Magnetic Control of Recombination Fluorescence and Tunability by Modulation of Radical Pair Energies in Rigid Donor–Bridge–Acceptor Systems

Synergistic Photochromism, Fluorescence Switching, and Photomagnetism of Three Mn(II) Complexes Based on a Thiazolothiazole Extended Viologen Derivative

Multifunctional photochromic hybrid materials have attracted great attention due to their wide prospects in information storage, molecular switches, and sensors. Herein, three new photochromic coordination polymers (CPs) with paramagnetic Mn2+ ions, namely [Mn2(TTVP)(m-BDC)2] (1), [Mn3(TTVP)(p-BDC)3(H2O)2]·0.5H2O (2), and [Mn(TTVP)(H2O)4]·(4,4'-BPC)·2H2O (3) have been synthesized (TTVP = 2,5-bis(pyridinium-4-yl)thiazolo[5,4-d]thiazole propionate, m-H2BDC = isophthalic acid, p-H2BDC = terephthalic acid, 4,4'-H2BPC = 4,4'-diphenyldicarboxylic acid). Interestingly, under Ultraviolet (UV) light irradiation, these compounds exhibit distinct photochromic performances due to photoinduced electron transfer (PIET) between aromatic carboxylic acids and TTVP, as validated by spectroscopic and structural analyses. The coloration kinetics and final states are finely tuned by modulating the number and strength of weak interactions between electron donors (EDs) and electron acceptors (EAs). Furthermore, these complexes exhibit photoinduced magnetization enhancement at room temperature, while complexes 2 and 3 exhibit reversible fluorescence modulation during the coloration-decoloration cycles. The introduction of photoregulated fluorescent and magnetism into PIET photochromic compounds presents a promising approach for the development of multifunctional materials, holding potential for a range of applications.

Microfluidic magnetic droplet-based chemiluminescence enzyme immunoassay for multiplex sepsis biomarker screening

Sepsis is a systemic inflammatory response syndrome caused by infection, requiring the joint detection of multiplex biomarkers for specific diagnosis. Here, we present a chemiluminescence enzyme immunoassay based on microfluidic magnetic droplets for multiplex sepsis biomarker screening. The droplet-based chemiluminescence enzyme immunoassay (CLIA) technology utilizes multicolor-encoded microspheres to distinguish biomarkers and mesoporous silica-loaded enzymes for signal amplification and catalytic fluorescent substrates. Additionally, digital immunoassays via Poisson distribution in the generated droplets provide a reliable quantitative strategy for detecting rare targets. This method achieves high sensitivity, low interference, and simultaneous detection with satisfactory specificity of various sepsis biomarkers, such as procalcitonin (PCT), interleukin-6 (IL-6), and C-reactive protein (CRP). These features demonstrate that the microfluidic droplet-based CLIA method has great potential for broader applications in multiplex biomolecule detection and early disease diagnosis.

Electrically Modulated Fluorescence in Single Rare-Earth Particles

Rare earth (RE)-based complexes, due to their unique electronic structures, exhibit excellent fluorescence properties with high quantum yield and a long lifetime. From an application perspective, exploring RE-based complexes in luminescent optoelectronic devices asks for effective modulation approaches that control the luminescent properties. Here we report an electrically modulated fluorescence phenomenon in an RE-based complex, namely Eu16(μ4-F)6(μ3-F)12(tBuCOO)18[N(CH2CH2O)3]4 (EuFC-16) particles, which effectively controls the optical behavior of individual particles. Frequency-dependent measurements and theoretical analysis reveal a charging mechanism on particles that rationalizes the voltage-modulated fluorescence. This charging-induced fluorescence modulation enables the localized capacitance mapping of individual RE particles at the single-pixel level. Moreover, modulation heterogeneity is observed within a single EuFC-16 particle, highlighting the importance of charge-distribution-controlled optical properties at the subparticle level. Our finding may offer a potential approach for controlling the luminescence of RE-based complexes with spatiotemporal controllability and potential scaling capability, which may enrich potential applications of RE-based electro-optical devices.

Molecular Engineering of Emissive Molecular Qubits Based on Spin-Correlated Radical Pairs

Spin chemistry of photogenerated spin-correlated radical pairs (SCRPs) offers a practical approach to control chemical reactions and molecular emissions by using weak magnetic fields. This capability to harness magnetic field effects (MFEs) paves the way for developing SCRPs-based molecular qubits. Here, we introduce a new series of donor-chiral bridge-acceptor (D-χ-A) molecules that demonstrate significant MFEs on fluorescence intensity and lifetime in solution at room temperature─critical for quantum sensing. By precisely tuning the donor site through torsional locking, distance extension, and planarization, we achieved remarkable control over key quantum properties, including field-response range and line width. In the most responsive systems, emission lifetimes increased by over 200%, and the total emission intensity was modulated by up to 30%. This level of tunability shows the power of synthetic spin chemistry. The rational design principle of optically addressable SCRP-based molecular systems, presented in this work, represents a major leap toward functional synthetic molecular qubits, advancing the field of molecular quantum technologies.

Amplifying Magnetic Field Effects on Upconversion Emission via Molecular Qubit-Driven Triplet-Triplet Annihilation

Triplet-triplet annihilation (TTA) enables photon upconversion by combining two lower-energy triplet excitons to produce a higher-energy singlet exciton. This mechanism enhances light-harvesting efficiency for solar energy conversion and enables the use of lower-energy photons in bioimaging and photoredox catalysis applications. The magnetic modulation of such high-energy excitons presents an exciting opportunity to develop molecular quantum information technologies. While the spin dynamics of triplet exciton pairs are sensitive to external magnetic fields, the magnetic field effects (MFEs) associated with these pairs are generally limited by spin statistics to at most 10% at low fields (<1 T), making them challenging to apply in technological advancements. In contrast, MFEs on spin-correlated radical pairs (SCRPs) can be significantly greater, surpassing those on triplet pairs. By using SCRPs-based molecular qubits as triplet sensitizers in the sensitized TTA scheme, we can magnetically modulate TTA and consequently, the delayed fluorescence of annihilators. In our current system, we have achieved more than 70% magnetic modulation of delayed fluorescence, effectively harnessing and even amplifying magnetic modulation within SCRPs to influence high-energy excitons. This work opens new opportunities for advancing spin-controlled chemical reactions and molecular quantum information technologies.

Photo-Induced Ultrafast Charge Transfer and Air-Stable Radical Formation in Tetraphenylpyrene Derivatives

Stable organic radicals generated by photo-excitation hold applications in molecular switching devices and information storage. It remains challenging to develop photo-generated radical materials with rapid response and air stability in the solid state. Here, we report a structure based on 1,3,6,8-tetraphenylpyrene derivative (Py-TTAc) displaying photo-induced radicals with air stability in the solid state. Photo-induced electron transfer, exposed to a 365 nm ultraviolet lamp for 1 minute, affords radicals in Py-TTAc powder as confirmed by electron paramagnetic resonance (EPR) spectroscopy. The maximum radical concentration reaches 2.21 % after continuous irradiation for 1 hour and recurs more than 10 times without any chemical degradation. The mechanistic study according to the femtosecond transient absorption (fsTA) and X-ray technology suggests that the radicals are derived from photo-induced symmetry-breaking charge separation (SB-CS) and stabilized through non-covalent interactions. The photo-generated stable radical system is employed in anti-counterfeiting paper and optoelectronic device applications. This study will provide insights into the development of photoactive organic radical materials.

Spin-related excited-state phenomena in photochemistry

The spin of electrons plays a vital role in chemical reactions and processes, and the excited state generated by the absorption of photons shows abundant spin-related phenomena. However, the importance of electron spin in photochemistry studies has been rarely mentioned or summarized. In this review, we briefly introduce the concept of spin photochemistry based on the spin multiplicity of the excited state, which leads to the observation of various spin-related photophysical properties and photochemical reactivities. Then, we focus on the recent advances in terms of light-induced magnetic properties, excited-state magneto-optical effects and spin-dependent photochemical reactions. The review aims to provide a comprehensive overview to utilize the spin multiplicity of the excited state in manipulating the above photophysical and photochemical processes. Finally, we discuss the existing challenges in the emerging field of spin photochemistry and future opportunities such as smart magnetic materials, optical information technology and spin-enhanced photocatalysis.

Magnetic manipulation of the reactivity of singlet oxygen: from test tubes to living cells

Although magnetism undoubtedly influences life on Earth, the science behind biological magnetic sensing is largely a mystery, and it has proved challenging, especially in the life sciences, to harness the interactions of magnetic fields (MFs) with matter to achieve specific ends. Using the well-established radical pair (RP) mechanism, we here demonstrate a bottom-up strategy for the exploitation of MF effects in living cells by translating knowledge from studies of RP reactions performed in vitro. We found an unprecedented MF dependence of the reactivity of singlet oxygen (1O2) towards electron-rich substrates (S) such as anthracene, lipids and iodide, in which [S ˙+ O2 ˙-] RPs are formed as a basis for MFs influencing molecular redox events in biological systems. The close similarity of the observed MF effects on the biologically relevant process of lipid peroxidation in solution, in membrane mimics and in living cells, shows that MFs can reliably be used to manipulate 1O2-induced cytotoxicity and cell-apoptosis-related protein expression. These findings led to a 'proof-of-concept' study on MF-assisted photodynamic therapy in vivo, highlighting the potential of MFs as a non-invasive tool for controlling cellular events.

Giant magneto-photoluminescence at ultralow field in organic microcrystal arrays for on-chip optical magnetometer

Optical detection of magnetic field is appealing for integrated photonics; however, the light-matter interaction is usually weak at low field. Here we observe that the photoluminescence (PL) decreases by > 40% at 10 mT in rubrene microcrystals (RMCs) prepared by a capillary-bridge assembly method. The giant magneto-PL (MPL) relies on the singlet-triplet conversion involving triplet-triplet pairs, through the processes of singlet fission (SF) and triplet fusion (TF) during radiative decay. Importantly, the size of RMCs is critical for maximizing MPL as it influences on the photophysical processes of spin state conversion. The SF/TF process is quantified by measuring the prompt/delayed PL with time-resolved spectroscopies, which shows that the geminate SF/TF associated with triplet-triplet pairs are responsible for the giant MPL. Furthermore, the RMC-based magnetometer is constructed on an optical chip, which takes advantages of remarkable low-field sensitivity over a broad range of frequencies, representing a prototype of emerging opto-spintronic molecular devices.

Single-Molecule Magnetoluminescence from a Spatially Confined Persistent Diradical Emitter

Luminescent radicals are an emerging class of materials that exhibit unique photofunctions not found in closed-shell molecules due to their open-shell electronic structure. Particularly promising are photofunctions in which radical's spin and luminescence are correlated; for example, when a magnetic field can affect luminescence (i.e., magnetoluminescence, ML). These photofunctions could be useful in the new science of spin photonics. However, previous observations of ML in radicals have been limited to systems in which radicals are randomly doped in host crystals or polymerized through metal complexation. This study shows that a covalently linked luminescent radical dimer (diradical) can exhibit ML as a single-molecular property. This facilitates detailed elucidation of the requirements for and mechanisms of ML in radicals and can aid the rational design of ML-active radicals based on synthetic chemistry.

Control and Modulation of Photoinduced Charge Transfer in a Rigid Donor-Bridge-Acceptor System by Electric Fields

This article theoretically studies the photoinduced charge transfer (CT) of rigid D-B-A molecules in two-photon absorption (TPA) adjusted by the external electric fields. Using a visualization method, the dynamic changes of light-induced CT in different channels of TPA are presented through a two-dimensional (2D) transition density matrix and a three-dimensional (3D) charge different density. Here, we prove the controllability of TPA on CT under the induction of a strong electric field. Adjusting the field direction and intensity significantly affects the position of the strong absorption peak in the TPA spectra, thereby further changing the electron-hole coherence length and the degree of dispersion. Our results can promote the recognition of the optical properties of the D-B-A system in synthetic molecules and provide an idea for increasing the proportion of excited states for CT in the molecule.

Readout of spin quantum beats in a charge-separated radical pair by pump-push spectroscopy

[Figure: see text].

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.