J . Phys. Chem. 1986, 90, 1455-1458
1455
Kinetics of Excited-State Proton Transfer in “Double” Benzoxazoles: 2,5-Bis( 2-benzoxazolyl)-4-methoxyphenol Andrzej Mordziiiski* Institute of Physical Chemistry, Polish Academy of Sciences, 44 Kasprzaka. 01 -224 Warsaw, Poland
and Wolfgang Kuhnle Max-Planck-Institut fur biophysikalische Chemie, Abteilung Spektroskopie, 0-3400 Gottingen, Federal Republic of Germany (Received: November 4 , 1985)
2,5-Bis(2-benzoxazolyl)-4-methoxyphenolexhibits dual luminescence arising from primarily excited and proton-transferred species. The temperature dependences of the relative fluorescence quantum yields allow us to determine the energy barriers controlling forward and back proton transfer. Excited-state intramolecular proton transfer was found to occur effectively even at very low temperatures; at 77 K, the proton-transfer rate constant kPT= (2 f 0.5) X 1O’O s-,. Nonlinear Arrhenius plots of the fluorescence intensity ratio strongly support the hypothesis that proton tunneling is effective in this particular excited-state reaction.
Introduction It is well-known that intra- and intermolecular hydrogen bonding can lead to drastic changes in the fluorescence lifetimes and quantum yields by comparison with non-H-bonded analogues. In most cases, the excited-state intramolecular proton transfer (ESIPT) plays a dominant role in the nonradiative dissipation of the excitation energy. As a result of phototautomerization and the ground-state back-reaction, the proton falls back to its original configuration without any photochemical change. Systems of this type are widely applied as effective light One of the fundamental problems is the question whether an energy barrier has to be overcome during the ESIPT reaction. As we have reported in our previous papers,“” 2,5-bis(2benzoxazoly1)hydroquinone (I, see Figure 1) reveals two fluorescence bands, F, and FZ,arising from the primary excited and the tautomeric form, linked by equilibration, which is established to be very fast; the excited-state reaction enthalpy AH* has been evaluated: AH* = -0.5 kcal/mol. It has been concluded that the two forms are separated by a very low energy barrier; the proton moves inside the excited-state double potential Ernstring et al.’ examined the isolated, cold molecule I in a supersonic jet and confirmed the existence of a barrier to ESIET. The height of the barrier in 2-MTHF glass was found to be Em = 0.35 kcal/mol or 121 cm-’. W e have also found dual fluorescence for the monomethoxy derivative of I, 2,5-bis(2-benzoxazolyl)-4-methoxyphenol(11), in nonpolar solvents (Figure 1). The close similarity of the spectral behavior of I and its monomethoxy analogue was considered to be one of the most convincing arguments supporting the assumption that, in system I, proton translocation in the excited state takes place on one of the alternative centers only. A deeper insight into the kinetics of single ESIPT can be achieved by a study of temperature effects on the dual luminescence. We report here some thermodynamic and kinetic parameters of the excited-state reaction of I1 in glasses of 3-methylpentane and 2-methyltetrahydrofuran. Dynamics of the intramolecular process, in both polar and nonpolar solvents, was completely described in terms of formal kinetic relations. Experimental Section Compound I was synthesized as d e ~ c r i b e d .2,5-bis(2-benz~ oxazolyl)-4-methoxyphenol (11) and 2,5-bis(2-benzoxazolyl)1,4-dimethoxybenzene (111) were prepared by the methylation of the sodium salts of I using CHJ. The compounds were separated
* Present address: Max-Planck-Institut fiir biophysikalische Chemie, Abteilung Laserphysik, D-3400 Gottingen, Federal Republic of Germany. 0022-3654/86/2090- 1455$01.50/0
chromatographically on SO2and finally purified by high-pressure chromatography (HPLC). Solvents. 3-Methylpentane (3-MP) and 2-methyltetrahydrofuran (2-MTHF) were purified by column chromatography; 2M T H F was additionally distilled from LiAIH.,. The temperature dependence of fluorescence intensities was measured between 295 and 12 K with Jasny’s spectrofluorimeter* and a helium cryostat, which was specially designed for the instrument. Fluorescence decay curves were recorded by single-photon counting. (We are indebted to Dr. B. Kozankiewicz for performing these measurements.) Fluorescence lifetimes were determined by least-squares fitting combined with a deconvolution of the excitation pulse.
Results Room temperature absorption and fluorescence spectra of molecules 1-111 in aprotic solvents are shown in Figure 1. For I and I1 two emission bands, F1 and F,, have been found. The emission F2, which is Stokes shifted by more than 8000 cm-I and arises from the proton-transferred species, is not observed in the dimethoxy derivative 111. Only the intense (aF = 0.35, at room temperature) normal fluorescence F, was found in this case. OH OCH3 substitution causes a blue shift of about 900 cm-’ per methoxy group, both in absorption and fluorescence. This is mainly due to consecutive elimination of intramolecular H bonds; however, steric effects of the methyl substituent may also contribute to a minor degree. For the remainder of this work we will concentrate on the photophysics of 11. One should notice that, contrary to our previous investigations where the spectroscopy of I1 was measured with the assistence of I, here the whole study was performed for the samples carefully separated by HPLC. In nonpolar solvents, e.g., 3-methylpentane (3-MP), room temperature excitation spectra of both emissions are in very good agreement with the absorption spectrum. Moreover, the fluorescence lifetimes of F, and F2 are equal, over a wide temperature region (293-153 K): T~~ = T F (3-MP, ~ 293 K) = 3.2 f 0.2 ns. This has been already stated for I in nonpolar solvents;6
-
(1) H.J. Heller and H.R. Blattmann, Pure Appl. Chem., 36, 141 (1973). (2) D.L. Williams and A. Heller, J . Phys. Chem., 74, 4473 (1970). (3) J. E.A. Otterstedt, J. Chem. Phys., 58, 5716 (1973). (4) A.Mordziiiski, A.Grabowska, W. Kuhnle, and A. Kr6wczyiiski, Chem. Phys. Lett., 101, 291 (1983). ( 5 ) A. Mordziiiski and A. Grabowska, J . Mol. Struct., 114, 337 (1984). (6) A. Mordziiiski, A. Grabowska, and K. Teuchner, Chem. Phys. Lett., 111, 383 (1984). (7) U.Brackmann, N.P. Ernsting, D. Ouw, and K. Schmitt, Chem. Phys. Lett., 110, 319 (1984). (8) J. Jasny, J . Lumin.,17, 149 (1978).
0 1986 American Chemical Society
Mordzifiski and Kiihnle
1456 The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986 j"10
14
I
H-G
/I i 0
1 15
I
1
20
25
30000 cm"
-2 5 e
-30-
* *
e
* * . . e *
-35-4 0
1030KIT 0'
' 15
I
20
/, 25
'
J
3C000cm.'
Figure 1. Room temperature absorption (in 2-MTHF) and fluorescence spectra (in 3-MP) of 2,S-bis(2-benzoxazolyl)hydroquinone (I), 2,s-bis(2-benzoxazolyl)-4-methoxyphenol(II), and 2,S-bis(2-benzoxazolyl)1,4-dimethoxybenzene(111).
however, it should be noted that F, and F, fluorescence lifetimes and intensities of I are much lower than those of its monomethoxy derivative 11. The strongly temperature-dependent nonradiative deactivation process (competing with ESIPT) which has been stated for the primary excited form of I6 is much less effective OCH, substitution. after OH In polar solvents, e.g., in 2-methyltetrahydrofuran (2-MTHF), the fluorescence band F2 (at room temperature) is red shifted (by about 700 cm-I or 2 kcal/mol) by comparison with nonpolar solvents (e.g., 3-MP). The transition energy of the tautomeric species must therefore be quite sensitive to the polarity of the environment. Upon increasing the polarity of the solvent we find that the fluorescence intensity ratio of F2 to F, increases. The fluorescence quantum yields in 3-MP at room temperature (dielectric constant E = 1.9) are qFl = 0.1 1 and ?&2 = 0.36. In 2-MTHF ( E = 6.2), however, qFl = 0.006 and qFz = 0.50, Le. the total fluorescence intensity, which in nonpolar solvents is distributed between the primary and the tautomeric species, now "flows" almost entirely into the final form exhibiting the red fluorescence F2. Moreover, the fluorescence lifetimes in 2-MTHF are no longer equal. At room temperatures, T ~ =, 0.7 f 0.3 ns is mainly determined by the fast excited-state proton transfer rate and is therefore much shorter than 7 ~ = 2 4.6 f 0.2 ns. In order to determine the photophysical parameters of the excited-state reaction, the temperature dependence of the fluorescence intensities in 3-MP and 2-MTHF was investigated. Figure 2 shows the quantum yields of F1 and F2 fluorescence of I1 in 2-MTHF as a function of temperature. As the sample was cooled, the quantum yield of the primary emission (qF1)decreased, reached a minimum, and again increased. The temperature dependence of the fluorescence quantum yield of the tautomeric
-
Figure 2. Temperature dependence of the quantum yields of F, and F2 emission of 11 in 2-MTHF and of the function In ( q F 2 / q F I - k,J. 'Ir
5045-
40-
qFi/?F1
k1
\ \
\
~
3530 1
1
2
3
1
5
6
7
-
--
100 ,< I T Figure 3. Temperature dependence of the ?7F2/qFI fluorescence intensity ratio of I1 in 2-MTHF below 100 K.
emission is much weaker; 7&2 increases slightly with decreasing temperature and reaches its highest value at low temperatures. Below 50 K, the fluorescence intensity ratio starts to be temperature independent (Figure 3). The high value of t7F2/?7F! even at very low temperatures strongly suggests that the description of the ESIPT reaction as a thermally activated process is incomplete in the present case. Discussion The general behavior of dual fluorescence of I found both for the various solvent^^-^ and for the isolated, jet-cooled molecule7 may be considered as direct evidence for the existence of a barrier in the ESIPT reaction. Compound I1 may be treated as another example of this type of reaction. For nonpolar solvents and at temperatures above 150 K the equality of F,and F, fluorescence lifetimes was stated. This proves that for I1 in 3-MP a very fast excited state equilibration between
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1457
Proton Transfer in "Double" Benzoxazoles 4
\s,
kPT
v
N- H-0 Figure 4. Kinetic scheme of the population and depopulation of the primary excited (X*) and the tautomeric ( Y * )form. The influence of a polar environment on potential energy curves is shown (dashed line).
the primary (X*) and tautomeric (Y*) species is achieved. Thus, both the forward and back proton-transfer rates should be taken into account in the kinetic scheme (Figure 4). The excited-state kinetics can be described by the following coupled differential equations: d[X*]/dt =
Iabs
- [X*l(kXO+ ZPT(T))
+ [y*liPT(T)
d[Y*]/dt = [x*liPT(T) - [y*l(kYo + iPT(T))
= kffPT( T )/ lkYoiPT( T ) + kXoEPT(T)]
?7FZ
= kFiPT(T)/[kYoiPT(T) ?7F2/?7F1
=
+ kXokcPT(T)l
kf'/kfKe(T)
(2)
(3)
(4) (5)
Since the radiative rate constants kf and kf' are in all probability rather insensitive to temperature, it follows from eq 5 that an increase of the ratio ?&2/qF1 upon cooling, as observed for I1 i,n 3 - y P , reflects the increase of the equilibrium constant K , = kPT/kPT.From the corresponding temp_erature_dependence,the excited-state enthalpy change AH* = EPT- E p T can be determined. According to the model of the excited-state reaction kinetics93l0 biexponential fluorescence decays may be expected, if the equiY* is established. librium X* The decay of the primary excited form and of the product of the ESIPT reaction has to be described by a sum and a difference of the corresponding exponentials. If the equilibrium is not e_sta_blishedwithin the lifetime of the excited state and the product kpTkpr can be neglected, the general kinetic relations are much
[X*] = [X*], exp(-Xlt) [y*1 = iPT[X*]O/(XZ
- Xl)(exp(-Xlt) - exp(-X2t))
-
(l)
where Iabs is the rate of excitation, kxo and kyo are the sums of radiative and radiationless rate constants for th_e depopulation of the primary and tautomeric forms, respectively; k,(T) and kn(T) denote rate constants of the forward and back proton transfer, carrying the main temperature dependence. As _over a brosd temperature range T F ~= 7F2,it is safe to assume that kpT and kpT >> kyo, kxo. Under these conditions the fluorescence quantum yields vFl of x * and ?7F2 of Y* are ?7Fl
temperature (which was already noted in Figure 3). At higher temperatures where according to relation 5 the linear portion represents a van? Hoff plot, the excited-state enthalpy change AH* can be determined. We have obtained AH* = -1.3 kcal/mol in 3-MP and AH* = -3.5 kcal/mol in 2-MTHF; so AH* in 2-MTHF is much larger (more negative) than in 3-MP. Since the polar solvent stabilities preferentially the tautomeric form, we conclude that the excited-state proton-transferred species has polar character (cf. Figure 4). In going from 3-MP to 2-MTHF solutions, the fluorescence F2shifts to the red by ca. 2 kcal/mol. This is almost completely accounted for by the enthalpy change for the ESIPT reaction noted above. Hence it appears that the ground-state tautomer has relatively little polar character. It is evident that both the temperature and polarity of environment control the kinetics of the ESIPT reaction. In the lowtemperature region and/or in highly polar solvents, the process X* Y* follows the simple, irreversible kinetics. Thus, at low temperatures in 2-MTHF, the slope of In (?7F2/?7F1 - k,) against T 1(Figure 2) gives the_temperature dependence of the protontransfer rate constant, kp? = f(T).This enables us to determine the height of the energy barrier of the ESIPT reaction: E p T = 1.1 kcal/mol (365 cm-I). It can be noted in this particular case that the energy required for proton translocation is definitely higher than the value reported by Ernsting Zt aL7 for I in the same glass. In the irreversible regime, where kPTC < kyo,the ESIPT rate constant can be evaluated by the following equation:
(6)
(7)
where X2 = kyo is the decay parameter for the tautomeric form. All the data required to determine the proton-transfer rate constant according to eq 8 can be obtained experimentally: kf = 2.3 x 108S-1 at 295 K and kf' = vF2/TF2 = 1.2 x loss-', qF2/qFl = 57, 7 ~ = 2 5.1 f 0.2 ns a t 77 K in 2-MTHF. In this way we get kpT = (2 f 0.5) X 1O'O sd. Thus, in the present case the ESIPT process occurs much more slowly than anJicipated. In numerous other examples reported in recent years kPT> 10" s-l, even at very low temperatures."-I6 The calcu1atio;s presented above are based on the assumption that kxo C C kPT. This is justified as at room temperature kxo can be estimated as C3.0 X lo9 s-l. At temperatures as low as 77 K, the ESIPT process occurs preferentially as the therqally activated reaction. Since the height of the energy barrier EpTfor I1 in 2-MTHF was determined to be 1.1 kcal/mol, one can estimate the temperatureindependent term. The preexponential factor is close to l O I 3 as expected for first-order chemical reaction of AS* 0. Thus, proton transfer from the primary excited into the tautomeric form has to be a spin-allowed, fast nonradiative transition. At room temperature, k , can be expected to be extremely fast, Le., of the order of 10l2 s-l. Let us now discuss the ?7F2/7&1 fluorescence intensity ratio at very low temperatures. The fluorescence intensity ratio shows that qFz/qF1 remains constant (cf. Figure 3) below 50 K. The high value of ko = ?7F2O/?&1' strongly suggests that a certain "initial" fraction of the molecules is placed into a proton-transferred form, which is a direct indication of proton tunneling in the excited-state reaction. We have also very recently found the dual fluorescence for the isolated molecule 11, cooled in a freely expanded neon jet. The observation of the tautomeric emission from excitation at the origin may be again attributed as an argument to support the hypothesis that proton penetration by tunneling plays an important role as a possible reaction route.
=
&.
Here, XI = kxo + iPT and A2 = ky0 + In this model it is obvious that the final product Y* cannot decay faster than its kinetic precursor, the primary species X*. This is the case for 11 in 2-MTHF as TFI C 7F2. As is shown in Figure 2 the plot In (?7F2/77F1- ko)vs. T I clearly displays two different regions of linearity. Here ko = T F ~ ~ / represents the temperature-independent, limiting term at very low
(1 1) K. K. Smith and K. J. Kaufmann, J . Phys. Chem., 82, 2286 (1978). (12) P. F. Barbara, L. E. Brus, and P. M . Rentzepis, J . Am. Chem. SOC., 102, 2786, 5631 (1980). (13) K. Ding, S. J. Courtney, A. J. Strandjord, S. Flom, D. Friedrich, and P. F. Barbara, J . Phys. Chem., 87, 1184 (1983). ~ F , ~ (14) G. J. Woolfe, M. Melzing, S. Schneider, and F. Dorr, Chem. Phys., 77, 213 (1983). (15) D. McMorrow, T. P. Dzugan, and T. J. Aartsma, Chem. Phys. Left., 103, 4 9 2 (1984). (16) H. Shizuka, M. Machii, Y.Higaki, M. Tanaka, and I. Tanaka, J . (9) A. Weller, 2. Phys. Chem. (Frankfurt um Main), 13, 335 (1957). Phys. Chem., 89, 320 (1985). (10) J. B. Birks, N o w . J . Chim., 1, 455 (1977).
J . Phys. Chem. 1986, 90, 1458-1464
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An alternative explanation is that the ESIPT process occurs from a different fraction of H-bonded structures. In the latter case only part of the molecules would require intramolecular rearrangement into a configuration which is most suitable for ESIPT. The other fraction of molecules, already “prepared” in the ground state, may tautomerize immediately, without any thermal activation. It should be noted that recently the importance of different conformers was excluded for the jet-cooled molecule I.I7 The author has also used the replacement of the proton on deuteron as a test of the barrier with the tunneling. The study of ESIPT we will continue with the isolated molecules in a supersonic jet.
Conclusion A new example of the fast reversible excited-state intramolecular proton transfer (ESIPT) was reported. The dual (17) N. P. Ernsting, J . Phys. Chem., 89, 4932 (1985).
fluorescence was tested over a large temperature region (300-8 K). The primary excited and the tautomeric form are separted by the energy barrier EPT= 365 cm-’ in 2-MTHF. The temperature and polarity of the environment control the excited-state reaction. The formal kinetic relations allow us to describe the process both in the reversible and irreversible regimes. At 77 K the proton-transfer rate constant is estimated as 2 X 1O’O SKI.For this particular case the excited-state reaction is much slower than that reported for many examples of ESIPT.
Acknowledgment. The authors thank Dr. Anna Grabowska and Dr. Niko Ernsting for many stimulating discussions and Prof. Z . R. Grabowski and Dr. K. H. Grellmann for critical reading of the manuscript. Dr. J. Herbich’s kind help in helium temperature experiments is very much appreciated. The work was carried out under project PAN 0.3.10.5. Registry No. I, 33450-1 1-2; 11, 88978-50-1; 111, 33450-14-5
Addition of Water to Premixed Laminar Methanol-Air Flames: Experimental and Computational Results Jim 0. Olsson, Lennart S. Karlsson, and Lars L. Andersson* Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden (Received: July 19, 1985; In Final Form: October 21, 1985)
Premixed laminar methanol-air flames at 100 torr were studied by experiment and computation. The composition CH30H/O2/N2/Ar (12.3%/ 18.4%/65.3%/4.0%) corresponds to a stoichiometric composition. The effect of water addition was studied in stoichiometric flames containing about 2.3 mol % water and 63 mol % nitrogen but with methanol, oxygen, and argon concentrations unchanged. The amount of water added corresponded to 10 wt % of the liquid phase. Species profiles were measured by using a modulated molecular beam mass spectrometer. They changed in an insignificant way when water was added. Detailed models for methanol-air combustion, including chemical kinetics and molecular diffusion, were used to compute the flame structure. The base mechanism used was a subunit of the mechanism developed by Westbrook, Dryer, and Schugh. Computations were also made with a mechanism developed by Warnatz. Water addition did not affect the computational concentration profiles significantly. Sensitivity analysis indicated the importance of HCO and CHzOH consumption reactions. Comparisons of experimental and computed concentration profiles supported a high value of the rate constant for the HCO decomposition reaction. This value, about a factor of 5 higher than the one used in the base mechanism, was in agreement with the value recommended by Warnatz.
Introduction Methanol is a possible alternative to gasoline or diesel fuels in the future. Today its main use is as a gasoline additive or as a reference fuel in both diesel and Otto engines. In practical use, methanol or fuels which contain methanol will either contain water from the manufacturing process or from contamination during transportation or storage. Sometimes water is also added to fuels in small quantities to reduce, for example, NO, and increase the octane number of the fuel.’ Water addition can affect combustion processes via reactions such as H 2 0 H and H 2 0 + M. It can also affect these processes in a more subtle way by its high efficiency as a third body, about 10-50 times more efficient than species such as nitrogen and oxygen, in decomposition and recombination Such important reactions are decomposition of the fuel and important intermediates as the formyl radical, and recombination of hydrogen and oxygen atoms. In a recent study of premixed methanol-air flames at 1 atm, Noda et al.’ found an increase in OH level, an unchanged H level,
+
(1) Rubin, M. B.; McLean, W. J. Combust. Sei. Technol. 1978, 18, 199. ( 2 ) Warnatz, J. In “Combustion Chemistry”: Gardiner, W. C., Jr., Ed.; Springer: New York, 1984. (3) Westbrook, C. K.; Dryer, F. L. Prog. Energy Combust. Sei. 1984, 10, 1. (4) Smooke, M. D.; Miller, J. A.; Kee, R. J. Combust. Sci. Technol. 1983, 34. 79.
0022-3654/8612090- 1458S01SO10
a decrease in 0 maximum, and a decrease in temperature when water was added. The water amount added corresponded to 2.15 mol % in the gas phase. They suggested as an explanation that water had a chemical effect through the reaction 0 + H 2 0 . However, in that study the equivalence ratio was also decreased from 0.75 to 0.69, complicating the analysis of the result. Measurements of species in laminar methanol flames have previously been made by Akrich et a1.,6 Pauwels et and Vandooren et a1.* Andersson et aL9 studied experimentally and theoretically a stoichiometric low-pressure methanol flame. Detailed computational studies of methanol combustion have been made by Westbrook and Dryer.’oJ’ Their mechanism is designed for handling different conditions in flow reactors, premixed flames, and shock tubes and it has been extensively used ( 5 ) Noda, S.; Demise, H.; Claesson, 0.;Yoshida, H. J . Phys. Chem. 1984, 88, 2552. (6) Akrich, R.; Vovelle, C.; Delbourgo, R. Combust. Flame 1978, 32, 17 1. (7) Pauwels, J. F.; Carlier, M.; Sochet, L. R. J . Phys. Chem. 1982, 86, 4330. (8) Vandooren, J.; Van Tiggelen, P. J. Symp. ( f n t . )Combust., [Proc.],18, 1981, 1982, 413. (9) Andersson L. L.; Christenson, 9.; Hoglund, A.; Olsson, J. 0.;Rosengren, L. G. Prog. Astronaut. Aeronaut. 1985, 95, 164. (10) Westbrook, C. K.; Dryer, F. L. Combust. Sei. Technol. 1979,20, 125. (11) Westbrook, C. K.; Dryer, F. L. Combust. Flame 1980, 37, 171.
0 1986 American Chemical Societv
