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The performance of a machine learning (ML) algorithm for chemistry is highly contingent upon the architect's choice of input representation. This work introduces the partial density of states (p-DOS) descriptor: a novel, quantum-inspired structural representation which encodes relevant electronic information for machine learning models seeking to simulate X-ray spectroscopy. p-DOS uses a minimal basis set in conjunction with a guess (non-optimised) electronic configuration to extract and then discretise the density of states (DOS) of the absorbing atom to form the input vector. We demonstrate that while the electronically-focused p-DOS performs well in isolation, optimal performance is achieved when supplemented with nuclear structural information imparted via a geometric representation. p-DOS provides a description of the key electronic properties of a system which is not only concise and computationally efficient, but also independent of molecular size or choice of basis set. It can be rapidly generated, facilitating its application with large training sets. Its performance is demonstrated using a wide variety of examples at the sulphur K-edge, including the prediction of ultrafast X-ray spectroscopic signal associated with photoexcited 2(5H)-thiophenone. These results highlight the potential for ML models developed using p-DOS to contribute to the interpretation and prediction of experimental results e.g. in operando measurements of batteries and/or catalysts and femtosecond time-resolved studies, especially those made possible by emergent cutting-edge technologies, especially X-ray free electron lasers. [ABSTRACT FROM AUTHOR]

Control of the delayed emission of organic compounds is an important factor in the development of new display technology and for the emerging use of organic emitters in sensing and fluorescence microscopy. In particular, there is a need to understand how the phenomena of room-temperature phosphorescence and thermally activated delayed fluorescence intersect. Here, we show that delayed fluorescence can be activated in room temperature phosphorescence emitters by quaternising the sp2-hybridised heterocyclic nitrogens. Furthermore by judicious positioning of a carbazole donor in the meta- or para-position with respect to the ring nitrogen atom, structural and sterical influences combine to tune the origins of the delayed fluorescence from triplet–triplet annihilation to thermally activated delayed fluorescence. Crucially, the quaternisation of nitrogen provides us with the means to fine-tune singlet and triplet states in a predictable manner, uncover the intersection between phosphorescence and delayed fluorescence and tip the balance in favour of delayed fluorescence. [ABSTRACT FROM AUTHOR]

Photoactive proteins typically rely on structural changes in a small chromophore to initiate a biological response. While these changes often involve isomerization as the "primary step", preceding this is an ultrafast relaxation of the molecular framework caused by the sudden change in electronic structure upon photoexcitation. Here, we capture this motion for an isolated model chromophore of the photoactive yellow protein using time-resolved photoelectron imaging. It occurs in <150 fs and is apparent from a spectral shift of ∼70 meV and a change in photoelectron anisotropy. Electronic structure calculations enable the quantitative assignment of the geometric and electronic structure changes to a planar intermediate from which the primary step can then proceed. [ABSTRACT FROM AUTHOR]

Several electronic-structure methods are available to study the photochemistry and photophysics of organic molecules. Among them, ADC(2) stands as a sweet spot between computational efficiency and accuracy. As a result, ADC(2) has recently seen its number of applications booming, in particular to unravel the deactivation pathways and photodynamics of organic molecules. Despite this growing success, we demonstrate here that care has to be taken when studying the nonradiative pathways of carbonyl-containing molecules, as ADC(2) appears to suffer from a systematic flaw. [ABSTRACT FROM AUTHOR]

Ideal emitters for organic light-emitting diodes (OLEDs) are capable of efficiently harvesting non-emissive triplet states, have high colour stabilities, and possess high photoluminescence quantum yields (PLQYs). Maintaining colour stability and PLQY is particularly challenging for multi-carbazole thermally activated delayed fluorescence (TADF) materials that form persistent dimers due to intermolecular interactions of their extended aromatic systems (with altered electronic states). Addressing this challenge, three new emitters are presented, which demonstrate that, somewhat counterintuitively, sterically uncrowded acceptor units can suppress these undesirable interactions. They do so by allowing the surrounding carbazole donors to be arranged with lower dihedral angles, which in turn limits their availability for dimerization. A new pyrazine-centered emitter 4CzPyz is contrasted directly with the cyanopyridine and terephthalonitrile analogues, 4CzCNPy and 4CzTPN respectively. The pyrazine derivative demonstrates enhanced colour stability in the solid-state compared to the cyanopyridine and terephthalonitrile acceptors, which we assign to its absence of intermolecular face-to-face aromatic interactions. This suppression of dimer formation is shared by two cyanopyrazine emitters 2Cz2CNPyz and 3CzCNPyz, each of which feature reduced steric pressure and flatter Cz-Pyz dihedral angles than non-heterocyclic analogues. Flatter dihedral angles consequently lead to C–H bonds of the Cz donors extending outwards at angles that prevent the stacking required for dimerization. This expanded understanding of dimer formation in TADF materials will guide future efforts to maintain colour stability in higher performance TADF materials by curbing the prevalence of face-to-face aromatic interactions. [ABSTRACT FROM AUTHOR]

Over the past decades, an important number of methods have been developed to simulate the nonadiabatic dynamics of molecules, that is, the dynamics of molecules beyond the Born–Oppenheimer approximation. These nonadiabatic methods differ in the way they approximate the dynamics emanating from the time-dependent molecular Schrödinger equation. In 1990, Tully devised a series of three one-dimensional model systems to test the approximations of the method called trajectory surface hopping. The Tully models were designed to probe different scenarios of nonadiabatic processes, such as single and multiple nonadiabatic (re)crossings. These one-dimensional models rapidly became the testbed for any new nonadiabatic dynamics strategy. In this work, we present a molecular perspective to the Tully models by highlighting a correspondence between these simple one-dimensional models and processes happening during the excited-state dynamics of molecules. More importantly, each of these nonadiabatic processes can be connected to a given exemplary molecular system, and we propose here three molecules that could serve as molecular Tully models, reproducing some of the key features of the original models but this time in a high-dimensional space. We compare trajectory surface hopping with the ab initio multiple spawning for the three molecular Tully models and highlight particular features and differences between these methods resulting from their distinct approximations. We also provide all the necessary information – initial conditions and all required parameters for the dynamics as well as the electronic structure – employed in our simulations such that the molecular Tully models can become in the future a unified and standardized test for ab initio nonadiabatic molecular dynamics methods. The molecular Tully models also offer an exciting link between the world of low-dimensional model systems for nonadiabatic dynamics and the excited-state dynamics of molecular systems in their full dimensionality. [ABSTRACT FROM AUTHOR]

An understanding of the initial photoexcited states of DNA is essential to unravelling deleterious photoinduced chemical reactions and the intrinsic ultrafast photoprotection of the genetic code for all life. In our combined experimental and theoretical study, we have elucidated the primary non-radiative relaxation dynamics of a model nucleotide of guanine and thymine (2′-deoxyguanosine 3′-monophosphate 5′-thymidine, d(GpT)) in buffered aqueous solution. Experimentally, we unequivocally demonstrate that the Franck–Condon excited states of d(GpT) are significantly delocalised across both nucleobases, and mediate d(G+pT−) exciplex product formation on an ultrafast (＜350 fs) timescale. Theoretical studies show that the nature of the vertical excited states is very dependent on the specific geometry of the dinucleotide, and dictate the degree of delocalised, charge-transfer or localised character. Our mechanism for prompt exciplex formation involves a rapid change in electronic structure and includes a diabatic surface crossing very close to the Franck–Condon region mediating fast d(G+pT−) formation. Exciplexes are quickly converted back to neutral ground state molecules on a ∼10 ps timescale with a high quantum yield, ensuring the photostability of the nucleotide sequence. [ABSTRACT FROM AUTHOR]

In this work, we investigate the factors influencing the shape of the low-energy tail of the absorption spectrum of a homoleptic biscyclometalated ruthenium complex with terdentate ligands [T. W. Rees et al., Inorg. Chem., 2017, 56, 9903] by combining an advanced theoretical strategy and the synthesis of an analogous osmium complex. The theoretical protocol merges relativistic linear-response time-dependent density functional theory and the nuclear ensemble approach, permitting to shed light on the influence of spin–orbit coupling and non-Condon effects on the theoretical absorption spectra of these rather large metal complexes. [ABSTRACT FROM AUTHOR]

Photodynamical simulations are increasingly used to explore photochemical mechanisms and interpret laser experiments. The vast majority of ab initio excited-state simulations are performed within semiclassical, trajectory-based approaches. Apart from underlying electronic-structure theory, the reliability of simulations is controlled by a selection of initial conditions for the classical trajectories. We discuss appropriate choices of initial conditions for simulations of different experimental arrangements: dynamics initiated by continuum-wave (CW) laser fields or triggered by ultrashort laser pulses. We introduce a new technique, CW-sampling, to treat the former case, based on the ideas of importance sampling, combined with the quantum thermostat approach based on the Generalized Langevin Equation (GLE) that allows for efficient sampling of both position and momentum space. The CW-sampling is particularly important for photodynamical processes initiated by absorption at the tail of the UV absorption spectrum. We also emphasize the importance of non-Condon effects for the dynamics. We demonstrate the performance of our approach on the photodissociation of the CF2Cl2 molecule (Freon CFC-12). A quantitative agreement with the experimental data is achieved with the use of empirical correlation energy correction (CEC) factor on top of FOMO-CASCI potential energy surfaces. [ABSTRACT FROM AUTHOR]

Dinuclear clathrochelate complexes are easily accessible by reaction of zinc(ii) triflate or cobalt(ii) nitrate with arylboronic acids and phenoldioximes. The utilization of brominated arylboronic acids and/or brominated phenoldioximes allows preparing clathrochelates with two, three, five or seven bromine atoms on the outside. These clathrochelates can undergo Pd-catalyzed cross-coupling reactions with 3- and 4-pyridylboronic acid to give new metalloligands featuring up to seven pyridyl groups. The pyridyl-capped clathrochelates display characteristics which make them interesting building blocks for structural supramolecular chemistry: they are rigid, large (up to 2.7 nm), luminescent (for M = Zn), and anionic. The pentatopic pyridyl ligands display an unusual trigonal bipyramidal geometry. [ABSTRACT FROM AUTHOR]

The use of electron-withdrawing substituents on the orthometalated phenyl ring is a common strategy to blue shift the emission of cyclometalated iridium complexes by stabilizing the highest occupied molecular orbital (HOMO), that is, increasing the oxidation potential of the complex. However, for application in blue organic light-emitting diodes (OLEDs), this approach imposes host materials with a deep HOMO, which negatively impacts the injection of charges, and hence the performance of the devices. In this context, we report new iridium complexes with an electron-donating substituent on the cyclometalated ligand to blue shift the emission while keeping a relatively low oxidation potential. As a result, bluish-green OLEDs based on host materials with shallow HOMOs (TCTA = 4,4′,4′′-tri(N-carbazolyl)-triphenylamine) display a higher performance than devices using FIrpic in the same architecture. The improvements are primarily attributed to the lower turn-on voltage (2.8 to 3 V) compared to those of FIrpic-device (3.6 V). White OLED was then prepared with a maximum brightness of 20 226 cd m−2 and current efficiency of 20.4 cd A−2 (at 100 cd m−2). Interestingly, a very small efficiency roll-off of about 1% at 1000 cd m−2 and high color stability were achieved. At a luminance level of 5000 cd m−2 the roll-off efficiency was still below 20%. The introduction of electron-donating substituents on a 2-phenylpyridine scaffold to obtain blue emitters with low oxidation potentials provides an alternative to strategies based on replacing the pyridine with imidazole, carbene, and pyrazole. [ABSTRACT FROM AUTHOR]

An ullazine unit was employed as a donor moiety in a donor–π–acceptor (D–π–A) motif, employing the porphyrin macrocycle as a π-system. Synthesis of this ullazine–porphyrin dyad containing sensitizer (SM63) was achieved and an investigation of the electrochemical and spectroscopic properties of this dye was performed. Introduction of the ullazine donor promoted significant enhancements in long and visible wavelength absorption, leading to panchromatic light harvesting. SM63 demonstrated a maximum absorption approaching 750 nm, a significant improvement compared to the model compound LD14-C8, which features a simpler donor component (4-(N,N-dimethylamino)phenyl) and an absorption onset at ∼700 nm. The dye SM63 was subjected to a rigorous ab initio investigation to gain further insight into its unique absorption and emission properties. Application of the molecular ullazine–porphyrin dyad SM63 into dye-sensitized solar cells afforded a device with significantly improved light harvesting abilities in both the visible region of the spectrum as well as NIR light (∼800 nm), demonstrating the value of ullazine unit in developing panchromatic dyes for light harvesting applications. [ABSTRACT FROM AUTHOR]

The computational elucidation and proper description of the ultrafast deactivation mechanisms of simple organic electronic units, such as thiophene and its oligomers, is as challenging as it is contentious. A comprehensive excited state dynamics analysis of these systems utilizing reliable electronic structure approaches is currently lacking, with earlier pictures of the photochemistry of these systems being conceived based upon high-level static computations or lower level dynamic trajectories. Here a detailed surface hopping molecular dynamics of thiophene and bithiophene using the algebraic diagrammatic construction to second order (ADC(2)) method is presented. Our findings illustrate that ring puckering plays an important role in thiophene photochemistry and that the photostability increases when going upon dimerization into bithiophene. [ABSTRACT FROM AUTHOR]

FIrpic is the most investigated bis-cyclometallated iridium complex in particular in the context of organic light emitting diodes (OLEDs) because of its attractive sky-blue emission, high emission efficiency, and suitable energy levels. In this Perspective we review the synthesis, structural characterisations, and key properties of this emitter. We also survey the theoretical studies and summarise a series of selected monochromatic electroluminescent devices using FIrpic as the emitting dopant. Finally we highlight important shortcomings of FIrpic as an emitter for OLEDs. Despite the large body of work dedicated to this material, it is manifest that the understanding of photophysical and electrochemical processes are only broadly understood mainly because of the different environment in which these properties are measured, i.e., isolated molecules in solvent vs. device. [ABSTRACT FROM AUTHOR]

While phosphorescent cyclometalated iridium(III) complexes have been widely studied, only correlations between oxidation potential EOX and Hammett constant σ, and between the redox gap (ΔEREDOX = EOX - ERED) and emission or absorption wavelength (λabs, λem) have been reported. We present now a quantitative model based on Hammett parameters that rationalizes the effect of the substituents on the properties of cyclometalated iridium(III) complexes. This simple model allows predicting the apparent redox potentials as well as the electrochemical gap of homoleptic complexes based on phenylpyridine ligands with good accuracy. In particular, the model accounts for the unequal effect of the substituents on both the HOMO and the LUMO energy levels. Consequently, the model is used to anticipate the emission maxima of the corresponding complexes with improved reliability. We demonstrate in a series of phenylpyridine emitters that electron-donating groups can effectively replace electron-withdrawing substituents on the orthometallated phenyl to induce a blue shift of the emission. This result is in contrast with the common approach that uses fluorine to blue shift the emission maximum. Finally, as a proof of concept, we used electron-donating substituents to design a new fluorine-free complex, referred to as EB343, matching the various properties, namely oxidation and reduction potentials, electrochemical gap and emission profile, of the standard sky-blue emitter FIrPic. [ABSTRACT FROM AUTHOR]

The non-relativistic quantum dynamics of nuclei and electrons is solved within the framework of quantum hydrodynamics using the adiabatic representation of the electronic states. An on-the-fly trajectory-based nonadiabatic molecular dynamics algorithm is derived, which is also able to capture nuclear quantum effects that are missing in the traditional trajectory surface hopping approach based on the independent trajectory approximation. The use of correlated trajectories produces quantum dynamics, which is in principle exact and computationally very efficient. The method is first tested on a series of model potentials and then applied to study the molecular collision of H with H2using on-the-fly TDDFT potential energy surfaces and nonadiabatic coupling vectors. [ABSTRACT FROM AUTHOR]

Dinuclear clathrochelate complexes are easily accessible by reaction of zinc(ii) triflate or cobalt(ii) nitrate with arylboronic acids and phenoldioximes. The utilization of brominated arylboronic acids and/or brominated phenoldioximes allows preparing clathrochelates with two, three, five or seven bromine atoms on the outside. These clathrochelates can undergo Pd-catalyzed cross-coupling reactions with 3- and 4-pyridylboronic acid to give new metalloligands featuring up to seven pyridyl groups. The pyridyl-capped clathrochelates display characteristics which make them interesting building blocks for structural supramolecular chemistry: they are rigid, large (up to 2.7 nm), luminescent (for M = Zn), and anionic. The pentatopic pyridyl ligands display an unusual trigonal bipyramidal geometry.

We perform quantum mechanics/molecular mechanics molecular dynamics simulations on the [Pt(2)(P(2)O(5)H(2))(4)](4-) (abbreviated PtPOP) complex; in water, dimethylformamide and ethanol. These are used to calculate the ground state X-ray absorption spectrum of the complex. The structural parameters from X-ray spectra are usually extracted using a fit of the experimental data. In such simulations the solvent is neglected meaning that any effect of the local environment will be compensated for by structural changes of the solute, leading to possible discrepancies in the extracted structural parameters. Our simulations show a significant solvent effect on the spectra, which has important implications for the structural analysis of molecules in solution.

A technique is described where an atmospheric pressure-thermal desorption (AP-TD) device and electrospray ionization (ESI)-mass spectrometry (MS) are coupled and used for the rapid analysis of Bacillus subtilis spores in complex matrices. The resulting AP-TD/ESI-MS technique combines the generation of volatile compounds and/or pyrolysis products with soft-ionization MS detection. In the AP-TD/ESI-MS approach, an electrospray solvent plume was used as the ionization vehicle of thermally desorbed neutrals at atmospheric pressure prior to mass spectrometric analysis using a quadrupole ion trap mass spectrometer. The approach is quantitative with the volatile standard dimethyl methylphosphonate (DMMP) and with the use of an internal standard (diethyl methylphosphonate, DEMP). A linear response was obtained as tested in the 1-50 ppm range (R(2) = 0.991) with a standard error of the estimate of 0.193 (0.9% RSD, n = 5). Bacterial spores were detected by performing pyrolysis in situ methylation with the reagent tetramethylammonium hydroxide (TMAH) for the detection of the bacterial spore biomarker dipicolinic acid (DPA) as the dimethylated derivative (2Me-DPA). This approach allowed spore detection even in the presence of growth media in crude lyophilized samples. Repetitive analyses could be performed with a duty cycle of less than 5 min total analysis time (including sample loading, heating and data acquisition). This strategy proved successful over other direct ambient MS approaches like DESI-MS and AP-TD/ESI-MS without the in situ derivatization step to detect the dipicolinic acid biomarker from spores. A detection limit for the dimethylated DPA biomarker was estimated at 1 ppm (equivalent to 0.01 mug of DPA deposited in the thermal desorption tube), which corresponded to a calculated detection limit of 10(5) spores deposited or 0.1% by weight spore composition in solid samples (assuming a 1 mg sample size). The AP-TD/ESI source used in conjunction with the in situ methylation step allowed the differentiation of bacterial spores from other 'suspicious white powders' using a single stage for mass analysis and with minimum sample preparation, making this approach suitable for simple field-portable MS instrumentation and pattern recognition data analysis.

New highly conductive, active and stable Ni steam reforming catalysts were prepared through a method consisting of the calcination of a hydrotalcite-like compound electrodeposited in a single step on FeCrAlloy foams.