Triplet State Research Articles

Design of organic crystals with lone pairs to study spin resonance and spin-lattice interactions

Lone pairs are widely prevalent in various types of molecules and have a significant impact on the band structure, carrier transport, and dielectric response, which are the key factors for exploring the underlying mechanism of phenomena in opto-spintronics. In this work, nitrogen substitutions with nonequivalent hybridization are adopted to obtain different types of organic charge transfer crystals, where lone pairs are generated to weaken the interactions between donors and acceptors, resulting in a blueshift in photoluminescence and a weaker electron-lattice coupling. Moreover, lone pairs could further strengthen the ability to transfer energy and spin angular momentum to the lattice vibration to enhance spin resonance. Additionally, due to the effect of lone pairs, the spin density inside the crystals is redistributed to tune the transition between the singlet and triplet states. Overall, crystals with lone pairs demonstrate a more diverse set of magnetic, optical, and spin-related properties.

Strain-Induced Photochemical Opening of Ferrocene[6]cycloparaphenylene: Uncaging of Fe2+ with Green Light.

We present the synthesis, structural analysis, and remarkable reactivity of the first carbon nanohoop that fully incorporates ferrocene in the macrocyclic backbone. The high strain imposed on the ferrocene by the curved nanohoop structure enables unprecedented photochemical reactivity of this otherwise photochemically inert metallocene complex. Visible light activation triggers a ring-opening of the nanohoop structure, fully dissociating the Fe-cyclopentadienyl bonds in the presence of 1,10-phenanthroline. This process uncages Fe2+ ions captured in the form of [Fe(phen)3]2+ complex in high chemical yield and can operate efficiently in a water-rich solvent with green light excitation. The measured quantum yields of [Fe(phen)3]2+ formation show that embedding ferrocene into a strained nanohoop boosts its photoreactivity by 3 orders of magnitude compared to an unstrained ferrocene macrocycle or ferrocene itself. Our data suggest that the dissociation occurs by intercepting the photoexcited triplet state of the nanohoop by a nucleophilic solvent or external ligand. The strategy portrayed in this work proposes that new, tunable reactivity of analogous metallamacrocycles can be achieved with spatial and temporal control, which will aid and abet the development of responsive materials for metal ion delivery and supramolecular, organometallic, or polymer chemistry.

Excited-State Rotational Dynamics of Amine-Functionalized Terephthalic Acid Derivatives as Linker Models for Metal-Organic Frameworks.

Understanding how structural modifications affect the photophysics of organic linkers is crucial for their integration into metal-organic frameworks (MOFs) for light-driven applications. This study explores the impact of varying the amine functional group position on two terephthalic acid derivatives─linker 1 and linker 2─by investigating their photophysics through a combination of steady-state and ultrafast laser spectroscopy and time-dependent density functional theory (TD-DFT) calculations. With tetrahydrofuran as the solvent, time-correlated single-photon counting revealed a 2-fold increase in the S1 excited-state lifetime of the molecule with the amine group at the meta position compared with that of the molecule with the amine group at the ortho position. This phenomenon can be attributed to restricted intramolecular twisting for the molecule with the amine group in the meta position. In this regime, an interplay of high-energy steric and conjugation barriers was revealed for the molecule with the amine group at the meta position by TD-DFT calculations. Moreover, femtosecond/nanosecond transient absorption spectroscopy revealed a reversible excited-state conformational change for the ortho isomer via intramolecular rotation that occurred within ∼110 ps, unlocking a triplet state manifold. This study underscores the importance of modifying organic emitters, either as free linkers or within MOFs, to increase their performance in sensing and light-emitting applications.

The chemical bonding and adsorption properties of the umbelliform B6Fe cluster

The interesting report [J. Am. Chem. Soc. 2016, 138, 5644−5651] is that the FeB6 monolayers have pronounced stabilities, in which planar hypercoordinate Fe atoms are located at the centre of six-membered boron rings. Here, we find that the potential energy surface of the FeB6 cluster has a single deep funnel that leads down to an umbelliform structure of the global minimum with the triplet state, which is 0.17 eV lower than that of the reported C2v plane. The chemical bonding analysis by the AdNDP reveals that the FeB6 has six localised two-centre two-electron (2c–2e) B–B bonds and six localised 2c–2e B–Fe bonds|, and the remaining two 7c–1e σ bonds are delocalised over the umbelliform structure, which is further confirmed by the ELF, LOL and bond order analyses. After the CO molecule is adsorbed on the FeB6, it prefers to be located at the top site of Fe atom. The adsorption energy is 1.34 eV that is due to the σ-donation from the CO 5σ orbital to the Fe. The charge transfer makes the C–O stretching frequency and bond length have a red-shift.

Molecular engineering charge transfer and triplet exciton formation in donor-acceptor cocrystals.

Organic donor-acceptor (D-A) cocrystals are gaining attention for their potential applications in optoelectronic devices. This study explores the dynamics of charge transfer (CT) and triplet exciton formation in various D-A cocrystals. By examining a series of D-A cocrystals composed of coronene (COR), peri-xanthenoxanthene (PXX), and perylene (PER) donors paired with N,N-bis(3'-pentyl)perylene-3,4:9,10-bis(dicarboximide) (PDI), naphthalene-1,4:5,8-tetracarboxy-dianhydride (NDA), or pyrene-4,5,9,10-tetraone (PTO) acceptors, using transient absorption microscopy and time-resolved electron paramagnetic resonance spectroscopy, we find that the strength of the CT interaction influences the nature and yield of triplet excitons produced by CT state recombination. In particular, in the PER-PDI, COR-PTO, and PER-PTO cocrystals, localized triplet excitons are lower in energy than the CT state. By contrast, no localized triplet excitons are available to the CT states of the PXX-NDA, PER-NDA, and PXX-PTO cocrystals, and as a result, the CT states rapidly decay to ground state with no triplet formation. Moreover, density functional theory calculations show that the transition between delocalized CT states to a triplet state localized to a single donor or acceptor unit provides the source of spin-orbit coupling necessary when the triplet states are energetically accessible. These findings provide insights into the design of molecular materials with tailored exciton properties for optoelectronic applications.

Proximal Oblique-Packing of Heptamethine Cyanines through Spiro-Connection Boosts Triplet State Generation in Near-Infrared.

Near-infrared (NIR) triplet dyes are the cornerstones of cutting-edge biomedical and material applications. The difficulty in rational development of triplet dyes increases exponentially as the absorption wavelength shifts deeper into the NIR range. Although classical H-/J-typed packing of NIR dyes has the potential to enhance intersystem crossing (ISC) compared with that in single-chromophore dyes, the triplet state quantum yields remain limited in such strategy. Herein, proximal oblique-packed (V-shaped) heptamethine cyanines (SZ780) through spiro-connection were achieved. Multi-channel ultrafast ISC were direct observed in SZ780 and a record high ISC rate constant (up to ~1011 s-1) is registered among all the reported NIR triplet dyes. SZ780 exhibits a triplet state quantum yield of 18.9% upon excitation at 750 nm, which is almost an order of magnitude higher than that of the monomer (IR780, 2.1%) and nearly threefold increase compared to that of the H-packed dimer (SC780) (6.7%). Moreover, SZ780 efficiently generates singlet oxygen under 808 nm light irradiation, inducing cancer cell apoptosis in vivo. These findings demonstrate that constructing V-aggregated dyes system by spiro-connection offers a powerful approach for the design of high-performance NIR triplet sensitizers.

Proximal Oblique‐Packing of Heptamethine Cyanines through Spiro‐Connection Boosts Triplet State Generation in Near‐Infrared

Near‐infrared (NIR) triplet dyes are the cornerstones of cutting‐edge biomedical and material applications. The difficulty in rational development of triplet dyes increases exponentially as the absorption wavelength shifts deeper into the NIR range. Although classical H‐/J‐typed packing of NIR dyes has the potential to enhance intersystem crossing (ISC) compared with that in single‐chromophore dyes, the triplet state quantum yields remain limited in such strategy. Herein, proximal oblique‐packed (V‐shaped) heptamethine cyanines (SZ780) through spiro‐connection were achieved. Multi‐channel ultrafast ISC were direct observed in SZ780 and a record high ISC rate constant (up to ~1011 s‐1) is registered among all the reported NIR triplet dyes. SZ780 exhibits a triplet state quantum yield of 18.9% upon excitation at 750 nm, which is almost an order of magnitude higher than that of the monomer (IR780, 2.1%) and nearly threefold increase compared to that of the H‐packed dimer (SC780) (6.7%). Moreover, SZ780 efficiently generates singlet oxygen under 808 nm light irradiation, inducing cancer cell apoptosis in vivo. These findings demonstrate that constructing V‐aggregated dyes system by spiro‐connection offers a powerful approach for the design of high‐performance NIR triplet sensitizers.

Donor/Acceptor Ligands Based on an o-Terphenyl Motif to Achieve Thermally Activated Delayed Fluorescence in Zn(II) Complexes.

The photophysical properties of six new luminescent tetrahedral Zn(II) complexes are presented that survey two electronic donor moieties (phenolate and carbazolate) and three electronic acceptors (pyridine, pyrimidine, and pyrazine). A unique ligand based on an o-terphenyl motif forms an eight-membered chelate, which enhances through-space charge-transfer (CT) interactions by limiting through-bond conjugation between the donor and acceptor. A single isomeric product was obtained in yields up to 90%. Single-crystal X-ray diffraction structures of Zn complexes incorporating either donor show complementary interligand π-π interactions. All of the Zn complexes display long-lived luminescence in the solid state consistent with emission involving the triplet state. The phenolate-based complexes show evidence of CT emission in the solid state only with the strongest (pyrazinyl) acceptor. In contrast, all carbazolate-based complexes show evidence of thermally activated delayed fluorescence (TADF) in the solid and solution state, with photoluminescent quantum yields of up to 39%. These ligands represent a new family of Zn coordination compounds demonstrating TADF/phosphorescent properties that expand upon and elucidate design principles in the pursuit of photoactive earth-abundant metal complexes.

Benchmark computations of nearly degenerate singlet and triplet states of N-heterocyclic chromophores. II. Density-based methods.

In this paper, we demonstrate the performance of several density-based methods in predicting the inversion of S1 and T1 states of a few N-heterocyclic triangulene based fused ring molecules (popularly known as INVEST molecules) with an eye to identify a well performing but cost-effective preliminary screening method. Both conventional linear-response time-dependent density functional theory (LR-TDDFT) and ΔSCF methods (namely maximum overlap method, square-gradient minimization method, and restricted open-shell Kohn-Sham) are considered for excited state computations using exchange-correlation (XC) functionals from different rungs of Jacob's ladder. A well-justified systematism is observed in the performance of the functionals when compared against fully internally contracted multireference configuration interaction singles and doubles and/or equation of motion coupled-cluster singles and doubles (EOM-CCSD), with the most important feature being the capture of spin-polarization in the presence of correlation. A set of functionals with the least mean absolute error is proposed for both the approaches, LR-TDDFT and ΔSCF, which can be more cost-effective alternatives for computations on synthesizable larger derivatives of the templates studied here. We have based our findings on extensive studies of three cyclazine-based molecular templates, with additional studies on a set of six related templates. Previous benchmark studies for subsets of the functionals were conducted against the domain-based local pair natural orbital-similarity transformed EOM-CCSD (STEOM-CCSD), which resulted in an inadequate evaluation due to deficiencies in the benchmark theory. The role of exact-exchange, spin-contamination, and spin-polarization in the context of DFT comes to the forefront in our studies and supports the numerical evaluation of XC functionals for these applications. Suitable connections are drawn to two and three state exciton models, which identify the minimal physics governing the interactions in these molecules.

A computational study on the effect of structural isomerism on the excited state lifetime and redox energetics of archetype iridium photoredox catalyst platforms [Ir(ppy)2(bpy)]+ and Ir(ppy)3.

This study investigates the impact of structural isomerism on the excited state lifetime and redox energetics of heteroleptic [Ir(ppy)2(bpy)]+ and homoleptic Ir(ppy)3 photoredox catalysts using ground-state and time-dependent density functional theory methods. While the ground- and excited-state reduction potentials differ only slightly among the isomers of these complexes, our findings reveal significant variations in the radiative and non-radiative decay rates of the reactivity-controlling triplet 3MLCT states of these closely related species. The observed differences in radiative decay rates could be traced back to variations in the transition dipole moment, vertical energy gaps, and spin-orbit coupling of the isomers. In [Ir(ppy)2(bpy)]+, transition dipole moment differences play a significant role in controlling the relative lifetime of the triplet states, which we rationalized by a vectorial analysis of permanent dipole moments of the ground and excited states. Regarding the two isomers of Ir(ppy)3, changes in radiative decay rates were primarily attributed to variations in vertical energy gaps and intensity borrowing from other singlet-singlet transitions driven by spin-orbit coupling. Non-radiative decay variations were assessed in terms of differences in reorganization energies, adiabatic energy gap, and spin-orbit coupling. For both complexes, reorganization energies associated with low-energy molecular vibrations and metal-ligand bond length changes following the de-excitation process were major contributors. These insights provide a deeper understanding of how molecular design can be leveraged to optimize the performance of iridium-based photoredox catalysts, potentially guiding the development of more efficient catalytic systems for future applications.

Localized and Excimer Triplet Electronic States of Naphthalene Dimers: A Computational Study

We perform DFT calculations with different hybrid (ωB97X-D and M05-2X) and double hybrid (B2PLYP-D3 and ωB2PLYP) functionals to characterize the lowest energy triplet excited states of naphthalene monomer and dimers in different stacking arrangements and to simulate their absorption spectra. We show that both excimer and localized triplet minima exist. In the former, the spin density is delocalized over the two monomers, adopting a face-to-face arrangement with a short inter-molecular distance. In the latter, the spin density is localized on a single naphthalene molecule, and different minima or pseudo-minima are possible, the most stable one corresponding to a slipped parallel arrangement. According to B2PLYP-D3 calculations, excimer minima are the most stable, in line with the indications of ADC(2) studies. However, the relative stability of the minima is reverted when including thermal and vibrational effects. Excimer minima exhibit a very intense absorption spectrum, peaking above 500 nm. The computed absorption spectra of localized minima significantly depend on the stacking geometry and do not coincide with that of isolated naphthalene. Hybrid functionals provide very accurate vibronic absorption spectra for naphthalene monomer, both in the singlet and in the triplet state, but underestimate the stability of the excimer triplet.

Impacts of β-diketonate terminal ligands on slow magnetic relaxation and luminescence thermometry in dinuclear DyIII single-molecule magnets.

Lanthanide-based Single-Molecule Magnets (SMMs) with optical and magnetic properties provide a means to understand intrinsic energy levels of 4f ions and their influence on optical and magnetic behaviour. Fundamental understanding of their luminescent and slow relaxation of the magnetization behaviour is critical for targeting and designing SMMs with multiple functionalities. Herein, we seek to investigate the role of DyIII coordination environment and fine electronic structure on the slow magnetic relaxation and luminescence thermometry. Our findings are illustrated through two distinct DyIII complexes, [Dy2(bpm)(hexd)6] (1) and [Dy2(bpm)(hpd)6] (2), (bpm = 2,20-bipyrimidine, hexd = 2,4-hexanedione, hpd = 3,5-heptanedione), by comparing their features with a family of DyIII dinuclear species bridged by bpm. These findings highlight that the hexd- and hpd- ligands exhibit a similar effective barrier to the reversal of magnetization (280-290 K). The values are among the highest for dinuclear DyIII complexes bridged by bpm, due to the low distortion of the DyIII coordination polyhedra and the long Dy-N equatorial bonds. Furthermore, the luminescence performance is affected by the triplet state energy of the terminal ligand, influencing ligand-to-DyIII energy transfer. The hpd- ligand's higher T1 state energy leads to poor ligand-to-DyIII energy transfer, limiting the use of 2 for luminescence thermometry. Conversely, this issue is absent in 1, which offers a relative thermal sensitivity of 0.1 to 0.7% K-1 (10 to 60 K) with a temperature uncertainty below 1 K. These findings contribute to our understanding of lanthanide-based SMMs and facilitate the design of multifunctional materials with tailored magnetic and luminescent properties for molecular electronics and beyond.

Carbon Dots@Upconversion Nanoparticles: Conjugate Manipulation and Luminophor Confinement Enabling Aqueous-Phase Orthogonal Multicolor Phosphorescence-Upconversion Luminescence.

Developing single-particle nanocomposite with aqueous-phase orthogonal multicolor phosphorescence or multimodal luminescence holds great significance for optical coding, anti-counterfeiting encryption, bioimaging, and biosensing. However, it faces challenges such as a limited range of emission wavelengths and difficulties in controlling the synthesis process. In this work, a conjugate structure manipulation integrated luminophor confinement strategy is proposed to prepare carbon dots@upconversion nanoparticles (CDs@UCNPs) featuring aqueous-phase orthogonal multicolor room-temperature phosphorescence-upconversion luminescence (RTP-UCL) through wet-chemical synthetic methods. Four types of CDs are synthesized by introducing molecules with varying degrees of conjugation, while the intersystem crossing process is enhanced by constructing charge-transfer states to narrow the energy gap between the excited singlet and triplet states. Aqueous-phase orthogonal multicolor RTP (green, yellow, and orange) and UCL (blue, green, yellow, and red) are achieved by confining CDs with different conjugation degrees within a NaBiF4 matrix doped with various lanthanide ions. Notably, NaBiF4 UCNPs can crystallize at low temperatures, serving as a matrix to immobilize CDs, thereby preventing their vibration and rotation, and minimizing interference from water and oxygen. Additionally, the versatility of this strategy is demonstrated by constructing multicolor CDs@Cs2NaGdCl6: 5%Yb3+,10%Er3+ perovskite nanocomposites. This strategy offers valuable guidance for the preparation of advanced aqueous-phase orthogonal multicolor materials.

Dynamic Phosphorescence Behavior of Carbene-Metal-Amide Complexes from the Perspective of Excited State Modulation.

Carbene-metal-amide (CMA) complexes have diverse applications in luminescence, imaging and sensing. In this study, we designed and synthesized a series of CMA complexes, which were subsequently doped into a PMMA host. These materials demonstrate light-induced dynamic phosphorescence, attributed to their long intrinsic triplet state lifetime (τP,int, in the μs-ms scale), high intersystem crossing (ISC) rate constant (kISC, up to 107 s-1), and bright phosphorescence. The extended τP,int, and elevated kISC facilitate efficient sensitization of singlet oxygen (1O2) under light irradiation, which is rapidly consumed by the host material, creating a localized anaerobic environment conducive to bright phosphorescence emission. The Sn-T1 process exhibits a large spin-orbital coupling matrix element (SOCME) value, while the SOCME value between T1 and S0 is comparatively smaller, resulting in a large kISC and long τP,int, Computational results indicate that the hole-electron configuration in the lowest triplet state exhibits low contributions from gold. Based on the dynamic phosphorescence properties, an encryption material capable of achieving a "burn after reading" effect was developed. This work illustrates that those phosphorescent emitters with minimal heavy atom contribution can produce dynamic phosphorescent phenomena, providing a novel strategy for designing stimuli-responsive phosphorescent materials.

Methacrylate-based copolymers as tunable hosts for triplet-triplet annihilation upconversion.

The ability to convert light to higher energies through triplet-triplet annihilation upconversion (TTA-UC) is attractive for a range of applications including solar energy harvesting, bioimaging and anti-counterfeiting. Practical applications require integration of the TTA-UC chromophores within a suitable host, which leads to a compromise between the high upconversion efficiencies achievable in liquids and the durability of solids. Herein, we present a series of methacrylate copolymers as TTA-UC hosts, in which the glass transition temperature (T g), and hence upconversion efficiency can be tuned by varying the co-monomer ratios (n-hexyl methacrylate (HMA) and 2,2,2-trifluoroethyl methacrylate (TFEMA)). Using the model sensitiser/emitter pair of palladium(ii) octaethylporphyrin (PdOEP) and diphenylanthracene (DPA), the upconversion quantum yield was found to increase with decreasing glass transition temperature, reaching a maximum of 1.6 ± 0.2% in air at room temperature. Kinetic analysis of the upconversion and phosphorescence decays reveal that increased PdOEP aggregation in the glassy polymers leads to a competitive non-radiative relaxation pathway that quenches the triplet state. Notably, the threshold intensity is highly sensitive to the glass transition temperature, ranging from 1250 mW cm-2 for PHMA90TFEMA10 (T g = -9.4 °C) to ∼200 mW cm-2 for more 'glassy' hosts, e.g. PHMA33TFEMA67 (T g = 20.1 °C), suggesting the TTA-UC mechanism switches from diffusion-based collisions to triplet exciton migration at localised sensitiser-emitter pairs.

Regioisomeric π-Extended Nanographene with Long-Lived Phosphorescence Afterglow.

The cutouts of graphene sheets, particularly those with a nonplanar topology, present vast opportunities for advancement. Even a slight deviation from the planar structure can lead to intriguing (chiro)optical features for helically twisted nanographenes. In this context, we introduce two regioisomeric π-extended nanographenes that exhibit distinct excited-state characteristics. The helicene structure and the photophysical features can be easily tuned by changing the connecting position of the nanographene to the carbazole core (2,7- and 3,6-). Single-crystal X-ray diffraction analysis confirmed the formation of nanographenes with bent and helical conformations. Both derivatives exhibited thermally activated delayed fluorescence at room temperature and phosphorescence at low temperatures. Notably, the nanographene with the bent structure displayed an impressive red afterglow lasting over 30 seconds, in contrast to the very weak afterglow observed in the helical structure. DFT calculations revealed the existence of an isoenergetic higher triplet state (T8) and comparatively weak spin-orbit coupling (T1-S0), thereby enabling the bent nanographene to exhibit a long-lived component and strong afterglow. Our findings highlight the significance of regioisomeric nanographenes with exceptional optical properties and offer a deeper understanding of the structure-property relationship in nonplanar nanographenes.

Probing the Design Rules for Optimizing Electron Spin Relaxation in Densely Packed Triplet Media for Quantum Applications.

Quantum technologies using electron spins have the advantage of employing chemical qubit media with tunable properties. The principal objective of material engineers is to enhance photoexcited spin yields and quantum spin relaxation. In this study, we demonstrate a facile synthetic approach to control spin properties in charge-transfer cocrystals consisting of 1,2,4,5-tetracyanobenzene (TCNB) and acetylated anthracene. We find that the extent and position of acetylation control the degree of charge-transfer and the optical band gap by modifying crystal packing and electronic structure. We further reveal that while the spin polarization of the triplet state is slightly reduced compared to prototypical Anthracene:TCNB, the phase memory (T m) and, for 9-acetylanthracene:TCNB spin-lattice relaxation (T 1) time, could be enhanced up to 2.4 times. Our findings are discussed in the context of quantum microwave amplifiers, known as masers, and show that acetylation could be a powerful tool for improving organic materials for quantum sensing applications.

Enhancing Reverse Intersystem Crossing in TSCT-TADF Emitters: Heavy Atom Modulation of Multiresonance Acceptors.

With the rapid development of thermally activated delayed fluorescence (TADF) materials, achieving efficient reverse intersystem crossing (RISC) to mitigate triplet-triplet annihilation has emerged as a prominent research focus. This study investigates five derivative molecules, featuring varied bridging atoms/groups (O, S, Se, -CH2-), designed from the reported TADF molecule AC-BO with through-space charge transfer (TSCT) properties. Utilizing time-dependent density functional theory coupled with a PCM solution model, their excited state behaviors were simulated in a toluene environment. Interestingly, it was observed that RISC in AC-BO and one derivative, AC-BCO, occurs predominantly via the T2 state rather than the typical T1 state (3LEB, where B denotes the fluorene bridge), distinguishing it from conventional TSCT-TADF compounds, where RISC typically involves transitions between the 3CT and 1CT states. This distinctive mode is attributed to reduced spin-orbit coupling (SOC) between 1CT and 3LEB, with T2 representing a significant contributor to the RISC process through its 3CT character. Introduction of heavy atoms enhances the electron-withdrawing ability of the acceptor unit, leading to the T1 transitions exhibiting 3MRCT characteristics and increased SOC, thereby favoring RISC via 3MRCT to 1CT transitions. This study not only deepens our understanding of transition mechanisms in TSCT-TADF compounds but also provides crucial insights into the molecular design and regulation of excited triplet states.

Dynamics of hydrogen shift reactions between peroxy radicals.

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.

Time-resolved transient optical and electron paramagnetic resonance spectroscopic studies of electron donor-acceptor thermally activated delayed fluorescence emitters based on naphthalimide-phenothiazine dyads.

The photophysics of naphthalimide (NI)-phenothiazine (PTZ) dyads were investigated as electron donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) emitters. Femtosecond transient absorption (fs-TA) spectra show that the photophysical processes in non-polar solvents are in singlet localized state (1LE, τ = 0.8 ps) → Franck-Condon singlet charge separation state (1CS, τ = 7.8 ps) → 1CS state (τ = 2.2 ns) → triplet state (3LE, τ = 16 μs). The 3LE state is formed via the spin-orbit charge transfer-intersystem crossing (SOCT-ISC) mechanism rather than the spin-orbit (SO)-ISC mechanism. In a polar solvent, the CS state has a much lower energy than the 3LE state; thus, the 3LE state is absent from the photophysical processes and no TADF was observed. Moreover, we found that the delayed fluorescence lifetime is related to the low-lying triplet state (3LE or 3CS states). When the 3CS state is the low-lying triplet state, the TADF lifetime is shorter than that of the 3LE state as the low-lying triplet state. In the time-resolved electron paramagnetic resonance (TREPR) spectra, both 3LE (zero field splitting parameter D = 2250 MHz, E = -150 MHz) and 3CS (D = 430 MHz, E = 0 MHz) states were observed. It is noteworthy that the electron spin polarization (ESP) phase pattern of the 3CS state was inverted at longer delay times as a consequence of the selective transition between the 3LE and 3CS states and a faster decay of one sublevel of the 3CS state. These results are strong and direct experimental evidence for the spin-vibronic coupling mechanism of TADF.