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PAPER 1213
Areno-Condensed Annulenes – Extended Discotic
MesogensAreno-Condensed AnnulenesH. Meier*Institute of Organic
Chemistry, Johannes Gutenberg University, Mainz, GermanyFax
+49(6131)3925396; E-mail: [email protected] 24
February 2002
Synthesis 2002, No. 9, 01 07 2002. Article Identifier:
1437-210X,E;2002,0,09,1213,1228,ftx,en;C10502SS.pdf. © Georg Thieme
Verlag Stuttgart · New YorkISSN 0039-7881
Abstract: [n]Annulenes (n �12) condensed with 2–4 aromatic
ringsystems (benzenes, naphthalenes, anthracenes,
phenanthrenes,chrysenes, pyrenes) can be prepared by
cyclocondensation reac-tions or ring transformations. Due to the
local arene aromaticity, themolecules can be regarded as aromatic
islands, which are connectedby olefinic bridges. The compounds are
non-planar, but the majorityof the systems shows a fast inversion
of the central macrocyclicring, so that the molecules appear on
average as large planar discs,which consist of extended � systems.
The aggregation tendency (�stacking) of the compounds can be
strengthened by the attachmentof flexible saturated chains on the
periphery. The discs representthen mesogens for columnar (or
nematic) discotic LC phases. Be-cause of the stilbenoid character,
the compounds show a variety ofinteresting photophysical and
photochemical properties. They canbe applied in photoconductive
liquid crystalline phases Colh and inradiation-induced imaging
techniques.
Key words: annulenes, condensation, ring closure,
aggregation,photochemistry
1 Introduction
Annulenes 1 (Figure1) represent a highly interesting classof
compounds in synthetic as well as in theoretical chem-istry. 1–6
However, because of their low thermal and pho-tochemical stability,
annulenes are hardly suitable forapplications in materials science,
although some promis-ing attempts have been made.7–9
Figure 1
The condensation of benzene or higher arene ring systemson the
annulene perimeter enhances significantly the ther-mal stability.
In the series of [18]annulene (2),10 a varietyof benzo-condensed
compounds has been studied, for ex-ample the monobenzo system 3,11
the dibenzo systems 412
and 5,13 the tetrabenzo system 6a,14 the hexabenzo system7,15
which represents a hexa-m-phenylene, the ‘nonaben-zo’ system 816
and the ‘dodecabenzo’ system 9,17 well-known as kekulene (Figure
2).
Figure 2 [18]Annulenes condensed with benzene ring systems
Apart from the fact, that all these hydrocarbons are disc-like
compounds with a central 18-membered ring, thechemical character of
the compounds is different. Themacrocyclic aromaticity
(diatropicity), present in 2, is re-duced in the monobenzo
derivative 311 and disappears to-tally in the higher condensed
systems 4, 5, 6a, etc. in favorof the local benzenoid
aromaticity.12–17
We were mainly interested in areno-condensed annuleneslike 6a,
which represent extended discotic mesogens andconsist of stilbenoid
units with interesting photophysicaland photochemical properties
for materials science, par-
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ticularly for columnar liquid crystals.18 The general for-mula
10/10� and the aromatic building blocks listed inTable 1
characterize these molecules. The annelation ofthe aromatic ring
systems with the central annulene ring isalways on the shorter
(concave) section of the circumfer-ence of the arene. The size of
the central annulene ringdiscussed in this article varies mainly
from 12 to 24; buteven in this range, many possible combinations of
10/10�with the aromatic building blocks shown in Table 1 werenot
yet synthesized.
2 Synthetic Methods
The synthetic strategy towards the areno-condensed annu-lenes
10/10� is mostly based on the preparation of appro-priately
functionalized arenes. The final step consists thenof the
generation of CC double bonds, which are formedin cyclocondensation
reactions (type A–B or AA–BB)(Equation 1).
Linear condensation products are always formed in com-petitive
processes. Due to the end groups, the linear prod-ucts have a
higher polarity and can be easily separated.Although the generation
of the linear products reduces the
yields of the cyclic products, cyclocondensation
reactionsrepresent a convenient route for the preparation of the
tar-get compounds. Alternatives are ring transformations
ofpreformed cyclic compounds, for example the final intro-duction
of olefinic double bonds by elimination reactions.
2.1 Siegrist Reactions
The majority of areno-condensed annulenes was preparedby
applying the Siegrist reaction,19–21 which is a conden-sation
process of N-arylimino groups and activated me-thyl groups in a
strongly alkaline medium. For theapplication discussed here, both
reactive groups are com-monly fixed on the same starting material
so that two,three or four such molecules can form a
cyclocondensa-
Equation 1
Table 1 Selection of Aromatic Building Blocks for the
Areno-Condensed Annulenes 10/10�
CondensationType
Benzene Naphthalene Anthracene Phenanthreneand Higher Arenes
[a]
[a,b]
[a,b,c]
[a,b,c,d]
[a,b,c,d,e]
[a,b,c,d,e,f]
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tion product. Due to the strict anti elimination of aniline,the
kinetically controlled reaction is highly stereoselec-tive. The
trans/cis ratio amounts to about 1000:1, which isfar beyond the
thermodynamic equilibrium.18 Scheme 1demonstrates that the size of
the generated central ring de-pends on the relative position of the
two reactive groupsand on the number of involved arene
molecules.
Figures 3 and Figure 4 summarize further examples
ofareno-condensed [16]-, [18]- and [24]annulenes whichwere prepared
by Siegrist ractions. The yields vary be-tween 67% for 6b and 3%
for 23, which is a byproduct inthe preparation of 17e.
2.2 McMurry Reactions
In the series of benzo-condensed [12]- and
[16]annulenes,terminal dialdehydes can be successfully applied for
ringclosure reactions. Scheme 2 shows three typical examplesof one-
and twofold intramolecular McMurry reactions.Compound 16a (Figure
3) is also accessible by a McMur-ry reaction.24
2.3 Pinacol Coupling Followed by the Corey–Win-ter Procedure
An alternative route for the intramolecular CC coupling
ofdialdehydes makes use of the pinacol formation. The re-action of
28 with VCl3(THF)3 and zinc leads to a 2:1 mix-ture of the
threo-diol 29 and the erythro-diol 30(Scheme 3). The threo isomer
can be converted into theerythro form by successive Swern oxidation
and reduc-tion with NaBH4. The reaction of 30 with
thiocarbonyldi-imidazole (TCDI) yields the thionocarbonate 31,
which istransformed to all-cis-tetrabenzo[a,e,i,m][16]annulene16c
by the reaction with
1,3-dimethyl-2-phenyl-1,3,2-di-azaphospholidine (DMPD) in refluxing
benzene.33 Theoverall yield for 28 � 16c is lower than for the
McMurrystep 26 � 16b, but the modified Corey–Winter procedure
Scheme 1 Preparation of tetrabenzo[ab,f,jk,o][18]annulene
(6a),14
trinaphtho[3,4,5-abc:3,4,5-ghi:3,4,5-mno][18]annulene (13a)22
andtriphenanthro[3,4,5,6-abcde:3,4,5,6-ijklm:3,4,5,6-qrstu][24]annule-ne
(15a):23 i) KOC(CH3)3, DMF
Figure 3 Further areno-condensed [16]-, [18]annulenes, which
were prepared by Siegrist reactions22, 24–28
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gives selectively the all-cis-configuration 16c – in con-trast
to 16b, which has the all-trans-configuration. Anal-ogous reaction
sequences can be used for the preparationof
all-cis-tribenzo[a,e,i][12]annulene 25d and all-cis-
pentabenzo[a,e,i,m,q][20]annulene 32a,33 which aredrawn in
Scheme 3 in the (also for 16c) more realisticcrown
conformation.
Figure 4 Further areno-condensed [18]- and [24]annulenes, which
were prepared by Siegrist reactions23, 24, 27–31
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2.4 Wittig–Horner Reactions
Intramolecular Wittig–Horner reactions represent
anotherwell-established method for the generation of
areno-con-densed annulenes. Interestingly, phosphonium salts
andphosphonates exhibit different cyclization tendencies.Whereas 33
yields 26% of the (Z,Z)-dimer 34 and onlytraces of the
(E,E,E)-trimer 25e,34 the related phosphonate35 gives a mixture of
all-(E)-configured cyclooligomersup to the 36-membered ring 39
(Scheme 4).24 The pre-ferred formation of (E)-configurations
excludes the gener-ation of 34; moreover, the [12]annulene 25e
cannot bedetected in the mixture of cyclooligomers.24 Figure 5shows
the MALDI-TOF spectrum of the benzo-con-densed [n]annulenes (n =
16, 20, 24, 28, 32, 36). The ma-jor products in both reactions have
linear structures; thepreparative value of these processes is
limited, since theseparation of the cyclooligomers is rather
laborious.
Instead of cyclocondensation reactions of the AB type,AA + BB
processes can be applied with superior
results.(E,E,E)-Tribenzo[a,e,i][12]annulene 25e can be obtainedby
the reaction of phthaldialdehyde 40 and the diphospho-nium salt
41.35 The additionally formed (E,E,Z)-isomerrearranges
spontaneously by an intramolecular [2� + 2� +2�] process to yield a
polycyclic compound, namely
tetra-cyclo[6.4.0.0.4,1205,9]dodeca-2,6,10-triene (See Scheme
9,vide infra).36 The Wittig reaction of 40 and 42
furnishestetrabenzo[a,e,i,m][16]annulene (16b).37 The two
an-thracene derivatives 43 and 44 yield the dianthra[14]an-nulene
4538 (Scheme 5).
2.5 Ring Transformations
Apart from cyclization reactions in the final step, the
syn-thesis of areno-condensed annulenes 10/10� can be
Scheme 2 Benzo-condensed [12]- and [16]annulenes prepared
byintramolecular McMurry reactions : i) TiCl3/Zn�Cu, DME
24, 32 Scheme 3 Preparation of the
tetrabenzo[a,e,i,m][16]annulene 16cby pinacol coupling followed by
the Corey–Winter procedure. (Ana-logous reaction sequences can be
used for the preparation of 25d and32a)
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achieved by transformation reactions of preformed (mac-rocyclic)
ring systems. The generation of three olefinicbridges –CH=CH– by
bromination and dehydrobromina-tion of three saturated bridges
–CH2–CH2– can be used forthe preparation of
(E,E,E)-tribenzo[a,e,i][12]annulene(25e).39 On the other hand,
acetylenic bridges –C�C– canbe partially hydrogenated with Lindlar
catalyst to olefinicbridges. Interestingly
(E,Z,Z)-tribenzo[a,e,i][12]annulene
Scheme 4 Benzo-condensed annulenes obtained by Wittig and
Hor-ner reactions of the AB type
Scheme 5 Wittig reactions of the AA-BB type for the
preparationof benzo- and anthra-condensed [12]-, [14]- and
[16]annulenes
Figure 5 MALDI-TOF spectrum of the cyclic products obtained by
the Horner reaction of phosphonate 35
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(25f) was obtained by this method (Scheme 6).40 Thermalor
photochemical isomerization of 25f yield the (E,E,Z)-configuration
which spontaneously rearranges to give thepolycyclic compound
mentioned above. An example,where the olefinic bridges already
exist, but the con-densed aromatic ring system has to be changed,
is pub-lished for the formation of 48 by the twofoldphotoyclization
of 47.40 The cyclic trimer 49 of benzocy-clobutadiene served as a
highly efficient precursor fortribenzo[a,e,i][12]annulene. The
all-cis arrangementpresent in 49 is almost completely retained in
the intramo-lecular [2� + 2� + 2�] process.41
3 Molecular Structures
The parent annulenes with (4n + 2) � electrons (n = 3, 4,5, 6)
exhibit a diatropic behavior. They have a planar oralmost planar
structure, which corresponds to a compro-mise between the repulsion
of the inner hydrogen atomsand the resonance stabilization of the
macrocyclic ring.4,5
The condensation of the macrocyclic rings with benzeneor higher
arene ring systems provokes a change of themacrocyclic aromaticitiy
to the ‘local’ aromalticity of thearenes. Thus, one can consider
the areno-condensed annu-lenes as molecules, which consist of
aromatic ‘islands’,which are connected by olefinic bridges. This
statementholds for all systems irrespective of the number of �
elec-trons in the central macrocyclic ring. A typical conse-
quence of the ‘local’ benzenoid aromaticity is the fact,that the
inner (olefinic) protons Hi have somewhat higher� values in the 1H
NMR spectra than the outer olefinicprotons Ho – provided that there
is no fast exchange be-tween Hi and Ho. The macrocyclic ring
current, which ischaracteristic for the parent annulenes1–6 and
permits aneasy destinction between 4n and (4n + 2) � electron
sys-tems, is not present in the areno-condensed annulenes
dis-cussed here.42
The existence of all-trans configurations in the areno-condensed
annulenes 10/10� is a necessary but not suffi-cient precondition
for on average planar, disc-like mole-cules. One or more cis
configurations as in 16c, 25d, 25f,32a and 48 cause crown- or
boatlike structures, which donot have the capability of
planarization. Concerning theconformation of the all-(E)-configured
compounds 10/10�, the trinaphtho-condensed [18]annulene 13a
mayserve here for a detailed discussion.22 The
conformationalmobility of 13a is guaranteed by torsions around the
CCsingle bonds on both sides of the olefinic double bonds.Figure 6
illustrates this mobility by the variation of thetorsion angle �.
The planar conformations A and E, eachpossessing C3h symmetry,
ensure maximum conjugation,yet incorporate high steric energy,
which is caused bystrong H–H interactions. According to force field
calcula-tions (MMX, Serena, PCM Version 4), A and E
representtransition regions, whereas conformer B (� = 32°) is
theglobal minimum (�Hf = 164.3 kcalmol
–1).22 The H–H in-teractions are minimized in the saddle region
C (� = 90°),but the conjugation is completely interrupted. Further
tor-sion can lead back to A via the energetically high lyingminimum
D (� = 140°, �Hf = 174.1 kcalmol
-1) and thetransition region E. Of course, the rotation does not
haveto be synchronous in all three positions indicated in A.The
force field calculation reveals a further minimum B�(�Hf = 165.3
kcalmol
–1) almost isoenergetic to B, inwhich two inner olefinic protons
are standing upwardsand one downwards. Since B (C3) and B� (C1) are
chiral,the corresponding enantiomers have to be included in
theconformational dynamics.
A part of the calculated energy hypersurface is shownin Figure
7.43 The contour plot of the energy illustrates,that smooth
diastereoisomerization routes B(�1 = �2 = �3 = –32°) � B� (�1 = �3
= –32°, �� = +32°)and B � B�� (�1 = �2 = +32°, �3 = –32°) and
enantiomeri-zation routes B� � B�� exist, which are characterized
bylow activation barriers (Ea 2 kcalmol
–1). Thus the com-pound 13a appears to be planar on average (de
facto C3hsymmetry). The inner olefinic protons of 13a show
theirresonance at lower field than the outer olefinic protons[�(Hi)
= 8.21, �(Ho) = 7.58]. The assignment was estab-lished by deuterium
labeling and NOE difference spec-troscopy including the
two-dimensional ROESYtechnique.22 Due to the macrocyclic
aromaticity, the un-substituted [18]annulene exhibits the opposite
behavior[�(Hi) = –2.99, �(Ho) = 9.28]
1. Moreover, inner and outerprotons do not exchange in 13a,
because conformation Dis too unfavorable to be populated.
Scheme 6 Preparation of areno-condensed annulenes by
differentring transformation reactions
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The other [18]annulenes condensed with phenanthrenes(17, 18)
pyrenes (19) or chrysenes (20) show very similarconformational
dynamics.22,27–31,43 Thus the areno-con-densed [18]annulenes are
specially suitable as disc-likemesogens with a great diameter.
Can the design of the mesogen be still improved, if
largerannulenes (n >18) are used? Let us consider the
areno-condensed [24]annulenes. They can be devided in
twocategories: the triareno-[abcde]-fused systems 15 and 21and the
tetraareno-[abc]-fused systems 22 and 23. Theconformational
dynamics of the first series includes C3and C1 species, and fast
flip processes like in the areno-condensed [18]annulenes lead on
average to a pseudo C3hsymmetry. The second series shows a
different behavior.The force field calculation reveals S4
conformations asglobal minima (Figure 8). The conformational
dynamicsinclude C1 and C2 species; however, the molecules cannever
adopt a plane of symmetry. Thus, the geminal pro-tons of the OCH2
groups in 22 and 23 are diastereotopic.The S4 conformation (the
lack of planarity) is not a resultof the steric repulsion of the
inner hydrogen atoms; thedifferent molecular structures and
dynamics in the seriesof the areno-condensed [24]annulenes are a
consequenceof different perimeter sequences of trans-fixed,
cis-fixedand flexible bonds of the 24-membered rings.28 The
1HNMR-spectra of the OCH2 protons provide in systemswith alkoxy
sidechains a proof for the fast planarization ofthe central ring or
its absence. Interestingly, the enan-tiotopic OCH2 protons in the
series 13, 15, 17–21 did notbecome diastereotopic on cooling, nor
did the diaste-
reotopic OCH2 protons of 22 and 23 become enantiotopicon
heating. 28,30 Consequently, only the ring systems of thefirst
series 15 and 21 of [24]annulenes are suitable as dis-cotic
mesogens.
Figure 8 S4 conformation of [24]annulene condensed with
fournaphthalene units (MMX calculation: Serena, PCM Version
4)28
An important point concerns the exchange of inner andouter
(olefinic) protons – a process that is well known forthe parent
annulenes. As mentioned above, this exchangedoes not occur in the
series of the areno-condensed[18]annulenes 13 and because of
similar reasons also notin the series 17–20. Nevertheless, such
molecular dynam-
Figure 6 Illustration of the conformational dynamics of
compound13a
Figure 7 Contour plot of the MMX energies (in kcal�mol–1) of
sel-ected conformations of 13a;43 (The torsional angles �1 and �2
weresystematically varied, �3 and the other molecular parameters
were op-timized without symmetrization.) Isomerization routes
between dia-stereoisomers (––) and between enantiomers (...)
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ics can be observed in the series 15, 16, 21 and 25.23,24,32
The exchange mechanism is independent of the pla-narization.
Whereas for example 15a and 25e exhibit bothfeatures, 16a and 16b
preserve diastereotopic OCH2 pro-tons, although the fast exchange
of inner and outer protonsprovokes a singlet signal for the
olefinic protons.
Semiemperical calculations (AM1, PM3) agree well withthe
conformations found in 1H NMR measurements; nev-ertheless, we
performed also an ab initio Hartree-Fockcalculation at the 3-21 G
level for the molecule 6a.14 Ac-cording to this calculation, the
free molecule has C2h sym-metry. The planar D2h geometry represents
the compoundin the ‘time average’. Concerning the bond lengths and
thebond angles, the parameters obtained in the crystal struc-ture
analysis of 6a show a very good agreement with thecalculated data.
However, the torsion angles are muchsmaller in the crystalline
state that means the molecule ismuch flatter than calculated. Table
2 gives a comparisonof the calculated parameters for the free
molecule and thedata measured in the crystal.
4 Aggregation
On the theoretical basis of the TURBOMOLE ab initioprogram
package,44 dimeric aggregates of 6a were calcu-lated.14 The most
stable molecular pairs have a C2h or a Cisymmetry (Figure 9). The
gas phase calculation indicatesin both cases an energy gain of 4.7
kcalmol–1 for the in-termolecular attraction.
Figure 9 Intrastack molecular pairs of 6a with C2h (left side)
and Cisymmetry (right side); The Å values indicate the distances
betweenneighbouring olefinic bonds.14
Within the C2h molecular pair, the upper molecule is shift-ed
with respect to the lower molecule so as to keep thecommon mirror
plane; in the Ci pair, the shift is mainly ina direction
perpendicular to that realized in the C2h pair.Both arrangements
are principally suitable for the genera-tion of higher aggregates,
for example in columnar me-sophases of compounds with long flexible
sidechains or inthe crystalline state, where a herringbone
design(Figure 10) was established by high resolution
electronmicroscopy and X-ray analysis.14 The observed
intrastackdimeric pair has Ci symmetry and an average distance
of3.58 Å between the olefinic bonds.
Table 2 Selected Molecular Parameters of Compound 6a: Ab initio
Calculation/Crystal Structure Analysis14
Bond Lengths/pm
2–3 3–4 4–5148.3/147.7 132.4/132.2 148.1/147.7
Bond Angles/°
1–2–3 2–3–4 3–4–5 4–5–6120.7/120.5 124.4/126.2 124.8/125.4
121.3/121.5
Torsion Angles/°
1–2–3–4 3–4–5–6–41.6/–20.9 +35.9/+11.1
Figure 10 Model structure obtained for the high-resolution
imagein the electron diffraction measurement of crystals of
6a14
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An aggregation tendency was found for the majority
ofareno-condensed annulenes. Long saturated chains at-tached to the
periphery of the disc-like moleculesstrengthen the aggregation. It
is not always easy to detectthe aggregation in solution
experiments. Concentration-dependent intensities in UV/Vis spectra
are reliable indi-cations – even when the effects are small as
shown inFigure 11 for compound 15b.30 A concentration – depen-dent
fluorescence can be due to ground state aggregates,which are
maintained in the first excited singlet state, orto excimers.
Apolar solvents like cyclohexane provoke a stronger ag-gregation
of the disc-like compounds with high � electrondensity than polar
solvents, but in some cases the aggre-gation can even be seen in
chloroform. Figure 12 exhibitsthe 1H NMR spectra of 15b at
different concentrations inCDCl3. The line broadening at higher
concentrations isdue to the formation (and dissociation) of
aggregates andthe restricted mobility of molecules and molecule
seg-ments in aggregates.45
Other proofs of aggregation make use of fluorescencespectra,
fluorescence excitation spectra or of dynamiclight scattering
experiments.28 However, all these mea-surements may exhibit
intriguing features. Let us considercompound 19a. Pyrene is an
excellent example for a dualfluorescence. Time-resolved
measurements revealed, thatadditionally to the monomer fluorescence
F, a fluores-cence band F� at longer wavelengths develops in
concen-trated solutions on a ns time scale after the
excitationpulse. After about 100 ns the measured spectrum
corre-sponds to the steady-state fluorescence.46 The delay in
theexcimer emission is due to the diffusion of a pyrene mol-ecule
in the excited state S1 to a ground-state molecule S0(Equation 2)
and to the geometrical change of the encoun-ter complex to an
excimer, which is characterized by aparallel arrangement of the
planar molecules in a distanceof about 3.0–3.5 Å.47
Equation 2
The tripyreno[18]annulene 19a shows also some depen-dence of the
fluorescence spectrum on the concentration.However, there is no
time-dependent evolution of theband shape.45 Despite the
aggregation tendency, there isvirtually only one emitting species
with an average life-time between 7.3 and 8.2 ns.45 The aggregates
exist in theground-state S0 as well as in the excited singlet state
S1,
Figure 12 400 MHz 1H NMR spectra of the
triphenanthro[24]annu-lene 15b in CDCl3: bottom: 6.0�10–4 M, top:
6.0�10–3 M solution (Thesignals in the range 9.5 > � > 7.5
belong to the aromatic ABX spin sy-stem and the superimposed
olefinic A2 singlet at 7.80 ppm, the signalat � = 4.3 to the OCH2
groups, the signals at 2.0 > � > 1.5 to the otherCH2 groups
and the signal at � = 0.9 to the CH3 groups.)
Figure 11 Absorbance A of the triphenanthro[24]annulene 15b in
methylcyclohexane; The measurements with constant c�d = A/� give
dif-ferent absorption curves (with four isosbestic points) and thus
reveal the influence of the concentration on the aggregation.
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but their electronic properties are similar to those of
themonomer (Equation 3). The slight dependence of the flu-orescence
spectra and the fluorescence lifetimes on theconcentration can be
explained by different intermolecu-lar interactions in different
loose aggregates. (DifferentFranck–Condon factors).45
5 Thermotropic Liquid Crystals
Although the areno-condensed annulenes adopt non-pla-nar
conformations, the majority of the compounds has, asdiscussed in
Section 3, a de facto symmetry with the mo-lecular plane as
symmetry plane. The inversion of the cen-tral macrocyclic rings is
in all these cases fast in terms ofthe NMR time scale.
Extended � electron systems with the shape of planardiscs
represent an ideal precondition for discotic me-sogens. The �
stacking is supported by van der Waals in-teractions of long
flexible chains attached on theperiphery: Moreover, alkoxy chains
enhance significantlythe solubility of the compounds. One has to
assume mi-crosegregation between the discotic mesogens and therange
of the saturated sidechains.
The association of discotic mesogens is crucial for the
for-mation of columnar liquid crystals, which we have inves-tigated
by X-ray small-angle diffraction, differentialscanning calorimetry
(DSC), and polarization microsco-py. In fact, many liquid
crystalline phases have beenfound in the class of areno-condensed
annulenes.14,22–32,43,48–52 The triphenanthro-condensed
[18]annulenes 17and 18 (Figure 3, Figure 4) may serve for a more
detaileddiscussion. It is evident, that the four methoxy
sidechainsin 17a are too short for the generation of a
mesophase;however, four hexyloxy groups, present in 17c, are
suffi-cient to form a nematic discotic phase ND. Figure 13 (up-per
part) shows the typical texture obtained in a polarizingmicroscope.
In the lower part of Figure 12 one can see forcomparison the
picture of the crystals of 17d, which con-tains nine hexyloxy
chains and forms crystals with a sharpmelting point at 328
°C.48
On the basis of X-ray scattering measurements, a model ofthe ND
phase was conceived, in which pairs of molecules
Equation 3
Figure 13 Measurements in the polarization microscope;48 Upper
part: LC texture caused by the birefringence of the ND phase of 17c
mea-sured at 190 °C; lower part: crystals of 17d, which melt at 328
°C (Scale 340:1)
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17c, double discs, are arranged in a preferred
orienta-tion.27,48 The model is closely related to the
arrangementof the molecules in the crystal. The unit cell contains
twoenantiomeric molecules 17c, which are twisted by 60°against each
other. The distance of the main molecularplanes amounts to 3.6 Å
and is enhanced to about 4.3 Å inthe molecular pairs of the ND
phase.
27 Figure 14 illustratesthe arrangement in the crystal and the
model for the liquidcrystal.
Figure 14 X-ray investigations of the crystalline phase and of
theND phase of compound 17c (The hexyloxy sidechains are
omitted.)
Compound 18a, which has three hexyloxy sidechains,does not form
an LC phase. Obviously the number, thelength and the position of
the sidechains are decisive forthe generation of LC phases.
From the viewpoint of materials science, the most inter-esting
discotic mesophases have a columnar arrangement.Colho and Colhd
phases are the most common LC phases inthe series of
areno-condensed annulenes. Since the X-raysignal for the distance
between the discs in a hexagonalcolumnar phase (h) is often broad,
it is a question of defi-nition, whether such phases are called
ordered (index o) ordisordered (index d). A schematic diagram for a
hexago-nal columnar phase based on areno-condensed annulenediscs is
shown in Figure 15.
Compound 17e, a triphenanthro[18]annulene with ninedodecyloxy
sidechains forms such a hexagonal discoticarrangment. Figure 16
shows the extremely broad temper-ature range, in which the
mesophase exists.50 The phasetransitions to the isotropic phase
(clearing points) are in
some cases above 300 °C, so that the decomposition
startsearlier.23
6 Interaction with Light and its Applications in Materials
Science
Stilbenoid chromophores are distinguished by their inter-esting
photophysics and photochemistry.18 The majorityof areno-condensed
annulenes is photostable in the crys-talline state. An exception
was found for compound 6a,which reacts in a topochemical control to
the ‘shifted’dimer 50.14,25 It could be assumed, that the
photodimeriza-tion corresponds to the calculated molecular pair
with C2hsymmetry (Figure 9). According to the electron
diffrac-tion, the crystals contain a similar arrangement with
ashortest distance of 3.46 Å between two olefinic doublebonds.14
The irradiation in solution as well the prolongedirradiation in the
solid state lead to oligomers of 6a. Thederivative 6b behaves
opposite; it forms oligomers in thesolid state, whereas the
solution photolysis yields the beltcyclophane 5125,51 (Scheme 7).
Obviously the geometryof excimers and/or pericyclic minima has a
decisive influ-ence on the photodimerization.
Scheme 7 Photodimer 50, obtained in a
topochemically-controlled
Figure 15 Schematic diagram of a hexagonal columnar phase
gene-rated by areno-condensed annulenes (The saturated side-chains
areomitted.)
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reaction of 6a in the crystalline state, and photodimer 51
obtainedfrom 6b in solution (0.4�10–3 M in benzene)
A D2h arrangement of a molecular pair 6b is a prerequisitefor
the formation of 51; however, such a pair does not cor-respond to
an exothermic molecular pair in the groundstate. Therefore the
formation of suitable excimers has tobe claimed.
Belt cyclophanes, a new phane type, in which the
originalannulene ring has generated the belt structure by
severalregio- and stereoselective photocycloaddition reactions,were
obtained from many areno-condensed
annu-lenes.14,23,25,28,31,43,45,50,51 The arene moieties attached
atthe edges of the belt are not in a complete face-to-face
ori-entation, because of the geometry, which is forced by
the4-membered rings; nevertheless, �-� interactions can beobserved
in the UV, the fluorescence and the 1H NMRspectra. The
photodimerization of trans stilbene works insolution at
concentrations c �10–2 M. Surprisingly, muchlower concentrations
down to 10–6 M can be used in theseries of the areno-condensed
annulenes. According tofluorescence lifetime measurements with the
single-pho-ton timing technique, the average lifetime � of the
in-volved singlet states S1 ranges between 2 and 16 ns.
28,45
The Smoluchowski equation for the diffusion indicates,that such
a lifetime is much too short to find a dimeriza-tion partner M(S0)
in diluted solutions (150 °C). A quantitativedimerization leads
then to the same belt cyclophanes,which can be obtained in solution
at room tempera-ture.28,49,50–52
How can this enormous temperature effect of the photore-action
be rationalized? As discussed above, the discs aretwisted against
each other and also somewhat laterallyshifted. Nevertheless, the
�-� interaction of the olefiniccenters has to occur within the
average lifetime � of the S1state in order to induce a
photocyclodimerization. Thiscondition demands high mobility of the
discs in the col-umn, particularly a fast uniaxial rotation. We
performed asolid-state 2H NMR investigation with compound
18k.51
The quadrupole splitting constant Q is directly related tothe
mobility of the discs. In the crystalline state at –73 °Ca Q value
of 123 kHz is measured in the pake spectrum.As soon as the phase
transition to the hexagonal columnarphase is reached at 0 °C, the
pake spectrum changes to aline spectrum. With increasing
temperature the lines be-come sharper and the Q values smaller. At
187 °C, where
Figure 16 DSC diagram of compound 17e with the phase transitions
between the crystalline phase c, the hexagonal discotic phase LC
andthe isotropic phase i; The phase transition enthalpies are given
in J�g–1.50
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an efficient photodimerization takes place, the Q value
isreduced to 19 kHz. This value corresponds to a fast uniax-ial
rotation, whereby the columnar axis is orthogonal tothe magnetic
field vector. Such an arrangement representsthe energy-lowest state
in the macroscopic orientation ofthe Colh phase in the magnetic
field.
53,54 From Q = 19 kHzcan be deduced, that the C–D bonds in 18k
form an angleof 60–65° with the axis of the column – a result,
whichagrees very well with the inverting pyramidal structure ofthe
compound. Thus, the temperature effect of the photo-dimerization is
an effect of the columnar topochemistryand its dynamics.
The application of columnar phases as photoconductivematerials
depends of course on the photostability. Thephotodimerization
results in an insulator – but fortunatelythe process does not work
at temperatures below 150 °C.
On the other hand, it is interesting to have LC systems,which
loose this property by irradiation – either irrevers-ibly for
imaging techniques or reversibly for opticalswitching.
Photodimerization and photocrosslinking reac-tions are processes,
which can serve for the first purpose.Figure 17 shows an
irreversible degradation of 18j by ir-radiation. The original
mosaic texture of the Colh phasemeasured in the polarization
microscope disappears. Theirradiated area becomes black, an
isotropic (non birefrin-gent) phase is isothermally formed by a
photochemicaltransformation.51 Monochromatic light ( = 366
nm),which is absorbed by the S0 � S1 transition, induces
thecyclophane formation. Prolonged irradiation periods orUV light
of short wavelengths ( = 254 nm) lead tocrosslinking in the
original columns and between the col-umns. The statistical CC bond
formation in stilbenoidcompounds is a radical process, whereby the
radicals havelifetimes, which are several orders of magnitude
longerthan the average S1 lifetimes in the concerted
dimeriza-tions.18
Figure 17 Mosaic texture of the LC phase of 18j obtained at 230
°Cand its photochemical degradation by irradiation in the right
upperarea (Scale 340:1)50
The formation of dimers or crosslinked oligomers in
thephotochemistry of stilbenoid compounds is an
irreversibleprocess.18 Open-chained stilbenoid compounds oftenshow
a reversible cis/trans photoisomerization, which canbe used for the
optical switching between LC phases andisotropic phases.49 Is it
possible to perform such switchingprocesses with areno-condensed
annulenes? Numerous ir-radiation experiments revealed that the
majority of thecompounds 6, 13, 15–23 does not show a trans � cis
pho-toisomerization, at most traces of cis isomers can be de-tected
in the 1H NMR spectra. An exception was found inthe series of the
tribenzo[a,e,i][12]annulenes.39,55 The(E,E,E)-configuration in 25e
is transformed by light to the(E,E,Z)-form of 25g, which rearranges
spontaneously tothe polycyclic hydrocarbon 52. Further irradiation
leadsagain in a sequence of a photochemical and a thermal
step(retro-Diels–Alder) to the cleavage of the compound
tonaphthalene (54) and anthracene (55) (Scheme 9). Thewhole process
however can not be applied to the LC-forming [12]annulene
25c.24
Scheme 8 Photochemical processes in solution and in columnar LC
phases at different temperatures
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Scheme 9 Consecutive photochemical and thermal steps for
thecleavage of tribenzo[a,e,i][12]annulenes (25e)
7 Conclusion and Outlook
Different cyclocondensation reactions represent the majorroutes
for the preparation of annulenes, which are con-densed with
benzene, naphthalene, anthracene, phenan-threne, chrysene or pyrene
ring systems. In contrast to theparent compounds, these
areno-condensed annulenes arethermally highly stable. They have
non-planar structures.Nevertheless, they appear to be planar on
average, if theinversion of the central macrocyclic ring has a low
activa-tion barrier – a condition, which is fulfilled for the
major-ity of the systems. Therefore the compounds are‘superdiscs’,
which represent extended � systems with ahigh aggregation tendency.
This property makes themuseful as discotic mesogens in columnar
liquid crystals.Due to the stilbenoid character, the
areno-condensed an-nulenes exhibit interesting photophysical and
photochem-ical properties. Special emphasis is put
onphotoconductive LC systems with high charge carrier mo-bilities
and on imaging techniques, in which birefringentLC phases are
transformed by light and without heating toisotropic phases. Regio-
and stereoselective photodimer-ization reactions in solution and at
high temperatures inLC phases lead to a new type of phanes, which
is calledbelt cyclophanes.
Recently an extension of this research area to
heterocyclicsystems was successful.56 The incorporation of three
ni-trogen atoms in the areno-condensed 1,7,13-triaza[18]an-
nulenes opens the door to molecular recognition and toswitching
processes with metal complexes. Moreover,many compounds 10/10� with
arene ring systems listed inTable 1 are still unknown and should be
studied with re-gard to applications in materials science.
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Areno-Condensed Annulenes – Extended Discotic Mesogens