Excited State Intramolecular Charge Transfer Research Articles

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

Fluorescence sensing of heparin and heparin-like glycosaminoglycans by stabilizing intramolecular charge transfer state of dansyl acid-labeled AG73 peptides with glutathione-capped gold nanoclusters

Sensors that can specifically and accurately detect glycosaminoglycans are rare. Here, a dual-mode platform for fluorescence intensity and lifetime sensing of plasma heparin and fluorescence imaging of heparan sulfate proteoglycan-expressed cancer cells was developed by stabilizing the intramolecular charge transfer (ICT) state of dansyl acid-labeling AG73 (DA-AG73) peptide with glutathione-capped gold nanoclusters (GSH-AuNCs). DA-AG73 peptides, including an electron-donor dimethylamino group and an electron-withdrawing sulfonamide moiety in the labeled DA molecules, emitted weak fluorescence due to the formation of the twisted ICT excited state. The complexation of heparin with DA-AG73 peptides followed by interacting with the GSH-AuNCs could restrict the rotation of the dimethylamino groups of the labeled DA molecules, triggering the transition from their twisted ICT state to ICT excited state. As a result, the fluorescence intensity and lifetime of the labeled DA molecules in DA-AG73 peptides were gradually enhanced with increasing the heparin concentration. The proposed platform provided excellent selectivity toward heparin and heparan sulfate and exhibited two linear calibration curves for quantifying 20–800 nM and 20–1000 nM heparin in the fluorescence intensity and lifetime modes, respectively. The proposed platform was practically applied for the fluorescence intensity and lifetime determination of plasma heparin and for the selective imaging of heparan sulfate proteoglycan-expressed cells.

Bicyclo[2.2.0]hexene derivatives as a proaromatic platform for group transfer and chemical sensing

Here we report the design, preparation, synthetic utility, and sensing application of a class of proaromatic structures, namely bicyclo[2.2.0]hexene (BCH) derivatives. Building on a valence isomerism concept, they feature modular and easy synthesis as well as high thermal stability, and can be oxidatively activated under mild conditions. New alkyl transfer reactions using BCHs as a radical donor have been developed to showcase the utility of their proaromaticity. Moreover, the redox-triggered valence isomerization of a quinoline-derived BCH led to colorimetric and fluorescent responses toward vapors of electrophilic reagents in solution and solid phase, respectively. This optical response was shown to involve a 1,3-cyclohexadiene structure that possesses an intramolecular charge transfer excited state with interesting aggregation induced emission (AIE) character. Thus, the potential of BCHs has been demonstrated as a versatile platform for the development of new reagents and functional materials.

Direct observation of excited state conversion in solid state from a TICT-Type mechanochromic luminogen

A coplanar donor-acceptor (D-A) structure in single crystal is directly observed for TICT-Type DPA-PYZ, which corresponds to a coplanar localized excited (LE) state. The conversion between coplanar LE and twisted intramolecular charge transfer (TICT) excited state in the solid state can be achieved through the change of the D-A twist angle between ~0° and ~30° during grinding, heating and solvent fumigation processes. This twist angle change between donor and acceptor was proved by the Raman, FT-IR spectroscopy and X-ray crystallographic analysis as well as theoretical calculation. This research provides the clear evidence for the conversion between coplanar LE and TICT excited states of D-A molecules in solid state and contributes much to the deep understanding of stimulus-response behavior of fluorescent D-A molecular solids from the viewpoint of excited state control.

The mechanisms of a bifunctional fluorescent probe for detecting fluoride and sulfite based on excited-state intramolecular proton transfer and intramolecular charge transfer.

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F−) and sulfite (SO32–) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F− ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623 nm, which agrees with the observed experimental value (634 nm) after adding F−. Therefore, the fluorescence red-shift (from 498 to 634 nm) of HBT-FS observed in experiment after adding F− is caused by ESIPT. The SO32– ion is added to the C5 site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404 nm. The experimentally measured fluorescence at 371 nm after adding SO32– is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO32–, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371 nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F− and SO32–.

Direct Tracking Excited-State Intramolecular Charge Redistribution of Acceptor-Donor-Acceptor Molecule by Means of Femtosecond Stimulated Raman Spectroscopy.

Symmetric quadrupolar molecules generally exhibit apolar ground states and dipolar excited states in a polar environment, which is explained by the excited state evolution from initial charge delocalization over all molecules to localization on one branch of the molecules after a femtosecond pulse excitation. However, direct observation of excited-state charge redistribution (delocalization/localization) is hardly accessible. Here, the intramolecular charge delocalization/localization character of a newly synthesized acceptor-donor-acceptor molecule (ADA) has been intensively investigated by femtosecond stimulated Raman scattering (FSRS) together with femtosecond transient absorption (fs-TA) spectroscopy. By tracking the excited state Raman spectra of the specific alkynyl (-C≡C-) bonds at each branch of ADA, we found that the nature of the relaxed S1 state is strongly governed by solvent polarity: symmetric delocalized intramolecular charge transfer (ICT) characters occurred in apolar solvent, whereas the asymmetric localized ICT characters appeared in polar solvent because of solvation. The solvation dynamics of ADA extracted from fs-TA is consistent with the time constants obtained by FSRS, but the FSRS clearly tracks the excited state intramolecular charge transfer delocalization/localization.

Carbazole-diazafluorene bipolar fluorophores: Synthesis, thermal stability, optical and electrochemical properties

Two bipolar fluorophores, namely CEDF and CPDF containing carbazole moiety as electron donor and 4,5-diazafluorene unit as electron acceptor were synthesized and characterized. Carbazole units attached to C-2, C-7 and C-9 positions of 4,5-dizafluorene posing an ideal model of donor-acceptor systems to investigate their photo-physical properties for application in organic light emitting diode (OLED) devices. Both compounds possess high thermal stabilities (Td > 450 °C) and high glass transition temperature (Tg), 182 °C and 180 °C for CEDF and CPDF respectively. These compounds revealed solvent polarity dependent emission spectra which confirm the excited state undergoes intramolecular charge-transfer (ICT) process in the molecule. Solid state emission is observed in the blue region at 447 nm for both the compounds. CPDF exhibit higher photoluminescence quantum yield (0.76) than CEDF (0.55) featuring its highly emissive ICT excited state. Using the oxidation and reduction potentials determined from cyclic voltammetry analysis, estimates of both the ionization potentials and electron affinity of the compounds are obtained under the same experimental conditions. HOMO and LUMO energy levels, singlet (ES) and triplet (ET) energy levels of the compounds were obtained through density functional theory (DFT) and time-dependent DFT calculations. A transient photoluminescence curve with a prompt fluorescent component gives an average lifetime of about 5.7 ns for both the compounds. These studies and the convenient synthetic methodologies starting from the 4,5-diazafluorene, make these molecules attractive opto-electronic materials for their application in OLED devices.

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.

Tuning relaxation pathways and aggregate-state fluorescence properties of 2-(2-hydroxyphenyl)benzothiazole derivatives by switching position of electron-withdrawing substituent

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) can occur either concertedly or sequentially, or even independently, thus harnessing the steady-state fluorescence features of a molecule. In this study, two 4-nitrostyryl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) isomers (o–NO2–HBT and m–NO2–HBT) are rationally designed, in which the 4-nitrostyryl group is located ortho and meso, respectively, to the phenolic hydroxyl. Unlike the para-isomer (p–NO2–HBT), which exhibits dual emission [solvatochromicenol-emission (EICT emission) and non-solvatochromic keto-emission (K-emission) based on a sequential ESICT-ESIPT process and aggregation-induced emission enhancement (AIEE) property, o–NO2–HBT shows only non-solvatochromic K-emission and anti-aggregation-caused quenching (ACQ), while m–NO2–HBT demonstrates EICT emission and ACQ. Theoretical calculations reveal that the substituent position and the resultant dipole moment induce differences in the relative energies of EICT* and K* and in the energy barriers for the forward and reverse ESIPT processes. Furthermore, based on the elucidated emission mechanisms, a red-emitting ESIPT-active AIEgen (o–CN–CF3-HBT), with a solid-state fluorescence efficacy of 41.0%, is strategically designed and successfully applied in lipid droplet imaging. Therefore, the results of this study provide important insights into designing high-performance AIEgens for future applications in various cutting-edge research areas.

Excited state dynamics of 7-deazaguanosine and guanosine 5'-monophosphate.

Minor structural modifications to the DNA and RNA nucleobases have a significant effect on their excited state dynamics and electronic relaxation pathways. In this study, the excited state dynamics of 7-deazaguanosine and guanosine 5'-monophosphate are investigated in aqueous solution and in a mixture of methanol and water using femtosecond broadband transient absorption spectroscopy following excitation at 267 nm. The transient spectra are collected using photon densities that ensure no parasitic multiphoton-induced signal from solvated electrons. The data can be fit satisfactorily using a two- or three-component kinetic model. By analyzing the results from steady-state, time-resolved, computational calculations, and the methanol-water mixture, the following general relaxation mechanism is proposed for both molecules, Lb → La → 1πσ*(ICT) → S0, where the 1πσ*(ICT) stands for an intramolecular charge transfer excited singlet state with significant πσ* character. In general, longer lifetimes for internal conversion are obtained for 7-deazaguanosine compared to guanosine 5'-monophosphate. Internal conversion of the 1πσ*(ICT) state to the ground state occurs on a similar time scale of a few picoseconds in both molecules. Collectively, the results demonstrate that substitution of a single nitrogen atom for a methine (C-H) group at position seven of the guanine moiety stabilizes the 1ππ* Lb and La states and alters the topology of their potential energy surfaces in such a way that the relaxation dynamics in 7-deazaguanosine are slowed down compared to those in guanosine 5'-monophosphate but not for the internal conversion of 1πσ*(ICT) state to the ground state.

Proton transfer inhibited charge transfer in a coumarinyl chalcone: Hassle free detection of chloroform vapor in alcohol medium and in neat solution

The photophysical aspects of a synthesized coumarinyl chalcone derivative 3-((2E, 4E)-5-(4-(dimethylamino) phenyl) penta-2, 4-dienoyl)-4-hydroxy-2H-chromen-2-one (DPPHC) were explored. DPPHC shows excited state intramolecular proton transfer (ESIPT) suppressed excited state intramolecular charge transfer (ESICT) as evidenced from steady state and time resolved spectroscopic analysis. Interestingly, DPPHC behaves as a strong red emitter solely in chloroform and dichloromethane semi-polar solvents exclusively. Using this property, on-spot detection of these two solvents was achieved in paper strips coated with DPPHC as well as in spiked alcohol samples by emission ratiometry change.

Adjusting the photophysical properties of AIE-active TADF emitters from through-bond to through-space charge transfer for high-performance solution-processed OLEDs

Intramolecular through-space charge transfer (TSCT) excited state has been exploited for developing thermally activated delayed fluorescence (TADF) emitters, while the adjustment of excited state dynamics by conformational engineering is still rare. Herein, we designed a series of AIE-active TADF emitters, bearing triphenylamine as donor and phenyl ketone as acceptor unit, and the evolution of charge transfer from through-bond to through-space is realized by controlling the relative position of the donor and acceptor units. Upon transforming the charge transfer categories from through-bond to through-space, the reverse intersystem crossing (RISC) rate is progressively improved by virtue of the narrower energy split between singlet and triplet states (ΔΕST), meanwhile, the restricted intramolecular motion suppresses the non-radiative decay effectively, resulting in obviously improved luminescence efficiency. Appointing a bipolar blue TADF material as host to sensitize these emitters, solution-processed organic light-emitting diodes (OLEDs) based on TSCT emitter 1TCPM-Cz achieve maximum current efficiency (CE) of 35.5 cd A−1 and external quantum efficiency (EQE) of 13.3%, respectively, which is higher than that of through-bond charge transfer (TBCT) analogues. The work demonstrates a convenient design strategy to achieve high-performance TSCT emitters by regulating the conformation of the donor and acceptor.

Dansyl Emits from a PICT Excited State.

Two derivatives of dansyl (1-dimethylamino-5-naphthalenesulfonyl) in which the amino group is constrained in a ring are prepared as neopentyl esters. Their photophysical behavior is compared with that of the dansyl ester. The solvatochromism and quantum yields are similar for all three. Since the two constrained derivatives cannot twist about the amino group, they must emit from a planar intramolecular charge-transfer excited state. The similar photophysical behavior suggests that dansyl also emits from a PICT excited state instead of a twisted intramolecular charge transfer state.

Counterion Migration Driven by Light-Induced Intramolecular Charge Transfer.

A series of D−π–A+ pyridinium compounds, in which D = −NPh2 and A+ = −PyMe+ are linked by various amounts of linear phenyl spacers, were strategically designed and synthesized. Their characterization revealed the presence of excited-state intramolecular charge transfer (ESICT) that triggers a corresponding response from the counterion. In medium and strong polar solvents, the fast solvent relaxation occurring after ESICT overwhelms the counterion effect, showing typical emission solvatochromism. In weakly polar solvents, ESICT induces counteranion migration for electrostatic stabilization, the time scale of which is dependent on the radius of the counteranion, the length of the π-linker, and the viscosity of the solvent. In low-viscosity organic solvents such as toluene, counteranion migration occurs within several tens to hundreds of picoseconds, resulting in a time-dependent continuous emission that can be resolved from the spectral temporal evolution. Concrete evidence for this is provided by the chemical synthesis of a D−π–A+ pyridinium–sulfur trioxide– zwitterion, where anion migration is restricted due to its internally locked ion pair. As a result, only a single emission band can be observed. These comprehensive studies prove that the ion migration process may be significant for a wide range of ESICT-type ionic fluorophores. Such an ionic movement, triggered by optically pumped ESICT of the D−π–A+ dyad, is similar to the molecular machine driven by the redox reaction, but with a facile access and fast response.

Deferasirox (ExJade): An FDA-Approved AIEgen Platform with Unique Photophysical Properties.

Deferasirox, ExJade, is an FDA-approved iron chelator used for the treatment of iron overload. In this work, we report several fluorescent deferasirox derivatives that display unique photophysical properties, i.e., aggregation-induced emission (AIE), excited state intramolecular proton transfer, charge transfer, and through-bond and through-space conjugation characteristics in aqueous media. Functionalization of the phenol units on the deferasirox scaffold afforded the fluorescent responsive pro-chelator ExPhos, which enabled the detection of the disease-based biomarker alkaline phosphatase (ALP). The diagnostic potential of these deferasirox derivatives was supported by bacterial biofilm studies.

Improving the fluorescence brightness of distyryl Bodipys by inhibiting the twisted intramolecular charge transfer excited state.

A new class of NIR distyryl Bodipy fluorescent dyes were developed with sulfone- and quaternary ammonium-modified piperidines as auxochromes instead of conventional dialkylamino auxochromes. Such modification markedly improved the fluorescence quantum yields due to the efficient inhibition of the twisted intramolecular charge transfer (TICT) state. Based on the dye platform, we developed a new fluorescent H2O2 probe via self-immolative chemistry, and confirmed its capability to sensitively and selectively sense H2O2in vitro and in vivo.

Disentangling Multiple Effects on Excited-State Intramolecular Charge Transfer among Asymmetrical Tripartite PPI-TPA/PCz Triads.

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron-hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.

Tumor tissues diagnosis with PIEE lipid droplet vesicles

A molecule ANB (6-(4-aminostyryl)-1,3-dioxo-1H-benzo[de]isoquinolin -2(3 H)-yl)-n-butane), was developed for fluorescence turn-on imaging of cancer cells lipid droplets (LDs). ANB exercised synergistically aggregation induced emission enhancement and photochromism, which are essential to construct a photoinduced emission enhancement (PIEE) LDs. The fluorescent nanostructures PIEE-LDs extremely improve intracellular imaging contrast with high emission intensity, high emission contrast to cytosol and high photostability. Following these unique optical properties, we statistically analyzed the differences of LDs between cancer and normal cells as well as successfully observed LDs progression and fusion in living cells over a long period. Consequently, the xenograft tumor tissues were favorably distinguished from normal based on this superior PIEE-LDs. It is known that ANB exhibited excited-state conformational conversion between the twist intramolecular charge transfer (TICT) and local excited (LE) states. Based on the results of our studies, we found that LE state became dominate and presented aggregation induced emission enhancement when ANB stay in LDs, which allows us to build a fluorescent nanovesicle model to explain the distinct fluorescence imaging of ANB in LDs. Furthermore, photochromism occurred when irradiation that could promoted emission behavior of bright nanovesicles LDs to further level, and achieve PIEE-LDs.

Pyrene appended terpyridine derivatives as electrochemiluminescence material for OLEDs: Characterization of photo-physical, thermal and electrochemical properties

Three new terpyridine appended pyrene derivatives ( Pyr-Tp , Pyr-Ph-Tp , and Pyr-Bp-Tp ) have been synthesized by condensation and Suzuki-Miyaura reaction. The photo-physical, electro-chemical and thermal properties of these compounds have been investigated in detail. DFT studies have been carried out to understand the structure-property relationships at the molecular level. Synthesized fluorophores displayed high quantum yield (70-96%) of fluorescence due to triplet to singlet intramolecular energy transfer, delayed fluorescence and positive solvatochromism indicating the presence of intra-molecular charge transfer excited state. In addition, they have high thermal decomposition temperature (T d : 390-450°C), melting temperature (T m : 180-220°C) and further with suitable oxidation and reduction potentials makes them favourable molecules for OLED fabrications.

Dual-channel ratiometric recognition of Al3+ and F- ions through an ESIPT-ESICT signalling mechanism.

An optical probe 1 has been synthesized comprising naphthalimide unit conjugated with Schiff base, exhibiting excited state intramolecular proton transfer and intramolecular charge transfer as a potential sensor for Al3+ and F- ions using standard spectroscopic techniques. The probe 1 exhibited local and charge-transfer excitation at 340nm and 460nm, respectively. On excitation at 460nm, probe 1 displayed two emission bands at 510nm and 610nm, accompanied by Stokes' shift of 50nm and 150nm, respectively. The solvatochromic effect and theoretical calculation depicted that the representative emissions resulted from the ESICT/ESIPT phenomenon. Upon addition of Al3+ ions, the charge transfer excitation at 460nm was enhanced ratiometrically to local excitation at 340nm and showed a color change from orange to yellow. Similarily, probe 1.Al3+ displayed emission enhancement at 540nm in H2O/CH3CN (1:9; v/v) and showed a color change from yellow to blue-green emission. Following the detection of Al3+ ions, hydrolysis of probe 1 to its reacting precursors was observed. The detection of Al3+ ions was also demonstrated in surfactant-containing water. The limit of detection (LOD) of probe 1 (H2O/CH3CN (1:9; v/v)) towards Al3+ ions was measured to be 3.2×10-8 M. The probe 1 displayed a ratiometric absorption response towards F- ions with a new peak at 570nm and showed a color change from orange to purple. The probe 1.F- displayed a decrease in emission at 635nm. The LOD of probe 1 (CH3CN) towards F- ions was measured to be 7.5×10-7 M.