Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron−phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.

Nanoporous adsorbents are a diverse category of solid-state materials that hold considerable promise for vehicular hydrogen storage. Although impressive storage capacities have been demonstrated for several materials, particularly at cryogenic temperatures, materials meeting all of the targets established by the U.S. Department of Energy have yet to be identified. In this Perspective, we provide an overview of the major known and proposed strategies for hydrogen adsorbents, with the aim of guiding ongoing research as well as future new storage concepts. The discussion of each strategy includes current relevant literature, strengths and weaknesses, and outstanding challenges that preclude implementation. We consider in particular metal-organic frameworks (MOFs), including surface area/volume tailoring, open metal sites, and the binding of multiple H 2 molecules to a single metal site. Two related classes of porous framework materials, covalent organic frameworks (COFs) and porous aromatic frameworks (PAFs), are also discussed, as are graphene and graphene oxide and doped porous carbons. We additionally introduce criteria for evaluating the merits of a particular materials design strategy. Computation has become an important tool in the discovery of new storage materials, and a brief introduction to the benefits and limitations of computational predictions of H 2 physisorption is therefore presented. Finally, considerations for the synthesis and characterization of hydrogen storage adsorbents are discussed. IntroductionStorage of hydrogen with sufficient gravimetric and volumetric capacity for vehicular use remains a significant obstacle to the widespread adoption of hydrogen fuel cell electric vehicles (FCEVs). Several FCEV models are now commercially available in limited locations around the world, and in these vehicles hydrogen is stored as a gas at room temperature with a fill Table 1 along with the current performance of 700 bar systems. These values pertain to the entire storage system, which includes the mass and volume of hydrogen in addition to the tank and associated balance-ofplant (BOP) components. Notably, it is physically impossible to meet the 2025 and ultimate volumetric capacity target with pressurized gas, as the density of H 2 gas at 700 bar and room temperature is just 40 g L À1 without accounting for the BOP.The search for solid-state H 2 storage materials that can supplant compressed gas systems has been ongoing for at least two decades. The development of a viable storage material Broader contextThe widespread use of hydrogen as a clean, sustainable energy carrier has the potential to provide several significant benefits, including a reduction in oil dependency and emissions, improved energy security and grid resiliency, and substantial economic opportunities across many sectors. Hydrogen-fueled vehicles are already appearing internationally, and one of the critical enabling technologies for increasing their availability is on-board hydrogen storage. Stakeholders in developing a hydrogen in...

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We investigate the accuracy of different formalisms within density functional theory in prediction of two-photon absorption (2PA) spectra for substituted oligophenylvinylenes compared to the experimental measurements. The quadratic response methods are compared with the recently proposed a posteriori Tamm−Dancoff approximation (ATDA) and previously published third-order coupled electronic oscillator results. Quadratic response is found to overestimate the cross sections in all cases. We trace the reasons to unreliable excited state description above the ionization threshold. In addition, quadratic response lacks the double excitations so that their contributions to the 2PA spectra are redistributed over the nearest single character excitations. This distorts the individual contributions to the 2PA response and affects the overall picture. For this reason, we do not recommend quadratic response for the essential state analysis, while ATDA can be used both for the 2PA predictions and the structure/property correlations. As an illustration for ATDA based essential state analysis, we report the mechanism of large 2PA in symmetric donor/acceptor substituted polyphenylvinylene (PPV) oligomers. While HOMO−LUMO transition provides the only bright intermediate state, the brightness of the one-photon absorption (1PA) to 2PA transition is associated with symmetric to asymmetric linear combination of the respective donor (HOMO − 1 to HOMO) or acceptor (LUMO to LUMO + 1) fragment orbitals of the donor or acceptor substituents. We also study the effect of the fraction of Hartree−Fock (HF) exchange on 2PA excitation energies and cross sections. Higher exchange (BMK and M05-2X) and range separated (CAM-B3LYP) hybrid functionals are found to yield rather inaccurate predictions both quantitatively and qualitatively. The results obtained with the long-range corrected functional LC-BLYP do not seem to be useful at all. This failure of the exchange-correlation functionals with the correct asymptotic is traced to inaccurate transition dipoles between the valence states, where only functionals with lower HF exchange succeed. A new sum over states (SOS) cutoff procedure is proposed to compensate for the collapse of the higher-lying excited states obtained with the hybrid functionals.

We use spectrally-resolved magneto-electroluminescence (EL) measurements to study the energy dependence of hyperfine interactions between polaron and nuclear spins in organic LEDs. Using layered devices based on Bphen/MTDATA -a well-known exciplex emitter -we show that the increase in EL emission intensity I due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (∆I/I ∼11%) than at the low-energy red end (∼4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.Several recent experiments have shown that small applied magnetic fields B on the order of 10-100 mT can induce substantial (∼10%) changes in the total light intensity emitted by organic light-emitting diodes (OLEDs) [1][2][3][4][5][6][7][8][9][10][11][12][13]. While initially surprising in view of the fact that the polymers and small molecules used in OLEDs are primarily composed of non-magnetic atoms (H, C, N), it was quickly appreciated that hyperfine spin interactions underpinned these phenomena. Specifically, the coupling of the electron and hole polaron spin to the many nuclear spins in the host material generates randomly-oriented local effective magnetic fields about which electron and hole polaron spins can precess. This precession leads to spin mixing between singlet and triplet polaron-pair states, which are precursors to exciton or exciplex formation in an OLED. Applied fields B suppress this hyperfineinduced mixing, altering the population balance between singlet and triplet excitons or exciplexes, which in turn modifies the electroluminescence (EL) efficiency.The detailed dependence of EL intensity on B allows direct insight into not only the rates of singlet and triplet exciton/exciplex formation, but also reveals the strength of hyperfine coupling and therefore provides a measure of the spatial extent (size) of the electron and hole polarons.In magneto-EL studies to date [1][2][3][4][5][6][7][8][9][10][11][12][13], only the total (spectrally-integrated) EL intensity was measured as a function of B. However, OLED emission spectra typically span a very broad wavelength range, reflecting the fact that excitons and exciplexes form over a wide range of energies, and with varying degrees of localization for which different hyperfine couplings may be expected. Here we spectrally resolve the magneto-EL from MTDATA/Bphen OLEDs -a well known exciplex emitter -and show that the increase in EL intensity due to B is significantly larger at the high-energy blue side of the spectrum than at the low-energy red side. Most importantly, the widths of the magneto-EL curves increase by over a factor of two from blue to red, directly revealing an increasingly strong hyperfine coupling and providing insight into the energy-dependent spatial extent and localization of the emitting states.Figu...

Five different Density Functional Theory (DFT) models (ranging from pure GGA to long-range-corrected hybrid functionals) were used to study computationally the nature of the self-trapped electronic states in oligophenylene vinylenes. The electronic excitations in question include the lowest singlet (S1) and triplet (T1(†)) excitons (calculated using Time Dependent DFT (TD-DFT) method), positive (P(+)) and negative (P(-)) polarons, and the lowest triplet (T1) states (computed with the Self-Consistent Field (SCF) scheme). The polaron formation (spatial localization of excitations) is observed only with the use of range-corrected hybrid DFT models including long-range electronic exchange interactions. The extent of localization for all studied excitations is found to be invariant with respect to the size of the oligomer chain in their corresponding optimal geometries. We have analyzed the interdependence between the extent of the geometrical distortion and the localization of the orbital and spin density, and have observed that the localization of the P(+) and P(-) charged species is quite sensitive to solvent polarization effects and the character of the DFT functional used, rather than the structural deformations. In contrast, the localization of neutral states, S1 and T1(†), is found to follow the structural distortions. Notably, T1 excitation obtained with the mean field SCF approach is always strongly localized in range-corrected hybrid DFT models. The molecular orbital energetics of these excitations was further investigated to identify the relationship between state localization and the corresponding orbital structure. A characteristic stabilization (destabilization) of occupied (virtual) orbitals is observed in hybrid DFT models, compared to tight-binding model-like orbital filling in semilocal GGA functionals. The molecular and natural orbital representation allows visualization of the spatial extent of the underlying electronic states. In terms of stabilization energies, neutral excitons have higher binding energies compared to charged excitations. In contrast, the polaronic species exhibit the highest solvation energies among all electronic states studied.

A fluorene-bridged squaraine dimer (SD-FLU-SD) was designed with the purpose of combining various chromophores in one molecule and enhancing its two-photon absorption properties using intra-and interchromophore transitions. Linear and nonlinear absorption properties of SD-FLU-SD were investigated with the goals of understanding the nature of one-and two-photon absorption spectra, determining the molecular optical parameters, and performing modeling of the photophysical processes. The optical behavior of this new SD-FLU-SD "hybrid" molecule was compared with its separate squaraine constituent moiety. Linear spectroscopic characterization includes absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements in solvents of different polarity and viscosity. Spectral positions of the absorption−fluorescence peaks and quantum yields of SD-FLU-SD and its separate squaraine moiety exhibited complex and nontrivial behavior as a function of solvent polarity. Comprehensive study of this unusual solvatochromism was conducted and interpreted using various models. Nonlinear spectroscopic studies included two-photon absorption measurements using the femtosecond Z-scan technique. The two-photon absorption spectrum of SD-FLU-SD was broad, covering the spectral range from 800 to 1400 nm with a maximum two-photon absorption cross section of 2 750 GM (1 GM = 1 × 10 −50 cm 4 s/photon). Quantum chemical analysis, based on time-dependent density functional theory, agreed with the experimental data and revealed details on the energy-level structure and origin of the linear and nonlinear absorption behavior of this novel SD-FLU-SD compound. These investigations advance the understanding of the nature of electronic transitions and the structure−property relations in long conjugated molecules, which are important for the rational design of new organic optical materials.

The two-photon absorption (2PA) spectrum of an organic single crystal is reported. The crystal is grown by self-nucleation of a subsaturated hot solution of acetonitrile, and is composed of an asymmetrical donor-π-acceptor cyanine-like dye molecule. To our knowledge, this is the first report of the 2PA spectrum of single crystals made from a cyanine-like dye. The linear and nonlinear properties of the single crystalline material are investigated and compared with the molecular properties of a toluene solution of its monomeric form. The maximum polarization-dependent 2PA coefficient of the single crystal is 52 ± 9 cm/GW, which is more than twice as large as that for the inorganic semiconductor CdTe with a similar absorption edge. The optical properties, linear and nonlinear, are strongly dependent upon incident polarization due to anisotropic molecular packing. X-ray diffraction analysis shows π-stacking dimers formation in the crystal, similar to H-aggregates. Quantum chemical calculations demonstrate that this dimerization leads to the splitting of the energy bands and the appearance of new red-shifted 2PA bands when compared to the solution of monomers. This trend is opposite to the blue shift in the linear absorption spectra upon H-aggregation.