Synthesis and characterization of new rhodamine dyes with large Stokes shift

Article abstract of DOI:10.1016/j.dyepig.2013.06.013

Two new rhodamine dyes (Rh Q-H, Rh Q-Me) containing 1, 4-diethyl-1, 2, 3, 4-tetrahydroquinoxaline as an effective electron donor are designed and synthesized. The structures of the novel compounds are confirmed by ¹H NMR, ¹³C NMR and ESI. Due to an excited-state intramolecular charge transfer (ICT), the new dyes exhibit longer absorption (>580 nm) and emission (>640 nm) compared with the model compounds, rhodamine 101 and rhodamine 6G. The new rhodamine dyes show large Stokes shift of 40-50 nm in commonly used solvents. Notably, when measured in a mixture of H₂O/EtOH solution, significant stokes shift of 65-68 nm are achieved, which is among the largest Stokes shifts ever reported for rhodamine dyes.

Full text of DOI:10.1016/j.dyepig.2013.06.013

Dyes and Pigments xxx (2013) 1e5
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Short communication
Synthesis and characterization of new rhodamine dyes with large
Stokes shift
*
Zhidan Tian, Baozhu Tian, Jinlong Zhang
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new rhodamine dyes (Rh Q-H, Rh Q-Me) containing 1, 4-diethyl-1, 2, 3, 4-tetrahydroquinoxaline as
an effective electron donor are designed and synthesized. The structures of the novel compounds
are confirmed by 1H NMR, 13C NMR and ESI. Due to an excited-state intramolecular charge transfer
(ICT), the new dyes exhibit longer absorption (>580 nm) and emission (>640 nm) compared with the
model compounds, rhodamine 101 and rhodamine 6G. The new rhodamine dyes show large Stokes shift
of 40e50 nm in commonly used solvents. Notably, when measured in a mixture of H2O/EtOH solution,
significant stokes shift of 65e68 nm are achieved, which is among the largest Stokes shifts ever reported
for rhodamine dyes.
Received 14 March 2013
Accepted 14 June 2013
Available online xxx
Keywords:
Rhodamine
1, 4-Diethyl-1, 2, 3, 4-tetrahydroquinoxaline
Fluorescence
Stokes shift
Ó 2013 Elsevier Ltd. All rights reserved.
Synthesis
Intramolecular charge transfer
1. Introduction
in the order of rhodamine 101 > rhodamine B > rhodamines 6G.
Besides, the absorption and emission bands are considerably shif-
Rhodamine dyes are widely used as laser dyes [1], fluorescence
standards [2], single-molecule imaging agents [3], fluorescent
markers in biological studies [4,5] and chemosensors [6,7] due to
their excellent photophysical properties, such as long absorption
and emission wavelength, high fluorescence quantum yield, large
extinction coefficient and high photostability. The absorption and
emission of the classic rhodamine dyes are only in the range of
500e600 nm and have a small Stokes shift of 20e40 nm. Their
fluorescence detection sensitivity is severely compromised by
background signals caused by biological autofluorescence. This
limitation renders their application for biological imaging in living
systems. Thus, it is highly desirable to develop rhodamine ana-
logues with absorption and emission beyond 600 nm. It is known
ted to the red if tetrafluorophthalic anhydride is used for rhoda-
mine synthesis instead of phthalic anhydride [10]. Moreover,
amidation [11] and esterification [12] of benzoic acid in rhodamine
dyes also result in a red shift in the absorption and emission bands.
Unfortunately, these typical chemical modifications of the rhoda-
mine dyes which lead to red-shifted absorption and emission are
not helpful in increasing the Stokes shift. Design and synthesis of
new rhodamine dyes simultaneously with long emission and large
Stokes shift is a challenging issue.
1, 4-Diethyl-1, 2, 3, 4-tetrahydroquinoxaline has been reported
as an effective electron donor in styryl [13], coumarin [14] and
squaraine dyes [15], which surprisingly shifts their absorption and
emission to longer wavelength and greatly increases their thermal
stability and Stokes shift. In this work, we present the synthesis
of two new rhodamine dyes based on 1, 4-diethyl-1, 2, 3, 4-
tetrahydroquinoxaline. Their UVevis absorption and fluorescence
emission spectroscopic behavior in different solvents have been
investigated. Compared with the traditional rhodamine dyes, these
new dyes exhibit unique properties.
that the addition of p-conjugation system, as well as the formation
of rigid rings in organic dyes can shift their absorption and emission
maxima to longer wavelength. Based on this hypothesis, some
groups have reported a novel class of highly fluorescent rhodamine
derivatives with absorption beyond 600 nm [8,9]. In another
important respect, the substituents on the amino group profoundly
affect the absorption and emission wavelength of the traditional
rhodamine dyes. Rhodamine with strong electron-donating amino
group has pronounced bathochromic shift of absorption and
emission bands, the emission wavelengths of these classic dyes are
2. Experimental
2.1. Chemicals and instruments
4-Methoxy-2-nitroaniline was purchased from Aladdin. The
other chemicals were of the highest grade available and were used
* Corresponding author. Tel./fax: þ86 21 64252062.
E-mail address: jlzhang@ecust.edu.cn (J. Zhang).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.013
Please cite this article in press as: Tian Z, et al., Synthesis and characterization of new rhodamine dyes with large Stokes shift, Dyes and Pigments
(2013), http://dx.doi.org/10.1016/j.dyepig.2013.06.013

2
Z. Tian et al. / Dyes and Pigments xxx (2013) 1e5
without further purification. All employed solvents were analyti-
cally pure and were employed without any further drying or
purification.
2.2.3. Synthesis of 1, 4-diethyl-6-hydroxy-1, 2, 3, 4-
tetrahydroquinoxalin (5)
1, 4-diethyl-6-methoxy-1, 2, 3, 4-tetrahydroquinoxaline
4
Reactions were monitored by TLC. Flash chromatography sepa-
rations were carried out using silica gel (200e300 mesh). 1H NMR
and 13C NMR spectra were recorded on Brucker AM-400 MHz in-
struments with tetramethylsilane as internal standard. ESI was
performed using a Waters LCT Premier XE spectrometer. Absorp-
tion spectra were carried out on an SHIMADZU UVeVis spectro-
photometer. Fluorescence spectra were measured on an SHIMADZU
RF-5301PC Fluorescence spectrophotometer.
(2.20 g, 0.01 mol), acetic acid (10 mL), hydrobromic acid (45%,
5 mL) were mixed, and then the ensuing mixture was stirred
under an inert atmosphere at reflux for 6 h. The solvent was
evaporated under reduced pressure. The crude product was un-
stable and directly used for the next step without further
purification.
2.2.4. Synthesis of Rh Q-H
1, 4-diethyl-6-hydroxy-1, 2, 3, 4-tetrahydroquinoxalin 5 (1.03 g,
5 mmol), ZnCl2 (0.68 g, 5 mmol) and phthalic anhydride (0.74 g,
5 mmol) were heated at 160 ꢀC for 2 h. After cooling, the reaction
mixture was dissolved in DMF (10 mL), and then the mixture was
added dropwise into the water (50 mL (10% NaCl, 1% HCl) w%) while
stirring. The resulting dark precipitate was collected and dried in
vacuo to give crude product. Rh Q-H furnished as a dark powder
was purified by silica gel chromatography (DCM/MeOH ¼ 20:1, V/
V), and then recrystallized from ethanol to get golden crystals
2.2. Synthesis
The synthesis of target compounds, Rh Q-H and Rh Q-Me were
achieved by the route outlined in Scheme 1.
2.2.1. Synthesis of 6-methoxyquinoxaline (3)
4-Methoxy-2-nitroaniline 1 (6.70 g, 25 mmol) and Raney Nickel
(1.00 g) in methanol (140 mL) were mixed and heated to 60 ꢀC.
After that, hydrazinehydrate (85%, 8.00 mL) was added dropwise
into the solution within 30 min. The reaction mixture was then
stirred at 60 ꢀC for 2 h. After cooling, the reaction mass was filtered
to separate the catalyst and then the filtrate concentrated with a
rotavapor to get 4-Methoxy-1, 2-phenylenediamine 2. Compound 2
dissolved in acetonitrile (100 mL) and glyoxal (40%, 13.00 mL) was
added to this solution. The reaction mixture was then stirred at
60 ꢀC for 6 h and cooled. The solvent was removed in a rotary
evaporator and the dark brown sticky solid obtained was purified
by silica gel chromatography (EtOAc) to get white crystals (5.40 g,
85%), m.p. 59e61 ꢀC.
(0.25 g, 17.8% yield). 1H NMR (400 MHz, DMSO-d6, TMS):
d 8.12 (s,
1H), 7.64 (m, 2H), 7.23 (d, J ¼ 6.56 Hz, 1H), 6.87 (s, 2H), 5.93 (s, 2H),
3.60 (m, 8H), 3.24 (m, 4H), 3.01 (m, 4H),1.21 (t, J ¼ 6.99 Hz, 6H), 0.86
(t, J ¼ 6.70 Hz, 6H). 13C NMR (100 MHz, DMSO-d6):
d 151.63, 144.58,
133.96, 130.57, 130.05, 129.08, 114.30, 103.05, 93.97, 47.11, 46.08,
44.77, 43.64, 10.46, 8.98. ESI found 525.3[M ꢁ Cl]þ calculated for
C
32H37N4Oþ3 : 525.29.
2.2.5. Synthesis of Rh Q-Me
Rh Q-H (0.10 g, 0.19 mmol), methanol (3 mL) and 0.1 mL
H2SO4 were refluxed under an inert atmosphere for 24 h. After
cooling, the reaction mixture was added dropwise into the
water (20 mL, 10% NaCl w%) while stirring. The resulting
golden precipitate was collected and dried in vacuo to give the
pure product (0.08 g, 78.0%). 1H NMR (400 MHz, DMSO-d6,
2.2.2. Synthesis of 1, 4-diethyl-6-methoxy-1, 2, 3, 4-
tetrahydroquinoxaline (4)
1, 4-diethyl-6-methoxy-1, 2, 3, 4-tetrahydroquinoxaline 4 was
synthesized according to reported method [16].
TMS):
d
8.23 (d, J ¼ 7.01 Hz, 1H), 7.91 (t, J ¼ 7.52 Hz, 1H),
Scheme 1. Synthetic route for Rh Q-H and Rh Q-Me.
Please cite this article in press as: Tian Z, et al., Synthesis and characterization of new rhodamine dyes with large Stokes shift, Dyes and Pigments
(2013), http://dx.doi.org/10.1016/j.dyepig.2013.06.013

Z. Tian et al. / Dyes and Pigments xxx (2013) 1e5
3
3. Results and discussion
3.1. UVeVis absorption and fluorescence spectra
The UVevis absorption spectra of Rh Q-H and Rh Q-Me in EtOH
are studied (Fig. 1). Close examination reveals that the shape of the
absorption spectrum of Rh Q-H is highly resemble to that of
rhodamine 6G and rhodamine 101. The intense absorption band
with maximum at 597 nm of Rh Q-H is assigned to the 0e0 band of
the S1)S0 transition, and a (less pronounced) shoulder peak on
the high-energy side (at around 560 nm) is attributed to the 0e1
vibrational band of the same transition. Rh Q-Me shows an intense
absorption band with maximum at 613 nm, which is assigned to
the 0e0 band of the S1)S0 transition. A less pronounced shoulder
peak on the high-energy side at around 570 nm is also observed,
which is attributed to the 0e1 vibrational band of the same tran-
sition. Compared to Rh Q-H, although the absorption spectrum is
similar, the absorption of Rh Q-Me shows 15 nm red shift, which is
attributed to the esterification of benzoic acid. Rh Q-H and Rh Q-
Me show a more red-shifted absorption than rhodamine 101 and
rhodamine 6G. Rh Q-H shows absorption maxima at 597 nm,
which is 33 nm red-shifted compared with rhodamine 101.
The absorption wavelength of rhodamine dyes is highly dependent
on the substituents on the amino group, and the absorption
maxima is also increased with an increasing electron-donating
ability. In Rh Q-H and Rh Q-Me, the electron-donating ability of
the 1, 2, 3, 4-tetrahydroquinoxaline substituent is much stronger
than julolidine.
Fig. 1. The normalized UVeVis absorption spectra of Rh Q-H, Rh Q-Me, rhodamine 101
and rhodamine 6G (ca. 10ꢁ5 mol/L) in EtOH.
The fluorescence emission profiles of Rh Q-H and Rh Q-Me in
EtOH are shown in Fig. 2. Rh Q-H exhibits an intense emission peak
with maximum at 644 nm, which extends to an onset of 780 nm.
Compared with rhodamine 101, the emission spectrum of Rh Q-H is
drastically bathochromic shifted for 50 nm while the spectrum
shape is maintaining the same. Similar emission bands are
observed for Rh Q-Me and maximum peak is 656 nm and further
extends to 790 nm. Both Rh Q-H and Rh Q-Me show a more red-
shifted absorption than rhodamine 101, which is attributed to a
much stronger electron-donating ability of the 1, 2, 3, 4-
tetrahydroquinoxaline substituent than julolidine.
Table 1 shows the Stokes shifts of Rh Q-H, Rh Q-Me, rhodamine
101 and rhodamine 6G. The traditional rhodamine dyes have small
Stokes shifts, as shown in Table 1, 30 nm for rhodamine 101 and
28 nm for rhodamine 6G. The Stokes shifts of Rh Q-H and Rh Q-Me
are increased to 47 nm and 43 nm respectively. This might be
attributed to an excited-state intramolecular charge transfer (ICT).
ICT effect is greatly enhanced in these new rhodamine dyes because
of the strong electron donors. To the best of our knowledge, this is
among the largest Stokes shifts ever reported for the rhodamine
dyes, without resorting to the FRET effect [17,18] and TBET effect
[19,20].
Fig. 2. The normalized fluorescence emission spectra of Rh Q-H, Rh Q-Me, rhodamine
101 and rhodamine 6G (ca. 10ꢁ5 mol/L) in EtOH excited with 597 nm, 613 nm, 594 nm,
527 nm respectively.
7.82 (t, J ¼ 7.66 Hz, 1H), 7.48 (d, J ¼ 7.56 Hz, 1H), 6.78 (s, 2H),
5.84 (s, 2H), 3.66 (m, 8H), 3.33(s, 3H), 3.29 (m, 4H), 3.06
(m, 4H), 1.24 (t, J ¼ 7.05 Hz, 6H), 0.88 (t, J ¼ 7.00 Hz, 6H).
13C NMR (100 MHz, DMSO-d6):
d 165.34, 151.80, 149.26,
144.95, 134.43, 134.34, 133.07, 130.56, 130.48, 130.09, 129.72,
114.20, 101.48, 94.18, 52.26, 47.09, 46.29, 44.74, 43.62, 10.52,
8.60. ESI found 539.3[M ꢁ Cl]þ calculated for C33H39N4O3þ:
539.29.
Table 1
lab and lem values of Rh Q-H, Rh Q-Me, rhodamine 101 and rhodamine 6G in various solvents.
Solvent
Rh Q-H
Rh Q-Me
Rhodamine 101
Rhodamine 6G
lab (nm)
lem (nm)
Dl (nm)a
lab (nm)
lem (nm)
Dl (nm)
lab (nm)
lem (nm)
Dl (nm)
lab (nm)
lem (nm)
Dl (nm)
CH2Cl2
MeCN
EtOH
MeOH
DMF
603
604
597
598
601
603
588
652
653
644
645
651
652
656
49
49
47
47
50
49
68
616
614
613
612
622
626
597
655
660
656
656
664
668
662
39
46
43
44
42
42
65
578
576
564
565
573
585
574
603
603
594
592
588
612
601
25
27
30
27
15
27
15
520
523
527
525
532
537
525
546
552
555
555
562
567
555
26
28
28
30
30
30
30
DMSO
H2O/EtOH
a
Dl(nm) ¼ Stokes shift.
Please cite this article in press as: Tian Z, et al., Synthesis and characterization of new rhodamine dyes with large Stokes shift, Dyes and Pigments
(2013), http://dx.doi.org/10.1016/j.dyepig.2013.06.013

4
Z. Tian et al. / Dyes and Pigments xxx (2013) 1e5
Fig. 4. UVevis absorption (a) and fluorescence emission spectra (b) of Rh Q-Me (ca.
Fig. 3. UVevis absorption (a) and fluorescence emission spectra (b) of Rh Q-H (ca.
10ꢁ5 mol/L) in different solvents.
10ꢁ5 mol/L) in different solvents.
3.2. Polarity sensitivity of the absorption and fluorescence
other solvents. The emission can be quenched by the ICT effect,
which is more significant in polar solvents. The Stokes shift of Rh Q-
Me is 3e10 nm smaller than that of Rh Q-H, but the Stokes shift still
have 65 nm in H2O/EtOH.
The absorption and fluorescence emission profiles of Rh Q-H in
distinct solvents (CH2Cl2, MeCN, EtOH, MeOH, H2O/EtOH (9/1, V/V))
are shown in Fig. 3. The absorption and emission of Rh Q-H depend
greatly on the polarity of solvents. In DMF, Rh Q-H shows very low
absorption and fluorescence for parts of Rh Q-H molecules transfer
into spirocyclic form which is essentially colorless and nonfluo-
rescent. The transformation mechanism in DMSO consists with that
in DMF, but to a less degree than in DMF. In water solution, Rh Q-H
shows a strong absorption but very weak fluorescence, while in
EtOH and MeOH, Rh Q-H shows stronger absorption and fluores-
cence than those in other solvents. These properties might be due
to hydrogen-bonding interaction between the solvents and the dye
molecules [21]. The Stokes shift in these solutions is 47e50 nm
except for H2O/EtOH in which 68 nm shift is achieved. Collectively,
it proved that an excited-state intramolecular charge transfer (ICT)
should take place in this case.
4. Conclusions
We have synthesized and characterized two new rhodamine
dyes (Rh Q-H, Rh Q-Me). This investigation constitutes the first use
of 1, 4-diethyl-1, 2, 3, 4-tetrahydroquinoxaline as an amino aux-
ochrome group in rhodamine dyes. These dyes show more excel-
lent photophysical performance than the traditional rhodamine
dyes with both absorption and emission in the long region. Espe-
cially, the Stokes shifts of these new dyes are 40e68 nm in different
solvents, which are significantly larger than those of the traditional
rhodamine dyes. The introduction of the two amino derivatives in
the same benzene of the rhodamine is an effective method to
change the character of the dyes.
The absorption and fluorescence emission profiles of Rh Q-Me in
distinct solvents (CH2Cl2, MeCN, EtOH, MeOH, H2O/EtOH (9/1, V/V))
are also studied (Fig. 4). Different from Rh Q-H, the absorption of Rh
Q-Me is not sensitive to the polarity of the solution. In particular, Rh
Q-Me still keep high absorption in DMF and DMSO, due to that the
esterification of benzoic acid restrains rhodamine change into
spirocyclic form which is essentially colorless. Extinction coefficient
and fluorescence intensity of Rh Q-Me are higher than those of Rh
Q-H, which could be attributed to the esterification of benzoic acid.
Fluorescence intensity in DMF, MDSO, H2O/EtOH is lower than in
Acknowledgements
This work was supported by Science and Technology Commis-
sion of Shanghai Municipality (11142201100, 10520709900), and
the Fundamental Research Funds for the Central Universities.
References
[1] Drexhage KH. What’s ahead in laser dyes. Laser Focus 1973;9:35e9.
Please cite this article in press as: Tian Z, et al., Synthesis and characterization of new rhodamine dyes with large Stokes shift, Dyes and Pigments
(2013), http://dx.doi.org/10.1016/j.dyepig.2013.06.013

Z. Tian et al. / Dyes and Pigments xxx (2013) 1e5
5
[2] Karstens T, Kobs K. Rhodamine B and rhodamine 101 as reference sub-
[13] Satam V, Rajule R, Bendre S, Bineesh P, Kanetkar V. Synthesis and application
of novel styryl dyes derived from 1, 4-diethyl-1, 2, 3, 4-tetrahydro-6-
methoxy-quinoxaline. J Heterocyclic Chem 2009;46:221e5.
stances for fluorescence quantum yield measurements.
1980;84:1871e2.
J Phys Chem
[3] Bossi M, Folling J, Belov VN, Boyarskiy VP, Medda R, Egner A, et al. Multicolor
far-field fluorescence nanoscopy through isolated detection of distinct mo-
lecular species. Nano Lett 2008;8:2463e8.
[4] Zhou ZG, Yu MX, Yang H, Huang KW, Li FY, Yi T, et al. FRET-based sensor for
imaging chromium(III) in living cells. Chem Commun 2008:3387e9.
[5] Liu W, Howarth M, Greytak AB, Zheng Y, Nocera DG, Ting AY, et al. Compact
biocompatible quantum dots functionalized for cellular imaging. J Am Chem
Soc 2008;130:1274e84.
[6] Dujols V, Ford F, Czarnik AW. A long-wavelength fluorescent chemodosimeter
selective for Cu (II) ion in water. J Am Chem Soc 1997;119:7386e7.
[7] Beija M, Afonso CAM, Martinho JMC. Synthesis and applications of rhodamine
derivatives as fluorescent probes. Chem Soc Rev 2009;38:2410e33.
[8] Liu JX, Diwu ZJ, Leung WY, Lu Y, Patcha B, Hauglanda RP. Rational design and
synthesis of a novel class of highly fluorescent rhodamine dyes that have
strong absorption at long wavelengths. Tetrahedron Lett 2003;44:4355e9.
[9] David E, Lejeune J, Pellet-Rostaing S, Schulz J, Lemaire M, Chauvin J, et al.
Synthesis of fluorescent rhodamine dyes using an extension of the Heck re-
action. Tetrahedron Lett 2008;49:1860e4.
[14] Jagtap AR, Satam VS, Rajule RN, Kanetkar VR. Synthesis of highly fluorescent
coumarinyl chalcones derived from 8-acetyl-1, 4-diethyl-1, 2, 3, 4-tetrahydro-
7H-pyrano-[2, 3-g]quinoxalin-7-one and their spectral characteristics. Dyes
Pigm 2011;91:20e5.
[15] Chandrasekaran Y, Dutta GK, Kanth RB, Patil S. Tetrahydroquinoxaline
based squaraines: synthesis and photophysical properties. Dyes Pigm
2009;83:162e7.
[16] Jagtap AR, Satam VS, Rajule RN, Kanetkar VR. The synthesis and character-
ization of novel coumarin dyes derived from 1, 4-diethyl-1, 2, 3, 4-tetrahydro-
7-hydroxy-quinoxalin-6-carboxaldehyde. Dyes Pigm 2009;82:84e9.
[17] Long LL, Lin WY, Chen BB, Gao WS, Lin Y. Construction of a FRET-based
ratiometric fluorescent thiol probe. Chem Commun 2011;47:893e5.
[18] Yu HB, Xiao Y, Guo HY, Qian XH. Convenient and efficient FRET platform
featuring a rigid biphenyl spacer between rhodamine and BODIPY: trans-
formation of ‘Turn-On’ sensors into ratiometric ones with dual emission.
Chem-Eur J 2011;7(11):3179e91.
[19] Lin WY, Yuan L, Cao ZM, Feng YM, Song JZ, Song JZ. Through-bond energy
transfer cassettes with minimal spectral overlap between the donor emission
and acceptor absorption: coumarin-rhodamine dyads with large pseudo-
Stokes shifts and emission shifts. Angew Chem Int Ed 2010;49(2):375e9.
[20] Jiao GS, Thoresen LH, Burgess K. Fluorescent, through-bond energy transfer
cassettes for labeling multiple biological molecules in one experiment. J Am
Chem Soc 2003;125(48):14,668e14,669.
[10] Nakata E, Nazumi Y, Yukimachi Y, Uto Y, Maezawa, Hashimoto T, et al. Syn-
thesis and photophysical properties of new SNARF derivatives as dual emis-
sion pH sensors. Bioorg Med Chem Lett 2011;21:1663e6.
[11] Nguyen T, Francis MB. Practical synthetic route to functionalized rhodamine
dyes. Org Lett 2003;5(18):3245e8.
[12] Chen X, Wu QZ, Henschke L, Weber G, Weil T. An efficient and versatile
approach for the preparation of a rhodamine B ester bioprobe library. Dyes
Pigm 2012;94:296e303.
[21] Peng XJ, Song FL, Lu EH, Wang YN, Zhou W, Fan JL, et al. Heptamethine cyanine
dyes with a large Stokes shift and strong fluorescence: a paradigm for excited-
state intramolecular charge transfer. J Am Chem Soc 2005;127:4170e1.
Please cite this article in press as: Tian Z, et al., Synthesis and characterization of new rhodamine dyes with large Stokes shift, Dyes and Pigments
(2013), http://dx.doi.org/10.1016/j.dyepig.2013.06.013