Approaches for Selective Synthesis of Ullazine Donor–Acceptor Systems

Article abstract of DOI:10.1002/chem.201704694

Methods for effective synthesis for the four possible isomeric 3,9-diphenylullazine carboxaldehydes and reactive halogen intermediates are described. Ullazine donor–acceptor (D–A) dyes were studied using UV/Vis, photoluminescence (PL) spectroscopy and cyclic voltammetry. X-ray single crystal diffraction analysis independently confirmed the structures of two key intermediates. A D–A dye based on ullazine with dihexylmalonate acceptor was tested as a dopant-free hole-transporting material (HTM) in a perovskite solar cell, exhibiting promising power conversion efficiency (PCE) reaching 13.07 %.

Full text of DOI:10.1002/chem.201704694

A Journal of
Accepted Article
Title: Approaches for selective synthesis of Ullazine donor-acceptor
systems
Authors: Mohammad Khaja Nazeeruddin, Nikita Drigo, Paek
Sanghyun, Aron Huckaba, Pascal. Schouwink, and Nouar
Tabet
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To be cited as: Chem. Eur. J. 10.1002/chem.201704694
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Approaches for selective synthesis of ullazine donor-acceptor
systems
Nikita A. Drigo,[a] Sanghyun Paek,[a] Aron J. Huckaba,[a] Pascal A. Shouwink,[b] Nouar Tabet,[c]and
Mohammad K. Nazeeruddin*[a]
Abstract: Methods for effective synthesis for the four possible
isomeric 3,9-diphenylullazine carboxaldehydes and reactive halogen
intermediates are described. Ullazine donor-acceptor (D-A) dyes
were studied using UV-Vis, photoluminescence (PL) spectroscopy
and cyclic voltammetry. X-ray single crystal diffraction analysis
independently confirmed the structures of two key intermediates. A D-
A dye based on ullazine with dihexylmalonate acceptor was tested as
a dopant-free hole-transporting material (HTM) in a perovskite solar
cell, exhibiting promising power conversion efficiency (PCE) reaching
13.07%.
synthesis of the ullazine scaffold and applying ullazine sensitizers
for dye-sensitized solar cells spurred significant interest in the
heteropolyaromatic system.[6–13] Even so, methods for selective
and effective modification of ullazine periphery remain scarce,
though publications claiming facile construction of ullazine
scaffolds still appear regularly.[14–25] Herein, we report approaches
for effective regioselective functionalization of 3,9-disubstituted
ullazines, which were used as π-extended and electron-rich
modular units for functional materials.
Due to their intriguing properties, research into polycyclic
heteroaromatic compounds (PHAs) have gained tremendous
momentum, especially towards organic electronics applications.
PHAs are numerous and vary in shapes and sizes from simple
structures to complex 3d-motifs with B, N, O, S, P or other
heteroatoms that can modulate key properties.[1–3] Ullazine, a
PAH exhibiting a fused aza-polycyclic core, is isoelectronic to
pyrene but combines the extended  system of the latter with the
electron-rich nature of indolizine-like heterocycles. Ullazine’s in-
plane N-atom is able to donate electron density to the periphery
to form a 14-π annulene perimeter with central positive iminium
moiety and electron rich periphery, thus promoting intermolecular
charge transfer (ICT) (Figure 1).[4]
Figure 2. Ullazines containing functional groups.
Since published procedures describe effective and high-yielding
preparation of 5-ullazinecarboxaldehydes as a major product in
Vilsmeier–Haack carbonylation and 6-chloroullazines as
a
Figure 1. Numbering and resonance structures of Ullazine.
reactant for cross-coupling reactions[7] we focused on exploring
routes towards the other possible core-substituted isomers
containing carbonyl or halogen derivatives (Figure 2). The
synthesis devised here enables simple derivatization of the
ullazine core or key intermediates to enable further selective
functional group transformation. Owing to the strong electron-
donating character of the ullazine periphery, we synthesized
donor-acceptor (D-A) systems to gain insight into photophysical
and electrochemical properties of each regioisomer. We chose
phenyl moieties as substituents at the 3 and 9 positions to impart
stability[4] to the polycyclic scaffold and to simplify synthesis and
characterization. Dihexylmalonate served as the solubilizing
acceptor moiety.
Ullazines, have been known since 1983 with the first reports on
preparation by Balli[5] and ion stability studies by Gerson
appeared.[4] However, tedious synthesis and lack of application
disfavored further investigation of the polycycle’s capabilities.
Almost 30 years later, seminal reports on the first feasible
[a]
Group for Molecular Engineering of Functional Materials, Institution of Chemical Sciences and
Engineering
Ecole Polytechnique Federale de Lausanne
CH-1951, Sion, Switzerland
E-mail: mdkhaja.nazeeruddin@epfl.ch
Institution of Chemical Sciences and Engineering
Ecole Polytechnique Federale de Lausanne
CH-1951, Sion, Switzerland
[b]
[c]
Our study began with the preparation of 6-bromo-substituted
ullazine (Scheme 1). Following the published route for
Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University (HBKU), Qatar
Foundation, Doha, 5825, Qatar.
Supporting information for this article is given via a link at the end of the document
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Scheme 1. Synthesis of ullazines 5a-5c. The upper route “b-d” is the published protocol, the lower “c-b” route is developed in this work. Reagents and conditions:
a) ICl, MeOH, reflux (for 1a); b) Clauson-Kaas reaction: 2,5-dimethoxytetrahydrofuran (for 2a, 2b, 4a, 4b) or 2,5-dimethoxy-3-tetrahydrofurancarboxaldehyde (for
4c), AcOH, DCE, reflux; c) PhCCH, CuI, Pd(Ph3P)4, Et3N, THF, RT (for 3a) or Et3N-neat, 90°C (for 3b); d) PhCCH, Pd(CH3CN)2Cl2, XPhos, Cs2CO3, CH3CN, 60°C
(for 4a); PhCCH, Pd(CH3CN)2Cl2, tBu3P, CuI, diisopropylamine, 1,4-dioxane, RT (for 4b); e) InCl3, toluene (for 5a, 5c) or o-xylene (for 5c), reflux.
3,9-disubstituted ullazines[7] we found the Sonogashira coupling
Table 1. Synthesis of dyes 7a-7d.
was ineffective with 2a despite the well-known reactivity of aryl
iodides: 2a reacted sluggishly and non-selectively (SI, Table 1).
We found swapping the order of reaction resulted in an increased
yield: obtaining the bisalkyne 3a from aniline 1a utilizing milder
reaction conditions and cheaper catalytic system, followed by
Clauson-Kaas ring rearrangement yielding pyrrole 4a. Finally,
InCl3-catalyzed cyclization completed the synthesis of 6-
bromosubstituted ullazine 5a (Scheme 1). Particularly noteworthy
in the optimized procedure are the high-yielding conditions
tolerant to the halogen atom placed in the ullazine’s 6-position
throughout all stages of the synthesis. Overall yield of the desired
6-bromoullazine 5a starting from 4-bromoaniline was 77% over
four steps.
Vilsmeier-Haack
formylation
Knoevenagel
condensation
R
Product
Yield, %
Product
Yield, %
-
-
8a
66[a]
One-pot combination of Bouveault aldehyde synthesis and
Knoevenagel condensation afforded desired dye 8a from 5a in
66% yield (Table 1). Unfortunately, corresponding 6-
ullazinecarboxaldehyde appeared to be sensitive to air and
oxidized readily being exposed to it, which was due to the lack of
electron donation in the 6 position.[7] However, 5a proved to be an
effective cross-coupling partner with 2-(tributylstannyl)pyridine
delivering the product 6 in 70% yield. (Scheme 2).
7b
5c
7d
56
32
6
8b
8c
8d
85
91
80
[a] Obtained from 5a via one-pot Bouveault aldehyde synthesis followed by
Knoevenagel condensation. Py = pyridine.
Scheme 2. Model coupling reactions for 5a. Reagents and conditions:
2-(tributylstannyl)pyridine, Pd(Ph3P)4, toluene, reflux.
Next, we sought the ullazine functionalized with the acceptor at
the 5 position. After optimization of Sonogashira coupling we
employed the same procedure as above to obtain the
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5b abs
5b PL
8a abs
8a PL
8b abs
8b PL
8c abs
8c PL
8d abs
8d PL
a)
c)
b)
~50A
20
15
10
5
1,0
0,5
5b
8a
8b
8c
8d
0,0
250
0
0,0
300
350
400
450
500
550
600
650
700
-3,5 -3,0 -2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0
0,2
0,4
0,6
0,8
1,0
1,2
Wavelength, nm
E, V vs NHE
Voltage, V
Figure 3. a) UV-vis (solid) and PL (dashed) spectra normalized at peak value of 5b and dyes 8a-d in cyclohexane; b) voltammograms of 5b and dyes 8a-d in a
0.1M Bu4NPF6 solution in dry deoxygenated DMF; c) current-voltage curve for the device with 8b as HTL.
Table 2 . Photophysical and electrochemical properties of 5b and dyes 8a-d.
Compound
λabs, nm[a]
λem, nm[a]
E(S/S-), V
E(S+/S), V
0,75
Eelch,eV
Eg, eV[b]
HOMO, eV[c]
LUMO, eV[c]
374, 391[b]
395
478, 513, 553
519
-2.22
-1.68
-1.43
-1.70
-1.36
2.97
2.45
2.27
2.69
2.27
2.71, (2.61)
2.61, (2.48)
2.30, (2.23)
2.63, (2.59)
2.64, (2.08)
-5.43
-5.45
-5.52
-5.67
-5.59
-2.46
-3.00
-3.25
-2.98
-3.32
5b
8a
8b
8c
8d
0,77
446, 466
416, 439
386, 401
530, 560
0,84
474, 502
0,99
492, 529, 570
0,91
[a] Measured in cyclohexane. Bold corresponds to the highest intensity in a band; [b] estimated from the intersection of the normalized absorbance and emission
spectra in acetonitrile or low-energy onset of absorption spectra in acetonitrile (values in parentheses); [c] converted to the NHE by adding +0.69V and converted
to vacuum scale by adding +4.44 eV.
unsubstituted ullazine with an increased overall yield of 79%
(Scheme 1). Interestingly, Vilsmeier–Haack formylation of 5b also
provided 5c in decent yield (32%) along with the reported minor
product 7d (6%). The structure of 5c was confirmed with single
crystal x-ray diffraction. To the best of our knowledge, formation
of the pyrrole carboxaldehyde regioisomer moiety hasn’t been
reported in literature so far. Unfortunately, other attempts to
synthesize 5-substituted ullazines directly were unsuccessful:
treatment of 5b with halogen or boron electrophiles in various
conditions didn’t yield the desired products. Thus, modification of
the 5 position in ullazine core still remains a challenging issue due
to strong electron donation at this position.[7,26]
observed to deactivate the pyrrole ring towards electrophiles.
Thus cyclization required higher temperatures (Scheme 1).
UV-Visible and PL spectroscopy was performed in solvents of
differing polarities. Photophysical properties of the unsubstituted
ullazine 5b and the dyes 8a-8d (Figure 3, Table 2) were studied.
Except for 8a and 8b, each dye exhibited vibronically-structured
absorption bands in cyclohexane. Upon increasing solvent
polarity to THF and acetonitrile, structural features were lost, apart
from the spectra of 8d, which remained unchanged regardless the
solvent polarity. Nonetheless, all high-energy absorption bands
were largely insensitive to changes in solvent polarities. Ullazine
5b displays strong absorption bands in the high energy region
(250-330) nm that were ascribed to π→π* transitions. Weaker
absorbance was observed between 350-450 nm and assigned to
ICT.
Ullazine dyes 8a and 8d exhibit similarly structured absorption
bands with slight bathochromic shift in ICT onset, while 8b and 8c
display weaker π→π* transition and a stronger ICT band, both of
which are broadened and red-shifted by 30 and 80 nm
correspondingly. The ICT band energy thus follows the expected
trend for electron donation strength at different ullazine periphery
sites (5>2>4>7).[7]
The reaction of the bisalkyne 4b with molecular iodine at 0°C
delivered monoiodoindolizine 9,
a
useful precursor for
4-derivatized ullazines (Scheme 3), and the structure was
confirmed by single crystal X-ray diffraction (Figure 4).[27] We
utilized 9 to obtain corresponding dye 8d stepwise but in selective
manner and with improved total yield. Interestingly, InCl3-
catalyzed cyclization of the iodoindolizine 9 selectively yielded 5b
albeit in low yield (~30%). installation of a deactivating carbonyl
group didn’t hinder the cyclisation, providing 7d in excellent yield.
Lastly, final step didn’t require elevated temperatures as was the
case for 4c (vide infra).
Similarly, vibronic structure was observed in the PL spectra
recorded in cyclohexane for ullazine 5b and dyes 8a, 8c, 8d, while
8b is broadened. Upon increasing solvent polarities from
cyclohexane to acetonitrile, emission of 5b and 8d
To reach the final isomer 5c, commercially available 2,5-
dimethoxy-3-oxotetrahydrofurane smoothly formed 4c,
a
precursor for the dye 8c. The installed carbonyl group was
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Scheme 3. Selective synthesis of 7d. Reagents and conditions: a) I2, NaHCO3, DCM, 0°C; b)n-BuLi, THF, -78°C, then dry DMF, -78°C→RT; c) InCl3, toluene, reflux.
experienced nearly no change bathochromic shifts by 5-10 nm
were observed with nearly no vibronic structure difference.
However, 8a-c exhibited: solvatochromism, displaying red-shifts
Acknowledgements
ranging from 20 to 70 nm. Interestingly, PL maximum of 8c is blue
The authors acknowledge the European Commission H2020-
shifted
by
ICT-2014-1,SOLEDLIGHT project, grant agreement N1: 643791
and the SwissState Secretariat for Education, Research and
Innovation (SERI) and CTI 15864.2 PFNM-NM, Solaronix,
Aubonne.
10nm in cyclohexane compared to the emission of 5b, that
presumably indicates destabilization of an excited state of 8c in
nonpolar medium.
The electrochemical properties of 5b and 8a-d were then
investigated by cyclic voltammetry. The ground-state oxidation
potential Es+/Es for the parental unsubstituted ullazine 5b was
observed to be -5.43 eV against vacuum level. Attachment of the
acceptor stabilized the HOMO level of the dyes 8a-d by 20-200
mV, which agreed well with photophysical data.
Dye 8b was deemed a promising candidate for use as a dopant
free hole-transporting material (HTM) for perovskite solar cells
because of strongest ICT characteristics and a well-matched
HOMO level (-5.52eV). Under 1 Sun illumination, the champion
devices containing 8b as an HTM attained a modest PCE of
13.08%, with Jsc = 18. 47 mAcm-2, Voc = 1.054 V, and FF = 0.671.
Keywords: heterocycles • polycycles • dyes • solar cells •
perovskites
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COMMUNICATION
From DSCs to PSCs: developed
approaches for site-selective
synthesis of ullazine dyes pave the
way to novel organic functional
materials. Simple ullazine donor-
acceptor system shows promising
results as a dopant-free HTM for
PSCs.
Nikita A. Drigo, Sanghyun Paek, Aron
J. Huckaba, Pascal A. Shouwink, Nouar
Tabet, and Mohammad K.
Nazeeruddin*
Page No. – Page No.
Approaches for selective synthesis
of ullazine donor-acceptor systems
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