Ultrafast bidirectional photoswitching of a spiropyran

Article abstract of DOI:10.1021/ja1062746

We report on bidirectional photochemical switching of 6,8-dinitro-1′, 3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open merocyanine form. This is studied by femtosecond three-color

Full text of DOI:10.1021/ja1062746

Published on Web 11/03/2010
Ultrafast Bidirectional Photoswitching of a Spiropyran
†
†
†
†
Johannes Buback, Martin Kullmann, Florian Langhojer, Patrick Nuernberger,
‡
‡
,†
Ralf Schmidt, Frank W u¨ rthner, and Tobias Brixner*
Institut f u¨ r Physikalische und Theoretische Chemie and Institut f u¨ r Organische Chemie,
UniVersit a¨ t W u¨ rzburg, Am Hubland, 97074 W u¨ rzburg, Germany
Received July 15, 2010; E-mail: brixner@phys-chemie.uni-wuerzburg.de
Abstract: We report on bidirectional photochemical switching of 6,8-dinitro-1′,3′,3′-trimethylspiro[2H-1-
benzopyran-2,2′-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open mero-
cyanine form. This is studied by femtosecond three-color pump-repump-probe experiments. Both ring
opening and ring closure are photoinduced. Completion of an entire cycle, consisting of opening and
subsequent closure, can be achieved within 40 ps. A much shorter time (<6 ps) is needed for the converse
cycle, consisting of initial ring closure and subsequent ring opening. Furthermore, we perform pump-probe
experiments with ultraviolet/visible pump and visible/mid-infrared probe pulses for an unambiguous
spectroscopic identification of the open and closed molecular forms. Following visible excitation of the
ring-open molecules, ultrafast ring closure is observed directly in the mid-infrared. The quantum efficiencies
for ring opening and ring closure starting from the respective equilibirum states are determined to be
approximately 9% and 40%. These results show that 6,8-dinitro-BIPS is an ultrafast bidirectional molecular
switch exhibiting a high quantum efficiency.
Introduction
Table 1. Comparison of Quantum Efficiencies of Exemplary
Molecular Switches Based on a 6π-Electrocyclic Reaction
Photochromic systems are defined by a controllable light-
induced change of color. They have a vast variety of applica-
quantum efficiency, %
e
e
1-3
derivative of
ring closure (trans f cis)
ring opening (cis f trans)
tions, from sunglasses to data storage.
Among the molecular
1
1
5
5
,
,
a
b
dithienylethene
dithienylethene
50
86
28
20
25
40
<2
0.15
14
12
50
9
switches, those with a reversible ring closure have drawn much
attention due to well-separated absorption bands of the two
isomers. Since the usability of photochromic molecular switches
depends further on the switching time scales and the respective
quantum yields for both directions, for an optimization of
molecular switches it is essential to understand the participating
photoreactions on a molecular level. Therefore, ultrafast studies
1
,
6 c
indolylfulgimide
17
furylfulgide
azobenzene
1
8
d
6,8-dinitro-BIPS
a
Bis(2,4-dimethyl-5-phenylthiophen-3-yl)perfluorocyclopentene substitu-
ted with benzonitrile: 50% not capable of ring closure. Polymer of
b
4
,5
have been performed, e.g., on dithienylethene derivatives,
fulgides and fulgimides, and spiropyrans.
bis(2,4-dimethyl-5-phenylthiophen-3-yl)perfluorocyclopentene: isomer not
6
,7
8-14
c
Table 1 shows
capable of ring closure suppressed in polymer. Isomer not capable of ring
d
e
quantum yields for dithienylethenes, fulgides, and fulgimides.
The quantum yields of the ring-closing and ring-opening
reactions can be chemically tuned. Either ring closure can be
achieved with very high quantum yield at the cost of the quantum
efficiency of ring opening, or vice versa. Typically though, the
achievable molecular quantum yield is higher for ring closure than
for ring opening (Table 1). Also, in some cases the reaction rates
are slow due to triplet-state pathways. However, for switching
applications, good quantum yields and high speed are desired for
switching in both directions. For spiropyrans, only photochemical
closure: < 3% Result of this work, see Discussion. Quantum efficiencies
refer to trans/cis isomerization for the case of azobenzene and to ring
closure/opening for all other examples.
8,9,13,14
ring opening has been directly demonstrated so far.
In this
work, we show that a suitable spiropyran system can be switched
photochemically in both directions with competitive quantum yields
on an ultrafast time scale.
The ring-closed form of a spiropyran (Figure 1, left structure)
consists of a pyran and an indoline moiety with orthogonal
1
9
orientation. Thus, both moieties are independent and absorb
in the ultraviolet only. Ring opening completely changes the
†
Institut f u¨ r Physikalische and Theoretische Chemie.
Institut f u¨ r Organische Chemie.
‡
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Braun, K.; Braun, M.; Zinth, W.; Gilch, P. J. Am. Chem. Soc. 2007,
129, 8577–8584.
(
1) D u¨ rr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems;
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10.1021/ja1062746  2010 American Chemical Society

Bidirectional Photoswitching of a Spiropyran
A R T I C L E S
Furthermore, although sketched in most spiropyran reaction
schemes in the literature, there is no direct proof that the
completely relaxed ring-open form can be switched back
10
photochemically at all. Only for the substituted compound with
a second nitro group in the 8-position (6,8-dinitro-BIPS) has it
been shown that irradiation with 400 nm light leads to a bleach
of the visible absorption band on the picosecond time scale.
1
2
This indirectly indicates formation of the ring-closed form.
In this work, we present the first direct evidence for pho-
tochemical switching of a spiropyran in both directions. We
investigate the bisubstituted 6,8-dinitro-BIPS and perform
pump-probe transient absorption experiments in the ultraviolet,
visible, and mid-infrared spectral regions. This combination of
measurements allows a direct observation of the product
formation. We then carry out three-pulse pump-repump-probe
Figure 1. Spiropyran and merocyanine main isomers (TTC, describing
the configuration around the three central double bonds) of 6,8-dinitro-
BIPS and their corresponding steady-state absorption spectra in chloroform.
The ring-closed spiropyran can be opened with light from the ultraviolet
spectral region, whereas the merocyanine can be closed with light from
either the ultraviolet or the visible spectral region. The spiropyran spectrum
was obtained after almost complete merocyanine bleaching in the spectrometer.
experiments, a scheme used to modify the course of a photo-
7,29-32
induced reaction for additional insight,
demonstrating the
complete open-closed-open as well as closed-open-closed
photocycle. The organization of the manuscript is as follows:
In the Experimental Section, we introduce our transient absorp-
tion setups and illustrate how the three-color pump-re-
pump-probe data are obtained. Steady-state measurements and
two-color pump-probe experiments on the ring-closing and the
ring-opening reactions are presented first in the Results section.
Those results represent a basis for understanding the three-color
pump-repump-probe data in the ultrafast switching subsection
at the end of the section. The insights obtained on the
photochemical ring-closing and ring-opening reactions are then
discussed and compared to results on related systems from the
literature. Finally, a summary of the photochemistry of the
spiropyran-merocyanine system is given.
molecule’s electronic and structural properties. In the ring-open
form (merocyanine, Figure 1, right structure), the two chro-
mophores are positioned such that one large planar π-system
emerges, leading to a strong absorption in the visible spectral
2
0
region. Hence, the angle between the chromophores is the
relevant switching reaction coordinate. The electronic changes
due to switching make spiropyrans potential candidates for
21
optical memories, whereas the structural changes could be used
as a switch of molecular or even biological properties, enhancing
2
2-24
or stopping processes.
The photochemical and thermal ring opening of several
spiropyrans has been investigated on the second to femtosecond
time scale. It has been shown that, for unsubstituted spiropyran
Experimental Section
(
1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline] ) BIPS),
6,8-Dinitro-BIPS was synthesized via the Knoevenagel conden-
sation reaction of commercially available 1,2,3,3-tetramethyl-3H-
indolium and 3,5-dinitrosalicylaldehyde, resulting in the merocy-
anine form of 6,8-dinitro-BIPS in 79% yield according to the
literature and characterized by HPLC, 400 MHz NMR spectros-
copy, and high-resolution mass spectrometry (see Experimental
Details below).
8
,9,25
the photoinduced reaction is fast (28 ps) but inefficient.
The efficiency is strongly enhanced if a nitro group is substituted
in the 6-position of the pyran moiety (6-nitro-BIPS). However,
for such a substituted compound, the reaction time scale
1
1
3,26-28
increases to nanoseconds due to a triplet pathway.
Steady-state absorption spectra were recorded in a 2 mm Suprasil
cuvette with a Hitachi U-2000 spectrophotometer. Merocyanine spectra
were recorded directly after the crystals were dissolved in chloroform.
(
(
(
(
11) Fidder, H.; Rini, M.; Nibbering, E. T. J. J. Am. Chem. Soc. 2004,
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The femtosecond laser system used for these experiments consists
of a home-built Ti:sapphire oscillator and a home-built regenerative
amplifier that generates pulses centered at 800 nm at a repetition rate
of 1 kHz with a duration of 80 fs full width at half-maximum (fwhm).
The visible (VIS) pump pulses were generated in a home-built non-
collinear optical parametric amplifier (NOPA), resulting in 2 µJ pulses
with about 50 fs fwhm duration tunable across the VIS spectral range.
The 800 nm light was also used to generate 400 and 267 nm pump
pulses using second-harmonic and sum-frequency generation in
nonlinear crystals. The energy of the 400 nm pulses was reduced to
2
003, 376, 214–219.
14) Holm, A.; Mohammed, O. F.; Rini, M.; Mukhtar, E.; Nibbering,
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(
18) Satzger, H.; Root, C.; Braun, M. J. Phys. Chem. A 2004, 108, 6265–
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271.
1
2 µJ in the mid-infared (MIR) experiments, and the energy of the
(
(
19) Tyer, N. W.; Becker, R. S. J. Am. Chem. Soc. 1970, 92, 1289–1294.
20) Heiligman-Rim, R.; Hirshberg, Y.; Fischer, E. J. Phys. Chem. 1962,
collinearly created 267 nm pulses was reduced to 1 µJ. The VIS and
6
6, 2465–2470.
(
21) Jiang, G.; Song, Y.; Guo, X.; Zhang, D.; Zhu, D. AdV. Mater. 2008,
2
0, 2888–2898.
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25) Aramaki, S.; Atkinson, G. H. J. Am. Chem. Soc. 1992, 114, 438–444.
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J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010 16511

A R T I C L E S
Buback et al.
the ultraviolet (UV) pump pulses can be delayed independently by
∼
3.7 ns via two motorized delay stages.
We have used different sources for the probe pulse in order to
cover wavelength regions from the MIR to the VIS spectral regime.
The MIR probe was generated in a home-built optical parametric
3
3,34
amplifier described elsewhere.
region, a small fraction of the residual 800 nm pump light leaving
the NOPA was focused in a linearly moving CaF disk, generating
a white-light continuum from 390 to 750 nm. Colored filter glass
Schott BG 40) was used to cut off the 800 nm pump light before
For probing in the VIS spectral
2
35
(
the sample. The probe light was spectrally dispersed by a spec-
trograph (Acton SP2500i) and digitized by a charge-coupled device
(
CCD) chip (Princeton Instruments Pixis 2K). By reading out the
CCD chip in a special way, high frame rates of 1 kHz were
achieved, making shot-to-shot measurements with the CCD chip
possible and a separate reference beam unnecessary. The wavelength
resolution of the VIS probe setup is ∆λ ) 1.4 nm, and the temporal
resolution is better than 50 fs.
For the pump-probe experiments, pump and probe pulses were
focused individually and non-collinearly overlapped spatially in a
Figure 2. Scheme of the pump-repump-probe experiments. Whereas the
UV pump (purple) and the VIS probe beam (red) remain at fixed positions
(tUV and tWL, respectively), the delay of the VIS pump (green) (tVIS) is varied
as indicated by the dashed green arrow. This results in the three different
delay regions with different pulse sequences (A, B, and C) shown at the
bottom.
2
00 µm flow cell. In the experiments with a VIS probe, the pump
diameter in the focus was 40 µm, and the probe beam diameter
was 15-30 µm (depending on the wavelength). In the MIR probe
experiments, the probe beam was substantially larger, with a
diameter of 400 µm, and therefore the pump pulse’s diameter was
increased to 450 µm. The angle between the linear polarizations
of pump and probe pulses was set to the magic angle (54.7°) by
turning the pump polarization with achromatic waveplates. For the
experiments with 267 nm pump pulses, the solution in the reservoir
was perpetually irradiated with an array of green light-emitting
diodes (LEDs) to shift the equilibrium to the spiropyran form.
For the pump-repump-probe experiments, the solution was also
converted to the spiropyran form with green LEDs. Pump and
repump pulses were polarized orthogonal to each other. In order
to isolate the dynamics induced by the pump-repump combination
from the single-pump dynamics, two optical choppers with 90°
phase difference were employed to block two consecutive pulses
and let the next two pass. Since this was done for both pump pulses,
this procedure gives rise to four pump-pulse combinations and
corresponding transmitted probe light intensities:
Figure 2 illustrates the scanning scheme used in the pump-
repump-probe experiments. The green pulse in this scheme
represents the VIS pump pulse that induces ring closure of present
merocyanine. The purple pulse represents the UV pump pulse that
mostly induces ring opening of spiropyran since almost no
merocyanine is present due to the constant illumination with the
LED array. The red pulse represents the white-light (WL) probe
pulse. UV and WL are both fixed in time and were set to 0 and
1
400 ps, respectively, on the time delay axis ∆t
For investigating both switching directions, the VIS pulse is
scanned from before the UV pulse to after the WL pulse (∆t
1.7 ns f ∆t ) +2 ns). Thus, the VIS pulse coincides with the
UV pulse at ∆t ) 0 ps and with the WL pulse at ∆t ) 1400 ps.
2
.
2
)
-
2
2
2
This gives rise to three delay regions, A, B, and C, with the different
pulse sequences given at the bottom of Figure 2.
Experimental Details: (Z)-2,4-Dinitro-6-[(E)-2-(1,3,3-trimethyl-
1,3-dihydroindol-2-ylidene)ethylidene]cyclohexa-2,4-dienone(1).1,2,3,3-
Tetramethyl-3H-indolium iodide (4.27 g, 14.2 mmol) was dissolved
in dry EtOH (150 mL) under an argon atmosphere. Piperidine was
added at 40 °C, and the solution was refluxed for 15 min. 3,5-
Dinitrosalicylaldehyde (3.00 g, 14.2 mmol) in dry EtOH (150 mL)
was added at room temperature, and the resulting mixture was
refluxed for 3 h. The solvent was removed under reduced pressure,
and the crude product was purified by column chromatography using
1
2
3
4
. VIS pump (IVIS
. VIS pump and UV pump (IUV+VIS
. UV pump (IUV
. no pump (Ino pump
)
)
)
)
Here, the transmitted probe intensities for a certain pump-probe
combination are denoted with I and the respective subscript. From
this we obtain three transient absorption signals (optical density
changes, ∆OD):
silica and CH
CH Cl /hexane and subsequent recrystallization from EtOH afforded
pure 1 as a greenish red crystalline powder (4.12 g, 11.2 mmol,
9%). Purity of the compound >99% was verified by HPLC (normal
2 2
Cl /MeOH ) 50:1 vol % as eluent. Precipitation from
2
2
IVIS
7
1
2
3
. ∆ODVIS ) -lg
Ino pump
1
phase, CHCl
(400 MHz, CDCl ): δ 8.52 (1 H, d, J ) 2.7 Hz, ArH), 8.11 (1 H,
3 4
3
/hexane 85:15). Mp: 262-265 °C (decomp). H NMR
4
IUV
3
4
. ∆ODUV ) -lg
d, J ) 2.8 Hz, ArH), 7.13 (1 H, td, J ) 7.8 Hz, J ) 1.3 Hz,
Ino pump
3
3
ArH), 7.03 (1 H, d, J ) 7.8 Hz, ArH), 6.96 (1 H, d, J ) 10.6 Hz,
IUV+VIS
3
4
CHmethine), 6.83 (1 H, td, J ) 7.4 Hz, J ) 1.0 Hz, ArH), 6.52
. ∆ODUV+VIS ) -lg
3
3
Ino pump
(1 H, d, J ) 7.2 Hz, ArH), 5.97 (1 H, d, J ) 10.5 Hz, CHmethine),
.69 (3 H, s, CH ), 1.28 (3 H, s, CH ), 1.13 (3 H, s, CH ). HRMS
ESI, positive mode, CH CN/CHCl ) 1:1 vol %): calcd, m/z
, 368.12410; found, 368.12410 ([M + H] ).
2
(
3
3
3
3
3
Recording all three combinations of transient absorption signals,
the cooperative absorption signal due to interactions with both pump
pulses (∆OD2p) is obtained via
+
19 18 3 5
C H N O
Results
Steady-State Spectra. The ring-open merocyanine form and
the ring-closed spiropyran form of 6,8-dinitro-BIPS differ
strongly in their absorption spectra (Figure 1). In contrast to
∆
OD2p ) ∆ODUV+VIS - ∆ODUV - ∆ODVIS
(1)
3
(
(
33) Hamm, P.; Kaindl, R. A.; Stenger, J. Opt. Lett. 2000, 25, 1798–1800.
34) Wolpert, D.; Schade, M.; Brixner, T. J. Chem. Phys. 2008, 129,
the majority of spiropyran-merocyanine systems, including
the unsubstituted and the 6-nitro-substituted variants, the
predominant form in thermodynamic equilibrium of 6,8-dinitro-
BIPS is the merocyanine, possessing a strong absorption in the
0
94504.
(
35) Nagura, C.; Suda, A.; Kawano, H.; Obara, M.; Midorikawa, K. Appl.
Opt. 2002, 41, 3735–3742.
1
6512 J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010

Bidirectional Photoswitching of a Spiropyran
A R T I C L E S
Figure 4. Single transients from Figure 3 at 633, 572, and 502 nm (top to
bottom). A global fit routine (red lines), obtained by fitting each individual
transient with two global time constants, results in τfast ) 98 ( 3 ps and
Figure 3. Spectrally resolved transient absorption data covering the visible
spectral range, recorded with 560 nm pump pulses. The blue regions denote
decreased absorption, thus revealing a permanent bleach at 560 nm and
stimulated emission at 630 nm, whereas at 445 nm excited-state absorption
τ
slow ) 830 ( 120 ps and the respective amplitudes afast and aslow for each
wavelength.
(
yellow-red) dominates the spectrum.
VIS range (540-610 nm). In chloroform the absorption band
Figure 1) is slightly red-shifted (λmax ) 559 nm) in comparison
(
1
2
to that in more polar solvents, which suggests a prevalence
3
6
of a zwitterionic ground state. Upon irradiation with green
light, the deeply purple merocyanine solution fades quickly. The
absorption spectrum of the formed (ring-closed) spiropyran
shown in Figure 1 exhibits two maxima (λ ) 340 nm and λ )
2
60 nm). The thermal ring opening occurs on a time scale of
minutes. After the spiropyran solution is illuminated with UV
light (267 nm), the solution turns colored again due to
merocyanine formation. Only slightly more than 50% of the
initial merocyanine concentration can be recovered in the
photostationary equilibrium because the formed merocyanine
also ring-closes upon 267 nm excitation.
Ultrafast Ring Closure. In order to prove that 6,8-dinitro-
BIPS can be photochemically switched on an ultrafast time scale,
we will first concentrate on the ring-closure direction. Transient
absorption with VIS pump pulses will demonstrate that the ring-
closed product is formed on a picosecond time scale after
merocyanine excitation. The reasoning is structured in two parts.
First we will prove that merocyanine is photobleached by
probing in the VIS region. Then we will show that the ring-
closed product is formed by probing in the MIR.
Figure 5. Merocyanine steady-state spectrum (green) in comparison with
the difference spectrum (blue dashed) after 3.4 ns of the ring-closure
experiments in the visible. The overall shape is quite similar. Thus the bleach
can be assigned to reacted merocyanine. The small differences at the edges
of the absorption band can be explained by changes in the distribution of
3
7
at least two merocyanine isomers in the solution.
merocyanine absorption in Figure 5 exhibits only small devia-
tions at the band edges.
b. Mid-Infrared Probing. Since the produced spiropyran does
not absorb in the VIS spectral region, the photobleach from
our VIS probing experiments only indirectly suggests that
spiropyran is formed. By contrast, we find direct evidence for
the formation of spiropyran by analyzing the photoreaction in
the MIR spectral region. For the structurally related 6-nitro-
BIPS, it is known that ring closure shifts the symmetrical
stretching mode of the nitro group to slightly higher frequencies
a. Visible Probing. After photoexcitation of 6,8-dinitro-BIPS
with a laser pulse centered at 560 nm, we observe the temporal
and spectral behavior depicted in Figures 3 and 4. The spectral
distribution consists of a strong bleach centered at 560 nm, excited-
state absorption around 450 nm, and stimulated emission above
5
90 nm. This is similar to the behavior measured after 400 nm
12
3
8
excitation of 1′-(2-carboxyethyl)-6,8-dinitro-BIPS. The overall
kinetics after 20 ps are best described by a sum of two
exponentials and an offset, where the same fast time constant
and the absorption band becomes much stronger. The top left
panel of Figure 6 shows a difference FTIR spectrum after
2
min of illumination of the 6,8-dinitro-BIPS merocyanine with
(
1
4
τ
fast ) 98 ( 3 ps) and slow time constant (τslow ) 830 (
20 ps) are used for all wavelengths (see resulting fit in Figure
). The dynamics during the first 20 ps are partly ascribed to
green continuous-wave (CW) light. The result is a similar shift
and intensity change of the absorption as observed for 6-nitro-
BIPS. Therefore, we used this absorption change and shift in
the transient absorption of 6,8-dinitro-BIPS as direct spectro-
scopic indications of ring closure.
1
1
vibrational cooling and therefore are ignored in the global fit
routine.
After a pump-probe delay of 3.4 ns, the ultrafast dynamics
are essentially over. It can be seen that the remaining bleach
after 3.4 ns is very similar to the merocyanine absorption band
in Figure 1. Direct comparison of the bleach at 3.4 ns and the
The transient absorption map for 6,8-dinitro-BIPS shown in
the bottom left panel of Figure 6 was obtained after pumping
with 400 nm light and probing between 1310 and 1360 cm .
-
1
(
37) Hobley, J.; Malatesta, V.; Millini, R.; Montanari, L.; Parker, W. O. N.,
(
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Int. Ed. 2008, 47, 4529–4532.
Jr. Phys. Chem. Chem. Phys. 1999, 1, 3259–3267.
(38) Schiele, C.; Arnold, G. Tetrahedron Lett. 1967, 8, 1191–1195.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010 16513

A R T I C L E S
Buback et al.
Figure 6. IR spectroscopy of 6,8-dinitro-BIPS. (Top left) FTIR difference spectrum after 2 min of irradiation of merocyanine with green CW light. Product
-
1
-1
absorption can be seen above 1335 cm , whereas reactant bleach can be seen below 1335 cm . (Bottom left) Transient absorption map of the same spectral
-
1
-1
region (∆ ν˜ ) 1.8 cm ) with a pump wavelength of 400 nm, showing merocyanine bleach and recovery centered from 1327 to 1346 cm and the partially
-
1
-1
overlapping spiropyran formation at 1344 cm (from 1336 to 1351 cm ). (Right) Single transients of the left map (blue, red, green), with the transients
obtained via the biexponential global fit routine in black (τ ) 12 ( 10 ps, τ ) 86 ( 5 ps). All transients are dominated by the merocyanine ground-state
1
2
bleach recovery (τ ) 98 ps in the VIS, see Figure 4, and τ ) 86 ps in the MIR, deviations due to product formation). Spiropyran formation is proven in
agreement with the FTIR spectrum by the remaining positive absorption at 1344 cm-
1
.
The main features of the map are a merocyanine bleach (from
-
1
1
327 to 1346 cm ) and an additional slightly blue-shifted
-
1
absorption band centered at 1344 cm (band from 1336 to
351 cm ) that can be attributed to spiropyran formation due
-1
1
to the similarities of the dynamics to those of the structurally
related 6-nitro-BIPS and the FTIR difference spectrum. We also
performed this experiment with pump pulses centered at
5
60 nm, resulting in the same characteristics.
All dynamics seen in the transient map decay during the first
00 ps. A global fit routine with two exponential decays and
5
an offset, taking into account only data points for times greater
than 2 ps, results in time constants of τ ) 12 ( 10 ps and τ
86 ( 5 ps. The time constant of 98 ps for the merocyanine
ground-state bleach recovery that was measured in the VIS
spectral range is in close agreement with the time constant of
1
2
)
Figure 7. Transient absorption spectrum of the ring opening with 267 nm
pump and visible probe. After excitation, a positive absorption band emerges
at 450 nm and decays with increasing time. Around 550 nm, different
absorption bands associated with merocyanine structures increase with time.
8
6 ps obtained from these measurements in the MIR. We
attribute the second time constant of 12 ps to vibrational cooling
1
1
of either the merocyanine or spiropyran ground state.
overlaid with the bleach recovery and the characteristic decay
changes from 98 to 86 ps, we conclude that spiropyran formation
is faster than merocyanine ground-state bleach recovery.
Ring Opening. Having shown that the ring closure is an
ultrafast reaction, we now concentrate on the other switching
direction, the ring opening. In these experiments the spiropyran
isomer is created by illumination of the reservoir with green
LEDs. Figure 7 shows the VIS transient absorption after
excitation with 267 nm laser pulses. Nonlinear optical processes
and two-photon absorption in the solvent strongly contribute
to the dynamics during the first picosecond after excitation. This
prevents a simple analysis of data from these early times. After
the decay of the initial dynamics, a broad transient absorption
band covering the whole VIS spectral range from 400 to
700 nm (λmax ) 500 nm) (Figure 7) is apparent. The difference
spectra at 500 and 2600 ps (see Figure 8a) reveal that the
absorption evolves into a spectrum with distinguishable shoul-
ders at 445, 525, and 560 nm (black arrows) and a flat positive
The spiropyran formation, on the other hand, can be seen in
the individual transients displayed in the three panels on the
right side of Figure 6. The obtained global fit transients are
shown in black to prove that all transients behave very similar.
As mentioned above, they all start with a negative absorption
change due to missing merocyanine ground-state absorption.
The dynamics thereafter are dominated by the corresponding
merocyanine ground-state bleach recovery, with only small
deviations at early times due to vibrational cooling. Nevertheless,
they differ in the remaining absorption change for delays
500 ps. The product formation is best seen in this remaining
absorption: the absorption change at 1344 cm is initially
negative, owing to the bleach, and becomes positive after
00 ps, due to the product formation. If there were no absorption
of the product at this position, a negative absorption change
would remain here, too, as seen in the transients at 1319 and
>
-1
1
-
1
1
331 cm . Since absorption due to spiropyran formation is
1
6514 J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010

Bidirectional Photoswitching of a Spiropyran
A R T I C L E S
Figure 8. (a) Difference spectra of the ring opening at 500 and 2600 ps
after excitation of the spiropyran form with 267 nm pulses. The development
of the shoulders at 445, 525, and 560 nm is indicated by the black arrows.
Further, an isosbestic point is identified at 485 nm. (b) Transients at two
Figure 9. Transient absorption signal (∆OD2p) at 560 nm (averaged from
550 to 570 nm) due to the combination of the two pump pulses. The transient
absorption can be divided into the three regions associated with the different
pulse orderings explained in Figure 2 (A, closure-reopening; B,
opening-reclosure; C, no pump-repump signal). (Top) Signal from -1.7
to 2 ns. The positions of the UV pulse and the WL probe pulse are indicated
by purple and red vertical lines, respectively. (Bottom) Measurement with
higher temporal resolution for the transition from region A to region B.
Here, the signal dynamics at the beginning of region B are emphasized.
The black line corresponds to a monoexponential fit that results in a time
constant of 34 ps.
(
445 nm, red open circles; 525 nm, blue dots) of the three shoulders of the
absorption band of Figure 7 and at the isosbestic point (green crosses) and
a monoexponential fit (τ ) 36 ps, black).
offset above 620 nm. While the absorption decreases at
4
45 nm, it increases at 525 and 560 nm. An isosbestic point
can be found at 485 nm. It should be noted that the shape and
wavelength do not match the steady-state merocyanine absorp-
tion band in Figure 1 completely. For a more quantitative
analysis of the dynamics, the transient absorptions at different
wavelengths are plotted in Figure 8b. The data after 500 ps show
increasing absorption at 525 nm (blue dots) and decreasing
absorption at 445 nm (red circles). In contrast, the isosbestic
point at 485 nm (green crosses) shows no dynamics after an
initial fast decay. A fit of a single exponential and an offset to
the data (black line) yields a time constant of 36 ps for this fast
decay. The bidirectional switching results from the next section
will help to interpret the dynamics found in the transient data
of Figure 8. Hence, the interpretation is deferred to the end of
the next section. There it will be shown that the broad VIS
absorption band in Figures 7 and 8 is at least partly due to
merocyanine in its electronic ground state.
over the wavelengths from 550 to 570 nm with respect to the
delay time axis ∆t in Figure 2. Note that the stronger noise
2
compared to the two-pulse experiments from, e.g., Figure 3 is
due to the fact that the cooperative three-pulse signal is 2 orders
of magnitude lower (∼0.4 mOD compared to ∼20 mOD).
Nevertheless, the measurement technique described in the
Experimental Section allows recovery of this transient with
excellent sensitivity.
Zero signal in Figure 9 corresponds to a situation where the
combination of both pump pulses yields the same result as the
sum of the signals of the two individual pump-probe experi-
ments (see eq 1). A positive value means that, due to the
combination of the two pump pulses, a higher absorption is
present at the time of the WL pulse. In the relaxed system
Ultrafast Switching. Whereas ring-closed, unsubstituted BIPS
can be opened to the ring-open ground state in a few tens of
(excluding reaction intermediates and thereby the dynamics seen
8
in the bottom of Figure 9), such a positive value can only result
from a higher spiropyran concentration at the time of the second
pump pulse (which then induces more ring opening than in the
individual experiment). Vice versa, a negative value can be
explained by a spiropyran concentration decrease, e.g., a
merocyanine concentration increase, at the time of the second
pump pulse (which then induces more ring closure than in the
individual experiment). Therefore, for an intuitive interpretation
of Figure 9, one can say that the transient absorption corresponds
to the change of the spiropyran concentration due to the first
pulse at the time of the second pulse.
picoseconds, it has been shown that the 6-nitro-BIPS is
transferred into a long-lived merocyanine triplet state within a
1
3,26
few picoseconds after excitation.
The question is therefore
how the bisubstituted 6,8-dinitro-BIPS investigated in this work
reacts and on what time scale it can be switched back and forth.
Our approach to showing that the reaction product after
excitation of the closed form, leading to the open form of 6,8-
dinitro-BIPS, is generated in its ground state is re-excitation of
the product. The experiments with direct excitation of the ring-
open compound serve as a reference for the re-excitation
7,39
experiments. For this purpose, pump-repump-probe experi-
ments were conducted as described in the Experimental Section.
a. Bleach. In Figure 9, we show the transient cooperative
absorption signal ∆OD2p(560 nm), as defined in eq 1, averaged
In region A, a small but nevertheless clearly positive
absorption is visible, corresponding to a positive spiropyran
concentration change. In this region of the time delay, the VIS
pulse comes before the UV pulse. Since the used LED array
does not completely close all merocyanine, the VIS pulse closes
some of the small portion that is still present in the solution
and thereby increases the spiropyran concentration by ∼1/50
(
39) Larsen, D. S.; Papagiannakis, E.; van Stokkum, I. H. M.; Vengris,
M.; Kennis, J. T. M.; van Grondelle, R. Chem. Phys. Lett. 2003, 381,
7
33–742.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010 16515

A R T I C L E S
Buback et al.
The blue curves in Figure 10 are taken from the delay region
B (averaged from 400 to 900 ps). In region B the spectral shape
of ∆OD2p (solid) is similar to the shape of ∆ODVIS (dashed).
This confirms our previous assignment of the underlying
processes.
The red curves were also taken from delay region B, however
at greater delay times such that the VIS pump is close in time
to the WL probe (averaged from 1200 to 1400 ps). The general
features that can be identified in the cooperative three-pulse
signal ∆OD2p (red solid line in Figure 10) correspond to the
dynamics in the transient map for the two-pulse ring closure of
merocyanine (Figure 3), and thus also the difference spectrum
(
red dashed line in Figure 10) agrees. These dynamics are
excited-state absorption (400-500 nm), ground-state bleach
Figure 10. Comparison of the absorption change of the pump-repump
combination (∆OD2p, solid lines) to the absorption changes of the single-
pump experiments (∆ODUV and ∆ODVIS, dashed lines). The data are scaled
for better comparison. To decrease the noise, the difference spectra were
averaged over 20 delay positions. The signals ∆OD2p and ∆ODUV, averaged
over the first nanosecond of region A, are shown in green. Spectra ∆OD2p
and ∆ODVIS of region B are averaged from 400 to 900 ps and shown in
blue. The red curves represent the signals ∆OD2p and ∆ODVIS at later times
of region B (delay times from 1200 to 1400 ps).
(550-600 nm), and stimulated emission (600-650 nm).
c. Speed. The experiment with higher temporal resolution of
the transition from region A to region B shown in the bottom
of Figure 9 allows us to determine the speed of merocyanine
as well as spiropyran formation.
Ring-Opening: AfB. Beginning with the ring-opening reac-
tion, i.e., merocyanine formation, we look at the dynamics of
(
∆ODUV ≈ 5 mOD, ∆OD2p ≈ 0.1 mOD). The dynamics of the
absorption change at 560 nm due to this ring closure are the
same as those recorded in the two-pulse experiment (∆ODVIS
the transition from region A to region B. Initially (at ∆t ) 0
2
in Figure 9), a sharp peak can be detected in the three-pulse
transient. Within approximately the first 100 ps after this sharp
peak, a decrease from positive to negative absorbance is
recorded. Whereas in delay region A the first pulse (VIS)
decreases merocyanine concentration, in delay region B the first
pulse (UV) increases merocyanine concentration. Therefore, the
negative slope of the absorption change is expected. The
decrease toward negative absorption change does not start at
,
see Figure 3). Therefore, these dynamics are canceled in the
cooperative signal ∆OD2p. However, the following UV pulse
acts on an increased spiropyran concentration (∼1/50 higher),
resulting in a slightly higher merocyanine production than
recorded by ∆ODUV and a small positive offset remains when
probing the effects of both pump and repump pulses.
Changing the pulse sequence to the delay region B (UV pulse
before VIS pulse), the UV pulse mostly excites the spiropyran,
since on the one hand the equilibrium is almost completely
shifted to this form and on the other hand at 267 nm the
extinction coefficient of the spiropyran is 2.5 times higher than
that of the merocyanine. Again, the absorption change due to
this process is not visible in ∆OD2p since it is the same as in
∆OD2p ) 0 for ∆t ) 0 because reaction intermediates or hot
2
spiropyrans are formed after excitation of the spiropyran. For
comparison see Figure 7 during the first 100 ps from 420 to
570 nm and Figure 8b. These intermediates absorb the VIS pulse
without being able to reclose afterward. This corresponds to an
effective reduction of VIS pump power that applies only to
∆ODUV+VIS and not to ∆ODVIS. Since ∆ODVIS at 560 nm is
negative, the subtraction of ∆ODVIS from ∆ODUV+VIS leads to
a positive ∆OD2p. Note also that the transient signal from Figure
9 cannot be explained by a dominating remaining merocyanine
triplet state. Such a merocyanine triplet state would have similar
effects to the ones observed for times slightly greater than zero
in Figure 9. Hence, the signal in delay region B would always
be positive.
∆
ODUV. However, the following VIS pulse acts on a higher
merocyanine concentration and therefore bleaches more of the
merocyanine compared to ∆ODVIS. This explains the negative
absorption change in delay region B of Figure 9.
When the VIS pump reaches the sample after the probe pulse
delay region C), the signal ∆OD2p is equal to zero and only
(
exhibits some noise, because the absorption change ∆ODUV+VIS
is the same as ∆ODUV, whereas ∆ODVIS is zero.
We conclude that the signal in Figure 9 is due to ultrafast
bidirectional switching.
For the following interpretation, we assume that the transient
absorption band from 420 to 570 nm during the first 100 ps
mentioned above in Figure 7 is not due to hot spiropyran
molecules but to reaction intermediates. This assumption is
supported by the fact that the absorption band is not continuous
from the VIS spectral range to the spiropyran absorption in the
UV spectral range but rather subsides at 420 nm. The decay of
these reaction intermediates and the population increase of the
merocyanine ground state have the same time constant. Thus,
b. Spectral Shape. Additional measurements for the spectral
shapes of the absorption changes in the delay regions A and B
are shown in Figure 10 to provide further proof for bidirectional
switching. One has to consider that different isomers/conformers
with different spectral shapes account for small deviations in
the following comparisons, because the conformeric distribution
after excitation is most certainly different from that in thermal
equilibrium.
The thin green line represents the difference spectrum ∆OD2p
of region A (averaged from -1700 to -700 ps). The dashed
green line represents the difference spectrum recorded with the
UV pump only (∆ODUV). Following the arguments given above,
the two transient spectra should be very similar, because the
UV pulse acts on a slightly increased concentration of the
spiropyran. This is indeed reflected by the comparison of
the two green lines in Figure 10.
we interpret the dynamics after ∆t ) 0 as the dynamics of
2
formation of merocyanine in the ground state. A monoexpo-
nential fit, shown as a black line in the bottom panel of Figure
9 starting at the temporal overlap of the two pulses, results in
a time constant of 34 ps for the merocyanine ground-state
formation. These dynamics are in agreement with the time
constant of 36 ps at 485 nm that was determined in Figure 8.
Changing the polarization angle between the two pump pulses
from 90° to 0° did not lead to a change in the observed time
constant.
1
6516 J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010

Bidirectional Photoswitching of a Spiropyran
A R T I C L E S
Ring Closure: BfA. An upper boundary for the ring closure,
i.e., spiropyran formation, reaction time can be extracted from
the transition from region B to A. In Figure 9, the signal is
b. Quantum Efficiency. The quantum efficiency of a photo-
reaction, Φ, can be calculated from the initial bleach, I
i
, and
the remaining bleach, I
e
, of an absorption that is characteristic
nearly constant (∼0.1 mOD) up to ∆t
2
) -6 ps. When the pump
for the reactant:
pulses cross (crossing at ∆t ) 0 from left to right in Figure 9),
2
the absorption changes abruptly. Spiropyran formation is
therefore very fast considering the large conformational change.
This very fast reaction, whose signal is also obscured by
vibrational cooling, does not allow an identification of putative
intermediate merocyanine conformers. Nevertheless, the high
speed of the ring closure is in good agreement with the MIR
transient absorption experiments (Figure 6), which disclosed that
ring closure is faster than the bleach recovery.
I
e
Φ )
(2)
Ii
We can apply this equation here, although vibrational cooling
and overlap with product bands might affect the results. Hence
the formula provides an estimate. In the present case we find
4
1
6% quantum efficiency for ring closure from the transient at
-1
322 cm in Figure 6.
In the VIS spectral region, bleach recovery is also observed
Figure 3), but here isomerization processes might have an
Discussion
(
influence on the result of eq 2. According to Figure 4, the slow
component is zero around 572 nm. If we assume that the bleach
in this spectral region is not superimposed by an excited-state
absorption, we obtain a quantum efficiency of 40% for isomer-
ization and ring closure. Since we do not observe significant
isomerization in the difference spectrum in Figure 5, the bleach
is mostly due to ring closure. The quantum efficiency of 40%
thus obtained from the experiments probing in the VIS spectral
range is in agreement with the 46% obtained from the experi-
ments with MIR probing.
Ring Closure. It is known that the investigated merocyanine
does not aggregate except in aliphatic solvents due to the steric
bulkiness of the 3,3′-dimethylmethylene unit of the electron-
3
7,40
donating indoline heterocycle.
We therefore exclude the
possibility that dimer aggregates could be responsible for any
observed signal. The two time constants (98 and 830 ps)
observed when pumping and probing in the VIS spectral region
are caused by two merocyanine isomers that coexist in thermal
equilibrium. The existence of two isomers in polar solvents has
3
7
been shown by NMR investigations in different solvents.
Ring Opening. For a better understanding of the dynamics
after UV excitation of ring-closed 6,8-dinitro-BIPS, we first
briefly review the known processes for the most investigated
similar spiropyrans, 6-nitro-BIPS and unsubstituted BIPS. For
These studies concluded that the two isomers involved are the
TTC and TTT isomers. Merocyanine isomers are named after
the trans-cis configuration of the methine bridge; see the
labeling in Figure 1. Participation of triplet states is excluded
1
2
6
-nitro-BIPS, it was found that the triplet reaction plays a crucial
because the photoreaction is too fast for a triplet pathway.
4
3
role and rivals a possible singlet route. It has been proposed
that, following S excitation of the closed form, a rather complex
succession of reaction intermediates takes place on the micro-
CASSCF simulations have shown that a pure singlet reaction
pathway via two conical intersections exists for the ring
1
4
1
closure.
4
3
second and millisecond time scales. At 435 nm a slowly
subsiding transient absorption has been found, and at 580 nm a
slowly increasing absorption has been found. These findings
have been assigned to the intersystem crossing of the mero-
cyanine triplet state to the merocyanine ground state. It has been
found that, 500 fs after excitation of the spiropyran in tetra-
chlorethene, the triplet of a vibrationally hot merocyanine isomer
a. Product Identification. To prove that photochemical ring
closure occurs on the picosecond time scale, we will now focus
on the MIR probe data to support our identification of the
product. Experiments on the structurally related, unsubstituted
and 6-nitro-substituted spiropyran compounds were published
8,11,13
by Fidder et al.
By probing the ring-opening reaction with
MIR pulses, they found that the two spiropyran absorption bands
which do not overlap with merocyanine absorption bands are
1
4
is formed. This hot triplet relaxes with a time constant of
17 ps to the vibrational ground state. The cooling process follows
an isomerization to a second isomer in the triplet state. In
contrast to 6-nitro-BIPS, no triplet pathway was found after
photoexcitation of the unsubstituted BIPS. According to transient
MIR absorption experiments, the ring-opening reaction of the
-1
centered at 1340 and 1610 cm . These frequencies are very
close to the vibrational modes assigned to the nitro group in
the 6-position. For the present molecule, 6,8-dinitro-BIPS in
-
1
chloroform, we do not see an isolated band at 1610 cm in the
ring-closure experiments. Instead we observe a strong ground-
state bleach at this spectral position (data not shown). This is
also confirmed by the FTIR data. A spiropyran absorption at
8
unsubstituted BIPS occurs on a time scale of 28 ps. In the gas
phase it has been shown that, after excitation with 267 nm (in
the chromene subunit), a cascade of three reactions for nitro-
-1
1
344 cm , on the other hand, is present in our species and can
4
4
substituted BIPS occurs. C-O bond breaking with 40 fs is
followed by isomerization to a cis-configured merocyanine
around the central methine bond with 250 fs. The last reaction
step, with a time constant of 12 ps, was attributed to the
cis-trans isomerization of the central methine bond. The authors
did not find any evidence for the two reaction mechanisms (one
triplet, one singlet) suggested by TD-DFT calculations. They
assumed a singlet process for all molecules on the basis of the
observed relaxation times.
be traced back to the same vibrational mode as the absorption
at 1340 cm-1 in 6-nitro-BIPS (see the steady-state experiments
3
8,42
and calculations in the literature
). Although the absorption
change at 1344 cm-1 after 2 ns due to the product formation is
very small (Figure 6, green transient), it is nevertheless clearly
positive. The direct conclusion from this positive absorption is
that spiropyran is formed on an ultrafast time scale after
photoexcitation of the merocyanine form of 6,8-dinitro-BIPS.
4
5
(
(
(
40) W u¨ rthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. J. Am.
Chem. Soc. 2002, 124, 9431–9447.
(43) Lenoble, C.; Becker, R. S. J. Phys. Chem. 1986, 90, 62–65.
(44) Poisson, L.; Raffael, K. D.; Soep, B.; Mestdagh, J.; Buntinx, G. J. Am.
Chem. Soc. 2006, 128, 3169–3178.
41) G o´ mez, I.; Reguero, M.; Robb, M. A. J. Phys. Chem. A 2006, 110,
3
986–3991.
42) Futami, Y.; Chin, M. L. S.; Kudoh, S.; Takayanagi, M.; Nakata, M.
(45) Sheng, Y.; Leszczynski, J.; Garcia, A. A.; Rosario, R.; Gust, D.;
Springer, J. J. Phys. Chem. B 2004, 108, 16233–16243.
Chem. Phys. Lett. 2003, 370, 460–468.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010 16517

A R T I C L E S
Buback et al.
We will now consider these summarized findings from the
literature with respect to our current findings on 6,8-dinitro-
BIPS. Analogous to the findings in the literature, we see a
decreasing absorption at 450 nm and an increasing absorption
centered at 550 nm (Figure 8). However, during the first
nanosecond the band changes much faster than the triplet
2
8
pathway that was observed in 6-nitro-BIPS. The broad,
constant background above 600 nm observed in Figure 7 might
suggest the possibility of a triplet or solvated electron
4
6,47
contribution,
yet in a spectral region and on a time scale
which has no impact on the interpretation of the ring-opening
and ring-closing dynamics. Hence, we can safely conclude that
the observed sub-nanosecond dynamics in 6,8-dinitro-BIPS do
not involve triplet states. A possible explanation for the observed
dynamics in Figures 8 and 9 is the existence of isomers/
conformers that differ in their absorption spectrum from the
completely relaxed ground state. Rotation/isomerization around
the double bonds in the methine bridge would be hindered
by the partial double bond character and the solvent, explaining
the observed increase of the lifetime of the conformers from
the 12 ps observed in the gas phase (observed for nitro-
substituted BIPS) to the 34 ps for 6,8-dinitro-BIPS in this work.
We do not detect any stimulated emission or excited-state
absorption in Figures 7 and 8, an indication that these processes
happen in the electronic ground state.
Figure 11. Investigated photoprocesses of 6,8-dinitro-BIPS. The ring-
opening pathway is depicted by the purple dashed arrows. The ring-closure
pathway is depicted by the green dashed arrows. * represents a vibrationally
excited state. “MC-cis” represents the reaction intermediates mentioned in
the interpretation of the ultrafast switching section; “MC-X” represents
unknown isomers and pathways that MC-cis relaxes to, as indicated by the
UV-pump-only experiments. These isomers are converted thermally on a
much longer time scale toward the MC-TTC isomer.
A fraction of the ring-opened product molecules can be closed
back to spiropyran. This conclusion is supported by the
observation that both the dynamics and the spectral shape of
the bleach (Figures 9 and 10) are very similar to the bleach
that results from simple excitation of the open merocyanine
form. Therefore, we conclude that there is a significant sub-
ensemble which does not react along a triplet route. The second
nitro group has a strong effect on the relative ground-state
energies of the merocyanine and spiropyran forms. In contrast
processes. Contributions of thermal ring closure and ring
opening are neglected in this approximation since those reactions
proceed on a time scale of minutes. Therefore, the extinction
coefficients at 267 nm of both forms and the quantum efficien-
cies for ring opening and ring closure determine the equilibrium.
The quantum efficiencies for ring closure of the merocyanine
1
2
excited to the S
1
(ΦRC560) and S
2
(ΦRC400
)
states seem to be
very similar (40%). Hence, we assume a similar quantum
efficiency for 267 nm excitation (ΦRC267 ) ΦRC560 ) 40%). In
this case we can roughly calculate the quantum efficiency for
ring opening after complete relaxation (ΦRO267) with
2
to 6-nitro-BIPS, 6,8-dinitro-BIPS is negatively photochromic.
Therefore, the electronic states of 6,8-dinitro-BIPS cannot be
directly compared to those of 6-nitro-BIPS, and one cannot
assume that intersystem crossing in the molecule studied here
is as efficient as that in 6-nitro-BIPS. The time constant of
ΦRO267εSp267
ΦRCεMe267
[
[
Me]
Sp]
)
(3)
3
4 ps for the ring opening is furthermore in good agreement
14
with the ring-opening speed of the unsubstituted BIPS (28 ps ),
for which it is known that ring opening does not proceed along
a triplet pathway.
where [Me]/[Sp] is the ratio of equilibrium concentrations. The
3
-1
-1
extinction coefficients εMe267 ) 14 000 dm mol cm and
Sp267 ) 32 900 dm mol cm-1 can be determined from Figure
, and the quantum efficiency for ring closure, ΦRC, is known
3
-1
ε
In conclusion, we ascribe the fast, broad absorption observed in
Figure 7 mostly to the ground state of transient ring-open forms
that are very hot and cannot be assigned to a certain isomer.
Afterward only the most stable isomers are formed, and therefore
the spectrum converges toward the steady-state absorption spectrum
of merocyanine. During this process the absorption between 440
and 490 nm decreases, whereas the absorption between 490 and
1
from the previous section.
The photostationary equilibrium while irradiating with
2
67 nm femtosecond laser pulses lies at approximately 50% of
the initial merocyanine concentration, and therefore we deter-
mine ΦRO267 ≈ 9%.
600 nm increases (Figure 8).
Summary
a. Quantum Efficiency. The quantum efficiency for ring-
By performing three-color pump-repump-probe experi-
ments in the visible range, we have shown that a spiropyran-
based molecular switch (6,8-dinitro-BIPS) can be switched
photochemically in both directions on a picosecond time scale.
We have furthermore reported the first direct observation of ring
closure of a spiropyran on the picosecond time scale. Figure
opening can be calculated in a good approximation from the
photostationary equilibrium, [Me]/[Sp], obtained while il-
luminating with 267 nm light. For this purpose, 267 nm
femtosecond laser pulses with a beam diameter of 3 mm were
-
6
used, leading to a decrease in intensity by a factor of 9 × 10
compared to the transient absorption experiments presented in
1
1 summarizes our findings on the photochemistry of 6,8-
Figures 7-10, thereby excluding contributions of two-photon
dinitro-BIPS.
After ring opening of spiropyran with 267 nm femtosecond
(
(
46) Kloepfer, J. A.; Vilchiz, V. H.; Lenchenkov, V. A.; Germaine, A. C.;
Bradforth, S. E. J. Chem. Phys. 2000, 113, 6288.
0 0
laser pulses [SP(S )], transient absorption experi-
) f MC-cis(S*
ments probing in the visible spectral range reveal a mixture of
isomers. The at least partial relaxation within 34 ps to the
47) Vogt, G.; Nuernberger, P.; Gerber, G.; Improta, R.; Santoro, F.
J. Chem. Phys. 2006, 125, 044513.
1
6518 J. AM. CHEM. SOC. 9 VOL. 132, NO. 46, 2010

Bidirectional Photoswitching of a Spiropyran
A R T I C L E S
merocyanine ground state MC-TTC(S
0
) (purple dashed arrows
(e6 ps) suggests that the reaction starts from the vibrationally
in Figure 11) was shown by three-color pump-repump-probe
experiments. The transient absorption experiments show that
pathways to other merocyanine isomers exist. The resulting
isomers of these pathways are denoted as “MC-X” since we
cannot identify them any further. The overall quantum efficiency
for ring opening obtained via the photostationary equilibrium
is approximately 9%.
From three-color pump-repump-probe experiments we
conclude that ring closure of the generated merocyanine ground-
0
state species MC-TTC(S ) can be induced with 560 nm laser
pulses (dashed green arrows in 11). Transient absorption in the
visible and mid-infrared spectral range provide proof for
identification of the ring-closed product. Furthermore, these
experiments show that the quantum efficiency for the ring-
closure reaction is 40%. The ring closure to the spiropyran
ground state completes the photocycle, as shown by the dashed
lines in Figure 11. The high speed of the ring-closure reaction
excited MC-TTC(S
vibrational cooling to the MC-TTC(S
1
*) state, because we see indications of
) taking up to 12 ps
1
(Figure 6). The other 60% of the excited merocyanine molecules
return to the merocyanine ground state with a time constant of
98 ps either via internal conversion or via fluorescence.
The results demonstrate that 6,8-dinitro-BIPS is a bidirectional
molecular switch that can be switched back and forth on a
picosecond time scale. Whereas common molecular switches
have a high quantum yield for one of the two switching
directions, the other direction is often much less efficient (Table
1). For the investigated system, however, the quantum efficiency
is high for both directions. Thus, the molecule possesses a high
potential for a multitude of applications.
Acknowledgment. J.B. acknowledges financial support from
the “Fonds der chemischen Industrie”.
JA1062746
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