All-Photonic Molecular Half-Adder
A R T I C L E S
tion, when neither input is on, the molecule is in state 1c2c2c,
and no outputs are on. When only one input is turned on, the
total merocyanine concentration rises to a steady-state value
determined by the light flux and the thermal reversion rates
(Figure 2). This gives rise to some absorption at 581 nm, but
the total absorbance is below a threshold level, and does not
trigger an on response (Figure 3b). When the second input, B,
is turned on instead, the same steady-state absorbance is
measured, and the gate remains off. However, when both inputs
are turned on concurrently, the rate of formation of merocyanine
increases, but the rate constants for thermal reversion remain
unchanged. The center of the isomer distribution rises farther
toward the top of the hexagon in Figure 2, and a new, higher
steady-state concentration of merocyanine is reached. The
absorbance at 581 nm increases above the threshold, and the
gate produces an on response (Figure 3b). These responses
follow the truth table of an AND gate.
Figure 4. Structures of the closed, spiro forms of model spiropyran 3c
and model quinoline-derived dihydroindolizines 4c and 5c. The structures
of the open forms (3o, 4o, 5o) are analogous to those of the corresponding
structures in 1o and 2o (Figure 1).
procedures. Additional information concerning the synthesis and
spectroscopic techniques for all compounds is given in the
Supporting Information.
The XOR output is merocyanine fluorescence at 690 nm in
1o2c2c. When 355-nm light is applied to the sample, some
1o2c2c is formed, leading to an increase in fluorescence at 690
nm. However, if the light flux is increased, 1o2o2c and finally
1o2o2o are generated from 1o2c2c, and these are essentially
nonfluorescent due to quenching by the open dihydroindolizine.
The result is that as the 355-nm light intensity is increased from
zero, the steady-state fluorescence at 690 nm initially increases,
goes through a maximum, and then decreases as the population
of quenched isomers increases. In XOR gate operation with two
355-nm inputs, the steady-state emission is therefore signifi-
cantly greater at the lower light levels generated by one input
than at the higher light levels that are produced when both inputs
are turned on (Figure 3c). The switching threshold for the XOR
gate is exceeded when either input A or input B is applied, but
not when both inputs are applied concurrently; this behavior
meets the requirements for an XOR gate. After any combination
of inputs, the gate can be reset to 1c2c2c thermally, or by
irradiating with red light.
From the description above, it is evident that a solution of
122 acts as a half-adder: two binary digits, represented by the
two inputs, are added (Table 1). The paragraphs below discuss
in detail the preparation of the molecule, the photochemical
properties of 122 and two model systems, and the performance
and resistance to photochemical degradation of the half-adder.
Synthesis. The core of 122 was prepared from 5-aminoisoph-
thalic acid, whose amino functionality was protected with a tert-
butoxycarbonyl group. The spiropyran acid was prepared as
previously described.19 The precursor of the quinoline-derived
dihydroindolizine moieties of 122, 6-hydroxy-spiro-[9H-fluo-
rene-9,3′(3′aH)-pyrrolo[1,2-a]-quinoline]-1′,2′-dicarbonitrile, was
prepared using procedures adapted from reported preparations
of related compounds.20 The quinoline-derived dihydroindoliz-
ines were coupled to amine-protected 5-aminoisophthalic acid
using 3-chloro-4,6-dimethoxy-1,3,5-triazine and N-methylmor-
pholine. After removal of the protecting group from the resulting
dyad, the spiropyran acid was attached using 3-chloro-4,6-
dimethoxy-1,3,5-triazine and N-methylmorpholine. Model com-
pounds 3 and 4 (Figure 4) were prepared using similar
Photochemistry of Model Compounds. To understand the
photochemical behavior of the triad, we investigated model
spiropyran 3 and model dihydroindolizine 4 spectroscopically.
The steady-state absorption and emission studies reported below
were performed on solutions (∼1 × 10-5 M) in 2-methyltet-
rahydrofuran at 22 °C, with optical path lengths of 1.0 cm and
sample volumes of 1.5 mL.
Spiropyran 3. As is the case with other spiropyrans, 3 is
thermally stable in the closed, spiro form 3c (Figure 4). The
absorption spectrum of 3c (Figure 5a) features maxima at 335
and 271 nm. There is no absorption in the visible region of the
spectrum. Irradiation at 355 nm causes isomerization to a
photostationary distribution containing mainly the merocyanine
form 3o (see Figure 1). The merocyanine has absorption maxima
at 593, 334, and 274 nm.
Spiro form 3c does not fluoresce significantly. Merocyanine
3o does fluoresce, with λmax ) 664 nm (Φf ) ∼0.03) (Figure
5a). Pump-probe transient absorption investigations were
performed on 3o. A 2-methyltetrahydrofuran solution of 3 was
constantly irradiated at 366 nm to maintain a photostationary
distribution containing mainly the open form, and the solution
was excited at 585 nm with ∼100 fs laser pulses. The transient
absorption was probed at 720 nm (Figure 6). Three components
were observed: a major component (67%) with a lifetime of
45 ( 1 ps, a 140 ( 10 ps component (19%), and a 0.42 ( 0.05
ps component with an amplitude of -14% (grow-in of
stimulated emission amplitude with time). Similar results
showing the presence of and interconversions between isomers
of merocyanines have been observed for other spiropyrans.18,21-28
On the basis of studies of a similar molecule,18 we can assign
the 45 ps lifetime in 3o to the major isomer, which absorbs at
593 nm and emits relatively strongly at 664 nm.
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Phys. Chem. A 2000, 104, 6103-6107.
(20) Du¨rr, H. Photochromism of dihydroindolizines and related systems. In
Organic Photochromic and Thermochromic Compounds; Crano, J. C.,
Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999; pp 223-266.
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