A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet

Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet–triplet energy gaps (ΔE_ST) over 0.59 kcal mol⁻¹ (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu₂-PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet–triplet energy gap ΔE_ST = 0.94 kcal mol⁻¹ (473 K). This is a rare example of stable diradicals with singlet–triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu₂-PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K2, single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.

We report a triplet diradical dianion in magnesium complex with ΔE_ST = 0.94 kcal mol⁻¹ (473 K). Its iron analog is the first single-molecule magnet bridged by a diradical dianion, and the SMM property is switched off through two-electron reduction.     

A cyclopenta-fused dibenzo[b,d]thiophene-co-phenanthrene macrocyclic tetraradicaloid

A cyclopenta-fused macrocyclic tetraradicaloid, MC4-S, containing alternating phenanthrene (Phen) and dibenzo[b,d]thiophene (DBTh) units was synthesized and isolated in single-crystal form. Compared with its all-carbon isoelectronic structure, CPTP-M, the incorporation of two sulfur atoms leads to a smaller radical character and a larger singlet–triplet energy gap. X-ray crystallographic analysis reveals that the spin–spin coupling through the DBTh unit is stronger than that through the Phen moiety. In addition, the electron-rich sulfur atoms also raise the energies of both the HOMO and LUMO in MC4-S, but the overall optical and electronic energy gaps are close to that of the CPTP-M. MC4-S displays global anti-aromaticity according to the NMR measurements and theoretical calculations (NICS, ACID and 2D ICSS), with a 36π ring current circuit along the all-carbon periphery excluding the two sulphur atoms. Its dication becomes globally aromatic due to the existence of a dominant 34π-conjugation pathway. This study sheds some light on the effect of heteroatoms on the electronic properties of open-shell polyradicaloids.

The first member of sulfur-heterocycloarene neutral tetraradicaloids, MC4-S, was synthesized in crystalline form, which displays strong global anti-aromaticity and unique properties.     

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells.     

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Summary Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the³ B _1u state takes place predominantly through vibronic coupling along thee _2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, best experimental value for the radiative lifetime of triplet benzene; 30 seconds.Dedicated to Inga Fischer-Hjalmars on her 75th birthday     

New Cy5 photosensitizers for cancer phototherapy: a low singlet–triplet gap provides high quantum yield of singlet oxygen

Highly efficient triplet photosensitizers (PSs) have attracted increasing attention in cancer photodynamic therapy where photo-induced reactive oxygen species (ROSs, such as singlet oxygen) are produced via singlet–triplet intersystem crossing (ISC) of the excited photosensitizer to kill cancer cells. However, most PSs exhibit the fatal defect of a generally less-than-1% efficiency of ISC and low yield of ROSs, and this defect strongly impedes their clinical application. In the current work, a new strategy to enhance the ISC and high phototherapy efficiency has been developed, based on the molecular design of a thio-pentamethine cyanine dye (TCy5) as a photosensitizer. The introduction of an electron-withdrawing group at the meso-position of TCy5 could dramatically reduce the singlet–triplet energy gap (ΔE_st) value (from 0.63 eV to as low as 0.14 eV), speed up the ISC process (τ_ISC = 1.7 ps), prolong the lifetime of the triplet state (τ_T = 319 μs) and improve singlet oxygen (¹O₂) quantum yield to as high as 99%, a value much higher than those of most reported triplet PSs. Further in vitro and in vivo experiments have shown that TCy5-CHO, with its efficient ¹O₂ generation and good biocompatibility, causes an intense tumor ablation in mice. This provides a new strategy for designing ideal PSs for cancer photo-therapy.

The electron-withdrawing group at the meso-position of Thio-Cy5 could dramatically reduce the singlet–triplet energy gap, and speed up the intersystem crossing process.     

Molecular ion LiHe: ab initio study

High level ab initio calculations are performed on the molecular ion LiHe⁺. Potential energy curves for the low-lying singlet and triplet electronic states are calculated using the multi-reference configuration interaction and single-reference coupled cluster methods with large basis sets. The corresponding dipole moments and transition dipole moments functions are also determined. The basic spectroscopic properties and excitation energies of the electronic states are derived from rovibrational bound state calculations.     

The phosphorescent state of dibenzothiophene

The ESR study of phosphorescent o,o'-bridged biphenyls has shown that the nature of the heteroatom has little effect on the molecular properties of the lowest triplet state of these molecules. This observation allows us to assign the symmetry of this state and to propose an electronic scheme for the phosphorescence in this family of aromatic compounds.     

Phosphorescence sensitization via heavy-atom effects in d10 complexes

Heavy atom-induced phosphorescence of organic chromophores that originates from spin?Corbit coupling (SOC) is always accompanied by fluorescence quenching concomitant with a reduction of the triplet excited state lifetime. However, such changes are typically manifest by fluorescence quenching at room temperature and phosphorescence sensitization at cryogenic temperatures. Herein we overview our efforts over the past decade in which both internal and external heavy-atom effects (HAEs) can trigger room temperature phosphorescence (RTP) with dramatic shortening of the phosphorescence radiative lifetime by several orders of magnitude. Such spectral properties render new classes of phosphorescent materials for potential use in organic light-emitting diodes (OLEDs). The molecular systems described in this paper are organic fluorophores that are ??-complexed or ??-bonded to a multinuclear d¹⁰ transition metal center, the presence of which leads to phosphorescence sensitization because of the significant SOC in such materials.     

Phosphorescence quenching by conjugated polymers

Energy transfer between phosphors and conjugated polymers was investigated using a fluorene trimer (F3) as a model conjugated material. The phosphors studied were bis-cyclometalated iridium complexes (FP, PPY, BT, PQ, and BTP), with triplet energies of 2.6, 2.4, 2.2, 2.1, and 2.0 eV, respectively (based on phosphorescence spectra). Stern-Volmer analysis of luminescent quenching shows that energy transfer from either FP or PPY to F3 is an exothermic process with Stern-Volmer quenching constants (kqSV) of near 109 M-1 s-1 while energy transfer from BT, PQ, and BTP is endothermic (kqSV = 107-106 M-1 s-1). On the the basis of above results, the triplet energy of F3 is estimated to be less than 2.3 eV (530 nm). This study suggests that conjugated polymers, which typically have lower T1 energies than F3, should also quench phosphorescent emission in thin films and organic light-emitting diodes (OLEDs) incorporating these and related phosphorescent dopants.     

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y_xSc_3−xN@C₈₀ (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁–T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Photoexcitation mechanism of Y_xSc_3−xN@C₈₀ metallofullerenes is studied by variable-temperature photoluminescence, advanced EPR techniques, and DFT calculations, revealing photoinduced rotation of the endohedral cluster.     

In silico prediction of annihilators for triplet–triplet annihilation upconversion via auxiliary-field quantum Monte Carlo

The energy of the lowest-lying triplet state (T1) relative to the ground and first-excited singlet states (S0, S1) plays a critical role in optical multiexcitonic processes of organic chromophores. Focusing on triplet–triplet annihilation (TTA) upconversion, the S0 to T1 energy gap, known as the triplet energy, is difficult to measure experimentally for most molecules of interest. Ab initio predictions can provide a useful alternative, however low-scaling electronic structure methods such as the Kohn–Sham and time-dependent variants of Density Functional Theory (DFT) rely heavily on the fraction of exact exchange chosen for a given functional, and tend to be unreliable when strong electronic correlation is present. Here, we use auxiliary-field quantum Monte Carlo (AFQMC), a scalable electronic structure method capable of accurately describing even strongly correlated molecules, to predict the triplet energies for a series of candidate annihilators for TTA upconversion, including 9,10 substituted anthracenes and substituted benzothiadiazole (BTD) and benzoselenodiazole (BSeD) compounds. We compare our results to predictions from a number of commonly used DFT functionals, as well as DLPNO-CCSD(T₀), a localized approximation to coupled cluster with singles, doubles, and perturbative triples. Together with S1 estimates from absorption/emission spectra, which are well-reproduced by TD-DFT calculations employing the range-corrected hybrid functional CAM-B3LYP, we provide predictions regarding the thermodynamic feasibility of upconversion by requiring (a) the measured T1 of the sensitizer exceeds that of the calculated T1 of the candidate annihilator, and (b) twice the T1 of the annihilator exceeds its S1 energetic value. We demonstrate a successful example of in silico discovery of a novel annihilator, phenyl-substituted BTD, and present experimental validation via low temperature phosphorescence and the presence of upconverted blue light emission when coupled to a platinum octaethylporphyrin (PtOEP) sensitizer. The BTD framework thus represents a new class of annihilators for TTA upconversion. Its chemical functionalization, guided by the computational tools utilized herein, provides a promising route towards high energy (violet to near-UV) emission.

Electronic structure theories such as AFQMC can accurately predict the low-lying excited state energetics of organic chromophores involved in triplet–triplet annihilation upconversion. A novel class of benzothiadiazole annihilators is discovered.     

Photosensitization mechanisms at the air–water interface of aqueous aerosols

Photosensitization reactions are believed to provide a key contribution to the overall oxidation chemistry of the Earth''s atmosphere. Generally, these processes take place on the surface of aqueous aerosols, where organic surfactants accumulate and react, either directly or indirectly, with the activated photosensitizer. However, the mechanisms involved in these important interfacial phenomena are still poorly known. This work sheds light on the reaction mechanisms of the photosensitizer imidazole-2-carboxaldehyde through ab initio (QM/MM) molecular dynamics simulations and high-level ab initio calculations. The nature of the lowest excited states of the system (singlets and triplets) is described in detail for the first time in the gas phase, in bulk water, and at the air–water interface, and possible intersystem crossing mechanisms leading to the reactive triplet state are analyzed. Moreover, the reactive triplet state is shown to be unstable at the air–water surface in a pure water aerosol. The combination of this finding with the results obtained for simple surfactant-photosensitizer models, together with experimental data from the literature, suggests that photosensitization reactions assisted by imidazole-2-carboxaldehyde at the surface of aqueous droplets can only occur in the presence of surfactant species, such as fatty acids, that stabilize the photoactivated triplet at the interface. These findings should help the interpretation of field measurements and the design of new laboratory experiments to better understand atmospheric photosensitization processes.

First-principles molecular dynamics simulations of imidazole-2-carboxaldehyde at the air–water interface highlight the role of surfactants in stabilising the reactive triplet state involved in photosensitisation reactions in aqueous aerosols.     

The electronic states of buta-1,3-diene studied by ab initio configuration interaction and DFT methods,and electron energy loss spectroscopy

The electronic vertical excitation energies for singlet and triplet valence, and Rydberg states of trans-buta-1,3-diene have been computed using ab initio multi-reference multi-root CI procedures with a [4s3p3d3f] set of Rydberg functions. Close numerical agreement between theory and experiment was found for a number of low-lying electronic states.     

Protein structure prediction using a combination of sequence homology and global energy minimization: II. Energy functions

A protein energy surface is constructed. Validation is through applications of global energy minimization to surface loops of protein crystal structures. For 9 of 10 predictions, the native backbone conformation is identified correctly. Electrostatic energy is modeled as a pairwise sum of interactions between anisotropic atomic charge densities. Model repulsion energy has a softness similar to that seen in ab initio data. Intrinsic torsional energy is modeled as a sum over pairs of adjacent torsion angles of 2-dimensional Fourier series. Hydrophobic energy is that of a hydration shell model. The remainder of hydration free energy is obtained as the energetic effect of a continuous dielectric medium. Parameters are adjusted to reproduce the following data: a complete set of ab initio energy surfaces, meaning one for each pair of adjacent torsion angles of each blocked amino acid; experimental crystal structures and sublimation energies for nine model compounds; ab initio energies over 1014 conformations of 15 small-molecule dimers; and experimental hydration free energies for 48 model compounds. All ab initio data is at the Hartree–Fock/6–31G* level. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 548–573, 1998     

Correlation of the zero-field splittings with the n,π* and π,π* triplet levels of benzaldehydes

The n,π^* and π,π^* triplet state energies of p-chlorobenzaldehyde and p-mathoxybenzaldehyde were determined in several hosts with the aid of phosphorescence and phosphorescence excitation spectra. A linear relationship expected from the theory considering spin-orbit interaction between the closely located n,π^* and π,π^* triplet states was found to be satisfied. The spin—orbit interaction parameter, <vb>G₂>vb>² was found to be 83 cm^?2 for benzaldehydes.     

Multivalent supramolecular assembly with ultralong organic room temperature phosphorescence,high transfer efficiency and ultrahigh antenna effect in water

Multivalent supramolecular assemblies have recently attracted extensive attention in the applications of soft materials and cell imaging. Here, we report a novel multivalent supramolecular assembly constructed from 4-(4-bromophenyl)pyridine-1-ium bromide modified hyaluronic acid (HABr), cucurbit[8]uril (CB[8]) and laponite® clay (LP), which could emit purely organic room-temperature phosphorescence (RTP) with a phosphorescence lifetime of up to 4.79 ms in aqueous solution via multivalent supramolecular interactions. By doping the organic dyes rhodamine B (RhB) or sulfonated rhodamine 101 (SR101) into the HABr/CB[8]/LP assembly, phosphorescence energy transfer was realized with high transfer efficiency (energy transfer efficiency = 73–80%) and ultrahigh antenna effect (antenna effect value = 308–362) within the phosphorescent light harvesting system. Moreover, owing to the dynamic nature of the noncovalent interactions, a wide-range spectrum of phosphorescence energy transfer outputs could be obtained not only in water but also on filter paper and a glass plate by adjusting the donor–acceptor ratio and, importantly, white-light emission was obtained, which could be used in the application of information encryption.

An ultralong lifetime supramolecular assembly was constructed via multivalent supramolecular interactions and achieved phosphorescence light harvesting. Multicolor (including white) broad-spectrum outputs could be achieved in water and also on filter paper and a glass plate.     

Electron spin resonance and phosphorescence of 4-phenylpyridine, 4,4′-bipyridine and their protonated cations in the triplet states

ESR and phosphorescence of phosphorescent triplet states of 4-phenylpyridine, 4,4′-bipyridine and their singly pretonated cations have been studied in methanol-water (4: 1 by volume) and in stretched poly(vinyl alcohol) films at 77 K. The stretched-film method is useful for ESR assignments of aromatic bases whose triplet states are influenced by pH.     

Highly sensitive photoalignment of calamitic and discotic liquid crystals assisted by axis-selective triplet energy transfer

Highly sensitive photoalignment of liquid crystals (LCs) can be realized by axis-selective triplet energy transfer. Addition of a triplet photosensitizer (phosphorescent donor) into a photocrosslinkable polymer tethering E-cinnamate side chains ensures dramatic enhancement of photosensitivity to generate the optical anisotropy of polymer film and surface-assisted LC photoalignment. Photoirradiation of triplet photosensitizer-doped polymer films with linearly polarized 365 nm light for the selective excitation of triplet sensitizer gives rise to optical anisotropy of cinnamates as a result of axis-selective triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin's formula, we find that triplet energy transfer is efficient within a radius of ～0.3 nm from the triplet photosensitizer. Such photoaligned polymer films can be used for the surface-assisted orientation photocontrol of not only calamitic LC, but also discotic LC, even for extremely low exposure energies. The present procedure would be greatly advantageous for high-throughput fabrication of optical devices by photoalignment techniques.     

Manipulating exciton dynamics of thermally activated delayed fluorescence materials for tuning two-photon nanotheranostics

Rational manipulation of energy utilization from excited-state radiation of theranostic agents with a donor–acceptor structure is relatively unexplored. Herein, we present an effective strategy to tune the exciton dynamics of radiative excited state decay for augmenting two-photon nanotheranostics. As a proof of concept, two thermally activated delayed fluorescence (TADF) molecules with different electron-donating segments are engineered, which possess donor–acceptor structures and strong emissions in the deep-red region with aggregation-induced emission characteristics. Molecular simulations demonstrate that change of the electron-donating sections could effectively regulate the singlet–triplet energy gap and oscillator strength, which promises efficient energy flow. A two-photon laser with great permeability is used to excite TADF NPs to perform as theranostic agents with singlet oxygen generation and fluorescence imaging. These unique performances enable the proposed TADF emitters to exhibit tailored balances between two-photon singlet oxygen generation and fluorescence emission. This result demonstrates that TADF emitters can be rationally designed as superior candidates for nanotheranostic agents by the custom controlling exciton dynamics.

Exciton dynamics can be manipulated rationally in the design of TADF materials for nanotheranostics. Regulating the ΔE_ST and f promises efficient energy flow for tailoring balances between singlet oxygen generation and fluorescence emission.     

Scanning‐Tunneling‐Spectroscopy‐Directed Design of Tailored Deep‐Blue Emitters

Frontier molecular orbitals can be visualized and selectively set to achieve blue phosphorescent metal complexes. For this purpose, the HOMOs and LUMOs of tridentate Pt^II complexes were measured using scanning tunneling microscopy and spectroscopy. The introduction of electron‐accepting or ‐donating moieties enables independent tuning of the frontier orbital energies, and the measured HOMO–LUMO gaps are reproduced by DFT calculations. The energy gaps correlate with the measured and the calculated energies of the emissive triplet states and the experimental luminescence wavelengths. This synergetic interplay between synthesis, microscopy, and spectroscopy enabled the design and realization of a deep‐blue triplet emitter. Finding and tuning the electronic “set screws” at molecular level constitutes a useful experimental method towards an in‐depth understanding and rational design of optoelectronic materials with tailored excited state energies and defined frontier‐orbital properties.