[8]Circulenes, a class of intriguing polycyclic aromatic hydrocarbons, have garnered considerable attention due to their aesthetically pleasing structures. Despite their unique structural features, the applications of these rigid pi-conjugated systems remain far from fully explored due to the inherent challenges in functionalizing them. In this study, we report the synthesis of a water-soluble tetrabenzotetraaza[8]circulene derivative (3) through the introduction of four 2,5,8,11-tetraoxatridecane substituents. The optical properties of 3 were systematically studied in THF/water mixtures, revealing its water-fraction-dependent fluorescence. At 77 K, compound 3 exhibits distinctive phosphorescence with an exceptionally long lifetime of 2.08 s and a phosphorescence quantum yield of 12.3%, both among the highest values of reported hetero[8]circulenes. Additionally, 3 displays a pronounced yellow-green afterglow luminescence that persists for 7.6 s, underscoring its potential for persistent emission applications. This study presents an effective method to modulate the optoelectronic properties of hetero[8]circulene derivatives, offering new opportunities for hetero[8]circulenes as promising functional materials.

Stable organic radicals exhibiting absorption capabilities in the visible and near-infrared regions (NIR) are appealing candidates for a wide range of applications, including photovoltaics, photothermal applications, anticounterfeiting and information encryption/decryption. Hetero[8]circulenes represent a promising pi-extended platform for stable delocalized radicals with rich possibilities for chemical tuning. In this paper, we present the stable cation radical of diazadioxa[8]circulene with reduced symmetry showing a sharp and strong peak (epsilon similar to 2 & times; 104 M-1 cm-1) in the NIR at 939 nm. The diazadioxa[8]circulene cation radical can be obtained via chemical doping using Magic Blue, or via electrochemical doping. It remains stable for several days without any significant change in the absorption spectra. Furthermore, according to magnetic criteria of aromaticity, it exhibits antiaromatic behaviour. Advanced theoretical calculations predict NIR emission for the radical cation through an anti-Kasha D4 -> D0 transition, which is experimentally not detectable due to non-radiative deactivation pathways. Our RI-CC2 theoretical calculations also corroborate the blue emission of neutral circulene from the bright lowest singlet excited state S1, which was previously unexplained due to TD-DFT artefact predicting the dark S1 state. These results open the opportunities for the practical application of diazadioxa[8]circulene cation radicals in photovoltaics, sensors and photothermal imaging.

We review Ag(I) complexes with organic ligands that show ambient-temperature phosphorescence or thermally activated delayed fluorescence (TADF). Criteria for the assignment of the emission character are given. Two materials are highlighted: [Ag(dmp)(DPEPhos)]PF6 A (dmp: 2,9-dimethyl-1,10-phenanthroline, DPEPhos: bis[(2-diphenylphosphino)phenyl]ether), featuring an exceptionally long decay time of τ(phos) = 110 ms at ΦPL(phos) ≈50%, shows phenanthroline ligand-centered 3LC → S0 phosphorescence. The resulting large ΔE(S1-T1) gap (640 meV) inhibits the efficient thermal population of the S1 state. In contrast, the rationally modified Ag(dbp)(P2-nCB) B (dbp: 2,9-dibutyl-1,10-phenanthroline, P2-nCB: strongly electron-donating diphosphine ligand comprising a negatively charged nido-carborane cage) features low-lying states of (ligand + metal)-to-ligand charge transfer (1,3(L + M)L'CT) character. This reduces ΔE(S1-T1) to 80 meV, enabling fast and bright ambient-temperature TADF (radiative decay time: 1.4 μs at ΦPL(TADF) = 100%) with the decay time of more than 5 orders of magnitude shorter than that of A. The radiative properties of both compounds are outstanding compared to other pseudotetrahedrally coordinated metal complexes. In a second part, we extend the reviewed work by investigating both compounds by detailed DFT and TD-DFT calculations, including spin-orbit coupling. The results reflect the experimental trends. A is a potential highly sensitive oxygen sensor, while B might represent a new type of efficient OLED emitter material.

We propose a hydrothermal synthesis route for highly chiral two-dimensional flakes of MoS2 doped with single nickel atoms (Ni:MoS2) for spin-controlled water oxidation. Organic enantiomers were added to the reaction mixture as chiral encoders. The resulting MoS2 material obtained in the 1T phase exhibited wide absorption bands throughout the visible spectrum. Circular dichroism measurements indicated a pronounced chiroptical response at both UV and visible wavelengths. After delamination, the chirality was retained, and the Ni:MoS2 flakes were subsequently tested for water oxidation under the assumption that the flakes could support chiral-induced spin alignment of the transiting electrons. The chiral-induced spin selectivity (CISS) effect facilitates water oxidation kinetics and tunes the reaction selectivity toward oxygen production. The results of the electrochemical experiments, subsequently confirmed by density functional theory calculations, showed a significant decrease in the water-oxidation offset and onset potentials for the optimal combination of doping and flake chirality. In turn, the CISS effect manifests itself as an apparent suppression of hydrogen peroxide production and enhancement of oxygen production. Our work provides clear evidence of how chiral transition metal dichalcogenides can be used to tune the reaction efficiency and selectivity in energy-related applications.

The harmonic approximation is commonly used to describe internal conversion in large organic molecules. Here, local X–H vibrations are represented by Morse oscillators, while cubic and quartic force constants explicitly...

The selective detection of chlorinated aromatic hydrocarbons (CAHs) in environmental samples is challenging due to matrix interference effects. We report a surface-enhanced Raman spectroscopy (SERS) sensor that combines mesoporous Au films with UiO-66-I metal-organic framework (MOF) coatings to achieve the selective detection of CAHs. We show that mesoporous Au films can be considered hyperuniform two-dimensional (2D) materials where long-range correlations and local disorder assist in electromagnetic hotspot formation for SERS. Infiltrating the mesoporous Au films with UiO-66-I serves dual functions critical to sensor performance: First, its iodine-functionalized linkers selectively recruit CAHs from complex matrices through halogen bonding (HaB), concentrating target molecules at SERS hotspots while excluding common interferents. Second, the high refractive index of the MOF enhances light coupling by limiting scattered light, concentrating optical energy on the adsorbed CAHs for SERS enhancement. At optimal MOF thickness, the sensor achieves a detection limit below 1 × 10-10 M for 1,4-dichlorobenzene and 4-chlorobiphenyl, surpassing environmental standards by several orders of magnitude. The sensor demonstrates excellent selectivity for CAHs over common interferents, including protein, polycyclic aromatic hydrocarbons, and complex environmental matrices. Furthermore, the sensor maintains performance through multiple adsorption-desorption cycles, enabling reuse. This approach combines reticular chemistry with self-assembled nanostructured metals to achieve both high sensitivity and selectivity in complex environmental samples.

Excited-state charge transfer (CT) and the formation of twisted conformers govern the optical properties of a wide range of dyes. Aminonaphthalimide (ANI) derivatives, popular photosensitizers for blue light, are no exception. The focus herein is on an ANI derivative with an aliphatic amine, which ensures its solubility in a broad variety of solvents including water. Increasing solvent polarity quenches ANI fluorescence because of the expected formation of a dark twisted intramolecular charge-transfer (TICT) state. Contrary to the expectations, however, viscous polar alcohols, which inhibit conformational changes leading to the TICT state, only marginally recover ANI emission. Our analysis reveals that the "solubilizing" aliphatic amine forms an intramolecular exciplex, providing alternative nonradiative deactivation pathways without significant conformational changes and eliminating the viscosity dependence of ANI fluorescence. The findings also show a substantial dependence of ANI photophysics on hydrogen bonding with the solvating media, considerably enhancing the polarity effects. As a result, the fluorescence quantum yield of ANI (exceeding 0.4 in moderately polar solvents) drops below 0.001 when transferred to aqueous media. This feature allows one to showcase the utility of such dyes for imaging bacterial cells, complementing their growing popularity for CT, spintronics, materials, and biomedical applications.

Peroxy radicals (RO 2 ) are ubiquitous intermediates in many oxidation processes, especially in the atmospheric gas phase.

We have developed a theoretical framework for calculating rate constants of internal conversion (kIC) in the Franck-Condon (FC) and Herzberg-Teller (HT) approximations. The method accounts for anharmonic vibrational contributions and the Duschinsky effect. Our approach employs recursive dynamic programming to sum over multiple vibrational quantum number combinations and uses a Lagrange-multiplier technique with dispersion broadening to improve the accuracy of the calculated rate constants. We validate the methods by performing calculations on dibenzoterrylene (DBT), which is a molecule emitting in the near-infrared spectral range. The calculations confirm that anharmonic vibrational effects are the main contribution to kIC, while the Duschinsky effect is significant only for molecules whose lowest excitation energy exceeds 22 000 cm-1. The contributions of the individual X-H bonds are quantified by using the X-H mode approximation (kIC-XH) and the XH bond approximation (kIC-proton). The calculations show that the CH bonds of the tetracene moiety of DBT have the largest contribution to kIC. Deuteration of these bonds leads to a significant decrease in kIC with complete deuteration resulting in the largest overall effect. The calculated rate constants highlight the important role of the X-H bonds as acceptors of electronic excitation energy, offering strategies for modulating the kIC through selective substitution of the hydrogen atoms with heavier atoms such as D, F or Cl.

We have developed a method to calculate orbital contributions to magnetically induced current density (MICD) susceptibilities in molecules using gauge-including atomic orbitals (GIAO). The methods implemented in the GIMIC program have been used for analyzing orbital contributions to magnetically induced ring-current (MIRC) strengths. We have studied five aromatic, one nonaromatic, and four antiaromatic molecules. We show here that the contributions to the MIRC strength of all orbitals belonging to a given irreducible representation of the molecular point group in the presence of an external magnetic field are divergence free, whereas the MICD susceptibility of the individual orbitals are generally not divergence free. The largest contribution to the MIRC strength of antiaromatic molecules originates from the transition between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), whereas aromatic molecules have significant contributions involving many occupied orbitals. The MIRC contributions of σ orbitals are significant for planar molecules with a strained molecular structure.

Two series of stable verdazyl radical derivatives, specifically 1,2,4,6-substituted-1,4-dihydro-1,2,4,5-tetrazin-3(2H)-ones (also called "alkylverdazyls" or "AlkVZs"), were systematically investigated in photoactivated C-N bond homolysis. In-depth evaluation of their mechanisms indicated that several factors played a pivotal role in the generation of radicals during photon absorbance. From quantum chemical calculations, it was found that the most influential factor is the generation of charge-separated excited states, where one electron is located at the verdazyl moiety and the second one is distributed at the alkyl part. The theoretical approach also allows one to predict the reaction rate constant based on the oscillator strengths of the S0 → S1 transition in AlkVZs. Surprisingly, the photon density of the LED source has a strong impact on controlling the reaction direction, and decreasing the light power could lower the yield of radicals by twofold with the full conversion of the starting materials in all cases. Our results delineate prospective approaches for achieving high yields in photochemical transformations via the variation of the wavelength of light, the careful design of the molecular structure and alterations in the LED power.

Peroxy radicals are key intermediates in many atmospheric processes. Reactions between such radicals are of particular interest as they can lead to accretion products capable of participating in new particle formation (NPF). These reactions proceed through a tetroxide intermediate, which then decomposes to a complex of two alkoxy radicals and O2, with spin conservation dictating that the complex must be formed in the triplet state. The alkoxy complex can follow different pathways e.g. hydrogen(H)-shift reactions, dissociation reactions etc., but the details of the full processes are not yet fully understood. This paper establishes the microscopic mechanisms of the H-shift and other associated pathways in the context of a self-reaction between methoxy radicals, with focus on the roles of the singlet and triplet states involved. Dynamics in time is explored by two methods: the multireference XMS-CASPT2 and very recently developed mixed reference spin-flip TDDFT (MRSF-TDDFT). The metadynamics method is used to compute energetics. The XMS-CASPT2 and the MRSF-TDDFT dynamics simulations yield similar results. This would be very encouraging for future simulations for large radicals, since MRSF-TDDFT simulations enjoy the advantages of linear response theory. Our calculations demonstrate that the reaction between methoxy radicals, though initiated on the triplet state, leads to products predominantly on the singlet surface, following efficient intersystem crossing (ISC). The computed branching ratio between H-shift and dissociation channels agrees well with experiment.

Quantum chemical calculations were employed to construct Jablonski diagrams for a series of phenolic carbonyls, including vanillin, iso-vanillin, 4-hydroxybenzaldehyde, syringaldehyde, and coniferyl aldehyde. These molecules can enter the Earth's atmosphere from forest fire emissions and participate in photochemical reactions within the atmospheric condensed phase, including cloud and fog droplets and aqueous aerosol particles. This photochemistry alters the composition of light-absorbing organic content, or brown carbon, in droplets and particles through the formation and destruction of key chromophores. This study demonstrates that following photon absorption, phenolic carbonyls efficiently transition to triplet states via intersystem crossings (ISC), with rate coefficients ranging from 109 to 1010 s-1. Despite the presence of multiple potential ISC pathways due to several lower-lying triplet states, a single channel is found to dominate for each system. We further investigated the dependence of the ISC rate constant (kISC) on the vibrational excitation energy of the first accessible (ππ*) singlet excited state (S1 or S2, depending on the molecule), and compared it with the measured wavelength dependence of the photochemical quantum yield (Φloss). Although our model only accounts for intramolecular nonradiative electronic transitions, it successfully captures the overall trends. All studied molecules, except coniferyl aldehyde, exhibit saturation in the dependence of both kISC and Φloss on the wavelength (or vibrational excitation energy). In contrast, coniferyl aldehyde displays a single maximum, followed by a monotonic decrease as the excitation energy increases (wavelength decreases). This distinct behavior in coniferyl aldehyde may be attributed to the presence of a double-bonded substituent, which enhances π-electron conjugation, and reduces the exchange energy and thus the adiabatic energy gap between the S1(ππ*) state and the target triplet state. For small energy gaps, the classical acceptor modes of the ISC process are less effective, leading to a low effective density of final states. Larger gaps enhance the effective density of states, making the wavelength dependence of the ISC more pronounced. Our calculations show that while all the studied phenolic carbonyls have similar acceptor modes, coniferyl aldehyde has a substantially smaller adiabatic gap (1700 cm-1) than the other molecules. The magnitude of the adiabatic energy gap is identified as the primary factor determining the energy/wavelength dependence of the ISC rate and thus Φloss.

Cyclo[n]carbons (Cn) have sparked substantial interest among experimentalists and theoreticians owing to their elusive geometric structures and unique aromaticity. Composed of two-coordinated sp-hybridized carbon atoms, Cn thus forms two perpendicular conjugated π-electron systems, i.e. out-of-plane and in-plane. Till now, on-surface generated cyclocarbons are either doubly aromatic or doubly anti-aromatic, as the number of electrons within out-of-plane and in-plane π systems was equal. Doping with heteroatoms allows one to create two π systems with different numbers of electrons, and to tune the aromaticity. Herein, we successfully generated two heteroatom-doped cyclocarbons, C12S and C12N, and characterized their chemical and electronic structures. Calculations show that C12S exhibits an out-of-plane (14 e) aromatic and in-plane (12 e) anti-aromatic character, resulting in a total non-aromaticity. For C12N, the out-of-plane (14 e) aromatic and in-plane (13 e) non-aromatic characters lead to total aromaticity. Doping with heteroatoms may open up the field of aromaticity engineering within cyclocarbons.

The oxidation of monoterpenes is one of the largest single sources of atmospheric secondary organic aerosol (SOA) significantly impacting the climate and air quality. Still, the autoxidation mechanisms converting these volatile precursors to low-volatility condensable products remain elusive even for the most abundant monoterpene α-pinene. We studied the ozonolysis of α-pinene by combining advanced isotopic labeling and state-of-the-art chemical ionization mass spectrometry supported by quantum chemical calculations. We reacted a full set of eight selectively deuterated α-pinene analogues separately in a flow reactor to probe the oxidation mechanisms on a molecular level. We found that surprisingly few carbon atoms participate in the autoxidation process when forming even the most oxygenated products. Additionally, prompt pinonic acid formation has likely been greatly overestimated, whereas the α-pinene-derived dioxirane appears more stable than previously thought. Importantly, we reveal that oxidation models should include multiple branching pathways rather than simple linear autoxidation progression from less to more oxygenated species. Correct modeling of the oxidation is crucial to enable accurate predictions in the changing climate and atmospheric conditions.

The contribution of anharmonicity to the accepting and promoting modes in the tetraoxa[8]circulene molecule has been estimated. The estimates were made for the characteristic modes corresponding to the stretching vibrations of the X-H bonds, deformation vibrations, and stretching vibrations of the C-C bond. It was determined that the contribution of anharmonicity corrections is most pronounced for the promoting modes. Tetraoxa[8]circulene has only 2 accepting modes and 8 promoting modes. For these modes, the contribution of the anharmonicity from the partial derivatives of the potential energy with respect to the normal coordinates is insignificant, allowing for the use of the approximation of non-interacting modes when calculating the internal conversion rate constant. Expressions for calculating the internal conversion rate constant have been derived within the framework of first-order perturbation theory, taking into account the contribution of anharmonicity.

We present a machine learning approach to predict internal conversion rate constants (kIC) based on molecular electronic and structural descriptors. Our approach addresses the challenge of accurately estimating kIC between the first excited singlet state (S1) and the ground state (S0), which is an important deactivation channel of photophysical processes. Using a cost-efficient method that assumes excitation-energy transfer to the vibrations of the X-H bonds (X = C, N, O), we generated a data set for kIC of more than 5000 molecules, including porphyrins and [8]circulenes using parameters calculated at the time-dependent density functional theory (TDDFT) level. We have used both the CatBoost (CB) model and a neural network (NN) including transformer-based and graph-based architectures. Using the CB and NN models we obtained high precision with R2 values of about 0.99. The models successfully predict kIC even for new molecules when they are trained with energy-augmented data sets based on electronic and structural descriptors. We also introduce graph-based X-H bond descriptors to predict rate constants for various chemical classes. The calculated rate constants indicate the promising potential of these descriptors to predict kIC using machine learning to bypass computationally demanding quantum mechanical calculations to determine the photophysical properties.

The spin-flip or intersystem crossing (ISC) process plays a main role in photophysics and photochemistry. ISC is the radiationless electronic transition between singlet and triplet states. ISC is responsible for phosphorescence and the chemical reaction which happens only in the electronic state with the specific spin. One specific and challenging case is the formation of accretion products in peroxy radical cross-reactions, recently demonstrated to be important in atmospheric chemistry. Here, the ISC or spin-flip occurs between triplet and singlet states of a complex of two alkoxy radicals (RO center dot & ctdot;R ' O-center dot). The complex is initially formed in a triplet state, while the formation of the ROOR ' accretion product can only happen on the singlet surface. Therefore, the ISC rate dictates the rate of this reaction. We developed a fast algorithm to calculate ISC rate constants (kISC) between the lowest electronic states of alkoxy radical pairs. The kISC calculation requires the spin-orbital coupled interaction matrix elements (SOCME) and the excitation energies (E) of the involved electronic states. The E and SOCME are calculated quickly using the CASSCF level of theory, and a novel analytical expression, respectively. Finally, the kISC calculation is performed efficiently; within 5-60 seconds even for radical pairs with the large substituents such as CH3(CO)CH2O(center dot) and HOCH2CH(O-center dot)CH2CH3. This algorithm is applied to a large number of radical pairs with different substituents, and the kISC is calculated for 95 875 radical pair conformers using this algorithm. It provides an opportunity to generate large amounts of data (input and output values, e.g. geometries and associated ISC rates) quickly. Therefore, we believe that the fast algorithm can be useful not only for photophysical calculations, but also for Big Data creation to implement machine learning methods. The spin-flip or intersystem crossing (ISC) process plays a main role in photophysics and photochemistry.

Molecular emitters with multi-emissive properties are in high demand in numerous fields, while these properties basically depend on specific molecular conformation and packing. For amorphous systems, special molecular arrangement is unnecessary, but it remains challenging to achieve such luminescent behaviors. Herein, we present a general strategy that takes advantage of molecular rigidity and S 1 -T 1 energy gap balance for emitter design, which enables fluorescence–phosphorescence dual-emission properties in various solid forms, whether crystalline or amorphous. Subsequently, the amorphism of the emitters based polymethyl methacrylate films endowed an in situ regulation of the dual-emissive characteristics. With the ratiometric regulation of phosphorescence by external stimuli and stable fluorescence as internal reference, highly controllable luminescent color tuning (yellow to blue including white emission) was achieved. There properties together with a persistent luminous behavior is of benefit for an irreplaceable set of optical information combination, featuring an ultrahigh-security anti-counterfeiting ability. Our research introduces a concept of eliminating the crystal-form and molecular-conformational dependence of complex luminescent properties through emitter molecular design. This has profound implications for the development of functional materials.

Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated ab initio levels of theory also show that 1 has a large inverted singlet-triplet (IST) gap, a non-vanishing spin-orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim. The calculated energy inversion of the first excited states of a heptazine phenylthiolate molecule is consistent with the intense blue fluorescence in the solid state and in toluene solution.