Results and Discussion
The straightforward synthesis of the azo-containing bichromophoric cyclophane 1 presented here is shown in [Scheme 1]. 1 is an azo-analogue of our recently described clamp structure, i.e. 2.[12] 3,3-Diiodoazobenzene (3)[24] was coupled with the monoprotected bisacetylene 4
[25] to give 5 in a yield of about 71% after purification by column chromatography and recycling
gel permeation chromatography (rec GPC). However, 5 contains small amounts of the Glaser side product that could not be removed at this
stage of the reaction sequence. Nevertheless, after statistical deprotection of 5, the mono-protected bisacetylene 6a (32%) along with the completely deprotected byproduct 6b (29%) were obtained in the pure form. Attempts to cyclodimerize 6b to 1 using CuCl and CuCl2 in pyridine were not successful and gave only the starting material, acyclic dimer,
and unknown byproducts of higher molecular weight. The route of success was first
a dimerization of the monoprotected “half-ring” 6a to 7 (48%), deprotection of the acetylenes (81%), and intramolecular ring-closure to 1 (50 – 60%) using Pd(II) and Cu(I) as a catalyst system, I2 as an oxidant and diisopropylamine as a base in THF. As expected, upon cyclization
the hydrodynamic radius of the compound decreases dramatically, leading to a large
change in the GPC-determined molecular weight (vs. polystyrene), although only two
hydrogen atoms are removed (see the Supporting Information, SI).
Scheme 1 a) Pd(PPh3)4, CuI, piperidine, THF, rt, 6 d, 71%; b) TBAF (1 M in THF), THF, H2O (5 vol%), rt, 5 h, 32%; c) Pd(PPh3)2Cl2, CuI, I2, HN(iPr)2, THF, rt, 16 h, 48%; d) TBAF (1 M in THF), THF, rt, 12 h, 81%; e) Pd(PPh3)2Cl2, CuI, I2, HN(iPr)2, THF, 50 – 60%.
While the NMR spectrum of the intermediates shows the presence of cis and trans azobenzene units, in the final macrocycle the azobenzene units cannot be switched
to the cis isomers, most probably due to steric reasons of the transition state of the isomerization
since space-filling models of trans/trans-1 and cis/cis-1 show no significant deformation of the chromophores. Thus, the proposed photoinduced
alteration of the chromophore–chromophore distance is not realized in this compound
1.
Although the absence of any switching behavior in a smaller macrocycle has been reported,[26] we expected that the large ring would give the system enough flexibility to allow
for a photoinduced change of the azo conformation.[27] Speculations about the large π-system being responsible for the stabilization of
the trans-conformation can be discarded, since 6b and 8 undergo reversible trans–cis switching (vide infra).
Self-assembled monolayers of 1 and 8 at the solid/liquid interface of the respective compound in 1,2,4-trichlorobenzene
(TCB) and highly oriented pyrolytic graphite (HOPG) were investigated by scanning
tunneling microscopy (STM).
At a concentration of 1 of 5 × 10−6 M in the supernatant liquid phase, 1 forms (after thermal annealing for 20 s to 80 °C, a routine procedure for enhancing
self-assembly and packing order) a densely covered self-assembled monolayer with domain
sizes of similarly oriented molecules in the range of 20 × 20 nm2 (see the overview STM image in the SI). In the high-resolution STM image shown in
[Figure 1]a, each molecule of 1 is imaged as a pair of bright lines (attributed to the rigid rod units) connected
by two medium-bright regions (attributed to the azobenzene units). The rigid rods
neither appear as perfect lines, nor do they all have the same shape. More precisely,
wide pairs of rods (as, e.g., those marked by arrow 1 in [Figure 1]a) should provide space for all (or most) of the alkoxy side chains being aligned
along the HOPG surface, while tight pairs of rods (e.g., those marked by arrow 2 in
[Figure 1]a) might correspond to alkoxy side chains pointing towards the solution phase (see
SI). Their random occurrence correlates with some degree of disorder of the otherwise
two-dimensionally (2D) crystalline domains, to which a unit cell of a = (5.1 ± 0.2) nm, b = (3.1 ± 0.2) nm, γ (a, b) = (78 ± 2)° and an orthogonal orientation of the backbones, c, to one of the HOPG main axis directions, d, is indexed. An idealized supramolecular model (of a geometry-optimized wide conformer
with all hexyloxy side chains adsorbed in parallel to the HOPG surface and along one
of its main axis directions, see SI) is shown in [Figure 1]c.
Figure 1 (a, b) Scanning tunneling microscopy images and (c, d) (supra-)molecular models of
(a), (c) 1 and (b), (b1), (b2), (d) 8 at the solid/liquid interface of highly oriented pyrolytic graphite (HOPG) and solutions
of the respective compounds in 1,2,4-trichlorobenzene. Image parameters: (a) 18 × 18 nm2, V
S = −0.8 V, I
t = 55 pA, c = 5 × 10−6 M, sample thermally annealed for 20 s to 80 °C prior to imaging; unit cell a = (5.1 ± 0.2) nm, b = (3.1 ± 0.2) nm, γ (a, b) = (78 ± 2)°; additional packing parameters: γ (c, d) = (90 ± 3)°, b ∥ d; (b): 30 × 30 nm2, V
S = −1.1 V, I
t = 117 pA; c = 1 × 10−5 M, sample thermally annealed for 20 s to 80 °C prior to imaging; unit cell a = (1.4 ± 0.1) nm; additional packing parameters: γ (c, d) = (83 ± 2)°, γ (a, d) = (7 ± 2)°; (b1), (b2): each 2.1 × 2.1 nm2. Red and white (black) lines as well as blue arrows indicate unit cell vectors, HOPG
main axis directions, and backbone directions, respectively.
At a concentration of 8 of 10−5 M in the supernatant liquid phase, 8 covers the surface densely (after thermal annealing for 20 s to 80 °C). An overview
STM image (see SI) shows domains of parallel-aligned backbones, however with disorder.
In the detailed STM image ([Figure 1]b), one dimensionally (1D) crystalline domains are observed, each consisting of a
few molecules. The image region in [Figure 1]b marked by the white dots is translated to the supramolecular model in [Figure 1]d. In this region, five molecules of 8 are aligned in parallel, and a unit vector of a = (1.4 ± 0.1) nm is indexed. Their azobenzene units adopt the trans/trans configuration, and are oriented anti relative to the central rigid rod. Moreover, the rigid rod units at each of the azobenzene
units are oriented in the anti-conformation (cf. Figure S8). The distance of the molecules is defined by intermolecularly
interdigitating hexyloxy side chains that are aligned along one of the HOPG main axis
directions, d. The rigid rods are oriented along c with γ (c, d) = (83 ± 2)° relative to one of the HOPG main axis directions, d, and the unit cell vector, a, is oriented relative to d with γ (a, d) = (7 ± 2)°. In the marked surface region, two more molecules of 8 are shifted in parallel (along c) by half a molecule length relative to the five previously discussed molecules, so
that seven trans-azobenzene units (each with two rigid rods in the anti-conformation) form a row. Notably, 8 in the periodic packing adopts a linear shape with two kinks, attributed to the azobenzene
units. One example of a trans-azobenzene unit is magnified in [Figure 1]b1 and is clearly distinguishable from the cis-azobenzene unit shown in [Figure 1]b2. Moreover (and despite the thermal annealing procedure), a single molecule of
cis/cis-8 is observed (the ends of which are marked by arrows 3/5 and 4/6 in [Figure 1]b/d), where the rigid rods adopt an angle of γ (c′, c′′) = (96 ± 2)°.
These results motivated us to investigate 1, its precursor 8 (containing three phenylene–ethynylene rods of two different lengths, connected by
two azobenzene units), and 6b (containing two “short” phenylene–ethynylene rods connected to one azobenzene unit)
by means of UV/vis spectroscopy (see SI). When a solution of 6 × 10−6 M of 1 in toluene was prepared and allowed to stand for 2 h, the UV/vis spectrum showed
two maxima at 319 and 421 nm. Irradiation with a light-emitting diode with 410 nm
maximum wavelength and a FWHM of 20 nm (see SI) did not lead to any discernible spectral
change (see SI). After preparing solutions of 8 and 6b under identical conditions, absorption maxima at 311, 319 and 416 nm (8) as well as 310, 319, and 399 nm (6b) were observed. An irradiation at 410 nm for 5 s (or 310 nm for 60 s, 8 and 6b) led to photostationary states with a minor reduction in absorbance in the spectral
range of 300 to 360 nm. Allowing each of the solutions to stand for 5 h in the dark
at rt led to a slow increase in absorbance in this region. Azobenzene[21] shows dramatic changes in intensities, and π-extended pyrene derivatives with four
azobenzene-ethynyl groups[28] still show significant variations in spectral intensities after irradiation (also
for a UV-light-induced back reaction). We attribute the minor intensity changes in
8 and 6b to the fact that their spectra (particularly at λ > 350 nm) are determined predominantly
by the absorption of the phenylene–ethynylene chromophores (cf. extinction coefficients
in Figure S11). The spectrum of 1 does not show any changes after irradiation. This fact is consistent with its 1H NMR spectrum, which does not contain any hint for the presence of the cis/cis isomer (Figure S1).
We then investigated macrocycle precursor 8 and the half-ring 6b by 1D and 2D thin-layer chromatography (TLC, see SI). Nominally, we would expect
trans/trans-8, trans/cis-8, and cis/cis-8 as well as trans-6b and cis-6b, and each set of compounds should have different R
f values. Handling the compounds and performing the TLC experiments (using Cy : DCM
1 : 2) in complete darkness led to single peaks for both species (R
f (8): 0.33; R
f (6b): 0.46). We conclude that these solutions consist of trans/trans-8 and trans-6b, respectively. Exposing 8 (and 6b) to daylight (either before and during its application to the TLC plate, or after
dissolving and depositing the compound and performing a first TLC run in darkness)
or 410 nm (while otherwise handling the compound in darkness) led to additional spots
(with R
f (8) = 0.21 and R
f (6b) = 0.24). Unexpectedly, a third spot, indicative of a third switching state, was
not observed for 8. This absence might be explained by either identical R
f values or concerted trans–cis-isomerization of both azobenzene units after photon absorption. This observation
is consistent with the STM experiment described before where trans/trans and cis/cis isomers were observed yet. Exposure of 6b during the TLC run led to smeared peaks, indicative of trans–cis (and cis–trans) isomerization on the TLC timescale.
Ensemble measurements to investigate the photophysical properties of 1 were performed in toluene solution. [Figure 2]a shows the normalized absorption (λ
Max
Abs ≈ 420 nm) and photoluminescence (PL) spectra (λ
Max
PL ≈ 470 nm) under excitation at 410 nm (green curves). In order to compare the results
with our previously synthesized structures, we also show the spectra of the rigid
clamp reference 2 ([Scheme 1]) with a chromophore spacing of approx. 0.7 nm (black).[12] Both the absorption and emission spectra of compound 1 are blue-shifted compared to this earlier bichromophoric structure. The blue-shift
in the absorption of 1 presumably originates from the inductive effects of the azobenzene clamps. [Figure 2]b shows the PL decay of 1 (green curve) in toluene solution, measured under pulsed laser excitation using time-correlated
single-photon counting (TCSPC) together with the instrument response function (IRF)
of the TCSPC system used (gray). By reconvolution of the signal with the IRF, we fit
the data using a biexponential function (red) with decay times of τ
1 = 25 ps and τ
2 = 648 ps, where the latter value is determined by a tail fit of the decay. This biexponential
decay profile of 1 is in stark contrast to that of the rigid clamp reference 2, where a monoexponential decay was observed with a PL lifetime of 660 ps.[12]
Figure 2 Ensemble spectroscopy in toluene solution: (a) normalized absorption (dotted lines)
and emission (solid lines) spectra of compound 1 (green) and the bichromophoric system 2 with rigid clamping units and a 0.7 nm interchromophoric spacing (black). (b) PL
intensity decay transient of 1 (green) together with the instrument response function (IRF; gray) of the TCSPC system
used. The biexponential fit (red) is achieved by reconvolution of the signal with
the IRF and by performing a tail fit of the signal to reveal the dominant time constants
τ
1 = (25 ± 1) ps and τ
2 = (648 ± 2) ps.
As we showed in earlier work, the dependence of the PL spectra and PL lifetime on
the interchromophore distance can be described in the framework of H-type electronic
coupling of molecular aggregates.[13],[29] As the chromophore separation decreases, the PL spectrum is shifted to lower photon
energies and the oscillator strength of the radiative transition decreases. The comparison
with the 0.7 nm-spaced rigid clamp reference 2 in [Figure 2] indicates a slightly increased chromophore spacing of the azobenzene bichromophore
1, and hence the PL spectrum of 1 is blue-shifted and the PL lifetime is slightly lowered to τ
2 = 648 ps with respect to 2. This observation agrees with the STM measurements shown in Figure S4, which suggest
a chromophore separation in 1 of between 0.8 and 1.6 nm. In order to identify the origin of the unexpectedly fast
PL lifetime component of 1, we measured the PL quantum yield (PLQY) using the commercial dye ATTO 390 as a reference
standard and found a value of (4 ± 0.4)%. This value is strongly reduced compared
to the previously reported rigid clamp bichromophoric system, where a value of > 60%
was measured.[10] Therefore, we conclude that the fast PL decay component of τ
1 = 25 ps follows a fast nonradiative recombination pathway, which is only present
in compound 1 but not in the rigidly clamped reference 2.
This conclusion raises the question of whether the multiexponential fluorescence decay
in solution is related to the occurrence of multiple subpopulations or conformations
of the molecules, or whether these dynamics can also be observed on the level of single
isolated molecules. Single-molecule spectroscopy offers direct access to answering
this question. By strongly diluting the analyte into a 2 wt% PMMA–toluene solution,
we can immobilize single molecules by spin coating this mixture onto microscope glass
cover slips to form a PMMA film of approx. 50 nm thickness. The concentration of the
analyte within the PMMA/toluene solution determines the density of immobilized molecules
observed in confocal scan images of the microscope as shown in [Figure 3]. We adjusted the analyte concentration of both the previously studied rigid clamp
reference 2 and of sample 1 prior to spin coating to the very same concentration and show the resulting confocal
microscopy scan images in [Figure 3]. For the same concentration, we find a strongly reduced fluorescence spot density
of (0.011 ± 0.004) µm−2 for compound 1 compared to the rigid clamp structures 2 with a spot density of (0.176 ± 0.008) µm−2.
Figure 3 Single molecule spectroscopy: (a) and (b) show confocal scan images with dimensions
of (20 × 20) µm2 of the 0.7 nm rigid clamp structures 2 and azobenzene-clamped bichromophore 1, respectively. From these images, we extract spot densities of (0.176 ± 0.008) µm−2 and (0.011 ± 0.004) µm−2, respectively. (c) and (d) show examples of single-molecule PL decays (black) together
with monoexponential fits (red lines).
[Figures 3](c, d) shows example TCSPC traces (black curves) of the rigid and azobenzene bichromophoric
clamp structures 2 and 1, respectively, together with single-exponential tail fits (red lines). The time constants
extracted are stated in the figure. Surprisingly, unlike in the ensemble measurements,
the average single-molecule PL lifetimes are almost identical for both samples ((0.83 ± 0.17)
ns for 1 vs. (0.89 ± 0.2) ns for 2), even though the lifetimes scatter within each sample. A statistical analysis of
this observation is shown in Figure S16 of the SI. The most important observation
is that all measured spots of 1 show a single-exponential PL decay without any fast lifetime component, unlike the
ensemble measurements in [Figure 2]. We therefore conclude that two subpopulations must be present within the ensemble
of compound 1. The first group exhibits photophysical properties comparable to the previously reported
rigid clamp reference 2. However, since the density of visible spots is approximately 16 times lower for
1 than for the rigid clamp reference 2, the results indicate that most single molecules of 1 show strong fluorescence quenching, i.e. nonradiative recombination, leading to the
low PL lifetime component τ
1 and the low PLQY in solution. Due to this low brightness, we cannot detect this subpopulation
at the single-molecule level. This strong quenching also agrees with the PLQY ratio
of the two compounds of approx. 60% to 4% (≈ 15). Since a trans–cis switching of the azo moieties in 1 can be excluded, it is conceivable that the azobenzene groups themselves directly
influence the nonradiative decay pathways of the chromophores. On the one hand, such
quenching could also conceivably be caused by the flexibility and mobility of the
clamp pieces, as has already been observed for molecular rotors such as BODIPY dye
molecules.[30],[31] On the other hand, it is also possible that energy or electron transfer occurs to
the azobenzene groups, for which low fluorescence quantum yields have already been
reported.[32] A possibility to test whether efficient energy transfer can occur within the clamped
bichromophore structure 2 is to measure the fluorescence photon statistics of single molecules as discussed
in Figure S16 of the SI. For both compound 1 and the analogous rigid clamp structure 2, we find nearly perfect single-photon emission, i.e. so-called photon antibunching,
in the single-molecule fluorescence. In a multichromophoric molecule such as 1, this effect can only occur if multiple excitons, which are present at the same time,
can diffuse in space and annihilate via bimolecular recombination pathways, i.e. singlet–singlet
annihilation.[32] The nearly perfect single-photon emission of 1 shows that efficient energy transfer indeed takes place between the two chromophores
within the molecule. It is also conceivable that, under certain circumstances, a transfer
process to the azobenzene clamp units might be possible and thereby quench the excited-state
population.
Experimental Section
Reagents and analytics. Reagents were purchased at reagent grade from commercial sources and used without
further purification. All air-sensitive reactions were carried out using standard
Schlenk techniques under argon. 2-Bromo-5-iodo hydroquinone (8) was prepared as described in Ref. [33].[33] [(3-Cyanopropyl) diisopropylsilyl] acetylene (CPDiPS-acetylene) and [(3-cyanopropyl dimethylsilyl] acetylene (CPDMS-acetylene) were
prepared as described in Ref. [34].[34] Diiodo azobenzene (2) was prepared as described in Ref. [35].[35] Reaction solvents (tetrahydrofuran, piperidine, dichloromethane, pyridine, triethylamine,
toluene) were dried, distilled, and stored under argon according to standard methods;
workup solvents were either used in “p. a.” quality or purified by distillation (dichloromethane,
cyclohexane). Prior to characterization and further processing, all solids and oils
were dried at rt under vacuum. 1H and 13C NMR spectra were recorded on a Bruker Avance I 300 MHz, a Bruker Avance I 400 MHz,
a Bruker Avance III HD 500 MHz Prodigy and a Bruker Avance III HD 700 MHz Cryo (300.1,
400.1, 500.1 and 700.1 MHz for 1H and 75.5, 100.6, 125.8 and 176.0 MHz for 13C). Chemical shifts are given in parts per million (ppm) referenced to residual 1H or 13C signals in deuterated solvents. All NMR spectra were recorded at rt unless otherwise
described. Mass spectra were measured on a Finnigan ThermoQuest MAT 95 XL (EI-MS),
a Sektorfeldgerät MAT 90 (EI-MS), a Bruker Daltonics micrOTOF-Q (ESI-MS, APCI), a
Bruker Daltonics autoflex TOF/TOF (MALDI-MS; matrix material: DCTB, no salts added)
and an ultrafleXtreme TOF/TOF of the Bruker Daltonik company (MALDI-MS; matrix material:
DCTB, no salts added). TLC was conducted on silica gel-coated aluminium plates (Macherey-Nagel,
Alugram SIL G/UV254, 0.25 mm coating with fluorescence indicator). Silica gel Kieselgel
60 (Merck, 0.040 – 0.063 mm) was used as the stationary phase for column chromatography.
UV/vis absorption spectra were recorded on a Perkin Elmer Lambda 18 and fluorescence
emission spectra on a Perkin Elmer LS-50B spectrophotometer using 10 mm quartz cuvettes.
Gel permeation chromatography (GPC). GPC was performed in THF (HPLC grade, stabilized with 2.5 ppm BHT) at rt. GPC analyses
were run on an Agilent Technologies system at a flow rate of 1 mL/min using an IsoPump
(G1310 A), a diode array UV detector (G1315B) and PSS columns (Polymer Standards Service;
Mainz, Germany; 102, 103, 105 and 106 Å, 5 µ, 8 × 300 mm). All molecular weights were determined vs. PS calibration (PS
standards from PSS, Mainz, Germany).
For the preparative separation, a Shimadzu rec GPC system, equipped with an LC-20
AD pump, an SPD-20 A UV detector and a set of three preparative columns from PSS (either
SDV 103 Å, 5 µ, 20 × 300 mm or SDV preparative linear S, 5 µ, 20 × 300 mm) with precolumn
(SDV, 5 µ, 20 × 50 mm) was employed. The system operated at a flow rate of 5 mL/min,
THF, 35 °C.
Scanning tunneling microscopy (STM). STM was performed under ambient conditions (rt) at the solution/solid interface,
using TCB as the solvent and HOPG as the substrate. In a typical experiment, 0.2 µL
of a 5 × 10−6 M to 1 × 10−5 M solution of the compound of interest was dropped onto a freshly cleaved HOPG substrate
at elevated temperature (80 °C), kept at this temperature for 20 s, and allowed to
cool to rt before the STM measurements were performed with the tip immersed into the
solution. Bias voltages between −0.8 and −1.2 V and tunneling current set points in
the range of 26 – 117 pA were applied to image the supramolecular adlayers shown here.
The experimental setup consists of an Agilent 5500 scanning probe microscope that
is placed on a Halcyonics actively isolated microscopy workstation. It is acoustically
shielded with a home-built box. Scissors cut Pt/Ir (80/20) tips were used and further
modified after approach by applying short-voltage pulses until the desired resolution
was achieved. HOPG was obtained from TipsNano (via Anfatec) in ZYB-SS and DS quality.
All STM images (unless otherwise noted) were calibrated by subsequent immediate acquisition
of an additional image at a reduced bias voltage, therefore the atomic lattice of
the HOPG surface is observed, which is used as a calibration grid. Data processing,
also for image calibration, was performed using the SPIP 5 (Image Metrology) software
package.
(Supra-)molecular modelling. (Supra-)molecular modelling was performed using Wavefunction Spartan ʼ18. Equilibrium
geometries shown in Figure S4 were obtained using molecular mechanics (based on the
Merck molecular force field) and a graphene monolayer with fixed atom positions as
the interaction partner. Equilibrium geometries shown in Figure S7(b – g) were obtained
with the same method, however using three different starting geometries. These were
manually created to match the shapes observed in the STM image shown in Figure S7a
(arrows 1 – 6). Moreover, dihedral angles of the azobenzene units were frozen to obtain
the trans or cis isomers. The molecular models shown in [Figure 1]c were obtained from the backbone structure shown in Figure S4(a, b) and subsequently
added all-trans-configured alkoxy side chains oriented along the HOPG main axis directions observed
in the STM image, and these molecules were used to create the supramolecular model.
The supramolecular model shown in [Figure 1]d was obtained in a similar procedure to match the structures observed in the STM
image shown in [Figure 1]b.
Optical spectroscopy. UV/vis absorption spectra shown in Figures S9–S11 were recorded on a Perkin Elmer
Lambda 18 spectrometer using 10 mm quartz cuvettes.
Ensemble absorption and PL spectra shown in [Figure 2] were recorded by dissolving the analyte in toluene solution and filling this into
a 10 mm quartz cuvette (Hellma Analytics, Quartz SUPRASIL). The data were recorded
using a Perkin Elmer spectrometer (Lambda 650) for absorption and a Horiba Jobin Yvon
Fluoromax 4 for PL. Spectra were background-corrected and normalized.
The PL decay of 1 in toluene solution was measured on an inverted confocal microscope as described
elsewhere (Figure S6). A frequency-doubled Ti : sapphire oscillator (Spectra Physics
Mai Tai BB) operating at 440 nm and 80 MHz repetition frequency was used for excitation.
Using a single-photon counting module (Picoquant-MPD-050-CTB), we recorded the signal
over a time period of 5 minutes. The extracted PL lifetimes shown in [Figure 2]b were confirmed by additional measurements using a picosecond streak-camera system
(data not shown).
Single-molecule measurements were performed under excitation at a wavelength of 440 nm
with a power density of approx. 750 Wcm−2 using two single-photon detectors (Picoquant-π-SPAD-20) in a Hanbury Brown and Twiss
detection geometry.
Synthetic procedures
Synthesis of 5: Under an Ar atmosphere, 3 (36.0 mg, 71 µmol)35, 4 (200.0 mg, 0.18 mmol), Pd(PPh3)4 (9.5 mg, 0.008 mmol), and CuI (1.0 mg, 0.004 mmol) in piperidine (15 mL) and THF
(5 mL) were stirred at rt for 6 d. Water and CH2Cl2 were added, the aqueous phase was extracted with CH2Cl2, and the organic phase was washed with aqueous HCl (2 M), water and brine and dried
over MgSO4. After evaporation of the solvent, column chromatographic purification (Cy : DCM = 1 : 1 → 2 : 3;
R
f = 0.53 (2 : 3)) gave 5 as a yellow film (139.8 mg, 58 µmol, 71%).
Formula: C156H216 N4O12Si2, molar mass: 2395.63 g/mol.
1H NMR (500 MHz, CDCl3, rt) δ [ppm]: 8.10 (t, 4
J
HH = 1.8 Hz, 2 H), 7.91 (ddd, 3
J
HH = 8.0 Hz, 4
J
HH = 2.0 Hz, 4
J
HH = 1.2 Hz, 2 H), 7.65 (dt, 3
J
HH = 7.7 Hz, 4
J
HH = 1.3 Hz, 2 H), 7.52 (t, 3
J
HH = 7.8 Hz, 2 H), 7.05 (s, 2 H), 7.04 (s, 2 H), 7.02 (s, 2 H), 7.01 (s, 2 H), 6.96
(s, 2 H), 6.93 (s, 2 H), 4.08 – 4.00 (m, 20 H), 3.96 (t, 3
J
HH = 6.4 Hz, 4 H), 2.44 (t, 3
J
HH = 7.0 Hz, 4 H), 1.94 – 1.81 (m, 28 H), 1.61 – 1.46 (m, 24 H), 1.43 – 1.24 (m, 48
H), 1.15 – 1.06 (m, 28 H), 0.94 – 0.82 (m, 40 H).
13C NMR (126 MHz, CDCl3, rt) δ [ppm]: 154.5, 153.9, 153.7, 153.4, 152.5, 134.2, 129.3, 125.9, 124.7, 123.2, 119.9,
118.0, 117.4, 117.4, 117.3, 116.6, 115.0, 114.8, 114.5, 114.4, 113.8, 113.4, 104.1,
95.3, 94.2, 91.8, 91.8, 91.7, 91.6, 87.1, 70.0, 69.9, 69.9, 69.8, 69.3, 31.8, 31.8,
29.9, 29.6, 29.5, 29.5, 29.5, 27.1, 26.0, 26.0, 25.9, 25.8, 25.8, 22.8, 22.8, 21.5,
20.9, 18.4, 18.2, 14.2, 14.2, 12.0, 9.8, 1.2.
MS (MALDI-TOF, DCTB) m/z: 2393.6 [M]+, 2643.7 [M + DCTB]+.
Calculated exact mass: 2393.60 g/mol.
Synthesis of 6a and 6b: Under an Ar atmosphere, TBAF (1 M in THF, 0.1 mL, 0.1 mmol) was added to 5 (130.0 mg, 54 µmol) in THF (6 mL) and water (0.24 mL). After 1 h, 1.5 h, and 2.5 h,
additional portions of TBAF (each 0.1 mL, 0.1 mmol) were added. After a total reaction
time of 5 h, water and CH2Cl2 were added, the aqueous phase was extracted with CH2Cl2, and the organic phase was washed with water and brine and dried over MgSO4. After evaporation of the solvent, column chromatographic purification (Cy : DCM = 2 : 3 → 1 : 2,
R
f = 0.64 (1 : 2)) and additional purification by rec GPC gave 6a as a yellow film (38.5 mg, 0.02 mmol, 32%). In addition, 6b was also obtained as a yellow film (32.5 mg, 16.0 µmol, 29%) (Cy : DCM = 2 : 3 → 1 : 2,
R
f = 0.70 (1 : 2)).
6a: Formula: C146H197 N3O12Si, molar mass: 2214.28 g/mol.
1H NMR (500 MHz, CDCl3, rt) δ [ppm]: 8.10 (t, 4
J
HH = 2.0 Hz, 2 H), 7.92 – 7.89 (m, 2 H), 7.65 (d, 3
J
HH = 7.6 Hz, 2 H), 7.52 (t, 3
J
HH = 7.8 Hz, 2 H), 7.07 – 6.91 (m, 12 H), 4.10 – 3.93 (m, 24 H), 3.34 (s, 1 H), 2.44
(t, 3
J
HH = 7.0 Hz, 2 H), 1.94 – 1.75 (m, 26 H), 1.62 – 1.42 (m, 24 H), 1.42 – 1.28 (m, 48 H),
1.16 – 1.04 (m, 14 H), 0.94 – 0.81 (m, 38 H).
13C NMR (126 MHz, CDCl3, rt) δ [ppm]: 154.5, 154.3, 153.9, 153.7, 153.5, 153.4, 152.5, 134.2, 129.3, 125.9, 124.7,
123.2, 119.9, 118.1, 117.9, 117.4, 117.3, 117.2, 116.6, 114.8, 114.4, 113.8, 113.4,
112.7, 104.1, 95.3, 94.2, 91.7, 91.7, 91.4, 87.1, 82.4, 80.2, 70.0, 69.9, 69.8, 69.8,
69.8, 69.7, 69.3, 31.8, 31.8, 31.8, 31.7, 29.9, 29.5, 29.5, 29.5, 29.4, 29.4, 29.4,
29.3, 26.0, 25.9, 25.9, 25.8, 25.8, 25.8, 25.8, 22.8, 22.8, 22.7, 21.5, 20.9, 18.4,
18.2, 14.2, 14.2, 14.2, 12.0, 9.8.
MS (MALDI-TOF, DCTB) m/z: 2212.4 [M]+, 2462.5 [M + DCTB]+.
Calculated exact mass: 2212.47 g/mol.
6b: Formula: C136H178 N2O12, molar mass: 2032.92 g/mol.
1H NMR (500 MHz, CDCl3, rt) δ [ppm]: 8.10 (t, 4
J
HH = 1.9 Hz, 2 H), 7.91 (dt, 3
J
HH = 8.0 Hz, 4
J
HH = 1.6 Hz, 2 H), 7.65 (dt, 3
J
HH = 7.7 Hz, 4
J
HH = 1.5 Hz, 2 H), 7.52 (t, 3
J
HH = 7.8 Hz, 2 H), 7.05 (s, 1 H), 7.04 (s, 1 H), 7.02 (s, 1 H), 7.01 (s, 1 H), 7.00
(s, 1 H), 6.98 (s, 1 H), 4.09 – 3.98 (m, 24 H), 3.34 (s, 2 H), 1.91 – 1.79 (m, 24 H),
1.61 – 1.46 (m, 24 H), 1.43 – 1.30 (m, 24 H), 1.30 – 1.17 (m, 24 H), 0.94 – 0.82 (m,
36 H).
13C NMR (126 MHz, CDCl3, rt) δ [ppm]: 154.3, 153.9, 153.7, 153.7, 153.5, 152.5, 134.2, 129.3, 125.9, 124.7, 123.2,
118.1, 117.4, 117.3, 117.2, 115.1, 114.8, 114.5, 114.4, 113.8, 112.7, 94.2, 91.8,
91.7, 91.7, 91.4, 87.1, 82.4, 80.2, 69.9, 69.9, 69.8, 69.8, 69.8, 32.1, 31.8, 31.8,
31.8, 31.7, 29.9, 29.5, 29.5, 29.5, 29.4, 29.4, 29.3, 26.0, 25.8, 25.8, 25.8, 25.8,
22.8, 22.8, 22.8, 22.7, 14.3, 14.2, 14.2, 14.2.
MS (MALDI-TOF, DCTB) m/z: 2031.3 [M]+, 2281.4 [M + DCTB]+.
Calculated exact mass: 2031.34 g/mol.
Synthesis of 7: Under an Ar atmosphere, 6a (78.5 mg, 36 µmol), Pd(PPh3)2Cl2 (7.5 mg, 0.01 mmol), CuI (5.1 mg, 0.03 mmol), and I2 (16.2 mg, 0.06 mmol) in HN(iPr)2 (5 mL) and THF (7 mL) were stirred at rt for 16 h. Water and CH2Cl2 were added, the aqueous phase was extracted with CH2Cl2, and the organic phase was washed with aqueous HCl (1 M), water, and brine and dried
over MgSO4. After evaporation of the solvent, column chromatographic purification (Cy : DCM = 2 : 3 → 1 : 2,
R
f = 0.45 (1 : 2)) and additional purification by rec GPC gave 7 as an orange film (37.7 mg, 8.5 µmol, 48%).
Formula: C292H392 N6O24Si2, molar mass: 4426.54 g/mol.
1H NMR (500 MHz, CDCl3, rt) δ [ppm]: 8.10 (s, 4 H), 7.91 (d, 3
J
HH = 8.0 Hz, 4 H), 7.65 (d, 3
J
HH = 7.6 Hz, 4 H), 7.52 (t, 3
J
HH = 7.8 Hz, 4 H), 7.06 – 6.92 (m, 24 H), 4.09 – 3.94 (m, 48 H), 2.43 (t, 3
J
HH = 6.9 Hz, 4 H), 1.92 – 1.76 (m, 52 H), 1.62 – 1.46 (m, 48 H), 1.41 – 1.28 (m, 96 H),
1.16 – 1.07 (m, 28 H), 0.95 – 0.80 (m, 76 H).
13C NMR (126 MHz, CDCl3, rt) δ [ppm]: 155.2, 154.5, 153.9, 153.7, 153.7, 153.5, 153.4, 152.6, 134.2, 129.3, 125.9,
125.7, 124.7, 123.2, 120.0, 118.0, 118.0, 117.4, 117.3, 117.3, 116.6, 115.6, 115.0,
114.8, 114.5, 114.4, 113.8, 113.4, 95.3, 94.2, 91.8, 91.8, 87.1, 70.0, 69.9, 69.9,
69.8, 69.3, 32.8, 32.1, 31.8, 31.8, 31.8, 31.7, 30.5, 29.9, 29.5, 29.5, 29.5, 29.4,
29.4, 29.3, 26.0, 26.0, 26.0, 25.8, 25.8, 25.8, 22.8, 22.8, 21.5, 20.9, 18.4, 18.2,
14.2, 12.0, 9.8, 1.2.
MS (MALDI-TOF, DCTB) m/z: 4422.9 [M]+, 4673.1 [M + DCTB]+, 4923.2 [M + 2DCTB]+.
Calculated exact mass: 4422.92 g/mol.
Synthesis of 8: Under an Ar atmosphere, TBAF (1 M in THF, 0.07 mL, 0.07 mmol) was added to 7 (50.0 mg, 11 µmol) in THF (7 mL) and stirred at rt for 12 h. Water and CH2Cl2 were added, the aqueous phase was extracted with CH2Cl2, and the organic phase was washed with water and brine and dried over MgSO4. After evaporation of the solvent, column chromatographic purification (Cy : DCM = 1 : 1,
R
f = 0.58) and additional purification by rec GPC gave 8 as a yellow film (37 mg, 9 µmol, 81%).
Formula: C272H354 N4O24, molar mass: 4063.83 g/mol.
1H NMR (500 MHz, CDCl3, rt) δ [ppm]: 8.11 – 8.09 (m, 4 H), 7.94 – 7.88 (m, 4 H), 7.67 – 7.64 (m, 4 H), 7.52 (t,
3
J
HH = 7.8 Hz, 4 H), 7.06 – 6.97 (m, 24 H), 4.09 – 3.98 (m, 48 H), 3.34 (s, 2H), 1.92 – 1.77
(m, 48 H), 1.61 – 1.46 (m, 48 H), 1.42 – 1.29 (m, 96 H), 0.96 – 0.85 (m, 72 H).
13C NMR (126 MHz, CDCl3, rt) δ [ppm]: 155.1, 154.3, 153.9, 153.7, 153.7, 153.7, 153.5, 153.5, 153.3, 152.5, 134.2,
130.9, 129.3, 128.8, 125.9, 124.7, 124.3, 123.2, 118.1, 118.0, 117.4, 117.3, 117.2,
117.2, 115.6, 115.1, 114.8, 114.6, 114.5, 114.4, 114.3, 113.8, 112.7, 94.2, 92.4,
91.8, 91.7, 91.7, 91.5, 91.4, 87.1, 82.4, 80.2, 79.8, 79.5, 69.9, 69.9, 69.8, 69.8,
69.8, 69.7, 31.8, 31.8, 31.8, 31.7, 31.7, 29.5, 29.5, 29.4, 29.4, 29.4, 29.3, 29.3,
26.0, 25.8, 25.8, 25.8, 25.8, 25.8, 22.8, 22.8, 22.8, 22.7, 14.2, 14.2, 1.2.
MS (MALDI-TOF, DCTB) m/z: 4060.7 [M]+, 4310.8 [M + DCTB]+, 4561.0 [M + 2DCTB]+.
Calculated exact mass: 4060.66 g/mol.
Synthesis of 1: Under an Ar atmosphere, 8 (10.0 mg, 2.5 µmol) in THF (20 mL) was purged with Ar for 1 h. By using a syringe
pump, this solution was slowly added (72 h) to Pd(PPh3)2Cl2 (17.3 mg, 24.6 µmol), CuI (2.3 mg, 12.3 µmol), and I2 (3.5 mg, 13.8 µmol) in THF (20 mL) and HN(iPr)2 (15 mL) at 50 °C and then additionally stirred for 72 h. After cooling to rt, water
and CH2Cl2 were added, the aqueous phase was extracted with CH2Cl2, and the organic phase was washed with water and brine and dried over MgSO4. After evaporation of the solvent, the crude product was dissolved in CH2Cl2, filtered through a plug of silica and purified by rec GPC to give 1 as a yellow film (5 – 6 mg, 1.23 – 1.48 µmol, 50 – 60%; variable yields of different
reactions).
Formula: C272H352 N4O24, molar mass: 4061.81 g/mol.
1H NMR (700 MHz, CDCl3, rt) δ [ppm]: 8.18 (d, 4
J
HH = 1.9 Hz, 4 H), 7.94 (dt, 3
J
HH = 8.1 Hz, 4
J
HH = 1.4 Hz, 4 H), 7.63 (dt, 3
J
HH = 7.5 Hz, 4
J
HH = 1.3 Hz, 4 H), 7.52 (t, 3
J
HH = 7.7 Hz, 4 H), 7.05 (s, 4 H), 7.04 (s, 4 H), 7.02 (s, 4 H), 7.01 (s, 4 H), 6.99
(s, 4 H), 6.99 (s, 4 H), 4.08 – 3.96 (m, 48 H), 1.89 – 1.75 (m, 48 H), 1.59 – 1.46
(m, 48 H), 1.44 – 1.26 (m, 96 H), 0.94 – 0.80 (m, 72 H).
13C NMR (176 MHz, CDCl3, rt) δ [ppm]: 155.1, 153.9, 153.7, 153.7, 153.7, 153.5, 152.3, 129.3, 124.8, 118.0, 117.4,
117.3, 117.3, 115.6, 114.8, 114.6, 114.4, 113.9, 112.7, 94.2, 92.5, 92.2, 91.6, 79.6,
69.9, 69.9, 69.9, 69.8, 45.4, 32.8, 31.8, 31.8, 31.7, 31.7, 31.2, 29.9, 29.5, 29.5,
29.4, 29.4, 29.4, 29.3, 26.7, 25.9, 25.9, 25.8, 25.8, 25.8, 25.8, 22.8, 22.8, 22.8,
22.7, 14.2, 14.2, 14.1, 14.1, 1.2.
MS (MALDI-TOF, DCTB) m/z: 4058.7 [M]+, 4308.8 [M + DCTB]+.
Calculated exact mass: 4058.64 g/mol.
Funding Information
The authors thank the Deutsche Forschungsgemeinschaft for funding through collaborative
grant no. 455 731 873, and RTG-2591 “TIDE-Template-designed Organic Electronics.”
