T1 State Research Articles

Conformation‐Controlled Double Phosphorescence Components From A Single Organic Molecule: Time‐Dependent and Excitation‐Dependent Wide‐Range Afterglow Color Change

AbstractDouble phosphorescence components from a single organic molecule is rarely reported before because most organic molecules must follow Kasha's rule and merely emit phosphorescence from T1 state. We discover a unique phenomenon of conformation‐controlled expression of double phosphorescence components. The green phosphorescence component and the red phosphorescence component are assigned to the dinaphthylamine moiety and the pyrene unit of N,N‐di(naphthalen‐2‐yl)pyren‐1‐amine (NaPy), respectively. NaPy shows time‐dependent and excitation‐dependent afterglow color change in copolymerized MMA film at room temperature due to simultaneous expression of the double phosphorescence components. However, in PMMA film and copolymerized MA film, NaPy just expresses the red phosphorescence component, suggesting that matrix rigidity greatly affects expression of the double phosphorescence components via altering conformation of NaPy. In low‐rigidity matrix, the NaPy conformation is dyanmic due to free rotation and triplet excitons relax to T1 state via inner conversion or vibration relaxation, leading to the sole expression of the red phosphorescence component. In high‐rigidity matrix, the highly twisted conformation of NaPy is stabilized due to limitation of rotation and triplet excitons at T1 state and T2 state return to S0 state separately, resulting in simultaneous expression of the double phosphorescence components. Thus, NaPy follows Kasha's rule in low‐rigidity matrix but disobeys Kasha's rule in high‐rigidity matrix. In high‐rigidity matrix, the dinaphthylamine moiety and the pyrene unit can function relatively independently, resulting in time‐dependent and excitation‐dependent wide‐range afterglow color change.

Cocrystallization-Induced Red Ultralong Organic Phosphorescence.

Organic cocrystals formed through multicomponent self-assembly have attracted significant interest owing to their clear structure and tunable optical properties. However, most cocrystal systems suffer from inefficient long-wavelength emission and low phosphorescence efficiency due to strong non-radiative processes governed by the energy gap law. Herein, an efficient long-lived red afterglow is achieved using a pyrene (Py) cocrystal system incorporating a second component (NPYC4) with thermally activated delayed fluorescence (TADF) and ultralong organic phosphorescence (UOP) properties. The cocrystal (NPYC4-Py) not only inherits the excellent luminescence of its monomeric counterparts, but also exhibits unique dual-mode characteristics, including persistent TADF and UOP emission with a high quantum yield of 58% and a lifetime of 362 ms. The precise cocrystal stacking distinctly reveals that intermolecular interactions lock the cocrystal formation and weaken the intermolecular π-π interactions between NPYC4 and Py, thereby stabilizing the excited triplet excitons. Furthermore, the favorable energy level of NPYC4 acts as a bridge, reducing the energy gap between the S1 and T1 states for Py, therefore activating its red phosphorescence from Py. This research provides direct insights into achieving efficient red UOP through co-crystallization.

Thermal Quenching Mechanism of Mn4+ in Na2SiF6, NaKSiF6, and K2SiF6 Phosphors: Insights from the First-Principles Analysis.

This study aims to identify the key factors governing the thermal quenching of Mn4+ ion luminescence in fluoride-based phosphor materials used as red emitters in modern-day phosphor-converted LED devices. Here, we employ first-principles calculations for Mn4+-doped Na2SiF6, NaKSiF6, and K2SiF6 hosts to explore how host properties and local coordination environments influence thermal quenching behavior. The ΔSCF method was used to model the geometric structures of the Mn4+4A2 (ground) and 2E, 4T2 (excited) states and the energies of the optical transitions between these states. Our results reveal that thermal quenching in Na2SiF6 and K2SiF6 phosphors occurs through thermally activated 2E → 4T2 → 4A2 crossover. In contrast, thermal quenching in NaKSiF6 is due to other nonradiative decay pathways. Investigations of the mechanical stability of these fluorides show that NaKSiF6 is mechanically unstable. We suggest that this property of the host limits the luminescence efficiency of the embedded Mn4+ ions. We also determined the reason for the difference in the intensity of the 2E → 4A2 emission transition (ZPL) in the systems. These findings advance our fundamental understanding of the thermal quenching mechanism of Mn4+ ion luminescence in fluorides, and the results can aid future discoveries of technologically useful phosphors through high-throughput design methodologies.

Evaluation of a Model of Transitional Care After Preterm Birth on Parents' Mental Health and Self-Efficacy: A Randomized Controlled Pilot Trial.

Parents of premature infants experience depression, anxiety, post-traumatic stress disorder, and increased stress, which can negatively impact parent-infant relationships and infant development. To reduce negative consequences and optimally support families, we developed the Transition to Home model (TtH). In this randomized controlled pilot trial (RCT), the feasibility of performing an experimental study to analyse the effects of TtH on parental mental health over time was evaluated. The following domains were assessed: recruitment, follow-up and study burden, outcome measures used and parental mental health outcomes. We included n = 22 parent couples with their preterm infants in the control group and n = 23 in the intervention group. Depression, anxiety and post-traumatic stress disorders, parenting stress, and parental self-efficacy were assessed at five timepoints. The study burden was evaluated once at the end of the study. The control and intervention groups had similar socio-demographic characteristics. The groups showed no differences in the mental health outcomes except for depression in mothers at T2 (p = 0.042) and T5 (p = 0.027) and state anxiety in fathers at T2 (p = 0.016). This pilot RCT established a framework for the evaluation of the TtH model of care and demonstrated the viability of the evaluation scheme. The results confirm the suitability of the RCT's structure and the feasibility of the methods and instruments used. Minor adjustments are recommended to include a more diverse sample in future studies.

Excited-State Decay and Photolysis of O-Nitrophenol after Proton Transfer. II: A Theoretical Investigation in the Microsolvated Atmospheric Environment.

Changes in atmospheric humidity affect the number of water molecules surrounding o-nitrophenol (ONP), creating an anisotropic chemical environment. It, in turn, influences the photodynamic behaviors of ONP, differing from those observed in the gas phase and in solution. Recently, we explored the excited-state decay and the generation of the hydroxyl (OH) radical before proton transfer of ONP in the microsolvated environment using the MS-CASPT2//CASSCF approach. As is well known, ONP is capable of converting to its aci-nitro isomer (aciONP) via an excited-state intramolecular proton transfer (ESIPT) process. In the present work, the photoinduced dynamics of aciONP, which can lead to an OH radical and nitrous acid (HONO), was studied using the same computational model. Our calculations demonstrated that increasing the number of water molecules affects the molecular geometries, particularly the key bond lengths and dihedral angles of the HONO group, while also reducing the relative energies of minima and intersections. Moreover, we identified two distinct types of minimum structures: one that retains the intramolecular hydrogen bond and the other that breaks the hydrogen bond with the H atom flipping outward. The latter structure, compared with the former, has a different electronic-state character and facilitates intersystem crossing processes. Subsequently, two major excited-state decay paths were proposed: (PATH I) ESIPT → S1 → S1S0 → S0; (PATH II) ESIPT → S1 → S1-2 → S1T1 → T1 → S0T1 → S0. Furthermore, the T1 state has a relatively long lifetime, allowing for the formation of the OH radical and HONO, and the corresponding energy barriers decrease as the number of water molecules increases. These theoretical findings provide valuable insights into the photodynamics of aciONP in the microsolvated atmospheric environment.

Fast singlet excited-state deactivation pathway of flavin with a trimethoxyphenyl derivative

Incorporation of the trimethoxyphenyl group at position 7 of flavin can drastically change the photophysical properties of flavin. We show unique fast singlet 1(π,π*) excited state deactivation pathway through nonadiabatic transition to the 1(n,π*) excited- state, and subsequent deactivation to the ground electronic state (S0), closing the photocycle. This mechanism explains the exceptionally weak fluorescence and the short excited–state lifetime for the flavin trimethoxyphenyl derivative and the lack of excited triplet T1 state formation. Full recovery of flavin in its ground state takes place within a 15 ps time window after photoexcitation in a polar solvent such as acetonitrile. According to quantum chemical calculations, the C(2)-O distance elongates by 0.16 Å in the 1(n,π*) state, with respect to the ground state. Intermediate–state structures are predicted by theoretical ab initio calculations and their dynamics are investigated using broadband vis-NIR time-resolved transient absorption and fluorescence up-conversion techniques.

Synergistic Effects of Unconventional Hydrogen Bonds and π-Stacking Interaction and Their Excited-State Dependence: The Origin of Unusual Photophysical Properties of Aromatic Thioketones in Acetonitrile and Hydrocarbons.

It has been established experimentally that aromatic thioketones possess several inherently unique photophysical properties, some of which are highly sensitive even to common hydrocarbon solvents. However, the deeper reasons and the underlying mechanisms remain unclear up to date. In this study, the multistate complete active space second-order perturbation theory (MS-CASPT2) has been utilized to investigate the five lowest-lying electronic states (S0, T1, S1, T2, and S2) of 4H-1-benzopyran-4-thione (BPT) in acetonitrile and hydrocarbons. The results show that the S1, T1, and T2 states of BPT are close in energy so that the T2-state-mediated S1 → T2 → T1 and T1 → T2 → S1 transitions could occur in tens of picoseconds, which exhibits little dependence on the formation of the BPT-solvent complexes and on the bulk-solvent effect. This explains why thermally activated delayed fluorescence from the S1 state has been observed for many aromatic thioketones in both inert media and hydrocarbons. Meanwhile, our calculations show that the intracomplex noncovalent interactions could be automatically adjusted by the redistribution of π-electrons in the flexible aromatic rings. This allows the S2 → S1 internal conversion to occur efficiently in the vicinity of the two-state conical intersection, which results in the remarkable changes in the S2-state lifetimes and fluorescence quantum yields of many aromatic thioketones from inert media to hydrocarbon solvents. The aforementioned inherent photophysical properties could be qualitatively understood by a simple model of frontier molecular orbitals. This model could be used to understand photophysical properties of other aromatic compounds (such as aldehydes, ketones, amines, and carboxylic acids) in different solvents.

Electron Spin Dynamics of the Intersystem Crossing in Aminoanthraquinone Derivatives: The Spectral Telltale of Short Triplet Excited States.

We studied the excited state dynamics of two bis-amino substituted anthraquinone (AQ) derivatives, with absorption in the visible spectral region, which results from the attachment of a electron-donating group to the electron-deficient AQ chromophore. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190-320 ps, and nanosecond transient absorption spectra demonstrated an unusually short triplet state lifetime (2.06-5.43 μs) for the two AQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at a longer delay time after laser flash. Spectral simulations show faster decay of the Ty sublevel than the other two sublevels (τx = 15.0 μs, τy = 1.50 μs, τz = 15.0 μs); theoretical computation predicts initial overpopulation of the Ty sublevel, and rationalizes the short T1 state lifetime and the ESP inversion. Theoretical computations taking into account the electron-vibrational coupling, i.e., the Herzberg-Teller effect, successfully rationalize the TREPR experimental observations.

ESIPT-induced intersystem crossing leads to tautomer fluorescence quenching for 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one molecule

The excited-state intramolecular proton-transfer (ESIPT) dynamics of 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF) and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) have been investigated using time-dependent density functional theory (TDDFT). Upon photoexcitation, 3NTF exhibits a single fluorescence emission while 3FTF is fluorescence quenched when dissolved in cyclohexane solution. The present study reveals that both species undergo barrierless ESIPT process, and the underlying reason for fluorescence quenching in 3FTF has been elucidated. Specifically, it is concluded that intersystem crossing (ISC) is responsible for the fluorescence quenching in the 3FTF molecule due to the energy gap between the S1 and T2 states is only 0.12 eV plus large S1 → T2 spin–orbit coupling resulting in a strong interaction between the singlet and triplet states. The present study provides a reference for the fluorescence quenching associated with thiol-hydrogen bond molecules, and it is helpful for further research on ESIPT reactions of sulfur-containing molecules.

Singlet oxygen generation by nanoassemblies containing porphyrin macrocycles: Steric and screening effects, energy transfer and competing processes

In this semi-review paper, we discuss some principal aspects that should be taken into account upon quantitative analysis of the interaction with molecular oxygen O2 and the singlet oxygen (1O2 or [Formula: see text] generation by various tetrapyrrolic photosensitizers (including monomers, chemical dimers, triads, pentads and organic-inorganic nanoassemblies). In all cases, the direct experimental measurements of 1O2 emission at [Formula: see text] = 1.27 μ need to be corrected to the solvent influence (refractive indexes, molecular polarizability) on the rate constant of the radiative transition [Formula: see text] in a 1O2 molecule. For porphyrin and chlorin chemical dimers, T1 states quenching by O2 followed by 1O2 generation depends on donor-acceptor interactions between the dimer halves, and extra-ligation effects as well as the spacer flexibility. In the case of self-assembled triads and pentads containing Zn-porphyrin dimers and pyridyl substituted porphyrin free bases (H2P) as extra-ligands, quenching rate constants of H2P T1 states by O2 are smaller compared to those found for individual monomeric H2P molecules which is explained by the spatial screening influence of a strongly quenched Zn-porphyrin dimer in multiporphyrin complexes. Finally, at 293 K for nanoassemblies of two types (semiconductor quantum dots CdSe/ZnS + coordinative linked tetra-pyridyl porphyrins, and semiconductor negatively charged quantum dots AgInS/ZnS + positively charged porphyrin molecules) it was quantitatively shown that non-radiative energy transfer FRET quantum dot [Formula: see text] porphyrin (competing with electron tunneling under conditions of quantum confinement) is only responsible for the singlet oxygen 1O2 generation by nanoassemblies.

Non-Radiative Deactivation in Isolated Quinoline.

The photophysics of the S2 1(ππ*) state of the polycyclic aromatic nitrogen-containing hydrocarbon (PANH) quinoline is investigated in a free jet using a picosecond laser system. A [1 + 1] multiphoton ionization spectrum yields the S2 origin at around 32 200 cm-1 and reveals several vibronic bands. In time-resolved experiments, quinoline is then excited between 312.2 and 279.7 nm. Probe wavelengths of 351 and 263.5 nm are employed. The dynamics is monitored by time-resolved photoelectron imaging. The images reveal a short-lived band at high electron kinetic energies with a ps lifetime and a band at lower electron kinetic energies that shows an offset at long delay times. In comparison with previous work, the offset is assigned to ionization from the T1 state. Lifetimes decrease from 45 ps at the S2 origin to 11 ps at +3550 cm-1. Most likely, the S2 1(ππ*) state deactivates by internal conversion to the S1 1(nπ*) state, followed by intersystem crossing to the triplet manifold.

Light-Induced Ring-to-Chain Transformations of Elemental Sulfur: Nonadiabatic Dynamics Simulations.

The emergence of high-sulfur content polymeric materials and their diverse applications underscore the need for a comprehensive understanding of the ring-to-chain transformation of elemental sulfur. In this study, we delve into the ultrafast transformation of the elemental sulfur S8 ring upon photoexcitation employing advanced nonadiabatic dynamics simulations. Our findings reveal that the bond breaking of the S8 ring occurs within tens of femtoseconds. At the time of bond breaking, most molecules are in the lowest singlet excited state S1. S1 survives for 40-450 fs before relaxing to the quasi-degenerate manifolds formed by the T1 and S0 states of the S8 chain. This suggests that upon photoexcitation the polymerization of the S8 chains might proceed before the chains relax to their lowest energy states. The derived temporal resolution provides a detailed perspective on the dynamics of S8 rings upon photoexcitation, shedding light on the intricate processes involved in its excited-state transformations.

Photostimulated Anticancer Activity of Mitochondria Localized Rhenium(I) Tricarbonyl Complexes Bearing 1H-imidazo[4,5-f][1,10]phenanthroline Ligands Against MDA-MB-231 Cancer Cells.

We have introduced Re(I) tricarbonyl complexes (ReL1 - ReL6) [Re(CO)3(N^N)Cl] where N^N = extensive π conjugated imidazo-[4,5-f]1,10-phenanthroline derivatives that helps in strong DNA intercalation, enhanced photophysical behavior, increase the 3π-π* character of T1 state for PDT and high value of lipophilicity for cell membrane penetration. These complexes exhibited prominent intraligand/ligand-centered (π - π*/ 1LC) absorption bands at λ 260 - 350 nm and relatively weak metal-to-ligand charge-transfer (1MLCT) bands within the λ 350 - 550 nm range. Among the six synthesized complexes, [(CO)3ReICl(K2-N,N-2-(4-(1-benzyl-1H-tetrazol-5-yl)phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline] (ReL6) exhibited outstanding potency (IC50 ~ 6 µM, PI > 9) under yellow light irradiation compared to dark conditions. Importantly, extremely lipophilic complex ReL6 showed effective penetration through the cell membrane and localized primarily in mitochondria (Pearson's correlation coefficient, PCC = 0.918) of MDA-MB-231 cells. Complex ReL6 exhibited more than 9 times higher photo-toxicity in normoxic and hypoxic environment of tumor by inducing 1O2 generation (type II PDT), radical generation triggered by NADH oxidation (type I PDT). This complex is a promising candidate for TNBC treatment in hypoxic tumors, with efficacy comparable to photofrin and have demonstrated CO release ability under UV light irradiation.

Ultralow Roll-off Thermally Activated Delayed Fluorescent Light-Emitting Diodes Based on Furo[2,3-b]quinoxaline Emitters.

Herein, a Y-type compound (67dMeOTPA-FQ) and a T-type compound (58dMeOTPA-FQ) based on furo[2,3-b]quinoxaline were synthesized. The theory calculation shows the S1 and T1 of both compounds own a charge-transfer feature while their T2 states have a local excitation feature. The calculated kRISC(T2-S1) is one to 2 orders of magnitude larger than kRISC(T1-S1). Thus, the nonadiabatic spin-vibronic mechanism involved in the T2 state is suggested to be responsible for the thermally activated delayed fluorescence (TADF) feature. Meanwhile, when 2-methyl-9,10-bis(naphthalen-2-yl)anthracene is selected as host, the maximum luminance of the device based on 67dMeOTPA-FQ is up to 104215 cd·m-2, and the external quantum efficiency (EQE) keeps in the 8.2-8.0% range with the luminance changed from 55.0 cd·m-2 to 90000 cd·m-2, only 2.4% efficiency roll-off. As for 58dMeOTPA-FQ, a slightly lower EQE of 7.1-6.7% with the luminance range of 1-40000 cd·m-2 was achieved for orange-red emission. Both the reverse intersystem crossing (RISC) and triplet-triplet annihilation mechanisms are supposed to concurrently contribute to the utilization of triplet excitons and suppress the notorious efficiency roll-off observed in TADF-based devices.

Substituents' Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate-Phosphine Cages.

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine R2PCH(R')PR2 ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R' = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the 31P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R' = H) scaffold. All copper(I) complexes exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. Importantly, the use of aliphatic substituents on the phosphorus atoms instead of aromatic ones leads to an almost double increase in the quantum efficiency (ΦPL) of photoluminescence by eliminating nonradiative decay from the 3LCPh states of the dppm aromatic rings. The higher donating ability of the substituents in the pyrazolate ligand (CF3 vs CH3) lowers the energy of the metal-centered excited state, allowing for a significant metal impact on the T1 state. Finally, the Ag(I) complex displays an emission efficiency of approximately 14%, being the highest among known trinuclear silver(I) pyrazolate homometallic derivatives.

Structural and Electronic Factors Controlling the Efficiency and Rate of Intersystem Crossing to the Triplet State in Thiophene Polycyclic Derivatives.

Thiophene polycyclic derivatives are widely used in organic light-emitting diodes, photovoltaics, and medicinal chemistry applications. Understanding the electronic and structural factors controlling their intersystem crossing rates is paramount for these applications to be successful. This study investigates the photophysical, electronic structure, and excited state dynamics of 1,2-benzodiphenylene sulfide, benzo[b]naphtho[1,2-d]thiophene, and benzo[b]naphtho[2,3-d]thiophene in polar aprotic and non-polar solvents. Steady-state absorption and emission spectroscopy, femtosecond transient absorption spectroscopy, and DFT and TD-DFT calculations are employed. Low fluorescence quantum yields of 1.2 to 2.7% are measured in acetonitrile and cyclohexene, evidencing that the primary relaxation pathways in these thiophene derivatives are nonradiative. Linear interpolation of internal coordinates calculations predict that an S-C bond elongation reaction coordinate facilitates the efficient intersystem crossing to the T1 state. Excitation of 1,2-benzodiphenylene sulfide and benzo[b]naphtho[1,2-d]thiophene at 350 nm or benzo[b]naphtho[2,3-d]thiophene at 365 nm, populates the lowest-energy 1ππ* state, which relaxes to the 1ππ* minimum in tens of picoseconds or intersystem crosses to the triplet manifold in ca. 500 ps to 1.1 ns depending on the position at which the benzene rings are added. Excitation at 266 nm does not affect the intersystem crossing rates. Laser photodegradation experiments demonstrate that the thiophene polycyclic derivatives are highly photostable.

Intense annihilation electrogenerated chemiluminescence from tris(2-phenylpyridinato)iridium(III) with organic radical cations and anions

The electrogenerated chemiluminescence (ECL) of 0.2 mM tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) in acetonitrile with a potential step method was dramatically enhanced by the electrochemically generated radical cation of 10-methylphenothiazine (MePT), thianthrene (TH), and dibenzo-1,4-dioxin (DD) or the radical anion of benzophenone (BP) and benzonitrile (BN). In the case of using the radical cation of phenothiazine (PT), no ECL was seen. Among the binary systems containing 0.2 mM Ir(ppy)3 and 10 mM parent molecule of the radical ions, intense annihilation ECL of Ir(ppy)3 was observed from a solution containing BN; the ECL intensity was about 250 times greater than that of the single system of 0.2 mM Ir(ppy)3. Among the cases using the radical cations, the ECL intensity with TH was the largest (about 90 times higher than that of the single system). The mechanism for enhancing the ECL is discussed based on the redox potential, energy level of the lowest triplet excited (T1) state, and electron-donating and −accepting characteristics. The electron transfer reaction between the reduced form of Ir(ppy)3 (Ir(ppy)3•−) and the radical cations can produce the T1 state of Ir(ppy)3 (3Ir(ppy)3*) and that of MePT, TH, and DD, whereas the electron transfer reaction between the oxidized form of Ir(ppy)3 (Ir(ppy)3•+) and the radical anions can produce 3Ir(ppy)3* but not the T1 state of BN or BP. The lower ECL intensity using the radical cations relative to that of using BN suggests that the triplet energy transfer from the parent molecule of the radical cations to Ir(ppy)3 is not efficient. It is also shown that the homogeneous electron transfer reaction between Ir(ppy)3 and the electrochemically generated radical cation of TH and radical anion of BN to produce Ir(ppy)3•+ and Ir(ppy)3•−, respectively, which facilitates an ion annihilation reaction between Ir(ppy)3•− and Ir(ppy)3•+, is also responsible for the intense ECL.

Roles of Nonadiabatic Processes, Reaction Mechanism, and Selectivity in Cu-Catalyzed [2 + 2] Photocycloaddition of Norbornene and Acetone to Oxetane.

Oxetane has been extensively studied for its applications in medicinal chemistry and as a reactive intermediate in synthesis. Experiments report a Cu-catalyzed [2 + 2] photocycloaddition of acetone and norbornene to oxetane, which is proposed to deviate from the conventional Paternò-Büchi reaction. However, its mechanism at the atomic level is not clear. In this study, we used a combination of multistate complete active space second-order perturbation theory (MS-CASPT2) and density functional theory to systematically investigate the reaction mechanism and elucidate the factors contributing to the diastereomeric selectivity. Initially, the formation of the TpCu(Norb) complex is achieved by strong interaction between tris(pyrazolyl)borate Cu(I) (TpCu) and norbornene in the ground state (S0). Upon photoexcitation, TpCu(Norb) eventually decays to the T1 state, in which TpCu(Norb) attacks acetone to initiate subsequent reactions and produces final endo- or exo-oxetane products. All these reactions initially involve the C-C bond formation in the T1 state thereto leading to a ring-opening intermediate. This intermediate then undergoes a nonradiative transition to the S0 state, producing a five-membered ring intermediate, from which the C-O bond is formed, leading to the experimentally dominant exo-product. In contrast, the endo-oxetane formation requires a rearrangement process after the C-C bond is formed because of the large steric effects. As a consequence, the different reaction pathways generating exo- and endo-products exhibit large differences in the free-energy barriers, which results in a diastereomeric selectivity observed experimentally. Additionally, the nonradiative transition is found to play an important role in facilitating these reaction steps. The present computational study provides valuable mechanistic insights into Cu-catalyzed photocycloaddition reactions.

Unimolecular net heterolysis of symmetric and homopolar σ-bonds

The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including SN1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners1–3. This means that if a σ-bond consists of two identical groups (that is, symmetric σ-bonds), its unimolecular fission from the S0, S1, or T1 states only occurs homolytically after thermal or photochemical activation1–7. To force symmetric σ-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary4. These tactics are only applicable to σ-bond constituents susceptible to such polarizing effects, and often suffer from inefficient chemoselectivity in polyfunctional molecules. Here we report the net heterolysis of symmetric and homopolar σ-bonds (that is, those with similar electronegativity and equal leaving group ability3) by means of stimulated doublet–doublet electron transfer (SDET). As exemplified by Se–Se and C–Se σ-bonds, symmetric and homopolar bonds initially undergo thermal homolysis, followed by photochemically SDET, eventually leading to net heterolysis. Two key factors make this process feasible and synthetically valuable: (1) photoexcitation probably occurs in only one of the incipient radical pair members, thus leading to coincidental symmetry breaking8 and consequently net heterolysis even of symmetric σ-bonds. (2) If non-identical radicals are formed, each radical may be excited at different wavelengths, thus rendering the net heterolysis highly chemospecific and orthogonal to conventional heterolyses. This feature is demonstrated in a series of atypical SN1 reactions, in which selenides show SDET-induced nucleofugalities3 rivalling those of more electronegative halides or diazoniums.

Isomeric Effects of Au28(S-c-C6H11)20 Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State.

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au28i(CHT)20 and Au28ii(CHT)20. Based on their structures, analysis of 3O2 (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au28i(CHT)20, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au28ii(CHT)20 isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S1)-triplet (T1) gap in Au28ii than that in Au28i, and the higher T2 state plays a critical role in both isomers' photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.