Photochemistry of linear-shaped phenylacetylenyl- and (phenylacetylenyl) phenylacetylenyl-substituted aromatic enediynes

Article abstract of DOI:10.1021/jp1054827

The series of linear-shaped phenylacetylenyl- and (phenylacetylenyl) phenylacetylenyl-substituted aromatic enediynes 1-3 were synthesized as pure trans and cis isomers and their photochemistry explored. With expansion of the π-electron system, the absorpt

Full text of DOI:10.1021/jp1054827

J. Phys. Chem. A 2010, 114, 10929–10935
10929
Photochemistry of Linear-Shaped Phenylacetylenyl- and (Phenylacetylenyl)phenylacetylenyl-
Substituted Aromatic Enediynes
Yoko Sugiyama,† Yoshihiro Shinohara,‡ Atsuya Momotake,† Kayori Takahashi,§ Yoko Kanna,|
Yoshinobu Nishimura,† and Tatsuo Arai*,†
Graduate School of Pure and Applied Sciences and Research Facility Center for Science and Technology,
UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, National Institute of AdVanced Industrial Science
and Technology (AIST), Tsukuba City, Ibaraki 305-8565, Japan, and Faculty of Science, UniVersity of the
Ryukyus, Nishihara, Okinawa 903-0213, Japan
ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: September 12, 2010
The series of linear-shaped phenylacetylenyl- and (phenylacetylenyl)phenylacetylenyl-substituted aromatic
enediynes 1-3 were synthesized as pure trans and cis isomers and their photochemistry explored. With
expansion of the π-electron system, the absorption spectra red-shifted and the molar extinction coefficients
dramatically increased up to 122000 M-1 cm-1 for trans-3. The absorption spectra of cis-2 and cis-3 consisted
of two independent absorption bands. The fluorescence quantum yields of the molecules were high, even for
the cis isomers (Φf ) 0.39-0.61). The fluorescence decay of each of the compounds was analyzed as a
single exponential and the wavelength dependence of time constants was not observed, indicating a single
emitting state in all cases. All isomers exhibited mutual cis-trans photoisomerization. The quantum yield of
both trans-to-cis and cis-to-trans photoisomerization considerably decreased in 2 and 3, presumably due to an
increased number of photochemical processes that yield nonreactive excited species and which result in
nonradiative deactivation. Three energy minima exist in the excited triplet state, where the energy of planar
conformation decreased with the extension of the phenyl acetylenyl chain, resulting in the promotion of
nonradiative processes without conformational change.
Introduction
showed a red shift of the fluorescence even though crystal-
lographic analysis suggests that the single bond connecting the
phenyl ring and carbon-carbon triple bond takes a twisted
conformation.7 Furthermore, both lipophilic8 and amphiphilic9
enediyne-cored dendrimers were prepared as pure cis and trans
isomers. All dendrimers were fluorescent with fluorescence
quantum yields of 0.10-0.66 and underwent efficient photo-
chemical isomerization in THF with quantum yields of 0.18-0.50.
Fluorescence and triplet lifetimes were strongly affected by
dendrimer generation and solvent polarity.
On the basis of these results, we focused our attention on
the photochemical properties of the linear-shaped aromatic
enediynes 1-3, where the π-electron system of the enediyne
should be expanded due to introduction of phenylacetylenyl or
(phenylacetylenyl)phenylacetylenyl units in the para position
(Figure 1c). The effects of extending the π-electron system on
the photochemistry of aromatic enediynes have not been studied
but are of interest from the viewpoint of construction of
molecules with new emissive and/or reactive properties.
Enediynes have been widely investigated, especially in the
area of antitumor antibiotics. The drugs esperamicin, calichemi-
cin, and dynemicin, for example, all possess an aliphatic endiyne
unit and produce a reactive 1,4-biradical aromatic intermediate
upon heating.1-4 Aromatic enediynes, on the other hand, are
usually thermally stable and have been known to exhibit unique
photochemical properties in organic solvents. Previously
we reported the photochemical reactivity of aromatic enediyne
bisphenylethynylethene (BEE) (Figure 1a).5 BEE gives efficient
fluorescence emission from both the cis and trans isomers and
undergoes intersystem crossing in addition to cis-trans pho-
toisomerization. Furthermore, three energy minima exist in the
triplet state potential surface where the planar triplet state
3
(3trans* and cis*) and the perpendicular triplet state (3p*) are
equilibrated. The fluorescence spectra, Stokes shift, fluorescence
lifetime, fluorescence quantum yield, and quantum yield of trans-
to-cis photoisomerization for the “push-pull” aromatic ene-
diynes trans- and cis-1-(4-dimethylaminophenyl)-6-(4-nitro-
phenyl)hex-3-ene-1,5-diynes (DANE) that contain both electron-
withdrawing and electron-donating groups on the phenyl rings
(Figure 1b) were found to exhibit a strong dependence on
solvent polarity in the less-polar region.6 The effects of methoxy
substituents on the phenyl ring of the cis isomer of enediynes
were also explored; ortho-substituted compounds (Figure 1b)
Experimental Section
Apparatus. Sample solutions were prepared in benzene
(Kanto Chemical) and deoxygenated by bubbling highly purified
argon (>99.999%) through a needle. Fluorescence and excitation
spectra were measured with an Hitachi F-4500 fluorescence
spectrophotometer using a 1 cm ×1 cm quartz cuvette. Absorp-
tion spectra were recorded with a Shimadzu UV-1600 spectro-
photometer. Fluorescence decay measurements were performed
using a time-correlated single-photon counting method. The
apparatus was assembled based on the method previously
described.10,11 Excitation at 410 nm was achieved using a diode
laser (PicoQuant, LDH-P-C- 405) driven by a power control
* To whom correspondence should be addressed.
† Graduate School of Pure and Applied Sciences, University of Tsukuba.
‡ Research Facility Center for Science and Technology, University of
Tsukuba.
§ National Institute of Advanced Industrial Science and Technology
(AIST).
| Faculty of Science, University of the Ryukyus.
10.1021/jp1054827  2010 American Chemical Society
Published on Web 09/27/2010

10930 J. Phys. Chem. A, Vol. 114, No. 41, 2010
Sugiyama et al.
Figure 1. Chemical structures of aromatic enediynes.
unit (PicoQuant, PDL 800-B) with a repetition rate of 2.5 MHz.
Temporal profiles of fluorescence decay were recorded using a
microchannel plate photomultiplier (Hamamatsu, R3809U)
equipped with a TCSPC computer board module (Becker and
Hickl, SPC630). The full width at half-maximum (fwhm) of
the instrument response function was 51 ps. Criteria for the best
fit included ꢀ2 values and Durbin-Watson parameters obtained
by nonlinear regression.12
organic layer was washed with brine, dried over MgSO4, filtered,
and evaporated. The residue was purified by silica gel column
chromatography (eluent, hexane/chloroform (90/10)) to give
1
trans-1 as a white powder (67 mg, 5%). H NMR (270 MHz,
CDCl3): 7.41-7.32 (m, 8H), 6.26 (s, 2H), 1.33 (s, 18H). 13C
NMR (67.5 MHz, CDCl3): 31.2, 34.9, 87.6, 94.9, 120.0, 120.4,
125.3, 131.2, 151.8. Anal. Calcd for C26H28: C, 91.71; H, 8.29.
Found: C, 91.94; H, 8.35.
Materials. cis-1,6-Bis(4-tert-butylphenyl)hexa-3-en-1,5-
diyne (cis-1). A solution of cis-1,2-dichloroethylene (456 mg,
4.71 mmol) and 1-tert-butyl-4-ethynylbenzene (2.43 g, 15.4
mmol) in benzene was added to a mixture of Pd(PPh3)4 (336
mg, 29 µmol), CuI (109 mg, 0.57 mmol), and n-BuNH2 (1.15
g, 15.7 mmol) in 20 mL of deaerated benzene which was cooled
at -78 °C. The mixture was allowed to warm to room
temperature and was stirred for 14 h. The reaction was then
quenched by addition of water and extracted with dichlo-
romethane. The organic layer was washed with brine, dried over
MgSO4, filtered, and evaporated. The residue was purified by
silica gel column chromatography (eluent, hexane/chloroform
cis-1,6-Bis(4-(2-(4-tert-butylphenyl)ethynyl)phenyl)hexa-3-
en-1,5-diyne (cis-2). A benzene solution of cis-1,2-dichloroet-
hylene (97 mg, 1.0 mmol) and 1-(2-(4-tert-butylphenyl)ethynyl)-
4-ethynylbenzene13 (580 mg, 2.2 mmol) was added to a mixture
of Pd(PPh3)4 (56 mg, 0.05 mmol), CuI (27 mg, 0.14 mmol),
and n-BuNH2 (217 mg, 3.0 mmol) in 20 mL of deaerated
benzene which was cooled at -78 °C. The mixture was allowed
to warm to room temperature and then stirred for 14 h, after
which time the reaction was quenched by addition of water and
extracted with dichloromethane. The organic layer was washed
with brine, dried over MgSO4, filtered, and evaporated. The
residue was purified by silica gel column chromatography
(eluent, hexane/chloroform (80/20)) to give cis-2 as a white
powder (110 mg, 20%). 1H NMR (270 MHz CDCl3): 7.48-7.45
(m, 12H), 7.39-7.36 (m, 4H), 6.12 (s, 2H), 1.33 (s, 18H). 13C
NMR (67.5 MHz, CDCl3): 31.2, 34.9, 88.4, 89.0, 91.8, 97.6,
119.5, 119.8, 122.5, 123.8, 125.3, 131.3, 131.5, 131.5, 151.7.
Anal. Calcd for C42H36: C, 92.98; H, 6.78. Found: C, 93.29; H,
6.71.
1
(80/20)) to give cis-1 as a pale yellow solid (1.08 g, 59%). H
NMR (270 MHz, CDCl3): 7.46 (d, J ) 8.1 Hz, 4H), 7.35 (d, J
) 8.1 Hz, 4H), 6.02 (s, 2H), 1.32 (s, 9H). 13C NMR (270 MHz,
CDCl3): 31.2, 34.9, 86.8, 97.7, 119.1, 120.1, 125.3, 131.2, 151.7.
Anal. Calcd for C26H28: C, 91.71; H, 8.29. Found: C, 91.57; H,
8.29.
trans-1,6-Bis(4-tert-butylphenyl)hexa-3-en-1,5-diyne (trans-
1). A benzene solution of trans-1,2-dichloroethylene (300 mg,
3.0 mmol) and 1-tert-butyl-4-ethynylbenzene (1.2 g, 7.0 mmol)
was added to a mixture of Pd(PPh3)4 (420 mg, 0.36 mmol),
CuI (95 mg, 0.50 mmol), and n-BuNH2 (650 mg, 9.0 mmol) in
30 mL of deaerated benzene which was cooled at -78 °C. The
mixture was allowed to warm to room temperature and then
stirred for 14 h, after which time the reaction was quenched by
addition of water and then extracted with dichloromethane. The
trans-1,6-Bis(4-(2-(4-tert-butylphenyl)ethynyl)phenyl)hexa-
3-en-1,5-diyne (trans-2). A solution of cis-2 (50.0 mg, 0.09
mmol) in 20 mL of chloroform was exposed to a hand-held
UV lamp (365 nm) for 12 h. After evaporation, the residue was
purified by GPC on a TSKgel G1000H column (TOHSO) by
elution with hexane to give trans-2 as a white powder (9.4 mg,
1
19%). H NMR (270 MHz,CDCl3): 7.49 (d, J ) 8.1 Hz, 4H),
7.47 (d, J ) 8.1 Hz, 4H), 7.43 (d, J ) 8.1 Hz, 4H), 7.38 (d, J

Photochemistry of Substituted Aromatic Enediynes
J. Phys. Chem. A, Vol. 114, No. 41, 2010 10931
Figure 2. Absorption spectra of the (a) trans and (b) cis isomers of BEE (thin line), 1 (open circles), 2 (closed circles), and 3 (solid line) in
benzene under argon. Fluorescence spectra of the (c) trans and (d) cis isomers of BEE (thin line), 1 (open circles), 2 (closed circles), and 3 (solid
line) in benzene under argon.
) 8.1 Hz, 4H), 6.30 (s, 2H), 1.33 (s, 18H). MALDI-TOFMS
(m/z) [M]+ calcd for C42H36, 540.28; found, 540.01.
cant. The UV absorption spectrum of trans-BEE in benzene is
reported to exhibit an absorption band around 300-360 nm.5
The absorption spectra of both trans-BEE and the tert-butyl
substituted analogue trans-1 and phenylacetylenyl and (phenyl-
acetylenyl)phenylacetylenyl substituted enediynes trans-2 and
trans-3 in benzene solution are shown in Figure 2a. The spectra
of trans-1 is slightly red-shifted compared to that of trans-BEE,
presumably due to the difference in electron-donating ability
of H and the tert-butyl group. Compared with the absorption
spectrum of trans-1, the spectrum of trans-2 is dramatically red-
shifted to 420 nm and its shape is also changed, indicating that
the π-electrons of the para-substituted phenylacetylenyl group
are delocalized into the core enediyne moiety. There is also a
shoulder at 395 nm in the spectrum for trans-2 that seems to
correspond to shoulders at 352 nm in the spectrum for BEE
and 360 nm in the spectrum for trans-1. Interestingly, the
absorption spectrum of trans-3 showed only slight red-shifting
compared to that of trans-2 despite further extension of the
π-conjugate system, while the extinction coefficient increased
cis-1,6-Bis(4-(2-(4-(2-(3,5-di-tert-butylphenyl)ethynyl)phe-
nyl)ethynyl)phenyl)hexa-3-en-1,5-diyne (cis-3). A benzene
solution of cis-1,2-dichloroethylene (20 mg, 0.2 mmol) and
1,3-di-tert-butyl-5-(2-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)-
ethynyl)benzene13 (203 mg, 0.49 mmol) was added to a mixture
of Pd(PPh3)2Cl2 (23 mg, 0.03 mmol), CuI (9.1 mg, 0.04 mmol),
and n-BuNH2 (50 mg, 0.68 mmol) in 10 mL of deaerated
benzene which was cooled at -78 °C. The mixture was allowed
to warm to room temperature and stirred for 5 h. After
evaporation, the residue was purified by silica gel column
chromatography (eluent, hexane/chloroform (85/15)) to give
cis-3 as a white powder (28 mg, 16%). 1H NMR (400
MHz,CDCl3): 7.54-7.46 (m, 16H), 7.41-7.40 (m, 2H),
7.38-7.37 (m, 4H), 6.12 (s, 2H), 1.33 (s, 36H). 13C NMR (67.5
MHz, CDCl3): 150.8, 131.5, 131.5, 131.5, 131.4, 125.8, 123.6,
122.9, 122.9, 122.4, 121.8, 119.6, 97.5, 92.6, 91.4, 90.7, 89.2,
87.8, 34.9, 31.4. MALDI-TOFMS (m/z) [M + H]+ calcd for
C66H60, 853.47; found, 853.47.
significantly to as high as 122000 M-1 cm-1
.
trans-1,6-Bis(4-(2-(4-(2-(3,5-di-tert-butylphenyl)ethynyl)phe-
nyl)ethynyl)phenyl)hexa-3-en-1,5-diyne (trans-3). A benzene
solution of trans-1,2-dichloroethylene (20 mg, 0.2 mmol) and
1,3-di-tert-butyl-5-(2-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)-
ethynyl)benzene (22 mg, 0.53 mmol) was added to a mixture
of Pd(PPh3)2Cl2 (22 mg, 0.03 mmol), CuI (7.8 mg, 0.04 mmol),
and n-BuNH2 (53 mg, 0.72 mmol) in 10 mL of deaerated
benzene which was cooled at -78 °C. The mixture was allowed
to warm to room temperature and stirred for 6 h. After
evaporation, the residue was purified by silica gel column
chromatography (eluent, hexane/chloroform (90/10)) to give
Some similarities and differences were also noted for the
corresponding cis isomers. Their absorption spectra can be seen
in Figure 1b. Although the extinction coefficients of the cis
isomers are almost half as much as the values for the trans
isomers, the spectral shape of cis- and trans-1 are quite similar
to each other. The shapes of the spectra for cis-2 and cis-3,
however, are different from those of the corresponding trans
isomers. The absorption band of cis-2 consists of the superposi-
tion of a broad band around 330-420 nm similar to that seen
in trans-1 and a band peaking at 315 nm, probably arising from
partial localization of the π-electron system at the phenyl-
acetylene moiety in the cis isomer. A similar trend was observed
in cis-3; the absorption band consists of two independent parts,
a broad band around 350-420 nm and a strong band peaking
at 336 nm, which may be ascribed to the band of the
origo(phenylacetylene) unit.14-16 The energies for the lowest
transition were calculated to be 81, 79, 71, and 70 kcal/mol for
both trans and cis isomers of BEE, 1, 2, and 3, respectively.
1
trans-3 as a white powder (18.4 mg, 10.8%). H NMR (270
MHz, CDCl3): 7.55-7.45 (m, 16H), 7.42-7.39 (m, 6H), 6.31
(s, 1H), 1.34 (s, 36H). 13C NMR (67.5 MHz, CDCl3): 150.8,
131.5, 131.5, 131.4, 125.8, 123.6, 123.3, 123.0, 122.6, 122.4,
121.8, 120.8, 94.9, 92.6, 91.4, 90.3, 90.0, 87.8, 34.9, 31.4.
MALDI-TOFMS (m/z) [M + H]+ calcd for C66H60, 853.47;
found, 853.48.
The phenylacetylene substituents also have an impact on the
fluorescence spectra, which are shown for the trans and cis
isomers in parts c and d of Figure 2, respectively, with some
relevant data also presented in Table 1. trans-BEE is reported
Results and Discussion
Steady State Absorption and Fluorescence Spectra. The
para-substituent effect of the phenylacetylenyl group is signifi-

10932 J. Phys. Chem. A, Vol. 114, No. 41, 2010
Sugiyama et al.
TABLE 1: Spectroscopic, Photophysical, and Photochemical Data (Benzene Solution, 298 K)
lowest singlet
Stokes
energy/kcal
Kq/109
λmax (abs) /nm
trans-BEE 328, 352
ꢀmax/M-1 cm-1
40100, 27100
52200, 35200
79700, 50000
122000, 66000
21100, 12600
23300, 13900
λmax (fl)/nm shift/cm-1
mol-1
Φf τS/ns Φtfc Φcft τT/ns M-1 s-1
362, 382
370, 392
411, 436
423, 448
364, 387
373, 394
413, 437
423, 448
780
750
990
1050
940
1050
1100
1050
81
79
71
70
81
79
71
70
0.42
0.17
0.37
0.49
2.45
trans-1
trans-2
trans-3
cis-BEE
cis-1
334, 360
365, 395
373, 405
328, 352
333, 359
0.61 0.69 0.10
0.61 0.61 0.007
0.47 0.52 0.004
0.31 0.88
0.39 1.13
0.60 1.42
4.22
3.25
2.88
2.86
0.27 0.37
0.19 0.48
0.03 2.47
0.02 2.86
4.16
2.94
3.04
cis-2
cis-3
315, 365, 395 62700, 38200, 19500
336, 375, 405 108800, 63000, 25000
0.42 1.28
to exhibit a fluorescence band with λmax ) 362 and 381 nm
and Φf ) 0.42 in benzene under argon.5 The fluorescence of
trans-1 is similar to that of trans-BEE but more intense (Φf )
0.61). The fluorescence spectrum of trans-2 is more than 40
nm red-shifted to 411 and 436 nm and exhibits a long tail at
low energy around 560 nm. The fluorescence quantum yield of
trans-2 (Φf ) 0.61) is the same value as trans-1, and the singlet
lifetime is similar: τs ) 0.69 ns for trans-1 and 0.61 ns for trans-
2, respectively. The fluorescence spectrum of trans-3 is slightly
(11 nm) red-shifted compared to that of trans-2 and exhibits a
long tail at low energy around 560 nm which is almost the same
as that of trans-2. The fluorescence quantum yield of trans-3
(Φf ) 0.48) is lower than those of trans-1 and trans-2, and the
singlet lifetime is shorter (0.47 ns), probably due to the
promotion of nonradiative decay processes.
tively.19-21 On the other hand, distinctive absorption bands for
phenylacetylenyl units cannot be observed in the spectra for
trans-2 and trans-3, indicating that the π-electron system
delocalizes over the whole molecule in these isomers.
Photoisomerization. Aromatic enediynes are known to
undergo a variety of photochemical processes including trans-cis
photoisomerization. The changes in the absorption spectra
observed upon irradiation with 365 nm light in benzene solution
under argon are shown in Figure 4, and some relevant data are
gathered in Table 1. Since the isosbestic point can be observed
during the photoirradiation (for 2 and 3 only) and the spectral
shapes at the photostationary state of the trans-starting and cis-
starting sample (solid line) are almost identical, the trans-cis
photoisomerization in the examined molecules is a clean process
The Stokes shift values (780, 990, and 1050 cm-1 for trans-
1, trans-2, and trans-3, respectively) only slightly increase with
expansion of the phenylacetylene chain, indicating that the
structural differences between the ground state and the excited
singlet state are not very significant among 1-3. In the ground
state, both planar and perpendicular conformations at the
phenylacetylene moiety should exist because of free rotation
of the single bonds, whereas in the excited singlet state an
immediate structural change to the planar conformation takes
place that results in fluorescence.17,18 In this case, the enediyne
having a longer phenylacetylene chain could cause more
significant conformational changes in the excited singlet state,15
resulting in a greater value of the Stokes shift than that observed
for the shorter chain. However, the calculated values of the
Stokes shift using the lowest energy transition band of the
absorption spectra is not very much different, suggesting that
the conformation of the fluorescent state should be almost similar
among 1-3.
Fluorescence lifetimes of the examined molecules are also
listed in Table 1. All compounds exhibited single exponential
decay, and the wavelength dependence of time constants was
not observed, indicating a single emitting state in all cases.
Excitation Spectra. Excitation spectra provided further
information about the substituted endiynes. Figure 3 shows
fluorescence excitation and absorption spectra. All of the trans
isomers and cis-1 exhibited similar absorption and excitation
spectral shapes, whereas the excitation spectra of cis-2 and cis-3
did not agree with their absorption spectra, especially at the
shorter wavelength region. This result indicates that interaction
between the phenylacetylenyl units and the enediyne chro-
mophore is not significant in cis-2 and cis-3. In addition, the
energy transfer from excited phenylacetylenyl units to the
enediyne chromophore does not occur efficiently, despite
the fact that the phenylacetylenyl units are substituted in the
para position. In other words, the excitation at 315 and 336 nm
for cis-2 and cis-3, respectively, can produce locally excited
states at the phenylacetylenyl units that mainly decay nonradia-
Figure 3. Absorption spectra (dashed-line) and fluorescence excitation
spectra (solid line) of (a) trans-1, (b) trans-2, (c) trans-3, (d) cis-1, (e)
cis-2, and (f) cis-3 in benzene under argon.

Photochemistry of Substituted Aromatic Enediynes
J. Phys. Chem. A, Vol. 114, No. 41, 2010 10933
The calculated Φnr values are 0.19, 0.38, and 0.52 for trans-1,
trans-2, and trans-3, respectively. Similar values were obtained
for the cis isomers. These values support above statements that
nonradiative decay processes are promoted with increasing chain
length.
Transient Absorption Spectra. Transient absorption spectra
were collected to gain further insight into the excited states of
compounds 1-3. Upon 355 nm laser excitation, each one of
the trans and cis isomers gave a clear transient absorption
spectrum (Figure 5). The observed transient species for all
compounds were quenched by oxygen with a rate constant of
(2.9-4.2) × 109 M-1 s-1 (Table 1), indicating that the observed
transients are assignable to the triplet state.22 The transient
absorption spectra of the trans and cis isomers are quite similar
to each other (Figure 5). In addition, the triplet lifetimes (τt)
are almost the same for the trans and cis isomers (Table 1),
suggesting that both undergo intersystem crossing to the triplet
3
state where the planar triplet state (3trans* and cis*) and the
perpendicular triplet state (3p*) are equilibrated. The potential
energy of 3p* does not change with introduction of substituents
on the phenyl ring,22 and thus the energies of 3p* are nearly the
same among all examined compounds (Figure 6), whereas the
Figure 4. Change in the absorption spectra of (a) cis-1, (b) cis-2, and
(c) cis-3 upon irradiation with 365 nm light in benzene under argon.
Inset shows change in the absorption spectra of the corresponding trans
isomer upon irradiation with 365 nm light under the same conditions.
3
3
energies of the planar conformations trans* and cis* depend
3
3
on the substituent. The energies of trans* and cis* of 1 and
BEE are nearly the same based on the similarities in their T-T
3
3
absorption spectra. The energies of trans* and cis* of 2 and
3, however, are expected to be lower than those of 1 based on
the fact that the triplet lifetimes of 2 and 3 are 5-6 times longer
than that of 1. The lower planar triplet energies in 2 and 3
probably promote a nonradiative deactivation process from the
planar triplet state (Figure 6), and therefore the quantum yield
of isomerization is much lower in 2 and 3 than it is in BEE
and 1. This result indicates that the photoisomerization of 1-3
partially proceeds from the excited triplet state.
and the photoproducts are either the trans or cis isomers. The
lack of observation of isosbestic point for BEE and 1 is due to
the similarity of the absorption spectra of trans and cis isomers
with smaller extinction coefficient around 300-350 nm in cis
isomer. From the absorption spectra of pure trans and cis
isomers, the trans/cis ratio at the photostationary state ([trans]/
[cis])pss can be calculated to be 48/52, 70/30, and 73/27 for 1,
2, and 3, respectively. This indicates that the photoismerization
from trans isomer is more difficult than that from cis isomer in
2 and 3. The quantum yields for trans-to-cis photoisomerization
(Φtfc) of trans-1 are calculated by the equation
Since the deactivation rate constant from 3trans* to the ground
state trans isomer is reasonably estimated to be the same as
3
that from cis* to the ground state cis isomer, the equilibrium
constant between the planar triplet state (3trans* and 3cis*) and
3p* can be estimated using the observed triplet lifetime. The
triplet lifetime (τt) can be described in the equation
Φtfc ) Φcftεcis/εtransx([trans]/[cis])pss
(1)
τt ) (1 + K)/(Kkd + kd′)
(3)
where εcis and εtrans are the molar extinction coefficients at 365
nm for cis-1 and trans-1, respectively, and Φcft is the quantum
yield for cis-to-trans photoisomerization obtained at a very early
stage.
where kd and kd′ are the rate constants for deactivation from
the planar triplet state and 3p*, respectively, and K is the
equilibrium constant between the planar triplet state and p*.
3
The Φtfc value obtained for trans-1 (0.10) is on the same
order as that obtained for trans-BEE (0.19), whereas those for
trans-2 (0.007) and trans-3 (0.004) are considerably smaller.
The much lower Φtfc values for trans-2 and trans-3 may be
due in part to steric interactions with surrounding solvent
molecules during rotation around the CdC double bond of the
phenylacetylene enediyne. It is more likely, though, to be related
to the promotion of alternative nonradiative decay processes
due to the lower excited state energy of trans-2 and trans-3 or
the presence of an increasing number of possible reaction
pathways from the emitting species to a nonemitting perpen-
dicular species in the excited state.19 Photoisomerization could
thus proceed mainly via the excited triplet state (see below).
The quantum yield for nonradiative decay other than photoi-
somerization (Φnr) can be calculated using the Φf and Φtfc
values
The kd and kd′ values were previously estimated to be 2 × 104
s-1 and 2 × 107 s-1, respectively.22 Using this information, the
equilibrium constants K were determined to be 0.11, 0.020, and
0.020 for 1, 2, and 3, respectively (Table 2). Furthermore, the
proportion of the planar and perpendicular excited triplet species
(3trans* and 3cis*) and 3p* was roughly estimated to be 89:11,
98:2, and 98:2 for 1, 2, and 3, respectively (Table 2). These
values support that the energies of planar triplet state in 2 and
3 are lower than that of 1 and the quantum yield of isomerization
is much lower in 2 and 3 due to the promotion of a nonradiateve
process from the planar triplet state.
Potential Energy Surfaces. On the basis of the experimental
results, the potential energy surfaces of 1-3 were estimated
and are depicted in Figure 6. The potential energy surfaces for
photoisomerization depend on the length of the phenyl acetyl-
enyl chain. In the triplet state, the three conformers 3trans* and
3cis* and p* are equilibrated and the photoisomerization may
3
Φnr ) 1 - (Φf + Φtfc
)
(2)
3
take place from the p* conformer. In particular, the energies

10934 J. Phys. Chem. A, Vol. 114, No. 41, 2010
Sugiyama et al.
Figure 5. Transient absorption spectra of (a) trans-1, (b) trans-2, (c) trans-3, (d) cis-1, (e) cis-2, and (f) cis-3 in benzene under argon.
of the planar conformations of 2 and 3 are lower than that of 1,
3
while those of p* are independent of the length of the phenyl
acetylenyl chain. The result is a higher activation energy for
excitation from the planar conformations to 3p* in 2 and 3 and
thus a decrease in the quantum yield of the photoisomerization
process. In addition, deactivation processes related to the
phenylacetylene group may also be a source of nonradiative
decay.19
Conclusion
In summary, a series of phenylacetylenyl- and (phenylacetyl-
enyl)phenylacetylenyl-substituted aromatic enediynes 1-3 were
synthesized as pure trans and cis isomers and their photochemi-
cal properties studied. With expansion of the π-electron system,
the absorption spectra red-shifted and the molar extinction
coefficients dramatically increased in both the trans and cis
isomers. The absorption spectra of cis-2 and cis-3 consist of
two distinctive absorption bands of which the band at the shorter
wavelength region is ascribed to the substituted phenylacetylenyl
units. The fluorescence excitation spectra of both cis-2 and cis-3
do not agree with those of the absorption spectra, indicating
Figure 6. Potential energy surfaces for the photoisomerization of
1-3.

Photochemistry of Substituted Aromatic Enediynes
J. Phys. Chem. A, Vol. 114, No. 41, 2010 10935
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Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research in a Priority Area “New Frontiers
in Photochromism (No. 471) and (No. 19550176) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan.
JP1054827