In the search for red-shifted small molecule emitters of circularly polarized light, a helically chiral N,N,O,O-boron chelated dipyrromethene (BODIPY) is described containing a meso-CF3 group, prepared in only two synthetic steps. The incorporation of an electron deficient substituent at the meso-position of a helically chiral BODIPY results in significant (similar to 50 nm) red-shift of both the absorption (615 -> 669 nm) and emission maxima (635 -> 686 nm) in comparison to the parent compound, whilst maintaining a high extinction coefficient (83,000 mol(-1) cm(-1)) and reasonable fluorescence quantum yield (0.30). Enantiomers were resolved by chiral stationary phase HPLC and absolute configuration assigned by the comparison of the experimental and calculated ECD spectra (divided by g(abs)divided by = 3.6 x 10(-3)). A luminescence dissymmetry factor (divided by g(lum)divided by = 2.3 x 10(-3)) is reported, well supported by calculation (divided by g(lum(calc))divided by = 2 x 10(-3)), resulting in an overall CPL brightness (B-CPL = 29 M-1 cm(-1)) that is consummate with similar helically chiral monomeric BODIPYs. This work demonstrates the use electron deficient meso-CF3 groups in the development of red-shifted circularly polarised luminescent small organic molecules (CPL-SOMs), whilst maintaining their key chiroptical properties.

The photoinduced Jahn-Teller (JT) switch in Mn(acac)3 provides a model for how ultrafast structural dynamics can be harnessed to control magnetic anisotropy in single-molecule magnets (SMMs). However, the role of excitation energy in governing the efficiency and pathway of this transformation remains unclear. Here, ultrafast transient absorption spectroscopy (380-940 nm) probes the excitation-energy dependence of the JT switch. Selective excitation of Q1, Q2, and Q3,4 states shows that all pathways converge to a long-lived compressed-state photoproduct, but with distinct relaxation timescales, vibrational dynamics, and yields. Vibrational coherence analysis reveals modes at 170, 208, and 254 cm-1, assigned to the elongated, Q1 excited, and compressed state configurations, respectively. The 208 cm-1 mode is observed only under direct Q1 excitation, indicating excitation-dependent access to the JT-reactive coordinate. The compressed-state yield exhibits a strong dependence on excitation energy. For excitation into the lowest Q1 state (940-700 nm), the yield increases and follows a one-dimensional Landau-Zener model, where higher wavepacket velocity along the JT coordinate enhances nonadiabatic transition probability. The yield plateaus between 700-640 nm as the Q2 state is accessed and excess energy is likely transferred into non-reactive modes. Upon excitation into the Q3,4 manifold, the yield increases sharply, reaching a maximum near 470 nm before decreasing at higher energies, revealing the onset of multidimensional dynamics and competing relaxation pathways. These results identify excitation energy as a control parameter for the nonadiabatic structural dynamics, providing design rules for optically steering magnetic anisotropy in SMMs.

Achieving ultrafast photomagnetic switching of single-molecule magnets (SMMs) could lead to simultaneous fast and dense data storage devices. To facilitate this, a thorough understanding of the ultrafast dynamics emerging after ultrashort laser pulse excitation is essential. However, the complex nature of these materials means there is a lack of established experimental techniques that can probe the spin dynamics in SMMs. Herein, we perform femtosecond time-resolved Mn K-edge X-ray emission spectroscopy on a Mn(III)-based trinuclear SMM (Mn3) and the model system Mn(acac)3. The spectral changes of Mn(acac)3 are consistent with switching between Jahn-Teller distorted structures expected after photoexcitation. A similar result is observed for Mn3; however, the Kβ signal also reveals insight into the distribution of spin states populated within 100 fs. The importance of using probes across the electromagnetic spectrum to gain a thorough understanding of the dynamics of exchange-coupled complexes is highlighted.

Quantum dots (QDs) exhibit unique properties arising from their reduced size and quantum confinement effects, including exceptionally bright and tunable photoluminescence. Among these, CuInS_2 QDs have gained significant attention owing to their remarkable broadband emission, making them highly desirable for various optoelectronic applications requiring efficient luminescent nanomaterials. However, maximizing radiative recombination in CuInS_2 QDs often necessitates minimizing intragap trap states. A common approach involves the introduction of Zn during the synthesis, which typically promotes the formation of a ZnS shell that passivates the QD surface. Despite its importance, the characterization and quantification of Zn incorporation using conventional techniques, such as optical spectroscopy or electron microscopy, remains challenging. In this study, we utilized X-ray absorption spectroscopy (XAS), in both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectral ranges, to investigate Zn incorporation into CuInS_2 QDs with element-specific precision. This approach allowed us to detect the formation of a ZnS surface shell and to resolve the spatial distribution of Zn atoms within the QD lattice, distinguishing between Zn as a substituent, or as an interstitial defect. Additionally, we explored the optical and dynamical properties of CuInS_2 QDs using time-resolved optical spectroscopies, particularly in the presence of electron and hole acceptors. These results provide deeper insights into the role and effectiveness of the Zn-induced passivating layer, paving the way for optimizing QD performance in photoluminescence applications.

Intramolecular charge transfer (ICT) is a fundamental chemical process whereby excitation moves charge from an electron donor to an electron acceptor within the same molecule. Thermally activated delayed fluorescence (TADF) exploits the ICT property to harvest triplet excited states, leading to extensive optoelectronic applications, including OLEDs. However, the highly twisted conformation of TADF molecules results in limited device lifetimes. Rigid molecules offer increased stability, yet their typical planarity and π-conjugated structures impede ICT. Herein, we introduce a new paradigm for achieving dispersion-free triplet harvesting in ICT molecules. Using fused indolocarbazole-phthalimide molecules, we demonstrate remarkably stable co-planar ICT states, yielding blue/green-TADF with good photoluminescence quantum yield and a small singlet-triplet energy gap (∆EST) <50 meV. The formation of ICT is dictated by the bonding connectivity between the donor and acceptor fragments, leading to excited-state conjugation breaking stabilising the planar ICT excited state, revealing a new criteria for designing efficient TADF materials.

Multi-resonant Thermally Activated Delayed Fluorescent (MR-TADF) materials have received significant research interest owing to their potential use as emitters in high-performance Organic Light Emitting Diodes (OLEDs). Despite their advantages, including narrow emission spectra leading to high colour purity, several challenges remain in optimising the performance of these materials. One key issue is the typically long delayed fluorescence lifetime which arises from a large gap and weak coupling between the lowest lying singlet and triplet states. To develop high-performing materials, in silico design is an important step and consequently it is crucial to develop and deploy computational methods that accurately model their excited state properties. Previous studies have highlighted the importance of double excitations, which are not accounted for within the framework of Linear Response Time-Dependent Density Functional Theory (LR-TDDFT), contributing to the poor performance of this method for these materials. Consequently, in this work, we employ Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT) to calculate the properties of MR-TADF materials. Our findings indicate that this approach accurately predicts the excited state properties including the crucial Delta E-ST, the energy difference between the lowest singlet (S-1) and triplet (T-1) excited states. We further use this method to explore the excited state properties of systems designed to enhance the coupling between singlet and triplet states by increasing the density of states and enhancing spin-orbit coupling through metal perturbation. The results in this work sets the foundation for computationally efficient in silico development high-performing MR-TADF materials within the framework of MRSF-TDDFT.

Computational spectroscopy has emerged as a critical tool for researchers looking to achieve both qualitative and quantitative interpretations of experimental spectra. Over the past decade, increased interactions between experiment and theory have created a positive feedback loop that has stimulated developments in both domains. In particular, the increased accuracy of calculations has led to them becoming an indispensable tool for the analysis of spectroscopies across the electromagnetic spectrum. This progress is especially well demonstrated for short-wavelength techniques, e.g. core-hole (x-ray) spectroscopies, whose prevalence has increased following the advent of modern x-ray facilities including third-generation synchrotrons and x-ray free-electron lasers. While calculations based on well-established wavefunction or density-functional methods continue to dominate the greater part of spectral analyses in the literature, emerging developments in machine-learning algorithms are beginning to open up new opportunities to complement these traditional techniques with fast, accurate, and affordable ‘ black-box ’ approaches. This Topical Review recounts recent progress in data-driven/machine-learning approaches for computational x-ray spectroscopy. We discuss the achievements and limitations of the presently-available approaches and review the potential that these techniques have to expand the scope and reach of computational and experimental x-ray spectroscopic studies.

We report the measurement of impulsive stimulated x-ray Raman scattering in neutral liquid water. An attosecond pulse drives the excitations of an electronic wavepacket in water molecules. The process comprises two steps: a transition to core-excited states near the oxygen atoms accompanied by transition to valence-excited states. Thus, the wavepacket is impulsively created at a specific atomic site within a few hundred attoseconds through a nonlinear interaction between the water and the x-ray pulse. We observe this nonlinear signature in an intensity-dependent Stokes Raman sideband at 526 eV. Our measurements are supported by our state-of-the-art calculations based on the polarization response of water dimers in bulk solvation and propagation of attosecond x-ray pulses at liquid density.

Abstract Intramolecular charge transfer (ICT) occurs when photoexcitation causes electron transfer from an electron donor to an electron acceptor within the same molecule and is usually stabilized by decoupling of the donor and acceptor through an orthogonal twist between them. Thermally activated delayed fluorescence (TADF) exploits such twisted ICT states to harvest triplet excitons in OLEDs. However, the highly twisted conformation of TADF molecules results in limited device lifetimes. Rigid molecules offer increased stability, yet their typical planarity and π-conjugated structures impedes ICT. Herein, we achieve dispersion-free triplet harvesting using fused indolocarbazole-phthalimide molecules that have remarkably stable co-planar ICT states, yielding blue/green-TADF with good photoluminescence quantum yield and small singlet-triplet energy gap < 50 meV. ICT formation is dictated by the bonding connectivity and excited-state conjugation breaking between the donor and acceptor fragments, that stabilises the planar ICT excited state, revealing a new criterion for designing efficient TADF materials.

Accurate analysis of the rich information contained within X-ray spectra usually calls for detailed electronic structure theory simulations. However, density functional theory (DFT), time-dependent DFT and many-body perturbation theory calculations increasingly require the use of advanced codes running on high-performance computing (HPC) facilities. Consequently, many researchers who would like to augment their experimental work with such simulations are hampered by the compounding of nontrivial knowledge requirements, specialist training and significant time investment. To this end, we present Web-CONEXS, an intuitive graphical web application for democratizing electronic structure theory simulations. Web-CONEXS generates and submits simulation workflows for theoretical X-ray absorption and X-ray emission spectroscopy to a remote computing cluster. In the present form, Web-CONEXS interfaces with three software packages: ORCA, FDMNES and Quantum ESPRESSO, and an extensive materials database courtesy of the Materials Project API. These software packages have been selected to model diverse materials and properties. Web-CONEXS has been conceived with the novice user in mind; job submission is limited to a subset of simulation parameters. This ensures that much of the simulation complexity is lifted and preliminary theoretical results are generated faster. Web-CONEXS can be leveraged to support beam time proposals and serve as a platform for preliminary analysis of experimental data.

Owing to ring strain, cyclic ketones exhibit complex excited state dynamics with multiple competing photochemical channels active on the ultrafast timescale. While the excited state dynamics of cyclobutanone after π* ← n excitation into the lowest-energy excited singlet (S1) state has been extensively studied, the dynamics following 3s ← n excitation into the higher-lying singlet Rydberg (S2) state are less well understood. Herein, we employ fully quantum multiconfigurational time-dependent Hartree (MCTDH) simulations using a model Hamiltonian as well as “on-the-fly” trajectory-based surface-hopping dynamics (TSHD) simulations to study the relaxation dynamics of cyclobutanone following 3s ← n excitation and to predict the ultrafast electron diffraction scattering signature of these relaxation dynamics. Our MCTDH and TSHD simulations indicate that relaxation from the initially-populated singlet Rydberg (S2) state occurs on the timescale of a few hundreds of femtoseconds to a picosecond, consistent with the symmetry-forbidden nature of the state-to-state transition involved. There is no obvious involvement of excited triplet states within the timeframe of our simulations (<2 ps). After non-radiative relaxation to the electronic ground state (S0), vibrationally hot cyclobutanone has sufficient internal energy to form multiple fragmented products including C2H4 + CH2CO (C2; 20%) and C3H6 + CO (C3; 2.5%). We discuss the limitations of our MCTDH and TSHD simulations, how these may influence the excited state dynamics we observe, and—ultimately—the predictive power of the simulated experimental observable.

We present a detailed investigation into the excited state properties of a planar D-3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of similar to 10(10) s(-1), comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield.

AbstractThe development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

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 predic- tion of experimental results made possible by emergent cutting-edge technologies, especially X-ray free electron lasers.

Controlling the architecture of organic molecules is an important aspect in tuning the functional properties of components in organic electronics. For purely organic thermally activated delayed fluorescence (TADF) molecules, design is focused upon orthogonality orientated donor and acceptor units. In these systems, the rotational dynamics around the donor and acceptor bond has been shown to be critical for activating TADF; however, too much conformational freedom can increase the non-radiative rate, leading to a large energy dispersion of the emitting states and conformers, which do not exhibit TADF. To date, control of the motion around the D-A bond has focused upon steric hindrance. In this work, we computationally investigate eight proposed donor-acceptor molecules, exhibiting a B-N bond between the donor and acceptor. We compare the effect of steric hindrance and noncovalent interactions, achieved using oxygen (sulfur) boron heteroatom interactions, in exerting fine conformational control of the excited state dynamics. This work reveals the potential for judiciously chosen noncovalent interactions to strongly influence the functional properties of TADF emitters, including the accessible conformers and the energy dispersion associated with the charge transfer states.

The intersystem crossing dynamics of two bis-meridional iron(II) complexes are studied by femtosecond transient M-edge X-ray near edge absorption spectroscopy (XANES) with a tabletop high-harmonic extreme ultraviolet (XUV) spectrometer. Visible-light photoexcitation of Fe(tpy)2(BF4)2 (where tpy = terpyridine) creates a metal-to-ligand charge transfer (MCLT) state that decays in 170 fs to a metal-centered triplet state (3MC), followed by 38 fs decay by intersystem crossing to a metal-centered quintet (5MC). Coherent oscillations on the 5MC surface are observed as a modulation in the XANES spectrum with a frequency of 103 cm-1 and a spectral shape that is characteristic of the symmetric Fe-N stretch. These dynamics and spectra are similar to those previously observed for Fe(phen)32+ (phen = phenanthroline). In contrast, transient spectroscopy of Fe[(4-CF3)2bpca]2 (bpca= bis(2-pyridylcarbonyl)amide) reveals a lower-frequency 42 cm-1 coherent oscillation. Ligand field multiplet calculations combined with ab initio ligand field theory identify this oscillation as a ligand bending mode, highlighting the ligand field sensitivity of M-edge XANES. The activation of different vibrational modes in Fe(tpy)2(BF4)2 and Fe[(4-CF3)2bpca]2 is explained by mapping their excited-state potential energy surfaces using density functional theory. In the latter complex, the nuclear trajectory follows initial expansion along the Fe-Naxial coordinate until reaching the 3MC/5MC seam. After intersystem crossing, the quintet state is significantly displaced along the coordinate corresponding to the ligand rocking mode, which therefore dominates the subsequent trajectory.

First principles simulations of excited state X-ray spectra are becoming increasingly important to interpret the wealth of electronic and geometric information contained within femtosecond X-ray absorption spectra recorded at X-ray Free Electron Lasers (X-FELs). However, because the transition dipole matrix elements must be calculated between two excited states (i.e. the valence excited state and the final core-excited state arising from the initial valence excited state) of very different energies, this can be challenging and time-consuming to compute. Herein using two molecules, protonated formaldimine and cyclobutanone, we assess the ability of n-electron valence state perturbation theory (NEVPT2), equation-of-motion coupled cluster theory (EOM-CCSD), linear-response time-dependent density functional theory (LR-TDDFT) and the maximum overlap method (MOM) to describe excited state X-ray spectra. Our study focuses in particular on the behaviour of these methods away from the Franck-Condon geometry and in the vicinity of important topological features of excited-state potential energy surfaces, namely conical intersections. We demonstrate that the primary feature of excited state X-ray spectra is associated with the core electron filling the hole created by the initial valence excitation, a process that all the methods can capture. Higher-energy states are generally weaker and more sensitive to the nature of the reference electronic wavefunction. As molecular structures evolve away from the Franck-Condon geometry, changes in the spectral shape closely follow the underlying valence excitation, highlighting the importance of accurately describing the initial valence excitation to simulate the excited state X-ray absorption spectra.

The importance of the hole-transport material (HTM) in perovskite solar cells (PSCs) is now very well-established, with state-of-the-art materials such as spiro-OMeTAD attracting significant attention in the last decade. The high cost of such materials still limits the commercialisation of these HTMs. To tackle this, the amide linker has recently been introduced into HTM systems viaEDOT-Amide-TPA, utilising condensation chemistry as a cheap and effective route to HTMs. EDOT-Amide-TPA is capable of a variety of intermolecular interactions such as dipole-dipole interactions and hydrogen bonding, both of which are beneficial for enhancing the film morphology and improving charge transport. However, the interplay between these different interactions is not trivial, and understanding how they affect each other is paramount to inform new HTM designs whilst minimising material waste. To date, studies investigating the combined effects of different intermolecular interactions within the HTL on the charge transport properties of these materials are lacking. Furthermore, dipole disorder within the film introduces a mobility 'penalty': mobility decreases with stronger overall dipole due to energetic disorder within the film, which hinders charge hopping. In this work, we investigate three amide-based HTM analogs with differing intermolecular interaction capabilities, and show that this penalty can be compensated by a preferentially increased dipole ordering, likely achieved through intermolecular hydrogen bonding. This effectively cancels out the dipole disorder while retaining the beneficial effects on the molecular packing. Our aim is that this work provides a good foundation for navigating the complex interplay between hydrogen bonding, dipole moments, conductivity, and film formation in small-molecule HTMs. Strong dipoles in molecular HTMs can cause energetic disorder, leading to reduced mobility. This can be counteracted by a preferential antiparallel dipole ordering brought about by intermolecular hydrogen bond-assisted pairing.

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)(3)[PMo12O40] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo12/n products which were shown to be mixtures of electron-rich PMo12Li x species. FTIR analysis revealed the lengthening/weakening of terminal Mo=O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at n similar to 8 and a significant structural change at n > 12. Successive Mo(VI )reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one Mo-IV-Mo(IV )bonded {Mo-IV (3)} triad together with MoV. Upon dissolution, the PMo12Li x species present in the solid PMo12/n products undergo electron exchange and single-peak 31P NMR spectra were observed for n = 1-12. For n >= 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with MoV-MoV and {MoIV 3} bonding in an epsilon-Keggin structure. Bonding between {Li(NCMe)}+ and 2-electron-reduced PMo 12 in (TBA)(4)[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n >= 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.