Dye-laser pulse shortening by transient absorption following excited-state intramolecular proton transfer

Molecules undergoing excited-state intramolecular proton transfer often show transient absorption after pulsed optical excitation, due to the tautomeric form in the singlet ground state. This absorption may be used to suppress, after a short time, the laser action of dyes emitting in the transient absorption band. The method is designed for high photochemical stability. Lasing from PBBO, pumped 10 times above threshold by an excimer pumped PTP dye laser, was suppressed after 3.3ns by the addition of 2-(2-hydroxyphenyl)-benzoxazole, which absorbed 30% of the pump energy. At higher concentrations, intense and stable 80ps PBBO laser pulses were obtained. The pulse evolution is simulated by model calculations. A hypothetical super-dye, consisting of chemically linked laser- and absorber-moities, is also discussed. Here Förster energy transfer should result in particularly efficient laser pulse shortening.     

Experimental test of a four-level kinetic model for excited-state intramolecular proton transfer dye lasers

The nanosecond pulses of a dye laser oscillator based on the excited-state intramolecular proton-transfer reaction (IPT) of salicylamide and 2-hydroxylphenyl benzimidazole dyes have been studied as a function of several experimental parameters. To explain the operation of this laser a numerical four-level kinetic model was developed until the lasing properties of these dyes, in the presence of a variable oxygen concentration and pumped with a double pulse technique, could be reproduced. This was possible only by assuming that the efficiency of the laser is controlled by the absorption cross-section of a transient state with a lifetime in the nanosecond-picosecond range, which was tentatively identified as a ground state tautomeric species.     

Theoretical insights into excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[a,h]phenazine

1,8-Dihydroxydibenzo[a,h]phenazine (DHBP) is a new synthetic compound possessing two intramolecular hydrogen bonds; however, it has been found to exhibit the excited-state intramolecular single proton transfer (ESSPT) behaviour, in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred (E), the single-proton-transferred (SK) and the double-proton-transferred (DK) forms. These studies suggest that the E form is the global minimum in the S₀ state, while the SK form is the most stable in the S₁ state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. The present results lend a good support to the experimental finding of single proton transfer (SPT).     

Investigation of excited-state proton transfer in 3-hydroxyflavone molecules

The dual fluorescence spectra of 3-hydroxyflavone molecules excited by electromagnetic radiation in the region of the S ₁ and S ₂ absorption bands in the temperature region of 20–80°C are studied using the dynamic quenching of the excited state. An analysis of the fluorescence parameters shows that heating the solution from room temperature to 60°C increases the proton transfer rate by a factor of 1.24 in the case of standard excitation into the main absorption band and even stronger (by a factor of 6.9) in the case of excitation into the second absorption band. The presence of a quencher reduces the yield of the two emission bands and noticeably increases the proton transfer rate, by a factor of 1.16 at room temperature and by a factor of 1.25 at 80°C. Upon excitation into the second singlet band, the transfer rate increases even more (especially at higher temperatures), by a factors of 1.24 and 3.5 for the same temperatures. The temperature dependences of the transfer rate constant allowed us to estimate the activation energies of the proton transfer reaction under different physical conditions and reach conclusions about the mechanism by which this reaction proceeds. It is found that the proton transfer activation energy decreases from 500 to 360 cm⁻¹ when measured in temperature ranges of 20–40 and 20–60°C. The introduction of a quencher with a concentration of 5 × 10⁻³ M increases the activation barrier to 534 and 471 cm⁻¹ in the same temperature ranges.     

1-Hydroxy-2-naphthaldehyde: A prospective excited-state intramolecular proton transfer (ESIPT) probe with multi-faceted applications

The present account aims at amassing and recounting on our series of spectroscopic studies with a potential excited state intramolecular proton transfer (ESIPT) probe 1-hydroxy-2-naphthaldehyde (HN12). After a detailed investigation from experimental as well as theoretical viewpoints, a deeper insight into the photophysics of the selected molecular system is provided from thorough spectral deciphering of the effects of solvent, medium pH and temperature. In the following sections, the ESIPT emission of HN12 has been documented to be a potential avenue wherefrom characterization of a wide variety of biological, biomimetic and supramolecular assemblies has been executed to commendable fruition. Efforts are also invested to delineate the location, distribution and strength of interaction of the probe with various microheterogeneous environments.     

Mathematical modeling of photoisomerization with intramolecular proton transfer

A system of equations describing time changes in the matrix elements of the density operator of a seven-level model of a molecule interacting with a light pulse taking into account spontaneous (including collective) decays of molecule excited states is suggested. Model parameters were selected to allow us to perform modeling of the photoisomerization of a molecule with two isomeric states with different stable proton positions on an intramolecular H-bond by numerically solving the suggested system of equations for density operator matrix elements. An analysis of the characteristic time dependences of the population of states of the model under consideration showed that proton phototransfer in the collective decay of various isomeric states of a molecule in an excited electronic state can be one of effective mechanisms of the photoisomerization of molecules whose structure is described by the model.     

Excited state intramolecular proton transfer mechanism of o-hydroxynaphthyl phenanthroimidazole

By utilizing the density functional theory(DFT) and the time-dependent density functional theory(TDDFT), the excited state intramolecular proton transfer(ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole(HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S_1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S_0 and S_1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S_1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves(PECs) calculated by the B3 LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S_1 state is smaller than in the S_0 state and the reverse proton transfer process in the S_1 state is more difficult than in the S_0 state, the ESIPT occurs in the S_1 state.     

Direct photoexcitation of intramolecular proton transfer states in 3-hydroxyflavone

It is shown that the photoexcitation of 3-hydroxyflavone at the red edge of its absorption spectrum can directly excite luminescing intramolecular proton-transfer forms of molecules of this compound. This offers new possibilities for studying the fundamental properties of molecular objects with intramolecular proton transfer, which are currently considered as promising multiparametric molecular sensors.     

激发态分子内质子转移有机分子HBT的三阶非线性光学特性

以532nm皮秒脉冲作抽运光,采用单光束Z-扫描技术对具有激发态分子内质子转移效应的有机分子2-(2′-羟基苯基)苯并噻唑(HBT)在其双光子吸收区的非线性光学特性进行了研究.实验结果表明,对532nm波长的光,HBT分子存在明显的双光子吸收.通过拟合开孔Z-扫描实验数据,求解了HBT分子在其双光子吸收区的非线性吸收系数,并探讨了抽运光强度对介质双光子吸收效应的影响.采用高斯分解法,通过拟合闭孔Z-扫描除以开孔Z-扫描数据,理论推导并计算了在介质对抽运光存在非线性吸收的情况下HBT分子的非线性折射率,以及不同入射光强度时HBT分子的三阶非线性极化率实部和虚部的值.计算结果表明,理论分析与实验结果较好地符合,这些结果为进一步研究和开发此类材料的应用提供了理论与实验依据.     

Photodynamics of intramolecular proton transfer in polar and nonpolar biflavonoid solutions

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within ??₁ = 0.6 ps. At the second stage, the second proton is transferred within ??₂ = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, ?? _f = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm^?1. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with ?? _max ^f = 460 nm and TTP phototautomers at ?? _max ^f = 610 nm.     

Manifestation of intramolecular proton transfer in imidazole in the electronic-vibrational spectrum

Electronic-vibrational spectra of both imidazole (I) and the intermediate molecular structure (II) in the intramolecular proton transfer process N₁H(I) → N₃H(III) have been calculated and analyzed theoretically. The geometries of the molecular structures of I and II in the first ππ* excited state were determined using semi-empirical correlations and the method of hybridized atomic orbitals. The difference in their spectra indicates that the intramolecular proton-transfer mechanism with imidazole (I ↔ III) tautomeric conversion can be identified by electronic-vibrational spectroscopy. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 164–169, March–April, 2008.     

2-Naphthol-phosphatidylethanolamine: A fluorescent phospholipid analogue for excited-state proton transfer studies in membranes

The fluorescence properties of the phospholipid derivative,N-[1-(2-naphthol)]-phosphatidylethanolamine (NAPH-PE), have been studied by steady-state and time-resolved fluorescence techniques. The new probe is a naphthol adduct of phosphatidylethanolamine. The emission spectrum of the fluorescent phospholipid depends on the pH and on the proton acceptor concentration as expected for a typical two-state excited-state proton transfer reaction. In ethanol solutions at an apparent pH of 6.7 and in the presence of acetate anion (0.14M), a biexponential decay is obtained from global analysis of the data. The lifetimes, ₁=3.9 ns and ₂=6.2 ns. are constant across the spectral region 350–460 nm. The decay-associated spectra and the species-associated spectra reproduce well the profiles reported for a two-state excited-state proton transfer reaction. The fluorescent phospholipid has been incorporated into dimyristoyllecithin and dipalmitoyllecithin vesicles. Although lower proton transfer is found, the reaction appears to be dependent on the gel-to-liquid-crystalline phase transition of the lipid membrane. In addition, the steady-state anisotropy of NAPH-PE measured as a function of temperature trace the phase transition of the two vesicle systems. Thus, it is shown that the physical state of the bilayer affects a reaction which takes place at the membrane surface. In the presence of acetate ions (0.3M), global analysis, performed in terms of fluorescence decay parameters, recovers preexponential coefficients that are consistent with an excited-state proton transfer reaction. The short lifetime drops from 3.9 to 0.44 ns without significant changes of the longer-lifetime component.     

On the possibility of measuring the temperature by the luminescence of a probe with intramolecular proton transfer

The temperature dependences of the fluorescence characteristics of three forms (normal, tautomeric, and anionic) of 3-hydroxyflavone in methanol upon selective excitation in different UV ranges are studied. The fluorescence of all the forms undergoes temperature quenching, whose efficiency depends on the excitation energy. It is found that the intensity ratio of the fluorescence of different forms also varies with temperature; the excitation regions in which these variations are most pronounced are determined. The highest temperature sensitivity is observed for the luminescence intensity ratio of the normal and tautomeric forms upon excitation in the region of 280 nm, this ratio continuously increasing from 0.5 to 1.0 in the temperature range of 20–70°C. The dependence found allows one to measure the temperature of the solution in some temperature regions with an error of ～1 ± 0.5% with the help of a standard Hitachi F-2500 spectrofluorimeter.     

Excited state intramolecular proton transfer of 1,2-dihydroxyanthraquinone by femtosecond transient absorption spectroscopy

1,2-Dihydroxyanthraquinone (alizarin) shows dual emission bands with a large Stokes shift from a “locally-excited (LE)” and “proton-transferred (PT)” tautomers in the excited state. Excited state intramolecular proton transfer (ESIPT) reaction of alizarin is tunable by changing concentration, solvent polarity, excitation wavelength, and etc. ESIPT reaction of alizarin in the excited state was investigated by steady-state absorption/emission spectroscopy and femtosecond transient absorption spectroscopy. In ethanol solution, the lifetime of PT tautomer of alizarin was measured as 87 ps, in addition to 0.35 and 8.3 ps vibrational cooling dynamics for the LE and PT tautomers of alizarin, respectively. In binary mixtures of ethanol and water, the excited state dynamics became more complicated; the LE and PT tautomers appeared to decay with 8.9 and 30.8 ps lifetimes, which is much shorter compared to the lifetime of the PT tautomer in ethanol. A long-lived nonradiative state in the excited states of alizarin was found as well, which was proposed as a “trapped” state with tightly hydrogen-bonded water molecules. The ESIPT reaction of alizarin was blocked in a 1:1 mixture of ethanol-water due to strong hydrogen bonding between water molecules and alizarin, which was further confirmed by the efficient coupling of alizarin to TiO₂ nanoparticles in the 1:1 binary mixture of ethanol-water.     

The substituent effect on the excited state intramolecular proton transfer of 3-hydroxychromone

The excited state intramolecular proton transfer of four derivatives(FM, BFM, BFBC, CCM) of 3-hydroxychromone is investigated.The geometries of different substituents are optimized to study the substituent effects on proton transfer.The mechanism of hydrogen bond enhancement is qualitatively elucidated by comparing the infrared spectra, the reduced density gradient, and the frontier molecular orbitals.The calculated electronic spectra are consistent with the experimental results.To quantify the proton transfer, the potential energy curves(PECs) of the four derivatives in S_0 and S_1 states are scanned.It is concluded that the ability of proton transfer follows the order: FM BFM BFBC CCM.     

Lead chloride-based layered perovskite incorporated with an excited state intramolecular proton transfer dye

We successfully fabricated thin films of organic–inorganic layered perovskite-type compound (AEHBA)₂PbCl₄, where AEHBA stands for N-(2-aminoethyl)-2-hydroxybenzamide, a blue fluorescent dye that shows excited state intramolecular proton transfer (ESIPT) reaction. The formation of its highly ordered structure was readily achieved by the spin-coating of N,N-dimethylformamide solution dissolving PbCl₂ and the AEHBA hydrogen chloride salt. X-ray diffraction analysis of the films revealed that organic AEHBA and inorganic PbCl₄ perovskite layers were alternately stacked parallel to the substrate. In the absorption spectrum, exciton formation within the perovskite layers was confirmed by the appearance of a characteristic sharp band, while the photoluminescence spectrum showed a large Stokes-shifted band of AEHBA, indicating that it underwent an ESIPT.     

A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

In the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ( r ) and Laplacian ?²ρ( r ) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O₁‐H₂?O₅ and O₃‐H₄?O₅) have been supported to be formed in the S₀ state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S₁ state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S₁ state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O₁‐H₂ and O₃‐H₄ coordinates demonstrate that the proton transfer process should be more likely to occur in the S₁ state via hydrogen bond wire O₁‐H₂?O₅ rather than O₃‐H₄?O₅ because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.     

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH₂) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S₀ and S₁ states were scanned with varying O-H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH₂ can undergo the ESDPT process in the S₁ state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.