Synthesis, physical properties and biradical characters of zethrene based polycylic hydrocarbons 2

1
Chapter 1 Introduction

1.1 Low band gap polycyclic hydrocarbons with either a closed-shell or an open-shell
singlet biradical ground state
Polycyclic hydrocarbons (PHs) with a low band gap (E
g
≤ 1.5 eV) represent a class of
interesting molecules with intriguing electronic and optical properties, which are of
fundamental importance in the field of structural organic chemistry and materials science.
1

The band gap of a PH molecule is dependant on both the molecular size and molecular shape,
when it is beyond a certain point, a low band gap can be achieved. Figure 1.1 lists a number
of low band gap PH candidates, such as acenes, rylenes, periacenes, anthenes, zethrenes,
bis(phenalenyl)s and indenofluorenes. Some of them, such as acenes and rylenes, are
extensively studied and actively participated in material sciences,
2
while the study of some
candidates, such as higher order periacenes, still stay at the theoretical level. Nevertheless,
those molecules are believed to be perfect models to link theoretical chemistry, synthetic
chemistry and material sciences, and have become a rising hot topic nowadays.

Figure 1.1 Examples of low band gap PHs.

In general, polycylic hydrocarbons exhibit two types of electronic states, closed-shell state
and open-shell state. Most π-conjugated molecules can be characterized by a closed-shell
electron configuration, accommodating their π electrons only in bonding orbitals. In contrast,
open-shell configuration refers to the existence of one or more unpaired electrons, also known
as radicals, in the molecular structures. This ground state often exists for low band gap PH

2
molecules. Particularly, the electronic states of open-shell systems with two unpaired
electrons can be further divided into open-shell singlet, when the unpaired electrons adopt
anti-parallel spin, or open-shell triplet, when the unpaired electrons adopt parallel spin.
Among all of the electronic states, the one with the lowest energy defines the ground state of
π-conjugated molecules. It is worth noting that the two-radical systems can either be termed
as diradicals (i.e. m-xylene) in which two radicals barely interact with each other, or biradicals
(i.e. p-xylene) in which two radicals weakly coupled with each other, leading to increased
biradical character and diminished intramolecular convalency compared to closed-shell
configuration. The intramolecular electron coupling of biradicals is always weaker than
closed-shell systems, but stronger than localized diradicals, or pure diradicals.
3
The origin of
the biradical character is a small HOMO-LUMO energy gap, which facilitates the admixing
of doubly excited configuration into the ground state configuration.
4
The ground state of
polycyclic hydrocarbons is dependant on the size (conjugation length) and shape (edge
structure) of the molecule. For example, molecules with only zigzag edges, such as
phenalenyl and triangulene, are intrinsically open-shell systems in that their structures cannot
be depicted in a closed-shell manner.
5
On the other hand, for systems possessing both zigzag
and armchair edges, such as anthenes and periacenes, there is a critical point on the
conjugation length (or the band gap) beyond which a singlet biradical ground state will
emerge. The properties of open-shell polycyclic hydrocarbons can be studied by a
combination of theoretical and experimental methods. Various computational calculation
methods provide informative parameters, such as biradical character index, LUMO
occupancy number, exchange interaction (K

H,L
), spin density and singlet-triplet energy gap
(ΔE
S-T
). On the other hand, experimental methods such as nuclear magnetic resonance (NMR),
electron paramagnetic resonance (ESR), superconducting quantum interference device
(SQUID), X-ray single crystal analysis, Raman spectroscopy and so on represent powerful
tools to investigate the magnetic properties, the biradical characters and the intermolecular
interactions.
The stability is a crucial issue for low band gap polycylic hydrocarbons, from both
synthetic and applications point of views. The low band gap polycylic hydrocarbons generally
possess less benzenoid aromatic sextet rings and tend to be more reactive, so different

3
substituents can be introduced to stabilize them, such as bulky groups for kinetic blocking of
reactive sites and electron-withdrawing groups for lowering the HOMO. Especially, when it
comes to those with singlet biradical character, delocalization of the radicals in the molecular
skeleton and kinetic blocking of the reactive sites should be taken into consideration. Once
the stability issue is properly addressed, many applications are opened up for low band gap
PHs. They are attractive candidates as near-infrared (NIR) dyes and semiconductors, which
open the door for diverse applications such as bio-imaging,
6
optical recording,
7
and
fabrication of electronic devices such as organic field effect transistors (OFETs) and solar
cells.
8
Moreover, recent theoretical and experimental studies on open-shell polycyclic
hydrocarbons have added new insights into their material applications for non-linear optics,

9

for the future photovoltaic devices
10
and for molecule-based spintronic devices.
11
All of these
studies create a rising hot topic for low band gap PHs in synthetic chemistry as well as in
materials science.

1.1.1 PHs with a closed-shell ground state
1.1.1.1 Acenes
Acenes refer to a series of laterally fused benzene rings which can serve as semiconducting
materials. The band gap of acenes will decrease with more benzene rings fused, so a low band
gap can be realized for the higher order candidates in this series. Interestingly, many
theoretical works are dedicated to reveal the open-shell character of higher order acenes, and
a singlet biradical ground state is commonly arrived using different calculation methods. This
result was further extended to polyradical for even larger acenes. Despite the theoretical
calculations, the experimental examination seems to be a tough task since the larger acenes
are normally very reactive and synthetically difficult. Fortunately, the development of modern
chemistry has allowed substitutions to be made on acenes and the stable derivatives can be
isolated and characterized (Figure 1.2). The method to stabilize acenes developed in
Anthony’s group is to introduce silyl ethynyl groups at the strategic positions, the carefully
chosen substituents allow them to obtain single crystals for hexacene and heptacene for the
first time.
12
More stable heptacene derivatives were synthesized in Wudl,
13
Miller
14

and Chi
15

group independently, by blocking more reactive sites at the zigzag edges using different

4
substituents (Compounds 1-13-1-15). It’s worth noting that all the heptacene derivatives
possess closed-shell configuration in the ground state as evidenced by sharp peaks in the
NMR spectra. Recently, two nonacene derivatives were prepared in Miller’s and Anthony’s
group, Miller’s nonacene 1-16a-b was protected by arylthio groups which by calculation
would eliminate the total spin when located at terminal rings, and hence led to a closed-shell
species. Indeed, the sharp NMR peaks seem to further support this conclusion.
16
But later,
Chen and Miller himself suggested an open-shell singlet biradical ground state for this
nonacene by unrestricted broken spin-symmetry density functional theory (UBS-DFT) at
B3LYP/6-31G* level, irrespective of the positions of the substituents.
17
On the other hand,
Anthony’s nonacene derivatives 1-17a-c were intensively protected by trialkylsilylethynyl
and bis(trifluoromethyl)phenyl groups on the zigzag edges and fluorine atoms on the outer
rings, and the structures of these nonacenes were unambiguously confirmed by single crystal
analysis. The nonacene featured a prominent S
0
-S
1
transitions at 1014 nm with an energy gap
of 1.2 eV based on the absorption onset, while no fluorescence was observed in the visible
region. Interestingly, the nonacene samples appear to be NMR silent and an ESR spectrum
was found at g

e
= 2.0060. Although the origin of the signal is not clear, there is a possibility
that it could be an intrinsic characteristic of open-shell nonacene.
18



5
Figure 1.2 Functionalized high order acenes.

1.1.1.2 Rylenes
Rylenes represent PHs with two or more naphthalene units peri-fused together. Only one
aromatic sextet benzenoid ring can be drawn for each naphthalene unit and the zigzag edges
exist at the terminal naphthalene units. On the basis of the number of fused naphthalenes,
rylenes can be termed as perylene, terrylene, quaterrylene and so on. In pursuit of stable dyes
with high extinction coefficients and long-wavelength absorption/emission, rylenes have
received a great deal of attention.
19
Among them, perylene and its imide derivatives have
shown obvious advantages, including outstanding chemical, thermal and photochemical
inertness, and have already been well investigated and documented.
20
Extension of
conjugation along the long molecular axis leads to higher order rylenes with low band gap
and NIR absorption and emission, which are promising dyes in various of applications. For
rylenes, the primary concern is the poor solubility arose from dye aggregation, so substituents
are introduced to improve the solubility. Four tert-butyl groups were firstly introduced to
solve the problem, but the scope was only limited to quaterrylene.
21
Later, dicarboxylic imide

group was proven to be a good solution and rylene diimide compounds up to hexarylene
(1-20–1-23) were prepared showing NIR absorption and high extinction coefficient.
22

Additional solubility can be achieved by substitution at the bay regions. Moreover,
core-expanded rylene diimides (1-24,1-25) were also synthesized as promising dyes for
bio-labeling or laser-induced applications.


6
Figure 1.3 High order rylenes and their diimide derivatives.

Another interesting modification concept of rylene dyes is N-annulation. New
opto-electronic properties are expected due to the electron-donating nature of amines. From
the year 2009, a series of poly(peri-N-annulated perylene) up to hexarylene 1-26, 1-27 were
achieved in Wang’s group
23
(Figure 1.4), and oxidative ring fusion driven by DDQ/Sc(OTf)
3

was found to be very effective for this system due to the electron-rich property of N-annulated
perylene core. A large dipole moment along the short molecular axis favoring formation of H
aggregates was reflected by decrease of intensity in absorption and a marked concentration
dependence of the spectrum. The large dipole moments may favor the formation of highly
ordered supramolecular structures, which may lead to enhanced charge carrier mobilities. In
parallel to these work, the carboximide derivative 1-28 was developed in our group, and the
presence of imide group not only enhanced the stability of the core by lowering the high-lying
HOMO energy level, but also allowed the introduction of bulky diisopropylphenyl group
which helped to increase the solubility together with the branched alkyl chain at the amine
site. An alternative cyclodehydrogenation strategy by mild base was applied to synthesize

1-28, due to the existence of electron-withdrawing imide group. Compound 1-28 exhibited
absorption at NIR region and emitted strong fluorescence with quantum yield up to 55% in
dichloromethane. Such a high quantum yield is remarkable given that many NIR absorbing
dyes usually exhibit low fluorescence quantum yield.
24


Figure 1.4 Structures of N-annulated rylenes.

1.1.1.3 Bisanthenes
Bisanthene refers to a class of PH with two anthracene units peri-fused together, it is an

7
unstable compound but can be stabilized by proper substitution. One example of stable
bisanthene derivative 1-29 was reported in Kubo’s group by introducing tert-butyl groups to
the periphery of the bisanthene, this method provides sufficient stability and solubility but
leave the most reactive meso-positions exposed.
25
One strategy developed in our group is to
block the meso-positions by different subsituents, such as imide,
26
aryl groups or
triisopropylsilylethynyl groups.
27
Bisanthene bisimide 1-30 was prepared using
base-promoted cyclization reaction as a key step. Compared to the parent bisanthene, 1-30
exhibited a red shift of 170 nm at the absorption maximum together with enhanced stability
and solubility, indicating 1-30 as a promising candidate for NIR absorbing materials. An
alternative approach was synchronously developed by means of meso-substitution with aryl or
alkyne substituents to block the most reactive sites. Based on this consideration, three

meso-substituted bisanthenes 1-32a-c were prepared by nucleophilic addition of aryl Grignard
reagent or alkyne organolithium reagent to the bisanthenequinone followed by
reduction/aromatization of the as-formed diol. The obtained compounds not only showed
enhanced stability and solubility, but also exhibited absorption and emission in the NIR
region as well as amphoteric redox behaviors, which qualified them as NIR dyes and
hole/electron transporting materials. The same synthetic strategy was also applied to prepare
quinoidal bisanthene 1-31, which can be regarded as a rare case of stable and soluble PAH
with a quinoidal character.
28


Figure 1.5 Stable bisanthene derivatives.

1.1.1.4 Indenofluorenes
Indenofluorene molecules are a class of π-conjugated molecules with five-membered rings.
These systems can be viewed as antiaromatic analogues of acenes, and are very interesting in

8
terms of their bonding pictures. Indenofluorene derivatives 1-33 and 1-34 were reported in
Haley’s group. Compounds 1-33 were prepared as stable indenofluorene derivatives from the
corresponding diketone precursors, and X-ray crystallographic analysis of the single crystals
allowed a rare glimpse of the p-quinodimethane (p-QDM) core. The bond length showed
alternation in the central p-QDM core but homogeneous for the peripheral benzenes, thus
those molecules should be described as fully conjugated 20-π-electron hydrocarbon with
fused s-trans 1,3-diene linkages across the top and bottom portions of the carbon skeleton.
29

In order to further explore how the substituents can influence the indenofluorene core, a
number of 6,12-diethylnylindenofluorenes 1-34 were prepared in the same group. The crystal
packing for 1-34b and 1-34h was observed in 1D stacks with contacts around 3.40 Ǻ, being

different from the herringbone packing mode of 1-33. Together with the optical and
electrochemical properties, these results suggest that these molecules can be promising
semiconductors.
30
Notably, due to the relatively large HOMO-LUMO energy gap, no
open-shell biradical ground state was observed for this system. Recently, a series of
diaryl-substituted indenofluorene derivatives 1-35 were prepared in Haley and Yamashita’s
group independently. The aryl substituents were found to have a profound impact on the
physical properties such as stability, HOMO-LUMO energies and redox properties. FET
device was fabricated on vapor-deposited thin films in Yamashita’s group, an interesting
ambipolar transporting behavior was observed for 1-35k with electron mobility of 8.2 × 10
-6

cm
2
V
-1
s
-1
and hole mobility of 1.9 × 10
-5
cm
2
V
-1
s
-1
. The relative low mobilities were due to
the less-ordered molecular arrangements in the thin films.
31

In parallel, a single crystal OFET
with 1-35j as active component was reported in Haley’s group, the device exhibited
ambipolar behavior with hole and electron mobilities as 7 × 10
-4
and 3 × 10
-3
cm
2
V
-1
s
-1
,
which represented one of very few single crystal OFETs from organic semiconductors.
32
The
24-π-electron antiaromatic system 1-36 possessing a bond-localized 2,6-naphthoquinone
dimethane unit was recently presented by the same group.
33
Although the structure of 1-36
could be drawn in a biradical form, the absence of line broadening in NMR, the silence in
ESR for both solid and solution samples together with a large bond alternation all lead to a
conclusion of a closed-shell ground state. Other isomers of indenofluorenes, such as 1-37 and
1-38 were also reported, but they are all proven to be closed-shell molecules from different

9
experimental measurements.
34,35

Figure 1.6 Indenofluorene derivatives.

1.1.2 PHs with an open-shell ground state
1.1.2.1 Higher order anthenes
Higher order anthenes refer to those with three or more anthracene units fused together, the
biradical character of anthenes will increase with more anthryl units fused. According to the
calculation at the CASSCF(2,2)6-31G level, the singlet biradical character (y) values are
estimated to be 0.07 for bisanthene, 0.54 for teranthene and 0.91 for quarteranthene.

Because
the unpaired electrons are fixed to the meso-positions of anthenes, the effect of delocalization
is minimized and the discussion of biradical character can be focused on the aromatic
stabilization effect. Therefore, anthenes represent excellent models to study how formation of
aromatic sextet rings affects biradical/polyradical characters in PAHs with Kekulé type
structures, and to investigate the spin-polarized state in zigzag edged GNRs. Inspiringly,
derivatives from bisanthene to quarteranthene are prepared and isolated in the crystalline form
in Kubo’s group, allowing a detailed examination of their molecular structure, chemical
behavior and physical properties (Figure 1.7).
36,37
Due to the solubility and stability problems,
tert-butyl substituents are introduced to the periphery of the anthenes and aryl groups are
introduced to the meso-positions to block the reactive sites. For teranthene and quarteranthene
derivatives 1-39 and 1-40, moderate to large biradical character and an edge localization of
unpaired electrons are confirmed by a combination of physical measurements and DFT
calculations. Both teranthene and quarteranthene derivatives 1-39 and 1-40 are NMR silent at
room temperature, and the peaks become sharp upon cooling for 1-39. However, the NMR
baseline of 1-40 was flat even when the temperature is lowered down to 183 K. The absence

10
of NMR signals is due to the large population of a thermally accessible triplet diradical

species for 1-40. The NMR results can also be explained by SQUID measurements, which
showed a small singlet-triplet gap for both compounds (1920 K for 1-39 and 347 K for 1-40a).
Single crystals suitable for X-ray analysis for both 1-30 and 1-40a were successfully obtained,
revealing a high planarity and symmetry for the anthene core. Moreover, as shown in the
resonance structures, the contribution from the biradical resonance will shorten the bond a
due to the enhanced double bond character. From the bond lengths information provided by
the X-ray analysis, the bond length of bond a for quarteranthene (1.412 Å) is much shorter
than that in teranthene (1.424 Å) and bisanthene (1.447 Å). Furthermore, the harmonic
oscillator model of aromaticity (HOMA) values of ring A is highest for quarteranthene and
lowest for bisanthene, indicating that quarteranthene has more benzenoid character for the
peripheral rings, hence, a larger biradical character. Another interesting property for
quarteranthene is the absorption behavior. The room temperature absorption spectrum located
at 920 nm derives from a mixture of triplet and singlet species, while at lower temperature
(183 K), a bathochromic shift to 970 nm was observed corresponding to the singlet biradical
ground state. The shape of the two spectra is quite similar due to their similar distribution of
the unpaired electrons at the zigzag edges. The investigations of the anthene series have paved
the way to understand the intrinsic properties of zigzag edged GNRs and the fabrications of
nanographene-based optical and magnetic devices.

Figure 1.7 Teranthene/quarteranthene derivatives with singlet biradical ground states.

1.1.2.2 Bis(phenalenyl)s
Connection of two phenalenyl radicals with π-conjugated systems will produce a series of
closed-shell quinoidal molecules 1-41–1-43 with biradical characters (Figure 1.8) and these

11
systems were systematically studied by Nakasuji and Kubo et al There are two factors that
account for the enhanced stability for these systems, one is the intrinsic delocalization of
phenalenyl moiety, and the other is the aromatic stabilization by recovery of one more sextet
benzenoid ring from quinoindal form to the biradical form. The first indacenodiphenalene

(IDPL) derivative 1-41b was reported in 1991 and various substituents were introduced since
then.
38
These compounds feature singlet biradical character in the ground state. The line
broadening in the NMR spectra at elevated temperature as well as a line sharpening at lower
temperature indicated a thermally accessible triplet species, and the small singlet-triplet
energy gap can be determined by solid state ESR and SQUID measurements. One of the most
salient features derived from a profound biradical character was the strong intermolecular
interactions in the solid state. In 2005, Kubo reported a phenyl-substituted IDPL 1-41d and its
packing motif in the solid state, initiating a hot discussion on the interacting motif between
and within these systems. The crystal structure of 1-41d demonstrated one-dimensional (1D)
chains in a staggered stacking mode with an average π-π distance of 3.137 Ǻ, which is
significantly shorter than the van der Waals contact of carbon atoms (3.4 Ǻ). This packing
mode will maximize the SOMO-SOMO overlapping between the radicals, leading to
stabilized intermolecular orbitals that corresponds to intermolecular covalency.
39
Further
evidence of the coexistence of inter- and intramolecular interactions was provided by Huang
from a theoretical perspective.
40
They found that the participation of unpaired electrons in
intermolecular π-π bonding made them partially localized on phenalenyl units but less
available for intramolecular delocalization, i.e., the intermolecular interaction is more
predominant. With an aim to better understand the inter- and intramolecular spin-spin
interactions, Shimizu et al. attempted to alter the magnitude of the interactions by varying the
external conditions such as molecular structure, temperature and pressure. Interestingly, they
found a larger intermolecular separation (3.225 Ǻ) when introducing a methyl group to the β
positions IDPL (1-41e), and a similar increase in the π-π distance was also observed when
increasing the temperature. A decreased π-π distance would improve the intermolecular
orbital overlap and strengthen the intermolecular bonding interaction, however, it would also

weaken the intramolecular interaction by making unpaired electrons more localized. As a
result, the electronic structure of the 1D chain can be depicted by the resonating valence bond

12
(RVB) model as a superposition of a resonance balance between intramolecular bonding and
intermolecular bonding.
41
A naphthalene-linked bis(phenalenyl) 1-42b with even larger
biradical character was synthesized by Kubo et al
42
The HOMO-LUMO energy gap of this
compound was determined by cyclic voltammetry as 1.04 eV, and the singlet-triplet gap was
estimated as 1900 K by SQUID measurements, both smaller than that of 1-41d (1.15 eV and
2200 K), in agreement with its larger biradical character. Compound 1-42a without tert-butyl
bulky group was prepared to minimize the steric hindrance and to study the intermolecular
interaction in the crystalline phase. The packing of 1-42a adopted similar stepped mode to
1-41d in the 1D chain, and the intermolecular bonding was stronger than the intramolecular
one due to the spin-localized nature on the phenalenyl moieties, which can be more
adequately described as muticenter bonding.
43
Very recently, bis(phenalenyl) linked by
anthracene unit (1-43) were synthesized in the same group.
44
The parent anthracene linked
bis(phenalenyl) molecule was featured by a larger biradical contribution (y = 0.68) compared
to its naphthalene (y = 0.50) and benzene (y = 0.30) analogues, and the molecules packed
more tightly in the 1D chain with a distance of 3.122 Ǻ, which resulted in a prominent
covalent bonding interaction between the molecules. The significant singlet biradical
characters are ascribed to the high aromatic stabilization energy of the anthracene linker.
Theoretical studies pointed out many intriguing potentialities of singlet biradical systems,

one of them being a large second hyperpolarizability, a quantity closely related to two-photon
absorption response. This prediction was later proved by experimental results, which showed
a maximum TPA cross-section values up to 424±64 GM at 1425 nm for 1-41d and 890±130
GM at 1500 nm for 1-42b, which are comparable to similar TPA chromophores with strong
donor and/or accepter peripheral groups and are among the best for pure hydrocarbons
without donor and acceptor substituents, thus providing new insights into the design criteria
of TPA materials.
45
Moreover, balanced ambipolar charge transport of the thin films of 1-41d
was reported by Chikamatsu et al., presumably due to the amphoteric redox properties and
strong intermolecular communications. Notably, despite the strong intermolecular interaction
as discussed above, only moderate electron and hole mobilities of up to 10
-3
cm
2
V
-1
s
-1
were
observed as a result of the amorphous structure and poor crystallinity of the film.
46
But still,
these exciting results indicate a bright future for these biradicals in materials science.

13

Figure 1.8 Bis(phenalenyl)s with singlet biradical ground states.

One attractive part of bis(phenalenyl) systems stems from the wide selection of the

aromatic linkers. Fusion of phenalenyl units to thiophene and alternative positions of benzene
leads to compounds 1-44 and 1-45 (Figure 1.9). The thiophene fused bis(phenalenyl) 1-44
was synthesized in 2004 and the X-ray crystallographic analysis revealed the formation of a
dimeric pair with a bended structure for each monomer.
47
Accommodation of a doubly excited
configuration into the ground state will stabilize this system by suppressing four-electron
repulsion arising from interaction between fully occupied orbitals. Compound 1-45 was found
to show even larger biradical character,
48
with the order of biradical character as 1-41a <
1-44a < 1-45a. An energy lowering of 7.08 kJ/mol and 35.38 kJ/mol from closed-shell form
to open-shell form were calculated for 1-44a and 1-45a, respectively, suggesting that the
ground state of these molecules is singlet biradical.

Figure 1.9 Bis(phenalenyl)s with different aromatic linkers.


14
1.1.2.3 Indeno[2,1-b]fluorene
Indeno[2,1-b]fluorene is an unique example with singlet biradical character while other
indenofluorene compounds usually feature a closed-shell ground state. Recently, an
indeno[2,1-b]fluorene 1-46 substituted by mesityl groups was reported by Tobe.
49
Moderate
singlet biradical character contributes to the ground state as indicated by NMR and X-ray
single crystal structures, and an extremely low energy light absorption was observed, which is
attributed to the S
0
-S

1
absorption.

Figure 1.10 Indenofluorene with a singlet biradical ground state.

1.2 Overview on zethrene-based PHs
Zethrene (1-47) refers to a Z-shaped polycyclic hydrocarbon with fixed double bonds in the
middle of the molecule (Figure 1.11). The structure of zethrene can be viewed as a
dibenzotetracene or a “head-to-head” fusion of two phenalenyl moieties. When the two
phenalenyl units are separated by benzene or naphthalene, the longitudinal homologues of
zethrene named heptazethrene 1-48 and octazethrene 1-49 will be obtained (Figure 1.11). On
a basis of the occupancy numbers of spin-unrestricted Hartree–Fock natural orbitals (UNOs),
the biradical character y value was calculated to be 0.407 for zethrene, 0.537 for
heptazethrene and 0.628 for octazethrene.
9c
The trend can be explained by the resonance
structures from a closed-shell Kekulé form to an open-shell biradical form. For zethrene, no
additional aromatic sextet ring is formed from the closed-shell form to the open-shell form,
while for heptazethrene onwards, one addition aromatic sextet ring will be gained. Therefore,
higher order zethrenes are more prone to exhibit biradical characters.


15

Figure 1.11 Resonance structures for zethrene and higher order zethrenes.

1.2.1 Synthesis and reactivity for zethrene-based PHs
The synthesis of zethrene was pursued back to 1955 when Clar et al. found a small amount
of deep red color hydrocarbon obtained by either catalytic dehydrogenation of acenaphthene
to acenaphthylene, thermolysis of acenaphthene, or treatment of acenaphthylene [or

bi(acenaphthylidene)] with NaCl and AlCl
3
at 110 °C.
50
The authentic sample was then
synthesized from 2,8-dicyanochrysene with dehydrogenation as a final step, and was
identified as zethrene (Scheme 1.1). This pioneering work represented the first synthesis of
zethrene, however, the overall yield is quite low and the parent zethrene was found to be
readily decomposed under ambient conditions.

Scheme 1.1 The first synthesis of zethrene by Clar.

In the 1960s, two leading groups in annulene chemistry led by Staab and Sondheimer
attempted to synthesize tetradehydrodinaphtho[10]annulene 1-53 while they accidentally
found the formation of zethrene.
51
The reason is probably the presence of proximate triple
bonds. A formation mechanism was suggested involving a diradical intermediate formed by
spontaneous transannular cyclization of 1-53, however, the mechanism has not been proved
yet. The zethrene was also obtained by other precursors such as 1-52 and 1-54. Since the

16
parent zethrene still suffers from the stability problems, people have tried to make stable
zethrene derivatives with substitutions. In 2009, Tobe et al. successfully isolated the
tetradehydrodinaphtho[10]annulene 1-53 in the crystalline phase. By treating 1-53 with iodine,
they were able to get 1-55 in 65% yield. The successive Sonogashira coupling afforded
7,14-bis(phenylethynyl) zethrene 1-56. Both 1-55 and 1-56 showed enhanced stability
compared to the parent zethrene.
52
The transformation of transannular cyclization could be

realized by either electrophilic, nuleophilic or reductive pathway. For example, as shown in
Scheme 1.3, the triple bond can be attacked by an electrophile, the formed vinylic cation
intermediate is then captured by a nucleophile to form zethrene derivative.
53


Scheme 1.2 Synthesis of zethrenes from trannsannular cyclization.


Scheme 1.3 Electrophile-induced transannular cyclization of 1-53.

Another method to construct zethrene is developed in Wu’s group by using Pd-catalyzed
cyclodimerization of 1-ethynyl-8-iodonaphthalenes 1-57.
54
This method allows the formation
of zethrene core and the substitution at the bay region to be achieved in one step, and the yield
can be improved to up to 73% (Scheme 1.4). The substituents can be a variety of aryl groups,
or even alkyl and trimethysilyl groups. A partially cyclized byproduct is also observed from

17
simultaneous cyclodehydrogenation. The authors also reported the first example of single
crystal for zethrene derivatives. The bond length alternation in the middle part clearly
indicated the butadiene character, and the substitution largely deviated from the planarity of
the parent zethrene. Moreover, the authors performed Pd-catalyzed hydrogenation to examine
the fixed double bond character, and the tetrahydrozethrene 1-50 was obtained as expected
(Scheme 1.4).

Scheme 1.4 Synthesis of zethrenes from cyclodimerization and reduction of zethrene.

In this year, a novel synthesis towards zethrene was reported in Miao’s group, which is

inspired by Sondheimer’s synthesis from precursor 1-52.
55
This sequence started from a
Wittig reaction between 1-58 and corresponding aldehyde to afford diene precursor 1-59,
followed by Heck reaction in presence of Pd(OAc)
2
to give zethrene in satisfactory yields.
The reactivity of zethrene was also examined by bromination and Diels-Alder reactions
(Scheme 1.5). The bromination of zethrene resulted in complicated products, indicating the
multiple reactive sites. Heating of zethrene with N-alkylmaleimide followed by oxidation
gave product 1-60, while further reaction with excess of N-alkylmaleimide followed by
oxidation by air gave more extended product 1-61, this reactivity clearly shows the diene
character in the bay region of zethrene.

18

Scheme 1.5 New synthetic route to zethrene and its Diels-Alder addition reaction.

In addition to the synthesis of zethrenes, the synthesis of higher order zethrene, namely
heptazethrene and octazethrene, is recently achieved in our group.
56
The synthesis of
triisopropylsilylethynyl substituted heptazethrene 1-63 and octazethrene 1-65 took advantage
of the corresponding diketone precursor 1-62 and 1-65 obtained by multiple step synthesis.
The precursors were then treated with Grignard reagents followed by reduction with SnCl
2

(Scheme 1.6). Interestingly, compound 1-63 featured a closed-shell ground state while 1-65
exhibited a singlet biradical ground state, which represented one of the open-shell PHs as
discussed in the previous sections in this chapter.

Scheme 1.6 Synthesis of heptazethrene/octazethrene derivatives.

1.2.2 Applications for zethrene-based PHs
Zethrene and its derivatives recently attracted increasing interest owing to their potentially
interesting properties that might qualify them as new opto-electronic materials for various
applications. Many theoretical calculations have been carried out to predict the properties of
this class of hydrocarbon. In 1995, Burt et al. predicted by PPP calculations that

19
zethrenebis(dicarboximide) would show substantial near-infrared (NIR) absorption and
emission,
57
the absorption wavelength of which is much larger than common diimide dyes. In
2006, Maksić et al. reported that zethrene as well as its longitudinal homologues would
exihibit large absolute proton affinity (APA) and second-order hyperpolarizability (γ) based
on semi-empirical AM1 calculations,
58
and this was further supported by Nakano’s
calculations that zethrene will possess a significant singlet biradical character at the ground
state.
9c
These predictions suggest that zethrene and its derivatives can be used as useful
building blocks for non-linear optical materials and NIR dyes.
Despite of all the predictions, the studies of the material applications of zethrene-based PHs
are scarce. There are a few patents for the use of zethrene derivatives in organic electronic
devices, but the preparative methods are not reported.
59
Recently, Miao et al. fabricated OTFT
devices for zethrene and its diimide derivative 1-61 (with C

6
H
13
).
55
The device was fabricated
by depositing a layer of gold on the films of zethrene through a shadow mask to form
top-contact source and drain electrodes. The resulting devices had highly doped silicon as the
gate electrode and a 300 nm-thick layer of SiO
2
as dielectrics. As measured from these
devices, zethrene functioned as a p-type organic semiconductor with field effect mobility in
the range of 0.01 to 0.05 cm
2
V
−1
s
−1
as shown in Figure 1.12. Under vacuum, the OTFTs of
1-61 exhibited n-channel field effect with electron mobility up to 2x10
-4
cm
2
V
-1
s
-1
and a
threshold voltage larger than 40V.

Figure 1.12 Drain current (IDS) versus gate voltage (VG) with drain voltage(VDS) at -50 V
for the best-performing OTFT of zethrene with the active channel of W = 1 mm and L = 150
Tm as measured in air.

20

1.3 Objectives
In light of this background, zethrene-based PHs represent excellent candidates to
investigate the fundamental structure-property relationship of singlet biradicaloid molecules.
Moreover, they are promising materials for the electronics, spintronics and non-linear optics.
However, the stability and proper functionalization are still major obstacles for these goals to
be realized. In order to investigate the intrinsic properties of zethrene-based molecules and to
seek the possibilities of using them as functional materials, the study in this thesis aim to
develop novel and facile synthetic methodologies to prepare soluble and stable zethrene
derivatives and homologues, and therefore study the physical properties of this interesting
class of PH.

21

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