Facile Incorporation of Pd(PPh3)2Hal Substituents into Polymethines, Merocyanines, and Perylene Diimides as a Means of Suppressing Intermolecular Interactions

Article abstract of DOI:10.1021/jacs.6b06361

Compounds with polarizable π systems that are susceptible to attack with nucleophiles at C-Hal (Hal = Cl, Br) bonds react with Pd(PPh₃)₄ to yield net oxidative addition. X-ray structures show that the resulting Pd(PPh₃)₂Hal groups greatly reduce intermolecular π-π interactions. The Pd-functionalized dyes generally exhibit solution-like absorption spectra in films, whereas their Hal analogues exhibit features attributable to aggregation.

Full text of DOI:10.1021/jacs.6b06361

Communication
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Facile Incorporation of Pd(PPh3)2Hal Substituents into Polymethines,
Merocyanines, and Perylene Diimides as a Means of Suppressing
Intermolecular Interactions
Iryna Davydenko,† Stephen Barlow,† Rajesh Sharma,‡ Sepehr Benis,‡ Janos Simon,† Taylor G. Allen,†
Matthew W. Cooper,† Victor Khrustalev,§,∥ Evgheni V. Jucov,§ Raul Castaneda,§ Carlos Ordonez,§
́
̃
Zhong’an Li,⊥ San-Hui Chi,† Sei-Hum Jang,⊥ Timothy C. Parker,† Tatiana V. Timofeeva,§
Joseph W. Perry,† Alex K.-Y. Jen,⊥ David J. Hagan,‡ Eric W. Van Stryland,‡ and Seth R. Marder*,†
†School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States
‡CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, United States
§Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701, United States
∥Department of Inorganic Chemistry, Peoples’ Friendship University of Russia, Moscow 117198, Russia
⊥Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States
S
* Supporting Information
films while generally maintaining absorption spectra similar to
ABSTRACT: Compounds with polarizable π systems that
those seen for dilute (10−100 μM) solutions.
are susceptible to attack with nucleophiles at C−Hal (Hal
= Cl, Br) bonds react with Pd(PPh3)4 to yield net oxidative
addition. X-ray structures show that the resulting Pd-
(PPh3)2Hal groups greatly reduce intermolecular π−π
interactions. The Pd-functionalized dyes generally exhibit
solution-like absorption spectra in films, whereas their Hal
analogues exhibit features attributable to aggregation.
We recently reported minimizing heptamethine aggregation
for third-order NLO applications by attaching bulky, rigid out-
of-plane groups to both the polymethine bridge and
heterocyclic end moieties.8 While attempting to use Pd-
catalyzed cross-couplings to introduce bulky aryl groups into
the central meso positions, we instead obtained the products of
oxidative addition of meso-Cl-substituted dyes to Pd(PPh3)4 as
stable compounds.
Dyes 1Cl−5Cl (see the Supporting Information (SI) for
syntheses) react cleanly with 1 equiv of Pd(PPh3)4 at room
temperature under an inert atmosphere to form 1Pd−5Pd,
respectively. The reaction is easily scalable (5Pd has been
prepared on a 1 g scale), and the products are readily purified
ighly polarizable organic π systems can exhibit properties
Hthat are relevant to organic electronics and nonlinear
optics (NLO). In some cases strong π−π interactions are
desirable, for example, for efficient charge transport within
organic semiconductors. In others π−π interactions can be
detrimental; for example, the third-order NLO properties of
certain polymethines in dilute solution are promising for all-
optical switching (AOS) applications yet are not easily
translated to the high-chromophore-density films required
because of strong intermolecular interactions that lead to
aggregate-like spectra in the solid state.1 Preventing aggregation
and π−π interactions can also be important for optical
limiting,2 dye-sensitized solar cells,3 and cases where strong
fluorescence is required.4 Approaches used to preclude
aggregation with varying degrees of success include host−
guest chemistry5 and attachment of dendrons4,6 and other
bulky groups to the chromophore.7
Here we report an effective and simple method of
introducing rigid steric bulk around π systems. Specifically,
C(sp3)−Hal (Hal = Cl, Br) bonds in a variety of species,
including cationic and anionic polymethines, neutral merocya-
nines, and perylene diimides (PDIs), react with Pd(PPh3)4 to
yield net oxidative addition products. Bulky out-of-plane PPh3
groups allow these chomophores, which otherwise exhibit
strong propensities to aggregate, to be spin-cast into 100 wt %
1
by column chromatography or recrystallization. H, 13C, and
31P NMR data (see the SI) indicate that the two PPh3 ligands
are trans (as shown in Scheme 1) but inequivalent, suggesting
that the plane formed by the Pd atom and its ligands is roughly
orthogonal to the cyanine π system, with the inequivalence of
t
the two PPh3 ligands arising from the Bu substituent. The
Pd(PPh3)2Cl dyes are stable toward air in the dark as crystalline
solids (no decomposition over 1 year was detectable by NMR
spectroscopy); studies of photobleaching in air and thermog-
ravimetric analysis indicated that incorporation of the organo-
metallic fragment does not significantly impair the stability of
the chromophores (see the SI).
The crystal structures of 2Cl, 1Pd, 5Pd, and 5′Pd were
determined (Figure 1). The phosphines of the Pd compounds
were confirmed to be trans, and the dihedral angles between the
square plane around Pd and the plane of the polymethine π
system were found to be ca. 90° (e.g., 88° for 1Pd). The
structure of 2Cl (Figure 1a,b) consists of centrosymmetric π
Received: June 20, 2016
© XXXX American Chemical Society
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DOI: 10.1021/jacs.6b06361
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Journal of the American Chemical Society
Communication
Scheme 1. Pd(PPh3)2Cl Functionalization of Heptamethines
Chart 1. Anionic and Neutral Polymethines Dyes
Figures 2 and S3 show absorption spectra of Cl- and
Pd(PPh3)2Cl-substituted dyes in solution and as 100 wt %
Figure 2. Absorption spectra of (a) 2Cl, (b) 1Pd, (c) 5Pd, and (d)
6Pd in CH2Cl2 solutions (red) and films of 100 wt % dye spin-cast
from CH2Cl2 blends (black). The blue spectrum in (c) is for a film of
100 wt % 5′Pd dye spin-cast from CH2Cl2.
Figure 1. Crystal structures of (a, b) 2Cl, (c, d) 1Pd, (e, f) 5Pd, and
(g, h) 5′Pd, showing for each the interactions of the π system of one
polymethine cation (colored by element) with that of its neighbors
(uniform colors). Selected interplanar and atom−plane distances in Å
are indicated. H atoms, counterions, solvent molecules, and disorder
have been omitted for clarity.
films. The maxima for the Pd(PPh3)2Cl species are blue-shifted
relative to those of their Cl analogues, suggesting that
Pd(PPh3)2Cl is a stronger π donor and/or weaker σ acceptor
than Cl.9 The solution fluorescence of 2Pd is also blue-shifted
relative to that of 2Cl (Figure S5). Their fluorescence quantum
yields (QYs) are fairly similar, indicating that the Pd(PPh3)2Cl
group does not lead to rapid intersystem crossing; indeed, the
QY for 2Pd is ca. 1.5× larger than that for 2Cl.
The absorption spectrum of a 100 wt % film of 2Cl (Figure
2a) shows a broad high-energy band that is likely due to H-like
or low-symmetry aggregates. On the other hand, the
interactions in the crystal (Figure 1a,b) more closely resemble
those of J-aggregates.10 Presumably the packing in the spin-cast
film differs from that in the crystal; regardless, there are
evidently strong interactions between adjacent π systems in
both solid environments. The “aggregate” feature is much less
prominent and less blue-shifted in the film spectrum of 5Cl
(Figure S3), perhaps as a result of the effect of the CMe2
portions of the terminal heterocycle. Nevertheless, the spectra
of spin-cast films of 100 wt % 1Pd and 5Pd (Figure 2b,c) retain
more of the characteristic spectral features of polymethines,
consistent with the large dye−dye displacements and reduced
π−π interactions induced by the Pd(PPh3)2Cl groups in the
crystals. However, the spectrum of a neat film of 2Pd suggests
J-aggregation (Figure S3). The absorption spectra of neat films
of 5Pd and 5′Pd are similar, indicating that in the presence of
the bulky Pd(PPh3)2Cl groups the anion has relatively little
effect (in contrast to 10 wt % films of related Cl dyes11).
dimers; a considerable portion of the π system of each dye is π-
stacked with that of its neighbor (3.57 Å average interplanar
distance). The terminal rings are also engaged in additional π
interactions with those of other cations. The structure of 1Pd
also consists of centrosymmetric π dimers; however, only the
naphthalene rings of one of the terminal heterocycles of each
dye are involved (3.47 Å interplanar distance). For 5Pd there
are no close intermolecular π−π interactions, perhaps partly
because of the out-of-plane CH3 groups in the terminal
heterocycles. The structure of 5′Pd (Figure 1g,h), in which the
BAr′4 (Ar′ = 3,5-(CF3)2C6H3) anion of 5Pd is replaced by BF4,
shows the impact of both anion and Pd functionalization on π
interactions: this structure consists of chains of cations with
close π interactions between terminal naphthalene rings.
The reaction is also applicable to a wide range of different
polymethines; for example, the anionic cyanine-like dyes 6Pd
and 6′Pd and the neutral merocyanine 7Pd (Chart 1) have also
been obtained from 6Cl, 6′Cl, and 7Cl, respectively. The
crystal structures of 7Cl and 7Pd (Figure S1) show that the
bulky Pd(PPh3)2Cl group also suppresses intermolecular π−π
interactions in the merocyanine.
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DOI: 10.1021/jacs.6b06361
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Journal of the American Chemical Society
Communication
Neat films of the anionic dyes 6Pd and 6′Pd and neutral
merocyanine 7Pd also exhibit spectra that more closely
resemble their solution spectra than do those of their Cl
analogues 6Cl12 and 7Cl, although for 6Pd the solid-state
absorption maxima are more red-shifted relative to those in
solution than for 6′Pd (Figures 2d and S3). In view of previous
work on 6′Cl and analogues with different cations,13 6Pd and
6′Pd provide an opportunity to examine the utility of
Pd(PPh3)2Cl functionalization for NLO applications; spin-
coated films of 50 wt % blends with amorphous polycarbonate
(APC) were studied to enable direct comparison to the
literature.13 Poor optical quality (Figure S13) precluded
acquisition of reliable data for 6Pd, but data acquired at 1550
nm for 50 wt % 6′Pd/APC (Figure S15) and 50 wt %
6′Cl:APC13a are compared in Table 1 along with the linear
Figure 3. Absorption spectra of (a) PDI-Br2 and (b) PDI-Pd2 in
CH2Cl2 solutions (red) and 100 wt % films spin-cast from CH2Cl2
(black).
cores found in the crystal structure of PDI-Pd2 (Figure S2) and
also that of II.14 Both the solution and film fluorescence spectra
(Figure S5) of PDI-Pd2 are consistent with molecule-based
emission; the QYs are 0.68 and 0.13, respectively. The structure
of PDI-Pd1 (Figure S2) consists of slipped π dimers, but films
also show a solution-like spectrum (Figure S4). The absorption
spectrum of a PDI-Pt1 film (Figure S4) indicates stronger
aggregation effects than for PDI-Pd1, consistent with less
effective prevention of approach of neighboring PDIs by cis-
PPh3 ligands. Given the rapid crossing to the triplet manifold
seen for Pt−acetylide-functionalized PDIs,16 studies of the
excited-state dynamics of the Pt−PDI dyes are underway.
In conclusion, Pd(PPh3)2Hal (Hal = Cl, Br) can readily be
introduced into a variety of polymethines and PDIs and can
effectively suppress the effects of intermolecular interactions on
the optical properties in high-chromophore-density films. In
contrast to our previous method of minimizing aggregation of
chalcogenopyrylium heptamethines,8 which required multistep
synthesis, the dyes obtained here can be readily assembled in a
few steps from commercial materials, with the bulky group
being introduced in the final step. The reaction of metal−
phosphine compounds and C−Hal bonds may prove useful in
other areas of organic photonics and electronics where control
of aggregation is required.
Table 1. NLO Properties of 50 wt % Blends of 6′Cl and 6′Pd
with APC at 1550 nm
6′Cl13a
−3.8
2.7
6′Pd
−4.2
7.2
Re(χ(3)) (10−11 esu)
|Re(χ(3))/Im(χ(3))|
linear loss (dB cm−1
a
)
7.5
5.6
a
Determined using a prism coupler; the loss for a neat APC film was
measured to be ca. 1.9 dB cm−1.
losses at the same wavelength. The magnitude of Re(χ(3)) for
the 6′Pd/APC film is comparable to that for APC blends of
6′Cl and analogues with different counterions,13a but the linear
and nonlinear optical losses are both significantly lower (the
latter corresponding to a larger value of the two-photon figure-
of-merit, |Re(χ(3))/Im(χ(3))|), indicating improved suitability
for AOS applications.
A related conversion of dibromo-PDI I to the Pd(PPh3)2Br
species II (Chart 2) using Pd2(dba)3 and PPh3 at 50 °C has
Chart 2. Pd and Pt PDI Derivatives
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b06361.
■
S
Experimental details, decomposition studies, and addi-
tional figures showing spectra and crystal structures
(PDF)
Crystallographic data for 1Pd, 2Cl, 5Pd, 5′Pd, 7Cl, 7Pd,
previously been described.14 We found that this reaction can
also be accomplished using the same conditions as for the
polymethines: direct reaction of PDI-Br2 and PDI-Br1 with
Pd(PPh3)4 at room temperature afforded compounds PDI-Pd2
and PDI-Pd1, respectively. Pt(PPh3)4 also reacts with bromo-
PDI derivatives but gives somewhat different products: PDI-
Pt1, which contains a cis-Pt(PPh3)Br substituent, and the
metallocycle PDI-cycloPt were obtained from PDI-Br1′ (Chart
2).
The absorption spectrum of a PDI-Br2 film is very different
from that of PDI-Br2 in solution (Figure 3a), as is typical for
N,N′-dialkyl-PDIs without large 1,7-substituents, where π
stacking is usually found in crystal structures (e.g., that of
I15). However, the spectra of PDI-Pd2 films (Figure 3b)
indicates minimal aggregation, consistent with other work on
PDIs with large 1,7-substituents, such as 2,6-diphenylphenoxy
groups,7a and with the effective separation of neighboring PDI
PDI-Pd1, and PDI-Pd2 (CIF)
AUTHOR INFORMATION
Corresponding Author
*seth.marder@chemistry.gatech.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Air Force Office of Scientific
Research through the COMAS MURI Program (Agreement
FA9550-10-1-0558).
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DOI: 10.1021/jacs.6b06361
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