Photochemical triode molecular signal transducer

Article abstract of DOI:10.1021/ja1019595

A molecular hexad in which five bis(phenylethynyl)anthracene (BPEA) fluorophores and a dithienylethene photochrome are organized by a central hexaphenylbenzene unit has been prepared. Singlet-singlet energy transfer among the BPEA units occurs on the 0.4 and 60 ps time scales, and when the dithienylethene is in the open form, the BPEA units fluoresce in the 515 nm region with a quantum yield near unity. When the dithienylethene is photoisomerized by UV light to the closed form, which absorbs in the 500-700 nm region, the closed isomer strongly quenches all of the excited singlet states of BPEA via energy transfer, causing the fluorescence quantum yield to drop to near zero. This photochemical behavior permits the hexad to function in a manner analogous to a triode vacuum tube or transistor. When a solution of the hexad is irradiated with steady-state light at 350 nm and with red light (>610 nm) of modulated intensity, the BPEA fluorescence excited by the 350 nm light is modulated accordingly. The fluorescence corresponds to the output of a triode tube or transistor and the modulated red light to the grid signal of the tube or gate voltage of the transistor. Frequency modulation, amplitude modulation, and phase modulation are all observed. The unusual ability to modulate intense, shorter-wavelength fluorescence with longer-wavelength light could be useful for the detection of fluorescence from probe molecules without interference from other emitters in biomolecular or nanotechnological applications.

Full text of DOI:10.1021/ja1019595

Published on Web 04/21/2010
Photochemical “Triode” Molecular Signal Transducer
Amy E. Keirstead,† James W. Bridgewater, Yuichi Terazono, Gerdenis Kodis,
Stephen Straight, Paul A. Liddell, Ana L. Moore,* Thomas A. Moore,* and
Devens Gust*
Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604
Received March 8, 2010; E-mail: gust@asu.edu
Abstract: A molecular “hexad” in which five bis(phenylethynyl)anthracene (BPEA) fluorophores and a
dithienylethene photochrome are organized by a central hexaphenylbenzene unit has been prepared.
Singlet-singlet energy transfer among the BPEA units occurs on the 0.4 and 60 ps time scales, and when
the dithienylethene is in the open form, the BPEA units fluoresce in the 515 nm region with a quantum
yield near unity. When the dithienylethene is photoisomerized by UV light to the closed form, which absorbs
in the 500-700 nm region, the closed isomer strongly quenches all of the excited singlet states of BPEA
via energy transfer, causing the fluorescence quantum yield to drop to near zero. This photochemical
behavior permits the hexad to function in a manner analogous to a triode vacuum tube or transistor. When
a solution of the hexad is irradiated with steady-state light at 350 nm and with red light (>610 nm) of
modulated intensity, the BPEA fluorescence excited by the 350 nm light is modulated accordingly. The
fluorescence corresponds to the output of a triode tube or transistor and the modulated red light to the grid
signal of the tube or gate voltage of the transistor. Frequency modulation, amplitude modulation, and phase
modulation are all observed. The unusual ability to modulate intense, shorter-wavelength fluorescence
with longer-wavelength light could be useful for the detection of fluorescence from probe molecules without
interference from other emitters in biomolecular or nanotechnological applications.
Introduction
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light, and this basic property has been used in the construction
of multistate molecular switches that employ chemicals and/or
light as inputs and outputs.1-33 Such molecules may act as
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† Present address: Department of Chemistry and Physics, University of
New England, 11 Hills Beach Road, Biddeford, ME 04005.
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6588 J. AM. CHEM. SOC. 2010, 132, 6588–6595
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Photochemical “Triode” Molecular Signal Transducer
A R T I C L E S
rescence is often employed as a particularly useful output for
these devices,38 and fluorescence switching has even been
observed at the single-molecule level.39 The use of light as the
only inputs for molecular switching is especially attractive, as
it does not rely on molecular diffusion for transport, can be
used in rigid media, requires no physical access for addition of
chemicals or wires, and does not produce byproducts that limit
the cycling ability of the system. For these reasons, many of
the molecular switches cited above employ photochromic
compounds (photochromes) that can be photoisomerized be-
tween two metastable species. The photochromes may be used
to control the photochemistry and photophysics of chemically
attached chromophores. Photochromes can quench excited states
via energy transfer or photoinduced electron transfer, act as
relays to mediate energy transfer between chromophores, or
modify the electronic coupling between chromophores, donors,
and acceptors where the coupling occurs either through the
linkage bonds or directly. These systems are usually studied as
binary “on-off” devices, in analogy with digital computers.
Although single molecules may exist only in discrete states,
ensembles of molecules may show analog properties that are
variable over a range of values. This suggests that molecular
systems incorporating photochromic switches should be capable
of acting as analog devices for controlling the properties of
molecular ensembles. This effect was recently demonstrated by
a self-regulating molecule40 that functionally mimics the non-
photochemical quenching mechanism by which plants down-
regulate photosynthesis in the presence of bright sunlight. The
molecule uses a photochrome to control the quantum yield of
photoinduced electron transfer by quenching the singlet excited
state of a porphyrin electron donor.
These results encouraged us to investigate new ways in which
photochrome-containing molecules can perform functions in-
spired by those of analog electronic devices. In a triode vacuum
tube, for example, the current flowing between a cathode and
an anode (device output) is modulated by a voltage applied to
a grid between the two electrodes. When a voltage waveform
(e.g., a sine wave) is applied to the grid voltage, the current
flowing between the cathode and anode is similarly modulated.
A transistor is a more modern implementation of this principle:
current flowing between the source and the drain is modulated
by a voltage applied to the gate.
Here we report a molecular photochemical functional ana-
logue of a triode tube or transistor, molecular hexad 1 (Figure
1). The hexad features five bis(phenylethynyl)anthracene (BPEA)
fluorophores and a dithienylethene (DTE) photochromic switch
organized by a central hexaphenylbenzene core. The hexaphe-
nylbenzene is a relatively rigid structure that constrains the
interchromophore distances and orientations. Hexad 1 may exist
either as 1o with the DTE in the open form, which absorbs
light only at short wavelengths, or as 1c with the DTE in the
Figure 1. Structures of hexad 1o, in which the DTE moiety is in the open
form that does not absorb visible light, and 1c, wherein the DTE is in the
closed, cyclic form that absorbs in the 600 nm region.
cyclic, closed form, which absorbs in the 500-700 nm region.
Excitation of 1c at 350 nm generates first excited singlet states
of the BPEA moieties, which exchange excitation energy rapidly
by singlet-singlet energy transfer. The 350 nm light also
photoisomerizes to 1c any 1o that may be present. Although
BPEA fluoresces in the 520 nm region, the fluorescence from
all of the BPEA moieties in 1c is strongly quenched by energy
transfer to the closed DTE moiety. When the sample is
concurrently irradiated with >610 nm light, some of the sample
is photoisomerized to 1o. The open DTE moiety in 1o has no
effect on the BPEA excited states, and strong fluorescence is
observed from BPEA. Under steady-state illumination, the
photostationary distribution of [1o] and [1c] (and therefore the
BPEA fluorescence intensity) is a function of the relative
intensities of the 350 nm and >610 nm illumination. Figure 2
shows the BPEA fluorescence intensity at 520 nm under
conditions where the intensity of 350 nm light was held constant
and the intensity of >610 nm light was sinusoidally modulated.
The fluorescence at 520 nm was modulated in concert with the
>610 nm light fluctuation. The molecule performed as a
photochemical triode (transistor), where the >610 nm light
played the role of the “grid” potential in a vacuum tube or the
gate voltage in a transistor and the 520 nm fluorescence acted
as the device output. The hexad is photochemically very unusual
in that the intensity of short-wavelength fluorescence (520 nm)
is modulated by irradiation into a transition at longer wavelength
(>610 nm). This behavior, which is impossible to realize with
a single chromophore, could be useful for signal discrimination
(34) Andre´asson, J.; Kodis, G.; Terazono, Y.; Liddell, P. A.; Bandyo-
padhyay, S.; Mitchell, R. H.; Moore, T. A.; Moore, A. L.; Gust, D.
J. Am. Chem. Soc. 2004, 126, 15926–15927.
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Keirstead et al.
Figure 2. Fluorescence emission (arbitrary units) at 520 nm (solid line)
from the BPEA moieties of 1 in 2-methyltetrahydrofuran solution (∼3 ×
10-6 M). The fluorescence was excited by a steady-state light beam at 350
nm (6.0 µW cm-2), which also drives photoisomerization of 1 toward a
photostationary distribution enriched in 1c. The sample was also irradiated
with a beam at >610 nm (dotted line) that was modulated as a sine wave
with a period of 3600 s.
in fluorescence detection for applications such as biomedical
imaging or long-distance detection.
Results and Discussion
Synthesis. Hexad 1 was prepared by functionalization of
hexakis(4-iodophenyl)benzene.41 First, a single DTE was at-
tached by palladium-catalyzed coupling of a suitable precursor.42
The five BPEA moieties were then linked via palladium-
catalyzed coupling of the DTE-bearing hexaphenylbenzene with
9-(3,5-dimethoxyphenyl)ethynyl-10-ethynylanthracene.43 Model
compounds were prepared using related methods. All of the new
Figure 3. (a) Absorption (solid line) and emission (λex ) 400 nm, dashed
line) spectra of a 2-methyltetrahydrofuran solution of model BPEA
chromophore 2. (b) Absorption spectra of a 2-methyltetrahydrofuran solution
of model DTE chromophore 3 in the open (3o, solid line) and closed (3c,
dashed line) isomeric forms.
1
compounds were characterized by H NMR spectroscopy and
MALDI-TOF mass spectrometry. The details are given in the
Supporting Information.
Absorption and Emission Spectra. Figure 3a shows the
absorption and emission spectra of model BPEA 2 (Figure 4)
in 2-methyltetrahydrofuran solution. The absorption spectrum
features maxima at 309, 319, 445, and 469 nm, whereas the
emission maxima are at 483, 515, and ∼547 (sh) nm. The
fluorescence quantum yield is 0.94.44 Figure 3b shows the
absorption spectra of model DTE 3 (Figure 4) in the same
solvent. The open form of the molecule, 3o, shows a maximum
at 305 nm with no absorption in the visible. Irradiation of the
solution of 3o with UV light leads to photoisomerization,
resulting in a solution with a photostationary distribution rich
in the closed form 3c, which features absorption in the visible
with λmax at 602 nm. Irradiation into the visible band converts
3c back to 3o.
Figure 5a shows absorption spectra of hexad 1 in 2-meth-
yltetrahydrofuran. The spectrum of 1o looks very much like
that of model BPEA 2, with maxima at 311, 320, 446, and 469
nm. The bands of 1o in the 310-320 nm region have slightly
different shapes than those of 2 because of the underlying
(41) Kobayashi, K.; Kobayashi, N.; Ikuta, M.; Therrien, B.; Sakamoto, S.;
Yamaguchi, K. J. Org. Chem. 2005, 70, 749–752.
(42) Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am.
Chem. Soc. 2002, 124, 7668–7669.
Figure 4. Structures of model BPEA 2, model DTE 3 in the open (3o)
and cyclic, closed (3c) isomeric forms, and BPEA-DTE dyad 4 with the
DTE portion in the closed form (4c).
(43) Terazono, Y.; Liddell, P. A.; Garg, V.; Kodis, G.; Brune, A.;
Hambourger, M.; Moore, T. A.; Moore, A. L.; Gust, D. J. Porphyrins
Phthalocyanines 2005, 9, 706–723.
absorption by the open DTE (see Figure 3a). After irradiation
of the solution at 365 nm to convert the sample to a photosta-
tionary distribution containing mostly 1c, the absorption spec-
(44) Kodis, G.; Terazono, Y.; Liddell, P. A.; Andre´asson, J.; Garg, V.;
Hambourger, M.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem.
Soc. 2006, 128, 1818–1827.
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Photochemical “Triode” Molecular Signal Transducer
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Figure 6. Transient absorption kinetics for hexad 1 and dyad 4 in
2-methyltetrahydrofuran solution. Samples were excited at 460 nm with
∼100 fs laser pulses and irradiated continuously at 365 nm in order to
maintain the samples in the closed forms (1c and 4c). (a) Transients for 1c
(0) and 4c (b) monitored at 630 nm. (b) Transients for 1c (0) and 4c (b)
monitored at 515 nm. The solid lines show best fits of exponential processes
to the data, and the dashed line in (a) shows the results of a simulation
using a kinetic model (see the text).
resolved studies were performed. As reported previously,44 the
lifetime of the first excited singlet state of the model BPEA
chromophore 2 is 2.80 ns, as determined by time-resolved
fluorescence studies. The energy of the excited state, as
estimated from the wavenumber average of the longest-
wavelength absorption maximum and shortest-wavelength emis-
sion maximum, is 2.61 eV. The lifetime of the first excited
singlet state of the closed form of the DTE model compound,
3c, was previously reported to be 2.9 ps.42
Figure 5. (a) Absorption and (b) fluorescence emission (λex ) 440 nm) of
a 2-methyltetrahydrofuran solution of hexad 1 in the open form 1o recorded
after >610 nm irradiation to achieve a photostationary distribution (solid
lines) and in a photostationary distribution highly enriched in the closed
form 1c (dashed lines).
Model BPEA-DTE dyad 4 (Figure 4) was investigated by
transient absorption spectroscopy in 2-methyltetrahydrofuran
solution. Figure 6 shows the transient absorption traces for hexad
1 and dyad model 4 in 2-methyltetrahydrofuran after excitation
with a 460 nm, ∼100 fs laser pulse. During these studies, the
sample was continuously irradiated at ∼365 nm in order to force
most of the sample into the 4c form. The data in Figure 6a
were obtained at 630 nm. The transient for dyad 4c shows
prompt formation of the BPEA excited state and then rapidly
decays to a negative value representing ground-state bleaching
of the closed-form DTE (DTEc) moiety. The kinetics for 4c at
515 nm show a prompt stimulated emission of BPEA (see Figure
trum shows some decrease in absorption in the 310-320 nm
region and an increase around 375 nm. Most striking is the
appearance of a broad new absorption band centered around
610 nm. These changes are all consistent with isomerization of
the DTE moiety from open to closed (see Figure 3). Irradiation
of the sample with visible (>610 nm) light causes the spectrum
to revert to that of 1o.
Figure 5b shows the fluorescence spectra in the 500-700
nm region for the solutions of 1 whose absorption spectra are
shown in Figure 5a. The emission of 1o, with a maximum at
514 nm and a shoulder around 547 nm, is essentially identical
in shape to the 500-700 nm portion of the emission spectrum
of model BPEA 2 shown in Figure 3a. After conversion of 1o
to a solution containing mainly 1c by irradiation at 365 nm,
the emission spectral shape did not change, but the intensity
decreased by a factor of 8.5. This decrease is ascribed to
quenching of the BPEA emission by the closed DTE moiety.
Clearly, this large decrease indicates that the closed DTE
quenches the emission from not only the two adjacent BPEA
moieties but also the more distant BPEA groups. This is
consistent with the occurrence of singlet-singlet energy transfer
among the BPEA chromophores.
1
6b) that decays to an induced absorption attributed to DTEc.
These data could be fitted to two exponential lifetimes of 1 and
3 ps. Similar studies on 4o (and 1o) obtained by irradiation of
the sample at λ > 610 nm simply yielded transients that did not
decay on the 16 ps time scale, which is consistent with the 2.80
1
ns lifetime of unperturbed BPEA. The results for 4c indicate
1
prompt formation of BPEA, which rapidly (1 ps) decays by
singlet-singlet energy transfer to DTEc to yield 1DTEc, which
in turn decays with τ ) 3 ps. This lifetime for 1DTEc is
consistent with that of model 3c. The extremely short lifetime
of 1BPEA in comparison with that of 4o shows that the quantum
yield of energy transfer is essentially unity.
We now turn to the results for hexad 1c, with the DTE moiety
held in the closed form by UV irradiation as described above
(Figure 6). The transients at 630 and 515 nm were in this case
Time-Resolved Spectroscopic Investigations. In order to learn
more about the interactions among the chromophores of 1, time-
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A R T I C L E S
Keirstead et al.
fitted by three exponential components with lifetimes of 1, 3
(fixed in the fitting process), and 13 ps. The transient in Figure
6a corresponds mainly to decay of 1BPEA with time constants
of 1 ps (70% of the decay) and 13 ps (30% of the decay). We
previously reported studies of the stimulated emission anisotropy
decay of BPEA in a model BPEA dyad whose structure was
similar to that of 2 but had the bromine atom replaced by a
second BPEA unit.44 We found that for 80% of the molecules,
singlet-singlet energy transfer between BPEA units occurred
with a time constant of 0.4 ps, whereas for 20% of the sample,
the energy-transfer time constant was ∼60 ps. The minor 60 ps
component was attributed to molecules in which the BPEA units
were in conformations that were not favorable for energy
transfer.
Given the results for the model compounds, we can interpret
the data for 1c. The majority of the BPEA excited states rapidly
exchange excitation energy by singlet-singlet energy transfer
on the subpicosecond time scale and then transfer the excitation
energy to the DTEc moiety with a time constant of 1 ps. The
DTEc excited state decays with a time constant of 3 ps. With
a minority of the BPEA excited states, exchange of excitation
energy is slower (∼60 ps) because of unfavorable orientations.
The 13 ps lifetime observed for 1c is attributed to decay of the
BPEA excited states by this exchange process followed by a 1
ps transfer to an ortho DTEc, and perhaps by direct energy
transfer to DTEc moieties meta or para to BPEA. This analysis
is likely oversimplified in that the observed lifetimes actually
represent data for a range of conformations and transfer time
constants that are satisfactorily fitted by the three decay times.
The dashed line in Figure 6a is the result of simulating the decay
of the excited states of the BPEA units in 1c with a kinetic
model in which the time constants are drawn from studies of
the model compounds: 0.4 ps for 70% of the BPEA exchange
excitation energy, 60 ps for 30% of the BPEA exchange
excitation energy, and 1 ps for transfer of singlet excitation to
DTEc from the two BPEA moieties ortho to DTEc. The fit is
well within the noise limits of the data. Thus, for hexad 1c, all
of the BPEA excited states decay by singlet-singlet energy
transfer to DTEc, and the quantum yield of this energy transfer
is essentially unity.
The results for 1c obtained at 515 nm (Figure 6b) are
consistent with the above interpretation. The majority of the
BPEA stimulated emission decays with, and the DTE induced
absorption appears to rise with, time constants of 1 and 3 ps.
However, some of the DTE induced absorption appears to decay
with a time constant of 13 ps. This unusual spectral behavior is
an example of inverted kinetics, where a fraction of the DTE
excited states decay more rapidly (3 ps) than they are formed
(13 ps). In summary, the transient spectroscopy shows that hexad
1o exhibits strong BPEA fluorescence with normal excited state
lifetimes but that BPEA singlet excitation energy in hexad 1c
migrates from BPEA to adjacent BPEA chromophores and is
ultimately quenched within a few picoseconds by singlet energy
transfer to DTEc. The BPEA fluorescence is thus very strongly
quenched in 1c, as observed by steady-state fluorescence
spectroscopy.
Figure 7. (a) BPEA fluorescence at 520 nm (O) of a sample of 1o in
2-methyltetrahydrofuran (∼3 × 10-6 M) as a function of time during
irradiation at 350 nm (6.0 µW cm-2). The data were fitted to an exponential
decay (solid curve) having a time constant of 328 s (R ) 0.999). (b)
Fluorescence at 520 nm (O) of a sample of 1 (∼3 × 10-6 M) highly enriched
in 1c in 2-methyltetrahydrofuran as a function of time during irradiation at
350 nm (6.0 µW cm-2) and >610 nm (117 µW cm-2). The data were fitted
to an exponential rise (solid curve) having a time constant of 247 s (R )
0.999). In each case, a photostationary distribution of 1o and 1c was nearly
achieved at the end of the irradiation period shown.
pound for 3, are 0.68 and 0.013, respectively, in organic
solvents.45 The time constants for cyclization and ring opening
of course depend on the irradiation wavelength and lamp
intensity, and in the experiments reported here, the rate of
stirring of the sample, which determines the percentage of time
that a given molecule is exposed to the excitation beam. Under
the experimental conditions employed in these studies, the
BPEA fluorescence at 520 nm of a sample of 1o in 2-meth-
yltetrahydrofuran irradiated at 350 nm (6.0 µW cm-2) decayed
exponentially with a time constant of 328 s (Figure 7a). This
decay represents the approach to a photostationary distribution
of 1o and 1c via photochemical cyclization of 1o. The BPEA
fluorescence is quenched by energy transfer in 1c, as discussed
above. When a similar solution enriched in 1c was irradiated at
both 350 and >610 nm (117 µW cm-2), a photostationary
distribution enriched in 1o was reached with τ ) 247 s (Figure
7b). Thus, for the experiments performed below, the time scale
for photoisomerization to a photostationary distribution from
either 1o or 1c was ∼5 min.
Photoisomerization. Photoisomerization of 1 due to the DTE
moiety was investigated in order to determine suitable time
scales for the photomodulation experiments. Thermal isomer-
ization of 3o and 3c (or 1o and 1c) does not occur on the time
scales of the experiments described here. The quantum yields
for photochemical cyclization and ring opening of 1,2-bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene, a model com-
Modulation Experiments. The experimental setup included
a cuvette containing a solution of 1 in 2-methyltetrahydrofuran
that had been degassed by at least five freeze-pump-thaw
cycles to remove oxygen and then sealed. The presence of
(45) Irie, M.; Lifka, T.; Uchida, K.; Kobatake, S.; Shindo, Y. Chem.
Commun. 1999, 747–748.
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Photochemical “Triode” Molecular Signal Transducer
A R T I C L E S
oxygen greatly accelerated sample photodecomposition. The
sample was continuously illuminated with UV light at 350 nm
from a fluorimeter in order to both induce fluorescence from
BPEA and photoisomerize 1o to 1c. Photoisomerization is much
more rapid with excitation in the 300 nm region (see Figure
3b), but photodecomposition45 is also more rapid in this
wavelength region. The sample was also illuminated at right
angles to the 350 nm light beam with visible light >610 nm in
order to induce photoisomerization of 1c to 1o. This light was
generated by a quartz-halogen bulb and focused through a 610
nm long-pass filter into a fiber-optic cable. The variable intensity
of this beam was computer-controlled via a custom-built
feedback circuit that utilized a linear response photodiode to
monitor the lamp intensity. This circuit ensured that the lamp
intensity was proportional to a computer-supplied voltage (see
the Supporting Information). The BPEA fluorescence was
monitored at right angles to the two excitation beams using the
fluorimeter monochromator at 520 nm and a 10 nm bandpass
filter at 520 nm to further exclude stray light. Sample concentra-
tions were ∼3 × 10-6 M. For each experiment, the intensities
of the 350 nm light and >610 nm light were adjusted to ensure
that a significant amount of 1o and 1c were both present in the
photostationary distribution.
Sine Wave: Frequency and Phase Modulation. One of the
ways that information can be transmitted using radio-frequency
radiation is via frequency modulation, in which information is
carried by changes in the frequency of the wave. Figure 2 shows
the BPEA fluorescence intensity at 520 nm as a function of
time obtained when the >610 nm light was modulated as a sine
wave with a period of 3600 s. The 350 nm light intensity was
6.0 µW cm2, and the >610 nm light was modulated between
117 and 269 µW cm-2. Emission and >610 nm irradiation data
were collected every 10 s. The fluorescence output is a sine
wave having the same period as the modulating beam. The
intensity variations of the maxima and minima were mainly due
to changes in the intensity of the 350 nm beam during the long
irradiation period. It should be noted that there is a small (22°)
phase shift between the modulation waveform and the emission
waveform. If the period of the sine wave had been long enough
that photoisomerization to a photostationary distribution had
been rapid relative to the modulation period, then this phase
shift would not have occurred. It is due to the fact that after
each change in >610 nm lamp intensity, the photoisomerization
to a new photostationary distribution does not quite “catch up”
with the new lamp intensity before the intensity is changed
again. This effect was more pronounced when the period of
the modulation was decreased relative to the time required for
photoisomerization to a photostationary distribution. Figure 8a
shows a sine wave with modulation having a period of 960 s
(350 nm irradiation at 5.6 µW cm-2), for which the phase shift
was 60°. Figure 8b shows a similar experiment using modulation
with a period of 480 s; the phase shift increased to 258°. When
the modulation period is short relative to the time required to
reach a photostationary distribution, changes in the intensities
at 350 or >610 nm can result in changes in the shape of the
fluorescence output waveform as well as a phase shift. In
communications, phase modulation is often used in remote-
control units, for example.
Figure 8. Frequency modulation. The fluorescence emission intensity (solid
line) at 520 nm of a solution of hexad 1 in 2-methyltetrahydrofuran (∼3 ×
10-6 M) is shown. The sample was excited by continuous irradiation at
350 nm (5.6 µW cm-2). The sample was also irradiated by a light beam
with λ > 610 nm whose intensity (dotted line) was modulated between 117
and 269 µW cm-2 by sine waves with periods of (a) 960 and (b) 480 s.
Figure 9. Amplitude modulation. The fluorescence emission intensity (solid
line) at 520 nm of a solution of hexad 1 in 2-methyltetrahydrofuran (∼3 ×
10-6 M) is shown. Fluorescence was excited by continuous irradiation at
350 nm (5.6 µW cm-2). The sample was also irradiated by a light beam
with λ > 610 nm whose intensity (dot-dashed line) was modulated with a
sine wave having a period of 240 s. The intensity range of the modulating
light was changed every five cycles to show the results of amplitude
modulation.
population of 1c and thus the emission intensity for a given
intensity of 350 nm steady-state irradiation. An example is
shown in Figure 9. In this experiment, the intensity of the 350
nm radiation was fixed at 5.6 µW cm-2, and the >610 nm light
was frequency-modulated with a period of 240 s. For the first
five cycles, the >610 nm light was amplitude-modulated between
82 and 269 µW cm-2, and then the modulation was changed to
Sine Wave: Amplitude Modulation. Information can also be
transmitted via radio-frequency radiation by amplitude modula-
tion of a sine wave. In hexad 1, the fluorescence amplitude can
be modulated by a unique mechanism: modulation of the >610
nm irradiation, which changes the average and waveform-driven
-2
82-223 µW cm-2 for five cycles, 82-166 µW cm for five
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A R T I C L E S
Keirstead et al.
Figure 11. Square-wave modulation. The fluorescence emission intensity
(upper line) at 515 nm of a solution of hexad 1 in 2-methyltetrahydrofuran
(∼3 × 10-6 M) is shown. The solution was illuminated with steady-state
350 nm excitation at 6.0 µW cm-2 and with >610 nm light whose intensity
was modulated between 0 and a high intensity (∼300 µW cm-2) with a
period of 1 min on followed by 1 min off (lower line).
Figure 10. Amplitude modulation. The fluorescence emission intensity
(solid line) at 520 nm of a solution of hexad 1 in 2-methyltetrahydrofuran
(∼3 × 10-6 M) is shown. Fluorescence was excited by continuous irradiation
at 350 nm (5.6 µW cm-2). The sample was also irradiated by a light beam
at >610 nm whose intensity (dot-dashed line) was modulated with the
product of two sine waves, one having a period of 200 s and the other
having a period of 2000 s.
light). Although it might at first seem that a similar result could
be easily accomplished by simply modulating the light beam
that excites fluorescence, the hexad operates in a fundamentally
different way. In 1, fluorescence excited through light absorption
by a shorter-wavelength electronic transition is modulated by
light absorbed by a second, longer-wavelength transition. Such
a result is thermodynamically precluded in a single-chromophore
system or a multichromophoric system in which energy is simply
transferred from one moiety to another. It can occur in 1 because
the long-wavelength transition modulates the population ratio
of the two isomeric forms of the DTE, only one of which
quenches BPEA fluorescence. This unique mode of operation
also allows the generation of an output waveform that is phase-
shifted relative to the modulating waveform or even has a
different shape. Simple modulation of the light exciting a
fluorophore cannot accomplish these things.
The phenomena described here might be useful in a variety
of detection and labeling applications. In biomedical imaging,
for example, fluorescence detection of a labeling chromophore
can be very sensitive, but interference and noise are often
introduced by extraneous background light at the emission
frequency or by emission (fluorescence, phosphorescence) from
materials other than the label that are excited by the probe light
(sometimes called “autofluorescence”). Although interference
from background light may be reduced by detecting fluorescence
in-phase with a modulated probe light, this does not discriminate
against emission from materials in the sample other than the
label. The fluorescence of a label having the properties of 1,
however, would be modulated by light of a wavelength longer
than that of the steady-state probe beam that excites fluores-
cence. Thus, the modulation and phase-sensitive detection would
discriminate not only against steady-state background light but
also against all fluorescence by interfering materials, because
any such materials would necessarily emit at wavelengths longer
than that of the modulating beam that excites them. (Two-photon
excitation of a fluorophore could partially discriminate against
interfering fluorescence to the extent that the interfering materi-
als had small cross sections for two-photon absorption.) Similar
advantages could be gained by using this approach for fluores-
cence detection of labeled micro- or nanoparticles or for
fluorescence detection at large distances in the presence of a
-2
cycles, and finally 82-117 µW cm for 5 cycles. With each
change in average modulated lamp intensity, the average
emission intensity decreased accordingly because of the decrease
in the amount of 1o present at and near the photostationary
distribution. In addition, the degree of amplitude modulation
of the emission sine wave decreased because the smaller
variation in intensity of the modulating light gave rise to a
smaller change in the isomer ratio in the photostationary
distribution.
Figure 10 shows another amplitude modulation experiment
that employed a waveform with a period of 2000 s modulating
a “carrier” waveform with a period of 200 s. The 350 nm steady-
state irradiation was at 5.6 µW cm-2, and the modulated
waveform at >610 nm varied from 117-166 µW cm-2. The
overall shape is reminiscent of an amplitude-modulated radio
transmission in which the information is carried by the amplitude
modulation of the fixed-frequency carrier wave.
Square-Wave Modulation. Square-wave modulation of the
>610 nm light beam featured “instantaneous” switching of the
intensity of this beam, and thus, the photoisomerization to a
photostationary distribution did not occur rapidly enough to track
the change in long-wavelength light intensity. Figure 11 shows
the results of an experiment in which the sample was irradiated
with steady-state 350 nm light at 6.0 µW cm-2 and with >610
nm light whose intensity was modulated between 0 and a high
intensity (∼300 µW cm-2) with a period of 1 min on followed
by 1 min off. When the >610 nm light was turned on, only 1c
absorbed, resulting in a net photoisomerization to increase the
concentration of 1o and therefore the fluorescence intensity at
515 nm. When the >610 nm light was then turned off, the
steady-state 350 nm radiation drove the system to an increased
concentration of 1c, with consequent quenching of fluorescence
at 515 nm. Changing the intensity of one of the irradiation beams
could alter the waveform to yield a variety of triangular or
sawtooth forms.
Unique Features, Limitations, and Possible Applications. It
is clear from the above results that hexad 1 can operate as a
kind of molecular photochemical “triode tube” or transistor
wherein modulation of one input (red light) modulates the
fluorescence output excited by a second, steady-state input (blue
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6594 J. AM. CHEM. SOC. VOL. 132, NO. 18, 2010

Photochemical “Triode” Molecular Signal Transducer
A R T I C L E S
variety of interferences. Although the experiments reported
above featured only modulation of the long-wavelength light
beam, various other combinations of frequency and amplitude
modulation could be achieved by modulating both the shorter-
and longer-wavelength input beams. For example, in an
amplitude modulation experiment, one beam could provide
carrier modulation and a second beam the signal modulation.
Also, in the experiments described here, the short-wavelength
(350 nm) input served to both photoisomerize the photochrome
and excite fluorescence of the BPEA moieties. This was done
only for convenience, and in the more general case, two inputs
would drive photoisomerization and a third would excite
fluorescence. Any combination of these could be modulated to
achieve complex data manipulation and transmission.
Although 1 illustrates possibilities for practical application
of the phenomena described, the hexad has several properties
that make it less than ideal. One of these is the time scale for
photoisomerization to a photostationary distribution, which
determines the upper limit to the modulation frequency. The
rate of approach to a photostationary distribution is a function
of the quantum yield of each photoisomerization, the extinction
coefficient of each isomer at the irradiation wavelength, and
the intensity of the two irradiation beams. In principle, the
irradiation beams could be made arbitrarily intense, in order to
drive the approach to a photostationary distribution arbitrarily
rapidly. In practice, this would require not only potentially very
intense irradiation but also very high photostability. The DTE
photochrome in 1 degrades relatively rapidly under UV irradia-
tion in the presence of oxygen. This can be minimized by
deoxygenation, but there is residual degradation by the UV light
due to photoisomerization that leads to an inert byproduct.45
This is not apparent in the data shown here but comes into play
when the modulation experiment is extended significantly. There
are other DTE chromophores that are much more resistant to
photodegradation than the one used in 1.46 In most photo-
chromes, UV light is much more likely to lead to photodegra-
dation than longer-wavelength irradiation. Thus, another solution
to the degradation problem would be to use a photochrome that
is thermally stable in and rapidly thermally isomerized to the
long-wavelength absorbing form. For such a system designed
with a fluorophore that is excited by visible irradiation, UV
irradiation could be avoided completely.
Acknowledgment. This work was supported by a grant from
the U.S. National Science Foundation (CHE-0846943).
Supporting Information Available: Experimental details of
the synthesis and spectroscopic investigations. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA1019595
(46) Irie, M. Chem. ReV. 2000, 100, 1685–1716.
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