Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Excited-State Proton Transfer in 3‑Cyano-7-azaindole: From Aqueous Solution to Ice Ting-Husn Tu, Yi-Ting Chen, Jiun-Yi Shen, Ta-Chun Lin, and Pi-Tai Chou* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *
ABSTRACT: We investigated the excited-state proton transfer (ESPT) reaction for 3-cyano-7-azaindole (3CAI) in aqueous solution and in ice. 3CAI undergoes water-catalyzed ESPT in the aqueous solution, giving normal (355 nm) and proton transfer tautomer (∼472 nm) emission bands. Detailed temperaturedependent studies showed that the values of activation free energy (ΔG‡) were similar between N−H and N−D isotopes. Therefore, water-catalyzed ESPT involves a stepwise mechanism incorporating solvation equilibrium (Keq) to form a 1:1 (molar ratio) water:3CAI cyclic hydrogen-bonded complex as an intermediate, followed by perhaps proton tunneling reaction. In sharp contrast, 3CAI in ice undergoes entirely different photophysical properties, in which 3CAI self-organizes to form a double-hydrogen-bonded dimers at the grain boundary of the polycrystalline. Upon excitation, the dimer proceeds with a fast excited-state double proton transfer reaction, giving rise to solely a tautomer emission (∼450 nm). The distinct difference in ESPT properties between water and ice makes azaindoles feasible for the investigation of water−ice interface property.
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INTRODUCTION
In one approach, antifreezing protein has been extensively studied. Various organisms such as bacteria,1,2 insects,3,4 polar fishes,5,6 and plants7,8 possess antifreeze proteins (AFPs) and/ or antifreeze glycoproteins (AFGPs),5,9,10 to protect freezing in a subzero environment. AFPs and AFGPs were adsorbed to the surface of small ice crystals seeds to inhibit growth of ice. The adsorption-inhibition mechanism11 operating at the ice surface leads to a nonequilibrium lowering of the freezing point below the melting point (thermal hysteresis). This motivated us to mull over the difference of photophysical properties for organic chromophores in water and ice such that relevant sensing molecules can be developed. In yet another approach, in 2001, we used 3-cyano-7-azaindole (3CAI) as a probe and demonstrated the first clear case among the azaindoles to resolve the excited-state proton transfer in pure water.12 Upon excitation at the lowest lying absorption band of 280 nm, 3CAI gives rise to dual emission bands maximized at 355 nm (the F1 band) and 472 nm (the F2 band) in neutral water at RT (see Figure 1). The study of the relaxation dynamics showed good correlation between the decay of the F1 band (∼900 ps) and the rise of the F2 band (∼905 ps), signifying a precursor− successor type of excited-state proton transfer (ESPT) reaction assisted by water molecules. We then adopted ESPT dynamics for 7-azaindole derived by Maroncelli et al. and Waluk et al.13−16 and proposed a mechanism that incorporates the excitation of a completely solvated, hydrogen-bonded (Hbonded) 3CAI (structure N* in Scheme 1), followed by a fast equilibrium between the N* and a specific 1:1 cyclic H-bonded © XXXX American Chemical Society
Figure 1. Steady-state emission spectra of 3CAI in (a) H2O and (b) D2O at various temperatures. In (a), the emission spectrum of 3CAI in a single crystal at RT is shown by a gray line with solid circles.
complex (C*). C* is suitable to for the intrinsic proton transfer with a rate kpt, forming a tautomer T* in the excited state (see Scheme 1). Standing on this basis, recently, we have Received: January 12, 2018 Revised: February 17, 2018 Published: February 19, 2018 A
DOI: 10.1021/acs.jpca.8b00379 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
fluorescence measurement was performed using a timecorrelated single photon counting system (TCSPC), in which the multichannel plates were used as the detector and the third harmonic generation (THG, at 285 nm) of pulse-selected femtosecond laser pulses at 855 nm (∼90 fs) was applied as the excitation source. The temporal resolution of the system is ∼15 ps. Data were analyzed by using the nonlinear least-squares procedure in combination with an iterative convolution method.
Scheme 1. Proposed ESPT Mechanism for 3CAI in Pure Water
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RESULTS AND DISCUSSION 3CAI in Water. According to Scheme 1, the overall ESPT rate constant krxn, using a steady-state approximation of C*, could be simply expressed as eq 1:27,28 synthesized unnatural tryptophan analogues based on the azaindole core and utilized the ESPT mechanism to successfully probe the water microsolvation in protein.17,18 Herein, we report the use of 3CAI as a prototype to probe the associated phtotophysics in water and ice. Interestingly, despite the wide studies of photochemistry of the pollutants in both phases,19,20 focus on the photophysics of organic emissive chromophores in ice is relatively rare, most of which are in the study of solvatochromism behavior.21 Two aims are to be focused in this study. First, despite the well-accepted ESPT mechanism depicted in Scheme 1, the rigorous proof of azaindoles such as 3CAI in water has not been firmly verified. In this study, in a comprehensive manner, we investigated the temperature-dependent kinetic isotope effect for ESPT in both H2O and D2O, which serves as a key to test the proposed mechanism in water (Scheme 1, vide infra). Second, in the solid ice phase, it is reasonable to expect that both random and specific water solvated 3CAI molecules in solution phase are disrupted. Therefore, what would be 3CAI−ice interaction and the associated photophysics in ice are of fundamental interest. As a result, 3CAI exhibits drastically different solvation effect and corresponding photophysics from water to ice. Details of the results and discussion are elaborated below.
Table 1. Normal and Tautomer Relaxation Dynamics of 3CAI in H2O (H) and D2O (D) at Different Temperatures F1 band (ns)a
F2 band (ns)b
temperature
τ(H)
τ(D)
τ(H)
τ(D)
318 K (45 °C)
0.71
2.75
298 K (25 °C)
0.91
3.50
287 K (14 °C)
1.09
4.25
276 K (3 °C)
1.29
5.07
0.72 (−), 3.44 0.93 (−), 3.78 1.10 (−), 4.05 1.31 (−), 4.33 11.77
2.74 (−), 6.15 3.51 (−), 7.64 4.23 (−), 8.08 5.07 (−), 8.59 38.62
263 K (−10 °C) a
Monitored at 360 nm. wavelength is 285 nm.
k rxn =
b
KIE ((D)/(H)) 3.87 3.85 3.90 3.93
Monitored at 480 nm The excitation
k1k pt k −1 + k pt
(1)
On the basis of eq 1, two simplified cases are discussed. First, if kpt > k−1 or kpt ≈ k−1, the overall ESPT rate constant in eq 1 is approximately equal to α k1 where α is a small proportional factor. For example, α = 1 and 1/2 for kpt > k−1 and kpt = k−1, respectively. In this case the energy required for breaking and reformation of hydrogen bonds should be H/D isotope dependent. Second, if k−1 > kpt, the rate constant in eq 1 is then simplified to eq 2
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EXPERIMENTAL SECTION 3CAI was synthesized according to the previously reported method.12,22,23 According to the 1H NMR, the N(1)-H proton peak (δ 12.09 ppm24 in DMSO-d6) decreases to its intensity value immediately after dissolving 3CAI in D2O, indicating its rapid N(1)-H and -D exchanging rate due to the high N(1)-H acidity.12 After twice evaporation of D2O and redissolving 3CAI in D2O, >98% of N(1)-D 3CAI could be obtained. Tripled distilled water was used for the aqueous solution. D2O is of spectral grade and was used right after received without further purification. The N(1) methylated derivative of 3CAI, namely 1MCAI, which represents the normal form of 3CAI with no proton transfer, was synthesized according to the previously reported method.12 Steady-state absorption and emission spectra were recorded using a Hitachi U-3310 spectrophotometer and an Edinburgh FS920 fluorometer, respectively. Temperature was monitored by a thermostat and controlled with variable temperature cell holder. The wavelength-dependent responsivity of monochromator and photomultiplier of the fluorometer have been calibrated by recording the scattered light spectrum of the corrected excitation light from a diffused cell in the 220−700 nm range. Time-resolved spectroscopic measurements were described previously.25,26 In brief, the picosecond time-resolved
k rxn = k pt
≠ k1 = k ptKeq = k pt e−ΔG / RT k −1
ln k rxn = ln k pt − ΔG≠/RT
(2) (3)
where Keq is the equilibrium constant between N* and C*. ≠ Thermodynamically, Keq = e−ΔG /RT; ΔG‡ is the difference in free energy between 1:1 (H2O:3CAI) cyclic H-bonded complex (C*) and the nonreactive, randomly solvated forms (N*). It can be described by solvation equilibrium (Keq) coupled with an intrinsic proton transfer rate kpt. The principle of eq 2 is similar to that derived from the transition state theory except that C* here can be treated as an intermediate so that ΔG‡ here is the true equilibrium, and there was no kBT/h factor deduced from the transition-state theory. Accordingly, the plot of ln krxn for 3CAI as a function of inverse of temperature gives a straight line, in which the slope presents −ΔG‡/R and intercept gives ln kpt. Because ΔG‡ is for the equilibrium and B
DOI: 10.1021/acs.jpca.8b00379 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 2. Arrhenius plot of 3CAI in H2O (black) and D2O (red) monitored at (a) 360 nm and (b) 480 nm (λex = 285 nm). The linear fit of ln krxn vs 1/T is shown in the corresponding color.
proton tunneling process and hence is deuterium isotope dependent. In an aim to verify the ESPT mechanism associated with eqs 2 and 3, we then carried out the temperature-dependent study for 3CAI in water (pH ∼ 7.0). Upon varying the temperatures from 318 to 243 K in H2O, as shown in Figure 1a, the steadystate emission, to a certain extent, revealed irregular ratiometric changes for the F1 (355 nm) versus F2 (472 nm) bands. When the temperature was well above 0 °C (287−318 K), no significant difference was observed for the F1 versus F2 intensity ratio. Upon decreasing the temperature near the freezing point such as 276 K, the F1 band decreases, accompanied by the increase of the F2 band. A similar trend on the ratiometric changes of emission intensity was observed for 3CAI in D2O (Figure 1b). Such an irregular change is due to the fact that population decay rates and hence the emission quantum yields for both F1 and F2 bands are temperaturedependent, which make systematic interpretation of ESPT dynamics via steady-state emission intensity difficult. Alternatively, the time-resolved measurement provides clear results. All pertinent time-resolved data in the temperature range of 318−276 K in H2O and D2O are listed in Table 1. Here, upon monitoring the decay rate of the 3CAI normal emission (360 nm of the F1 band), the decay rate of N(1) methylated 3CAI, 1MCAI, emission has been used to simulate the overall nonproton transfer decay rate of 3CAI. The overall proton transfer rate krxn(T) could be extracted by eq 4
Figure 3. Steady-state absorption (black line) and excitation spectrum (monitored at 400 nm emission) of 3CAI in water solution (blue line). Red line: the excitation spectrum (monitored at 450 nm emission) of 3CAI in ice. (a) In H2O; (b) in D2O.
k rxn(T ) = kN *(3CAI) − kN *(1MCAI)
(4)
The plot of ln krxn, which was calculated from normal decay, versus 1/T for 3CAI in aqueous solution reveals a straight line in both H2O and D2O (see Figure 2a). As shown in Figure 2a, the Arrhenius plot for the reaction reveals two parallel lines, indicating that the activation energy is the same which is H/D isotope independent. According to the slope the ΔG‡ value for ESPT was deduced to be 2.63 ± 0.06 kcal mol−1 in H2O and 2.75 ± 0.08 kcal mol−1 in D2O, respectively, which, within experimental error, is identical. Similar results were also obtained by monitoring the rise time constant of the tautomer emission (Figure 2b), in which two parallel straight lines are obtained for H2O and D2O with an average ΔG‡ value of 2.69 kcal/mol. On the other hand, according to the intercept of the plot in Figure 2, kpt is deduced to be 8.66 × 1010 s−1 and 2.27 × 1010 s−1 in H2O and D2O, respectively. The deuterium isotope dependent kpt and isotope independent ΔG‡ firmly support the latter ESPT mechanism, k−1 > kpt, we proposed for 3CAI (Scheme 1). In theory, the assumption of k−1 > kpt is not unreasonable because any slight
Figure 4. A qualitative illustration of polycrystalline formation of ice in which the 3CAI dimer is formed along the grain boundary. The gray and light blue spheres represent H2O (or D2O) molecules in different layers.
hence is independent of deuterium isotope effect, the slope is expected to be the same between N−H and N−D in H2O and D2O, respectively, while the difference in krxn lies in the intercept ln kpt, for which kpt is conventionally assumed to be a C
DOI: 10.1021/acs.jpca.8b00379 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Scheme 2. Proposed 3CAI Arrangement in Aqueous Solution and Ice and Its Corresponding Excited-State Proton Transfer Reaction
for the reference. Nevertheless, the excitation spectrum of 3CAI in ice can be well resolved, which is obviously much redshifted with respect to that in water (H2O) by >10 nm. As shown in Figure 3b, a significant difference in the excitation spectrum is also observed between 3CAI solution and ice in D2O. It should be noted that upon increasing temperature above the freezing point dual emission (355 and 472 nm bands) behavior in aqueous solution was recovered. This process is fully reversible; therefore, any possible presence of impurity to account for the changes can be eliminated. Since the H-bond formation induces certain degree of redistribution of electron density and hence electronic transition, the red-shift of 3CAI absorption (excitation) spectrum from water to ice infers different types of H-bonding formation along the N(1)-H and N(7) sites (see Scheme 1) from water to ice environment. Furthermore, owing to the large shift in the spectral onset between excitation and emission spectra, i.e., 320 nm (31 250 cm−1) versus 375 nm (26 666 cm−1), we expect the occurrence of anomalous photophysical behavior for 3CAI in ice as well. The high solubility of 3CAI in water makes unlikely the insolubility of 3CAI and hence the precipitate of aggregates in ice. This viewpoint is also supported by a concentration study in which the appearance of 446 nm emission in ice is independent of 3CAI concentrations within 10−7−10−4 M in water. The possibility of forming H-bonded species between 3CAI and ice surface −OH group in the grain boundaries, so that the ESPT may take place from this cyclic species, has been eliminated by observing similar proton transfer emission in the frozen acetonitrile (see Supporting Information) where no proton donor can be provided. Alternatively, in light of the tendency of solute molecules aggregated to the grain boundary, forming highly concentrated solute molecules21 and not incorporated into the solid polycrystalline ice,29−32 it is reasonable to expect the concentration-enhancing effect33,34 along the grain boundary of ice, which may cause self-assembly of 3CAI, most likely via dual H-bonds to form a symmetric dimer shown in Figure 4 and Scheme 2.
Figure 5. (a) The dual H-bonded dimer formation for 3CAI in single crystal, in which the N(1)−N(7) distance is 2.875 Å. (b) View of the packing of 3CAI in the unit cell.
distortion of H-bonds of the intermediate, i.e., the 1:1(H2O: 3CAI) cyclic H-bonded complex (C*), will lead to downhill of the reaction toward the reactant. This even does not require breakage of the associated water solvates. Therefore, k−1 can be in the order of (few picoseconds)−1 or faster. 3CAI in Ice. As the temperature approaches the freezing point of 0 °C at 1 atm, forming ice, drastic changes of the emission properties take place in both H2O and D2O, in which the 355 nm (F1) emission band completely vanishes by showing only a F2 band-like emission maximized at 446 nm (see Figure 1). We also observed significant changes of the excitation spectrum. For comparison, Figure 3a shows the absorption and excitation spectrum of 3CAI in water (H2O) at RT. Apparently, the excitation spectrum monitored at the emission wavelength of 400 nm, which is the overlap of F1 and F2 bands (∼400 nm), resembles the absorption spectrum, indicating the same origin for the F1 and F2 bands in water. In ice, unfortunately, we were not able to obtain the absorption spectrum due to the difficulty in preparing identical ice sample D
DOI: 10.1021/acs.jpca.8b00379 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A The feasibility of dual H-bonded dimer formation for 3CAI along the grain boundary of ice can be supported by the X-ray analysis of the 3CAI single crystal and its associated emission spectrum. The single crystal structure and the packing of 3CAI in the unit cell are shown in Figure 5, which clearly reveals a dimer-like intermolecular arrangement. The distances between N(1)···N(7) are estimated to be 2.875 Å, strongly supporting the existence of dual intermolecular H-bonded dimer formation. The colorless single crystal of 3CAI exhibits a sky blue emission maximized at 450 nm, which well matches the emission of 3CAI in ice (see Figure 1a for comparison). For the parent compound, namely 7-azaindole (7AI), it has been well established that the H-bonded dimer formed at high concentration in nonpolar solvents such as cyclohexane, from which ultrafast excited-state double proton transfer (ESDPT) takes place, giving a tautomer emission.35 Unfortunately, for lower N(1)-H acidity, 7AI could not be used as an ideal ESPT model in water due to its lack of any tautomer emission.12,14,36 Accordingly, we propose that upon electronic excitation the 3CAI dual H-bonded dimers in ice undergo ESDPT, giving rise to the dimeric tautomer 446 nm emission (Scheme 2). We then further monitored the rise time of the purported 446 nm tautomer emission of the 3CAI dimer to probe the rate of ESDPT. The results showed that the rise time constant of the 446 nm tautomer emission for the 3CAI dimer is beyond the system response time of ∼15 ps in ices formed by either H2O and D2O, followed by a 11.8 and 38.7 ns population decay time, respectively (see Table 1). The results lead us to conclude an ultrafast rate of ESDPT, which is consistent with the lack of observing any normal emission for 3CAI in ice from the steadystate measurement (see Figure 1). Note that the overall rate of ESDPT for 7AI H-bonded dimer in cyclohexane has been reported to be
