Excited-state intramolecular proton transfer in five-membered hydrogen-bonding systems: 2-Pyridyl pyrazoles

Article abstract of DOI:10.1021/ja035382y

The excited-state intramolecular proton transfer (ESIPT) reaction in five-membered N-H···N hydrogen-bonding systems has been explored through design and syntheses of a series of 5-(2-pyridyl) 1-H-pyrazoles 1a-d. The ESIPT mechanism was confirmed through s

Full text of DOI:10.1021/ja035382y

Published on Web 08/14/2003
Excited-State Intramolecular Proton Transfer in Five-Membered
Hydrogen-Bonding Systems: 2-Pyridyl Pyrazoles
†
†
†
‡
‡
,‡
Wei-Shan Yu, Chung-Chih Cheng, Yi-Ming Cheng, Pei-Chi Wu, Yi-Hwa Song, Yun Chi,* and
,
†
Pi-Tai Chou*
Department of Chemistry, National Taiwan UniVersity, 106, Taipei, Taiwan R.O.C., and Department of Chemistry,
National Tsing Hua UniVersity, 300, Hsinchu,Taiwan R.O.C.
Received March 31, 2003; E-mail: chop@ntu.edu.tw
Since the seminal studies on the excited-state intramolecular
1
proton transfer (ESIPT) reaction in salicylic deivatives, numerous
ESIPT molecules have been discovered and investigated to shed
2
light on their corresponding spectroscopy and dynamics. Most
ESIPT molecules possess six-membered ring types of a strong
intramolecular hydrogen bond between O-H (or N-H) and CdO
(
or pyridinic nitrogen) groups, in which the intramolecular proton
1
3
transfer in the ππ* state commonly reveals a negligible barrier
in nonpolar solvents and may proceed during the period of low-
frequency vibrational motions associated with the hydrogen bond.4
From the fundamental viewpoint, searching for the ESIPT
reaction with a finite, well-defined barrier is of great importance
to gain detailed insights into the reaction potential energy surface.
-7
-
5
Figure 1. The absorption and emission spectra of (a) 1a (-), 1.0 × 10
M), (b) 1a-1Me (- - -) and (c) 1a-1′Me (‚‚‚) in cyclohexane. The excitation
To achieve this goal, conjugated pyrrole-pyridine systems such as
wavelength is 280 nm.
8
1
-H-pyrrolo[3, 2-h]quinolines possessing an N-H‚‚‚N type of five-
Scheme 1. Structures and Proposed ESIPT Mechanism for the
Studied Systems
membered hydrogen bond have received much attention. Under
similar proton donor/acceptor strengths, the intramolecular hydrogen-
bonding (HB) strength is empirically on the order of 6 > 5 . four-
membered systems due to the steric and orientation effects. Unlike
the O-H site that has certain rotational degrees of freedom, the
orientation effect of the pyrrolic N-H site being restricted toward
a specific direction becomes more critical for the HB formation.
Furthermore, the photoacidity of the pyrrolic hydrogen is in general
weaker than that of the phenolic hydrogen. As a result, ESIPT in
five-membered HB pyrrole-pyridine systems may be associated
with an appreciably large barrier. Unfortunately, to our knowledge,
none of the five-membered N-H‚‚‚H HB systems have yet been
reported to exhibit intrinsic ESIPT. Instead, similar to the four-
membered proton donor/acceptor systems such as the 7-azaindoles,9
this class of molecules is well-known to undergo a guest molecule
2 0 1
F bands are identical, and are effectively the same as the S f S
absorption spectral feature, indicating that both emission bands
originate from the same ground-state precursor. In a comparative
study, 1a-1Me (see Scheme 1), which serves as a nonproton-transfer
model, exhibits a normal Stokes shifted emission maximum at ∼318
nm. Conversely, the proton-transfer tautomer analogue 1a-1′Me
reveals a single fluorescence maximum at ∼570 nm of which the
(
including the protic solvents)-catalyzed excited-state proton-transfer
8
reaction.
In this study, we have alternatively designed and synthesized
the conjugated 2-pyridyl pyrazole systems where the N(1)-H
acidity is enhanced via the electron-withdrawing property of the
proximal nitrogen atom (N(2), see Scheme 1) in the pyrazole ring.
Scheme 1 shows the molecular drawings of 5-(2-pyridyl) 1-H-
pyrazoles 1a-d, possessing various substituents at the -3 posi-
tion.10 Both X-ray structural analyses and molecular modeling
indicate that the N(1)-H‚‚‚‚N(1′) HB isomer is the predominant
species for 1a-d in the ground state (see Supporting Information).
Prototypical absorption and emission spectra of 1a in thoroughly
2
spectral feature is similar to that of the F band. We thus conclude
that ESIPT takes place in 1a, resulting in an anomalously large
-1
Stokes-shifted (>10 000 cm ) proton-transfer tautomer emission.
Similar dual emissions were resolved in 1b-d, of which the
corresponding photophysical properties are listed in Table 1.
In contrast to the exclusive tautomer emission resulting from
4
the ultrafast proton-transfer reaction for most ESIPT molecules,
the appearance of dual emissions in 1a-d in which the normal
fluorescence is prevailing is remarkable. It implies the existence
of an appreciably high barrier associated with ESIPT so that the
decay dynamics of the locally excited S f S relaxation is
1 0
competitive with the rate of proton transfer. Further insight into
the reaction mechanism was gained from the dynamic studies.
0 1
dry cyclohexane are depicted in Figure 1. Upon S f S (ππ*)
4
-1
-1
excitation (λmax ≈ 290 nm, ꢀ290 ≈ 1.2 × 10 M cm ), dual
fluorescence was observed maximized at 320 nm (the F band) and
band). The excitation spectra monitored at F and
1
585 nm (the F
2
1
†
Department of Chemistry, National Taiwan University, 106, Taipei, Taiwan
R.O.C.
1
Single-exponential decay kinetics were resolved for the F band of
2
1
a, and a lifetime was fitted to be ∼85 ps (ø ) 1.02) at 298 K,
‡
Department of Chemistry, National Tsing Hua University, 300, Hsinchu,Taiwan
R.O.C.
while the F
2
band is apparently composed of rise and decay compo-
10800
9
J. AM. CHEM. SOC. 2003, 125, 10800-10801
10.1021/ja035382y CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Table 1. Photophysical Parameters of 1a-d Measured in
tunneling mass. As a result, a negligible H/D isotope effect would
be observed. We further carried out a series of temperature-
dependent studies regarding reaction dynamics. As shown in Figure
2, the reaction rate for 1a monitored by the decay dynamics of the
Cyclohexane at 298 K
F1
max
(Φa)a
F2
max
(Φ)a
b
b
∆Ed
λ
λ
F
1
F
2
c
1
1
1
1
a
b
c
320 (19)
320 (21)
320 (25)
322 (9)
585(1.5)
577(0.5)
577(0.2)
590(0.3)
85
115
125
74
80 (-0.071)
2.03
2
75 (0.065)
1 2
F band or equivalently the rise dynamics of the F band revealed
120 (-0.017)
42 (0.021)
130 (-0.042)
13 (0.035)
76 (-0.070)
75 (0.061)
2.22
2.62
1.76
12
significant temperature dependence in the range of 298-203 K.
The logarithm plot for the ESIPT rate versus 1/T is sufficiently
linear, from which a nearly deuterium isotope independent barrier
2
2
1
1 -1
d
(∆E ≈ 2.03 kcal/mol) and frequency factor (∼3.8 × 10
s ) were
1
deduced. Negligible N-D isotope ESIPT dynamics were also
observed for 1b-d, and their corresponding ∆E are listed in Table
a
3
b
c
Φ × 10 . Picoseconds Numbers in the parenthesis are preexpo-
-
k t
-k t
d
b
1. Upon increasing the electron-withdrawing ability in R (see
nential factors of the fitting eq F(t) ) a1 e
mol) measured in methylcyclohexane. λ in nm.
1
+ a2 e
2
. ∆E (in kcal/
Scheme 1) the acidity of the N(1)-H proton should increase, and
hence faster ESIPT dynamics is expected. In contrast, Table 1 shows
a lack of correlation between ∆E and donating/accepting properties
of R
ESIPT process. Further support was given by a similar Arrhenius
plot for 1a in dry CH
CN, and ∆E was deduced to be ∼2.90 kcal/
b
, indirectly supporting the skeletal reorganization facilitating
3
mol, consistent with that obtained in methylcyclohexane.
In conclusion, the results demonstrate a novel and unique system
among ESIPT molecules where the intrinsic proton transfer is
associated with a substantial energy barrier. The nature of the
reaction potential surface in 1a-d may be described by certain
skeletal reorganization and hence is of a great theoretical challenge.
This, in combination with the structural simplicity and diversity,
makes the 5-(2-pyridyl)-1-H-pyrazole system an ideal model for
probing ESIPT dynamics, which are believed to bring up a broad
spectrum of interests in the proton-transfer field.
Figure 2. The plot for ln kobs versus the reciprocal of temperatures in
methylcyclohexane: 1a-h (b) and 1a-d (4) (monitored at the F1 band).
Supporting Information Available: Detailed experimental pro-
cedures, absorption, emission, time-resolved, and X-ray studies (PDF/
CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
2
nents that were fitted to be 80 and 275 ps, respectively (ø ) 1.01).
The rise time of the F
2
band, within experimental error, is identical
with the decay time of the F
1
band, further supporting the pre-
cursor-successor type of reaction mechanism. ESIPT dynamics
were also obtained for 1b-d at 298 K and the results are listed in
Table 1.
References
(
1) Weller, A. Z. Elektrochem. 1956, 60, 1144.
(
2) For recent reviews, see: (a) Scheiner, S. J. Phys. Chem. A 2000, 104,
5898. (b) Waluk, J. Conformational aspects of intra- and intermolecular
excited state proton transfer. In Conformational Analysis of Molecules in
Excited States; Waluk, J., Ed.; Wiley-VCH: 2000. (c) Chou, P. T. J. Chin.
Chem. Soc. 2001, 48, 651.
The rate constants of 70-130 ps- measured for 1a-d are ∼2
1
orders of magnitude smaller than that reported for typical ESIPT
4
molecules in nonpolar solvents. Slower proton-transfer reaction
(
3) The prohibition of ESIPT in the nπ* state has been reported in several
may be in some way associated with its rather weak hydrogen-
bonding strength, and hence a long HB distance. Thus, one might
initially suspect that this is simply due to a larger barrier along the
reaction coordinate with only the proton motion involved. To test
this possibility, the ESIPT rates in deuterated (N(1)-D) versions
of 1a-d were investigated. If only proton motion is involved in
the reaction coordinate, a large deuterium isotope effect would be
predicted. However, as shown in Figure 2, within experimental
errors, our results reveal a negligible isotope effect.
ESIPT molecules. For example, see ref 2a.
(4) For example, see: (a) Chudoba, C.; Riedle, E.; Pfeiffer, M.; Elsaesser, T.
Chem. Phys. Lett. 1996, 263, 622. (b) Lochbrunner, S.; Wurzer, A. J.;
Riedle, E. J. Chem. Phys. 2000, 112, 10699. (c) Chou, P. T.; Chen, Y.
C.; Yu, W. S.; Chou, Y. H.; Wei, C. Y.; Cheng, Y. M. J. Phys. Chem. A
2001, 105, 1731. (d) Ameer-Beg, S.; Ormson, S. M.; Brown, R. G.;
Matousek, P.; Towrie, M.; Nibbering, E. T. J.; Foggi, P.; Neuwahl, F. V.
R. J. Phys. Chem. A 2001, 105, 3709. (e) Stock, K.; Bizjak, T.;
Lochbrunner, S. Chem. Phys. Lett. 2002, 354, 409.
(
5) One exceptional case is 3-hydroxyflavones possessing a five-membered-
ring dO‚‚‚H-O hydrogen bond. Ultrafast ESIPT was reported for
4
d
3
-hydroxyflavone (3HF) in nonpolar solvents. Barrierless excited-state
proton transfer in 3HF was also reported through the cyclic hydrogen
Accordingly, more than proton motion must be involved in the
reaction coordinate. Molecular modeling of 1a renders a relatively
long N(1)H‚‚‚N(1′) HB distance of ∼2.49 Å with an N-H‚‚‚N
6
bond with protic solvents such as methanol.
6) Schwartz, B. J.; Peteanu, L. A.; Harris, C. B. J. Phys. Chem. 1992, 96,
591
(
(
(
3
7) The ESIPT from phenol O-H to a â-carbon atom is not included in this
category, see: Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2002, 124, 9458.
8) For recent examples, see: (a) Kyrychenko, A.; Herbich, J.; Izydorzak,
M.; Wu, F.; Thummel, R. P.; Waluk, J. J. Am. Chem. Soc. 1999, 121,
11179. (b) Herbich, J.; Kijak, M.; Zieli n´ ska, A.; Thummel, R. P.; Waluk,
J. J. Phys. Chem. A 2002, 106, 2158.
11
angle of ∼93°. These results, in combination with the restricted
orientation of the pyrazole N-H bond, lead us to propose that the
reaction coordinate does not couple directly with the N-H
stretching mode. Rather, it involves other skeletal motions such as
in-plane bending modes, which change the relative position of atoms
associated with the hydrogen bond, and hence channel into the
proton-transfer process. In this case, the resulting effective tunneling
mass should be greatly increased so that the deuteration of amino
proton results in only a very small fractional increase in the
(
9) Taylor, C. A.; El-Bayoumi, A. M.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A.
1969, 65, 253. Also, see refs 2b and 2c for recent reviews.
(
10) See Supporting Information for detailed syntheses and characterization.
11) The calculation was based on the HF/6-31G(d′,p′) level. X-ray data were
not applied here due to the dimeric structure in a single crystal.
(
(12) Further decreasing temperatures resulted in microcrystals interferences.
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