Rational synthesis of bis(hexyloxy)-tetra(hydroxy)-triphenylenes and their derivatives

Article abstract of DOI:10.1039/c4ra06503d

A straightforward, reliable, and scalable synthesis of rationally designed, mixed-substituent triphenylene derivatives from ortho-terphenyl precursors is described. Three isomers of bis(hexyloxy)-tetrahydroxy triphenylenes were synthesized and functionali

Full text of DOI:10.1039/c4ra06503d

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PAPER
Rational synthesis of bis(hexyloxy)-tetra(hydroxy)-
triphenylenes and their derivatives†
Cite this: RSC Adv., 2014, 4, 38281
*
Merry K. Smith, Natalia E. Powers-Riggs and Brian H. Northrop
A straightforward, reliable, and scalable synthesis of rationally designed, mixed-substituent triphenylene
derivatives from ortho-terphenyl precursors is described. Three isomers of bis(hexyloxy)-tetrahydroxy
triphenylenes were synthesized and functionalized with monomethyl di(ethylene glycol) chains to
provide new amphiphilic, mixed substituent triphenylenes. Oxidative triphenylene annulation, tetra-ol
formation, and subsequent functionalization were supported by significant changes in phase and melting
point, and confirmed by mass spectrometry, differential scanning calorimetry, and UV/Vis, 1H, and 13C
NMR spectroscopies. The thermal phase properties of amphiphilic mixed-substituent triphenylene
derivatives were found to vary between the different isomers, demonstrating how small changes in
substitution pattern can result in significant differences in mesogenic behavior. The controlled synthetic
route to de novo designed triphenylene derivatives is dependable, wide in scope, and can be applied to
the synthesis of a vast array of other mixed-substituent triphenylene derivatives, thus enabling the
preparation of libraries of novel triphenylene and triphenylene-containing materials.
Received 1st July 2014
Accepted 15th August 2014
DOI: 10.1039/c4ra06503d
www.rsc.org/advances
prepared from the demethylation of 2,3,6,7,10,11-hexame-
thoxytriphenylene (Scheme 1).1
Introduction
While synthetic routes to symmetric triphenylenes are well
established, there are few reliable and scalable synthetic routes
to asymmetric or mixed-substituent triphenylene derivatives
that are not complicated by the formation of closely related
byproducts. This is despite the fact that such compounds are
increasingly desirable as varying the substituents at precise
locations on the triphenylene core can affect their mesogenic
behavior and lead to new and unique materials properties.14
Synthetic routes reported to produce mixed-substituent 2, 3, 6,
7, 10, 11 substituted triphenylenes typically rely on oxidative
trimerization, dimerization,15 or annulation16 of substituted aryl
systems (Table 1). Originally, mixed substituent or asymmetric
triphenylenes were prepared by oxidative trimerization of
substituted catechols using chloranil and acid,10b,15 but the low
yields of desired products prompted the development of new
methods. Oxidative Scholl annulations of differently
substituted catechols (Table 1, entry A) or catechols and
biphenyls (Table 1, entry B) using agents such as FeCl3 and
MoCl5 have provided routes to mixed-substituent tripheny-
lenes.17,18 These pathways, however, require a large stoichio-
metric excess of oxidant and oen result in undesirable side
products that can be difficult or impossible to separate. Tricy-
clic ortho-terphenyl precursors (Table 1, entry C) eliminate
triphenylene-based side products and require fewer equivalents
of oxidant,4a,18 making them attractive target compounds for
asymmetric triphenylene synthesis. Ortho-terphenyl precursors
of mixed-substituent triphenylene derivatives, however, typi-
cally require a greater number of synthetic steps to prepare.
Polycyclic aromatic triphenylene derivatives are well-known for
their mesogenic behavior.1 Examples of over 500 discotic liquid
crystals incorporating a triphenylene core have been reported
in the literature.1c,2 The delocalized 18 p-electron system and
high thermal and chemical stabilities3 of triphenylenes make
them well-suited as components in optoelectronic,4 photo-
conductive,5 and electroluminescent6 materials, with potential
applications as liquid crystalline semiconductors.7 Typically,
mesogenic triphenylenes are functionalized with alkyl groups
at their periphery to facilitate columnar phase assembly
through a combination of aliphatic van der Waals interactions
and aromatic p–p stacking of triphenylene cores.1,3a,8 In
particular, triphenylene assembly is enhanced by the six-fold
substitution of medium length alkyloxy chains at the 2, 3, 6,
7, 10, and 11 positions.9 Synthetic routes toward six-fold
substituted triphenylenes are well-established,3b,10 and
symmetric triphenylenes have been prepared with a variety of
substituents, including alkyl chains,1c esters,2a,11 thioesters,7a,12
and benzyl ether2b,13 moieties. Most commonly, symmetric
hexa-substituted triphenylenes have been prepared by the
alkylation of 2,3,6,7,10,11-hexahydroxytriphenylene, which is
Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459, USA.
E-mail: bnorthrop@wesleyan.edu
† Electronic supplementary information (ESI) available: Full experimental
procedures and spectroscopic characterization (1H and 13C NMR, mass, and
UV/Vis spectra) of newly reported compounds and differential scanning
calorimetry (DSC) traces of compounds 4–6 and 25 and 26. See DOI:
10.1039/c4ra06503d
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Scheme 1 Common retrosynthetic route to symmetrically substituted triphenylene mesogens: (i) alkylation, (ii) O-demethylation, (iii) oxidative
trimerization.
An alternative route to mixed triphenylene derivatives can Triphenylene tetra-ol 1, for example, can serve as a precursor to
involve the selective alkylation of triphenylene poly-ols in a a discrete analogue of the widely studied COF-5 framework.25 In
manner analogous to the synthesis of symmetric triphenylene developing a convenient synthetic route to 1 it became evident
derivatives from triphenylene hexaol as shown in Scheme 1. that similar synthetic routes could be used to prepare a variety
To that end, selectively substituted triphenylene mono and of differently substituted triphenylene tetra-ols. Herein we
poly-ols are valuable precursors to mixed-substituent triphe- report the reliable synthesis of three isomers of tetra(hydroxy)
nylenes. Care must be taken to design appropriate synthetic triphenylene derivatives (1–3) as well as their functionalization
routes to triphenylene poly-ols as catechol protecting groups with monomethyl di(ethylene glycol) chains to give three new
must be both stable to oxidative conditions used in the amphiphilic, mixed substituent triphenylenes (4–6), which
construction of the central triphenylene core, and easily and provide an opportunity to investigate the inuence of regioiso-
orthogonally removed to reveal the desired triphenylene poly- merism on the thermal properties of mixed substituent tri-
ol. As such, examples of routes to non-saturated triphenylene phenylene derivatives.
poly-ols are limited in scope and methodology. Mono-, di-, and
tri-ols have been reported, but many have been synthesized
through non-selective means.19 Ringsdorf and co-workers, for
example, have prepared penta-substituted triphenylene mono-
ols by the partial cleavage of alkoxy groups from hexa-
substituted triphenylenes using 9-Br-BBN.20 The alkyl
Table 1 Summary of general synthetic routes to mixed-substituent
triphenylene derivatives including oxidative trimerization of different
catechol derivatives (entry A), oxidative dimerization of a catechol
derivative with substituted biphenyls (entry B), and oxidative annu-
cleavage, however, was non-selective. Bushby and Lu reported
a rational route to mono- and di-hydroxy triphenylenes by
using isopropyl substituents to mask hydroxyl groups in
the FeCl3-oxidized dimerization of selectively substituted
catechols and biphenyls.21 Another selective route reported by
Kumar and Manickam relied upon bromocatecholborane to
cleave one, two, or three pentyl substituents from hex-
akis(pentyloxy) triphenylene resulting in the desired mono-,
di-, and tri-ol products but yields were variable (17–70%) and
the authors noted difficult purications.22 In general, it is
evident that the synthetic methods used to produce
unsaturated triphenylene poly-ols tend to be harsh and non-
selective, making the controlled synthesis of triphenylene
lation of ortho-terphenyl derivatives bearing different substituents
(entry C)a
tetra-ols particularly challenging.
A reliable, controlled,
scalable route to prepare rationally designed triphenylene
poly-ol compounds is needed to fully explore the full vari-
ability and utility of mixed-substituent mesogenic tripheny-
lene materials.
Our interest in the synthesis of triphenylene tetra-ols arose
from our desire to prepare triphenylene derivative 1 (Fig. 1) for
use in dynamically assembled boronate ester materials. The
dynamic self-assembly of catechol derivatives with boronic
acids to form boronate esters has recently received increasing
attention due to the development of structurally precise, highly
porous covalent organic frameworks (COFs).23 Along similar
lines we have sought to design and self-assemble a variety of
discrete, soluble analogues of COFs.24 Such analogues would
allow us to investigate the mechanism of COF assembly in
solution and the properties of boronate ester mesogens.
a
Oxidative trimerization and dimerization reactions are commonly
carried out using oxidants such as chloranil or Lewis acids such as
FeCl3, MoCl5, VoCl3, etc. The annulation of ortho-terphenyl
derivatives has been carried out using these transition metal
oxidants or anion-catalyzed TBAF ring closure.
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Fig. 1 Chemical structures of target bis(hexyloxy)-tetra(hydroxy) triphenylene derivatives 1–3 and their corresponding amphiphilic derivatives
4–6, which have been functionalized with monomethyl di(ethylene glycol) substituents.
poly-ols, namely (i) it avoids the production of alternative tri-
phenylene byproducts, (ii) the number and location of hydroxyl
functionalities in the product are controlled precisely, and (iii)
Results and discussion
Synthesis
the route can be easily and reliably scaled to gram quantities.
Given the reliability of the synthetic route to triphenylene
tetra-ol 1 it became apparent that the synthesis could be readily
adapted to the preparation of additional triphenylene tetra-ol
isomers 2 and 3 (Fig. 1). The key synthetic intermediates
along the routes to tetra-ols 2 and 3 are uniquely substituted
ortho-terphenyl derivatives, which can be similarly prepared
from Suzuki–Miyaura couplings of aryl pinacolboranes and aryl
dihalides that are different variations of compounds 7 and 8
(Scheme 2). The choice of substituents in the pinacolborane
and dihalide precursors precisely determines the substituent
pattern in their nal triphenylene tetra-ols. Scheme 4 summa-
rizes the synthetic routes to the three synthetic precursor
compounds 14, 18, and 19. Aryl pinacolboranes 14 and 18 are
isomers of each other that differ only in the placement of their
hexyloxy and tert-butyldimethylsilyloxy (OTBDMS) groups: in
compound 14 the hexyloxy and OTBDMS substituents are meta
and para to the pinacolborane, respectively, while in compound
Initial attempts at the synthesis of 1 involved the oxidative
dimerization of symmetric and asymmetric substituted
biphenyl compounds with functionalized catechols, employing
a variety of protecting groups, transition metal oxidants, and
reaction conditions. In all cases the desired product was either
not observed or obtained in low yield and purication was
complicated by the prevalence of undesired byproducts. Atten-
tion was therefore turned to the synthesis of ortho-terphenyl
compounds that would likely undergo more selective and
controlled annulation to desired triphenylene derivatives.
Toward this end ortho-terphenyl derivative 9 was prepared by
Suzuki–Miyaura26 coupling of 4,5-dibromo-1,2-bishexyloxy
benzene 7 and bis(tert-butyldimethylsilyl) (TBDMS) protected
aryl pinacolborane 8 (Scheme 2). Precursors 7 and 8 were
prepared from 4,5-dibromoveratrole and 4-bromoveratrole in
two and three steps, respectively.
Kumar and coworkers have previously reported that treat-
ment of a related methoxy-substituted ortho-terphenyl deriva-
tive with tetra-butyl ammonium uoride (TBAF) results in the
sequential deprotection of the TBDMS moieties and subsequent
annulation to give tetra(hydroxy) triphenylene derivatives.27
Attempts to adapt this TBAF-promoted deprotection/
annulation, while promising at preparative scales and when
hexyloxy chains were replaced with methoxy substituents, were
unsuccessful when run at larger scales or when applied to
compound 9. Alternative conditions for oxidative annulation
were then explored. Rathore and others have shown that
oxidative cyclodehydrogenation of various Scholl precursors can
be carried out efficiently and in high yields using a mixture of
dichlorodicyano-p-benzoquinone (DDQ) and an acid.28 Indeed,
reacting ortho-terphenyl compound 9 and stoichiometric DDQ
in a 10 : 1 mixture of dichloromethane/TFA gave annulated
triphenylene derivative 10 in good yield (Scheme 3). Subsequent
deprotection of the four TBDMS groups at positions 6, 7, 10, and
11 with KF and HBr resulted in the desired 2,3-bis(hexyloxy)-
6,7,10,11-tetrahydroxy triphenylene 1. The overall synthetic
route to triphenylene tetra-ol 1 outlined in Schemes 2 and 3 has Scheme 2 Synthesis of ortho-terphenyl 9 from the palladium-cata-
lyzed cross-coupling of bishexyloxy-substituted benzene dibromide 7
and bis(tert-butyldimethylsilyl) protected aryl pinacolborane 8.
several notable advantages over previous routes to triphenylene
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18 the hexyloxy substituent is para to the pinacolborane and the with KF in HBr to provide 2,7-bis(hexyloxy)-3,6,10,11-
OTBDMS is meta. The synthesis of compound 14 (Scheme 4a) tetrahydroxy triphenylene 2. Likewise, palladium-catalyzed
requires statistical alkylation of catechol (11) followed by coupling of 18 and 19 gives ortho-terphenyl derivative 22. DDQ
bromination (12),29 protection with TBDMS (13), and ultimately oxidation of 22 in TFA/dichloromethane gives annulated tet-
borylation using conditions developed by Buchwald.30 ra(tert-butyldimethylsilyloxy) product 23. The TBDMS protect-
Compound 18 was synthesized along a related but slightly ing groups of 23 are deprotected with KF and HBr to give 3,6-
different route (Scheme 4b), starting with the alkylation31 of 5- bis(hexyloxy)-2,7,10,11-tetrahydroxy triphenylene 3. While the
bromosalicylaldehyde to give 15, Baeyer–Villiger rearrange- syntheses of isomeric triphenylene tetra-ols 1–3 each require 8
ment32 to give the alcohol 16, protection with TBDMS to provide linear synthetic steps the reactions proceed, with one excep-
17, and nally borylation.30 The third key precursor shown in tion,33 in good to excellent yields (59–99%) and are completely
Scheme 4c is di(t-butyldimethylsilyloxy) dibromide 19, which is selective, providing highly controlled synthetic routes to
easily prepared in two steps from dibromoveratrole.12
precisely functionalized triphenylene tetra-ols.
With compounds 14, 18, and 19 at hand the preparation of
Triphenylene tetra-ols 1–3 are able to serve as versatile
triphenylene tetra-ols 2 and 3 (Scheme 5) follows the same platforms for the preparation of mixed-substituent triphenylene
general method as the preparation of isomeric triphenylene derivatives. As representative examples of the ease with which
tetra-ol 1. Suzuki–Miyaura coupling of 14 and 19 gives ortho- hydroxyl functionalities of compounds 1–3 can be functional-
terphenyl 20. Oxidative annulation of 20 with DDQ in the ized we have prepared mixed-substituent amphiphilic triphe-
presence of TFA results in tetra(tert-butyldimethylsilyloxy) tri- nylene derivatives 4–6 (Scheme 6). In each case, all four hydroxyl
phenylene derivative 21, which is subsequently deprotected groups were successfully substituted with di(ethylene glycol)
Scheme 3 Successful annulation of ortho-terphenyl derivative 9 to triphenylene derivative 10 followed by subsequent TBDMS deprotection to
give triphenylene tetra-ol 1.
Scheme 4 Synthetic routes to two isomeric aryl pinacolboranes 14 (a) and 18 (b), each of which are catechol derivatives possessing one hexyloxy
substituent and one tert-butyldimethylsilyloxy group. Shown in (c) is the synthesis of bis(tert-butyldimethylsilyloxy) dibromide 19.
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monomethyl ether tosylate34 in moderate yields. The versatility
of the synthetic route presented herein should be reiterated, as
any substituent with suitably electrophilic character could be
used at this stage to provide libraries of mixed-substituent tri-
phenylene derivatives. With rational routes to amphiphilic tri-
phenylenes 4–6 the relative locations of hexyl and monomethyl
di(ethylene glycol) substituents along the triphenylene core are
completely controlled. Di(ethylene glycol) substituents were
chosen because they are known to crystallize less readily than
comparable length alkyl substituents. By substituting triphe-
nylene cores with two substituents that favor crystallization (i.e.
hexyloxy) and four that disfavor crystallization (i.e. monomethyl
di(ethylene glycol)) we are able to investigate how the thermal
and physical properties of mixed-substituent triphenylenes vary
between different regioisomers.
Experimental
Materials. Chemicals were obtained from commercial sour-
ces and used as received. Reagent-grade solvents were used as
obtained from commercial sources. Anhydrous solvents were
dried using an Innovative Technologies SPS-400-5 solvent
purication system.
Scheme
6
Functionalization of triphenylene tetra-ols 1–3 with
hydrophilic monomethyl di(ethylene glycol) substituents to give
mixed-substituent triphenylene derivatives 4–6.
Instrumentation. 1H and 13C NMR spectra were recorded
with a Varian Mercury (300 MHz and 75 MHz, respectively)
spectrometer using residual solvent as the internal standard. All
chemical shis are quoted using the d scale and all coupling
constants are expressed in Hertz (Hz). UV/Vis spectroscopy was
recorded on a Varian Cary 100 Bio UV-Visible spectrophotom-
eter. Differential scanning calorimetry (DSC) was performed on
a TA Instruments DSC Q20. The DSC is equipped with an RCS90
cooling system. DSC traces were acquired at rates of 10 ꢀC
minꢁ1 (heating) and 5 ꢀC minꢁ1 (cooling) in the temperature
range of (ꢁ50)–100 ꢀC. ESI/APCI and APCI-MS analysis was
carried out at the University of California, Riverside, Mass
Spectrometry Facility.
General ortho-terphenyl preparation. To a heavy-walled glass
reaction vessel was added aryl dihalide (1 eq.), aryl pinacolborane
(3 eq.), and potassium phosphate (4 eq.). The vessel was ushed
with nitrogen, and SPhos Buchwald ligand (4 mol%) and palla-
dium acetate (2 mol%) were added in that order. The vessel was
further evacuated and backlled with nitrogen (3ꢂ), and
degassed 10 : 1 toluene–water mixture was added. The vessel was
quickly sealed with a Teon screw cap and was heated to 100 ꢀC
overnight. The dark reaction mixture was allowed to cool, diluted
with ether, and passed through a pad of Celite. The ltrate was
concentrated under reduced pressure and puried by column
chromatography to afford pure product.
Compound 9. Reaction scale: compound 7 (1.5 g, 3.44
mmol). The pure product eluted from the column with 20%
dichloromethane in hexanes, and was isolated as a pale yellow
oil (3.0 g, 92%). APCI-MS (m/z) [MH]+ calculated for
C
54H95O6Si4, 951.6200: found 951.6180. 1H NMR (300 MHz,
CDCl3): d 6.85 (s, 2H), 6.61–6.67 (m, 4H), 6.52–6.57 (dd, J ¼ 8.8,
2.6 Hz, 2H), 4.04 (t, J ¼ 6.6 Hz, 4H), 1.79–1.90 (m, 4H), 1.42–1.53
(m, 4H), 1.31–1.39 (m, 8H), 0.98 (m, 18H), 0.94 (s, 18H), 0.90 (t, J
¼ 5.3 Hz, 6H), 0.19 (s, 12H), 0.08 (s, 12H) ppm. 13C NMR (CDCl3,
75 MHz): 147.9, 146.1, 145.3, 135.1, 132.7, 122.8, 122.5, 120.2,
116.1, 69.3, 31.6, 29.3, 26.1, 25.9, 25.7, 22.6, 18.4, 14.1, ꢁ4.1,
ꢁ4.2 ppm.
Compound 20. Reaction scale: compound 19 (144 mg, 0.291
mmol). The pure product eluted from the column with 10%
Scheme 5 Synthesis of isomeric triphenylene tetra-ols 2 and 3 from precursors 14 and 19 or 18 and 19, respectively, following the synthetic
route involving Suzuki–Miyaura coupling, oxidative annulation, and TBDMS deprotection.
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dichloromethane in hexanes, and was isolated as a white solid 148.7, 146.6, 124.0, 123.2, 114.0, 106.8, 69.3, 31.7, 29.2, 26.1,
(199 mg, 72%). Mp ¼ 146.0–147.8 ꢀC. ESl/APCI (m/z) [MH]+ 26.1, 25.9, 25.8, 22.7, 18.7, 14.1, ꢁ4.0, ꢁ4.1 ppm.
calculated for C54H95O6Si4, 951.6200: found 951.6200. 1H NMR
Compound 21. Reaction scale, ortho-terphenyl 20 (199 mg,
(CDCl3, 300 MHz): d 7.67 (s, 2H), 7.11 (d, J ¼ 8.2 Hz, 2H), 6.99 (d, 0.209 mmol). The organic extracts were a ruby red, and the
J ¼ 7.9 Hz, 2H), 6.89 (s, 2H), 4.00 (t, J ¼ 6.5 Hz, 4H), 2.11–2.02 product was isolated as a pale yellow solid (56 mg, 29%). Mp ¼
(m, 4H), 1.83–1.74 (m, 4H), 1.75–1.66 (m, 8H), 1.43 (s, 18H), 1.40 132.7–134.2 ꢀC. ESI/APCI (m/z) [MH]+ calculated for
(s, 18H), 1.31 (t, J ¼ 6.5 Hz, 6H), 0.66 (s, 12H), 0.54 (s, 12H) ppm.
C
54H93O6Si4, 949.6044: found 949.6031. 1H NMR (CDCl3, 300
13C NMR (CDCl3, 75 MHz): 149.6, 145.6, 143.2, 135.1, 133.6, MHz): d 7.86 (s, 2H), 7.80 (s, 2H), 7.71 (s, 2H), 4.16 (t, J ¼ 6.5 Hz,
122.8, 121.5, 120.2, 115.1, 68.2, 31.6, 29.3, 26.0, 26.0, 25.7, 25.7, 4H), 1.98–1.89 (m, 4H), 1.63–1.53 (m, 4H), 1.44–1.34 (m, 8H),
22.6, 18.4, 14.1, ꢁ4.0, ꢁ4.7 ppm.
1.10 (s, 18H), 1.08 (s, 18H), 0.95 (t, J ¼ 5.6 Hz, 6H), 0.32 (s, 12H),
Compound 22. Reaction scale: compound 19 (110 mg, 0.223 0.27 (s, 12H) ppm. 13C NMR (CDCl3, 75 MHz): 150.2, 146.5,
mmol). The pure product eluted from the column with 10% 144.9, 124.2, 123.8, 123.2, 114.1, 105.8, 68.6, 31.7, 29.4, 26.1,
dichloromethane in hexanes, and was isolated as a colorless 25.9, 25.8, 22.7, 18.7, 14.1, ꢁ4.0, ꢁ4.1 ppm.
semi-solid (124 mg, 59%). ESl/APCI (m/z) [MH]+ calculated for
Compound 23. Reaction scale, ortho-terphenyl 22 (200 mg,
C
54H95O6Si4, 951.6200: found 951.6196. 1H NMR (CDCl3, 300 0.210 mmol). The organic extracts were deep purple, and the
MHz): d 6.81 (s, 2H), 6.67–6.57 (m, 6H), 3.89 (t, J ¼ 6.5 Hz, 4H), product was isolated as a violet grey solid (144 mg, 72%). Mp ¼
1.83–1.74 (m, 4H), 1.51–1.43 (m, 4H), 1.36–1.28 (m, 8H), 1.02 (s, 131.4–133.6 ꢀC. ESI/APCI (m/z) [MH]+ calculated for
18H), 0.96 (s, 18H), 0.92 (t, J ¼ 7.0 Hz, 6H), 0.25 (s, 12H), 0.07 (s,
C
54H93O6Si4, 949.6044: found 949.6034. 1H NMR (CDCl3, 300
12H) ppm. 13C NMR (CDCl3, 75 MHz): 149.1, 145.5, 144.1, 134.1, MHz): d 7.80 (s, 2H), 7.79 (s, 2H), 7.76 (s, 2H), 4.19 (t, J ¼ 6.5 Hz,
133.2, 123.0, 122.4, 112.3, 68.3, 31.7, 29.5, 26.0, 25.7, 22.6, 18.4, 4H), 1.87–1.99 (m, 4H), 1.65–1.56 (m, 4H), 1.44–1.35 (m, 8H),
14.1, ꢁ4.0, ꢁ4.7 ppm.
1.09 (s, 18H), 1.07 (s, 18H), 0.95 (t, J ¼ 6.3 Hz, 6H), 0.32 (s, 12H),
General triphenylene preparation. Two methods were used 0.26 (12H) ppm. 13C NMR (CDCl3, 75 MHz): 150.3, 146.6, 145.0,
to prepare tetra(hydroxy) triphenylene derivatives from their 124.1, 123.7, 123.3, 114.2, 113.9, 106.2, 68.8, 31.7, 29.5, 26.1,
appropriate ortho-terphenyl precursors. In a two-step procedure 25.8, 22.7, 18.5, 14.1, ꢁ4.1, ꢁ4.6 ppm.
(Method A) tetra-TBDMS protected triphenylene intermediates
General deprotection procedure. To a 0.1 M solution of
10, 21, and 23 were isolated and puried prior to TBDMS TBDMS-protected triphenylene derivatives 10, 21, and 23 in 1 : 2
deprotection to allow full characterization of the tetra-TBDMS dimethylformamide–tetrahydrofuran was added potassium
protected triphenylene derivatives. Alternatively, a one-step uoride (8 equivalents), and aqueous hydrogen bromide (0.12
procedure (Method B) can be used wherein the intermediate equivalents). The solution was stirred overnight with periodic
is not isolated but rather carried directly through to depro- monitoring by TLC. Aqueous potassium carbonate (1 M) was
tection following annulation. Method B was observed to both added, and the mixture stirred for an hour. The solution was
maximize the yield of the desired tetra-ol product and simplify slowly acidied with aqueous hydrochloric acid (1 M), and
the synthesis of 1–3.
extracted with diethyl ether (3ꢂ). The combined organic layers
Method A. General annulation procedure: to a 0.01 M solu- were washed with brine, dried over MgSO4, and concentrated in
tion of ortho-terphenyl in dry dichloromethane was added neat vacuo. The crude material was puried by column chromatog-
triuoroacetic acid (10% with respect to volume of solvent), and raphy, affording pure triphenylene tetra-ol derivatives 1–3.
the solution stirred for 30 minutes at room temperature.
Compound 1. Reaction scale: TBDMS-protected triphenylene
Oxidant 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1.1 equiv- derivative 10 (245 mg, 0.258 mmol). The crude product was
alents) was added at 0 ꢀC, and the solution was allowed to return puried by column chromatography, eluting with ethyl acetate,
slowly to room temperature over 3 hours; accompanied by a to afford the pure product as a lavender solid (101 mg, 79%). Mp
color change from pale lime green to emerald. Water was added ¼ >200 ꢀC. ESI/APCI (m/z) [MH]+ calculated for C30H37O6,
slowly, and the crude product was extracted with dichloro- 493.2585: found 493.2575. 1H NMR (300 MHz, acetone-d): d 8.35
methane (3ꢂ). The combined red or purple organic layers were (s, 2H) 8.17 (s, 2H) 7.94 (s, 2H), 7.87 (s, 2H), 7.85 (s, 2H), 4.25 (t, J
washed with saturated sodium bicarbonate (3ꢂ) and brine, ¼ 6.5 Hz, 4H), 1.84–1.96 (m, 4H), 1.55–1.67 (m, 4H), 1.34–1.48
dried over MgSO4, and concentrated in vacuo. The resulting (m, 8H), 0.91–0.98 (m, 6H) ppm. 13C NMR (acetone-d6, 75 MHz):
material was puried through a short pad of silica, eluting with 149.7, 146.1, 124.2, 123.9, 109.1, 108.8, 108.7, 69.7, 32.5, 30.5,
2.5% ethyl acetate in hexanes to afford the pure triphenylene 26.7, 23.4, 14.4 ppm.
derivatives.
Compound 2. Reaction scale: TBDMS-protected triphenylene
Compound 10. Reaction scale, ortho-terphenyl 9 (700 mg, derivative 21 (70 mg, 0.074 mmol). The crude product, while
0.736 mmol). The organic extracts were red, and the product isolated as a relatively pure solid, was further puried by
was isolated as a pale pink semi-solid (323 mg, 46%). APCI-MS column chromatography eluting with 100% diethyl ether to
(m/z) [MH]+ calculated for C54H93O6Si4, 949.6044: found obtain a pale orange solid (33 mg, 92%). Mp ¼ 157 ꢀC. ESI/APCI
949.6027. 1H NMR (300 MHz, CDCl3): d 7.83 (s, 2H), 7.79 (s, 2H), (m/z) [MH]+ calculated for C30H37O6, 493.2585: found 493.2583.
7.74 (s, 2H), 4.22 (t, J ¼ 6.4 Hz, 4H), 1.99–1.88 (m, 4H), 1.33–1.26 1H NMR (300 MHz, acetone-d): d 8.26 (s, 2H), 7.99 (s, 2H), 7.89
(m, 12H), 1.08 (s, 18H), 1.07 (s, 18H), 0.93 (t, J ¼ 6.74 Hz, 6H), (s, 2H), 7.83 (s, 2H), 7.79 (s, 2H), 4.28 (t, J ¼ 6.5 Hz, 4H), 1.84–
0.31 (s, 12H) 0.31 (s, 12H) ppm. 13C NMR (CDCl3, 75 MHz): 1.93 (m, 4H), 1.50–1.62 (m, 4H), 1.32–1.44 (m, 8H), 0.92 (t, J ¼
6.2 Hz, 6H) ppm. 13C NMR (acetone-d6, 75 MHz): 147.9, 147.2,
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146.0, 124.4, 124.2, 123.7, 109.0, 108.7, 105.9, 69.6, 32.5, 30.4, equivalents), catalytic lithium bromide, and 18-crown-6 under
26.6, 23.4, 14.4 ppm. inert conditions. The system was purged with nitrogen again, and
Compound 3. Reaction scale: TBDMS-protected triphenylene the reaction stirred at 80 ꢀC overnight. The solution was allowed to
derivative 23 (140 mg, 0.147 mmol). The crude product, while cool, water was added, and the crude product was extracted with
isolated as a relatively pure solid, was further puried by diethyl ether (3ꢂ). The combined ethereal extracts were washed
column chromatography eluting with 100% diethyl ether to give with aqueous hydrochloric acid (1 M) and brine, and the
a violet solid (38 mg, 52%). Mp ¼ 130 ꢀC. ESI/APCI (m/z) [MH]+ combined aqueous layers back-extracted again with ether. The
calculated for C30H37O6, 493.2585: found 493.2584. 1H NMR combined organic layers were dried over MgSO4, concentrated
(300 MHz, acetone-d6): d 8.18 (s, 2H), 7.95 (s, 2H), 7.88 (s, 2H), under reduced pressure, and puried by column chromatography,
7.87 (s, 2H), 7.80 (s, 2H), 4.31 (t, J ¼ 6.4 Hz, 4H), 1.85–1.96 (m, eluting with 10% acetone in dichloromethane.
4H), 1.51–1.64 (m, 4H), 1.32–1.45 (m, 8H), 0.93 (t, J ¼ 6.5 Hz, 6H)
Compound 4. Reaction scale: compound 1 (65 mg, 0.132
ppm. 13C NMR (acetone-d, 75 MHz): 147.5, 147.0, 146.0, 124.4, mmol). Pure product was isolated as a brown oil that gradually
123.8, 123.4, 108.7, 108.6, 106.1, 69.5, 32.4, 26.4, 23.1, 14.2 ppm. solidied (70 mg, 59%). Mp ¼ 37 ꢀC. ESI/APCI (m/z) [MNa]+
Method B. To a 0.01 M solution of ortho-terphenyl derivatives calculated for C50H76O14Na, 923.5127: found 923.5148. 1H NMR
9, 20, and 22 in dry dichloromethane was added neat tri- (300 MHz, CDCl3): d 7.91 (s, 2H), 7.89 (s, 2H), 7.82 (s, 2H), 4.40–
uoroacetic acid (10% with respect to volume of solvent), and 4.44 (m, 8H), 4.23 (t, J ¼ 6.5 Hz, 4H), 3.98–4.02 (m, 8H), 3.78–
the solution stirred for 30 minutes at room temperature. 3.83 (m, 8H), 3.60–3.63 (m, 8H), 3.42 (s, 12H), 1.90–2.00 (m, 4H),
Oxidant DDQ (1.1 equivalents) was added at 0 ꢀC, and the 1.55–1.61 (m, 4H), 1.37–1.45 (m, 8H), 0.94 (t, J ¼ 6.6 Hz, 6H)
solution allowed to return slowly to room temperature over 3 ppm. 13C NMR (CDCl3, 75 MHz): 149.1, 148.5, 124.0, 123.8,
hours. Water was added slowly, and the intermediate extracted 123.4, 108.2, 107.8, 106.9, 71.9, 70.7, 69.9, 69.8, 69.5, 69.1, 69.0,
with dichloromethane (3ꢂ). The combined organic layers were 59.0, 31.6, 29.4, 25.8, 22.6, 14.0 ppm.
washed with saturated sodium bicarbonate (3ꢂ) and brine,
Compound 5. Reaction scale: compound 2 (121 mg, 0.246
dried over MgSO4, and concentrated in vacuo. To a 0.1 M solu- mmol). Pure product was isolated as a brown oil (132 mg, 60%).
tion of the resulting residue in 1 : 1 dimethylformamide–tetra- ESI/APCI (m/z) [MNa]+ calculated for C50H76O14Na, 923.5127:
hydrofuran was added potassium uoride (8 equivalents), and found 923.5137. 1H NMR (300 MHz, CDCl3): d 7.95–7.88 (m,
aqueous hydrogen bromide (0.12 equivalents). The solution was 4H), 7.83 (s, 2H), 4.42 (t, J ¼ 4.9 Hz, 8H), 4.23 (t, J ¼ 6.5 Hz, 4H),
stirred overnight. Aqueous potassium carbonate (1 M) was 3.98–4.04 (m, 8H), 3.79–3.85 (m, 8H), 3.59–3.65 (m, 8H), 3.41 (s,
added, and the mixture stirred for one hour. The solution was 12H), 1.89–1.98 (m, 4H), 1.53–1.62 (m, 4H), 1.37–1.45 (m, 8H),
slowly acidied with aqueous hydrochloric acid (1 M), and 0.95 (t, J ¼ 6.5 Hz, 6H) ppm. 13C NMR (CDCl3, 75 MHz): 149.5,
extracted with diethyl ether (3ꢂ). The combined organic layers 149.0, 145.9, 124.3, 123.9, 108.3, 107.6, 72.02, 70.88, 70.80,
were washed with brine, dried over MgSO4, and concentrated in 69.88, 69.56, 69.30, 69.15, 59.08, 31.67, 29.39, 25.82, 22.64, 14.04
vacuo. The crude material was puried by column chromatog- ppm.
raphy, or by recrystallization in ether–hexanes, affording pure
triphenylene tetra-ol derivatives 1–3.
Compound 6. Reaction scale: compound 3 (93 mg, 0.189
mmol). Pure product was isolated as a brown oil that gradually
Compound 1. Reaction scale: ortho-terphenyl 9 (500 mg, solidied (97 mg, 57%). Mp ¼ 33 ꢀC. ESI/APCI (m/z) [MNa]+
0.525 mmol). The pure product was isolated by recrystallization calculated for C50H76O14Na, 923.5127: found 923.5153. 1H NMR
from diethyl ether and hexanes (171 mg, 66%). This reaction (300 MHz, CDCl3): d 7.90 (s, 2H) 7.88 (s, 2H) 7.82 (s, 2H), 4.42 (t,
has also been run at larger scales up to 2.8 grams of ortho-ter- J ¼ 4.2 Hz, 8H), 4.22 (t, J ¼ 6.5 Hz, 4H), 3.88–4.03 (m, 8H), 3.78–
phenyl 9, giving tetraol 1 in similar yields. Characterization 3.84 (m, 8H), 3.59–3.63 (m, 8H), 3.41 (s, 12H), 1.89–2.00 (m, 4H),
matched the data provided for compound 1 as synthesized 1.53–1.64 (m, 4H), 1.34–1.46 (m, 8H), 0.94 (t, J ¼ 6.6 Hz, 6H)
using the two-step procedure (Method A).
ppm. 13C NMR (CDCl3, 75 MHz): 149.0, 148.5, 124.0, 123.7,
Compound 2. Reaction scale: ortho-terphenyl 20 (848 mg, 123.4, 108.3, 107.3, 106.7, 72.0, 71.9, 70.8, 69.9, 69.8, 69.4, 69.3,
0.891 mmol). The pure product was isolated by column chro- 69.1, 59.0, 31.6, 29.4, 25.8, 22.6, 14.0 ppm.
matography eluting with 10% acetone in dichloromethane (332
mg, 76%). Characterization matched the data provided for
compound 2 as synthesized using the two-step procedure
Spectroscopic characterization
(Method A).
The key step in our synthesis of triphenylene tetra-ols 1–3 is the
Compound 3. Reaction scale: ortho-terphenyl 22 (718 mg, oxidative annulation of ortho-terphenyl derivatives to their
0.754 mmol). The pure product was isolated by column chro- corresponding triphenylene derivatives by DDQ in the presence
matography eluting with 10% acetone in dichloromethane (251 of acid (TFA). This transformation is easily observed by 1H NMR
mg, 67%). Characterization matched the data provided for spectroscopy as diagnostic proton signals in the aromatic
compound 3 as synthesized using the two-step procedure region of the spectra of ortho-terphenyl compounds 9, 20, and
(Method A).
22 shi substantially downeld upon annulation to tripheny-
General procedure for the preparation of amphiphilic tri- lene derivatives 10, 21, and 23, respectively. Fig. 2 provides a
phenylenes. To a 0.1 M solution of triphenylene tetra-ol in representative example of these spectral changes highlighting
dimethylformamide, was added 2-(2-methoxy-ethoxy)-ethyl-tolue- the spectroscopic shis observed upon annulation of ortho-
nesulphonate33 (6 equivalents), potassium carbonate (8 terphenyl 9 to triphenylene 10. Singlet Ha of the dihexyloxy ring
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shis downeld from 6.85 ppm in ortho-terphenyl derivative 9 signals in the aromatic region of 10, 21, and 23 can be grouped
to 7.74 ppm in triphenylene derivative 10. Proton signals Hb and into three clusters of three peaks each. The quaternary carbons
Hc of the di(tert-butyldimethylsilyloxyl) rings, which overlap in of the central six-member ring of each triphenylene, labeled
the region from 6.60–6.67 ppm in 9, separate into two distinct C1–C3 in Fig. 3, are the farthest downeld (144–150 ppm) due to
singlets at 7.79 and 7.83 ppm in annulated product 10. Lastly, greater deshielding in this central ring. The peripheral quater-
the doublet at 6.54 ppm that corresponds to proton Hd of ortho- nary carbon atoms (C4–C6 in Fig. 3) fall within a tighter range of
terphenyl derivative 9 is no longer present in the annulated 123–125 ppm. Lastly, the methine carbons (C7–C9 in Fig. 3) are
triphenylene derivative. Accurate mass APCI mass spectro- found between 106–114 ppm.
metric analysis further supports the loss of two hydrogen atoms
As can be seen in Fig. 3, the carbon signals C1–C3 of triphe-
upon annulation: m/z ¼ 951.6180 [M + H]+ for ortho-terphenyl 9 nylene derivatives 21 and 23 are highly similar while those of 10
and 949.6027 [M + H]+ for triphenylene 10 (Dm/z9–10 ¼ 2.0153) are more distinct. This distinction comes from the fact that
compared with calculated values of 951.6200 and 949.6044 (Dm/ carbon C1 of compound 10 at 148.7 ppm is contained within an
z ¼ 2.0156), respectively. Cleavage of the four TBDMS protecting aryl ring bearing two hexyloxy substituents, a feature not present
groups of triphenylene derivative 10 with KF and HBr (Scheme in any of the aryl rings of isomers 21 or 23. Furthermore, carbons
3) is accompanied by the loss of peaks at 0.98, 0.94, 0.19 and C2 and C3 of compound 10 both appear at 146.6 ppm as they
0.08 ppm as well as a signicant change in compound solu- occupy almost identical positions within aryl rings bearing two
bility: the deprotected triphenylene tetra-ol 1 displays very OTBDMS groups. Three distinct signals are observed for carbon
limited solubility in chloroform but is well solvated in more atoms C1–C3 of compounds 21 and 23: one assigned to a carbon
polar solvents such as acetone and tetrahydrofuran. Accurate atom within a di(OTBDMS) ring, one assigned to a carbon atom
mass APCI mass spectrometric analysis of triphenylene tetra-ol proximal to the hexyloxy group of a mixed hexyloxy/OTBDMS
1 reveals an [M + H]+ signal at m/z ¼ 493.2575, which is in ring, and one assigned to a carbon atom proximal to the
agreement with the calculated value of 493.2585 and OTBDMS group of a mixed hexyloxy/OTBDMS ring. Given this
commensurate with the loss of four TBDMS groups. Similar similarity between isomers 21 and 23 the signals for carbons
changes in the 1H NMR spectra and APCI mass spectra C1–C3 appear almost indistinguishable.
accompany the oxidative annulation and KF deprotection of
In the middle cluster of signals, corresponding to carbon
ortho-terphenyl 20 to TBDMS-protected triphenylene 21 and atoms C4–C6, isomer 10 again displays a distinct pattern while
ultimately tetra-ol 2, as well as from regioisomeric ortho-ter- isomers 21 and 23 are signicantly more similar. Carbon
phenyl 22 to TBDMS-protected triphenylene 23 and tetra-ol 3.
signals for compound 10 in this region appear at two chemical
While proton signals in the 1H NMR spectra of 10, 21, and 23 shis: one isolated peak corresponding to C4 at 123.2 ppm and
support the successful formation of triphenylene derivatives, two overlapping peaks corresponding to carbon atoms C5 and
the three regioisomers cannot be distinguished by proton C6 at 124.0 ppm. Carbon atom C4 is distinct as it is substituted
spectra alone. Similarly, mass spectroscopic analyses of the with hexyloxy groups whereas C5 and C6, while symmetrically
three TBDMS-protected triphenylene derivatives are, within inequivalent, are both substituted with an OTBDMS group and
error, identical (m/z ¼ 946.6027, 949.6031, and 949.6034 [M + are observed at the same chemical shi. Isomers 21 and 23
H]+ for 10, 21, and 23, respectively). 13C NMR spectroscopy, again display three distinct peaks for C4–C6 following the same
however, does provide a means of distinguishing between the reasons as discussed above for distinguishing their C1–C3
three different isomers. As shown in Fig. 3, the nine carbon signals. Lastly, isomers 21 and 23 can be distinguished from
each other by the shis of methine carbon signals in the region
spanning 106–114 ppm. Within this region the carbon atom
alpha to a hexyloxy-substituted peripheral carbon is found
farthest upeld and at a unique chemical shi: 106.8 for C7 of
10, 105.8 for C8 of 21, and 106.2 for C9 of 23. Furthermore, in
compound 23, methine carbon atoms C7 and C8 are in subtly
distinct chemical environments such that their signals appear
close (114.2 and 113.9, respectively) but do not overlap. For
isomers 10 and 21, however, signals for the methine carbon
atoms alpha to OTBDMS-substituted peripheral carbon atoms
are sufficiently similar that they do overlap and cannot be
resolved. Collectively, the nine aromatic carbon signals in the
13C NMR spectra of triphenylene isomers 10, 21, and 23 provide
a means of distinguishing each isomer.
Functionalization of triphenylene tetra-ols 1–3 to give
amphiphilic, mixed-substituent triphenylenes 4–6 was conrmed
by the disappearance of hydroxyl peaks and concomitant
Fig. 2 Representative partial 1H NMR spectrum (300 MHz, CDCl3, 298
K) of ortho-terphenyl derivative 9 (top) and TBDMS-protected tri-
appearance of ethylene glycol peaks in the region extending from
3.4 to 4.5 ppm of the 1H NMR spectra of each species. Func-
phenylene derivative 10 (bottom) indicating characteristic shifts of
aromatic signals upon oxidative annulation.
tionalization with monomethyl di(ethylene glycol) chains was
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Fig. 3 Partial 13C NMR spectrum (75 MHz, CDCl3, 298 K) of TBDMS-protected triphenylene derivatives 23 (top), 21 (middle), and 10 (bottom)
highlighting the differences in chemical shift that distinguish each regioisomer.
also accompanied by a notable increase in the solubility of each between those of triphenylenes 25 and 26, and provide a means
compound and a phase change from high melting solid mate- of assessing the inuence of alkyl versus ethylene glycol regio-
rials to low melting solids (4 and 6) and one liquid (5). Mass chemistry on triphenylene phase behavior.
spectroscopic analysis conrmed the addition of four mono-
Shown in Table 2 are the phase transitions of triphenylene
methyl di(ethylene glycol) substituents to compounds 1–3, derivatives 4–6 and 25–26. Also shown in Table 2 are schematic
revealing [M + Na]+ signals at m/z ¼ 923.5148, 923.5137, and representations of each triphenylene derivative that aid in
923.5153 for mixed-substituent triphenylene derivatives 4–6, understanding how substituent regiochemistry in amphiphilic
respectively, compared to the calculated value of 923.5127. The triphenylenes inuences thermal phase transitions. Of the
three triphenylene tetra-ols 1–3 and amphiphilic triphenylenes three amphiphilic triphenylenes studied, compound 4, bearing
4–6 were also characterized by UV/Vis spectroscopy. Spectra of all hexyloxy substituents at positions 2 and 3, was found to behave
six compounds display nearly identical absorption maxima most similar to hexakis(hexyloxy) triphenylene 25. Compound 4
(lmax ¼ 345 ꢃ 1 nm) with extinction coefficients ranging from 3 ¼ shows a sharp crystallization at a lower temperature than all-
3.2–4.9 ꢂ 104 Mꢁ1 cmꢁ1 (see Fig. S1 and S2 of the ESI†). These hexyloxy 25 (39 ꢀC versus 51 ꢀC) and similarly transitions to
absorption characteristics closely mirror those of fully symmetric isotropic at a lower temperature than compound 25 (57 ꢀC
hexakis(hexyloxy) triphenylene (lmax ¼ 346 nm, 3 ¼ 5.2 ꢂ 104 Mꢁ1 versus 64 ꢀC), as would be expected with the introduction of
cmꢁ1), hexa(hydroxy) triphenylene (lmax ¼ 346 nm, 3 ¼ 4.0 ꢂ 104 monomethyl di(ethylene glycol) chains. It is interesting to note
Mꢁ1 cmꢁ1), and hexakis(monomethyl di(ethylene glycol)) tri- that amphiphilic triphenylene 4 exhibits a broader mesophase
phenylene (lmax ¼ 345 nm, 3 ¼ 4.0 ꢂ 104 Mꢁ1 cmꢁ1).
(37–57 ꢀC) than all-hexyloxy triphenylene 25 (56–64 ꢀC). By
Lastly, the thermal properties of new amphiphilic tripheny- contrast, amphiphilic triphenylene derivative 5, with hexyloxy
lene derivatives 4–6 were investigated by differential scanning substituents at the 2 and 7 positions, does not show a sharp
calorimetry (DSC) and compared to symmetric control melt (or crystallization) but rather a broad transition around
compounds hexakis(hexyloxy) triphenylene (25) and hex- ꢁ6 ꢀC. A second transition is observed at 28 ꢀC, likely indicating
akis(monomethyl di(ethylene glycol)) triphenylene (26). As the formation of a nematic mesophase rather than a columnar
noted earlier, monomethyl di(ethylene glycol) substituents were phase more typical of compound 25. Lastly, compound 6 with
chosen because ethylene glycol chains are less crystalline than hexyloxy substituents at positions 3 and 6 exhibits a narrower
comparable length alkyl chains. Indeed, DSC analysis of hex- mesophase between 33 and 44 ꢀC. Compound 6, therefore,
akis(hexyloxy) triphenylene 25 reveals a sharp crystallization at becomes isotropic at a temperature below triphenylene deriva-
51 ꢀC whereas crystallization is suppressed for hex- tives 4, 25, and 26 yet above derivative 5. Similar to hex-
akis(monomethyl di(ethylene glycol)) triphenylene 26 (Table 2, akis(monomethyl di(ethylene glycol)) triphenylene 26, no
Fig. S3 and S4 of the ESI†). Furthermore, alkyl-substituted 25 distinct crystallization could be observed for amphiphilic tri-
exhibits a mesophase between 56 and 64 ꢀC, in good agreement phenylene derivative 6.
with the reported formation of a columnar hexagonal (Colh)
The results presented in Table 2 clearly show that the relative
liquid crystalline phase.35 Hexakis(monomethyl di(ethylene placement of hexyloxy chains in amphiphilic triphenylene
ꢀ
glycol))-substituted 26, by contract, becomes isotropic at 47 C derivatives 4–6 signicantly inuences their phase behavior.
with no evidence of mesophase formation. The thermal prop- Given the observed results, we hypothesize that the primary
erties of amphiphilic triphenylenes 4–6 may be expected to vary factor inuencing phase behavior in compounds 4–6 is the
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Table 2 Phase transition temperatures (ꢀC) of substituted triphenylenes. Transition temperatures are based on the 1st cooling run (5 ꢀC minꢁ1
)
and 2nd heating run (10 ꢀC minꢁ1). TM ¼ transition to mesophase, TI ¼ clearing temperature (isotropic melt). In the schematic representations of
compounds 4–6 and 25–26, hexyloxy substituents are represented by angular black lines and monomethyl di(ethylene glycol) substituents are
represented by blue helices. Dashed lines indicate divisions between hydrophobic and hydrophilic sections of triphenylene derivatives 4–6
Compound
TM
TI
Mesophase
56
64
Colh
37
55
Colh
ꢁ6
33
44
Colh
—
47
—
28
a
a
Compound 5 exhibits a broad mesophase between ꢁ6 and 28 ꢀC. Dissimilarity from the characteristic columnar hexagonal mesophase prevents
conclusive mesophase characterization other than a possible ordered nemetic phase.
relative spacing of their two hexyloxy substituents. Alkyl chains triphenylene derivatives provide a versatile platform for further
are known to promote crystallinity and long-range order in tri- synthetic modications, as demonstrated here by their func-
phenylene mesogens.1–3 As such, placement of the two hexyloxy tionalization with monomethyl di(ethylene glycol) chains to
chains as close to each other as possible – i.e. 2,3-bis(hexyloxy) provide three regioisomers of amphiphilic triphenylenes
derivative 4 – results in the amphiphilic derivative with the bearing two hexyloxy and four monomethyl di(ethylene glycol)
highest clearing temperature of 55 ꢀC along with a well-dened substituents. Furthermore, the importance of regioisomerism
crystallization (see Fig. S3 and S4 of the ESI†). The clearing on the physical properties of triphenylene mesogens was
temperature of the amphiphilic 2,6-bis(hexyloxy) derivative 6, demonstrated in differences in the thermal properties of the
with its hexyloxy substituents spaced slightly further apart than three amphiphilic triphenylene isomers as compared to each
ꢀ
ꢀ
in compound 4, is observed 11 C lower at 44 C. Amphiphilic other and to hexakis(hexyloxy) triphenylene and hex-
derivative 5 is notably different than derivatives 4 and 6 because akis(monomethyl di(ethylene glycol)) triphenylene.
its hexyloxy substituents are almost diametrically opposed at
The adaptability of the synthetic routes presented herein is
positions 2 and 7. As such, monomethyl di(ethylene glycol) evident in the precursor design: functional groups and their
substituents fully segregate the two hexyloxy substituents from regiochemistry may be easily varied by small changes in
each other (as indicated by dashed curves in Table 2) whereas precursor substituent patterns. Similarly, multiple different
hexyloxy substituents are not similarly segregated from each functionalities can be introduced at several points in the
other in derivatives 4 and 6. This greater separation of hexyloxy synthesis, providing facile routes to mixed triphenylene deriv-
substituents further inhibits crystallization and depresses the atives with two – or more – types of substituents. The synthetic
clearing temperature of derivative 5 to 28 ꢀC. Further investi- routes demonstrated herein can likely be adapted to the prep-
gation of mixed hexyloxy and monomethyl di(ethylene glycol) aration of heterocyclic triphenylene derivatives such as aza-
substituted triphenylenes that vary in both the stoichiometry triphenylenes,36–39 which are known to exhibit different
(1 : 5 through 5 : 1) and relative positioning of the different electronic and physical properties36,37 than triphenylene deriv-
substituents will be necessary to determine if this preliminary atives but their development has been limited by the current use
trend is more broadly applicable. Such investigations are of toxic and costly transition metal catalysts in their
currently underway.
synthesis.38,39 In general, we anticipate triphenylene derivatives
will continue to play vital roles in the development of multi-
functional mesogens with implications in such areas as organic
electronic and photovoltaic materials, and rational routes to
multifunctional triphenylene derivatives, such as those
described herein, will allow the full potential of these unique
compounds to be explored and applied.
Conclusion
The synthesis of hydroxy-functionalized triphenylene deriva-
tives via oxidative annulation of ortho-terphenyl compounds is
reliable, facile, scalable, and opens innumerable routes to the
synthesis of structurally precise mixed-substituent triphenylene
derivatives. In the current study, three isomers of rationally-
designed tetrahydroxy triphenylene derivatives were synthe-
sized. The good to excellent yields and straightforward puri-
cations of the synthetic route presented herein offer a valuable
Acknowledgements
alternative to the current harsh, non-selective methods that are We thank Wesleyan University for nancial support of this
typical of triphenylene poly-ol syntheses. The tetrahydroxy research.
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12 A. N. Cammidge and H. Gopee, J. Mater. Chem., 2001, 11,
2773.
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Paper
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38292 | RSC Adv., 2014, 4, 38281–38292
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