Connectivity matters-ultrafast isomerization dynamics of bisazobenzene photoswitches

Article abstract of DOI:10.1039/c6cp00603e

We have investigated the ultrafast dynamics of o-, m- and p-bisazobenzenes, which represent elementary building blocks for photoswitchable multiazobenzene nanostructures. The connectivity pattern within bisazobenzenes and the ensuing complex interactions between the individual azobenzene units determines the ultrafast dynamics of these compounds and their photochemical properties. While retaining a relatively high E → Z isomerization quantum yield, o-bisazobenzene exhibits a very high thermal relaxation rate (half-life of 1.6 ms). Our theoretical calculations reveal that the geometry allows intramolecular excitonic interaction between the azobenzene units, which is reflected in the femtosecond transient absorption data via the simultaneous bleaching of the two excitonic bands. In contrast, the properties of m-bisazobenzene are very similar to the monomeric azobenzene, with the two units acting nearly independently from each other. The highest degree of π conjugation extending over the two azobenzene units was observed for p-bisazobenzene, which results in strong planarity of the molecule, reduced excited state lifetime and relatively low isomerization quantum yield. Multiphotochromic systems bridge the gap between molecular photoswitches and macroscopic function and thus, understanding the properties of bisazobenzenes opens the way to the design and development of new structures with extensive and versatile applications.

Full text of DOI:10.1039/c6cp00603e

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Connectivity matters - Ultrafast isomerization dynamics of
bisazobenzene photoswitches†
Chavdar Slavov,a Chong Yang,b Luca Schweighauser,c Chokri Boumrifak,a Andreas Dreuw,b
Hermann A. Wegner,c,* and Josef Wachtveitla,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We have investigated the ultrafast dynamics of o-, m- and p-bisazobenzenes, which represent elementary building blocks
for photoswitchable multiazobenzene nanostructures. The connectivity pattern within bisazobenzenes and the ensuing
complex interactions between the individual azobenzene units determines the ultrafast dynamics of these compounds and
their photochemical properties. While retaining a relatively high E→Z isomerization quantum yield, o-bisazobenzene
exhibits a very high thermal relaxation rate (half-life of 1.6 ms). Our theoretical calculations reveal that the geometry
allows intramolecular excitonic interaction between the azobenzene units, which is reflected in the femtosecond transient
absorption data via the simultaneous bleaching of the two excitonic bands. In contrast, the properties of m-bisazobenzene
are very similar to the monomeric azobenzene, with the two units acting nearly independently from each other. The
highest degree of π conjugation extending over the two azobenzene units was observed for p-bisazobenzene, which
results in strong planarity of the molecule, reduced excited state lifetime and relatively low isomerization quantum yield.
Multiphotochromic systems bridge the gap between molecular photoswitches and macroscopic function and thus,
understanding the properties of bisazobenzenes opens the way to the design and development of new structures with
www.rsc.org/
extensive
and
versatile
applications.
UV-VIS absorption spectrum of E-AB (Fig. 1) shows a weak
Introduction
n
→
π* transition band at ~450 nm (S1 state) and a strong
π* transition band at ~315 nm (S2 state).21 The
π→
corresponding bands of Z-azobenzene are located at ~440 nm
Molecular photoswitches can be transformed with light
between at least two states with different physicochemical
properties (e.g. molecular conformation, absorption spectrum,
dipole moment, etc.).1,2 The use of light offers a highly
selective, spatiotemporally precise and non-invasive way for
control of such systems. Thus, photoswitches are widely
applied in fields like bio(chemical) science and technology,3,4
nanotechnology,5-7 nanomedicine,8-10 renewable energy,11,12 to
name a few.
and 260 nm. Excitation in the S2 state leads to isomerization
quantum yields (QYE
~0.13, QYZ
~0.23) that are nearly
E
→
→
Z
half of the yields measured after S1 excitation (QYE
~0.25,
→
Z
QYZ
~0.5), which is a violation of Kasha’s rule21,22 and
→
E
implies the presence of different relaxation pathways.
The photoinduced E Z isomerization of AB occurs on the
subpicosecond to picosecond timescale after n π* as well as
π* excitation. The decay of the S1 state is described by two
→
→
π
→
Azobenzenes (AB) are
compounds widely employed as photoswitches. Their
photochromism is based on the E Z isomerization of the
N=N bond. The isomerization is accompanied by a substantial
length change ( 3.4 Å) – a property that has been extensively
a major class of photochromic
lifetimes - ~300-400 fs and 1-3 ps – assigned, correspondingly,
to a direct and a diffusive motion from the Franck-Condon
region to the conical intersection with the ground state.23-26 An
additional ultrashort (~100 fs) component is observed after
↔
∼
π
→
π* excitation and ascribed to the S2
→
S1 transition.26,27
exploited.13-20 The E-isomer is thermodynamically more stable,
whereas the Z-isomer, in addition to the photoinduced
isomerization, can undergo also thermal isomerization. The
The Z E photoisomerization is ultrafast with a main lifetime
→
of ~150 fs and a minor contribution of a ~1-2 ps lifetime.23,25,26
In both isomerization directions, the internal conversion to the
ground state is followed by vibrational cooling dynamics of the
hot molecules (10-20 ps lifetime).23,26,28,29 It should be noted
that although we follow in the current work the above-
described, established interpretation of the time-resolved data
for the E
→
Z isomerization kinetics23-26, a recent study puts
forward a new interpretation where the 10-20 ps component
was attributed to the isomerization process, while the shorter
lifetimes were linked to excited state dynamics30.
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Two isomerization mechanisms are typically discussed for AB:
i) in-plane inversion (one N=N–C angle increases to 180°, the
C–N=N–C dihedral angle remains at 0°) and ii) rotation (torsion
of the N=N bond resulting in a change of the C-N=N-C dihedral
angle to 120°).31 Theoretical calculations give preference to
the rotation mechanism for the E
↔
Z isomerization after both
π*
n
→
π* and π →
→
π* excitation.32-37 The lower QYs after π
excitation is explained by the presence of an ultrafast internal
conversion channel from the S2 state32,33 (potentially involving
a crossing with a higher singlet state38-40). The most recent
theoretical studies support a modified two-step rotation
mechanism, particularly important for isomerization of AB in
the condensed phase, where the torsion of the N=N bond is
followed by rotation of both phenyl rings.36,41-47 Rotation38,48
and inversion49,50 have both been proposed for the mechanism
Fig. 1. Structures of the investigated compounds. 1 - azobenzene; o-2 - o-bisazobenze;
m-2 - m-bisazobenzene; p-2 - p-bisazobenzene.
of thermally activated Z
→ E isomerization.
Although single AB units are well characterized, little is known
about the behaviour of connected ABs. The interactions
between the individual AB units in multiphotochromic systems
offer new properties and thus, such oligoazobenzene
derivatives are of great interest for the development of
advanced photocontrollable materials. ABs have been
introduced into photoresponsive oligomers,51-54 polymers55,56
and other materials20,57,58. Bis(Azo) compounds, in which two
ABs share one phenyl ring (Fig. 1), represent one of the
Results and discussion
Synthesis
All three bis(AB)s (Fig. 1) were prepared relying on the Bayer-
Mills coupling of the corresponding aniline with
nitrosobenzene. In case of o-2, a one-pot procedure starting
from 1,2-diaminobenzene developed in the Wegner group was
applied.61 The one-pot method was not applicable to access m-
2
and p-2, therefore, a three-step procedure was employed.
elementary
building
blocks
for
photoswitchable
The corresponding nitroanilines were subjected to the Bayer-
Mills coupling. After selective reduction of the nitro-group a
second Bayer-Mills coupling yielded the desired product
(Scheme 1).
multiazobenzene nanostructures. Previous studies on m- and
p-bis(AB)s showed that the two compounds have different
spectral properties and E
→
Z switching QY.59 Theoretical
calculations explain the observed difference by the degree of
electronic coupling between the two azo moieties.59,60 In
contrast,
while
m-
and
p-bis(AB)s
were
both
photoisomerizable, o-bis(AB) surprisingly showed no
accumulation of the Z-form.61,62 Clearly, the connectivity
pattern of bis(AB)s plays a crucial role in the photochromic
properties of these oligoazobenzenes and thus deeper
understanding of the photophysics and photochemistry of the
building blocks is necessary to achieve intelligent design of
complex photoswitchable nanostructures. However, at
present, no characterization of the ultrafast reactions of o-, m-
and p-bis(AB)s exists. Hence, we have performed femtosecond
transient absorption experiments in a pursuit to gain further
understanding of the bis(AB)s photochemistry. The study is
complemented by theoretical calculations taking into account
the solvent environment.
Scheme 1. Synthesis of m-2 and p-2. See ESI for further details.
Spectroscopic characterization
All three bis(AB) compounds deviate in their stationary
absorption spectra from the parent AB (Fig. 2, Table 1). The
π
→π* band of the E,E-isomer of m-2 shows a minor red-shift
and -broadening (similarly to [59]). Under identical irradiation
conditions, the spectrum of the UV light induced
photostationary state (PSSUV) of m-2 has a more pronounced
contribution of the E-isomer than the PSSUV spectrum of AB.
Nevertheless, despite the apparently lower E
level, the E-isomer depletion in m-2 is still very high indicating
that double E Z isomerization in this bis(AB) is possible if
→Z isomerization
→
sufficient irradiation power is provided. The spectra of o-2 and
p-2 show significant differences compared to AB (Fig. 2,
Table 1). The π
→π* band of the E,E -isomer of o-2 is broader
than the corresponding band of the other studied compounds
and has an absorption maximum at ~303 nm with a strong
shoulder at ~340 nm. The spectrum of the Z-isomer (PSSUV) of
o-2 is not shown as it was not possible to accumulate a
detectable amount of this isomer even after prolonged
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irradiation.61 The absorption spectra of p-2 show the strongest methods72-74. The theoretical absorption spectra for AB in E-
spectral shift compared to the parent AB (Fig. 2, Table 1). The and Z-isomerization states, as well as for the bis(AB)s in E,E-
maximum of the π
~358 nm, whereas the n
The connection pattern in bis(AB)s affects also the thermal different possible isomers, however a significant effect on the
E relaxation rates after UV irradiation (Table 1). While m-2 spectral properties was observed only in the case of o-2 (Fig. 3,
→
π* band of the E,E-isomer is located at and E,Z-isomerization states are shown in Fig. 3. The
→
π* band peaks at ~450 nm. absorption spectra of the E,E-bis(AB)s were calculated for two
Z
→
has a half-life similar to that of AB, the half-life of p-2 is see Fig. S4 for m-2 and p-2). The attachment and detachment
shortened to ~10 h. Interestingly, although no Z-form of o-2 densities75,76 for the energetically lowest singlet states of the
was accumulated during UV irradiation, the ultrafast time- different bis(AB)s (Fig. S5) indicate the character of the
resolved experiments (vide infra) clearly demonstrated that electronic transitions as well as at the level of interaction
this compound is indeed capable of E
Therefore, nanosecond flash photolysis63 was used to The theoretical spectra of
determine the thermal Z E relaxation rate of o-2 (ESI, with the experimental data (Fig. 2). The position of the
Fig. S12) which yielded a half-life of ~1.6 ms. The E theoretically predicted π π* transition band of the E,E-m-2
→
Z isomerization. between the two AB moieties.
1,
m-2 and p-2 (Fig. 3) are consistent
→
→
Z
→
isomerization QY of o-2 and m-2 is about 6-8% (Table 1). As coincides roughly with the position of the same transition of E-
previously observed,59 the isomerization QY of p-2 (~1-2%) is AB (Fig. 3, ~306 nm). However, the intensity of the band in m-2
significantly lower than that of the other compounds (Table 1). is two times larger than in AB, which accounts for the presence
of two AB moieties. Overall, the two AB moieties of m-2 are
largely decoupled from each other as suggested previously60.
The E,E-m-2 has two optically forbidden n
located above 400 nm. E Z isomerization of one of the azo
groups of m-2 leads to the rise of the characteristic Z-isomer
π* transition band at ~424 nm and the concomitant
decrease in the intensity of the π π* transition band (Fig. 3).
→π* transitions
→
n
→
→
Nevertheless, the latter band does not disappear completely,
but is only reduced to half of the original intensity and thus the
E,Z-m-2 is nearly a sum of the E- and Z-AB spectra.
The strong red-shift (~50 nm) of the π
→π* transition band of
p-2 (Fig. 2) as compared to AB is reflected also in the
theoretical spectra (Fig. 3). Furthermore, similarly to m-2, the
intensity of this band is about twice as large as the one of AB,
accounting for the presence of two AB moieties. The results
show that the conjugated π-system extends over the complete
Fig. 2. Absorption spectra of the investigated compounds in the photostationary states
(PSS) after VIS and UV irradiation. The transparent rectangulars (UV and VIS) mark the
corresponding illumination wavelength ranges. See Fig. S13 for the 200-250 nm range.
molecule (Fig. S5), which decreases the energy of the π
transition. Isomerization of one of the AB units of E,E-p-2 leads
to the disappearance of the π π* transition band at ~354 nm
and simultaneous rise of the Z-isomer n π* transition at
→π*
→
Table 1. Spectral properties of the studied compounds in ethanol.
→
ππ*
absorption
λmax [nm]
315
nπ*
absorption
λmax [nm]
442
ππ*
Z → E
thermalb
τ1/2
~430 nm. The isomerization breaks the planar conformation of
the molecule and destroys the extended π-system
delocalization, which results in the observation of a single E-
irradiation
a
QYE → Z
E-1
~12-14%
~6-8%
~2 d
isomer π
AB (the E,Z-p-2 spectrum is nearly a sum of the E-AB and Z-AB
spectra). The small red shift of the π π* transition band of
E,Z-p-2 indicates that even after E Z isomerization a certain
level of conjugation between the two AB units remains.
→π* transition band at ~320 nm similar to the one of
E,E-o-2
E,E-m-2
E,E-p-2
303
424
1.6 ms
>2 d
320
437
~8-11%
→
358
450
~1.5-2.5%
~10 h
→
aQY as determined from the femtosecond transient absorption data (Fig. 4 and
Fig. S7) based on the ratio of the initial (150 fs) and the final (1.3 ns) ground state
bleach intensity; bmeasured at 22°C.
Theoretical calculations
Quantum chemical calculations using time-dependent density
functional theory (TDDFT)64-66 in combination with the BHLYP67
exchange-correlation functional and the 6-31G* basis set68
(see ESI) were performed to understand the molecular origin
of the spectral properties of the investigated bis(AB)s. The
calculations were performed with and without conductor-like
polarizable continuum model (C-PCM)69-71 for ethanol
solvation and have been validated against higher-level ab initio
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contributes to the red-shift of the low energy band. The
theoretical spectra of the isomerized E,Z-o-2 are different for
the two isomers. The isomerization destroys the excitonic
coupling between the two AB units, and the corresponding
π
π
→
→
π* transition excitonic bands are replaced by the typical AB
π* transition band (~310 nm). The isomerization is
accompanied also by a rise of the associated Z- isomer n→π*
transition band above 400 nm. In the twisted configuration
(E,Z-o-2(1)), an interaction between the electron clouds of the
azo groups leads to splitting of the n
→π* transition band. In
contrast, the spectrum of the planar E,Z-o-2(2) is similar to the
sum of the E- and Z-AB spectra. The Boltzmann population for
the twisted and the planar isomers of the different E,E-bis(AB)s
was computed based on the BHLYP/6-31G* results to
determine their contribution to the experimental spectra. In
the case of m-2 and p-2 the energy difference between the
different isomers is negligible as is their spectral difference.
However, in the case of o-2 the planar E,E-o-2(2) isomer is
~0.05 eV lower in energy than the twisted E,E-o-2(1) isomer.
Thus, the experimental spectrum of o-2 (Fig. 3) is determined
predominantly by the planar isomer (~87%) with a minor
contribution of the twisted isomer (~13%).
Ultrafast E→Z isomerization after π→π* excitation
The ultrafast E
investigated by
spectroscopy (Fig. 4). The isomerization process was initiated
by laser pulses (~100-130 fs) with peak wavelength
coinciding either with the main maximum of the π π* band
→
Z isomerization kinetics in bis(AB)s was
femtosecond transient absorption
a
→
Fig. 3. Molecular structures of E- and Z-AB, as well as the E,E and E,Z isomers of the o-,
m- and p-bis(AB) (left side) together with the corresponding simulated spectra using
TDDFT/BHLYP/6-31G* with a PCM for ethanol (right).
of the E,E-isomers (Fig. 4) or with the corresponding red-
shifted shoulder (ESI, Fig. S6) of the same band. The transient
absorption data of the compounds are characterized by the
presence of strong ground state bleach (GSB) signal (negative
transient absorption signal, blue) at the
The theoretical spectra of o-2 deviate largely from AB (Fig. 3).
The spectra of E,E-o-2 are characterized by a dual band feature
in the π→π* transition range (at 278 nm and 313 nm for E,E-o-
(1), and at 293 nm and 336 nm for E,E-o-2(2)), which
2
resembles the experimental absorption spectra (Fig. 2). A key
result from the theoretical calculations on o-2 is that the dual
band structure of the absorption spectrum of the E,E-isomer is
due to the interaction between the two AB moieties. The o-2
bis(AB) molecule can be regarded as composed of two
identical, non-conjugated AB units with strongly interacting
transition dipole moments oriented along different axes of the
molecule. As a consequence, intramolecular excitonic coupling
occurs between the π→π* transition dipole moments of two
AB units, which causes excitonic splitting of the corresponding
band into a higher and a lower transition. In this respect, the
two isomers of o-2 have slightly different theoretical spectra.
The non-planar configuration of E,E-o-2(1) has a more parallel
orientation of the π
→π* transition dipole moments, which
results in a higher intensity of the blue-shifted excitonic band.
This effect might be enhanced also by potential interactions
between the electron clouds of the two N=N groups.
Conversely, in the planar E,E-o-2(2) the π
→π* transition dipole
moments are less parallel, which leads to a more equal
intensity of the two excitonic bands. Furthermore, the planar
configuration allows for certain
π delocalization, which
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(Figs. 6A and S7A-B) shows that not all of the initially excited E-
isomers return, thus it is used as a marker for the E
→ Z
isomerization QY (~13%).
Fig. 5. Single transient absorption traces at selected detection wavelengths (as
indicated in the plots) of the investigated compounds obtained after excitation in
the maximum of the π→π* absorption band of the E,E-isomers. A-B) AB, λexc =
320 nm; C-D) o-2, λexc = 305 nm; E-F) m-2, λexc = 320 nm; G-H) p-2, λexc = 355 nm.
As expected, based on the stationary absorption spectra
(Fig. 2) and the theoretical analysis, the transient absorption
data from the E,E-isomer of m-2 (Figs. 4E, 6C and S6C-D) is
similar to the one of AB. The spectral positions of the GSB
band (~320 nm) and the main ESA band (~400 nm) are
identical. Nevertheless, a major difference is present in the
structure of the ESA bands. While in AB a clear S2 ESA is
detected (~475 nm) such a well-defined feature is missing in
Fig. 4. Femtosecond transient absorption data (left) and lifetime density maps (right) of
the investigated compounds obtained after excitation in the maximum of the π→π*
absorption band of the E,E-isomers. A-B) AB, λexc = 320 nm; C-D) o-2, λexc = 305 nm; E-F)
m-2, λexc
= 320 nm; G-H) p-2, λexc = 355 nm. The negative-amplitude lifetime
distributions at ~1 ns (left column) corresponds to the non-decaying component and
thus to the final absorbance difference spectrum in the transient absorption data. For a
comprehensive comparison between excitation in the maximum and in the red-shifted
shoulder of the π→π* absorption band see ESI, Fig. S6-8.
the transient absorption of m-2. Instead, on the early
timescale (<300 fs) the ESA contribution in m-2 appears to
cover the complete spectral range above 350 nm (a weak ESA
band is found at ~540 nm and a strong one at ~400 nm). The
broadening of the ESA is most likely due to the increased
number of higher excited states and thus available electronic
transitions in the bis compound. After the decay of the initially
excited state, the band typical for AB S1 ESA (~400 nm) remains
dominant also in m-2. Nevertheless, the decay time of this ESA
band, and thus of the S1 excited state, is increased in m-2
compared to AB. The decay of the S1 excited state of m-2 is not
followed by a complete recovery of the GSB band, which
position of the corresponding stationary absorption spectra
(Fig. 2). At longer wavelengths (>400 nm) the data shows
pronounced excited state absorption (ESA) contributions
(positive transient absorption signal). Representative kinetics
at detection wavelengths within the GSB and the ESA signals
are shown for each of the compounds in Fig. 5.
The transient absorption data from the E-isomer of AB
(Figs. 4A and S6A-B) is identical to previous studies26. The GSB
signal is located at ~315 nm and two pronounced ESA bands
are observed at ~475 nm and ~400 nm. The ESA at ~475 nm is
associated with the initially excited S2 state which decays and
populates the S1 excited state (ESA band at ~400 nm) on the
200-300 fs timescale. The S1 state decay (2 ps timescale) leads
to the population of a vibrationally hot ground state (Figs. 4A,
6A and S7A-B, positive transient absorption signal at ~365 nm,
after 2 ps). The cooling of the vibrationally hot molecules
proceeds on the 10 ps timescale and is accompanied by the
partial recovery of the GSB signal. The cooling dynamics is
identical for all compounds. The remaining GSB signal at 1.3 ns
indicates that the E
→ Z isomerization QY (~10%) is slightly
lower than in AB. This QY is in the same range as detected
previously59 and in contrast to the recent theoretical
calculations, predicting higher QY for m-2 as compared to AB.60
In contrast to m-2, the transient absorption data of the E,E-
isomers of o-2 and p-2 differ significantly from that of AB
(Fig. 4C and G, Fig. S6). The GSB of o-2 is characterized by a
dual band feature reflecting the ground state absorption
spectrum (Fig. 2). The simultaneous bleach of the two bands
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Fig. 6. Transient absorption spectra at different delay times (as indicated in the
plots) of the investigated compounds obtained after excitation in the maximum
after excitation signifies the interaction between the two AB
moieties of the o-2 molecule. Similarly to m-2, the initial ESA
signal is not structured and no well-defined S2 ESA band can be
identified. Rather the ESA is represented by a spectrally broad
band with a peak at ~405 nm. The decay of this initial ESA is
slightly slower than in m-2 but it is followed similarly by the
dominant S1 ESA at ~405 nm. The S1 decay in o-2 appears on
of the π
→π* absorption band of the E,E-isomers. A-B) AB, λexc = 320 nm; C-D) o-
2
, λexc = 305 nm; E-F) m-2, λexc = 320 nm; G-H) p-2, λexc = 355 nm.
Additional transient absorption experiments were performed
with excitation in the red-shifted shoulder of the E,E-isomers
π
→
reveal any major difference in the overall kinetics of m-2
π* band (ESI, Figs. S6 and S5). These experiments did not
the same timescale as in m-2. Although in stationary
1
,
experiments the Z-isomer could not be accumulated61, the
transient abortion data of o-2 (Figs. 4C, 6B and S7C-D) clearly
show that after the relaxation of the excited state (<10 ps) the
GSB band recovers only to ~7%. Therefore, contrary to the
and p-2. In contrast, on the early timescale (<500 fs) and
>400 nm the kinetics of o-2 appears to be excitation
wavelength dependent (Fig. S7C-D). In this spectral range,
excitation at 340 nm leads to an increased ESA contribution,
which decays until ~500 fs. The decay is associated with an
absorption rise below 420 nm. Based on the theoretical
calculations, which indicated the presence of excitonic splitting
in o-2, this early blue shift of the ESA could be linked to
presence of a barrier on S2 potential energy surface for
excitations in the lower energy excitonic band. Onwards, the
kinetics of the E,E-isomer are identical for the two excitation
wavelengths.
previous belief,61 o-2 is indeed capable of E
but as shown by the flash photolysis experiments (Fig. S12 and
Table 1) has a very high thermal back relaxation rate (t1/2
→ Z isomerization,
=
1.6 ms), which prevents accumulation of the Z-isomer in
stationary experiments.
In the transient absorption data of p-2 (Figs. 4G and S6G-H) the
GSB (~360 nm) and ESA (~550 nm) bands are strongly red-
shifted compared to AB, o-2 and m-2. Only one, very broad
ESA band can be discerned both after π→π* (Figs. 4G and 6D)
and n π* excitation (Fig. 7) and thus it was assigned to the
All datasets were analysed using lifetime density analysis
(LDA)77. To facilitate the comparison of the kinetics, the results
are shown as lifetime density maps (LDM) (Fig. 4 and Fig. S8).
The LDMs are kinetic footprints77 for each sample under a
given condition and thus reveal even minor kinetic differences.
In each LDM (Fig. 4) four lifetime ranges can be distinguished –
~100 fs; 0.4-3 ps; ~10-20 ps; > 1 ns. In AB, the S2 excited state
decay (ESA at ~475 nm, Fig. 4A) is described by the positive-
amplitude lifetime distribution at 100-200 fs (Fig. 4B). The
distribution is complemented by negative-amplitude
distributions (<450 nm) accounting for the rise of the S1 state
(ESA at ~400 nm, Fig. 4A) and the partial recovery of the
ground state (<380 nm, Fig. 4A). The decay of the S1 state
(~400 nm, Fig. 4A) is reflected by a strong positive-amplitude
lifetime distribution with a peak at ~600-700 fs (Fig. 4B and
Fig. S8A-B). This distribution is accompanied by negative-
amplitude distributions <380 nm (Fig. 4B) due to the rise of the
absorption of the vibrationally hot ground state molecules
(~400 nm, Fig. 4A). Previously, global lifetime analyses77 of the
transient absorption data indicated the presence of two
lifetime components describing the decay of the AB S1 excited
state (~300-400 fs and 1-3 ps)23-26, which were assigned to a
direct and a diffusive relaxation pathway from the Franck-
Condon region. However, as evident from the LDM (Fig. 4B),
those two lifetimes fall on the short and the long side of a
broad lifetime distribution (ESA decay, ~400 nm) with a peak
at ~600-700 fs. The presence of a single lifetime distribution
shows that there is no abrupt temporal distinction between
the pathways along the different relaxation coordinates.
Rather the variation in the ground state geometries of the
→
absorption of the S1 excited state. Consequently, in p-2 the
decay of the initially excited S2 state most likely occurs on the
<100 fs timescale. A similar conclusion was reached in a recent
theoretical study.60 In the range <450 nm a positive intensity
contribution can be detected, which rises with the decay of the
ESA band and consequently is attributed to the absorption of
the vibrationally hot ground state molecules. The decay of this
band associated with the cooling of the hot molecules is
accompanied by simultaneous decay of the GSB due to the
repopulation of the initial ground state. The contribution of
the GSB signal at longer timescale (1.3 ns) is minor, indicating
that despite the accumulation of the Z-isomer of p-2 in
stationary experiments (Fig. 1) the E → Z isomerization
efficiency (up to ~2% QY) is significantly lower than that of the
other bis(AB)s.59,60
initially excited E-AB molecules leads to
a probability
distribution for the search of the conical intersection from the
Franck-Condon region.
The cooling dynamics of the vibrationally hot AB molecules
generated after the relaxation of the excited state is described
by a pair of lifetime distributions at ~10 ps (Fig. 4B): i) negative
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at ~315 nm, accounting for the recovery of the GSB; ii) positive distribution at ~350 nm linked to the recovery of the ground
at ~365 nm, accounting for the decay of the absorption of the state. Those two lifetime distributions of p-2 are characterized
hot molecules. Since the cooling process does not follow a by the largest relative lifetime width amongst the four
mono-exponential kinetics, the corresponding lifetime compounds. This can, most likely, be attributed to the
distributions have a tilted elliptic shape - a signature typical for delocalization of the excitation over the entire molecule which
cooling dynamics.78,79
results in an increased number of degrees of freedom for
The LDMs of all investigated bis(AB)s show differences relaxation.
compared to the LDM of AB (Fig. 4 and Fig. S8). The differences
Ultrafast E→Z isomerization after n→π* excitation
Previous studies21,22 have shown that the QY of the E
isomerization of AB is twofold lower after π π* excitation as
compared to n π* excitation, which implies the presence of
different S2 and S1 relaxation pathways. In this respect, the
are mainly related to the excited state kinetics (<5 ps), while
the cooling dynamics (>5 ps) appears to be identical. The LDM
of m-2 (Fig. 4F) is the least different from the LDM of AB. In m-
→
Z
→
→
2
the positive-amplitude lifetime distribution at 100-200 fs,
representing the decay of the initially excited state, covers the
complete range >350 nm. This is in contrast to AB, where the
distribution is accompanied by a strong negative-amplitude
distribution (350-450 nm) related to the rise of the S1 state. As
discussed above, in m-2 the ESA of the initially excited state is
spectrally very broad and covers also the range of S1
absorption, hence no negative-amplitude 100-200 fs lifetime
kinetics of the
E
→
investigated also after n
Z
isomerization of bis(AB)s was
→
π* excitation (~430 nm) (Fig. 7 and
Figs. S7A,C,E, Figs. S10A,C,E). In AB, the transient absorption
data after n π* excitation (Fig. 7A) lack the pronounced,
short-lived ESA band (~475 nm) that is present after π π*
→
→
excitation (Fig. 4A) and is associated with the decay of the S2
state. This effect of the excitation wavelength is detectable
also when comparing the corresponding LDMs (Fig. 4A,
Fig. 7B). The kinetics on longer timescale is identical for the
distribution can be found at ~400 nm. Nevertheless,
a
negative-amplitude distribution can be detected in the range
of the GSB (<360 nm) accounting for the repopulation of the
ground state. At longer lifetimes (~2 ps) a strong positive-
amplitude lifetime distribution is present in m-2, which is
similar to the 600-700 fs distribution in AB associated with the
S1 decay. Thus, evidently, the S1 excited state decay in m-2 is
delayed.
two experiments, although in the n
→π* excitation a clear
500 nm ESA band can be seen, which decays a bit faster than
the ~400 nm band (Fig. 7A). This difference could potentially
account for the difference between the direct and diffusive
relaxation pathways. The cooling of the vibrationally hot
ground state (~10 ps) is followed by the repopulation of the
initial ground state and photoproduct formation (Z-isomer).
The LDM of o-2 (Fig. 4D) shares similarities with that of m-2
.
However, the ~200 fs positive-amplitude lifetime distribution
(maximum at 430 nm) is significantly more intense. In
addition, the maximum of this lifetime distribution does not
coincide spectrally with the maximum of the ESA band
(~405 nm). Instead, it is slightly red-shifted, which indicates
that the decay of ESA at longer wavelengths is compensated
by a rise of absorption at ~ 405 nm (location of the S1 ESA).
Therefore, the ~200 fs lifetime distribution can be assigned to
the decay of the initially excited state to the S1 state or
repopulation of the ground state. The assignment is further
confirmed by the following positive-amplitude distribution at
~405 nm (~2 ps) accounting, similarly to the other compounds,
for the S1 decay. Noteworthy, o-2 was the only compound that
showed excitation wavelength dependence of the transient
absorption data (see above, and Fig. S7C-D). This dependence
is also mirrored in the LDMs (Fig. S8C-D), where after 340 nm
excitation (in the shoulder of the π→π* band) a much weaker
~200 fs positive amplitudes distribution is observed at
~430 nm. As discussed above, in o-2 the excitonic splitting of
the π→π* band may lead to an energy barrier on the S2
potential energy surfaces which is overcome on the sub-500 fs
timescale. The process leads to a blue shift of the ESA band
and appears as a reduced positive amplitude at 430 nm of the
~200 fs distributions in the LDM.
The LDM of p-2 is dominated by a very broad, in both
wavelength and in lifetime, positive-amplitude lifetime
distribution, with a peak at ~550 nm and ~700 fs (Fig. 4H). The
Fig. 7. Femtosecond transient absorption data (left) and lifetime density maps (right) of
the investigated compounds obtained after excitation in the n→π* absorption band
(430 nm) of the E,E-isomers. A-B) AB; C-D) m-2; E-F) p-2; For
a comprehensive
distribution is related to the decay of the S1 excited state of p- comparison between the compounds see ESI, Figs. S7-9.
2, as assigned above and is matched by a negative-amplitude
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The transient absorption datasets recorded after n
π* excitation of m-2 are very similar and differ slightly only
on the sub-500 fs timescale where in the case of the π π*
→π* and
π
→
→
excitation a stronger ESA decay is observed (Fig. 4C and
Fig. S10). This difference is revealed also by comparing the
corresponding LDMs, where the positive-amplitude lifetime
distribution at ~100-200 fs (>350 nm), which is present in the
π
→
π* excitation case, is not observed after n
(Fig. 7C-D). No significant difference between the datasets of
the n π* excitation of AB and m-2 are observed, which
means that after S1 excitation the E Z isomerization of the
two compounds proceeds via an identical pathway. In contrast
to and m-2, where the n π* and the π π* excitation have
→π* excitation
→
→
1
→
→
an effect on the isomerization kinetics, such an influence is not
detected for p-2. The two transient absorption datasets as well
as the corresponding LDMs are identical (Fig. 4G, 7D, 8E-F).
The results confirm the conclusion that the relaxation of the
initially π→π* excited state in p-2 occurs on the sub-100 fs
timescale populating to a high degree the S1 excited state.
Consequently, the photoinduced dynamics in p-2 proceeds
without violation of Kasha’s rule, while the dynamics in both
and m-2 violates this rule, albeit the effect seems to be less
pronounced for m-2
1
.
Fig. 8. Femtosecond transient absorption data (left) and lifetime density maps (right) of
Ultrafast Z→E isomerization after n→π* excitation
Finally, the photoinduced Z
bis(AB)s after n π* excitation was also studied (Fig. 8,
Fig. S9B,D,F and Fig. S10B,D,F). The overall ultrafast dynamics
(<1 ps) is slightly accelerated as compared to the one of E
isomerization (n π* excitation).
In and m-2, after the decay of the ESA band <400 nm, a clear
photoproduct formation band (π π* absorption of the E-
the investigated compounds obtained after excitation in the n→π* absorption band
→
E isomerization kinetics of the (430 nm) of the E,Z-isomers. A-B) AB; C-D) m-2; E-F) p-2; For a comprehensive
comparison between comparison between the compounds see ESI, Figs. S7-9.
→
→
Z
Conclusions
→
1
Multiphotochromic systems, bridging the gap between
molecular photoswitches and macroscopic function gain
increasing importance in (bio)chemical research and
technology. Nevertheless, the complex interactions between
the building blocks of such systems is not well-studied and
understood. Here we provide thorough insight into the
molecular photoswitching behaviour of connected ABs, which
represent one of the most important model systems for
coupled photoswitches.
→
isomer), can be observed at the lower wavelength detection
limit (~350 nm). The band is accompanied by a strong GSB
feature present at longer delay times > 10 ps (~430 nm).
Interestingly, the timescale of the excited state relaxation of p-
2
is very similar for both E
↔Z isomerization directions (E→Z
and Z E), which is confirmed also by the identical LDMs
→
(Fig. 7-8F).
We reveal a strong dependence of the isomerization dynamics
of these systems on the connectivity pattern. While retaining
relatively high E
→Z isomerization QY, o-bis(AB)s show a very
high thermal relaxation rate (half-life on the ms scale). Thus
such compounds are extremely suitable for applications where
high-temporal resolution of the switching process is required
(quick and repeated operation of the photoswitch).
Furthermore, the geometry of o-bis(AB)s allows intramolecular
excitonic interaction between the AB units, which provides
another opportunity for control, modification and fine-tuning
of their function. In contrast,
m-bis(AB)s show a behaviour that
is very similar to the monomeric AB, with the two AB units
acting nearly independently from each other. This behaviour
permits double isomerization of the
under sufficiently high irradiation. In effect, unique
substitution of the two AB units within -bis(AB)s will permit
m-bis(AB) compound
m
selective addressing of the two units and thus unprecedented
control of the isomerization state of a single molecule double
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21 H. Rau, in Photochromism: Molecules and Systems, eds. H.
Dürr and H. Bouas-Laurent, Elsevier, Amsterdam, 2003, ch. 4,
pp. 165-192.
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photoswitch. Finally, p-bis(AB)s showed the highest degree of
π conjugation extending over the two AB units, which results
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Despite the lower isomerization QY, p-bis(AB)s are interesting
for molecular electronics applications. Their high degree of
conjugation allows conductance, which can be precisely
controlled (ON/OFF) via fast and reversible light-induced
switching. Overall, the current study delivers essential
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and versatile application.
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