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[
11]
isomerization with visible light (l > 500 nm). Furthermore,
the thermal half-life of the Z isomer has been reported to be
as high as two years at room temperature for certain
derivatives. Such azobenzenes offer the unique opportunity
to achieve the desired molecular properties, since the
Z isomer can be readily generated in high yield by using
visible light, and its thermal stability facilitates the study of
the sensitized two-photon-triggered Z!E isomerization. A
triarylamine moiety, which is known to be an excellent two-
420 nm [(Z)-1] are assigned to the typical n!p* band of the
ortho-fluorinated azobenzene, while the maxima at about
320 nm [(E)-1] and 270 nm [(Z)-1] are attributed to the
azobenzene p!p* band. The strong absorption bands at
300 nm and 380 nm are associated with the antenna and
exhibit no spectral change upon photoisomerization, thus
further illustrating decoupling of the two chromophore
subunits in the ground state in both isomers. Interestingly,
the parent azobenzene 3 exhibits significant solvatochrom-
[12]
[15]
photon-absorbing chromophore,
was chosen as the
ism, which however only caused a bathochromic shift of the
antenna. To allow selective excitation of the antenna, both
a thiophene unit and a meta-phenylene linkage were intro-
duced into the targeted triarylamine–azobenzene dyad
p!p* band upon increasing solvent polarity, while the n!p*
band remained unaltered. As a consequence, the shape of the
absorption spectrum of dyad 1 changes upon varying solvent
polarity, that is, upon going from n-hexane (Figure 2 and
Figure S4 in the Supporting Information) to
1
(Scheme 1, top). While the thiophene and the phenylene
acetonitrile (Figure S5) solution. Note that
the antenna itself (2) does not display notable
[
14]
solvatochromism.
Dyad 1 as well as the parent azobenzene 3
exhibit fully reversible photochromism. Upon
irradiation with green light (l > 500 nm), the
n!p* band of the E isomer is selectively
excited, thereby leading to high photoconver-
sion with 92% and 96% Z isomer content in
the photostationary state (PSS) for 1 and 3,
respectively. These values are similar to those
observed for other previously reported ortho-
[11]
fluorinated azobenzenes.
It can thus be
assumed that the meta-linked antenna has no
electronic influence over the n!p* band of
the azobenzene, thus ensuring the possibility
to selectively address each individual compo-
nent of the molecular dyad 1.
Scheme 1. Triarylamine–azobenzene dyad 1 (top) and the antenna (2) and azobenzene
reference (3) compounds.
In contrast to their similar spectroscopic
properties, the Z isomers of compounds 1 and
3
display substantially different thermal sta-
bility. Interestingly, (Z)-1 exhibits a markedly
units elongate the p-conjugated system of the antenna to shift
its absorption band to the 380 nm region, where the azoben-
zene exhibits negligible absorption, the meta-configured
elongated thermal half-life (t1/2 = 522 days at room temper-
ature, see Table S2) compared to (Z)-3 (t1/2 = 48 days).
Detailed analysis involving experimental determination of
the activation parameters for the thermal Z!E isomerization
of 1 and 3 suggests that the observed effect is mostly enthalpic
in origin (Table S2).
[13]
linkage ensures decoupling in the ground state and hence
selective excitation and spectroscopic identity of each indi-
vidual component. Importantly, overlap of the emission of the
[
14]
antenna
with the n!p* absorption band of the (Z-
In the excited state, efficient communication between the
antenna and azobenzene moieties in the dyad was observed,
as manifested by the relative fluorescence quantum yields (in
configured) azobenzene should facilitate the envisioned
energy-transfer mechanism (Figure 1).
À3
[14]
The synthesis of the desired triarylamine–azobenzene
dyad 1 was accomplished in a convergent fashion. Initial
n-hexane) of 1 (F = 3.9 10 ) compared to 2 (F = 0.12).
The negligible fluorescence emission of the antenna in the
dyad is clear evidence for an efficient intramolecular quench-
ing process. Assuming energy transfer from the excited
antenna to the azobenzene moiety to be the origin of the
observed quenching, Z!E isomerization experiments were
conducted with compounds 1 and 3 upon 380 nm irradiation
(see Figure S7). Indeed, when the highly Z-enriched PSS
mixtures were subjected to identical irradiation conditions,
Z!E isomerization in the dyad 1 proceeded much faster
[14]
halogen–metal exchange of antenna 2 with nBuLi at À788C
followed by borylation with B(OiPr) and reaction under
3
Suzuki cross-coupling conditions with the corresponding 4-
bromo-2,2’,6,6’-tetrafluoroazobenzene afforded target com-
pound 1. Further details of the synthesis and compound
characterization are provided in the Supporting Information.
The UV/Vis absorption spectra of dyad 1, in the E as well
as Z configuration, resemble the spectral signatures of both
individual components, as illustrated by comparison with the
sum spectra of reference compounds 2 and 3 (Figure 2). The
long-wavelength absorption maxima at 460 nm [(E)-1] and
(F
= 0.38 for 1 vs. F
= 0.09 for 3, see Table S1) and
Z!E
Z!E
a significantly higher E content in the PSS was achieved
compared to the parent azobenzene 3 (E/Z = 76:24 for 1 vs.
Angew. Chem. Int. Ed. 2016, 55, 1544 –1547
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1545