Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Molecular Orientation and Structural Characterization of Ultrathin Films of C12AzoNaph(1,4)C6N-SDS Studied by FT-IR and NIR-SERS Spectroscopies

IntroductionAzobenzene- containing long- chain fatty acidsand their ammonium amphiphiles have recentlyaroused a great interest of some researchersbecause of their promising photochromicproperties[1— 6 ] . In order to understand theinteresting physical properties the LB films withazo chromophores show,a structure- functionrelationship of the films must be explored.Thusfar,infrared spectroscopy has been usedextensively for the investigations of molecularaggregation,orientation and structuralch…     

An alternate proton acceptor for excited-state proton transfer in green fluorescent protein: rewiring GFP

The neutral form of the chromophore in wild-type green fluorescent protein (wtGFP) undergoes excited-state proton transfer (ESPT) upon excitation, resulting in characteristic green (508 nm) fluorescence. This ESPT reaction involves a proton relay from the phenol hydroxyl of the chromophore to the ionized side chain of E222, and results in formation of the anionic chromophore in a protein environment optimized for the neutral species (the I* state). Reorientation or replacement of E222, as occurs in the S65T and E222Q GFP mutants, disables the ESPT reaction and results in loss of green emission following excitation of the neutral chromophore. Previously, it has been shown that the introduction of a second mutation (H148D) into S65T GFP allows the recovery of green emission, implying that ESPT is again possible. A similar recovery of green fluorescence is also observed for the E222Q/H148D mutant, suggesting that D148 is the proton acceptor for the ESPT reaction in both double mutants. The mechanism of fluorescence emission following excitation of the neutral chromophore in S65T/H148D and E222Q/H148D has been explored through the use of steady state and ultrafast time-resolved fluorescence and vibrational spectroscopy. The data are contrasted with those of the single mutant S65T GFP. Time-resolved fluorescence studies indicate very rapid (< 1 ps) formation of I* in the double mutants, followed by vibrational cooling on the picosecond time scale. The time-resolved IR difference spectra are markedly different to those of wtGFP or its anionic mutants. In particular, no spectral signatures are apparent in the picosecond IR difference spectra that would correspond to alteration in the ionization state of D148, leading to the proposal that a low-barrier hydrogen bond (LBHB) is present between the phenol hydroxyl of the chromophore and the side chain of D148, with different potential energy surfaces for the ground and excited states. This model is consistent with recent high-resolution structural data in which the distance between the donor and acceptor oxygen atoms is < or = 2.4 A. Importantly, these studies indicate that the hydrogen-bond network in wtGFP can be replaced by a single residue, an observation which, when fully explored, will add to our understanding of the various requirements for proton-transfer reactions within proteins.     

Studies on molecular structure of Schiff base derivatives in LB films

The present paper investigated the molecular configurations of the two novel amphiphiles with Schiff base moiety as headgroup in LB films. FTIR-ATR spectra and UV-vis electronic absorption spectra revealed the different isomers between the two amphiphiles LB film. Since z.sbnd;OH groups situate in different position of the aromic rings in Schiff base amphiphiles, the proton transfer by different ways leads to different isomers, which were reflected by monolayer and LB films behavior.     

Luminescent Langmuir-Blodgett films of platinum(II) complex [Pt(L18)Cl](PF6) (L18 = 2,6-bis(1-octadecylbenzimidazol-2-yl)pyridine)

A novel amphiphilic Pt complex containing 2,6-bis(1-octadecylbenzimidazol-2-yl)pyridine (L18), [Pt(L18)Cl](PF6), has been synthesized. The complex exhibits concentration-dependent absorption and emission spectra in solution. With increasing the concentration of the Pt complex, we observed a new absorption band centered at 550 nm derived from a metal-metal d sigma* to ligand pi* charge transfer (MMLCT) transition and the corresponding broad emission centered at 650 nm. The Pt complex is surface-active, and the surface pressure-area isotherm reveals three phase transitions. The three phases correspond to one liquid-expanding phase and two solid-condensed phases, respectively, with different intermolecular overlap in the "flat-on" orientation at the air-water interface. Without additives such as fatty acids, the complex forms a stable and reproducible Langmuir-Blodgett (LB) multilayer film above a surface pressure of 15 mN m-1. Strong emission from the LB films, even monolayer, was observed. Comparing the relative emission intensity of the MMLCT band for transferred LB monolayer film with that for cast films, we concluded that Pt-Pt interactions are suppressed in the LB film. Instead, the emission at 600 nm arising from the ligand-ligand pi-pi interacted excited state became dominant. The results would provide the insight into the control of molecular ordering for planar Pt complexes from the viewpoint of characteristic excited states.     

Excited-state dynamics of phenol-pyridinium biaryl

The excited-state dynamics of a donor-acceptor phenol-pyridinium biaryl cation was investigated in various solvents by femtosecond transient absorption spectroscopy and temperature dependent steady-state emission measurements. After excitation to a near-planar Franck-Condon delocalized excited S(1)(DE) state with mesomeric character, three fast relaxation processes are well resolved: solvation, intramolecular rearrangement leading to a twisted charge-shift (CSh) S(1) state with localized character, and excited-state proton transfer (ESPT) to the solvent leading to the phenoxide-pyridinium zwitterion. The proton transfer kinetics depends on the proton accepting character of the solvent whereas the interring torsional kinetics depends on the solvent polarity and viscosity. In nitriles, ESPT does not occur and interring twisting arises with no significant intrinsic barrier, but still slower than solvation. The CSh state is notably fluorescent. In alcohols and water, ESPT is faster than the solvation and DE → CSh relaxation processes and yields the zwitterion hot ground state, which strongly quenches the fluorescence. In THF, solvation and interring twisting occur first, leading to the fully relaxed, weakly fluorescent CSh state, followed by slow ESPT towards the zwitterion. At low temperature (77 K), the large viscous barrier of the solvent inhibits the torsional relaxation but ESPT still arises to some extent. Strong emission from the DE geometry and planar zwitterion is thus observed. Finally, quantum chemical calculations were performed on the ground and excited state of model phenol-pyridinium and phenoxide-pyridinium compounds. Strong S(1) state energy stabilization is predicted upon twisting in both cases, consistent with a fast relaxation towards the perpendicular geometry. A substantial S(0)-S(1) energy gap is still present for the twisted cationic species, which can explain the long-lived emission of the CSh state in nitriles. A quite different situation arises with the zwitterion for which the S(0)-S(1) energy gap predicted at the twisted geometry is very small. This suggests a close-lying conical intersection and can account for the strong fluorescence quenching observed in solvents where the zwitterion is produced by ESPT.     

A theoretical prediction about harnessing ESPT process for HBO derivatives

The different excited-state behaviors involved in excited-state proton transfer (ESPT) process of a series of 2-(2-hydroxyphenyl)benzoxazole (HBO) derivatives have been theoretically investigated. The primary bond lengths and bond angles were analyzed. Coupling with the infrared (IR) vibrational spectra, we confirmed that the intramolecular hydrogen bond O–H···N should be strengthened in the S₁ state, which might provide the possibility for ESPT reaction, whereas introducing the fused rings may weaken the hydrogen bond in excited state. By investigating the vertical excitation process, the charge redistribution was explored. It is found that the electron-accepting –NO₂ and –COOH would facilitate the ESPT reaction. With adding fused rings to HBO, less charge transfer exists in the transition process, which can reasonably explain the weakening hydrogen bond phenomenon in excited states. Via constructing the potential energy curves of both S₀ and S₁ states, we further confirm that electron-accepting substitutions could promote the ESPT process for HBO systems. And fused rings do inhibit ESPT reaction to a great extent. We believe this work not only elaborates the different excited-state proton transfer behaviors for a series of HBO derivatives but also presents a new harnessing ESPT process through substitutional effects.     

Fluorescence characteristics of protonated form of 6-hydroxyquinoline in Nafion film

Fluorescence characteristics of 6-hydroxyquinoline (6-HQ) have been studied at room temperature in Nafion(R) film by steady state and nano-second time-resolved fluorescence spectroscopy. The fluorescence spectrum exhibits single emission band corresponding to the protonated form of 6-HQ in this matrix. However, the decay fits with two or three exponential functions depending on the emission wavelength monitored. At blue edge of the emission, the decay fits to three-exponential function, whereas at longer wavelengths, the decay fits to bi-exponential function. Two tentative mechanisms have been proposed to explain the experimental data, viz. a closely lying charge transfer state (CT) or an excited state proton transfer (ESPT) process. The photophysical parameters appear to be sensitive to the change in microstructure due to swelling of the membrane by the solvents.     

A theoretical investigation about the excited state behavior for 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole: The water‐assisted excited state proton transfer process

In this work, based on density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods, we theoretically investigate the excited‐state process of the 2‐(6'‐hydroxy‐2'‐pyridyl)benzimidazole (2HPB) system in acetonitrile and water solvents. Since acetonitrile is an aprotic solvent, it has no effect on the solvent‐assisted excited‐state proton transfer (ESPT) process. Therefore, the 2HPB molecule cannot transfer the proton in acetonitrile, which is consistent with previous experimental observation. On the other hand, 2HPB can combine one water molecule (which is a protic solvent), forming the 2HPB–H₂O complex in the S₀ state. After photoexcitation, the intermolecular hydrogen bonds O1 H2···O3 and O3 H4···N5 both get strengthened in the S₁ state, which leads to the possibility of a water‐assisted ESPT process. Further, the charge redistribution reveals the tendency of ESPT. By exploring the potential energy curves for the 2HPB–H₂O complex in water, we confirm that a stepwise double proton transfer process occurs in the S₁ state. Water‐assisted ESIPT can occur along O1 H2···O3 or O3 H4···N5 because of their similar potential barriers. Based on the stepwise ESPT mechanism, we reinterpret the absorption and fluorescence spectra mentioned in the experiments and confirm the rationality of the water‐assisted ESPT process.     

Regular two-dimensional molecular array of photoluminescent Anderson-type polyoxometalate constructed by Langmuir-Blodgett technique

A regular two-dimensional photoluminescent array of Anderson-type polyoxometalates (POMs) was constructed as built-up Langmuir-Blodgett (LB) films. LB films of hexatungstoantimonate (SbW(6)) and -manganate (MnW(6)) were successfully fabricated by using dimethyldioctadecylammonium (DODA) as cationic partner, while hexamolybdochromate (CrMo(6)) was unsuccessful. Specular X-ray reflectivity (SXR) and grazing incidence X-ray diffraction (GIXD) measurements revealed that both SbW(6)/DODA and MnW(6)/DODA LB films exhibited well-ordered layers consisting of periodic arrangement of the planar-structured Anderson-type molecules. Surprising periodicity was observed in SbW(6)/DODA LB film, in which the distance between SbW(6) and DODA layers was 4.40 nm, and SbW(6) anions would form a two-dimensional square lattice with a length of 1.4 nm. SbW(6)/DODA LB films exhibited photoluminescence at 77 K, while emission spectra were observed at room temperature for SbW(6) solid.     

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited‐state proton transfer (ESPT) can be used to probe protein binding sites. For 2‐(2′‐hydroxy‐4′‐aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active‐site conformation. The emission properties are tuned by hydrogen‐bonding interactions of HABT with the neighboring amino acid T766 and with active‐site water. The use of ESPT tuners opens the possibility of creating two‐color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.     

Embedding Photoacids into Polymer Opal Structures: Synergistic Effects on Optical and Stimuli-Responsive Features

Opal films with their vivid structural colors represent a field of tremendous interest and obtained materials offer the possibility for many applications, such as optical sensors or anti-counterfeiting materials. A convenient method for the generation of opal structures relies on the tailored design of core-interlayer-shell (CIS) particles. Within the present study, elastomeric opal films were combined with stimuli-responsive photoacids to further influence the optical properties of structurally colored materials. Starting from cross-linked polystyrene (PS) core particles featuring a hydroxy-rich and polar soft shell, opal films were prepared by application of the melt-shear organization technique. The photoacid tris(2,2,2-trifluoroethyl) 8-hydroxypyrene-1,3,6-trisulfonate (TFEHTS) could be conveniently incorporated during freeze-drying the particle dispersion and prior to the melt-shear organization. Furthermore, the polar opal matrix featuring hydroxylic moieties enabled excited-state proton transfer (ESPT), which is proved by spectroscopic evaluation. Finally, the influence of the photoacid on the optical properties of the 3-dimensional colloidal crystals were investigated within different experimental conditions. The angle dependence of the emission spectra unambiguously shows the selective suppression of the photoacid’s fluorescence in its deprotonated state.     

Excited-state proton transfer via hydrogen-bonded acetic acid (AcOH) wire for 6-hydroxyquinoline

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogen-bonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N- group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N- group in 6HQ upon excitation to the S(1) state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogen-bonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N- group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.     

具有不同取代链长的卟啉衍生物LB膜的结构研究

本文研究了三种羧酸取代的四苯基卟啉衍生物在空气/Cd^2^+水溶液界面上所形成的单层膜及LB膜。这三种卟啉衍生物中, 一种没有脂链, 另外两种具有不同长度的脂链。由π-A等温线得到的平均表观分子面积相差很大。紫外-可见光谱表明, LB膜中卟啉的Soret吸收带相对于溶液的吸收均红移, 但红移程度不同。LB膜的偏振紫外-可见光谱表明, LB膜中三种卟啉衍生物的卟啉环具有基本一致的取向。运用亚相降低法得到了三种卟啉衍生物单层LB膜, 其紫外-可见光谱与用垂直提拉法得到的LB膜的紫外-可见光谱具有一致的特征。这些结果表明: 卟啉衍生物有无取代链及取代链长的不同对平均表观分子面积的大小和膜中环间的距离有影响, 但对环的取向没有影响。环的取向由环本身及环上的亲水取代基来确定。气/液界面上三种卟啉衍生物的单层膜中环也具有一致的取向, 且与LB膜中环的取向相差不大。提拉不会对膜中环的取向及膜的结构造成大的改变。     

ESPT of 2-(2'-pyridyl)benzimidazole at the Micelle-Water interface: selective enhancement and slow dynamics with sodium dodecyl sulfate

The effect of micellar environment on the excited state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) has been investigated by steady state and time resolved fluorescence spectroscopy. The ESPT, which occurs to a rather small extent at pH 7, is found to be enhanced remarkably at the interface of sodium dodecyl sulfate (SDS) micelles and water. Such an enhancement is not observed for the cationic cetyl trimethyl ammonium bromide (CTAB) or neutral Triton X-100 micelles. This selective enhancement is explained in the light of a modification of pK(a) and a more acidic local pH in the micelle-water interface. A rise time of about 890 ps is observed in the region of tautomer emission. The origin of this rise time is explored, considering three factors, namely, diffusion controlled protonation of the normal form of 2PBI, slow and possibly incomplete solvation of the transition state, leading to a slowing down of the proton transfer process and a similar slow dynamics of the tautomeric excited state.     

强绿荧光铽－异丁酰基吡唑啉酮配合物的合成、荧光性能及LB膜

强绿荧光铽－异丁酰基吡唑啉酮配合物的合成、荧光性能及ＬＢ膜李琴，周德建，姚光庆，黄春辉Ｓ．ＵｍｅｔａｎｉＭ，Ｍａｔｓｕｉ（北京大学稀土材料化学及应用国家重点实验室，北京，１００８７１）（日本京都大学化学研究所）关键词铽，１－苯基－３－甲基－４－异丁酰...     

Charge Transfer in 1,8-Naphthalimide: A Combined Theoretical and Experimental Approach

Photophysics of 1,8-naphthalimide (NAPMD) in different solvents has been delineated in this paper. Theoretically calculated bond distance of N–H and C=O groups rule out any intramolecular proton transfer in the excited state. Concomitant increase in negative charge on O atom compared to N atom and dipole moment hints at possible intramolecular charge transfer. Progressive redshift with polarity of solvents in emission and absorption spectra also confirms the theoretical prediction. Weakening of N–H bond helps hydrogen abstraction and anion formation in water with decay time of 2.54 ns through intermolecular proton transfer. This was corroborated from the ground state photoexcitation of laboratory synthesized anion of NAPMD. Amide hydrolysis in higher pH and excess proton availability at low pH are responsible for anion emission quenching. A possible electron transfer diminishes phosphorescence at 77 K with changing pH.     

Ultrafast excited-state intermolecular proton transfer of cyanine fluorochrome dyes

Steady-state and time-resolved emission spectroscopy techniques were employed to study the excited-state proton transfer (ESPT) to water and D(2)O from QCy7, a recently synthesized near-infrared (NIR)-emissive dye with a fluorescence band maximum at 700 nm. We found that the ESPT rate constant, k(PT), of QCy7 excited from its protonated form, ROH, is ~1.5 × 10(12) s(-1). This is the highest ever reported value in the literature thus far, and it is comparable to the reciprocal of the longest solvation dynamics time component in water, τ(S) = 0.8 ps. We found a kinetic isotope effect (KIE) on the ESPT rate of ~1.7. This value is lower than that of weaker photoacids, which usually have KIE value of ~3, but comparable to the KIE on proton diffusion in water of ~1.45, for which the average time of proton transfer between adjacent water molecules is similar to that of QCy7.     

Role of solvation dynamics in excited state proton transfer of 1-naphthol in nanoscopic water clusters formed in a hydrophobic solvent

Excited state proton transfer (ESPT) in biologically relevant organic molecules in aqueous environments following photoexcitation is very crucial as the reorganization of polar solvents (solvation) in the locally excited (LE) state of the organic molecule plays an important role in the overall rate of the ESPT process. A clear evolution of the two photoinduced dynamics in a model ESPT probe 1-naphthol (NpOH) upon ultrafast photoexcitation is the motive of the present study. Herein, the detailed kinetics of the ESPT reaction of NpOH in water clusters formed in hydrophobic solvent are investigated. Distinct values of time constants associated with proton transfer and solvent relaxation have been achieved through picosecond-resolved fluorescence measurements. We have also used a model solvation probe Coumarin 500 (C500) to investigate the dynamics of solvation in the same environmental condition. The temperature dependent picosecond-resolved measurement of ESPT of NpOH and the dynamics of solvation from C500 identify the magnitude of intermolecular hydrogen bonding energy in the water cluster associated with the ultrafast ESPT process.     

Surface enhanced fluorescence and Raman imaging of Langmuir-Blodgett azopolymer films

The spectroscopic properties and surface-enhanced spectra of Langmuir-Blodgett (LB) films of methacrylic homopolymer (HPDR13) are presented. It is shown that LB film displays strong fluorescence attributed to the spatial restrictions imposed by its structure. The emission is observed in conjunction with photoisomerization, a process clearly demonstrated by the formation of surface-relief gratings in the LB film [C.R. Mendon?a et al., Macromolecules 32 (1999) 1493]. Surface-enhanced Raman scattering (SERS), Surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF) were observed for LB films of HPDR13 deposited onto silver island films. SERS measurements were also carried out on a sample fabricated with one monolayer LB film deposited onto silver islands followed by one overlayer of silver (LB sandwiched between two layers of silver islands). The polymer interacts very weakly with the metal surface (physisorption), and the enhancement effect is determined by the local electric field enhancement. The strong SERS and SERRS signals were suitable for micro-Raman imaging. Line, area mapping and global images of the LB monolayer on silver island are reported. The transfer ratio in the fabrication of the LB suggests a homogeneous coating of the silver islands, thereby the chemical images show the variation of the SERS intensity due to surface enhancement.