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Table 2 Redox potentials of 1–4 in CH2Cl2 measured by cyclic voltam-
metrya and estimated energy levels of CSS and ÀDG1 for charge separation
Compound E1/2OX1/V E1/2RED1/V E1/2RED2/V CSS/eV ÀDG1 (eV)
subporphyrin cation radical generated by chemical oxidation
of A with NOSbF6 displayed absorption bands at 388 and
502 nm (Fig. S6-1 in the ESI†). After searching the literature,
the electrochemically generated radical anion of nitrobenzene
showed absorption bands at 435 and 465 nm with a tail reach-
ing out to 600 nm.11 The TA spectra of 1 displayed very rapid
formation and decay of the CS state showing broad absorbance
in the range of 500–630 nm consisting of contributions from
the subporphyrin cation radical and the nitrobenzene anion
1
2
3
4
A
0.78
0.72
0.70
0.71
0.71
À1.36b
À1.74
À1.79
À1.69
À1.97
À1.56b
1.86
2.18
2.21
2.12
À0.49
À0.19
À0.21
À0.25
À1.96
a
Conditions: scan rate: 0.05 V sÀ1, supporting electrolyte: 0.1 M nBu4NPF6,
working/counter electrodes: Pt/Pt wire, reference electrode: Ag/0.01 M
radical with a CS rate of (3.7 ps)À1 and a CR rate of (330 ps)À1
.
b
AgClO4 in MeCN. Determined by differential pulse voltammetry.
The TA spectra of 2 and 3 also revealed quantitative CS and CR
with kCS = (3.0 ps)À1 and kCR = (530 ps)À1 for 2, and kCS = (2 ps)À1
and kCR = (210 ps)À1 for 3, respectively. The fluorescence life-
times of 1–3 (340, 510, and 200 ps, respectively) were all well
matched with the lifetimes for CR processes, indicating that
their red-shifted fluorescence in CH2Cl2 is originating from
the CS state (Fig. S7-3 in the ESI†). It is worth noting that such
CR-associated emissions12 have been very rare for porphyrins.
The TA spectra of 4, which is held in an orthogonal conforma-
tion, also indicated the rise and decay of broad absorbance
The first oxidation waves of 1, 2, and 3 were observed at
0.78, 0.72, and 0.70 V, respectively, reflecting the decreasing
influence of the nitro group due to the installed methyl groups,
in this order. In the reduction part, 1 showed two reversible
reduction waves at À1.36 V and À1.56 V. By searching the existing
literature, these two reduction waves have been assigned to the
reduction of the freely rotating 4-nitrophenyl group and of
the electron deficient subporphyrin core, respectively.10 In 2,
the installed methyl group mitigates communication between
the nitro group and aryl group. Judging from the first oxidation
and reduction potentials of 0.72 and À1.74 V, the nitro group in
2 is exerting a modest influence on the subporphyrin core. The
influence of the nitro group in 3 is even less due to the two
methyl groups flanking the nitro group. Finally, the oxidation
potential of 4 was observed at 0.71 V, whilst the reduction
potentials were observed at À1.69 and À1.96 V. These values
confirm that the 2-methyl-4-nitrophenyl group in 4 is electro-
nically decoupled from the subporphyrin core, although the
nitro group can still influence its respective aryl group due to
the absence of methyl groups at the 3 and 5 positions. On the
basis of the above electrochemical data, the energy levels of the
charge separated states (CSS), consisting of a subporphyrin
cation radical and a nitrobenzene anion radical, have been
estimated to be 1.86, 2.18, 2.21, and 2.12 eV for 1–4, respectively
(Coulombic correction term was taken into account when
calculating these values (Table 2)). These considerations made
it possible to estimate the driving force for CS to be À0.49,
À0.19, À0.21, and À0.25 eV for 1–4, respectively.
in the range of 480 nm to 660 nm due to the CS state with kCS
=
(5.0 ps)À1 and kCR = (40 ps)À1. Very fast CR in 4 may explain
its non-fluorescence nature in the red-shifted region due to
its short lived CS state. Therefore, it may be inferred that weak
CR-associated emission is only allowed for subporphyrins
bearing freely rotating meso-aryl groups.
In summary, A2B-type meso-(4-nitrophenyl)-substituted sub-
porphyrins 1–4 have been synthesized and have been shown
to undergo very fast photoinduced intramolecular CS and CR
reactions between the subporphyrin core and the meso-4-
nitrophenyl group in CH2Cl2. Characteristically, broad and
red-shifted emissions were detected from the CS states of 1–3
as a rare case for porphyrinoids.
The work at Kyoto was supported by JSPS KAKENHI Grant
Numbers (25220802 and 25620031). The work at Yonsei was
supported by Global Research Laboratory (GRL) Program
(2013K1A1A2A02050183) through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Science, ICT
(Information and Communication Technologies) and Future
Planning. GC thanks JSPS for a Postdoctoral Fellowship.
To reveal the CS and CR events in 1–4, femtosecond
transient absorption (TA) spectra were recorded in CH2Cl2
(Fig. 4 and Fig. S7-2 in the ESI†). As a control experiment, a
Notes and references
1 Y. Inokuma, J. H. Kwon, T. K. Ahn, M.-C. Yoo, D. Kim and A. Osuka,
Angew. Chem., Int. Ed., 2006, 45, 961.
2 (a) T. Torres, Angew. Chem., Int. Ed., 2006, 45, 2834; (b) Y. Inokuma
and A. Osuka, Dalton Trans., 2008, 2517; (c) A. Osuka, E. Tsurumaki
and T. Tanaka, Bull. Chem. Soc. Jpn., 2011, 84, 679; (d) C. G. Claessens,
´
´
´
D. Gonzalez-Rodrıguez, M. S. Rodrıguez-Morgade, A. Medina and
T. Torres, Chem. Rev., 2014, 114, 2192; (e) N. Kobayashi, Y. Takeuchi
and A. Matsuda, Angew. Chem., Int. Ed., 2007, 46, 758.
3 Y. Inokuma, Z. S. Yoon, D. Kim and A. Osuka, J. Am. Chem. Soc.,
2007, 129, 4747.
4 (a) Y. Inokuma, S. Easwaramoorthi, S. Y. Jang, K. S. Kim, D. Kim and
A. Osuka, Angew. Chem., Int. Ed., 2008, 47, 4840; (b) Y. Inokuma,
S. Easwaramoorthi, Z. S. Yoon, D. Kim and A. Osuka, J. Am.
Chem. Soc., 2008, 130, 12234.
5 (a) M. Kitano, S. Hayashi, T. Tanaka, N. Aratani and A. Osuka,
Chem. – Eur. J., 2012, 18, 8929; (b) W.-Y. Cha, J. M. Lim, K. H. Park,
M. Kitano, A. Osuka and D. Kim, Chem. Commun., 2014, 50, 8491.
Fig. 4 TA spectra and decay profile (inset) of 1 in CH2Cl2 (excitation at
490 nm).
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