Photochemical properties of multi-azobenzene compounds

Article abstract of DOI:10.1039/c2pp25291k

A systematic study is reported of the photochemical properties of the multi-azobenzene compounds bis[4-(phenylazo)phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA) compared to the parent molecule 4-aminoazobenzene (AAB). The bis- and tris-azobenzenes were synthesised by a variant of the Ullmann reaction and exist in their stable all-E forms at room temperature. Striking changes in the spectral positions and intensities of their first ππ* absorption bands compared to AAB reveal strong electronic coupling between the AB units. The nature of the excited states was explored by quantum chemical calculations at the approximate coupled-cluster (CC2) level. Upon UV/VIS irradiation, the molecules isomerise to the Z-isomer (AAB), ZE- and ZZ-isomers (BPAPA), and ZEE-, ZZE- and ZZZ-isomers (TPAPA), respectively. The photoswitching behaviours were investigated by UV/VIS and NMR spectroscopies. All individual isomers were detected by one-dimensional (1D) ¹H NMR spectroscopy (BPAPA) and two-dimensional (2D) HSQC NMR spectroscopy (TPAPA). A kinetic analysis provided the isomer-specific thermal lifetimes. The variance of the thermal lifetimes demonstrates a dependence of the Z-E isomerisation on the chromophore size and number of AB units.

Full text of DOI:10.1039/c2pp25291k

Photochemical &
Photobiological Sciences
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PAPER
Photochemical properties of multi-azobenzene
compounds†
Cite this: DOI: 10.1039/c2pp25291k
a
a
a
a
Julia Bahrenburg, Claudia M. Sievers, Jan Boyke Schönborn, Bernd Hartke,
a
a
b
c
Falk Renth,* Friedrich Temps,* Christian Näther and Frank D. Sönnichsen*
A systematic study is reported of the photochemical properties of the multi-azobenzene compounds bis-
4-(phenylazo)phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine (TPAPA) compared to the
[
parent molecule 4-aminoazobenzene (AAB). The bis- and tris-azobenzenes were synthesised by a variant
of the Ullmann reaction and exist in their stable all-E forms at room temperature. Striking changes in the
spectral positions and intensities of their first ππ* absorption bands compared to AAB reveal strong elec-
tronic coupling between the AB units. The nature of the excited states was explored by quantum chemi-
cal calculations at the approximate coupled-cluster (CC2) level. Upon UV/VIS irradiation, the molecules
isomerise to the Z-isomer (AAB), ZE- and ZZ-isomers (BPAPA), and ZEE-, ZZE- and ZZZ-isomers (TPAPA),
respectively. The photoswitching behaviours were investigated by UV/VIS and NMR spectroscopies. All
Received 22nd August 2012,
Accepted 24th November 2012
1
individual isomers were detected by one-dimensional (1D) H NMR spectroscopy (BPAPA) and two-
dimensional (2D) HSQC NMR spectroscopy (TPAPA). A kinetic analysis provided the isomer-specific
thermal lifetimes. The variance of the thermal lifetimes demonstrates a dependence of the Z–E isomerisa-
tion on the chromophore size and number of AB units.
DOI: 10.1039/c2pp25291k
www.rsc.org/pps
1
4–19
systems, including dendrons and dendrimers,
molecular
1
. Introduction
2
0,21
22,23
24
wires
The reversible E ⇄ Z photoisomerisation of azobenzene (AB) materials,
and its derivatives upon irradiation with visible (VIS) or ultra- holographic information storage.
and helices,
emulsions, and polymer films and
which are suitable for surface patterning
2
5–28
29–31
or
3
2,33
To reach the ambitious
violet (UV) light forms the basis for a wide range of appli- design goals for functional AB devices, however, it is man-
cations as light-triggered molecular switches or optical datory to acquire detailed knowledge about the ensuing molecu-
1
–6
memory devices.
The high application potential of AB as a lar dynamics under different circumstances.
In functional systems, photochromic molecular switches
photoswitchable element rests on the reversible, large changes
in size, shape and dipole moment between the thermodynami- are typically embedded in complex environments, where the
cally favoured, stretched E-isomer and the energetically higher, close proximity of the chromophores leads to cooperative
more compact Z-isomer and on the low photochemical fatigue phenomena, e.g., steric interactions, excitonic coupling,
of the chromophore. These properties have been exploited for, charge-transfer (CT), or direct electronic coupling in π-conju-
7
,8
e.g., single molecule optomechanical research, photoregula- gated systems. All of these mechanisms may compete with the
9
–12
tion of biomolecules,
photoswitching of magnetic bistabil- desired photoisomerisation. To elucidate the ensuing influe-
1
3
ity, and photocontrol of macromolecular and supramolecular nces, we initiated an investigation of two prototypical photo-
switchable multi-azobenzene compounds, bis[4-(phenylazo)-
phenyl]amine (BPAPA) and tris[4-(phenylazo)phenyl]amine
a
Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel,
Olshausenstr. 40, D-24098 Kiel, Germany. E-mail: renth@phc.uni-kiel.de,
temps@phc.uni-kiel.de; http://www.uni-kiel.de/phc/temps
(TPAPA), where the AB units are connected via an amino linker
to enable electronic coupling between the chromophores. The
chemical structures of both molecules in their all-E forms and
the parent 4-aminoazobenzene (AAB) as a reference compound
are given in Scheme 1.
b
Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel,
Otto-Hahn-Platz 6-7, D-24098 Kiel, Germany
c
Otto Diels-Institut für Organische Chemie, Christian-Albrechts-Universität zu Kiel,
Otto-Hahn-Platz 4, D-24098 Kiel, Germany. E-mail: fsoennichsen@oc.uni-kiel.de
The influence of chromophore–chromophore interactions
and electronic coupling between several addressable AB
moieties connected by a central unit on the photoswitching
behaviour has rarely been investigated to date. Several studies
†
Electronic supplementary information (ESI) available: Chemical syntheses of
BPAPA and TPAPA, X-ray diffraction structure of TPAPA (cif file), tabulated
1
results of excited-state calculations, H and 2D-HSQC NMR data for TPAPA.
CCDC 897971. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2pp25291k
3
4–37
34
involve bis-azobenzenes.
Work by Cisnetti et al. on the
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and the thermal lifetimes of the individual EE-, EZ-, ZZ-
isomers of BPAPA and the EEE-, ZEE-, ZZE-, ZZZ-isomers of
TPAPA were investigated in acetonitrile solution by convention-
1
al H NMR and by two-dimensional (2D) HSQC NMR spec-
troscopy, respectively. The thermal lifetimes of the different
isomers are significantly longer for TPAPA compared to
BPAPA, which indicates a dependence of the Z–E isomerisation
on the chromophore size and number of AB units. Large differ-
ences in the spectral band positions and in the intensities of
the absorption spectra of AAB, BPAPA and TPAPA hint at
sizable chromophore–chromophore interactions and electronic
couplings between the AB moieties.
Scheme 1
photo- and electrochemical properties of a meta- and a para-
substituted bis-azobenzene showed a difference in the elec-
tronic coupling for the two systems. However, it was not pos-
sible to determine the composition in the photostationary
2. Experimental and computational
states (PSS), which should contain three isomers in both cases. methods
The thermal isomerisation of a conjugated meta-substituted
The protocols for the chemical syntheses of BPAPA and TPAPA
bis-azobenzene derivative has been investigated by Robertus
37
are given in the ESI.† All commercially available compounds
were purchased from Merck, Sigma-Aldrich, Deutero and Lan-
caster and used without further purification, except for AAB,
which was recrystallised from ethanol. Solvents were dried
et al. Both AB units seemed to switch independently of each
other, and the thermal behaviour was comparable to that of
the single AB derivative. Considering much larger systems,
1
6
Puntoriero et al. determined the photoisomerisation yields
of a fourth generation dendrimer containing 32 trans-AB
units and its complexes with eosin hosted in the dendrimer.
1
13
according to standard procedures. H-, C-, COSY-, HSQC-
and HMBC NMR data were acquired with Bruker AC-200 and
Bruker AV-600 spectrometers. UV/VIS absorption spectra were
taken on a Shimadzu UV-2401 desktop spectrometer. Three
light emitting diodes (Nichia) with emission maxima at λ =
1
8
Franckevičius et al. presented a fluorescence and femto-
second transient absorption study of the excited-state relaxation
of dendrimers containing cyano-AB end groups, which indi-
cated rather little effect by the environment. Last but not least,
365 nm (NCSU033A, 400 mW), 385 nm (NCSU034A, 400 mW)
3
0
and 455 nm (NS4C107E, 400 mW) were used for the photo-
isomerisation experiments. The light intensities were attenu-
ated to 100 mW. The thermal back-isomerisations were
Takahashi et al. reported the effect of surface relief grating
SRG) formation by TPAPA. With each AB unit capable of E–Z
(
isomerisation, this structurally well defined tris-azobenzene
may exist in four distinct isomeric forms (EEE, ZEE, ZZE, ZZZ).
Thus, great potential would arise if the AB units could be indi-
vidually addressed, a functionality that might be reached by
chemical substitutions. Moreover, if TPAPA shows similar hole
1
followed by UV/VIS and H and 2D-HSQC NMR spectroscopies.
Quantum chemical calculations of the structures of EE-BPAPA
and EEE-TPAPA in their electronic ground states were per-
4
3
formed by density functional theory (DFT) using Gaussian09.
AAB had been calculated for another purpose before using the
3
8
conductivity as, e.g., the related triphenylamine, it may
4
4
OM2-MRCISD method. The photoexcited electronic states
9,40 were explored using the second-order approximate coupled-
provide access to logical devices for applications in molecular
3
electronics or to organic light emitting diodes (OLEDs)
cluster with the resolution-of-the-identity (RI-CC2) model of
that could be photoswitched to emit at different wavelengths.
However, the central question of whether and how the close
proximity and electronic interactions of two or more AB moie-
ties in a molecule influence the photoswitching has remained
unanswered.
4
5,46
Turbomole.
3. Results
In the present paper, we report on a systematic investigation
of the photoswitching properties of the bis- and tris-azobenz-
3.1. Molecular structures
enes BPAPA and TPAPA in comparison to the reference com- The calculated B3LYP/6-31+G(d,p) equilibrium structures of
pound AAB by means of UV/VIS absorption and NMR EE-BPAPA and EEE-TPAPA are displayed in Fig. 1. As can be
spectroscopy. The study is a prelude to subsequent time- seen, the molecules adopt propeller-like configurations. The
resolved dynamics measurements of the molecules. The target out-of-plane dihedral angles of the AB units in the electronic
compounds were obtained by a modified, less harsh variant of ground state are ≈21° for BPAPA and ≈42° for TPAPA.
4
1,42
the Ullmann coupling reaction
between AAB and 4-iodo-
The X-ray diffraction structure of TPAPA (see ESI†) con-
azobenzene (IAB). To begin with, the nature of the photoexcited firmed the calculated propeller-like configuration of the AB
electronic states was explored by quantum chemical calcu- units around the central nitrogen. Quite similar configurations
lations. The photostationary states for each system were deter- are well known for related molecules with a triphenylamine
4
7
48
mined by UV/VIS spectroscopy. The relative concentrations core or triphenylamine itself.
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Fig. 1 Calculated structures of EE-BPAPA and EEE-TPAPA at the B3LYP/6-31+G-
(d,p) level of theory.
Fig. 2 UV/VIS absorption spectra of E-AAB (blue), EE-BPAPA (green) and EEE-
TPAPA (red; solid lines) and in their photostationary states (dashed lines) after
irradiation for ≈30 s in the strong first absorption bands at λ = 365 nm (AAB),
3
85 nm (BPAPA) and 455 nm (TPAPA), respectively, in n-hexane (T = 30 °C).
3
.2. Stationary UV/VIS absorption spectra
The UV/VIS absorption spectra of AAB, BPAPA and TPAPA in
their all-E forms in n-hexane as the solvent are depicted in
Fig. 3 UV/VIS absorption spectra of AAB (top), BPAPA (middle) and TPAPA
(bottom) in n-hexane (black), chloroform (blue), acetonitrile (green) and ethanol
Fig. 2. As for unsubstituted AB, AAB shows a strong ππ* band (red). The absorption maxima were normalized for better comparison.
4
in the near UV with a maximum at λ = 361 nm (ε
M
= 2.7 × 10
max
−
1
−1
cm ) and a weak nπ* band at λ ≈ 450 nm in the visible.
In contrast, the spectra of BPAPA and TPAPA exhibit pro- 360 nm compared to the pure E isomer and increased absorp-
nounced bathochromic and hyperchromic shifts, which tion around 450 nm. In the PSS385 for BPAPA and PSS455 for
increase with the number of AB units. The maxima for BPAPA TPAPA, the ππ* absorptions are much lower as well. At the
4
−1
−1
and TPAPA are at λ = 412 nm (εmax = 5.6 × 10 M cm ) and same time, the presence of several isomers in both cases
32 nm (εmax = 7.0 × 10 M cm−1), respectively. The corres- results in a significant broadening of the bands. Unfortu-
4
−1
4
ponding nπ* transitions are obscured by the intense ππ* nately, the lack of clear characteristic spectral signatures of the
bands, but may be located in the extended wings to the red. different isomers in the course of the thermal back-reactions
The red-shift is more pronounced for BPAPA compared to AAB hampers further analysis of the absorption spectra. However,
than for TPAPA compared to BPAPA. Thus, the intramolecular the photoisomerisation yields and isomer-resolved Z–E back-
coupling of two (three) AB chromophores in BPAPA (TPAPA) reactions could be monitored by NMR (below).
has drastic consequences on the resulting excited electronic
states and their energies and spectra.
Fig. 3 displays the UV/VIS spectra of the molecules in
n-hexane, chloroform, acetonitrile and ethanol as solvents.
Also given in Fig. 2 are the absorption spectra of the mole- The absorption maximum of AAB can be seen to shift from
cules in the photostationary states PSS365 (AAB), PSS385 λmax = 361 nm in n-hexane to 371 nm in chloroform and
(BPAPA) and PSS455 (TPAPA) after irradiation at the specified 380 nm in acetonitrile and ethanol in line with the increasing
wavelengths. Similar to unsubstituted AB, where the Z-isomer solvent polarity. The bands in chloroform, acetonitrile and
shows a blue-shifted and much weaker ππ* absorption but a ethanol are broadened compared to n-hexane. Likewise, the
stronger nπ* absorption than the E-isomer, the spectrum of absorption maxima of BPAPA are at λmax = 412 nm in
AAB in the PSS365 exhibits decreased absorption around n-hexane, 424 nm in chloroform, 429 nm in acetonitrile and
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4
55 nm in ethanol. As for AAB, the bathochromic shift is stron-
ger in the more polar solvents. The pronounced red-shift of
45 nm in ethanol compared to n-hexane can be rationalised
≈
by hydrogen bond formation between the solute and solvent.
In the case of TPAPA, the influence of the solvent polarity on
the absorption behaviour of TPAPA is not as distinctive as for
AAB or BPAPA. The absorption maxima are at λmax = 432 nm in
n-hexane, 444 nm in chloroform and 434 nm in acetonitrile
and ethanol. Compared with the very broad and unstructured
band shapes in the more polar solvents, the spectra of BPAPA
and TPAPA in n-hexane show some very weak structure. The
variances in the band shifts and shapes may be partly related
to the existence of two resp. three transitions in BPAPA and
TPAPA and/or to different basicities and polarities of the
primary, secondary and tertiary amine.
3
.3. Excited-state calculations
The characters of the photoexcited states were studied using
4
5,46
49
the RI-CC2 model
with the def2-TZVPP basis for AAB
5
0
and BPAPA and the def2-SVP basis for TPAPA at the afore-
mentioned optimised B3LYP/6-31G+(d,p) structures. The calcu-
lations gave the first two excited states of AAB, the first four of
BPAPA, and the first six of TPAPA. Table S1 in the ESI† lists the
respective excitation energies, oscillator strengths and domi-
nant excited determinants for these states.
The relevant molecular orbitals are displayed in Fig. 4. As
can be seen, the energetic order of the states (nπ* < ππ*)
remains conserved despite the combination of two resp. three
AB units. The first excited state of AAB, the first two of BPAPA
Fig. 4 Relevant molecular orbitals involved in the electronic transitions in the
and the first three of TPAPA are of nπ* character, while the fol- UV/VIS absorption spectra for AAB, BPAPA and TPAPA.
lowing ones are of ππ* character. All nπ* states carry very low
oscillator strengths from S . For BPAPA, the calculated excited
0
(
def2-TZVPP). The first two are virtually degenerate and carry
states appear in pairs of one with higher and one with lower
oscillator strength, as one would expect for an excitonic
system. While the two nπ* states (S , S ) are virtually degener-
large oscillator strengths, the third is calculated to be about
0
.65 eV higher in energy and has very low oscillator strength.
1
2
3 4
ate, the two ππ* states (S , S ) are split by about 0.6 eV. The nπ*
states are calculated to have about the same energy as in AAB,
3.4. NMR measurements
while the first ππ* state (S ) is lowered compared to AAB by
3
about 0.75 eV. Since this state carries the bulk of the oscillator The isomer-specific photoisomerisation yields in the photosta-
strength, its energetic lowering explains the experimentally tionary states and the thermal Z–E back-reactions as a function
1
observed red-shift of the first strong UV/VIS absorption band of time were determined by H NMR spectroscopy (BPAPA) and
of BPAPA relative to AAB. In contrast, the S
sition of BPAPA is only rather weakly allowed. The results for
4
0
(ππ*) ← S tran- by 2D-HSQC NMR spectroscopy (TPAPA).
1
3
Fig. 5 displays H NMR spectra of BPAPA in CD CN after
TPAPA are not quantitatively comparable to the others because excitation to the photostationary state PSS385 as a function of
of the smaller basis, but qualitative conclusions can neverthe- time for T = 15 °C. As can be seen, the three singlet peaks of
less be drawn. Similar to BPAPA, the nπ* states (S , S , S ) are the chemically different NH protons of the three isomers (EE,
1 2 3
virtually degenerate, but the n orbitals remain localised on ZE, ZZ), which co-exist in the PSS, are readily assigned on the
one of the AB units each instead of taking the form of linear basis of their chemical shifts and their time dependence. The
combinations delocalised over the molecule. This difference NH signal of the ZZ-isomer disappears most rapidly, while the
can be rationalized on the grounds of the larger out-of-plane NH signal of the ZE-isomer shows a slight initial rise due to
dihedral angles of the AB chromophores. The calculated ener- the ZE-production from the ZZ-isomer before its subsequent
4 5 6
gies of the ππ* states (S , S , S ) are comparable to those of slower decay to zero. The NH signal of the EE-isomer slowly
BPAPA. Judged by a comparison to results with the same rises to its final equilibrium value. All other proton signals can
smaller basis set for BPAPA, the excitation energies for TPAPA be assigned accordingly (see ESI, section 1.2†). Similar spectra
obtained with the def2-SVP basis are likely slightly overesti- were taken at a slightly elevated temperature (T = 35 °C), where
mated compared to the expected outcome at higher level the kinetics were correspondingly faster.
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1
Fig. 5 H NMR spectra of BPAPA in CD
3
CN at T = 15 °C as a function of time after excitation to the photostationary state PSS385 at t = 0. The circles in the bottom
trace highlight the NH proton signals of the three different isomers.
The isomer ratio in the PSS385 was found to be 39% EE,
4
1% ZE, and 20% ZZ. Taking these data, the isomer-specific
thermal lifetimes could be determined by a kinetic analysis of
the respective integrated singlet signals for the three NH
protons using a consecutive kinetic model including the back-
reactions of the ZZ- and EZ-isomers according to the scheme
τZZ
τZE
ZZ ꢀ! ZE ꢀ! EE
The experimental time profiles for the three isomers and
the best-fit time profiles for T = 15 °C are shown in Fig. 6.
The resulting isomer-specific thermal lifetimes are τZZ = 1.9 ±
0
2
.2 h, τZE = 7.5 ± 0.6 h (T = 15 °C) and τZZ = 0.5 ± 0.1 h, τZE
.4 ± 0.2 h (T = 35 °C).
Because of the much slower back-reaction compared to
=
Fig. 6 Time profiles of the ZZ-, ZE- and EE-isomers of BPAPA in the thermal
back-reaction starting from the photostationary state PSS385 in CD CN at T =
5 °C. The data points (cf. Fig. 5) are given by open symbols, the fitted time
profiles by solid curves.
3
1
BPAPA, similar data for TPAPA were taken only at slightly ele-
vated temperature (T = 35 °C). As for BPAPA, the proton signals
belonging to the all-E isomer in the spectrum at the longest
time are readily recognisable from the time dependence. ZZE-, and ZZZ-isomers, respectively. The recorded HSQC
However, the spectrum of the PSS (see Fig. S1, ESI†) is more spectra of the all-E isomer, the photostationary state (PSS455),
congested than in the BPAPA case so that a direct identifi- and the photoisomer mixture at three selected times after
cation of the individual photoisomers was not easily possible. preparation of the PSS are presented in the ESI (cf. Fig. S2†). As
The thermal back-reactions were therefore followed by HSQC shown, the isomer-specific cross-peaks belonging to the aro-
1
NMR spectroscopy to resolve the severe overlap in the H NMR matic rings next to the central amino-N atom are well separ-
spectrum and to assign related cross-peaks to the EEE-, ZEE-, ated, the respective concentrations are therefore easily
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interactions, e.g., excitonic coupling or charge-transfer (CT),
between the different AB units. It has to be taken into account
in addition that the twist angle between the AB units is larger
in TPAPA than in BPAPA.
The calculated RI-CC2 excitation energies for the nπ* and
the optically bright ππ* excited states of BPAPA and TPAPA are
in reasonable agreement with the recorded spectra. The
observed strong 412 nm absorption band of BPAPA is attribu-
ted to the lower of the two ππ* states, the second ππ* state 0.6
eV higher lacks high oscillator strength. In contrast, the strong
432 nm absorption of TPAPA appears to arise from two degen-
erate ππ* components with comparable oscillator strengths.
The next ππ* state is significantly higher in energy and has
only low oscillator strength. The calculated nπ* transitions are
Fig. 7 Time profiles of the ZZZ-, ZZE-, ZZE-, and ZZZ-isomers of TPAPA in the
3
thermal back-reaction starting from the photostationary state PSS455 in CD CN
at T = 35 °C. The data points are given by open symbols, the fitted time profiles very weak, but may contribute to the absorptions in the
by solid curves.
extended wings in the spectra to the red.
Both BPAPA and TPAPA are efficient photoswitches. Starting
from the all-E isomers, the prepared photostationary states
were found to contain ≈20% all-Z isomer in the BPAPA case
and ≈12% all-Z isomer in the TPAPA case. It should be noted
that the transformation of the molecules from the all-E to the
all-Z forms requires two sequential photoisomerisation steps
for BPAPA and three such steps for TPAPA. In the photostation-
ary state, a TPAPA molecule on average has nearly 1.5 of its AB
units switched to the Z-form.
accessible by peak integration. In this way, the photoisomer
ratio in the PSS455 was found to be 16% EEE, 37% ZEE, 36%
ZZE, and 12% ZZZ.
Starting from these values, the respective thermal lifetimes
were determined by a kinetic analysis of the time profiles
using a consecutive kinetic model including the back-reactions
of the ZZZ-, ZZE-, and ZEE-isomers and formation of the EEE-
isomer of the form
The isomer-specific concentrations in the photostationary
states and the thermal back-isomerisation lifetimes for BPAPA
τZZZ
τZZE
τZEE
1
ZZZ ꢀ! ZZE ꢀ! ZEE ꢀ! EEE
and TPAPA were determined by means of H NMR and by
2
D-HSQC NMR spectroscopy, respectively. In the BPAPA case,
the lifetimes with respect to the thermal back-reactions in
CD CN at T = 35 °C were found to be τZZ = 0.5 h and τZE = 2.4 h
The experimental time profiles and the fitted curves for the
four isomers are given in Fig. 7. The resulting time constants
at T = 35 °C for the ZZZ-, ZZE-, and ZEE-isomers are τZZZ = 4.0
3
for the ZZ- and ZE-isomers, respectively. For TPAPA, the corres-
ponding lifetimes at T = 35 °C were found to be τZZZ = 4.0 h,
±
0.4 h, τZZE = 6.4 ± 0.9 h and τZEE = 12 ± 1 h, respectively. For
τ
ZZE = 6.4 h and τZEE = 12 h for the ZZZ-, ZZE-, and ZEE-
isomers, respectively. The thermal lifetime of Z-AAB under
similar conditions is only of the order of a few minutes (τ
min). Thus, the Z-isomer lifetimes increase considerably
reference, the thermal lifetime of the Z-isomer of AAB (τ
Z
)
under similar conditions determined by UV/VIS spectroscopy
is of the order of just ≈5 min.
Z
=
5
with increasing overall chromophore size and number of AB
units. This behaviour is related to the changes in the geo-
metric and electronic structures of the molecules, although it
cannot be explained unambiguously at the moment.
4. Discussion and conclusions
The photoswitching properties of two multi-azobenzene com-
Towards applications, it is important to understand the
pounds, the secondary amine bis[4-(phenylazo)phenyl]amine intramolecular couplings between the AB units in more detail.
BPAPA) and the tertiary amine tris[4-(phenylazo)phenyl]- Both BPAPA and TPAPA are therefore currently under investi-
(
amine (TPAPA), have been investigated by means of UV/VIS gation in our laboratory by femtosecond time-resolved absorp-
and NMR spectroscopy in comparison to the primary amine tion and fluorescence spectroscopy. Ongoing polarisation-
4
-aminoazobenzene (AAB). As expected, the UV/VIS absorption dependent decay measurements are expected to provide
spectra of BPAPA and TPAPA show drastic changes which are insight into possible excitonic energy migration processes.
understandable on the grounds of the larger π-electron
systems by the linkage of two resp. three AB chromophores via
the central amino-N core, despite the non-coplanar orientation
of the AB moieties in the molecules. The observed red-shift of
the absorption maximum and the increase in absorbance are
Acknowledgements
more pronounced for BPAPA compared to AAB than for TPAPA The support of this work by the Deutsche Forschungsge-
compared to BPAPA, which stresses the non-additive effects of meinschaft through the Sonderforschungsbereich 677 “Func-
the added AB units. This may hint at specific intramolecular tion by Switching” is gratefully acknowledged.
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with peripheral azobenzene groups, J. Am. Chem. Soc.,
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