Computational design of boron-free triangular molecules with inverted singlet-triplet energy gap.

A novel, computationally designed, class of triangular-shape organic molecules with an inverted singlet-triplet (IST) energy gap is investigated with electronic structure methods. The considered molecular systems are cyclic oligomers and their common feature is electronic conjugation along the molecular rim. Vertical excitation energies from the electronic ground state to the lowest singlet and triplet excited states were computed, as well as vertical emission energies from these states to the ground state.

Photo-oxidation of methanol in complexes with pyrido[2,3-]pyrazine: a nonadiabatic molecular dynamics study.

Excited-state Proton-Coupled Electron Transfer (PCET) constitutes a key step in the photo-oxidation of small, electron-rich systems possessing acidic hydrogen atoms, such as water or alcohols, which can play a vital role in green hydrogen production. In this contribution, we employ quantum-chemical methods and on-the-fly nonadiabatic molecular dynamics simulations to study the mechanism and the photodynamics of PCET in 1 : 1 pyrido[2,3-]pyrazine complexes with methanol. We find the process to be ultrafast and efficient when the intramolecular hydrogen bond is formed with one of the β-positioned nitrogen atoms.

Harnessing Proton-Coupled Electron Transfer for Hydrogenation of Aza-Arenes: Photochemistry of Quinoxaline Derivatives in Methanol.

Three quinoxaline derivatives are investigated both experimentally and theoretically to assess their ability for the methanol oxidation and harvesting of hydrogen. In inert solvents, the nonplanar compounds exhibit very weak fluorescence from the lowest excited singlet state, whereas the planar and rigid chromophore emits non-Kasha fluorescence from the S(ππ*) state despite the proximity of the S(nπ*) state. In methanol, hydrogen-bonded complexes with solvent molecules are formed, and in all chromophores, the lowest singlet state is populated after excitation of the S(ππ*) state.

Excited-state singlet-triplet inversion in hexagonal aromatic and heteroaromatic compounds.

The inversion of the energies of the lowest singlet (S) and lowest triplet (T) excited states in violation of Hund's multiplicity rule is a rare phenomenon in stable organic molecules. S-T inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, wave-function based computational methods were employed to explore the possibility of S-T inversion in hexagonal polycyclic aromatic and heteroaromatic compounds.

Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[]coumarins.

Photosensitizers that display "unusual" emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha's rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe group around the core of benzo[]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states.

Local Hydrogen Bonding Determines Branching Pathways in Intermolecular Heptazine Photochemistry.

Heptazine is the molecular core of the widely studied photocatalyst carbon nitride. By analyzing the excited-state intermolecular proton-coupled electron-transfer (PCET) reaction between a heptazine derivative and a hydrogen-atom donor substrate, we are able to spectroscopically identify the resultant heptazinyl reactive radical species on a picosecond time scale. We provide detailed spectroscopic characterization of the tri-anisole heptazine:4-methoxyphenol hydrogen-bonded intermolecular complex (TAHz:MeOPhOH), using femtosecond transient absorption spectroscopy and global analysis, to reveal distinct product absorption signatures at ∼520, 1250, and 1600 nm.

Saddle-shaped aza-nanographene with multiple odd-membered rings.

A saddle-shaped aza-nanographene containing a central 1,4-dihydropyrrolo[3,2-]pyrrole (DHPP) has been prepared a rationally designed four-step synthetic pathway encompassing intramolecular direct arylation, the Scholl reaction, and finally photo-induced radical cyclization. The target non-alternant, nitrogen-embedded polycyclic aromatic hydrocarbon (PAH) incorporates two abutting pentagons between four adjacent heptagons forming unique 7-7-5-5-7-7 topology. Such a combination of odd-membered-ring defects entails a negative Gaussian curvature within its surface with a significant distortion from planarity (saddle height ≈ 4.

Migration of Subtalar Implants in Paediatric Flatfoot Correction: A Pilot Study in Synthetic Bone Models.

Background: Subtalar implant migration as a complication following subtalar arthroeresis has been described in the scientific literature. However, clinical studies do not allow for unequivocally determining the underlying causes. The aim of the study is to determine the risk of migration of two geometric types of subtalar implants.

Water Oxidation and Hydrogen Evolution with Organic Photooxidants: A Theoretical Perspective.

In this Perspective, we discuss a novel water-splitting scenario, namely the direct oxidation of water molecules by organic photooxidants in hydrogen-bonded chromophore-water complexes. In comparison with the established scenario of semiconductor-based water splitting, the distance of electron transfer processes is thereby reduced from mesoscopic scales to the Ångström scale, and the time scale is reduced from milliseconds to femtoseconds, which suppresses competing loss processes. The concept is illustrated by computational studies for the heptazine-HO complex.

Photochemical Hydrogen Storage with Hexaazatrinaphthylene.

When irradiated with violet light, hexaazatrinaphthylene (HATN) extracts a hydrogen atom from an alcohol forming a long-living hydrogenated species. The apparent kinetic isotope effect for fluorescence decay time in deuterated methanol (1.56) indicates that the lowest singlet excited state of the molecule is a precursor for intermolecular hydrogen transfer.

Tuning the aromatic backbone twist in dipyrrolonaphthyridinediones.

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores.

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core.

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND).

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview.

The excited-state intramolecular proton transfer (ESIPT) phenomenon is nowadays widely acknowledged to play a crucial role in many photobiological and photochemical processes. It is an extremely fast transformation, often taking place at sub-100 fs timescales. While its experimental characterization can be highly challenging, a rich manifold of theoretical approaches at different levels is nowadays available to support and guide experimental investigations.

Are Heptazine-Based Organic Light-Emitting Diode Chromophores Thermally Activated Delayed Fluorescence or Inverted Singlet-Triplet Systems?

Two chromophores derived from heptazine, HAP-3MF and HAP-3TPA, were synthesized and tested as emitters in light-emitting diodes (OLEDs) by Adachi and co-workers. Both emitters were shown to exhibit quantum efficiencies which exceed the theoretical maximum of conventional fluorescent OLEDs. The enhanced emission efficiency was explained by the mechanism of thermally activated delayed fluorescence (TADF).

Triangular boron carbon nitrides: an unexplored family of chromophores with unique properties for photocatalysis and optoelectronics.

It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited state (S1) is below the energy of the lowest triplet excited state (T1).

Bowl-Shaped Pentagon- and Heptagon-Embedded Nanographene Containing a Central Pyrrolo[3,2-b]pyrrole Core.

A bowl-shaped nitrogen-doped nanographene composed of a pyrrolo[3,2-b]pyrrole core substituted with six arene rings circularly bonded with one another has been prepared via a concise synthetic strategy encompassing the multicomponent tetraarylpyrrolopyrrole (TAPP) synthesis, the Scholl reaction, and intramolecular direct arylation. This synthesis represents the first case of programmed sequential intramolecular direct arylation reactions utilizing the different reactivity of C-Br and C-Cl bonds. The target compound contains two central pentagons confined between two adjacent heptagons-the inverse Stone-Thrower-Wales topology.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry.

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules.

Aggregation controlled photoluminescence of hexaazatri-naphthylene (HATN) - an experimental and theoretical study.

Photophysics of hexaazatrinaphthylene (HATN) in solution and in the solid state is determined by the nπ* character of its lowest excited singlet state and by the ππ* character of the first triplet state. The relaxation of an electronically excited state is dominated by nonradiative relaxation channels and only very weak fluorescence is observed for HATN monomers in solution. In liquid solution, the fluorescence quantum yield is 3-7 × 10 and the triplet formation yield ranges from 0.

Photoinduced water oxidation in pyrimidine-water clusters: a combined experimental and theoretical study.

The photocatalytic oxidation of water with molecular or polymeric N-heterocyclic chromophores is a topic of high current interest in the context of artificial photosynthesis, that is, the conversion of solar energy to clean fuels. Hydrogen-bonded clusters of N-heterocycles with water molecules in a molecular beam are simple model systems for which the basic mechanisms of photochemical water oxidation can be studied under well-defined conditions. In this work, we explored the photoinduced H-atom transfer reaction in pyrimidine-water clusters yielding pyrimidinyl and hydroxyl radicals with laser spectroscopy, mass spectrometry and trajectory-based ab initio molecular dynamics simulations.

Brain tissue oxygenation during transnasal endoscopic skull base procedures.

Purpose: We aimed to study brain tissue oxygenation during the period of controlled reduction of arterial blood pressure - a maneuver often used in extended endoscopic skull base surgery for bloodless operative field.

Methods: Intracranial pressure, arterial blood pressure and the resultant cerebral perfusion pressure were measured during extended endoscopic skull base surgery in 5 patients with diagnosed tumors of the skull base and arterial hypertension. Simultaneously, in those patients, we measured partial pressure of oxygen in the brain parenchyma (PbtO).

Highly Polarized Coumarin Derivatives Revisited: Solvent-Controlled Competition Between Proton-Coupled Electron Transfer and Twisted Intramolecular Charge Transfer.

Linking a polarized coumarin unit with an aromatic substituent via an amide bridge results in weak electronic coupling that affects the intramolecular electron-transfer (ET) process. As a result of this, interesting solvent-dependent photophysical properties can be observed. In polar solvents, electron transfer in coumarin derivatives of this type induces a mutual twist of the electron-donating and -accepting molecular units (TICT process) that facilitates radiationless decay processes (internal conversion).

Photophysical transformations induced by chemical substitution to salicylaldimines.

A series of different electron-deficient aromatic substituents were used to investigate the role of the electron-acceptor strength on the photophysical properties of salicylaldimine derivatives. These molecules were synthesised and characterised through X-ray diffraction, absorption and emission spectroscopies. Their photochemical reaction mechanisms and properties were explored with the aid of ab initio methods of quantum chemistry.