Quantum Yield Switching of Fluorescence by Selectively Bridging Single and Double Bonds in Chalcones: Involvement of Two Different Types of Conical Intersections

Two-Photon Absorption and Multiphoton Excited Fluorescence of Acetamide-Chalcone Derivatives: The Role of Dimethylamine Group on the Nonlinear Optical and Photophysical Properties

This work studied the effect of different electron-withdrawing and electron-donating groups on the linear and nonlinear optical properties of acetamide-chalcone derivatives. The results showed that the addition of the dimethylamine group led to a large fluorescence emission (71% of fluorescence quantum yield in DMSO solution) that can be triggered by two and three-photon excitations, which is essential for biological applications. Furthermore, dimethylamine also red-shifts the lower energy state by approximately 90 nm, increasing the two-photon absorption cross-section of the lower energy band by more than 100% compared with the other studied compounds. All compounds presented two-electronic states observed through one and two-photon absorption spectroscopy and confirmed by Quantum Chemistry Calculations (QCCs). QCC results were also used to model the experimental two-photon absorption cross-sectional spectrum by the Sum-Over-States (SOS) approach, revealing a dependence between the coupling of the ground state with the first excited state and the transition dipole moment between these states.

Rationally designed far-red emitting styryl chromones and a magnetic nanoconjugate for strip-based 'on-site' detection of metabolic markers

The global burden of liver damage and renal failure necessitates technology-aided evolution towards point-of-care (POC) testing of metabolic markers. Hence in the prevalence of current health conditions, achieving on-site detection and quantifying serum albumin (SA) can contribute significantly to halting the increased mortality and morbidity rate. Herein, we have rationally designed and synthesized far-red emitting, solvatofluorochromic styryl chromone (SC) derivatives SC1 and SC2, and SC2-conjugated fluorescent magnetic nanoparticles (SCNPs) for sensing SA with a fluorogenic response via interacting at an atypical drug binding site. In solution, the highly sensitive and selective fluorogenic response was evaluated by the prominent amplification and blue-shift in the emission maxima of the probes from deep red to dark yellow through an intermediate orange emission. The transformation of the fluorogen into a fluorophore was manifested through spectroscopic measurements. The stabilization of the probes at protein pockets was ascribed to the non-covalent interactions, such as H-bonding, cation-π, and hydrophobic interactions, as unveiled by docking studies. The practical applications revealed the novelty of SC derivatives through (a) the capability to detect SA isolated from real blood samples via a turn-on fluorescence response; (b) the design of a simple, cheap, and portable test-strip using a glass-slide loaded with solid-state emissive SC2, which provided differential emission color of the SC2-HSA complex in solution and the solid-state with increasing concentration of HSA. Moreover, a smartphone-based color analysis application was employed to obtain the ratio of green and red (G/R) channels, which was utilized for quantitative detection of HSA; (c) the biocompatibility of the SC1 was ascertained through confocal laser scanning microscopic imaging (CLSM). Detailed investigation showed that SC1 could entirely localize in the mitochondria and evolve as a promising biomarker for distinguishing cancer cells from normal cells. Additionally, the validation of uncommon binding of SC1 and SC2 between domains I and III was determined using competition experiments with a known site-specific binder and molecular docking studies. This unique property of the probes can be further exploited to understand the cellular intake of HSA-drug complexes in the multifaceted biological system. These results find the utility of SC derivatives as small molecule-based chemosensors for at-home SA detection and as a biomarker for cancer.

Structural and theoretical study of copper(ii) complex incorporating chalcone and 2,2′-bipyridine mixed ligands: a probable candidate for optical material

We report the synthesis, structural and photophysical data for a Cu(ii) complex, [Cu(L·bpy)(NO3)], with mixed chalcone and 2,2′-bipyridine ligands. Calculations are used to consider prospective applications in optoelectronic devices.

Electronic and Geometric Characterization of TICT Formation in Hemithioindigo Photoswitches by Picosecond Infrared Spectroscopy

Deciphering the exact electronic and geometric changes of photoexcited molecules is an important task not only to understand the fundamental atomistic mechanisms but also to rationally design molecular properties and functions. Here, we present a combined experimental and theoretical study of the twisted intramolecular charge transfer (TICT) process in hemithioindigo photoswitches. Using ultrafast transient IR spectroscopy as the main analytical method, a detailed understanding of the extent and direction of charge transfer within the excited molecule is obtained. At the same time, the geometrical distortion is monitored directly via changes of indicative vibrational modes over the time course of the photoreaction. These high-resolution data deliver a detailed molecular movie of the TICT process in this important class of chromophores with picosecond time resolution.

The mechanisms of boronate ester formation and fluorescent turn-on in ortho-aminomethylphenylboronic acids

ortho-Aminomethylphenylboronic acids are used in receptors for carbohydrates and various other compounds containing vicinal diols. The presence of the o-aminomethyl group enhances the affinity towards diols at neutral pH, and the manner in which this group plays this role has been a topic of debate. Further, the aminomethyl group is believed to be involved in the turn-on of the emission properties of appended fluorophores upon diol binding. In this treatise, a uniform picture emerges for the role of this group: it primarily acts as an electron-withdrawing group that lowers the pKa of the neighbouring boronic acid thereby facilitating diol binding at neutral pH. The amine appears to play no role in the modulation of the fluorescence of appended fluorophores in the protic-solvent-inserted form of the boronic acid/boronate ester. Instead, fluorescence turn-on can be consistently tied to vibrational-coupled excited-state relaxation (a loose-bolt effect). Overall, this Review unifies and discusses the existing data as of 2019 whilst also highlighting why o-aminomethyl groups are so widely used, and the role they play in carbohydrate sensing using phenylboronic acids.

Modification of Spectral Behavior of Ketocyanine Dyes by Silver Nanoparticles of Different Sizes

The spectral behavior of two ketocyanine dyes 2,5-di[(E)-1-(4-dimethylaminophenyl) methylidine]-1-cyclopentanone(2,5-DMAPMC) and 2,5-di[(E)-1-(4-diethylaminophenyl) methylidine]-1-cyclopentanone (2,5-DEAPMC) is investigated in the presence of silver nanoparticles (AgNPs) of sizes 32[Formula: see text]nm, 52[Formula: see text]nm and 60[Formula: see text]nm using absorption and fluorescence spectroscopy. The absorption spectral changes of dyes with the addition of AgNPs of different sizes suggest their possible interaction with AgNPs. Fluorescence quenching has been observed for both the dyes with the addition of AgNPs of all the sizes. The possible quenching mechanisms including energy transfer and electron transfer are discussed. The fluorescence quenching rate parameter is found to increase with increase in the size of AgNPs and is explained based on the varying ability of silver AgNPs of different sizes to accommodate dye molecules. The effect of functional groups on quenching rate parameters is also discussed.

Femtosecond electronic relaxation and real-time vibrational dynamics in 2′-hydroxychalcone

Electronic relaxation, proton transfer and instantaneous vibrational frequency change after the impulsive excitation by a deep ultraviolet 9 fs pulse were studied.

Impacts of hydroxylation on the photophysics of chalcones: insights into the relation between the chemical composition and the electronic structure

A combined theoretical/experimental study of the photoreactivity of two flavylium-derived chalcones, 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, at the multiconfigurational wavefunction level of theory (CASSCF//CASPT2) in vacuo and in an implicit solvent (water, treated as a polarisable continuum) and by means of linear absorption spectroscopy is presented. The photosensitivity of flavium salts is expressed in the ability of their chalcone form to undergo a cis-trans isomerisation which has found application in logical networks. Despite a considerable amount of experimental data documenting the dependence of the isomerisation on solvent, pH and temperature, the knowledge of how chalcones process energy under various conditions at the molecular level is still scarce. On the example of 2,4,4'-trihydroxychalcone we unravel the complex excited state deactivation mechanism in vacuo involving ultrafast decay through conical intersections, formation of twisted intramolecular charge transfer species, intramolecular proton transfer and inter system crossings. Furthermore, we rationalise the observed discrepancies in the linear absorption spectra of 2,4,4'-trihydroxychalcone and 2,4'-dihydroxychalcone, thereby establishing a link between the functionalisation pattern and the observed spectral properties.

Effect of TiO2 nanoparticles on some photophysical characteristics of ketocyanine dyes

The effect of titanium dioxide (TiO₂ ) nanoparticles (NPs) on photophysical characteristics of 2,5-di[(E)-1-(4-dimethylaminophenyl) methylidine]-1-cyclopentanone (2,5-DMAPMC) and 2,5-di[(E)-1-(4-diethylaminophenyl)methylidine]-1-cyclopentanone (2,5-DEAPMC) ketocyanine dyes has been studied using absorption, steady-state and time-resolved fluorescence spectroscopy. The magnitudes of association constants determined based on modified absorption spectrum of dyes due to the presence of TiO₂ NPs indicate the interaction of TiO₂ NPs with dye molecules. The quenching of fluorescence intensity of dyes by TiO₂ NPs is observed and it follows linear Stern-Volmer (S-V) equation. The magnitude of quenching rate parameter suggests the involvement of static quenching mechanism. The involvement of electron transfer process in reducing fluorescence intensity of dyes has been discussed. Also, varying influence of TiO₂ NPs on two dyes is explained based on the presence of different alkyl substituent in two dyes.

Ingredients to TICT Formation in Donor Substituted Hemithioindigo

Twisted intramolecular charge transfer (TICT) formation in hemithioindigo photoswitches has recently been reported and constitutes a second deexcitation pathway complementary to photoisomerization. Typically, this behavior is not found for this type of photoswitches, and it takes special geometric and electronic conditions to realize it. Here we present a systematic study that identifies the molecular preconditions leading to TICT formation in donor substituted hemithioindigo, which can thus serve as a frame of reference for other photoswitching systems. By varying the substitution pattern and providing an in-depth physical characterization including time-resolved and quantum yield measurements, we found that neither ground-state pretwisting along the rotatable single bond nor the introduction of strong push-pull character across the photoisomerizable double bond alone leads to formation of TICT states. Only the combination of both ingredients produces light-induced TICT behavior in polar solvents.

Experimental and theoretical studies of the influence of solvent polarity on the spectral properties of two push-pull oxazol-5-(4H)-one compounds

Spectral and photophysical properties of two derivatives of the 2-phenyl-1,3-oxazol-5(4H)-ones were studied in 17 solvents of different polarity. These compounds have either push-pull non-centrosymmetric or C₃-symmetric structures with electron-withdrawing groups (2-phenyl-oxazolone) introduced onto the triphenylamine. It has been found that their spectral and photophysical properties depend on the structure of the compounds and on the solvent polarity. The non-radiative relaxation process is facilitated by an increase of the solvent polarity. The changes in the electronic absorption and fluorescence maximum positions with solvent polarity were analyzed applying different solvent polarity parameters based on Lippert-Mataga, McRae, Bakhshiev and Kawski theories or E_T^N scale. The long-wavelength absorption band positions exhibit a slight dependence on the solvent, whereas the fluorescence spectra demonstrate substantial positive solvatochromism. It was found that the position of the electronic absorption band depends mainly on the solute polarizability (related to the solvent refraction index function f(n²)=(n²-1)/(2n²+1)), whereas the solvent polarity influences the position of the fluorescence band. Quantum chemical calculations of the transition energies and dipole moments at the DFT level have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be 10.8 D and 13.0 D according to Bakhshiev's model. The experimental values of Δμ were compared to that one obtained from theoretical calculations for various solvents.

Design, synthesis and photophysical studies of styryl-based push–pull fluorophores with remarkable solvatofluorochromism

A library of 20 styryl-based push–pull dyes derived from 6-amino substituted benzothiazoles were prepared by an efficient and practical synthetic route from low-cost starting materials.

Aggregation-induced emission enhancement in halochalcones

Decorating the periphery of chalcones with halogen atoms increased their aggregation-induced emission enhancement properties, even if their optoelectronic properties are dominated by the other substituents.

Heteroaryl chalcone allied triazole conjugated organosilatranes: synthesis, spectral analysis, antimicrobial screening, photophysical and theoretical investigations

A series of heteroaryl tethered triazole conjoined organosilatranes were synthesized and studied for their solvatochromism experimentally and theoretically followed by antimicrobial screening.

Optical properties and fluorescence quenching of biologically active ethyl 4-(4-N,N-dimethylamino phenyl)-2-methyl-5-oxo-4,5-dihydro-1H-indeno[1,2-b]pyridine-3-carboxylate (DDPC) dye as a probe to determine CMC of surfactants

4-(4-N,N-Dimethylamino phenyl)-2-methyl-5-oxo-4,5-dihydro-1H-indeno[1,2-b]pyridine-3-carboxylate (DDPC) was prepared via the multi-component reaction of indane-1,3-dione with 4-(dimethylamino)benzaldehyde, ethyl acetoacetate and ammonium acetate.

Synthesis, spectral behaviour and photophysics of donor-acceptor kind of chalcones: Excited state intramolecular charge transfer and fluorescence quenching studies

The spectral and photophysical properties of two chalcones containing electron donating and accepting groups with intramolecular charge transfer characteristics were synthesized and characterized by (1)H NMR, (13)C NMR and X-ray crystallography. Both compounds show very strong solvent polarity dependent changes in their photophysical characteristics, namely, remarkable red shift in the emission spectra with increasing solvent polarity, large change in Stokes shift, significant reduction in the fluorescence quantum yield; indicating that the fluorescence states of these compounds are of intramolecular charge transfer (ICT) character. The solvent effect on the photophysical parameters such as singlet absorption, molar absorptivity, oscillator strength, dipole moment, fluorescence spectra, and fluorescence quantum yield of both compounds have been investigated comprehensively. For both dyes, Lippert-Mataga and Reichardt's correlations were used to estimate the difference between the excited and ground state dipole moments (Δμ). The interactions of dyes with colloidal silver nanoparticles (Ag NPs) were also studied in ethanol using steady state fluorescence quenching measurements. The fluorescence quenching data reveal that dynamic quenching and energy transfer play a major role in the fluorescence quenching of dyes by Ag NPs.

Spectroscopic investigation, effect of solvent polarity and fluorescence quenching of a new D-π-A type chalcone derivative

A new chalcone derivative 3-(1-methyl-1H-pyrrol-2-yl)-1-naphthalen-2-yl propenone (MPNP) with electron donor-acceptor group has been synthesized and characterized by IR, (1)HNMR, (13)C NMR and X- ray crystallography. Electronic absorption and emission spectra of MPNP have been studied in solvents of different polarity. A remarkable red shift was observed in the emission spectrum of MPNP compared to the absorption spectrum upon increasing the solvent polarity, indicating a higher dipole moment in the excited state than in the ground state and the transition involved are π-π* with charge transfer character. Lippert-Mataga and Reichardts correlations were used to estimate the change in dipole moments (Δμ); suggest that the emissive state of MPNP is of strong ICT character. Fluorescence quantum yield (ϕf) of MPNP was correlated with empirical solvent polarity parameter ET(30), and it is observed that ϕf increases with increase in solvent polarity of polar aprotic solvents and decrease in alcoholic solvents. The interaction of MPNP with colloidal silver nanoparticles (AgNPs) was also studied in ethanol and ethylene glycol using steady state emission measurements. The fluorescence quenching data reveal that dynamic quenching and energy transfer play a major role in the fluorescence quenching of MPNP by Ag NPs.

Simple Syntheses of Two New Benzo-Fused Macrocycles Incorporating Chalcone Moiety

Simple syntheses of the benzo-fused 26-membered macrocyclic bischalcone (19E,43E)-2.11.27.36-tetroxaheptacyclo[44.4.0.0^4,9.0^12,17.0^21,26.0^29,34.0^37,42]pentaconta-1(46),4(9),5,7,12(17),13,15,19,21,23,25,29,31,33,37,39,41,43,47,49-icosaene-18,45-dione (3) and the benzo-fused 13-membered macrocyclic chalcone (19E)-2.11-dioxatetracyclo[19.4.0.0^4,9.0^12,17]pentacosa-1(25),4(9),5,7,12(17),13,15,19,21,23-decaen-18-one (5) using very common starting materials and reagents are described. The compounds are new and they have been characterized from their analytical and spectral data.

Probing deep into the interaction of a fluorescent chalcone derivative and bovine serum albumin (BSA): an experimental and computational study

In the present manuscript, a novel fluorescent chalcone derivative is synthesized and its photophysical properties are fully characterized. The designed fluorophore is applied as a probe to study protein-dye interactions with bovine serum albumin. Circular dichroism gave interesting results on the thermodynamics of the interaction. NMR spectroscopy, especially relaxation measurements, revealed the atoms in the chalcone derivative that interacts with the protein upon binding. Molecular docking calculations indicate that the most favourable binding sites are near the two tryptophan residues. Furthermore, ab initio and DFT calculations offer insights into the reactivity and physicochemical properties of this novel fluorophore.

Impact of water on the cis-trans photoisomerization of hydroxychalcones

The photochromism of a 2-hydroxychalcone has been studied in CH3CN and H2O/CH3OH (1/1, v/v), as well as in analogous deuterated solvents using steady-state (UV-vis absorption, (1)H and (13)C NMR) and time-resolved (ultrafast transient absorption and nanosecond flow flash photolysis) spectroscopies. Whereas the irradiation of trans-chalcone (Ct) under neutral pH conditions leads to the formation of the same final chromene derivative (B) in both media, two distinct photochemical mechanisms are proposed in agreement with thermodynamic and kinetic properties of the chemical reaction network at the ground state. Following light excitation, the first steps are identical in acetonitrile and aqueous solution: the Franck-Condon excited state rapidly populates the trans-chalcone singlet excited state (1)Ct* (LE), which evolves into a twisted state (1)P*. This excited state is directly responsible for the photochemistry in acetonitrile in the nanosecond time scale (16 ns) leading to the formation of cis-chalcone (Cc) through a simple isomerization process. The resulting cis-chalcone evolves into the chromene B through a tautomerization process in the ground state (τ = 10 ms). Unlike in acetonitrile, in H2O/CH3OH (1/1, v/v), the P* state becomes unstable and evolves into a new state attributed to the tautomer (1)Q*. This state directly evolves into B in one photochemical step through a consecutive ultrafast tautomerization process followed by electrocyclization. This last case represents a new hypothesis in the photochromism of 2-hydroxychalcone derivatives.

A fluorescence study of the solute–solvent interactions of aminochalcones in a room-temperature ionic liquid

Unlike most other electron donor–acceptor (EDA) molecules, aminochalcones exhibit unusual solvent polarity-dependent fluorescence behavior. The photophysical behavior of two aminochalcones, namely, 4-aminochalcone (AC) and 4-dimethylaminochalcone (DMAC), has been studied in a viscous room-temperature ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], by steady-state and time-resolved fluorescence techniques. The observation of a single emission band in viscous IL, which is similar to the one observed in less viscous polar conventional solvents, suggests no twisting is necessary for the formation of the charge-transfer state from which the emission of aminochalcones originates. The fluorescence decay profiles, solvation dynamics, and excitation-wavelength-dependent emission behavior of AC are found to be quite different from those of DMAC in the IL. The observed difference is attributed to specific H-bonding interaction between AC and [bmim][PF6].

Photophysical properties and electronic structure of bacteriochlorin-chalcones with extended near-infrared absorption

Synthetic bacteriochlorins enable systematic tailoring of substituents about the bacteriochlorin chromophore and thereby provide insights concerning the native bacteriochlorophylls of bacterial photosynthesis. Nine free-base bacteriochlorins (eight prepared previously and one prepared here) have been examined that bear diverse substituents at the 13- or 3,13-positions. The substituents include chalcone (3-phenylprop-2-en-1-onyl) derivatives with groups attached to the phenyl moiety, a "reverse chalcone" (3-phenyl-3-oxo-1-enyl), and extended chalcones (5-phenylpenta-2,4-dien-1-onyl, retinylidenonyl). The spectral and photophysical properties (τs, Φf, Φ(ic), Φ(isc), τT, k(f), k(ic), k(isc)) of the bacteriochlorins have been characterized. The bacteriochlorins absorb strongly in the 780-800 nm region and have fluorescence quantum yields (Φf) in the range 0.05-0.11 in toluene and dimethylsulfoxide. Light-induced electron promotions between orbitals with predominantly substituent or macrocycle character or both may give rise to some net macrocycle ↔ substituent charge-transfer character in the lowest and higher singlet excited states as indicated by density functional theory (DFT) and time-dependent DFT calculations. Such calculations indicated significant participation of molecular orbitals beyond those (HOMO - 1 to LUMO + 1) in the Gouterman four-orbital model. Taken together, the studies provide insight into the fundamental properties of bacteriochlorins and illustrate designs for tuning the spectral and photophysical features of these near-infrared-absorbing tetrapyrrole chromophores.

Interaction of alkali, alkaline earth and transition metal ions with a ketocyanine dye: a comparative electronic spectroscopic study

Interaction of a dye which is structurally similar to a ketocyanine dye with metal ions (alkali, alkaline earth and transition metal) has been studied by monitoring the electronic absorption, steady state and time resolved fluorescence parameters of the dye. The dye (S(0) state) forms a 1:1 complex with cations as indicated by the appearance of a new band at a longer wavelength. Equilibrium constant and other thermodynamic parameters for complexation have been determined. The interaction between the dye and the cation is mostly electrostatic in nature. Spectroscopic results have been supplemented by DFT calculation. For very low concentration of cations, where complexation is insignificant, the absorption band of the dye undergoes a slight blue shift. Enhancement of fluorescence intensity has been observed in the same concentration range. Both phenomena have been explained in terms of formation of a weak association complex where one/more cation replace equivalent solvent molecules in the cybotatic region around the dye. The binding constant of the weak association complex involving cation and the dye (S(1) state) has been determined and has been found to depend on the charge-to-size ratio of the cations. Measurement of fluorescence lifetime of the dye indicates that the association complex is slowly decaying relative to solvated dye. At higher concentration of metal ions, however, fluorescence of the dye is quenched by the metal ions. A red shift of fluorescence maximum has also been observed in this concentration range.

Spectral characteristics of 4-(p-N,N-dimethyl-aminophenylmethylene)-2-phenyl-5-oxazolone (DPO) in different media

The absorption and fluorescence characteristics of 4-(p-N,N-dimethyl-aminophenylmethylene)-2-phenyl-5-oxazolone (DPO) have been investigated in different solvents. DPO dye exhibits a large red shift in both absorption and emission spectra as solvent polarity increases, indicating a large change in the dipole moment of dye molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents. Crystalline solids of DPO gave strong red emission at 605nm due to the excitation of molecular aggregates. The absorption and fluorescence emission spectral properties of DPO have also been investigated in organized media of aqueous micellar and β-cyclodextrin (β-CD) solutions. The critical micelle concentrations (CMCs) of SDS and CTAB as well as the binding constants of DPO in micellar solution and β-cyclodextrin have been also determined.

Chemistry and applications of flavylium compounds: a handful of colours

Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host-guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).