Synthesis and redox and photophysical properties of Benzodifuran-Spiropyran ensembles

Article abstract of DOI:10.1002/chem.201204043

Two benzodifuran (BDF)-coupled spiropyran (SP) systems and their BDF reference compounds were obtained in good yields through Huisgen-Meldal- Sharpless click chemistry and then subjected to investigation of their electrochemical and photophysical properties. In both SP and merocyanine (MC) forms of the coupled molecules, the BDF-based emission is quenched to around 1 % of the quantum yield of emission from the BDF reference compounds. Based on electrochemical data, this quenching is attributed to oxidative electron-transfer quenching. Irradiation at 366 nm results in ring opening to the MC forms of the BDF-coupled SP compounds and the SP reference compound with a quantum efficiency of about 50 %. The rate constants for the thermal ring closing are approximately 3.4×10^-3 s^-1. However, in the photostationary states the MC fractions of the coupled molecules are substantially lower than that of the reference SP compound, attributed to the observed acceleration of the ring-closing reaction upon irradiation. As irradiation at 366 nm invariably also excites higher-energy transitions of the BDF units in the coupled compounds, the ring-opening reaction is accelerated relative to the SP reference, which results in lower MC fractions in the photostationary state. Reversible photochromism of these BDF-coupled SP compounds renders them promising in the field of molecular switches. Copyright

Full text of DOI:10.1002/chem.201204043

FULL PAPER
DOI: 10.1002/chem.201204043
Synthesis and Redox and Photophysical Properties of Benzodifuran–
Spiropyran Ensembles
Hui Li,[a] Jie Ding,[b] Songjie Chen,[a] Christoph Beyer,[c] Shi-Xia Liu,*[a]
Hans-Achim Wagenknecht,[c] Andreas Hauser,*[b] and Silvio Decurtins[a]
Abstract: Two benzodifuran (BDF)-
coupled spiropyran (SP) systems and
their BDF reference compounds were
obtained in good yields through Huis-
gen–Meldal–Sharpless “click” chemis-
try and then subjected to investigation
of their electrochemical and photo-
physical properties. In both SP and
merocyanine (MC) forms of the cou-
pled molecules, the BDF-based emis-
sion is quenched to around 1% of the
quantum yield of emission from the
BDF reference compounds. Based on
electrochemical data, this quenching is
attributed to oxidative electron-transfer
quenching. Irradiation at 366 nm re-
sults in ring opening to the MC forms
of the BDF-coupled SP compounds
and the SP reference compound with a
quantum efficiency of about 50%. The
rate constants for the thermal ring clos-
ing are approximately 3.4ꢀ10À3 sÀ1.
However, in the photostationary states
the MC fractions of the coupled mole-
cules are substantially lower than that
of the reference SP compound, attrib-
uted to the observed acceleration of
the ring-closing reaction upon irradia-
tion. As irradiation at 366 nm invaria-
bly also excites higher-energy transi-
tions of the BDF units in the coupled
compounds, the ring-opening reaction
is accelerated relative to the SP refer-
ence, which results in lower MC frac-
tions in the photostationary state. Re-
versible photochromism of these BDF-
coupled SP compounds renders them
promising in the field of molecular
switches.
Keywords: asymmetric synthesis ·
click chemistry · electron transfer ·
photochromism · redox chemistry ·
spiro compounds
Introduction
response to external stimuli, such as UV light, changes in
pH, and addition of metal ions.[2] Recently, various fluores-
cence-coupled SP compounds have attracted a lot of interest
within the context of energy-transfer switching.[3,4] In these
systems, fluorescent chromophores are either covalently
bonded to or physically mixed with SP derivatives. Fluores-
cence emission from the fluorophore is observed only in the
SP form, whereas it is severely quenched in the MC form
due to fluorescence resonance energy transfer (FRET) from
the excited state of the fluorophore to the MC form of the
SP unit. Upon irradiation with visible light, the transforma-
tion of the MC back to the SP form of the system or simply
thermal ring closing leads to recovery of the fluorescence.
No benzodifuran (BDF)-coupled photochromism has been
reported so far, despite the fascinating features and promis-
ing applications of SP and BDF compounds. BDF deriva-
tives are not only excellent components in organic field-
effect transistors (OFETs) and organic light-emitting diodes
(OLEDs) by virtue of their p-type semiconductor character-
istics and high hole mobility (up to 10À3 cm2 VÀ1 sÀ1 for
amorphous films),[5] but also they can undergo a reversible
oxidation forming stable cation-radical salts that can be iso-
lated in crystalline form.[6] Moreover, the BDF core can be
functionalized to modulate spectral properties or to serve as
precursors in polymerization reactions.[7] Despite these at-
tractive aspects, functionalized BDF and SP derivatives were
quite rarely reported in the literature owing to the limited
preparative accessibility and the lack of efficient and con-
venient synthetic methodologies.
Incorporating photochromic molecules into organic or
hybrid organic–inorganic materials leads to a large number
of photoresponsive systems that show potential applications
in a variety of fields, such as chemical or biological sensors,
optical memory, fluorescent switches for photonic materials,
reversible optical data storage, molecular-level logic gates,
and holography.[1,2] Spiropyran (SP) molecules play a promi-
nent role in this field as they can undergo reversible ring
opening to the corresponding merocyanine (MC) forms in
[a] H. Li, S. Chen, Dr. S.-X. Liu, Prof. S. Decurtins
Departement fꢁr Chemie und Biochemie
Universitꢂt Bern
Freiestrasse 3, 3012 Bern (Switzerland)
Fax : (+41)31-631-3995
E-mail: liu@iac.unibe.ch
[b] Dr. J. Ding, Prof. A. Hauser
Dꢃpartement de Chimie Physique
Universitꢃ de Genꢄve
30 Quai Ernest-Ansermet, 1211 Genꢄve 4 (Switzerland)
Fax : (+41)22-379-6103
E-mail: andreas.hauser@unige.ch
[c] Dr. C. Beyer, Prof. H.-A. Wagenknecht
Karlsruher Institut fꢁr Technologie (KIT)
Institut fꢁr Organische Chemie
Fritz-Haber-Weg 6, 76131 Karlsruhe (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204043.
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the symmetrical compounds,
the copper(I)-catalyzed 1,3-di-
polar cycloaddition of 2,6-bis-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
dicarbonitrile (9)[9] and the cor-
responding azides gave the
target compounds 1 and 3 in 72
and 66% yields, respectively,
whereby two SPs are linked to
the BDF chromophore through
the 1,2,3-triazole linkers. For
the asymmetrical compounds, a
Pd/Cu-catalyzed
coupling reaction was used as
key reaction to build the
Sonogashira
a
ethynylene-linked asymmetrical
molecule 7. Starting reagent 6
was obtained from 5[9] in 40%
yield by reaction with one
Scheme 1. Chemical structures of the targets 1, 2 and the reference compounds 3, 4.
Herein, we present two BDF-based chromophores 1 and
2 symmetrically and asymmetrically functionalized with SP,
respectively, together with two reference compounds 3 and
4 (Scheme 1). The 1,2,3-triazole linkers between the central
BDF core and the SP unit(s) or the hexyl chain(s) were
formed by “click” chemistry, namely the Huisgen 1,3-dipolar
cycloaddition of azides and acetylenes, to generate all four
compounds. Our study was mo-
equivalent of 3-hydroxy-3-methylbutyne under standard So-
nogashira conditions. Subsequent treatment of 6 with one
equivalent of phenylacetylene in the presence of CuI, Pd-
AHCTNUGERTNG(NNU PPh3)2Cl2, and Et3N afforded intermediate 7 in 70% yield.
The monoacetylene 8 was obtained as a yellow solid in 55%
yield by deprotection of the propargylic compound 7 with
powdered sodium hydroxide in dry toluene at reflux temper-
tivated by the question of
whether the reversible intercon-
version between the SP and
MC forms of the SP unit(s) can
be exploited to modulate light-
induced electron- and energy-
transfer processes; this topic
has not been fully explored in
the literature.
Results and Discussion
Synthesis: Recently, we have
developed facile and efficient
synthetic routes to 1’-(3-azido-
propyl)-1’,3’-dihydro-3’,3’-di-
methyl-6-nitrospiro-[2H-1-ben-
zopyran-2,2’[2H]-indole] (10)[8]
and the fully functionalized
BDF derivatives[9]
5 and 9
(Scheme 2). With these key pre-
cursors in hand, a variety of
BDF-coupled SP systems were
obtained through Huisgen–
Meldal–Sharpless click chemis-
try for studies of photoinduced
intramolecular electron- and/or
Scheme 2. Synthesis of the targets 1, 2 and the reference compounds 3, 4. TBTA= tris[(1-benzyl-1H-1,2,3-tria-
energy-transfer processes. For zol-4-yl)methyl]amine.
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Benzodifuran–Spiropyran Ensembles
FULL PAPER
ature. Surprisingly, the homocoupling product 11 was also
formed as an orange solid under these conditions. For 5 and
9, the incorporation of the hexyl substituents prevents low-
ered solubility at later steps in the synthesis. All new com-
pounds were characterized unambiguously by spectroscopic
analyses (see the Experimental Section).
Electrochemistry: The electrochemical properties of 1–4 and
10 in CH2Cl2/CH3CN (1:4 v/v) were investigated by cyclic
voltammetry. Their electrochemical data are collected in
Table 1. As expected, the reference compounds 3 and 4 un-
Table 1. Redox potentials [V versus Fc/Fc+] of compounds 1–4 together
with those of the reference compound 10.
ox1
ox2
1
2
Ered1
Ered2
1
Compound
E =
E =
2
1
SP
MC
SP
0.30
0.28
0.30
0.31
0.67
À1.67[b]
À1.68[b]
À1.70[b]
À1.48[b]
0.69[a]
0.66
2
MC
0.71[a]
0.66
À1.70[b]
À1.75[b]
3
0.32
4
10
0.34
0.68
SP
MC
0.64[b]
0.66[b]
À1.69[b]
À1.40[b]
[a] Quasi-reversible peak. [b] Irreversible peak.
dergo two well-separated and reversible single-electron oxi-
dation processes leading to the BDF-centered radical cation
and dication. Upon incorporation of the SP unit(s), com-
pounds 1 and 2 show two reversible anodic peaks, which are
attributed to the sequential oxidation of the BDF and SP
cores to their radical cation species. When they are convert-
ed to the corresponding MC forms upon irradiation at
375 nm, the oxidation potentials are slightly shifted and the
second oxidation process becomes quasi-reversible (Fig-
ure S1, Supporting Information). In each case, a visual in-
spection of the solution at the working electrode surface
during the second oxidation shows the formation of oxidized
MC, confirmed by the presence of a red color. These obser-
vations are consistent with the electro-oxidation characteris-
tics of 10-SP and its analogues,[10] which exhibit a one-elec-
tron oxidation at the indoline nitrogen atom only slightly af-
fected by the photoinduced structural changes.
In contrast, 10-SP exhibits only one irreversible cathodic
wave at À1.69 V whereas 10-MC undergoes two irreversible
reductions at À1.40 and À1.75 V. It can therefore be de-
duced that 10 is thermodynamically more easily reduced
upon conversion to the MC form. For compounds 1 and 2 in
both the SP and MC forms, the cathodic wave patterns are
similar to those of 10. The fact that the redox potentials of
the BDF and SP unit(s) are only negligibly shifted relative
to those of the reference compounds 3, 4, and 10 suggests
that the electronic interaction between the BDF and SP
moieties of 1 and 2 in the ground state is limited.
Figure 1. a) Absorption spectra of 1-SP, 3, and 10-SP, and emission and
excitation spectra of 3; b) absorption spectra of 2-SP, 4, and 10-SP, and
emission and excitation spectra of 4, all in DMF at room temperature.
compound 1-SP is close to the sum of the weighted absorp-
tion spectra of the BDF reference compound 3 and 10-SP.
The latter exhibits its typical absorption maximum at
around 347 nm (28800 cmÀ1), the former at a slightly lower
energy of 367 nm (27250 cmÀ1). The spectrum of 1-SP has a
corresponding maximum at 360 nm (27800 cmÀ1). The ab-
sorption spectrum of 2-SP shown in Figure 1b is likewise
equal to the sum of the spectra of the BDF reference com-
pound 4 and 10-SP. The lowest-energy band of 4 is at
413 nm (24200 cmÀ1). The redshift of this absorption band
relative to 3 is ascribed to the more extended p system origi-
nating from the BDF-ethynylbenzene unit, quite analogous
to the corresponding pyridine derivatives reported earlier.[9a]
As a consequence, the absorption spectrum of 2-SP now
shows two distinct maxima at 357 nm (28000 cmÀ1) and
413 nm (24200 cmÀ1), which can be associated with the 10-
SP and BDF units, respectively, plus some higher-energy
bands also associated with the latter.
According to previous studies,[7b,9] solutions of BDF com-
pounds typically exhibit strong fluorescence in the energy
range of 25000–16700 cmÀ1 (400–600 nm). As shown in Fig-
ure 1a and b, the reference compounds 3 and 4 do indeed
Photophysical study: The electronic absorption spectra of
compounds 1-SP and 3 together with that of 10-SP are de-
picted in Figure 1a. The optical absorption spectrum of
show strong luminescence at 23260 cmÀ1 (430 nm, Fem
=
0.85) and 21050 cmÀ1 (475 nm, Fem =0.90), respectively, in
DMF solution at room temperature, upon excitation at the
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S.-X. Liu, A. Hauser et al.
maximum of the respective BDF-centered absorption band.
The excitation spectra of both compounds follow the corre-
sponding absorption spectra closely. In compounds 1-SP and
2-SP the emission associated with the respective BDF units
is quenched to about 1% of the intensities observed for the
reference compounds 3 and 4 (Figure S2, Supporting Infor-
mation). This quenching cannot be due to an intramolecular
energy-transfer process because the lowest-energy transition
associated with the 10-SP unit is at higher energy than the
emission of the BDF units. It can, however, be ascribed to
an oxidative intramolecular electron transfer. The thermo-
dynamic driving forces for the charge-recombination
(ÀDGCR) and charge-separation (ÀDGCS) processes were
calculated by using the following expressions: ÀDGCR
=
(EoxÀEred)+DGS; ÀDGCS =DE0–0 (ÀDGCR), in which Eox is
ÀACHTUNGTRENNUG
Figure 2. UV/Vis absorption spectra in DMF of 1 and 2 (c1 =5ꢀ10À6 m,
c2 =1ꢀ10À5 m) before irradiation and in the photostationary state upon ir-
radiation at 27320 cmÀ1 ((366Æ10) nm, Xe arc lamp, power ꢀ2.8 mW
over an area of 10ꢀ5 mm2 stirred solution) for 5 min in comparison with
those of the reference compound 10 (c=1ꢀ10À5 m), and the correspond-
ing luminescence spectra for excitation at 17540 cmÀ1 (570 nm).
the first oxidation potential, Ered is the first reduction poten-
tial, DGS is the static Coulomb energy (approximately
0.05 eV in polar DMF), and DE0–0 is the energy of the 0–0
transition (the energy gap between the lowest excited state
and the ground state of the donor–acceptor system).[11] Ac-
cording to the redox potentials (Table 1) and the energy of
the BDF-centered excited state, the values of the driving
forces for charge recombination processes, ÀDGCR, for 1
and 2 are calculated as 2.02 and 2.05 eV, respectively. To-
As mentioned above, the closed forms of 1 and 2 show
quenched luminescence from the BDF core, and also no lu-
minescence from the SP units either. For their open forms,
the BDF-centered luminescence is still quenched (Figure S3,
see Supporting Information), but a quite strong lumines-
cence associated with a new low-energy absorption band ap-
pears (Figure 2). This emission can be used to monitor the
ring-opening kinetics of the SP units. (Of course this can
also be monitored by absorption spectroscopy; see Figure S4
in the Supporting Information.) The corresponding lumines-
cence intensities for the three compounds as a function of ir-
radiation time are shown in Figure 3. They follow first-order
kinetics in all cases with observed rate constants for the
ring-opening reaction kro of 2.9(1)ꢀ10À2, 3.1(1)ꢀ10À2, and
3.5(1)ꢀ10À2 sÀ1 for 1, 2, and 10, respectively. With the light
intensities used (ꢀ2.8 mW at (366Æ10) nm over an area of
1
gether with the energy of the BDF* state derived from the
optical spectra (3.10 and 2.79 eV for 1 and 2, respectively),
the values of the driving forces for the charge-separation
processes, ÀDGCS, are calculated as 1.08 and 0.74 eV, thus
1
suggesting that electron transfer from BDF* to SP is ther-
modynamically feasible. For the closed forms of 1 and 2 no
additional luminescence from the SP units is observed, as
indeed 10-SP is not emissive either.
Irradiation of the closed forms 1, 2, and 10 at 27320 cmÀ1
(366 nm), the lowest-energy SP-centered absorption band,
rapidly turns the colorless or slightly yellow solutions
purple. This color change is due to SP ring-opening reac-
tions, with dramatic changes in the UV/Vis spectra
(Figure 2) manifesting themselves in the appearance of a
strong absorption band at 17520 cmÀ1 (570 nm), typical for
MC forms.[8] For the corresponding experiments compara-
tively low concentrations were used, such that the absorb-
ance at the irradiation wavelength did not exceed 0.2, and
the concentration of 1 was chosen at half the concentrations
of 2 and 10 to correspond to the same effective concentra-
tion of the SP chromophore. The spectra shown in Figure 2
were taken after 5 min of irradiation from when the photo-
stationary state was reached; see below. The appearance of
the new intense absorption band at 17520 cmÀ1 (570 nm) is
in good agreement with that of the analogous system in
which 3-azidopropyl is replaced with a methyl group.[4a] In
that case, it was stated that the photostationary state upon
irradiation at 366 nm corresponds to nearly 100% conver-
sion from its SP to MC form. Thus, we may assume that this
also holds for 10. Therefore, for 1 and 2 the photostationary
state corresponds to approximately 55 and 65% conversion,
respectively.
Figure 3. Evolution of the luminescence intensity in DMF of 1, 2 (c1 =5ꢀ
10À6 m, c2 =1ꢀ10À5 m), and 10 (c=1ꢀ10À5 m) during irradiation at
27320 cmÀ1 ((366Æ10) nm, Xe lamp, ꢀ2.8 mW) starting from the closed
forms; white lines: exponential fits.
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Benzodifuran–Spiropyran Ensembles
FULL PAPER
10ꢀ5 mm2 into 2 mL of a stirred solution of optical density
(OD) <0.1 at the excitation wavelength in a 1 cm cell, see
Experimental Section) and an extinction coefficient of 7ꢀ
103 mÀ1 cmÀ1 of the SP-centered transition at the excitation
wavelength, the average excitation rate constant kex is esti-
mated to be about 0.07 sÀ1. Thus, the quantum efficiency for
the light-induced ring opening is of the order of 50% for
both 1 and 2 as well as for 10.
At the low concentrations used, the reduced stationary-
state conversions for 1 and 2 are not due to an overlapping
of absorption bands from BDF-centered transitions that
simply reduce the available light intensity for the photocon-
version. This in itself would only change the time needed to
reach the stationary state but not the stationary-state frac-
tions themselves. It could, however, be due to either faster
thermal ring closing for 1 and 2 compared to 10, or to addi-
tional photophysically accelerated ring closing. To test the
former hypothesis, the ring-closing kinetics in the dark and
under irradiation into the MC-centered transition at
17540 cmÀ1 (570 nm) was studied, this time by following the
intensity of the MC-centered transition itself (Figure S5,
Supporting Information). Figure 4 shows the corresponding
relaxation curves for 2 in the dark and as a function of irra-
diation intensity at 532 nm, on the high-energy side of the
MC-centered transition. (For the corresponding results for 1
and 10, see Figure S6 in the Supporting Information.) In the
dark the rate constant for ring closing, krc, for 2 is 3.4(1)ꢀ
10À3 sÀ1. The corresponding values of 3.4(1) and 3.3(1)ꢀ
10À3 sÀ1, respectively, for 1 and 10 are, within experimental
accuracy, equal to the value for 2. The values are consistent
with an analogous system in which 3-azidopropyl is replaced
with the methyl group.[12,13]
Given that the ring-closing rate constants in the dark for
all three compounds are almost equal and an order of mag-
nitude smaller than the rate constant for the light-induced
ring-opening reaction for the light intensity at 366 nm, one
would expect for all three compounds a similar MC fraction
in excess of 90% for the photostationary state. This not the
case, and the second reason for the reduced photostationary
MC fraction needs to be tested. This can best be done with
compound 2, for which the lowest-energy BDF-centered ab-
sorption at 24200 cmÀ1 (413 nm) is lower in energy and well
separated from the SP-centered absorption at 28000 cmÀ1
(357 nm). Accordingly, Figure 4b shows the ring-closing ki-
netics for 2 in the dark and under irradiation into the BDF-
centered absorption band by using a Xe arc lamp in con-
Figure 4. a) Thermal ring closing for 2 (c=3ꢀ10À5 m) monitored by the
optical density (OD) of the absorption band at 17540 cmÀ1 (570 nm) and
its acceleration through irradiation on the high-energy side of this band
at 532 nm (defocused laser beam). b) Thermal ring closing for 2 (c=3ꢀ
10À5 m) and its acceleration through irradiation at (450Æ25) nm from a
Xe lamp and bandpass filter (2.5 mW, Ø=20 mm). Inset in both cases:
rate constant for the ring-closing process as a function of irradiation in-
tensity.
the ring closing in analogy to direct excitation of the MC-
centered transitions. An additional control experiment was
performed by irradiating 2 and 10 in their SP forms at
(450Æ25) nm. This did not result in any ring opening, thus
supporting the conclusion that in this case the BDF-centered
luminescence in 2 is quenched by electron transfer.
The photochromic processes are reversible. By alternating
irradiation in the UV region at 366 nm and under visible
light (Figures S8 and S9, Supporting Information) both 1
and 2 can be switched back and forth between the SP and
the MC forms, which suggests that both 1 and 2 could func-
tion as molecular switch devices. Even though in DMF solu-
tion some degradation seems to occur, it could probably be
inhibited by incorporating the active molecules into an inert
polymer matrix.[2b,14]
junction with
a
50 nm bandpass filter at 450 nm
(22200 cmÀ1, ꢀ2.5 mW, Ø=20 mm). Irradiation into the
BDF-centered absorption clearly accelerates the ring-closing
process. For a control, the irradiation experiment was also
performed with compound 10 (Figure S7, Supporting Infor-
mation). Even though there is also a slight acceleration for
compound 10, it is much smaller and can be simply attribut-
ed to irradiation into the extended tail of the higher-energy
MC-centered absorption. We may thus conclude that in the
MC form, excitation into BDF-centered transitions results
in energy transfer to the MC unit and therefore accelerates
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S.-X. Liu, A. Hauser et al.
III spectrometer for the MALDI ionization method and HRMS data
were obtained in the electrospray ionization (ESI) mode.
Conclusion
Synthesis of compound 6: Compound 5 (826 mg, 1 mmol), PdACHTUNGTRENNUNG(PPh3)2Cl2
As a result of facile and efficient synthetic routes, the BDF-
coupled SP systems 1 and 2 and the reference compounds 3
and 4 were obtained in good yields through Huisgen–
Meldal–Sharpless click chemistry. The electrochemical and
photophysical properties of the compounds were investigat-
ed, and a detailed study with regard to the regulation of
electron- and energy-transfer processes with light was per-
formed. In comparison with the high quantum yield of emis-
sion of the reference BDF compounds 3 and 4, compounds
1-SP and 2-SP show that the emission associated with the re-
spective BDF units is quenched to around 1%. Based on
(70 mg, 0.1 mmol), and CuI (10 mg, 0.05 mmol) were added to an oven-
dried round-bottom flask. The atmosphere was removed by vacuum ex-
traction and replaced with dry Ar (3ꢀ). Triethylamine (40 mL) and piper-
idine (2 mL) were added to the mixture, and then at 1108C a solution of
2-methyl-3-butyn-2-ol (84 mg, 1 mmol) in triethylamine (10 mL) was
added dropwise. The mixture was stirred at 1108C for 8 h. Upon cooling
to RT, the solvent was removed in vacuum; the resulting residue was
poured onto a silica gel column and eluted with hexane/ethyl acetate
(10:1) to obtain 6 (312 mg, 40%) as an orange oil. 1H NMR (300 MHz,
CDCl3, 258C): d=3.56 (t, J=7.81 Hz, 8H), 3.06 (s, 1H), 1.71 (m, 8H),
1.62 (s, 6H), 1.34 (m, 24H), 0.89 ppm (t, J=7.12 Hz, 12H); 13C NMR
(75.5 MHz, CDCl3, 258C): d=162.4, 161.4, 144.7, 142.9, 122.8, 121.9,
116.2, 115.4, 103.3, 94.4, 70.8, 64.6, 63.3, 61.8, 60.2, 49.3, 30.5, 29.8, 27.5,
27.4, 25.2, 21.6, 13.1, 13.0 ppm; HRMS, ESI (positive): m/z calcd for
[C41H59IN4O3]+: 782.3626; found: 782.3623.
1
the redox potentials and the energy of BDF*, this is attrib-
uted to intramolecular oxidative electron-transfer quench-
ing. The light-induced ring opening upon irradiation at
366 nm has a quantum efficiency of approximately 50% for
both BDF-coupled molecules 1-SP and 2-SP as well as for
the reference compound 10-SP. However, in the photosta-
tionary state, the MC fractions of molecules 1 and 2 are sub-
stantially lower than that of compound 10. In the dark, the
rate constants for thermal ring closing krc are approximately
3.4ꢀ10À3 sÀ1 for all three compounds. Irradiation into the
MC-centered transition accelerates the ring-closing reaction.
For 2, for which the lowest-energy BDF-centered absorption
band is well separated from the SP- and MC-centered ones,
irradiation into this BDF-centered transition also accelerates
the ring-closing process, thus indicating that ring closing can
also be induced through energy transfer from the BDF to
the MC units. This explains why the photostationary MC
fraction in 1 and 2 is lower than that in 10, as irradiation at
366 nm invariably also excites higher-energy transitions of
the BDF unit.
Synthesis of compound 7: Compound 6 (782 mg, 1 mmol), PdACHTUNGTRENNUNG(PPh3)2Cl2
(70 mg, 0.1 mmol), and CuI (10 mg, 0.05 mmol) were added to an oven-
dried round-bottom flask. The atmosphere was removed by vacuum ex-
traction and replaced with dry Ar (3ꢀ). Phenylacetylene (102 mg,
1 mmol), triethylamine (40 mL), and THF (40 mL) were added to the
mixture. The resulting solution was stirred at 608C for 1.5 h. Upon cool-
ing to RT, the solvent was removed in vacuum; the residue was poured
onto a silica gel column and eluted with hexane/ethyl acetate (10:1) to
afford 7 (529 mg, 70%) as a yellow solid. 1H NMR (300 MHz, CDCl3,
258C): d=7.65 (m, 2H), 7.35 (m, 3H), 3.60 (t, J=7.50 Hz, 8H), 3.12 (s,
1H), 1.77 (m, 8H), 1.65 (s, 6H), 1.36 (m, 24H), 0.89 ppm (t, J=7.12 Hz,
12H); 13C NMR (75.5 MHz, CDCl3, 258C): d=163.1, 162.9, 145.0, 144.7,
131.6, 128.4, 123.4, 123.1, 117.3, 116.4, 104.7, 99.7, 96.1, 95.6, 79.7, 72.4,
65.7, 62.6, 62.1, 50.2, 31.5, 30.9, 28.5, 26.2, 22.6, 13.9 ppm; MS (MALDI-
TOF): m/z calcd for C49H64N4O3 [M]+: 756.50; found: 756.42.
Synthesis of compound 8: NaOH (400 mg, 10 mmol) was added to a solu-
tion of 7 (756 mg, 1 mmol) in dry toluene (40 mL). The atmosphere was
removed by vacuum extraction and replaced with dry Ar (3ꢀ). The mix-
ture was heated at reflux for 1.5 h. Upon cooling to RT, the crude prod-
uct was poured into water and extracted with dichloromethane (3ꢀ). The
combined organic phase was evaporated under reduced pressure, and the
residue was poured onto a silica gel column and eluted with hexane/di-
chloromethane (2:1) to give 8 (383 mg, 55%) as a yellow solid. M.p.
Furthermore, the MC fluorescence in compounds 1 and 2
can be reversibly switched on and off by alternating UV and
visible-light irradiation. Such reversible photochromic pro-
1
108.2–108.38C; H NMR (300 MHz, CDCl3, 258C): d=7.67 (m, 2H), 7.35
(m, 3H), 3.66 (s, 1H), 3.62 (m, 8H), 1.77 (m, 8H), 1.36 (m, 24H),
0.89 ppm (m, 12H); 13C NMR (75.5 MHz, CDCl3, 258C): d=163.1, 146.0,
145.0, 131.6, 128.7, 128.3, 127.9, 123.5, 123.0, 116.4, 116.0, 100.0, 96.8,
94.6, 87.9, 73.3, 62.6, 62.5, 50.2, 50.0, 31.5, 28.5, 26.2, 22.7, 14.1 ppm;
HRMS, ESI (positive): m/z calcd for [C46H58N4O2]+: 698.4554; found:
698.4550. The homocoupling product 11 was also obtained as an orange
solid (53 mg, 7%) under these reaction conditions. M.p. 189.7–189.88C;
1H NMR (300 MHz, CDCl3, 258C): d=7.67 (m, 4H), 7.36 (m, 6H), 3.65
(t, J=6.99, 16H), 1.74 (m, 16H), 1.36 (m, 48H), 0.89 ppm (m, 24H);
13C NMR (75.5 MHz, CDCl3, 258C): d=163.15, 162.98, 146.31, 144.94,
131.61, 128.33, 124.65, 123.02, 116.43, 115.31, 100.42, 97.09, 84.41, 79.77,
63.13, 62.48, 50.17, 49.89, 31.91, 31.52, 29.68, 29.34, 28.52, 28.36, 26.23,
26.08, 22.62, 22.56, 14.08, 13.95 ppm; MS (MALDI-TOF): m/z calcd for
C92H114N8O4 [M]+: 1394.90; found: 1394.81; HRMS, ESI (positive): m/z
calcd for [C92H114N8O4 +2H]2+: 698.4554; found: 698.4555.
ACHTUNGTRENNUNGcesses render these compounds promising in the field of mo-
lecular switches. Hopefully these results may offer new op-
portunities to design a more rational fabrication of multi-
functional molecular sensors and devices.
Experimental Section
General synthesis: 1-Azidohexane[15] and compounds 5, 9,[9] and 10[8]
were synthesized according to literature procedures. Commercially avail-
able reagents were used throughout without purification unless otherwise
stated. Analytical thin-layer chromatography was carried out on alumi-
num-backed plates coated with silica gel and visualized under UV light
at 254 and/or 360 nm. Chromatography was carried out on silica gel. 1H
and 13C NMR spectra were recorded at 300 and 75.5 MHz, respectively.
Chemical shift in ppm is quoted relative to residual solvent signals cali-
brated as follows: CDCl3 dH (CHCl3)=7.26 ppm, dC =77.0 ppm. Multi-
plicities in the 1H NMR spectra are described as: s=singlet, d=doublet,
t=triplet, m=multiplet; coupling constants are reported in hertz. Melt-
ing points were obtained on a Bꢁchi Melting Point B-540 instrument and
not corrected. Elemental analyses were performed on a Carlo Erba EA
1110 CHNS apparatus. Mass spectra were recorded on a Bruker Autoflex
General procedure for the click reaction: The ethynyl compound
(2.0 equiv of 8 or 1.0 equiv of 9), CuSO4·5H2O (15 mol% based on azido
precursor), tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (5 mol%
based on azido precursor), and (+)-sodium l-ascorbate (35 mol% based
on azido precursor) were added to a solution of azido precursor (2 equiv)
in a mixture of DMF and water (15/5 mL). The mixture was stirred at RT
(ꢀ24 h), diluted with EtOAc, and washed with brine and water. The
aqueous phase was again extracted with CH2Cl2. The combined organic
layers were dried over Na2SO4 and evaporated. The crude product was
purified by flash chromatography on silica gel.
6464
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Chem. Eur. J. 2013, 19, 6459 – 6466

Benzodifuran–Spiropyran Ensembles
FULL PAPER
Synthesis of compound 1: The crude product was eluted with CH2Cl2 to
afford
either a standard laboratory UV lamp (Benda, NU-4 KL) at 366 nm or
the light from a Xe arc lamp was used. The latter proved particularly
convenient for monitoring the ring opening by measuring the intensity of
the MC luminescence with the Fluorolog 3 instrument. For this, the exci-
tation monochromator was set to 366 nm with a bandpass of 10 nm,
which resulted in 2.8 mW light power over an irradiated area of 10ꢀ
5 mm2 at the sample. As concentrations were kept low enough so that
OD<0.1 at 366 nm, intensity gradients in the sample were negligible and
the excitation rate constant and the quantum efficiency of the process
could be estimated (see Supporting Information). For the acceleration of
the ring-closing reaction by irradiation, light from a diode-pumped solid-
state laser at 532 nm (defocused Ø=20 mm, 0–5 mW at the sample) for
irradiation into the MC-centered transition, or from a Xe arc lamp in
conjunction with a (450Æ25) nm bandpass filter and a lens (spot size Ø=
20 mm, 2.5 mW at the sample) for irradiation into the BDF-centered
transition of compound 2, was used.
1 (72%) as a
dark red solid. M.p. 242.7–242.88C; 1H NMR
(300 MHz, CDCl3, 258C): d=7.98 (m, 2H), 7.84 (m, 2H), 7.18 (t, J=
7.17 Hz, 2H), 7.08 (d, J=7.35 Hz, 2H), 6.91 (d, 2H, J=7.50 Hz), 6.86 (d,
J=10.14 Hz, 2H), 6.75 (d, J=9.0 Hz, 2H), 6.61 (d, J=7.74 Hz, 2H), 5.90
(d, J=10.14 Hz, 2H), 4.54 (m, 4H), 3.50 (t, J=7.17 Hz, 8H,), 3.34 (m,
4H), 2.43 (m, 2H), 2.32 (m, 2H), 1.65 (m, 8H), 1.27 (m, 27H), 1.20 (s,
3H), 0.85 ppm (t, J=6.57 Hz, 12H); 13C NMR (75.5 MHz, CDCl3, 258C):
d=163.9, 159.4, 146.7, 143.4, 140.9, 138.3, 136.1, 128.5, 127.9, 125.8, 124.1,
122.8, 122.7, 121.8, 121.6, 121.3, 119.9, 118.5, 117.4, 115.4, 106.9, 106.7,
104.2, 61.3, 52.6, 49.9, 48.1, 40.6, 31.4, 28.4, 26.1, 22.6, 19.9, 13.8 ppm;
HRMS, ESI (positive): m/z calcd for [C82H96N14O8]+: 1404.7536; found:
1404.7554; elemental analysis calcd (%) for C82H96N14O8·0.5H2O:
C
69.62, H 6.91, N 13.86; found: C 69.78, H 7.04, N 13.36.
Synthesis of compound 2: The crude product was eluted with CH2Cl2/
hexane (50–100%) to give 2 (76%) as a dark red solid. M.p. 179.5–
179.68C; 1H NMR (300 MHz, CDCl3, 258C): d=8.01 (m, 1H), 7.90 (m,
1H), 7.84 (s, 1H), 7.71 (m, 2H), 7.37 (m, 3H), 7.19 (t, J=8.43 Hz, 1H),
7.11 (d, J=6.81 Hz, 1H), 6.91 (m, 2H), 6.75 (d, J=8.94 Hz, 1H), 6.59 (d,
J=7.80 Hz, 1H), 5.90 (dd, J1 =10.65, J2 =2.28 Hz, 1H), 4.54 (m, 2H),
3.50 (m, 8H), 3.31 (m, 2H), 2.43 (m, 1H), 2.30 (m, 1H), 1.70 (m, 8H),
1.34 (m, 27H), 1.20 (s, 3H), 0.88 ppm (m, 12H); 13C NMR (75.5 MHz,
CDCl3, 258C): d=163.7, 163.4, 159.4, 146.7, 145.6, 142.9, 141.0, 138.2,
136.1, 128.5, 128.3, 127.8, 125.8, 124.1, 123.2, 122.7, 121.8, 121.6, 120.3,
119.9, 118.5, 117.4, 116.6, 115.5, 106.9, 106.7, 99.2, 98.2, 62.5, 61.4, 52.6,
49.9, 48.1, 40.7, 31.5, 28.5, 28.2, 26.3, 26.0, 22.6, 19.9, 14.0 ppm; HRMS,
ESI (positive): m/z calcd for [C67H79N9O5]+: 1089.6204; found: 1089.6212;
elemental analysis calcd (%) for C67H79N9O5·0.5H2O: C 73.20, H 7.33, N
11.47; found: C 73.22, H 7.64, N 11.18.
Acknowledgements
Financial support for this research by the Swiss National Science Founda-
tion (Grant Nos. 200020-130266 and 200020-125175), the Sino Swiss Sci-
ence and Technology Cooperation, as well as by KIT and the University
of Regensburg is gratefully acknowledged.
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CH2Cl2/hexane (1:1) and then with CH2Cl2/methanol (10:1) to afford 3
(66%) as a brown solid. M.p. 232.4–232.58C; 1H NMR (300 MHz, CDCl3,
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found: 876.6459.
Synthesis of compound 4: The crude product was initially eluted with
CH2Cl2/hexane (1:1) and then with CH2Cl2/methanol (10:1) to give 4
(78%) as a brown solid. M.p. 140.6–140.78C; 1H NMR (300 MHz, CDCl3,
258C): d=7.89 (s, 1H), 7.68 (m, 2H), 7.34 (m, 3H), 4.45 (t, J=7.50 Hz,
2H), 3.58 (m, 8H), 1.98 (m, 2H), 1.71 (m, 8H), 1.36 (m, 30H), 0.89 ppm
(m, 15H); 13C NMR (75.5 MHz, CDCl3, 258C): d=163.5, 163.1, 145.4,
142.8, 137.7, 131.4, 131.3, 128.6, 128.2, 128.1, 123.9, 123.0, 120.3, 117.0,
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30.0, 28.3, 28.1, 26.1, 25.9, 13.7 ppm; HRMS, ESI (positive): m/z calcd for
C52H71N7O2 [M]+: 825.5669; found: 825.5668.
Electrochemistry: Cyclic voltammetry was conducted on a VA-Stand 663
electrochemical analyzer. An Ag/AgCl electrode containing 2m LiCl (in
ethanol) served as reference electrode, a glassy carbon electrode as coun-
ter electrode, and a Pt disk as working electrode. Cyclic voltammetric
measurements were performed at RT under Ar in CH2Cl2/CH3CN
(1:4 v/v; 1ꢀ10À4 m) with 0.1m nBu4NPF6 as supporting electrolyte at a
scan rate of 100 mVsÀ1. All the potentials were calibrated with the stand-
+
1
ard ferrocene/ferrocenium redox couple (Fc/Fc : E = = +0.50 V meas-
2
ured under identical conditions).
Spectroscopic measurements: Solutions (2 mL) of appropriate concentra-
tions for either absorption (5ꢀ10À6–5ꢀ10À5 m) or emission (5ꢀ10À6–2ꢀ
10À5 m) of the compounds in spectroscopic-grade dimethylformamide
(DMF) were prepared directly in 1 cm quartz cells. UV/Vis absorption
spectra were measured on a Cary 5000 spectrophotometer. Emission
spectra were recorded on a Fluorolog 3 spectrophotometer. For the ki-
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minescence intensity of the MC form or the corresponding absorption in-
tensity was monitored. For the former the Fluorolog 3 instrument, and
for the latter a custom-built system consisting of a Spex 270M monochro-
mator and a CCD camera were used. For irradiation in the UV region
Chem. Eur. J. 2013, 19, 6459 – 6466
ꢅ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Chem. Eur. J. 2013, 19, 6459 – 6466