Multiple switching and photogated electrochemiluminescence expressed by a dihydroazulene/boron dipyrromethene dyad

Article abstract of DOI:10.1002/anie.200501573

Optical control of the photo- and electrochemiluminescence of a brightly fluorescent reporter group is possible by switching between the photochromic units of dyads 1a and 1b. Irradiation, that is, writing in information, with UV light converts the highly luminescent dihydroazulene 1a into the weakly emissive vinylheptafulvene 1b. Readout of the system can be performed by either electrochemically generated or photogenerated luminescence. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200501573

Angewandte
Chemie
Optical Switches
DOI: 10.1002/anie.200501573
Multiple Switching and Photogated
Electrochemiluminescence Expressed by a
Dihydroazulene/Boron Dipyrromethene Dyad**
Christian Trieflinger, Holger Röhr, Knut Rurack,* and
Jörg Daub*
Optical signal generation, control, and transduction are key
processes in natural and technological systems that essentially
rely on fast and efficient processing of information.[1]
[*] Dr. C. Trieflinger, Prof. J. Daub
Institut für Organische Chemie
Universität Regensburg
Universitätsstrasse 31, 93053 Regensburg (Germany)
Fax: (+49)941-943-4984
E-mail: joerg.daub@chemie.uni-regensburg.de
Dipl.-Phys. H. Röhr, Dr. K. Rurack
Div. I.3
Bundesanstalt für Materialforschung und -prüfung (BAM)
Richard-Willstätter-Strasse 11, 12489 Berlin (Germany)
Fax: (+49)30-8104-5005
E-mail: knut.rurack@bam.de
Dipl.-Phys. H. Röhr
Associated with: Fachbereich Biologie, Chemie, Pharmazie
Freie Universität Berlin
[**] This research was supported by the German Research Foundation
(DFG), the German National Academic Foundation (C.T.), and
BAM’sPhD program (H.R.). Thiswork ispart of the Graduate
College “Sensory Photoreceptors in Natural and Artificial Systems”
supportedby the DFG (GRK 640, University of Regensburg).
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Prominent examples in nature are the vision process, photo-
tropism, photomorphogenesis, and circadian rhythms where
the response to light is realized by a variety of enzymatic
photoreceptors, including rhodopsins, phototropins, phyto-
chromes, cryptochromes, and BLUF domains.[2] All of these
photoreceptors rely on a functional chromophore as the
photoactive element, which initiates a highly sophisticated
signaling cascade with, for example, an electron transfer or a
photochromic reaction as the primary step after light
absorption.
In studying these biological systems chemists have recog-
nized the superiority of signal processing by light in terms of
velocity and efficiency, and they have strived to use light-
driven and -controlled processes in artificial chromophoric
ensembles at the molecular level for technological purposes
such as molecular electronics and photonics, and artificial
photosynthesis. Especially photochromic compounds have
gained strong popularity here, for instance, in high-density
optical data storage and multimode photoswitching.[3] Using
light as a trigger medium and as a tool for readout, the latter
typically by fluorescence, offers the advantages of the non-
destructive control and processing of information. However,
up to now, only a few systems that show photochromically
modulated luminescence have been realized, and the effi-
ciency of the modulation and the overall brightness of the
systems is often rather poor.[4] Furthermore, to our knowl-
edge, a photochromic ensemble in which the switching states
are photocontrolled but the luminescence readout signal can
be sampled by either optical or electrochemical excitation,
i.e., by photo- or electrochemiluminescence (ECL), has not
yet been reported. The latter feature would be a key link
between OLEDs (organic light-emitting diodes) and conven-
tional optical-gate technologies.
We realized our aim of a bright photochromic luminescent
switch that can be either photo- or electrochemically excited
by combining the photochromic system dihydroazulene/vinyl-
heptafulvene (DHA/VHF; Scheme 1a) with the boron dipyr-
romethene (BDP) fluorophore in dyad 1 (Scheme 1b; for
synthetic details see the Supporting Information). The DHA/
VHF system shows UV/Vis photochromism that is partic-
ularly suitable for our present purposes and has been
employed successfully in various molecular switches.[5] The
photochromic conversion of the alternant p system DHA into
the non-alternant VHF occurs by means of a 10-electron
rearrangement of the carbon framework, which strongly
influences the electronic properties of the substituents
attached to the photochromic system.[6] The BDP chromo-
phore provides favorable signaling features, as BDPs show
green emission of high brightness and are also fluorophores
that display photogenerated as well as electrochemically
generated luminescence.[7] Furthermore, the spectral and
redox properties of both subunits complement each other
well for the intended performance.
Scheme 1. The photochromic system dihydroazulene/vinylheptafulvene
DHA/VHF (a) iscombined with the fluorophore boron dipyrromethene
(BDP) to give dyads 1a and 1b (b). The nonswitchable derivative 2
and the model fluorophore 3 (c) were used in control experiments.
Table 1: Absorption (in CHCl3) and redox data (in MeCN) of 1a and 1b
(E in mV vs. Fc+/Fc).
red
1=2
ox
1=2
labs [nm]
E
Ecpred
[mV]
E
Eapox
[mV]
([lg(e)]/[lg(mꢀ1 cmꢀ1)])
[mV]
[mV]
1a
1b
527 (4.86), 376 (4.60)
527 (4.90), 495 (4.69)
ꢀ1615
ꢀ1610
ꢀ1920
ꢀ1325
615
630
1025
760
UV/Vis-spectrophotometric measurements of 1a in
chloroform clearly reveal the independence of both subchro-
mophores, with main bands centered at 376 nm for the DHA
and 527 nm for the BDP moiety (Table 1, Figure 1). The
substantial difference between the lowest energy absorptions
of DHA and BDP guarantees exclusive excitation of the
readout module, the BDP fluorophore, without initiating the
“write-in process”, the photoreaction of DHA. Cyclic vol-
tammetry supports these findings. The reversible reduction
(E1/2 = ꢀ1615 mV vs. Fc+/Fc) and oxidation (E1/2 = + 615 mV
vs. Fc+/Fc) of BDP and the irreversible reduction (Ep =
ꢀ1920 mV vs. Fc+/Fc) and oxidation of DHA (Ep =
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chemiluminescence (ECL) (Figure 2, discrete signals).[8] The
strong fluorescence emission and electrochemically generated
luminescence let us thus define 1a as the ON state of the
system.
Irradiation of 1a at 366 nm induces the uniform photo-
conversion to 1b, which is apparent from the isosbestic points,
the decrease of the band at 376 nm, and the appearance of a
new band at 476 nm in the absorption spectrum (Figure 1).
The BDP absorption remains virtually unaffected, stressing
the fact that both modules of dyad 1b are also decoupled in
the ground state. Again, cyclic voltammetry provides support:
the BDP reduction (E1/2 = ꢀ1610 mV vs. Fc+/Fc) and oxida-
tion (E1/2 = + 630 mV vs. Fc+/Fc) are negligibly shifted
relative to values for 1a. Moreover, the typical irreversible
reduction (Ep = ꢀ1325 mV vs. Fc+/Fc) and oxidation (Ep =
+ 760 mV vs. Fc+/Fc) of the VHF unit of 1b are found at far
lower potentials than those of DHA in 1a.[3c,5d] Photoconver-
sion of 1a to 1b occurs with a quantum yield of ca. 0.01. This is
quite low in comparison to other DHA/VHF systems and can
Figure 1. Absorption and emission spectra of 1a and absorption
spectrum of 1b in CH2Cl2 (c=5”10ꢀ6 m).
+ 1025 mV vs. Fc+/Fc) are centered in the expected potential
regions thus indicating electronically decoupled ground
states.[5d,7b] After excitation of 1a in dichloromethane at l >
500 nm, the typical BDP emission with a maximum at 540 nm,
a fluorescence quantum yield of 0.67, and a single exponential
decay time of 4.06 ns is observed (Figure 2). Changing the
!
be attributed to an overlap of the oscillator-weak S2 S0
!
transition localized on BDP[7e] and the intense S1 S0
transition of DHA. Furthermore, analysis of the fluorescence
excitation spectra revealed that despite the very weak
fluorescence of DHA at room temperature (Ff ꢁ 10ꢀ4),[6a]
energy transfer from DHA to BDP might play an additional,
competitive role here. The OFF state 1b is thermally
reconverted to 1a with t1/2 = 350 min at room temperature.
In contrast to 1a, 1b shows a strongly reduced, typical
green BDP emission upon excitation at l > 500 nm.[9] Ff drops
remarkably from 0.67 to 0.013 (Figure 3) and the fluorescence
Figure 2. Emission spectrum of 1a (c=1”10ꢀ6 m) in CH2Cl2. Discrete
signals: ECL spectrum of 1a (c=1”10ꢀ3 m) in acetonitrile/0.2m
Bu4NPF6. Applied potentialsare marked in the cyclic voltammogram in
the inset.[8]
polarity of the solvent does not alter the spectroscopic
properties significantly (Table 2). Furthermore, as the poten-
tial separation of BDP reduction and oxidation (DEox-red
ꢁ 2.30 mV) is also sufficient to populate the excited state of
the BDP in 1a electrochemically, the dyad shows electro-
Figure 3. Change of the emission intensity during the photoreaction of
1a (c=1”10ꢀ6 m in CH2Cl2) to 1b with excitation of the BDP at
505 nm. Inset: Reversibility of the switch as shown by several cycles of
irradiation and back-reaction.
Table 2: Fluorescence properties of 1a, 1b, and 2.
Solvent
MeCN[c]
Ff[a]
tf [ns][b]
lifetime is shorter (Table 2). The quenching mechanism can
be rationalized in terms of intramolecular fluorescence
resonance energy transfer (FRET). The broad absorption
spectrum of the VHF fragment, which stretches well beyond
550 nm,[6] and the narrow emission band of the BDP unit,
centered between 530 and 540 nm, show considerable over-
lap. Analysis of the FRET process and its features yields a
spectral overlap integral J = 2.6 ” 1014 mꢀ1 cmꢀ1 and an effec-
tive interaction radius R0 = 39 Š.[10] Assuming that no other
processes contribute significantly to the deactivation of the
excited state of the BDP, we calculate the donor–acceptor
distance of the FRET partners to be 15.1 Š,[11] which
1a
1b
2
0.62
0.67
0.66
0.01
0.01
0.01
0.16
0.27
0.67
3.96
4.06
3.87
0.03
0.02
0.03
0.88
1.72
4.13
[d]
CH2Cl2
Et2O[e]
MeCN[c]
[d]
CH2Cl2
Et2O[e]
MeCN[c]
[d]
CH2Cl2
Et2O[e]
[a] Relative to fluorescein 27 in 0.1n NaOH (0.90Æ0.03),[14] Æ5% (1a,
2) Æ15% (1b). [b] At 298 K, Æ0.003 ns. [c] labs/lem =523/536 nm.
[d] labs/lem =526/540 nm. [e] labs/lem =524/536 nm.
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[8] The ECL measurements were carried out with an unstirred
solution (c = 1 ” 10ꢀ3 m; supporting electrolyte: 0.1m Bu4NPF6) in
an electrochemical cell: Stefan Hien, PhD Thesis, University of
Regensburg, 1995. The potential was switched with alternation
between the oxidation and reduction potentials (see inset of
Figure 2, electrochemical switching frequency 1 sꢀ1, scan rate of
the spectrometer: 240 nmminꢀ1). The spectrum consisting of
discrete signals is the result of the relatively slow switching
frequency. The bathochromic shift of ca. 20 nm between the
fluorescence and the ECL spectrum is well known and can be
attributed to reabsorption effects as a result of the higher dye
concentration used in the ECL experiment and the different
dielectric properties of the supporting electrolyte.
[9] VHF was reported to be entirely nonfluorescent, see ref. [6a].
[10] For a description of the analysis of FRET processes, see P. Wu, L.
Brand, Anal. Biochem. 1994, 218, 1 – 13. The following equations
apply: kFRET = (1/tD)(R0/r)6 with r being the distance between the
FRET partners and tD the lifetime of the unperturbed donor 3
(5.15 ns in CH2Cl2). R06 = 8.875 ” 10ꢀ5(k2FDJ/n4) mit J =
sFD(l)eA(l)l4dl, where J is the overlap integral between the
fluorescence of the donor (i.e. 3 in CH2Cl2) and the absorption of
the acceptor, obtained after deconvolution of the absorption
spectrum of 1b, k2 = 2/3 is a geometry factor for random
orientation of the two subunits, FD is the fluorescence quantum
yield of the unperturbed donor (here, for model 3: FD = 0.95 in
CH2Cl2), and n is the refractive index of the solvent.
corresponds within Æ 0.2 Š to the distance between the BDP
core and the center of the VHF unit as obtained for the
ground-state geometry optimized by semiempirical AM1
calculations. Moreover, the absence of solvatokinetic behav-
ior of 1b, that is, that the quenching process is not significantly
accelerated upon increasing the polarity of the solvent, led us
to assume that photoinduced electron transfer (PET) plays a
negligible role here. Control experiments with 2, where the
DHA/VHF moiety has been replaced with a nonswitchable
dihydro-VHF group that isolates the p-dicyanovinylbiphenyl
from the cycloheptatrienyl fragment (Scheme 1c), support
these findings. The spectral characteristics of the BDP
fluorophore in 2 are very similar to those of 1a/1b
(Table 2). Furthermore, the fluorescence of 2 in nonpolar
solvents is as bright as that of 1a. Upon increasing solvent
polarity, a slight quenching of the BDP emission in 2 is
observed. The latter is apparently connected to the moderate
PETactivity of the para-cyanovinylbiphenyl group,[12] empha-
sizing the fact that the entire VHF unit is mandatory for the
performance of 1b. As expected, after the photoreaction to
1b, the latter remains ECL-silent, and 1b thus represents the
OFF state of the system.
In conclusion, we have presented a photochromic pair of
BDP-DHA/VHF dyads as a versatile all-optically control-
lable switch, for which irradiation with near-UV light serves
as the input and luminescence at 540 nm as the output.
Furthermore, the readout signal of the ON state 1a can be
generated by conventional photoexcitation as well as electro-
chemically; in other words, the system shows photogated
electrochemiluminescence. Light-induced conversion of 1a to
1b is accompanied by a 50-fold decrease of luminescence,
defining the system as a true ON/OFF photoswitch. We have
demonstrated here that the outstanding photo- and electro-
chemical properties of both modules, DHA/VHF and BDP,
can be combined synergistically for highly interesting all-
optical photochromic molecular switches. The ECL findings
also indicate potential applications in gated organic light-
emitting diodes (OLEDs).[13]
Received: May 9, 2005
Published online: October 10, 2005
Keywords: boron dipyrromethene · electrochemiluminescence ·
.
fluorescence · molecular switches · photochromism
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[11] If no other deactivation processes are present, kFRET can be also
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