Discrete Dimers of Redox-Active and Fluorescent Perylene Diimide-Based Rigid Isosceles Triangles in the Solid State

Article abstract of DOI:10.1021/jacs.8b11201

The development of rigid covalent chiroptical organic materials, with multiple, readily available redox states, which exhibit high photoluminescence, is of particular importance in relation to both organic electronics and photonics. The chemically stable, thermally robust, and redox-active perylene diimide (PDI) fluorophores have received ever-increasing attention owing to their excellent fluorescence quantum yields in solution. Planar PDI derivatives, however, generally suffer from aggregation-caused emission quenching in the solid state. Herein, we report on the design and synthesis of two chiral isosceles triangles, wherein one PDI fluorophore and two pyromellitic diimide (PMDI) or naphthalene diimide (NDI) units are arranged in a rigid cyclic triangular geometry. The optical, electronic, and magnetic properties of the rigid isosceles triangles are fully characterized by a combination of optical spectroscopies, X-ray diffraction (XRD), cyclic voltammetry, and computational modeling techniques. Single-crystal XRD analysis shows that both isosceles triangles form discrete, nearly cofacial PDI-PDI π-dimers in the solid state. While the triangles exhibit fluorescence quantum yields of almost unity in solution, the dimers in the solid state exhibit very weak - yet at least an order of magnitude higher - excimer fluorescence yield in comparison with the almost completely quenched fluorescence of a reference PDI. The triangle containing both NDI and PDI subunits shows superior intramolecular energy transfer from the lowest excited singlet state of the NDI to that of the PDI subunit. Cyclic voltammetry suggests that both isosceles triangles exhibit multiple, easily accessible, and reversible redox states. Applications beckon in arenas related to molecular optoelectronic devices.

Full text of DOI:10.1021/jacs.8b11201

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Article
Discrete Dimers of Redox-Active and Fluorescent Perylene
Diimide-Based Rigid Isosceles Triangles in the Solid State
Siva Krishna Mohan Nalluri, Jiawang Zhou, Tao Cheng, ZHICHANG LIU, Minh T. Nguyen, Tianyang Chen,
Hasmukh A. Patel, Matthew D. Krzyaniak, William A. Goddard, Michael R. Wasielewski, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11201 • Publication Date (Web): 11 Dec 2018
Downloaded from http://pubs.acs.org on December 12, 2018
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Discrete Dimers of Redox-Active and Fluorescent Perylene
Diimide-Based Rigid Isosceles Triangles in the Solid State
†
†,‡
║
†
†
Siva Krishna Mohan Nalluri, Jiawang Zhou, Tao Cheng, Zhichang Liu, Minh T. Nguyen, Tianyang
†
†
‡
║
*,†,‡
Chen, Hasmukh A. Patel, Matthew D. Krzyaniak, William A. Goddard III, Michael R. Wasielewski
and J. Fraser Stoddart*,†,§,♯
†
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA
Institute for Sustainability and Energy at Northwestern, Northwestern University, 2145 Sheridan Road,
‡
Evanston, Illinois 60208, USA
║
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125,
USA
§
Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Nankai District, Tianjin
300072, China.
♯
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia.
*E-mail: m-wasielewski@northwestern.edu; stoddart@northwestern.edu.
MAIN TEXT
*
Correspondence Address
Professor J. Fraser Stoddart
Department of Chemistry
Northwestern University
2
145 Sheridan Road
Evanston, IL 60208 (USA)
Email: stoddart@northwestern.edu
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■
ABSTRACT
The development of rigid covalent chiroptical organic materials, with multiple, readily available
redox states, which exhibit high photoluminescence is of particular importance in relation to both
organic electronics and photonics. The chemically stable, thermally robust and redox-active
perylene diimide (PDI) fluorophores have received ever-increasing attention owing to their
excellent fluorescence quantum yields in solution. Planar PDI derivatives, however, generally
suffer from aggregation-caused emission quenching in the solid state. Herein, we report on the
design and synthesis of two chiral isosceles triangles wherein one PDI fluorophore and two
pyromellitic diimide (PMDI) or naphthalene diimide (NDI) units are arranged in a rigid cyclic
triangular geometry. The optical, electronic and magnetic properties of the rigid isosceles triangles
are fully characterized by a combination of optical spectroscopies, X-ray diffraction, cyclic
voltammetry, and computational modeling techniques. Single-crystal X-ray diffraction analysis
shows that both isosceles triangles form discrete, nearly cofacial PDI-PDI π-dimers in the solid
state. While the triangles exhibit fluorescence quantum yields of almost unity in solution, the
dimers in the solid state exhibit very weak — yet at least an order of magnitude higher — excimer
fluorescence yield in comparison with the almost completely quenched fluorescence of a reference
PDI. The triangle containing both NDI and PDI subunits shows superior intramolecular energy
transfer from the lowest excited singlet state of the NDI to that of the PDI subunit. Cyclic
voltammetry suggests that both isosceles triangles exhibit multiple, easily accessible and reversible
redox states. Applications beckon in arenas related to molecular optoelectronic devices.
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■
INTRODUCTION
The molecular design and development of redox-active organic materials displaying efficient
photoluminescence (PL) is a fundamental prerequisite for the fabrication of optoelectronic and
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photonic devices. Organic fluorophores, exhibiting intense fluorescence in dilute solution, often
suffer from emission quenching9-10 in the solid state because of strong intermolecular interactions
involving many molecules. Although it is often difficult to predict the fluorescence quantum yield
of a particular fluorophore, several strategies11-21 have been proposed in the literature to enhance
their fluorescence efficiency. These strategies include (i) the introduction of bulky substituents
into the parent fluorophore to prevent the detrimental intermolecular interactions between the
neighboring fluorophores,11-14 (ii) the restriction of intramolecular rotations of the fluorophore side
groups to minimize radiationless deactivation, (iii) the enforcement of a conformational change
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from a twisted conformation in solution to a planar one in the solid state, (iv) the formation of J-
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type fluorophore aggregates, and (v) the enhancement of intramolecular charge transfer (ICT)
character within donor-π-acceptor systems.17-18 Also, the development of chiral emissive organic
molecules exhibiting circularly polarized luminescence (CPL) has attracted increasing attention in
the recent past.22-23 Despite these significant advances, the development of novel molecular
designs for rigid covalent chiral organic materials with multiple, readily available reversible redox
states, exhibiting photoluminescence with high quantum yields both in solution and solid state,
remains a formidable challenge.24
In the past two decades, perylene diimide (PDI) derivatives have emerged as one of the
most extensively investigated model organic fluorophores because of their near unity fluorescence
quantum yields in solution in addition to their excellent chemical, thermal and photochemical
stabilities.25-27 The unique redox-active characteristics associated with the high electron mobilities
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of PDIs renders them attractive candidates28-29 for applications in a wide variety of domains, such
as organic light-emitting diodes30 (OLEDs), organic light-emitting field-effect transistors31-32
(OFETs) and organic photovoltaics33-35 (OPVs). Selective substitution at the core positions of the
PDI fluorophores has been well-established36-38 as a means of improving their optical properties.
The fluorescence of PDIs is quenched in the solid state on account of their planarity which favors
mostly H-type aggregation by dint of consecutive π-π interactions and/or dipole-dipole interactions
between the neighboring fluorophores.39-40 Nevertheless, Würthner and co-workers41-42 have
shown that the introduction of bulky substituents onto the PDI fluorophore is an effective strategy
to favor the formation of discrete PDI-PDI π-dimers and undistorted planar PDI fluorophores,43-44
which inhibit long-range PDI aggregation in the solid state, giving rise to variable structural and
optical properties, depending on the nature of the substituents. It is noteworthy that the
conformational flexibility offered by the core- or bay-substituents on the PDI fluorophores in most
cases, however, leads to inevitable intramolecular rotations or twists, which presumably reduce
their fluorescence quantum yields as a result of enhanced nonradiative decay.45-46 Therefore, we
envision that the incorporation of a core- or bay-unsubstituted PDI fluorophore— with solubilizing
bulky and chiral substituents at the imide positions—into a rigid cyclic geometry that blocks one
side of the PDI plane would not only prevent long-range intermolecular interactions but would
restrict also intramolecular rotations.
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In the past few years, we have focused our attention on the design, synthesis, characterization
and applications of a series of enantiopure chiral rigid diimide-based oligomeric macrocycles,47
including dimers,48-49 trimers,50-56 and tetramers, of which the cyclic trimers are observed to be
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particularly attractive. Initially, we reported a rigid equilateral triangle (−)-3NDI-Δ, comprising
three equivalent naphthalene diimide (NDI) units which gave rise to six individually accessible
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one-electron redox states. Extensive characterization of the symmetric (−)-3NDI-Δ by single-
crystal X-ray diffraction revealed51, 54 its distinct packing arrangements in the solid state,
depending on the nature of its encapsulated guest50 and solvent51 molecules, as well as its
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chirality and electronic state.
In summary, the assembly of (−)-3NDI-Δ resulted in the
formation of (i) one-dimensional helical superstructures driven by anion-π induced face-to-face π-
π stacking of two of the NDI units of (−)-3NDI-Δ in the presence of an encapsulated linear I3−
anion in CHCl , as well as (ii) finite and infinite supramolecular nanotubes in the presence of
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encapsulated dihaloethane and -ethene (DXE) driven by the columnar stacking of (−)-3NDI-Δ
with cooperative weak [C–H···O] interactions along the direction of [X···X]-bonded solvent
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chains. On the other hand, the monoradical anion 3NDI assembled into a K structure driven
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by the intermolecular face-to-face π−π stacking interactions between the NDI radical anions in the
solid state, while the trisradical trianion 3NDI3(•−) strongly associated with three cobaltocenium
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(CoCp ) cations assembled into infinite one-dimensional channels by dint of electrostatic and
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hydrogen bonding interactions.
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Furthermore, we recently reported two isosceles triangles [(−)-2PMDI-1NDI-Δ and (−)-
2
NDI-1PMDI-Δ], which are obtained by replacing one of the redox-active units with another in
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the case of the equilateral triangles [(−)-3PMDI-Δ and (−)-3NDI-Δ] without disrupting the
triangular geometry. Unlike the equilateral triangles, these isosceles triangles have lower
symmetries which lack the ability to form extended 1D tubular (super)structures but give rise to
2D layer-like (super)structures in the solid state. Based on the knowledge gained from the
intramolecular cyclical through-space electron sharing properties and the distinct solid-state
packing arrangements associated with all of the PMDI- and NDI-based trimers, we now intend to
include the PDI derivatives in these cyclic systems in order to achieve efficient optoelectronic and
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photonic properties in the realm of small-molecule organic materials. Considering the synthetic
challenges associated with the poor solubility of core- or bay-unsubstituted PDI fluorophores, we
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recently reported the one-pot synthesis of a molecular triangle composed of three bulky 1,6,7,12-
tetra(phenoxy)-substituted PDIs. Unlike the previously reported rigid molecular triangles to date,
this PDI-based equilateral triangle containing as many as 12 flexible phenoxy substituents could
neither be crystallized nor encapsulate suitable guest molecules. Surprisingly, we found that the
fluorescence emission of this PDI triangle is quenched, even in dilute CH Cl with a fluorescence
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quantum yield (Φ ) of about 0.2%, as a result of highly efficient nonradiative decay by means of
f
an ultrafast photoinduced intramolecular symmetry-breaking charge separation process. In
contrast with the six distinct one-electron redox waves observed for the PMDI- or NDI-based
equilateral triangles, cyclic voltammetry reveals only two distinct reversible reduction waves,
involving a total of six electrons for this PDI triangle. Therefore, we anticipated that the design
and synthesis of rigid core- or bay-unsubstituted PDI-based isosceles, rather than equilateral,
triangles would be of particular value in an attempt to improve the structural, optical, electronic
and magnetic properties of the diimide-based triangles. These unique rigid chiral cyclic systems
may serve as model platforms for the investigation of (i) the through-space electron
communication as well as of (ii) the solid-state packing arrangements with respect to the
competitive intermolecular π-π stacking interactions between the non-identical redox-active units,
with different dimensions, present in confined environments.
Herein, we report the design, synthesis and the full characterization of two chiral rigid N-
substituted PDI-based isosceles triangles — namely, (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ
—
in both solution and solid states, and compare the results with those of the related monomeric
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reference compounds, Ref-PMDI, Ref-NDI and Ref-PDI. The properties of the isosceles
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triangles are characterized by a combination of spectroscopies [steady-state and transient
absorption, fluorescence, circular dichroism (CD), electron paramagnetic resonance (EPR),
electron-nuclear double resonance (ENDOR)], variable temperature powder X-ray diffraction
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(VT-PXRD), single-crystal X-ray diffraction (XRD), thin film XRD, cyclic voltammetry (CV) and
computational modeling techniques. XRD Analysis shows that the rigid triangular geometry of the
macrocycles suppresses the conventional global π-π stacking into discrete nearly cofacial PDI-PDI
π-dimers. The optical properties of the compounds have been investigated by steady-state and
transient absorption, and fluorescence spectroscopies both in solution and solid state. Although the
fluorescence quantum yields of the isosceles triangles are almost unity in solution, they exhibit
very weak excimer emission in the solid state when compared to the almost completely quenched
photoluminescence of the monomeric Ref-PDI. The electronic properties investigated by CV
suggest that the isosceles triangles exhibit multiple reversible redox states implicating a total of up
to six electrons. The magnetic properties studied by EPR and ENDOR spectroscopies, supported
by density functional theory calculations, indicate that the behavior of the unpaired electron on the
singly reduced PDI subunit is indeed dependent on the nature of the adjacent PMDI/NDI subunits
present within the isosceles triangles.
■
RESULTS AND DISCUSSION
Syntheses of PDI-based isosceles triangles. In contrast with the synthesis of the PMDI/NDI-
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based isosceles triangles reported recently by us, here we outline the synthesis of the rigid cyclic
PMDI/NDI/PDI trimers involving non-identical redox-active aromatic subunits with different
dimensions. The two rigid PDI-based isosceles triangles were prepared (Figure 1) by stepwise
condensations between commercially available (RR)-trans-1,2-cyclohexanediamine and three
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different
dianhydride
derivatives—namely,
pyromellitic
dianhydride
(PMDA),
naphthalenetetracarboxylic dianhydride (NDA) and perylenetetracarboxylic dianhydride (PDA).
Starting with the condensation between (RR)-trans-1,2-cyclohexanediamine and an excess of
either PMDA in AcOH at 120 °C or NDA in DMF at 140 °C gave the corresponding
monoimide‒monoanhydride dimers—namely, (−)-2PMIA55 and (−)-2NIA —as previously
reported by us. The subsequent condensation of the dimers with an excess of mono-N-Boc-(RR)-
trans-1,2-cyclohexanediamine in DMF at 130 °C afforded their carbamate derivatives, (−)-
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PMDI-2NHBoc and (−)-2NDI-2NHBoc, respectively, in good yields. Removal of the Boc
groups using trifluoroacetic acid (TFA) gave the corresponding diamines, (−)-2PMDI-2NH and
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(−)-2NDI-2NH . The cyclocondensation of these diamines with 1 mol equiv of PDA in the
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presence of Zn(OAc) in molten imidazole at 140 °C afforded (Figure 1) the desired highly rigid
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isosceles triangles (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ in 31 and 22% yields, respectively.
See the Supporting Information for more details.
+
Detection of peaks at m/z = 1063.2917 and 1163.3213 in the gas phase for [M + H] ions
by electrospray ionization high-resolution mass spectrometry (ESI-HRMS) confirmed the
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existence of both (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ, respectively. Both the H and C
NMR spectra (Figure 2 and Figures S1-S5) of the isosceles triangles (−)-2PMDI-1PDI-Δ and (−)-
2
NDI-1PDI-Δ were in agreement with them having rigid cyclic structures with lower symmetry
(
C point group) when compared to the higher symmetry (D point group) of the equilateral
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triangles, (−)-3NDI-Δ and (−)-3PMDI-Δ. The assignments of all of the resonances corresponding
to aromatic, methine and methylene protons of the triangles were confirmed by two-dimensional
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H‒ H and H‒ C correlation spectroscopies. In particular, the H NMR spectrum of (−)-2PMDI-
1PDI-Δ shows (Figure 2 and Figure S1) three sets of signals for the eight PDI protons, a sharp
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singlet for the four PMDI protons and three sets of signals for the six methine protons, while that
of (−)-2NDI-1PDI-Δ shows (Figure 2 and Figure S3) three sets of signals for the eight PDI protons,
two sets of signals for the eight NDI protons and two sets of signals for the six methine protons.
Furthermore, the conformational rigidity of these isosceles triangles could be established
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by dynamic H NMR experiments by probing the rates of rotation of the aromatic subunits around
their C–N•••N–C bond axes. Based on our previous observations,54-55 we thought it would be
unlikely to surmount the high free energy of activation necessary for the rotation of the bulky NDI
1
and PDI subunits around their C–N•••N–C bond axes within the triangles on the H NMR time
scale. In any event, the accidental chemical shifts of the resonances for the heterotopic PMDI
protons of (−)-2PMDI-1PDI-Δ, affording a sharp singlet (Figure S1) even at room temperature,
restricted our attempts (Figure S6) to probe the rates of rotation of the PMDI subunits with
increasing temperature. The downfield shift of the resonances observed for the aromatic protons
of (−)-2PMDI-1PDI-Δ (Figure S6) and (−)-2NDI-1PDI-Δ (Figure S7) can be attributed to the
weakening of the π-stacking intermolecular interactions between the aromatic subunits upon
increasing the temperature.
Quantum mechanical (QM) calculations. In order to investigate the conformational rigidity of
the aromatic subunits within the isosceles triangles, we carried out quantum mechanical
calculations (Figures S8 and S9) to map the potential energy surface as a function of the rotational
barriers of the dihedral angle ∠(H–C–N–C) for all the aromatic PDI, NDI and PMDI subunits
(Figure S9) present in all the compounds. In the case of Ref-PDI, the activation barrier to the
−1
rotation of PDI is only 6.85 kcal mol . This barrier increases dramatically to 25.4 and 25.1 kcal
−
1
mol for (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ, respectively. This observation suggests that
their rigid cyclic structures restrict the free intramolecular rotation of the PDI components.
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After investigating the rigidity of the isosceles triangles, we conducted thermogravimetric
analyses under a nitrogen atmosphere in order to determine their thermal stabilities. Although an
initial mass loss of up to 4% below 250 °C was observed (Figure S12) for (−)-2PMDI-1PDI-Δ
and (−)-2NDI-1PDI-Δ, when compared to that of Ref-PDI, the triangles exhibited high thermal
stability up to 410 °C whereupon they began to decompose. Moreover, variable-temperature
powder X-ray diffraction (VT-PXRD) studies were performed on the as-synthesized powder
samples of the isosceles triangles to obtain insights into their thermal stability, crystallinity and
phase purity. The PXRD patterns of (−)-2PMDI-1PDI-Δ at room temperature indicate (Figure
S10) that it is semi-crystalline, while the sharpening of peaks with the rise in temperature up to
443 K suggests its increased crystallinity. The original degree of crystallinity is, however, obtained
upon subsequent cooling to room temperature. On the other hand, PXRD patterns of (−)-2NDI-
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PDI-Δ at room temperature reveal (Figure S11) its crystalline structure which is, not only
thermally stable up to 473 K, but also retains the same crystalline structure upon subsequent
cooling to room temperature. In addition to their superior structural and conformational rigidity,
the remarkable thermal stability of (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ is promising for
practical applications in organic electronic devices.
Single-crystal X-ray diffraction (XRD) analyses. Single-crystal X-ray diffraction analyses were
carried out in order to gain insights into structural details and packing arrangements of these rigid
isosceles triangular macrocycles. Single crystals of (−)-2PMDI-1PDI-Δ were obtained by slow
vapor diffusion of n-hexane into a 3 mM solution in 1,2-dichloroethane (DCE) over the course of
3
days, while single crystals of (−)-2NDI-1PDI-Δ were obtained by slow evaporation of a 6 mM
solution in CHCl over the course of 7 days. The crystal structures of both (−)-2PMDI-1PDI-Δ
3
and (−)-2NDI-1PDI-Δ reveal (Figure 3a and b) the strained rigid geometries of the isosceles
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triangular hollow prisms which are characterized by similar vertex angles (Figure 3c and e) of
~88° — a value which is much greater than the 60° observed for the equilateral triangle (−)-3NDI-
Δ50-51, 54 — as a result of the stretching of the long and curved PDI subunits. On account of the fact
that the PMDI units (9.6 Å) are slightly shorter than the NDI ones (10.0 Å), the base-to-apex
distance (8.8 Å) of (−)-2PMDI-1PDI-Δ is shorter than that (9.5 Å) of (−)-2NDI-1PDI-Δ. The
angle (102°) between the PDI and the triangular planes in (−)-2PMDI-1PDI-Δ (Figure 3d) is larger
than that (86°) of (−)-2NDI-1PDI-Δ (Figure 3f) as a result of the less bulky PMDI when compared
with the NDI units. In addition, the PDI plane of (−)-2PMDI-1PDI-Δ is more bent than the PDI
plane of (−)-2NDI-1PDI-Δ. These observations indicate that the triangular structure of (−)-
2PMDI-1PDI-Δ is relatively more strained than that of (−)-2NDI-1PDI-Δ. In both crystal
superstructures, every two (−)-2PMDI-1PDI-Δ and every two (−)-2NDI-1PDI-Δ form (Figure 3c-
f) nearly face-to-face π‒π dimers by means of π‒π stacking interactions (~3.4 Å) between nearly
parallel PDI units, respectively. Although both PDI units in the π‒π dimer of (−)-2PMDI-1PDI-Δ
are almost parallel, a longitudinal offset of 0.6 Å and a transverse offset of 1.3 Å are present (Figure
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c and d and see supplementary movies of the π‒π dimers) between the two overlaying PDI units.
By contrast, in the π‒π dimer of (−)-2NDI-1PDI-Δ, the longitudinal (4.0 Å) and transverse (2.0
Å) offsets of two PDI units are much greater (Figure 3e and f), an observation which indicates that
the π‒π stacking interactions between PDI units of (−)-2PMDI-1PDI-Δ are more efficient than
those in (−)-2NDI-1PDI-Δ. This result can be ascribed to the fact that the more bent PDI units of
(−)-2PMDI-1PDI-Δ results in them exposing larger π-surfaces for π‒π stacking while at the same
time weakening the steric barriers from the cyclohexano groups and thus favor the overlap of the
larger areas provided by the two PDI units between adjacent (−)-2PMDI-1PDI-Δ. In the extended
superstructures, every four π‒π dimers of (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ pack in
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different orientations in the presence of solvent molecules within a unit cell. Notably, no
significant noncovalent bonding interactions were found; namely, π‒π stacking and [C‒H∙∙∙O]
interactions between PMDI/PMDI, PMDI/PDI, NDI/NDI, or NDI/PDI pairs which are very
commonly observed previously in the cases of (−)-3NDI-Δ,50-51, 54 (−)-1PMDI-2NDI-Δ, and (−)-
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PMDI-1NDI-Δ triangles . In addition, the X-ray structures reveal that the nanoporous cavities
of both isosceles triangles are indeed encapsulated by the solvent molecules as evidenced by (−)-
PMDI-1PDI-Δ forming (Figure S14) a 2:1 host-guest complex with n–hexane as a result of
multiple [C−H···π] interactions, while CHCl molecules are bound (Figure S15) to the cavities of
2
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(−)-2NDI-1PDI-Δ stabilized by multiple [Cl···π] interactions (∼3.4 Å) with the π-surfaces of NDI
and PDI subunits.
64
On the other hand, Hirshfeld surface analyses performed on the structures of the two
isosceles triangles confirmed (Figure S18) that the reciprocal [π···π] interactions, which contribute
about 12.2 and 12.1% to the Hirshfeld surfaces of (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ,
respectively, are the most significant interactions between the PDI units in the case of both π‒π
dimers. In order to assess the robustness of such discrete nearly cofacial π···π dimer packing motifs,
several attempts have been made to crystallize these triangles from a range of different solvents.
Although single crystals suitable for XRD analysis could not be obtained in most cases, (−)-
2
PMDI-1PDI-Δ did crystallize from the slow vapor diffusion of n-hexane into a 3 mM solution
of (−)-2PMDI-1PDI-Δ in CHCl over the course of several days. Single-crystal XRD analysis
3
reveals (Figure S16) that (−)-2PMDI-1PDI-Δ also forms discrete nearly cofacial PDI-PDI π-
dimers which exhibit identical packing arrangements and the unit cell parameters to those observed
for the DCE/n-hexane system. See Figure 3 and details of the crystallographic characterization in
the Supporting Information. Also, in order to understand the role of solvents on the formation of
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such discrete PDI-PDI π-dimers in the solid state, solvent-free single crystals of (−)-2PMDI-
1
PDI-Δ, suitable for XRD analysis, were obtained by air drying the single crystals grown by slow
vapor diffusion of n-hexane in CHCl solution. It was observed (Figure S17) that the discrete PDI-
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PDI π-dimers remained intact, with no change in the unit cell parameters before and after the
evaporation of CHCl from the single crystals. Based on all of these observations, it can be
3
concluded that the geometries of the triangular hollow prisms of (−)-2PMDI-1PDI-Δ and (−)-
2
NDI-1PDI-Δ facilitate specifically the formation only of the discrete nearly cofacial π‒π dimers
involving two PDI units by preventing the PDI-PDI π-dimers from further long-range π‒π stacking
6
5
and aggregation, which otherwise occurs very easily in most of the PDI derivatives. Therefore,
we anticipated that the presence of the geometrically protected rigid discrete PDI-PDI π-dimers of
the triangles may be reflected in their solid-state photoluminescence properties.
Photophysical studies in solution. Motivated by the variation in the structural properties and the
unusual packing arrangements of the rigid isosceles triangles in the solid state, as evidenced by
single-crystal XRD (Figure 3), we set out to investigate (i) the optical properties of all three
compounds by steady-state UV/Vis absorption and fluorescence spectroscopies (Table 1 and
Figure 4), and (ii) the chiroptical behavior of the isosceles triangles by CD spectroscopy in solution.
The absorption spectra (Figure 4a, b and c) of all three compounds recorded in CH Cl displayed
2
2
three well-defined vibronic bands with maxima between 456‒459, 487‒490 and 524‒527 nm,
corresponding to the characteristic S  S electronic transition of the PDI derivatives. This
1
0
observation suggests that the imide substituents of PDI subunits present in all three compounds
have negligible impact on their electronic transitions. Additionally, the absorption spectra (Figure
4
c) of (−)-2NDI-1PDI-Δ displayed two vibronic progressions, centered on 360 and 380 nm,
corresponding to the characteristic S  S electronic transition of the NDI subunits. The molar
1
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extinction coefficients of both the isosceles triangles are smaller (Table 1) than that of Ref-PDI.
Consequently, the CD spectra of both chiral isosceles triangles displayed (Figure S13) prominent
negative exciton Cotton effects in the 225‒250, 350‒400 and 450‒550 nm regions, corresponding
to the electronic transitions of the PMDI, NDI and PDI subunits, respectively, where the sign of
the peaks is consistent with the absolute (RRRRRR)-configuration of the triangles. The
fluorescence spectra (Figure 4a, b and c) of all three compounds showed similar monomeric
emission bands with mirror-image vibronic patterns to their absorption spectra. The fluorescence
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quantum yield measured in CH Cl relative to that of Ref-PDI (Φ = 100%) is almost unity (Φ
f
2
2
f
=
100%) for (−)-2PMDI-1PDI-Δ, while that for (−)-2NDI-1PDI-Δ is only slightly lower (Φ =
f
88%). Similarly, all three compounds also exhibited excellent fluorescence quantum yields (Φ ~
f
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0-100%) in other organic solvents, such as MeCN and PhMe, except the partially quenched
fluorescence of (−)-2NDI-1PDI-Δ (Φf, PhMe = 63%) in PhMe which can be attributed to
aggregation-caused quenching.
The excitation spectra (Figure 4a, b and c) of all three compounds match well with their
absorption spectra, suggesting that the emission arises from only one excited species. The
excitation spectrum (Figure 4c) of (−)-2NDI-1PDI-Δ exhibited, however, additional peaks in the
region 350‒390 nm which match perfectly with the vibronic progressions corresponding to the
electronic transition localized on the NDI subunits in its absorption spectrum. This observation
suggests that there is an efficient intramolecular energy transfer from the lowest excited singlet
state of NDI to that of the PDI subunit within the isosceles triangle (−)-2NDI-1PDI-Δ upon
photoexcitation of the NDI subunits (Figure S19). Furthermore, the time-resolved fluorescence
spectra (Figure 3d) of all three compounds in CH Cl displayed mono-exponential decay curves
2
2
with similar lifetimes <τ > of 4.0, 4.5 and 5.0 ns for Ref-PDI, (−)-2PMDI-1PDI-Δ and (−)-
em
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NDI-1PDI-Δ, respectively. Additional insights into the excited state dynamics of all three
compounds were obtained (Figures S20‒S22) using femtosecond transient absorption
spectroscopy (fsTA). An excitation wavelength of 493 nm was chosen, which is resonant with the
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← 1 vibronic band for all three samples. For Ref-PDI, excited-state PDI is formed (Figure
S20a) within the instrument response time, as characterized by positive bands around 350, 700 and
00 nm, which can be assigned to the excited-state absorption (ESA). Meanwhile, negative bands,
which are observed at 456, 486, 525, 575 and 615 nm, can be ascribed to the ground state bleach
GSB) and stimulated emission (SE) based on the steady-state measurements. In the subsequent 7
9
(
ns, all transient features decay back to the ground state simultaneously. Global fits to the fsTA data
with a species-associated model gave two time constants values of 124 ± 2 ps and 3.50 ± 0.03 ns,
which can be assigned (Figure S20) to conformational relaxation and decay of the first singlet
excited state S to the ground state S , respectively. Interestingly, the two isosceles triangles
1
0
exhibited (Figures S21 and S22) similar excited state dynamics to those of Ref-PDI. The observed
conformational relaxation timescales are 181 ± 6 and 58 ± 2 ps, and the singlet lifetimes are 3.92
±
0.05 and 3.44 ± 0.04 ns, for (−)-2PMDI-1PDI-Δ (Figure S21) and (−)-2NDI-1PDI-Δ (Figure
S22), respectively. All of these observations reflect that the functionalization of the PDI subunit
with chiral cyclic N-substituents does not have any significant effect on the excited state dynamics
of the PDIs in solution.
Photophysical studies in the solid state. In contrast with the optical properties observed in
solution, the optical properties of all three compounds in the solid state differ (Table 2) from one
another. The UV/Vis absorption spectra (Figure 5a-c) of thin films and diffuse reflectance spectra
(Figure S23) of powder samples were recorded for all three compounds. The UV/Vis absorption
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spectrum of Ref-PDI displays (Figure 5a) typical signatures of excitonically coupled aromatic
subunits which can be attributed to strong - overlap between the closely arranged neighboring
molecules. The UV/Vis absorption spectra of (−)-2PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ,
however, show (Figure 5b and c) well-resolved vibronic patterns similar to those found in solution,
indicative of only weak interactions between the aromatic subunits in the solid state. It is
noteworthy that the intermolecular π-π stacking between the neighboring molecules in the case of
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Ref-PDI is consecutive and uninterrupted in the solid state, while the π-π stacking in the case of
both the isosceles triangles is suppressed, as a result of the formation of discrete π-dimers lacking
any additional long-range noncovalent bonding interactions, on account of their unique rigid
triangular geometries as evidenced (Figure 3) by their solid-state (super)structures.
Unlike the excellent fluorescence properties observed in solution, the photoluminescence
(PL) of all three compounds led (Table 2 and Figure 5) to aggregation-caused quenching in the
solid state. The PL spectra (Figure 5a, b and c) of all three compounds displayed weak, red-shifted
excimer emission bands at 652, 670 and 602 nm for Ref-PDI, (−)-2PMDI-1PDI-Δ and (−)-2NDI-
1
PDI-Δ, respectively. This variation in the emission maxima of all three compounds reflects the
difference in electronic coupling between the two PDIs in the solid-state structure that leads to
excimer state stabilization. The greater - overlap of the PDI molecules observed in the crystal
structure of (−)-2PMDI-1PDI-Δ relative to that of (−)-2NDI-1PDI-Δ is consistent with the larger
red shift of the excimer emission from (−)-2PMDI-1PDI-Δ. As a consequence of long, continuous

- stacking between neighboring Ref-PDI molecules, negligible photoluminescence quantum
yields are observed for Ref-PDI, in both powder form (Φpowder = 0.1%) and thin film (Φfilm = 0.2%).
In contrast, the photoluminescence behavior is somewhat improved (Figure S24) for the isosceles
triangles but the quantum yields are still low in the solid state for (−)-2PMDI-1PDI-Δ (Φpowder
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% and Φfilm = 2%) and (−)-2NDI-1PDI-Δ (Φpowder = 4% and Φfilm = 2%). The photographs (Figure
e, f and g) of all three compounds in powder form, as well as drop-casting films coated on a glass
substrate, taken under 365 nm light irradiation reveal that (−)-2PMDI-1PDI-Δ and (−)-2NDI-
PDI-Δ exhibit red and orange emissions, respectively, while the emission of Ref-PDI is quenched
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almost completely. Although the X-ray diffraction patterns obtained using the powder (Figures
S10 and S11) and thin film (Figure S25) samples of both the isosceles triangles at room
temperature may suggest the peaks corresponding to intermolecular π-π stacking within the range
of 3.2 to 3.7 Å between the aromatic units, this level of information was not sufficient to determine
and compare specific molecular packing arrangements in the bulk samples with those observed
(Figure 3) by single-crystal XRD. It should be noted that the single-crystal XRD analyses reveal
the random orientation of the discrete PDI-PDI π-dimers throughout the (super)structures in the
case of both isosceles triangles and thus, we suppose it may be challenging to experimentally
observe the XRD peaks characteristic of packing arrangements associated with π-π stacking in the
powder or thin film samples. Based on the presence and the robustness of the nearly cofacial PDI
π-π dimer packing motifs of the isosceles triangles in the presence of different solvents (Figure 3
and Figures S14-S16) and even under solvent-free conditions (Figure S17), we believe that the
improvement in the photoluminescence quantum yields of (−)-2PMDI-1PDI-Δ and (−)-2NDI-
1
PDI-Δ — by about 10 to 40 times that of Ref-PDI — in the solid state is, however, a consequence
of suppressing the global π-π stacking interactions defined by their unique structurally rigid
triangular geometries.
The solid-state excitation spectra of all three compounds were recorded (Figure 5d) in
powder form. Unlike the excitation spectra of Ref-PDI and (−)-2PMDI-1PDI-Δ, the presence of
NDI vibronic patterns in the region 340‒390 nm for (−)-2NDI-1PDI-Δ suggests that efficient
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energy transfer occurs between the excited singlet state of NDI and that of the PDI subunit upon
photoexcitation of the NDI subunits, even in the solid state. The excimer fluorescence of Ref-PDI
and (−)-2NDI-1PDI-Δ in the solid state display (Figure S26a and S26c) multi-exponential decays
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with average lifetimes of <τ > = 1.5 and 3.0 ns, respectively, while the corresponding excimer
em
fluorescence decay of (−)-2PMDI-1PDI-Δ exhibits (Figure S26b) a longer multi-exponential
decay with an average lifetime of <τ > = 13.4 ns, which again is consistent with the more highly
em
constrained cofacial geometry of the PDI-PDI π-dimers in (−)-2PMDI-1PDI-Δ.
Moreover, fsTA was likewise applied to film samples to interrogate the excited state
dynamics. While the poor solubility of Ref-PDI and (−)-2NDI-1PDI-Δ in CH Cl prevented our
2
2
attempts to prepare their homogeneous thin film samples, a suitable drop-casting film sample of
−)-2PMDI-1PDI-Δ was prepared. It is evident that the spectra at the early time delays correspond
(
to the S state (Figure 6a). Significantly, after about 3 ps, the peaks in the 600‒800 nm ESA region
1
are flattened, which is a signature67-68 of excimer formation. All transient signals then decay back
to S in 7 ns. Global fits to the fsTA data with a species-associated model gave (Figure 6b) three
0
time constant values of 0.8 ± 0.3, 16.2 ± 0.3 and 382 ± 8 ps, respectively. Figures 6c and 6d
illustrate the corresponding multiple-wavelength fits and populations of kinetic states of the
species, respectively. The first timescale can be readily attributed to excimer formation from the
PDI S state, whereas the latter two timescales correspond to excimer decay back to S .
1
0
Frontier molecular orbitals of isosceles triangles. In order to obtain deeper insight into the
electronic properties of these fluorescent isosceles triangles, we performed density functional
theory (DFT) quantum mechanics (QM) calculations at the M06-2X level of theory using the 6-
3
11G(d,p) basis set and including Poisson Boltzmann Continuum Solvation with CH Cl as the
2 2
solvent. All the calculations (Figure 7) include post-stage D3 van der Waals corrections.69-70 Our
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calculations on (−)-2PMDI-1PDI-Δ reveal that the HOMO−1 and HOMO−2 are primarily located
(
Figure 7a and b) at the diimide-based junctions of the cyclohexano linkers and the PDI subunit,
while the LUMO and HOMO are located only on the PDI subunit (Figure 7c and d). In the case of
−)-2NDI-1PDI-Δ, both HOMO−1 and HOMO−2 are located (Figure 7e and f) on the two NDI
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(
subunits. On the other hand, the HOMO of (−)-2NDI-1PDI-Δ is located on the PDI subunit, while
the LUMO is located mostly on the PDI subunit but with partial delocalization onto the two NDI
subunits (Figure 7g and h). The electronic transitions for both the isosceles triangles were
calculated using a tight-binding approximation of time-dependent density functional theory (TD-
DFTB). Additionally, the simulated UV/Vis spectra (Figure S27) show that the S  S electronic
1
0
transitions around 560 nm correspond to transitions from the HOMO to LUMO located on the PDI
subunits. Additionally, the graphical representations obtained from our calculations (Figure 7i and
j) show that the PDI planes of both isosceles triangles are significantly curved when compared to
the fully relaxed PDI component of Ref-PDI, presumably as a consequence of rigidity and strain
−1
(
∆E ~ 11.8 kcal mol ) associated with their cyclic triangular geometries, an observation which is
consistent (Figure 3) with the single-crystal XRD data.
Cyclic voltammetry (CV). In order to evaluate the potential applicability of the isosceles triangles
in optoelectronic devices, we investigated the electrochemical characteristics of (−)-2PMDI-
1
PDI-Δ and (−)-2NDI-1PDI-Δ by CV (Figure 8 and Figure S28), and compared these results with
those of the monomeric reference compounds, Ref-PMDI, Ref-NDI and Ref-PDI. The CVs of
the monomeric reference compounds in CH Cl exhibited two distinct reversible one-electron
2
2
redox waves with half-wave potentials at (i) −925 and −1527 mV vs Ag/AgCl for Ref-PMDI, (ii)
709 and −1131 mV for Ref-NDI, and (iii) –605 and −794 mV for Ref-PDI, corresponding to the
–
formation of radical anions and dianions, respectively. Strikingly, the CV of (−)-2PMDI-1PDI-Δ
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displayed (Figure 8) four distinct redox processes involving a total of six-electrons — namely, (i)
two sequential and distinct reversible one-electron waves at −566 and –832 mV, corresponding to
•−
2−
the formation of PDI and PDI , respectively, (ii) a reversible two-electron wave at –1008 mV,
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•−
corresponding to the formation of two PMDI radical anions and (iii) finally, a quasi-reversible
2
−
two-electron process at –1764 mV, corresponding to the formation of two PMDI dianions. In
addition, the CV of (−)-2NDI-1PDI-Δ exhibited (Figure 8) five distinct redox processes involving
six electrons in total — namely, (i) a reversible one-electron wave at −572 mV, corresponding to
•−
the formation of a singly reduced PDI radical anion, (ii) a reversible two-electron wave at –749
•−
mV, corresponding to the formation of two singly reduced NDI species, (iii) a reversible one-
2
−
electron wave at −960 mV, corresponding to the formation of PDI , and (iv) finally, two
reversible one-electron waves at –1331 and –1479 mV, corresponding to the formation of two
2
−
NDI dianions. The number of electrons involved in each of these redox processes is confirmed
by differential pulse voltammetry (DPV, Figure S29). It is worth noting that the unambiguous
assignment of the peaks corresponding to the mono radical anionic states of all PDI and NDI
subunits present in (−)-2NDI-1PDI-Δ was confirmed by spectroelectrochemistry (Figure S30). In
addition, all PDI-containing compounds can also be oxidized quasi-reversibly (Figure S28) to their
corresponding radical cationic states at around +1700 mV. Thus, the availability of multiple easily
accessible reversible redox states for both isosceles triangles demonstrates their potential
applicability as electron accumulation materials.
EPR and ENDOR spectroscopies. Since the CV data (Figure 8) indicated that the monoradical
states of PDIs could be accessed much more easily than those of the NDIs or PMDIs, we confirmed
this trend by EPR and ENDOR spectroscopies. We also investigated whether the unpaired electron
of the PDI subunit is shared among the adjacent PMDI or NDI subunits within the isosceles
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•−
triangles. While the EPR spectrum (Figure S31a) of [(−)-2PMDI-1PDI-Δ] is nearly identical to
•−
that of [Ref-PDI] with similar spectral widths and number of lines, the EPR spectrum (Figure
•−
•−
S31a) of [(−)-2NDI-1PDI-Δ] is narrow compared to that of [Ref-PDI] , corresponding to a
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decrease in the magnitude of the nuclear hyperfine interactions. These results indicate that the
•−
unpaired electron is localized only on the PDI subunits in the case of [(−)-2PMDI-1PDI-Δ] but
•−
partially shared with the adjacent NDI subunits in the case of [(−)-2NDI-1PDI-Δ] . Similarly, the
ENDOR spectrum (Figure S31b) of [(−)-2PMDI-1PDI-Δ]•− has equal isotropic hyperfine
•−
coupling constants (a ) compared to that of [Ref-PDI] , once again indicating that the unpaired
H
electron is isolated within the PDI subunits. On the other hand, the ENDOR spectrum (Figure S31b)
•−
of [(−)-2NDI-1PDI-Δ] exhibits a decrease in its hyperfine coupling constant by about 15%
•−
•−
compared to those of [Ref-PDI] and [(−)-2PMDI-1PDI-Δ] , suggesting a small degree of
7
−1
electron sharing with the adjacent NDI subunits on the time scale (>10 s ) of the electron-nuclear
hyperfine interaction. All these observations indicate strongly that (i) the sequential reductions of
the PDI subunit of (−)-2PMDI-1PDI-Δ into its monoradical and the dianionic states can be easily
accessed without being interrupted by its cyclic N-substituents, and (ii) the unpaired electron in
•−
[
(−)-2PMDI-1PDI-Δ] is completely localized on the PDI subunit in a fashion similar to that of
monomeric Ref-PDI.
■
CONCLUSIONS
In summary, we have demonstrated the synthesis of two isosceles triangles—namely, (−)-2PMDI-
PDI-Δ and (−)-2NDI-1PDI-Δ—based on a one of the first of its kind designs, wherein one large
1
and two small planar π-conjugated aromatic diimides are introduced into rigid chiral cyclic
structures, incorporating three (RR)-trans-1,2-cyclohexanediamines. The two PDI-based isosceles
triangles have rigid geometries with lower symmetries (C point groups), relative to those (D point
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groups) of the equilateral triangles [(−)-3NDI-Δ and (−)-3PMDI-Δ], as evidenced by the expected
differences in their H and 13C NMR spectra. Their solid-state (super)structures show that
geometrically protected PDI fluorophores of the isosceles triangles can only undergo
intermolecular PDI-PDI π-stacking to form dimers because of the absence of any additional long-
range noncovalent bonding interactions. Quantum mechanical calculations reveal that both the
isosceles triangles consist of (i) conformationally rigid structures as indicated by their high
intramolecular rotational barriers and (ii) significantly curved PDI planes relative to the fully
relaxed PDI plane of Ref-PDI. This unusual formation and packing arrangement, associated with
the molecular rigidity, of the isolated PDI-PDI π-dimers of the isosceles triangles has a significant
influence on their photoluminescence properties in the solid state. The solid-state
photoluminescence quantum yields observed for the excimer states of (−)-2PMDI-1PDI-Δ and
1
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(−)-2NDI-1PDI-Δ are about 10 to 40-fold larger compared with that of Ref-PDI. The fluorescence
emission spectra also suggest that efficient intramolecular energy transfer occurs between the
adjacent NDI and PDI subunits of (−)-2NDI-1PDI-Δ. Such variations in the photophysical
properties observed between the monomeric reference compound and the two isosceles triangles
could form a basis for the rational design of highly efficient fluorescent organic materials for
applications in optoelectronic and photonic devices. Also, both (−)-2PMDI-1PDI-Δ and (−)-
2
NDI-1PDI-Δ are chiral molecules with strong fluorescence emissions, and hence they would be
2
3
expected to exhibit circularly polarized luminescence (CPL). Moreover, the electrochemical
properties investigated by CV indicate that Ref-PDI can only produce two redox states, while (−)-
2
PMDI-1PDI-Δ and (−)-2NDI-1PDI-Δ produce multiple easily accessible redox states,
suggesting their potential use as electron accumulation or transport materials. The EPR and
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ENDOR spectra show that the unpaired electron in (−)-2PMDI-1PDI-Δ is localized on the PDI
subunit, while it is partially shared between the NDI and PDI subunits in (−)-2NDI-1PDI-Δ.
Based on this strategy of synthesizing PMDI/NDI/PDI cyclic trimers, it should be possible to
prepare higher-order macrocyclic oligomers by introducing various other redox-active functional
aromatic diimides, including anthracene diimides (ADIs), coronene diimides (CDIs), terrylene
diimides (TDIs) and quaterrylene diimides (QDIs). The general features of this synthetic strategy
are to make use of smaller aromatic diimides, such as PMDIs and NDIs, as solubilizers so as to
incorporate larger insoluble aromatic diimides, without any bulky substituents, easily into these
cyclic systems exhibiting multi-functional structural, optical, electronic and magnetic properties
associated with their chirality, rigidity, accessible cavities, through-space electron sharing and
several readily available redox states. This approach may hold great promise for the design and
synthesis of new active materials, as well as establishing their structure-performance relationships,
in organic optoelectronics, energy storage and energy harvesting devices.
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■
ASSOCIATED CONTENT
Supporting Information
Materials and methods, synthetic procedures and characterization, NMR spectroscopic data,
PXRD and thin film XRD data, photophysical data, computational details and supportive figures.
This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
m-wasielewski@northwestern.edu; stoddart@northwestern.edu
Notes
*
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The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
■
We would like to thank Yilei Wu for his assistance with spectroscopy experiments, Charlotte L.
Stern for performing single-crystal XRD studies and Sumit Kewalramani for performing thin film
XRD. This research is a part of the Joint Center of Excellence in Integrated Nano-Systems (JCIN)
at King Abdulaziz City for Science and Technology (KACST) and Northwestern University (NU).
The authors would like to thank both KACST, NU, Tianjin University and the University of New
South Wales for their continued support of this research. This work was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-
FG02-99ER14999 (M.R.W.). The quantum mechanics (QM) calculations used the resources of the
Extreme Science and Engineering Discovery Environment (XSEDE) which is supported by
National Science Foundation grant number ACI-1053575.
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