Excited-state Intramolecular Proton Transfer Molecules Research Articles

AI-Powered Mining of Highly Customized and Superior ESIPT-Based Fluorescent Probes.

Excited-state intramolecular proton transfer (ESIPT) has attracted great attention in fluorescent sensors and luminescent materials due to its unique photobiological and photochemical features. However, the current structures are far from meeting the specific demands for ESIPT molecules in different scenarios; the try-and-error development method is labor-intensive and costly. Therefore, it is imperative to devise novel approaches for the exploration of promising ESIPT fluorophores. This research proposes an artificial intelligence approach aiming at exploring ESIPT molecules efficiently. The first high-quality ESIPT dataset and a multi-level prediction system are constructed that realized accurate identification of ESIPT molecules from a large number of compounds under a stepwise distinguishing from conventional molecules to fluorescent molecules and then to ESIPT molecules. Furthermore, key structural features that contributed to ESIPT are revealed by using the SHapley Additive exPlanations (SHAP) method. Then three strategies are proposed to ensure the ESIPT process while keeping good safety, pharmacokinetic properties, and novel structures. With these strategies, >700 previously unreported ESIPT molecules are screened from a large pool of 570000 compounds. The ESIPT process and biosafety of optimal molecules are successfully validated by quantitative calculation and experiment. This novel approach is expected to bring a new paradigm for exploring ideal ESIPT molecules.

AIE-ESIPT Photoluminescent Probe Based on 3-(3-Hydroxypyridin-2-yl)isoquinolin-4-ol for the Detection of Intracellular Hydrogen Peroxide.

Excited-state intramolecular proton transfer (ESIPT) molecules, which feature large Stokes shifts to avoid self-absorption, play an essential role in photoluminescent bioimaging probes. Herein, we report the development of an ESIPT molecule 3-(3-hydroxypyridin-2-yl)isoquinolin-4-ol (PiQ).PiQnot only undergoes a distinct ESIPT process unlike the symmetrical 2,2'-bipyridyl-3,3'-diol but also exhibits aggregation-induced emission (AIE)characteristics. PiQself-assembles into aggregates with an average size of 241.0 ± 51.9 nm in aqueous solutions, leading to significantly enhanced photoluminescence. On the basis of the ESIPT and AIE characteristics ofPiQ, the latter is functionalized with a hydrogen peroxide-responsive 4-pinacoratoborylbenzyl group (B) and a carboxylesterase-responsive acetyl group (A) to produce a photoluminescent probeB-PiQ-A. The potential ofPiQfor applications in bioimaging and chemical sensing is underscored by its efficient detection of both endogenous and exogenous hydrogen peroxide in living cells.

Amphiphilic Molecules Exhibiting Zwitterionic Excited-State Intramolecular Proton Transfer and Near-Infrared Emission for the Detection of Amyloid β Aggregates in Alzheimer's Disease.

Chromophores with zwitterionic excited-state intramolecular proton transfer (ESIPT) have been shown to have larger Stock shifts and red-shifted emission wavelengths compared to the conventional π-delocalized ESIPT molecules. However, there is still a dearth of design strategies to expand the current library of zwitterionic ESIPT compounds. Herein, a novel zwitterionic excited-state intramolecular proton transfer system is reported, enabled by addition of 1,4,7-triazacyclononane (TACN) fragments on a dicyanomethylene-4H-pyran (DCM) scaffold. The solvent-dependent steady-state photophysical studies, pKa measurements, and computational analysis strongly support that the ESIPT process is more efficient with two TACN groups attached to the DCM scaffold and not affected by polar protic solvents. Impressively, compound DCM-OH-2-DT exhibits a near-infrared (NIR) emission at 740 nm along with an uncommonly large Stokes shift. Moreover, DCM-OH-2-DT shows high affinity towards soluble amyloid β (Aβ) oligomers in vitro and in 5xFAD mouse brain sections, and we have successfully applied DCM-OH-2-DT for the in vivo imaging of Aβ aggregates and demonstrated its potential use as an early diagnostic agent for AD. Overall, this study can provide a general molecular design strategy for developing new zwitterionic ESIPT compounds with NIR emission in vivo imaging applications.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence.

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

One/Two‐Photon‐Excited ESIPT‐Attributed Coordination Polymers with Wide Temperature Range and Color‐Tunable Long Persistent Luminescence

AbstractThe multiple metastable excited states provided by excited‐state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature‐dependent and color‐tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D‐π‐A structure to obtain both one/two‐photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM‐106) from ESIPT ligand to achieve the above goals. By comparing LIFM‐106 with the counterparts, we established a temperature‐regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM‐106 shows outstanding one/two‐photon excited LPL performance with wide temperature range (100–380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti‐counterfeiting systems.

Regulating the ESIPT processes and photophysical properties of indole derivates by introducing the appropriate functional groups

In this work, the substitution effect of different electron-donating and electron-withdrawing groups on the electron spectra and excited state intramolecular proton transfer (ESIPT) properties of the chromophore 2-(2-hydroxy-5-methylphenyl)benzo[d]oxazole-5-sulfonic acid (BOM-1) has been studied via the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. All the compounds exhibit the dual-fluorescence emission, and the fluorescence spectra of the derivatives show redshift emission compared with that of the original molecule, resulting in the larger Stokes shift as expected. The results were cross-verified by the molecular geometry, frontier molecular orbitals, infrared spectroscopy and other auxiliary calculations. These results show that the electron-withdrawing group greatly promote the occurrence of the forward ESIPT process and increase the Stokes shift. What is more special is that the Stokes shift and the energy barrier of the forward ESIPT are significantly enlarged by introducing both electron-withdrawing and electron-donating groups. The energy barrier, absorption and fluorescence spectra of luminescent molecules can be adjusted effectively by introducing appropriate functional groups. In addition, the emission spectra from the ultraviolet region (359 nm) to the orange region (601 nm) are obtained. This work would contribute to the understanding of the mechanism of ESIPT reactions and improves the luminescence properties of ESIPT molecules.

ESIPT mechanism of triple emission with hydroxy-oxadiazole compound in DMSO: A theoretical reconsideration

The compound in solvents with triple fluorescence feature of excited state intramolecular proton transfer (ESIPT) has a broad prospect in fluorescent probes, dye sensors and molecular synthesis of photosensitive dyes. An ESIPT molecule hydroxy-bis-2,5-disubstituted-1,3,4-oxadiazoles (compound 1a) emits two fluorescence peaks in dichloromethane (DCM) and three fluorescence peaks in dimethyl sulfoxide (DMSO). [Dyes and Pigments 197 (2022) 109927]. Two longer peaks were attributed to enol and keto emission in both solvents and the shortest third peak in DMSO was just attributed simply. However, there is a significant difference in proton affinity between DCM and DMSO solvents which has influence on the position of emission peaks. Therefore, the correctness of this conclusion needs to be further verified. In this research, density functional theory and time-dependent density functional theory method are used to explore ESIPT process. Optimized structures indicate ESIPT occurs through molecular bridge assisted by DMSO. The calculated fluorescence spectra demonstrate two peaks indeed originated from enol and keto in DCM, while interestingly three peaks are originated from enol, keto and intermediate in DMSO. Infrared spectrum, electrostatic potential and potential energy curves further prove existence of three structures. We reveal the mechanisms that compound 1a molecule occurs ESIPT in DCM solvent and undergoes an ESIPT through assisted by DMSO molecular bridge. Additionally, three fluorescence peaks in DMSO are reattributed. Our work is expected to provide an insight for understanding intra- and intermolecular interactions and synthesis of efficient organic lighting-emitting molecule.

Excited-state intramolecular proton transfer emitter for efficient violet-blue organic light-emitting diodes with hybridized local/charge transfer channel

Excited-state intramolecular proton transfer (ESIPT) materials have attracted much interest, attributed to their metastable excited states which provide suitable platforms for efficient intersystem crossing (ISC) and reverse intersystem crossing (rISC). However, the studies on the utilization of ESIPT to achieve effective hybridized local/charge transfer (HLCT) emitters are rarely reported, due to their ubiquitous issue of low photoluminescence quantum yields (ΦPLs). In this research, a novel ESIPT molecule with HLCT characteristic, 4′-(diphenylamino)-4-(1,4,5- triphenyl-1H-imidazol-2-yl)-[1,1′-biphenyl]-3-ol (HIT-TPA), is designed and synthesized. The hydrogen atom in this molecule can transfer on the way of an intramolecular hydrogen bond from a proton-donating part (–OH/–NH–) to the accepting one (CO/CN–). The HLCT and /or color-tuned fluorescence are observed by the irradiation of specific wavelength, which followed the guidance of ESIPT. It is consistent with the results of transient absorption and time-resolved photoluminescent spectroscopy. By the synergistic interaction of ESIPT and HLCT, the film based on the target molecule presents a high ΦPL value of ∼75.5%. After employing HIT-TPA as the emitter for doped/undoped organic light-emitting diodes (OLEDs), a maximum external quantum efficiency of ∼10.08% is achieved, which proves the efficient triplet harvesting in the ESIPT-based HLCT material. Noticeably, the device exhibits a violet-blue emission with the CIE(x,y) of (0.152, 0.051). It is among the highest reported EQE values on the ESIPT-HLCT materials for violet-blue devices. These results not only open a novel pathway to design effective ESIPT attributed HLCT materials, but also provide a strategy to efficiently utilize ESIPT emitters in achieving high-performance OLEDs.

Mechanism of fluorescence enhancement of HClO detected by excited-state intramolecular proton transfer based HBT-OMe molecule

The molecule with excited-state intramolecular proton transfer (ESIPT) has wide applications in fluorescent probe, biology imaging, light-emitting materials, etc. Biologically active oxygen hypochlorite (HClO) exists widely in the biological and chemical environment, which can pose a great threat to human health. Design of HClO-sensitive molecules in solvents is very important. Recently, Wu et al. [Wu L L, Yang Q Y, Liu L Y, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1039/C8CC03717E">2018 <i>Chem. Commun.</i> <b>54</b> 8522</ext-link>] designed an ESIPT-based HBT-OMe probe molecule, which can detect HClO due to its methoxy-hydroxy-benzothiazole. They found that the fluorescence intensity of the system gradually increases with HClO increasing. However, the microscopic mechanism of this highly efficient fluorescent probe is not well understood. Therefore, in this work, we theoretically investigate the ESIPT mechanism of the HBT-Ome and its product molecule by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model (PCM) with the integral equation formalism variant (IEFPCM) and Becke’s three-parameter hybrid exchange function with the Lee-Yang-Parr gradient-corrected functional (B3LYP) as well as the TZVP basis, the optimized structures are obtained. The structures show that the HBT-Ome product molecules tend to undergo proton transfer in the excited state but HBT-OMe molecules cannot undergo the proton transfer process. The analysis of frontier molecular orbitals not only explains the reason why the fluorescence of the HBT-Ome product is enhanced, but also demonstrates that the HBT-Ome fluorescence intensity is diminished owing to twisted intramolecular charge transfer in the excited state. It is twisted intramolecular charge transfer that leads smaller charge density to be overlapped and the fluorescence intensity of HBT-OMe molecule to be further weakened. Infrared vibrational spectrum shows the enhancement of intramolecular hydrogen bond of O—H, which indicates the tendency of proton transfer. The molecular covalent interaction analysis shows that the intramolecular interactions of HBT-OMe remain largely unchanged clearly. The intramolecular O—H bonding interaction is weakened, and the N—H bonding interaction is increased for HBT-OMe product molecule. The enhancement of intramolecular hydrogen bond of N—H further illustrates the trend of proton transfer. The calculated potential energy curve provides direct evidence for the occurrence of ESIPT in the HBT-Ome product molecule. Our work is of great significance in designing and synthesizing the HClO fluorescent probes based on ESIPT molecules.

Exploring Solvent Effects on the Proton Transfer Processes of Selected Benzoxazole Derivatives by Femtosecond Time-Resolved Fluorescence and Transient Absorption Spectroscopies.

Excited-state intramolecular proton transfer (ESIPT) is of great importance due to the large Stokes shift emission that can be observed in some ESIPT molecules. Although steady-state spectroscopies have been employed to study the properties of some ESIPT molecules, their excited-state dynamics have not been examined directly with time-resolved spectroscopy methods yet for a number of systems. Here, an in-depth investigation of the solvent effects on the excited-state dynamics of two prototypical ESIPT molecules, 2-(2'-hydroxyphenyl)-benzoxazole (HBO) and 2-(2'-hydroxynaphthalenyl)-benzoxazole (NAP), have been accomplished by using femtosecond time-resolved fluorescence and transient absorption spectroscopies. Solvent effects affect the excited-state dynamics of HBO more significantly than that of NAP. Particularly in the presence of water, the photodynamics pathways of HBO are changed, while only small changes can be found in NAP. An ultrafast ESIPT process that occurs within our instrumental response is observed for HBO, and this is followed by an isomerization process in ACN solution. However, in aqueous solution, the obtained syn-keto* after ESIPT can be solvated by water in about 3.0 ps, and the isomerization process is totally inhibited for HBO. The mechanism of NAP is different from HBO and is determined to be a two-step excited-state proton transfer process. Upon photoexcitation, NAP is deprotonated first in the excited state to generate the anion*, which can transfer to the syn-keto* form followed by an isomerization process.

Third-order nonlinear optical manifestations in an intramolecular proton transfer fluorophore due to Tamm-plasmon based broadband optical absorbers

The imidazole ring has proven to be a very promising candidate for realizing molecular systems that facilitate efficient generation of broadband optical radiation using an excited-state intramolecular proton-transfer (ESIPT) mechanism. To obtain a targeted RGB-color composition, the conventional route is to create optimum mixtures of dyads and triads of non-interacting ESIPT molecules that cover the entire visible spectral band as well as restrict any energy transfer between wide bandgap donors and narrow bandgap acceptors. However, there is a need for alternate routes that can facilitate dynamic control over color composition during operation. The nonlinear optical (NLO) response of ESIPT fluorophores and configurations derived out of them could be explored from this perspective. In the present work, we design a T i O 2 / S i O 2 based distributed-Bragg-reflector (DBR) configuration with a terminating layer consisting of a film of an ESIPT fluorophore, namely, 2-(4,5-diphenyl-1-(4-(trifluoromethyl)phenyl)-1H-imidazol-2yl)phenol (DTIP). In the presence of thin Ag-film, the DTIP-DBR multilayer geometry supports a Tamm-plasmon-polariton (TPP) mode within the photonic bandgap of DBR, which exhibits an unusual spectral bandwidth greater than 50 nm. The open-aperture and closed-aperture Z-scan measurements reveal the strong TPP based broadband absorption that leads to reversal of third-order NLO characteristics, i.e., the multilayer geometry exhibits saturation absorption behavior at wavelengths where the bare DTIP fluorophore (in solution and film) shows a two-photon absorption signature.

Excited state intramolecular proton transfer of 2-phenyl,3-hydroxybenzo[g]quinolones in solution and in G4 supramolecular hydrogels

The 2-Phenyl,3-Hydroxybenzo[g]quinolone (2P3HBQ) fluorophore and its derivative, 2-(4′-hydroxyphenyl)-3-hydroxybenzo[g]quinolone (2HP3HBQ),exhibit well-separated dual emission peaks attributable to Excited State Intramolecular Proton Transfer (ESIPT), thus offering promise as useful visible-absorbing ratiometric fluorescence probes. Time-resolved emission measurements in polar and H-bonding solvents reveal very long time-constants extending up to ∼2 ns, ostensibly associated with the ESIPT process, although the latter has traditionally been known to be ultra-fast for ordinary ESIPT molecules. These unusually long time-constants suggest that ESIPT of 2P3HBQ and 2HP3HBQ involves a large activation barrier, resulting from intermolecular H-bonding with the solvent. This is supported by temperature-dependent fluorescence studies, which show that the ESIPT rates in a polar solvent differ significantly over a range of temperatures, implying that solvent‧‧‧fluorophore intermolecular H-bonding is prevalent at lower temperatures but get disrupted at higher temperatures. Our studies were extended up to 2P3HBQ molecules incorporated inside G4 Supramolecular hydrogels via host–guest type non-covalent binding. ESIPT observed in the gels differ from that in solution state on several counts, reflecting the effect of confinement on the photophysics of the gel-bound fluorophores.

Investigation of a Series of 2-(2'-Hydroxyaryl)benzazole Derivatives: Photophysical Properties, Excited-State Intramolecular Proton-Transfer Reactions, and Observation of Long-Lived Triplet Excited States.

Excited state intramolecular proton transfer (ESIPT) has drawn much attention for its important applications in a variety of areas. Here, the steady-state and time-resolved absorption spectroscopic experiments as well as DFT/TD-DFT calculations are employed to study the photophysical properties and photochemical reaction mechanisms of 2-(2'-hydroxyphenyl) benzoxazole (HBO) and selected derivatives (compounds 1-3). Because of their larger π-conjugation framework, compounds 1-3 display red-shifted absorbance but blue-shifted fluorescence compared with HBO. A fast ESIPT process is observed directly for HBO while compound 3 has an enol/keto equilibrium type of ESIPT that exhibits dual emission. Interestingly, only the emission of the enol form is observed for compounds 1 and 2 which suggests that the ESIPT process is strongly inhibited. These results indicate the decoration with electron-withdrawing groups such as thiadiazol and pyrazine on the hydroxyphenyl ring (compounds 1 and 2) apparently suppresses the proton-transfer processes in their excited states. Whereas the ESIPT process is rarely increased for compound 3 that modified with the phenanthrol ring, because the effective conjugation is reduced for compound 3 compared with HBO. The work here provides fundamental insights that may be useful for designing novel ESIPT molecules in the future.

Solvent Effect on Excited-State Intramolecular Proton-Coupled Charge Transfer Reaction in Two Seven-Membered Ring Pyrrole-Indole Hydrogen Bond Systems

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g-PPDBI and e-PPDBI, by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.

Correlation between Kinetics and Thermodynamics for Excited-State Intramolecular Proton Transfer Reactions.

Finding the relation between thermodynamics and kinetics for a reaction is of fundamental importance. Here, the thermodynamics and kinetics correlation of excited-state intramolecular proton transfer (ESIPT) was investigated by the TD-DFT calculation under the CAM-B3LYP/6-311+G** level. We choose the family 2-(2'-aminophyenyl)benzothiazole and its amino derivatives as paradigms, which all possess the NH-type intramolecular hydrogen bond (H-bond), and investigate the corresponding ESIPT reaction. The H-bond strength can be systematically tuned, so both activation energy ΔG‡ and free energy difference between proton transfer tautomer (T*, product) and normal species (N*, reactant) ΔGT*-N* can be varied. To minimize the environmental interference such as solvent external H-bond and polarity perturbation, a nonpolar solvent such as cyclohexane is chosen as a bath with a polarizable continuum solvation model for the calculation. As a result, the comprehensive computational approach reveals a linear relationship between ΔGT*-N* and ΔG‡, which can be expressed as ΔG‡ = ΔG0 + αΔGT*-N*. The fundamental insight is reminiscent of the Bell-Evans-Polanyi (BEP) principle where α represents the character of the position of the transition state along the proton motion coordinate. In other words, the more exergonic the ESIPT reaction is, the faster the proton transfer rate can be observed. To verify that such a correlation is not a sporadic event, another ESIPT family with an -OH proton, 1-hydroxy-11H-benzo[b]fluoren-11-one and its derivatives, was also investigated and proved to follow the BEP principle as well. Unlike the quantum mechanics description of proton transfer where either proton tunneling is dominant or solute/solvent is coupled in ESIPT, this work demonstrates that reaction kinetics and thermodynamics are strongly correlated within the same class of ESIPT molecules with an intrinsic barrier free from solvent perturbation, being faster with the more exergonic reaction.

A pH-sensitive ESIPT molecule with aggregation-induced emission and tunable solid-state fluorescence multicolor for anti-counterfeiting and food freshness detection

Achieving stimulus-responsive multicolor solid-state photoluminescence based on a single fluorescence compound has potential applications in anti-counterfeiting labels and visual detection. However, few such organic chromophores have these features and been used in this fields. In this study, multiple pH-sensitive sites were incorporated into a single molecule of succinimide substituted 2-(2′-hydroxyphenyl)Benzothiazole derivative (BTSA) that was Aggregation induced emission (AIE) and Excited state intramolecular proton transfer (ESIPT) active, exhibiting multilevel and highly secured anti-counterfeiting abilities. Under ammonia atmosphere, BTSA exhibits reversible switching between the green and blue fluorescence. By rationally manipulating two stimuli (acid and base), BTSA labels incredibly shows four different emission colors. The multicolor fluorescence changes are irreversible and evidently depend on the order of acid-base added. Together with smartphone that can quickly and accurately identify the multiple colors, BTSA affords a new covert fluorescent anti-counterfeiting technique with high security through the combination of reversible and irreversible fluorescence multicolor changes, as well as order and stimuli-dependent luminescent patterns. In addition, BTSA labels are successfully applied for real-time and visual monitoring the food freshness of pork and shrimp. The molecular design rationale presented here provide a new design strategy for the purposes of multicolor anti-counterfeiting and food freshness detection.

A butterfly-shaped ESIPT molecule with solid-state fluorescence for the detection of latent fingerprints and exogenous and endogenous ONOO− by caging of the phenol donor

The latent fingerprints (LFPs) at the crime scene are unique and stable, which are considered as an important clue in criminal justice and forensic identification. Herein, a butterfly-shaped molecule DPTS with solid fluorescence plus excited-state intramolecular proton transfer (ESIPT) properties was used to develop for enhancing the visualization of the LFPs. Considering the solid fluorescence of DPTS, the color and efficiency of DPTS with a large Stokes shift (216 nm) can be tuned by changing the morphology of its aggregates, and gradually red-shifted (green-yellow-red) with increasing water content. Furthermore, its effectiveness for the detection of LFPs was demonstrated on various different substrates including paper box, tinfoil and weighting paper. The emissive fingerprint of DPTS obtained gave good fluorescence images with high contrast and resolution such as the core, delta, bifurcation, ridge termination, independent ridge and pores. Caging of the phenol donor of DPTS with a sensitive biomarker group provided DPTS-ONOO-, which had high sensitive with detection limit of 5 nM and the quantification limit of 21 nM toward ONOO−. Modularly derived DPTS-ONOO- was synthesized and demonstrated specific fluorescence imaging of exogenous and endogenous peroxynitrite (ONOO−) in living macrophage cells.

Imidazole-based solid-state fluorophores with combined ESIPT and AIE features as self-absorption-free non-doped emitters for electroluminescent devices

Excited-state intramolecular proton transfer (ESIPT) fluorophores with an eye-catching aspect of a large Stokes shift in optical properties have been exceptionally considered as prime candidates for numerous applications. Nevertheless, as non-doped emitters for electroluminescent devices, the device performance is still far behind the traditional fluorescence emitters. Herein, two imidazole-based ESIPT-aggregate induced emission (AIE) fluorophores (HITPE and HPITPE) are designed and synthesized by covalently linked the ESIPT cores of 2-(2-hydroxyphenyl)-1,4,5-triphenylimidazole (HI) and 2-(2-hydroxylphenyl)-1-phenylphenanthroimidazole (HPI) with AIE-active luminogen of tetraphenylethene (TPE) at 5-position of the 2-hydroxyphenyl unit. The ESIPT, AIE, and photophysical properties are theoretically and experimentally studied. Both molecules display ESIPT and AIE characters with intense sky-blue/green-blue color emissions from a pure keto form in the solid-state and photoluminescence quantum yields of 57–64%. They possess high thermal and electrochemical stabilities. They are successfully fabricated as non-doped emitters in organic light-emitting diodes (OLED), and all devices exhibit strong keto-form emissions with low turn-on voltages (3.0–3.2 V). Especially, HPITPE-based OLED achieves a high luminance of 10680 cd m−2, LE value of 3.67 cd A−1, and external quantum efficiency (EQE) of 3.26% with a slight efficiency roll-off. Importantly, this represents an advance in the development of ESIPT molecules as a non-doped emitter for fluorescent OLEDs.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis.

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH2) and electron acceptor (-NO2) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH2 and -NO2 groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Enhanced solid-state photoluminescence and fluorescence spectral behaviors for an ESIPT molecule: An experimental and theoretical investigation

3-Dimethylamino-1-(2-hydroxy-phenyl)-propenone (DHP) molecule with excited-state intramolecular proton transfer (ESIPT), was good candidate as active organic laser dyes. In this work, the enhanced solid-state photoluminescence and the structure-property relationship for photophysical properties are investigated by time-dependent density functional theory (TD-DFT) method and the experimental spectra. The calculated potential energy surface (PES) curve of the ground-state S0 indicates that the planar DHP-cis-enol (α isomer) conformer with innate intramolecular H-bond is the global energy minimum. The PES of the lowest excited-state S1 indicate that the photoexcitated DHP-cis-enol (α* isomer) and DHP-cis-keto (β isomer) conformers are achieved as well, corresponding to dual-peak fluorescence emission in solution. Especially, in addition to the planar DHP-cis-keto isomer of DHP deriving from ESIPT, a barrierless twist process of phenyl hydroxide group brings out another stable excited-state isomer (γ isomer), which dramatically influences the photophysical properties. Furthermore, the low-frequency modes of phenyl hydroxide group rotation with large Huang-Rhys factors for the twisted isomer result in a noteworthy conical intersection of the S0 and S1 energy curves, which dissipate the excited-state energy nonradiatively for DHP in solution. However, in solid-state, the fluorescence spectrum of DHP features only ESIPT emission band which demonstrates the β isomer mainly exist in S1 state to promote the radiative decay processes with suppressed population of the α* and γ isomers, which can explain the phenomenon of the enhanced solid-state photoluminescence.