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

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 mann

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

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 (KH,L), spin density and singlet-triplet energy gap (ΔES-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

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,9for the future photovoltaic devices10 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 Miller14 and Chi15group independently, by blocking more reactive sites at the zigzag edges using different

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 S0-S1 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 ge = 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

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

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 group23

(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

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

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

cm2 V-1 s-1 and hole mobility of 1.9 × 10-5 cm2 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 cm2 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

experimental measurements

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

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

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

(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 cm2 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

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

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

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 AlCl3 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

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

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

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 SnCl2

(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

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 C6H13).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 SiO2 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 cm2V−1s−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 cm2V-1s-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

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

1.4 Reference

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Chapter 2: Zethrene bis(dicarboximide) and its unexpected oxidation

2.1 Introduction

Zethrene (2-1, Figure 2.1), a hydrocarbon whose synthesis was established a long time ago,

has been forgotten for a long time The first synthesis of zethrene was achieved by Clar in

1955,1 and a more convenient access to zethrene was found accidentally by Staab and Sondheimer in 1960s during their independent attempts to synthesize tetradehydrodinaphtho[10]annulene, which was highly unstable and could be automatically

transformed into zethrene via transannular cyclization.2 It was not until 2009 that the pure tetradehydrodinaphtho[10]annulene, the precusor to zethrene, was isolated by Tobe’s group

and they also managed to synthesize stable 7,14-disubstituted zethrene derivatives (2-2,

Figure 2.1).3 Later, more 7,14-disubstituted zethrene derivatives was prepared by Wu et al

via a Pd-catalyzed cycloaddition reaction, with better yield and more choices for bay

substitution Very recently, a facile access to zethrene was reported in Miao’s group and the semiconducting properties of zethrene was discussed for the first time.4

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.5 Many theoretical predictions have been done to predict the properties of this

class of hydrocarbon In 1995, Burt et al predicted by PPP calculations that

zethrenebis(dicarboximide)s (e.g 2-3 in Figure 2.1) would show substantial near-infrared

(NIR) absorption and emission,6 the absorption wavelength of which is much larger than common diimide dyes.7 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, and this was

further supported by Nakano’s calculations that zethrene will possess a significant singlet biradical character at the ground state These predictions suggest that zethrene and its derivatives can be used as useful building blocks for non-linear optical materials and NIR dyes.8

Despite all of these attracting properties and promising applications, zethrene and its derivatives were seldom synthesized and studied deeply due to their low accesibility and high sensitivity in the presence of oxygen and light especially in dilute solution.3a Tobe et al have

successfully synthesized 7,14-substituted zethrene by blocking the most reactive 7,14-positions In parallel to that work, we have been working on the synthesis of electron-withdrawing dicarboxylic imide group substituted zethrene derivative Such an approach will not only stabilize the highly reactive zethrene by lowering its HOMO energy level, but also result in obvious red-shift of the absorption and emission spectra to far-red or NIR region owing to the acceptor-donor-acceptor structure This concept has also been proved

to be efficient to prepare soluble and stable NIR dyes by using very unstable hydrocarbons such as bisanthene as building block.9 In addition, according to calculations, zethrene exihibits a central butadiene moiety flanked by two naphthalene rings and the central butadiene unit shows significant bond length alternation (1.368 and 1.468 Å).5 So we can also study the reactivity of the butadiene subunit and perform further modifications at the reactive 7,14- positions In the following part of this chapter, the synthesis, reactivity, photophysical and electrochemical properties and theoretical calculations will be discussed

Figure 2.1 Structures of zethrene 2-1, 7,14-disubstituted zethrene 2-2 and zethrene bis(dicarboximide) 2-3

2.2 Results and Disccusion

2.2.1 Synthesis and mechanism study

Different from previous work, we chose 4,6-dibromo-1,8-naphthalimide (2-6) as precursor

with bulky 2,6-diisopropylphenyl as substituent, which can improve the solubility as well as suppress aggregation of the zethrene chromophore As shown in Scheme 2.1, 1,8-dibromo-naphthoic anhydride (2-5) was firstly prepared by oxidation of

1,8-dibromoacenaphthenedione (2-4)10 with oxone,9a and subsequent imidization of 2-5 with

2,6-diisopropylaniline afforded 4,6-dibromo-1,8-naphthalimide (2-6). Cross coupling reaction12 between 2-6 and bis(tributylstannyl)acetylene (1:1 ratio) and subsequent in situ

transannular cyclization reaction gave the desired zethrenebis(dicarboximide) 2-3 in one pot.

It is essential to carefully control the reaction conditions to obtain the target compound This reaction must be performed in dilute solution to favor intramolecular cyclization over intermolecular polymerization.13 Oxygen has to be strictly excluded from the reaction system The optimized temperature is 80 oC to avoid incomplete conversion at lower temperature or complicated products at higher temperature Although we did rigid control on the experimental conditions, the separation yield for this step was still low (13-20%) due to the existence of other oligomers, which complicated the column chromatography purification

Scheme 2.1 Synthetic route towards 2-3

In order to further modify the zethrene at the 7,14-positions, bromination of 2-3 was

attempted by using N-bromosuccinimide (NBS) in DMF.14 Interestingly, the oxidized product

zethrenebis(dicarboximide) quinone (2-8) rather than brominated product (2-9) was formed The structure of 2-8 was confirmed by 1H NMR, 13C NMR, HR-ESI MS, MALDI-TOF MS and FT-IR spectra For example, in the MALDI-TOF MS spectrum the molecular weight

corresponding to the 2-8 was observed (Figure 2.2), and in the 13C NMR spectrum, two

resonances at 186.18 ppm and 163.15 ppm were observed, which can be assigned to carbonyl carbon of the quinone and the imide group, respectively (see Appendix) Furthermore, in the

FT-IR spectra of 2-8, besides a C=O vibration band at 1675 cm-1 which is correlated to the imide unit, an additional C=O stretching band at 1715 cm-1 related to the quinone structure was also observed (Figure 2.2) It is worth noting that no oxidized product but some complicated mixture was obtained if the bromination was conducted in other systems such as NBS/CHCl3 and NBS/DMSO This unusual transformation could be related to the central butadiene character in zethrene.1,15

Scheme 2.2 Unexpected oxidation reaction under bromination conditions

A reasonable mechanism is proposed in Scheme 2.4 Electrophilic addition of 2-3 with

NBS takes place and subsequent quenching with trace amount of water in the DMF generates

the intermediate 2-10 with simultaneous formation of succinimide This is followed by nucleophilic attack of C-Br by water to afford the diol 2-11 This process is favorable because

the Br atom is attached to a benzylic and allylic carbon In addition, the as-formed HBr molecule can be trapped by the DMF solvent This also highlights the importance of using

basic solvent such as DMF in this conversion Further bromination of 2-11 by NBS results in either a substituted intermediate (2-12) or hypobromite intermediate (2-13),16 both would turn

into a zethrene quinone (2-8) after removal of HBr in presence of DMF.

4000 3000 2000 1000

wavenumber

1693 1660 imide C=O

(b)

wavenumber

1675 1645 1715

imide C=O

C=O at 7,14-position

(c)

Figure 2.2 (a) MALDI-TOF Mass spectrum of 2-8, (b) FT-IR spectrum of 2-3, (c) FT-IR spectrum of 2-8

Scheme 2.4 Proposed mechanism for the formation of 2-8

2.2.2 Theoretical calculations

Time-dependent density function theory calculations were conducted for 2-3 and 2-8 and

their optimized molecular structures and frontier molecular orbital profiles are shown in

Figure 2.3 For 2-3, the carbon-carbon bond lengths of the central part show significant alternation (1.371 and 1.467 Å), indicative of a butadiene-like structure HOMO of 2-3 shows

electron delocalization through the zethrene core to a large extent, and the central two double bonds possess the largest HOMO coefficient, suggesting that the 7,14-positions can easily undergo electrophilic attack, consistent with our observations in the bromination reaction On

the other hand, the LUMO of quinone 2-8 has high coefficient along the central quinone unit,

thus it has a high tendency to accept electrons The calculations also predict that compounds

2-3 and 2-8 will show absorption maximum at 645.8 nm and 590.6 nm, which are in

agreement with the experimental data

Figure 2.3 Optimized molecular structures and frontier molecular orbital profiles of 2-3 and 2-8 Some bond lengths are indicated by arrows (Å) The blue and red color denote negative

and positive charges, respectively

2.2.3 Photophysical and electrochemical properties

Both compounds 2-3 and 2-8 are soluble in common organic solvents and 2-3 exhibits a blue color while 2-8 appears to be red The UV-Vis absorption and fluorescence spectra recorded in chloroform are shown in Figure 2.4 Compound 2-3 has a major absorption band

in far-red region with maximum at 648 nm (ε = 18580 M-1cm-1) together with a shoulder at

596 nm Compared to the maximum absorption of zethrene (550 nm)1 and 7,14-bis(phenylethynyl)zethrene (576 nm)3a, a significant bathochromic shift was observed due to attachment of electron-withdrawing dicarboxylic imide groups, which leads to a convergence of HOMO-LUMO energy gap.6 In contrast to 2-3, compound 2-8 displays a

hypsochromic shift with absorption maximum at 477 and 528 nm, indicating that the

conjugation of zethrene core is partially disrupted in the central part Both 2-3 and 2-8 show

small stokes shift due to their rigid backbone The photoluminescence quantum yeild (Φ) of

2-3 was determined by using Rhodamine B17a and cardiogreen17b as standards and the Φ values were obtained as 0.52 and 0.54, respectively, showing significant enhancement compared with 7,14-bis(phenylethynyl)zethrene (Φ = 0.07).3a Compound 2-8 exhibits very weak fluorescence due to the existence of the quinone structure The photostability of 2-3 in

chloroform was also investigated under irradiation of UV light (4W), white light (100W) and

ambient light, and the half-life times were determined as 35, 600 and 4320 min, respectively (Figure 2.5) Such large enhancement on photostability can be explained by the introduction

of the electron-withdrawing dicarboxylic imide groups The quinone 2-8 showed even higher

photo-stability and the absorption spectra of its solution did not change after standing under ambient conditions for weeks

0.0 0.2 0.4 0.6 0.8 1.0

1.2

2-3 UV-Vis 2-3 PL 2-8 UV-Vis 2-8 PL

1.0

Under ambient light Under UV light Under white light

Figure 2.5 Photostability measurements for 2-3 UV spectra change under irradiation of (a)

UV lamp (254 nm), (b) white bulb and (c) ambient condition (d) Change of optical density of

2-3 at the absorption maximum wavelength with the irradiation time All experiments are

carried out under presence of oxygen

The electrochemical properties of compounds 2-3 and 2-8 were investigated by cyclic

voltammetry (CV) in dry DCM (Figure 2.6) and the data was tabulated in Table 2.1 The

cyclic voltammogram of 2-3 exhibits one quasi-reversible oxidation wave with half-wave

potential (E ox) at 0.93 (vs Fc/Fc+) but no obvious oxidation wave was observed for compound

2-8 The HOMO energy level of 2-3 was estimated to be -5.50 eV based on the onset of the

oxidation wave.18 The oxidation potential of 2-3 is much larger than that of

7,14-bis(phenylethynyl)zethrene (0.29 V in CH2Cl2 vs Fc/Fc+),3a as well as common semiconductor pentacene (0.3 V vs Fc/Fc+ in C6H4Cl2)19a and its bis(triiso-propylsilyl)ethynyl derivative (0.38 V vs Fc/Fc+ in THF),19b suggesting that a better stability of zethrene derivatives can be achieved by attaching electron-withdrawing imide group, in accordance

with our design at the first place Compound 2-3 has shown three reversible reduction waves

with half-wave potentials at -0.84 V, -0.99 V, -1.30 V, respectively, and there are four

reduction potentials for compound 2-8 at -0.55 V, -0.87 V, -1.21 V and -1.67 V Accordingly, LUMO energy level of -3.96 eV was calculated for 2-3 and -4.31 eV for 2-8 based on the

onset of the first reduction wave,18 indicating that the additional carbonyl groups in 2-8 increase the electron affinity compared with 2-3 It is also worth noting that 2-3 has a small

band gap (1.54 eV), which is in agreement with the optical bandgap (1.81 eV) from the absorption spectra

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

2-3 2-8

Figure 2.6 Cyclic voltammograms of compounds 2-3, 2-8 in dichloromethane with 0.1 M

Bu4NPF6 as supporting electrolyte, Ag/AgCl as reference electrode, Au disk as working electrode, Pt wire as counter electrode, and scan rate at 50 mV/s

LUMO (eV)

Table 2.1 Photophysical and electrochemical properties of compounds 2-3, 2-8 E ox and E red

are half-wave potentials for respective redox waves with Fc/Fc+ as reference HOMO and

LUMO energy levels were calculated according to equations: HOMO = - (4.8 + E ox

In conclusion, a new zethrene derivative, the zethrenebis(dicarboximide) 2-3 was

successfully synthesized through an in-situ Cross coupling/transannular cyclization reaction

Its quinone form 2-8 was also obtained by chance under typical bromination condition This

unusual chemical transformation can be explained by the existence of reactive butadiene

subunit in compound 2-3 Some remarkable chemical and optical properties of 2-3 were

observed such as excellent solubility, good chemical and photo-stability, and high

fluorescence quantum yield Compound 2-8 shows even better stability and solubility All of

the properties make them promising candidates as dyes and semiconductors in the materials

sciences

2.4 Experimental section

2.4.1 General experimental methods

All reagents were purchased from commercial suppliers and used as received without further purification Anhydrous dichloromethane (DCM) was distilled from CaH2

1,8-dibromoacenaphthenedione (2-4) was prepared according to literature.20 The 1H NMR and

13

C NMR spectra were recorded in solution of CDCl3 on Bruker DPX 300 or DRX 500 NMR spectrometers with tetramethylsilane (TMS) as the internal standard The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet MALDI-TOF mass spectra (MS) were recorded on a Bruker Autoflex instrument using anthracene-1, 8, 9-triol as matrix HR-EI mass spectra were recorded on Agilent 5975C DIP/MS mass spectrometer and HR-ESI mass spectra were recorded on Finnigan LCQ quadrapole ion trap mass spectrometer UV-vis absorption and fluorescence spectra were recorded on a Shimadzu UV-1700 spectrometer and a RF-5301 fluorometer, respectively IR spectra were obtained on a Varian 3100 FT-IR instrument by using pressed KBr pellets The electrochemical measurements were carried out in anhydrous DCM with 0.1M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at a scan rate of 0.05 V/s at room temperature under the protection of nitrogen A gold disk was used as working electrode, platinum wire was used as counting electrode, and Ag/AgCl (3M KCl solution) was used as reference electrode The potential was calibrated against the ferrocene/ferrocenium couple The fluorescence quantum yields were measured by optical dilute method (A < 0.05) using cardiogreen (λabs,

max = 780 nm, Ф = 0.13 in DMSO) and Rhodamine B (λabs = 543 nm, Φ = 0.7 in Ethanol) as references

2.4.2 Characterization data

Synthesis of 2-5: 1,8-Dibromoacenaphthenedione 2-4 (2.00 g, 5.92 mmol) was suspended in

methanol (150 mL) and oxone (2.50 g, 16.4 mmol) was added The mixture was stirred at reflux for 72 h After cooling, the mixture was poured into water and the orange precipitate

was washed with water and then methanol and dried to afford an orange solid 4 (1.88 g) in

90% yield This compound is very insoluble and the H NMR and C NMR spectra were not

easy to record HR-MS (EI) m/z = 353.8527, calcd for C12H4Br2O3 m/z = 353.8500

Synthesis of 2-6: To the suspension of 1,8-dibromonaphthoic anhydride 2-5 (1.00 g, 2.83

mmol) in acetic acid (50 mL) was added 2, 6-diisopropylaniline (1.74 g, 9.81 mmol, 3.5 equiv.) The mixture was stirred at reflux for 24 h under nitrogen atmosphere The resulting brown mixture was poured into ice water The precipitate was filtered by suction filtration, and followed by washing with water for 3-5 times The crude product was then purified by column chromatography on silica gel (chloroform: hexane= 1:1) to give a yellow solid

compound 5 (420 mg) in 29% yield 1H NMR (CDCl3, 300 MHz) δ: 8.47 (d, J = 8.0 Hz, 2H), 8.27 (d, J = 8.0 Hz, 2H), 7.48 (t, J = 7.2 Hz, 1H), 7.32 (d, J = 7.7 Hz, 2H), 2.61-2.71 (m, 2H), 1.14 (d, J = 6.9 Hz, 12H). 13C NMR (CDCl3, 75 MHz) δ: 163.2, 145.5, 136.3, 132.0, 129.8,

128.6, 124.1, 123.1, 29.2, 23.9 HR-MS (EI): m/z = 512.9939, calcd for C24H21Br2NO2: m/z

= 512.9900

Synthesis of 2-3: 4,6-Dibromo-1,8-naphthalimide 2-6 (100 mg, 0.190 mmol), Pd(PPh3)4 (22.0

mg, 0.017mmol) were dissolved in toluene (200 mL) Oxygen was strictly excluded by Freeze-thaw-Pump cycle for three times Bis(tributylstannyl)acetylene (0.10 mL, 0.19 mmol,

1 equiv.) was added to the solution by syringe, the mixture was heated to 80 oC and maintained at 80 oC for 24 h under nitrogen atmosphere in the dark The resulting dark blue solution was washed with water After removal of solvent the residue was purified by column chromatography on silica gel (DCM as eluent, Rf = 0.3) to yield a blue solid in 13-20% yield

1

H NMR (CDCl3, 300 MHz) δ: 8.76（d, J = 8.0 Hz, 2H）, 8.68 (d, J = 3.6 Hz, 2H), 8.65 (d, J = 3.0 Hz, 2H), 8.36 (s, 2H), 7.88 (d, J = 7.7 Hz, 2H), 7.49 (t, J = 7.7 Hz, 2H), 7.35 (d, J = 7.7

Hz, 4H), 2.73-2.83 (m, 4H), 1.18 (d, J = 6.7 Hz, 12H) 13C NMR (CDCl3, 125 MHz) δ: 163.6, 145.7, 136.8, 132.6, 131.2, 129.62, 129.6, 127.8, 126.6, 124.2, 124.1, 124.07, 122.5, 122.4,

29.7, 24.0 HR-MS (ESI): m/z = 760.3307, calcd for C52H44N2O4: m/z = 760.3276

Synthesis of 2-8: Zethrene bis(dicarboximide) 2-3 (20 mg, 0.026 mmol),

N-bromosuccinimide (NBS, 10 mg, 0.058 mmol) was dissolved in N, N-dimethylformamide

(DMF, 10 mL) The mixture was stirred under nitrogen atmosphere in the dark for 12 h The resulting red solution was poured into water, the precipitate was filtered and washed with water to remove DMF, and further purified by column chromatography on silica gel

(chloroform as eluent, Rf = 0.4) to give a red solid (17 mg) in 82% yield 1H NMR (CDCl3,

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