Tetrahedral rigid core antenna chromophores bearing bay-substituted perylenediimides

Article abstract of DOI:10.1016/j.tet.2015.10.083

Two new representative methane- and adamantane-centered 'antenna' tetramers bearing bay-substituted π-conjugated phenylethynyl-perylenediimides (PDICCPh) as chromophoric subunits, tetrakis-[1-(4-ethynylphenyl)-N,N′-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide]-methane (1) and tetrakis-1,3,5,7-[1-(4-ethynylphenyl)-N,N′-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide]-adamantane (2), have been synthesized and their structural aspects have been thoroughly investigated by NMR spectroscopy. These PDI tetramers (1 and 2) represent the first successful example of incorporating the bay-substituted phenylethynyl-perylenediimides into the large rigid core tetrahedral frameworks. In these PDI tetramers, dynamic NMR experiments revealed the existence of perylene-centered conformational dynamic equilibrium (ΔG^≠=15-17 kcal/mol), the primary cause of the observed spectral broadening in conventional ¹H NMR spectra (295 K). In addition, PDI tetramers 1 and 2 were found to possess exceptional (photo)chemical stability, and their corresponding photophysical properties (ε_max～180,000; τ_FL=6.9 ns; Φ_FL～60%) make them viable candidates for various photonic applications and are in good agreement with other related multichromophoric PDI-based systems.

Full text of DOI:10.1016/j.tet.2015.10.083

Tetrahedron 71 (2015) 9519e9527
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tetrahedral rigid core antenna chromophores bearing
bay-substituted perylenediimides
*
Mykhaylo Myahkostupov, Felix N. Castellano
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new representative methane- and adamantane-centered ‘antenna’ tetramers bearing bay-
Received 4 September 2015
Received in revised form 27 October 2015
Accepted 29 October 2015
Available online 2 November 2015
substituted
p-conjugated phenylethynyl-perylenediimides (PDICCPh) as chromophoric subunits, tetra-
kis-[1-(4-ethynylphenyl)-N,N0-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide]-methane
(1) and tetrakis-1,3,5,7-[1-(4-ethynylphenyl)-N,N0-bis(1-hexylheptyl)-perylene-3,4:9,10-tetracarboxylic
diimide]-adamantane (2), have been synthesized and their structural aspects have been thoroughly
investigated by NMR spectroscopy. These PDI tetramers (1 and 2) represent the first successful example
of incorporating the bay-substituted phenylethynyl-perylenediimides into the large rigid core tetrahe-
dral frameworks. In these PDI tetramers, dynamic NMR experiments revealed the existence of perylene-
Keywords:
Perylenediimide
Rigid core chromophores
Sonogashira coupling
Dynamic NMR
centered conformational dynamic equilibrium (
D
Gs¼15ꢀ17 kcal/mol), the primary cause of the ob-
served spectral broadening in conventional 1H NMR spectra (295 K). In addition, PDI tetramers 1 and 2
were found to possess exceptional (photo)chemical stability, and their corresponding photophysical
Conformational exchange
properties (
3
maxw180,000; sFL¼6.9 ns;
FFLw60%) make them viable candidates for various photonic
applications and are in good agreement with other related multichromophoric PDI-based systems.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
conjugation in the bay region as building blocks to construct three-
dimensional, electronically coupled molecular systems (tetrameric
Over the years, perylenediimide-based (PDI) chromophoric sys-
tems have been the subject of immense research efforts thanks to
their unique combination of chemical, photochemical, and photo-
physical properties.1e6 Multichromophoric systems incorporating
a broad spectrum of PDI molecules proved to be excellent models for
exploring fundamental and applied aspects of diverse photonic,
electronic, and photovoltaic materials.7e16 In addition, various
carefully designed PDI-based molecular compositions have been
utilized as probes of photoinduced ultrafast processes (e.g., those
occurring in natural photosynthetic systems), thus leading to a bet-
ter understanding of underlying quantum coherence, excitation
energy flow and excitonic coupling effects.17e25 Similarly, molecular
PDI acceptor chromophores have been exploited in numerous
upconversion photochemistry compositions.10,12,26
‘antennas’) that would exhibit relatively strong intramolecular
electronic interactions and desirable photophysics as a result of
being held together by a tetrahedral rigid core. In this particular
case, the extent of intramolecular electronic coupling can be ma-
nipulated by varying the size of the rigid core: methane vs. ada-
mantane. In addition, as prototype photochemically robust antenna
chromophores and potentially viable alternatives to fullerenes,
these PDI tetramers may be of particular interest for photovoltaic
applications as well as in photochemical upconversion.7,30e32
Noteworthy, the rigid cores of this nature have been extensively
explored in a variety of visible light sensitizers (both pure organic
and transition metal-based) designed for the covalent surface
functionalization of wide bandgap semiconductor nanomaterials
(e.g., TiO2, ZnO).33e37
The synthetic accessibility of PDI functionalization chemistry
(either at the imide position or in the bay region) allows one to
construct multichromophoric assemblies with a desired set of
functional properties.8,27e29 The goal of the present work is to
PDI-based molecular systems are also well known to display
solvent-dependent aggregation behavior, thus making them per-
fect candidates for probing various energy and electron transfer
pathways in both aggregated and non-aggregated forms.38e42
However, we note that any detailed discussion of the associated
excited state photochemistry of synthesized PDI tetramers is well
beyond the scope of the present work and will be the subject of
a separate in-depth contribution. To date, there is only a handful of
(photophysical) studies on PDI-containing tetrahedral assemblies
utilize the highly emissive PDI chromophores with extended
p-
* Corresponding author. Tel.: þ1 919 515 3021; fax: þ1 919 515 8920; e-mail
address: fncastel@ncsu.edu (F.N. Castellano).
http://dx.doi.org/10.1016/j.tet.2015.10.083
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

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M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
and, in most cases, these PDI architectures have been synthesized
utilizing a relatively straightforward imide condensation chemistry
between the amine-functionalized rigid core(s) and the corre-
sponding perylene dianhydride(s).1,41,43 Moreover, the successful
examples of tetrameric structures incorporating the bay-
substituted PDIs have been quite scarce in the literature and the
few available reports are mainly based on studying the properties
of phenyl-PDI-containing structures.44,45 In our case, it is expected
that the incorporation of phenylethynyl-perylenediimides
frameworks. We also note that the adequate interpretation of the
associated NMR structural characterization data of these large PDI
assemblies can be quite challenging due to the substantial broad-
ening of 1H resonances (at RT) and is oftentimes overlooked in the
literature. In some cases, it was argued that oligomeric PDIs exhibit
such behavior as a result of aggregation-induced effects occurring
at the typical NMR sample concentrations (mM range).46 To fully
address these questions, dynamic NMR experiments as well as
concentration-dependent NMR studies have been performed to
understand the origin of observed spectral broadening associated
with single temperature (295 K) 1H NMR experiments in the target
PDI tetramers 1 and 2.29,47,48
(PDICCPh) bearing extended
p-conjugation into tetrahedral rigid
core structures would result in the substantial enhancement of
their corresponding light-harvesting properties in a broader spec-
tral window and thus may be advantageous in various photonic
applications as compared to other related PDI-based oligomeric
structures.
In this manuscript, we report the synthesis, NMR structural
characterization and photophysical properties of two representa-
tive methane- and adamantane-centered tetramers bearing bay-
2. Results and discussion
2.1. Synthesis
substituted
p
-conjugated
phenylethynyl-perylenediimides
The monomeric PDI precursors (PDI and PDIBr) were synthe-
sized via modified literature procedures and the representative
synthetic methodology is presented as Supplementary data in
Scheme S1.49,50 In general, the symmetric 1-hexylheptylamine
was obtained in 95% yield via a facile reductive amination of
commercially available dihexylketone in the presence of NaBH3CN.
In the next step, 1-hexylheptylamine was condensed under ambi-
ent conditions with commercially available 3,4,9,10-perylene-tet-
racarboxylic dianhydride (PDA) to obtain the corresponding
perylenediimide (PDI) in a nearly quantitative yield (95%). PDI was
then selectively brominated in the 1-position of its bay region with
elemental Br2 to afford 1-bromoperylenediimide (PDIBr) in 65%
yield. Subsequently, PDIBr was used as a departure point to gain
access to the target PDI tetramers 1 and 2 (Schemes 1 and 2), as well
as the model monomer 3 (Scheme S1). Monomer 3 was obtained
via a standard Sonogashira cross-coupling protocol of PDIBr with
(PDICCPh) as chromophoric subunits that also incorporate solubi-
lizing 1-hexylheptyl functionalities at the imide positions, tetrakis-
[1-(4-ethynylphenyl)-N,N0-bis(1-hexylheptyl)-perylene-3,4:9,10-
tetracarboxylic diimide]-methane (1) and tetrakis-1,3,5,7-[1-(4-
ethynylphenyl)-N,N0-bis(1-hexylheptyl)-perylene-3,4:9,10-
tetracarboxylic diimide]-adamantane (2). To delineate the elec-
tronic and structural effects associated with the tetrahedral as-
sembly of four PDICCPh subunits into a large tetrameric framework,
1 and 2 are compared to the model monomer 3, a single structural
subunit of each target PDI tetramer. The corresponding chemical
structures of the newly synthesized PDI tetramers 1 and 2, as well
as the model PDI monomer 3 are presented in Fig. 1. To the best of
our knowledge, this work represents the first example when the
bay-substituted phenylethynyl-perylenediimides (PDICCPh) have
been successfully incorporated into the large rigid core tetrameric
Fig. 1. Structures of tetrahedral rigid core perylenediimide (PDI) tetramers and a model monomer: methane-centered PDI tetramer 1 (red), adamantane-centered PDI tetramer 2
(blue), and PDICCPh 3 (green). R denotes a hexyl (C6H13) group.

M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
9521
Scheme 1. Synthetic route towards PDI tetramer 1. Reagents and conditions: (i) Br2; rt; 1 h; (ii) Sonogashira: TMSA; Pd(PPh3)4; CuI; (iPr)2NH; DCM (anh.); 60 ꢁC; 24 h; (iii) K2CO3
(aq); CHCl3/MeOH; rt; 24 h; (iv) Sonogashira: PDIBr (4.4 equiv); Pd(PPh3)4; CuI; (iPr)2NH; DCM (anh.); 65 ꢁC; 48 h. R denotes a hexyl (C6H13) group.
Scheme 2. Synthetic route towards PDI tetramer 2. Reagents and conditions: (i) benzene (anh.); AlCl3; tert-BuBr; reflux; 3 h; (ii) I2; BFIB; CHCl3; 40 ꢁC; 4 days; (iii) Sonogashira:
TMSA; Pd(PPh3)4; CuI; (iPr)2NH; DCM (anh.); 60 ꢁC; 24 h; (iv) K2CO3 (aq); CHCl3/MeOH; 40 ꢁC; 24 h; (v) Sonogashira: PDIBr (4.4 equiv); Pd(PPh3)4; CuI; (iPr)2NH; DCM (anh.); 65 ꢁC;
48 h. R denotes a hexyl (C6H13) group.
phenylacetylene (PhCCH) in 75% yield (44% overall) as a dark red
solid (see Supplementary data).51e53
tetrakis(4-bromophenyl)methane (M2) in 80%. In the next step, M2
was Sonogashira cross-coupled in a facile fashion with trime-
thylsilylacetylene (TMSA) using a standard reaction protocol to
obtain tetrakis(4-trimethylsilylethynylphenyl)methane (M3) in 82%
yield. M3 was then deprotected with aqueous K2CO3; however, the
removal of TMS group did not proceed very cleanly and typical
product yields of tetrakis(4-ethynylphenyl)methane (M4) were
around 50%. At this point, any other alternative deprotection
strategies, such as treating with TBAF, have not been explored. In
the final step, tetrakis(4-ethynylphenyl)methane core (M4) was
Sonogashira cross-coupled with slight excess of PDIBr (4.4 equiv)
to ensure the complete conversion into the tetrasubstituted target
PDI tetramer 1. Afterward, 1 was carefully purified via several
rounds of recrystallization (liquid diffusion) from MeOH/DCM¼3/1
and isolated in 64% yield (25% overall) as a dark red solid. We note
2.1.1. PDI tetramer 1. The synthetic transformation sequence to-
wards PDI tetramer 1 is shown in Scheme 1. The tetrahedral
methane-centered core with four
p-conjugated phenylethynyl
(CCPh) arms (M4) was constructed starting from commercially
available tetraphenylmethane (M1).54 The below described ap-
proach is advantageous over other possible synthetic strategies as it
allows one to avoid using perylenediimide-acetylene (PDICCH) in
final Sonogashira cross-coupling reaction steps and thus circum-
vents any possible formation of undesirable PDI homodimeric
products via Glazer-type coupling.55
In the first step, tetraphenylmethane (M1) was conveniently
brominated with elemental Br2 under ambient conditions to yield

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M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
that purification of PDI tetramers by column chromatography (sil-
ica gel) can be challenging as we previously experienced un-
controllable product degradation with other related PDI-based
oligomeric compounds.
to 1, thus affording the PDI tetramer 2 in 75% yield (10% overall) as
a dark red solid.
2.2. NMR structural characterization
Representative 1H NMR spectra (measured in CDCl3) of PDIBr,
PDI tetramers 1 and 2, and PDI monomer 3 are displayed in Fig. 2.
The corresponding atom-labeling scheme is presented in Fig. 3. The
assignment of 1H chemical shifts was based on the comprehensive
analysis of structurally related and previously reported acetylene-
bridged PDI dimers.29
2.1.2. PDI tetramer 2. The synthetic route towards PDI tetramer 2 is
presented in Scheme 2. Similar to PDI tetramer 1, we started from
constructing the tetrahedral adamantane-centered core bearing
four p-conjugated phenylethynyl (CCPh) arms (A5). In the first step,
1,3,5,7-tetraphenyladamantane (A2) was obtained as an insoluble
white solid in 95% yield from commercially available 1-
Fig. 2. Comparison of 1H NMR spectra (400 MHz; CDCl3; 295 K) for PDI tetramers 1 and 2, monomer 3 and PDIBr. For clarity, only the regions of interest are shown. All chemical
shifts are referenced to the residual solvent peak (CDCl3, 7.26 ppm). The representative chemical structures are shown in Fig. 1 and Supplementary data.
d
bromoadamantane (A1) following the procedure developed by
Reichert and Mathias.56 In the next step, A2 was converted into
1,3,5,7-tetrakis(4-iodophenyl)adamantane (A3) via treatment with
elemental I2 and [bis(trifluoroacetoxy)iodo]benzene (BFIB) in
chloroform.56 Due to the intrinsically heterogeneous nature of this
reaction, in our hands the yields of A3 never exceeded 35% and
usually required extended reaction times and moderate heating (4
days, 40 ꢁC). Apparently, this reaction step appears to be limiting in
terms of overall yield of PDI tetramer 2. Alternatively, we thought of
overcoming this issue by synthesizing 1,3,5,7-tetrakis(4-
bromophenyl)adamantane; however, all our attempts (reactions
with either elemental Br2 or Fe powder/Br2) always produced
complex, difficult to separate mixtures of products.57 In the next
step, 1,3,5,7-tetrakis(4-iodophenyl)adamantane (A3) was Sonoga-
shira cross-coupled with trimethylsilylacetylene (TMSA) to obtain
1,3,5,7-tetrakis(4-trimethylsilylethynylphenyl)adamantane (A4) in
80% yield. Subsequently, A4 was deprotected with aqueous K2CO3.
As discussed in the previous section, the deprotection of TMS group
typically produced the yields of 1,3,5,7-tetrakis(4-ethynylphenyl)
adamantane (A5) around 50%. In the final step, 1,3,5,7-tetrakis(4-
ethynylphenyl)adamantane core A5 was Sonogashira cross-
coupled with slight excess of PDIBr (4.4 equiv) to yield the target
PDI tetramer 2. The purification of 2 was carried in a fashion similar
At room temperature (295 K), both PDI tetramers (1 and 2) and
PDI monomer 3 displayed very similar 1H NMR patterns with par-
tial broadening of aromatic resonances: two well separated P12 and
P2 protons and a broad chemical shift envelope comprising of five
protons (P5eP8 and P11). This splitting pattern is quite character-
istic and has been observed in other related PDI-based assemblies.
Structurally, the positioning of acetylene group induces a strong
deshielding effect on its spatially most proximate proton (P12) that
appears as the most downfield shifted doublet (d 10.30 ppm). This
effect is further evidenced upon the comparison of 1H NMR spectra
(Fig. 2): the attachment of a phenylethynyl (CCPh) arm to the 1-
position of PDI causes the substantial downfield shift of P12 pro-
ton (w0.5 ppm) relative to PDIBr, whereas all other remaining
perylene-based protons experience relatively minor changes. An-
other accompanying effect is the downfield shift of the phenyl
group resonances (w0.4 ppm) in PDI tetramers 1 and 2 as com-
pared to their rigid core precursors; for instance, in 1 vs. M4
(Supplementary data Fig. S10). In addition to that, in PDI tetramer 2
the methylene protons of adamantane core (d 2.39 ppm) also ex-
perience deshielding effect and are downfield shifted by
a
w0.65 ppm relative to unsubstituted adamantane (Supplementary
data Fig. S12). At room temperature (295 K), P12 proton is a rela-
tively well resolved doublet (
d
10.30 ppm, J¼8.2 Hz) while P2

M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
9523
Table 1
Calculated conformational exchange kinetic parameters for PDI tetramers 1 and 2,
and monomer 3
PDI compound
Temperature (K)
kex (sꢀ1
)
D
Gs (kcal/mol)
1
303ꢂ0.1
308ꢂ0.1
313ꢂ0.1
318ꢂ0.1
323ꢂ0.1
328ꢂ0.1
303ꢂ0.1
308ꢂ0.1
40.65ꢂ2.8
59.75ꢂ3.5
84.92ꢂ4.1
159.79ꢂ4.8
213.99ꢂ5.7
315.44ꢂ7.3
48.21ꢂ2.3
77.73ꢂ3.4
16.6ꢂ0.2
2
3
313ꢂ0.1
318ꢂ0.1
323ꢂ0.1
328ꢂ0.1
298ꢂ0.1
105.7ꢂ5.1
155.73ꢂ4.8
220.37ꢂ6.1
338.53ꢂ7.6
33.26ꢂ2.2
15.0ꢂ0.3
16.7ꢂ0.2
308ꢂ0.1
318ꢂ0.1
328ꢂ0.1
85.76ꢂ3.8
194.44ꢂ4.9
440.61ꢂ6.4
used as integration reference points. Based on our previous studies of
structurally relevant acetylene-bridged PDI dimers using multidi-
mensional NMR spectroscopy (e.g., heteronuclear 1He13C HSQC ex-
periments), protons H2 (d d 1.92e1.82 ppm)
2.40e2.22 ppm) and H20 (
were found to exhibit a pronounced diastereotopic splitting accom-
Fig. 3. Atom-labeling scheme used for the chemical shift assignment in 1H NMR
spectra of PDI tetramers 1 and 2, and monomer 3. PDI monomer 3 is shown as
a representative example.
panied by a substantial chemical shift difference (w0.4 ppm).29
2.2.1. Perylene-centered conformational dynamics. In order to un-
ravel the origin of spectral broadening in conventional 1H NMR
spectra (Fig. 2) and higher than expected number of 13C resonances
(see Supplementary data) observed at room temperature (295 K) in
the synthesized monomeric and tetrameric PDIs (1, 2 and 3), we
recorded the corresponding variable temperature (VT) 1H NMR
spectra (CDCl3) in 253e328 K range (Fig. 4 and Supplementary
data). As a representative example, the temperature-dependent
behavior of PDI tetramer 2 will be discussed here.
proton appears as a broad singlet (e.g.,
d 8.96e8.87 ppm for 2)
stemming from the underlying chemical exchange and therefore
can be used as a convenient probe to evaluate the kinetic param-
eters of this phenomenon (Fig. 4 and Table 1).
The aliphatic 1-hexylheptyl group remains pseudo-symmetric
even when the perylene core symmetry is perturbed upon the in-
corporation of CCPh arm in 1-position of its bay region. The H1 pro-
As clearly evident from the temperature-induced changes in the
aromatic regions (especially for protons P2 and P12), both PDI
tons nexttotheimide nitrogen(d 5.24e5.16 ppm) are characteristicof
a wide variety of PDI molecules in general and can be conveniently
Fig. 4. Temperature-dependent 1H NMR spectra of PDI tetramer 2 (400 MHz, CDCl3) measured in 253e328 K range. For clarity, only the regions of interest are displayed.

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M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
tetramers (1 and 2) and monomer 3 exist in a dynamic equilibrium
arising primarily from the conformational dynamics of the per-
ylene core (Fig. 4 and Supplementary data).29,58,59 On the other
hand, the 1H signals in the aliphatic regions (H1eH7) were found to
undergo very minor temperature-induced changes and, conse-
quently, any possible rotational conformers could not be suffi-
ciently resolved under the employed experimental conditions.
range of 0.1e2 mM and thus confirmed that the observed broad-
ening of 1H NMR signals in PDI tetramers 1 and 2 stems solely from
the conformational dynamics of twisted perylene core(s) within
their corresponding phenylethynyl-perylenediimide (PDICCPh)
subunits. Noteworthy, chlorinated solvents (such as CH2Cl2 or
CHCl3) are well known to be ‘good’ solvents for PDI-based systems
as they usually tend to prevent these chromophores from aggre-
gating. On a separate note, it is quite interesting that the observed
perylene-centered conformational exchange phenomenon seems
to be amplified when PDI-based chromophoric assemblies in-
corporate long 1-hexylheptyl solubilizing groups whereas no
spectral broadening (at RT) was observed with shorter (e.g., 1-
ethylpropyl) substituents (data not shown). However, the exact
origin of this effect is not yet well understood.
The P2 proton (d 8.96e8.87) is structurally unique as it is the
only uncoupled aromatic proton within the perylene core and
therefore can best serve as a probe of the dynamic behavior of 2.
Based on the obtained VT NMR data, the dynamic behavior of all
synthesized PDI molecules (1, 2 and 3) can be rationally described
by a two-state conformational equilibrium model with nearly equal
population distributions.60 At temperatures below RT (253e283 K),
P2 proton appears as a set of two singlets of nearly equal intensity
with a peak separation of w17.5 Hz. This represents a textbook
example of a dynamic spin system that exists in the slow exchange
regime and two different conformers are clearly resolved. The co-
alescence point emerges near RT (295 K) and the P2 peak separa-
tion is no longer resolved, thus indicating a crossover into the fast
exchange regime. Above this point (>300 K), the system shifts
completely into the fast exchange mode and the PDI conforma-
tional dynamics becomes too fast to be resolved into two separate
components. Consequently, the P2 proton gradually becomes
a well-defined ensemble-average singlet in complete accordance
with its structural placement.
2.3. Photophysical properties
The photophysical properties of target PDI molecules (1e3) have
been measured in dichloromethane (DCM). In general, PDI tetra-
mers 1 and 2 possess exceptional (photo)chemical stability and
their photophysical properties are characteristic of a wide variety of
PDI-based molecular systems.1,2 The steady-state absorption spec-
tra are presented in Fig. 6 (left). The absorption bands in the near-
UV region (300e400 nm) are assigned to the
the phenylethynyl (CCPh) arm whereas PDI-centered
p
e
p
* transitions of
* tran-
pep
sitions span the visible portion of the spectrum (400e600 nm).
Compared to PDIBr (see Supplementary data Fig. S17), the attach-
ment of the phenylethynyl (CCPh) group to the 1-position of PDI
The residual exchange-induced line broadening of P2 proton can
be conveniently used to estimate the rate constants (kex) and the
activation barrier (
D
G
s) for the observed conformational exchange
bay region results in a bathochromic shift of PDI-centered pep*
(Fig. 5). The estimated activation energy for both PDI tetramers (1
and 2) and monomer 3 was found to be within 15e17 kcal/mol
(Table 1) that falls right in the ballpark of literature reported values
for structurally relevant molecules.29,47,59,61e63 Furthermore, the
origin of the observed conformational exchange has been often
attributed to the induced out-of-plane twist of the perylene core
upon the chemical modification of its bay region, with dihedral
angles ranging from 13ꢁ to 37ꢁ depending on the number and na-
ture of the substituent groups.29,64e66 In other words, the origin of
the observed conformational dynamics in PDI tetramers 1 and 2,
and monomer 3 comes from the local fluctuations in the twist
(dihedral) angle of the perylene core. It is important to note here
that the conformational dynamics of PDI tetramers is only ob-
servable by solution NMR spectroscopy. In addition, the
concentration-dependent NMR studies (Supplementary data Figs.
S15 and S16) showed no evidence of any aggregation-induced ef-
fects (which, if existed, would be typically accompanied by the
substantial chemical shift changes) in a measured concentration
transitions in the visible and enhancement of the light-harvesting
properties of 3 in near-UV region. However, the delocalization of
electron density over the entire PDICCPh moiety inadvertently
leads to a decrease of the oscillator strength of the PDI-centered
pep* transitions, as compared to the unsubstituted PDI (used
here as a reference): 53,000 Mꢀ1 cmꢀ1 for 3 vs. 88,000 Mꢀ1 cmꢀ1
for PDI (Table 2).1,50 Compared to monomer 3, the absorption
spectra of PDI tetramers 1 and 2 are only marginally red-shifted
(w2 nm) and somewhat broadened on the lower energy side (see
Supplementary data Fig. S18). However, integration of the data
presented in Fig. 6 (left) between 300 and 600 nm indicates a non-
linear enhancement (larger than a factor of 4) of electronic transi-
tions in PDI tetramers 1 and 2, as compared to monomer 3 (Table 2).
Both lower energy, PDI-centered and higher energy, phenylethynyl-
centered pep* transitions contribute to the enhancement of light-
harvesting properties in the newly synthesized PDI tetramers.
Based on our recent photophysical studies of acetylene-bridged PDI
dimers using coherent two-dimensional electronic spectroscopy,
we hypothesize that the observed absorption enhancement effect
may stem from intramolecular excitonic coupling between the
adjacent PDI subunits within the tetramer framework and thus will
be the subject of in-depth photophysical investigation in a separate
future contribution.19 Interestingly, this enhancement is the largest
in PDI tetramer 1 (Table 2) and may indicate a stronger degree of
intramolecular interactions between neighboring PDI subunits
when they are attached to a smaller, methane-centered rigid core.
The photoluminescence of PDI tetramers 1 and 2, and monomer
3 originates from the singlet excited state and is characteristic of
other related PDI-based chromophores (Fig. 6, right).41 The emis-
sion spectra of both PDI tetramers (1 and 2) are nearly superim-
posable and their emission maxima (lmax w574 nm) are slightly
red-shifted (w8 nm) as compared to monomer 3 (lmax¼566 nm).
The apparent loss of the vibronic structure in the emission spectra
is likely due to the delocalization of electron density over the entire
PDICCPh subunit. As compared to monomer 3, the tetrahedral as-
sembly of four PDICCPh subunits into PDI tetramers 1 and 2 causes
an enhancement of non-radiative decay pathways as evidenced by
Fig. 5. Arrhenius analysis of conformational exchange (dynamic NMR) in PDI tetra-
mers 1 and 2, and monomer 3.

M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
9525
Fig. 6. Steady-state absorption (left) and uncorrected emission (right, lex¼480 nm) spectra of PDI tetramers 1 (red) and 2 (blue), and monomer 3 (green) measured in
dichloromethane.
Table 2
tetramers 1 and 2, and monomer 3 were found to exhibit mono-
exponential fluorescence decays, with each PDI tetramer possess-
ing slightly longer excited state lifetime than monomer 3: 6.9 ns vs.
6.1 ns (Supplementary data Fig. S19 and Table 2). To summarize, the
Photophysical properties of PDI tetramers 1 and 2, and monomer 3 measured in
dichloromethane
PDI compound
photophysical properties of these newly synthesized PDI tetramers
1 and 2 appear to be consistent with other related PDI-based as-
semblies; however, as we noted before, a more detailed photo-
physical investigation of these PDI tetramers will be reported in
a separate contribution.
1
2
3
Extinction coefficient
181,000ꢂ2000
169,000ꢂ2000
53,000ꢂ3000
at lmax (Mꢀ1 cmꢀ1
Relative absorption
integrated area (a.u.)
Fluorescence
)
4.48ꢂ0.01
6.9ꢂ0.1
4.16ꢂ0.01
6.9ꢂ0.1
1.0ꢂ0.06
6.1ꢂ0.1
lifetime (
Fluorescence quantum
yield (FFL
sFL, ns)
0.59ꢂ0.03
0.58ꢂ0.03
0.71ꢂ0.03
3. Conclusions
)
kr (sꢀ1
)
knr (sꢀ1
kr/knr
0.82ꢃ108
0.57ꢃ108
1.44
0.84ꢃ108
0.61ꢃ108
1.38
1.16ꢃ108
0.48ꢃ108
2.42
We have synthesized and thoroughly structurally characterized
two new representative tetrahedral rigid core ‘antenna’ tetramers
bearing phenylethynyl-perylenediimide (PDICCPh) subunits. The
developed synthetic methodology is expected to be a valuable
addition to the synthetic toolbox geared towards the design of
novel complex multichromophoric systems. Dynamic NMR exper-
iments revealed the existence of perylene-centered dynamic con-
)
the substantial drop (>10%) of their corresponding fluorescence
quantum yields (FFL, Table 2). In addition, time-resolved photo-
luminescence measurements were performed to further charac-
terize the photophysical behavior of PDI tetramers 1 and 2 (Fig. 7).
Under optically dilute conditions in dichloromethane, PDI
formational exchange (
D
Gs¼15ꢀ17 kcal/mol), the primary cause of
substantial spectral broadening of the aromatic resonances in
conventionally acquired (RT) 1H NMR spectra. Interestingly, the
observed perylene-centered conformational exchange phenome-
non appears to be a common theme for a variety of related PDI-
based chromophoric assemblies that incorporate long 1-
hexylheptyl solubilizing groups. Preliminary photophysical char-
acterization of PDI tetramers 1 and 2 pointed towards an un-
derlying intramolecular interaction (likely excitonic coupling)
between the neighboring PDICCPh subunits and will be the subject
of our future research efforts.
4. Experimental
4.1. General
Most synthetic manipulations were carried out under inert at-
mosphere (nitrogen) using standard Schlenk techniques unless
otherwise noted. All solvents (ACS reagent grade) and reagents
were purchased from commercial sources and used as received.
When necessary, anhydrous solvents were obtained from a stan-
dard solvent purification system equipped with activated alumina
columns. To ensure the reproducibility of reaction procedures, all
reaction sequences were repeated at least two times.
Fig. 7. Fluorescence intensity decays of PDI tetramers 1 (red) and 2 (blue), and monomer
3
(green) measured in optically dilute dichloromethane solutions
(lex¼480 nm;
lem¼570 nm).

9526
M. Myahkostupov, F.N. Castellano / Tetrahedron 71 (2015) 9519e9527
4.2. Characterization and instrumentation
4.3.1. Tetrakis-[1-(4-ethynyl-phenyl)-N,N0-bis(1-hexylheptyl)-per-
ylene-3,4:9,10-tetracarboxylic diimide]-methane (1). 50 mg
The structures of synthesized phenylethynyl-perylenediimide
tetramers 1 and 2 along with monomer 3 were confirmed by
high-resolution 1H and 13C NMR spectroscopy, MALDI-TOF mass
spectrometry (MALDI-MS) and elemental analysis. Mass spectra
were acquired using a standard MALDI-TOF spectrometer (MSU
Mass Spectrometry Facility). Elemental analysis was performed by
Atlantic Microlab, Inc. 1H (400 MHz) and 13C (100 MHz and
176 MHz) were recorded at 295 K on 400 MHz Varian Inova
(0.12 mmol) of tetrakis(4-ethynylphenyl)methane (M4), 440 mg
(0.53 mmol) of 1-bromoperylenediimide (PDIBr), 18 mg (0.1 mmol)
of CuI and 100 mg (0.1 mmol) of Pd(PPh3)4 were placed under ni-
trogen in a 100 mL Schlenk flask equipped with a reflux condenser,
followed by the addition of 10 mL of N2-degassed diisopropylamine
(anh.) and 20 mL of dichloromethane (anh.). The reaction mixture
was additionally degassed with N2 for 20 min and then heated to
65 ꢁC for 48 h (under nitrogen). The reaction progress was moni-
tored by steady-state absorption spectroscopy. After completion,
the reaction mixture was cooled to RT and then quenched with 10%
HCl. Afterward, the product was extracted with dichloromethane,
combined organic fractions dried over Na2SO4 (anh.), filtered and
concentrated under vacuum. The obtained solid was recrystallized
at least twice from dichloromethane/methanol (1:3) via liquid
diffusion to afford analytically pure dark red solid in 64% yield
(263 mg). MALDI-MS: m/z¼3429.7 ([MþH]þ). Anal. Calcd for
spectrometer and 700 MHz Bruker Avance
1 spectrometer.
Variable-temperature (VT) 1H NMR (400 MHz) experiments were
carried out in 253e328 K range with sample temperature equili-
bration time of 10 min. NMR spectral analysis was performed using
MestReNova 9.0.1 software. All chemical shifts were referenced to
the residual solvent peak (CDCl3, d
7.26 ppm for 1H and 77.23 ppm
for 13C) and splitting patterns were assigned as s (singlet),
d (doublet), and m (multiplet). The conformational exchange rate
constants (kex) were calculated from the lineshape broadening
deconvolution of an uncoupled aromatic proton (P2) in 298e328 K
temperature range (fast exchange regime) assuming a two-state,
equal population exchange model and using the following
equation:
C233H260N8O16: C, 81.62; H, 7.64; N, 3.27. Found: C233H260N8O16: C,
81.53; H, 7.51; N, 3.26. 1H NMR (400 MHz, CDCl3, 295 K): 10.31 (4H,
d, J¼8.3 Hz); 8.92e8.86 (4H, broad s); 8.77e8.61 (20H, m);
7.78e7.70 (4H, dd, J¼8.2 Hz); 7.56e7.48 (4H, dd, J¼8.2 Hz);
5.22e5.11 (8H, m); 2.30e2.15 (16H, m); 1.93e1.75 (16H, m);
1.42e1.12 (128H, m); 0.87e0.73 (48H, m). 13C NMR (176 MHz,
CDCl3, 295 K): 164.9; 164.7; 164.6; 164.3; 163.8; 163.7; 163.6;
163.2; 147.2; 138.9; 138.2; 134.5; 134.1; 132.0; 131.9; 131.6; 131.0;
130.8; 129.3; 128.7; 127.4; 127.2; 126.9; 124.3; 124.1; 123.8; 123.7;
123.5; 123.3; 123.1; 123.0; 122.3; 121.1; 120.9; 120.3; 99.3; 92.0;
65.7; 55.1; 54.9; 32.5; 32.0; 31.9; 29.4; 27.1; 27.0; 22.8; 22.7; 14.3;
14.2.
p
ð
nA
ꢀ
nBÞ2
kex
¼
2ðLWAB ꢀ LWREF
Þ
where: (nA
ꢀ
nB) is the exchangeable uncoupled proton peak (P2)
separation (in Hz) measured at 253 K (slow exchange regime); LWAB
is a linewidth (FWHM, in Hz) of the uncoupled exchangeable pro-
ton peak (P2) at a given temperature; LWREF is the linewidth
(FWHM, in Hz) of a reference non-exchangeable proton peak (re-
sidual solvent peak, CDCl3) at a given temperature. The activation
4.3.2. Tetrakis-1,3,5,7-[1-(4-ethynyl-phenyl)-N,N0-bis(1-
hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide]-adamantane
(2). 50 mg (0.093 mmol) of 1,3,5,7-tetrakis(4-ethynylphenyl)ada-
mantane (A5), 375 mg (0.45 mmol) of 1-bromoperylenediimide
(PDIBr), 18 mg (0.1 mmol) of CuI and 100 mg (0.1 mmol) of
Pd(PPh3)4 were placed under nitrogen in a 100 mL Schlenk flask
equipped with a reflux condenser, followed by the addition of
10 mL of N2-degassed diisopropylamine (anh.) and 20 mL of
dichloromethane (anh.). The reaction mixture was additionally
degassed with N2 for 20 min and then heated to 65 ꢁC for 48 h
(under nitrogen). The workup and purification protocols were
identical to 1. After recrystallization, PDI tetramer 2 was isolated as
analytically pure dark red solid in 75% yield (247 mg). We also note
that any residual (emissive) impurities in PDI tetramers 1 and 2 can
be removed by careful repetitive precipitation of concentrated DCM
solution(s) into excess hexane. MALDI-MS: m/z¼3549.9 ([MþH]þ).
Anal. Calcd for C242H272N8O16 , CH2Cl2: C, 80.32; H, 7.60; N, 3.08.
Found: C, 80.08; H, 7.45; N, 3.06. 1H NMR (400 MHz, CDCl3, 295 K):
10.33 (4H, d, J¼8.2 Hz); 8.96e8.87 (4H, broad s); 8.81e8.63 (20H,
m); 7.83e7.70 (16H, m); 5.24e5.16 (8H, m); 2.40e2.22 (28H, m);
energy (DG
s) for the conformational exchange was estimated from
the Arrhenius analysis (ln(kex) vs. 1/T).
The steady-state absorption spectra were measured in optically
dilute dichloromethane solutions on Shimadzu UV-3600
UVeViseNIR spectrophotometer. The extinction coefficients were
calculated as an average of two independent runs. The uncorrected
steady-state emission spectra were measured in optically dilute,
air-saturated dichloromethane solutions on Edinburgh Instruments
FLS980 fluorescence spectrometer. The fluorescence quantum
yields (FFL) were measured in dichloromethane using N,N0-bis(1-
hexylheptyl)-perylene-3,4:9,10-tetracarboxylic diimide (PDI) as
a reference. The excited state lifetimes (sFL) were measured using
the gated second-harmonic (4 MHz, w1 nJ/pulse) output of a tun-
able ultrafast Ti-Sapphire oscillator (Coherent, w120 fs) as an ex-
citation source and a time-correlated single photon counting
(TCSPC) detection system (LifeSpec II, Edinburgh Instruments,
w100 ps IRF), and estimated from the first-order decay fit of single
wavelength emission transients using Igor Pro 6.36 software. The
radiative (kr) and non-radiative (knr) excited state decay rate con-
stants were calculated as follows:
1.92e1.82 (16H, m); 1.40e1.15 (128H, m); 0.84e0.79 (48H, m). 13
C
NMR (176 MHz, CDCl3, 295 K): 164.9; 164.7; 164.3; 163.8; 163.6;
163.2; 150.8; 139.0; 138.3; 134.5; 134.1; 132.4; 131.7; 131.6; 130.9;
130.8; 129.3; 128.6; 127.4; 127.1; 126.8; 126.0; 124.2; 124.0; 123.8;
123.6; 123.5; 123.3; 123.2; 123.1; 122.9; 122.2; 120.6; 120.4; 100.0;
91.4; 55.1; 54.9; 47.1; 40.0; 32.6; 32.0; 29.5; 27.2; 22.8; 22.7; 14.3.
FFL
sFL
1
sFL
kr
¼
knr
¼
ꢀ kr
4.3. Synthesis
Tetrahedral methane- and adamantane-centered rigid cores
were synthesized via multi-step procedures using modified litera-
ture protocols. All intermediates and target products were isolated
using standard workup and purification techniques. All synthesized
rigid core intermediates and PDI synthons produced satisfactory
analytical characterization data that were found to be consistent
with previously reported values.
Acknowledgements
This work was financially supported by the Air Force Office of
Scientific Research (FA9550-13-1-0106). We thank Dr. Valentina
Prusakova (BGSU) for valuable input at the preliminary stages of
this project, and Dr. Xiaoyan Sun (NCSU) for the assistance with
dynamic NMR experiments.

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9527
Supplementary data
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Representative synthetic scheme and synthetic protocol for
a model monomer 3 (PDICCPh); 1H and 13C NMR spectra of PDI
tetramers 1 and 2, and monomer 3; MALDI-TOF spectra of 1 and 2;
1H NMR spectra of rigid core intermediates in the synthetic se-
quences towards M4 and A5; temperature-dependent 1H NMR
spectra of 1 and 3; concentration-dependent 1H NMR spectra of 1
and 2; additional steady-state and time-resolved photophysical
data.
Supplementary data associated with this article can be found in
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