Digital processing with a three-state molecular switch

Article abstract of DOI:10.1021/jo0340455

Certain molecular switches respond to input stimulations producing detectable outputs. The interplay of these signals can be exploited to reproduce basic logic operations at the molecular level. The transition from simple logic gates to complex digital circuits requires the design of chemical systems able to process multiple inputs and outputs. We have identified a three-state molecular switch that responds to one chemical and two optical inputs producing two optical outputs. We have encoded binary digits in its inputs and outputs applying positive logic conventions and demonstrated that this chemical system converts three-digit input strings into two-digit output strings. The logic function executed by the three-state molecular switch is equivalent to that of a combinational logic circuit integrating two AND, two NOT, and one OR gate. The three states of the molecular switch are a colorless spiropyran, a purple trans-merocyanine, and its yellow-green protonated form. We have elucidated their structures by X-ray crystallography, ¹H NMR spectroscopy, COSY and NOE experiments, as well as density functional calculations. The three input stimulations controlling the interconversion of the three states of the molecular switch are ultraviolet light, visible light, and H⁺. The two outputs are the absorption bands in the visible region of the two colored states of the molecular switch. We have monitored the switching processes and quantified the associated thermodynamic and kinetic parameters with the aid of ¹H NMR and visible absorption spectroscopies.

Full text of DOI:10.1021/jo0340455

Digita l P r ocessin g w ith a Th r ee-Sta te Molecu la r Sw itch
Franc¸isco M. Raymo* and Silvia Giordani
Center for Supramolecular Science, Department of Chemistry, University of Miami, 1301 Memorial Drive,
Coral Gables, Florida 33146-0431
Andrew J . P. White and David J . Williams*
Department of Chemistry, Imperial College, South Kensington, London SW7 2AY, UK
fraymo@miami.edu; d.williams01@ic.ac.uk
Received J anuary 15, 2003
Certain molecular switches respond to input stimulations producing detectable outputs. The
interplay of these signals can be exploited to reproduce basic logic operations at the molecular
level. The transition from simple logic gates to complex digital circuits requires the design of
chemical systems able to process multiple inputs and outputs. We have identified a three-state
molecular switch that responds to one chemical and two optical inputs producing two optical outputs.
We have encoded binary digits in its inputs and outputs applying positive logic conventions and
demonstrated that this chemical system converts three-digit input strings into two-digit output
strings. The logic function executed by the three-state molecular switch is equivalent to that of a
combinational logic circuit integrating two AND, two NOT, and one OR gate. The three states of
the molecular switch are a colorless spiropyran, a purple trans-merocyanine, and its yellow-green
protonated form. We have elucidated their structures by X-ray crystallography, 1H NMR
spectroscopy, COSY and NOE experiments, as well as density functional calculations. The three
input stimulations controlling the interconversion of the three states of the molecular switch are
ultraviolet light, visible light, and H+. The two outputs are the absorption bands in the visible
region of the two colored states of the molecular switch. We have monitored the switching processes
and quantified the associated thermodynamic and kinetic parameters with the aid of 1H NMR and
visible absorption spectroscopies.
In tr od u ction
puters, where networks of interconnected gates execute
complex logic operations relying on the interplay of
electrical inputs and outputs. To reproduce the functions
of these devices with organic compounds, strategies to
transduce input stimulations into detectable outputs at
the molecular level must be developed. Certain molecules
switch between two or more states in response to chemi-
cal, electrical, or optical signals. Often, the stereoelec-
tronic properties of the various states are sufficiently
different to produce distinct spectroscopic or voltammet-
ric responses. It follows that the behavior of these
molecular switches4,5 is equivalent to that of conventional
logic gates. Both can transduce input signals into output
signals. Indeed, their analogy has led to the development
of chemical systems able to execute the three basic logic
The rapid progress of information technology continues
to stimulate the exploration of innovative materials and
architectures for digital processing. Organic molecules
appear to be potential candidates for the realization of
future data-elaboration, -storage, and -communication
devices.1 Their nanoscale dimensions and the power of
chemical synthesis offer access to ultraminiaturized
building blocks with tailored structures and properties.
These promising features are encouraging the design of
practical strategies to integrate molecular components
into functioning electronic, optoelectronic, and all-optical
devices. Indeed, examples of molecular memories, modu-
lators, rectifiers, switches, transistors, and wires are
starting to be developed adapting the operating principles
of conventional devices.2
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Logic gates convert input signals into output signals
according to precise protocols.3 Their processing ability
ensures the elaboration of information in present com-
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10.1021/jo0340455 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/01/2003
4158
J . Org. Chem. 2003, 68, 4158-4169

Digital Processing with a Three-State Molecular Switch
functions (AND, NOT, OR) and simple combinations of
them (EOR, Half-Adder, INH, NOR, XNOR, XOR).6-25
Conventional logic circuits are assembled connecting
the input and output terminals of multiple gates.3 The
logic operations of the individual components and the
interconnection scheme determine the logic function
executed by the entire circuit. Thus, designed logic
functions can be implemented in combinational circuits
by selecting the number, type, and configuration of the
individual operators. At the molecular level, however,
relatively complex logic functions resulting from the
combination of multiple gates can be programmed into
a single switch. It is necessary, however, to design
chemical systems that can respond to multiple inputs
producing multiple outputs. In this paper, we demon-
strate that a combinational logic function can be imple-
mented with a three-state molecular switch able to
transduce one chemical and two optical inputs into two
optical outputs. In particular, we have selected for our
studies a member of a well-known family of photochromic
switches, namely a spiropyran.2g,5b,f,26
Resu lts a n d Discu ssion
Syn th esis. The spiropyran (R/ S)-SP (Figure 1) was
synthesized in three steps starting from the commercially
available compound 2,3,3-trimethyl-3H-indole. Alkylation
of 2,3,3-trimethyl-3H-indole with 2-bromoethanol gave
the bromide salt 1 in a yield of 69%. Treatment of 1 with
KOH afforded the oxazole derivative (R/ S)-2 in a yield
of 88%. Reaction of (R/ S)-2 with 2-hydroxy-5-nitrobenz-
aldehyde gave the target molecule (R/ S)-SP in a yield
of 79%.
Nu clea r Ma gn etic Reson a n ce Sp ectr oscop y. The
chiral spirocenter of (R/ S)-SP imposes two different
environments (A and B in Table 1) on the methyl protons
Ho and Hp. Similarly, it imposes the two environments
C and D on the methylene protons Hj and Hk and the
two environments E and F on the methylene protons Hl
and Hm. As a result, two distinct singlets (1.24 and 1.14
ppm) for the methyl protons Ho/Hp and two complex
multiplets (3.72-3.47 and 3.38-3.14 ppm) for the two
pairs of methylene protons Hj/Hk and Hl/Hm are observed
in the 1H NMR spectrum (Figure 2a). The methylene
protons Hj and Hk are coupled to the adjacent hydroxy
proton Hn, which resonates as a pseudotriplet at 2.80
ppm. Consistently, the COSY spectrum (Figure S-1,
Supporting Information) shows cross-peaks between the
pseudotriplet at 2.80 ppm and the multiplet at 3.72-3.47
ppm. The methylene protons Hl and Hm are close to the
aromatic proton Hh. In fact, a positive NOE of the doublet
for this proton at 6.65 ppm (Figure 3a) is observed after
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J . Org. Chem, Vol. 68, No. 11, 2003 4159

Raymo et al.
protons Hb and Hg were assigned following the coupling
pattern in the COSY spectrum. Cross-peaks between (1)
the doublet for Ha at 8.06 ppm and the doublet of doublets
for Hb at 7.99 ppm and (2) this doublet of doublets and
the doublet for Hg at 6.71 ppm can be identified (Figure
S-1, Supporting Information).
1
The high-field H NMR spectrum of a CD3CN solution
of (R/ S)-SP equilibrated in the dark shows an additional
weak singlet at 1.77 ppm for the six equivalent methyl
protons of trans-ME (Figure 4). The ratio between the
two isomers is (26 ( 3) × 10 in favor of (R/ S)-SP and
corresponds to an equilibrium constant (K1) of (40 ( 5)
× 10-4 at 298 K (Table 2). After ultraviolet irradiation,27
the singlet at 1.77 ppm increases in intensity (Figure
S-3a, Supporting Information), and additional signals can
1
be observed in the H NMR spectrum. In particular, a
doublet for one of the olefinic protons of trans-ME can
be identified at 6.50 ppm. Its large coupling constant (16
Hz) confirms the trans configuration of the associated
double bond.28
The relatively fast thermal reisomerization of trans-
ME to (R/ S)-SP prevents an accurate quantification of
1
the photostationary state by H NMR spectroscopy. The
ratio between (R/ S)-SP and trans-ME appears to be ca.
49, after the delay required to record the spectrum, and
changes back to the original value in less than 5 min in
the dark. The addition of 1 equiv of CF3CO2H im-
mediately after ultraviolet irradiation, however, gener-
ates the protonated form trans-MEH (Figure 4) slowing
the formation of (R/ S)-SP . Consistently, the 1H NMR
spectrum recorded at this point reveals the resonances
of (R/ S)-SP and trans-MEH in a ratio of ca. 9.
The addition of 1 equiv of CF3CO2H to a CD3CN
F IGURE 1. Synthesis of the spiropyran derivative (R/ S)-SP .
solution of (R/ S)-SP maintained in the dark produces
1
significant changes in the H NMR spectrum. The two
irradiation at 3.26 ppm (Figure S-2a, Supporting Infor-
mation). The signals for the other three aromatic protons
Hc, Hf, and Hd on the indoline fragment were assigned
following the coupling pattern in the COSY spectrum.
Cross-peaks between (1) the doublet for Hh at 6.65 ppm
and the pseudotriplet for Hc at 7.14 ppm, (2) this
pseudotriplet and another one for Hf at 6.82 ppm, and
(3) the pseudotriplet for Hf and a doublet for Hd at 7.10
ppm can be identified (Figure S-1, Supporting Informa-
tion). This assignment was confirmed exploiting the
proximity of Hd to the methyl protons Ho and Hp. A
positive NOE of the doublet for Hd is observed after
irradiation at either 1.24 or 1.14 ppm (Figure S-2b,c,
Supporting Information). The irradiation at 1.14 ppm
also produces a positive NOE of the doublet for Hi at 5.98
ppm. This effect is not observed after irradiation at 1.24
ppm. In fact, only the methyl protons in the environment
B are sufficiently close to the olefinic proton Hi to induce
a NOE. This proton is coupled to the other olefinic proton
He and, consistently, the COSY spectrum (Figure S-1,
Supporting Information) shows cross-peaks between the
doublet for Hi at 5.98 ppm and that for He at 7.02 ppm.
The cis arrangement of the two olefinic protons is
confirmed by the positive NOE of the doublet for He at
7.02 ppm observed after irradiation at 5.98 ppm (Figure
S-2d, Supporting Information). Irradiation at 7.02 ppm
produces, in turn, a positive NOE of the doublet for the
aromatic proton Ha at 8.06 ppm (Figure S-2e, Supporting
Information). The signals for the other two aromatic
singlets for the methyl protons Ho and Hp coalesce (Figure
2b), and the two multiplets for the two pairs of methylene
protons Hj/Hk and Hl/Hm resolve into two triplets. These
changes indicate that, under these conditions, the three
sets of protons Ho/Hp, Hj/Hk, and Hl/Hm exchange rapidly
on the 1H NMR time scale between the environments
A/B, C/D, and E/F (Table 1), respectively. Their fast
exchange is a result of the acid-promoted interconversion
between the two enantiomers of (R/ S)-SP , presumably
through the formation of cis-MEH.
Additional signals appear in the 1H NMR spectrum
(Figures 2c and 3c) if the acidified solution of (R/ S)-SP
is maintained in the dark. Indeed, (R/ S)-SP switches to
trans-MEH under these conditions (Figure 4). Over the
course of ca. 3 d, the ratio between (R/ S)-SP and trans-
MEH decreases gradually to a stationary value of (25 (
5) × 10-2, which corresponds to an equilibrium constant
(K2) of (8 ( 3) × 102 M-1 at 298 K (Table 2). Upon
exposure of the solution to ambient light, trans-MEH
reverts to (R/ S)-SP . Consistently, the ratio between (R/
S)-SP and trans-MEH increases (Figures 2d and 3d).
1
After ca. 4 d, the H NMR spectrum (Figures 2e and 3e)
shows almost exclusively the signals of (R/ S)-SP .
(27) The solution was irradiated for 5 min with a Mineralight UVGL-
25 lamp.
(28) Pretsch, E.; Clerc, T.; Seibl, J .; Simon, W. Tables of Spectral
Data for Structure Determination of Organic Compounds; Springer-
Verlag: Berlin, 1989.
4160 J . Org. Chem., Vol. 68, No. 11, 2003

Digital Processing with a Three-State Molecular Switch
TABLE 1. 1H NMR Sp ectr oscop ic Da ta (300 MHz, 298 K, CD3CN) for (R/S)-SP a a n d tr a n s-MEHb
(R/ S)-SP
trans-MEH
protons
δ (ppm)
8.06
7.99
7.14
7.10
7.02
6.82
6.71
no. of protons
multiplicity
J (Hz)
protons
δ (ppm)
8.78
8.27
7.66-7.61
7.74
8.47
7.66-7.61
7.20
7.74
7.78
4.04
4.04
no. of protons
multiplicity
J (Hz)
Ha
1
1
1
1
1
1
1
1
1
1
1
1
1
1
3
3
d
3
HA
1
1
1
1
1
1
1
1
1
1
1
1
1
d
dd
m
d
d
m
d
d
d
t
3
Hb
dd
pt
d
d
pt
d
9 and 3
7
7
10
7
9
8
10
HB
9 and 3
Hc
HC f
HD g
HE h
HF f
HG
Hd
7
17
He
Hf
Hg
9
8
17
5
5
Hh
6.65
5.98
d
d
HH g
HI h
HJ i
HK i
HL j
HM j
HN k
HOl
HP l
Hi
Hj c
Hk c
Hl d
Hm d
Hn
3.72-3.47
3.72-3.47
3.38-3.14
3.38-3.14
2.80
m
m
m
m
pt
s
t
t
t
4.66
4.66
5
5
6
Ho/Hp e
Hp/Hï e
1.24
1.14
1.80
1.80
3
3
s
s
s
a
b
1
The concentration of (R/ S)-SP was 3 × 10-2 M. The data reported for trans-MEH were determined recording the H NMR spectrum
of a solution prepared from (R/ S)-SP (3 × 10-2 M) and 10 equiv of CF3CO2H in CD3CN and maintained in the dark at ambient temperature
for 12 d. c The resonances for the protons Hj and Hk overlap and appear as a multiplet at 3.72-3.47 ppm. The resonances for the protons
d
Hl and Hm overlap and appear as a multiplet at 3.38-3.14 ppm. e The singlets at 1.24 and 1.14 ppm correspond to the environments A
and B, respectively. f The resonances for the protons HC and HF overlap and appear as a multiplet at 7.66-7.61 ppm. The resonances
g
for the protons HD and HH overlap and appear as a multiplet at 7.76-7.72 ppm. Structural considerations suggest that the proton HE
h
should resonate at a chemical shift greater than that of the proton HI. As a result, we have assigned tentatively the doublet at 8.47 ppm
to HE and that at 7.78 ppm to HI. The protons HJ and HK are equivalent and appear as a triplet at 4.04 ppm. The protons HL and HM
i
j
are equivalent and appear as a triplet at 4.66 ppm. No signals are observed for the proton HN. The protons HÃ and HP are equivalent
k
l
and appear as a singlet at 1.80 ppm.
1
The H NMR spectrum (Figures 2f and 3f) of a CD3CN
solution prepared by dissolving (R/ S)-SP and 10 equiv
of CF3CO2H and maintained in the dark for ca. 12 d
shows almost exclusively the signals of trans-MEH. The
loss of the chiral spirocenter simplifies the sets of signals
for the methyl and methylene protons of trans-MEH.
Only one singlet for the methyl protons HÃ/HP (Table 1)
and two triplets for the methylene protons HJ /HK and HL/
HM are observed at 1.80, 4.04, and 4.66 ppm, respectively.
The COSY spectrum (Figure S-4, Supporting Informa-
tion) confirms the coupling of the two pairs of methylene
protons showing cross-peaks between the two triplets at
4.66 and 4.04 ppm. The methylene protons HL and HM
are close to the aromatic proton HH, and a positive NOE
of the doublet for this proton at 7.74 ppm is observed after
irradiation at 4.66 ppm (Figure S-5a, Supporting Infor-
mation). Instead, the methyl protons HÃ and HP are close
to the other aromatic proton HD, and a positive NOE of
the doublet for this proton at 7.74 ppm is observed after
irradiation at 1.80 ppm (Figure S-5b, Supporting Infor-
mation). The two doublets for HD and HH overlap and
appear in the H NMR spectrum as a multiplet at 7.76-
1
7.72 ppm. The resonances of the other two aromatic
protons HC and HF on the indolium fragment were
identified from cross-peaks in the COSY spectrum. They
overlap and appear as a multiplet at 7.66-7.61 ppm.
Cross-peaks in the COSY spectrum allowed also the
identification of the signals corresponding to the protons
HA, HB, and HG on the p-nitrophenol fragment. They
resonate at 8.78, 8.27, and 7.20 ppm, respectively.
The COSY spectrum reveals cross-peaks between the
remaining two doublets at 8.47 and 7.78 ppm. They
correspond to the olefinic protons HE and HI.29 Their large
coupling constant of 17 Hz (Table 1) suggests that these
(29) Structural considerations suggest that the proton HE should
resonate at a chemical shift larger than that of the proton HI. As a
result, we have assigned tentatively the doublet at 8.47 ppm to HE
and that at 7.78 ppm to HI.
J . Org. Chem, Vol. 68, No. 11, 2003 4161

Raymo et al.
F IGURE 2. Partial 1H NMR spectra of (R/ S)-SP (3 × 10-2 M, CD3CN, 298 K, 300 MHz) recorded before (a) and after the addition
of 1 equiv of CF3CO2H (b), after storing the acidified solution in the dark for 1 d (c), and after the subsequent exposure of the
same solution to ambient light for 1 (d) and 4 d (e). 1H NMR spectrum of trans-MEH (CD3CN, 298 K, 300 MHz) prepared by
maintaining a solution of (R/ S)-SP (3 × 10-2 M) and 10 equiv of CF3CO2H in the dark for 12 d (f). The proton labels are assigned
in the diagram of Table 1.
two protons are in a trans arrangement relative to the
interposed double bond.28 In fact, the coupling constant
for the olefinic protons of (R/ S)-SP , where they are forced
in a cis configuration, is only 10 Hz. This interpretation
is consistent with the solid-state geometries (vide infra)
and with NOE experiments. Indeed, NOEs are observed
only between the cis olefinic protons of (R/ S)-SP (Figures
S-2d, Supporting Information).
The relative simplicity of the 1H NMR spectrum (Figures
2f and 3f), however, suggests that the two or more
1
conformers exchange rapidly on the H NMR time scale.
1
The H NMR spectrum of a CD3CN solution prepared
dissolving (R/ S)-SP and one equivalent of CF3CO2H and
maintained in the dark for ca. 16 d shows the signals of
(R/ S)-SP and trans-MEH. The addition of 1 equiv of
Et3N results in the deprotonation of trans-MEH with the
formation of trans-ME (Figure 4). Consistently, the
resonances of trans-MEH are replaced by those of trans-
ME. In particular, a new singlet for the six equivalent
methyl protons of trans-ME and a new doublet for one
of its olefinic protons can be observed at 1.77 and 6.48
ppm respectively (Figure S-3b, Supporting Information).
The large coupling constant (16 Hz) of the doublet
confirms the trans configuration of the associated double
bond.28 Immediately after the addition of the base, the
ratio between (R/ S)-SP and the newly formed trans-ME
In the solid-state structure of trans-MEH (vide infra),
the aromatic proton HA is close to the olefinic proton HI
but far from HE. Irradiation at 8.47 ppm, however,
produces positive NOEs on both HE and HI (Figure S-5d,
Supporting Information). Similarly, the irradiation of the
singlet for the methyl protons HO and HP produces
positive NOEs on both HE and HI (Figure S-5b, Support-
ing Information). The four observed NOEs indicate that
at least two (I and III or II and IV) of the four
conformations illustrated in Figure 5 coexist in solution.
4162 J . Org. Chem., Vol. 68, No. 11, 2003

Digital Processing with a Three-State Molecular Switch
F IGURE 3. Partial 1H NMR spectra of (R/ S)-SP (3 × 10-2 M, CD3CN, 298 K, 300 MHz) recorded before (a) and after the addition
of 1 equiv of CF3CO2H (b), after storing the acidified solution in the dark for 1 d (c), and after the subsequent exposure of the
same solution to ambient light for 1 (d) and 4 d (e). 1H NMR spectrum of trans-MEH (CD3CN, 298 K, 300 MHz) prepared by
maintaining a solution of (R/ S)-SP (3 × 10-2 M) and 10 equiv of CF3CO2H in the dark for 12 d (f). The proton labels are assigned
in the diagram of Table 1.
is ca. 6. Over time, trans-ME isomerizes to the more
stable (R/ S)-SP (Figure 4) and the H NMR spectrum
recorded after ca. 2 h shows almost exclusively the
signals of (R/ S)-SP .
X-r a y Cr ysta llogr a p h y. Single-crystal analysis es-
tablished unequivocally the spiropyran structure of (R/
S)-SP (Figure 6), the compound crystallizing in a cen-
trosymmetric space group containing equal numbers of
R and S molecules. The geometry of the indoline/
benzopyran core is essentially identical to those observed
for the methyl,30 n-propyl,31 and benzyl32 analogues. In
all of these structures, the benzopyran unit has the
spirocenter lying slightly out of the plane of its remaining
atoms [by 0.07 Å in (R/S)-SP ], and the indoline has a ca.
30° fold about the N-C(isopropylidene) vector such that
its phenylene ring is folded toward the pyran oxygen
atom. In (R/S)-SP , the pyrrole nitrogen atom is pyrami-
dalized and lies 0.24 Å out of the plane of its substituents.
The terminal hydroxy group is disordered, and two
alternative orientations were identified in the crystals.
A feature of the solid-state structure of (R/S)-SP not
observed in any of the above-mentioned analogues is the
formation of a channel structure containing substantial,
essentially rectangular, pores of free cross-section ca. 4.5
by 10.0 Å (Figure 7). The inner walls of each of these
channels are bounded on two opposite sides by C-H‚‚‚O
and C-H‚‚‚π hydrogen-bonded molecules (Figure 8) and
on the remaining sides by end-to-end hydrogen-bonded
-CH2CH2OH chains (O‚‚‚O 2.82 Å). The channels are
filled by diffuse disordered solvent, which has been
attributed as comprising a mixture of ethanol and n-
1
(30) (a) Karaev, K. Sh.; Furmanova, N. G.; Belov, N. V. Dokl. Akad.
Nauk SSSR 1982, 262, 877-880. (b) Clegg, W.; Norman, N. C.; Flood,
T.; Sallans, L.; Kwak, W. S.; Kwiatkowski, P. L.; Lasch, J . G. Acta
Crystallogr., Sect. C 1991, 47, 817-824.
(31) Godsi, O.; Peskin, U.; Kapon, M.; Ezra, N. A.; Eichen, Y. Chem.
Commun. 2001, 2132-2133.
(32) (a) Karaev, K. Sh.; Belov, N. V. Kristallografiya 1983, 28, 855-
861. (b) Karaev, K. Sh.; Furmanova, N. G. J . Struct. Chem. 1984, 25,
168-170.
J . Org. Chem, Vol. 68, No. 11, 2003 4163

Raymo et al.
F IGURE 4. Three states (R/ S)-SP , trans-ME, and trans-MEH of the molecular switch.
TABLE 2. Equ ilibr iu m a n d Ra te Con sta n ts for th e Th er m a l In ter con ver sion of (R/S)-SP a n d tr a n s-ME a n d of (R/S)-SP
a n d tr a n s-MEH in MeCN a t 298 Ka
N
probe
λ (nm)
ꢀλ× 103 (M-1 cm-1
41 ( 5
32 ( 1
)
KN
kN
k-N (s-1
)
1
2
trans-ME
trans-MEH
563
401
(40 ( 5) × 10-4
(23 ( 1) × 10-6s-1
(52 ( 2) × 10-4
(8 ( 3) × 102M-1
(69 ( 2) × 10-5M-1s-1
(17 ( 8) × 10-7
a
The equilibria (1) and (2) illustrate the thermal processes and the associated rate constants (kN and k-N). The corresponding equilibrium
constants (KN) were determined by 1H NMR spectroscopy. The molar extinction coefficients (ꢀλ), kN and k-N were determined by absorption
spectroscopy.
(R/ S)-SP yk z trans-ME
(1)
(2)
1
k-1
(R/ S)-SP + CF3CO2H yk z trans-MEH
2
k-2
heptane (see the Experimental Section). An unusual
physical feature of these crystals was that, although they
were initially colorless, a distinct straw/magenta dichro-
ism developed after a period of X-ray irradiation, with
the magenta coloration being at a maximum when viewed
along the crystallographic 100 direction. This direction
coincides with that of the channels and lies within the
planes of the bounding benzopyran ring systems.
A single-crystal X-ray analysis also established the
trans-olefinic configuration of MEH (Figure 9) and
showed the molecule to have an approximately planar
conformation, there being only a ca. 4° torsional twist
about the trans-olefinic linkage and a ca. 5° angle
between the two ring systems. The cation is fully ordered
but the trifluoroacetate anion has 50:50 rotational dis-
order of its CF3 group. Analysis of the bond lengths in
trans-MEH shows there to be a pattern of conjugation
that extends from the p-nitrophenol ring, via the CdC
double bond, to include the pyrrole nitrogen atom; the
indole is planar to within 0.024 Å. Both the bonding and
conformation observed here are very similar to those
observed in, for example, the related zwitterionic complex
2-(2-(1,5-dinitro-2-oxyphenyl)ethenyl)-1,3,3-trimethyl-3H-
indolinium.33 The molecules are linked to form chains
that extend in the crystallographic 11h1h direction. Cen-
trosymmetrically related pairs of molecules are linked
by O-H‚‚‚O hydrogen bonds (a and b in Figure 10)
between (a) the phenolic OH group and one of the
trifluoroacetate oxygen atoms and (b) between the -CH2-
CH2OH hydroxy and the other trifluoroacetate oxygen
atom. These interactions are supplemented by a π‚‚‚π
stacking interaction (c) between adjacent p-nitrophenol
rings. These “dimer pairs” are then linked end-to-end by
a further π‚‚‚π stacking interaction (d) between the indole
rings of lattice translated “dimers”.
Molecu la r Mod elin g. The spirocenter of (R/ S)-SP
locks the olefinic fragment in a cis configuration. In ME
and MEH, four different conformations with a trans
configuration (Figure 5) plus four with a cis geometry are
possible. To assess their relative stabilities, model struc-
tures with a methyl group in place of the 2-hydroxyethyl
appendage of (R/ S)-SP , ME, and MEH were optimized
at the B3LYP/6-31G(d) level. The density functional
calculations identified minima on the potential energy
surfaces for all four trans geometries of the merocyanine
(33) Hobley, J .; Malatesta, V.; Millini, R.; Montanari, L.; Parker,
W. O. N., J r. Phys. Chem. Chem. Phys. 1999, 1, 3259-3267.
4164 J . Org. Chem., Vol. 68, No. 11, 2003

Digital Processing with a Three-State Molecular Switch
F IGURE 5. Four possible conformations of trans-MEH and
the NOEs observed experimentally.
F IGURE 7. Ball and stick (a) and space-filling (b) representa-
tions of the channel structure formed by (R/S)-SP in the solid
state.
F IGURE 6. Molecular structure of (R)-SP in the solid state.
forms and for two of their four cis structures. In the case
of ME (Figures 11a and S-7 (Supporting Information)),
the trans forms I-IV are at least 3.7 kcal mol-1 lower in
energy than the cis forms V and VI, but at least 5.1 kcal
mol-1 higher in energy than (R/ S)-SP . This trend is
consistent with the nuclear magnetic resonance spectro-
scopic analyses, which revealed (1) only trans isomers
for ME and (2) (R/ S)-SP to be the predominant species
in the dark. In the case of MEH (Figures 11b and S-8
(Supporting Information)), the difference in energy be-
F IGURE 8. Pattern of C-H‚‚‚O and C-H‚‚‚π hydrogen-
bonding interactions between molecules of (R/S)-SP that bind
to the sides of the channels in the crystal. The C-H‚‚‚O
hydrogen bonds have [C‚‚‚O], [H‚‚‚O] (Å), and [C-H‚‚‚O]
(deg): (a) 3.37, 2.45, 163; (b) 3.37, 2.43, 170. The C-H‚‚‚π
interactions have [H‚‚‚π] (Å) and [C-H‚‚‚π] (deg): (c) 3.02, 140;
(d) 2.98, 150.
tween the trans and cis forms is larger and exceeds 8.8
kcal mol-1. Furthermore, the energies of the trans forms
J . Org. Chem, Vol. 68, No. 11, 2003 4165

Raymo et al.
F IGURE 9. Molecular structure of trans-MEH in the solid
state. Selected bond lengths (Å): (a) 1.440(3), (b) 1.343(3), (c)
1.424(3), (d) 1.322(3).
F IGURE 11. Zero-point corrected B3LYP/6-31G(d) energies
of the trans and cis geometries of ME (a) and MEH (b). The
energies are reported relative to that of (R/ S)-SP (a) and II
(b). The geometries are illustrated in Figures S-6, S-7, and S-8
(Supporting Information).
bond appears to be significantly longer in trans-ME (calcd
) 1.400 Å). In fact, the three bonds linking the indolium
and p-nitrophenolate fragments of trans-ME are almost
equal in length,34 suggesting a pronounced electronic
delocalization between them.
Absor p tion Sp ectr oscop y. The absorption spectrum
of a relatively concentrated MeCN solution of (R/ S)-SP
reveals a band at 563 nm corresponding to the purple
isomer trans-ME. The absorbance at this wavelength
decreases linearly with the total concentration of the two
isomers (Figure 12a). From this correlation and the
F IGURE 10. Part of one of the chains of molecules present
in the crystal of trans-MEH. The hydrogen bonds have [O‚‚‚
O], [H‚‚‚O] (Å), and [O-H‚‚‚O] (deg): (a) 2.58, 1.69, 169; (b)
2.72, 1.83, 171. The π‚‚‚π stacking interactions have centroid‚
‚‚centroid and mean interplanar separations (Å) of (c) 3.55,
3.44; (d) 4.06, 3.53. The figure also shows the disorder in the
CF3 groups of the trifluoroacetate anions.
1
equilibrium constant (K1) determined by H NMR spec-
troscopy, the molar extinction coefficient of trans-ME can
be estimated to be (41 ( 5) × 103 M-1 cm-1 (Table 2).
At a total concentration of ca. 1 × 10-4 M, the
absorption band for trans-ME cannot be detected. After
ultraviolet irradiation,35 however, (R/ S)-SP isomerizes
to trans-ME (Figure 4) and the absorbance at 563 nm
increases dramatically. Its value at the photostationary
state suggests that the ratio between (R/ S)-SP and
trans-ME is 27 ( 4. The reisomerization of the purple
trans-ME to the colorless (R/ S)-SP occurs in the dark
(Figure 4). Consistently, the band at 563 nm fades over
the course of 20 min (Figure 12b,c). The time dependence
of the absorbance indicates that the rate constant (k1)
for the thermal interconversion of (R/ S)-SP into trans-
ME and that (k-1) for the opposite process are (23 ( 1)
× 10-6 and (52 ( 2) × 10-4 s-1, respectively, at 298 K
(Table 2). The reisomerization of trans-ME to (R/ S)-SP
can be accelerated by irradiating the sample with visible
light. Complete decoloration occurs in ca. 5 min under
constant irradiation at ca. 562 nm.36
I-IV lie within 2.1 kcal mol-1. Both observations are in
agreement, once again, with the nuclear magnetic reso-
nance spectroscopic analyses, which (1) did not detect cis
isomers for MEH and (2) suggested the coexistence of
several trans forms in solution (Figure 5).
For both merocyanine forms, the trans structures I and
II are slightly lower in energy than III and IV (Figure
11). The examination of the corresponding geometries
(Figures S-7 and S-8) reveals that steric interactions
between the alkyl group on the indolium nitrogen atom
and the adjacent olefinic fragment force III and IV to
deviate from planarity. Interestingly, I appears to be
more stable than II for trans-ME, while II is the lowest
energy structure for trans-MEH and happens to be the
one observed in the solid state (Figure 9).
The calculated bond lengths for (R/ S)-SP are in good
agreement with the experimental values. The largest
difference is observed for the double bond of the olefinic
bridge (calcd ) 1.340 Å, expt ) 1.320 Å). A similar trend
emerges from the comparison of the calculated bond
lengths for II of trans-MEH and the experimental values.
Once again, the largest difference is observed for the
olefinic bridge (calcd ) 1.360 Å, expt ) 1.340 Å). This
(34) It is important to note that the lack of diffuse functions in the
basis set employed might have resulted in an underestimation of the
zwitterionic character of trans-ME.
(35) The solution was irradiated with a liquid light guide coupled
to a 150 W Xe arc lamp and a 341 ( 10 nm interference filter. A
photostationary state was reached in ca. 5 min.
(36) A 562 ( 10 nm interference filter, coupled to the illumination
system described in ref 35, was employed to irradiate the solution.
4166 J . Org. Chem., Vol. 68, No. 11, 2003

Digital Processing with a Three-State Molecular Switch
F IGURE 13. Evolution of the absorption spectrum (a) and
time dependence of the absorbance at 401 nm (b) of a solution
(1 × 10-4 M, MeCN, 298 K) of (R/ S)-SP and CF3CO2H (1 ×
10-2 M) in the dark.
Bin a r y Logic An a lysis. The spectroscopic analyses
revealed that the interconversion of (R/ S)-SP , trans-ME,
and trans-MEH can be controlled with external stimula-
tions (Figure 4). Ultraviolet irradiation induces the
transformation of the colorless (R/ S)-SP into the purple
trans-ME. At a total concentration of ca. 1 × 10-4 M,
complete decoloration occurs either if the ultraviolet
source is turned off or if the sample is irradiated with
visible light. The addition of CF3CO2H either before or
after ultraviolet irradiation promotes the formation of the
yellow-green trans-MEH. Complete decoloration occurs
after either the addition of Et3N or visible irradiation.
In summary, three input stimulations (ultraviolet light,
visible light, and H+) control the proportions of the three
states of the molecular switch and determine the absor-
bance at the two wavelengths (401 and 563 nm) corre-
sponding to the yellow-green and purple colors.
F IGURE 12. Concentration dependence of the absorbance at
563 nm of a MeCN solution of (R/S)-SP and trans-ME
equilibrated in the dark at 298 K (a). Evolution of the
absorption spectrum (b) and time dependence of the absor-
bance at 563 nm (c) of a solution (1 × 10-4 M, MeCN, 298 K)
of (R/ S)-SP and trans-ME in the dark after irradiation at ca.
341 nm for 15 min.
The absorption spectrum of an acidified solution of (R/
S)-SP maintained in the dark changes over the course
of ca. 1 d (Figure 13a). The gradual formation of the
protonated merocyanine trans-MEH (Figure 4) produces
an increase in absorbance at 401 nm. The time depen-
dence of this absorption band (Figure 13b) reveals that
the molar extinction coefficient of trans-MEH is (41 (
1) × 103 M-1 cm-1 (Table 2). From the same analysis,
the rate constant (k2) for the transformation of (R/ S)-
SP into trans-MEH and that (k-2) for the opposite
process can be estimated to be (54 ( 2) × 10-5 M-1 s-1
and (5 ( 2) × 10-6 s-1, respectively, at 298 K.
The ultraviolet light (I1), visible light (I2), and H+ (I3)
inputs can be either off or on. Binary digits can be
encoded in them applying positive logic conventions (off
) 0, on ) 1). The absorbance at 401 nm (O1) and that at
563 nm (O2) are below 0.02, when (R/ S)-SP is the
dominant species in solution. They are above this thresh-
old when the concentrations of trans-MEH or trans-ME,
respectively, are significant. Binary digits can be encoded
in them applying, once again, positive logic conventions
(low ) 0, high ) 1). Thus, this chemical system responds
to an input string of three binary digits (I1 I2 I3)
producing an output string of two binary digits (O1 O2).
The protonated form trans-MEH switches back to (R/
S)-SP under visible irradiation (Figure 4). Complete
decoloration occurs in less than 5 min, if the sample is
irradiated at ca. 413 nm.37 Instead, the addition of Et3N
results in the deprotonation of trans-ME H to produce
trans-ME (Figure 4).37 Consistently, the characteristic
absorption band of trans-ME can be observed at 563 nm
immediately after the addition of Et3N. However, trans-
ME isomerizes to (R/ S)-SP in the dark and only the
absorption bands of (R/ S)-SP can be detected after ca. 2
h from the addition of Et3N.
(37) A MeCN solution of (R/ S)-SP (1 × 10-4 M) and CF3CO2H (1 ×
10-2 M) was maintained in the dark for 15 h. After this time, the
absorption spectrum (Figure 13a) revealed the band for trans-MEH
at 401 nm. Part of the solution (0.5 mL) was irradiated with the
illumination system described in ref 35 using
a 413 ( 10 nm
interference filter. After 5 min of continuous irradiation, the absorption
band at 401 nm could not be detected. Another aliquot of the same
solution (0.5 mL) was treated with Et3N (2 × 10-2 M). Complete
decoloration occurred in ca. 2 h.
J . Org. Chem, Vol. 68, No. 11, 2003 4167

Raymo et al.
TABLE 3. Tr u th Ta ble of th e Th r ee-Sta te Molecu la r
Sw itch a
The ratio between the colorless and colored isomers is
ca. 260 at equilibrium in the dark. Consistently, molec-
ular modeling indicated that the spiropyran state is at
least 5.1 kcal mol-1 lower in energy than the merocyanine
isomer. Upon ultraviolet irradiation, the ratio decreases
to a photostationary value of ca. 27. The colored form
reisomerizes to the colorless species in the dark or under
visible irradiation. At a total concentration of ca. 1 × 10-4
M, complete decoloration occurs in less than 20 min. In
this concentration range, the addition of acid produces a
yellow-green protonated merocyanine over the course of
ca. 15 h in the dark. In the presence of at least 1 equiv
of acid, the protonated form becomes the predominant
species at equilibrium. However, visible light irradiation
or the addition of a base regenerates quantitatively the
spiropyran state in ca. 5 min or 2 h, respectively. X-ray
crystallography revealed that the olefinic bridge linking
the indolium and p-nitrophenol fragments of the yellow-
green form has a trans configuration. Nuclear magnetic
resonance spectroscopy, however, indicated that at least
two trans isomers exchange rapidly on the 1H NMR time
scale in solution. Density functional calculations con-
firmed that the four trans geometries possible for both
merocyanines lie within 3.1 kcal mol-1 but are up to 12.6
kcal mol-1 lower in energy that the cis forms.
input datab
output datac
O2
I1
I2
I3
O1
0
0
0
1
0
1
1
1
0
0
1
0
1
0
1
1
0
1
0
0
1
1
0
1
0
1
0
0
0
1
0
1
0
0
0
1
0
0
1
0
a
The switching processes are illustrated in Figure 4. b The three
inputs I1, I2, and I3 are ultraviolet light, visible light, and H+,
respectively. A positive logic convention (off ) 0, on ) 1) was
applied to encode the binary digits in the three inputs. The input
strings where I1 and I2 are both 1 imply that ultraviolet and
visible light are both on. The nature of the light sources determines
the output under these conditions. c The two outputs O1 and O2
are the absorbances at 401 and 563 nm, respectively. A positive
logic convention (off ) 0, on ) 1) was applied to encode the binary
digits in the two outputs. The outputs are determined at the
stationary states.
In summary, the interconversion between a colorless
spiropyran, a purple merocyanine and its yellow-green
protonated form, can be (1) induced addressing the
system with ultraviolet light, visible light, and/or H+ and
(2) monitored measuring the absorbance of the two
colored forms in the visible region. The interplay of the
three input stimulations and the two optical outputs can
be exploited to implement logic operations at the molec-
ular level. Indeed, the logic function executed by this
particular molecular switch is equivalent to that of a
combinational logic circuit incorporating two AND, two
NOT, and one OR gate. Similar logic conventions can be
employed to encode binary digits in the inputs and
outputs of any multistable chemical or biochemical
system. In principle, it should be possible to program a
specific logic function into an ensemble of interconverting
molecules by imposing the required number of inputs and
outputs and designing their transduction protocol. This
approach can lead to the development of a wealth of
chemical systems for digital processing. At this stage, it
is certainly unclear if practical applications in informa-
tion technology can emerge for these chemical processors.
It is worth noting, however, that our central nervous
system relies on the ability of molecular components to
transduce input stimulations into specific outputs.38 We
do not have sophisticated semiconductor-based transis-
tors in our brain, but we are capable of much more than
present computers.39 Perhaps, future data-processing,
-storage, and -communication devices might share the
“chemical” operating principles of neurotransmission
rather than continuing to rely on silicon and electronics.
F IGURE 14. Combinational logic circuit equivalent to the
three-state molecular switch.
For example, the input string 0 0 0 indicates that all
three input stimulations are off. Under these conditions,
the colorless (R/ S)-SP is the dominant species and the
absorbance in the visible region is low. The output string
is 0 0. Instead, an input string 1 0 1 indicates that the
ultraviolet and H+ inputs are on. Under these conditions,
the yellow-green trans-MEH is the dominant species. The
absorbance at 401 nm is high while that at 563 nm is
low. The output string is 1 0. Following similar consid-
erations, the eight output strings corresponding to the
eight possible combinations of input strings can be
determined (Table 3). It is interesting to note that the
output string 1 1 is excluded by the molecular processors.
In fact, it would correspond to a condition that cannot
be satisfied experimentally, namely the simultaneous
presence of relatively large amounts of trans-MEH and
trans-ME in solution.
The logic behavior of the three-state molecular switch
corresponds to that of a combinational logic circuit
integrating two AND, two NOT, and one OR gate (Figure
14). One portion of the logic circuit combines the three
inputs I1, I2, and I3 through NOT, OR, and AND
operations producing the output O1. The other combines
the two inputs I1 and I3 through NOT and AND
operations producing the output O2. Comparison of the
two independent parts of the logic circuit reveals that
the input I2 has no influence on the output O2.
Exp er im en ta l Section
Con clu sion s
Gen er a l Meth od s. MeCN was distilled over CaH2. Alumi-
A spiropyran was synthesized in three steps, and its
structure was confirmed unequivocally by X-ray crystal-
lography. Nuclear magnetic resonance and visible ab-
sorption spectroscopies revealed that this compound is
in equilibrium with a purple trans-merocyanine in MeCN.
num plates coated with silica gel 60 F254 were used for thin-
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4168 J . Org. Chem., Vol. 68, No. 11, 2003

Digital Processing with a Three-State Molecular Switch
layer chromatography (TLC). Melting points (mp) were deter-
mined with an Electrothermal 9100. Fast atom bombardment
mass spectra (FABMS) were recorded with a VG Mass Lab
Trio-2 using a 3-nitrobenzyl alcohol matrix.
(R/ S)-SP to trans-ME by difference 1H NMR spectroscopy.
Quartz NMR tubes were employed for these experiments. The
samples were irradiated for 5 min with a Mineralight UVGL-
25 lamp.
1-(2-Hyd r oxyeth yl)-2,3,3-tr im eth yl-3H-in d oliu m Br o-
m id e 1. A solution of 2,3,3-trimethyl-3H-indole (2.61 g, 16
mmol) and 2-bromoethanol (2.46 g, 20 mmol) in MeCN (20 mL)
was maintained under N2 and heated under reflux for 1 d.
After the mixture was cooled to ambient temperature, the
solvent was distilled off under reduced pressure. The residue
was suspended in hexane (25 mL), and the mixture was
sonicated and filtered. The resulting solid was crystallized from
CHCl3 (35 mL) to afford 1 (2.95 g, 69%) as a pink solid: mp )
195 °C; FABMS m/z 204 [M - Br]+; 1H NMR (CD3CN) δ 7.83-
7.75 (1H, m), 7.72-7.65 (1H, m), 7.64-7.54 (2H, m), 4.71 (1H,
t, J ) 6 Hz), 4.54 (2H, t, J ) 5 Hz), 4.02-3.94 (2H, m), 2.81
(3H, s), 1.57 (6H, s); 13C NMR (CD3CN) δ 198.84, 142.95,
142.42, 130.69, 130.00, 124.29, 116.50, 58.79, 55.63, 51.79,
22.89, 15.54.
X-r a y Cr yst a llogr a p h y. Cr yst a l d a t a for (R/S)-SP :
20H20N2O4‚0.5EtOH‚0.5C7H16, M ) 425.5, monoclinic, I2/a
C
(no. 15), a ) 11.379(1) Å, b ) 14.139(1) Å, c ) 28.767(1) Å, â
) 93.66(1)°, V ) 4618.8(3) Å3, Z ) 8, Dc ) 1.224 g cm-3, µ(Cu
KR) ) 0.68 mm-1, T ) 183 K, colorless/straw-magenta dichroic
plate-like needles; 3433 independent measured reflections, F2
refinement, R1 ) 0.052, wR2 ) 0.138, 2248 independent
observed reflections [|Fo| > 4σ(|Fo|), 2θ e 120°], 244 param-
eters. CCDC 206822.
Cr ysta l d a ta for tr a n s-MEH: [C20H21N2O4](CF3CO2), M
) 466.4, triclinic, P1h (no. 2), a ) 8.778(1) Å, b ) 11.494(2) Å,
c ) 11.637(1) Å, R ) 77.59(1)°, â ) 71.22(1)°, γ ) 82.62(1)°, V
) 1083.4(2) Å3, Z ) 2, Dc ) 1.430 g cm-3, µ(Mo KR) ) 0.12
mm-1, T ) 293 K, orange diamond plates; 3807 independent
measured reflections, F2 refinement, R1 ) 0.048, wR2 ) 0.120,
2888 independent observed reflections [|Fo| > 4σ(|Fo|), 2θ e
50°], 334 parameters. CCDC 206823.
Molecu la r Mod elin g. A methyl group was used in place
of the 2-hydroxyethyl appendage of (R/ S)-SP , ME, and MEH.
Four coplanar geometries with trans configurations and four
with cis were generated for the merocyanine forms. All
structures were optimized at the B3LYP/6-31(d) level using
Gaussian 98.40 The configuration of two of the four cis
geometries changed to trans upon optimization. Frequency
calculations on the stationary points found did not reveal
imaginary frequencies confirming that the optimized geom-
etries of Figures S-6-S-8 (Supporting Information) correspond
to minima on the potential energy surface. The resulting zero-
point energies and a scaling factor41 of 1.0119 were applied to
compute the energy differences of Figure 11.
(R/S)-9,9,9a -Tr im eth yl-2,3,9,9a -tetr a h yd r ooxa zolo[3,2-
a ]in d ole (R/S)-2. A solution of 1 (2.93 g, 10 mmol) and KOH
(0.92 g, 16 mmol) in H2O (50 mL) was stirred at ambient
temperature for 10 min. Then, it was extracted with Et2O (3
× 20 mL). The organic phase was dried (MgSO4), filtered, and
concentrated under reduced pressure to afford (R/ S)-2 (1.84
1
g, 88%) as a yellow oil: FABMS m/z 204 [M + H]+; H NMR
(CD3CN) δ 7.14-7.03 (2H, m), 6.90-6.82 (1H, m), 6.76 (1H,
d, J ) 8 Hz), 3.81-3.65 (2H, m), 3.57-3.45 (1H, m), 3.40-
3.30 (1H, m), 1.35 (3H, s), 1.31 (3H, s), 1.11 (3H, s); 13C NMR
(CD3CN) δ 152.11, 140.97, 128.35, 123.24, 122.21, 112.76,
109.61, 63.50, 50.37, 47.51, 28.17, 20.92, 17.47.
(R/S)-2-(3′,3′-Dim eth yl-6-n itr o-3′H-sp ir o[ch r om en e-2,2′-
in d ol]-1′-yl)eth a n ol (R/S)-SP . A solution of 2-hydroxy-5-
nitrobenzaldehyde (1.05 g, 6 mmol) and (R/ S)-2 (0.87 g, 4
mmol) in EtOH (10 mL) was maintained under N2 and heated
under reflux for 3 h. After being cooled to ambient tempera-
ture, the mixture was filtered. The resulting purple solid was
washed with EtOH (2 mL) and purified by semipreparative
HPLC to afford (R/ S)-SP (1.18 g, 79%): HPLC (analytical) tR
) 9.4 min, PA ) 3.0, APP ) 239.4 ( 0.1 nm; mp ) 171 °C;
FABMS m/z 353 [M + H]+; 1H NMR (CD3CN) δ 8.06 (1H, d, J
) 3 Hz), 7.99 (1H, dd, J 1 ) 9 Hz, J 2 ) 3 Hz), 7.14 (1H, pt, J )
7 Hz), 7.10 (1H, d, J ) 7 Hz), 7.02 (1H, d, J ) 10 Hz), 6.82
(1H, pt, J ) 7 Hz), 6.71 (1H, d, J ) 9 Hz), 6.65 (1H, d, J ) 8
Hz), 5.98 (1H, d, J ) 10 Hz), 3.72-3.47 (2H, m), 3.38-3.14
Absor p tion Sp ectr oscop y. Absorption spectra were re-
corded with
irradiated with a Thermo Oriel liquid light guide coupled to a
Spectral Energy LH 150/1 light source equipped with
a Varian Cary 100 Bio. The samples were
a
Spectral Energy 150 W Xe arc lamp and Thermo Oriel 341,
413 or 562 ( 10 nm interference filters.
Ack n ow led gm en t. We thank the National Science
Foundation (CAREER Award CHE-0237578) for finan-
cial support and the University of Miami for a Robert
E. Maytag Fellowship to S.G.
(2H, m), 2.80 (1H, pt, J ) 6 Hz), 1.24 (3H, s), 1.14 (3H, s); 13
C
NMR (CD3CN) δ 160.43, 148.35, 142.10, 136.97, 128.80,
128.66, 126.56, 123.70, 123.25, 122.70, 120.36, 120.04, 116.24,
108.03, 107.77, 60.89, 53.49, 46.90, 26.15, 20.07.
Su p p or tin g In for m a tion Ava ila ble: COSY, NOE, and
1H NMR difference spectra; thermal ellipsoid plots, CIF files;
calculated structures; determination of molar extinction coef-
ficients and rate constants. This material is available free of
charge via the Internet at http://pubs.acs.org.
High -P er for m a n ce Liqu id Ch r om a togr a p h y. A Varian
ProStar high-performance liquid chromatography (HPLC)
system, equipped with a ProStar 330 photodiode array detec-
tor, was used to purify (R/ S)-SP . Analytical (column dimen-
sions ) 4.6 × 250 mm, flow rate ) 1.0 mL min-1, injection
volume ) 20 µL, sample concentration ) 1 × 10-3 M) and
semipreparative (column dimensions ) 21.4 × 250 mm, flow
rate ) 10.0 mL min-1, injection volume ) 1.0 mL, sample
concentration ) 2 × 10-2 M) Varian Microsorb BDS columns
were employed. The mobile phase was MeCN. The retention
time (tR) and the peak asymmetry (PA) reported were deter-
mined at a wavelength of 253.6 nm. The average purity
parameter (APP) reported was calculated for the peak heart
in the wavelength range 200-700 nm.
Nu clea r Ma gn etic Reson a n ce Sp ectr oscop y. A Bruker
DPX 300 (300 MHz) was used to record 1H- and 13C-nuclear
magnetic resonance (NMR) spectra and to perform correlated
spectroscopy (COSY) and nuclear Overhauser enhancement
(NOE) experiments. Bruker Avance 400 (400 MHz) and 500
(500 MHz) were employed to study the photoisomerization of
J O0340455
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