O-Annulation to Polycyclic Aromatic Hydrocarbons: A Tale of Optoelectronic Properties from Five- To Seven-Membered Rings

Article abstract of DOI:10.1021/acs.orglett.0c01331

We take advantage of the Pummerer oxidative annulation reaction to extend PAHs through the formation of an intramolecular C-O bond with a suitable phenol substituent. Depending on the peripheral topology of the PAH precursor (e.g., pyrene, boron-dipyrromethene, or perylene diimide) five-, six-, and seven-membered O-containing rings could be obtained. The effect of the O-annulations on the optoelectronic properties were studied by various methods with the pyrano-annulated pyrene and BODIPY derivatives depicting quantitative emission quantum yields.

Full text of DOI:10.1021/acs.orglett.0c01331

pubs.acs.org/OrgLett
Letter
O‑Annulation to Polycyclic Aromatic Hydrocarbons: A Tale of
Optoelectronic Properties from Five- to Seven-Membered Rings
́
Luka Đorđevic, Domenico Milano, Nicola Demitri, and Davide Bonifazi*
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ABSTRACT: We take advantage of the Pummerer oxidative
annulation reaction to extend PAHs through the formation of an
intramolecular C−O bond with a suitable phenol substituent.
Depending on the peripheral topology of the PAH precursor (e.g.,
pyrene, boron−dipyrromethene, or perylene diimide) five-, six-,
and seven-membered O-containing rings could be obtained. The
effect of the O-annulations on the optoelectronic properties were
studied by various methods with the pyrano-annulated pyrene and
BODIPY derivatives depicting quantitative emission quantum
yields.
he π-extension of polycyclic aromatic hydrocarbons
(PAHs) is a powerful approach in the chemical toolbox
molecular C−C bond formation of biaryl ethers.13 As depicted
in Scheme 1, the preparation of five- and six-membered O-
annulated PAHs were successfully reported through, for
example, base-promoted cyclization of o-ethynylphenol sub-
structures,14 acid-mediated cyclization of alkynes with alcohols
T
for tailoring properties of organic molecules and materials.1
Amid the synthetic approaches, fusing and replacing benzene
rings with heterocycles have emerged as attractive routes to
tailor the band gaps, redox properties, self-assembly properties,
aromaticity, and chemical stability of PAHs.2 Traditionally,
PAHs have been synthesized through multistep protocols
building on polyarylated precursors, followed by a planariza-
tion reaction to obtain the fused aromatic scaffold.
Representative examples of this approach are the hexa-peri-
hexabenzocoronene (HBC) and its heteroatom-doped ana-
logues.3 Another approach comprises the annulative π-
extension (APEX) reactions of PAHs, which can be selective
at their bay- or k-regions.4 A recent example accomplishes
APEX on thiophene, benzofuran, and indole derivatives, using
a Pd-catalyzed dual C−H annulation with π-extensive
dibenzosiloles and dibenzogermoles.5 Another general strategy
includes the (benz)annulation reactions of o-ethynylbiaryl
precursors.6 Ethynylbiaryls can also be combined with
anthranil, through Au-catalyzed π-extension, for preparing N-
doped PAHs.7 π-Extension through alkyne annulation has
recently attracted attention for the synthesis of nonplanar
PAHs. Examples include PAHs prepared through electro-
catalysis with accessible boronic acids8 as well as Rh-catalysis
for synthesizing N- and S-doubly doped and cationic
thiazoloquinolinium scaffolds.9
Scheme 1. Common Synthetic Approaches toward O-
Annulated PAHs
Received: April 16, 2020
Compared to the five- or six-membered N- or S-containing
heteroaromatics, O-based extended π-systems are less
common, with 7-membered rings being very rare, i.e., oxepin,10
and five-membered furans11 the most popular rings. The two
main synthetic approaches include intramolecular carbon−
heteroatom bond formation of biaryl alcohols12 and intra-
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or ketones,15 acid-mediated condensation between ketones
and phenols,16 Friedel−Crafts reactions (followed by cyclo-
dehydrogenation),17 Claisen condensation between arylalde-
hydes and naphthols (followed by oxidation),18 cross-
condensation between hydroquinone and arylaldehyde de-
rivatives for the simultaneous formation of C−C and C−O
bonds,19 and Rh-catalyzed C−H activation/annulations of
arylaldehydes with diaryl alkynes.20 In this context, our group
has recently applied the Pummerer oxidative cyclization21 as a
planarization reaction for the synthesis of O-doped nano-
graphenes22 and molecular ribbons with various peripheral
topologies (armchair and zigzag),23 as well as for the
cyclization of 2,2′-binaphthol derivatives for the preparation
of π-extended and imide-based peri-xanthenoxanthenes
(PXX).24 A similar Cu-catalyzed C−H/C−O cyclization of
2,2′-binaphthol derivatives was recently used to develop chiral
PXXs, exhibiting intense circularly polarized luminescence
both in solution and the solid state.25
a
Scheme 2. Synthesis of O-Annulated PAHs
Now we report on π-extension through O-fusion on different
classes of PAHs. Specifically, we used the Cu-mediated ring
closure of relevant phenol precursors to obtain five-, six-, and
seven-membered O-containing heterocycles. Different ring
sizes were accessible through the appropriate choice of
substituted PAHs, which include pyrene,26 boron−dipyrrome-
thene (BODIPY),27 and perylene diimide (PBI).28 The
synthetic routes are depicted in Scheme 2.
To obtain furan-bearing 1fused (Scheme 2, i−iv), pyrene was
first borylated, through Ir-catalyzed C−H activation, in the 2-
position.29 Suzuki−Miyaura Pd-catalyzed cross-coupling with
5-tert-butyl-2-iodoanisole afforded anisole-substituted pyrene
1OMe. After demethylation with BBr3, annulation of phenol 1OH
with CuI and PivOH in DMSO gave furan derivative 1fused in
fair yield. For pyran-embedded heteroarene 2fused, regioselec-
tive bromination of pyrene with NBS and cross-coupling with
the relevant boronic acid, led to anisole derivative 2OMe
(Scheme 2, v−vii). Demethylation and C−O ring closure
(with CuO in boiling nitrobenzene) gave product 2fused in
excellent yield.
a
Reagents and conditions: (i) [Ir(μ-OMe)cod]2, dtbpy, B2pin2,
hexane, N2, 80 °C; (ii) 5-tert-butyl-2-iodoanisole, [Pd(PPh3)4],
Na2CO3, toluene/H2O, N2, 120 °C; (iii) BBr3, CH2Cl2, N2, 0 °C
→ rt; (iv) CuI, PivOH, DMSO, 140 °C; (v) NBS, CH2Cl2, rt; (vi) (4-
tert-butyl-2-methoxyphenyl)boronic acid, [Pd(PPh3)4], Na2CO3,
toluene/H2O, N2, 120 °C; (vii) CuO, PhNO2, 240 °C; (viii) TFA;
(ix) (step 1) DDQ, CH2Cl2, Ar, rt; (step 2) Et3N, BF3OEt2, CH2Cl2,
Ar, rt; (x) Cu(OAc)2, Cs2CO3, PivOH, DMSO, 140 °C; (xi) BnBr,
K2CO3, DMF, N2, 70 °C; (xii) 3-aminopentane, imidazole, N2, 160
°C; (xiii) Br2, CH2Cl2, N2, rt; (xiv) 6, (4-tert-butyl-2-methoxyphenyl)-
boronic acid, [Pd(PPh3)4], K2CO3, dioxane/H2O, N2, 100 °C.
For the BODIPY derivative, it was possible to obtain the
phenol-substituted boron−dipyrromethene precursor 3OH
through TFA-catalyzed condensation of salicylaldehyde and
pyrrole, followed by DDQ oxidation to dipyrromethene and its
treatment with BF3 in the presence of Et3N (Scheme 2, xii−xiv,
x). Molecule 3OH was successfully converted into 3fused in
excellent yield, using Cu(OAc)2, Cs2CO3, and PivOH in
DMSO. BODIPY derivative 3OH was also benzylated to 3OBn
for recording reference electrochemical data.
compounds crystallize in centrosymmetric space groups with
one (α,β-1fused, 2fused) or two (3fused) crystallographically
independent molecules. In all crystals, the aromatic cores are
flat (RMSD of atoms from their mean plane are 0.06(5) Å for
α-1fused, 0.09(7) Å for β-1fused, 0.03(3) Å for 2fused, and 0.05(3)
Å for 3fused). Crystal packing of molecules α-1fused and 2fused
(Figures 1b1,d) show the presence of dimers linked by
crystallographic inversion centers with π···π pairing (dπ···π
3.36(9) Å and 3.30(3) Å in 2fused and α-1fused, respectively).
On the other hand, in phase β the molecules of 1fused lack
significant π···π stacking contacts, preferring a herringbone
arrangement governed by C−H···π interactions (Figure 1b2).
Finally, molecule 3fused arranges in π···π stacked pillars, with
average interplanar distances between the aromatic cores of
3.38(10) Å (Figure 1f).
Finally, molecule 4fused annulated at the bay region with an
oxepin ring was prepared (Scheme 2, xii−xiv, x). In the
synthetic strategy, perylene bisanhydride was converted into
the relevant PDI with 3-aminopentane (in molten imidazole),
followed by bromination with Br2. Subsequent Pd-catalyzed
Suzuki cross coupling and deprotection with BBr3 of the
corresponding anisole gave phenol-bearing PDI precursor 4OH
.
Finally, 4OH was converted into 4fused, using Cu(OAc)2,
Cs2CO3, and PivOH in DMSO in an excellent yield. The
structure of 4fused was confirmed by bidimensional NMR
investigations (Figures S52−S54) and excluded the presence of
any arene-oxide tautomer.10b
Single crystals for X-ray diffraction were obtained for
compounds 1fused, 2fused, and 3fused (Figure 1 and Supporting
Information Section 6). For molecule 1fused, two different
monoclinic crystalline phases were found (α, β). The
Final compounds, along with their precursors, were
characterized, and their photophysical, electrochemical, and
computational data are summarized in Table 1 (see also
Supporting Information Sections 3−5). From the Table, it is
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chromophore/luminophore. For instance, furan-embedded
1fused exhibits red-shifted UV−vis absorption and emission
profiles (λmax = 399 nm and λem = 401 nm, respectively), along
with a stronger emission (Φem = 33%) if compared to anisole-
pyrene 1OMe (λmax = 339 nm and λem = 396 nm, with Φem
=
4%). The O-annulation reaction yielding pyrenes 1fused and
2fused is accompanied also by a narrowing of the Stokes shift,
compared to anisole-substituted pyrenes 1OMe and 2OMe, from
57 and 39 nm to 2 and 17 nm, respectively. Previously
reported benzoheterocycle-fused pyrenes, pyrenebenzo[b]-
phosphole and -benzo[b]silole, showed λmax = 414 (λem
=
414 nm) and 354 nm (λem = 406 nm), with Φem = 30 and 4%,
respectively.30 Similarly, pyrenebenzo[b]thiophene showed
absorption and emission maxima at λmax = 338 and λem
=
373 nm with Φem = 25%.31 On the other hand, pyrano-fused
heteroarene 2fused showed a stronger bathochromic shift in the
absorption (λmax = 433 nm) and emission (λem = 450 nm)
profiles with the fluorescence quantum yield value exception-
ally reaching unity (Φem = 100%). Previously reported
thiopyran-fused pyrene31 showed a similar lowest electronic
transition (λmax = 433 nm) but a wider Stokes’ shift (λem = 480
nm) and a significantly lower Φem value (60%). In the case of
boron−dipyrromethene 3fused (Figure 2a), however, the pyrano
annulation resulted in a blue-shifted absorption of the optical
properties and emission profiles (λmax = 472 nm and λem = 489
nm) when compared to 3OBn (λmax = 505 nm and λem = 522
nm), with quantitative emission quantum yield (Φem
=
quantitative). Mayer bond order analysis for molecules 3OH
and 3fused (Figure 2b) suggests that the pyranyl annulation
perturbates the aromatic character of the boron-dipyrrome-
thene skeleton, reducing the degree of π-conjugation. Similar
analyses and observations were reported for BODIPYs bearing
electron donating groups, with the rings being less aromatic.32
As observed for pyrene derivative 2fused, the dramatic
Figure 1. X-ray crystal structures of α,β-1fused, 2fused, and 3fused. Left:
(a1, a2, c, e) ellipsoid representation of X-ray structures (50%
probability). Right: crystal packing; (b1) view along the [401]
crystallographic direction; (b2) view along [501] crystallographic
direction; (d) view along [301] crystallographic direction (solvent
molecules, THF, are shown with cyan sticks); (f) view along [0−13]
crystallographic direction. Atom colors: red O, gray C, yellow B,
green, F.
enhancement of the emission quantum yield (Φem
=
quantitative) is attributed to the pyranyl ring that, restricting
the rotational degrees of freedom, disfavors any nonradiative
deactivation of the excited state.33
For the annulated PDI derivative, the presence of the oxepin
ring in 4fused caused minor shifts to the absorption and
emission spectra (λmax = 538 nm and λem = 611 nm for 4fused
compared to λmax = 530 nm and λem = 617 nm for 4OMe) and a
apparent that the O-annulation impacted the optoelectronic
properties, with the extent of the effect depending on the type
of the O-ring as well as the structure of the precursor
Table 1. Photophysical, Electrochemical, and Computational Data of Compounds Reported Herein
a
d
g
photophysical
electrochemical
calcd
b
c
compd
λmax (nm)
E00 (eV) λem,max (nm)
Φem (%)
τem (ns) Eox,1 (V)
Ered,1 (V)
EHOMO (eV) ELUMO (eV) Egap (eV)
Egap (eV)
e
1OMe
1fused
2OMe
2fused
3OH
3fused
4OMe
4fused
339
399
345
433
505
472
530
538
3.13
3.09
3.23
2.76
2.38
2.54
2.14
2.20
396
401
384
450
522
489
617
611
4
33
25
11.7
7.3
16.7
4.5
2.8
5.7
+0.85
+0.76
+0.68
+0.44
+1.06
+1.14
+1.08
+1.10
−2.28
−5.65
−5.56
−5.48
−5.24
−5.86
−5.94
−5.88
−5.90
−2.52
−2.47
−2.25
−2.48
−3.44
−3.44
−3.65
−3.72
3.13
3.09
3.23
2.76
2.42
2.50
2.23
2.18
3.41
3.26
3.41
2.86
2.67
2.75
2.20
2.14
e
−2.33
e
−2.55
e
quant.
24
quant.
20
−2.32
f
f
−1.36
−1.36
−1.15
−1.08
1.6
6.0
4
a
b
c
Spectra were measured in CH2Cl2 (spectroscopic grade) at rt. Calculated optical gap as E00 = 1240/λem. Relative fluorescence quantum yields
d
measured using either Quinine sulfate, Coumarin 153 or Rhodamine 6G as references. Electrochemical data obtained from differential pulse
voltammetry experiments in anhydrous CH2Cl2, containing 0.1 M nBu4PF6 using glassy carbon working electrode, Pt wire counter-electrode and Ag
wire reference electrode. All potentials are referenced versus the Fc+/Fc couple used as internal standard. HOMO and LUMO energy levels were
e
approximated using the equations HOMO = −(4.80 + Eox,1) and LUMO = −(4.80 + Ered,1). Estimated as Ered,1 = Eox,1 + E00, since no reduction
f
g
peaks were observed. Measured using benzylated derivative 3OBn
.
Calculated from TD-DFT calculations, performed at the CAM-B3LYP/6-
31G(d,p) level of theory.
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derivative, in full agreement with the optical data. As far as
3fused is concerned, the reduction potential is virtually the same
as that of 3OBn, whereas the oxidation process unexpectedly
occurs at a slightly higher potential (ΔEox,1 = 80 mV). These
observations suggest the presence of a destabilization effect of
the O-annulation on the first oxidative event. In agreement
with the blue-shift observed in the absorption profile, these
electrochemical data indicate that the widening of the
electrochemical bandgap is likely caused by an energy lowering
of the HOMO level. This behavior contrasts to recent reports
describing widening of molecular bandgap in meso,β-hetero-
aryl-fused BODIPYs caused by the raising of the LUMO
energy level.35 At last, a 50 meV shrinking of the electro-
chemical bandgap was observed when passing from 4OMe to
4fused, in contrast to the optical investigations, which suggested
a 60 meV blue shift of the lowest energy electronic transition.
We have also performed theoretical calculations in order to
gain a deeper insight into different heteroatom-extended
PAHs. After optimization of the structures, time-dependent
(TD)-DFT calculations (CAM-B3LYP/6-31G**)36 repro-
duced fairly well the experimental UV−vis spectra (Figures
S21−S23). Nucleus-independent chemical shifts (NICS)37
were used as a criterion to access aromaticity in the planar
1fused and 2fused molecules (Figure 3 and Supporting
Figure 2. (a) Absorption (left) and emission (right) spectra for
BODIPY derivatives 3OH and 3fused in CH2Cl2 (inset: solutions upon
irradiation with a hand-held UV lamp). (b) Mayer bond-order
analysis for the two optimized structures (red and blue values show a
decrease and increase in bond order values, respectively).
notable decrease of the emission quantum yield (from 20% to
4%). Considering the increase of +5.6° of the core-twist in the
O-annulated PDI derivative deriving from the shallow
envelope structure of the oxepin substructure (Figure S20),
one could hypothesize that the decrease of the singlet emission
intensity is accompanied by an increase of the population of
the triplet excited state.34 This phenomenon is triggered by an
enhancement of the intersystem crossing following a
perturbation of the singlet−triplet energy gap.34 As no
phosphorescence signal was detected either at rt or in frozen
medium for both PBI derivatives, we indirectly measured the
Figure 3. NICSπzz-XY-scan for pyrene, 1fused, and 2fused
.
Information). The new formed furan ring is slightly aromatic
and does not influence the aromaticity of the pyrene ring.
Previously reported five-membered ring benzoheterocycle-
fused pyrenes, pyrene-benzo[b]phosphole, and -benzo[b]silole
derivatives, on the other hand, were shown to be slightly
antiaromatic and nonaromatic, respectively.30
In summary, we have decorated three classes of PAHs
(pyrene, boron−dipyrromethene, and perylene diimide) with
phenol substituents. Then we applied the Pummerer oxidative
cyclization for the intramolecular formation of C−O bond,
which led to the formation of O-embedded five-, six-, and
seven-membered rings. The annulations led to substantial
changes in the photophysical and electrochemical properties,
compared to the parent compounds. While annulation with the
pyranyl ring depicts the strongest bathochromic shifts with the
emission yields reaching quantitative values, the oxepin
annulation has a detrimental effect on the emission properties.
1
triplet generation by studying the photosensitization of O2 in
a cycloaddition reaction. Thus, we performed the oxidation of
1O2-acceptor 9,10-dimethylanthracene (DMA) in air-equili-
brated CH2Cl2 using either 4OMe or 4fused as photosensitizers
and monitored the formation of 9,10-endoperoxianthracene
(Figure S5). As clearly appears from the inset of Figure S5, the
oxygenation of DMA is faster in the presence of 4OMe than with
4fused. These observations suggest that, in contrast to the
pyranyl ring, the oxepin ring has a detrimental effect on the
singlet radiative properties as well as on the triplet population.
We conjectured that the nonradiative deactivation is caused by
a rapid inversion of the envelope-like conformation of the
oxepin ring.
CV and DPV electrochemical investigations (Table 1,
Supporting Information Section 4) suggest that, in the case
of the pyrene derivatives, both 1fused and 2fused feature lower
oxidation potentials if compared to precursors 1OMe and 2OMe
fused
(ΔEox,1 = 90 and 240 mV, respectively, where ΔEox,1 = Eox,1
ASSOCIATED CONTENT
* Supporting Information
■
− Eox,1precursor). As far as the reductive processes are concerned,
the reduction of 1fused is cathodically shifted of 50 mV when
compared to 1OMe, whereas that of 2fused is anodically shifted of
230 mV (−2.32 V vs Fc+/Fc) with respect to that of 2OMe. This
results in Egap values for both 1fused and 2fused that are lower
(ΔEgap = 40 and 470 meV) than those of their precursors, with
the strongest shrinking effect observed with the pyranyl
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01331.
Synthetic protocols and characterizations, photophysical
data, electrochemical data, crystal structures and
computational details (PDF)
D
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Organic Letters
Accession Codes
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CCDC 1952301−1952304 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/structures, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Davide Bonifazi − School of Chemistry, Cardiff University,
Cardiff CF10 3AT, United Kingdom; Institute of Organic
Chemistry, Faculty of Chemistry, University of Vienna, 1090
Vienna, Austria; orcid.org/0000-0001-5717-0121;
Email: bonifazid@cardiff.ac.uk, davide.bonifazi@univie.ac.at
(8) Kong, W. J.; Finger, L. H.; Oliveira, J. C. A.; Ackermann, L.
Rhodaelectrocatalysis for Annulative C-H Activation: Polycyclic
Aromatic Hydrocarbons through Versatile Double Electrocatalysis.
Angew. Chem., Int. Ed. 2019, 58, 6342−6346.
Authors
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cations via a direct double C-H activation strategy: Rh-N,S-
heterocyclic carbene is the key. Chem. Commun. 2019, 55, 854−857.
́
Luka Đorđevic − School of Chemistry, Cardiff University,
Cardiff CF10 3AT, United Kingdom; Department of Chemical
and Pharmaceutical Sciences, Trieste, University of Trieste,
Trieste 34127, Italy; orcid.org/0000-0002-8346-7110
Domenico Milano − Department of Chemical and
Pharmaceutical Sciences, Trieste, University of Trieste, Trieste
34127, Italy
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Nicola Demitri − Elettra − Sincrotrone Trieste, 34149
Basovizza, Trieste, Italy; orcid.org/0000-0003-0288-3233
Complete contact information is available at:
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Notes
(12) (a) Rossignon, A.; Bonifazi, D. O-Annulation Leading to Five-,
Six-, and Seven-Membered Cyclic Diaryl Ethers Involving C−H
Cleavage. Synthesis 2019, 51, 3588−3599. (b) Elbert, S. M.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
̈
Reinschmidt, M.; Baumgartner, K.; Rominger, F.; Mastalerz, M.
D.B. gratefully acknowledges the EU through the ERC Starting
Grant “COLORLANDS” and MC-RISE “INFUSION” proj-
ects, the MIUR through the FIRB (“SUPRACARBON”,
Contract No. RBFR10DAK6), and the School of Chemistry
at Cardiff University for financial support. We thank Andrea
Fermi and Tommaso Battisti (Cardiff University) for the
lifetime measurements, the attempts to measure any
phosphoresce signals and the 1O2-oxidation experiment of
DMA.
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conjugated molecules with fused thiophene and pyran units for
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