Two-photon-induced cycloreversion reaction of coumarin photodimers

Article abstract of DOI:10.1016/S0009-2614(03)00535-9

Photochemical reactions induced by two-photon-absorption processes offer several advantages over common one-photon initiated photoreactions, e.g., three-dimensional spatial control. We present the photocleavage reaction of coumarin photodimers via a two-photon process using pulsed frequency-doubled Nd:YAG-laser light. The two-photon-induced cycloreversion reaction leads selectively to the cleavage of the coumarin photodimers resulting in the formation of monomeric coumarin molecules. The two-photon cross section of the coumarin photodimer was determined to be of 1.6×10^-52 cm⁴ s photon^-1. The presented reaction is of interest, e.g., for the photo-triggered release of chemicals in areas which cannot be directly optically addressed due to cover layers which have a high absorption at the single-photon-absorption wavelength.

Full text of DOI:10.1016/S0009-2614(03)00535-9

Chemical Physics Letters 372 (2003) 899–903
www.elsevier.com/locate/cplett
Two-photon-induced cycloreversion reaction
of coumarin photodimers
a
a
a,b,
*
H-C. Kim , S. Kreiling , A. Greiner a,b, N. Hampp
a
Institute of Physical, Nuclear and Macromolecular Chemistry, University of Marburg, Marburg D-35032, Germany
b
Material Science Center, University of Marburg, Marburg D-35032, Germany
Received 18 March 2003; in final form 31 March 2003
Abstract
Photochemical reactions induced by two-photon-absorption processes offer several advantages over common one-
photon initiated photoreactions, e.g., three-dimensional spatial control. We present the photocleavage reaction of
coumarin photodimers via a two-photon process using pulsed frequency-doubled Nd:YAG-laser light. The two-pho-
ton-induced cycloreversion reaction leads selectively to the cleavage of the coumarin photodimers resulting in the
formation of monomeric coumarin molecules. The two-photon cross section of the coumarin photodimer was deter-
mined to be of 1:6 ꢀ 10ꢁ52 cm4 s photonꢁ1. The presented reaction is of interest, e.g., for the photo-triggered release of
chemicals in areas which cannot be directly optically addressed due to cover layers which have a high absorption at the
single-photon-absorption wavelength.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
diation with light of shorter wavelengths [2–6]
(Fig. 1). Recently applications of this reaction
were explored where coumarin and its derivates
have been used as photoreactive materials for ap-
plications such as photorecording and photoresists
[7–9], and as photolabile protecting groups for
biological applications [10]. We introduce here
coumarin photodimers as suitable multiphoton-
cleavable linkers for photo-triggered release of
molecules e.g., from a polymeric backbone in cases
where one-photon absorption photochemistry is
not desired or not possible.
Since the first photochemical [p2s þ p2s] cyc-
lodimerization of coumarin was reported by Cia-
mician and Silber [1], the photochemical and
photophysical properties of coumarin and its
derivates have been intensively investigated. It is
well known that direct irradiation (k > 300 nm) of
coumarin leads exclusively to photochemical di-
merization resulting in cyclobutane-type dimers
which revert to the starting compound upon irra-
Since the availability of high power pulsed la-
sers, intensive studies on multiphoton processes
appeared in the areas of two-photon spectroscopy
[11–13], opto-electronics [14] and photonics [15,16].
* Corresponding author. Fax: +49-6421-2825798.
E-mail address: hampp@mailer.uni-marburg.de (N. Hampp).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00535-9

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H-C. Kim et al. / Chemical Physics Letters 372(2003) 899–903
Fig. 1. Reversible photodimerization of coumarin under irradiation with UV light.
However, only few contributions were made con-
cerning multi-photon organic photoreactions, e.g.,
two-photon induced photoisomerization [17,18],
photolysis [10] and photodimerization [19]. A
quadratic nonlinear dependence of the probability
for a two-photon process versus the light intensity
results as two-photon processes require the quasi-
simultaneous absorption of two photons via a
virtual state [20].
spectrofluorimeter model RF-1502. The incident
light intensity at 266 nm was 1:086 ꢀ 1014
photons sꢁ1 cmꢁ2 as determined by azobenzene as
a chemical actinometer [21]. The exposure area
was 2 cm2.
2.3. Two-photon-absorption (TPA) induced photo-
cleavage
The use of light in the VIS region allows the
three-dimensional spatially controlled cyclorever-
sion of di-coumarin behind of layers of UV-ab-
sorbing materials, a situation which is relevant in
several medical applications. This Letter describes
for the first time the two-photon-induced cleavage
of coumarin dimers in acetonitrile.
5 mM solutions of coumarin dimer in acetoni-
trile were irradiated with 532 nm pulses of 3 ns
duration from an Infinity 40–100 (Coherent)
mode-locked Nd:YAG laser operating at 20 Hz.
The beam diameter was 3 mm.
2.4. Analysis of the photoconversions
The photocleavage of coumarin dimers was
monitored using UV/VIS absorption spectroscopy
and HPLC analysis. UV/VIS absorption spectra
were recorded on an UVIKON922 (Kontron)
spectrophotometer. HPLC analyses were per-
formed on a Hewlett–Packard Model 1050 system
equipped with a diode array detector. A RP-18
column (250 mm ꢀ 3 mm i.d., Bischoff) was used
with a linear water–acetonitrile gradient eluant.
2. Experimental
2.1. Preparation of coumarin dimers
Photodimers of coumarin were obtained in
isopropanol solution by benzophenone-sensitized
photodimerization. The products were recrystal-
lized twice in ethanol prior to further use. The
purity of the coumarin dimer was >99% as
checked by UV–VIS and HPLC analyses. Single-
photon and two-photon-induced photocleavage of
coumarin dimers were done in acetonitrile solution
using 10 mm path length quartz cuvettes at room
temperature.
3. Results and discussion
3.1. SPA-induced photocleavage of coumarin dimer
The absorption spectra of coumarin and cou-
marin dimer are given in Fig. 2. Coumarin dimers
show only one absorption band at about 270 nm,
but monomeric coumarin has two strong absorp-
tion bands at 270 and 310 nm which are assigned
to p–pÃ- and n–pÃ-transitions [22]. Photocleavage
2.2. Single-photon-absorption (SPA) induced photo-
cleavage
2 mM solutions of coumarin dimer in acetoni-
trile were irradiated at 266 nm using a Shimadzu

H-C. Kim et al. / Chemical Physics Letters 372(2003) 899–903
901
Fig. 2. UV spectra of coumarin and coumarin dimer. Both
compounds were measured in a 10 mm pathlength cell at a
concentration of 0.14 mM in acetonitrile.
Fig. 3. Single-photon-induced cycloreversion reaction of cou-
marin dimer (2 mM in acetonitrile). Energy-dependent UV
difference spectra of coumarin dimer during the course of ir-
radiation with 266 nm light (1:086 ꢀ 1014 photons sꢁ1 cmꢁ2).
of the coumarin dimers was analyzed by measur-
ing the increase of absorbance at kmax ¼ 310 nm
assumed that all the incident light is initially ab-
sorbed by the dimer. In the experiments described
here the optical density at 266 nm was ꢂ2, so only
about 1% of the actinic light is not absorbed. A
value of v0 ¼ 9:1 ꢀ 1013 molecules sꢁ1 was mea-
sured and the quantum yield of the SPA-induced
photocleavage of coumarin dimers was determined
to be USPA ꢂ 0:21.
employing
a molar extinction coefficient of
.
e310ðcoumarinÞ ¼ 5170 Mꢁ1 cmꢁ1
Consistent with literature data [2–6], a cyclore-
version reaction of coumarin dimers in solution
upon irradiation with 266 nm light is observed.
HPLC analyses confirmed that the SPA photoc-
leavage reaction leads selectively to the formation
of monomeric coumarin molecules. Absorption
spectra of di-coumarin solutions in dependence on
the light energy are shown in Fig. 3. The overall
efficiency of the SPA induced photocleavage is
characterized by the quantum yield USPA which is
defined as the ratio of the number Ndimer of mole-
cules transformed to the number Q of photons
absorbed [23].
3.2. TPA-induced photocleavage of coumarin dimer
Upon irradiation of coumarin dimer solutions
with intense 532 nm pulses the photocleavage of
the cyclobutane ring is observed. Fig. 4 shows the
change in the absorption spectrum of the reaction
mixture during the course of irradiation. The in-
crease in absorbance at kmax ¼ 310 nm in the UV
spectrum is characteristic for the formation of
monomeric coumarin. HPLC analyses confirmed
that coumarin is the only product. Of course, due
to the different mechanism and the intrinsically
lower absorption probability the photocleavage
rate is much smaller compared to that of the one-
photon-reaction.
Ndimer
Q
USPA
¼
:
ð1Þ
For low degrees of photoconversion of cou-
marin dimers Eq. (1) can be expressed in terms of
the initial rate of photoconversion vm0 onomer in
molecules sꢁ1, and the actinic power I0 of the light
in photons sꢁ1 cmꢁ2 multiplied by the area A
which is irradiated, here 2 cm2.
In order to verify that a two-photon reaction is
observed the expected quadratic dependence of the
rate of coumarin formation on the irradiation
power [20] was analyzed. We performed cyclore-
version experiments in dependence on the intensity
ð1=2Þ ꢃ vmonomer
I0 ꢃ 0A
Eq. (2) holds for high initial optical densities at
the wavelength of the actinic light because it is
USPA
ꢂ
:
ð2Þ

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H-C. Kim et al. / Chemical Physics Letters 372(2003) 899–903
Fig. 4. Two-photon-induced cycloreversion reaction of cou-
marin dimer (5 mM in acetonitrile). UV difference spectra under
irradiation with 3 ns pulses at 20 Hz having a diameter of 3 mm.
The light intensity at 532 nm was 50 mJ pulseꢁ1
.
of the laser pulses in the range from 10 to
80 mJ pulseꢁ1 (Fig. 5a). Considering the quadratic
dependence on the irradiation power and em-
ploying again the measurement of the initial rates
[24], the following equation describes the relation
between the square of the photon density F and
the initial rate of coumarin monomer formation v0,
which increases at twice the speed the dimer de-
creases:
v0monomer ¼ 2 ꢃ C0dimer ꢃ UTPA ꢃ rTPA ꢃ F 2;
ð3Þ
Fig. 5. Two-photon induced photocleavage of coumarin dimers
in acetonitrile. (a) Coumarin formation measured as the in-
crease of absorption at 310 nm in dependence on the incident
laser power. (b) The TPA-induced nature of the photocleavage
of coumarin dimer corresponds with the slope of 1.8 when ln v0
is plotted against ln F .
where cd0imer is the initial concentration of couma-
rin dimer and UTPA is the quantum yield of the
TPA-induced photocleavage and rTPA is the TPA
cross-section of coumarin dimer. From Eq. (3)
follows that in a plot of the logarithm of the initial
rate of coumarin formation ln v0 versus the loga-
rithm of the photon density ln F a linear depen-
dence with a slope of 2 is expected. As shown in
Fig. 5b an experimental value of 1.8 is measured.
This confirms that we observe a two-photon in-
duced cycloreversion of coumarin dimers. The
deviation from the theoretical value of 2 is pre-
sumably due to the occurrence of additional ab-
sorption processes via excited states which cannot
be excluded completely.
for the fluorescence excited by one-photon ab-
sorption [25]. In order to derive the TPA cross-
section of coumarin dimer rTPA we assume that the
quantum yield of the TPA-induced photocleavage
of coumarin dimer is the same as that for
the corresponding SPA-induced process, i.e.,
USPA ¼ UTPA. With this assumption the two-pho-
ton cross-section can be derived from Eq. (3). The
photocleavage rate v0 was measured by UV- and
HPLC-methods. Using the data shown in Fig. 5a
the two-photon cross-section of coumarin dimer is
determined to be rTPA ¼ 1:6ðꢄ0:27Þ ꢀ 10ꢁ52 cm4 s
In most cases reported in the literatures, the
quantum yield of the two-photon excited fluores-
cence has been assumed to be the same as that
photonꢁ1
.

H-C. Kim et al. / Chemical Physics Letters 372(2003) 899–903
903
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The experiments reported analyze the single-
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