A new class of N-H proton transfer molecules: Wide tautomer emission tuning from 590 nm to 770 nm via a facile, single site amino derivatization in 10-aminobenzo[h]quinoline

Article abstract of DOI:10.1039/c5cc06633f

Facile derivation of 10-aminobenzo[h]quinoline via replacing one of the N-H hydrogen atoms by various substituents generates a new series of excited-state intramolecular N-H proton-transfer molecules, for which the proton-transfer emission can be widely t

Full text of DOI:10.1039/c5cc06633f

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. Chou, H.
Tseng, T. Lin, C. Chen, T. Lin, Y. Chen, J. Liu, C. Hung, C. Chau and K. Liu, Chem. Commun., 2015, DOI:
10.1039/C5CC06633F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

Please do not adjust margins
Page 1 of 5
ChemComm
View Article Online
DOI: 10.1039/C5CC06633F
Journal Name
COMMUNICATION
A New Class of N-H Proton Transfer Molecules; Wide Tautomer
Emission Tuning from 590 nm to 770 nm via a Facile, Single Site
Amino Derivatization in 10-Aminobenzo[h]quinoline
Received 00th July 2015,
Accepted 00th August 2015
Huan-Wei Tseng,a,† Ta-Chun Lin,b,† Chi-Lin Chen,a,† Tzu-Chieh Lin,a Yi-An Chen,a Jun-Qi Liu,b Cheng-
DOI: 10.1039/x0xx00000x
Hsien Hung,b Chi-Min Chao,b,c,* Kuan-Miao Liu,b,c,* and Pi-Tai Choua,*
www.rsc.org/
Facile derivation of 10-aminobenzo[h]quinoline via replacing one mutual interference such as reabsorption, energy transfer and
of the N-H hydrogen atoms by various substituents generates a electron transfer.4-7 This may lead to white light generation with a
new series of excited-state intramolecular N-H proton-transfer more straightforward manner, and accordingly makes highly
molecules, for which the proton-transfer emission can be widely desirable the wide tunability of the proton transfer emission.
tuned from 590 nm to 770 nm simply by harnessing the electron-
donating/withdrawing strength of the substituents.
Recently, we have reported a series of amino- (NH-) type
hydrogen bonding (H-bonding) compounds comprising 2-(2’-
aminophenyl)benzothiazole and its derivatives.8 Upon derivation on
the amino- or phenyl- moieties, we were able to tune both the
normal and tautomer emissions throughout the visible range. This
series of molecules have close-lying normal and tautomer excited
states. In addition, the relatively weak intramolecular H-bond
requires geometry adjustments prior to ESIPT, inducing a barrier.
Therefore the ratiometric fluorescence for the normal versus
tautomer emission is sensitive to the environmental perturbation.
This, together with the rather weak emission due to the cis-trans
isomerization, makes this class of 2-(2’-aminophenyl)benzothiazoles
less potential in colour applications.
Bearing the above viewpoint in mind, we have thus screened
through –OH type of ESIPT molecules and strategically selected 10-
hydroxybenzo[h]quinoline (HBQ) as a prototype.9-11 Owing to the
intrinsic six-membered ring hydrogen bond, HBQ serves as perhaps
the most prominent ESIPT system that undergoes an ultrafast
proton transfer free from the solvent perturbation.12, 13 Its great
photostability and rigid fused rings lead to further derivations
meaningful. Replacing –OH by –NHR group at the C(10) position of
benzo[h]quinoline (see Scheme 1) we herein report on a new class
of N-H proton transfer molecules, for which the tautomer emission
can widely tuned from 590 nm to 770 nm simply by a single site
amino derivatization.
Introduction
The excited-state intramolecular proton transfer (ESIPT) reaction
commonly incorporates transfer of a hydroxyl proton to the
carbonyl oxygen (or pyridyl nitrogen) through a pre-existing
hydrogen bonding configuration in the excited state.1-3 The
resulting proton-transfer tautomer reveals drastically different
electron density distribution from its corresponding normal species.
As a result, most of the ESIPT molecules show the S0  S1 transition
band around near UV, while upon excitation, the occurrence of
ESIPT gives rise to visible emissions with a large Stokes shift (peak-
to-peak difference between absorption and emission) of > 8000 cm-
1. Unlike the excited-state charge transfer or structural relaxation,
in which the resulting large Stokes shifted emission is greatly
perturbed by the environment such as polarity, viscosity/rigidity,
etc., the proton-transfer tautomer emission is virtually perturbation
free except for external hydrogen bonding interferences.3 In other
words, the spectral profile of the tautomer emission in terms of
peak wavelength and intensity are relatively insensitive to the
external perturbation and is thus beneficial to a number of cutting-
edge applications. One perspective is to apply an UV LED to excite a
combination of ESIPT systems containing RGB colours without
Synthesis and Characterization
a.Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan. E-
mail: chop@ntu.edu.tw
All the titled compounds are synthesized and characterized with
1H/13C NMR, IR and mass spectroscopies (see ESI for detailed
synthesis and characterizations). The parent compound III (10-
aminobenzo[h]quinolone) was prepared with a high yield from 10-
nitrobenzo[h]quinoline under iron/ammonium chloride reduction.
As depicted in Scheme 1, starting from III, on the one hand, we
were able to replace either one or two of the –NH2 proton by
b.School of Medical Applied Chemistry, Chung Shan Medical University, Taichung
40201, Taiwan. E-mail: chimin.chau@gmail.com (C.-M. Chao);
lkm@csmu.edu.tw (K.-M. Liu)
c. Department of Medical Education, Chung Shan Medical University Hospital,
Taichung 40201, Taiwan.
† The first three authors have equal contribution.
Electronic Supplementary Information (ESI) available: [experimental details, X-ray
crystal structure & data (CIF, CCDC 1421361), additional spectroscopy results]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 5
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C5CC06633F
Scheme 1. The molecular structures and the syntheses of the titled compounds. Inset : The ESIPT process in the titled molecules.
electron-donating groups to give compounds I, II, IX and X. On the protons in −NH2 are much weaker in acidity than that of –OH.
other hand, substitution of one of the –NH2 protons by various Therefore, unless one of the –NH2 protons is replaced by an
electron-withdrawing groups forms compounds IV-VIII. The X-ray electron withdrawing group to increase the N-H acidity, ESIPT is
crystal structure of a representative compound VI is provided in Fig generally considered to be prohibited. In fact, to our knowledge,
S1 and the pertinent data are listed in Table S1 and S2.
We first introduced electro-donating groups such as a benzyl or ever been report so far to proceed with ESIPT.
none of the primary amino (–NH2) hydrogen bonding system has
8, 14
To our surprise,
methyl group to replace the amino hydrogen in III. Upon adding however, compound III exhibits remarkable proton transfer
1.1-1.2 equivalent of iodomethane or benzyl bromide with respect phenomenon in the excited state. As shown in Fig. 1 and Table 1,
to III into the reaction, we were able to obtain the mono N- compound III in cyclohexane shows the lowest lying absorption
substituted compounds I & II with good yields (78% & 58%), band maximum around 400 nm. Upon excitation, an exceedingly
accompanied by the isolation of a small amount (< 5%) of the long wavelength emission maximized at 740 nm is resolved. The
doubly N-substituted compounds. When the quantity of Stokes shift was calculated to be as large as 11,500 cm-1. The
iodomethane or benzyl bromide was increased to exceed two folds corresponding excitation spectrum is identical with the absorption
with respect to III, the doubly N-substituted compounds could be spectrum (see Fig. S2), eliminating any artefact caused by trace of
readily synthesized.
impurity. The result thus unambiguously concludes the occurrence
To increase the acidity of the N(R)-H proton, various electron- of ESIPT for compound III, which demonstrates for the first time
withdrawing groups, R, with different strength (pivaloyl, acetyl, that the primary amino (–NH2) H-bonded system is able to execute
benzoyl, tosyl and trifluoroacetyl) could also be introduced onto the ESIPT. The acidity of amino proton was generally considered to be
amino nitrogen. The prototype III was easily transformed into a too weak to provide driving force for ESIPT. The ability of ESIPT for
series of amide compounds IV, V, VI and VIII through acyl III may plausibly be relevant to the strong six-membered ring
substitution reactions by reacting with pivaloyl chloride, acetic hydrogen bonding formation that is constrained in a fused, rigid
anhydride, benzoyl chloride and trifluoroacetic anhydride, benzo[h]quinolone moiety (see Scheme 1). The resulting shorter
respectively. The sulfonamide compound VII was prepared by hydrogen bonding distance thus acts as the driving force for
reacting III with toluenesulfonyl chloride. For these reactions we efficient ESIPT (vide infra).
could only obtain the mono N-substituted compounds. The
One great advantage of the –NH2 H-bonded system over that of
introduction of the first electron-withdrawing substituent onto the the –OH class lies in the fact that one of the –NH2 proton can be
amino group may lower the nucleophilicity of the N (of the resulting chemically modified by an R group to systematically tuning the
amide) and consequently prevents the second substitution.
N(R)-H proton acidity. We then demonstrate that the tautomer
emission can be widely tuned by simply altering the electronic
property of the R group, which is otherwise inaccessible in –OH
ESIPT systems that have been ubiquitously adopted in both
fundamental and application approaches.
Absorption and Emission Spectroscopy
From conventional wisdom, we do not anticipate the parent
molecule, compound III, with only a primary amino group to be the
potential ESIPT system. This viewpoint is based on the fact that the
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
ChemComm
Please do not adjust margins
Journal Name
COMMUNICATION
I and II being weaker than that of III, a pronounVcieewd AtrtaiculetoOmnliener
DOI: 10.1039/C5CC06633F
emission maximized at 770 nm (I) and 750 nm (II) was observed.
Also, none of any normal emission, which is expected to be in the
range of < 500 nm could be resolved, concluding the occurrence of
fast and highly exergonic type of ESIPT for I-VIII (vide infra).
Moreover, remarkably, the tautomer emission peak is strongly
dependent on the electronic property of the N-R group, being blue
shifted as R increases the electron withdrawing ability (i.e., the
greater acidity and hence the H-bond strength). Theoretically, this
can be rationalized by the ESIPT reaction product, i.e., a proton
transfer tautomer for which the lowest lying excited state inherits a
HOMO  LUMO charge transfer character from the imino (HOMO)
to the quinolone (LUMO) moiety (see Scheme 2). Thus, adding an
electron withdrawing substituent R at the amino site results in the
lowering of HOMO energy, hence the increase of energy gap of the
imino-tautomer emission. As a result, depending on the electron
withdrawing/donating strength a wide and tuneable range of the
proton-transfer tautomer emission is achieved from 590 nm (in VIII)
to 770 nm (in I) (see Fig. 1b) via a facile, single amino derivatization
in 10-aminobenzo[h]quinolone. To show the clear contrast, the
emission of doubly N-substitution of I and II, forming IX and X,
respectively (see Scheme 1), is also depicted in Fig. 1b. Due to the
lack of N-H proton, IX and X exhibit normal emission maximized at
467 and 458 nm, respectively.
The tautomer emission yield reveals a decreasing trend upon
lowering the emission gap. We then compared their emission
intensity by assessing the quantum yield (Φem) of the tautomer
emission. This approach becomes possible by assuming that the
reaction thermodynamics is highly exergonic and the efficiency of
ESIPT is unity. The latter is implied by lack of resolving any normal
emission in the steady-state manner. The former is supported by
the computational approach (see ESI for detail methodology). The
resulting ΔE* (computed energy differences between the normal
and tautomer form species in the lowest excited state) shown in
Table 1 (and details in Table S3) indicates that thermodynamics of
ESIPT for compounds I-VI are exergonic by at least 2 kcal/mol. It is
worthy of note that ΔE* cannot be obtained for VII and VIII because
the minimum energy of the excited-state normal form was unable
to be located, implying more exergonic ESIPT thermodynamics.
Fig. 1 (a) Absorption and (b) normalized emission spectra for the titled compounds in
cyclohexane at room temperature. The concentrations are 8.8 x 10-6 − 2.2 x 10-5 M for
the emission spectra measurements. Photos in the middle: The photos of the titled
compounds in cyclohexane under room light and the emission hue under 365 nm UV
irradiation.
Firstly we began with substitution of the N-H proton by an
electron donating methyl or benzyl group (relative to H), forming
compound I and II. In an opposite synthetic strategy, we then
substitute one of the –NH2 hydrogen atom by electron-withdrawing
groups, yielding compounds IV-VIII. In this approach we
intentionally enhance the acidity of the N-H proton by increasing
the electron withdrawing ability of the R group. As a result, the
downfield shift of the N-H proton (in CDCl3) for I-VIII is in the order
of I (δ ~ 10.72 ppm) < II (δ ~11.55 ppm) < IV (δ ~14.91 ppm) ~ V (δ
~14.83 ppm) < VI (δ ~15.67 ppm) ~ VII (δ ~15.28 ppm) < VIII (δ
~16.68 ppm). Note that we only compare the mono-substituted
compounds and exclude compound III (-NH2) for its distinct
chemical environment. For the same class of H-bonded molecules,
the intramolecular H-bonding strength can empirically correlate
with the corresponding proton shift, with larger downfield shift
leading to stronger H-bond. Since I-VIII possess the same proton
acceptor, i.e., the benzo[h]quinoline nitrogen, the stronger H-bond
thus infers the increase of the proton donating strength, i.e., the
increase of the N(R)-H acidity from I to VIII.
Table 1. Photophysical Data for the Titled Compounds
#
λabs/nm
λem
/nm
770
750
740
684
682
670
640
590
467
458
Φem
τobs
knr
ΔE*
(kcal/mol)
-2.07
(ε/M−1cm−1
)
(x 1010 s−1
)
a
a
I
II
419(10907)
415(10444)
401(8920)
383(8020)
382(7980)
385(11100)
372(5720)
370(11700)
390(5016)
386(4760)
5.09 x 10-5
9.74 x 10-4
1.19 x 10-3
2.92 x 10-3
3.46 x 10-3
6.61 x 10-3
1.98 x 10-2
0.11
-3.47
III
IV
V
17.1 ps
61.4 ps
59.5 ps
95.7 ps
247 ps
1.08 ns
1.28 ns
1.33 ns
5.84
1.63
1.67
1.03
0.39
0.08
-3.31
-5.43
-5.84
VI
VII
VIII
IX
X
-7.15
b
b
0.07
0.07
Note: a Emission too weak to be monitored using fluorescence upconversion technique.
b The energy difference between excited normal and tautomer states, ΔE*, could not
be calculated due to the failure of locating energy minimum of the excited normal state
(see text for detail) .
Experimentally, as shown in Fig. 1b, ESIPT takes place for all titled
10-aminobenzo[h]quinoline derivatives. This is evidenced by the
appearance of a unique, large Stokes shifted (> 8000 cm-1) tautomer
emission for I-VIII. Again, to our surprise, despite the N-H acidity for
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 5
COMMUNICATION
Journal Name
withdrawing groups, forming I-VIII. Compounds I-VII all exhibit S0 →
View Article Online
DOI: 10.1039/C5CC06633F
S1 absorption at around 400 nm, while ESIPT apparently takes place,
resulting in a large Stokes-shifted tautomer emission, for which the
peak wavelength can be widely tuned from 590 nm to 770 nm. The
results also unveil a correlation that the stronger the electron
withdrawing R group is, the bluer the shift of the emission. This has
been rationalized by the decrease of HOMO energy of the tautomer
due to the electron withdrawing properties of the −NR group,
enlarging the energy gap. The intensity of the emission also
correlates with the peak wavelength, with reduction of the
emission quantum yield upon increasing the emission to the near
infrared, which is well explained by the operation of the energy gap
law. In summary, a new class of N-H proton transfer dyes has been
generated, for which the tautomer emission is spanning from
yellow to the near infrared region via a facile and single site
derivation of 10-aminobenzo[h]quinolone. The result should attract
a broad spectrum of interests in the fields of proton transfer
research and functional organic materials for optoelectronics.
Scheme 2. The calculated HOMO and LUMO molecular orbitals of normal and tautomer
form of III.
ACKNOWLEDGMENTS
We gratefully acknowledge funding support from the Ministry
of Science and Technology, Taiwan.
The nearly unitary ESIPT efficiency implies fast rate of ESIPT,
which can be probed by early reaction dynamics via femtosecond
fluorescence upconversion technique. Using III, VII and VIII as the
prototypes, we applied 370 nm femtosecond pulses (~150 fs)
excitation and then monitoring at 680, 640 and 600 nm tautomer
emission. The results clearly showed system response limited (~150
fs) rise time of the tautomer emission (see Fig. S3), supporting the
ultrafast rate of ESIPT. Furthermore, the tautomer population decay
for III-VIII was resolved and pertinent data (τobs) are listed in Table
1. With QY and population decay rate kobs provided (Table 1), the
radiative decay rate constant kr and nonradiative decay rate
References
1
2
3
P.-T. Chou, J. Chin. Chem. Soc., 2001, 48, 651-682.
J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3615-3642.
A. P. Demchenko, K.-C. Tang and P.-T. Chou, Chem. Soc. Rev.,
2013, 42, 1379-1408.
4
5
6
7
8
S. Kim, J. Seo, H. K. Jung, J. J. Kim and S. Y. Park, Adv. Mater.,
2005, 17, 2077-2082.
T. Mutai, H. Tomoda, T. Ohkawa, Y. Yabe and K. Araki, Angew.
Chem. Int. Edit., 2008, 47, 9522-9524.
H. Shono, T. Ohkawa, H. Tomoda, T. Mutai and K. Araki, ACS
Appl. Mater. Inter., 2011, 3, 654-657.
T. Mutai, H. Shono, Y. Shigemitsu and K. Araki,
CrystEngComm., 2014, 16, 3890-3895.
H.-W. Tseng, J.-Q. Liu, Y.-A. Chen, C.-M. Chao, K.-M. Liu, C.-L.
Chen, T.-C. Lin, C.-H. Hung, Y.-L. Chou, T.-C. Lin, T.-L. Wang
and P.-T. Chou, J. Phys. Chem. Lett., 2015, 6, 1477-1486.
M. L. Martinez, W. C. Cooper and P.-T. Chou, Chem. Phys.
Lett., 1992, 193, 151-154.
constant knr can thus be deduced from the relationship kr = kobs

Φem and knr = kobs –kr, respectively. The knr value listed in Table 1
clearly shows a decreasing trend, which is in the order of III > IV ~ V
> VI > VII > VIII. For the deactivation process between two states
(generally S1 and S0 states) with low energy gap and in the absence
of a zero-order surface crossing, an empirical energy gap law,15
specifies that the rate constant for the radiationless deactivation knr
(generally denotes the internal conversion process) can be assessed
by knr ~ νe-αΔE where α is a proportionality constant and ΔE denotes
the emission energy gap, respectively. As ΔE decreases knr increases
with an exponential manner, which also correlates well with the
decrease of the tautomer emission yield (Φem) from 0.11 in VIII (590
nm) to 5.09  10-5 in I (770 nm). For the new ESIPT systems I-VIII,
the great shift of the tautomer emission to the near IR region thus
clearly witnesses the energy gap law.
9
10 P.-T. Chou and C.-Y. Wei, J. Phys. Chem., 1996, 100, 17059-
17066.
11 K.-Y. Chen, C.-C. Hsieh, Y.-M. Cheng, C.-H. Lai and P.-T. Chou,
Chem. Commun., 2006, DOI: Doi 10.1039/B610274c, 4395-
4397.
12 P.-T. Chou, Y.-C. Chen, W.-S. Yu, Y.-H. Chou, C.-Y. Wei and Y.-
M. Cheng, J. Phys. Chem. A, 2001, 105, 1731-1740.
13 S. Takeuchi and T. Tahara, J. Phys. Chem. A, 2005, 109
,
10199-10207.
Conclusion
14 T. P. Smith, K. A. Zaklika, K. Thakur and P. F. Barbara, Journal
of the American Chemical Society, 1991, 113, 4035-4036.
10-aminobenzo[h]quinoline (compound III) forms a geometry
favorable, strong six-membered ring NH2···N intramolecular
hydrogen bond, from which we report for the first time the
occurrence of ESIPT in the primary amino- (−NH2) H-bonded system.
Otherwise, the N-H H-bonded systems require secondary NRH
derivation, in which R has to be electron withdrawing group, to
boost ESIPT. We are thus able to replace one of the amino protons
in III by various R groups spanning from to electron donating to
15 W. Siebrand, The Journal of Chemical Physics, 1967, 47
,
2411-2422.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
ChemComm
Please do not adjust margins
Journal Name
COMMUNICATION
View Article Online
DOI: 10.1039/C5CC06633F
Table of Contents Graphics
Tuning the ESIPT Emission from Visible to Near IR
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins