Strongly red-fluorescent novel donor-π-bridge-acceptor-π-bridge-donor (D-π-A-π-D) type 2,1,3-benzothiadiazoles with enhanced two-photon absorption cross-sections

Article abstract of DOI:10.1039/b410016f

Novel donor-π-bridge-acceptor-π-bridge-donor (D-π-A-π-D) type 2,1,3-benzothiadiazole fluorescent dyes connected to the N,N-diarylamino terminus via various type π-conjugate spacers exhibits large two-photon absorption cross-sections and high fluorescent quantum yields in orange-red color.

Full text of DOI:10.1039/b410016f

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Strongly red-fluorescent novel donor–p-bridge–acceptor–p-bridge–
donor (D–p–A–p–D) type 2,1,3-benzothiadiazoles with enhanced
two-photon absorption cross-sections{
a
Shin-ichiro Kato, Taisuke Matsumoto, Tsutomu Ishi-i, Thies Thiemann, Motoyuki Shigeiwa,
b
b
b
c
c
c
d
Hideki Gorohmaru, Shuichi Maeda, Yoshiro Yamashita and Shuntaro Mataka*
b
a
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koh-en,
Kasuga 816-8580, Japan. E-mail: mataka@cm.kyushu-u.ac.jp; Fax: 181-92-583-7894;
Tel: 181 92 583 7811
b
c
d
Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, 6-1 Kasuga-koh-en,
Kasuga 816-8580, Japan
Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida-cho,
Aoba-ku, Yokohama 227-8502, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta,
Midori-ku, Yokohama 226-8502, Japan
Received (in Cambridge, UK) 2nd July 2004, Accepted 2nd August 2004
First published as an Advance Article on the web 7th September 2004
Novel donor–p-bridge–acceptor–p-bridge–donor (D–p–A–p–
were prepared via Still coupling reaction of 1 by Lin and Tao’s
13
D) type 2,1,3-benzothiadiazole fluorescent dyes connected to
the N,N-diarylamino terminus via various type p-conjugate
spacers exhibits large two-photon absorption cross-sections
and high fluorescent quantum yields in orange-red color.
group as emitting materials in OLED. Olefinic and acetylenic
spacers were introduced by the Heck and Sonogashira reactions,
respectively. The Heck reaction of 1 with 4-(N,N-diphenyl-
amino)styrene afforded 9 having the C–C double bond. The
Sonogashira coupling reaction of 1 with trimethylsilylacetylene and
the subsequent deprotection gave 4,7-diethynyl-2,1,3-benzothia-
diazole, which was derived to 10 by reaction with 4-(N,N-
diphenylamino)iodobenzene.
Organic molecules with large two-photon absorption (TPA) cross-
sections have been the subject of much research in recent years
because of their potential applications for optical power limitation,
1
2
3
microfabrication, three-dimensional optical data storage, photo-
dynamic therapy, and two-photon laser scanning fluorescence
4
5
imaging. In the case of two-photon laser scanning fluorescence
imaging which has gained widespread popularity in the biological
community, fluorophores are required to have both large TPA
cross-sections and high fluorescence quantum yields in the red-NIR
region, in order to image at an increased penetration depth in
tissues with less photodamage. Although a number of compounds
including donor-acceptor-donor (D–A–D) type molecules, donor–
p-bridge–acceptor (D–p–A) type molecules, donor–p-bridge–
donor (D–p–D) type molecules, macrocycles, polymers, and
multibranched compounds are known to have large TPA cross-
sections, their accessible fluorescent color is mostly restricted to
6–9
blue and green. Recently, Fr e´ chet and Prasad’s group designed
bichromophoric molecules with both TPA chromophore and the
red-fluorescence emitting moiety and achieved the intense red-color
emission indirectly via efficient intramolecular fluorescent reso-
1
0
nance energy transfer. However, molecules capable of strong red
emission via direct two-photon excitation are scarce at present, to
the best of our knowledge. Such emission will be realized in the
molecule having the enhanced intramolecular charge transfer (ICT)
and large stokes shift. In our molecular design, electron-donating
N,N-disubstituted amino groups are connected via various
p-conjugate spacers to electron-withdrawing 2,1,3-benzothiadiazole
The single-photon absorption and fluorescence spectral proper-
ties of 2–10 are summarized in Table 1. BTDs 2–10 emitted
orange–red fluorescence at the wavelength region of 550–689 nm.
The fluorescence shifted to longer wavelength (28–107 nm) with the
increase of the solvent polarity. The fluorescent quantum yields (wf)
in toluene solution (0.41–0.98) are higher than those in
dichloromethane solution (0.13–0.52). The solvatochromism
observed above indicates ICT between the terminal amino
groups and the BTD core and a more polar molecular structure
in the excited state than that in the ground state. The pronounced
ICT character of 2–10 was supported by X-ray crystallographic
analysis of 4 (Fig. 1).{ The bond length alternation in the spacer
benzene rings can be expressed by the quinoid character (dr) of the
(BTD) fluorophore, as heteroaromatic molecules including 2,1,3-
benzothiadiazoles and 1,2,5-oxadiazolo[3,4-c]pyridines exhibited
In this communication, we report an example
of D–p–A–p–D type molecules exhibiting red emission with TPA
ability.
The synthesis of BTD dyes 2–10 was carried out by means of
palladium-catalyzed coupling reactions of 4,7-dibromo-2,1,3-
benzothiadiazole (1). Compounds 2–8, which possessed aromatic
spacers such as phenyl, biphenyl, and thienylphenyl, were obtained
via Suzuki reaction of 1 with arylboronic acids. Recently, 4 and 8
11,12
large stokes shift.
14
ring defined by:
dr ~ {(a 2 b ) 1 (c 2 b )}/2
n
(2-1 )
n
n
n
n n n
where a , b , and c (n ~ 1–8) are C–C bond length in the benzene
ring (see Fig. 1). The normalized dr value (0.0185) of 4 is higher
by one order of magnitude compared to that (0.00194) of
{ Electronic supplementary information (ESI) available: Experimental
section. See http://www.rsc.org/suppdata/cc/b4/b410016f/
2
3 4 2
C h e m . C o m m u n . , 2 0 0 4 , 2 3 4 2 – 2 3 4 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 One- and two-photon properties of 2,1,3-benzothiadiazoles 2–10 and AF-50
23
a
c
l
max absorbance/nm (10 e)
l
max-Em/nm (w
f
)
f
g
Compd
Toluene
CH Cl
2
2
Toluene
CH
2
Cl
2
TPF lmax-Em/nm
s/GM
d
624 (0.41 )
d
680 (0.13 )
h
2
3
4
5
6
7
8
9
472 (14)
468 (17)
473 (13)
464 (15)
458 (18)
458 (19)
461 (21)
422 (22)
529 (35)
511 (42)
484 (38)
628
614
599
599
599
552
670
642
586
43
59
d
612 (0.50 )
d
660 (0.23 )
h
b
462 (20) [459 ]
d
592 (0.62 ) [584 ]
b
d
b
640 (0.43 ) [639 ]
h
83
66
d
588 (0.64 )
d
634 (0.52 )
h
462 (20)
465 (21)
426 (23)
d
591 (0.64 )
d
646 (0.40 )
i
94
96
e
550 (0.98 )
e
657 (0.14 )
h
b
d
661 (0.56 ) [653 ]
b
d
689 (0.15 ) [698 ]
b
j
530 (32) [529 ]
512 (42)
480 (39)
196
211
d
617 (0.65 )
d
683 (0.23 )
h
d
568 (0.79 )
d
663 (0.19 )
h
10
175
45
k
AF-50
a
25
b
c
26
d
Measured in 1 6 10 M solution. Reported value in ref. 13. Measured in 1 6 10 M solution. Fluorescence quantum yield relative to
rhodamine B in ethanol (0.65). Fluorescence quantum yield relative to Fluorescein in ethanol (0.97). In toluene solution. 1 GM (G o¨ ppert–
e
f
g
250
4
cm s photon molecule
21
21
h
23
i
23
Mayer) ~ 1 6 10
.
Measured in 5 6 10 M toluene solution. Measured in 1 6 10 M toluene solution.
Measured in 1.2 6 10 M toluene solution. Measured in 3.3 6 10 M toluene solution.
j
23
k
23
between the amino groups and the BTD core, and extended
p-system by introduction of a planar p-conjugate spacer.
Notes and references
{
Crystal data of 4,7-diphenyl-2,1,3-benzothiadiazole: C18
H
12
˚
N
2
S, M ~
2
88.37, triclinic, a ~ 9.597 (7), b ~ 12.05 (1), c ~ 13.959 (9) A, U ~ 1371
3
˚
¯
1) A , T ~ 123 K, space group P1 (no. 2), Z ~ 4, m (Mo–Ka) ~
21
(
Fig. 1 (a) ORTEP representation of 4,7-diphenyl-2,1,3-benzothiadiazole.
˚
2.29 mm , 54140 reflections measured, 6222 unique (Rint ~ 0.064) which
Selected bond lengths/A: a
.389, a 1.397, b 1.394, c
.393, c 1.393, a 1.402, b
.400, b 1.396, c 1.387. (b) ORTEP representation of 4. Selected bond
1.397, b 1.388, c 1.397, a 1.399, b 1.378, c 1.399, a 1.402,
1.399, a 1.402, b 1.388, c 1.397.
1
1.397, b
1.396, a
1.387, c
1
1.399, c
1.404, b
1.388, a
1
1.388, a
1.397, c
1.403, b
2
1.404, b
1.387, a
1.392, c
2
1.396, c
1.406, b
1.389, a
8
2
2
were used in all calculations. The final wR(F ) was 0.075. Crystal data of 4:
1
1
1
3
3
3
4
4
4
5
5
42 30 4
C H N S, M ~ 622.78, triclinic, a ~ 7.6837 (9), b ~ 9.991 (1), c ~
3
5
6
6
6
7
7
7
˚ ˚
¯
0.947 (2) A, U ~ 1604.5 (1) A , T ~ 123 K, space group P1 (no. 2), Z ~
1
2
8
˚
8
2
2, m(Mo–Ka) ~ 1.39 mm , 76820 reflections measured, 9313 unique
2
(Rint ~0.049)whichwereusedinallcalculations.ThefinalwR(F )was0.067.
lengths/A: a
1.382, c
1
1
1
2
2
2
3
b
3
3
4
4
4
CCDCnumbers244163and244164.Seehttp://www.rsc.org/suppdata/cc/b4/
b410016f/ for crystallographic data in .cif or other electronic format.
1
5
4,7-diphenyl-2,1,3-benzothiadiazole.
having ICT effect and strong red-emission were created.
Thus, compounds 2–10
§ The femtosecond laser measurements provide a more accurate evaluation
of the true TPA cross-sections with respect to nanosecond laser pulses.
The two-photon absorption and fluorescence spectral properties
of 2–10 also are summarized in Table 1. By the two-photon
excitation of 800 nm laser pulses, BTDs 2–10 emitted, as expected,
frequency upconverted orange-red fluorescence at the wavelength
region of 552–670 nm. In particular, red-emission was obtained
from 2, 8, and 9, in which the value of two-photon excitation
fluorescence (TPF lmax-Em) was present at 628 nm, 670 nm, and
1
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9
fluorescence (lmax-Em) as found in the precedent report. TPA
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pulses in toluene solution.§ The experimental uncertainty of s value
amounts to about ¡12%. Compound AF-50 with D–p–A
structure displays a large s value and is usually referred to as a
7
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16
TPA benchmark. Compared to AF-50, all BTD derivatives 2–10
showed large s values derived from ICT. The s value (84 GM) of 4
with terminal N,N-diphenyamino groups was about two times
larger than that (45 GM) of AF-50. The substituents such as
methyl, phenyl, 1-naphthyl, and 2-naphthyl group on the nitrogen
atom slightly affect the s value in 2–6. As a spacer, the thiophene
ring is effective as well as the C–C double bond. The s values of 8
and 9 are almost doubled, compared to that of 4. The C–C triple
bond is also effective, but to less extent; 10 showed the s value of
2
144–2145.
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1
1
1
175 GM. Introduction of the benzene ring at the spacer moiety of 7
was not effective, probably due to the non-planar biphenyl
structure. The results mentioned above indicate that the high TPA
ability in 8, 9, and 10 is attributable to the extended p-system
caused by introducing an efficient p-conjugate spacer.
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BTD 8 and 9 emitted strong red-fluorescence on the TPA process.
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C h e m . C o m m u n . , 2 0 0 4 , 2 3 4 2 – 2 3 4 3
2 3 4 3