Aromatization of Benzannulated Perylene-3,9-diones: Unexpected Photophysical Properties and Reactivity

Article abstract of DOI:10.1021/acs.orglett.6b01559

Highly unusual properties of acene-based quinones 1A and 1T are reported. They undergo an unexpected combination of Michael and carbonyl additions of aryllithium leading to new triarylated benzoperylenes 3A and 3T. Uncharacteristically for quinones, 1A and 1T display vibronically split absorption bands and small Stokes shifts. The absorption/emission spectra of the highly emissive 1A are almost indistinguishable from those of the aromatized 3A. Additional benzene rings cause a counterintuitive blue shift of the aromatic derivatives (2T/3T), but an expected red shift was observed for the quinone (1T). This behavior is fully supported by DFT calculations and rationalized by considering the longest conjugation path.

Full text of DOI:10.1021/acs.orglett.6b01559

Letter
pubs.acs.org/OrgLett
Aromatization of Benzannulated Perylene-3,9-diones: Unexpected
Photophysical Properties and Reactivity
M. Rajeswara Rao, Shea Johnson, and Dmitrii F. Perepichka*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 0B8
*
S Supporting Information
ABSTRACT: Highly unusual properties of acene-based
quinones 1A and 1T are reported. They undergo an
unexpected combination of Michael and carbonyl additions of
aryllithium leading to new triarylated benzoperylenes 3A and
3
T. Uncharacteristically for quinones, 1A and 1T display
vibronically split absorption bands and small Stokes shifts. The
absorption/emission spectra of the highly emissive 1A are
almost indistinguishable from those of the aromatized 3A.
Additional benzene rings cause a counterintuitive blue shift of
the aromatic derivatives (2T/3T), but an expected red shift was observed for the quinone (1T). This behavior is fully supported
by DFT calculations and rationalized by considering the longest conjugation path.
cenequinones are key building blocks used to construct
functionalized acenes, which are important semiconducting
Scheme 1. Synthesis of Benzannulated Perylenes 2 and 3
A
1
,2
materials for optoelectronic applications. The carbonyl groups
allow for convenient placement of various substituents on the
acene core, thereby improving its solubility and chemical
3
inertness as well as amending the solid-state packing. Such a
substitution strategy has greatly benefited the field of organic
4
semiconducting devices by providing materials with an
outstanding performance and stability, notably 6,13-bis-
5
+
2
(
V
triisopropylsilylethynyl)pentacene (hole mobility μ > 1 cm
−1
−1
6
+
2
−1 −1
s ) and rubrene (μ ∼ 15 cm V s ). It has also enabled
isolation and studies of long oligoacenes, which are otherwise
7
extremely reactive because of closed-shell structure instability.
In designing such materials, the common expectations are that
i) reductive aromatization of quinones and (ii) extending the π
(
conjugation of the resulting polycyclic aromatics will lead to
contraction of the HOMO−LUMO gap and red shift of
absorption. None of these premises hold true for the anthracene-
and tetracene-based quinones 1A and 1T described here. These
diones show unexpected chemical reactivity, affording trisub-
stituted polycyclic aromatic hydrocarbons 3A and 3T, and
possess highly unusual spectroscopic properties that we report
and rationalize in this letter.
8
In our recent study of diaryldibenzoperylene 2A, prepared by
arylation of the known dibenzo[a,j]perylene-8,16-dione 1A, we
noticed an unusually low yield (12%) of the isolated product
along with the formation of a triarylated derivative (5%), which is
now identified as triaryldibenzoperylene 3A. Reaction of 1A with
an excess of aryllithium (20 equiv) followed by reductive
aromatization of the resulting tertiary alcohol using SnCl2
afforded 3A as the major product in 34% isolated yield (with a
The formation of 3A and 3T can thus be rationalized as a
combination of one Michael (1,4-) addition to the benzene ring
and two nucleophile-to-carbonyl (1,2-) additions. Nucleophilic
1
,4-addition of organometallic reagents to an aromatic system is
not unprecedented but generally requires that the carbonyl
1% trace of 2A, Scheme 1). This behavior is general: the
tetracene-based quinone 1T, prepared from acid-catalyzed
cyclization of tetracene lactone (TetL), also produces triarylated
Received: May 30, 2016
3
T in 30% yield (along with a 3% yield of 2T).
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01559
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
electrophile is deactivated either sterically or electronically (e.g.,
reduction with SnCl . Also, the low yield of the reaction is
explained by 1A playing a role of a sacrificial electron acceptor,
oxidizing the deprotonated 1AMC trianion.
2
9
10
“
masked” as an oxazoline ). Organometallic 1,4-addition is also
1
1
not uncommon for benzo- and naphtho-1,4-quinones.
However, such chemical behavior is uncharacteristic for fully
benzannulated quinones lacking aliphatic CC bonds, and a
standard 1,2-addition is considered a highly reliable method for
To support such a mechanistic hypothesis, we performed DFT
calculations of Michael (1AMC) and carbonyl (1A ) inter-
C2
mediates at the B3LYP/6-31G(d) level, applying a continuum
solvent model (THF). Although the reaction is not expected to
be under thermodynamic control, following the Bell−Evans−
Polanyi principle, the difference of the activation energies within
each series is expected to be proportional to the difference of the
corresponding reaction enthalpies. The enthalpy of formation of
7
,12
their derivatization. The only exception of which we are aware
is the fourfold Michael addition of arylmagnesium to fused
13,17
bispentacenequinone reported by Wu’s group,
which was
attributed to low reactivity of the carbonyl groups due to the
extended conjugation and activation of the CC bonds by a ring
distortion. In their case, no 1,2-addition to carbonyls was
observed; in contrast, we have noticed no higher Michael
addition products across a broad range of reaction conditions
1AMC from 1A was found to be 17 kcal/mol more favorable than
C
that for the formation of 1A , supporting the observed
C2
selectivity. To validate this finding, we performed the same set
of calculations with anthraquinone (AQ), which showed that the
corresponding Michael addition leading to AQ-MC is less
(aryllithium concentration, temperature, reaction time). Thus,
1
A and 1T present a rare case where an aromatic ketone is
2
amenable to combined nucleophilic attacks on both the benzene
ring and the carbonyl group.
favorable than the carbonyl addition forming AQ-C by 8 kcal/
mol, in agreement with the known reactivity of AQ and other
7
A tentative mechanism accounting for the formation of di- and
triarylated species starts with an aryllithium attack on the
acenequinones.
Such unusual reactivity of quinones 1A and 1T also echoes the
14
carbonyl group to afford monoadduct 1A (Scheme 2). This can
reported earlier facile photooxidation of 1A, which in our
experience decomposes readily upon exposure to laboratory
light. We have considered a biradical form of 1A as an origin of
this behavior. However, DFT calculations suggest that the triplet
state of 1A lies above a closed-shell singlet by 1.0 eV, and no
paramagnetic broadening was observed in its NMR spectrum up
to 140 °C.
C
Scheme 2. Proposed Mechanism of Arylation of 1A; the Inset
Shows the DFT-Calculated Energy Diagram for Michael/
Carbonyl Addition to 1A in Comparison with 9,10-
Anthraquinone (AQ)
Deep-purple single crystals of 3A suitable for X-ray diffraction
analysis were grown via slow diffusion of diethyl ether into
CH Cl solution of 3A (under nitrogen, protected from light).
2
2
The perylene backbone has a highly contorted structure with a
dihedral angle of 28.7° between the two anthracene planes
(
Figure 1). Steric congestion of the adjacent aryl substituents
Figure 1. Crystal structure of 3A.
bestows a slightly higher distortion (5.5°) to the corresponding
anthracene moiety compared with its monosubstituted counter-
part (1.9°). A considerable bond length elongation (0.01−0.03
Å) is also noticed for 3A compared with 2A in the Michael
acceptor benzene ring.
be followed by either an attack on the second carbonyl to
produce 1AC2 or a Michael addition to afford 1AMC. With an
excess of aryllithium present, the latter can be deprotonated, and
the formed trianion is sufficiently electron-rich to be oxidized
Quinones 1 and their corresponding aromatic derivatives 2
and 3 are readily soluble in common organic solvents, and their
photophysical properties were analyzed in CH Cl solution
(most likely by electron transfer to quinone 1A), regenerating
2
2
the carbonyl group, which is subject to subsequent nucleophilic
addition to produce 1AMC2. Reduction of the thus-formed
(Figure 2 and Table 1). 1A displays a sharp vibronically
abs
structured absorption band at λ = 575 nm (2.16 eV). Quite
max
diolates (1AC2 and 1AMC2) with SnCl produces the isolated
dibenzoperylenes 2A and 3A. The proposed mechanism explains
a different ratio of products 2A and 3A upon variation of the
surprisingly, the absorption spectrum of this quinone 1A is barely
distinguishable from those of its diarylated derivative 2A and
triarylated 3A (Figure 2). Furthermore, 1A is significantly more
2
8
CF PhLi concentration. When only 2 equiv of aryllithium is used,
emissive (Φ = 58%) than its aromatized derivatives 2A and 3A
3
f
15
1
AMC could still be the major product, but it is not able to convert
(Φ = 8 and 13%, respectively), which is uncharacteristic for
f
into 1AMC2 because of the insufficient amount of aryllithium and
quinones because of usually fast intersystem crossing. As
thus cannot aromatize into a stable isolable compound upon
expected, the extended π conjugation in quinone 1T leads to a
B
DOI: 10.1021/acs.orglett.6b01559
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. Absorption (left) and emission (right) spectra of anthracene-
based 1A/3A (top) and tetracene-based 1T/3T (bottom) in CH Cl .
2
2
The insets show photographs of CH Cl solutions of 1 and 3.
2
2
Figure 3. DFT-calculated HOMO−LUMO energies of quinones 1 and
their corresponding aromatics 2.
significantly red-shifted (vs 1A) absorption centered at 675 nm
(1.84 eV).
A very different and counterintuitive trend was observed for
the aromatized di/triarylated derivatives (Table 1; discussed
below for triarylated compounds). Both the absorption and
calculated (and X-ray-measured) bond lengths shows 0.06 Å
contraction and 0.05 Å elongation of bonds a and b, respectively,
going from 1A to 2A, in accord with the drawn resonance
structures (Figure 3). Additional fused benzene rings in 2T and
2P break the acene structure, resulting in a small blue shift,
abs
em
emission bands of 3A (λ = 579 nm, λ = 620 nm) undergo a
max
max
abs
blue shift upon extension of the conjugation in 3T (λ = 551
max
em
nm, λ = 582 nm).
max
In fact, the divergent evolution of the energy gaps for the
aromatized structures and their convergent evolution for the
quinones was predicted by DFT at the outset of this work,
similar to that observed upon [a]benzannulation of anthracene
abs
(λ = 378 nm vs 364 nm for tetraphene).
max
Analysis of the electrochemical properties by cyclic
voltammetry reveals the same trends established above by
DFT and photophysical measurements (Figure 4). All of the
compounds exhibit amphoteric multistage redox behavior: two
reversible reduction waves and one reversible oxidation wave for
the quinones and two reversible oxidations and one reversible
reduction for the aromatized derivatives. Extending the π-
electron system in quinones (1T vs 1A) lowers the oxidation and
reduction potentials by 0.2 and 0.3 V, respectively (Table 1).
Aromatization of the quinones leads to expected negative shift
of both oxidation (by 0.4−0.6 V) and reduction (by 0.5−0.8 V)
16
prompting us to synthesize the tetracene derivatives. Figure 3
shows that as the conjugation is extended in the quinone series
from 1A to 1P, the HOMO−LUMO gap decreases from 2.37 to
1.59 eV. On the contrary, the same extension of conjugation in
the aromatic series leads to an increase in the HOMO−LUMO
gap from 2.24 to 2.41 eV. The same trend was also calculated for
unsubstituted (Ar = H) and triarylated (3A, 3T) aromatic series
(
see the Supporting Information), thus ruling out the Ar-induced
twist as an origin of this unusual behavior.
To rationalize these observations, one should consider the
longest conjugation path in these compounds (highlighted in
bold in Figure 3). Within this framework, quinones 1 are best
viewed as acenes substituted at their peri positions, explaining the
large red shift/contracting HOMO−LUMO gap in going from
potentials. The CF Ph substituents, being orthogonal to the π-
3
conjugated system, have a very small electron-withdrawing effect,
noticed as ∼0.01 V more positive redox potentials in triarylated
3A/3T versus diarylated 2A/2T. Within the di- and triarylated
series, the extension of the π-electron system from an
anthracene-based to a tetracene-based core leads to slightly
higher oxidation (by 0.06 V) and lower reduction (by 0.05 V)
1
A to 1T and 1P. On the contrary, such a central acene moiety
does not exist in the aromatized derivatives, and 2A/3A can be
viewed as two connected anthracene moieties. Analysis of
a
Table 1. Electrochemical and Optical Data for 1−3
E1/2 (V)
b
E1/2
b
(V)
red
Eg (eV)
c
λabs (nm)
λem (nm)
Φ (%)
Eg (eV)
d
compd
ox
1
1
2
3
2
3
A
0.94
−1.29, −1.50
−0.98, −1.22
−1.82
−1.81
−1.77
2.24
1.72
2.11
2.11
2.13
2.13
575
675
571
579
548
551
607
737
595
620
570
582
58
−
13
8
2.16
1.84
2.08
2.14
2.26
2.25
T
0.74, 1.18
0.29, 0.83
0.30, 0.81
0.35, 0.82
0.36, 0.83
8
A
A
T
T
19
9
−1.77
a
b
c
1/2
ox
1/2
d
Measured in CH Cl . Redox potentials are reported with respect to ferrocene (Fc) oxidation. Electrochemical gap, E = E
− E
. Optical
red
2
2
g
max
abs
gap, E = 1240/λ
.
g
C
DOI: 10.1021/acs.orglett.6b01559
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ACKNOWLEDGMENTS
This work was supported by NSERC of Canada. MRR thanks
FQRNT for PBEEE fellowship.
■
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1
A/1T (by 0.52 eV) and slightly expanded in the aromatized
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(
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of aryllithium. The proposed mechanism, supported by DFT,
involves two carbonyl additions and one Michael addition with
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physical properties, except for a higher fluorescence quantum
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in the π-electron system in the aromatic series 3A/2A to 3T/2T
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bands, while a “normal” red shift is observed for the
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3
and aromatic systems was predicted by DFT and rationalized by
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(
8
(15) A smaller Φ of 0.9% was mistakenly reported for 2A earlier. The
f
current datum was confirmed by absolute quantum yield measurements
in an integrating sphere and reference measurements versus cresyl violet
(
Φ = 54%).
ASSOCIATED CONTENT
Supporting Information
f
■
(
16) To date, our attempts to extend the series to the pentacene
*
S
derivatives have failed: contrary to 1A and 1T, quinone 1P could not be
prepared by intramolecular cyclization of the corresponding lactone
PenL, presumably because of its instability under the reaction
conditions.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01559.
Synthetic procedures and characterization data for the
synthesized compounds; computational protocols and the
detailed results on the photophysical properties (PDF)
Crystallographic data for 3A (CIF)
(17) Note added in proof: after submission of this manuscript another
example of Michael addition to fully benzannulated quinone was
reported by Tykwinski’s group: Reus, C.; Lechner, M. P.; Schulze, M.;
Lungerich, M.; Diner, C.; Gruber, M.; Stryker, J. M.; Hampel, F.; Jux, N.;
Tykwinski, R. R. Chem. - Eur. J. 2016, 22, 9097.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: dmitrii.perepichka@mcgill.ca.
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.6b01559
Org. Lett. XXXX, XXX, XXX−XXX