- Email: [email protected]
Excited-state proton transfer of 7-hydroxyquinoline in a non-polar medium: mechanism of triple proton transfer in the hydrogen-bonded system
7 January 2000
Chemical Physics Letters 316 Ž2000. 88–93 www.elsevier.nlrlocatercplett
Excited-state proton transfer of 7-hydroxyquinoline in a non-polar medium: mechanism of triple proton transfer in the hydrogen-bonded system Shigeru Kohtani ) , Akira Tagami, Ryoichi Nakagaki Department of Physical and Chemical Biodynamics, Faculty of Pharmaceutical Sciences, Kanazawa UniÕersity, Takara-machi, Kanazawa 920-0934, Japan Received 17 August 1999; in final form 20 October 1999
Abstract Excited-state proton transfer ŽESPT. of 7-hydroxyquinoline Ž7HQ. has been investigated in hexane solutions containing a small amount of alcohol Žmethanol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol. by means of picosecond time-resolved fluorescence spectroscopy. Formation of hydrogen-bonded complexes with 1:2 stoichiometry, 7HQ P Žalcohol. 2 , is concluded from measurements and analysis of steady-state absorption spectra. The formation of cyclic complexes in the ground state is supported by semi-empirical MO calculations. The rate constants of ESPT within the complexes are found to increase with increasing acidity of the alcohols. The mechanism of triple proton transfer is discussed on the basis of the Kamlet–Taft acidity and basicity scales of alcohols. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Multiple proton transfer in a hydrogen-bonded system has been of great interest in chemistry and biochemistry for a long time. Recently, Schowen reviewed and classified this reaction into concerted and stepwise mechanisms w1x. He also claimed that multiple proton transfer obeys Jencks’s principle w2x that if an intermediate along a stepwise route has a very high-energy structure then it is likely that a lower-energy structure will be possible for a concerted reaction transition state. If this is the case, the reaction will proceed along the concerted route. According to this concept, alcohol-mediated double pro) Corresponding author. E-mail: [email protected]
ton transfer of excited 7-azaindole Ž7-AI. proceeds in a concerted manner because generation of an oxonium or alkoxide ion is unfavorable as the intermediate in the stepwise route w3x. On the other hand, the isomerization of dimeric excited 7AI in the gas phase occurs in a stepwise manner in which a zwitterionic intermediate is stabilized by charge delocalization in the aromatic rings w4x. Tautomerization reactions of 7-hydroxyquinoline Ž7HQ. in the excited and ground states are well known as multiple proton transfer processes mediated by protic solvents w5–17x. Lee and Jang reported that ESPT of 7HQ in a neutral aqueous solution takes place in the stepwise manner w11x. Douhal and co-workers suggested that stepwise ESPT may occur in 1:1 hydrogen-bonded 7HQrglycerol or 7HQrethylene glycol systems w14x. Chou and co-workers
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 2 4 7 - 6
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
proposed a mechanism of ESPT incorporating rotational diffusion dynamics of the protic solvent within a 1:1 7HQracetic acid complex w15x. On the other hand, Fang has investigated the formation of hydrogen-bonded 7HQ P Žmethanol. 2 Ž n s 1,2. and 7HQ P ŽH 2 O. n Ž n s 1–3. complexes and predicted multiple proton transfer within the cyclic complexes on the basis of ab initio quantum-mechanical calculations w16,17x. The two-step model has been considered for the isomerizations of 7HQ in neat alcohols w8,10x. The first step is thermally activated formation of a specific cyclic hydrogen-bonded 7HQ P Žalcohol. 2 intermediate Žsolvent reorganization. and the second step is a proton transfer event. Varma and co-workers reported that two fluorescence rise signals from the excited 7HQ tautomer were observed in neat alcohols w8x. They assigned the fast component Žminor. to the direct ESPT within a cyclic 7HQ P Žalcohol. 2 complex and the slow one Žmajor. to the two-step process in which solvated 7HQ P Žalcohol. n complex Ž n ) 2. is thermally reorganized to a cyclic intermediate in the excited state, followed by ESPT. Thus, the reorganization process strongly affects the overall isomerization rates of 7HQ. Consequently, it is desirable for a better understanding of ESPT mechanism to investigate the tautomerization within cyclic 7HQ P Žalcohol. 2 complexes already formed in the ground state. In this study, we first confirm the formation and 1:2 stoichiometry of the 7HQ P Žalcohol. 2 complexes in hexane on the basis of analysis of absorption spectra. The alcohols used here are methanol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol. The optimized structure of the ground-state 7HQ P Žalcohol. 2 complexes have been calculated by semiempirical MO methods ŽAM1 and PM3.. We next measure the rate constants of ESPT occurring within the complexes Žsee Scheme 1. by picosecond time-
Scheme 1.
89
resolved fluorescence measurements and finally the mechanism of the ESPT is discussed.
2. Materials and methods 2.1. Materials 7HQ ŽAcross Organics. was purified by sublimation under reduced pressure. Hexane and 2-propanol were fractionally distilled once before use. Methanol and ethanol were purified by distillation after refluxing for 2 h over magnesium oxide and calcium oxide, respectively. tert-Butanol was distilled after refluxing for 24 h over calcium hydride and further distilled over benzoic acid. 2,2,2-Trifluoroethanol was purified by distillation over sodium hydrogen carbonate. The concentration of 7HQ was adjusted to 10y4 –10y5 M. 2.2. Measurements Absorption spectra were measured using a 10 cm optical pathlength cell and a spectrophotometer ŽHITACHI U-3210.. Steady-state fluorescence spectra were obtained on a fluorescence spectrophotometer ŽHITACHI F-4500.. The time-resolved fluorescence measurements were carried out using a photon-counting streak camera system with picosecond excitation pulses Žexcitation wavelength of 345 nm and a pulse energy of approximately 50 mJ at 10 Hz repetition rate.. The overall instrument response function was found to be 10 ps. The details of experimental setup have been reported elsewhere w12x. 2.3. Calculations AM1 and PM3 semi-empirical MO calculations were performed using the MOPAC 97 software package ŽFujitsu.. Geometry optimization of the complexes were first performed in vacuum and then carried out in relative dielectric constant of hexane Ž1.89. by the COSMO method in which NSPA Žnumber of segment per atom. and RSOLV Žradius of solvent. parameters were set to values of 60 and 2.0, respectively.
90
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
3. Results and discussion 3.1. Steady-state absorption and fluorescence spectra Fig. 1 shows steady-state absorption and fluorescence spectra of 7HQ in hexane containing a 0–25 mM concentration of methanol. The spectral shape apparently varies with increasing methanol concentration: broadening and intensification of the absorption band observed in 300–360 nm region on addition of methanol is attributed to the hydrogenbonding interaction of 7HQ and methanol. Dual fluorescence in the 360–450 nm region and above 500 nm are ascribed to emission from the excited enol 7HQ and tautomer generated via ESPT as previously assigned by Itoh and co-workers, respectively w7x. The tautomeric fluorescence with a large Stokes shift is intensified on increasing the methanol concentration. No other fluorescence can be detected. Similar spectral changes in absorption spectra were observed for all alcohols; the broad absorption band appears in the same wavelength region Ž300– 360 nm. with addition of alcohol. Therefore, the ground-state hydrogen-bonded complexes are formed in the same manner. Furthermore, two common features are observed in fluorescence spectra for all alcohols: the first is that the intensity of tautomer fluorescence Ž l ) 500 nm. increases with increasing alcohol concentration and the second is that no other
fluorescence Žemission from cation or anion. was detected at 238C. Fluorescence from the excited enol 7HQ in 2,2,2-trifluoroethanolrhexane solution was not observed even at high alcohol concentration Ž50 mM. because of a very efficient ESPT for 2,2,2-trifluoroethanol Žvide infra.. The 1:2 stoichiometry of the 7HQralcohol complex was confirmed from the absorbance change at 345 nm Žarrow in Fig. 1... The equilibrium of the complex formation is expressed as follows: K
7HQ q 2 alcohol ~ 7HQ P Ž alcohol. 2 where K is a corresponding equilibrium constant. The K value is obtained by the following equation w12x: 1 C
2
s Ka Ž ´AB y ´A .
10 d y d0
yK
where d and d 0 denote the absorbance measured at the alcohol concentration C and 0 mM, respectively, a is the initial concentration of 7HQ, and ´A and ´AB are the molar extinction coefficients of the uncomplexed and complexed 7HQ, respectively. If the squared reciprocals of alcohol concentration Ž1rC 2 . are linearly correlated with the reciprocal of the absorbance change at 345 nm Ž1rd–d 0 ., the 1:2 stoichometry of 7HQralcohol is apparent and the K values can be determined from intercepts of the straight lines. Fig. 2 shows linear relationships between Ž1rC 2 . and Ž1rd–d 0 .. Consequently, the hydrogen-bonded complexes are composed of 7HQ and two alcohol molecules. Since the K values are more than 5000 My2 for all alcohols, the 7HQ P Žalcohol. 2 complexes are readily formed in hexane. ŽThe K value for the formation of 7HQ P Žmethanol. 2 complex was erroneously printed as 900 My2 in a previous paper w12x and is actually 9000 My2 . The present result is in good agreement with the previous data.. 3.2. Semi-empirical MO calculations
Fig. 1. Absorption and fluorescence spectra of 7HQ Ž1=10y5 M. in hexane containing a small amount of methanol at 238C. Con. 0 mM, ŽPPPPP. 5 mM, centrations of methanol were Ž Ž- - -. 10 mM, Ž-P-. 15 mM, Ž-PP-. 20 mM and Ž . 25 mM.
The formation of 7HQ P Žalcohol. 2 complexes in hexane is confirmed by AM1 and PM3 semi-empirical MO methods. Fig. 3a,b show the energyminimized structure of the hydrogen-bonded complex of 7HQ P Ž tert-butanol. 2 calculated by the AM1
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
Fig. 2. Plots of the squared reciprocals of alcohol concentration Ž1r C 2 . versus the reciprocals of the absorbance change at 345 nm Ž1r d – d 0 .. Ža. Methanol Ž-`-., ethanol ŽPIP., 2-propanol Ž-PP D P P-.; Žb. 2,2,2-trifluoroethanol Ž —e— ., tert-butanol Ž-P=P-.. The values of equilibrium constants were obtained for methanol Ž13 600"900 My2 ., 2,2,2-trifluoroethanol Ž10 500" 1000 My2 ., ethanol Ž10 100"200 My2 ., 2-propanol Ž5000"300 My2 . and tert-butanol Ž5700"400 My2 ..
method. Fig. 3b indicates the structure viewed from the arrow in Fig. 3a. As is seen from Fig. 3, three
91
hydrogen bonds are located nearly in the plane of the quinoline ring, whereas tert-butyl groups are located above and below the plane. This geometry is commonly obtained for all the 7HQ P Žalcohol. 2 complexes by AM1 and PM3 optimizations. The three hydrogen-bond distances are calculated to be less ˚ for all the 7HQ P Žalcohol. 2 complexes. than 3 A Although our values of the hydrogen-bond distances of the 7HQ P Žmethanol. 2 complex in vacuum are somewhat longer than those calculated by ab initio methods w16x, a similar cyclic structure is obtained. The cyclic hydrogen-bonded complex has been found for other 7HQrprotic solvent systems. The formation of cyclic hydrogen-bonded 7HQ P ŽH 2 O. n Ž n s 2,3. have been experimentally observed under jet-cooled conditions w18,19x and further supported by theoretical calculations w17x. The cyclic hydrogen-bonded structure has also been confirmed in the 1:1 7HQrglycerol and 7HQrethylene glycol complexes using the PM3 semi-empirical MO method w14x. In the present study, we further demonstrate the formation of a 1:2 cyclic hydrogen-bonded complex between 7HQ and several alcohols in hexane by employing both AM1 and PM3 semi-empirical MO methods. 3.3. Time-resolÕed fluorescence measurements The overall tautomerization rates were determined by measuring tautomer fluorescence rise times Žtr . after excitation pulse irradiation. Fig. 4 shows fluo-
Fig. 3. Ground-state optimized structure of the cyclic 7HQ P Ž tert-butanol. 2 complex in hexane Žrelative dielectric constants 1.89. obtained ˚ by the semi-empirical method AM1. Žb. The structure viewed from the arrow in Ža.. Distances are given in Angstroms.
92
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
in hexane; this is due to the cyclic formation of the complexes. On the other hand, the overall reaction rates in neat alcohols are significantly slower at low temperature because a thermally activated solvent reorganization process is involved in this reaction. 3.4. Mechanism of ESPT in the hydrogen-bonded system
Fig. 4. Fluorescence rise signals of 7HQ tautomer in hexane solutions containing 50 mM 2,2,2-trifluoroethanol Ž1., methanol Ž2., ethanol Ž3., 2-propanol Ž4. and tert-butanol Ž5. at 238C. The rise signals were detected at the tautomer fluorescence band Ž l ) 500 nm..
rescence rise signals of the tautomer in hexane solutions containing 50 mM of each alcohol at 238C. These time profiles are well fitted by a sum of two exponential functions Žrise and decay components., which suggests that ESPT in these solutions occurs only within the cyclic 7HQ P Žalcohol. 2 complexes. The obtained tr values and tautomer fluorescence lifetimes are summarized in Table 1. Apparently the tautomerization rate depends on the alcohol. Very fast ESPT takes place for 2,2,2-trifluoroethanol, and the tr value Ž14 ps. is more than 10 times faster than that for 2-propanol Ž146 ps. and tert-butanol Ž185 ps.. The tr value for methanol Ž68 ps. is similar to that reported by Varma and coworkers Ž50 ps. w8x. ESPT within the cyclic 7HQ P Žalcohol. 2 complexes in hexane has been concluded from a comparison of temperature dependence of the tr values in hexane and neat alcohol. The value in hexane containing 50 mM methanol Ž68 ps. is 2.5 times faster than that in neat methanol Ž170 ps. at 238C. Furthermore, the tr values in hexane show little temperature dependence Ž70 ps at y608C. while those in neat methanol slow dramatically with decreasing temperature Ž600 ps at y608C.. In other words, the tr value is increased by a factor of 9 on going from the hexane solution to neat methanol at y608C. Similarly, the tr values in hexane varies slightly from 94 ps Ž238C. to 128 ps Žy608C. for ethanol. Thus, fast tautomerization is observed even at low temperature
From the facts described above, the solvent reorganization effects are negligible and our observation is directly related to the ESPT rates. Thus, the reciprocal of the tr value is regarded as the proton transfer rate constants Ž k pt . if the deactivation rate constant Ža sum of radiative and non-radiative rate constants. of excited 7HQ without ESPT is of the same order of magnitude as that for 8-methyl-substituted 7HQ Ž2.8 = 10 8 sy1 .. It should be noted that 8-methyl-substituted 7HQ shows no ESPT w12x. The k pt values are listed in Table 2 with the Kamlet–Taft acidity Ž a . and basicity Ž b . scales of the alcohol w20x. These a and b values, determined by solvatochromic comparison methods w21x, provide explicit measures of the hydrogen-bond donating and accepting power of the solvent. It is evident that the k pt values tend to increase on increasing the a scale of the alcohol. Therefore, the proton-donating ability of alcohol is crucial for this proton transfer. The proton transfer rate for 2,2,2-trifluoroethanol is much faster than the others because of the greatest a
Table 1 Double-exponential fit parameters a for the tautomer fluorescence rise Žtr . and decay Žtd . curves of 7HQ in hexane solutions containing a small amount of the alcohols at 238C Alcohol
tr Žps.
td Žps.
2,2,2-Trifluoroethanol Methanol Ethanol 2-Propanol tert-Butanol
14 68 Ž"20. b 94 Ž"25. c 146 Ž"10. d 185
1920 2040 Ž"50. b 1640 Ž"110. c 1790 Ž"100. d 2290
a
Time profiles are fit to the functional form AwexpŽy trtd .yexpŽy trtr .x. b An average for five separate measurements. c An average for four separate measurements. d An average for three separate measurements.
FŽ t . s
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93 Table 2 Proton transfer rate constants Ž k pt . at 238C and Kamlet–Taft acidity Ž a . and basicity Ž b . scales of alcohols Alcohol
k pt Ž10 9 sy1 .
aa
ba
2,2,2-Trifluoroethanol Methanol Ethanol 2-Propanol tert-Butanol
70 15 11 6.9 5.4
1.51 0.93 0.83 0.76 0.68
0 0.62 0.77 0.95 1.01
a
Ref. w20x.
value, in spite of the zero value on the b scale. Conversely, proton transfer rates are slow for tertbutanol and 2-propanol despite having significant b values. Thus, the basicity of the alcohol is not important for this proton transfer, suggesting that the reaction rate cannot be determined by the transfer of a hydroxyl proton on 7HQ to the alcohol. The results described above can be explained as follows. Ž1. The reaction is initially triggered by the transfer of the alcoholic proton hydrogen-bonded to the ring nitrogen of 7HQ, which is compatible with the observed dependence of reaction rates on the a values. Ž2. The subsequent transfer of two other hydroxylic protons from cyclic complexes must be very fast because the 7HQ cation that could be generated as a counter-ion of alkoxide is not detected at all. Schowen has pointed out that the alcoholmediated tautomerization of 7-AI proceeds through a concerted mechanism, because a stepwise proton transfer may involve an intermediate of higher energy w1x. Since a similar situation also holds in the present study, the quasi-concerted proton transfer can reasonably be concluded for the cyclic 7HQ P Žalcohol. 2 complexes. More detailed information on the mechanism will be obtained by observing and analyzing isotope effects on proton transfer kinetics for the 7HQ P Žalcohol. 2 complexes one or two hydroxylic protons of which are site-selectively deuterated.
4. Concluding remarks The 1:2 stoichiometry and formation of cyclic 7HQ P Žalcohol. 2 complexes in hexane were obtained
93
from an analysis of the absorbance change at 345 nm and semi-empirical MO calculations. The comparison of the temperature dependence of the tr values in hexane and neat alcohols shows that ESPT in hexane containing 50 mM alcohol occurs only within the cyclic 7HQ P Žalcohol. 2 complexes. The rate constants of ESPT were found to increase with increasing acidity of the alcohols, which suggests that the reaction can be triggered by the transfer of the alcoholic proton hydrogen-bonded to the nitrogen atom of 7HQ.
Acknowledgements The authors are indebted to Professor M. Itoh Žthe Kitasato Institute. and Dr. K. Tokumura ŽKanazawa University. for their helpful comments and advice.
References w1x R.L. Schowen, Angew. Chem. Int. Ed. Eng. 36 Ž1997. 1434. w2x W.P. Jencks, Acc. Chem. Res. 13 Ž1980. 161. w3x Y. Chen, F. Gai, J.W. Petrich, J. Am. Chem. Soc. 115 Ž1993. 10158. w4x A. Douhal, S.K. Kim, A.H. Zewail, Nature 378 Ž1995. 260. w5x S.F. Mason, J. Philip, B.E. Smith, J. Chem. Soc. A, 1968, 3051. w6x P.J. Thistlethwaite, P.J. Corkill, Chem. Phys. Lett. 85 Ž1982. 317. w7x M. Itoh, T. Adachi, K. Tokumura, J. Am. Chem. Soc. 106 Ž1984. 850. w8x J. Konijnenberg, G.B. Ekelmans, A.H. Huizer, C.A.G.O. Varma, J. Chem. Soc., Faraday Trans. II 85 Ž1989. 39. w9x A. Lavin, S. Collins, Chem. Phys. Lett. 204 Ž1993. 96. w10x P.-T. Chou, F. Martinez, Chem. Phys. Lett. 256 Ž1995. 415. w11x S.I. Lee, D.J. Jang, J. Phys. Chem. 99 Ž1995. 7537. w12x T. Nakagawa, S. Kohtani, M. Itoh, J. Am. Chem. Soc. 117 Ž1995. 7952. w13x K. Tokumura et al., Chem. Phys. Lett. 271 Ž1997. 320. w14x I. Garcia-Ochoa et al., J. Phys. Chem. A 102 Ž1998. 8871. w15x P.-T. Chou, C.-Y. Wei, C.-R.C. Wang, F.-T. Hung, C.-P. Chang, J. Phys. Chem. A 103 Ž1999. 1939. w16x W.-H. Fang, J. Am. Chem. Soc. 120 Ž1998. 7586. w17x W.-H. Fang, J. Phys. Chem. A 103 Ž1999. 5567. w18x F. Lahmani, A. Douhal, E. Breheret, A. Zehnacker-Rentien, Chem. Phys. Lett. 220 Ž1994. 235. w19x A. Bach, S. Leutwyler, Chem. Phys. Lett. 299 Ž1999. 381. w20x M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 Ž1983. 2877. w21x R.W. Taft, M.J. Kamlet, J. Am. Chem. Soc. 98 Ž1976. 2886.
Chemical Physics Letters 316 Ž2000. 88–93 www.elsevier.nlrlocatercplett
Excited-state proton transfer of 7-hydroxyquinoline in a non-polar medium: mechanism of triple proton transfer in the hydrogen-bonded system Shigeru Kohtani ) , Akira Tagami, Ryoichi Nakagaki Department of Physical and Chemical Biodynamics, Faculty of Pharmaceutical Sciences, Kanazawa UniÕersity, Takara-machi, Kanazawa 920-0934, Japan Received 17 August 1999; in final form 20 October 1999
Abstract Excited-state proton transfer ŽESPT. of 7-hydroxyquinoline Ž7HQ. has been investigated in hexane solutions containing a small amount of alcohol Žmethanol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol. by means of picosecond time-resolved fluorescence spectroscopy. Formation of hydrogen-bonded complexes with 1:2 stoichiometry, 7HQ P Žalcohol. 2 , is concluded from measurements and analysis of steady-state absorption spectra. The formation of cyclic complexes in the ground state is supported by semi-empirical MO calculations. The rate constants of ESPT within the complexes are found to increase with increasing acidity of the alcohols. The mechanism of triple proton transfer is discussed on the basis of the Kamlet–Taft acidity and basicity scales of alcohols. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Multiple proton transfer in a hydrogen-bonded system has been of great interest in chemistry and biochemistry for a long time. Recently, Schowen reviewed and classified this reaction into concerted and stepwise mechanisms w1x. He also claimed that multiple proton transfer obeys Jencks’s principle w2x that if an intermediate along a stepwise route has a very high-energy structure then it is likely that a lower-energy structure will be possible for a concerted reaction transition state. If this is the case, the reaction will proceed along the concerted route. According to this concept, alcohol-mediated double pro) Corresponding author. E-mail: [email protected]
ton transfer of excited 7-azaindole Ž7-AI. proceeds in a concerted manner because generation of an oxonium or alkoxide ion is unfavorable as the intermediate in the stepwise route w3x. On the other hand, the isomerization of dimeric excited 7AI in the gas phase occurs in a stepwise manner in which a zwitterionic intermediate is stabilized by charge delocalization in the aromatic rings w4x. Tautomerization reactions of 7-hydroxyquinoline Ž7HQ. in the excited and ground states are well known as multiple proton transfer processes mediated by protic solvents w5–17x. Lee and Jang reported that ESPT of 7HQ in a neutral aqueous solution takes place in the stepwise manner w11x. Douhal and co-workers suggested that stepwise ESPT may occur in 1:1 hydrogen-bonded 7HQrglycerol or 7HQrethylene glycol systems w14x. Chou and co-workers
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 2 4 7 - 6
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
proposed a mechanism of ESPT incorporating rotational diffusion dynamics of the protic solvent within a 1:1 7HQracetic acid complex w15x. On the other hand, Fang has investigated the formation of hydrogen-bonded 7HQ P Žmethanol. 2 Ž n s 1,2. and 7HQ P ŽH 2 O. n Ž n s 1–3. complexes and predicted multiple proton transfer within the cyclic complexes on the basis of ab initio quantum-mechanical calculations w16,17x. The two-step model has been considered for the isomerizations of 7HQ in neat alcohols w8,10x. The first step is thermally activated formation of a specific cyclic hydrogen-bonded 7HQ P Žalcohol. 2 intermediate Žsolvent reorganization. and the second step is a proton transfer event. Varma and co-workers reported that two fluorescence rise signals from the excited 7HQ tautomer were observed in neat alcohols w8x. They assigned the fast component Žminor. to the direct ESPT within a cyclic 7HQ P Žalcohol. 2 complex and the slow one Žmajor. to the two-step process in which solvated 7HQ P Žalcohol. n complex Ž n ) 2. is thermally reorganized to a cyclic intermediate in the excited state, followed by ESPT. Thus, the reorganization process strongly affects the overall isomerization rates of 7HQ. Consequently, it is desirable for a better understanding of ESPT mechanism to investigate the tautomerization within cyclic 7HQ P Žalcohol. 2 complexes already formed in the ground state. In this study, we first confirm the formation and 1:2 stoichiometry of the 7HQ P Žalcohol. 2 complexes in hexane on the basis of analysis of absorption spectra. The alcohols used here are methanol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol. The optimized structure of the ground-state 7HQ P Žalcohol. 2 complexes have been calculated by semiempirical MO methods ŽAM1 and PM3.. We next measure the rate constants of ESPT occurring within the complexes Žsee Scheme 1. by picosecond time-
Scheme 1.
89
resolved fluorescence measurements and finally the mechanism of the ESPT is discussed.
2. Materials and methods 2.1. Materials 7HQ ŽAcross Organics. was purified by sublimation under reduced pressure. Hexane and 2-propanol were fractionally distilled once before use. Methanol and ethanol were purified by distillation after refluxing for 2 h over magnesium oxide and calcium oxide, respectively. tert-Butanol was distilled after refluxing for 24 h over calcium hydride and further distilled over benzoic acid. 2,2,2-Trifluoroethanol was purified by distillation over sodium hydrogen carbonate. The concentration of 7HQ was adjusted to 10y4 –10y5 M. 2.2. Measurements Absorption spectra were measured using a 10 cm optical pathlength cell and a spectrophotometer ŽHITACHI U-3210.. Steady-state fluorescence spectra were obtained on a fluorescence spectrophotometer ŽHITACHI F-4500.. The time-resolved fluorescence measurements were carried out using a photon-counting streak camera system with picosecond excitation pulses Žexcitation wavelength of 345 nm and a pulse energy of approximately 50 mJ at 10 Hz repetition rate.. The overall instrument response function was found to be 10 ps. The details of experimental setup have been reported elsewhere w12x. 2.3. Calculations AM1 and PM3 semi-empirical MO calculations were performed using the MOPAC 97 software package ŽFujitsu.. Geometry optimization of the complexes were first performed in vacuum and then carried out in relative dielectric constant of hexane Ž1.89. by the COSMO method in which NSPA Žnumber of segment per atom. and RSOLV Žradius of solvent. parameters were set to values of 60 and 2.0, respectively.
90
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
3. Results and discussion 3.1. Steady-state absorption and fluorescence spectra Fig. 1 shows steady-state absorption and fluorescence spectra of 7HQ in hexane containing a 0–25 mM concentration of methanol. The spectral shape apparently varies with increasing methanol concentration: broadening and intensification of the absorption band observed in 300–360 nm region on addition of methanol is attributed to the hydrogenbonding interaction of 7HQ and methanol. Dual fluorescence in the 360–450 nm region and above 500 nm are ascribed to emission from the excited enol 7HQ and tautomer generated via ESPT as previously assigned by Itoh and co-workers, respectively w7x. The tautomeric fluorescence with a large Stokes shift is intensified on increasing the methanol concentration. No other fluorescence can be detected. Similar spectral changes in absorption spectra were observed for all alcohols; the broad absorption band appears in the same wavelength region Ž300– 360 nm. with addition of alcohol. Therefore, the ground-state hydrogen-bonded complexes are formed in the same manner. Furthermore, two common features are observed in fluorescence spectra for all alcohols: the first is that the intensity of tautomer fluorescence Ž l ) 500 nm. increases with increasing alcohol concentration and the second is that no other
fluorescence Žemission from cation or anion. was detected at 238C. Fluorescence from the excited enol 7HQ in 2,2,2-trifluoroethanolrhexane solution was not observed even at high alcohol concentration Ž50 mM. because of a very efficient ESPT for 2,2,2-trifluoroethanol Žvide infra.. The 1:2 stoichiometry of the 7HQralcohol complex was confirmed from the absorbance change at 345 nm Žarrow in Fig. 1... The equilibrium of the complex formation is expressed as follows: K
7HQ q 2 alcohol ~ 7HQ P Ž alcohol. 2 where K is a corresponding equilibrium constant. The K value is obtained by the following equation w12x: 1 C
2
s Ka Ž ´AB y ´A .
10 d y d0
yK
where d and d 0 denote the absorbance measured at the alcohol concentration C and 0 mM, respectively, a is the initial concentration of 7HQ, and ´A and ´AB are the molar extinction coefficients of the uncomplexed and complexed 7HQ, respectively. If the squared reciprocals of alcohol concentration Ž1rC 2 . are linearly correlated with the reciprocal of the absorbance change at 345 nm Ž1rd–d 0 ., the 1:2 stoichometry of 7HQralcohol is apparent and the K values can be determined from intercepts of the straight lines. Fig. 2 shows linear relationships between Ž1rC 2 . and Ž1rd–d 0 .. Consequently, the hydrogen-bonded complexes are composed of 7HQ and two alcohol molecules. Since the K values are more than 5000 My2 for all alcohols, the 7HQ P Žalcohol. 2 complexes are readily formed in hexane. ŽThe K value for the formation of 7HQ P Žmethanol. 2 complex was erroneously printed as 900 My2 in a previous paper w12x and is actually 9000 My2 . The present result is in good agreement with the previous data.. 3.2. Semi-empirical MO calculations
Fig. 1. Absorption and fluorescence spectra of 7HQ Ž1=10y5 M. in hexane containing a small amount of methanol at 238C. Con. 0 mM, ŽPPPPP. 5 mM, centrations of methanol were Ž Ž- - -. 10 mM, Ž-P-. 15 mM, Ž-PP-. 20 mM and Ž . 25 mM.
The formation of 7HQ P Žalcohol. 2 complexes in hexane is confirmed by AM1 and PM3 semi-empirical MO methods. Fig. 3a,b show the energyminimized structure of the hydrogen-bonded complex of 7HQ P Ž tert-butanol. 2 calculated by the AM1
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
Fig. 2. Plots of the squared reciprocals of alcohol concentration Ž1r C 2 . versus the reciprocals of the absorbance change at 345 nm Ž1r d – d 0 .. Ža. Methanol Ž-`-., ethanol ŽPIP., 2-propanol Ž-PP D P P-.; Žb. 2,2,2-trifluoroethanol Ž —e— ., tert-butanol Ž-P=P-.. The values of equilibrium constants were obtained for methanol Ž13 600"900 My2 ., 2,2,2-trifluoroethanol Ž10 500" 1000 My2 ., ethanol Ž10 100"200 My2 ., 2-propanol Ž5000"300 My2 . and tert-butanol Ž5700"400 My2 ..
method. Fig. 3b indicates the structure viewed from the arrow in Fig. 3a. As is seen from Fig. 3, three
91
hydrogen bonds are located nearly in the plane of the quinoline ring, whereas tert-butyl groups are located above and below the plane. This geometry is commonly obtained for all the 7HQ P Žalcohol. 2 complexes by AM1 and PM3 optimizations. The three hydrogen-bond distances are calculated to be less ˚ for all the 7HQ P Žalcohol. 2 complexes. than 3 A Although our values of the hydrogen-bond distances of the 7HQ P Žmethanol. 2 complex in vacuum are somewhat longer than those calculated by ab initio methods w16x, a similar cyclic structure is obtained. The cyclic hydrogen-bonded complex has been found for other 7HQrprotic solvent systems. The formation of cyclic hydrogen-bonded 7HQ P ŽH 2 O. n Ž n s 2,3. have been experimentally observed under jet-cooled conditions w18,19x and further supported by theoretical calculations w17x. The cyclic hydrogen-bonded structure has also been confirmed in the 1:1 7HQrglycerol and 7HQrethylene glycol complexes using the PM3 semi-empirical MO method w14x. In the present study, we further demonstrate the formation of a 1:2 cyclic hydrogen-bonded complex between 7HQ and several alcohols in hexane by employing both AM1 and PM3 semi-empirical MO methods. 3.3. Time-resolÕed fluorescence measurements The overall tautomerization rates were determined by measuring tautomer fluorescence rise times Žtr . after excitation pulse irradiation. Fig. 4 shows fluo-
Fig. 3. Ground-state optimized structure of the cyclic 7HQ P Ž tert-butanol. 2 complex in hexane Žrelative dielectric constants 1.89. obtained ˚ by the semi-empirical method AM1. Žb. The structure viewed from the arrow in Ža.. Distances are given in Angstroms.
92
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93
in hexane; this is due to the cyclic formation of the complexes. On the other hand, the overall reaction rates in neat alcohols are significantly slower at low temperature because a thermally activated solvent reorganization process is involved in this reaction. 3.4. Mechanism of ESPT in the hydrogen-bonded system
Fig. 4. Fluorescence rise signals of 7HQ tautomer in hexane solutions containing 50 mM 2,2,2-trifluoroethanol Ž1., methanol Ž2., ethanol Ž3., 2-propanol Ž4. and tert-butanol Ž5. at 238C. The rise signals were detected at the tautomer fluorescence band Ž l ) 500 nm..
rescence rise signals of the tautomer in hexane solutions containing 50 mM of each alcohol at 238C. These time profiles are well fitted by a sum of two exponential functions Žrise and decay components., which suggests that ESPT in these solutions occurs only within the cyclic 7HQ P Žalcohol. 2 complexes. The obtained tr values and tautomer fluorescence lifetimes are summarized in Table 1. Apparently the tautomerization rate depends on the alcohol. Very fast ESPT takes place for 2,2,2-trifluoroethanol, and the tr value Ž14 ps. is more than 10 times faster than that for 2-propanol Ž146 ps. and tert-butanol Ž185 ps.. The tr value for methanol Ž68 ps. is similar to that reported by Varma and coworkers Ž50 ps. w8x. ESPT within the cyclic 7HQ P Žalcohol. 2 complexes in hexane has been concluded from a comparison of temperature dependence of the tr values in hexane and neat alcohol. The value in hexane containing 50 mM methanol Ž68 ps. is 2.5 times faster than that in neat methanol Ž170 ps. at 238C. Furthermore, the tr values in hexane show little temperature dependence Ž70 ps at y608C. while those in neat methanol slow dramatically with decreasing temperature Ž600 ps at y608C.. In other words, the tr value is increased by a factor of 9 on going from the hexane solution to neat methanol at y608C. Similarly, the tr values in hexane varies slightly from 94 ps Ž238C. to 128 ps Žy608C. for ethanol. Thus, fast tautomerization is observed even at low temperature
From the facts described above, the solvent reorganization effects are negligible and our observation is directly related to the ESPT rates. Thus, the reciprocal of the tr value is regarded as the proton transfer rate constants Ž k pt . if the deactivation rate constant Ža sum of radiative and non-radiative rate constants. of excited 7HQ without ESPT is of the same order of magnitude as that for 8-methyl-substituted 7HQ Ž2.8 = 10 8 sy1 .. It should be noted that 8-methyl-substituted 7HQ shows no ESPT w12x. The k pt values are listed in Table 2 with the Kamlet–Taft acidity Ž a . and basicity Ž b . scales of the alcohol w20x. These a and b values, determined by solvatochromic comparison methods w21x, provide explicit measures of the hydrogen-bond donating and accepting power of the solvent. It is evident that the k pt values tend to increase on increasing the a scale of the alcohol. Therefore, the proton-donating ability of alcohol is crucial for this proton transfer. The proton transfer rate for 2,2,2-trifluoroethanol is much faster than the others because of the greatest a
Table 1 Double-exponential fit parameters a for the tautomer fluorescence rise Žtr . and decay Žtd . curves of 7HQ in hexane solutions containing a small amount of the alcohols at 238C Alcohol
tr Žps.
td Žps.
2,2,2-Trifluoroethanol Methanol Ethanol 2-Propanol tert-Butanol
14 68 Ž"20. b 94 Ž"25. c 146 Ž"10. d 185
1920 2040 Ž"50. b 1640 Ž"110. c 1790 Ž"100. d 2290
a
Time profiles are fit to the functional form AwexpŽy trtd .yexpŽy trtr .x. b An average for five separate measurements. c An average for four separate measurements. d An average for three separate measurements.
FŽ t . s
S. Kohtani et al.r Chemical Physics Letters 316 (2000) 88–93 Table 2 Proton transfer rate constants Ž k pt . at 238C and Kamlet–Taft acidity Ž a . and basicity Ž b . scales of alcohols Alcohol
k pt Ž10 9 sy1 .
aa
ba
2,2,2-Trifluoroethanol Methanol Ethanol 2-Propanol tert-Butanol
70 15 11 6.9 5.4
1.51 0.93 0.83 0.76 0.68
0 0.62 0.77 0.95 1.01
a
Ref. w20x.
value, in spite of the zero value on the b scale. Conversely, proton transfer rates are slow for tertbutanol and 2-propanol despite having significant b values. Thus, the basicity of the alcohol is not important for this proton transfer, suggesting that the reaction rate cannot be determined by the transfer of a hydroxyl proton on 7HQ to the alcohol. The results described above can be explained as follows. Ž1. The reaction is initially triggered by the transfer of the alcoholic proton hydrogen-bonded to the ring nitrogen of 7HQ, which is compatible with the observed dependence of reaction rates on the a values. Ž2. The subsequent transfer of two other hydroxylic protons from cyclic complexes must be very fast because the 7HQ cation that could be generated as a counter-ion of alkoxide is not detected at all. Schowen has pointed out that the alcoholmediated tautomerization of 7-AI proceeds through a concerted mechanism, because a stepwise proton transfer may involve an intermediate of higher energy w1x. Since a similar situation also holds in the present study, the quasi-concerted proton transfer can reasonably be concluded for the cyclic 7HQ P Žalcohol. 2 complexes. More detailed information on the mechanism will be obtained by observing and analyzing isotope effects on proton transfer kinetics for the 7HQ P Žalcohol. 2 complexes one or two hydroxylic protons of which are site-selectively deuterated.
4. Concluding remarks The 1:2 stoichiometry and formation of cyclic 7HQ P Žalcohol. 2 complexes in hexane were obtained
93
from an analysis of the absorbance change at 345 nm and semi-empirical MO calculations. The comparison of the temperature dependence of the tr values in hexane and neat alcohols shows that ESPT in hexane containing 50 mM alcohol occurs only within the cyclic 7HQ P Žalcohol. 2 complexes. The rate constants of ESPT were found to increase with increasing acidity of the alcohols, which suggests that the reaction can be triggered by the transfer of the alcoholic proton hydrogen-bonded to the nitrogen atom of 7HQ.
Acknowledgements The authors are indebted to Professor M. Itoh Žthe Kitasato Institute. and Dr. K. Tokumura ŽKanazawa University. for their helpful comments and advice.
References w1x R.L. Schowen, Angew. Chem. Int. Ed. Eng. 36 Ž1997. 1434. w2x W.P. Jencks, Acc. Chem. Res. 13 Ž1980. 161. w3x Y. Chen, F. Gai, J.W. Petrich, J. Am. Chem. Soc. 115 Ž1993. 10158. w4x A. Douhal, S.K. Kim, A.H. Zewail, Nature 378 Ž1995. 260. w5x S.F. Mason, J. Philip, B.E. Smith, J. Chem. Soc. A, 1968, 3051. w6x P.J. Thistlethwaite, P.J. Corkill, Chem. Phys. Lett. 85 Ž1982. 317. w7x M. Itoh, T. Adachi, K. Tokumura, J. Am. Chem. Soc. 106 Ž1984. 850. w8x J. Konijnenberg, G.B. Ekelmans, A.H. Huizer, C.A.G.O. Varma, J. Chem. Soc., Faraday Trans. II 85 Ž1989. 39. w9x A. Lavin, S. Collins, Chem. Phys. Lett. 204 Ž1993. 96. w10x P.-T. Chou, F. Martinez, Chem. Phys. Lett. 256 Ž1995. 415. w11x S.I. Lee, D.J. Jang, J. Phys. Chem. 99 Ž1995. 7537. w12x T. Nakagawa, S. Kohtani, M. Itoh, J. Am. Chem. Soc. 117 Ž1995. 7952. w13x K. Tokumura et al., Chem. Phys. Lett. 271 Ž1997. 320. w14x I. Garcia-Ochoa et al., J. Phys. Chem. A 102 Ž1998. 8871. w15x P.-T. Chou, C.-Y. Wei, C.-R.C. Wang, F.-T. Hung, C.-P. Chang, J. Phys. Chem. A 103 Ž1999. 1939. w16x W.-H. Fang, J. Am. Chem. Soc. 120 Ž1998. 7586. w17x W.-H. Fang, J. Phys. Chem. A 103 Ž1999. 5567. w18x F. Lahmani, A. Douhal, E. Breheret, A. Zehnacker-Rentien, Chem. Phys. Lett. 220 Ž1994. 235. w19x A. Bach, S. Leutwyler, Chem. Phys. Lett. 299 Ž1999. 381. w20x M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, J. Org. Chem. 48 Ž1983. 2877. w21x R.W. Taft, M.J. Kamlet, J. Am. Chem. Soc. 98 Ž1976. 2886.