Fluorescence Yield Research Articles

Water availability and atmospheric dryness controls on spaceborne sun-induced chlorophyll fluorescence yield

Climate change is amplifying the duration, frequency, and intensity of droughts, harming global ecosystems. During droughts, plants can close their stomata to save water, at the expense of a reduced carboxylation rate. When in a carboxylation-limited regime, plants benefit from an increase in water availability, as it increases their photosynthetic rate. The sun-induced chlorophyll fluorescence (SIF) signal, measurable from satellites, is mechanistically linked to this rate. Like canopy photosynthesis, SIF carries an imprint from the available irradiation (PAR) as well as the canopy structure and the efficiency of the photosynthesis at the photosystem level. Normalizing the global TROPOMI SIF observations with TROPOMI reflectance and MODIS Normalised Difference Vegetation Index (NDVI) data, we extracted the fluorescence quantum yield (ϕF), which lab-scale experiments have found to be linked to the photosynthetic electron transport. Plant physiologists have long proved the photosynthetic electron transport to be sensitive to plant water status. Here, the plant water status is controlled by the soil moisture (SM) and the vapour pressure deficit (VPD). Combining data from the TROPOMI, AIRS and SMAP satellite sensors, this study describes how SM and VPD control the ϕF at the global scale. We identify a VPD range (VPD<1.5 kPa) in which the ϕF is mainly controlled by VPD, and another (VPD>1.5 kPa) in which the ϕF is co-regulated by SM and VPD. The precise values of this range, as well as the magnitude of ϕF values, are modulated by the plant isohydricity. To gain a deeper understanding of the link between ϕF and photosynthetic efficiency at large scale, we used the link between ϕF and data on the canopy conductance (Gs), which were calculated using remote sensing data-driven models. A comparison found that the ϕF-Gs relationship at large scale is in line with the ϕF-Gs relationship described in plant-level studies.

Roadmap of electrons from donor side to the reaction center of photosynthetic purple bacteria with mutated cytochromes

In photosynthetic bacteria, the absorbed light drives the canonical cyclic electron transfer between the reaction center and the cytochrome bc1 complexes via the pools of mobile electron carriers. If kinetic or structural barriers hinder the participation of the bc1 complex in the cyclic flow of electrons, then the pools of mobile redox agents must supply the electrons for the multiple turnovers of the reaction center. These conditions were achieved by continuous high light excitation of intact cells of bacterial strains Rba. sphaeroides and Rvx. gelatinosus with depleted donor side cytochromes c2 (cycA) and tetraheme cytochrome subunit (pufC), respectively. The gradual oxidation by ferricyanide further reduced the availability of electron donors to pufC. Electron transfer through the reaction center was tracked by absorption change and by induction and relaxation of the fluorescence of the bacteriochlorophyll dimer. The rate constants of the electron transfer (~ 3 × 103 s‒1) from the mobile donors of Rvx. gelatinosus bound either to the RC (pufC) or to the tetraheme subunit (wild type) were similar. The electrons transferred through the reaction center dimer were supplied entirely by the donor pool; their number amounted to about 5 in wild type Rvx. gelatinosus and decreased to 1 in pufC oxidized by ferricyanide. Fluorescence yield was measured as a function of the oxidized fraction of the dimer and its complex shape reveals the contribution of two competing processes: the migration of the excitation energy among the photosynthetic units and the availability of electron donors to the oxidized dimer. The experimental results were simulated and rationalized by a simple kinetic model of the two-electron cycling of the acceptor side combined with aperiodic one-electron redox function of the donor side.

Photochemical Stability of 5-Methylcytidine Relative to Cytidine: Photophysical Insight for mRNA Therapeutic Applications.

5-Methylcytidine (5mCyd) has recently been investigated with renewed interest in its utilization in mRNA therapeutics. However, its photostability following exposure to electromagnetic radiation has been overlooked. This Letter compares the photostability and excited-state dynamics of 5mCyd with those of the canonical RNA nucleoside, cytidine (Cyd), using steady-state and femtosecond transient absorption spectroscopy under physiologic conditions. 5mCyd is shown to have a 5-fold higher fluorescence yield and a 5-fold longer 1ππ* excited-state decay lifetime. Importantly, however, the excited-state population in 5mCyd decays primarily by internal conversion, with a photodegradation rate 3 times smaller than that in Cyd. In Cyd, the population of a 1nπ* state with a lifetime of ca. 45 ps is implicated in the formation 6-hydroxycytidine and other photoproducts.

The carbonyl-lock mechanism underlying non-aromatic fluorescence in biological matter

Challenging the basis of our chemical intuition, recent experimental evidence reveals the presence of a new type of intrinsic fluorescence in biomolecules that exists even in the absence of aromatic or electronically conjugated chemical compounds. The origin of this phenomenon has remained elusive so far. In the present study, we identify a mechanism underlying this new type of fluorescence in different biological aggregates. By employing non-adiabatic ab initio molecular dynamics simulations combined with a data-driven approach, we characterize the typical ultrafast non-radiative relaxation pathways active in non-fluorescent peptides. We show that the key vibrational mode for the non-radiative decay towards the ground state is the carbonyl elongation. Non-aromatic fluorescence appears to emerge from blocking this mode with strong local interactions such as hydrogen bonds. While we cannot rule out the existence of alternative non-aromatic fluorescence mechanisms in other systems, we demonstrate that this carbonyl-lock mechanism for trapping the excited state leads to the fluorescence yield increase observed experimentally, and set the stage for design principles to realize novel non-invasive biocompatible probes with applications in bioimaging, sensing, and biophotonics.

Monitoring low O2 stress in apples – What we can learn from chlorophyll fluorescence and respiratory characteristics

Dynamic controlled atmosphere (DCA) is becoming a storage method of choice for apple producers seeking to store long-term. There is limited prior work comparing DCA monitoring chlorophyll fluorescence yield (DCA-CF) and respiratory characteristics (DCA-RQ), particularly for the popular UK-grown apple cultivars ‘Gala’ and ‘Braeburn’. However, these techniques can give an insight into the physiological response of apples to low O2 storage, which could help to design more efficient and sustainable storage regimes. In this study the chlorophyll fluorescence (CF) and respiratory quotient (RQ) responses of ‘Gala’ and ‘Braeburn’ are compared in typical and exaggerated DCA conditions. Results show that not only do the two techniques differ in their detection of low O2 stress but the two cultivars studied also differ noticeably in their behaviour. CF increase was confirmed to be a reaction to a stress signal from inside the fruit. Respiration rate curves were found to reflect the internal structure of the fruit, with ‘Gala’’s more homogeneous tissues and greater gas permeability resulting in a strongly bimodal O2 consumption rate where the change in rate hypothetically corresponds with the majority of the fruit tissue switching to anaerobic respiration at the same time. CF and RQ showed noticeable differences between cultivars, agreeing with previous observations that ‘Braeburn’ is less tolerant of low O2 and elevated CO2 than ‘Gala’ (CF Lower O2 Limit (LOL) for Braeburn of approximately 0.75 kPa O2 relative to 0.5 kPa O2 for Gala). Additionally, the CF and RQ responses indicated different LOLs for ‘Gala’, at approximately 0.5 kPa O2 for CF and 0.25 kPa O2 for RQ. Elevated CO2 depressed both the CF and RQ response, with a strong depressive effect on the rate of CO2 evolution but little effect on the rate of O2 consumption.

Time-resolved spectroscopic and computational study of the initial events in doxazosin photochemistry

Doxazosin is a quinazoline derivative widely used in medicine as a drug. In this study, a combined experimental and computational approach based on the time-dependent density functional theory was used to elucidate the primary events following the photoexcitation of DOX upon interaction with light. The photophysical properties and photochemical reactivity of DOX were investigated by steady-state and time-resolved absorption and fluorescence spectroscopy. DOX in H2O in S0 is present in two prototropic forms, with the protonated form dominating (∼91 %, pKa = 6.75). The computations indicated that the most basic quinazoline nitrogen is at the position 1. Upon excitation, DOX deprotonates in the singlet excited state (pKa* = 1.31), and the decay times from the singlet excited state of 5 ns and 13 ns are attributed to the non-protonated and protonated forms of DOX, respectively. The quantum yield of fluorescence in H2O is 0.51 and 0.64 in basic media. The quantum yield of intersystem crossing along with triplet–triplet molar absorption coefficient at 520 nm and the lifetime of the triplet excited state were obtained by LFP, ΦISC = 0.17, ε520 = 11600 ± 100 M−1 cm−1 and τ = 11 μs, respectively. Furthermore, LFP enabled detection of DOX radical formed by the photoinduced intramolecular electron transfer from the benzodioxane-carbamoyl to the protonated aminoquinazoline. Computations were used to back up the assignments of the detected transients and to construct an energy diagram with all plausible photophysical and photochemical pathways. These results elucidated the mechanisms of DOX photochemistry leading to DOX photodegradation which is relevant to environmental studies. They also provided insights into the potential use of such a quinazoline derivative in other applications such as push–pull chromophores or fluorescent probes.

X-Ray Fluorescence Spectrometry: Current Status and Prospects of Development

This review covers characteristics and potential applications of various versions of the X-ray fluorescence (XRF) spectrometry for analyzing both liquid and solid samples. Particular emphasis is given to research published within the past decade, as information on XRF’s previous applications can be found in earlier reviews and monographs. The results of experiments on determining fundamental atomic parameters, such as mass absorption coefficients, fluorescence yields, transition probabilities for the emission of specific lines of elements, and nonradiative transition probabilities. Additionally, the review addresses the capabilities of newly designed models of XRF spectrometers developed in recent years. The application of total reflection X-ray fluorescence spectrometry for diverse samples is examined in greater detail. Furthermore, the document presents data on the utilization of XRF in investigating nanoparticles of some typical materials. These particles exhibit qualitatively novel properties and have become a focal point of nanotechnology, an area rapidly developing in the last few decades.

USE OF CYTOKININS AND COMPOSTING TO IMPROVE THE AGRONOMIC AND PHYSIOLOGICAL TRAITS OF CAIGUA (CYCLANTHERA PEDATA L.)

The scarcity of nutrients and synthetic fertilizers reduced crop productivity, increasing production costs and prompting scientists to seek new technologies to ensure high-quality output. In this context, using cytokinins with organic fertilizers ensued to assess their effects on caigua (Cyclanthera pedata L.) production. For this purpose, land preparation for planting comprised two levels of cytokinins (100 and 150 ml, Anthesis Plus per 200 L of water) and organic compost at 10 t/ha, then spread across the land. Results showed that adding cytokinins and compost manure, alone and in combination, significantly improved the agronomic and physiological characteristics of caigua. However, the combined application of compost manure and cytokinins dramatically enriched the caigua plant length, diameter, fruits per plant, and fruit dry and fresh weight per plant up to 85.0%, 46.9%, 81.8%, 80.6%, and 83.2%, respectively, in comparison with the control treatment. Similarly, chlorophyll contents, quantum yield, photosynthetically active radiation, fluorescence yield, and electron transport reaction increased at 68.8%, 66.4%, 79.2%, 51.1%, and 74.0%, respectively, with combined application as compared with control. Furthermore, the co-addition of composting and cytokinins also upgraded the biochemical composition versus the control. The presented results suggested that applying a mixture of compost manure and cytokinins may help enhance caigua plant growth, yield, and quality and improve soil characteristics.

Organomediated electrochemical fluorosulfonylation of aryl triflates via selective C–O bond cleavage

Although aryl triflates are essential building blocks in organic synthesis, the applications as aryl radical precursors are limited. Herein, we report an organomediated electrochemical strategy for the generation of aryl radicals from aryl triflates, providing a useful method for the synthesis of aryl sulfonyl fluorides from feedstock phenol derivatives under very mild conditions. Mechanistic studies indicate that key to success is to use catalytic amounts of 9, 10-dicyanoanthracene as an organic mediator, enabling to selectively active aryl triflates to form aryl radicals via orbital-symmetry-matching electron transfer, realizing the anticipated C–O bond cleavage by overcoming the competitive S–O bond cleavage. The transition-metal-catalyst-free protocol shows good functional group tolerance, and may overcome the shortages of known methods for aryl sulfonyl fluoride synthesis. Furthermore, this method has been used for the modification and formal synthesis of bioactive molecules or tetraphenylethylene (TPE) derivative with improved quantum yield of fluorescence.

Fluorescence enhancement induced by supramolecular J-aggregation of pyrene dye containing a flexible rotation group

A pyrene dye bearing a flexibly rotated hydrophobic group (3,4,5-tri(dodecyloxy)benzoyl-) was synthesized and fully characterized. In cooling processes for the dye monomer in CHCl3/CH3OH, the monomer was transformed into the J-aggregates which were characterized to possess a typically nanowired shape. A thermodynamic hysteresis of the molar fraction of the aggregates was observed explicitly between the cooling and heating processes. A pronounced self-absorption was involved in the aggregate solutions of the dye at high concentrations. Clearly, the J-aggregation conferred a few intriguing attributes such as a red-shift of maximum absorption peak, a higher quantum yield of absolute fluorescence and a shorter fluorescence lifetime than the monomer. Collectively, these attributes brought about a phenomenon of aggregation-induced emission enhancement (AIEE). In addition, the HOMO and LUMO energies were calculated by DFT for both the monomeric and aggregated states. Furthermore, the stacking parameters in the J-aggregates were evaluated according to the molecular exciton theory. It can be concluded that the J-aggregation enhanced substantially the emission of the dye. Undoubtedly, the strategy for the development of the AIEE molecules containing a flexible rotation group via J-aggregation as revealed in this work will be very useful in the future design of new organic light-emitting dyes.

Understanding the synergistic effect of hydrated electron generation from argon plasma catalysis over Bi2O3/CeO2 for perfluorooctanoic acid dehalogenation: Mechanism and DFT study

Pseudo-photocatalysis driven by argon-plasma-system (AP) is a new approach toward the promotion of reactive species production for water remediation. Here, we investigated the synergistic effect between AP and catalyst by altering the oxygen vacancies (OV) concentration of CeO2/Bi2O3 for stimulating the hydrated electrons (eaq–) production for PFOA removal. The soft X-ray total fluorescence yield (TFY) analysis and DFT calculation revealed the formation of the built-in electric field in the Bi/Ce0.43 interface can enhance interfacial electron migration with direction from Bi2O3 toward CeO2, simultaneously promoting the eaq– generation. Notably, AP-Bi/Ce0.43 (0.1488 min−1, EEO = 0.43 kW mg−1) exhibited excellent PFOA removal kinetic performance with almost 5.7 times faster and 72.6% lower energy consumption than sole AP (0.0261 min−1, EEO = 1.57 kW mg−1), respectively. The multiple-plasma-jet continuous-flow-experiments results illustrated the scalability of AP-Bi/Ce0.43 for PFOA destruction. Our findings demonstrate fundamental insights into the synergistic effect of PFOA removal in AP catalysis.

Electrical Signals at the Plasma Membrane and Their Influence on Chlorophyll Fluorescence of Chara Chloroplasts in vivo.

Action potentials of plant cells are engaged in the regulation of many cell processes, including photosynthesis and cytoplasmic streaming. Excitable cells of characean algae submerged in a medium with an elevated K+ content are capable of generating hyperpolarizing electrical responses. These active responses of plasma membrane originate upon the passage of inward electric current comparable in strength to natural currents circulating in illuminated Chara internodes. So far, it remained unknown whether the hyperpolarizing electrical signals in Chara affect the photosynthetic activity. Here, we showed that the negative shift of cell membrane potential, which drives K+ influx into the cytoplasm, is accompanied by a delayed decrease in the actual yield of chlorophyll fluorescence F' and the maximal fluorescence yield Fm' under low background light (12.5 µmol m-2 s-1). The transient changes in F' and Fm' were evident only under illumination, which suggests their close relation to the photosynthetic energy conversion in chloroplasts. Passing the inward current caused an increase in pH at the cell surface (pHo), which reflected high H+/OH- conductance of the plasmalemma and indicated a decrease in cytoplasmic pH due to the H+ entry into the cell. The shifts in pHo arising in response to the first hyperpolarizing pulse disappeared upon repeated stimulation, thus indicating the long-term inactivation of plasmalemmal H+/OH- conductance. Suppression of plasmalemmal H+ fluxes did not abolish the hyperpolarizing responses and the analyzed changes in chlorophyll fluorescence. These results suggest that K+ fluxes between the extracellular medium, cytoplasm, and stroma are involved in the functional changes of chloroplasts reflected by transients of F' and Fm'.

Relationship between Photosynthetic CO2 Assimilation and Chlorophyll Fluorescence for Winter Wheat under Water Stress.

Solar-induced chlorophyll fluorescence (SIF) has a high correlation with Gross Primary Production (GPP). However, studies focusing on the impact of drought on the SIF-GPP relationship have had mixed results at various scales, and the mechanisms controlling the dynamics between photosynthesis and fluorescence emission under water stress are not well understood. We developed a leaf-scale measurement system to perform concurrent measurements of active and passive fluorescence, and gas-exchange rates for winter wheat experiencing a one-month progressive drought. Our results confirmed that: (1) shifts in light energy allocation towards decreasing photochemistry (the quantum yields of photochemical quenching in PSII decreased from 0.42 to 0.21 under intermediate light conditions) and increasing fluorescence emissions (the quantum yields of fluorescence increased to 0.062 from 0.024) as drought progressed enhance the degree of nonlinearity of the SIF-GPP relationship, and (2) SIF alone has a limited capacity to track changes in the photosynthetic status of plants under drought conditions. However, by incorporating the water stress factor into a SIF-based mechanistic photosynthesis model, we show that drought-induced variations in a variety of key photosynthetic parameters, including stomatal conductance and photosynthetic CO2 assimilation, can be accurately estimated using measurements of SIF, photosynthetically active radiation, air temperature, and soil moisture as inputs. Our findings provide the experimental and theoretical foundations necessary for employing SIF mechanistically to estimate plant photosynthetic activity during periods of drought stress.

Experimental measurements of X-rays emission cross sections induced by photons

Abstract The energy-dispersive X-ray fluorescence (ED-XRF) technique has been used for measurements of K α , K β , and total K X-rays emission cross sections induced by photons, as well as K-shell fluorescence yields for pure elements with 22 ⩽ Z ⩽ 49. These metals have been irradiated at 59.54 KeV excitation energy from an 241Am annular radioactive source. The characteristic K X-rays emitted by the targets have been detected with a Silicon drift detector (SDD); the net areas of the K α and K β spectral peaks have been determined using the fitting process of PYMCA software. The obtained experimental values of the K X-ray fluorescence cross sections and the K-shell fluorescence yields were compared to the theoretical calculated ones and also to the experimental results obtained from a different experimental setup using a point source. The obtained values were between (6.84 ± 0.55 b/at.) and (891.0 ± 67.5 b/at.) for pure metals form Scandium to Indium. Experimental values were in good agreement with theoretical values within limits of uncertainty, as well as with the data reported in the literature. Moreover, the experimental determination of K X-ray fluorescence cross-section of scandium (Sc) at 59.54 KeV has not been preceded as far as we know. The value obtained is (6.84 ± 0.55 b/at). This new result obtained maybe contribute to the enrichment of the available databases.

Origin of Enhancement of Orbital Magnetic Moment in SiO2-Coated Fe3O4 Nanocomposites Studied by X-ray Magnetic Circular Dichroism.

In this study, magnetic Fe3O4 nanoparticles (NPs) were dispersed uniformly by varying the thickness of the SiO2 coating, and their electronic and magnetic properties were investigated. X-ray diffraction confirmed the structural configuration of monophase inverse-spinel Fe3O4 NPs in nanometer size. Scanning electron microscopy revealed the formation of proper nonporous crystallite particles with a clear core-shell structure with silica on the surface of Fe3O4 NPs. The absorption mechanism studied through the zeta potential indicates that SiO2-coated Fe3O4 nanocomposites (SiO2@Fe3O4 NCs) possess electrostatic interactions to control their agglomeration in stabilizing suspensions by providing a protective shield of amorphous SiO2 on the oxide surface. High-resolution transmission electron microscopy images demonstrate a spherical morphology having an average grain diameter of ∼11-17 nm with increasing thickness of SiO2 coating with the addition of a quantitative presence and proportion of elements determined through elemental mapping and electron energy loss spectroscopy studies. Synchrotron-based element-specific soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) techniques have been involved in the bulk-sensitive total fluorescence yield mode to understand the origin of magnetization in SiO2@Fe3O4 NCs. The magnetization hysteresis of Fe3O4 was determined by XMCD. At room temperature, the magnetic coercivity (Hc) is as high as 1 T, which is about 2 times more than the value of the thin film and about 5 times more pronounced than that of NPs. For noninteracting single-domain NPs with the Hc spread from 1 to 3 T, the Stoner-Wohlfarth model provided an intriguing explanation for the hysteresis curve. These curves determine the different components of Fe oxides present in the samples that derive the remnant magnetization involved in each oxidation state of Fe and clarify which Fe component is responsible for the resultant magnetism and magnetocrystalline anisotropy based on noninteracting single-domain particles.

Pure Polycyclic Aromatic Hydrocarbon Isomerides with Delayed Fluorescence and Anti-Kasha Emission: High-Efficiency Non-Doped Fluorescence OLEDs.

Pure polycyclic aromatic hydrocarbons (PAHs) consisting solely of carbon-hydrogen or carbon-carbon bonds offer great potential for constructing durable and cost-effective emitters in organic electroluminescence devices. However, achieving versatile fluorescence characteristics in pure PAHs remains a considerable challenge, particularly without the inclusion of heteroatoms. Herein, an efficient approach is presented that involves incorporating non-six-membered rings into classical pyrene isomerides, enabling simultaneous achievement of full-color emission, delayed fluorescence, and anti-Kasha emission. Theoretical calculations reveal that the intensity and distribution of aromaticity/anti-aromaticity in both ground and excited states play a crucial role in determining the excited levels and fluorescence yields. Transient fluorescence measurements confirm the existence of thermally activated delayed fluorescence in pure PAHs. By utilizing these PAHs as emitting layers, electroluminescent spectra covering the entire visible region along with a maximum external quantum efficiency of 9.1% can be achieved, leading to the most exceptional results among non-doped pure hydrocarbon-based devices.

Solar-induced chlorophyll fluorescence captures the effects of elevated ozone on canopy structure and acceleration of senescence in soybean.

Solar-induced chlorophyll fluorescence (SIF) provides an opportunity to rapidly and non-destructively investigate how plants respond to stress. Here, we explored the potential of SIF to detect the effects of elevated O3 on soybean in the field where soybean was subjected to ambient and elevated O3 throughout the growing season in 2021. Exposure to elevated O3 resulted in a significant decrease in canopy SIF at 760 nm (SIF760), with a larger decrease in the late growing season (36%) compared with the middle growing season (13%). Elevated O3 significantly decreased the fraction of absorbed photosynthetically active radiation by 8-15% in the middle growing season and by 35% in the late growing stage. SIF760 escape ratio (fesc) was significantly increased under elevated O3 by 5-12% in the late growth stage due to a decrease of leaf chlorophyll content and leaf area index. Fluorescence yield of the canopy was reduced by 5-11% in the late growing season depending on the fesc estimation method, during which leaf maximum carboxylation rate and maximum electron transport were significantly reduced by 29% and 20% under elevated O3. These results demonstrated that SIF could capture the elevated O3 effect on canopy structure and acceleration of senescence in soybean and provide empirical support for using SIF for soybean stress detection and phenotyping.

Ultrafast internal conversion and photochromism in gas-phase salicylideneaniline.

Salicylideneaniline (SA) is an archetypal system for excited-state intramolecular proton transfer (ESIPT) in non-planar systems. Multiple channels for relaxation involving both the keto and enol forms have been proposed after excitation to S1 with near-UV light. Here, we present transient absorption measurements of hot gas-phase SA, jet-cooled SA, and SA in Ar clusters using cavity-enhanced transient absorption spectroscopy (CE-TAS). Assignment of the spectra is aided by simulated TAS spectra, computed by applying time-dependent complete active space configuration interaction (TD-CASCI) to structures drawn from nonadiabatic molecular dynamics simulations. We find prompt ESIPT in all conditions followed by the rapid generation of the trans keto metastable photochrome state and fluorescent keto state in parallel. Increasing the internal energy increases the photochrome yield and decreases the fluorescent yield and fluorescent state lifetime observed in TAS. In Ar clusters, internal conversion of SA is severely hindered, but the photochrome yield is unchanged. Taken together, these results are consistent with the photochrome being produced via the vibrationally excited keto population after ESIPT.

Supramolecular assembly of coumarin 7 with sulfobutylether-β-cyclodextrin for biomolecular applications.

Coumarins, in general, exhibit a wide range of photophysical characteristics and are highly sensitive to their microenvironment, and, therefore, their fluorescence characteristics have attracted immense attention as sensors in chemical and biological systems. In the present study, the supramolecular interaction of a bichromophoric coumarin dye, namely, Coumarin 7 (C7) with sulfobutylether-β-cyclodextrin (SBE7βCD) macrocyclic host at different pH conditions has been investigated by using optical spectroscopic techniques such as absorption, steady-state and time-resolved emissions, and circular dichroism measurements and compared with that of βCD. Considerable enhancement in the fluorescence intensity and lifetime of C7 on complexation with SBE7βCD proposes that non-radiative processes like TICT behavior are strictly hindered due to the confinement in the host cavity experienced by the C7 dye. The increase in the rotational correlation time evaluated from the fluorescence anisotropy decay kinetics further confirms the formation of tightly bound inclusion complexes. The binding constant values reveal that the monocationic form of dye at pH 3 shows ∼3 times stronger interaction with SBE7βCD than the neutral form of dye at pH 7 due to strong electrostatic cation-anion interaction. SBE7βCD:C7 exhibits an improved photostability and an upward pK a shift of 0.4 unit compared to the contrasting downward pK a shift of 0.5 with the βCD. The enhanced fluorescence yield and increased photostability have been exploited for bioimaging applications, and better images were captured by staining the Drosophila fly gut with the SBE7βCD:C7 complex. The enhancement in the binding interaction and the emission intensity were found to be responsive to external stimuli such as small competitive binders or metal ions and nearly quantitative dissociation of the complex was demonstrated to release the dye and would find stimuli-responsive applications.

Fluorescent pH sensors based on BODIPY structure sensitive in acidic media

Two BODIPY phenolic esters 1 and 2 were synthesized and their acid-catalyzed hydrolytic and esterase enzymatic stability was demonstrated. Their photophysical properties were investigated in solvents of different polarity. Their spectral properties are the typical of BODIPY dyes with narrow absorption and emission spectra in the visible spectral region and large molar absorption coefficients (≈500 nm, ε = 40000-50000 M−1cm−1). The derivative with methyl substituted BODIPY core 2 shows quantum yields of fluorescence one magnitude higher (Φf = 0.30–0.55) compared to the BODIPY without methyl groups. Solvents affect very weakly spectral properties, with more pronounced effect of the solvent polarizability than the solvent polarity. Both BODIPY dyes show pH responsive fluorescence in aqueous solution (pKa = 2.72 ± 0.08 and 4.18 ± 0.05), with quenching of fluorescence by protonation due to PET from the BODIPY to phenol, rendering them applicable for intracellular pH measurements. The feasibility of the PET in the protonated form of 1 was corroborated by electrochemical measurements and calculation of ΔETGo for PET, and (time-dependent) density functional theory computations. This less frequently encountered quenching mechanism where the BODIPY is an electron donor provides new opportunities in the rational design of new generations of fluorescent sensors. The applicability for intracellular measurements was demonstrated by confocal fluorescence microscopy on H460 human cancer cell line.