Intramolecular proton transfer and tunnelling reactions of hydroxyphenylbenzoxazole derivatives in Xenon at 15 K

Chemical Physics 312 (2005) 177–185 www.elsevier.com/locate/chemphys

Intramolecular proton transfer and tunnelling reactions of hydroxyphenylbenzoxazole derivatives in Xenon at 15 K Peter J. Walla

a,b,*

, Bernhard Nickel

a

a

b

Max-Planck-Institute for Biophysical Chemistry, Department 010, Spectroscopy and Photochemical Kinetics, Am Faßberg 11, D-37077 Go¨ttingen, Germany Department for Biophysical Chemistry, Technical University of Brunswick, Institute for Physical and Theoretical Chemistry, Hans-Sommerstr. 10, D-38106 Braunschweig, Germany Received 17 May 2004; accepted 29 November 2004 Available online 11 February 2005

Abstract We investigated the site dependence and the tunnelling processes of the intramolecular proton and deuteron transfer in the triplet state of the compounds 2-(2 0 -hydroxy-4 0 -methylphenyl)benzoxazole (m-MeHBO) and 2-(2 0 -hydroxy-3 0 -methylphenyl)benzoxazoles (o-MeHBO) and their deuterio-oxy analogues in a solid xenon matrix. After singlet excitation there occurs an ultrafast intramolecular enol ! keto proton transfer and subsequent intersystem crossing mainly to the keto triplet state. In the triplet state of m-MeHBO, the proton transfer back to the lower enol triplet state is governed by tunnelling processes. In o-MeHBO, however, the enol triplet state is higher and therefore normally no tunnel reaction can be observed. Because of the external heavy atom-effect in a xenon matrix, we were able to investigate the reverse enol–keto-tunnelling after exciting directly the enol triplet state of deuterated o-MeHBO. The time constants of the reverse enol–keto tautomerization are similar to those of the normal keto–enol tautomerization. In a xenon matrix, the observed site-selective phosphorescence spectra are very well-resolved vibrationally. This allowed the study of the tunnel rates in different well-defined sites. The vibrational energies obtained in the spectra are in good agreement with vibrational energies found in resonant Raman and IR spectra of 2-(2 0 -hydroxyphenyl)benzoxazole (HBO). Ó 2004 Elsevier B.V. All rights reserved. Keywords: Tunnelling; Proton transfer; Matrix isolation; Triplet state

1. Introduction For the study of proton or deuterium transfer processes, 2-(2 0 -hydroxy-phenyl)benzoxazoles (HBO) and derivatives are very interesting compounds, because they can exist in two tautomeric forms, the enol tautomer (E) and the keto tautomer (K), whose relative stabilities depend on the derivative and its electronic state [1–7]. Fig. 1 shows a schematic energy level diagram of unsubstituted HBO. In the electronic ground state of HBO,

*

Corresponding author. Tel.: +551 201 1087; fax: +531 391 5352. E-mail address: [email protected] (P.J. Walla).

0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.11.039

the enol form, 1E, is the stable tautomer. After excitation into the first excited singlet state 1E ! 1E*, a very rapid adiabatic proton transfer takes place, yielding the first excited singlet state of the keto tautomer, 1 E* [ 1K* [8]. The primary intersystem crossing from 1 K* to the triplet manifold is almost exclusively due to the process 1K* [ 3K* and not to 1K* [ 3E* [9]. Like many proton transfer reactions, the transitions between the keto and enol triplet state, 3K* 3E*, are governed by tunnel effects [4,10–17]. At low temperatures, the time constant of the keto–enol tautomerization is almost temperature independent and shows a deuterium isotope effect of a factor of about three orders of magnitude. By chance, the two triplet states 3K* and 3E* of the

178

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185

zero-point level of 3K* and the nearest vibronic level of 3E*, v. In the present work, we have investigated the compounds m-MeHBO and o-MeHBO and their deuterated analogues m-MeDBO and o-MeDBO in a solid xenon matrix at 15 K. The use of a xenon matrix [22–25] is advantageous for the following aims of this investigation:

1

E*

3

1

K*

3

K* kISC

E*

hv

1

K

1

E

N

H O N

O

O

HO

Enol form

Keto form

Fig. 1. Schematic representation of the energy levels of HBO in liquid solution.

unsubstituted HBO are isoenergetic in liquid solution [18]. The experimentally observed kinetics provide evidence for equally populated 3K* and 3E* states. Its meta-methyl derivative 2-(2 0 -hydroxy-4 0 -methylphenyl)benzoxazole (m-MeHBO) has a higher state 3K* (
450 cm1 in solid solution, see Fig. 2), and therefore a unidirectional tunnel reaction 3K* ! 3E* can be observed. Conversely, its ortho-methyl derivative 2-(2 0 -hydroxy-3 0 -methylphenyl)benzoxazole (o-MeHBO) has a lower state 3K* (
550 cm1 in solid solution), and normally no tunnel reaction can be observed at all. At very low temperatures, the tunnel kinetics in an alkane glass is multiexponential. The time constants are varying by a factor of five. This can be explained by a broad distribution of the time constants for individual sites [19–21]. In previous work [21], we suggested that in the case of weak exothermicity the tunnel time constant at low temperatures was not only very sensitive to small changes of intramolecular parameters like the tunnel distance, but also depended strongly on the probability of generating a phonon with an energy corresponding to the energy gap, xv, between the vibronic

3 3 *

E

kKE

K*

kISC 450 cm–1

3

E*

kEK

kEK

3

K*

kISC 550 cm

–1

kKE

H O N O

m-MeHBO

CH3

H O N

CH3

O

o-MeHBO

Fig. 2. Schematic representation of the triplet energies of m-MeHBO and o-MeHBO in solid solution.

1. In order to verify the general reversibility of the proton tunnelling, we wanted to measure the reverse enol–keto tunnelling 3E* [ 3K* after direct excitation to the enol triplet state 1E ! 3E*. In a xenon matrix, this spin forbidden transition is enhanced due to the external heavy atom effect. The direct excitation 1E ! 3E* also enables us to determine the unknown energy of 3E* in o-MeHBO. 2. Our model described in [21] suggests a strong dependence of the tunnelling time constant on very small differences in the keto–enol energy gap. We wanted to confirm this prediction by measuring the time constants for individual sites with well-defined, slightly different xv. Because of the highly ordered structure of a xenon matrix, different sites can be distinguished optically with their well-resolved zero phonon lines. In an alkane glass, a subpopulation of sites with a distribution of xv, which again, shows multiexponential kinetics, can be excited only. 3. Finally, we wanted to measure the site-selective, vibrationally resolved phosphorescence spectra of the compounds. The detailed vibronic structure of the molecules in the triplet state provides valuable insights into the triplet-state geometry, which is important for understanding the origin of the tunnelling character of the reaction.

2. Experimental 2-(2 0 -Hydroxy-4 0 -methylphenyl)benzoxazole (mMeHBO) was synthesized and purified as described in [21]. 2-(2 0 -Hydroxy-3 0 -methylphenyl)benzoxazole (oMeHBO) was prepared by analogy to m-MeHBO by using 2-hydroxy-3-methylbenzoic acid instead of 2-hydroxy-4-methylbenzoic acid; giving colourless needles of o-MeHBO (m.p. 128.4 °C, yield 16.4%). Xenon (6.0) was purchased from Messer Griesheim. The deuteration of the compounds will be described together with the preparation of the xenon matrices. The fluorescence and phosphorescence spectra of the doped xenon matrices and their excitation spectra as well as the time dependence of the emissions were measured with a low-light-level detection setup having a maximum detection efficiency of about 15% (for details see [26]). Laser excitation was carried out with a

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185

pulsed dye laser (Lambda Physik, Scanmate 2, spectral bandwidth 0.15 cm1), which was pumped by an excimer laser (Lambda Physik, LPX 100, pulse duration 12–20 ns). The highly efficient collecting part is a home-built combination of an ellipsoidal mirror and a small plane aluminium mirror (collection efficiency of about 80%). The plane mirror is situated in the first focus of the ellipsoidal mirror with the reflecting side oriented towards the ellipsoidal mirror. A thin layer of the xenon matrix (thickness roughly 200 lm) is deposited on the plane mirror (preparation is described later). The dye-laser beam is sent through a little hole in the centre of the ellipsoidal mirror onto the matrix deposited on the plane mirror. The ellipsoidal mirror is mounted on the tip of a closed-cycle cryostat (Air Products and Chemicals) where it can be cooled down to about 14 K. The vacuum inside the cryostat was better than 107 mbar. To suppress prompt luminescence, the slit of a fast mechanical chopper (home-built, dead time 2 ls) [27] is situated in the second focus of the ellipsoidal mirror outside the cryostat. With an adjustable combination of two plane mirrors and a spherical mirror, this focus is projected onto the entrance slit of a spectrograph/monochromator (Chromex, 500IS/SM, maximum spectral resolution 0.07 nm). In spectrograph mode, the dispersed light is projected onto a CCD-array (Photometrics, SDS 9000/EEV15-11B, maximum quantum efficiency
80%). In the monochromator mode, the light is registered with a Peltier-cooled GaAs photomultiplier (RCA, C31034A-02, maximum quantum efficiency
30%), which is used under photon counting conditions (Stanford Research Systems, fast amplifier SR445, multichannel Scaler/Averager SR430).

179

For the preparation of xenon matrices with the HBO derivatives, a home-built deposition unit was used (Fig. 3). A feature of this unit is two separate, coaxial streams – a core stream with dopant molecules and a outer stream with xenon atoms. The distances of the outlets for both streams can be varied to find the optimum distance for mixing both streams and guiding them through the small hole of the ellipsoidal mirror to prepare an optically clear matrix on the small plane mirror. To mix a sufficient amount of HBO molecules into the stream of xenon, it is necessary to heat up the HBO derivatives to 40–60 °C, depending on the desired concentration. By the twostream technique, it is possible to prepare a thin layer of pure xenon on the plane mirror first, to add a thicker doped xenon layer by opening the oven valve, and finally to add again a thin layer of pure xenon by closing the oven valve before the xenon stream is stopped completely. The advantage of this technique is very little contamination of the cryostat with dopant molecules, so that cleaning the whole cryostat or stopping the vacuum pump for changing the dopant molecules and preparing matrices doped with a new compound are not required. Only the small oven chamber has to be cleaned with organic solvents and refilled with the new compound. For preparing matrices with the deuterated compounds, the compounds were first dissolved completely in a large amount of boiling deutero-ethanol. The major part of the deutero-ethanol was then evaporated. Only a minor part remained, so that the crystallized DBO derivative was still completely covered in deuteroethanol. Then the oven chamber was washed several times with deutero-ethanol and filled with some of the

Fig. 3. Deposition unit for the preparation of doped xenon matrices.

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185 1,5 1

1

E→ E*

–1

rel. extinction coefficient/ M cm–1 rel. intensity / a.u.

180

1

1

K*→ K

1,0

3

1

E*→ E

0,5

0,0 15000

20000 25000 30000 wavenumber / cm–1

35000

Fig. 4. Absorption spectrum and luminescences of m-MeHBO in solid xenon at 15 K.

deutero-HBO derivative covered in deutero-ethanol. This small amount of deutero-ethanol was then pumped off with the turbo-molecular pump.

3. Results and discussion 3.1. Steady-state absorption and emission spectra The absorption spectrum and emission spectra of mMeHBO in xenon are presented in Fig. 4. As expected, the absorption spectrum does not show any vibrational structure, even in a xenon matrix at low temperatures. This is due to the very fast adiabatic proton transfer from the primarily populated enol state to the keto state, 1 E* [ 1K*, leading to spectral line-broadening. The fluorescence is strongly Stokes-shifted (
7000 cm1). It also shows no vibrational structure indicating a fast back transfer of the proton to the ground state. The phosphorescence is a vibrationally well-resolved pure enol phosphorescence. Therefore, the enol triplet state must be below the keto triplet state, as in liquid solution or in a solid alkane glass. The well-resolved strong zero phonon line of the electronic origin indicates that the

geometry of the enol triplet state 3E* is very similar to the ground-state geometry [28,29]. Due to the heavyatom effect, all spin-forbidden transitions are enhanced. Therefore, the phosphorescence of m-MeHBO in xenon is much stronger than in a solid alkane glass. As a consequence, it was necessary to use a shutter closing shortly after the laser excitation to obtain the pure fluorescence spectrum in Fig. 4. By running the laser with a low repetition rate, it was possible to determine the relative quantum yields of phosphorescence and fluorescence, /P//Fl, as 0.3 by comparing the intensities of the resulting spectra with or without using the shutter. The absorption spectrum or fluorescence spectrum of o-MeHBO in xenon is very similar to that of m-MeHBO. Minor differences are comparable to the differences of the spectra between both compounds in liquid solution [30]. In Fig. 5, spectra of the delayed luminescences of a xenon matrix doped with o-MeHBO are presented. A fresh sample is exhibiting a nearly pure keto phosphorescence (Fig. 5(a)), but as in liquid solution an ingrowing enol-like phosphorescence is observed. The intensity of this enol-like phosphorescence is depending on the duration of the laser irradiation (Fig. 5(b)). However, the comparison of the phosphorescence spectrum of the photoproduct with the phosphorescence spectrum of m-MeHBO in xenon clearly shows that it is not enol phosphorescence. Fortunately, the generation of the photoproduct is much slower in xenon than in an alkane solution and therefore it is possible to extract the spectra of the pure compounds by subtracting spectra measured after different times of laser irradiation. The weak emission in the spectral region between 18,000 and 20,000 cm1 of the pure o-MeHBO phosphorescence could be caused by the radiative keto–enol-transition 3K* ! 1E postulated in [30]. However, this is an assumption only considering the uncertainties of the subtracting procedure. Nevertheless, the spectra show that in most sites of o-MeHBO in xenon the keto triplet state is below the enol triplet state. 3.2. Kinetics and site-dependent deuterium tunnelling

relative intensity / a.u.

1.0 0.8

(a) 0.6 0.4

(b)

0.2 0.0 12000 14000

16000 18000 20000 wavenumber / cm–1

22000 24000

Fig. 5. Phosphorescences of o-MeHBO (a) and its photoproduct (b) in solid xenon at 15 K.

The phosphorescence lifetime of m-MeHBO in xenon is about 40 ms (Table 1), which is about eight times shorter than the phosphorescence lifetime of m-MeHBO in a solid alkane glass (315 ms at 80 K). Certainly, this is a consequence of the external heavy atom effect of xenon. No deviation from a monoexponential phosphorescence decay can be observed after excitation at 32,500 cm1 (308 nm). This indicates that all sites of m-MeHBO in xenon have a similar phosphorescence lifetime. This result is in striking contrast to the multiexponentially decaying phosphorescence observed in a solid alkane glass [19]. From this fact, we conclude that for m-MeHBO in xenon the radiative decay and not a non-radiative deac-

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185

181

Table 1 Resulting parameters s1 and s2 fitting Eq. (1) to the observed phosphorescence kinetics of the samples with excitation of various electronic states and detection of various sites Sample

Excitation

mExc (cm1)

Detection of 3 E* ! 1E at site

mDet (cm1)

s1 (ms)

s2 (ms)

Corresponding phosphorescence spectra and/or kinetics

m-MeHBO

1

32,500

1, 2, 3

–

40 ± 2

Fig. 4

m-MeHBO m-MeDBO m-MeDBO m-MeDBO m-MeDBO o-MeDBO o-MeDBO

1

22,379 ± 5 32,500 32,500 32,500 32,500 22,453 ± 1 22,470 ± 1

1 1, 2, 3 1 2 3 1 2

22,300–22,500 (22,415, 22,401 and 22,379) 22,379 ± 5 22,300–22,500 22,372 ± 5 22,394 ± 5 22,408 ± 5 22,453 ± 5 22,470 ± 5

– 1.5 ± 0.6 1.27 ± 0.2 2.02 ± 0.4 1.06 ± 0.4 1.44 ± 0.025 1.14 ± 0.025

40 ± 2 40 ± 2 40 ± 2 40 ± 2 40 ± 2 – –

Fig. 8 Fig. 6 Fig. 7 Fig. 7 Fig. 7 Figs. 9 and 10 Fig. 10

E ! 1E*

E ! 3E* E ! 1E* 1 E ! 1E* 1 E ! 1E* 1 E ! 1E* 1 E ! 3E* 1 E ! 3E* 1

mExc and mDet are the corresponding excitation and detection energies, respectively.

100

0

10

20

30

40 50 time / ms

60

70

80

Fig. 6. Time dependence of the phosphorescence of m-MeDBO/mMeHBO in xenon at 15 K.

bonds in the excited state. This leads to a different shift of the zero-point vibrational energy in the excited state and causes the blue shift. It seems, however, that in mMeHBO the hydrogen atom is bonded stronger to the oxygen in the triplet state than in the ground state.

ð1Þ

The difference between the amplitude of the partial rise, A1, and the decay, A2, reflects the degree of deuteration. We found that deuteration degrees higher than 90% could be reached with the method described in Section 2. With high spectral resolution, three different zerophonon lines can be distinguished in the phosphorescence spectrum of m-MeHBO. The spectrum of m-MeDBO almost looks the same, but it is shifted about 7 cm1 to the red (Table 1, Fig. 7). It can be expected that the sites in the xenon crystals are the same for m-MeHBO and m-MeDBO. So the difference observed is caused only by different zero-point vibrational energies of the two compounds. In aromatic hydrocarbons, a blue shift often is obtained by replacing a hydrogen atom by a deuterium atom, as the latter has weaker

3 τ1

2,02 time constant / ms rel. intensity / a.u.

IðtÞ
A1 et=s1 þ A2 et=s2 :

1000

time

tivation, as in an alkane glass is the dominant decay process, because the radiative decay usually is not very sensitive to different environments. Another indication for this assumption is the lifetime of the deuterated m-MeDBO in xenon: the phosphorescence lifetimes of m-MeHBO and m-MeDBO in xenon are nearly the same. Without a heavy atom effect, like in an alkane glass, the triplet lifetime of m-MeHBO is about five times faster than that for m-MeDBO. This difference is typical for non-radiative decays of deuterated and protonated compounds [31,32]. The fact, that the phosphorescence quantum yield is considerably greater than in an alkane glass also supports that the radiative deactivation as well as the intersystem-crossing 1K* [ 3K* is enhanced. The time dependence of the 3E*-phosphorescence intensity (Fig. 6) allows an estimate of the deuteration degree of a sample. Only the slower 3K* ! 3E* tunnelling in m-MeDBO can be resolved as initial phosphorescence rise with our instrument. The much faster tunnelling in m-MeHBO appears as fast instant rise. Therefore, we fitted the sum of two exponentials to the data of the (partially) deuterated samples

1,27

spectrum detection

2 1,06 1

0 22300

22350

22400

22450

–1

wavenumber / cm

Fig. 7. Zero-phonon lines of different sites of m-MeDBO in xenon (solid line). Tunnelling time constants measured at various wavenumbers (circles with error bars). Resolution of the detection for the measurement of site-selective kinetics (dotted line).

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P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185

1.0

1.0

1.0

22379

22379

0.8

0.8

0.6 0.4

0.6

0.2 0.0 22300

22350

22400

22450

0.4 0.2 0.0 20500 21000

3

1

E* → E

1

3

E → E*

21500 22000 22500 23000 23500 wavenumber / cm–1

Fig. 8. Phosphorescence excitation spectrum and site-selective phosphorescence spectrum of m-MeHBO observed by direct excitation in the triplet state 1E ! 3E*. The inset shows a zoom of the zero-phonon line of the phosphorescence spectrum together with gaussian fit. The fitted linewidth is 15 cm1.

The kinetic curve of Fig. 6 was measured with detection of all zero-phonon lines together. The tunnel time constant, s1, therefore is an average value over all sites only. The differences in s1 of the individual sites seem to be smaller than in an alkane glass, because a monoexponential rise (Eq. (1)) can be fitted very well to the curve in Fig. 6. However, with a spectral detection window of about 10 cm1 FWHM (dotted line in Fig. 7), it was possible to measure the individual time constants for A1 of three different sites: 1.17, 2.02 and 1.06 ms. This result indicates that indeed the time constant depends strongly on the probability for generating a phonon corresponding to the energy gap, xv, between the vibronic zero-point level of 3K* and the nearest vibronic level of 3E*, v, as predicted by our model described in detail in [21]. 3.3. Excitation in the triplet state and reverse enol–keto-tautomerization Site-selective excitation in the first singlet state E ! 1E* is not possible, because of the large overlap of the absorption of the different sites. Nevertheless, we succeeded in direct exciting site-selectively the enol triplet state 1E ! 3E*. Fig. 8 shows the site-selective phosphorescence of the dominant site of m-MeHBO in xenon (excitation at 22,379 cm1). The vibrational energies and intensities of all sites are nearly identical. The excitation spectrum of the transition 1E ! 3E* shows mirror symmetry to the phosphorescence spectrum. With an excitation 1E ! 3E*, it is, for the first time, possible to determine the energy of the enol triplet state 3 E* of o-MeDBO and investigate the reverse enol–keto tautomerization. Fig. 9 shows the phosphorescence of the dominant site of o-MeDBO. (It is very difficult to find the same spectrum for o-MeHBO, because of a fast reverse proton transfer 3E* [ 3K* in the microsecond range). 1

rel. Intensity / a.u.

r e l. in t e n s it y / a . u .

0.8

0.6 0.4 0.2 0.0 20500

21000

21500 22000 wavenumber / cm–1

22500

Fig. 9. Site-selected phosphorescence spectrum of o-MeDBO observed after direct excitation in the triplet state 1E ! 3E*.

Surprisingly, there was only little difference between the energies of the 3E* of m-MeHBO (22,379 cm1) and o-MeDBO (22,453 cm1), i.e., only about 80 cm1. We expected a larger difference, because a keto– enol gap of about 550 cm1 was observed for o-MeHBO and 450 cm1 for m-MeHBO (Fig. 2). (In liquid solution both values are about 150 cm1 higher due to solvent relaxation of 3K*) [19]. This indicates that the methyl substitution mainly affects the energy of the keto triplet state, but not of the enol triplet state. This is also supported by the facts, that 3E* of HBO has an energy of 22,398 cm1 in xenon and that the maximum of the unstructured keto-band of o-MeHBO and HBO in liquid 3-methylpentane differs by 700 cm1, which exactly corresponds to the energy difference of 550 cm1 + 150 cm1 of 3K* in liquid solution. The vibrational energies of m-MeHBO and o-MeDBO observed from the phosphorescence spectrum are very similar to the energies for HBO observed from resonance Raman and infrared measurements of Pfeiffer et al. (see Table 2) [33,34]. As expected, the low frequency vibrational energies of m-MeHBO in general are smaller than those of HBO. Fig. 10 shows the phosphorescence excitation spectrum of o-MeDBO in comparison with the phosphorescence excitation spectrum of m-MeHBO. The zero-phonon lines are not resolved very well for two reasons: 1. For measuring a signal after exciting the very weak transition 1E ! 3E* a higher concentration was necessary. This usually leads to stronger inhomogeneous broadening in xenon matrices. 2. The phonon sidebands are blue-shifted in the excitation spectrum. The phonon side bands of dominant zero phonon lines in the red are overlapping with weaker zero-phonon lines in the blue. Nevertheless, the zero-phonon lines of two sites can be identified. The enol triplet state of o-MeDBO has an origin of about 22,460 cm1. The kinetics of the

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185

183

Table 2 Vibrational energies of m-MeHBO, HBO and o-MeDBO. mv: vibrational energies observed from the phosphorescence spectra after direct excitation into the triplet state 1E ! 3E* Number

m-MeHBO (Fig. 10)

HBO

mv (cm1)

This work

Iv

o-MeDBO (Fig. 11) 1

mv (cm ) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

0

1.00

262 289

0.03 0.03

629

0.04

663 705 732 808 897 938

0.01 0.02 0.08 0.01 0.05 0.09

1113

0.01

1151

0.02

1253

0.09

1343 1366

0.04 0.03

1465

0.05

1558 1649

0.23 0.15

Iv

0

mv (cm1)

Data observed by Pfeiffer et al. [33,34] 1

mR (cm )

IR

1

mIR (cm )

IIR

1.00

318

0.11

121 280 310 480 524 572 629

0.27 0.10 0.28 w w 0.28 vw w

699

0.07

673

908 943

0.08 0.13

844 899 942 1034

0,23 0.14 0.32 vw

1105

vw

1147

m

1238 1250 1310 1334 1357 1425 1455

0.13 0.42 0.19 0.25 w w 0.34

1488 1548 1635

0.30 1.00 0.40

1257

0.12

1561 1646

0.29 0.27

Iv

519 566 620 672

m m w s

891 934 1027 1045 1101 1120 1132 1144 1152

s s s vs m s vs m vs

1284 1303

vs vs

1346 1423 1454 1478

s s vs vs

1627 1637

vs vs

0

1.00

629

0.05

840

0,02

950 993

0.05 0.03

1251

0.04

1335

0.02

1464

0.03

1562 1634

0.12 0.15

Iv, heights of the peaks in the phosphorescence spectra. mR, IR, vibrational energies and intensities in the resonance Raman spectrum observed by Pfeiffer et al. mIR, IIR, vibrational energies and intensities in the infrared spectrum observed by Pfeiffer et al. [33,34]. 1,5

lifetime / ms rel. intensity / a.u.

1,44

1,14

1,0 m-MeHBO, 1 3 E→ E*

o-MeDBO, 1 3 E→ E*

0,5

0,0 22300

m-MeHBO, 3 1 E*→ E

22400 22500 –1 excitation wavenumber / cm

Fig. 10. Phosphorescence excitation of the zero-phonon lines of mMeHBO and o-MeDBO observed by direct excitation in the triplet state 1E [ 3E* (thick lines). Phosphorescence spectrum of m-MeHBO observed after excitation in the singlet state 1E ! 1E*. Tunnelling time constants of the ‘‘reverse’’ enol-keto tautomerization 3E* [ 3K* of oMeDBO after direct excitation in the triplet state 1E ! 3E* measured at various wavenumbers (squares with error bars).

reverse enol–keto tautomerization was measured after excitation at these zero-phonon lines. The decay after excitation of the two dominant sites is monoexponential, with the exception of a very rapid decay in the first microseconds. It is only possible to measure the slower kinetics of o-MeDBO. The time constants are very similar to the time constants of the keto–enol tautomerization of m-MeDBO. In Fig. 11, these time constants are shown together with the temperaturedependent constants measured by Stephan and co-workers [21,35] and AlSoufi et al. [4]. Stephan determined the time constant for the reverse enol–keto tautomerization of o-MeDBO in liquid solution indirectly by triplet energy transfer experiments. This was possible only in a small temperature range around 130 K which is still in the thermally activated tunnelling regime. However, the data measured at 15 K in

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P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185 6

10

5

ra te c o n s ta n t / s

–1

10

o -M eD BO (k EK ) 10

4

10

3

10

2

θ1

k1 m -M eD BO (k KE ) k2 1

10

100 T/K

Fig. 11. Comparison of the tunnelling rates of the ‘‘normal’’ enol–keto tautomerization in m-MeDBO, kKE, with the ‘‘reverse’’ keto-enol tautomerization in o-MeDBO, kEK. Squares: enol–keto tunnelling of m-MeDBO in liquid solution [4], Open symbols: biexponential rates k1 and k2 which where used in [21] to approximate the multiexponential enol–keto tunnelling kinetics of m-MeDBO in an alkane glass. Up triangles: ‘‘reverse’’ keto–enol tautomerization rate constants in oMeDBO in Xenon (this work), Down triangles: ‘‘reverse’’ keto–enol tautomerization in o-MeDBO determined indirectly by triplet energy transfer experiments [40].

the enol triplet state. Deuteration of m-MeHBO shifts the enol triplet state to the red by 7 cm1 only. Thus it seems that in m-MeHBO the hydrogen atom is bonded stronger to the oxygen in the triplet state than in the ground state. The site-selective phosphorescence spectra are very well resolved vibrationally. The vibrational energies are very similar to the values measured by Pfeiffer et al. [33,34] in resonant Raman and IR-experiments with 2-(2 0 -hydroxyphenyl)benzoxazole (HBO). For the first time, the kinetics of the reverse enol–keto tautomerization with direct excitation of the enol triplet state 1E ! 3E* of o-MeDBO at low temperatures could be measured. We found the values 1.44 and 1.14 ms for two sites with an enol triplet energy of 22,453 and 22,470 cm1. These values are very similar to the values for the normal keto–enol-tautomerization of m-MeDBO. Together with values determined indirectly in triplet energy transfer experiments [21,35] in liquid solution at about 130 K our results provide strong indications that the reverse enol–keto tautomerization of o-MeDBO is governed by very similar tunnelling processes.

Acknowledgements xenon is in the temperature-independent regime of the reaction. All data sets together (see Fig. 11) show, that the rate of the reverse enol–keto tautomerization has a very similar temperature dependence like the normal keto–enol tautomerization. This clearly indicates that the reverse enol–keto tautomerization is also governed by tunnel processes.

We thank Professor J. Troe for generous support and for his interest in this work. We also thank Professor B. Dick and Dr. U. Kensy for their kind introduction of the technique of matrix isolation and Prof. N. Ernsting for using his CCD camera.

References 4. Conclusion In this study, we reported the results of the spectroscopic properties and deuterium tunnel effects of the meta- and ortho-methylderivatives of HBO in a solid xenon matrix at 15 K. We confirmed a sensitive dependence of the time constants of deuterium tunnelling of m-MeDBO in the triplet state on slightly different environments in different sites. For the keto–enol tautomerization, we found time constants of 1.27, 2.02 and 1.06 ms for the three dominant sites with enol triplet energies of 22,372, 22,394 and 22,408 cm1. We determined for the first time the energy of 3E* of o-MeDBO (22,453 cm1) by direct, site-selective excitation of the spin-forbidden transition 1E ! 3E*. The energy of this state was unknown, because 3E* of o-MeDBO was never populated after ‘‘normal’’ excitation into the singlet state. The values for m-MeHBO and HBO are 22,379 and 22,398 cm1, respectively. This result shows that methyl substitution mainly affects the energy of the keto triplet state, but not the energy of

[1] A. Mordzinski, A. Grabowska, Chem. Phys. Lett. 90 (1982) 122. [2] A. Mordzinski, K.H. Grellmann, J. Phys. Chem. 90 (1986) 5503. [3] K.H. Grellmann, A. Mordzinski, A. Heinrich, Chem. Phys. 136 (1989) 201. [4] W. Al-Soufi, K.H. Grellmann, B. Nickel, J. Phys. Chem. 95 (1991) 10503. [5] S. Lochbrunner, A.J. Wurzer, E. Riedle, J. Phys. Chem. A 107 (2003) 10580. [6] R. de Vivie-Riedle, V. De Waele, L. Kurtz, E. Riedle, J. Phys. Chem. A 107 (2003) 10591. [7] J. Weiss, V. May, N.P. Ernsting, V. Farztdinov, A. Muhlpfordt, Chem. Phys. Lett. 346 (2001) 503. [8] T. Arthen-Engeland, T. Bultmann, N.P. Ernsting, M.A. Rodriguez, W. Thiel, Chem. Phys. 163 (1992) 43. [9] H. Eisenberger, B. Nickel, A.A. Ruth, W. Al-Soufi, K.H. Grellmann, Mercedes Novo, J. Phys. Chem. 95 (1991) 10509. [10] A. Jarczewski, C.D. Hubbard, J. Mol. Struct. 649 (2003) 287. [11] W. Siebrand, Z. Smedarchina, M.Z. Zgierski, A. FernandesRamos, Int. Rev. Phys. Chem. 18 (1999) 5. [12] A. Kohen, J.P. Klinman, Chem. Biol. 6 (1999) R191. [13] P. Bertrand, J. Biol. Inorg. Chem. 9 (2004) 2. [14] Q.A. Qui, M. Karplus, J. Phys. Chem. B 106 (2002) 7927. [15] A.J. Horsewill, C.J. McGloin, H.P. Trommsdorff, M.R. Johnson, Chem. Phys. 291 (2003) 41. [16] V.A. Benderskii, E.V. Vetoshkin, E.I. Kats, H.P. Trommsdorff, Phys. Rev. E 6702 (2003) 2003.

P.J. Walla, B. Nickel / Chemical Physics 312 (2005) 177–185 [17] A.L. Sobolewski, W. Domcke, Ab initio reaction paths and potential-energy functions for excited-state intra- and intermolecular hydrogen-transfer processes, in: T. Elsaesser, H.J. Bakker (Eds.), Understanding Chemical Reactivity, vol. 23, Kluwer Academic Publishers, Dordrecht, Netherlands, 2002, p. 93. [18] M.F. Rodrı´guez Prieto, B. Nickel, K.H. Grellmann, A. Mordzinski, Chem. Phys. Lett. 146 (1988) 387. [19] B. Nickel, M.F. Rodrı´guez Prieto, Chem. Phys. Lett. 95 (1988) 393. [20] B. Nickel, A.A. Ruth, Chem. Phys. 184 (1994) 261. [21] B. Nickel, K.H. Grellmann, J.S. Stephan, P.J. Walla, Ber. Bunsenges. Phys. Chem. 102 (1998) 436. [22] E.D. Whittle, A. Dows, G.C. Pimentel, J. Chem. Phys. 22 (1954) 1943. [23] E.D. Becker, G.C. Pimentel, J. Chem. Phys. 25 (1956) 224. [24] G.C. Pimentel, Spectrochim. Acta 12 (1958) 94. [25] M.J. Almond, A.J. Downs, Spectroscopy of Matrix Isolated Species, in: R.J.H. Clark, R.E. Hester (Eds.), Advances in Spectroscopy, vol. 17, Wiley, Chichester, New York, Brisbane, Toronto, Singapore, 1989.

185

[26] P.J. Walla, F. Jelezko, Ph. Tamarat, B. Lounis, M. Orrit, Chem. Phys. 233 (1998) 117. [27] B. Nickel, A.A. Ruth, J. Phys. Chem. 95 (1991) 2027. [28] S. Nagaoka, A. Itoh, K. Mukai, U. Nagashima, J. Phys. Chem. 97 (1993) 11385. [29] L. Lavtchieva, V. Entchev, Z. Smedarchina, J. Phys. Chem. 97 (1993) 306. [30] B. Nickel, P.J. Walla, Chem. Phys. 237 (1998) 371. [31] E.W. Schlag, S. Schneider, S.F. Fischer, Annu. Rev. Phys. Chem. 22 (1971) 465. [32] N.J. Turro, Modern Molecular Photochemistry, University Science Books, Mill Valley, CA, 1991, p. 189. [33] M. Pfeiffer, K. Lenz, A. Lau, T. Elsaesser, T. Steinke, J. Raman Spec. 28 (1997) 61. [34] M. Pfeiffer, K. Lenz, A. Lau, T. Elsaesser, J. Raman Spec. 26 (1995) 607. [35] J. Stephan, Kinetische Untersuchungen von Wasserstoff-Tunneleffekten und von cis–trans-Isomerierungen methylsubstituierter Hydroxybenzoxazole, dissertation, Georg-August-Universita¨t Go¨ttingen, 1996.