Systematic investigations on fused π-system compounds of seven benzene rings prepared by photocyclization of diphenanthrylethenes

Article abstract of DOI:10.1039/c7pp00040e

We studied the photoproducts of 1-(n-phenanthryl)-2-(m-phenanthryl)ethenes (nEm; n, m = 1, 3 and 9) for understanding photocyclization patterns based on NMR spectroscopy. The crystal structures of the photoproducts were analyzed by X-ray crystallography, and the photophysical features of the photocyclized molecules were investigated based on emission and transient absorption measurements. Phenanthrene derivatives substituted at the 1- and 3-positions were prepared for synthesizing nEm by photocyclization of stilbene derivatives. We obtained four types of primary photoproducts (n@m) from the corresponding nEm. Two of them were found to have racemic molecular structures in the single crystal determined by X-ray crystallography. Besides the primary photoproducts, two types of secondary photoproducts (n@mPP) were isolated. Fluorescence quantum yields and lifetimes of the obtained photoproducts were determined in solution whereas the definite fluorescence quantum yields were obtained in the powder. Observation of the triplet-triplet absorption spectra in solution by laser photolysis techniques showed that intersystem crossing to the triplet state competes with the fluorescence process.

Full text of DOI:10.1039/c7pp00040e

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Page 1 of 23
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DOI: 10.1039/C7PP00040E
Systematic investigations on fused πꢀsystem compounds of seven
benzene rings prepared by photocyclization of diphenanthrylethenes
Shota Fujino,1 Minoru Yamaji,2,* Hideki Okamoto,3 Toshiki Mutai,4 Isao Yoshikawa4, Hirohiko
Houjou4 and Fumito Tani5
1
Education Program of Materials and Bioscience, Graduate School of Science and Engineering,
Gunma University, Kiryu, Gunma 376ꢀ8515, Japan
2
Division of Molecular Science, Graduate School of Science and Engineering, Gunma University,
Kiryu, Gunma 376ꢀ8515, Japan
3
Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Sciences and
Technology, Okayama University, Okayama 700ꢀ8530, Japann
4 Department of Materials and Environmental Science, Institute of Industrial Science, the University of
Tokyo, Meguro, Tokyo 153ꢀ8505, Japan
5 Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819ꢀ0395, Japan
Abstract
We studied photoproducts of 1ꢀ(nꢀphenanthryl)ꢀ2ꢀ(mꢀphenanthryl)ethenes (nEm; n, m = 1, 3 and 9) for
understanding the photocyclization patterns based on NMR spectroscopies. The crystal structures of
the photoproducts were analyzed by Xꢀray crystallography, and the photophysical features of the
photocyclized molecules were investigated based on emission and transient absorption measurements.
Phenanthrene derivatives substituted at the 1ꢀ and 3ꢀpositions were prepared for synthesizing nEm by
photocyclization of stilbene derivatives. We obtained four types of primary photoproducts (n@m) from
the corresponding nEm. Two of them were found to have racemic molecular structures in the single
crystal determined by Xꢀray crystallography. Besides the primary photoproducts, two types of
secondary photoproducts (n@mPP) were isolated. Fluorescence quantum yields and lifetimes of the
obtained photoproducts were determined in solution whereas the definite fluorescence quantum yields
were obtained in the powder. Observation of the tripletꢀtriplet absorption spectra in solution by laser
photolysis techniques showed that intersystem crossing to the triplet state competes with the
fluorescence process.
1

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DOI: 10.1039/C7PP00040E
1. Introduction
Polycyclic aromatic hydrocarbons (PAH), such as helicene, acene and phenacene are attractive not
only to organic chemists but also to material chemists from the viewpoint of the synthetic strategy,
geometric interests and applications to organic electronic materials. Since the first synthesis of
helicene molecules reported in 1909,1 a large number of synthetic and application investigations on a
varieties of helicenes have been reported, which can be found in recent reviews.2ꢀ7 A variety of
synthetic procedures for preparing helicenes, especially, [7]helicene have been proposed8ꢀ14 as it is the
base beginning the helical turn. One of the synthetic strategies is photocyclizing diarylethenes in
solution. This procedure has been used for forming the phenanthrene skeleton from stilbene derivatives
in the presence of oxygen and iodine (Scheme 1).15
Scheme 1. Photocyclization process of stilbene to phenanthrene.
Because helicenes are generally constructed of ortho annulation of benzene, they are considered as
phenanthrene derivatives to which the photocyclization process is applicable. Recently, oneꢀstep
photochemical synthesis of [16]helicene using this process was reported.16 Conversely, the
photocyclization process cannot be applied to acenes having linearly aligned benzene rings, such as
anthracene, tetracene and pentacene for expanding the πꢀelectron systems. In these two decades,
acenes have been paid much attention17 because of the potentials for organic semiconductor materials,
that is, active layers of organic fieldꢀeffect transistors (OFET)18ꢀ21 although they are generally unstable
upon exposure to oxygen and light.22ꢀ24 There have been reported a number of synthetic investigations
including photochemical approaches to produce higher acenes.22ꢀ29 In contrast to linearly arranged
acenes, less attention has been paid to phenacenes, such as phenanthrene, chrysene, picene, fulminene,
etc. bearing benzene rings fused in a zigzag array. Large phenacenes had not been subjected for
application to functional materials because of the difficulties in the organic synthesis processes until
2

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our recent finding of a facile preparation method for phenacenes using photochemical reactions of
diarylethanes sensitized with aromatic carbonyls (photosensitized cyclization method).30 With this
technique, we have shown facile formation of phenacenes (chrysene ([4]phenacene), picene
([5]phenacene) and fulminene ([6]phenacene).30, 31 We also reported efficient preparation of a variety
of phenacene derivatives by means of the direct photoirradiation of diarylethenes using a homeꢀmade
flow photoreactor (direct method).32 We have unveiled that some of phenacenes show excellent OFET
performance33, 34 and that potassiumꢀdoped picene shows superconductivity at 20 K.35 Because
phenacenes are quite stable on exposing to oxygen and light, they are promising organic electronic
materials.36 It is of an additional interest in the photophysical aspects that picene provides fluorescence
emission from the second lowest excited singlet (S2) state in the vapor phase.37
The synthetic procedure of photolyzing diarylethenes for forming cyclized PAHs, as mentioned
above, seems to be useful for preparing varieties of helicenes and phenacenes. In the present research,
we prepared 1ꢀ(nꢀphenanthryl)ꢀ2ꢀ(mꢀphenanthryl)ethenes (nEm; n, m = 1, 3 and 9, see Chart 1) as
precursors, and conducted the photocyclization for understanding the photocyclization patterns. The
direct irradiation method was employed for preparing the phenanthrene component. Some of the
photocyclized compounds were succeeded in analyzing the crystal structures. The photophysical
features of the photocyclized PAHs were investigated based on fluorescence and transient absorption
measurements with the aid of theoretical computations.
Chart 1. Molecular structures and the abbreviations of 1ꢀ(nꢀ
phenanthryl)ꢀ2ꢀ(mꢀphenanthryl)ethenes (nEm) used in this
study.
3

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2. Experimental
Analytical Instruments. NMR spectra were recorded on 400 and 600 MHz NMR spectrometers.
Melting points were obtained with YANACO micro melting point apparatus.
Materials. Phenanthreneꢀ9ꢀaldehyde was commercially purchased. Diphenanthrylethenes (nEm) were
synthesized according to the procedures described in the Electronic Supplementary Information (ESI).
Cyclohexane (spectroscopic grade from Kanto), chloroform (spectroscopic grade from Kanto) and
ethanol (EtOH, spectroscopic grade from Kishida) were used as the solvent for the spectral
measurements without further purification.
Absorption, emission and transient absorption measurements. Absorption spectra were recorded
on a Vꢀ550 (JASCO) spectrophotometer whereas emission spectra were on a Hitachi Fꢀ4010
fluorescence spectrophotometer. Nanosecond fluorescence lifetimes (τf) were determined using a timeꢀ
correlated singleꢀphoton counting fluorimeter (C12132 from Hamamatsu Photonics). The fluorescence
quantum yields (Φf) were determined by using an absolute luminescence quantum yield measurement
system (C9920ꢀ02) from Hamamatsu Photonics. Samples in a quartz cell with a 1 cm path length were
prepared in the dark, and Arꢀpurged when necessary. Fourth harmonics (266 nm) from a Nd:YAG
laser system (9 mJ / pulse, LotisꢀTII, LTꢀ2137) was used as the excitation laser light sources. The
details of the detection system for the time profiles of the transient absorption have been reported
elsewhere.38 Transient absorption spectra were obtained using a Unisoku USPꢀT1000ꢀMLT system,
which provides a transient absorption spectrum with one laser pulse. The obtained transient spectral
data were analyzed using the leastꢀsquares bestꢀfitting method.
Theoretical calculations. The calculation was carried out at the DFT level, using the Gaussian 09
software package.39 The geometries of the compounds were fully optimized by using the 6ꢀ31+G(d)
basis set at the B3LYP method in vacuum. Atom coordinates for the optimized geometries are
deposited in the ESI.
4

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3. Result and Discussion
Photochemical preparations
for
diphenanthrylethenes.
Synthetic
routes to the
phenanthrenecarbaldehydes (PhenCHOs) and (phenathrylmethyl)phosphonium bromides (PhenPBs) as
the precursors to nEms are shown in Scheme 2.
Scheme 2. Preparation of phenanthrenecarbaldehydes (PhenCHOs) and phosphonium bromides
(PhenPBs).
1ꢀ And 3ꢀmethylstilbenes (MSs) were prepared by Wittig reaction of benzaldehyde with
(tolylmethyl)triphenylphosphonium bromides (TolPBs). Subsequently, MSs were subjected to
photocyclization to afford methylphenanthrenes (MPs). The methyl group of MPs was brominated by
Nꢀbromosuccinimide (NBS) and benzoylperoxide (BPO) to give (bromomethyl)phenanthrenes
(BMPhens). The bromomethyl functionality was converted to phosphonium salts (PhenPBs) by
substitution reaction with PPh3 and to phenathrenecarbaldehydes (PhenCHOs) by Sommelet reaction.
Commercially available 9PhenCHO was reduced by NaBH4 to 9PhenM. The hydroxyl moiety was
brominated to 9BMPhen which was then substituted by PPh3 to afford phosphonium salt (9PhePB).
5

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1ꢀ(nꢀPhenanthryl)ꢀ2ꢀ(mꢀphenanthryl)ethenes (nEm; n, m = 1, 3 and 9) were prepared via Wittig
reaction between the phenanthrenecarbaldehyde and the (phenanthrylmethyl)triphenylphosphonium
bromides (Scheme 3).
Scheme 3. Preparation of nEm via Wittig reaction between phenanthrenecarbaldehyde
(PhenCHO) and (phenanthrylmethyl)triphenylphosphonium bromide (PhenPB).
Photoproducts of diphenanthrylethenes (nEm). We performed photolysis of nEm with the use of a
homeꢀmade flow photoreactor which could suppress photodecomposition of the primary
photoproduct.32, 40ꢀ42 Photolysis of 1E1 using the photoreactor provided the unique photoproduct, 1@1
([7]phenacene, Scheme 4) with a moderate yield (59 %).
1E1
1@1 ([7]Phenacene)
Scheme 4. Photochemical formation of 1@1 from 1E1.
Similarly, photolysis of 1E9 using the photoreactor provided the unique photoproduct, 1@9 (Scheme
5) with a moderate yield (55 %).
1E9
1@9
Scheme 5. Photochemical formation of 1@9 from 1E9.
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We found that photolysis of 1E1 and 1E9 provides the corresponding unique photoproduct for the first
time. Therefore, photolysis of 9E9 was expected to produce 9@9 (see Scheme 6), but the secondary
photoproduct (9@9PP, see also Scheme 6) of 9@9 was actually isolated with a good yield (89 %) by
using the flow photolysis.
9E9
9@9
9@9PP
Scheme 6. Photochemical formation of 9@9PP via 9@9.
The photochemical formation of 9@9PP from 9E9 is reported in the literature.15 The formation of
9@9PP indicates that the secondary photocyclization of 9@9 readily proceeded immediately after the
formation of 9@9.
Formation of two types of photocyclized steric isomers (1@3 and 1@3ꢀSI, see Scheme 7) were
expected upon photolysis of 1E3. We, however, obtained 1@3PP as the solo product (Scheme 7).
Scheme 7. Plausible photochemical processes for forming 1@3PP via 1@3.
The steric isomer of 1@3 (1@3ꢀSI) was not formed.
The expected photoproducts, 1@3 and 1@3ꢀSI were not obtained during the photolysis of 1E3, but
1@3PP was obtained in 22% yield. The formation of 1@3PP convinces that the preferable
photocyclization process of 1E3 is forming not 1@3ꢀSI but 1@3.
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The [5]helicene skeleton can be seen in the molecular structures of 1@3 and 9@9, both of which
underwent further photocyclization leading to the photoproducts, 1@3PP and 9@9PP, respectively.
These considerations predicted that the primary photocyclization process preferentially occurs for
forming the helicene skeletons. We, however, had controversial results against the above criterion for
the photocyclization pattern of diphenanthrylethenes by using 3E3 and 3E9 (Scheme 8).
3E3
3@3
3@3-SI1
3@3-SI2
([7]Helicene)
3E9
3@9
3@9-SI
Scheme 8. Photochemical processes for forming 3@3 (upper) from 3E3 and 3@9 from
3E9 (lower). The steric isomers of 3@3 (3@3ꢀSI1 and ꢀSI2) and 3@9 (3@9ꢀSI) were not
formed.
Photochemical formation of 3@3 ([7]helicene) from 3E3 is reported in the literature15 while that of
3@9 is seen in another literature.43
X-ray crystallography for 1@9 and 3@9. We succeeded in preparing single crystals that enabled to
collect the crystallographic data. Figure 1 shows an ORTEP drawing and molecular packing of 1@9
(Pbca, Z = 8).
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Figure 1. Crystal structure of 1@9. An ORTEP drawing (a), molecular packing viewed from ab
plane (a) and molecular arrangement in the layer A and B (c).
The detailed crystallographic data are summarized in Table S1 in the ESI. A molecule forms twisted
conformation with a dihedral angle of ca. 34° between the edge ring (X) and the outꢀofꢀplane ring (Y)
(Figure 1a). As shown in Figures 1b and c, molecules formed ABABꢀtype layered structure. They were
arranged similarly in headꢀtoꢀtail, herringꢀbone like manner in each layer, and stacking of the layer A
and B cancelled out their nonꢀsymmetric nature. It is remarkable that πꢀπ stacking motif was not found
in the crystal.
Xꢀray crystallographic analysis was also performed on 3@3 (P21/n, Z = 4) and 3@9 (P21, Z = 2). It
was found that the obtained crystal structure of 3@3 was the same as the previous report.44 ORTEP
drawings of both compounds are illustrated in Figures S1b and c in the ESI. For both compounds, it is
clear that molecular conformations in crystal are very similar to the optimized one in vacuum, which
will be mentioned later, showing absence of packing effect in the crystal. In crystal of 3@9, molecules
were arranged in antiꢀparallel manner in the ac plane (Figure 2a), and formed columnar structures
along b axis (Figure 2b). The distance between πꢀplanes was 3.13 Å, but no πꢀπ interaction was
observed because of the largely slipped stacking.
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Figure 2. Crystal structure of 3@9 viewed from ac plane (a) and along b axis (b).
Molecular structures optimized with DFT calculations.
Figure 3 shows molecular configurations of 1@1([7]phenacene) in the ground state optimized by DFT
calculations.
Figure 3. Molecular configurations of 1@1([7]phenacene) in the ground state
optimized by DFT calculations.
The molecular configuration of 1@1 is calculated to be planar.
The molecular configuration of 1@9 in the ground state optimized by DFT calculations is shown in
Figure 4.
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Figure 4. Molecular configuration of 1@9 in the ground state optimized by
DFT calculation.
The conformation of the fulminene ([6]phenacene) skeleton in 1@9 is no more planar, but distorted
because of steric interaction between the benzo moiety and the fulminene framework. The
experimentally determined conformation in crystal (Figure S1a in the ESI) was nearly identical with
the DFTꢀcalculated structure in vacuum (Figure 4). These results indicate that 1@9 might not suffer
from obvious packing stress in the crystalline state.
Molecular geometries of 9@9 and 9@9PP in the ground state were optimized by DFT calculations.
The results are deposited in the ESI as Figure S3. Interestingly, the molecular geometry of 9@9 is
twisted whereas that of 9@9PP is planar. The difference in the geometries between 9@9 and 9@9PP
indicates that during the photocyclization, the helical structure of 9@9 changed to be planar in 9@9PP.
Molecular geometry of 1@3 and 1@3PP in the ground state optimized by DFT calculations can be
seen in the ESI as Figure S4. Similar to the case of 9@9 vs. 9@9PP, the molecular geometry of 1@3
that is the plausible initial photoproduct of 1E3 is calculated to be tilted whereas that of 1@3PP to be
unambiguously planar. The helical structure of 1@3 also changed into the planar one in 1@3PP during
the photocyclization.
Molecular geometry of 3@3 and 3@9 in the ground state optimized by DFT calculations are shown
in Figure S5 in the ESI. The conformation of 3@9 in crystal was nearly identical with the DFTꢀ
simulated one, indicating that 3@9 might not suffer from obvious packing stress in crystalline state as
in the case of 1@9.
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Absorption and emission measurements. Figure 5 shows absorption and emission spectra of the
photocyclized compounds obtained in the present work.
Figure 5. Absorption (black) and fluorescence (blue) spectra in
cyclohexane for 1@3PP, 1@9, 3@3 and 3@9, and in CHCl3 for
1@1 and 9@9PP at 295 K, and phosphorescence spectra (red) in
ethanol at 77 K. The emission spectra are not corrected.
Phosphorescence from 1@3PP was absent in ethanol at 77 K. We
were unable to prepare a sample for measuring phosphorescence
from 1@1 because of the low solubility in ethanol.
The absorption and fluorescence spectra of the studied compounds have wellꢀresolved vibrational
structures. No phosphorescence from 1@3PP was seen in ethanol at 77 K while we were unable to
prepare a sample for observing phosphorescence from 1@1 because of the low solubility in ethanol.
Observation of phosphorescence indicates that intersystem crossing (ISC) from the lowest excited
singlet (S1) state to the lowest triplet (T1) state is efficient in 1@9, 3@3, 3@9 and 9@9PP. The ISC
process for 1@1 and 1@3PP was investigated by observing the triplet absorption with lasse photolysis
techniques, which will be mentioned later.
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Based on the quantum yields, Φf and lifetimes, τf of fluorescence determined in cyclohexane and
CHCl3, the fluorescence rates, kf, quantum yields, Φnr and rates, knr for nonradiative processes were
evaluated by eqs 1–3, respectively.
kf = Φfτf−1 (1)
Φnr = 1 − Φf (2)
knr = Φnrτf−1 (3)
The obtained photophysical parameters are listed in Table 1 along with the absorptivities, ε in
cyclohexane. The S1 and T1 energies, ES and ET were determined from the origins of the absorption
and fluorescence, and phosphorescence spectra, respectively, and also listed in Table 1.
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Table 1. Photophysical parameters obtained for the studied compounds in cyclohexane.
ε
e
ES
a
b
d
f
Powder g
τT
kf
knr
ET
λflu
/ nm
τf
λflu
/ nm
/ kcal molꢀ
c
/ dm3 molꢀ1 cmꢀ1
(λabs / nm)
Φf
Φnr
ΦfPowder g
Compound
/ 106 sꢀ1
/ 107 sꢀ1
/ kcal molꢀ1
/ ns
/ ꢀs
1
N/A
393a
432
395
446
411
414a
0.12a
0.32
0.15
0.02
0.18
0.04a
13.1a
34.3
23.6
12.2
26.5
13.4a
9.9 a
9.3
0.88a
0.68
0.85
0.98
0.82
0.96a
6.7
2.0
3.6
8.0
3.1
7.2a
73.1a
68.5
77.0
74.7
71.5
75.3a
N/A
N/A
53.5
50.1
46.6
53.4
424
590
426
481
471
530
0.14
0.03
0.07
0.02
0.10
0.04
N/A
3.3
6.2
1.2
4.3
5.3
1＠1
1@3PP
1@9
63000 (314)
103000 (305)
86000 (268)
120000 (318)
N/A
6.4
3@3
1.6
3@9
6.8
3.0 a
9@9PP
a) In CHCl3. b) Determined by the equation, kf = Φf τfꢀ1. c) Determined by the equation, Φnr = 1 − Φf. d) Determined by the equation, knr = Φnr τf−1.
e) Averaged energies determined from origins of the absorption and fluorescence spectra at 295 K. f) Determined from the origins of the phosphorescence
spectra obtained in ethanol at 77 K. g) In powder.
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The Φf values of the studied compounds ranged from 0.02 to 0.32 whereas the kf values were
in the same magnitude of order, 106 sꢀ1. This consideration indicates that the studied
condensed compounds of seven benzene rings show similar kf values irrespective of the
molecular configurations, probably, because of little distortion in the πꢀelectron systems, In a
foregoing paper, we reported photophysical properties of phenanthrene ([3]phenacene),
chrysene([4]phenacene), picene ([5]phenacene) and fulminene ([6]phenacene).31 The Φf
values of these small phenacenes are as small as 0.1 while that (0.12) of 1@1 ([7]phenacene)
is also close to them. The kf values of [n]phenacenes in CHCl3 were 2.6 × 106 sꢀ1, 4.4 × 106 sꢀ
1, 5.2 × 106 sꢀ1 and 7.5 × 106 sꢀ1 on going from n = 3 to 6. Since the kf value of [7]phenacene
(1@1) was determined to be 9.9 × 106 sꢀ1, we can say that as increasing the number of the
benzene rings in the phenacene skeleton, the kf value of [n]phenacene increases.
As with the excited state energies, we have formulated the ES and ET values of
[n]phenacene with eqs 4 and 5, respectively,31
Es = –2.6n + 89.1
ET = –1.8n + 66.2
(kcal mol–1)
(kcal mol–1)
(4)
(5)
According to eqs 4 and 5, the ES and ET for [7]phenacene (1@1) are evaluated to be 70.9 kcal
mol–1 and 53.6 kcal mol–1, respectively. The former value is close to the experimentally
determined one (73.1 kcal mol–1).
Figure 6 shows fluorescence spectra in the solid state (powder) of the studied compounds.
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Figure 6. Fluorescence spectra obtained in the solid state (powder) at 295 K.
΄ΝΥΒΓΠ
ΔΚΣ
It is noteworthy that all the fluorescence maxima (
ꢀ
) in the solid state, listed in Table
1, are redꢀshifted compared with those (λflu) in solution. The fluorescence quantum yields (Φ
Powder ) in the solid state were determined, and also listed in Table 1. The Φ fPowder values are
f
equivalent to or smaller than the Φf values in solution. Especially, 1@3PP shows a drastically
reduced fluorescence quantum yield in the solid state compared to that in solution.
Transient absorption measurements. We have carried out laser flash photolysis of the
photocyclized compounds in CHCl3 to investigate the deactivation processes from the S1
state. Figure 7 shows transient absorption spectra obtained upon 308 nm laser pulsing in the
CHCl3 solution.
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Figure 7. Transient absorption spectra obtained at 200 ns
upon 308 nm laser pulsing in the Arꢀpurged CHCl3
solution) at 295 K.
Because of the small solubility of 1@1 in CHCl3, we were unable to carry out laser
photolysis of 1@1. Transient absorption spectra of the studied compounds were obtained in
the wavelength region, 380–690 nm. The intensities of the transient absorption signals
decreased with decay lifetimes (τT) in the microsecond time domain, which are tabulated in
Table 1. In the presence of the dissolved oxygen, the decay lifetimes were shortened,
indicating that the transient signals are ascribable to the tripletꢀtriplet (TꢀT) absorption. The
observation of the triplet state in solution indicates that ISC from the S1 to the T1 state
competes with the fluorescence process. Thus, the evaluated Φnr and knr values using eqs 2
and 3, respectively, are for ISC.
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4. Conclusion
We studied photoproducts of 1ꢀ(nꢀphenanthryl)ꢀ2ꢀ(mꢀphenanthryl)ethenes (nEm; n, m = 1, 3
and 9) for understanding the photocyclization pattern based on the product analysis data, and
investigated the photophysical features of the πꢀextended molecules, n@m and n@mPP
based on emission and transient absorption measurements. Four types of the primary
photoproducts (1@1([7]phenacene), 1@9, 3@3([7]helicene) and 3@9) from the
corresponding nEm and two types of secondary photoproducts (1@3PP and 9@9PP) that
might be formed upon sequential photocyclization of the corresponding primary
photoproducts (1@3 and 9@9, respectively). Two (1@9 and 3@9) of the primary
photoproducts were found to have racemic molecular structures in the single crystal although
the optimized molecular structures by DFT calculations for the secondary photoproducts
(1@3PP and 9@9PP) were planar. Fluorescence quantum yields and lifetimes of the
photoproducts in solution were determined as listed in Table 1. Fluorescence from all the
obtained photoproducts was also observed in the powder with definite fluorescence quantum
yields. Laser flash photolysis of the photoproducts provided the tripletꢀtriplet absorption
spectra in solution, indicating that intersystem crossing to the triplet state competes with the
fluorescence process. The present work showed advantages of utilizing photocyclization of
diarylethenes for forming condensed PAHs of seven benzene rings. Preparing diarylethenes
of [n] and [m]phenacene skeletons (n + m > 7) would be clue to systematically form
condensed PAHs of benzene rings more than seven. The condensed PAHs, thus prepared by
photocyclization, could be applicable to organic semiꢀ and superꢀconductor materials in the
near future.
18

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Electronic Supplementary Information (ESI) available: The precise synthesis procedures
and analytical data, Xꢀray crystallographic data, decay profiles of fluorescence, results of
DFT calculation including tables of atom coordinates for the optimized geometries, and 1H and
13C NMR spectra for the compounds used in this work are provided.
Acknowledgment
This work has been supported by a GrantꢀinꢀAid for Scientific Research (JP26288032) from
the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese
Government to MY. MY acknowledges the technical staffs at Kyushu University for
performing the HRMS spectrometry of the new compounds under Cooperative Research
Program of the Network Joint Research Center for Materials and Devices. A part of this work
(Xꢀray single crystallographic structure analysis) was conducted at Advanced
Characterization Nanotechnology Platform of the University of Tokyo, supported by
"Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan
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Molecular and crystal structures of photoproducts (n@m and n@mPP) from
diphenanthrylethenes (nEm) were studied to understand the photocyclization patterns,
and the photophysical properties were investigated in solution and the solid state.