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Hybridized local and charge-transfer (HLCT) excited-state fluorophores, which enable full exciton utilization through a reverse intersystem crossing from high-lying triplet states to singlet state,...

The development of high-performance organic materials-based devices requires a careful selection of molecular structures as well as detailed understanding of the fundamental mechanisms and processes upon photo excitation of single molecules. The employment of intramolecular charge transfer type molecules, typically containing electron accepting and electron donating molecular fragments connected through π-conjugated linker, offers an extensive possibility to manipulate singlet and triplet excited states both through environmental and molecular-structure related parameters. The control of singlet or singlet and triplet states, on the other hand, is an optimal tool to achieve the desired excited state lifetimes for targeted applications. Thus, this dissertation, presented as a collection of articles, is devoted to the photophysical study of different intramolecular charge transfer molecular series by employing steady state and time-resolved photoluminescence experimental techniques in combination with theoretical modelling. The aim of this work is to understand, control and optimize photophysical properties and excited-state processes of studied organic molecules, related to singlet or singlet and triplet excited states altogether and to provide a perspective towards possible applications, such as polarity probes, cation, or temperature sensors, TADF-based OLEDs and information recording tags.

Graphite carbon nitride (g-C3N4) is regarded as a potential candidate to settle environmental and energy issues, but its activity remains unsatisfactory due to the huge exciton effect and delayed charge transfer. Herein, the effects of phosphorus-doping and two-dimensional (2D) porous structures on the transition among singlet excitons, triplet excitons, and carriers and consequently the regulation mechanism of reactive oxygen species have been intensively studied. The calculations of the density functional theory and experimental results confirmed that P-doping could induce a dual built-in electric field to provide a driving force (up to 0.3 eV) for exciton dissociation. The light response capability, exciton dissociation, and carrier migration efficiency were significantly enhanced by the synergistic effect of the dual electric field and 2D porous structure. The optimal sample exhibited excellent H2 evolution rate and tetracycline degradation rate under visible-light irradiation which were approximately 25 and 7.1 times higher than those of bulk g-C3N4, respectively. The improved photocatalytic activity can also be demonstrated by a 2.9-fold and 3.2-fold increase in ·O2– and H2O2 production, respectively. This work provides a new perspective and approach to improve the performance of polymer photocatalysts and the targeted regulation of active species using exciton engineering.

Blue-efficient thermally-activated delayed fluorescence emitters are widely desired in organic light-emitting diodes due to their advantages in both improving display resolution and providing better pixels. However, both the species and amounts of blue-TADF molecules are far from meeting the requirement for practical applications. Thus, systematic studies are desired to study the relationship between molecular structures and luminescent properties. Herein, a total of 15 molecules with different donor substitutions (DMAC, PXZ, PTZ, DPA and Cz) and different position substitutions (ortho, meta and para) are designed and their photophysical properties are studied in detail utilizing density functional theory (DFT) and time-dependent DFT methods. The conformational isomerization, intramolecular interactions, energy gaps, spin–orbit couplings, and fluorescence efficiencies are analyzed. Moreover, the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are investigated using the thermal vibration correlation function method. Results indicate that molecules with m-substitutions possess the largest energy gap between the lowest singlet and triplet excited states. However, molecules with DPA and Cz substitutions can bring remarkable spin–orbit coupling effects, excellent radiative decay rate and efficient ISC and RISC rates. By analyzing the calculated quantum efficiencies, five promising blue-TADF molecules (o-DPA-QL, m-DPA-QL, p-DPA-QL, o-Cz-QL and m-Cz-QL) with stable nearly planar conformations are theoretically proposed. Our work gives reasonable explanations for experimental measurements and provides a molecular design strategy for efficient blue-TADF molecules.

A pair of thermally activated delayed fluorescence (TADF) emitters with symmetric and asymmetric D-A-D structure are investigated. Despite displaying near-identical photoluminescence spectrum and quantum yields, the symmetric material possesses significantly better delayed fluorescence characteristics and OLED performance. Building on a previous study of analogous D-A materials we are able to explain these differences in terms of different strengths of electronic interactions between the two donor units. This interaction lowers the energy of the TADF-active triplet state in the asymmetric molecule, increasing its singlet–triplet energy gap and leading to worse performance. This result therefore demonstrates a new strategy to selectively control the triplet states of TADF molecules, in contrast to established control of singlet states using host environment. These results also show that multi-donor TADF emitters cannot be understood simply as the sum of their isolated parts; these parts have different electronic interactions depending on their relative positions, even when there is no scope for steric interaction.

The synthesis, structural characterization and photophysical properties of a series of platinum(II) complexes of benzannulated pincer-type diarylamido ligands are described. The ligands all contain tricyclic phenanthridine (3,4-benzoquinoline) rings as donor arms, which were elaborated into N∧N−∧O-coordinating β-enaminoketonato chelates via partial condensation with acetylacetone. The proligands are easily deprotonated, and metallation can be achieved under mild conditions using simple Pt(II) salts and Ag2O as a base. The resulting Pt(II) complexes exhibit strong metal-to-ligand charge-transfer absorptions in the region of ∼450–575 nm and are phosphorescent in solution at room temperature, emitting bright orange light (λmax ∼ 600 nm) with quantum yields of up to 16% and excited-state lifetimes on the order of ∼20 μs, representing significant improvements to these photophysical properties compared with many previously reported N∧N∧O or N∧N∧N-ligated systems. Computational modelling reveals that the lowest-lying triplet state is populated efficiently thanks to strong coupling between singlet and triplet excited state manifolds, as in cyclometallated compounds of Pt(II). Substituents (CH3, tBu, or CF3) in the 2-position of the phenanthridinyl unit are found to have little influence on the optical properties, but the emission is severely quenched when a methyl substituent is introduced ortho to the coordinating nitrogen. Molecular distortions in the excited state are shown to be primarily responsible for the quenching in this complex.

The conversion of visible light into high-energy UV photons via sensitized triplet–triplet annihilation (TTA-UC) could pave the way for several energy-demanding applications. However, only a handful of annihilator chromophores for Vis-to-UV upconversion are known and the current limit for the excited-state energy of the UV emitter in TTA-UC schemes is below 4 eV, i.e., in the spectral region (>310 nm) where the sun still provides a considerable photon flux. Herein, we present a novel biphenyl-based annihilator with a highly fluorescent singlet state at 4.04 eV. This annihilator can be combined with a widely-used TADF sensitizer to yield an efficient blue-light (2.77 eV) driven upconversion system with unprecedented properties.

We investigate triplet pair dynamics in pentacene dimers that have varying degrees of coplanarity (pentacene-pentacene twist angle). The fine-tuning of the twist angle was achieved by alternating connectivity at the 1-position or 2-positions of pentacene. This mix-and-match connectivity leads to tunable twist angles between the two covalently linked pentacenes. These twisted dimers allow us to investigate the subtle effects that the dihedral angle between the covalently linked pentacenes imparts on singlet fission and triplet pair recombination dynamics. We observe that as the dihedral angle between the two bonded pentacenes is increased, the rates of singlet fission decrease, while the accompanying decrease in triplet recombination rates is stark. Temperature-dependent transient optical studies combined with theoretical calculations show that the triplet pair recombination proceeds primarily through a direct multiexciton internal conversion process. Calculations further show that the significant decrease in recombination rates can be directly attributed to a corresponding decrease in the magnitude of the nonadiabatic coupling between the singlet multiexcitonic state and the ground state. These results highlight the importance of the twist angle in designing systems that exhibit rapid singlet fission, while maintaining long triplet pair lifetimes in pentacene dimers.

Summary In this work, we describe a highly efficient and reabsorption-free X-ray-harvesting system using luminescent metal-organic framework (MOF)-fluorescence chromophore composite films. The ultrafast time-resolved experiments and density functional theory calculations demonstrate that a nearly 100% energy transfer from a luminescent MOF with a high atomic number to an organic chromophore with thermally activated delayed fluorescence (TADF) character can be achieved. Such an unprecedented efficiency of interfacial energy transfer and the direct harnessing of singlet and triplet excitons of the TADF chromophore led to remarkable enhancement of radioluminescence upon X-ray radiation. A low detection limit of 256 nGy/s of the fabricated X-ray imaging scintillator was achieved, about 60 times lower than the MOF and 7 times lower than the organic chromophore counterparts. More importantly, this detection limit is about 22 times lower than the standard dosage for a medical examination, making it an excellent candidate for X-ray radiography.

Photocatalytic generation of singlet oxygen (1O2) is an attractive strategy to convert organic chemicals to high value-added products. However, the scarcity of suitable active sites in photocatalysts commonly leads to the poor adsorption and activation of oxygen molecules from a triplet state to a singlet state. Here, we report single atomic Cu-N3 sites on tubular g-C3N4 for the production of singlet oxygen. X-ray absorption fine spectroscopy, in combination with high-resolution electron microscopy techniques, determines the existence of atomically dispersed Cu sites with Cu-N3 coordination mode. The combined analysis of electron spin resonance and time-resolved optical spectra confirmed that a single atomic Cu-N3 structure facilitates a high concentration of 1O2 generation due to charge transport, electron-hole interaction, and exciton effect. Benefiting from the merits, a single atomic photocatalyst yields nearly 100% conversion and selectivity from thioanisole to sulfoxide within 2.5 h under visible light irradiation. This work deeply reveals the design and construction of catalysts with specific active sites, which are helpful to improve the activation efficiency of oxygen.

The delta self-consistent field (ΔSCF) method was applied to the simulation of low-lying singlet and triplet excited states of multiple-resonance (MR)-type thermally activated delayed fluorescence (TADF) molecules, which form a promising group for organic light-emitting diode (OLED) emitters. A comparison with the experimental values of 13 emitters from the literature showed that ΔSCF gave fairly accurate S1 and T1 excitation energies (mean absolute errors (MAEs) of 0.092 and 0.055 eV, respectively) as well as quite accurate ΔEST (S1-T1 gap, MAE of 0.041 eV), which could not be calculated with sufficient accuracy by the conventional time-dependent density functional theory (TDDFT). ΔSCF also demonstrated its utility for the analysis of photophysical properties through a simulation of the reverse intersystem crossing (RISC) process of an MR-type emitter.

Acridone and its derivatives have potential application as emitters for highly efficient blue organic light-emitting diodes (OLEDs). In this paper, we demonstrated ultrafast access of a solvent-independent singlet-triplet equilibrium state in acridone solutions by using femtosecond time-resolved spectroscopy. Our spectral data show that due to highly effective forward and reverse intersystem crossing (both kISC and krISC over 1010 s-1), a singlet-triplet equilibrium state is always populated in acridone in all solvents studied. However, the lifetimes of the equilibrium state varied a lot in different solvent environments and the final decay pathway of this state can switch between high quantum yield fluorescence emission and further internal conversion to the lowest triplet state. These findings provide direct experimental evidence to understand the distinct photophysical behaviors of acridone and also provide guidance for further design of acridone and its derivatives as blue OLED emitters.

Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates. In this study, the excited state chemistry of two small Criegee intermediates, formaldehyde oxide (CH2 OO) and acetaldehyde oxide (CH3 CHOO), was modeled to compare their specific dynamics and mechanisms following excitation to the bright ππ* state and to assess the involvement of triplet states to the excited state decay process. Following excitation to the bright ππ* state, the photoexcited population exclusively evolves to form oxygen plus aldehyde products without the involvement of triplet states. This occurs despite the presence of a more thermodynamically stable triplet path and several singlet/triplet energy crossings at the Franck-Condon geometry and contrasts with the photodynamics of related systems such as acetaldehyde and acetone. This work sets the foundations to study Criegee intermediates with greater molecular complexity, wherein a bathochromic shift in the electron absorption profiles may ensure greater removal via solar photolysis.

Conventional semiconductor photocatalysts produce superoxide and hydroxyl radicals as the major reactive oxygen species (ROS). Here, we report that Ag/Ag3PO4 catalyst efficiently generates singlet ...

High-color purity organic emitters with a simultaneous combination of aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) characteristics are in great demand due to their excellent comprehensive performances toward efficient organic light-emitting diodes (OLEDs). In this work, two D-π-A-structure emitters, ICz-DPS and ICz-BP, exhibiting AIE and TADF properties were developed, and both the emitters have narrow singlet (S1)-triplet (T1) splitting (ΔEST) and excellent photoluminescence (PL) quantum yields (ΦPL), derived from the distorted configurations and weak intra/intermolecular interactions, suppressing exciton annihilation and concentration quenching. Their doped OLEDs based on ICz-BP provide an excellent electroluminescence external quantum efficiency (ηext) and current efficiency (ηC) of 17.7% and 44.8 cd A-1, respectively, with an ηext roll-off of 2.9%. Their nondoped OLEDs based on ICz-DPS afford high efficiencies of 11.7% and 30.1 cd A-1, with pure-blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.08) and a low roll-off of 6.0%. This work also shows a strategy for designing AIE-TADF molecules by rational use of steric hindrance and weak inter/intramolecular interactions to realize high ΦPL values, fast reverse intersystem crossing process, and reduced nonradiative transition process properties, which may open the way toward highly efficient and small-efficiency roll-off devices.

The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully's fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH3O(2E) + OH(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O-H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0 leading to ground-state products CH3OO(X 2A″) + H(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO(1 2A') + H(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm.

A simple experimental procedure for scaling carbene Bronsted basicity is described. The results highlight the strong basicity of pyrazol-4-ylidenes, a type of mesoionic carbene, also named cyclic-bentallenes (CBA). They are more basic (pKaH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P4 (t Bu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases.

The National Natural Science Foundation of China is acknowledged, project approval number: 21673071. The One Hundred Talents Foundation of Hunan Province are gratefully acknowledged for the support of S.J. and S.R.K. The effect of a static electric ( field and an unchirped and chirped laser pulse field on the cycl[3.3.3]azine molecule was investigated using next generation quantum theory of atoms in molecules (NG QTAIM). Despite the magnitude of the E field of the laser pulses being an order of magnitude lower than for the static E field, the variation of the energy gap between the lowest lying singlet (S1) and triplet (T1) excited states was orders of magnitude greater for the laser pulse than for the static E field. Insights into the response of the electronic structure were captured by NG QTAIM, where differences in the inverted singlet triplet gap due to the laser pulses were significant larger compared to those induced by the static E field. The response of the S1 and T1 excited states, as determined by NG QTAIM, switched discontinuously between weak and strong chemical character for the static E field. In contrast, the response to the laser pulses, determined by NG QTAIM, is to induce a continuous range of chemical character, indicating the unique ability of the laser pulses to induce polarization effects in the form of ‘mixed’ bond types. Our analysis demonstrates that NG QTAIM is a useful tool for understanding the response to laser irradiation of the lowest lying singlet S1 and triplet T1 excited states of emitters exhibiting thermally activated delayed fluorescence (TADF). The chirped laser pulse led to more frequent instances of the desired outcome of an inverted singlet triplet gap than the unchirped pulse, indicating its usefulness as a tool to design more efficient OLED devices. Postprint Postprint

Two categories of absorption features, singlet and doublet, have been observed in semiconductive II–VI magic-size clusters (MSCs) experimentally; however, the underlying mechanism resulting in the ...