitative data according to the three main risk components sensu the Intergovernmental Panel on Climate Change (IPCC), i.e., hazard, exposure, and vulnerability, with a structured participatory approach involving stakeholders and experts. Our results illustrate the islands with high potential for improvement in terms of adapting capacity and, by indicating the contribution of the different risk components, highlight the main environmental and socio-economic elements that affect the islands’ vulnerability and risk under climate change. The approach’s potentials and constraints are discussed, suggesting that the method can be handily used to point out the priorities that must be addressed by mitigation and adaptation policies and measures at the island level. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

ConspectusExcitons and polarons play a central role in the electronic and optical properties of organic semiconducting polymers and molecular aggregates and are of fundamental importance in understanding the operation of organic optoelectronic devices such as solar cells and light-emitting diodes. For many conjugated organic molecules and polymers, the creation of neutral electronic excitations or ionic radicals is associated with significant nuclear relaxation, the bulk of which occurs along the vinyl-stretching mode or the aromatic-quinoidal stretching mode when conjugated rings are present. Within a polymer chain or molecular aggregate, nuclear relaxation competes with energy- and charge-transfer, mediated by electronic interactions between the constituent units (repeat units for polymers and individual chromophores for a molecular aggregate); for neutral electronic excitations, such inter-unit interactions lead to extended excited states or excitons, while for positive (or negative) charges, interactions lead to delocalized hole (or electron) polarons. The electronic coupling as well as the local coupling between electronic and nuclear degrees of freedom in both excitons and polarons can be described with a Holstein Hamiltonian. However, although excitons and polarons derive from similarly structured Hamiltonians, their optical signatures are quite distinct, largely due to differing ground states and optical selection rules.In this Account, we explore the similarities and differences in the spectral response of excitons and polarons in organic polymers and molecular aggregates. We limit our analysis to the subspace of excitons and hole polarons containing at most one excitation; hence we omit the influence of bipolarons, biexcitons, and higher multiparticle excitations. Using a generic linear array of coupled units as a model host for both excitons and polarons, we compare and contrast the optical responses of both quasiparticles, with a particular emphasis on the spatial coherence length, the length over which an exciton or polaron possesses wave-like properties important for more efficient transport. For excitons, the UV-vis absorption spectrum is generally represented by a distorted vibronic progression with H-like or J-like signatures depending on the sign of the electronic coupling, Jex. The spectrum broadens with increasing site disorder, with the spectral area preserved due to an oscillator strength sum rule. For (hole) polarons, the generally stronger electronic coupling results in a mid-IR spectrum consisting of a narrow, low-energy peak (A) with energy near a vibrational quantum of the vinyl stretching mode, and a broader, higher-energy feature (B). In contrast to the UV-vis spectrum, the mid-IR spectrum is invariant to the sign of the electronic coupling, th, and completely resistant to long-range disorder, where it remains entirely homogeneously broadened. Even in the presence of short-range disorder, the width of peak A remains surprisingly narrow as long as |th| remains sufficiently large, a property that can be understood in terms of Herzberg-Teller coupling. Unlike for excitons, for polarons, the absorption spectral area decreases with increasing short-range disorder σ (i.e., there is no oscillator sum rule) reflective of a decreasing polaron coherence length. The intensity of the low-energy peak A in relation to B is an important signature of polaron coherence. By contrast, for excitons, the absorption spectrum contains no unambiguous signs of exciton coherence. One must instead resort to the shape of the steady-state photoluminescence spectrum. The Holstein-based model has been highly successful in accounting for the spectral properties of molecular aggregates as well as conjugated polymers like poly(3-hexylthiophene) (P3HT) in the mid-IR and UV-vis spectral regions.

Advances in protein design and engineering have yielded peptide assemblies with enhanced and non-native functionalities. Here, various molecular organic semiconductors (OSCs), with known excitonic up- and down-conversion properties, are attached to a de novo-designed protein, conferring entirely novel functions on the peptide scaffolds. The protein-OSC complexes form similarly sized, stable, water-soluble nanoparticles that are robust to cryogenic freezing and processing into the solid-state. The peptide matrix enables the formation of protein-OSC-trehalose glasses that fix the proteins in their folded states under oxygen-limited conditions. The encapsulation dramatically enhances the stability of protein-OSC complexes to photodamage, increasing the lifetime of the chromophores from several hours to more than 10 weeks under constant illumination. Comparison of the photophysical properties of astaxanthin aggregates in mixed-solvent systems and proteins shows that the peptide environment does not alter the underlying electronic processes of the incorporated materials, exemplified here by singlet exciton fission followed by separation into weakly bound, localized triplets. This adaptable protein-based approach lays the foundation for spectroscopic assessment of a broad range of molecular OSCs in aqueous solutions and the solid-state, circumventing the laborious procedure of identifying the experimental conditions necessary for aggregate generation or film formation. The non-native protein functions also raise the prospect of future biocompatible devices where peptide assemblies could complex with native and non-native systems to generate novel functional materials.

Polaritons in an ensemble of permutationally symmetric chromophores confined to an optical microcavity are investigated numerically. The analysis is based on the Holstein–Tavis–Cummings Hamiltonian which accounts for the coupling between an electronic excitation on each chromophore and a single cavity mode, as well as the coupling between the electronic and nuclear degrees of freedom on each chromophore. A straightforward ensemble partitioning scheme is introduced, which, along with an intuitive ansatz, allows one to obtain accurate evaluations of the lowest-energy polaritons using a subset of collective states. The polaritons include all three degrees of freedom—electronic, vibronic, and photonic—and can therefore be described as exciton–phonon polaritons. Applications focus on the limiting regimes where the Rabi frequency is small or large compared to the nuclear relaxation energy subsequent to optical excitation, with relaxation occurring mainly along the vinyl stretching coordinate in conjugated organic chromophores. Comparisons are also made to the more conventional vibronic polariton approach, which does not take into account two-particle excitations and vibration–photon states.

Aggregates of quadrupolar DAD-type chromophores are investigated theoretically using the essential states model (ESM) enhanced to include vibronic coupling. On the basis of vibronic signatures in the absorption and photoluminescence spectra, important connections are made to the Frenkel exciton model, which is the basis for defining conventional H- and J-aggregates. In contrast to the exciton theory, the ESM allows for an unusual type of the red-shifted H-aggregate, driven by the large quadrupole–quadrupole interaction. Except for the red shift of the main absorption peak, such aggregates display all of the characteristics of conventional H-aggregates, including suppressed radiative decay rates and all of the vibronic signatures in the absorption and photoluminescence spectra consistent with Frenkel exciton models. Red-shifted H-dimers occur for a range of slip angles, which separate conventional H- and J-aggregate behavior, with the slip range diminishing with increasing intermolecular separation. Davydo...

For the prototypical two-dimensional hybrid organic–inorganic perovskites (2D HOIPs) (AE4T)PbX4 (X = Cl, Br, and I), we demonstrate that the Frenkel–Holstein Hamiltonian (FHH) can be applied to describe the absorption spectrum arising from the organic component. We first model the spectra using only the four nearest neighbor couplings between translationally inequivalent molecules in the organic herringbone lattice as fitting parameters in the FHH. We next use linear-response time-dependent density functional theory (LR-TDDFT) to calculate molecular transition densities, from which extended excitonic couplings are evaluated based on the atomic positions within the 2D HOIPs. We find that both approaches reproduce the experimentally observed spectra, including changes in their shape and peak positions. The spectral changes are correlated with a decrease in excitonic coupling from X = Cl to X = I. Importantly, the LR-TDDFT-based approach with extended excitonic couplings not only gives better agreement with the experimental absorption line shape than the approach using a restricted set of fitted parameters but also allows us to relate the changes in excitonic coupling to the underlying geometry. We accordingly find that the decrease in excitonic coupling from X = Cl to Br to I is due to an increase in molecular separation, which in turn can be related to the increasing Pb–X bond length from Cl to I. Our research opens up a potential pathway to predicting optoelectronic properties of new 2D HOIPs from ab initio calculations and to gain insight into structural relations from 2D HOIP absorption spectra.

The origin of the mid-infrared (IR) spectral features for hole absorption in doped poly(3-hexylthiophene) (P3HT) films is investigated theoretically and experimentally. Using a Holstein-style Hamil...

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.

Small conjugated molecules are of great interest as promising alternatives to semiconducting polymers in organic photovoltaics (OPV). In this work, we introduce a more accurate assignment of the excited state of a promising squaraine (SQ) targeted for OPV application. From this assignment, we conclude that a mixed population of monomers and aggregates exists in spin-cast SQ:PC 61 BM bulk heterojunction (BHJ) films, where monomers indicate the presence of amorphous regions that could act as traps. Since crystallinity is critically important for efficient charge transport and exciton diffusion in the BHJ, we thermally anneal the as-cast films to reduce the amorphous regions. Our analysis of annealed films demonstrates a delicate trade-off between increased crystallinity and larger domain sizes. Crystallinity improves but often at the expense of larger crystal size, as supported by XRD and TEM study. Therefore, to achieve optimal OPV efficiency, we controlled the tradeoff to improve the crystallinity while maintaining a small, highly mixed BHJ morphology. We thus highlight the importance of chemical compatibility when designing small molecules for use in high efficiency BHJ devices. Significantly, we have connected theoretically validated spectroscopic assignment with the first full study of morphology and domain size control as they affect small molecule OPV active layers.

The importance of spatial coherence in energy and charge transfer processes in biological systems and photovoltaic devices has been hotly debated over the past several years. While larger spatial coherences are thought to benefit transport, a clear correlation has yet to be established, partly because a simple and accurate measure of the coherence length has remained elusive. Previously, it was shown that the number of coherently connected chromophores, N Coh , can be determined directly from the ratio ( S R ) of the 0-0 and 0-1 vibronic line strengths in the photoluminescence (PL) spectrum. The relation N Coh = λ 0 2 S R , where λ 0 2 is the associated monomeric Huang–Rhys parameter, was derived in the Frenkel exciton limit. Here, it is shown that S R remains a highly accurate measure of coherence for systems characterized by significant charge transfer interactions (e.g. conjugated π-stacked systems). The only requirement is that the exciton band curvature must be positive, as in a J-aggregate.

A theory describing the spatial coherence length of polarons in disordered conjugated polymer films is presented, revealing a simple relationship between oscillator strength of the mid-IR absorption band and the polaron coherence function. Applications are made to positively charged polarons or holes in poly(3-hexylthiophene) π-stacks where polaron delocalization occurs in essentially two dimensions: along the polymer axis (x-axis) and along the interchain stacking axis (y-axis). Based on a lattice model consisting of a 2D array of thiophene units linked electronically along the polymer axis (tintra) and between adjacent chains (tinter), a disordered 2D Holstein-style Hamiltonian is developed. The Hamiltonian includes vibronic coupling involving the polaron-forming vibrational mode responsible for aromatic/quinoidal oscillations. The hole absorption spectrum for light polarized along each axis is calculated along with the corresponding hole coherence function, CP(r). For diagonal disorder arising from a r...

A vibronic exciton model is introduced to describe the excited state band structure and associated absorption spectra of low bandgap donor-acceptor conjugated polymers. The Hamiltonian is represented in a diabatic basis consisting of Frenkel-like donor and acceptor fragment excitations as well as charge-transfer (CT) excitations between neighboring fragments. States are coupled to each other through electron and hole transfer as well as Coulombically, through interacting fragment transition dipole moments. Local vibronic coupling involving the prominent aromatic-quinoidal vibrational mode, which is responsible for pronounced vibronic progressions in most conjugated oligomers and polymers, is also included. The DAD repeat unit is shown to behave like a J-aggregate trimer, driven by both the sizable in-phase electron and hole transfer integrals between donor and acceptor fragments as well as negative Coulomb coupling between donor and acceptor fragment excitations. The J-aggregate behavior is enhanced in the polymer limit through inter-repeat unit coupling, with the 0-0 vibronic peak significantly enhanced in the lowest-energy near-IR band. In addition, the radiative rate is enhanced by the number of coherently connected repeat units. The near-IR band is shown to possess roughly equal admixtures of CT and Frenkel-like excitations. Applications are made to the polymer PffBT4T-2DT, with the simulated absorption spectrum quantitatively capturing the salient features of the measured spectrum.

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings., Singlet fission is desirable for highly efficient solar cells, yet the details of the process remain elusive. Here, Broch et al. show that the changes in bulk intermolecular interactions can have little effect on the singlet fission rate in a mixture of pentacene and weakly interacting spacer molecules.

The polarized absorption spectra of crystalline pentacene are obtained for excitation normal to the ab herringbone plane by measuring transmitted light in ultrathin crystals. The spectral line shapes for excitation polarized along b and orthogonal to b are analyzed theoretically using a Holstein-like Hamiltonian which includes both Frenkel and charge transfer (CT) excitons represented in a multiparticle basis set. The model agrees with prior estimates regarding the strong CT contribution (≈45%) of the exciton responsible for the b-polarized lower Davydov component. The polarization resolution allows one to also establish the nature of the upper Davydov component, which is found to contain far less CT content (≈15%), as well as the natures of the higher-energy vibronic excitons, which are found to consist of a complex mixture of Frenkel one- and two-particle states and CT excitons. Generally, the spectrum polarized along b displays J-aggregate-like vibronic signatures while the spectrum polarized orthogona...

Squaraines offer significant potential for organic photovoltaics because of their broad absorbance and high extinction coefficients as well as their expected use as mechanistic probes in such devices. In this work steps are taken to develop a comprehensive understanding of the excited-state properties of squaraines on the basis of their molecular structure and the resulting solid-state packing. Accurate assignments of the absorption spectral peaks are made on the basis of an essential states model, expanded to include intermolecular charge transfer (ICT). A comparison of simulated spectra with spectra for two symmetric squaraine derivatives confirms that ICT has a major influence on the optical and electronic properties of squaraine aggregates, fully accounting for the origin of the strong panchromaticity of these systems.

Sequential doping is a promising new doping technique for semicrystalline polymers that has been shown to produce doped films with higher conductivity and more uniform morphology than conventional doping processes, and where the dopant placement in the film can be controlled. As a relatively new technique, however, much work is needed to understand the resulting polymer–dopant interactions upon sequential doping. A combination of infrared spectroscopy and theoretical simulations shows that the dopants selectively placed in the amorphous regions in the film via sequential doping result in highly localized polarons. We find that the presence of dopants within the amorphous regions of the film leads to an increase in conjugation length of the amorphous chains upon doping, increasing film connectivity and hence improving the overall conductivity of the film compared with the conventional doping processes.

The interaction of organic molecules and molecular aggregates with electromagnetic fields that are strongly confined inside optical cavities within nanoscale volumes, has allowed the observation of exotic quantum regimes of light-matter interaction at room temperature, for a wide variety of cavity materials and geometries. Understanding the universal features of such organic cavities represents a significant challenge for theoretical modelling, as experiments show that these systems are characterized by an intricate competition between coherent and dissipative processes involving entangled nuclear, electronic and photonic degrees of freedom. In this review, we discuss a new theoretical framework that can successfully describe organic cavities under strong light-matter coupling. The theory combines standard concepts in chemical physics and quantum optics to provide a microscopic description of vibronic organic polaritons that is fully consistent with available experiments, and yet is profoundly different from the common view of organic polaritons. We show that by introducing a new class of vibronic polariton wave functions with a photonic component that is dressed by intramolecular vibrations, the new theory can offer a consistent solution to some of the long-standing puzzles in the interpretation of organic cavity photoluminescence. Throughout this review, we confront the predictions of the model with spectroscopic observations, and describe the conditions under which the theory reduces to previous approaches. We finally discuss possible extensions of the theory to account for realistic complexities of organic cavities such spatial inhomogeneities and the multi-mode nature of confined electromagnetic fields., Review Article, 16 Pages, 9 Figures

Organic microcavities can be engineered to reach exotic quantum regimes of strong and ultrastrong light-matter coupling. However, the microscopic interpretation of their spectroscopic signals can be challenging due to the competition between coherent and dissipative processes involving electrons, vibrations and cavity photons. We develop here a theoretical framework based on the Holstein-Tavis-Cummings model and a Markovian treatment of dissipation to account for previously unexplained spectroscopic features of organic microcavities consistently. We identify conditions for the formation of dark vibronic polaritons, a new class of light-matter excitations that are not visible in absorption but lead to strong photoluminescence lines. We show that photon leakage from dark vibronic polaritons can be responsible for enhancing photoluminescence at the lower polariton frequency, and also explain the apparent breakdown of reciprocity between absorption and emission in the vicinity of the bare molecular transition frequency. Successful comparison with experimental data demonstrates the applicability of our theory., Comment: 25 pages, 10 figures

Absorption and photoluminescence properties of terrylene derivative 7,8,15,16-tetraazaterrylene (TAT) in its solution and crystal phases have revealed rather unusual spectral characteristics that defy classification in terms of simple H- or J-aggregate-coupled systems. TAT readily forms crystalline aggregates by either self-assembly in solution or physical vapor deposition, based on π stacks aligned roughly along the crystallographic a axis. Using a Holstein-style Hamiltonian including both Frenkel and charge-transfer (CT) excitons, the crystal absorption and steady-state photoluminescence (PL) spectra/line shapes are shown to be determined by a competition between long-range Coulombic coupling, which induces H-aggregate behavior, and short-range charge-transfer-mediated coupling, which induces J-like behavior. Such “HJ” aggregates display J-aggregate signatures in the low-energy region of the absorption spectrum and H-aggregate signatures at higher energies, which are in excellent agreement with our expe...

The impact of chain bending on the photophysical properties of emissive conjugated polymers (CPs) is studied theoretically using Holstein-style Hamiltonians which treat vibronic coupling involving the ubiquitous vinyl/ring stretching mode nonadiabatically. The photophysical impact of chain bending is already evident at the level of an effective Frenkel Hamiltonian, where the positive exciton band curvature in CPs translates to negative excitonic coupling between monomeric units, as in J-aggregates. It is shown that the absorption and photoluminescence (PL) spectral line shapes respond very differently to chain bending. The misalignment of monomeric transition dipole moments with bending selectively attenuates the 0-0 PL peak intensity while leaving the 0-1 intensity practically unchanged, a property which is ultimately due to the uniquely coherent nature of the 0-0 peak. Hence, the 0-0/0-1 PL ratio, as well as the radiative decay rate, decrease with chain bending, effects that are more pronounced at lower temperatures where exciton coherence extends over a larger portion of the chain. Increasing temperature and/or static disorder reduces the exciton coherence number, Ncoh, thereby reducing the sensitivity to bending. In marked contrast, the absorption vibronic progression is far less sensitive to morphological changes, even at low temperatures, and is mainly responsive to the exciton bandwidth. The above results also hold when using a more accurate 1D semiconductor Hamiltonian which allows for electron-hole separation along the CP chain. The findings may suggest unique ways of controlling the radiative properties of conjugated polymer chains useful in applications such as organic light emitting diodes (OLEDs) and low-temperature sensors.

Aggregates of conjugated polymers exhibit two classes of fundamental electronic interactions: those occurring within a given chain and those occurring between chains. The impact of such excitonic interactions on the photophysics of polymer films can be understood using concepts of J- and H-aggregation originally developed by Kasha and coworkers to treat aggregates of small molecules. In polymer assemblies, intrachain through-bond interactions lead to J-aggregate behavior, whereas interchain Coulombic interactions lead to H-aggregate behavior. The photophysics of common emissive conjugated polymer films are determined by a competition between intrachain, J-favoring interactions and interchain, H-favoring interactions. We review formalisms describing absorption and photoluminescence lineshapes, based on intra- and intermolecular excitonic coupling, electron-vibrational coupling, and correlated energetic disorder. Examples include regioregular polythiophenes, pheneylene-vinylenes, and polydiacetylene.

We report the polarized absorption spectra of high-quality, thin crystals of a perylene diimide (PDI) species with branched side chains (B2). The absorption spectrum shows exemplary polarization-dependent H-like and J-like aggregate behavior upon orthogonal excitation, with a sizable Davydov splitting (DS) of 1230 cm–1 and peak to peak splitting of 3040 cm–1. The experimental results are compared to theoretical calculations with remarkable agreement. The theoretical analysis of the polarized absorption spectra shows evidence of a high degree of intermolecular charge transfer, which, along with Coulombic coupling, conspires to create the unprecedented DS for this family of dye molecules. The large polarization dependence of the electronic spectra is attributed to the unique twisted crystal structure, in which a substantial rotational displacement exists between neighboring chromophores within a π-stack. These results highlight the strong sensitivity of the Davydov splitting to intermolecular geometry in PD...

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.

The absorption line shapes of a series of linear and star-shaped perylene diimide (PDI) complexes are evaluated theoretically and compared to experiment. The cyclic trimer and tetrahedral complexes are part of the symmetric series, characterized by a single interchromophoric coupling, J(0), between any two PDI chromophores. The measured spectra of all complexes show pronounced vibronic progressions based on the symmetric ring stretching mode at ~1400 cm(-1). The spectral line shapes are accurately reproduced using a Holstein Hamiltonian parametrized with electronic couplings calculated using time-dependent density functional transition charge densities. Although the "head-to-tail" linear complexes display classic J-aggregate behavior, the star-shaped complexes display a unique photophysical response, which is neither J- nor H-like. In the symmetric N-mers (N = 2-4), absorption and emission are polarized along N - 1 directions in contrast to linear complexes where absorption and emission remain polarized along the long molecular axis. In the symmetric complexes the red-shift of the 0-0 peak with increasing |J(0)|, as well as the initial linear rise of the 0-0/1-0 oscillator strength ratio with increasing |J(0)|, are independent of the number of PDI chromophores, N, and are markedly smaller than what is found in the linear series, where the shifts and ratios depend on N. Moreover, whereas the radiative decay rate, γ(r), scales with N and is therefore superradiant in linear complexes, γ(r) scales with N/(N - 1) in the symmetric complexes. Vibronic/vibrational pair states (two-particle states) are found to profoundly affect the absorption line shapes of both linear and symmetric complexes for sufficiently large coupling.

Organic microcavities are photonic nanostructures that strongly confine the electromagnetic field, allowing exotic quantum regimes of light-matter interaction with disordered organic semiconductors. The unambiguous interpretation of the spectra of organic microcavities has been a long-standing challenge due to several competing effects involving electrons, vibrations and cavity photons. Here we present a theoretical framework that is able to describe the main spectroscopic features of organic microcavities consistently. We introduce a class of light-matter excitations called dark vibronic polaritons, which strongly emit but only weakly absorb light in the same frequency region of the bare electronic transition. Successful comparison with experimental data demonstrates the applicability of our theory. The proposed microscopic understanding of organic microcavities paves the way for the development of optoelectronic devices enhanced by quantum optics., 5 pages, 2 figures [APS format]

In recent years, higher power conversion efficiencies have been measured using "push-pull" or Donor-Acceptor (D-A) type compounds designed to specifically address bandgap and energy level requirements. Yet, a strong prescription is fundamentally lacking that improves materials for the set of all critical properties (including exciton diffusion rate and charge transport/ mobility) that combine to provide optimal performance. We will present our newest theoretical models that simulate the morphology-based spectroscopy for a series of squaraines, compounds representative of the total set of D-A type OPV-targets. The theory will describe how morphological and molecular structure influences i) the absorption spectrum, ii) the excited states and iii) the intermolecular charge transfer integral (ICTI). In particular, the ICTI's role in exciton diffusion and carrier mobility will be explored. Using device data that correlates with the ICTI variation, we will explain how this parameter must be considered in future design of new easily-purified, consistently processable, low-band-gap small molecules targeted for large scale OPV manufacture.

© 2016 American Chemical Society. Coupling among chromophores in molecular assemblies is responsible for phenomena such as resonant energy transfer and intermolecular charge transfer. These processes are central to the fields of organic photovoltaics and photocatalysis, where it is necessary to funnel energy or charge to specific regions within the system. As such, a fundamental understanding of these transport processes is essential for developing new materials for photovoltaic and photocatalytic applications. Recently, photocatalytic systems based on photosensitizing perylene monomimide (PMI) chromophore amphiphiles were found to show variation in hydrogen gas (H2) production as a function of nanostructure crystallinity. The 2D crystalline systems form in aqueous electrolyte solution, which provides a high dielectric environment where the Coulomb potential between charges is mitigated. This results in relatively weakly bound excitons that are ideal for reducing protons. In order to understand how variations in crystalline structure affect H2generation, two representative PMI systems are investigated theoretically using a modified Holstein Hamiltonian. The Hamiltonian includes both molecular Frenkel excitations (FE) and charge-transfer excitations (CTE) coupled nonadiabatically to local intramolecular vibrations. Signatures of FE/CTE mixing and the extent of electron/hole separation are identified in the optical absorption spectrum and are found to correlate strongly to the observed H2production rates. The absorption spectral signatures are found to sensitively depend on the relative phase between the electron and hole transfer integrals, as well as the diabatic energy difference between the Frenkel and CT exciton bands. Our analysis provides design rules for artificial photosynthetic systems based on organic chromophore arrays.

The demonstration of strong and ultrastrong coupling regimes of cavity QED with polyatomic molecules has opened new routes to control chemical dynamics at the nanoscale. We show that strong resonant coupling of a cavity field with an electronic transition can effectively decouple collective electronic and nuclear degrees of freedom in a disordered molecular ensemble, even for molecules with high-frequency quantum vibrational modes having strong electron-vibration interactions. This type of polaron decoupling can be used to control chemical reactions. We show that the rate of electron transfer reactions in a cavity can be orders of magnitude larger than in free space, for a wide class of organic molecular species., Comment: 4 pages, 3 figures, 12 pages SM