New AB2 type two-photon absorption dyes for well-separated dual-emission: Molecular preorganization based approach to photophysical properties

Article abstract of DOI:10.1016/j.tet.2016.04.024

A variety of AB2 type new target dyes containing two proton transfer segments (o-hydroxy-phenyl-imino) are presented in this study. Furthermore, the corresponding reference molecules absent of hydroxy or imino groups are provided as well. Dual intramolecular hydrogen bonds in the target dyes are demonstrated by ¹H NMR spectra, X-ray single crystal analysis as well as one-photon absorption spectra. Linear emission properties (static and transient emission nature) and two-photon absorption optical properties of these target and reference dyes are determined in various media such as organic solvents, solid phase state as well as PMMA substrate. The results show that the new AB2 type target dyes can efficiently undergo internal proton transfer in the excited states under one-photon irradiation and near-infrared femtosecond laser two-photon irradiation, respectively, and thus they exhibit well-separated dual-emission. The molecular geometry optimization is performed for the analysis of the occurrence of internal proton transfer in the excited states of the target AB2 chromophores.

Full text of DOI:10.1016/j.tet.2016.04.024

Accepted Manuscript
New AB2 type two-photon absorption dyes for well-separated dual-emission:
Molecular preorganization based approach to photophysical properties
Ge Ding, Yao Lu, Yulong Gong, Li Ma, Ziping Luo, Shengtao Zhang, Fang Gao,
Hongru Li
PII:
S0040-4020(16)30277-0
10.1016/j.tet.2016.04.024
TET 27667
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 22 February 2016
Revised Date: 7 April 2016
Accepted Date: 8 April 2016
Please cite this article as: Ding G, Lu Y, Gong Y, Ma L, Luo Z, Zhang S, Gao F, Li H, New AB2 type two-
photon absorption dyes for well-separated dual-emission: Molecular preorganization based approach to
photophysical properties, Tetrahedron (2016), doi: 10.1016/j.tet.2016.04.024.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
New AB2 type two-photon absorption dyes for well-separated dual-emission: molecular
preorganization based approach to photophysical properties
Ge Ding1, Yao Lu1, Yulong Gong1, Li Ma1, Ziping Luo1,2, Shengtao Zhang*1,2, Fang Gao*1,2, Hongru Li*1,2
1College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, China, 400044
2National-municipal Joint Engineering Laboratory for Chemical Process Intensification and Reaction, Chongqing University, Chongqing,
China, 400044
Tel & Fax: 86-23-65106479, Emails: stzhang@cqu.edu.cn, fanggao1971@gmail.com, hrli@cqu.edu.cn
ABSTRACT
A variety of AB2 type new target dyes containing two proton transfer segments (o-hydroxy-phenyl-imino) are
presented in this study. Furthermore, the corresponding reference molecules absent of hydroxy or imino groups are
1
provided as well. Dual intramolecular hydrogen bonds in the target dyes are demonstrated by H-NMR spectra,
X-ray single crystal analysis as well as one-photon absorption spectra. Linear emission properties (static and
transient emission nature) and two-photon absorption optical properties of these target and reference dyes are
determined in various media such as organic solvents, solid phase state as well as PMMA substrate. The results
show that the new AB2 type target dyes can efficiently undergo internal proton transfer in the excited states under
one-photon irradiation and near-infrared femtosecond laser two-photon irradiation respectively, and thus they
exhibit well-separated dual-emission. The molecular geometry optimization is performed for the analysis of the
occurrence of internal proton transfer in the excited states of the target AB2 chromophores.
Keywords: dual emission; internal proton transfer; excited state; hydrogen bond
1. Introduction
Photoinduced occurrence of excited state intramolecular proton transfer (ESIPT) based on
internal hydrogen bond is one of the most significant phenomena in life and nature such as the
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phototautomerization of amine-imino or enol-keto.1-5 As a consequence, the investigation of
internal proton transfer in the excited states of the organic molecules receives considerable
experimental and theoretical interests,6-10 which in turn stimulates the synthesis of new ESIPT
molecules.
Strong intramolecular hydrogen bond between proton donor-acceptor groups could lead to
the decrease of energy barriers of internal proton transfer in the excited state of an organic
molecule. For instance, it was proposed firstly in 1956 that internal hydrogen bond between
o-hydroxy-phenyl and imino segments in salicylidene methylamine was the critical factor for
occurrence of ESIPT,11 which undergoes internal proton transfer in the excited state via a
four-level cycle phototautomerization mechanism (E→E*→K*→K→E, E, enol, K, keto).
Normally, such a four-level cycle process gives rise to well-separated dual fluorescence
emission bands, in which the second emission band possesses an exceptionally large Stokes
red-shift without its self-absorption since it is produced by the decay of tautomer K*→K, while the
absorption is assigned to tautomer E→E*. 12-15 The energy loss due to the intramolecular proton
transfer process leads to the remarkable Stokes red-shift of the second emission band. While the
first emission band derived from the E*→E deactivation process yields the normal Stokes red-shift.
This unique nature of ESIPT molecule makes its great application potentials such as fluorescence
sensors of cation or anion ions, optical devices and vitro or vivo bioassay. 16-20
There are two real tautomerization chemical reactions in enol-keto type ESIPT including
E*→K* and K→E. To such an organic molecule, small energy barrier or even energy barrierless
of E*→K* in the excited state is required to achieve ESIPT. It is thought that the potential energy
barrier of such an intramolecular E*→K* phototautomerization could be increased by the increase
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in molecular size. Thus, it is considered that the insertion of π-conjugation units could play the
inhibition effect on the occurrence of ESIPT to an organic chromophore.21 This interprets why the
most reported ESIPT studies were concentrated on quite small size organic molecules,22-24 which
could favor the decreased energy barrier of enol-keto phototatumerization.
However, the enlarged organic dyes can be conveniently applied in the greater fields. For
instance, the extended π-conjugated organic dyes could possess the larger two-photon absorption
(TPA) cross sections under near-infrared (near-IR) femtosecond laser providing various superior
advantages over common light resources such as pulsed photons, deep penetration and negligible
damage to life cells.25-28 While for the organic dye chemists, the molecular preorganization and
huge theoretical calculations of the extended ESIPT molecules are the great challenges. So far,
few experimental and theoretical studies on near-IR TPA ESIPT of π-extension enlarged organic
dyes have been presented.
The key challenge is how to get ideal target molecules through the rational molecular
preconstruction. One representative example is the preparation of conjugated organic dye
containing 2-(2’-hydroxy-phenyl)-benzothiazole part.29 However, this π-extended dye shows only
sign of ESIPT, and the characteristic tautomer keto emission band is not well distinguished from
the normal enol emission. It could be caused by the large potential energy barrier of enol-keto
phototatuomerization in the excited state of this chromophore.
In order to further strength intramolecular hydrogen bond between hydroxy and imino groups,
we propose that it could be efficient approach to pre-construct π-extended TPA dyes bearing
salicylidene methylamine-like segments through a covalent double bond. Such molecular design
affords the unrestricted imino double bond, and thus the rotation of imino group could not be
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restricted. This could lead to the stronger intramolecular H-bond between the adjacent phenolic
hydroxy and imino groups. Therefore, a quasi six-membered ring transition state form of eno-keto
phototautomerization could be more easily generated.
To achieve well-separated dual-emission under one-photon exposure and near-IR two-photon
irradiation, we studied AB type organic dyes carrying one o-hydroxy-phenyl-imino part such as
2-((4'-methoxyl-diphenylethylene-4''-ylimino)methyl)phenol and 2-((3', 4'-dimethoxyl-diphenyl-
ethylene-2''-ylimino)methyl)phenol.30 However, these molecules showed quite weak ESIPT
emission under one-photon irradiation and near-IR two-photon excitation due to the presence of
remarkable energy barrier of enol-keto phototautomerization and small TPA cross sections.
In this study, we further propose to synthesize AB2 or ABn type organic dyes, which carry
two or multi o-hydroxy-phenyl-imino parts. The number increase of proton transfer segments
could increase the possibility of ESIPT, and thus the well-separated dual-emission could be
observed. In addition, the reference dyes absent of either proton donor or acceptor segments are
prepared to reveal the origin of the dual-emission. The molecular geometry optimization is also
performed to find the fundamental reason of intramolecular proton transfer in the excited states.
2. Results and discussion
2.1 Design and synthesis of the target dyes
It is considered that the free imino bond could increase the coplanarity of quasi
six-membered ring (O-H-N-C-C-C) in o-phenyl-hydroxy-imino part of the target dyes, and thus
intramolecular hydrogen bond between phenolic hydroxy and imino groups may be improved.31 In
particular, the presence of dual intramolecular hydrogen bonds in the AB2 target dyes could
efficiently lower the potential energy barrier of E* to K*, and thus the occurrence of ESIPT is
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increased. As a result, the well-separated dual-emission could be determined.
It is well accepted that the extension of π conjugation favors the improvement of two-photon
absorption of organic molecules.32 Hence, the π-conjugated molecular structure could increase
TPA cross sections of the target dyes C1~C3, which further favors the occurrence of
intramolecular proton transfer under fetmosecond near-IR laser irradiation.
Carbanion was produced by the hydrogen-abstracting of methylene in nitro substituted
phenylacetic acid at the basic condition,33 which underwent the addition reaction with carbonyl
group in benzaldehyde derivatives to yield trans C-C double bond of the intermediate of the first
step. It was found that the strong reduction such as Fe/AcOH was unfavorable for the synthesis of
the intermediate in the second step, namely 2’, 4’-diaminyl-diphenylethylene derivatives, which
were more efficiently produced by using the mild reduction condition (Na2S in base solution).
However, the easy oxidization of the diaminyl intermediate in air and the serious adsorption on gel
chromatography gave rise to its low isolated yield in the second step.
The target dyes were prepared by the condensation reaction between the freshly prepared
doubly-aminly intermediate and salicylaldehyde derivatives in the final step, which could be
further purified by the combination of silicon gel or aluminum oxide column chromatography and
recrystallization due to the good chemical stability.
2.2 X-ray single crystal diffraction analysis
X-ray single crystal diffraction analysis may afford the direct evidence of internal hydrogen
bond in the target molecules. The single crystals of C1 and C3 were prepared by the slow
evaporation of mixed organic solvents. We particularly focus on the analysis of distances between
phenolic hydroxy and imino groups and the corresponding dihedral angles in these single crystals
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since they reflect the strength of intramolecular hydrogen bonds.
Dual intramolecular hydrogen bonds are found in the both of 4’- and 2’- arms of C1 and C3
(Figure 1). It is noticed that the distance of H1-N1 in 2’-arm of C1 or C3 is a little longer than
that in the 4’-arm (such as to C1, 1.873 Ǻ for 2’-arm and 1.847 Ǻ for 4’-arm, C3, 1.964 Ǻ for
2’-arm and 1.867 Ǻ for 4’-arm ), which is mostly due to the presence of the steric hindrances in
4’-arm. It also causes the larger dihedral angles in 2’-arms comparing to those in 4’-arms (such as
C1, 2’-arm ∠C24-O2-H2-N2, 13.81˚, 4’-arm ∠C17-O1-H1-N1, 7.57˚). As a consequence,
intramolecular hydrogen bond in para-arms could be a little stronger than that in ortho-arms of C1
or C3.
Figure 1 ORTEP drawing of C1 and C3 with 30% possibility of thermal ellipsoids
Additionally, the distances of H-N in two arms of C1 are slightly shorter than those of C3
respectively (such as in 2’-arm, for C1, 1.873 Ǻ and for C3, 1.964 Ǻ). It could be ascribed to the
diminished negative charge of imino group by the electron-donating role of the methoxyl group.
Furthermore, the corresponding dihedral angles of ∠C24-O2-H2-N2 and ∠C17-O1-H1-N1 in C1
are also lower than those of ∠C26-O4-H4-N2 and ∠C19-O3-H3-N1 of C3 respectively (such as
in 2’-arm for C1, ∠C24-O2-H2-N2, 13.81˚, and for C3, ∠C26-O4-H4-N2, 15.45˚). Hence, the
two internal hydrogen bonds of C1 could be stronger than those of C3.
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X-ray single crystal diffraction analysis also demonstrates that C-C double band and C-N
double band in the target dyes C1 and C3 show the good trans-configuration. The small dihedral
angles around double bonds suggest that two π conjugation parts in the target molecules possess
the great coplanarity respectively. It is also found that the coplanarity around imino bond in the
4’-arm of C1 or C3 is better than that in the 2’-arm due to the steric hindrances (such as in C1,
∠C3-N2-C22-C23, 4.56°, ∠C1-N1-C15-C16, 0.25°), which further reflects the stronger
intramolecular hydrogen bond in 4’-arm. It is observed that the dihedral angle around imino bond
of C1 is smaller than that of C3 (such as 4’-arm, C1, 0.25°, C3, 2.64°), indicating a stronger
internal hydrogen bond in C1.
Overall, X-ray single crystal diffraction of C1 and C3 suggests the presence of two
intramolecular hydrogen bonds. The factors including intramolecular hydrogen bond and the
coplananrity around C-N and C-C double bonds in these dyes are favorable for the formation of a
quasi six-membered ring (such as in C1, O2-H2-N2-C24-C23-C22). Hence, the double internal
hydrogen bonds in C1 and C3 could increase the possibility of intramolecular proton transfer in
the excited state. It is worthy pointing out that there is no intermolecular hydrogen bond between
the target molecules based on X-ray single crystal diffraction analysis.
2.3 1H-NMR spectral analysis
1
The clean H-NMR spectra of the target dyes were obtained in this study. The typical
1H-NMR spectra of the target dye C1 and its reference molecules C4 and C7 in D6-DMSO are
1
shown in Figure 2. It is accepted that H-NMR chemical shift of hydroxy group can be greatly
influenced by intramolecular H-bond.34 Figure 2 shows that the 1H-NMR chemical peak values of
phenolic hydroxy in the target molecules increase comparing to the reference molecule C4 (such
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as C1, 13.005 ppm and 13.058 ppm versus C4, 9.403 ppm, 9.459 ppm).
1
The H-NMR chemical shift of imino group in the target molecules also moves to the lower
magnetic field comparing to the reference molecule C7 (such as C1, 9.110 ppm and 9.046 ppm
versus C7, 8.751 ppm and 8.763 ppm, Figure 2). These data further show that there is an intensive
intramolecular H-bond in C1.34
Figure 2 The 1H-NMR spectra in D6-DMSO of C1, C4 and C7
1
It is further observed that the H-NMR shift of hydroxy group in the equal molar C7/phenol
mixture is almost identical to that of phenol (Figure S1, Supporting information, SI). In addition,
the 1H-NMR peaks of two phenolic hydroxy groups in C1 move to the much down magnetic field
as comparison with that of phenol (such as in C1, 13.005 ppm and 13.058 ppm, phenol, 9.350
ppm). The results suggest that the internal hydrogen bond in the target molecules could be the
predominant factor leading to the downfield shift of 1H-NMR peak of hydroxy group.
1
In addition, the H-NMR chemical shift of phenolic group in 4’-arm shows a little more
down-shifted magnetic field than that in 2’-arm of these target dyes, which could be due to the
stronger internal hydrogen bond in 4’-arm. It is also found that both phenolic hydroxy and imino
groups in C1, C2 and C3 exhibited the split 1H-NMR peaks respectively (also see Figure S2, SI).
It demonstrates that the two o-hydrox-phenyl-imino segments are located in slightly different
intramolecular chemical environments.
It is also noticed that the reported dye containing the restricted imino group shows the greater
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1
upfield H-NMR peak of phenolic hydroxy than the target molecules studied in this work (~10
ppm).29 The results demonstrate that the free imino group could yield the stronger internal
1
hydrogen bond with phenolic hydroxy group. In a word, H-NMR spectra of new AB2 dyes
C1~C3 demonstrate the presence of dual internal hydrogen bonds.
2.4 Effect of intramolecular hydrogen bond on one-photon UV/visible spectral properties
One-photon UV/visible spectra of the target dyes and the reference molecules are studied in
various organic solvents. The target C1 and its references C4 and C7 are used as the examples to
illustrate one-photon absorption spectra. The representative UV/visible spectra of C1, C4 and C7
in benzene and DMF are given in Figure 3. The typical UV/visible spectral parameters of C1, C4
and C7 in various solvents are presented in Table 1.
It is found that the peak absorption wavelength of C1 shows the bathochromic shift with
respect to that of C4 and C7 respectively in various solvents (such as in DMF, C1, 363 nm, C4,
345 nm, C7, 355 nm). Furthermore, the UV/visible absorption spectra of the mixture of C7/phenol
are almost identical to those of C7 in various solvents. The other target dyes C2 and C3 display
the similar absorption spectral properties as C1 (typically shown in Figure S3 & Table S1 in SI),
the absorption maxima of C2 and C3 show the red-shift comparing to the corresponding reference
dyes as well.
C1
C4
C7
(a)
C1
C4
C7
(b)
0.4
0.2
0.0
0.4
0.2
0.0
300
350
400
450
500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 3 One-photon UV/visible absorption spectra of C1, C4 and C7 in benzene (a) and in DMF (b), the
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concentration is 1×10-5 mol/L for the absorption spectra
Table 1 The one-photon spectral parameters of C1, C4 and C7 in various solvents, λa,max: the absorption maximum
(nm), λf,max: the emission maximum (nm), εmax: the maximal molar extinction coefficient (105 cm-1·mol-1·L), Φ: the
fluorescence quantum yield
C1
C4
λa,max λf,max
C7
λa,max λf,max
Solvents
λa,max λf,max
Φ
εmax
Φ
εmax
Φ
εmax
Benzene
368
527
532
532
533
535
535
532
525
542
529
0.0152
0.364
364
363
360
352
363
360
350
351
345
354
438
449
453
450
456
442
463
468
466
461
0.220
0.323
0.134
363
356
361
359
352
359
356
358
355
352
426
427
430
431
445
430
447
435
430
435
0.170
0.155
0.347
0.295
1,4-dioxane 364
0.00794 0.454
0.00840 0.360
0.00697 0.435
0.00551 0.424
0.00961 0.369
0.0997
THF
363
360
365
364
361
359
363
359
0.0 596 0.351
0.0312 0.279
0.129 0.231
EtOAc
CH2Cl2
CHCl3
ACN
0.0636
0.155
0.0684
0.159
0.264
0.232
0.115
0.216
0.140
0.162
0.0930
0.108
0.194
0.105
0.0737 0.238
0.0560 0.280
0.0821 0.319
0.0376 0.257
0.0312 0.279
0.0445 0.244
0.0110
0.0403
0.392
0.359
DMSO
DMF
0.00839 0.360
0.00500 0.135
Ethanol
The further study shows that the maximal absorption peaks of C1 are blue-shifted in
hydrophilic organic solvents such as 1, 4-dixoane and DMF as NaOH or K2CO3 alkali aqueous
solution is added (363 nm versus 341 nm, Figure S4, SI), while the absorption maxima of C1 are
recovery as dilute HCl solution is further added. This could be due to the neutralization of
phenolic hydroxy groups by the alkali solution, and internal hydrogen bond in C1 is lowered. It is
further observed that the addition of NaOH alkali aqueous solution into C4 and C7 DMF solutions
produces only slight variation of the maximal absorption peak (Figure S5 in SI).
In contrast, the maximal absorption wavelength of C1 in DMF also exhibits the blue-shift as
HCl or acetic acid aqueous solution is added. However, the further addition of NaOH or K2CO3
alkali aqueous solution yields the original absorption maxima of C1 (Figure S6, SI). The results
suggest that imino groups in C1 could be protonated by acid, and intramolecular hydrogen bond is
diminished.
Hence, the above studies show that the bathochromic shift of the peak wavelength of the
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target dyes with respect to that of the reference molecules could be caused by the intramolecular
hydrogen bonds,35 which enhances intramolecular charge transfer in the ground state of these
target dyes.
2.5 Dual-emission in organic solvents under one-photon excitation
In general, intramolecular H-bond in the ground state of an organic molecule is accepted as
the necessary condition to undergo internal proton transfer in the excited state.36 The investigation
1
including X-ray crystallization analysis, H-NMR spectra and UV/visible spectra suggests the
presence of the strong internal H-bonds in the ground state of the target dyes C1~C3. Hence, the
intramolecular proton transfer in the excited singlet state of these target dye molecules could occur.
In order to get the direct experimental evidence, one-photon fluorescence emission spectra of the
target dyes and the reference molecules are investigated in various organic solvents.
500
400
300
200
100
0
250
200
150
100
50
C1
C4
C7
(a)
(b)
C1
C4
C7
normal emission
normal emission
ESIPT emission
ESIPT emission
0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength
(nm)
Wavelength (nm)
Figure 4 One-photon emission spectra of C1, C4 and C7 in benzene (a) and in DMF (b), excitation wavelength is
350 nm, the concentration is 5×10-6 mol/L
It is found that the target dyes C1, C2 and C3 show the well-separated dual fluorescence
bands in the most of organic solvents such as benzene and DMF (typically shown in Figure 4,
also see Figure S7, SI). In contrast, the corresponding reference dyes such as C4 and C7 produce
only single emission band in various solvents. In the followings, C1 and its reference dyes C4 and
C7 are used as the typical examples to analyze and discuss the experimental results.
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Figure 4 (b) shows that the first emission band of C1 peaked around 450 nm possesses the
similar maximal emission wavelength as that of C4 and C7 respectively, which exhibits the
normal Stokes red-shift in various solvents (such as ~ 80 nm in benzene). We further observe that
the maximal wavelength of the second emission band of C1 is remarkably red-shifted with respect
to that of the first emission band (such as C1, the second emission band, ca. 550 nm in DMF).
The titration of NaOH or K2CO3 alkali water solution into C1 hydrophilic organic solution
such as DMF solution could quench the second emission band. Meanwhile, the emission
intensities of the first fluorescence band increase. On the other hand, the second emission band is
regenerated as dilute HCl aqueous solution is further added (Figure S8, SI). In contrast, the
second emission band of C1 in DMF solution could be quenched by the addition of dilute HCl
aqueous solution, and it is reproduced by the further addition of NaOH or K2CO3 alkali water
solution.
Furthermore, the addition of NaOH aqueous solution into C4 and C7 DMF solution does not
cause the remarkable variation of the emission spectra (Figure S8, SI). The fluorescence spectra
of C4 and C7 are also not greatly varied by the added phenol in DMF (Figure S9, SI).
Hence, the studies show that the first emission band of C1 is yielded by the tautomer enol* to
enol decay, which is effectively the same as the tautomer enol S0→S1 absorption spectral feature.37
However, the second emission band of C1 is produced by the decay of tautomer keto* to keto. As a
consequence, the energy of the excited state in C1 is consumed by internal proton transfer,37 and
its fluorescence quantum yields are much lower than the corresponding reference molecules C4
and C7 in various solvents (Table 1, C1, 0.0152, C4, 0.220, C7, 0.170, in benzene).
The results also suggest that C7 is not able to undergo internal proton transfer in the excited
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state due to the absence of active phenolic hydroxy groups. And, internal proton transfer between
secondary amine and hydroxy groups in the excited state of C4 could not occur either.
*
*
The ratios of the maximal ESIPT intensity (IE ) to the normal emission intensity (IN ) of the
organic molecules reflect the extent of intramolecular proton transfer in the excited state.38 It is
*
found that the order of IE /IN* is C1>C2>C3 (such as in DMF, C1, 0.944, C2, 0.732, C3, 0.602),
which could be ascribed to reduced internal hydrogen bond by electron-donating nature of the
methoxyl group in C2 and C3.
*
*
It is noticed that IE /IN of C2 and C3 is larger than that of the dyes carrying single
o-hydroxy-phenyl-imino part in 4’-arm in various solvents (such as in benzene, C2, 0.453, C3,
0.328, 2-((4'-methoxyl-diphenylethylene-4''-ylimino)methyl)phenol, 0.100).30 Hence, the above
results demonstrate that double o-hydroxy-phenyl-imino moieties are favorable for intramolecular
proton transfer in the excited state.
We further observe that ESIPT emission of C1~C3 is more pronounced in alcohol solvents
*
(Figure S10, SI). As a result, the IE /IN* increases in the protic alcohol solvents (such as in ethanol,
C1, 1.483). The results show that intermolecular H-bond between the solutes and the alcohol
solvents could improve the intramolecular proton transfer in the excited states of these dyes.39
2.6 The radiative transition rates of the tautomers enol and keto decay in the excited states
of the target dyes
Because the fluorescence emission of the target dyes consists of the normal emission and
ESIPT emission, the fluorescence quantum yield of the target dye is the sum of the normal
fluorescence quantum yield and the ESIPT fluorescence quantum yield, as shown in the following
equation:
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Φf (sum) = Φf (normal) + Φf (esipt)
(1)
Since the absorption of the target dye is assigned to tautomer enol to enol*, thus the ratio of the
integrated area of the ESIPT emission band or the normal emission band to that of the total
emission represents the percent of the ESIPT emission or the normal emission to the total
fluorescence quantum yield respectively. Consequently, the ESIPT fluorescence quantum yield
and the normal fluorescence quantum yield of the target dyes can be calculated respectively.
Accordingly, the radiative transition rate (kr) of the tautomer enol decay and keto decay in the
excited state can be calculated by the following equation：
kr =Φf /τf
(2)
wherein Φf is the fluorescence quantum yield and τf shows the lifetime determined at the maximal
wavelength of fluorescence band.
Table 2 Radiative transition rates (s-1) of the tautomers enol and keto decay in the excited state of the targets C1
and C3
Solvents
Dyes
Benzene
DMF
Φf (normal)
0.0118
2.521
0.00419
1.254
τf (normal) × 10-9 (s)
kr (normal) × 107 (s-1)
Φf (esipt)
0.466
0.334
C1
0.00348
0.545
0.00420
0.122
τf (esipt) × 10-9 (s)
kr (esipt) × 108 (s-1)
Φf normal
0.0638
0.00846
2.287
0.351
0.00631
1.952
τf (normal) × 10-9 (s)
kr (normal) × 107 (s-1)
Φf (esipt)
0.370
0.323
C3
0.00243
0.433
0.00203
0.089
τf (esipt) × 10-9 (s)
kr (esipt) × 108 (s-1)
0.0561
0.228
Table 2 lists the radiative transition rates of the tautomers enol and keto decay in the excited
state of C1 and C3. The calculation shows that ESIPT emission of C1 possesses the much larger
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radiative transition rates than the normal emission in various organic solvents (such as in DMF C1,
kr(esipt)=0.351×108 s-1, kr(normal)=0.334×107 s-1 ), suggesting that K-tautomer emission decay is
strong competitive in the excited state of C1. It is further found that the radiative transition rate of
the tatutomer K in C1 is larger than C3 (C1, kr(esipt) =0.351×108 s-1, C3, kr(esipt) =0.228 ×108 s-1 in
DMF), which further demonstrates the effect of the substituted group on ESIPT.
The transient emissive species of the tautomers enol and keto forms of the target
chromophores are well distinguished from each other in the time-resolved fluorescence spectra
obtained by the decay kinetics at the emission wavelength dependence. Figure 5 shows that a
competitive tautomer enol emission band of target C1 appears at 50 ps, and the intensities of
tautomer enol species decrease with the decay time. The transient tautomer keto species emission
band is yielded at the same while, and the maximal transient peak is formed at 200 ps decay time.
After then, the emission intensities of the transient keto species decrease. The transient emissive
spectra of C1 further demonstrate that ESIPT occurs in ultrafast sub-femtosecond time range.
3
0.05ns
0.10ns
0.15ns
2
1
0
0.20ns
0.30ns
0.50ns
450
500
550
600
650
Wavelength(nm)
Figure 5 Time-resolved fluorescence spectra of C1 in CH2Cl2, the concentration is 5×10-5 mol/L
2.7 Dual-emission in organic solvents under near-IR two-photon excitation
Near-IR Ti:sapphire femtosecond laser were utilized to detect TPA optical properties of the
target dyes and the corresponding reference dyes in various organic solvents. C1 and its references
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C4 and C7 are utilized the representative examples to show TPA emission results. TPA emission
spectra of C1 and C4 and C7 in benzene and DMF are shown in Figure 6.
1000
800
600
400
200
0
750
600
450
300
150
0
C1
C4
C7
(a)
(b)
C1
C4
C7
normal emission
normal emission
ESIPT emission
ESIPT emission
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 6 TPA fluorescence emission spectra of C1, C4 and C7 in benzene (a) and DMF (b) under 740 nm near-IR
femtosecond laser irradiation, the concentration is 3×10-5 mol/L respectively
It is observed that C1 shows the greatly separated dual emission bands, while C4 and C7
display the single TPA emission band in DMF under 740 nm femtosecond laser irradiation. The
mixture of C7/phenol shows the quite similar TPA spectra as C7 in various solvents. Furthermore,
the TPA emission maxima of C4 and C7 are close to the emission peak of the first TPA emission
band of C1, and the peak wavelength of the second TPA emission band of C1 is much close to
that of one-photon ESIPT emission (ca. 550 nm).
The titration of NaOH or K2CO3 alkali aqueous solution to hydrophilic organic solvents such
as DMF solution of C1 leads to the disappearance of its second TPA emission band, while the
intensity of the first TPA emission band increases (Figure S11, SI). The further addition of dilute
HCl solution reproduces the second TPA emission band.
It is further found that the second TPA emission band of C1 in DMF solution disappears as
HCl aqueous solution is added. While, the further titration of NaOH aqueous solution leads to the
reappearance of the second TPA emission band, and the TPA emission intensity of the first band
decreases.
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C2 and C3 produce the similar TPA optical properties as C1 (typically shown in Figure S12,
SI). These results suggest that the first TPA emission band of the target dyes C1, C2 and C3 is
produced by the normal TPA E*→E decay, and the second emission band is yielded by TPA
K*→K process. Therefore, the results demonstrate that the target dyes are able to undergo TPA
ESIPT under near-IR femtosecond laser irradiation, while the reference molecules can not process
TPA internal proton transfer due to the absence of proton donor or acceptor.
It is found that TPA ESIPT emission of the target dyes are more competitive in the decay
*
*
process of the excite state under near-IR laser irradiation. IE /IN in TPA emission spectra of C1,
C2, and C3 is larger than that in one-photon emission spectra (such as in benzene, C1, 1.446, C2,
1.098, C3, 0.931). This could be ascribed to the good match of time scale between near-IR
femtosecond pulsed laser and ultrafast internal proton transfer in the excited state. As a
consequence, ESIPT becomes the predominant pathway to deactivate the excited states of the
target dyes under near-IR laser irradiation.
Table 3 Two-photo cross-sections (GM, 1GM=10-50 cm·s-1·photon-1) of C1~C9 under 740 nm femtosecond laser
irradiation in DMF solution
Target Dyads
C1
C2
C3
C4
C5
C6
C7
C8
C9
TPA Cross-sections
116.452 215.867 373.675 37.989 42.432 50.265 29.561 35.431 39.780
The TPA cross-sections of the target molecules was further determined based on TPA
fluorescent methods. It is found that the target dyes display the larger near-IR TPA cross-sections
than the corresponding reference molecules (see Table 3, such as C1, 116.452 GM, C4, 37.989
GM, C7, 29.561 GM). The extent of the internal charge transfer and molecular aggregation could
be increased by intramolecular H-bond in the target molecules.40 As a consequence, the TPA
absorption sections are improved.
We shall point out that the near-IR TPA cross-sections of target dyes C2~C3 bearing double
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o-hydroxy-phenyl-imino parts are above ten times as large as those of the dyes containing single
o-hydroxy-phenyl-imino moiety (such as in DMF, C3, 373.675 GM, 2-((3',4'-dimethoxyl-
diphenyle-thy-lene-2''-ylimino)methyl)phenol, 33.545 GM). It could be attributed to the more
efficient π extension of the molecular geometry in space and more intensive internal H-bonds in
the target molecules.41Hence, a remarkable well-separated dual emission of the target dyes C1~C3
is observed under near-IR laser irradiation.
2.8 Dual-emission of the target dyes in the solid phase and polymer matrix under one-photon
excitation and near-IR two-photon irradiation
The typical one-photon and two-photon emission spectra of C1, C4 and C7 in solid phase
state and polymer matrix are shown in Figure 7. It is found that C1 shows the greatly separated
dual fluorescence bands including the normal emission band and the internal proton transfer
emission band in the solid phase state and PMMA matrix under one-photon excitation and near-IR
two-photon irradiation respectively. In contrast, the corresponding reference dyes such as C4 and
C7 yield only the single normal emission band.
Figure 7 shows that a strong competitive second emission band with the large Stokes
red-shift is presented. The results further suggest that intramolecular proton transfer in the excited
states of these target dyes occurs in solid phase state and polymer matrix on the basis of
*
one-photon excitation and near-IR laser irradiation respectively. IE /IN* of C1 is higher than that of
C2 and C3 in solid phase state and polymer matrix as well (such as in solid phase, C1, 1.898, C2,
1.653, C3, 1.432 under one-photon irradiation), showing the electron-donating substituted groups
are unfavorable for the occurrence of ESIPT of the target dyes.
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1000
750
500
250
0
2000
C1
(b)
C4
C1
C4
C7
(a)
C7
1500
1000
500
0
400
450
500
550
600
650
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength(nm)
1000
750
500
250
0
2000
C1
C4
C7
C1
C4
C7
(c)
(d)
1500
1000
500
0
400
450
500
550
600
650
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Figure 7 One-photon emission spectra (a) and near-IR two-photo emission spectra (b) of C1, C4 and C7 in solid
phase state; One-photon emission spectra (c) and near-IR two-photo emission spectra (d) of C1, C4 and C7 in
polymer matrix. The excited wavelength of one-photon emission spectra is 350 nm, The excited wavelength of
TPA emission is under 740 nm
2.9 Molecular geometry optimization
2.9.1 Geometrical parameters
Molecular modeling of the target molecules is carried out to reveal the dual-emission. The
calculation shows that the transition state form (TS) of the tautomerization are presence in the
ground and excited states of the target dyes C1~C3. In contrast, the reference molecules such as
C4 and C7 do not possess transition state form due to the absence of either proton accepter or
donor. The results reflect that it is impossible to undergo internal proton transfer in the ground and
excited states of the reference molecules, while it is possible to process intramolecular proton
transfer in the target dyes.42
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O
H
(a)
N
N
H
O
TS
x
O
N
O
N
H
H
N
N
H
O
O
H
*
2K
*
2E
O
N
O
N
H
H
H-transfer
N
x
N
O
H
H
O
2K
2E
(b)
O
H
N
N
HO
*
O
EK
H
O
H
N
N
N
N
H
O
HO
TS2
TS1
O
N
H
O
H
N
N
H
O
N
*
O
2K
H
*
O
H
O
N
N
H-transfer
H
X
N
O
N
H
H
O
2E
2K
Scheme 1 (a) The transition states for the simultaneous occurrence of ESIPT (2E→TS→2K) or (b) step-by-step
occurrence of ESIPT (2E→TS1→EK→TS2→2K)
It is noticed that the targets C1, C2 and C3 contain double o-hydroxy-pheyl-imino parts,
however, the optimized transition states of the double phototautomerization could not be obtained
either through the simultaneous occurrence (2E→TS→2K) or step-by-step occurrence
(2E→TS1→EK→TS2→2K) (In Scheme 1 ). This means that the internal double proton transfer
in the excited states of the target dyes could not occur. In contrast, the target dyes only exhibit the
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optimized transition states of the single phototautomerization in 2’-arm or 4’-arm (such as C1,
typically shown in Figure 8), which shows that they could undergo single intramolecular proton
transfer in the excited states either through in 2’-arm or 4’-arm (In Scheme 2 ). In the followings,
C1, C4 and C7 are used as the examples to analyze the molecular geometry optimization data.
E*
C1-4’ arm TS*
C1-4’ arm K*
E*
C1-2’ arm TS*
C1-2’ arm K*
Figure 8 The optimized geometries of enol, TS and keto forms in the excited state of C1
(a)
O
O
H
H
N
H-transfer
N
N
N
HO
HO
*
*
K
E
O
O
N
H
H
N
H-transfer
N
N
HO
HO
E
K
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(b)
HO
N
HO
N
H-transfer
N
N
O
H
O
H
K*
E*
HO
N
HO
N
H-transfer
N
N
H
O
H
O
E
K
Scheme 2 (a) A four-level cycle of enol-keto phototautomerization during internal proton transfer in the excited
state either in 2’-arm of C1 or (b) in 4’-arm of C1
First of all, the distances of H-O and H-N (in o-hydroxy-phenyl-imino part) in the transition
state forms of C1 in S0 and S1 states are between those of tautomers forms (such as H-O in S1,
4’-arm of C1, E, 0.956 Å, TS, 1.237 Å, K, 1.810 Å, in Table 4). This indicates that intramolecular
proton transfer of C1 could be required to be through an energy-barrier pathway. Additionally, the
distances of O-N in the transition state forms are much shorter than those of E and K forms in both
S0 and S1 states of C1, which suggests that the transition state forms possess the stronger internal
hydrogen bond than E and K forms in both S0 and S1 states of C1.43 As a consequence, internal
proton transfer could occur in C1 if the potential energy barrier is overcome.
Table 4 The most important bond lengths (Å) association with ESIPT
Ground state
TS
Excited state
TS*
Structural
parameters
E
K
E*
K*
RO-H
0.954
1.891
2.713
0.954
1.891
2.713
1.272
1.869
1.005
2.657
1.871
1.006
2.656
0.956
1.891
2.719
0.954
1.889
2.710
1.237
1.810
1.011
2.648
1.804
1.006
2.631
RN-H
RO-N
RO-H
RN-H
RO-N
1.185
1.214
C1,4’-arm
2.384
2.391
1.274
1.261
1.183
1.189
C1,2’-arm
C4,4’-arm
2.384
2.384
RO-H
0.911
0.948
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RN-H
RO-N
RO-H
RN-H
RO-N
2.133
2.872
0.946
2.118
2.895
1.823
2.637
0.948
2.136
2.906
C4,2’-arm
The important dihedral angles of
∠CCCN in o-hydroxy-phenyl-imino part of the target
molecules were also calculated (in Table 5, Atomic number see Scheme 3). The small dihedral
angle in S0 state of C1 means the excellent coplanarity in o-hydoxy-phenyl-imino part
(∠
C6-C7-C8-N2, 0.801○). It is further found that the coplanarity in o-hydoxy-phenyl-imino part is
more remarkable in S1 states of the target dyes (such as C1,
∠
C6-C7-C8-N2, 0.474○). The good
coplanarity in o-hydroxy-phenyl-imino part in S0 and S1 states of C1 is favorable for internal
proton transfer.44
11
10
13
12
1
2
H
9
4
H
1O
1
N
1
2 N
O 2
6
5
8
2
3
7
C1
11
11
10
4
10
4
13
13
12
2
12
2
1
1
2
1
2
H
9
9
1O
1
O 2
6
H
N
N
HN
2 NH
1
5
5
6
1
8
8
3
3
7
7
C4
C7
Scheme 3 The atomic number of C1, C4 and C7 in computation
In contrast, the corresponding dihedral angles of
C7 are much larger than C1 in S0 and S1 states (C4,
S1; C7,
C6-C7-C8-N2, 1.418○ for S0 and 1.342○ for S1), which suggests that the modified
∠CNCC in the reference molecules C4 and
∠
C6-C7-C8-N2, 54.102○ for S0 and 40.133○ for
∠
molecular structure of o-hydoxy-phenyl-imino group lowers the coplanarity, so thus internal
proton transfer could not occur in C4 and C7.
Table 5 The most important dihedral angel (○): ground state and excited state
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Ground state
Excited state
TS*
Structural
Parameters
E
TS
K
E*
K*
∠C6-O2-H2-N2
2.119
-42.280
-2.274
-0.801
0.883
-61.687
-2.274
-0.317
2.626
-1.224
30.139
-2.243
0.433
1.741
-52.200
-2.347
-0.918
4.141
-24.810
-2.305
0.071
4.677
-53.222
-2.278
-1.032
1.135
-6.753
0.359
0.474
8.028
-45.683
0.359
0.282
10.772
0.812
1.843
-0.536
-0.773
0.505
6.832
-27.234
-1.574
2.686
∠C8-N2-C9-C5
∠C10-C11-C12-C13
∠C6-C7-C8-N2
∠C1-O1-H1-N1
∠C3-N1-C4-C5
∠C10-C11-C12-C13
∠C1-C2-C3-N1
∠C6-O2-H2-N2
-0.531
-0.449
0.170
C1,4’-arm
C1,2’-arm
4.628
-32.031
-2.894
1.667
∠C8-N2-C9-C5
∠C10-C11-C12-C13
∠C6-C7-C8-N2
∠C1-O1-H1-N1
∠C3-N1-C4-C5
∠C10-C11-C12-C13
∠C1-C2-C3-N1
∠C8-N2-C9-C5
∠C10-C11-C12-C13
∠C6-C7-C8-N2
-14.925
2.751
-16.452
8.942
C4,4’-arm
C4,2’-arm
54.102
-0.534
9.192
40.133
-0.823
6.463
2.751
8.942
56.959
-42.086
-1.661
1.418
57.297
-15.129
2.302
C7,4’-arm
C7,2’-arm
1.342
∠C3-N1-C4-C5
∠C10-C11-C12-C13
∠C1-C2-C3-N1
-48.733
-1.661
-2.184
-45.506
2.302
-3.509
The dihedral angles of
also analyzed. A large torsion of
transfer in the ground state could be very difficult (such as 4’-arm of C1,
-42.280○). However, the much decreased torsion of
CNCC in S1 state of C1 means the more
possibility of intramolecular proton transfer in the excited state than that in the ground state (such
as 4’-arm of C1,
C8-N2-C9-C5, -6.753○). It is also found that the dihedral angles of
C10-C11-C12-C13 in the excited states of the target dyes tend be to zero degree (such as C1,
C10-C11-C12-C13, 0.359○), which further favors internal proton transfer in S1 states.45
∠
CNCC attached to diphenylethylene part of the target molecules are
CNCC in S0 states of C1 indicates that the internal proton
C8-N2-C9-C5,
∠
∠
∠
∠
∠
∠
In addition, the coplanarity in o-hydoxy-phenyl-imino part 4’-arm is better than that in
24

ACCEPTED MANUSCRIPT
2’-arm in the target dyes (such as C1 in S1 of E form, ∠C6-O2-H2-N2, 1.135○ in 4’-arm,
∠C1-O1-H1-N1, 8.028○ in 2’-arm). These parameters suggest that intramolecular hydrogen bond
in 4’-arm is stronger than that in 2’-arm in the target dyes.
We also notice that H-N distances in two arms of C1 are shorter than those in C3
correspondingly (such as in S1, 4’-arm of C1, 1.891 Å, 4’-arm of C3, 1.753 Å). Furthermore, the
coplanarity in two o-hydoxy-phenyl-imino parts of C1 is better than that in C3 correspondingly
(such as in S1 of E form, 4’-arm of C1, ∠C6-O2-H2-N2, 1.135○, 4’-arm of C3, ∠C6-O2-H2-N2,
2.374○). These results further suggest that intramolecular hydrogen bond in C1 is stronger than
that in C3.
It is further found that H-N distance in 4’-arm of C3 is shorter than that in the dye carrying
single o-hydoxy-phenyl-imino part (such as 2-((3', 4'-dimethoxyl-diphenylethylene-2''-
ylimino)-methyl)phenol, 1.851 Å, in excited of
E form), while the coplanarity in
o-hydoxy-phenyl-imino part in 4’-arm of C3 is greater than that in 2-((3', 4'-
dimethoxyl-diphenylethylene-2''-ylimino)methyl)phenol, ∠C1-O1-H1-N1, 1.302○ , in S1 of E form).
Consequently, the intramolecular hydrogen bond in the target dyes is stronger than that in the ones
including single o-hydoxy-phenyl-imino part.
2.9.2. Frontier orbitals, Mulliken atomic charge and dipole moment
The target and reference molecules display nice π-symmetric characteristic of HOMO and
LUMO orbitals in the ground and excited states (typically shown in Figure 9). In HOMOs of the
target molecules, the electron cloud density is mainly distributed in dye part, while there is a part
of electron cloud density in ortho-hydorx-phenyl-imino group (such as C1, in Figure 9). But as
the target molecules are excited from HOMO to LUMO orbitals, the electron cloud density is
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ACCEPTED MANUSCRIPT
remarkably shifted to ortho-hydorx-phenyl-imino part. The occurrence of electron transfer from
HOMO to LUMO causes the larger charge density distribution in the phenolic ring, which is the
driving force for the internal proton transfer in S1 state of the target molecules.46
C1
HOMO
LUMO
C1*
HOMO
LUMO
Figure 9 Electron cloud density distribution of frontier orbitals in the ground and excited states of enol tautomers
of C1
It is further found that as E is excited to E*, the part of negative charge is transferred from the
hydroxy group to the imino group in these target molecules. As a consequence, the negative
charge in proton donor (O atom) of the target molecules is lowered (such as 4’-arm of C1, in S0,
-0.675 e, in S1, -0.669 e in Table 6), which means the less basicity and the more acidity of hydoxy
group.47 In contrast, the negative charge in proton receptor (N atom) of the target molecules
increases (such as 4’-arm of C1: S0, -0.683 e, S1, -0.742 e), which reflects the less acidity and the
more basicity of imino group.48 Such charge variation is favorable for intramolecular proton
transfer in the excited state of the target dyes.48
We further notice that the negative charge in N atom of the reference molecules reduces
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(such as 4’-arm of C4, -0.665 e, 4’-arm of C7, -0.615 e), which in turn demonstrates that the
internal proton transfer from hydroxy to imino group in the excited states of the target molecules
(such as C1) is much association with the charge variation of N atom.
Table 6 The calculated Mulliken net atomic charges (e) of the key atoms of the molecules C1, C4 and C7
E
TS
K
Mulliken
Charge
C
O
H
N
C
O
H
N
C
O
H
N
0.267
0.272
0.399
0.339
-0.117
-0.116
0.438
0.428
0.327
0.343
-0.109
-0.111
-0.675
-0.673
-0.680
-0.681
0.403
0.403
0.381
0.380
-0.683
-0.685
-0.779
-0.803
-0.561
-0.576
-0.742
-0.709
-0.665
-0.673
-0.615
-0.584
0.535
0.534
-0.785
-0.785
0.525
0.529
-0.815
-0.820
0.562
0.560
-0.698
-0.704
0.393
0.391
-0.776
-0.747
C1,4’-arm
C1,2’-arm
C4,4’-arm
C4,2’-arm
C7,4’-arm
C7,2’-arm
C1,4’-arm
C1,2’-arm
C4,4’-arm
C4,2’-arm
C7,4’-arm
C7,2’-arm
Ground
States
-0.669
-0.670
-0.655
-0.651
0.407
0.408
0.378
0.405
0.314
0.310
-0.777
-0.797
0.545
0.540
-0.880
-0.853
0.287
0.282
-0.724
-0.724
0.407
0.408
-0.796
-0.800
Excited
States
The charge changes of the atoms are the indication of the dipole moment changes of the
entire target molecules. The computation shows that the dipole moments of the transition state
forms are between the enol and keto tautomers in the ground and excited states of the target
molecules (such as the enol of C1, 4’-arm, S1, 2.962 D, the transition state of C1, 4’-arm, S1,
3.265 D, the keto of C1, 4’-arm, S1, 4.127 D), further suggesting the tautomerization of the target
molecules could be required to get over the energy barrier channel. The reduced dipole moments
of the tautomers in the excited states of the target dyes indicate that it is more possible to undergo
intramolecular proton transfer in S1 state than in S0 state (such as the enol of C1, S0, 3.207 D, S1,
2.962 D in Table 7). In contrast, the dipole moments of the reference molecules show the
enhancement in the excited states (such as C4, S0, 2.123 D, S1, 2.610 D, C7, S0, 1.610 D, S1, 2.260
D, in Table 7).
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Table 7 The dipole moment (D) of each tautomer in the ground state and excited state of the molecules C1, C4 and
C7
Ground state
ꢀE ꢀK
Excited state
ꢀE ꢀK
Dipole moment
C1, 4’-arm
C1, 2’-arm
C4
3.207 5.135
3.207 5.590
2.123
2.962 4.127
2.962 4.440
2.610
C7
1.610
2.260
2.9.3 Energy barriers of enol-keto tautomerization of in S0 and S1 states
Since enol-keto tautomerization is typical organic chemical reaction, the potential energy
curves of intramolecular proton transfer of the target molecules are drawn intensive attentions.
The target C1 could not undergo double proton transfer because the optimized transition states of
double phototautomerization could be obtained. Hence, it is considered that single intramolecular
proton transfer could occur in 2’-arm or 4’-arm of C1. The potential energy curves of
intramolecular proton transfer in the either of two o-hydroxy-phenyl-imino groups in S0 and S1
states of C1 are computed respectively.
Figure 10 shows the typical plots of the potential energy curves of intramolecular proton
transfer as a function of O-H distance in S0 and S1 states of the target molecules C1 and C3
respectively. It is obvious that the there are presence of energy barriers to intramolecular proton
transfer in S0 and S1 states of C1 and C3 respectively. This demonstrates that the attached
diphenylethylene part does increase the occurrence difficulty of internal proton transfer of the
target dyes.
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