Synthesis, Photophysical and Electronic Properties of New Red-to-NIR Emitting Donor–Acceptor Pyrene Derivatives

Article abstract of DOI:10.1002/chem.201904219

We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin-Day class-II and delocalized Robin-Day class-III species.

Full text of DOI:10.1002/chem.201904219

A Journal of
Accepted Article
Title: Synthesis, Photophysical and Electronic Properties of New Red
to NIR Emitting Donor-Acceptor Pyrene Derivatives
Authors: Julia Merz, Maximilian Dietz, Yvonne Vonhausen, Frederik
Wöber, Alexandra Friedrich, Daniel Sieh, Ivo Krummenacher,
Holger Braunschweig, Michael Moos, Marco Holzapfel,
Christoph Lambert, and Todd B. Marder
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To be cited as: Chem. Eur. J. 10.1002/chem.201904219
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Synthesis, Photophysical and Electronic Properties of New Red to NIR
Emitting Donor-Acceptor Pyrene Derivatives
Julia Merz,[a] Maximilian Dietz,[a] Yvonne Vonhausen,[a] Frederik Wöber,[a] Alexandra Friedrich,[a]
Daniel Sieh,[a] Ivo Krummenacher,[a] Holger Braunschweig,[a] Michael Moos,[b]
Marco Holzapfel,[b] Christoph Lambert,[b] Todd B. Marder*[a]
[a] Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron (ICB), Julius-
Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg
[b] Institut für Organische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg
Abstract
We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at
the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess
a strong intramolecular charge transfer, leading to unusual properties such as emission in the
red to NIR region (700 nm), which has not been reported before for monomeric pyrenes.
Detailed photophysical studies reveal very long intrinsic lifetimes of > 100 ns for the new
compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes.
The incorporation of strong donors and acceptors leads to very low reduction and oxidation
potentials, and spectroelectrochemical studies show that the compounds are on the borderline
between localized Robin-Day class-II and delocalized Robin-Day class-III species.
Introduction
The polycyclic aromatic hydrocarbon (PAH) pyrene is among the most widely studied
chromophores and possesses some unique properties such as intense blue emission and an
exceptionally long-lived excited singlet state together with excimer and exciplex formation.[1] In
addition to its photophysical properties, it has high chemical stability and charge-carrier mobility.
Therefore, pyrene derivatives have been used in a broad range of applications in diverse
scientific fields such as organic light emitting diodes (OLEDs), organic field-effect transistors
(OFETs) and organic photovoltaic cells (OPVs).[1,2] Furthermore, they have been used for
sensing of temperature,[3] pressure[4] or pH,[5] or to detect guest molecules such as O2 or NH3,[6]
organic molecules,[7] or metals[1,8] and to construct covalent organic frameworks.[9] Its monomer
and excimer fluorescence have also been used to study the properties such as dynamics or
interactions of macromolecules[10] and lipids.[11] Its remarkably long fluorescence lifetime of
354 ns, compared to other PAHs, makes pyrene exceptionally well suited for further applications
such as the determination of cellular oxygen concentrations or reactive oxygen species (ROS) in
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biological systems.[12] Furthermore, 2‐phenylethynylpyrenes have also been used as
fluorescence labels for DNA.[13]
To adjust the properties for a specific application, electron donors and/or acceptors are often
introduced onto a PAH core as they strongly modulate the frontier orbitals. Substituting PAHs
with donors and acceptors gives derivatives with properties such as a permanent dipole
moment, charge transfer (CT) excited states, strong solvatochromism, environmentally-
influenced photophysics, the possibility for energy or electron transfer, and narrowed energy
gaps.[14,15] Furthermore, such chromophores can absorb and emit in the near-infrared (NIR)
region, which is in demand for applications such as bioimaging and cell recognition, as NIR light
penetrates deeper into biological tissues, is less damaging than visible or UV light, and gives
minimum interference from background autofluorescence by biomolecules.[16] In general,
materials with high HOMO energies, such as the compound N,N’-diphenyl-N,N’-bis(3-
methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD), are especially useful for hole transport.[17]
Common π-donors that have been used in dyes to boost HOMO energies include amines, with a
lone pair on the nitrogen, such as diarylamino, diethylamino, dimethylamino or carbazolyl
moieties.[18] Diarylamines are among the strongest π-electron donors and have been employed
in diverse applications,[19] due to their outstanding physical, photochemical and electrochemical
properties, and they are easy to synthesize and handle.[20] A methoxy group at the position para
to the nitrogen not only increases the electron donating strength of diarylamines, but enables
reversible oxidations.[20,21] Nevertheless, a significant drawback can be energy loss due to the
rotational motion of the phenyl rings which can result in a reduction in luminescence
efficiency.[22] In addition, the possibility of rotation around the N-C(π) bond can lead to twisted
intramolecular charge transfer (TICT) excited states.[15] The compound 1,1,7,7-tetramethyl-
julolidine is known to be an even stronger π-donor than diarylamines. The julolidine moiety has
been thoroughly studied since its discovery in 1892 by Pinkus,[23] and is used in a wide range of
dyes.[15,24] Its nitrogen lone pair is conformationally restricted to remain parallel to the aromatic
system,[25] in our case, the pyrene moiety, in both the ground and excited states, and our
previous studies revealed a significantly enhanced electron donating effect on the pyrene core
compared to diarylamines.[15]
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Figure 1. a) Atom numbering system in pyrene with the three sets of chemically inequivalent sites; b) the four frontier
orbitals of pyrene.
Pyrene exhibits 10 peripheral reactive positions, which can be classified into three sets of
chemically inequivalent sites (Figure 1, a). Thus, the position of substitution is very important
and, typically, the 1-,3-,6- and 8-positions are functionalized by electrophilic substitution
reactions as the HOMO has its largest coefficients at these positions (Figure 1, b).[1,26] Therefore,
π-orbitals of substituents at the 1-,3-,6-,8-positions mix very efficiently with pyrene’s
HOMO/LUMO orbitals. However, unsymmetrical substitution at these positions is rather
challenging due to the numerous possible isomers (Scheme 1). Niko, Konishi and co-workers
synthesized D-π-A pyrene systems with donors and acceptors at the 1-,3-,6- and 8-positions,
which displayed strong solvatochromism with emissions into the red region (557-648 nm, in
MeOH) and high quantum yields (ϕ > 0.75).[27]
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Scheme 1. Schematic representation of reported D/A-pyrene patterns.
Substituents at the 2,7-positions do not interact with the HOMO/LUMO orbitals of pyrene as they
lie on the nodal plane (Figure 1, b). However, they can interact strongly with the HOMO-1 and
LUMO+1 of pyrene that have nonzero contributions at these positions. We previously reported
that the photophysical properties of pyrenes with substituents at the 2-position significantly differ
from those with substituents at the 1-position.[28] Moreover, previous studies demonstrated that
strong π-donors/acceptors are able to switch the energetic ordering of its HOMO/HOMO-1 and
LUMO/LUMO+1, respectively, which results in greatly influenced photophysical and redox
properties of these derivatives.[15,29–32] Unsymmetrical substitution at these positions is
straightforward via iridium-catalyzed C−H borylation,[33–35] and the introduction of julolidine-like
donors at the 2,7-positions (I, Scheme 2) even resulted in an unusual green luminescence.[15,29–
32,36]
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The K-region (positions 4,5,9,10, Figure 1) has large HOMO/LUMO as well as HOMO-
1/LUMO+1 contributions. Ottonelli and co-workers showed[37] that the contribution to the
HOMO/LUMO increases in the order: 2,7 < 4,5,9,10 < 1,3,6,8, while contributions to the HOMO-
1/LUMO+1 increase in the order 4,5,9,10 < 1,3,6,8 < 2,7. The K-region possesses alkene-like
rather than aromatic character. Hence, reagents such as osmium tetroxide, which are known to
react with alkene double bonds, selectively react with pyrene at the 4,5,9,10-positions. Müllen
and co-workers reported a protocol for the asymmetric substitution at these sites and presented
a few examples with emission into the orange region of the spectrum (613 nm in THF) (Scheme
1).[14,38]
There are also reports on pyrenes that have donor and acceptor moieties on alternative
positions, besides the typical three categories (vide supra). Hu and co-workers developed
unsymmetric pyrenes with donors at the 1,3-positions and acceptors at the 5,9-positions that
emit in the blue-green region (Scheme 1).[39] Sutherland and co-workers combined the 2,7- and
1,8-positions with the K-region (V and VI, Scheme 2), respectively, and obtained an S1←S0
absorbance up to 900 nm, as it was their objective to obtain strongly redshifted absorptions with
high molar absorptivities. However, their derivatives are not emissive and thus studies are
missing on the influence on the excited state properties and how additional K-region substituents
influence 2,7-substituted pyrenes.[40,41]
In our previous reports we showed that the influence of a second amine donor at the 2,7-
positions on the occupied orbitals is larger than that of a cyano or even Bmes2 acceptor on the
unoccupied orbitals and thus emission from the D-π-D derivative (I, Scheme 2) is further
redshifted than the D-π-A compound (II, Scheme 2). Therefore, our initial objective was to utilize
two julolidine-like donors at the 2,7-positions and add additional acceptors to the pyrene core to
obtain a new class of D-π-A pyrene derivative with unparalleled photophysical properties such
as red to NIR emission and interesting redox behavior. Moreover, we were interested to see how
much more the S1 state can be influenced by the addition of acceptors to the K-region.
For the acceptor unit, we chose n-azaacenes fused to the K-region of pyrene that were reported
by Mateo-Alonso and co-workers and which possess strong π-accepting properties (VII and VIII,
Scheme 2). N-azaacenes have attracted much attention for their outstanding electronic
properties and application in organic electronic devices and as anion radicals.[42] In particular,
pyrene-fused azaacenes are even more stable than their n-azaacene analogues and emit
strongly in the blue to green region of the electromagnetic spectrum.[43,44]
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Scheme 2. Schematic overview of some important pyrene derivatives that possess either the same acceptor or
similar donor moieties as our derivatives 6, 7 or 8.
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Table 1. Selected optical and electrochemical properties of the pyrene derivatives I to VIII.
λabs / nm
λem / nm
E1/2 [V] [red]a)
E1/2 [V] [ox]1 a)
E1/2 [V] [ox]2 a)
I[15]
475 (hexane)
481 (THF)
450 (hexane)
462 (THF)
-
511 (hexane)
525 (THF)
-
-0.18 (CH2Cl2)
+0.26 (CH2Cl2)
II[15]
464 (hexane) -2.51 (THF)
497 (THF)
+0.29 (THF)
III[32]
IV[28]
V[41]
-
-
-
+0.14 (CH2Cl2) +0.38 (CH2Cl2)
453 (toluene)
630 (CHCl3)
575 (CHCl3)
455 (CH2Cl2)
482 (toluene)
-
-
-
-
-
-
-
-0.67 (CHCl3) +0.39 (CHCl3)
-1.04 (CHCl3) +0.33 (CHCl3)
VI[40]
VII[45]
-
538 (CH2Cl2)
471 (CH2Cl2)
-1.43 (THF)
-
-
-
VIII[46] 435 (CH2Cl2)
a) Redox potentials were measured with the addition of 0.1 M [n-Bu4N][PF6] and are referenced vs. the
Fc/Fc+ couple.
Results and Discussion
Synthesis and structural characterization
The procedures used to synthesize the three key compounds are summarized in Schemes 3
and 4. At the beginning of this project, we aimed for derivative 7’ (Scheme 3) and its analogue
with ketone moieties at all four K-region positions (Scheme S1). The selective RuO4-catalyzed
oxidation of pyrene at its 4- and 5-positions is the starting point for the synthesis route and gives
the dione 1. This one-step reaction was reported by Harris and co-workers in 2005 as an
alternative to the toxic OsO4-catalyzed oxidation of pyrene.[47,48] In this regard, RuO4 is
generated in situ from RuCl3·3H2O and NaIO4, and the oxidation takes place in a solvent mixture
of MeCN, CH2Cl2 and H2O (1:1:1.25). In 1981, Sharpless and co-workers reported that addition
of MeCN to the RuO4-catalyzed oxidation greatly improves the effectiveness of the reaction.
However, this procedure suffers from several disadvantages. Thus, the reaction can only be
performed on a small scale and, furthermore, there are various side products such as
dialdehydes or acids, which Nowicka et al. identified in detail, that complicate the workup and
reduce the yield.[49] Bodwell and co-workers recently introduced an improved RuO4-catalyzed
oxidation procedure to obtain 1, which is scalable and eases the workup. This method includes
the additive N-methylimidazole, and the solvent MeCN is replaced by THF.[50] In order to perform
the Ir-catalyzed C-H borylation,[33–35] the ketone groups had to be protected as a diketal to
prevent unwanted reactions with the Ir-catalyst. The lack of reactivity of oxidized pyrene in other
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metal-catalyzed reactions has been reported before.[51,52] Compound 2 was easily obtained by
refluxing 1 in toluene with an excess of ethylene glycol for 20 h in the presence of p-
toluenesulfonic acid. The C–H borylation was performed according to our reported procedure[33–
35] and takes place at the 2,7-positions selectively providing 3.
Scheme 3. Synthesis of derivatives 1-3 and 4’-7’.
The Bpin moieties were further converted to azide moieties using a procedure reported by
Chang and co-workers.[53] They converted arylboronic acids to arylazides in MeOH by using
1.50 eq of NaN3 as the azide source and 5 mol% of Cu(OAc)2 as the catalyst. Subsequent
reduction of the azido species 4’ with Pd/C and H2 gave the corresponding primary amine 5’.
The reaction was stirred at room temperature until monitoring by IR spectroscopy confirmed the
complete disappearance of azide moieties (in the region 2000-2270 cm-1) and the appearance of
primary amine bands at 3450 cm-1, 3363 cm-1 and 1620 cm-1 (Figure S26). The amine moieties
were alkylated in MeCN with 1-chloro-3-methylbutene in the presence of the base K2CO3,
according to the approach used in our previously reported synthesis of I.[15] The alkylation
reaction was complete after 48 h and gave the desired product 6’ as a bright yellow solid (42%).
Unfortunately, the final step to obtain the desired derivative 7’ was unsuccessful. Our aim was to
achieve ring closure and simultaneously deprotect the acetal groups in order to regenerate the
two ketone moieties at the K-region. However, using a diverse range of acids, the acetal
protecting groups were removed first, which most likely generated a pyrene core that was too
deactivated to permit the electrophilic ring closure to take place. Therefore, we decided to
change our julolidine-like donor moiety to a diarylamine. The new synthesis route is depicted in
Scheme 4. Compound 3 was transformed into the corresponding dibromo species 4 by a
bromodeboronation reaction.[54]
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Scheme 4. Synthesis of derivatives 3-8.
Thus, 3 and CuBr2 were suspended in a mixture of THF/MeOH/H2O (1:1:1) and the reaction
mixture was stirred at 90 °C for 4 d. The Buchwald-Hartwig amination, which we performed to
obtain 5 from 4 using Pd2(dba)3·CHCl3 as the catalyst precursor and XPhos as the ligand, was
achieved in a yield of 58%. The deprotection of 5 in a trifluoroacetic acid and water mixture (6:1)
is straightforward, and compound 6 was obtained in 81% yield. The cyclocondensation reactions
between dione 6 and 2,3-diaminomaleonitrile to give 7, or benzene-1,2-diamine to give 8, were
performed in an ethanol/acetic acid mixture (1:1) at 80 °C for 15 h according to the procedure of
Mateo-Alonso and co-workers.[43,51,55]
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Figure 2. Solid-state molecular structures of compounds 2, 5', 6', and 7 as determined by single-crystal X-ray
diffraction at 100 K. Hydrogen atoms, solvent molecules, and disordered parts of lower occupancies are omitted for
clarity. Atomic displacement ellipsoids are shown at the 50% probability level. Element color: carbon (white), nitrogen
(blue), oxygen (red). Representative bond labeling is shown on compound 2. Aryl rings are labeled R1 to R4 in 7.
The solid-state structures of compounds 2, 5', 6', and 7 were determined via single-crystal X-ray
diffraction (Figure 2). In compound 7, the biphenyl unit of the pyrene moiety exhibits typical
aromatic C−C bond lengths ranging from 1.391(4) to 1.418(3) Å (bonds a, b, f, g, i, and j and
their symmetrical equivalents, Figure 2 and Table S2), as is also observed in pyrene. The c and
e bonds on the unsubstituted site of the pyrene moiety are slightly longer (1.444(3) and 1.431(4)
Å), and the d bond (1.355(3) Å) is typical of a C=C double bond. This means that it can be
viewed as a biphenyl unit constrained to be planar by a −CH=CH− group. This has also recently
been observed for many other 2-, and 2,7-substituted pyrenes by Marder and co-workers.[15,29,30]
Similar bond lengths have also been reported for both sides of the pyrene moiety of 2,7-
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bis(dianisylamino)pyrene,[32] the azaacene-free analogue of our compound 7. However, in 7 the
c, d, and e bonds on the 4,5-azaacene-substituted side of the pyrene moiety are all longer than
the equivalent bonds on the unsubstituted side, the d bond (1.426(3) Å) being shorter than the c
and e bonds (1.451(3) and 1.453(3) Å) (Table S2). Hence, the incorporation of the azaacene
moiety at the 4,5-positions has a bond-lengthening effect, and the d bond can no longer be
compared with a C=C double bond but rather has aromatic character. Interestingly, the
azaacene-moiety also effects the central bond h which links the two phenyl rings, and is slightly
longer (1.435(3) Å) than in the azaacene-free analogue compound (1.416(2) Å).[32]
A similar, even more pronounced effect is observed in the unsymmetric compounds with acetals
substituted at the 4,5-positions of pyrene, i.e. compounds 2, 5', and 6'. While the unsubstituted
side of the pyrene core still shows bond distances similar to those of pyrene or compound 7, the
4,5-substituted side shows long bond distances for the c, d, and e bonds (1.515(2)–1.555(3) Å),
which are typical of C–C single bonds (Table S2). The central h bond is also further elongated
(1.440(3)–1.447(3) Å) with respect to 7.
As is usually observed for aromatic amines, in compound 7 the N−C(pyrene) bond lengths
are significantly shorter (1.406(3) and 1.415(3) Å) than the other N−C bonds of the amine
moieties (1.422(3) – 1.435(2) Å). Similar distances were also reported for the azaacene-free
pyrene with amines substituted at the 2- and 7- positions.[32] The nitrogen atoms have a nearly
ideal trigonal planar configuration with the sum of the C−N−C angles being between 359.0(6)
and 360.0(3)°. The interplanar angles between the NC3 and pyrene planes (37.45(10) for N1 and
32.3(2)–33.2(4)° for the disordered groups bonded to N2) are in a similar range as those
between NC3 and the terminal phenyl rings (35.2(4)–47.5(2)°) (Table S2). Again, this is in
agreement with the NC3–pyrene angle (31°) which was reported for the analogous azaacene-
free 2,7-substituted pyrene compound.[32] The methoxyphenyl groups R3 and R4 of 7 are
strongly disordered and show a higher degree of rotational freedom than the groups R1 and R2.
Indeed, the interplanar angles between the NC3 planes and the terminal rings vary between
38.2(4) and 47.5(2)° for the disordered parts of R3 and between 35.2(4) and 39.0(6)° for those
of R4. In addition, important intermolecular interactions involving the methoxyphenyl groups are
only present for the R1 and R2 groups but are not observed for the R3 and R4 groups (Table
S4).
In the crystal structure of 7, the molecules form π-stacked dimers related by inversion symmetry.
Hence, a π-stacking interaction with an interplanar separation of ca. 3.41–3.47 Å is present
between the pyrene core and the azaacene group (Table S4). Dimers are arranged edge-to-face
in a sandwich-herringbone packing, which is typically observed for pyrene itself and its
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derivatives (Figure 3).[56] Parallel and inverted molecules that are offset along the a axis exhibit
C···C intermolecular interactions between their CN end groups. The molecular packing is further
determined by the large steric demand of the amine moieties and the presence of
tetrahydrofuran solvent molecules in the crystal lattice. Intermolecular C−HN and HH
interactions are present between the R1 and R2 methoxyphenyl groups and the pyrene-4,5-
azaacene core, while C−HO interactions exist with the tetrahydrofuran molecule (Table S4). A
Hirshfeld surface analysis was performed in order to quantify the nature and type of
intermolecular interactions in 7.[57] The disorder of the methoxyphenyl groups was not taken into
account, but only the parts of highest occupancies were considered in the analysis. Fingerprint
analysis and its breakdown to the individual relative contributions,[58] shows a major contribution
from H···H interactions (42%), followed by a significant amount from C···H (22%), N···H (15%),
and O···H (10%) interactions (Figures S44 and S45); 5% C···C interactions and ca. 3.6% N···C
interactions are observed as well.
Figure 3. Crystal structure of compound 7 projected along the a axis, at 100 K. Hydrogen atoms and disordered parts
of lower occupancies are omitted for clarity. Compound 7 crystallizes in a sandwich-herringbone packing wherein the
herringbone is made up of sandwich-like diads with both edge-to-face and offset face-to-face interactions of π stacked
molecules that have an interplanar distance of X = 3.41-3.47 Å. Atomic displacement ellipsoids are shown at the 50%
probability level. Element color: carbon (white), nitrogen (blue), oxygen (red).
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Photophysical and redox properties
Figure 4. Absorption (left) of 6, 7 and 8 recorded in toluene and emission (right) spectrum of 8 recorded in toluene
and CH2Cl2.
The absorption spectra of derivatives 6, 7 and 8 are depicted in Figure 4 and are generally
similar to that of pyrene, in that the S1←S0 absorption is comparably weak with extinction
coefficients of ε = 2 700-4 000 M-1 cm-1. However, for 2,7-substituted pyrenes, these are the
largest extinction coefficients reported so far. Thus, the acceptor moieties at the K-region
increase the allowedness of this transition. Furthermore, the S1←S0 absorption is strongly
bathochromic shifted in the order 8 < 7 < 6, and remarkably broad, covering a range of
6 000 cm-1, with no vibrational progression, providing an indication of strong charge transfer (CT)
character. In particular, dione 6 has a very pronounced bathochromic shift (λmax (abs) = 658 nm),
which is significantly stronger compared to the analogous dione VI reported by Sutherland and
co-workers (Scheme 2).[40] However, the donor moieties in their derivative are further separated
from the pyrene core by alkyne and phenyl spacers.[59] Hence, the donating ability through the
2,7-positions is reduced and, as a result, a weaker CT character is obtained. Thus, derivative VI
has a smaller bathochromic shift with λmax (abs) = 575 nm compared to derivative 6. It is
interesting to observe that the substituents at the 2,7-positions in these derivatives have such a
significant effect on the S1←S0 absorption, which is unusual. The CT character of these
derivatives is even more evident when comparing them to their analogues without the donor
moieties at the 2,7-positions. Mateo-Alonso and co-workers reported[45] that λmax (abs) = 455 nm
for VII, the analogue of our derivative 7, which demonstrates that the resulting CT character
introduced via the additional donors causes a bathochromic shift of 3 500 cm-1 of the S1←S0
absorption in our systems. Compound VIII, the analogue of our derivative 8, has
λmax (abs) = 435 nm according to Sahoo et al. Hence, the CT character introduced through our
additional donors at the 2,7-positions shifts the S1←S0 absorption by 3 700 cm-1.
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Table 2. Selected photophysical data of the derivatives 6, 7 and 8 recorded under argon at room temperature.
a)
cpd Medium λabs /nm
(ε /103 M-1 cm-1)
λem
Apparent
Stokes[60]
shift /cm-1
τ b)
/ns
φ
τ0
/ns
kr
knr
/nm
/107 s-1
/107 s-1
658 (2.7), 408 (7.0), 344
(41), 299 (31)
6
7
8
8
8
toluene
toluene
-
-
-
-
-
-
-
-
-
-
542 (3.1), 427 (8.5), 348
(48), 300 (42)
-
-
-
-
519 (4.0), 417 (21), 399
toluene (17), 354 (42), 333 (76),
317 (78)
652
700
679
3930
4908
-
32.3
8.8
0.31 104 1.0
0.03 293 0.3
<0.01
2.1
11.0
521, 416, 397, 352, 332,
CH2Cl2
316
solid
-
a) Excited at the respective λmax (abs) of S1←S0.
Compounds 6 and 7 are not emissive; however, 8 emits in the red to NIR region of the
electromagnetic spectrum with λmax (em) = 652 nm in toluene, which is intriguing for pyrenes as
they usually emit in the blue region and, to the best of our knowledge, such a red-shifted
emission has not been reported before for monomeric pyrenes. Even pyrene-fused azaacenes
reported previously do not have such a red-shifted emission.[55] Furthermore, compound 8
exhibits significant solvatochromism, as the emission shifts bathochromically with increasing
solvent polarity from toluene to CH2Cl2 by 1 051 cm-1 (48 nm), confirming the CT nature of the
lowest energy excited state and a significant change in dipole moment between ground state
and excited state. The large CT nature of the excited state becomes more evident when
comparing the emission of 8 with previously reported analogues that only possess a donor or
acceptor moiety. Derivative VIII emits in the blue region with λmax (em) = 471 nm and compound
IV (unfortunately, there is no report of an emission of compound III) emits in the green region
with λmax (em) = 520 nm (both in CH2Cl2) while we observe a λmax (em) = 700 nm for 8 in
CH2Cl2.[32,46] Nevertheless, the non-radiative decay rates are significantly increased in CH2Cl2
(knr = 11·107 s-1), which is fully in line with the energy gap law as the reorganization energy is
enhanced in polar solvents.[61] Therefore, the quantum yield is strongly decreased in the polar
solvent. The apparent Stokes shifts are very large for pyrenes with values ranging from 3 900 to
4 900 cm-1 and are much larger compared to pyrenes that have D/A groups at the 2,7- or
1,3,6,8-positions only.[15,27,28,30] However, the pyrene derivatives reported by Müllen and co-
workers that have D/A moieties at the K-region exhibit even larger Stokes shifts ranging from
4 500 to 5 400 cm-1.[14] Interestingly the radiative decay rates are rather slow (kr = 0.3-
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1.0·107 s-1), which is a result of the strong CT character. However, such slow radiative rate
constants are also typical of forbidden transitions (Strickler-Berg relation) and, thus, the lifetimes
remain quite long (τ0 = 104-293 ns) in this derivative, which is a typical property of 2,7-
substituted pyrenes.[28] There are not many reports on lifetimes of K-region substituted pyrenes,
but they are typically in the range of (τ0 = 13-16 ns), which we recently reported.[62] Müllen and
co-workers do not give lifetimes for their K-region D/A derivatives; however, the S1←S0
absorptions are significantly more allowed with ε > 7 000 M-1 cm-1 and, therefore, shorter
lifetimes than for 2,7-substituted pyrenes can be assumed.[14,63] Compared to compound 8,
derivative 7 has a more pronounced CT nature, hence its energy gap is even smaller and, thus,
it is possible that the non-radiative decay rates are largely increased, so that fluorescence
becomes too weak to detect.
Electrochemistry
In order to determine the impact of the acceptor groups on the K-region in combination with the
donors at the 2,7-positions, cyclic voltammetry was performed. The cyclic voltammograms are
shown in Figure 5 and the respective reduction and oxidation potentials are given in Table 3. All
derivatives exhibit one reversible reduction, whereas compound 6 has the lowest reduction
potential with E1/2 = -1.13 V and 8 the highest with E1/2 = -1.84 V vs. Fc/Fc+. Hence, the
accepting strength of our derivatives is quite strong and decreases in the order 6 > 7 > 8.
Furthermore, all three derivatives can be reversibly oxidized twice; derivative 8 is the easiest to
be oxidized with E1/2 = 0.17 and 0.39 V, and derivatives 6 and 7 have the same first (E1/2 = 0.28)
and second (0.45 V) oxidation potentials vs. Fc/Fc+. Hence, our derivatives possess quite strong
donors and the donating strength in 8 is only minimally influenced by the additional acceptor as
compound III has oxidation potentials of E1/2 = 0.14 and 0.38 V vs. Fc/Fc+. A similar trend is
observed in our derivative 6, as the reduction potential is comparable to that of the analogous
derivative VI, which was reported by Sutherland[40] (Table 1). Consequently, the different
donating abilities at the 2,7-positions do not influence the accepting effect of acceptors at the K-
region. This observation is also in line with the reduction potential of our derivative 7, which is
very similar to that of compound VII which has no donors at the 2,7-positions (Table 1).
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Figure 5. Cyclic voltammograms of 6, 7 and 8 in CH2Cl2/0.1 M [n-Bu4N][PF6] with a scan rate of 250 mV / s.
Table 3. Cyclic voltammetry results for 6, 7 and 8 in CH2Cl2/0.1 M
[n-Bu4N][PF6] relative to the Fc/Fc+ couple.
cpd E1/2 [V] [red]1
E1/2 [V] [ox]1
E1/2 [V] [ox]2
6
7
8
-1.13
-1.42
-1.84
0.28
0.28
0.17
0.47
0.45
0.39
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Spectroelectrochemistry
To gain further information about the electronic structure of the corresponding pyrene 7+, 72+ and
8+, 82+ derivatives, we performed UV/Vis/NIR spectroelectrochemical measurements in
CH2Cl2/0.1 M [n-Bu4N][PF6].
Figure 6. Spectroelectrochemical measurements of the stepwise oxidation process of 7 and 8 in CH2Cl2/0.1 M [n-
Bu4N][PF6]. Top left: absorption spectrum of 7+ (magenta); top right: of 72+ (cyan); bottom left: of 8+ (dark blue) and
bottom right: of 82+ (green).
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Upon oxidation to the respective monocations 7+ and 8+, a band rises in the NIR that is very
broad, covering a range of ca. 7 000 cm-1. The observed plateau may be caused by two
overlapping peaks of a vibronic progression. Thus, the peak maximum depends on the relative
intensities of the first and second overtones. For 7+, the lowest energy band has a maximum at
퐼푉퐶푇
푣̃
= 6 400 cm-1 (1 563 nm,  = 15 160 M-1 cm-1), but for 8+, the maximum is
= 5 350 cm-1 (1 869 nm,  = 28 830 M-1 cm-1) with some asymmetry in its shape (Figure
푚푎푥
퐼푉퐶푇
푚푎푥
푣̃
S1). The absorption spectra of the monocations 7+ and 8+ are very similar to that of compound
III+, which was reported by Ito and co-workers, for which the lowest energy band has a maximum
at 5260 cm-1 (1900 nm) and is also slightly asymmetric.[32] In general, delocalized (Robin-Day
class-III) derivatives possess an asymmetric and narrow lowest energy band, while in localized
(Robin-Day class-II) derivatives the lowest energy band results fron an intervalence charge
transfer (IV-CT) with a well-defined symmetric Gaussian shape.[64] For that reason, Ito and co-
workers concluded that compound III+ is a fully delocalized Robin-Day class-III derivative and
퐼푉퐶푇
푣̃
푚푎푥
assumed an electronic coupling V of
= 2608 cm-1.[32] However, our TD-DFT computations
2
on the monocations 7+ and 8+, with a specially adjusted functional (BLYP with 35% exact HF
exchange[65]), show that the lowest energy excitation is a result of the promotion of an electron
from the β-HOMO to the β-LUMO orbital and these orbitals show the expected phase behavior
for localized Robin-Day class-II compounds (Figure 7). Further analysis of this lowest energy
band enables the calculation of the electronic coupling V between the localized mixed valence
states according to the Mulliken-Hush theory[66] (equation (1)) where µab is the transition moment
(evaluated by integration of the IV-CT band), and the diabatic dipole moment difference Δµ12
was evaluated using the DFT calculated adiabatic dipole moment difference Δµab.[67] According
to these calculations, both derivatives possess a large dipole moment change between their
ground and excited states of Δµab = 42 D (7+) and 43 D (8+), which is typical for Robin-Day class-
II compounds.
퐼푉퐶푇
푣̃
푉 = µ
with ∆µ12
=
∆µ푎푏 + 4µ푎푏
(1)
푎푏 푚푎푥
2
2
√
∆µ
12
Hence, the electronic coupling V between the localized mixed valence states yields
V = 1 279 cm-1 for 7+ and 1 278 cm-1 for 8+. Thus, the electronic coupling V of both derivatives is
퐼푉퐶푇
푣̃
smaller than 푚푎푥 , which shows that these compounds are class-II compounds, but lie close to
2
the borderline between localized class-II and delocalized class-III compounds, as V is quite
large. This result is also in line with the observed broad and asymmetric band shape. Upon
further oxidation to the respective dications, the IV-CT band decreases and the band at 790-
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792 nm (12 660-12 630 cm-1) rises significantly (72+: εmax = 57 000 M-1 cm-1, 82+: εmax = 54 000 M-
1
cm-1). Furthermore, both spectra show bands rising at 727 nm (13 800 cm-1) and 415-421 nm
(24 000 cm-1) while the bands at 320-350 nm (29 000-31 000 cm-1) decrease, which is also very
similar to the observations made for the oxidation to the monocation of compound III.[32]
Table 4. TD-DFT results of the compounds 7+, 8+ and III+ using ublyp with 35% exact exchange admixture, a
SVP basis set and a polarizable continuum model accounting for solvent effects (CH2Cl2).
Sn
E / eV (E / nm)
f
Configuration (major contribution > 10%)
7+
8+
III+
S1
S2
S3
S4
0.96 (1286)
1.55 (799)
1.84 (675)
2.01 (616)
0.383
0.020
0.271
0.007
-H→-L (97%)
-H-1→-L (94%)
-H-4→-L (97%)
-H-3→-L (18%), α-H→α-L+1 (11%),
-H-1→-L+2 (10%)
-H-3→-L (56%), α-H→α-L+1 (14%)
-H→-L (97%)
-H-1→-L (95%)
-H-4→-L (91%)
-H-2→-L (68%)
-H-2→-L (15%), α-H-2→α-L+1 (11%),
α-H→α-L (11%)
-H→-L (97%)
-H-1→-L (98%)
-H-4→-L (98%)
-H-3→-L (31%), α-H-2→α-L (27%),
-H-1→-L+1 (23%)
S5
S1
S2
S3
S4
S5
2.13 (582)
0.87 (1429)
1.37 (902)
1.91 (651)
1.95 (637)
2.07 (599)
0.137
0.505
0.005
0.231
0.074
0.047
S1
S2
S3
S4
0.87 (1419)
1.13 (1102)
1.92 (646)
1.98 (626)
0.478
0.003
0.265
0.033
S5
2.11 (587)
0.087
-H-3→-L (53%), α-H-2→α-L (18%)
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Figure 7. The UDFT computed MOs of 7+, 8+ and III+ that are responsible for the lowest energy excitation.
Consequently, the acceptor moieties at the K-region of the pyrene bridge do not have a large on
the influence the electronic coupling between the two amine donor moieties and thus we prefer
to classify compounds 7+ and 8+ as being at the borderline between class-II and class-III.
DFT and TD-DFT calculations
To rationalize the observed trends and properties we performed DFT and TD-DFT calculations.
The ground state structures were first optimized in the gas-phase at the B3LYP/6-31+G* level of
theory. Previous studies[28] have shown that range-separated hybrid functionals are necessary to
obtain a reliable picture of the nature and relative energetic ordering of the excited states in
pyrenes. We have thus used the CAM-B3LYP functional for the subsequent TD-DFT
calculations.
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Figure 8. Molecular orbital diagram with orbital depictions of pyrene, 7 and 8.
The nitrogen 2pz orbitals in both pyrene derivatives 7 and 8 mix very efficiently with the HOMO-1
of the pyrene core. This leads to a drastic destabilization of the HOMO-1 (black, Figure 8) by ca.
1.51 eV in 7 and 1.82 eV in 8, which consequently switches the order of the HOMO and
HOMO-1 orbitals. The pyrene-like HOMO (black, Figure 8), on the other hand, mixes with the
acceptor unit at the K-region. Hence, the pyrene bridge and the acceptor unit are fully
delocalized and thus, this orbital (now HOMO-2) is stabilized by ca. 0.54 eV in 7 and 0.08 eV in
8. A new orbital (blue, Figure 8) of non-bonding character with large coefficients at the nitrogens
of the dianisylamine donors is energetically positioned between the new HOMO and HOMO-2
orbitals, which was also observed for derivative IX. The LUMOs of both 7 and 8 are greatly
delocalized over the acceptor units and the pyrene bridge. Hence, these orbitals are
considerably stabilized, with the LUMO of 7 stabilized by 1.08 eV, and of 8 by 0.52 eV compared
to the one in pyrene. This pronounced stabilization of the LUMOs in 7 and 8 is reflected in our
cyclic voltammetry studies by their remarkably low reduction potentials of -1.42 V and -1.84 V vs.
Fc/Fc+, respectively. The pyrene LUMO and LUMO+1-like orbitals (grey, Figure 8) are not
significantly affected in 7 (now LUMO+2 and LUMO+3, respectively) and 8 (now LUMO+1 and
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LUMO+2, respectively). The pyrene LUMO-like orbital is delocalized over the acceptor unit and
the pyrene core, while the pyrene LUMO+1-like is delocalized over the dianisylamine donors and
the pyrene core.
The TD-DFT calculations show that the nature of the S1←S0 transition changes in such a way
that it is no longer a nearly 50:50 weighted configuration interaction of HOMO-1→LUMO and
HOMO→LUMO+1 as in pyrene (Table 4). In 7 and 8, the S1←S0 transitions are nearly pure
HOMO→LUMO transitions, which have significant CT nature (Figure 8). They are strongly
bathochromically shifted in the order 7 > 8 with low oscillator strengths, which matches well with
the absorption maxima and extinction coefficients that were measured for this band (7:
λmax (abs) = 542 nm, ε = 3 100 M-1 cm-1 and 8: λmax (abs) = 519 nm, ε = 4 000 M-1 cm-1).
Table 5. TD-DFT results (CAM-B3LYP/6-31+G*) for the five vertical transitions of pyrene, 7 and 8.
FC-Sn E [eV] (E [nm])
f
Configuration (major contribution
10%)
>
pyrene
S1
S2
S3
S4
S5
S1
S2
S3
S4
S5
S1
S2
S3
S4
S5
3.99 (311)
4.02 (309)
4.90 (253)
5.10 (243)
5.11 (243)
2.69 (461)
2.76 (449)
3.40 (365)
3.43 (361)
3.57 (348)
2.93 (423)
3.19 (398)
3.63 (342)
3.64 (341)
3.76 (330)
0.000
0.323
0.000
0.000
0.414
0.071
0.047
0.059
0.005
0.167
0.120
0.006
0.390
0.002
0.000
H-1→L (49%), H→L+1 (50%)
H→L (67%), H-1→L+1 (23%)
H-2→L (21%), H→L+2 (67%)
H-2→L (66%), H→L+2 (19%)
H-1→L (50%), H→L+1 (49%)
H→L (90%)
H→L+1 (84%)
H-1→L (66%), H-2→L (18%)
H-1→L+1 (79%)
H-2→L (72%), H-1→L (20%)
H→L (90%)
H→L+1 (66%), H-2→L (12%)
H-2→L (75%), H→L+1 (15%)
H-8→L (90%)
7
8
H-1→L (82%)
Conclusions
We have reported the synthesis and structural characterization of new pyrene derivatives with
donor moieties at the 2,7-positions and an acceptor group at the K-region. In general,
unsymmetrically substituted pyrene derivatives remain rare due to their challenging synthesis.
The influence of the donors at the 2,7-positions and the acceptors at the K-region on the
photophysical and electrochemical properties of the compounds is remarkable. The new
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derivatives possess very broad absorptions with maxima at 519-658 nm, and tails up to 800 nm,
and compound 8 emits in the red to NIR region, which has not previously been reported for
monomeric pyrenes. The intrinsic lifetimes remain rather long (τ0 = 104-293 ns), which is a
typical property of 2,7-substituted pyrenes, whereas the excited states of most K-region
substituted pyrenes have lifetimes (τ0) of only ca. 13-16 ns. Cyclic voltammetry studies reveal
two reversible one-electron oxidations and one reversible reduction for all three derivatives 6, 7
and 8, with very low potentials showing the unique donating and accepting properties of our
derivatives. Spectroelectrochemical measurements suggest a strong electronic coupling
between the substituents at the 2,7-positions for 7 and 8. Our DFT and TD-DFT calculations
indicate that these properties are the result of the very strong donors and acceptors which
couple very well with the pyrene orbitals, resulting in a reordering of the occupied orbitals.
Consequently, the S1 state of these derivatives has strong CT nature giving rise to unparalleled
properties.
Experimental Section
General considerations
[68]
[69]
The catalyst precursors [Ir(COD)(OMe)]2 and Pd2(dba)3·CHCl3 were prepared according to
literature procedures, B2pin2 was a gift from AllyChem Co. Ltd. while other starting materials
were purchased from commercial sources and used as received. Solvents used for synthesis
were HPLC grade, further treated to remove trace water using a commercial solvent purification
system from Innovative Technology Inc. and deoxygenated using the freeze-pump-thaw method.
1
13
11
15
The H, C{1H}, B{1H}, N HMBC NMR spectra were recorded at room temperature in CDCl3
or CD2Cl2 solution either on a Bruker Avance I HD 500 (1H, 500 MHz; 13C, 125 MHz; 11B, 160
MHz), a Bruker Avance III HD 300 (1H, 300 MHz; 13C, 75 MHz; 11B, 96 MHz) or a Bruker Avance
1
13
200 (1H, 200 MHz). H NMR and C spectra are referenced to the residual protonated solvent
CHCl3 (1H, δ = 7.26, CDCl3) and CHDCl2 (1H, δ = 5.32, CD2Cl2). 11B{1H} NMR signals were
referenced to external BF3∙OEt2 and 15N HMBC signals were referenced to MeNO2 + 10%CDCl3.
Chemical shifts are listed in parts per million (ppm) and coupling constants in Hertz (Hz).
HRMS were recorded using a Thermo Scientific Exactive Plus Orbitrap MS system with either an
Atmospheric Sample Analysis Probe (ASAP) or by Electrospray Ionization (ESI). Elemental
analyses were performed on an Elementar vario MICRO cube elemental analyzer.
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Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600
potentiostat. A standard three-electrode cell configuration was employed using a platinum disk
working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip,
serving as the reference electrode. Formal redox potentials are referenced to the
ferrocene/ferrocenium ([Cp2Fe]+/0) redox couple by using decamethylferrocene ([Cp*2Fe]; E1/2
=
−0.532 V in CH2Cl2) as an internal standard. Tetra-n-butylammonium hexafluorophosphate ([n-
Bu4N][PF6]) or [n-Bu4N][Al(OC(CF3)3)4] were employed as supporting electrolytes. Compensation
for resistive losses (iR drop) was employed for all measurements.
Crystals suitable for single-crystal X-ray diffraction were selected, coated in perfluoropolyether
oil, and mounted on MiTeGen sample holders. Diffraction data were collected on a Bruker X8
Apex II 4-circle diffractometer with a CCD area detector using Mo-Kα radiation monochromated
by graphite (2, 6’, 11, 12) or multi-layer focusing mirrors (5’, 10). Diffraction data of 7 were
collected on a Bruker D8 Quest 4-circle diffractometer with a CMOS area detector (Photon II)
and multi-layer mirror monochromated Mo-K radiation. The crystals were cooled using Oxford
Cryostream or Bruker Kryoflex II low-temperature devices. Data were collected at 100 K. The
images were processed and corrected for Lorentz-polarization effects and absorption as
implemented in the Bruker software packages. The structures were solved using the intrinsic
phasing method (SHELXT)[70] and Fourier expansion technique. All non-hydrogen atoms were
refined in anisotropic approximation, with hydrogen atoms ‘riding’ in idealized positions, by full-
matrix least squares against F2 of all data, using SHELXL[70] software. In compound 11, the
coordinates of the hydrogen atom of the water molecule, which lies on a two-fold rotation axis,
were refined freely, but restraints were applied to the O−H and H−H distances. In compound 5’
the coordinates of the hydrogen atoms bonded to nitrogen were refined freely. The
methoxyphenyl groups as well as the tetrahydrofuran solvent molecule are strongly disordered in
compound 7. Hence, several restraints had to be applied in the refinement. Diamond[71] software
was used for graphical representation. Hirshfeld surfaces were calculated and analyzed using
the CrystalExplorer[72] program. Other structural information was extracted using Mercury[73] and
OLEX2[74] software. Crystal data and experimental details are listed in Table S1; full structural
information has been deposited with the Cambridge Structural Database. CCDC-1917153 (2),
1917154 (5’), 1917155 (6’), 1917156 (7), 1917157 (10), 1917158 (11), and 1917159 (12).
General photophysical measurements
All photophysical measurements were carried out under an argon atmosphere. All solution state
measurements were performed in standard quartz cuvettes (1 cm x 1 cm cross section). UV/Vis
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absorption spectra were recorded using an Agilent 1100 diode array UV/Vis spectrophotometer.
Excitation, emission, lifetime and quantum yield measurements were recorded using an
Edinburgh Instruments FLSP920 spectrometer equipped with a 450 W Xenon arc lamp, double
monochromators for the excitation and emission pathways, and a red-sensitive photomultiplier
(PMT-R928P) and a near-IR PMT as detectors. The measurements were made in right-angle
geometry mode and all spectra were fully corrected for the spectral response of the instrument.
All solutions used in photophysical measurements had a concentration lower than 10-5 M.
Fluorescence quantum yield measurements
Fluorescence quantum yields of the samples were measured using a calibrated integrating
sphere (150 mm inner diameter) from Edinburgh Instruments combined with the FLSP920
spectrometer described above. For solution-state measurements, the longest wavelength
absorption maximum of the compound in the respective solvent was chosen for the excitation. In
order to exclude self-absorption, the emission spectra were measured with dilute samples (ca.
0.1 OD at the excitation wavelength).
Fluorescence lifetime measurements
Lifetime measurements were conducted using the time-correlated single-photon counting
method (TCSPC) on the FLSP920 spectrometer equipped with a high-speed photomultiplier
tube positioned after a single emission monochromator. Measurements were made in right-angle
geometry mode, and the emission was collected through a polarizer set to the magic angle.
Solutions were excited with either a 315 (pulse width 932.5 ps), 376 (pulse width 72.6 ps) or a
472 nm (pulse width 90.6 ps) pulsed diode laser at repetition rates of 1-5 MHz and were
recorded at emission maxima. Decays were recorded to 10 000 counts in the peak channel with
a record length of at least 4 000 channels. The band-pass of the monochromator was adjusted
to give a signal count rate of <20 kHz. Iterative reconvolution of the IRF with one decay function
and nonlinear least-squares analysis were used to analyze the data. The quality of all decay fits
was judged to be satisfactory, based on the calculated values of the reduced χ2 and Durbin-
Watson parameters and visual inspection of the weighted and autocorrelated residuals.
Spectroelectrochemical measurements
Spectroelectrochemical experiments in reflection mode were performed using an Agilent Cary
5000 Spectrometer in combination with a designed sample compartment consisting of a
cylindrical PTFE cell with an Infrasil® wedge window with an angle of 0.5° and an adjustable
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three-in-one electrode (6 mm platinum disc working electrode, 1 mm platinum counter electrode
and pseudo reference electrode). The potentials were adjusted with a Gamry 600 potentiostat
and all experiments were measured at room temperature under an argon atmosphere.
Theoretical studies
All calculations (DFT and TD-DFT) were carried out with the Gaussian 09 (Rev. E.01) program
package[75] and were performed on a parallel cluster system. GaussView 5.0.9 was used to
visualize the results, to measure calculated structural parameters, and to plot orbital surfaces
-3 1/2
(isovalue: ± 0.02 [e a0 ] ). The ground-state geometries were optimized using the B3LYP
functional[76] in combination with the 6-31G(d) basis set.[77] The optimized geometries were
confirmed to be local minima by performing frequency calculations and obtaining only positive
(real) frequencies. Based on these optimized structures, the lowest-energy gas-phase vertical
transitions were calculated (singlets, 10 states) by TD-DFT, using the Coulomb-attenuated
functional CAM-B3LYP[78] in combination with the 6-31G+(d,p) basis set. DFT calculations on the
monocations of 7 and 8 were carried out, using UBLYP with 35% exact-exchange admixture, the
SVP basis set and a polarizable continuum model accounting for solvent effects. The time
dependent (TD-DFT) calculations were performed at the same level of theory.[65,79]
Synthesis of the pyrene derivatives
Pyrene-4,5-dione (1)
In a round bottom flask pyrene (10.0 g, 49.4 mmol, 1.00 eq) and RuCl3 • x H2O (987 mg,
4.74 mmol, 0.096 eq) were dissolved in 200 mL of MeCN. NaIO4 (42.3 g, 198 mmol, 4.00 eq)
was dissolved in 250 mL of hot water and carefully added to the pyrene solution. Another
200 mL of CH2Cl2 were added and the reaction mixture was vigorously stirred for 18 h at room
temperature. After filtration of the suspension through Celite®, the filtrate was extracted with
CH2Cl2 (3 x 100 mL) and the combined organic phases were washed with saturated aqueous
Na2S2O3 and H2O and then dried over Na2SO4. After removing the solvent under reduced
pressure the product was obtained as a dark orange solid (7.56 g, 66%). The spectroscopic data
match those reported previously.[47,50,80]
1H NMR (200 MHz, CDCl3): δ = 8.51 (dd, J = 7 Hz, J = 1 Hz, 2H), 8.19 (dd, J = 8 Hz, J = 1 Hz,
2H), 7.87 (s, 2H), 7.77 (t, J = 8 Hz, 2H) ppm.
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Pyrene-4,5-di(ethyleneglycol)ketal (2)
Pyrene-4,5-dione (1) (4.92 g, 21.2 mmol, 1.00 eq) was suspended in 200 mL toluene and
ethylene glycol (148 mL, 2.65 mol, 125 eq) and p-toluenesulfonic acid (1.81 g, 9.53 mmol,
0.45 eq) were added. The reaction mixture was refluxed at 125 °C for 20 h. Toluene was
removed under reduced pressure and 400 mL of water. The crude product was isolated by
filtration, washed with water and purified by flash chromatography (silica, cyclohexane/ethyl
acetate 9:1). The product was isolated as a white solid (3.46 g, 10.8 mmol, 51%). The
spectroscopic data match those reported previously.[81]
1H NMR (500 MHz, CD2Cl2): δ = 7.98 (dd, J = 7 Hz, J = 1 Hz, 2 H), 7.97 (dd, J = 8 Hz, J = 1 Hz,
2 H), 7.82 (s, 2 H), 7.73 (dd, J= 8 Hz, J = 7 Hz, 2 H), 4.29 (br, 4 H), 3.75 (br, 4 H) ppm.
13C{1H} NMR (125 MHz, CD2Cl2): δ = 133.5, 131.8, 129.2, 127.6, 127.1, 126.9, 124.6, 93.9, 62.3.
HRMS (ASAP+): m/z calcd for [C20H16O4+H]+: 321.1121; found: [M+H]+ 321.1115 (IΔI = 1.87
ppm).
Elemental analysis calcd (%) for C20H16O4: C 74.99, H 5.03; found: C 74.90, H 5.08.
2,7-Bis(Bpin)-4,5-di(ethyleneglycol)ketal-pyrene (3)
In an argon-filled glovebox, a Young’s tube was filled with pyrene-4,5-di(ethyleneglycol)ketal (2)
(2.91 g, 9.08 mmol, 1.00 eq), B2pin2 (4.61 g, 18.2 mmol, 2.00 eq), [Ir(COD)(OMe)]2 (301 mg,
0.45 mmol, 0.05 eq), 4,4’-di-tert-butyl-2,2’-dipyridine (dtbpy) (260 mg, 0.91mmol, 0.10 eq) and
50 mL of MTBE. The Young’s tube was sealed, and the reaction mixture was stirred at 80 °C for
20 h. After cooling to room temperature, the crude product was passed through a pad of silica
using toluene as the eluent. The solvent was removed under reduced pressure and the crude
product was purified via flash chromatography (silica, cyclohexane/ethyl acetate 95:5). The
product was obtained as a white solid (4.58 g, 8.00 mmol, 88%).
1H NMR (300 MHz, CD2Cl2): δ = 8.40 (d, J = 1 Hz, 2H), 8.28 (d, J = 1 Hz, 2H), 7.85 (s, 2H), 4.33-
4.28 (m, 4H), 3.76-3.71 (m, 4H), 1.41 (s, 24H) ppm.
13C{1H} NMR (75 MHz, CD2Cl2): δ = 136.6, 133.0, 131.6, 129.7, 128.6, 127.4, 94.0, 84.6, 62.3,
25.2 ppm (one C not observed, likely that attached to B).
11B{1H} NMR (96 MHz, CD2Cl2): δ = 31.5 ppm.
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10.1002/chem.201904219
Chemistry - A European Journal
HRMS (ASAP+): m/z calcd. for [C32H38B2O8+H]+ 573.2826; found: [M+H]+ 573.2799 (IΔI = 4.71
ppm).
Elemental analysis calcd. (%) for C32H38B2O8: C 67.16, H 6.69; found: C 66.89, H 6.97.
2,7-Bis(bromo)-4,5-di(ethyleneglycol)ketal-pyrene (4)
The compound 2,7-bis(Bpin)-4,5-di(ethyleneglycol)ketal-pyrene (3) (4.58 g, 7.99 mmol, 1.00 eq)
was diluted in 100 mL THF and heated to 50 °C. Afterwards, 80 mL of MeOH was added and
the heating bath temperature was elevated to 90 °C. A solution of CuBr2 (8.90 g, 40.0 mmol,
6.00 eq) in 80 mL of H2O was slowly added to this reaction mixture via a dropping funnel. The
reaction mixture was stirred at 90 °C for 96 h and an orange solid precipitated. The precipitate
was isolated by filtration and washed with water and EDTA (0.1 M) solution. The crude product
was purified via flash chromatography (silica, hexane/ethyl acetate 4:1) to give a white solid
(1.02 g, 2.13 mmol, 27%).
1H NMR (500 MHz, CDCl3): δ = 8.08 (s, 4H), 7.71 (s, 2H), 4.32 (br, 4H), 3.78 (br, 4H) ppm.
13C{1H} NMR (125 MHz, CDCl3): δ = 135.0, 133.1, 131.5, 127.9, 127.1, 125.1, 121.9, 93.2, 62.1
ppm.
HRMS (APCI+): m/z calcd. for [C20H14Br2O4+H]+ 476.9332; found: [M+H]+ 476.9330 (IΔI = 0.42
ppm).
2,7-Bis(DPA)-4,5-di(ethyleneglycol)ketal-pyrene (5)
In an argon-filled glovebox,
a
Young’s tube was filled with 2,7-bis(bromo)-4,5-
di(ethyleneglycol)ketal-pyrene (4) (3.00 g, 6.27 mmol, 1.00 eq), bis(4-methoxyphenyl)amine
(3.02 g, 13.2 mmol, 2.10 eq), Pd2(dba)3·CHCl3 (130 mg, 126 μmol, 0.02 eq), XPhos (120 mg,
251 μmol, 0.04 eq), KOtBu (2.11 g, 18.8 mmol, 3.00 eq) and toluene (200 mL), which was then
sealed. Afterwards, the reaction mixture was stirred at 115 °C for 15 h. The solvent was
removed under reduced pressure and the crude product was purified by flash chromatography
(Biotage® KP-NH, cyclohexane/CH2Cl2 4:1 → 4:1). Product (5) was obtained as a yellow solid
(2.83 g, 3.65 mmol, 58%).
1H NMR (300 MHz, CD2Cl2): δ = 7.49 (d, J = 2 Hz, 2H), 7.31 (s, 2H), 7.24 (d, J = 2 Hz, 2H),
7.14–7.09 (m, 8H), 6.90–6.86 (m, 8H), 4.11 (br, 4H), 3.80 (s, 12H), 3.65 (br, 8H) ppm.
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10.1002/chem.201904219
Chemistry - A European Journal
13C{1H} NMR (75 MHz, CD2Cl2): δ = 156.6, 147.6, 141.3, 133.3, 132.0, 127.0, 126.8, 121.3,
119.2, 118.0, 115.1, 93.9, 62.1, 55.9 ppm.
15N (from 15N 1H HMBC) NMR (300 MHz, CD2Cl2): -285.8 ppm.
HRMS (APCI+): m/z calcd. for [C48H42N2O8+H]+ 775.3014; found: [M+H]+ 775.3000
(|Δ| = 1.80 ppm).
Elemental analysis calcd. (%) for C48H42N2O8: C 74.40, H 5.46, N 3.62; found: C 74.00, H 6.07,
N 3.34.
2,7-Bis(DPA)-4,5-dione (6)
In a round bottom flask, 2,7-bis(DPA)-4,5-di(ethyleneglycol)ketal-pyrene (5) (500mg, 645 μmol)
was suspended in 10 mL of water. Then, 60 mL of TFA was added dropwise and the reaction
mixture was stirred at room temperature for 15 h. The solution was then neutralized with an
aqueous solution of NaHCO3 and extracted with ethyl acetate. The organic phases were dried
over NaSO4, the resulting crude product was washed with methanol and the solvents were
removed under reduced pressure. The product 2,7-bis(DPA)-4,5-dione (6) was obtained as a
dark green solid (360 mg, 534 μmol, 81%).
1H NMR (300 MHz, CD2Cl2): δ = 7.88 (d, J = 3 Hz, 2H), 7.43 (d, J = 3 Hz, 2H), 7.39 (s, 2H),
7.15–7.10 (m, 8H), 6.93–6.88 (m, 8H), 3.82 (s, 12H) ppm.
13C{1H} NMR (75 MHz, CD2Cl2): δ = 181.1, 157.2, 148.1, 140.2, 132.9, 130.3, 127.5, 127.2,
123.5, 123.1, 122.7, 115.4, 55.9 ppm.
15N (from 15N 1H HMBC) NMR (300 MHz, CD2Cl2): -284.9 ppm.
HRMS (ASAP+): m/z calcd. for [C44H34N2O6+H]+ 687.2417; found [M+H]+ 687.2479
(|Δ| = 9.02 ppm).
Elemental analysis calcd. (%) for C44H34N2O6: C 76.95, H 4.99, N 4.08; found: C 77.00, H 4.96,
N 4.25.
2,7-Bis(DPA)-4,5-azaacene-CN (7)
In a round bottom flask, 2,7-bis(DPA)-4,5-dione (6) (200 mg, 291 μmol), 2,3-diaminomaleonitrile
(188 mg, 1.74 mmol), ethanol (35 mL) and acetic acid (35 mL) were stirred at 80 °C for 15 h.
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