Mercury (Hg2+) Sensing Using Coumarin-Derived Fluorescent Chemo-Sensors: An Intuitive Development from 2015 to 2023

Mercury is known as a highly toxic metal that is poisonous even if present in a trace amount. Generally, it enters in the food chain (especially fish) and water resources via different pathways and leads to harmful effects. Owing to the detrimental nature of the metal, traditionally several methods were employed by researchers for regular monitoring of the mercury metal ions. However, these methods are associated with many limitations like high cost of technical expertise, and intricacy of the detection procedure. So, using these methods to detect mercury ions in real time is challenging. Therefore, in recent years fluorescent-based analytical tools emerged rapidly. Among the various fluorescent organic scaffolds, coumarin has been scorching, owing to quick response, light stability, high sensitivity, good selectivity, excellent fluorescence intensity, and fluorescence quantum yield. This review provides a deep dive into the coumarin-derived chemo-sensors development throughout 2015-2023. We anticipate that the review will assist to broad scientific community as a reference document to design more interesting sensors.

Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display "unusual" emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha's rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe₂ group around the core of benzo[g]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how "seemingly inconsequential" auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to "anti-Kasha" photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.

Nature of Linear Spectral Properties and Fast Relaxations in the Excited States and Two-Photon Absorption Efficiency of 3-Thiazolyl and 3-Phenyltiazolyl Coumarin Derivatives

Coumarin-based fluorescent agents play an important role in the manifold fundamental scientific and technological areas and need to be carefully studied. In this research, linear photophysics, photochemistry, fast vibronic relaxations, and two-photon absorption (2PA) of the coumarin derivatives, methyl 4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]butanoate (1) and methyl 4-[4-[2-(7-methoxy-2-oxo-chromen-3-yl)thiazol-4-yl]phenoxy]butanoate (2), were comprehensively analyzed using stationary and time-resolved spectroscopic techniques, along with quantum-chemical calculations. The steady-state one-photon absorption, fluorescence emission, and excitation anisotropy spectra, as well as 3D fluorescence maps of 3-hetarylcoumarins 1 and 2 were obtained at room temperature in solvents of different polarities. The nature of relatively large Stokes shifts (∼4000-6000 cm^-1), specific solvatochromic behavior, weak electronic π → π* transitions, and adherence to Kasha's rule were revealed. The photochemical stability of 1 and 2 was explored quantitatively, and values of photodecomposition quantum yields, on the order of ∼10^-4, were determined. A femtosecond transient absorption pump-probe technique was used for the investigation of fast vibronic relaxation and excited-state absorption processes in 1 and 2, while the possibility of efficient optical gain was shown for 1 in acetonitrile. The degenerate 2PA spectra of 1 and 2 were measured by an open aperture z-scan method, and the maximum 2PA cross-sections of ∼300 GM were obtained. The electronic nature of the hetaryl coumarins was analyzed by quantum-chemical calculations using DFT/TD-DFT level of theory and was found to be in good agreement with experimental data.

Synthesis, Optical Characterization in Solution and Solid-State, and DFT Calculations of 3-Acetyl and 3-(1'-(2'-Phenylhydrazono)ethyl)-coumarin-(7)-substituted Derivatives

Intramolecular charge transfer (ICT) effects are responsible for the photoluminescent properties of coumarins. Hence, optical properties with different applications can be obtained by ICT modulation. Herein, four 3-acetyl-2H-chromen-2-ones (1a–d) and their corresponding fluorescent hybrids 3- (phenylhydrazone)-chromen-2-ones (2a–d) were synthesized in 74–65% yields. The UV-Vis data were in the 295–428 nm range. The emission depends on the substituent in position C-7 bearing electron-donating groups. Compounds 1b–d showed good optical properties due to the D-π-A structural arrangement. In compounds 2a–d, there is a quenching effect of fluorescence in solution. However, in the solid, an increase is shown due to an aggregation-induced emission (AIE) effect given by the rotational restraints and stacking in the crystal. Computational calculations of the HOMO-LUMO orbitals indicate high absorbance and emission values of the molecules, and gap values represent the bathochromic effect and the electronic efficiency of the compounds. Compounds 1a–d and 2a–d are good candidates for optical applications, such as OLEDs, organic solar cells, or fluorescence markers.

Coumarins into Polyurethanes for Smart and Functional Materials

Polyurethanes are of undoubted interest for the scientific community and the industry. Their outstanding versatility from tailor-made structures turns them into major polymers for use in a wide range of different applications. As with other polymers, new, emerging molecules and monomers with specific attributes can provide new functions and capabilities to polyurethanes. Natural and synthetic coumarin and its derivatives are characterised by interesting biological, photophysical and photochemical properties. Then, the polyurethanes can exploit those features of many coumarins which are present in their composition to achieve new functions and performances. This article reviews the developments in the proper use of the special properties of coumarins in polyurethanes to produce functional and smart materials that can be suitable for new specific applications.

The Interplay between Solvation and Stacking of Aromatic Rings Governs Bright and Dark Sites of Benzo[g]coumarins

Coumarins are classic, strongly polarized fluorophores with multiple applications, and significant efforts have been put into modifying their emission characteristics and elucidating their photophysics. Expecting that π-expansion of these donor-acceptor chromophores could modify their ground- and excited-state characteristics, the authors performed combined, detailed photophysical and computational studies of linearly π-expanded coumarins, that is, 8-dialkylamino-3-carboxyalkyl-benzo[g]coumarins. The investigation led to the conclusion that emission is only possible thanks to the stabilizing effect of the solvent and that breaking of the lactone ring leads to the conical intersection with the ground state and induces the radiationless decay of the electronic excitation. Aiming at the fine-tuning the excited state properties through the construction of covalently linked dye assemblies, the authors designed and synthesized a new bis(benzo[g]coumarin), built from two similar moieties that exhibit different degrees of polarization due to the electron donor at position 8: one possesses a dialkylamino, and the other a weaker amide donor. Comprehensive studies have shown that the observed weak fluorescence of the system is the result of the interplay between the solvation-induced separation of the benzo[g]coumarin moieties, which stabilizes the emitting locally excited singlet state and the π-stacking interactions, favoring their sandwiched orientation and leading to the non-emissive charge-transfer state.

Accessing [g]-Face π-Expanded Fluorescent Coumarins by Scholl Cyclization

[g]-Face π-expanded coumarins are synthesized by employing the Scholl cyclization method. These new arene-annulated dipolar coumarins display interesting absorption and fluorescent properties. The large Stokes shifts, tuneable fluorescent quantum yields, and high photostability reveal promise in bioimaging application.

Building Molecular Complexity from Quinizarin: Conjoined Coumarins and Coronene Analogs

The double Knoevenagel condensation of 1,4-dibenzoyloxyanthraquinone with methyl esters of arylacetic acids affords a series of compounds based upon a previously unknown 1,8-dioxa-benzo[e]pyrene-2,7-dione heterocyclic core. The aryl groups incorporated in the 3- and 6-positions can be oxidatively coupled to the π-expanded backbone to produce a further new heterocyclic core: 1,10-dioxa-dibenzo[dj]coronene-2,9-dione. The intriguing optical properties of these π-expanded coumarin derivatives are discussed and rationalized through quantum chemical calculations. The broad absorption bands of 1,8-dioxa-benzo[e]pyrene-2,7-dione-based dyes are attributed to both HOMO-1→LUMO and HOMO→LUMO transitions, which have a similar energy. Weakly coupled electron-donating aryl substituents result in a moderate bathochromic shift of both the absorption and emission by 30-60 nm in toluene. The emissive properties of these compounds are in part determined by the oscillator strength of the main transition, lifetimes of the excited state, and by the energy match of the excited state with a triplet state of a similar energy. The 1,10-dioxa-dibenzo[dj]coronene-2,9-dione displays a much smaller Stokes shift, yet a markedly increased fluorescence quantum yield of 90 % owing to the increased rigidity compared with the 1,8-dioxa-benzo[e]pyrene-2,7-dione core.