Modulation Of Intramolecular Charge Transfer Research Articles

Tuning Intramolecular Charge Transfer in Antimony(V) Porphyrin through Axial Fluorination.

Modulation of intramolecular charge transfer (ICT) has been tested in two antimony(V) porphyrins, SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, where the meso-positions are occupied by 3,5-dimethoxyphenyl (DMP), and the axial positions are linked with either methoxy (OMe) or trifluoroethoxy (OTFE) units, respectively. The presence of the Sb(+5) ion makes the porphyrin center electron poor. Under this situation, placing electron-rich units in the meso-position creates a condition for push-pull type ICT in the SbT(DMP)P(OMe)2·PF6. Remarkably, it is shown that the ICT character can be further enhanced in SbT(DMP)P(OTFE)2·PF6 with the help of electron-withdrawing TFE units in the axial position, which makes the porphyrin center even more electron scarce. The steady-state and transient studies as well as solvatochromism studies establish the ICT in SbT(DMP)P(OMe)2·PF6 and SbT(DMP)P(OTFE)2·PF6, and the strength of the ICT can be modulated by exploiting the structural properties of antimony(V) porphyrin. The existence of ICT is further supported by density functional theory calculations. The transient studies show that upon excitation of these porphyrin, their charge-transfer states convert to a full charger-separated states with appreciable lifetimes.

Modulation of intramolecular charge transfer in D-A functionalized pyrrolopyrazine and their application in luminescent solar concentrators

In this study, we fabricated LSCs using pyrrolopyrazine-based organic fluorophore (PP2) with an intramolecular charge transfer (ICT) function and aggregation-induced emission. PP2 incorporates unique features, including an intramolecular charge transfer (ICT) function and aggregation-induced emission. The pyrrolopyrazine core of PP2 is decorated with an electron-rich dimethylamine group and two electron-poor cyano groups, enhancing its potential for light harvesting in LSC applications. In the polymethylmethacrylate (PMMA) matrix, PP2 showed strong absorption near ultraviolet (395 nm–406 nm), emitted green-colored visible light (474 nm–478 nm), and exhibited a photoluminescence quantum efficiency (PLQY) of 67%. The external photon efficiency (ηext) and internal photon efficiency (ηint) of the fabricated LSC under AM 1.5G illumination (1 sun) were 3.36% and 39.1%, respectively. Subsequently, we conducted an analytical simulation to estimate the ηext and the ηint as the dimensions of the LSC were scaled up. In addition, the photovoltaic performance measurement under 1 sun showed a power conversion efficiency (ηPCE) of 1.074% with a scattering background and 0.633% without a scattering background. Thus, this research introduces PP2 as a novel fluorophore with promising prospects for advancing the field of LSC technology.

Exploring binding chemistry of alkali/alkaline earth cations in solution through modulation of intramolecular charge-transfer in an excited ambidentate organic fluorophore.

Studying the metal-ligand monoligation of alkali/alkaline earth metals (AMs) in solution represents a significant challenge due to the low stabilization of their complexes and the absence of an effective strategy to identify this type of weak binding. Herein, we show that the modulation of the intramolecular charge-transfer (ICT) in an excited ambidentate organic fluorophore is a convenient strategy to characterize the binding chemistry of AM cations in solution through simple steady-state fluorescence and fluorescence lifetime measurements. The key points of the fluorophore as a metal-binding probe were the location of diverse coordination functionalities with different binding abilities (ionic-, pseudo-covalent- and non-covalent-probes) along the donor-acceptor (D-A) chain and the occurrence of an intramolecular charge-transfer (ICT) mechanism upon excitation. The binding of these functionalities with AM-cations generated selective and specific fluorescence responses, which were quantifiable and allowed us to recognize the primary, secondary and tertiary interactions for all the AM cations in the solution. The relative binding affinities for each one of the functionalities with AM cations was estimated, and a general and consistent perspective of the binding of AMs as a function of their location in the Periodic Table, hardness property and ionic radius was established. The binding preferences of the AM cations were supported by DFT calculations. Our strategy allowed us to validate the binding dynamics of AMs in solution for three types of key ligations, which opens a new perspective to recognize weak intermolecular interactions derived from acidic species and rationally design selective AM-cation probes using an ICT-based ambidentate organic fluorophore.

A sensitive zinc probe operating via enhancement of excited-state intramolecular charge transfer.

Novel highly sensitive fluorescent probes for zinc cations based on the diketopyrrolopyrrole scaffold were designed and synthesized. Large bathochromic shifts (≈80 nm) of fluorescence are observed when the Zn2+-recognition unit (di-(2-picolyl)amine) is bridged with the fluorophore possessing an additional pyridine unit able to participate in the coordination process. This effect originates from the dipolar architecture and the increasing electron-withdrawing properties of the diketopyrrolopyrrole core upon addition of the cation. The new, greenish-yellow emitting probes, which operate via modulation of intramolecular charge transfer, are very sensitive to the presence of Zn2+. Introduction of a morpholine unit in the diketopyrrolopyrrole structure induces a selective six-fold increase of the emission intensity upon zinc coordination. Importantly, the presence of other divalent biologically relevant metal cations has negligible effects and typically even at a 100-fold higher concentration of Mg2+/Zn2+, the effect is comparable. Computational studies rationalize the strong bathochromic shift upon Zn2+-complexation. Decorating the probes with the triphenylphosphonium cation and morpholine unit enables selective localization in the mitochondria and the lysosome of cardiac H9C2 cells, respectively.

Investigation of ultrafast broadband optical nonlinearity and intensity-dependent nonlinear absorption in novel hydrazone derivatives

Two novel hydrazone derivatives (BTH-1 and BTH-2) were designed and synthesized to investigate the modulation of intramolecular charge transfer (ICT) and planar π conjugation on optical nonlinearities. Femtosecond Z-scan measurements show that trithiophene-substituted BTH-2 exhibits stronger broadband (600–900 nm) reverse saturable absorption (RSA) resulting from two-photon absorption (TPA)-induced excited state absorption (ESA). Combined with quantum chemical calculations, the results indicate that the local excitation and spatial delocalization enhanced by planarized trithiophene are more conducive to promoting the nonlinear optical (NLO) response of hydrazone molecules than the ICT introduced by twisted triphenylamine. In addition, the intensity-dependent transition from saturable absorption to RSA of BTH-2 at 532 nm under multiple pulse widths (fs, ps, ns) was explored. Transient absorption spectroscopy provided information on the excited state dynamics of the compounds. This work extends NLO research on hydrazone derivatives and provides a reference for the design of hydrazone molecules with high nonlinearity. The results also suggest that BTH-2 is a promising candidate for optical limiting applications.

A colorimetric and fluorescent dual-modal displacement probe based on host-assisted modulation of intramolecular charge transfer and deaggregation.

A new colorimetric and fluorescent dual-modal displacement probe was developed based on a supramolecular host-guest complex of cucurbit[8]uril (CB8) and a new functionalized perylene derivative involving macrocycle encapsulation that modulated intramolecular charge transfer and deaggregation.

A Fluorescent Probe for Hydrogen Peroxide in Vivo Based on the Modulation of Intramolecular Charge Transfer.

Endogenous hydrogen peroxide in vivo is related to many diseases, including cancer, diabetes, cardiovascular disease, and neurodegenerative disorders. Although many probes for detection of H2O2 have been explored, rapid response probes are still expected for in vivo application. Here, a new probe (PAM-BN-PB) was designed based on an intramolecular charge transfer (ICT) process with three parts: phenanthroimidazole, benzonitrile, and phenyl boronate. By modulation ICT process of PAM-BN-PB, H2O2 in solution systems can be detected with good selectivity. The exogenous and endogenous H2O2 in normal living cells, ischemia-reperfusion injury cells, and animals all can be imaged by PAM-BN-PB.

Thiophene bridged aldehydes (TBAs) image ALDH activity in cells via modulation of intramolecular charge transfer.

Aldehyde dehydrogenases (ALDHs) catalyze the oxidation of an aldehyde to a carboxylic acid and are implicated in the etiology of numerous diseases. However, despite their importance, imaging ALDH activity in cells is challenging due to a lack of fluorescent imaging probes. In this report, we present a new family of fluorescent probes composed of an oligothiophene flanked by an aldehyde and an electron donor, termed thiophene-bridged aldehydes (TBAs), which can image ALDH activity in cells. The TBAs image ALDH activity via a fluorescence sensing mechanism based on the modulation of intramolecular charge transfer (ICT) and this enables the TBAs and their ALDH-mediated oxidized products, thiophene-bridged carboxylates (TBCs), to have distinguishable fluorescence spectra. Herein, we show that the TBAs can image ALDH activity in cells via fluorescence microscopy, flow cytometry, and in a plate reader. Using TBA we were able to develop a cell-based high throughput assay for ALDH inhibitors, for the first time, and screened a large, 1460-entry electrophile library against A549 cells. We identified α,β-substituted acrylamides as potent electrophile fragments that can inhibit ALDH activity in cells. These inhibitors sensitized drug-resistant glioblastoma cells to the FDA approved anti-cancer drug, temozolomide. The TBAs have the potential to make the analysis of ALDH activity in cells routinely possible given their ability to spectrally distinguish between an aldehyde and a carboxylic acid.

A selective fluoride sensor and a digital processor with "Write-Read-Erase-Read" behaviour.

An easy-to-synthesize Schiff base as a selective and colorimetric fluoride sensor via modulation of intramolecular charge transfer (ICT) has been demonstrated. A typical dual-ion binding property along with its distinct reversibility has been explored for YES, NOR and INH logic functions and a potential "Write-Read-Erase-Read" mimic.

Spectral modulation of a charge transfer reaction of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde inside cyclodextrin nanocage

This article reports modulation of intramolecular charge transfer (ICT) reaction of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde (2-MDMABA) encapsulated within the cyclodextrin nanocavities investigated by steady state and time resolved measurements. The ICT emission, absent in bulk water, originates in the presence of α-, β- and γ-CD with the huge enhancement of local emission. From the Benesi–Hildebrand plot, the stoichiometry of the host–guest inclusion complex is found to be 1:1 for β- and γ-CD whereas 1:1 and 1:2 guest to host complexation occur at low and high concentration of α-CD, respectively. The association constants of the inclusion complexes have also been estimated from the Benesi–Hildebrand plot. The greater binding capability of 2-MDMABA with β-CD than that of other two CDs is further supplemented by time resolved study.

A ratiometric fluorescent probe for bisulphite anion, employing intramolecular charge transfer

4-(1H-benzimidazol-2-yl)benzaldehyde (1) has been developed as a new ratiometric fluorescent probe for bisulphite, based on the modulation of intramolecular charge transfer (ICT). Upon mixing with bisulphite in aqueous ethanol, an aldehyde-bisulphite adduct was formed and the ICT of the probe was switched off, which resulted in a ratiometric fluorescence response with an enhancement of the ratios of emission intensities at 368 and 498 nm. The detection range of the probe for bisulphite is in the 2.0-200 µmol/L concentration range and the detection limit is 0.4 µmol/L. Probe 1 produces a ratiometric fluorescent response to bisulphite with a marked emission wavelength shift (130 nm) and displays high selectivity for bisulphite over other anions.

Non-covalent interactions of coumarin dyes with cucurbit[7]uril macrocycle: modulation of ICT to TICT state conversion

Non-covalent interaction of coumarin laser dyes, namely coumarin-1 (C1), coumarin-481 (C481) and coumarin-6H (C6H), with a versatile macrocyclic host molecule cucurbit[7]uril (CB7), has been investigated in aqueous solution using photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the fluorescence yields, lifetimes and spectral features of C1, C481 and C6H on interaction with CB7, and are assigned to 1 : 1 complex formation between the dyes and the CB7 host. The complex formation is mainly driven by charge-dipole interaction, as evident from the binding constant values (K ~ 10(4)-10(5) M(-1)). The large changes in the excited state behaviour of C1 and C481 as compared to C6H in the presence of CB7 indicate that CB7 binds C1 and C481 through the encapsulation of the 7-N,N'-diethylamino group of the dyes and the structural rigidity imposed by this interaction dramatically alters the excited state properties of the dyes by preventing the conversion of their emissive intramolecular charge transfer (ICT) state to the non-radiative twisted intramolecular charge transfer (TICT) state. The present results direct towards the probable supramolecular approach using water soluble macrocyclic CB7, in the development of aqueous dye laser systems in the blue-green region.

Temperature-induced phase transition and intramolecular charge-transfer process based on poly( N-isopropylacrylamide) hydrogel with covalently attached D–π–A type dye

A thermoresponsive poly(NIPAM- co-dye) copolymer with covalently attached D–π–A type dye was prepared by typical radical copolymerization. Software was used to calculate the electron density distribution of the push–pull, intramolecular charge transfer (ICT) operating in donor–π-conjugation–acceptor (D–π–A) configurations of dye monomer 3. It can be constructed an acid/base-induced molecular switch by modulation of intramolecular charge transfer with protonation/deprotonation. The lower critical solution temperature (LCST) behavior was investigated by means of UV–vis spectroscopy that allows the measurement of the phase transition from 25 to 40 °C in aqueous solution. The poly(NIPAM- co-dye) copolymer also exhibited color change when used an acid/base-induced molecular switch via control of intramolecular charge transfer (ICT). The morphology of the internal microstructure of the poly(NIPAM- co-dye) hydrogel was observed by scanning electron microscopy (SEM). The reversible switch could be obtained by thermal and acid/base stimuli.

Thermo- and Acid/base-induced Spectral Switching of a Poly(N-isopropylacrylamide) Copolymer Containing Benzopyran-based D-π-A type Dye Units

A thermoresponsive poly(NIPAM-co-dye) labeled with benzopyran-based D-<TEX>$\pi$</TEX>-A type dye was prepared by typical radical copolymerization. It can be also constructed a acid/base-induced molecular switch by modulation of intramolecular charge transfer with protonation/deprotonation. The lower critical solution temperature behavior was investigated by means of UV-vis spectroscopy which allows the measurement of the phase transition from <TEX>$25^{\circ}C$</TEX> to <TEX>$45^{\circ}C$</TEX> in aqueous DMSO solution. The morphology of the internal microstructure of the poly(NIPAM-co-dye) hydrogel was observed by scanning electron microscopy. The reversible switch could be obtained by thermal and acid/base stimuli.

New Type of Dual Solid-State Thermochromism: Modulation of Intramolecular Charge Transfer by Intermolecular π−π Interactions, Kinetic Trapping of the Aci-Nitro Group, and Reversible Molecular Locking

When heated above room temperature, some crystalline polymorphs of the 1,3-bis(hydroxyalkylamino)-4,6-dinitrobenzenes (BDBn, n = 2-5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6-dinitrobenzene, exhibit "dual" thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10(-7)-10(-6) s(-1) for BDB2 to about 0.1 s(-1) in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of "locking" and "unlocking" of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened pi-pi interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid-solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto-enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular pi-pi interactions and thermal intramolecular proton transfer to aromatic nitro group.

D–π–A solvatochromic charge transfer dyes containing a 2-cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran acceptor

The preparation and solvatochromic behavior of novel, intramolecular charge transfer dyes obtained by the condensation of 2-cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran with 4-dimethylamino benzaldehyde and 9-formyljulolidine are described. The absorption and fluorescence emission spectra of the dyes were studied in solvents of differing polarity. The dyes exhibited positive solvatochromism and their solvatochromic properties are discussed with the aid of semiempirical calculations. The HOMO and LUMO values of the dyes were obtained using both cyclic voltammetry and theoretical calculations; the electrochemical results were in agreement with both observed values and theoretical calculations. pH molecular switching was achieved by modulation of intramolecular charge transfer by means of protonation/deprotonation in DMSO solution.

A rational approach to emission ratio enhancement of chemodosimeters via regulation of intramolecular charge transfer

The sensitivity as well as dynamic range of a ratiometric probe is determined by the ratio of emission intensities at two wavelengths. Thus, it is highly desirable to acquire a large ratiometric fluorescence response at two wavelengths. However, ratiometric fluorescent signals are intrinsic characteristics of the particular probe–analyte interactions. The design for fluorescent probes with a large ratiometric signal remains a challenging task. There is still a lack of a proper approach to enhance the ratiometric fluorescence response for fluorescent chemodosimeters. Herein, we introduced a novel strategy to increase the emission ratios of a chemodosimeter via modulation of intramolecular charge transfer.

Photophysical and electrochemical properties of D–π–A type solvatofluorchromic isophorone dye for pH molecular switch

The photophysical and electrochemical properties of a donor-π-accepter (D–π–A) type isophorone dye, 2-(3-(4-((2-hydroxyethyl)(methyl)amino)styryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile with asymmetrical structure, was investigated. It showed interestingly solvatofluorchromic property in various solvents based on intramolecular charge transfer mechanism. Its LUMO and HOMO values were obtained by electrochemical measurement and theoretical calculation. In addition, it can be also constructed a pH fluorescent molecular switch by modulation of intramolecular charge transfer with protonation/deprotonation.