A. R. Tuktarov et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
compounds 3–8 and the efficiency of the photochromic response
phot
estimated by the
D
D
B
value were dependent on the nature of
substituents on the second thiophene ring. However, no stable
structure–property relationships could be elucidated. Fulleropy-
rrolidine 3 was colored upon UV irradiation but was virtually not
bleached under visible light (see Fig. 1sp in supporting informa-
tion), which was attributed to the presence of the Cl atom on the
thiophene ring.
The decrease in the efficiency of the photochromic transforma-
tions of fullerene adducts 3–8 with respect to the initial dithieny-
lethene could be interpreted as being due to the transfer of
excitation energy from the diarylethene moiety to the fullerene
cage caused by their proximate positions. Therefore, it appears that
it is pertinent to synthesize hybrid compounds having spacers of
different lengths between the fullerene and photochromic
moieties.
The study of thermal stability of the photoinduced forms of the
synthesized hybrid compounds showed that unlike diarylethene 2
they are thermally stable from 5 to 20 or more hours of storage in
the dark (Fig. 6sp in supporting information). During the first 5 h of
storage for all compounds, except for 4, a decrease in the value of
photoinduced absorbance due to aggregation and precipitation of
hybrid molecules in photoinduced merocyanine form is observed.
The degree of aggregation depends on the nature of the sub-
stituents. The highest efficiency of aggregation was found for
chloro-substituted compound 3.
Figure 1. Absorption spectra of dithienylethene 2 in toluene before (curve 1) and
after UV irradiation through the UFS-1 filter for 40 s (curve 2) and after subsequent
visible light irradiation through the ZhS-16 filter (curve 3). The other curves
correspond to intermediate UV exposure.
The initial absorption spectrum showed a band with a maxi-
mum at 314 nm. After UV irradiation, two bands appeared with
maxima at 375 nm 584 nm. Upon irradiation with visible light,
these disappeared in order to restore the initial absorption spec-
trum. These photoinduced reversible spectral changes were indica-
tive of the inherent photochromism of percyclopentene
An important issue is analysis of the kinetic curves for pho-
todegradation of the hybrid compounds upon exposure to unfil-
tered light (Figs. 3 and 4).
2
diarylethene derivatives. These compounds are thermally irre-
As seen from Figure 3 (curves 1 and 2), upon irradiation with fil-
tered UV light, the photoinduced absorbance in the photoequilib-
rium state did not exceed D = 0.02. A comparison of these curves
with the kinetic curves measured for compound 2 in toluene shows
that they are caused by photochromic transformations on diary-
lethene fragments in 4.
versible and undergo reversible photoinduced interconversions
between the ring-open and ring-closed forms only upon photoex-
citation (Scheme 3).
Hybrid compound 4 showed much less efficient phototransfor-
mations (Fig. 2).
The photoinduced changes in the absorption spectra upon
exposure to UV and visible light indicated that this hybrid com-
pound showed photochromic behavior. The differential absorption
spectrum indicated that the absorption band of the closed isomer
of diarylethene was located at 557 nm. This was hypsochromically
shifted by 27 nm relative to the initial compound 2. The other
hybrid compounds, 3 and 5–8, also displayed similar photoinduced
spectral changes (Table 1, Fig. 1sp–5sp in supporting information).
It follows from Table 1 that the positions of the closed form
absorption bands of the dithienylethene moieties of hybrid
Exposure to unfiltered light gave rise to a complex kinetic curve
(Fig. 3, curve 3). At the beginning of irradiation, the photoinduced
absorbance showed slight changes and increased by an order of
magnitude (to D = 0.2). In the first seconds of irradiation, a yel-
low-brown precipitate appeared on the cell walls. Similar kinetic
curves were observed for all fulleropyrrolidines 3–8.
It was found that [60]fullerene in toluene was photochemically
unstable (Fig. 3, curve 4). The kinetic curve of [60]fullerene similar
Figure 3. Kinetic curves for photocoloring of compound 4 in toluene under the
action of UV radiation through the UFS-1 filter (curve 1), photobleaching on
exposure to visible light through the ZhS-16 filter (curve 2), photodegradation of 4
(curve 3) and C60 (curve 4) upon irradiation with unfiltered light. The ordinate axis
for curves 1 and 2 is on the left and that for curve 3 and 4 is on the right.
Figure 2. Absorption spectra of compound 4 in toluene before (curve 1) and after
UV irradiation through the UFS-1 filter for 90 s (curve 2) and after subsequent
visible light irradiation through the ZhS-16 filter (curve 3).