The Empirical Correlation between Hydrogen Bonding Strength and

Apr 10, 2012 - Manojkumar Jadhao , Oinam Romesh Meitei , Ritika Joshi , Himank Kumar , Chayan Das , Sujit Kumar Ghosh. Journal of Photochemistry and ...
0 downloads 0 Views 914KB Size
Download PDF
Article pubs.acs.org/JPCA

The Empirical Correlation between Hydrogen Bonding Strength and Excited-State Intramolecular Proton Transfer in 2-Pyridyl Pyrazoles Tsung-Yi Lin,†,∥ Kuo-Chun Tang,†,∥ Shen-Han Yang,‡ Jiun-Yi Shen,† Yi-Ming Cheng,† Hsiao-An Pan,† Yun Chi,*,‡ and Pi-Tai Chou*,† †

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan

‡

S Supporting Information *

ABSTRACT: 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 fivemembered-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 1H 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.

1. INTRODUCTION Hydrogen bonds exist ubiquitously in living and synthetic systems. The strength of a hydrogen bond and the associated proton (or hydrogen atom) motion plays an important role from the viewpoint of fundamental and application. This can be exemplified by the groundwork of the DNA double helix structure established by A−T and G−C double and triple hydrogen bonds, respectively, of the purine bases, from which the photoinduced proton transfer may take place, resulting in mutation.1 Other representative case can be demonstrated by the emission of the core chromophore of green fluorescence protein via an excited-state proton transfer reaction in a hydrogen-bonding (HB) network involving the relay of water and certain amino acids.2 Along this line, studies of the hydrogen bonding systems and the corresponding excited-state proton transfer phenomena have been a subject of intense studies for decades. According to our long-term experience in this field, an issue of prime importance may lie in the in-depth insight into the correlation of hydrogen bonding strength versus proton transfer dynamics, particularly, in the electronically excited state. Note that the term “hydrogen bonding strength” used in the text refers to hydrogen bonding strength in the electronic ground state. In this regard, while proton transfer in solute− solvent hydrogen bonded systems may provide a facile way to probe this fundamental issue, the resulting proton transfer to © 2012 American Chemical Society

the surrounding solvent is often complicated by multiple solute−solvent hydrogen bonding configuration and inner-shell solvent perturbation.3 Alternatively, systems possessing intramolecular hydrogen bonds, from which the proton transfer takes place and is dubbed as the excited-state intramolecular proton transfer (ESIPT), seem to be a case in point. Unfortunately, relevant studies are challenging, mainly because most of the ESIPT molecules exhibit a strong hydrogen bond between the O−H (or N−H) proton donor and pyridyl−N or carbonyl oxygen acceptor. Therefore, the electronic coupling matrix between the reactant (normal species) and product (proton-transfer tautomer) along the reaction potential energy surfaces (PESs) in the excited state is expected to be rather strong. As a result, for most of the ESIPT molecules in nonpolar solvents, ESIPT may be either barrierless, i.e., a coherent motion, or take place during the period of low frequency vibrational motions associated with the hydrogen bond.4 The rate of ESIPT is commonly in ultrafast time scale, and is limited by the instrument response for most cases. Note that in early attempts, although the rate of certain ESIPT systems might have been claimed to be finite and resolvable after convolution of data, studies based on more advanced, Received: January 11, 2012 Revised: March 29, 2012 Published: April 10, 2012 4438

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

become feasible if one can strategically design a series of pyridyl-pyrazoles for which the hydrogen bonding strength is able to be fine-tuned so that the corresponding ESIPT dynamics can be probed with a systematic manner. Herein, we report on the design and synthesis of a series of pyridylpyrazolate derivatives conferred with different acid/base behavior on the pyrazole and pyridyl sites, respectively, while the parent pyridyl-pyrazolate moiety remains unchanged. We then carried out comprehensive spectroscopic and dynamic studies to probe the HB formation as a function of proton donor/acceptor strength as well as the ESIPT dynamics versus hydrogen bonding strength. Details of the results and discussion are elaborated as follows.

shorter time-resolution always rendered faster ESIPT dynamics that required further convolution for resolution.5 This makes the exploration of correlation between hydrogen bonding strength and ESIPT dynamics not feasible. Also noteworthy is that, for some systems, dubbed as excited-state proton coupled charge transfer reactions, ESIPT is associated with substantial changes of the dipole moment between the normal and protontransfer tautomer. As a result, in polar solvents, solvent reorganization energy plays a key role and channels into the reaction coordinate.4,6 In other words, the reaction incorporates both proton transfer and solvent polarization coordinates. For this case, the overall rate of ESIPT could be finite and thus resolvable due to a rise in the solvent polarity-induced barrier. However, once the reactant and product are brought to resonance along the solvent coordinate, the intrinsic rate of ESIPT regarding the proton transfer coordinate is still ultrafast. This also makes the exploration of intrinsic proton motion versus hydrogen bonding strength not feasible. From our viewpoint, an ideal model to probe the correlation for HB strength versus ESIPT dynamics lies in those latent ESIPT molecules possessing weak intramolecular hydrogen bonding interaction. The rather weak coupling matrix along the proton (hydrogen atom) transfer coordinate is expected to induce an appreciable barrier, resulting in slow dynamics of ESIPT that could be resolved experimentally. Moreover, the strength of the intramolecular hydrogen bond may be subject to the substituent effect. As a result, via the strategic chemical modification, studies of ESIPT dynamics fine-tuned by the HB strength become possible. In an aim to search for weak HB cases that exhibit slow, resolvable ESIPT dynamics, we have reported a 2-pyridyl pyrazole system, in which a relatively weak intramolecular hydrogen bond exists between pyrazolate N−H proton and pyridyl nitrogen.7 Using 2-(4-tert-butyl-1H-pyrrol-2-yl)pyridine (compound 5; see Scheme 1) as a prototype, a slow rate of

2. EXPERIMENTAL SECTION General Information. All reactions requiring anhydrous conditions were conducted in a flame-dried apparatus under an atmosphere of argon or nitrogen. The methyl and t-butyl derivatives were synthesized following the procedures described in the literature,10 using ethyl picolinate and 3,3-dimethyl-2butanone (or acetone) as reactants. 1H and 13C NMR spectra in acetone-d6 were recorded using a Varian (Unity Plus 400) spectrometer. Synthetic Procedures. 3-Trifluoromethyl-5-(2-pyridyl)pyrazole (1). 2-Acetylpyridine (3.14 mL, 28 mmol) was added dropwise to a stirred suspension of sodium ethoxide (3 g, 54 mmol) in dry tatrahydrofuran (THF) solution (60 mL) at room temperature. Ethyl trifluoroacetate (5.24 mL, 54 mmol) was added slowly, and the resulting mixture was brought to reflux for 20 h. After stopping the reaction, the content was neutralized with 2 M HCl solution, and the mixture was extracted with ethyl acetate. The combined organic phase was washed with brine and dried over Na2SO4, and the ethyl acetate solvent was then removed in vacuo to yield the crude 1,3-dione. The resulting solid material was then treated with 98% hydrazine hydrate (4.5 mL, 92 mmol) in 60 mL of refluxing ethanol (12 h). Finally, the residue was purified by silica gel column chromatography using ethyl acetate/hexane = 1:1 as eluent. Recrystallization from ethyl acetate/hexane gave 1 (3.13 g, 52%). MS (EI, 70 eV), observed m/z, (actual), [assignment]: 213, (213), [M+]. 1H NMR (400 MHz, acetone-d6): δ 13.45 (br, NH), 8.65 (d, J = 4.8 Hz, 1H), 7.91−7.94 (t, J = 6.8 Hz, 2H), 7.28−7.40 (m, 2H). 3-Trifluoromethyl-5-[5-(1,3-dioxolan-2-yl)-2-pyridyl]pyrazole (1a). A solution of 2-bromo-5-(1,3-dioxolan-2-yl)pyridine (4.3 g, 25.0 mmol) in THF (100 mL) was cooled to −78 °C, and n-butyl lithium (11.0 mL, 2.5 M in hexane, 27.5 mmol) was added dropwise over 10 min. After the solution was stirred for 30 min at this temperature, N,N-dimethylacetamide (5.8 mL, 62.5 mmol) was added, and the solution was stirred at room temperature overnight. The reaction was quenched by water and extracted with diethyl ether. Then the organic layer was washed with water and brine. The organic extracts were dried over anhydrous MgSO4 and evaporated to dryness to yield 1-(5-(1,3-dioxolan-2-yl)pyridin-2-yl)ethanone. It was added dropwise to a stirred suspension of sodium ethoxide (2.6 g, 37.5 mmol) and ethyl trifluoroacetate (3.88 mL, 32.5 mmol) in dry THF solution (50 mL) at room temperature. The mixture was heated to reflux for 4 h. Afterward, the content was neutralized with concentrated HCl and extracted with diethyl ether. The resulting solid material was then dissolved in ethanol (100 mL) and treated with 98% hydrazine hydrate (10.0 mL, 200 mmol) in refluxing ethanol for 12 h. Subsequently, it was

Scheme 1. (A) Molecular Structures of 2-Pyridyl-pyrazoles in this Study; (B) Structure of the Proposed Proton Transfer Tautomer

(125 ps)−1 at 298 K and appreciable activation energy (2.62 kcal/mol) for ESIPT were resolved. Note that the rate constant is 2 orders of magnitude smaller than that reported for typical ESIPT molecules in nonpolar solvents and can thus be clearly resolved with picosecond time-resolved systems.2b,8 The slow ESIPT dynamics, together with the lack of N−D isotope effect on the observed kinetics, leads us to propose that bending of the molecular framework brings close the N−H···N distances, inducing the transfer of the proton (hydrogen atom). This viewpoint was then firmly supported by an elegant jet-cooled experiment of the analogue 2-(2′-pyridyl)pyrrole, in which the bending motion actively involved in ESIPT has been resolved.9 Encouraged by the above results, the goal of probing the correlation for HB strength versus ESIPT dynamics may 4439

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

decays were fitted by the sum of exponential functions with a temporal resolution of ∼300 ps by deconvolution of instrument response function. The setup of picosecond dynamical measurements consists of a femtosecond Ti-Sapphire oscillator (Tsunami, Spectra-Physics) and a time-correlated single photon counting (TCSPC) system (OB 900-L, Edinburgh). The fundamental (800 nm) pulse train at ∼82 MHz was pulsepicked using a pulse selector (model 3980, Spectra-Physics) to reduce the repetition rate down to typically 8 MHz. Third harmonics (267 nm) was generated by combining second harmonics and residual fundamental in a β-barium borate (BBO) crystal and then used as the excitation light source. A polarizer was placed in front of the detector to ensure that the polarization of the fluorescence was set at the magic angle (54.7°) with respect to that of the excitation laser pulse to eliminate fluorescence anisotropy. The measured time-dependent fluorescence data were fitted by the sum of exponential functions with a temporal resolution of ∼30 ps after deconvolution. Computational Methodology. Ground-state geometries of cis and trans forms (pyrazolate N−H relative to pyridyl N along C5−C2′ axis; see inset of Figure S1 in the Supporting Information) in cyclohexane were optimized by the density functional theory (DFT) with B3LYP hybrid functional13 and the polarizable continuum model (PCM).14 The 6-311+G(2df,2dp) basis sets were employed for all atoms.15 Vibrational frequencies were then calculated based on their optimized geometries to verify that each of the calculated geometries is the global optimized structure. All calculations were carried out using Gaussian 03 program.16 The hydrogen bonding strength was approximated by the energy differenced between cis and trans forms.

cooled, and the solvent was removed under vacuum. The residue was extracted with diethyl ether, dried with MgSO4, desolvated in vacuo, and purified by column chromatography on silica gel using a hexane/ethyl acetate mixture as eluent to provide 1a as a white solid in 55% yield. MS (EI, 70 eV): m/z 285.2 [M]+. 1H NMR (400 MHz, acetone-d6): δ 13.46 (br, NH), 8.69 (s, 1H), 7.94−7.98 (m, 2H), 7.29 (s, 1H), 5.86 (s, 1H), 4.03−4.15 (m, 4H). 13C NMR (125 MHz, acetone-d6):δ 149.1, 148.7, 144.4, 136.6, 135.2, 124.1, 121.1, 120.9, 102.8, 102.3, 66.2. 5-(2-Pyridyl) Pyrazole (3). 2-Acetylpyridine (3 mL, 26 mmol) were introduced in a round-bottomed flask. N,Ndimethylformamide dimethyl acetal (6 mL, 45 mol) was then added, and the resulting reaction mixture was refluxed overnight. The dark brown solution obtained was evaporated to dryness. The resulting dark brown crystalline material was washed with hexane (3 × 100 mL) and with diethyl ether (3 × 100 mL), yielding 2 g of pure, bright yellow solid. The resulting solid material was then dissolved in ethanol (10 mL) and treated with 98% hydrazine hydrate (8.7 mL, 180 mmol) at 60 °C and stirred for 30 min at this temperature. After cooling to room temperature, 30 mL of distilled water was added, producing a light yellow precipitate. This solid was washed on a glass filter with hexane (2 × 100 mL) and diethyl ether (2 × 100 mL) to give a colorless solid (3.33 g, 81%). MS (EI, 70 eV), observed m/z, (actual), [assignment]: 145, (145), [M+]. 1 H NMR (400 MHz, acetone-d6): δ 12.61 (br, NH), 8.58 (d, J = 4.4 Hz, 1H), 7.75−8.02 (m, 3H), 7.26 (t, J = 6.8 Hz, 1H), 6.87 (s, 1H). Photophysical Measurement. Absorption spectra were recorded at room temperature (25 °C) from 250 to 500 nm with a Hitachi (U-3310) spectrophotometer. Stock solutions of 2 × 10−5 M of the title compounds were prepared in methanol/ water mixture (1:1, v/v) initially, and 0.1, 0.5, 1, 3, and 5 M NaOH aqueous solutions were used to deprotonate the N−H of pyrazole (or change the pH value of the solution) in a microtitration manner. During the UV−vis spectral measurement, the entire cell was capped to avoid solvent loss from vaporization, so that the change of methanol (or water) content during the titration process was negligible. The pH value was measured with a Scott micro-pH combination electrode in combination with a Clean PH500 pH meter. Cyclohexane used for photophysical measurement was spectroscopic grade (Merck Inc.) and refluxed several hours over sodium metal under a nitrogen atmosphere. It was then transferred, prior to use, through distillation to the sample cuvette. Steady-state absorption and emission spectra were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorimeter, respectively. The excitation light source of the fluorimeter was corrected by a Rhodamine B spectrum. In addition, the wavelength-dependent characteristics of the monochromator and photomultiplier were calibrated by recording the scattered light spectrum of the corrected excitation light from a diffuser cell in the range of 220−700 nm. The fluorescence quantum yield (QY, Φ) was determined by comparing the fluorescent emission of quinine sulfate aqueous solution (QY ∼ 0.577 in 0.1 M H2SO4), which was used as a standard reference.12 Detailed fluorescence lifetime measurement has been described elsewhere.11 In brief, nanosecond lifetime studies were performed with an Edinburgh FL 900 photon-counting system with a hydrogen- or a nitrogen-filled lamp as the excitation source with 40 kHz repetition rate. The emission

3. RESULTS AND DISCUSSION A series of pyridyl-pyrazoles with various R1 substituents, such as trifluoromethyl (1), phenyl (2), hydrogen (3), methyl (4), and tert-butyl (5), have been synthesized as illustrated in Scheme 1. As a result, for compounds 1−5, the acidity (protondonating power) of N−H proton is designated to be tuned down by changing the functional R1 from the electron withdrawing group (1) to the donating group (5). On the other hand, the basicity (proton-accepting power) of the pyridyl nitrogen is expected to increase by adding the electron donating group 1,3-dioxolan-2-yl at the R2 position in 1a (see Scheme 1) in the same manner. To explore the proposed correlation between the hydrogen bonding strength and ESIPT dynamics, the following approaches, such as 1H NMR, titration via absorption spectroscopy, steady-state absorption/emission spectroscopy, time-resolved fluorescence dynamics, and a computational approach, were performed. It has been well established that within the same class of moiety with different substitution, stronger intramolecular hydrogen bonding causes more downfield chemical shift of the 1H NMR signal.17 Therefore, the hydrogen bonding strength between N−H proton and pyridyl nitrogen among 1−5 were first investigated with 1H NMR spectroscopy. As shown in Table 1, the chemical shift of the N−H proton is upfield shifted from δ = 13.45 ppm to 12.09 ppm as the electron-donating strength of the substituent increases (1−5). The results suggest that the hydrogen bonding strengths of 1− 5 are in a descending order of 1 > 2 > 3> 4 > 5, which correlates well with electron withdrawing strength of 1 > 2 > 3> 4 > 5. Notably, 1a exhibits the largest downfield chemical 4440

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

Table 1. Ground-State Properties and Activation Energy for ESIPT of the Title Molecules properties of ground state

1a 1 2 3 4 5

ESIPT

pKa

pKb

1 H NMR δ (ppm)a

ΔH (kcal/mol)b

HB bond length (Å)c

10.88 10.90 11.57 11.60 12.10 12.27

8.77 8.85 8.87 8.85 8.83 8.87

13.50 13.45 12.84 12.61 12.15 12.09

4.00 4.02 3.72 3.76 3.67 3.61

2.485 2.483 2.494 2.496 2.502 2.503

Ea′ (kcal/mol) 1.63d 1.67d 1.76e 2.01d 2.24e 2.61e

Figure 1. The pH titration curves of 1−5 and 1a in methanol/water 1:1 mixture (v/v) at room temperature. The absorption is monitored at the peak wavelength of the anionic species.

a

Data obtained in acetone-d6. bThe hydrogen bonding energy (ΔH) is calculated via the subtraction between the optimized energies of the trans and cis forms in the ground state in cyclohexane by B3LYP/6311+G(2df,2dp)/ PCM. (see the Computational Methodology section) cThe intramolecular hydrogen bond length obtained from our DFT calculation. dEa′: activation energy obtained experimentally in this work. eEa′: activation energy obtained experimentally from a previous report.7

titration curve (see Figure 1) for N1−H with a pKa value of 10.88, within experimental error, is identical with that (10.90) of compound 1. On the other hand, the pH titration for basicity (pKb) of the pyridyl nitrogen on 1−5 and 1a was also carried out. While the pKb of 1−5 within 8.83−8.87 is about the same (see Table 1), The value of 8.77 for 1a is apparently lower, manifesting the increase of basicity due to the addition of 1,3dioxolan-2-yl at the R2 position. Comparing 1, stronger HB strength is expected for 1a, consistent with the above 1H NMR measurement. Figure 2 shows the steady-state absorption and emission spectra of all title compounds in cyclohexane at room

shifts (13.50 ppm) among the title compounds, supporting the original design that the introduction of 1,3-dioxolan-2-yl at the R2 position increases the basicity of pyridyl nitrogen, enhancing the HB strength. In a qualitative manner, the intramolecular HB strength can be approximated by computing the energetics difference between cis- and trans- geometries (pyrazolate N−H relative to pyridyl N along the C5−C2′ axis) of the title compounds. Figure S1 (in the Supporting Information) representatively demonstrates the calculated energy of the optimized structures of cis and trans forms for 1 and 5 in cyclohexane. The calculation results revealed two forms whose pyrazole and pyridine rings were essentially coplanner (i.e., the dihedral angle ∠N(1)−C(5)−C(2′)−N(1′) is either 0 or 180 degrees). The difference in ground state energy between these two geometries (trans- subtracts cis-) is then used as a hydrogen bonding scale. Additionally, the intramolecular hydrogen bonding length was also obtained from the optimized ground state structure (cis-form). All of the calculated energies of hydrogen bonding strength and HB bond length are listed in Table 1. As a result, the intramolecular hydrogen bonding strength decreases in a trend of 1a ≈ 1 (4.02 kcal/mol) > 2 > 3 > 4 > 5 (3.61 kcal/mol), well correlating with the increase of hydrogen bond length in a trend of 1a ≈ 1 (2.483 Å) < 2 < 3 < 4 < 5 (2.503 Å). To have more quantitative insight into the acidity of N−H proton of the title compounds, their pKa values were determined by titration with NaOH basic solution along with the absorption spectroscopy. The results were also tabulated in Table 1. Evidently, the pKa value increases from 10.90 (1) to 12.27 (5) as the functional R1 changes from trifluoromethyl to tert-butyl (see Figure 1 for the obtained pH titration curves). The trend of pKa is in the order of 1 < 2 < 3 < 4 < 5. Several earlier reports have established the empirical correlation between the intramolecular hydrogen bond and pKa.18,19 Under the same base (pyridine) site, the decrease of pKa, i.e., the increase of proton acidity, results in the increase of the HB strength. This trend is mutually in good agreement with the order of HB strength deduced from 1H NMR data (N−H proton, vide supra), namely, the stronger the hydrogen bond, the more acidic the value (lowering pKa) and the more downfield of the N1−H (see Scheme 1) 1H NMR signal for 1− 5. The pH titration of 1a was also performed; the resulting

Figure 2. Absorption and emission spectra at room temperature of 1a and 1−5 in cyclohexane solution. Inset: emission spectra of 2−5 with intensity magnified in the range of 500−800 nm.

temperature. The lowest-lying absorption band with peak wavelength located in the region of 270−290 nm is primarily attributed to the π → π* electronic transition, for which the shift of electron density is mainly from the pyrazolate moiety (highest occupied molecular orbital (HOMO)) to the pyridyl moiety (lowest unoccupied molecular orbital (LUMO)). Thus, similar to most of the ESIPT molecules, the ππ* excitation is expected to increase the basicity (acidity) of pyridyl nitrogen (pyrazolate proton), triggering the proton transfer reaction. Unfortunately, the measurement of pKa (or pKb) in the excited state based on the fluorescence titration is not feasible due to the lack of both anion and cation fluorescence for all the title compounds. The nonluminescent property of both ionic forms may not be surprising, and is rationalized by the dominant C5− C2′ (see Scheme 1 for definition) rotational quenching process, which is drastically reduced in the neutral forms due to the constraint imposed by the intramolecular hydrogen bond. All the title compounds exhibit dual emissions, consisting of a mirror-imaged (c.f. the absorption band) emission band with regular Stokes shift (F1) together with another enormously redshifted one (F2, Stokes shift >10 000 cm−1). The possibility of the F2 band is simply the result of the second order (2λ) of the F1 band being promptly eliminated by two experimental facts: 4441

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

(1) The F2 band was resolved by placing a 400 nm long-pass filter in front of the monochromator to block the F1 band. (2) The spectral profile of the F2 band is very different from that of the F1 band. More firm support is given by the relaxation dynamics, where the population decay time of F1 and F2 band is completely different. The concentration dependence was also examined. In this study, the concentration of the title compounds prepared is sufficiently low (10−6−10−5 M) to avoid the possible aggregation or dimerization via intermolecular hydrogen bonds. The lack of aggregation effect is evidenced by the fact that the intensity ratio for F1 versus F2 bands remains unchanged between 10−6 M and 2 × 10−5 M. In addition, the excitation spectra monitored at both F1 and F2 emission bands are found to be identical to their absorption spectra, indicating that both emissions are from the same ground-state origin. It is thus reasonable to assign the F1 emission originating from a locally relaxed excited state with nonproton transfer characteristics (referred to as normal excited state hereafter). Accordingly, the F2 band is ascribed to the tautomer emission resulting from ESIPT, where proton (or hydrogen atom) transfers from the pyrazolate to the pyridyl site (see Scheme 1). In Figure 2, note that the F2 band of 1 and 1a is obviously blueshifted with respect to that of 2−5, indicating that the lowering of the HOMO level by the strong withdrawing group CF3 on the pyrazole moiety largely increases the tautomer S0′−S1′ (prime denotes the tautomer state) energy gap. Figure 3 depicts two representative time-resolved decay curves for 1 and 5, together with the measured instrument

Scheme 2

denote the time-dependent concentrations of normal and proton-transferred tautomer species, respectively, in the excited state. kpt is the rate constant of the ESIPT reaction, while kN* and kT* are population decay rate constants for normal and tautomer species, respectively. The corresponding rate equations can thus be expressed as follows: d[N*]t = −kN *[N*]t − k pt[N*]t dt

(1a)

d[T*]t = k pt[N*]t − k T *[T*]t dt

(1b)

With the initial conditions of [N*] = [N*]0 and [T*] = 0 at t = 0, the time dependent concentration of N* and T* can be expressed as [N*]t = [N*]0 e−(kN*+ k pt)t [T*]t =

k pt[N*]0 k T * − kN * − k pt

(2)

[e−(kN*+ k pt)t − e−k T*t ] (3)

Table 2 lists the results of relaxation dynamics of the normal F1 band and the tautomer F2 band fitted by eqs 2 and 3, Table 2. Photophysical Properties of All Studied Molecules Recorded in Cyclohexane at Room Temperature in This Work λem [nm] (Φ) 1a 1 2

Figure 3. The fluorescence decay dynamics in cyclohexane of (a) 1 and (b) 5. Open circles (blue) and squares (red) denote data points obtained for normal and tautomer emissions, respectively, with monitored wavelengths as depicted, while solid lines correspond to nonlinear fitting curves (blue and red, see Table 2 for fitting results) and instrument response function (IRF, black). Insets: the residuals of the fitting.

3 4 5

N: 321(0.0018) T: 443(0.0007) N: 321(0.0029) T: 443(0.0015) N: 322(0.0085) T: 590(0.0008) N: 322(0.054) T: 588(0.002) N: 320(0.020) T: 577(0.001) N: 322(0.020) T: 590(0.005)

fitted time constant, τ (χ2)a 350 480 350 480 350 620 350 620 350 620 350 620

nm nm nm nm nm nm nm nm nm nm nm nm

[τdecay: 45 ps] (χ2: 1.21) [τrise: 46 ps, τdecay: 2.1 ns] (χ2: 1.12) [τdecay: 50 ps] (χ2: 1.18) [τrise: 50 ps, τ2:2.3 ns] (χ2: 1.09) [τdecay:74 ps]b [τrise: 76 ps, τdecay:180 ps]b [τdecay: 98 ps] (χ2: 1.26) [τrise: 100 ps, τdecay: 500 ps] (χ2: 1.16) [τdecay: 120 ps]b [τrise:120 ps, τdecay: 240 ps]b [τdecay: 130 ps]b [τrise: 130 ps, τdecay: 220 ps]b

χ : chi-square obtained from the fitting. bThe fitted time constants were from a previous report (ref 7). a 2

response signal. A similar relaxation pattern is resolved for all the title compounds, in which the F1 emission band consists of an instant, irresolvable rise component accompanied by a single-exponential decay. The F2 band, except for 1a, reveals a resolvable rise component followed by a relatively long population decay time of a few hundred picoseconds to nanoseconds. The results suggest a precursor-successor type of ESIPT dynamics, in which the observed rising signal monitored at the F2 emission is regarded as the proceeding of the ESIPT reaction. Schematically, a kinetic model for ESIPT of the title compounds is depicted in Scheme 2, where [N*]t and [T*]t

respectively. The fitted time constants of the rising component of 1−5, within experimental error, are found to correspond well to the decay time constants fitted for the F1 emission decays. As for 1a, after deconvolution, the fitted decay of the F1 band is 45 ps, and the fitted rise of the F2 band is 48 ps. Our time-resolved system typically renders a reliable temporal resolution of ∼30 ps. Thus, it is reasonable to expect that 1a should fall into the same category of 1−5, i.e., a correlation between F1 decay and 4442

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

mainly attributed to the fast ESIPT depopulation reaction, while the low quantum yield of the F2 band should be attributed to rather long radiative lifetime of the excited tautomer so that the decay is governed by the radiationless deactivation, perhaps via those vibrations associated with hydrogen bond or the rotation between pyrazole and pyridine rings. There seems to be a lack of trend in quantum yield due to the variation in both radiative and nonradiative decay rate constants for each compound.

F2 rise components. Assuming that ESIPT is the dominant process for the normal excited state, the time constant of ESIPT is in the order of 1a (45 ps) < 1 (50 ps) < 2 (75 ps) < 3 (100 ps) < 4 (120 ps) < 5 (130 ps). Figure 4 then summarizes

4. CONCLUSION To summarize, the proof of concept regarding the correlation between the proton acidity, hydrogen bond strength, and ESIPT is provided via studies of a series of 2-pyridyl pyrazole derivatives. Various experimental approaches have drawn the conclusion that the withdrawing groups on the pyrazolate C(3) moiety affectively increase the proton acidity and the intramolecular hydrogen bond strength, and then accelerate the rate of ESIPT. On the other hand, substituting the donating group on the pyridine increases the nitrogen basicity, and hence enhances the hydrogen bonding strength and facilitates the ESIPT reaction. Such a hydrogen bonding structure versus proton-transfer relationship is of fundamental importance and can be exploited in strategic design of proton-transfer systems, facilitating their applications in several fields. For example, the intrinsic ratiometric normal versus proton transfer emission may be fined tuned via hydrogen bonding strength to achieve a broad wavelength coverage suited for light-emitting devices.20

Figure 4. The correlation between the time constant of ESIPT versus pKa and 1H NMR for 1a and 1−5.

the correlation between rate of ESIPT versus pKa and 1H NMR for the title compounds. Although for the adjacent compounds (e.g., 4 and 5) the difference in ESIPT rate may not be very prominent, the trend of increasing ESIPT rate upon increasing the HB strength (vide supra) is evident. In our previous report,7 compounds 2, 4, and 5 showed no kinetic isotope effect (KIE). It is proposed that the ESIPT reaction takes place along with the molecular skeletal motions, such as in-plane bending modes associated with the hydrogen bond, instead of direct N−H stretching motion. Therefore, negligible or no KIE should be expected. In this work, we also studied KIE by deuterating 1a, 1, and 3, and no KIEs were observed. To gain more insight into the ESIPT dynamics, we further performed temperature-dependent studies on the decay of the normal emission (F1 band) in methylcyclohexane. Figure 5 presents

■

ASSOCIATED CONTENT

S Supporting Information *

Additional synthetic procedure, computational result, and complete ref 16 are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.C.); [email protected] (P.T.C.). Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

Figure 5. The Arrhenius plot of 1a and 1−5 (as depicted) in methylcyclohexane solution. Temperature was varied between 240 and 298 K. kobs is the population decay rate of the F1 band, which is dominated by ESIPT. Error bars put on 2 representatively demonstrate the error range. Straight lines depict the linear fits (see text and Table 1 for deduced activation energies).

ACKNOWLEDGMENTS This work was supported by the National Science Council of Taiwan. We are also grateful to the National Center for HighPerformance Computing for computer time and facilities.

■

results for the logarithm of decay rate of the normal emission versus reciprocal of the temperature (1/T) for 1a and 1−5 from 298 to 240 K. The slope of the Arrhenius plot renders an activation energy of 1.67 kcal/mol for 1, as an example. As listed in Table 1, the resulting activation energies for the title compounds, though being with small difference, do reveal an ascending trend from 1a and then 1 to 5, confirming the empirical relationship for the rate of ESIPT (also see Table 2) versus the hydrogen bonding strength. From the viewpoint of quantum yields, the measured F1 and F2 band fluorescence QYs of all investigated compounds in Table 2 were low. For the F1 band, the low quantum yield is

REFERENCES

(1) (a) Watson, J. D. H.; Crick, F. A. Nature 1953, 171, 737. (b) Steczkiewicz, K.; Zimmermann, M. T.; Kurcinski, M.; Lewis, B. A.; Dobbs, D.; Kloczkowski, A.; Jernigan, R. L.; Kolinski, A.; Ginalski, K. Proc. Natl. Acad. Sci. U.S.A. 2011, 108 (9443), 1015399108. (c) Nakagawa, M.; Takahashi, S.; Tanaka, A.; Narumi, I.; Sakamoto, A. N. Plant Physiol. 2011, 155, 414. (d) Rinaldi, C. N.; Doudney, C. O. Mutat. Res. 1989, 217, 33. (e) Fu, P. K.-L.; Abuzakhm, S.; Turro, C. Photochem. Photobiol. 2005, 81, 89. (2) (a) Hsieh, C.-C.; Chou, P.-T.; Shih, C.-W.; Chuang, W.-T.; Chung, M.-W.; Lee, J.; Joo, T. J. Am. Chem. Soc. 2011, 133, 2932. (b) Chen, K.-Y.; Cheng, Y.-M.; Lai, C.-H.; Hsu, C. -C.; Ho, M.-L.; Lee, G.-H; Chou, P.-T. J. Am. Chem. Soc. 2007, 129, 4534. 4443

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444

The Journal of Physical Chemistry A

Article

(3) (a) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1984, 106, 850. (b) Lahmani, F.; Douhal, A.; Breheret, E.; ZehnackerRentien. Chem. Phys. Lett. 1994, 220, 235. (c) Presiado, I.; Erez, Y.; Gepshtein, R.; Huppert, D. J. Phys. Chem. C 2009, 113, 20066. (d) Kwon, O.-H.; Jang, D.-J. J. Phys. Chem. A 2005, 109, 20479. (e) Wu, P.-W.; Hsieh, W.-T.; Cheng, Y.-M.; Wei, C.-Y.; Chou, P.-T. J. Am. Chem. Soc. 2006, 128, 14426. (4) Hsieh, C.-C.; Jiang, C.-M.; Chou, P.-T Acc. Chem. Res. 2010, 43, 1364. (5) (a) Becker, R. S.; Lenoble, C.; Zein, A. J. Phys. Chem. 1987, 91, 3509. (b) Frey, W.; Laermer, F.; Elsaesser, T. J. Phys. Chem. 1991, 95, 10391. (c) Lochbrunner, S.; Wurzer, A. J.; Riedle, E. J. Chem. Phys. 2000, 112, 10699. (d) Barbatti, M.; Aquino, A. J. A.; Lischka, H.; Schriever, C.; Lochbrunner, S.; Riedle, E. Phys. Chem. Chem. Phys. 2009, 11, 1406. (6) (a) Lin, C.-C.; Chen, C.-L.; Chung, M.-W.; Chen, Y. -J.; Chou, P. -T. J. Phys. Chem. A 2010, 114, 10412. (b) Douhal, A.; Sanz, M.; Carranza, M. A.; Orangero, J. A.; Santos, L. Chem. Phys. Lett. 2004, 394, 54. (c) Ameer-Beg, S.; Ormson, S. M.; Poteau, X.; Brown, R. G.; Foggi, P.; Bussotti, L.; Neuwahl, F. V. R. J. Phys. Chem. A 2001, 114, 10412. (d) Swinney, T. C.; Kelly, D. F. J. Chem. Phys. 1993, 99, 211. (e) Chou, P.-T.; Pu, S.-C.; Cheng, Y.-M.; Yu, W. -S.; Yu, Y.-C.; Hung, F.-T.; Hu, W.-P. J. Phys. Chem. A 2005, 109, 3777. (f) Peters, K. Acc. Chem. Res. 2008, 42, 89. (7) Yu, W.-S.; Cheng, C.-C.; Cheng, Y.-M.; Wu, P.-C.; Song, Y.-H.; Chi, Y.; Chou, P.-T. J. Am. Chem. Soc. 2003, 125, 10800. (8) (a) Chen, K.-Y; Hsieh, C.-C.; Cheng, Y.-M.; Lai, C.-H.; Chou, P.T. Chem. Commun. 2006, 4395. (b) Chou, P.-T.; Wei, C.-Y. J. Phys. Chem. 1996, 100, 17059. (c) Chou, P.-T.; Chen, Y.-C.; Yu, W.-S.; Chou, Y.-H; Wei, C.-Y.; Cheng, Y.-M. J. Phys. Chem. A 2001, 105, 1731. (9) Kijak, M.; Nosenko, Y.; Singh, A.; Thummel, R. P.; Waluk, J. J. Am. Chem. Soc. 2007, 129, 2738. (10) Satake, A.; Nakata, T. J. Am. Chem. Soc. 1998, 120, 10391. (11) Chou, P.-T.; Chen, Y.-C.; Yu, W.-S.; Chou, Y.-H.; Wei, C.-Y.; Cheng, Y.-M. J. Phys. Chem. A 2001, 105, 1731. (12) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer Science+Bussiness Media: New York, 2006. (13) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (14) (a) Mineva, T.; Russo, N. Int. J. Quantum Chem. 1997, 61, 665. (b) Houjou, H.; Inoue, Y.; Sakurai, M. J. Am. Chem. Soc. 1998, 120, 4459. (15) Clark, T.; Andzelm, J.; Elkin, B. C.; Wimmer, E.; Dobbs, K. D.; Dixon, D. A. J. Comput. Chem. 1983, 4, 294. (16) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (17) (a) Yamauchi, K.; Kuroki, S.; Fujii, K.; Ando, I. Chem. Phys. Lett. 2000, 324, 435. (b) Kuroki, S. Mod. Magn. Reson. 2008, 31. (18) Chou, P.-T.; Liu, Y.-I; Wu, G.-R.; Shiao, M.-Y.; Yu, W.-S.; Cheng, C.-C.; Chang, C.-P. J. Phys. Chem. B 2001, 105, 10674. (19) Kanamori, D.; Furukawa, A.; Okamura, T.; Yamamoto, H.; Ueyama, N. Org. Biomol. Chem. 2005, 3, 1453. (20) Tang, K.-C.; Chang, M.-J.; Lin, T.-Y.; Pan, H.-A.; Fang, T.-C.; Chen, K.-Y.; Hung, W.-Y.; Hsu, Y.-H.; Chou, P.-T. J. Am. Chem. Soc. 2011, 133, 17738.

4444

dx.doi.org/10.1021/jp300340t | J. Phys. Chem. A 2012, 116, 4438−4444