Photochemical synthesis and electronic spectra of fulminene ([6]phenacene)

Article abstract of DOI:10.1007/s11164-012-0639-1

Facile synthesis of fulminene ([6]phenacene) was achieved through the Mallory reaction of 1-(1-naphthyl)-2-(1-phenanthryl)ethene or the 9-fluorenone-sensitized photo-ring-closure of 1-(1-naphthyl)-2-(1-phenanthryl) ethane. The electronic spectral properties of fulminene were investigated for the first time using photoluminescence as well as transient absorption spectroscopy. The spectral features were compared with those of a series of lower phenacene homologs such as phenanthrene ([3]phenacene), chrysene ([4]phenacene), and picene ([5]phenacene). For the [n]phenacene series, both the fluorescence and phosphorescence bands linearly red-shifted with an increase in the number of the benzene rings (n). Trends in the energy levels of the excited singlet (E _S) and the triplet (E _T) states were expressed as E _s = -2.6n + 89.1 (kcal mol^-1) and E _T = -1.8n + 66.2 (kcal mol^-1), respectively. In the case of fulminene, laser flash photolysis displayed a transient spectrum with an absorption maximum (λ _max ^T-T) at 675 nm, which was assigned as the triplet fulminene excited state. The λ _max ^T-T values for the [n]phenacene series showed a linear correlation as a function of the ring number n, given by an equation, λ _max ^T-T = 60n + 318 (nm).

Full text of DOI:10.1007/s11164-012-0639-1

Res Chem Intermed (2013) 39:147–159
DOI 10.1007/s11164-012-0639-1
Photochemical synthesis and electronic spectra
of fulminene ([6]phenacene)
•
Hideki Okamoto Minoru Yamaji Shin Gohda
•
•
•
Kaori Sato Hisako Sugino Kyosuke Satake
•
Received: 30 September 2011 / Accepted: 31 October 2011 / Published online: 14 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Facile synthesis of fulminene ([6]phenacene) was achieved through the
Mallory reaction of 1-(1-naphthyl)-2-(1-phenanthryl)ethene or the 9-fluorenone-
sensitized photo-ring-closure of 1-(1-naphthyl)-2-(1-phenanthryl)ethane. The elec-
tronic spectral properties of fulminene were investigated for the first time using
photoluminescence as well as transient absorption spectroscopy. The spectral fea-
tures were compared with those of a series of lower phenacene homologs such as
phenanthrene ([3]phenacene), chrysene ([4]phenacene), and picene ([5]phenacene).
For the [n]phenacene series, both the fluorescence and phosphorescence bands
linearly red-shifted with an increase in the number of the benzene rings (n). Trends
in the energy levels of the excited singlet (ES) and the triplet (ET) states were
expressed as Es = -2.6n ? 89.1 (kcal mol-1) and ET = -1.8n ? 66.2 (kcal
mol-1), respectively. In the case of fulminene, laser flash photolysis displayed a
transient spectrum with an absorption maximum (kTm–aTx) at 675 nm, which was
assigned as the triplet fulminene excited state. The kTm–aTx values for the [n]phenacene
series showed a linear correlation as a function of the ring number n, given by an
equation, k
T–T
max
= 60n ? 318 (nm).
This article is dedicated to Professor Kazuhiko Mizuno in memory of his retirement.
H. Okamoto (&) ꢀ K. Satake
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology,
Okayama University, Tsushima-Naka 3-1-1, Okayama 700-8350, Japan
e-mail: hokamoto@cc.okayama-u.ac.jp
M. Yamaji (&)
Department of Chemistry and Chemical Biology, Graduate School of Engineering,
Gunma University, Kiryu, Gunma 376-8515, Japan
e-mail: yamaji@gunma-u.ac.jp
S. Gohda ꢀ K. Sato ꢀ H. Sugino
Material Science Research Group, NARD Institute, Ltd., Nishinagasu-cho 2-6-1, Amagasaki,
Hyogo 660-0805, Japan
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H. Okamoto et al.
Keywords Phenacene ꢀ Fulminene ꢀ Photocyclization ꢀ Mallory reaction ꢀ
Electronic spectra ꢀ Transient absorption spectra
Introduction
Aromatic molecules with an extended p-conjugated electronic system have attracted
considerable attention from the theoretical and experimental fields [1]. Polycyclic
aromatic compounds are divided into two major structural categories. The first
category comprises phenacenes, which contain benzene rings fused in a zigzag
manner; the second comprises acenes, which contain linearly bound benzene rings.
The structures of the phenacene and the acene conformations as well as those of the
phenacenes used in this study are shown in Fig. 1.
Recently, polycyclic aromatic molecules have been used as the active layer in
organic thin-film field-effect transistors (OFETs). The use of acenes, typified by
pentacene, in OFETs has been extensively investigated [2, 3]. However, acenes are
generally not stable enough to use under exposure of light and air, making the
practical application in electronic devices difficult. Thus, unsubstituted higher
acenes with n [ 5 were generated and characterized in a polymer film or in a noble-
gas matrix [4–6]. Therefore, an aromatic electronic material, which is stable under
atmospheric conditions, remains highly desired [7, 8].
Phenacenes are more stable compared with acene structures [9]. Therefore, the
phenacene structure has been expanded to an [11]phenacene skeleton, which was
reported to be stable [10–12]. Recently, phenacenes have become an important class
of compounds in the field of organic electronics. It was shown that picene 3
([5]phenacene) could be used as an active layer of a p-channel OFET with high
field-effect mobility, l = ca. 5 cm2 V-1 s-1 [13]. [7]Phenacene also served as an
active layer of OFET, showing a field-effect mobility of l = 0.75 cm2 V-1 s-1
[14]. As the picene and [7]phenacene thin-film OFETs showed sensitivity toward O2
and humidity, they were potentially applicable to practical devices such as gas
Fig. 1 General structures of phenacene and acene (upper), and the structures of [n]phenacenes (n = 3–6)
used in this study (lower)
123

Photochemical synthesis and electronic spectra of fulminene
149
sensors [15, 16]. Furthermore, picene 3 doped with an alkaline metal such as
potassium displayed superconductivity with a high superconducting transition
temperature (Tc = 18 K). The potassium-doped picene was the first aromatic
hydrocarbon-derived superconductor [17]. Very recently, phenanthrene 1 also
showed superconductivity (Tc = 5 K) upon potassium doping [18, 19]. These
findings clearly indicate that phenacenes and their derivatives are a novel and
promising class of compounds for organic electronics [20].
Phenacenes were discovered decades ago (i.e., picene 3 and fulminene 4 were
isolated from coal tar in the 1950s [21]). However, compared with acenes,
considerably less effort has been made to synthesize such higher phenacenes and
fabricate an electronic device utilizing them. Therefore, insufficient strategies for
systematic synthesis of higher phenacene skeletons and only limited information
about their electronic features, such as absorption spectra [21, 22], are currently
available. Therefore, it would be highly desirable to establish an efficient synthetic
pathway to a series of higher phenacenes, and to systematically elucidate their
electronic features. Such studies on phenacenes would initiate new chemistry,
physics, and material-science progress in the area of expanded aromatic organics
from the viewpoint of their application to electronic devices.
Conventionally, phenacene frameworks (e.g., chrysene 2 and picene 3) have been
constructed by photochemical ring closure of a stilbene-like diarylethene precursor
followed by an oxidative aromatization, also referred to as the Mallory reaction
(Scheme 1, path A) [23]. Previously, it was reported that phenacene skeletons
such as picene 3 ([5]phenacene) could be readily prepared through a 9-fluorenone
(9F)-sensitized photolysis of a diarylethane (Scheme 1, path B) [24]. These two
methodologies have yet to be applied to the preparation of fulminene 4
([6]phenacene). Furthermore, detailed electronic features of fulminene 4 have not
been experimentally investigated. Thus, preparation and electronic spectral
characterization of fulminene 4 would contribute to the design and development
of phenacene-based organic electronic devices.
In the present study, we investigated the preparation of fulminene 4 via both
the Mallory reaction and the 9F-sensitized photolysis. The characterization of
fulminene 4 using photoluminescence and transient absorption spectroscopy is also
reported here. Furthermore, the spectral features of fulminene 4 were compared with
those of lower phenacenes to establish correlation between the spectral behavior and
the structure of the phenacenes as related to the number of the benzene rings, n.
Scheme 1 Photochemical synthesis of chrysene 2 and picene 3. Reagents: a hm, I2, O2; b 9F, hm, CHCl3
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H. Okamoto et al.
Experimental
Instruments
1H and 13C nuclear magnetic resonance (NMR) spectra were collected on a Varian
400MR (400 MHz) or Varian NMR System 600 MHz (600 MHz) spectrometer.
Infrared (IR) spectra were measured using a Shimadzu IR Prestige-21 spectropho-
tometer. Absorption spectra were recorded on a Jasco U-best 50 or Jasco V-530
spectrophotometer. Emission spectra were recorded on a Hitachi fluorescence
spectrometer F-7000. Fluorescence quantum yields were determined by using an
absolute photoluminescence quantum yield measurement system (Hamamatsu
Photonics C9920-02). Fluorescence lifetimes were determined by using a time-
correlated single-photon counting fluorimeter (Hamamatsu Photonics TAU System).
Elemental analyses were performed using a PerkinElmer 2400II Analyzer in
the Micro Elemental Analysis Laboratory of Okayama University. Steady-state
photolysis was carried out using a Rayonet photoreactor (Southern New England
Ultraviolet Company, RPR-100) equipped with 350-nm fluorescent lamps (RPR-
3500, 16 9 16 W).
A Lextra 50 XeCl excimer laser (308 nm, Lambda Physik) was used as the
excitation light source for transient absorption spectral measurements. The details of
the detection system for the time profiles of the transient absorption have been
reported elsewhere [25]. The transient absorption spectra were obtained using a
Unisoku USP-554 system, which provided a transient absorption spectrum with one
laser pulse. The obtained transient spectral data were analyzed using the least-
squares best-fitting method.
CHCl3 (fluorimetry grade from Kanto Chemical) was used as solvent for
absorption and fluorescence measurements, while a mixture of methylcyclohexane
(UVasol, Dojin) and 2-methylbutane (Fluka) (MP, 3:1 v/v) was used for
phosphorescence measurements as a glass matrix at 77 K. All the samples (as
CHCl3 solution) in a quartz cell with 1-cm path length for measurements of transient
absorption, fluorescence quantum yields, and lifetimes were deaerated by bubbling
with extrapure argon gas for 20 min at room temperature. The optical density of the
samples was adjusted to be ca. 0.8 at 308 nm.
Materials
Compounds 9, 10, and 11 were prepared by the previously reported procedures [10].
Phenanthrene 1 and chrysene 2 were purchased from Aldrich and Wako Pure
Chemical Industries, Ltd., respectively. Picene 3 was prepared according to the
previously reported procedure [24, 26].
Preparation of fulminene 4 by the Mallory reaction To a solution of
phosphonium salt 11 (533 mg, 1.0 mmol) and 1-naphthaldehyde (156 mg,
1.0 mmol) in 20 mL CH2Cl2 was dropwise added a 50 % aqueous soln. of NaOH
(ca. 1 mL). The mixture was vigorously stirred at room temperature for 1 h. To the
resulting mixture was added small amount of anhyd. Na2SO4, and the salt was
123

Photochemical synthesis and electronic spectra of fulminene
151
filtered off. The filtrate was concentrated under reduced pressure, and the residue
was chromatographed on silica gel (hexane) to afford naphthylphenanthrylethene 12
as an E/Z mixture (314 mg). The mixture was used in the following Mallory
reaction without further separation or purification. A solution of compound 12
(314 mg) and iodine (240 mg) in toluene (80 mL) was irradiated at 350 nm under
aerated conditions for 13 h. The precipitated product 4 was collected and washed
with toluene (147 mg, 45 % from 11). An analytical sample was obtained by
sublimation under reduced pressure.
Colorless plates, mp 479 °C (determined by differential scanning calorimetry,
1
Lit. [21] 466–469 °C). H NMR (600 MHz, CDCl3, cf. Fig. 2) d 9.05 (d, 2H,
J = 9.1 Hz, H8,16), 9.00 (d, 2H, J = 9.1 Hz, H7,15), 8.89 (d, 2H, J = 7.6 Hz, H1,9),
8.88 (d, 2H, J = 9.1 Hz, H6,14), 8.08 (d, 2H, J = 9.1 Hz, H5,13), 8.04 (d, 2H,
J = 7.6 Hz, H4,12), 7.77 (t, 2H, J = 7.6 Hz, H2,10), 7.69 (t, 2H, J = 7.6 Hz, H3,11).
IR (neat) mmax 3,086, 3,050, 3,027, 1,601, 1,439, 1,428, 1,278, 1,269, 807, 762,
741 cm-1. Anal. Found: C, 95.02; H, 4.59 %. Calc. for C26H16: C, 95.09; H, 4.91.
1-(1-Naphthyl)-2-(1-phenanthryl)ethane 13 A mixture of compound 12 (247 mg,
0.75 mmol) and 5 % Pd/C (38 mg) in an AcOEt–EtOH mixture (1:1 v/v, 100 mL)
was stirred under an atmosphere of hydrogen for 18 h. The Pd/C was filtered off,
and the filtrate was concentrated under reduced pressure. The residue was
chromatographed on silica gel (hexane) to afford naphthylphenanthrylethane 13
(235 mg, 94 %).
1
Colorless crystals, mp 131–132 °C. H NMR (600 MHz, CDCl3) d 8.76 (d, 1H,
J = 8.4 Hz), 8.67 (d, 1H, J = 8.4 Hz), 8.19 (d, 1H, J = 6.0 Hz), 8.10 (d, 1H,
J = 9.0 Hz), 7.94 (m, 2H), 7.84 (d, 1H, J = 9.0 Hz), 7.80 (d, 1H, J = 7.4 Hz), 7.70
(ddd, 1H, J = 8.4, 7.8, 1.3 Hz), 7.65 (t, 1H, J = 7.8 Hz), 7.62-7.54 (m, 3H), 7.49
(d, 1H, J = 7.2 Hz), 7.44 (t, 1H, J = 7.2 Hz), 7.38 (d, 1H, J = 7.2 Hz), 3.59 (m,
4H). 13C NMR (150 MHz, CDCl3) d 138.7, 137.9, 133.9, 131.8, 131.6, 130.8, 130.7,
130.1, 128.9, 128.4, 127.1, 126.9, 126.8, 126.6, 126.5, 126.2, 125.95, 125.93, 125.6,
125.5, 123.6, 122.9, 122.3, 121.2, 34.5, 34.4. IR (neat) mmax 3,045, 2,937, 1,597,
1,508, 1,465, 1,456, 1,396, 827, 204, 794, 779, 756 cm-1. Anal. Found: C, 93.83; H,
5.89 %. Calc. for C26H20: C, 93.94; H, 6.06.
Fig. 2 1H NMR spectrum (aromatic region) of fulminene 4 (600 MHz, CDCl3)
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H. Okamoto et al.
Preparation of fulminene 4 by the 9F-sensitized photolysis of naphthylphenan-
thrylethane 13 A solution of naphthylphenanthrylethane 13 (99.7 mg, 0.30 mmol)
and 9F (162 mg, 0.90 mmol) in 15 mL CHCl3 was purged with nitrogen and
irradiated at 350 nm for 3 days. The precipitate formed was collected and washed
with CHCl3 to afford fulminene 4 (7.4 mg, 7.5 %). The 1H NMR spectral data were
identical to those observed for the product obtained in the above-mentioned
photolysis of naphthylphenanthrylethene 12.
Results and discussion
Preparation of fulminene 4
The synthetic pathways for fulminene 4 are shown in Scheme 2. Phosphonium
bromide 11 was prepared by the previously reported procedure [10]: 1-Methylph-
enanthrene 9 was brominated with N-bromosuccinimide (NBS) in the presence of
benzoylperoxide (BPO) to afford 1-bromomethylphenanthrene 10, which was
treated with triphenylphosphine to form phosphonium salt 11. A Wittig reaction of
the phosphonium salt 11 and 1-naphthaldehyde using NaOH as a base in a CH2Cl2-
1
H2O mixture produced naphthylphenanthrylethene 12. The H NMR spectrum of
the obtained product 12 displayed complex signals, presumably because the product
contained both E- and Z-isomers. Therefore, the structure of compound 12 was only
confirmed after hydrogenation of the double bond: Compound 12 was reacted
with hydrogen in the presence of Pd/C to afford naphthylphenanthrylethane 13 in
1
94 % yield. The structure of compound 13 was confirmed by H and 13C NMR
spectroscopy as well as elemental analysis. From the characterization of
naphthylphenanthrylethane 13, compound 12 was established to possess the
naphthylphenanthrylethene framework.
First, the Mallory photocyclization [23] of compound 12 was investigated
(Scheme 2, path A). A solution of compound 12 and iodine in aerated toluene was
irradiated at 350 nm and room temperature. After 13 h of photolysis, fulminene 4
was obtained as a precipitate in 45 % yield based on phosphonium salt 11. The
structure of fulminene 4 was confirmed by 1H NMR and IR spectroscopy as well as
elemental analysis. The analytical sample was obtained by sublimation under
reduced pressure [26]. IR spectrum of fulminene 4 was identical to the reported data
[21]. It has been noted that, due to fulminene’s poor solubility in organic solvents,
1H NMR measurement was not possible [22]. However, by using a high-field NMR
spectrometer (600 MHz), we successfully obtained the 1H NMR spectrum; the
results are shown in Fig. 2. The signals of the protons at the bay regions (H1,9, H6,14,
H7,15, H8,16) appear at lower field (8.9–9.1 ppm), while the protons at both edges of
the molecule (H2,10, H3,11) show two triplet signals at higher field (*7.7 ppm).
These features are consistent with the reported 1H NMR spectra of related
phenacenes [22, 24]. Unfortunately, the 13C NMR spectrum of fulminene 4 could
not be observed due to poor solubility.
Subsequently, the 9F-sensitized photocyclization [24, 26] of naphthylphenan-
thrylethane 13 was investigated (Scheme 2, path B). When a CHCl3 solution of
123

Photochemical synthesis and electronic spectra of fulminene
153
Scheme 2 Synthesis of fulminene 4. Reagents: (a) NBS, BPO, CCl4, 74 %; (b) PPh3,
dimethylformamide (DMF), 91 %; (c) 1-naphthaldehyde, NaOH aq., CH2Cl2; (d) H2, Pd/C, AcOEt–
EtOH, 94 %; (e) hm, I2, O2, 45 % from 11; (f) 9F, hm, CHCl3, 7.5 %
compound 13 and 9F was irradiated at 350 nm and room temperature for 3 days,
fulminene 4 was obtained as a precipitate in 7.5 % yield. The spectral data of
the fulminene 4 obtained by the 9F-sensitized photocyclization were identical to
those prepared by the Mallory reaction. Although the yield was not high, it was
discovered that the 9F-sensitized photocyclization of 1,2-diarylethane [24] was
useful for systematic synthesis of [n]phenacene skeletons.
Electronic spectra of [n]phenacenes
Figure 3 shows the electronic absorption, fluorescence, and phosphorescence
spectra of a series of [n]phenacenes (n = 3–6) in nonpolar media. All the spectra
consist of well-resolved vibrational structures. The 0–0 origins of the absorption and
fluorescence spectra are clearly seen with a Stokes shift of \300 cm-1. The
excitation spectra for the fluorescence and phosphorescence were identical to the
corresponding absorption spectrum of phenacenes.
The lowest excited singlet state energies (ES) in the phenacenes were determined
from the averaged energies of the 0–0 origins of the corresponding absorption and
fluorescence spectra while the excited triplet state energies (ET) were estimated
from the 0–0 origins of the phosphorescence spectra. The obtained values of ES
ABS FL
and ET are listed in Table 1 along with the 0–0 transition wavelengths (k , k0–0
)
0–0
and the Stokes shift. Also, the lifetimes (sf), quantum yields (Uf), and rate constants
(kf) of fluorescence determined in the present work are listed. Additionally, the
photophysical parameters of the related acenes are summarized in Table 2. The rate
constants of fluorescence (kf) were obtained by Eq. 1.
kf ¼ Ufsꢁf 1
:
ð1Þ
123

154
H. Okamoto et al.
Fig. 3 Absorption (black line)
and fluorescence (blue line)
spectra in CHCl3 at 295 K and
phosphorescence (red line)
spectra (normalized) of
phenanthrene 1 (a), chrysene 2
(b), picene 3 (c), and fulminene
4 (d) in a 3:1
methylcyclohexane:2-
methylbutane mixture at 77 K.
The intensity of the absorption
spectra in the longer wavelength
region was arbitrarily enlarged
for clear expression. (Color
figure online)
The estimated kf values are all in the magnitude of 106 s-1, which indicates that
the fluorescence process is of p,p* type.
Transient absorption spectra of the phenacenes were obtained by means of flash
photolysis techniques using a nanosecond laser pulse. Figure 4 compares the
transient absorption spectra obtained at 100 ns upon 308-nm laser photolysis of
degassed CHCl3 solution of phenacenes at 295 K. The intensity of all the transient
absorption decreased within tens of microseconds, and the decay was accelerated by
the presence of dissolved oxygen. From these observations, the obtained transient
signals can be ascribable to the triplet–triplet (T–T) absorption of [n]phenacenes.
The absorption maximum wavelengths (kTm–aTx) of the obtained T–T absorption
spectra are listed in Table 1.
It would be of interest to find the relationship between the obtained spectroscopic
features and the number of benzene rings, n, in the phenacene series. Additionally,
this relationship was compared with that of an [n]acene series reported previously to
clarify the difference in the spectral behavior between phenacene and acene series.
In Fig. 5a, b, the ES and ET, and kTm–aTx values are, respectively, plotted as a function
T–T
max
of n. With increasing n values, the ES and ET values linearly decrease, and the k
T–T
max
values increase. Therefore, ES, ET, and k are expressed using Eqs. 2–4 and 5–7
as a function of n for the [n]phenacene and [n]acene series, respectively.
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Photochemical synthesis and electronic spectra of fulminene
155
Table 1 Photophysical parameters of [n]phenacenes (n = 3–6)a
ABS
0–0
FL
0–0
c
d
T–T
k
max
(nm)
Compd. (n)
k
k
Stokes
(nm) (nm) shift
(cm)-1
UF
sf
(ns) (106 s-1
kfb
ES
(kcal mol-1
ET
(kcal mol-1
)
)
)
Phenanthrene 347
1 (3)
350
365
247
0.049 18.9 2.6
0.045 10.3 4.4
81.9
78.5
61.8
57.0
490
570
Chrysene 2
(4)
362
227
Picene 3 (5)
376
382
380
386
280
271
0.088 17.0 5.2
0.120 16.0 7.5
75.5
74.3
57.3
55.6
615
675
Fulminene 4
(6)
a
In CHCl3 at 295 K
b
c
d
Estimated by kf = Ufsf-1
Estimated from the 0–0 origins of the absorption and fluorescence spectra
Estimated from the 0–0 origins of phosphorescence spectra in a mixture of methylcyclohexane and
2-methylbutane (3:1 v/v) at 77 K
Table 2 Photophysical parameters of [n]acenes (n = 3–5)a
kfb (106 s-1
)
ES (kcal mol-1
)
ET (kcal mol-1
)
(nm)
T–T
k
max
Compd. (n)
UF
sf (ns)
Anthracene (3)
Tetracene (4)
Pentacene (5)
a
0.30
0.17
0.08
5.3
6.4
–
56
26
–
77.2
58.6
49.0
42.5
29.3
17.9
433
465
505
Data taken from [27]
b
Estimated by kf = Ufsf-1
For the [n]phenacene series:
À
Á
Es ¼ ꢁ2:6n þ 89:1 kcal molꢁ1
;
ð2Þ
ð3Þ
ð4Þ
À
Á
ET ¼ ꢁ1:8n þ 66:2 kcal molꢁ1
;
TꢁT
k
¼ 60n þ 318 ðnmÞ:
max
For the [n]acene series:
À
Á
Es ¼ ꢁ13:6n þ 116 kcal molꢁ1
;
ð5Þ
ð6Þ
ð7Þ
À
Á
ET ¼ ꢁ12:4n þ 79:4 kcal molꢁ1
;
TꢁT
k
¼ 36n þ 324 ðnmÞ:
max
According to the plots of ES and ET versus n, the slopes for the phenacene series
were smaller than for the acene series. This finding indicates that, on increasing the
number of benzene rings in acenes and phenacenes, the energy levels of the excited
states of phenacenes are less influenced than those of acenes. The decreased
influence is attributed to the difference in the molecular structures, i.e., zigzag
versus linear. The slopes for the shift in the kTm–aTx values were appreciably the same
123

156
H. Okamoto et al.
Fig. 4 Transient absorption
spectra obtained at 100 ns after
308-nm laser photolysis of
phenanthrene 1 (a), chrysene 2
(b), picene 3 (c), and fulminene
4 (d) in CHCl3
Fig. 5 a Plots of the excited singlet (ES) and triplet (ET) state energies for [n]phenacenes and [n]acenes
as a function of n; ES (filled circle) and ET (filled square) of [n]phenacenes, ES (open circle) and ET (open
T–T
max
square) of [n]acenes. b k
of [n]phenacenes (filled square) and [n]acenes (open square) plotted as a
function of n. Data for [n]acenes quoted from Ref. [27]
between the phenacene and acene series. However, the value for phenacene at a
certain n number tends to drift towards longer wavelength regions compared with
the corresponding acene series. The relationship between ES, ET, kTm–aTx, and
n expressed using Eqs. 2–7 will be useful for estimating those values for phenacenes
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Photochemical synthesis and electronic spectra of fulminene
157
and acenes with n values greater than 7, because they are anticipated to have very
low solvent solubility. Moreover, acenes with n [ 5 are known to be too unstable to
undergo spectroscopic measurements in solution at room temperature [4–6].
It has been noted that, for the phenacene and acene series, the absorption band
corresponding to the highest occupied molecular orbital (HOMO)–lowest unoccu-
pied molecular orbital (LUMO) gap linearly red-shifted as a function of n as
expressed by Eqs. 8 and 9 [11, 28].
For the [n]phenacene series:
kmax ¼ 9n þ 279 ðnmÞ:
ð8Þ
ð9Þ
For the [n]acene series:
kmax ¼ 98n þ 88 ðnmÞ:
Thus, it can be concluded that, for phenacenes with higher n values, the
electronic properties concerning absorption, fluorescence (ES), phosphorescence
(ET), and T–T absorption (kTm–aTx) can be predicted by Eqs. 2–4 and 8.
Conclusions
Fulminene 4 ([6]phenacene) was prepared by both the Mallory reaction of
naphthylphenanthrylethene 12 and the 9F-sensitized photocyclization of naphthyl-
phenanthrylethane. The photoluminescence spectra for the series of [n]phenacenes
(n = 3–6) were obtained and compared under the same conditions. The fluores-
cence and phosphorescence spectra red-shifted with an increase of n, thus linear
relationships between n and the excited state energy levels were determined as
Es = -2.6n ? 89.1 (kcal mol-1) and ET = -1.8n ? 66.2 (kcal mol-1). The triplet
excited state for fulminene, which displayed a T–T absorption band at 675 nm, was
detected using transient absorption measurements. The kTm–aTx also linearly red-shifted
T–T
max
depending on n, with the relationship k
= 60n ? 318 (nm). These results
indicate that the photoluminescence and T–T absorption properties of higher
phenacenes could be predicted. The linear correlations reported here could provide
useful property estimations for future phenacenes and provide guidance in designing
organic electronic molecular devices using higher phenacene skeletons.
Acknowledgments Financial support through Grants-in-Aid for Scientific Research (KAKENHI) from
JSPS (no. 23350059 to M.Y.), the Adaptable and Seamless Technology Transfer Program (A-STEP), FS
Stage, Exploratory Study from JST (AS231Z01256D to H.O.) and the‘‘Element Innovation’’ Project by
the Ministry of Education, Culture, Sports, Science, and Technology, Japan (to M.Y.) are gratefully
acknowledged. The authors thank the SC-NMR Laboratory of Okayama University for the NMR spectral
measurements.
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