Long‐Lived Charge‐Transfer State Induced by Spin‐Orbit Charge Transfer Intersystem Crossing (SOCT‐ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

Superior Circularly Polarized Luminescence Brightness Achieved with Chiral Heteroleptic Nine-Coordinate Coumarin-Based Tb3+ Complexes

In order to facilitate the practical application of circularly polarized luminescence (CPL) active molecules, the CPL brightness (BCPL) must be optimized. We have applied a binary modular strategy to synthesize two chiral organo-Tb3+ complexes, [Tb(Coum)3(1R,2R-Ph-PyBox)] (2) and [Tb(Coum)3(1S,2S-Ph-PyBox)] (5), combining 3-acetyl-4-hydroxy-coumarin (Coum) and enantiopure 2,6-bis(4-phenyl-2-oxazolin-2-yl) pyridine (1R,2R/1S,2S-Ph-PyBox). The photophysical properties of these novel complexes have been fully characterized. The combined point-chiral induction capability of chiral bis(oxazoline) derivatives and the outstanding photophysical properties of the coumarin-derived ligand have resulted in an intense excited-state chiroptical activity (|glum| = 0.097-0.103) for both Tb3+ enantiomers, with a bright Tb3+-centered high-purity green emission (ΦPL = 74%) and enhanced antenna-centered absorption behavior (ε320 nm = 47820-47940 M-1 cm-1). A superior BCPL (1132.7-1205.8 M-1 cm-1 at 5D4 → 7F5) has been established for complexes 2 and 5. The strategy adopted in this work provides a new route to chiroptical organo-Tb3+ luminophores with outstanding comprehensive performance.

Solvent Effects on Spin-Orbit Charge-Transfer Intersystem Crossing in Aryl-Substituted Boron-dipyrromethene Donor-Acceptor Dyads

In heavy-atom-free organic molecules, the rate of triplet generation through charge recombination, as dictated by the El-Sayed rule, can be enhanced by 101-102 times compared with the rate of spontaneous spin flipping between π-π* orbitals. This mechanism is known as the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Within the framework of the SOCT-ISC mechanism, facilitating the generation of charge-separated (CS) states and suppressing the spin-allowed direct charge recombination to the ground state are pivotal for maximizing the efficiency of generating localized triplet states. Herein, a series of orthogonal aryl-substituted boron-dipyrromethene dyads were studied by time-resolved spectroscopy to unravel the multichannel competitive relationships in the SOCT-ISC mechanism. The energy level of the electron donor and the stabilization of the solvent effect to the charge-transfer state are reflected in the Gibbs free energy changes of the electron transfer and recombination reactions, leading to significantly different triplet quantum yields. Additionally, solvation-induced electronic coupling changes in excited states lead to the fact that the spin-allowed charge recombination rate cannot be well simply predicted by the Marcus inverted region but has to consider the specific excited-state dynamics in optimizing the proportion of triplet generation channels based on charge recombination.

Earth Mover's Charge Transfer Distance: A General and Robust Approach for Describing Excited State Locality

A novel approach for assessing the extent of electron displacement in optical transitions is proposed by implementing the Earth Mover's Distance (EMD) method, which quantifies the spatial dissimilarity between ground and excited state electron density distributions. In contrast to previous descriptors, this index provides a representative and intuitively understandable distance under a robust and computationally efficient scheme for all possible forms of locality, even in the most difficult to dissect topological cases. The theoretical differences among the existing indices and our method are first illustrated with the help of a simplified model system, followed by a benchmarking of several partial atomic charge models using experimentally relevant push-pull compounds with diverse symmetries. These same molecules are finally employed to further demonstrate the principal advantages of the EMD index and its capabilities in rationalizing charge transfer phenomena.

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone

In this work, we prepared a series of electron donor-acceptor systems based on spiro[fluorene-9,7'-dibenzo[c,h]acridine]-5'-one (SFDBAO). Our SFDBAOs consist of orthogonally positioned fluorenes and aromatic ketones. By fine-tuning the substitution of electron-donating pyrenes, the complex interplay among different excited-state decay channels and the overall impact of solvents on these decay channels were uncovered. Placing pyrene, for example, at the aromatic ketones resulted in a profound solvatochromism in the form of a bright charge-transfer (CT) emission spanning from yellow to red-NIR. In contrast, a dark non-emissive CT was noted upon pyrene substitution at the fluorenes. In apolar solvents, efficient triplet-excited state generation was observed for all SFDBAOs. Either charge transfer was concluded to mediate the intersystem crossing (ISC) in the case of pyrene substitution or the El-Sayed rule was applicable when lacking pyrene substitution as in the case of SFABAO. In polar solvents, charge separation is the sole decay upon pyrene substitution. Moreover, competition between ISC and CT lowered the triplet-excited state generation in SFDBAO.

A Silicon-Rhodamine-Based Heavy-Atom-Free Photosensitizer for Mitochondria-targeted Photodynamic Therapy

Silicon-xanthene derivatives (SiXs) have gained popularity in the field of bioimaging due to their advantageous far-red to near-infrared (NIR) absorption and emission wavelengths, notable brightness (ε × Φ), inherent mitochondrial targeting properties and high photo-stability, making them an excellent candidate for photodynamic therapy (PDT). Nevertheless, the utilization of SiXs as photosensitizers (PSs) for PDT in cancer treatment remains largely unexplored, primarily due to their limited capacity to generate cytotoxic reactive oxygen species (ROS). However, the potential of SiXs in PDT warrants further investigation. In this study, utilizing the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC) mechanism, we reported one novel heavy-atom-free, mitochondria-targeted, silicon-rhodamine-based photosensitizer (SiR-PXZ), which demonstrated excellent biocompatibility, minimal dark toxicity, favorable water-solubility and stability, and considerable singlet oxygen quantum yield under 660 nm light irradiation (ΦΔ = 0.16 in air-saturated PBS). Moreover, SiR-PXZ could be rapidly taken up by the mitochondria and efficiently induced apoptosis of cancer cells with an IC50 value of 1.2 μM. The in vivo studies showed that SiR-PXZ exhibited excellent anti-tumor effects, making it potentially valuable for clinical application. This study offers a source of ideas for the construction of SiXs-based photosensitizers for photodynamic cancer treatment in the future.

One-Pot Synthesis and Excited-State Dynamics of Null Exciton-Coupled Diketopyrrolopyrroles Oligo-Grids

Exciton coupling in molecular aggregates plays a vital role in impacting and fine-tuning optoelectronic materials and their efficiencies in devices. A versatile platform to decipher aggregation-property relationships is built around multichromophoric architectures. Here, a series of cyclic diketopyrrolopyrrole (DPP) oligomers featuring nanoscale gridarene structures and rigid bifluorenyl spacers are designed and synthesized via one-pot Friedel-Crafts reaction. DPP dimer [2]Grid and trimer [3]Grid, which are cyclic rigid nanoarchitectures of rather different sizes, are further characterized via steady-state and time-resolved absorption and fluorescence spectroscopies. They exhibit monomer-like spectroscopic signatures in the steady-state measurements, from which null exciton couplings are derived. Moreover, in an apolar solvent, high fluorescence quantum yields and excited-state dynamics that resembled DPP monomer are gathered. In a polar solvent, the localized singlet excited state on a single DPP dissociates into the adjacent null coupling DPP with charge transfer characteristics. This pathway facilitates the evolution of the symmetry-broken charge-separated state (SB-CS). Notable is the fact that the SB-CS of [2]Grid is, on one hand, in equilibrium with the singlet excited state and promotes, on the other hand, the formation of the triplet excited state with a yield of 32% via charge recombination.

Highly Twisted Conformation Thiopyrylium Photosensitizers for In Vivo Near Infrared-II Imaging and Rapid Inactivation of Coronavirus

Despite significant effort, a majority of heavy-atom-free photosensitizers have short excitation wavelengths, thereby hampering their biomedical applications. Here, we present a facile approach for developing efficient near-infrared (NIR) heavy-atom-free photosensitizers. Based on a series of thiopyrylium-based NIR-II (1000-1700 nm) dyads, we found that the star dyad HD with a sterically bulky and electron-rich moiety exhibited configuration torsion and significantly enhanced intersystem crossing (ISC) compared to the parent dyad. The electron excitation characteristics of HD changed from local excitation (LE) to charge transfer (CT)-domain, contributing to a ≈6-fold reduction in energy gap (ΔEST ), a ≈10-fold accelerated ISC process, and a ≈31.49-fold elevated reactive oxygen species (ROS) quantum yield. The optimized SP@HD-PEG2K lung-targeting dots enabled real-time NIR-II lung imaging, which precisely guided rapid pulmonary coronavirus inactivation.

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor-Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

The photophysical processes involved in the electron donor-acceptor thermally activated delayed fluorescence (TADF) emitters are complicated and controversial. The recent consensus is that at least three states are involved, i. e. the singlet charge transfer state (¹ CT), the triplet localized excited state (³ LE) and the triplet CT state (³ CT). It is clear the very often used steady state and time-resolved luminescence spectroscopic methods are unable to present direct evidence for the dark states, i. e. the ³ LE and ³ CT states, as well as the interconversion of these states. Concerning this aspect, the femtosecond-nanosecond transient absorption spectroscopic methods are in particular interests. Both the emissive state and the dark state can be detected in these spectra, and interconversion of the states involved in TADF process can be also revealed. This review article focuses on the recent development of using the transient absorption spectra to study the photophysics of the TADF emitters.

Regioisomeric Benzidine-Fullerenes: Tuning of the Diverse Hole-Distribution to Influence Charge Separation Patterns

Understanding the influence of molecular structure on charge distribution and charge separation (CS) provides essential guidance for optoelectronic materials design. Here we propose a regioisomeric strategy to tune the diverse hole-distribution, and probe the influence on CS patterns. Para-, meta- and ortho-substituted benzidine-fullerene, named 1 p, 1 m and 1 o are designed. Following CS, hole-delocalization occurs in 1 p, while hole-localization exists in 1 m and 1 o. The rates of charge separation (4.02×10¹¹  s^-1 ) and recombination (9.8×10⁹  s^-1 ) of 1 p is about 20 and 12 times faster than 1 m and 1 o, indicating that para-determined delocalization promotes ultrafast CS, while meta- and ortho-generated localization contributes to long-lived CS states. Computational analysis further implies that localization results from the destruction of electronic conjugation for 1 m, and limitation of conformational relaxation for 1 o. Given that the universality and simplicity of regional isomerism, this work opens up new thoughts for molecular design with tunable charge separation patterns.

Charge Transfer, Intersystem Crossing, and Electron Spin Dynamics in a Compact Perylenemonoimide-Phenoxazine Electron Donor-Acceptor Dyad

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor-acceptor dyad (PMI-PXZ) to study the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S₀ → ¹CT transition, was observed (ε = 2840 M^-1 cm^-1 at 554 nm, FWHM: 2850 cm^-1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI-PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τ_T = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (Φ_Δ = 76%) is higher than that of the previously reported analogue dyad PMI-PTZ (Φ_Δ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI-PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor-acceptor dyads.

Near-Unity Triplet Generation Promoted via Spiro-Conjugation

An intersystem crossing (ISC) rate constant of 1.0×10¹¹  s^-1 was previously registered with a spiro-bis-benzophenone scaffold. Triplet generation efficiency could be further enhanced by stabilizing the spiro-charge-transfer (CT) state and rationally designing spiro-compounds (SCTs) that consist of electron-rich diphenyl ether as the spiro-CT donor and electron-deficient dinaphthyl ketone as the spiro-CT acceptor. Through fine-tuning of the energy level between the CT and high energy triplet states, near-unity triplet generation quantum yield was achieved and the underlying ISC mechanism is revealed by using ultrafast spectroscopy and quantum chemical calculations. Potential triplet sensitizing application was demonstrated in SCTs. Our findings suggest that a spiro-bichromophoric molecular system with an enhanced spiro-charge transfer warrants efficient triplet generation and is a powerful strategy of heavy-atom-free triplet sensitizers with predictable ISC properties.

Intersystem Crossing and Electron Spin Selectivity in Anthracene-Naphthalimide Compact Electron Donor-Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (Φ_Δ =92 %, τ_T =438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (¹ CT→S₀ , takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (Φ_Δ =40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the ³ An and ³ NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet-triplet annihilation upconversion (TTA UC). High UC quantum yield (Φ_UC =12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.