The efficient photoinduced electron transfer from a stilbene derivative incarcerated within a negatively charged organic nanocapsule to positively charged acceptors (methyl viologen and a pyridinium salt) adsorbed outside and the back electron transfer were controlled by supramolecular effects

Jayathirtha Rao, V.

Jockusch, Steffen

Parthasarathy, Anand

Porel, Mintu

Ramamurthy, V.

Turro, Nicholas J.

Chemical Communications

English

Publications of the IAS Fellows

2710 Chem. Commun., 2012, 48, 2710–2712 This journal is c The Royal Society of Chemistry 2012
Cite this: Chem. Commun., 2012, 48, 2710–2712
Photoinduced electron transfer between a donor and an acceptor
separated by a capsular wallw
Mintu Porel,a Steffen Jockusch,b Anand Parthasarathy,a V. Jayathirtha Rao,a
Nicholas J. Turrob and V. Ramamurthy*a
Received 19th December 2011, Accepted 16th January 2012
DOI: 10.1039/c2cc17938e
The efficient photoinduced electron transfer from a stilbene
derivative incarcerated within a negatively charged organic
nanocapsule to positively charged acceptors (methyl viologen
and a pyridinium salt) adsorbed outside and the back electron
transfer were controlled by supramolecular effects.
Recent establishment of spin and energy communication between
two molecules, one trapped within an organic capsule and the
other free in solution, led us to investigate the feasibility of
electron transfer between such molecules.1 The electron donor
used in this study, 4,40-dimethyl stilbene (DMS), is enclosed
within a capsule made of two octa acid (OA) molecules and the
electron acceptors explored are N-methyl pyridinium iodide
(Py+) and 4,40-dimethyl viologen chloride (MV2+) (Scheme 1).
The choice of the pair was made based on well-established
literature reports that photoinduced electron transfer between
stilbene and the above electron acceptors is exothermic and
occurs in solution.2 Based on excitation energy and oxidation
potential of DMS and reduction potentials of Py+ and MV2+
the electron transfer in both systems is expected to be exothermic
(B1.3 eV in the case of MV2+ andB0.5 eV in the case of Py+).3
In this report we present results demonstrating (1) that the
electron transfer between excited DMS and the above cationic
acceptors takes place despite their separation by the atoms of the
capsular wall and (2) the control of the back electron transfer
process by judicious choice of electron acceptors.
We have established previously that DMS formed a 1 : 2
(guest to host) complex with OA in aqueous borate buffer
solution (pHB9.0).4 In Fig. S1 in the ESIw 1H NMR spectra
of free DMS, Py+ and MV2+ and DMS@OA2 alone (one
molecule of guest included within two molecules of host OA)
and in the presence of Py+ and MV2+ are provided. Based on
the large upfield shift of 1H NMR signals of the 4-methyl
group of DMS and the corresponding small shifts of the
methyl group(s) of Py+ and MV2+ in the presence and
absence of OA we concluded that DMS is encapsulated inside
the OA host and Py+ and MV2+ are located outside the
capsule. The DOSY spectra presented in Fig. S2 and S3 in
ESIw indicated reduced mobility of the cationic guests Py+
and MV2+ in the presence of DMS@OA2. For example, the
diffusion constant for Py+ in the presence of OA was reduced to
4.8 1010 m2 s1 from that in water (8.9 1010 m2 s1). Guest
MV2+ had an identical diffusion constant (1.2  1010 m2 s1)
to that of DMS@OA2 suggesting that DMS, OA and MV
2+
move together in aqueous solution. Based on DOSY data we
conclude that the cationic acceptors Py+ and MV2+ remain
closely associated with DMS@OA2 due to electrostatic inter-
action between their cationic pyridyl parts and the carboxylate
anion groups of OA. Apparently, Py+ is not associated to the
capsule as strongly as MV2+. The preferential intracapsular
location of the neutral stilbene and cationic guests in proximity
of the exterior walls of OA is consistent with our previous
observations with cationic and neutral nitroxides.1a–d
The first indication of interaction between OA-trapped
excited DMS and free Py+ and MV2+ came from fluorescence
spectra of DMS@OA2 in their presence. As illustrated in Fig. 1a
and Fig. S4 (ESIw) addition of Py+ or MV2+ to a solution of
DMS@OA2 resulted in quenching of the fluorescence of DMS.
Stern–Volmer plots that include Io/I and to/t vs. concentration
shown in Fig. 1b and Fig. S5 (ESIw) suggested that the quenching
was entirely static for MV2+, and mostly static for Py+. Note,
had the quenching been dynamic, the Stern–Volmer plots based
on steady state fluorescence intensity and lifetime measurements
should have fully overlapped, but this was not the case.
The origin of the quenching became clearer from the absorption
spectra of the transient intermediates of DMS@OA2 in the
presence of Py+ and MV2+ recorded by laser flash photolysis.
In both cases transient absorptions at 510 and >700 nm were
observed (Fig. 2), which were assigned to the radical cation of
Scheme 1 Structures of host and guest molecules; counter anions of
Py+ and MV2+ are I and Cl respectively.
aDepartment of Chemistry, University of Miami, Coral Gables,
Florida 33124, USA
bDepartment of Chemistry, Columbia University, New York,
New York 10027, USA
w Electronic supplementary information (ESI) available: Experimental
details, NMR, DOSY, fluorescence and transient absorption data. See
DOI: 10.1039/c2cc17938e
ChemComm Dynamic Article Links
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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 2710–2712 2711
DMS (DMS+) based on previously reported transient absorption
spectra of DMS+.2 Both absorptions showed the same decay
kinetics. This suggests that the quenching is due to electron
transfer from the singlet excited state of DMS to Py+ and
MV2+ that are associated to the capsule. The excellent
correlation observed between bleaching of DMS (observed at
320 nm) and generation of DMS+ proved that the observed
transient spectra are not artifacts.
Further support for the assignment of the observed transient
at 510 nm to a radical cation was provided by the absence of
quenching by dissolved oxygen (oxygen saturated solution;
Fig. S6 in ESIw). Most importantly, the methyl viologen
monocation radical (MV+) spectrum (Fig. 2b)5 provided
unequivocal support for electron transfer across the capsular
wall. Thus we have been able to directly identify both the
products of electron transfer in the case of MV2+, namely
DMS+ and MV+. However, the spectrum of an N-methyl
pyridinium radical (Py generated from Py+) could not be
detected because it does not possess detectable absorption at
350 to 800 nm.6 Based on the above data we conclude that
photoinduced electron transfer between excited DMS@OA2
and Py+ and MV2+ occurs under our experimental conditions.
Examination of Stern–Volmer plots shown in Fig. 1b suggests
that the electron transfer is much more efficient in the case of
MV2+ compared to Py+. The lower efficiency in the case of
Py+ is probably due to the weaker binding of Py+ (+1 charge)
compared to MV2+ (+2 charge) to the negatively charged
exterior walls of OA as suggested by DOSY data and/or the
electron transfer process is much less exothermic than in the
case of MV2+.3
Support for the hypothesis that the electron transfer is
indeed between a molecule within a capsule and the other
adjacent to it came from experiments involving cucurbit[7]uril
(CB7) as the second host molecule. As illustrated in Fig. 3 the
quenched fluorescence of DMS by Py+ could be fully recovered
upon addition of CB7 to the solution. Consistent with this, in the
presence of CB7 there was no transient absorption due to DMS+
(510 nm) and bleaching of the DMS ground state (320 nm) did
not occur. Similar observations were made withMV2+ (Fig. S7 in
ESIw). From 1H NMR spectra (Fig. S8 and S9 in ESIw) we
inferred that both Py+ and MV2+ complexed to CB7 under our
conditions. This is consistent with the known high binding
constant of MV2+ to CB7.7 The above stated fluorescence
quenching by Py+ and MV2+ and fluorescence recovery by
CB7 suggested that removal of electrostatically held Py+ and
MV2+ from the capsular wall inhibited the electron transfer.
Given that the OA capsular assembly–disassembly occurs in the
microsecond time scale,8 this process is unlikely to play a role in
the above observed electron transfer process that takes place in
the sub-nanosecond time scale. Based on the above results we
conclude that generation of DMS+ is due to electron transfer
from DMS inside the OA capsule to the acceptors Py+ and
MV2+ that are closely associated to the external capsular wall.
Our most important observation relates to the back electron
transfer process to regenerate the ground states of DMS, Py+
and MV2+ (Scheme 2). As illustrated in Fig. 2, DMS+ had
lifetimes of 4.6 ms and o20 ns when generated via electron
transfer to Py+ and MV2+, respectively. This variation in
lifetimes is understandable on considering the products of
Fig. 1 (a) Fluorescence spectra of DMS@OA2 at different amounts of
MV2+. (b) Stern–Volmer plots of fluorescence quenching with Py+ (red)
and MV2+ (blue) using steady-state fluorescence intensity (solid circles)
and fluorescence lifetime (hollow circles); [DMS] = 1.25  105 M,
[OA] = 2.5  105 M, lex = 320 nm.
Fig. 2 Transient absorption spectra after laser excitation of DMS@OA2
in the presence of (a) Py+ and (b) MV2+; right: kinetic traces at different
observation wavelengths. [DMS]= 1.25 105M, [OA]= 2.5 105 M
and [Py+] = 31.25  105 M and [MV2+] = 2.5  105 M in 10 mM
sodium tetraborate buffer; laser pulse: 308 nm, pulse width: 15 ns.
Fig. 3 (a) Fluorescence spectra of DMS@OA2 in the absence and
presence of Py+ and CB7 (lex = 320 nm); transient absorption decay
traces of (b) bleaching and recovery of DMS and (c) decay of DMS+
in the absence and presence of CB7; [DMS] = 1.25  105 M,
[OA] = 2.5  105 M, [Py+] = [CB7] = 31.25  105 M in 10 mM
sodium tetraborate buffer; laser pulse: 308 nm, pulse width: 15 ns.
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2712 Chem. Commun., 2012, 48, 2710–2712 This journal is c The Royal Society of Chemistry 2012
electron transfer to these acceptors; Py+ upon acceptance of
an electron yielded Py (no charge) while MV2+ generated
MV+ (positively charged). The former is not expected to stick
to the capsule while the latter with a positive charge would still
be associated with the capsule. This difference would make the
rates of back electron transfer to be different in these two
cases. A comparison of DMS+ decay provided in Fig. S10
(ESIw) reveals that the intensity of the signal due to DMS+ is
distinctly weaker in the case of MV2+ than in Py+. We believe
that the observed weaker DMS+ signal intensity in the
presence of MV2+ is caused by fast back electron transfer
occurring in the nanosecond timescale during the laser pulse
(laser pulse width 15 ns).
Balzani and co-workers, in their pioneering studies on
electron transfer in supramolecular assemblies, reported that
a biacetyl triplet included within a hemicarcerand is quenched
by aromatic amines with rate constants in the range of 104 to
108 M1 s1.9 The low rate constants were attributed to small
electronic interaction between the incarcerated biacetyl acceptor
and free donor amines. Due to the static nature of the quenching,
we were unable to measure the exact rate constant of electron
transfer. However, we believe that the quenching rate constant in
our system must be higher than the fluorescence decay constant
of DMS (>109 s1). In our systems, the weak electronic coupling
between the excited donor and the acceptor through the capsular
wall is most likely compensated by the strong association of the
acceptor (Py+ or MV2+) to the negatively charged external wall
of the capsule (that contains the donor) through electrostatic
attraction.
The final point relates to the ability of the host OA itself to
act as an electron donor. Closer examination of Fig. 2b reveals
that even after the complete decay of DMS+, some amount of
MV+ is left in solution (compare the spectra at 0 and 100 ns).
This suggested the possibility of OA itself acting as a donor.
This was probed by exciting a solution of OA/MV2+ (free of
DMS) with laser pulses of 308 nm. As illustrated in Fig. S11
(ESIw) MV+ is detected even in the absence of DMS.
However, the signal intensity of MV+ was weaker than when
DMS is present in solution. Since DMS has a much higher
absorption co-efficient than OA, we believe that the direct
electron transfer between OA and Py+ andMV2+ plays only a
minor role under our conditions (Fig. S12 in ESIw). However,
at present we do not clearly understand why MV+ generated
via direct electron transfer from OA has a long lifetime. We are
currently investigating this aspect in more detail.
The above observations suggest that electron transfer can
occur between incarcerated and free molecules and the back
electron transfer rates in photoinduced electron transfer processes
can be controlled by applying supramolecular concepts. Photo-
induced electron transfer between cyclodextrin, cucurbituril
and hemicarcerand enclosed dyes and TiO2 in the context of
dye-sensitized solar cells has in fact attracted considerable
interest in recent years.10 We are currently examining the
photoinduced electron transfer phenomenon of guest@OA2
adsorbed on TiO2 surfaces. We envision that the current study,
establishing the feasibility of electron transfer across molecular
walls, will lay the ground work for exploration of OA and
related deep cavity cavitands as supramolecular hosts in
controlling dye aggregation and the back electron transfer
process in solar energy capture and release.
The authors thank the National Science Foundation for
its generous support of this research through grants NSF-
CHE-08-48017 (VR) and NSF-CHE-11-11398 (NJT).
Notes and references
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V. Ramamurthy, Can. J. Chem., 2011, 89, 203.
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10 (a) C. Pagba, G. Zordan, E. Galoppini, E. L. Piatnitski, S. Hore,
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Scheme 2 Forward and back electron transfer from DMS@OA2 to
(a) Py+ and (b) MV2+.
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Photoinduced electron transfer between a donor and an acceptor separated by a capsular wall

Photoinduced electron transfer between a donor and an acceptor separated by a capsular wall

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