An anthracene-incorporated [8]Cycloparaphenylene derivative as an emitter in Photon upconversion

Article abstract of DOI:10.1021/acs.joc.7b02590

The anthracene-incorporated [8]cycloparaphenylene 2 has been synthesized and its optoelectronic properties studied by UV-vis spectroscopy, cyclic voltammetry, and DFT calculations. NMR and computational studies indicate that the new cycloparaphenylene derivative possesses a C_s point group symmetry. The new CPP 2 exhibits peculiar optoelectronic properties: (i) fluorescence emission is blue-shifted with respect to [8]cycloparaphenylene 1, and its quantum yield is higher; (ii) in the presence of an octaethylporphyrin Pd complex, as sensitizer, it undergoes a visible light upconversion. This is the first case in which a cycloparaphenylene derivative is involved as an emitter in low power light frequency conversion.

Full text of DOI:10.1021/acs.joc.7b02590

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Article
An Anthracene-Incorporated [8]Cycloparaphenylene
Derivative as Emitter in Photon Upconversion
Paolo Della Sala, Amedeo Capobianco, Tonino Caruso, Carmen Talotta,
Margherita De Rosa, Placido NERI, Andrea Peluso, and Carmine Gaeta
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02590 • Publication Date (Web): 12 Dec 2017
Downloaded from http://pubs.acs.org on December 15, 2017
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An AnthraceneꢀIncorporated [8]Cycloparaphenylene
Derivative as Emitter in Photon Upconversion
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Paolo Della Sala, Amedeo Capobianco, Tonino Caruso, Carmen Talotta, Margherita De Rosa,
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Placido Neri, Andrea Peluso and Carmine Gaeta
Dipartimento di Chimica e Biologia “A. Zambelli”, Università di Salerno, Via Giovanni Paolo II 132,
Iꢀ84084 Fisciano (Salerno), Italy,
eꢀmail: cgaeta@unisa.it
,
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT. The anthraceneꢀincorporated [8]cycloparaphenylene 2 has been synthesized and its
optoelectronic properties studied by UVꢀvis spectroscopy, cyclic voltammetry, and DFT calculations.
NMR and computational studies indicate that the new cycloparaphenylene derivative possesses a Cs
point group symmetry. The new CPP 2 exhibits peculiar optoelectronic properties: i) fluorescence
emission is blue shifted with respect to [8]cycloparaphenylene 1 and its quantum yield is higher; ii) in
the presence of a octaethylporphyrin Pd complex, as sensitizer, it undergoes a visible light upconversion.
This is the first case in which a cycloparaphenylene derivative is involved as emitter in low power light
frequency conversion.
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INTRODUCTION
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CycloParaPhenylenes (CPPs) are conjugated macrocycles consisting of paraꢀlinked benzene units
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(
e.g.: 1, Figure 1), which have originally raised much interest as potential seeds for growing uniform
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singleꢀwalled carbon nanotubes and as supramolecular hosts for the recognition of fullerene and
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pyridinium guests. Aside from those potential applications, [n]CPPs (n stands for the number of
phenylene rings of the nanohoop) also proved to possess intriguing size dependent optical and electronic
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properties, arising from a delicate interplay between the macrocycle strain energy and the πꢀelectron
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g
conjugation: i) [n]CPPs can host up to four electrons (for n = 8) without breaking the hoop; ii) they
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have high optical absorbance, which increases with the number of phenylene rings; iii) although they
exhibit a common absorbance maximum independent of n, their emission is significantly size
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dependent, being redꢀshifted as the number of benzene rings decreases; iv) quantum efficiency also
decreases as n decreases, so that while [n]CPPs with n ≥ 10 have high emission quantum yields (> 0.6)
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those with n = 5,6 do not emit at all.
The sizeꢀtunable properties of [n]CPPs make them attractive for several applications in
optoelectronics. Understanding how chemical substituents can further affect the optoelectronic
properties of those compounds, widening their field of potential applications, is still an almost
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unexplored field, with a few notable exceptions. Recently, Jasti and coworkers reported the synthesis
of a 1,4ꢀanthraceneꢀincorporated [12]CPP, exploring its potential as intermediate to synthesize novel
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carbon materials. Incorporation of strongly fluorescent groups into a [n]CPP skeleton could pave the
way to obtain CPPꢀbased optoelectronic materials with intriguing properties. Following this line, we
report herein the synthesis and the optical properties of 9,10ꢀanthraceneꢀincorporated [8]CPP 2.
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9,10-DPA
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Figure 1. [8]Cycloparaphenylene 1, palladium(II) octaethylporphyrin
diphenylanthracene.
complex and 9,10ꢀ
We show that this new compound exhibits unique optical properties with respect to its parent [8]CPP
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and affords bimolecular upꢀconversion (UC) of noncoherent, low intensity light, in the presence of a
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proper sensitizer. This is an important process for converting low energy photons into light adequate
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for photovoltaic, photocatalysis, and bioimaging. The most common method for tripletꢀtriplet
annihilation (TTA) UC involves the use of acceptor/annihilator molecules based on anthracene
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derivatives. At this regard, the 9,10ꢀDiPhenylAnthracene unit (DPA) (Figure 1) is a known acceptor in
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TTA photon UC processes. Upꢀconversion abilities of 9,10ꢀDPAꢀbased nanoparticles have been
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exploited in lymph node inꢀvivo imaging in mouse, while recently, it has been showed that 9,10ꢀDPA
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also retains the upꢀconversion properties in supramolecular gels. It is known that cycloparaphenylenes
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are excellent chromophores exhibiting high fluorescence quantum yields, whereas, at the best of our
knowledge, no literature data is available about their abilities as emitters into upꢀconversion schemes.
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Scheme 1. Synthesis of 2. (a) (4ꢀBromophenyl)lithium, THF, −78 °C, 2 h. (b) NaH, MeI, dry THF, 0 °C
→ 25 °C, 18 h. (c) nꢀBuLi, dry THF, −78 °C. (d) isopropyl pinacol borate, 5 h. (e) Pd(OAc) , SꢀPhos,
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K PO , DMF/H O, 19 h, 100 °C. (f) Sodium Naphthalenide, dry THF, −78 °C, 3 h. (g) I , THF, 45 min.
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Prompted by these consideration, we envisioned that the incorporation of a 9,10ꢀDPA unit into a
[8]CPP skeleton could afford the first example of a CPPꢀbased emitter in photon upꢀconversion, thus
widening the scope of this class of compounds. In the present work, we describe the synthesis of 9,10ꢀ
DPAꢀincorporated [8]CPP macrocycle 2 (Scheme 1) and a study of its optoelectronic and upꢀconversion
properties.
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RESULTS AND DISCUSSION
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Synthesis and NMR Characterization. CPP derivatives are highly curved nanohoops in which the
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Consequently, Justi and coworkers reported a synthetic strategy which consists of a SuzukiꢀMiyaura
crossꢀcoupling/macrocyclization between rigidlyꢀcurved elements incorporating cyclohexaꢀ2,5ꢀdiene
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moieties as masked benzene rings. The precursor macrocycle thus obtained, is successively aromatized
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to CPP by reduction in the presence of sodium napthalenide. On this basis, we have designed the
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synthesis of 2 by adapting the gramꢀscale protocol of Jasti and coworkers. In details, we have selected
derivative 6 (Scheme 1), bearing the 9,10ꢀdimethoxyꢀ9,10ꢀdihydroanthracene curvedꢀmoiety as a
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masked anthracene unit, to be coupled by SuzukiꢀMiyaura reaction with derivative 7. The synthesis of
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is outlined in Scheme 1. Anthraquinone 3 was reacted with (4ꢀbromophenyl)lithium yielding the synꢀ
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diol 4, in accordance with the electrostatic model proposed by Justi and coworkers. Successive
deprotonation of 4 with sodium hydride in the presence of MeI afforded derivative 5, which was
converted in diboronate 6 by treatment with isopropyl pinacol borate. Derivative 6 underwent
cyclization with 7 in the presence of Pd(OAc) , SꢀPhos, and K PO in DMF/H O, to give macrocycle 8.
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This was confirmed by a HR MALDIꢀFT-ICR mass spectrum, which showed a molecular ion peak at
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m/z 894.3895 (Calcd. for C H O , 894.3921). The H NMR spectrum of 8 in CDCl (600 MHz, 298 K)
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showed three OMe singlets at 3.46, 3.41, and 3.15 ppm, in a 1:1:1 ratio, which correlated in the HSQC
spectrum (SI) with carbon signals at 52.0 and 52.2 ppm.
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Figure 2. Significant portion of the H NMR spectrum of 2 (600 MHz, CDCl , 298 K). Inset:
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Significant portion of the isotope pattern in HR MALDI FT ICR mass spectrum of 2, calculated (in blu)
and experimental (in red).
Regarding the cyclohexadienyl moieties of 8, an AB system was present at 6.03 and 6.11 ppm (J =
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0.3 Hz, 4H each), which correlated in the HSQC spectrum (SI) with two resonances at 133.0 and 133.5
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ppm. In addition, the aromatic region of the H NMR spectrum of 8 evidenced the presence of two AB
systems at 7.44/7.38 ppm (J = 8.5 Hz, 4H each) and 7.17/6.92 ppm (J = 8.5 Hz, 4H each) attributable to
benzene rings A and B in Scheme 1. Finally, a singlet was present at 7.50 ppm attributable to ArH of
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ring C (4H). The C NMR spectrum of 8 evidenced the diagnostic presence of three singlets (DEPTꢀ
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35) at 80.3, 74.6, and 74.3 ppm, attributable to the oxygenated carbon atoms.
The treatment of 8 with sodium naphthalenide at −78 °C for 3 h in the dark, in degassed dry THF,
followed by quenching with I , afforded [8]CPP 2 in 11 % yield. The HR MALDIꢀFT-ICR mass
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spectrum of 2 (inset of Figure 2) showed a molecular ion peak at m/z 708.2812 (Calcd. for C H ,
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708.2811). A close inspection of the H NMR (Figure 2) and DQFꢀCOSY spectra of 2 (SI) in CDCl3
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(600 MHz, 298 K) revealed two distinct AA'BB' spin systems attributable to the aromatic hydrogen
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atoms 1/2 and 1'/2' (Scheme 1) of the anthracenyl group. In details, the two AA'BB' systems were
detected at 7.89/7.39 and 7.72/7.41 ppm (DQFꢀCOSY), and correlated in the HSQC spectrum with four
distinct carbon resonances at 127.72, 128.64, 127.64, and 127.12 ppm, respectively.
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Figure 3. Schematic representation of 2 indicating the dihedral angles τ and τ and the deformation and
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bent angles α and β.
This result clearly indicates that the two peripheral benzene rings of the anthracenyl group in 2 are
nonequivalent because of the their different in/out orientation with respect to the CPP cavity, which
implies a hindered rotation around τ (Figure 3). Because of this restricted rotation, the benzene ring A,
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adjacent to the anthracenyl group, adopts an inclined orientation, and the two 3/3' (and 4/4') protons
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(Scheme 1) become nonequivalent. Consequently, in the H NMR spectrum of 2 (SI) two AB systems
were detected for the orthoꢀcoupled 3/4 and 3'/4' (Scheme 1) protons at 7.11/7.01 (J = 6.1 Hz, 4H) and
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.00/7.08 ppm (J = 9.0 Hz, 4H). VT H NMR experiments (400 MHz, C D Cl ) evidenced no hint of
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coalescence up to 493 K for the 1/1', 2/2', 3/3', and 4/4' protons. In addition, the H NMR and DQFꢀ
COSY spectra of 2 evidenced the presence of an AB system at 7.42/7.23 ppm (J = 8.4 Hz, 8H)
attributable to 5/6 Hꢀatoms of ring B (Scheme 1). In fact, because of the fast (with respect to the NMR
time scale) rotation around the CꢀC bond connecting A and B rings, the protons 5 and 5' (and 6/6')
become equivalent. Finally, the ArH of rings C and D resonate as two sharp singlets at 7.47 and 7.49
ppm in 2:1 ratio (Figure 2).
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Figure 4. (Top) Potential energy profile (PCM/B97D/TZVP) of 2 along the τ coordinate. Energy is
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referred to the most stable conformer I. (bottom) The most stable conformers of 2 and their relative
ꢀ1
energies (E / kcal mol ).
Structural and optoelectronic properties. Since the NMR spectra of 2 suggest the occurrence of a
plane of symmetry, geometry optimizations of 2 were initially restricted to the C point group. To that
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end, we computed the optimized potential energy profile along the Lagrangian variable τ (Figure 3), by
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fixing in each step the two dependent dihedral angles and performing a full optimization of all the other
nuclear coordinates.
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Figure 5. Different views of the optimized structure (DFT calculations, PCM/B97D/TZVP) of 2 (I in
Figures 4).
The resulting potential energy profile is reported in Figure 4 (top); four minimum energy structures (Iꢀ
IV in Figure 4, bottom) have been found, the most relevant of which fall within an energy range of ca 1
kcal/mol. The statistically relevant conformers at room temperature are all characterized by an
alternating zigꢀzag conformation of the phenylene rings, the dihedral angle between neighboring
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phenylene units being ca 28 degrees, close to the optimal value of 30° found for [n]CPP systems.
Figure 6. Normalized UV/vis absorption (black) and emission (red) spectra of 2 in CH Cl and
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computed (PCM/TDCAMꢀB3LYP/6ꢀ31+G**) vertical electronic transitions (vertical lines) of
conformers I (blue), II (violet), and III (green).
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The dihedral angle τ is roughly 60° in both conformers I and II, reaching the expected value for free
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9,10 DPA (≈ 77°) only in III. Different stereochemical views of the optimized structure
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PCM/B97D/TZVP) of the most stable conformer of 2 are shown in Figure 5. Deformation and bent
angles (α and β in Figure 3) amount to 6.2° and 14.2°, respectively, close to the values of 9.3° and 13.7°
observed for α and β in the phenyl rings of [8]CPP. These indicate that the steric hindrances between the
two peripheral benzene rings (1 and 1' positions in Scheme 1) of the 9,10ꢀDPA core and the adjacent
phenyl rings causes a substantial distortion of the central unit of the anthracene ring, as already found in
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1,4ꢀanthracene incorporated [12]CPP’s. To ascertain that all the relevant conformations have been
actually located, we have also carried out: i) geometry optimizations by scanning the τ variable of
2
Figure 3 (SI), and ii) geometry optimizations starting from random conformations generated either
preserving or relaxing the C symmetry. No additional minimum energy conformations were found.
s
Cyclic voltammetry measurements showed that the first oxidation peak of 2 is quasiꢀreversible (SI), with
+
a half wave potential falling at 0.53 V vs Fc /Fc redox couple, similar to that found for [8]CPP (0.59
1
5
V), but significantly different from that of 1,4ꢀanthraceneꢀincorporated [12]CPP (0.65 V). The first
+
oxidation potential of the most stable conformer (I, SI) is predicted to be at +0.52 V vs Fc /Fc by
(
RO)CAMꢀB3LYP computations, in excellent agreement with the experimental value. Mulliken spin
·+
density analysis shows that the hole in the radical cation 2 is mainly, but not fully (80%), localized on
the anthracene moiety, in line with previous results reported for the 1,4ꢀanthraceneꢀincorporated
5
[
12]CPP derivative (1,4ꢀDPAꢀ[12]CPP). Indeed, DFT calculations at PCMꢀCAMꢀB3LYP/6ꢀ31+G**
level predict that the HOMO and LUMO of 2 are mainly localized on the anthracene moiety (Figure 6).
The predicted HOMO/LUMO energy gap of 2 is 2.2 eV, significantly lower than that calculated for 1,4ꢀ
5
DPAꢀ[12]CPP derivative (3.06 eV). The absorption and the emission spectra of 2 are reported in Figure
4
−1
6
. The absorption spectrum is characterized by an intense band peaked at 320 nm (ε = 1.7 × 10 M
−
1
cm ) and by a less intense absorption extending from 360 to 450 nm. The former signal is characteristic
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of [n]CPP systems, while the latter resembles very closely that of 9,10ꢀDPA, even though it lacks of its
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
peculiar vibronic structure.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7. Isodensity surface plots of the frontier KohnꢀSham orbitals of the most populated conformers
of 2.
On the whole, the absorption spectrum of 2 is mainly given by the superposition of the signals of
isolated 9,10ꢀDPA and [8]CPP, suggesting that the two molecular moieties maintain in the ground state
their own electronic structure, at least as concerns frontier orbitals. The above hypothesis has been
checked by theoretical calculations carried out at PCM/TDCAMꢀB3LYP/6ꢀ31+G** level of theory.
Predicted lowest energy transitions are reported in Table S1 and sketched in Figure 6, where the
computed transition strengths, averaged over Boltzmann conformer populations, are reported as a
function of the absorption wavelength and are superimposed to the experimental UV/vis spectrum.
Computed optical transitions agree well with the observed absorption spectrum. The broad spectral
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absorption observed in the region 350ꢀ450 nm corresponds to the absorptions predicted at 400, 410, and
80 nm for conformers I, II, and III, respectively (Table S1). The lower symmetry of 2 with respect to
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
unsubstituted [8]CPP, reflects in a reduced σ-π orbital mixing which makes the HOMOꢀLUMO
17
absorption, symmetry forbidden in [8]CPP, a bright transition (f ≈ 0.3, see Table S1). The S ← S
1
0
transitions predicted at ca 400 nm (Table S1) result from the excitation of one electron from the HOMO
to the LUMO, which are both KohnꢀSham π orbitals localized over the 9,10ꢀDPA moiety, as shown in
Figure 7. The electronic transitions, which are predicted to fall between 290 and 340 nm, are reported in
Table S1 for the three low energy conformers; the involved KohnꢀSham orbitals are sketched in Figure 7
for the two most populated conformers. For conformer I, there are two transitions with significantly
higher electric dipole transition moments, falling at 330 and 301 nm. The former transition mainly
consists of HOMOꢀ1 → LUMO+1 electron excitation, both levels being mainly localized on the five
phenyl rings of CPP which do not belong to DPA, so that electron density changes upon excitation does
not significantly involve the DPA moiety, see Figure 7. The transition at 301 nm mainly consists of
HOMOꢀ2 → LUMO+1 and HOMOꢀ1 → LUMO+2 electron excitations. The HOMOꢀ2 and LUMO+2
are mainly localized on the six phenylene rings surrounding the DPA moiety and the opposite phenyl
ring, thus also this transition does not involve the DPA moiety. The same holds true also for conformers
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
8,19,1e,20
II and III.
The fluorescence spectrum of 2 is broad, extending from 350 up to 650 nm, with a peak at 485 nm.
The slightly structured signal occurring at shorter wavelengths, see Figure 6, is somewhat characteristic
of the 9,10ꢀDPA unit, whose emission spectrum in cyclohexane is characterized by vibronic peaks in the
2
1
region 400ꢀ450 nm. By contrast, the emission peak is significantly blueꢀshifted with respect to that of
1e
[
8]CPP, whose emission spectrum is peaked at ca. 530 nm, being more similar to those of [n]CPP
hoops, with n>9. It is known that [n]CPPs exhibit a large increase in fluorescence quantum efficiencies
(Ф) for increasing value of n, for [16]CPP Ф=0.9 while for [8]CPP Ф=0.1. The fluorescence quantum
22
yield of 2 has been measured by using the relative method reported by Miller and coꢀworkers. Using
quinine sulphate as standard, the fluorescence quantum yield of 2 is Ф = 0.47, which is significantly
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higher than that observed for [8]CPP and closer to the values observed for larger [9ꢀ12]CPP
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
e
derivatives. Summarizing, both the emission spectrum and the result of quantum yield measurements
reveal that the emission properties of 2 are more similar to that of nanohoops of higher dimensions.
According to TDDFT computations, the electronic excitations of 2 involve KohnꢀSham orbitals which
are mainly localized either on the 9,10ꢀDPA moiety or on the remaining five CPP rings (Figure 7). The
formation of an excitonic state localized on five phenylene rings, which characterizes the excited state
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
6
geometries of [n]CPPs with n>8, can also likely occur in 2, making its emission properties more
similar to those of higher size nanohoops.
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Figure 8. Upconverted emission spectra of 2 (1 mM) and PdOEP in CH Cl , λ
= 540 nm. Blue:
exc
2
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[PdOEP]= 10 ꢀM; red: [PdOEP]= 100 ꢀM; green: [PdOEP]= 1.0 mM. Light source was passed through
a 295 nm long pass cutꢀoff emission filter prior to incidence on samples.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
9
,10ꢀDPA (Figure 1) has been already successfully used as emitter in upconversion of nonꢀcoherent
8
a
light of low intensity, in the presence of a phosphorescent molecule, usually Pd or Pt complexes. The
peculiar properties of 2, which retains most of spectral features of 9,10ꢀDPA, could indeed make it a
potential emitter in low intensity upconversion process. We have thus investigated the optical response
of 2 in DCM solution, employing as sensitizer palladium(II) octaethylporphyrin (PdOEP). A few
solutions of 2 and PdOEP at different concentration ratios were prepared in anhydrous CH Cl under
2
2
nitrogen atmosphere and then transferred into a quartz cuvette and sealed. Excitation of PdOEP at 540
nm produced the antiꢀStokes emissions shown in Figure 8. The shape of the upconverted emission
depends on the concentration of the sensitizer, because of the partial overlap with the absorption
spectrum of PdOEP. Indeed, at low [PDOEP]/[CPP] concentration ratios the emission profile is much
more similar to the normal excitation spectrum, cfr. Figure 6. The slight shoulder appearing at 460 nm
is reasonably due to partial reabsorption by the sensitizer in the region 470ꢀ520 nm, see Figure S20 of
S.I., as also confirmed by the fact that at higher concentration ratios the peak at longer wavelength
disappears and the emission band appears flat in the region 460ꢀ490 nm, see figure 8. The emission
spectrum obtained by upꢀconversion is significantly red shifted with respect to that previously reported
8
a
for 9,10ꢀDPA, demonstrating that emission involves the first excited state of the whole nanoꢀhoop.
Reasonably, the antiꢀStokes emission at λ = 540 nm takes place via sensitized TTA process. As first
exc
2
3
pointed out by Parker and Hatchard and deeper investigated by Castellano and coworkers, light
upconversion occurring via TTA is characterized, at least at low power density of the incident light, by
a quadratic dependence of the emission intensity upon the power of the incident light. To have a better
mechanistic insight of the observed antiꢀStokes fluorescence, we have investigated the dependence of
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the emission signal upon excitation light power in the region of low power density. The results are
presented in Figure 9, where the upconverted emissions of a solution of 2 (0.5 mM) and PdOEP (10 µM)
in anhydrous CH Cl prepared under nitrogen atmosphere and irradiated at different incident power
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2
densities (maximum power density ≈ 0.8mW/cm2) are shown.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 9. Upconverted emission intensity profiles of a solution of 2 (0.5mM) and PdOEP (10 µM) in
dichloromethane at different incident power density (top) and the corresponding normalized integrated
2
emission as a function of the normalized incident power bottom. Dashed line y=x, full line y=x ;
ꢀ2
maximum incident power 0.8 mWcm .
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The emission band shape is independent of the excitation power. In Figure 9b, the integrated emission
intensities of Figure 9a is reported as a function of the power density of the incident light, both
normalized at their highest values. Figure 9b clearly indicates that the emission intensity depends on the
2
3b,c
square of the power of the incident light, in agreement with previous studies,
the upconversion occurs via sensitized TTA.
thus confirming that
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Conclusions
9,10 anthraceneꢀincorporated [8]cycloparaphenylene has been synthesized adapting the gramꢀscale
synthesis recently reported by Jasti. The compound has been characterized by HR MALDI mass
spectroscopy and 1D and 2D NMR spectroscopy, evidencing a Cs symmetry. The absorption spectrum
is characterized by two low energy transitions, one at longer wavelength, attributable to the 9,10ꢀDPA,
the other falling at wavelengths characteristic of [n]CPP systems. The emission spectrum is however
significantly different from [8]CPP one, better resembling that of larger size nanoꢀhoops, both for the
peak wavelength and quantum yield. In the presence of the octaethylporphyrin Pd(II) complex as
sensitizer, 9,10ꢀanthraceneꢀincorporated [8]CPP undergoes visible light upconversion, the first case
reported in the literature in which a cycloparaphenylene derivative is involved as emitter in low power
light frequency conversion.
Experimental Section
General Information. HR MALDI mass spectra were recorded on a FTꢀICR mass spectrometer
equipped with a 7T magnet. The samples were ionized in positive ion mode using ESI positive mode
and MALDI ion source. The mass spectra were calibrated externally, and a linear calibration was
applied. The samples recorded in MALDI were prepared by mixing 10 ꢁL of analyte in dichloromethane
(1 mg/mL) with 10 ꢁL of solution of dithranol (10 mg/mL in THF). Tetrahydrofuran was dried by
heating under reflux over sodium wire in the presence of benzophenone as indicator while
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dimethylformamide were dried by activated 3 Å molecular sieves. All chemicals reagents grade were
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
d
1a
used without further purification and were used as purchased. Derivative 7 and sodium naphthalenide
1
d
were prepared in accord to reported procedures. Reaction temperatures were measured externally.
Reactions were monitored by TLC silica gel plates (0.25 mm) and visualized by UV light 254 nm, or by
1
13
spraying with H SO −Ce(SO ) . NMR spectra were recorded on a 600 [600 ( H) and 150 MHz ( C)],
2
4
4 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
13
1
13
1
4
00 [400 ( H) and 100 MHz ( C)], 300 MHz [300 ( H) and 75 MHz ( C)] or 250 MHz [250 ( H) and
1
3
62.5 MHz ( C) ] spectrometer. Chemical shifts are reported relative to the residual solvent peak.
Computational details. Constrained and unconstrained geometry optimizations were carried at the
density functional level of theory (DFT), by using the B97D functional including the empirical
2
4
correction for dispersion energy proposed by Grimme, in conjunction with the density fitting
approximation and the TZVP basis set. The nature of the stationary points was verified by computing
the eigenvalues of the Hessian matrix. B97D was chosen because it has been proven to be the most
2
5
accurate method in predicting torsional barriers of substituted biphenyls. Time dependent (TD) DFT
2
6
computations were carried out by using the CAMꢀB3LYP rangeꢀseparated hybrid functional. Indeed, it
is well known that electronic excitations in conjugated oligomers have a charge transfer nature, with
structural distortions that can be modelled only with the use of rangeꢀcorrected hybrid DFT models
1
6,27
including long range electronic exchange interactions
The same functional was adopted for the
computation of the first oxidation potential. The restricted open shell (RO) formalism was used because
it yields more accurate oxidation potentials when used with the CAMꢀB3LYP functional. The absolute
+
28
potential of Fc /Fc in dichloromethane has been set to 4.98 V. The 6ꢀ31+G(d,p) basis set was adopted
for all the computations employing the CAMꢀB3LYP functional. Effects due to solvent
(dichloromethane) polarization were included in all computations by the polarizable continuum model
2
9
30
(
PCM). DFT and TDDFT computations were carried out by using the Gaussian program.
Optoelectronic characterization. Optical absorption spectra were measured on a Cary 50 UVꢀvis
spectrophotometer from Varian. Steadyꢀstate luminescence spectra were recorded on a modular
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fluorescence spectrometer from Acton Research, equipped with a xenon lamp. Light source was passed
through a 295 nm long pass cutꢀoff emission filter prior to incidence on the sample. All solution samples
were deaerated with nitrogen. Relative quantum yields of 2 in dichloromethane were determined as
described by Williams using quinine sulfate (0.1 M H2SO4) as standard. Excitation occurred at 319 nm
and the fluorescence of 2 was integrated from 350 – 650 nm, both for 2 and for quinine sulphate. In upꢀ
conversion measurements an additional longpass filter cutting at ca. 300 nm has been employed to
completely cut down possible emission produced by excitation of second order diffraction. The quantum
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
23
yield measurements were performed in accord with the method reported by Miller and coꢀworkers. The
CPP 2 solutions were diluted to give absorbance values in the range 0.02ꢀ0.35, within the expected
linear calibration range of fluorescence emission versus concentration. The quantum yields were
calculated according to equation reported by Miller
.
2
X
2
IX AR
η
η
Φ = Φ
X
R
IR A
X
R
Φ is the quantum yield of the standard, A is the absorbance of the solution, I is the integrated
R
intensity of the exciting light and
η is the average refractive index of the solution. Subscripts R and X
refer to the reference and unknown compound, respectively. Cyclic voltammetry measurements were
performed in CH Cl with TBAP as electrolyte. All potentials were referenced to the platinum quasiꢀ
2
2
+
reference electrode. The potential has been calibrated with ferrocene/ferrocenium (Fc/Fc ) redox couple.
Derivative 5. 1,4ꢀDibromobenzene (10.0 g, 42.4 mmol) was dissolved in 450 mL of dry THF and
cooled to –78 °C. To this solution was added a 2.5 M solution of nꢀbutyllithium in nꢀhexane (18.2 mL,
4
5.5 mmol) over 15 min and the reaction mixture was stirred for 30 min. Anthraquinone (4.31 g, 20.7
mmol) was added to the reaction mixture in three equal portions over 30 min. The solution was stirred
for 2 h at –78 °C and then added to 180 mL of water. The mixture was further diluted by the addition of
1
80 mL of diethyl ether and the organic layer was separated. The aqueous layer was extracted with
diethyl ether (3 x 70 mL) and the combined organic layers were washed with a saturated brine solution
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The Journal of Organic Chemistry
(
100 mL). The organic phase was then dried over sodium sulfate, filtered and evaporated under reduced
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
pressure to give the diol product 4 which was used without further purification. The diol 4 was dissolved
in 70 mL of dry THF and sodium hydride (2.07 g, 51.7 mmol, 60% in mineral oil) was added at 0 °C.
The mixture was stirred for 30 min and methyl iodide (5.2 mL, 83 mmol) was added. The mixture was
allowed to warm to room temperature and stirred for an additional 18 h. The excess of sodium hydride
was then quenched by addition of 100 mL of water and filtered. The white solid was washed with 120
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
mL of ethyl ether and dried in vacuum for 12 h, to give 5 in 53% of yield (6.04 g). M. p. > 310 °C dec.
1
H NMR (400 MHz, 298 K, CDCl ): δ (ppm) 7.49 (m, 4H, ArꢀH), 7.30 ꢀ 7.33 (overlapped, 8H, ArꢀH),
3
1
3
7
.19 (d, J = 8.5 Hz, 4H, ArꢀH), 2.93 (s, 6H, OMe); C NMR (100 MHz, CDCl ): δ (ppm) 147.9, 138.2,
3
+
131.0, 128.7, 128.2, 121.1, 78.7, 51.3. HRMS (ESI+): m/z [M+Na] calculated for C H Br NaO :
2
8
22
2
2
5
72.9864, found 572.9859.
Derivative 6. 5 (2.53 g, 4.77 mmol) was dissolved in 280 mL of dry THF and cooled to –78 °C. Then
a 2.5 M solution of nꢀbutyllithium in hexane (9 mL, 22.5 mmol) was added over 10 min. Then isopropyl
pinacol borate (8.50 mL, 42.0 mmol) was added rapidly and the solution was stirred for 5 h under argon.
2
00 mL of water and 100 mL of ethyl acetate were added to the solution and the biphasic mixture was
stirred for 30 min at room temperature. The organic phase was extracted and the aqueous layer was
washed with ethyl acetate (3 x 70 mL). The combined organic layers were dried over sodium sulfate,
filtered and concentrated in vacuum to give a colorless oil that was purified by chromatography on silica
gel (hexane/ethyl acetate, 7:3). The product 6 was obtained as a white solid in 41% of yield (1.30 g).
1
M.p.: 252.2ꢀ253.3°C. H NMR (400 MHz, 298 K, CDCl ): δ (ppm) 7.68 (d, J = 8.1 Hz, 4H, ArꢀH), 7.48
3
(m, 4H, ArꢀH), 7.43 (d, J = 8.1 Hz, 4H, ArꢀH), 7.24 (m, 4H, ArꢀH), 2.94 (s, 6H, OMe), 1.30 (s, 24H,
1
3
CH ); C NMR (62.5 MHz, CDCl ): δ (ppm) 152.1, 138.5, 134.5, 128.5, 128.4, 126.2, 83.8, 79.0, 51.2,
3
3
+
2
5.0. HRMS (ESI+): m/z [M+Na] calculated for C H B NaO : 667.3378, found 667.3380.
40 46 2 6
Derivative 8. Dibromide 7 (1.99 g, 3.00 mmol), diboronate 6 (1.93 g, 3.00 mmol), SꢀPhos (246 mg,
0
.61 mmol) and K PO (2.55 g, 12.0 mmol) in a mixture of DMF/H O (600 mL/60mL) was mixed and
3
4
2
degassed twice through freeze/pump/thaw technique. Then Pd(OAc) (471 mg, 2.11 mmol) was added
2
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and the mixture was stirred at 100 °C for 19 h under argon. After cooling to room temperature, the
mixture was filtered through a short plug of Celite® and added to 150 mL of water and 100 mL of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
6
CHCl . The biphasic mixture was stirred for 30 min and separated. The aqueous layer was washed with
3
CHCl (3 x 80 mL). The combined organic layers were washed with 200 mL of a saturated brine
3
solution and dried over sodium sulfate. After filtration the solvent was evaporated under vacuum, the
crude mixture was purified by column chromatography on silica gel (Hexane/CH Cl /Et O = 29:70:1) to
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
1
give the macrocycle 8 as a white solid in 10 % of yield (0.27 g). M. p. > 305 °C dec. H NMR (600
MHz, 298 K, CDCl ): δ (ppm) 7.85 (m, 4H, ArꢀH), 7.50 (s, 4H, Ar), 7.47 (m, 4H, ArꢀH), 7.44 (d, J =
3
8
.5 Hz, 4H, CH=CH), 7.38 (d, J = 8.5 Hz, 4H, CH=CH), 7.17 (d, J = 8.5 Hz, 4H, CH=CH), 6.92 (d, J =
8.5 Hz, 4H, CH=CH), 6.11 (d, J = 10.3 Hz, 4H, CH=CH), 6.03 (d, J = 10.3 Hz, 4H, CH=CH), 3.46 (s,
1
3
6
1
5
H, OMe), 3.41 (s, 6H, OMe), 3.15 (s, 6H, OMe). C NMR (150 MHz, 298 K, CDCl ): δ (ppm) 145.0,
3
43.4, 142.8, 139.8, 139.5, 138.6, 133.5, 133.0, 128.3, 127.5, 126.9, 126.3, 126.1, 80.3, 74.6, 74.3, 52.2,
2.0. DEPTꢀ135 (150 MHz, CDCl ): δ (ppm) 133.5, 133.0, 128.3, 127.5, 126.9, 126.3, 126.1, 52.2,
3
52.0. HRMS (MALDI): m/z M+ calculated for C H O : 894.3921, found 894.3895.
6
2
54
6
Derivative 2. Derivative 8 (225 mg, 0.25 mmol) was dissolved in 18 mL of dry THF, under argon,
and the solution was degassed through freeze/pump/thaw technique and cooled to –78 ℃. Then, a
solution of sodium naphthalenide in dry THF, freshly prepared (2.7 mL, 2.7 mmol) was added and the
reaction mixture was stirred for 3 h at –78 ℃. Then I was added (3 mL of a 1.0 M solution in THF) and
2
the reaction mixture was warmed at room temperature. Subsequently a saturated solution of sodium
thiosulfate (50 mL) and CHCl (50 mL) were added, and the reaction mixture was stirred for 10 min.
3
The organic layer was extracted and the aqueous phase was washed with CHCl (3 x 50 mL) and finally
3
the combined organic layers was washed with a saturated brine solution (60 mL) and dried over sodium
sulfate and filtered. The solvent was removed under reduced pressure, and the crude product was
purified by chromatography column on silica gel under nitrogen flow (nꢀhexane/CH Cl = 3:7), to give
2
2
1
the product 2 as a yellow solid in 11 % of yield (19 mg). M. p. > 285 °C dec. H NMR (600 MHz, 298
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K, CDCl ): δ (ppm) 7.89 (m, 2H, ArꢀH), 7.72 (m, 2H, ArꢀH), 7.49 (s, 4H, ArꢀH), 7.47 (s, 8H, ArꢀH),
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
.42ꢀ7.40 (overlapped, 8H, ArꢀH), 7.22 (d, J = 8.3 Hz, 4H, ArꢀH), 7.11ꢀ7.08 (overlapped, 4H, ArꢀH),
1
3
7.02ꢀ7.00 (overlapped, 4H, ArꢀH). C NMR (150 MHz, CDCl ): δ (ppm) 146.6, 146.5, 142.7, 140.3,
3
1
1
1
39.7, 138.8, 138.4, 137.2, 137.0, 136.9, 128.6, 128.3, 128.0, 127.9, 127.8, 127.7, 127.6, 127.5, 127.5,
27.4, 127.2, 127.1, 127.1. DEPTꢀ135 (150 MHz, CDCl ): δ (ppm) 128.6, 128.3, 128.0, 127.9, 127.8,
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
27.7, 127.6, 127.5, 127.5, 127.4, 127.2, 127.10, 127.12. HRMS (MALDI): m/z M calculated for
C H : 708.2811, found 708.2812.
5
6
36
Acknowledgements
The authors acknowledge the Regione Campania (POR CAMPANIA FESR 2007/2013 O.O.2.1,
B46D14002660009, “Il potenziamento e la riqualificazione del sistema delle infrastrutture nel settore
dell’istruzione, della formazione e della ricerca”), for the FTꢀICR mass spectrometer facilities, the
Centro di Tecnologie Integrate per la Salute (CITIS, Project PONa3_00138), Università di Salerno, for
the 600 MHz NMR facilities.
Supporting Information: Copies of 1D, 2D NMR and HR MS spectra of new compounds, UV−vis
absorption and fluorescence spectra of 2, cyclic voltammetry of 2, emission spectrum of 2 in
dichloromethane at λexc = 390 nm, emission spectrum of 2 in dichloromethane at λexc = 484 nm,
cartesian coordinates of the conformer I and III (Figure 4), this material is available free of charge via
the Internet at http://pubs.acs.org.
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