Novel 2,1,3-benzothiadiazole-based red-fluorescent dyes with enhanced two-photon absorption cross-sections

DOI: 10.1002/chem.200500921

Source and publish data:

Chemistry - A European Journal p. 2303 - 2317 (2006)

Update date:2022-08-02

Topics: Synthetic route Density functional theory (DFT) Photostability UV-Vis Spectroscopy Fluorescence Spectroscopy Stokes Shift Quantum Yield Solvatochromism Chromophore π-conjugation Fluorescent dyes Fluorophore Nonlinear optics (NLO)

Authors:

Kato, Shin-Ichiro

Matsumoto, Taisuke

Shigeiwa, Motoyuki

Gorohmaru, Hideki

Maeda, Shuuichi

Ishi-i, Tsutomu

Mataka, Shuntaro

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Article abstract of DOI:10.1002/chem.200500921

This paper reports the two-photon absorbing and orange-red fluorescence emitting properties of a series of new 2,1,3-benzothiadiazole (BTD)-based D-π-A-π-D-type and star-burst-type fluorescent dyes. In the D-π-A-π-D-type dyes 1-6, a central BTD core was connected with two terminal N,N-disubstituted amino groups via various.π-conjugated spacers. The star-burst-type dyes 8 and 10 have a three-branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine-containing BTD branches. All the BTD-based dyes displayed intense orange-red color fluorescence in a region of 550-689 nm, which was obtained by single-photonexcitation with good fluorescent quantum yield up to 0.98 as well as by two-photon excitation. Large two-photon absorption (TPA) cross-sections (110-800 GM) of these BID dyes were evaluated by open aperture Z-scan technique with a femtosecond Ti/sapphire laser. The TPA cross-sections of D-π-A-π-D-type dyes 2-6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional π-conjugated spacer are about 1.5-2.5 times larger than that of 1_c with only a benzene spacer. The TPA cross-sec- lions significantly increased in three-branched star-burst-type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3-5 times larger than those of the corresponding one-dimensional sub-units 9 (170 GM) and 11 (230 GM). respectively. The ratios of σ/ e_π between three-branched and one-dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11). which are larger than those predicted simply on the basis of the chromophore number density (1:1). according to a cooperative enhancement of the two-photon absorbing nature in the three-branched system.

Full text of DOI:10.1002/chem.200500921

FULL PAPER

DOI: 10.1002/chem.200500921

Novel 2,1,3-Benzothiadiazole-Based Red-Fluorescent Dyes with Enhanced

Two-Photon Absorption Cross-Sections

Shin-ichiro Kato,^[a]Taisuke Matsumoto,^[b]Motoyuki Shigeiwa,^[c]Hideki Gorohmaru,^[c]

Shuuichi Maeda,^[c]Tsutomu Ishi-i,^[b]and Shuntaro Mataka*^[b]

Abstract: This paper reports the two-

photon absorbing and orange-red fluo-

rescence emitting properties of a series

of new 2,1,3-benzothiadiazole (BTD)-

based D–p–A–p–D-type and star-

burst-type fluorescent dyes. In the D–

excitation with good fluorescent quan-

tum yield up to 0.98 as well as by two-

photon excitation. Large two-photon

absorption (TPA) cross-sections (110–

800 GM) of these BTD dyes were eval-

uated by open aperture Z-scan tech-

nique with a femtosecond Ti/sapphire

laser. The TPAcross-sections of D– p–

A–p–D-type dyes 2–6 with a benzene,

thiophene, ethene, ethyne, and styrene

moiety, respectively, as an additional p-

conjugated spacer are about 1.5–2.5

times larger than that of 1c with only a

benzene spacer. The TPAcross-sec-

tions significantly increased in three-

branched star-burst-type BTDs

(780 GM) with a benzene core and 10

(800 GM) with a triphenylamine core,

which are about 3–5 times larger than

those of the corresponding one-dimen-

sional sub-units 9 (170 GM) and 11

(230 GM), respectively. The ratios of s/

e_pbetween three-branched and one-di-

mensional dyes were 6.5:3.8 (for 8 and

9) and 6.0:4.0 (for 10 and 11), which

are larger than those predicted simply

on the basis of the chromophore

number density (1:1), according to a

cooperative enhancement of the two-

photon absorbing nature in the three-

branched system.

p–A–p–D-type dyes 1–6,

a central

BTD core was connected with two ter-

minal N,N-disubstituted amino groups

via various p-conjugated spacers. The

star-burst-type dyes 8 and 10 have a

three-branched structure composed of

a central core (benzene core in 8 and

triphenylamine core in 10) and three

triphenylamine-containing

BTD

branches. All the BTD-based dyes dis-

played intense orange-red color fluo-

rescence in a region of 550–689 nm,

which was obtained by single-photon

Keywords: donor–acceptor

sys-

tems · fluorescence · two-photon

absorption

Introduction

ing molecules have attracted considerable attention in the

last decade, although two-photon absorption (TPA) was pre-

dicted theoretically by Gçppert-Mayer in 1931.^[1]Photoexci-

tation as well as frequency-upconverted fluorescence of mol-

ecules on the TPAprocess are confined to the focal point,

since the probability of TPAis proportional to the square of

the light intensity. These features produced potential appli-

cations for optical power limitation,^[2]microfabrication,^[3]

three-dimensional optical data storage,^[4]photodynamic ther-

apy,^[5]and two-photon laser scanning fluorescence imaging.^[6]

Among these applications, two-photon laser scanning fluo-

rescence imaging that uses near IR photons as the excitation

source has gained widespread popularity in the biological

community, which provides images at an increased penetra-

tion depth in tissue with less photodamage. Recently, much

progress has been made in two-photon laser scanning fluo-

rescence imaging by using two-photon confocal laser scan-

Organic molecules exhibiting significant nonlinear optical

nature have been the subject of various scientific studies in

recent years. In particular, highly active two-photon absorb-

[a] S.-i. Kato

Interdisciplinary Graduate School of Engineering Sciences

Kyushu University, 6-1 Kasuga-koh-en

Kasuga 816-8580 (Japan)

[b] T. Matsumoto, Dr. T. Ishi-i, Prof. Dr. S. Mataka

Institute for Materials Chemistry and Engineering (IMCE)

Kyushu University, 6-1 Kasuga-koh-en

Kasuga 816-8580 (Japan)

Fax : (+81)92-583-7894

E-mail: mataka@cm.kyushu-u.ac.jp

[c] M. Shigeiwa, Dr. H. Gorohmaru, Dr. S. Maeda

Mitsubishi Chemical Group Science

ning microscopy,^[7]which has promoted synthetic studies to

and Technology Research Center, Inc.

1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502 (Japan)

[8]

develop TPAfluorophore for biomolecular tags.

There-

Chem. Eur. J. 2006, 12, 2303 – 2317

ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

2303

fore, it is important to understand the nature of the relation-

ship between the molecular structure and the TPAproperty

to provide guidelines for the development of materials with

large TPAcross-sections at the wavelength of available laser

sources. BrØdas, Marder, Perry, and Webbꢁs group designed

bis(styryl)benzene derivatives with electron donor (D), elec-

tron acceptor (A), and p-spacer (p), and achieved large

TPAcross-sections on the basis of a concept that the TPA

nature is ascribed to intramolecular charge transfer (ICT).^[9]

Subsequently, a number of compounds including D–A–D-

type molecules,^[10,11]D–p–A-type molecules,^[12–15]A–D–A-

type molecules,^[16–18]D–p–D-

emitting moiety, which can avoid the difficulty of adding a

red-emission property to TPAdyes, and achieved intense

red-color emission indirectly by efficient intramolecular flu-

orescent resonance energy transfer.^[32a,b]Recently, we report-

ed the first example of red-fluorescent TPAdyes by direct

two-photon excitation using 2,1,3-benzothiadiazole (BTD)-

based fluorescent dyes.^[33]At almost the same time, Müllen

and his co-workers synthesized water-soluble and red-fluo-

rescent perylene diimides,^[32c]which can be developed to

living cell imaging by direct two-photon excitation by Hofk-

ens and Schryverꢁs group.^[32d]More recently, an excellent

type molecules,^[19–21]macrocy-

cles,^[22–24]multi-branched mole-

cules,^[25–30]and organometallic

complexes^[31]were synthesized,

and their TPAproperties were

investigated. Then, it has been

pointed out that expansion of

p-electron conjugation is crucial

for large TPAcross-sections. In

addition, a significant increase

in the TPAability has been dis-

covered in the multi-branched

structures, indicating a coopera-

tive enhancement.^{[25a,c,g,27b,c,30]}

The fluorescent color emitted

from TPAdyes has mostly been

restricted to blue and green;

nevertheless various types of

compounds exhibiting large

TPAcross-sections are now ac-

cessible as described above.

The blue and green colors

would interfere with the self-

absorption wavelength of the

cell. In order to achieve effi-

cient two-photon laser scanning

fluorescence

imaging, two-

photon absorbing fluorophores

are required to emit fluores-

cence in the red region. Howev-

er, strong red emission with

two-photon excitation is scarce

compared with the two colors

mentioned above, because an

elongated p-system in the two-

photon absorbing fluorophores

is required to obtain red-emis-

sion by direct two-photon exci-

tation.^[32]Therefore, more effort

is needed to achieve promising

red-fluorescent TPAmaterials.

FrØchet and Prasadꢁs group de-

signed bichromophoric mole-

cules with the TPAchromo-

phore and the red-fluorescence

2304

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Chem. Eur. J. 2006, 12, 2303 – 2317

Red-Fluorescent Dyes

FULL PAPER

III

TPAdye of Eu complex with high purity red emission was

created by Zhang and Wang group.^[32e]The structures of D–

p–A–p–D-type and star-burst-type 2,1,3-benzothiadiazoles

1–11 are depicted below.

4,7-Diphenyl-2,1,3-benzothiadiazole and its derivatives

are known to be strongly fluorescent dyes, which exhibit

large Stokes shifts.^[34,35]Furthermore, an electron-withdraw-

ing 2,1,3-benzothiadiazole (BTD) unit was used as an impor-

tant spacer in semiconducting oligomers.^[36]Recently, we

have developed BTD-based dyes to functional materials

such as dichroic fluorescent materials in liquid crystal dis-

plays,^[37]and a light-harvesting antenna as well as an energy-

transfer reagent in a fullerene-dyad system.^[38]We expected

that a strong red emission by direct two-photon excitation

would be achieved by using the strong fluorescent and elec-

tron-accepting nature of the BTD dye. BTD-based TPA

dyes, so-called donor–p-bridge–acceptor–p-bridge–donor

(D–p–A–p–D)-type dyes, were designed and prepared on

the basis of a combination of the BTD moiety and two ter-

minal electron-donating amino groups by p-conjugated

spacers to enhance the ICT character.^[33]As a part of our

continuing work to develop BTDs as red-fluorescent TPA

materials, we report here the detailed results of six kinds of

D–p–A–p–D-type dyes 1–6 (see above) with different p-

conjugate length. The TPAability is discussed on the basis

of absorption and fluorescent solvatochromism, X-ray crys-

tallographic analysis, as well as Z-scan TPAcross-section

measurement, whereas their red-fluorescent nature is dis-

cussed based on single- and two-photon excited fluorescence

spectroscopy. In addition, two kinds of BTD-based star-

burst-type derivatives 8 and 10 have been synthesized to en-

hance TPAability by a cooperative effect. Significantly

large TPAcross-section up to 800 GM is achieved in the

start-burst structures together with the attractive red-fluo-

rescence. Finally, correlation between the two parameters

“TPAcross-section and fluorescent quantum yield” is dis-

cussed to characterize molecular ability for the development

to the two-photon laser scanning fluorescence imaging.

Results and Discussion

Scheme 1. Preparation of D–p–A–p–D-type 2,1,3-benzothiadiazoles 1–6.

Synthesis: The synthesis of D–p–A–p–D-type BTDs 1–6

was achieved by means of palladium-mediated cross-cou-

pling reactions of 4,7-dibromo-2,1,3-benzothiadiazole (12) as

shown in Scheme 1. To observe the influence on the photo-

physical properties of the substituents such as methyl,

phenyl, and naphthyl groups on the nitrogen atom, 1a–e

with various N,N-disubstituted amino groups with only ben-

zene spacers between the BTD moiety and the amino nitro-

gen atoms were synthesized by Suzuki coupling reactions of

12 with the corresponding arylboronic acids 13a–d in 50–

70% yields in the presence of a palladium(0) catalyst. To

enhance p-conjugation relative to 1c, other BTD derivatives

2–6 having a benzene, thiophene, ethene, ethyne, and sty-

rene moiety, respectively, as an additional spacer between

the BTD ring and the amino group were prepared. Com-

pound 2 with the additional benzene ring was prepared by

Suzuki coupling reaction of 12 with 4’-(N,N-diphenylamino)-

biphenyl-4-yl-boronic acid (13e) in 18% yield according to

the procedure described above. Similarly, 3 with the thio-

phene ring was obtained by a Suzuki coupling reaction of

4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (14) with

13b in 33% yield. The introduction of the additional ethene

spacer in 4 was performed by Heck reaction of 12 with 1-

(N,N-diphenylamino)-4-vinylbenzene (15) catalyzed by pal-

ladium acetate/tris(o-tolyl)phosphine in the presence of trie-

thylamine in 17% yield. Sonogashira coupling reaction of

4,7-diethynyl-2,1,3-benzothiadiazole (17) with 4-(N,N-diphe-

nylamino)iodobenzene (18) gave 5 with the additional

ethyne spacer in 11% yield. The synthetic intermediate 17

Chem. Eur. J. 2006, 12, 2303 – 2317

ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

2305

S. Mataka et al.

was derived from 12 by coupling reaction with trimethyl-

silylacetylene followed by treatment with potassium hydrox-

ide. BTD 6 with the additional styrene spacer was obtained

from the bishydroxymethyl derivative 19 in three steps. Bro-

mination of 19 by phosphorus tribromide afforded the di-

bromide 20 in 44% yield, which was converted to the di-

phosphate 21 by treatment with trimethyl phosphite in high

yield. Finally, double Horner–Emmons reaction of 21 with

triphenylamine aldehyde 22 in the presence of potassium

tert-butoxide afforded the desired 6 in 61% yield.

To enhance the TPAability, three-branched star-burst-

type BTDs 8 and 10 were designed and prepared as shown

in Scheme 2. The star-burst-type BTD 8 with a benzene core

was obtained from monoaldehyde of BTD 23 and triphos-

phate 27 under triple Horner–Emmons reaction conditions

in 63% yield. Similarly, another star-burst-type BTD 10

with a triphenylamine core was obtained from monophos-

phate of BTD 26 and trialdehyde 29 under Horner–

Emmons reaction conditions in 59% yield. The key synthet-

ic intermediate 26 was derived from 23 in three steps (re-

duction, bromination, and phosphorylation) according to a

similar procedure in the synthesis of 6. One-dimensional

BTD-based dye 9 was obtained by Wittig reaction of 23 as

reference of 8 with the benzene core. Similarly, 11 was pre-

pared as reference of 10 with the triphenylamine core. All

new dyes have been fully characterized by spectroscopy and

elemental analysis.

ties of D–p–A–p–D-type BTDs 1–6 are summarized in

Table 1; the data of star-burst-type BTDs 8 and 10 are

shown in Table 2 along with their references 9 and 11. The

single-photon absorption (SPA) and single-photon excited

fluorescence (SPEF) spectra were measured in toluene and

dichloromethane to investigate the influence of solvent po-

larity on optical properties of BTDs. The SPAspectra of

BTDs have two bands around 300–380 and 420–550 nm aris-

ing from p–p* and charge-transfer transitions, respective-

ly.^[35]The SPAmaxima ( l_max) of all the BTDs were scarcely

changed between nonpolar toluene and polar dichlorome-

thane (Tables 1 and 2). In the D–p–A–p–D-type BTDs 1a–

e, the absorption coefficients (e) depended on the substitu-

ents on the nitrogen atom (13000–21400). Compared with

1c with only a benzene spacer (19500–20500), 3–6 with the

expanded p-spacer largely increased the e values (32300–

49300), except for 2 (22300–23200) (Figure 1 and Table 1).

More significant enhancement of the e values was observed

in three-branched star-burst-type BTDs 8 (70000–71400)

and 10 (115400–115900). The e values of 8 and 10 were

about 3.5 times larger than those of the one-dimensional

sub-units 9 and 11, respectively (Figure 2 and Table 2). The

multidimensionality in the star-burst systems 8 and 10 leads

the increase of oscillator strength.

All the BTD derivatives emitted orange-red fluorescence

at the wavelength region of 550–689 nm and exhibited sig-

nificant solvatochromism contrary to the absorption process.

With increasing solvent polarity, the SPEF maxima (l_SPEF

)

Single-photon absorption and emission properties: The

single-photon excited absorption and fluorescence proper-

showed remarkable bathochromic shifts. As a typical exam-

ple of this phenomenon, the SPEF spectra of D–p–A–p–D-

type BTD 4 and star-burst-type

BTD 10 are shown in Figure 3,

which provide the differences

of l_SPEFbetween toluene and

dichloromethane of 66 nm in 4

and 55 nm in 10. Furthermore,

the fluorescent quantum yields

(f_f) in dichloromethane solu-

tion of all the BTDs were

smaller than those in toluene

solution (Table 1). Considerably

high f_fvalues of all the BTDs

are maintained in toluene solu-

tion among the orange-red

color region. In order to exam-

ine in detail the solvent effect,

the SPEF spectra of 1c were

measured in various media

from nonpolar to polar solvents.

With increasing solvent polarity

from cyclohexane, toluene,

THF, dichloromethane, PhCN,

to DMF, the l_SPEFvalues of 1c

shifted significantly to longer

wavelength region by 100 nm

(from 557 nm in cyclohexane,

to 657 nm in DMF) along with

Scheme 2. Preparation of star-burst-type 2,1,3-benzothiadiazoles 8–11.

2306

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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2006, 12, 2303 – 2317

Red-Fluorescent Dyes

FULL PAPER

Table 1. Single- and two-photon properties of D–p–A–p–D-type 2,1,3-benzothiadiazoles 1–6.

groups and 1b with one dime-

thylamino group have smaller

f_fvalues (0.13 in 1a and 0.23 in

1b) in dichloromethane. In 2–6,

the additional spacer between

the BTD ring and the amino

groups tends to decrease the f_f

values. For example, the f_f

value (0.23) of 4 with the addi-

tional ethene spacer in di-

[f]

l_max/nm (10^ꢀ2e)^[a]

Toluene CH₂Cl₂

l_SPEF/nm^[b](f_f)

Toluene CH₂Cl₂

f_fe^[e]

f_fs^[e]

/GM

l_TPEF

s/GM^[g]

/nm

472

(148)

468

(172)

462

(205)

462

(203)

465

(214)

426

(232)

530

(323)

512

473

(130)

464

(156)

458

(195)

458

(190)

461

(210)

422

(223)

529

(364)

511

624

680

6070

(1690)

8600

(16)

(27)

(48)

628

614

599

552

670

642

586

569

120^[h]

(760)

120^[h]

(780)

130^[h]

(780)

120^[h]

(780)

110^[i]

(780)

200^[j]

(780)

280^[k]

(780)

330^[l]

(780)

200^[l]

(780)

330^[m]

(780)

(0.41^[c]

612

)

(0.13^[c]

660

)

(0.50^[c]

592

(0.23^[c]

640

(3590)

12700

(7020)

13000

(9880)

13700

(8400)

22700

(3120)

18100

(5460)

27100

(9570)

31000

(7220)

30900

(2470)

(0.62^[c]

588

(0.36^[c]

634

(0.64^[c]

591

(0.52^[c]

646

(60)

chloromethane

solution

(0.64^[c]

550

(0.40^[c]

657

(44)

190

(27)

160

(42)

220

(77)

160

(38)

210

(17)

almost two-thirds of that (0.36)

of 1c with only the benzene

spacer (Table 1). In star-burst-

type BTDs, the f_fvalue (0.07 in

dichloromethane) of 10 with a

triphenylamine core is signifi-

cantly lower than that (0.39 in

dichloromethane) of 8 with a

benzene core. In spite of this

disadvantage, 10 is still a good

emitter, because the e value of

10 is quite large (115400) com-

pared with other BTD deriva-

tives without star-burst struc-

(0.98^[d]

661

)

(0.14^[d]

689

)

(0.56^[c]

617

)

(0.15^[c]

683

)

(417)

480

(392)

443

(416)

484

(380)

437

(0.65^[c]

568

(0.23^[c]

663

(0.79^[c]

564

(0.19^[c]

654

(490)

(493)

(0.63^[c]

(0.05^[c]

[a] Measured in a 110^ꢀ5m solution. [b] Measured in a 110^ꢀ6m solution. [c] Fluorescent quantum yield rela-

tive to rhodamine B in ethanol (0.65). [d] Fluorescent quantum yield relative to fluorescein in ethanol (0.97).

[e] In toluene solution, whereas the value in parentheses is in dichloromethane solution. [f] l_ex=800 nm in tol-

uene solution. [g] The maximal TPAcross-sections ( s) in toluene were estimated by using AF-50 (61 GM at

780 nm) (3.310^ꢀ3m) as a reference. The value in parentheses is absorption wavelength. 1 GM (Gçppert–

Mayer)=110^ꢀ50cm⁴s per photon per molecule. [h] Measured in a 510^ꢀ3m toluene solution. [i] Measured in

a 110^ꢀ3m toluene solution. [j] Measured in a 3.710^ꢀ3m toluene solution. [k] Measured in a 0.9410^ꢀ3m tolu-

ene solution. [l] Measured in a 2.010^ꢀ3m toluene solution. [m] Measured in a 1.610^ꢀ3m toluene solution.

ture. Aparameter of

ef_f

(8080 in dichloromethane) in 10

was higher than that (2470) of 6

decreasing the fluorescence in-

tensity (Figure 4 and Table 3).

Table 2. Single- and two-photon properties of star-burst-type 2,1,3-benzothiadiazoles 8–11.

The fluorescent quantum yields

(f_f) from 0.85 in cyclohexane to

0.22 in DMF were reduced

indeed with increasing the sol-

vent polarity (Table 3). On the

other hand, the SPAmaxima

(l_max) of 1c were independent

[e]

Compd

l_max/nm (10^ꢀ2e)^[a]

l_SPEF/nm^[b](f_f)^[c]

f_fe^[d]

f_fs^[d]

/GM

l_TPEF

s/GM^[f]

/nm

s/e_p

Toluene

CH₂Cl₂

Toluene

CH₂Cl₂

/nm

451

446

577

636

50400

(27800)

15900

(9840)

81100

(8080)

23000

(5090)

560

(310)

120

(82)

560

(56)

170

591

581

587

586

780^[g]

(720)

170^[h]

(720)

800^[i]

(780)

230^[j]

(780)

6.5

3.8

6.0

4.0

(700)

448

(714)

445

(0.72)

577

(0.39)

636

(227)

459

(205)

455

(0.70)

583

(0.48)

638

(1159)

452

(1154)

447

(0.70)

584

(0.07)

635

the

solvent

polarity

(Table 3). These results indicate

that the structure of the excited

state (assumed to be the first

excited state S₁) in BTD dyes is

more polar than that of the

ground state. One can conclude

that in the BTD dyes the ICT

actually occurs between the ter-

minal amino group and the

BTD ring.

(319)

(318)

(0.72)

(0.16)

(37)

[a] Measured in a 110^ꢀ5m solution. [b] Measured in a 110^ꢀ6m solution. [c] Fluorescent quantum yield rela-

tive to rhodamine B in ethanol (0.65). [d] In toluene solution, whereas the value in parentheses is in dichloro-

methane. [e] l_ex=800 nm in toluene solution. [f] The maximal TPAcross-sections ( s) in toluene were estimat-

ed by using AF-50 (61 GM at 780 nm) (3.310^ꢀ3m) as a reference. The value in parentheses is absorption

wavelength. 1 GM =110^ꢀ50cm⁴s per photon per molecule. [g] Measured in a 1.510^ꢀ3m toluene solution.

[h] Measured in a 2.010^ꢀ3m toluene solution. [i] Measured in a 1.010^ꢀ3m toluene solution. [j] Measured in a

3.310^ꢀ3m toluene solution.

As described above, the present BTD dyes can emit

orange-red light. In polar dichloromethane, the emitting

ability significantly changed depending on the molecular

structure of the BTD dyes. In 1a–e with only a benzene

spacer, the f_fvalue was affected by the substituents on the

nitrogen atom. The highest f_fvalue was observable in 1d

with 2-naphthyl groups to be 0.64 in nonpolar toluene and

0.52 in polar dichloromethane solution. Compared with 1c–

e with two diarylamino groups, 1a with two dimethylamino

whereas it is comparable to that (7020) of a good emitter 1c

(Tables 1 and 2).^[18]Star-burst-type BTD 8 with a rigid ben-

zene core exhibits the highest ef_fvalue (27800) among all

the BTD derivatives in dichloromethane. In contrast, in tol-

uene the order of f_fvalues was appropriately reflected on

that of ef_fvalues because of considerably high f_fvalues

of all the BTD derivatives. Thus, 10 shows the highest ef_f

value (81100), reflecting both high e and f_fvalues in tolu-

ene.

Chem. Eur. J. 2006, 12, 2303 – 2317

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2307

S. Mataka et al.

Figure 1. Single-photon absorption spectra of 1c and 2–6 in toluene at 1

10^ꢀ5m.

Figure 4. Single-photon excited fluorescence spectra of 1c in cyclohexane,

toluene, THF, dichloromethane, benzonitrile, and DMF at 110^ꢀ6m.

Table 3. Charge-transfer absorption and emission wavelength of 1c in

different solvents.

Cyclo

Toluene THF

CH₂Cl₂PhCN

DMF

hexane

l_max/nm

460

(189)

557

461

(205)

592

460

(207)

619

459

(195)

639

467

(197)

643

460

(187)

657

(10^ꢀ2e)^[a]

l_SPEF/nm^[b]

(f_f^[c]

)

(0.85)

(0.62)

(0.51)

(0.36)

(0.34)

(0.22)

[a] Measured in a 110^ꢀ5m solution. [b] Measured in a 110^ꢀ6m solution.

[c] Fluorescent quantum yield relative to rhodamine B in ethanol (0.65).

Figure 2. Single-photon absorption spectra of 8–11 in toluene at 1

10^ꢀ5m.

Figure 3. Single-photon excited fluorescence spectra of 4 and 10 in tolu-

ene and dichloromethane at 110^ꢀ6m.

Figure 5. ORTEP drawings of a) 1c and b) 7.

Structural properties of BTDs: The pronounced ICT charac-

ter of BTD derivatives was supported by X-ray crystallo-

graphic analysis. Single crystals of 1c and 7^[34a]with and

without the donating amino groups, respectively, were

grown by slow diffusion of hexane into chloroform solutions

(Figure 5). The bond-length alternation in the spacer ben-

zene rings is a good indication for the ICT from the amino

groups (donor) to the BTD ring (acceptor), which can be

expressed by the quinoid character (d_r) of the ring defined

by^[11]

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Chem. Eur. J. 2006, 12, 2303 – 2317

Red-Fluorescent Dyes

FULL PAPER

d_r¼ fða_nꢀb_nÞ þ ðc_nꢀb_nÞg=2

ꢀ

where a_n, b_n, and c_n(n=1–8) are the C C bond lengths in

the benzene ring (see Figure 5 and Table 4). As expected,

the normalized d_rvalue (0.0185) of 1c is higher by one

order of magnitude compared with that (0.00194) of 7. The

ICT character is reflected also in the bond length between

the BTD ring and the spacer benzene ring. In 7, the bond

lengths are 1.498(4), 1.495(4), 1.500(4), and 1.497(4) in

ꢀ

C3 C7, C6 C13, C21 C25, and C24 C31, respectively. In

ꢀ

contrast, the corresponding bond lengths of C3 C7 and C6

C25 in 1c are reduced to 1.475(3). These results indicate

that the connection of the electron-donating amino group to

the electron-accepting BTD ring via the p-conjugated

spacer is crucial for the highly-enhanced ICT character in

BTD derivatives.

Figure 6. Dependence of the output fluorescence intensity on the input

laser density in 1a ( ), 1c ( ), and AF-50 ( ).

Two-photon absorption and emission properties: Two-

photon excited fluorescence (TPEF) spectra of BTD deriva-

tives were recorded in toluene solution at 298 K. To reduce

the possibility of excited state absorption, a femtosecond Ti/

sapphire laser with the pulse width of 120 fs was adopted as

the excitation source in this TPEF experiment. Upon excita-

tion at 800 nm laser pulses, all the BTDs emitted frequency-

upconverted orange-red fluorescence at the wavelength

region of 552–670 nm (Tables 1 and 2). For the TPEF mea-

surement, the dependence of the output fluorescence inten-

sity on the input laser density fits to square parabolas. Se-

lected examples 1a and 1c are shown in Figure 6 together

with AF-50 having D–p–Astructure, which is a typical TPA

material developed by Prasadꢁs group.^[12a]The result shows

that the orange-red fluorescence observed in BTD deriva-

tives was derived from two-photon excitation. When the

input laser density is above a certain threshold, for example,

94 GWcm^ꢀ2for 1c, the square-dependence begins to devi-

ate, implying that some uncertain photophysical processes

which cause fluorescence saturation are involved as reported

previously.^[13b]The TPEF bands (l_TPEF) of BTD derivatives

shifted to longer wavelength by 2–25 nm compared with the

SPEF bands (l_SPEF) (Tables 1 and 2). The red-shift is attrib-

uted to the re-absorption of the fluorescence under the con-

Figure 7. Single-photon absorption (SPA, at 110^ꢀ5m), single-photon ex-

cited fluorescence (SPEF, at 110^ꢀ6m), and two-photon excited fluores-

cence (TPEF, at 210^ꢀ3m) spectra of 4 in toluene.

Table 4. Selected bond length [] in 1c and 7.

centrated solution conditions

(~10^ꢀ3m).^[18,39]As an example, a

ꢀ

C7 C8 (a₁)

1.399(3)

1.398(3)

1.402(3)

1.475(3)

C8 C9 (b₁)

1.378(2)

1.388(2)

1.383(3)

1.388(3)

1.477(3)

C9 C10 (c₁)

1.400(3)

1.397(2)

1.399(3)

1.396(3)

set of SPA, SPEF, and TPEF

ꢀ

C7 C12 (a₂)

C11 C12 (b₂)

C10 C11 (c₂)

spectra of was shown in

ꢀ

C25 C26 (a₃)

C26 C27 (b₃)

C27 C28 (c₃)

ꢀ

Figure 7, in which the shorter

wavelength region of the SPEF

band partially overlaps with the

tail of the linear absorption

peak to effect the re-absorp-

tion. Considering this finding,

one can conclude that SPEF

and TPEF come from the same

fluorescence excited state, al-

though the energy level of the

two-photon excited state is

higher than that of the single-

C25 C30 (a₄)

C29 C30 (b₄)

C28 C29 (c₄)

ꢀ

C3 C7

C6 C25

ꢀ

C7 C8 (a₁)

1.402(3)

1.406(4)

1.403(4)

1.400(3)

1.402(4)

1.397(4)

1.397(5)

1.406(4)

1.498(4)

1.500(4)

C8 C9 (b₁)

1.386(4)

1.393(4)

1.396(4)

1.397(4)

1.398(4)

1.393(4)

1.397(4)

1.495(4)

1.497(4)

C9 C10 (c₁)

1.390(5)

1.392(3)

1.389(3)

1.386(5)

1.390(5)

1.388(4)

1.396(4)

1.387(5)

ꢀ

C7 C12 (a₂)

C11 C12 (b₂)

C10 C11 (c₂)

ꢀ

C13 C14 (a₃)

C14 C15 (b₃)

C15 C16 (c₃)

ꢀ

C13 C18 (a₄)

C17 C18 (b₄)

C16 C17 (c₄)

ꢀ

C25 C26 (a₅)

C26 C27 (b₅)

C27 C28 (c₅)

ꢀ

C25 C30 (a₆)

C29 C30 (b₆)

C28 C29 (c₆)

ꢀ

C31 C32 (a₇)

C32 C33 (b₇)

C33 C34 (c₇)

ꢀ

C31 C36 (a₈)

C35 C36 (b₈)

C34 C35 (c₈)

ꢀ

C3 C7

C6 C13

ꢀ

C21 C25

C24 C31

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2309

S. Mataka et al.

photon excited state due to the different spectral selection

rule in centro-symmetrical molecules. The l_TPEFvalues of all

the BTD derivatives are independent of the excitation

wavelength from 700 to 800 nm of the tunable laser, but the

TPEF intensity evidently depends on the wavelength.

TPAcross-sections ( s) of all the BTD derivatives at 700–

800 nm were determined by open aperture Z-scan tech-

nique.^[33]As a whole, plots of TPA cross-sections against the

excitation wavelength exhibited two maxima around 720

and 780 nm corresponding to the p–p* and CT transitions

observed in the SPAspectra (Figures 8–10), which indicate

Figure 9. Two-photon absorption spectra of 1c ( ) and 2–6 (2: , 3: , 4:

, 5: , 6: ) in toluene. The s values of 3 at 700 and 720 nm (1300 and

670 GM, respectively) are out of range of the plot area to keep clarity of

other data.

which probably is correlated with the less steric hindrance

between the BTD core and the thiophene ring as well as the

high electron-donating ability of the thiophene ring. These

observations suggest that the planarity of p-conjugated

spacers affects also the TPAcross-sections. In 3 and 4, the

significant increase of s values observed below 740 nm is in-

terpreted as one-photon resonance enhancement on two-

photon transition at a short wavelength near single-photon

transition, as reported in the precedent report (Figure 9).^[40]

In particular, 3 provides the extremely large s values of

1300 GM at 700 nm and 670 GM at 720 nm.

Figure 8. Two-photon absorption spectra of 1a–e (a: , b: , c: , d: ,

e: ) and AF-50 ( ) in toluene.

The influence of solvent polarity on the TPAcharacteris-

tics of BTD derivatives was investigated by measuring TPA

cross-sections of 1c in various solvents by 800 nm excitation.

The s values of 1c are almost the same in toluene, THF,

and DMF (83, 84, and 69 GM, respectively), although the

value (110 GM) in dichloromethane was slightly larger than

in other solvents (Table 5). The result implies that the exci-

the spectral similarity between SPAand TPA. The TPA

maxima are observed at shorter wavelength positions com-

pared to the SPAmaxima, because the TPA-allowed excited

state exists above the SPAexcited state for this kind of cen-

trosymmetrical molecules.

As summarized in Tables 1 and 2, all BTD derivatives ex-

hibited large s values around the near IR region derived

from ICT compared to AF-50 used as a TPA benchmark.^[12a]

Figure 8 shows the plots of TPAcross-sections of 1a–e with

various substituents on the nitrogen atom. The maximal s

values of 1a–e (110–130 GM) were about two times larger

than that of AF-50 (61 GM) (Table 1). The substituents such

as methyl, phenyl, 1-naphthyl, and 2-naphthyl groups on the

nitrogen atom slightly affect the maximal s values and TPA

peaks.

Table 5. Dependence of TPAcross-sections

( s) and TPEF maxima

(l_TPEF) of 1c on the solvent polarity by excitation at 800 nm.

Toluene

THF

CH₂Cl₂

DMF

s/GM^[a]

l_TPEF/nm

599

617

110

648

673

[a] TPAcross-sections ( s) were estimated by using AF-50 (45 GM at

800 nm, in toluene) as a reference.

Contrary to the effect of substituents on the nitrogen

atom in 1a–e, the expansion of p-conjugation between the

BTD unit and the amino groups is crucial in increasing TPA

cross-sections. The maximum s values of 2–6 with the addi-

tional p-conjugated spacers are about 1.5–2.5 times larger

than that of 1c (Figure 9). The most significant TPAen-

hancement is achieved by the additional ethene spacers in 4

and 6 with s value of 330 GM. By contrast, the introduction

of the additional benzene spacer in 2 (200 GM) and the

ethyne spacer in 5 (200 GM) provides moderate TPAen-

hancement. Interestingly, the introduction of the thiophene

spacer in 3 (280 GM) moderately increases the s value,

tation process in TPAis hardly affected by the solvent polar-

ity. The finding can be rationalized by the less polar struc-

ture of the ground state compared with that of the excited

state as found in SPA(Table 3). The TPEF spectra of 1c dis-

play a significant red-shift from 599 to 673 nm proportional

to the solvent polarity as observed in SPEF (Table 5).

Figure 10 shows the plots of TPAcross-sections of three-

branched star-burst-type BTDs 8 and 10, and their one-di-

mensional sub-units

9 and 11. The maximal s value

(780 GM) of 8 with a benzene core is about 4.6 times larger

than that (170 GM) of 9. Similarly, the maximal s value

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Chem. Eur. J. 2006, 12, 2303 – 2317

Red-Fluorescent Dyes

FULL PAPER

contrast, 10 shows low f_fs value of 56 GM because of its

low f_fvalue.

Conclusion

In conclusion, we have demonstrated that a series of D–p–

A–p–D-type and star-burst-type benzothiadiazole-based

dyes possessing terminal N,N-disubstituted amino groups

provide high fluorescence quantum yields among the

orange-red color region as well as large two-photon absorp-

tion cross-sections in the near IR region. The two-photon

absorption activity is attributed to the electron-accepting

nature of the benzothiadiazole unit, which causes significant

intramolecular charge-transfer even in the ground state by

the combination with the electron-donating amino groups

connected with the p-conjugated spacers. Further, the ex-

tended p-conjugate system including the donor and acceptor

moieties lowers the excited state to emit the attractive red-

orange fluorescence. In the D–p–A–p–D system, the length

and planarity of the p-conjugated spacers between the ben-

zothiadiazole unit and the amino groups play an important

role in increasing the two-photon absorption activity. In ad-

dition, the combination of dimensionality and extended p-

system leads to significant increase of the two-photon ab-

sorption cross-sections in star-burst structure. The present

benzothiadiazole-based two-photon absorbing dyes with

red-orange fluorescence emitting ability are attractive candi-

dates as a new material toward two-photon laser scanning

microscopy. However, we need more efforts to create a

useful optical emission window in near IR region (650–

900 nm) for fluorescence bioimaging.^[42,43]We believe that

the present study provides valuable information for the syn-

thesis of new two-photon absorbing materials.

Figure 10. Two-photon absorption spectra of 8–11 (8: , 9: &, 10: , 11:

) in toluene.

(800 GM) of 10 with a triphenylamine core is about 3.4

times larger than that (230 GM) of 11. One can conclude

that the increasing p-conjugation length as well as multidi-

mensionality leads to cooperatively increasing the TPA

cross-sections in 8 and 10. In order to precisely discuss the

TPAenhancement, the s values were normalized to the

number of p-electrons (e_p) to obtain a parameter of s/e_p.^[30]

The ratio of s/e_pvalues between three-branched and one-di-

mensional BTD derivatives are 6.5:3.8 (1.7-fold enhance-

ment) in benzene core system 8 and 9, and 6.0:4.0 (1.5-fold

enhancement) in triphenylamine core system 10 and 11

(Table 2). The observed trend of 1.5–1.7-fold enhancement

is larger than that predicted simply on the basis of the chro-

mophore number density (i.e., 1:1) according to the increas-

ing of branch numbers from one to three. Similar TPAen-

hancement in the star-burst structure was reported for tri-

phenylamine core^[25a,c,g]and triazine core systems.^[27b,c]Com-

pared with the previous examples, the present BTD deriva-

tives provide a comparable cooperative effect in the three-

branched structure, although a significant TPAenhancement

of 13–23-fold was achieved in a polar pyridinium core

system.^[30]It can be emphasized that the TPAenhancement

achieved in 8 is the first example of a benzene core, which

has weaker electron-donating and accepting abilities com-

pared with the previous triphenylamine and triazine cores,

respectively.

For the development to two-photon laser scanning fluo-

rescence imaging, the product of f_fs in two-photon ab-

sorbing fluorophore is an important factor,^{[8a,e,19a,41]}when

the f_fvalues in both SPAand TPAprocess are assumed to

be the same. As summarized in Tables 1 and 2, the order of

s values was appropriately reflected on that of f_fs values

in toluene solution because of the relatively high f_fvalues

of all the BTD derivatives. Thus, star-burst 8 and 10 exhibit

quite large f_fs values (560 GM) compared with any other

D–p–A–p–D-type derivatives, for example, 1c (83 GM) and

4 (220 GM). In dichloromethane, the f_fs value (310 GM)

of 8 is still larger by one order of magnitude than those (16–

77 GM) of other D–p–A–p–D-type derivatives, even under

the unfavored polar conditions for fluorescence emitting. In

Experimental Section

General methods: All melting points are uncorrected. IR spectra were

recorded on a JASCO FT/IR-470 plus Fourier transform infrared spec-

trometer and measured as KBr pellets. ¹H NMR spectra were determined

in CDCl₃or [D₆]DMSO with a JEOL EX-270, and GSX-270 spectrome-

ter. Residual solvent protons were used as internal standard and chemical

shifts (d) are given relative to tetramethylsilane (TMS). The coupling

constants (J) are given in Hertz. Elemental analysis was performed at the

Elemental Analytical Center, Kyushu University. EI-MS spectra were re-

corded with a JEOL JMS-70 mass spectrometer at 70 eV using a direct

inlet system. FAB-MS spectra were recorded with a JEOL JMS-70 mass

spectrometer with m-nitrobenzyl alcohol as a matrix. Matrix assisted

laser desorption ionization time-of-flight (MALDI-TOF) mass spectrom-

etry was performed on a PerSeptive Biosystems Voyager-DE spectrome-

ter by using delayed extraction mode and with an acceleration voltage of

20 keV. Samples were prepared from a solution of dichloromethane by

using dithranol as the matrix. UV/Vis spectra were measured on a

JASCO V-570 spectrophotometer in a 1 cm width quartz cell. Fluores-

cence spectra were measured on a HITACHI F-4500 fluorescence spec-

trophotometer in a 1 cm width quartz cell. Gel-permeation chromatogra-

phy (GPC) was performed with a Japan Analytical Industry Co., Ltd.

LC-908 using JAIGEL-1H column (20600 mm) and JAIGEL-2H

column (20600 mm) eluting with chloroform (3.5 mLmin^ꢀ1). Analytical

TLC was carried out on silica gel coated on aluminum foil (Merck 60

Chem. Eur. J. 2006, 12, 2303 – 2317

ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

2311

S. Mataka et al.

F₂₅₄). Column chromatography was carried out on silica gel (Wako C-300

or KANTO 60N). THF was distilled from sodium and benzophenone

under an argon atmosphere just before use. DMF was distilled from cal-

cium hydride under reduced pressure just before use. Benzene was dried

with 4 molecular sieve over night before use. 4-Dimethylaminophenyl-

boronic acid (13a) and 4-diphenylaminobenzaldehyde (22) are commer-

cially available. 4,7-Dibromo-2,1,3-benzothiadiazole (12),^[44]N-(4-bromo-

phenyl)-1-naphthylphenylamine,^[45]4-(N,N-diphenylamino)phenylboronic

acid (13b),^[46]4-[N-(2-naphthyl)-N-phenyl]phenylboronic acid (13d),^[47]4-

(N,N-diphenylamino)biphenyl-4’-boronic acid (13e),^[48]4,7-bis(5-bromo-

2-thienyl)-2,1,3-benzothiadiazole (14),^[35]1-(N,N-diphenylamino)-4-vinyl-

benzene (15),^[49]4-diphenylaminoiodobenzene (18),^[45]4,7-bis[(4-hydroxy-

methyl)phenyl]-2,1,3-benzothiadiazole (19),^[37]4-[4-(N,N-diphenylamino)-

phenyl]-7-(4-formylphenyl)-2,1,3-benzothiadiazole (23),^[38]1,3,5-tris(dime-

thoxyphosphorylmethyl)benzene (27),^[50]and tris(4-formylphenyl)amine

(29)^[51]were prepared according to methods described previously. Benzyl-

triphenylphosphonium bromide (28) was prepared from benzyl bromide

and triphenylphosphine according to common method. Preparation of 1c

and 3 were reported by another group by using Stille coupling reac-

tions.^[35]

258C, TMS): d=3.05 (s, 6H; CH₃), 6.89 (d, J=8.6 Hz, 2H; ArH), 7.05 (t,

J=7.3 Hz, 2H; ArH), 7.16–7.32 (m, 10H; ArH), 7.70 (d, J=7.6 Hz, 1H;

ArH), 7.73 (d, J=7.6 Hz, 1H; ArH), 7.88 (d, J=8.6 Hz, 2H; ArH),

7.93 ppm (d, J=8.6 Hz, 2H; ArH); IR (KBr): n˜ =1610, 1590, 1489, 1349,

1282, 1197, 887, 813 cm^ꢀ1; EI-MS (70 eV): m/z: 498 [M ⁺]; elemental

analysis calcd (%) for C₃₂H₂₆N₄S (498.6): C 77.08, H 5.26, N 11.24;

found: C 76.81, H 5.28, N 11.16.

4,7-Bis[4-(N,N-diphenylamino)phenyl]-2,1,3-benzothiadiazole (1c):^[35]

solution of 13b (112 mg, 0.18 mmol) in ethanol (3 mL) and aqueous 2m

Na₂CO₃(10 mL) were added at 608C under an argon atmosphere to a

mixture of 12 (46 mg, 0.16 mmol) and tetrakis(triphenylphosphine)palla-

dium(0) (5.5 mg, 0.0048 mmol) in benzene (20 mL). After the mixture

was heated at 858C for 12 h, the reaction mixture was poured into water

(50 mL) and extracted with toluene (30 mL3). The combined organic

layers were dried over anhydrous magnesium sulfate and evaporated in

vacuo to dryness. The residue was separated by silica gel column chroma-

tography (KANTO 60N) eluting with chloroform/hexane 1:2 to give an

orange powder (50 mg), which was the mixture of 1c and 4-bromo-7-[4-

(N,N-diphenylamino)phenyl]-2,1,3-benzothiadiazole. To a mixture of this

orange powder (50 mg) and tetrakis(triphenylphosphine)palladium(0)

(3.8 mg, 0.0033 mmol) in benzene (15 mL) were added a solution of 13b

(124 mg, 0.21 mmol) in ethanol (1 mL) and aqueous 2m Na₂CO₃(7 mL)

at 608C under an argon atmosphere. After the mixture was heated at

858C for 12 h, the reaction mixture was poured into water (50 mL) and

extracted with toluene (20 mL3). The combined organic layers were

dried over anhydrous magnesium sulfate and evaporated in vacuo to dry-

ness. The residue was purified by silica gel column chromatography

(KANTO 60N) eluting with chloroform/hexane 1:2 to give 1c (52 mg,

0.084 mmol, 53%) as a red powder. An analytical sample was obtained

by gel-permeation chromatography eluting with chloroform. M.p. 236–

2388C; ¹H NMR (270 MHz, CDCl₃, 258C, TMS): d=7.06 (t, J=8.5 Hz,

4H; ArH), 7.17–7.32 (m, 20H; ArH), 7.74 (s, 2H; ArH), 7.88 ppm (d, J=

8.5 Hz, 4H; ArH); IR (KBr): n˜ =1590, 1513, 1484, 1328, 1278, 825,

754 cm^ꢀ1; FAB-MS (NBA): m/z: 622 [M ⁺]; elemental analysis calcd (%)

for C₄₂H₃₀N₄S (622.8): C 81.00, H 4.86, N 9.00; found: C 80.79, H 4.87, N

9.02.

4-Bromo-7-[4-(N,N-dimethylamino)phenyl]-2,1,3-benzothiadiazole: Aso-

lution of 13a (400 mg, 3.40 mmol) in ethanol (5 mL) and aqueous 2m

Na₂CO₃(25 mL) were added at 608C under an argon atmosphere to a

mixture of 12 (1.00 g, 3.40 mmol) and tetrakis(triphenylphosphine)palla-

dium(0) (118 mg, 0.10 mmol) in benzene (50 mL). After the mixture was

heated at 858C for 12 h, the reaction mixture was poured into water

(120 mL) and extracted with toluene (40 mL3). The combined organic

layers were dried over anhydrous magnesium sulfate and evaporated in

vacuo to dryness. The residue was purified by silica gel column chroma-

tography (Wako C-300) eluting with chloroform/hexane 2:5 and recrystal-

lization from chloroform/hexane to give 4-bromo-7-[4-(N,N-dimethylami-

no)phenyl]-2,1,3-benzothiadiazole (772 mg, 2.31 mmol, 68%) as orange

prisms. M.p. 179–1808C; ¹H NMR (270 MHz, CDCl₃, 258C, TMS): d=

3.05 (s, 6H; CH₃), 6.86 (d, J=8.9 Hz, 2H; ArH), 7.51 (d, J=7.6 Hz, 1H;

ArH), 7.83–7.88 ppm (m, 3H; ArH); IR (KBr): n˜ = 1609, 1513, 1479,

1362, 1200, 884, 810 cm^ꢀ1; EI-MS (70 eV): m/z: 333, 335 [M ⁺]; elemental

analysis calcd (%) for C₁₄H₁₂N₃BrS (334.2): C 50.31, H 3.62, N 12.57;

found: C 50.31, H 3.60, N 12.55.

4,7-Bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-2,1,3-benzothiadiazole

(1d): According to a method similar to the preparation of 1c, 1d was ob-

tained in 73% yield from 12 and 13c. The crude product was purified by

silica gel column chromatography (KANTO 60N) eluting with chloro-

form/hexane 3:2. An analytical sample was obtained by gel-permeation

chromatography eluting with chloroform as a red powder. ¹H NMR

(270 MHz, CDCl₃, 258C, TMS): d=7.10 (t, J=7.3 Hz, 2H; ArH), 7.22–

7.45 (m, 18H; ArH), 7.54–7.66 (m, 4H; ArH), 7.75–7.80 (m, 4H; ArH),

7.78 (s, 2H; ArH), 7.92 ppm (d, J=8.6 Hz, 4H; ArH); IR (KBr): n˜ =

1592, 1508, 1480, 1279, 811, 747 cm^ꢀ1; FAB-MS (NBA): m/z: 722 [M ⁺];

elemental analysis calcd (%) for C₅₀H₃₄N₄S·0.1CHCl₃(734.8): C 81.89, H

4.68, N 7.62; found: C 82.18, H 4.85, N 7.35.

4,7-Bis[4-(N,N-dimethylamino)phenyl]-2,1,3-benzothiadiazole (1a):

A n

solution of 13a (138 mg, 0.83 mmol) in ethanol (2 mL) and aqueous 2m

Na₂CO₃(25 mL) were added at 608C under an argon atmosphere to a

mixture of 4-bromo-7-[4-(N,N-dimethylamino)phenyl]-2,1,3-benzothiadi-

azole (200 mg, 0.60 mmol) and tetrakis(triphenylphosphine)palladium(0)

(21 mg, 0.018 mmol) in benzene (20 mL). After the mixture was heated

at 858C for 12 h, the reaction mixture was poured into water (100 mL).

The precipitates were collected by filtration and washed well with water

and benzene to give 1a (130 mg, 0.35 mmol, 58%) as red needles. An an-

alytical sample was obtained by gel-permeation chromatography eluting

with chloroform. M.p. 217–2188C; ¹H NMR (270 MHz, CDCl₃, 258C,

TMS): d=3.04 (s, 12H; CH₃), 6.89 (d, J=8.5 Hz, 4H; ArH), 7.68 (s, 4H;

ArH), 7.91 ppm (d, J=8.5 Hz, 2H; ArH); IR (KBr): n˜ =1611, 1528, 1478,

1444, 1364, 1229, 1202, 1113, 811 cm^ꢀ1; FAB-MS (NBA): m/z: 374 [M ⁺];

elemental analysis calcd (%) for C₂₂H₂₂N₄S (374.5): C 70.56, H 5.92, N

14.96; found: C 70.31, H 5.68, N 15.06.

4,7-Bis{4-[N-(2-naphthyl)-N-phenylamino]phenyl}-2,1,3-benzothiadiazole

(1e): According to a method similar to the preparation of 1c, 1e was ob-

tained in 53% yield from 12 and 13d. The crude product was purified by