Excitation-Dependent Multiple Fluorescence of a Substituted 2-(2'-Hydroxyphenyl)benzoxazole

Citric Acid-Based Intrinsic Band-Shifting Photoluminescent Materials

Citric acid, an important metabolite with abundant reactive groups, has been demonstrated as a promising starting material to synthesize diverse photoluminescent materials including small molecules, polymers, and carbon dots. The unique citrate chemistry enables the development of a series of citric acid-based molecules and nanomaterials with intriguing intrinsic band-shifting behavior, where the emission wavelength shifts as the excitation wavelength increases, ideal for chromatic imaging and many other applications. In this review, we discuss the concept of "intrinsic band-shifting photoluminescent materials", introduce the recent advances in citric acid-based intrinsic band-shifting materials, and discuss their potential applications such as chromatic imaging and multimodal sensing. It is our hope that the insightful and forward-thinking discussion in this review will spur the innovation and applications of the unique band-shifting photoluminescent materials.

Excitation Energy-Dependent, Excited-State Intramolecular Proton Transfer-Based Dual Emission in Poor Hydrogen-Bonding Solvents

2-(2'-Hydroxyphenyl)benzoxazole (HBO) substituted at the 5'-position with bipyridylvinylene phenylenevinylene (compound 2) produces both normal and, via an excited-state intramolecular proton transfer (ESIPT) reaction, tautomer emissions in solvents that preserve intramolecular hydrogen bonds. The abundance of the tautomer emission from compound 2 in a poor hydrogen-bonding solvent increases in response to the application of a higher excitation energy. Based on quantum chemical calculations, the excitation-dependent dual emission is consistent with a model in which the ESIPT reaction is more favored in the S₂ than in the S₁ electronically excited state.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (). Part II: Emission from Anions

This paper is the second part of a study on the effects of a substituted 5'-phenylenevinylene (PV) functionality on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO)─a dye that is known for excited-state intramolecular proton transfer. The topical compounds are referred to as PVHBOs, each of which is a structural fusion of HBO and a 4-hydroxy-4'-R-stilbene fluorophore that occurs at the hydroxyphenyl moiety. Therefore, the resulting fusion fluorophore manifests the properties of one component or the other, as governed by its interactions with the environment. In part I (the preceding paper), PVHBOs are divided into two groups depending on whether the R substituent is electron-donating/neutral (group I) or electron-withdrawing (group II). The difference in absorption and emission properties between groups I and II is explained based on observations from spectroscopic experiments (both steady-state and time-resolved) and quantum chemical calculations. In the current paper, the same set of tools is applied to characterize the photophysical properties of the conjugate bases─that is, the anions─of PVHBOs. The emission energy of the anion of any group I compound, where the R substituent is either electron-donating or neutral, is situated between those of the neutral enol and keto forms. The emission of the anion of any given group II compound, on the other hand, has a lower energy than both the enol and keto emissions. The frontier molecular orbitals (i.e., HOMO, LUMO, and LUMO + 1) of a PVHBO localized on either HBO or stilbenoid are impacted by the substituent R and the solvent/additive differently, which leads to the differences in the optical properties of group I and II PVHBOs in both neutral and anion forms.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Fine-tuning directionality of ESIPT behavior of the asymmetric two proton acceptor system via atomic electronegativity

The excited state intramolecular proton transfer (ESIPT) processes and photophysical features of 3-(benzo[d]oxazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BOHMB) and 3-(benzo[d]selenazole-2-yl)-2-hydroxy-5-methoxy benzaldehyde (BSeHMB) molecules were investigated in detail by using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The strengthened excited state hydrogen bonds (H-bond) of the title compounds are favorable to ESIPT process according to the analyses of structural parameter, infrared vibration frequency, electron density and reduced density gradient. The atomic substitution changes the intramolecular H-bond O₁-H₂…O₃ and O₁-H₂…N₄ and the fluorescence emission peaks of BOHMB-N and BSeHMB-N in normal and tautomer forms. The potential energy curves indicate that the ESIPT energy barriers of BOHMB-O, BTHMB-O and BSeHMB-O increase as the electron-withdrawing abilities of atoms (from O to S and Se) are gradually weakened. However, the ESIPT energy barriers of BOHMB-N and BTHMB-N follow the totally opposite order. For BOHMB and BSeHMB, ESIPT process prefers to occur in the direction from O-H group to the O atom.

Rhodium-catalyzed coupling-cyclization reaction of isocyanides and 2-azidophenyloxyacrylates: synthesis of N-(3-substituted benzo[d]oxazol-2(3H)-ylidene)amines and dihydrobenzo[d]oxazoles

A rhodium-catalyzed coupling cyclization of isocyanides with 2-azidophenyloxyacrylates has been developed. The reaction provides a new method for the synthesis of N-(3-substituted benzo[d]oxazol-2(3H)-ylidene)amines and dihydrobenzo[d]oxazoles.

Spectroscopic and mechanistic insights into solvent mediated excited-state proton transfer and aggregation-induced emission: introduction of methyl group onto 2-(o-hydroxyphenyl)benzoxazole

2-(2-Hydroxy-5-methylphenyl)benzoxazole(HBO-pCH3), a solvatochromic benzoxazole-based probe, exhibited a typical dual fluorescence phenomenon, high fluorescence quantum yield, red-shifted emission and large Stokes’ shift via the ESIPT in solvents.

Structural Features of a Family of Coumarin-Enamine Fluorescent Chemodosimeters for Ion Pairs

A family of coumarin-enamine chemodosimeters is evaluated for their potential use as fluorescent molecular probes for multiple analytes [cadmium(II), cobalt(II), copper(II), iron(II), nickel(II), lead(II), and zinc(II)], as their chloride and acetate salts. These fluorophores displayed excellent optical spectroscopic modulation when exposed to ion pairs with different Lewis acidic and basic properties in dimethyl sulfoxide (DMSO). The chemodosimeters were designed to undergo excited-state intramolecular proton transfer (ESIPT), which leads to significant Stokes shifts (ca. 225 nm) and lower-energy fluorescence emission (ca. 575 nm). A more basic anion, e.g., acetate, inhibited the ESIPT mechanism by deprotonation of the enol, producing a binding pocket (N^O^- chelate) that can coordinate to an appropriate metal ion. Coordination of the metal ions enhances the fluorescent intensity via the chelation-enhanced fluorescence emission mechanism. Subjecting the spectroscopic data to linear discriminant analysis provided insights into the source of these systems' markedly different behavior toward ion pairs, despite the subtle structural differences in the organic framework. These compounds are examples of versatile, low-molecular-weight, dual-channel fluorescent sensors for ion-pair recognition. This study paves the way for using these probes as practical components of a sensing array for different metal ions and their respective anions.

Trace-Level Humidity Sensing from Commercial Organic Solvents and Food Products by an AIE/ESIPT-Triggered Piezochromic Luminogen and ppb-Level "OFF-ON-OFF" Sensing of Cu2+: A Combined Experimental and Theoretical Outcome

Selective and sensitive moisture sensors have attracted immense attention due to their ability to monitor the humidity content in industrial solvents, food products, etc., for regulating industrial safety management. Herein, a hydroxy naphthaldehyde-based piezochromic luminogen, namely, 1-{[(2-hydroxyphenyl)imino]methyl}naphthalen-2-ol (NAP-1), has been synthesized and its photophysical and molecular sensing properties have been investigated by means of various spectroscopic tools. Owing to the synergistic effect of both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) along with the restriction of C=N isomerization, the probe shows bright yellowish-green-colored keto emission with high quantum yield after the interaction with a trace amount of water. This makes NAP-1 a potential sensor for monitoring water content in the industrial solvents with very low detection limits of 0.033, 0.032, 0.034, and 0.033% (v/v) from tetrahydrofuran (THF), acetone, dimethyl sulfoxide (DMSO), and methanol, respectively. The probe could be used in the food industry to detect trace moisture in the raw food samples. The reversible switching behavior of NAP-1 makes it suitable for designing an INHIBIT logic gate with an additional application in inkless writing. In addition, an Internet of Things-(IoT) based prototype device has been proposed for on-site monitoring of the moisture content by a smartphone via Bluetooth or Wi-Fi. The aggregated probe also has the ability to recognize Cu²⁺ from a purely aqueous medium via the chelation-enhanced quenching (CHEQ) mechanism, leading to ∼84% fluorescence quenching with a Stern-Volmer quenching constant of 1.46 × 10⁴ M^-1 and with an appreciably low detection threshold of 57.2 ppb, far below than recommended by the World Health Organization (WHO) and the United States Environmental Protection Agency (U.S. EPA). The spectroscopic and theoretical calculations (density functional theory (DFT), time-dependent DFT (TD-DFT), and natural bond orbital (NBO) analysis) further empower the understanding of the mechanistic course of the interaction of the host-guest recognition event.

Photodriven Charge Accumulation and Carrier Dynamics in a Water-Soluble Carbon Nitride Photocatalyst

Charge accumulation in photoactive molecules and materials holds great promise in solar energy conversion as it allows for decoupling solar-driven charging from (dark) redox reactions. In this contribution, light-driven charge accumulation was investigated for a recently reported novel water-soluble carbon nitride [K,Na-poly(heptazine imide); K,Na-PHI] photocatalyst, which exhibits excellent activity and stability in highly selective photocatalytic oxidation of alcohols and concurrent reduction of dioxygen to H2 O2 under quasi-homogeneous conditions. An excellent charge storage ability of the K,Na-PHI material was demonstrated, showing an optimal density of accumulated electrons (32.2 μmol of electrons per gram) in the presence of 10 vol % MeOH as a sacrificial electron donor. The long-lived electrons accumulated under anaerobic conditions as K,Na-PHI.- radical ions were utilized in interfacial electron transfer to O2 or methyl viologen in a subsequent dark reaction. Ultrafast time-resolved spectroscopy was employed to reveal the kinetics of charge-carrier recombination and methanol oxidation. Geminate recombination of electrons and holes within approximately 100 ps was followed by trap-assisted recombination. The presence of methanol as a sacrificial electron donor accelerated the decay of the transient absorption signal when a static sample was used. This behavior was ascribed to the faster charge recombination in the presence of the radical anions generated after hole extraction. The work suggests that photodriven electron storage in the water-soluble carbon nitride is enabled by localized trap states, and highlights the importance of the effective electron donor for creating long-lived photo-generated carbon nitride radicals.

Hydroxyaromatic Fluorophores

Many fluorophores that are widely used in analytical biochemistry and in biological microscopy contain a hydroxyaromatic component. One could also find fascinating chemistries of hydroxyaromatic dyes, especially those capable of excited state proton transfer (ESPT) to produce dual emission, in the literature of materials and physical chemistry. The ESPT-capable compounds have attracted interest based on their fundamental intellectual values in molecular photophysics and their potential utilities as light emitters in organic light-emitting diodes (LEDs) or fluorescent sensors. The hydroxyaromatic dyes could undergo either intra- or intermolecular proton transfer in either electronic ground or excited states. Although having long been applied for various purposes, some of their absorption and emission properties have not always been clearly described because of the insufficient attention given to proton transfer equilibria in either the ground or excited state and the challenges in computationally modeling the true emitters of these dyes under any given conditions. In this article, an attempt is made to summarize the spectroscopic properties of a few common hydroxyaromatic dyes that have been studied for both fundamental and practical purposes, with the help from quantum chemical calculations of the absorption and emission energies of these dyes in neutral and anion forms. The goal of this article is to provide readers some clarity in the optical properties of these compounds and the tools to understand and to predict the photon-initiated behaviors of hydroxyaromatic fluorophores.

A 2-(benzo[d]thiazol-2-yl)phenol based on conjugated polymer: Highly selective colorimetric fluorescent chemosensor for F-depending on Si–O bond cleavage reaction

Design and synthesis of highly selective fluorine ion probes become particularly important owing to the specific role of fluorine ion in chemical and biomedical progresses. As a new-type of fluorescent material, conjugated polymers with unique photometric properties have been widely researched by scientists in the field of the fluorescence sensors. In this study, the polymer PBTPV-OSi containing benzothiazole moiety is synthesized via palladium-catalyzed Heck coupling reaction. This polymer not only exhibits good solubility in organic solvents but also shows high selectivity for fluorine ion detection in comparison to other anions. Upon addition of F− to PBTPV-OSi solution, Si–O cleavage of PBTPV-OSi leads to the fluorescence quenching of the polymer in tetrahydrofuran dramatically, and the detection limit is 8 × 10−6 mol/L. Moreover, besides detecting fluorine ion from organic phase, the probe can also effectively detect potassium fluoride from inorganic phase. More importantly, a naked-eye detectable chromogenic and fluorogenic dual response to fluorine ion (F−) can be visibly noted and the detection process of fluorine ion is relatively fast.

A theoretical elucidation of the mechanism of tuneable fluorescence in a full-colour emissive ESIPT dye

We reinvestigate with ab initio tools the origin of the diverse colours in a complex multi-ESIPT dye, and we propose a new assignment for the blue fluorescence.