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

Radical enhanced intersystem crossing mechanism, electron spin dynamics of high spin states and their applications in the design of heavy atom-free triplet photosensitizers

Heavy atom-free triplet photosensitizers (PSs) can overcome the high cost and biological toxicity of traditional molecular systems containing heavy atoms (such as Pt(II), Ir(III), Ru(II), Pd(II), Lu(III), I, or Br atoms) and, therefore, are developing rapidly. Connecting a stable free radical to the chromophore can promote the intersystem crossing (ISC) process through electron spin exchange interaction to produce the triplet state of the chromophore or the doublet (D) and quartet (Q) states when taking the whole spin system into account. These molecular systems based on the radical enhanced ISC (REISC) mechanism are important in the field of heavy atom-free triplet PSs. The REISC system has a simple molecular structure and good biocompatibility, and it is especially helpful for building high-spin quantum states (D and Q states) that have the potential to be developed as qubits in quantum information science. This review introduces the molecular structure design for the purpose of high-spin states. Time-resolved electron paramagnetic resonance (TREPR) is the most important characterization method to reveal the properties of these molecular systems, generation mechanism and electron spin polarization (ESP) of the high spin states. The spin polarization manipulation of high spin states and potential application in the field of quantum information engineering are also summarized. Moreover, molecular design principles of the REISC system to obtain long absorption wavelength, high triplet state quantum yield and long triplet state lifetime are introduced, as well as applications of the compounds in triplet-triplet annihilation upconversion, photodynamic therapy and bioimaging. This review is useful for the design of heavy atom-free triplet PSs based on the radical-chromophore molecular structure motif and the study of the photophysics of the compounds, as well as the electron spin dynamics of the multi electron system upon photoexcitation.

Construction of Boron Difluoride Complexes with Asymmetric N,N'-Bidentate Ligands

Boron difluoride (BF2) complexes with asymmetrical N,N'-bidentate ligands have received increasing attention due to their fascinating properties and broad applications. They are generally constructed in two steps: ligand formation, followed by boron complexation. This review focuses on categorizing these BF2 complexes based on the key synthetic strategies that have been applied in the ligand formation steps. The post-functionalization, properties and applications of different types of BF2 complexes are presented. Their challenges and opportunities are also discussed. This should help the future rational design and synthesis of BF2 complexes with intriguing properties and practical applications.

Helical BODIPY Dyes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Despite of having many advantages over the popular cancer therapies, photodynamic therapy still is not widely acceptable for clinical applications. Scarcity of efficient triplet photosensitizers (PSs) is one of the main bottlenecks for this. Although numerous heavy atom attached PSs are developed as PDT agents, but these are not suitable for clinical applications due to several reasons. Thus, development of heavy-atom-free organic PSs is urgently required. In this context, development of new type of helical BODIPYs as PSs for PDT is discussed. BODIPYs have rich photophysical properties and structural helicity further enhances their triplet conversion rates. This new concept of structural helicity to enhance the triplet conversion of BODIPYs is discussed with reported helical BODIPYs. Helical geometries of these dyes are checked by X-ray crystallography studies and their high triplet conversions as compared to planar BODIPYs are also confirmed. Importantly, these dyes have high triplet lifetimes and are capable of generating high singlet oxygens even in hypoxia condition as compared to conventional heavy atom attached BODIPYs. All these make the helical BODIPYs excellent candidates as PDT agents. Finally, their successful applications as PDT agents in killing of various types of cancer cells are also discussed. The results are encouraging which indicate that helical BODIPYs could be next generation heavy-atom-free PSs for PDT applications.

Driving Triplet State Population in Benzothioxanthene Imide Dyes: Let's twist!

Controlling the formation of photoexcited triplet states is critical for many (photo)chemical and physical applications. Here, we demonstrate that a permanent out-of-plane distortion of the benzothioxanthene imide (BTI) dye promotes intersystem crossing by increasing spin-orbit coupling. This manipulation was achieved through a subtle chemical modification, specifically the bay-area methylation. Consequently, this simple yet efficient approach expands the catalog of known molecular engineering strategies for synthesizing heavy atom-free, dual redox-active, yet still emissive and synthetically accessible photosensitizers.

Unsymmetrical Benzothieno-Fused BODIPYs as Efficient NIR Heavy-Atom-Free Photosensitizers

Heavy-atom-free photosensitizers are potentially suitable for use in photodynamic therapy (PDT). In this contribution, a new family of unsymmetrical benzothieno-fused BODIPYs with reactive oxygen efficiency up to 50% in air-saturated toluene was reported. Their efficient intersystem crossing (ISC) resulted in the generation of both 1O2 and O2-• under irradiation. More importantly, the PDT efficacy of a respective 4-methoxystyryl-modified benzothieno-fused BODIPY in living cells exhibited an extremely high phototoxicity with an ultralow IC50 value of 2.78 nM. The results revealed that the incorporation of an electron-donating group at the α-position of the unsymmetrical benzothieno-fused BODIPY platform might be an effective approach for developing long-wavelength absorbing heavy-atom-free photosensitizers for precision cancer therapy.

Synthesis, Structures and Photophysical Properties of Asymmetric Fulvene-[b]-fused BODIPYs

Herein, we developed a one-pot procedure to synthesize novel fulvene-[b]-fused BODIPYs from α-(2-alkynylphenyl)-pyrrole and acylpyrrole, using 5-exo cyclization as the key transformation. Compared to benzene-[b]-fused BODIPYs, although they have similar chemical compositions, their structures and properties significantly differ from each other, which can be attributed to the less aromaticity of the fulvene linker than benzene. Notably, fulvene-[b]-fused BODIPY 1 b exhibits helical-twisted core skeleton, intensified red-shifted absorption, and peak fluorescence. In addition, the pathway of this one-pot reaction and the mechanism of POCl3 mediated 5-exo cyclization have been proposed by a combining experimental and computational study.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

Kinetic effects in singlet oxygen mediated oxidations by immobilized photosensitizers on silica

Singlet oxygen (1O2) mediated photo-oxidations are important reactions involved in numerous processes in chemical and biological sciences. While most of the current research works have aimed at improving the efficiencies of these transformations either by increasing 1O2 quantum yields or by enhancing its lifetime, we establish herein that immobilization of a molecular photosensitizer onto silica surfaces affords significant, substrate dependant, enhancement in the reactivity of 1O2. Probing a classical model reaction (oxidation of Anthracene-9, 10-dipropionic acid, ADPA or dimethylanthracene, DMA) with various spectrofluorimetric techniques, it is here proposed that an interaction between polar substrates and the silica surface is responsible for the observed phenomenon. This discovery could have a direct impact on the design of future photosensitized 1O2 processes in various applications ranging from organic photochemistry to photobiology.

Porphyrin- and Bodipy-helicene conjugates: syntheses, separation of enantiomers and chiroptical properties

The synthesis of several new compounds containing a chromophore and a helicenic moiety is reported. The preparation, characterisation and some physico-chemical studies are detailed. In particular, the two enantiomers of several chiral molecules of this type were separated by chiral HPLC (both analytically and in a preparative way) and their racemisation rates were determined for short-lived species. Electronic circular dichroism (ECD) and circular polarised luminescence (CPL) measurements were performed for the compounds with a very long racemisation half-life. Chiral porphyrins and Bodipys both gave ECD and CPL responses over a large area of the visible spectrum.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

Pictet-Spengler synthesis of twisted quinoline-fused BODIPYs as heavy-atom-free photosensitizers

Singly and doubly quinoline-fused BODIPYs were effectively synthesized through a reaction sequence consisting of the reduction of nitrophenyl-substituted BODIPYs and subsequent Pictet-Spengler cyclization. The combination of the BODIPY core and fused quinoline rings imposed significantly twisted conformations in the quinoline-fused BODIPYs (around 20.0° deviation from coplanarity obtained from X-ray crystal structure analysis). These twisted BODIPYs showed significantly reduced LUMO, redshifted absorption/emission bands, high molar extinction coefficients and satisfactory reactive oxygen species generation efficiency up to 0.56, indicating potential use as heavy-atom-free photosensitizers.

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.

Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C-N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin-spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ_Δ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE_S1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ_Δ is increased to 40%. The improved ISC yield is rationalized by the presence of a T₂ state, localized on the anthryl unit, with energy close to that of the S₁ state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T_z sublevel of the T₁ state overpopulated. The small zero-field splitting D parameter (-1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S₁/T_n energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.

Heavy Atom-Free Triplet Photosensitizers: Molecular Structure Design, Photophysical Properties and Application in Photodynamic Therapy

Photodynamic therapy (PDT) is a promising method for the treatment of cancer, because of its advantages including a low toxicity, non-drug-resistant character, and targeting capability. From a photochemical aspect, a critical property of triplet photosensitizers (PSs) used for PDT reagents is the intersystem crossing (ISC) efficiency. Conventional PDT reagents are limited to porphyrin compounds. However, these compounds are difficult to prepare, purify, and derivatize. Thus, new molecular structure paradigms are desired to develop novel, efficient, and versatile PDT reagents, especially those contain no heavy atoms, such as Pt or I, etc. Unfortunately, the ISC ability of heavy atom-free organic compounds is usually elusive, and it is difficult to predict the ISC capability of these compounds and design novel heavy atom-free PDT reagents. Herein, from a photophysical perspective, we summarize the recent developments of heavy atom-free triplet PSs, including methods based on radical-enhanced ISC (REISC, facilitated by electron spin-spin interaction), twisted π-conjugation system-induced ISC, the use of fullerene C₆₀ as an electron spin converter in antenna-C₆₀ dyads, energetically matched S₁/T_n states-enhanced ISC, etc. The application of these compounds in PDT is also briefly introduced. Most of the presented examples are the works of our research group.

Core-Twist Reduces the Triplet Formation Efficiency in Brominated Perylene Diimides

Bromination is a recent approach to achieve intersystem crossing (ISC) in perylene diimides (PDIs). Herein, we explore the triplet formation dynamics in two tetrabrominated PDI (PDI-Br₄) positional isomers with planar (P-PDI) and twisted (T-PDI) π-conjugated frameworks. In contrast to the known effect where the planar geometry favors fluorescence, T-PDI shows higher fluorescence (ϕ_f = 0.64) than the planar counterpart P-PDI (ϕ_f = 0.42). P-PDI possesses near-degenerate S₁ and T₃/T₄ states and a larger spin-orbit coupling (SOC). Core-twisting has a pronounced effect on the absorption spectra due to symmetry breaking and would open up multiple ISC pathways, albeit with a lower SOC. Low-energy singlet-triplet state crossings within the Franck-Condon region would facilitate ultrafast triplet generation via the S₁-T₃/T₄ ISC pathway in P-PDI. In comparison, such crossings occur at relatively higher energy, reducing the triplet formation efficiency in T-PDI.

Synthesis and fluorescence properties of red-to-near-infrared-emitting push-pull dyes based on benzodioxazole scaffolds

Fluorescence imaging with high temporal and spatial resolution has emerged as one of the most promising techniques to monitor biomolecules and biological processes in living systems. Among many kinds of small molecular fluorescent dyes, 2,1,3-benzoxadiazole (BD) derivatives have been widely applied in many chemical and biological applications due to their excellent photophysical properties. However, only a limited number of BD dyes with long emission wavelengths were reported. Herein, we have reported a new class of red-to near-infrared-emitting small molecular dyes 2a-3a based on benzodioxazole scaffolds, which are named VBDfluors. To bathochromically shift both absorption and emission, the conjugation system was extended by introducing electron-withdrawing group-substituted vinyl groups at position 7 via a Knoevenagel condensation reaction. The basic photophysical properties of VBDfluors were detected and summarized. The VBDfluors display excellent photophysical properties, including emission in the red-to-NIR region, large Stokes shifts, good stability/photostability and cell permeability. The geometry of the molecules was optimized by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Bioimaging results indicated that 2a and 3a exhibited excellent cell permeability and could be utilized for visualization of lipid droplets in living cells.