Femtosecond dynamics on 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole: solvent polarity in the excited-state proton transfer.

Detailed insights into the excited-state enol(N*)-keto(T*) intramolecular proton transfer (ESIPT) reaction in 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (HABT) have been investigated via steady-state and femtosecond fluorescence upconversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2'-hydroxyphenyl)benzothiazole (>2.9+/-0.3 x 10(13) s(-1)), HABT undergoes a relatively slow rate (approximately 5.4+/-0.5 x 10(11) s(-1)) of ESIPT. In polar aprotic solvents competitive rate of proton transfer and rate of solvent relaxation were resolved in the early dynamics. After reaching the solvation equilibrium in the normal excited state (N(eq)*), ESIPT takes place with an appreciable barrier. The results also show N(eq)*(enol)<-->T(eq)*(keto) equilibrium, which shifts toward N(eq)* as the solvent polarity increases. Temperature-dependent relaxation dynamics further resolved a solvent-induced barrier of 2.12 kcal mol(-1) for the forward reaction in CH(2)Cl(2). The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between N(eq)* and T(eq)*, while the dipolar vector for the enol form in the ground state (N) is in between that of N(eq)* and T(eq)*. Upon N-->N* Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the solvation relaxation process. Upon reaching equilibrium configurations N(eq)* and T(eq)*, forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between N(eq)* and T(eq)*.     

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: formation of the anion in polar solvents

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole (HBT) dissolved in pyridine is investigated. New absorptions and fluorescence bands are detected after addition of water or NaOH to the solution. The spectra are identical to the HBT anion absorption and fluorescence. Picosecond spectroscopy is used to determine the kinetics of electronically excited states. The excited-state lifetime of the anion is 3.5 ns. The tautomeric fluorescence of HBT after proton transfer builds up within 4 ps after excitation and decays with a time constant of 20 ps.     

Matching-pursuit split-operator Fourier-transform simulations of excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)-oxazole

The excited-state intramolecular proton-transfer dynamics associated with the keto-enolic tautomerization reaction in 2-(2(')-hydroxyphenyl)-oxazole is simulated according to a numerically exact quantum-dynamics propagation method and a full-dimensional excited-state potential energy surface, based on an ab initio reaction surface Hamiltonian. The reported simulations involve the propagation of 35-dimensional wave packets according to the recently developed matching-pursuit/split-operator-Fourier-transform (MP/SOFT) method by Wu and Batista. The underlying propagation scheme recursively applies the time-evolution operator as defined by the Trotter expansion to second order accuracy in dynamically adaptive coherent-state expansions. Computations of time-dependent survival amplitudes, photoabsorption cross sections, and time-dependent reactant(product) populations are compared to the corresponding calculations based on semiclassical approaches, including the Herman-Kluk semiclassical initial value representation method. The reported results demonstrate the capabilities of the MP/SOFT method as a valuble computational tool to study ultrafast reaction dynamics in polyatomic systems as well as to validate semiclassical simulations of complex (nonintegrable) quantum dynamics in multidimensional model systems.     

Rotamerism, tautomerism, and excited-state intramolecular proton transfer in 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazoles: novel benzimidazoles undergoing excited-state intramolecular coupled proton and charge transfer

The solvent and temperature dependence of the phototautomerization of 1-methyl-2-(2'-hydroxyphenyl)benzimidazole (4) and the novel compounds 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2), and 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3), together with the ground-state rotamerism and tautomerism of these new compounds, have been studied by UV-vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. A solvent-modulated rotameric and tautomeric equilibrium is observed in the ground state for 1, 2, and 3. In cyclohexane, these compounds mainly exist as a planar syn normal form, with the hydroxyl group hydrogen-bonded to the benzimidazole N3. In ethanol, the syn form is in equilibrium with its planar anti rotamer (for 1 and 2), with the phenyl ring rotated 180 degrees about the C2-C1' bond and with a nonplanar rotamer for compound 3. In aqueous solution, a tautomeric equilibrium is established between the anti normal form (or the nonplanar rotamer for 3) and the tautomer (with the hydroxyl proton transferred to the benzimidazole N3). The syn normal form of these compounds undergoes in all the solvents an excited-state intramolecular proton-transfer process from the hydroxyl group to the benzimidazole N3 to yield the excited tautomer. The tautomer fluorescence quantum yield of 2, 3, and 4 shows a temperature-, polarity-, and viscosity-dependent radiationless deactivation, connected with a large-amplitude conformational motion. We conclude that this excited-state conformational change experienced by the tautomer is associated with an intramolecular charge transfer from the deprotonated dialkylaminophenol or phenol (donor) to the protonated benzimidazole (acceptor), affording a nonfluorescent charge-transfer tautomer. Therefore, these compounds undergo an excited-state intramolecular coupled proton- and charge-transfer process.     

The empirical correlation between hydrogen bonding strength and excited-state intramolecular proton transfer in 2-pyridyl pyrazoles

A series of 2-pyridyl pyrazoles 1a and 1-5 with various functional groups attached to either pyrazole or pyridyl moieties have been strategically designed and synthesized in an aim to probe the hydrogen bonding strength in the ground state versus dynamics of excited-state intramolecular proton transfer (ESIPT) reaction. The title compounds all possess a five-membered-ring (pyrazole)N-H···N(pyridine) intramolecular hydrogen bond, in which both the N-H bond and the electron density distribution of the pyridyl nitrogen lone-pair electrons are rather directional, so that the hydrogen bonding strength is relatively weak, which is sensitive to the perturbation of subtle chemical substitution and consequently reflected from the associated ESIPT dynamics. Various approaches such as (1)H NMR (N-H proton) to probe the hydrogen bonding strength and absorption titration to assess the acidity-basicity property were made for all the title analogues. The results, together with supplementary support provided by a computational approach, affirm that the increase of acidity (basicity) on the hydrogen bonding donor (acceptor) sites leads to an increase of hydrogen-bonding strength among the title 2-pyridyl pyrazoles. Luminescence results and the associated ESIPT dynamics further reveal an empirical correlation in that the increase of the hydrogen bonding strength leads to an increase of the rate of ESIPT for the title 2-pyridyl pyrazoles, demonstrating an interesting relationship among N-H acidity, hydrogen bonding strength, and the associated ESIPT rate.     

一个新的七元环吡咯-吡啶氢键分子激发态分子内质子转移的理论研究

运用含时密度泛函理论(TD-DFT)方法和以环己烷为溶剂的可极化连续模型(PCM),研究了2-[2-(1 H-pyrrol-2-yl)-cyclopent-1-enyl]-pyridine(7-HB)发生激发态分子内质子转移(ESIPT)的反应机制.结果表明,7-HB分子被光激发到Franck-Condon区域后,在第一光学亮态(S_1)上会发生一个超快的从Normal(N)式到Tautomer(T)式的质子转移反应,其反应的能垒仅有0.05eV.此外,在相同计算水平下,还研究了7-HB分子的吸收和发射光谱,所得结果与实验数据吻合得很好.     

Fluorescence spectroscopy and amplified spontaneous emission (ASE) of phenylimidazoles: predicted vibronic coupling along the excited-state intramolecular proton transfer in 2-(2'-hydroxyphenyl)imidazoles

Methylation at the 1N position of 2-phenylimidazole provides the shortest wavelength for a liquid-state laser dye reported to date; that is, the 1-methyl-2-phenylimidazole molecule in cyclohexane solution yields amplified spontaneous emission (ASE) with a peak wavelength at 314.5 nm and a constant laser gain value of 5 cm(-1) from 310 to 317 nm. Methyl substitution in this case favors the appearance of laser action (owing to a torsion-vibrational mechanism) in cyclohexane as compared with the nonmethylated species which does not exhibit ASE in this solvent. The 2-(2'-hydroxyphenyl)imidazole molecules give rise to ASE with high gain values (ca. 9 cm(-1)) at 450 and 466 nm. The mechanism of population inversion is understood in terms of a vibronic coupling between the hydroxyl stretching motion and the torsional vibration of the phenyl and imidazole rings. The proton-transfer spectroscopy of 2-(2'-hydroxyphenyl)imidazoles is studied in dioxane, cyclohexane, dimethyl sulfoxide, methanol, and water. The greater the acidity of the solvent the greater the disruption of the intramolecular hydrogen bond; solvent acidity is the main parameter which favors formation of the open-form species in the ground electronic state. Methyl substitution at the 1N position favors formation of the open species for 2-hydroxyphenylimidazoles in the ground electronic state, which decreases their own capacity to undergo ASE. Low-temperature absorption spectroscopy confirms aggregation processes for 2-(2'-hydroxyphenyl)imidazoles in solution. In accordance with X-ray analyses in the solid phase, these molecules form associations through intermolecular chains of the type N-H...O or O-H...N.     

Unraveling solvent-dependent hydrogen bonding interaction and excited-state intramolecular proton transfer behavior for 2-(benzo[d]thiazol-2-yl)-4-(9H-carbazol-9-yl)phenol: A theoretical study

Organic chemosensors with excited-state intramolecular proton transfer (ESIPT) behavior have attracted much attention because it has great potential in a wide range of applications. Considering the paramount behavior of excited-state relaxation, in this work, we mainly focus on deciphering photo-induced hydrogen bonding effects and ESIPT mechanism for the novel 2-(benzo[d]thiazol-2-yl)-4-(9H-carbazol-9-yl)phenol (mCzOH) dye. Considering the effects of different solvents on excited-state dynamics of mCzOH flurophore, we adopt four solvents with different polarities. Analyses of fundamental structural changes, infrared (IR) vibrational spectra, and core valence partition index between S₀ and S₁ state, we confirm hydrogen bond O H···N of mCzOH should be enhanced via photoexcitation. Especially, the increase of solvent polarity could promote hydrogen bonding strengthening degree. Intramolecular charge transfer (ICT) resulting from photoexcitation qualitatively facilitates the ESIPT occurrence to a large extent. For further checking and probing into ESIPT mechanism, via constructing potential energy curves (PECs) in four solvents, we clarify the ESIPT behavior for mCzOH. Most worthy of mention is that polar solvent plays critical roles in lowering potential barrier of ESIPT reaction and in facilitating ESIPT process. We not only clarify the detailed excited-state process, but also present the solvent-polarity-dependent ESIPT mechanism for mCzOH fluorophore.     

The effect of hydrogen bonding on oligoleucine structure in water: A molecular dynamics simulation study

Molecular dynamics simulations were conducted in order to improve our understanding of the forces that determine polyleucine chains conformations and govern polyleucine self-assembly in aqueous solutions. Simulations of 10 repeat unit oligoleucine in aqueous solution were performed using the optimized potential for liquid simulations (OPLS) - all atom force field using the canonical ensemble for a minimum of 1.3 ns. These simulations provided information on conformations, chain collapse and intermolecular aggregation. Simulations indicate that single isotactic oligoleucine chains in dilute solution assume tightly packed, regular hairpin conformations while atactic oligoleucine assumes a much less regular and less compact structure. The regular, compact collapsed isotactic chain exhibited a greater degree of intramolecular hydrogen bonding and an increased level of hydrophobic t-butyl functional group aggregation compared to the atactic chain. This occurs at the expense of reduced leucine-water hydrogen bonding.     

Role of the intermolecular and intramolecular hydrogen bonding on the excited-state proton transfer behavior of 3-aminophthalimide (3AP) dimer

In this work, both the intermolecular and intramolecular hydrogen bonding of 3-aminophthalimide (3AP) dimer complex in the electronically excited state have been investigated theoretically using the time-dependent density functional theory (TDDFT) method. The calculated infrared spectrum of the hydrogen-bonded 3AP dimer complex for the S₁ state shows that the CO and H–N bonds involved in the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ which are markedly red-shifted compared with those predicted for the ground state. The calculated length of the two hydrogen bonds C₃O₅?H₈–N₆ and C₁O₄?H_7′–N_2′ are significantly shorter in S₁ state than in the ground state. However, the bond lengths of the intramolecular hydrogen bond C_3′O₅?H_8′–N_6′ and intermolecular hydrogen bond C_1′O_4′?H₇–N₂ nearly unchanged upon electronic excitation to the S₁ state. Thus, the intramolecular hydrogen bond C₃O₅?H₈–N₆ and intermolecular hydrogen bond C₁O₄?H_7′–N_2′ of the hydrogen-bonded 3AP dimer complex are stronger in the electronically excited state than in the ground state. Moreover, it has been demonstrated that the excited-state proton transfer reaction is facilitated by the electronic excited-state hydrogen bond strengthening.     

Aggregation-induced emission enhancement of 2-(2'-hydroxyphenyl)benzothiazole-based excited-state intramolecular proton-transfer compounds

A novel class of 2-(2'-hydroxyphenyl)benzothiazole-based (HBT-based) excited-state intramolecular proton-transfer (ESIPT) compounds, N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]isophthalic amide (DHIA) and N,N'-di[3-Hydroxy-4-(2'-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) has been feasibly synthesized and the properties of their nanoparticles in THF/H2O mixed solvent were investigated. Both compounds were found to exhibit aggregation-induced emission enhancement (AIEE) due to restricted intramolecular motion and easier intramolecular proton transfer in solid state. On identical experimental conditions, the emission of DHBIA aggregates increased more remarkably than that of DHIA. Different aggregation forms of these two organic compounds, due to the steric hindrance of a single tert-butyl group, could be responsible for the notably different degrees of the fluorescence enhancement. Their aggregation modes were investigated on the basis of time-dependent absorption, scanning electron microscope (SEM) images, and molecular modeling with theoretical calculation. The photophysical dynamics were also depicted based on the extremely fast ESIPT four-level cycle.     

Excited-state intramolecular proton transfer in 2-(2′,6′-dihydroxyphenyl)benzoxazole: effect of dual hydrogen bonding on the optical properties

2-(2′,6′-Dihydroxyphenyl)benzoxazole (DHBO) has been synthesized by using palladium-catalyzed oxidative cyclization. The compound utilizes both O-H···N and O-H···O bonds to ensure a coplanar structure between the benzoxazole and phenol fragments. Optical comparison with the parent compound 2-(2′-hydroxyphenyl)benzoxazole (HBO) reveals that the dual hydrogen bonding in DHBO plays an essential role in raising the desirable keto emission for ESIPT and tuning the polarity sensitivity toward the molecular environment. DHBO also exhibits a higher quantum yield (?_fl = 0.108 in methanol) than HBO (?_fl = 0.0025) in the same solvent.     

Role of nitrogen substitution in phenyl ring on excited state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl)benzimidazole: a theoretical study

A comparative study of 2-(2'-hydroxy-3'-pyridyl)benzimidazole (2',3'-HPyBI), 2-(3'-hydroxy-4'-pyridyl)benzimidazole (3',4'-HPyBI), 2-(4'-hydroxy-3'-pyridyl)benzimidazole (4',3'-HPyBI), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (3',2'-HPyBI), and 2-(5'-hydroxy-4'-pyrimidinyl)benzimidazole (5',4'-HPymBI) with 2-(2'-hydroxyphenyl)benzimidazole (HPBI) was performed theoretically to evaluate the effect of nitrogen substitution in the phenolic ring on the photophysics and rotamerism of HPBI. Density functional theory (DFT) and configuration interaction singles (CIS) combined with time-dependent DFT were employed for ground and excited state studies, respectively. Different possible molecular forms were considered for each molecule viz., cis-enol, trans-enol, open-enol, and keto forms. The computational results revealed that cis-enol is the most stable form in the ground state for all the molecules except in 2',3'-HPyBI. In 2',3'-HPyBI, K-2 keto is the most stable form. Water molecule assisted interconversions between different forms of 2',3'-HPyBI were examined theoretically. Excitation and emission energies for all the forms have been calculated theoretically and the values are in good agreement with the available experimental data. The calculations show that intramolecular proton transfer (ESIPT) is endothermic in the ground state while it is exothermic in the first excited singlet state (except 5',4'-HPymBI). The barrier for the excited state ESIPT reaction increases with nitrogen substitution. Torsional rotation between the benzimidazole and the pyridinyl∕pyrimidinyl rings in the S(1) state depicts that twisted-keto structures involve charge transfer from the hydroxypyridinyl∕hydoxypyrimidinyl to the benzimidazole ring. However, the formation of twisted-keto is not energetically favored in these systems.     

Barrier for excited-state intramolecular proton transfer and proton tunneling in bis-2,5-(2-benzoxazolyl)-hydroquinone

The excitation spectra of dual fluorescence for isolated bis-2,5-(2-benzoxazolyl)-hydroquinone at low temperatures in a supersonic jet is reported. The vibronic structure near the electronic origin for the 410 nm band is attributed to proton transfer. Proton transfer was observed for the vibrationally cold excited state. From the relative fluorescence quantum yields in organic glasses below 100 K, a barrier for the excited-state proton transfer or 121 ± 17 cm^?1 is obtained. It is concluded that proton tunneling occurs. The relative yield of the usual Stokes fluorescence in an organic glass, as a function of temperature. is compared with the relative yield in the supersonic jet as a function of excitation energy. This leads to estimates of the temperature of the isolated molecule in the excited state.     

2, 5-二间氮杂氧茚氢醌激发态质子转移反应机理的研究

用AM1+INDO/SDCI方法对2, 5-二间氮杂氧茚氢醌分子内激发态质子转移反应进行了理论研究, 求得了基态和激发态反应的位能面、势垒、过渡态, 并对有关化合物的光谱进行了理论指认, 所有理论计算结果均与实验结果符合较好。在此基础上对反应机理进行了探讨, 认为有利的是单质子转移反应不是双质子转移反应。     

Ab initio molecular dynamics of excited-state intramolecular proton transfer around a three-state conical intersection in malonaldehyde

Excited-state potential energy surface (PES) characterization is carried out at the CASSCF and MRSDCI levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S2(pipi*) state in malonaldehyde. The proton-transfer transition state lies close to an S2/S1 conical intersection, leading to substantial coupling of proton transfer with electronic relaxation. Proton exchange proceeds freely on S2, but its duration is limited by competition with twisting out of the molecular plane. This rotamerization pathway leads to an intersection of the three lowest singlet states, providing the first detailed report of ab initio dynamics around a three-state intersection (3SI). There is a significant energy barrier to ESIPT on S1, and further pyramidalization of the twisted structure leads to the minimal energy S1/S0 intersection and energetic terminal point of excited-state dynamics. Kinetics and additional mechanistic details of these pathways are discussed. Significant depletion of the spectroscopic state and recovery of the ground state is seen within the first 250 fs after photoexcitation.     

Ultrafast branching of reaction pathways in 2-(2'-hydroxyphenyl)benzothiazole in polar acetonitrile solution

In a combined study on the photophysics of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in polar acetonitrile utilizing ultrafast infrared spectroscopy and quantum chemical calculations, we show that a branching of reaction pathways occurs on femtosecond time scales. Apart from the excited-state intramolecular hydrogen transfer (ESIHT) converting electronically excited enol tautomer into the keto tautomer, known to be the dominating mechanism of HBT in nonpolar solvents such as cyclohexane and tetrachloroethene, in acetonitrile solution twisting also occurs around the central C-C bond connecting the hydroxyphenyl and benzothiazole units in both electronically excited enol and keto tautomers. The solvent-induced intramolecular twisting enables efficient internal conversion pathways to both enol and keto tautomers in the electronic ground state. Whereas relaxation to the most stable enol tautomer with twisting angle Θ = 0° implies full ground state recovery, a small fraction of HBT molecules persists as the keto twisting conformer with the twisting angle Θ = 180° for delay times extending beyond 120 ps.