INTRODUCTION
Molecules (and supermolecules)[1'2] form the smaller size range of nano-objects, especially those that allow a degree of rational design including control of size or other properties and those that possess some useful function. This article puts the spotlight on molecules whose usefulness stems from the human comprehensibility of light signals. When a molecule is empowered with light absorption/emission, its small size can be an advantage to operate in tiny spaces but yet remain under a degree of human remote control. Most of these are sensing and switching devices,[3-5] the latter including some logic capabilities. Some of the design principles governing these will be outlined below. These are classified in terms of the formatting of chromophore/fluorophore and receptor components.[6] Chromophores and fluorophores are dyes that give light absorption signals and in the latter case, light emission signals as well. As their name suggests, receptors serve to receive species which are chemical in our cases. Thus chromo/fluorophores and receptors allow physical and chemical transactions, respectively.
"CHROMOPHORE-RECEPTOR" SYSTEMS
The most famous optical molecular devices of this kind are the pH indicators1-7-1 known to every student of high school chemistry. Occupation of the receptor by a proton disturbs the electron distribution of the former. Because of the direct coupling between the chromophore and the receptor, it is therefore natural that the electron distribution of the chromophore itself is disturbed (Fig. 1). The consequence is a significant change of the absorption spectrum of the chromophore. Such acid-induced color changes have brightened up many a chemistry class around the world. A common example would be 1. In general, such p-electron systems have electron donor and electron acceptor terminals, which leads to charge separation in the excited state. Such fractionally charged regions in these internal charge transfer (ICT) excited states can easily lead to observation of spectral wave length shifts as a result of electrostatic interactions with the newly arrived target ion.
As simple as these indicators are, from a chemical standpoint, they also show a logic activity that has hitherto gone unnoticed. For instance, 2[8] shows a simple blue shift of its absorption spectrum upon interaction with Ca2+. However, this blue shift can be examined as a series of optical transmittance values obtained at different wavelengths of observation. As Fig. 2 shows, four wavelengths can be picked out to show clear Ca2+-induced transmittance changes of the ”low-high,” ”high-low,” ”low-low,” and ”high-high” variety. When ”high” is coded as binary 1 and ”low” is coded as binary 0, these digital input-output patterns can be identified as arising from single-input logic devices of the YES, NOT, PASS 0, and PASS 1 types, respectively. Furthermore, all of these logic behaviors can be simultaneously observed because light signals are readily multiplexed. So it is clear that humble ion indicators can show superposed logic behavior, which is unknown in the semiconductor device world.
"RECEPTOR-i-CHROMOPHORE-RECEPTOR2" SYSTEMS
It is only logical to add another receptor to a ”chromo-phore-receptor” system to develop more sophisticated formats (Fig. 3). This can be particularly productive when the two receptors are chosen to be selective, each to its own target species. For instance, 3[8] takes in H+ and Ca2+ at its quinoline nitrogen and amino acid receptors, respectively. The p -electron system again develops a dipole in the excited state with the positive pole being near the amino acid nitrogen and the negative end being close to the quinoline nitrogen. Thus admission of Ca2+ causes a destabilization of the excited state and hence a blue shift of the absorption spectrum (Fig. 4). On the other hand, the entry of H+ causes a stabilization of the excited state and hence a red shift of the absorption spectrum. Of course, the simultaneous treatment of 2 with H+ and Ca2+ gives a near cancellation of these spectral shifts. So an interesting situation arises, where the spectral effect caused by two target ions is nearly the same as what is seen in their absence. Hence we can choose a monitoring wavelength where the transmittance of light is low (coded as binary 0) when the input target species are both low (H+ and Ca2+ both coded as 0) or both high (H+ and Ca2+ both coded as 1). Furthermore, each target ion on its own causes an absorption spectral shift away from the ion-free position. So now the transmittance of light is high (coded as binary 1) when the one input target species is low and the other high (H+ coded as 1 and Ca2+ coded as 0 or its permutation). When these results are cast into a logic truth table (Fig. 3), we see that 3 behaves as a two-input XOR gate.
Fig. 1 The general format of a ”Chromophore (C)-Receptor (R)” system.
"FLUOROPHORE-RECEPTOR" SYSTEMS
Fluorescent versions of ion indicators1-9-1 also have a long history and their mode of action borrows extensively from their absorption-based cousins. One of the significant deviations of ”fluorophore-receptor” systems arises as a result of the relative temporal delay before fluorescence emerges from an excited molecule (Fig. 5). Electrostatic repulsion between the photo-produced charge separations and the receptor-incumbent target species during this time period can cause decoordination of the target. Thus the target-induced spectral change will also dissolve away. Fluorescence emission spectra are therefore weakly influenced by target binding in many ”fluorophore-receptor” systems known so far, although several exceptions are available. Of course, the target-induced changes survive in the fluorescence excitation spectra, which are related to the absorption spectra anyway. Grynkiewicz et al.’s[10] excellent Ca2+ sensor 4 illustrates this very well. Here is an iconic optical molecular device that has served the cellular physiology community for nearly two decades now by imaging Ca2+ populations within living cells.
Fig. 2 Simultaneous observation of all four single-input logic types from a single experiment with a Ca2+ indicator.
Fig. 3 The general format of a ”Receptor1 (R1)-Chromophore (C)-Receptor2 (R2)” system and the logic truth table for the corresponding XOR gate.
”FLUOROPHORE-SPACER-RECEPTOR” SYSTEMS
The apparently trivial addition of a spacer between a fluorophore and a receptor (Fig. 6) can completely change the device characteristics of the system. The spacer brings with it the ability to isolate components from the influence of short-range forces that normally abound in the chemical world. So the fluorophore and the receptor are forced to communicate via long-range interactions alone. These are few, and in many cases, can be reduced to one. Pho-toinduced electron transfer (PET), the celebrated mecha- nism of green plant photosynthesis, is the commonest controller of optical molecular devices of the ”fluoro-phore-spacer-receptor” type. The fluorescence emission capability of the fluorophore is arrested by PET successfully competing for the energy of the excited state. Thus the device output is initially held in the ”low” state (coded as 0). However, PET can be electrostatically stamped out, especially by charged target species when they take up residence in the receptor. Now excitation of the system will lead to no competition for the energy of the excited state. Consequently, the excited state returns to ground by emitting fluorescence as most fluorophores do. The device output is now ”high” (coded as 1). Such target-induced fluorescence switching is logically a single-input YES gate. An example is the fluorescent sensor 5[11] for Na+, which is marketed by Roche Diagnostics for blood analysis in hospital critical care units. Cases such as 5 use more than electrostatics to enhance the fluorescence switching. The receptor within 5 is a N-(2-methoxyphenyl)monoaza-15-crown-5 ether, which suffers a major change in conformation upon capturing Na+. This act reduces the electron delocalization within p-system of the receptor, which, in turn, makes the PET process more difficult and the fluorescence emission stronger. An extra feature within systems such as 5 is the ease with which components can be substituted for, in order to change the species being targeted or even its concentration range. So K+-selective sensors with the same optical parameters found in 5 become available by simply replacing the receptor. Similarly, the Na+-selective relatives of 5 such as 6,[12] which communicate with different colors, of absorption and emission, are obtained by changing the fluorophore. Of course, the feasibility of PET must be conserved during such module replacements.
Fig. 4 The realization of general XOR logic behavior in the transmittance output at 390 nm of the UV-Vis absorption spectra set of 3.
Fig. 5 The general format of a ”Fluorophore (F)-Receptor (R)” system.
Fig. 6 The general format of a ”Fluorophore (F)-Spacer (S)-Receptor (R)” system.
"FLUOROPHORE-SPACER^RECEPTOR!-SPACER2-RECEPTOR2" SYSTEMS
As observed above, a spacer ensures a high degree of modularity of systems so that individual components are somewhat autonomous. This not only makes PET switch system design a predictive activity but also makes PET system expansion thoroughly logical. Addition of new modules will bring with them possibilities of PET, which are each predictable (provided that electron transfer data are available). Then it becomes possible to arrange situations in which two target species arrive at suitable receptors, either alone or together. Of course, we need to have adequate selectivity within the chosen receptors so that cross-talk of target species will be minimized. Now we have two-input, one-output devices that employ the same foundations as discussed above for one-input, one-output systems. In the simplest cases, ”fluorophore-spac-er1-receptor1-spacer2-receptor2” systems will have two possible PET paths originating from each receptor and finishing at the fluorophore, unless each is blocked by the correct target species. So fluorescence emerges unchallenged only if both receptors are blocked by the two target species being applied as inputs. The condition of Inputj=1 and Input2=1 is required before a "fluorophore-spacerj-receptori-spacer2-receptor2” PET system will pass an output=1. This is clearly AND logic (Fig. 7). The first example of this, and the first molecular logic gate of any kind in the primary literature, was 7.[13] This uses H+ and Na+ as the two inputs. There are several excellent ways of arriving at molecular AND gates now,[14-18] some of which have led to more complex logical behavior.[14,19-21] ”Fluorophore-spacer1-receptor1-spacer2-receptor2” systems can also be put to uses that do not depend on binary logic. For instance, receptor1 can be chosen as an electron donor amine, whereas receptor2 can be chosen to be poorly electroactive. A pyridine is the choice for practical reasons, where the fluorophore is an anthracene unit within 8.[22] Naturally, amines lose their electron donor activity upon binding to a proton. PET processes are suppressed. On the other hand, pyridines become good electron acceptors upon proton binding. PET processes are created. So the proton target species has opposite effects upon arrival at the two receptors, each with its own concentration threshold for reception. We note that a single-input species causes a single fluorescence output to be controlled in a relatively complex way. At low proton concentrations, both receptors are free and the amine launches a PET process to destroy fluorescence. At mid-range proton concentrations, the more avid amine receptor picks up a proton, thereby closing its PET channel. Fluorescence flares up as a consequence. At high proton concentrations, both receptors are protonated. The pro-tonated amine remains PET-disabled, but the newly formed pyridinium launches its own PET channel and extinguishes the fluorescence. Thus the fluorescence output follows a ”off-on-off” pattern in response to mono-tonically ramping proton concentrations (Fig. 8).[22-24] Such systems are useful in being direct optical indicators of pH conditions of enzyme activity or even cellular activity. After all, the principle of ”the happy medium” or ”the middle way” affects everyone.
Fig. 7 The general format of a ”Fluorophore (F)-Spacer1 (S1)-Receptor1 (R1)-Spacer2 (S2)-Receptor2 (R2)” system and the logic truth table for the corresponding AND gate.
"RECEPTOR^SPACER^FLUOROPHORE-SPACER2-RECEPTOR2” SYSTEMS
Realizable permutations arise when a sufficiently large number of modules are contained in a system. This is the case with ”fluorophore-spacer1-receptor1-spacer2-re-ceptor2” systems. A realizable permutation is to shift the fluorophore to the center of the system (Fig. 9). This act has an advantage for chemical design, because fluorescence switching efficiencies can be improved via accelerated PET processes arising from the shorter fluorophore-receptor distances involved. An example is 9.[25] Even ”receptor1-spacer1-fluorophore-spacer2-re-ceptor2-spacer3-receptor3” systems are now in the hands of designers[26] to perform increasingly complex tasks with deceptively small molecules such as 10.
Fig. 8 The ‘off-on-off’ fluorescence-pH profile of 8.
Fig. 9 The general format of a ”Receptor1 (R1)-Spacer1 (S1)-Fluorophore (F)-Spacer2 (S2)-Receptor2 (R2)” system.
The cases discussed above involved separate target species such as Na+ and H+ arriving essentially simultaneously at their respective receptors. Of course, these target species can be independently controlled to test all the input combinations for setting up truth tables to assign logic behavior. It is also feasible to build, say, two target species into separate sites of a bifunctional molecule. Now the real target becomes the bifunctional molecule itself. Naturally, such bifunctional reception can lead to enhanced selectivity of binding and detection. For example, 11[27,28] selectively targets amino acid zwitterions with a specified number of carbon atoms in between the ammonium and carboxylate functionalities. While the binding is enhanced, the fluorescence signaling suffers from a weakness. Indeed, the binding of the ammonium group leads to PET suppression and fluorescence enhancement. However, the capture of the carboxylate moiety reaps no such fluorescence reward owing to the lack of sufficient PET activity in the guanidinium group. Nevertheless, a nice case with two PET-active receptors is available in the form of 12 from Cooper Protonated glucosamine is the valuable target. The aza-18-crown-6 ether receives an ammonium group as in the case of 11. Additionally, a diol feature is held by the aminomethylboronic acid receptor, which leads to PET suppression. So now both PET channels are blocked upon arrival of the glucosamine species in an AND logical manner. This application of AND logic systems for the enhanced binding and optical signaling is a very promising avenue of research.
CONCLUSION
Dyes (fluorescent or not), receptors, and spacers are the building blocks that designers of optical molecular devices can play with. Combinations, or even some permutations, of these blocks can lead us to sensors, logic gates, and ”off-on-off” systems already. Considering that at least some of these are demonstrably useful here and now, the number of players is bound to increase. The result will be even more interesting systems in the future.
