Photocleavage of dimers of coumarin and 6-alkylcoumarins

Article abstract of DOI:10.1016/j.jphotochem.2009.11.018

The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1), the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV-vis and IR spectroscopy and HPLC upon direct 254nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is Φ_sp=0.1-0.3 in various solvents and the effects of structure and solvent polarity are small. In certain solvents some of the dimers produced CO₂ along with the monomers in the splitting reaction. Electron transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was observed in acetonitrile.

Full text of DOI:10.1016/j.jphotochem.2009.11.018

Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 219–223
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocleavage of dimers of coumarin and 6-alkylcoumarins
Thomas Wolffa, Helmut Görnerb,∗
a Technische Universität Dresden, Physikalische Chemie, D-01062 Dresden, Germany
b Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The cleavage of four coumarin dimers, the syn-head-to-tail (ht) dimer of parent coumarin (syn-ht-CC1),
the anti- and syn-hh dimers of 6-methylcoumarin (anti-hh-CC2 and syn-hh-CC2, respectively) and the
anti-hh dimer of 6-dodecylcoumarin (anti-hh-CC3), was studied by UV–vis and IR spectroscopy and HPLC
upon direct 254 nm irradiation as well as sensitized excitation. The quantum yield of dimer splitting is
Received 27 August 2009
Received in revised form
18 November 2009
Accepted 24 November 2009
Available online 2 December 2009
˚
sp = 0.1–0.3 in various solvents and the effects of structure and solvent polarity are small. In certain
solvents some of the dimers produced CO2 along with the monomers in the splitting reaction. Electron
transfer from dimers to the triplet state of sensitizers, such as benzophenone or 9,10-anthraquinone, was
observed in acetonitrile.
Keywords:
Coumarin
Dimer
© 2009 Elsevier B.V. All rights reserved.
Quantum yield
Photodimerization
Photocleavage
Sensitizer
CO2-formation
1. Introduction
have maxima at 410–430 nm and the influence of substitution on
the quantum yield of intersystem crossing ˚isc in a given solvent
The photodimerization properties of parent coumarin (C1) in
solution as well as in solid systems have been intensively inves-
tigated [1–9]. Four photodimers (CC), syn-head-to-head (syn-hh),
anti-hh, syn-head-to-tail (ht) and anti-ht, were identified for C1
and a series of 6-alkyl coumarins, e.g. 6-methylcoumarin (C2) and
6-dodecylcoumarin (C3) [10], see Scheme 1. Essentially syn-hh and
anti-hh dimers and virtually no anti-ht dimers are formed upon
direct excitation of C1 in solution. The anti-hh dimers of C1 and
C2 were found after benzophenone sensitization in both polar and
non-polar solvents and anti-hh dimers are favoured in solvents of
low polarity, but syn dimer formation is enhanced in polar sol-
vents and micellar solutions of cationic cetyltrimethylammonium
bromide and anionic sodium dodecyl sulfate [10].
The photophysical properties of coumarins are in favour of
an overall triplet mechanism rather than of separate singlet and
triplet pathways for the non-sensitized and sensitized photodimer-
izations of coumarins, respectively [11–16]. Fluorescence and
phosphorescence of C1 were detected at −196 ◦C, but virtually
no emission appears in fluid solution, the quantum yield of fluo-
rescence is ˚f < 10−3 [13]. The triplet state is observable at room
temperature [14–16], the T–T absorption spectra of C1, C2 or C3
is small at room temperature. ˚isc = 0.054 for C1 in water [15],
smaller for C1 and C2 in most other solvents and largest in 2,2,2-
trifluoroethanol (TFE) [13]. The dimerization quantum yield (˚dim
)
upon irradiation at 300–350 nm is very low in most cases, e.g. for
C1 (0.3 M) in dichloromethane and acetonitrile (for syn- and anti-
hh) ˚dim = 1 × 10−3 and 9 × 10−4, respectively [5,8]. On the other
hand, ˚dim is large, up to 0.8, for the BF3/OEt2 catalyzed reaction
[9]. Recently, several points in the mechanism of photodimeriza-
tion were addressed, e.g. the roles of the sensitizer, solvent and
molecular structure, and reasons for the differing triplet reactivities
were discussed [12]. The dimers are photocleaved into monomers
upon irradiation at 200–300 nm (Scheme 1). The dimer splitting of
coumarins is efficient [12,17–20] and for dimers of C1, which were
prepared by benzophenone-sensitized irradiation in 2-propanol, a
quantum yield of ˚sp = 0.2 has been reported upon irradiation at
266 nm [17].
Photoswitched storage and release of guest molecules in the
pore void of coumarin-modified MCM-41 nanoparticles has been
reported [21]. Reversible photocleavage and crosslinking was
achieved when 4-methylcoumarin was functionalized in polyester
[22]. With respect to the wavelength dependence of photodimer-
ization versus photocleavage, the features of coumarins appear to
be analogous to the thymine and other pyrimidine moieties. The
photosensitized splitting of thymine dimers has been achieved by
using a variety of sensitizers, mostly electron acceptors, such as
∗
Corresponding author.
E-mail address: goerner@mpi-muelheim.mpg.de (H. Görner).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.11.018

220
T. Wolff, H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 219–223
Scheme 1.
quinones or flavins [23–30]. Photoinduced DNA repair and split-
ting of pyrimidine model dimers can also be initiated by electron
transfer [23]. The photoinduced electron transfer of monomers has
been reviewed [31]. For cyanine dyes it has been shown that the
triplet state of both, monomer and dimer, can be quenched by elec-
tron donors and acceptors [32].
Here, we aim at a deeper insight with regard to the photo-
chemical dimer splitting of parent C1 and alkylcoumarins. For
these purposes, syn-ht-CC1, anti-hh-CC2, syn-hh-CC2 and anti-hh-
CC3 dimers were studied by both UV and IR spectroscopy in the
absence and presence of appropriate sensitizers using illumination
at 248/254 and 308 nm, respectively. As sensitizers benzophenone,
4-carboxybenzophenone, duroquinone (Me4BQ), chloranil (Cl4BQ)
and 9,10-anthraquinone (AQ) were used.
from dimer splitting are as low as 0.2 mM. Photoproduct analy-
ses after 366 nm irradiation of AQ or Cl4BQ, in acetonitrile–water
(1:1) in the presence of syn-ht-CC1 in using HPLC were per-
formed, but not further carried out since they revealed several
peaks related to quinone photochemistry and overlapping with
the coumarin monomer and dimer signals. The molar absorp-
tion coefficient of monomeric C1 in acetonitrile at 310 nm is
ε310 = 5.2 × 103 M−1 cm−1 [20]. The value of C2 in several sol-
vents at the maximum is typically ε275 = (0.8–1.2) × 104 M−1 cm−1
and for the dimers anti-hh-CC2 and syn-hh-CC2 in chloro-
form ε280 = 3.1 × 103 and ε280 = 2.5 × 103 M−1 cm−1, respectively
[10]. The IR spectra were recorded on a FTIR spectrometer
(Bruker IFS66); the concentrations were adjusted to Aexc = 0.1–2
in 0.05 cm CaF2 cells. The molar absorption coefficient of
anti-hh-CC2 in methylcyclohexane (MCH) was estimated as
ε1722 = 5 × 103 M−1 cm−1. For photolysis with UV–vis detection
two excimer lasers (ꢀexc = 248 and 308 nm, rise time 10–20 ns),
two transient digitizers (Tektronix 7912AD and 390AD) and an
Archimedes 440 computer for data handling were used as in pre-
vious work [11]. The measurements refer to 25 ◦C.
2. Experimental details
The coumarins were from previous work [12] and the sen-
sitizers as commercially available. The solvents (Merck) were
checked for impurities. For irradiation at 254 and 280/366 nm
a Hg lamp and a 1000 W Hg–Xe lamp with a monochromator
was used, respectively. The UV–vis spectra were recorded on a
diode array (HP 8453). They are presented for 0.5 mm pathlength,
otherwise performed in 1 cm cells. The quantum yield ˚sp was
measured using the absorption at the maximum versus irradia-
tion time. As reference uridine in water was used, ˚d = 0.002 [33].
For HPLC analyses a 125 × 4.6 mm Inertsil ODS-3 5 ␮m column
was used with MeOH–water 1:2 or 2:1 as eluents. Redimeriza-
tion can be excluded since the monomer concentrations derived
3. Results and discussion
3.1. Direct photocleavage of dimers
The absorption spectrum of syn-ht-CC1 in MCH (Fig. 1b), carbon
tetrachloride or more polar solvents has a maximum at 275 nm
and no band above 300 nm. Continuous UV irradiation results in
an absorption increase at 250–350 nm and an isosbestic point at
248 nm. The spectral changes with stronger absorbance at a major
maximum around 275 nm and minor maximum at 312 nm are due
to monomer formation. Corresponding spectra were obtained in
solvents of low and large polarity for the other coumarin dimers, the
changes are shown for anti-hh-CC2 in acetonitrile and anti-hh-CC3
in MCH, Figs. 2b and 3b, respectively.
The IR spectra of monomeric C1 and dimeric syn-ht-CC1 in
MCH exhibit major peaks at ꢁ˜m = 1730 cm−1 and ꢁ˜d = 1757 cm−1
,
respectively (Fig. 1a). Anti-hh-C2 in air-saturated MCH shows a
major dimer peak at 1780 cm−1, an isosbestic point at 1769 cm−1
and the monomer band centered at ꢁ˜m = 1722 cm−1 (not shown).
Additional monomer peaks appear at 1575, 1497, 1170, 1125 and
1044 cm−1. The main absorption peak of anti-hh-CC2 in carbon
tetrachloride is similar and shifted to ꢁ˜d = 1768 cm−1 in more
polar solvents, acetonitrile (Fig. 2a). For anti-hh-CC3 in MCH the
peak at ꢁ˜d = 1779 cm−1 converts to the monomer peak with ꢁ˜m
=
1722 cm−1 (Fig. 3a). Corresponding spectra were obtained for other
cases (Table 1). The peak to peak ratio 2 × εm/εd of monomers
at ꢁ˜m (after complete cleavage) to dimers at ꢁ˜d of 0.4–0.9 was
obtained. The photoproduct of anti-hh-CC2 in carbon tetrachlo-
ride and dichloromethane, however, is not fully in line with the
Fig. 1. (a) IR and (b) corresponding UV spectra of syn-ht-CC1 in air-saturated MCH
prior to (full) and after 2 (dashed) and 10 min (dotted) irradiation at 254 nm.

T. Wolff, H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 219–223
221
Table 1
Monomer and dimer IR maxima of 1–3 and qantum yield of splittinga.
Dimer
Solvent
MCH
˚
ꢁ˜d (cm−1
)
ꢁ˜m (cm−1
)
sp
syn-ht-CC1
1768
1757
1757
1760
1722
1730
1732
1735
b
CCl4
0.15
Dichloromethane
Acetonitrile
anti-hh-CC2
Cyclohexane
CCl4
Chloroform
Dichloromethane
Acetonitrile
0.28
1780c
1778
1766
1766
1768
1722c
1746
1726
1726
1730
0.2
0.2
0.2
anti-hh-CC3
MCH
CCl4
Dichloromethane
1779
1776
1766
1722
1741
1727
0.18
a
In air-saturated solution, ꢀirr = 254 nm.
Using ꢀirr = 290 nm.
Using MCH.
b
c
Fig. 2. (a) IR and (b) UV spectra of anti-hh-CC2 in air-saturated acetonitrile prior to
(full) and after 2 (dashed) and 10 min (dotted) irradiation at 254 nm.
anism of direct cleavage of the dimer could occur via a biradical
(Scheme 1) or, less likely, via radical ions.
monomers in other solvents, indicating a side reaction with the
solvent.
3.3. Side reaction
Irradiation at 254 nm generally gives rise to a decrease in dimer
absorption and formation of new peaks of the monomers (and/or
other minor photoproducs). For dimeric anti-ht-CC1 or anti-hh-
CC2 in MCH or CCl4 (and to a smaller extent for anti-ht-CC1
in acetonitrile) also a photoinduced peak at 2340 cm−1 appears.
It is formed in proportion to irradiation time in a similar man-
ner to the monomer peaks and is assigned to CO2 formation.
The estimated yield is smaller than 15%, taking an absorption
coefficient of ε2240 = 1.5 × 104 M−1 cm−1 for CO2 with respect to
ε1722 = 5 × 103 M−1 cm−1 for C1. For anti-ht-CC1 in acetonitrile
under otherwise the same conditions the CO2 peak is 20 times
smaller than in MCH. The photodecarboxylation of coumarin
dimers concomitant to photocleavage is new and without prece-
dence in the literature. Speculative reactions of hh and ht dimers
leading to CO2 are proposed in Scheme 2, but these need further
investigations for verification. It should be mentioned that a new
type of photodimerization reaction with release of CO2 has been
reported for certain coumarin derivatives, which, however, are
carboxy-substituted [34], in contrast to parent C1 or C2, where CO2
is only part of the skeleton. This cannot support photodecarboxy-
lation of coumarin dimers as a general phenomenon.
3.2. Quantum yield
Examples of the photoconversion as a function of irradiation
time are shown in Fig. 4 using ꢀirr = 254 nm. The quantum yield
is ˚sp = 0.15–0.28 (Table 1). A value of ˚sp = 0.21 in acetonitrile
has been reported upon irradiation at 266 nm of a coumarin
dimer mixture [20]. The initial absorption at 270 nm is much
weaker than that of the two monomers, as expressed by the ratio
εd(UV)/2 × εm(UV) = 0.2–0.3. The identity of the coumarin dimers
has been documented previously [10]. The postulated excited state
as intermediate in direct excitation is a non-fluorescent singlet
state since an observation of the triplet state in the ns–␮s range
failed. Note that the triplet state of monomers has been detected
[11,12,14]. Fast dimer splitting bypassing the lowest triplet state
is supported by the observation that the change in absorbance at
280–320 nm, measured by laser flash photolysis upon direct exci-
tation at 248 nm, is completed within the pulse width of 10 ns
(not shown). The splitting probably takes place in picoseconds, i.e.
instantaneously and without detectable intermediate. The mech-
Fig. 4. Plots of (a) absorption at 320 nm versus time of irradiation at 254 nm for
splitting of syn-ht-CC1 and (b) monomer (full) and dimer (open) concentrations
using HPLC analyses in argon-saturated cyclohexane (circles), carbon tetrachloride
(triangles) and acetonitrile (squares), pathlength 1 cm.
Fig. 3. (a) IR and (b) UV spectra of anti-hh-3 in air-saturated MCH prior to (full) and
after 2 (dashed) and 10 min (dotted) irradiation at 254 nm.

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T. Wolff, H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 219–223
Scheme 2.
3∗S + CC → 3∗S· · ·CC → S + CC or 2C
3∗S + CC → S• + CC•
(3)
(4)
(5)
(6)
3.4. Photoreactions of coumarin monomers
−
+
The photodimerization properties of C1 and 6-alkylcoumarins
were recently discussed [11]. Variation of the concentration
changes the triplet lifetime (ꢂT) and reveals a quenching reaction
of the triplet state of the monomer (3*C) by the ground state (1)
in competition to intrinsic decay. The postulated complex 3*C· · ·C
which is not observable under our conditions is the precursor of
dimers and ˛ is a fraction between 0 and 1.
+
+
CC• → C + C•
S• + C•+ → S + C
−
When benzophenone in dry acetonitrile was applied (Fig. 5a),
S•− could not be detected probably due to too fast electron back
transfer in the solvent cage. This is in contrast to the alkaline
medium (Fig. 5b). An example of inefficient quenching is naph-
thalene, where the triplet is kinetically observed, but no quenching
reactivity by anti-hh-CC2 was found (not shown).
Quinones are also appropriate electron acceptors. The T–T
absorption spectrum of AQ has ꢀTT = 360 nm and the triplet
lifetime in acetonitrile is ca. 10 ␮s. In the presence of anti-
hh-CC2 ꢂT becomes shorter (Fig. 6a) and the rate constant is
kq = 3 × 109 M−1 s−1. With AQ in acetonitrile–water (1:1) a transient
with maximum at 500 nm was formed (Fig. 6b) which is assigned
to the radical anion (AQ•−) [36]. The lack of AQ•− formation in dry
acetonitrile (Fig. 6a) is suggested to be due to electron back trans-
3∗C + C → 3∗C· · ·C → ˛CC + (1 − ˛)2 × C or other products
(1)
The low quantum yield of dimerization of C1 or C2 is
partly due to the small ˚isc value [5,8,11]. ˚isc = 0.03–0.06 in
dichloromethane, smaller in benzene, acetonitrile or methanol and
larger in TFE and water [11]. For reaction (1) to initiate non-
sensitized dimerization, a concentration of 1 mM is necessary for
50% triplet quenching in dichloromethane or acetonitrile, but in
water 0.03 mM coumarin is sufficient to populate the 3*C state
and to successfully compete with triplet decay without quenching.
Changes in the product pattern could be due to larger preorienta-
tion of complex-hh versus complex-ht in competition to intrinsic
decay. We propose three products pathways, taking that for anti-ht
as absent. In these three cases, the final dimer formation competes
with non-reactive decay into two coumarin molecules (Scheme 1).
Calculations support the triplet route into the anti-hh dimer [35].
3.5. Oxidative photosensitized dimer cleavage
On addition of anti-hh-CC2 the triplet lifetime ꢂT of ben-
zophenone in acetonitrile at the maximum ꢀTT = 520 nm becomes
shorter, the T–T absorption spectrum is not markedly changed,
but no acceptor triplet was found (Fig. 5a). The rate constant
for triplet quenching, obtained from the slope of the linear
dependence of 1/ꢂT versus the anti-hh-CC2 concentration, is
kq = 0.8 × 109 M−1 s−1. In the presence of water at pH 12 the triplet
lifetime is also shorter, however, the radical anion of the sensi-
tizer (S•−) with a major band at 320 nm and a weaker band at
600 nm was detected as long-lived species (triangles in Fig. 5b).
The results for 4-carboxybenzophenone are kq = 1.1 × 109 M−1 s−1
in acetonitrile–water (1:1) and the spectra in alkaline solution
are similar (Fig. 5c). The photosensitized cleavage of coumarin
dimers could occur via energy transfer from 3*S (3). With benzophe-
none and 4-carboxybenzophenone, however, this can be excluded.
Instead, electron transfer (4) takes place, i.e. formation of S•− and
the cationic dimer radical, CC•+. The likely possibility is instanta-
neous cleavage of the dimeric radical cation (5) and back electron
transfer via (6) see Scheme 3.
Fig. 5. Transient absorption spectra (under argon) in the presence of anti-hh-CC2
(0.3 mM) of benzophenone (a) in acetonitrile and (b) in acetonitrile–water (1:1) at
pH 12 (b) and (c) of 4-carboxybenzophenone in aqueous solution at pH 12 at 20 ns
(ꢀ), 1 ␮s (ꢁ) and 10 ␮s (ꢀ) after the 308 nm pulse; insets: kinetics as indicated.
S + hꢁ → 1∗S → 3∗S
(2)
Scheme 3.

T. Wolff, H. Görner / Journal of Photochemistry and Photobiology A: Chemistry 209 (2010) 219–223
223
the triplet state of an acceptor, was achieved with ketones, such as
benzophenone or quinones, e.g. chloranil or 9,10-anthraquinone.
CO2 formation upon direct irradiation was observed along with the
expected monomers for anti-ht-CC1 or anti-hh-CC2 in MCH or CCl4
and for anti-ht-CC1 in acetonitrile.
Acknowledgments
We thank Professor Wolfgang Lubitz for his support and Mrs.
Corinna Kesseler, Mr. Leslie J. Currell, Horst Selbach and Christian
Lemsch for technical assistance.
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4. Conclusions
The photochemical cleavage mechanism of coumarin dimers
was studied. The direct cleavage of the dimers is characterized
by a relatively high quantum yield ˚sp = 0.2 and a fast reaction
upon direct irradiation at 248 and 254 nm. The photocleavage is
suggested to occur via a non-fluorescent short-lived singlet state.
The oxidative cleavage, due to electron transfer from the dimer to
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