Highly Efficient Conformer-Specifif Adiabatic Cis -> Trans Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene in S1

Article abstract of DOI:10.1021/ja951630h

Emission from cis-1-(2-anthryl)-2-phenylethene, c-APE, in toluene cosists primarily (up to 76percent) of fluorescence from adiabatically formed ¹t-APE_B^*.In this respect, the behavior of c-APE is analogous to that of the na

Full text of DOI:10.1021/ja951630h

J. Am. Chem. Soc. 1996, 118, 2811-2817
2811
Highly Efficient Conformer-Specific Adiabatic Cis f Trans
Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene in S1
Jack Saltiel,* Yuxin Zhang, and Donald F. Sears, Jr.
Contribution from the Department of Chemistry, The Florida State UniVersity,
Tallahassee, Florida 32306-3006
ReceiVed May 19, 1995X
Abstract: Emission from cis-1-(2-anthryl)-2-phenylethene, c-APE, in toluene consists primarily (up to 76%) of
fluorescence from adiabatically formed 1 t-APEB*. In this respect, the behavior of c-APE is analogous to that of the
1
naphthyl analogue, c-NPE. However, the conformer specific adiabatic photoisomerization,1c-APEB* f t-APEB*,
1
is much more efficient in c-APE than in c-NPE (g44% vs g2%) although the lifetime of c-APEB* is 1000-fold
longer than the lifetime of 1c-NPEB* (4.5 ( 0.5 ns vs 4 ( 1 ps). Resolution of the spectra of 1c-APE* and 1t-APEB*
was achieved by application of principal component analysis on a matrix of sets of fluorescence spectra measured
in the presence of Ar, air, and O2. The known fluorescence spectrum of t-APEB and the known Stern-Volmer
constant for O2 quenching of the fluorescence of t-APEB serve as the constraints for this resolution. Sequential
1
1
quenching by O2 of c-APEB* and t-APEB* is reflected in a quadratic Stern-Volmer plot for the latter.
Introduction
somerization of cis-stilbene8 is enhanced at least 10-fold (∼2%)
when a 2-naphthyl group is substituted for one of the phenyl
groups10 and is rendered highly efficient (∼61%) when an even
larger aryl group, the 1-pyrenyl, is used instead9 raise doubts
concerning the validity of the conclusion that the c* f t*
pathway does not contribute to the photoisomerization of
c-APE.1-3 Following the submission of this work we became
aware of a brief report of observations on c-APE in methylcy-
clohexane/3-methylpentane (MCH-3MP) solutions by Maz-
zucato and co-workers that led to the proposal that the singlet
adiabatic photoisomerization pathway is dominant at least for
temperatures higher than room temperature.11,12
cis-1-(2-Anthryl)-2-phenylethene, c-APE, has attracted a great
deal of attention because it belongs to a family of 1,2-
diarylethenes that exhibits one way cis f trans adiabatic
photoisomerization on the lowest triplet energy surface.1 In
contrast to the triplet state where the transoid geometry
represents a readily accessible global energy minimum, the
energy surface of the lowest excited singlet state has been
postulated to have deep minima at transoid and cisoid geometries
and to undergo no photoisomerization in either direction.1-3 This
was presumably consistent with different fluorescence spectra2,4
and large fluorescence quantum yields from solutions of the
two isomers.2,3 Following direct excitation, photoisomerization
1
1
Experimental Section
1
in the cis f trans direction was assumed to involve c-APE*
f 3c-APE* intersystem crossing as the initial key step.2,3 This
assumption was based in part on transient triplet-triplet
absorption measurements that have led to φis ) 0.17 as the
Materials. Toluene (Fisher, HPLC grade) was washed with several
portions of concentrated sulfuric acid, several portions of water, and
aqueous sodium bicarbonate. It was then dried over sodium sulfate
and distilled. Quinine sulfate (Matheson, Coleman and Bell, reagent)
was recrystallized three times from water. A mixture of c- and t-APE
was synthesized by a Wittig reaction as previously described.13
Separation of the two isomers was achieved by column and rotary
chromatography and the structure of c-APE was confirmed by 1H NMR
(300 MHz, DCCl3) δ 6.70-6.83 (dd, 2 H, J ) 12.5 Hz, vinyl), 7.22-
7.25 (m, 3 H), 7.31-7.34 (m, 3 H), 7.44-7.46 (m, 2 H), 7.78-7.81
(d, 1 H, J ) 9 Hz), 7.90 (s, 1 H), 7.96-7.98 (m, 2 H), 8.30 (s, 1 H),
8.34 (s, 1 H).
estimated intersystem crossing yield of c-APE*.3 Semiem-
1
pirical quantum mechanical calculations suggest that the
perpendicular geometry, 1p*, in the APEA conformer is a
transition state along the 1c* f 1t* reaction coordinate that lies
16 and 23 kcal/mol higher than 1c* and 1t*, respectively.5
Relatively large energy barriers for c* f p* and t* f p*
torsional motion were attributed to confinement of the electronic
excitation largely in the anthracenyl moiety of the two isomers.1,5
1
1
1
1
Spectroscopic Measurements. Absorption spectra were measured
using a Perkin-Elmer Lambda-5 spectrophotometer and fluorescence
This study was prompted by recent reports showing that
1
1
adiabatic c* f t* isomerization of aryl substituted olefins is
much more common than previously imagined.6-10 Observa-
tions showing that the inefficient (∼0.2%) adiabatic photoi-
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) For a review see Arai, T.; Tokumaru, K. Chem. ReV. 1993, 93, 23-
39.
(7) (a) Sandros, K.; Sundahl, M.; Wennerstro¨m, O.; Norinder, U. J. Am.
Chem. Soc. 1990, 112, 3082-3086. (b) Sandros, K.; Sundahl, M.;
Wennerstro¨m, O. J. Phys. Chem. 1993, 97, 5291-5294. (c) Sandros, K.;
Sundahl, M. J. Phys. Chem. 1994, 98, 5705-5708.
(8) (a) Saltiel, J.; Waller, A.; Sun, Y.-P; Sears, D. F., Jr. J. Am. Chem.
Soc. 1990, 112, 4580-4581. (b) Saltiel, J.; Waller, A. S.; Sears, D. F., Jr.
J. Photochem. Photobiol. A: Chem. 1992, 65, 29-40. (c) Saltiel, J.; Waller,
A. S.; Sears, D. F., Jr. J. Am. Chem. Soc. 1993, 115, 2453-2465.
(9) Spalletti, A.; Bartocci, G.; Mazzucato, U.; Galiazzo, G. Chem. Phys.
Lett. 1991, 186, 297-302.
(2) Karatsu, T.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett.
1985, 115, 9-15.
(3) Arai, T.; Karatsu, T.; Tsuchiya, M.; Sakuragi, H.; Tokumaru, K.
Chem. Phys. Lett. 1988, 149, 161-166.
(4) Castel, N.; Fischer, E. J. Photochem. Photobiol. A 1989, 48, 109-
(10) Saltiel, J.; Tarkalanov, N.; Sears, D. F., Jr. J. Am Chem. Soc. 1995,
117, 5586-5587.
114.
(5) Bartocci, G.; Masetti, F.; Mazzucato, U.; Spalletti, A.; Orlandi, G.;
Poggi, G. J. Chem. Soc., Faraday Trans. 2 1988, 84, 385-399.
(6) (a) Sandros, K.; Becker, H.-D. J. Photochem. 1987, 39, 301-315.
(b) Go¨rner, H. J. Photochem. Photobiol. A 1988, 43, 263-288. (c) Sandros,
K.; Becker. H.-D. J. Photochem. Photobiol. A 1988, 43, 291-292.
(11) Mazzucato, U.; Spalletti, A.; Bartocci, G. Coord. Chem. ReV. 1993,
125, 251-260.
(12) We thank Professor U. Mazzucato for bringing ref 11 to our
attention.
(13) Maercker, A. Org. React. 1965, 14, 270-490
0002-7863/96/1518-2811$12.00/0 © 1996 American Chemical Society

2812 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Saltiel et al.
Figure 1. Absorption spectra of c-APE (s) and t-APE (- -) and
fluorescence excitation spectrum (λem ) 415 nm) of c-APE (- - -) and
contributions of t-APEB* (- -) and c-APE* (×2) (s) emissions in
c-APE fluorescence (λexc ) 366 nm) (- - -) in toluene at 20 °C. All
fluorescence spectra are corrected for nonlinearity in instrumental
response. Inset: Global Stern-Volmer plots for the O2 effect on
resolved fluorescence areas of c-APE and t-APEB.
Figure 3. Adherence of the time evolution of fluorescence areas in
Figure 2 to eq 1.
1
1
t-APE with a corresponding 50% increase in fluorescence
spectrum intensity. The time dependence of the mixture
fluorescence quantum yield, φhf, is given by
φhf(t) ) hfc(t)φhf + hft(t)φhf ) φhf + (φhf - φhf )hft(t)
(1)
c
t
c
t
c
where φhf and φhf are the effective fluorescence quantum yields
t
c
of t- and c-APE at the λexc employed and
jꢀt(λexc)cj
hft(t) )
t ) 1 - hfc(t)
(2)
jꢀ(λexc)cj
represents the fraction of t-APE molecules excited at time t.
Following correction for the very small increases in absorbance
at 366 nm with time, the plot of fluorescence area vs hft(t) obeys
eq 1, Figure 3. Fluorescence area extrapolation to hft values of
0 and 1 gives φhf /φhf ) 1.80. Bars over the symbols indicate
t
c
that the quantities reflect the behavior of mixtures of conformers
for each isomer.
Figure 2. Absorption and fluorescence spectra of a solution of c-APE
in toluene after irradiation at 20 °C in a static cell at 366 nm for 0, 5,
10, 30, and 40 min, bottom to top.
Flow Cell Experiments. To minimize interference by t-APE
photoproduct formed in the course of the measurements,
fluorescence spectra were also recorded at 20.0 °C in toluene
using a flow cell system as described for cis-stilbene.8 Under
these conditions, it was shown with the aid of neutral density
filters (up to a factor of 5 attenuation) that the dependence of
fluorescence intensity from a c-APE solution on incident
excitation intensity is strictly linear. Ar-, air- and O2-saturated
solutions ([c-APE] ) 3.4 × 10-5 M, 150 mL) were employed
and a minimum number of fluorescence spectra recorded at λexc
) 374, 384, 395, 399, 403, 405, 407, and 409 nm. The small
c-APE absorbances at λexc g 399 nm gave rise to relatively
weak and noisy fluorescence spectra that contain large scattered
light contributions. These spectra were not of sufficiently good
quality to be employed in the analysis. A second experiment
was therefore carried out employing a higher c-APE concentra-
tion, 1.4 × 10-4 M. Approximately 200 mL of this solution,
saturated successively with Ar, O2, and air, was circulated
through the flow cell system and spectra were recorded for 370
e λexc e 410 nm. Better quality fluorescence spectra were
obtained at the expense of onset distortion due to self-absorption.
In a similar experiment an APE solution composed of 5% t-APE
and 95% c-APE was employed, [APE] ) 1.1 × 10-4 M, in
order to evaluate the effect of t-APE contamination on the
fluorescence spectra. Pure solvent background spectra were
employed for baseline correction. Negligible buildup of t-APE
was established by comparing absorption and fluorescence-
excitation spectra of working solutions before and after mea-
surement of fluorescence spectra. The fluorescence spectra of
Ar-saturated c-APE solutions bear a strong similarity to the
spectra were measured using an extensively modified Hitachi/Perkin-
Elmer MPF-2A spectrophotometer as previously described.14 1H NMR
spectra were obtained on a Gemini 300 MHz instrument.
Fluorescence Lifetimes. Fluorescence lifetimes were measured
using a SPEX Fluorolog fluorimeter.15
Data Analysis. Principal component analyses with self-modeling
(PCA-SM) calculations were performed on a PC’s limited Dell 80486/
87 (25 MHz) microcomputer as previously described.14
Results
Static Cell Experiments. The UV spectra of c- and t-APE
in toluene are compared in Figure 1. Figure 2 shows absorption
and fluorescence spectra of a solution of c-APE in Ar-saturated
toluene irradiated at 20.0 °C with 366-nm (nearly an isosbestic
point of the APE isomers) light in a 1-cm static cell in our
fluorimeter for different time intervals. While the buildup of
t-APE is obvious in the absorption spectra, the corresponding
change in the fluorescence spectra is an increase in intensity
with no readily discernible difference in spectral shape. Quan-
titative treatment of the changes in absorption in Figure 2 was
achieved using PCA. No self-modeling is involved as both pure
APE isomer spectra are known, Figure 1. Irradiation for 40
min in the fluorimeter resulted in 69% conversion of c-APE to
(14) Saltiel, J.; Sears, D. F., Jr.; Choi, J.-O.; Sun, Y.-P.; Eaker, D. W. J.
Phys. Chem. 1994, 98, 35-46.
(15) We thank Professor M. Kasha for the use of this instrument and
Dr. Juan-Carlos Del Valle for assistance in carrying out the measurements.

Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2813
Figure 5. A typical set of fluorescence spectra of c-APE (1.4 × 10-4
M) in Ar- (s), air- (- -) and O2-saturated (- - -) toluene solution
obtained for λexc ) 390 nm. The spectrum for the O2-saturated solution
is also shown multiplied by a factor of 6.64 (the area ratio). The spectra
are corrected for background and self-absorption, but not for nonlin-
earity of instrumental response.
Figure 4. Eigenvectors of the global matrix consisting of fluorescence
spectra from all three flow cell experiments and the resolved t-APEA
and t-APEB spectra from ref 18: (a) VR (s), Vâ (- -); (b) Vγ (s), Vδ
(- -). The six largest eigenvalues were 5.407 × 10-1, 4.632 × 10-3
6.442 × 10-4, 8.507 × 10-5, 7.617 × 10-6, and 5.310 × 10-6
respectively.
,
,
resolved fluorescence spectrum of t-APEB,5,16-18 whereas
fluorescence-excitation spectra roughly track the absorption
spectrum of c-APE, Figure 1. The fluorescence spectrum
broadens with increasing [O2].
Spectral Resolution. Fluorescence spectra were corrected
for self-absorption based on the effective transmittance of each
APE solution. Satisfactory background corrections were achieved
by subtraction of the corresponding solvent background spec-
trum, provided that the Rayleigh scattered-light peak from the
solvent did not overlap the region of the fluorescence spectrum,
λexc e 375 nm. Spectra for longer λexc were corrected by least-
squares fitting of eigenvectors obtained from a matrix of spectra
for λexc e 375 nm to the regions of each spectrum free of
scattered-light peaks. This procedure allows reconstruction of
the spectrum over the entire wavelength range. The spectral
region requiring correction was then substituted into the
experimental spectrum and the corrected spectrum was added
to the matrix.18a Since each spectrum potentially consists of at
least four components (c- and t-APEA and c- and t-APEB), the
final least-squares fit correction employed the four significant
eigenvectors, Figure 4, from a matrix consisting of the flow
cell spectra described above and the previously resolved pure
component t-APEA and t-APEB fluorescence spectra.18a A set
of typical spectra from this matrix (corrected for self-absorption
and background emission, but not for nonlinearity of instru-
mental response) clearly shows the pronounced effect of oxygen
on the fluorescence profile, Figure 5. Examination of a plot of
the combination coefficients from this global matrix in R, â, γ
space, Figure 6, shows that most spectra from the two pure
c-APE solutions fall along a line that includes the combination
coefficients of the spectrum of t-APEB at one end. Spectra
corresponding to the solution containing 5% t-APE show strong
systematic deviations from this line with points for spectra at
Figure 6. Combination coefficients for the global matrix corresponding
to the eigenvectors in Figure 4 in R, â, γ space. Experimental spectra
for [c-APE] ) 3.4 × 10-5 and 1.4 × 10-4 M solutions are designated
by squares and circles, respectively; spectra for the c-APE solution
contaminated by 5% t-APE are designated by open diamonds, and their
projection on the three-component plane by shaded diamonds. Projec-
tions are on the normals to the plane; the apparent angle of the lines to
the plane is an optical illusion due to coordinate scale selection. The
cross within the symbol designates spectra for λexc g 400 nm and closed
circles and squares correspond to the 18 spectra employed in the two-
component treatment.
the longest λexc and the largest [O2] falling closest to the point
defined by the combination coefficients of t-APEA. Based on
this behavior the PCA treatment was confined to the subset of
18 spectra from the pure c-APE solutions obtained with λexc e
398 nm. The matrix spanned the 360-580-nm range in 1-nm
increments. Eigenvalues and eigenvectors from this matrix are,
to a very good approximation, consistent with a two component
system, Figure 7. Using target analysis to define the combina-
tion coefficients of the known t-APEB spectrum as dot products
of the spectrum with the eigenvectors in Figure 7 defines a point
that falls on the normalization line, Figure 8, and corresponds
(16) (a) Spalletti, A.; Bartocci, G.; Masetti, F.; Mazzucato, U.; Cruciani,
G. Chem. Phys. 1992, 160, 131-144. (b) Bartocci, G.; Mazzucato, U.;
Spalletti, A.; Elisei, F. Spectrochim. Acta, A 1990, 46, 413-418. (c)
Bartocci, G.; Masetti, F.; Mazzucato, U.; Baraldi, I.; Fischer, E. J. Mol.
Struct. 1989, 193, 173-183.
(17) (a) Ghiggino, K. P.; Skilton, P. F.; Fischer, E. J. Am. Chem. Soc.
1986, 108, 1146-1149. (b) Wismontski-Knittel, T.; Das, P. K.; Fischer, E.
J. Phys. Chem. 1984, 88, 1163-1168. (c) Fischer, G.; Fischer, E. J. Phys.
Chem. 1981, 85, 2611-2613.
(18) (a) Saltiel, J.; Zhang, Y.; Sears, D. F., Jr.; Choi, J.-O. Res. Chem.
Intermed. 1995, 21, 899-921. (b) Adjustments for deviations from unity
of the intercepts of Stern-Volmer plots have led to small increases in the
Stern-Volmer constants reported in ref 18a.
c
to KSV ) 175 ( 7 M-1 (see eq 4 below). It is in reasonable
agreement with KSVc ) 145 ( 16 M-1 based on our fluorescence
lifetime of 1c-APE* and the assumption of identical O2-
1
quenching rate constants, 3.22 × 1010 M-1 s-1 18
,
for c-APE*
1
and t-APEB*.
Two extreme possibilities can be envisioned that would lead
to strictly two-component fluorescence spectra from c-APE
1
solutions. In both, t-APEB* fluorescence arises through its
adiabatic formation from 1c-APEB*, Scheme 1; however, in the
first possibility (case a) the second component is due entirely

2814 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Saltiel et al.
Scheme 1
φfo
) (1 + KSVc-B[O2])(1 + KSVt-B[O2])
(3)
( )
φf
t-B
c-B
t-B
where KSV
and KSV
are the Stern-Volmer constants of
1
t-B
1c- and t-APEB*, respectively. The value KSV ) 900 ( 25
M-1 was determined previously.18 However, the value obtained
c
above for KSV could, in principle, correspond to either
1c-APEA* or 1c-APEB*, or to both. The search for the
combination coefficients of c-APE* fluorescence was based
1
on reproducing the oxygen effect defined by eq 3 (see inset,
c-B
Figure 1), assuming, tentatively, that KSV ) 160 M-1. The
resulting resolved fluorescence spectrum is shown in Figure 1.
If the two-component case b applied exclusively this spectrum
could be assigned to 1c-APEB*. That this is not strictly correct
is revealed by the λexc dependence of the combination coef-
ficients on the normalization line in Figure 8. This topic is
addressed in detail in the Discussion section.
Figure 7. Eigenvectors of the matrix consisting of 18 fluorescence
spectra of c-APE in toluene (3.4 × 10-5 and 1.4 × 10-4 M) obtained
in the flow cell as a function of [O2] for λexc e 398 nm. The four
largest eigenvalues were 0.1366, 0.1964 × 10-3, 0.2330 × 10-4, and
Fluorescence Quantum Yields. Fluorescence quantum
yields were determined for c-APE under flow cell conditions
and for t-APE under static cell conditions using quinine bisulfate
in 0.1 N H2SO4 as the fluorescence standard, φf ) 0.546.19 Ar-
saturated toluene solutions of the APE’s at 20.0 °C were
employed and absorbances of standard and unknown were
matched at λexc. Slits were set at 3.0 nm for both excitation
and emission monochromators. Fluorescence quantum yields,
corrected for differences in index of refraction, are 0.52, 0.53,
0.49, 0.46, and 0.12 for c-APE excited at λexc ) 340, 350, 360,
390, and 410 nm, respectively, and 0.89 for t-APE excited at
either 340 or 344 nm. Estimated uncertainties in the quantum
yields are (3%.
0.9124 × 10-5
.
Fluorescence Lifetimes. Fluorescence lifetimes of an Ar-
outgassed toluene solution of c-APE, ∼2.5 × 10-5 M, were
measured at ambient temperature, ∼27 °C, using the phase
modulation method. Duplicate determination using short and
long exposure times yielded clean biexponential decays corre-
sponding to lifetimes of 27.9 ( 0.2 (95.0%) and 4.5 ( 0.5 ns
(5.0%), for the major and minor components, respectively, ø2
) 1.5. These measurements were made with excitation and
emission monochromators set at 325 and 420 nm, respectively.
Figure 8. Normalization line based on the eigenvectors in Figure 7.
Experimental combination coefficients and pure component combination
coefficients are also shown (see Figure 6 for symbols).
1
to c-APEA* fluorescence, whereas in the second possibility
(case b) it is due entirely to 1c-APEB* fluorescence. Based on
case a, an attempt to find the combination coefficients for the
fluorescence spectrum of c-APE by applying the Stern-Volmer
constant criterion14 on t-APEB fluorescence yielded an anoma-
Discussion
The UV absorption spectrum of c-APE in Figure 1 is in good
agreement with an approximate spectrum of its methyl deriva-
tive, cis-1-(2-anthryl)-2-p-toluylethene (c-ATE), in MCH that
was estimated by Castel and Fischer by subtracting the spectrum
of t-ATE from the spectrum of a nearly equimolar mixture of
t-B
lously large apparent KSV ) 1191 ( 20 M-1 value and a
highly structured spectrum. The larger than expected O2 effect
1
on t-APEB* fluorescence intensity suggests prior quenching
of the precursor 1c-APEB* excited state as shown in Scheme 1.
Based on Scheme 1 the dependence of the fluorescence intensity
1
of t-APEB* on [O2] should follow the quadratic relationship
(19) Meech, S. R.; Phillips, D. J. Photochem. 1983 23, 193-217.

Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2815
c- and t-ATE.20 It is in excellent agreement with the spectrum
of c-APE measured by Mazzucato, Spalletti, and Bartocci in
methylcyclohexane/3-methylpentane (MCH-3MP) 9/1 (v/v).11
Examination of Figures 1 and 2 shows that in toluene at 20.0
°C the fluorescence spectra of c- and t-APE are nearly identical,
exhibiting only subtle differences. Since, provided λexc e 395
nm, the fluorescence of t-APE is due primarily to the t-APEB
conformer, this preliminary examination suggests t-APEB as the
major source of fluorescence following c-APE excitation.
Support for this conclusion is provided by the observation that
the lifetime, 27.9 ( 0.2 ns, of the major decay component of
c-APE fluorescence in toluene corresponds exactly to the
lifetime of the fluorescence of t-APEB.5,16a The lifetime of the
minor fluorescence component, 4.5 ( 0.5 ns, is nearly a factor
of 2 smaller than the fluorescence lifetime of t-APEA, 8.7 (
0.5 ns.5,16a Thus, adiabatic 1c-APEA* f 1t-APEA* isomerization
can be discounted as a significant process. Analogous observa-
Figure 9. c-APE fluorescence spectra based on three-spectra sets
for λexc ) 374 nm (s, [c-APE] ) 3.4 × 10-5 M) and λexc ) 402 nm
(- -, [c-APE] ) 1.4 × 10-4 M).
1
tions led Mazzucato et al. to postulate adiabatic c-APEB* f
1t-APEB* isomerization in MCH-3MP.11 However, in MCH-
3MP the fluorescence spectrum of c-APE at 25 °C differs
sharply from that of t-APE, whereas the spectra of the two
isomers are very similar at 81 °C.11
normalization) and âo and â are the corresponding coefficients
of the normalized spectra in the absence and in the presence of
O2, respectively.14 However, as pointed out in the Results
section, if the spectra consisted only of 1c-APEB* and 1t-APEB*
fluorescence, their combination coefficients would define only
one point for each [O2] on the normalization line, i.e., their
position would be independent of λexc. This is based on the
PCA Resolution of c-APE Fluorescence. As described in
the Results section, our resolution of c-APE fluorescence spectra
in toluene into pure component spectra rests, in no small
measure, on prior knowledge of an accurate t-APEB fluorescence
spectrum and of the corresponding Stern-Volmer quenching
constant, KSVt-B. Two t-APEB fluorescence spectra have been
reported, both based on PCA-SM resolutions of matrices of
t-APE fluorescence spectra in toluene at 20 °C.16a,18a They
appear to differ mainly in the degree of definition of vibrational
structure. We have traced the reason for the discrepancy to
the difference in SM criteria employed in the two studies.18a
Bartocci et al. relied on the Lawton and Sylvestre nonnegativity
constraint21 that gives a broad range of acceptable spectra for
solutions,16a whereas our resolution was based on the Stern-
Volmer constant14 that gives a nearly unique solution, well
within the range of acceptable spectra from the Lawton and
Sylvestre approach.18a Since the outer limit Lawton and
Sylvestre combination coefficients that were assigned by
Bartocci et al. to t-APEB are further out on the normalization
line than the actual coefficients, their spectrum includes a
substantial negative contribution of t-APEA fluorescence leading
to the impression of better-resolved vibronic structure. The
consequence of using such a difference spectrum in the
quantitative analysis of c-APE fluorescence spectra would be
the erroneous conclusion that t-APEA is present as a third
component in the fluorescence of c-APE solutions. This
conclusion is inconsistent with the PCA results and with the
biexponential fluorescence decays observed here in toluene and
in the Mazzucato et al. work in MCH-3MP.11
1
reasonable assumption that the efficiency of the adiabatic c-
APEB* f 1t-APEB* process is λexc independent. It follows that
the λexc-induced dispersion of the points on the normalization
line in Figure 8 into well-defined regions for each [O2] can be
1
accounted for only if c-APEA* fluorescence is present as a
component whose contribution varies with λexc. We conclude
that despite the apparent behavior of the spectral matrix as a
two-component system, each experimental spectrum contains
fluorescence contributions from three excited species (1c-APEA*,
1
1c-APEB*, and t-APEB*), but that c-APEA and c-APEB have
closely related fluorescence spectra and similar fluorescence
lifetimes. The contribution of 1c-APEA* fluorescence is espe-
cially enhanced in spectra for λexc g 400 nm. Combination
coefficients for these spectra fall closest to the c-APE corner
(located by target analysis using the vectors in Figure 4 and
the c-APE spectrum in Figure 1) of the c-APE/t-APEB side of
the triangle in Figure 6. Inclusion of these spectra into the
spectral matrix does lead to eigenvectors and eigenvalues
indicating the presence of a minor third component. When
spectra from the solution contaminated with t-APE, λexc g 400
nm, are included, the spectral matrix becomes a four-component
system. This is evident in Figure 6 where most relevant points
lie in front of the three-component plane (triangle) and are
shown together with their projections on that plane. Deviations
from the plane increase with increasing [O2]. They are a
measure of the contribution of c-APEA* fluorescence in the
spectra.
For a two-component system, the location of the combination
coefficients (Rt-B,ât-B) of the fluorescence spectrum of t-APEB
on the normalization line in Figure 8 defines the Stern-Volmer
constant of the cis component according to
To test the conclusion that the fluorescence spectra and
lifetimes of the cis rotamers are nearly identical the PCA
treatment was applied separately to each set of three fluorescence
spectra obtained for each λexc. The results for λexc ) 374 nm
(low [c-APE]) and λexc ) 402 nm (high [c-APE]) are typical.
They are described here because based on the location of the
combination coefficients of the two sets of spectra in the global
plot in Figure 6 they represent nearly the widest variation in
c-APEA/c-APEB fluorescence ratio that was achieved in the
experimental spectra. Defining the location of (Rt-B,ât-B) on the
normalization line of each matrix by target analysis resulted in
Fo(âo - ât-B)
c
) 1 + KSV [O2]
(4)
F(â - ât-B)
where Fo and F are areas of the fluorescence spectra (prior to
(20) Castel, N.; Fischer, E. J. Photochem. Photobiol. A 1989, 48, 109-
114.
c
KSV ) 152 and 158 M-1 for λexc ) 374 and 402 nm,
(21) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 13, 617-
633.
respectively. Applying the quadratic relationship, eq 3, for the
(22) This conclusion is independent of the value of φf employed. Use
t-B
1
of the earlier lower value16a,18a for φf in the above analysis would lead to
quenching of t-APEB* fluorescence gave the c-APE spectra
t-B
1
c
a corresponding decrease in φf
.
shown in Figure 9. Clearly, increasing the contribution of c-
t-B

2816 J. Am. Chem. Soc., Vol. 118, No. 12, 1996
Saltiel et al.
APEA* fluorescence in the spectra has little effect on KSVc and
on the appearance of the c-APE fluorescence spectrum. Though
subtle differences are discernible, the spectra in Figure 9 are
was used to estimate φf ) 0.81 and φf ) 1.00 as the
t-B
t-A
conformer specific fluorescence quantum yields.16a,18a However,
it was pointed out that a significant deviation of the φhf ’s from
t
1
-1
nearly identical to the c-APE* spectrum in Figure 1.
the plot of φhf vs xA could reflect either uncertainty in the
t
18a
c-B
φh ’s or a λexc dependence of φ and, possibly, of φ
.
ft-A
We are justified therefore in using eq 3 with KSV ) 160
ft
ft-B
t-B
M-1 and KSV ) 900 M-1, based on the sequential excited
ft
Viewed in this context, our experimental φh ) 0.89 at λexc
)
340 and 344 nm leads to a tentative preference for the former
state quenching shown in Scheme 1, to guide the search for the
1
explanation. Used in eq 5 together with the earlier φhf ’s for
coefficients of the c-APE* fluorescence spectrum. The fit to
t
λexc g 400 nm we obtain φf ) 1.00, as before,16a,18a and a
the expected quadratic Stern-Volmer plot (inset) and the
corresponding resolved 1c-APE* fluorescence spectrum are
shown in Figure 1. The broad appearance of the spectrum is
consistent with room temperature fluorescence spectra of other
cis-1,2-diarylethenes,8,10 and its somewhat blue-shifted origin
relative to the fluorescence spectrum of t-APEB is consistent
with the blue-shifted origin of the absorption spectrum of the
c-APE conformer mixture. In rigid media that preclude
photoisomerization the fluorescence spectra of c-ATE (decalin,
-150 °C)20 and c-APE (MCH-3MP, -153 °C)11 show resolved
vibronic structure and are relatively red-shifted. Since for Ar-
saturated solutions the major portion of the fluorescence at 20.0
°C originates from adiabatically formed 1t-APEB*, the fluores-
cence excitation spectrum of c-APE should track primarily the
absorption spectrum of c-APEB. It is not unexpected, therefore,
that in toluene our fluorescence excitation spectrum in Figure
1, though similar, does not track the absorption spectrum of
c-APE exactly. Especially large deviations, observed in the
second absorption band in the 290-320-nm region, are probably
not significant due to the large absorbance of the c-APE solution
in that region (A ) 0.8 at 296 nm). Interestingly, the reported
fluorescence excitation spectrum of c-APE in MCH-3MP is
almost identical to the c-APE absorption spectrum.11 The
fluorescence excitation spectra are expected to depend on the
emission wavelength monitored since, based on our fluorescence
spectra, the onset of c-APEA absorption should be at somewhat
longer wavelengths than the onset of c-APEB absorption.
Fluorescence Quantum Yields. The ratio of the fluores-
cence quantum yields of the APE isomers, φhf /φhf ) 1.80 at 366
t-A
somewhat larger φf ) 0.88, independent of λexc. Since xA )
t-B
0.12 for λexc ) 366 nm,18a eq 5 predicts φhf ) 0.893 which leads
t
to φhf ) 0.496 based on the analysis of the spectra in Figure 2.
c
Conformer-Specific Photochemistry. The fluorescence
spectra of c-APE in Ar-saturated toluene obtained for the
quantum yield determinations for λexc ) 340, 350, and 360 nm
are nearly identical. Based on the resolved spectra in Figure
1, they correspond to 74.5 ( 0.3% 1t-APEB* and 25.5 ( 0.3%
1c-APE* fluorescence contributions. It follows that the effective
quantum yields of the two components are φf ) 0.131 ( 0.002
tc
and φf ) 0.383 ( 0.013 for these three excitation wavelengths.
t-B
As pointed out above, significantly larger contributions for 1c-
APE* emission are observed as λexc approaches the onset (∼410
nm) of c-APE absorption indicating that excitation of c-APEA,
which does not undergo detectable adiabatic 1c* f 1t* isomer-
ization, is relatively more effective at higher λexc. The inef-
ficiency of the 1c* f 1t* isomerization process for c-APEA can
be appreciated by considering the location in Figure 6 of the
combination coefficients of the fluorescence spectrum of the
Ar-saturated solution excited at 410 nm. It is reflected in the
deviation of points from the three-component plane defined by
the triangle in Figure 6. Despite the fact that, due to larger
absorptivities of the t-APE’s,18 fluorescence spectra obtained
for this λexc are especially sensitive to the presence of t-APE as
an impurity (see points in Figure 6 for the solution contaminated
with 5% t-APE), the point for the pure c-APE solution falls
very close to the 1c-APE*/1t-APEB* side of the triangle. Based
on 0.88 as the fluorescence quantum yield of 1t-APEB*,
competitive absorption of incident light by c-APEA ensures that
at least 44% of 1c-APEB* undergoes adiabatic isomerization to
1t-APEB*.22 Intersystem crossing from 1t-APEB* should, there-
fore, contribute significantly to the yield of triplets observed
on excitation of c-APE.3 Thus, substitution of the 2-anthryl
group for one of the phenyl groups in stilbene markedly
enhances the efficiency of adiabatic isomerization on the lowest
excited singlet state surface of only one of the APE conformers
in toluene, Scheme 1. Aside from the larger enhancement of
the conformer specific adiabatic 1c* f 1t* pathway, c-APE and
c-NPE exhibit entirely analogous behavior.10 The analogy with
c-APE can be extended further if we consider photocyclization
to dihydrophenanthrenes. Conformer-specific photocyclization
t
c
nm, based on the static cell measurements, Figures 2 and 4, is
in good agreement with the independently measured quantum
yields using quinine bisulfate as fluorescence standard. Our
φhf values, varying between 0.53 and 0.46 for λexc in the 340-
c
390-nm range, are somewhat smaller than φhf ) 0.57 reported
c
earlier.2,3 This discrepancy may be due to the use of the flow
cell system that eliminates the small buildup of t-APE that
occurs in the course of spectral acquisition under static cell
conditions. The substantially smaller φhf ) 0.12 obtained for
c
λexc ) 410 nm, the onset of c-APE absorption, is consistent
with substantial c-APEA absorption at that wavelength. The
low φhf value reflects the low fluorescence quantum yield of
c
1c-APEA* (we estimate φf = 0.025, as an upper limit23 ) and
c-A
the failure of this conformer to undergo adiabatic isomerization
1
to the highly fluorescent t-APEA*.
Earlier extensive measurements of φhf under the same condi-
t
tions (toluene, 20 °C) had yielded values in the 0.76-0.83 range
for 340 e λexc e 372 nm.16a As xA, the fractional contribution
of t-APE in the fluorescence spectrum, increases at λexc g 400
nm, the reported φhf ’s approach unity.16a The relationship of
t
these φhf to xjA
t
1
1
1
1
)
+
-
xA(λexc
)
(5)
of c-NPEA has been reported by several workers.24 Conformer-
specific photocyclization of c-APEA can be inferred from a
single report on the ATE derivative. Prolonged irradiation of
φf
(
φf
φf
)
φhf (λexc
)
t-B
t-A
t-B
t
(23) It is assumed that absorption at λexc ) 374 nm is primarily by
c-APEB. It follows that the ratio of resulting 1c-APEB* to 1t-APEB*
fluorescence is at least 1:3.1. Subtraction of the 1t-APEB* (60%) and 1c-
APEB*(19%) contributions from the c-APE spectrum for λexc ) 410 nm
gives 0.025 as the quantum yield of the residual 1c-APEA* fluorescence.
(24) See the following reviews and references cited therein: (a) Muszkat,
K. A. Top. Curr. Chem. 1980, 88, 89-144. (b) Mallory, F. B.; Mallory, C.
W. Org. React. 1984, 30, 1-456. (c) Laarhoven, W. H. Photochromism,
Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier:
Amsterdam, 1990; pp 270-313.

Photoisomerization of cis-1-(2-Anthryl)-2-phenylethene
J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2817
t-ATE under oxidative conditions gives a 15% yield of the
phenanthrene corresponding to the cyclization of 1c-ATEA*, as
the only isolable product.25 Assuming the usual mechanism
for the formation of the above phenanthrene derivative reveals
further that the photoisomerization of ATE cannot be exclusively
one way. A minor pathway must exist in the t f c direction.
Preliminary observations indicate that, under the conditions
described for Figure 2, the photostationary state, though not
exactly 100% t-APE, is >99% t-APE.
increase of τf to 90 ns in MCH-3MP at 77 K can probably be
c
attributed in part to the suppression of torsional motion in the
glassy medium.11 It is surprising, however, that the estimated
kf ) 5 × 106 s-1 under these conditions11 is nearly six times
c
smaller than our value for toluene at 20 °C. The usual
dependence of kf on the refractive index cannot account for such
a large change.26 It appears that either most of 1c-APE*
fluorescence originates from a different conformer in fluid
solution than in the glassy medium or, if the same conformer
persists as the major source of the fluorescence, its emission
reflects the coupling of more than one excited state. Diminished
coupling with a state with a more allowed radiative transition
could account for the decrease in the effective radiative rate
constant at low temperature. Strong evidence demonstrating
coupled decay of the two lowest excited singlet states of t-APEB
has been presented.16
We recently obtained a rough estimate of the fluorescence
lifetime, τf = 4 ( 1 ps, for 1c-NPE* based on its fluorescence
c
quantum yield and an estimated radiative rate constant, kf .10
c
The fact that the fluorescence lifetime of 1c-APE* is three orders
of magnitude longer is, no doubt, a reflection of the greater
degree of localization of the excitation in the larger aryl
substituent. Such localization might diminish the ability of 1c-
APE* to undergo radiationless decay via the large-amplitude
torsional motions of the olefinic portion of the molecule that
are essential for cis f trans isomerization. Other radiationless
decay paths, such as intersystem crossing, might be expected
to compete more effectively in 1c-APE* than in 1c-NPE*. The
much higher efficiency of the 1cB* f 1tB* adiabatic reaction in
1c-APE* is therefore all the more noteworthy.
1
The effective radiationless decay rate constant of c-APE*
in toluene at 20 °C is knr ) 2 × 108 s-1. Since this rate constant
c
1
can be assigned mainly to torsional motion along the cB* f
1tB* adiabatic reaction coordinate, the activation energy barrier
along this coordinate may be as high as 7 kcal/mol.27 Whether
this barrier is on the way to a shallow minimum at the
1
1
perpendicular geometry, pB*, or whether pB* corresponds to
the geometry of the transition state for the 1cB* f 1tB* process,
as appears to be the case on the lowest triplet state energy
surface,1-3 remains to be established. Transient absorption
measurements and medium and temperature effects on the
fluorescence and photochemistry of c-APE are in progress.
The fluorescence lifetime of c-APE is highly medium
dependent. The decrease from 4.5 ns in toluene at 20 °C to 2
ns in fluid MCH-3MP is difficult to interpret because φfc is
unknown under the latter conditions.11 The estimated low value,
∼0.025, for the effective quantum yield of 1c-APEA*, even when
preferential excitation of this conformer is achieved by employ-
ing λexc at the onset of c-APE absorption, suggests that the bulk
of c-APE fluorescence can be assigned to 1c-APEB*. It follows
Acknowledgment. This research was supported by NSF
Grant No. CHE 93-12918.
that φf = 0.13 ( 0.01, although a lower limit, should be very
c
close to the fluorescence quantum yield of c-APEB. If we assign
JA951630H
our τf to this conformer, we obtain kf ) 2.9 × 107 s-1 for the
c
c
radiative rate constant of 1c-APE* in toluene. A dramatic
(26) Saltiel, J.; Waller, A. S.; Sears, D. F., Jr.; Garrett, C. Z. J. Phys.
Chem. 1993, 97, 2516-2522 and references cited therein.
(27) The Arrhenius A factor of 1c-NPE* 10 was employed in this
estimation.
(25) Laarhoven, W. H.; Cuppen, Th. J. H. M.; Nivard, R. J. F.
Tetrahedron 1970, 4865-4881.