Upper excited state photochemistry: Solution and gas phase photochemistry and photophysics of 2- and 3-cyclopropylindene

Article abstract of DOI:10.1021/ja981397y

The cyclopropylcarbinyl radical rearrangement has been used to probe the photochemistry and photophysics of S₂ and S₁ in 2-cyclopropylindene (2CPI) and 3-cyclopropylindene (3CPI). Studies in solution and the gas phase are described. Population of S₂ with 254 nm light excitation in the gas phase produces the anticipated ring expansion products 2,3,3a,8- tetrahydrocyclopenta[a]indene (1, Φ₁ = 0.1) and 1,3,3a,8- tetrahydrocyclopenta[a]indene (2, Φ₂ = 0.06) from 2CPI and 3CPI, respectively (Scheme 3). Direct excitation into the S₂ state (254 nm) of 2CPI in solution also produces compound 1. The efficiency of the solution phase chemistry is a function of excitation wavelength (Φ₁ = 0.022 and 0.006 for 254 and 280 nm, respectively). The solution phase excitation spectrum of 2CPI shows an anomalous dependency on monitoring wavelength which is attributed to a conformational equilibrium. The S₁ singlet lifetimes of the cyclopropylindenes are quite short (0.39 and 1.0 ns for 2CPI and 3CPI, respectively) relative to the previously measured values for the corresponding methylindenes (2.3 and 13.9 ns). These shortened lifetimes are attributed to cyclopropyl ring opening in S₁ with rates of 2.1 x 10⁹ and 9.2 x 10⁸ s^-1 for 2CPI and 3CPI, respectively. Semiempirical excited state calculations support descriptions of the S₂ and S₁ states of alkylindenes as biradicals.

Full text of DOI:10.1021/ja981397y

J. Am. Chem. Soc. 1999, 121, 3083-3092
3083
Upper Excited State Photochemistry: Solution and Gas Phase
Photochemistry and Photophysics of 2- and 3-Cyclopropylindene1
Tim Waugh and Harry Morrison*
Contribution from the Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
ReceiVed April 23, 1998
Abstract: The cyclopropylcarbinyl radical rearrangement has been used to probe the photochemistry and
photophysics of S2 and S1 in 2-cyclopropylindene (2CPI) and 3-cyclopropylindene (3CPI). Studies in solution
and the gas phase are described. Population of S2 with 254 nm light excitation in the gas phase produces the
anticipated ring expansion products 2,3,3a,8-tetrahydrocyclopenta[a]indene (1, Φ1 ) 0.1) and 1,3,3a,8-
tetrahydrocyclopenta[a]indene (2, Φ2 ) 0.06) from 2CPI and 3CPI, respectively (Scheme 3). Direct excitation
into the S2 state (254 nm) of 2CPI in solution also produces compound 1. The efficiency of the solution phase
chemistry is a function of excitation wavelength (Φ1 ) 0.022 and 0.006 for 254 and 280 nm, respectively).
The solution phase excitation spectrum of 2CPI shows an anomalous dependency on monitoring wavelength
which is attributed to a conformational equilibrium. The S1 singlet lifetimes of the cyclopropylindenes are
quite short (0.39 and 1.0 ns for 2CPI and 3CPI, respectively) relative to the previously measured values for
the corresponding methylindenes (2.3 and 13.9 ns). These shortened lifetimes are attributed to cyclopropyl
ring opening in S1 with rates of 2.1 × 109 and 9.2 × 108 s-1 for 2CPI and 3CPI, respectively. Semiempirical
excited state calculations support descriptions of the S2 and S1 states of alkylindenes as biradicals.
Introduction
Scheme 1. Proposed Mechanism of the Methylindene
Transposition Reaction
Alkylindenes are a remarkably fruitful class of substrates for
photochemical study, since they exhibit an unusual skeletal
rearrangement in solution from S1 and a novel series of hydrogen
and/or alkyl shifts from S2 in the gas phase.2,3 Related chemistry
is observed with dihydronaphthalenes.4 The S0 f S2 transition
for these indenes and dihydronaphthalenes is centered at ca.
260 nm, while the λmax for S0 f S1 is ca. 290 nm.3,4 As shown
in Scheme 1, irradiation into the S1 or S2 states of alkylindenes
in solution results in a net transposition of carbons 1 and 2.
The reaction involves sequentially a [2 + 2] cycloaddition, a
[1,3] sigmatropic shift, a retro [2 + 2] ring opening to the
isoindene, and a [1,5] hydrogen shift from C2 to C1 or C3.
Shown in Scheme 1 are also the reaction quantum efficiencies.
Population of the S2 state of alkylindenes in the Vapor phase
results in the formation of solution phase products plus
additional alkyl and hydrogen shift products. Excitation into S2
in the vapor results in virtually no S1 fluorescence, in contrast
to the facile S1 emission produced in solution. Introduction of
an inert gas (butane) quenches formation of the additional alkyl/
hydrogen shift products and restores fluorescence, while retain-
ing the transposition product. On the basis of these data and
the published emission spectral characteristics of styrene deriva-
tives,5 a mechanism unique to the gas phase was proposed which
invoked conversion of the alkene into a 1,2-biradical as the
chemical manifestation of either S2 or S1vib. Multiple, sequential
1,2 shifts could then explain the scrambling observed in the
products (see Scheme 2).
Recent ab initio calculations on indene8 indicate that the S2
state possesses a frequency mode that involves displacement
(5) Styrene derivatives exhibit dual fluorescence that has been attributed
to emission from a low energy, short-lived, planar S1 species and a higher
energy, longer lived species having twisted π bond character.6 An analogy
was drawn between this latter species and the biradical S2 state of indene
proposed by Suarez.3a Subsequent theoretical studies7 have characterized
the twisted species as a combination of triplet ethylene and triplet benzene
and located this state 16 kcal mol-1 higher in energy than the planar
minimum. This state is described as “a combination of triplet quinoid
benzene with triplet ethylene coupled to a singlet overall.” 7
(6) (a) Steer, R. P.; Swords, M. D.; Crosby, P. M.; Phillips, D.; Salisbury,
K. Chem. Phys. Lett. 1976, 43, 461-464. (b) Ghiggino, K. P.; Phillips, D.;
Salisbury, K.; Swords, M. D. J. Photochem. 1977, 7, 141-146. (c) Ghiggino,
K. P.; Hara, K.; Mant, G. R.; Phillips, D.; Salisbury, K.; Steer, P.; Swords,
M. D. J. Chem. Soc., Perkin Trans. 2 1978, 88-91.
(1) Organic Photochemistry. Part 116. Part 115: Morrison, H.; Lu, Y.;
Carlson, D. J. Phys. Chem. A 1998, 102, 5421-5432.
(2) (a) Morrison, H.; Palensky, F. J. Am. Chem. Soc. 1977, 99, 3507-
3508. (b) Morrison, H.; Giacherio, D. J. Am. Chem. Soc. 1978, 100, 7109-
7110. (c) Morrison, H.; Giacherio, D.; Palensky, F. J. J. Org. Chem. 1982,
47, 1051-1058. (d) Morrison, H.; Giacherio, D. J. Org. Chem. 1982, 47,
1058-1063.
(3) (a) Suarez, M. L.; Duguid, R. J.; Morrison, H. J. Am. Chem. Soc.
1989, 111, 6384-6391. (b) Duguid, R. J.; Morrison, H. J. Am. Chem. Soc.
1991, 113, 3519-3525.
(4) (a) Duguid, R. J.; Morrison, H. J. Am. Chem. Soc. 1991, 113, 1265-
1271. (b) Duguid, R.; Morrison, H. J. Am. Chem. Soc. 1991, 113, 1271-
1281.
(7) Bearpark, M. J.; Olivucci, M.; Wilsey, S.; Bernardi, F.; Robb, M. A.
J. Am. Chem. Soc. 1995, 117, 6944-6953.
(8) Zilberg, S.; Kendler, S.; Haas, Y. J. Phys. Chem. 1996, 100, 10869-
10874.
10.1021/ja981397y CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/17/1999

3084 J. Am. Chem. Soc., Vol. 121, No. 13, 1999
Waugh and Morrison
Scheme 2. Proposed Biradical Mechanism for Methylindene
Rearrangements in the Gas Phase
Scheme 3. Potential Ring Expansion Products from the Gas
Phase Photolyses of 2CPI and 3CPI
Table 1. Photophysical Data of the Indenes Studieda
abs λmax
nm (log
ꢀmax
,
fluor
λmax Φf (soln)
τf
kf
indene
)
(nm) (×10-3
)
(ns)
13.9b
(×106)
3MI (soln)
3MI (gas)
252 (4.00) 307 77b
248 (3.91)
5.5b
4.7c
3CPI (soln) 253 (3.96) 311 4.7 ( 0.3c 1.0 ( 0.06c
3CPI (gas)
2MI (soln)
248 (3.99)
257 (4.07) 312d 24b
2.3b
10.4b
9.6c
23 ( 0.3c
2.4 ( 0.1c
of carbons 1, 2, and 3 out of the molecular plane, an observation
noted by the authors as being consistent with the characterization
of S2 as a biradical.
2MI (gas)
250 (4.15) 319f
2CPI (soln) 264 (4.21) 334d 2.4 ( 0.2
3.8 ( 0.2c
0.39 ( 0.04c
9.7c
The intent of the current study was to provide more direct
evidence for such a biradical intermediate. The facile rearrange-
ment of cyclopropylcarbinyl radicals has proven useful in this
regard and has been used to probe both thermal9 and photo-
chemical10 reactions. In particular, Caldwell and Zhou10c used
the cyclopropylcarbinyl radical clock reaction to test the model
of triplet alkenes as 1,2 biradicals. Substrates studied by these
workers included cis- and trans-â-cyclopropylstyrene, R-cyclo-
propylstyrene, R-cyclopropylindene (i.e., 3-cyclopropylindene,
3CPI), and â-cyclopropylindene (i.e., 2-cyclopropylindene,
2CPI). The indene triplets were created using thioxanthone and
xanthone photosensitization, and the 1,3-biradicals formed by
cyclopropyl ring opening were observed by transient spectros-
copy. The rate constants for ring opening were measured as
3.5 × 106 and >5 × 108 s-1 for 3CPI and 2CPI, respectively.
No direct photolysis of these substrates was reported.
2CPI (gas)
253 (4.07) 331e
2ISOI (soln) 258 (4.07) 312 20 ( 0.2
2.5 ( 0.1c
8.0c
a Unless otherwise noted, spectra were acquired in cyclohexane at
25 ( 1 °C with 254 nm excitation. b See ref 2c. c 266 nm excitation.
d As can be seen in Figure 1, the apparent Stokes shift for 2CPI relative
2MI is exaggerated by using λmax. The midpoints of the broad maxima
in solution come at 328 and 319 nm for 2CPI and 2MI, respectively.
e The midpoints of the broad maxima in the gas phase come at 335
and 319 nm for 2CPI and 2MI, respectively.
We have therefore prepared 2CPI and 3CPI and studied the
solution and gas phase photophysics and photochemistry of these
molecules. The anticipated photoproducts included those result-
ing from the transposition reaction outlined in Scheme 1 and
the ring expansion products, 1 and 2, shown in Scheme 3.
Results
Photophysics of 2CPI and 3CPI. Absorption, fluorescence,
and fluorescence excitation spectra were recorded for the two
cyclopropylindenes. Unless otherwise noted, the spectra were
recorded at room temperature (ca. 25 °C). These data, together
with reference data for the corresponding 2- and 3-methylin-
denes (2MI, 3MI) and 2-isopropylindene (2ISOI), are compiled
in Table 1. The solution phase spectra of the methyl- and
cyclopropylindenes are displayed in Figures 1 and 2. The
fluorescence and excitation spectra were multiplied by scalars
to normalize them to the same scale as the absorption spectra.
Figure 1. Solution phase absorption (thin line), fluorescence (bold
line; λexc ) 254 nm), and excitation (dashes; λem ) fluor λmax) spectra
of (a) 2MI and (b) 2CPI.
(9) For some representative examples, see: (a) Griller, D.; Ingold, I. U.
Acc. Chem. Res. 1980, 13, 317-323. (b) Effio, A.; Griller, D.; Ingold, I.
U.; Beckwith, A. K. J.; Serelis, A. J. J. Am. Chem. Soc. 1980, 102, 1734-
1736.
(10) Some selected examples: (a) Smart, R. P.; Peelen, T. J.; Blankespoor,
R. L.; Ward, D. L. J. Am. Chem. Soc. 1997, 119, 461-465. (b) Hu, S.;
Neckers, D. C. J. Org. Chem. 1997, 62, 755-757. (c) Caldwell, R. A.;
Zhou, L. J. Am. Chem. Soc. 1994, 116, 2271-2275. (d) Rudolph, A.;
Weedon, A. C. Can. J. Chem. 1990, 68, 1590-1597.
The excitation spectra were obtained monitoring at the fluo-
rescence λmax. It is noteworthy that the excitation spectra for
both 2CPI and 3CPI are slightly red-shifted relative to the
absorption spectra. This is clearly not the case for the methyl-
indenes.

Upper Excited State Photochemistry
J. Am. Chem. Soc., Vol. 121, No. 13, 1999 3085
Table 2. Wavelength Dependence of Room-Temperature
Fluorescence Quantum Yields of 2CPI in Cyclohexane
excitation
excitation
a
a
wavelength (nm) Φf (×10-3
)
wavelength (nm) Φf (×10-3
)
254
260
266
2.4 ( 0.2
270
275
280
2.8 ( 0.1
2.0 ( 0.1
3.8 ( 0.2
4.2 ( 0.1
5.5 ( 0.5
a Relative to naphthalene12 (Φf ) 0.19).13
Figure 2. Solution phase absorption (thin line), fluorescence (bold
line; λexc ) 254 nm), and excitation (dashes; λem ) fluor λmax) spectra
of (a) 3MI and (b) 3CPI.
Figure 4. Comparison of the solution (thin line) and the gas-phase
absorption spectra (dashes) of (a) 2MI and (b) 2CPI.
Figure 3. Comparison of the solution phase absorption spectrum (thin
line) of 2CPI with its solution phase excitation spectra monitored at
310 nm (dashes) and 330 nm (bold line).
The solution phase excitation spectra of 2CPI were also
collected as a function of monitoring wavelength and were found
to be wavelength dependent (see Figure 3). Note that the
excitation spectra were collected monitoring sequentially at 310
nm, 330 nm, and then again at 310 nm. No difference was
observed between the two spectra collected at 310 nm, indicating
that there was no degradation of the emission signal in the time
span of the fluorescence experiments.11
The excitation spectra for 2MI and 3CPI exhibited no such
dependence on monitoring wavelength. The excitation spectrum
for 2CPI was also collected at 77 K in a methylcyclohexane
glass and found to be independent of the monitoring wavelength.
The shifts in the solution phase fluorescence excitation spectra
relative to the absorption spectra for the cyclopropylindenes led
us to specifically examine the possible wavelength dependence
of the fluorescence quantum efficiency of 2CPI in solution. The
data are presented in Table 2.
Figure 5. Comparison of the fluorescence spectra of 2CPI (λexc
254 nm) collected in solution at room temperature (bold line), in a
glass at 77 K (thin line), and in the gas phase (circles).
)
the solution spectra relative to those seen in the vapor phase,
with the effect magnified for 2CPI.
In Figure 5 we compare the fluorescence spectra of 2CPI
collected in solution at room temperature, in a methylcyclo-
hexane glass at 77 K, and in the gas phase. The low-temperature
spectrum appears at the blue edge of the room-temperature
fluorescence while the gas phase fluorescence spectrum is red-
shifted. In contrast with 2CPI, there is no difference between
the solution and the gas phase fluorescence spectra of 2MI.
Photolysis of 2CPI in Solution. Photolysis of 2CPI in
cyclohexane using 254 nm light gave three products as shown
in eq 1. The major photoproduct is assigned as the ring
expansion product 1 on the basis of its NMR and UV absorption
spectra. Thus, one observes a single vinyl proton at δ 5.58, two
benzylic protons at δ 3.44, and one benzylic proton appearing
as a multiplet at δ 4.15-4.07. The UV spectrum is similar to
A comparison of the absorption spectra of 2CPI and 2MI in
solution with those in the vapor phase is presented in Figure 4.
The spectra for both compounds show bathochromic shifts for
(11) All indenes were purified by preparative GLC prior to analysis. The
UV spectra of 2CPI before and after the fluorescence experiments were
identical.

3086 J. Am. Chem. Soc., Vol. 121, No. 13, 1999
Waugh and Morrison
Table 3. Solution Phase Reaction Quantum Yields for the
Photolysis of 2CPI as a Function of Excitation Wavelengtha,b
λexc
Φ
dis (×10-2
)
Φ1 (×10-2
)
Φ
3CPI (×10-2
)
Φ
1CPI (×10-2
)
250
254
266
280
5.8
4.1
0.25
0.38
4.4 ( 0.2
4.4 ( 0.3
3.1 ( 0.3
2.2 ( 0.3
1.8 ( 0.2
0.6 ( 0.1
0.20 ( 0.04
0.25 ( 0.01
c
c
0.25d
0.38d
a Experiments were conducted in cyclohexane at ca. 25 °C. b Values
without error bars correspond to only one experiment. c 3CPI and 1CPI
are undetectable in these experiments. d The experiment was repeated
in triplicate, but this product was only detected from one experiment.
that for o-xylene, an indication that a double bond is no longer
in conjugation with the aromatic ring.
NMR spectroscopy and a comparison of GLC retention time
with authentic material have been used to identify 3CPI as one
of the minor photoproducts. The assignment of the other
photoproduct as the expected transposition product, 1CPI, is
based on a comparison on its mass spectrum to the spectra of
the other indene isomers. In addition to a molecular ion at m/z
156, the product shows principal fragments at 141 (loss of
methyl), 128 (loss of ethylene), and 115 (loss of cyclopropyl),
a pattern identical with that observed for 2CPI and 3CPI.
The effect of excitation wavelength on the photolysis of 2CPI
in dilute cyclohexane solutions (3-8 mM) was studied using
250, 254, 266, and 280 nm light (254 nm light was provided
by a low-pressure mercury lamp, while the other wavelengths
were provided by lasers). The results are presented in Table 3.
No quenching of photoproduct formation was observed in
the presence of piperylene, nor are these photoproducts reported
from triplet sensitization.10c
Photolysis of 2CPI in the Gas Phase. The photolysis of
2CPI in the gas phase with 254 nm light typically gave two
photoproducts, one of which was isolated and shown by NMR
spectroscopy and GLC retention time to be the ring expansion
product 1. The second product was assigned as 1CPI on the
basis of GLC analysis. Quantum efficiencies for disappearance
of 2CPI and appearance of the products were determined using
the conversion of 2MI to 3MI as a secondary actinometer (Φ3MI
) 0.03).3a The results are shown in eq 2.14
Table 4. GLC Percentages of Unreacted Starting Material and of
Photoproducts from the Gas Phase Photolysis of 2CPI (λexc ) 254
nm) with and without Butane
butane pressure
(Torr)
%
unreacted
2CPI
conversiona
1
3CPI 1CPI 1/1CPI
0b
14
13-25
84
85
89
96
4.2
1.1
3.7
2.6
0.5
4.0
1.1
1.6
6.0
18
7
5.9 5.2
4.2 4.2
3.0 0.5
400
infinitec
5
a Photolysis times were as follows: 0 Torr, ca. 5 s; 400 Torr, ca. 15
s. b Indene was observed as a photoproduct in several, but not all, of
the photolyses. c Infinite pressure corresponds to the solution phase.
Photolysis of 3CPI in the Gas Phase. Photolysis of 3CPI
vapor with 254 nm light gave the compounds shown in eq 3.
The major photoproduct is assigned as the ring expansion
product 2 on the basis of its NMR and UV absorption spectra.
One observes a single vinyl proton at δ 5.79-5.78 split into
multiplet and a multiplet at δ 2.80-2.60 corresponding to the
two benzylic hydrogens. The methine proton appears as a pentet
at δ 2.28-2.22 and is split by the two benzyl protons and the
two methylene protons. The UV spectrum is similar to that for
indene, with the two absorption maxima at 256 and 290 nm, an
indication that the double bond is in conjugation with the phenyl
ring. 2CPI was identified by GLC co-injection with an authentic
sample, and the identity of 1CPI was established by co-injection
of the 3CPI gas phase reaction mixture with the reaction mixture
from a solution phase photolysis of 2CPI. Quantum efficiencies
for loss of 3CPI and formation of the photoproducts were
determined using the conversion of 2MI to 3MI3a as a secondary
actinometer.
Introduction of butane into the sample chamber resulted in
the appearance of 3CPI and the formation of relatively higher
levels of 1CPI. The relative amounts of these products, as a
function of butane pressure, are summarized in Table 4.
Solution Phase Photolysis of 3CPI. Prolonged photolysis
of 3CPI in cyclohexane using 254 nm light gave only a trace
amount of the ring expansion product 2. A quantum efficiency
for loss of 3CPI was measured as Φdis ) 0.016; a corresponding
value for the formation of 2 could only be determined at high
conversions and is estimated as 0.0002.
The effect of butane on the photolysis was measured, and
the data are reported in Table 5. In several static photolyses we
have found evidence by GLC for formation of indene.
Discussion
(12) For dilute solutions, the fluorescence quantum yield for naphthalene
is reported to be wavelength independent over the range of wavelengths
studied, see: (a) Becker, R. S.; Dolan, E.; Balke, D. E. J. Chem. Phys.
1969, 50, 239-245. (b) Birks, J. B.; Conte, J. C.; Walker, G. J. Phys. B.
1969, 2, 934-945.
Major Observations. The purpose of this work was to
validate the representation of upper or vibrationally excited
electronic excited states of alkylindenes as biradical-like species.
Consistent with our prediction, ring expansion chemistry is
observed in the gas phase photolyses of 2CPI and 3CPI. The
presence of ring expansion chemistry and the concomitant virtual
elimination of transposition chemistry observed for 2CPI in
solution were unexpected. 3CPI (like 3MI2c) is essentially inert
in the solution phase. Both 2CPI and 3CPI have shortened S1
(13) Birks, J. B. Photophysics of Aromatic Molecules; John Wiley &
Sons: New York, 1969; p 126.
(14) The observed photoproducts account for ca. 65% of the loss of
starting material in the gas phase photolysis. We have found evidence by
GLC for the trace formation of indene in several photolyses of 2CPI. Indene
has also been found, on occasion, to be a photoproduct in the gas phase
photolysis of the methylindenes. See ref 3b.

Upper Excited State Photochemistry
J. Am. Chem. Soc., Vol. 121, No. 13, 1999 3087
Table 5. GLC Percentages of Unreacted Starting Material and of
Photoproducts from the Gas Phase Photolysis of 3CPI (λexc ) 254
nm) with and without Butane
butane pressure
(Torr)
%
unreacted
3CPI
conversiona
2
1CPI 2CPI 2/1CPI
0b
8
15
12
15
50
93.1
86.4
99.1
99.5
3.8 2.2
3.2 4.2
0.0 0.4
0.5
0.9
3.8
0.5
1.7
0.8
0.0
18
infinitec
a Photolysis times were as follows: 0 Torr, ca. 5 s; 18 Torr, ca. 15
s. b Indene was observed as a photoproduct in several, but not all, of
the photolyses. c Infinite pressure corresponds to the solution phase.
lifetimes relative to their methyl analogues. 2CPI exhibits a
number of unusual spectral characteristics, most notable of
which are absorption spectra which are phase dependent, an
excitation spectrum in solution which is red-shifted relative to
the absorption spectrum, and solution phase fluorescence spectra
which vary with excitation wavelength.
Nature of the Excited States. The absorption spectra of
styrenes, as well as the indenes studied in this work, consist of
an intense short wavelength absorption band (ca. 250 nm) and
a weaker longer wavelength band from 280 to 300 nm. The
lower energy band corresponds to the symmetry forbidden
1
transition to S1 and is related to the Lb transition of benzene.
The higher energy band corresponds to a transition to the S2
state and is associated with the symmetry allowed 1La transition
of benzene. The S0 f S2 absorption spectrum is diffuse,15 an
indication of efficient nonradiative transitions.16
Analyzing the excited states of the substituted indenes
computationally provides further insight into the nature of S2
and S1. Figure 6 contains MO coefficient plots of the HOMO-
1, HOMO, LUMO, and LUMO+1 for 2CPI in an s-trans
conformation (see further discussion of the conformations of
2CPI below). These molecular orbitals are qualitatively similar
to the semiempirically generated molecular orbitals of 1-phen-
ylpropene.17
The MOs in Figure 6 are representative of the orbitals
calculated for the range of substituted indenes discussed in this
paper. Note that the HOMO has substantial π character at C2/
C3 while a π* (antibonding) relationship exists for the corre-
sponding atoms in the LUMO. Thus, excited state configurations
which incorporate a substantial percentage of a HOMO f
LUMO transition would indeed be well represented as being
radical-like at C2 and, to a lesser extent, at C3. This is exactly
the case for the S0 f S2 transition in all of the indenes studied,
as may be seen in Table 6 (the extensive involvement of the
HOMO f LUMO transition in S2 is analogous to that found in
earlier studies of 1-phenylpropene17 and indene.8
Figure 6. Molecular orbital plots of the frontier orbitals of s-trans
2CPI.
As has been previously observed,8 depopulation of the HOMO
is also involved in the formation of S1. Table 7 shows that
involvement of the HOMO dominates the S0 f S1 transition,
albeit through a HOMO f LUMO+1 excitation. Likewise, there
is a major component involving population of the LUMO by
excitation from the HOMO-1. The net consequence is a similar,
though more modest, element of biradical character created in
S1, with an overtone of net charge transfer from the double bond
to the benzene ring. Thus, the S2 state can be chemically
represented as a 1,4 biradical while the S1 state can be
Table 6. Percent Contribution of Singly Excited Configurations to
the S2 CI State of Substituted Indenes
indene
HOMO f LUMO
HOMO f LUMO+1
s-trans 2CPI
3CPI
2MI
3MI
s-trans 2ISOI
90.1
91.5
89.0
89.2
91.1
6.4
2.6
6.5
5.5
4.9
represented as a 1,4 biradical slightly polarized to account for
the element of charge transfer (see Figure 7).
(15) (a) Hemley, F. J.; Leopold, D. G.; Vaida, V.; Karplus, M. J. Chem.
Phys. 1985, 82, 5379-5397. (b) Zeigler, L. D.; Varotsis, C. Chem. Phys.
Lett. 1986, 123, 175-181. (c) Leopold, D. G.; Hemley, R. J.; Vaida, V.;
Roebber, J. L. J. Chem. Phys. 1981, 75, 4758-4769.
(16) Byrne, J. P.; Ross, I. G. Aus. J. Chem. 1971, 24, 1107-1141.
(17) Lewis, F. D.; Bassani, D. M.; Caldwell, R. A.; Unett, D. J. J. Am.
Chem. Soc. 1994, 116, 10477-10485.
Photochemistry in the Gas Phase. A combination of isotopic
scrambling and butane quenching experiments, plus the obser-
vation of minimal gas phase fluorescence, led to the proposal
that three discreet species are involved in the gas phase

3088 J. Am. Chem. Soc., Vol. 121, No. 13, 1999
Waugh and Morrison
Table 7. Percent Contribution of Singly Excited Configurations to
the S1 CI State of Substituted Indenes
ns) is significantly reduced relative to the reported lifetime of
2MI (2.3 ns).2c Since the radiative lifetimes of these two
compounds are nearly identical, we conclude that the shortened
fluorescence lifetime of 2CPI is entirely a consequence of the
indene
HOMO f LUMO+1
HOMO-1 f LUMO
s-trans 2CPI
3CPI
2MI
3MI
s-trans 2ISOI
60.0
56.9
59.1
55.4
62.4
28.3
33.8
29.9
34.7
28.1
new chemistry at C2 (kr).18 One can therefore calculate kr by
-1
the expression (τ2CPI
)
) kr + ∑kI ) kr + (τ2MI)
-1. Insertion
of the lifetimes gives kr ) 2.1 × 109 s-1 for 2CPI. (An
analogous calculation for 3CPI yields a kr ) 9.2 × 108 s-1).
However, insertion of this kr for 2CPI into the quantum
efficiency expression gives a calculated quantum yield of 0.82,
while the measured value of Φ1 at 266 nm is only 0.018. Clearly,
a concerted process proceeding with the rate constant needed
to explain the reduced lifetime is inconsistent with the low
measured quantum efficiency, i.e., the majority of the excited
state species must be returning nonproductively to the initial
ground state. If one assumes that a photoinitiated ring expansion
occurring by a 1,3 sigmatropic shift would proceed in a
concerted manner,19 we are led to conclude that S1 reacts through
a reversible9b cyclopropylcarbinyl homoallyl radical rearrange-
ment that provides a nonproductive radiationless decay pathway
for the cyclopropylindene excited state. The rate constants for
ring opening would then be the kr values calculated above. The
somewhat slower rate for 3CPI might be explained by the
resonance stabilization of the benzylic position of the pseudo
radical at C3 in S1, akin to the explanation invoked to rationalize
the slower rate of ring opening of the 3CPI triplet relative to
that of 2CPI.10c A particularly interesting feature of the
photochemistry arising from S1 in the cyclopropylindenes is the
dominance of ring expansion over transposition photochemistry.
Clearly, ring opening is competing with the [2+2] ring closure
necessary for transposition chemistry.
Figure 7. Chemical representation of the S2 and S1 states of 2CPI.
photochemistry of alkylindenes.3 Transposition chemistry occurs
0
from lower vibrational levels of S1 or S1 , hydrogen shifts from
higher vibrational levels of S1 (S1vib), and methyl shifts from
either the highest vibrational levels of S1 or from S2. It was
vib
suggested that S2 and S1 could be represented as biradical
species.
Consistent with this view, we have observed that the ring
expansion products 1 and 2 are indeed formed from the gas
phase photolyses of both cyclopropylindenes. As with the gas
phase photochemistry of alkylindenes3 and dihydronaphtha-
lenes,4 2CPI also produces net [1,2] shift photoproducts (1CPI
and 3CPI) as well as traces of indene. The addition of butane
(Table 4) results in a relative increase in the amount of 1CPI
and the appearance of 3CPI. These products can arise either
from a series of [1,2] alkyl and hydrogen shifts or from a skeletal
rearrangement. The small amount of these one observes in
solution, where only the skeletal rearrangement is expected,2
argues strongly for the [1,2] shift mechanism in the gas phase.
Interestingly, the fact that 3CPI is formed only when butane is
present is reminiscent of the gas phase photolysis of dihy-
dronaphthalenes where, again, one observes unique products
being formed in the presence of a quencher gas.4 Apparently,
collision with butane is relaxing the initially formed 2CPI
excited state to a level (presumably S1vib) that is not accessible
by direct excitation and is capable of alkyl shift chemistry.
Though this same species is likewise relatively more efficient
in producing 1CPI, it more effectively forms the 3-isomer than
S1 does in solution. With the more extensive collisional
deactivation occurring in solution, the [1,2] shift mechanism
disappears and the small, comparable, amounts of 1CPI and
3CPI now seen can be attributed to the skeletal rearrangement
mechanism.
Solution Phase Photochemistry. The formation of ring
expansion photoproducts in solution requires that S2 is active
in solution and/or that S1 must also be capable of this chemistry.
We will return to the issue of S2 involvement below. What can
certainly be said is that the greatly shortened singlet lifetimes
for S1 in 2CPI and 3CPI strongly indicate that this singlet is
involved in the solution phase photochemistry. The question
then is by what mechanism? We have already noted that the S2
state is dominated (ca. 90%) by a HOMO f LUMO transition
that, through the electronic distribution in the LUMO, in effect
creates a cyclopropylcarbinyl radical at C2. Though the S1 state
likewise includes a ca. 28% contribution of a transition involving
the population of the LUMO, its more modest role raises the
question of whether a concerted, i.e., a [1,3] sigmatropic shift,
mechanism may be operating in S1.
We return then to the question of a possible role for S2 in
the solution phase photochemistry. There are two sets of
observations that could, in the absence of other data, be
interpreted as support for ring opening occurring from S2. First,
the observations that the excitation spectra of the cycloprop-
ylindenes are red-shifted relative to the absorption spectra
(Figure 1) and that Φf therefore increases with longer wave-
length excitation (Table 2) raise the possibility that S2 is
incompletely communicating with S1. This could clearly be a
result of competing photochemistry in the upper excited state.
Second, there is a decrease in photochemical quantum efficiency
for 2CPI on the red-edge of its absorption (i.e., in the potential
region of the S0 f S1 transition) (cf. Table 3). One is tempted
to attribute the effect to more efficient chemistry arising from
S2 relative to S1. However, there are experimental data and
literature precedents that provide an additional perspective
involving the existence of conformational isomers. These are
now discussed in some detail.
Conformations of 2CPI. Numerous computational20 and
experimental21 studies have determined that the ground state
energy surface of vinylcyclopropane contains two higher energy
minima corresponding to an enantiomeric pair of gauche
(18) The nearly identical lifetimes of the isopropyl- and the methylindenes
indicate that the diminution in the singlet lifetime of 2CPI is due to the
presence of the cyclopropane ring.
(19) It is possible that the reaction coordinate for the [1,3] sigmatropic
shift involves a conical intersection that allows the excited state to decay
unproductively to the 2CPI ground state. Future theoretical analyses in
collaboration with Professor Yehuda Haas of the Hebrew University are in
progress. The involvement of conical intersections on the excited state
surface of indene has been previously discussed, see: Bearpark, M. J.;
Bernardi, F.; Olivucci, M.; Robb, M. A. J. Phys. Chem. 1997, 101, 8395-
8401.
In fact, there is good reason to believe this is not the case.
Recognizing that Φ1 ) kr1τ (where kr is the rate of the
rearrangement), we note that the singlet lifetime for 2CPI (0.39
(20) (a) Klahn, B.; Dyczmons, V. J. Mol. Struct. (THEOCHEM) 1985,
122, 75-94. (b) Huang, M.-B.; Pan, D.-K. J. Mol. Struct. (THEOCHEM)
1984, 108, 49-58.

Upper Excited State Photochemistry
J. Am. Chem. Soc., Vol. 121, No. 13, 1999 3089
Table 8. Selected Proton NMR Resonances (ppm) of 2CPI and
2MI at 25 and 2 °C
solvent
T (°C)
H1
H2
H3
CH3
2CPI
2CPI
2MI
2MI
2CPI
2CPI
CDCl3
CDCl3
CDCl3
CDCl3
hexane
hexane
25
2
25
2
25
2
6.480
6.490
6.467
6.468
6.389
6.397
3.178
3.165
3.275
3.276
3.098
3.083
1.818
1.809
2.141
2.140
1.737
1.746
1
Figure 9. Numbering scheme of 2CPI and 2MI used for H NMR
analysis.
above the cyclopropyl ring. No such effect was seen for the
allylic proton resonances of the 1-methyl analogue, where the
methyl group is thought to destabilize the s-trans rotamer relative
to the gauche form. Mazzocchi23 used proton NMR to determine
the s-trans to gauche conformer ratio for trans-2-cyclopropyl-
1-phenylethylene to be 3:1. These literature precedents suggest
that knowledge of the conformational equilibrium in the
cyclopropylindenes may likewise be critical in understanding
the photochemistry and photophysics of these compounds.
Figure 8. Structures of the rotamers of vinylcyclopropane. The torsion
angles (θ) are defined by the atoms in black.
Ab initio calculations were therefore performed on 2CPI at
the HF/6-31G27 level. These calculations indicate that the indene
has two gauche minima, an s-trans minimum, and a maximum
on the ground-state potential energy surface corresponding to
the s-cis rotamer. The conformations are analogous to those
shown in Figure 8. In contrast to vinylcyclopropane, the energy
difference between the s-trans and gauche rotamers of 2CPI
was calculated to be 0.04 kJ/mol. At equilibrium at 298 K in
the gas phase, and including entropic effects, 2CPI is predicted
to consist of 67% gauche and 33% s-trans isomers.
rotamers, a lower energy minimum corresponding to the s-trans
rotamer, and an energy maximum corresponding to the s-cis
rotamer. Figure 8 presents the structures of these rotamers. The
calculated and experimental energy differences between the
s-trans and gauche rotamers are 5.17 kJ/mol20a and 5.98,21c
respectively.
The s-trans and gauche rotamers are known to have markedly
different photochemical and photophysical properties. For
example, Jorgenson22 and Mazzocchi23 have argued that in the
photoinduced vinylcyclopropane-cyclopentene rearrangement,
cyclopentene formation preferentially occurs from the gauche
conformation. In contrast, the auxochromic effects of the
cyclopropyl group on the UV spectra of cyclopropyl ketones
and olefins are maximized in the s-trans conformation.24,25
Conformationally rigid molecules containing a vinylcyclopro-
pane moiety which approximate the s-trans rotamer have their
UV absorption band (λmax ) 201.0 nm, ꢀ ) 14 850) shifted to
the red of those compounds which model the gauche rotamer
(λmax ) 193.0 nm, ꢀ ) 8740).25 In addition, the s-trans
conformer manifests itself in the chemical shifts of protons
which are affected by the shielding of the cyclopropyl ring. In
a study of the proton NMR spectra of 2-propenylcyclopropane
and 1-methyl-2-propenylcyclopropane, Schrumpf26 observed that
the resonances for the allylic protons of 2-propenylcyclopropane
appear at higher frequencies relative to those of typical allylic
protons. The upfield shift was explained by postulating that the
dominant rotamer is s-trans, in which the allylic hydrogens lie
1
We have also utilized H NMR experiments to observe the
preferred conformations of 2CPI in solution. The data given in
Table 8 are consistent with the s-trans conformer being prevalent
at room temperature (see Figure 9 for the numbering scheme).
We compared the 500 MHz proton NMR spectra of 2CPI
and 2MI at 2 °C and at 25 °C. The resulting chemical shift
data for several nonaromatic protons are shown in Table 8. It
is evident that lowering the temperature has a minimal effect
on the proton resonances of 2MI, but a significant effect on
the proton resonances of 2CPI.28 The fact that analogous
observations were made for 2CPI in deuteriohexane as well as
deuteriochloroform indicate that the observed shifts are not
solvent induced. We ascribe the shifts at lower temperatures to
a progressive stabilization of the s-trans conformer. The upfield
shift of the benzylic (H2) protons would then be a result of their
increased exposure to the anisotropy of the cyclopropyl ring in
this conformer. The downfield shift of the vinyl H1 resonance
at lower temperature would be consistent with this interpreta-
tion.29
(21) (a) Traetteberg, M. J. Mol. Struct. (THEOCHEM). 1988, 189, 357-
371. (b) De Mare´, G. R.; Lapaille, S. Org. Magn. Reson. 1980, 13, 75-76.
(c) Carreira, L. A.; Towns, T. G.; Malloy, T. B. J. Am. Chem. Soc. 1978,
100, 385-388.
(22) Jorgenson, M. J. J. Am. Chem. Soc. 1969, 91, 6432-6443.
(23) Mazzochi, P. H.; Ladenson, R. C. J. Chem. Soc., Chem. Commun.
1970, 469-470.
(24) (a) Heathcock, C. H.; Poulter, S. R. J. Am. Chem. Soc. 1968, 90,
3766-3768. (b) Pete, J. P. Bull. Soc. Chim. Fr. 1967, 357-370.
(25) (a) Van-Catledge, F. A.; Boerth, D. W.; Kao, J. J. Org. Chem. 1982,
47, 4096-4106. (b) Van-Catledge, F. A. J. Am. Chem. Soc. 1973, 95, 1173-
1178.
(27) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
56, 2257-2266. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta. 1973,
28, 213-222. (c) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163-168.
(28) Temperature effects were also observed for protons H4 and H5, but
the complex splitting patterns make interpretations difficult. In deuterio-
hexane, the 2CPI aromatic protons’ chemical shifts were observed to deviate
by 0.001 ppm as a function of temperature.
(29) NOESY experiments were attempted, but H1 and H3 have T1 values
of 9 and 2 s, respectively. This large difference precludes the observation
of a dipole-dipole interaction between the protons.
(26) Schrumpf, G. Tetradron Lett. 1970, 30, 2571-2574.

3090 J. Am. Chem. Soc., Vol. 121, No. 13, 1999
Waugh and Morrison
Table 9. Summary of the Conformational Equilibrium for 2CPI
phase
gas
experimental observation
qualitative equilibrium
MO calculations
UV spectrum (λmax ) 257 nm)
s-trans u gauche
solution 1H NMR spectra
s-trans U gauche
Wavelength dependent
excitation spectra for S1
emission
UV spectrum (λmax ) 264 nm)
Wavelength independent
excitation spectra
glass
deformed s-trans r gauche
Figure 10. Hypothetical deconvolution of the solution phase UV
absorption spectrum of 2CPI. See text for definitions of the convolutes.
conclude that, at 77 K, the conformation could be a modestly
deformed s-trans rotamer. In such a case, the conjugation of
the cyclopropane with the indene would be reduced, resulting
in hypsochromicity and hypochromicity in the excitation spectra.
Table 9 summarizes our conclusions concerning the ground state
conformational equilibria of 2CPI in the three phases studied
in this work.
We return then to the solution phase absorption and excitation
spectra of 2CPI and the wavelength dependence of the
photochemistry. The fact that the solution phase excitation
spectra of 2CPI are dependent upon monitoring wavelength
(while those for 2MI are not) is consistent with the presence of
one or more conformations which involve the cyclopropane ring.
There is ample precedent for conformers which have different
absorption and emission properties to exhibit wavelength-
dependent fluorescence and excitation spectra.30,31 The marked
red-shift in the 2CPI absorption spectrum, relative to that
observed for 2MI (cf. Table 1), is consistent with Van-
Catledge’s observations and our NMR experiments and provides
further support for our conclusion that the s-trans rotamer is
prevalent.
By analogy with Van-Catledge’s UV spectroscopy of vinyl-
cyclopropanes, we would expect that the absorption spectrum
of gauche 2CPI would be to the blue of the absorption spectrum
for the s-trans rotamer. In fact, the greater blue shift in the gas
phase absorption maximum of 2CPI relative to solution (11
nm, Table 1), versus that observed for 2MI (7 nm) is consistent
with the gauche conformation having become prevalent in the
gas phase. This interpretation of the absorption spectra is also
consistent with the ground state MO calculations (see above).
Figure 10 presents the S0 f S2 absorption band of 2CPI in
solution, together with hypothetical gauche and s-trans convo-
lutes. The hypothetical gauche spectrum, in which the cyclo-
propyl group is minimally auxochromic, is approximated by
the solution phase absorption spectrum of 2MI. The hypothetical
s-trans spectrum is approximated by the solution phase excitation
spectrum of 2CPI monitored at 330 nm.
Figure 10 provides us with a rationale for the observations
that ring expansion chemistry in solution increases in efficiency
with shorter wavelength excitation (cf. Table 3). Such wave-
lengths of light would preferentially excite more of the gauche
rotamer, the rotamer that has been shown to be generally more
efficient in ring expansion chemistry (see above). The greater
efficiency of ring expansion in the gas phase (relative to
solution) can also be partially attributed to the greater gauche
population in the gas phase.
The excitation spectrum of 2CPI collected at 77 K in a
methylcyclohexane glass is not dependent upon monitoring
wavelength, an indication that only a single rotamer is present.
In a related system, Nijveldt32 determined that the X-ray
structure of vinylcyclopropane at 100 K “slightly deviates” from
the s-trans structure. By analogy with Nijveldt’s work, we
A comment about the positions of the S2 and the S1 transitions
of 2CPI is in order. The solution phase absorption and excitation
spectra of 2CPI, and the wavelength dependence of the
photochemistry, involve changes in quantum efficiencies that
culminate in a maximal effect at 280 nm (Tables 2 and 3). The
wavelength effects that fall clearly within the S0 f S2 transition
(i.e., 250-266 nm) are nicely explained by the existence of a
mixture of conformational isomers. The specific maximum for
Φf, and minimum for Φ1, with 280 nm excitation deserves more
comment. If 280 nm corresponds to excitation of the red edge
of the S0 f S2 transition, then one can assume that S2 is not, in
of itself reactive, but rather uniformly decays through S1.
However, if 280 nm excitation corresponds to the S0 f S1
transition, then the data of Tables 2 and 3 can only be
rationalized by assuming that S2 is reacting in solution and, in
fact, doing so more efficiently than S1. This would explain the
higher reaction quantum yields with 250-266 nm light, and
also explain why Φf increases upon directly exciting into S1.
Differentiation between these two possibilities requires further
spectroscopic studies.
Finally, it is interesting that the apparent Stokes shift for the
absorption and emission of 2CPI in the gas phase (82 nm) is
significantly higher than that (64 nm) observed in solution (Table
1). The solution value is similar to those observed for 2MI in
both the solution and gas phases (62 and 69 nm, respectively).
Large Stokes shifts are normally associated with the conversion
of a state created by a Franck-Condon transition into one in
which a substantial geometric change has occurred. The result
is red-shifted radiative decay to a high-energy ground state
species. Examples include the formation of twisted intra-
molecular charge transfer states (TICT)33,34 through adiabatic
processes, which have been termed horizontal radiationless
transitions.35 One may speculate that the large apparent Stokes
shift for 2CPI in the gas phase involves such an adiabatic
formation of an emissive, higher energy rotamer (possibly the
s-cis rotamer).
Conclusion
Excitation of 2CPI and 3CPI in the gas and solution phases
results in cyclopropane ring opening and shortened singlet
(30) Haas, E.; Fischer, G.; Fischer, E. J. Phys. Chem. 1978, 82, 1638-
1643.
(31) Lewis, F. D.; Yoon, B. A.; Arai, T.; Iwasaki, T.; Tokumaru, K. J.
Am. Chem. Soc. 1995, 117, 3029-3036.
(32) Nijveldt, D.; Vos, A. Acta Crystallogr., Sect. B: Struct. Sci. 1988,
B44, 281-289.
(33) (a) Rettig, W. Angew. Chem., Int. Ed Engl. 1986, 25, 971-988. (b)
Rettig, W. Topics Curr. Chem. 1994, 169, 254-299.
(34) (a) Grabowski, Z. R.; Dobkowski, J. Pure Appl. Chem. 1983, 55,
245-252. (b) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Chem.
Phys. Lett. 1973, 19, 315-318.
(35) Birks, J. B. Chem. Phys. Lett. 1978, 54, 430-434.

Upper Excited State Photochemistry
J. Am. Chem. Soc., Vol. 121, No. 13, 1999 3091
wave functions comprising each eigenvector were identified from the
calculated UV spectrum. The 400 electron spin up, singly excited
configurations contributing to each wave function were identified from
the CI matrix, squared, multiplied by 2 (to account for the spin down
electrons), and multiplied by 100 to convert to a percentage.
Ground-state MO calculations were conducted using Gaussian 94.40
All geometries were optimized at the HF/6-31G27 level of theory.
Frequency calculations at this level showed the s-trans and gauche
rotamers to be minima while the s-cis rotamer was shown to be a
maximum. Energy differences between the conformers are the HF
energies.
lifetimes. Isolation of ring expansion products in the gas phase
provides evidence of a biradical reactive state, the origin of
which is either S2 or S1vib. There is also unexpected ring
expansion chemistry of 2CPI in solution, accompanied by a
significant diminution in transposition chemistry. This suggests
that S1 also has diradical character. Rates of ring opening in S1
in solution are 2.9 × 109 and 9.2 × 108 s-1 for 2CPI and 3CPI,
respectively. The photochemistry and photophysics of 2CPI are
governed by an s-trans h gauche conformational equilibrium,
with ring expansion likely to be occurring preferentially from
the gauche rotamer. This rotamer dominates the equilibrium in
the gas phase, while the s-trans rotamer is prevalent in solution.
The wavelength-dependent effects observed for photochemistry
and fluorescence of 2CPI in solution are consistent with either
S2 decaying to S1 prior to reaction or emission or S2 photo-
chemistry competing with internal conversion to the lower
excited state.
Solution Phase Photochemistry. Experiments were done with
matched quartz tubes (1 cm o.d.) in a Rayonet Model RPR-100 reactor
(New England Ultraviolet Company) equipped with two or four 254
nm low-pressure mercury lamps or with laser light sources. Quantum
efficiencies at 254 nm were measured using 1-phenyl-2-butene acti-
nometry41 with column A (80 °C) to analyze the E/Z isomerization.
The E isomer (Aldrich) was purified by GLC using column B (100
°C). All solution experiments were conducted in cyclohexane and
degassed with argon for 30 min prior to photolysis. Photolyses
conducted at 266 nm were performed using a Continuum NY-61 Nd:
YAG laser equipped with a frequency quadrupler (10 Hz, 3.0-3.3 mJ/
pulse). Irradiations at 280 and 250 nm employed a YAG-pumped dye
laser (Continuum ND-60, Rhodamine 590 or Coumarin 500, respec-
tively). A 2× beam enlarger was used in front of the photolysis cell to
avoid cell damage. Singlet lifetimes were collected using either a time-
correlated photon counting apparatus with 266 nm excitation from a
frequency quadrupled mode locked Quantronix Nd:YAG laser with a
pulse width of fwhm 120 ps or using a PTI LS-1 spectrophotometer
exciting with either 254 or 266 nm light.
Gas Phase Photochemistry. The apparatus has been described in
detail elsewhere.3a Exploratory and preparative experiments were
performed under flowing conditions. Briefly, a liquid sample was
distilled (typically at pressures of 20 mTorr at 25 °C) through a quartz
tube (33 cm in length × 14.7 cm in circumference) surrounded by
sixteen 254 nm low-pressure mercury lamps. The photolyzed vapor
was collected in a liquid nitrogen trap. After warming to room
temperature, the trap was washed twice with 10 mL of pentane and
the combined washings were concentrated under a stream of nitrogen.
Exploratory experiments utilized 10-20 mg of the appropriate indene,
while preparative work employed 30-300 mg. Isolation of photoprod-
ucts employed either silica gel chromatography (hexane as eluent) or
separation by GLC using column B.
Qualitative and quenching studies were performed under static
conditions which involved filling a cylindrical quartz vessel (length
34 cm × 4.7 cm o.d.) with the desired vapor to pressures of 150-800
mTorr, depending on the indene. Unless otherwise noted, the pressures
were not corrected for instrument response. The vessel was removed
from the vacuum line and placed in a modified RPR-100 Rayonet
reactor containing one 254 nm lamp for photolysis times of 5-15 s at
25 °C. Collection of the photolyzed sample required immersion of the
vessel in liquid nitrogen and the injection of 5 mL of pentane containing
an internal standard. After the vessel warmed to room temperature,
the solution was removed and the vessel was again washed with
pentane. The combined pentane washings were concentrated under a
stream of nitrogen and analyzed on column A (120 °C). The gas phase
apparatus was designed with apertures to allow the introduction of a
Experimental Section
Materials. Diethyl ether was dried over sodium/benzophenone prior
to use. Cyclopropyl bromide (Aldrich, Lancaster), 1-indanone (Aldrich),
and iodomethane (Aldrich) were used as received. 2-Indanone (Aldrich)
was purified by flash chromatography (CH2Cl2) and recrystallized from
hexane. Spectranalyzed cyclohexane (Fisher) was used for all spectro-
scopic and photochemical studies. Naphthalene was purified by the
method of Perrin.36 (E)-1-Phenyl-2-butene (Aldrich) was purified by
GLC prior to use.
Instrumentation. 1H and 13C NMR spectra were obtained using
Varian Gemini 200 MHz or Varian 500 MHz spectrometers. Chemical
shifts are reported in deuteriochloroform in parts per million relative
to tetramethylsilane (TMS) or residual chloroform. For the variable-
temperature 1H NMR experiments GLC pure 2CPI and 2MI was
dissolved in deuteriochloroform or deuteriohexane and analyzed at 2
and 25 °C utilizing the Varian 500 MHz spectrometer equipped with
a cryostatically controlled temperature unit. These samples were ca.
e5% v/v in the solvents used.
Mass spectra were recorded on a Finnigan 4000 mass spectrometer
interfaced to a gas chromatograph containing either packed or capillary
columns. Electron impact (EI) and chemical ionization (CI) mass spectra
were recorded at 70 eV. High-resolution mass spectra were recorded
on a Kratos Model MS-50 instrument operated at 70 eV for EI.
Ultraviolet absorption spectra were collected in matched 1 cm2 quartz
cells using a Cary 100 spectrometer interfaced to a Pentium 100 MHz
PC controlled by the Cary Scan Package software. Corrected steady
state fluorescence and excitation spectra were recorded on an SLM
Aminco Model SPF-500 C spectrofluorometer using the A/B mode in
all experiments. Low-temperature fluorescence spectra were collected
using a liquid nitrogen cooled Dewar equipped with a quartz window.
Analytical GLC was performed on Varian Model 3700 FID and Varian
Model 3500 FID capillary gas chromatographs with Hewlett-Packard
3390A digital integrators. Preparative GLC was performed on a Varian
Model 3300 TCD gas chromatograph with a Hewlett-Packard 3390A
digital integrator. Two columns were used: A (J & W DB-1, 15 m ×
0.25 mm id., 0.25 µm) and B (10 ft × 0.25 in, 5% OV-17 on
Chromosorb W).
Computations. Excited state calculations were performed using the
HyperChem 5.0 (Hypercube Inc.) program package. Geometries of the
indenes were optimized in Cs symmetry using the AM137 or ZINDO/
138 Hamiltonians. The molecular orbital plots were generated from
ZINDO/S-CI39 calculations using 20 occupied and 20 unoccupied
orbitals. The multiplicity of each eigenvector (excited state) was
determined from the state dipole moments. All the single determinant
(39) (a) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111-
134. (b) Ridley, J.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223-236.
(c) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T.
J. Am. Chem. Soc. 1980, 102, 589-599.
(40) M. J. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G.
A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; J. Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA,
1995.
(36) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980; pp 349-
350.
(37) Dewar, M. J. S. J. Am. Chem. Soc. 1985, 107, 3902-3909.
(38) (a) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta. 1979, 53, 21-
54. (b) Anderson, W. P.; Edwards, W. D.; Zerner, M. C. Inorg. Chem.
1986, 25, 2728-2732.
(41) Morrison, H.; Pajak, J.; Peiffer, R. J. Am. Chem. Soc. 1971, 93,
3978-3985.

3092 J. Am. Chem. Soc., Vol. 121, No. 13, 1999
Waugh and Morrison
quencher gas (butane). Samples were degassed prior to flowing and
static experiments by at least five freeze-pump (5 mTorr)-thaw cycles.
Gas phase absorption and fluorescence studies utilized two modified
matched 1 cm2 rectangular fluorescence cells fitted with vacuum
attachments.
Syntheses. 2-Cyclopropylindene (2CPI) and 3-cyclopropylindene
(3CPI) were synthesized following the literature procedure and spectral
assignments were consistent with published data.7c 2CPI: MS (EI) m/z
(relative intensity) 156 (M+) (100), 141 (84.89), 128 (41.03), 115
(64.36). 3CPI: MS (EI) m/z (relative intensity) 156 (M+) (88.29), 141
(100), 128 (43.66), 115 (96.70).
2-Methylindene (2MI). The above literature procedure was modified
by using iodomethane and gave a 50% yield of 2MI. Spectral
assignments were consistent with the literature.42
2-Isopropylindene (2ISOI). 2-Isopropylidene-1-indanone was pre-
pared from the aldol condensation of 1-indanone with acetone as
described in the literature.43 The reduction of the ketone with sodium
borohydride in pyridine44 afforded 2-isopropyl-1-indanol, which was
dehydrated with acid to give 2ISOI. Spectral data were consistent with
those in the literature.45
Spectroscopy. The cyclopropyl and methylindenes were purified
by preparative GC using column B (170 °C, 3CPI: Rt 6.1 min, 2CPI:
Rt 8.5 min; 160 °C, 3MI: Rt 3.25 min, 2MI: Rt 3.30) prior to analysis.
Relative fluorescence quantum yields of 2CPI and 3CPI in cyclohexane
were determined with 254 nm excitation relative to toluene. The
fluorescence wavelength study was conducted on 2CPI using naph-
thalene as the fluorescence reference. Separate solutions of 2CPI in
cyclohexane were prepared having absorbances at 254, 260, 266, 270,
275, and 280 nm of 0.10 ( 0.01. Quantum efficiencies were corrected
for differences in absorbance. Cyclohexane solutions were prepared
having absorbances of ca. 0.25 at 266 nm for singlet lifetime
determinations. The data were fit to first-order decay kinetics. Gas phase
absorption and fluorescence spectra were collected in the modified
fluorescence cells, and extinction coefficients were calculated using
corrected pressures.46
Solution Phase Photolysis of 2CPI. Cyclohexane solutions of 3-8
mM 2CPI were irradiated at 254 nm to yield a reaction mixture
containing one major and two minor photoproducts. The solvent was
removed and the residue chromatographed over silica gel (230-400
mesh) using hexane as eluent to give the ring expansion product 1: 1H
NMR (CDCl3, 500 MHz) δ 7.31-7.14 (m, 4H), 5.58-5.57 (s, 1H),
4.15-4.07 (m, 1H), 3.44 (s, 2H), 2.78-2.61 (m, 1H), 2.56-2.41 (m,
2H), 1.85-1.68 (pentet, 1H); 13C NMR (500 MHz, CDCl3) δ 150.34,
146.43, 144.76, 126.58, 126.27, 124.81, 123.80, 120.79, 55.56, 35.32,
32.42; UV (cyclohexane) λmax (log ꢀ) 252 (2.87), 259 (3.05), 265 (3.16),
272 (3.16); MS (EI) m/z (relative intensity) 156 (M+) (100), 141 (47.6),
128 (64.57), 115 (31.77); HRMS (EI) calcd for C12H12 (M+) 156.1017,
found 156.1012.
A minor photoproduct was isolated using column B (170 °C) and
identified by H NMR as 3CPI. The third photoproduct was found to
1
be inseparable from the starting material by flash chromatography and
was unobtainable in sufficient amounts for identification using prepara-
tive GC. It was assigned as 1CPI on the basis of a comparison of its
GLC mass spectrum to that of 2CPI and 3CPI: MS (EI) m/z (relative
intensity) 156 (M+) (72.34), 141 (36.31), 128 (43.48), 115 (100).
Solution Phase Quantum Yields. Cyclohexane solutions (3 mL)
containing 3 mM 2CPI and either 3 mM tridecane (for determining
starting material disappearance) or 0.5 mM tridecane (for photoproduct
formation) were placed in matched quartz tubes. Two actinometer
solutions, 15 mM in cyclohexane, were placed into matched quartz
tubes and used to give a time average of the flux during the course of
the photolysis. The solutions were and irradiated in the Rayonet reactor
using four 254 nm lamps for 5 min. Solution quantum yields for 3CPI
were determined in like fashion. For laser studies, sample solutions in
cyclohexane were placed in matched Vycor laser cells and irradiated
at 250 nm for 14 min, at 266 nm for 2.7 min, and at 280 nm for 9.0
min.
Gas Phase Photolysis of 3CPI. Product 2 was isolated by silica
1
gel chromatography: H NMR (CDCl3, 200 MHz) δ 7.44-7.42 (m,
1H), 7.26-7.15 (m, 3H), 5.79-5.78 (AB q, 1H), 3.4-3.2 (m, 1H),
3.11-3.03 (m, 1H), 2.8-2.6 (m, 2H), 2.56-2.45 (m, 1H), 2.28-2.22
(pentet, 1H), 1.69-1.45 (pentet, 1H); 13C NMR (200 MHz, CDCl3) δ
153.37, 150.24, 127.51, 126.53, 125.66, 121.75, 117.22, 51.69, 38.02,
36.97, 33.12; UV (cyclohexane) λmax (log ꢀ) 256 (3.66), 290 (2.57);
MS (EI) m/z (relative intensity) 156 (M+) (100), 141 (71.41), 128
(58.48), 115 (59.43); HRMS (CI) calcd for C12H13 (M+ + H) m/z
157.1017, found m/z 157.1012.
Butane Quenching Studies. Using static conditions, 2CPI vapor
was distilled into the reaction vessel to a pressure of 150 mTorr and
photolyzed for 5 s using one 254 nm lamp. The product mixture was
analyzed by column A (120 °C) and found to contain compound 1 and
1CPI. 2CPI vapor was again distilled into the reaction vessel to a
pressure of 150 mTorr with the addition of butane (8 and 400 mTorr)
and photolyzed for 5-15 s. The product mixture was analyzed by
column A at 120 °C and found to contain the ring expansion product
1, 1CPI, and 3CPI in the product ratios reported in Table 4. Quenching
studies were repeated for 3CPI and the results are in Table 5.
Gas Phase Quantum Yields. Absorbances for the methyl- and
cyclopropylindenes were measured in 1 cm rectangular cells and
corrected for the longer optical path length of the cylindrical reaction
vessel by the expression L ) πd/4, where L is the effective path length
of a cylinder. The fraction of light absorbed by the methylindenes was
ca. 60%, while the absorbances for 3CPI and 2CPI were 10% and
7%, respectively. Prior to photolysis the mass of the indene vapor was
determined by GLC analysis at gauge pressures of 400 mTorr for 2MI,
800 mTorr for 3MI, 325 mTorr for 3CPI, and 150 mTorr for 2CPI.
These single point calibrations were precise to (5% and were used to
determine the quantum yields of disappearance. The vapor was
photolyzed for 5-10 s and resulted in up to 25% loss of starting material
depending on the indene. Due to the approximations made and the low
absorbances, quantum yields are only reported to one significant figure.
(42) Marchal, E.; Basselier, J.-J.; Sigwalt, P. Bull. Soc. Chim. Fr. 1964,
1740-1749.
(43) Plattner, P. A.; Fu¨rst, A.; Wyss, J.; Sandrin, R. HelV. Chim. Acta.
1947, 30, 689-694.
(44) Jackson, W. R.; Zurgiyan, A. J. Chem. Soc. 1965, 5280-5287.
(45) Edlund, U Org. Magn. Reson. 1978, 11, 516-519.
(46) Gauge pressure correction factors were determined using the mass
of the vapor and assuming ideality. These were used to calculate the vapor
concentration in the vacuum adapted fluorescence cell: 1 Torr gauge
pressure ) 0.4 Torr 2MI, 3MI calculated pressure; 1 Torr gauge pressure
) 0.1 Torr 2CPI, 3CPI calculated pressure. Note that a correction factor
of 0.67 Torr was used for the methylindenes in the previous work. See ref
3a.
Acknowledgment. This work was supported by the National
Science Foundation Grant (Grant CHE9530650).
JA981397Y