Photoinduced Electron Transfer Involving a Naphthalimide Chromophore in Switchable and Flexible [2]Rotaxanes

Article abstract of DOI:10.1002/chem.201904155

The interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron-donor and electron-acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers.

Full text of DOI:10.1002/chem.201904155

A Journal of
Accepted Article
Title: Photoinduced Electron Transfer Involving a Naphthalimide
Chromophore in Switchable and Flexible [2]Rotaxanes
Authors: Benoit Colasson, Alberto Credi, and Barbara Ventura
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Photoinduced Electron Transfer Involving a Naphthalimide
Chromophore in Switchable and Flexible [2]Rotaxanes
Benoit Colasson,*,[a],[b] Alberto Credi*,[c],[d],[e] and Barbara Ventura*,[e]
Dedicated to Jean-Pierre Sauvage on his 75th birthday
Abstract: The interlocking of ring and axle molecular components in
rotaxanes provides a way to combine chromophoric, electron-donor
and electron-acceptor moieties in the same molecular entity, in order
to reproduce the features of photosynthetic reaction centers. To this
aim, the photoinduced electron transfer processes involving a 1,8-
naphthalimide chromophore embedded in several rotaxane-based
dyads were investigated by steady-state and time-resolved
absorption and luminescence spectroscopic experiments in the 300
fs-10 ns time window. Different rotaxanes built around the
dialkylammonium / dibenzo[24]crown-8 ether supramolecular motif
were designed and synthesized to decipher the relevance of key
structural factors, such as the chemical deactivation of the
ammonium-crown ether recognition, the presence of a secondary
site for the ring along the axle, and the covalent functionalization of
the macrocycle with a phenothiazine electron donor. Indeed, the
conformational freedom of these compounds gives rise to a rich
dynamic behavior induced by light, and may provide opportunities for
investigating and understanding phenomena that take place in
complex (bio)molecular architectures.
lived charge separation.[1] Remarkably, the transfer of charge
between the various D and A units follows a non-covalent
pathway that allows for the optimization of the thermodynamic
and kinetic coupling, and a structural reorganization after the
electron transfer events.[2] In the frame of research on artificial
photosynthesis, both covalently linked and self-assembled
multicomponent RC model systems have been investigated. The
former have the advantage of a precise relative positioning of
the subunits, but are often synthetically challenging and the lack
of flexibility can limit their performance. The latter, on the other
hand, are stable under specific experimental conditions that do
not necessarily coincide with those optimal for photoinduced
charge separation.
An interesting possibility to combine D and A units in a
molecular species with the aim to reproduce the features of
natural RCs is to exploit the mechanical bond in interlocked
molecular architectures such as rotaxanes.[3] Minimally,
a
rotaxane consists of a molecular ring that encircles a molecular
axle stoppered at both extremities such that a mechanical link is
established between the components. When the D-A pair is
installed in the rotaxane in such a way that D (or A) is attached
to the ring and A (or D) is on the axle, the mechanical bond
guarantees enough flexibility to enable many conformations of
the dyad (i.e. mutual D-A arrangements), while not only
preventing the dissociation of the components but also limiting
their degree of freedom in the three directions of space. In
addition, providing that the interaction between the ring and the
axle can be modulated by an external stimulus, the conformation
of the dyad can be modified accordingly. Different D-A pairs
embedded in a rotaxane were synthesized and studied as
models of the photosynthetic center in the context of artificial
photosynthesis.[4] For instance, Sauvage and co-workers
reported on multiporphyrinic rotaxanes.[5-7] C60 has also been
exploited for its photophysical and redox properties as an
Introduction
Photoinduced electron transfer (PET) is one of the key
processes that occur during the photosynthesis. It takes place in
the photosynthetic reaction center (RC), in which the optimized
organization of the donor-acceptor (D-A) pairs enables
a
directional electron migration and eventually leads to a long-
[a]
[b]
[c]
Dr. B. Colasson, Université de Paris, UMR 8601, LCBPT, CNRS, 45
rue des Saints-Pères, F-75006 Paris, France.
E-mail : benoit.colasson@parisdescartes.fr
Dr. B. Colasson, Photochemical Nanosciences Laboratory,
Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via
Selmi 2, 40126 Bologna, Italy.
Prof. A. Credi, Dipartimento di Scienze e Tecnologie Agro-
alimentari, Università di Bologna, viale Fanin 50, 40127 Bologna,
Italy.
electron acceptor in rotaxanes developed by Sauvage et al., [8]
[9]
Ito at al.
and Schuster, Guldi, et al.[10-12] Although the high
flexibility of such large systems often prevents
a clear
interpretation of the results, these studies highlight the
importance of the mutual arrangement between the
chromophores in the efficiency of the PET process.
E-mail: alberto.credi@unibo.it
In principle, a better control of the distance is expected when D
and A are incorporated in the ring and axle components of a
bistable molecular shuttle, i.e. a rotaxane that has two different
macrocycle recognition sites (stations) on the axle (Figure 1).
For instance, a fine tuning of the electron-transfer kinetics was
demonstrated in a C60-ferrocene rotaxane in which the distance
between A and D is dictated by the polarity of the solvent. [13]
Nevertheless, our recent studies on bistable rotaxanes based on
[d]
[e]
Prof. A. Credi, CLAN – Center for Light Activated Nanostructures,
Università di Bologna and Consiglio Nazionale delle Ricerche, via P.
Gobetti 101, 40129 Bologna, Italy.
Prof. A. Credi, Dr. B. Ventura, Istituto ISOF-CNR, via P. Gobetti 101,
40129 Bologna, Italy.
E-mail: barbara.ventura@isof.cnr.it
Supporting information for this article is given via a link at the end of
the document.
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ammonium and viologen stations[14] unraveled details on the
effective control on the distance when a flexible macrocycle
such as dibenzo[24]crown-8 (DB24C8) is part of the structure.[15]
In particular, by incorporating spin-labels on the DB24C8 ring
and at one stopper extremity of the thread, a combination of
electron-electron double resonance spectroscopy and molecular
dynamics calculations indicated that the stimuli-induced shuttling
Scheme 1. The acid-base triggered rearrangement of the rotaxanes studied in
this work. Two selected conformations for the flexible ring are represented to
of the ring along the axle is accompanied by
conformational rearrangement of the ring, eventually leading to a
limited modification of the distance between the two spin-labels.
a large
emphasize the fact that shuttling does not necessarily result in
a
net
D
modification of the distance between (naphthalimide stopper) and
A
[phenothiazine (PTZ) unit appended to the DB24C8 macrocycle]. Other ring
conformations and relative ring-axle orientations are possible for both states.
Results and Discussion
Design and synthesis
Figure 1. The stimuli-induced ring shuttling between two stations along the
axle in a bistable rotaxane can be used to modulate the efficiency of distance-
dependent processes such as electron transfer.
Rotaxanes based on a DB24C8 macrocycle surrounding an axle
composed of a dialkyl-ammonium (amH+) and a triazolium (tria+)
stations have recently been proposed as acid-base switchable
molecular devices.[19-30] These rotaxanes can be readily
synthesized and easily functionalized either at the ring or at the
axle. Hence, we designed a [2]rotaxane bearing a phenothiazine
More specifically, the DB24C8 ring can exist in two extreme
conformations: (i) a V-shaped form in which the two aromatic
units are folded on the same side of the macrocycle, and (ii) a Z-
shaped form in which the two catechol rings are anti-parallel
(see Scheme 1; the conformational changes were transposed to
the rotaxanes studied in this work, vide infra). These two
conformations were previously observed in the X-ray crystal
structure of rotaxane-type compounds containing the DB24C8
ring.[16-18] In the context of electron transfer, the main
consequence of this rearrangement (not to mention the flexibility
of the axle) is that the triggered translational motion of the ring
does not guarantee the operator to have a perfect control over
the distance change between A and D. Here we report on an in-
depth study aiming at deciphering the influence of the flexibility
and relative arrangement of the molecular parts on the
photophysical properties of an acid-base switchable rotaxane
designed to behave as a dyad for photoinduced electron transfer.
Moreover, we explore the role of the bare DB24C8 macrocyle –
(PTZ) donor unit linked to the crown ether ring and
a
photoactivable naphthalimide (NI) acceptor unit as a stopper
(Scheme 1). These two groups have been previously associated
in several D-bridge-A-type dyads in which the photo-induced
charge separated state NI●−−PTZ●+ was observed.[31-36] In the
protonated state of the rotaxane, the ring encircles the amH+
station and interacts with it via hydrogen bonds. After
deprotonation, the affinity of the ring for this station vanishes and
the ring translates and resides on the tria+ station. Protonation of
the amine resets the system back to its initial situation (Scheme
1). This operation should in principle modify the spatial
arrangement between A and D and, as a consequence, tune the
PET characteristics of the dyad.
The rotaxane dyads, the molecular components and the model
compounds considered in this study are represented in Figure 2.
Rotaxanes with two stations (amH+ and tria+), in which the ring
surrounds the amH+ site, are denoted as 2s; the corresponding
deprotonated species, wherein the ring is located on the
remaining tria+ station, are identified with a prime. Rotaxanes
labelled 1s possess only the amH+ station (the tria+ station is
lacking because the triazole unit is not methylated). The
representation of the DB24C8 macrocycle in Figure 2 has no
implication regarding its conformation in the rotaxanes.
a highly popular ring for mechanically interlocked molecules[3]
–
in photoinduced electron transfer processes that occur between
the interlocked ring and axle components.
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Figure 2. Model compounds, ring components and rotaxanes employed in this work.
NI, NI-am, NI-amH+, Rot1s, Rot2s and Rot'2s were synthesized
and fully characterized (1H, 13C, COSY and ROESY NMR
spectroscopy, high-resolution mass spectroscopy) in a previous
work.[37] The synthesis and characterization (1H and 13C NMR
spectroscopy, high-resolution mass spectroscopy) of DB24C8-
PTZ, Rot1s-PTZ, Rot2s-PTZ and Rot'2s-PTZ are reported in the
Supporting Information. The synthesis of the PTZ-containing
rotaxanes follows the procedures used for the PTZ-free
equivalents. By comparison with Rot2s, the shuttling of the ring
by quantitative deprotonation of Rot2s-PTZ to yield Rot'2s-PTZ
was unambiguously attested by 1H NMR.
PTZ to produce the secondary station for Rot2s-PTZ has almost
no effect on the measured potentials.
Table 1. Halfwave reduction potentials in V vs SCE of the NI and PTZ units.
The numbers in parenthesis are the difference between the anodic and
cathodic peaks, in mV.[a]
PTZ●+/PTZ
NI/NI●−
NI-amH+
DB24C8-PTZ
Rot1s-PTZ
Rot2s-PTZ
–1.421 (57)
–
–1.371 (55)
–1.391 (61)
–
+0.724 (70)
+0.742 (66)
+0.752 (64)
Electrochemistry
The redox properties of NI and PTZ embedded in the rotaxanes
Rot1s-PTZ and Rot2s-PTZ were studied by cyclic voltammetry in
CH3CN with 0.1 M Bu4NPF6 as the supporting electrolyte (see
Figures S20 to S25). The reduction potentials of the NI and PTZ
units in these molecules and in pertinent models are listed in
Table 1. Both electroactive units in NI-amH+, DB24C8-PTZ,
Rot1s-PTZ and Rot2s-PTZ exhibit a quasi-reversible behavior.
The incorporation of NI and PTZ in the rotaxane architecture
Rot1s-PTZ affects very moderately the potential values [∆E1/2(NI)
= +50 mV and ∆E1/2(PTZ) = –20 mV], indicating a good
electronic insulation of both units from the structural components
of the rotaxane. The methylation of the triazolyl moiety in Rot1s-
[a] In CH3CN, 0.1 M Bu4NPF6. For more details see SI.
Absorption and luminescence properties
The electronic absorption spectra were measured in CH3CN. For
all the compounds (except DB24C8-PTZ), a band centered at ca.
350 nm with
ε ≈
14000 mol-1.L.cm-1 typical of the NI
chromophore is observed.[38-41] In all the rotaxanes, the
absorption band of the DB24C8 macrocycle (λmax = 275 nm) is
also detected. For the three PTZ-containing rotaxanes, an
overlap of the absorption bands due to the PTZ (maximum at
310 nm) and NI units is observed until ca. 350 nm; for longer
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wavelengths the absorption due to PTZ becomes negligible.
Interestingly, the NI absorption band in all the protonated
rotaxanes has a 2 nm bathochromic shift compared to the band
in NI-amH+ (see Figure 1 for a comparison between NI-amH+,
Rot1s and Rot1s-PTZ). Previous studies on NI-containing
chromophores pointed out that the maximum of this absorption
band is sensitive to the presence of a hydrogen bond donor in
the medium.[32, 33, 42] In our case, this shift is attributed to the fact
that in NI-amH+ an intramolecular hydrogen bond can be
established between the ammonium unit and a carbonyl group
of the naphthalimide in the ground state, while in the protonated
rotaxanes the ammonium is engaged in a stronger interaction
with the crown ether. When excited in the NI absorption band
(325 nm), all the compounds exhibit an emission band centered
at ca. 390 nm (Figures 3 and S26 to S34). The excitation
spectra of the rotaxanes for the emission centered on the NI unit
are not entirely superimposable with the corresponding
absorption spectra. In particular, the absorption band of DB24C8
centered at 275 nm is absent in the excitation spectra (with the
exception of Rot1s where the band is partially recovered, Figures
S26 to S34), indicating the absence of energy transfer from the
crown ether to the NI chromophore in the rotaxanes.
In the ns scale, fluorescence lifetimes of 1.53 ns and 2.66 ns are
measured for NI and NI-amH+, respectively. With the ultrafast
apparatus, a short rise time of ca. 30 ps is detected only for NI-
amH+ (Figure S47). The latter can be ascribed to a fast process
involving a species that rapidly converts into 1NI, presumably an
excited state proton transfer favored by the presence of the
ammonium group. Indeed, the difference in the ns lifetimes of NI
and NI-amH+ has been previously ascribed to the presence of
an intramolecular hydrogen bond between the ammonium unit
and a carbonyl group of the naphthalimide, both in the ground
and in the excited state.[33]
Differently from NI and NI-amH+, the fluorescence decay of NI-
am is dominated by a short lifetime of 7 ps, detected with the
streak camera apparatus. This lifetime accounts for 95% (as
pre-exponential fraction) of the decay, whereas the remaining
5% is fitted by a lifetime of 1800 ps (Figure S48). The latter
component, detected also with the TCSPC system (Tables 2
and S2), can be ascribed to the presence of a very small (≤ 5%)
amount of NI-amH+. The quenched lifetime of 7 ps of the NI
singlet accounts for the very low emission quantum yield
measured for NI-am (< 0.1 %, Table 2).
The presence of multiple emission decays for Rot1s and Rot2s
was discussed in a previous work.[37] The longer lifetime in the
two-station rotaxane Rot2s was ascribed to the presence of the
minor (ca. 0.03 %) co-conformational isomer in which the
DB24C8 ring is encircling the triazolium station instead of the
ammonium station. Note that this longer component is also
found for Rot2s-PTZ. The ultra-fast analysis confirmed the multi-
exponential nature of the fluorescence decay of the two
rotaxanes, and evidenced a short component on the order of 2
ps and 6 ps for Rot1s and Rot2s, respectively (Figure S49). The
decays can be reasonably fitted by a tri-exponential function,
with τ2 and τ3 of the order of 200 ps and 600-700 ps, matching
the values obtained with the TCSPC analysis (Table 2). It is
evident that for Rot2s a longer component exceeding the time
scale of the streak camera is also present (Figure S49b).
For Rot’2s, the streak camera analysis shows that the decay is
dominated by a 10 ps component (80% as pre-exponential
fraction) followed by components of ca. 100 and 600 ps, and by
a longer tail extending over the time scale of the apparatus
(Figure S50), well matching with the distribution detected in the
TCSPC analysis.
1,2
16000
1,0
0,8
0,6
0,4
0,2
0,0
14000
12000
10000
8000
6000
4000
2000
0
250
300
350
400
450
500
λ (nm)
Figure 3. Absorption (―) and emission (− − −) spectra of NI-amH+ (black),
Rot1s (blue) and Rot1s-PTZ (red) in CH3CN (300 K, λexc = 325 nm) (absorption,
excitation and emission spectra of all the explored compounds can be found in
the SI, Figures S26-S35).
The PTZ containing rotaxanes Rot1s-PTZ and Rot2s-PTZ show
TCSPC distributions similar to their parents Rot1s and Rot2s
(Table 2). The ps analysis confirmed lifetimes in the order of 200
ps and 550 ps for both rotaxanes but also evidenced a prompt
decay close to the time resolution of the system (1 ps, not
processable for this reason) (Figure S51). In Rot'2s-PTZ the
decay is dominated by a short lifetime of 5 ps (Figure S52) with
two residual lifetimes of ca. 200 ps and > 600 ps that could be
better detected on the ns range (Table 2).
Overall, the detailed fluorescence analysis evidenced that in all
rotaxanes a fast process (1-10 ps) is responsible for the
quenching of the NI chromophore. The detected longer lifetime
components can be ascribed to the decay of the NI excited
singlet in different conformations of the ring around the
ammonium site (vide infra). At this stage, the influence of the
The fluorescence quantum yields were measured in aerated
CH3CN at 298 K (Table 2). The low quantum yields of
fluorescence of the NI chromophore in all the protonated
rotaxanes, compared with the naphthalimide models NI and NI-
amH+, suggest that the luminescence is strongly quenched by
the presence of the DB24C8 ring (with or without
functionalization by PTZ). Moreover, the deprotonation of the
ammonium also leads to a strong quenching of the fluorescence
in NI-am, Rot'2s and Rot'2s-PTZ.
Lifetimes were measured both in the ns range with a time-
correlated single-photon counting (TCSPC) apparatus (time
resolution: 100 ps, see Figures S36 to S45) and in the ps time
regime with a streak camera apparatus and a femtosecond
oscillator as excitation source (time resolution: 1 ps).
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functionalization of DB24C8 by the donor unit PTZ and of the
position of the ring on the axle, as key structural factors in the
quenching of the luminescence of NI, cannot be unambiguously
demonstrated. Further information and evidences regarding the
quenching mechanisms in the different compounds were
obtained by transient absorption spectroscopy.
species can be ascribed to a protonated form of the NI
chromophore.
A completely different scenario is observed for model NI-am.
The end-of-pulse signal, ascribable to the NI singlet, shows a bi-
exponential decay dominated by a short component of the order
of 7 ps, followed by a longer decay of ca. 1600 ps accounting for
about 1 % of the total band intensity (Figure S55). In the 470-
480 nm region a further slow signal rise is detected (Figure
S55b). The data can be interpreted on the basis of a fast
quenching of 1NI due to an electron transfer process from the
amine group. The spectral features of the naphthalimide anion,
to be expected in the 700-800 nm region (see below), cannot be
identified,[47] most likely because of an ultrafast (≤ 300 fs)
charge-recombination process. The second minor decay
component (1500 ps) is consistent with the longer lifetime
observed from luminescence measurements and it evolves in
the population of the NI triplet state, as confirmed by the signal
rise at 470-480 nm.
The two DB24C8-containing rotaxanes with the ring encircling
the ammonium station, Rot1s and Rot2s, show a similar behavior.
The time evolution of ∆A in the 450-540 nm region is satisfactory
fitted by three exponentials, with lifetimes of 2.5 ps, 100 ps and
620 ps for Rot1s and 5 ps, 250 ps and 740 ps for Rot2s (Figures
S56 and S57). Two bands, at ca. 735 nm and 790 nm, form with
a rise time consistent with the shortest decay observed in the
450-540 nm region and then decay in 740 ps for Rot1s and 820
ps for Rot2s. These bands can be safely ascribed to the
naphthalimide anion, since the spectrum of NI-am obtained by
spectroelectrochemistry (Figure S46) clearly evidences their
Table 2. Fluorescence data (quantum yields Φ, lifetimes τi) for some selected
compounds in aerated CH3CN at 298 K.
τi / ps
[a]
[b]
[b]
[b]
Φ / %
τ1
τ2
τ3
τ4
NI
–
–
7
2
6
10
1
1
–
–
–
–
620
760
610
580
540
–
1530
2660
1750
–
2620
2080
–
18.4 ± 1
20.2 ± 1.2
< 0.1
1.0 ± 0.1
1.8 ± 0.2
0.1
0.6 ± 0.1
1.3 ± 0.1
0.3 ± 0.1
NI-amH+
NI-am
–
[c]
Rot1s
Rot2s
Rot'2s
Rot1s-PTZ
Rot2s-PTZ
Rot'2s-PTZ
190
220
140
170
170
270
2430
1720
5
[a] From streak-camera analysis (time resolution: 1 ps); λexc = 350 nm; [b]
from TCSPC analysis (time resolution: 100 ps); λexc = 340 nm, λem = 415 nm.
The pre-exponential factors measured by TCSPC analysis are given in SI. All
the fluorescence intensity decays are shown in the SI; [c] a decay component
shorter than the time resolution of the TCSPC apparatus is detected (see
Figure S38).
presence together with an intense peak at ca. 420 nm, usually
48-50]
reported for NI anions.[31,
The radical cation of 1,2-
Transient absorption spectroscopy
dimethoxybenzene, that can be considered as a model for the
DB24C8 ring, has been reported to show absorption features at
ca. 400-430 nm, a region outside the spectral range of our
analysis.[51-53] The presence of two longer components in the
region of absorption of the singlet state can be ascribed to
different conformations of the rotaxane, where the distance
between the dioxybenzene moieties of the crown ether and the
NI unit is larger, thus hindering the electron transfer process.
The observed lifetimes are in good agreement with those
measured for the luminescence decays (Table 2) and can be
tentatively assigned to two different conformations of the ring
around the ammonium site, characterized by the presence of
intramolecular hydrogen bonds in the ground state[54-56] and/or
proton-transfer processes in the excited-state. On long time
Pump-and-probe experiments with femtosecond resolution were
carried out on all compounds in aerated CH3CN. Excitation at
350 nm was performed in order to achieve almost exclusive
excitation of the NI component also in the PTZ containing
rotaxanes.
Model compound NI shows an end-of–pulse spectrum with
maximum at 481 nm and a weak and broad absorption in the
700-800 nm region, which evolves into a species absorbing at
474 nm (Figure S53). The observed process is identified as the
S1 →T1 intersystem crossing of the NI chromophore, confirmed
by the lifetime of 1500 ps measured over the whole spectral
range (Figure S53b). The decay shows a second component,
almost constant in the time range of the experiment, due to the
formed triplet. The observed transient spectra are in reasonable
agreement with those reported for singlet and triplet excited
states of NI chromophores.[39, 40, 43-45] Very similar features are
observed for NI-amH+, where the spectral evolution is consistent
with intersystem crossing (Figure S54a). The absorption maxima
of the singlet and triplet excited states in this case are 474 nm
and 477 nm, respectively. The time evolution is dominated by a
decay of 2400 ps, consistent with the fluorescence lifetime[46] but,
at a difference with NI, a prompt decay of 20 ps is detected in
the 480-550 nm region (Figure S54b). This behavior is in
agreement with the luminescence outcome and the short-lived
3
scales, a weak signal at 478 nm, attributable to NI, is detected
(Figures S56a and S57a).
When the ring resides on the tria+ station, its interaction with the
NI unit is negligible. Indeed, the transient absorption behavior of
Rot'2s is very similar to that of NI-am. The signal evolves mainly
with a short decay of the order of 10 ps, followed by a longer
one of ca. 1000 ps (accounting for ca. 20 %, Figure S58). In the
470-480 nm region a further slow signal rise is detected. The
main decay can be ascribed to a PET process from the amine
group to the NI chromophore.
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a)
excited state which can be populated by the unquenched NI
singlet. The other two spectra, associated with lifetimes of 23 ps
and 1100 ps, show features ascribable to a PTZ+-NI– state: the
bands in the 700-800 nm identify the naphthalimide anion and
the bands in the 500-550 nm region are ascribable to the
phenothiazine cation, whose absorption is reported at 520 nm.[31,
35, 57]
Interestingly, two different lifetimes are found for the decay
of the CS state. A plausible explanation is the existence of two
different conformations of the ring on the axle governing the
distance between the units. Due to the overlap of the signals, we
have no clear evidence for the formation of the CS state crown+-
NI– prior to formation of PTZ+-NI–.
Very similar transient features are observed for Rot2s-PTZ
(Figure S60). Creation of the charge separated states occurs
within the formation of the signal, with a kinetic faster or on the
order of the instrumental time resolution (300 fs). From global
analysis, three lifetimes (and an infinite component) were
identified (Figure S60b). One component has a lifetime of 204 ps
and is associated with an unquenched 1NI; it corresponds to the
main component of the luminescence decay (170 ps, Table 2).
Two lifetimes of 24 ps and 1090 ps are distributed with features
ascribable to a PTZ+-NI– CS state. The long (infinite) lifetime can
be associated with 3NI.
b)
The behavior of Rot'2s-PTZ is more peculiar. At 470 nm a multi-
exponential decay is detected, dominated by a first component
of ca. 5 ps (Figure 5a and S61). This short lifetime can be
attributed to the quenching of the NI singlet by the amine group
through a PET process, in a conformation where the PTZ unit is
not in close proximity to the NI chromophore. Weak features of
the PTZ+-NI– CS state are observed, and their formation occurs
within the formation of the signal. The decay in the CS spectral
region (500-800 nm) is described by a bi-exponential function
with lifetimes of ca. 25 ps and 570 ps, with 70 % and 30 % in
weight, respectively. From a global analysis, three lifetimes (and
an infinite lifetime component) with the spectral distribution
shown in Figure 5b are identified. The dominating component in
the 450-500 nm region is a short lifetime of ca. 5 ps, associated
Figure 4. a) Transient absorption spectra of Rot1s-PTZ in CH3CN measured at
the end of pulse and at various delays. b) Spectral distribution of the
amplitudes of the calculated lifetimes (indicated in the legend) from global fit
analysis of the transient absorption matrix (obtained with four principal
components and three expected lifetimes, plus an infinite lifetime). λexc= 350
nm (A350 = 0.2, 2 mm optical path).
1
with NI quenched by the amine group. Two lifetimes of 30 ps
and 570 ps are distributed with features ascribed to the PTZ+-NI–
state. For similarity with the other rotaxanes, they are associated
to two different conformations of the ring. The infinite lifetime is
associated with triplet features.
Transient absorption analysis of the PTZ containing rotaxanes
evidences additional features, with a similar behavior for Rot1s-
PTZ and Rot2s-PTZ. Figure 4 shows the time evolution of the
transient absorption of Rot1s-PTZ. In the 450-500 nm region
multi-exponential decays are detected, with an initial short
component of ca. 1 ps. At 780-790 nm a short rise in the order of
1 ps is detected and the 500-800 nm region is characterized by
a bi-exponential decay with lifetimes of ca. 25 ps and 1000 ps
with equal weight (Figure 4a and S59 for time evolution). To get
further insights in the comprehension of the transient data, a
global fit analysis has been performed. Figure 4b shows the
spectral distribution of the amplitudes of the calculated lifetimes.
1
A lifetime of 160 ps has a spectral distribution typical of NI and
can be associated to the decay of a NI unit not quenched by
PET processes but affected by the environment (it corresponds
to a detected luminescence lifetime of 170 ps). The spectral
distribution of the infinite lifetime is consistent with a triplet
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a)
towards to the NI acceptor. It can be noticed that the kinetics of
the various processes are different: the am→NI PET process
occurs in a time scale of 5-10 ps, the crown→NI PET is slightly
faster, with time constants of 2-5 ps, whereas the PTZ→NI
reaction is very fast, with τ below or around 1 ps. The latter,
indeed, is the reaction with the highest driving force (figure 6).
Conclusions
Six rotaxanes comprising an electron-donor macrocycle and
bearing an electron-acceptor naphthalimide chromophore (NI) at
one extremity of the axle have been synthesized, characterized
and submitted to a comprehensive photophysical study. These
rotaxanes were designed to evaluate several factors which can
govern the events following the photogeneration of the NI singlet,
namely: i) the presence of a secondary station for the ring along
the axle; ii) the flexibility of the components and the presence of
several (co-)conformations; iii) the functionalization of the
DB24C8 ring with a phenothiazine (PTZ) unit; iv) the position of
the ring along the axle.
b)
The comparison of two dyads containing one primary
ammonium station and either an innocent triazolyl moiety (Rot1s)
or a triazolium unit that can act as a secondary station for the
ring (Rot2s) rules out a significant role of the secondary station in
the photophysical behavior, and confirms that the macrocycle is
surrounding the amH+ site in the vast majority of the Rot2s
species. [37] In both cases, the multiexponential decay of the NI
singlet excited state indicates the presence of different
conformations in which the excited NI unit either generates the
charge separated state crown+-NI–, which recombine with similar
lifetimes (ca. 800 ps) in the two rotaxanes, or is not quenched by
PET processes. We hypothesize that intramolecular hydrogen
bonding and/or excited-state proton transfer processes between
the ammonium center and the NI unit may be at the basis of the
latter behavior.
In the PTZ-functionalized rotaxanes Rot1s-PTZ and Rot2s-PTZ,
photoexcitation of NI produces the charge separated state PTZ+-
NI– within 1 ps, while there is no evidence for the participation of
the crown ether as an electron donor in PET processes. The
charge recombination is characterized in both cases by two very
different lifetimes (ca. 25 ps and 1 ns), which can be attributed to
the presence of two distinct conformations in the CS state.
Rotaxanes wherein the NI singlet excited state is not quenched
by PET are also observed, as discussed for the parent Rot1s
and Rot2s. Finally, the study of the two deprotonated rotaxanes
Rot'2s and Rot'2s-PTZ, in which the ring interacts with the
triazolium station and a free amine is located close to the NI
chromophore, highlights the strong involvement of the amine as
an electron donor moiety in PET. Indeed, in Rot'2s, the NI
quenching is dominated by PET from the free amine, while in
Rot'2s-PTZ an electron transfer process from the PTZ unit to the
excited 1NI level is also detected.
Figure 5. a) Transient absorption spectra of Rot'2s-PTZ in CH3CN measured
at the end of pulse and at various delays. b) Spectral distribution of the
amplitudes of the calculated lifetimes (indicated in the legend) from global fit
analysis of the transient absorption matrix (obtained with four principal
components and three expected lifetimes, plus an infinite lifetime). λexc= 350
nm (A350 = 0.2, 2 mm optical path).
Overall, the transient absorption data evidence excited state
deactivation kinetics in good agreement with those measured in
the fluorescence analysis, shining light on the nature of the
electron transfer processes involved in the different systems.
The closeness of the deprotonated amine group is responsible
for the fast quenching of the NI unit in NI-am, Rot'2s and partially
in Rot'2s-PTZ (the latter being the rotaxanes with the ring shifted
on the tria+ station) through a PET process from the amine group
to NI. In Rot1s and Rot2s, where the ring encircles the amH+
station and is in close proximity of the NI unit, the identified PET
reaction is from the DB24C8 ring to NI. Finally, in the PTZ
containing rotaxanes Rot1s-PTZ and Rot2s-PTZ, the PET
process from the PTZ unit to the NI chromophore appears to be
predominant. In Rot'2s-PTZ, both am→NI and PTZ→NI electron
transfer processes are detected, probably due to the presence
of conformations where the PTZ unit is pointing either away or
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Figure 6. Simplified energy-level diagram with a schematic representation of the main photophysical processes that occur in the different rotaxanes. The different
arrows departing from the same 1NI excited state denote processes that occur in different conformations of the rotaxane and are not comptetitive processes that
can occur in the same conformation. The reported lifetimes are those measured by transient absorption spectroscopy (from global analysis, in particular, when
performed; see the text). The energy values of the NI-localized singlet and triplet excited states have been taken from the literature, [31] while those of the charge-
separated states have been estimated from redox potentials determined in this work or reported in the literature. [14]
This work was supported by the Italian Ministero dell’Istruzione,
In summary, this study clearly shows that in rather simple
rotaxane-based dyads the intra- and inter-component mobility
leads to a complex situation in which multiple photoinduced
pathways are active. Chemical switching – namely,
Università e Ricerca (FARE grant No. R16S9XXKX3), the
University of Bologna, the University Paris Descartes and the
Centre National de la Recherche Scientifique (CNRS) with the
“délégation” program (B. C.), and the Italian CNR (Project
“PHEEL”).
deprotonation/protonation – affects the photophysical behavior
of the two-station rotaxanes, in a way which is not exclusively
related to the donor-acceptor distance change caused by the
modified ring-axle arrangement. These findings suggest that
rotaxanes with more rigid components are needed in order to
exploit stimuli-controlled ring shuttling for modulating
intercomponent PET reactions. On the other hand, our work
highlights that flexible multichromophoric rotaxanes can enable
the occurrence of through-space electron transfer in spatially
organized chromophores and can provide new challenges for
the experimental investigation of complex dynamic processes,
thereby revealing useful to probe the functional sophistication of
natural architectures.
Keywords: charge separation
•
electron transfer
•
photochemistry
spectroscopy
•
rotaxanes
•
time-resolved
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Electron traffic: multiple light-induced
Benoit Colasson,* Alberto Credi* and
Barbara Ventura*
electron-transfer
processes
are
enabled by mechanically interlocking
molecular components that comprise
chromophoric, electron-donor and
electron-acceptor units.
Page No. – Page No.
Photoinduced Electron Transfer
Involving a Naphthalimide
Chromophore in Switchable and
Flexible [2]Rotaxanes
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