Published on Web 11/05/2004
Design of Mesomorphic Diarylethene-Based Photochromes
Michel Frigoli, Chris Welch, and Georg H. Mehl*
Department of Chemistry, UniVersity of Hull, Hull HU6 7RX, U.K.
Received May 28, 2004; E-mail: g.h.mehl@hull.ac.uk
Table 1. Photochromic Properties of Compounds 1, 2, and 3
The structural requirements for bistable photochromic (PC)
systems, of which diarylethene derivatives are excellent examples,1
and that for liquid crystal (LC) phase behavior in molecular systems
seem so far to be almost noncompatible.2 Diarylethene-based
systems require alkyl or perfluorinated groups placed at strategic
positions in the molecules to enable reversibility of the switching
and to prevent degradation; thermotropic calamitic LC phase
formation is in most cases associated with molecules with an overall
cylinder shape. So far, bistable photochromic LCs have been
obtained only by the connection of photochromic groups and
mesogens by alkyl spacers in a modular concept or in dendrimers.3
The combination of photochromic and mesomorphic function-
alities in a single center has, however, intrinsic advantages, as it
has the potential to be synthetically much more efficient than the
modular approach. Compared to modular LC photochromes, the
volume density of switchable groups in the condensed phase
(enhanced interactions in smectic layers2a,c) is increased, being
comparable to that of azobenzene-based photochromes,4 while
exhibiting PC bistability and a high differentiation of the electronic
properties (e.g., colorless-colored) of both forms.
To achieve this, the structural parameters determining the LC
and the PC behavior have to be correlated to explore the functional
scope of this approach. From an LC design point of view,
diarylethenes can be viewed as dimers whose properties can be
modulated by extending the length of the aromatic systems and/or
terminal alkyl chains and by variation of the symmetry of the dimer.
This is particularly effective where dipole groups are involved.5
Melting behavior can be addressed by disrupting the packing of
the aromatic groups; for that, fluorine groups are very suitable.6
For photochromes the extension of the aromatic system should lead
to bathochromic shifts of the absorption behavior. The variation in
molecular symmetry is expected to have a strong effect too. The
photoconversion processes, the extinction coefficients, and the
fluorescence behavior should be modulated by this structural change
as well. To investigate this, the symmetric “dimeric” systems 1
and 3 and the nonsymmetric molecule 2, shown in Scheme 1, were
prepared, all linked by a hexafluorocyclopentene group, ensuring
the photochromic bistability of the systems.1a,7 The final systems
are based on the precursors 4 and 6, which were obtained by two
successive cross-coupling reactions. The preparation of the sym-
metric photochromes 1a and 3a was achieved by reacting the
lithiated derivatives of 4 and 6 on a semi-equivalent of octafluo-
rocyclopentene. The nucleophilic condensation of the lithiated
derivative of 6 on 5 afforded the nonsymmetric photochrome 2a.8
The photochromic behavior of the systems 1-3 was investigated
in cyclohexane solutions (2.05 × 10-5 mol L-1). Figure 1a-c shows
the absorption spectral changes of the systems under UV irradiation;
313 nm light was used for 1 and 366 nm light for 2 and 3. The
molecular variation of the systems altered significantly the absorp-
tion behavior of both forms. 1a absorbs below 370 nm with an
absorption maximum at 319 nm (ꢀ ) 7.8 × 104 mol-1 L cm-1),
while 3a absorbs below 450 nm with an absorption maximum at
cyclization
cycloreversion
compd
Φaf
(
λirr
)
conversion
Φbf (λirr)
a
b
1
2
3
0.33 (313 nm)
0.13 (366 nm)
0.006 (366 nm)
>99
63
26
0.008 (546 nm)
0.15 (546 nm)
386 nm (ꢀ ) 5.4 × 104 mol-1 L cm-1). For the 2-thienyl structure
3a, the π-conjugation is extended over six aromatic groups, leading
to a bathochromic shift of 65 nm when compared with the 3-thienyl
structure 1a, where the full π-conjugation is located only in each
set of aromatic rings. An intermediate absorption behavior is
observed for the hybrid structure 2a; the foot band is located at
400 nm and the absorption maximum is centered at 321 nm (ꢀ )
5.7 × 104 mol-1 L cm-1). Irradiation of 1a, 2a, or 3a with UV
light resulted in solutions that were blue, red, or yellow, respec-
tively. The formation of 1b, 2b, and 3b is responsible for the change
of the absorption behavior. Absorption spectra of the closed-ring
isomers were calculated from the collected data.3c,d 1b absorbs at
597 nm (ꢀ ) 2.7 × 104 mol-1 L cm-1) as the π-conjugation is
extended throughout the molecule. For 3b, the formation of the
C-sp3 in the 3-position (at the thiophene group) disrupts the
conjugation throughout the molecule, leading to an insignificant
shift of the absorption bands (444 nm; ꢀ ) 8.5 × 103 mol-1
L
cm-1). The absorption behavior of the mixed system 2b (518 nm;
ꢀ ) 2.4 × 104 mol-1 L cm-1) is between those of 1b and 3b. The
position of the absorption bands of the open forms 1a, 2a, and 3a
and of the closed forms 1b, 2b, and 3b can be viewed as a function
of their π-conjugation lengths.7 The photochromic properties are
listed in Table 1. In this series, the quantum yields of photocol-
oration Φafb decrease with increasing π-conjugation of the open-
ring isomers. The quantum yield of the cyclization reaction of 1a
was determined to be 0.33, while the yield of 2a decreased to 0.13.
For 3a, Φafb was as low as 0.006. The same effect was observed
for the quantum yields of the cycloreversion reaction Φbfa.9 For
1b, Φbfa was very low, with a value of 0.008. For 2b, Φbfa
increased strongly to 0.13. Φbfa of 3b could not be determined
with our experimental setup8 but is expected to be higher than the
values of the other photochromes.9b The conversion value from the
open form to the closed form at the photostationary state (PS),
determined by HPLC,8 was strongly dependent on the chemical
structure of the photochromes. At 313 nm, the PS for 1 was reached
in 1 min and the conversion from the open form 1a to the closed
form 1b was almost 100%. The conversion value of 2 decreased
to 63% and further to 26% for 3 (2a, 3a irradiation at 366 nm).
Only the 2-thienyl derivative 3a exhibited fluorescence at 486 nm,
with a fluorescence quantum yield of 0.021 (excitation at 366 nm).
The fluorescence decreased only slightly over time as photocon-
version from 3a to 3b is low, as shown in Figure 1d. The LC
behavior of the open-ring isomers 1a, 2a, and 3a was investigated
using differential scanning calorimetry (DSC) and optical polarizing
microscopy (OPM). The results are given in Table 2. The melting
9
15382
J. AM. CHEM. SOC. 2004, 126, 15382-15383
10.1021/ja0468269 CCC: $27.50 © 2004 American Chemical Society
Frigoli, Michel
