Direct observation of the nuclear motion during ultrafast intramolecular proton transfer

The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(2′-hydroxyphenyl)benzothiazole, 2-(2′-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV–visible pump-probe spectroscopy with 30 fs resolution. The proton transfer is observed in real time and a characteristic ‘ringing’ of the molecule in a small number of vibrational modes is found after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor–acceptor distance and an electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the acceptor site and tunneling has not to be considered.     

The interplay of skeletal deformations and ultrafast excited-state intramolecular proton transfer: Experimental and theoretical investigation of 10-hydroxybenzo[h]quinoline

The excited-state intramolecular proton transfer in the aromatic polycycle 10-hydroxybenzo[h]quinoline is investigated by means of transient absorption experiments with 30 fs time resolution, classical dynamics and wavepacket dynamics. The experiments establish the ultrafast transfer after UV excitation and show signatures of coherent vibrational motion in the keto product. To elucidate details of the proton transfer mechanism, the classical dynamics is also performed for 2-(2′-hydroxyphenyl)benzothiazole and the results are compared. For both systems the proton transfer takes place on the ultrafast scale of 30–40 fs, with good agreement between the theoretical investigations and the measurements. The dynamics simulations show that for both molecules the proton is handed over by means of skeletal deformation of the molecule. Due to the more rigid structure of 10-hydroxybenzo[h]quinoline the hydrogen migration mode participates more actively than in 2-(2′-hydroxyphenyl)benzothiazole.     

Synthesis and reactions with <Emphasis Type="Italic">N</Emphasis>-nucleophiles of 1-(thien-2-yl)- and 1-(4-hydroxyphenyl)-3,4,4-trichloro-3-buten-1-ones

Acylation of thiophene and phenol with 3,4,4-trichloro-3-butenoyl chloride afforded the corresponding 1-(thien-2-yl)- and 1-(4-hydroxyphenyl)-3,4,4-trichloro-3-buten-1-ones, whose reaction with amines led to the formation of 3-amino-1-(thien-2-yl, 4-hydroxyphenyl)-4,4-dichloro-2-buten-1-ones The heterocyclization of the initial ketones into pyrazole structure was not observed, and the reaction with hydrazine hydrate provided bispyrazole products, N,N′-bis(5-thien-2-yl)- and N,N′-bis[5-(4-hydroxyphenyl)-1H-pyrazol-3-ylmethylene]hydrazines.     

The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine

The excited-state dynamics of the excited-state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time-dependent density functional theory. The excited-state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen-bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited-state double proton transfer (ESDPT) along the hydrogen-bonded bridge. Importantly, the previous proposed ESDPT-triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.     

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.     

Synthesis and some transformations of 2- and 2,2′-substituted bis(ethylenedioxy)biphenyls containing cyclopropane fragments

3,4: 3′,4′-Bis(ethylenedioxy)biphenyl undergoes bromination, nitration, and cyclopropylcarbonylation only at the 2-position. Analogous reactions with 2-substituted bis(ethylenedioxy)biphenyls occur regioselectively at the 2′-position. The reactions of 2-cyclopropylcarbonyl- and 2,2′-bis(cyclopropylcarbonyl)bis(ethylenedioxy)biphenyls with complex metal hydrides afforded the corresponding arylcyclopropylcarbinols which tended to undergo intramolecular alkylation of the aromatic ring with conservation of the cyclopropane fragment (monosubstituted derivatives) and formation of cyclopropyl-containing cyclic ethers (disubstituted ethylenedioxybiphenyls). The reduction of the nitro group in 2′-cyclopropylcarbonyl-2-nitro-4,5: 4′,5′-bis(ethylenedioxy)biphenyl was accompanied by intramolecular cyclization involving spatially close functional groups, the cyclopropane fragment remaining intact.     

Femtosecond dynamics of intramolecular photoinduced proton transfer in <Emphasis Type="Italic">N</Emphasis>-substituted 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones

Femtosecond dynamics of processes in the excited state of 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones has been studied by femtosecond absorption spectroscopy. The rate constants of intramolecular photoinduced proton transfer (IPPT) have been determined for the N-substituted derivatives (0.7–11 ps⁻¹). The IPPT rate constant depends on the inductive constant of the substituent and the potential barrier height, which was calculated by a quantum-chemical method (TDDFT). The multiexponential character of the kinetics of photoinduced absorption by the compounds with a low inductive constant of the N-substituent in the spectral region of the S₁ → S_N absorption and the stimulated emission of the IPPT product is explained by the rapid (∼10 ps⁻¹) relaxation process preceding the IPPT.     

Unexpected formation of 6,7-dihydrobenzo[4′,5′]imidazo-[1′,2′: 1,6]pyrimido[5,4-a]indolizine derivative in the alkylation of 2-amino-1-(benzimidazol-2-yl)-3-(4-methoxybenzoyl)indolizine

An attempt to effect exhaustive alkylation of 2-amino-1-(benzimidazol-2-yl)-3-(4-methoxybenzoyl) indolizine with alkyl iodides in boiling acetone led to the formation of 6,6-dimethyl-8-(4-methoxybenzoyl)-6,7-dihydrobenzo[4′,5′]imidazo[1′,2′: 1,6]pyrimido[5,4-a]indolizine instead of expected N-alkyl derivatives. The product structure was proved by X-ray analysis.     

Three Modes of Proton Transfer in One Chromophore: Photoinduced Tautomerization in 2‐(1H‐Pyrazol‐5‐yl)Pyridines,Their Dimers and Alcohol Complexes

Studies of 2‐(1H‐pyrazol‐5‐yl)pyridine (PPP) and its derivatives 2‐(4‐methyl‐1H‐pyrazol‐5‐yl)pyridine (MPP) and 2‐(3‐bromo‐1H‐pyrazol‐5‐yl)pyridine (BPP) by stationary and time‐resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited‐state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited‐state intermolecular double‐proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent‐assisted double‐proton transfer in hydrogen‐bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited‐state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground‐state syn–anti equilibria are detected and discussed. The fraction of the higher‐energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen‐bond donor or acceptor abilities.     

Theoretical study on proton tautomerism of chryosphanol and its analogues in the ground and excited states

Proton tautomerism of 1,8-dihydroxy-3-methyl-anthraquinone and its analogues were studied using HF and CIS methods with 6-31g(d,p) basis set for the ground and singlet excited states. The calculations indicate that the compound exists two strong intramolecular hydrogen bonds (IHB), and shows similar characters in its proton transfer processes considering the geometries and Mulliken charge population. Calculation results further show that intramolecular proton transfer (IPT) is not favored in view of the energy trend for chryosphanol, which has two hydrogens of hydroxyl groups bond with a common oxygen of carbonyl group and exists two IHBs in the peri region. However, it exhibits normal intramolecular proton transfer for the derivatives of chryosphanol, which have only one pair of adjacent hydroxyl group and carbonyl group existing in the peri region. Hereby, it can be conjectured from a theoretical point of view that IPT is absent in the similar structure such as hypericin’s peri region. Calculation results on the photophysical process show that the isomerization process is competitive with the intersystem crossing process, which facilitates the increase of triplet state quantum efficiency and photosensitive activity.     

UV/Vis Spectroscopic Properties of <Emphasis Type="Italic">N</Emphasis>-(2′-Hydroxy-4′-<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethyl-aminobenzylidene)-4-nitroaniline in Various Solvents and Solid Environments

Summary.  N-(2′-Hydroxy-4′-N,N-dimethylaminobenzylidene)-4-nitroaniline [HDBN] has been used as a model for investigating intra- and intermolecular D–A (donor–acceptor) interactions in various environments by means of UV/Vis spectroscopy. UV/Vis spectra of HDBN have been measured in various solvents, ethanolic solutions of different pH, adsorbed on silica, and in the solid state. A bathochromic shift of ν_max is observed with increasing the dipolarity/polarizability and HBD (hydrogen bond donor) capacity of the solvent, which is described by means of a multiple LSE (linear solvation energy) relationship in terms of the empirical Kamlet-Taft solvent polarity parameters. The adsorption of HDBN on Aerosil^? 300-silica particles in non-HBA (hydrogen bond acceptor) solvents is explained in the same sense. Mobile protons and sol–gel entrapping cause a hypsochromic shift due to protonation of the lone electron pair of the 4′-N,N-dimethylamino group. Hydroxide ions attack the 2′-hydroxy group which causes a bathochromic shift. A strong intramolecular hydrogen bond between the 2′-hydroxyl hydrogen and the imine nitrogen atom is present in the solid-state structure causing an unprecedented bathochromic shift. Corresponding author. E-mail: stefan.spange@chemie.tu-chemnitz.de Received July 8, 2002; accepted (revised) September 30, 2002     

Spirocyclohexadienones: XII. Dienone-phenol rearrangement of 1-substituted 2-azaspiro[4.5]undeca-1,6,9-trienes and their analogs

Hydrolytic cleavage of 1-substituted 2-azaspiro[4.5]undeca-1,6,9-trienes in acid medium is accompanied by dienone-phenole rearrangement with formation of substituted N-[2-(p-hydroxyphenyl)ethyl] carboxylic acid amides. 1,2-Dimethoxy-3-oxo-15-phenyl-14-azadispiro[5.1.5.2]pentadeca-1,4,14-triene and 2′-R-7a′-methyl-3a′,4′,5′,6′,7′,7a′-hexahydrospiro[cyclohexa[2,5]diene-1,3′-indol]-4-ones undergo analogous cleavage.     

Rotamerisation and intramolecular proton transfer of 2-(2′-hydroxyphenyl)oxazole, 2-(2′-hydroxyphenyl)imidazole and 2-(2′-hydroxyphenyl)thiazole: a theoretical study

Semi-empirical (AM1-SCI) calculations have been performed on 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) to rationalise the photophysical behaviour of the compounds exhibiting intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The calculations reveal that there is a gradual variation in the properties from HPO to HPT through HPI so far as the existence of the rotational isomers in the ground state is concerned. While HPO gives rise to two stable rotamers (I and II) in all the common solvents, there is only one stable species for HPT in the S₀ state. For HPI, rotamer II is possible only in the isolated state and/or in solvents of low polarity, but in high polar solvents it gives rise to the normal form (I) only. For all the molecules in the series, however, intramolecular proton transfer (IPT) takes place in the lowest excited singlet (S₁) and the triplet (T₁) states. Combination of the rotamerism and ESIPT gives rise to multiple fluorescence bands for the fluorophores. Theoretical assignments have been made for the excitation, fluorescence and phosphorescence bands. Simulated potential energy curves (PEC) in different electronic states reveal that the IPT process is feasible in either of the S₁ and T₁ states but not in the ground state. The ESIPT reaction has been found to be favoured both thermodynamically and kinetically in these electronic states compared to the ground state. However, quantum mechanical tunnelling has been proposed for the prototropic reaction to proceed in the excited states.     

Binuclear and polynuclear transition metal complexes with macrocyclic ligands 7. Directed step-by-step synthesis of novel unsymmetric macrocyclic ligands

Novel asymmetric macrocyclic Schiff bases were synthesized by the condensation of N,N′-bis(2-aminophenyl)-3,4-diphenylthiophene-2,5-dicarboxamide (1) with diformyl derivatives of phenol, furan, difurans, pyridine, pyrrole, and dipyrroles. The reaction proceeds in high yields and without by-products in methanol in the presence of inorganic and organic acids (proton-template condensation). In the case of monocyclic diformyl derivatives and di(5-formylfuran-2-yl) sulfide, the reaction occurs in 1,4-dioxane (templateless synthesis). The synthesized macrocycles were characterized by elemental analysis data and NMR and mass spectra. For Part 6, see Ref. 1. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2152–2156, September, 2005.     

NMR study of conformational exchange and double-well proton potential in intramolecular hydrogen bonds in monoanions of succinic acid and derivatives

We present a (1)H, (2)H, and (13)C NMR study of the monoanions of succinic (1), meso- and rac-dimethylsuccinic (2, 3), and methylsuccinic (4) acids (with tetraalkylammonium as the counterion) dissolved in CDF(3)/CDF(2)Cl at 300-120 K. In all four monoanions, the carboxylic groups are linked by a short intramolecular OHO hydrogen bond revealed by the bridging-proton chemical shift of about 20 ppm. We show that the flexibility of the carbon skeleton allows for two gauche isomers in monoanions 1, 2, and 4, interconverting through experimental energy barriers of 10-15 kcal/mol (the process itself and the energy barrier are also reproduced in MP2/6-311++G** calculations). In 3, one of the gauche forms is absent because of the steric repulsion of the methyl groups. In all four monoanions, the bridging proton is located in a double-well potential and subject, at least to some extent, to proton tautomerism, for which we estimate the two proton positions to be separated by ca. 0.2 ?. In 1 and 3, the proton potential is symmetric. In 2, slowing the conformational interconversion introduces an asymmetry to the proton potential, an effect that might be strong enough even to synchronize the proton tautomerism with the interconversion of the two gauche forms. In 4, the asymmetry of the proton potential is due to the asymmetric substitution. The intramolecular H-bond is likely to remain intact during the interconversion of the gauche forms in 1, 3, and 4, whereas the situation in 2 is less clear.     

N−H‐Type Excited‐State Proton Transfer in Compounds Possessing a Seven‐Membered‐Ring Intramolecular Hydrogen Bond

A series of compounds containing 5‐(2‐aminobenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one ( o ‐ABDI ) as the core chromophore with a seven‐membered‐ring N?H‐type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N?H proton and thus the hydrogen‐bond strength can be fine‐tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron‐withdrawing strength of R, that is, in the order H<COCH₃<COPh<Tosyl<COCF₃. The tosyl and trifluoroacetyl derivatives undergo ultrafast, irreversible excited‐state intramolecular proton transfer (ESIPT) that results in proton‐transfer emission solely in the red region. Reversible ESIPT, and hence dual emission, involving the normal and proton‐transfer tautomers was resolved for the acetyl‐ and benzyl‐substituted counterparts. For o ‐ABDI , which has the weakest acidity, ESIPT is prohibited due to its highly endergonic reaction. The results clearly demonstrate the harnessing of ESIPT by modifying the proton acidity and hydrogen‐bonding strength in a seven‐membered‐ring intramolecular hydrogen‐bonding system. For all the compounds studied, the emission quantum yields are weak (ca. 10^?3) in dichloromethane, but strong in the solid form, ranging from 3.2 to 47.4 %.     

Fluorescent sensing of anions based on excited state intramolecular proton transfer in <Emphasis Type="Italic">N</Emphasis>-(3-hydroxy-2-naphthamido)-<Emphasis Type="Italic">N</Emphasis>′-phenylthiourea

A neutral N-amidothiourea-based excited state intramolecular proton transfer (ESIPT) anion receptor bearing an o-hydroxynaphthamide fluorophore and a thiourea binding site, N-(3-hydroxy-2-naphthamide)-N′-phenylthiourea (1a), was designed and synthesized. Fluorescence and absorption response of 1a toward anions were assessed in acetonitrile. IR and NMR experiments indicated that the “OH⋯O=C” intramolecular hydrogen bond (IHB) in 1a was weak so that it only exhibited the short-wavelength normal emission other than ESIPT fluorescence. Due to the high anion binding affinity of the N-amidothiourea binding site and the formation of a hydrogen binding network in the 1a-anion complex, 1a underwent structural change upon anion binding that strengthens the “OH⋯O=C” IHB, leading to the ESIPT and the observation of the long-wavelength ESIPT emission whereas the normal fluorescence is quenched. On the basis of NMR and fluorescence titrations and control experiments with model compounds, a sensing mechanism of the anion-binding-induced ESIPT was proposed. Supported by the National Natural Science Foundation of China (Grant Nos. 20425518, 20675069 & 20835005) and the National Fund for Fostering Talents of Basic Science (Grant No. J0630429)     

2-(benzimidazol-2-yl)-3-hydroxychromone derivatives: spectroscopic properties and a possibile alternative intramolecular proton phototransfer

Three 2-(benzimidazol-2-yl)-3-hydroxychromone derivatives were synthesized. Their spectroscopic and fluorescent properties, due to excited state intramolecular proton transfer (ESIPT) from OH to carbonyl, were studied. Theoretical possibility of an alternative intramolecular H-bonding and experimental evidence for such behavior are discussed.      

Reductive Coupling Reactions of 2-Nitrochalcones and their β-Hydroxy-analogues: New Syntheses of 2-Arylquinoline and 2-Aryl-4-hydroxyquinoline Derivatives

Summary. A one-pot synthesis of novel 2-arylquinolines and 2-aryl-4-hydroxyquinolines was developed from the intramolecular reductive coupling reactions of 2-nitrochalcones and 3-hydroxy-1-phenyl-3-(2-nitrophenyl)-2-propen-1-ones. Depending on the reduction method and on the presence of electron donating substituents on the A ring of 2-nitrochalcones one can modulate the formation of 2-arylquinolines, their N-oxides, and of 2-aminochalcones. The reduction of 3-hydroxy-1-(2-hydroxyphenyl)-3-(2-nitrophenyl)-2-propen-1-ones with stannous chloride in hydrochloric acid gave 2′-aminoflavones and with ammonium formate and Pd/C yielded 2-(2-hydroxyaryl)-4-hydroxyquinolines.