On the mechanism of intramolecular sensitization of photocleavage of the 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) protecting group

Article abstract of DOI:10.1021/ja072355p

A spectroscopic study of a variety of covalently linked thioxanthone(TX)-linker-2-(2-nitrophenyl)-propoxycarbonyl(NPPOC)-substrate conjugates is presented. Herein, the TX chromophore functions as an intramolecular sensitizer to the NPPOC moiety, a photolabile protecting group used in photolithographic DNA chip synthesis. The rate of electronic energy transfer between TX and NPPOC was quantified by means of stationary fluorescence as well as nanosecond and femtosecond time-resolved laser spectroscopy. A dual mechanism of triplet-triplet energy transfer has been observed comprising a slower mechanism involving the T₁(ππ*) state of TX with linker-length-dependent time constants longer than 20 ns and a fast mechanism with linker-length-dependent time constants shorter than 3 ns. Evidence is provided that the latter mechanism is due to energy transfer from the T ₂(nπ*) state which is in fast equilibrium with the fluorescent S₁(ππ*) state. In the case of direct linkage between the aromatic rings of TX and NPPOC, the spectroscopic properties are indicative of one united chromophore which, however, still shows the typical NPPOC cleavage reaction triggered by intramolecular hydrogen atom transfer to the nitro group.

Full text of DOI:10.1021/ja072355p

Published on Web 09/18/2007
On the Mechanism of Intramolecular Sensitization of
Photocleavage of the 2-(2-Nitrophenyl)propoxycarbonyl
(NPPOC) Protecting Group
Dominik Wo¨ll,†,§ Stefan Laimgruber,‡ Marina Galetskaya,† Julia Smirnova,†
Wolfgang Pfleiderer,† Bjo¨rn Heinz,‡ Peter Gilch,‡ and Ulrich E. Steiner*,†
Contribution from the Fachbereich Chemie, UniVersita¨t Konstanz, 78465 Konstanz, Germany,
and Department fu¨r Physik, Ludwig-Maximilians-UniVersita¨t, Oettingenstr. 67,
80538 Mu¨nchen, Germany
Received April 4, 2007; E-mail: ulrich.steiner@uni-konstanz.de
Abstract: A spectroscopic study of a variety of covalently linked thioxanthone(TX)-linker-2-(2-nitrophenyl)-
propoxycarbonyl(NPPOC)-substrate conjugates is presented. Herein, the TX chromophore functions as
an intramolecular sensitizer to the NPPOC moiety, a photolabile protecting group used in photolithographic
DNA chip synthesis. The rate of electronic energy transfer between TX and NPPOC was quantified by
means of stationary fluorescence as well as nanosecond and femtosecond time-resolved laser spectroscopy.
A dual mechanism of triplet-triplet energy transfer has been observed comprising a slower mechanism
involving the T1(ππ*) state of TX with linker-length-dependent time constants longer than 20 ns and a fast
mechanism with linker-length-dependent time constants shorter than 3 ns. Evidence is provided that the
latter mechanism is due to energy transfer from the T2(nπ*) state which is in fast equilibrium with the
fluorescent S1(ππ*) state. In the case of direct linkage between the aromatic rings of TX and NPPOC, the
spectroscopic properties are indicative of one united chromophore which, however, still shows the typical
NPPOC cleavage reaction triggered by intramolecular hydrogen atom transfer to the nitro group.
1. Introduction
Several of these compounds showed excellent performance
as ppg’s with enhancements of the light sensitivity (expressed
as the value of the product ꢀ × φ of molar absorption coefficient
ꢀ and photochemical quantum yield φ) by up to a factor of 20
at 366 nm and of 140 at 405 nm. Recently, the concept of using
intramolecular antennae to enhance the efficiency of photore-
actions was also applied by Corrie and co-workers,9,10 who
studied triplet-sensitizer/1-acyl-7-nitroindoline conjugates and
by Dietliker and co-workers11 who reported on covalently linked
triplet-sensitizer/photoinitiator conjugates.
Intramolecular energy transfer in general represents a research
field of high current interest. Numerous examples and applica-
tions, some of which are reviewed by Speiser,12 have been
published. A major line of such research is driven by the
motivation to develop artificial photosynthetic systems.13,14
Otherareasconcernphotonicdevicesformolecularelectronics15-18
Recently it was shown that the light-sensitivity of photolabile
protecting groups (ppg’s) of the 2-(2-nitrophenyl)propoxycar-
bonyl (NPPOC) type1-3 used in photolithographic DNA chip
synthesis4 can be considerably enhanced by spectral sensitization
with triplet sensitizers.5 In the case of NPPOC as a ppg,
thioxanthone turned out to be a particularly good choice6
because it showed the least extent of side reactions. In a further
attempt to enhance the efficiency of the sensitizer, we synthe-
sized a series of ppg’s wherein the NPPOC moiety is covalently
linked to thioxanthone which thus adopts the function of an
intramolecular antenna (cf. Scheme 1).7,8
† University of Konstanz.
‡ University of Munich.
§ Present address: Katholieke Universiteit Leuven, Celestijnenlaan 200F,
3001 Heverlee, Belgium.
(1) Hasan, A.; Stengele, K.-P.; Giegrich, H.; Cornwell, P.; Isham, K. R.;
Sachleben, R. A.; Pfleiderer, W.; Foote, R. S. Tetrahedron 1997, 53, 4247-
4264.
(2) Giegrich, H.; Eisele-Bu¨hler, S.; Hermann, C.; Kvasyuk, E.; Charubala, R.;
Pfleiderer, W. Nucleosides Nucleotides 1998, 17, 1987-1996.
(3) Bu¨hler, S.; Lagoja, I.; Giegrich, H.; Stengele, K.-P.; Pfleiderer, W. HelV.
Chim. Acta 2004, 87, 620-659.
(4) An overview of photoremovable protecting groups used in DNA chip
synthesis has been given by Pirrung, M. C.; Rana, V. S., Photoremovable
Protecting Groups in DNA Synthesis and Microarray Fabrication. In
Dynamic Studies in Biology: Phototriggers, Photoswitches, and Caged
Compounds; Givens, R. S., Goeldner, M., Eds.; J. Wiley & Sons: New
York, 2005; p 341.
(7) Steiner, U.; Wo¨ll, D. Efficient photolithographic synthesis of DNA-chips
by photosensitization. DE 10315772(A1), WO 2004089529, 2004 [Chem.
Abstr. 2004, 141, 366034].
(8) Wo¨ll, D.; Smirnova, J.; Pfleiderer, W.; Steiner, U. E. Angew. Chem., Int.
Ed. 2006, 45, 2975-2978.
(9) Papageorgiou, G.; Lukeman, M.; Wan, P.; Corrie, J. E. T. Photochem.
Photobiol. Sci. 2004, 3, 366-373.
(10) Papageorgiou, G.; Ogden, D.; Corrie, J. E. T. J. Org. Chem. 2004, 69,
7228-7233.
(11) Dietliker, K.; Broillet, S.; Hellrung, B.; Rzadek, P.; Rist, G.; Wirz, J.;
Neshchadin, D.; Gescheidt, G. HelV. Chim. Acta 2006, 89, 2211-2225.
(12) Speiser, S. Chem. ReV. 1996, 96, 1953-1976.
(5) Wo¨ll, D.; Walbert, S.; Stengele, K.-P.; Albert, T. J.; Richmond, T.; Norton,
J.; Singer, M.; Green, R. D.; Pfleiderer, W.; Steiner, U. E. HelV. Chim.
Acta 2004, 87, 28-45.
(6) Other sensitizers investigated, such as acridone or xanthone, showed
considerable photochemical side reactions with the solvent.
(13) Gust, D.; Moore, T. A. Science 1989, 244, 35-41.
(14) Sykora, M.; Maxwell, K. A.; DeSimone, J. M.; Meyer, T. J. Proc. Natl.
Acad. Sci. U.S.A. 2000, 97, 7687-7691.
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Chem. 1992, 43, 719-754.
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J. AM. CHEM. SOC. 2007, 129, 12148-12158
10.1021/ja072355p CCC: $37.00 © 2007 American Chemical Society

Intramolecular Sensitization
A R T I C L E S
Figure 1. NPPOC conjugates investigated in this work.
Scheme 1. Intramolecular Sensitized Photocleavage of a Protecting Group of the NPPOC Typea
a Reproduced, with the publisher’s permission, from ref 8.
or bioanalytical applications like fluorescence resonance energy
transfer (FRET).19-22 While in most of these reports the effects
of intramolecular energy transfer are reversible, the number of
case studies with irreversible chemical change as a consequence
of the energy transfer are less numerous. The work of Morrison
and colleagues,23 who showed that Z/E photoisomerizations,24-26
Norrish type II cleavages,27 and photoreductions28 can be
brought about by intramolecular energy transfer in nonconju-
gated aryl olefins and keto olefins, has figured prominently in
this respect.
In this paper we present a detailed account of the mechanism
of intramolecular energy transfer in a series of thioxanthone/
NPPOC conjugates as revealed by stationary and time-resolved
spectroscopic investigations. Our study not only contributes to
the general aspects of intramolecular energy transfer but also
sheds new light on the photophysics of the established sensitizer
thioxanthone.
2. Results
The compounds investigated in this work are shown in Figure
1. Normally, thymidine, protected at the 5′-OH position and
connected to the 2-(2-nitrophenyl)propyl moiety through a
carbonate linkage, was used as a test substrate. The sensitizer
antenna was connected to the 2-nitrophenyl chromophore at
various positions and through various types of linkers. The letter/
number code for these compounds is of the general structure
TmLn-OH for the free alcohols and TmLn-O(CO)Thy for the
protected thymidines. The first letter codes for the antenna
moiety (T for thioxanthone), and the number m gives the
position at the NPPOC moiety to which the linker is attached
(1-6 aromatic positions, 7 benzylic position). The letter L
characterizes the type of chain linking NPPOC and the sensitizer
(S, saturated carbon chain; D, single bond carbon chain including
(16) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804.
(17) Heath, J. R.; Ratner, M. A. Physics Today 2003, 43-49.
(18) James, D. K.; Tour, J. M. Top. Curr. Chem. 2005, 257, 33-62.
(19) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press:
New York, 1999.
(20) Valeur, B. Molecular Fluorescence: Principles and Applications; Wiley-
VCH: Weinheim, 2002.
(21) Andrews, D. L.; Demidov, A. A. Resonance Energy Transfer; John Wiley
& Sons: Chichester, U.K., 1999.
(22) Wieb Van Der Mer, B.; Coker, G.; Simon Chen, S.-Y. Resonance Energy
Transfer: Theory and Data; VCH: New York, 1994.
(23) Morrison, H. ReV. Chem. Intermed. 1987, 8, 125-45.
(24) Morrison, H. Tetrahedron Lett. 1964, 3635-3656.
(25) Morrison, H.; Peiffer, R. J. Am. Chem. Soc. 1968, 90, 3428-32.
(26) Agyin, J. K.; Timberlake, L. D.; Morrison, H. J. Am. Chem. Soc. 1997,
119, 7945-7953.
(27) Morrison, H.; Brainard, R. J. Am. Chem. Soc. 1971, 93, 2685-92.
(28) Wu, Z. Z.; Morrison, H. Photochem. Photobiol. 1989, 50, 525-30.
9
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A R T I C L E S
Wo¨ll et al.
Figure 3. Fluorescence spectra of investigated NPPOC conjugates excited
at 355 nm. Line labels are ordered in parallel to the fluorescence intensities
at 440 nm. The concentrations were adjusted such as to yield an absorbance
of 0.05 at 355 nm for each sample.
Figure 2. Absorption spectra of the thioxanthone/NPPOC conjugates. For
better recognition, the labels to the various spectra are ordered in parallel
to the absorbance values at 320 nm.
one double bond; T, single bond carbon chain including one
triple bond; E, ester bridge). The number n is the length of the
chain connecting the aromatic rings of NPPOC and thioxan-
thone.
The synthesis of most of the protected thymidines is described
in ref 29. In order to study the length effect of the linker more
systematically, the protected thymidine T7S2-O(CO)Thy and
the alcohols T7Sn-OH (n ) 5, 6, 9) have also been synthesized.
Their syntheses are described in the Supporting Information.
All spectroscopic experiments were carried out in MeOH as
solvent.
2.1. Absorption and Fluorescence Spectra. Figure 2 shows
the absorption spectra of the new protecting groups in com-
parison to the unmodified NPPOC chromophore. Since thy-
midines have negligible absorption above 300 nm, the thiox-
anthone chromophore dominates the picture of the conjugates
in this region of wavelengths. If thioxanthone is linked to
NPPOC through an aliphatic chain with more than two carbon
atoms, the spectrum is essentially identical to a superposition
of the spectra of 2-alkyl-thioxanthone and NPPOC (T7S3-
O(CO)Thy to T7S9-OH). In the case of a very short aliphatic
linker (T7S2-O(CO)Thy), small deviations from additivity are
noticeable, indicating a weak interaction of the chromophores
in the ground state.30
A weak spectral change is also observed for the ester-group-
linked T4E2-O(CO)Thy, which is probably due to the effect of
the benzoyloxy substituent at ring position 2 in thioxanthone.
More pronounced spectral effects are associated with a double
or triple bond in the linker in conjugation with the thioxanthone
chromophore (T7D4-O(CO)Thy and T7T4-O(CO)Thy). These
compounds exhibit a bathochromic shift of the first thioxanthone
absorption band and the appearance of a new strong UV
absorption band around 340 nm. Both of these effects can be
attributed to the extension of the π-system of thioxanthone. Here
a double bond has a stronger effect than a triple bond. The
largest spectral change is observed for T5S0-O(CO)Thy where
the aromatic systems of thioxanthone and NPPOC are directly
linked. Obviously, the interaction is very strong, so that the
Table 1. Fluorescence Quantum Yields φf and Wavelengths λmax
of Fluorescence Maxima
compound
φf
λ
max (nm)
ETa
0.22
0.13
0.12
0.10
0.09
0.08
0.06
0.05
0.04
0.04
0.01
440
438
440
440
440
440
444
438
436
458
406
T4E2-O(CO)Thy
T7S9-OH
T7S4-O(CO)Thy
T7S6-OH
T7S5-OH
T7T4-O(CO)Thy
T7S3-O(CO)Thy
T7S2-O(CO)Thy
T7D4-O(CO)Thy
T5S0-O(CO)Thy
a 2-Ethylthioxanthone.
linked moieties essentially form a joint chromophore with a
strong absorption maximum at 325 nm and a pronounced
shoulder around 380 nm, the position of the absorption
maximum of the parent thioxanthone chromophore. It should
be noted that relative to the unmodified NPPOC group, the
absorbances of the new protecting groups are increased by a
factor of 12 to 33 at 366 nm and by a factor of 100 to 190 at
405 nm, to mention two convenient mercury lines. No significant
differences between the free alcohols of the protecting groups
and the protected thymidines were observed above 300 nm.
The fluorescence spectra are shown in Figure 3. Fluorescence
quantum yields and wavelengths of fluorescence maxima are
listed in Table 1. The spectral shapes and positions are in
agreement with Kasha’s mirror image rule.31 The fluorescence
maxima are centered at about 440 nm. In accordance with the
bathochromic shifts of the absorption spectra of compounds
T7D4-O(CO)Thy and T7T4-O(CO)Thy, there is also a corre-
sponding shift of their fluorescence spectra. The fluorescence
maximum of the unified chromophore of T5S0-O(CO)Thy
appears at the shortest wavelength. In all cases, the fluorescence
quantum yields are lower than that in the reference compound
2-ethylthioxanthone. Within the series T7Sn of compounds with
an aliphatic linker of variable length between the two chro-
mophores, there is a systematic decrease of the fluorescence
quantum yield with decreasing length n of the linker. In this
(29) Smirnova, J.; Wo¨ll, D.; Pfleiderer, W.; Steiner, U. E. HelV. Chim. Acta
2005, 88, 891-904.
(30) A similar observation with linked benzyol/styryl energy donor acceptor
systems was made by Cowan, D. O.; Baum, A. A. J. Am. Chem. Soc. 1971,
93, 1153-1162.
(31) Kasha, M. Discuss. Faraday Soc. 1950, 14-19.
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Intramolecular Sensitization
A R T I C L E S
Table 2. Triplet Lifetimes τ and Rate Constants kq,O of
Quenching of Thioxanthone Triplet in the Investigate2d
Thioxanthone/NPPOC Conjugates by Molecular Oxygena
τ
/ ns
N2-saturated
MeOH
1
aerated MeOH
kq,O / 109 M-1 s-
2
compound
R
)
H
R
)
(CO)Thy
R
)
H
R
)
(CO)Thy
R
)
H
R
)
(CO)Thy
T7S2-OR
T7S3-OR
T7S4-OR
T7S5-OR
T7S6-OR
T7S9-OR
T7T4-OR
T5S0-OR
19
38
109
110
66
242
914
---
23
36
102
19
32
64
72
58
195
185
---
23
30
68
---
---
2.65
2.33
2.35
3.07
2.28
1.00
0.47
1.92
---
1020
134
3640
245
117
217
1.48
0.52
2.06
T4E2-OR 3070
215
2.06
a A concentration of 2.1 × 10-3 M for oxygen in air-saturated methanol
was used (cf. p. 291 in ref 35).
Figure 4. Transient absorption spectra of thioxanthone in deaerated MeOH
after excitation with a 355 nm nanosecond laser pulse.
accurately determined because this band is superimposed by
the bleaching of the ground state absorption band.32 Immediately
after the laser flash, the shapes of the transient spectra of
thioxanthone and T7S4-O(CO)Thy are practically identical,
although the magnitude of the latter is only about half of the
former. A second characteristic difference is the decay time of
the transients. At 600 nm, the decay kinetics of the free
thioxanthone triplet had to be fitted with a mixed first- and
second-order kinetic model taking triplet-triplet annihilation
(second-order rate constant kTT) into account. This yielded a
first-order rate constant of 3.8 × 104 s-1 and a second-order
rate constant of 4 × 109 M-1 s-1. The decay of the thioxanthone/
NPPOC conjugate was single exponential with a decay constant
of 9.8 × 107 s-1. This indicates a very efficient triplet energy
transfer in the conjugate. For T7S4-O(CO)Thy, the transient
absorption below 450 nm undergoes a weak bathochromic shift,
the negative absorption due to the ground state bleaching is
compensated by a stronger absorption with a maximum at about
410 nm and a decay constant of 1.4 × 104 s-1. This transient
can be assigned to the aci-nitro form of NPPOC,33 indicating
that energy transfer from the thioxanthone triplet to NPPOC is
followed by the well-known intramolecular hydrogen atom
transfer from the benzylic position to the nitro group which is
characteristic for 2-nitrobenzyl compounds.34 Corresponding
transient absorption spectra and decay curves for the other
compounds in this class can be found in the Supporting
Information. As far as lifetimes (cf. Table 2) and initial
intensities of the transient spectra of the triplet moieties are
concerned, there is little difference between the protected
thymidines T7Sn-O(CO)Thy and the free alcohols T7Sn-OH.
However, as is borne out by Figure 6, both of these parameters
are significantly affected by the length n of the linker. In general,
the initial transient absorption, i.e., the yield of detected
thioxanthone triplet, decreases as the chain length of the linker
is decreased. The same applies to the triplet lifetime (for a
compilation of the triplet lifetimes, cf. Table 2). Exceptions to
the general trend are compound T7S4-O(CO)Thy with a
relatively high triplet yield and compound T7S6-O(CO)Thy with
a relatively short triplet decay time.
Figure 5. Transient absorption spectra of T7S4-O(CO)Thy in deaerated
MeOH after excitation with a 355 nm nanosecond laser pulse.
respect it does not seem to matter whether free alcohols T7Sn-
OH or protected thymidines T7Sn-O(CO)Thy are considered.
The only irregular case in the series is T7S4-O(CO)Thy which
ranges between T7S9-OH and T7S6-OH. Introducing a double
or triple bond in conjugation with the thioxanthone chromophore
in a chain of four carbon atoms (T7D4-O(CO)Thy and T7T4-
O(CO)Thy) further decreases the fluorescence quantum yield,
whereby the effect of the double bond is more pronounced. By
far the lowest fluorescence quantum yield is found for the
directly linked conjugate T5S0-O(CO)Thy, a fact which,
together with the relatively large hypsochromic shift, emphasizes
the special role of the unified chromophore.
2.2. Nanosecond Laser Flash Photolysis. The new com-
pounds were systematically examined by nanosecond laser flash
spectroscopy measuring their transient absorption spectra at
variable delay times with a CCD camera and recording kinetic
decay curves at selected wavelengths.
2.2.1. Compounds with an Oligomethylene Chain Linker
(T7Sn-OR). For comparison, the transient spectra of the parent
thioxanthone and compound T7S4-O(CO)Thy as a typical
representative of this class of conjugates are shown in Figures
4 and 5. The triplet-triplet absorption spectrum of thioxanthone
is characterized by a strong band at 600 nm and a weaker band
somewhat below 450 nm, the maximum of which cannot be
(32) Amirzadeh, G.; Schnabel, W. Makromol. Chem. 1981, 182, 2821-2835.
(33) Walbert, S.; Pfleiderer, W.; Steiner, U. E. HelV. Chim. Acta 2001, 84, 1601-
1611.
(34) Pelliccioli, A. P.; Wirz, J. Photochem. Photobiol. Sci. 2002, 1, 441-458.
9
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A R T I C L E S
Wo¨ll et al.
Figure 6. Triplet decay curves of compounds T7Sn observed at 600 nm
in deaerated MeOH after excitation with a 355 nm nanosecond laser pulse.
Figure 7. Transient absorption spectra of T5S0-O(CO)Thy in deaerated
MeOH after excitation with a 355 nm nanosecond laser pulse.
Table 3. Decay Times τ of aci-Nitro Species in the Investigated
Thioxanthone/NPPOC Conjugates
to the case of T7S2-OR, is still observable (cf. Figure 3). Very
likely, in this case a photochemical trans/cis-isomerization of
the vinyl thioxanthone triplet impedes the observation of a
transient on the nanosecond time scale.
τ / µs
N2-saturated
MeOH
aerated MeOH
compound
R
)
H
R
)
(CO)Thy
R
)
H
R
)
(CO)Thy
2.2.3. Compounds with an Ester Linkage (T4E2-OR). The
transient spectra of these compounds show a triplet-triplet
absorption maximum at 600 nm. The triplet lifetime of 3.6 µs
is the longest of all thioxanthone/NPPOC conjugates investigated
here, but it is still by almost 1 order of magnitude shorter than
that for the parent thioxanthone. Thus energy transfer takes
place, and the aci-nitro product is formed as can be seen from
a long-lived transient absorption with a maximum at about 420
nm and a lifetime of about 80 µs.
T7S2-OR
T7S3-OR
T7S4-OR
T7S5-OR
T7S6-OR
T7S9-OR
T7T4-OR
T5S0-OR
T4E2-OR
38
39
32
26
23
25
30
68
78
99
118
71
39
40
32
16
23
27
36
84
60
101
99
63
123
129
85
123
122
64
2.2.4. Compounds with a Direct Connection of the Chro-
mophores (T5S0-OR). The transient spectrum of T5S0-O(CO)-
Thy differs significantly from the spectra of the other protecting
groups (cf. Figure 7). It shows an absorption peak at about 420
nm and a broad absorption increasing from 470 to 700 nm
without reaching a maximum. The broad absorption has a decay
time of 134 ns, and the peak at 420 nm decays in about 130 µs.
In an aerated solution, the former is quenched to a value of
117 ns whereas the latter is not affected. Therefore, we assume
that the unstructured, broad absorption can be attributed to a
triplet state of the conjugated thioxanthone/nitrophenyl chro-
mophore and the absorption at 420 nm is due to both the triplet
state and the aci-nitro form that is formed during the decay of
the triplet. A slight hypsochromic shift of the band maximum
observed while the triplet is decaying indicates the transforma-
tion of the triplet to the aci-nitro form.
In order to study the isolated T5S0 chromophore, the model
compound T5S0-M was synthesized, which yielded the transient
absorption spectrum shown in Figure 8. It is very similar in
shape, although about 3 times higher in intensity to the initial
spectrum observed with T5S0-O(CO)Thy, but its decay is
spectrally uniform; i.e., it does not show the band due to the
aci-nitro form, which cannot be formed in such a compound.
The whole spectrum decays with a lifetime of 4 µs. Its
assignment to a triplet-triplet absorption is supported by
quenching experiments with oxygen, yielding a quenching
constant of 3.5 × 108 M-1 s-1. Comparing Figures 7 and 8, we
may thus conclude that the triplet of the T5S0-chromophore,
even if the excitation encompasses a much larger electronic
It is remarkable that, although the initial triplet yield detected
after the laser flash in the T7Sn-O(CO)Thy series is strongly
dependent on the chain length of the linker, the observed initial
intensity of the aci-nitro band in all these compounds is fairly
independent of the linker length (cf. Discussion). The decay
time of the aci-nitro band does show some, but not a very
pronounced, dependence on the linker length (cf. Table 3).
However, it is significantly shorter for the free alcohols than
for the protected thymidines.
2.2.2. Compounds with Unsaturated Alkyl Linker (T7T4-
OR, T7D4-OR). The triplet-triplet absorption of compounds
T7T4-OR is centered at 630 nm (cf. Figure S7 in the Supporting
Information). Compared to the compounds with saturated linker,
this corresponds to a bathochromic shift by 30 nm. The band is
also slightly broader. These changes are likely due to the
extension of the thioxanthone chromophore by the conjugated
triple bond. The ground state bleaching around 400 nm observed
with the saturated linkers can also be observed in the transient
spectra of T7T4-OR. In this compound, the thioxanthone triplet
decays relatively slowly with a lifetime of 1.02 µs. Its decay is
concomitant with the appearance of an absorption band at 400
nm that can be associated with the corresponding aci-nitro form.
It has a lifetime of 123 µs. However, the yield of this
intermediate is only half the yield of the compounds with a
saturated linker. This fits with the observation of a competing
photochemical reaction which does not result in the formation
of the aci-nitro intermediate.8
For the compounds T7D4-OR, it was not possible to detect
any transient signal, although a weak fluorescence, comparable
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Intramolecular Sensitization
A R T I C L E S
the nanosecond LFP experiments with thioxanthone (cf. Figure
4). They have been assigned to the lowest triplet state of the
latter chromophore. The deviations between the spectrum at 2
ns in Figure 9A and the spectrum at 20 ns in Figure 4 are mainly
due to the fact that the population of the triplet state, that grows
in with a time constant of about 2 ns, is not yet completed at 2
ns but fully completed at 20 ns (cf. Discussion). At earlier delay
times, a strong, negative transient “absorption” (marked in blue
in Figure 9A) is observed. This negative spectral contribution
is due to stimulated emission and results from the excited singlet
state. It decays with a time constant of ∼2 ns, matching the
reported fluorescence lifetime.38
The effect of a potential triplet energy acceptor on the
observed excited states of 2-ethylthioxanthone was studied by
adding 1-methylnaphthalene at high concentrations (1.0 M) (cf.
Figure 9B). Under such conditions, the lifetime of the stimulated
emission and the transient absorption around >600 nm is
reduced. This reduction cannot be described by simple expo-
nential kinetics since dynamic and static components contribute
to the quenching (see discussion in ref 39). A biexponential
approximation yields time constants of ∼50 ps and ∼1.5 ns. It
has to be stressed that either constant is shorter than the
fluorescence lifetime of 2.6 ns.38 In addition to the decay of
the stimulated emission and the transient absorption at >600
nm, a sharp absorption band at 415 nm is seen to grow in. This
band can be assigned to the triplet state of 1-methylnaphtha-
lene.39
Finally, femtosecond photoexcitation of the thioxanthone
moiety linked to NPPOC in T7S2-OH results in a transient
absorption pattern (Figure 9C) which, at early delay times, is
identical to that of 2-ethylthioxanthone. Later on, a quenching
of the stimulated emission and the induced absorption around
600-650 nm is observed. The decay time of the stimulated
emission is ∼500 ps as compared to 2 ns for 2-ethylthioxan-
thone. The decay time of the absorption peak at 600 nm cannot
be determined precisely but may be extrapolated to some
10 ns.
Figure 8. Transient absorption spectra of T5S0-M in deaerated MeOH
after excitation with a 355 nm nanosecond laser pulse.
system, shows a similar reactivity toward benzylic hydrogen
as that of NPPOC.
2.2.5. Quenching of Thioxanthone Triplet in the Thiox-
anthone/NPPOC Conjugates by Molecular Oxygen. The
triplet lifetime of the thioxanthone chromophore in the thiox-
anthone/NPPOC conjugates is reduced in the presence of
oxygen. However, in comparison to free thioxanthone, this
sensitivity of the triplet lifetime to oxygen is greatly reduced
because of the competing intramolecular energy transfer process
that shortens the triplet lifetime of the thioxanthone chro-
mophore. Denoting the triplet decay constants in aerated and
deaerated solutions by kair and knitrogen, respectively, the quench-
ing rate constant is given by
kair - knitrogen
kq,O
)
(1)
2
[O2]
The values of kq,O determined for the investigated compounds
2
are listed in Table 2. In general, the observed values are in the
expected order of magnitude, that is about one-ninth of the
diffusion controlled rate constant.36,37 Quenching of the T5S0-
3. Discussion
O(CO)Thy triplet by oxygen (kq,O ) 0.52 × 109 M-1 s-1) is
The energetic requirements for fast energy transfer demand
that the energy of the excited donor state be higher than that of
the excited acceptor state. For NPPOC, the exact energy of the
lowest triplet state is not known, but the value of ET ) 252
kJ/mol for nitrobenzene in polar solvents35 may be a good
estimate. Since it lies by 13 kJ/mol below the triplet energy of
thioxanthone (ET ) 265 kJ/mol),35 electronic energy transfer
should proceed at a considerable rate. This expectation is in
accord with data for energy transfer between unlinked thiox-
anthone triplet and NPPOC-protected substrates.5 Here the rate
constants were found in a range between 109 M-1 s-1 and 4 ×
109 M-1 s-1, showing some variation with solvent and substrate
linked to NPPOC. This range of values is close to, but yet below,
the diffusion controlled limit. Wirz and co-workers40 studied
intramolecular triplet energy transfer between thioxanthone and
naphthalene (ET ) 253 kJ/mol). In these systems, establishment
2
relatively slow. Similar (unexplained) deviations from the rule
have been reported, e.g., for the coronene triplet.36
2.3. Femtosecond Transient Absorption. In order to clarify
the mechanism of energy transfer in further detail, compound
T7S2-OH showing the shortest triplet decay time and the
strongest intramolecular fluorescence quenching was investi-
gated by femtosecond (fs) laser spectroscopy. As a suitable
reference, not linked to the NPPOC chromophore, 2-ethylth-
ioxanthone was chosen.
When a solution of 2-ethylthioxanthone is photoexcited by
femtosecond laser pulses, at delay times of 2 ns (approximately
the longest time accessible with our femtosecond equipment),
one observes a transient spectrum as shown in Figure 9A with
an intense band at 600 nm and a weaker band at about 420 nm.
These features are in good agreement with those observed in
(35) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker Inc.: New York, 1993.
(36) Saltiel, J.; Atwater, B. W. Spin-Statistical Factors in Diffusion-Controlled
Reactions. In AdVances in Photochemistry; Gollnick, K., Volman, D. H.,
Hammond, G. S., Eds.; John Wiley & Sons: 1988; pp 1-90.
(37) Stevens, B.; Algar, B. E. Ann. N.Y. Acad. Sci. 1970, 171, 50-60.
(38) Ley, C.; Morlet-Savary, F.; Jacques, P.; Fouassier, J. P. Chem. Phys. 2000,
255, 335-346.
(39) Satzger, H.; Schmidt, B.; Root, C.; Zinth, W.; Fierz, B.; Krieger, F.;
Kiefhaber, T.; Gilch, P. J. Phys. Chem. A 2004, 108, 10072-10079.
(40) Bieri, O.; Wirz, J.; Hellrung, B.; Schutkowski, M.; Drewello, M.; Kiefhaber,
T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 9597-9601.
9
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A R T I C L E S
Wo¨ll et al.
Figure 9. Femtosecond transient absorption spectra of (A) 2-ethylthioxanthone, (B) 2-ethylthioxanthone in the presence of 1-methylnaphthalene (1.0 M),
and (C) T7S2OH. The smaller signal magnitude here is due to a lower concentration of T7S2-OH. The samples were excited with laser pulses at 388 nm,
and absorption changes were probed by a white light continuum in multichannel fashion. Lower panels: contour representation of the time dependence of
the transient absorption spectra. The color code for the absorbance change is indicated at the bottom. The time axis is linear until 1 ps and logarithmic
thereafter. Upper panels: transient absorption spectra at representative delay times of 1 ps (solid line), 100 ps (dash-dotted), and 2 ns (dashed). For comparison,
these delay times are also marked in the lower panels.
of an equilibrium between the two triplets is observed,
demonstrating that in polar solvents the triplet energy of
thioxanthone is only slightly above the value of naphthalene
which is close to that of nitrobenzene.
As was found here, triplet energy transfer from the thioxan-
thone chromophore to NPPOC leads to a strongly reduced triplet
lifetime of the thioxanthone moiety (cf. Table 2) as compared
to free 2-ethylthioxanthone (τ ) 26 µs in deaerated MeOH).
Even in the slowest case (T4E2-O(CO)Thy), it is 7 times shorter,
which means that the triplet lifetime is essentially determined
by the energy transfer process. Values of the pertinent rate
constant kT are obtained by the equation
acceptor are linked by a linear aliphatic chain, trends of kT are
particularly interesting to analyze. Figure 10 shows how kT
varies as a function of the chain length parameter n. The kT
values range from 5.3 × 107 s-1 for the shortest linker (n ) 2)
to 4.1 × 106 s-1 for the longest (n ) 9).
In between, the decrease is monotonic except for a “hump”
at n ) 6. For this length of the linker, the rate constant is found
between those of n ) 3 and 4. Such irregularities are not unusual
in the study of the interactions of reactive ends of aliphatic
chains (for examples, cf. the references given by Wagner and
co-workers41 for electron-transfer reactions, exciplex formation,
as well as spin-orbit and exchange interaction effects in
biradicals). It must be assumed that they reflect the ease at which
specific conformations of the linker chain bring the reactive
ends into a relative position suitable for reaction. In experiments
on triplet energy transfer in alkane linked benzoyl/aryloxy
moieties, a second relative maximum in the chain length
1
τ
1
τ0
kT )
-
(2)
where τ is the triplet lifetime of the NPPOC-linked thioxanthone
and τ0 is the triplet lifetime of 2-ethylthioxanthone.
3.1. Energy Transfer in the Series T7Sn-OR. In the series
of thioxanthone/NPPOC conjugates where the energy donor and
(41) Vrbka, L.; Kla´n, P.; Kriz, Z.; Koca, J.; Wagner, P. J. J. Phys. Chem. A
2003, 107, 3404-3413.
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12154 J. AM. CHEM. SOC. VOL. 129, NO. 40, 2007

Intramolecular Sensitization
A R T I C L E S
Figure 10. Decay constants of thioxanthone triplet (kT, b) and singlet (kS,
O) as a function of linker length in the series T7Sn-OH.
dependence of the triplet energy transfer rate from the benzoyl
to the aryloxy chromophore has been observed. The maximum
was located at linker lengths between 7 and 10 atoms and was
sensitive to the position at which the linkers were connected to
the chromophores.42,43 These findings have been rationalized
by means of molecular modeling.41 For excimer formation in
1,n-bis(1-pyrenylcarboxy)alkanes, a pronounced peak at n ) 5
was found, whereas a pronounced minimum appeared at n ) 7
in corresponding 1,n-bis(1-pyrenyl)alkanes.44,45 Altogether, these
examples demonstrate that the exact length at which intermediate
maxima or minima occur in the chain-length dependence of the
rate of an end-to-end interaction differ with the structure of the
reactive ends and with the position to which the linker ends are
attached.
In principle, the rate constant of energy transfer is a function
of distance and the relative orientation of the energy donor and
acceptor. It depends on an overlap integral of the respective
wavefunctions (where through space and through bond contri-
butions have to be distinguished) and a Franck-Condon (FC)
factor. Depending on the particular coupling mechanism, the
overlap integral will decay exponentially with distance whereby
the length parameter of this decay is shorter for the through-
space than for the through-bond mechanism. The FC factor is
a function of the free energy difference ∆G between the excited
donor and excited acceptor and a reorganization energy λ. Ιn
general, these latter two parameters are only slightly dependent
on the distance. On excitation of the donor, the conformation
of the linker chain will be represented by a statistical distribution.
Since the rate constant of energy transfer is faster at close
distances of donor and acceptor moiety, the triplet population
in such close-to-contact conformations will decrease more
rapidly and part of the energy transfer kinetics will involve
repopulation of such favorable conformations through the
dynamics of the linker chain (chain diffusion). Depending on
which of the two processes is faster, the rate constant of energy
transfer will be closer to one of the two limiting kinetic cases:
(i) “static” energy transfer control or (ii) chain diffusion control.
Figure 11. Correlation of initial triplet absorbance (∆AT,0) and initial
absorbance of aci-nitro form (∆Aaci-N,0) with fluorescence quantum yield
φf (the symbols n-OH and n-T represent the compounds T7n-OH and T7n-
O(CO)Thy, respectively, and ET-TX represents 2-ethylthioxanthone).
We note that, for the corresponding linker length, the rate
constants observed by us are by about 2 orders of magnitude
smaller than those found for triplet-triplet energy transfer in
linked benzoyloxy/aryloxy conjugates42,43 or for electron transfer
in 1-pyrenyl-n-dimethylaniline alkanes.46 Thus it should be clear
that triplet-triplet energy transfer in our case is not controlled
by chain diffusion dynamics but that it is limited by the rate of
the “static” triplet energy process itself. It is noteworthy that
triplet energy transfer in polypeptide linked thioxanthone/
naphthalene pairs40 occurs at about the same rate as those in
our systems which supports the authors’ conclusion that the
observed time constant of 20 ns represents the slowest temporal
component of the folding kinetics of a short oligopeptide with
as little as three peptide bonds.47
When investigating the triplet decay curves for the various
thioxanthone/NPPOC conjugates we noticed that the initial
amplitude of the triplet signal showed a systematic correlation
with the length of the linker and, hence, with the triplet decay
time (cf. Figure 6). We also found a significant dependence of
the fluorescence quantum yield on the linker length. A plot of
the initial triplet absorbance ∆AT,0 versus the fluorescence
quantum yield φf shown in Figure 11 reveals that both quantities
are linearly correlated. This means that the quenching of S1 of
the thioxanthone chromophore prevents formation of the triplet
(46) Weller, A. Chain Effect and Magnetic Field Effect on the Photoinduced
Electron Transfer Reactions of Polymethylene-Linked Donor Acceptor
Systems. In Supramolecular Photochemistry; Balzani, V., Ed.; D. Reidel
Publishing Company: Dordrecht, The Netherlands, 1987; pp 343-354.
(47) Recently, using intramolecular triplet-triplet energy transfer between the
xanthone triplet and naphthalene, it has been confirmed that time constants
of about 20 ns do indeed represent the slowest temporal components of
the folding kinetics which otherwise extend to the 50-500 ps regime. Cf.
Fierz, B.; Satzger, H.; Root, C.; Gilch, P.; Zinth, W.; Kiefhaber, T. Proc.
Natl. Acad. Sci. U.S.A. 2007, 104, 2163-2168.
(42) Kla´n, P.; Wagner, P. J. J. Am. Chem. Soc. 1998, 120, 2198-2199.
(43) Wagner, P. J.; Kla´n, P. J. Am. Chem. Soc. 1999, 21, 9626-9635.
(44) Zachariasse, K. A.; Macanita, A. L.; Ku¨hnle, W. J. Phys. Chem. B 1999,
103, 9356-9365.
(45) Zachariasse, K.; Ku¨hnle, W. Z. Phys. Chem. (Munich) 1976, 101, 267-
76.
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A R T I C L E S
Wo¨ll et al.
state normally formed at yields above 50%.48 The apparent rate
constant kS of intramolecular singlet quenching can be evaluated
as
case, too, the experiment with the addition of the triplet energy
acceptor 1-methylnaphthalene (Figure 9B) gives clear evidence
that triplet energy transfer between the photoexcited 2-ethylth-
ioxanthone and the triplet energy acceptor takes place. This
requires that at least partially a triplet state, which is not the
lowest 3ππ* state, is populated well before 2 ns. As in the case
of xanthone,50 the establishment of an equilibrium between the
φf,0
1
kS )
- 1
(3)
(
)
τS,0 φf
1
3
primarily excited ππ* singlet state and an upper nπ* triplet
state must be invoked. Within this mechanism, the time constant
of 2 ns is assigned to the simultaneous decay of the equilibrated
which follows from the usual Stern-Volmer equation. Here φf
and φf,0 are the fluorescence quantum yields of the thioxanthone/
NPPOC conjugate and of free ethyl thioxanthone, respectively,
and τS,0 is the fluorescence lifetime of the thioxanthone
chromophore, for which a value of 2.6 ns in MeOH38 was used.
In Figure 10, the values obtained for kS are plotted versus the
linker length n together with the values of kT. Both rate constants
exhibit a similar dependence on n, but the absolute values of kS
are larger than those of kT by almost 2 orders of magnitude.
The observations regarding the linker-length dependence of
fluorescence quantum yield and initial triplet yield indicate that
intramolecular quenching of the singlet excited thioxanthone
chromophore by the NPPOC moiety precludes the formation
of observable thioxanthone triplets on the nanosecond time scale.
However, it is important to note that the yield of the aci-nitro
intermediate generated through this quenching process is rather
independent of the linker length (cf. Figure 11). The same
applies to the overall photochemical quantum yield.8 In search
of a quenching mechanism that can account for all these
observations, singlet-singlet energy transfer from thioxanthone
(ES1 ≈ 285 kJ/mol)49 to NPPOC (ES1 ≈ 314 kJ/mol)49 can be
discarded, since it is too strongly endothermic to occur at such
a fast rate. Singlet quenching through a charge transfer
intermediate might be invoked since electron transfer from
singlet excited thioxanthone to nitrobenzene is estimated to be
exergonic by about -38 kJ/mol (cf. Supporting Information).
However, the femtosecond experiments provide conclusive
evidence that a different quenching pathway is followed in the
linked thioxanthone/NPPOC conjugates. Recently, for xanthone
in water, it was shown that its first excited singlet state
undergoes an ultrafast intersystem crossing in the picosecond
domain but that the fluorescence decay time is 700 ps.39,50 This
“delayed” fluorescence is due to a rapidly established equilib-
rium between the 1ππ* state and the 3nπ* state which are nearly
isoenergetic. The observed “delayed” fluorescence decay reflects
the relatively slow internal conversion process between the 3nπ*
3
3
1ππ* and nπ* states. This decay populates the lowest ππ*
state. During the first 20-30 ps, the stimulated emission slightly
decays in magnitude and experiences a red shift. A global
analysis yields a time constant of ∼10 ps for this process (for
all three samples). This constant represents the characteristic
time of the equilibration between the 1ππ* and the 3nπ* states.
The femtosecond time-resolved experiment with the thiox-
anthone/NPPOC conjugate T7S2-OH (cf. Figure 9C) shows that
the behavior of this system is essentially similar to that of the
combination of the free thioxanthone chromophore and a triplet
energy acceptor in high concentration. The observed shortening
of the excited singlet decay time of T7S2-OH must be assigned
to an intramolecular triplet energy transfer from the upper (3nπ*)
state of the thioxanthone moiety to the NPPOC group. The 3nπ*
1
state (ET ≈ 295 kJ/mol)51 is close in energy to the ππ* state,
and therefore, a fast and reversible intersystem crossing between
these states occurs. Hence, when this higher triplet state is
quenched by the attached NPPOC group, the fluorescence of
thioxanthone will be also quenched. Thus, fluorescence quench-
ing occurs through triplet-triplet energy transfer from the 3nπ*
state. This process is by 2 orders of magnitude faster than the
energy transfer from the lower 3ππ* state. The increase can be
related to the Gibbs free energy ∆G released in either process.
3
The nπ* state is expected to lie higher in energy by ∼30 kJ/
mol.51 This value matches those derived for the closely related
compounds xanthone (35 kJ/mol)50 and benzophenone (25 kJ/
mol).52 Therefore, the free energy release ∆G for energy transfer
to the NPPOC group is expected to be more negative by 25-
35 kJ/mol for the transfer from the 3nπ* than that from the 3ππ*
state. Provided that the process shows normal behavior in a
Marcus theory sense, this should result in an increased rate, as
observed. The transfer from the 3nπ* state leads to direct
formation of the NPPOC triplet, which reacts further to the aci-
3
3
3
nitro compound. No intermediate formation of the ππ* state
state and the ππ* state. The nπ* state can be quenched by
high concentrations of 1-methylnaphthalene with concomitant
formation of the triplet state of the quencher. As documented
by the results of the femtosecond time-resolved experiments
with 2-ethylthioxanthone and T7S2-OH presented in Figure 9,
this scenario is also fully applicable to thioxanthone.
of thioxanthone is observable for this energy transfer channel.
Hence the amplitude of the 3ππ* absorption in the nanosecond
time-resolved experiment is decreased. The mechanism is
summarized in Scheme 2.
The present findings on the decay mechanism of the first
excited singlet state and the two lowest triplet states of
thioxanthone reveal a remarkable parallelism to the behavior
of xanthone: namely, the detectability of a fast S1(ππ*) a T2-
(nπ*) equilibrium and the fairly slow internal conversion from
T2(nπ*) to T1(ππ*). In a previous investigation by Ley et al.38
employing laser spectroscopy with about 30 ps time resolution,
a fast (∼1011 s-1) and a slow component of intersystem crossing
were observed. The latter was closely correlated with the
solvent-dependent singlet lifetime. Based on the experimental
According to Figure 9A, the growing-in time of about 2 ns
of the transient absorption of the 3ππ* state matches the decay
time of the stimulated emission from the fluorescing 1ππ* state.
This seems to imply that intersystem crossing takes place in
∼2 ns. However, as can be learned from the example of the
xanthone case,50 such a conclusion may be premature. In our
(48) Allonas, X.; Ley, C.; Bibaut, C.; Jacques, P.; Fouassier, J. P. Chem. Phys.
Lett. 2000, 322, 483-490.
(49) These singlet energies were estimated from the onsets of the first absorption
bands in MeOH yielding 420 nm for thioxanthone and 380 nm for NPPOC.
(50) Heinz, B.; Schmidt, B.; Root, C.; Satzger, H.; Milota, F.; Fierz, B.;
Kiefhaber, T.; Zinth, W.; Gilch, P. Phys. Chem. Chem. Phys. 2006, 8,
3432-3439.
(51) Suga, K.; Kinoshita, M. Bull. Chem. Soc. Jpn. 1981, 54, 1651-1657.
(52) Yabumoto, S.; Sato, S.; Hamaguchi, H. Chem. Phys. Lett. 2005, 416, 100-
103.
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Intramolecular Sensitization
A R T I C L E S
Scheme 2. Deactivation Pathways of Photoexcited Thioxanthone Linked to NPPOC as Energy Acceptor Group
findings at that time these processes have been assigned to the
S2(nπ*) to T1(ππ*) and the S1(ππ*) to T2(nπ*) intersystem
crossing, respectively. In light of the new results reported here,
this interpretation ought to be reconsidered.
O(CO)Thy the yield of photorelease of thymidines is only 33%
or less;8 i.e., other photochemical reaction channels involving
the triple bond are dominant. Since these reactions probably
compete with electronic energy transfer to the NPPOC group,
the triplet lifetime of the thioxanthone moiety as given above
is a lower limit to the time constant of the triplet energy transfer
process.
3.2.4. T5S0-OR. In this case, the notion of separated
chromophores and energy transfer between them is no longer
valid. Stationary UV/vis and transient absorption spectra clearly
reveal that it is the combined chromophore which is spectro-
scopically active. In view of this situation, and considering the
example of the methylnitropiperonyloxycarbonyl (MeNPOC)
group where the methylenedioxy-substituent leads to enhanced
absorption but also to a concomitant decrease of the photo-
chemical quantum yield,5 it is fortuitous that the combined
T5S0-chromophore is still very reactive toward benzylic hy-
drogen to form the aci-nitro intermediate. In fact, this process
seems to determine the lifetime of the excited triplet state in
this chromophore, as can be seen by comparison with the model
chromophore T5S0-M that does not have an abstractable
benzylic hydrogen.
3.2. Other Linkers. 3.2.1. T4E2-OR. Although the oxycar-
bonyl group in this compound represents a very short linker
between the two chromophores, energy transfer is very slow.
Nevertheless, it should be noted that, for the observed triplet
lifetime of the thioxanthone chromophore of 3 µs, the efficiency
of energy transfer is still close to 90%. One reason for the slow
energy transfer might be a possible unfavorable shift of the
triplet energy of thioxanthone by the substitution with a
2-benzoyl function. In order to check this possibility, we
performed triplet quenching experiments with the unlinked
chromophore, viz. the benzoic acid ester TOBz using NPPOC-
Thy as a quencher. The triplet quenching constant was found
Conclusions
to be 6.4 × 108 M-1 s-1, whereas for unsubstituted thioxanthone
it was 1.0 × 109 M-1 s-1. This difference is not so pronounced
as to explain why intramolecular energy transfer should be so
much slower in T4E2-OR than in the compounds T7Sn-OR.
The reason for the slow energy transfer must be probably
sought in the rigidity of the ester linkage which only allows
free rotation of the two chromophores in a coaxial fashion
around the σ-bonds through which they are connected to the
-O(CO)- bridge. This does not change the distance between
them. Obviously, a through-bond mechanism involving this
linkage is not effective either.
Intramolecular electronic energy transfer in a novel class of
photoremovable protecting groups based on the NPPOC group
but enhanced by thioxanthone, covalently linked as an intramo-
lecular antenna, has been studied. Triplet-triplet energy transfer
has been demonstrated to occur between the thioxanthone and
NPPOC chromophores. For long linkers, or for inflexible short
linkers forcing the chromophores into relative conformations
with weak electronic coupling, energy transfer occurs from the
relaxed 3ππ* state of the thioxanthone moiety. From this state,
energy transfer is slow enough as to leave it detectable by
nanosecond time-resolved laser flash spectroscopy. For short,
flexible linkers, fast triplet energy transfer to NPPOC and
fluorescence quenching is observed. Based on femtosecond time-
resolved experiments, evidence has been provided that in this
case triplet-triplet energy transfer occurs from the higher 3nπ*
3.2.2. T7D4-OR. In these compounds, no transient absorption
3
from a ππ* state is detectable. However, from the fact that
T7D4-O(CO)Thy does show some photodecomposition involv-
ing reaction of the NPPOC group,8 one may conclude that some
energy transfer occurs which, according to the reasoning above,
should involve the 3nπ* state. Trans/cis-isomerization occurring
in either the singlet or triplet state competes with energy
transfer.8 Hence the quantum yield of the NPPOC reaction is
low.
3.2.3. T7T4-OR. With a value of 0.9 µs, this compound
shows the second longest triplet decay time among the
investigated thioxanthone/NPPOC conjugates. This indicates a
relatively slow triplet-triplet energy transfer, which may be
explained by the relatively long, inflexible linear part of the
linker chain extending over three C-C σ-bonds including the
triple bond. It must be pointed out, though, that for T7T4-
1
state which is in dynamic equilibrium with the ππ* state of
the thioxanthone moiety. This situation is completely analogous
to what was recently reported for xanthone.39,50 For the acceptor
and donor in contact, the rate constant for energy transfer from
the 3nπ* state of thioxanthone is by about 2 orders of magnitude
larger than that from the 3ππ* state, a fact which is assigned to
the higher energy of the former. However, to what extent each
of the two triplet-triplet energy transfer channels actually
contributes depends on the time it takes to bring the energy
donor and acceptor close enough together, that is, on the
conformational dynamics of the linker chain. For long linkers,
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A R T I C L E S
Wo¨ll et al.
stimulated Raman spectroscopy, but when blocking the Raman
pump beam it can also deliver transient absorption data. The pertinent
parameters for the present experiments were the following. Excitation
pulses at 388 nm were derived from the output of a CLARK CPA
2001 laser/amplifier (775 nm, 1 kHz) by frequency doubling. The
pulse energy at the sample location was ∼300 nJ. The focal spot
had a diameter of ∼150 µm there. Excitation induced absorption
changes were monitored by a white light continuum generated by
focusing the CPA fundamental onto a CaF2 plate. The relative
polarization of excitation and probe light was set to the magic angle.
After passing the sample the white light was dispersed and detected
with a diode array. The time resolution of the experiment was ∼200
fs. The excitation light was chopped at 500 Hz to yield induced
absorption differences. The timing between excitation and probe was
adjusted with a delay line. At each delay position, data were ac-
cumulated for 2-4 s and the results of two scans of the delay were
averaged.
Sample solutions were pumped through fused silica flow cells (path
length 1 mm) at a rate sufficient to exchange the sample between
subsequent laser shots. The concentration of the thioxanthones dissolved
in methanol (Merck UVASOL) was adjusted to 1-3 mM which
corresponds to an optical density of ∼1 OD at the excitation wavelength.
The volume of the sample solution was 5-10 mL and high enough to
avoid signal contributions from photoproducts.
this time is longer than the time for internal conversion from
the 3nπ* to the 3ππ* state, and thus the slower energy transfer
process from the 3ππ* state dominates. For short linkers, contact
formation of the energy donor and acceptor is faster than the
internal conversion process, and thus energy transfer occurs from
3
the nπ* state (cf. Scheme 2).
Direct linkage of the aromatic ring systems of donor and
acceptor chromophores yields a united chromophore as evi-
denced by the greatly changed absorption spectra in the ground
state and in the triplet state. In this situation, the reactivity of
the nitro group toward the benzylic hydrogen atom is by several
orders of magnitude slower than that in the case of the locally
3
excited nπ* state of NPPOC. Nevertheless, it seems that the
H-atom transfer still determines the lifetime of the combined
chromophore’s triplet state since a high yield of photocleavage
is observed in this case, too.
4. Experimental Section
4.1. Spectroscopy. 4.1.1. Absorption and Fluorescence Spectra.
Absorption spectra were measured on a Cary 50 or a Perkin-Elmer
Lambda 18 spectrometer. Fluorescence spectra were recorded on a
Perkin-Elmer LS 50 spectrometer. Fluorescence quantum yields were
determined for an excitation wavelength of 355 nm using acridone in
ethanol as a reference (φf ) 0.98).53 The concentrations of all sample
solutions were adjusted to an absorbance of 0.05 in order to keep inner
filter effects and reabsorption low.
4.1.4. NMR and MS Spectra. NMR Spectra were measured with a
JEOL GX 400 FT-NMR or a Bruker DRX 600, respectively. EI and
FAB mass spectra were recorded with a Varian MAT 312 mass
spectrometer. The FAB ion source was from AMD.
4.1.2. Nanosecond Laser Flash Spectroscopy. Samples were
excited by a frequency tripled Nd:YAG-laser (Spectra Physics Quanta
Ray GCR 150, 355 nm, repetition rate 5 Hz, pulse width 4-6 ns). The
pulse energy of 100 mJ was reduced to about 30 mJ by neutral density
filters. The optical detection system for transient absorption involved
a xenon lamp (Osram XBO 150), a monochromator (SpectraPro) and
a photomultiplier (Hamamatsu R 955) or an intensified gated CCD
camera (PI-MAX 1024 HQ-Blue, Princeton Instruments). The photo-
multiplier signal was recorded by a digital oscilloscope (LeCroy
9354A), and the data were transmitted to a personal computer. The
CCD data were processed using the Labview-based program “TOs-
KaNa” developed in our laboratory. For measurements in oxygen-free
media, the solutions were purged with N2. A flow system was used to
exchange the probed sample volume in the cuvette between any two
successive laser pulses.
4.2. Materials. The syntheses of most of the protected thymidines
are described in ref 29. The alcohols T7Sn-OH (n )2, 5, 6, 9), the
protected thymidine T7S2-O(CO)Thy, and the reference compounds
TOBz and T5S0-M were synthesized as described in the Supporting
Information.
Acknowledgment. This work was carried out within the
Research Unit 434 (Oligosaccharide and DNA chips - recogni-
tion of secondary gene products) funded by the Deutsche
Forschungsgemeinschaft (DFG Ste 283/7). B. Heinz acknowl-
edges a scholarship donated by the Fonds der Chemischen
Industrie.
Supporting Information Available: 1. Transient spectra and
decay curves. 2. Estimation of energy of charge transfer state
[TX]-[NPPOC]+. 3. Syntheses. 4. NMR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
4.1.3. Femtosecond Transient Absorption Spectroscopy. Femto-
second transient absorption spectra were recorded with the setup
detailed in ref 54. The setup as described there is used for femtosecond
JA072355P
(53) Siegmund, M.; Bendig, J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1061-
(54) Laimgruber, S.; Schachenmayr, H.; Schmidt, B.; Zinth, W.; Gilch, P. Appl.
Phys. 2006, 85, 557-564.
1067.
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12158 J. AM. CHEM. SOC. VOL. 129, NO. 40, 2007