Fast photo-processes in triazole-based push-pull systems

Article abstract of DOI:10.1039/b921322h

Electron donor-acceptor compounds 1 (asymmetrical push-pull derivative) and 2 (symmetrical push-pull-push derivative) were studied in which one (push-pull) or two aniline units (push-pull-push) are connected to a biphenyl group via triazole linkers, made

Full text of DOI:10.1039/b921322h

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Fast photo-processes in triazole-based push–pull systems
Peter D. Zoon,a Ivo H. M. van Stokkum,b Manuel Parent,cd Olivier Mongin,cd
Mireille Blanchard-Desce*cd and Albert M. Brouwer*a
Received 13th October 2009, Accepted 7th January 2010
First published as an Advance Article on the web 27th January 2010
DOI: 10.1039/b921322h
Electron donor–acceptor compounds 1 (asymmetrical push–pull derivative) and 2 (symmetrical
push–pull–push derivative) were studied in which one (push–pull) or two aniline units
(push–pull–push) are connected to a biphenyl group via triazole linkers, made by ‘‘click’’
chemistry. Steady-state and time-resolved spectroscopies indicate that highly dipolar charge
separated excited states are populated in moderately polar solvents. The very similar
photophysical behavior of both compounds implies symmetry breaking in the excited state of 2.
The polarity of the solvent determines the efficiency of formation of the charge separated state.
While in toluene it is very low, it becomes very high in acetonitrile. The bis-triazole substituted
biphenyl unit in 2 behaves as a better electron acceptor than the mono-triazole substituted
biphenyl in 1, which leads to a more facile charge separation in 2. Rates of charge separation
are of the order of 1011–1012
s
ꢀ1, and increase with solvent polarity.
response in the visible region and stronger fluorescence but
Introduction
its fluorescence characteristics were found to be only slightly
solvent dependent. In both cases, the significant TPA responses
can be related to pronounced intramolecular charge redistribution
upon excitation. The different solvatochromic behavior,
however, points to different excited state nature for the two
compounds in relation with the different nature of their core
(biphenyl—which has some electron-withdrawing characterw—
versus triphenylamine donor).
In the past decades, many new compounds have been designed
and synthesized for optimal two photon absorption (TPA) for
various applications.1 Among the most efficient two photon
absorbers are molecules that combine electron-donating (D)
and electron-accepting (A) groups in a conjugated system in
such a way that there is no net dipole moment. This is the
case, for example in an arrangement D–A–D (quadrupolar
derivatives),2,3 as well as in octupolar derivatives4,5 or in
branched systems.6
When changing the solvent from toluene to acetonitrile
(ACN), the emission maximum of 2 shifts from 385 nm to
545 nm, which is a rather large solvatochromic shift compared
with that observed for representative push–pull systems. For
example, Nile Red shows a shift from 548 nm in n-hexane to
626 nm in ACN9 and the emission maxima of push–pull
pyrrolidine and piperidine substituted peryleneimides shift
from about 630 nm to 740 nm.10 Still larger solvatochromic
shifts are found in the fluoroprobe (donor–saturated-
bridge–acceptor) class of compounds (n-hexane 420 nm;
ACN 650 nm).11
Recently, Parent et al. described the spectroscopic properties
of two triazole-based chromophores made by click chemistry
(Scheme 1: chromophores 2 and 3).7,8 These chromophores
were found to combine full transparency and significant TPA
in the visible region in addition to photoluminescence properties.
Chromophore 2 is based on the symmetrical grafting of two
dialkylanilino (donor) moieties on a biphenyl core via triazole
units built by click chemistry. Interestingly, this symmetrical
push–push derivative was found to show major solvent
dependency of its fluorescence properties (both emission
wavelength and lifetime) on solvent polarity.8 No such effect
was observed for chromophore 3 which is based on the
grafting of three dialkylanilino (donor) moieties on a triphenyl-
amine core via triazole units also built by click chemistry.
Chromophore 3 was found to exhibit both larger TPA
The major solvatochromic shift of the fluorescent band of
chromophore 2 with increased solvent polarity indicates that
the emissive excited state is strongly polar. This points to a
charge transfer character of the emissive excited state, which
implies that symmetry breaking occurs in the excited state.
Recently, in several symmetrical quadrupolar type systems
solvent polarity has been shown to play an important role in
favoring symmetry breaking.12 Interestingly, chromophore 2
shows larger solvatochromic shifts than the related fully
conjugated quadrupolar derivative where the triazole moieties
are replaced by ethynylene moieties3 (compound Q in Scheme 1).
This strongly suggests that charge separation occurs after
excitation prior to emission. The marked variation of the
a van’t Hoff Institute for Molecular Sciences, University of
Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam,
The Netherlands. E-mail: a.m.brouwer@uva.nl;
Tel: +31 20 5255491
b Division of Physics and Astronomy, Faculty of Sciences,
Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV
Amsterdam, The Netherlands. E-mail: ivo@few.vu.nl
c
´
CNRS, Chimie et Photonique Moleculaire (CPM),
35042 Rennes, France
´
Universite de Rennes 1, CPM, Campus de Beaulieu, Batiment 10A,
F-35042 Rennes, France
d
ˆ
w Biphenyl has indeed been reported to behave as an electron-
E-mail: mireille.blanchard-desce@univ-rennes1.fr
withdrawing group in push–pull derivatives.5
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Table 1 Absorption and emission maxima (nm) of 1 and 2 in a series
of solvents
1
2
Absorption Emission Absorption Emission
Solvent
Toluene
Tetrahydrofuran 309
310
387
415
471
503
326
326
325
325
385
435
514
545
Butyronitrile
Acetonitrile
310
305
binned to 10 nm and for every bin the decay trace was
abstracted and fitted with a convolution of a Gaussian system
response and a mono-exponential or bi-exponential decay
function. The results from this analysis show that there is a
clear difference between decay profiles extracted at different
wavelengths in toluene for compound 1 (Fig. 1).
The decay extracted at 406 nm shows only a fast component
of 0.19 ns. The decay trace extracted at 453 nm shows a clear
bi-exponential behavior, with the same fast component of
0.19 ns, but also a longer component that decays with a time
constant of 3.0 ns.
The short lifetime in the blue part of the spectrum in toluene
is an indication that there might be fast processes occurring. In
an attempt to extract more information from the data, the
streak images were fitted globally. To reach a satisfactory fit,
two or three components were used. The time constants that
emerge from the global analysis of the streak images are given
in Table 2. The wavelength dependent amplitudes shown in
Fig. 2 represent evolution associated spectra,13 that is, a
kinetic model A - B (- C) was assumed, in which the decay
times of A, B, and C increase.
Scheme 1 Structure of triazole-based molecules 1–3 and reference
compound Q.
The lifetime values (Table 2) show that for most solvents a
fast component is present at short wavelength. A clear trend
can be seen upon increasing solvent polarity: the red-shifted
longer lifetime components (blue and red traces in Fig. 2)
become more important with increasing polarity. The time
constants associated with the fast fluorescence components
in PrCN and ACN are unreliable, because of the small
lifetimes with solvent polarity (which increase by about one
order of magnitude from toluene to ACN)8 also suggests
profound structural modifications in connection with photo-
induced charge separation.
To gain more insight into the nature of the excited states of
chromophore 2 and the dynamics of their interconversion, we
performed time-resolved fluorescence and transient absorption
experiments. In order to investigate the role of the different
constituting units in symmetrical chromophore 2 in the intra-
molecular charge separation process, we also investigated
the related dissymmetrical push–pull chromophore 1 which
retains the biphenyl ‘‘acceptor’’ moiety (Scheme 1).w
Results
Absorption and emission spectra
In Table 1 we present the steady state absorption and emission
maxima of 1 and 2 in a series of solvents representing low,
medium, high and very high polarity. The absorption spectra
are almost solvent independent, but the emission is strongly
red-shifted with increased polarity. Both absorption and
emission for 2 occur at longer wavelength than for 1 in the
same solvent and its emission solvatochromism is more
pronounced. From measurements with a streak camera the
fluorescence lifetimes and emission spectra were acquired
simultaneously. For analysis, the streak images were horizontally
Fig. 1 A. Streak camera image of 1 fluorescence in toluene. Time on
the vertical axis, wavelength on the horizontal axis and color for the
intensity. B. Extracted decay trace (with fit) at 406 nm. C. Extracted
decay trace (with bi-exponential fit) at 453 nm.
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Table 2 Global analysis results of the fluorescence decays of 1 and 2
in different solvents. Time constants in ns. In parentheses the maxima
of the evolution associated spectra (nm)
keep the presentation as simple as possible this artefact is not
shown in Fig. 4.
We will first look at 1 and 2 in butyronitrile, because in this
solvent all relevant electronic excited states are most clearly
visible. Subsequently we will consider the other solvents. For 1
in PrCN, a broad absorption band is seen (magenta line in
Fig. 4C), extending from 470 to 550 nm, which according to
our working hypothesis is due to the LE state. This decays
with a time constant of 1.2 ps to an intermediate (black line in
Fig. 4C) with a narrower band. We tentatively assign this as a
relaxed LE state, in which excess vibrational energy has been
dissipated. The clear ‘‘dip’’ in the band near 440 nm is due
to stimulated LE emission, which shifts to the red upon
vibrational and solvent relaxation. At the same time,
vibrational cooling leads to a blue shift of the excited state
absorption. Subsequently, the charge separation process takes
place with a time constant of 5.3 ps, and leads to a CS species
(red line in Fig. 4C) with two absorption bands, a strong one
at 450 nm, a weaker one at 750 nm. This species decays with a
time constant of 0.2 ns to the final one (blue line in Fig. 4C),
which has an almost identical spectrum. Only the relative
intensity of the 750 nm band (vs. the 450 nm band) is lower.
It is not obvious what kind of process is occurring on this time
scale. Due to the presence of a number of single bonds in the
molecule, in the initial state different conformations will be
populated. Possibly, the relatively slow but minor spectral
evolution is caused by conformational relaxation processes or
by different kinetics in the different conformers. The final CS
state then has a lifetime of 1.8 ns, which is in acceptable
agreement with the 0.7 ns and 3.0 ns time constants found for
the fluorescence decay. The transient absorption measurement
window is only 1 ns wide. The estimates of lifetimes from
several hundred ps to a few ns can be obtained accurately only
when the signal to noise ratio is very good. An overview of all
the different components and their time constants is given in
Table 3.
Solvent
Toluene
1
2
0.19 (430)
3.0 (453)
—
1.0 (459)
3.4 (488)
0.70 (494)
3.0 (494)
—
o0.05 (418)
0.50 (424)
3.1 (453)
0.47 (471)
2.3 (471)
0.27 (450)
2.3 (524)
4.6 (520)
0.26 (shoulder)
4.6 (548)
—
Tetrahydrofuran
Butyronitrile
Acetonitrile
0.23 (440)
0.57 (527)
4.1 (527)
amplitudes and the widths of the system response functions,
which were B0.6 ns in these cases. The transient absorption
experiments presented below show that the conversion from
the LE state to the CS state occurs on a picosecond time scale.
Femtosecond transient absorption spectra
The time-resolved fluorescence spectra are dominated by
emission on the nanosecond time scale. In order to probe
dynamics at shorter timescales, and in order to have independent
spectroscopic data that can provide information on the nature
of the excited states involved, transient absorption (TA)
spectra were obtained with sub-picosecond time resolution.
Fig. 3 shows the spectral data obtained in the first 30 picoseconds
after excitation of 2 in butyronitrile (PrCN). As can be seen
from this figure on the very short time scale (o10 ps)
significant changes occur. To fully comprehend this dataset a
global analysis was performed, in which the decays at all
wavelengths were fitted simultaneously. For a satisfactory fit,
five kinetic components were required. In order to interpret
the associated wavelength dependent amplitudes, a kinetic
model is needed. A simple model that often turns out to allow
a meaningful interpretation of the results assumes sequential
processes, i.e. A - B - C - D - E, with gradually
increasing decay times.13 This leads to the ‘‘evolution associated
difference spectra’’ of 1 and 2 in toluene, tetrahydrofuran,
butyronitrile and acetonitrile that are shown in Fig. 4.
For the symmetrical system (2) in PrCN (Fig. 4G), the
overall picture is similar to that of 1 in the same solvent: after
decay of the very short-lived artefact (not shown), a broad
band is found (magenta, maximum 575 nm) that can be
attributed to the hot LE state, which relaxes in 0.9 ps to an
intermediate state (relaxed LE), with a maximum at 530 nm
(black in Fig. 4G). This is converted into the CS state with a
time constant of 2.8 ps. The CS state (red line in Fig. 4G) has
its maximum at 510 nm and it relaxes further in 40 ps.
The final CS state (blue), however, has the same spectral
characteristics as its precursor, with a final lifetime of 2.8 ns.
The fluorescence decays in a bi-exponential fashion, with time
constants of 2.3 and 4.6 ns. In summary, the data in PrCN
reveal the expected LE and CS species, but also some
additional complexity. Interestingly, the spectra of the relaxed
LE and CS states of 1 and 2 are quite similar, although
somewhat shifted in wavelength.
Because the electronic absorption spectra of 1 and 2 are
practically solvent independent, it is reasonable to assume that
initial excitation produces a locally excited state (LE), which
does not have a large dipole moment. The large solvatochromic
shifts in the fluorescence spectra clearly show that the relaxed
excited state, which has a nanosecond decay time, is very
polar, i.e. it is a charge separated state (CS). It is therefore
natural to attribute the spectral evolution to the process
LE - CS. The global analysis, however, reveals more than
just these two expected components. There is a very fast decay
in almost all cases, with a spectral maximum near 420 nm, and
a time constant of o0.2 ps, which is beyond the time resolution
of our setup (ca. 0.2 ps). It is not certain what the significance
of this component is. It could be a kind of ‘‘coherent artefact’’,
but because the excitation wavelength is relatively close
to absorption bands of the solvent, it could also be due
to a non-resonant process involving the solvent. In order to
Turning to toluene, we see a rather different behavior. The
hot LE state of 1 (Fig. 4A, magenta) has an absorption
maximum at 570 nm and a decay time of 1 ps. This species
evolves into a relaxed LE state that is a little blue-shifted
(lmax = 550 nm, black line in Fig. 4A) and has a decay
constant of 0.21 ns. The spectral narrowing and simultaneous
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Fig. 2 Evolution associated spectra from the global analysis of the fluorescence data of chromophores 1 (A–D) and 2 (E–H) in four different
solvents. Black lines are components associated with LE emission, blue and red lines show emission from charge separated states.
blue shift are probably due to vibrational relaxation. Then the
final species is formed, which has an absorption maximum
around 400 nm (blue line). The fitted decay constant for
this final species is 6 ns. When we compare the last two
species with the fluorescence data we see that the intermediate
species with an absorption maximum at 550 nm has a decay
time in excellent agreement with the fluorescence decay of
0.19 ns. The final species found in the transient absorption
decays with a time constant of 6 ns. In fluorescence, we find a
bi-exponential decay with a component of 3.0 ns. This second
component only has very small amplitude (see Fig. 3). It seems
unlikely that the long-lived species in transient absorption is
the same as the 3 ns fluorescence component. A reasonable
hypothesis is that the 3 ns fluorescence component (red-shifted
with respect to the LE emission, see Fig. 4) is the CS state,
produced in only low yield due to the low polarity of toluene,
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at 430 nm (black line). With t = 1.2 ps, this decays to the CS
species, with an absorption maximum at 460 nm and stimulated
emission at B600 nm. In this case the relaxed LE state is not
observed. For the CS species, we find three subsequent time
constants of 5.2 ps, 129 ps and 3.2 ns, with very little spectral
evolution when going from the first CS species to the third.
For the symmetrical compound 2, directly after excitation
we observe a species with a maximum at 570 nm and a decay
constant of 0.3 ps. Based on the spectrum, this can be assigned
to the LE state. This is the only case of the eight studied in
which the short-lived component due to the artefact is not
detected, because it is overwhelmed by a TA band of the LE
state in the same time window. After this, we see a state with a
decay constant of 0.5 ps and an absorption maximum at
525 nm, which might be a vibrationally relaxed LE state, with
the characteristic dip at 450 nm due to stimulated emission.
Then the final species is formed with a maximum at 500 nm
and stimulated emission at B620 nm, although also in this
case we can see three subsequent time constants of 7.1 ps,
340 ps and 2.7 ns. However, as in the case of 1, almost no
spectral evolution occurs after the first of the CS states has
been populated. The fluorescence decay time of 4.6 ns is again
comparable with the 2.7 ns of the last species. In summary, in
ACN the formation of the CS state is much faster than in
PrCN and is faster for 2 than for 1. This faster formation of
the CS state in the case of the symmetrical derivative 2 might
be related to larger driving force for electron transfer from the
electron-donating moiety (dialkylaninilo) to the core due to
the presence of two triazole units grafted on the biphenyl core
which reinforce its electron-withdrawing ability. This is also
consistent with the red-shifted absorption of the CS state of
the symmetrical compound.
Fig. 3 Femtosecond transient absorption matrix of 2 in PrCN. The
color represents the absorbance difference. lexc = 320 nm.
while the long-lived TA component is due to a local
triplet state.
For the symmetrical species 2 (Fig. 4E) we observe the LE
state with a broad band from 520 to 620 nm, which seems to
have two maxima (magenta line). This species decays with a
time constant of 2.6 ps to a state with an almost identical
spectral signature but with a lifetime of 0.38 ns. After this, a
state is populated showing a broad band ranging from 400 to
500 nm. The precise maximum is obscured by the stimulated
emission at 440 nm, which matches the emission maximum of
the fast component in the fluorescence data (Fig. 4E, blue
line). This species decays with a fitted time constant of 8.4 ns.
It is not certain how this final spectrum should be assigned.
There may be two components, which are not resolved because
of low amplitudes and insufficient sampling time. The peak
near 500 nm is probably due to the CS state, being populated
with low efficiency. The band rising on the short-wavelength
edge of the spectrum seems to correspond to the long-lived
species detected in the case of 1 in toluene. For the fluorescence
we find a major component with a time constant of 0.50 ns,
which agrees well with the 0.38 ns found in TA, which is
attributed to the LE state. The weak fluorescence component
of 3.1 ns may be due to the CS state. This may also be
responsible for the contribution near 500 nm in the weak ns
component in TA.
THF is more polar than toluene, but less polar than PrCN.
We expect that 1 and 2 will show ‘‘intermediate’’ behavior. For
1, after excitation there is a fast component of 0.2 ps similar to
that in the other solvents. After this a broad band with
maxima at 480 and 570 nm appears (magenta in Fig. 4B)
and this settles with t = 0.9 ps into a narrower band (black in
Fig. 4B) with a single maximum at 550 nm and stimulated
emission at B425 nm. Further narrowing and a shift of the
maximum to 520 nm occur with a time constant of 5.2 ps. This
‘relaxed’ LE state decays with a time constant of 0.8 ns
towards the final species. This corresponds well with the 1.0 ns
observed in fluorescence. The final species (blue in Fig. 4B)
shows a maximum at 415 nm, much like 1 and 2 in toluene.
This is likely to be due to a triplet state. In addition there is a
shoulder at 500 nm, which may be a contribution from the CS
state. The 3.4 ns fluorescence decay component with small
amplitude indicates that the transient species is a CS state.
For 2 in THF (Fig. 4F), the final species is different than for
1. After excitation, the LE state (magenta curve) has a
maximum around 600 nm. This relaxes with a time constant
of 0.7 ps to 570 nm. After another 4.4 ps, the CS state is
populated and this has its absorption maximum at 540 nm (red
spectrum). The CS state relaxes further with a time constant of
500 ps to the final excited state species. This also has its
maximum at 540 nm and decays in 2.4 ns to the ground state.
The final species has a decay constant that is in agreement with
the 2.3 ns found in the fluorescence measurements.
In summary, we observe some processes occurring on a very
fast time scale in toluene. The main bands are due to LE states,
with decay times in agreement with those from fluorescence.
Based on the fluorescence data, formation of the CS species in
toluene does occur, but with much lower efficiency than in
PrCN. In TA the CS species are not detected with certainty.
In ACN, the situation is very different. In this case, the data
indicate that the LE state is very short lived. This is not so
surprising because the solvent is very polar, thus favoring the
formation of a CS state.
For 1 (Fig. 4D), directly after excitation, there is the
commonly observed very fast artefact, which decays with a
time constant of 0.1 ps (not shown). After this, the LE state is
populated with a maximum at 520 nm and stimulated emission
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Fig. 4 Evolution associated decay spectra of 1 (A–D) and 2 (E–H) in four different solvents. Black, magenta and green lines mainly represent
locally excited states, whereas blue, orange and red lines mainly represent charge separated states.
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Table 3 Time constants and maxima of the different species found for
1 and 2 from the global analysis of the transient absorption data. Time
constants in parentheses are the fluorescence lifetimes
Importantly, at energies that are only slightly higher
additional states are found in both molecules. Mixing of the
two or three low-energy states along a reaction coordinate
for relaxation of the molecular geometry and the solvent
coordinates can give rise to the two very different states that
are experimentally observed. In this respect compounds 1
and 2 are similar to other push–pull systems, of which
N,N-dimethylaminobenzonitrile is a prototype.16
Compound 1
Compound 2
Time constant/ns
lmax/nm Time constant/ns
lmax/nm
Solvent
Toluene 1.0 ꢂ 10ꢀ3
570
550
420
2.6 ꢂ 10ꢀ3
520–620
520–620
Broad
600
570
540
540
—
575
0.21 (0.19)
0.38 (0.5)
>3 (3)
>3 (3.1)ꢀ3
THF
0.9 ꢂ 10ꢀ3
5.2 ꢂ 10ꢀ3
B0.8 (1.0)
B2.7 (3.4)
—
480, 570 0.7 ꢂ 10
550
520
4.4 ꢂ 10ꢀ3
0.50 (0.47)
B2.4 (2.3)
—
Discussion
410
500(sh)
In the preceding section, the results of the time-resolved
experiments have been presented together with an assignment
of the transient species. For the two-armed compound (2), the
final species (blue line in Fig. 4) with a maximum at 500 nm in
acetonitrile is a CS species. A clear shift to longer wavelength
can be observed when the polarity of the solvent is decreased.
The band around 520 nm (black line) is attributed to the
relaxed LE state. Finally, the broad band around 580 nm
(magenta line) is the (hot) LE state. This band does not shift
with solvent polarity, but it changes shape, the contribution
at shorter wavelength decreasing with increasing polarity.
Similar transient spectral signatures can be recognized for 1.
In this case the long-lived CS state shows a peak at shorter
wavelength than for 2 in the same solvent. An additional band
in the near-infrared range can be seen, of which the maximum
seems to be near the edge of our spectral window. A similar
band for 2 is red-shifted, and we can just see a small rise up to
750 nm.
PrCN
ACN
1.2 ꢂ 10ꢀ3
5.3 ꢂ 10ꢀ3
B0.20 (0.7)
B1.8 (3.0)
1.2 ꢂ 10ꢀ3
470(sh) 0.9 ꢂ 10ꢀ3
550
520
2.8 ꢂ 10ꢀ3 (0.27)a 530
40 ꢂ 10ꢀ3
510
510
570
525
450, 750 B2.8 (2.ꢀ3,3 4.8)
520
0.3 ꢂ 10
5.2 ꢂ 10ꢀ3 (0.23)a 460
0.5 ꢂ 10ꢀ3
B0.13 (0.57)
B3.2 (4.1)
—
460
460
—
7.1 ꢂ 10ꢀ3 (0.26)a 500
B0.34
500
500
B2.7 (4.6)
a
Fluorescence decay time unreliable due to limited time resolution.
To summarize, in THF compounds 1 and 2 do not behave
identically, as they did in the other solvents. Compound 1 is
showing behavior similar to that of 2 in toluene. Compound 2,
however, behaves more like 1 in PrCN. This can be related to
the difference in driving force for electron transfer, which is
more favorable in 2 due to the presence of two triazole units
contiguous to the biphenyl electron acceptor.
The time constants associated with the formation and decay
of the CS state are listed in Table 5. The results in Table 5
show a clear internal consistency: charge separation is more
rapid in 2 than in 1, and more rapid in more polar solvents.
This follows the trend in the driving force. Interestingly, the
decay times of the CS states do not seem to depend much on
the solvent and the structure of the molecules.
Quantum chemical calculations
The geometries and electronic structures of 1 and 2 (with
methyl groups instead of the hexyl groups) were calculated
using the standard B3LYP/6-31G(d) approach (Table 4).14
Time-dependent DFT shows that the lowest excited states are
essentially described by HOMO–LUMO excitations. The
HOMO has a large contribution on the dialkylaniline donor,
and the LUMO is more on the biphenyl acceptor fragment,
while both are very strongly delocalized, as illustrated in
Fig. 5. This indicates that the lowest energy transition has a
partial intramolecular charge transfer (ICT) character (from
the donor end-group to the biphenyl moiety in push–pull
chromophore 1 and from the peripheral donors to the biphenyl
core in the symmetrical push–pull–push chromophore 2)
showing that the triazole moiety although partially disrupting
the conjugation between the donor and acceptor moieties
indeed promotes ICT both in dipolar and quadrupolar
derivatives.
The quantum chemical calculations indicate that the LE
state that is reached upon absorption of UV light can be
characterized by a strongly allowed HOMO to LUMO excitation.
Although the dipole moment of the LE state of chromophore
1 is probably somewhat larger than that of the ground
state, the absorption spectrum shows no solvatochromicity.
Fluorescence from the LE state is observed only in non-polar
solvents. The red-shifted emission for 1 in THF vs. toluene,
which according to the TA experiments is an LE emission, is
an indication that the LE fluorescence indeed is solvatochromic,
as expected from the calculations which reveals partial ICT
character of the LE state.
The detailed nature of the CS state is not revealed by the
experimental data. The TA results indicate that it is clearly
different from the LE state. In one way or another, a
large dipole must be formed, which leads to the strongly
solvatochromic emission. In N,N-dimethylaminobenzonitrile,
twisting of the dimethylamino electron donor group with
respect to the benzene ring helps to localize the positive charge
in the twisted intramolecular charge transfer state on the
amino group, while the negative charge is delocalized over
the benzonitrile fragment.17 In 1 and 2 twisting could also
occur about the C–N bond linking the phenyl ring to the
Interestingly, the transition in push–pull chromophore 1 is
stronger (but significantly blue-shifted) than that observed in a
recently reported triazole-based push–pull chromophore
bearing the same donor but a much stronger acceptor.15
The computed lowest excited state energies are slightly
lower than what is found experimentally, but the agreement
is satisfactory. The bathochromic and hyperchromic shift of
the absorption band in symmetrical compound 2 as compared
to push–pull compound 1 is also well reproduced.
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a
Table 4 Calculated excitation wavelengths for 10 and 20
Compound
Excitation wavelength/nm and oscillator strength (in parentheses)
10
317 (0.88)
285 (0.35)
282 (0.055)
338 (1.94)
315 (0.0032)
295 (0.0045)
20
a
Compounds 10 and 20 are derived from 1 and 2 by replacing the n-hexyl groups by methyl groups.
Fig. 5 HOMO (C,D) and LUMO (A,B) of model systems 10 (A,C) and 20 (B,D) (B3LYP/6-31G(d), no solvent).
Table 5 Time constants for formation of the CS state from transient
absorption and (in parentheses) the decay of the CS state from
fluorescence
seen by the lack of solvatochromic shift in absorption. In
sufficiently polar solvents, the locally excited state decays into
a highly dipolar one, as can be seen from the solvatochromic
behavior in fluorescence and the changes in the transient
absorption spectra. For 1 in THF the emission is still mainly
from the LE state, but in PrCN and ACN it stems almost
completely from the CS state. For 2, charge separation occurs
already in THF. For both compounds a small extent of charge
separation in toluene can be inferred from the fluorescence
decays.
1
2
Toluene
THF
PrCN
ACN
(3.0 ns)a
0.8 ns (3.4 ns)
5.3 ps (3.0 ns)
1.2 ps (4.1 ns)
(3.1 ns)a
4.4 ps (2.3 ns)
2.8 ps (2.3, 4.6 ns)
0.5 ps (4.6 ns)
a
Because of the low yield of the CS state, the time constant for its
formation could not be determined.
The nature of the excited state can be tuned: non-polar
solvents, like toluene, will predominantly give rise to the
locally excited state while polar solvents, like acetonitrile, will
yield the charge separated excited state. This charge separated
excited state shows highly solvatochromic emission and strong
absorption in the visible region as well as a lifetime of several
nanoseconds. This behavior may have interesting applications
in different fields. Both chromophores may be used as sensitive
fluorescent probes of environment polarity. The fast formation
of the CS excited state (in medium to high polarity solvents)
which strongly absorbs in the visible region and has several
nanoseconds lifetime may have interesting implications for
optical limiting applications. Indeed chromophore 2 is a good
candidate for combination of two-photon excitation from the
ground state and excited state absorption from the CS state.
triazole. The electron-accepting group comprises the biphenyl
unit. Replacement of the biphenyl in 1 by a phenyl ring
completely suppresses electron transfer.18
The 1,2,3-triazole ring has recently been used as a linker
between functional units in numerous molecular architectures.
Active involvement of the triazole unit in the photophysical
and redox properties of conjugated systems is not common.8,15,19
In the present case, the triazole ring(s) clearly help to enhance
the electron affinity of the conjugated aromatic systems. Overall,
the behavior of 1 and 2 is similar, indicating that symmetry
breaking occurs in the excited state of 2. The differences in the
behaviors of 1 and 2 can be attributed to the enhanced
electron-accepting ability of the bis-triazole biphenyl unit in 2.
Experimental
Conclusion
Synthesis
This work shows that with ‘click’ chemistry interesting
photoactive compounds can be made in which the triazole
linkers play a pivotal role as both non-conjugating connectors
and electron-withdrawing units favoring and stabilizing
charge separation. The exact localization of the charges in
the charge separated states of 1 and 2 remains somewhat
ambiguous. It is clear, however, that the state populated
directly after excitation is of a ‘‘local’’ nature as can be
Fluorophores 1 and 2 were synthesized in good yields as single
regioisomers, by 1,3-dipolar cycloaddition of azide 4 with
alkyne 5 and bis-alkyne 6, respectively (Scheme 2).
4-(4-[1,10-Biphenyl]-4-yl-1H-1,2,3-triazol-1-yl)-N,N-dihexyl-
benzenamine (1). Air was removed from a solution of
4-azido-N,N-dihexylbenzenamine 4 (0.547 g, 1.81 mmol) and
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resolution of ca. 2 nm. The spectra were recorded in rect-
angular 1 cm quartz cells.
Steady-state fluorescence
The steady state fluorescence spectra (Table 1) were recorded
on a Spex Fluorolog 3 spectrometer, equipped with double
grating monochromators in the excitation and emission
channels. The excitation light source was a 450W Xe lamp
and the detector a Peltier cooled R636-10 (Hamamatsu)
photomultiplier tube. The fluorescence spectra were corrected
for the wavelength response of the detection system. The
fluorescence of the solution was detected in a right angle
geometry using solutions with low absorbances (o0.10).
Scheme 2 Synthesis of triazole-based molecules 1 and 2. Reagents and
conditions: CuI, DIPEA, THF, 35 1C.
4-ethynylbiphenyl 5 (0.153 g, 0.86 mmol) in a mixture of THF
(1.2 mL) and diisopropylethylamine (0.5 mL) by blowing
argon for 20 min. Then CuI (15.2 mg, 80 mmol) was added,
and deaeration was continued for 10 min. Thereafter the
mixture was stirred at 35 1C for 21 h. The solvent was removed
under reduced pressure, and the crude product was purified by
column chromatography (heptane–CH2Cl2, gradient from
50 : 50 to 25 : 75) and crystallized in heptane–dichloromethane
to yield 0.408 g (86%) of 1; 1H NMR (200.13 MHz, CDCl3) d
8.08 (s, 1H), 7.98 and 7.69 (AA0XX0, JAX = 8.1 Hz, 4H), 7.55
and 6.69 (AA0XX0, JAX = 9.0 Hz, 4H), 7.65 (d, J = 8.8 Hz,
2H), 7.42 (m, 3H), 3.31 (t, J = 7.5 Hz, 4H), 1.61 (4H), 1.33
(m, 12H), 0.91 (t, J = 6.0 Hz, 6H); 13C NMR (50.32 MHz,
CDCl3) d 148.3, 147.4, 140.7, 140.5, 129.6, 128.7, 127.4, 127.3,
126.9, 126.0, 125.5, 122.1, 117.6, 111.4, 51.1, 31.6, 27.0, 26.7,
22.6, 14.0; HRMS (EI) calcd for C32H40N4K ([M + K]+) m/z
519.2890, found 519.2880. Anal. calcd for C32H40N4
(480.6991): C, 79.96; H, 8.39; N, 11.66%. Found: C, 79.81;
H, 8.44; N, 11.57%.
Time-resolved measurements
Time resolved fluorescence was measured with a Hamamatsu
streak camera system (details described elsewhere20). As
excitation source a Spectra-Physics Hurricane Titanium:sapphire
regenerative amplifier system was used. The excitation
wavelength of 320 nm was made with an optical parametric
amplifier (Spectra-Physics OPA 800). The excitation light was
imaged onto the cell with a mirror and the emission light was
collected with a fiber coupled to the spectrograph of the streak
setup. The streak camera was run in ‘‘dump mode’’. In this
mode, the sync signal from the Hurricane is combined with the
trigger signal from a triggering diode to gain optimal resolution.
Unfortunately, there is still electronic jitter, which limits the
instrument response width to ca. 0.2 ns, depending on the
measurement time (longer measurements giving broader
instrument response). The instrument responses (FWHM)
used in the global analysis of the streak measurements were:
0.37, 0.26, 0.64 and 0.63 ns for 1 and 0.15, 0.56, 0.58 and
0.61 ns for 2, respectively, in the series of solvents toluene–
THF–PrCN–MeCN.
4,40-[[1,10-Biphenyl]-4,40-diylbis(1H-1,2,3-triazol-4,1-diyl)]bis-
[N,N-dihexyl-benzenamine] (2). Air was removed from a
solution of 4-azido-N,N-dihexylbenzenamine 4 (0.1882 g,
0.662 mmol) and 4,40-diethynyl-1,10-biphenyl 6 (0.0498 g,
0.246 mmol) in a mixture of THF (1.2 mL) and diisopropyl-
ethylamine (0.5 mL) by blowing argon for 20 min. Then CuI
(6.1 mg, 32 mmol) was added, and deaeration was continued
for 10 min. Thereafter the mixture was stirred at 35 1C for
15 h. The solvent was removed under reduced pressure, and
the crude product was purified by column chromatography
(CH2Cl2 then CH2Cl2–AcOEt 95 : 5) and crystallized in
heptane–dichloromethane to yield 0.171 g (86%) of 2;
1H NMR (200.13 MHz, CDCl3) d 8.09 (s, 2H), 7.99 (d,
J = 8.3 Hz, 4H), 7.73 (d, J = 8.4 Hz, 4H), 7.56 (d, J = 9.0
Hz, 4H), 6.70 (d, J = 9.2 Hz, 4H), 3.31 (t, J = 7.8 Hz, 8H),
1.61 (m, 8H), 1.34 (m, 24H), 0.91 (t, J = 6.5 Hz, 12H); 13C
NMR (75.48 MHz, CDCl3) d 148.4, 147.4, 140.1, 129.8, 127.3,
126.2, 125.7, 122.1, 117.7, 111.6, 51.2, 31.7, 27.1, 26.8, 22.7,
14.1; HRMS (ESI) calcd for C52H70N8Na [M + Na]+ m/z
829.56211, found 829.5634; calcd for C52H70N8K [M + K]+
m/z 845.53605, found 845.5344.
For the transient absorption measurements, the same
excitation source was used. The residual fundamental light
from the Hurricane was used for the white light generation,
which was transmitted through the sample and detected with a
CCD spectrograph (Ocean Optics). Typical laser output was
5 mJ per pulse (130 fs FWHM) with a repetition rate of 1 kHz.
The samples were placed into 2 mm quartz cells and stirred
with a ‘finger’. The sample used had a ground state absorbance
of around 0.2. Steady state absorbance measurements were
performed before and after the laser experiments, which showed
that degradation of the sample during the measurements is
insignificant.
Global analysis
A global analysis was performed using a sequential scheme
with increasing lifetimes. This results in exactly the same
lifetimes and residuals as a global analysis using parallel
decays.13 The estimated evolution associated (difference)
spectra (EADS, or EAS in the case of the emission data) are
a linear combination of the decay associated (difference)
spectra that result from the model with parallel decays. We
assume that the sequential scheme corresponds reasonably
well to the underlying photophysical chemistry, and thus we
attempt to interpret the EADS.
Steady-state absorption
The electronic absorption spectra presented in Table 1 were
recorded on a single beam HP 8453 diode array spectrometer
with a spectral range from 190 nm to 1100 nm and a spectral
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N. Leonczek, N. Nicolaou, E. Badaeva, O. Mongin and
M. Blanchard-Desce, J. Phys. Chem. B, 2007, 111, 9468–9483.
7 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
Ed., 2001, 40, 2004–2021; V. V. Rostovtsev, L. G. Green,
V. V. Fokin and K. B. Sharpless, Angew. Chem., Int. Ed., 2002,
41, 2596–2599.
8 M. Parent, O. Mongin, K. Kamada, C. Katan and M. Blanchard-
Desce, Chem. Commun., 2005, 2029–2031.
9 R. S. Moog, D. D. Kim, J. J. Oberle and S. G. Ostrowski, J. Phys.
Chem. A, 2004, 108, 9294–9301.
10 P. D. Zoon and A. M. Brouwer, ChemPhysChem, 2005, 6,
1574–1580; P. D. Zoon and A. M. Brouwer, Photochem. Photobiol.
Sci., 2009, 8, 345–353.
11 R. M. Hermant, N. A. C. Bakker, T. Scherer, B. Krijnen and
J. W. Verhoeven, J. Am. Chem. Soc., 1990, 112, 1214–1221;
T. Scherer, W. Hielkema, B. Krijnen, R. M. Hermant,
C. Eijckelhoff, F. Kerkhof, A. K. F. Ng, R. Verleg, E. B. van
der Tol, A. M. Brouwer and J. W. Verhoeven, Recl. Trav. Chim.
Pays-Bas, 1993, 112, 535.
12 F. Terenziani, A. Painelli, C. Katan, M. Charlot and
M. Blanchard-Desce, J. Am. Chem. Soc., 2006, 128,
15742–15755; S. Amthor, C. Lambert, S. Dummler, I. Fischer
and J. Schelter, J. Phys. Chem. A, 2006, 110, 5204–5214.
13 I. H. M. van Stokkum, D. S. Larsen and R. van Grondelle,
Biochim. Biophys. Acta, Bioenerg., 2004, 1657, 82–104; I. H.
M. van Stokkum, D. S. Larsen and R. van Grondelle, Biochim.
Biophys. Acta, Bioenerg., 2004, 1658, 262–262.
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN
03, Revision B.3, Gaussian Inc., Pittsburgh PA, USA, 2003.
15 P. D. Jarowski, Y.-L. Wu, W. B. Schweizer and F. Diederich,
Org. Lett., 2008, 10, 3347–3350.
Acknowledgements
We thank John van Ramesdonk and Michiel Groeneveld for
their help with the streak camera and the fs-setup and Loig
Kergoat for his help with the TA measurements. This
research was supported by the Netherlands Research School
Combination Catalysis (NRSCC). MBD thanks Region
Bretagne ()Renouvellement des Competences* Program) for
the fellowship to MP.
Notes and references
1 G. S. He, L.-S. Tan, Q. Zheng and P. N. Prasad, Chem. Rev., 2008,
108, 1245–1330; F. Terenziani, C. Katan, E. Badaeva, S. Tretiak
and M. Blanchard-Desce, Adv. Mater., 2008, 20, 4641–4678;
H. M. Kim and B. R. Cho, Chem. Commun., 2009, 153–164.
2 J. E. Ehrlich, X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. Rockel,
S. R. Marder and J. W. Perry, Opt. Lett., 1997, 22, 1843–1845;
B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard,
J. C. Bhatt, R. Kannan, L. Yuan, G. S. He and P. N. Prasad,
Chem. Mater., 1998, 10, 1863–1874; M. Albota, D. Beljonne,
J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu, A. A. Heikal, S. E. Hess,
T. Kogej, M. D. Levin, S. R. Marder, D. McCord-Maughon,
J. W. Perry, H. Rockel, M. Rumi, G. Subramaniam, W. W. Webb,
X.-L. Wu and C. Xu, Science, 1998, 281, 1653–1656; L. Ventelon,
L. Moreaux, J. Mertz and M. Blanchard-Desce, Chem. Commun.,
1999, 2055–2056; O.-K. Kim, K.-S. Lee, H. Y. Woo, K.-S. Kim,
G. S. He, J. Swiatkiewicz and P. N. Prasad, Chem. Mater., 2000,
12, 284–286; M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry,
S. Barlow, Z.-Y. Hu, D. McCord-Maughon, T. C. Parker,
H. Rockel, S. Thayumanavan, S. R. Marder, D. Beljonne and
J.-L. Bredas, J. Am. Chem. Soc., 2000, 122, 9500–9510;
L. Ventelon, S. Charier, L. Moreaux, J. Mertz and
M. Blanchard-Desce, Angew. Chem., Int. Ed., 2001, 40,
2098–2101; P. K. Frederiksen, M. Jørgensen and P. R. Ogilby,
J. Am. Chem. Soc., 2001, 123, 1215–1221; A. Abbotto, L. Beverina,
R. Bozio, A. Facchetti, C. Ferrante, G. A. Pagani, D. Pedron and
R. Signorini, Org. Lett., 2002, 4, 1495–1498; W. J. Yang,
D. Y. Kim, M.-Y. Jeong, H. M. Kim, S.-J. Jeon and B. R. Cho,
Chem. Commun., 2003, 2618–2619; O. Mongin, L. Porres,
M. Charlot, C. Katan and M. Blanchard-Desce, Chem.–Eur. J.,
2007, 13, 1481–1498.
3 O. Mongin, L. Porres, L. Moreaux, J. Mertz and M. Blanchard-
Desce, Org. Lett., 2002, 4, 719–722.
4 S.-J. Chung, K.-S. Kim, T.-C. Lin, G. S. He, J. Swiatkiewicz and
P. N. Prasad, J. Phys. Chem. B, 1999, 103, 10741–10745; G. S. He,
J. Swiatkiewicz, Y. Jiang, P. N. Prasad, B. A. Reinhardt, L.-S. Tan
and R. Kannan, J. Phys. Chem. A, 2000, 104, 4805–4810;
B. R. Cho, K. H. Son, S. H. Lee, Y.-S. Song, Y.-K. Lee,
S.-J. Jeon, J. H. Choi, H. Lee and M. Cho, J. Am. Chem. Soc.,
2001, 123, 10039–10045; W.-H. Lee, H. Lee, J.-A. Kim, J.-H. Choi,
M. Cho, S.-J. Jeon and B. R. Cho, J. Am. Chem. Soc., 2001, 123,
10658–10667; D. Beljonne, W. Wenseleers, E. Zojer, Z. Shuai,
H. Vogel, S. J. K. Pond, J. W. Perry, S. R. Marder and
J.-L. Bredas, Adv. Funct. Mater., 2002, 12, 631–641; L. Porres,
O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz and
M. Blanchard-Desce, Org. Lett., 2004, 6, 47–50; C. Le Droumaguet,
O. Mongin, M. H. V. Werts and M. Blanchard-Desce, Chem.
Commun., 2005, 2802–2804.
16 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 2003,
103, 3899–4032; S. I. Druzhinin, N. P. Ernsting, S. A. Kovalenko,
L. W. Lustres, T. A. Senyushkina and K. A. Zachariasse, J. Phys.
Chem. A, 2006, 110, 2955–2969; W. Fuss, W. E. Schmid,
K. K. Pushpa, S. A. Trushin and T. Yatsuhashi, Phys. Chem.
Chem. Phys., 2007, 9, 1151–1169.
17 S. Murali, P. Changenet-Barret, C. Ley, P. Plaza, W. Rettig,
M. M. Martin and R. Lapouyade, Chem. Phys. Lett., 2005, 411,
192–197; S. Carlotto, A. Polimeno, C. Ferrante, C. Benzi and
V. Barone, J. Phys. Chem. B, 2008, 112, 8106–8113; S. Cogan,
S. Zilberg and Y. Haas, J. Am. Chem. Soc., 2006, 128, 3335–3345;
I. Gomez, M. Reguero, M. Boggio-Pasqua and M. A. Robb,
J. Am. Chem. Soc., 2005, 127, 7119–7129.
18 M. Parent, PhD thesis, University of Rennes, 2008.
19 R. M. Meudtner, M. Ostermeier, R. Goddard, C. Limberg and
S. Hecht, Chem.–Eur. J., 2007, 13, 9834–9840; K. Sivakumar,
F. Xie, B. M. Cash, S. Long, H. N. Barnhill and Q. Wang, Org.
Lett., 2004, 6, 4603–4606; Z. Zhou and C. J. Fahrni, J. Am. Chem.
Soc., 2004, 126, 8862.
5 F. Terenziani, C. Le Droumaguet, C. Katan, O. Mongin and
M. Blanchard-Desce, ChemPhysChem, 2007, 8, 723–734.
6 S.-J. Chung, T.-C. Lin, K.-S. Kim, G. S. He, J. Swiatkiewicz,
P. N. Prasad, G. A. Baker and F. V. Bright, Chem. Mater., 2001,
13, 4071–4076; A. Abbotto, L. Beverina, R. Bozio, A. Facchetti,
C. Ferrante, G. A. Pagani, D. Pedron and R. Signorini, Chem.
Commun., 2003, 2144–2145; J. Yoo, S. K. Yang, M.-Y. Jeong,
H. C. Ahn, S.-J. Jeon and B. R. Cho, Org. Lett., 2003, 5, 645–648;
C. Katan, S. Tretiak, M. H. V. Werts, A. J. Bain, R. J. Marsh,
20 X. Y. Lauteslager, I. H. M. van Stokkum, H. J. van Ramesdonk,
A. M. Brouwer and J. W. Verhoeven, J. Phys. Chem. A, 1999, 103,
653–659.
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