Benzannulation through Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition: Precision Synthesis and Photophysical Characterization of Soluble Diindenoperylenes

Article abstract of DOI:10.1002/chem.202001731

The first application of ruthenium(0)-catalyzed 1,2-dione-diyne [2+2+2] cycloaddition to PAH construction is achieved by the precision synthesis of soluble diindenoperylenes (DIPs), the electronic structures of which were investigated using steady-state a

Full text of DOI:10.1002/chem.202001731

Accepted Article
Accepted Article
Title: Benzannulation via Ruthenium(0)-Catalyzed Transfer
Hydrogenative Cycloaddition: Precision Synthesis and
Photophysical Characterization of Soluble Diindenoperylenes
Authors: Michael Joseph Krische, Sankar Rao Suravarapu, Sai
Prathima Parvathaneni, Jon A. Bender, and Sean T. Roberts
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To be cited as: Chem. Eur. J. 10.1002/chem.202001731
Link to VoR: https://doi.org/10.1002/chem.202001731
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1/2020

Chemistry - A European Journal
10.1002/chem.202001731
Benzannulation via Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition:
Precision Synthesis and Photophysical Characterization of Soluble Diindenoperylenes
†
†
†
Sankar Rao Suravarapu, Sai Prathima Parvathaneni, Jon A. Bender,
Sean T. Roberts,*and Michael J. Krische*
University of Texas at Austin, Department of Chemistry
1
05 E 24th St. (A5300), Austin, TX 78712-1167 (USA)
<Graphical Abstract>
Green Chemistry, Red Dyes. The first application of ruthenium(0)-catalyzed 1,2-dione-diyne
2+2+2] cycloaddition to PAH construction is achieved by the precision synthesis of soluble
[
diindenoperylenes (DIPs), whose electronic structures were investigated using steady-state
absorption and emission, transient absorption, cyclic voltammetry and time-dependent density
functional theory.
Keywords: Ruthenium, Benzannulation, Cycloaddition, Transfer Hydrogenation.
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Chemistry - A European Journal
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Benzannulation via Ruthenium(0)-Catalyzed Transfer Hydrogenative Cycloaddition:
Precision Synthesis and Photophysical Characterization of Soluble Diindenoperylenes**
†
†
†
Sankar Rao Suravarapu, Sai Prathima Parvathaneni, Jon A. Bender, Sean T. Roberts,*and
Michael J. Krische*
[
[
*]
Dr. S. R. Suravarapu, Dr. S. P. Parvathaneni, Mr. J. A. Bender, Prof. S. T. Roberts and Prof. M. J. Krische
University of Texas at Austin, Department of Chemistry
1
05 E 24th St. (A5300), Austin, TX 78712-1167 (USA)
Email: roberts@cm.utexas.edu
Email: mkrische@mail.utexas.edu
**]
The Welch Foundation (F-1885, F-0038), the National Science Foundation (CHE-1855744), and the W. M. Keck
Foundation (Grant 22605) are acknowledged for partial support of this research. SRS and SPP acknowledge postdoctoral
fellowship support from the Swiss National Science Foundation and Fulbright-Nehru post-doctoral fellowship programs,
respectively.
[†
]
These authors contributed equally to this work.
Among polycyclic aromatic hydrocarbons (PAHs), perylene derivatives have been
intensively investigated due to their utility in wide-ranging device applications and, perhaps of
[
1]
equal significance, their commercial availability and low cost. Diindenoperylenes (DIPs), also
known as “periflanthenes,” represent a subset of perylene-based materials which, despite their
[
2]
promising photophysical properties, have received far less attention as there is no commercial
source. Existing strategies for the synthesis of diindenoperylenes all involve Diels-Alder
[3]
cycloadditions and the reported routes do not deliver highly soluble materials. In connection with
our efforts toward the development of catalytic alcohol-mediated C-C bond formations,[4]
ruthenium(0)-catalyzed cycloadditions of vicinally dioxygenated hydrocarbons (diols, ketols,
diones) were discovered,[5] including an unusual ruthenium(0)-catalyzed [2+2+2] cycloaddition
wherein 1,6-diynes with 1,2-diones (or diols/ketols).[5f] Whereas numerous examples of [2+2+2]
cycloadditions that occur via π-bond
Figure 1. [2+2+2] Cycloaddition for benzannulation using
diones as 22π components: construction of diindenoperylenes.
insertion-reductive
elimination
of
metallacyclopentadienes exist,[6]
the
[
2+2+2] cycloadditions we report are the
first examples of the use of diones as 22π
components (Figure 1).[5f] In related diol-
diene cycloadditions, the reaction
products were found to engage in acid-
catalyzed dehydration to form aromatic
rings, enabling access to diverse PAH
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materials, including acenes,[7a] triple helical oligo(phenylene) cages,[7b] nanographenes, [7c] as well
as an all-aryl caged fac-Ir(ppy)
analog.[7d,8] Inspired by these capabilities, an effort to expand the
3
lexicon of diol-mediated benzannulations was undertaken. Here, the first application of
ruthenium(0)-catalyzed 1,2-dione-diyne [2+2+2] cycloaddition[5f] to PAH construction is
demonstrated by the precision synthesis of soluble diindenoperylenes (DIPs), whose electronic
structures were investigated using steady-state absorption and emission, transient absorption,
cyclic voltammetry and time-dependent density functional theory. While the optical and
electrochemical properties of these materials in solution are akin to unsubstituted DIP, the pendant
aryl groups at the 4,4’,7,7’-positions enforce a unique face-to-face packing structures in the solid
state that has not been demonstrated for any previously reported DIP meriting further investigation
of their properties within thin film electronics.
The synthesis of diindenoperylenes 5a-5e begins with the ruthenium(0)-catalyzed 1,6-
diyne-dione [2+2+2] cycloaddition (Scheme 1).[5f] The requisite fluorene-containing 1,6-diynes
1
a-1c and the diethyl malonate-containing 1,6-diynes 1d and 1e are readily prepared via
Sonogashira coupling.[9] The ruthenium catalyzed [2+2+2] cycloaddition can be conducted in
oxidative, redox neutral or reductive modes, using diols, ketols or diones respectively. As our
targeted cycloadducts incorporate an acenaphthylene moiety (vide supra), it was most convenient
to utilize commercially available acenaphthylene-1,2-dione 2, also known as acenaphthoquinone.
For this reductive cycloaddition 2-propanol was used as the hydrogen source. To our delight,
exposure of equimolar quantities of 1,6-diynes 1a-1e and acenaphthylene-1,2-dione 2 to 2-
propanol (300 mol%) and the ruthenium(0) catalyst modified by bis(diphenylphosphino)ethane
(
dppe) (6 mol%) led to the formation of cycloadducts 3a-3e in good yield after isolation by silica
gel chromatography.
A procedure for diol deoxydehydration (DODH) was sought to convert cycloadducts 3a-
e to the corresponding fluoranthenes 4a-4e.[10] Although rhenium-catalyzed DODH reactions are
well-precedented, the requisite catalyst, (C Me )ReO
, is rather expensive.[11] Several inexpensive
oxovanadium complexes have been shown to catalyze DODH.[10a] Among these,
Bu N](dipic)VO with sodium sulfite as reductant was most effective, however, this protocol
requires high temperatures (180 C) and delivered only modest yields of fluoranthene.
3
5
5
3
[
4
2
o
[12]
Eastwood’s orthoester-mediated DODH protocol utilizes inexpensive reactants, but also requires
high temperatures and modest yields of fluoranthene were again obtained.[13] Finally, among the
DODH conditions that were evaluated, TiCl
4
-NEt was identified as the method of choice in terms
3
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of operational simplicity, mildness, scalability and isolated yield.[14] Using TiCl
fluoranthenes 4a-4e were accessible in good yield after isolation by silica gel chromatography
Scheme 1).
-NEt ,
4
3
(
Scheme 1. Ruthenium(0)-catalyzed [2+2+2] cycloaddition of 1,6-diynes 1a-1e with
acenaphthoquinone 2 to form cycloadducts 3a-3e and titanium-mediated DODH to form
a
fluoranthenes 4a-4e.
aYields are of material isolated by silica gel chromatography. See Supporting Information for further experimental details.
Dehydrogenative dimerization of fluoranthenes 4a-4e to form
Scheme 2. Dehydrogenative dimerization
of fluoranthenes 4a-4e to form
a
diindenoperylenes 5a-5e proved to be highly sensitive to both
temperature and reaction time. Following literature
precedent,[3c] the use of superstoichiometric cobalt trifluoride
was attempted, but products of Scholl-type dimerization were
diindenoperylenes 5a-5e.
not detected. Exposure of fluoranthenes 4a to FeCl in
3
dichloromethane-nitromethane[3d] at 25 C for 5 minutes
provided the highly fluorescent compound hemi-5a, for which
only a single C-C bond had been formed in 77% yield (eq. 1).
To identify optimal conditions for diindenoperylenes
formation, reactions were monitored by MALDI-TOF mass
o
spectroscopy. For the fluorene-containing fluoranthenes 4a-
o
4
c, reactions were conducted at 35 C for 20 minutes and for
aYields of material isolated by silica gel
chromatography. See Supporting
Information for further experimental
details.
diethyl malonate-containing fluoranthenes 4d and 4e,
o
reactions were conducted at 25 C for 5 minutes (Scheme 2).
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Under these conditions, good to excellent isolated yields of the targeted diindenoperylenes 5a-5e
were obtained.
To assess the potential utility of DIPs 5a-5e for molecular photovoltaics and related
applications, their electronic structure using absorption and emission spectroscopy was
characterized. Figure 2 plots absorption spectra of 5a and 5d dissolved in dichloromethane while
spectra for 5b, 5c and 5e are shown in the Supporting Information (Figure S1). Spectra for
compounds 5a-5e were found to be nearly identical, reflecting their structural similarity. Like
unsubstituted DIP, the lowest energy absorption transition of 5a-5e display a distinct vibronic
-1
progression with a peak spacing of ~170 meV (1370 cm ) that is associated with carbon-carbon
stretching transitions of the DIP backbone. DFT calculations suggest this transition consists
predominantly of HOMO  LUMO character (vide infra, Table 3). The 0-0 transition of this band
peaks at 560 nm, which is notably redshifted from
the corresponding absorption peak of unsubstituted
DIP (540 nm),[15,16] and likely reflects some small
degree of HOMO and LUMO extension onto the
flanking phenyl groups attached to the DIP core of
5
a-5e.[17] At higher energy, a cluster of transitions
appear that we largely attribute to higher lying
transitions of the DIP core of each molecule based
on DFT calculations. Also present in spectra of 5a,
5
b, and 5c, is a weak transition peaked at 307 nm
that we assign to their terminal fluorene groups
based on this peak’s similarly to the absorption
onset of unsubstituted fluorene.[18]
Figure 2. Absorption (solid red), emission (dot
dashed blue, λEx 520 nm), and
photoluminescence (PL) excitation spectra (grey
dotted, λEm = 600 nm) of 5a and 5d in
dichloromethane.
=
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Emission spectra are plotted alongside absorption data in Figure 2. While these spectra
display a vibronic progression similar to the HOMO-LUMO transition, the peak spacing,
linewidth, and intensity pattern of these transitions are distinct from those seen in the absorption
spectra of 5a-5e. To characterize the strength of the emission from 5a-5e, the emission quantum
yield relative to a structurally related compound, N,N-dioctyl-3,4,9,10-perylenedicarboximide
(C8-PDI) was measured. Perylenediimide dyes featuring N-alkyl substituents, such as C8-PDI,
have been reported to have quantum yields of unity.[19,20] Results from these measurements are
summarized in Table 1, which shows that emission from 5a-5e is more than 2 orders of magnitude
weaker than that of C8-PDI. Weak emission intensity and an emission lineshape that significantly
differs from the absorption lineshape has also been noted for unsubstituted DIP in solution, and
assigned to rapid relaxation of the initially photoexcited state into a weakly emissive state that
predominantly decays to the ground state via internal conversion.[15,16] Our DFT calculations below
identify a dark state consisting of excitation from the HOMO-1  LUMO state with energy below
that of the lowest energy bright transition, supporting this hypothesis. Photoluminescence
excitation (PLE) spectra are also included in Figure 2 and largely track absorption features seen
in the ground state absorption spectrum. Interestingly, our PLE data show an increase in the
emission of 5a-5e when exciting into higher energy states that absorb in the UV spectral range.
However, a detailed explanation of this behavior is beyond the scope of this report.
To better assess deviations in the photophysics of 5a-5e monomers relative to DIP in
solution, transient absorption (TA) spectra of 5a, 5c, and 5d were measured. Experimental
conditions and procedures for recording TA spectra are described in the Supporting Information.
TA spectra of 5a and 5d are shown in Figure 3 while spectra of 5c appear in Figure S2. These
spectra were collected following excitation of 5a, 5c and 5d at 560 nm with the mutual polarization
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10.1002/chem.202001731
of the pump and probe pulses set to the
magic angle (54.7º) to remove TA signal
contributions from molecular reorientation.
Upon photoexcitation, a broad negative
signal arises between ~450 – 575 nm
corresponding to the ground state bleach of
each molecule while induced absorption
bands appear at other wavelengths. All of
these features decay to baseline for 5a, 5c,
and 5d, suggesting the decay corresponds
to the relaxation of each molecule to the
ground state. This rate of relaxation is well-
reproduced by a single exponential decay
convolved with a gaussian instrument
response function, yielding decay
timescales of 410 ps, 413 ps and 576 ps,
Figure 3. TA spectra of 5a and 5d in dichloromethane
following excitation at 560 nm. The mutual orientation of
the Pump and probe pulses was set at the magic angle
(
54.7º).
respectively (Table 1 & Figure S3). Over the time range we probe, we see negligible changes in
the spectral features of each molecule, suggesting we observe a single excited state as it returns to
the ground state. This state is most likely the weakly emissive state inferred from our emission
data and predicted on the basis of our DFT calculations. Given our TA time-resolution of ~100 fs,
this state must be populated within this time window following excitation of 5a, 5c, and 5d. This
is consistent with prior conclusions drawn from TA spectra of unsubstituted DIP in solution.[16]
Interestingly, the decay rates we find for 5a, 5c, and 5d are approximately twice that of
unsubstituted DIP. This difference in rates could reflect the increased vibrational degrees of
freedom of 5a, 5c and 5d relative to unsubstituted DIP and suggests these materials could exhibit
enhanced photostability by rapidly dissipating their excitation energy to their surroundings.
To explore the electronic properties of DIPs 5a, 5b, and 5e, cyclic voltammetry (CV) was
used to characterize their oxidation and reduction potentials in solution. Details describing these
experiments including a representative CV curve, are included in the Supporting Information.
CV curves for each species displayed reversible oxidation waves and two reversible reduction
waves corresponding to formation of their radical cation, radical anion, and diradical anion forms,
respectively. Table 2 summarizes the results of these measurements, listing the potentials for
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10.1002/chem.202001731
formation of each of these species relative to a ferrocenium/ferrocene redox couple. Like other
members of the perylene family, such as perylenediimides (PDIs), the reduction waves for
formation of the radical anion and dianion states of 5a, 5b, and 5e are closely spaced. The
electrochemical gap separating the anion and cation states of 5a, 5b, and 5e is found to be ~2.2
eV, consistent with absorption and emission spectra shown in Figure 2. Comparison of the
oxidation and redox potentials for 5a-5e to that of PDI (+1.21 and -0.98 V relative to
ferrocenium/ferrocene)[20] show that while the HOMO/LUMO energy separation of 5a-5e is
similar to that of PDI (2.36 eV),[21] these states lie closer to vacuum in 5a-5e than in PDI.
To better understand the character of the excited states involved in the absorption and
emission spectra of 5a-5e, TDDFT calculations within the Tamm-Dancoff approximation were
used to examine the excited states of 5a and 5d. The electronic structure computed at this level of
theory suggests the excited states of the frontier orbitals of these two materials are of similar
character. In Figure 4, we plot the frontier orbitals of 5a. Examining these states, we find that most
of the electron density is localized on the DIP core, with minimal electron density extending onto
the attached phenyl rings and terminal fluorene groups. The lack of strong participation by these
groups, which can in part be attributed to their orthogonality to the DIP core, points to the similarity
of the optical spectra of 5a-5e to one another and to unfunctionalized DIP. This suggests a variety
of directing groups can be placed at the periphery of the DIP monomer with minimal perturbation
to the electronic structure of its frontier molecular orbital. Such groups could be used to guide the
self-assembly of DIP structures in the solid-state for electronics applications, similar to approaches
used to control the solid-state structure of related perylenediimide compounds.[22-25]
In addition to computing the energies of different excited states of 5a and 5d, we have also
computed their participation in different optical transitions. The oscillator strength and orbital
participation in different transitions are listed in Table 3. We find these calculations predict a
bright transition at 2.42 eV for 5a and 2.45 eV for 5d, which consist predominately of HOMO-to-
LUMO character and reasonably agree with their absorption onsets near ~2.2 eV (Figure 2).
Importantly, this transition also contains mixed contributions from other frontier orbitals that serve
to raise its energy above a dark HOMO-1  LUMO transition, suggesting the existence of an
excited singlet state that exists below the bright transition at 2.2 eV. This lower energy dark state
has previously been hypothesized to provide both the origin of the low emission quantum yield
from DIP and the lack of symmetry between its absorption and emission lineshape.[15,16] Moving
to higher energy, we identify a second set of bright transitions that fall close to the onset of the UV
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absorption band seen in Figure 2. Our calculations suggest this band consists of an admixture of
transitions associated with the central DIP unit of 5a and 5d that are polarized along their short
axis along with excitation of the terminal fluorene units of 5a.
In addition to examining spin-singlet states associated with optical absorption and
emission, we have also calculated the spin-triplet excited states of 5a and 5d. We compute the
energy of the lowest exited triplet state of each molecule to be ~1.46 eV, which is larger than the
previously reported value for unsubstituted DIP (1.27 eV).[15] However, we note that applying the
same scaling ratio (0.87) needed to align the energy of our lowest computed bright singlet state to
that which we experimentally measure to our computed triplet state energy provides good
agreement with the experimentally reported value for DIP. This suggests that the lowest triplet
state energy of 5a and 5d is lower than that of perylene (1.53 eV),[26] but higher than that of
perylenediimide (1.19 eV)[21,27,28] despite the more extended π-system of 5a and 5d. This reduction
in singlet-triplet splitting for 5a and 5d relative to perylenediimide could reflect a higher degree
of biradical character in the former due to the inclusion of a 5-membered ring in their central π-
system.[29]
Unfunctionalized DIP molecules have been extensively studied in the solid state, both as
single crystals and as thin films.[16,30-34] In single crystals, DIP forms a herringbone structure akin
to unsubstituted tetracene and pentacene (Figure 5).[35] DIP crystallites have also been identified
in thin DIP films investigated for their use in organic electronics.[5,20-23] However, for applications
wherein electron transport throughout the solid is of importance, obtaining structures where
molecules exhibit a face-to-face packing can potentially improve this transport by allowing for
improved orbital overlap between neighboring molecules. This is seen for example in rubrene,
which unlike its tetracene parent, adopts a face-to-face packing structure
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Figure 5. Single crystal X-ray diffraction of diindenoperylene 5d with displacement ellipsoids
3
5
scaled at 50% probability level. Comparison of single crystal structure of DIP and that of 5d
substituents omitted for clarity). The 5d structure involves molecules in a unique face-on
(
geometry with the long axis of neighboring molecules twisted by 73.4º. See Supporting
Information for further details regarding 5d’s crystal structure.
that allows for improved carrier transport in single crystals along these face-to-face packing
directions.[40,41] This suggests creation of DIP derivatives that exhibit face-to-face packing
structures can enable a range of applications.
We have grown single crystals of 5d and characterized their solid-state structure via single
crystal x-ray diffraction analysis (Figure 5). We find 5d crystals adopt a face-to-face packing motif
in contrast to DIP. This motif, which we term slip-twist, displays an unusual structure in which 5d
molecules form one-dimensional columns wherein the long axis of neighboring molecules adopts
a twist angle of 73.4º. Moreover, looking at one of these columns from its side, we see that the
molecular planes of neighboring 5d molecules are not fully parallel, but are canted by a shallow
angle of 13.7º. This effect decreases the spacing between the π-systems of neighboring molecules
of 5d to less than 3 Å, indicating the potential for strong intermolecular electronic coupling needed
for carrier transport in thin film electronics. This unique crystal structure also suggests 5d may
possess birefringent properties[42] that could be of use for optoelectronics applications.
In summary, the first application of ruthenium(0)-catalyzed 1,2-dione-diyne [2+2+2]
cycloaddition to PAH construction was demonstrated by modular syntheses of highly soluble and
structurally unique derivatives of diindenoperylene. Notably, the terminal fluorene groups and
4
,4’,7,7’-tetraaryl moieties of the present DIP derivatives not only enhance their solubility relative
to the parent unsubstituted DIP, but also direct the formation of solid-state structures wherein
neighboring DIPs adopt a unique face-to-face arrangement. We expect these structures to exhibit
unique carrier transport properties, meriting investigation of their excited state and electrical
properties within thin film electronics.
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