Triplet Photosensitizers Research Articles

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

Synthesis of Heavy-Atom-Free Triplet Photosensitizers Based on Organoboron Complexes

AbstractA new class of organoboron complexes have been developed as heavy-atom-free triplet photosensitizers. A methodology was developed for the synthesis of an indolocarbazole–imine boron difluoride (IIBD) dye and its dimer from a 6-formylindolocarbazole. The IIBD dye was then coupled with a BODIPY dye through the C-2 or C-8 position of the latter to synthesize two dyads. Both dyads showed superior photophysical properties to those of the IIBD dyes. The relative triplet conversion efficiencies of these dyes were determined by measuring their singlet-oxygen (1O2) generation capacities. All the synthesized dyes showed high 1O2 generation compared with the BODIPY dye PM567. The dyad linked through the C-2 position of the BODIPY core showed the highest 1O2 generation efficiency, which could be useful for photodynamic therapy of cancers.

Multifunctional broadband visible-light absorbing selenophene modified bodipy photosensitizers

Large π-conjugated organic systems with effective photosensitization are highly desirable for many photochemical applications. Herein, a series of π-conjugated selenophene-substituted boron dipyrromethene (known as bodipy) dyes (MB-Se, 2B–Se, MB-Se-2Br) were prepared by appending selenophene electron-donor to the core of bodipy acceptors via alkynyl bridge and their excited state properties were investigated. Optical spectroscopy revealed that although these three selenophene-substituted bodipy dyes have nearly identical molecular profiles, their photophysical properties and excited state dynamics are drastically different. Higher triplet quantum efficiency (ΦΔ = 82%) was observed for MB-Se, derivative in which α,β-positions of bodipy decorated with methyl substituents, compared to another dye 2B–Se, which showed inefficient triplet population (ΦΔ = 25%). Employing steady-state absorption and emission spectroscopies, we demonstrated that the designed selenophene-substituted bodipy dyes has excellent J-aggregation capabilities. Notably, the J-aggregates of selenophene-substituted bodipy dyes display NIR emission. The potential of devised selenophene-substituted bodipy dyes in triplet fusion upconversion was also demonstrated; a blue upconverted emission was realized upon irradiating the mixture of bodipy dyes and TIPS-AC acceptor with 589 nm CW-laser. The selenophene-substituted bodipy dyes were also confirmed to be a potent photodynamic therapeutic reagent; with an IC50 value of 0.76 μM, nearly 6 times lower than that of the iodide-appended bodipy photosensitizer (BDP-2I, 6.0 μM). This study hopes to provide valuable guidelines for the future development of multipurpose triplet photosensitizers.

Long‐lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis‐acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

AbstractWe prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×104 M−1 cm−1 at 570 nm), and the singlet oxygen quantum yield (ΦΔ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S1 and T1 states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet‐triplet energy transfer (TTET) and triplet‐triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.

Panchromatic Light-Absorbing [70]Fullerene-Perylene-BODIPY Triad with Cascade of Energy Transfer as an Efficient Singlet Oxygen Sensitizer.

A panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C70-P-B) was synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. The photophysical processes were comprehensively investigated by the methods of steady-state spectroscopy, time-resolved spectroscopy, as well as theoretical calculations. C70-P-B shows a strong absorption ability from 300-620 nm. Efficient cascading intramolecular singlet-singlet energy transfer in C70-P-B was confirmed by the luminescence study. The backward triplet excited state energy transfer from C70 moiety to perylene then occurs to populate 3perylene*. Thus, the triplet excited states of C70-P-B are distributed on both C70 and perylene moiety with lifetimes of 23 ± 1 μs and 175 ± 17 μs, respectively. C70-P-B exhibits excellent photooxidation capacity, and its yield of singlet oxygen reaches 0.82. The photooxidation rate constant of C70-P-B is 3.70 times that of C70-Boc and 1.58 times that of MB, respectively. The results in this paper are useful for designing efficient heavy atom-free organic triplet photosensitizers for practical application in photovoltaics, photodynamic therapy, etc.

Assessing Intersystem Crossing Rates in Donor- and/Acceptor-Functionalized Corroles: A Computational Study

Small singlet-triplet gap (ΔES-T) and large spin-orbit coupling (SOC) between the low-lying excited spin singlet and triplet states greatly promote the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes that are keys to harvest the triplet population. The electronic structure of a molecule, which strongly depends on its geometry, governs the ISC/RISC. Herein, we have studied visible-light-absorbing freebase corrole and its electron donor/acceptor functional derivatives to explore and understand the effect of homo/hetero meso-substitution in the modulation of corrole photophysical characteristics using time-dependent density functional theory implementing optimally tuned range-separated hybrid. Dimethylaniline and pentafluorophenyl are considered as the representative donor and acceptor functional groups, respectively. Solvent effects are accounted for using a polarizable continuum model with the dichloromethane dielectric. Calculations reproduce the experimentally measured 0-0 energies for some of the functional corroles studied here. Importantly, results reveal that both the homo- and hetero-substituted corroles including the unsubstituted one show substantial ISC rates (∼108 s-1) that are comparable to the fluorescence rates (∼108 s-1). On the other hand, while homo-substituted corroles exhibit modest RISC rates (∼104 - 106 s-1), hetero-substituted ones show relatively lower RISC rates (∼103 - 104 s-1). These results together suggest that both homo- and hetero-substituted corroles could act as triplet photosensitizers, which is also evident from some experimental reports on modest singlet oxygen quantum yield. Calculated rates are analyzed with respect to the variation of ΔES-T and SOC, and their dependence on the molecular electronic structure was evaluated in detail. The research findings reported in this study will add to understanding rich photophysical properties of functional corroles and also aid in devising molecular-level design strategies for developing heavy-atom free functional corroles or related macrocycles for applications in lighting, photocatalysis, photodynamic therapy, etc.

Enhanced Triplet–Triplet Annihilation Upconversion Luminescence through Conformational Restriction Based on Donor–Acceptor–Heavy Atom Molecules

AbstractDesigning optimized systems for triplet‐triplet annihilation upconversion (TTA‐UC) with high UC emission efficiency at low power densities remains challenging. To improve the lifetime of triplet photosensitizers for TTA‐UC, donor‐acceptor‐heavy atoms were used to synthesize rotationally restricted PY‐MBD‐BR and unrestricted PY‐BD‐BR photosensitizers. The singlet oxygen quantum yields of both photosensitizers was ≈0.60 due to their structural similarity, except for the degree of rotation between the donor and acceptor. The triplet lifetime of PY‐BD‐BR and PY‐MBD‐BR was 352 μs and 1,503 μs, respectively. The latter may be among the highest reported for BODIPY photosensitizers. The prolonged triplet lifetime enabled excellent green‐to‐blue upconversion emission of PY‐MBD‐BR, with a high upconversion emission efficiency (ηUC) of 8.8 % and low threshold intensity (Ith) of 90 mW/cm2 compared to PY‐BD‐Br (low ηUC of 4.0 % and high Ith of 381 mW/cm2). The rigid molecular design of triplet photosensitizers based on donor‐acceptor‐heavy atom systems has potential for efficient TTA‐UC.

A novel heavy-atom-free lysosome-targeted BODIPY as triplet photosensitizer based on SOCT-ISC mechanism for photodynamic therapy

As important organelles in biological cells, lysosomes are closely associated with cell signaling, necrosis and apoptosis. Herein, we report a novel lysosome-targeted boron dipyrromethene (BODIPY) photosensitizer, Lyso-BDP, which has the advantage of lower side effects and higher phototoxicity for 4T1 cells in photodynamic therapy (PDT). Lyso-BDP possesses the lysosomal targeting treatment through the production of the singlet oxygen under the light excitation at 600 nm. We combine the anthracenyl group and BODIPY to form a D-A binary compound. The orthogonal configuration is confirmed by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The molecular orbitals and the energies of the triplet state explain the spin-orbit charge transfer inter-system crossing (SOCT-ISC) mechanism. Due to SOCT-ISC mechanism, Lyso-BDP has the high singlet oxygen (1O2) quantum yield of 62% in DCM without heavy atoms. In this way, the dark toxicity of photosensitizer can be reduced. Meanwhile, the 1O2 ″afterglow” can be achieved to enhance the phototherapeutic effect because of the anthracene group. Compared to classical photosensitizers, Lyso-BDP shows the low half maximum inhibitory concentration (IC50) of 2.5 μM for 4T1 cells in vitro cytotoxicity assays. Therefore, this work provides a strategy to solve the problem of high cellular dark toxicity and no organelle targeting of classical 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/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.

NI-BODIPY-GO Nanocomposites for Targeted PDT.

Three multifunctional targeted NI-BODIPYs (10-12) and GO-(10-12) nanocarriers were fabricated. NI-BODIPYs are designed to facilitate non-covalent interaction with graphene oxide (GO) and target toward cancer cells for specific recognition with glucose moieties while efficiently producing singlet oxygen. We probed detailed characterization, fundamental photophysical/photochemical properties, and interactions with GO of such triplet photosensitizers and nanocarriers. The effect of the formation of nanohybrids with GO on singlet oxygen formation as well as on the efficacies of the molecules in terms of in vitro killing of cancer cells was evaluated with K562 human chronic myelogenous leukemia cells. Amazingly, it was observed that GO exhibited favorable interactions with the NI-BODIPY dyads and promoted the formation of singlet oxygen, while not showing any dark toxicity.

Tuning the triplet population of arylselanyl-BODIPY photosensitizers through substituents engineering for triplet-triplet annihilation photon upconversion with perylene

Organic triplet-triplet annihilation photon upconversion (TTA-UC) systems have attracted considerable attention owing to their promising applicability in solar energy harvesting, optoelectronic devices, photocatalysts, and bio-imaging. In this study, a series of BODIPYs prepared by incorporating substituted/ unsubstituted arylselenium groups, 1 (R = H), 2 (R = OMe), and 3 (R = F), were evaluated as triplet photosensitizers for TTA-UC. Direct Se-C bond formation on the BODIPY core provided a facile intersystem crossing (ISC) channel from the excited singlet state to the metastable triplet state, being the most effective in 2, as inferred from the singlet oxygen generation experiments, possibly because of the n-electron arising from the OMe group. The UC behavior of systems comprising the sensitizers and perylene as the acceptor in deaerated toluene was investigated using a 524 nm-wavelength laser to detect upconverted emission at 449 nm; thus, the UC yield decreased in the order of 21% for 3 > 16% for 2 > 12% for 1. This trend is consistent with the Stern-Volmer constants calculated from the quenched triplet state lifetimes of the sensitizers as a function of the concentration of the perylene quencher. This suggests that the UC efficiency was mainly governed by the intermolecular triplet-triplet energy transfer (TTET) process between arylselanyl-BODIPY photosensitizers and the perylene acceptor. This result was rationalized by the efficient population of the long-lived triplet excited state of the sensitizer, which is advantageous for diffusion-controlled TTA-UC behavior.

Highly Efficient Triplet–Triplet Annihilation Upconversion with a Thermally Activated Delayed Fluorescence Molecule as Triplet Photosensitizer

Thermally activated delayed fluorescence (TADF) molecules have great potential for practical applications as triplet photosensitizers, owing to their long triplet lifetimes, almost negligible energy loss in intersystem crossing, and excellent light-harvesting ability. Recently, several triplet–triplet annihilation (TTA) upconversion systems using the TADF photosensitizer have been reported. To provide comprehensive dynamic information for designing an optimized system in the future, more experiments with the new TADF photosensitizers are necessary. Herein, we conducted a new TTA upconversion system with a recently developed TADF molecule, DMACPDO, as the triplet photosensitizer and 9,10-diphenylanthracene (DPA) as the annihilator. The overall upconversion yield was determined to be relatively high, e.g., 22.3% in toluene, 21.9% in benzene, and 8.1% in chlorobenzene. By examining elementary photophysical and photochemical processes with time-resolved transient absorption and fluorescence emission spectroscopy, we revealed the influence of solvent polarity and viscosity on each step. Our conclusion highlights the elusive solvent effect in TTA upconversion application of the TADF photosensitizer.

Photon Upconversion in the Visible Wavelengths with ZnSe/InP/ZnS Nanocrystals

Nanocrystals that can absorb strongly in the near-infrared (NIR) wavelengths for conversion to visible light are of great interest for biological imaging applications. In this work, we examine an inverse Type-I heterostructure with an inner InP shell for triplet–triplet annihilation-based photon upconversion. The InP-based nanocrystals are earth-abundant, benign, and can be synthetically tuned to absorb in the entire NIR window I. Here, a two monolayer ZnS shell was used to passivate the surface defects on inverse Type-I ZnSe/InP core/shell particles. We show that this ZnS shell increases the InP photoluminescence quantum yield (QY) by a factor of 16 to 0.43 and the transmitter triplet lifetime from 138 to 451 μs, but decreases the rate of triplet energy transfer by a factor of 3. This results in the ZnSe/InP and ZnSe/InP/ZnS nanocrystal triplet photosensitizers producing a similar photon upconversion QY of about 4.0% (out of a maximum of 50%) when paired with 9,10-diphenylanthracene as the blue emitter. This work suggests that the ZnS shell can be further tuned to increase the photon upconversion QY. This work also shows that ZnSe/InP/ZnS nanocrystals are promising candidates for a hybrid organic–inorganic nanostructure that can convert NIR photons to visible light.

Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study.

Photochemical activation by triplet photosensitizers is highly expedient for a green focus society. In this work, we have theoretically probed excited state characteristics of thioxanthone and its derivatives for their triplet harvesting efficiency using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Absorption and triplet energies corroborate well with the available experimental data. Our results predict that both the S1 and T1 states are π-π* in nature, which renders a high oscillator strength for S0 to S1 transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S1 (1 π-π* ) and high energy 3 n- π* state. Apart from that, there is both radiative and non-radiative channel from S1 to S0 , which competes with the ISC channel and reduces the triplet harvesting efficiency. For thioxanthones with -OMe (Me=Methyl) or -F substitution at 2 or 2' positions, the ISC channel is not energetically feasible, causing sluggish intersystem crossing quantum yield (ΦISC ). For unsubstituted thioxanthone and for isopropyl substitution at 2' position, the S1 -T1 gap is slightly positive ( ), rendering a lower triplet harvesting efficiency. For systems with -OMe or -F substitution at 3 or 3' position of thioxanthone, because of buried π state and high energy π* state, the S1 -3 nπ* gap becomes negative. This leads to a high ΦISC (>0.9), which is key to being an effective photocatalyst.

Photosensitive damage of dipeptides: mechanism and influence of structure.

We illustrate the influence of the dipeptide structure on photosensitive damage and the kinetic mechanism was investigated using acenaphthenequinone (ACQ) as a triplet photosensitizer. With tyrosine (Tyr) serving as the core structure, two classic dipeptides with double (trptophan-tyrosine, Trp-Tyr) and single (tyrosine-alanine, Tyr-Ala and Ala-Tyr) active reaction sites were constructed, and the underlying photodamage mechanisms were investigated carefully. According to the experimental results, the proton-coupled electron transfer processes between ACQ and numerous Trp-Tyr reaction sites have independent reaction properties. The bimolecular quenching rate (kq) value is roughly equivalent to the sum of the rates of two amino acid monomers, and a novel intramolecular dynamic channel between Trp/N˙-Tyr and Trp-Tyr/O˙ was observed. The ACQ/Tyr-Ala system demonstrated the key role of steric hindrance on the kq in bimolecular reactions.

Hydrazone-based boron difluoride complexes as triplet photosensitizers for singlet oxygen generation.

Due to the highly selective nature of singlet oxygen as an oxidant, it has received considerable interest in various areas of (organic) chemistry. Two green light activated hydrazone-based boron difluoride triplet photosensitizers possessing high quantum yields for 1O2 formation are reported. These photostable complexes are promising in applications in synthesis and catalysis.

Visible-light-initiated nickel-catalyzed amination of aryl halides using thioxanthen-9-one as a photocatalyst

An inexpensive thioxanthen-9-one as a triplet photosensitizer could significantly accelerate this Ni-catalyzed amination of aniline with aryl halides, providing various diarylamines with good yields at room temperature.

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.

Truxene-linked di-coumarin-mono-corrole tetrad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

Triplet−triplet annihilation upconversion (TTA-UC) has drawn widespread interest because of its great potential applications. However, it is still a challenge to design and synthesize efficient triplet photosensitizers without heavy atoms for TTA-UC. In this work, a triplet photosensitizer tetrad 2C-T-Cor based on two coumarin moieties and one corrole moiety with a truxene as the bridge was synthesized and used in TTA-UC as a novel heavy atom-free organic photosensitizer (perylene bisimide (PBI) was used as triplet acceptor). The photophysical properties were investigated by means of steady-state UV–Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, three-dimensional excitation emission matrix fluorescence spectra, and cyclic voltammetry. With photo-excitation of the coumarin unit at 420 nm, the intramolecular singlet–singlet energy transfer takes place from the coumarin moiety to the corrole unit with a transfer efficiency of 63.9%. Electrochemical study indicated that the intramolecular electron transfer was thermodynamically prohibited in toluene owing to the positive ΔGCS for charge-separation. A triplet excited state was investigated for 2C-T-Co via nanosecond time-resolved transient absorption and the triplet lifetime is 118 μs. 2C-T-Co can act as the triplet photosensitizer for the application of TTA-UC, and the upconversion quantum yield of 0.98% was observed.

Effects of Heavy Iodine Atoms and π-Expanded Conjugation on Charge Transfer Dynamics in Carboxylic Acid BODIPY Derivatives as Triplet Photosensitizers.

Borondipyrromethene (BODIPY) chromophores are composed of a functional-COOH group at meso position with or without a biphenyl ring, and their compounds with heavy iodine atoms at -2, -6 positions of the BODIPY indacene core were synthesized. The photophysical properties of the compounds were studied with steady-state absorption and fluorescence measurements. It was observed that the absorption band is significantly red-shifted, and fluorescence signals are quenched in the presence of iodine atoms. In addition to that, it was indicated that the biphenyl ring does not affect the spectral shifting in the absorption as well as fluorescence spectra. In an attempt to investigate the effect of π-expanded biphenyl moieties and heavy iodine atoms on charge transfer dynamics, femtosecond transient absorption spectroscopy measurements were carried out in the environment of the tetrahydrofuran (THF) solution. Based on the performed ultrafast pump-probe spectroscopy, BODIPY compounds with iodine atoms lead to intersystem crossing (ISC) and ISC rates were determined as 150 ps and 180 ps for iodine BODIPY compounds with and without π-expanded biphenyl moieties, respectively. According to the theoretical results, the charge transfer in the investigated compounds mostly appears to be intrinsic local excitations, corresponding to high photoluminescence efficiency. These experimental findings are useful for the design and study of the fundamental photochemistry of organic triplet photosensitizers.