The rate of excited-state proton transfer to solvent from trifluoromethylphenols in water

Article abstract of DOI:10.1246/cl.2009.312

The proton-transfer reactions to solvent from electronically excited o-, m-, and p-(trifluoromethyl)phenols (TFOHs) in water have been investigated by picosecond time-resolved fluorescence measurements. The rate constants for the proton dissociation of o-, m-, and p-TFOH are obtained to be 2.2 × 10 ⁹, 8.6 × 10⁸, and 2.5 × 10⁸ s ^-1, respectively. On the basis of the rate constants, the effects of substituent and deuterium isotope effects on the proton-transfer reactions are revealed. copyright

Full text of DOI:10.1246/cl.2009.312

312
Chemistry Letters Vol.38, No.4 (2009)
The Rate of Excited-state Proton Transfer to Solvent from Trifluoromethylphenols in Water
Shigeo Kaneko, Toshitada Yoshihara, and Seiji Tobitaꢀ
Department of Chemistry and Chemical Biology, Gunma University, Kiryu 376-8515
(Received December 24, 2008; CL-081213; E-mail: tobita@chem-bio.gunma-u.ac.jp)
The proton-transfer reactions to solvent from electronically
excited o-, m-, and p-(trifluoromethyl)phenols (TFOHs) in
water have been investigated by picosecond time-resolved fluo-
rescence measurements. The rate constants for the proton disso-
ciation of o-, m-, and p-TFOH are obtained to be 2:2 ꢁ 109,
8:6 ꢁ 108, and 2:5 ꢁ 108 sꢂ1, respectively. On the basis of the
rate constants, the effects of substituent and deuterium isotope
effects on the proton-transfer reactions are revealed.
Wavelength/nm
600 500
(a) PhOH
400 350
300
250
Abs.
Fluo.
H2O
D2O
(b) o–TFOH
Hydroxyarenes such as 1- and 2-naphthols are well known
to undergo excited-state proton-transfer (ESPT) to solvent in wa-
ter because of drastic enhancement of the acidity in the first ex-
cited singlet (S1) state.1 The occurrence of ESPT is usually sub-
stantiated by appearance of dual fluorescence originating from
the protonated and deprotonated species. In the case of phenol
(PhOH) however, the extremely small fluorescence quantum
yield (ꢀf ¼ 8:0 ꢁ 10ꢂ4 in water) and short lifetime (ꢁf ¼
18 ps) of the phenolate anion render difficult the direct observa-
tion of the proton-transfer process in the excited state, and only a
few papers can be found on ESPT reactions of phenols in the lit-
erature.2–4 In the present study, we found that the introduction of
an electron-withdrawing trifluoromethyl (CF3) group into the
aromatic ring of PhOH enhances significantly the fluorescence
quantum yield and lifetime of the deprotonated forms, and the
fluorescence spectrum exhibits dual fluorescence due to the
protonated and deprotonated forms (Scheme 1). The acidity of
PhOH is increased both in the ground and excited states by intro-
ducing a CF3 group in the aromatic ring.
We report here the first direct measurement of the rate of
proton-transfer to solvent from excited o-, m-, and p-TFOHs
in water. Picosecond fluorescence lifetime measurements were
made by using a femtosecond laser system which was based
on a mode-locked Ti:sapphire laser pumped by a CW green laser
(Spectra-Physics).5 The third harmonic (266 nm, FWHM:
ꢃ250 fs) was used as the excitation source. The instrument re-
sponse function had a half-width of about 25 ps. The fluores-
cence time profiles were analyzed by deconvolution with the
instrument response function.
(c) m–TFOH
(d) p–TFOH
20
30
Wavenumber/103cm–1
40
Figure 1. Absorption and fluorescence spectra of PhOH, o-
TFOH, m-TFOH, and p-TFOH in H2O (pH 4.7, solid line) and
D2O (pD 4.7, broken line).
m-TFOH in H2O exhibit dual fluorescence bands: normal and
large Stokes-shifted fluorescence bands with maxima at around
310 and 345 nm, respectively. The fluorescence spectrum of p-
TFOH in H2O also shows a similar feature, although the inten-
sity of the longer-wavelength band is much weaker than those
of o- and m-TFOH, and both bands appear at shorter wave-
lengths. The shorter-wavelength bands of TFOHs resembled
their fluorescence spectrum in acetonitrile (CH3CN), and the
longer-wavelength bands almost coincided with the fluorescence
spectrum of the deprotonated forms (anions) in H2O. The exci-
tation spectrum of the longer- and shorter-wavelength bands
agreed with the absorption spectrum. These indicate that both
bands originate from the parent molecule and suggest the occur-
rence of ESPT to solvent. Another interesting feature of Figure 1
is remarkably large isotope effects on the fluorescence spectra;
the longer-wavelength fluorescence bands of TFOD in D2O
almost disappear in all the isomers. This suggests significant
decreases in the proton-transfer rate by deuterium substitution.
Since the decay time constant of the deprotonated anion
TFOꢂꢀ is relatively short (260, 450, and 290 ps for o-, m-,
and p-TFOꢂ, respectively), the excited-state protonation reac-
tion (krec[H3Oþ] in Scheme 1) can be neglected under moder-
ately acidic conditions ([H3Oþ] < 10ꢂ4 M).6 The fluorescence
Figure 1 illustrates the absorption and fluorescence spectra
of PhOH and TFOHs in H2O (pH 4.7) and D2O (pD 4.7) at
293 K. It is noteworthy that the fluorescence spectra of o- and
kdis
TFOH*
TFO-*
H3O+
+
+
H2O
+
+
krec
'
k0
hν
k0
H3O+
H2O
TFOH
TFO-
Scheme 1.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.4 (2009)
313
Table 1. Proton-dissociation rate (kdis) and (kdisH/kdisD) at
293 K, activation energy (Ea), and frequency factor (A) for excit-
ed-state proton-transfer reactions of TFOHs in H2O (pH 4.7) and
D2O (pD 4.7)
3
0
-3
1000
500
0
(a) o-TFOH
χ2 = 1.122
kdis
Ea
A
τdecay = 360ps
D
Compound Solvent
kdisH/kdis
/108 sꢂ1 /kJ molꢂ1 /1011 sꢂ1
o-TFOH
m-TFOH
p-TFOH
H2O
D2O
H2O
D2O
H2O
D2O
22
6.6
8.6
2.2
2.5
0.44
9.8
13
12
15
14
21
1.2
1.4
1.1
1.2
0.87
2.9
0
500
1000
1500
2000
3.3
3.9
5.7
3
0
-3
τrise = 240ps
(b) o-TFO-
χ2 = 1.102
1000
500
0
τdecay = 360ps
tive effect acting through the intervening ꢃ bonds. According to
Hynes et al.,9 the differences in ground-state acidity among sev-
eral substituted phenols are due mainly to effects in the pheno-
late anions, the effects in the corresponding phenols being of
minor importance. Our results reveal that a similar effect of
the CF3 group is also applicable for the excited-state acidity of
TFOHs.
0
500
1000
Time (ps)
1500
2000
Figure 2. Fluorescence time profiles of o-TFOH monitored at
(a) 300 and (b) 360 nm in H2O at 293 K.
kinetics of 1TFOHꢀ and 1TFOꢂꢀ on excitation by a ꢂ-pulse
follows the monoexponential and biexponential laws, respec-
tively:
The Arrhenius plot of the kdis values of TFOHs in H2O and
D2O gave straight lines in the temperature range between 278
and 323 K, and the activation energy and the frequency factor
were obtained as shown in Table 1. Although the magnitude
of the frequency factor is nearly constant among TFOHs, the
activation energy of o-, m-, and p-TFOH increases in that order,
and all TFOHs shows larger Ea values in D2O than in H2O. The
ESPT rates (ꢃ108{109 sꢂ1) of TFOHs at the temperature range
between 275 and 323 K are significantly slower than that
(ꢃ1011 sꢂ1) of the solvent relaxation rate of water.10 These indi-
cate that at the above temperature range, the solvent relaxation is
sufficiently fast and the rate-determining step in the ESPT reac-
tion is the actual proton-transfer step. The activation energy and
IfTFOHðtÞ ¼ A0eꢂðk þk
IfTFO ðtÞ ¼ A0 ½eꢂðk þk ꢂ eꢂk
ð1Þ
ð2Þ
disÞt
0
0t
0
ꢂ
0
disÞt
0
ꢄ
In order to determine the proton dissociation rate constant (kdis
1
)
of TFOHsꢀ, the fluorescence time profiles of TFOHs in H2O
and D2O were taken as shown in Figure 2.
The fluorescence time profiles of the shorter-wavelength
band (monitored at 300 nm) and the longer-wavelength band
(monitored at 360 nm) of o-TFOH are displayed along with
the least-square fitting curves based on eqs 1 and 2. The solid
lines represent the deconvoluted best fit of (a) a single-exponen-
tial function (ꢁdecay ¼ 360 ps) and (b) a two-exponential func-
tion (ꢁrise ¼ 240 ps and ꢁdecay ¼ 360 ps) superimposed on the
experimental data points. The rise time (240 ps) of the time pro-
file in Figure 2b is in fair agreement with the lifetime of the de-
protonated anion (260 ps). This demonstrates that the reaction
sequence from TFOHꢀ to TFOꢂ (see Scheme 1) follows a
D
the deuterium isotope effect (kdisH/kdis = 3.0–6.0) are ascribed
to the proton motion along the proton-transfer coordinate.
References
1
2
N. Agmon, J. Phys. Chem. A 2005, 109, 13.
E. L. Wehry, L. B. Rogers, J. Am. Chem. Soc. 1965, 87,
4234.
0
consecutive reaction with the rate constants kdis and k0
0
3
4
P. Seiler, J. Wirz, Helv. Chim. Acta 1972, 55, 2693.
S. G. Schulman, W. R. Vincent, W. J. M. Underberg, J. Phys.
Chem. 1981, 85, 4068.
(kdis < k0 ). Since the decay rate (ꢁdecay) of 1TFOHsꢀ corre-
sponds to (k0 þ kdis), the magnitude of kdis can be estimated from
ꢂ1
(ꢁdecay ꢂ ꢁ0ꢂ1). We assumed that ꢁ0 (=1=k0) is equal to the
5
J. Oshima, S. Shiobara, H. Naoumi, S. Kaneko, T.
Yoshihara, A. K. Mishra, S. Tobita, J. Phys. Chem. A
2006, 110, 4629.
fluorescence lifetime of TFOHs in CH3CN where no dissocia-
tion occurs.7 From the above kinetic analyses the rate constants
for the proton-dissociation reaction of o-, m-, and p-TFOH were
6
7
S. Shiobara, S. Tajima, S. Tobita, Chem. Phys. Lett. 2003,
380, 673.
K. M. Solntsev, D. Huppert, N. Agmon, Phys. Rev. Lett.
2001, 86, 3427.
obtained to be 2:2 ꢁ 109 sꢂ1, 8:6 ꢁ 108 sꢂ1, and 2:5 ꢁ 108 sꢂ1
,
respectively, at 293 K. The kdis values of o- and m-TFOHs are
much larger than that of PhOH which can be evaluated to be less
than 2:0 ꢁ 108 sꢂ1 from the fluorescence lifetime in H2O (3.1 ns)
and CH3CN (6.4 ns). The enhancement of the ESPT rate in
TFOHs can be attributed to the electron-withdrawing effect of
the TF group. The electronic effects of substituents are com-
posed of two main parts: a field/inductive component and a res-
onance component.8 In the ESPT of TFOHs, the CF3 group is an-
ticipated to accelerate the proton-dissociation rate by the induc-
8
9
C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165.
a) G. Granucci, J. T. Hynes, P. Millie, T.-H. Tran-Thi, J. Am.
Chem. Soc. 2000, 122, 12243. b) J. T. Hynes, T.-H. Tran-Thi,
G. Granucci, J. Photochem. Photobiol., A 2002, 154, 3.
˚
10 a) C. Rønne, P.-O. Astrand, S. R. Keiding, Phys. Rev. Lett.
1999, 82, 2888. b) C. Rønne, S. R. Keiding, J. Mol. Liq.
2002, 101, 199.
Published on the web (Advance View) February 28, 2009; doi:10.1246/cl.2009.312