Intense Two-photon Absorption Research Articles

Phosphorus(V) Tetraazaporphyrin with an Intense, Broad CT Band in the Near-IR Region.

In phosphorus tetraazaporphyrins (PTAPs), the Q- and charge-transfer (CT) bands appear as a result of configuration interaction between their excited states. On the basis of this concept, a PTAP with an intense, broad CT band in the near-IR region has been rationally designed and realized by introducing eight diphenylaminophenyl (dPAP) groups. The order of the CT and Q-bands in ascending energy was supported by magnetic circular dichroism (MCD) spectroscopy and theoretical calculations. An intense two-photon absorption was also found in the deep near-IR region.

Computational design of ratiometric two-photon fluorescent Zn2+ probes based on quinoline and di-2-picolylamine moieties

To improve two-photon absorption (TPA) response of a newly synthesized probe, a series of ratiometric two-photon fluorescent Zn2+ sensors based on quinoline and DPA moieties have been designed. The one-photon absorption, TPA, and emission properties of the experimental and designed probes before and after coordination with Zn2+ are investigated employing the density functional theory in combination with response functions. The design consists of two levels. In the first level of design, five probes are constructed through using several electron acceptors or donors to increase accepting or donating ability of the fluorophores. It shows that all the designed probes have stronger TPA intensities at longer wavelengths with respect to the experimental probe because of the increased intra-molecular charge transfer. Moreover, it is found that the probe 4 built by adding an acyl unit has the largest TPA cross section among the designed structures due to the form of longer conjugated length and more linear backbone. One dimethylamino terminal attached along the skeleton can improve TPA intensity more efficiently than two side amino groups. Therefore, in the second level of design, a new probe 7 is formed by both an acyl unit and a dimethylamino terminal. It exhibits that the TPA cross sections of probe 7 and its zinc complex increase dramatically. Furthermore, the fluorescence quantum yields of the designed probes 4 and 7 are calculated in a new way, which makes use of the relation between the computed difference of dipole moment and the measured fluorescence quantum yield. The result shows that our design also improves the fluorescence quantum yield considerably. All in all, the designed probes 4 and 7 not only possess enhanced TPA intensities but also have large differences of emission wavelength upon Zn2+ coordination and strong fluorescence intensity, which demonstrates that they are potential ratiometric two-photon fluorescent probes.

Responsive mechanism and coordination mode effect of a bipyridine-based two-photon fluorescent probe for zinc ion**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM026).

The properties of one-photon absorption (OPA), emission and two-photon absorption (TPA) of a bipyridine-based zinc ion probe are investigated employing the density functional theory in combination with response functions. The responsive mechanism and coordination mode effect are explored. The structural fluctuation is illustrated by molecular dynamics simulation. The calculated OPA and emission wavelengths of the probe are consistent with the experimental data. It is found that the red-shift of OPA wavelength and the enhancement of TPA intensity are induced by the increased intra-molecular charge transfer mechanism upon metal binding. The structural fluctuation could result in the blue-shift of TPA wavelength and the decrease of the TPA cross section. The TPA properties are quite different among the zinc complexes with different coordination modes. The TPA wavelength of the complexes with two ligands is close to that of the probe, which is in agreement with the experimental observation.

Switching of Two-Photon Optical Properties by Anion Binding of Pyrrole-Based Boron Diketonates through Conformation Change.

Two-photon absorption (TPA) dyes with intense fluorescence can be used to detect small chemical species and as sensors and bioimaging probes for specific analytes. Various TPA dyes responding to a number of external stimuli have been reported. Among them, biologically important anionic species have not been used as agents to control TPA properties because their direct electronic influences on the transition dipole moments of dyes are typically small. In this study, dipyrrolyldiketone BF2 complexes substituted with π-extended units exhibited efficient TPA properties that could be regulated by conformation changes induced by anion binding. The TPA intensity decreased to 1/5 of the original intensity upon anion binding, which was much larger than that observed for one-photon absorption. Anion detection was achieved by a change in the emission intensity of spatially resolved spots of two-photon-excited fluorescence (TPEF) in the sample. Experimental and theoretical studies were performed to understand the mechanism of the TPA property control and showed that the drastic changes in the transition dipole moments upon conformation changes between the straight and bending forms of the π-electronic systems caused the TPA and TPEF intensities drop.

Two-photon absorption property of Cl-substituted diphenylacetylenes by optical-probing photoacoustic spectroscopy.

Two-photon absorption spectra and two-photon absorption cross sections of Cl-substituted diphenylacetylenes (ClDPAs) were investigated by optical-probing photoacoustic spectroscopy and quantum chemical calculations for the first time. The two-photon absorption spectra of ClDPAs exhibited intense two-photon absorption bands at around 480 nm, which are forbidden by one-photon absorption. The two-photon absorption cross sections σ(2) of o-, m-, and p-ClDPAs at 476 nm were determined to be 22 ± 1, 23 ± 1, and 38 ± 2 GM, respectively. Compared with diphenylacetylene (DPA) (27 GM at 472 nm), the σ(2) values of o- and m-ClDPAs were lower, while that of p-ClDPA was higher. Simulated two-photon absorption spectra using the TD-B3LYP/6-311+G(d,p) level of calculations within the Tamm-Dancoff approximation, based on the three-state model, well agreed with the experimental results. The difference in the σ(2) values of DPA and ClDPAs was responsible for those in the transition dipole moments between the intermediate and the final states.

Low-energy/pulse response and high-resolution-CMOS camera for spatiotemporal femtosecond laser pulses characterization @ 1.55 μm.

In this work, we present a commercial CMOS (Complementary Metal Oxide Semiconductor) Raspberry Pi camera implemented as a Near-Infrared detector for both spatial and temporal characterization of femtosecond pulses delivered from a femtosecond Erbium Doped Fiber laser (fs-EDFL) @ 1.55 µm, based on the Two Photon Absorption (TPA) process. The capacity of the device was assessed by measuring the spatial beam profile of the fs-EDFL and comparing the experimental results with the theoretical Fresnel diffraction pattern. We also demonstrate the potential of the CMOS Raspberry Pi camera as a wavefront sensor through its a nonlinear response in a Shack-Hartmann array and for the temporal characterization of the femtosecond pulses delivered from the fs-EDFL through TPA Intensity autocorrelation measurements. The direct pulse detection and measurement, through the nonlinear response with a CMOS, is proposed as a novel and affordable high-resolution and high-sensitivity alternative to costly detectors such as CCDs, wavefront sensors and beam profilers @ 1.55 µm. The measured fluence threshold, down to 17.5 µJ/cm2, and pJ/pulse energy response represents the lowest reported values applied as a beam profiler and a TPA Shack-Hartmann wavefront sensor, to our knowledge.

Effects of donor position and number on two-photon absorption properties of tetraphenylethylene derivatives

Organic materials with strong two-photon absorption response and aggregation induced emission have aroused a great deal of interest in recent years, for their many potential applications such as two-photon fluorescence microscopy, up-conversion laser, photodynamic therapy, etc. The tetraphenylethylene units are usually employed in two-photon absorption and aggregation induced emission materials because of their good electron-donating capability and special propeller starburst structures. Theoretical study on the relationship between molecular structure and two-photon absorption property is of great importance for guiding the experimental design and synthesis of functional materials. In this paper, the two-photon absorption properties of a series of organic molecules containing tetraphenylethylene and cyano groups are studied by employing the density functional response theory in combination with the polarizable continuum model. The molecular geometries are optimized at a hybrid B3LYP level with 6-31g(d, p) basis set in the Gaussian 16 program. The two-photon absorption cross sections are calculated by response theory through using the CAM-B3LYP functional with 6-31g(d) basis set in the Dalton program. The effect of donor position and number on two-photon absorption properties are investigated. In addition, by increasing the planarity and conjugated length of the molecule, as well as by enhancing the strength of the electron donor, we design three molecular structures and calculate their two-photon absorption properties. The results show that the donor position and number have important effects on two-photon absorption properties. The methoxy donor at the end of the molecule can increase the two-photon absorption intensity effectively. As the number of substituents increases, the position of the two-photon absorption peak is red-shifted. The effects of adding electron donor groups on different side positions have a significant difference in the two-photon absorption property. Comparing with the experimental molecules, the two-photon absorption cross sections of the designed molecules are greatly enhanced. When the tetraphenylethylene group is replaced by the triphenylamine group, the two-photon absorption peak is greatly red-shifted, and the two-photon absorption intensity is significantly increased. Since all of these molecules contain tetraphenylethylene or triphenylamine group with propeller structure, they can have both two-photon absorption and aggregation induced emission properties. This study provides theoretical guidelines for synthesizing a new type of active two-photon absorption and aggregation induced emission material.

Assessment of Functionals for TDDFT Calculations of One- and Two-Photon Absorption Properties of Neutral and Anionic Fluorescent Proteins Chromophores

Performance of DFT functionals with different percentages of exact Hartree-Fock exchange energy (EX) is assessed for recovery of the CC2 reference one- (OPA) and two-photon absorption (TPA) spectra of fluorescent proteins chromophores in vacuo. The investigated DFT functionals, together with their EX contributions are BLYP (0%), B3LYP (20%), B1LYP (25%), BHandHLYP (50%), and CAM-B3LYP (19% at short range and 65% at long range). Our test set consists of anionic and neutral chromophores as naturally occurring in the fluorescent proteins. For the first time, we compare TDDFT and CC2 methods for higher excited states than the S1 state, exhibiting relatively large TPA intensity. Our TDDFT results for neutral chromophores reveal an increase in excitation energies as well as TPA and OPA intensities errors, compared to CC2-derived results, as the DFT functional contains less exact exchange. The long-range-corrected CAM-B3LYP functional performs the best, closely followed by BHandHLYP, while BLYP usually significantly underestimates all investigated spectral properties, hence being the worst in reproducing the reference CC2 results. The hybrid B3LYP and B1LYP functionals can be roughly placed in between. We propose that TDDFT may underestimate the TPA intensities for neutral chromophores of fluorescent proteins due to underestimated oscillator strengths between some excited states. In the case of anionic chromophores, we find that B3LYP and B1LYP functionals overcome others in terms of reproducing CC2 excitation energies. On the other hand, however, TPA intensity is usually significantly underestimated, and in this respect, CAM-B3LYP functional seems to be again superior. In contrast to the case of neutral chromophores, it seems that a large magnitude of excited-state dipole moments or changes in dipole moments upon excitation may be the driving force behind high TPA transition moments.

Two-photon absorption properties of D-A-D type chromophores containing tetraphenylethylene and triphenylamine moieties: a close look at the effects of the strength, position and number of donors

ABSTRACTThe two-photon absorption (TPA) properties of a series of D-A-D type chromophores containing tetraphenylethylene (TPE) and triphenylamine moieties have been investigated employing the density functional response theory in combination with the polarisable continuum model. Special emphases are placed on the effects of the strength, position and number of donors on TPA. Our calculations demonstrate that the TPA intensity enhances with the increase of the electron-donating ability of the substituents on the triphenylamine unit, which is consistent with experimental observations. It is shown that the position of donors has an important effect on TPA. The donor on the side position of TPE unit decreases the TPA cross section, thereby more donors could not necessarily produce stronger TPA. The cis-trans isomerism effect is also explored and it is found that all the cis structures have lower TPA intensities than those of corresponding trans forms. By considering a different number of donors on TPE and triphenylamine unit respectively, it is interesting to find that the structure with one terminal donor on TPE and two donors on triphenylamine unit has the largest TPA cross section. The few-state model and the natural bond orbital charge analysis are used to explore the reason behind it.

Isomerism and coordination mode effects on two-photon absorption of tris(picolyl)amine-based fluorescent probes for zinc ions**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM026) and the Taishan Scholar Project of Shandong Province, China.

One-photon absorption and two-photon absorption (TPA) properties of three tris(picolyl)amine-based zinc ion sensors are investigated by employing the density functional response theory in combination with the polarizable continuum model. The different isomer and coordination geometry of each probe are taken into account. Special emphasis is placed on the effects of isomerism and the coordination mode on the optical properties. The intra-molecular charge transfer (ICT) properties are specified by natural bond orbital charge analysis. It is shown that the isomerism has non-negligible effects on TPA properties of free ligands. It is found that both the TPA wavelength and the cross section are highly dependent on the coordination mode. When the zinc ion connects with the picolyl unit in the middle of a ligand, the zinc complex has a large TPA intensity in a long wavelength range due to the increased ICT mechanism.

Site-Specific Photodecomposition in Conjugated Energetic Materials.

Nonadiabatic excited-state molecular dynamics (NEXMD) has been used to study photodecomposition in a class of recently synthesized bicyclic conjugated energetic materials (CEMs) composed of fused tetrazole and tetrazine derivatives with increasing oxygen substitutions. Modification by oxygen functionalization has already been demonstrated to increase the two-photon absorption intensity in the target CEMswhile simultaneously improving oxygen balance. Photodecomposition mechanisms in materials that undergo nonlinear absorption could be used to achieve controlled, direct optical initiation. Here, we use NEXMD simulations to model the nonradiative relaxation and photodecomposition in CEMs following photoexcitation by a simulated Nd:YAG laser pulse. Excess electronic energy is quickly converted into vibrational energy on a sub-100 fs time scale resulting in bond dissociation. We find that, for the studiedtetrazine derivatives, the bicyclic framework is an important structural feature that enhances the photochemical quantum yield and the high atomic oxygen content increases the relaxation lifetime and opens additional photodissociation pathways targeting the oxygen-substituted sites. The presented analysis scheme based on bond orders in the swarm of NEXMD trajectories is a useful tool for determining photochemical reactions.

Responsive mechanism and molecular design of di-2-picolylamine-based two-photon fluorescent probes for zinc ions**Project supported by the Shandong Provincial Natural Science Foundation, China (Grant No. ZR2014AM026), the National Natural Science Foundation of China (Grant Nos. 11374195 and 11404193), and the Taishan Scholar Project of Shandong Province, China.

The properties of one-photon absorption (OPA), emission and two-photon absorption (TPA) of a di-2-picolylamine-based zinc ion sensor are investigated by employing the density functional theory in combination with response functions. The responsive mechanism is explored. It is found that the calculated OPA and TPA properties are quite consistent with experimental data. Because the intra-molecular charge transfer (ICT) increases upon zinc ion binding, the TPA intensity is enhanced dramatically. According to the model sensor, we design a series of zinc ion probes which differ by conjugation center, acceptor and donor moieties. The properties of OPA, emission and TPA of the designed molecules are calculated at the same computational level. Our results demonstrate that the OPA and emission wavelengths of the designed probes have large red-shifts after zinc ions have been bound. Comparing with the model sensor, the TPA intensities of the designed probes are enhanced significantly and the absorption positions are red-shifted to longer wavelength range. Furthermore, the TPA intensity can be improved greatly upon zinc ion binding due to the increased ICT mechanism. These compounds are potential excellent candidates for two-photon fluorescent zinc ion probes.

Coherent source interaction, third-order nonlinear response of synthesized PEG coated magnetite nanoparticles in polyethylene glycol and its application

Third-order nonlinear response of synthesized polyethylene glycol coated Fe3O4 nanoparticles dispersed in a suitable solvent, polyethylene glycol has been studied. The structural characterization of the synthesized magnetite nanoparticles were carried out. The linear optical property of the synthesized magnetite nanoparticles was investigated using UV-visible technique. Both closed and open aperture Z-scan techniques have been performed at 532nm with pulse width 5ns and repetition rate 10Hz. It was found that polyethylene glycol coated magnetite exhibits reverse saturable absorption, with significant nonlinear absorption coefficient. Two-photon absorption intensity dependent positive nonlinear refraction coefficients indicate self focusing phenomena. Results show that higher concentration gives better nonlinear and optical limiting properties.

Dimerization and Isomerism Effects on Two-Photon Absorption of Tetraphenylethene Derivatives and Molecular Design for Two-Photon Absorption Materials.

The two-photon absorption (TPA) properties of a new tetraphenylethene derivative and its covalent dimers have been calculated employing the density functional response theory. It is found that linear arrangement of branches can give rise to a cooperative TPA behavior. Partial planarity and linear arrangement are the possible reasons for the observed aggregation-induced TPA enhancement. On the basis of the model molecule, we have designed a series of tetraphenylethene derivatives which differ by donor moieties, connection modes, or central bridges after taking the structure-property relationship of TPA mechanism into account. The TPA spectra of the designed molecules have been calculated, and their TPA properties are analyzed at length. Our results suggest that the change of the connection mode of the carbazole group and the introduction of a vinylene or ethynylene linkage into a molecule can enhance TPA intensity greatly. It can be expected that all of the designed molecules could possess high TPA features. This research is helpful for the design of efficient TPA materials.

Chelation-Induced Quenching of Two-Photon Absorption of Azacrown Ether Substituted Distyryl Benzene for Metal Ion Sensing.

Imaging of metal ion concentration, distribution, and dynamics can pave the way to diagnose a number of diseases and to identify the normal functioning of the human body. Recently, two-photon microscopy-based imaging of metal ions has become popular due to several favorable factors as compared to fluorescence-based imaging. However, much has to be investigated in order to design probes with large two-photon absorption cross sections and yet with selective binding affinity toward metal ions. In particular, it is crucial to recognize the mechanisms of metal ion-induced changes of the two-photon absorption intensity. The present paper contributes to this effort and reports on the results of extensive studies carried out to define a reliable computational protocol that can account for sampling, solvent, and finite temperature effects for one- and two-photon properties of metal probes, using azacrown ether substituted distyrylbenzene embedded in solvents as a testbed. We employ a selection of theoretical approaches to model the structure of the probe alone and in the presence of Mg(2+) ion in acetonitrile solvent, including static quantum-chemical calculations, rigid- and flexible-body molecular dynamics, and hybrid QM/MM molecular dynamics. For a set of solute-solvent configurations, the one- and the two-photon properties are computed using the recently developed polarizable embedding response approach. It is found that the hybrid QM/MM molecular dynamics based approach is the most successful one among other employed computational strategies, viz. reproduction of the metal ion-induced blue shift in the absorption wavelength and decrease in the two-photon absorption cross section, which actually is in excellent agreement with experimental data. The mechanism for such metal ion-induced changes in the optical properties is put forward using a few-state model. Possible design principles to tune the two-photon absorption properties of probes are also discussed.

Solvent effect on the nonlinear absorption of 5,10-A2B2meso substituted porphyrins

The effect of the solvent on the nonlinear absorptive properties of two series of 5,10-A2B2 porphyrins was investigated with an open Z-scan technique in the ns time regime. The recorded responses, which varied between compounds and solvents, were fitted to a four-level model where the one-photon excited state absorption is followed by a two-photon process arising from the higher excited states. For most of the compounds the positive nonlinear absorption in toluene was stronger than that in DMF and chloroform. This was attributed to enhanced two-photon absorption in toluene. For DMF and chloroform the solvent effects were most likely to be compound specific. It was demonstrated that the high saturation intensity of two-photon absorption shifts the RSA/SA turnover into a higher fluence range, which is desirable for optical limiting applications. This saturation intensity of two-photon absorption varied between compounds and solvents. Additionally, nonlinear scattering contributed strongly to the open Z-scan responses for many compounds in chlorobenzene and chloroform-chlorobenzene solutions. This was associated with the photodegradation of chlorobenzene.

Optical Limiting Using the Two-Photon Absorption Electrical Modulation Effect in HgCdTe Photodiode

The electrical modulation properties of the output intensity of two-photon absorption (TPA) pumping were analyzed in this paper. The frequency dispersion dependence of TPA and the electric field dependence of TPA were calculated using Wherrett theory model and Garcia theory model, respectively. Both predicted a dramatic variation of TPA coefficient which was attributed into the increasing of the transition rate. The output intensity of the laser pulse propagation in the pn junction device was calculated by using function-transfer method. It shows that the output intensity increases nonlinearly with increasing intensity of incident light and eventually reaches saturation. The output saturation intensity depends on the electric field strength; the greater the electric field, the smaller the output intensity. Consequently, the clamped saturation intensity can be controlled by the electric field. The prior advantage of electrical modulation is that the TPA can be varied extremely continuously, thus adjusting the output intensity in a wide range. This large change provides a manipulate method to control steady output intensity of TPA by adjusting electric field.

Impact of Diradical Character on Two-Photon Absorption: Bis(acridine) Dimers Synthesized from an Allenic Precursor

The two-photon absorption (TPA) of a bis(acridine) dimer (8) having singlet diradical character in its ground state was found to be enhanced by more than 2 orders of magnitude as compared to its closed-shell counterpart (12), which has the same structural backbone and atom composition. The dimer, a tetracation species consisting of two connected acridinium cation moieties with high coplanarity, was obtained during our attempts to synthesize triplet carbenes by double oxidation of an allenic precursor (3b). High conjugation over the two aromatic rings connected by dimerization was revealed by X-ray analysis, and a small HOMO-LUMO gap was found in the visible-near-infrared one-photon absorption spectrum in solution and in the crystalline state, exhibiting that the ground state of 8 has singlet diradical nature. Ab initio molecular orbital calculations of the ground state also suggested that 8 has an intermediate diradical character (y) of 0.685. Interconversion between diradical tetracation dimer 8 and closed-shell dication dimer 12 was achieved by oxidation/reduction in good yields and was accompanied by formation of monoradical trication dimer 13 as an intermediate. TPA measurements at near-infrared wavelengths revealed that diradical dimer 8 has large TPA cross sections (3600 GM at 1200 nm), while closed-shell 12 has TPA cross sections of <21 GM. This result represents a straightforward comparison between the TPA activity of molecules with the same structural backbone and atom composition but with different degree of the diradical character, supporting the theoretical prediction that enhanced TPA intensity can be observed in the intermediate y region (0 < y < 1).

Theoretical study of two-photon absorption properties and up-conversion efficiency of new symmetric organic π-conjugated molecules for photovoltaic devices

Organic material with high intensity of two-photon absorption (TPA) induced fluorescence can be used as the frequency up-converter materials for improving efficiency of the solar cells. In this work, the organic molecular structures were designed by symmetrically grafting two elongated conjugated linkers on a conjugated core and then adding donor groups at both terminals. Fluorene derivatives as a core acceptor, phenylethynyl as the conjugated linker and diphenylamino or methyl-9H-carbozole as the donor end groups were selected. Quantum mechanical modeling techniques were applied to investigate the molecular electronic structure and properties. Absorption properties of these novel π-conjugated organic molecules were studied. The TPA cross-sections of these derivatives were calculated using few-states models, respectively. The effects of donor and acceptor groups on the TPA behaviors of these designed molecules were investigated. The up-conversion efficiency of designed molecules was also calculated.

Editorial: Recent Progress in Theoretical and Computational Chemistry

Theoretical and Computational Chemistry has seen a remarkable development in the past decades. While it started as a discipline of a few experts in the 1960s, the field has grown enormously due to advances in computational resources and software packages. In addition to the dramatic increase in computer power, a wide range of methods with predictive capabilities such as density functional theory (DFT) turned out to be extremely valuable tools in the hands of those who understand the chemistry of the systems under study and the limitations of the methods. As a result, increasingly larger molecular systems are nowadays routinely studied and interpreted according to the fundamental principles of quantum mechanics, including studies of electronic structure and reactivity of complex molecular processes. At the same time, the development of new methods and density functionals continues to be a vibrant field of research in a sustained effort to develop reliable approaches for almost any problem in chemistry. The availability of user-friendly software packages has popularized the field well beyond the realm of theoretical and computational chemistry groups. Today, almost any experimental research group in the world has an active member, or close collaborator, involved in computational modeling. The software packages are so popular and easy to use that calculations can be performed even by researchers with very limited knowledge of the underlying theories or approximations involved in the computations. While the calculations are easy to run, the interpretation of the resulting data, the range of validity of the approximations involved, and the range of error of the calculations performed with these popular software packages are often difficult to assess even by experienced users. These difficulties are exacerbated by the rapid emergence of new methods, acronyms, and software packages for general-purpose applications. It is, therefore, essential to complement efforts in software development with a comprehensive literature on new and existing methods, readily accessible to graduate students and researchers with an interest in current advances in the field. The present issue of ChemPhysChem is devoted to recent progress in a wide range of areas of Theoretical and Computational Chemistry by world-leading experts, reflecting the broad scope and rapid development of the field. For example, methodological developments for the treatment of excited states, heavy element systems, and non-covalent interactions are included. Another main topic is the computation of spectroscopic properties such as Raman optical activity, hyperfine coupling constants, NMR parameters or two-photon absorption intensities. The computation of these more complex properties has emerged as a research area that is of outstanding importance for the interpretation of modern spectroscopic experiments. Many of the applied papers in this issue furthermore reflect a current trend to model multicomponent systems of increasing complexity. The theoretical treatment of such inorganic/organic/bio-hybrids, which often include phase boundaries and require descriptions with periodic-boundary conditions, needs new approaches for the description of the basic electronic structure and for handling a huge number of degrees of freedom—some of which have already been proposed. This issue of ChemPhysChem reflects all these developments and we hope that the reader will find this selection useful, enjoyable and stimulating for her or his own work. 1 1 1