J.-C. Micheau et al. / Dyes and Pigments 106 (2014) 32e38
33
Scheme 1.
assumes that the excited state produced by the photon absorption
leads to the photochemical reaction. Indeed, on small DAE’s, calcu-
lations have shown a very efficient barrierless evolution of the
excited state toward the ring closure [8]. This interpretation was
also used for the ring opening reaction, usually quite inefficient
compared to the ring closure [9]. However the presence of sub-
stituents or larger conjugated moieties can result in a complex
arrangement of the molecule’s various excited states resulting in
overlap of several transitions [10].
Quantum yields measurement in diarylethene series has taken a
rather long period to be performed extensively [11]. The majority
were determined for the classical perfluorocyclopentene de-
rivatives of diarylethenes. For instance, the 2000 Irie’s review [2a]
only mentions the quantitative values for 11 diarylethene com-
pounds. Only very few supplementary values are provided in the
next 2004 He Tian’s review [2c]. As most of the measured photo-
cyclization quantum yields were restricted at less than 50% [12], it
was commonly considered that the two parallel and anti-parallel
conformations of the open form were in equal concentrations.
Nowadays, thanks to the very active Japanese and Chinese teams, a
lot of new compounds have been quantified [13], photocyclization
quantum yields higher than 50% were claimed for asymmetric
derivatives [14] as well as for symmetric structures [15]. More
recently, even higher photocyclization quantum yields have been
reported thanks to some structural improvements. Two main ways
have been explored either by keeping the perfluorocyclopentene
center cycle and adding chlorine atoms on thiophenic terminal
phenyl groups [16] or by exploiting hydrogen bonding when
replacing thiophenes by thiazole derivatives [17] and central
cyclopentene by various other cycles or heterocycles such as
indenones [18], coumarine [19], phenylthiazole, azaindole and
benzothiophene [20]. The result of such structural changes is a
better accumulation of the photo-active anti-parallel conformation.
Although no clearcut rules were established until now, it is sug-
gested that these high yields could be the result of the influence of
the nature and the position of the various substituents [21]. Other
types of diarylethenes, in particularly perhydrocyclopentene [22]
siloles and phospholes [23] and dithiolthiones and dithiolones
[24] derivatives have been also synthesized in order to check the
influence of a structural change on the quantum yields of photo-
chromic transformations. High quantum yields (>50%) were also
reported for the photobleaching reaction too.
Fig. 1. Generic A and detailed (I-1 to III-9) structures of the 18 diarylethenes. Cycles A
are variously substituted benzothiophenes, thiophenes or thiazoles. Cycle B are either
cyclopentenic, oxazolinic or lactonic with or without exocyclic double bond. Cycle C are
substituted benzothiophenes or thiophenes.
werenotdegassed.Synthesis ofcyclopenteniccompoundsof theIeN
series (N ¼ 1 to 5) has been described in [25], oxazolones of the II-N
series (N ¼ 1 to 4) in [26a] and lactones of the III-N series (N ¼ 1 to 9)
in [27]. All experiments were carried out within the 1 ꢀ 10ꢁ5 to
5 ꢀ 10ꢁ5 M concentration range. Spectral and kinetic characteristics
were measured on a diode array spectrophotometer 8452A (Hewlett
Packard).
All of the experiments and manipulations were performed
protecting the solutions against room light. 2.4 mL of solution was
irradiated in a stirred and thermostated (25 ꢂC) 1 cm ꢀ 1 cm quartz
cuvette by filtered light from a 250 W high pressure mercury arc
selecting lines at wavelengths 313, 365, 405, 436 and 546 nm. Two
irradiation wavelengths are used successively, a shorter (l1) to
obtain a photocoloration and a longer (l2) to bleach the obtained
colored solution. Quantum yields of photo-cyclization and photo-
cycloreversion and molar absorption coefficient of the closed
forms were determined simultaneously from Absorbance vs time
curves recorded under continuous monochromatic irradiation us-
ing a kinetic modeling method [28a,b]. In order to assure the
quantum yield values, we used Parker’s ferrioxalate as chemical
actinometer in an identical geometry and a calibrated home-made
silicon photodiode photometer to determine the intensity of the
light used for photochromic reactions. The accuracy for the deter-
mination of the quantum yields and molar absorption coefficients
was estimated at ꢃ10%. (see Supplementary Information part for
further details).
By portraying a large panel of structural changes, this work tries
to meet the recurrent challenge for new molecules with highly effi-
cient photo-transformations. Therefore, this paper is devoted to the
determination of quantum yields for photocoloration and photo-
bleaching processes of 18 thermally irreversible diarylethenes de-
rivatives obtained by changing the bridging ring that stabilizes the
central ethenic double bond in the cis configuration: from perhy-
drocyclopentene[25], oxazolone[26]andlactones[27]andthelateral
aryl moieties from thiophene to benzothiophene and thiazoles.
3. Results and discussion
The generic and detailed structures of the 18 diarylethenes are
displayed on Fig. 1. The set has been divided into three main groups
depending on the nature of the central ring B. Compounds I are
cyclopentenic (I-1 to I-5), II oxazolinic (II-1 to II-4) and III lactonic
(III-1 to III-9). Among them, 10 bring at less a thiophenic aryl group
2. Experimental section
Solvents of the highest purity grade (toluene (Rectapur) and
acetonitrile (H.P.L.C.) were used without further purification. They
Micheau
