Toward fluorescent memories with nondestructive readout: Photoswitching of fluorescence by intramolecular electron transfer in a diaryl ethene-perylene bisimide photochromic system

Article abstract of DOI:10.1002/anie.200802007

(Chemical Equation Presented) Flicking a switch: Photoinduced electron transfer (PET) occurs between the perylene and the closed (but not the open) form of the diarylethene unit in a prototype optical memory system based on a photochromic diarylethene-perylene bisimide dyad (see picture). This allows three different wavelengths to be used for the read, write, and erase steps without destructive readout occurring by photochromic cycloreversion.

Full text of DOI:10.1002/anie.200802007

Communications
DOI: 10.1002/anie.200802007
Molecular Switches
Toward Fluorescent Memories with Nondestructive Readout:
Photoswitching of Fluorescence by Intramolecular Electron Transfer in
a Diaryl Ethene-Perylene Bisimide Photochromic System**
Martin Berberich, Ana-Maria Krause, Michele Orlandi, Franco Scandola,* and Frank Würthner*
Photochromic compounds change their optical and electronic
properties by reversible photochemical reactions upon irra-
diation and are therefore suitable for many tasks.[1] Very
recently, the application of photochromic compounds to the
imaging of living cells by single-molecule photoswitching has
been demonstrated.[2] In the last few years diaryl ethenes
(DAEs) have evolved as highly promising photochromic
molecules for optical data storage because of their durable
persistency and the thermal irreversibility of their closed and
open forms.[3]
Fluorescence photoswitching of DAEs linked to fluores-
cent dyes has been demonstrated even at the single-molecule
level.[4] In these systems, the readout is based on the observed
Figure 1. Schematic energy diagram for the fluorescent open form
(dashed line)and the nonfluorescent closed form (dotted line)of a
DAE and CT states of the DAE–dye conjugate.
fluorescence of the open form of diaryl ethene (DAEO),
whilst fluorescence quenching from the excited dye to the
closed form of the diaryl ethene (DAEC) takes place by an
intramolecular Förster resonance energy transfer (FRET).
The drawback of such energy-transfer-based memory systems
is that FRET induces photochromic cycloreversion during the
readout step, and thus destroys the memory.
sources of three different wavelengths without destructive
readout.
A photochromic DAE system based on the above-
mentioned optimized concept is, to the best of our knowledge,
unknown.[10] Thus, we have designed the photochromic
system 1o based on a perfluorinated DAE derivative and a
bay-substituted perylene bisimide (PBI) dye (Scheme 1). The
fluorescent 1,7-dipyrrolidinylperylene bisimide was chosen
because of its low oxidation potential and its absorption
maximum in the NIR region (around 700 nm).[11] The photo-
chromic compound 1o was synthesized by imidization of the
Such destructive readout may be circumvented by fluo-
rescence switching through intramolecular photoinduced
electron transfer (PET)[5] if the open and closed forms of
the photochromic moieties were to possess different redox
properties.[6] The optimal case would be if the electron
transfer involves oxidation of the dye and reduction of the
DAE[7,8] and the solvent-dependent driving force for PET is
exergonic for the DAECꢀ–dye+ but not for the DAEOꢀ–dye+
charge transfer (CT) state (Figure 1). Furthermore, the
emission spectrum of the dye should not overlap with the
absorption spectrum of any isomer of the DAE, as in FRET-
based systems, but needs to be at a higher wavelength.[9] This
would facilitate writing, erasing, and reading data with light
[*] M. Berberich, A.-M. Krause, Prof. Dr. F. Würthner
Universität Würzburg, Institut für Organische Chemie
Am Hubland, 97074 Würzburg (Germany)
Fax: (+49)931-888-4756
E-mail: wuerthner@chemie.uni-wuerzburg.de
M. Orlandi, Prof. Dr. F. Scandola
Università di Ferrara, Dipartimento di Chimica
44100 Ferrara (Italy)
Fax: (+39)0532-240-709
E-mail: snf@unife.it
[**] Financial support from MUR (PRIN 2006 030320)is gratefully
acknowledged. We thank S. Carli for preliminary synthetic work and
C. Chiorboli for help with the femtosecond studies.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802007.
Scheme 1. Photochromic compounds 1o and 1c, and reference com-
pounds 2 (perylene bisimide dye)and 3o (DAE).
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respective amino-substituted diaryl ethene with 1,7-dipyrro-
lidinylperylene monoanhydride monoimide (see the Support-
ing Information for details). Compound 1o can be switched
between two photostationary states. Upon irradiation with
UV light (lmax = 350 nm) a photostationary state containing
66% of the closed form of the PBI–DAE conjugate 1c
(Scheme 1) was obtained,[12] which could be isolated by
column chromatography. The open form can be regenerated
almost quantitatively (99%) by irradiation of a solution of the
closed form with visible light (l > 400 nm).
Eox(D) and Ered(A) are the first oxidation potential of the
donor perylene bisimide and first reduction potential of the
acceptor DAE, respectively. E00 signifies the spectroscopic
excited state energy, while RCC is the distance between the
centers of the donor and acceptor moieties. The effective
ionic radii of the donor radical cation and acceptor radical
anion are labeled as r+ and rꢀ, respectively. The dielectric
constant of the reference solvent used in electrochemistry is
denoted as eref, while eS is the dielectric constant of the given
solvent.[13]
The fluorescence quantum yields for 1o and 1c were
determined in a series of solvents of different polarity, with
dielectric constants (er) ranging from 2.24 (tetrachlorome-
thane) to 36.71 (dimethylformamide). These values, divided
by the quantum yield of the reference PBI 2 (without a DAE
moiety), are shown as a function of the er values of the
solvents in Figure 2 (see also Table S1 in the Supporting
Compounds 1o and 1c exhibit the characteristic UV/Vis
spectra of 1,7-dipyrrolidinylperylene bisimides, with an
absorption maximum at around 700 nm. The spectrum of 1c
displays some additional bands (in particular, a broadened
absorption band between 450 and 550 nm, see Figure 3) that
are attributed to the closed form of DAE. From the UV/Vis
and fluorescence spectra recorded in dichloromethane an
S0–S1 excitation energy of 1.71 eV was estimated for 1o and
1c.
Figure 2. Fluorescence quantum yields of 1o and 1c (f )divided by
fl
the quantum yield of the reference perylene bisimide 2 (ꢀrflef)without
the DAE moiety as a function of dielectric constants of solvents; open
Figure 3. UV/Vis spectra of 1o (solid line)and 1c (dashed line)in
dichloromethane.
*
*
form ( ), closed form ( ).
Information). In low-polarity solvents such as chloroform
(er = 4.89), the quantum yields of 1o and 1c are nearly
identical to that of the reference dye 2, which indicates that no
fluorescence quenching by the DAEC or DAEO moieties
occurs. Interestingly, the quantum yields of 1o and 1c differ
significantly at higher solvent polarity, and in dimethylforma-
mide (highly polar) almost complete quenching of the closed
form 1c is observed. Fluorescence quenching by FRET from
the excited dye to the closed form of the DAE can be
excluded as a deactivation path. Thus, the increase in the
fluorescence quenching of 1c as the solvent polarity increases
is highly indicative of a preferential PET from the dye to the
closed form of DAE.
The redox potentials of 1o and 1c and the reference
compounds (see Table S2 in the Supporting Information)
were obtained by cyclic voltammetry measurements in
dichloromethane, with ferrocene used as an internal reference
(Figure 4). The cyclic voltammogram of 1o is identical to that
of the reference dye 2 (see Figure S1 in the Supporting
Information). Thus, additional redox processes associated
with the DAEO moiety are apparently not observable within
the available potential window. The first half-wave oxidation
potentials of the PBImoiety were observed at 0.22 V and
0.21 V for 1o and 1c, respectively. The closed form 1c showed
additional waves for the DAE moiety at ꢀ1.60 V and 1.00 V,
in agreement with the reference DAE compound 3c bearing a
phthalic imide residue (see Figure S3 in the Supporting
Information). The first reduction potential of 3o was
observed at ꢀ1.98 V while the respective wave for 1o was
not observable within the available potential range (up to ca.
ꢀ2.0 V). Donor–acceptor distances of 1.32 nm and 1.24 nm
were estimated from the optimized geometries of 1o and 1c
conjugates, respectively.
The Gibbs free energy (DG8) for an intramolecular
charge-separated state of a covalently bound donor–acceptor
system in a given solvent can be estimated using the Rehm–
Weller Equation (1).
DGꢁ ¼ e½EoxðDÞꢀEredðAފꢀE00
ꢀ
ꢁꢀ
ꢁ
e2
e2
1
1
1
1
Since all experimental values were determined in
dichloromethane, the solvent-related term in the Rehm–
Weller equation can be neglected in the present case, and by
ð1Þ
ꢀ
ꢀ
þ
ꢀ
eref eS
4pe0eS Rcc 8pe0 rþ rꢀ
Rehm ꢀ Weller equation
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Figure 5. Ultrafast transient absorption spectra of a) 1o and b) 1c in
dimethyl sulfoxide; lex =680 nm.
Figure 4. Cyclic voltammograms of a) 1o (2.7”10ꢀ4 m)and b) 1c
(3.8”10ꢀ4 m)in dichloromethane with tetrabutylammonium hexa-
fluorophosphate (0.1m)as the supporting electrolyte, scan rate
100 mVsꢀ1. Fc/Fc+ =ferrocene/ferrocenium couple.
former is the rate-determining step in *PBI–DAEC deactiva-
tion. Clear evidence for the occurrence of photoinduced
electron transfer is obtained from femtosecond transient
absorption experiments by excitation at 400 nm (the S0!S2
transition of the dye). In this case, while the behavior of 1o is
virtually identical to that observed upon excitation at 680 nm
(see Figure S4 in the Supporting Information), an additional
short-lived positive absorption band with a maximum cen-
tered at 613 nm is observed for 1c (Figure 6). The latter can
be assigned to the radical cation of the 1,7-pyrrolidinylper-
ylene bisimide moiety,[15] which was verified by spectroelec-
trochemistry (see Figure S5 in the Supporting Information).
In this case, population of the DAECꢀ–PBI+ state from the
upper S2 state of the dye, rather than from the S1 state, is fast
applying the obtained data in this equation, a DG8 value of
0.37 eV for 1o and a slightly exergonic value of ꢀ0.03 eV for
1c in dichloromethane can be calculated. These energy values
imply that the electron-transfer process, clearly endergonic
for the open form, is thermodynamically feasible only for the
closed form in dichloromethane. In more polar solvents such
as dimethyl sulfoxide (er = 46.68), the CT state for 1c
(DAECꢀ–PBI+) becomes more stable[14] and PET is more
likely to take place (Figure 2). Fluorescence lifetime meas-
urements in dimethyl sulfoxide solution provided values of
1.9 ns for 1o and 0.23 ns for 1c, thus indicating that the
excited fluorophore is subject to an additional quenching
process when the photochrome is in its closed form (see
Table S3 in the Supporting Information).
Photochromic compounds 1o and 1c were also inves-
tigated by femtosecond transient absorption spectroscopy in
dimethyl sulfoxide by excitation at 680 nm (the S0!S1
transition of the perylene bisimide dye). An intense bleaching
between 650 and 770 nm that arises from the depopulation of
the ground-state molecules and the stimulated emission
between 770 and 800 nm is observed as a combined negative
signal in the spectrum of 1o (Figure 5a) that decays with a
lifetime of approximately 1.5 ns. Compound 1c shows the
same qualitative behavior (Figure 5b), but with a greatly
reduced lifetime of 230 ps. The fact that the quenching of the
dye S1 state is not accompanied by a distinctive accumulation
of a transient charge-separated species suggests that charge
separation is slower than charge recombination and the
Figure 6. Ultrafast transient absorption spectra of 1c in dimethyl
sulfoxide; lex =400 nm.
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Received: April 29, 2008
Published online: July 24, 2008
Keywords: electron transfer · fluorescence · molecular devices ·
.
perylene bisimides · photochromism
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