Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

Article abstract of DOI:10.1021/jacs.5b08562

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S₀ →S₁ (π-π) absorption at ～400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA and TB, respectively. The first proton transfer (DHNA → TA) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA → TB) and backward (TA ← TB) proton transfer were determined to be (1.7 ps)^-1 and (3.6 ps)^-1, respectively. The fast equilibrium leads to identical population lifetimes of ～54 ps for both TA and TB tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA (560 nm) and TB (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.

Full text of DOI:10.1021/jacs.5b08562

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Article
Optically Triggered Stepwise Double Proton Transfer in an Intramolecular
Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde (DHNA)
Chia-Yu Peng, Jiun-Yi Shen, Yi-Ting Chen, Pei-Jhen Wu, Wen-Yi Hung, Wei-Ping Hu, and Pi-Tai Chou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08562 • Publication Date (Web): 23 Oct 2015
Downloaded from http://pubs.acs.org on October 23, 2015
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Optically Triggered Stepwise Double Proton Transfer in an Intramo-
lecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-
naphthaldehyde (DHNA)
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¶,†
¶, ‡
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‡
§
†,
Chia-Yu Peng, Jiun-Yi Shen, Yi-Ting Chen, Pei-Jhen Wu, Wen-Yi Hung, Wei-Ping Hu, * and
Pi-Tai Chou *
‡,
†
Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi 62102, Taiwan, R.O.C.
‡Department of Chemistry and Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei
1
0617, Taiwan, R.O.C.
§
Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan, R.O.C.
KEYWORDS excited state intramolecular proton transfer, double proton transfer, proton relay, stepwise, asynchronous
ABSTRACT: 1,8-Dihydroxy-2-naphthaldehyde (DHNA) having doubly intramolecular hydrogen bonds was strategically designed
and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double proton
transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S S (*) absorption at ~400 nm. Upon excitation,
0
1
two large Stokes shifted emission bands maximized at 520 nm and 650 nm are resolved, which are ascribed to the tautomer emis-
sion resulting from the first and second proton transfer products, denoted by TA* and TB* respectively. The first proton transfer
(
DHNA*TA*) is ultrafast (< system response of 150 fs), while the second proton transfer is reversible, for which the rates of
1 1
forward (TA*  TB*) and backward (TA*  TB*) proton transfer were determined to be (1.7 ps) and (3.6 ps) , respectively.
The fast equilibrium leads to identical population lifetimes of ~54 ps for both TA* and TB* tautomers. Similar excited-state double
proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual tautomer emission. A
comprehensive 2-D plot of reaction potential energy surface further proves that the sequential, two-step proton motion is along the
minimum energetic pathway, firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unam-
biguously a stepwise, proton-relay type of intramolecular double proton transfer reaction in the excited state, which should gain
fundamental understanding of the multiple proton transfer reactions.
Introduction
exclude those systems for which the proton transfer in the
excited state is catalyzed by the protic solvent molecules.
Accordingly, one famous example may be ascribed to the hy-
drogen bonded (H-bonded) dimer of 7-azaindole (see Scheme
6,20-28
Most excited-state intramolecular proton transfer (ESIPT)
reactions refer to the single proton transfer event. Upon elec-
tronic excitation, the transfer of a proton (or hydrogen atom)
takes place via the existence of an intramolecular hydrogen
bond between proton donor (OH or NH) and acceptor (car-
1
). 7-Azaindole H-bonded dimer undergoes a switch of two
symmetric protons in the excited state, denoted as excited-
state double proton transfer (ESDPT), the associated double
proton transfer reaction dynamics of which has been a core
1
bonyl oxygen or nitrogen). In most cases this process results
in a tautomer species possessing a drastically different electron
density distribution (compared to the normal form). The tau-
tomer thus exhibits large Stokes shifted emission, which has
2
9-32
subject during the past several decades.
Unfortunately,
such an external H-bonded dimer has an ill-defined structure.
This, together with the structure perturbation by solvent mole-
cules, made the associated mechanism unclear, and it has been
the subject of a long-standing debate regarding synchronous
2-
received intensive attention in both fundamental exploration
1
0
10-18
and potential applications.
However, such a single proton transfer event may not be
(concerted) versus asynchronous (non-concerted and/or step-
sufficient to mimic the proton transfer reaction in biological
systems, which may frequently be associated with multiple
proton transfer reactions, incorporating the migration of pro-
tons via a series of proton relays bridged by the hydrogen
bonds, among which the proton transfer in hydroxylic solvents,
the so-called Grotthus mechanism is a prototype. Unfortu-
nately, up to this stage, ESIPT systems capable of incorporat-
ing more than a single proton transfer are very rare. Here we
wise) types of double proton-transfer. The other representative
33-36
case is [2,2’-bipyridyl]-3,3’-diol (BP(OH)2)
that contains
two channels of ESDPT with concerted (one-step) and/or se-
quential (two-step) protons transfer. The latter has been con-
cluded to be a more favorable pathway according to the com-
1
9
36
putational approach. A very recent example is credited to 7-
hydroxyquinoline-8-carboxylic acid, in which intramolecular
double proton transfer takes place via the assistance of the
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built-in carboxylic acid group. Unfortunately, since all these
cases did not show distinct emission associated with first and
second proton transfer, experimentally, whether it is one-step
or stepwise process for the transfer of two protons remains
unclear.
DHNA. Chemically, the stronger H-bond leads to more down-
field H-NMR shift. In accord with the hydrogen bonding dis-
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tance revealed in X-ray structure analysis, the 14.2 ppm and
9.4 ppm proton peaks are assigned to the O(2)H and O(3)H
proton H-bonded to carbonyl O(1) and hydroxyl O(2), respec-
tively (see Chart 1 and Scheme 1). This assignment is further
supported by the comparison with the controlled compound, 1-
hydroxy-2-naphthaldehyde (HN12, see Scheme 2B), which
shows a sharp proton peak at 12.6 ppm (Figure S2), pointing
to a stronger intramolecular H-bond formation. Details of the
synthesis and characterization are elaborated in the Experi-
mental Section.
Experimentally, to the best of our knowledge, none of the
systems so far have provided definitive reaction pathways
regarding the double or multiple proton transfer in the excited
state. In an aim to probe this crucial fundamental issue regard-
ing the possible stepwise double proton transfer in the excited
3
8,39
state, 1,8-dihydroxy-2-naphthaldehyde
(DHNA, Scheme 1)
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possessing double intramolecular hydrogen bonds and its de-
rivatives were strategically designed and synthesized. The
results and discussion elaborated below clearly demonstrate
for the first time a case in point of stepwise, relay type of in-
tramolecular double proton transfer in the excited state.
Scheme 2. (A) The synthesis of DHNA and (B) the struc-
ture of HN12.
Scheme 1. Excited-state double proton transfer (ESDPT)
in (A) 7-azaindole dimer and (B) DHNA presented in this
study.
Chart 1. (A) The X-ray structure of DHNA thermal ellip-
soids drawn at 50% probability level. (B) View of the
packing of DHNA in the unit cell.
Results and Discussion
Syntheses and Structural Characterization In this work,
compound DHNA was prepared from commercially available
starting material naphthalene-1,8-diol (1) following the litera-
3
8,39
ture
(Scheme 2). 1 was reacted with NaH and chloromethyl
methyl ether by protecting the hydroxyl protons to form 1,8-
bis(methylmethoxy)naphthalene (2). The addition of the alde-
hyde group on 2, forming 1,8-bis(methoxymethoxy)-2-
naphthaldehyde (3), was accomplished using n-BuLi and
TMEDA, followed by DMF with good yield. Deprotection of
3
under an acid condition gave the designated compound
DHNA. Definitive proof of DHNA is given by the X-ray
structure analysis. Chart 1 depicts the single crystal structure
and the packing of DHNA in the unit cell. Monoclinic DHNA
single crystals (Tables S1~S3 and Figure S1) belong to the
space group of P2(1)/c, with cell parameters of a = 8.4818(10)
Å , b = 6.7611(8) Å , c = 14.9995(17) Å , α = γ = 90°, and β =
Steady-state and Dynamics Studies in Solution
DHNA
exhibits the lowest lying absorption band maximized at 400
4
1
1
nm (~1.1  10 M cm ), which is reasonably attributed to a
ππ* transition. Upon electronic excitation, DHNA clearly ex-
hibits dual emission bands maximized at 520 and 650 nm in
cyclohexane (Figure 1). Monitoring the 520 nm and 650 nm
emission regions showed the corresponding excitation spectra
to be identical (see Figure S3), which are also the same as the
absorption spectrum, indicating that the two emission bands
share the same ground-state origin. The differences in peak
frequency between absorption and emission, defined as the
1
05.17°. According to Chart 1, the distances between the two
hydrogen bonds of the O(2)H…O(1) and the O(3)H…O(2) are
estimated to be 1.73 and 1.90 Å , respectively, strongly sup-
porting the existence of dual intramolecular hydrogen bonds
under a double six-membered ring configuration. In good
1
1
1
agreement with this observation, the H-NMR spectrum of
Stokes shift, are as large as 5800 cm and 9615 cm for 520
nm and 650 nm bands, respectively. For comparison, proton
transfer is prohibited in 3 (see Scheme 2) due to the lack of a
proton donor. Accordingly, 3 exhibits solely one emission
DHNA in CDCl (see Figure S2) showed two sharp proton
3
peaks at 14.2 and 9.4 ppm, assigned to the two OH protons,
manifesting the dual intramolecular H-bonding formation for
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TB* emissions show identical population decay times of ~54
band (max~ 450 nm, see Figure 1), showing a normal Stokes
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ps. The result clearly leads to the conclusions that TA* and
TB* have the precursor-successor type of relationship and that
these two species are in fast pre-equilibrium in the excited
state. Since the rise component of TA* cannot be resolved, we
conclude that the time scale of the first ESIPT from N* to TA*
is beyond the instrument response function (IRF) of our cur-
rent fluorescence upconversion setup (~150 fs, monitored by
Raman scattering signal at 428 nm (ex = 400 nm) in cyclo-
hexane). This viewpoint is also supported by monitoring the
emission at, e.g., close to 450 nm, which is supposed to be in
the normal emission region and is obscure in the steady-state
measurement; the upconverted signal is irresolvable from IRF
shift and mirror image with respect to the lowest lying absorp-
tion band (λmax = 365 nm). Note that this 450 nm emission
band is apparently missing in the case of DHNA.
1
.0
1.0
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0.4
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(see Figure S4).
3
00
400
500
600
700
Wavelength (nm)
Figure 1. Absorption (dash) and emission (solid) spectra of
DHNA (black) and 3 (blue) in cyclohexane. (Red) The emis-
sion spectrum of single crystal DHNA at room temperature.
The above steady state spectroscopic results lead to the pro-
posal that excited state intramolecular proton transfer takes
place in DHNA, resulting in two types of proton transfer tau-
tomers (isomers) and hence dual emission maximized at 520
nm and 650 nm. The molecular structure of each isomer can
be inferred from the controlled compound, HN12, which has
been reported to undergo ESIPT from -O(2)H to the carbonyl
4
0,41
oxygen (O(1)),
yielding a distinct O(2)-keto tautomer
emission at ~480 nm. The spectral similarity makes it reason-
able to ascribe the 520 nm emission in DHNA to an O(2)-keto
like tautomer (TA*, * denotes the electronically excited state).
Accordingly, the 650 nm emission most plausibly originates
from the O(3)-keto tautomer (TB*) possessing extensive elon-
gation of the -conjugation (see Schemes 1 and 3). Since the
excitation of DHNA is mainly from an aldehyde form config-
ured with a relay of O(2)-H and O(3)-H in H-bond (denoted
by N, see Scheme 3), double proton transfer must take place in
the excited state to rationalize the TB* generation. The inter-
play between one proton and two proton transfer in the excited
state, resulting in TA* and TB*, is then of prime fundamental
interest. Single (TA*) and double (TB*) proton transfer may
proceed via two separated pathways with a distinct branching
ratio, or a stepwise two proton transfer may take place, se-
quentially yielding TA* and TB*, or even more complicated
multiple pathways. Insight into this important fundamental is
gained from the time-resolved emission kinetics and theoreti-
cal approach elaborated below.
Figure 2. Time-resolved femtosecond fluorescence up-
conversion of DHNA in cyclohexane monitored at 520 nm
(black open square, □), 650 nm (blue open square, □) and
deuterated DHNA in cyclohexane monitored at 520 nm (green
open circle, ○) and 650 nm (orange open circle, ○). Solid
(black and blue) and dash lines (green and orange) depict the
corresponding fitting curves and instrument response function
(red). Inset: the depiction of relaxation dynamics within 10 ps.
The experimental results lead us to conclude a stepwise ex-
cited-state double proton transfer reaction, consisting of an
ultrafast, system irresolvable (<150 fs) ESIPT from N* to TA*,
followed by a second forward and reverse proton transfer be-
tween TA* and TB*. As a result, the kinetic relationship
among N*, TA* and TB* can be described by the coupling
reaction model depicted in Scheme 3 (see also Scheme S1 in
SI). Assuming at t~0 (< 150 fs) N* has been depopulated to
~zero, under instant population of TA* and pseudo equilibri-
um between TA* and TB*, the time-resolved fluorescence
intensity of TA* and TB*, denoted as [TA*] and [TB*] can
We made attempts to further resolve the dynamics of
ESDPT for DHNA by using the femtosecond fluorescence
upconversion technique. The results shown in Figure 2 and
Table S4 reveal that the TA* emission monitored at 520 nm
f
f
be expressed by (see Eqs. S1 in SI for detailed derivation):




kpt2
kpt2




TA*
r
t /1
t /2
[TA*]  I k [TA*]t0
e

e
f
0
(ex = 400 nm, 120 fs) consists of a fast relaxation decay (1.1 
kpt2  kpt2
kpt2  kpt2
0
.2 ps) and a longer decay component of 53  3.6 ps (an aver-
TB*
r
I0k
kpt2[TA*]
t0
age of four replicas). When monitored at 650 nm, assigned to
the TB* emission, the time trace apparently consists of a rise
time of 1.1  0.3 ps (a negative pre-exponential factor) and a
decay component of 54  3.2 ps (an average of four replicas).
On the one hand, within experimental error, the shorter decay
component of the TA* emission matches well to the rise com-
ponent of the TB* emission. On the other hand, both TA* and
t /1
t /2
[
TB*] 

e
 e

f
1
1
(2 )  (1 )
TA*
TB*
k f  k f Keq
1
1
(1 )

, (2 )  kpt2  kpt2
(I)
1
 Keq
where I is a proportional constant incorporating instrument
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TA *
TB*
(A)
factor.
and
are the fluorescence radiative decay rate
303K
283K
263K
kr
kr
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TA*
kf
TB*
constant of TA* and TB*, while
and
are the sum of
k f
2
43K
non-ESIPT decay rate constants for TA* and TB*, respective-
ly. kpt2 denotes forward (TA*  TB*) proton transfer rate and
k−pt2 represents reverse (TB*  TA*) proton transfer rate. The
2
2
and  are the observed decay time constants of the fast and
0
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1
00
500
600
700
slow decay components, respectively. As a result, the equilib-
rium constant K = k /k can be obtained by the ratio of the
pre-exponential factor (at t = 0) in Eq. (I), which is deduced to
be 2.13 (see Figure 2), corresponding to a ΔG of 0.45
kcal/mol from TA* to TB*. Since 1/ is equivalent to k
k−pt2, the forward and backward proton transfer rate constants
can be further deduced to be k = (1.7 ps) and k−pt2 = (3.6
ps) . In a steady state manner, the TA* versus TB* rati-
1
.0 (B)
eq
pt2 −pt2
333K
98K
223K
23K
2
1
0
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0
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9
0
1
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3
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6
7
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9
0
0.5
+
2
pt2
1
pt2
0.0
4
1
00
500
600
700
Wavelength (nm)
ometric emission was also studied as a function of temperature.
Upon lowering of the temperature from 303-243 K in methyl-
cyclohexane, the results clearly show that TA* 520 nm emis-
sion gradually decreases, accompanied by an increase in the
TB* 650 nm emission, supporting the thermally more stable
TB* species (see Figure 3A).
Figure 3. Temperature dependent emission spectra of DHNA
A) in methylcyclohexane (B) in a single crystal. Note: For
comparison, the emission is normalized at the maximum of the
TA* emission.
(
ESDPT in Single Crystal To make a direct correlation with
the molecular structure, we also performed spectroscopy and
dynamics measurement in a single crystal. The emission of
DHNA in a single crystal showed a similar double proton
transfer property, with the peak wavelengths located at 560 nm
(
TA*) and 650 nm (TB*), respectively (see Figure 1). In the
current stage, it is not feasible to apply fluorescence upconver-
sion measurement for a single crystal of DHNA because we
could not achieve the homogeneous excitation by the rotating
sample cell to avoid thermal heating. Alternatively, the time-
resolved studies were performed using the time correlated
single photon counting (TCSPC) technique coupled with a
femtosecond (~120 fs) laser pulse and a microchannel plate
(MCP) detection system (see experimental section). This com-
bination allows a system response time of ~20 ps. The results
shown in Figure S5 reveal that the TA* emission monitored at
5
20 nm consists of a fast relaxation decay (< system response
time of 20 ps) and a longer decay component of 228  20 ps.
When monitored at 650 nm, assigned to the TB* emission, the
time trace apparently consists of a system irresolvable rise
component (< 20 ps) and a long decay component of 223  22
ps. Within experimental errors, the population decay times of
TA* and TB* are identical, supporting the fast equilibrium
relationship concluded in solution. Interestingly, however, the
thermodynamic relationship between TA* and TB* seems to
be opposite to that observed in solution. Upon lowering of the
temperature from 333-123 K in a single crystal, the results
show that TA* 520 nm emission gradually increases, accom-
panied by the decrease of the TB* 650 nm emission (see Fig-
ure 3B). The result leads to a more stable TA* species in the
crystal, which is opposite to the more stable TB* species con-
cluded in cyclohexane/methylcyclohexane. We thus believe
that despite the establishment of equilibrium, the relative
thermodynamics is sensitive to the environmental perturbation
such as solvent polarity (in solution) or lattice (in crystal) sta-
bilization.
Scheme 3. The proposed double proton transfer model for
DHNA in cyclohexane.
We also replaced the two protons of DHNA with deuterium
and then studied the deuterium isotope effect on ESDPT. The
result shown in Figure 2 reveals negligible changes of the fast
decay (rise) component (~1.2 ps) of TA* (TB*). The deuteri-
um isotope-independent kinetics may, on the one hand, indi-
cate that ESDPT is not directly coupled with the OH stretch-
ing but rather induced by the motion of the molecular frame-
work, most likely the low frequency bending motion associat-
ed with changes in the angle and distance of the H-bond. On
the other hand, it may imply that the energy barrier is rather
low and that the entropic or the recrossing effects
42,43
44
dominate.
According to our transition state theory (TST)
calculation (see page S23 of SI for detail), if the excited-state
proton transfer reaction was dominantly controlled by the en-
ergy barrier and without recrossing the transition state region,
the deuterium kinetic isotope effect (KIE) would have been as
high as 3.02 for the TA*  TB* reaction, and kpt2 would have
been at least 7 times higher at 303 K.
Theoretical Investigation We now present the computation-
al approach in an aim to support the experimental observations.
The calculated relative energies on the ground-state (S ) are
listed in Table 1. The calculated molecular geometry and atom
numbering are shown in Figure 4. The calculated H-bond
0
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Journal of the American Chemical Society
sual that some of the proton-transfer tautomers exist only on
lengths of O(2)…H(3) and O(1)…H(2) of DHNA (Figure 4A)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the electronic excited state but not on the ground state if the
in cyclohexane are ~0.12 Å shorter than those obtained by X-
ray. Some of the calculated bond lengths are listed in Table S5.
4
0
9
normal form or other tautomers are much more stable on S .
0
5
In the ground-state (S ), the normal form (N) is 1.5 kcal/mol
The calculated relative energies on the S
1
at the TD -
0
B3LYP/6-31+G(d,p) level in cyclohexane are shown in Table
lower in energy than tautomer A (TA), and the barrier of N
st
4
5
46
2
. In the 1 singlet excited-state (S ), the tautomer A (TA*)
TA was 2.6 kcal/mol at the B3LYP /6-31+G(d,p) level in
1
4
7
48
became the most stable isomer, and it was 2.4 and 1.8
kcal/mol lower in energy than the normal form (N*) and the
tautomer B (TB*), respectively. The calculated barriers of N*
cyclohexane. The MP2 and M06-2X calculations predicted
only slightly higher barriers and energies of reactions. The
calculated results in other solvents were very similar, as shown
in Table 1.

TA* and TA*  TB* were 0.3 and 2.1 kcal/mol, respec-
tively. The results of the small energy difference (1.8 kcal/mol)
between TA* and TB* support the existence of thermal equi-
librium on the one hand; on the other, the calculated TA* en-
ergy being lower than that of TB* is opposite to the experi-
mental results. We attribute this discrepancy to the experi-
mental and theoretical uncertainty, with the latter affected and
limited by the theoretical level of the current computational
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
1
approach. However, our EOM-CCSD calculation suggests
(Table 2) that TB* is 1.2 kcal/mol lower in energy than TA*,
which is more consistent with the value deduced from the ex-
periment. A similar discrepancy can be seen in the ground
state. The tautomer TA is calculated to be only 1.5 kcal/mol
higher in energy than that of the normal form (N), indicating
the existence of trace TA under equilibrium. Experimentally,
however, we could not resolve any corresponding TA spec-
trum. Nevertheless, even if it existed, due to the ultrafast N*

TA* ESIPT process, any direct excitation of trace TA
Figure 4. Calculated structures of (A) the normal form (N and
N*), (B) tautomer A (TA and TA*) and (C) tautomer B (TB*)
for the DHNA molecule.
should not affect the aforementioned double proton transfer
dynamics. In fact, the calculated energy of the transition state
between N and TA is quite similar to that of TA (See Table 1).
This suggests that with the inclusion of vibrational zero-point
energy or at higher-level calculation, N and TA might merge
into a single potential energy well centered near N.
st
Table 1. Calculated ground-state (S ) energetics (kcal/mol) of
0
DHNA system in various solvents
Table 2. Calculated 1 excited state (S ) energetics (kcal/mol)
1
cyclohexane CH Cl CH CN
2
2
3
of the DHNA system in various solvents
a
N
0.00
2.59
1.54
0.00
2.60
1.53
0.00
2.60
1.52
cyclohexane CH Cl CH CN
2
2
3
B3LYP/6-31+G(d,p)
TS1
TA
a
N*
0.00
0.27
0.00
0.30
0.00
0.31
TS1*
N
0.00
2.93
1.73
0.00
3.14
1.78
0.00
3.15
1.77
TD-B3LYP/6-31+G(d,p) TA*
2.38
0.28
0.56
2.57
0.84
1.76
2.61
0.94
2.06
B3LYP/6-311+G(2df,2pd)//
B3LYP/6-31+G(d,p)
TS1
TA
TS2*
TB*
N
TS1
TA
0.00
4.56
4.29
0.00
4.51
4.27
0.00
4.50
4.26
N*
0.00
0.54
0.00
0.67
0.00
0.71
MP2/aug-cc-pVTZ//
B3LYP/6-31+G(d,p)
TS1*
TD-B3LYP/6-311+G(2df,2pd)//
TA*
TS2*
TB*
2.30
0.04
2.20
0.02
1.03
2.15
0.08
1.10
TD-B3LYP/6-31+G(d,p)
N
0.00
3.16
2.67
0.00
3.15
2.65
0.00
3.14
2.65
M06-2X/6-31+G(d,p) TS1
TA
0.45
N*
TS1*
0.00 
0.55 
0.00 
0.51 
0.00 
0.50 
a
N, TA, and TS1 represent the normal form, tautomer A, and
EOMCCSD/6-311+G(d,p)//
TA*
transition state of N  TA, respectively.
2.07 
1.01 
3.26 
2.11  2.13 
1.14  1.23 
4.60  4.96 
TD-B3LYP/6-31+G(d,p)
TS2*
TB*
While it is true that TB can be drawn as a resonance Lewis
structure of the normal form, the potential energy curve on S0
a
N*, TA*, TS1*, and TS2* represent the normal form*, tau-
tomer A*, and transition states of N*  TA* and TA*  TB*,
respectively.
along the TA  TB path (obtained from S ), as shown in Fig-
1
ure S8 in the Supporting Information, is monotonously uphill
and does not form a localized potential energy well around TB.
Thus with the electron density distribution of S , the TB is a
0
Despite the above discrepancies, the theoretical approach is
consistent with the ultrafast first proton transfer and the slower
second proton transfer indicated by experiment. The calcula-
tion using the larger basis set 6-311+G(2df,2dp) gave very
similar results, though with slightly higher barriers. The TD-
B3LYP/6-31+G(d,p) calculation predicted slightly lower bar-
highly unstable structure. This has also been checked by MP2
st
calculation. However, on the 1 singlet excited-state (S ) sur-
1
face, the carbonyl group (C(7)=O(2)) of TA* becomes more
electron-rich which induces the second proton transfer and
makes the TB structure a local energy minimum. It is not unu-
5
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Page 6 of 10
riers in dichloromethane and acetonitrile. A schematic plot
showing the calculated relative energies on both S and S and
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
the wavelengths of vertical excitation and emission in cyclo-
hexane solvent at the B3LYP/6-31+G(d,p) and TD-B3LYP/6-
31+G(d,p) levels are shown in Figure 5. It is noted that the
solvent effects need to be taken into account to find the TB*
structure. In the gas phase calculation, the TB* is not an ener-
gy minimum on the S potential energy surface (PES). The
1
calculated absorption wavelength (407 nm) of N was in good
agreement with the experimental data. The calculated emission
wavelengths from TA* and TB* were slightly shorter than the
peak values of the experiments. It is, however, understandable
because experimentally, the emission corresponds to energy
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
relaxation from the zero-point level of S to various vibrational
1
states on S , while the calculated values correspond to the dif-
0
ferences in the underlying Born-Oppenheimer energies be-
tween S and S at the molecular geometries calculated on S .
1
0
1
All the transitions between S and S were found to be of the
0
1
HOMO  LUMO type. Similar schematic plots in dichloro-
methane and acetonitrile are included in the Supporting Infor-
mation. Except for a 20 nm red shift in the emission of TB*
from cyclohexane to acetonitrile, the calculated relative ener-
gies and the absorption/emission wavelengths were very simi-
lar in different solvents. Figure 6 shows a comprehensive 2-D
PES plot both on S and S in cyclohexane calculated at the
0
1
B3LYP/6-31+G(d,p) level. It clearly proves that on S , the
1
sequential, two-step proton motion is subject to the minimum
energetic pathway, consistent with the experimental results. It
also showed that the region between the TA* and TB* was
very smooth and low in energy on the S surface. The calcu-
Figure 6. Calculated two-dimensional potential energy maps
st
1
lated minimum energy path on S indicated that the proton H(2)
of both the ground state (S ) and the 1 excited state (S ) for
0 1
1
moves towards the O(1) in the first step, which is followed by
the transfer of H(3) to O(2). The electron density difference
the proton transfer reactions in the DHNA system in cyclo-
hexane. The minimum energy path (the solid purple circle) is
also shown. The energy difference between neighboring dotted
lines or color shades is 1 kcal/mol.
(
EDD) maps between S and S as well as the HOMO and
0 1
LUMO involved in the S S transitions for N and TA are
0
1
plotted in Figure 7. As expected, the transitions are of the
ππ* type. The EDD maps in Figure 7 shows that upon excita-
tion from S to S , there is a net electron density shift from the
0
1
hydroxyl groups to the carbonyl oxygen (O(1) in N and to O(2)
in TA). This suggests that after the excitation, a driving force
was induced to facilitate the proton transfer reactions. This is
consistent with the physical picture obtained from the HOMO-
LUMO maps.
Figure 7. Calculated electron density difference (EDD) maps
between S and S and the HOMO (π)-LUMO (π*) maps of
1
0
the normal form (N) and tautomer A (TA) of the DHNA sys-
tem in cyclohexane. In the EDD maps, the regions with in-
creased electron density from S to S are shown in violet
0
1
Figure 5. Calculated relative energies (kcal/mol) and wave-
lengths (nm) of vertical excitation and emission of the DHNA
system in cyclohexane at the B3LYP/6-31+G(d,p) and TD-
B3LYP/6-31+G(d,p) levels.
while the regions with decreased density from S to S are
0
1
shown in yellow.
Conclusion
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In summary, we have clearly shown that molecule DHNA
tography on silica gel (DCM/hexanes/EA) to yield the 3 (1.06
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
containing dual intramolecular H-bonds undergoes a stepwise
double proton-transfer. The first proton transfer (N*TA*)
is ultrafast, resulting in an O(2)-keto like tautomer emission
maximized at 520 nm, while the second proton transfer (TA*
TB*) is fast but reversible, giving an O(3)-keto like tauto-
mer form emission at 650 nm. The experimental results are
firmly supported by the theoretical calculation, which provides
a comprehensive 2-D PES plot associated with the double
proton transfer reaction, leading to the conclusion that the
sequential, two-step proton motion pathway is indeed along
the minimum energy path.
During the past half century, excited-state intramolecular
proton transfers have been the recognized models for finding
fundamental regularities of the proton transfer reactions that
are basic to chemistry and biology. However, researchers have
met with great difficulties in studying the case of multiple
proton transfers to mimic the biological proton-transfer system
involving proton relay through hydrogen bonds. In this regard,
the targeted synthesis of new doubly hydrogen bonded DHNA
renders the paradigm to demonstrate unequivocally a stepwise,
proton relay type of excited-state intramolecular double proton
transfer reaction.
g, 70%) as an ochre solid. H NMR (400 MHz, CDCl ): δ
3
10.57 (s, 1 H), 7.79 (d, J = 8.4 Hz, 1 H), 7.54 (d, J = 8.8 Hz, 1
H), 7.44-7.40 (m, 2H), 7.14 (m, 1H), 5.30 (s, 2 H), 5.18 (s, 2
1
3
H), 3.55 (s, 3 H), 3.52 (s, 3 H). C NMR (100 MHz, CDCl ):
3
δ 190.9, 159.1, 154.1, 140.2, 129.3, 127.0, 125.0, 122.5, 122.4,
+
119.4, 111.5, 101.9, 95.6, 57.9, 56.4. ESI-MS: m/z 276 (M ).
Synthesis of 1,8-dihydroxy-2-naphthaldehyde (DHNA). A
solution of compound 3 (0.28 g, 1.0 mmol) in CH Cl (25 mL)
2
2
at 0 °C was added dropwise to a mixture of isopropanol (6.4
mL) and HCl (4.8 mL). After stirring for 6 h, the solvent was
removed under reduced pressure. The residue was purified
using column chromatography on silica gel (DCM/hexanes) to
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
yield DHNA (0.14 g, 75%) as a brownish-yellow solid. H
NMR (400 MHz, CDCl ): δ 14.29 (s, 1 H), 9.85 (s, 1 H), 9.40
3
(s, 1 H), 7.53 (t, J = 8.0 Hz, 1 H), 7.38 (d, J = 8.4 Hz, 1 H),
7.31 (d, J = 8.8 Hz, 1 H), 7.25 (d, J = 8.0 Hz, 1 H), 6.93 (d, J =
1
3
7.6 Hz, 1 H). C NMR (100 MHz, CDCl ) δ 195.9, 163.7,
3
157.7, 139.3, 132.7, 125.8, 120.5, 118.9, 113.2, 111.9. ESI-
+
MS: m/z 189 (M+H) .
Spectroscopic Measurements Steady-state absorption spectra
were recorded using a Hitachi U-3310 Spectrophotometer, and
emission spectra were obtained using an Edinburgh FS920
Fluorimeter. Detailed time-resolved spectroscopic measure-
Experimental Section
Synthesis and characterization. All solvents were distilled
from appropriate drying agents prior to use. Commercially
available reagents were used without further purification un-
less otherwise stated. All reactions were monitored by TLC.
Column chromatography was carried out using silica gel from
52
ments have been reported previously. In brief, nanosecond
time-resolved experiments were performed by using an Edin-
burgh FLS920 time-correlated single photon-counting
(
TCSPC) system with a pulsed hydrogen-filled lamp as the
excitation source. Data were fitted with the sum of exponential
functions using a nonlinear least-squares procedure in combi-
nation with a convolution method.
1
13
Merck (230-400 mesh). H and C NMR spectra were record-
ed on a Varian Unity 400 spectrometer at 400 MHz and 100
MHz, respectively. Chemical shifts (δ) are recorded in parts
per million (ppm) and coupling constants (J) are reported in
Hertz (Hz). Mass spectra were obtained using a Gas Chro-
matograph-Mass Spectrometer (Finnigan MAT TSQ-46C
GC/MS/MS/ DS).
Sub-nanosecond to nanosecond time-resolved studies were
performed using another TCSPC system (OB-900 L lifetime
spectrometer, Edinburgh) with an excitation light source from
the second harmonic generation (SHG, 400 nm) of pulse-
selected femtosecond laser pulses at 800 nm (Tsunami, Spec-
tra-Physics) and a microchannel plate (MCP) detector. The
fluorescence was collected at a right angle with respect to the
pump beam path and passed through a polarizer, which was
located in front of the detector. The polarization was set at a
magic angle (54.7º) with respect to the pump polarization di-
rection to eliminate anisotropy. Similar data analysis and fit-
ting procedures were applied. The temporal resolution, after
partial removal of the instrumental time broadening, was ~20
ps.
Synthesis of 1,8-bis(methylmethoxy)naphthalene (2). To a
solution of naphthalene-1,8-diol (1) (1.60 g, 10 mmol) in DMF
(12 mL) at 0 °C was added NaH (30 mmol) and stirred for 1h.
A solution of chloromethyl methyl ether (2.0 mL, 25 mmol) in
Et O (10 mL) was added to the mixture. After stirring over-
2
night, the reaction mixture was added to water and extracted
with Et O (20 mL × 3). The organic extracts were combined,
2
dried over anhydrous MgSO , filtered and concentrated. The
4
residue was purified using column chromatography on silica
gel (DCM/hexanes) to yield 2 (2.11 g, 85%) as a white solid.
1
Ultrafast spectroscopic study of the titled compounds was
H NMR (400 MHz, CDCl ): δ 7.48 (dd, J = 8.0, 1.2 Hz, 2 H),
3
performed by
a
femtosecond photoluminescence up-
7
(
1
2
.33 (t, J = 8.0 Hz, 2 H), 7.07 (dd, J = 8.0, 1.2 Hz, 2 H), 5.27
13
conversion (uPL) system pumped at 400 nm. In this measure-
ment, fluorescence from a rotating sample cell was focused in
a BBO crystal and its frequency was summed along with an
interrogation gate pulse at a designated delay time with respect
to the pump pulse. A half-wave plate was used to set the pump
polarization at a magic angle (54.7º) with respect to the gate
pulse to prevent the fluorescence anisotropy contributed by
solute reorientation. Fluorescence up-conversion data were
fitted to the sum of exponential functions convoluted with the
instrument response function (IRF). The IRF was determined
from the Raman scattering signal and its profile was fitted to a
Gaussian function with a full width at half maximum of ~220
fs.
s, 4 H), 3.57 (s, 6 H). C NMR (100 MHz, CDCl ): δ 153.5,
3
37.7, 126.1, 122.7, 119.1, 112.7, 96.4, 56.3. ESI-MS: m/z
+
48 (M ).
Synthesis of 1,8-bis(methoxymethoxy)-2-naphthaldehyde (3).
A solution of compound 2 (1.36 g, 5.5 mmol) and Et O (10
2
mL) at 0 °C was added dropwise to a mixture of n-BuLi (2.5
M in hexane, 2.86 mL, 7.1 mmol) and TMEDA (1.06 mL, 7.1
mmol) in Et O (50 mL) and stirred for 6 h at this temperature.
2
DMF (0.85 mL, 11 mmol) was added to the mixture for stir-
ring overnight (0 °C to r.t.). Water was added to the mixture
and the pH7 adjusted using 1.0 N HCl(aq). After extraction with
Et O, the organic layer was dried over MgSO , filtered and
2
4
evaporated. The residue was purified using column chroma-
7
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Computational methodology The structures and energies of
Author Contributions
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the 1,8-Dihydroxy-2-naphthaldehyde (DHNA) molecule and
its tautomers in solvents were calculated by the hybrid density
functionals B3LYP with the 6-31+G(d,p) basis set for the
¶
These authors contributed equally to this work.
Notes
ground-state (S ) potential energy surface (PES), and by the
0
The authors declare no competing financial interests.
time-dependent (TD) B3LYP with the 6-31+G(d,p) basis set
st
for the 1 singlet excited state (S ) PES and the vertical S 
1
0
ACKNOWLEDGMENT
P.-T. Chou and W.-P. Hu thank the Ministry of Science and
Technology, Taiwan for the financial support.
S excitation and the S  S emissions. The above B3LYP/6-
1
1
0
3
1+G(d,p) calculation on S were checked by the single-point
0
calculation at B3LYP/6-311+G(2df,2pd) and MP2/aug-cc-
pVTZ level on the B3LYP/6-31+G(d,p) geometries and by the
full calculation using the M06-2X functional with 6-31+G(d,p)
basis set. The solvation effects in the solvent of cyclohexane,
dichloromethane, and acetonitrile were considered by using
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
REFERENCES
(1)
For exceptional case, see: Parada, G. A.; Markle,
T. F.; Glover, S. D.; Hammarstrom, L.; Ott, S.; Zietz, B.
Chem. Eur. J. 2015, 21, 6362.
53
the polarizable continuum model (PCM). EOM-CCSD/6-
3
11+G(d,p) calculation was also performed to calculate the
(2)
(3)
Waluk, J. Acc. Chem. Res. 2003, 36, 832.
Hsieh, C.-C.; Jiang, C.-M.; Chou, P.-T. Acc. Chem.
relative energies on the S1 PES using the TD-B3LYP/6-
3
1+G(d,p) geometries.
Res. 2010, 43, 1364.
4) Demchenko, A. P.; Tang, K. C.; Chou, P. T. Chem.
Soc. Rev. 2013, 42, 1379.
5) Tomin, V. I.; Demchenko, A. P.; Chou, P.-T. J.
Photochem. Photobiol., C 2015, 22, 1.
The transition state (TS) search was started from an educat-
(
ed guess on the TS structure by placing the hydrogen atom
being transferred between the hydrogen donor and acceptor
and by adjusting the bond lengths that are being affected by
the hydrogen transfer. Then a hessian calculation (second de-
rivatives of energies with respect to the spatial coordinates)
was performed at the guessed TS structure to obtain the force
constants. Based on the force constants, the TS search was
(
(
(
6)
7)
Chou, P.-T. J. Chin. Chem. Soc. 2001, 48, 651.
Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res.
2
002, 35, 19.
5
4
(8)
Kwon, O. H.; Jang, D. J. J. Phys. Chem. B 2005,
performed by the Berny optimization method. The obtained
TS was confirmed by performing a frequency calculation to
see if only one imaginary frequency was obtained and that its
vibrational eigenvector points to the correct direction. The rate
constants and kinetic isotope effects (KIEs) were obtained
using the transition state theory (TST) with the calculated ge-
ometries, vibrational frequencies, and barrier heights at the
B3LYP/6-31+G(d,p) level. The 2-D potential energy map of
Figure 6 was calculated by setting the bond distances of
O(2)H(2) and O(3)H(3) to various values that span the en-
tire double proton transfer processes. The other geometrical
degrees of freedoms were optimized to achieve lowest possi-
ble energies (partial optimization). These energies thus ob-
tained were used to construct the map in Figure 6. The justifi-
cation of this approach is that the two bond lengths are the
most interesting and relevant coordinates in the proton transfer
processes, and plotting the PES this way gave a clearer picture
of the stepwise nature of the reactions. The other coordinates
were allowed to relax during the calculation to obtain more
reliable 2-D PES. The full minimum energy paths (the IRC
calculation) of the N  TA, N*  TA*, and TA*  TB*
reactions also have been calculated and the correctness of the
calculated TS were thus confirmed. The geometry on the paths
were mapped to the 2-D PES (Figure 6) marked as the solid
purple circles. All the calculations were performed using the
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Table of Content
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Compound DHNA presents the paradigm to
demonstrate unequivocally a stepwise, relay type
of excited-state intramolecular double proton
transfer reaction.
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