(Aza)Acenes Share the C2 Bridge with (Anti)Aromatic Macrocycles: Local vs. Global Delocalization Paths

Article abstract of DOI:10.1002/anie.202011848

A strong conjugation present in fused systems plays a crucial role in tuning of the properties that would be showing a dependence on the efficiency of π-electrons coupling. The π-cloud available in the final structure can be drastically influenced by a side- or a linear fusion of unsaturated and conjugated hydrocarbons. The linear welding of naphthalene/anthracene or quinoxaline/benzo[g]quinoxaline with triphyrin(2.1.1) gives structures where the competition between local and global delocalization is distinguished. The aromatic character observed in skeletons strongly depends on the oxidation state of the macrocyclic flanking and is either extended over the whole system or kept as a composition of local currents (diatropic and paratropic) of incorporated units. The hybrid systems show the properties derived from the π-conjugations that interlace one another but also show a significant independence of (aza)acene subunits reflected in the observed spectroscopic properties.

Full text of DOI:10.1002/anie.202011848

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9063–9070
International Edition: doi.org/10.1002/anie.202011848
German Edition: doi.org/10.1002/ange.202011848
Acenes
(Aza)Acenes Share the C2 Bridge with (Anti)Aromatic Macrocycles:
Local vs. Global Delocalization Paths
Krzysztof Bartkowski and Miłosz Pawlicki*
th
´
Dedicated to Professor Lechosław Latos-Graz˙ynski on the occasion of his 70 birthday
Abstract: A strong conjugation present in fused systems plays
a crucial role in tuning of the properties that would be showing
a dependence on the efficiency of p-electrons coupling. The p-
cloud available in the final structure can be drastically
influenced by a side- or a linear fusion of unsaturated and
conjugated hydrocarbons. The linear welding of naphthalene/
was efficiently modulated by merging with other p-extended
molecules including strongly coupled macrocycles (e.g. por-
phyrinoids). The merging process of both structural motifs
involves two modes of interaction: the linear extension on the
C2 bridge and b-positions[5] and the edge-fashion (e.g. 1).[6]
Both types of interactions lead to different final outcome and
substantially dissimilar contribution of local and global
conjugations.
anthracene
or
quinoxaline/benzo[g]quinoxaline
with
triphyrin(2.1.1) gives structures where the competition between
local and global delocalization is distinguished. The aromatic
character observed in skeletons strongly depends on the
oxidation state of the macrocyclic flanking and is either
extended over the whole system or kept as a composition of
local currents (diatropic and paratropic) of incorporated units.
The hybrid systems show the properties derived from the p-
conjugations that interlace one another but also show a sig-
nificant independence of (aza)acene subunits reflected in the
observed spectroscopic properties.
A modulation of delocalization in structures merging
acenes and macrocycles can be also realized by a controlled
linking of isolated motifs[7] simultaneously masking a specific
behavior that after revealing via a fundamental process (e.g.
redox)[8] strongly influence final response changing the
observed p-delocalization path (e.g. 2).[8,9] The redox process
is one of the post-synthetic modification with a substantial
influence on the conjugated p-cloud opening a path of
switching between aromatic (4n + 2 p-electrons)[2] and anti-
aromatic (4n p-electrons) character[10] observed for porphyr-
inoids[11] but also (aza)acenes.[3] The modulation of aromatic
character of both motifs needs using strong oxidizing or
reducing agents what limits a possibility of exploring of the
mutual influence of subunits incorporated in the hybrids. On
the other hand a triphyrin(2.1.1) (e.g. 3)[14] has been reported
as the skeleton with the accessible aromatic/antiaromatic
switching behavior.[15] The ethylene linker of triphyrins(2.1.1)
(Figure 1) can be naturally used for welding with acene/
heteroacene in the linear fashion. Potential hybrids are
eventually formed with two well defined p-clouds and
a potential for changing the oxidation state in one unit to
form antiaromatic current that influence the p-cloud of
(aza)acene.
Introduction
Unsaturated and strongly p-conjugated hydrocarbons are
extensively explored as skeletons important for fundamental
studies but also with a significant potential for practical
applications. A cyclic construction of unsaturated hydro-
carbons introduces the very fundamental phenomenon of
aromaticity resulted from strongly delocalized p-electrons
that give a diatropic current essential for observed properties
and reactivity.[1] The gradual increase of the number of p-
electrons allows a specific control of the spectroscopic
response derived from a global conjugation.[2] Archetypal
skeletons with a potential for increasing the p-conjugation by
gradual fusion are acenes and azaacenes.[3] The extended
conjugation of an (aza)acene is described as the equilibrium
of several contributors where the Clarꢀs sextet[4] shifts from
one side to another significantly influencing the final out-
come[3] but also having a substantial effect on the observed
reactivity.[2] Beside the linear extension the p-cloud of acenes
[*] K. Bartkowski, Prof. M. Pawlicki
Department of Chemistry, University of Wrocław
F. Joliot-Curie 14, 50383 Wrocław (Poland)
Prof. M. Pawlicki
Faculty of Chemistry, Jagiellonian University
Gronostajowa 2, 30387 Kraków (Poland)
E-mail: pawlicki@chemia.uj.edu.pl
Homepage: http://mjplab.org
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202011848.
Figure 1. Acene-Macrocycle Hybrids.
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Following these observations here we report on the
modulation of the p-conjugation within hybrid skeletons
built of acene/azaacene and macrocycle (Figure 2) and
controlled by the aromatic-antiaromatic switch in the macro-
cyclic contributor. The observed optical response is depen-
dent on the degree of planarization and results in competition
between local and global delocalization with a substantial
contribution of local effects documented in spectroscopy.
pyrrole boronic acid followed by deprotection gave 6a (58%)
and 6b (12%) (Scheme 1, path a). Both azaacenes requested
an application of dipyrrole derivative 5 that after condensa-
tion (Scheme 1, path b) with ortho-phenylenediamine and 2,3-
diaminonaphthalene gave 6c[17] and 6d with 67% and 96%
yield, respectively. The subsequent condensation with 2,5-
bis(3,5-di-tbutylphenyl)-hydroxymethylfuran applied for 6a
and 6b gave 7a and 7b, respectively (Scheme 1, path c),
eventually isolated as cationic forms. Importantly both acene
derivatives were obtained with good yields of 47% (7a) and
39% (7b) what shows a generality of applied approach but
also effectiveness of condensation. The cationic character was
indirectly confirmed by HRMS experiments where m/z peaks
consistent with expected atomic composition (m/z observed
723.4219 expected 723.4309 for C52H55N2O (7a); m/z ob-
served 773.4364 expected 773.4465 for C56H57N2O (7b)) were
observed. The condensation of 6c/6d required significantly
different conditions with an excess of stronger acid
(Scheme 1, path d). Even then the observed yields were
substantially lower (6% (7c) and 5% (7d)) showing a dissim-
ilar character of both linear systems of acene and azaacene.
Similarly to the acene derivatives 7c and 7d were isolated as
cations as documented in HRMS experiments where the m/z
signals (observed 725.4219 expected 725.4214 for C50H53N4O
(7c); observed 775.4382 expected 775.4370 for C54H55N4O
(7d)) consistent with expected atomic composition were
Figure 2. Acene-Switchable Macrocycle Edge Fused Hybrid.
Results and Discussion
The modulation of acene/azaacene p-cloud has been
realized on the way of welding with a triphyrin(2.1.1) skeleton
via a sequence of reactions requiring formation of proper
derivatives of both acenes and azaacenes (Scheme 1). Acene
derivatives were efficiently obtained on the way of Suzuki– observed. In all four derivatives the unidentified counter ion
Miyaura coupling starting from a commercially available
dibromonaphthalene 4a or obtained according to the pre-
viously reported procedure dibromoanthracene 4b.[16] Both
derivatives, subjected to catalytic reaction with N-BOC-
was replaced with a tetrafluoroborane that increased the
solubility and allowed a detailed spectroscopic analysis of all
derivatives. All further experiments were performed on
systems with tetrafluoroborate anion.
Scheme 1. Synthetic approach for formation of acene/azaacene-triphyrin(2.1.1) hybrids. Conditions: a) 1. (N-Boc-pyrrol-2-yl)boronic acid, K2CO3,Pd-
(PPh3)4, THF, Toluene, H2O, Aliquat 336, 958C, 24 h (6a)/18 h (6b). 2. (CH2OH)2 reflux, 1 h. b) ortho-Phenylenediamine (6c)/ Naphthalene-2,3-
diamine (6d), AcOH, reflux, o/n; c) 1. 2,5-bis(3,5-di-tbutylphenyl)hydroxymethylfuran, BF3·Et2O, CH2Cl2, 4 h; 2. DDQ, 30 minutes, 3. NaHCO3,
HBF4(aq). d) 1. 2,5-bis(3,5-di-tbutylphenyl)hydroxymethylfuran, HBF4*Et2O, CH2Cl2, 48 h; 2. DDQ, 1 h, 3. NaHCO3, HBF4(aq). e) Zn/Hg, CD2Cl2;
f) air or DDQ; g) BPhCl2, Et3N, Toluene, reflux 1 h (9a/b).
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The extension of the p-cloud in conjugated systems
absorbance at l = 458 nm and l = 452 nm, respectively and
significantly influence the UV/Vis absorption as documented
for PAHs (e.g. (aza)acenes)[3] or macrocycles[5–8] including
triphyrins(2.1.1).[14,15] The UV/Vis spectra of 7a and 7c
(Figure 3A) with intense Soret bands (l = 434 nm and l =
421 nm, respectively) accompanied by a set of Q-bands with
the most red-shifted band at l = 662 nm (7a) and l = 588 nm
(7c) (Figure 3A) show the picture consistent with the
significant contribution of globally extended delocalization
involving both subunits of the hybrid systems and character-
istic for aromatic porphyrinoids.[13,15] Substantially different
optical properties were recorded for 7b and 7d with the sharp
the Q-region tailing to l = 850 nm (7b) and l = 800 nm (7d).
It suggests more complex behavior of both hybrids 7b/7d
and a potential contribution of local effects. A mutual
interaction of both subunits was deeply modified in the redox
switched structures 8x obtained quantitatively with a zinc
amalgam (Scheme 1, path e). The presence of paratropic
current modulates the global delocalisation what finds
a reflection in the UV/Vis spectra.[15,18] The four hybrids
(8a–d) show significantly different UV/Vis spectra supporting
a substantial modification of available p-electrons and con-
sistent with their 4n number (Figure 3B). Nevertheless, the
observed complex shapes show dissimilar pattern to other 4n
systems based on triphyrin(2.1.1),[15] where the intense Soret
band is accompanied by bathochromically shifted absorption
with a small oscillator strength.[18] The reduced acene
derivatives 8a/8b were efficiently entrapped as boron(III)
complexes 9a/9b (Scheme 1, path g) which prevents from
back oxidation to 7 which occurs efficiently as documented in
time-evolved experiments monitored with UV/Vis
(Scheme 1, path f, Figure S57-60). 9a and 9b showed nicely
marked molecular peaks in HRMS (m/z observed 841.4863
expected 841.4909 for C59H62BN2O2 (9a), m/z observed
891.4997 expected 891.5066 for C63H64BN2O2 (9b)) consistent
with the expected atom composition confirmed also by the
isotopic pattern (Figure S49,50). We were unable to obtain
boron(III) complexes for 7c/7d. The oxidation state of
macrocyclic part in 8 and 9 is similar but the pattern of
observed spectra shows substantial differences (Figure 3B,D)
suggesting a behaviour dependent on expected planarity. The
reduced forms of macrocycles entrapped in boron(III) com-
plexes 9x showed a significantly different absorbance while
comparing to 7x (Figure 3D) with a sharp band in the l =
350–470 nm region and the broad absorption at l = 500–
750 nm with the pattern observed for partially antiaromatic
compounds.[15,18]
The crucial information about the behaviour and mutual
influence of both subunits was derived from the observed
emission spectra. As expected for a dominating contribution
of globally p-extended delocalisation in 7a a single maximum
at lem = 708 nm and Stokes shift of ꢀ 45 nm, characteristic for
triphyrin derivatives was recorded.[14,15] In the same experi-
ment 7c and 7d showed emissions shifted hipsochromically
(lem = 450 and lem = 500 nm, respectively (Figure 3A, inset,
blue and green)) while 7b has been documented as non-
emissive. The emission wavelength of 7a supported by the
excitation experiment performed at the lem (Figure S61)
suggest that the red shifted band of fluorescence is associated
with the global delocalisation. A surprising emission recorded
for 7c and 7d at the shorter wavelengths and comparable to
those recorded for 6x (Figure 3C) suggests a substantial
contribution of local effects assigned to (aza)acene incorpo-
rated to final structure in addition the excitation spectra
recorded show the pattern with maxima recorded for 6c/6d.
The local effect of conjugation resulting in the shortwave
emission overlapping with the absorption spectrum has been
recorded for complex structures for example in corrole
derivatives.[19] The local delocalisation effects has been even
more marked for the reduced structures 8x that, in contrast to
Figure 3. Absorbance and emission spectra of 7x (A, CH2Cl2), 8x (B,
CH2Cl2), 6x (C, CH2Cl2) and 9x (D, CH2Cl2). All spectra measured at
298 K. Colour Scheme applied: orange (naphthalene, Ya), wine (anthra-
cene, Yb), blue (quinoxaline, Yc), green (benzo[g]quinoxaline, Yd).
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typical 4n p-electrons systems,[18] showed emission. The
observed fluorescence (Figure 3B, inset, lem = 430 nm (8a),
lem = 494 nm (8b), lem = 482 nm (8c)) was recorded in the
spectral region observed previously for dipyrrolic derivatives
6x (Figure 3C, inset). It suggests an independent character of
(aza)acene(s) in the final molecule as the excitation spectra of
8a–c (Figure S72–76) fairly reproduce those observed for 6x.
The emission for 8d was observed at lem = 381 nm what is
substantially blue shifted even while compering to 6d (lem
ꢀ 520 nm) suggesting even deeper isolation of delocalization
paths of benzo[g]quinoxaline and reduced triphyrin(2.1.1).
The both boron(III) complexes 9a and 9b showed lack of
emission consistently with the predicted more efficient para-
tropic current.[15,18]
consistent with a strong hydrogen bond documented in crystal
structure(s)[21] that competes with the diatropic current
influence.[22] The two-electron reduced derivatives that
showed a significant disturbance of the global delocalisation
1
what has been also reflected in H chemical shifts range for
the b-lines (Figure 4C 8a, Figures S45-47) recorded between
d = 5.2–6.2 ppm and consistent with a contribution of local
effects.[20] The NH resonances were shifted upfield while
compering to 7x (d = 12.51 ppm (8a); d = 12.06 ppm (8b);
d = 13.54 ppm (8c); d = 12.70 ppm (8d)) showing a significant
influence of N-H-N hydrogen bond unavailable for non-
planar geometry of 8x. Boron(III) complexes shifts b-lines
upfield (9a d = 6.37 ppm (4,15), d = 5.99 ppm (5,14), d =
5.74 ppm (9,10) (Figure 4D), and for 9b d = 6.61 ppm
(4,15), d = 6.15 ppm (5,14), d = 5.94 ppm (9,10)), locating
those systems on the border between non-aromatic and
antiaromatic structures.[15,20] The chemical shifts of the axial
aryl group attached to the central boron inside the macrocycle
are sensitive probes for assessing the aromatic character of
the macrocycle.[15,23] The downfield shifted resonances of the
axial phenyl group (Figure 4D; 9a d = 8.22 ppm ortho, d =
7.48 ppm meta, d = 7.42 ppm para; 9b d = 7.93 ppm ortho, d =
7.39 ppm meta, d = 7.34 ppm para) confirm that the macro-
cycle is slightly antiaromatic.[15,20,24]
The presence of diatropic or paratropic current was
expected to substantially affect the fused (aza)acene reso-
nances crucial for assessing the contribution of global or local
delocalisation(s). Actually the range of chemical shifts (d) is
an intuitive tool helpful in assessing the contribution of
a global and local diatropic p-expansion that de-shields the
perimeter lines shifting them down-field.[7,8,19] A typical
behavior of the paratropic current is the up-field relocation
of the margin resonances.[15,18,20] The lack of global effect
either diatropic or paratropic will leave the influence of local
conjugation characteristic for unsaturated carbocycles or
heterocycles. Thus, it is expected that changes in the character
of analyzed molecules and being a consequence of switching
from diatropic to paratropic current will significantly influ-
ence the observed d range. The observed chemical shifts of
naphthalene subunit in 7a (d = 9.94 ppm (11,21), d = 8.59 ppm
(13,23), d = 8.10 ppm (14,24)) show a noticeable down-field
shift while comparing to 6a (d = 7.92 ppm (11,21), d =
7.79 ppm (13,23), d = 7.44 ppm (14,24)) consistently with effi-
cient global delocalization and significant contribution of 22p
electrons path (Figure 4A,B, green arrow). The two-electron
reduction and formation of 8a shifts the discussed resonances
up-field (d = 7.72 ppm (11,21), d = 7.60 ppm (13,23), d =
7.32 ppm (14,24)) to positions comparable to 6a, consistently
with breaking the global extension of delocalization (Fig-
ure 4C) and stabilizing the local effects. The correlation of the
chemical shift with the oxidation state is also observed for
boron(III) complexes (d = 7.23 ppm (11,21), d = 7.37 ppm
(13,23), d = 7.14 ppm (14,24) 9a) comparable with 6a (Fig-
ure 4D) and consistent with the lack of global effect and
stabilization of both characters (diatropic and paratropic).
Similar trend was recorded for quinoxaline derivatives 7c
(Figure S46). The longer (aza)acenes of 7b and 7d show the
perimeter protons downfield shifted by D ꢀ 1.1–1.5 ppm while
comparing with 6b and 6d supporting a less efficient global
Both types of the currents— diatropic or paratropic— can
be diagnosed following the magnetic criterion of aromaticity
1
based on H NMR spectra.[20] The series of oxidized deriva-
1
tives 7x showed the H chemical shifts of the b-lines of all
heterocyclic subunits located in the region of 9.0–7.5 ppm
consistent with a noticeable contribution of global delocali-
sation (Figure 4B 7a, Figures S45–47)[20] but also underlining
the structural modifications. The most significant difference
can be observed for resonances assigned to positions 4,15
significantly downfield shifted in azaacene derivatives (d =
9.10 ppm 7c and d = 8.96 ppm 7d) while comparing to acene
hybrids (d = 8.48 ppm 7a, d = 8.30 ppm 7b) and caused by the
proximity of nitrogens.[12f] In contrast to typical behavior in
aromatic macrocycles the inner hydrogens of 7a–d showed
a
substantial downfield shift (d = 17.01 ppm (7a); d =
18.54 ppm (7b); d = 15.62 ppm (7c); d = 17.08 ppm (7d))
Figure 4. 1H NMR spectra (600 MHz, 300 K) for 6a (A, CDCl3), 7a (B,
CD2Cl2), 8a (C, CDCl3) and 9a (D, CD2Cl2). A shift of 11/21 naphthalene
resonances marked with green and red arrows shows a change in
chemical shifts and modulation of the observed currents.
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delocalization concluded from the electronic spectra (Fig-
marked in two electron reduced structures 8x and confirmed
ure S45 and S47). The reduced skeletons of 8b–d showed
similar trends presenting a good correlation of chemical shifts
assigned to (aza)acenes subunits with linear substrates 6b, 6c,
6d consistent with a lack of global effect deduced from
electronic properties (see SI). It also proves that the two-
electron reduction and conversion from 7x to 8x blocks the
macrocyclic delocalization and leaves an (aza)acene to
operate as an independent subunit.
by the recorded emission(s) at lmax comparable to 6x.[7,8,19,25]
The lack of local of emission in boron(III) complexes 9a/9b is
consistent with a contribution of paratropic current.
To get a deeper insight into the observed behaviour and
modulable delocalization paths we conducted theoretical
analyses for all presented motifs with the B3LYP functional
and at the 6-31G(d,p) level of theory. The optimized models
were compared with available crystal structures showing
a good correlation of geometries crucial for reliable of
analysis conducted on conjugated macrocycles.[14,15,20] The
theoretical models of 7x show a significant planarity (Fig-
ure S77–80) while the geometries of reduced forms (8x) are
strongly disturbed from planarity because of steric repulsion
of two NH groups present in the limited space of macrocycle
cavity (see SI, Figure S77–80). It is consistent with the
observed spectroscopic behavior and strongly disturbed
global delocalization. Similarly to the crystal structure of
9a/9b all geometries optimized for boron(III) complexes
showed a substantial planarity responsible for efficient p-p
overlap that supports a presence of a paratropic current. The
GIAO predictions of the 1H NMR chemical shifts reproduced
the recorded patterns for all analyzed structures (see the
Supporting Information, Table S3/S4). The fully optimized
structures of 7x, 8x, and 9x have been used for NICS(1)
(nucleus independent chemical shift)[27] and AICD (aniso-
tropy induced current density)[28] analyses (Figure 6). It can
be easily noticed that the fusion of 10p electrons of
naphthalene or quinoxaline in 7a and 7c, respectively opens
a global 22p delocalization as observed in AICD profiles
(Figure 6, A1/B1) efficiently switching off the local effects of
separated units. In contrast to that the longer (aza)acenes—
anthracene and benzo[g]quinoxaline in 7b and 7d, respec-
tively— show the competition between two local effects
(Figure 6, C1/D1) keeping the 14p delocalization of (aza)a-
cene and diminished global contribution of potential 26p
path. The NICS(1) calculated within macrocylic motifs
(points a and e) show noticeably negative values for 7a and
7c that are significantly lowering for 7b and 7d (Figure 6)
consistently with recorded spectroscopic behaviour and sub-
stantially different global aromatic character. The (aza)a-
cenes subunits incorporated in all oxidized derivatives were
characterized as strongly aromatic based on observed NICS-
Changes in delocalization paths can be also derived from
structural parameters (bond lengths or angles) available from
crystal structures. As observed in the X-Ray structure of 7a
(Figure 5A) the main motif of the final molecule is planar
with the hydrogen entrapped inside of the cavity.[14,15,21] The
NON cavity creates an environment with the distances (N-N
2.526 Š, N-O 2.521 Š) consistent with strong three-center
hydrogen entrapment within the confined cavity of triphyrinic
flanking.[15] Importantly the formation of linear extension of
acene p-cloud with a macrocyclic ending does not change the
planarity of acene and in addition the whole molecule creates
strongly planarized conformation. The X-ray analyses of 7a/
9a couple (Figure 5A,B) showed a noticeable trend in
changing of the C(1)-C(2) bond length as the most crucial
for discussing global vs. local effects competition. The C(1)-
C(2) bond lengths in 7a (1.485 Š) is significantly longer while
comparing to 6a (1.431 Š, Figure 5A, inset) consistent with
a contribution of global delocalization in strongly extended p-
cloud of acene-triphyrin hybrid. The C(1)-C(2) bond length in
9a is shorted (1.439 Š) suggesting a presence of two local
effects as observed in spectroscopy. The crystal structure of
9b (Figure 5C) confirmed a better interaction between acene
and macrocyclic motif increasing the efficiency of p-p overlap
significantly influencing the optical response.[19] Both com-
plexes showed a different packing dependent on the type of
the employed acene. Naphthalene derivative 9a showed
a head-to-tail organization (Figure 5B, inset) with naphtha-
lene edge pointing to the furan subunit while in 9b a head-to-
head of anthracenes is observed in addition stabilized by the
p-p interaction (Figure 5C, inset). Thus, all experimental
evidences show a difference in observed quality of conjuga-
tion depending on the type of involved (aza)acene and
a significant contribution of global (7a/7c) or local (7b/7d)
delocalization(s). The local contributors are significantly
Figure 5. Crystal structures for 7a (A) 9a (B) and 9b (C). Thermal ellipsoids present 50% (A,B) and 30% (C) probability. Inset in trace A presents
the crystal structure for linear naphthalene-pyrrole structure (50% thermal ellipsoids). In traces B and C the mutual interaction between acene-
macrocycle hybrids are presented showing a p-p interaction. Meso-aryls are omitted for clarity.[26]
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Figure 6. AICD profiles for analysed derivatives (isovalue 0.035) (A-D/1–3) and NICS(1) values calculated in points a–e. Traces E and F present
possible resonance contributions of the oxidized and reduced variants of (aza)acene-macrocycle hybrids. Red arrows show clockwise current
characteristic for aromatic systems and blue arrows show anticlockwise current typical for paratropic delocalization in analysed structures.
(1) values (Figure 6, points b,c,d). The AICD profiles
obtained for the two-electron reduced oxidation systems 8x
showed a presence of paratropic current in the macrocyclic
motif significantly influencing the diatropic current within
PAH and opening an opportunity for having a contribution of
local effects of 6p (8a/8c) and 10p (8b/8d) (Figure 6, A2/B2/
C2/D2). It suggests a potential for stabilization of two
opposite currents aromatic in a carbocycle and antiaromatic
in a macrocyclic part. The NICS(1) values obtained for 8x
series with noticeably positive values in macrocyclic part
(Figure 6, points a and e) consistently support separation of
delocalization paths of a weak paratropic current in macro-
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cyclic part and a diatropic flow in an acene unit. The
Supercomputer Centre (KDM WCSS) is kindly acknowl-
significantly lowered NICS(1) values observed at point b
especially for 8c and 8d with strongly negative values of
NICS(1) recorded for the 6p and 10p fused system (points c,
d), respectively consistently support substantial contribution
of local effects. The drastic change of spectroscopic behavior
assigned to observed co-planarity of both components in
boron(III) complexes 9a/9b finds further confirmation in
theoretical analysis (Figure 6, A3 and C3). The AICD profiles
and NICS(1) values obtained for both boron(III) complexes
show more efficient paratropic current in macrocyclic motif
followed by further separation of 6p (9a) and 10p (9b)
aromatic paths in acenes.
edged for sharing computation resources necessary for DFT
calculations.
Conflict of interest
The authors declare no conflict of interest.
Keywords: acene ·antiaromaticity ·aromaticity ·azaacene ·
triphyrin
[1] a) R. Gleiter, G. Haberhauer, Aromaticity and Other Conjuga-
tion Effects, Wiley-VCH, Weinheim, 2012; b) T. M. Krygowski,
´
M. K. Cyranski, Z. Czarnocki, G. Häfelinger, A. R. Katritzky,
Conclusion
Tetrahedron 2000, 56, 1783 –1796.
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We have carefully planned the synthesis of hybrid
material opening a possibility for modulation of acene/
azaacene properties by merging with a redox switchable
macrocycle of triphyrin(2.1.1). Both components share the C2
bridge that allows a transmission of global aromatic delocal-
ization over the whole molecule observed for 4n + 2 p-
electrons entrapped in the system. Depending on the
(aza)acene a 22p delocalization (naphthalene/quinoxaline)
and 26p (anthracene/benzo[g] quinoxaline) global path is
potentially available. The two electron reduction of the redox
sensitive fragment of the hybrid systems introduces a 4n p-
electrons motif and converts the globally delocalized struc-
ture to the non-planar system where the local effect of
(aza)acene deeply modifies observed properties as docu-
mented spectroscopically. The planarization of 4n p-electrons
system via a boron(III) insertion increases an influence of the
paratropic current of macrocyclic part switching off the
behavior recorded for ruffled skeleton(s). Importantly a sig-
nificant difference in efficiency of observed effects can be
assigned to different linker between acene and macrocycle.
Considerably more efficient influence can be assigned to
nitrogen linkers in both types of skeletons— oxidized and
reduced— where more efficient global effects of diatropic
current and better separation of two opposite delocalization
was observed for 7c/8c and 7d/8d. Thus, the hybrid material
constructed of (aza)acene and a modifiable macrocycle can be
treated as a skeleton where several effects are interlacing one
another masking the local behavior while forming a planar
structure of fully delocalized and aromatic 22p/26p system.
The redox switch of the (aza)acene side-decoration strongly
influences the final system with the paratropic current of
boron(III) complex switching off the fluorescence. Signifi-
cantly the two-electron reduction entrapping strongly ruffled
geometry revels the local effects as documented in the
fluorescence behavior and confirmed with the theoretical
analysis.
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Acknowledgements
Financial support from the National Science Centre, Poland
(2015/17/B/ST5/01437) is kindly acknowledged. The Wrocław
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Manuscript received: August 30, 2020
Accepted manuscript online: January 6, 2021
Version of record online: March 5, 2021
9070 www.angewandte.org
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Angew. Chem. Int. Ed. 2021, 60, 9063 –9070