Charge Transfer from Donor to Acceptor in Conjugated Microporous Polymer for Enhanced Photosensitization

Photosensitization associated with light absorption and energy/electron-transfer represents the central processes for photosynthesis. However, it's still a challenge to develop a heavy-atom-free (HAF) strategy to improve the sensitizing ability of polymeric photosensitizers. Herein, we propose a new protocol to significantly improve the photosensitization by decorating mother conjugated microporous polymer (CMP-1) with polycyclic aromatic hydrocarbons (PAHs), resulting in a series of CMPs (CMP-2-4). Systematic study reveals that covalent modification with PAHs can transfer charge to Bodipy in CMP to further facilitate both intersystem crossing and electron-hole separation, which can dramatically boost energy-/electron-transfer reactions. Remarkably, CMP-2 as a representative CMP can efficiently drive the photosynthesis of methyl phenyl sulfoxide with 92 % yield, substantially higher than that of CMP-1 (32 %). Experiments and theory calculations demonstrate the structure-property-activity relationship of these CMPs, opening a new horizon for developing HAF heterogeneous photosensitizers with highly efficient sensitizing activity by rational structure regulation at a molecular level.

The tumor phototherapeutic application of nanoparticles constructed by the relationship between PTT/PDT efficiency and 2,6- and 3,5-substituted BODIPY derivatives

BODIPY dyes have recently been used for photothermal and photodynamic therapy of tumors. However, complex multi-material systems, multiple excitation wavelengths and the unclear relationship between BODIPY structures and their PTT/PDT efficiency are still major issues. In our study, nine novel BODIPY near-infrared dyes were designed and successfully synthesized and then, the relationships between BODIPY structures and their PTT/PDT efficiency were investigated in detail. The results showed that modifications at position 3,5 of the BODIPY core with conjugated structures have better effects on photothermal and photodynamic efficiency than the modifications at position 2,6 with halogen atoms. Density functional theory (DFT) calculations showed that this is mainly due to the extension of the conjugated chain and the photoinduced electron transfer (PET) effect. By encapsulating BDPX-M with amphiphilic DSPE-PEG₂₀₀₀-RGD and lecithin, the obtained NPs not only show good water solubility and biological stability, but also could act as superior agents for photothermal and photodynamic synergistic therapy of tumors. Finally, we obtained BODIPY NPs that exhibited excellent photothermal and photodynamic effects at the same time under single irradiation with an 808 nm laser (photothermal conversion efficiency: 42.76%, A/A₀: ∼0.05). In conclusion, this work provides a direction to design and construct phototherapeutic nanoparticles based on BODIPY dyes for tumor treatment.

Strategic Construction of Sulfur-Bridged BODIPY Dimers and Oligomers as Heavy-Atom-Free Photosensitizers

An efficient strategy for building sulfur-bridged oligo-BODIPYs based on the S_NAr reaction is described. These oligo-BODIPYs showed broadband and strong visible-near-infrared (NIR) light absorption, strong intramolecular exciton coupling, and efficient intersystem crossing (ISC). Generation of ¹O₂ as well as O₂^•- under irradiation was found to give high reactive oxygen species generation efficiencies for those oligomers.

Photocatalysis in Supramolecular Fluorescent Metallacycles and Metallacages

The utilization of photocatalytic techniques for achieving light-to-fuel conversion is a promising way to ease the shortage of energy and degradation of the ecological environment. Fluorescent metallacycles and metallacages have drawn considerable attention and have been used in widespread fields due to easy preparation and their abundant functionality including photocatalysis. This review covers recent advances in photocatalysis in discrete supramolecular fluorescent metallacycles and metallacages. The developments in the utilization of the metallacycles skeletons and the effect of fluorescence-resonance energy transfer for photocatalysis are discussed. Furthermore, the use of the ligands decorated by organic chromophores or redox metal sites in metallacages as photocatalysts and their ability to encapsulate appropriate catalytic cofactors for photocatalysis are summarized. For the sake of brevity, macrocycles and cages with inorganic coordination complexes such as ruthenium complexes and iridium complexes are not included in this minireview.

Controlling the triplet states and their application in external stimuli-responsive triplet-triplet-annihilation photon upconversion: from the perspective of excited state photochemistry

The property of organic light-responsive materials is determined by their electronic excited states to a large extent, for instance, the radiative decay rate constants, redox potentials, and lifetimes. Tuning the excited state properties with external stimuli will lead to versatile functional materials; a representative example is the fluorescence molecular probes, in which the singlet excited states are controlled by the external stimuli, i.e., by interaction with the analytes. In comparison, controlling the triplet excited state with external stimuli has been rarely reported, although it is also crucial for the development of novel materials for targeted photodynamic therapy (PDT) reagents and phosphorescent molecular probes. The reported results show that the principles used in singlet excited state tuning are unable to be simply applied to the triplet excited state. In this review article, we summarized the recent results on controlling the triplet excited states by the external stimuli (chemical or light), and the application of the triplet state tuning in the chemical/light controllable triplet-triplet-annihilation upconversion (TTA UC). We discussed the methods for the control of the triplet states, as well as singlet excited state, for the purpose of controlling the TTA UC. Both successful and unsuccessful methods are discussed. This information is helpful for understanding the photophysical processes in which the triplet excited state is involved, and the development of novel external stimuli-responsive triplet photosensitizers.

Turn-On Photocatalysis: Creating Lone-Pair Donor-Acceptor Bonds in Organic Photosensitizer to Enhance Intersystem Crossing

There is growing interest in developing triplet photosensitizers in terms of implementing photochemical strategies in synthetic chemistry. However, synthesis of stable triplet organic photosensitizers is nontrivial and often requires the use of heavy atoms. Herein, an alternative strategy is demonstrated to enhance the triplet generation efficiency by implanting lone-pair donor-acceptor bonds in the conjugated covalent organic frameworks (COFs). This powerful method is validated using COFs that host triazine, a moiety that has been extensively investigated in photocatalysis. Spectroscopic analysis and theoretical calculations reveal substantial improvements in the photoabsorptivity and triple-state photogeneration efficiency, consistent with catalytic tests concerning industrially relevant sulfide oxidation. These systems represent a promising addition to the rapidly increasing arsenal of synthetic photocatalytic systems.

BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of α,β-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O₂ ^-. ) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S₁ ) to triplet (T₁ ) states via an intermediate triplet (T₂ ) state. The low reduction potential and ultralong lifetime of the T₁ state facilitate the efficient generation of O₂ ^-. by inter-molecular charge transfer to molecular oxygen. The energy gap of T₁ -S₀ is smaller than that between ³ O₂ and ¹ O₂ thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.

Rationalizing the effect of benzo-fusion at [a] and [b] positions of BODIPY on fluorescence yields

The origin of the large difference of fluorescence yields between benzo[a] and benzo[b] BODIPY derivatives was investigated. The benzo[a]-BODIPY chromophore shows high fluorescence yields while the totally quenched fluorescence of benzo[b]-BODIPYs is observed. Quantum calculations indicated that larger spin-orbit coupling (SOC) and smaller singlet-triplet energy gaps result in non-fluorescence for benzo[b]-BODIPY. Benzo[b]-fusion makes a partial contribution to the HOMO but a full contribution to the HOMO-1, and thus the S₁→ S₀ and T₂→ S₀ transitions, involved in HOMO-LUMO and HOMO-1-LUMO, have different characteristics, which leads to spin flipping for intersystem crossing (ISC) and increases the SOC to 1.70 cm^-1. However, benzo[a] contributes to HOMO and HOMO-1 equally, and minimizes the SOC between S₁ and T₂, leading to slow ISC from S₁, thus possessing strong fluorescence. These results are useful for the rational design of heavy-atom-free triplet organic chromophores.

The switchable phosphorescence and delayed fluorescence of a new rhodamine-like dye through allenylidene formation in a cyclometallated platinum(ii) system

A new rhodamine-like alkyne-substituted ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. The chemo-induced “ON–OFF” switching capabilities on the spirolactone ring of the Rhodyne ligand with an end-capping platinum(ii) metal centre can modulate the interesting acetylide–allenylidene resonance. The long-lived ³IL excited state of Rhodyne in its ON state as an optically active opened form was revealed via steady-state and time-resolved spectroscopy studies. Exceptional near-infrared (NIR) phosphorescence and delayed fluorescence based on a rhodamine-like structure were observed at room temperature for the first time. The position of the alkyne communication bridge attached to the platinum(ii) unit was found to vary the lead(ii)-ion binding mode and also the possible resonance structure for metal-mediated allenylidene formation. The formation of a proposed allenylidene resonance structure was suggested to rationalize these phenomena.

A new rhodamine-like ligand (Rhodyne) was designed to coordinate a cyclometallated platinum(ii) system. Allenylidene formation could trigger NIR phosphorescence at 740 nm originating from Rhodyne ³IL, as well as delayed fluorescence at 620 nm.

Bodipy-Containing Porous Microcapsules for Flow Heterogeneous Photocatalysis

Photocatalysis is a facile strategy for complex chemical transformations. Heterogeneous photocatalysis, especially in the flow system, has attracted much attention as it avoids the separation of catalysts. Herein, a kind of a Bodipy-containing porous microcapsule heterogeneous photocatalyst was rationally constructed with modulation on a multiscale. The diiodo-Bodipy with methacrylate (MA-2IBDP) was synthesized as a polymerizable photosensitizer. After immobilization in a polymer matrix, the intersystem crossing rate constant of MA-2IBDP increased to 2.7 × 10¹⁰ s^-1 and its triplet excited-state lifetime prolonged to ∼1 ms. Porous structures in microcapsules were created to facilitate mass transfer. A flat plate flow reactor was constructed to fix the catalytic microcapsules and improve light utilization. With the combination of all the above benefits, the reaction rate constant (0.896 s^-1) is 10 times faster than that of MA-2IBDP in a homogeneous system for juglone synthesis. The continuous production can last for 30 h without yield decrease. The photocatalyst can also be used in aza-Henry reaction, Alder-Ene reaction, and oxidation of thiols to disulfides with conversion rates above 95%. This study provides a means for the construction of heterogeneous catalysts and the flow reaction system.

Electron spin dynamics in excited state photochemistry: recent development in the study of intersystem crossing and charge transfer in organic compounds

Electron spin dynamics are crucial to photochemical and photophysical processes. However, to a large extent, they are neglected in routine photochemistry studies. Herein, we summarized the recent developments of electron spin dynamics in organic molecular systems. The electron-spin selective intersystem crossing (ISC) as well as charge separation (CS) and charge recombination (CR) of the organic molecular system are discussed, including ISC of the compounds with twisted π-conjugation frameworks and CR-induced ISC in compact orthogonal electron donor-acceptor dyads. We found that the electron spin polarization (ESP) of the triplet state formed in these systems is highly dependent on the molecular structure and geometry. The zero-field-splitting (ZFS) D and E parameters of the triplet state of series chromophores determined with time-resolved electron paramagnetic resonance (TREPR) spectroscopy are presented. Some unanswered questions in related areas are raised, which may inspire further theoretical investigations. The examples demonstrate that the study of electron spin dynamics is not only important in fundamental photochemistry to attain in-depth understanding of the ISC and the charge transfer processes, but is also useful for designing new efficient organic molecular materials for applications including photodynamic therapy, organic light-emitting diodes, and photon upconversion.

A Family of Highly Fluorescent and Membrane-Permeable Bis(BF2) Acyl-Pyridinylhydrazine Dyes with Strong Solid-State Emission and Large Stokes Shifts: The BOAPH Fluorophores

Organic small-molecule fluorescent chromophores have become essential to modern chemical, biological, and materials related investigations. Herein, a straightforward synthesis and subsequent borylation were presented to form a novel family of bisBF₂-anchoring acyl-pyridinylhydrazine, which we named BOAPH. The chromophore enjoys outstanding structural diversities owing to varied acyl chlorides and N-heteroarenylhydrazides. These resultant BOAPH dyes are confirmed by NMR, HRMS, and single-crystal X-ray structure analysis. Their spectroscopic properties were studied, and most of the strong absorbance and bright fluorescence with maximum wavelengths centered in the range of 400 and 650 nm. More importantly, they exhibit promising fluorescence quantum yields up to 0.79 in solution and solid states, good photostability, and large Stokes shifts. Furthermore, a respective BOAPH dye with a para-dimethylaminophenyl group exhibited the interesting ability of ultrafast staining and two-photon imaging, which can specifically label lipid droplets of living cells immediately without the need for incubation.

Riboflavin-based carbon dots with high singlet oxygen generation for photodynamic therapy

Photodynamic therapy, as an effective treatment for superficial tumors, has attracted more and more attention. The development of safe, biocompatible and in vivo photosensitive materials is helpful to promote photodynamic therapy. Here we report green fluorescent carbon quantum dots prepared from a natural vitamin, riboflavin (VB2), as a photosensitizer. The VB2-based carbon dots have excellent water solubility and biocompatibility, and their singlet oxygen generation ability is much stronger than that of riboflavin itself. Through endocytosis, the carbon dots can easily enter the cells and show bright green fluorescence. In vivo experiments show that after photodynamic therapy the carbon dots can significantly inhibit the growth of tumors, and will not have toxic and side effects on other organs.

An Approach to Developing Cyanines with Simultaneous Intersystem Crossing Enhancement and Excited-State Lifetime Elongation for Photodynamic Antitumor Metastasis

Heavy-atom-based photosensitizers usually exhibit shortened triplet-state lifetimes, which is not ideal for hypoxic tumor photodynamic therapy. Although several heavy-atom-free photosensitizers possess long triplet-state lifetimes, the clinical applicability is limited by their short excitation wavelengths, poor photon capture abilities, and intrinsically hydrophobic structures. Herein we developed a novel NIR heavy-atom-free photosensitizer design strategy by introducing sterically bulky and electron-rich moieties at the meso position of the pentamethine cyanine (Cy5) skeleton, which simultaneously enhanced intersystem crossing (ISC) and prolonged excited-state lifetime. We found that the ¹O₂ generation ability is directly correlated to the electron-donating ability of the meso substituent in cyanine, and the excited-state lifetime was simultaneously much elongated when the substituents were anthracene derivatives substituted at the 9-position. Our star compound, ANOMe-Cy5, exhibits intense NIR absorption, the highest ¹O₂ quantum yield (4.48-fold higher than Cy5), the longest triplet-state lifetime (9.80-fold longer than Cy5), and lossless emission intensity (nearly no change compared with Cy5). Such excellent photophysical properties coupled with its inherently cationic and hydrophilic nature enable the photosensitizer to realize photoablation of solid tumor and antitumor lung metastasis. This study highlights the design of a new generation of NIR photosensitizers for imaging-guided photodynamic cancer treatment.

Oxygen Self-Supplying Nanotherapeutic for Mitigation of Tissue Hypoxia and Enhanced Photodynamic Therapy of Bacterial Keratitis

Hypoxia, a common characteristic of bacterial infections, is known to be closely associated with the emergence of multidrug-resistant bacteria, which hastens the need to develop advanced microbicides and antibacterial techniques. Photodynamic therapy is a promising strategy to reduce bacterial antibiotic resistance and employs photosensitizers, excitation light sources, and sufficient oxygen to generate toxic reactive oxygen species (ROS). The inherent limitation of PDT is that the generation of ROS is restricted by the hypoxic microenvironment in infection sites. Here, an oxygen self-supplying nanotherapeutic is developed to enhance antibacterial activity against multidrug-resistant bacteria on the basis of fluorinated boron dipyrromethene (BODIPY)-based glycomimetics. The nanotherapeutic not only could capture the bacteria efficiently but also was able to act as an oxygen carrier to relieve the hypoxic microenvironment of bacterial infections, thus achieving enhanced PDT efficacy. In a Pseudomonas aeruginosa infection of a rat cornea, typical administration of the nanotherapeutic decreased the infiltrate and showed a faster healing capacity in comparison with BODIPY-based glycomimetics. Self-supplying oxygen nanotherapeutics that relieve the hypoxic microenvironment and interfere with bacterial colonization have been shown to be a promising candidate for the management of drug-resistant microbial keratitis.

Aromatic [b]-fused BODIPY dyes as promising near-infrared dyes

Far-red and near-infrared (NIR) absorbing/emitting dyes have found diverse applications in biomedicine and material science. However, the absorption and emission of classical BODIPY chromophores at short wavelength hamper their applications. Several strategies have been adopted to modify the structure of the BODIPY core to design NIR dyes. Among these, the most efficient approach to expand the π-conjugation of the BODIPY core is via fusion of aromatic rings. So far, many novel BODIPY skeletons fused to aromatic hydrocarbons and heterocycles at the b bond have been reported. This review comprehensively describes the recent advances regarding the development of aromatic [b]-fused BODIPY dyes with the focus on the design and synthesis, the relationships between their photophysical/spectroscopic properties and molecular structures, and the potential applications in bioassays and optoelectronic devices.

Dirhodium-Based Supramolecular Framework Catalyst for Visible-Light-Driven Hydrogen Evolution

The direct conversion of solar energy to clean fuels as alternatives to fossil fuels is an important approach for addressing the global energy shortage and environmental problems. Here, we introduce a new dirhodium-complex-based framework assembly as a heterogeneous molecule-based photocatalyst for hydrogen evolution using visible light. Two dirhodium complexes bearing visible-light-harvesting BODIPY (boron dipyrromethene, BDP) moieties were newly designed and synthesized. The obtained complexes were self-assembled to framework structures (supramolecular framework catalysts), which are stabilized intermolecular noncovalent interactions. These frameworks retained excellent visible-light-harvesting properties of BDP moieties. Investigation of the catalytic performance of the supramolecular framework catalysts revealed that the supramolecular framework catalyst with heavy atoms at BDP moieties exhibited excellent performance in the formation of hydrogen with a reaction rate of 275.8 μmol g^-1 h^-1 under irradiation of visible light, whereas the supramolecular framework catalyst without heavy atoms at BDP moieties was inactive. Moreover, the system has the additional benefits of high durability (up to 96 h), reusability, and facile removal from the reaction mixture. We also disclosed the effect of heavy atoms at BDP moieties on the catalytic activity and proposed a reaction mechanism.

Tuning Photodynamic Properties of BODIPY Dyes, Porphyrins' Little Sisters

The photodynamic properties of a series of non-halogenated, dibrominated and diiodinated BODIPYs with a phthalimido or amino end modification on the phenoxypentyl and phenoxyoctyl linker in the meso position were investigated. Halogen substitution substantially increased the singlet oxygen production based on the heavy atom effect. This increase was accompanied by a higher photodynamic activity against skin melanoma cancer cells SK-MEL-28, with the best compound reaching an EC50 = 0.052 ± 0.01 µM upon light activation. The dark toxicity (toxicity without light activation) of all studied dyes was not detected up to the solubility limit in cell culture medium (10 µM). All studied BODIPY derivatives were predominantly found in adiposomes (lipid droplets) with further lower signals colocalized in either endolysosomal vesicles or the endoplasmic reticulum. A detailed investigation of cell death indicated that the compounds act primarily through the induction of apoptosis. In conclusion, halogenation in the 2,6 position of BODIPY dyes is crucial for the efficient photodynamic activity of these photosensitizers.

Quantitative Structure-Property Relationship Modelling for the Prediction of Singlet Oxygen Generation by Heavy-Atom-Free BODIPY Photosensitizers*

Heavy-atom-free sensitizers forming long-living triplet excited states via the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process have recently attracted attention due to their potential to replace costly transition metal complexes in photonic applications. The efficiency of SOCT-ISC in BODIPY donor-acceptor dyads, so far the most thoroughly investigated class of such sensitizers, can be finely tuned by structural modification. However, predicting the triplet state yields and reactive oxygen species (ROS) generation quantum yields for such compounds in a particular solvent is still very challenging due to a lack of established quantitative structure-property relationship (QSPR) models. In this work, the available data on singlet oxygen generation quantum yields (ΦΔ ) for a dataset containing >70 heavy-atom-free BODIPY in three different solvents (toluene, acetonitrile, and tetrahydrofuran) were analyzed. In order to build reliable QSPR model, a series of new BODIPYs were synthesized that bear different electron donating aryl groups in the meso position, their optical and structural properties were studied along with the solvent dependence of singlet oxygen generation, which confirmed the formation of triplet states via the SOCT-ISC mechanism. For the combined dataset of BODIPY structures, a total of more than 5000 quantum-chemical descriptors was calculated including quantum-chemical descriptors using density functional theory (DFT), namely M06-2X functional. QSPR models predicting ΦΔ values were developed using multiple linear regression (MLR), which perform significantly better than other machine learning methods and show sufficient statistical parameters (R=0.88-0.91 and q2 =0.62-0.69) for all three solvents. A small root mean squared error of 8.2 % was obtained for ΦΔ values predicted using MLR model in toluene. As a result, we proved that QSPR and machine learning techniques can be useful for predicting ΦΔ values in different media and virtual screening of new heavy-atom-free BODIPYs with improved photosensitizing ability.

Highly Efficient Near-Infrared Photosensitizers with Aggregation-Induced Emission Characteristics: Rational Molecular Design and Photodynamic Cancer Cell Ablation

Photosensitizers (PSs) that play a decisive role in effective photodynamic therapy (PDT) have attracted great research interest. PSs with aggregation-induced emission (AIE) characteristics could overcome the deficiencies of traditional PSs that usually suffer from the aggregation-caused fluorescence quenching (ACQ) effect in applications and show enhanced emission and high singlet oxygen (¹O₂) generation efficiency in aggregates; therefore, they are outstanding candidates for imaging-guided PDT, and the development of AIE PSs with both excellent photophysical properties and ¹O₂ generation ability is highly desirable. Herein, three AIE fluorogens (AIEgens), BtM, ThM, and NaM, with a donor-π-acceptor (D-π-A) structure were designed and synthesized, and the photosensitizing ability was adjusted by π-linker engineering. All of the three AIEgens showed excellent photostability and high molar absorption coefficients, and their emission edges were extended to the near-infrared (NIR) region, with peaks at 681, 678, and 638 nm, respectively. NaM demonstrated the smallest ΔE_S1-T1, which was ascribed to its better separation degree of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The AIEgens were fabricated into nanoparticles (NPs) by amphipathic mPEG3000-DSPE encapsulating, and thus the obtained NaM NPs exhibited the best ¹O₂ generation efficiency under white light irradiation, which was almost 3 times that of the renowned PS rose bengal (RB). Furthermore, under white light irradiation, the cell killing efficiency of NaM NPs was also much better than those of the other two AIE PSs and RB. Therefore, NaM NPs revealed great potential to treat superficial diseases as a PS for PDT.

BODIPY-vinyl dibromides as triplet sensitisers for photodynamic therapy and triplet-triplet annihilation upconversion

We devised a new generation of halogen-based triplet sensitisers comprising geminal dibromides at the vinyl backbone of a BODIPY fluorophore. Incorporating geminal dibromides into the π-conjugation of BODIPY enhanced intersystem crossing due to the heavy atom effect, which in turn improved the extent of excited triplet states.

Sterically Protected and Conformation-Restricted BOBHY Dyes with Bright Near-Infrared Fluorescence: N2O-type Expanded BOPHY Dyes Derived from Boronic Acids

A new family of N₂O-type hydrazine-containing bipyrrole boron complexes has been developed via a one-pot condensation of formylisoindole, hydrazine, and various organoboronic acids. Because of the conformation-restricted coplanar structure and the axial-substituted aryl groups, these novel dyes show deep-red absorption, bright near-infrared (NIR) fluorescence in both solution and solid states, and good solubility in organic solvents. The derivative with pyridinium ions also has been synthesized as an NIR mitochondrially targetable fluorescent probe.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

Does Twisted π-Conjugation Framework Always Induce Efficient Intersystem Crossing? A Case Study with Benzo[b]- and [a]Phenanthrene-Fused BODIPY Derivatives and Identification of a Dark State

The photophysical properties, especially the intersystem crossing (ISC) of two heavy-atom-free BODIPY derivatives with twisted π-conjugated frameworks (benzo[b]-fused BODIPY, BDP-B; and [a]phenanthrene-fused BODIPY, BDP-P), are studied with steady-state and time-resolved optical and electron paramagnetic resonance (TREPR) spectroscopic methods as well as with ADC(2) theoretical investigations. Interestingly, BDP-B has a planar π-conjugation framework, but it displays weaker UV-vis absorption (ε = 3.8 × 10⁴ M^-1 cm^-1 at 569 nm) and fluorescence (Φ_F < 0.1%), a short-lived singlet-excited state (fluorescence lifetime, τ_F = 0.2 ns), and a long-lived triplet state (τ_T = 132.3 μs). In comparison, the more twisted BDP-P shows stronger UV-vis absorption (ε = 9.8 × 10⁴ M^-1 cm^-1 at 640 nm) and fluorescence (Φ_F = 70%), longer singlet-excited-state lifetime (τ_F = 6.4 ns), and shorter triplet-state lifetime (τ_T = 18.9 μs). In contrast to helicenes (Φ_T = ca. 90%), the ISC of BDP-P and BDP-B is nonefficient (Φ_T < 23%). The electron spin selectivity of the ISC of the derivatives is different, manifested by the phase pattern of the TREPR spectra as AAEAEE and EEEAAA for BDP-B and BDP-P, respectively. The spatially confined T₁ state wave function of the twisted molecule keeps the T₁ state energy high (1.44-1.61 eV). A dark S₁ state was identified for BDP-B. This work demonstrated that the twisted π-conjugated framework does not necessarily induce efficient ISC and we found a dark singlet state for BODIPY, which is rare.

Torsion-Induced Nonradiative Relaxation of the Singlet Excited State of meso-Thienyl Bodipy and Charge Separation, Charge Recombination-Induced Intersystem Crossing in Its Compact Electron Donor/Acceptor Dyads

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties of the compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as density functional theory (DFT) computations. Different from the meso-phenyl Bodipy analogues, the meso-thienyl Bodipy are weakly fluorescent. Based on femtosecond transient absorption and DFT computations, we propose that the torsion of the thienyl group and the distortion of the Bodipy core (19.7 ps) in the S₁ state lead to a conical intersection on the potential energy surface as an efficient nonradiative decay channel (408 ps), which is responsible for the observed weak fluorescence as compared to the meso-phenyl analogue. The increased fluorescence quantum yield (from 5.5 to 14.5%) in viscous solvents supports this hypothesis. With the electron donor 4'-hydroxylphenyl moiety attached to the meso-thienyl unit, the fast charge separation (CS, 15.3 ps) and charge recombination (CR, 238 ps) processes outcompete the torsion-induced nonradiative decay and induce fast ISC through the SOCT-ISC mechanism. The triplet quantum yield of the electron donor/acceptor dyad is highly dependent on solvent polarity (Φ_T = 1.9-45%), which supports the SOCT-ISC mechanism, and the triplet-state lifetime is up to 247.3 μs. Using the electron donor-acceptor dyad showing SOCT-ISC as a triplet photosensitizer, efficient triplet-triplet annihilation (TTA) upconversion was observed with a quantum yield of up to 6.0%.

An octabrominated Sn(iv) tetraisopropylporphyrin as a photosensitizer dye for singlet oxygen biomedical applications

Two novel Sn(iv) tetraisopropylphenylporphyrins have been synthesized to explore the effect of octabromination at the β-pyrrole positions on their photophysical properties and photodynamic activity. The lower energy Q band of an octabrominated complex lies at 675 nm well within the therapeutic window. The octabrominated dye has a relatively high singlet oxygen quantum yield of 0.78 in DMF and exhibits favorable photodynamic activity against MCF-7 cells with an IC50 value of 10.7 μM and a 5.74 log reduction value (5 μM) towards S. aureus under illumination at 660 nm for 60 min with a Thorlabs M660L3 LED (280 mW cm-2).

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.

Spiro Rhodamine-Perylene Compact Electron Donor-Acceptor Dyads: Conformation Restriction, Charge Separation, and Spin-Orbit Charge Transfer Intersystem Crossing

Spiro rhodamine (Rho)-perylene (Pery) electron donor-acceptor dyads were prepared to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC) in these rigid and sterically congested molecular systems. The electron-donor Rho (lactam form) moiety is attached via the N-C bond to the electron acceptor at either 1- or 3-position of the Pery moiety (Rho-Pery-1 and Rho-Pery-3). Severe torsion of the Pery moiety in Rho-Pery-1 was observed. The fluorescence of the two dyads is significantly quenched in polar solvents, and the singlet oxygen quantum yields (Φ_Δ) are strongly dependent on solvent polarity (4-36%). Femtosecond transient absorption spectra demonstrate that charge separation (CS) takes 0.51 ps in Rho-Pery-1 and 5.75 ps in Rho-Pery-3, and the charge recombination (CR)-induced ISC is slow (>3 ns). Nanosecond transient absorption spectra indicate that the formation of triplet states via SOCT-ISC takes 24-75 ns for Rho-Pery-1 and 6-15 ns for Rho-Pery-3, and the distorted π-framework of the Pery moiety results in a shorter triplet lifetime of 19.9 vs 291 μs for the planar analogue. Time-resolved electron paramagnetic resonance spectroscopy confirms the SOCT-ISC mechanism.

BODIPY-attached zinc(II) complexes of curcumin drug for visible light assisted photo-sensitization, cellular imaging and targeted PDT

Boron-dipyrromethene (BODIPY) based photosensitizers as porphyrinoids and curcumin as natural product possess exciting photophysical features suitable for theranostic applications, namely, imaging and photodynamic therapy (PDT). Limited aqueous solubility and insufficient physiological stability, however, reduce their efficacy significantly. We have designed a novel strategy to deliver these two unusable cytotoxins simultaneously in cancer cells and herein, report the synthesis, characterization and imaging-assisted photocytotoxicity of three zinc(II) complexes containing N³-donor dipicolylamine (dpa) ligands (L^1-3) and O,O-donor curcumin (Hcur) viz. [Zn(L¹)(cur)]Cl (1), [Zn(L²)(cur)]Cl (2) and [Zn(L³)(cur)]Cl (3), where L² and L³ have pendant fluorescent BODIPY and non-emissive di-iodo-BODIPY moieties. Metal chelation imparted remarkable biological stability (pH ∼7.4) to the respective ligands and induces significant aqueous solubility. These ternary complexes could act as replacements of the existing metalloporphyrin-based PDT photosensitizers as their visible-light photosensitizing ability is reinforced by the dual presence of blue light absorbing curcumin and green light harvesting BODIPY units. Complex 2 having emissive BODIPY unit L² and curcumin, showed mitochondria selective localization in HeLa, MCF-7 cancer cells and complex 3, the di-iodinated analogue of complex 2, exhibited type-I/II PDT activity via inducing apoptosis through mitochondrial membrane disruption in cancer cells while being significantly nontoxic in dark and to the healthy cells.

Amphiphilic BODIPY dye aggregates in polymeric micelles for wavelength-dependent photo-induced cancer therapy

Near-infrared (NIR) light-responsive nanoparticles of organic small-molecule dyes hold great promise as phototherapeutic dyes (PDs) for clinical translation due to their intrinsic merits, including well-defined structure, high purity, and good reproducibility. However, they have been explored with limited success in the development of photostable NIR PDs with extraordinary photoconversion for highly effective phototherapy. Herein, we have described amphiphilic BODIPY dye aggregates within the polymeric micelles (Micelles) as potent bifunctional PDs for dually cooperative phototherapy under NIR irradiation. Micelles possessed an intensive NIR absorption, high photostability, and favorable non-radiative transition, thereby exhibiting both remarkable singlet oxygen generation and photothermal effect under NIR light irradiation. Besides, Micelles had preferable cellular uptake, effective cytoplasmic drug translocation as well as enhanced tumor accumulation. Owing to the combined virtues, Micelles showed clinical potential as bifunctional PDs for photo-induced cancer therapy. This work thus provides a facile strategy to exploit advanced PDs for practical phototherapeutic applications.

Enzymatic enhancing of triplet-triplet annihilation upconversion by breaking oxygen quenching for background-free biological sensing

Triplet-triplet annihilation upconversion nanoparticles have attracted considerable interest due to their promises in organic chemistry, solar energy harvesting and several biological applications. However, triplet-triplet annihilation upconversion in aqueous solutions is challenging due to sensitivity to oxygen, hindering its biological applications under ambient atmosphere. Herein, we report a simple enzymatic strategy to overcome oxygen-induced triplet-triplet annihilation upconversion quenching. This strategy stems from a glucose oxidase catalyzed glucose oxidation reaction, which enables rapid oxygen depletion to turn on upconversion in the aqueous solution. Furthermore, self-standing upconversion biological sensors of such nanoparticles are developed to detect glucose and measure the activity of enzymes related to glucose metabolism in a highly specific, sensitive and background-free manner. This study not only overcomes the key roadblock for applications of triplet-triplet annihilation upconversion nanoparticles in aqueous solutions, it also establishes the proof-of-concept to develop triplet-triplet annihilation upconversion nanoparticles as background free self-standing biological sensors.

A Supramolecular Strategy to Engineering a Non-photobleaching and Near-Infrared Absorbing Nano-J-Aggregate for Efficient Photothermal Therapy

The design of organic photothermal agents (PTAs) for in vivo applications face a demanding set of performance requirements, especially intense NIR-absorptivity and sufficient photobleaching resistance. J-aggregation offers a facile way to tune the optical properties of dyes, thus providing a general design platform for organic PTAs with the desired performance. Herein, we present a supramolecular strategy to build a water-stable, nonphotobleaching, and NIR-absorbing nano-PTA (J-NP) from J-aggregation of halogenated BODIPY dyes (BDP) for efficient in vivo photothermal therapy. Multiple intermolecular halogen-bonding and π-π stacking interactions triggered the formation of BDP J-aggregate, which adsorbed amphiphilic polymer chains on the surface to provide PEGylated sheetlike nano-J-aggregate (J-NS). We serendipitously discovered that the architecture of J-NS was remodeled during a long-time ultrafiltration process, generating a discrete spherical nano-J-aggregate (J-NP) with controlled size. Compared with J-NS, the remodeled J-NP significantly improved cellular uptake efficiency. J-aggregation brought J-NP striking photothermal performance, such as strong NIR-absorptivity, high photothermal conversion efficiency up to 72.0%, and favorable nonphotobleaching ability. PEGylation and shape-remodeling imparted by the polymer coating enabled J-NP to hold biocompatibility and stability in vivo, thereby exhibiting efficient antitumor photothermal activities. This work not only presents a facile J-aggregation strategy for preparing PTAs with high photothermal performance but also establishes a supramolecular platform that enables the appealing optical functions derived from J-aggregation to be applied in vivo.

Spin-Orbit Charge-Transfer Intersystem Crossing in Anthracene-Perylenebisimide Compact Electron Donor-Acceptor Dyads and Triads and Photochemical Dianion Formation

Perylenebisimide (PBI)-anthracene (AN) donor-acceptor dyads/triad were prepared to investigate spin-orbit charge-transfer intersystem crossing (SOCT-ISC). Molecular conformation was controlled by connecting PBI units to the 2- or 9-position of the AN moiety. Steady-state, time-resolved transient absorption and emission spectroscopy revealed that chromophore orientation, electronic coupling, and dihedral angle between donor and acceptor exert a significant effect on the photophysical property. The dyad PBI-9-AN with orthogonal geometry shows weak ground-state coupling and efficient intersystem crossing (ISC, Φ_Δ =86 %) as compared with PBI-2-AN (Φ_Δ =57 %), which has a more coplanar geometry. By nanosecond transient absorption spectroscopy, a long-lived PBI localized triplet state was observed (τ_T =139 μs). Time-resolved EPR spectroscopy demonstrated that the electron spin polarization pattern of the triplet state is sensitive to the geometry and number of AN units attached to PBI. Reversible and stepwise generation of near-IR-absorbing PBI radical anion (PBI^-⋅ ) and dianion (PBI^2- ) was observed on photoexcitation in the presence of triethanolamine, and it was confirmed that selective photoexcitation at the near-IR absorption bands of PBI^.- is unable to produce PBI^2- .

Synthesis, Structure and Photophysical Properties of a New Class of Inherently Chiral Boron(III) Chelates-The tert-Leucine Complexes

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O-salicylaldehyde-based derivative shows pronounced intramolecular charge-transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2-methylamino-benzaldehyde-derived N,N,O-chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature-dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)-based chelates, this derivative undergoes intersystem crossing to form a meta-stable triplet-excited state. X-ray crystal structures are reported for both compounds, along with simulated ECD spectra.

Weakened Triplet-Triplet Annihilation of Diiodo-BODIPY Moieties without Influence on Their Intrinsic Triplet Lifetimes in Diiodo-BODIPY-Functionalized Pillar[5]arenes

The triplet-triplet annihilation (TTA) effect of sensitizers themselves can lead to the additional quenching of lifetimes of triplet states; therefore, how to weaken the TTA effect of sensitizers is an urgent issue to be resolved for their further applications. Besides, it remains a tremendous challenge for constructing supramolecular systems of photosensitizers based on photosensitizer-functionalized pillararenes because there have been very few investigations on them. Thus, 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (DIBDP) and ethoxy pillar[5]arene (EtP5) were utilized to synthesize a DIBDP-functionalized pillar[5]arene (EtP5-DIBDP), a cyano-containing DIBDP (G) used as a guest molecule was also prepared, and they were used to investigate the electron-transfer mechanism between EtP5 and DIBDP moieties and weaken the TTA effect of DIBDP moieties. The theoretical computational results of frontier molecular orbitals and isosurfaces of spin density preliminarily predicted that the cavities of the EtP5 moiety had influence on the fluorescence emission of DIBDP units but not on their triplet states in EtP5-DIBDP. The fluorescence emission intensities in a variety of solvents with different polarities and electrochemical studies revealed that there was electron transfer from EtP5 to the DIBDP units, and the electron-transfer process had influence on the fluorescence emission but not on the triplet states of DIBDP moieties in EtP5-DIBDP, which verified the results of density functional theory calculations. The triplet state lifetimes of EtP5-DIBDP were longer than those of DIBDP and G and the photooxidation abilities of EtP5-DIBDP were better than those of DIBDP and G at a high concentration (1.0 × 10^-5 M) in various solvents; in contrast, the intrinsic triplet state lifetimes and singlet oxygen quantum yields (Φ_Δ) of DIBDP, G, and EtP5-DIBDP were very similar. This was because the steric hindrance of EtP5 moieties could weaken the TTA effect of DIBDP moieties without influencing their intrinsic triplet state lifetimes in EtP5-DIBDP.

Exploiting a Neutral BODIPY Copolymer as an Effective Agent for Photodynamic Antimicrobial Inactivation

We report the synthesis and photophysical properties of a neutral BODIPY photosensitizing copolymer (poly-8-(4-hydroxymethylphenyl)-4,4-difluoro-2,6-diethynyl-4-bora-3a,4a-diaza-s-indacene) containing ethynylbenzene links between the BODIPY units. The copolymer absorbs further towards the red in the UV-vis spectrum compared to the BODIPY precursor. Photolysis of the polymer produces a singlet excited state which crosses to the triplet surface in less than 300 ps. This triplet state was used to form singlet oxygen with a quantum yield of 0.34. The steps leading to population of the triplet state were studied using time-resolved spectroscopic techniques spanning the pico- to nanosecond timescales. The ability of the BODIPY polymer to generate a biocidal species for bactericidal activity in both solution- and coating-based studies was assessed. When the BODIPY copolymer was dropcast onto a surface, 4 log and 6 log reductions in colony forming units/ml representative of Gram-positive and Gram-negative bacteria, respectively, under illumination at 525 nm were observed. The potent broad-spectrum antimicrobial activity of a neutral metal-free copolymer when exposed to visible light conditions may have potential clinical applications in infection management.

BODIPY-based monofunctional Pt (II) complexes for specific photocytotoxicity against cancer cells

Photodynamic therapy (PDT) has attracted extensive attention in cancer treatment because of its minimum trauma, less side effects, and so on. Photosensitizers, as one of the core elements of PDT, usually have to face problems such as poor water solubility and light stability, lack of targeting, and other problems, which seriously affect the therapeutic effect. In this work, two BODIPY (boron-dipyrromethene)-based monofunctional Pt (II) complexes, 1a and 2a, were designed and synthesized, and their PDT effect was studied. The Pt atom improved the singlet oxygen quantum yield (0.19 for 1a and 0.14 for 2a, respectively), which effectively improves the efficiency of PDT. MTT assay confirmed that the short time photo-irradiation distinctly promoted antitumor cytotoxicity of Pt (II) compounds against different cell lines. For 1a under irradiation, the IC₅₀ value of cancer cell lines were 13.1 μM for HeLa cells and 7.6 μM for MCF-7 cells, while those of normal cell lines were 32.4 μM for HBL-100 cells and 48.6 μM for L02 cells. The results demonstrated that 1a showed specific phototoxicity to cancer cells. This specific selectivity could be attributed to the synergistic effect of increased cellular uptake (determined by ICP-MS) and higher ROS generation (detected by Cell ROX Deep Red) in cancer cells after irradiation. This study laid the foundation for the future design and synthesis of effective PDT photosensitizers.

NIR halogenated thieno[3, 2-b]thiophene fused BODIPYs with photodynamic therapy properties in HeLa cells

Commonly, an efficient photosensitizer usually requires a number of excellent properties, such as a larger molar absorption coefficient in the tissue transparency window, a high intersystem spin-crossing (ISC) probability induced by heavy atom and low dark toxicity as well as high photostability. In this study, NIR tetra-bromo thieno[3,2-b]thiophene-fused BODIPYs derivatives 3 was prepared, and fully characterized. Their photophysical properties have been well investigated including absorption, fluorescence profiles and photostability. The novel BODIPYs 2-3 possess long wavelength absorptions of maximum up to 720 nm with large molar absorption coefficients due to extend the effect of π-conjugation system via fusion the thieno[3,2-b]thiophene group. Especially, BODIPY 3 containing heavy atoms (four bromine atoms) exhibits photocytotoxicity upon irradiation with light NIR laser based on the results of MTT assays and flow analyses in living HeLa cells, in the meanwhile, it features lower cytotoxic in the dark. The current research work will contribute to the development of functional dyes and new organic NIR photosensitizer agents.

Recent progress in photosensitizers for overcoming the challenges of photodynamic therapy: from molecular design to application

Photodynamic therapy (PDT), a therapeutic mode involving light triggering, has been recognized as an attractive oncotherapy treatment. However, nonnegligible challenges remain for its further clinical use, including finite tumor suppression, poor tumor targeting, and limited therapeutic depth. The photosensitizer (PS), being the most important element of PDT, plays a decisive role in PDT treatment. This review summarizes recent progress made in the development of PSs for overcoming the above challenges. This progress has included PSs developed to display enhanced tolerance of the tumor microenvironment, improved tumor-specific selectivity, and feasibility of use in deep tissue. Based on their molecular photophysical properties and design directions, the PSs are classified by parent structures, which are discussed in detail from the molecular design to application. Finally, a brief summary of current strategies for designing PSs and future perspectives are also presented. We expect the information provided in this review to spur the further design of PSs and the clinical development of PDT-mediated cancer treatments.

Manipulating the Dynamics of Dark Excited States in Organic Materials for Phototheranostics

Manipulating the dynamics of dark excited states (DESs), such as higher excited singlet or excited triplet states with no or small radiative decay, are of both fundamental and practical interests, an important application being photoactivated diagnosis and therapy (phototheranostics), which include photoacoustic (PA) imaging, photodynamic therapy (PDT), and photothermal therapy (PTT). However, the current understanding of DESs in organic structures is rather limited, thus making any rational manipulation of DES in organic materials very challenging.A DES decays primarily by radiationless transition through two pathways: (i) singlet-to-triplet intersystem crossing (ISC) and (ii) internal conversion (IC) relaxation. The deactivation of a DES via ISC can generate cytotoxic reactive oxygen species (ROS) for PDT, while IC could convert photons into heat for PA imaging and PTT. In this Account, we highlight our research on developing a fundamental understanding of structure-property relationships for manipulation of DESs in organic materials in relation to phototheranostic applications. We describe the application of femtosecond transient absorption (fs-TA) spectroscopy for obtaining valuable insights into the DES dynamics. Afterward, we present our work on DESs in nonrigid molecules that revealed greatly enhanced ISC through geometry twisting, which leads to an innovative pathway to develop organic materials exhibiting external stimuli-responsive reversible switching of ISC. We introduce the concept of smart PDT where highly efficient ISC imparted by geometry twisting in the acidic environment specific to tumors leads to very efficient and highly localized PDT, thus leaving surrounding healthy tissues at a different pH unaffected. This insightful understanding of ISC can lead to the development of more advanced photosensitizers for PDT. Two other emergent concepts from our work presented here are (1) significantly enhanced IC producing strong local heating by combining two-photon absorption with excited state absorption for cumulative multiphoton absorption, thus greatly increasing the strength of the PA signal for nonlinear PA imaging, and (2) shown by an example of an organic molecule, BODIPY, nanoscale charge-transfer state mediated strong IC in aggregate nanoparticles resulting in exceptionally high photothermal conversion efficiency of 61% for both PA and PTT. Some in vivo results of the phototheranostic studies using BODIPY are presented, providing an elegant example of nanoscale manipulation of the excited state dynamics.This Account concludes with a summary and discussion of future perspectives. We hope this Account will deepen the understanding of molecular and nanoscale control of excited state dynamics in organic materials, hopefully enticing a broad range of scientists within different disciplinary areas.

Stable π-radical nanoparticles as versatile photosensitizers for effective hypoxia-overcoming photodynamic therapy

We report the first demonstration using a stable π-radical as a versatile photosensitizer for hypoxia-overcoming photodynamic therapy. After self-assembling the radical molecules into radical nanoparticles (NPs), the NPs show good water dispersibility, good biocompatibility, broad near-infrared (NIR) absorption and emission at ∼800 nm. Significantly, the radical NPs remain stable in various biological mediums, after 100 days exposure to the ambient environment, and even after long-term laser irradiation, which is superior to many reported radical-based materials. More importantly, upon 635 nm laser irradiation, sufficient superoxide radical (O₂^-˙) generation and in vitro cytotoxicity were observed addressing the most important hurdle for successful PDT in the oxygen-deficient tumor microenvironment. In addition, the radical NPs are also demonstrated to have effective in vivo PDT efficacy, and excellent biosafety.