Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: Photophysical properties and photoadditions of alcohols

Article abstract of DOI:10.1021/jo052123d

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styre > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (μ_e = 13.2 D).

Full text of DOI:10.1021/jo052123d

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes:
Photophysical Properties and Photoadditions of Alcohols
Jeffrey C. Roberts† and James A. Pincock*
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3
james.pincock@dal.ca
ReceiVed October 10, 2005
The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been
investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-
trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from
photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products
formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent
atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes
and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE
proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient
carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-
hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation
of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation
intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large
quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic
solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation
of a highly polarized charge-transfer excited state (µe ) 13.2 D).
Introduction
about the central CdC bond. This activated bond torsion process
results in formation of a short-lived species which possesses a
perpendicular geometry. Subsequent internal conversion to the
ground-state surface yields either the original trans isomer (no
net reaction) or the isomer cis-1a (net trans-cis isomerization)
in approximately equal amounts. As a result of the highly
efficient trans-cis photoisomerization of trans-1a in room-
temperature alkane solvents (φtc ) 0.40),2 the substrate displays
only weak fluorescence (φf ) 0.04).3 Many investigations have
focused on hindering the twisting motion by immobilizing the
substrate in high-viscosity solvents4 or in low-temperature glass
The photochemistry of trans-stilbene (E-1,2-diphenylethene,
trans-1a) has been extensively investigated by many research
groups and has been fundamental in the understanding of the
excited-state behavior of arylalkenes.1 Following excitation to
S1, the main deactivation pathway for this substrate is twisting
† Current address: Department of Chemistry, Queen’s University, Kingston,
Ontario, Canada K7L 3N6.
(1) (a) Saltiel, J.; Charlton, J. L. In Rearrangements in Ground and
Excited States; de Mayo, P., Ed.; Academic Press: New York, 1980; Vol.
3, pp 25-89. (b) Saltiel, J., Sun, Y.-P., Eds. Photochromism, Molecules
and Systems; Elsevier: Amsterdam, 1990. (c) Waldeck, D. H. Chem. ReV.
1991, 91, 415-436. (d) Go¨rner, H.; Kuhn, H. J. AdV. Photochem. 1995,
19, 1-117.
(2) Gu¨sten, H.; Klasinc, L. Tetrahedron Lett. 1968, 26, 3097-3101.
(3) Saltiel, J.; Waller, A. S.; Sears, D. F. J. Am. Chem. Soc. 1993, 115,
2453-2465.
10.1021/jo052123d CCC: $33.50 © 2006 American Chemical Society
Published on Web 01/26/2006
1480
J. Org. Chem. 2006, 71, 1480-1492

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
matrixes5sthese changes lead to smaller quantum yields of
isomerization and larger quantum yields of fluorescence.
Analogues of trans-1a in which excited-state bond torsion is
completely inhibited by bridging covalent bonds are highly
fluorescent.6
SCHEME 1. Products Detected Following Irradiation of
trans-1a-e in TFE
adducts were also detected following irradiation of the substrates
in methanol, the photoaddition reaction proceeded much more
rapidly in TFE.10 Our results indicated that the solvent addition
reaction occurred more readily for substrates possessing m-
methoxy substituents. Furthermore, the more reactive substrates
(trans-1c and trans-1d) also displayed very high quantum yields
of fluorescence and exceptionally long singlet lifetimes in
acetonitrile solution. We rationalized this connection between
photophysical properties and photochemical reactivity as being
due to differences in the heights of the barriers for excited-
state bond torsion, i.e., a m-methoxy effect. According to this
explanation, those substrates with longer singlet lifetimes are
more reactive toward TFE because the excited stilbene ef-
fectively has more time to enter into a bimolecular reaction. A
corollary to this theory is that the very short lifetimes of the
corresponding cis isomers should preclude their ability to
undergo photoaddition chemistry.11
Although our initial studies were successful in describing
many aspects of the photochemical reactivity of methoxystil-
benes, some important issues still deserved closer investigation.
In the first place, a better understanding of this apparent
m-methoxy effect seemed desirable. We now report detailed
photophysical characterization (absorption maxima, extinction
coefficients, fluorescence maxima, singlet lifetimes, fluorescence
quantum yields, and trans-cis isomerization quantum yields)
for trans-1a-e in six solvents of varying polarity. In addition,
these same experiments have been performed for the analogous
methoxy-substituted styrenes 8b-e and trans-1-arylpropenes
trans-9b-e. Taken together, these results provide a much better
understanding of the factors that influence the excited-state
properties of methoxy-substituted arylalkenes.
Another mechanism for reducing excited-state bond torsion
has been established by the detailed investigations of amino-
stilbene photochemistry by Lewis and co-workers.7 The “meta-
amino effect” is best demonstrated by the very different
photophysical properties of trans-4-aminostilbene (trans-2: φtc
) 0.52, φf ) 0.03) and trans-3-aminostilbene (trans-3: φtc )
0.23, φf ) 0.40) in hexane solution.7b The reduced quantum
yield of isomerization of trans-3 compared to either trans-1a
or trans-2 was taken as evidence that electron-donating meta
substituents provide electronic inhibition of the CdC twisting
motion. This hypothesis was further substantiated by the
experimentally determined activation energies for the excited-
state bond torsion process. Whereas the energy barrier for
twisting of the trans-2 excited state is identical to that of
unsubstituted trans-1a (Eact ) 3.5 kcal/mol), the barrier for the
m-amino derivative trans-3 is much higher (Eact ) 7.0 kcal/
mol).7b Even more recent work by Lewis and co-workers has
demonstrated that an analogous meta effect exists for methoxy-
and hydroxystilbenes.8
Our own interest in stilbene photochemistry was initiated by
a study of the photoaddition of alcohols to trans-1a and the
methoxy-substituted stilbene derivatives trans-1b-d.9 The
products resulting from the irradiation of trans-1a-e in 2,2,2-
trifluoroethanol are displayed in Scheme 1. Although solvent
(4) Gegiou, D. S.; Muszkat, K. A.; Fischer, E. J. Am. Chem. Soc. 1968,
90, 12-18.
(5) Gegiou, D. S.; Muszkat, K. A.; Fischer, E. J. Am. Chem. Soc. 1968,
90, 3907-3918.
(6) Saltiel, J. S.; Zafiriou, O. C.; Megarity, E. D. J. Am. Chem. Soc.
1968, 90, 4759-4760.
The second focus of the current research was the mechanism
of the TFE photoaddition reactions. The acid-catalyzed photo-
addition of water to a variety of substituted styrene derivatives
(7) (a) Lewis, F. D.; Yang, J.-S. J. Am. Chem. Soc. 1997, 119, 3834-
3835. (b) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am. Chem. Soc.
1999, 121, 12045-12053. (c) Lewis, F. D.; Kalgutkar, R. S. J. Phys. Chem.
A. 2001, 105, 285-291. (d) Lewis, F. D.; Weigel, W.; Zuo, X. J. Phys.
Chem. A. 2001, 105, 4691-4696. (e) Lewis, F. D.; Weigel, W. J. Phys.
Chem. A. 2000, 104, 8146-8153.
(10) Earlier results from our laboratory (Roberts, J. C.; Pincock, J. A.
Can. J. Chem. 2003, 81, 709-722) have shown that solvent adducts 4 and
5 cannot interconvert during the steady-state irradiation experiments.
(11) Although the photophysical measurements indicate that the trans
isomers are the reactive substrates, rapid trans-cis isomerization does occur
during the steady-state irradiation experiments; see ref 9 for the yield-versus-
time plots for the irradiations of trans-1a-e and cis-1a-e in TFE.
(8) (a) Lewis, F. D.; Crompton, E. M. J. Am. Chem. Soc. 2003, 125,
4044-4045. (b) Crompton, E. M.; Lewis, F. D. Photochem. Photobiol. Sci.
2004, 3, 660-668. (c) Lewis, F. D.; Sinks, L. E.; Weigel, W. Sajimon, M.
C.; Crompton, E. M. J. Phys. Chem. A 2005, 109, 2443-2451.
(9) Roberts, J. C.; Pincock, J. A. J. Org. Chem. 2004, 69, 4279-4282.
J. Org. Chem, Vol. 71, No. 4, 2006 1481

Roberts and Pincock
TABLE 1. Summary of Photophysical Data for Stilbene Derivatives trans-1a-e in Acetonitrile
b
substrate
λ
max (abs) (nm)
ꢀmax (M-1 cm-1
)
λ
max (fluor) (nm)
λ
0,0 (nm)
τs (ns)
æfa
ætc
trans-1a
trans -1b
trans -1c
trans -1d
trans -1e
295, 307
302, 317
295
299, 306
301, 321
28500, 27600
28700, 27700
25800
30100, 29800
20600, 24700
350
374
359
390
385
328
344
336
341
353
0.07c
0.04f
0.023d
0.007g
0.16i
0.32
0.035
0.45e
0.53
0.39
0.28
0.59
0.93h
16.9
<0.5j
a 295 nm excitation, relative to φf ) 0.0433 for trans-1a in hexanes, ref 3. b Determined by steady-state irradiation at 300 nm relative to φtc ) 0.40 for
trans-1a in cyclohexane, ref 2. c From ref 15 (in hexane). d Literature value φf ) 0.016, ref 16. e Literature value φtc ) 0.45, ref 16. f From ref 8b (in THF).
g Literature values φf ) 0.031 (cyclohexane) and 0.040 (THF), ref 8b. h Literature value τs ) 0.78 ns (THF), ref 8b. i Literature values φf ) 0.23 (cyclohexane)
and 0.26 (THF), ref 8b. j Below the time resolution of the instrument.
has been extensively investigated by Yates and co-workers.12
For methoxy-substituted styrenes, the established mechanism
involves photoprotonation of a zwitterionic excited state,
followed by nucleophilic trapping of a carbocation intermediate
by the solvent. These reactions proceed more rapidly for
3-methoxystyrene than for 4-methoxystyrene.13 In work that is
particularly relevant to our own studies, McClelland and co-
workers employed styrene photoprotonation as a method for
the generation and subsequent observation of arylmethyl car-
bocations in TFE and in 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP).14 Although the photoaddition of TFE to methoxystil-
benes can also be explained by a photoprotonation mechanism,9
Laarhoven and co-workers have reported that the photoaddition
of methanol to stilbene does not proceed via carbocation
intermediates.15 In light of this disagreement, we have performed
a variety of mechanistic experiments, including laser flash
photolysis studies, designed to probe the nature of any reactive
intermediates involved in the photoaddition reactions.
Results
FIGURE 1. Fluorescence spectra of trans-1a-e in acetonitrile solution.
Photophysical Properties: Stilbenes. A compilation of the
given substrate to that of trans-1a in hexanes, which was chosen
photophysical properties of trans-1a-e in acetonitrile solution
as the reference system for these studies.3 The values in Table
is provided in Table 1. Many of these values were reported in
our earlier communication,9 but have been included now for
ease of reference. The absorption spectra (Supporting Informa-
tion) are all quite similar and are typical of substituted stilbene
derivatives: absorption maxima (λmax (abs)) in the range of
295-320 nm and extinction coefficients (ꢀmax) on the order of
25000-30000 M-1 cm-1. Closer examination of the λmax (abs)
values in Table 2 indicates that the para-methoxy substrates
trans-1b and trans-1e absorb at slightly longer wavelengths than
trans-1c and trans-1d, which possess only meta substituents.
The fluorescence spectra of trans-1a-e in acetonitrile solution
are shown in Figure 1. In contrast to the absorption spectra, the
fluorescence behavior of the substrates is not uniformsaltering
the position of the methoxy substituents induces significant
changes in both the wavelength and intensity of fluorescence
from the stilbene chromophore. A comparison of the mono-
substituted compounds shows that the para isomer (trans-1b,
1 (and the spectra in Figure 1) clearly indicate that substrates
possessing m-methoxy substituents are highly fluorescent in
comparison to the parent trans-1a. The very strong fluorescence
of the di-m-methoxy derivative trans-1d is particularly striking
(φf ) 0.32). In contrast, the p-methoxy substrate trans-1b
displays only weak fluorescence. That the fluorescence of trans-
1e (3,4-dimethoxy, φf ) 0.04) is intermediate between that of
trans-1b (4-methoxy (φf ) 0.007) and trans-1c (3-methoxy, φf
) 0.16) appears to indicate a conflict between the effects of p-
and m-methoxy substituents.
Quantum yields of trans-cis isomerization were obtained by
performing steady-state irradiations of the trans isomers in
acetonitrile solution. At low conversions (generally <10%), the
increase in the yields of the cis isomers were linear in time as
determined by GC-FID analysis; the yield versus time plots
for these irradiations are shown in Figure 2. The slopes of these
plots were assumed to be proportional to the quantum yield of
trans-cis isomerization for each substrate. Again, the unsub-
λ
max (fluor) ) 374 nm) fluoresces at a longer wavelength than
the meta isomer (trans-1c, λmax (fluor) ) 359 nm). Surprisingly,
the two disubstituted compounds fluoresce at very nearly the
same wavelength (trans-1d, λmax (fluor) ) 390 nm; trans-1e,
λmax (fluor) ) 385 nm). Fluorescence quantum yields were
determined by comparing the intensity of fluorescence for a
stituted compound trans-1a was taken as the reference (φtc
)
0.40 in cyclohexane at room temperature using 313 nm
excitation).2 The reliability of this method is indicated by the
fact that our value for trans-1a in acetonitrile (φtc ) 0.45) is an
exact match with the measurement reported by Muzzucato.16
The φtc values in Table 2 are clearly dependent on the position
of the methoxy substituents. In comparison to the unsubstituted
compound, the p-methoxy derivative trans-1b displays a larger
(12) Wan, P.; Yates, K. ReV. Chem. Intermed. 1984, 5, 157-181.
(13) McEwen, J.; Yates, K. J. Phys. Org. Chem. 1991, 4, 193-206.
(14) Cozens, F. L.; Kanagasabapathy, V. M.; McClelland, R. M.;
Steenken, S. Can. J. Chem. 1999, 77, 2069-2082.
(15) Woning, J.; Oudenampsen, A.; Laarhoven, W. H. J. Chem. Soc.,
Perkin Trans. 2 1989, 2147-2154.
(16) Muzzucato, U. Pure Appl. Chem. 1982, 54, 1704-1721.
1482 J. Org. Chem., Vol. 71, No. 4, 2006

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
TABLE 2. Fluorescence Maxima (λmax (fluor)) for trans-1a-e in a
Series of Six Solvents
solvent
trans-1a trans-1b trans-1c trans-1d trans-1e
cyclohexane
dibutyl ether
diethyl ether
ethyl acetate
2-propanol
349
349
349
349
348
350
364
367
366
366
365
374
358
358
357
358
357
359
364
372
374
382
382
390
372
375
373
382
374
385
acetonitrile
TABLE 3. Ground- and Excited-State Dipole Moments for
trans-1a-e
compd
µga (D)
µeb (D)
trans -1b
trans -1c
trans -1d
trans -1e
1.6
1.3
1.3
0.6
6.8
3.2
13.2
8.7
a Calculated using the Gaussian 98 software package (B3LYP/6-31G(d),
optimized geometries). b Determined from solvatochromic analysis as
described in the Supporting Information.
FIGURE 2. Yield of cis isomer formation as a function of time during
the irradiation (300 nm) of trans-1a-e in acetonitrile solution: trans-
1a (diamonds), trans-1b (squares), trans-1c (triangles), trans-1d
(crosses), trans-1e (circles).
played a significant solvent dependence; the values for trans-
1a-e are displayed in Table 2. Note that the amount of variation
in the fluorescence wavelength with solvent polarity is not the
same for all five stilbenes; the fluorescence maximum of trans-
1a is essentially the same in all six solvents, while the
fluorescence maximum for trans-1d undergoes a bathochromic
shift of 26 nm in acetonitrile compared to cyclohexane.
The solvatochromic behavior displayed by some substrates
in Table 2 has been observed previously for many other
compounds, including substituted stilbene derivatives.7,8 In those
cases, the results have been successfully analyzed by using the
Lippert-Mataga treatment.18 This technique provides values for
the dipole moments of the emissive excited states (µe), which
can possess substantial charge-transfer character. We suspected
that the involvement of charge-transfer excited states might play
an important role in the photochemistry of trans-1a-e, so the
Lippert-Mataga treatment of these substrates was pursued.
Table 3 provides values of µg (calculated) and µe (experimental)
for trans-1b-e; complete details of this analysis are provided
in the Supporting Information. Our values for the excited-state
dipole moments of trans-1b and trans-1c are reasonably close
to the values previously reported by Crompton and Lewis (µe
) 5.4 and 3.6 D, respectively).8b The results in Table 3 indicate
that the emissive excited states of the methoxy-substituted trans-
stilbenes have larger dipole moments than do the corresponding
ground states. The very large change in dipole moment that
accompanies excitation of the trans-1d is the most obvious
example of this effect (µg ) 1.3 D, µe ) 13.2 D).19
quantum yield of trans-cis isomerization (φtc ) 0.53). The
presence of m-methoxy substituents has the opposite effect: both
trans-1c (φtc ) 0.39) and trans-1d (φtc ) 0.28) isomerize less
readily than trans-1a. Given these trends, the very high quantum
yield of isomerization for trans-1e (3,4-dimethoxy, φtc ) 0.59)
is somewhat surprising.
Importantly, the values in Table 1 support the established1
relationship between φf and φtc: those substrates with higher
quantum yields of isomerization are more weakly fluorescent
than those which display smaller values of φtc. In this regard,
the differences between the values for the 4-methoxy derivative
trans-1b (φf ) 0.007, φtc ) 0.52) and the 3-methoxy derivative
trans-1c (φf ) 0.16, φtc ) 0.39) are particularly striking. The
singlet lifetimes of the five substrates, determined by single
photon counting techniques, also corroborate the quantum yield
measurements. The two substrates with the smallest quantum
yields of isomerization display the longest singlet lifetimes
(trans-1c, τs ) 0.9 ns; trans-1d, τs ) 17 ns). The singlet
lifetimes of trans-1a, trans-1b, and trans-1e could not be
determined using our single photon counting apparatus (time
resolution limit 0.5 ns), but the very short lifetimes of these
three substrates are consistent with their high quantum yields
of isomerization (φtc g 0.45) and low quantum yields of
fluorescence (φf e 0.04). The singlet lifetimes of trans-1a (0.07
ns in hexane)17 and trans-1b (0.05 ns in THF)8b have been
reported in the literature, and these values are included in Table
1 as points of reference.
To investigate the effect of solvent polarity on the excited
states of trans-1a-e, the photophysical properties of these
substrates were determined in a series of six solvents of varying
polarity (cyclohexane, di-n-butyl ether, diethyl ether, ethyl
acetate, 2-propanol, and acetonitrile). The absorption maxima
and extinction coefficients of trans-1a-e were essentially
unchanged through the series of six solvents; the values in
acetonitrile (Table 1) are representative. In contrast, the
fluorescence maxima (λmax (fluor)) of several substrates dis-
In addition to their solvatochromic fluorescence maxima,
trans-1a-e display a variety of other solvent-dependent pho-
tophysical properties; the fluorescence quantum yields of these
substrates in the six solvents are given in Table 4. The quantum
yields of isomerization were only determined in cyclohexane
and acetonitrile, and these values are provided in Table 5.
Notably, our experimentally determined isomerization quantum
(18) (a) Lippert, E. Z. Elektrochem. 1957, 61, 962. (b) Mataga, N.; Kaifu,
Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465.
(19) The substantial increase in the dipole moment of the trans-1d excited
state has been reported previously: Hara, M.; Tojo, S.; Majima, T. J.
Photochem. Photobiol. A: Chem. 2004, 162, 121-128. However, we note
that these authors reported values for other photophysical properties of trans-
1d in acetonitrile (φf ) 0.023, τs ) 4 ns) that are not consistent with our
measurements. At this time, we have no explanation for these differences.
(17) Kim, S. K.; Courtney, S. H.; Fleming, G. R. Chem. Phys. Lett. 1989,
159, 543-548.
J. Org. Chem, Vol. 71, No. 4, 2006 1483

Roberts and Pincock
TABLE 4. Fluorescence Quantum Yields (Of) for trans-1a-e in a
Series of Six Solvents
photophysical properties of the styrenes are provided in Table
6, and the data for the arylpropenes are given in Table 7.
solvent
trans -1a trans -1b trans -1c trans -1d trans -1e
cyclohexane
dibutyl ether
diethyl ether
ethyl acetate
2-propanol
0.038
0.042
0.035
0.032
0.030
0.023
0.022a
0.020
0.015
0.013
0.011
0.007
0.18b
0.20
0.18
0.18
0.15
0.16
0.18
0.26
0.27
0.29
0.27
0.32
0.060
0.050
0.051
0.046
0.041
0.035
acetonitrile
a Literature value φf ) 0.031, ref 8b. b Literature value φf ) 0.23, ref
8b.
TABLE 5. Isomerization Quantum Yields (Otc) for trans-1a-e in
The extinction coefficients for the S0 f S1 transitions of the
eight arylalkenes are about an order of magnitude lower in
magnitude than those of the methoxy-substituted stilbene
derivatives (absorption and fluorescence spectra are provided
as Supporting Information). This observation is consistent with
the extended π-conjugation that is present in the stilbenes but
not in the arylalkenes. Note that the data in Tables 6 and 7
were obtained using 295 nm excitation, so as to facilitate
comparison with the stilbene results (vide supra, Table 1).
Neither of the two unsubstituted compounds (8a and trans-9a)
absorb significantly at 295 nm, and so these substrates have
not been included in Tables 6 and 7.22 For those cases where
literature values23-25 are available for comparison with our
results, the agreement between the two values is very good.
The results in Tables 6 and 7 corroborate the prediction that
the styrenes and trans-1-arylpropenes should display very similar
photophysical properties. For example, the fluorescence maxima
of identically substituted styrenes and trans-1-arylalkenes are
quite similar (i.e., λmax (fluor) 8x ≈ λmax (fluor) trans-9x). The
same statement can be made for many of the other measure-
ments. Perhaps not surprisingly, the properties of the corre-
sponding stilbenes (trans-1b-e, Table 2) are quite different from
those of either the styrenes or the trans-1-arylpropenes. In
general, the stilbene derivatives display longer wavelength
emission maxima, shorter singlet lifetimes, and smaller quantum
yields of fluorescence when compared to the styrenes and trans-
1-arylpropenes. The quantum yields for trans-cis isomerization
are also much greater for the stilbenes than for the trans-1-
arylpropenes.
The fluorescence maxima of the eight substrates (8b-e and
trans-9b-e) appear at shorter wavelengths when compared to
the corresponding methoxystilbenes; this observation is con-
sistent with the shorter-wavelength absorption maxima of the
former compounds. For any given arylalkene, the position of
λmax (fluor) is also quite insensitive to the polarity of the
medium; no attempts were made to determine the solvatochro-
mic behavior of these substrates. The fluorescence quantum yield
measurements also deserve some mention. In particular, the 3,5-
dimethoxy substrates 8d and trans-9d display the lowest
fluorescence intensities for either set of compounds (in aceto-
nitrile, φf ) 0.15 and 0.05, respectively). These results are
especially interesting given that the fluorescence of trans-3,5-
dimethoxystilbene (trans-1d) was the highest of the five
stilbenes investigated (vide supra, Table 2). Apart from the low
Cyclohexane and Acetonitrile
solvent
trans -1a trans -1b trans -1c trans -1d trans -1e
cyclohexane
acetonitrile
0.40a
0.45d
0.39b
0.53
0.32c
0.39
0.29
0.28
0.41
0.59
a Reference value from ref 2. b Literature value φtc ) 0.40, ref 2.
c Literature value φtc ) 0.31, ref 2. d Literature value φtc ) 0.45, ref 16.
yields for trans-1b and trans-1c in cyclohexane (φtc ) 0.39 and
0.32, respectively) are excellent matches with the literature
values reported by Gu¨sten and Klasinc (φtc ) 0.40 and 0.31,
respectively).2 The results in Tables 4 and 5 indicate that the
excited states of trans-1b, trans-1c, and trans-1e all undergo
more efficient trans-cis isomerization in more polar solvents
(i.e., φtc (cyclohexane) < φtc (acetonitrile)) while also becoming
less fluorescent (i.e., φf (cyclohexane) > φf (acetonitrile)). The
important exception to this trend is trans-3,5-dimethoxystilbene,
trans-1d: the quantum yield of isomerization for this substrate
is essentially the same in either solvent (φtc ) 0.29), and the
fluorescence quantum yield of trans-1d is increased substantially
in more polar solvents. Note that trans-1c and trans-1d display
identical fluorescence quantum yields in cyclohexane (φf )
0.18). However, whereas the fluorescence of trans-1c falls to
φf ) 0.16 in acetonitrile, the fluorescence of trans-1d increases
to φf ) 0.32. The solvent dependence of the singlet lifetimes
of these substrates is also intriguing. The singlet lifetime of
trans-1c is virtually solvent-independent (τs ) 0.7 ns in
cyclohexane, 0.9 ns in acetonitrile), while the singlet lifetime
of trans-1d is much longer in the more polar solvent (τs ) 3.7
ns in cyclohexane, 16.9 ns in acetonitrile).20
Photophysical Properties: Arylalkenes. The results de-
scribed in the previous section clearly demonstrate that there
are significant differences between the photophysical properties
of trans-1a-e. To determine whether these differences are
general for all methoxy-substituted arylalkene derivatives, the
corresponding styrenes (8a-e) and trans-1-arylpropenes (trans-
9a-e) have also been investigated.21 Based on the fact that these
two classes of substrates possess the same chromophore, they
were expected to have very similar photophysical properties.
The utility of the arylpropenes lies in the potential to determine
isomerization quantum yields; photochemical isomerization of
the styrenes represents a degenerate rearrangement (i.e., refor-
mation of starting material). The experimentally determined
(20) An important note regarding the fluorescence decay of trans-1d in
acetonitrile is that the mathematical fit to the data is greatly improved
through the use of a biexponential model: τs (short) ) 3.8 ns (36%), τs
(long) ) 18.6 ns (64%). We are now in the process of characterizing this
interesting behavior in much greater detail.
(21) Note that for all three classes of substrate (stilbenes, styrenes, and
arylpropenes), the methoxy substitution pattern is provided by the letter in
the structure label (i.e., trans-1b, 8b, and trans-9b all possess a 4-methoxy
substituent, and so forth).
(22) We have repeated all photophysical measurements of 8a-e and
trans-9a-e using 254 nm excitation; these results are provided in the
Supporting Information.
(23) Condirston, D. A.; Laposa, J. D. Chem. Phys. Lett. 1979, 63, 313-
317.
(24) Lewis, F. D.; Bassani, D. M.; Caldwell, R. A.; Unett, D. J. J. Am.
Chem. Soc. 1994, 116, 10477-10485.
(25) Lewis, F. D.; Zuo, X. J. Am. Chem. Soc. 2003, 125, 8806-8813.
1484 J. Org. Chem., Vol. 71, No. 4, 2006

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
TABLE 6. Summary of Photophysical Data for Methoxy-Substituted Styrenes 8b-e in Cyclohexane and Acetonitrile
compd
solvent
λ
nax (abs)a (nm)
ꢀmax (M-1 cm-1
)
λ
nax (fluor)a (nm)
τsb (ns)
φfb,c
8b
cyclohexane
acetonitrile
cyclohexane
acetonitrile
cyclohexane
acetonitrile
cyclohexane
acetonitrile
259, 290 (sh), 302 (sh)
19000, 2500, 1500
9400, 2300
322
323
327
330
341
350
331
334
6.6
6.3
6.8
7.8
3.8
6.9
3.3
3.2
0.33
0.21
0.32
0.28
0.12
0.5
8c
8d
8e
249, 295
256, 299
9800, 2000
261, 293, 309 (sh)
14300, 5300, 3300
0.32
0.29
a Shoulders of absorption spectra are labeled (sh). b Values obtained following excitation at 295 nm. c Standard value: φf ) 0.0433 for trans-1a in
hexane, ref 3.
TABLE 7. Summary of Photophysical Data for Methoxy-Substituted Arylpropenes trans-9b-e in Hexane and Acetonitrile
d
compd
solvent
λ
max(abs)a (nm)
ꢀmax (M-1 cm-1
)
λ
max (fluor)b (nm)
τsb (ns)
æfb,c
ætc
trans-9be
hexane
acetonitrile
hexane
acetonitrile
hexane
acetonitrile
hexane
258, 292 (sh), 305 (sh)
19800, 2100, 1200
11800, 2400
328
330
325
327
338
343
334, 333
7.9f
8.7i
5.9
6.5
1.5
2.9
3.7
4.0
0.35g
0.27
0.31
0.24
0.04
0.05
0.35
0.28
0.10h
0.12j
0.07
0.10
0.03
0.05
0.12
0.14
trans-9c
trans-9d
trans-9e
252, 293
257, 296
12100, 1900
259, 290, 308 (sh)
16300, 5200, 3200
acetonitrile
a Shoulders of absorption spectra are labeled (sh). b Values obtained following excitation at 295 nm. c Standard value: φf ) 0.0433 for trans-1a in
hexane, ref 3. d Determined by steady-state irradiation at 300 nm relative to φtc ) 0.40 for trans-1a in cyclohexane, ref 2. e Literature values obtained using
281 nm excitation, ref 20. f Literature value 8.5 ns. g Literature value 0.42. h Literature value 0.12. i Literature value 7.2 ns. j Literature value 0.13.
TABLE 8. Percent Conversion of Styrenes (8b-e), Arylpropenes
(trans-9b-e), and Stilbenes (trans-1a-e) to the Corresponding TFE
Adducts
SCHEME 2. Products Detected Following Irradiation of
8b-e and trans-9b-e in TFE
structure type
styrene
substituent
compda
% conversionb,c
4-methoxy
3-methoxy
3,5-dimethoxy
3,4-dimethoxy
4-methoxy
8b
8c
8d
8e
trans-9b
trans-9c
trans-9d
trans-9e
trans-1a
trans-1b
trans-1c
trans-1d
trans-1e
33
57
82
96
20
3
48
29
5
arylpropene
stilbene
3-methoxy
3,5-dimethoxy
3,4-dimethoxy
unsubstituted
4-methoxy
3-methoxy
3,5-dimethoxy
3,4-dimethoxy
fluorescence quantum yields of the 3,5-dimethoxy substrates,
the methoxystyrenes and trans-1-arylpropenes show remarkably
similar fluorescence intensities. Such behavior differs signifi-
cantly from trans-1a-e, for which the quantum yields of
fluorescence spanned nearly 2 orders of magnitude.
4
20
39
7
a The unsubstituted styrene (8a) and arylpropene (trans-9a) derivatives
do not absorb appreciably at 300 nm, so the photoaddition chemistry of
these substrates was not investigated. b Percent conversion of starting
material to solvent adducts following 300 nm irradiation of substrate for 2
min in TFE. c For trans-1b-e and trans-9b-e, the values correspond to
conversion of the mixture of trans and cis isomers to solvent adducts.
Photoaddition of TFE to Arylalkenes. To completely assess
the photochemistry of 8b-e and trans-9b-e, these substrates
were irradiated in TFE solution (Scheme 2). The very low
absorbance of the unsubstituted compounds (styrene 8a and
trans-1-phenylpropene trans-9a) at the maximum output of the
lamps (300 nm) once again prevented detailed investigations
of these substrates. As was expected based on previous studies
by McClelland and co-workers,14 the major product detected
following irradiation of styrenes 8b-e in TFE was the corre-
sponding Markovnikov ether 10.26 Irradiation of trans-9b-e
using the same conditions led to formation of the corresponding
cis isomers and the Markovnikov solvent adducts 11b-e.
Surprisingly, small amounts (<5%) of the anti-Markovnikov
ethers 13 were detected following irradiation of trans-9c and
trans-9e (12 or 13 were not detected for any other substrates).
The values for percent conversion of arylalkene to solvent
adducts are provided in Table 8, and the yield versus time plots
are included in the Supporting Information. To facilitate
comparisons between the reactivity of the different structure
types (styrenes, trans-1-arylpropenes, and trans-stilbenes) to-
ward TFE, Table 8 also includes values for percent conversion
of stilbenes trans-1a-e.27 The data in Table 8 indicate that, for
a given substituent position, the order of substrate reactivity in
TFE is styrene > arylpropene > stilbene. This trend appears to
hold for all twelve substrates, although the very low reactivity
of trans-9c is a notable exception. For the stilbene and
arylpropene structure types, the 3,5-dimethoxy substrates are
the most reactive compounds; for the styrenes, the 3,4-
dimethoxy substrate (8e, 96% conversion) is slightly more
reactive than the 3,5-dimethoxy substrate (8d, 82% conversion).
(26) Small amounts (<5%) of unidentified compounds were also detected
by GC-MS. Based on their long retention times and large molecular weights
(m/z > 300), these products are tentatively assigned to dimers and oligomers
of the arylalkenes.
(27) Note that the percent conversion values reported in Table 8 are for
2 min of irradiation in TFE, whereas the values in ref 9 were for 10 min of
irradiation in TFE.
J. Org. Chem, Vol. 71, No. 4, 2006 1485

Roberts and Pincock
TABLE 9. Quenching of Fluorescence Quantum Yields Of and
Singlet Lifetimes τs by TFE for Styrenes (8b-e), Arylpropenes
(trans-9b-e), and Stilbenes (trans-1a-e)
unsubstituted compound trans-1a is not as reactive as would
be expected based on the fluorescence quenching results.
Surprisingly, the quenching of styrenes 8b-e does not match
the solvent addition reactivity: 4-methoxystyrene 8b reacts with
the slowest rate of the four substrates (33%) but displays the
highest quenching ratio (φf ratio ) 12.2). In addition, the
quenching ratios are not consistent between different structure
types: trans-9d undergoes solvent addition more rapidly than
trans-1d (48% and 39%, respectively), but the fluorescence of
the latter substrate is more easily quenched by TFE (φf ratios
) 1.8 and 11.1, respectively).
Laser Flash Photolysis. Previous investigations by McClel-
land and co-workers employed laser flash photolysis (LFP) of
a variety of substituted styrenes in solutions of TFE and
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) to generate the cor-
responding arylethyl cations. These cations displayed absorption
maxima in the range of 315-345 nm and decayed with rate
constants on the order of 102-105 s-1 in HFIP. The transient
species were rapidly quenched by nucleophiles, such as azide
and bromide ions. Only those cations possessing highly stabiliz-
ing p-methoxy substituents were successfully observed in TFEs
this observation was attributed to the higher nucleophilicity of
TFE relative to HFIP,28 which results in more rapid quenching
of cations in the former solvent. Indeed, the stabilized 4-meth-
oxyphenethyl cation underwent much faster decay in TFE (kTFE
) 4 × 105 s-1) than in HFIP (kHFIP ) 2 × 102 s-1). With hopes
of directly observing the reactive intermediates involved in the
photochemical addition of TFE to trans-1b-e, we subjected
these four substrates to LFP excitation at 308 nm using a
nanosecond laser system. Based on the structures of the major
products from the irradiations in TFE,9 cations 14b-e were
suspected to be the likely reactive intermediates. To compare
the properties of all four cations in the same solvent, HFIP was
employed for the LFP experiments.
æf
τs
compda æf (AcN)b æf (TFE)b ratioc τs (AcN) τs (TFE) ratioc
8b
8c
8d
8e
trans-9b
trans-9c
trans-9d
trans-9e
trans-1a
trans-1b
trans-1c
trans-1d
trans-1e
0.21
0.28
0.15
0.29
0.27
0.24
0.05
0.28
0.023
0.007
0.16
0.32
0.035
0.02
0.15
0.02
0.05
0.20
0.23
0.03
0.20
0.01
0.006
0.08
0.029
0.026
12.2
1.8
9.6
6.4
1.4
1.0
1.8
1.4
2.3
1.1
1.9
11.1
1.4
6.3
7.8
6.9
3.2
8.7
6.5
2.9
4.0
<0.5d
4.6
0.9
0.7
6.6
6.3
2.3
3.1
> 12.6
1.7
8.1
4.5
1.3
1.0
1.3
1.3
<0.5d
<0.5d
0.93
16.9
<0.5d
<0.5d
<0.5d
0.63
1.64
<0.5d
1.5
10.3
a The unsubstituted styrene (8a) and arylpropene (trans-9a) derivatives
do not absorb appreciably at 295 nm, so these substrates were not
investigated. b 295 nm excitation, relative to φf ) 0.0433 for trans-1a in
hexanes, ref 3. c Calculated as value (TFE)/value (acetonitrile). d Shorter
than the time resolution of the instrument.
Quenching of Excited States by TFE. An important
observation from our previous investigations was the quenching
of the strong fluorescence of trans-1d in mixtures of TFE and
acetonitrile.9 The strong upward curvature of a Stern-Volmer
plot for fluorescence quenching versus the percentage of TFE
in acetonitrile (0-100%) indicated that the quenching mecha-
nism does not follow a simple bimolecular process. We
estimated that the quenching of trans-1d occurs with a rate
constant of 9 × 106 M-1 s-1 in the range of 0-20% TFE and
1 × 108 M-1s-1 in the range of 80-100% TFE. Similar
experiments with deuterated TFE (TFE-OD) provided a quench-
ing ratio of kTFE/kTFE-OD ) 3:1 in the high concentration regime.
Finally, irradiations of trans-1d in mixtures of TFE and
acetonitrile indicated a strong correlation between the fluores-
cence quenching process and the formation of photoaddition
products 4d and 5d (vide supra, Scheme 1).
To establish that the excited states of the other substrates are
also quenched by TFE, fluorescence quantum yields and singlet
lifetimes were determined for all thirteen compounds in TFE
solution. These results are displayed in Table 9, along with the
same values for the substrates in acetonitrile solution. Quenching
ratios were calculated by dividing the quantum yield or singlet
lifetime in TFE by the corresponding value in acetonitrile. For
those cases where both quantum yields and singlet lifetimes
are available for both solvents, the quenching ratios determined
by either method are remarkably consistent. Taking the stilbenes
as an example, the ratios determined by fluorescence quantum
yields (1.9 for trans-1c and 11.1 for trans-1d) are quite close
to those determined using singlet lifetimes (1.5 for trans-1c and
10.3 for trans-1d). This implies that the quantum yield ratios
should be reliable for those substrates possessing very short
singlet lifetimes (<0.5 ns).
The quenching ratios in Table 9 indicate that the singlet
excited states of the styrenes, arylpropenes, and stilbenes are
quenched by TFE. However, there is not always a good
correlation between the magnitude of fluorescence quenching
(Table 9) and the yield of solvent adduct from the steady-state
irradiations (Table 8). Considering each structure type separately,
the order of fluorescence quenching matches the order of
reactivity quite well for the arylpropenes trans-9b-e. The same
is true for the methoxystilbenes trans-1b-e, although the
A representative kinetic trace of the change in optical density
(∆OD) at 320 nm following 308 nm excitation of trans-1d in
HFIP is provided in Figure 3. The intense signal bleaching is
attributed to the efficient trans-cis isomerization process; the
cis isomers possess much smaller extinction coefficients at the
monitoring wavelengths for all LFP experiments reported herein.
This bleaching effect, which is general to all four methoxy-
substituted stilbene derivatives, complicates the use of LFP for
these substrates. Clearly, the substituted styrenes employed by
McClelland and co-workers were much better suited to LFP
studiessphotoisomerization of those substrates would result in
re-formation of the same compounds. Regardless of the signal
bleaching effect for the stilbene experiments, rate constants for
transient decay were successfully determined for all four
substrates, Table 10. The inset in Figure 3 displays a plot of
(28) NOTs ) 0.06 for ethanol, -3.07 for TFE, and -4.27 for 97% HFIP.
Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17, 121-
158.
1486 J. Org. Chem., Vol. 71, No. 4, 2006

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
1
FIGURE 4. Newman projection for photoproduct 4a and H NMR
analysis of the ABX protons of the product formed in TFE (left) or
TFE-OD (right).
FIGURE 3. Kinetic decay trace at 320 nm following 308 nm LFP of
trans-1d (inset: rate constant for cation decay versus concentration of
tetrabutylammonium bromide, TBAB).
rearrangement of the trans-1a singlet state via a 1,2-hydride
shift, followed by insertion of the resulting carbene intermediate
into the O-H bond of the solvent.29 Observations that led the
authors to these conclusions included the failure of excess acid
to either quench the fluorescence of trans-1a or increase the
yield of solvent adduct upon steady-state irradiation in methanol.
However, the most important experiment in these studies was
the irradiation of trans-1a in deuterated methanol (CH3OD).
1H NMR analysis of the product mixture indicated that both
the deuterium and the methoxy nucleophile were incorporated
on the same carbon of the resulting solvent adduct.
TABLE 10. Compilation of Data for Cations 14b-e in HFIP
Solution: Absorption Maxima, Rate Constants for Decay in Pure
HFIP, and Bimolecular Rate Constants for Quenching by Bromide
Ion
compd
cation
λ
max (abs) (nm)
kHFIP (s-1
)
kBr (M-1 s-1
)
trans-1b
trans-1c
trans-1d
trans-1e
14b
14c
14d
14e
340
315
320
350
4 × 103
3 × 106
3 × 106
1 × 104
7.8 × 108
1.1 × 109
2.9 × 109
7.4 × 108
the rate constant for decay of cation 14d versus the concentration
of bromide ion in HFIP. Similar experiments for the other three
substrates provided the bimolecular rate constants for quenching
of cations 14b-e by bromide ion, also included in Table 10.
The results in Table 10 are fully consistent with a reaction
mechanism that proceeds via photoprotonation of the methoxy-
stilbene excited states, followed by nucleophilic trapping of a
carbocation intermediate by the solvent. The transient intermedi-
ates display all the important characteristics of carbocations:
more facile observation in HFIP than in TFE, rapid quenching
by nucleophiles, and absorption maxima that are quite similar
to those reported by McClelland and co-workers.14 The data
also indicate that the cations derived from trans-1b and trans-
1e are more highly stabilized than those derived from trans-1c
and trans-1d. Indeed, cations 14b and 14e have longer
wavelength absorption maxima, and react more slowly with
either HFIP or bromide ion. These observations support the
cation structure assignments (14b-e) that were made on the
basis of the major products from the irradiations of trans-1b-e
in TFE, especially with regard to the highly stabilizing p-
methoxy substituents that are present in cations 14b and 14e.
Although the steady-state irradiations and LFP experiments were
performed in two different solvents (TFE and HFIP, respec-
tively), the same solvent photoprotonation mechanism appears
to operate in both cases.
To examine the possibility that formation of cations 14a-e
might occur in competition with a carbene insertion mechanism,
we performed steady-state irradiations of trans-1a-e in TFE-
OD. The results obtained for trans-1a provide a good introduc-
tion to these experiments, which are not straightforward to
interpret. Figure 4 displays a Newman projection of the preferred
conformation for photoproduct 4a, with the two phenyl rings
at a dihedral angle of 180°. Also provided are the chemical shifts
and coupling constants for the three protons, as well as the
relevant portions of the 1H NMR spectra of 4a following
irradiation of trans-1a in either TFE or TFE-OD. The geminal
protons are diastereotopicsthe chemical shifts and coupling
1
constants for 4a are consistent with previous H NMR studies
of 1,2-diphenylethanol by Kingsbury and Thornton.30 The
product formed by irradiation of trans-1a in TFE also gives
the expected 1:1:1 ratio for the integrations of the three protons.
1
Following irradiation of trans-1a in TFE-OD, the H NMR
spectrum of 4a is significantly modified (Figure 4, spectrum
on right). The signals of all three protons are reduced to doublets,
and the integrations are altered as well. These changes indicate
that incorporation of deuterium does occur, but also that the
location of incorporation is not always the same. Conceptually,
the photochemical addition of TFE-OD to trans-1a might result
in any one of the three protons in 4a being “replaced” by
deuterium.31 Thus, there are three isomers for the structure of
Irradiations in TFE-OD. Although the LFP results support
the proposed photoprotonation mechanism for addition of TFE
to the methoxystilbenes, Laarhoven and co-workers have
previously determined that the photochemical addition of
methanol to trans-1a does not occur via protonation of excited
states.15 Instead, these researchers proposed a photoaddition
mechanism involving two competitive pathways: (1) direct
addition of methanol across the central alkene unit and (2)
(29) The carbene-insertion mechanism is well-established for the pho-
toaddition of alcohols to simple aliphatic alkenes. This process is thought
to involve (π, R(3s)) Rydberg states, which react by processes that are
characteristic of radical cations. Kropp, P. J. In CRC Handbook of
Photochemistry and Photobiology; Horspool, W., Lenci, F., Eds.; CRC
Press: Boca Raton, 2004; pp 9-1-9-11.
(30) Kingsbury, C. A.; Thornton, W. B. J. Org. Chem. 1966, 31, 1000-
1004.
J. Org. Chem, Vol. 71, No. 4, 2006 1487

Roberts and Pincock
FIGURE 5. Three isomers of 4a that may be formed by photoaddition
of TFE-OD to trans-1a.
FIGURE 6. Observed GC-MS fragmentation of photoproduct 5d
when formed by irradiation in TFE (left) or in TFE-OD (right).
TABLE 11. 1H NMR Analysis of the Products Formed by
Irradiation of trans-1a-e in TFE and TFE-OD
spectra obtained for the TFE/TFE-OD irradiations are far
superior to those from the methanol reactions.
substrate
producta
% insertionb
% anti
% syn
The results for the irradiations in TFE-OD support all aspects
of our proposed9 mechanism for addition of the solvent to the
substituted stilbene derivatives. The relative importance of the
carbene insertion product is much lower for the irradiation of
trans-1a in TFE-OD (12%) than in methanol-O-d (39%). The
calculated amounts of carbene insertion product are even less
for the methoxystilbenes trans-1b-e (6-0%) and are essentially
zero considering the error involved with determining accurate
integrations for the three peaks. Therefore, the carbene insertion
mechanism, which accounts for 40-50% of the product formed
in methanol solution, appears to be of negligible importance
for the TFE irradiations. This solvent dependence may be
attributed to the ability of TFE to act as a photoprotonation
reagent and thereby react with an excited substrate prior to the
1,2-hydride shift that initiates the carbene insertion mechanism.
The relationship between the relative amounts of syn and anti
addition products and the position of the methoxy substituent
is also quite interesting. In particular, the relative amount of
syn addition is lower for the substrates possessing p-methoxy
substituents compared to those with m-methoxy substituents.
For example, syn ) 53% for trans-1b (4-methoxy) and 73%
for trans-1d (3,5-dimethoxy).
The absence of substantial carbene insertion product following
irradiation of trans-1a-e in TFE-OD was also supported by
GC-MS analysis of the reaction mixtures. As indicated in our
previous communication,9 the solvent adducts 4 and 5 lend
themselves particularly well to GC-MS techniques. The mo-
lecular ions of these products undergo preferential fragmentation
to generate the arylmethyl cations that are alkoxy-substituted,
presumably due to the enhanced stability of these ions over the
alternatives. Importantly, the mass-to-charge ratios (m/z) for
these ions are characteristic of the regiochemistry of the
photoaddition reactions. Following irradiation of a given
substrate in TFE-OD, the molecular ion of the product is
increased by one mass unit, but the mass of the trifluoroethoxy-
substituted arylmethyl cation fragment is unchanged. In those
cases where the other possible arylmethyl cation is also detected
in the GC-MS spectrum, a one unit increase in the mass-to-
charge ratio of that fragment is observed. The observed
fragmentation of 5d provides a good example of these results,
Figure 6.
trans-1ac
trans-1a
trans-1b
trans-1c
trans-1d
trans-1e
4ac
4a
4b
5c
5c
39
12
0
22
28
47
27
26
43/33
39
60
53
67
73
57/67
6
2
4e/5e
0d
a See Scheme 1 for product structures. b Highest possible yield of carbene
insertion product; see text for details. c Data for irradiations in methanol
and methanol-O-d (which result in formation of the corresponding methanol
adducts). d The HX signals for the two products formed by irradiation of
trans-1e overlap, so the relative amount of carbene insertion product cannot
be determined.
deuterated 4a (Newman projections shown in Figure 5), each
of which should result from a distinct mechanistic pathway.
Structure 15 corresponds to the product that would be formed
via the rearrangement-carbene insertion mechanism proposed
by Laarhoven and co-workers. The other two isomers represent
either anti (16) or syn (17) addition of TFE-OD across the alkene
bond of trans-1a. That the experimentally determined integration
of HA (0.8H) is larger than that of HB (0.4H) indicates a
preference for syn addition, i.e., the ratio of HA to HB provides
a method for determining the relative amounts of 16 and 17.
The relative amount of 15 is more difficult to assesssformation
of this product results in a decrease in the integration of HX
relative to those of HA and HB. Table 11 provides the calculated
yields of the carbene insertion, anti addition, and syn addition
products for the irradiations in TFE-OD (complete details for
these calculations are included in the Supporting Information).
The first entry in Table 11 is from the reaction of trans-1a
in methanol and methanol-O-d, the reaction investigated by
Laarhoven and co-workers in their original study. The spectra
obtained from this reaction were easily the most difficult to
analyze of all the experimentssthe very low yield of the solvent
adduct and the overlapping signal from product 7a (bibenzyl)
made reliable integration of the important peaks problematic.
Despite these difficulties, the calculated percentages of the three
reaction pathways (39% insertion, 22% anti addition, 39% syn
addition) are in fact very close to the values reported Laarhoven
and co-workers (47% insertion, 24% anti addition, 29% syn
addition). The reasonable agreement between these results is
an excellent indication that the measurements reported in the
current work are reliable, especially given the fact that the
Irradiations of styrenes 8b-e and arylpropenes trans-9b-e
in TFE-OD were also performed. In all eight cases, the GC-
MS spectra of the deuterated (TFE-OD) photoaddition products
were consistent with incorporation of the deuterium and the
trifluoroethoxy nucleophile on different carbons of the original
arylalkene double bond. For the photoproducts derived from
styrenes 8b-e, these assignments were also confirmed by
(31) McClelland and co-workers have reported that methoxy-substituted
aromatics are capable of undergoing ring-protonation following irradiation
in HFIP (Mathihvanan, N.; Cozens, F.; McClelland, R. A.; Steenken, S. J.
1
Am. Chem. Soc. 1992, 114, 2198-2203). However, none of the H NMR
spectra discussed in the current report showed evidence for ring-incorpora-
tion of deuterium, and attempts to observe cyclohexadienyl cations (λmax
> 350 nm) by LFP were unsuccessful.
1
1
analysis of the H NMR spectra. Unfortunately, the H NMR
1488 J. Org. Chem., Vol. 71, No. 4, 2006

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
TABLE 12. Excited-State Dipole Moments µe and Quantum Yields
of Isomerization Otc for trans-1a-e in Cyclohexane (Cyc) or
Acetonitrile (AcN)
spectra of the photoproducts derived from trans-9b-e were
complicated by the additional coupling to the terminal methyl
group, and also by the presence of several conformations of
similar energy. As a result of these issues, the peaks of interest
in the 1H NMR spectra consisted of multiplets that were difficult
to integrate reliably. Thus, analysis of the arylpropene products
rests solely on the GC-MS spectra, which are included in the
Supporting Information.
compd
µea (D)
φtc(Cyc)b
φtc(AcN)b
φtc(AcN) - φtc(Cyc)
trans-1a
trans-1b
trans-1c
trans-1e
trans-1d
0
0.40
0.39
0.32
0.41
0.29
0.45
0.53
0.39
0.59
0.28
0.05
0.14
0.07
0.18
0.00
6.8
3.2
8.7
13.2
a Calculated from analysis of solvatochromic data, Table 3. b Determined
by steady-state irradiation at 300 nm, Table 5.
Discussion
m-Methoxy Effect: Photophysical Characteristics. As
discussed in the Introduction, Lewis and co-workers have
established that m-amino substituents are capable of increasing
the barrier for torsional deactivation of the trans-stilbene singlet
excited state. By examining the temperature dependence of the
singlet lifetimes for trans-stilbene (trans-1a) and trans-3-
aminostilbene (trans-3) in hexane solution, the activation
energies for this process have been estimated as 3.5 and 7.5
kcal/mol, respectively.7b Because of these differences, the
photochemical trans-cis isomerization of trans-3 in cyclohexane
is quite inefficient (φtc ) 0.09) when compared to trans-1a (φtc
) 0.40). The lower quantum yield of isomerization for trans-3
also results in a very long-lived, highly fluorescent excited
singlet state (τs ) 7.5 ns, φf ) 0.78). By way of comparison,
measurements for trans-4-aminostilbene in cyclohexane (φtc )
0.49, φf ) 0.05) indicated that the barrier for excited-state bond
torsion of this substrate is quite similar to that of trans-1a. The
properties displayed by methoxystilbenes trans-1b-e are con-
sistent with a similar perturbation of the torsional barrier by
m-methoxy substituents. In cyclohexane, trans-3-methoxystil-
bene (trans-1c) does have a smaller quantum yield of trans-
cis isomerization (φtc ) 0.32) and larger quantum yield of
fluorescence (φf ) 0.18) when compared to trans-stilbene itself.
However, the effect of the 3-methoxy substituent on the stilbene
chromophore is clearly not as large as for the 3-amino
substituent; these differences are likely due to the more potent
electron-donating ability of the amino group.8b
The photophysical properties of substituted stilbene deriva-
tives also depend on solvent polarity. Waldeck and co-workers
have determined that the barrier for excited-state bond torsion
of trans-1a is decreased from 3.5 kcal/mol in hexane1 to 2.6
kcal/mol in polar nitrile solvents.32 This effect, which has been
explained by a model involving polarization of the torsional
barrier, appears to explain the observation by Muzzucato16 that
trans-1a isomerizes more efficiently in acetonitrile (φtc ) 0.45)
than in hexanes (φtc ) 0.40). We were intrigued by the
possibility of observing similar solvent dependence on the
properties of trans-1a-e. A comparison between the properties
of trans-3 (3-amino)7b and trans-1c (3-methoxy) is interesting
in this regard: upon increasing solvent polarity, the increase
FIGURE 7. Plot of the difference between isomerization quantum
yields in acetonitrile and cyclohexane versus the excited-state dipole
moment for trans-1a, trans-1b, trans-1c, and trans-1e.
closely, Table 12 provides a compilation of the excited-state
dipole moments (µe) and quantum yields of isomerization for
trans-1a-e. Four of the five substrates display a clear relation-
ship between excited-state dipole moments and the increase in
isomerization efficiency upon changing from nonpolar cyclo-
hexane to polar acetonitrile. As shown in Figure 7, a plot of
the difference in the isomerization quantum yields (i.e., φtc(AcN)
- φtc(Cyc)) versus excited state dipole moment is linear for
these four substrates, with a surprisingly high correlation
coefficient of r2 ) 0.961.33 Given that these two properties were
determined using very different experimental techniques (vide
supra), incidental correlation would appear to be quite unlikely.
Although the mathematical relationship that governs this cor-
relation is not clear, these trends do support the hypothesis that
the magnitude of µe dictates the influence of solvent polarity
on the isomerization process.
An important point in the data analysis discussed above is
that the 3,5-dimethoxy substrate trans-1d, which behaves
differently in almost all respects from the other stilbenes, has
not been included in Figure 7. Although this substrate has the
highest dipole moment of the five compounds (µe ) 13.2 D),
the quantum yield of isomerization is the same in both
cyclohexane and acetonitrile (φtc ) 0.29). Furthermore, the
excited state of trans-1d becomes more fluorescent in polar
solvents (φf ) 0.18 in cyclohexane, 0.32 in acetonitrile). Given
these very different results, the failure of trans-1d to fit the
trend in Figure 7 is not surprising. The very large values of µe,
in isomerization efficiency is much larger for trans-3 (φtc
)
0.09 in cyclohexane, 0.23 in acetonitrile) than for trans-1c (φtc
) 0.32 in cyclohexane, 0.39 in acetonitrile). A possible
explanation for these differences may lie in the experimentally
determined excited-state dipole moments (µe ) 11.9 D for trans-
3, µe ) 3.2 D for trans-1c). Assuming that similar differences
in polarization may exist for the excited-state torsional barriers
of these two substrates, solvent polarity should have a greater
effect on the properties of trans-3. To investigate this idea more
(32) Sivakumar, N.; Hoburg, E. A.; Waldeck, D. H. J. Chem. Phys. 1989,
90, 2305-2316.
(33) Removing trans-1a from Figure 7 improves the correlation to r2 )
0.997. Alternatively, the data may indicate an upward curvature.
J. Org. Chem, Vol. 71, No. 4, 2006 1489

Roberts and Pincock
φf, and τs that have been measured for trans-1d are character-
istics of stilbene derivatives which possess charge-transfer
excited states.1d These unusual excited-state properties make
trans-1d an interesting substrate for further investigations; we
are presently completing experiments designed to provide a
better understanding of the photochemistry of this substrate in
particular and also of a number of substituted trans-3,5-
dimethoxystilbenes.
(i.e., in the stilbene system). Although the much longer singlet
lifetime of trans-1d in polar acetonitrile (τs ) 16.9 ns) compared
to nonpolar cyclohexane (τs ) 3.6 ns) supports this hypothesis,
further experiments are required to fully characterize the excited-
state behavior of this interesting substrate.20
m-Methoxy Effect: Photoaddition Chemistry. Two main
aspects of the TFE photoaddition reactions need to be consid-
ered: the reason(s) for the differences in substrate reactivity
and the actual mechanism that converts excited states to solvent
adducts. In our original studies, the relative reactivities of trans-
1a-e were assumed to depend solely on the singlet lifetime of
the substrate in question. The more detailed experiments
provided in the current report demonstrate that this hypothesis
is generally correct, but that additional factors are also important.
Fluorescence quenching results confirm that the excited singlet
states are reactive toward TFE, and there is no evidence that
TFE reacts with any other excited species. The relative order
of reactivity observed for the three main structure types (styrenes
> arylpropenes > stilbenes, see Table 8) is consistent with a
strong correlation between the photoaddition chemistry and the
magnitude of the singlet lifetimes. The low reactivity of the
stilbenes is due to these substrates having smaller barriers for
excited state bond torsion when compared to the styrenes or
arylpropenes. As a result of these differences, the stilbenes
undergo rapid trans-cis isomerization in competition with the
solvent addition reaction. In addition to providing a mechanism
for deactivating the reactive excited state, the trans-cis isomer-
ization process obviously results in formation of the cis isomers.
Importantly, the cis and trans isomers of all five stilbenes are
expected to have quite different photophysical properties. In
particular, the lifetimes of the cis isomers are even shorter than
the corresponding trans isomers; the values for trans-1a (τs )
0.07 ns)17 and cis-1a (τs ) 0.001 ns)34 should be representative.35
Based on these differences, the cis isomers are estimated to be
an order of magnitude less reactive toward TFE than the
corresponding trans isomers. Thus, efficient trans-cis isomer-
ization not only provides a mechanism for deactivation of the
reactive excited state of the trans isomers, but also produces
the cis isomers, which have attenuated reactivity toward TFE.
Eventual cis-trans isomerization allows a pathway for re-
formation of the more reactive trans isomers during the steady-
state irradiations. Essentially the same argument can be used
to explain the observation that photoaddition of TFE to
arylpropenes trans-9b-e occurs more slowly than for styrenes
8b-e, even though both structure types have quite similar
photophysical properties. Excited-state bond torsion leads to
trans-cis isomerization of the arylpropenes but results in
reformation of the starting material in the case of the styrenes.
The reported24 singlet lifetimes of trans-9a (τs ) 11.8 ns) and
cis-9a (τs ) 2.6 ns) suggest that the cis-arylpropenes should be
less reactive toward photoaddition of TFE.
The photophysical properties of substituted styrenes and trans-
1-arylpropenes are also dependent on the height of the barrier
for excited-state bond torsion. Lewis and co-workers have
determined the barriers for both styrene 8a (6.6 kcal/mol in
methylcyclohexane)25 and trans-1-phenylpropene trans-9a (8.8
kcal/mol in hexane, 8.0 kcal/mol in acetonitrile).24 The larger
barriers for excited-state bond torsion in these substrates lead
to less-efficient singlet state bond torsion (compared to trans-
1a). Instead, isomerization occurs to a large extent by intersys-
tem crossing to the triplet state, followed by barrierless bond
torsion. Although the rate constants of intersystem crossing for
trans-1a (kisc ) 3.9 × 107 s-1 1a
)
and trans-9a (kisc ) 4.7 × 107
s-1 24
) are similar, the larger barrier for bond rotation in trans-
9a makes intersystem crossing the major decay pathway for
this compound (φisc ) 0.60). The effects of various aryl
substituents on the photophysical properties of arylpropenes have
also been determined previously. The presence of either electron
donating or electron withdrawing substituents appears to lower
the barrier, particularly in polar solvents; trans-1-(4-methox-
yphenyl)propene (5.6 kcal/mol in hexane, 4.2 kcal/mol in
acetonitrile) and trans-1-(4-cyanophenyl)propene (5.6 kcal/mol
in hexane, 2.6 kcal/mol in acetonitrile) are representative
examples.24 The absorption and fluorescence spectra and singlet
lifetimes of substituted styrenes, including 8b and 8c, have been
reported by Yates and McEwen in their study of the acid-
catalyzed photohydration of these compounds in water.13
Our photophysical measurements involving methoxystyrenes
8b-e and arylpropenes trans-9b-e are consistent with most
aspects of the literature results described in the preceding
paragraphs. As a result of the larger torsional barriers that these
substrates should possess, the singlet lifetimes, quantum yields
of fluorescence, and quantum yields of isomerization are more
uniform than those of the stilbene derivatives. In general, these
substrates display longer singlet lifetimes, larger quantum yields
of fluorescence, and (in the case of the trans-1-arylpropenes)
smaller quantum yields of isomerization. The minimal effect
of solvent polarity on these properties is another consequence
of the larger barriers for excited state bond torsion. One
interesting observation that is not explained by the literature
data is the unusually low quantum yields of fluorescence for
the 3,5-dimethoxy substrates (8d and trans-9d). For the aryl-
propenes, most substrates displayed quite strong fluorescence
in either hexanes or acetonitrile (0.35 g φf g 0.24) except for
trans-9d (φf ) 0.05). This large difference is particularly
intriguing, given that the 3,5-dimethoxy substrate in the stilbene
series (trans-1d) displayed the highest quantum yield of
fluorescence (φf ) 0.32) of those five substrates. In addition to
the low fluorescence displayed by trans-9d, the substrate also
gives the smallest isomerization quantum yield of the trans-1-
arylpropenes. Taken together, these observations may imply that
the 3,5-dimethoxy system undergoes a deactivation process that
results in return to the ground state for the styrene and
arylpropene systems, but instead forms a highly fluorescent
charge transfer state in the presence of a second phenyl ring
The high reactivity of trans-1d (3,5-dimethoxy) toward TFE
is clearly related to the unique photophysical properties dis-
played by this substrate. The very long singlet lifetime (τs )
16.9 ns) and very high excited-state dipole moment (µe ) 13.2
(34) (a) Abrash, S.; Repinec, S.; Hochstrasser, R. M. J. Chem. Phys.
1990, 93, 1041-1053. (b) Todd, D. C.; Jean, J. M.; Rosenthal, S. J.;
Ruggiero, A. J.; Yang, D.; Fleming, G. R. J. Chem. Phys. 1990, 93, 8658-
8668.
(35) Lewis and co-workers have demonstrated that the m-amino effect
does extend the lifetime of the cis-stilbene chromophore. However, the
lifetimes of these species still do not exceed those for the corresponding
trans isomers. For example, in alkane solvents at 200 K, τs ) 7.3 ns for
trans-3 (ref 7b) and τs ) 1.9 ns for cis-3 (ref 7c).
1490 J. Org. Chem., Vol. 71, No. 4, 2006

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes
SCHEME 3. Energy State Diagram for the Deactivation of
Excited-State trans-Arylalkenes
D) for trans-1d are ideally suited to rapid photoprotonation and
solvent adduct formation. That the reactivity of the methox-
ystilbenes depends in part on the dipole moment of the excited
state is not surprising; excited states possessing substantial
charge transfer should be rapidly quenched by protic solvents,
such as TFE. However, those substrates which possess large
excited state dipole moments also display higher isomerization
quantum yields (particularly in polar solvents, vide supra), so
there is certainly a competition between the two effects. This
competition is best demonstrated by trans-1e (3,4-dimethoxy),
which has both a high excited-state dipole moment (µe ) 8.7
D) and a high trans-cis isomerization quantum yield (φtc
)
essentially the same reactivity as 14c and 14d. These ideas are
supported by the very similar amounts of syn addition for all
three products (5c 67% syn, 5d 73% syn, 5e 67% syn).
0.59 in acetonitrile). As a result of the conflicting effects of µe
on the reactivity of trans-1e in TFE, the reactivity of this
substrate (7% conversion after 2 min) is intermediate between
that of trans-1c (20%, µe ) 3.2 D, φtc ) 0.39) and trans-1b
(4%, µe ) 6.8 D, φtc ) 0.52). These results indicate that,
although the quantum yield of isomerization of trans-1e is larger
than that of trans-1b, the higher dipole moment of the former
substrate makes it more reactive toward TFE. Conversely, the
low quantum yield of isomerization (and longer singlet lifetime)
of trans-1c makes this substrate more reactive in the photoad-
dition experiments, even though it possesses a less polarized
excited state.
Conclusions
Scheme 3 provides a summary for the connection between
the photophysical properties and photoaddition chemistry of the
substrates discussed in this report. Excitation of the ground-
state trans isomer results in formation of a short-lived singlet
excited state (tS0 f tS1). In aprotic solvents, the preferred
deactivation pathway for tS1 is excited-state bond torsion to a
perpendicular intermediate (tS1 f pS1) followed by internal
conversion to the ground-state cis or trans isomer. Deactivation
of tS1 can also occur by fluorescence. In ionizing solvents such
as TFE and HFIP, tS1 may undergo protonation to yield a
carbocation intermediate C+, which is in turn trapped by solvent-
derived nucleophiles. Substrates possessing meta-methoxy sub-
stituents display a propensity toward photoprotonation, likely
due to higher barriers for torsional deactivation of tS1. The
photochemically generated carbocation intermediates react in
the same manner as those cations produced by ground-state
chemistry. Finally, we propose that tS1 is the sole precursor to
C+; alternative reaction pathways involving either pS1 or excited
states of the corresponding cis isomer (not shown) are not
supported by the available data.
The position of methoxy substituents also dictates the relative
rates at which the benzylic carbocation intermediates undergo
nucleophilic attack by solvent. The LFP experiments in HFIP
are consistent with the cations (14b-e) that would be formed
by photoprotonation of trans-1b-e en route to the corresponding
solvent adducts (4b, 5c, 5d, 4e). Those cations possessing
p-methoxy substituents (14b and 14e) react more slowly with
nucleophiles than those cations that are unsubstituted (14c and
14d). Inspection of the products formed by irradiation of trans-
1a-e in TFE-OD showed that very little (if any) solvent adduct
is formed via a competitive carbene insertion mechanism, which
was originally proposed for the photoaddition of methanol to
trans-1a.15 Further analysis of the 1H NMR spectra of the
deuterated products provided the relative amounts of syn and
anti addition products. These results are consistent with the
relative reactivity of cations 14b-e, as determined by the LFP
experiments. The more reactive cations 14c and 14d gave higher
yields of the syn addition product (≈70% syn) when compared
to the less reactive cations 14b and 14e (≈50-60% syn). The
equal amounts of syn and anti addition product from 14b and
14e are consistent with the formation of free carbocations in
solution, with little diastereoselectivity for reaction with the
nucleophile. In contrast, the increased amount of syn addition
for 14c and 14d implies that these less-stabilized cations undergo
rapid nucleophilic attack by the trifluoroethoxy anion derived
from the same molecule of TFE involved in the initial
photoprotonation step. An alternative explanation for these
results is that the “anti” addition product is actually formed via
syn addition to the cis isomers, which would be formed in
greater yield from those substrates with high quantum yields
of trans-cis isomerization (i.e., trans-1b and trans-1e). How-
ever, the second explanation is inconsistent with the majority
of results presented in this report, and also fails to explain the
different amounts of syn addition for the two products derived
from trans-1e (4e 57% syn, 5e 67% syn). The first mechanistic
explanation easily accounts for this observation: the cation
required for formation of addition product 5e does not possess
a stabilizing 4-methoxyphenyl substituent, and should display
Experimental Section
Syntheses. The syntheses of trans-1a-e have been reported
previously.9 Of the five styrene derivatives selected for the current
investigations, only 3,5-dimethoxystyrene 8d was not commercially
available. This compound was prepared by combination of the
methylmagnesium iodide Grignard reagent with 3,5-dimethoxy-
benzaldehyde, followed by dehydration of the alcohol intermediate.
The unsubstituted trans-1-phenylpropene trans-9a was also com-
mercially available. The syntheses of the four methoxy-substituted
trans-1-arylpropenes were accomplished using the same route as
for 8d, but using ethylmagnesium iodide with the appropriate
methoxyphenyl aldehydes. Separation of the trans-1-arylpropenes
from their cis isomers by flash chromatography techniques was not
possible, but fortunately GC-FID analysis indicated that the
reaction mixtures contained g90% of the trans isomers in all cases.
Complete details regarding the synthesis and characterization of
these substrates (8d, trans-9b, trans-9c, trans-9d, and trans-9e) are
provided in the Supporting Information.
Photophysical Measurements. Fluorescence spectra were de-
termined using solutions for which A ) 0.25 at the incident
wavelength (either 295 or 254 nm). All fluorescence spectra were
corrected, and quantum yields of fluorescence were obtained by
comparing the integrated area of a given sample to that of the
appropriate reference. Many values of φf for trans-1a have appeared
in the literature, with slight variations depending on the excitation
wavelength, solvent, and temperature.1 The chosen reference value
J. Org. Chem, Vol. 71, No. 4, 2006 1491

Roberts and Pincock
for the samples excited at 295 nm is φf ) 0.0433, obtained by
Saltiel and co-workers for trans-1a in hexane solution at 26.6 °C
using an excitation wavelength of 294 nm.3 Slight differences
between excitation wavelengths (295 nm versus 294 nm) and
temperature (25.0 °C versus 26.6 °C) should not affect our results
greatly.36 For those samples excited at 254 nm (specifically the
styrenes and arylalkenes reported in the Supporting Information),
the reference value is φf ) 0.25 for styrene 8a in cyclohexane.25
Our method for calculating quantum yields of fluorescence also
accounted for differences in the absorbance of the sample and
reference solutions, although the experimental differences were
generally quite small (less than 0.02 absorbance units, or 10%).
Singlet lifetimes were measured by monitoring fluorescence decay
using a time-correlated single photon counting apparatus with a
hydrogen flash lamp of pulse width about 1.8 ns. All fluorescence
data (steady-state and time-resolved) were obtained using a path
length of 1 cm and samples that were degassed by three freeze-
pump-thaw cycles before being thermostated at 25 °C. As indicated
in the Results, quantum yields of trans-cis isomerization were
obtained by steady-state irradiation of optically dense solutions (1
× 10-2 M) using one 300 nm low-pressure mercury lamp in a
photochemical reactor (solutions thermostated at 25 °C). The rate
of cis isomer formation was linear in time for low conversion
irradiations (<10% conversion of trans isomer), and the quantum
yields were determined using trans-1a in cyclohexane (φtc ) 0.40)
as a reference.2
Steady-State Irradiations. A 1 × 10-3 M solution (50 mL) of
the substrate of interest in TFE was thermostated at 25 °C in a
quartz reaction vessel and purged with nitrogen for 30 min.
Irradiations were performed in a photochemical reactor with 10
low-pressure mercury lamps (300 nm emission). Aliquots taken
during the reaction were analyzed by GC-FID in order to determine
peak areas for each component in the reaction mixture. The peak
areas were converted to percentages based on the initial peak area
of the starting material, and these percentages were then normalized
for each sample. Identification of many photoproducts was achieved
by analysis of their GC-MS spectra; these data are included in
the Supporting Information.
laser (308 nm, 75 mJ/pulse, 8 ns pulse width) with solutions having
an absorbance of 0.4 at the incident wavelength. These solutions
were prepared by adding a small volume (≈5 µL) of concentrated
acetonitrile stock solution to a 0.7 cm cuvette with 2 mL of the
fluorinated solvent.37 This method required good mixing of each
sample prior to irradiation in order to ensure homogeneous solutions.
Further mixing of the sample was performed before every laser
flash, but even with this technique the high reactivity of the
substrates toward trans-cis isomerization restricted every sample
to less than three flashes each. For the experiments involving
quenching of the cations by bromide ion, portions of a concentrated
solution of tetrabutylammonium bromide in HFIP (0.01 M) were
added to the sample before mixing. The quenching rate constants
(kBr) were determined from the slopes of the plots of kd versus [Br-]
for six different concentrations of bromide ion.
Acknowledgment. We gratefully acknowledge Dr. Norman
Schepp (Dalhousie University) for assistance with laser flash
experiments, Dr. Heidi Muchall and the Centre for Research in
Molecular Modeling (Concordia University, Montreal, Quebec)
for assistance with Gaussian calculations, and Sepracor Canada
Ltd. (Windsor, Nova Scotia) for donation of chemicals. Financial
support for this research was provided by the Natural Sciences
and Engineering Research Council of Canada (NSERC). J.C.R.
also thanks NSERC for a postgraduate scholarship.
Supporting Information Available: The synthesis and char-
acterization of 8d and trans-9b-e, the absorption spectra for all
15 substrates (trans-1a-e, 8a-e, and trans-9a-e), the experimental
Stokes’ shifts and details regarding the Lippert-Mataga analysis
for trans-1a-e, the fluorescence spectra of 8a-e and trans-9a-e,
the GC-MS characterization of the solvent adducts, the yield versus
time plots for the irradiations of 8b-e and trans-9b-e, details
regarding the analysis of TFE/TFE-OD irradiation mixtures by 1H
NMR, and the Cartesian coordinates and absolute energies for the
Gaussian calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO052123D
LFP Experiments. LFP was performed using a XeCl excimer
(37) Surprisingly, the absorbance spectra of all substrates were slightly
blue-shifted (5-10 nm) in TFE compared to acetonitrile solution. Although
this change did not have a large effect on the laser experiments, the
fluorescence quenching studies required the use of more concentrated TFE
solutions in order to keep the same absorbance (0.25) for the quantum yield
determinations. The difference in concentration is not expected to alter the
results significantly.
(36) The φf values reported in ref 9 were determined using a different
fluorescence standard. The current values are preferred on the basis of the
very close match between our experimental conditions and those in ref 3
(excitation wavelength, temperature, etc.). Importantly, either set of data
provides the same mechanistic conclusions.
1492 J. Org. Chem., Vol. 71, No. 4, 2006