near-IR Phosphorescence Research Articles

Bathochromically shifted absorption and near IR emission in bismuth(III) porphyrins with meso-fluorophenyl and methoxyphenyl substitutions

A series of bismuth(III) porphyrins with varying degrees of electron-donating and electron-withdrawing groups in their meso-positions have been synthesized. All the porphyrins were synthesized successfully and characterized spectroscopically. The absorption studies reveal bathochromically shifted spectra, up to [Formula: see text]60 nm, compared to their corresponding free-base porphyrins which is common for [Formula: see text]-type hyperporphyrins. Interestingly, all the samples exhibited broad near-IR phosphorescence at room temperature. The spectroscopic studies indicate that the demetallation occurs in both solution and solid states to form corresponding free-base porphyrin and this process is much faster in meso-trimethoxyphenyl substituted bismuth(III) porphyrin. Nevertheless, the study reveals interesting optical properties of bismuth(III) porphyrins, particularly red-shifted absorption and near IR emission bands.

Photoexcited triplet state and singlet oxygen generation of quinine, an antimalarial drug.

Energy transfer from the lowest excited triplet (T1) state of quinine (QN) to ground-state molecular oxygen produces singlet oxygen. In aqueous solutions, a neutral form QN, a singly protonated cation QNH+ and doubly protonated cation QNH22+ are present according to their pKa values. To the best of our knowledge, the pH dependence of QN-photosensitized singlet oxygen generation has not been reported. In the present study, the quantum yields of photosensitized singlet oxygen generation (ΦΔ) by QN, QNH+ and QNH22+ have been determined through the measurements of time-resolved near-IR phosphorescence. ΦΔ decreases in the following order: ΦΔ (QNH+) > ΦΔ (QNH22+) > ΦΔ (QN). The nature of the T1 states of QN, QNH+ and QNH22+ has been studied through the measurements of transient absorption, phosphorescence and EPR by changing the pH of the medium. This is the first report of EPR for the T1 state of QN. The photoexcited T1 state of 6-methoxyquinoline (6-MeOQL), a closely related component, has been studied for comparison. The observed zero-field splitting parameters, phosphorescence spectra and triplet lifetimes suggest that the nature of the T1 state of QN can be regarded as a locally excited 3ππ*state within 6-MeOQL. The two unpaired electrons localize mainly on 6-MeOQL. The nature of the T1 state of QN scarcely changes when the quinuclidine nitrogen site is protonated. Applying the Förster cycle to the T1 states of QN and its protonated cations, it was found that QNH+ becomes more basic when excited to its T1 state.

A Novel Characteristic of a Water-soluble UV-B Absorber, 2-Phenylbenzimidazole-5-sulfonic Acid: Suppression of Riboflavin-sensitized Singlet Oxygen Generation

Abstract 2-Phenylbenzimidazole-5-sulfonic acid (PBSA) is a water-soluble UV-B absorber used in cosmetic sunscreens. Riboflavin (RF) is a water-soluble vitamin B2. RF is an efficient singlet oxygen photosensitizer. The effects of PBSA on RF-photosensitized singlet oxygen generation have been studied through measurements of transient absorption and time-resolved near-IR phosphorescence in phosphate buffer (pH 7.4). PBSA suppresses the RF-photosensitized singlet oxygen generation. The observed suppression can be ascribed to the quenching of the lowest excited triplet state of RF by PBSA.

Long-lived, near-IR emission from Cr(III) under ambient conditions.

Bis-terdentate (N^N^N) ligands coordinated to Cr(III) yield complexes that display near-IR emission under aerated solvent conditions at room temperature.

Singlet fission in nanoaggregate of bis(phenylethynyl) derivative of benzene (BPEB): High energy triplet exciton generation with >100 % yield

1,4-bis(phenylethynyl)benzene (BPEB) is observed to show a prominent ∼0.17 eV red shift of absorption band due to monomer-aggregate conversion. Along with the absorption band shift it also causes nearly one order reduction in both emission yield and singlet lifetime. The ultrafast transient absorption measurements show annihilation free decay of singlet excitons with ∼40 ps lifetime for this nanoaggregate. Concomitant growth of triplet excitonic absorption and ground state bleach signal with similar time constant ensures presence of singlet fission (SF) process in the present system. Flash photolysis study indicates separated triplet exciton yield of ∼139 % and its decay with ∼4.4 ms lifetime. Observation of triplet-triplet annihilation (TTA) and resultant delayed fluorescence process in this NA also confirms the near-isoergic or endoergic nature of SF process. It also indicates the presence of high energy triplet excitonic state which can be utilized to boost the silicon solar cell efficiency or sensitize other materials having application in photodynamic therapy or near-IR phosphorescence based bio-imaging.

Interparticle Delivery and Detection of Volatile Singlet Oxygen at Air/Solid Interfaces.

An interparticle system has been devised, allowing airborne singlet oxygen to transfer between particle surfaces. Singlet oxygen is photogenerated on a sensitizer particle, where it then travels through air to a second particle bearing an oxidizable compound-a particulate-based approach with some similarities to reactive oxygen quenching in the atmosphere. In atmospheric photochemistry, singlet oxygen is generated by natural particulate matter, but its formation and quenching between particles has until now not been determined. Determining how singlet oxygen reacts on a second surface is useful and was developed by a three-phase system (particle-air-particle) interparticulate photoreaction with tunable quenching properties. We identify singlet oxygen quenching directly by near-IR phosphorescence in the airborne state and at the air/particle interface for total quenching rate constants (kT) of adsorbed anthracene trapping agents. The air/solid interface kT of singlet oxygen by anthracene-coated particles was (2.8 ± 0.8) × 107 g mol-1 s-1 for 9,10-dimethylanthracene and (2.1 ± 0.9) × 107 g mol-1 s-1 for 9,10-anthracene dipropionate dianion, and the lifetime of airborne singlet oxygen was measured to be 550 μs. These real-time interactions and particle-induced quenching steps open up new opportunities for singlet oxygen research of atmospheric and particulate processes.

Amphiphilic Rhenium-Oxo Corroles as a New Class of Sensitizers for Photodynamic Therapy.

A set of rhenium(V)-oxo meso-triarylcorroles bearing ester and carboxylic acid functionalities were synthesized with a view to determining their potential for photodynamic therapy. Toward this end, we measured their near-IR phosphorescence and their ability to sensitize singlet oxygen formation. The two esters studied, ReVO 5,10,15-tris(meta-carbomethoxyphenyl)corrole and ReVO 5,10,15-tris(para-carbomethoxyphenyl)corrole, were found to exhibit phosphorescence quantum yields of around 1% and fairly long phosphorescence lifetimes of about 60 μs in toluene. The corresponding carboxylic acids, which were examined in ethanolic/aqueous media, in contrast, showed much lower phosphorescence quantum yields on the order of 0.01% and somewhat shorter phosphorescent lifetimes. The quantum yields for singlet oxygen formation, on the other hand, turned out to be equally high (0.72 ± 0.02) for the esters and corresponding carboxylic acids. For the two carboxylic acids, we also carried out photocytotoxicity measurements on rat bladder cancer cells (AY27) and human colon carcinoma cells (WiDr). Cell viability measurements (MTT assays) indicated 50% cell death (LD50) for AY27 cells upon 5 min of blue light exposure with the meta carboxylic acid and upon 7 min of exposure with the para carboxylic acid; complete cell death resulted after 20 min for both compounds. The WiDr cells proved less sensitive, and LD50 values were reached after 8 and 12 min illumination with the meta and para carboxylic acids, respectively.

Short-lived and Nonphosphorescent Triplet state of Mexoryl SX, a UV-A Sunscreen

Mexoryl SX (terephthalylidene-3,3′-dicamphor-10,10′-disulfonic acid, Ecamsule) is a water-soluble UV-A absorber. The near-IR phosphorescence spectrum of singlet oxygen generated by photosensitization with Mexoryl SX was not observed in air-saturated water. On the other hand, the time-resolved near-IR phosphorescence spectrum was observed in oxygen-saturated phosphate buffer (pH 7.4). The quantum yield of the singlet oxygen generation (ΦΔ) was determined to be 0.0021 ± 0.0005. The ability of Mexoryl SX as a photosensitizer is quite low. The question arises as to the quite low ΦΔ value. No phosphorescence was detectable from Mexoryl SX in ethanol at 77 K. We elucidated the nature of the lowest excited triplet (T1) state of Mexoryl SX using a time-resolved EPR technique, because this technique is powerful for the study of short-lived and nonphosphorescent T1 molecules. The strong time-resolved EPR signals were observed. This fact shows that a considerable proportion of the lowest excited singlet (S1) state molecules undergoes intersystem crossing (ISC) to the T1 state and the deactivation process of the T1 state is mainly radiationless. The observed zero-filed splitting parameters, T1 lifetime, and S1 → T1 ISC anisotropy suggest that the T1 state can be regarded as a 3nπ*–3ππ* mixing state in character and the two unpaired electrons in the T1 state do not localize on (4-methylbenzylidene)camphor, a closely related component. Although the shorter T1 lifetime (47 ns) prevents T1 state quenching by ground-state oxygen, the 3nπ* character may contribute something to the low ΦΔ value.

Ambient-temperature near-IR phosphorescence and potential applications of rhenium-oxo corroles.

Presented herein is a first photophysical investigation of a series of rhenium(v)-oxo tris(p-X-phenyl)corroles, Re[TpXPC](O), where X = CF3, F, H, CH3, and OCH3. The complexes all exhibit near-IR phosphorescence in anoxic toluene at room temperature, with emission maxima at 780 ± 10 nm, phosphorescence decay times of 56-74 μs and quantum yields of 1.07-1.52%, the highest reported to date for phosphorescent corrole derivatives. We have also demonstrated that the dyes may serve as indicators in optical oxygen sensors and as sensitizers in triplet-triplet-annihilation-based upconversion. Although the ReO corroles are not superior to benchmark benzoporphyrin complexes that are currently used in these applications, they may prove useful in certain areas such as optical multianalyte sensing owing to improved flexibility in the available spectral properties of the dyes. The high thermal and photochemical stability of the complexes also bodes well for their deployment as new, phosphorescent sensitizers in photodynamic therapy.

Stable Platinum(IV) Corroles: Synthesis, Molecular Structure, and Room-Temperature Near-IR Phosphorescence.

A series of stable Pt(IV) corrole complexes with the general formula PtIV[TpXPC](m/p-C6H4CN)(py), where TpXPC3– is the trianion of a tris(p-X-phenyl)corrole and X = CF3, H, and CH3, has been synthesized, affording key physicochemical data on a rare and elusive class of metallocorroles. Single-crystal X-ray structures of two of the complexes revealed very short equatorial Pt–N distances of 1.94–1.97 Å, an axial Pt–C distance of ∼2.03 Å, and an axial Pt–N distance of ∼2.22 Å. The complexes exhibit Soret maxima at ∼430 nm, which are essentially independent of the meso-aryl para substituents, and strong Q bands with the most intense peak at 595–599 nm. The substituent-independent Soret maxima are consistent with an innocent PtIV–corrole3– description for the complexes. The low reduction potentials (−1.45 ± 0.08 V vs saturated calomel reference electrode) also support a highly stable Pt(IV) ground state as opposed to a noninnocent corrole•2– description. The reductions, however, are irreversible, which suggests that they involve concomitant cleavage of the Pt–aryl bond. Unlike Pt(IV) porphyrins, two of the complexes, PtIV[TpXPC](m-C6H4CN)(py) (X = CF3 and CH3), were found to exhibit room-temperature near-IR phosphorescence with emission maxima at 813 and 826 nm, respectively. The quantum yield of ∼0.3% is comparable to those observed for six-coordinate Ir(III) corroles.

Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations.

Presented herein is a comprehensive account of the electronic structure of corrole derivatives. Our knowledge in this area derives from a broad range of methods, including UV-vis-NIR absorption and MCD spectroscopies, single-crystal X-ray structure determination, vibrational spectroscopy, NMR and EPR spectroscopies, electrochemistry, X-ray absorption spectroscopy, and quantum chemical calculations, the latter including both density functional theory and ab initio multiconfigurational methods. The review is organized according to the Periodic Table, describing free-base and main-group element corrole derivatives, then transition-metal corroles, and finally f-block element corroles. Like porphyrins, corrole derivatives with a redox-inactive coordinated atom follow the Gouterman four-orbital model. A key difference from porphyrins is the much wider prevalence of noninnocent electronic structures as well as full-fledged corrole•2- radicals among corrole derivatives. The most common orbital pathways mediating ligand noninnocence in transition-metal corroles are the metal(dz2)-corrole("a2u") interaction (most commonly observed in Mn and Fe corroles) and the metal(dx2-y2)-corrole(a2u) interaction in coinage metal corroles. Less commonly encountered is the metal(dπ)-corrole("a1u") interaction, a unique feature of formal d5 metallocorroles. Corrole derivatives exhibit a rich array of optical properties, including substituent-sensitive Soret maxima indicative of ligand noninnocence, strong fluorescence in the case of lighter main-group element complexes, and room-temperature near-IR phosphorescence in the case of several 5d metal complexes. The review concludes with an attempt at identifying gaps in our current knowledge and potential future directions of electronic-structural research on corrole derivatives.

Antioxidant capacity of (+)-catechin visible-light photoirradiated in the presence of vitamin B2

ABSTRACTObjectives: Catechins are important components of human diet and have received special attention due to their antioxidant capacity. The purpose of this paper was to study the antioxidant action of (+)-catechin (CTQ) in the presence of vitamin B2 (riboflavin) as light-absorbing agent. Furthermore, two model compounds, catechol (CTC) and resorcinol (RSC), were selected in order to elucidate the reactive target of the CTQ molecule. The influence of pH-medium was investigated.Methods: Stationary photolysis, polarographic detection of dissolved oxygen, reactive oxygen species (ROS)-scavengers, time-resolved near-IR phosphorescence detection, stationary, and time-resolved fluorescence and laser flash photolysis techniques were employed.Results: CTQ interacts with riboflavin under visible-light photoirradiation as well as with different ROS which are generated in this mechanism. Radical-scavenging activity increases with increasing of pH-medium.Discussion: pH-effect of the medium on radical-scavenging activity comes from the increased electron-donating ability of CTQ upon deprotonation. These results are very interesting due to the fact that the pH of the food products displays important variations.The -scavenging ability of CTQ, would be equal to the additive contribution of each reactive center, CTC, and RSC, present at the molecule of CTQ. However, CTQ would have a moderate ability to removal of -species at pH 7.

Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells.

Two amphiphilic corroles-5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])-and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 μs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm(-2), a fill factor of 0.72, and an efficiency of 3.5%.

Triplet Energy Transfers in Well-Defined Host-Guest Porphyrin-Carboxylate/Cluster Assemblies.

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)], dppm = (Ph2P)2CH2) via ionic CO2(-)···Pd3(2+) interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd3(2+)] (∼8190 cm(-1)) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680-700 nm arising from a T1-T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1-T1 energy transfer ((3)dye*···[Pd3(2+)] → dye···(3)[Pd3(2+)]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 10(5) s(-1) on the basis of transient absorption measurements at 298 K.

Synthesis of π-extended platinum porphyrins

We report the efficient synthesis of tetrabicycloporphyrin platinum under mild conditions (60°C). The platinum complexes were heated under vacuum to give π-extended platinum porphyrins in 100% yield. The porphyrins are expected to have interesting properties such as high-quantum-efficiency near-IR phosphorescence and large absorptivity in the near-IR region.

Developments of Porphyrin Derivatives As Photosensitizers for Photodynamic Therapy

Photodynamic therapy (PDT) is accepted as one of the promising treatments for cancers. In PDT, three individually nontoxic components (photosensitizer, light, and molecular oxygen) are combined only in tumor tissue to produce reactive oxygen species (ROS), particularly singlet oxygen. Thus the patient can improve the quality of life by PDT different from conventional therapies, such as radiotherapy, surgery or chemotherapy. To develop PDT, improvements of the photosensitizing drug are required. Mainly, there are two methods to improve photosensitizers. The first is improvement of photodynamic activity. The second method is to reduce side effects. Tetraphenylporphyrin (TPP) is one of the most fundamental photosensitizers for PDT and thus we have tried to improve its activity and also tried to suppress its side effects. First, we have tried to improve the activity. Factors controlling the activity involve accumulation efficiency to tumors, cellular uptake efficiency, light absorption efficiency in the red to near-IR region, and quantum yield of singlet oxygen sensitization. We substituted silyl groups to water-soluble TPP derivatives, because silicon is considered to have potential for improving pharmacological and/or photochemical efficiencies. Quantum yield of singlet oxygen sensitization has evaluated by monitoring near-IR phosphorescence from singlet oxygen sensitized, and found that silylation improve the quantum yield. Cell-culture studies have been carried out to evaluate pharmacological properties. We found that silylation also improve cellular uptake efficiency. We carried out in vivo study by using tumor-bearing mice. Silylation also improve selective accumulation efficiency. As a results of these improvements, in vivo PDT activity was found to be enhanced by silylation significantly [1]. Second, we have tried to suppress the side effects. Although the photosensitizer used for PDT possesses tumor selectivity, its concentration is not negligibly small in normal tissue such as skin. Photosensitizers themselves show no side effects in the dark, but under the sun or room light, the side effect will be induced. Thus patients have to remain in a dark room until almost all the photosensitizers injected are eliminated. These long periods are one of the reasons for hesitation in selecting PDT as a cancer treatment. To suppress the side effects, we introduced ON/OFF switching mechanism to photosensitizers. We selected very simple modification i.e. just introduction of amino group(s) to phenyl moiety of TPP derivative to switch photosensitization efficiency by acidity. In the neutral condition, the photosensitization efficiency was low (OFF state) because of photo-induced electron transfer quenching from the aromatic amine moiety to porphyrin moiety. In the acidic condition, the photosensitization efficiency recovered (ON state) by the protonation of amino group. This system is expected to be as a pH-activatable photosensitizer. [1] H. Horiuchi et al., Chem. Eur. J., (2014), 20, 6054.

Photoexcited triplet states of UV-B absorbers: ethylhexyl triazone and diethylhexylbutamido triazone.

The excited states of UV-B absorbers, ethylhexyl triazone (EHT) and diethylhexylbutamido triazone (DBT), have been studied through measurements of UV absorption, fluorescence, phosphorescence, triplet-triplet absorption and electron paramagnetic resonance spectra in ethanol. The energy levels of the lowest excited singlet (S1) and triplet (T1) states and quantum yields of fluorescence and phosphorescence of EHT and DBT were determined. In ethanol at 77 K, the deactivation process of EHT and DBT is predominantly fluorescence, however, a significant portion of the S1 molecules undergoes intersystem crossing to the T1 state. The observed phosphorescence spectra, T1 lifetimes and zero-field splitting parameters suggest that the T1 state of EHT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid, while the T1 state of DBT can be assigned to a locally excited (3)ππ* state within p-(N-methylamino)benzoic acid or p-amino-N-methylbenzamide. The quantum yields of singlet oxygen generation by EHT and DBT were determined by time-resolved near-IR phosphorescence measurements in ethanol at room temperature. EHT and DBT did not exhibit significantly antioxidative properties by quenching singlet oxygen, in contrast to the study by Lhiaubet-Vallet et al.

Singlet molecular oxygen quenching by the antioxidant dimethylmethoxy chromanol in solution and in ex vivo porcine skin

Singlet-oxygen is a non-radical reactive oxygen species believed to play a major role in many photooxidation processes in connection with diverse photo-biological processes such as skin ageing or photocarcinogenesis. Dimethylmethoxy chromanol (3,4-dihydro-6-hydroxy-2,2-dimethyl-7-methoxy-1(2H)-benzopyran) is a potent antioxidant used in cosmetic and pharmaceutical formulations. We have assessed the singlet oxygen quenching ability of dimethylmethoxy chromanol, by monitoring the near-IR phosphorescence of singlet-oxygen in solution and in ex vivo porcine skin samples. Dimethylmethoxy chromanol quenches singlet oxygen with a rate constant of (1.3 ± 0.1) × 10⁸ M⁻¹ s⁻¹ in solution. Consistent with this, a clear reduction in the singlet oxygen lifetime and emission intensity was observed when ex vivo porcine skin samples were treated with dimethylmethoxy chromanol.

Red-light-absorbing diimine Pt(ii) bisacetylide complexes showing near-IR phosphorescence and long-lived 3IL excited state of Bodipy for application in triplet–triplet annihilation upconversion

Bodipy is used for the preparation of Pt(II) bisacetylide complexes which show strong absorption of visible light and long-lived triplet state. Room temperature (RT) near-IR phosphorescence of Bodipy was observed. The π-conjugation framework of visible light-harvesting Bodipy ligand was connected to the Pt(II) center by the C≡C bond. The complexes were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion.

Metal Coordination Induced π-Extension and Triplet State Production in Diketopyrrolopyrrole Chromophores

Triplet state photophysics has been generated in two distinct diketopyrrolopyrrole (DPP) chromophores terminated with either phenyl (1) or thienyl (2) spacers, when sandwiched between two Ir(III) complexes using bipyridyl linkers. Upon coordination of the bpy-DPP-bpy subunit resulting in its planarization, the π-conjugation in the DPP chromophore formally extends and was manifested as a substantial red shift in the absorption and fluorescence profiles of 1 and 2. Low energy excitation of these dinuclear metal complexes produced strongly quenched singlet fluorescence, generated quite intense long-lived (τ ∼ 3 μs) absorption transients in the red, sensitized (1)O(2) photoluminescence centered at 1270 nm in aerated solutions, and yielded low temperature near-IR phosphorescence in 1 centered at 950 nm.