Photophysics of 10-Hydroxybenzo[h]quinoline in Aqueous Solution

Jinfeng Zhao , Hongbin Yao , Jianyong Liu , and Mark R. Hoffmann. The Journal .... Pi-Tai Chou, Guo-Ray Wu, Yun-I Liu, Wei-Shan Yu, and Chau-Shuen Chi...
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J. Phys. Chem. 1996, 100, 17059-17066

17059

Photophysics of 10-Hydroxybenzo[h]quinoline in Aqueous Solution Pi-Tai Chou* and Ching-Yen Wei Department of Chemistry, The National Chung-Cheng UniVersity, Chia Yi, Taiwan, R.O.C. ReceiVed: May 13, 1996; In Final Form: July 15, 1996X

Various 7,8-benzoquinoline (BQ) derivatives have been synthesized and studied in order to determine the fluorescence species and dynamics of excited-state proton transfer (ESPT) of 10-hydroxybenzo[h]quinoline (HBQ) in water. The acidity of the hydroxyl proton and protonated benzoquinolinic nitrogen for various BQ derivatives in both ground and excited states has been measured by pH-dependent absorption and emission spectroscopies. The results, in combination with fluorescence decay dynamics, conclude that for HBQ the keto-tautomer formation in the excited state is highly exergonic and a dominant process in a broad pH range, regardless of whether HBQ is in a neutral, protonated, or deprotonated form. This process is only limited dynamically by the proton-donating or -accepting rate associated with free water molecules. The results also indicate that the excited-state resonance charge transfer between the hydroxyl oxygen and benzoquinolinic nitrogen acts as a driving force for the ESPT reaction.

Introduction 10-Hydroxybenzo[h]quinoline (HBQ)1 has found important application as a reagent in the preparation of optical filter agents in photographic emulsion for a long time.2,3 However, it is not until recently that the orange-red fluorescence species was recognized as a keto-tautomer emission resulting from the intramolecular excited-state proton transfer (ESPT) reaction4 (Scheme 1). On the basis of the >10 000 cm-1 Stokes’ shifted emission, HBQ has been suggested as a suitable radiation-hard scintillate.5 Recently, Sytnik et al. have applied HBQ to probe enzyme kinetics, and they concluded that HBQ can distinguish static solvent-cage polarity from dynamical solvent dielectric relaxation and other solvent-cage effects (e.g. mechanical restriction of molecular conformation).6,7 Robert et al. used HBQ as a fluorescence probe to examine the influence of organized media, especially cyclodextrins in aqueous solution.8 In neutral water HBQ exhibits a dominant fluorescence band maximum at 585 nm, which was tentatively ascribed to the proton transfer tautomer emission.6-8 This assignment seems to be reasonable due to the proposed ultrafast rate of ESPT for HBQ in both aprotic and protic, nonaqueous solution.4,6 However, intramolecular ESPT taking place exclusively in the aqueous solution is intriguing in the field of proton transfer spectroscopy. Although the ESPT reaction has been the subject of numerous investigations,9-14 the observation of a solvent perturbation-free intramolecular ESPT reaction in the aqueous solution is rare. The strong solvent (H2O) intermolecular hydrogen bonding perturbation usually leads to the prohibition of the intramolecular proton transfer during the life span in the excited state. In many cases, ESPT molecules either exhibit normal Stokes’ shifted (non-proton-transfer) emission or undergo excited-state proton dissociation in water, forming an anion species. Owing to the increasing importance of using HBQ in biochemical applications, we have synthesized various 7,8benzoquinoline (BQ) derivatives (see Figure 1) and studied their photophysical properties in order to elucidate the details concerning the photophysics of HBQ in water. The acidity of the hydroxyl proton and protonated benzoquinolinic nitrogen for various BQ derivatives in both ground and excited states has been measured by pH-dependent absorption and emission X

Abstract published in AdVance ACS Abstracts, September 15, 1996.

S0022-3654(96)01368-8 CCC: $12.00

Figure 1. Structures of various 7,8-benzoquinoline derivatives.

SCHEME 1: Excited-State Intramolecular Proton Transfer of HBQ Normal Species (HBQ(N)), Forming a Keto-Tautomer Species (HBQ(T))

spectroscopies. The results in combination with fluorescence decay dynamics conclude that for HBQ the formation of a ketotautomer in the excited state is a dominant process in a broad pH range, regardless of whether HBQ is in a neutral, protonated, or deprotonated form. This process is only limited by the proton-donating or -accepting rate of free water, which competes with the non-proton-transfer decay dynamics in the excited state. The results also indicate that the intramolecular hydrogen bonding effect in HBQ is not a key factor for the ESPT reaction in water. Instead, the excited-state charge transfer between the hydroxyl oxygen and benzoquinolinic nitrogen, resulting in © 1996 American Chemical Society

17060 J. Phys. Chem., Vol. 100, No. 42, 1996 extremely strong acidic and basic sites simultaneously, acts as a driving force for the reaction. Experimental Section Materials. BQ was purchased from Aldrich (99.0%) and used without further purification. HBQ (TCI Inc.) was twice recrystallized from cyclohexane followed by purification using column chromatography (2:1 ethyl acetate/cyclohexane). The excitation at the S0-S1 absorption region (320-400 nm) of HBQ gives only proton transfer tautomer emission (λmax ) 620 nm) in cyclohexane. However, there is a trace of impurity in which the excitation at 5 × 109 s-1. e kobs is measured in 10 M HCl.

Figure 2. pH-dependent emission spectra of OBQ in (1) pH ) 4.2, (2) pH ) 11, (3) [NaOH] ) 0.9 M, (4) [NaOH] ) 1.8 M, (5) [NaOH] ) 2.7 M, (6) [NaOH] ) 4.5 M. Inset: Absorption spectra of OBQ at varying pH values of (a) pH ) 6.4, (b) pH ) 7.3, (c) pH ) 7.5, (d) pH ) 7.8, (e) pH ) 10.2.

(C), according to the pH titration curve at 380 nm, was determined to be 7.5. In comparison, if we take off the methoxyl substituent at the 10th position from OBQ, forming the parent molecule BQ (see Figure 1), the pKa of its Nprotonated form, BQ(C), was determined to be 4.2, which is 103 times more acidic than that of OBQ(C). Since OBQ has an additional electron-donating methoxyl group with respect to BQ, we thus propose that the drastic decrease of the OBQ(C) acidity with respect to BQ(C) is mainly due to the resonance charge transfer from the methoxyl oxygen to the benzoquinolinic nitrogen, which greatly stabilizes the OBQ(C) form, resulting in an increase of pKa. This hypothesis can be further supported by another synthesized compound, EBQ, in which the charge resonance between two ester oxygens significantly reduces the charge transfer ability to the benzoquinolinic nitrogen. As a result, EBQ has a similar pKa of 4.3 with respect to that of BQ. In contrast to a unique 470 nm emission observed in NOBQ, OBQ shows pH-dependent fluorescence spectra consisting of two bands with maxima at 390 and 470 nm (Figure 2). The latter one has a spectral profile and decay dynamics (see Table 1) similar to those of NOBQ and was therefore ascribed to the cationic form OBQ(C) emission. Consequently, the 390 nm emission originates from the excited neutral species, OBQ*. This

assignment was supported by the observation of a unique 380 nm OBQ emission band in cyclohexane, where the excitedstate protonation to the benzoquinolinic nitrogen doesn’t take place. The pKa* value of OBQ(C) was measured to be 13.8, which is 6 orders of magnitude larger than that of 7.5 in the ground state. We thus conclude that with respect to the ground state the degree of charge transfer in the excited state is significantly enhanced in OBQ, forming a strong base on the nitrogen site. NBQ in Water. At this point, we attempted to measure the acidity of the hydroxyl proton using the model compound NBQ, in which the hydroxyl group is the only proton-donating site. Unfortunately, similar to the results observed for NOBQ, the measurement of the ground-state pKa value for NBQ is not feasible because the methyl cation dissociated at pH > 8, prior to the dissociation of the hydroxyl proton. However, at pH < 5 NBQ is stable with a S0-S1 absorption maximum at 380 nm and spectral features similar to those of NOBQ and OBQ(C) (Table 1), indicating that at pH < 5 the dominant species is NBQ (i.e. a cationic form). However, in contrast to OBQ(C), which exhibits a unique 470 nm cationic emission in the acidic solution, the emission of NBQ is dominated by a 575 nm emission band from pH ) 5 to 0. Further increasing the acid

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Figure 3. Emission spectra of HBQ in various HCl concentrations of (1) 1.0 M, (2) 3.72 M, (3) 4.63 M, (4) 5.54 M, (5) 6.45 M, (6) 7.36 M, (7), 8.27 M. Inset: Absorption spectra of HBQ at various pH values of (a) 2.05, (b) 2.85, (c) 3.67, (d) 4.17, (e) 4.61, (f) 5.13, (g) 6.58, (h) 7.0, (i) [NaOH] ) 1.0 M, (j) [NaOH] ) 2.0 M, (k) [NaOH] ) 5.0 M.

concentration from 1 M HCl to 10 M HCl leads to the growth of a new emission band maximum at 470 nm accompanied by the decrease of the 575 nm keto-tautomer emission. The 470 nm emission has similar spectral features and decay dynamics with respect to that of OBQ(C) and NOBQ (see Table 1) and is therefore ascribed to the emission of the excited cationic form, NBQ(C)*. As a result, the 575 nm band is unambiguously assigned to the emission of the excited keto-tautomer of NBQ, the NBQ(T)*, which is formed through the adiabatic deprotonation of NBQ*. Since both 470 and 575 nm emission bands have identical excitation spectra, they originate from the same excited species, NBQ*. In other words, NBQ* undergoes two competitive deactivation pathways, the NBQ* f NBQ relaxation (470 nm emission) and NBQ* f NBQ*(T) intermolecular ESPT (deprotonation of the hydroxyl proton to water), resulting in a keto-tautomer emission. Since similar results were observed in the case of HBQ in acidic solution, to save the length of the text, detailed excited-state spectroscopy and dynamics will be presented and discussed in a later section. In 1.0 M HCl the spectrum of NBQ is still dominated by the 575 nm keto-tautomer emission, indicating that the acidity of the hydroxyl proton in the excited state is very high. In comparison, for the case of 1-hydroxyphenanthrene (HP, Figure 1), where no such quinolinic nitrogen acts as an electron acceptor, the pKa* of the hydroxyl proton was measured to be 3.0, indicating that in 1.0 M HCl negligible deprotonation occurs in the excited state. Therefore, for NBQ the drastic increase of the acidity of the hydroxyl proton in the excited state must be associated with the charge transfer between the hydroxyl oxygen and benzoquinolinic nitrogen. The results obtained from OBQ and NBQ lead to an important conclusion that the resonance charge transfer is a key factor to

account for the formation of a strong acid in the hydroxyl site of NBQ and a strong base in the nitrogen site of OBQ in the excited state. Therefore, when the acid and base sites are resonantly coupled, such as in the case of HBQ, in which both hydroxyl group and benzoquinolinic nitrogen coexist in a resonance configuration, there should be a simultaneous formation of a super strong acid and base in each site. This conclusion turns out to be crucial in elucidating the photophysical properties of HBQ in water. HBQ in Water. pH-dependent absorption spectra of HBQ in aqueous solution are depicted in the inset of Figure 3. Three ground-state species exist in the studied pH ranges of 2.0-5.0 M NaOH: the cationic form (HBQ(C), λmax ) 390 nm), neutral form (HBQ, λmax ) 365 nm), and the deprotonated anion form (HBQ(A), λmax ) 390 nm). The ground-state pKa value for the hydroxyl proton of HBQ was determined to be 14.2. Apparently, the hydroxyl proton of HBQ dissociates only at very high pH values owing to its strong intramolecular hydrogen bonding with the benzoquinolinic nitrogen. We will focus on the issue of this unusually large hydrogen-bonding effect in the Discussion section. Although similar to OBQ(C), a resonance charge transfer configuration can also be drawn in the case of HBQ(C); the pKa of the protonated benzoquinolinic nitrogen was measured to be only 3.8, which is 3 orders of magnitude smaller than that of OBQ(C). This difference can be rationalized by two factors: (1) the methoxyl group in OBQ(C) is a better electrondonating group than the hydroxyl group in HBQ(C); (2) the proton dissociation product of HBQ(C), forming HBQ, can be further stabilized by a strong intramolecular hydrogen-bonding formation. Therefore, dissociation of the benzoquinolinic proton in HBQ(C) to form HBQ is expected to be more exergonic than

Photophysics of 10-Hydroxybenzo[h]quinoline that of OBQ(C), resulting in the decrease of the pKa. Since the acidity of the hydroxyl proton (pKa ) 14.2) is ∼10 units less than that of the >N+-H proton (pKa ) 3.8), the results clearly indicate that the formation of a ground-state HBQ keto-tautomer, HBQ(T), in aqueous solution is highly unfavorable. This is quite different from the hydroxyl-substituted quinolines, in which, depending on the position of the substitution, a certain degree of equilibrium between the normal species (enol form) and tautomer species (keto or zwitterionic form) exists in water.21-23 In the acidified solution special caution has to be taken in the fluorescence titration study. It was found that HBQ decomposes in H2SO4 and HClO4 solution, which are commonly used in preparing concentrated acidic solution. A high concentration of H2SO4 leads to the sulfonation of HBQ. While an N-oxide product was formed when dissolving HBQ in HClO4 solution. This N-oxide product with an absorption maximum at 350 nm exhibits a fluorescence maximum at 440 nm. Since the emission of HBQ is very weak in aqueous solution (Φf ≈ 0.005, see Table 1), this interfering 440 nm emission makes the spectral analysis complicated even at HClO4 concentration as low as 10-5 M (pH ) 5), in which the yield of the N-oxide is less than 1.0%. Therefore, in this study the acidic solution was prepared by adding various concentrations of HCl, in which no decomposition of HBQ was observed throughout the measurement. On the basis of a pKa of 14.2 for HBQ and 3.8 for HBQ(C), at pH ) 7 the neutral species HBQ should predominate in the ground state. The emitted fluorescence of HBQ at pH ) 7 consists of a dominant orange-red emission maximum at 585 nm, with an overall quantum yield as low as 5.0 × 10-3. At pH ) 1, the emission is still dominated by the 585 nm band and the intensity remains unchanged with respect to that measured at pH ) 7, disregarding that in the ground state >99.9% of HBQ exists in the cationic form (HBQ(C)). Further increasing the acid concentration from 1 M to 10 M HCl, a new emission band maximum at 480 nm gradually increases, accompanied by the decrease of the 585 nm emission. Since the excitation spectrum of the 480 nm emission is identical with the absorption spectrum of HBQ(C), the possibility that the 480 nm emission resulting from the decomposed product of HBQ has been eliminated. Furthermore, the emission peak wavelength resembles that of OBQ(C), NOBQ, and NBQ (see Table 1). Therefore, the 480 nm emission unambiguously originates from the excited cationic form, HBQ(C)*. Consequently, the 585 nm band is assigned to the keto-tautomer, HBQ(T), emission. This assignment can be further rationalized on the basis of its similar peak wavelength (585 nm) with respect to NBQ(T) (575 nm). At pH < 1.0, where only HBQ(C) exists in the ground state, both 480 and 585 nm emission bands have the same excitation spectra, which are identical with the HBQ(C) absorption spectrum, indicating that they originate from the same excited species, the HBQ(C)*. In other words, similar to the dynamics of the relaxation of NBQ*, HBQ(C)* undergoes two competitive deactivation pathways, the HBQ(C)* f HBQ(C) relaxation (480 nm emission) and HBQ(C)* f HBQ*(T) intermolecular ESPT (deprotonation of the hydroxyl proton), resulting in a 585 nm keto-tautomer emission. The acidity of the hydroxyl proton in the excited state is extremely high, as indicated by the observation of only 50% HBQ(C) emission; that is, half still undergoes excited-state proton transfer, forming HBQ(T), in 7.5 M HCl. At 10 M HCl the 585 nm emission disappears and the intensity of the 480 nm band dominates, indicating that the rate of intermolecular ESPT in competition

J. Phys. Chem., Vol. 100, No. 42, 1996 17063

Figure 4. Emission spectra of HBQ in various NaOH concentration of (a) 8.0 M, (b) 10 M, (c) 12 M, (d) 15 M.

with the rate of HBQ(C)* f HBQ(C) relaxation is negligible in the 10 M HCl solution. In the basic solution the dominant 585 nm emission intensity remains constant from the pH value of 7.0 to even 5.0 M NaOH, where the anion form, HBQ(A), is a predominant species in the ground state. Upon further increasing the basicity from 5 M to 15 M NaOH, the 585 nm emission intensity slightly decreased, accompanied by the appearance of a new band maximum at 520 nm (Figure 4). This 520 nm emission band is the mirror image of the absorption spectrum of HBQ(A) and is unambiguously assigned to the anion HBQ(A) emission. Similar to the conclusion made in acidic solution, the 585 nm emission originates from an excited keto-tautomer species HBQ(T)*. More details of the proton transfer dynamics in both concentrated HCl and concentrated NaOH will be discussed in the following section. Discussion Intramolecular Hydrogen-Bonding Effect. The unusually strong hydrogen-bonding formation for HBQ can be verified in the proton NMR study, where the hydroxyl proton exhibits a large downfield chemical shift (δ ) 14.25) in CCl4. If we assume that the chemical shift of the hydroxyl proton in HBQ correlates with that measured in aromatic alcohols, the hydrogenbonding energy can be roughly estimated by introducing Shaefer’s correlation18

∆δ ) -0.4 ( 0.2 + E

(1)

where ∆δ is given in parts per million relative to phenol and E denotes the hydrogen-bonding energy in kcal/mol. A δ value for phenol in CCl4 was measured to be 4.29. Thus, according to eq 1, we obtain a very strong intramolecular hydrogen bonding energy of 10.3 ( 0.2 kcal/mol for HBQ. To rationalize this result, we also performed an ab initio calculation for the hydrogen-bonding energy of HBQ based on the 6-31G** basis set. This energy was calculated as the change in total molecular energy in the conversion of the optimized intramolecular hydrogen bonded species into a conformer in which the O-H group is freely rotated around the C-O bond axis until forming a non-hydrogen-bonded local minimum. The calculation indicates that there exists a non-hydrogen-bonded conformer (Figure 5b) in which the O-H group rotates about 180° with respect to that of the hydrogen-bonded conformer (Figure 5a). It is certainly necessary to check whether the calculated energies for both conformers are at energy minima, transition states, or higher order saddle points. Therefore, the Hessians, and hence vibration frequencies, were calculated, and the result shows no negative vibrational frequency, confirming the existence of local

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Figure 6. Decay of HBQ(C) 480 nm emission, kobs, as a function of the free H2O activity in concentrated HCl.31

Figure 5. Optimized geometry (only bond distances (in angstroms) are shown) of the ground states of (a) intramolecular hydrogen bonded and (b) nonintramolecular hydrogen bonded HBQ.

energy minima for both conformers. The difference of energies between two conformers gives a hydrogen-bonding energy of 12.1 kcal/mol. In addition, a very short hydrogen-bonding distance of 1.81 Å was calculated in the hydrogen-bonded HBQ. In comparison to the hydrogen-bonding energy of OBQ > HBQ because the electrondonating ability is in the order -O- > -OCH3 > -OH. As a result, for HBQ the excited-state resonance charge transfer effect induces a driving force for the dissociation of the hydroxyl proton, forming an oxoanion, which will simultaneously enhance the basicity of quinolinic nitrogen where the protonation takes place. Therefore, charge transfer and proton transfer are strongly coupled in the case of HBQ, forming a keto-tautomer species. Thermodynamically, this process must be highly exergonic, as indicated by the super strong acidity and basicity in hydroxyl and nitrogen sites measured by the fluorescence titration study. This viewpoint can be supported by the

observation of a dominant HBQ(T) 585 nm emission in a broad pH range from 1 M HCl to 5 M NaOH, regardless of whether HBQ is in the neutral, protonated, or deprotonated form. The results also imply that the equilibrium between HBQ(T) and other forms of HBQ may not be established in the excited state. Once the keto-tautomer is formed, the reverse proton transfer is prohibited because the back reaction is highly endergonic, and deactivation is therefore solely dominated by the HBQ(T)* f HBQ(T) nonradiative process. The prohibition of HBQ(T)* formation only at concentrated acid or base solution leads us to propose that the process of forming HBQ(T)* is limited dynamically by the proton-donating or -accepting rate associated with free water molecules in the solution, which competes with the overall non-proton-transfer decay dynamics. Recently, a similar mechanism has been proposed by Bardez et al.24 based on the studies of 6-hydroxyquinoline in aqueous solution. This proposed mechanism can be supported by the decay dynamics of HBQ(C) in concentrated HCl solution. Figure 6 shows the decay of HBQ(C) 480 nm emission, kobs, as a function of the free H2O concentration in terms of water activity, [aH2O].31 The plot can be fitted by the equation expressed as

kobs ) kpt[aH2O]n + kd where kpt is the proton transfer rate and kd denotes the nonproton decay rate. On the basis of the best fit n was calculated to be equal to 2.7 (see Figure 6). This value is close to several theoretical and experimental approaches in which the proton is solvated on average by 3 to 5 water molecules.25 The fitted kd of 1.6 × 108 s-1 is close to the decay rate observed for the cationic form of BQ derivatives such as NOBQ (kobs ) 1.11 × 108 s-1) and OBQ(C) (kobs ) 1.15 × 108 s-1, see Table 1), in which no ESPT takes place. The calculated kpt value of 3.0 × 109 s-1 is in good agreement with the kpt value of 1.0 × 108 to 5.0 × 1011 s-1 measured for a variety of proton transfer molecules in water.26-28 A similar proton transfer mechanism is proposed in the case of the anionic form, HBQ(A), in concentrated NaOH solution, where the rate of proton transfer (protonation) at the quinolinic nitrogen site should depend on the proton-donating ability of free H2O. Unfortunately, since the decay rate of the 520 nm HBQ(A) emission is faster than our instrument response of 5.0 × 109 s-1 even in 15 M NaOH, definitive proof of this proposed mechanism is not feasible at this stage. However, the result indicates that protonation at the quinolinic nitrogen of HBQ(A)*, forming HBQ(T)*, is still a dominant process with respect to the HBQ(A)* f HBQ(A) relaxation even in 15 M NaOH. This viewpoint can be further supported by the observation of an appreciable 585 nm HBQ-

Photophysics of 10-Hydroxybenzo[h]quinoline (T)* emission in 15 M NaOH (Figure 4), where only HBQ(A) exists in the ground state. It is worthy to note that in highly acidic and basic solution where only the cation or anion exists in the ground state the proton transfer reaction (either protonation or deprotonation) is an intermolecular process associated with the solvent (H2O) molecule, whereas in medium pH ranges of 5.0-10, where most applications are studied5-8 due to the enormously high pKa (14.2) of the hydroxyl proton and pKa of 3.8 for the protonated quinolinic nitrogen, HBQ is believed to exist as the normal form (i.e. HBQ(N)) consisting of a strong intramolecular hydrogen bond. Therefore, the excited-state proton transfer takes place intramolecularly. For this case, the rate of ESPT should be ultrafast, which cannot be resolved by our current photoncounting instrument with a system response of 200 ps. It is intriguing to compare pKa and pKa* values for HBQ with two other prototype excited-state proton transfer molecules, 3-hydroxyflavone (3HF) and o-hydroxyacetophenone (HAP), in which the hydroxyl proton and carbonyl oxygen serve as a proton donor and acceptor, respectively. In these two cases, although the basicity of the carbonyl oxygen increases in the excited state, it is still considered as a very weak base due to the pKa* of 1.229 and 2.830 for protonated 3HF and HAP, respectively. In fact, for most of the HAP and 3HF type of proton transfer molecules the pKa* of the protonated carbonyl oxygen is usually on the magnitude of 1.0-3.0, while the pKa* of the hydroxyl proton is normally 0.2, except for BQ) in the aqueous solution, regardless of whether they are in protonated or deprotonated forms, the quantum yield of HBQ(T) emission is very weak (Φf ≈ 0.005). The weak ketotautomer emission is parallel to many other ESPT molecules and can be generally interpreted in terms of two possible deactivation mechanisms: (1) the rapid intersystem crossing to the triplet state, and (2) other fast nonradiative pathways. To resolve mechanism 1, we have attempted to study the ketotautomer triplet state by measuring the phosphorescence of HBQ(T) at 77 K in methylcyclohexane glass using a redsensitive, intensified charge coupled detector. Unfortunately, no phosphorescence can be observed at this stage. The lack of keto-tautomer triplet-state population was further supported by the negligible triplet-triplet absorption in the range 400-700 nm. Therefore, the low quantum yield resulting from the

J. Phys. Chem., Vol. 100, No. 42, 1996 17065 TABLE 2: Photophysical Properties of HBQ Keto-Tautomer Emission in Various Solvents (λex ) 365 nm) n-hexane CCl4 ethyl acetate acetonitrile C2H5OH CH3OH CH3OD CD3OD H2O D2O a

kobs × 10-9

Φf

knr × 10-9

kr × 10-6

1.90 1.52 1.92 1.80, 2.37a 1.75 1.85, 2.01a 0.55 0.50 2.85 0.63

5.0 × 4.88 × 10-3 5.43 × 10-3 5.52 × 10-3 7.74 × 10-3 8.00 × 10-3 3.0 × 10-2 3.5 × 10-2 5.0 × 10-3 2.3 × 10-2

1.89 1.51 1.90 1.79 1.74 1.84 0.53 0.48 2.84 0.62

9.50 7.41 10.4 10.0 14.0 14.8 16.5 17.5 14.3 14.5

10-3

Data are taken from ref 7.

quenching mechanism 1 can be excluded. Mechanism 2 may incorporate multi-deactivation processes. One widely accepted deactivation pathway is through the hydrogen-bonding interaction. The hydrogen-bonding interaction at least partially involved in the depopulation of the HBQ(T) S1 state is supported by the fact that NBQ (T), which has an electronic configuration similar to HBQ(T) but lacks the intramolecular hydrogen bond, fluoresces 10 times stronger (see Table 1). On the other hand, since the S1-S0 emission of the keto-tautomer species is small (∼15 000 cm-1), other nonradiative pathways such as quenching through electronic energy transfer coupled with the O-H stretching vibrations of water molecules are plausible. Highfrequency vibrations are good acceptors because fewer quanta are required in S0 than for a lower frequency vibration, thus providing a more favorable Franck-Condon factor for the S1S0 radiationless process. Apparently, deuterium substitution lowers the frequency of the acceptor mode and hence decreases the Franck-Condon factor and the nonradiative rate. To elucidate this mechanism, we have performed a series of experiments to study the deuterium isotope effect and found that there is considerable solvent isotope effect on HBQ in protic solvents. Comparison of quantum yield data in H2O and D2O (see Table 2) shows that the keto-tautomer emission in D2O is 4.6 times stronger than that in H2O. This result is consistent with the observed decay rate of 6.35 × 108 s-1 for HBQ(T) in D2O, which is 4.5 times less than that in H2O (2.85 × 109 s-1), indicating that the deuterium isotope effect is directly coupled to the nonradiative deactivation pathway. However, from the thermodynamics viewpoint HBQ has formed DBQ through H/D isotope exchange in D2O. Therefore, it is necessary to distinguish the solvent deuterium isotope effect from the intramolecular hydrogen bonding effect due to deuterium substitution. Fortunately, the radiationless deactivation induced by the intramolecular hydrogen bonding effect is not sensitive to the deuterium isotope substitution. This can be supported by the results that measured Φf and kobs values are nearly identical between HBQ and DBQ in various aprotic solvents such as in cyclohexane, where D/H exchange is prohibited. In the case of CH3OD, similar to the results of the H2O/D2O experiment, the observed decay rate is ∼3.5 times smaller than that in CH3OH. This value correlates very well with the ∼4 times increase of the fluorescence yield. Interestingly, Φf and kobs are nearly the same between CH3OD and CD3OD, indicating that only the hydroxyl proton (or deuterium) of methanol that forms an intermolecular hydrogen bond with HBQ plays a major role in the radiationless deactivation. Other high-frequency modes, such as C-H stretching, that do not directly involve hydrogen-bonding formation are not coupled in the deactivation pathways. To explain the weak keto-tautomer emission of 6-hydroxyquinoline in aqueous solution, Bardez et al.24 recently proposed a major deactivation mechanism incorporating a keto-tautomer

17066 J. Phys. Chem., Vol. 100, No. 42, 1996 (Q*) f zwitterionic-tautomer (Z) nonradiative decay process. The subtle change of the molecular geometry due to the different electronic configuration between Q and Z forms induces the nonradiative decay. On the basis of spectroscopic information obtained in this study, we could not definitively conclude that HBQ(T) in the excited state exists as a keto form (Q*) at this stage. However, previous studies have shown that the wavelength of the emission maximum of HBQ(T) is blue-shifted when the polarity of solvent increases.7,8 Assuming that the zwitterionic form has a larger dipole moment than that of the keto form, and should thus be more stable in the polar solvent, the blue shift of the HBQ(T) emission in more polar solvent may indicate that the favorable structure of HBQ(T) in the excited state and the ground state is the keto form and the zwitterionic form, respectively, in H2O. If the keto-tautomer (Q*) f zwitterionic-tautomer (Z) nonradiative decay process is a major deactivation channel for HBQ(T), the fluorescence yield and the dynamics of decay should be nearly solvent isotope independent, which contradicts the large solvent deuterium isotope effect measured for HBQ(T). In addition, the quantum yield of HBQ(T) in various solvents was measured (see Table 2), and the result shows that the keto-tautomer fluorescence yield is weak (Φf < 0.01) in both nonpolar and polar solvents. In cyclohexane, where the formation of a large dipolar zwitterionictautomer (Z) in the ground state seems to be highly unfavorable due to the lack of solvent stabilization, nevertheless, the quantum yield of HBQ(T) is still very weak (Φf ) 0.005), indicating that the proposed Q* f Z nonradiative process for 6-hydroxyquinoline is at least not a major deactivation channel in the case of HBQ(T). Conclusion We have studied the absorption and fluorescence species of various BQ derivatives in aqueous solution. The results have elucidated the photophysical properties of HBQ in water. The conclusions are as follows. 1. The result suggests a cooperative change in the electronic configuration of HBQ mediated by the intramolecular hydrogen bonding formation, resulting in a large stabilization energy for the intramolecular hydrogen bonded conformation. 2. For HBQ, the excited-state resonance charge transfer triggers the proton transfer reaction, which simultaneously enhances the degree of resonance charge transfer effect. In other words, there is a strong coupling between charge and proton transfer in the excited state for the case of HBQ. 3. The formation of a proton transfer keto-tautomer for HBQ is a highly exergonic process in the excited state, regardless of whether HBQ is in the neutral, protonated, or deprotonated form. This process is only limited by either the proton-donating or -accepting rate of the free water molecules at various pH. Acknowledgment. We thank Professor Chen-Pin Chang for stimulating discussions and graciously providing us access to his laboratory facilities. Professor Shannon Studer Martinez

Chou and Wei provided many helpful suggestions for synthetic and purification procedures. Startup support from the National Chung-Cheng University is graciously acknowledged. This work was supported by the National Science Council, Taiwan, R.O.C. (Grant No. NSC 84-2113-M-194-010). References and Notes (1) Alternatively, the name 4-hydroxy-5-azaphenanthrene (HAP) has been used by other authors.5,6 However, Chemical Abstracts shows that the name of HBQ has been used more commonly. In addition, the name 5-hydroxy-4-azaphenanthrene instead of 4-hydroxy-5-azaphenanthrene is adopted in Chemical Abstracts. (2) Inazu, T. J. Am. Chem. Soc. 1966, 39 (5), 1065. (3) Idelson, E. M. U.S. Patent 3,920 667, 1975, Chem. Abstr. 1976, 84, 84. (4) Martinez, M. L.; Cooper, W. C.; Chou, P. T. Chem. Phys. Lett. 1992, 193 (1-3), 151. (5) Chou, P. T.; Martinez, M. L. Radiat. Phys. Chem. 1993, 41, 373. (6) Sytnik, A.; Gormin, D.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11968. (7) Sytnik, A.; Del Valle, J. C. J. Phys. Chem. 1995, 99, 13028. (8) Roberts, E. L.; Chou, P. T.; Alexander, T. A.; Agbaria, R. A.; Warner, I. M. J. Phys. Chem. 1995, 99, 5431. (9) Klopffer, W. Intramolecular Proton Transfer in Electronically Excited Molecules. In AdVances in Photochemistry; Pitts, J. N., Jr., Hammond, G. S., Gollnick, G., Eds.; John Wiley and Sons: New York, 1977; Vol. 10, pp 311-358. (10) Kasha, M. J. Chem. Soc., Faraday, Trans. 2 1986, 82, 2379. (11) Barbara, P. F.; Walsh, P. K.; Brus, L. E. J. Phys. Chem. 1989, 93, 29. (12) Barbara, P. F.; Trommsdorff, H. P., Eds. Spectroscopy and dynamics of elementary proton transfer in polyatomic system. Chem. Phys. 1989, 136, 153-360. (13) Special Issue (M. Kasha Festschirift) J. Phys. Chem. 1991, 95, 10215-10524. (14) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol. A: Chem. 1993, 75, 21. (15) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024. (16) O’Connor, D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic Press: New York, 1984. (17) Chou, P. T.; Martinez, M. L.; Studer, S. L. J. Phys. Chem. 1991, 95, 10306. (18) Schaefer, T. J. Phys. Chem. 1975, 79, 1888. (19) The Hydrogen Bond; Pimentel, G. C., McClellan, A. L., Eds.; W. H. Freeman and Co.: New York, 1960; p 368. (20) Chou, P. T.; Wei, C. Y.; Chang, C. P.; Kuo, M. S. J. Phys. Chem. 1995, 99, 11994. (21) Mason, S. F. J. Chem. Soc. 1957, 5010. (22) Mason, S. F. J. Chem. Soc. 1958, 674. (23) Mason, S. F.; Philp, J.; Smith, B. E. J. Chem. Soc. (A) 1968, 3051. (24) Bardez, E.; Chatelain, A.; Larry, B.; Valeur, B. J. Phys. Chem. 1994, 98, 2357. (25) Lee, J. Robinson, G. W.; Webb, S. P.; Phillips, L. A.; Clark, J. H. J. Am. Chem. Soc. 1986, 108, 6538. (26) Bardez, E.; Goguillon, B. T.; Keh, E.; Valeur, B. J. Phys. Chem. 1984, 88, 1909. (27) Webb, S. P.; Phillips, L. A.; Yeh, S. W.; Tolbert, L. M.; Clark, J. H. J. Phys. Chem. 1986, 90, 5154. (28) Pines, E.; Huppert, D. Chem. Phys. Lett. 1986, 126, 88. (29) Wolfbeis, O. S.; Leiner, M.; Hochmuth, P. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 759. (30) Chou, P. T.; Wei, C. Y.; Chang, C. P. Unpublished results. (31) (a) Paul, M. A. Chem. ReV. 1957, 57, 1. (b) Randall, M.; Young, L. E. J. Am. Chem. Soc. 1928, 50, 989.

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