Electronic spectroscopy and photoisomerization of trans-Urocanic acid in a supersonic jet

Article abstract of DOI:10.1021/ja003261h

trans-Urocanic acid (trans-UA), a component of the epidermal layer of skin, exhibits wavelength-dependent photochemistry. The quantum efficiency of isomerization to cis-UA is greatest when the molecule is excited on the long wavelength tail of its absorption profile in solution (300-320 nm). However, exciting the molecule where it absorbs UV light most efficiently (260-285 nm) causes almost no isomerization. We have used fluorescence excitation and dispersed emission methods in a supersonic jet to investigate the electronic states involved in this complex and interesting photochemistry. Three distinct regions are present in the excitation spectrum. Region I, which is below the isomerization barrier, contains sharp, well-resolved peaks that upon excitation emit from the S₁ state of trans-UA. Region II exhibits peaks that increase in broadness and decrease in intensity with increasing excitation energy. Upon excitation these peaks produce dual emission from the S₁ states of both trans- and cis-UA. The trans to cis isomerization barrier is estimated to be 1400 cm^-1. Region III exhibits excitation to the S₂ electronic state and has a broad structure that spans 3000 cm^-1 and occurs 4000 cm^-1 above S₁. S₂ excitation results in essentially no trans to cis isomerization.

Full text of DOI:10.1021/ja003261h

J. Am. Chem. Soc. 2001, 123, 961-966
961
Electronic Spectroscopy and Photoisomerization of trans-Urocanic
Acid in a Supersonic Jet
Wendy L. Ryan and Donald H. Levy*
Contribution from the Department of Chemistry and James Franck Institute, UniVersity of Chicago,
Chicago, Illinois 60637
ReceiVed September 5, 2000
Abstract: trans-Urocanic acid (trans-UA), a component of the epidermal layer of skin, exhibits wavelength-
dependent photochemistry. The quantum efficiency of isomerization to cis-UA is greatest when the molecule
is excited on the long wavelength tail of its absorption profile in solution (300-320 nm). However, exciting
the molecule where it absorbs UV light most efficiently (260-285 nm) causes almost no isomerization. We
have used fluorescence excitation and dispersed emission methods in a supersonic jet to investigate the electronic
states involved in this complex and interesting photochemistry. Three distinct regions are present in the excitation
spectrum. Region I, which is below the isomerization barrier, contains sharp, well-resolved peaks that upon
excitation emit from the S1 state of trans-UA. Region II exhibits peaks that increase in broadness and decrease
in intensity with increasing excitation energy. Upon excitation these peaks produce dual emission from the S1
states of both trans- and cis-UA. The trans to cis isomerization barrier is estimated to be 1400 cm-1. Region
III exhibits excitation to the S2 electronic state and has a broad structure that spans 3000 cm-1 and occurs
4000 cm-1 above S1. S2 excitation results in essentially no trans to cis isomerization.
Introduction
emission peak near 370 nm when the molecule is excited at
308 nm, on the red tail of the absorption curve. This maximum
shifts only slightly when excitation is in the 300 to 320 nm
region. Excitation at the absorption maximum (266 nm) results
in emission that peaks at 354 nm; this is constant as the
excitation wavelength is varied from 260 to 285 nm. The
fluorescence quantum yield of trans-UA excited at 266 nm is
estimated to be 10-4, which is 10 times smaller than the quantum
yield resulting from excitation at 310 nm.
trans-Urocanic acid (trans-UA) is formed in the epidermal
layer of the skin upon deamination of the amino acid histidine.
trans-UA absorbs in the UV-A and UV-B regions and is thought
to act as a natural sunscreen to protect DNA from UV damage.
It was added to sunblocks for a period until the discovery that
UV absorption also results in isomerization to cis-UA, which
has been shown to exhibit immunosuppressive behavior.1
This finding generated a number of investigations into the
photochemistry and photobiology of UA,2 which has turned out
to be a complex and interesting problem. Urocanic acid exhibits
wavelength-dependent photochemistry; the quantum efficiency
of isomerization has its maximum value of 0.5 when the
molecule is excited on the long wavelength tail of the absorption
curve. However, exciting the molecule where it absorbs UV
light most efficiently causes almost no isomerization.3 A handful
of organic molecules have this type of wavelength-dependent
photochemistry, and two explanations have been proposed to
explain this behavior. The molecule may have multiple ground-
state rotamers that have distinct electronic structures, or the
molecule may have several near lying electronic excited states
that have different photophysics.
Transient absorption experiments6 show that the state popu-
lated when pumping at 308 nm is likely a short-lived (<40 ps)
singlet state. Since isomerization occurs in this region, the
authors suggest a minimum rate of isomerization of 1.2 × 1010
s-1 based on the transient signal observed. The authors conclude
that the state populated via pumping at 266 nm is also a very
short-lived singlet state that decays with a rate of 1.4 × 1011
s-1 by intersystem crossing to a nearby triplet state. Photoa-
coustic calorimetry experiments5 have shed further insight into
the nature of the electronic states involved by measuring how
much energy is released nonradiatively when the molecule is
excited in the two regions of interest. At 308 nm, all of the
excitation energy is released to the solvent as heat. However,
excitation at 266 nm causes 50% of the excitation energy to be
retained in the molecule when the solvent is saturated with
argon. This suggests the presence of a long-lived (>1 µs) state.
When the solvent is saturated with oxygen, 75% of the energy
is released implying that the long-lived state is a triplet state.
The energy of the triplet state was calculated to be 230 kJ/mol,
which is in good agreement with triplet sensitization experiments
performed by Morrison et al.3 The above experiments indicate
that there are two different excited states involved at the different
excitation wavelengths.
The Simon group has performed several experiments in the
condensed phase that indicate that the wavelength-dependent
photochemistry of urocanic acid is due to multiple electronic
excited states.4-6 Fluorescence emission studies5 show an
(1) De Fabo, E. C.; Noonan, F. P. J. Exp. Med. 1983, 158, 84-98.
(2) Mohammad, T.; Morrison, H.; HogenEsch, H. Photochem. Photobiol.
1999, 69, 115-135.
(3) Morrison, H.; Avnir, D.; Bernasconi, C.; Fagan, G. Photochem.
Photobiol. 1980, 32, 711-714.
(4) Simon, J. D. Acc. Chem. Res. 2000, 33, 307-313.
(5) Hanson, K. M.; Li, B.; Simon, J. D. J. Am. Chem. Soc. 1997, 119,
2715-2721.
The electronic structures of both trans- and cis-UA have also
been investigated in recent theoretical calculations.7 The authors
perform CASPT2 calculations on the lowest electronic excited
(6) Li, B.; Hanson, K. M.; Simon, J. D. J. Phys. Chem. A 1997, 101,
969-972.
10.1021/ja003261h CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/13/2001

962 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Ryan and LeVy
Figure 1. Fluorescence excitation spectrum and structure of trans-urocanic acid.
states of the two isomers. For the trans isomer, the calculated
vertical excitation energies of the first two electronic excited
states correspond to a π-π* state at 39 760 cm-1 and an n-π*
state at 41 292 cm-1 with oscillator strengths of 0.1203 and
0.0005, respectively. At the equilibrium geometries of the n-π*
and π-π* states, the origin transitions were calculated to be
33 000 and 37 600 cm-1, respectively. Note that the n-π*
transition is predicted to occur at lower energy in this case,
unlike the vertical excitation energy calculations, indicating a
surface crossing. The n-π* state was predicted to involve a
large change in geometry from the ground to the excited state,
with its emission maximum shifted 9600 cm-1 from the origin
transition. Similarly, the n-π* and π-π* states for the cis
isomer were calculated to occur at 33 800 and 27 200 cm-1
.
Again, the emission maximum for the n-π* state was shifted
significantly, indicating a large change in geometry from the
ground to the excited state.
Figure 2. Expanded fluorescence excitation spectrum of trans-UA in
Region I.
The prototype molecule for studying trans/cis photoisomer-
ization in a supersonic jet is trans-stilbene (1,2-diphenylethyl-
ene). Syage et al.8,9 measured fluorescence lifetimes upon
excitation of vibronic levels in S1. They found the fluorescence
lifetimes were constant for excitation energies below the
isomerization barrier, but the lifetimes rapidly decreased with
increasing energy beyond the isomerization barrier. The decrease
in lifetimes above the isomerization barrier was attributed to
the competition of the nonradiative isomerization channel with
radiative decay. In the fluorescence excitation spectrum which
is more relevant to our study, the shortening of vibronic lifetimes
due to isomerization was manifested as an increase in broaden-
ing and decrease in intensity of the peaks with increasing
excitation energy.
sufficient vapor pressure and the nozzle temperature was kept about
20 °C higher than the sample temperature. The molecules were seeded
into helium gas preheated to a temperature of 200 °C and a stagnation
pressure of 3-4 atm, and the mixture was expanded into a vacuum
chamber through a 50-100 µm orifice. Excitation spectra were
measured by monitoring the total fluorescence as a function of excitation
wavelength, using a photomultiplier tube. The dispersed fluorescence
spectra were measured by dispersing the emission obtained at fixed
excitation wavelength through a 1.0 m monochromator and detecting
the fluorescence with a photomultiplier tube. Resonance-enhanced
multiphoton ionization was also performed to ensure that all of the
features occurring in the fluorescence excitation spectrum were due to
trans-UA.
Results
To gain further insight into urocanic acid’s complicated
photochemistry, we have performed fluorescence excitation and
dispersed emission studies of trans-UA in a supersonic jet.
The fluorescence excitation spectrum and structure of trans-
UA are shown in full in Figure 1. It is useful for discussion
purposes to divide the spectrum into three regions: Region I
from 32 500 to approximately 34 000 cm-1, Region II from
34 000 to 35 500 cm-1, and Region III from 35 500 to 39 000
Experimental Section
trans-Urocanic acid was purchased from Aldrich and used without
further purification. The spectra presented were recorded with use of
fluorescence excitation and dispersed fluorescence techniques described
in detail elsewhere.10 The molecules were heated to 270 °C to attain a
cm-1
.
Region I. The first region, expanded in Figure 2, is comprised
of sharp, well-resolved peaks. The strongest peak in this
spectrum is at 32 626 cm-1 (307 nm), and is assigned as the S1
origin. The less intense features to the blue of the origin are
vibrational progressions. The emission spectrum resulting from
excitation of the origin transition is shown in Figure 3a. The
most intense feature occurs at the excitation wavelength, with
several less intense vibrational peaks to the red. The fact that
the origin transition is the strongest feature in both the excitation
(7) Page, C. S.; Merchan, M.; Serrano-Andres, L.; Olivucci, M. J. Phys.
Chem. A 1999, 103, 9864-9871.
(8) Syage, J. A.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1984, 81,
4685-4705.
(9) Syage, J. A.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1984, 81,
4706-4723.
(10) Sharfin, W.; Johnson, K. E.; Wharton, L.; Levy, D. H. J. Chem.
Phys. 1979, 71, 1292.

Photoisomerization of trans-Urocanic Acid in a Supersonic Jet
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 963
Figure 3. Single vibronic level emission of trans-UA excited at (A)
origin (32 636 cm-1) at 40 cm-1 spectral resolution and (B, C, D) origin,
+475, and +1214 cm-1, respectively, at 120 cm-1 spectral resolution.
The x axis is in cm-1, relative to the origin transition 32 626 cm-1
.
Features marked with asterisks are entirely due to scattered laser light.
Less than 15% of the intensity at the excitation wavelength in parts A
and B is due to scattered laser light.
and emission spectra implies that the geometry of the S1 state
is not significantly different from that of the ground state.
The spectrum in Figure 3a was taken at 40-cm-1 resolution.
To obtain sufficient signal upon exciting the less intense features,
the monochromator slits were widened such that the resolution
in the rest of the emission spectra to be discussed is 120 cm-1
;
the origin emission spectrum at this resolution is shown in Figure
3b. Exciting the other vibrational peaks in Region I results in
almost identical emission spectra (see the +475-cm-1 peak in
Figure 3C, for example), but a noticeable broadening occurs at
higher energies, as illustrated in Figure 3d. This broadened
emission appears at excitation energies around +1100 cm-1
from the origin; note that the peak in the excitation spectrum is
still sharp at this point.
The majority of the vibrations described above exhibited
relaxed emission. That is, energy originally contained in a single
vibrational mode (or a few vibrational modes in the case of
combination bands) has been distributed to many lower fre-
quency modes prior to emission. However, two peaks at +620
and +1180 cm-1 above the origin did not relax, as shown in
Figure 4a, where a significant portion of the intensity at the
excitation wavelength is fluorescence. Figure 4b shows the
overlay of the unrelaxed peaks and origin emission, where it is
clear that the unrelaxed peaks are vibrational peaks built upon
the originally excited level.
Figure 4. (a) Single vibronic level emission spectra of trans-UA
excited at (A) origin, (B) +620 cm-1, and (C) +1180 cm-1. Less than
15% of the emission intensity at the excitation wavelengths is due to
scattered laser light. (b) Overlays of the +620 (B) and +1180 cm-1
(C) emission spectra from Figure 4A with origin emission (A). Spectra
B and C were shifted -630 and -1195 cm-1, respectively, to depict
overlap with origin emission.
onset of emission spectra exhibiting dual emission first occurs
for excitation around 34 000 cm-1 (origin + 1400 cm-1). Note
that there are several sharp peaks in the excitation spectrum
around the onset of the broad peaks.
Region III. The bluest region begins with the continuing
broad peaks from Region II with a growing in of an extremely
broad feature covering over 3000 cm-1. We believe that this is
due to the S2 electronic state. The maximum absorption occurs
about 4000 cm-1 above the S1 origin (273 nm). Such a broad
absorption signifies that vertical excitation is to a point on the
S2 potential energy surface that contains a large density of states.
Furthermore, absorption to a large density of states implies that
the S2 state is shifted in one or more coordinates relative to the
S0 state since absorption to the lowest vibrational level would
appear sharp and broadened only by radiationless processes.
Emission resulting from excitation of this feature is shown in
Figure 7. It appears to be an even further broadened version of
Region II. The second region, shown in greater detail in
Figure 5, involves peaks that decrease in intensity and increase
in width as the excitation energy is increased. Excitation of these
peaks results in dual emission, as shown in Figure 6. The
emission is comprised of the same features, now broadened,
observed in Region I. In addition there are blue-shifted
components, the bluest occurring at about 33 800 cm-1. The

964 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Ryan and LeVy
Figure 7. Emission spectra of trans-UA excited at (A) +1690 cm-1
Figure 5. Expanded fluorescence excitation spectrum of trans-UA in
Region II.
in Region II and (B) +2974 cm-1 in Region III. The x axis is in cm-1
,
relative to the origin transition 32 626 cm-1. Features marked with
asterisks are entirely due to scattered laser light.
tion in the isolated molecule under the collision-free conditions
achieved in the jet.
As seen above, the fluorescence excitation spectrum of trans-
UA features three distinct regions. The first region, covering
about 1400 cm-1, is comprised of many sharp, well-resolved
peaks. The second region is characterized by many broad but
resolvable peaks that increase in width and decrease in intensity
with increasing energy. The third region involves the broad
features from Region 2 overlaid on a broad, structureless
absorption covering about 3000 cm-1. The three regions will
be discussed, followed by comparisons to condensed-phase
experiments and theoretical calculations.
Region I. As presented in the Results section, the fact that
the ∆V ) 0 transition is the strongest in both the excitation and
emission spectra implies that the ground and first excited state
potential energy surfaces are not shifted much relative to each
other. This behavior is indicative of a π-π* transition because
in large, delocalized π-systems, a change in one delocalized
electron usually has little effect on the geometry or force
constants of the molecule. High-level ab initio calculations have
suggested that the S1 state is an n-π* transition of miniscule
oscillator strength localized on the carbonyl oxygen, with a very
different geometry from that of the ground electronic state.7 If
the transition observed here were to an n-π* state, we would
expect not only a much less intense transition but also a
broadened emission spectrum with a large shift relative to the
excitation spectrum.
Figure 6. Single vibronic level emission spectra of trans-UA excited
at (A) +1213 cm-1 in Region I and (B, C, D) +1690, +1810, and
+1840 cm-1 in Region II. The x axis is in cm-1, relative to the origin
transition 32 626 cm-1. Features marked with asterisks are entirely due
to scattered laser light.
It is difficult to assign the vibrations in the ground and excited
state. Infrared studies have been performed on imidazole and
propionic acid separately, but not on urocanic acid as a whole.
Furthermore, the electronic spectrum of the imidazole chro-
mophore has not been studied in a jet. Much of the vibrational
activity in the emission spectrum of trans-UA may be attributed
to the normal mode vibrations of the imidazole chromophore.
Most of the observed features correlate well with published
infrared spectra,11,12 as seen in Table 1. A few features appear
in the emission spectrum that cannot be imidazole vibrations,
so we can look to the propionic acid moiety. The feature at
226 cm-1 in the emission spectrum is identical to the ethylene
bend in propionic acid.13 There is a corresponding feature in
the excitation spectrum at 237 cm-1. Several features in the
trans-UA fluorescence emission and excitation spectra appear
the emission spectrum produced by excitation of the S1 origin
transition. A very small amount of emission is observable at
wavelengths where the blue features occur in the emission
spectra from Region II, but the ratio of blue -shifted emission
to origin emission has decreased significantly in this region.
Discussion
trans-Urocanic acid (trans-UA) exhibits somewhat rare
wavelength-dependent photochemistry in solution. Excitation
at low energies results in maximum isomerization to cis-UA
while excitation where trans-UA absorbs most efficiently results
in minimal isomerization. The objectives of this supersonic jet
study were to elucidate the nature of the electronic states buried
under the broad, structureless absorption spectrum of urocanic
acid observed in solution. Furthermore, we were very interested
in the manifestation of wavelength-dependent photoisomeriza-
(11) King, S. T. J. Phys. Chem. 1970, 74, 4.
(12) Van Bael, M. J. Phys. Chem. A 1997, 101, 2397-2413.
(13) Umemura, J. J. Mol. Struct. 1977, 36, 35-54.

Photoisomerization of trans-Urocanic Acid in a Supersonic Jet
J. Am. Chem. Soc., Vol. 123, No. 5, 2001 965
Table 1. Vibronic Features in Excitation (S1) and Emission (S0) Spectra of trans-UA, Compared to a Published Imidazole Infrared Spectrum
published imidazole
a
S0 (cm-1
226
)
IR spectrum11 (cm-1
)
S1 (cm-1
)
assignments
ethylene bend
237
506
606
736
538/551
636/631
735/728
476, 514 (Y)
620
719
1066
1281
1376
1576
1056/1074/1120/1130
924/954/987
imidazole normal
modes
1252
1325
1518
1214/1180 (Z)
1338/1366 (X)
1519
1786
1876
1694 (Z + Y)
1814/1842 (X + Y)
combination bands?
other propionic acid vibrations?
a The letters X, Y, and Z in parentheses represent possible fundamentals contributing to combination bands, e.g. X + Y.
between 1600 and 1800 cm-1, but the imidazole infrared
spectrum does not contain any peaks between 1600 and 3000
cm-1. These features may be combination bands of some of
the imidazole fundamental vibrations or vibrations arising from
the rest of the molecule, like the carbonyl stretch. A few peaks
in the excitation spectrum, labeled X and Z in the table, appear
to be combination bands with the +476 cm-1 mode, labeled Y.
The emission spectra resulting from excitation of the vibra-
tional peaks in this region are very similar and resemble the
emission spectrum obtained by exciting the origin. At higher
energies, the spectra become broadened, but retain similar
features. This is likely a consequence of intramolecular vibra-
tional relaxation (IVR). The molecule is initially excited to a
Franck-Condon allowed bright state. As the excitation energy
is increased, the density of dark states originating from other
vibrational modes of the molecule increases, which may mix
with the bright state via anharmonic couplings. Each of these
dark states has large Franck-Condon factors with corresponding
ground-state levels, i.e., large ∆V ) 0 Franck-Condon factors.
The many overlapping transitions with slightly different fre-
quencies result in a broadened emission spectrum. The IVR
broadening of the emission spectra occurs around +1000 cm-1
above the origin, when the first two features in the emission
spectrum appear to merge together. However, the unrelaxed
vibrational peaks at +620 and +1180 cm-1 suggest that IVR
in this region is mode selective.
Region II. The second region in the fluorescence excitation
spectrum involves an abrupt appearance of features with
increasing width and decreasing intensity as the excitation
energy increases. We will discuss two arguments as to why we
believe that we are observing trans to cis isomerization in this
region.
Initially, the broad excitation features alternate with sharp
features, then at about 1600 cm-1 all features become broad.
Excitation of these broad features results in dual emission which
contains the standard IVR broadened trans-UA emission with
some blue-shifted components. Dual emission signifies coupling
to another electronic state, which we believe is the cis-UA S1
state. The blue-shifted emission would then be fluorescence of
the cis-UA S1 state back to the cis-UA ground state. The
presence of some sharp peaks initially means that at the onset
of level mixing of the trans-UA S1 with the cis-UA S1, the
density of states of acceptor levels has not reached the statistical
limit, so accidental degeneracies are observed.
energy increased. Quantitative studies were performed to
measure the lifetimes of each vibrational peak, and the conclu-
sion was that the lines broadened due to competition of the
radiationless isomerization channel with radiative decay. Fur-
thermore, the onset of IVR occurred a few hundred cm-1 before
isomerization, which led to the conclusion that the isomerization
was not mode selective.
The assumption that isomerization is occurring in Region II
allows us to make a few comments on the isomerization process
in urocanic acid. The onset of the broad features exhibiting dual
emission occurs around 1400 cm-1, or 4 kcal/mol. We ap-
proximate this as the isomerization barrier. Emission spectra
exhibiting IVR broadened features occurs around 1100 cm-1
,
which means that like stilbene, isomerization of trans-UA is
not mode selective. The bluest emission peak of the cis-UA
emission occurs at 33 800 cm-1. This is likely the cis-UA origin
transition which is about 1200 cm-1 higher in energy than the
trans-UA origin. Several calculations have shown that the cis-
UA ground state is more stable than the trans-UA ground state
because of the intramolecular hydrogen bond that may form
between the ring imidazole and the carbonyl oxygen. The
calculation of Page et al.7 assigns a difference of 3 kcal/mol, or
about 1000 cm-1, between the cis-UA and trans-UA ground
states. If this is the case, the S1 state will be about 200 cm-1
higher in energy than the trans S1 state, which means that the
cis-to-trans isomerization barrier is a few hundred cm-1 lower
than the trans-to-cis barrier.
Region III. The third region is comprised of the increasingly
broadened peaks from Region II, with a growing in of a broad,
structureless feature that covers about 3000 cm-1. The broad
structure is likely due to excitation of the S2 electronic state of
trans-UA. The emission spectrum resulting from excitation of
this state is S1 emission minus the blue-shifted components
characteristic of cis-UA emission, which seems to indicate that
isomerization is occurring to a significantly lesser extent in
Region III than in Region II. According to Kasha’s rule, S2
emission rarely occurs, because the density of states available
at higher electronic states is great enough that some other
radiationless process will occur much faster than fluorescence.
The most likely radiationless processes are internal conversion
(IC) to the S1 state and intersystem crossing (ISC) to a triplet
state.
We do not believe we are observing ISC because the emission
is shifted at most a hundred cm-1 from S1 emission. The triplet
state would have to lie coincidentally close to the S1 state for
ISC to be occurring. Furthermore, we attempted to measure a
lifetime for this state but it was shorter than our instrument
response of 10-20 ns, which is too short to be a triplet state.
Ruling out ISC would lead us to believe that we are observing
IC to the S1 state. The problem with this idea is that if S2 is
The second argument we have for the occurrence of isomer-
ization in this region is the similarity of this portion of the
spectrum to the isomerization region of trans-stilbene.8,9 In the
frequency domain, the excitation spectrum of stilbene showed
an abrupt broadening of peaks at the isomerization barrier that
increased in width and decreased in intensity as the excitation

966 J. Am. Chem. Soc., Vol. 123, No. 5, 2001
Ryan and LeVy
Unfortunately, the geometry of trans-UA in the second π-π*
electronic state has not been calculated.
The photodynamics observed in the gas phase are similar to
those in solution. In both cases, some radiationless process
competes with isomerization at high energies to lower the
isomerization quantum yield. Unlike the solution results, we
do not observe ISC to a triplet state. In addition, in the jet
isomerization only seems to be occurring over about 1500 cm-1
of excitation energy, while it occurs over at least 3000 cm-1 in
solution. It is possible that the solvent may lower the isomer-
ization barrier and increase the rate of isomerization. We have
begun studies on van der Waals clusters of trans-UA with water
molecules to address these issues.
As far as comparison with the calculated electronic structure,
we do not observe the n-π* state that is supposed to occur at
33 100 cm-1. The origin of the first π-π* state is predicted to
be 36 700 cm-1, which is close in energy to our S2 state.
However, the origin transitions for higher lying π-π* states
were not calculated. The difference in vertical excitation energy
between the two lowest lying π-π* states is calculated to be
around 4000 cm-1, which is close to the difference in vertical
excitation energy of our S1 and S2 states.
Figure 8. Schematic of electronic states involved in the photodynamics
of urocanic acid. The potential energy surfaces of trans-UA are shown
with respect to two coordinates: the isomerization coordinate (right-
hand side) and another coordinate (left-hand side) in which S2 changes
with respect to S0 and S1. The arrows labeled I, II, and III depict
excitation into Region I (below the isomerization barrier), Region II
(isomerization region), and Region III (S2 electronic state where IC to
S1 competes with isomerization), respectively.
Conclusion
The rare wavelength-dependent photochemistry of trans-
urocanic acid has been observed in the gas phase. Isomerization
of trans-UA to cis-UA occurs in the S1 electronic excited state
after overcoming a barrier of approximately 1400 cm-1. The
isomerization quantum yield decreases significantly in the region
of the S2 electronic state, possibly due to fast internal conversion
back to S1 with little excitation in the reaction coordinate.
The S1 origin of trans-UA occurs at 32 626 cm-1 and is likely
a π-π* state. From the blue-shifted emission characteristic of
cis-UA, we can predict that the cis-UA S1 origin occurs
approximately 1200 cm-1 higher in energy than the trans-UA
origin. The S2 electronic excited state of trans-UA occurs about
4000 cm-1 above S1, and is extremely broad, suggesting a
change in geometry relative to the S0 state.
coupling to a large density of states high in the S1 state, i.e.,
above the isomerization barrier, we would expect that isomer-
ization would occur after IC. This seems unlikely as the blue-
shifted emission characteristic of cis-UA is miniscule. However,
recall that the broad absorption of S2 led us to believe that its
geometry is shifted from that of the ground state. This being
the case, excitation would occur to some high energy vibrational
levels in the S2 manifold, and vibrational relaxation could occur
within S2 before IC to S1. If the relaxed vibrational state had
vibrational energy distributed among many vibrational modes
with little or no excitation of the vibrations involved in the trans
to cis isomerization reaction coordinate, S2 would couple to a
density of states that would result in only trans-UA emission.
Figure 8 illustrates this idea. This scheme requires that the
coordinate along which S2 shifts relative to S1 is different than
that of the coordinate change in trans-to-cis isomerization.
Acknowledgment. We are grateful to John Simon for helpful
discussions regarding the solution photochemistry of trans-UA
and to Chris Page and Massimo Olivucci for helpful discussions
regarding the electronic structure calculations.
JA003261H