Comparison of the Absorption, Emission, and Resonance Raman

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Comparison of the Absorption, Emission, and Resonance Raman Spectra of 7-Hydroxyquinoline and 8-Bromo-7-Hydroxyquinoline Caged Acetate Jiani Ma,† Shun Cheung Cheng,† Huiying An,† Ming-De Li,† Chensheng Ma,† Adam C. Rea,‡ Yue Zhu,‡ Jameil L. Nganga,‡ Timothy M. Dore,*,‡ and David Lee Phillips*,† † ‡

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., People's Republic of China Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556, United States

bS Supporting Information ABSTRACT: To better understand the deprotection reaction of the new promising phototrigger compound BHQ-OAc (8-bromo-7-hydroxyquinoline acetate), we present a detailed comparison of the UVvis absorption, resonance Raman, and fluorescence spectra of BHQ-OAc with its parent compound 7-hydroxyquinoline in different solvents. The steady-state absorption and resonance Raman spectra provide fundamental information about the structure, properties, and population distribution of the different prototropic forms present under the different solvent conditions examined. The species present in the excited states that emit strongly were detected by fluorescence spectra. It is shown that the ground-state tautomerization process of BHQ-OAc is disfavored compared with that of 7-HQ in aqueous solutions. The observation of the tautomeric form of BHQ-OAc in neutral aqueous solutions demonstrates the occurrence of the excited-state proton-transfer process, which would be a competing process for the deprotection reaction of BHQ-OAc in aqueous solutions.

’ INTRODUCTION Proton transfer is a common elementary step involved in a wide range of chemical reactions and is an important aspect of many chemical and biological processes.17 Bifunctional molecules that contain both acid and basic groups (hydroxyquinoline compounds possessing a proton donor group and a proton acceptor group are examples) can undergo various interesting photoinduced processes like intramolecular proton transfer via H-bonded vicinal groups, concerted biprotonic transfer within a doubly H-bonded dimer or relayed by a bridge of solvent molecules between two distinct groups in the molecule, double proton transfer between distant groups, and coupled proton and electron transfers.8 There have been many studies reported on excitedstate proton transfer (ESPT) over the last two decades.9 The 7-HQ (7-hydroxyquinoline) molecular system among hydroxyquinoline studies has received particular attention over the past several decades.1017 This work determined that there are four protropic species of 7-HQ (with chemical structures shown in Scheme 1). Most of the studies were based on stationary and time-resolved absorption and emission spectroscopic techniques. Since the electronic spectra of many compounds in solution are typically broad and display little or no fine structure, it is not easy to obtain detailed structural information from the electronic spectra in solutions. To our knowledge, no study has yet reported work utilizing resonance Raman spectroscopy to obtain structural information on 7-HQ from the vibrational spectra.18 r 2011 American Chemical Society

To better investigate the temporal and spatial changes taking place during physiological processes, it is useful to have a range of probes that are able to selectively release a biological effector at specific times and places within a biological system. 1924 Chromophoreeffector conjugates are often referred to as caged compounds since the activity of the effector is caged by the connection of the chromophore with the effector through a single covalent bond. Photoexcitation by light causes the covalent bond to break and uncage the effector in its active form. In several recent papers, BHQ (8-bromo-7-hydroxyquinoline) was reported to be a good phototrigger compound that could efficiently release a caged biological effector in physiological environments with enough sensitivity for both 1PE (one-photon excitation) and 2PE (two-photon excitation) applications.2529 Although much effort has been made to characterize and understand this new and promising phototrigger compound, there are still some questions left to be answered on the deprotection reaction mechanism of BHQ-OAc (BHQ-protected acetate). Some previous studies on BHQ-OAc proposed that the singlet anionic (A) form (Scheme 2) was the precursor for the deprotection reaction in neutral aqueous solution.29 However, the Q u (quantum efficiency) for deprotection following photolysis of Received: July 5, 2011 Revised: September 6, 2011 Published: September 12, 2011 11632

dx.doi.org/10.1021/jp2063172 | J. Phys. Chem. A 2011, 115, 11632–11640

The Journal of Physical Chemistry A CHQ-OAc (8-chloro-7-hydroxyquinoline acetate) was found to be noticeably lower than that for BHQ-OAc.28 The introduction of a heavy atom like bromine often significantly promotes an intersystem crossing (ISC) process. If the deprotection reaction really takes place via the singlet state, one would expect that the Q u of CHQ-OAc would be significantly higher than that of BHQ-OAc because depletion of the singlet would be more efficient through the ISC process in the BHQ-OAc molecule. A second issue that needs to be addressed is that as a derivative of 7-HQ , the tautomerization process for BHQ-OAc probably takes place in aqueous solution with the formation of the tautomeric (T) form, but the role of this form is not yet known. A third issue to be studied is the ESPT process, which has not been examined in detail for BHQ-OAc in aqueous solution. In comparison to 7-HQ , which has been extensively studied, the work on BHQ-OAc is still in a preliminary stage. To increase our understanding of the mechanism of the deprotection reaction of BHQ-OAc in aqueous solution environments, we present here a comparison of the absorption, emission, and resonance Raman Scheme 1. Chemical Structures of the Different Forms of 7-HQ

Scheme 2. Chemical Structures of the Different Forms of BHQ-OAc

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spectroscopy of the BHQ-OAc phototrigger compound with the parent benchmark compound 7-HQ. As a derivative of 7-HQ , it is conceivable that BHQ-OAc has some similar properties. A cationic (quinolinium) form of BHQOAc (C), an anionic (quinolinate) form of BHQ-OAc (A), a neutral form of BHQ-OAc (N), and a tautomeric form of BHQOAc (T) are likely to be the major forms found in aqueous solution environments (Scheme 2). Density functional theory (DFT) calculations using the B3LYP method and a 6-311G** basis set found that the distance between the proton donor and the proton acceptor for BHQ-OAc is similar to that of 7-HQ (see Figure 1). Nevertheless, there are some significant differences between BHQ-OAc and 7-HQ. The pKa of the phenolic moiety of BHQ-OAc has been reported to be 6.8,29 which is much lower than the 9.0 value of 7-HQ.9 Also, it is reported that almost no significant ESPT takes place in a methanol solution of 8-Me-7HQ , and this may be attributable to the trans conformer of the 7-OH group of this compound.9 These facts indicate that the distribution of the forms of BHQ-OAc in aqueous solutions would likely be different from those of 7-HQ because of the presence of the 8-bromo and the 2-methylacetate substituents. The characterization of the ground-state species of the different forms will provide the groundwork for additional study of the photophysics and the photochemistry of BHQ-OAc. In our earlier publications, the populations of the different forms of the ground-state BHQ-OAc in MeCN (acetonitrile) and H2O/MeCN mixed solvents were studied by UVvis absorption and resonance Raman spectroscopy and were compared with some previous studies for 7-HQ by other groups that were performed in pure water. Because water plays an important role in the tautomerization process, a study of 7-HQ in the same solvents is needed to have a more direct comparison with BHQ-OAc and a greater insight into the effects of the Br and CH2OAc substituents. In this paper, we report a detailed comparison of the UVvis absorption, resonance Raman, and fluorescence spectra of 7-HQ with BHQ-OAc in different solvents.

’ EXPERIMENTAL AND COMPUTATIONAL METHODS BHQ-OAc (8-bromo-7-hydroxyquinoline caged acetate) was prepared as described previously in the literature.25,29 7-HQ (7hydroxyquinoline) was purchased from Sigma and was used without further purification. Samples for the experiments described in this work were prepared using spectroscopic grade MeCN (acetonitrile) and deionized water. NaOH and HClO4 were used as needed to control sample pH.

Figure 1. The optimized structures of the ground state of 7-HQ (left) and BHQ-OAc (right) obtained from B3LYP/6-311G** DFT calculations. Selected bond lengths (in Å) are labeled in the structures. 11633

dx.doi.org/10.1021/jp2063172 |J. Phys. Chem. A 2011, 115, 11632–11640

The Journal of Physical Chemistry A A. Absorption, Emission, and Resonance Raman Experiments. Steady-state absorption spectra were recorded on a

PerkinElmer Lambda 19 UVvis spectrometer using a 1 cm path length quartz cuvette. Resonance Raman experiments were performed for 7-HQ in MeCN and NaOHH2O (pH 1112) and HClO4H2O (pH 23) solutions. The concentration was about 3 mM 7-HQ for the sample solutions. Fluorescence emission spectra were obtained on an FL 600 spectrometer using the excitation wavelength of 266 nm to be consistent with the excitation wavelength used in the resonance Raman experiments. The resonance Raman experimental apparatus and methods used for these experiments have been described elsewhere,30 and only a brief description will be given here. The resonance Raman experiments were conducted using 266 nm excitation (the fourth harmonic from a Nd:YAG laser) with about 0.9 mW laser power. The excitation laser beam was focused to about a 0.5 mm diameter spot size onto a flowing liquid stream of sample. A backscattering geometry was employed for sample excitation and for collection of the Raman scattered light by reflective optics. The Raman signal detected by a liquid-nitrogen-cooled charged-coupled device (CCD) detector was acquired for 30 s before being read out to an interfaced personal computer, and 10 of these readouts were added together to obtain the resonance Raman spectrum. The Raman bands of the MeCN solvent were employed to calibrate the resonance Raman spectra with an estimated accuracy of 5 cm1 in absolute frequency. The Raman spectrum of the sample was obtained by removing the Raman spectrum of the corresponding solvent with a proper scaling factor. Because water has no Raman signal in the 6001800 cm1 region, the subtraction work is not needed for the experiments performed in water solvents. B. HPLC-MS (High Performance Liquid ChromatographyMass Spectrometry) Product Analysis Experiments. A 10 mM solution of BHQ-OAc (0.5 mL) in a mixed H2O/MeCN (3:2, v/v, pH 67) solvent was irradiated at 254 nm in a Rayonet photochemical reactor and then was analyzed by HPLC within a few minutes. The products and starting materials were stable in the photolysis solvent and in the HPLC mobile phase. The initial and final composition of the reaction was determined on a Varian ProStar HPLC system with a Microsorb C-18 reverse phase column, a 70:30 H2O (0.1% TFA)/MeCN mobile phase at 1 mL/min, and a diode array UV detector. The HPLC system was interfaced with a Perkin-Elmer Sciex API I plus quadrupole mass spectrometer. Electrospray ionization (ESI) was used, and the ionization was set so that little or no fragmentation of the starting material occurred in the instrument. The starting materials, reactions, and standards were analyzed by injecting 20 μL aliquots onto the HPLC-MS. C. DFT (Density Functional Theory) Calculations. The optimized geometry, vibrational modes, and vibrational frequencies for the singlet ground state of the neutral form, the anionic form, the cationic form, and the tautomeric form of 7-HQ were obtained from (unrestricted) B3LYP/6-311G** DFT calculations. No imaginary frequency modes were observed at any of the optimized structures shown here. A Lorentzian function with a 15 cm1 bandwidth was used with the computed Raman vibrational frequencies and their computed relative intensities to determine the (unrestricted) B3LYP/6-311G** calculated Raman spectra to compare with the corresponding experimental resonance Raman spectra. All of the calculations were executed using the Gaussian 03 program suite.31

’ RESULTS AND DISCUSSION Previous studies suggested that there are mainly four prototropic species of 7-HQ that may exist in different solvents, and

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the chemical structures of these four forms are shown in Scheme 1. Similarly, BHQ-OAc also appears to exist as the normal neutral molecular form BHQ-OAc (N) in inert organic solvents that do not contain water. The pKa value of the phenolic moiety in BHQOAc is reported to be 6.8 in aqueous solutions.29 Therefore, BHQOAc exists almost exclusively as the anionic form BHQ-OAc (A) at pH values larger than 8.8. At pH values around 7 in aqueous solutions, BHQ-OAc (N) is probably still one of the major species but other forms, such as BHQ-OAc (T), are also likely to coexist to some extent. In this study, the steady-state UVvis absorption, resonance Raman, and fluorescence spectra for 7-HQ were compared with those obtained for BHQ-OAc in varying solvents such as MeCN, H2O/MeCN, and pure water. A. Comparison of Absorption Spectra of 7-HQ and BHQOAc in Acetonitrile, Pure Water, and Mixed H2O/MeCN Solvents. The lowest absorption bands of the different equilibrium forms of 7-HQ in aqueous solutions have been reported to be spectrally distinguishable from one another.1 The different forms of 7-HQ, namely, 7-HQ (N), 7-HQ (C), 7-HQ (A), and 7-HQ (T), can be distinguished by the position of their lowest energy absorption bands, which are, respectively, at ∼330, 350, 360, and 400 nm. To make an accurate and more comprehensive comparison, the UVvis absorption spectra of BHQ-OAc with different pH values were measured in both pure water and mixed H2O/ MeCN solvents (these spectra are shown in Figure 3). With the addition of water, the maximum absorption wavelength of BHQOAc (N) in H2O/MeCN mixed solvents shows a slight blue shift compared to the signal seen in MeCN, and this can be explained on the basis of the environmental effect in the mixed solvent systems. The absorption spectra of BHQ-OAc in pure water at pH 12 is similar to those observed in the mixed H2O/MeCN solvents at pH 12, where the BHQ-OAc (C) is located at 320 nm. Since hydrolysis reaction of BHQ-OAc takes place in alkaline solution even in a dark environment to make a BHQ-OH product, the species observed in the absorption spectra under the experimental condition at pH 1112 solution should be BHQOH (A) instead of BHQ-OAc (A) as inadvertently proposed in some previous studies.26,27 The comparison of the absorption spectra of BHQ-OAc in alkaline solution at different times after the sample solution was prepared with the spectrum of BHQ-OH obtained in an alkaline solution is displayed in Figure 1S of the Supporting Information. Examination of Figure 1S provides evidence for the assignment as BHQ-OH (A) in alkaline solution. In neutral pure water, the absorption spectrum shows two main bands of BHQ-OAc (N) at 320 nm and BHQ-OAc (T) at 420 nm. This is different from the absorption spectrum of BHQOAc in a neutral mixed H2O/MeCN solvent, where there is only one absorption band assigned to the BHQ-OAc (N). It is suggested that the water concentration in the aqueous solution affects the population of BHQ-OAc (T), and thus, the conclusion that the amount of the tautomeric species is very small in aqueous solutions needs further clarification.26 In the mixed H2O/MeCN solvents, the band associated with the BHQ-OAc (T) could not be easily detected by the UVvis study, while the population of the BHQ-OAc (T) in the neutral pure water was observed to be much higher with an obvious absorption band seen around 420 nm. The UVvis absorption results in the mixed solvents and in pure water for both 7-HQ and BHQ-OAc suggest that water plays an important role in the tautomerization process. It can be concluded that in pure water the population of BHQ-OAc (T) is larger than that in the H2O/MeCN (3:2, v/v) mixed solvents. 11634


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Figure 2. UVvis absorption spectra of 7-HQ in (a) water and pure MeCN and in (b) H2O/MeCN (3:2) mixed solvents at different pH values.

Figure 3. UVvis absorption spectra of BHQ-OAc in (a) water and in (b) H2O/MeCN (v:v, 3:2) at different pH values indicated in the insets and in pure MeCN.

In the latter solvent, the absorption spectra do not exhibit an obvious absorption band for the BHQ-OAc (T) species. For the case of 7-HQ , this is obviously different. Although the population of 7-HQ (T) decreases significantly in H2O/MeCN (3:2, v/v) mixed solvents compared to the population in pure water, the absorption associated with the 7-HQ (T) species can still be detected. The higher population of the tautomeric form of both 7-HQ and BHQ-OAc in pure water than that in H2O/MeCN mixed solvent may be caused by the different polarity of water and MeCN. A comparison of the data in Figures 1 and 2 shows that in a neutral pH condition for either the H2O/MeCN mixed solvent or pure water, the population of BHQ-OAc (T) is much lower than the population of 7-HQ (T) in the corresponding solutions. These results suggest that the tautomerization process for BHQ-OAc is not as efficient as that of 7-HQ. B. Resonance Raman Spectra of 7-HQ in Pure Water and Comparison of Resonance Raman Spectra of 7-HQ with BHQ-OAc in H2O/MeCN Mixed Solvents. The UVvis results for 7-HQ are consistent with those found previously by other research groups. Comparison of the UVvis spectra of BHQOAc in MeCN and in neutral, basic, and acidic water with those

found for 7-HQ under corresponding conditions suggests that in the neutral aqueous solution the population of BHQ-OAc (T) decreases substantially compared to 7-HQ (T) under the corresponding conditions examined. To further test the absorption study results and to supply vibrational information for the different forms of 7-HQ , resonance Raman experiments were conducted for 7-HQ in pure water. The experimental spectra of the different forms of 7-HQ were compared to the normal Raman spectra predicted from (U)B3LYP/6-311G** DFT calculations (Figure 4). There is good agreement between the experimental resonance Raman spectra of 7-HQ (N), 7-HQ (C), and 7-HQ (A) with their respective calculated normal Raman spectra. The differences in the relative intensities between the experimental and calculated spectra are within expectations because the former are obtained with resonance enhancement, whereas the latter correspond to the normal Raman spectra. Comparison of the resonance Raman spectra to the predicted normal Raman spectra confirms the assignments of the predominant species in MeCN, basic water, and acidic water to the singlet ground state of the 7-HQ (N), 7-HQ (C), and 7-HQ (A) species, respectively. A brief comparison of the resonance Raman spectra of BHQ-OAc 11635



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Figure 4. Comparison of 266 nm resonance Raman spectra of 7-HQ obtained in different solvents with their respective calculated singlet ground-state normal Raman spectra. (i) In MeCN, (ii) in HClO4H2O (pH = 12), and (iii) in NaOHH2O (pH = 1112). The dashed lines indicate the correlation of the calculated Raman bands to the experimental resonance Raman bands.

(N) (shown in Figure 4S of the Supporting Information) and 7-HQ (N) is described here. Most of the Raman bands observed for the neutral form of 7-HQ and BHQ-OAc are due to vibrations associated with the ring CdC stretching, CC stretching, and CN stretching motions for the 6001800 cm1 region. For instance, the CdC stretching and CN stretching modes cause the Raman bands of BHQ-OAc (N) at 1607 and 1564 cm1 and shift to higher wave numbers at 1621 and 1572 cm1 for 7-HQ (N). The bands relating to the CdC stretching and CN stretching modes in the calculated Raman spectra for BHQOAc (N) and 7-HQ (N) show the same tendency as the experimental result. Also, the CBr stretching mode was observed for BHQ-OAc with Raman band at 830 cm1. The absorption and resonance Raman spectroscopy results support the conclusion that 7-HQ exists almost exclusively as 7-HQ (C), 7-HQ (A), and 7-HQ (N) in acidic water (pH = 12), basic water (pH = 1112), and MeCN solutions, respectively. Since 7-HQ (T) coexists with the other forms like 7-HQ (N) and its resonance Raman spectrum could not be easily obtained directly, we did some subtractions from the Raman spectra obtained in neutral water solution to obtain a resonance Raman spectrum of the 7-HQ (T) species. Because water plays an important role in the tautomerization process for different ground-state species of 7-HQ, it is reasonable to assume that the population of 7-HQ (T) grows as the water concentration increases. To avoid any further oversubtraction during the data treatment, a resonance Raman study for 7-HQ

was conducted in neutral aqueous solutions with different water concentrations in the mixed MeCNH2O solvents, and these data are shown in Figure 5. Examination of Figure 5 indicates that the marked peaks (*) are the resonance Raman signals of the 7-HQ (T) species, and these Raman bands can be used as criteria during the data subtraction process. Since the signal of the Raman spectrum of 7-HQ (A) is simpler than that of 7-HQ (C) and the latter was not detected in the absorption study under neutral water conditions, the population of 7-HQ (C) in neutral pure water was omitted in the analysis of the resonance Raman data, and an appropriately scaled spectrum of the Raman spectra of 7-HQ (A) and 7-HQ (N) was subtracted from the Raman spectrum taken under neutral water conditions. The subtracted resonance Raman spectrum of 7-HQ (T) and comparison with its calculated spectrum is shown in Figure 6. The absorption study demonstrated that the population of 7-HQ (T) changes in the mixed aqueous solution compared to that observed in pure water. To further characterize the different forms of 7-HQ in the H2O/MeCN mixed solutions and to make a comparison with the previous resonance Raman results of BHQ-OAc in mixed H2O/MeCN solvents,26 resonance Raman experiments for 7-HQ were conducted in mixed H2O/MeCN solvents at different pH values. The resonance Raman spectra of 7-HQ in mixed H2O/MeCN solvents at pH 1112 and pH 12 were compared with the spectra obtained in acidic water and basic water solutions, and these comparisons are shown in Figures 2S and 3S of the Supporting Information, respectively. 11636



Figure 5. The 266 nm resonance Raman spectra of 7-HQ obtained in neutral MeCNH2O mixed solutions with different MeCNH2O volumetric ratios indicated at the top of the figure.

Figure 6. The 266 nm resonance Raman spectrum of 7-HQ obtained in (a) pure water (pH = 67) and (b) basic water (pH = 1112), (c) the bands remaining after subtraction of an appropriately scaled b from a, (d) the spectrum in MeCN, (e) the bands remaining after subtraction of an appropriately scaled d from c, and (f) the DFT calculated normal Raman spectrum of the singlet ground state of 7-HQ (T). The dashed lines indicate the correlation of the two spectra (e and f) compared to each other.

In both acidic and basic mixed aqueous solutions, the resonance Raman spectra do not show obvious differences with the results observed under pure water conditions. This is consistent with the

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Figure 7. The 266 nm resonance Raman spectra of 7-HQ obtained in (a) neutral MeCN/H2O mixed solution and in (b) neutral pure water are shown. The dashed lines indicate the correlation of the resonance Raman bands in the two solvents.

UVvis absorption results and further supports the conclusion that at pH 12 and pH 1112, the main species for 7-HQ is, respectively, 7-HQ (C) and 7-HQ (A) in both pure water and mixed H2O/MeCN solvents. However, the resonance Raman spectrum taken in a mixed H2O/MeCN solvent at pH 7 differs obviously from the spectrum obtained in pure water, and a comparison of these spectra is shown in Figure 7. The peaks marked with an * belong to the 7-HQ (T) in spectrum b and are more intense than those observed in spectrum a. Compared with our previous resonance Raman study for BHQOAc in neutral aqueous solvents,26 the relative intensity of the resonance Raman signals for 7-HQ (T) is much stronger than that of BHQ-OAc (T). We conclude that the population of BHQ-OAc (T) in neutral aqueous solutions is appreciably lower than that of 7-HQ (T) and that the ground-state tautomerization process is not so efficient for BHQ-OAc in aqueous solution as it is for 7-HQ. This is probably due to the introduction of the heavy atom bromine into the 8-position of the hydroxyquinoline molecule (just between the proton donor and the proton acceptor), and this will not only restrain the formation of a water bridge which transfers the proton but will also disfavor the formation of a positively charged nitrogen in the molecule because of the bromine atom’s strong electron-withdrawing nature. C. Comparison of the Fluorescence Spectra of 7-HQ and BHQ-OAc in MeCN, Water, and Mixed H2O/MeCN Solvents. On the basis of previous studies, the contribution from the phosphorescence to the emission spectra of 7-HQ is negligible for any species excited.1 It can hereby be assumed that the signals in the emission spectra are almost exclusively due to fluorescence emission. Figure 8 shows the fluorescence emission spectra of 7-HQ and BHQ-OAc obtained in MeCN and pure water at different pH values. The fluorescence study was also conducted for 7-HQ in MeCN/H2O mixed solvents (see Figure 5S of the Supporting Information); the results appeared similar to those obtained in pure water. Figure 8a shows that in pure MeCN only one emission signal emerges around 360 nm, and it can be assigned to the emission from the first singlet excite state species of 7-HQ (N).1,13 In 70% HClO4 water solutions and basic (pH 13.8) water solutions, the fluorescence peaks (see Figure 6S in the Supporting Information) are at 443 and 478 nm, respectively, which can be assigned to the 11637



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Figure 8. Fluorescence spectra (λex = 266 nm) of (a) 7-HQ obtained in MeCN (λem = 360 nm) and in pure water with pH values of 2, 7, and 12 (λem = 510 nm) and of (b) BHQ-OAc in MeCN (λem = 400 nm) and pure water with pH values of 2 (λem = 538 nm), 7 (λem = 500 nm), and 12 (λem = 494 nm).

emission from the singlet excited state of the 7-HQ (C) and 7-HQ (A) species.1 In pure water at pH 2 and 12, the fluorescence spectra are almost the same as those obtained in neutral water with the emission peak appearing around 510 nm arising from the singlet excited state of 7-HQ (T).1,13 The steady-state absorption and the resonance Raman study demonstrate that the main species in pure water at pH 2 and 12 is the ground state of 7-HQ (C) and 7-HQ (A), respectively. This suggests that at different pH values, the excited state of 7-HQ (C) or 7-HQ (A) undergoes a deprotonation reaction or a protonation reaction in water (see eqs 1 and 214) with the final product of the excited state being 7-HQ (T). In other words, the hydroxyquinoline becomes more acidic in the excited state just as described in several previous studies.1017 Figure 7S of the Supporting Information displays the fluorescence emission spectra of 7-HQ obtained in neutral H2O/ MeCN mixed solvents with different volumetric ratios of H2O and MeCN. As the concentration of water in the mixed solvents increases, the intensity of the emission spectra around 510 nm becomes larger. This indicates that as the water concentration increases the tautomerization level becomes greater. C þ H2 O f Z þ H3 Oþ

ð1Þ

A þ H2 O f Z þ OH

ð2Þ

Figure 8b exhibits the fluorescence spectra of BHQ-OAc in pure water at different pH values and also in MeCN solvent. The fluorescence spectra obtained for BHQ-OAc in MeCN/H2O mixed solvents (see Figure 8S of the Supporting Information) appear almost the same as those acquired in pure water. In pure MeCN, the fluorescence signal at around 400 nm has its contribution mainly from the excited state of BHQ-OAc (N). On the basis of the absorption results, the emission peak at 494 nm in alkaline water (pH 12) is proposed to have its contribution mainly from the excited state of BHQ-OH (T). The fluorescence spectrum for BHQ-OH was also conducted in the same condition and supports the assignment. The fluorescence spectrum of BHQOAc in water at pH 2 displays a large Stokes shift at 538 nm and is unusual. Comparison with the results in neutral aqueous solution suggests that the species in pH 2 water is not the excited-state BHQ-OAc (T). The fluorescence spectrum of BHQ-OAc in 70% HClO 4 water solutions (see Figure 9S in the Supporting

Information) shows emission from the corresponding excitedstate species of the ground-state species, that is, BHQ-OAc (C). This therefore indicates that there is another reaction route that leads to the fluorescence band around 540 nm in moderate acidic aqueous solutions. In addition to the ground-state tautomerization process, the excited-state protonation and deprotonation reactions, and the ESPT process that may be associated with the hydroxylquinoline structure, the chemical structure of BHQOAc that includes a carbonhalogen bond suggests that a dehalogenation reaction may be able to occur appreciably under some conditions. A fluorescence spectrum was acquired for hydrobromic acid (HBr) in water solution (see Figure 10S of the Supporting Information), and a comparison of this spectrum to the fluorescence spectra obtained for BHQ-OAc in pH 2 acid aqueous solution shows that the spectra are almost identical. This suggests that the debromination reaction may be an effective process and may be competitive with the deprotonation reaction (perhaps also with the deprotection reaction) in the pH 2 aqueous solution. Also, the debromination reaction was detected by the HPLC-MS study. The initial and final composition upon the photolysis of BHQ-OAc in a mixed H2O/MeCN (3:2, pH 67) solvent were analyzed by HPLC-MS (Figures 11S and 12S of the Supporting Information, respectively). The photoproduct BHQ-OH was detected quickly upon photolysis (e.g., after 1 min) along with the debromination product 7-hydroxyquinolin-2-ylmethyl acetate (HQ-OAc, Figure 13S of the Supporting Information), which suggests that the dehalogenation might be a competitive process with the photodeprotection in the aqueous solution. The fluorescence spectra of BHQ-OAc in the buffer solution KMOPS reported by Dore and co-workers28,29 resemble the results obtained in neutral aqueous solutions and demonstrate that the singlet excited state of BHQ-OAc (T) is also the main species responsible for the emission spectra in KMOPS. Since the steady-state fluorescence study gives average emission results in a certain short-time region, it is reasonable to assume that the singlet excited state of BHQ-OAc (T) is the predominant species in the early time period for BHQ-OAc in neutral aqueous solutions. The observation that the deprotection of OAc takes place in neutral aqueous solutions suggests that the ESPT process can be a competing process of the deprotection reaction of BHQ-OAc. Accordingly, the mechanism of the 11638


The Journal of Physical Chemistry A deprotection reaction reported25 and especially the identification that the excited singlet BHQ-OAc (A) is the precursor for the deprotection reaction of BHQ-OAc needs further study and consideration using explicit time-resolved spectroscopy measurements.

’ CONCLUSIONS The steady-state absorption and resonance Raman spectra were obtained for the ground-state species of 7-HQ and BHQ-OAc, and these spectra provided fundamental information about the structure, properties, and population distribution of the different prototropic forms present under the different solvent conditions examined. Fluorescence spectra were also obtained, and these spectra provided some information about the species present in the excited states that emit strongly. The population of the different forms of 7-HQ and BHQ-OAc in water containing mixed solvents mainly depends on the proton-donor ability (protic or aprotic), the pH value of the aqueous environment, and the concentration of water in the mixed aqueous solvents. In an aprotic solvent like MeCN, both 7-HQ and BHQ-OAc exist almost exclusively as the neutral form of the molecule. In water containing solvents, BHQ-OAc behaves differently from 7-HQ. In acidic pure water and mixed aqueous solutions (pH = 12), the cationic form is the main species for 7-HQ and BHQ-OAc. In alkaline pure water and mixed aqueous solutions (pH = 1112), the anionic form is the main species for 7-HQ while BHQ-OH (A) exists in the alkaline solution of BHQ-OAc resulting from the hydrolysis reaction. In neutral aqueous solution, the concentration of water in the solvent plays an important role in determining the population of the neutral form and the tautomer form of 7-HQ and BHQ-OAc systems although the degree of this effect is different for 7-HQ and BHQ-OAc. In pure water, BHQ-OAc exists mainly as the BHQOAc (N) with a detectable amount of the BHQ-OAc (T), while in neutral H2O/MeCN (3:2, v/v) mixed solvents, the population of BHQ-OAc (T) is too low to be detected in both the UVvis absorption and the resonance Raman spectroscopy experiments used to study the samples here. 7-HQ (N) and 7-HQ (T) are the main species that are observed in both pure water and H2O/ MeCN (3:2, v/v) mixed solvents, and the population of 7-HQ (T) is much lower than in the latter condition. It can be concluded that the tautomerization process of BHQ-OAc is disfavored compared with that of 7-HQ in both pure water and H2O/MeCN (3:2, v/v) mixed solvents. This may be caused by the steric or electronic effects of the 8-bromo group substituent or by competitive hydrogen bonding between the 8-bromo group and the water molecules, which hinder the formation of a cyclic BHQOAcwater complex. From the fluorescence study performed here, it appears that excited states of different forms of BHQ-OAc exhibit noticeably different activity than that of 7-HQ. In MeCN, the singlet excited state of the neutral form for both 7-HQ and BHQ-OAc is the only species contributing appreciably to the emission spectra. In neutral pure water and mixed solutions, another emission spectrum emerged to be the main and even the only apparent emission signal around 510 and 500 nm for 7-HQ and BHQOAc, respectively. On the basis of previous studies reported in the literature,1 these signals are assigned to the emission from the singlet excited states of the tautomer form, 7-HQ (T) and BHQOAc (T), respectively. This demonstrates that the ESPT process is involved in neutral aqueous solution for the bifunctional molecules

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7-HQ and BHQ-OAc with the main emissive species being the excited state of the T form in a certain time region. The study reported here indicates that the concentration of water in mixed aqueous solution noticeably affects the population of the different forms of BHQ-OAc, especially the amount of BHQ-OAc (T), which is obviously larger in pure water than in mixed neutral aqueous solutions (like MeCN:H2O = 3:2, v/v). In addition, the singlet BHQ-OAc (T) was found to be the main excited-state species in neutral aqueous solutions after an ESPT process; this suggests that ESPT may be a competing process for the deprotection reaction of BHQ-OAc in aqueous solutions. From the absorption study, BHQ-OH (A) is the main species existing in alkaline aqueous solution (pH 1112) of BHQ-OAc resulting from the hydrolysis reaction. The fluorescence study for BHQ-OAc in a pH 2 aqueous solution and the HPLC-MS results in mixed neutral aqueous solution and suggest that the debromination reaction is likely a competitive process of the deprotection reaction under some conditions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Comparison of the absorption spectra of BHQ-OAc in alkaline solution at different times after the sample solution was prepared with the spectrum of BHQ-OH in alkaline solution (Figure 1S). Comparison of 266 nm resonance Raman spectra of 7-HQ in (a) basic water (pH = 1112) and in (b) basic mixed MeCN/H2O (pH = 1112) solvents (Figure 2S). Comparison of 266 nm resonance Raman spectra of 7-HQ (a) in acid water (pH = 12) and (b) in acid mixed MeCN/H2O (pH = 12) solvents (Figure 3S). Resonance Raman spectrum of BHQ-OAc (N) obtained in MeCN under 266 nm excitation (Figure 4S). Fluorescence of 7-HQ in mixed H2O/MeCN (1:1) solutions at different pH values (λex = 266 nm) (Figure 5S). Fluorescence of 7-HQ in 70% HClO4 water solutions (λem = 443 nm) and pH 13.8 water (λem = 478 nm) (Figure 6S). Corrected fluorescence spectra of 7-HQ (λex = 266 nm) in different volumetric ratios of H2O/MeCN solvents at neutral pH value. The spectra are normalized at the same emission at wavelengths around (a) 510 nm and (b) 380 nm (Figure 7S). Fluorescence of BHQ-OAc (λex = 266 nm) in mixed H2O/ MeCN (1:1) solutions at different pH values (Figure 8S). Fluorescence of BHQ-OAc in 70% HClO4 water solutions (λem= 468 nm) and pH 13.8 water (λem = 480 nm) (Figure 9S). Fluorescence of HBr (λex = 266 nm) in water solution (λem = 537 nm) (Figure 10S). HPLC-MS product analysis experimental data obtained from photolysis of BHQ-OAc under selected conditions (Figures 11S and 12S) and a standard sample of 7-hydroxyquinolin-2-ylmethyl acetate (Figure 13S). Cartesian coordinates, total energies, and zero-point energies found from the (U) B3LYP/6-311G** density functional theory (DFT) calculations for the 7-HQ (N), 7-HQ (C), 7-HQ (A), 7-HQ (T), and BHQ-OAc (N) species. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: 852-2859-2160 (D.L.P.) 1-706-583-0423 (T.M. D.); fax: 852-2957-1586 (D.L.P.) 1-706-542-9454 (T.M.D.); e-mail: [email protected] (D.L.P.) and [email protected] (T.M.D.). 11639



’ ACKNOWLEDGMENT This work was supported by a grant from the Research Grants Council of Hong Kong (HKU 7039/07P) and the University Grants Committee Special Equipment Grant (SEG-HKU-07) to D.L.P. and by grants from the National Science Foundation (CHE-0349059 and CHE-1012412) to T.M.D. Support from the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08) is also gratefully acknowledged. ’ REFERENCES (1) Lee, S. I.; Jang, D. J. J. Phys. Chem. 1995, 99, 7537–7541. (2) Kim, T. G.; Topp, M. R. J. Phys. Chem. A 2004, 108, 10060–10065. (3) Mehata, M. S. Chem. Phys. Lett. 2007, 436, 357–361. (4) Park, S.; Kwon, J. E.; Kim, S. H.; Seo, J.; Chung, K.; Park, S.-Y.; Jang, D.-J.; Medina, B. M.; Gierschner, J.; Park, S. Y. J. Am. Chem. Soc. 2009, 131, 14043–14049. (5) Henderson, J. N.; Gepshtein, R.; Heenan, J. R.; Kallio, K.; Huppert, D.; Remington, S. J. J. Am. Chem. Soc. 2009, 131, 4176–4177. Park, S. Y.; Jang, D. J. J. Am. Chem. Soc. 2010, 132, 297–302. (6) Park, S. Y.; Jang, D. J. J. Am. Chem. Soc. 2010, 132, 297–302. (7) Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2002, 124, 9458–9464. (8) Bardez, E.; Chatelain, A.; Larrey, B.; Valeur, B. J. Phys. Chem. 1994, 98, 2357–2366. (9) (a) Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol., A 1993, 75, 1–20. (b) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol., A 1993, 75, 21–48. (c) Nakagawa, T.; Kohtani, S.; Itoh, M. J. Am. Chem. Soc. 1995, 117, 7952–7957. (d) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19–27. (10) Itoh, M.; Adachi, T.; Tokumura, K. J. Am. Chem. Soc. 1984, 106, 850–855. (11) Chou, P. T.; Wei, C. Y.; Wang, C. R. C.; Huang, F. T.; Chang, C. P. J. Phys. Chem. A 1999, 103, 1939–1949. (12) Tokumura, K.; Natsume, M.; Nakagwa, T.; Hashimoto, M.; Yuzawa, T.; Hamaguchi, H.; Itoh, M. Chem. Phys. Lett. 1997, 271, 320–326. (13) Kang, W. K.; Cho, S. J.; Lee, M.; Kim, D. H.; Ryoo, R.; Jung, K. H.; Jang, D. J. Bull. Korean Chem. Soc. 1992, 13, 140–145. (14) Mason, S. F.; Philp, J.; Smith, B. E. J. Chem. Soc. A 1968, 3051–3056. (15) Lim, H.; Park, S. Y.; Jang, D. J. J. Phys. Chem. A 2010, 114, 11432–11435. (16) Kwon, O. H.; Kim, T. G.; Lee, Y. S.; Jang, D. J. J. Phys. Chem. A 2006, 110, 11997–12004. (17) Bardez, E.; Fedorov, A.; Berberan-Santos, M. N.; Martinho, J. M. G. J. Phys. Chem. A 1999, 103, 4131–4136. (18) Myers, A. B.; Rizzo, T. R. Laser techniques in chemistry; John Wiley & Sons, Inc., New York, 1995. (19) Dynamic Studies in Biology: Phototriggers, Photoswitches, and Caged Biomolecules; Goeldner, M., Givens, R. S., Eds.; Wiley-VCH: Weinheim, Germany, 2005. (20) Mayer, G.; Heckel, A. Angew. Chem., Int. Ed. 2006, 45, 4900–4921. (21) Young, D. D.; Deiters, A. Org. Biomol. Chem. 2007, 5, 999–1005. (22) Ellis-Davies, G. C. R. Nat. Methods 2007, 4, 619–628. (23) Lee, H. M.; Larson, D. R.; Lawrence, D. S. ACS Chem. Biol. 2009, 4, 409–427. (24) Photosensitive Molecules for Controlling Biological Function; Kramer, R. H., Chambers, J. J., Eds.; Humana Press: New York, 2011. (25) Zhu, Y.; Pavlos, C. M.; Toscano, J. P.; Dore, T. M. J. Am. Chem. Soc. 2006, 128, 4267–4276. (26) An, H. Y.; Ma, C. S.; Nganga, J. L.; Zhu, Y.; Dore, T. M.; Phillips, D. L. J. Phys. Chem. A 2009, 113, 2831–2837. (27) An, H. Y.; Ma, C. S.; Li, W.; Harris, K. T.; Dore, T. M.; Phillips, D. L. J. Phys. Chem. A 2010, 114, 2498–2505. (28) Davis, M. J.; Kragor, C. H.; Reddie, K. G.; Wilson, H. C.; Zhu, Y.; Dore, T. M. J. Org. Chem. 2009, 74, 1721–1729. (29) Fedoryak, O. D.; Dore, T. M. Org. Lett. 2002, 4, 3419–3422. (30) (a) Li, Y. L.; Leung, K. H.; Phillips, D. L. J. Phys. Chem. A 2001, 105, 10621–10625. (b) Zhu, P. Z.; Ong, S. Y.; Chan, P. Y.; Leung, K. H.;

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Comparison of the Absorption, Emission, and Resonance Raman

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