Fluorescent Photoinduced Electron Transfer Research Articles

The Anthracen-9-ylmethyloxy Unit: An Underperforming Motif Within the Fluorescent PET (Photoinduced Electron Transfer) Sensing Framework

Compound, which was designed to act as a fluorescent sensor for calcium according to the PET (Photoinduced Electron Transfer) principle, shows a relatively small Ca2+-induced fluorescence enhancement factor (FE) of 1.8 whereas its close relative is known to display a far higher FE value of 16. Though designed as fluorescent PET sensors for solvent polarity, compounds and also show negligible fluorescence enhancement as their environments are made progressively less polar even though their relatives and show limiting FE values of 53 and 3, respectively. Indeed, and are useful since they are fluorescent sensors for solvent polarity without being affected by Bronsted acidity. The poor sensory performance of and relative to their cousins is attributed to the presence of an oxygen proximal to the 9-position of an anthracene unit, which opens up a CT (charge transfer) channel. Normal PET sensing service is resumed when the offending oxygen is deleted.

Fluorescent Anion Sensor Derived from Cholic Acid: The Use of Flexible Side Chain

[structure: see text] The fluorescent photoinduced electron transfer (PET) chemosensors 1-3 were synthesized from cholic acid. 1 and 2 containing amidothiourea groups as anion receptive sites demonstrated much higher affinity toward anions than 3 containing traditional thiourea H-bond donating group. Comparative studies on their binding affinity toward carboxylates, dihydrogen phosphate, and halides revealed that the amidothiourea moiety on the C17 side chain could work cooperatively with H-bond donating groups on C7 and C12 to bind spherical halogen anions. An unexpected specific fluorescence enhancement of 1 by coordinating bromide ion was observed.

Fluorescent Photoinduced Electron Transfer (PET) Sensors for Anions; From Design to Potential Application

This mini review highlights the synthesis and photophysical evaluation of anion sensors, for nonaqueous solutions, that have been developed in our laboratories over the last few years. We have focused our research mainly on developing fluorescent photoinduced electron transfer (PET) sensors based on the fluorophore-spacer-anion receptor principle using several anthracene (emitting in the blue) and 1,8-naphthalimide (emitting in the green) fluorophores, with the aim of targeting biologically and industrially relevant anions such as acetates, phosphate and amino acids, as well as halides such as fluoride. The receptors and the fluorophore are separated by a short methyl or ethyl spacer, where the charge neutral anion receptors are either aliphatic or aromatic urea (or thiourea) moieties. For these, the anion recognition is through hydrogen bonding, yielding anion:receptor complexes. Such bonding gives rise to enhanced reduction potential in the receptor moieties which causes enhancement in the rate of PET quenching of the fluorophore excited state from the anion:receptor moiety. This design can be further elaborated on by incorporating either two fluorophores, or urea/thiourea receptors into the sensor structures, using anthracene as a fluorophore. For the latter design, the sensors were designed to achieve sensing of bis-anions, such as di-carboxylates or pyrophosphate, where the anion bridged the anthracene moiety. In the case of the naphthalimide based mono-receptor based PET sensors, it was discovered that in DMSO the sensors were also susceptible to deprotonation by anions such as F(-) at high concentrations. This led to substantial changes in the absorption spectra of these sensors, where the solution changed colour from yellow/green to deep blue, which was clearly visible to the naked eye. Hence, some of the examples presented can act as dual fluorescent-colorimetric sensors for anions. Further investigations into this phenomenon led to the development of simple colorimetric sensors for fluorides, which upon exposure to air, were shown to fix carbon dioxide as bicarbonate.

A fluorescent “off–on–off” proton switch derived from natural products and further studies of first-generation fluorescent photoinduced electron transfer (PET) systems

Two new second generation fluorescent switches that are based on the chromophore–spacer–receptor architecture are reported in this paper. The fluorescence modulation of these switches is a result of quenching or initiating photoinduced electron transfer (PET) between the chromophore and the receptors due to cation binding. These two molecules utilize an alkaloid, anabasine, as the proton receptor, and function as fluorescent off–on–off switches with decreasing pH. One of these molecules also utilizes a coumarin as the chromophore, making it the first rationally designed fluorescent PET system based on natural products. Further studies of two well-known first generation fluorescent switches to achieve more complex fluorescence modulation are also reported in this paper.

Synthesis and photophysical evaluation of charge neutral thiourea or urea based fluorescent PET sensors for bis-carboxylates and pyrophosphate

The synthesis of the fluorescent photoinduced electron transfer (PET) chemosensors 1-3 for bis-anions such as bis-carboxylates and pyrophosphate in organic solvents is described herein. These sensors are based on the receptor-spacer-fluorophore-spacer-receptor motif where the receptors are charge neutral aromatic thiourea or urea receptors and the fluorophore is anthracene. The anion recognition was evaluated using 1H NMR as well as absorption and fluorescence spectroscopy in DMSO. For simple anions such as acetate or fluoride, the recognition was shown to be through hydrogen bonding of the corresponding anion to the receptors. This gave rise to only minor changes in the absorption spectra, but significant changes in the fluorescence emission spectra, which was substantially (70-95%) quenched. Analysis of these recognition events implied a 1 : 2 (sensor : anion) binding and ideal PET behaviour for ions such as AcO- and H2PO4-. For F-, the luminescent quenching indicated a 1 : 1 binding, but we deduced that this was due more to complete quenching of the excited state after the addition of one equivalent of the anion. For all of the anions, the quenching contributed to enhanced efficiency of PET from the receptors to the excited state of the fluorophore. In the case of the bis-anions (ambient), such as di-carboxylates, similar fluorescence quenching was observed. However, here either a 1 : 1 or a 1 : 2 binding was observed depending on the length of the spacer separating the two carboxylate moieties and the nature of the receptor. Whereas both pyrophosphate and malonate gave rise to a 1 : 1 binding, glutarate gave rise to approximately 1 : 2 binding for the thiourea sensors 1 and 2. However, for the urea based sensor 3, the binding was found to be 1 : 1 for all the bis-anions. For such a 1 : 1 binding we propose that the anion most likely bridges the fluorophore moiety. This was also evident from the 1H NMR (DMSO-d6) spectrum where the anthracene resonances were significantly affected. By simply modifying the electronic structure of the receptor, the sensitivity of the recognition process could also be modified; e.g. compound 1, bearing the trifluoromethyl substituent, showed stronger binding to the bis-anions than 2, which possessed a simple phenyl moiety.

Design, synthesis and photophysical studies of simple fluorescent anion PET sensors using charge neutral thiourea receptors.

The synthesis of four fluorescent photoinduced electron transfer (PET) chemosensors 1-4 for anions is described. These are all based on a simple design employing charge neutral aliphatic or aromatic thiourea anion receptors connected to an anthracene fluorophore via a methylene spacer. Here the anion recognition occurred through 1 : 1 hydrogen bonding between the thiourea protons and the anion, as demonstrated by observing the changes in the (1)H NMR in DMSO-d(6) where the two thiourea protons were shifted downfield upon addition of anions. Whereas 1-3 were designed for the detection of anions such as fluoride, acetate or phosphate, 4 was made for the recognition of N-protected amino acids. All the sensors showed 'ideal' behaviour where only the fluorescence emission was quenched upon anion recognition, due to enhanced efficiency of electron transfer quenching from the receptor to the excited state of the fluorophore. By simply varying the nature of the thiourea substituent it was possible to modulate, or tune, the acidity of the thiourea receptor moiety, altering the sensitivity of the anion recognition. For, the anion selectivity and the degree of the fluorescence quenching were in the order of F(-) > AcO(-) > H(2)PO(4)(-), with Cl(-) or Br(-) not being detected.

A computational study of a fluorescent photoinduced electron transfer (PET) sensor for cations

AbstractAb initio molecular orbital theory and density functional theory with the 6‐31G(d,p) basis set have been used to calculate the structural parameters of a fluorescent photoinduced electron transfer (PET) sensor for cations and its zinc complex. The optimized geometries are compared with the X‐ray crystal structures of N‐(9‐anthracenylmethyl)–N‐[(2‐pyridinyl)methyl]‐2‐pyridinemethanamine and [N‐(9‐anthracenylmethyl)‐N‐[(2‐pyridinyl‐κN)methyl]‐2‐pyridinemethanamine‐κN1, κN2]dichlorozinc. Although the X‐ray studies are based on solid‐phase structures and the computational studies are based on gas‐phase structures, comparisons of the data show significant agreement between the two sets of geometric parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Dual responsive chemosensors for anions: the combination of fluorescent PET (Photoinduced Electron Transfer) and colorimetric chemosensors in a single molecule

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of ‘ fluorophore-spacer-(anion)receptor’. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F − and AcO − in DMSO, the fluorescence emission of 1 and 2 was ‘ switched off’, with no significant changes in the UV–vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F −, further additions of the anion, gave rise to large changes in the UV–vis spectra, where the λ max at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6 . Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.

Detecting microdamage in bone.

Fatigue-induced microdamage in bone contributes to stress and fragility fractures and acts as a stimulus for bone remodelling. Detecting such microdamage is difficult as pre-existing microdamage sustained in vivo must be differentiated from artefactual damage incurred during specimen preparation. This was addressed by bulk staining specimens in alcohol-soluble basic fuchsin dye, but cutting and grinding them in an aqueous medium. Nonetheless, some artefactual cracks are partially stained and careful observation under transmitted light, or epifluorescence microscopy, is required. Fuchsin lodges in cracks, but is not site-specific. Cracks are discontinuities in the calcium-rich bone matrix and chelating agents, which bind calcium, can selectively label them. Oxytetracycline, alizarin complexone, calcein, calcein blue and xylenol orange all selectively bind microcracks and, as they fluoresce at different wavelengths and colours, can be used in sequence to label microcrack growth. New agents that only fluoresce when involved in a chelate are currently being developed--fluorescent photoinduced electron transfer (PET) sensors. Such agents enable microdamage to be quantified and crack growth to be measured and are useful histological tools in providing data for modelling the material behaviour of bone. However, a non-invasive method is needed to measure microdamage in patients. Micro-CT is being studied and initial work with iodine dyes linked to a chelating group has shown some promise. In the long term, it is hoped that repeated measurements can be made at critical sites and microdamage accumulation monitored. Quantification of microdamage, together with bone mass measurements, will help in predicting and preventing bone fracture failure in patients with osteoporosis.

Fluorescence and NMR binding studies of N-aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives.

N-Aryl-N'-(9-methylanthryl)diaza-18-crown-6 derivatives perform as fluorescent photoinduced electron-transfer (PET) sensors with very selective response toward Ca(2+) versus Mg(2+), Na(+), and K(+). The fluorescence intensity was increased by a factor of up to 170 in the presence of Ca(ClO(4))(2). (1)H NMR studies show that metal cations affect these molecules very differently: Ca(2+) has a global effect on each molecule, while Mg(2+) affects part of each molecule, and K(+) and Na(+) affect each molecule moderately, which is very consistent with the fluorescence response.

Fluorescent sensing of pyrophosphate and bis-carboxylates with charge neutral PET chemosensors.

[reaction: see text] The fluorescent photoinduced electron transfer (PET) chemosensors 2 and 3 were designed for the recognition of anions possessing two binding sides such as dicarboxylates and pyrophosphate; the anion recognition in DMSO takes place through the two charge neutral thiourea receptor sites with concomitant PET quenching of the anthracene moiety. The anion binding of acetate, phosphate, and pyrophosphate to 2 and 3 was also evaluated by using 1H NMR in DMSO-d6.

A novel fluorescent photoinduced electron transfer (PET) sensor for lithium

The chiral diaza-9-crown-3 derivative 1 is the first example of a fluorescent PET chemosensor designed specifically for detecting lithium ions (Li +). 1 shows ‘off–on’ switching of fluorescence when treated with various lithium salts in organic solvents such as CH 3CN, with binding constant log β=5.4, at the same time as discriminating against a variety of group I and II metal ions. In water the emission is quenched by anions due to the heavy-atom effect.

Conversion of thioureas to fluorescent isothiouronium-based photoinduced electron transfer sensors for oxoanion sensing

A convenient conversion is described of thiourea-based receptors to fluorescent isothiouronium-based photoinduced electron transfer (PET) sensors for oxoanion sensing. Naphthalene- or anthracene-functionalized mono-isothiouroniums are synthesized from the corresponding thioureas, in which the fluorophore is connected to the sulfur atom of the thiourea moiety by a methylene or an ethylene spacer. Even though all of the isothiouroniums with a methylene spacer readily decompose in MeOH upon excitation of the fluorophore moiety, the isothiouronium with an ethylene spacer shows good stability under identical conditions. The naphthyl isothiouronium with an ethylene spacer shows a significant fluorescence enhancement upon formation of a 1 ∶ 1 complex with oxoanions in MeOH, and the selectivity follows the order of hydrogen phosphate > acetate ≫ dihydrogen phosphate ≫ chloride. The results in the present work indicate that various types of fluorescent PET sensors might be readily obtainable from non-fluorescent thiourea-based receptors by the introduction of appropriate fluorophores at the sulfur atom of thiourea-binding sites.

4-Amino-1,8-dicyanonaphthalene derivatives as novel fluorophore and fluorescence switches: efficient synthesis and fluorescence enhancement induced by transition metal ions and protons

A new electronic push–pull fluorophore, 4-amino-1,8-dicyanonaphthalene, and its derivatives have been synthesized efficiently. They can be used as photo-induced electron transfer fluorescence switches which exhibit considerable fluorescence enhancement induced by transition metal ions Cr 3+ and Fe 3+ and also respond to pH values very sensitively.

Modular fluorescence sensors for saccharides

Modular and modular polymer supported fluorescence photoinduced electron transfer (PET) sensors 2 and 3 with two boronic acid receptor units, a pyren-1-yl fluorophore, and hexamethylene linker show selective saccharide binding in aqueous methanolic solution at pH 8.21.

Synthesis and evaluation of D-glucosamine-selective fluorescent sensors

Fluorescent PET (photoinduced electron transfer) sensors 11a and 11b with monoaza-18-crown-6 or monoaza-15-crown-5 ether and boronic acid receptor units are synthesised.

Synthesis and complexation behavior of N-(1-naphthylmethyl)-1,4,7,10,13-pentaoxa-16-azacyclooctadecane

A fluorescent photoinduced electron transfer (PET) sensor ( 3) that consists of 1-aza-18-crown-6 and a 1-naphthyl pendant shows a fluorescent increase with bivalent metal ions and a fluorescent decrease with monovalent metal ions. 1H and 13C NMR analyses of this 1:1 complexation behavior revealed that Ba 2+ strongly coordinates with the azacrown nitrogen to cause a dramatic decrease in the intramolecular charge-transfer character. In addition, it was suggested that not only the metal ion-dependent coordinated structure of the resulting complex but also the interaction between the thiocyanate anion and the naphthyl group play a role in controlling the extent of the emission-intensity increase and decrease that was observed in the presence of a given metal salt.

Arenedicarboximide Building Blocks for Fluorescent Photoinduced Electron Transfer pH Sensors Applicable with Different Media and Communication Wavelengths

A whole family of modular photoinduced electron transfer (PET) sensors has been constructed from a single simple reaction type with commercially available materials. This family, one member of which is shown here, possesses amine receptors for protons and arenedicarboximide fluorophores addressable at different wavelengths. Its pH-sensing behaviour is examined in solution and in polymer membrane matrices, which are crucial for the use of fluorescent PET sensors as optode pH monitors.

Selective Fluorescence Signalling of Saccharides in Their Furanose Form

Abstract A boronic acid binding site and biphenyl fluorophore has been used to create a fluorescent PET (photoinduced electron transfer) sensor for saccharides in aqueous solution at pH 7.77. Formation of five or six membered cyclic boronate esters can be “read out” by the relative fluorescence intensity.

Fluorescent PET(Photoinduced Electron Transfer) reagents for thiols

Michael addition of thiois to an electron deficient alkene functional group in reagents (1)-(3) give rise to fluorescence quantum yield enhancements. In the absence of the thiol, photoinduced electron transfer (PET) takes place from the fluorophore to the alkene moiety. Such electron transfer is suppressed after the thiol reaction.