Synthesis and radiation sensitivity of phenoxazine type color formers including thiol ester protective group

Article abstract of DOI:10.1080/15421400590947126

3,7-Bis(N,N-diethylamino)-10-(phenylthio)carbonylphenoxazine (1a) and its analogs (1b-f) were synthesized and the color change after γ irradiation was investigated. The color change of acetonitrile solutions of 1a ([1a] ₀ = 1.0 × 10^-3 M) after γ irradiation was identified by naked eyes at the dose of 20 Gy. Phenylthio-substituted compound la showed more significant absorbance increase than phenoxy-substituted compound, 3,7-bis(N,N-diethylamino)-10-phenoxycarbonylphenoxazine (2a). The C-S cleavage in la by γ irradiation was revealed to occur more easily than the C-O cleavage in 2a. The sensitivity of the color formers 1a-f) to γ rays was correlated with the stability of the thiyl radicals generated in the reaction.

Full text of DOI:10.1080/15421400590947126

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Synthesis and Radiation
Sensitivity of Phenoxazine
Type Color Formers Including
Thiol Ester Protective Group
Tatsuya Tachikawa a , Yohei Sato a & Sumio Tokita a
a Department of Applied Chemistry , Faculty of
Engineering, Saitama University , Saitama, Japan
Published online: 31 Aug 2006.
To cite this article: Tatsuya Tachikawa , Yohei Sato & Sumio Tokita (2005) Synthesis
and Radiation Sensitivity of Phenoxazine Type Color Formers Including Thiol Ester
Protective Group, Molecular Crystals and Liquid Crystals, 431:1, 461-466
To link to this article: http://dx.doi.org/10.1080/15421400590947126
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Mol. Cryst. Liq. Cryst., Vol. 431, pp. 161=[461]–166=[466], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590947126
Synthesis and Radiation Sensitivity of Phenoxazine Type
Color Formers Including Thiol Ester Protective Group
Tatsuya Tachikawa
Yohei Sato
Sumio Tokita
Department of Applied Chemistry, Faculty of Engineering, Saitama
University, Saitama, Japan
3,7-Bis(N,N-diethylamino)-10-(phenylthio)carbonylphenoxazine (1a) and its ana-
logs (1b–f) were synthesized and the color change after c irradiation was investi-
gated. The color change of acetonitrile solutions of 1a ([1a]0 ¼ 1.0 ꢀ 10ꢁ3 M)
after c irradiation was identified by naked eyes at the dose of 20 Gy. Phenylthio-
substituted compound 1a showed more significant absorbance increase than
phenoxy-substituted compound, 3,7-bis(N,N-diethylamino)-10-phenoxycarbonyl-
phenoxazine (2a). The CꢁS cleavage in 1a by c irradiation was revealed to occur
more easily than the CꢁO cleavage in 2a. The sensitivity of the color formers (1a–f)
to c rays was correlated with the stability of the thiyl radicals generated in the
reaction.
Keywords: c rays; color former; dosimetry system; phenoxazine dyes; substituent effect
INTRODUCTION
c rays are useful in various fields such as radiometric analysis, food
hygiene or medical treatment, but they can ruin our health and cause
cancer and even death. We cannot detect c rays by any of our senses.
The visualization of c rays using leuco dyes [1,2], which change their
color by c irradiation, must be important to study.
The CꢁS cleavage by c irradiation would occur more easily than that
of CꢁO bonds [3,4]. In out previous study [5], tris(4-N,N-dimethylamino-
phenyl)methanethiol could produce the colored form (kmax ¼ 587.5 nm)
by CꢁS cleavage initiated by c rays, and the absorbance change at
Address correspondence to Tatsuya Tachikawa, Department of Applied Chemistry,
Faculty of Engineering, Saitama University, 255 Shimo-Ohkubo, Sakura-ku, Saitama,
Saitama 338-8570, Japan. E-mail: tachikaw@apc.saitama-u.ac.jp
161=[461]

162=[462]
T. Tachikawa et al.
587.5 nm after c irradiation of 10 Gy was about 100 times larger than
that of a CꢁO analog, tris(4-N,N-dimethylaminophenyl)methanol. The
introduction of a CꢁS bond into a cleavage position by c irradiation will
be a key concept to improve the sensitivity to c rays. The leuco phenox-
azine color formers, which consisted of a precursor of phenoxazine dye
and a protective group, were revealed to give the colored form 3
(cmax ¼ 644 nm) by c irradiation in our previous study [6]. As leuco
phenoxazine type color formers are more stable to heat or light than tri-
arylmethane type color formers, the leuco phenoxazine type color
formers with a CꢁS bond at a cleavage position will be an excellent
component of c rays detection system.
In this paper, synthesis and sensitivity to c rays of phenoxazine
type color formers including a thiol ester protective group (1a–f),
and their substituent effect on the sensitivity to c rays will be reported.
RESULTS AND DISCUSSION
Phenoxazine type color formers including a thiol ester protective
group (1a–f) were prepared by the analogous method with our pre-
vious paper [7] as shown in Scheme 1. 3,7-Bis(diethylamino)-10-chlor-
oformylphenoxazine (4) was obtained by reduction of basic blue 3
ꢁ
ꢂ
(3 Cl ) using sodium dithionite, followed by addition of toluene
solution of triphosgen. Sodium thiolates (RSNa) were prepared from
SCHEME 1

Synthesis and Radiation Sensitivity of Phenoxazine
163=[463]
sodium and corresponding thiols. Reaction between the chloroformyl-
phenoxazine (4) and the sodium thiolates gave the compounds 1a–f as
white or pale yellow crystals. The yields were improved by using
dimethoxyethane (DME) instead of tetrahydrofurane (THF) as solvent
for the preparation of 1a–f [8,9].
c rays were irradiated to dry acetonitrile solutions of leuco dyes
(1.0 ꢀ 10ꢁ3 M) with a dose rate of 600 Gy=h using 60Co source. The elec-
tronic spectra of the solutions after c irradiation were recorded on a
spectrometer. The absorbance at 644 nm increased gradually as the
irradiated dose increased. As the peak at the first absorption band of
basic blue 3 was at 644 nm, generation of colored form 3 after c
irradiation was obvious. This color change to blue can be recognized
by naked eyes after c irradiation of 20 Gy.
The absorbance change observed in 1a was compared with that of
phenoxy compound 2a (Fig. 1). The absorbances at 644 nm obtained
for 1a were larger than those for 2a after irradiation from 10 Gy to
1100 Gy and the absorbance after irradiation of 1100 Gy was about
1.4 times larger than that for 2a. Introduction of a sulfur atom into
a cleavage position was effective to increase the sensitivity to c
rays of leuco phenoxazine type color formers. However, the introduc-
tion of a sulfur atom was less effective to increase the sensitivity to
c rays of leuco phenoxazine type color formers than we expected. By
FIGURE 1 Absorbance changes at 644 nm observed in acetonitrile solutions
of 1a (^) or 2a (.) to give
[1a]0 ¼ [2a]0 ¼ 1.0 ꢀ 10ꢁ3 M.
3
(kmax ¼ 644 nm) after
c irradiation,

164=[464]
T. Tachikawa et al.
the effect of the neighboring carbonyl group, the bond order of CꢁS
bond in the thiol ester became larger than CꢁS bond [10,11], and
the CꢁS cleavage must become more difficult than that in the triaryl-
methanethiol type color formers.
The effect of the substituents was investigated using phenylthio
derivative (1a), 1-naphthylthio derivative (1e), and t-butylthio deriva-
tive (1f). The absorbance change at 644 nm observed for 1a, 1e or 1f
after c irradiation is shown in Figure 2 (A). The order of the sensitivity
to c rays was 1e > 1a > 1f. The order was explained by the stability of
the thiyl radicals generated by c irradiation (Fig. 3 (A)). A free electron
is stabilized by the delocalization in the conjugated system. The order
of the stability of the thiyl radicals was 5e > 5a > 5f, judging from the
extent of conjugation. This order coincided with that of the sensitivity
to c rays of the leuco dyes. Generation of the thiyl radicals by c
irradiation would be made certain by MS spectra. Fragment peaks
at the same mass number position with the corresponding thiyl radi-
cals were detected in MS spectra of the color formers.
The effect of the substituent at p-position of the phenyl group in
leuco dyes was investigated using 1a–d. The absorbance change at
644 nm observed for 1a–d is shown in Figure 2 (B). The order of the
sensitivity to c rays of 1a–d was 1b > 1a > 1c > 1d. This order could
be explained by the same consideration as mentioned above for 1a, 1e,
and 1f. The order was same with the electron withdrawing ability of
FIGURE 2 (A) Absorbance changes at 644 nm observed in acetonitrile sol-
ution of 1a (^), 1e (.) or 1f (&) to give 3 (kmax ¼ 644 nm) after c irradiation,
[1a]0 ¼ [1e]0 ¼ [1f]0 ¼ 1.0 ꢀ 10ꢁ3 M; (B) Absorbance changes at 644 nm
observed in acetonitrile solution of 1a (^), 1b (.), 1c (&) or 1d (~) to give 3
(kmax ¼ 644 nm) after c irradiation, [1a–d]0 ¼ 1.0 ꢀ 10ꢁ3 M.

Synthesis and Radiation Sensitivity of Phenoxazine
165=[465]
FIGURE 3 Structures and orders of stability of the thiyl radicals (5a–f).
the substituent at p-position of the phenyl group. The delocalization of
a free radical to the phenyl ring increases as the electron withdrawing
ability of the substituents on the phenyl ring increases. Thus, the
order of the stability of the thiyl radicals was 5b > 5a > 5c > 5d
(Fig. 3 (B)). This order coincided with that of the sensitivity to c rays
of the leuco dyes.
The correlation between the absorbances at 644 nm observed in the
solution of 1a–d after c irradiation of 15 and 40 Gy and Hammett
sigma constants at para position is shown in Figure 4. The absor-
bances and Hammett sigma constants showed a linear relationship
and the correlation constants (R2) were above 0.99 in both case. These
correlations indicated that the sensitivity to c rays of the color formers
concerned with the stability of the thiyl radicals.
CONCLUSION
3,7-Bis(N,N-diethylamino)-10-(phenylthio)carbonylphenoxazine (1a)
and its analogs (1b–f) were synthesized and the color changes after
c irradiation were investigated. The c irradiation to acetonitrile solu-
tions of 1a ([1a]0 ¼ 1.0 ꢀ 10ꢁ3 M) was identified by color change at
the dose of 20 Gy. Phenylthio substituted compound 1a showed more
significant absorbance increase than phenoxy substituted compound,
3,7-bis(N,N-diethylamino)-10-phenoxycarbonylphenoxazine (2a). The
absorbance change at 644 nm for 1a was about 1.4 times larger than

166=[466]
T. Tachikawa et al.
FIGURE 4 Correlation between absorbance at 644 nm observed in acetoni-
trile solutions of 1a–d after c irradiation (^: 15 Gy, .: 40 Gy) and Hammett
sigma constants for substituents at p-position of phenyl ring in 1a–d.
that for 2a after irradiation of 1100 Gy. The sensitivity of the color for-
mers (1a–f) to c rays was correlated with the stability of the thiyl radi-
cals (5a–f) generated in the reaction.
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