Intramolecular Charge Transfer System Research Articles

Crystallography, Charge Transfer, and Two-Photon Absorption Relations in Molecular Cocrystals for Two-Photon Excited Fluorescence Imaging.

Two-photon excited fluorescence imaging requires high-performance two-photon absorption (TPA) active materials, which are commonly intramolecular charge transfer systems prepared by traditional chemical synthesis. However, this typically needs harsh conditions and new methods are becoming crucial. In this work, based on a collaborative intermolecular charge transfer (inter-CT) strategy, three centimeter-sized organic TPA cocrystals are successfully obtained. All three cocrystals exhibit a mixed stacking arrangement, which can effectively generate inter-CT between the donor and acceptor. The ground and excited state characterizations compare their inter-CT ability: 1,2-BTC > 2D-BTC > 1D-BTC. Transient absorption spectroscopy detects TCNB•-, indicating that the TPA mechanism arises from molecular polarization caused by inter-CT. Meanwhile, 1,2-BTC exhibits the highest excited-state absorption and the longest excited-state lifetime, suggesting a stronger TPA response. First-principles calculations also confirm the presence of inter-CT interactions, and the significant parameter Δµ which can assess the TPA capability indicates that inter-CT enhances the TPA response. Besides, cocrystals also demonstrate excellent water solubility and two-photon excited fluorescence imaging capabilities. This research not only provides an effective method for synthesizing TPA crystal materials and elucidates the connection between inter-CT ability and TPA property but also successfully applies them in the fields of multi-photon fluorescence bioimaging.

Transformation of organic afterglow mechanism from room-temperature phosphorescence to thermally-activated delayed fluorescence in intramolecular charge transfer systems

Organic afterglow materials have emerged as an important class of luminescent materials. These afterglow systems feature small rate constants of key photophysical processes, such as phosphorescence decay and nonradiative decay of triplet excited states. When coupled to other photophysical processes, these afterglow systems would give rise to intriguing photophysical behaviors. Here we report the transformation of afterglow mechanism from room-temperature phosphorescence (RTP) to thermally-activated delayed fluorescence (TADF) in intramolecular charge transfer (ICT) systems. Detailed studies reveal that the increase of the acceptor strength of ICT molecules can reduce singlet-triplet splitting energy and consequently open reverse intersystem crossing (RISC) pathway with moderate kRISC of 1–10 s−1 to harvest triplet energies, leading to the RTP-to-TADF afterglow transformation. Such observations provide deep understanding on the behaviors of organic triplets with small photophysical rate constants. Besides, these organic ICT molecules have been incorporated into aqueous emulsion systems to exhibit promising applications.

Caffeic acid, a natural extract, as an activatable molecular probe for viscosity detection in a liquid system.

Liquids, functioning as nutrients and energy systems, regulate various functions during storage programs. Microenvironmental viscosity is one of the most important physical parameters associated with the extent of deterioration, and it is crucial to monitor the mutation of viscosity at a molecular level. Herein, we utilized caffeic acid (CaC), a natural product extracted from thistles, as a molecular probe for viscosity sensing. CaC contains phenol hydroxyl (electron-donor) and carboxyl (electron-acceptor) groups, with both moieties connected by conjugated single and double bonds, forming a typical twisted intramolecular charge transfer system. The fluorescent probe CaC, obtained from a natural product without any chemical processing, exhibits high sensitivity (x = 0.43) toward viscosity, with an obvious visualized turn-on signal. Moreover, it displays good photostability, selectivity, and wide universality in commercial liquids. Utilizing CaC, we have successfully visualized viscosity enhancement during the spoilage process, with a positive correlation between the degree of liquid spoilage and microenvironmental viscosity. Thus, this study will provide a convenient and efficient molecular probe for food safety inspection across the boundaries of traditional biological applications.

Describing Chemical Reactivity with Frontier Molecular Orbitalets.

Locality in physical space is critical in understanding chemical reactivity in the analysis of various phenomena and processes in chemistry, biology, and materials science, as exemplified in the concepts of reactive functional groups and active sites. Frontier molecular orbitals (FMOs) pinpoint the locality of chemical bonds that are chemically reactive because of the associated orbital energies and thus have achieved great success in describing chemical reactivity, mainly for small systems. For large systems, however, the delocalization nature of canonical molecular orbitals makes it difficult for FMOs to highlight the locality of the chemical reactivity. To obtain localized molecular orbitals that also reflect the frontier nature of the chemical processes, we develop the concept of frontier molecular orbitalets (FMOLs) for describing the reactivity of large systems. The concept of orbitalets was developed recently in the localized orbital scaling correction method, which aims for eliminating the delocalization error in common density functional approximations. Orbitalets are localized in both physical and energy spaces and thus contain both orbital locality and energy information. The FMOLs are thus the orbitalets with energies highest among occupied orbitalets and lowest among unoccupied ones. The applications of FMOLs to hexadeca-1,3,5,7,9,11,13,15-octaene in its equilibrium geometry, inter- and intra-molecular charge-transfer systems, and two transition states of a bifurcating reaction demonstrate that FMOLs can connect quantum mechanical treatments of chemical systems and chemical reactivities by locating the reactive region of large chemical systems. Therefore, FMOLs extend the role of FMOs for small systems and describe the chemical reactivity of large systems with energy and locality insight, with potentially broad applications.

Carbon dots as an electron extractant for enhanced photocatalytic antibacterial activity of covalent organic frameworks

An intramolecular charge transfer system was constructed to extract π-electrons from photosensitive D–A COF semiconductors to CDs through hydrogen bonding for more ROS evolution, which demonstrated high-efficiency bactericidal mechanism.

An Integration Strategy to Develop Dual-State Luminophores with Tunable Spectra, Large Stokes Shift, and Activatable Fluorescence for High-Contrast Imaging

An Integration Strategy to Develop Dual-State Luminophores with Tunable Spectra, Large Stokes Shift, and Activatable Fluorescence for High-Contrast Imaging

Synthesis and Properties of a Water-soluble Fluorescent Probe Based on ICT System for Detection of Ultra-trace SO2 Derivatives.

SO2 and its derivatives are widely present in the environment and living organisms, endangering the environment and human health. Therefore, it is of great significance for the effective detection of sulfur dioxide (SO2) and its hydrated derivatives (HSO3- /SO32-). In this study, based on the mechanism of intramolecular charge transfer (ICT), a water-soluble colorimetric fluorescent probe (E)-2-(4-acetamidostyryl)-3-(5-carboxypentyl)-1, 1-dimethyl-1H-benzo[e]indol-3-ium (ABI) for the detection of SO2 derivatives was successfully synthesized from p-acetaminobenzaldehyde by connecting the benzo[e]indoles cationic fluorophore containing highly activated methyl via C = C double bond, and the ABI structure was characterized by HRMS and 1H NMR, 13C NMR. Studies have shown that the ABI probe has some advantages such as good selectivity for SO2 derivatives, high sensitivity (detection limit LOD = 14.9 nM), and fast reaction rate. After adding HSO3-, the color of the probe solution changed from light yellow to colorless within 10 s, which provides a simple way to identify bisulfite with the naked eye. Studies on the effect of pH on the fluorescence performance of ABI showed that fluorescence performance of ABI was stable in the range of pH (7.0-10.26). Therefore, ABI is expected to become an effective tool for detecting SO2 derivatives in cells and organisms in the future.

Development of Turn-On Probes for Acids Triggered by Aromaticity Enhancement Using Tricyclic Amidine Derivatives.

Two fluorophores consisting of tricyclic amidine derivatives (DHIm and DHPy) were prepared as selective turn-on probes for acids, which were triggered by an aromaticity enhancement. Both amidine derivatives were expanded rings prepared by condensed reactions between the corresponding dibromoalkanes and an aminonaphthyridine analogue. In X-ray analyses, DHIm, in which the dihydroimidazole ring was condensed into aminonaphthyridine, showed high planarity, compared to DHPy, with condensed dihydropyrimidine. The fluorescence properties of DHIm exhibited a higher quantum yield than DHPy due to the difference in planarity. Under acidic conditions, such as in the presence of H+ and M(II), protonations and complexations occurred, exhibiting a higher quantum yield than the neutral DHX (X = Im or Py). The nucleus-independent chemical shift values from the density functional theory calculations suggested that the protonations and complexations caused an enhancement of the aromaticity within the frameworks. These aromaticity changes led to intense fluorescence, and DHX behaved as a selective turn-on probe for acids and metal ions. Interestingly, this fluorescence turn-on system triggered by the aromaticity-based enhancement is not a typical system, such as the photoinduced electron transfer, aggregation-induced enhanced emission, and twisted intramolecular charge transfer systems, but is classified as a novel turn-on system.

Discrete and continuum modeling of solvent effects in a twisted intramolecular charge transfer system: The 4-N,N-dimethylaminobenzonitrile (DMABN) molecule

The 4-N,N-dimethylaminobenzonitrile (DMABN) molecule is a prototypical system displaying twisted intramolecular (TICT) charge transfer effects. The ground and the first four electronic excited states (S1–S4) in gas phase and upon solvation were studied. Charge transfer values as function of the torsion angle between the donor group (dimethylamine) and the acceptor moiety (benzonitrile) were explicitly computed. Potential energy curves were also obtained. The algebraic diagrammatic construction method at the second-order [ADC(2)] ab initio wave function was employed. Three solvents of increased polarities (benzene, DMSO and water) were investigated using discrete (average solvent electrostatic configuration - ASEC) and continuum (conductor-like screening model - COSMO) models. The results for the S3 and S4 excited states and the S1–S4 charge transfer curves were not previously available in the literature. Electronic gas phase and solvent vertical spectra are in good agreement with previous theoretical and experimental results. In the twisted (90°) geometry the optical oscillator strengths have negligible values even for the S2 bright state. Potential energy curves show two distinct pairs of curves intersecting at decreasing angles or not crossing in the more polar solvents. Charge transfer and electric dipole values allowed the rationalization of these results. The former effects are mostly independent of the solvent model and polarity. Although COSMO and ASEC solvent models mostly lead to similar results, there is an important difference: some crossings of the excitation energy curves appear only in the ASEC solvation model, which has important implications to the photochemistry of DMABN.

A simple dihydrophenazine-based chemosensor for the detection of sulfite with turn-on fluorescence

A simple novel dihydrophenazine-based chemosensor R has been developed for the detection of SO32− in DMSO-H2O solution with turn-on fluorescence. Upon the addition of sulfite, the ICT (intramolecular charge transfer) system of the probe was destroyed owing to the nucleophilic addition of sulfite to the formyl group of R. Thus a charming orange-red fluorescence centered at 603 nm appeared, which is favorable for potential biosensing. The reaction mechanism was confirmed by MS and 1H NMR titration. In addition, the detection limit for sulfite was determined as low as 71.4 nM, which is superior to many reported probes. All above results promote the probe as a promising candidate in practical and biological applications.

DFT and TDDFT investigation of the Schiff base formed by tacrine and saccharin.

Schiff bases have many chemical and biological applications in medicine and pharmaceuticals due to the presence of an imine group (-C=N-). These bases are used in many different fields of technology, and in photochemistry because of their photochromic properties. Here, the structural and electronic properties of the Schiff base formed by tacrine and saccharin (TacSac) were explored using density functional theory with the B3LYP, M06-2X, M06L, and ωB97XD functionals in combination with the 6-311++G(d,p) basis set. The time-dependent formalism was used at the B3LYP/6-311++G(d,p) level to obtain electronic transitions. The calculations were repeated in an implicit solvent model mimicking water, using the polarizable continuum model in conjunction with a solvation model based on a density approach. The results indicate that TacSac cannot form spontaneously, but can be obtained in mild reactions. However, the resulting Schiff base displays different characteristics to its monomers. It also has the potential for use in photochemical intramolecular charge-transfer systems. Graphical Abstract Intramolecular charge transfer between HOMO and LUMO of TacSac.

Characteristics of Guajazulene Based Chemosensor Toward CN− and F− Anions

A chemosensor having electron pushing donor and pulling acceptor material based on conjugation bridges, namely D-π-A structural chemosensor, was herein designed and synthesized. This chemosensor 3 contained guajazulene and indole moieties, which was synthesized through a one-step condensation reaction. The synthesized material showed very selective sensing properties towards anions. Upon the addition of CN- and F-, the remarkable changes in absorption spectra were monitored. However, each of others did not show any noticeable optical changes. The original intramolecular charge transfer system of chemosensor 3 was altered by the formation of N- at iindole moiety with anion reaction: caused by the deprotonation of NH at the indole moiety of chemosensor. With intermolecular charge transfer system between chemosensor 3 and anions, the electron density and distribution were determined by frontier calculation method.

Intramolecular Charge Transfer Effects on the Diradical Character and Second Hyperpolarizabilities of Open-Shell Singlet X−π–X (X = Donor/Acceptor) Systems

We investigate the effect of the quadrupole-type intramolecular charge transfer (ICT) in open-shell singlet donor-π-donor (D-π-D) molecules on the singlet open-shell (diradical) character and the longitudinal second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale). For this investigation we used the para-quinodimethane (PQM) with point charges (pc's) model calculated with the unrestricted coupled cluster method including single and double excitations with a perturbative treatment of the triple excitations (UCCSD(T)). In this model, the diradical character y and the amount of the ICT, that is, the D-π-D nature, can be varied primarily by changing the exocyclic carbon-carbon bond (C-C) lengths and the external pc's Q, respectively. It turns out that the increase in the D-π-D nature decreases the y values, moves the y values (ymax) giving the maximum γ (γmax) to the large y region, and enhances the γmax values, for example, the γmax of the singlet diradical PQM with Q = -2.8 au reaches twice that of the singlet diradical PQM without any pc's. This result indicates that open-shell singlet D-π-D systems with ICT are promising candidates for a new class of third-order NLO molecules, whose γ values are more enhanced than those of conventional closed-shell D-π-D systems and of symmetric open-shell singlet systems without the ICT. To confirm this tendency, we examine the boron-disubstituted PQM dianion model, which is found to exhibit further enhancement of γ as compared to the PQM model with intermediate diradical character due to the synergy effects of the intermediate open-shell singlet nature and the strong field-induced ICT nature in the dianionic state of the D-π-D system. Further investigation of the acceptor-π-acceptor (A-π-A) type ICT effect in the PQM-pc model shows that both D-π-D and A-π-A type symmetric ICTs give similar effects on the relationship between y and γ, though there are some differences originating in the orbital contraction and extension induced by the pc's. The present results contribute to understanding the third-order NLO properties of open-shell symmetric ICT systems and thus to constructing new design guidelines for highly efficient third-order NLO systems.

An OFF–ON–OFF type of pH fluorescent sensor: Benzo[c,d]indole-based dimethine cyanine dye-synthesis, spectral properties and density functional theory studies

We synthesized a novel OFF–ON–OFF type of pH-dependent fluorescent sensor: benzo[c,d]indole-based dimethine cyanine dye D1, with donor-π-acceptor (D-π-A) structure based on intramolecular charge transfer system (ICT), which employed dimethine cyanine dye as a fluorophore and pentavalent nitrogen NH+ group as a pH modulator, respectively. The product was identified by 1H NMR, 13C NMR, IR, UV–vis and HRMS. The investigation of spectral properties found that dye D1 showed excellent spectroscopic properties and its absorption maxima and fluorescence quantum yield were basically larger in protic solvent than in aprotic solvent. Meanwhile, the absorption spectra of D1 were revealed to hypochromatic-shift and the absorption intensity was gradually decreased along with the increase of pH value. Interestingly, dye D1 showed remarkable fluorescence when pH value was in the range of 6.00–9.80 with the peak at 8.21, which was defined as an OFF–ON–OFF type of pH-dependent fluorescent sensors based on ICT. In addition, dye D1 exhibited a high selectivity for H+ over other common ions, such as Cl−, K+, Fe2+ etc. Theoretical calculations based on density functional theory (DFT) were employed to provide a better understanding of this particular dye sensor. These results indicated that D1 would be able to act as an efficient pH-sensor and had a potential to play an important role in biological and medical study.

Looking for the species responsible of the anomalous fluorescence band in ortho-, meta- and para-(di-tert-butylamino)benzonitrile

In spite of the amount of effort devoted to the study of the photophysics of intramolecular charge transfer systems, there are still controversial points of debate. One of them is the geometrical and electronic structure of the species responsible of the anomalous fluorescence band shown by some compounds. DTABN is a derivative of the prototype system, ABN, that can shed some light in this controversy due to the geometrical constrains that its bulky substituents must impose. We have studied here the ortho-, meta- and para- isomers of DTABN using the strategy of combining QM/MM (CASSCF/UFF) and CASPT2 methods. Our results show that the steric repulsions of the bulky amino substituents impose a non-planar geometry even in the ground state. This constrain favors the twisted structure of the charge transfer excited state relative to the locally excited state in all three isomers, predicting anomalous fluorescence for all of them. Our results are in good agreement with the experimental data available for the para- and meta-isomers.

Synthesis and Solvatofluorochromism Behaviors on Intramolecular Charge Transfer System of Novel D-π-A Dyes

Solvatochromic donor-π-acceptor (D-π-A) charge transfer dyes have been great attention from the viewpoint of their applicability as probes for the determination of solvent polarity and colorimetric chemosensors. In this work, novel solvatochromic fluorophore dyes based on D-π-A charge transfer system were designed and synthesized containing a strong electron-donating unit as a carbazole moiety and three different strong electron-withdrawing units. The corresponding molecular design and the optical properties of the D-π-A dyes were examined in various solutions with widely different polarity. The prepared dyes were found to exhibit a positive fluorescence solvatochromism. Furthermore, the energy properties of these dyes were also computationally optimized and calculated by computational simulation approaches using Material Studio 4.3 and electrochemical measurement.

J-aggregate structure in a chloroform solvate of a 2,3-dicyanopyrazine dye – Separation of two-dimensional stacking dye layers by solvate formation

Here, we report the J-aggregate structure of a chloroform solvate of 5-t-butyl-2,3-dicyano-6-[4-(dimethylamino)styryl]-pyrazine with a strong intramolecular charge transfer system. The dye was found to form a two-dimensional brick-wall structure separated by chloroform molecules in the solvated crystals, which exhibited intense red fluorescence. The absorption maximum of the solvate was also found to show a bathochromic spectral shift. These observed optical characteristics were interpreted in terms of excitonic intermolecular interaction based on two-dimensional brick-wall structure which is one of the proposed structures for J-aggregates.

Molecular orbital assistance in the design of intramolecular and photoinduced electron transfer systems

A theoretical approach is described for the design of donor–acceptor intramolecular charge transfer (ICT) systems and for photoinduced electron transfer (PET) hybrids of fullerene, based on orbital level diagrams of the separate donor and acceptor moieties. Minimization of the HOMO–LUMO (highest occupied–lowest unoccupied orbital) gap in ICT systems, translates to a requirement for near degeneracy of the HOMO of the donor and LUMO of the acceptor, determined separately for the two moieties by density functional theory calculations. Similarly, near degeneracy of the LUMO of the donor and LUMO of the acceptor moieties would indicate the possibility of PET in the combined hybrid.

Novel triazacarbocyanine dye sensor synthesis: pH switching effect

Design, synthesis and pH switching effects of the intramolecular charge transfer (ICT) dye sensor chromophore prepared by the coupling reaction of 6-methoxy-2-aminobenzothiazole (donor moiety) with 4-nitroaniline (acceptor moiety) were carried out and discussed. Optical absorption properties were monitored with the addition of increasing alkali/acid equivalence. In this determination, the prepared pH dye sensor chromophore showed reversible optical switching functions. The levels of energy potential and the electron density distribution of this dye sensor such as HOMO and LUMO values were calculated by computational simulation approaches using Material Studio 4.3. The functions of molecular switching sensor with pH stimulation of alkali-acid addition were determined in DMSO:H2O, which was operated by deprotonation/protonation effects based on ICT system.

Synthesis and property of solvatochromic fluorophore based on D-π-A molecular system: 2-{[3-Cyano-4-(N-ethyl-N-(2-hydroxyethyl)amino)styryl]-5,5-dimethylfuran-2(5H)-ylidene}malononitrile dye

2-[[3-Cyano-4-(N-ethyl-N-(2-hydroxyethyl)amino)styryl]-5,5-dimethylfuran-2(5H)-ylidene]malononitrile styryl dye was prepared by the condensation of 4-[(2-hydroxy-ethyl)-methyl-amino]-benzaldehyde (donor moiety) with 2-cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (acceptor moiety). The corresponding design, synthesis and solvatochromic characteristics of the intramolecular charge-transfer (ICT) dye chromophore were discussed and determined. Optical properties such as absorption and fluorescence emission spectra were monitored in several solvent media with different polarity. In this determination, the prepared dye chromophore showed positive solvatochromism effect and the resulting solvatochromic characteristics were studied with semiempirical calculations. The energy potentials of this dye chromophore such as HOMO and LUMO values were calculated by computational simulation approaches using Material Studio 4.3. Furthermore, the functions as a molecular switching sensor with pH stimulation of alkali-acid addition were determined in DMSO, which was operated by deprotonation/protonation effects based on intramolecular charge-transfer system.