Photoinduced intramolecular charge transfer in 9-aminoacridinium derivatives assisted by intramolecular H-bond

Fluorochromic dyes derived from 9-aminoacridinium containing a vinylene function with electron withdrawing groups such as diethyl [(acridinium-9-ylamino)methylene]malonate (I), ethyl [(acridinium-9-ylamino)methylene]cyanoacetate (II), [(acridinium-9-ylamino)methylene]malononitrile (III), are prepared and studied in their monoprotonated form. Absorption spectra of the new dyes are red shifted compared to that of the precursor dye. The observed dual fluorescence and multiexponential decay are ascribed to normal emission from the acridinium chromophore in addition to excited-state intramolecular charge transfer (ESICT) process. However, biexponential decay character is observed only for the dicyano derivative (compound III), whereas for the two other systems, more complex kinetics and a three-component decay is recovered. The analysis of the fluorescence decays in different solvents for the first two compounds reveals two short-lived components in the range of 160-350 ps and 1.1-3.0 ns, related to formation and decay of the ESICT state, plus a third one with decay time of about 9 ns, which is ascribed to the normal emission from the acridinium chromophore as an enol tautomer or as an intramolecular H-bond conformer (closed form tautomer). For the dicyano derivative, in which the absence of carbonyl group precludes the H-bond interaction, the biexponential fitting reveals a slightly fast formation rate of the ESICT state with values on the order of 10(10) s(-1), whereas its decay time is between 0.6 and 3.2 ns, depending on the solvent used.     

Excited-state intramolecular proton transfer in 2,4,5-triarylimidazole molecules

Excited-state intramolecular proton transfer (ESIPT) in the 2,4,5-triarylimidazole molecules was studied by spectral-luminescent technique. For 4,5-diphenyl-(2-hydroxyphenyl)imidazoles, the ESIPT occurs in both liquid and glassy matrices at 77 K. For 4,5-diphenyl-(2-hydroxynaphthyl)imidazole, the ESIPT requires rotation of molecular fragments and is not observed at 77 K.     

Excited-state intramolecular proton transfer in hydroxyoxime-based chemical sensors

The electronic structure of a series of β-hydroxy-oximes, with different aromatic cores (naphthalene, pyrene, coumarin, pyridine) between the oxime and the hydroxyl groups, has been investigated by time-dependent density functional theory (TDDFT) and of the naphthalene-based oxime, in addition, by resolution-of-identity second-order perturbative coupled cluster (RICC2) calculations with basis sets up to augmented triple-ζ quality. The particular systems have been proposed as fluorescent sensors of organophosphorus (OP) nerve agents, with enhancement of fluorescence accompanying the sensing of OP agents. It is found that the experimentally observed fluorescence quenching of the oxime sensors in their initial form can be attributed to intramolecular proton transfer upon excitation from the β-hydroxyl group to the nitrogen atom, thus forming a weakly emitting hydroxylaminoquinoid.     

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones

Nonradiative decay pathways associated with vibronically coupled S₁(ππ*)–S₂(nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck–Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S₁(ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S₂(nπ*) within a few femtoseconds while the other decays to S₁ minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S₂(nπ*), competing with the traditional pathway occurring on S₁(ππ*).     

Piezofluorochromism of triphenylamine-based triphenylacrylonitrile derivative with intramolecular charge transfer and aggregation-induced emission characteristics

New triphenylacrylonitrile derivative (DPPA) with triphenylamine moiety as electron donor group has been synthesized. Its emission wavelengths were strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. It was found that DPPA was almost non-emissive in tetrahydrofuran (THF), and the emission could be intensified obviously when a great amount of H₂O was added, illustrating aggregation induced emission property. It is interesting that the pristine crystal of DPPA emitted green light, and the ground made its emitting color to change into orange, which could be recovered under fumed with dichloromethane (DCM) vapor. The piezofluorochromic behavior might be switched reversibly under the stimulus of external force because of crystalline-amorphous phase transformation, which could be confirmed from the results of PXRD patterns and DSC curves in different solid states. This work might open a new strategy to provide a broad perspective for the development of AIE PFC materials.     

Excited-state intramolecular proton transfer distinguishes microenvironments in single- and double-stranded DNA

Herein, the efficient interaction of an environment-sensitive fluorophore that undergoes excited-state intramolecular proton transfer (ESIPT) with DNA has been realized by conjugation of a 3-hydroxychromone (3HC) with polycationic spermine. On binding to a double-stranded DNA (dsDNA), the ratio of the two emission bands of the 3HC conjugates changes up to 16-fold, so that emission of the ESIPT product increases dramatically. This suggests an efficient screening of the 3HC fluorophore from the water molecules in the DNA complex, which is probably realized by its intercalation into dsDNA. In sharp contrast, the 3HC conjugates show only moderate changes in the dual emission on binding to a single-stranded DNA (ssDNA), indicating a much higher fluorophore exposure to water at the binding site. Thus, the 3-hydroxychromone fluorophore being conjugated to spermine discriminates the binding of this polycation to dsDNA from that to ssDNA. Consequently, ESIPT-based dyes are promising for monitoring the interaction of polycationic molecules with DNA and probing the microenvironment of their DNA binding sites.     

Conformationally controlled intramolecular charge transfer complexes

Trans-1-acceptor-2-donor-substituted cyclohexanes (1), as well as their 4- (or 5-)methyl-substituted homologues (2), have been prepared and are shown to form intramolecular charge-transfer (donor-acceptor) complexes. These weak complexes are turned on and off by the chair-chair interconversion of the cyclohexane ring. The CT absorptions have been measured and the equilibrium constants for the ring reversal have been determined by UV/vis spectroscopy at 298 K, as well as by NMR spectroscopy at two temperatures: at 183 K, by direct comparison of signals due to the two chair conformations, and at 300 K, by comparison of calculated and measured widths of the alpha-proton signals. The Gibbs free energies assigned to the donor-acceptor interactions range between 0 and -1 kcal mol(-1). A crystal structure of one of the complexes (1b) confirms the intramolecular donor-acceptor alignment and interaction. The regioisomers of the methyl-substituted complexes were characterized by NOE interaction between the methyl and an alpha-proton cis to it.     

Excited-state intramolecular proton transfer in five-membered hydrogen-bonding systems: 2-pyridyl pyrazoles

The excited-state intramolecular proton transfer (ESIPT) reaction in five-membered N-H...N hydrogen-bonding systems has been explored through design and syntheses of a series of 5-(2-pyridyl) 1-H-pyrazoles 1a-d. The ESIPT mechanism was confirmed through spectroscopy, relaxation dynamics, and corresponding methylated analogues. The results demonstrate for the first time a unique system among ESIPT molecules, in which ESIPT incorporates an appreciably large energy barrier fine-tuned by the skeletal reorganization. This makes 1a-d systems ideal models for probing the reaction potential energy surface.     

Excited-state intramolecular proton transfer in o-hydroxybiaryls: a new route to dihydroaromatic compounds

An excited-state intramolecular proton transfer (ESIPT) from the phenol OH to the 7'-carbon on the naphthyl ring in o-(1-naphthyl)phenol (3) and 1-(1'-naphthyl)-2-naphthol (4) leads to efficient (Phi = 0.1-0.2) formation of the corresponding dihydrobenzoxanthenes (5 and 7) via quinone methide intermediates. This new reaction represents a clean, efficient, and high-yielding route to benzoxanthenes and dihydrobenzoxanthenes. A related ESIPT of similar efficiency has been detected at the 2'-aromatic position in these systems, by deuterium labeling studies.     

Highly twisted triarylamines for photoinduced intramolecular charge transfer

9-(N,N-Dianisylamino)anthracene (9DAAA), 9-(N,N-dianisylamino)dinaphth([1,2-a:2'-1'-j]-anthracene (9DAAH), and 9,10-bis(N,N-dianisylamino)anthracene (910BAA) were synthesized as highly twisted triarylamines with potential for photoexcited internal charge transfer. Crystallography of 9DAAA shows its dianisylamino group to be twisted nearly perpendicular to its anthracene unit, similar to a report for 910BAA. The solution fluorescence spectra show strong bathochromic shifts for each of the three molecular systems with strongly decreased quantum efficiency in higher polarity solvents. Solution-phase (ensemble) time-resolved photoluminescence measurements show up to 4-fold decreases in fluorescence lifetime in acetonitrile compared to hexane. The combined results are consistent with photoinduced, transient intramolecular charge-transfer from the bis-anisylamine unit to the polycyclic aromatic unit. Computational modeling is in accord with intramolecular transfer of electron density from the bis-anisylamino unit to the anthracene, based on in comparisons of HOMO and LUMO.     

Guest-responsive intramolecular charge transfer of viologen-crown ether conjugate

Viologen-benzocrown ether conjugate (1) was prepared, in which a phenyl unit in benzocrown ether was incorporated into the nitrogen atom of a bypiridinium unit through a sigma bond. 1 showed a yellow color associated with an intramolecular charge transfer (CT) that responds to alkali and alkaline earth metal ions, especially to Ca²⁺.     

电化学方法用于分子内电荷转移特性的研究

设计合成一系列具有不同取代基的苯甲酰萘（苯）胺衍生和，测定它们在非极性溶剂环己烷中的荧光发射光谱，发现系列合成产物具有双重荧光，其长滤发射具有电荷转移特性，其长波发射态能量与不同取代基的苯甲酰萘（苯）胺衍生物的失（得）电子能力，即给（受）体的氧化还原电位之间符合Weller方程。     

分子内扭转电荷转移的胶束效应

研究了水溶液中对二甲氨基苯甲(DMABA)分子内扭转电荷转移(TICT)的胶束效应.胶束能促进DMABA的TICT过程,并导致DMABA的总荧光量子产率提高和TICT荧光峰的显著蓝移,DMABA的TICT荧光强度与正常荧光带的强度之比与DMABA的浓度之间有线性关系,胶束存在时该直线的斜率提高,认为DMABA的TICT激发态涉及两个DMABA分子,并且影响DMABA的TICT激发态相对布居的主要因素是其与相应的三重态和基态的能隙.讨论了DMABA的TICT激发态的可能结构及胶束效应的本质.     

Excited-state intramolecular proton transfer molecules bearing o-hydroxy analogues of green fluorescent protein chromophore

o-Hydroxy analogues, 1a-g, of the green fluorescent protein chromophore have been synthesized. Their structures and electronic properties were investigated by X-ray single-crystal analyses, electrochemistry, and luminescence properties. In solid and nonpolar solvents 1a-g exist mainly as Z conformers that possess a seven-membered-ring hydrogen bond and undergo excited-state intramolecular proton transfer (ESIPT) reactions, resulting in a proton-transfer tautomer emission. Fluorescence upconversion dynamics have revealed a coherent type of ESIPT, followed by a fast vibrational/solvent relaxation (<1 ps) to a twisted (regarding exo-C(5)-C(4)-C(3) bonds) conformation, from which a fast population decay of a few to several tens of picoseconds was resolved in cyclohexane. Accordingly, the proton-transfer tautomer emission intensity is moderate (0.08 in 1e) to weak (～10(-4) in 1a) in cyclohexane. The stronger intramolecular hydrogen bonding in 1g suppresses the rotation of the aryl-alkene bond, resulting in a high yield of tautomer emission (Φ(f) ≈ 0.2). In the solid state, due to the inhibition of exo-C(5)-C(4)-C(3) rotation, intense tautomer emission with a quantum yield of 0.1-0.9 was obtained for 1a-g. Depending on the electronic donor or acceptor strength of the substituent in either the HOMO or LUMO site, a broad tuning range of the emission from 560 (1g) to 670 nm (1a) has been achieved.     

Excited-state charge coupled proton transfer reaction via the dipolar functionality of salicylideneaniline

Based on design and synthesis of salicylideneaniline derivatives(1a-1d),we demonstrate a prototypical system to investigate the excited-state intramolecular charge transfer(ESICT) coupled excited-state intramolecular proton transfer(ESIPT) reaction via the dipolar functionality of the molecular framework. In solid and aprotic solvents 1a-1d exist mainly as E conformers that possess an intramolecular sixmembered -ring hydrogen bond.Compounds 1a-1c exhibit a unique proton-transfer tautomer emission, while compound 1d exhibits remarkable dual emission due to the different solvent-polarity environment between ESICT and ESIPT states.Time-dependent density functional theory(TDDFT) calculations are reported on these Schiff bases in order to rationalize their electronic structure and absorption spectra.     

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.     

Kinetics of intramolecular charge transfer with N-phenylpyrrole in alkyl cyanides

For the electron acceptor/donor molecule N-phenylpyrrole (PP), the fast intramolecular charge transfer (ICT) reaction accompanied by dual fluorescence from a locally excited (LE) and an ICT state is investigated in alkyl cyanide solvents as a function of temperature. After a comparison of the X-ray crystal structure of PP with calculations from the literature, absorption and fluorescence spectra of PP in a series of solvents over a wide polarity range are discussed. ICT with PP strongly depends on solvent polarity and starts to appear in solvents more polar than diethyl ether. From an analysis of the ICT/LE fluorescence quantum yield ratio Phi'(ICT)/Phi(LE), approximate data for the change in enthalpy -DeltaH of the ICT reaction of PP are obtained, ranging from 9 kJ/mol in acetonitrile (MeCN) to 4 kJ/mol in n-butyl cyanide (BuCN). From ICT and LE fluorescence decays of PP measured as a function of temperature, the forward (Ea = 9 kJ/mol in ethyl cyanide (EtCN) and 6 kJ/mol in MeCN) and backward (Ed = 16 kJ/mol in EtCN and MeCN) ICT reaction barriers are determined. From these data, -Delta H (7 kJ/mol (EtCN); 10 kJ/mol (MeCN)) is calculated, in good agreement with the results coming from Phi'(ICT)/Phi(LE). The data for Ea show that the forward ICT barrier becomes smaller with increasing solvent polarity, whereas the absence of change for Ed comes from the compensating increase of -DeltaH. Both observations are indicative of a late transition state for the LE --> ICT reaction. For PP in EtCN and MeCN, the ICT radiative rate constant k'(f)(ICT) increases with temperature. This is caused by the ICT low transition dipole moment and hence does not contain information on the molecular structure (twisted or planar) of the ICT state. The fast ICT observed with PP supports our previous conclusion, based on a comparison of PP with its planarized derivative fluorazene, that the pyrrole and phenyl moieties in the ICT state of PP are coplanar and possess substantial electronic coupling.