Altitudinal Molecular Motors
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their H NMR spectra (Figure S1 in the Supporting Informa-
with UV light (lmax =365 nm) led to a redshift and broaden-
ing of their UV/Vis absorptions, thus indicating the photo-
chemically induced formation of the unstable isomers
(Schemes 4 and 5, dashed lines). The shifted bands are cen-
tered at 412 and 405 nm for cis-16 and cis-17, and 407 and
400 nm for trans-16 and trans-17, respectively. During irradi-
ation, clear isosbestic points were observed in all cases, thus
indicating that the photochemical isomerization from the
stable to unstable form proceeds cleanly. Samples were irra-
diated until no further changes were observed, thus indicat-
ing that the photostationary state had been reached. Allow-
ing the solutions to warm to room temperature led to rever-
sion to the original spectra consistent with thermal relaxa-
tion to the corresponding stable
tion)[37] with that of previously reported structurally related
molecular motors.[38–41]
Characterization of the rotary cycle: To show that 16 and 17
operate as molecular motors,[24] photochemical and thermal
isomerization studies were carried out in solution by using
low-temperature UV/Vis and 1H NMR spectroscopy. The
UV/Vis spectra of a sample of the stable forms of cis-16 and
trans-16 in MeOH at ꢀ208C show absorption bands cen-
tered at 395 nm, whereas under identical conditions the
major absorption band of the cis and trans isomers of alkene
17 are centered at 388 nm (Schemes 4 and 5, solid line).
isomers (Schemes 4 and 5).
The thermal isomerization of
unstable-cis-16 and unstable-
trans-16 to the corresponding
stable isomers was monitored
by UV/Vis spectroscopy at
450 nm as a function of time at
four different temperatures
(253, 258, 263, and 268 K).
From the first-order rate con-
stants the Gibbs free energy of
activation (D°G8) for the ther-
mal helix inversion could be de-
termined with the Eyring equa-
tion; they were calculated to be
82.2 kJmolꢀ1 for the unstable-
cis-16!stable-cis-16
and
82.5 kJmolꢀ1 for the unstable-
trans-16!stable-trans-16 proc-
ess (Figure S2 in the Supporting
Information).[37] By extrapola-
tion of the kinetic data, half-
lives (t1/2) at RT of 50 and 57 s
were calculated for the unsta-
Scheme 4. Photochemical and thermal isomerization studies of stable-cis-16 and stable-cis-17. a) UV/Vis spec-
tra of stable-cis-16 (solid line), the PSS mixture of stable-cis-16 and unstable-trans-16 after irradiation at lmax
=
365 nm (dashed line), and the mixture of stable-cis-16 and stable-trans-16 after thermal conversion upon stand-
ing at RT for 15 min (~). b) UV/Vis spectra of stable-cis-17 (solid line), the PSS mixture of stable-cis-17 and
unstable-trans-17 after irradiation at lmax =365 nm (dashed line), and the mixture of stable-cis-17 and stable-
trans-17 after thermal conversion upon standing at RT for 15 min (~).
ble-cis-16!stable-cis-16
and
unstable-trans-16!stable-trans-
16 processes, respectively. In
the same manner the conver-
sion of unstable-cis-17 and un-
Compared to the parent motor without substituents on the
rotor part (major absorption band at 378 nm),[28] introduc-
tion of the substituents resulted in a slight redshift of the
UV/Vis absorption. Such shifts have already been observed
upon the introduction of electron-donating or electron-with-
drawing groups to molecular motors,[33] azobenzenes, and
stilbenes.[42–44] In the case of the cis- and trans-cyano-substi-
tuted motor 16, the maxima of the major absorption band of
the stable isomers in MeOH are more redshifted than those
of the fluoroalkyl motors 17. This shift is attributed to the
stronger electron-withdrawing character of the cyano group
than the perfluoroalkyl chain.[33]
stable-trans-17 to the corresponding stable isomers was
monitored by UV/Vis spectroscopy at 440 nm as a function
of time at different temperatures that ranged from 253 to
273 K. The Gibbs free energy of activation (D°G8) was cal-
culated to be 83.3 kJmolꢀ1 for the unstable-cis-17!stable-
cis-17 and 84.0 kJmolꢀ1 for unstable-trans-17!stable-trans-
17 thermal conversions (Figure S3 in the Supporting Infor-
mation).[37] By extrapolation of the kinetic data, half-lives
(t1/2) at RT of 81 and 102 s were calculated for the unstable-
cis-17!stable-cis-17 and unstable-trans-17!stable-trans-17
processes, respectively. These values are similar to those ob-
tained for structurally related motors,[38–41] thus indicating
that the introduction of the functional groups does not have
significant influence on the thermal isomerization steps.
Irradiation of a sample of stable forms of solutions of cis-
16 and cis-17 and trans-16 and trans-17 in MeOH at ꢀ208C
Chem. Eur. J. 2013, 00, 0 – 0
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