ISSN 19950780, Nanotechnologies in Russia, 2012, Vol. 7, Nos. 5–6, pp. 280–287. © Pleiades Publishing, Ltd., 2012. Original Russian Text © M.F. Budyka, N.I. Potashova, T.N. Gavrishova, V.M. Li, 2012, published in Rossiiskie Nanotekhnologii, 2012, Vol. 7, Nos. 5–6.
Design of Fully Photonic Molecular Logic Gates Based on the Supramolecular Bisstyrylquinoline Dyad M. F. Budyka, N. I. Potashova, T. N. Gavrishova, and V. M. Li Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Academician Semenov 1, Chernogolovka, Moscow oblast, 142432 email:
[email protected] Received November 17, 2011; in final form, February 2, 2012
Abstract—The design algorithm of fully photonic twoaddress molecular logic gates (MLGs) based on the supramolecular bisstyrylquinoline S3S dyad was considered in which two pieces of 2(4oxystyryl)quinoline are linked to each other with a trimethylene “bridge.” The reaction of the photoisomerization of styrylquin oline fragments is used to switch between the states, and the reading of the output signal is done by the absorp tion and luminescence of these fragments. In the latter case, both input and output signals for MLGs are pho tons; i.e., homogeneity of input and output signals is achieved, which is necessary for the construction of more complex circuits from these gates. It is shown that the S3S dyad simulates the action of logic gates “AND,” “OR,” “NOTAND” (“NAND”), “NOTOR” (“NOR”), “IMPLICATION” (“IMP”), and “INHIBIT” (“INH”). DOI: 10.1134/S1995078012030032
INTRODUCTION The requirement for the miniaturization of semi conductor systems for information processing is inev itably limited by physical and technological limits; below them, a reduction in the size of devices is impossible. In this regard, a variety of molecular sys tems is being actively investigated now that can serve as signal conductors, switches, diodes, memory ele ments, and logic devices. Many molecular systems can perform logic operations in an appropriate choice of input and output signals, i.e., function as molecular logic gates (MLGs) [1–7]. We have previously shown that 2styrylquinoline (2SQ) simulates the effect of twoaddress MLGs, for which the input signals are light exposure and proto nation [8, 9]. The disadvantage of this MLG is the need to add extra chemicals: acids (which play the role of the input signal) or base (to return the MLG to its original state). Photonic devices are completely devoid of this shortcoming; to switch them from one state to another only light exposure is necessary [10]. Diarylethylenes are convenient objects for modeling the action of the photon MLG, because trans and cis isomers of these compounds are thermally stable and, after light expo sure, they undergo a reversible photoisomerization reaction with high quantum yields. For example, 2(4 etoxystyryl)quinoline has quantum yields of trans–cis and cis–trans photoisomerization ϕtc = 0.58 and ϕct = 0.52, respectively [11]. Twoaddress MLGs must have two photoactive groups that change their properties under irradiation.
There are examples of twoaddress logic gates that operate on the basis of the photocyclization reaction of dithienylethene and fulgimid [12–14], photochro mic dihydroindolizines and dihydropyrene [15–17], or spiropyran [18]. We hypothesized that combining two fragments of styrylquinoline in a covalently linked supramolecular system will make it possible to create a model of two address MLGs operating through the photoisomeriza tion reaction and using only the irradiation of light as input signal. An example of such a system is a bis1,3 (4(2quinoline2vinyl)phenoxy)propane–bis styrylquinoline dyad S3S, in which fragments of 2(4 oxystyryl)quinoline (SQ) are linked by a trimethylene “bridge” with each other. According to quantum chemical calculations (method B3LYP/631G*) [19], the dyad in a maximally unfolded conformation has a van der Waals size of 3.49 nm along the long axis. Due to the presence of two photoisomerized groups, each of which exists in the form of a trans (E) or cis(Z) isomer, a S3S dyad can exist in four isomeric forms as EE, EZ, ZE, and ZZ isomers; interconver sions between them are shown in Scheme 1. Recent studies have shown that each piece of a SQdyad is isomerized regardless of the presence and condition of another fragment, while high quantum yields of photoisomerization stay in the dyad, which is characteristic for the reactivity of the model mono functional styrylquinoline [20]. This fact favors the use of a S3S dyad as a model of an MLG. Indeed, as is shown in this paper, the S3S dyad is able to simulate the effect of different twoaddress logic gates and the type of operation performed by
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I, rel. un 1
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4
I, rel. un 100 80
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3
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3
0.6
2 1
60 4
40 0.3
0.2 0
400 λ, nm
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500
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400 λ, nm
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Fig. 1. Absorption spectra (left axis) of isomer forms of bis styrylquinoline S3S dyad: (1) EE isomer, (2) EZ and ZE isomers, and (3) ZZ isomer; (4) luminescence spectra (right axis) of EE isomer, excitation with 390 nm.
Fig. 2. Spectra of (1–3) absorption and (4–6) lumines cence of bisstyrylquinoline S3S dyad: (1, 4) photostation ary state PS365; light exposure with 313 nm for (2, 5) 170 s and (3, 6) 340 s.
MLGs depends on the choice of isomer (form) of the S3S dyad as the initial state of the logic gate (0,0); input and output signals are photons.
0.3 mW cm2 (measured by a PP1 cavitary receiver or ferrioxalate actinometer).
EXPERIMENTAL The synthesis of the S3S dyad is described in [20]. Electronic absorption spectra were recorded on a Specord M400 spectrophotometer; the emission spectra were taken on a PerkinElmer LS55 spectrof luorimeter. A DRSh500 mercury lamp was used as a source of UV light; spectral lines of 313 and 365 nm were isolated by set of glass filters. Studies were per formed at room temperature in airsaturated solutions in ethanol. Quartz cuvettes with optical path length l = 1 cm were used; the intensity of acting light was 0.05–
RESULTS AND DISCUSSION The absorption spectra of four isomeric forms of the S3S dyad are shown In Fig. 1. The spectrum of the EE isomer was measured experimentally; the spectra of other isomers were calculated by the method of Fisher [21] using the spectra of photostationary mix tures obtained by irradiation with 313 and 365nm light. Due to the symmetry of dyad, the spectra of the EZ isomer and ZE isomer are indistinguishable (Fig. 1, spectrum 2). The EE isomer has a longwavelength absorption band with a maximum at 360 nm (Fig. 1,
hν
N
N O
N O
O
EE hν
O
N
O
N
hν
N N
O
EZ
hν
O
N
ZE
ZZ
Scheme 1. Photochemical reactions of S3S bisstyrylquonline dyad.
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Table 1. Correspondence between the state of the MLG and the S3S isomer depending on the chosen initial isomer Input MLG state in1
in2
0 1 0 1
0 0 1 1
Isomer of the dyad
0,0 1,0 0,1 1,1
EE ZE EZ ZZ
EZ ZZ EE ZE
ZE EE ZZ EZ
ZZ EZ ZE EE
Table 2. Truth table of logic gates for which the states (1,0) and (0,1) or (0,0) and (1,1) are indistinguishable Input
Output
in1
in2
AND
OR
NAND
NOR
XOR
XNOR
INH
IMP
0 1 0 1
0 0 1 1
0 0 0 1
0 1 1 1
1 1 1 0
1 0 0 0
0 1 1 0
1 0 0 1
0 1 0 0
1 0 1 1
spectrum 1), which is shifted hypsochromically to 340 nm for the ZZ isomer with a decrease of more than two times in the intensity of the band (Fig. 1, spectrum 3). In addition, Fig. 1 also shows the luminescence spec trum of EE isomer (spectrum 4), which has an emis sion band at 400–550 nm with a maximum at 440 nm. As can be seen from Fig. 1, the spectra of isomeric forms of the S3S dyad differ markedly, which makes it possible to judge the change in the composition of the reaction mixture using the spectral changes when exposed to the system by taking measurements of the optical density and luminescence intensity. Let’s consider an algorithm of doubleaddress MLG design. As an example, take ZZ isomer as an ini tial state of the MLG (0,0). In a symmetric S3S dyad, both SQfragments are indistinguishable, so in the process of light irradiation the left fragment can be isomerized equally (forming an EZ isomer) as can the right fragment (forming a ZE isomer). However, for definiteness, we assume that the signal applied to input 1 (in1) acts on the left SQfragment; i.e., it transforms the ZZ isomer into the EZ isomer, which in this case corresponds to the MLG state (1,0). The signal fed to input 2 (in2) affects the right SQfragment and turns the ZZ isomer into a ZE isomer, which corresponds to the MLG (0,1). The impact on both inputs of MLG leads to the isomerization of both SQfragments; herewith an EE isomer is formed which corresponds to the final state of the MLG (1,1). ZZ
in2
in1
EZ
ZE in1
in2
EE
Scheme 2.
Under this correspondence between the input sig nals and isomerized fragments, the structural transfor mations occurring under the action on the dyad can be represented in the form of Scheme 2. We considered the ZZ isomer as the initial state of the MLG (0,0). Since transformations between four isomeric forms of the dyad are completely reversible, any of these forms can be selected as the initial state of the MLG. Carrying out a comparison between the input signals and the isomerized fragments in each case similarly to the above, we obtain a table of corre spondence between the state of MLG and the isomeric form of the S3S dyad, depending on the choice of the initial form of the dyad as the (0,0) state of the MLG (Table 1). The EZ isomer and the ZE isomer are spectrally indistinguishable that restricts the range of logic oper ations that can be performed using an MLG based on the bisstyrylquinoline dyad. States of the logic gates corresponding to these two isomers can have only the same signals at the output (“0” or “1”). As can be seen from Table 1, if the initial state of the MLG is an EZ isomer (or ZE isomer), the final state will be a ZE iso mer (or EZ isomer), and therefore, states of MLGs (0,0) and (1,1) corresponding to these isomers must have the same output signals. If the initial state of the MLG is an EE isomer (or ZZ isomer) and the final state is a ZZ isomer (or EE isomer), then the ZE isomer and EZ isomer are intermediate states, so the same output signals must have (1,0) and (0,1) MLG states. Taking into account these limitations, Table 2 shows the truth table of twoaddress logic gates for which pairwise output signals in the states (1,0) and (0,1) or (0,0) and (1,1) coincide.
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Table 3. The states of a molecular logic gate based on S3S. The initial state of the system is PS365; input signals in1 = in2 are a 313nm light exposure Input in1
Output* in2
hν313
hν313
0 1 0 1
0 0 1 1
A360
I440
0.51 0.65 0.65 0.72
54 82 82 93
OR
AND
A = 0.60, I = 70
A = 0.70, I = 88
0 1 1 1
0 0 0 1
* Experimental and threshold values of optical density and luminescence intensity on given wavelengths and the corresponding values of output signals are presented.
Another limitation on the type of logic operations that can be performed using the bisstyrylquinoline dyad is due to the fact that the spectral properties of the EE isomer and ZZ isomer, by contrast, are differ ent, so the states of the MLG state corresponding to these isomers should have different output signals (0 and 1). Two pairs of isomers, EEand ZZ, as well as EZ and ZE, occupy opposite corners of the reaction square in Scheme 2; therefore, they may be pairwise either initial (0,0) or final (0,1) MLG states or inter mediate (1,0) and (0,1) states. If the initial and final MLG states are the same (by output signal), two inter mediate states should differ, and vice versa, if output signals of the intermediate states of MLG are the same, the initial and final states should differ. As can be seen from Table 2, taking into account this limita tion, one cannot transmit a function of logic gates of XOR and XNOR using the S3S dyad. However, there are six logic operations that can be done. In each case it is necessary to select the isomer of the dyad for the initial MLG state, the wavelength of acting light for the input signal, and the threshold value of the output signal to convert the analog signal (the magnitude of the optical density or intensity of luminescence) to digital (0 or 1). It should be noted that in reality it is impossible to obtain “clean” states of an MLG corresponding to the specific compounds listed in the Table 1 since, due to the reversibility of the photoisomerization reaction upon irradiation with light, a cis isomer cannot be fully converted into a trans isomer and vice versa. In real conditions, the photostationary state PSλ is reached under the influence of light consisting of a mixture of cis and trans isomers, the relative content of which ηλ depends on the wavelength of acting light λ. The com position of the photostationary state can be expressed by the content of the cis isomer: ηλ = [cis]ps/([trans]ps + [cis]ps), where [i]ps is the concentration of the i isomer in the mixture. If the initial reactant is a trans isomer, then η is the degree of conversion during the irradiation of the system. NANOTECHNOLOGIES IN RUSSIA
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The calculation for the S3S dyad by the Fischer method has shown that, in photostationary states PS313 and PS365 (formed by irradiation with a light having a wavelength of 313 and 365 nm), η313 = 54% and η365 = 80%. From these data it is clear that the photostationary state PS365 is enriched by the cis iso mer; it can be roughly compared with the ZZ isomer of the dyad. The input signal for MLG in this case will be a light exposure with a wavelength of 313 nm; more over, both input signals are identical; i.e., in1 = in2. In the photostationary state of PS313, the proportion of the trans isomer is increased; it can be roughly com pared with the EE isomer of the dyad. The input signal in this case will be a light exposure with a wavelength of 365 nm, and both input signals are also the same (in1 = in2). The state which is intermediate between PS313 and PS365 can be associated with an asymmetric ZE isomer (or EZ isomer), which under the influence of input signals must be converted to the EE or ZZ iso mer. To obtain an EE isomer, irradiation with a light of 313 nm is necessary; a light of 365 nm is needed to obtain a ZZ isomer. Therefore, in this case the input signals are different: one of them is irradiation with a light of 313 nm; the other is with a light of 365 nm. We now consider a simulation of MLG operation when using the photostationary state of the PS365 dyad as the initial state (0,0) of a logic gate. This state is associated with the ZZ isomer of the dyad, and struc tural transformations occurring under the influence of the dyad are shown in Scheme 2. Input signals (in1 = in2) are the irradiation of light with a wavelength of 313 nm. Fig. 2 shows spectral changes occurring upon the irradiation of the dyad in the photostationary state PS365 by light with 313 nm. Spectrum 1 (and 4) (Fig. 2) corresponds to the MLG (0,0) state. Upon irradiation, there was an increase in optical density and lumines cence intensity. An investigation into the kinetics of the reaction showed that the necessary spectral changes at the light intensity that was used were achieved in 170 s; this exposure time should be taken as the input signal. The single impact to one of the 2012
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I440 in1
reset
in2
in1 + in2 reset reset
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in1 + in2 reset reset
in2
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AND NOR
100 80 OR 80
NAND
60
40
in1: hν 313 in2: hν 313 reset: hν 365
in1: hν 365 in2: hν 365 reset: hν 313
60
Stimuli
Stimuli
Fig. 3. The change in the luminescence intensity of the S3S dyad at a wavelength of 440 nm as a response to exter nal stimuli: the initial state is a PS365 photostationary state, input signals (in1 = in2) are light exposure with 313 nm, and reset is light exposure with 365 nm. Horizontal dashed lines indicate the threshold values of luminescence inten sity for logic gates “OR” and “AND.”
Fig. 4. The change in the luminescence intensity of the dyad S3S at a wavelength of 440 nm as a response to exter nal stimuli: the initial state is the photostationary state PS313, input signals (in1 = in2) are light exposure with 365 nm, and the reset is light exposure with 313 nm. Horizon tal dashed lines indicate threshold values of luminescence intensity for logic gates “NAND” and “NOR.”
inputs (in1 or in2) translates MLG into a (1,0) or (0,1) state, to which the spectrum 2 (and 5) corresponds (Fig. 2). The twofold impact at both inputs (in1 and in2) translates MLG into the state (1,1), to which spectrum 3 (and 6) corresponds (Fig. 2). From any of these three states, (1,0), (0,1), or (1,1), when irradi ated with a light of 365 nm, the system returned to the state (0,0), the spectrum of which coincided with the initial spectrum 1 (and 4) (Fig. 2). Therefore, in this case, irradiation with a light of 365 nm corresponds to the operation of “reset.”
experimental values of optical density (A360) and lumi nescence intensity (I440) after exposure to the system when using the photostationary state PS313 as the ini tial state of MLG (0,0). The response of MLGs to input signals (in1 and/or in2)—irradiation with a light of 313 nm—and the function of reset—irradiation with a light of 365 nm—is shown in Fig. 3. The output signal of luminescence was read at a wavelength of the maxi mum emission of the 440nm EE isomer. A similar form (with a different scale of ordinates) has a graph of the optical density change, the reading of which was performed at an absorption maximum wavelength of 360 nm for the EE isomer. The last stage of the modeling of MLG operation is the conversion of the received analog signal—the optical density or the intensity of luminescence—into
Note that the study of the kinetics of spectral changes allows one to select the optimal dose of expo sure to switch the MLG, but for modeling its work the kinetics has no value and it is necessary to know only the final optical density or luminescence intensity in various states of the logic gate. Table 3 shows the
Table 4. The states of a molecular logic gate based on S3S. The initial state of the system is PS313; input signals in1 = in2 are a light exposure with 365 nm Input in1
Output* in2 A360
NAND
NOR
A = 0.62, I = 70
A = 0.70, I = 100
I440
hν365
hν365
0
0
0.80
110
1
1
1
0
0.65
77
1
0
0
1
0.65
77
1
0
1
1
0.59
63
0
0
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Table 5. The states of the MLG based on S3S. The initial state of the system is 0.5(PS313 + PS365), the input signal in1 is light exposure with 365 nm, and the input signal in2 is light exposure with 313 nm Input in1
Output* A360
INH
A = 0.60, I = 70
A = 0.68, I = 83
I440
hν365
hν313
0
0
0.65
77
1
0
1
0
0.56
57
0
0
0
1
0.70
89
1
1
1
1
0.65
77
1
0
digital. For this it is necessary to establish a threshold value below which the output signal is assumed to be “0” and above which it is “1.” In Fig. 3 two threshold values of luminescence intensity, 70 and 88, are shown by horizontal dashed lines. It is clear from Fig. 3 that at the threshold value 70 the intensity of luminescence I440 < 70 only in the ini tial state of the dyad. Consequently, only in the MLG state (0,0) the output signal is “0.” After any impact on the MLG (to one of the inputs or both), the lumines cence increases to I440 > 70; i.e., the output signal is set to “1.” As can be seen from Table 2, such a ratio between the input and output signals corresponds to the logic gate “OR.” I440 100 in2 in1
reset
reset in1 + in2
INH
80 IMP
60
IMP
in2
in1: hν 365, reset: hν313 in2: hν 313, reset: hν 365
Stimuli
At a threshold value of 88, the luminescence inten sity I440 > 88 only after exposure to both inputs of the logic gate (Fig. 3). Consequently, only in the MLG (1,1) state is the output signal set to “1”; in all other states I440 < 88; i.e., the output signal has a value “0.” As can be seen from Table 2, such a ratio between the input and output signals corresponds to the logic gate “AND.” The established threshold values of optical densities (0.60 and 0.70) and the luminescence inten sities (70 and 88) for the corresponding logic gates are shown in Table 3. Note that the function “OR” and “AND” are oper ations of logic addition and logic multiplication, respectively. On the basis of these functions, comple mented by a function of the negation “NOT,” one can construct any other logic function. Another option for the simulation of MLG opera tion may be obtained by using the photostationary state of the dyad PS313 as the initial state (0,0) of the logic gate. This state was matched with the EE isomer of the dyad; therefore, structural transformations occurring under the influence on the dyad can be rep resented as Scheme 3. Input signals (in1 = in2) in this case are the irradi ation of light with a wavelength of 365 nm, and the reset function is performed by irradiation with a light of 313 nm. The exposure required to switch MLG from one state to another is achieved for 35 s of expo sure. The experimental and established threshold values of optical density and luminescence intensity are shown in Table 4; the response of the MLG to input signals and the function of the reset are shown in Fig. 4. It can be seen that, at a threshold intensity of a
Fig. 5. The change in the luminescence intensity of the S3S dyad at a wavelength of 440 nm as a response to exter nal stimuli: the initial state (the photostationary state) is 0.5 (PS313 + PS365), input signal in1 is light exposure with 365 nm for 40 s (reset is light exposure with 313 nm), and in2 is light exposure with 313 nm for 125 s (reset is light exposure with 365 nm). The horizontal dashed lines indi cate the threshold values of the luminescence intensity for logic gates “IMP” and “INH.” NANOTECHNOLOGIES IN RUSSIA
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in2
in1
ZE
EZ in1
in2
ZZ
Scheme 3. 2012
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in2
in1
ZZ
EE in1
in2
ZE
Scheme 4.
luminescence of 70, the S3S dyad simulates the oper ation of logic gate “NOTAND” (“NAND”), while, increasing the threshold value to 100, it simulates the action of logic gate “NOTOR” (“NOR”). When reading the output signal on the optical density, the corresponding threshold values are 0.62 and 0.70, Table 4. As was shown above, one can choose an intermedi ate state of the dyad between the PS313 and PS365, which is comparable with the asymmetric EZ isomer, as an additional version of the initial state of MLG (0,0). Conventionally, this state can be defined as 0.5 (PS313 + PS365), and structural transformations occur ring under the influence to the dyad are shown in Scheme 4. In this case, there are two input signals: in1 corre sponds to irradiation with 365nm light (exposure is 40 s) and reset corresponds to irradiation with 313nm light and in2 corresponds to 313nm light (exposure is 125 s) and reset to irradiation with 365nm light. Note that, with the sequential operation on both inputs of the MLG, one input signal, in fact, grades the effect of the other, so the final state of the dyad coincides with the original (EZ and ZE isomers are indistinguishable); i.e., the logic gate without the reset is ready for the next cycle of work. Table 5 shows the measured experimental and threshold values of optical density and luminescence intensity, and Fig. 5 shows the response of the MLG to input signals and the function of the reset. It is seen that at the threshold value of the optical density 0.60 and the luminescence intensity 70, the S3S dyad simulates the operation of the logic gate “IMPLICATION” (“IMP”). When the threshold value of the optical density is 0.68 and the luminescence intensity is 83, the dyad S3S simulates the operation of logic gate “INHIBIT” (“INH”) (Table 5). CONCLUSIONS Thus, a variety of logic operations (“AND,” “OR,” “NAND,” “NOR,” “IMP,” and “INH”) can be per formed on the basis of the bisstyrylquinoline S3S dyad. The input signal for the MLG is a light exposure of 313 and/or 365 nm. The output signal is an optical density and luminescence. The type of operation per formed by the MLG depends on the choice of isomers (forms) of the S3S dyad as the initial state of the logic gate (0,0), as well as on the threshold value of the opti cal density or the intensity of luminescence below and
above which the output is assumed to be “0” or “1,” respectively. The S3S dyad demonstrates a unique property that is typical specifically for molecular systems: the com patibility of multiple logic devices in one or the tun ability of logic devices on a different type of work. This property is essentially inaccessible in semiconductor elements that are currently in use, where for each logic operation a separate device is required. An MLG based on a S3S dyad allows one to read the output signal upon luminescence. In this case, both inputs and outputs are photons; i.e., homogene ity of input and output signals is observed. Due to compliance with the conditions of homogeneity for input and output signals, there are prerequisites for the construction of more complex circuit element based on MLGs where the output signal of one element of the circuit is an input signal for another element. The creation of such chains is, perhaps, the next step in the replacement of modern semiconductor elements with molecular analogues. ACKNOWLEDGMENTS This work was performed as part of the program of fundamental research of the Presidium of the Russian Academy of Sciences and supported by the Russian Foundation for Basic Research (project 100300751). REFERENCES 1. A. P. de Silva and S. Uchiyama, Nature Nanotechnol. 2, 399 (2007). 2. F. M. Raymo and M. Tomasulo, Chem.Eur. J. 12, 3186 (2006). 3. D. H. Qu, F. Y. Ji, Q. C. Wang, and H. Tian, Adv. Mater. 18, 2035 (2006). 4. D. Gust, T. A. Moore, and A. L. Moore, Chem. Com mun., 1169 (2006). 5. A. Credi, Angew. Chem., Int. Ed. Engl. 46, 5472 (2007). 6. U. Pischel, Angew. Chem. Int. Ed. 46, 4026 (2007). 7. K. Szacilowski, Chem. Rev. 108, 3481 (2008). 8. M. F. Budyka, N. I. Potashova, T. N. Gavrishova, and V. M. Li, Izv. Akad. Nauk, Ser. Khim., 2535 (2008). 9. M. F. Budyka, N. I. Potashova, T. N. Gavrishova, and V. M. Lee, J. Mat. Chem. 19, 7721 (2009). 10. M. F. Budyka, High Energy Chem. 44, 121 (2010). 11. M. F. Budyka, N. I. Potashova, T. N. Gavrishova, and V. M. Li, High Energy Chem. 42, 446 (2008). 12. S. D. Straight, P. A. Liddell, Y. Terazono, T. A. Moore, A. L. Moore, and D. Gust, Adv. Funct. Mater. 17, 777 (2007). 13. J. Andreasson, S. D. Straight, T. A. Moore, A. L. Moore, and D. Gust, J. Am. Chem. Soc. 130, 11122 (2008).
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