1,1-Dimethylnaphthalenon-dimers as photocleavable linkers with improved two-photon-absorption efficiency and hydrolytic stability

Article abstract of DOI:10.1016/j.jphotochem.2009.11.005

Coumarins are well known for reversible dimer formation with wavelengths greater than 300. nm and dimer cleavage below 300. nm. In a photochemical [2+2]-cycloaddition a cyclobutane ring is formed. Formation as well as cleavage of the cyclobutane ring may be accomplished by a single-photon-absorption as well as by a two-photon-absorption triggered reaction. The coumarin system is of interest for various kinds of applications, ranging from drug delivery for ophthalmic implants to optical data storage. However, the two-photon-absorption coefficient of coumarin dimers is rather low falling in the range of 1. GM in the visible range. We present here a substitute for the coumarin dimer system which not only has an about one order of magnitude higher two-photon-absorption coefficient, but also overcomes several other problems of the coumarin dimer system. Coumarines and in particular coumarine dimers have a very limited solubility in common solvents and are susceptible to hydrolysis of the lactone ring, which leads to an undesired complexity in the photochemical cleavage reaction. The 1,1-dimethylnaphtalenone dimers introduced here show excellent stability, lead only to a single cleavage product, and have a two-photon-absorption coefficient of about 10. GM at 532. nm. These properties make the 1,1-dimethylnaphtalenone dimers a superior substitute over the well-known coumarin dimers in particular in applications where two-photon-absorption induced photocleavage is required.

Full text of DOI:10.1016/j.jphotochem.2009.11.005

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 128–134
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
1,1-Dimethylnaphthalenon-dimers as photocleavable linkers with
improved two-photon-absorption efficiency and hydrolytic stability
Julia Liesea, Norbert A. Hamppa,b,∗
a Department of Chemistry, University of Marburg, D-35032 Marburg, Germany
b Material Sciences Center, D-35032 Marburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Coumarins are well known for reversible dimer formation with wavelengths greater than 300 nm and
dimer cleavage below 300 nm. In a photochemical [2+2]-cycloaddition a cyclobutane ring is formed. For-
mation as well as cleavage of the cyclobutane ring may be accomplished by a single-photon-absorption
as well as by a two-photon-absorption triggered reaction. The coumarin system is of interest for vari-
ous kinds of applications, ranging from drug delivery for ophthalmic implants to optical data storage.
However, the two-photon-absorption coefficient of coumarin dimers is rather low falling in the range
of 1 GM in the visible range. We present here a substitute for the coumarin dimer system which not
only has an about one order of magnitude higher two-photon-absorption coefficient, but also over-
comes several other problems of the coumarin dimer system. Coumarines and in particular coumarine
dimers have a very limited solubility in common solvents and are susceptible to hydrolysis of the
lactone ring, which leads to an undesired complexity in the photochemical cleavage reaction. The 1,1-
dimethylnaphtalenone dimers introduced here show excellent stability, lead only to a single cleavage
product, and have a two-photon-absorption coefficient of about 10 GM at 532 nm. These properties make
the 1,1-dimethylnaphtalenone dimers a superior substitute over the well-known coumarin dimers in
particular in applications where two-photon-absorption induced photocleavage is required.
© 2009 Elsevier B.V. All rights reserved.
Received 1 September 2009
Received in revised form 29 October 2009
Accepted 9 November 2009
Available online 13 November 2009
Keywords:
Two-photon-absorption
Coumarin dimers
Asymmetric cleavage
Lactone ring
Dimethylnaphthalenone
1. Introduction
Of particular relevance for the process is the molecular sta-
bility in aqueous surroundings, as they are given in modern
The photochemistry of coumarins has been extensively stud-
ied throughout the last century [1–4], due to its wide range of
applications, i.e., laser dyes [5], fluorescent markers [6], therapeu-
tic properties [7], and data storage [8]. Coumarins are able to form
dimers upon irradiation with wavelengths above 300 nm, which
was first discovered by Ciamician and Silber in 1902 [9]. The dimers
formed may occur in one out of four different isomeric configura-
tions, syn and anti head-to-head and syn and anti head-to-tail [10].
This reaction is reversible at wavelengths below 300 nm and was
already employed in the photopolymerisation of functionalized
coumarin monomers [11]. Recently, dicoumarin has been evalu-
ated as a functional building block for drug delivery devices to
treat posterior capsule opacification [12,13]. The coumarin dimer
forms the functional unit for the two-photon-absorption photocon-
trolled separation of polymer backbone and the cytostatic drug in
the intraocular lens (IOL).
hydrophilic IOL materials, and a high and efficient two-photon-
absorption (TPA) cross-section of the dimer. However, this is
difficult if not impossible with coumarine dimers. The lactone
ring motives in the dimers are easily hydrolyzed by nucleophilic
attack in water, which leads to an unsymmetric cleavage in the
applied head-to-head dimer, not yielding the desired release of
the cytotoxic compound. In this study, we present 7-methoxy-
1,1-dimethylnaphthalenone dimer as a replacement for coumarin
dimer which is based on the same principle, but shows better stabil-
ity, better solubility, well separated absorption bands of monomer
and dimer, and a significantly improved two-photon-absorption
coefficient.
2. Materials and methods
All chemicals were bought from commercial suppliers and used
without further purification. Organic solvents were received in
technical quality and purified by filtration and distillation. The
coumarin dimers were prepared according to Härtner [14] and
2-iodoxybenzoic acid was prepared according to Frigerio [15]. As
aqueous eluent for HPLC, ultra pure water (0.05 ␮S/cm) was acidi-
fied with 300 ␮L H3PO4 per liter. Buffers were prepared with ultra
∗
Corresponding author at: Department of Chemistry, University of Marburg,
Hans-Meerwein-Str. Bldg. H, D-35032 Marburg, Germany. Tel.: +49 6421 2825775;
fax: +49 6421 2825798.
E-mail address: hampp@staff.uni-marburg (N.A. Hampp).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.11.005

J. Liese, N.A. Hampp / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 128–134
129
pure water and the desired pH values were adjusted with a HI 9017
pH-meter (Hanna Instruments).
3.03 (t, 2H, J = 6.5 Hz, J = 7.1 Hz), 2.65 (t, 2H, J = 6.5 Hz, J = 7.1 Hz), and
1.42 (s, 6H).
Analytical RP-HPLC measurements were done on an UltiMate
3000 System (Dionex) equipped with a Nucleosil column (RP18,
5.0 ␮m, 250 mm × 4 mm, Bischoff Chromatography) using acetoni-
trile and acidified water as eluents. For preparative RP-HPLC a
YMC-Pack ODS-A column (C18, 5.0 ␮m, 250 mm × 4 mm, YMC) was
used, an AD 25 absorbance detector and a P680 HPLC pump (both
Dionex). All NMR spectra were recorded on a Bruker Avance 300B
spectrometer (300 MHz). For UV/VIS spectroscopy a sample was
dissolved in acetonitrile and measured against the solvent in 1 cm
quartz cuvettes. All UV/VIS spectra were recorded on an Uvikon 922
(Kontron Instruments). GC–MS measurements were done on a GC-
MS-QP5050 (Shimadzu) on a SE-54-cb column (Chromatography
Service).
Monomers were dimerized in an UV-reactor equipped with 12
fluorescent tubes Eversun 40W/79, 25X (Osram). Photocleavage of
dimers in acetonitrile was either done at 266 nm in a fluorescence
spectrophotometer type RF 1502 (Shimadzu) or at 512 nm using a
frequency-doubled Nd:YAG pulse-laser (Infinity 40-100, Coherent).
Photon densities at 266 nm were determined using a photodiode
1337-1010 BQ (Hamamatsu).
13C NMR (75 MHz, CDCl3): ı/ppm = 213.5, 157.8, 143.9, 128.0,
126.4, 111.2, 110.4, 54.3, 46.9, 36.4, 26.7, and 25.8.
2.4. Synthesis of 7-methoxy-1,1-dimethylnaphthalenone
7-Methoxy-1,1-dimethyltetralone (475 mg, 2.33 mmol) was
dissolved in 23 mL of a mixture of DMSO:toluene (1:2) and 829 mg
2-iodoxybenzoic acid (2.69 mmol, 1.3 equiv.) were added. The reac-
tion was heated to 70 ◦C and stirred for 5 days. The solution was
diluted with diethyl ether and washed with 5% NaHCO3 (2×), H2O
(1×) and brine (1×). The solvents were removed and the crude
product was purified using preparative RP-HPLC with acetonitrile
and acidified water (35:65) as the eluent. A yellowish solid was
obtained (355 mg, 1.76 mmol, 76%).
1H NMR (300 MHz, CDCl3): ı/ppm = 7.39 (d, 1H, J = 9.8 Hz), 7.26
(d, 1H, J = 8.4 Hz), 7.09 (d, 1H, J = 8.2 Hz), 6.98 (d, 1H, J = 2.4 Hz), 6.04
(d, 1H, J = 9.8 Hz), 3.86 (s, 3H), 3.04 (t, 2H, J = 6.4 Hz, 7.3 Hz), 2.66 (t,
2H, J = 6.5 Hz, 7.3 Hz), and 1.46 (s, 6H).
13C NMR (75 MHz, CDCl3): ı ppm = 144.6, 131.0, 130.0, 127.0,
122.0, 115.2, 115.0, 113.0, 111.3, 55.4, 55.2, and 28.1
2.1. Hydrolysis of the lactone ring in coumarin dimers
2.5. Dimerization of 7-methoxy-1,1-dimethylnaphthalenone
The coumarin dimers were dispersed in MeOH and stirred
over night at 80 ◦C. The solvent was removed in vacuo and the
dimer was extracted with chloroform, dried and analyzed by
NMR.
7-Methoxy-1,1-dimethyl-naphthalenone (100 mg, 0.50 mmol)
and benzophenone (18 mg, 0.10 mmol) were dissolved in 2 mL
chloroform and degassed with argon. The solution was irradiated
with UV light for 8 days and the dimerization was monitored by
HPLC. The solvent was removed, the crude product was dissolved
in acetonitrile and purified via isocratic preparative RP-HPLC with
acetonitrile and acidified water (50:50) as the eluent. The main
product, the anti-head-to-head dimer, was obtained as a white solid
(71 mg, 0.18 mmol, 72%).
1H NMR (300 MHz, CDCl3): ı/ppm = 6.82 (d, 2H, J = 8.5 Hz), 6.22
(d, 2H, J = 2.3 Hz), 6.16 (d, 1H, J = 2.3 Hz), 6.14 (d, 1H, J = 2.3 Hz), 4.78
(d, 2H, J = 8.9 Hz), 3.86 (d, 2H, J = 9.0 Hz), 3.26 (s, 6H), 0.80 (s, 18H),
and 0.00 (s, 12H).
13C NMR (75 MHz, CDCl3): ı/ppm = 173.3, 155.2, 154.5, 127.4,
117.9, 111.9, 107.2, 51.3, 43.3, 37.7, 25.2, 17.8, and −4.9.
1H NMR (300 MHz, CDCl3): ı ppm = 7.40 (d, 2H, J = 8.5 Hz), 6.91
(dd, 2H, J = 2.6 Hz, 8.5 Hz), 6.87 (d, 2H, J = 2.6 Hz), 4.05 (d, 2H,
J = 8.7 Hz), 3.81 (s, 6H), 3.64 (d, 2H, J = 8.7 Hz), 1.48 (s, 6H), and 1.33
(s, 6H).
2.2. Photocleavage of coumarin dimers
2.5 mL of a solution of dimer in acetonitrile (1 mol L−1) was irra-
diated with 266 nm. Three different products were obtained and
were purified using preparative HPLC. The products were identified
as dimethyl fumarate and (E) and (Z)-6,6ꢀ-(ethene-1,2-diyl)bis(3-
(tert-butyldimethyl-silyloxy)phenol).
13C NMR (75 MHz, CDCl3): ı ppm = 214.0, 160.2, 146.0, 130.2,
129.6, 118.2, 113.8, 111.4, 55.9, 48.5, 42.9, 28.5, and 21.4.
3. Results
E-isomer: 1H NMR (300 MHz, CDCl3): ı/ppm = 7.17 (d, 2H,
J = 8.3 Hz), 6.97 (s, 2H), 6.20–6.16 (m, 6H), 0.77 (s, 18H), and 0.00
(s, 12H).
3.1. Cycloreversion of dicoumarin derivatives with hydrolyzed
lactone rings
Z-isomer: 1H NMR (300 MHz, CDCl3): ı/ppm = 6.88 (d, 2H,
J = 8.4 Hz), 6.53 (s, 2H), 6.18 (dd, 2H, J = 8.4 Hz, J = 2.0 Hz), 0.95 (s,
18H), and 0.18 (s, 12H)
Nucleophilic agents, e.g. methanol, water, mixtures thereof,
cause ester cleavage of the lactone rings of coumarin dimers
only. Coumarin monomers do not show any lactone ring open-
ing at the same conditions [16]. This reaction may be easily
monitored by UV/VIS spectroscopy. For our experiments we
used the tert-butyldimethylsilyl (TBS) protected 7-hydroxy-
dicoumarin. The TBS-protected 7-hydroxy-coumarin dimer (CD)
and the resulting dimethyl-3,4-bis(4-(tert-butyldimethylsilyloxy)-
2-hydroxyphenyl)cyclobutane-1,2-dicarboxylate (DBCD) show
significant spectral differences between 200 and 300 nm (Fig. 1).
The hydrolysis of the lactone ring in TBS-protected dicoumarin
was accomplished by MeOH and analyzed via NMR. The results
show opening of the lactone rings on both sides in the dimer, prob-
ably due to steric restrains of the cyclobutane ring.
GC–MS (EI, m/z): retention times of the isomers = 29.4 min,
35.4 min, fragmentation pattern: 470 (M+), 413, 385, 207, 178, and
73.
2.3. Synthesis of 7-methoxy-1,1-dimethyltetralone
7-Methoxy-2-tetralone (250 mg, 1.42 mmol) was dissolved in
10 mL THF under argon and stirred at −78 ◦C. Butyllithium (1.42 mL,
2.84 mmol) was added slowly followed by methyl iodide (0.18 mL,
2.84 mmol) after 30 min. The solution was stirred for another
30 min at −78 ◦C and then warmed to room temperature over night.
The solvent was removed in vacuo. The oily residue was dissolved
in chloroform and washed with brine four times. The organic phase
was dried with Na2SO4, filtered and the solvent was removed.
270 mg (1.32 mmol, 93%) of a deeply red colored oil were obtained.
1H NMR (300 MHz, CDCl3): ı/ppm = 7.09 (d, 1H, J = 8.3 Hz), 6.88
(d, 1H, J = 2.6 Hz), 6.73 (dd, 1H, J = 2.6 Hz, J = 8.3 Hz,), 3.81 (s, 3H),
The various isomers, i.e. head-to-head and head-to-tail, in com-
bination with either closed or opened lactone rings, cause that
upon dimer photocleavage a number of different products might be
obtained (Table 1). An intact lactone ring structure leads for head-
to-head as well as for head-to-tail isomers exclusively to symmetric

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J. Liese, N.A. Hampp / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 128–134
dimers the bonds along the C1–C4 and C2–C3 axes are more strained
than those along the C1–C2 and C3–C4 bonds. An asymmetric cleav-
age reduces this strain and therefore represents the main reaction
path [17].
Obviously the lactone ring cleavage is a problem in cases
where the coumarin dimers are used as photocleavable linker
molecules because up to four different products might be
obtained.
3.2. Photocleavage of TBS-protected dicoumarin by
single-photon-absorption
DBCD was irradiated at 266 nm and the resulting products were
analyzed via NMR and GC/MS. Two products were obtained which
were identified as dimethyl fumarate (DF) and (Z)-6,6ꢀ-(ethene-1,2-
diyl)bis(3-(tert-butyldimethylsilyloxy)phenol) (EBP). In daylight
the E-product is formed from the Z-product by photo-induced cis-
trans isomerization, which does not occur when kept in the dark.
The photocleavage of DBCD was found to be exclusively asymmetric
and the aromatic part of the linker molecule remains intact. In cases
where the aromatic positions are used to attach, e.g., a linker to a
polymer backbone or a drug, the material would loose its function
as a photocleavable linker.
Fig. 1. Absorption spectra of the TBS-protected coumarin dimer (CD, solid line) and
of dimethyl-3,4-bis(4-(tert-butyldimethylsilyloxy)-2-hydroxyphenyl)cyclobutane-
1,2-dicarboxylate (DBCD, dashed line) which is obtained after dissolving CD in
methanol.
dimer cleavage. With hydrolyzed lactone rings the cleavage pat-
tern differs between head-to-head and head-to-tail isomers. The
head-to-head isomer can cleave both, symmetric and asymmetric,
yielding different products [15]. In these lactone opened coumarin
Table 1
Photocleavage products of head-to-head and head-to-tail coumarin dimers in dependance of lactone ring opening.
Cleavage
Products
Head-to-head dimers
Symmetric
Asymmetric
Head-to-tail dimers
Symmetric
Symmetric
Modified from [17].

J. Liese, N.A. Hampp / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 128–134
131
Fig. 2. Synthetic route to 7-methoxy-1,1-dimethylnaphthalenone-dimer (MND).
Table 2
An option seems to be to use solely head-to-tail dicoumarin
to overcome this problem. The products of both cleavage direc-
tions would be identical. However, to the authors knowledge, no
investigations regarding the cycloreversion of lactone-hydrolyzed
head-to-tail dimers have been reported so far. Maybe the reason
is the very low yield of the head-to-tail coumarin dimerization
[18–20].
Comparison of the stabilities of coumarin (TBS-C) and MN monomer and TBS-CD
and MND dimer.
Stabilities
TBS-C
7-MN
TBS-CD
7-MND
Distilled water
Methanol
H2O:MeOH (1:1)
pH 4–10
+
+
+
+
+
+
+
+
−
−
−
−
+
+
+
+
Thus we decided to synthesize a new linker molecule basi-
cally using the well established reversible [2+2]-cycloaddition,
but including the required stability and having a single cleavage
product, and most of all, having a higher two-photon-absorption
coefficient. In order to achieve the higher stability against nucle-
ophilic attack the removal of the lactone ring is required.
After considering several possible alternatives, we have cho-
sen 7-methoxy-1,1-dimethyl-2(1H)-naphthalenone as a promising
candidate meeting the discussed requirements. Photo-induced
dimerization similar to coumarin has been reported earlier [20,21].
A synthetic route for 1,1-dimethyl-2(1H)-naphthalenone has been
developed by Wenkert [22], but did not include any functional
group in the molecule, which is required for using it as a photo-
cleavable linker.
3.5. Measurement of photophysical properties
Linker cleavage via two-photon-absorption processes requires
high photon densities, which require use of a pulse-laser system.
The quadratic power dependence of the TPA process provides pre-
cise spatial control since the cyclobutane cleavage is only induced
in the very confined focal area.
For the determination of the two-photon cross-section an
extinction coefficient of a characteristic band for monomer for-
mation and the single-photon-absorption quantum yield were
employed.
3.5.1. Extinction coefficient of the monomer
3.3. Synthesis
Cycloreversion of MND causes an absorption band with a maxi-
mum at ꢀ = 331 nm, which results from the conjugated ␲-system in
MN between the carbonyl group and the double bond formed by the
cycloreversion of the cyclobutane ring. The MND dimer possesses
no absorption in this region. Thus, it is possible to use the 331 nm
band for monitoring and quantization of the reaction (Fig. 3).
The mean value of the extinction coefficient of this band was
determined to be 10,250 L mol−1 cm−1, which lies in the same
region as coumarins do (11,347 L mol−1 cm−1) [25].
To synthesize
a molecule like 7-methoxy-1,1-dimethyl-
2(1H)naphthalenone (7-MN) the commercially available 7-
methoxy-2-tetralon is a suitable precursor. Having no oxygen
in the ring system and preventing the target molecule from
rearrangement into an aromatic system, the introduction of two
methyl-groups at the C1-atom is required (see Fig. 2).
This was carried out in a first step, using two equivalents of butyl
lithium, followed by two equivalents of methyl iodide. In the next
step the molecule was oxidized with 2-iodoxybenzoic acid (IBX) in
toluene/dimethylsulfoxide [23] to introduce a double bond in the
3,4-position. Finally, the molecule was dimerized according to the
procedure described for coumarins with benzophenone as a pho-
tosensibilisator [24]. Two photo-products were detected in a ratio
of 10:1 and the dominating product was isolated and identified as
the anti-head-to-head dimer.
3.5.2. Quantum yield of single-photon-absorption induced
cleavage of 7-methoxy-1,1-dimethylnaphthalenone-dimer
For single-photon-absorption (SPA), a dimer sample dissolved in
acetonitrile was irradiated at 266 nm. Absorption spectra of MND
as a function of irradiation time are shown in Fig. 4. The light energy
of the lamp was measured to be 1.172 × 1015 photons s−1
.
3.4. Hydrolytic stability of the monomer and the dimer
Several tests were run to verify the stability of 7-methoxy-
1,1-dimethylnaphthalenone
and
7-methoxy-1,1-
dimethylnaphthalenone dimer (MND) in aqueous solutions,
at different pH values and against various nucleophiles. Monomer
and dimer were dispersed in pure methanol, pure water, and a
50:50 mixture of water and methanol and stored at room tem-
perature for 17 days. The samples were monitored by analytical
HPLC. For the pH tests, three different buffer systems were used,
an acetate buffer for pH 4.0–5.5, NaH2PO4/NaOH for pH 5.5–8.0
and glycine/NaOH for pH 8.5–10.0. A series of measurements
from pH 4 to pH 10 in half pH steps at room temperature were
done and monitored by analytical HPLC for four days. In all
samples the MN and MND did not undergo any decomposition
(Table 2).
Fig. 3. UV/VIS spectra of 7-methoxy-1,1-dimethylnaphthalenone monomer (MN)
and dimer (MND).

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Fig. 6. Difference spectra at 331 nm of the TPA cleavage of 7-MND with Nd:YAG
laser pulses at 532 nm (59.3 mJ, 20 Hz, 3 ns).
Fig. 4. UV/Vis difference spectra during the course of SPA cleavage of MND.
The efficiency of the SPA-induced photocleavage is character-
dimers neff in the beam volume. The two-photon-absorption cross-
section is derived from Eq. (2), where neff represents the cleaved
cyclobutane rings, ФSPA the cleavage efficiency following single-
ized by the quantum yield ФSPA which is defined as the ratio of the
number of molecules nMol cleaved by the number of photons nPhot
[25]:
photon excitation, F the laser photon density, and cDimer the initial
0
number of cyclobutane rings in the solution.
nMol
˚
SPA
=
(1)
neff
nPho
ꢁTPA
=
(2)
ꢂSPAF2c0Dimer
The number of cleaved molecules is calculated from the initial
reaction rate v0 (t → 0) according to Lambert–Beers law.
The initial reaction rate was measured to be v0 = 1.454 ×
1014 molecules s−1 as shown in Fig. 5 and a SPA quantum yield
of ФSPA = 0.12, i.e. 12%, was calculated. For coumarin dimers this
value is three times higher, i.e. ФSPA = 0.36.
To verify the TPA characteristic of the cycloreversion, the rela-
tion between the initial reaction rate and the square of the photon
density F, which is dependant on the square of the irradiation
power, is considered. The following equation (3) gives a mathe-
matical representation [26,27]:
The absorption at 512 nm, which is of interest for excitation of
the TPA process, is for both compounds practically zero.
vM0 onomer = 2c0Dimer
˚
TPAꢁTPAF2
(3)
The initial reaction rate v0 increases twice as fast as the dimer
concentration c0Dimer decreases. ˚TPA is the quantum yield of the
two-photon induced photocleavage and ꢁTPA the TPA cross-section
of the dimer. In most cases reported in the literature the quantum
yield of the TPA has been assumed to be the same as for SPA [28].
From Eq. (3) follows that in a double logarithmic plot of the
initial reaction rate versus the light intensities a linear relation with
a slope of 2 should result. The measured rates v0 were plotted versus
the laser intensities P in Fig. 8 for MND.
The slope of the double logarithmic plot of the pulse inten-
sities against the initial reaction rate is found to be closely to 2
which indicates the two-photon nature of the process. The calcu-
lated TPA cross-section is ∼9.8 GM (10−50 cm4 s photon−1) for all
energies. This value is one order of magnitude higher than the TPA
cross-section of coumarin dimers, which is in the range of ∼1 GM
for CD [13]. To ensure that exclusively monomer is formed during
cycloreversion, we analyzed the solution for exposure to 59.3 mJ
pulses after each irradiation period via HPLC (Fig. 9). Only a rising
3.5.3. Two-photon-absorption cross-section of the of
7-methoxy-1,1-dimethylnaphthalenone-dimer
Irradiation of the MND in acetonitrile with intense 532 nm
pulses (20 Hz, 3 ns) causes TPA-induced cleavage of the dimer sim-
ilar to CD. The formation of monomer is monitored at ꢀ = 331 nm in
the UV spectrum (Fig. 6) and a constant increase in the absorption
indicates the cleavage of the linker.
The change in the absorption should be linearly proportional to
the applied irradiation energy. To verify this, the TPA cross-section
was measured at four different energies (28.9 mJ, 38.8 mJ, 50.0 mJ
and 59.3 mJ). The results are shown in Fig. 7.
The TPA cross-section ꢁTPA was calculated according to the
procedure reportedearlier[13]. Thechangein monomer concentra-
tions is plotted against the irradiation times for the different pulse
energies in order to obtain the initial reaction rates v0. The initial
reaction rates give the number n of cleaved cyclobutane rings in the
irradiated solution. This number together with the diameter of the
laser beam allows to calculate the number of the effectively cleaved
Fig. 7. TPA-induced cleavage of the 7-MND at different energies. Monomer forma-
tion measured as the increase of the absorption at 331 nm in dependence on the
incident laser power.
Fig. 5. Plot of the absorption at 331 nm versus time for the rising monomer concen-
trations.

J. Liese, N.A. Hampp / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 128–134
133
Table 3
Comparison of the photochemical properties of 7,7-(tert-butyldimethylsilyloxy)-
coumarin and 7-methoxy-1,1-dimethylnaphthalenone monomer and dimer [26].
Coumarin dimer (CD)
Methylnaphtalenone
dimer (MND)
Hydrolysis
In nucleophilic solvents
0.36
None
0.12
˚
SPA
ꢁTPA (532 nm)
0.87 GM
9.8 GM
Melting point
Monomer
Dimer
228–234 ◦C*
179–181 ◦C*
94.5 ◦
C
54 ◦C**
Spectral separation
Monomer
ꢀmax = 316 nm,
ꢀmax = 331 nm,
ε = 11,347 L mol−1 cm−1
ꢀmax = 266 nm,
ε = 10,250 L mol−1 cm−1
ꢀmax = 266 nm,
Dimer
Fig. 8. Double logarithmic plot of the applied laser intensities P versus the ini-
tial rates v0 of the linker cleavage. The TPA-induced nature of the photocleavage
corresponds with the slope of 2 when ln v is plotted versus ln P.
ε = 4729 L mol−1 cm−1
ε = 1,507 L mol−1 cm−1
The ꢁTPA is given for 532 nm excitation wavelength.
*
Taken from Krauch et al., Photo-C4-cyclodimerisation von Cumarin, Chem. Ber.
99 (1966) 625–633.
**
Anti head-to-tail.
Acknowledgment
This work was supported by grant FKZ 13N8978 from the Fed-
eral Ministry of Education and Research (BMBF).
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