Intramolecular Charge Transfer Character Research Articles

Estimation of Electric Dipole Moment by Solvatochromism, Computational Method, and Study of the Effect of Solvents by Preferential Solvation of 6 - Methoxy-4-(4-Nitro-Phenoxy Methyl)-Chromen-2-One (6mnpm).

At room temperature, the absorption and fluorescence properties of coumarin 6-Methoxy-4-(4-nitro-phenoxy methyl)-chromen-2-one (6MNPM) are investigated in pure organic solvents and a combination of acetonitrile (ACN) and tetrahydrofuran (THF). The pure solvents' influence on spectral characteristics is examined by applying theories such as Kamlet and Catalan's multiple linear regression techniques, Reichardt's microscopic solvent polarity parameter, and the Lippert-Mataga polarity function. The significant role of solute-solvent interactions in pure solvents, particularly dielectric interaction and hydrogen bonding. However, hydrogen bonding interactions dominate the contribution of dielectric interactions. The electric dipole moments of both the ground as well as excited states had been calculated using the Solvatochromic method. The value of the excited state electric dipole moment and the redshifts of the emission spectra show that the emitting singlet state has an intramolecular charge transfer (ICT) character. From Catalan's linear regression, we found that di-polarity has a much smaller influence than polarizability. By solvation study, we conclude that Tetrahydrofuran solvent is preferred over Acetonitrile.

Intramolecular Charge Transfer and Spin-Orbit Coupled Intersystem Crossing in Hypervalent Phosphorus(V) and Antimony(V) Porphyrin Black Dyes.

Porphyrin dyes with strong push-pull type intramolecular charge transfer (ICT) character and broad absorption across the visible spectrum are reported. This combination of properties has been achieved by functionalizing the periphery of hypervalent and highly electron-deficient phosphorus(V) and antimony(V) centered porphyrins with electron-rich triphenylamine (TPA) groups. As a result of the large difference in electronegativity between the porphyrin ring and the peripheral groups, their absorption profiles show several strong charge transfer transitions, which in addition to the porphyrin-centered π → π* transitions, make them panchromatic black dyes with high absorption coefficients between 200 and 800 nm. Time-resolved optical and electron paramagnetic resonance (EPR) studies show that the lowest triplet state also has ICT character and is populated by spin-orbit coupled intersystem crossing.

Imaging of intracellular mitochondrial membrane potential with a highly photostable NIR fluorescent probe

Mitochondrial membrane potential (ΔΨm) participated in important physiological processes such as ATP synthesis, ion exchange, and plays an important role in maintaining normal life activities of cells. Herein, we synthesized a fluorescent probe EDXB (3-ethyl-2-(2-(6-methoxy-2,3-dihydro-1H-xanthen-4-yl) vinyl) benzothiazol-3-ium iodide) with positive charge, which targets mitochondria by electrostatic attraction. The probe has twisted intramolecular charge transfer (TICT) characteristics, resulting in near-infrared fluorescence emission enhanced with the increase of viscosity, when the probe falls off from the mitochondrial membrane to the low-viscosity cytoplasmic aqueous phase, the fluorescence emission intensity is weakened. Therefore, the EDXB exhibited favorable targeting. In addition, EDXB also has good anti-bleaching and pH stability, which enables it to accurately feedback the change of ΔΨm. In the experiment, the intracellular imaging of EDXB certificated mitochondrial depolarization (decrease of ΔΨm) during various cell injury events including cell inflammation, apoptosis and mitophagy. We expect EDXB to be a useful tool for the analysis of ΔΨm changes.

Molecular Design Motifs of Non‐Fullerene Acceptors for Mitigating All Inherent Non‐Ideal Energy Losses in Organic Photovoltaics

AbstractNon‐fullerene acceptors (NFAs) have demonstrated a great potential for the transcendence of performance limits in organic photovoltaics. Despite the tremendous efforts, however, synthetic control of non‐ideal recombination losses on the photovoltage are still elusive and thus lag far behind the inorganic counterparts. Here, a rational design strategy of non‐fullerene acceptors is presented to overcome the limitations derived from the inherent excited state properties of organic molecules. Spectroscopic studies on a series of chemical analogs of ladder‐type fused acceptors reveal that the radiative efficiency of non‐fullerene acceptors is predominantly determined by the inherently strong electron‐vibration coupling of their bound excited states, rather than the redistribution of molecular orbitals itself. In this sense, regardless of intramolecular charge transfer characteristics, the incorporation of rigid, and planar moiety with wide π plane is found to promote the dominance of long‐lived and weakly‐bound excited states in the photophysical process, in which the mitigated non‐radiative decays of NFAs can be directly translated into that of charge‐transfer states. Furthermore, the incorporation of such chemical moiety can minimize the evolution of subgap broadening in the bulk heterojunction blends, lowering the contribution of non‐ideal radiative losses. As a result, the model systems demonstrate the non‐radiative loss down to 180–190 meV, without sacrificing photocurrent generations, reaching that of inorganic counterparts.

A long-wavelength fluorescent probe based on carbazole with AIE characteristics for the detection of ONOO− and its application

Peroxynitrite (ONOO−) is an important reactive oxygen species, and excess ONOO− in the body can lead to a range of diseases. Addressing the need for sensitive detection ONOO−, in this work, a highly sensitivity and selectivity long-wavelength fluorescent probe (named CIS) was designed for tracking ONOO− in biological cells. Probe CIS featured carbazole as the fluorophore and the CC double bond with two electron withdrawing –CN groups as the recognition site. The probe CIS exhibited the typical aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) characteristics with the bright red fluorescent emission in high water fraction (fw ≥ 50 %) and negligible emission in low water fraction. The fluorescence emission wavelength of probe CIS is 631 nm, with a large Stokes shift (151 nm), which can sensitively detect ONOO− in DMSO: H2O (7:3, v/v) environment. Notably, the probe CIS exhibited a wide pH detection range (pH = 3–10), a low detection limit of 36.66 nM, high selectivity and minimal interference. Furthermore, probe CIS demonstrated excellent biocompatibility and low cytotoxicity, and can be used for the detection of exogenous ONOO− in 4T1 cells (Mouse breast cancer cells). These research results indicate that probe CIS has potential application prospects in the biological aspects.

Red/near-infrared (NIR) difluoroboron β-diketonate derivatives with reversible mechanochromism for cellular imaging

Near-infrared (NIR) fluorophores have promoted the development of materials for bioimaging, but traditional NIR dyes usually suffer from aggregation-caused quenching (ACQ), impeding their applications. Herein, we propose two difluoroboron β-diketonate complexes TBO and TBS, consisting a donor–acceptor (D-A) structure with triphenylamine (TPA) moiety as an electron donors and difluoroboron as well as furan or thiophene building block as an electron acceptor. The theoretical calculation and optical data shows that both of them have intramolecular charge transfer (ICT) characteristics. Such ICT characteristics endow them with both solvatochromism and dual-state emission (DSE) properties. In the solvent CH2Cl2, the emission wavelength of TBO ranges from 550 nm to 750 nm, with a low fluorescence quantum yield (Φ = 7.0 %). However, in the less polar solvent hexane, the emission wavelength blue-shifts, with an increased Φ reaching up to 18 %. Moreover, TBO and TBS exhibit mechanochromic characteristics and rare multi-channel fluorescence emission phenomena at solid-state. Their solid-state samples can emit fluorescence in four spectral bands with maximum emission wavelengths at 300 nm, 400 nm, 600 nm, and 770 nm under excitation at 240 nm. These unique optical properties are expected to be utilized for detecting polarity of system and deformation. Moreover, according to the results of cell imaging and flow cytometry, TBO molecular were easily internalized into Hela cells and distributed in the cytoplasm with strong red fluorescence. Therefore, this research inspires more insight into development of NIR luminogens for biomedical imaging.

Ring Rearrangement Reactions of 4-Alkenylisocoumarins and Photophysical Evaluation of Multi-Substituted Anthracene Products.

Herein we report that readily available 4-alkenylisocoumarins can be regarded as potent dienolate equivalents. For example, lactol silyl ethers derived from 4-alkenylisocoumarins were selectively converted to the corresponding benzo-homophthalates through a fluoride-induced ring opening step that was followed by a ring closure through a vinylogous intramolecular aldol condensation. Likewise, nucleophilic activation of 4-alkenylisocoumarins directly yields diversely poly-substituted naphthalenes and anthracenes without formation of any regioisomer. Photophysical evaluation of a set of thus obtained 1,3-di- and 1,3,4-trisubstituted anthracenes reveals their distinct intramolecular charge transfer (ICT) character during light absorption in polar solutions and excimer emission from the solid state when a face-to-face π-stacked molecular assembly is present in the crystal packing.

An azaryl-ketone-based thermally activated delayed fluorophore with aggregation-induced emission for efficient organic light-emitting diodes with slow efficiency roll-offs.

Achieving the concurrent manifestation of thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) within a single molecular system is highly sought after for organic light-emitting diodes (OLEDs), yet remains rare. In this study, we present a novel TADF-AIE dye, named PQMO-PXZ, which has been designed, synthesized, and systematically characterized. Our comprehensive investigation, which includes structural analysis, theoretical calculations, and optical studies, evaluates the potential of PQMO-PXZ for integration into OLEDs. Unlike existing azaryl-ketone-based emitters, PQMO-PXZ exhibits red-shifted emission and enhanced luminescence efficiency, due to its rigid structure and strong intramolecular charge transfer characteristics. Significantly, PQMO-PXZ demonstrates pronounced AIE properties and TADF with a short delayed lifetime. When utilized as the emissive core, OLED devices based on PQMO-PXZ achieve a respectable external quantum efficiency of up to 11.8% with minimal efficiency roll-off, underscoring PQMO-PXZ's promise as a highly efficient candidate for OLED applications.

A Photoredox Thiol-yne Reaction for the Synthesis of Vinyl Sulfide-Based Coumarins and its Effect on Fluorescence Properties.

Coumarins still remain one of the most widely explored fluorescent dyes, with a broad spectrum of applications spanning various fields, such as molecular imaging, bioorganic chemistry, materials chemistry, or medical sciences. Their fluorescence is strongly based on a push-pull mechanism involving an electron-donating group (EDG), mainly located at the C7 or C8 positions of the dye core. Unfortunately, up to now, these positions have been very limited to hydroxyl or amino groups. In this study, we present in detail the synthesis of the first series of coumarins bearing a vinyl sulfide as the EDG at the C7 position. These derivatives were prepared by thiol-yne reaction, promoted by ruthenium- or porphyrin-based photoredox catalysis, enabling rapid late-stage diversification. We also functionalized coumarins with short peptides, and BSA protein as a proof-of-concept study, in a single-step process. This strategy, capable of proceeding under aqueous conditions, overcomes the protection/deprotection steps usually required by traditional methods, which also use strong bases and organic solvents. Moreover, the photophysical properties such as absorption and emission of obtained coumarins (for 3-CF3, 3-benzothiazole, 6-8-difluoro derivatives), predominantly exhibited large Stokes shifts (up to 204 nm) and maintained intramolecular charge transfer (ICT) characteristics.

Study on four pyridine-based difluoroboron compounds for mechanofluorochromic behaviors, the ink-free writing and detection of latent fingerprints

With the increasing demand for multifunctional aggregation-induced emission materials, four pyridine-based difluoroboron compounds (1, 2, 3, and 4) with the anthryl unit as an electron donor and difluoroboron moiety as the electron acceptor were designed and successfully synthesized. It found that four compounds displayed significantly intramolecular charge transfer characteristics and solvatochromism phenomena. The other compounds possessed mechanofluorochromic behaviors under external mechanical stimulation except compound 1, owing to its lesser change value of the dipole moment between the ground and excited states. More importantly, they exhibited potential application in the field of ink-free writing due to reversible mechanochromism during the grinding and fumigation process. Additionally, four compounds were used for detection of latent fingerprints on the glass surface due to their excellent aggregation effects in the THF/water mixtures. This research provides a reference for exploring multifunctional aggregation-induced emission difluoroboron compounds with brightness luminescence.

Emission Color Tuning of Inverse Type Diarylethene Crystals

AbstractOrganic luminescent solid materials have attracted much attention due to practical applications such as sensor materials and optical waveguides. We have previously reported that inverse type diarylethenes exhibit strong emission in crystal without causing aggregation‐caused quenching. However, the emission color was limited to mainly green. To tune the emission color, in this work, we newly synthesized inverse type diarylethenes having a shortened π‐conjugation length or a polar substituent and investigated their fluorescence properties in solutions and crystals. The crystals exhibited various emission colors from blue, green, yellow to red depending on the molecular structure. The emission color changes of the crystals were induced by the intermolecular interactions such as CH‐π interactions in addition to the shortened π‐conjugation length and the intramolecular charge transfer character.

Mechanofluorochromic behaviors and latent fingerprint detection of triphenylamine-based compounds with mono-/bis-BF2 fluorophores

To better understand the relationship between molecular structure of the mono-/bis-BF2-core compounds and mechanofluoroboron behaviors, two pyridine-based difluoroboron compounds with triphenylamine group (TPA-ts-BF2 and TPA-ts-2BF2) were designed and successfully synthesized, which TPA-ts-BF2 including a BF2 fluorophore and TPA-ts-2BF2 containing the bisBF2 fluorophores. Based on the photophysical properties measurements results, it was found that TPA-ts-2BF2 had more excellent intramolecular charge transfer characteristics than that of TPA-ts-BF2, and exhibited significant aggregation-induced emission activity, however, TPA-ts-BF2 displayed typical aggregation-caused quenching phenomenon. Meanwhile, the emission spectrum of the solid powders of TPA-ts-2BF2 was red-shifted 52 nm after grinding, that of TPA-ts-BF2 was red-shifted 46 nm, which was resulted from crystalline state switching to amorphous state. According to the theoretical calculations, we conjectured that TPA-ts-BF2 with uncoordinated amide linkage moiety had a tendency to forming a more twisted conformance and higher molecular polarity, which made that mechanofluorochromic behavior was worse than that of TPA-ts-2BF2. Additionally, TPA-ts-2BF2 was applied to latent fingerprint detection due to its prime aggregation-induced emission property.

Tailoring Emission Color Shifts in Mechanofluorochromic-Active AIE Systems of Carbazole-Based D-π-A Conjugates: Impact of π Spacer Unit Variants.

The expanding field of organic electronics has led to the emergence of donor-π-acceptor (D-π-A) systems as promising candidates for various optoelectronic applications. Recent advancements in the design of organic fluorophores allow precise modulation of their optical properties in the solution and solid states, broadening their utility. Here, we synthesize and characterize two series of D-π-A systems based on carbazole-cyanostilbene compounds (3a-c and 4a-c), which feature differences in π-spacer groups and substitution on the acceptor cyanostilbene units. These compounds exhibit intramolecular charge transfer (ICT) characteristics, with 3c possessing a phenyl spacer and -CF3 substitution displaying notable solvatochromism and polarity-enhanced emission (PEE). Additionally, both series show aggregation-induced emission (AIE), with emission color shifts controlled by spacer groups. These compounds demonstrate diverse emissive behaviors, including locally excited (LE), twisted intramolecular charge transfer (TICT), and π-π stacked aggregated states, contributing to the AIE phenomenon. Compounds with phenyl spacers (3a-c) exhibit higher solid-state quantum yields than those of thiophene (4a-c). Furthermore, the impact of spacer groups and acceptor substitutions on the solid-state emissive properties and mechanofluorochromism (MFC) are highlighted. Compounds with phenyl spacers and substitution (3a, 3c) show a blue-shifted MFC, while those with thiophene (4a, 4c) display a red-shifted MFC. These findings offer insights into the development of advanced materials with tunable emission properties and high thermal stability for optoelectronic applications.

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.

Fluorescent response mechanism based on ESIPT and TICT of novel probe H2QJI: A TD-DFT investigation

Recently, synthesized N-linked-disalicylaldehyde H2QJI probes have been used to detect heavy metal ions in the experiment conveniently. Nevertheless, there needs to be a more in-depth examination of the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of the probe. This work remedied it based on quantum chemistry calculations. We contained due hydrogen bond (O1-H2 ⋯ N3 and O4-H5 ⋯ O6) and then analyzed bond parameters, IR vibration spectra, and non-covalent interaction. The bond strength is enhanced under photoexcitation, and the former is significantly stronger. The calculated electron spectra are in agreement with the experimental values. The results of the S0 and S1 potential energy curves and IRC calculations also confirm the unique ESIPT behavior, which isan excited stated stepwise double proton transfer. The fluorescence, internal conversion, and intersystem crossing rate of KD molecules (twisted-, double proton transfer) were calculated respectively to reveal the radiative and non-radiative pathways. It proved that the corresponding spectra are not obtained since the electrons are mainly deactivated by the ISC (S1->T1). Furthermore, the interfragment charge transfer (IFCT) approach indicates that the molecule possesses twisted intramolecular charge transfer (TICT) characteristics, which lead to the quenching of fluorescence introduction.

Intramolecular charge transfer and the function of vibronic excitons in photosynthetic light harvesting.

A widely discussed explanation for the prevalence of pairs or clusters of closely spaced electronic chromophores in photosynthetic light-harvesting proteins is the presence of ultrafast and highly directional excitation energy transfer pathways mediated by vibronic excitons, the delocalized optical excitations derived from mixing of the electronic and vibrational states of the chromophores. We discuss herein the hypothesis that internal conversion processes between exciton states on the <100fs timescale are possible when the excitonic potential energy surfaces are controlled by the vibrational modes that induce charge transfer character in a strongly coupled system of chromophores. We discuss two examples, the peridinin-chlorophyll protein from marine dinoflagellates and the intact phycobilisome from cyanobacteria, in which the intramolecular charge-transfer (ICT) character arising from out-of-plane distortion of the conjugation of carotenoid or bilin chromophores also results in localization of the initially delocalized optical excitation on the vibrational timescale. Tuning of the ground state conformations of the chromophores to manipulate their ICT character provides a natural photoregulatory mechanism, which would control the overall quantum yield of excitation energy transfer by turning on and off the delocalized character of the optical excitations.

An All-In-One Integrating Strategy for Designing Platinum(II)-Based Supramolecular Polymers for Photocatalytic Hydrogen Evolution.

Organic polymer photocatalysts have achieved significant progress in photocatalytic hydrogen evolution, while developing the integrated organic polymers possessing the functions of photosensitizer, electron transfer mediator, and catalyst simultaneously is urgently needed and presents a great challenge. Considering that chalcogenoviologens are able to act as photosensitizers and electron-transfer mediators, a series of chalcogenoviologen-containing platinum(II)-based supramolecular polymers is designed, which exhibited strong visible light-absorbing ability and suitable bandgap for highly efficient photocatalytic hydrogen evolution without the use of a cocatalyst. The hydrogen evolution rate (HER) increases steadily with the decrease in an optical gap of the polymer. Among these "all-in-one" polymers, Se-containing 2D porous polymer exhibited the best photocatalytic performance with a HER of 3.09mmolg-1h-1 under visible light (>420nm) irradiation. Experimental and theoretical calculations reveal that the distinct intramolecular charge transfer characteristics and heteroatom N in terpyridine unit promote charge separation and transfer within the molecules. This work could provide new insights into the design of metallo-supramolecular polymers with finely tuned components for photocatalytic hydrogen evolution from water.

Photochemical synthesis and solvatochromic fluorescence behavior of imide-fused phenacenes

Chrysenes, picene, fulminene, modified with imide, bromo, and amino functionalities, were synthesized through Mallory photoreaction as the key step, and their electronic spectra were investigated. Fluorescence spectra of chrysene-diimide CHRDI and bromo-substituted phencanene-imides, BrCHRI, BrPICI, BrFULI were dependent on solvent polarity to display appreciable fluorescence color changes. The solvatofluorochromic behavior was analyzed by conventional relationships between Stokes shift and solvent polarity parameters, such as Lippert-Mataga and Bilot-Kawski equations. The results indicated that the solvatofluorochromism was derived from the intramolecular charge transfer (ICT) nature in the excited state. Theoretical studies using time-dependent density-functional theory revealed that the phenacene-imide molecules in the fluorescent state possessed ICT characters between the strongly electron-withdrawing imide moiety and moderately electron-donating phenacene cores. Amino-substituted chrysene-imide NH2CHRI showed fluorescence band in a red region (λFL = 618 nm) in toluene with a very large Stokes shift (Δν∼ = 7630 cm−1) suggesting that the molecule in the fluorescent state was highly polarized. The present results indicate that phenacenes would provide potential platforms for constructing future functional fluorophores through an appropriate functionalization.

Easy but Efficient: Facile Approach to Molecule with Theoretically Justified Donor-Acceptor Structure for Effective Photothermal Conversion and Intravenous Photothermal Therapy.

To accelerate the pace in the field of photothermal therapy (PTT), it is urged to develop easily accessible photothermal agents (PTAs) showing high photothermal conversion efficiency (PCE). As a proof-of-concept, hereby a conventional strategy is presented to prepare donor-acceptor (D-A) structured PTAs through cycloaddition-retroelectrocyclization (CA-RE) reaction, and the resultant PTAs give high PCE upon near-infrared (NIR) irradiation. By joint experimental-theoretical study, these PTAs exhibit prominent D-A structure with strong intramolecular charge transfer (ICT) characteristics and significantly twisting between D and A units which account for the high PCEs. Among them, the DMA-TCNQ exhibits the strongest absorption in NIR range as well as the highest PCE of 91.3% upon irradiation by 760-nm LED lamp (1.2W cm-2). In vitro and in vivo experimental results revealed that DMA-TCNQ exhibits low dark toxicity and high phototoxicity after IR irradiation along with nude mice tumor inhibition up to 81.0% through intravenous therapy. The findings demonstrate CA-RE reaction as a convenient approach to obtain twisted D-A structured PTAs for effective PTT and probably promote the progress of cancer therapies.

Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms.

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3 - for CF3 -groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.