Twisted Intramolecular Charge Transfer Fluorescence Research Articles

Rotamer-Controlled Dual Emissive α-Amino Acids.

The synthesis and photoluminescent properties of novel α-amino acids are described in which the biaryl benzotriazinone-containing chromophores were found to display dual emission fluorescence via locally excited (LE) and twisted intramolecular charge transfer (TICT) states. The intensity of each emission band could be controlled by the electronics and position of the substituents, and this led to the design of a 2-methoxyphenyl analogue that, due to twisting, displayed bright TICT fluorescence, solvatochromism, and pH sensitivity.

Substituent Effect on AIE mechanism of two coumarin derivatives: uncommon TICT fluorescence in aggregation state.

Two coumarin derivatives, 7-diethylamino-3-(4-nitrophenyl)coumarin (DNC) and 7-hydroxy-3-(4-nitrophenyl)coumarin (HNC), were synthesized via Knoevenagel condensation of salicylaldehyde derivatives with 4-nitrophenylacetonitrile and then cyclization reaction. Both of them were characterized by single-crystal X-ray diffraction. The molecules of DNC are stacked via π-π interaction, while the hydrogen bond interactions instead of π-π interaction were observed in the crystal packing of HNC. Both of DNC and HNC showed solvatochromic properties and aggregation-induced emission (AIE) activities, but the AIE characteristics of them were entirely different. HNC exhibited an AIE phenomenon as the result of the restriction of twisted intramolecular charge transfer (TICT), while DNC emitted peculiar dual fluorescence which was assigned to the emission based on the inhibition of TICT state formation and the emission from the TICT state respectively.

Study of refolding of a denatured protein and microenvironment probed through FRET to a twisted intramolecular charge transfer fluorescent biosensor molecule

In the present study, Fӧrster Resonance Energy Transfer (FRET) from Tryptophan residues (Trp) of bovine serum albumin (BSA) to a nongenotoxic and nontoxic molecule, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) resulting in fluorescence from the twisted intramolecular charge transfer (TICT) state of the latter has been explored to probe the gemini surfactant induced unfolding and β-cyclodextrin (β-CD) induced refolding of the protein. DMASBT as a potential acceptor in FRET process with its polarity-sensitive TICT fluorescence properties is found to be an efficient biosensor molecule to mark the changes in microenvironment as well as conformations of the protein. λmax of TICT fluorescence band of DMASBT in native protein at 486 nm is red-shifted to 503 nm in unfolded protein which is blue-shifted to 487 nm upon refolding of the protein. Process of BSA fluorescence quenching through FRET to DMASBT has been utilized to compare between the hydrophobic area and fractional accessibility to quencher in native and refolded protein. Fluorescence quenching data have also been used to explain the binding affinity of DMASBT with native, refolded and unfolded BSA. Study reveals the roles of spacer group of gemini surfactants, 12-4-12,2Br− and 12-8-12,2Br− on refolding and unfolding of BSA. A gemini surfactant with a longer spacer is found to be easily stripped off by β-CD resulting in better refolding of protein gaining a larger hydrophobic area. Results obtained corroborate well with the findings from other experimental measurements on circular dichroism, dynamic light scattering and Hg2+ ion-induced fluorescence quenching. An easy and efficient method to study the microenvironment, and unfolding and refolding of a protein is presented here.

Buried Liquid Interfaces as a Form of Chemistry in Confinement: The Case of 4-Dimethylaminobenzonitrile at the Silica-Aqueous Interface.

Time-resolved fluorescence emission and resonance-enhanced second harmonic generation (SHG) spectra were collected from 4-dimethylaminobenzonitrile (DMABN) adsorbed to the aqueous-silica interface in order to identify how strongly associating solvent-substrate interactions change DMABN's photoisomerization properties. In bulk polar solution, DMABN forms an excited twisted intramolecular charge-transfer (TICT) state that emits with a distinctive, solvatochromic fluorescent signature. At the silica-aqueous interface, the TICT fluorescence disappears, similar to DMABN's behavior in nonpolar environments. SHG spectra confirm that the interface is, in fact, polar, and DMABN's unusual fluorescence emission acquired from the interface is attributed to strong hydrogen bonding associations between the water molecules and the silica surface that prevent adsorbate isomerization. Additionally, SHG spectra show a strong resonance at long wavelengths that is unexpected based on bulk absorbance spectra and selection rules for nonlinear hyperpolarizabilities. Using both Zerner's INDO semiempirical and TD-DFT calculations, this spectroscopic behavior is attributed to a combination of strong electric fields present at the aqueous-silica interface and surface-induced changes to DMABN's ground-state molecular structure.

Temperature-Controlled Locally Excited and Twisted Intramolecular Charge-Transfer State-Dependent Fluorescence Switching in Triphenylamine-Benzothiazole Derivatives.

Triphenylamine–benzothiazole derivatives, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (1), N-(4-(benzo[d]thiazol-2-yl)-3-methoxyphenyl)-N-phenylbenzenamine (2), and 2-(benzo[d]thiazol-2-yl)-5-(diphenylamino)phenol (3), showed unusual temperature-controlled locally excited (LE) and twisted intramolecular charge-transfer (TICT) state fluorescence switching in polar solvents. The detailed photophysical studies (absorption, fluorescence, lifetime, and quantum yield) in various solvents confirmed polarity-dependent LE and TICT state formation and fluorescence tuning. 1 and 2 exhibited strong fluorescence with short lifetime in nonpolar solvents compared to polar solvents. 1, 2, and 3 in dimethylformamide (DMF) at room temperature showed low-energy weak TICT state fluorescence, whereas high-energy strong LE state fluorescence was observed at −196 °C. Interestingly, further increasing the temperature from 20 to 100 °C, the DMF solution of 1 and 2 exhibited rare fluorescence enhancement with a slight blue shift of λmax via activating more vibrational bands of the TICT state. Thus, 1 and 2 showed weak TICT state fluorescence at room temperature, strong LE state fluorescence at −196 °C, and activation of TICT state at 100 °C. Moreover, molecular conformation and aggregation in the solid state influenced strongly on the fluorescence properties of 1, 2, and 3. Solid-state fluorescence and pH-responsive imidazole nitrogen have been exploited for demonstrating halochromism-induced fluorescence switching. Computational studies provided further insights into the fluorescence tuning and switching. The present studies provide understanding and opportunity to make use of D–A organic molecules in the LE and TICT states for achieving fluorescence switching and tuning.

Photophysics of 2-(4'-amino-2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine and its analogues: intramolecular proton transfer versus intramolecular charge transfer.

Photophysical characteristics of 2-(4'-amino-2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (AHPIP-c) have been studied in various aprotic and protic solvents using UV-visible, steady state fluorescence and time-resolved fluorescence spectroscopic techniques. To comprehend the competition between the intramolecular charge transfer (ICT) and the excited state intramolecular proton transfer (ESIPT) processes, the photophysical properties of 2-(4'-amino-2'-methoxyphenyl)-1H-imidazo-[4,5-c]pyridine (AMPIP-c) and 2-(4'-aminophenyl)-1H-imidazo-[4,5-c]pyridine (APIP-c) were also investigated. Though APIP-c displays twisted ICT (TICT) emission in protic solvents, AHPIP-c exhibits normal and tautomer emissions in aprotic as well as in protic solvents due to ESIPT. However, the methoxy derivative, AMPIP-c, emits weak TICT fluorescence in methanol.

A twisted-intramolecular-charge-transfer (TICT) based ratiometric fluorescent thermometer with a mega-Stokes shift and a positive temperature coefficient.

The fluorescence intensity of N,N-dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline exhibits an unusual intensification with increasing temperature, by activating more vibrational bands and leading to stronger TICT emissions upon heating in dimethyl sulfoxide. Based on the different temperature dependence at various wavelengths, as shown in the TICT fluorescence spectrum, this dye can be employed to ratiometrically detect temperature.

The Role of Normal Versus Twisted Intramolecular Charge Transfer Fluorescence in Predicting the Forms of Inclusion Complexes of Ethyl-4-dialkylaminobenzoate with α-Cyclodextrin in Aqueous Solution

An evidence is introduced through the b- and the twisted intramolecular charge transfer (TICT) fluorescence of ethyl-4-(N,N-dimethylamino)benzoate (EDMAB) and ethyl-4-(N,N-diethylamino)benzoate (EDEAB), confirming the role of donor size on the formation and emission of various inclusion complexes formed between these probes and α-CD in aqueous solution. A large variation in the b-fluorescence band of EDEAB as compared to that of EDMAB and a large variation in the TICT-fluorescence band of EDMAB as compared to that EDEAB, as the concentration of α-CD is increased in their aqueous solutions are observed. These variations are supported by time resolved fluorescence (TRF) spectra, fluorescence decay lifetimes and red edge effect (REE) results.

TICT fluorescence of N-borylated 2,5-diarylpyrroles: a gear like dual motion in the excited state

A significantly red-shifted fluorescence and a high fluorescence quantum yield for the emission from the twisted intramolecular charge transfer (TICT) state are attained in new aminoboranes, N-borylated 2,5-diarylpyrroles. A gear like dual structural motion in the excited state is responsible for their large Stokes shifts.

Exploration of twisted intramolecular charge transfer fluorescence properties of trans-2-[4-(dimethylamino)styryl]benzothiazole to characterize the protein–surfactant aggregates

The characterization of aggregates of an anionic surfactant, sodium dodecyl sulphate (SDS) with bovine serum albumin (BSA) in various regions of binding isotherm of SDS to BSA with increasing concentration of the former have been done by exploring the twisted intramolecular charge transfer (TICT) fluorescence properties of a probe, trans-2-[4-(dimethylamino)styryl] benzothiazole (DMASBT). The TICT fluorescence, steady-state fluorescence anisotropy and time-resolved fluorescence of DMASBT, and the fluorescence resonance energy transfer (FRET) study reveal the characteristics of the native protein as well as the protein–surfactant aggregates viz., micropolarity, microviscosity, locations of probe, denaturation of protein in various regions of binding isotherm, and also the validation of necklace-bead model. The changes in the polarity and the viscosity of the microenvironment around the probe from one binding region of SDS to other have been reflected in the highly sensitive fluorescence properties of DMASBT. The study of FRET between the DMASBT and the tryptophan residue (Trp) of BSA has identified the locations of the probe molecule in the native protein as well as that in various BSA–SDS aggregates. The energy transfer efficiency decreases, whereas the distance between the DMASBT and the Trp residue increases with increasing concentration of SDS. The significant change in the conformations of protein molecules during the non-cooperative binding region of SDS is evidenced by the fluorescence anisotropic behavior of DMASBT in the same region.

Study on premicellar and micellar aggregates of gemini surfactants with hydroxyl substituted spacers in aqueous solution using a probe showing TICT fluorescence properties

The aggregation properties of two gemini surfactants, 12-3(OH)-12,2Br − and 12-4(OH) 2-12,2Br − with hydroxyl substituted spacer group have been studied. The changes in photophysical properties of a single probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) showing dipolar nature in its twisted intramolecular charge transfer (TICT) excited state have been exploited rather than using multiple probes to describe various properties of micellar aggregates. Formation of a number of premicellar aggregates has been demonstrated in addition to the description of the micropolarity and the microviscosity of environment using steady-state fluorescence spectroscopy and fluorescence anisotropy of DMASBT. Conductometric measurements have been carried out to determine degree of micellar ionization ( α) and to verify critical micelle concentration (CMC) values estimated by fluorescence method. Hydroxyl substituted spacer group induces the formation of premicellar aggregates. The micropolarity of environment around probe molecules increases on going from premicellar to micellar aggregates. The growth of micellar aggregates has been demonstrated by a continuous increase in the microviscosity of environment. The micropolarity of micellar environment of 12-4(OH) 2-12 is found to be less than that of 12-3(OH)-12. The microviscosity of premicellar and micellar aggregates of 12-4(OH) 2-12 are higher than that of 12-3(OH)-12. CMC increases, whereas α decreases with increasing spacer chain length as well as number of hydroxyl substitution of a spacer group.

A ratiometric TICT-type dual fluorescent sensor for an amino acid

A novel ratiometric fluorescent sensor 1 having the 4-(N,N-dimethylamino)benzoate (DMAB) group as a twisted intramolecular charge transfer (TICT) dual fluorescence site and the guanidinium group as a binding site for an amino acid at the 12- and 3-positions of cholic acid, respectively, was designed and synthesized. The sensor 1 showed characteristic dual fluorescence and it was shown that the guanidinium group in the proximity of the DMAB group influenced largely the TICT formation process in 1. Liquid/liquid extraction experiments demonstrated that the intensity ratio of the TICT fluorescence to the locally-excited (LE) fluorescence (I(TICT)/I(LE)) observed from the organic layer decreased with an increase in the concentration of N-acetyl-d-phenylalanine (AcPhe) in the water phase. Such fluorescence titration experiments afforded the apparent binding constant between 1 and AcPhe to be 7.0 x 10(5) M(-1). Temperature dependence of the fluorescence spectrum of 1 in the absence and presence of AcPhe indicated that the primary origin of the change in the I(TICT)/I(LE) value upon recognition of the amino acid was the increase in the activation energy of the TICT formation process in 1.

Characterization of guest molecule concentration dependent nanotubes of β-cyclodextrin and their secondary assembly: Study with trans-2-[4(dimethylamino)styryl]benzothiazole, a TICT-fluorescence probe

The formation of nanotubes of β-cyclodextrins (β-CD) and their secondary assembly induced by trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) as a guest molecule has been explained using twisted intramolecular charge transfer (TICT) fluorescence, steady state fluorescence anisotropy, time-correlated single-photon counting fluorescence decay of DMASBT, atomic force microscopy and also transmission electron microscopy. It has been demonstrated that although at lower concentration of β-CD, DMASBT molecules form simple adduct of 1:1 stoichiometry but they form extended nanotubes at relatively high concentration of β-CD which further lead to the secondary assembly through intertubular hydrogen bondings forming rod-like structures. The changes in fluorescence properties of DMASBT yield critical aggregation concentration (CAC) of β-CD very close to 2.5 mM. The extent of formation of these supramolecular structures is dependent on the concentration of guest molecule. It has been observed that perchlorate ion (ClO 4 −) reduces the stability of supramolecular structures at higher concentration of it but at low concentration (<7 mM) it enhances the stability by providing the anchor sites for intermolecular hydrogen bonding between neighboring β-CD molecules. The polarity and viscosity dependent TICT fluorescence characteristics of DMASBT are exploited to determine the micropolarity and microviscosity of environment around it inside the nanotubes.

Excited state isomerization and effect of viscosity- and temperature-dependent torsional relaxation on TICT fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole

Effect of viscosity and temperature on twisted intramolecular charge transfer (TICT) fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) have been studied. TICT fluorescence quantum yield in glycerol solution is found to be ∼23-fold greater than that in ethylacetate as a non-viscous solvent. For high-viscosity solvent, the fluorescence quantum yield increases at low temperature due to the decrease in free-volume of the solvent, which favors the decrease in torsional relaxation of the molecule that induces the radiationless decay. However, the free-volume concept is not meaningful at a temperature much above the glass transition temperature when the free-volume is highly abundant. The temperature-dependent phenomenon provides a more accurate description compared to the viscosity-dependent study in a series of solvents of varying viscosity at a given temperature. The stabilization of the TICT state, as a consequence of the restricted motion of the N(CH 3) 2 group in DMASBT, results in a large Stokes shifted fluorescence band. The TICT fluorescence characteristics of DMASBT are found to be different from that of molecular rotors. In solvents of low polarity, where TICT is practically zero, the molecule exhibits excited state temperature-induced cis– trans isomerization showing fluorescence emissions from both the isomers. However, temperature-induced TICT fluorescence quenching is observed in polar viscous medium without any isomerization. Results indicate that DMASBT can be a potential microsensor for biomimicking as well as real biological systems.

Electronic Structure Characteristics of ESIPT and TICT Fluorescence Emissions and Calculations of Emitting Energies

A scheme of time-dependent density functional theory (TDDFT) combined with the single-excitation configuration interaction (CIS) approach was employed to investigate the first excited singlet state ( S 1) for eight salicylanilide derivatives and analogues, which have similar structural formulas. The results showed that fluorescence-emitting mechanisms of these molecules were in two distinct manners (excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT)), which agreed with the well-known experiments. For ESIPT compounds with inconspicuous charge transfer (CT) during electron transition, pure functionals without Hartree-Fock (HF) exchange energy, such as OLYP and BLYP, were suitable to calculate emitting energies. For TICT compounds with large CT during electron transition, hybrid functionals with about 37% HF exchange energy, such as mPW1B95 and MPW1K, performed well. On condition that the exchange-correlation (XC) functionals were chosen properly according to the rules above, reliable emitting energies for salicylanilide derivatives and analogues could be obtained at the TDDFT/6-31G( d)//CIS/3-21G( d) level. The average accuracy reached about 0.2 eV. For the salicylanilides with both proton transfer (PT) and CT reaction channels, only one channel occurred actually according to the principle of energy minimum. This actual reaction decided proper XC functionals, whereas the reaction that did not occur actually was trivial. Eight appendent compounds were calculated to prove that this successful scheme is expected to be suitable for other ESIPT and TICT compounds.

Photophysical characterization and effect of pH on the twisted intramolecular charge transfer fluorescence of trans-2-[4-(dimethylamino)styryl]benzothiazole

Experimental and theoretical photophysical characterization of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) has been carried out in solutions of different polarities. Steady state fluorescence emission and excitation spectra of DMASBT in the solvents of varying polarities suggest the existence of dual emitting states where locally excited state is responsible for fluorescence in nonpolar solvents; however, in polar solvents fluorescence is from twisted intramolecular charge transfer (TICT) state. Solvatochromic comparison method suggests that the dipolar interactions make the major contribution to the stability of ground as well as excited states with more effect on the latter. Semi-empirical AM1 singly excited configuration interaction (AM1-SCI) calculations support the formation of the TICT state of DMASBT. The twist of the –N(CH 3) 2 group and the change in its hybridization in the excited state develops a high dipole moment in the S 3 state and thereby stabilizing it to give the TICT fluorescence in polar solvents. The TICT fluorescence of DMASBT decreases above the pH value of 3.7 suggesting that hydrogen bonding ability of water molecules becomes efficient above this pH. Results indicate that DMASBT can be a potential sensor based on its remarkably high fluorescence sensitivity towards polarity, viscosity and pH of the medium.

Effects of cyclodextrins and saccharides on dual fluorescence of N,N-dimethyl-4-aminophenylboronic acid in water

N,N-Dimethyl-4-aminophenylboronic acid (DMAPB) showed pH-dependent dual fluorescence at 360 and 462 nm originating from locally excited (LE) and twisted intramolecular charge transfer (TICT) states, respectively, in aqueous solutions. Upon complexation with α-CD, LE fluorescence was markedly increased while TICT fluorescence was decreased. In contrast, both LE and TICT fluorescence were increased when DMAPB was complexed with β-CD. The fluorescence variations enabled us to determine the 1:1 and 1:2 binding constants of the DMAPB/α-CD complex to be 10 and 40 M−1, respectively, and the 1:1 binding constant of the DMAPB/β-CD complex to be 635 M−1. The dual fluorescence of DMAPB alone was found to be a good indicator of saccharide sensing. Under weakly alkaline conditions, saccharides suppressed TICT fluorescence while increasing LE fluorescence. Among the saccharides investigated, d-fructose induced the largest fluorescence change, followed by d-ribose and d-glucose. This order is consistent with the stability of the boronate esters of DMAPB with saccharides. In the presence of β-CD, saccharide selectivity was unchanged, while fluorescence was amplified. These results demonstrate the superiority of the supramolecular DMAPB/β-CD complex to DMAPB alone as a ratiometric fluorescence sensor for saccharides in water.

Electronic spectroscopy and photophysics of 2-(N-methyl-N-isopropylamino)-5-cyanopyridine and related compoundsDedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

In polar solvents, dual fluorescence is observed for three electron donor–acceptor molecules, consisting of the m-cyanopyridine electron acceptor and one of three slightly different N,N-dialkylamino donor groups: N-methyl-N-isopropyl, N,N-diethyl, and N,N-dimethyl. A low energy “anomalous” emission is assigned to a twisted intramolecular charge transfer (TICT) state. Kinetic and steric factors account for variations of the TICT vs. primary emission intensity ratio in the three compounds. Comparison with the “TICT paradigm”, N,N-dimethylaminobenzonitrile is also provided. In alcohol solutions, relative enhancement of the TICT fluorescence, as well as a fast radiationless process from the initially excited state are observed. Possible sources of these phenomena are discussed.

Fluorescent Cyclodextrin/Peptide Hybrids with a Novel Guest-Responsive Chemosensor in the Peptide Side Chain

Peptides bearing β-cyclodextrin and 4-(N,N-dimethylamino)benzoyl (DMAB) units in the peptide side chains were prepared as chemosensors for molecule detection. The DMAB unit was expected to be included into the cyclodextrin cavity intramolecularly. However, these peptides exhibited no twisted intramolecular charge transfer fluorescence and the normal fluorescence intensity decreased upon the addition of 1-adamantanol as an exogenous guest, indicating that the DMAB units are shallowly included in the cyclodextrin cavities.

Photophysical studies of substituted 1,2-diarylethenes: twisted intramolecular charge transfer fluorescence in dimethoxycyano-substituted 1,2-diarylethene

1,2-Diarylethenes, namely (E)-1-(4-cyanophenyl)-2-phenylethene (1), (E)-1-(4-methoxyphenyl)-2-phenylethene (2), (E)-1-(3,4-dimethoxyphenyl)-2-phenylethene (3), (E)-1-(4-cyanophenyl)-2-(4-methoxyphenyl)ethene (4) and (E)-1-(4-cyanophenyl)-2-(3,4-dimethoxyphenyl)ethene (5), have been synthesized and their absorption and fluorescence properties at room temperature in different organic solvents and also in 1,4-dioxane–water binary mixtures have been investigated. Additionally, the fluorescence of these compounds has been examined at 77 K in an ethanol–methanol (1∶1 v/v) matrix. Photophysical parameters like absorption, excitation and fluorescence spectra, fluorescence quantum yields, excited state dipole moment changes, and correlation of solvatochromic fluorescence with solvent parameters like ET(30)-values and the π*-scale have been made. Compound 5, with one cyano and two methoxy substituents, has been found to exhibit solvent polarity-dependent dual fluorescence. The shorter wavelength fluorescence is attributed to an initially excited delocalized planar state, while the longer wavelength fluorescence is attributed to a non-planar twisted intramolecular charge transfer excited state.