•Anion receptors are important in various biological processes and ecology.•The urea or thiourea are often incorporated into anion receptors and sensors.•Urea and thiourea receptors selectively bind ...anions of complementary geometry.•This critical review contains 386 references.•Review gives an overview of sensing mechanisms in complex photophysical systems.•Receptors are classified with respect to the complexity and the number of urea groups.
The importance of anion receptors is a reflection of the ubiquitousness of anions, their functions in biological and industrial processes, as well as their behaviour as pollutants. In the immense field of anion receptor chemistry which has expanded greatly in the past few decades, one of the most prominent binding sites is (thio)urea. The simple synthesis of diverse (thio)urea derivatives, their possibility to form two H-bonds with anions and a promising selectivity with anions of complementary geometry are factors that gave rise to a plethora of studies reporting new organic molecules with different properties and applicability. This review includes the most important examples of (thio)urea receptors from the very beginning, until present. Many authors have used (thio)urea moieties in the computer-aided design of anion sensors, which was then followed by their synthesis, and utilization in the photophysical characterization of host–guest systems, and studies in the transport of ions and ion pairs through membranes. These endeavours led to their successful utilization in the fields of crystal engineering and development of functional materials. Characterization of supramolecular systems and investigations of the complexation thermodynamics has been conducted by use of different analytical and physico-chemical methods. Consequently, it is highly beneficial to summarize all aspects of anion recognition by (thio)ureas within one review article. This critical review containing 386 references classifies (thio)urea derivatives, published from 1990 to 2015, with respect to the complexity of the receptors and the number of urea groups in the molecule.
Photoremovable protecting groups (photocages) 6b–6i based on 1-amino-2-hydroxymethylnaphthalene were developed, and their applicability to release alcohols and carboxylic acids in photohydrolysis was ...investigated. Compound 6b cannot release alcohol since N-demethylation takes place instead. However, the photorelease of carboxylic acids from 6c–6i was demonstrated on caged substrates, including some nonsteroidal drugs and a neurotransmitter. A simultaneous use of aniline and aminonaphthalene cages allows for the chromatic orthogonality and selective deprotection by UV-B or near-visible and UV-A light, respectively. The photochemical reaction mechanism of decaging was investigated by fluorescence measurements and laser flash photolysis, indicating that the heterolysis and elimination of carboxylic acids take place in the singlet excited state, delivering carbocation as an intermediate. The photoheterolysis in the singlet excited state, which directly releases caged substrates, is highly applicable for the photocages and has advantages compared to hitherto used nitrobenzyl derivatives.
3-Hydroxymethyl-2-aminonaphthalene photocage (photoremovable protecting group) 2 was synthesized and transformed to different ethers and esters to investigate the applicability to decage alcohols and ...carboxylic acids, respectively. The photoelimination of carboxylic acids takes place relatively efficiently (ΦR = 0.11) upon excitation with near-visible light, contrary to the elimination of alcohols. The scope of the decaging of both alcohols and esters was demonstrated on several examples, including aliphatic and aromatic substrates, carbohydrates, and nonsteroidal anti-inflammatory drugs. The photophysical properties of the photocage and its models, methyl ether 4a and acetyl ester 5a, were investigated. The fluorescence quantum yields (Φf = 0.40–0.002) were found to be reversely proportional to the efficiency of elimination of OH, alcohols, or carboxylic acids. The decaging photochemical reaction mechanism was investigated experimentally by transient absorption techniques with time scales from femtoseconds to seconds and computationally on the TD-DFT level of theory. The photoelimination of carboxylates takes place directly in the singlet excited state by a homolytic cleavage producing a radical pair within 1 ns. The subsequent electron transfer gives rise to aminonaphthalene carbocation and the carboxylate. A wide scope of substrates that can be decaged relatively efficiently with near-visible light and the chromo-orthogonal compatibility of aminonaphthalene and aniline derivatives render these photocages potentially applicable in organic synthesis or biology.
Excited state intramolecular proton transfer (ESIPT) has been documented from an amino NH2 group to a carbon atom of an adjacent aromatic ring. This finding changes the paradigm, as hitherto such ...processes have not been considered as plausible due to slow protonation of carbon and low (photo)acidity of the NH2 group. The ESIPT was studied by irradiation of 2-(2-aminophenyl)naphthalene in CH3CN–D2O, whereupon regiospecific incorporation of deuterium takes place at the naphthalene position 1, with a quantum yield of Φ = 0.11. A synergy of experimental and computational investigations completely unraveled the mechanism of this important photochemical reaction. Upon excitation to the photoreactive S2(La) state, a favorable redistribution of charge sets the stage for ESIPT to the carbon atom in naphthalene position 1. H2O molecules are needed, as they increase the excitation energy and oscillator strength for the population of the S2(La) state. The gain in energy is used to surmount a small energy barrier on the pathway from the Franck–Condon geometry to the conical intersection with the S0, delivering aza-quinone methide.
Femtosecond time-resolved transient absorption spectroscopy experiments and density functional theory computations were done for a mechanistic investigation of 3-(1-phenylvinyl)phenol (1) and ...3-hydroxybenzophenone (2) in selected solvents. Both compounds went through an intersystem crossing (ISC) to form the triplet excited states Tππ* and Tnπ* in acetonitrile but behave differently in neutral aqueous solutions, in which a triplet excited state proton transfer (ESPT) induced by the ISC process is also proposed for 2 but a singlet ESPT without ISC is proposed for 1, leading to the production of the triplet quinone methide (QM) and the singlet excited QM species respectively in these two systems. The triplet QM then underwent an ISC process to form an unstable ground state intermediate which soon returned to its starting material 2. However, the singlet excited state QM went through an internal conversion process to the ground state QM followed by the formation of its final product in an irreversible manner. These differences are thought to be derived from the slow vinyl C–C rotation and the moderate basicity of the vinyl C atom in 1 as compared with the fast C–O rotation and the greater basicity of the carbonyl O atom of 2 after photoexcitation. This can account for the experimental results in the literature that the aromatic vinyl compounds undergo efficient singlet excited state photochemical reactions while the aromatic carbonyl compounds prefer triplet photochemical reactions under aqueous conditions. These results have fundamental and significant implications for understanding of the ESPT reactivity in general, as well as for the design of molecules for efficient QM formation in aqueous media with potential applications in cancer phototherapy.
New photoactivable BODIPY fluorescent dyes, which are quinone methide (QM) precursors, were synthesized. The series of BODIPY dyes has a photoreactive group linked to the BODIPY core by vinyl or ...ethynyl spacers. Molecules have bathochromic shifted absorption and emission maxima and higher fluorescence quantum yields compared to the previously reported BODIPY QM precursors. They show anti-Kasha photochemical reactivity and undergo photomethanolysis reaction upon photoexcitation at λ < 350 nm. Although UV light is needed for the photochemical generation of QMs and attachment to potential bio-targets, bathochromic shifted absorption and emission has advances in sensing and fluorescence labeling. Antiproliferative investigation was conducted for the vinyl derivative against three human cancer lines with the cells kept in dark or irradiated. Upon excitation with visible light antiproliferative activity was enhanced with IC50 values in low micromolar concentration.
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Derivatives of p-cresol 1–4 were synthesized, and their photochemical reactivity, acid–base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from ...that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9–11) or diamine is in the monocationic form (pH 7–9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.
Formation of quinone methides (QMs) by photoelimination of an ammonium salt from cresol derivatives was investigated by femtosecond transient absorption spectroscopy (fs-TA) and computationally by ...time-dependent density functional theory using the PCM(water)/(TD-)B3LYP/6-311++G(d,p) level of theory. The photoelimination takes place in an adiabatic ultrafast reaction on the S1 potential energy surface delivering the corresponding QMs(S1), which were detected by fs-TA. Computations predicted a high-energy cation intermediate in the pathway between the Franck–Condon state of a monoammonium salt and the corresponding QM(S1) that was not detected by fs-TA. On the other hand, elimination from a disalt in H2O takes place in one step, giving directly the QM(S1). The combined experimental and theoretical investigation fully disclosed the formation of QMs by the deamination reaction mechanism, which is important in the application of cresols or similar molecules in biological systems.
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