A study of the competitive multiple hydrogen bonding effect and its associated excited-state proton transfer tautomerism

Article abstract of DOI:10.1039/c7cp05002j

1,8-Dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA) has been strategically designed and synthesized with the aim to study the competitive multiple hydrogen bonding (H-bonding) effect and the associated excited-state intramolecular proton transfer reaction (ESIPT). In nonpolar solvents such as cyclohexane, equilibrium exists between the two H-bonding isomers DHDA-23-OO and DHDA-23-OI, both of which possess double intramolecular H-bonds. In polar, aprotic solvents such as CH₂Cl₂, DHDA-23-OO becomes the predominant species. Due to various degrees of H-bond induced changes of electronic configuration each isomer reveals a distinct absorption feature and excited-state behavior, in which DHDA-23-OI in cyclohexane undergoes double ESIPT in a stepwise manner, giving the first and second proton-transfer tautomer emissions maximized at ～500 nm and 660 nm, respectively. As for DHDA-23-OO both single and double ESIPT are prohibited, resulting in an intense normal 450 nm emission band. In a single crystal DHDA-23-OI is the dominant species, which undergoes excited state double proton transfer, giving intense emission bands at 530 nm and 650 nm. The mechanism associated with competitive multiple H-bonding energetics and ESIPT was underpinned by detailed spectroscopy/dynamics and computational approaches.

Full text of DOI:10.1039/c7cp05002j

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A Study of Competitive Multiple Hydrogen Bonding Effect and Its
Associated Excited-State Proton Transfer Tautomerism
Yi-Ting Chen,¶,a Pei-Jhen Wu,¶, a Chia-Yu Peng, ¶,b Jiun-Yi Shen,a Cheng-Cheng Tsai,b Wei-Ping Hu,b,*
and Pi-Tai Choua,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
1,8-Dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA) has been strategically designed and synthesized in a aim to study
the competitive multiple hydrogen bonding (H-bonding) effect and the associated excited-state intramolecular proton
transfer reaction (ESIPT). In nonpolar solvents such as cyclohexane, equilibrium exists between two H-bonding isomers
DHDA–23_OO and DHDA–23_OI (Scheme 1), which all possess double intramolecular H-bonds. In polar, aprotic solvents
such as CH2Cl2, DHDA–23_OO becomes the predominant species. Due to various degrees of H-bond induced changes of
electronic configuration each isomer reveals distinct absorption feature and excited-state behavior, in which DHDA–23_OI
in cyclohexane undergoes double ESIPT in a stepwise manner, giving the first and second proton-transfer tautomer
emissions maximized at ~500 nm and 660 nm, respectively. As for DHDA–23_OO both single and double ESIPT are
prohibited, resulting in an intense normal 450 nm emission band. In a single crystal DHDA–23_OI is the dominant species,
which undergoes excited state double proton transfer, giving intense 530 nm and 650 nm emission bands. The mechanism
associated with competitive multiple H-bonding energetics and ESIPT was underpinned by detailed spectroscopy/dynamics
and computational approaches.
www.rsc.org/
emission bands maximized at 520 nm and 650 nm are
resolved, which are ascribed to the tautomer emission
resulting from the first and second proton transfer products,
1. Introduction
Recently, excited-state intramolecular proton transfer
(ESIPT) involving multiple protons has been attracting
considerable attention.1-6 On the one hand, it offers the
opportunity to mimic the ubiquitous proton transfer
phenomena in biological systems7-11 that incorporating moving
multiple protons across bridging hydrogen bond (H-bond). On
the other hand, from viewpoint of lighting and display,
multiple ESIPT may consequently lead to multiple emission
bands that warrant white light generation with a single type of
molecule.12-14 However, probing the mechanism of multiple
proton transfer, including the energetics of reaction pathways,
the simultaneous proton transfer versus stepwise relocation,
etc., is challenging because the structures of the intermediates
could be ill-defined, particularly when environment
perturbations are also involved.1-5,10, 15, 16
denoted by TA* and TB* respectively (see Scheme 1(a), *
denotes the excited state). 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 forward (TA*
proton transfer were determined to be (1.7 ps)− and (3.6
→ TB*) and backward (TA* ← TB*)
1
1
ps)− , respectively. The fast equilibrium leads to identical
population lifetimes of ~54 ps for both TA* and TB*
tautomers. Thus, the napthaldahyde molecule provides
valuable fundamental insight into the multiple ESIPT reaction.
In an aim to gain more insight into the single versus double
excited-state intramolecular proton transfer, in this study, we
synthesized
dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA, see Scheme
1b). Similar to DHNA DHDA is expected to form two
intramolecular H-bonds free from intermolecular H-bond
a
new DHNA derivative, namely 1,8-
,
Very recently, we have judiciously designed a molecule 1,8-
dihydroxy-2-naphthaldehyde (DHNA, see Scheme 1a) that
shows a distinct stepwise double proton transfer in the excited
state17. Upon UV electronic excitation, two large Stokes shifted
19
perturbation.18,
The difference, however, lies in the
existence of different types of H-bond conformers. For
example, shown in Scheme 1(b), the H-bond isomer DHDA–
23_OI possesses C=O(1)---HO(2)---HO(3) sequential dual H-
bonds while the other isomer DHDA–23_OO consists of a pair
of C=O(1 or 4)---HO(2 or 3) symmetric H-bonds. Various
degrees of H-bond strength and position are expected to have
distinctive electron distribution and hence different
photophysical properties for various DHDA isomers. Scheme
1(b) also depicts the possible proton-transfer tautomer
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Figure 1 depicts the single crystal structure and the packing of
DHDA in the unit cell. Monoclinic DHDA single crystals belong to
the space group of P2(1)/n, with cell parameters of a = 3.7960(3) Å,
b = 18.3816(13) Å, c = 13.3342(10) Å, α = γ = 90°, and β = 90.367(5)°
(see Tables S1~S3 and Fig. S1 in detail). According to Fig. 1(a), the
hydrogen bonds of DHDA are asymmetric. The distances between
two hydrogen bonds of O(2)H … O(1) and O(3)H … O(2) are
estimated to be 1.78 and 1.90 Å, respectively, strongly supporting
the existence of dual intramolecular hydrogen bonds in the
crystalline.
structures under the occurrence of first and second ESIPT for
DHDA–23_OI and DHDA–23_OO without energetic concerns.
Details of ground-state isomerization and excited-state single
versus double proton transfer are elaborated in the following
sections.
(a)
H
H
*
*
O
H
H
H
H
*
O
O
O
O
O
O
O
O
1st PT
2nd PT
1.7 ps
3.6 ps
H
H
H
< 150 fs
DHNA
*
TB
TA
*
*
1
In solvents such as CDCl3, the H-NMR spectrum of DHDA (see Fig.
H
H
(b)
*
O
O
O
O
H
H
H
H
S2) exhibits one relatively broad peak at 12.6 ppm and the other
sharp peak at 10.1 ppm, which is assigned to the −OH and CHO
protons, respectively. This assignment is supported by the
computational approach (vide infra) as well as the comparison with
the controlled compound, 1-hydroxy-8-methoxy-2-naphthaldehyde
(HN, see Scheme 2) which reveals two proton peaks at 12.6 and 9.9
ppm (Fig. S3) and are unambiguously assigned to −OH and CHO
protons, respectively. The results manifest two symmetric H-
bonding sites for DHDA in CDCl3, which is in sharp contrast to the
asymmetric O(2)H…O(1) and O(3)H…O(2) H-bonds in crystal. The
difference of H-bond configuration between solution and solid lies
in the strong polar-polar intermolecular HC=O(4) … HC=O(4)
interaction in the DHDA crystal, which reveals an intermolecular
C(12)H(12)O(4)…H(11)O(1)C(11) distance of 2.62 Å between two
vicinal monomers (see Fig. 1 (b), green dashed line), constructing a
zigzag chain parallel to the b axis direction. Such a crystal packing
makes the CH(12)O(4) group slightly twisted out of the naphthalene
plane without involvement on intramolecular H-bonds.
*
*
O
O
O
O
O
O
O
O
H
H
hv
H
H
H
H
proton transfer?
proton transfer?
DHDA-23_OI*
DHDA-13_II*
DHDA-12_II*
or
?
O
H
H
*
H
H
O
O
O
H
H
*
O
O
O
O
O
O
O
O
H
H
hv
H
H
H
H
proton transfer?
proton transfer?
DHDA-23_OO*
DHDA-13_IO*
DHDA-14_II*
Scheme 1. (a) Structure of DHNA and its associated proton
transfer reaction.17 (b) Two proposed dual H-bonded
structures of aldehyde DHDA and their possible proton-
transfer isomers after single and double ESIPT reaction.
2. Results and discussion
2.1 Syntheses and Structural Characterization
Compound DHDA was prepared from naphthalene-1,8-diol (1)
17, 20, 21
following the literature
(Scheme 2). In brief, 1 was reacted
with NaH and chloromethyl ethyl ether by protecting the hydroxyl
protons to form 1,8-bis(ethoxymethoxy)naphthalene (2). The
addition of the aldehyde group on 2, forming 1,8-
bis(ethoxymethoxy)naphthalene-2,7-dicarbaldehyde
(3),
was
accomplished using n-BuLi and tetramethylethylenediamine
(TMEDA), followed by addition of two stoichiometric DMF.
Deprotection of 3 under an acid condition gave DHDA in 75% yield
(see supporting information for detailed synthetic methods and
Figures S1-S4 for characterization).
Scheme 2. The synthetic routes of DHDA and the structure of
HN and HMN.
Figure 1. (a) The X-ray structure of DHDA thermal ellipsoids drawn
at 50% probability level. (b) View of the packing of DHDA in the unit
cell.
O
O
O
O
O
O
OH
OH
O
O
O
O
2.2 Photophysical study in solution
1) NaH, DMF
1) n-BuLi, TMEDA
2) DMF
H
H
2) CH3CH2OCH2Cl
As shown in Fig. 2, DHDA exhibits the lowest lying absorption band
maximized at 400 nm (ε~1.1 × 104 M-1cm-1), which is reasonably
attributed to a ππ* transition. Upon electronic excitation, DHDA
clearly exhibits a dominant emission band maximized at 445 nm in
cyclohexane, accompanied by a small, large Stokes shifted emission
maximum at 660 nm (Fig. 2). Upon monitoring at the 450 nm and
650 nm emission regions, the respective excitation spectra are
slightly different, in which the excitation spectrum monitored at
450 nm emission shows a characteristic vibronic progression with a
peak at 400 nm and an onset at ~430 nm, while that for 660 nm
shows less structural feature with the onset being red shifted to
1
2
3
O
OH
OH
1
O
R
OH
1
O
7
7
HCl(aq), IPA
CH2Cl2
6
6
H
H
8
9
H
8
2
3
2
9
3
5
5
10
4
4
10
R= H: HN
R=OCH3: HMN
DHDA
2 | J. Name., 2012, 00, 1-3
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500 nm. Moreover, upon excitation at e.g. 440 nm, bypassing the notice that 1-hydroxyl-2-naphthaldehyde (HN, scheme 2), which
onset of the 400 nm excitation spectral band, weak dual emission possesses a single O-H---O=CH H-bond, exhibits both normal and
bands maximized at 505 nm and 660 nm were resolved (see inset of proton-transfer tautomer emissions due to the existence of thermal
Fig. 2). Upon dissolving DHDA in polar solvents such as CH2Cl2 and equilibrium between normal and tautomer in the excited states.22,
23
CH3CN. The emission character in CH2Cl2 is similar to that in
Therefore, it is reasonable that the excited-state equilibrium
cyclohexane, but the 660 nm intensity is much decreased. In highly would be perturbed via substituent effect of HN. On this basis, we
polar solvent such as CH3CN, we observed solely the 480 nm then examined the derivative HN, namely HMN (Scheme 2), which
emission band and no emission bands at ~510 and 660 nm could be possesses an O-H---O=C H-bond, supported by the 1H-NMR (Fig. S3).
resolved (see Fig. S5). The solvent polarity dependence in spectral As shown in Fig. S7, HMN in cyclohexane exhibits only one emission
changes is reversible, indicating that the multiple emission bands band maximized at 450 nm that is unambiguously assigned to the
observed in cyclohexane are not from impurity but an intrinsic normal emission. Therefore, for HMN that mimics the single H-
property.
bonding site of DHDA–23_OO isomer, a single proton transfer in
the excited state seems to be thermally inaccessible. Likewise, it is
reasonable for us to propose that single proton transfer is
prohibited in DHDA–23_OO. The result further infers that stepwise
double proton transfer cannot take place in DHDA–23_OO. On the
other hand, our computational approach elaborated in the later
section (vide infra) also concludes that concerted double proton
transfer in DHDA–23_OO is highly endergonic. Both viewpoints
support the experimental results that only normal emission was
observed for DHDA–23_OO.
According to the above steady state spectroscopy, DHDA in
nonpolar solvents clearly exist at least two types of H-bonding
isomers, tentatively assigned to be DHDA–23_OO and DHDA–23_OI
(Scheme 1(b)) in the ground state, in which the major isomer, upon
excitation, gives rise to normal 450 nm emission, while the minor
populated isomer exhibits both 510 nm and 660 nm emissions.
Increasing the solvent polarity leads to prevailing population of
DHDA–23_OO. Prior to the structural assignment, it is noteworthy
that the proposed DHDA–23_OI isomer possesses the same dual H-
bonding sites as DHNA (cf. Scheme 1(a)). Therefore, it is reasonable
to assume that similar to DHNA,17 DHDA–23_OI undergoes
stepwise double proton transfer in the excited state, resulting in
DHDA–13_II* and DHDA–12_II* isomers (see Scheme 1(b)), which
exhibit ~500 nm and 660 nm emission bands, respectively (see inset
of Fig. 2). The other conformer DHDA–23_OO possesses a pair of
C=O---HO symmetric H-bonds and is the predominant species in
polar solvent such as CDCl3 according to 1H-NMR results (vide
supra). DHDA–23_OO exhibits only normal 445 nm emission, for
which the emission spectral feature and peak wavelength are
similar to that of non-ESIPT compound 3 (see Scheme 2 and Fig. S6).
Accordingly, both single and double proton transfers are prohibited
for DHDA–23_OO.
We then made attempts to further resolve the dynamics of
ESIPT for DHDA by using the femtosecond fluorescence
upconversion technique. A typical result in cyclohexane is shown in
Fig. 3 while the pertinent data are listed in Table 1. Upon 400 nm
excitation and monitoring at the blue emission region of e.g. 460
nm, the fluorescence upconverted signal clearly reveals dual decay
components, consisting of an ultrafast system-response (~150 fs)
decay and a much longer population decay component that was
further resolved to be 788±15 ps by the time-correlated single
photon counting (TCSPC) measurement. In accordance with the
steady state emission the long 788 ps decay component can
reasonably be assigned to the population decay from the symmetric
DHDA–23_OO conformer, for which ESIPT is prohibited (vide supra).
On the other hand, the emission monitored at e.g. 650 nm consists
of a 1.1±0.3 ps rise component and a decay time of 35±3.2 ps (Fig. 3
and Table 1). The rise component (1.1 ps) of the emission at 650 nm
is well-matched with the decay component (1.1 ps) of emission
monitored at 520 nm upon 420 nm excitation. Another result worth
to note is the identical population decay times for both 520 and 650
nm emission bands (∼35 ps), indicating equilibrium between two
excited-state species. This, together with the an ultrafast decay
component (< 150 fs) monitoring at 460 nm emission (vide supra), is
reminiscent of the ESIPT mechanism of DHNA.17 Therefore, similar
to DHNA, a stepwise double proton transfer takes place in DHDA–
23_OI where upon excitation a < 150 fs single proton transfer takes
1.0
ex380 nm
ex440 nm
1.0
0.8
0.6
0.8
0.4
0.2
0.6
0.0
500
600
700
800
Wavelength (nm)
0.4
0.2
0.0
place, forming DHDA
–
13_II*, followed by second proton transfer
13_II* and DHDA 12_II* are in fast
to DHDA 12_II*. DHDA
–
–
–
300
600
Wavelength (nm)
900
equilibrium, giving 505 nm and 660 nm emission bands, respectively,
with identical population decay time of 35 ps. The result clearly
Figure 2. Absorption (black), emission (dotted black, excited at 380
nm) and excitation (monitored at 450 nm (red) and 660 nm (blue)
emissions) spectra of DHDA. Inset: the enlarged emission at 660 nm
(black, λex = 380 nm) and the emission at > 500 nm (green, λex = 440
nm).
leads to the conclusion of the precursor (DHDA
–12_II*)-successor
(DHDA 13_II*) reversible type proton-transfer relationship. As a
–
result, the equilibrium constant Keq = kpt2/k−pt2 (Scheme 3, see SI for
derivation) can be obtained by the ratio of the pre-exponential
factor at t = 0, which is deduced to be 4.72 (see Fig. 2),
corresponding to a ΔG of -0.92 kcal/mol from DHDA
–13_II* to
DHDA 12_II*. The forward and backward proton transfer rate
–
Despite having double intramolecular H-bonds, the prohibition
of ESIPT for DHDA–23_OO is uncommon, which is of fundamental
interest to shed light on its H-bond/ESIPT relationship. First, we
constants can be further deduced to be kpt2 = (1.3 ps)-1 and k−pt2
=
(6.3 ps)-1. Lastly, despite the ground-state equilibrium between
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Table 1. The photophysical properties of DHDA
DHDA–23_OO and DHDA–23_OI their interconversion does not
2.3 Excited-state double proton transfer in single crystal
take place during the excited-state life span (see Scheme 3).
We also performed spectroscopy and dynamics measurement in a
single crystal in an attempt to make a direct correlation with the
molecular structure. The x-ray disclosed crystal structure of DHDA
1.0
(a)
to be solely in the DHDA–23_OI isomer. Therefore, we expect that
the ESIPT dynamics, if there is any, should be similar to that
observed in the DHNA crystal14. As a result, the emission of DHDA
in the single crystal showed a dual emission maximized at 530 nm
0.5
0.0
and 650 nm, (see Fig. 4), which is reasonable to assign as DHDA
13_II* and DHDA 12_II* proton-transfer tautomer emissions. At
the current stage, it is not feasible to apply fluorescence
upconversion measurement for single crystal of DHDA.
–
0
100
200
300
400
500
–
1.0
0.5
(b)
a
Alternatively, the time-resolved studies were performed using the
TCSPC technique with a system response time of ~20 ps. The results
0.0
1.0
(c)
shown in Fig. S8 reveal that the DHDA–13_II* emission monitored
at 520 nm consists of a fast relaxation decay (< system response
0.5
0.0
time of 20 ps) and a longer decay component of 134 ± 5 ps. When
monitored at 700 nm, assigned to the DHDA–12_II* emission, the
0
20
40
60
80
100
time trace apparently consists of a system irresolvable rise
Time (ps)
component and a long decay component of 139 ± 5 ps. Within
experimental errors, the population decay times of DHDA
–13_II*
Figure 3. Time-resolved femtosecond fluorescence up-conversion of
DHDA in cyclohexane monitored at (a) 460 nm, (b) 520 nm and (c)
and DHDA 12_II* are identical, supporting the fast equilibrium
–
relationship concluded in solution. The results reaffirm the
650 nm. Solid red lines depict the corresponding fitting curves. λex
400 nm for (a) and (c), λex = 420 nm for (b).
=
dominant DHDA–23_OI conformer in the crystal form, in which
ultrafast first proton transfer takes place, forming DHDA
which then undergoes second proton transfer to yield DHDA
DHDA 13_II* and DHDA 12_II* are in fast thermal equilibrium,
–
13_II*,
–
12_II*.
–
–
giving rise to 520 and 650 nm emission bands with identical
population decay time.
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
Wavelength (nm)
700
800
Figure 4. The comparative emission spectra of DHDA in cyclohexane
(black) and in solid (red) at room temperature. λex = 380 nm
Scheme 3. The proposed double proton transfer model for DHDA in
cyclohexane.
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2.4 Theoretical Investigation We then present the computational
approach in an aim to confirm the experimental observations. The
calculated relative energies for various isomers in cyclohexane on
the ground state (S0) and the first excited state (S1) are listed in Fig.
In summary, combining the spectroscopy/dynamics and
computational approaches we have clearly shown that molecule
DHDA containing at least two conformers in the ground state, in
which DHDA
transfer, giving the first and second proton transfer tautomers
DHDA 13_II* and DHDA 12_II* that are in equilibrium. The other
H-bonded isomer is ascribed to DHDA 23_OO, for which both
–23_OI isomer undergoes a stepwise double proton-
5. As shown in the Figure, on the ground state the DHDA-23_OO is
found to be the lowest-energy isomer with DHDA-23_OI and DHDA-
13_II about 0.9 and 2.2 kcal/mol, respectively, being higher in
energies. The isomer DHDA-12_II is predicted to be not an energy
minimum. The absorptions of DHDA-23_OO and DHDA-23_ OI are
predicted at 383 and 396 nm, respectively. This is consistent with
the experimental observation mentioned in previous sections. On
the first excited state S1, the DHDA-23_OI* is predicted to be not an
–
–
–
single and double proton transfer are energetically forbidden. As a
result multiple emission bands are observed, consisting of normal
DHDA
–
23_OO* emission (450 nm), first proton-transfer DHDA
–
energy minimum whereas the DHDA-12_II* is predicted to be a
minimum-energy structure. As shown in Fig. 5, excitation from
DHDA-23_OI would immediately cause an excited-state proton
transfer and form the DHDA-13_II* isomer. A small intramolecular
proton transfer barrier (~1 kcal/mol) separates DHDA-13_II* and
DHDA-12_II* where the two excited-state isomers are almost
isoenergetic. The small energy difference (-1.0 kcal/mol) is almost
identical to the experimentally derived value (-0.9 kcal/mol). The
calculated tautomer emissions from DHDA-13_II* and DHDA-12_II*
are 491 and 610 nm, respectively, which are also in reasonable
agreement with the experiment. It is also apparent that excitation
from DHDA-23_OO would not lead to intramolecular proton
transfer within the lifetime of the excited state due to the high
barriers at TS2* and TS3* (see Fig. 5). Note that both TS3 (or TS3*)
13_II* emission (500 nm) and second proton-transfer DHDA
–
12_II*
emission (650 nm) in cyclohexane. The computational approach
calculation also predicts very minor ground-state population for
DHDA–13_II* (< 5%). Unfortunately, we cannot confirm this
prediction by either steady-state or dynamic approaches at current
stage because the associated ESIPT dynamics, in theory, is the same
as DHDA
–
23_OI that undergoes ultrafast (< 150 fs) first proton
23_OI isomer
transfer. Due to the molecular packing, only DHDA
–
exists in the crystal form, rendering a white light-like emission
originating from first and second proton transfer tautomers that are
in equilibrium in the excited state.
takes two steps from DHDA-23_OO (or DHDA-23_OO*) to DHDA
–
ASSOCIATED CONTENT
23_OI (or DHDA 23_OI*), incorporating the C(1)-O(3)-H(3) (see Fig.
–
Supporting Information. Additional crystallographic (including CIF)
and spectroscopic data are provided. This material is available free
of charge via the Internet at http://pubs.acs.org.
1) rotation on the right phenyl ring and aldehyde rotation step
(C(2)-C(12-O(4), see Fig. 1). The normal emission of DHDA-23_OO is
predicted at 403 nm in cyclohexane, which is in good agreement
with experimental results shown in Fig. 2 and Scheme 3. Finally, in
CH3CN DHDA-23_OO was calculated to be more stable by 1.3
kcal/mol than DHDA-23_OI, which is 0.4 kcal/mol more stable than
that in cyclohexane. The result, in a qualitative manner, rationalizes
the dominant normal emission upon increasing the solvent polarity.
Also, according to the calculation, the normal emission red shifts
AUTHOR INFORMATION
Corresponding Author
Email:chop@ntu.edu.tw (P.-T. Chou.)
Email: chewph@ccu.edu.tw (W.-P. Hu)
from 403 nm to 426 nm in CH3CN, which is consistent with the Author Contributions
experiments (~30 nm in red shift, see Fig. S4)).
¶These authors contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
P.-T. Chou and W.-P. Hu thank the Ministry of Science and
Technology, Taiwan for the financial support.
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