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/Tn 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.

Novel Water-Soluble Chlorin-Based Photosensitizer for Low-Fluence Photodynamic Therapy.

Photodynamic therapy (PDT), performed with low-fluence rates, can improve antitumor responses and prevent adverse effects. However, photosensitizers (PSs) for low-fluence PDT treatment are rarely reported. Herein, we exploited an amphiphilic chlorin-based PS, named DYSP-C34, which has a variety of beneficial biological properties, such as improved water solubility, better cellular permeability, specific localization and enhanced phototoxicity under low light dose irradiation. In addition, DYSP-C34 could effectively accumulate in a mouse subcutaneous xenograft tumor and exhibit substantial tumor regression after irradiation with an extremely low light fluence (6 J/cm2). Meanwhile, the excellent phototoxicity could stimulate the host immune system and lead to a strong inhibition of tumor growth synergistically. These results indicated the potential value of DYSP-C34 as a chlorin-type PS for low-fluence PDT application.

Ru(ii) and Ir(iii) phenanthroline-based photosensitisers bearing o-carborane: PDT agents with boron carriers for potential BNCT.

Four novel transition metal-carborane photosensitisers were prepared by Sonogashira cross-coupling of 1-(4-ethynylbenzyl)-2-methyl-o-carborane (A-CB) with halogenated Ru(ii)- or Ir(iii)-phenanthroline complexes. The resulting boron-rich complexes with one (RuCB and IrCB) or two carborane cages (RuCB2 and IrCB2) were spectroscopically characterised, and their photophysical properties investigated. RuCB displayed the most attractive photophysical properties in solution (λem 635 nm, τT 2.53 µs, and φp 20.4%). Nanosecond time-resolved transient absorption studies were used to explore the 3MLCT nature of the triplet excited states, and the highest singlet oxygen quantum yields (ΦΔ) were obtained for the mono-carborane-phenanthroline complexes (RuCB: 52% and IrCB: 25%). None of the complexes produce dark toxicity in SKBR-3 cells after incubation under photodynamic therapy (PDT) conditions. Remarkably, mono-carboranes RuCB and IrCB were the best internalised by the SKBR-3 cells, demonstrating the first examples of tris-bidentate transition metal-carborane complexes acting as triplet photosensitisers for PDT with a high photoactivity; RuCB or IrCB killed ∼50% of SKBR-3 cells at 10 µM after irradiation. Therefore, the high-boron content and the photoactive properties of these photosensitisers make them potential candidates as dual anti-cancer agents for PDT and Boron Neutron Capture Therapy (BNCT).

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.

Twisted BODIPY derivative: intersystem crossing, electron spin polarization and application as a novel photodynamic therapy reagent.

The photophysical properties of a heavy atom-free BODIPY derivative with a twisted π-conjugated framework were studied. Efficient intersystem crossing (ISC quantum yield: 56%) and an exceptionally long-lived triplet state were observed (4.5 ms in solid polymer film matrix and 197.5 µs in solution). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations confirmed the delocalization of the triplet state on the whole twisted π-conjugated framework and the zero-field-splitting (ZFS) D parameter of D = -69.5 mT, which is smaller than that of 2,6-diiodoBODIPY (D = -104.6 mT). The electron spin polarization (ESP) phase pattern of the triplet state TREPR spectrum of the twisted BODIPY is (a, a, e, a, e, e), which is different from that of 2,6-diiodo BODIPY (e, e, e, a, a, a), indicating that the electron spin selectivity of the ISC of the twisted structure is different from that of the spin orbital coupling effect. According to the computed spin-orbit coupling matrix elements (0.154-1.964 cm-1), together with the matched energy of the S1/Tn states, ISC was proposed to occur via S1→T2/T3. The computational results were consistent with TREPR results on the electron spin selectivity (the overpopulation of the TY sublevel of the T1 state). The advantage of the long-lived triplet state of the twisted BODIPY was demonstrated by its efficient singlet oxygen (1O2) photosensitizing (ΦΔ = 50.0%) even under a severe hypoxia atmosphere (pO2 = 0.2%, v/v). A high light toxicity (EC50 = 1.0 µM) and low dark toxicity (EC50 = 78.5 µM) were observed for the twisted BODIPY, and thus the cellular studies demonstrate its potential as a novel potent heavy atom-free photodynamic therapy (PDT) agent.

Heavy-Atom-Free Photosensitizers: From Molecular Design to Applications in the Photodynamic Therapy of Cancer.

Photodynamic therapy (PDT) is a clinically approved therapeutic modality that has shown great potential for the treatment of cancers owing to its excellent spatiotemporal selectivity and inherently noninvasive nature. However, PDT has not reached its full potential, partly due to the lack of ideal photosensitizers. A common molecular design strategy for effective photosensitizers is to incorporate heavy atoms into photosensitizer structures, causing concerns about elevated dark toxicity, short triplet-state lifetimes, poor photostability, and the potentially high cost of heavy metals. To address these drawbacks, a significant advance has been devoted to developing advanced smart photosensitizers without the use of heavy atoms to better fit the clinical requirements of PDT. Over the past few years, heavy-atom-free nonporphyrinoid photosensitizers have emerged as an innovative alternative class of PSs due to their superior photophysical and photochemical properties and lower expense. Heavy-atom-free nonporphyrinoid photosensitizers have been widely explored for PDT purposes and have shown great potential for clinical oncologic applications. Although many review articles about heavy-atom-free photosensitizers based on porphyrinoid structure have been published, no specific review articles have yet focused on the heavy-atom-free nonporphyrinoid photosensitizers.In this account, the specific concept related to heavy-atom-free photosensitizers and the advantageous properties of heavy-atom-free photosensitizers for cancer theranostics will be briefly introduced. In addition, recent progress in the development of heavy-atom-free photosensitizers, ranging from molecular design approaches to recent innovative types of heavy-atom-free nonporphyrinoid photosensitizers, emphasizing our own research, will be presented. The main molecular design approaches to efficient heavy-atom-free PSs can be divided into six groups: (1) the approach based on traditional tetrapyrrole structures, (2) spin-orbit charge-transfer intersystem crossing (SOCT-ISC), (3) reducing the singlet-triplet energy gap (ΔEST), (4) the thionation of carbonyl groups of conventional fluorophores, (5) twisted π-conjugation system-induced intersystem crossing, and (6) radical-enhanced intersystem crossing. The innovative types of heavy-atom-free nonporphyrinoid photosensitizers and their applications in cancer diagnostics and therapeutics will be discussed in detail in the third section. Finally, the challenges that need to be addressed to develop optimal heavy-atom-free photosensitizers for oncologic photodynamic therapy and a perspective in this research field will be provided. We believe that this review will provide general guidance for the future design of innovative photosensitizers and spur preclinical and clinical studies for PDT-mediated cancer treatments.

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low-Dose Photodynamic Therapy.

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×105 m-1 cm-1 at 630 nm), satisfactory triplet quantum yield (ΦT =52 %), and long-lived triplet state (τT =492 µs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 µg kg-1 ), which is several hundred times lower than that of the existing PDT reagents.

Interactive Aggregation-Induced Emission Systems Controlled by Dynamic Covalent Chemistry.

Aggregation-induced emission (AIE) molecules show all kinds of application in biological research, chemical sensing, and medical study. However, most of the reported molecules are based on the performance of the single molecular entity. In this paper, a molecular system for real-time sensing through combination of dynamic covalent chemistry and aggregation-induced emission was rationally designed and tested. The aggregated particles exhibit different fluorescence emission colors upon the addition of various kinds of chemical reagents. The LC-MS analysis reveals that the breakage, formation, and exchange of the disulfide bonds in the molecular system occur spontaneously upon different reagents (base/acid and cysteine), which leads to a change in the proportion of different components in the system accordingly. Meanwhile, the fluorescence emission of the AIE system exhibits blue/red shift accompanied by intensity changes. Moreover, the particle size of the aggregated molecules gradually increased with the change of the chemical environment, which could be the result of the nucleus growing through intermolecular hydrogen bonding among molecular components. Thus, the chemical environment change results in the interactions of molecules, which further leads to the variation of dynamic fluorescence emission and morphology. The result represents a promising future for a dynamic AIE molecular system in the bioimaging and sensing study.

Efficient Radical-Enhanced Intersystem Crossing in an NDI-TEMPO Dyad: Photophysics, Electron Spin Polarization, and Application in Photodynamic Therapy.

A compact naphthalenediimide (NDI)-2,2,6,6-tetramethylpiperidinyloxy (TEMPO) dyad has been prepared with the aim of studying radical-enhanced intersystem crossing (EISC) and the formation of high spin states as well as electron spin polarization (ESP) dynamics. Compared with the previously reported radical-chromophore dyads, the present system shows a very high triplet state quantum yield (ΦT =74 %), a long-lived triplet state (τT =8.7 µs), fast EISC (1/kEISC =338 ps), and absorption in the red spectral region. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy showed that, upon photoexcitation in fluid solution at room temperature, the D0 state of the TEMPO moiety produces strong emissive (E) polarization owing to the quenching of the excited singlet state of NDI by the radical moiety (electron exchange J>0). The emissive polarization then inverts into absorptive (A) polarization within about 3 µs, and then relaxes to a thermal equilibrium while quenching the triplet state of NDI. The formation and decay of the quartet state were also observed. The dyad was used as a three-spin triplet photosensitizer for triplet-triplet annihilation upconversion (quantum yield ΦUC =2.6 %). Remarkably, when encapsulated into liposomes, the red-light-absorbing dyad-liposomes show good biocompatibility and excellent photodynamic therapy efficiency (phototoxicity EC50 =3.22 µm), and therefore is a promising candidate for future less toxic and multifunctional photodynamic therapeutic reagents.

Controllable Photodynamic Therapy Implemented by Regulating Singlet Oxygen Efficiency.

With singlet oxygen (1O2) as the active agent, photodynamic therapy (PDT) is a promising technique for the treatment of various tumors and cancers. But it is hampered by the poor selectivity of most traditional photosensitizers (PS). In this review, we present a summary of controllable PDT implemented by regulating singlet oxygen efficiency. Herein, various controllable PDT strategies based on different initiating conditions (such as pH, light, H2O2 and so on) have been summarized and introduced. More importantly, the action mechanisms of controllable PDT strategies, such as photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT) and some physical/chemical means (e.g. captivity and release), are described as a key point in the article. This review provide a general overview of designing novel PS or strategies for effective and controllable PDT.

Enhancing Photodynamic Therapy through Resonance Energy Transfer Constructed Near-Infrared Photosensitized Nanoparticles.

Photodynamic therapy (PDT) is an important cancer treatment modality due to its minimally invasive nature. However, the efficiency of existing PDT drug molecules in the deep-tissue-penetrable near-infrared (NIR) region has been the major hurdle that has hindered further development and clinical usage of PDT. Thus, herein a strategy is presented to utilize a resonance energy transfer (RET) mechanism to construct a novel dyad photosensitizer which is able to dramatically boost NIR photon utility and enhance singlet oxygen generation. In this work, the energy donor moiety (distyryl-BODIPY) is connected to a photosensitizer (i.e., diiodo-distyryl-BODIPY) to form a dyad molecule (RET-BDP). The resulting RET-BDP shows significantly enhanced absorption and singlet oxygen efficiency relative to that of the acceptor moiety of the photosensitizer alone in the NIR range. After being encapsulated with biodegradable copolymer pluronic F-127-folic acid (F-127-FA), RET-BDP molecules can form uniform and small organic nanoparticles that are water soluble and tumor targetable. Used in conjunction with an exceptionally low-power NIR LED light irradiation (10 mW cm-2 ), these nanoparticles show superior tumor-targeted therapeutic PDT effects against cancer cells both in vitro and in vivo relative to unmodified photosensitizers. This study offers a new method to expand the options for designing NIR-absorbing photosensitizers for future clinical cancer treatments.

Photodynamic effect of light-harvesting, long-lived triplet excited state Ruthenium(II)-polyimine-coumarin complexes: DNA binding, photocleavage and anticancer studies.

Two coumarin based RuII-polyimine complexes (Ru-1 and Ru-2) showing intense absorption of visible light and long-lived triplet excited states (~12-15µs) were used for study of the interaction with DNA. The binding of the complexes with CT-DNA were studied by UV-vis, fluorescence and time-resolved nanosecond transient absorption (ns-TA) spectroscopy. The results suggesting that the complexes interact with CT-DNA by intercalation mode of binding, showing the binding constants (Kb) 6.47×104 for Ru-1 and 5.94×104 M-1 for Ru-2, in contrast no such results were found for Ru-0. The nanosecond transient absorption spectra of these systems in the presence of CT-DNA showing a clear perturbation in the bleaching region was observed compare to buffer alone. Visible light photoirradiation DNA cleavage was investigated for these complexes by treating with the supercoiled pUC19 DNA and irradiated at 450nm. The reactive species produced upon irradiation of current agents is singlet oxygen (1O2), which results in the generation of other reactive oxygen species (ROS). The complexes shown efficient cleavage activity, converted complete supercoiled DNA to nicked circular at as low as 20µM concentration in 30min of light irradiation time. Significant amount of linear form was generated by Ru-1 at the same conditions. Even though Ru-0 has significant 1O2 quantum yield but shown lower cleavage activity compared to other two analogs is due the miserable interaction (binding) with DNA. The cytotoxicity in vitro of the complexes toward HeLa, BEL-7402 and MG-63 cells was assessed by MTT assay. The cellular uptake was observed on BEL-7402 cells under fluorescence microscope. The complexes shown appreciable cytotoxicity towards the cancer cell lines.

Ultralow-Power Near Infrared Lamp Light Operable Targeted Organic Nanoparticle Photodynamic Therapy.

Tissue penetration depth is a major challenge in practical photodynamic therapy (PDT). A biocompatible and highly effective near infrared (NIR)-light-absorbing carbazole-substituted BODIPY (Car-BDP) molecule is reported as a class of imaging-guidable deep-tissue activatable photosensitizers for PDT. Car-BDP possesses an intense, broad NIR absorption band (600-800 nm) with a remarkably high singlet oxygen quantum yield (ΦΔ = 67%). After being encapsulated with biodegradable PLA-PEG-FA polymers, Car-BDP can form uniform and small organic nanoparticles that are water-soluble and tumor-targetable. Rather than using laser light, such nanoparticles offer an unprecedented deep-tissue, tumor targeting photodynamic therapeutic effect by using an exceptionally low-power-density and cost-effective lamp light (12 mW cm-2). In addition, these nanoparticles can be simultaneously traced in vivo due to their excellent NIR fluorescence. This study signals a major step forward in photodynamic therapy by developing a new class of NIR-absorbing biocompatible organic nanoparticles for effective targeting and treatment of deep-tissue tumors. This work also provides a potential new platform for precise tumor-targeting theranostics and novel opportunities for future affordable clinical cancer treatment.

Highly selective detection of 2,4,6-trinitrophenol and Cu(2+) ions based on a fluorescent cadmium-pamoate metal-organic framework.

A luminescent cadmium-pamoate metal-organic framework, [Cd2 (PAM)2 (dpe)2 (H2 O)2 ]⋅0.5(dpe) (1), has been synthesized under hydrothermal conditions by using π-electron-rich ligands 4,4'-methylenebis(3-hydroxy-2-naphthalenecarboxylic acid) (H2 PAM) and 1,2-di(4-pyridyl)ethylene (dpe). Its structure is composed of both mononuclear and dinuclear Cd(II) building units, which are linked by the PAM and dpe ligands, resulting in a (4,8)-connected 3D framework. The π-conjugated dpe guests are located in a 1D channel of 1. The strong emission of 1 could be quenched efficiently by trace amounts of 2,4,6-trinitrophenol (TNP), even in the presence of other competing analogues such as 4-nitrophenol, 2,6-dinitrotoluene, 2,4-dinitrotoluene, nitrobenzene, 1,3-dinitrobenzene, hydroquinone, dimethylbenzene, and bromobenzene. The high sensitivity and selectivity of the fluorescence response of 1 to TNP shows that this framework could be used as an excellent sensor for identifying and quantifying TNP. In the same manner, 1 also exhibits superior selectivity and sensitivity towards Cu(2+) compared with other metal ions such as Zn(2+) , Mn(2+) , Mg(2+) , K(+) , Na(+) , Ni(2+) , Co(2+) , and Ca(2+) . This is the first MOF that can serve as a dual functional fluorescent sensor for selectively detecting trace amounts of TNP and Cu(2+) .

Switching of the triplet excited state of rhodamine-C60 dyads.

Acid-switching of the triplet excited state in rhodamine-C60 dyads was achieved. The rhodamine moiety acts as an acid-activated visible light-harvesting antenna and C60 as the singlet energy acceptor and the spin converter, and production of the triplet state was enhanced in the presence of acid.

Cyclometalated Ir(iii) complexes with styryl-BODIPY ligands showing near IR absorption/emission: preparation, study of photophysical properties and application as photodynamic/luminescence imaging materials.

Heteroleptic C^N cyclometalated iridium(iii) complexes incorporating a monostyryl/distyryl BODIPY ligand via acetylide bonds of 2,2'-bipyridine (bpy) with both absorption (ca. ε = 8.96 × 104 M-1 cm-1, 9.89 × 104 M-1 cm-1, and 7.89 × 104 M-1 cm-1 at 664 nm, 644 nm, and 729 nm for Ir-2, Ir-3 and Ir-4, respectively) and fluorescence emission bands (ca. 624-794 nm for Ir-1, Ir-2, Ir-3 and Ir-4) in the near infra-red region (NIR) and exceptionally long-lived triplet excited states (τ = 156.5 µs for Ir-2) have been reported. Ir(ppy)3 (Ir-0; ppy = 2-phenylpyridine) was used as reference, which gives the typical weak absorption in visible range (ε = 1.51 × 104 M-1 cm-1 M-1 cm-1 at 385 nm). The nanosecond time-resolved transient absorption and DFT calculations proposed that styryl BODIPY-localized long lived 3IL states were populated for Ir-1, Ir-2, Ir-3 and Ir-4 (τT = 106.6 µs, 156.5 µs, 92.5 µs and 31.4 µs, respectively) upon photoexcitation. The complexes were used as triplet photosensitizers for singlet oxygen (1O2) mediated photooxidation of 1,5-dihydronaphthalene to produce juglone. The 1O2 quantum yields (ΦΔ) of Ir-1 (0.53) and Ir-2 (0.81) are ca. 9-fold of Ir-3 (0.06) and 40-fold of Ir-4 (0.02), respectively. Ir-2 has high molar absorption coefficient at 664 nm, moderate fluorescence in the NIR region, and high singlet oxygen quantum yield (ΦΔ = 0.81), exhibits predominate photocytotoxicity over dark cytotoxicity in LLC cells (lung cancer cells) upon irradiation, making it potentially suitable for use in in vivo photodynamic therapy (PDT). Our results are useful for preparation of transition metal complexes that show strong absorption of visible light in the NIR region with long-lived triplet excited states and for the application of these complexes in photocatalysis and theranostics such as simultaneous photodynamic therapy (PDT) and luminescent bioimaging.

Structure-property relationships and (1)O)2) photosensitisation in sterically encumbered diimine Pt(II) acetylide complexes.

A series of sterically encumbered [Pt(L)(σ-acetylide)2 ] complexes were prepared in which L, a dendritic polyaromatic diimine ligand, was held constant (L=1-(2,2'-bipyrid-6-yl)-2,3,4,5-tetrakis(4-tert-butylphenyl)benzene) and the cis ethynyl co-ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron-withdrawing phenyl-CF3 substituents (4) with electron-donating pyrenes (5) resulted in a red shift of both the lowest-energy absorption (ΔE=3300 cm(-1) , 61 nm) and emission bands (ΔE=1930 cm(-1) , 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited-state lifetimes, switched from being (3) MMLL'CT-derived (mixed metal-ligand-to-ligand charge transfer) when phenyl/polyphenylene substituents (3, 4, 6) were present, to ligand-centred (3) ππ* when the substituents were more conjugated aromatic platforms [pyrene (5) or hexa-peri-hexabenzocoronene (7)]. The novel Pt(II) acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited-state lifetimes suggested they would be good candidates for use as singlet-oxygen photosensitisers. Determined by in situ photooxidation of 1,5-dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl-5 as sensitiser (kobs =39.3×10(-3)  min(-1) ) was over half that of the known (1) O2 sensitiser tetraphenylporphyrin (kobs =78.6×10(-3)  min(-1) ) under the same conditions. Measured (1) O2 quantum yields of complexes 5 and 7 were half and one-third, respectively, of that of TPP, and thus reveal an efficient triplet-triplet energy-transfer process in both cases.

Porous material-immobilized iodo-Bodipy as an efficient photocatalyst for photoredox catalytic organic reaction to prepare pyrrolo[2,1-a]isoquinoline.

Iodo-Bodipy immobilized on porous silica was used as an efficient recyclable photocatalyst for photoredox catalytic tandem oxidation-[3+2] cycloaddition reactions of tetrahydroisoquinoline with N-phenylmaleimides to prepare pyrrolo[2,1-a]isoquinoline.

Intramolecular RET enhanced visible light-absorbing bodipy organic triplet photosensitizers and application in photooxidation and triplet-triplet annihilation upconversion.

Resonance energy transfer (RET) was used for the first time to enhance the visible light absorption of triplet photosensitizers. The intramolecular energy donor (boron-dipyrromethene, Bodipy) and acceptor (iodo-Bodipy) show different absorption bands in visible region, thus the visible absorption was enhanced as compared to the monochromophore triplet photosensitizers (e.g., iodo-Bodipy). Fluorescence quenching and excitation spectra indicate that the singlet energy transfer is efficient for the dyad triplet photosensitizers. Nanosecond time-resolved transient absorption spectroscopy has confirmed that the triplet excited states of the dyads are distributed on both the energy donor and acceptor, which is the result of forward singlet energy transfer from the energy donor to the energy acceptor and in turn the backward triplet energy transfer. This 'ping-pong' energy transfer was never reported for organic molecular arrays, and so it is useful to study the energy level of organic chromophores. The triplet photosensitizers were used for singlet oxygen ((1)O2) mediated photooxidation of 1,5-dihydroxylnaphthalene to produce juglone. The visible light absorption of the new visible light-absorbing triplet photosensitizers are higher than the conventional monochromophore based triplet photosensitizers, as a result, the (1)O2 photosensitizing ability is improved with the new triplet photosensitizers. Triplet-triplet annihilation upconversion with these triplet photosensitizers was also studied. Our results are useful to design the triplet photosensitizers showing strong visible light absorbance and for their applications in photocatalysis and photodynamic therapy.

Observation of the long-lived triplet excited state of perylenebisimide (PBI) in C

Perylenebisimide (PBI) was used to prepare C^N cyclometalated Ir(III) complexes that show strong absorption of visible light and it is the first time the long-lived triplet excited state of PBI chromophore was observed in a transition metal complex (τT = 22.3 µs). Previously, the lifetime of the triplet state of PBI in transition metal complexes was usually shorter than 1.0 µs. Long-lived triplet excited states are useful for applications in photocatalysis or other photophysical processes concerning triplet-triplet-energy-transfer. PBI and amino-PBI were used for preparation of cyclometalated Ir(III) complexes (Ir-2 and Ir-3), in which the PBI chromophore was connected to the coordination center via C≡C π-conjugation bond. The new complexes show strong absorption in visible region (ε = 34,200 M(-1) cm(-1) at 541 nm for Ir-2, and ε = 19,000 at 669 nm for Ir-3), compared to the model complex Ir(ppy)(bpy)[PF6] Ir-1 (ε < 5000 M(-1) cm(-1) in the region beyond 400 nm). The nanosecond time-resolved transient absorption and DFT calculations indicated that PBI-localized long-lived (3)IL states were populated for Ir-2 and Ir-3 upon photoexcitation. The complexes were used as triplet photosensitizers for (1)O2-mediated photooxidation of 1,5-dihydronaphthalene to produce juglone, an important intermediate for preparation of anti-cancer compounds. (1)O2 quantum yields (Φ(Δ)) up to 91% were observed for the new Ir(III) complexes and the overall photosensitizing ability is much higher than the conventional Ir(III) complex Ir-1, which shows the typical weak visible light absorption in visible region. Our results are useful for preparation of transition metal complexes that show strong absorption of visible light and long-lived triplet excited state and for the application of these complexes in photocatalysis.