Photochemical cleavage of the axial group attached to the central carbon atom of triazatriangulene

Article abstract of DOI:10.1246/cl.140897

Photocleavage reaction of triazatriangulene (TATA) leuco derivatives bearing axial substituents proceeded quantitatively in polar solvents with quantum yields (Φ_r) of up to 0.56. The photoreaction can easily be detected by the red color and yel

Full text of DOI:10.1246/cl.140897

CL-140897
Received: September 25, 2014 | Accepted: October 22, 2014 | Web Released: October 29, 2014
Photochemical Cleavage of the Axial Group Attached
to the Central Carbon Atom of Triazatriangulene
Soichi Yokoyama, Takashi Hirose, and Kenji Matsuda*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510
(E-mail: kmatsuda@sbchem.kyoto-u.ac.jp)
Photocleavage reaction of triazatriangulene (TATA) leuco
attached to the central carbon atom. Unlike the photochromism
of Malachite Green leucocyanide, the photoreaction of the TATA
leuco derivatives was irreversible because of the stability of the
photogenerated TATA cation and irreversible protonation of the
carbanion of the axial group upon the photocleavage reaction
(Scheme 1). It was found that the photoreaction quantum yield
of TATA leuco derivatives strongly depends on the axial group.
The difference in the photoreactivity was discussed in view of the
substituent effect, solvent effect, and density functional theory
(DFT) calculations.
derivatives bearing axial substituents proceeded quantitatively in
polar solvents with quantum yields (Φr) of up to 0.56. The
photoreaction can easily be detected by the red color and yellow
fluorescence originating from the photogenerated TATA cation.
Electron-withdrawing groups on the axial substituent effectively
suppress the photoreaction, in a particular case, Φr became less
than 0.01.
Triphenylmethanes (TPMs) have attracted interest for a long
time because of their brilliant colors and photophysical properties,
which can be applicable to organic dyes and stains for tissues.1-5
Indeed, this skeleton has been used for many applications,
including organic dyes like Crystal Violet, fluorescent chromo-
phores like fluorescein, and pH indicators like phenolphthalein
(Figure 1). Owing to the π-conjugation of TPMs, not only the
parent form but also all of its radical, cation, and anion forms are
stable due to the delocalization of charge or spin to the phenyl
rings.6,7 Photochromic dyes like Malachite Green using this
skeleton also have been developed. The leucocyanide of the TPM
dye undergoes photocleavage of the cyano group upon UV
irradiation and recombines via a thermal back reaction.8-10
Triazatriangulene (TATA) cation is a derivative of the
triphenylmethyl cation, in which nitrogen atoms link the three
phenyl rings together to provide a rigid and planar geometry, while
phenyl rings of the triphenylmethyl cation flexibly rotate. The
resonance structures involving the nitrogen atoms stabilize the
carbon-centered cation.11-13 Because of its stable cationic planar
structure, applications to the surface chemistry14-17 and supra-
molecular chemistry18,19 are attracting interest. Inspired by the
photochromic reaction of Malachite Green, we investigated the
photoreactivity of TATA leuco derivatives that have an axial group
The studied compounds bearing different axial groups 1-6 are
shown in Figure 2. Compounds 1, 2, and 6 are known com-
pounds.20 The leuco derivatives can be easily synthesized through
the nucleophilic attack of the carbanion to the center of the TATA
cation (see the Supporting Information for details). The structures
of 1-6 were characterized by NMR spectroscopy and mass
spectrometry.
The absorption spectra of compounds 1 and 2, and the absorp-
tion and fluorescence spectra of the TATA cation tetrafluoroborate
¹
salt (TATA+BF4 ) were measured in acetonitrile (Figure S1). The
¹
solution of TATA+BF4 shows a characteristic structured absorp-
tion band at 525 nm and a fluorescence band at 562 nm, and shows
a red color and a yellow emission under UV irradiation.21 On the
other hand, the solutions of the leuco derivatives 1 and 2 appeared
to be colorless owing to the change of the orbital hybridization from
X
A
R
R
HX hν
N
N
R
+
H A
N
N
N
R
N
R
R
Scheme 1. Irreversible photocleavage of TATA leuco derivative.
N
N
N
C3H7
N
C8H17
C8H17
N
N
N
Cl
N
N
N
N
N
C3H7
C8H17
C8H17
C3H7
C8H17
C8H17
N
triphenylmethane
1
2
3
Crystal Violet
O
HO
HO
O
O
O
OH
−
2H
N
CN
N
COOH
COO
+2H
O
O
fluorescein
phenolphthalein
R
C8H17
C8H17
C8H17
N
N
N
N
N
N
N
N
CN
hν
N
N
N
X
N
N
N
C8H17
C8H17
C8H17
CN
C8H17
C8H17
C8H17
N
N
Δ
R
R
4
5
6
triazatriangulene (TATA) cation
Malachite Green-leucocyanide
Figure 2. TATA leuco derivatives bearing different axial groups at
the central carbon atom.
Figure 1. Triphenylmethane-based compounds.
76 | Chem. Lett. 2015, 44, 76–78 | doi:10.1246/cl.140897
© 2015 The Chemical Society of Japan

2.0
1.5
1.0
0.5
0
2.0
1.5
1.0
0.5
0
C8H17
N
(a)
(b)
hν
EtOD
C8H17
N
D
OEt
N
N
N
N
C8H17
C8H17
C8H17
C8H17
2
(a)
(b)
(c)
(d)
(e)
(f)
2 before UV irrad.
UV 3 min
300 400 500 600
800
300 400 500 600
700
700 800
Wavelength / nm
Wavelength / nm
2.0
1.5
1.0
0.5
0
2.0
1.5
1.0
0.5
0
UV 10 min
(c)
(d)
UV 20 min
–
TATA +BF
4
phenylacethylene
3.5 3.0 2.5
300 400 500
800
300 400 500
600 700
600 700 800
Wavelength / nm
Wavelength / nm
8.5 8.0 7.5 7.0 6.5
4.5
4.0
1.0
0.5
0
1.0
0.5
0
δ / ppm
(e)
(f)
Figure 4. 1H NMR spectra of compound 2 in ethanol-d6 (a) before
and after UV irradiation (365 nm) for (b) 3, (c) 10, and (d) 20 min.
¹
Reference spectra of (e) TATA+BF4 and (f) phenylacetylene. Filled
circle denotes the proton peaks of compound 2, and open circle and
¹
triangle denote those of the photogenerated TATA+BF4 and phenyl-
300 400 500
300 400 500
600 700 800
Wavelength / nm
600 700 800
Wavelength / nm
acetylene, respectively. The small difference in chemical shift upon
UV irradiation may be attributed to the change in polarity of medium
or intermolecular interactions caused by the concentration change of
components.
0.5
0.4
0.3
0.2
0.1
0.0
compound
compound
compound
2
5
6
(g)
tion band of the photogenerated 9-ethynylanthracenene was also
observed at the wavelength range from 300 to 400 nm. It is noted
that the absorption spectrum of 2 after UV irradiation did not
change at least for 24 h at room temperature (Figure S2),
0
50 100 150 200 250 300
time / min
Figure 3. Absorption spectral change of compounds (a) 1, (b) 2, (c)
3, (d) 4, (e) 5, and (f) 6 in ethanol before (black solid line) and after
(blue dashed line) UV irradiation (313 nm). The adsorption spectra of
5 and 6 are recorded at concentration of 2 © 10¹5 M, that is lower than
compounds 1-4 (c = 5 © 10¹5 M), because of the low solubility of 5
suggesting thermal stability of the TATA+EtO salt.
¹
The formation of side products was more prominent when the
photoreaction was carried out in an aprotic solvent (Figure S3).
Upon UV irradiation of 1 in acetonitrile, the absorption bands not
only at 600 nm but also at 400 nm appeared. When acetonitrile
containing 5-10 vol % of water was used for solvent, the
magnitude of the band at 400 nm decreased and the band at
525 nm increased simultaneously upon irradiation. However, the
band at 600 nm was not influenced by the presence of water
(Figure S3a). In the case of compound 2, the appearance of the
absorption bands around 300-400 nm was effectively suppressed
and the quantitative production of the TATA cation was observed
by the addition of 5 vol % of water (Figure S3b). The presence of
protic solvent makes the photoreaction cleaner, suggesting that
a photoinduced heterolysis of the axial group affords the TATA
cation and the carbanion of axial group. The difference in the
formation of side products between 1 and 2 can be attributed to the
difference in the acidity between phenylacetylene and benzene;
phenylacetylene has higher acidity, so that the phenylethynyl
group is considered to dissociate cleanly.
¹
and 6. The red dotted lines denote the spectrum of TATA+BF4 at the
same concentration. (g) The plot of the absorbance of 2 (black square),
5 (blue triangle), and 6 (red circle) at 525 nm against the time of UV
irradiation.
sp2 to sp3 at the central carbon atom. This feature was clearly seen
in the absorption spectra; the absorption bands of 1 and 2 were
observed at only less than 370 nm. An absorption band correspond-
ing to the axial substituent was clearly observed for compounds 4
and 5 at ca. 400 nm, and 6 at ca. 350 nm in ethanol (Figure 3).
Photocleavage reactions of 1-6 were investigated in ethanol
(Figure 3). For the phenyl-substituted compound 1, two absorption
bands were generated upon irradiation with UV light (313 nm):
one is the structured band appearing at 525 nm, corresponding to
the TATA cation and the other is a weaker band at ca. 600 nm
(Figure 3a). As the absorption band at 525 nm increased, the
colorless solution turned red. The shape of the absorption band
at 525 nm is in good agreement with that of the TATA cation;
however, the weaker band around 600 nm is absent in the spectrum
of the TATA cation, suggesting generation of side products upon
UV irradiation of the phenyl derivative 1. On the other hand, the
ethynylene derivatives 2-4 showed cleaner photoreactions in the
solvent (Figures 3b-3d). In these cases, the band at ³600 nm was
not observed. Not only the shape but also the magnitude of the
absorption band at 525 nm corresponds to the TATA cation at the
same concentration, suggesting a quantitative photoreaction of
2-4. For the 9-anthrylethynyl derivative 4, the structured absorp-
For the phenylethynyl derivative 2, the photoreaction affording
the TATA cation was monitored by H NMR in ethanol-d6. Upon
1
UV irradiation, the signals corresponding to the TATA cation and
phenylacetylene appeared as expected (Figure 4). No acetylenic
proton was observed around 3.4 ppm after UV irradiation probably
due to exchange with deuterium (red arrow, in Figure 4). Since no
other peaks were observed after UV irradiation, the quantitative
photocleavage reaction of 2 was also supported by 1H NMR.
In a marked contrast to 1-4, the photoreactions of compounds
5 and 6 were significantly slow (Figures 3e-3g). According to
Chem. Lett. 2015, 44, 76–78 | doi:10.1246/cl.140897
© 2015 The Chemical Society of Japan | 77

Table 1. Quantum yields of photocleavage reaction upon UV
irradiationa
decreased by introducing an electron-withdrawing group into the
axial group. It is noted that there is a rough correlation between the
LUMO energy and the photoreactivity; the quantum yield of the
photocleavage reaction is small when the acceptor ability of the
axial group is high (Table 1).
In conclusion, TATA leuco derivatives bearing different axial
groups at the central carbon atom were synthesized and their
photoreactivity was investigated. Owing to the good donor
property of the TATA framework, a typical CT character was
observed upon photoexcitation. It was found that the photo-
cleavage reaction becomes clean and effective in the following
conditions: (i) in polar media, (ii) in the presence of a protic
solvent, and (iii) with a high LUMO energy of the axial group.
These observations are of importance for the design of smart
photoresponsive materials that can release small molecular
components upon photoirradiation, especially on a 2-dimensional
surface, via a photochemical reaction.
Orbital energyc/eV
Quantum yieldb
(Φr)
HOMO-LUMO
gap/eV
Compd
HOMO
LUMO
1
2
3
4
5
6
0.013
0.56
0.45
0.35
<0.01
<0.01
¹5.25
¹5.29
¹5.22
¹5.28
¹5.33
¹5.30
¹0.75
¹1.22
¹0.88
¹2.26
¹2.84
¹2.76
4.50
4.07
4.34
3.02
2.49
2.54
aBy irradiation with 313 nm light. bMeasured in ethanol. cCalculated at
B3LYP/6-311g(2d,p)-SCRF(PCM)// 6-31g(d) level.
control experiments under dark condition, the process of thermal
cleavage reaction from the TATA leuco derivative to the TATA
cation was found to compete with the photoreaction for 5 and 6
(Figures S6-S8). Analysis by the Eyring plot gave the activation
free energy of the thermal reaction for 5 as ¦G‡ = 101 kJ mol¹1 at
25 °C (¦H‡ = 95 kJ mol¹1, ¦S‡ = ¹20 J K¹1 mol¹1) (Figure S9).
On the basis of these parameters, the halflife (t1/2) of 5 in ethanol is
14 h at 25 °C, which is significantly shorter than that of 2 (t1/2 = 10
days, Figure S10). In the case of compounds 1-4, the photo-
chemical process was dominant, so that the thermal process was
not easily discerned. For compound 6, a small change in the
absorption spectrum was also detected around 350 nm, which is
due to the cis-trans photoisomerization of the azobenzene moiety.
The photoisomerization of 6 became more distinct in toluene20
(Figure S11).
The quantum yield of the photocleavage reaction was inves-
tigated in ethanol by monitoring the increase of absorbance at
525 nm upon UV irradiation (Table 1). Compared to the quantum
yield of the phenyl derivative 1 (Φr = 0.013), those of the
arylethynyl derivatives 2-4 were significantly large (Φr = 0.35-
0.56). On the other hand, the quantum yields of the 10-cyano-9-
anthrylethynyl derivative 5 and the 4-(phenyldiazenyl)phenyl-
ethynyl derivative 6 were notably small (Φr < 0.01). The differ-
ence in Φr between 1 and 2 can be explained by the presence or
absence of the ethenyl group. The difference in Φr among 2-6
should be caused by the substituent effect and difference in the
aromatic framework, i.e., benzene or anthracene.
In order to investigate the difference in the photoreactivity,
time-dependent density functional theory (TD-DFT) calculations
were performed on 1-6 at the B3LYP/6-311g(2d,p)// 6-31g(d)
level. The solvent effect was taken into account by using the self-
consistent reaction field (SCRF) with the polarizable continuum
model (PCM). HOMOs of the leuco derivatives mainly localize at
the TATA moiety while LUMOs localize at the axial groups
(Figure S12). According to the contributions of molecular orbitals
(MOs) calculated by TD-DFT, the lowest energy excited state has
an intramolecular charge-transfer (CT) property from the TATA
moiety to the axial groups. The LUMO energies were significantly
lowered as the acceptor ability of axial groups increased; the
LUMO energies of the 10-cyano-9-anthrylethynyl derivative 5
(¹2.84 eV) and 4-(phenyldiazenyl)phenylethynyl derivative 6
(¹2.76 eV) are lower than that of the 9-anthrylethynyl derivative
4 (¹2.26 eV) and the phenylethynyl derivative 2 (¹1.22 eV).
In contrast, the HOMO energies of 1-6 were almost constant
(ca. ¹5.3 eV), which is attributed to the localization of HOMOs
at the TATA moiety. Therefore, the HOMO-LUMO gap efficiently
This work was supported by NEXT program (No. GR062)
from JSPS and Grant-in-Aid for Scientific Research on Innovative
Areas “Photosynergetics” (No. 26107008) from MEXT, Japan.
Supporting Information is available electronically on J-STAGE.
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78 | Chem. Lett. 2015, 44, 76–78 | doi:10.1246/cl.140897
© 2015 The Chemical Society of Japan