PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: tuning photosensitizing ability by dihedral angles

Aggregation Behavior of CHR-bis(BODIPY) Bichromophores in THF-water Mixtures: Effect of Linking Positions and Aryl-spacer Substituents

Aggregation-caused quenching effect (ACQ) greatly limits the practical use of many organic luminophores in biomedicine, optics and electronics. The comparative analysis of aggregation characteristics of CHR-bis(BODIPY) bichromophores 1-6 with R = H, Ph, MeOPh and various linking positions (α,α-; α,β-; β,β- and β',β'-) in THF-water mixtures with different water fractions or dye concentrations is first presented in this article. Both the linking style 1-4 and the arylation of the spacer with phenyl (Ph-) 5 or methoxyphenyl (MeOPh-) 6 substituents strongly affect the formation of luminophore aggregated forms in binary THF-water mixtures. The α,α-and β,β-isomers (1 and 3) form non-fluorescent H-type aggregates in THF-water mixtures with fw > 70%. The α,β-; β',β'-isomers (2, 4) and the MeOPh-substituted β,β-bichromophore 6 are characterized by predominant formation fluorescent aggregates. All bichromophores are characterized by the presence of residual amounts of non-aggregated forms in binary mixtures with maximum water content. The results are useful for controlling the aggregation behavior and spectral characteristics of CHR-bis(BODIPY) bichromophores in aqueous-organic media, which is important in the development of biomarkers and PDT agents.

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.

Theoretical study on the photophysical properties of thiophene-fused-type BODIPY series molecules in fluorescence imaging and photodynamic therapy

As a class of photosensitizers (PSs) with dual functions of photodynamic therapy (PDT) and fluorescence imaging, the relationship between the structure and dual-function of thiophene-fused-type BODIPY dyes has not been studied in depth before. We found that the thiophene-fused-type BODIPY triplet photosensitizer is produced according to the energy level matching rule and the introduction of the thiophene ring significantly reduces the energy gap ΔEST between singlet and triplet states, as revealed by our investigation of the excited state structures and energies of thieno-fused BODIPY dyes. At the same time, a tiny ΔEST also results in a greatly enhanced intersystem crossing (ISC) rate, kISC. The kISC value of MeO-BODIPY, having the highest singlet oxygen quantum yield (ΦΔ), is the largest. Substitution with a strong electron donor N,N-dimethylaminophenyl (DMA) leads to the vertical configuration in the T1 state. The small ΔE (0.0029 eV) between the HOMO and HOMO-1 triggers the photo induced electron transfer (PET) of inhibiting ISC and fluorescence. When thieno-fused BODIPYs react with pyrrole, the increase of π-conjugation and smaller ΔEHOMO-LUMO explain the redshift in emission wavelength of thieno-pyrrole-fused BODIPY. The more planar configuration of the S1 state and the stronger oscillator intensity reflect a higher fluorescence quantum yield (ΦF). The extension of π-conjugation can cause molecules to transition to higher-level singlet excited states (Sn states, n ≥ 1) after absorbing energy and reduce the energy level of the excited state, resulting in multiple channels and favoring 1O2 production for thieno-pyrrole-fused BODIPYs with electron-withdrawing groups at the para-position of the phenyl groups. Due to ΔES0-T1 < 0.980 eV, the substitution of electron-donating groups cannot produce 1O2. In this work, we have revealed the mechanism of ISC and the fluorescence emission process in the thiophene-fused-type BODIPY dye, which has provided a theoretical foundation and guidance for the future design of BODIPY-based heavy-atom-free PSs for molecular applications in PDT.

Triphenylphosphonium-functionalized dimeric BODIPY-based nanoparticles for mitochondria-targeting photodynamic therapy

The dimerization of boron dipyrromethene (BODIPY) moieties is an appealing molecular design approach for developing heavy-atom-free triplet photosensitizers (PSs). However, BODIPY dimer-based PSs generally lack target specificity, which limits their clinical use for photodynamic therapy. This study reports the synthesis of two mitochondria-targeting triphenylphosphonium (TPP)-functionalized meso-β directly linked BODIPY dimers (BTPP and BeTPP). Both BODIPY dimers exhibited solvent-polarity-dependent singlet oxygen (1O2) quantum yields, with maximum values of 0.84 and 0.55 for BTPP and BeTPP, respectively, in tetrahydrofuran. The compact orthogonal geometry of the BODIPY dimers facilitated the generation of triplet excited states via photoinduced charge separation (CS) and subsequent spin-orbit charge-transfer intersystem crossing (SOCT-ISC) processes and their rates were dependent on the energetic configuration between the frontier molecular orbitals of the two BODIPY subunits. The as-synthesized compounds were amphiphilic and hence formed stable nanoparticles (∼36 nm in diameter) in aqueous solutions, with a zeta potential of ∼33 mV beneficial for mitochondrial targeting. In vitro experiments with MCF-7 and HeLa cancer cells indicated the effective localization of BTPP and BeTPP within cancer-cell mitochondria. Under light irradiation, BTPP and BeTPP exhibited robust photo-induced therapeutic effects in both cell lines, with half-maximal inhibitory concentration (IC50) values of ∼30 and ∼55 nM, respectively.

Spin-orbit charge-transfer intersystem crossing in heavy-atom-free orthogonal covalent boron-dipyrromethene heterodimers

Boron-dipyrromethene (BODIPY) derivatives are prospective organic-based triplet photosensitizers. Since the triplet generation yield of the parent BODIPY is low, heavy atoms are widely used to improve the triplet yield. However, the dimerization of BODIPYs can also significantly improve their ability to produce triplets. Through a comparative study of the triplet formation dynamics of two heavy-atom-free orthogonal covalent BODIPY heterodimers that differ in their dihedral angles, we have demonstrated that the mechanism of spin-orbit charge-transfer intersystem crossing (SOCT-ISC) promotes the triplet generation of BODIPY heterodimers in solution. Different from the general understanding of SOCT-ISC, the heterodimer with a smaller dihedral angle and low structural rigidity showed better triplet generation due to (a) the stronger inter-chromophoric interaction in the heterodimer, which promoted the formation of a solvent-stabilized charge-transfer (CT) state, (b) the more favorable energy level alignment with sizeable spin-orbit coupling strength, and (c) the balance between the stabilized singlet CT state and limited direct charge recombination to the ground state in a weakly polar solvent. The complete spectral characterization of the triplet formation dynamics clarified the SOCT-ISC mechanism and important factors affecting the triplet generation in BODIPY heterodimers.

Formylation as a Chemical Tool to Modulate the Performance of Photosensitizers Based on Boron Dipyrromethene Dimers

Heavy-atom-free photosensitizers are envisioned as the next generation of photoactive molecules for photo-theragnosis. In this approach, and after suitable irradiation, a single molecular scaffold is able to visualize and kill tumour cells by fluorescence signalling and photodynamic therapy (PDT), respectively, with minimal side effects. In this regard, BODIPY-based orthogonal dimers have irrupted as suitable candidates for this aim. Herein, we analyse the photophysical properties of a set of formyl-functionalized BODIPY dimers to ascertain their suitability as fluorescent photosensitizers. The conducted computationally aided spectroscopic study determined that the fluorescence/singlet oxygen generation dual performance of these valuable BODIPY dimers not only depends on the BODIPY-BODIPY linkage and the steric hindrance around it, but also can be modulated by proper formyl functionalization at specific chromophoric positions. Thus, we propose regioselective formylation as an effective tool to modulate such a delicate photonic balance in BODIPY-based dimeric photosensitizers. The taming of the excited-state dynamics, in particular intramolecular charge transfer as the key underlying process mediating fluorescence deactivation vs. intersystem crossing increasing, could serve to increase fluorescence for brighter bioimaging, enhance the generation of singlet oxygen for killing activity, or balance both for photo-theragnosis.

Modification of a SOCT-ISC type triphenylamine-BODIPY photosensitizer by a multipolar dendrimer design for photodynamic therapy and two-photon fluorescence imaging

In this study, a series of multipolar triphenylamine-BODIPY photosensitizers T-BDP_n (n = 1, 2, 3) was synthesized. Compared with T-BDP₁ of D-A configuration, the multipolar T-BDP₃ dendrimer have higher singlet oxygen efficiency (44%), better fluorescence quantum yield (7.45%), and could be used in the simulated photodynamic therapy in A-549 cells and two-photon fluorescence imaging in zebrafish. The theoretical calculation and fs-transient absorption spectra indicated that the reason of its higher singlet oxygen efficiency was that the multipolar T-BDP₃ dendrimer could generate more nearly degenerate charge transfer (CT) states and triplet states, which could further increase the possibility of spin-orbit charge-transfer intersystem crossing (SOCT-ISC) process. In the simulated photodynamic therapy of A-549 cells, T-BDP₃ shows good cytocompatibility, great phototoxicity with its IC₅₀ value of 3.17 μM, and could kill cancer cells effectively with the dosage of 5 μM under 10 min irradiation in the AO/EB double-staining experiment. In the fluorescence imaging of zebrafish, the experiment results indicate that T-BDP₃ could generate superoxide radical (O₂˙^-) in the body of zebrafish and could be applied to the two-photon fluorescence imaging under 800 nm excitation. The above experiment results shown that the multipolar dendrimer design was an effective approach to improve the key parameters of SOCT-ISC-type BODIPY photosensitizer and was ready for further two-photon photodynamic therapy in organisms.

Solvent-Dependent Fluorescence Properties of CH2-bis(BODIPY)s

Biocompatible luminophores based on organic dyes, which have fluorescence characteristics that are highly sensitive to the properties of the solvating medium, are of particular interest as highly sensitive, selective, and easy-to-use analytical agents. We found that BODIPY dimers (2,2'-, 2,3'-3,3'-CH₂-bis(BODIPY) (1-3)) demonstrate fluorescence characteristics with a high sensitivity to the presence of polar solvents. The intense fluorescence of 1-3 in nonpolar/low-polarity solvents is dramatically quenched in polar media (acetone, DMF, and DMSO). It has been established that the main reason for CH₂-bis(BODIPY) fluorescence quenching is the specific solvation of dyes by electron-donating molecules (Solv) with the formation of stable supramolecular CH₂-bis(BODIPY)·2Solv structures. Using steady-state absorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, and computational modeling, the formation mechanism, composition, and structure of CH₂-bis(BODIPY)·2Solv supramolecular complexes have been substantiated, and their stability has been evaluated. The results show the promise of developing fluorescent probes based on CH₂-bis(BODIPY)s for detecting toxic N/O-containing compounds in solutions.

Halogenated BODIPY photosensitizers: Photophysical processes for generation of excited triplet state, excited singlet state and singlet oxygen

We have systematically examined the formation of singlet oxygen O₂(¹Δ_g), the excited triplet state (T₁), and excited singlet state (S₁) for halogenated BODIPY photosensitizers (halogen = Cl, Br, and I) in eight solvents to understand how halogen atoms and solvent affect these properties. The phosphorescence spectra and lifetimes of singlet oxygen generated by these halogenated BODIPYs have been measured by steady state/time resolved NIR emission, while the formation quantum yield of singlet oxygen (Φ_Δ) has been determined by chemical method using diphenylisobenzofuran (DPBF) as the trapping agent. The formation quantum yield Φ_Δ of singlet oxygen can be as high as 0.96 for iodinated BODIPY and 0.71 for brominated BODIPY. The triplet state T₁ absorption spectra of brominated and iodinated BODIPYs have been recorded by laser flash photolysis method, in which T₁ shows high formation efficiency and long lifetime. The formation and decay of excited singlet state S₁ of four BODIPYs have been measured by ground state (S₀) absorption and steady state/time resolved fluorescence. The results show that larger halogen atoms on BODIPY core lead to smaller fluorescence quantum yield, shorter fluorescence lifetime and higher singlet oxygen formation quantum yield due to heavy atom effect that promotes the formation of triplet state. On the other hand, higher solvent polarity causes lower singlet oxygen formation quantum yield, smaller fluorescence quantum yield, and shorter fluorescence lifetime. This solvent effect is explained by the presence of photoinduced charge transfer (ICT) process from halogen atoms to BODIPY. The ICT efficiency has been estimated and the results are agreed with ICT theory. ICT process in halogenated BODIPYs has never been revealed in literature. HOMO/LUMO obtained from DFT calculation also supports the presence of ICT. The involvement of ICT in the photosensitizing process of halogenated BODIPYs provides new insights for designing BODIPY photosensitizers for photodynamic therapy of tumor.

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.

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.

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

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long‐lived triplet charge‐transfer (³CT) state, based on the electron spin control using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT‐ISC takes 8 ns to produce the ³NI* state. Then the slow secondary CS (125 ns) gives the long‐lived ³CT state (0.94 μs in deaerated n‐hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as ¹NI*→¹CT→³NI*→³CT. With time‐resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron‐spin polarization pattern was observed for the naphthalimide‐localized triplet state. Our spiro compact dyad structure and the electron spin‐control approach is different to previous methods for which invoking transition‐metal coordination or chromophores with intrinsic ISC ability is mandatory.

Exploring BODIPY Derivatives as Singlet Oxygen Photosensitizers for PDT

This minireview is devoted to honoring the memory of Dr. Thomas Dougherty, a pioneer of modern photodynamic therapy (PDT). It compiles the most important inputs made by our research group since 2012 in the development of new photosensitizers based on BODIPY chromophore which, thanks to the rich BODIPY chemistry, allows a finely tuned design of the photophysical properties of this family of dyes to serve as efficient photosensitizers for the generation of singlet oxygen. These two factors, photophysical tuning and workable chemistry, have turned BODIPY chromophore as one of the most promising dyes for the development of improved photosensitizers for PDT. In this line, this minireview is mainly related to the establishment of chemical methods and structural designs for enabling efficient singlet oxygen generation in BODIPYs. The approaches include the incorporation of heavy atoms, such as halogens (iodine or bromine) in different number and positions on the BODIPY scaffold, and also transition metal atoms, by their complexation with Ir(III) center, for instance. On the other hand, low-toxicity approaches, without involving heavy metals, have been developed by preparing several orthogonal BODIPY dimers with different substitution patterns. The advantages and drawbacks of all these diverse molecular designs based on BODIPY structural framework are described.

Electronic coupling and spin-orbit charge transfer intersystem crossing (SOCT-ISC) in compact BDP-carbazole dyads with different mutual orientations of the electron donor and acceptor

In order to study the spin-orbit charge transfer induced intersystem crossing (SOCT-ISC), Bodipy (BDP)-carbazole (Cz) compact electron donor/acceptor dyads were prepared. Charge transfer (CT) emission bands were observed for dyads showing strong electronic coupling between the donor and the acceptor (coupling matrix elements V_DA, 0.06 eV-0.18 eV). Depending on the coupling magnitude, the CT state of the dyads can be either dark or emissive. Equilibrium between the ¹LE (locally excited) state and the ¹CT state was confirmed by temperature-dependent fluorescence studies. Efficient ISC was observed for the dyads with Cz connected at the meso-position of the BDP. Interestingly, the dyad with non-orthogonal geometry shows the highest ISC efficiency (Φ_Δ = 58%), which is different from the previous conclusion. The photo-induced charge separation (CS, time constant: 0.7 ps) and charge recombination (CR, ∼3.9 ns) were studied by femtosecond transient absorption spectroscopy. Nanosecond transient absorption spectroscopy indicated that the BDP-localized triplet state was exceptionally long-lived (602 µs). Using pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the SOCT-ISC mechanism was confirmed, and we show that the electron spin polarization of the triplet state is highly dependent on the mutual orientation of the donor and acceptor. The dyads were used as triplet photosensitizers for triplet-triplet-annihilation (TTA) upconversion, and the quantum yield is up to 6.7%. TTA-based delayed fluorescence was observed for the dyads (τ_DF = 41.5 µs). The dyads were also used as potent photodynamic therapy reagents (light toxicity of IC₅₀ = 0.1 µM and dark toxicity of IC₅₀ = 70.8 µM).

Elucidating heterogeneous photocatalytic superiority of microporous porphyrin organic cage

The investigation on the catalytic properties of porous organic cages is still in an initial stage. Herein, the reaction of cyclohexanediamine with 5,15-di[3',5'-diformyl(1,1'-biphenyl)]porphyrin affords a porphyrin tubular organic cage, PTC-1(2H). Transient absorption spectroscopy in solution reveals much prolonged triplet lifetime of PTC-1(2H) relative to monomer reference, illustrating the unique photophysical behavior of cagelike photosensitizer. The long triplet lifetime ensures high-efficiency singlet oxygen evolution according to homogeneous photo-bleach experiment, electron spin-resonance spectroscopy, and aerobic photo-oxidation of benzylamine. Furthermore, microporous supramolecular framework of PTC-1(2H) is able to promote the heterogeneous photo-oxidation of various primary amines with conversion efficiency above 99% under visible light irradiation. These results indicate the great application potentials of porous organic cages in heterogeneous phase.

Halogen Bonding Facilitates Intersystem Crossing in Iodo-BODIPY Chromophores

BODIPY chromophores can serve as organic-based triplet photosensitizers for a wide range of applications. To perform this function, the formation of the triplet state is critical, and a better understanding of how to modulate the formation of the triplet state could lead to further advances in BODIPY-based sensitizers for solar energy conversion and photodynamic therapy. In this work we investigate the ability of halogen bonding, a noncovalent solvent interaction, to facilitate intersystem crossing in a diiodo-BODIPY. Ultrafast transient absorption spectroscopy is applied to diiodo-BODIPY in the presence of pyridine-based halogen bonding solvent molecules to determine the rate constants for intersystem crossing. We find that halogen bonding facilitates the formation of the triplet state by increasing the intersystem crossing rate constant of diiodo-BODIPY. The results are interpreted in terms of the Marcus expression for intersystem crossing. Quantum chemical calculations show that halogen bonding acts to alter both the spin-orbit coupling terms and the relative energetics of the singlet and triplet states.

Making triplets from photo-generated charges: observations, mechanisms and theory

Photo-excitation of electron donor–acceptor systems can lead to the generation of a charge separated state (CT). Sometimes the charge recombination occurs mainly to the local triplet excited state (T₁). How does the spin flip?

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

Recently varieties of Bodipy derivatives showing intersystem crossing (ISC) have been reported as triplet photosensitizers, and the application of these compounds in photocatalysis, photodynamic therapy (PDT), and photon upconversion are promising. In this review we summarized the recent development in the area of Bodipy-derived triplet photosensitizers and discussed the molecular structural factors that enhance the ISC ability. The compounds are introduced based on their ISC mechanisms, which include the heavy atom effect, exciton coupling, charge recombination (CR)-induced ISC, using a spin converter and radical enhanced ISC. Some transition metal complexes containing Bodipy chromophores are also discussed. The applications of these new triplet photosensitizers in photodynamic therapy, photocatalysis, and photon upconversion are briefly commented on. We believe the study of new triplet photosensitizers and the application of these novel materials in the abovementioned areas will be blooming.

Engineering pH-Responsive BODIPY Nanoparticles for Tumor Selective Multimodal Imaging and Phototherapy

It is a challenge to develop multifunctional theranostic agents in one molecule, which simultaneously possesses tumor imaging ability with a high signal-to-noise ratio and excellent therapeutic activity. In this work, we synthesized and screened a series of BODIPY (BDP) with various absorption and fluorescence. The interplay of the molecular structure, pH-sensitive absorption and emission, and photodynamic and photothermal activities was well studied in detail. Photoinduced electron transfer, intramolecular charge transfer, and heavy atom effect were leveraged to engineer BDP with tumor imaging and therapeutic functions. The BDP nanoparticle formulations possessed multifunctional biological features, including selective treatment of cancer cells, near-infrared fluorescence, photoacoustic and computed tomography imaging, and photodynamic and photothermal therapy, as validated by cellular and animal experiments. These results not only give a new horizon to multifunctional BDP for biological applications but also show a new way to design the organic dye for tumor imaging and phototherapy.

Balance between Triplet States in Photoexcited Orthogonal BODIPY Dimers

The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electron spin polarization (ESP) of the triplet state of the dimers, accessed with spin-orbit charge-transfer ISC, is different from that of the monomer (spin-orbit coupling-induced ISC). The TREPR spectra show that the triplet state initially formed by charge recombination is localized on either of two subunits, with different preference and ESP patterns. On the basis of the relative orientation of the respective zero field splitting principal axes, the T_x state on one subunit and the T_z state on another subunit in the dimer are overpopulated. The balance between the two triplet states is confirmed by the temperature dependency of the population ratio. No quintet state was detected with TREPR down to 20 K; thus, the recently proposed singlet fission ISC mechanism is excluded.

Luminescent properties of new 2,2-, 2,3- and 3,3-CH2-bis(BODIPY)s dyes: Structural and solvation effects

In this paper the synthesis and spectral properties of three new dimeric bis(BODIPY)s with two indacene domains connected by a methylene (-CH₂-) spacer at 2,2-, 2,3- or 3,3- positions were reported. It was found bis(BODIPY)s exhibit a high sensitivity of fluorescence to the medium properties. To interpret solvatochromic effects of bis(BODIPY)s, a multilinear correlation analysis of bis(BODIPY)s fluorescence quantum yields with respect to solvent different parameters was carried out. To understand the features of the spectral properties of bis(BODIPY)s, we carried out a thorough quantum-chemical analysis of the structural, conformational, and spectral characteristics of bis(BODIPY)s. The obtained bis(BODIPY)s have a high potential for application as sensors of medium polarity.

Methylated Unsymmetric BODIPY Compounds: Synthesis, High Fluorescence Quantum Yield and Long Fluorescence Time

We show that unsymmetric BODIPY compounds with one, two, and three methyl groups can be synthesized easily and efficiently by the unsymmetric reaction method. Their steady state and time-resolved fluorescence properties are examined in solvents of different polarity. These compounds show high fluorescence quantum yields (0.87 to 1.0), long fluorescence lifetimes (5.89 to 7.40 ns), and small Stokes shift (199 to 443 cm^-1). The methyl substitution exhibits influence on the UV-Vis absorption and fluorescence properties, such as the blue shift in emission and absorption spectra. It is the number rather than the position of methyls that play major roles. Except for 3 M-BDP, the increase in the number of methyls on BODIPY core leads to the increase in both fluorescence quantum yield and radiative rate constant, but causes the decrease in fluorescence lifetime. H-bonding solvents increase both the fluorescence lifetime and quantum yields. The methylated BODIPYs show the ability to generate singlet oxygen (¹Δ_g) which is evidenced by near-IR luminescence and DPBF chemical trapping techniques. The formation quantum yield of singlet oxygen (¹Δ_g) for the compounds is up to 0.15 ± 0.05.

Rational Design of Conjugated Photosensitizers with Controllable Photoconversion for Dually Cooperative Phototherapy

High-performance photosensitizers are highly desired for achieving selective tumor photoablation in the field of precise cancer therapy. However, photosensitizers frequently suffer from limited tumor suppression or unavoidable tumor regrowth due to the presence of residual tumor cells surviving in phototherapy. A major challenge still remains in exploring an efficient approach to promote dramatic photoconversions of photosensitizers for maximizing the anticancer efficiency. Here, a rational design of boron dipyrromethene (BDP)-based conjugated photosensitizers (CPs) that can induce dually cooperative phototherapy upon light exposure is demonstrated. The conjugated coupling of BDP monomers into dimeric BDP (di-BDP) or trimeric BDP (tri-BDP) induces photoconversions from fluorescence to singlet-to-triplet or nonradiative transitions, together with distinctly redshifted absorption into the near-infrared region. In particular, tri-BDP within nanoparticles shows preferable conversions into both primary thermal effect and minor singlet oxygen upon near-infrared light exposure, dramatically achieving tumor photoablation without any regrowth through their cooperative anticancer efficiency caused by their dominant late apoptosis and moderate early apoptosis. This rational design of CPs can serve as a valuable paradigm for cooperative cancer phototherapy in precision medicine.

Singlet Fission Mediated Photophysics of BODIPY Dimers

The photodynamics of an orthogonal BODIPY dimer, particularly the formation of triplet states, has been explored by femtosecond and nanosecond transient absorption measurements. The short time scale data show the appearance of transient features of triplet character that, according to quantitative analysis of their intensities, account for more than 100% of the initially excited molecules, which reveals the occurrence of a singlet fission process in the isolated dimers. The formation rate of the triplet correlated state ¹(TT) is found to depend on the solvent polarity, pointing to the mediation of a charge transfer character state. The dissociation of the ¹(TT) state into pairs of individual triplets determines the triplet yield measured in the long time scales. The kinetic model derived from the results provides a comprehensive view of the photodynamics of BODIPY dimers and permits rationalization of the photophysical parameters of these systems.

Spin–orbit charge transfer intersystem crossing in perylenemonoimide–phenothiazine compact electron donor–acceptor dyads

Efficient triplet excited state production (57%) was observed for perylenemonoimide–phenothiazine compact electron donor–acceptor dyads based on spin–orbit charge transfer ISC.

Recent progress in heavy atom-free organic compounds showing unexpected intersystem crossing (ISC) ability

The intersystem crossing (ISC) of the heavy atom-free triplet photosensitizers was summarised, including the spin–orbit charge transfer ISC mechanism.

Control of triplet state generation in heavy atom-free BODIPY–anthracene dyads by media polarity and structural factors

Heavy atom-free BODIPY–anthracene dyads show triplet excited state formation via PeT, controlled by molecular rotation and environmental polarity.