was estimated to be 45% from the integral ratio of the methyl
group at the top of the azobenzene moiety. To clarify the
mechanism underlying this unique photoisomerization behaviour,
the TD-DFT calculation was carried out (Fig. S4). The result
indicates that band II is assignable to a transition from the
metalladithiolene(p) to the azobenzene(p*) level. Therefore the red
shift of the trans-to-cis isomerization wavelength in complex 1 is
induced in quite a different way from that of donor/acceptor-
substituted azobenzenes.4b Such an MLCT-like transition, which
induces trans-to-cis photoisomerization, is also observed in
ferrocenylazobenzenes.2
Moreover, unlike the other azobenzene-conjugated bipyridine
complexes, complex 2 showed cis-to-trans isomerization by the
excitation of the MMLL9CT band with 578 nm yellow light, as
complex
1 did. Quenching of phosphorescence from the
3MMLL9CT state was also observed (Fig. S5c{), implying the
consumption of the excitation energy for the isomerization of
the azobenzene moiety.
In summary, the intrinsic trans-to-cis photoisomerization
behaviour of the azobenzene moieties on the metalladithiolene3
and bipyridine5 sides was almost retained in complexes 1 and 2
despite the existence of the low-lying MMLL9CT bands.
Complex 3, bearing an azobenzene on both the metalladithio-
lene and bipyridine sides, showed a convincing photoresponse. The
relationship between photoirradiation wavelengths and cis-isomer
percentages in the PSS is given in Table 1. By irradiation with
405 nm purple light, complex 3 showed spectral changes similar to
Table 1 presents in detail the relationship between photoirradia-
tion wavelengths and cis-isomer percentages in the PSS of complex
1. Irradiation with 436 nm blue light, corresponding to band III,
also afforded a cis-conversion to some extent. This phenomenon is
elucidated with the MO calculation (Fig. S4{), which describes that
a main transition in band III is quite similar to that in band II. It
seems possible that band I, assignable to a p–p* transition of the
azobenzene moiety, participates in the trans-to-cis photoisomeriza-
tion. However, a high cis-isomer percentage was not attained upon
irradiation with 312 nm UV light. Excitation of band IV,
assignable to an MMLL9CT band with 578 nm yellow light,
resulted in cis-to-trans isomerization. Phosphorescence from the
3MMLL9CT state was thoroughly quenched (Fig. S5b). This is
consistent with the consumption of the MMLL9CT excitation
energy for the cis-to-trans conversion. This is the first report that
cis-to-trans isomerization was held by excitation of such the low-
lying CT band.
1
those of complex 1 (Fig. 3(A)(b)). In accordance with H-NMR
spectroscopy, the cis-isomer percentages of the azobenzenes in the
PSS were 44% for that on the metalladithiolene side and 9% for
that on the bipyridine side (Fig. S8{). On the other hand, on
irradiation with 365 nm UV light, complex 3 displayed almost the
same changes that complex 2 showed (Fig. 3(B)(b)). 1H-NMR
spectra indicated that the cis-isomer percentages in the PSS were
22% on the metalladithiolene side and 45% on the bipyridine side
(Fig. S9{).
Furthermore, by excitation of the MMLL9CT band with 578 nm
yellow light, both of the azobenzenes almost returned to the trans-
state (Table 1). The phosphorescence from the 3MMLL9CT state
was quenched as it was in complexes 1 and 2 (Fig. S5d{). Thus, the
trans-to-cis photoisomerization behaviour of complex 3 was almost
a superposition of those of complexes 1 and 2. As illustrated in
Fig. 4, by irradiation with three monochromic lights in the UV
region and across the whole visible region three out of four possible
states, the exception being the (cis, cis)-state, can be reversibly
switched.
In the electronic spectrum of complex 2{ in dichloromethane,
three characteristic bands, V–VII, were observed (Fig. 2(B)(a)).
Bands V–VII were intuitively assignable to the p–p* bands of the
azobenzene and bipyridine moieties, an n–p* band of the
azobenzene moiety, and an MMLL9CT band, respectively, and
the TD-DFT calculation favored this speculation (Fig. S6{).
Among the other azobenzene-conjugated bipyridine complexes,5
complex 2 showed orthodox photoisomerization behaviour except
for upon excitation of the MMLL9CT band. Table 1 presents in
detail the relationship between photoirradiation wavelengths and
cis-isomer percentages in the PSS of complex 2. This complex
displayed trans-to-cis photoisomerization upon irradiation with
365 nm UV light (Fig. 2(B)(b)), and the percentage of cis-isomer in
the PSS was 23%, estimated by the same method as in the case of
complex 1 (Fig. S7{). In contrast, upon irradiation with 436 nm
blue light, the PSS was biased in the trans-rich direction.
The photo-controllable tristability of complex 3 found in this
study will enlarge the application area of photochromic molecules
for molecular devices such as molecular memories.
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 15033215 (area 417), and 14204066), and by a
grant from The 21st Century COE Program for Frontiers in
Fundamental Chemistry from MEXT, Japan.
Table 1 The relationship between photoirradiation wavelengths and
cis-isomer percentages in the PSS of 1–3
Proportion of cis-isomer (%)
l/nm
1
2
Metalladithiolene-3
Bipyridine-3
312
334
365
405
436
546
578
a
17a
20a
21a
45b
37a
15a
14a
16a
24a
23b
7a
21a
21a
22b
44b
32b
7b
33a
50a
45b
9b
8a
10b
6b
Fig. 3 (A) Electronic spectra of 3 in dichloromethane (a, solid line) and
in the PSS upon irradiation with 405 nm light (b, dotted line). The inset
shows the difference in the spectra, (b) 2 (a). (B) Electronic spectra of 3
in dichloromethane (a, solid line) and in the PSS upon irradiation with
365 nm light (b, broken line). The inset shows the difference in the spectra,
(b) 2 (a).
5a
5a
8b
4b
Calculated from the UV-vis spectra with the values from the
1H-NMR spectra as the standard. Calculated from the 1H-NMR
spectra.
b
1216 | Chem. Commun., 2005, 1215–1217
This journal is ß The Royal Society of Chemistry 2005