Low Fluorescence Quantum Yield Research Articles

Elaborating the fluorescence regulation and quenching mechanism of sulfur-for-oxygen replacement for fluorophores.

Thio-caged fluorophores can be effectively desulfurized into their oxygenated derivatives through visible light, thereby restoring the strong emission, and are applied inlive cell super-resolution imaging. Herein, we theoretically investigated the reasons for the low fluorescence quantum yields of a series of thio-caged fluorophores and the underlying reasons for the differences in fluorescence quantum yields of their oxygenated derivatives. The calculation results show that the S atom on the thiocarbonyl group is more likely to excite n electrons to form the nπ* state, which reduces the energy of the nπ* state and leads to fluorescence quenching. In contrast, oxygenated derivatives is more likely to excite π electrons to form ππ* state, which is the main reason for restoring the strong emission of fluorophore. Meanwhile, the calculation results show that the difference of fluorescence intensity caused by oxygenated derivatives is determined by the number of the carbonyl group, which affects the vibronic coupling between ππ* and nπ* states and thereby leads to fluorescence quenching. These results can effectively reveal the fluorescence quenching mechanism of thio-caged fluorophores and the luminescence mechanism of their oxygenated derivatives, and provide correct and guiding design strategies for the development of new thio-caged fluorophores.

Responsive Organic Fluorescent Aggregates Based on Ion‐π Interactions Away from Fluorescent Conjugated Groups

Responsive organic luminescent aggregates have a wide range of application fields, but currently there is still a lack of reasonable molecular design strategies. Introducing ion‐π interactions into molecules can effectively alter their luminescent properties. However, current research typically focuses on ion localization at luminescent conjugated groups with the strong interaction forces. In this work, we introduce the flexible alkoxy chain spacers between fluorescent conjugated groups and ion‐π interaction sites, and then adjust the fluorescence performance of the molecule by changing the strength of ion‐π interactions. Bromine ion‐based molecules with strong ion‐π interactions exhibit high and stable fluorescence quantum yields in crystals and amorphous powders under the external stimuli. Hexafluorophosphate ion‐based molecules with weak ion‐π interactions have the high fluorescence quantum yield in crystals and very low fluorescence quantum yield in amorphous powders, showing variable fluorescence intensities under external stimuli. This demonstrates a new class of responsive organic luminescent solid‐state materials.

Responsive Organic Fluorescent Aggregates Based on Ion-π Interactions Away from Fluorescent Conjugated Groups.

Responsive organic luminescent aggregates have a wide range of application fields, but currently there is still a lack of reasonable molecular design strategies. Introducing ion-π interactions into molecules can effectively alter their luminescent properties. However, current research typically focuses on ion localization at luminescent conjugated groups with the strong interaction forces. In this work, we introduce the flexible alkoxy chain spacers between fluorescent conjugated groups and ion-π interaction sites, and then adjust the fluorescence performance of the molecule by changing the strength of ion-π interactions. Bromine ion-based molecules with strong ion-π interactions exhibit high and stable fluorescence quantum yields in crystals and amorphous powders under the external stimuli. Hexafluorophosphate ion-based molecules with weak ion-π interactions have the high fluorescence quantum yield in crystals and very low fluorescence quantum yield in amorphous powders, showing variable fluorescence intensities under external stimuli. This demonstrates a new class of responsive organic luminescent solid-state materials.

γ-glutamyl transpeptidase activatable NIR photosensitizer for visualization and selective killing of liver cancer cells

Photodynamic therapy (PDT) is a treatment model that involves using light sources to excite photosensitizers (PSs), generating reactive oxygen species. This method is considered a potential treatment for tumors. However, the non-selective toxicity of traditional PSs to both tumors and normal tissues hinders the advancement of PDT. This lack of selectivity is a major obstacle in the development of effective PDT. The activatable PS can only function in a specific environment, which enhances the selectivity and safety of PDT. In this paper, we focus on GGT (γ-glutamyltranspeptidase) overexpressed in liver cancer as the biological target. We designed and synthesized the activatable PS BrCy-Glu by modifying the substrate of GGT (l-glutamic acid) onto the brominated hemicyanine PS (BrCy-OH). The intramolecular charge transfer (ICT) effect of BrCy-Glu is inhibited, which leads to low fluorescence quantum yield and singlet oxygen yield. However, in the presence of GGT, it can undergo selective hydrolysis and be converted into BrCy-OH through a series of elimination reactions. This process is accompanied by a significant increase in fluorescence quantum yield and singlet oxygen yield. In vitro studies demonstrate that BrCy-Glu is suitable for precise fluorescence identification of liver cancer and selective eradication of cancer cells under light conditions.

Prediction of the Feasibility of Using the ≪Gold Standard≫ Thioflavin T to Detect Amyloid Fibril in Acidic Media.

Ordered protein aggregates, amyloid fibrils, form toxic plaques in the human body in amyloidosis and neurodegenerative diseases and provide adaptive benefits to pathogens and to reduce the nutritional value of legumes. To identify the amyloidogenic properties of proteins and study the processes of amyloid fibril formation and degradation, the cationic dye thioflavin T (ThT) is the most commonly used. However, its use in acidic environments that induce amyloid formation in vitro can sometimes lead to misinterpretation of experimental results due to electrostatic repulsion. In this work, we show that calculating the net charge per residue of amyloidogenic proteins or peptides is a simple and effective approach for predicting whether their fibrils will interact with ThT at acidic pH. In particular, it was shown that at pH 2, proteins and peptides with a net charge per residue > +0.18 are virtually unstained by this fluorescent probe. The applicability of the proposed approach was demonstrated by predicting and experimentally confirming the absence of ThT interaction with amyloids formed from green fluorescent (sfGFP) and odorant-binding (bOBP) proteins, whose fibrillogenesis was first carried out in an acidic environment. Correct experimental evidence that the inability to detect these fibrils under acidic conditions is precisely because of the lack of dye binding to amyloids (and not their specific structure or the low fluorescence quantum yield of the bound dye) and that the number of ThT molecules associated with fibrils increases with decreasing acidity of the medium was obtained by using the equilibrium microdialysis approach.

An all-electron acceptor-based semiconducting polymer nanoprobe for in vivo high brightness NIR-II fluorescence imaging

In vivo fluorescence imaging within the second near-infrared window (NIR-II, 1000−1700 nm) has advantages of a higher spatial resolution, and deeper tissue penetration depth than that in traditional near-infrared (NIR-I, 750−950 nm). However, currently reported most of imaging materials showed very low fluorescence quantum yield. Here, we developed a new all-acceptor strategy to design and synthesize a new diketopyrrolopyrrole-based semiconducting polymer nanoparticles (PDP NPs) with high quantum yield (∼1.3 %–0.13 %) by Stille reaction of weak and moderate electron acceptor. Such high fluorescence quantum yield may be attributed to the all-acceptor strategy effectively reducing the polymeric intrachain charge-transfer effect. The fluorescence quantum yield (0.13 %) of PDP NPs was higher than currently reported most of semiconducting polymer nanoparticles. Thus, the PDP NPs-enhanced NIR-II imaging exhibited clear microvascular networks in living mice as well as xenograft 4T1 tumor with high signal-to-background ratio and spatial resolution. This study not only introduced an all-acceptor strategy to fabricate diketopyrrolopyrrole-based semiconducting polymer nanoparticles, but also provided a convenient and effective approach to amplify the effectiveness of imaging in vivo.

Steric protection of near-infrared fluorescent dyes for enhanced bioimaging.

Near-fluorescent (NIR) dyes that absorb and emit light in the wavelength range of 650-1700 nm are well-suited for bioimaging due to the improved image contrast and increased penetration of the long-wavelength light through biological tissue. However, the imaging performance of NIR fluorescent dyes is limited by several inherent photophysical and physicochemical properties including, low fluorescence quantum yield, high chemical and photochemical reactivity, propensity to self-aggregate in water, non-specific association with off-target biological sites, and non-optimal pharmacokinetic profiles in living subjects. In principle, all these drawbacks can be alleviated by steric protection which is a structural process that surrounds the fluorophore with bulky groups that block undesired intermolecular interactions. The literature methods to sterically protect a long-wavelength dye can be separated into two general strategies, non-covalent dye encapsulation and covalent steric appendage. Illustrative examples of each method show how steric protection improves bioimaging performance by providing: (a) increased fluorescence brightness, (b) higher fluorophore ground state stability, (c) decreased photobleaching, and (d) superior pharmacokinetic profile. Some sterically protected dyes are commercially available and further success with future systems will require experts in chemistry, microscopy, cell biology, medical imaging, and clinical medicine to work closely as interdisciplinary teams.

C9-Aryl-substituted berberine derivatives with tunable AIE properties for cell imaging application.

Berberine (BBR), a widely used isoquinoline alkaloid derived from natural sources, exhibits aggregation-induced emission (AIE) characteristics and has biological applications such as in selective lipid droplet imaging and photodynamic therapy. However, natural BBR suffers from low fluorescence quantum yield (ΦF) and monotonous emission wavelength. In this paper, a series of C9-position-aryl-substituted berberine derivatives with a D-A structure were designed and synthesized. The electronic effect of the substitution groups can tune the intramolecular charge transfer (ICT) effect of the berberine derivatives, resulting in bluish green to NIR (508-682 nm) luminescence with AIE characteristics and enhanced ΦF up to 36% in the solid state. Interestingly, berberine derivatives containing an amino or a pyridyl group can exhibit fluorescence response to TFA. Cell imaging of the berberine derivatives was conducted using Caco-2 cancer cells, demonstrating their multi-color and efficient wash-free imaging capabilities. This work presents a new strategy for developing novel berberine derivatives with tunable AIE properties for application in biological imaging.

Deciphering the photophysical properties of naphthalimide derivatives using ultrafast spectroscopy.

Naphthalimide derivatives composed of donor-acceptor type structures hold significant promise across a wide range of applications. Here, the solvent polarity and viscosity controlled excited-state dynamics of a naphthalimide derivative with a donor-acceptor structure were studied using multiple spectroscopic techniques. From the stationary spectroscopic investigations, large Stokes shift and low fluorescence quantum yield were observed with increasing the solvent polarity, suggesting a more polar excited state relative to the ground state, which is evidenced by the Lippert-Mataga relationship. We also observe an enhanced fluorescence with a prolonged lifetime in a more viscous solution due to the restriction of excited-state molecular rearrangement. These observations result from the emerged twisted intramolecular charge transfer (TICT) state. The ultrafast spectroscopy studies further unravel a solvent polarity dependent excited state evolution from the intramolecular charge transfer state to the TICT state, revealing that the TICT state can be populated only in strong polar solvents. Control experiments by tuning the solvent viscosity in ultrafast experiments were employed to verify the excited state molecular rearrangement subsequently. These observations collectively emphasize how fine-tuning the photophysical properties of naphthalimide derivatives can be achieved through strategic manipulation of solvent polarity and viscosity.

The Balance Effect of π-π Electronic Coupling on NIR-II Emission and Photodynamic Properties of Highly Hydrophobic Conjugated Photosensitizers.

Deep NIR organic phototheranostic molecules generally have large π-conjugation structures and show highly hydrophobic properties, thus, forming strong π-π stacking in the aqueous medium, which will affect the phototheranostic performance. Herein, an end-group strategy is developed to lift the performance of NIR-II emitting photosensitizers. Extensive characterizations reveal that the hydrogen-bonding interactions of the hydroxyl end group can induce a more intense π-π electronic coupling than the chlorination-mediated intermolecular forces. The results disclose that π-π stacking will lower fluorescence quantum yield but significantly benefit the photodynamic therapy (PDT) efficiency. Accordingly, an asymmetrically substituted derivative (BTIC-δOH-2Cl) is developed, which shows balanced phototheranostic properties with excellent PDT efficiency (14.6 folds of ICG) and high NIR-II fluorescence yield (2.27%). It proves the validity of the end-group strategy on controlling the π-π interactions and rational tuning the performance of NIR-II organic phototheranostic agents.

N-Type Fullerene-Carbon Dots: Synthesis and Electrochemical and Photophysical Properties.

The covalent incorporation of C60 and C70 derivatives of the well-known n-type organic semiconductor PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) onto carbon dots (CD) is described. Morphological and structural characterization reveal combined features of both pristine starting materials (CD and PCBM). Electrochemical investigations evidenced the existence of additional reduction processes to that of CD or PCBM precursors, showing rich electron-acceptor capabilities, with multistep processes in an affordable and narrow electrochemical window (ca. 1.5 V). Electronic communication in the obtained nanoconjugated species were derived from steady-state absorption and emission spectroscopies, which showed bathochromically shifted absorptions and emissions well entering the red region. Finally, the lower fluorescence quantum yield of CD-PCBM nanoconjugates, compared with CD, and the fast decay of the observed emission of CD, support the existence of an electronic communication between both CD and PCBM units in the excited state.

Solvent-free one-pot two component synthesis. Tuning of optoelectronic properties of fluorescent phloroglucinol-aryl Schiff base compounds

A series of new aryl-Schiff base derivatives were synthetized from the one-pot approach: two component condensation reaction in absence of solvent, in quantitative yields with a high atom economy under green conditions. All of the derivatives were fully characterized by NMR, vibrational absorption analysis, UV/vis absorption and fluorescence spectroscopy, and high-resolution mass spectrometry. The aryl-Schiff bases predominantly present the enol-imine tautomer in acetone-d6, while both enol-imine and keto-imine tautomer readily concur in dimethyl sulphoxide-d6. The emission of the series could be tailored by substituting electron donor or electron acceptor groups in the aryl-Schiff bases from fluorescent to not fluorescent molecules, respectively. Fluorescent molecules emit blue, cyan and yellow color with quantum yields in the 2.4–6.2 % range and average lifetime of ∼0.2 ns. The large Stokes’ shift for the molecule bearing a methylalcoxy (3248 cm−1) and diethyl amine (5017 cm−1) suggests a possible intramolecular charge transfer process as also corroborated by cyclic voltammetry, and explaining their low fluorescence quantum yield. The aryl-Schiff base substituted with the methylalcoxy (2b) was used as an electron donor material in photovoltaic devices in configuration: ITO/PEDOT:PSS/2b:PC61BM/Fields metal. The device built under atmospheric conditions displayed an intrinsic power conversion efficiency of PCE ∼0.4 %, with a Voc of 420 mV and Jsc 4.28 mA/cm2.

High-performance fluorinated D-A-D type electrochromic polymers with different structural types of thiophene donor units

In this work, two fluorinated D-π-A-π-D type electrochromic polymer precursors were synthesized by Stille coupling reactions that using mono-fluorine atom substituted benzothiadiazole (F-BT) as acceptor units (A), 3-dodecylthiophene as the π-bridge, and thiophene (Th) and 3,4-ethoxylenedioxythiophene (EDOT) as donor units (D), respectively, and their optical properties were characterized by various analytical techniques. All of them were electropolymerized into the corresponding fluorinated electrochromic polymer films (P(F-Th) and P(F-EDOT)). F-EDOT has red-shifted absorption spectra and emission spectra but with relatively lower fluorescence quantum yield, which is due to the strong electron donating ability of EDTO unit. The lower oxidation potential of F-EDOT (0.72 V) is very beneficial to improve the optoelectronic properties of its electropolymerized P(F-EDOT) with favorable redox activity and better redox stability (remained 73 % redox activity after 1000 cycles). Due to the enhanced π-π* conjugation effect, P(F-EDOT) has shown a red-shifted absorption spectra compared to P(F-Th) and a decreased optical band gap (1.6 eV for P(F-Th); 1.38 eV for P(F-EDOT)). Although P(F-Th) has a higher coloration efficiency (370.4 cm2C−1 at 894 nm) and faster response time (0.04–0.37 s), the transparent green P(F-EDOT) should have more satisfied electrochromic properties due to its favorable coloration efficiency (192 cm2C−1 at 458 nm), higher optical contrast (20.6 % at 458 nm), and also very fast response time (0.2–0.86 s), especially the enhanced optical stability and better memory effect. Overall, these properties highlight the potential applications of the current fluorinated D-π-A-π-D type polymers in displays and smart windows.

Synthesis and Characterization of a One‐Component Water‐Soluble Photoinitiator Based on Thioxanthone Derivatives

AbstractThioxanthone derivatives with additional amino groups (TX‐S‐NH3) are synthesized via a simple esterification reaction between TX‐S‐CH2‐COOH and 2‐methylaminoethanol hydrocarbons in a p‐toluene sulfonic acid medium. The TX‐S‐NH3 is characterized by 1H NMR, 13C NMR, FT‐IR, HRMS, ultraviolet‐visible spectrophotometry, and fluorescence spectroscopy. TX‐S‐NH3 has strong absorption in water and ethanol, and the molar extinction coefficients at 395 nm are 2800 and 2780 mol−1L cm−1, respectively. TX‐S‐NH3 shows a low fluorescence quantum yield of 0.109 in deionized water. The maximum mobility of TX‐S‐NH3 in the photocuring product in ethanol is 12.5%. The double bond conversion of TMPTA cured by TX‐S‐NH3 is 60.7% without co‐initiator. TX‐S‐NH3 has excellent curing effects in UV‐curable ink with a light curing time of only 2.3 s.

Thioxanthonation of 2-naphthalene-thioacetic acid as a visible-light photoinitiator: The investigation of photophysical and photochemical properties

Thioxanthonation of naphthalene thioacetic acid (TXNPSCH2COOH) resulted a visible light photoinitiator with the high molar absorptivity (ε405: 5680 L.mol−1.cm−1) in the visible spectral region and made this compound an attractive photoinitiator for free radical polymerization reactions. Low fluorescence quantum yield (ΦF: 0.06 in 2-MeTHF) and comparatively low triplet energy (186 kJ/mol in 2-MeTHF) was in favorable for efficient intersystem crossing to the triplet state which is necessary for production of initiating radicals. Indeed, photopolymerization of methyl methacrylate along with TXNPSCH2COOH (1 × 10−3 M) and a tertiary amine namely MDEA yielded nearly 25% conversion of monomer to polymer. All spectrophotometric studies including laser flash photolysis which gives triplet lifetime of initiator which is very important information for initiation mechanism of photoinitiators, indeed a long enough triplet lifetime of TXNPSCH2COOH (τT: 1000 ns) allows for efficient reactions with co-initiators to generate radicals.

A fluorescent probe based on the ESIPT (excited state intramolecular proton transfer) mechanism for rapid detection of endogenous and exogenous H2O2 (hydrogen peroxide) in cells

Hydrogen peroxide (H2O2) is one of the important reactive oxygen species in the body and can be used as a marker of some diseases such as cancer and neurodegenerative diseases. Therefore, it is of great significance to develop fluorescent probes that can detect H2O2 in living organisms for early diagnosis of diseases. However, slow response time and low fluorescence quantum yield limit the application of many probes. In this study, using 2-(2-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, the introduction of weakly absorbing bromine atoms can accelerate the speed of electron transfer during the recognition process. Three ESIPT (excited state intramolecular proton transfer) fluorescent probes JLO/JLM/JLP were designed and synthesized. The detection of H2O2 can be achieved with all three probes, and we screened a probe JLO with the fastest response time (30 min) and highest fluorescence quantum yield (Ф = 0.731). The probe also has a large Stokes shift, which can reduce fluorescence self-absorption and background interference, and also has a high sensitivity, which is designed to accurately detect endogenous and exogenous H2O2 in living cells, which has great potential for biological applications.

Evaluation of intracellular lipid droplets viscosity by a probe with high fluorescence quantum yield

BackgroundLipid droplets (LDs) are an important organelle as the main energy storage site in cells. LDs viscosity controls the material and energy exchange between it and other organelles. Furthermore, the LDs metabolic abnormalities, cell dysfunction, some diseases may be attributed to the singular LDs viscosity. Currently, the fluorescent probes for sensing the variations of LDs viscosity are still scarce and expose some drawbacks of low fluorescence quantum yield, low sensitivity and LDs polarity interference. Thus, the development of high performance probes is significant to detect LDs viscosity. ResultsWe hereby provide a lipophilic fluorescent probe (TPE-BET) with high fluorescence quantum yield (Φf, 0.91 in glycerol) for imaging LDs viscosity in living cells. With the increase of viscosity from 0.54 cp to 934 cp, the fluorescence at λex/λem = 405/520 nm and the fluorescence quantum yield of TPE-BET linearly increased by 64.9 and 128.5 folds, respectively. Meanwhile, the outstanding LDs staining capability of TPE-BET may provide a high spatial resolution for LDs imaging. The cell imaging of TPE-BET not only successfully observed the viscosity variations of LDs in cell stress models, e.g., ferroptosis, inflammation and mitophagy, but also revealed the increased viscosity and extracellular delivery of LDs in heavy metal cell injury models (Hg/As) for the first time, which may supply concrete evidence for understanding the structure and function of LDs. SignificanceThis represents a new fluorescent probe TPE-BET with high fluorescence quantum yield for imaging LDs viscosity, which may decrease the dose of probe and excitation light intensity along with the improvement on signal noise ratio (S/N). The imaging results of TPE-BET clarified that LDs viscosity may be an appraisal index on cell differentiation, state evaluation and drug screening.

Oligomerization Strategy of D-A-Type Conjugated Molecules for Improved NIR-II Fluorescence Imaging.

Fluorescence imaging (FI) in the second near-infrared (NIR-II) window has emerged as a promising imaging method for cancer diagnosis because of its superior properties such as deep penetration depth and high signal-to-background ratio. Despite the superiorities of organic conjugated nanomaterials for NIR-II FI, the issues of low fluorescence quantum yield, weak metabolic capability, undefined molecular structure for conjugated polymers, weak light-harvesting ability, short emission wavelength, and high synthetic complexity for conjugated small molecules still remain to be concerned. We herein propose an oligomerization strategy by facilely adjusting the oligomerization time to balance the advantages and disadvantages between conjugated polymers and small molecules, obtaining the candidate (CO1, oligomerization time: 1 min) with the optimal NIR-II optical performance. Then the CO1 is further prepared into water-dispersed nanoparticles (CON1) via a nanoprecipitation approach. By virtue of the suitable size, excellent NIR-II optical properties, low toxicity, and strong cell-labeling ability, the CON1 is successfully employed for in vivo NIR-II imaging, permitting the real-time visualization of blood vascular system and tumors with high sensitivity and resolution. This work thus not only provides a personalized organic conjugated nano-agent for NIR-II FI, but also highlights the molecular strategy for the development of organic conjugated systems with optimal performance for bio-imaging.

Overcome the "Buckets Effect": Integration of AIEgens into Proteins for Fluorescence-Enhanced Two-Photon Imaging.

Luminogens with aggregation-induced emission characteristics (AIEgens) are considered good options for two-photon (2P) probes, owing to their flexibility of design, heavy-metal-free composition, and resistance to photobleaching. However, the design principles for large 2P absorption cross-section (δ) generally require high coplanarity, strong donor-acceptor (D-A) interactions, and long conjugation, which can severely weaken the brightness of AIEgens at the aggregated state and undermine their potential in 2P fluorescence imaging (2PFI). Exploration of a feasible approach to overcome the "Buckets Effect" of AIEgen-based 2P probes is thus a fascinating yet challenging task. Herein, an AIEgen, namely (Z)-2-(4-aminophenyl)-3-(5-(4-(bis(4-methoxyphenyl)amino)phenyl)thiophen-2-yl)acrylonitrile (MTAA) is designed to have a big δ according to the calculation result and a low fluorescence quantum yield (QY) of 2.2% in dimethyl sulfoxide (DMSO). Impressively, upon integrating into bovine serum albumin (BSA), the protein-sized MTAA@BSA dots exhibit a 25-fold higher fluorescence QY compared to MTAA molecules, contributing to an imaging depth of 818µm in the brain vasculature. The retention and clearance of MTAA@BSA dots in the liver and kidney are also studied using 2PFI. Overall, this work provides a facile approach to overcome the "Buckets Effect" of AIEgen to generate highly efficient, reliable, and biocompatible 2P probes.

Improved brightness of multiarylpyrroles in the near-infrared region via coordination-induced supramolecular system

Organic fluorescent dyes that emit in the near-infrared region offer many benefits in biological imaging; nevertheless, their low fluorescence quantum yield remains problematic. In this study, we synthesized an aggregation-induced emission dye from multiarylpyrroles (MAPs) and continuously increased the fluorescence intensity by coordinating the MAPs with tris(pentafluorophenyl)borane (BCF). The coordination between MAPs and BCF improved the fluorescence quantum yields without changing the emission wavelength by increasing the intramolecular interactions and restricting the molecular motion. Moreover, experimental and theoretical calculations verified that the ability to produce reactive oxygen species decreased after coordination, which was beneficial for long-term fluorescence imaging tracking. The coordination between near-infrared dyes and BCF not only provides general guidance for improving the quantum yield but also regulates the charge transfer between the singlet and triplet states, which influences the production of reactive oxygen species.