Photophysical properties of octupolar chromophore based on triazine core and fluorene divinylene conjugated bridge

Article abstract of DOI:10.1016/j.cclet.2012.04.023

A novel octupolar chromophore with 1,3,5-triazine as core, 2,7-divinylene-9,9-dimethylfluorene as extended π-conjugated bridge, triarylamine as the electron-donating end-groups was successfully synthesized and characterized. Their linear photophysical and two-photon absorption (TPA) properties were investigated by UV absorption, excited fluorescence (SPEF) spectra and nonlinear transmission method, respectively. The absorption cut-off of the chromophore is below 520 nm and it has stronger fluorescence emission in a nonpolar solvent. In addition, the chromophore exhibits larger TPA cross-section (226.0 GM) in the femtosecond regime at 800 nm.

Full text of DOI:10.1016/j.cclet.2012.04.023

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 867–870
www.elsevier.com/locate/cclet
Photophysical properties of octupolar chromophore based on
triazine core and fluorene divinylene conjugated bridge
a
Li Wei , Jie Ping Shi , Zhi Qiang Zhou , Yi Ping Cui ,
a
b
b
a
Hong Wen Hu , Guo Yuan Lu
a,
*
a
Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
b
Advanced Photonics Center, Southeast University, Nanjing 210096, China
Received 24 February 2012
Available online 9 June 2012
Abstract
A novel octupolar chromophore with 1,3,5-triazine as core, 2,7-divinylene-9,9-dimethylfluorene as extended p-conjugated
bridge, triarylamine as the electron-donating end-groups was successfully synthesized and characterized. Their linear photo-
physical and two-photon absorption (TPA) properties were investigated by UVabsorption, excited fluorescence (SPEF) spectra and
nonlinear transmission method, respectively. The absorption cut-off of the chromophore is below 520 nm and it has stronger
fluorescence emission in a nonpolar solvent. In addition, the chromophore exhibits larger TPA cross-section (226.0 GM) in the
femtosecond regime at 800 nm.
#
2012 Guo Yuan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Octupolar molecules; Synthesis; Horner–Wadsworth–Emmons reaction; Photophysical properties; TPA cross-section
The materials with intense two-photon absorption (TPA) have attracted increasing attention owing to their potential
applications in the photo-electronic materials [1]. The coplanarity of the system and length of the conjugation bridges
plays an important role in enhancing the molecular TPA properties [2]. The 1,3,5-triazine is a stronger p-electron
deficient group than other heterocyclic ring and unlike benzene ring, connecting three large branches at the 2, 4 and 6
position of triazine will not create the steric interactions of the ortho-hydrogens. Accordingly, triazine-cored octupolar
molecules have good coplanarity, strong intramolecular charge-transfer (ICT) [3]. Fluorene has extended conjugation
length through the planar structure, excellent thermo and photostability, which has been used widely in electronic and
photonic fields [4]. We have demonstrated in previous works [5] that the chromophores with triphenylamino as end-
groups have higher TPA cross-section than other groups and good solubility in ordinary organic solvents. Therefore, in
this work, we employed 2,7-divinylene-9,9-dimethylfluorene as extended p-conjugated bridge and connected it to
1,3,5-triazine core with triphenylamino as electron-donating end-groups. Thus tribranched octupolar molecule 1 with
strong ICT from end-groups to core was constructed (see Scheme 1).
The compound 2 and 2,4,6-trimethyl-1,3,5-triazine 5 were synthesized according to literature [6,7]. The acetal 3
was prepared by the condensation of 2 and ethylene glycol in the presence of p-toluenesulfonic acid in refluxing
*
Corresponding author.
E-mail address: lugyuan@nju.edu.cn (G.Y. Lu).
1001-8417/$ – see front matter # 2012 Guo Yuan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.04.023

868
L. Wei et al. / Chinese Chemical Letters 23 (2012) 867–870
Scheme 1. Synthetic route of chromophore. Reagent and condition: (a) HOCH
2
CH
78 8C, 56.3%; (c) 2,4,6-trimethyl-1,3,5-triazine 5, Bu OK, THF, reflux, 48 h, 91.5%; (d) TsOH, CH
e) compound 10, NaH, THF, reflux, 48 h, 58.4%. (f) KBH , CH OH, reflux, 6 h, 95%; (g) P(OC
2
OH, TsOH, toluene, reflux, 12 h, 98%; (b) n-BuLi/THF, DMF,
COCH -THF (v/v = 3:1), reflux, 20 h, 98.5%;
, I , 258C, 20 h, 70.2%.
t
ꢀ
3
3
(
4
3
2
H
5
)
3
2
toluene, and then 3 was transformed into 4 with n-BuLi and DMF at ꢀ78 8C. The 2,4,6-trimethyl-1,3,5-triazine 5 was
condensed with 4 under alkaline condition in THF to give compound 6 in high yield (91.5%), followed by acetal
deprotection with p-toluenesulfonic acid in acetone to afforded 7 in 98.5% yield. The intermediate phosphonate 10
was prepared by a very simple one-pot method, in which 9 was treated in triethyl phosphite with one equivalent of
iodine. Horner–Wadsworth–Emmons reaction was used between 4 equivalent phosphonate 10 and 1 equivalent
trialdehyde 7 in the presence of NaH in THF at refluxing temperature. The crude product was purified by column
chromatography on silica gel using petroleum ether and chloroform (v/v = 2:1) to give 1 in 58.4% yield, mp 150–
1
13
1
are in accord with the assigned structures [8].
52 8C. The chromophore 1 and intermediates 3, 4, 6, 7 are new compounds and the H NMR, C NMR, MS spectra
The linear absorption and excited fluorescence (SPEF) spectra of 1 in toluene (Tol), tetrahydrofuran (THF),
dichloroform (DCM) and acetonitrile (ACN) are shown in Fig. 1. The corresponding spectroscopic parameters are
0
0
0
0
0
.8
.6
.4
.2
.0
1000
a
a
8
6
4
2
00
00
00
00
0
b
c
d
b
c
d
3
00
350
400
450
500
550
450
500
550
600
650
700
750
Wavelength / nm
Wavelength / nm
CN(1.0 ꢁ 10ꢀ mol/L). Right: Emission spectra of 1 in
6
Fig. 1. Left: Linear absorption spectra of 1 in (a) toluene, (b) THF, (c) CH
2
Cl
2
and (d) CH
3
CN (1.0 ꢁ 10ꢀ mol/L) (lex = 400 nm).
6
2 2 3
(a) toluene, (b) THF, (c) CH Cl and (d) CH

L. Wei et al. / Chinese Chemical Letters 23 (2012) 867–870
869
Table 1
The spectroscopic parameters of linear single-photon absorption and excited fluorescence (SPEF) of 1.
abs
max
a
ꢀ1
ꢀ1
SPEF
max
b
c
Solvents
l
(nm)
e (mol L cm
)
l
(nm)
K
5
Toluene
THF
429
428
425
423
2.0 ꢁ 10
496
574
600
0.53
5
1.5 ꢁ 10
0.035
5
CH
CH
2
Cl
CN
2
1.4 ꢁ 10
5
3
1.0 ꢁ 10
a
b
c
ꢀ6
ꢀ6
Measured in toluene, THF, CH
Measured in toluene, THF, CH
2
Cl
Cl
2
, CH
,CH
3
CN (1.0 ꢁ 10 mol/L) solution.
2
2
3
CN (1.0 ꢁ 10 mol/L) solution (lex = 400 nm).
3
Fluorescent quantum yield were determined with N-methyl-ANBD in CH CN (F = 0.38, lex = 458 nm) as a reference.
summarized in Table 1. The chromophore 1 has absorption cut-off below 520 nm, indicating that it will be no linear
absorption when laser source of 800 nm is used for TPA applications. In addition, there is no appreciable shift in the
absorption spectra to longer wavelength with increasing solvent polarity (ACN > DCM > THF > Tol). By contrast, a
marginal blue shift in the absorption maximum is observed. This phenomenon is similar to some other star-shaped
molecules [9]. However, the fluorescence of 1 has shown significant positive solvatochromism, which is arised from a
large charge separation on the excited-state in the octupolar molecules [10]. The fluorescent quantum yield (K ) is also
F
influenced by the polarity of the solvents. The K value of 1 in a nonpolar solvent is much larger than that in a polar
F
solvent. This phenomenon is also similar to other tribranched chromophores [9].
TPA cross-section was measured using the nonlinear transmission method [11]. The input–output intensity relation
ꢀ
2
of chromophore 1 in THF solution (c = 1.0 ꢁ 10 mol/L) was measured using the femtosecond (fs) laser source at
0
8
00 nm. TPA cross-section s can be obtained based on calculation [5b] from the nonlinear absorption coefficient
2
measured in experiments, which is 226.0 GM. It is worth noting that the effective TPA cross section which was
measured in the femtosecond (fs) regime can be more than two orders of magnitude smaller than the intrinsic cross
section value obtained via a nanosecond regime [12]. So 1 has fairly large TPA cross-section via fs regime.
In summary, a novel octupolar chromophore 1 was successfully synthesized and characterized. The absorption cut-
off of the chromophore is below 520 nm and it has stronger fluorescence emission in a nonpolar solvent. TPA
measurement by nonlinear transmission method indicates that the chromophore has larger TPA cross-section
(
226.0 GM) in the femtosecond regime.
Acknowledgments
This work was supported financially by National Natural Science Foundation of China (Nos. 20774039, 20872061)
and the National Basic Research (No. 2007CB925103).
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13
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(
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3
1
5
4
3
)
2
3
90 6
H N
1
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3
13
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2
CH
2
O), 1.49 (s, 6H, 2 ꢁ CH
3
3

870
L. Wei et al. / Chinese Chemical Letters 23 (2012) 867–870
+
1
2
3
1
26.0, 123.1, 121.1, 121.0, 120.5 (Ar), 103.7 (OCHO), 65.4 (OCH ), 47.0 (C(CH
2
3 2 3
) ), 26.9 (CH ). ESI-MS (m/z): 295.08 [M+1] , Calcd.
1
95.34. 6: H NMR (300 MHz, CDCl
ꢁ OCHO), 4.07–4.25 (m, 12H, 3 ꢁ OCH
39.6, 137.4, 134.9, 128.0, 125.9, 125.8, 122.1, 120.9, 120.7, 120.4 (Ar), 104.0 (OCHO), 65.4 (OCH
3
): d 8.41 (d, 3H, J = 15.9 Hz), 7.48–7.83 (m, 18H, ArH), 7.26 (d, 3H, J = 15.9 Hz), 5.90 (s, 3H,
1
3
2
CH
2
O), 1.56 (s, 18H, 6 ꢁ CH
3
). C NMR (75 MHz, CDCl
3
): d 171.3, 154.6, 154.4, 141.9, 140.8,
), 27.2 (CH ). ESI-MS
2
), 47.0 (C(CH
3
)
2
3
+
1
(
m/z): 953.42 [M+1] , Calcd. 953.14. 7: H NMR (300 MHz, CDCl
3
): d 10.08 (s, 3H, 3 ꢁ CHO), 8.41 (d, 3H, J = 15.9 Hz), 7.73–8.01 (m, 18H,
13
ArH), 7.29 (d, 3H, J = 15.9 Hz), 1.60 (s, 18H, 6 ꢁ CH
3
). C NMR (75 MHz, CDCl ): d 192.1 (CHO), 171.2, 155.6, 154.8, 144.8, 141.6, 139.6,
3
), 27.0 (CH ). ESI-MS (m/z): 821.50 [M+1] , Calcd. 820.99.
3
+
136.2, 135.9, 130.7, 128.1, 126.6, 123.2, 122.3, 121.7, 120.7 (Ar), 47.0 (C(CH
3
)
2
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