Photonic switching of photoinduced electron transfer in a dithienylethene-porphyrin-fullerene triad molecule

Article abstract of DOI:10.1021/ja026327c

A dithienylethene (DTE)-porphyrin (P)-fullerene (C₆₀) triad molecule in which intramolecular photoinduced electron transfer is controlled by the photochromic DTE moiety has been prepared. Irradiation of the molecule with visible light gives the

Full text of DOI:10.1021/ja026327c

Published on Web 06/06/2002
Photonic Switching of Photoinduced Electron Transfer in a
Dithienylethene-Porphyrin-Fullerene Triad Molecule
Paul A. Liddell, Gerdenis Kodis, Ana L. Moore,* Thomas A. Moore,* and Devens Gust*
Department of Chemistry and Biochemistry, Center for the Study of Early EVents in Photosynthesis,
Arizona State UniVersity, Tempe, Arizona 85287
Received March 26, 2002
Loosely modeled after photosynthetic reaction centers, synthetic
molecules capable of intramolecular photoinduced electron transfer
are promising entries into the realm of molecular-scale optoelec-
tronics. A subset of such molecules consists of porphyrin chro-
mophores covalently linked to electron donors and/or acceptors.1
Excitation of the porphyrin initiates electron transfer, generating
charge-separated states. Although this molecular photovoltaic effect
could be serviceable in device applications, the ability to switch
the photoinduced electron transfer behavior on or off with light
pulses would make these molecules even more interesting and
potentially useful.
Figure 1. Absorption spectra of 2 after VIS (-) and UV (- -) irradiation,
One approach employs a photochromic molecule, which under-
goes reversible light-induced isomerization, as a switching
component.2-4 Recently,5 we suggested that an electron-transfer
switch might be made by linking to a porphyrin-electron acceptor
dyad an auxiliary photochromic moiety that could be isomerized
between a short-wavelength-absorbing form (incapable of affecting
the porphyrin excited singlet state) and a long-wavelength-absorbing
form able to accept singlet excitation energy from the porphyrin,
thereby quenching porphyrin-initiated photoinduced electron transfer
to the acceptor. We reported photonic control of a porphyrin first
excited singlet state lifetime via an attached photochromic spiro-
pyran, but did not demonstrate control of photoinduced electron
transfer via this effect.
and fluorescence spectra (λex ) 550 nm) after VIS (O) and UV (b)
irradiation. The inset shows absorption spectra of model DTE 3 after VIS
(-) and UV (- -) irradiation, and the fluorescence spectrum
(λex ) 600 nm) after UV irradiation (b).
anthracene radical cation was observed, and quenching was
attributed to energy transfer to DTEP.
Encouraged by these findings, we have prepared dithienylethene
(DTE)-porphyrin (P)-fullerene (C60) triad 1 as an electron-transfer
switch. When the DTE is in the open, colorless form (DTEo), the
porphyrin excited state donates an electron to C60, giving DTEo-
P•+-C60•- with a quantum yield of unity. Irradiation of 1 with UV
light converts the DTE into its closed, colored isomer (DTEc). In
this form, the porphyrin excited state is quenched by energy transfer
to DTEc in ∼2 ps, precluding significant electron transfer to the
fullerene. Visible irradiation of DTEc returns it to its colorless form.
The syntheses of 1 and model compounds 2 and 3 were carried
out by using modifications of known procedures.7,10 The absorption
spectrum of model DTEo-P dyad 2 in 2-methyltetrahydrofuran
features porphyrin Q-bands in the 500-650 nm region and a Soret
band at 417 nm (Figure 1). Irradiation with UV light (∼360 nm,
1.5 mW/cm2, 3 min) yields DTEc-P, in which the broad DTEc
absorption at 600 nm underlies the porphyrin bands. Visible (∼600
nm, 1.5 W/cm2, 10 min) irradiation converts the DTEc back to the
open form. Figure 1 also shows the fluorescence of 2. DTEo-P
features typical porphyrin fluorescence. DTEc-P exhibits strongly
quenched porphyrin fluorescence superimposed on a slight amount
of DTEc fluorescence at 717 nm. Single-photon-timing experiments
(λex ) 600 nm) gave a lifetime of 10.5 ns for DTEo-1P. The
fluorescence decay of 2 at 650 nm after UV irradiation features
two significant components with lifetimes of ∼3 ps (90%, ascribed
to DTEc-P) and 10.4 ns (9%, ascribed to a small amount of
DTEo-P).
Dithienylethenes (DTE) are useful and well-studied photochro-
mics that have been used in various molecular switching applica-
tions2,6,7 and shown to quench porphyrin excited states.3,8 In
addition, Effenberger and co-workers have recently reported a dyad
consisting of a dithienylethenepyridinium (DTEP) unit linked to
anthracene.9 In the open form of the DTEP, the anthracene excited
singlet state is quenched by electron donation to DTEP to form a
charge-separated state. When DTEP is in the closed form, the
anthracene excited singlet state is quenched as well, but no
Additional information was obtained from transient absorption
measurements of DTEc-P with porphyrin excitation at 420 nm
(Figure 2). At 0.42 ps, the spectrum is characteristic of the porphyrin
first excited singlet state. At 3 ps, a broad bleaching band due to
formation of the DTEc excited singlet state is superimposed upon
* Corresponding author. E-mail: gust@asu.edu.
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C O MMU N I C A T I O N S
Figure 2. Transient absorption spectra (∼100 fs laser pulse) of 2 after
UV irradiation taken 0.42 (0), 3.0 (b), and 50 (4) ps after excitation.
Inset: kinetic behavior of ∆A at 590 nm, and exponential fit giving time
constants in the text (-).
the porphyrin spectrum. At 50 ps, the DTEc band has disappeared,
leaving only porphyrin bands. These changes are due to formation
of DTEc-1P with the pulse, singlet-singlet energy transfer to the
1
DTEc to yield DTEc-P, and decay of this species, leaving only
Figure 3. (a) Transient absorption of 1 (λex ) 550 nm, 100 ps after laser
excitation) following VIS (-) and UV (- -) irradiation. (b) Transient
absorption at 1000 nm showing electron-transfer switching due to cycling
between DTEc-P-C60 and DTEo-P--C60 by VIS (V) and UV (U) light.
transient absorption of residual DTEo-1P. The inset in Figure 2
shows the kinetic trace at 590 nm. The porphyrin excited state
(formed with the pulse) decays exponentially (τ ) 2.3 ps), with
concomitant formation of bleaching due to the DTEc excited state
•-
photoinduced electron transfer to give DTEo-P•+-C60 with a
1
generated by energy transfer. The DTEc-P bleach decays in turn
quantum yield of charge separation (Φcs) of unity. UV illumination
converts the molecule to DTEc-P-C60, wherein quenching of DTEc-
1P-C60 by energy transfer to the closed dithienylethene in 2.3 ps
precludes significant electron transfer (Φcs ) 0.09). Of course, some
1DTEc-P-C60 formed by energy transfer will be converted to DTEo-
P-C60, with the fraction converted depending on the quantum yield
of isomerization.
Photonic switching of photoinduced electron transfer in molecular
photovoltaic molecules, as demonstrated by 1, could be extended
to more complex molecular systems, and could prove useful in
construction of molecular-scale optoelectronic devices for digital
logic and memory applications.
with a time constant of 2.9 ps, leaving only nondecaying transient
absorption due to residual DTEo-1P. The 1DTEc state of 3 decays
with the same time constant.
The results for 2 show that linking DTE to the porphyrin does
not inhibit its photochromic behavior. They also demonstrate that
although the open form of DTE does not affect the excited-state
properties of the porphyrin, DTE in the closed form quenches the
porphyrin excited singlet state in 2.3 ps by singlet-singlet energy
transfer. These results allowed design of 1 as a molecular switch.
Triad 1 in the open and closed forms has absorption spectra
similar to those of 2, with the addition of weak fullerene absorption
throughout the visible to ∼705 nm. Triad 1 does not show
significant porphyrin fluorescence in either open or closed form.
Acknowledgment. This work was supported by a grant from
the National Science Foundation (CHE-0078835).
1
Studies of a model porphyrin-fullerene dyad show that P-C60 is
quenched by photoinduced electron transfer to form P•+-C60•- with
a time constant of 25 ps.11 Thus, electron transfer from DTEo-1P-
Supporting Information Available: Experimental details for the
synthesis of all new compounds and associated spectroscopic informa-
tion (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
C
60 to the fullerene in 1, yielding DTEo-P•+-C60•-, should be facile.
The much more rapid quenching of the porphyrin singlet state by
the DTEc moiety in 2 suggests that the yield of electron transfer in
DTEc-P-C60 should be reduced by a factor of ∼10.
References
Transient absorption measurements of 1 in 2-methyltetrahydro-
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P-C60 generates DTEo-1P-C60, which decays with a time constant
of 25 ps to yield DTEo-P•+-C60•-, characterized by absorption of
the fullerene radical anion at ∼1000 nm (Figure 3a). DTEo-P•+-
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•-
C60 decays to the ground state with a time constant of 3 ns.
Irradiation of the sample with UV light generates DTEc-P-C60. The
transient spectrum of this species at 100 ps shows no significant
fullerene radical anion absorption in the 930-1050 nm region. No
detectable electron transfer has occurred due to quenching of the
porphyrin first excited singlet state by energy transfer to the
dithienylethene moiety.
The photonic switching of photoinduced electron transfer in 1
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measured after irradiation of the sample with either UV or VIS
light. Visible illumination forms DTEo-P-C60, which undergoes
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