铜离子配位调控的TICT荧光化合物的双重荧光发射

设计合成了2个分子内扭转电荷转移（TICT）荧光体（1和2）,铜离子的配位作用可开关其双重荧光发射。2在乙腈/水溶液（1：1,V/V）中的双重荧光发射随着铜离子的加入以类似电子转移机制“开-关”形式猝灭,而1在铜离子与其计量比为1之内的乙腈/水溶液（1：1,V/V）中,其双重荧光发射随着铜离子的加入逐渐猝灭;在计量比之后其长波长的TICT荧光发射随着铜离子的加入逐渐增强。即1的TICT荧光发射以“开-关-开”的机制被铜离子诱导。同时还获得了铜离子与1形成的配合物的晶体结构以及配合物的荧光性质。¹H NMR波谱滴定实验表明荧光体的电荷转移程度是影响TICT发射的主要因素,1是一个新的且其TICT发射可以被铜离子调制为“开-关-开”的TICT荧光体。     

Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes

Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an ‘off-on’ photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H⁺ and 2/H⁺), the fluorescing species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation process.     

Radiative decay channel assessment to understand the sensing mechanism of a fluorescent turn-on Al3+ chemosensor

The turn-on luminescent chemosensor [2-Hydroxy-1-naphthaldehyde-(2-pyridyl) hydrazone] (L), selective to Al³⁺ ions, was studied by means of density functional theory (DFT) and time-dependent-DFT quantum mechanics calculations. The UV-Vis absorption and the radiative channel from the adiabatic S₁ excited state were assessed in order to elucidate the selective sensing mechanism of L to Al³⁺ ions. We found that twisted intramolecular charge transfer (TICT) and photoelectron transfer (PET), which alter the emissive state, are responsible for the luminescence quenching in L. After coordination with Al³⁺, the TICT is blocked, and PET is no longer possible. So, the emission of the coordination complex is activated, and a fluorescence effect enhanced by chelation is observed. For compounds with Zn²⁺ and Cd²⁺, the luminescence quenching is caused by PET, while for Ni²⁺, ligand to metal charge transfer is the prominent mechanism. To go into more detail, the metal-ligand interaction was analyzed via the Morokuma-Ziegler energy decomposition scheme and the natural orbital of chemical valence.     

Twisted internal charge transfer fluorescence of para-N,N-dimethylaminobenzonitrile in rigid matrix at room temperature

The dual fluorescence of para-N,N-dimethylaminobenzonitrile is observed for the first time in a rigid matrix at room temperature (polyvinylalcohol). A relationship between the twisted internal charge transfer (TICT) fluorescence and the formation of a ground-state hydrogen bonded complex is shown.     

Restriction of Photoinduced Twisted Intramolecular Charge Transfer

An intensive investigation of structure–property relationships in the aggregation‐induced enhanced emission (AIEE) of luminescent compounds is essential for the rational design of highly emissive solid‐state materials. In the AIEE‐active compounds N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]isophthalamide and N,N′‐bis[3‐hydroxy‐4‐(2′‐benzothiazolyl)phenyl]‐5‐tert‐butylisophthalamide, fast photoinduced twisted intramolecular charge transfer (TICT) of the enol excited state is found to be mainly responsible for the weak emission of their dilute solutions. The photoinduced TICT enol excited state is formed with a greatly distorted configuration, due to the large rotation about the C? N single bond. This facilitates nonradiative TICT decay from the normal enol excited state to the highly twisted enol excited state, rather than proton‐transfer decay to the keto excited state. In aggregates, photoinduced nonradiative deactivation of TICT is strongly prohibited, so that excited‐state intramolecular proton transfer (ESIPT) becomes the dominant decay, and hence contributes greatly to the subsequent emission enhancement of the keto form. Molecular design and investigation of analogous single‐armed compounds further verifies this kind of AIEE mechanism.     

A dual role of phenylboronic acid as a receptor for carbohydrates as well as a quencher for neighboring pyrene fluorophore

A simple amino acid based compound (1) containing a phenyl boronic group and pyrene fluorophore showed an enhanced fluorescence in aqueous solutions at physiological pH through suppression of the photoinduced electron transfer from pyrene to boronic acid on carbohydrate binding. The compound exhibited an interesting fluorescence change depending on pH with decreased emission intensity at acidic pH but enhanced emission intensity at basic pH unlike the fluorescent carbohydrate chemosensors using a PET process with amine and aryl-boronic acid. We have characterized a dual role of phenylboronic acid as a receptor for carbohydrates as well as a quencher for the fluorescence of pyrene fluorophore.     

Solvent interactions in the excited state of non-planar donor—acceptor molecules

In order to probe solute—solvent interactions in the intramolecular charge transfer (TICT) state of 2,6,N,N-tetramethylcyanoaniline (TMCA) fluorescence spectra, quantum yields and lifetimes of this compound were measured in binary mixtures of n-hexane with chlorinated hydrocarbons. An isoemissive point in the fluorescence spectra was observed as the composition of the solvent was varied. Evidence for both clustering of polar solvent molecules around the excited solute and for non-specific long-range interactions with the polar bulk of the solution was found. The properties of the solvent affect primarily the efficiency of non-radiative deactivation. Fluorescence quenching by saturated amines gives good evidence that TMCA emission arises from a highly polar state in both polar and non-polar solutions.     

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.     

A luminescence molecular switch via modulation of PET and ICT processes in DCM system

A novel versatile dicyanomethylene-4H-pyran (DCM) based derivative bearing ferrocenyl group (DCM-N-Fc) is designed as modulator to construct “off-on” logic operation. The optical properties of DCM-N-Fc are characterized by absorption and steady-state fluorescence technique, showing that the fluorescence from DCM chromophore via intramolecular charge transfer (ICT) is strongly quenched by photoinduced electron transfer (PET) process from ferrocene moiety. In contrast with the references (DCM-N and DCM-Fc), the fluorescence of DCM-N-Fc can be triggered by oxidizing ferrocenyl unit either chemically or electrochemically, exhibiting a characteristic emission modulation at around 610 nm with an electrofluorochromic behavior. Furthermore, the free energy and the fluorescence lifetime in the PET path verify the thermodynamic feasibility. Cyclic voltammetry, absorption spectroscopy, time-resolved fluorescence as well as DFT calculation have been used to elaborate the manipulation via both PET and ICT processes.     

Synthesis and properties of covalently linked thiaporphyrin-ferrocene conjugates

Four examples of ferrocene-thiaporphyrin conjugates in which the ferrocenyl group was covalently connected either directly at meso-position of thiaporphyrin or to meso-phenyl group of thiaporphyrin via ethyne bridge were prepared by coupling bromo- or iodo thiaporphyrin with α-ethynylferrocene under mild Pd(0) coupling conditions. NMR, absorption and electrochemical studies indicated that the thiaporphyrin and ferrocenyl units interact strongly in ethyne bridged porphyrin-ferrocene conjugates but the interaction is very weak in phenyl ethyne bridged porphyrin-ferrocene conjugates. The steady state fluorescence studies indicated that the fluorescence yields are reduced to 50% in phenyl ethyne conjugates but the fluorescence is completely quenched in ethyne bridged conjugates. The partial or complete quenching of porphyrin fluorescence in these conjugates is due to electron transfer from ferrocene unit to excited state of porphyrin sub-unit. Oxidation of ferrocene to ferrocenium ion with an oxidizing agent in ethyne bridged conjugates resulted in a recovery of porphyrin fluorescence.     

Photoinduced single- versus double-bond torsion in donor-acceptor-substituted trans-stilbenes

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), and fluorescence lifetimes of trans-4-(N-arylamino)-4'-cyanostilbenes (2H, 2Me, 2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4'-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4'-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4'-nitrostilbene (3MP), and three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) < 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) for judging the importance of a TICT state for trans-stilbenes is provided. Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4'-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS).     

Thermodynamics and Conformations in the Formation of Excited States and Their Interconversions for Twisted Donor‐Substituted Tridurylboranes

We synthesized a series of donor‐substituted tridurylboranes containing different types and number of chromophores including 1‐pyrene (PB1–3), 3‐carbazole (CBC1–3), or substituted p‐carbazol‐N‐phenyl (CBN3a–c) as various donor–acceptor (D–A) molecules. The photophysical and electrochemical properties of these twisted D–A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge‐separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge‐transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited‐state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited‐state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes.     

Twisting dynamics in the excited singlet state of Michler's ketone

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.     

Hypochlorous acid turn-on boron dipyrromethene probe based on oxidation of methyl phenyl sulfide

A boron dipyrromethene (BODIPY)-based fluorometric probe, HCS, has been successfully developed for the highly sensitive and selective detection of hypochlorous acid (HOCl). The probe is based on the specific HOCl-promoted oxidation of methyl phenyl sulfide. The reaction is accompanied by a 160-fold increase in the fluorescent quantum yield (from 0.003 to 0.480). The fluorescent turn-on mechanism is accomplished by suppression of photoinduced electron transfer (PET) from the methyl phenyl sulfide group to BODIPY. The fluorescence intensity of the reaction between HOCl and HCS shows a good linearity in the HOCl concentration range 1–10 μM. The detection limit is 23.7 nM (S/N = 3). In addition, confocal fluorescence microscopy imaging using RAW264.7 macrophages demonstrates that the HCS probe could be an efficient fluorescent detector for HOCl in living cells.     

Review: fluorescent cyclodextrins for molecule sensing

Cyclodextrins (CDs), which are spectroscopically inert, were converted into fluorescent CDs by modification with one or two fluorophores. Many fluorescent CDs changed the fluorescent intensities upon addition of guest compounds, causing the locational change of the fluorophore mostly from inside to outside of the CD cavities. On this basis, the fluorescent CDs were used as fluorescent chemosensors for molecule recognition. Modified CDs bearing two naphthalene or pyrene moieties exhibit intramolecular excimer emission and their guest-responsive excimer intensity variations were used for molecule sensing. Fluorescent CDs bearing a dansyl moiety decreased the fluorescence intensity upon guest addition, reflecting the environmental change around the fluorophore from the hydrophobic interior of the CD cavities to bulk water solution. Modified CDs bearing a p-N,N-dimethylaminobenzoyl (DMAB) moiety exhibit dual emissions from nonpolar planar (NP) and twisted intramolecular charge transfer (TICT) excited states, and the TICT emission intensity was useful for sensing molecules. A biotin-bound DMAB system was also constructed, and the presence of the protein (avidin) was found to enhance the NP fluorescence. This avidin-bound DMAB system showed higher sensitivities and stronger binding ability for guest species than the system without avidin.     

Substituent-dependent photoinduced intramolecular charge transfer in N-aryl-substituted trans-4-aminostilbenes

The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes.     

On twisted intramolecular charge transfer (TICT) states in N-aryl carbazoles

By comparing fluorescence behaviour of carbazole derivatives carb-N-R with R = phenyl (CB). 4-cyanophenyl (CBN). 1-naphthyl (CIN) and 9′-phenanthryl (C9P) theoretical presuppositions for efficient TICT state population are exemplified. While CB fluoresces from a locally excited state, CBN, CIN and C9P exhibit exclusively TICT fluorescence in polar solvents, with an unusually lugh quantum yield.     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Novel chemosensor for alkaline earth metal ion based on 9-anthryl aromatic amide using a naphthalene as a TICT control site and intramolecular energy transfer donor

Novel fluorescent chemosensors 1 and 2 with two different fluorophores (naphthalene and anthracene) at the both ends of polyether was synthesized. These compounds based on 9-anthryl aromatic amide adopted a naphthalene as a TICT controller and an intramolecular energy transfer source. Compound 1 shows high fluorescence efficiency upon complexation with metal ion, and the fluorescence efficiency of 2 is regulated by metal ionic size.     

Spectral Properties and Solubilization Location of 2'-Ethylhexyl 4-(N,N-Dimethylamino)benzoate in Micelles

Dual fluorescence and UV absorption of 2'-ethylhexyl 4-(N,N-dimethylamino)benzoate (EHDMAB) were investigated in cationic, non-ionic and anionic micelles. When EHDMAB was solubilized in different micelles, the UV absorption of EHDMAB was enhanced. Twisted intramolecular charge transfer (TICT) emission with longer wavelength was observed in ionic micelles, whereas TICT emission with shorter wavelength was obtained in non-ionic micelles. In particular, dual fluorescence of EHDMAB was significantly quenched by the positively charged pyridinium ions arranged in the Stern layer of cationic micelles. UV radiation absorbed mainly decays via TICT emission and radiationless deactivation. The dimethylamino group of EHDMAB experiences different polar environments in ionic and non-ionic micelles according to the polarity dependence of TICT emission of EHDMAB in organic solvents. In terms of the molecular structures and sizes of EHDMAB and surfactants, each individual EHDMAB molecule should be buried in micelles with its dimethylamino group toward the polar head groups of different micelles and with its 2'-ethylhexyl chain toward the hydrophobic micellar core. Dynamic fluorescence quenching measurements of EHDMAB provide further support for the location of EHDMAB in different micelles.