Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
286
J.J. Dressler et al. / Tetrahedron 71 (2015) 283e292
soluble in all solvents compared to lactone bridge-flipped 2 and 3.
Fortunately, crankshafts 1e3 were soluble enough to fully charac-
terize by NMR spectroscopy. In all cases, 1H NMR analysis strongly
supports the rigidity of compounds 1e3. The chemical shifts in the
aromatic region of 1, 2, and 3 all display a significant downfield shift
due, in part, to increased steric compression resulting from lacto-
nization relative to 7, 12, and 18 (see Supplementary data). The
general downfield chemical shift in the aromatic region of the 1H
NMR spectra due to steric compression has been observed in our
previous work7 and for numerous highly conjugated compounds
relative to their uncyclized precursors.13g,21b,34 Compounds 1e3
also exhibit bright fluorescence in the solid state upon irradiation
with a UV lamp while compound 3 displays visible fluorescence
under direct sunlight in solution.
Terphenyl dilactones 1e3 and their unlactonized precursors 7,
12, and 18 were analyzed by UVevis spectroscopy (Fig. 3) and
fluorescence spectroscopy (Fig. 4). As expected, the rigidity of 1, 2,
and 3 results in a dramatic increase in the molar absorptivity (Fig. 3
and Table 1) relative to their unlactonized precursors, 7, 12, and 18.
Quinquephenyl 3 also displays a bathochromic shift and increased
molar absorptivity relative to terphenyls 1 and 2 resulting from the
longer conjugation pathway. The absorption spectra of 1e3 are
typical of polyaromatic hydrocarbons with low rotational freedom
Fig. 4. Normalized emission spectra of compounds 1, 2, 3, 7, 12, and 18 dissolved in
CH2Cl2. All compounds were excited at 350 nm.
Table 1
Electronic absorption and emission data for 1, 2, 3, 7, 12, and 18
Compd
Lowest E abs
lmax [nm] (
Em lmaxb [nm]
Ffc
3
[Mꢁ1 cmꢁ1])a
1
2
3
7
12
18
335 (37,983)
364 (36,832)
384 (65,080)
319 (7713)
326 (10,609)
337 (22,801)
439
385
394
414
409
424
0.24
0.23
0.92
0.05
0.04
0.09
in that multiple sharp peaks are observed due to the distinct p/p
*
transitions, whereas the absorption spectra of 7, 12, and 18 display
broader peaks, typical of a compound with a higher degree of bond
rotational freedom. Compounds 1, 2, 3, 7, 12, and 18 are all fluo-
rescent (Fig. 4 and Table 1) when excited at 350 nm. However, di-
rect comparison of the cyclized 1e3 with their non-lactonized
precursors (7, 12, and 18) illustrates how the rigidity of these
compounds affects relative fluorescence quantum yield (Table 1).35
All three crankshaft compounds have much higher relative quan-
tum yields (1¼0.24, 2¼0.23, and 3¼0.92) compared to their much
less rigid precursors (7¼0.05, 12¼0.04, 18¼0.09). These quantum
yield values of 1e3 are similar to other crankshaft bridged ter-
phenyls with different bridging groups.13f,16f Increasing the conju-
gation length also increases the quantum yield as crankshaft 3 and
quinquephenyl 18 have the highest quantum yields relative to 1, 2
and 7, 12, respectively. Overall, the two bridging lactone units in
between the arene rings results in a 3-fold to 5-fold increase in
molar absorptivity and 5-fold to 10-fold increase in fluorescence
quantum yield of 1e3 compared to 7, 12, and 18.
a
UVevis spectral data from compounds dissolved in CH2Cl2.
b
Emission spectral data from compounds dissolved in CH2Cl2 and excited at
350 nm.
c
Calculated relative to quinine sulfate dissolved in 0.5 M H2SO4 and excited at
350 nm.36
(1ae3a) with attenuated photophysical properties relative to 1e3.
Re-acidification with an organic soluble acid, such as trifluoroacetic
acid (TFA), should result in a return to 1e3. The interconversion of 1
and 1a should be unaffected by the presence of oxygen during the
switching process. However, spontaneous oxidation of aryl lactones
similar to 2 and 3 has been reported upon base-promoted lactone
cleavage resulting in immediate quinone formation.37 Therefore
compounds 2a and 3a could become readily oxidized to 2b and 3b,
halting the reversibility of the switching in 2 and 3.
Our previous studies of the pH-driven geometry switching in
analogous DeA biaryl lactones were performed with the biaryl
lactones dissolved in acetonitrile while employing TBA-OH and TFA
as switching stimuli.7 In these studies, both the rigid lactonized
switch states and the ring-opened anionic switch states were sol-
uble in acetonitrile, thus lending to characterization of the
switching process with UVevis spectroscopy, 1H NMR spectros-
copy, and by visible fluorescence monitoring.
In the current study, crankshaft compounds 1e3 are completely
insoluble in acetonitrile and thus preliminary switching studies
were required to be performed in dichloromethane. The differing
solubility of both 1e3 and 1ae3a in dichloromethane resulted in
inconclusive results while monitoring the switching with UVevis
spectroscopy, even when attempting additional solvents, including
biphasic solvent systems. In all cases, addition of TBA-OH to 1e3
resulted in partial precipitation and/or decomposition of the
products making quantitative UVevis measurements unreliable.
However, the samples irradiated with an UV lamp display directly
visible fluorescence switching upon addition of TBA-OH followed
by TFA (Fig. 6). All three compounds instantly become non-
fluorescent upon addition of TBA-OH. However, only crankshaft
compounds 1 and 2 displayed fluorescence switching after TBA-OH
treatment and a subsequent TFA addition. Unexpectedly, after
Fig. 3. Absorption spectra of compounds 1, 2, 3, 7, 12, and 18 dissolved in CH2Cl2.
We were interested in exploring how crankshaft compounds
1e3 would behave as pH-driven rigidity switches (Fig. 5). Based on
our previous studies,7 cleavage of the lactone bridges with an or-
ganic soluble base, such tetrabutylammonium hydroxide (TBA-OH),
should result in significantly less rigid anionic ring-opened analogs