Photochemical and photophysical characteristics in the excited state properties of methoxy-substituted enediynes

Article abstract of DOI:10.1246/cl.2008.174

Effects of methoxy substituent on the phenyl ring of cis isomer of enediynes were explored. All the cis isomers exhibited fluorescence emission with moderate efficiency. Furthermore, the ortho-substituted compounds showed red shift of the fluorescence emission even though crystallographic analysis suggests that the single bond connecting the phenyl ring and carbon-carbon triple bond takes considerably twisted conformation. Copyright

Full text of DOI:10.1246/cl.2008.174

174
Chemistry Letters Vol.37, No.2 (2008)
Photochemical and Photophysical Characteristics in the Excited State Properties
of Methoxy-substituted Enediynes
Nao Yoshimura,1 Atsuya Momotake,1 Yoshihiro Shinohara,2 Yoshinobu Nishimura,1 and Tatsuo Araiꢀ1
1Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
2Research Facility Center for Science and Technology, University of Tsukuba, Tsukuba 305-8571
(Received October 25, 2007; CL-071186; E-mail: arai@chem.tsukuba.ac.jp)
Effects of methoxy substituent on the phenyl ring of cis
o-MEY
isomer of enediynes were explored. All the cis isomers exhibited
fluorescence emission with moderate efficiency. Furthermore,
the ortho-substituted compounds showed red shift of the fluores-
cence emission even though crystallographic analysis suggests
that the single bond connecting the phenyl ring and carbon–
carbon triple bond takes considerably twisted conformation.
Front view
Side view
m-MEY
Stilbenoid compounds and azobenzenes are known to iso-
merize mutually between trans and cis isomers by photo-irradi-
ation.1–4 Although the trans-stilbene gives fluorescence with the
quantum yield of 0.04 in hexane at room temperature, cis-stil-
bene and both cis- and trans-azobenzene do not exhibit fluores-
cence at room temperature owing to the existence of the fast
deactivation processes from the singlet excited state to give
the corresponding isomers or photocyclization product.
In contrast, not only trans isomer but also cis isomer of
aromatic enediyne is highly fluorescent. Thus, they deactivate
through fluorecence emission, cis–trans photoisomerization, or
intersystem crossing processes from the excited singlet state.5,6
Since the introduction of methoxy substituent at the phenyl ring
of stilbene increased the fluorescence quantum yield and the
fluorescence lifetime,7–9 we have investigated the effect of
methoxy substituent on the excited state properties of enediyne
compounds.
The methoxy-substituted aromatic enediynes on 2,20,6,60
(o-MEY), 3,30,5,50 (m-MEY), and 4,40 positions (p-MEY) of
the phenyl ring were newly synthesized (Figure 1) and identified
by NMR and elemental analysis.10
Substitution of the methoxy group at the ortho positions
should strongly affect the ground state structure of cis aromatic
enediyne. Figure 2 shows ORTEP diagrams of o-MEY and
m-MEY. While the terminal phenyl rings of m-MEY takes a
Figure 2. X-ray crystal structures of o-MEY (top) and m-MEY
(bottom).
coplanar structure, those of o-MEY takes a nonplanar structure
due to the steric hindrance caused by two methoxy groups at
ortho positions.11 The dihedral angle between the phenyl
ring and the enediyne part is calculated to be more than 55ꢁ
(Table S2 in Supporting Information).10
Figure 3 shows the absorption and fluorescence spectra of
cis-enediyne in benzene at room temperature. The introduction
2.0
1.0
BEE
0
O
O
2.0
O
O
1.0
m-MEY
0
O
O
2.0
cis-Azobenzene
cis-Stilbene
cis-Enediyne
1.0
p-MEY
N
N
0
2.0
Fluorescent
No
No
No
Yes
Yes
O O
Thermally stable
Yes
O
O
1.0
0
o-MEY
BEE : R = H
R
R
o-MEY : R = 2,2',6,6'-Tetramethoxy
m-MEY : R = 3,3',5,5'-Tetramethoxy
p-MEY : R= 4,4'-Dimethoxy
300
350
400
450
500
550
Wavelength/nm
Figure 3. Absorption (solid lines) and fluorescence (dashed
lines) spectra of BEE, m-, p-, and o-MEY in benzene under Ar.
Figure 1. Chemical structures of cis aromatic enediynes.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.2 (2008)
Table 1. Spectroscopic parameters of o-, m-, p-MEY, and BEE
175
ꢀmax (abs)
/nm
"
ꢀmax (fl) Stokes shift
/nm
ES
/kcal molꢂ1
max
/cmꢂ1 molꢂ1 dm3
T
Substrate
ꢀf
ꢀisc ꢀc!t (ꢀc!tS, ꢀc!t
)
ꢁs/ns ꢁT/ns
/cmꢂ1
o-MEY
m-MEY
p-MEY
BEE
346, 376
334, 357
343, 369
328, 352
20400, 9700
20500, 11900
21100, 13700
21100, 13300
399, 421
375, 396
382, 404
364, 387
1506
1143
1023
937
0.35 0.35 0.30
0.22 0.25 0.16
0.48 0.23 0.27
0.31 0.25 0.27
(0.12, 0.18)
(0.04, 0.12)
(0.15, 0.12)
(0.14, 0.13)
1.25
0.91
1.32
—
980
530
520
370
73.9
78.2
76.2
79.6
of the methoxy group resulted in the red-shift in the absorption
and fluorescence spectra of o-MEY, m-MEY, and p-MEY com-
pared to the parent endiyne compound BEE. While the spectra
of m-MEY and p-MEY showed similar vibronic features to
BEE, the spectra of o-MEY showed broadened vibronic features
probably because of the twisting of the single bond connecting
the phenyl ring and carbon–carbon triple bond.
ꢀiso ¼ 2 ꢃ ꢀc!t
where ꢀc!t is the efficiency of cis-to-trans photoisomerization,
ꢀc!t is that of cis-to-trans photoisomerization only from the
;
ð3Þ
S
singlet excited state, and ꢀiso is that of all photoisomerization
processes. These results suggest that [ꢀf þ ꢀiso] of o-MEY
and p-MEY was 0.95 and 1.02, and was about 1 within experi-
mental uncertainty, indicating that deactivation by nonradiative
process other than intersystem crossing and isomerization proc-
ess hardly occur. However, that of m-MEY was 0.54, indicating
that large contribution of nonradiative process exists on the
deactivation from the excited singlet state.
In summary, all cis-enediynes showed fluorescence emis-
sion with relatively high quantum efficiency. They underwent
cis–trans photoisomerization and intersystem crossing. The
maximum wavelengths in the absorption and fluorescence spec-
tra and the triplet lifetime were the longest, and the quantum
yield of fluorescence emission was the highest in o-MEY.
All cis-enediynes showed fluorescence emission with
relatively high quantum efficiency (ꢀf) of 0.35, 0.22, and 0.48
for o-MEY, m-MEY, and p-MEY, respectively (Table 1). The
lifetimes of the excited singlet state for o-MEY, m-MEY, and
p-MEY were determined to be 1.25, 0.91, and 1.32 ns, respec-
tively. These lifetimes were much longer than that of cis-stilbene
(less than 2 ps) in agreement with the strong fluorescence emis-
sion from BEE.2 The energy of the excited singlet state was
estimated from the crossing point of the absorption spectra and
fluorescence spectra to be 73.9, 78.2, and 76.2 kcal molꢂ1, for
o-MEY, m-MEY, and p-MEY, respectively. The energy increas-
ed in the order of o-, p-, and m-MEY, and these values are lower
than that of BEE (79.6 kcal molꢂ1). Although o-MEY takes a
nonplanar conformation as revealed by the dihedral angle
between the phenyl ring and the enediyne part to be more
than 55ꢁ, the excited singlet energy is the lowest in o-MEY.10
These results suggest that the conjugation among C=C and
C=C may be extended or almost the same even by the twisting
of the single bond connecting these two chromophores.
On irradiation in benzene, all derivatives underwent
cis–trans isomerization to give the photostationary state isomer
ratio ([cis]/[trans])pss to be 76/24, 40/60, 52/48, and 44/56
for o-MEY, m-MEY, p-MEY, and BEE, respectively.
On 308 nm laser excitation, all derivatives gave transient
absorption spectra decaying with single-exponential function
(Table 1). The observed transients were quenched by oxygen
with the diffusion-controlled reaction and are assigned to the
triplet excited state. The quantum efficiency (ꢀisc) of intersys-
tem crossing to give the excited triplet state was estimated
to be 0.35, 0.25, and 0.23 for o-MEY, m-MEY, and p-MEY,
respectively. The lifetimes of the excited triplet state were
determined to be 980, 530, and 520 ns for o-MEY, m-MEY,
and p-MEY, respectively, and longer than that of BEE. Thus,
the methoxy substitution increased the triplet lifetime probably
shifting the equilibrium between the planar triplet and the
perpendicular triplet to the planar triplet side. One can estimate
the efficiency of cis-to-trans isomerization from the triplet
excited state (ꢀc!tT) by following equation:
This work was supported by a Grant-in-Aid for Science
Research in a Priority Area ‘‘New Frontiers in Photochromism
(No. 471)’’ from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan.
References and Notes
1
2
3
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G. Sudesh Kumar, D. C. Neckers, Chem. Rev. 1989, 89,
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H. Sakakibara, M. Ikegami, K. Isagawa, S. Tojo, T. Majima,
T. Arai, Chem. Lett. 2001, 1050.
Y. Miki, A. Momotake, T. Arai, Org. Biomol. Chem. 2003,
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H. Gruen, H. Gorner, J. Phys. Chem. 1989, 93, 7144.
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4
5
6
7
8
¨
9
10 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/.
11 Crystallographic data reported in this manuscript have been
deposited with Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-659191 and 659192.
Copies of the data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html.
T
ꢀc!t ¼ 1=2 ꢃ ꢀisc
;
ð1Þ
ð2Þ
S
T
ꢀc!t ¼ ꢀc!t þ ꢀc!t
;
Published on the web (Advance View) January 19, 2008; doi:10.1246/cl.2008.174