5,6,12,13-Tetraazaperopyrenes as Unique Photonic and Mechanochromic Fluorophores

Article abstract of DOI:10.1002/anie.201914900

5,6,12,13-Tetraazaperopyrenes with different number of tert-butyl groups (c-TAPP-T, c-TAPP-H) were synthesized, via four-fold Bischler–Napieralski cyclization as the key step. As deduced from the single-crystal structures and optical properties, N-doping

Full text of DOI:10.1002/anie.201914900

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Accepted Article
Title: 5,6,12,13-Tetraazaperopyrenes as Unique Photonic and
Mechanochromic Fluorophores
Authors: Wei Yuan, Junjie Cheng, Xiaopei Li, Mengjiao Wu, Yi Han,
Chunmei Yan, Gang Zou, Klaus Müllen, and Yulan Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914900
Angew. Chem. 10.1002/ange.201914900
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5,6,12,13-Tetraazaperopyrenes as Unique Photonic and
Mechanochromic Fluorophores
Wei Yuan,[a], Junjie Cheng,[b], Xiaopei Li,[a] Mengjiao Wu,[a] Yi Han,[a] Chunmei Yan,[a] Gang Zou,*[b]
Klaus Müllen,[c] Yulan Chen*[a]
Abstract: 5,6,12,13-Tetraazaperopyrenes with different number of
tert-butyl groups (c-TAPP-T, c-TAPP-H) were synthesized, via four-
fold Bischler-Napieralski cyclization as the key step. As deduced from
the single crystal structures and optical properties, N-doping and
transport, which appears to be facilitated by the π-π stacking of
the perylene central unit, their photonic properties have hardly
been explored. Apart from the above difficulties of crystal
engineering, this is due to the low fluorescence quantum yields
because of aggregation induced quenching. Further, mechano-
chromic fluorescence (MCF) has not been detected in these
reported azaperopyrenes, which might be explained by an
unfavorable packing.[7,1c]
substitution type allow for
a precise control of intermolecular
interactions. Compared to the reported 1,3,8,10-tetraazaperopyrenes,
significantly different packing modes were found in 5,6,12,13-
tetraazaperopyrenes. Going from c-TAPP-T to c-TAPP-H, two
additional tert-butyl groups lead to different preferential growth
directions, affording 1D and 2D microcrystals, respectively. Most
importantly, both microcrystals exhibit excellent optical waveguide
properties with extraordinarily low loss coefficients and unique
polarization features. Although c-TAPP-H possesses a rigid and
planar core, its crystals display an exceptional mechanochromic
fluorescence, which, again, depends on the mode of molecular
packing. The resulting properties qualify these new peropyrenes as
advanced optical and photonic materials.
The access to multi-functional semiconductors with controllable
flow of photons at the microscale and tunable fluorescence under
mechanical force can pave the way to new photonic devices and
innovative sensors.[1] Mostly, device performance can be boosted
by the fabrication of semiconductor microcrystals with well-
defined shapes and dimensions, but their rational design by the
inherent molecular structures and packing modes remains
challenging.[2]
Figure 1. Examples of peropyrene derivatives.
Our motivation was to seek structural modifications of the
peropyrene core which would allow different preferential growth
directions towards fluorescent microstructures, photonic and MCF
properties. We therefore target azaperopyrene structures with
different intermolecular recognition sites, e. g, by varying the
positions of N-doping and the type of substitution. In contrast to B
In this regard, peropyrenes (structure A in Figure 1), a member
of the polycyclic aromatic hydrocarbon (PAH) family, have
attracted immense attention due to their rich photochemistry and
structural versatility.[3] The synthesis of peropyrene derivatives
has mainly targeted the degree of π-extension, geometry, and
edge topologies.[4] Among these endeavors, chemical doping of
the peropyrene skeleton with nitrogen atoms opens a new avenue
for tailor-made properties, as is evidenced by recent progress on
functional azaperopyrenes.[5] The known azaperopyrenes
comprise N-doping at the 2,9 or 1,3,8,10 positions of the core
(structures B and C in Figure 1).[6] In contrast to charge carrier
and
C (Figure 1) with N-atom mediated intermolecular
interactions along the long molecular axis, we envisaged that
peropyrenes with multiple N-atoms and substitution groups
located at the bay positions of the perylene core could provide
multiple and different directional intermolecular recognitions.[5a,5b]
However, up to now, the core-doped 5,6,12,13-tetraaza-
peropyrerenes (Figure 1) have never been explored which follows
from the lack of suitable synthesis protocols.
Herein, we describe the synthesis of core-doped
tetraazaperopyrenes substituted with tert-butyl (t-Bu) groups (c-
TAPP-T, c-TAPP-H, Scheme 1). Interestingly, different t-Bu
substitution led to intermolecular interactions favorable for either
1D or 2D molecular alignment. We thus obtained highly
fluorescent 1D and 2D microcrystals displaying excellent optical
waveguide performance with either 1D or 2D light propagation
and unique polarization features. To the best of our knowledge,
these microcrystals offer the lowest optical loss coefficients
among PAH-based waveguide materials. Although possessing
the same rigid aza-substituted PAH-core with seven fused
aromatic rings, c-TAPP-H showed unique MCF properties.
Regioselective N-doping and substitution with bulky side groups
thus, appear as efficient design principles for photonic and MCF
materials.
[a]
[b]
W. Yuan, X. Li, M. Wu, Y. Han, C. Yan, Prof. Y. Chen
Tianjin Key Laboratory of Molecular Optoelectronic Science,
Department of Chemistry, Tianjin University, Tianjin 300354, China
E-mail: yulan.chen@tju.edu.cn
J. Cheng, Prof. G. Zhou
CAS Key Laboratory of Soft Matter Chemistry, Department of
Polymer Science and Engineering, iChEM, University of Science
and Technology of China, Hefei, Anhui 230026, China
E-mail: gangzou@ustc.edu.cn
[c]
[]
Prof. K. Müllen
Max-Planck-Institut für Polymerforschung, Ackermannweg 10,
55128 Mainz, Germany
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
The structures and synthetic protocols of the target molecules
c-TAPPs are presented in Scheme 1. We previously found that
the Bischler-Napieralski cyclization allowed for the facile incorpo-
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ration of N-atoms into PAHs. A good case could be referred to the
synthesis of diazapyrenes and thiophene fused phenanthrol-
llines.[8] Now we extend this reaction to the fourfold N-doping of
peropyrene and also introduce different numbers of t-Bu groups.
Starting from 4,7-diphenylbenzo-[c][1,2,5]thiadiazole-5,6-diamine
(1), the amide precursor 2 was obtained through an amidation
reaction with pivaloyl chloride, followed by a Bischler-Napieralski
ring closure reaction in P2O5/POCl3 to furnish the t-Bu substituted
dibenzo[c,i][1,2,5]thiadiazolo[3,4-f][1,10] phenanthroline (3),
followed by reduction with zinc powder to obtain 4. N,N'-
(Dibenzo[c,i][1,10]phenanthroline-13,14-diyl)bis(2,2-dimethylpro-
panamide) 5a and 5b then became readily accessible by a
second amidation reaction. Finally,
a Bischler-Napieralski
cyclization of 5a and 5b afforded c-TAPP-T and c-TAPP-H,
respectively. The bulky t-Bu groups were readily incorporated
along with the amide precursors. All the compounds were
unambiguously characterized by mass and NMR (1H and 13C)
spectroscopy. Good thermal stability of c-TAPP-T and c-TAPP-H
was confirmed by TGA measurements (Figure S1).
Scheme 1. The synthesis of substituted 5,6,12,13-tetraazaperopyrenes c-TAPP-T and c-TAPP-H.
Single crystals of c-TAPP-T and c-TAPP-H for X-ray diffraction
measurements were obtained by slow evaporation of carefully
chosen solvent mixtures (see SI). Remarkably enough, the
crystals exhibit quite different shapes: c-TAPP-T grows into 2D
microsheets while c-TAPP-H is obtained as 1D microrods. Both
c-TAPP-T and c-TAPP-H possess planar structures, with no π-π
stacking of the peropyrene cores (Figure 2), as is supported by
the observation of either perpendicular alignment of c-TAPP-T
molecules or parallel aromatic cores far away from each other
(9.475 Å) for c-TAPP-H. This will prove relevant for the solid-state
emission (see below). Notably, slight structure modulation of the
core by changing the number of t-Bu side groups affords profound
changes of the packing mode. As shown in Figure S2, driven by
multiple intermolecular interactions such as C-H (t-Bu)···N (2.73
Å) and C-H (t-Bu)···C (C5* of the peropyrene ring) (2.84 Å), c-
TAPP-T molecules are arranged orthogonally to each other to
form a 3D packing pattern in the Im-3 cubic space group. In
contrast, c-TAPP-H crystallizes in the Pbca orthorhombic space
group with preferential growth directions (Table S2). As a result,
a 1D alignment is realized, which is mainly connected by C-H (t-
Bu)···C (C3* of the peropyrene ring) (2.83 Å) and C-H (t-Bu)···C
(C3 of the peropyrene ring) (2.79 Å) interactions.
at 497 nm and quantum yields of 76% and 86% (relative to
coumarin 6 whose ΦF is 76% in DCM[9]), respectively. Their FL
spectra both feature small Stokes shifts (ca. 18 nm) and a
characteristic vibrational progression with mirror images of their
absorption bands, reflecting a highly rigid backbone. The FL
decay curves of c-TAPP-T and c-TAPP-H estimate their FL
lifetime as 3.38 and 2.98 ns, respectively (Figure S4). In solid
state, the c-TAPP-T and c-TAPP-H crystals emit bright green-
yellow fluorescence (Figure S5), with absolute quantum yield of
11.4% and 11.8%, respectively (Table S4).
In toluene, both compounds displayed three sets of absorption
bands as typical feature of peropyrenes (Figure 3). The peak
assignments were in accordance with DFT calculations (Figure
S3 and Table S3). The absorption peak at longest wavelength for
c-TAPP-T and c-TAPP-H is at 456 nm (εc-TAPP-T = 5.6 × 104 M-
1cm-1) and 457 nm (εc-TAPP-H = 6.7 × 104 M-1cm-1), respectively. The
bathochromic shift by comparison with the reported 1,3,8,10-
tetraazaperopyrene analogues (437 nm for compound C)[5b]
reflects the profound changes of the optical properties brought
about by core modification of tetraazaperopyrene. Both c-TAPP-
T and c-TAPP-H gave rise to green fluorescence with a maximum
Figure 2. X-ray crystal structure and stacking mode of (a) c-TAPP-T and (b) c-
TAPP-H.
Cyclic voltammograms (CV) of c-TAPP-T in DCM displayed
two irreversible one-electron oxidation and two quasi-reversible
reduction steps (Figure S6). From the first oxidation and reduction
potentials, the HOMO and LUMO energy levels were calculated
to be -5.48 and -3.59 eV, respectively (Table S4). Similar CV
curves for c-TAPP-H were obtained, manifesting that
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incorporation of four N-atoms into the peropyrene core decreases
the LUMO energy remarkably compared with the peropyrene
derivatives A.[3c] DFT calculations revealed that LUMO and
HOMO were delocalized over the π-scaffold, without
intramolecular charge transfer effects (Figure S7) which stands in
good agreement with experimental results (Figure S8).
lowest one among the rarely explored PAH-based waveguide
crystals so far,[10] indicating that PL energy can propagate
efficiently along the axial direction of the microrods.
Besides the low propagation loss, photon flow along the
microrods appears to be sensitive to the polarization of the
exciting light. The set-up for polarization measurements is shown
in Figure S10. With the excitation polarizer oriented with β at 0°,
30°, 60° and 90°, the fluorescence intensity at tip spot has a
periodic function with the polarization angle θ. The highest
fluorescence intensity occurred at θ of about 0° and 180°, and the
lowest intensity was all localized at 90° and 270° regardless of the
angle β. Therefore, the unique polarization features of the
fluorescence emission can be estimated from both the excitation
and the emission point.[11] An emission dichroic ratio (Rd = Imax/Imin
)
up to 4.2 is found when β is 0°, corresponding to a polarization
ratio (ρ = (Rd - 1)/(Rd + 1)) of 0.62. The polarization-dependent
emission behavior is highly reproducible and all cos2 curves can
be perfectly fitted to these data at various angle θ (Figure 4d).
These observations indicate good alignment of molecules in the
microrods.[12]
Figure 3. UV-vis absorption and fluorescence spectra of c-TAPP-T and c-
TAPP-H (λex = 428 nm) in toluene (2.0 × 10-5 M).
Since the 1D microrods and 2D microsheets grown from c-
TAPP-H and c-TAPP-T respectively, both possess regular
shapes, smooth surfaces and emit bright light with small self-
absorption (Figure S5), their shape-dependent optical
propagation were studied to explore their application as photonic
materials. Microrods with 5-20 μm in width and 200-400 μm in
length were obtained by slow evaporation of a toluene solution of
c-TAPP-H between two pieces of glass (Figure 4a). The flow of
fluorescence was visualized by gray-color far-field microscopy.
When the microrods were excited at 450 nm, bright luminescence
spots were detected at the tip (Figure 4b). The FL spectra of the
excitation site (body) and the tip site (tip) are shown in Figure S9.
The intensity of out-coupled light decreases exponentially as a
Due to the limited control of 2D microstructures, the
development of 2D optical waveguide crystals lags behind their
1D counterparts, despite their great potential in chip-scale planar
photonics system for direction controllable waveguides.[13]
Interestingly, unlike c-TAPP-H that tends to aggregate along a
single direction, a higher degree of intermolecular weak bonds
prevails along different directions in c- TAPP-T crystals, resulting
in a 2D growth of c-TAPP-T microsheets with an average edge
length of 80 µm (Figure 4e). When the laser beam is focused on
the sheet, photons are confined and actively propagate along two
distinct directions, which is characteristic for a 2D waveguide
(Figure 5). The optical loss coefficients (R) are determined to be
0.0088 (Figure 4g, toward direction ① in Figure 5b) and 0.0072
dB µm -1 (Figure S11, for direction ② labelled in Figure 5c)
respectively. Notably, this is a record value in 2D optical
waveguide studies. It represents an improvement by an order of
magnitude compared to the current minimum (0.033 dB µm -1).[13d]
function of propagation distance (Figure 4c) which is
a
characteristic feature of optical waveguides. The fluorescence
intensities at different sites (Iboby and Itip), together with the optical
loss coefficient (R) were fitted by a single exponential equation
(Iboby/Itip = A × e(-RX), where X is the distance from the excited site
to the emitting tip). R is thus determined as 0.0086 dB µm -1 which
is lower than those of most organic materials, in particular, the
Figure 4. (a) Bright-field optical and fluorescence microscope images of c-TAPP-H microrods. (b) Top: bright field optical image; bottom: gray-color far-field
fluorescence images of c-TAPP-H microrod upon laser excitation at different positions, scale bar is 50 µm. (c, g) -Ln (Itip/Ibody) vs distance (X) and the exponential fit
to the observed data, and (d, h) polar image of the peak intensities at different β (0°, 30°, 60°, 90°) for c-TAPP-H microrod (c,d) and c-TAPP-T microsheet (g, h).
The solid curve shows the cos2 fit. (e) Fluorescence microscope image and (f) gray-color fluorescence microscopy image of c-TAPP-T microsheet under excitation
(λ = 450 nm), scar bar is 20 µm.
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Polarization-dependent emission was also observed in these
microsheets, and the highest FL intensity occurred at a cross
angle of ca. 90° (Figure 4h). The emission dichroic ratio Rd is
calculated to be 2.4 where β is 0°, corresponding to polarization
ratio ρ of 0.41. Overall, these excellent waveguide performances
are mainly derived from well-defined packing modes and high
quality crystals which have been tailored by the substitution
number of t-Bu and N-incorporation. Besides, prominent optical
properties featured with intense fluorescence emission from these
crystals upon excitation at 450 nm with low self-absorption effect
may also make great contributions.[14]
molecules perpendicularly arranged to each other. Such knitted-
like structures are known to be stable and resistant to mechanical
force.[15] In contrast, the looser molecular stacking and weaker
intermolecular interactions prevailing in c-TAPP-H crystals
causes a sensitive response to external forces. This interpretation
is in agreement with powder XRD analyses. Both pristine samples
displayed intense and sharp diffraction peaks indicating an
ordering and crystalline-like state (Figure 6b). After grinding,
some of the sharp peaks for c-TAPP-H disappeared or became
weaker, suggesting that the crystalline structure was destroyed
and became amorphous.[8b] In contrast, the PXRD curves for c-
TAPP-T crystals before and after grinding remained unchanged
(Figure S13). Herein, the t-Bu substitution and regioselective N-
doping offer a practical molecular design strategy to tune MCF
property of azaperopyrenes.
Figure 6. (a) Normalized fluorescence spectra (λex = 450 nm) and (b) powder
XRD curves of c-TAPP-H in different solid states. .
Figure 5. (a) Fluorescence microscope images and (b,c) gray-color far-field
fluorescent images showing optical waveguide of c-TAPP-T microsheet upon
laser excitation at different positions along direction ① (b) and ② (c).
In conclusion,
the first 5,6,12,13-tetraazaperopyrene
derivatives with four N-atoms doped at the juxtapositions of the
central perylene unit have been synthesized (c-TAPP-T and c-
TAPP-H). Although this azaperopyrene core with seven fused
aromatics is planar and rigid, it has been demonstrated as a
multifunctional chromophore exhibiting versatile abilities to
propagate photons efficiently and respond to mechanical force
sensitively. In particular, the microsheets of c-TAPP-T show a
direction controlled symmetric waveguide with the optical loss
coefficients of 0.0086 and 0.0072 dB µm-1. These are the lowest
values to date for 2D optical waveguide crystals. Moreover, c-
TAPP-H exhibits pronounced MCF properties. Altogether, the
present findings greatly extend our knowledge of the structures
and functionality of aza-PAHs. Our molecular design strategy and
synthetic concepts will be beneficial for the facile access to
different aza-PAH-based microcrystals. This will contribute to the
development of organic optical devices, miniaturized lasers, and
other advanced photonics at the microscale.
Controlling the aligned aggregation of fluorophores by external
stimuli, e. g. mechanical force, is a powerful approach to modulate
the fluorescence of organic materials. Generally, such mechano-
responsiveness is related to the mode of molecular alignment.
Considering the remarkably different molecular packing modes in
crystal structures, we further explored the MCF properties of
azaperopyrenes. As shown in Figure 6a, the freshly crystallized
c-TAPP-H displayed green fluorescence with an emission
maximum at 498 nm. After grinding, the emission band became
broad and less-resolved. The fluorescence color varied to yellow
with the emission maximum shifting bathochromically to 530 nm
and the absolute FL quantum yield changed from 11.8% to 9.7%
(±0.2%). Upon addition of acetone followed by evaporation, the
recrystalized sample recovered its original emission color,
supporting
a reversible MCF process. The switching of
fluorescence by applying a mechanical force and acetone could
be repeated many times without fatigue (Figure S12). By contrast,
c-TAPP-T crystals were not sensitive to mechanical force (Figure
S13).
Acknowledgements
XRD measurements were perfomed to reveal the MCF
mechanism. According to the single crystal structures of c-TAPP-
T and c-TAPP-H, neither of them possesses π-π interactions
within adjacent azaperopyrenes as discussed above. This is
supposed to favor external perturbation of molecular arrangement
in comparison with reported peropyrenes with well-established π-
π stacking.[3] Specifically, c-TAPP-T possesses less steric
repulsion with neighboring molecules. As a result, the packing in
its crystal cell is denser as is evidenced by the higher crystal cell
density for c-TAPP-T (1.246 molecule/Å3) than that of c-TAPP-H
(1.120 molecule/Å3). Furthermore, there are also multiple
intermolecular interactions stemming from the t-Bu groups and
the adjacent aromatic core, affording 3D structures with
Financial support by National Key Research and Development
Program
of
China
(Grants
2017YFA0204503
and
2017YFA0207800), National Natural Science Foundation of
China (Grants 217340062, 21975178 and 21522405), Thousand
Youth Talents Plan and Natural Science Foundation of Tianjin
(Grant 17JCJQJC44200) is gratefully acknowledged
Keywords: Bischler-Napieralski cyclization • peropyrene •
microcrystals • optical waveguide • mechanochromic
fluorescence
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10.1002/anie.201914900
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Wei Yuan, Junjie Cheng, Xiaopei Li,
Mengjiao Wu, Yi Han, Chunmei Yan,
Gang Zou,* Klaus Müllen, Yulan Chen*
Page 1. – Page 5.
5,6,12,13-Tetraazaperopyrenes as
Unique Photonic and
Mechanochromic Fluorophores
Regioselective N-doping of peropyrenes for fluorescent tailored microcrystals
were achieved. The first 5,6,12,13-tetraazaperopyrene derivatives were synthesized,
whose crystals showed excellent optical waveguide properties and exceptional
mechanochromic fluorescence.
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