The Quest of Excited-State Intramolecular Proton Transfer via Eight-Membered Ring π-Conjugated Hydrogen Bonding System

Article abstract of DOI:10.1002/asia.201701057

Searching for eight-membered ring π-conjugated hydrogen bonding (8-MR H-bonding) systems with excited-state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π-conjugated molecules (8-HB-1,

Full text of DOI:10.1002/asia.201701057

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Accepted Article
Title: The Quest of Excited-State Intramolecular Proton Transfer via
Eight-Membered Ring π-Conjugated Hydrogen Bonding System
Authors: Fan-Yi Meng, Yen-Hao Hsu, Zhiyun Zhang, Pei-Jhen Wu, Yi-
Ting Chen, Yi-An Chen, Chi-Lin Chen, Chi-Min Chau, Kuan-
Miao Liu, and Pi-Tai Chou
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The Quest of Excited-State Intramolecular Proton Transfer via
Eight-Membered Ring -Conjugated Hydrogen Bonding System
Fan-Yi Meng,[a]§ Yen-Hao Hsu,[a]§ Zhiyun Zhang,*[a] Pei-Jhen Wu,[a] Yi-Ting Chen,[a] Yi-An Chen,[a] Chi-
Lin Chen,[a] Chi-Min Chao,[b] Kuan-Miao Liu,[b] and Pi-Tai Chou*[a]
Dedication ((optional))
Abstract: Searching for eight-membered ring -conjugated hydrogen
bonding (8-MR H-bonding) systems with excited-state intramolecular
proton transfer (ESIPT) property is seminal and synthetically
challenging. In this work, a series of -conjugated molecules (8-HB-1,
8-HB-L1 and 8-HB-2) potentially possessing 8-MR H-bond are
strategically designed, synthesized and characterized. The
configurations of these three potential molecules are checked by their
X-ray structures, among which 8-HB-L1 (a structurally locked 8-HB-1
core chromophore) is proved to be an 8-MR H-bonding system,
whereas 8-HB-1 and 8-HB-2 are too sterically hindered to form the 8-
MR intramolecular H-bond. The ESIPT property of 8-HB-L1 is
confirmed by the dual fluorescence consisting of normal and proton-
transfer tautomer emissions. The insight into the ESIPT process of 8-
HB-L1 is provided by femtosecond fluorescence upconversion
measurements together with computational simulation. The results
demonstrate for the first time a successful synthetic route to attain an
8-MR H-bonding molecule 8-HB-L1 with ESIPT property.
induced by electronic excitation, resulting in the proton relocation,
i.e., ESIPT, forming a proton transfer tautomer.
Most intramolecular H-bonding systems involve hydroxyl or
pyrrolic proton (–OH or –NH) as the proton donor while carbonyl
oxygen or pyridyl nitrogen serves as an acceptor.[1] Along this line,
a representative for five-membered ring (5-MR) intramolecular H-
bond can be ascribed to 3-hydroxyflavone[2] or 2-pyridylpyrazole,[3]
forming C=O···H–O and N···H–N H-bond, respectively, from
which ESIPT takes place. Relative to the absorption peak
wavelength, the resulting proton-transfer tautomer exhibits
anomalously large Stokes shifted emission that is pertinent for
potential optoelectronic and sensing applications.[4] Upon
increasing the H-bonding size, the six-membered ring (6-MR) H-
bonding systems are more common,[5],[6],[7],[8] the favorable
geometry of which leads to a rather strong hydrogen bond and
hence a shorter H-bonding distance, driving a fast ESIPT.
Prototypical
examples
are
methylsalicylate,[5]
10-
hydroxybenzoquinoline,[6] 2-hydroxybenzo-thiazole[7] and 7-(2-
pyridyl)indole,[8] covering various different types of H-bond.
Introduction
Searching for
a new type of -conjugated intramolecular
hydrogen bonding (H-bonding) system associated with unique
photophysical properties such as the excited-state intramolecular
proton transfer (ESIPT) reaction is always important and
fascinating.[1] Herein we define the -conjugated H-bond as the
H-bonding from which the switch of proton from donor to acceptor
site may induce the -electron rearrangement and hence the
delocalization of the -conjugation. Vice versa, the H-bond
properties may be affected by the redistribution of the  electrons
Scheme 1. The representative seven-membered ring (7-MR) H-bonding
systems with ESIPT.
[a]
F.-Y. Meng,§ Y.-H. Hsu,§ Dr. Z. Zhang, P.-J. Wu, Y.-T. Chen, Y.-A.
Chen, C.-L. Chen, Prof. P.-T. Chou
Department of Chemistry
It then becomes obscure when the -conjugated H-bonding
system is greater than 6-MR. Needless to say that the system is
designated to undergo ESIPT. To our best knowledge, the first
seven-membered ring (7-MR) H-bonding system undergoing
ESIPT should be ascribed to the cis-1-(2-pyrrolyl)-2-(2-
quinolyl)ethane (cis-2, Scheme 1).[9] Another example is an ortho-
GFP core chromphore[10] (o-HBDI, Scheme 1), which is stable as
a Z-form, possessing a 7-MR intramolecular H-bond between –
OH and immidazole nitrogen. Upon electronic excitation (~400
nm), o-HBDI undergoes intramolecular proton transfer, resulting
in a large Stokes shifted tautomer emission at 605 nm. Recently,
National Taiwan University
Taipei, 10617, Taiwan, R.O.C.
E-mail: zhangzhiyun@hotmail.com
chop@ntu.edu.tw
Prof. C.-M. Chao, Prof. K.-M. Liu
Department of Medical Applied Chemistry and Department of
Medical Education
[b]
Chung Shan Medical University
Taichung, 40201, Taiwan, R.O.C.
§These authors contributed equally
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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we have reported a structurally locked o-HBDI core chromophore,
o-LHBDI[11] (Scheme 1). Owing to the inherent inhibition of
rotational motion, o-LHBDI rendered an intense tautomer
emission capable of generating amplified spontaneous emission
via four electronic levels. Subsequently, another prototype (7-
HB,[8b] see Scheme 1) possessing 7-MR pyridine-pyrrole H-bond
has also been reported to exhibit ESIPT property.
(Scheme 2). Details of results and discussion are elaborated
below.
Results and Discussion
Synthesis
As depicted in Scheme 3a, with a green synthetic methodology,[14]
8-HB-1 could be obtained by reacting 1H-indene-1,3(2H)-dione
(ID) and 2-hydroxybenzaldehyde in water. However, a similar
protocol using ID and 7-hydroxy-1-indanone (7-HI) as the starting
material in an attempt to synthesize 8-HB-L1 unfortunately failed
(see Scheme 3b, route 1). Amid the reaction, monitored by the
thin-layer chromatography (TLC), the result showed that 7-HI
remained intact while ID disappeared. With a harsher approach,
we then used TiCl4 (in THF) to chelate the carbonyl oxygen of 7-
HI, which should increase the neucleophilicity of 7-HI and thus
facilitates the condensation reaction (see Scheme 3b, Route 2).
This modification was unsuccessful either. Alternatively, we then
reacted 7-HI with PhNH2 in the presence of catalytic formic acid
to form the Schiff-base type precursor (E)-3-(phenylimino)-2,3-
dihydro-1H-inden-4-ol (PDI) (Scheme 3b, Route 3). Schiff base
has been commonly used as a base for deprotonation via the
formation of an iminium cation that facilitates the reactivity of
nucleophicity of C=N. As expected, PDI reacted with ID in
acetonitrile successfully, affording 8-HB-L1 with a moderate yield
Scheme 2. The proposed potential eight-membered ring (8-MR) H-bonding
systems with ESIPT.
For further extension, it is then of great interest to search for
any eight-membered ring (8-MR) intramolecular H-bonding
systems endowed with ESIPT property. Theoretically, the 8-MR
H-bonded systems can effectively extend the -conjugated length
and may thus serve as a case in point to develop deep-red and
near infrared fluorescent materials by exploiting the anomalously
large Stokes shifted tautomer emission. The 8-MR intramolecular
H-bond can be easily realized in the flexible chain molecules,
such as peptides of aminoxy acids as foldamers.[12] However,
development of -conjugated molecules with 8-MR intramolecular
H-bond is of great challenge, which, to our knowledge, is
unprecedented. From the viewpoint of chemical structure, we first
mulled over the E isomer of o-HBDI (Scheme 2) as a case in point,
which shows an 8-MR, C(2)=O(1)···H–O(2) H-bonding
configuration. Unfortunately, due to the steric hindrance and
weaker H-bond, the E isomer is thermally unfavorable compared
with that of the Z form (see Scheme 1).[13] To lift the E,Z-restriction,
we then utilized the 1H-indene-1,3(2H)-dione to replace the
imidazole moiety of o-HBDI, and synthesized a potential 8-MR H-
bonding molecule, 2-(2-hydroxybenzylidene)-1H-indene-1,3(2H)-
dione (8-HB-1) (Scheme 2). In yet another approach, inspired by
7-HB (Scheme 1), we utilized phenol to replace the pyrrole moiety
and strategically designed and synthesized another potential 8-
MR H-bonding system 2-(2-(pyridin-2-yl)cyclopent-1-enyl)phenol
(8-HB-2) (Scheme 2). Unfortunately, further crystal structure
investigations of 8-HB-1 and 8-HB-2 were found to lack
intramolecular H-bond (vide infra). To ascertain the 8-MR H-
bonding -conjugated system, we then intentionally locked the
free rotational single bond C10−C11 of 8-HB-1 with the 2-phenol
ring, yielding the third potential 8-MR H-bonding molecule 7-
hydroxy-2,3-dihydro-[1,2'-biindenylidene]-1',3'-dione (8-HB-L1)
(33%). In yet
a
separated route (Scheme 3c), 1,2-
dibromocyclopent-1-ene was used as the starting material, which
was then coupled with (2-methoxyphenyl)boronic acid and 2-
(tributylstannyl)pyridine by sequential Suzuki and Stille couplings,
followed by demethoxylation with BBr3 in CH2Cl2, to form another
potential 8-MR H-bonding system, 8-HB-2. All new compounds
were fully verified with 1H NMR, 13C NMR and HRMS, and detail
of synthetic route is elaborated in the Experimental Section.
Scheme 3. Synthetic routes of the titled compounds.
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side-view to illustrating the planarity of the structures. Overlap packing of (c) 8-
HB-1 and (d) 8-HB-L1 with the cell axes.
Crystal Structures
Chemically, under the same proton donating and accepting
strength, the more planar H-bonding configuration should result
in a stronger H-bond. Accordingly, nearly planar H-bonding
geometry was frequently found in 5- and 6-MR H-bonding
systems and was also recently confirmed in the 7-MR H-bonding
cases.[10] Extending to 8-MR H-bonding configuration, simple
maneuver on the geometry constraint indicates that the formation
of a planar 8-MR H-bonding is nontrivial. For example, we
anticipated that 8-HB-1 and 8-HB-2 may potentially form planar
8-MR H-bond. Conversely, as shown in the X-ray structure (see
Figure 1a for 8-HB-1), apart from the 8-MR H-bonding
configuration drawn in Scheme 2, the phenol ring of 8-HB-1
rotates ~171° (C12–C11–C10–C1) along the C10–C11 single
bond that serves as the linker for the proton donor and acceptor
aryl rings. It is also noteworthy that 8-HB-1 takes a nearly planar
structure, with a small torsional angle of 6.2° (C11–C10–C1–
C2). On the one hand, this nearly planar structure without
involving intramolecular H-bond is believed to result from the
strong electron push-pull interactions together with two
intermolecular O–H···O H-bonds in a dimeric formation (Figure
1c). On the other hand, careful examination of the crucial bond
angles of 8-HB-1 indicates that both O3–C12–C11 and C12–
C11–C10 are around 117° (Table 1), which is much smaller than
135°, a theoretical interior angle calculated for a regular polygon
with eight sites. The result implies the difficulty to overcome the
steric hindrance upon forming the 8-MR intramolecular H-bond in
a flexible structure such as 8-HB-1. The steric hindrance of 8-MR
intramolecular H-bonding formation is even larger for 8-HB-2. The
X-ray structure of 8-HB-2, shown in Figure S1 in Supporting
Information, reveals two twisted isomers with different torsional
angles, forming two types of intermolecular O−H···N H-bonding
arrangement in the crystal lattice (Figure S2 in Supporting
Information). Obviously, all bond angles involved in the potential
H-bonding formation are substantially smaller than 135° (see
Table S1 in Supporting Information for detail), rationalizing its lack
of intramolecular H-bonding formation.
Upon locking the C10–C11 bond of 8-HB-1, forming 8-HB-L1,
the C(2)=O(1)···H–O(3) 8-MR intramolecular H-bonding
configuration was then firmly evidenced by its crystal structure
(Figure 1b), in which C10 and C16 are linked by the ethane bridge
(C17–C18). This brings two advantages for forming the 8-MR H-
bond. Firstly, the rotation of C10–C11 is inhibited due to its lock
with the phenol ring. Secondly, as listed in Table 1, the C16–
C11–C10 angle is much reduced from ~125° in 8-HB-1 to ~108°
in 8-HB-L1, and correspondingly C12–C11–C10 angle is
increased from ~117° in
8-HB-1 to ~135° in 8-HB-L1.
Nevertheless, the steric hindrance cannot be completely
suppressed, such that the corresponding H-bonding geometry for
8-HB-L1 is not fully planar, evidenced by the torsional angle of
C11–C10–C1–C2 ~4.3° (Figure 1b and Table 1), which is larger
than that in 7-MR H-bonding systems such as o-HBDI
(C5−C4−C3−N2 ~1.5°)[10] and o-LHBDI (~0°)[11] (see Scheme 1
and Figure S3 in Supporting Information). The H-bonding
distance in the C(2)=O(1)···H–O(3) 8-MR configuration of 8-HB-
L1 is measured to be as short as 1.583 Å via X-ray analyses.
Accordingly, whether ESIPT can take place in 8-HB-L1 is of great
interest.
Table 1. The Values of Selected Angles in Crystal Structures of 8-HB-1 and 8-
HB-L1
Selected Angles
O3−C12−C11
C12−C11−C10
C11−C10−C1
C10−C1−C2
C1−C2−O1
8-HB-1
117.4°
117.4°
134.2°
133.5°
129.2°
-
8-HB-L1
126.4°
134.9°
135.4°
132.2°
130.4°
114.8°
195.8°
109.5°
108.7°
-4.3°
C2−O1−H
O1−H−O3
-
H−O3−C12
-
C16−C11−C10
C11−C10−C1−C2
C12−C11−C10−C1
125.4°
6.2°
-171.1°
-1.7°
ESIPT Property
Figure 2 shows optical properties of 8-HB-L1 in cyclohexane (red
line), in which the absorption spectrum displays an intense peak
at 364 nm. Upon excitation (e.g. 350 nm) 8-HB-L1 displays a
rather weak emission band maximized at 490 nm, accompanied
by a second band appearing at ~700 nm (see Figure 2). The
overall dual-band emission yield was estimated to be as low as
(2.20.5) x 10-5 in cyclohexane. In comparison, non-H-bonded 8-
HB-1 is considered to be a non-proton transfer prototype, which
shows a single emission band maximized at ~475 nm (see black
line in Figure 2) with a low emission yield of (4.30.5) x 10-5. On
the basis of time-dependent DFT (B3LYP/6-31+G(d,p)) with
geometry optimization of 8-HB-L1 in the ground state, the S0 → S1
was calculated to be 420 nm. The geometry optimization of the S1
Figure 1. Single crystal structures of (a) 8-HB-1 and (b) 8-HB-L1: the top
images show a front view of the structures and the bottom pictures show the
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state always relaxes to the proton-transfer tautomer S’1 state
(prime sign denotes the tautomer), indicating that a rather small
or even negligible barrier for ESIPT of 8-HB-L1. The vertical S’1
→ S’0 transition of the proton-transfer tautomer is calculated to be
1.56 eV (793 nm, see Figure 3). As a result of calculation, ESIPT
is thermodynamically favorable by ~18 kcal/mol in cyclohexane
(see Figure 3). This value is considered to be an upper limit of the
energy difference because the optimized S1 state cannot be
obtained under current computational level. The matched
experimental and computational data supports the occurrence of
ESIPT in 8-HB-L1 in cyclohexane, giving normal (490 nm) and
proton-transfer tautomer (700 nm) emission bands. It’s deserved
to mention that the tautomeric emission band ~700 nm is located
at the region of near infrared emission. The steady-state optical
properties of 8-HB-L1 and 8-HB-1 were also investigated in
dichloromethane (polar aprotic solvent) and tert-butyl alcohol
(polar protic solvent). As shown in Figure S3 in Supporting
Information, the absorption spectra of 8-HB-L1 in various solvents
are almost identical. Unlike the resolvable dual emission in
cyclohexane, the tautomer band of 8-HB-L1 cannot be well
resolved in dichloromethane because of the relatively week
tautomer emission. Similar to a number of ESIPT dyes in protic
solvents, 8-HB-L1 only showed normal emission in tert-butyl
alcohol plausibly due to the stronger intermolecular H-bonding
formation that ruptures the weak intramolecular H-bond. As
shown in Figure S4, 8-HB-1 exhibited more obvious charge
transfer absorption band (~400 to 500 nm) from nonpolar
cyclohexane to polar tert-butyl alcohol as well as small red-shifted
emission. Also note that 8-HB-2 is virtually non-emissive in
various solvents, due perhaps to the much flexible molecular
structure that induces drastic nonradiative quenching.
Figure 3. Relative energy of the normal and tautomer species for 8-HB-L1 in
ground and excited states (in cyclohexane), calculated using B3LYP/6-31g*
level. Also depicted are the calculated HOMO and LUMO for normal and
tautomer species.
The rather low emission yield for 8-HB-L1 in cyclohexane
leads us to propose dominant non-radiative decay channels for
both normal and tautomer emissions. The femtosecond
fluorescence upconversion measurements were then carried out
to gain further insight into the relaxation properties of 8-HB-L1.
Upon 400 nm laser excitation and monitoring at either the blue
(e.g. 500 nm) or red (e.g. 700 nm) emission region, both
corresponding emissions revealed system limited response of
~220 fs (see Figure S4 in Supporting Information). The results, on
the one hand, indicate an ultrafast (< 220 fs) ESIPT for 8-HB-L1
in cyclohexane, similar to that of the 7-MR H-bonding systems
such as o-HBDI[10] and o-LHBDI,[11] giving an extremely weak
normal emission. On the other hand, in sharp contrast to the
exclusively tautomer emission observed in 5, 6 and 7-MR ESIPT
molecules, the tautomer emission in 8-HB-L1 is also very weak,
leading to simultaneous observation for both normal and tautomer
emissions. The ultra-weak tautomer emission may be rationalized
by its large exocyclic distortion. The results of calculation for 8-
HB-L1 show that C11–C10–C1–C2 angle changes largely from
~16.8° (normal form) to ~0° (tautomer form) during ESIPT (see
Figure 4). Such a torsional motion associated with H-bond may
play a key role to vastly deactivate the tautomer emission. This
process may only require very small amplitude motion such as a
slight bending associated with H-bond, which seems to be
operative in the solid as well because very weak emission was
observed for 8-HB-L1 in the solid powder. Nevertheless, as
shown in Figure 2 the ~700 nm proton-transfer tautomer emission
is obviously resolvable.
Absorption
Emission
8-HB-L1 in CyH
8-HB-1 in CyH
8-HB-L1 in CyH
8-HB-L1 as solid
8-HB-1 in CyH
1.0
0.5
0.0
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 2. The normalized absorption (red dash) and emission (red solid) spectra
of 8-HB-L1 in cyclohexane (CyH) as well as emission spectrum in solid state
(blue solid). Also shown are the absorption (black dash) and emission (black
solid) spectra of 8-HB-1 in cyclohexane. ex = 355 nm. A > 400 nm band passed
filter was inserted in front of the emission monochromator to eliminate the
second-order signal.
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To a solution of 7-hydroxy-1-indanone (100 mg, 0.61 mmol) and aniline
(57 mg, 0.61 mmol) in ethanol (30 mL), a few drops of formic acid was
added. Then the mixture was stirred at 80 ºC for 6 h. The mixture was
evaporated under vacuum. The crude product was purified by flash
chromatography eluting with 1:5 EtOAc/hexane to afford PDI (120 mg,
88%) as yellow solid. 1H NMR (400 MHz, CDCl3): δ 11.08 (br, 1H), 7.41–
7.32 (m, 3H), 7.19–7.15 (m, 1H), 7.06–7.03 (m, 2H), 6.87–6.81 (m, 2H),
3.08 (t, J = 6.0 Hz, 2H), 2.81–2.78 (m, 2H). 13C NMR (100 MHz, CDCl3): δ
178.12, 157.64, 150.29, 149.04, 133.80, 128.86, 124.41, 123.69, 120.87,
115.78, 113.08, 29.33, 28.42. MS (EI, 70 eV): m/z (relative intensity) 223
(M+, 100); HRMS calcd. for C15H13NO 223.0997, found 223.1002.
Figure 4. The front and side views of DFT optimized geometries for (a) normal
of 8-HB-L1 in the S0 ground state and (b) tautomer forms of 8-HB-L1 in the S1
excited state.
Synthesis of 7-hydroxy-2,3-dihydro-[1,2'-biindenylidene]-1',3'-dione
(8-HB-L1)
A solution of PDI (30 mg, 0.13 mmol) and 1,3-indan-dione (20 mg, 0.13
mmol) in acetonitrile (30 mL) was stirred at 80 ºC for 6 h. The mixture was
evaporated under vacuum. The crude product was purified by flash
chromatography eluting with 1:6 EtOAc/hexane to afford 8-HB-L1 (12 mg,
33%) as orange solid. 1H NMR (400 MHz, CDCl3): δ 10.78 (s, 1H), 7.96–
7.89 (m, 2H), 7.79–7.74 (m, 2H), 7.51–7.47 (m, 1H), 7.01–6.95 (m, 2H),
3.77–3.75 (m, 2H), 3.11 (t, J = 5.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ
195.51, 191.65, 171.67, 159.58, 157.54, 140.67, 140.28, 137.31, 135.63,
134.59, 128.88, 123.54, 122.72, 121.35, 118.98, 117.25, 38.26, 31.53. MS
(EI, 70 eV): m/z (relative intensity) 276 (M+, 100); HRMS calcd. for
C18H12O3 276.0786, found 276.0781.
Conclusions
In conclusion, we have carried out a strategic design, synthesis
and characterization of three potential 8-MR H-bonding systems.
As a result, all titled molecules have been thoroughly examined
by X-ray structure analyses, among which a structurally locked 8-
HB-L1 is proved to be in an 8-MR H-bonding configuration per se,
while the flexible molecules 8-HB-1 and 8-HB-2 failed to render
8-MR intramolecular H-bond due to their high steric hindrance.
While ESIPT takes place in 8-HB-L1, the resulting tautomer
emission is subject to dominant non-radiative deactivation,
induced perhaps by the structure deformation that couples with
the H-bonding motion or touches the conical intersection.[15] By
and large, the results demonstrate for the first time a successful
synthetic route to attain an 8-MR molecule 8-HB-L1, which so far
possesses the highest atom numbers involved in the
intramolecular H-bond ring, adding a new family to the-
conjugated intramolecular H-bonding systems.
Synthesis of 1-(2-bromocyclopent-1-en-1-yl)-2-methoxybenzene (1)
Under an nitrogen atmosphere, to a solution of 1,2-dibromocyclopent-1-
ene (300 mg, 1.33 mmol), (2-methoxyphenyl)boronic acid (201 mg, 1.33
mmol),[16] and Na2CO3 (1.4 mL, 2 M, 2.66 mmol) in toluene/ethanol (30
mL:7.5 mL), Pd(PPh3)4 (77 mg, 0.06 mmol) was added. The resulting
mixture was stirred at 100 ºC for 12 h. After cooling to room temperature,
the solution was evaporated under vacuum. The residue was extracted
sequentially with water (10 mL), CH2Cl2 (3 × 10 mL), dried (MgSO4) and
evaporated under vacuum. The crude product was purified by flash
chromatography eluting with 1:4 EtOAc/hexane to afford 1 (204 mg, 61%)
as white solid. 1H NMR (400 MHz, CDCl3): δ 7.29–7.22 (m, 2H), 6.97–6.93
(m, 1H), 6.88 (d, J = 8.0 Hz, 1H), 3.82 (s, 3H), 2.83–2.78 (m, 2H), 2.74–
2.69 (m, 2H), 2.09–2.02 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 156.21,
138.49, 129.86, 128.42, 125.48, 119.87, 117.85, 110.62, 55.50, 41.03,
36.18, 22.78. MS (EI, 70 eV): m/z (relative intensity) 252 (M+, 100); HRMS
calcd. for C12H13BrO 252.0150, found 252.0156.
Experimental Section
Instrumentation and Materials
NMR spectra were recorded on Varian Unity 400. High resolution mass
spectra were recorded by Gas Chromatograph-Mass Spectrometer
(Finnigan MAT TSQ-46C GC/MS/MS/DS). The X-ray diffraction intensity
data were collected at 200 K or 150 K on a Rigaku RAXIS RAPID IP
imaging plate system with MoKα radiation (λ = 0.71073 Å). UV-visible
Synthesis of 2-(2-(2-methoxyphenyl)cyclopent-1-en-1-yl)pyridine (2)
Under a nitrogen atmosphere, to a solution of compound 1 (100 mg, 0.4
mmol), 2-(tributylstannyl)pyridine[17] (189 mg, 0.48 mmol) in toluene (30
mL), Pd(PPh3)4 (23 mg, 0.02 mmol) was added. The reaction mixture was
refluxed for 12 h, and then the solution was evaporated under vacuum.
The crude product purified by flash chromatography eluting with 1:4
EtOAc/hexane to afford 2 (70 mg, 70%) as white solid. 1H NMR (400 MHz,
CDCl3): δ 8.52–8.50 (m, 1H), 7.30–7.20 (m, 2H), 7.01–6.94 (m, 2H), 6.87–
6.91 (m, 3H), 3.69 (s, 3H), 3.09–3.05 (m, 2H), 2.90–2.85 (m, 2H), 2.10–
2.04 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 156.79, 148.65, 140.26,
138.66, 135.07, 129.87, 128.40, 128.09, 122.79, 120.85, 120.73(×2),
110.06, 55.29, 39.89, 36.25, 22.36. MS (EI, 70 eV): m/z (relative intensity)
251 (M+, 100); HRMS calcd. for C17H17NO 251.1310, found 251.1303.
absorption spectra were recorded by
a
Hitachi (U-3310)
spectrophotometer, and steady-state emission spectra were taken from an
Edinburgh (FS920) fluorimeter. Ultrafast dynamics was studied by the
femtosecond fluorescence up-conversion (FOG100, CDP). Commercially
available reagents and solvents were used without further purification.
Synthesis of 2-(2-hydroxybenzylidene)-2H-indene-1,3-dione (8-HB-1)
According to literature procedure,[14] 8-HB-1 was prepared as a yellow
solid. 1H NMR (400 MHz, CDCl3): δ 9.61 (s, 1H), 8.04-8.02 (m, 3H), 7.87–
7.80 (m, 4H), 7.49–7.45 (m, 1H), 7.07 (d, J = 8.0 Hz, 1H), 7.01 (t, J = 8.0
Hz, 1H).
Synthesis of 2-(2-(pyridin-2-yl)cyclopent-1-en-1-yl)phenol (8-HB-2)
Synthesis of (E)-3-(phenylimino)-2,3-dihydro-1H-inden-4-ol (PDI)
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Gryko, Chem. Asian J.,2012, 7, 2656-2661; g) C. Zhao, X. Li,
F. Wang, Chem. Asian J., 2014, 9, 1777-1781.
Under a nitrogen atmosphere, to a solution of compound 2 (60 mg, 0.24
mmol) in dried CH2Cl2 (20 mL) at 0 ºC, BBr3 (0.31 mL, 1M, 0.31 mmol) was
added slowly with a syringe. After stirring for 6 h, the solution was
evaporated under vacuum. The residue was extracted sequentially with
water (10 mL), CH2Cl2 (3 × 10 mL), dried (MgSO4) and evaporated under
vacuum. The crude product purified by flash chromatography eluting with
1:2 EtOAc/hexane to afford 8-HB-2 (32 mg, 56%) as white solid. 1H NMR
(400 MHz, CDCl3): δ 12.01 (br, 1H), 8.43 (d, J = 5.2 Hz, 1H), 7.71–7.66 (m,
1H), 7.32 (d, J = 8.0 Hz, 1H), 7.18–7.04 (m, 4H), 6.86–6.82 (m, 1H), 3.04–
3.01 (m, 2H), 3.00–2.88 (m, 2H), 2.11–2.04 (m, 2H). 13C NMR (100 MHz,
CDCl3): δ 155.43, 154.74, 146.70, 140.39, 137.49, 135.44, 128.83, 128.49,
128.19, 122.49, 121.45, 120.40, 119.89, 43.02, 37.79, 22.69. MS (EI,
70eV): m/z (relative intensity) 237 (M+, 100); HRMS calcd. for C16H15NO
237.1154, found 237.1146.
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Acknowledgements
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P.T.C. thanks the Ministry of Science and Technology, Taiwan for
financial support.
Keywords: proton transfer • hydrogen bonds • single crystal •
fluorescence • eight-membered ring
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Fan-Yi Meng, Yen-Hao Hsu, Zhiyun
Zhang,* Pei-Jhen Wu, Yi-An Chen, Yi-
Ting Chen, Chi-Lin Chen, Chi-min
Chao, Kuan-Miao Liu, Pi-Tai Chou*
In searching for seminal eight-
-conjugated
hydrogen bonding (8-MR H-bonding)
systems we strategically designed
and synthesized a series of
compounds (8-HB-1, 8-HB-L1, 8-
HB-2) potentially possessing 8-MR
H-bond, among which the
structurally locked 8-HB-L1 is
affirmed to be the authentic 8-MR H-
bonding system with prominent
excited-state intramolecular proton
transfer.
Page No. – Page No.
The Quest of Excited-State
Intramolecular Proton Transfer via
Eight-Membered Ring -Conjugated
Hydrogen Bonding System
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Title
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