Synthesis, structures, and physical properties of benzo[k]fluoranthenebased linear acenes

Article abstract of DOI:10.1002/chem.200902695

This work describes the syntheses, crystal structures, photophysical properties, and electro-chemical analyses of benzo[k]fluoranthene-based linear acenes, together with ab initio density functional theory computations on them. The molecules were prepared

Full text of DOI:10.1002/chem.200902695

FULL PAPER
DOI: 10.1002/chem.200902695
Synthesis, Structures, and Physical Properties of Benzo[k]fluoranthene-
Based Linear Acenes**
Yun-Hua Kung,[a] Yu-Sung Cheng,[a] Chia-Cheng Tai,[a] Wei-Szu Liu,[b]
Chien-Chueh Shin,[a] Chih-Chung Ma,[a] Yi-Chan Tsai,[b] Tsun-Cheng Wu,[a]
Ming-Yu Kuo,*[b] and Yao-Ting Wu*[a]
Abstract: This work describes the syn-
theses, crystal structures, photophysical
properties, and electro-chemical analy-
ses of benzo[k]fluoranthene-based
linear acenes, together with ab initio
density functional theory computations
on them. The molecules were prepared
in generally moderate to good yields
through Pd-catalyzed cycloadditions
between 1,8-diethynylnaphthalene de-
rivatives and aryl iodides. This protocol
is simpler and more efficient than con-
ventional methods. The scope and limi-
tations of this reaction were examined.
The structures of compounds 4hb,
15ac, 17ab, 19ac, and 24je were deter-
mined by X-ray analysis; they are
either bent or twisted, rather than
planar. The photophysical and electro-
chemical properties of these cycload-
ducts were also investigated and com-
pared with computational predictions
based on density functional theory.
Keywords: cross-coupling
· cyclo-
addition · luminescence · palladium ·
polycyclic aromatic hydrocarbons
Introduction
in optoelectronics,[4] as conducting materials,[5] in solar
cells,[6] and in other functions. The benzo[k]fluoranthenes 4
(Scheme 1) have recently stimulated our interest because of
their special properties: modifications by peripheral func-
tional groups can allow compounds 4 to act as luminescent
materials,[7] useful molecular sensors,[8] or organic semicon-
ductors.[9] The unsubstituted benzo[k]fluoranthene 4 (R=
Polycyclic aromatic hydrocarbons (PAHs) might be the larg-
est class of organic molecules.[1,2] They exhibit a wide range
of molecular sizes and structural forms, causing them to
have versatile physical properties and allowing them to be
employed as organic materials, including as liquid crystals,[3]
ACTHNUTRGNEUNG
H)[10] and naphtha[2,3-k]fluoranthene 5 (R=H)[11] are com-
mercially available and numerous approaches for preparing
them have been described. However, the methods for gener-
ating their functionalized derivatives are relatively few. Two
protocols based on the Diels–Alder reaction have been de-
veloped (Scheme 1). Reactions between the reactive dieno-
[a] Y.-H. Kung, Y.-S. Cheng, C.-C. Tai, C.-C. Shin, C.-C. Ma, T.-C. Wu,
Prof. Y.-T. Wu
Department of Chemistry, National Cheng Kung University
No. 1 Ta-Hsueh Rd., 70101 Tainan (Taiwan)
Fax : (+886)6-2740552
E-mail: ytwuchem@mail.ncku.edu.tw
[b] W.-S. Liu, Y.-C. Tsai, Prof. M.-Y. Kuo
Department of Applied Chemistry
National Chi Nan University, No. 1 University Rd.
54561 Puli, Nantou (Taiwan)
E-mail: mykuo@ncnu.edu.tw
[**] Metal-Catalyzed Reactions of Alkynes. Part V. Part IV: C.-C. Cheng,
C.-S. Chang, Y.-L. Hsu, T.-Y. Lee, L.-C. Chang, S.-H. Liu, Y.-T. Wu,
Eur. J. Org. Chem. 2010, 672. Part III: Y.-T. Wu, M.-Y. Kuo, Y.-T.
Chang, C.-C. Shin, T.-C. Wu, C.-C. Tai, T.-H. Cheng, W.-S. Liu,
Angew. Chem. 2008, 120, 10039; Angew. Chem. Int. Ed. 2008, 47,
9891.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902695.
Scheme 1. Conventional and new methods for the synthesis of benzo[k]-
fluoranthenes (4) and naphthoACTHNUGRTNEUNG[2,3-k]fluoranthenes (5).
Chem. Eur. J. 2010, 16, 5909 – 5919
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phile acenaphthylene (3)[7,12] and either the isobenzofurans 1
or the naphtho[2,3-c]furans 2, followed by aromatization
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
under acidic conditions, yield the desired products 4 or 5, re-
spectively (Route A, or Bergmannꢁs protocol).[13] The con-
venience and efficiency of this protocol are heavily depen-
dent on the furan derivatives 1 and 2. 1,3-Diphenylisobenzo-
furan (1, R=Ph), for example, is commercially available,
whereas other substituted furan derivatives, such as 5,6-di-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
G
ACHTUNGTRENNUNG
ated in moderate to good yields (Scheme 2 and Table 1). A
methyl-1,3-diphenylisobenzofuran[14]
diphenylnaphtho
[2,3-c]furan (2, R=Ph),[15] have to be pre-
or
1,3-
ACHTUNGTRENNUNG
pared by lengthy synthetic processes. 7,10-Diphenylben-
zo[k]fluoranthene (4, R=Ph)[7] is hence easily accessible,
but the synthesis of other substituted derivatives such as
9,10-dimethyl-7,12-diphenylbenzo[k]fluoranthene[16] or 7,14-
diphenylnaphthoACHTUNGTRENNUNG
[2,3-k]fluoranthene[15b] is inconvenient.
Benzo[k]fluoranthenes 4 can also be obtained through cy-
cloadditions between the cyclopenta[a]acenaphthylen-8-ones
10 and benzyne (9) with subsequent elimination of carbon
monoxide (Route B, or Allenꢁs protocol).[17] The cyclopenta-
dienone derivatives 10 are generated either through the
Knoevenagel condensation from acenaphthenequinone and
a ketone[18] or by the metal-catalyzed carbonylation of
diynes 8.[19] The former method cannot be employed with
simple ketones such as acetone or butan-2-one[17] or with ke-
tones that contain acid- or base-sensitive functional groups.
The metal-catalyzed carbonylation of diynes 8 can more
readily be performed when bulky silyl substituents are pres-
ent as the R moieties.[19]
Scheme 2. Preparation of benzo[k]fluoranthene-based linear acenes. For
details see Table 1.
diyne and an aryl iodide provide a four-carbon building
block and a two-carbon building block, respectively, and
these undergo formal [(2+2)+2] cycloaddition. The aryl io-
dides 6 and 7 reacted here as “pre-arynes”. In this newly de-
veloped protocol, many substituents can easily be intro-
duced into these cycloadducts through variation of R1, R2,
R3, and R4. This feature can be exploited for screening and
tuning of the photophysical properties of the desired mole-
cules. Some functional groups, including aryl bromides,
esters, and ethers, can be tolerated under the reaction condi-
tions. Except in the case of diynes 4, reactions performed in
toluene or in p-xylene at 110 or 1308C do not differ signifi-
cantly. The amounts of AgOAc used in these reactions
depend on the number of iodo substituents in the aryl io-
dides: one equivalent for iodobenzene and two equivalents
for diiodoarenes. The presence of an additional triphenyl-
phosphine ligand makes the reaction inefficient. The reactiv-
ities of several aryl iodides in cycloadditions either with 1,8-
bis(phenylethynyl)naphthalene (8a) or with 5,6-bis(phenyle-
thynyl)acenaphthalene (11a) were examined. Unlike o-diio-
doarenes, monoiodoarenes could not be utilized in this reac-
tion because they afforded the products in unsatisfactory
yields and with low regioselectivities. 3-Iodotoluene, for ex-
ample, produced a mixture of 8- and 9-methyl-7,12-diphe-
nylbenzo[k]fluoranthenes in low yields (less than 10%). 9-
Iodophenanthrene was also only poorly active in this reac-
tion, giving the cycloadduct 21 in 9% yield (entry 25 in
Table 1). Fortunately, the expensive 1,2-diiodobenzene[25]
can be replaced with iodobenzene (6a). 1,2-Diiodo-4,5-di-
methylbenzene (6b), 1,2-diiodo-4,5-dimethoxybenzene (6c),
and 2,3-diiodonaphthalene (7a) generated the correspond-
ing cycloadducts in moderate yields (entries 2, 3, and 6,
Table 1).
Fluoranthenes can be prepared through RhI-catalyzed
[(2+2)+2] cycloadditions between 1,8-diethynylnaphtha-
lenes 8 (Scheme 1) and norbornadiene or a molecule of an
alkyne.[20] Similarly, the replacement of an alkyne with a
benzyne derivative in this reaction should yield a benzo[k]-
fluoranthene 4. Benzyne, however, is very active, and it
reacts with 1,8-diethynylnaphthalene (8, R=H) to yield
benzo[a]pyrene in the absence of metal catalysts, even at
room temperature.[21] Accordingly, stabilized benzynes or
pre-benzynes are suitable starting materials in the protocol
designed here. Fortunately, in the presence of Pd catalysts,
iodobenzenes[22] and 1,2-diiodobenzenes[23] can be formally
treated as pre-arynes, which have been utilized in the syn-
thesis of highly substituted naphthalenes through cycloaddi-
tion with two alkyne units. In the light of these metal-cata-
lyzed protocols, the target molecules 4 and 5 were observed
to be obtainable through Pd-catalyzed cycloadditions be-
tween 1,8-diethynylnaphthalenes 8 and either iodobenzene
(6a) or 2,3-diiodonaphthalene (7a), respectively. Here this
reaction is elucidated for the construction of benzo[k]fluor-
anthene-based linear acenes. Their structures and physical
properties are analyzed and compared with corresponding
computational results.
1,2,4,5-Tetraiodobenzene (6e) was able to undergo two-
fold cycloadditions; treatment of the diyne 11a with excess
1,2,4,5-tetraiodobenzene (6e) furnished the diiodo-substitut-
ed benzo[k]fluoranthene 15ae, which was used to prepare
the cycloadduct 22 in 65% yield (entries 11 and 26, Table 1).
Results and Discussion
Synthesis: Upon heating of diynes 8 or 11–14 with aryl io-
dides 6 or 7 in the presence of catalytic amounts of Pd-
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Benzo[k]fluoranthene-Based Linear Acenes
FULL PAPER
Table 1. Preparation of benzo[k]fluoranthene-based linear acenes.[a]
Entry
Diyne
Aryl iodide
T [8C]
Product
Yield [%]
1
2
3
4
5
6
8a: R1 =Ph
6a: X=R2 =R3 =H
6b: X=I; R2 =R3 =Me
6c: X=I; R2 =R3 =OMe
6a
110/85
110
110
110
110
4aa
4ab
4ac
73/85
48[b]
62[c]
55
8a
8a
8b: R1 =2-Br-C6H4
8c: R1 =SiMe3
8a
4ba
6b
4cb: R1 =H
5aa
33
7a: X=I; R2-R3 =C4H4
110
58[b]
7
8
9
10
11
12
13
14
15
16
11a: R1 =R2 =Ph
6a: X=R3 =R4 =H
130/110
130
130
130
110
110
110
110
110
15aa
15ab
15ac
15ad
15ae
15 db
15dc
15eb
15ec
16aa
61/55
53
64
traces
56[d]
84
75
69
63
52
11a
11a
11a
11a
6b: X=I; R3 =R4 =Me
6c: X=I; R3 =R4 =OMe
6d: X=Br; R3 =OMe, R4 =H
6e: X=R3 =R4 =I
11d: R1 =R2 =4-OMe-Ph
6b
6c
6b
6c
11d
11e: R1 =4-OMe-Ph, R2 =4-CO2Me-Ph
11e
11a
7a: X=I; R3-R4 =C4H4
130
17
18
19
20
21
22
23
24
12a: R1 =Ph, R2 =Me, R3 =H
6b: X=I; R4 =R5 =Me
6c: X=I; R4 =R5 =OMe
6a: X=R4 =R5 =H
130
130
110
110
130
130
130/110
130/110
17ab
17ac
18aa
18 fc
19aa
19ab
19ac
20aa
47
58
<33
39
60
51
68/71
63/61
12a
13a: R1 =R2 =R3 =Ph
13 f: R1 =4-O-nC8H17-Ph, R2 =R3 =Ph
6c: X=I; R4 =R5 =OMe
6a: X=R4 =R5 =H
14a: R1 =Ph, R2 =Me, R3 =CO2Me
14a
14a
14a
6b: X=I; R4 =R5 =Me
6c: X=I; R4 =R5 =OMe
7a: X=I; R4-R5 =C4H4
25
11a
8a
130
110
21
22
9[e]
26
15ae
65
[a] Diynes (0.25 mmol), iodoarenes (0.50 mmol), PdACTHUNRGTNEUNG(OAc)2 (12.5 mmol, 5 mol%), AgOAc (0.50 mmol; 0.25 mmol for iodobenzene), and p-xylene (or tol-
uene, 4 mL) were used in this reaction, if not otherwise mentioned. The isolated yields are based on the diynes. [b] Acetonitrile was used as the solvent.
[c] The reaction was carried out in a mixture of p-xylene (3 mL) and acetonitrile (1 mL). [d] 1,2,4,5-Tetraiodobenzene (0.75 mmol) was used. [e] Com-
pound 11a (36%) was recovered.
Unlike the o-diiodoarenes, 3-bromo-4-iodoanisole (6d)
did not yield the desired cycloadduct (entries 10, Table 1).[26]
1,2,3,4-Tetrafluoro-5,6-diiodobenzene, 3,6-dichloro-1,2,4,5-
tetraiodobenzene, and 2-iodopyridine also did not undergo
cycloadditions.
Unlike other diynes, the 1,8-bis(arylethynyl)naphthalenes
8 easily undergo cyclization to form the corresponding 7-
phenylbenzo[k]fluoranthene products,[27] presumably be-
cause of the short distances between the pairs of parallel al-
kynyl moieties in compounds 8.[28] 7,8,9,10-Tetraphenyl-3,4-
bis(phenylethynyl)fluoranthene (13a) gave the cycloadduct
18aa in low yield; its low solubility in common organic sol-
vents makes its purification difficult (entry 19, Table 1). This
inconvenience was overcome by using the dioctoxy-substi-
tuted diyne 13 f (entry 20, Table 1).
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Y.-T. Wu, M.-Y. Kuo et al.
Diaryl-substituted diynes appear to be more suitable than
trimethylsilyl- and alkyl-substituted analogues for use in this
reaction. The cycloaddition between the sterically congested
1,8-bis(trimethylsilylethynyl)naphthalene (8c) and 1,2-
As mentioned above, the asymmetric benzo[1,2-k:4,5-
k’]difluoranthene derivative 22 was prepared by the devel-
oped protocol in a two-step process (entries 11 and 26,
Table 1). The symmetric benzo[1,2-k:4,5-k’]difluoranthenes
24 and their analogues are important molecules because
they are (potential) photoelectronic materials[30,31] and ex-
hibit interesting conformations on metal surfaces.[32] Synthe-
ses of 7,9,16,18-tetraphenylbenzo[1,2-k:4,5-k’]difluoranthene
diiodo-4,5-dimethylbenzene (6b) formed
a mixture of
mono- and bis(trimethylsilyl)-substituted cycloadducts in
low yield. These two compounds were converted into 9,10-
dimethylbenzo[k]fluoranthene (4cb) by treatment with
acetic acid (entry 6, Table 1).
(24ae),[30]
7,9,16,18-tetrakisACHTUNGETRN[UNG 3,5-di(tert-butyl)phenyl]ben-
The reactions of the alkyl-substituted 1,8-diethynylnaph-
thalenes 8g and 8h with diiodoarenes 6 were inefficient
(Table 2), although 1,2,3,4-tetraACTHUNTGRNEUNG(n-propyl)naphthalene can
be obtained in excellent yield from 1,2-diiodobenzne and
oct-4-yne under the same reaction conditions.[23] The diynes
8g and 8h generated the desired cycloadducts 4, accompa-
nied by the byproducts 4’ and the fluoranthenes 23.[29] The
low yields of the alkyl-substituted benzo[k]fluoranthenes 4
and 4’ perhaps follow from poor stability of one intermedi-
ate in this reaction (Scheme 3). Notably, the 4’/4 ratio is de-
pendent on the substituents in the diynes 8 and the o-diio-
doarenes 6, with the former being more important than the
latter. The amounts of 4’ became significant when the di-
alkyl-substituted diyne 8g was utilized. Use of a lower reac-
tion temperature (808C) did not improve the selectivity of
the formation of 4gc and 4’gc. Separation of compounds 4
from compounds 4’ was very difficult, so the byproducts 4’
were converted into compounds 4 by palladium-catalyzed
hydrogenation. Hydrogenation in methanol was unsuccess-
ful, because of the low solubilities of compounds 4’ under
these conditions, but addition of dichloromethane to the re-
action mixtures ensured excellent yields.
zo[1,2-k:4,5-k’]difluoranthene (24ie),[33] 8,17-di-n-hexylben-
zo[1,2-k:4,5-k’]difluoranthene (25),[34] and 6,8,15,17-tetra-
phenyl-1.18,4.5,9.10,13.14-tetrabenzoheptacene (26)[35] have
been reported elsewhere. Compounds 24ie and 25 were pre-
pared by a lengthy synthetic process and the method for the
generation of 7,9,16,18-tetraphenylbenzo[1,2-k:4,5-k’]difluor-
anthene (24ae) has not been clearly elucidated. Elaboration
of the presented protocol offers the potential for the synthe-
sis of symmetric benzo[1,2-k:4,5-k’]difluoranthene deriva-
tives through twofold cycloadditions between 1,2,4,5-tetraio-
dobenzene (6e) and diynes 8, 11, or 13 in a one-pot proce-
dure (Table 3). A test was performed with diyne 11a
(2.5 equiv) in the presence of PdACTHNUTRGNEUNG(OAc)2 (10 mol%) and
AgOAc (4 equiv) [Variant A,
entry 5 in Table 3]. Unfortu-
nately, the desired product
27ae was obtained in only
35% yield, accompanied by
the monoadduct 15ae (37%).
The results were almost identi-
cal, however, when the reac-
tion was performed either
under more dilute conditions
or
with
more
catalyst
(40 mol%), as well as for a
longer reaction time (three
days). Increasing the amount
of AgOAc to 6 equiv slightly
improved the yield of 27ae
(46%), but 15ae (17%) was
also obtained, possibly indicat-
ing that the lower solubilities
of 6e and 15ae under these
conditions made the cycloaddi-
tions inefficient and caused
some of the AgOAc to be de-
graded (Scheme 3). This prob-
lem could be resolved by addi-
tion of two equal portions of
Scheme 3. Proposed reaction mechanism for the formation of cycloadducts 4 and 4’.
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Benzo[k]fluoranthene-Based Linear Acenes
FULL PAPER
Table 2. Reactions between the alkyl-substituted 1,8-diethynylnaphthalenes 8 and the diiodoarenes 6.[a]
AgOAc (2ꢂ2 equiv) at an inter-
val of 24 h apart; 27ae was then
isolated in 61% yield (Var-
iant B, entry 7 in Table 3).
Under these optimized condi-
tions, the more extended linear
acenes 28 were straightforward-
ly prepared through reactions
between the 3,4-diethynylfluor-
anthenes 13 and 6e (entries 8
Entry
Diyne
R1
Aryl iodide
R2
Product
Ratio (4/4’)[b]
Yield [%][c] and 9 in Table 3). Like that of
18aa, the separation of cycload-
duct 28ae from impurities is
difficult because it is only
weakly soluble in common or-
ganic solvents, but the tetraoc-
1
2
3
4
8g
8g
8h
8h
nBu
nBu
Ph
6b
6c
6b
6c
Me
OMe
Me
4gb+4’gb
4gc+4’gc
4hb+4’hb
4hc+4’hc
50:50
52:48
79:21
83:17
22 (92)
39 (94)
24 (89)
34 (90)
Ph
OMe
[a] The cycloaddition reactions were carried in thick-walled sealable tubes. Diynes (0.25 mmol), diiodoarenes
(0.50 mmol), PdACHTUNGTRENNUNG(OAc)2 (12.5 mmol), AgOAc (0.50 mmol), and toluene (4 mL) were used in these reactions.
toxy-substituted
cycloadduct
[b] Determination by 1H NMR analysis. [c] Isolated yields for the mixtures of 4 and 4’. Yields for hydrogena-
28 fe can be purified. In the
synthesis of cycloadducts 24,
however, the reactions per-
formed with Variants A and B
tion are reported in parentheses.
did not differ significantly. The unsatisfactory results arise
from competition between the cycloaddition and the cycliza-
tion of diynes 8. Crystals of 24ae could not be grown, and
so the tetrakis(n-butylphenyl)-substituted derivative 24je
with higher solubility in organic solvents was synthesized for
X-ray structural analysis.
Table 3. Preparation of the benzo[1,2-k:4,5-k’]difluoranthene derivatives
24, 27, and 28.[a]
Scheme 3 presents a putative literature-based mechanism
for the generation of the benzo[k]fluoranthenes 4. Initially,
PdACTHNUGTRENNG(U OAc)2 oxidizes p-xylene to produce 2,5-dimethylphenyl
acetate and 4-methylbenzyl acetate.[36,37] The thus-generated
palladium(0) species undergoes oxidative addition with the
o-diiodobenzene 6 to give the complex 29. A syn-addition of
À
the Ar Pd bond in 29 to one triple bond of the diyne 8
yields the alkenylpalladium derivative 30. This process is fol-
lowed by the insertion of the other alkynyl moiety to afford
the arylbutadienylpalladium intermediate 31. Cyclization of
31 would furnish the palladabenzocycloheptatriene 32,[38]
which would subsequently undergo reductive elimination to
Entry Diyne
Variant Product (yield,
%)
1
2
3
4
5
6
7
8a: R1 =Ph, R2 =H
A
4ae (35)[b]+24ae
(31)
8a
B
4ae (35)[b]+24ae
(33)
produce the benzo[k]fluoranthene 4 and PdI2. PdACTHNUTRGNE(UNG OAc)2 is
8j: R1 =4-nBu-Ph, R2 =H
A
B
4je (36)[b]+24je
(30)
eventually regenerated by the reaction of PdI2 with the addi-
tional AgOAc. However, when the diiodoarene [or 1,2,4,5-
tetraiodobenzene (6e)] is inactive under these reaction con-
ditions, AgOAc can be consumed oxidizing Pd0 species. This
reduction/oxidation cycle is responsible for the inefficiency
of the twofold cycloadditions. As stated above, the dialkyl-
substituted diyne 8 (R2 =nBu) yielded the desired cycload-
ducts 4, as well as the byproducts 4’ (Table 2). Compound 31
(R2 =nBu) in a crowded environment preferentially under-
goes b-hydride elimination to generate the allene 33, rather
than ring-closure to form the complex 32 (R2 =nBu). Palla-
dium-catalyzed cyclization of 33 gives the byproducts 4’
after b-hydride elimination. The low stability of the allene
intermediate 33 under the reaction conditions here might be
the cause of the unsatisfactory yields of these alkyl-substitut-
ed cycloadducts.
8j
4je (38)[b]+24je
(31)
11a: R1 =Ph, R2–R2 =(CH)2
A
A[c]
B
15ae (39)+27ae
(34)
15ae (17)+27ae
(46)
15ae (0)+27ae
(61)
11a
11a
8
9
13a: R1 =Ph, R2–R2 =C
13 f: R1 =4-O-nC8H17-Ph, R2–R2 =C-
(CPh)4C
(CPh)4C
B
B
28ae (<23)
28 fe (25)
ACHTUNGTRENNUNG
[a] Variant A: diynes (0.48 mmol), tetraiodobenzene (0.19 mmol), Pd-
ACHTUNGTRENNUNG(OAc)2 (19.0 mmol), AgOAc (0.76 mmol), and p-xylene (4 mL) were used
in this reaction. Variant B: similar to Variant A, but AgOAc (0.38ꢂ
2 mmol) was added in two equal portions at a 24 h interval. [b] An insep-
arable mixture of the 7-arylbenzo[k]fluoranthene and the compound 4
1
was obtained. The yield of the latter is calculated from H NMR analysis.
[c] AgOAc (6 equiv) was used.
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Y.-T. Wu, M.-Y. Kuo et al.
X-ray crystallographic structures: X-ray-quality crystals of
4hb, 15ac, 17ab, 19ac, and 24je were obtained by slow
evaporation of CH2Cl2/MeOH solvent mixtures at ambient
temperature (Table 4).[39] In the solid state, these molecules
have either bent or twisted conformations. The degree of
bending can be determined by analyzing the dihedral angle
between the two peripheral benzene or naphthalene planes
along the longest axis of the molecule. Compounds 4hb,
15ac, 17ab, and 19ac[40] have bent structures, with angles of
bending of 7.78, 8.98, 10.28, and 15.98, respectively. These
angles of bending increase with the molecular length. How-
ever, compound 24je,[41] the longest molecule in Table 4, is
twisted rather than bent in the solid state (Figure 1). The
torsion angle of the central anthracene core, measured at
C7-C7A-C13A-C13, and the dihedral angle associated with
the two naphthalene planes in 24je are 12.48 and 21.18, re-
spectively, whereas 8,17-di-n-hexylbenzo[1,2-k:4,5-k’]difluor-
anthene (25) has an almost planar structure.[34] p–p stacking
is an usual intermolecular interaction in PAHs.[42] In the five
compounds listed in Table 4, the skeletons and substituents
display different forms of packing and only 17ab exhibits
significant p–p interaction. Molecules of 17ab form two
nonequivalent p-stacks in a cross-herringbone-like packing
pattern. The slightly bent structure of 17ab causes the mole-
cules to pack in a concave-convex manner and to exhibit
four intermolecular interactions, the associated distances of
which are in the 3.54–3.91 ꢃ range.[43] Notably, molecules of
4hb are not well organized, and each reveals the presence
of three alkyl–p interactions.[44]
Figure 1. Molecular structure of cycloadduct 24je: top) showing 50%
probability ellipsoids, and bottom) in space-filling mode.
and 17ac/18 fc are all benzoannelated cycloadducts of 4aa,
in which an additional benzo ring is present at the 9,10-posi-
tions and at the 3,4-positions, respectively. Their absorption
spectra, but not their emission bands, are very similar.
Geometrically, cycloaaducts 24 and 28 fe can be regarded
as quasi-dimeric molecules of 4aa and 18 fc, respectively.
The shapes of their absorption bands are very similar to
those of their “monomers”, but their positions are batho-
chromically shifted to approximately 90–100 nm. Com-
pounds 17ac and 18 fc are weakly fluorescent, as revealed
by the low quantum yields.
Photophysical properties: The photophysical properties of
numerous acenes were measured (Figure 2 and Table 5),[45]
and the effects of the substituents and of the extension of
the aromatic system were analyzed. The effects of the latter
exceed those of the former. Increasing extension of the aro-
matic p system should cause the absorption bands to shift
bathochromically, and the results here support this expecta-
tion with shifts in the order 4<5<24<28. Compounds 5aa
Notably, dilute solutions of
24ae and 24je are brightly
green-fluorescent even under
ambient lighting.
Table 4. Crystal structure data for cycloadducts 4hb, 15ac, 17ab, 19ac, and 24je.
4hb
15ac
17ab
19ac
24je
formula
MW
C32H28
412.54
C72H52O4
981.14
C42H30
534.66
C70H48O9
1033.08
C37H33
477.63
The substituents in these cy-
cloadducts can be grouped
into two classes: those directly
attached to the central aromat-
ic core and those in the phenyl
side arm. The former class has
a greater influence over photo-
physical properties than the
latter. The two methoxy
groups in 4ac and the ethylene
bridge in 15aa, for example,
slightly red-shift the absorp-
tion and emission bands and
increase the quantum yields
solvent
T [K]
crystal system
space group
CH2Cl2/MeOH CH2Cl2/MeOH CH2Cl2/MeOH CH2Cl2/MeOH CH2Cl2/MeOH
296(2)
triclinic
P1
296(2)
triclinic
P1
295(2)
monoclinic
P21/n
296(2)
orthorhombic
Pbcm
296(2)
monoclinic
C2/c
Z
2
2
4
8
8
unit cell dimensions
a [ꢃ]
b [ꢃ]
c [ꢃ]
a [8]
b [8]
10.6072(3)
10.6637(3)
11.8750(4)
89.211(2)
75.182(2)
61.934(2)
1136.86(6)
bent (7.5)[a]
10.3250(5)
11.8629(6)
23.5306(11)
104.513(3)
100.927(3)
91.007(3)
16.167(3)
8.4914(14)
22.917(4)
90
91.398(3)
90
17.2064(13)
23.5573(18)
31.957(2)
90
90
90
17.8939(6)
20.3823(8)
15.5372(6)
90
105.478(2)
90
g [8]
V [ꢃ3]
2733.1(2)
3145.1(9)
12953.2(16)
5461.2(4)
conformation ([8])
bent (7.7)[a]
bent (10.2)[a]
bent (15.9)[a]
twisted (21)[a]
[a] Determination from the dihedral angle between two peripheral benzene or naphthalene planes along the
longest axis of the molecules.
5914
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Chem. Eur. J. 2010, 16, 5909 – 5919

Benzo[k]fluoranthene-Based Linear Acenes
FULL PAPER
cyclic voltammetry and are summarized in Table 6.[47,48] The
first oxidation potentials are dependent on the substituents
and the extension of the aromatic systems in the central
cores. As was determined with regard to the photophysical
properties, substituents directly bound to the central aromat-
ic cores have a stronger effect on redox properties than
those in the phenyl side arms. The two methoxy groups in
4ac (0.91 V) and the ethylene bridge in 15aa (0.83 V) result
in lower oxidation potentials than in the case of 4aa (1.06 V
vs. Fc/Fc+). Substituents in the two side-arm phenyl moiet-
ies can also slightly change the electrochemical properties,
as revealed by comparison of 15dc (0.69 V) with 15ec
(0.75 V) and of 24ae (0.62 V) with 24je (0.58 V). The exten-
sion of the p system in the central aromatic core typically
reduces the oxidation potential of the compound—the order
4>5>24>28 is consistent with this fact—but the potentials
of 24je (0.58 V) and 28 fe (0.59 V) are very similar to each
other. Moreover, the first oxidation potentials of compounds
5 and 17/18, which all are benzo-annelated cycloadducts of
4aa, differ: that of 5aa (0.73 V) is significantly less than
those both of 17ac (1.03 V) and of 18 fc (0.87 V). These un-
usual results can be explained by analysis of the isosurface
distribution of the HOMOs
Figure 2. Photoluminescence spectra of benzo[k]fluoranthene-based
linear acenes (ca. 10À5 m) in CH2Cl2.
(see DFT calculations and
Figure 4, below).
Table 5. Photophysical properties of benzo[k]fluoranthene-based linear acenes.[a]
Entry Cpd la [nm]
lf [nm]
Ff [%][b]
1
2
3
4
5
6
7
8
4aa
4ac
5aa
300, 311, 330, 354, 368, 387, 410 [310, 330, 345, 375, 385, 408][c] 424, 448 [418, 444][d] 48
DFT calculations: Density
functional theory (DFT) calcu-
lations were performed to in-
vestigate the effects of sub-
stituents and the conjugation
lengths of the aromatic sys-
tems on their electronic prop-
erties. The geometric optimiza-
tions and frequencies of the
molecules were calculated at
the B3LYP/6–31G** level with
the aid of the Gaussian 03 pro-
gram.[49] The octoxy groups of
compounds 18 fc and 28 fe
were all simplified by treating
them as methoxy groups. More
reliable energy levels of the
307 (s), 320, 340 (s), 391, 413
323, 338, 372 (s), 395, 419, 445
435, 457
86
61
75
73
83
54
7
460, 489, 528 (s)
429, 452, 482 (s)
439, 462
439, 462, 493 (s)
437, 460, 493 (s)
547, 567
15aa 306, 318, 373, 394, 417
15dc 311, 324, 375 (s), 397, 419
15eb 308, 321, 376, 397, 420
15ec 313 (s), 326, 396, 417
17ac 317 (s), 331, 412, 436, 464
18 fc 324 (s), 340, 419, 446, 473
24ae 343 (s), 359, 426, 454, 485
24je 343 (s), 359, 429, 456, 488
28 fe 310, 373, 393, 425 (s), 450 (s), 486, 522, 564
9
544
6
10
11
12
494, 529, 570 (s)
496, 531, 574 (s)
580, 626 (s)
52[e]
44
35
[a] All samples (ca. 10À5 m) measured in CH2Cl2. Only absorption bands over 300 nm are listed in this table.
Excitation and emission maxima are italicized. Shoulders are indicated by (s). [b] Standards for the determina-
tion of quantum yields: quinine sulfate for entries 1–7; rhodamine 110 for entries 8–11; rhodamine 101 for
compound 28 fe. [c] Measurement in acetonitrile from ref. [46]. [d] Measurement in benzene from ref. [7b].
[e] Because of its low solubility in common organic solvents, the purity of this compound was not determined
by HPLC.
relative to 4aa. The effects of the substituents in the phenyl
side arms should not be important because the X-ray struc-
tures demonstrate that the two phenyl rings are twisted
from the central aromatic core. This prediction is verified by
comparing compounds 15dc/15ec and 24ae/24je, which all
exhibit essentially identical absorption and fluorescence
spectra, but different quantum yields (Figure 2). The addi-
tion of functional groups in products 4 and 15 did not signif-
icantly change their photophysical properties, except for
quantum yields, and so offers the potential for fine-tuning of
their electroluminescence properties.
highest occupied molecular orbitals (HOMOs) and the
lowest unoccupied molecular orbitals (LUMOs) were ob-
tained at the B3LYP/6–311+G**//B3LYP/6–31G** level.[50]
In Figure 3 the redox potentials determined by CV are com-
pared with the HOMO and LUMO levels generated from
the DFT computations, and they exhibit similar tendencies.
To provide insight into the effects of substituents and conju-
gation lengths of the aromatic systems on the electrochemi-
cal properties, Figure 4 plots the HOMO and LUMO surfa-
ces of some important compound classes. The optimized
structure of compound 4aa has a planar backbone with a
frontier orbital that is distributed throughout the benzo[k]-
fluoranthene framework but not on the two perpendicular
phenyl rings.
Electrochemistry: The redox properties of the benzo[k]-
fluoranthene-based linear acenes were characterized by
Chem. Eur. J. 2010, 16, 5909 – 5919
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5915

Y.-T. Wu, M.-Y. Kuo et al.
Table 6. Cyclic voltammetric (CV) analyses for benzo[k]fluoranthene-based linear acenes.[a]
extended as efficiently as
might be expected (see rectan-
gle in Figure 4). The nonbond-
ing characteristics of, for ex-
ample, the LUMOs (indicated
by the arrow can see one for
24ae but not for 5aa) of 5aa
and 24ae suggest that the ef-
fective lengths of the isosur-
face distributions of their
LUMOs correspond to those
of anthracene and quasi-ben-
zo[k]fluoranthene, respectively,
and their LUMO potentials
are analogous to that of 4aa.
Similarly, the isosurface and
the potential of the HOMO of
17ac are very close to those of
4ac. Moreover, the HOMO
and LUMO of 28 fe are not
distributed throughout the
entire skeleton; its HOMO is
like that of benzo[1,2-k;4,5-
k’]difluoranthene 24ae and its
LUMO is akin to that of 17ac.
Qualitatively, these computa-
tional analyses of the HOMOs
and/or LUMOs of compounds
5, 17, 24, and 28 fit with the
results obtained by CV.
[b]
Entry
Cpd
Oxidation potential (Epa)[b]
Reduction potential (Epa)
1
2
3
4
5
6
7
8
4aa
4ac
5aa
1.06 (Ep) [1.42 vs. SCE][c]
0.91
0.73
0.83, 1.19 (Ep)
0.69, 1.08 (Ep)
0.78, 1.17 (Ep)
0.75, 1.08 (Ep)
1.03
0.87
0.62
0.58, 0.91 (Ep)
0.59
À2.36
À2.41
À2.36
15aa
15dc
15eb
15ec
17ac
18 fc
24ae
24je
28 fe
À2.50
À2.56
À2.52, À2.73 (Ep)
À2.51, À2.78
À1.96, À2.32
À1.94, À2.27
9
10
11
12
À2.38 (Ep), À2.60 (Ep), À2.98 (Ep)
À2.35 (Ep), À2.60 (Ep), À3.01 (Ep)
À1.87 (Ep), À2.30 (Ep), À2.57 (Ep)
[a] Experimental conditions: compounds (concentration ca. 10À3 m) in background electrolyte solution of
TBAPF6 (tetrabutylammonium hexafluorophosphate, 0.1m) in benzene/acetonitrile (9:1). Working electrode:
glass carbon (GC) electrode, reference electrode: Ag/AgNO3 [AgNO3 (0.1m) and NACHTNUGTRENUNG(nBu)4ClO4 (0.1m) in ace-
tonitrile], counter electrode: platinum wire. Scan rate: 50 mVs.À1 Internal standard: ferrocene. [b] Units: volt
vs. Fc/Fc+. [c] Measurement in benzene/acetonitrile (1:1) from ref. [7b].
Conclusion
Figure 3. Redox potentials determined by CV (in gray) and HOMO and LUMO levels generated from the
DFT computations (in black).
The
Pd-catalyzed
formal
[(2+2)+2] cycloadditions be-
tween diynes and aryl iodides
Substituents that are directly bound to the central aromat-
ic core therefore influence the photophysical and electro-
chemical properties more strongly than those positioned in
the phenyl side arm. However, with increasing conjugation
length of the central aromatic core, some p orbitals are not
offer a simple method for preparing highly substituted ben-
zo[k]fluoranthene-based linear acenes. The advantages of
the developed synthetic protocols include: 1) relatively
small numbers of synthetic steps, 2) easy introduction of
functional groups, and 3) linear extension of aromatic cores.
Figure 4. Isosurface plots of the frontier orbitals of 4aa, 4ac, 5aa, 17ac, 24ae, and 28 fe calculated at the B3LYP/6-311+G**//B3LYP/6-31G** level. Rec-
tangles and arrows indicate the effective lengths of frontier orbital distributions and nonbonding characteristic, respectively.
5916
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Chem. Eur. J. 2010, 16, 5909 – 5919

Benzo[k]fluoranthene-Based Linear Acenes
FULL PAPER
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These differences might be useful for screening for efficient
electroluminescence materials. Further studies of the elec-
troluminescence of these molecules are currently underway,
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light emitting diodes (OLEDs).[51]
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Experimental Section
Instruments: 1H and 13C NMR: Bruker 300 (300 and 75.5 MHz) and 400
(400 and 100 MHz). MS: Bruker Daltonics Apex II30. X-ray crystal
structure determination: the data were collected with a Stoe–Siemens-
AED diffractometer. Melting points were determined with a Bꢄchi B545
melting point apparatus and are uncorrected. The purities (>97%) of
samples for photophysical studies were determined by HPLC [Shimadzu
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This work was supported by the National Science Council of Taiwan
(NSC 96-2113M-006-008-MY2 and NSC-96-2113M-260-006-MY2). We
also thank Prof. S.-L. Wang and Ms. P.-L. Chen (National Tsing-Hua Uni-
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for High-performance Computing for providing computational resources.
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5917

Y.-T. Wu, M.-Y. Kuo et al.
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graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ACHTUNGTRENNUNG(OAc)2,
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5918
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Chem. Eur. J. 2010, 16, 5909 – 5919

Benzo[k]fluoranthene-Based Linear Acenes
FULL PAPER
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Received: September 30, 2009
Revised: January 9, 2010
Published online: April 15, 2010
Chem. Eur. J. 2010, 16, 5909 – 5919
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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