Remarkable difference between five- and six- number-membered ring transition states for intramolecular proton transfer in excited state

Article abstract of DOI:10.1016/j.jphotochem.2017.02.016

In this study, a range of organic dyes were prepared to investigate difference between five- and six- number-membered ring transition states for internal proton transfer in excited state. Different strength intramolecular hydrogen bond in the target dyes

Full text of DOI:10.1016/j.jphotochem.2017.02.016

Accepted Manuscript
Title: Remarkable difference between five- and six-
number-membered ring transition states for intramolecular
proton transfer in excited state
Authors: Xiaozhuan Qin, Ge Ding, Zhenqiang Wang,
Shengtao Zhang, Hongru Li, Ziping Luo, Fang Gao
PII:
S1010-6030(16)31141-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.02.016
JPC 10542
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
29-11-2016
14-2-2017
19-2-2017
Please cite this article as: Xiaozhuan Qin, Ge Ding, Zhenqiang Wang, Shengtao
Zhang, Hongru Li, Ziping Luo, Fang Gao, Remarkable difference between five-
and six- number-membered ring transition states for intramolecular proton transfer
in excited state, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2017.02.016
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Remarkable difference between five- and six- number-membered ring transition states for
intramolecular proton transfer in excited state
Xiaozhuan Qin1, 2, Ge Ding1, Zhenqiang Wang1, Shengtao Zhang1, Hongru Li1*, Ziping Luo1*, Fang Gao1*
1College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
2College of Chemical and Food Engineering, Zhengzhou Institute of Technology, Zhengzhou, Hennan, 450044, China,
Tel/Fax, 86-23-65102531, Emails, hongruli1972@gmail.com, zpluo@126.com, fanggao1971@gmail.com
1

Graphic Abstract
Highlights
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Preparation of dyes with different ring transition states for ESIPT
Demonstration of different strength internal hydrogen bond in dyes
Evidence for occurrence or no occurrence of ESIPT of dyes
Remarkable different potential energy barriers of ESIPT of dyes
Abstract
In this study, a range of organic dyes were prepared to investigate difference between five- and six-
number-membered ring transition states for internal proton transfer in excited state. Different strength
intramolecular hydrogen bond in the target dyes was demonstrated by X-ray crystallography, 1H-NMR
spectroscopy and ultraviolet/visible spectroscopy. The fluorescence measurement suggested excited state
intramolecular proton transfer occurred via six number-membered ring transition state, while it could not occur
through five number-membered ring transition state. The molecular geometry optimization of the target dyes
further accounted for the difference of internal proton transfer in excited state occurred by five- or six-
2

number-membered ring transition state.
Keywords: Ring transition sate; Internal proton transfer; Excited state; Phototautomerization; Molecular geometry
optimization
1. Introduction
Amongst photoinduced processes, proton transfer in excited state is of a special interest as
fundamental processes for many important biochemical reactions in life and nature such as in
DNA and protein [1-4]. Photoinduced proton transfer occurs from one atom to another atom
within a single molecular framework, and proton is normally reversibly transferred to the original
atom in the end, which is called as excited state intramolecular proton transfer (ESIPT) [5, 6]. To
such an organic molecule, this process can produce unique spectroscopic nature including a
normal emission band with its absorption and an abnormal emission band without its absorption [7,
8]. Owing to energy loss caused by proton transfer reaction, this abnormal emission band is
greatly shifted and characterized by large Stokes shift, which makes ESIPT molecules as advanced
materials for various uses such as fluorescence sensors and electronic optical devices [9, 10].
Therefore, synthesis of new ESIPT organic molecules becomes one of main concerns. It is
found that organic molecules undergo internal proton transfer in excited state mainly through
quasi six number-membered ring transition states [11-13], while there are few successful reports
of organic molecules displaying intramolecular proton transfer in excited state via five
number-membered ring transitions [14]. This really confuses us because five number-membered
ring possesses quite conformation stability in chemistry.
3

Hence, it is considered that there are certain other factors determining proton transfer process
in excited state. For instance, a typical four-level cycle enol-keto ESIPT process
(E→E*→K*→K→E, E, enol, K, keto) includes two actual chemical reactions E*-K* and K-E [15].
So, potential energy barrier of internal enol-keto phototautomerization must be small or even
barrierless for ESIPT [16]. This means that internal hydrogen bond between proton donor and
proton acceptor must be strong enough so that proton transfer tunnel can be shortened. Thus,
formation and stability of transition state of internal phototautomerization could be determined by
intramolecular hydrogen bond between proton and proton segments, and as a consequence, ESIPT
process could be controlled by varying intramolecular hydrogen bond. For example, internal
hydrogen bond strength between neighbouring hydroxy and imino groups could play a significant
role in an eno-keto intramolecular phototautomerization.
So far, no comprehensive contrastive study of intramolecular phototautomerization in organic
dyes through five- or six- number-membered ring transition state has been carried out. It is
necessary to deal with this bewildering scientific question because it would guide us to prepare
new organic dyes with ESIPT nature, which leads to an efficient production of dual-emission
bands of target organic chromophores.
In this study, we attempt to reveal crucial contributing factors influencing intramolecular
phototautomerziation through five- and six- number-membered ring transition states. For this
purpose, a variety of enol-keto type target dyes carrying similar chemical structure which provides
diversities of transition states for intramolecular proton transfer in excited state are synthesized.
We conduct combinational experimental and theoretical investigations to analyze effect of internal
hydrogen bond as well as potential energy barrier on occurrence of ESIPT of the target dyes. To
4

our best knowledge, this is the first successful attempt to find out remarkable difference between
five- and six- number-membered ring transition states for intramolecular phototautomerization
process. This study would be beneficial for design and synthesis of new ESIPT organic
chromophores based on efficient molecule preconstruction.
2. Experimental
2.1. Reagents and characterization
The organic solvents that were further processed by the standard laboratory method were
purchased from Aldrich Chemical Corporation [17]. The target organic dyes C1-C12 were
prepared in our own laboratory (Scheme 1), which have been reported elsewhere [18]. The
melting points of the studied molecules were determined by a Beijing Fukai melting point
apparatus, China. 1H and 13C nuclear magnetic resonance (NMR) spectra were measured by Bruke
apparatus (400 MHz, 500MHz and 600 MHz) in standard NMR tubes by using tetramethylsilane
(TMS) as an internal reference at 25 °C. A CE440 elemental analysis meter from Exeter Analytical
Inc. was utilized to perform elemental analysis of the target molecules. Fourier transform infrared
spectra of the samples were acquired by KBr pellet method with Nicolet 550II IR spectrometer
(Nicolet Instrument Corporation, USA). A UPLC-Q-ToF MS spectrometer (Waters, USA) was
utilized to record high resolution mass spectra (HRMS) under electro spray ionization (ESI).
A suitable single crystal of the target molecule that was obtained by slow volatilization of
organic solvents in NMR tubes was put inside a glass fiber capillary for measuring molecular
structure analyzed by direct methods and refined by full-matrix least squares on F2. XRD data
were collected on a Bruker-AXS CCD area detector equipped with diffractometer with Mo Ka
5

(λ=0.71073 Å) at 298 K. All the hydrogen atoms were introduced in the calculated positions
respectively and all the non-hydrogen atoms were advanced by anisotropic temperature factors.
2.2. Steady and transient Ultraviolet/visible and fluorescence spectroscopic determination
Spectral grade organic solvents were utilized to the spectroscopic measurement. A TU1901
spectrophotometer from Beijing PUXI General Equipment Limited Corporation was used to
measure ultraviolet/visible (UV/visible) absorption spectra of the samples. The fluorescence
emission spectra of the samples were recorded by Shimadzu RF-531PC spectrofluorophotonmeter.
Quinine sulfate in 0.5 mol/L H2SO4 (Φ, 0.546) was used as the reference to determine fluorescence
quantum yields of the studied dyes [19]. The absorption at the excited wavelength is below 0.1 for
removal of the experimental errors of fluorescence quantum yields.
The fluorescence lifetimes of the samples were measured by a ultrafast laser system.
Time-resolved laser measurement was performed through a spectrograph using a connected
intensified double photodiode array. The pulses were focused onto a sample cuvette containing
100 µm thick fused-silica window and a 250 pm optical path length. The transient emission
spectra were recorded by a ultrafast white-light continuum.
2.3. Molecular geometry optimization computation
Molecular geometry computation of the target molecules was carried out by 6-31G** basis set
in Gaussian 09 program package. The phototautomers enol and keto geometry optimization in
ground state (S0) was performed by using B3LYP method on basis of Hartree-Fock (HF) level [20],
while single-excitation configuration interaction (CIS) was employed to optimize the geometries
6

in the first singlet excited state (S1). The energies of optimized geometries of S0 and S1 states were
computed by density functional theory (DFT) and time-dependence DFT (TD-DFT) method at HF
and CIS levels respectively such as DFT//HF or TDDFT//CIS (the single-point
calculation//optimization method) [21]. The organic solvents were used as the media for
theoretical computation.
2.4. Synthesis of the target dyes
The synthesis approaches of the target dyes are depicted in Scheme 1. The preparation
procedure and chemical structural characterization of the studied dyes are described in the
followings.
2.4.1. General procedure for carbonylation of o-hydroxyaniline and o-methoxyaniline
derivatives
A solution of 6 mmol of the corresponding aldehyde and 5 mmol of o-hydroxyaniline or
o-methoxyaniline in 30 mL of anhydrous ethanol was stirred at 25 °C for 24 h. The target dyes
could be further purified by silicon gel column chromatography and recrystallization.
1
C1, 2-(benzylideneamino)-phenol White solid (yield, 91%), m.p., 89.5-90.1 °C. H-NMR
(500 MHz, DMSO-d6) δ (ppm): 9.010 (s, Ar-OH, 1H), 8.712 (s, N=CH, 1H), 8.046-8.027 (m,
Ar-H, 2H), 7.530-7.508 (m, Ar-H, 3H), 7.219-7.200 (m, Ar-H, 1H), 7.109-7.075 (m, Ar-H, 1H),
6.920-6.904 (t, J = 8 Hz, Ar-H, 1H), 6.863-6.832 (t, J = 7.8 Hz, Ar-H, 1H). 13C-NMR (151 MHz,
DMSO-d6) δ (ppm): 161.285, 156.165, 145.241, 137.164, 131.329, 130.476, 129.382, 128.864,
126.613, 124.246, 122.463, 118.183, 114.519. IR (KBr, cm-1): 3374.6, 3326.6, 2915.0, 1625.1,
7

1482.5, 1381.0, 1250.9, 1195.2, 850.9, 765.8, 689.5, 646.1, 502.5. Elementary analysis, Anal.
Calcd for C13H11NO, C, 79.16, H, 5.62, N, 7.10; Found, C, 79.24, H, 5.55, N, 7.23. HRMS (ESI)
m/z: calcd for C15H16N2O [M+H]+ 198.0914, found, 198.0918.
C2, 2-(4-methoxybenzylideneamino)-phenol White solid (yield, 88%), m.p., 86.5-87.5 °C.
1H-NMR (500 MHz, DMSO-d6) δ (ppm): 8.885 (s, Ar-OH, 1H), 8.615 (s, N=CH, 1H),
7.987-7.970 (d, J = 8.5 Hz, Ar-H, H), 7.177-7.158 (m, Ar-H, 1H), 7.068-7.035 (m, Ar-H, 3H),
6.889-6.873 (t, J = 8 Hz, Ar-H, 1H), 6.835-6.805 (t, J = 15 Hz, Ar-H, 3H), 3.835 (s, Ar-OMe, 3H).
13C-NMR (151 MHz, DMSO-d6) δ (ppm): 161.999, 159.842, 151.586, 140.973, 133.028, 131.016,
129.125, 128.874, 122.876, 119.987, 114.562, 113.218, 112.563, 56.087. IR (KBr, cm-1): 3341.8,
3016.2, 2836.1, 1595.5, 1486.5, 1422.6, 1258.4, 1168.0, 1028.0, 833.7, 756.4, 643.3. Elementary
analysis, Anal. Calcd for C14H13NO2, C, 73.99, H, 5.77, N, 6.16; Found, C, 74.15, H, 5.8, N, 6.25.
HRMS (ESI) m/z: calcd for C14H13NO2 [M+H]+ 228.1019, found, 228.1021.
C3, 2-(4-cyanobenzylideneamino)-phenol Pale yellow solid (yield, 82%), m.p.,
133.5-134.5 °C. 1H-NMR (500 MHz, DMSO-d6) δ (ppm): 9.191 (s, Ar-OH, 1H), 8.842 (s, N=CH,
1H), 8.239-8.223 (d, J = 8 Hz, Ar-H, 2H), 7.996-7.980 (d, J = 8 Hz, Ar-H, 2H), 7.298-7.280 (m,
Ar-H, 1H), 7.156-7.122 (m, Ar-H, 1H), 6.935-6.850 (m, Ar-H, 2H). 13C-NMR (151 MHz,
DMSO-d6) δ (ppm): 159.917, 152.554, 141.762, 133.245, 131.297, 129.092, 128.341, 127.654,
125.971, 123.041, 120.091, 118.814, 118.782, 115,782. IR (KBr, cm-1): 3398.5, 2222.9, 1625.3,
1567.0, 1484.5, 1380.6, 1256.6, 1149.7, 950.5, 833.2, 747.7, 626.8, 547.5. Elementary analysis,
Anal. Calcd for C14H10N2O, C, 75.66, H, 4.54, N, 12.60; Found, C, 75.73, H, 4.33, N, 12.72.
HRMS (ESI) m/z: calcd for C14H10N2O [M+H]+ 223.0866, found, 223.0869.
C4, 2-methoxy-N-(benzylidene)-aniline White solid (yield, 87%), m.p., 82.9-84.9 °C.
8

1H-NMR (400 MHz, DMSO-d6) δ (ppm): 8.619 (s, N=CH, 1H), 7.908-7.886 (m, Ar-H, 2H),
7.495-7.480 (t, J = 6 Hz, Ar-H, 3H), 7.293-7.271 (d, J = 8.8 Hz, Ar-H, 2H), 6.977-6.955 (d, J =
8.8 Hz, Ar-H, 2H), 3.761 (s, Ar-OMe, 3H). 13C-NMR (151 MHz, DMSO-d6) δ (ppm): 159.867,
153.682, 145.638, 141.457, 138.827, 134.396, 132.196, 129.782, 128.372, 124.682, 121.187,
117.291, 116.652, 56.824. IR (KBr, cm-1): 1637.2, 1618.3, 1508.3, 1400.4, 1248.6, 1026.5, 739.5,
616.3, 474.6. Elementary analysis, Anal. Calcd for C14H13NO, C, 79.59, H, 6.20, N, 6.63; Found,
C, 79.67, H, 6.11, N, 6.75. HRMS (ESI) m/z: calcd for C14H13NO [M+H]+ 212.107, found,
212.1077.
C5, 2-methoxy-N-(4-methoxybenzylidene)-aniline White solid (yield, 93%), m.p.,
48.6-50.1 °C. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 8.398 (s, N=CH, 1H), 7.853-7.833 (d, J =
8 Hz, Ar-H, 2H), 7.160-7.123 (m, Ar-H, 1H), 7.057-6.909 (m, Ar-H, 5H), 3.818 (s, Ar-OMe, 3H),
3.763 (s, Ar-OMe, 3H). 13C-NMR (151 MHz, DMSO-d6) δ (ppm): 161.231, 159.005, 153.512,
139.126, 132.536, 131.001, 129.874, 126.472, 125.489, 121.016, 115.305, 114.652, 113.987,
56.071, 55.628. IR (KBr, cm-1): 2836.3, 1683.0, 1604.3, 1511.2, 1462.9, 1263.8, 1162.9, 1027.7,
833.8, 746.2, 642.2, 607.4, 518.4. Elementary analysis, Anal. Calcd for C15H15NO2, C, 74.67, H,
6.27, N, 5.81; Found, C, 74.73, H, 6.14, N, 5.90. HRMS (ESI) m/z: calcd for C15H15NO2 [M+H]+
242.1176, found, 242.1181.
C6, 2-methoxy-N-(4-cyanobenzylidene)-aniline Pale yellow solid (yield, 88%), m.p.,
73.1-74.3 °C. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 8.642 (s, N=CH, 1H), 8.080-7.957 (m,
Ar-H, 4H), 7.211 (s, Ar-H, 1H), 7.095-7.064 (t, Ar-H , J = 12.4 Hz, 2H), 6.969 (s, Ar-H, 1H),
3.792 (s, Ar-OMe, 3H). 13C-NMR (151 MHz, DMSO-d6) δ (ppm): 160.152, 153.016, 141.231,
140.862, 139.982, 133.627, 131.267, 127.241, 123.561, 121.735, 119.874, 116.724, 115.421,
9

114.992, 55.876. IR (KBr, cm-1): 2834.6, 2225.2, 1637.8, 1582.1, 1493.3, 1434.8, 1250.0, 1171.2,
1112.9, 1022.6, 977.6, 861.1, 831.5, 781.5, 757.0, 568.2, 535.7. Elementary analysis, Anal. Calcd
for C15H12N2O, C, 76.25, H, 5.12, N, 11.86; Found, C, 76.31, H, 5.22, N, 11.97. HRMS (ESI) m/z:
calcd for C15H12N2O [M+H]+ 237.1023, found, 237.1030.
2.4.2. General procedure for amination of salicylaldehyde and o-methoxybenzaldehyde
derivatives
A solution of 6 mmol of the corresponding amine and 5 mmol of salicylaldehyde or
o-methoxybenzaldehyde in 30 mL of anhydrous ethanol was stirred at 25 °C for 24 h. The target
molecules could be further purified by silicon gel column chromatography as well as
recrystallization.
C7, 2-((phenylimino)methyl)-phenol White solid (yield, 86%), m.p., 50-51°C. 1H-NMR (600
MHz, DMSO-d6) δ (ppm): 13.074 (s, Ar-OH, 1H), 8.941 (s, N=CH, 1H), 7.651-7.638 (d, J = 7.8
Hz, Ar-H, 1H), 7.463-7.389 (m, Ar-H, 5H), 7.312-7.287 (t, J = 7.5 Hz, Ar-H, 1H), 6.981-6.945 (m,
Ar-H, 2H). 13C-NMR (151 MHz, DMSO-d6) δ (ppm): 161.009, 159.651, 153.981, 136.238,
135.016, 134.365, 130.094, 127.859, 125.372, 123.253, 121.587, 121.274, 118.372. IR (KBr,
cm-1): 3414.0, 2884.1, 1616.2, 1571.6, 1483.8, 1455.3, 1400.8, 1359.1, 1276.6, 1186.7, 1150.0,
1074.5, 981.2, 867.0, 845.4, 755.7, 692.4, 547.8. Elementary analysis, Anal. Calcd for C13H11NO,
C, 79.16, H, 5.62, N, 7.10; Found, C, 79.27, H, 5.55, N, 7.23. HRMS (ESI) m/z: calcd for
C13H11NO [M+H]+ 198.0914, found, 198.0909.
C8, 2-((4-methoxyphenylimino)methyl)-phenol White solid (yield, 89%), m.p., 71.5-72.3 °C.
1H-NMR (600 MHz, DMSO-d6) δ (ppm): 13.263 (s, Ar-OH, 1H), 8.910 (s, N=CH, 1H),
10

7.592-7.579 (d, J = 7.8 Hz, Ar-H, 1H), 7.402-7.345 (m, Ar-H, 3H), 7.002-6.987 (d, J = 9 Hz, Ar-H,
2H), 6.951-6.910 (m, Ar-H, 2H), 3.776 (s, Ar-OMe, 3H). 13C-NMR (151 MHz, DMSO-d6) δ
(ppm): 161.005, 160.997, 159.874, 144.537, 135.379, 133.024, 123.069, 122.106, 121.058,
120.145, 118.762, 116.594, 115.617, 55.078. IR (KBr, cm-1): 3411.2, 2964.3, 2841.7, 1621.2,
1571.1, 1509.7, 1492.3, 1406.0, 1276.5, 1247.8, 1187.9, 1033.6, 983.2, 910.5, 838.7, 748.6, 632.0,
545.4. Elementary analysis, Anal. Calcd for C14H13NO2, C, 73.99, H, 5.77, N, 6.16; Found, C,
74.13, H, 5.63, N, 6.27. HRMS (ESI) m/z: calcd for C14H13NO2 [M+H]+ 228.1019, found,
228.1026.
C9, 2-((4-cyanophenylimino)methyl)-phenol Pale yellow solid (yield, 80%), m.p.,
83.5-84.5 °C. 1H-NMR (600 MHz, DMSO-d6) δ (ppm): 12.386 (s, Ar-OH, 1H), 8.933 (s, N=CH,
1H), 7.894-7.880 (d, J = 8.4 Hz, Ar-H, 2H), 7.681-7.668 (d, J = 7.8 Hz, Ar-H, 1H), 7.528-7.514 (d,
J = 8.4 Hz, Ar-H, 2H), 7.441-7.415 (t, J = 7.8 Hz, Ar-H, 1H), 6.983-6.952 (t, J = 9 Hz, Ar-H, 2H) .
13C-NMR (151 MHz, DMSO-d6) δ (ppm): 163.065, 157.063, 151.983, 135.682, 134.013, 132.673,
131.786, 124.035, 125.484, 122.531, 121.087, 119.643, 117.835, 111.153. IR (KBr, cm-1): 3416.8,
3067.7, 2981.8, 2227.4, 1616.6, 1565.0, 1503.8, 1454.0, 1393.8, 1359.7, 1273.9, 1172.7, 1150.1,
1031.5, 972.6, 908.1, 858.5, 823.5, 760.5, 622.8, 558.9, 543.3. Elementary analysis, Anal. Calcd
for C14H10N2O, C, 75.66, H, 4.54, N, 12.60; Found, C, 75.71, H, 4.48, N, 12.73. HRMS (ESI) m/z:
calcd for C14H10N2O [M+H]+ 223.0866, found, 223.0872.
C10, N-(2-methoxybenzylidene)-aniline Pale yellow solid (yield, 90%), m.p., 41-42 °C.
1H-NMR (600 MHz, DMSO-d6) δ (ppm): 8.816 (s, N=CH, 1H), 8.005-7.989 (d, J = 9.6 Hz, Ar-H,
1H), 7.519-7.493 (t, J =7.8 Hz, Ar-H, 1H), 7.399-7.373 (t, J = 7.8 Hz, Ar-H, 2H), 7.218-7.137 (m,
Ar-H, 4H), 7.058-7.033 (t, J = 7.5 Hz, Ar-H, 1H), 3.869 (s, Ar-OMe, 3H). 13C-NMR (151 MHz,
11

DMSO-d6) δ (ppm): 157.764, 153.268, 139.167, 133.254, 131.215, 130.261, 128.574, 127.563,
125.346, 123.365, 122.915, 119.821, 112.326, 56.091. IR (KBr, cm-1): 2836.8, 1616.8, 1586.0,
1489.2, 1467.7, 1363.6, 1264.7, 1252.6, 1187.4, 1159.6, 1105.6, 1045.6, 1021.6, 970.2, 880.7,
758.3, 690.9, 516.6. Elementary analysis, Anal. Calcd for C14H13NO, C, 79.59, H, 6.20, N, 6.63;
Found, C, 79.65, H, 6.13, N, 6.75. HRMS (ESI) m/z: calcd for C14H13NO [M+H]+212.107, found,
212.1062.
C11, 4-methoxy-N-(2-methoxybenzylidene)-aniline White solid (yield, 91%), m.p.,
58.6-59.1 °C. 1H-NMR (600 MHz, DMSO-d6) δ (ppm): 8.867 (s, N=CH, 1H), 7.784-7.778 (d, J =
3.6 Hz, Ar-H, 1H), 7.591-7.577 (d, J = 8.4 Hz, Ar-H, 2H), 7.389-7.334 (m, Ar-H, 4H),
7.276-7.247 (t, J = 9 Hz, Ar-H, 1H), 3.872 (s, Ar-OMe, 3H), 3.862 (s, Ar-OMe, 3H). 13C-NMR
(151 MHz, DMSO-d6) δ (ppm): 159.876, 157.034 156.983, 145.002, 133.012, 132.615, 128.051,
127.632, 125.174, 123.031, 122.093, 121.168, 116.513, 56.037, 55.318. IR (KBr, cm-1): 2837.9,
1621.7, 1577.0, 1505.9, 1400.3, 1268.2, 1248.4, 1191.7, 1110.5, 1030.2, 957.1, 834.9, 754.7,
616.8, 550.2. Elementary analysis, Anal. Calcd for C15H15NO2, C, 74.67, H, 6.27, N, 5.81; Found,
C, 74.71, H, 6.19, N, 5.96. HRMS (ESI) m/z: calcd for C15H15NO2 [M+H]+ 242.1176, found,
242.1183.
C12, 4-cyano-N-(2-methoxybenzylidene)-aniline yellow solid (yield, 84%), m.p.,
63.1-64.3 °C. 1H-NMR (600 MHz, DMSO-d6) δ (ppm): 8.859 (s, N=CH, 1H), 8.352-8.331 (d, J =
8.4 Hz, Ar-H, 2H), 8.173-8.152 (d, J = 8.4 Hz, Ar-H, 2H), 7.351-7.310 (t, J = 8.2 Hz, Ar-H, 1H),
6.911-6.848 (m, Ar-H, 3H), 3.855 (s, Ar-OMe, 3H). 13C-NMR (151 MHz, DMSO-d6) δ (ppm):
157.871, 156.437, 155.653, 133.213, 133.216, 131.086, 127.042, 125.091, 123.146, 122.071,
120.948, 118.456,112.099, 110.097, 55.672. IR (KBr, cm-1): 2227.4, 1608.7, 1560.7, 1345.8,
12

1214.6, 1108.3, 1027.3, 977.6, 858.1, 781.5, 652.3, 538.2. Elementary analysis, Anal. Calcd for
C15H12N2O, C, 76.25, H, 5.12, N, 11.86; Found, C, 76.38, H, 5.06, N, 11.98. HRMS (ESI) m/z:
calcd for C15H12N2O [M+H]+ 237.1023, found, 237.1032.
3. Results and discussion
3.1. The target dyes
In this work, the designed target dyes shown in Scheme 1 contain a similar proton transfer
skeleton as a representative ESIPT molecule salicylidene methylamine [22], which are divided
into two groups: (i) the dyes could undergo enol-keto phototautomerizaiton via a five
number-membered ring transition state (C1-C3), (ii) the dyes could process excited state
intramolecular proton transfer via a six number-membered ring transition state (C7-C9). The
methylated studied dyes are used as the references for comparing spectroscopic properties to those
of the target ones. Electron-donating and withdrawing groups are introduced to vary
intramolecular hydrogen bond strength of the target dyes, and thus internal proton transfer in
excited state could be tuned.
3.2. Intramolecular hydrogen bond based on analysis of 1H-NMR spectra
The higher melting points of the target dyes with respect to those of the corresponding
reference dyes indicate the presence of special internal interaction in the target dyes. It is
1
well-accepted that hydrogen bond leads to an increase of H-NMR chemical shift of relevant
hydrogen atoms, which means a detectable downshift to magnetic field due to the deshielding
effect [23]. In order to remove NMR peak interference of deuterated chloroform (CDCl3) and
13

increase the solubility, DMSO-d6 was chosen as the deuterated solvent in NMR spectral
measurement. In particular, 1H-NMR peak of phenolic hydroxy group in target dyes could be well
identified in DMSO-d6.
The typical 1H-NMR spectra of the target molecules C1 and C7 as well as the corresponding
reference molecules C4 and C10 in DMSO-d6 are shown in Figure 1 and Figure S1 respectively
(Figures S and Tables S are included in Supplementary Materials). As shown in Figure 1,
1H-NMR chemical shifts of imino groups in the references are located at the higher magnetic field
comparing to those in targets (such as C1, 8.712 ppm versus C4, 8.619 ppm, in Figure S1). In
1
addition, H-NMR chemical shift of CH=N in C1 or C4 also switch to the higher magnetic field
comparing to those in C7 or C10 correspondingly (such as C1, 8.712 ppm versus C7, 8.941 ppm,
Figure 1). These indicate that there is an intramolecular H-bond between phenolic and imino
groups in C1 and C7.
1
It is remarkable that H-NMR chemical shift of phenolic group in C7 moves to much lower
magnetic field comparing to that in C1 (such as C1, 9.010 ppm versus C7, 13.074 ppm, Figure 1),
which suggests that there is a stronger intramolecular hydrogen bond between adjacent phenolic
hydroxy and imino groups in C7 than that in C1. Hence, it is deduced that formation and stability
of six number-membered ring transition state in C7 are greater than five number-membered ring
one in C1 [24], and as a consequence, ESIPT can be more easily proceeded in C7.
The other target dyes show similar nuclear magnetic resonance spectral characteristics. It is
1
further observed that H-NMR chemical shift of phenolic group in C1-C3 shifts to more
down-magnetic fields with the substitution of electron-accepting groups (C1, 9.010 ppm, C2,
8.659 ppm, and C3, 9.257 ppm). In contrast, phenolic group in C7-C9 moves to lower magnetic
14

field by substituting electron-donating groups (C7, 13.074 ppm, C8, 13.263 ppm, C9, 12.386
ppm). This means that internal hydrogen bond in the target dyes can be also tuned by varying
substituents.
3.3. Intramolecular hydrogen bond based on X-single crystal diffraction analysis
The molecular structures of the target dyes are further confirmed by X-single crystal
diffraction analysis. The atomic visualization in single crystal authenticates molecular structures
of the target dyes. Figure 2 shows that there is an intramolecular hydrogen bond between the
neighboring phenolic hydroxy and imino groups in C1 and C7 respectively. It is given from X-ray
single crystal diffraction that the C1 and C7 both exist as E isomers of C=N double bond, which
offers great coplanarity in C1 and C7 respectively.
X-ray single crystal diffraction shows that the distance between phenolic hydrogen and imino
groups in C7 is 1.902 Å, while it is shorter in C1 (N1-H1, 2.129 Å for C1), which demonstrates a
stronger intramolecular hydrogen bond in C7. The data further suggest that C1 presents longer
N1-O1 (2.616 Å for C1, 2.613 Å for C7) and larger dihedral angle of ∠C-O1-H1-N1 (∠
C2-O1-H1-N1 for C1, -0.79˚; ∠C3-O1-H1-N1 for C7, -0.46˚) in phenolic hydroxy and imino
groups than C7. This further indicates that there is a harder internal hydrogen bond in C7
comparing to that in C1.
X-ray single crystal diffraction suggests that formation of five number-membered ring
transition state in C1 could be more difficult as comparing to that of six number-membered ring
one in C7. In addition, the stability of five number-membered transition state in C1 could be
weaker than that of six number-membered ring one in C7. Hence, it is much possible that the
15

formation of five number-membered ring transition state in C1 display higher energy barrier than
that of six number-membered one in C7. As a consequence, it is more difficult to undergo
intramolecular proton transfer through a five number-membered ring transition state in excited
state of C1.
3.4. Effect of intramolecular hydrogen bond on steady ultraviolet/visible and emission
spectra
Ultraviolet/visible spectra of the target and the reference molecules were studied in various
organic solvents. The typical UV/visible spectra of the studied dyes in 1,4-dioxane and DMF are
given in Figure 3, and the representative UV/visible spectral parameters of are presented in Table
1. (also see Figure S2 & Table S1). It is found that the studied dyes show similar UV/visible
shapes in various solvents, while the absorption maxima exhibit variations with electron
donor/accepting substituents. This suggests that the long-wavelength peak could be assigned to (π,
π*) transition with intramolecular charge transfer nature due to π-conjugation of entire molecular
framework.
It is noticed that the target dyes exhibit a bathochromic shift of the absorption peak
wavelength with respect to the methylated reference ones correspondingly (such as in DMF, C1,
348 nm, C4, 336 nm), although methoxyl group is a stronger electron-donating group. The results
indicate that internal hydrogen bond increases intramolecular charge transfer of entire molecules,
and the energy of (π, π) transition decreases accordingly.
Therefore, it is considered that intramolecular proton transfer in excited state of these target
dyes could occur [25]. The fluorescence emission spectra of the studied dyes were investigated in
16

various organic solvents to get the direct experimental evidence. The typical fluorescence spectra
of the studied dyes in 1,4-dioxane and DMF are depicted in Figure 4. The representative
fluorescence spectral parameters are provided in Table 2 (also see Figure S2 & Table S1).
It is found that C7 exhibits well-separated dual emission bands, while C10 shows single
fluorescence band. In addition, the peak wavelength of the first emission band of C7 is close to the
emission maximum of C10 with normal Stokes shift (such as in DMF, C10, 405 nm, C7, 409 nm,
Stokes shift, about 4 nm). In contrast, the second emission band of C7 is remarkably red-shifted
and characterized with huge Stokes shift (such as in DMF, C7, 517 nm, Stokes shift, about 112
nm). The similar fluorescence emission experimental phenomena are yielded to the partners
C8/C11 and C9/C12. The results show that the first emission band of C7-C9 is assigned to the
normal enol form decay of single excited state, while the second emission band is produced by the
excited keto tautomer produced by intramolecular proton transfer in excited state through a
six-membered ring transition state (Scheme 2(a)).
In contrast, the partners C1/C4, C2/C5 and C3/C6 show almost identical single emission
band shape with normal Stokes shift respectively in various solvents. Hence, the results show that
the fluorescence emission of these partners is produced by the normal decay of S1→S0, which
means that C1-C3 can not undergo internal proton transfer in excited state through a five
number-membered ring transition state, and the keto tautomers are not yielded (Scheme 2(b)). It is
found that there is an emission shoulder of the partner C3/C6 that could be assigned to internal
charge transfer due to strong electron-accepting effect of cyano group.
It is also found that the targets C7-C9 exhibits strong competitive ESIPT emission in solid
state or polymer matrix, while the solids C1-C3 only show normal emission that is almost
17

identical to the corresponding reference dyes (Figure S4). The results further demonstrate that it is
more easily to undergo ESIPT through a six number-membered ring transition state than a five
number-membered ring one (Scheme 2), which could be ascribed to the stronger internal
hydrogen bond in C7-C9 comparing to that in C1-C3. It is also discovered that the emission
intensity ratios of keto to enol forms and the wavelengths of keto tautomers of the targets C7-C9
can be tuned by varying the substituents and the media.
3.5. Time-resolved fluorescence emission spectra
Time-resolved fluorescence decay of 2-((phenylimino)methyl)-phenol (C7) have ever been
reported [26, 27]. So, we performed an investigation of time-dependent fluroescence decay
profiles of C2 and C8 in this study. It is found that C2 displays a similar fluorescence lifetime as
its methylated reference (C5), which is also approximate to the first emission band of C8 (such as
in 1,4-dioxane, C2, 2.63×10-11 s at 456 nm, C5, 2.34×10-11 s at 452 nm, C8, 2.26×10-11 s at 393
nm). It is found that the methylated molecule C11 also shows the similar fluorescence lifetime
(such as in 1,4-dioxane, C11, 2.07×10-11 s at 392 nm). It is also noticed that the other emission
wavelengths of C2 show almost identical fluorescence lifetime as the maximal emission
wavelength.
While in contrast, the second emission band of C8 exhibits a shorter fluorescence lifetime
than the first one in various solvents (such as in 1,4-dioxane, 1.78×10-12 s at 504 nm). The results
suggest that the fluorescence band of C2 and the first fluorescence band of C8 are assigned to the
excited enol forms producing typical normal S1→S0 emission decay. However, the excited keto
form of C2 yielded by phototatutomerization could show a more competitive radiative transition
18

than the excited enol form.
The radiative and non-radiative transition constants (kr, knr) in excited state of a dye can be
calculated according to the following equations [28-30]:
kr = Φf/τf
(1)
(2)
knr = (1-Φf)/τf
wherein Φf shows fluorescence quantum yield of a dye and τf represents fluorescence lifetime
obtained at the maximal emission wavelength of a dye. Since C8 yields the enol fluorescence band
and the keto fluorescence band, the fluorescence quantum yield of C8 can be considered as the
sum including the normal fluorescence quantum yield and ESIPT fluorescence quantum yield,
which can be described as:
Φf (sum) = Φf (enol) + Φf (keto) (3)
Wherein Φf (sum) shows the fluorescence quantum yield of the studied dyes, Φf (enol) represents the
fluorescence quantum yield of the enol forms of the studied dyes, Φf (keto) is the fluorescence
quantum yield of the keto forms of the studied dyes.
Because UV/visible absorption of C2 is produced by tautomer enol to enol*, the integrated
area ratio of the enol emission band or the keto emission band to the total emission represents the
percent of the normal fluorescence quantum yield or the ESIPT fluorescence quantum yield
respectively. As a result, the normal fluorescence quantum yield and ESIPT fluorescence quantum
yield of C8 can be obtained respectively. The radiative and non-radiative transition constants of
the tautomers in excited state of C8 can be calculated.
Radiative and non-radiatibe transition constants of C2/C5 and C8/C11 are given in Table 3.
The data suggest that keto emission band of C8 show much larger radiative transition constant
19

than the enol emission band (such as in DMF, C8, kr(keto), 1.17×1010 s-1, kr(enol), 5.01×109 s-1 ),
which indicates that the excited keto form of C8 is more competitive in excited state. It is further
found that the nonradiative transition rate of C2 is greater than that of the normal emission band of
C8, reflecting that internal proton transfer decreases the transition decay of the excited enol form.
Time-dependence transient fluorescence spectra of C8 registered by decay kinetics basing on
dependence of emission wavelength are used to distinguish excited keto form from excited enol
tautomer. The peak transient tautomer enol emissive species of C8 in 1,4-dioxane is yielded at
2.0×10-11 s (Figure 5). However, its intensity decreases with the decay time course. The peak
transient keto band appears at 6.0×10-11 s decay time range. It is further found that both of
transient species exhibit the diminished emission intensity with decay process. Overall,
time-resolved fluorescence emission profiles of C8 further demonstrate intramolecular proton
transfer in excited state. In contrast, only presence of transient tautomer enol emissive peak of C2
means that internal proton transfer in excited states does not occur (Figure S5).
3.6. Molecular geometry optimization
3.6.1. Frontier orbitals, Mulliken atomic charge and dipole moment
Frontier orbitals of excited enol tautomer associating with enol-keto phototautomerization of
the target dyes are shown in Figure 6. To excited enol form of C1, electron cloud density in
HOMO is mainly located at o-hydroxy-phenyl-imino part, but it shifts to conjugated phenyl ring
in LUMO. The absence of electron cloud density distribution in o-hydroxy-phenyl-imino part in
LUMO of excited enol form indicates that proton transfer part could be inactive in excited enol
form of C1.
20

In contrast, electron cloud density is mainly distributed in conjugated phenyl part in HOMO
of excited enol tautomer of C7, while it is primarily located at o-hydroxy-phenyl-imino part in
LUMO of excited enol form. This means that proton transfer segment can be more reactive in
LUMO of excited enol form of C7. Thus, electron cloud density distribution in frontier orbitals of
excited enol tautomers suggests that C7 is more able to process internal proton transfer in excited
state than C1.
We further analyze molecular geometry optimization parameters of enol and keto tautomers
and transition state (TS) forms of the target dyes are shown in Figure 7 (also see Figure S6). First,
it is found that transition state of phototautomerization of C1 can not be acquired, which indicates
that TS form is unstable in excited state of C1. This means that intramolecular proton transfer in
excited state of C1 could be hard. While the presence of stable transition state of enol-keto
phototautomerization of C7 means a large possibility of ESIPT.
Table 4 shows that the distances of O-H and N-H in TS forms are between those of tautomers
enol and keto in S0 and S1 states (such as in S1 of C7, O-H, E, 0.944 Å, TS, 1.179 Å, K, 1.856 Å,
Table 4). The smallest O-N distance in TS forms in the ground and excited states respectively
suggests that there is the strongest hydrogen bond in TS (such as in S1 of C7, O-N, E, 2.826 Å, TS,
2.401 Å, K, 2.673 Å ). The results in turn suggest that enol-keto tautomerization in S0 and S1 is
required to go through a transition state. It is noticed the distance of N-H in excited enol tautomer
of C1 is longer than that of C7 (2.495 Å for C1, 2.084 Å for C7), indicating a longer internal
proton transfer tunnel in C1.
As shown in Table 5, dihedral angles of ∠COHN and ∠CCNC in o-hydroxy-phenyl-imino
part of the target dyes are given. A large variation of dihedral angle of∠COHN from S0 to S1 in
21

enol form of C1 (∠C1-O1-H1-N1, enol form, 5.301˚ in S0 to 1.737˚ in S1) reflects that phenolic
hydroxy group becomes loose in S1, which facilitates proton transfer occurrence. In addition, the
smaller dihedral angle in TS form of C7 shows the greater coplanarity in o-hydoxy-phenyl-imino
part in TS in excited state (∠C1-O1-H1-N1, TS form, 1.111˚ in S0 to -0.009˚ in S1), which is
favorable for internal proton transfer. While the corresponding dihedral angle of ∠COHN in enol
form of C1 are lower in S1 (∠C1-O1-H1-N1, enol form, 5.301˚ in S0 to 1.737˚ in S1), implying that
phenolic hydroxy group tends to be tight in excited state. Hence, it is more difficult for C1 to
undergo intramolecular proton transfer in excited state comparing to C7.
It is further found that as E of C7 is excited to E*, oxygen atom of phenolic group shows a
decrease in negative charge, but hydrogen atom of phenolic group presents a decrease in positive
charge (Table 6, -0.672 e to -0.651 e for O atom, 0.403 e to 0.384 e for H atom). In contrast, a
reversible tendency is shown during E to E* transition of C1 (-0.659 e to -0.666 e for O atom,
0.349 e to 0.369 e for H atom). The results indicate that phenolic group in C7 is more acidity and
more active in excited state, and thus proton in phenolic group in C7 is more reactive accordingly.
It is also found that C7 shows a quite lower dipole moment change during tautomerization in
ground and excited states than C1 (Table 7, such as enol form of C1, 3.235 D to 5.710 D; such as
enol form of C7, 2.660 D to 5.015 D). This means a smaller energy to achieve such a dipole
moment change for C7 comparing to C1.
3.6.2. Energy barriers of enol-keto tautomerization of in S0 and S1 states
Plots of potential energy curves (PEC) of tautomerization in S0 and S1 are acquired by
specifying a reaction coordinate of the distance from O atom to H atom. Figure 8 suggests that
22

there is an absence of TS form in excited state of C1 and K form is more unstable in excited state
in C1. The energy of enol-keto tautomerization in excited state of C1 keeps increasing, indicating
that it is impossible for undergoing four-level cycle process (E→E*→K*→K→E). However, the
energy of K form in excited state of C7 is much lower than that of enol form, which suggests that
K form is more stable in excited state of C7. Furthermore, there is a quite small energy barrier of
E-K tautomerization in excited state of C7 (about 6.50 kcal/mol).
As a matter of fact, the other target partners C2/C8 and C3/C9 show the similar contrasting
molecular geometry optimization results as C1/C7. The calculated results reflect that the targets
C7-C9 show a greater capacity to undergo intramolecular proton transfer in excited state through a
reasonable six number-membered ring transition state, while it is difficult for C1-C3 to proceed
enol-keto phototautomerization through a five number-membered ring transition state.
4. Conclusions
In summary, this study presents a variety of dyes that could undergo internal proton transfer
in excited state through five- and six- number-membered ring transition states respectively. X-ray
1
single crystal diffraction and H-NMR spectroscopy show that there is different strength
intramolecular hydrogen bond between five number-membered ring transition states dyes and six
number-membered ring ones. Based on the effect of internal hydrogen bond on UV/visible spectra,
the fluorescence emission spectra demonstrate excited state intramolecular proton transfer
occurred via six number-membered ring transition state. However, no solid experimental evidence
suggests the occurrence of ESIPT through five number-membered ring transition states of the
studied dyes. The remarkable difference in molecular geometry optimization of the studied dyes
23

leads to the diversities between five- and six- number-membered ring transition states for internal
proton transfer in excited state. This study suggests that if we develop new dyes with ESIPT
nature occurred via five number-membered ring transition states, an enhanced intramoleuclar
hydrogen bond could be required.
Acknowledgements
The authors thank Chongqing Natural Science Foundation CSTC2012jjB50007 and
CSTC2010BB0216 greatly. H. Li appreciates the Postdoctoral Science Foundation of China for
financial supporting (Grant No. 22012T50762 & 2011M501388).
Supplementary materials
The 1H-NMR spectra of the major products of C4 and C10 are included in Supplementary
materials. The absorption spectra and steady and transient fluorescence emission spectra of the
studied dyes in various media are also provided in Supplementary materials. The optimized
geometries of enol, TS and keto forms in the ground state of the target dyes, the spectral
parameters of the dyes and the crystallographic data are further contained in Supplementary
materials. The document is available free of charge from the website of this journal.
The supplementary crystallographic data for this work can be also found in files CCDC
numbered
by
1502866
and
1518272,
which
are
available
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge crystallographic data
center, 12, Union Road, Cambridge CB2 1EZ, UK; fax: 44-1223-336033.
24

References
[1] A. S. Klymchenko, V. V. Shvadchak, D. A. Yushchenko, N. Jain, Y. Mély, Excited-state
intramolecular proton transfer distinguishes microenvironments in single-and double-stranded
DNA, J. Phys. Chem. B 112 (2008) 12050-12055.
[2] K. C. Tang, M. J. Chang, T. Y. Lin, H. A. Pan, T. C. Fang, K. Y. Chen, W. Y. Hung, Y. H. Hsu, P.
T. Chou, Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT):
white light generation in a single ESIPT system, J. Am. Chem. Soc. 133 (2011) 17738-17745.
[3] A. P. Demchenko, K. C. Tang, P. T. Chou, Excited-state proton coupled charge transfer
modulated by molecular structure and media polarization, Chem. Soc. Rev. 42 (2013)
1379-1408.
[4] D. K. Rana, S. Dhar, A. Sarkar, S. C. Bhattacharya, Dual intramolecular hydrogen bond as a
switch for inducing ground and excited state intramolecular double proton transfer in
doxorubicin: an excitation wavelength dependence study, J. Phys. Chem. A 115 (2011)
9169-9179.
[5] M. Jadhao, O. R. Meitei, R. Joshi, H. Kumar, C. Das, S. K. Ghosh, ESIPT reaction of potential
bioactive heterocyclic Schiff base: Atomic visualization coupled with in vitro spectroscopy, J.
Photochem. Photobiol. A: Chem. 326 (2016) 41-49.
[6] R. Hu, J. Fang, D. Hu, S. Wang, S. Li, Y. Li, G. Yang, A rapid aqueous fluoride ion sensor with
dual output modes, Angew. Chem. Int. Ed. 49 (2010) 4915-4918.
[7] A. I. Ciuciu, K. Skonieczny, D. Koszelewski, D. T. Gryko, L. Flamigni, Dynamics of
Intramolecular Excited State Proton Transfer in Emission Tunable, Highly Luminescent
Imidazole Derivatives, J. Phys. Chem.C 117 (2013) 791-803.
25

[8] R. Daengngern, N. Kungwan, Dynamics simulations of photoinduced proton transfer reactions
of 2-(2′-hydroxyphenyl)benzoxazole in the gas phase and its hydrated clusters, Chem. Phys.
Lett. 609 (2014) 147-154.
[9] (a) X. D. Jin, C. Z. Liu, X. M. Wang, H. Huang, X. Q. Zhang, H. J. Zhu, A flavone-based
ESIPT fluorescent sensor for detection of N2H4 in aqueous solution and gas state and its
imaging in living cells, Sens. Actuators B 216 (2015) 141-149. (b) X. D. Jin, X. L. Sun, X. Y.
Di, X. Q. Zhang, H. Huang, J. N. Liu, P. W. Ji, H. J. Zhu, Novel fluorescent ESIPT probe
based on flavone for nitroxyl in aqueous solution and serum, Sens. Actuators B 224 (2016)
209-216.
[10] Y. Qian, S. Y. Li, Q. Wang, X. H. Sheng, S. K. Wu, S. Q. Wang, J. Li, G. Q. Yang, A
nonpolymeric highly emissive ESIPT organogelator with neither dendritic structures nor long
alkyl/alkoxy chains, Soft Mater. 8 (2012) 757-764.
[11] Z. Y. Zhang, Y. H. Hsu, Y. A. Chen, C. L. Chen, T. C. Lin, J. Y. Shen, P. T. Chou, New six-and
seven-membered ring pyrrole–pyridine hydrogen bond systems undergoing excited-state
intramolecular proton transfer, Chem. Commu. 50(95) (2014) 15026-15029.
[12] (a) Y. L. Gong, Y. Lu, H. Y. Ma, G. Ding, S. T. Zhang, Z. P. Luo, H. R. Li, F. Gao, Efficient
enhancement of internal proton transfer of branched π-extended organic chromophore under
one-photon and near-infrared two-photon irradiation, Chem. Phys. Lett. 619 (2015) 201-207.
(b) G. Ding, Y. Lu, Y. L. Gong, L. Ma, Z. P. Luo, S. T. Zhang, F. Gao, H. R. Li, New AB2 type
two-photon absorption dyes for well-separated dual-emission: molecular preorganization
based approach to photophysical properties, Tetrahedron 72 (2016) 3040-3056.
[13] (a) J. Zhou, R. Y. Shi, J. X. Liu, R. Wang, Y. F. Xu, X. H. Qian, An ESIPT-based fluorescent
26

probe for sensitive detection of hydrazine in aqueous solution, Org. & Biomol. Chem. 13
(2015) 5344-5348. (b) S. Goswami, A. Manna, S. Paul, A.K. Das, P.K. Nandi, A.K. Maity, P.
Saha, A turn on ESIPT probe for rapid and ratiometric fluorogenic detection of homocysteine
and cysteine in water with live cell-imaging, Tetrahedron Lett., 55(2) (2014) 490-494.
[14] (a) W. S. Yu, C. C. Cheng, Y. M. Cheng, P. C. Wu, Y. H. Song, Y. Chi, P. T. Chou,
Excited-state intramolecular proton transfer in five-membered hydrogen-bonding systems:
2-pyridyl pyrazoles, J. Am. Chem. Soc. 125 (2003) 10800-10801. (b) R. B. Singh, S. Mahanta,
N. Guchhait, Photophysical properties of 1-acetoxy-8-hydroxy-1, 4, 4a, 9a
-
tetrahydroanthraquinone: Evidence for excited state proton transfer reaction, Chem. Phys. 331
(2007)189-199.
[15] A. I. Ciuciu, K. Skonieczny, D. Koszelewski, D. T. Gryko, L. J. Flamigni, Dynamics of
Intramolecular Excited State Proton Transfer in Emission Tunable, Highly Luminescent
Imidazole Derivatives, J. Phys. Chem. C, 117 (2013) 791-803.
[16] J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, Photophysics of Schiff bases:
theoretical study of salicylidene methylamine, ChemphysChem. 13 (2012) 4287-4294.
[17] D. D. Perrin, W. L. F. Armarego, D. R. Perrin, Purification of laboratory chemicals; Pergamon:
New York, NY, 1966.
[18] C. M. da Silva, D. L. da Silva, C. V. Martins, M. A. de Resende, E. S. Dias, T. F. Magalhães,
L. P. Rodrigues, A. A. Sabino, R. B. Alves, Â. de Fátima, Synthesis of aryl aldimines and their
activity against fungi of clinical interest, Chem. Biol. Drug Des. 78 (2011) 810-815.
[19] (a) L. Ilies, Y. Sato, C. Mitsui, H. Tsuji. E. Nakamura, Modular Synthesis of Polybenzo [b]
silole Compounds for Hole-Blocking Material in Phosphorescent Organic Light Emitting
27

Diodes, Chem. Asian J. 5 (2010) 1376-1381. (b) G. W. Men, C. R. Chen, S. T. Zhang, C. S.
Liang, Y. Wang, M. Y. Deng, H. X. Shang, B.Yang, S. M. Jiang, A real-time fluorescent sensor
specific to Mg2+: crystallographic evidence, DFT calculation and its use for quantitative
determination of magnesium in drinking water, Dalton T. 44 (2015) 2755-2762.
[20] (a) M. Z. Zgiershi, A. J. Grabowska, Theoretical approach to photochromism of aromatic
Schiff bases: A minimal chromophore salicylidene methylamine, J. Chem. Phys. 113 (2000)
7845-7852. (b) P. Yi, Y. H. Liang, C. Z. Cao, Intramolecular proton or hydrogen-atom
transfer in the ground-and excited-states of 2-hydroxybenzophenone: a theoretical study,
Chem. Phys. 315 (2005) 297-302.
[21] (a) Y. H. Liang, P. G. Yi, Theoretical studies on structure, energetic and intramolecular proton
transfer of alkannin, Chem. Phys. Lett. 438 (2007) 173-177. (b) Z. N. Yang, S. Y. Yang, J. P.
Zhang,
Ground-and
excited-state
proton
transfer
and
rotamerism
in
2-(2-Hydroxyphenyl)-5-phenyl-1, 3, 4-oxadiazole and its O/“NH or S”-substituted derivatives,
J. Phys. Chem. A 111 (2007) 6354-6360.
[22] (a) E. Bardez, I. Devol, B. Larrey, B. Valeur, Excited-State Processes in 8-Hydroxyquinoline:
Photoinduced Tautomerization and Solvation Effects, J. Phys. Chem. B 101 (1997) 7786-7793.
(b) W. S. Yu, C. C. Cheng, Y. M. Cheng, P. C. Wu, Y. H. Song, Y. Chi, P. T. Chou, J. Am.
Chem. Soc. 125 (2003) 10800-100801.
[23] (a) J. Casanovas, A. M. Namba, S. León, G. L. Aquino, G. V. José da Silva, C. Alemán,
Calculated and experimental NMR chemical shifts of p-menthane-3,9-diols. A combination of
molecular dynamics and quantum mechanics to determine the structure and the solvent effects,
J. Org. Chem. 66 (2001) 3775-3782. (b) G. Wagner, A. Pardi, K. Wuethrich, Hydrogen bond
28

length and proton NMR chemical shifts in proteins, J. Am. Chem. Soc. 105 (1983) 5948-5949.
[24] R. Casadesús, M. Moreno, J. M. Lluch, Theoretical study of the photoinduced intramolecular
proton transfer and rotational processes in 2-(2′ hydroxyphenyl)-4-methyloxazole in gas phase
and embedded in β-cyclodextrin, J. Photochem. Photobiol. A: Chem. 173 (2005) 365-374.
[25] (a) G. A. Parada, T. F. Markle, S. D. Glover, L. Hammarström, S. Ott, B. Zietz, Control over
Excited State Intramolecular Proton Transfer and Photoinduced Tautomerization: Influence of
the Hydrogen‐Bond Geometry, Chem.-A Eur. J. 21 (2015) 6362-6366. (b) H. W. Tseng, J. Q.
Liu, Y. A. Chen, C. M. Chao, K. M. Liu, C. L. Chen, T. C. Lin, Harnessing excited-state
intramolecular proton-transfer reaction via a series of amino-type hydrogen-bonding
molecules, J. Phys. Chem. Lett. 6 (2015) 1477-1486.
[26] M. Ziółek, J. Kubicki, A. Maciejewski, R. Naskręcki, A. Grabowska, An ultrafast excited
state intramolecular proton transfer (ESPIT) and photochromism of salicylideneaniline (SA)
and its “double” analogue salicylaldehyde azine (SAA), A controversial case, Phys. Chem.
Chem. Phys. 6(19) (2004) 4682-4689.
[27] M. Sliwa, N. Mouton, C. Ruckebusch, L. Poisson, A. Idrissi, S. Aloïse, S. Aloïse, L. Potier, J.
Dubois, O. Poizata, G. Buntinx, Investigation of ultrafast photoinduced processes for
salicylidene aniline in solution and gas phase: toward a general photo-dynamical
scheme, Photoch. Photobio. Sci. 9(5) (2010) 661-669.
[28] J. B. Birks, Photophysics of Aromatic Molecules ~Wiley-Interscience, New York, 1970.
[29] R. S. Becker, Theory and Interpretation of Fluorescence and Phosphorescence
~Wiley-Interscience, New York, 1969.
[30] J. R. Lakowicz, Principles of Fluorescence Spectroscopy ~Plenum, New York, 1983.
29