Dipicolinate as acceptor in D-π-A chromophores: synthesis, characterization and fluorescence following single- and two-photon excitation

Article abstract of DOI:10.1016/j.tetlet.2008.09.073

Three novel donor-π-acceptor type compounds with dipicolinate as acceptor have been synthesized. Their absorption, photoluminescence as well as their two-photon absorption properties have been investigated. Two of them show strong two-photon absorption and two-photon excited up-conversion fluorescence.

Full text of DOI:10.1016/j.tetlet.2008.09.073

Tetrahedron Letters 49 (2008) 6819–6822
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Dipicolinate as acceptor in D–p–A chromophores: synthesis, characterization
and fluorescence following single- and two-photon excitation
a
b
b
a
a
Haibo Xiao a, , Xiaoming Tao , Yaochuan Wang , Shixiong Qian , Guanghao Shi , Hui Li
*
a Department of Chemistry, Shanghai Normal University, Shanghai 200234, PR China
b Department of Physics, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2008
Revised 5 September 2008
Accepted 12 September 2008
Available online 17 September 2008
Three novel donor–
p
–acceptor type compounds with dipicolinate as acceptor have been synthesized.
Their absorption, photoluminescence as well as their two-photon absorption properties have been inves-
tigated. Two of them show strong two-photon absorption and two-photon excited up-conversion
fluorescence.
Ó 2008 Elsevier Ltd. All rights reserved.
Research into molecular two-photon absorption (TPA) has
received increasing attention owing to numerous applications in
various areas such as three-dimensional optical data storage,
up-conversion lasing, optical power limiting, microfabrication,
photodynamic therapy and two-photon laser scanning fluores-
cence imaging.1–4 Among them, two-photon-excited fluorescence
(TPEF) has gained widespread popularity in the biology community
owing to its various advantages. The main advantage is that inci-
dent wavelengths are located in the region of infrared (700–
1200 nm). In this spectral domain, the transparency of biological
samples limits absorption and diffusion by the media and thus
b
a
N
2
N
3
N
H3C
CH3 HOOC
COOH H3COOC
COOCH3
1
CHO
CH2OH
c
d
N
N
H3COOC
COOCH3
H3COOC
COOCH3
H3COOC
5
4
Ph
Ph
N
allows deeper probing (500
tonic laser excitation is confined in a small focal volume (1
l
m). In addition, the bi- or multipho-
-I,+Ph3P
-I,+Ph3P
H2N
Ph
Ph
N
l
m3),
N
e
f
H3COOC
H3COOC
which permits 3D resolved imaging while avoiding global irradia-
tion of the sample and thus limiting photodamage to the tissue and
photobleaching of the probe. The sensitization of LnIII by two-pho-
6
7
CH3
CH3
N
N
5
CH3
CH3
ton absorption ligand may lead to high-purity LnIII (Eu3+, Er3+, Nd3+
,
N
Tb3+) emission and less-harmful, deep-penetrating bioimaging
applications.5–7 Upon the coordination of TPA dyes to metal ions,
the mechanical properties and stability in environment of two-
photon absorption materials can be improved.8–13
H3COOC
H3COOC
CH3
CH3
N
g
N
CH3
CH3
N
N
Pyridine-2,6-dicarboxylic(dipicolinic) acid and its derivatives
are highly useful tridentate ligands, they can form nine-coordinat-
ing complexes with lanthanides.14–17 To the best of our knowledge,
there has not been any report on dipicolinate and its derivatives as
H3COOC
8
Scheme 1. Reagents and conditions: (a) KMnO4, H2O, reflux, 4 h, 80%; (b) CH3OH,
H2SO4, reflux, 20 h, 85%; (c) CH3OH, H2O2, FeSO4, l5–30 °C, 1 h, 30%; (d) CrO3,
pyridine, CH2Cl2, 25–30 °C,5 h, 75%; (e) NaH, THF, rt, 24 h, 50%; (f) NaH, THF, rt,
24 h, 45%; (g) CH3OH, reflux, 4 h, 85%.
an acceptor in D–
synthesis (Scheme 1) and the optical properties (absorption, fluo-
rescence and TPA) of a serial of D– –A chromophores containing
p–A chromophores. In this Letter, we describe the
p
dipicolinates. The arylamine group in chromophore functions as
an electron donor (D), the dipicolinate moiety acts as an electron
acceptor (A) and C@C or C@N double bond as conjugation bridge.
character, which is known to be the structural basis for an efficient
two-photon absorber.18 The structures were determined by 1H
NMR, mass spectrometry, and IR spectrum.19
Therefore, chromophore is a polar molecule that shows D–p–A
The normalized UV–vis absorption spectra of objective com-
pounds 6–8 in CH2Cl2 solutions at a concentration of around
1 ꢀ 10ꢁ5 M are shown in Figure 1. The maximum peaks of one pho-
* Corresponding author.
E-mail address: xiaohb@shnu.edu.cn (H. Xiao).
ton absorption corresponding to p-conjugated structures are at
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.073

6820
H. Xiao et al. / Tetrahedron Letters 49 (2008) 6819–6822
redistribution occurring upon excitation, prior to emission, and
potentially large TPA cross sections.1 Comparing 6 and 7, by chang-
ing the arylamino in 6 with alkylamino, there is a 12 nm blue-shift
in emission spectra. Comparing the structures of 7 and 8, it can be
seen that they are only different from the conjugated bridge. Com-
pound 8 shows very weak emission. The fluorescence quench of 8
may be attributed to the nitrogen atom in the conjugated bridge:
(i) The C@N double bond is a polar bond, which may have an effect
of reducing charge transfer from donor to acceptor and reduce the
molecular polarity. (ii) photoisomerization is one source of nonra-
diative deactivation.23 The C@N double bond can be isomerized to
the higher-energy and less-stable isomer by irradiating with UV–
vis light because of the presence of lone electron pair in nitrogen
atom. This photoreaction competes with fluorescence.23
The TPA induced emission spectra of 10ꢁ3 M solutions of 6 and
7 in CH2Cl2 were detected. When the solutions are pumped with a
laser pulse at 800 nm, strong fluorescence emission can be de-
tected. The kmax of the two-photon induced fluorescence for 6
and 7 are at ca. 569 and 551 nm, respectively. The shape are similar
to that data obtained by one-photon excitation in the same sol-
vents (Fig. 3). The interesting feature is again the solvatochromism
that occurs even on two-photon excitation.
Figure 1. Absorption spectra of compounds 6–8 in CH2Cl2 solutions (around
1 ꢀ 10ꢁ5 M).
But these fluorescences are red-shifted compared with the one-
photon induced fluorescences. The fluorescence spectra in the
short wavelength sides and the one-photon absorption spectra in
the long wavelength sides overlap. For one-photon absorption in-
duced emission measurements, we used dilute solutions, thus
the reabsorption of the fluorescence within the sample can be ne-
glected. In the case of two-photon excitation, concentrated solu-
tions were used. Since the 800 nm laser beam can pass through
the whole solution length without depletion, the fluorescence
emission is not only from the surface layer, but also from inside
the solution sample. The reabsorption of the shorter wavelength
fluorescence by the concentrated sample can no longer be ne-
glected. The blue side of the two-photon fluorescence was reab-
sorbed by the solution and red-shifts of the fluorescence spectra
were observed.22 Tian et al.24 reported the synthesis and photo-
physical properties of 4-[4-(diethylamino)styryl]pyridine (DEASP).
We note that the absorption and the fluorescence maximal wave-
lengths of compound 7 show bathochromic shifts and the Stokes
shift increases in comparison to DEASP (Table 1). This spectro-
scopic behavior reflects the fact that the presence of two elec-
tron-withdrawing methoxycarbonyl groups in pyridine ring
413 nm for compound 6 and at 410 nm for compound 7. Note that
the kmax of 7 appears at a shorter wavelength than that of 6. The
result could be explained by noting that the lone pair electrons
on the nitrogen of 6 may have delocalized onto the terminal phen-
yls.20 The maximum UV spectral absorption of compound 8 shifts
from 410 nm of 7 to 445 nm when the conjugation bridge varies
from C@C to C@N double bond, hinting that the p-electron conju-
gation bridge has significant effect on their ground state electron
absorption spectra of molecules and compound 8 possesses larger
p
-electron delocalized degree than 7.21 Simultaneously, we found
in Figure 1 that compounds 6–8 show absorptions in less than
300 nm wavelength, which are assigned to the n–
p
* transition. Sig-
nificantly, compounds 6–8 show no linear absorption in the wave-
length range from 500 to 1000 nm. Therefore, any emission
induced by excitation at this wavelength range must be attributed
to a multi-photon absorption process.22
Compounds 6 and 7 exhibit green fluorescence in CH2Cl2. As
illustrated in Figure 2 for compound 6, a pronounced positive sol-
vatochromism is observed in emission. A parallel increase of the
Stokes shift and of the half bandwidth of the fluorescence spectra
is observed. Such behaviour is indicative of significant charge
Figure 2. Normalized fluorescence emission spectra of compound 6 in solvents of
different polarity.
Figure 3. One-photon fluorescence and two-photon fluorescence spectra of com-
pound 6.

H. Xiao et al. / Tetrahedron Letters 49 (2008) 6819–6822
6821
Table 1
It is also interesting to note that , value increase upon replacing
dimethylamino group in compound 7 with diphenylamino group
(compound 6). The result could be explained by noting that20 (i)
NPh2 has a better cation stabilizing ability and (ii) the additional
degree of spatial delocalization of the mobile electrons for 6 is pos-
sible. To elaborate point (i), it should be noted that the diphenyl-
amino group is a much weaker electron donor than the former,
as indicated by the smaller pKa value of Ph2NH (0.9) than Et2NH
(10.8) and by the more positive oxidation potential. Hence, the
ground state of 6 would be located at slightly lower energy than
7. On the other hand, a larger stabilization of the first excited state
would be provided by the diphenylamino group, in which the po-
sitive charge can be stabilized. Consequently, the energy gap be-
tween the ground and the first excited state would be decreased.
Consistent with this interpretation is the larger Stokes shift ob-
served for 6 (Table 1). This would enhance the intramolecular
charge transfer from the donor to the accepter to increase , value.
The work of compounds 6 and 7 chelating with lanthanide ions
is in progress.
Linear absorption and emission parameters of compounds 6, 7 and DEASP in CH2Cl2
kc
ud
kmax (TPEF)/nm
a
b
e
Compd
kmax (abs)/nm
kmax (fl)/nm
D
6
7
299, 413
290, 410
267, 376.5
544
532
464
131
122
87.5
0.85
0.63
569
551
DEASP
480 (in DMF)
a
kmax of the one-photon absorption spectra in nm.
kmax of the one-photon emission spectra in nm.
Stokes shift.
Fluorescence quantum yield.
kmax of the two-photon absorption spectra in nm.
b
c
d
e
enhances the acceptor strength and the acceptor stabilizes the
lowest excited state more than the ground state.8
The TPA cross sections of molecules 6 and 7 have been mea-
sured by open aperture Z-scan experiments performed with a fem-
tosecond (fs) laser source. Figure 4 shows the Z-scan data of dyes 6
and 7 in DMSO, measured in a 1 mm cell, with 0.749 lJ pulse. In
Figure 4, the normalized transmittance (i.e., I(z)/I(1), with I(1)
being the linearly transmitted intensity far from the focal plane)
is reported as a function of the sample position (z). Compounds 6
and 7 show deep dip typical of nonlinear absorption. Table 2 re-
ports the fs TPA cross-section coefficients of compounds 6 and 7
compared to two representative literature examples measured
with fs pulses. N,N-diphenyl-7-[2-(4-pyridl)ethenyl]-9,9-di-n-de-
cyl-9H-fluoren-2-amine (AF50) and (7-(7-benzothiazol-2-yl-9,9-
diethylfluoren-2-yl)-9,9-diethylfluoren-2-yl)diphenylamine (AF250)
are two of the most representative TPA dyes so far reported in
the literature.25 It can be seen that the TPA cross-section coefficient
of 6 is larger than AF50 and 7 is larger than AF250. However, it
should be considered that the TPA measurements of 6 and 7 have
been carried out at the nonoptimized 800 nm wavelength position.
An enhancement of , value is expected at optimized parameters.25
Acknowledgments
This work was financially supported by Shanghai Municipal
Education Committee (No. 06DZ010) and Shanghai Normal Univer-
sity (No. SK200839). H.B. Xiao thanks Professor J.Y. Chen (Depart-
ment of Physics, Fudan University, Shanghai/China) for his help
in the measurements of the two-photon fluorescent spectra.
References and notes
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17. Shelkov, R.; Melman, A. Eur. J. Org. Chem. 2005, 1397–1401.
18. Albota, M.; Beljonne, D.; Bredas, J.-L., et al. Science 1998, 281, 1653–1656.
19. Compound 5: A mixture of CrO3 (1 g, 1mmo1), pyridine (1.58 g, 2 mmo1) and
CH2Cl2 (100 ml) was stirred at room temperature for 20 min. A solution of 4
(0.45 g, 2 mmol) in CH2C12 (10 ml) was added dropwise over 30 min at 25–
30 °C. The mixture was stirred for 5 h at room temperature. The organic phase
was washed with 1 mol/L HC1, water successively and dried over MgSO4. The
crude product was purified by silica gel column chromatography eluting with
petroleum/ethyl acetate to afford 5 (0.33 g), yield 75%. Mp: 186–188 °C. 1H
NMR (CDC13, d ppm):10.2 (s, 1H, CHO), 8.7 (s, 2H), 4.1 (s, 6H, OCH3). EI-MS: m/z
223 (M+).
Figure 4. Z-Scan experimental data of compounds 6(a) and 7(b) in DMSO (10ꢁ2 M).
Table 2
TPA cross-section coefficient of compounds 6 and 7 in DMSO is compared with those
of different compounds in the literature data
Compound 6: To
a 50 ml two-necked flask, 0.115g 5 (0.5 mmol), 0.323 g
phosphonium salt (0.6 mmol) and 100 ml anhydrous THF were added under a
nitrogen atmosphere. The reaction mixture was cooled to 0 °C in an ice bath. A
THF solution (15 ml) of 0.024 g NaH was dropped into the flask. After the
reaction mixture was stirred for 24 h at room temperature, the mixture was
washed with water. The organic phase was dried over MgSO4, then evaporated
to give yellow solid. The crude product was purified by chromatograph on
silica gel, using CH2Cl2/hexane as the eluent to give pure product in 50% yield
(119 mg). Mp 193–195 °C. IR (KBr) 2953.9, 2925.3, 2851.7, 1711.6, 1585.0,
Compd
Pulse laser
r [GM]
k (nm)
Fwhm (fs)
6
7
800
800
796
796
140
140
150
150
35
23
22
30
AF50
AF250
1491, 1356, 1278.5, 1249.9, 1115.1, 992.5, 967.9, 759.6, 694.2 cmꢁ1 1H NMR
.

6822
H. Xiao et al. / Tetrahedron Letters 49 (2008) 6819–6822
2806.8, 1752.5, 1719.8, 1625.8, 1576.8, 1442.0, 1339.8, 1245.8, 1204.9, 1160.0,
(CDC13, d ppm): 8.35(s, 2H), 7.4–7.46 (m, 2H), 7.28–7.34 (m, 4H), 7.15 (d,
J = 8.4 Hz, 4H), 7.03–7.13 (m, 4H), 6.95–7.0 (d, J = 16.8 Hz, 2H), 4.0 (s, 6H). EI-
996.6, 816.8, 780.0 cmꢁ1 1H NMR (CDC13, d ppm): 8.8 (s, 2H), 8.6 (s, 1H), 7.4 (d,
.
MS: m/z 464 (M+).
J = 8.8 Hz, 4H), 4.1 (s, 6H), 3.1 (s, 6H). EI-MS: m/z 341 (M+).
20. Cho, B. R.; Son, K. H.; Lee, S. H., et al. J. Am. Chem. Soc. 2001, 123, 10039–
10045.
Compound 7: Use the same method of 6, 45%, mp 200–202 °C. IR (KBr): 2953.9,
2921.2, 2855.8, 2802.7, 1752.5, 1711.6, 1609.5, 1589.1, 1446.1, 1258.1, 1237.7,
1184.6, 1155.9, 992.5, 955.7, 804.5 cmꢁ1
.
1H NMR (CDC13, d ppm): 8.35 (s, 2H),
21. Guang, S. Y.; Yin, S. C.; Xu, H. Y., et al. Dyes Pigment 2007, 73, 285–291.
22. Tang, X.-J.; Wu, L.-Zh.; Tung, Ch.-H., et al. Phys. Chem. Chem. Phys. 2002, 4,
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7.4–7.5 (m, 3H), 6.7–7.0 (m, 3H), 4.0 (s, 6H), 3.0 (s, 6H). EI-MS: m/z 340 (M+).
Compound 8: To a 100 ml flask, 50 mg (0.224 mmol) 5 and 30 ml anhydrous
ethanol were added. After the mixture was stirred for 30 min at room
temperature, 30.5 mg (0.224 mmol) N,N-dimethylaniline was added. The
reaction mixture was refluxed for 4 h and then cooled to room temperature.
The resulting precipitate was filtered and recrystallized from ethanol to give
yellow crystals in 85% yield (65 mg). Mp 192–193 °C. IR (KBr): 2949.8, 2884.5,
23. Strehmel, B.; Sarker, A. M.; Detert, H. ChemPhysChem. 2003, 4, 249–259.
24. Ma, J.; Yang, J.-X.; Tian, Y. P., et al. J. Anhui University Natural Sci. Ed. 2004, 28,
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