Multivalued Logic with a Tristable Fluorescent Switch - The Journal of

J. Phys. Chem. C 2009, 113, 5805–5811

5805

Multivalued Logic with a Tristable Fluorescent Switch Rita Ferreira, Patricia Remo´n, and Uwe Pischel* Department of Chemical Engineering, Physical Chemistry, and Organic Chemistry, Faculty of Experimental Sciences, UniVersity of HuelVa, Campus de El Carmen s/n, E-21071 HuelVa, Spain ReceiVed: October 28, 2008; ReVised Manuscript ReceiVed: January 29, 2009

A fluorophore1-spacer-receptor-spacer-fluorophore2 dyad was prepared and photophysically characterized. The two spectrally well-differentiated fluorophores are 1,8-naphthalimide and its 4-amino-substituted derivative, the latter showing strong ICT emission. The receptor unit consists of a tertiary amine, which leads to siteselective PET (only the 1,8-naphthalimide part gets quenched). This process can be blocked by protonation of the tertiary amine, which, on the other hand, results in enhanced singlet-singlet energy transfer between the 1,8-naphthalimide and 4-amino-1,8-naphthalimide parts (ΦEET ) 0.27 and 0.59 for neutral and monoprotonated form, respectively). In acetonitrile, the monoprotonated form can be deprotonated by sufficiently basic anions like fluoride in micromolar concentration (1 mM leads to deprotonation of the 4-amino group of one of the naphthalimides, which is accompanied by more fluorescence quenching (ca. 93%). The resulting double-sigmoidal titration curve enables the implementation of ternary logic. Using fluoride anions as degenerate inputs results in a ternary NOR logic gate, which is demonstrated for the first time. Introduction The design of functional molecular and supramolecular entities continues to be the focus of intense research activities in the chemistry community.1,2 Molecular architectures containing photoactive building blocks have enjoyed strong preference in this respect,2 because of the possibility to activate, control, and read-out using photons as tool in a variety of excited-state processes like for example photoinduced electron transfer (PET), electronic energy transfer (EET), and photochromism.3 Many photoactive molecular devices are designed to facilitate supramolecular recognition of external chemical inputs or involve in such fundamental processes like acid-base interactions,4 thereby modulating for instance absorption or fluorescence characteristics, which can be used as optical output signals. This has led to numerous molecular sensors, machines, memories, switches, etc.1,2,5-8 Most reported molecular fluorescent switches distinguish between high and low optical output signals, characterized by fluorescence ON-OFF or OFF-ON switching. This bistable situation matches perfectly with the concept of binary logic, the basis of modern information technology, where every bit is coded by 0 or 1, corresponding to a low or high signal (positive logic convention), respectively. Therefore it is not surprising that the molecular realization of binary logic operations with fluorescent switches has attracted wide attention, resulting in examples for simple or combinational molecular logic gates,9-12 calculators,13,14 password entry devices,15-18 multiplexers/demultiplexers,19-21 or encoders/decoders.22 The relevance of such research activities lies in the fact that molecular information processing has been pointed out as one possible alternative for overcoming the current size-resolution impasse in lithographybased conventional chip fabrication.23 * To whom correspondence [email protected].

should

be

addressed.

E-mail:

However, implementing molecular logic operations in the context of the intensively debated long-term objective of molecular computing may go beyond the imitation of existing protocols in the binary macroscopic world of silicon circuitry. A strength of the molecular approach, besides its obvious sizerelated advantage, could be the realization of alternative concepts related to logics. Such a concept is the extension of predominating binary logic to its multivalued variations.24,25 Ternary logic for instance defines three different states for input and output signals, which might correspond for example to logic values of 0 (low signal), 1 (medium signal), and 2 (high signal). Other related notations define a 0 as low, a 1 as high, and an X for an unknown signal value. Given the availability of three states, it was noted by several authors that ternary logic could facilitate processing of higher information densities than binary systems.26,27 Noteworthy is the idea that multivalued logic was found recently to be a versatile approach in molecular computational identification (MCID) of small solid objects.28 In line with these thoughts, the objective of the present work was the implementation of a defined ternary molecular logic operation, as common for the many reported molecular binary logic gates. For this purpose, a tristable molecular switch, the monoprotonated form of dyad 1 (Chart 1), was developed. Multistable fluorescence switching, involving more than two distinguished optical output levels of the same signal, is limited to just a handful of examples, and it has been generally acknowledged that their realization is an intricate task.27,29-32 Such switches may be useful for logic operations beyond binary notation. However, until now the description of a fully assigned ternary logic gate operation remains elusive to the best of our knowledge. Toward this aim we demonstrate the implementation of a molecular ternary NOR gate. Importantly, the herein discussed multistable switching of one output channel is conceptually different from systems, which often involve photochromic compounds and where the distinct states have spectrally different output signatures.33

10.1021/jp809527d CCC: $40.75  2009 American Chemical Society Published on Web 03/11/2009

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CHART 1: Structures of Dyad 1 and Corresponding Model Compounds 2-4

SCHEME 1: Synthesis of Dyad 1

Recently, microfluidic fluorescent logic gate devices with “wet” chemical inputs (among them degenerate inputs) have been reported.34 This shows that solution logic gates, like the one realized in this work, might have interesting applications, which are not only limited to the ambitious target of computing, but can be easily imagined in the context of sensing.35 The increased information density, which characterizes ternary logic, is also a surplus for combinational sensing. Experimental Section Materials. Commercial reagents for synthesis (Fluka or Aldrich) were of highest purity available and used as received. Acetonitrile (Scharlau) was of spectroscopic quality. Dimethyl sulfoxide-d6 (Aldrich) had a deuteration grade of >99.9 atom% D. Trifluoroacetic acid (Riedel-de-Haen, Spectranal, 99.8%) and tetra-n-butylammonium fluoride trihydrate (TBAF, Fluka, >97%) were used as received. Compounds 236 and 337 are known compounds. Compound 3 has been synthesized for this work (see the Supporting Information), whereas 2 was available from a previous project.38 Quinine sulfate was from Fluka and used as received. N-Propyl-1,8-naphthalimide (4) was available from a previous project.38 Synthesis of Dyad 1. The reaction sequence is shown in Scheme 1. An ethanol solution (10 mL) of 290 mg (0.89 mmol) of N-[3-[(3-aminopropyl)methylamino]propyl]-1,8-naphthalimide (synthesis reported in ref 38) and 155 mg (0.73 mmol) of 4-amino-1,8-naphthalic anhydride was refluxed for 4 h. Subsequently, the mixture was cooled and the formed precipitate was filtrated. The obtained solid was washed successively with acetonitrile and n-hexane and finally dried under vacuum (170 mg, 44% yield, orange-brown powder). 1 H NMR (400 MHz, DMSO-d6): 8.59 (d, J ) 8.5 Hz, 1H, arom. H), 8.46 (d, J ) 7.2 Hz, 2H, arom. H), 8.43 (d, J ) 8.1 Hz, 2H, arom. H), 8.36 (d, J ) 7.2 Hz, 1H, arom. H), 8.14 (d, J ) 8.4 Hz, 1H, arom. H), 7.84 (t, J ) 7.8 Hz, 2H, arom. H), 7.62 (t, J ) 7.8 Hz, 1H, arom. H), 7.41 (s, 2H, NH2), 6.82 (d, J ) 8.4 Hz, 1H, arom. H), 4.08 (t, J ) 7.3 Hz, 2H, NimideCH2), 4.00 (t, J ) 7.3 Hz, 2H, NimideCH2), 2.41 (t, J ) 6.8 Hz, 2H, NamineCH2), 2.38 (t, J ) 6.8 Hz, 2H, NamineCH2), 2.16 (s, 3H, NCH3), 1.74 (m, 4H, CH2). HRMS (CI) calcd for C31H29N4O4 (1H+) 521.2189, found 521.2176. Measurements. Absorption measurements were done with a UV-1603 spectrometer from Shimadzu. Fluorescence spectra were recorded with a Cary Eclipse instrument from Varian. All measurements were done at ambient temperature (23 °C) in

quartz cuvettes of 1 cm optical path length. Fluorescence titrations were performed by successive addition of aliquots of stock solutions of trifluoroacetic acid or TBAF to ca. 10 µM solutions of 1 or 1H+ in acetonitrile. The samples were either excited at 332 nm (>90% of absorbed photons in 1,8-naphthalimide) or 416 nm (100% of absorbed photons in 4-amino-1,8naphthalimide). Optically matched solutions of dyad 1 and the respective reference compound were used for the determination of fluorescence quantum yields, according to published standard procedures.39 For the 1,8-naphthalimide part (λexc ) 332 nm), N-propyl-1,8-naphthalimide (4) served as reference (Φf ) 0.016 in acetonitrile).40 The quantum yield was corrected for the small absorption of the 4-amino-1,8-naphthalimide part at 332 nm. Quinine sulfate (Φf ) 0.55 in 0.05 M H2SO4)41 was used as reference for fluorescence quantum yield determination of the 4-amino-1,8-naphthalimide part in dyad 1 (direct excitation) or model 3. Quantum yields have been corrected for refractive index differences, where necessary. Time-resolved picosecond fluorescence measurements were performed using the single-photon timing method with laser excitation. The setup42,43 consisted of a mode-locked Coherent Innova 400-10 argon-ion laser that synchronously pumped a cavity dumped Coherent 701-2 dye (Rhodamine 6G or DCM) laser, delivering fundamental or frequency-doubled 5 ps pulses at a repetition rate of 3.4 MHz. Alternatively, a pulse picked Spectra-Physics Tsunami Ti:sapphire laser, which was pumped by a Spectra-Physics Millenia Xs Nd:YVO4 diode pumped laser, was used as excitation source. This configuration delivered 100 fs frequency-doubled pulses at a repetition rate of 4 MHz. Intensity decay measurements were made by alternate collection of impulse and decay, with the emission polarizer set at the magic angle position. The instrument impulse was recorded slightly away from excitation wavelength with a scattering suspension (1 cm cell). For the decays, a cutoff filter was used, effectively removing all excitation light. Excitation/emission wavelengths were 332/380 and 416/510 nm for 1,8-naphthalimide and 4-amino-1,8-naphthalimide chromophores, respectively. The emission signal passed through a depolarizer, a JobinYvon HR320 monochromator with a grating of 100 lines/mm, and was detected with a Hammamatsu 2809U-01 microchannel plate photomultiplier. Time scales of 1.3-2.6 ps/channel were used for 1,8-naphthalimide emission decays and 20-48 ps/ channel for 4-amino-1,8-naphthalimide emission decays. The instrument response function had an effective fwhm of 35 ps. The fitting of the decay traces was done by deconvolution taking

Multivalued Logic with a Tristable Fluorescent Switch

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SCHEME 2: Fluorescence Switching upon Addition of Fluoridea

a

Fluorescence intensity is color coded.

into account the instrument impulse. The goodness of the fit was characterized by χ2 values between 1.00 and 1.31. Results and Discussion Molecular Design. Fluorophore-spacer-receptor architectures integrating redox-active fluorophores and electron-donating amino receptors, which are prone to engage in PET processes, form the well-established fundament of the chosen molecular design.5 The herein applied general fluorophore1spacer-receptor-spacer-fluorophore2 format has been used in other reports for the purpose of pH or metal cation fluorescence sensing.44-46 However, tristable fluorescence switching behavior was never observed with such molecular architecture. The bifluorophoric dyad 1 integrates a PET-active and electronically isolated tertiary amino group and two well-established photoactive aromatic dicarboximides: 1,8-naphthalimide36,38,47 and 4-amino-1,8-naphthalimide.48-51 It is noteworthy that this chromophore pair allows practically selective excitation of each entity, due to sufficiently separated absorption bands (see below). The general photophysical design is based on the interplay of two thermodynamically allowed processes: photoinduced electron transfer (PET) and electronic energy transfer (EET). Upon selective excitation of the 1,8-naphthalimide part, EET between 1,8-naphthalimide as energy donor and 4-amino1,8-napthalimide as acceptor is in competition with PET from the tertiary aliphatic amine to 1,8-naphthalimide. On the other hand, the analogue PET to 4-amino-1,8-naphthalimide, which shows internal charge transfer (ICT) character in the excited state, is rather inefficient. This behavior was reasoned in recent studies by invoking molecular electric field effects48 and calculations of electronic coupling matrix elements.52 Competition between PET and EET depends on their actual rate constants. For our purpose it is desirable that both processes have comparable rates, without the total predominance of one of them. In this scenario, EET should lead to fluorescence sensitization of the 4-amino-1,8-naphthalimide part and blocking of PET by protonation of the tertiary amine would increase fluorescence further due to more efficient EET from 1,8-naphthalimide. The monoprotonated state of dyad 1, i.e., 1H+ (Scheme 2, left structure), is the starting point for the realization of the tristable switch. Tertiary amine protonation can be reversed by additionofbasicanions,likefluoride(F-)inµMconcentration,38,46,53 thereby diminishing again the fluorescence signal of 4-amino1,8-naphthalimide. This is the indirect result of PET reactivation, causing a drop of EET efficiency due to the renewed competition between both excited-state processes in neutral 1 (Scheme 2, middle structure). Further, the ICT fluorophore contains a relatively acidic NH2 group (electron-withdrawing effect of the imide function), which can be deprotonated at high F- concentrations (mM range) leading to (1-H)- and additional fluorescence quenching, as observed for other derivatives of 4-amino-

Figure 1. Absorption spectra of 1 (solid line) and respective model compounds 2 (dotted line) and 3 (dashed line) in acetonitrile.

1,8-naphthalimide.54 Thus, three different states are involved (Scheme 2), showing pronounced differences for 4-amino-1,8naphthalimide fluorescence: the monoprotonated dyad 1H+ as starting point, neutral dyad 1, and the deprotonated dyad (1H)-. Steady-State Photophysical Properties of Dyad 1 and their Modulation by Protonation. In acetonitrile, dyad 1 shows the typical UV/vis absorption spectrum corresponding to the two chromophores (Figure 1). The 1,8-naphthalimide part exhibits a band with maxima at 332 nm (
) 12900 M-1 cm-1) and 346 nm (12200 M-1 cm-1), whereas the 4-amino-1,8-naphthalimide site is characterized by a UV band at 259 nm (22400 M-1 cm-1) and a broad band in the visible range with a maximum at 416 nm (11400 M-1 cm-1). The global spectrum of 1 corresponds to the absorption spectra of model compounds 2 and 3, which suggests the absence of significant ground-state interactions between the chromophore units. Furthermore, as can be seen in Figure 1, selective excitation of each chromophore in the dyad is feasible: at 332 nm 1,8-naphthalimide accounts for ca. 90% of the absorbed photons, whereas at 416 nm 4-amino-1,8naphthalimide absorbs exclusively. In agreement with the above outlined photophysical design of dyad 1, its fluorescence behavior upon selective excitation at 332 nm is dictated by the competition between PET and EET processes. PET from the spacer-integrated tertiary amino function to singlet-excited 1,8-naphthalimide as electron acceptor (path a, Scheme 3) is thermodynamically feasible. The free energy of this process was estimated as ∆GPET ) -1.78 eV with the Rehm-Weller equation (eq 1).55,56 For the sake of clarity it should be mentioned at this point that in the further discussion two other PET processes (path b and c) will be invoked, which are summarized in Scheme 3 to avoid their confusion. However, the mentioning of PET without further indications in the following text refers always to path a, the only one with importance for the herein developed switching mechanism.

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SCHEME 3: Nomenclature of PET Pathways in 1a

a

The electron donor and acceptor are color-coded.

∆GPET ) Eox - Ered - E* + C

(1)

In dyad 1 the PET fluorescence quenching pathway (path a) has to compete with singlet-singlet energy transfer (EET) from 1,8-naphthalimide to 4-amino-1,8-naphthalimide, which is exergonic as well: ∆GEET ) -0.73 eV, estimated with eq 2.55 Dipole-dipole energy transfer (Fo¨rster) has been assumed as the mechanism. This is corroborated by the significant spectral overlap (Jdipole-dipole ) 1.6 × 10-11 cm6 mol-1, eq 3) between donor fluorescence and acceptor absorption spectrum and a critical energy transfer radius of 19 Å, estimated with eq 4.57 In eq 4 the orientation factor κ2 was taken as 2/3, Φdonor is 0.016 (for 4),40 the refractive index n for acetonitrile is 1.3441,58 and NA is Avogadro’s number. Modeling of dyad 1 at the semiempiricalAM1levelyieldedanestimatefortheenergydonor-acceptor distance R of 14.5 Å (see Supporting Information), well below the critical radius. In agreement with EET the experimentally observed fluorescence spectrum (λexc ) 332 nm) is clearly dominated by the intrinsically strong ICT emission of the 4-amino-1,8-naphthalimide part with a maximum at 511 nm. The fluorescence of 1,8-naphthalimide is seen as minor band with a maximum at 380 nm and a shoulder at 368 nm. Further, the excitation spectrum of 1 (monitored at λobs ) 511 nm) shows clearly the absorption features of 1,8-naphthalimide. According to a published method,39 the comparison of excitation and absorption spectra allowed the estimation of the EET quantum yield (ΦEET). In the present case a moderate value of ΦEET ) 0.27 ( 0.06 was obtained.

∆GEET ) E*acceptor - E*donor Jdipole-dipole )

R06 )

∫0∞

FD(ν)εA(ν) dν

9 ln 10κ2Φdonor 128π5n4NA

ν4

Jdipole-dipole

(2)

(3)

(4)

The fluorescence quantum yield (Φf) of the 1,8-naphthalimide part is given as upper limit of e10-3, because the rather weak fluorescence signal of this chromophore is measured with higher uncertainty. However, it is clear that the compared to model 4 (Φf ) 0.016 in acetonitrile)40 significantly reduced fluorescence relates to the efficient deactivation through PET and EET. The fluorescence quantum yield for the direct excited 4-amino-1,8napthalimide part is Φf ) 0.08 ( 0.02. Also this value is lower than for other 4-amino-1,8-naphthalimide derivatives (e.g., Φf ) 0.45 ( 0.05 for 3 in acetonitrile). This reduction might be the result of a third pathway, which includes singlet-excited 4-amino-1,8-naphthalimide as electron donor in PET (path b,

Figure 2. Fluorescence titration of 1 (9 µM in acetonitrile) with trifluoroacetic acid at λexc ) 332 nm. (a) Spectral evolution, (b) titration curve at λobs ) 520 nm.

Scheme 3) to 1,8-naphthalimide as acceptor (not shown in Scheme 2 for the sake of simplicity). Such pathway is thermodynamically feasible (∆G ) -0.56 eV).55 Similar observations have been made for other dyads containing 4-amino-1,8-naphthalimide motifs.59,60 However, it must be stressed that its occurrence does not compromise the above outlined photophysical design principle. As anticipated from the competitive PET in the unprotonated dyad, the protonation of the tertiary amine (formation of 1H+) leads to enhanced EET in consequence of PET blocking. Upon addition of 1 equiv of trifluoroacetic acid (TFA), corresponding to ammonium ion formation, an increase of EET-sensitized green fluorescence of 4-amino-1,8-naphthalimide was observed (Figure 2). The fluorescence enhancement (FE) was ca. 1.7. The moderate FE value relates to the competition between PET and EET. On the other hand, absence of fluorescence enhancement (FE ) 0.8) of selectively excited (λexc ) 416 nm) 4-amino1,8-naphthalimide upon protonation of the tertiary amine corroborates that PET to this chromophore from a distal amine linked to the imide N is not favorable for the reasons outlined above.48,52 The enhanced EET in 1H+ is clearly reflected by the increased quantum yield (ΦEET), which was estimated as 0.59 ( 0.06. Based on eq 5, a larger value for ΦEET (0.84) could be expected for 1H+. Two reasons can be invoked for explaining the discrepancy: (a) uncertainty in the estimation of donor-acceptor distance for the highly flexible spacer, (b) thermodynamically possible involvement of a competitive PET pathway (path c,

Multivalued Logic with a Tristable Fluorescent Switch

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Scheme 3) from 4-amino-1,8-naphthalimide to singlet-excited 1,8-naphthalimide (∆G ) -1.29 eV).55

ΦEET )

1 1+

( ) R R0

6

(5)

Competition between PET (Path a) and EET Processes. From the steady-state fluorescence measurements discussed above, it is clear that PET and EET processes are in competition in dyad 1. In order to obtain a more global picture, time-resolved fluorescence data were measured as well. PET involving the spacer-integrated tertiary amine as electron donor (path a) and EET are the decisive processes for the observed proton-induced fluorescence modulation (left and middle structure in Scheme 2), in both of which singlet-excited 1,8-naphthalimide is involved. The following discussion will concentrate mainly on these two paths. Hence, measurements of the fluorescence lifetime of the 1,8-naphthalimide part upon selective excitation at 332 nm and observation at 380 nm were performed. For both forms, 1 and 1H+, biexponential decay kinetics resulted, which might be traced back to different conformations of the flexible spacer unit. For the sake of simplicity, average lifetimes τav, which have been obtained with eq 6 (Ri is the weighted preexponential factor), are compared herein. For 1 τav ) 454 ps [τ1 ) 201 ps (41%), τ2 ) 629 ps (59%)] resulted, while 1H+ gave rise to a significant longer average lifetime τav ) 733 ps [τ1 ) 273 ps (35%), τ2 ) 981 ps (65%)] for the 1,8naphthalimide part. This hints on a strong participation of PET in 1.

Figure 3. Fluorescence titration of 1H+ (9.7 µM in acetonitrile) with TBAF at λexc ) 332 nm. (a) Spectral evolution and (b) titration curve at λobs ) 520 nm.

ΦPET is verified by comparing the fluorescence lifetimes (see above) of 1,8-naphthalimide in 1 and 1H+; application of eq 8 yields ΦPET ) 0.38.

ΦPET ) 1 n

τav )

∑ Riτi i)1

n

with

∑ Ri ) 1

ΦEET(1) ΦEET(1H+)

(7)

(6)

i)1

ΦPET ) 1 On the other hand, direct excitation into the 4-amino-1,8naphthalimide band (λexc ) 416 nm) and selective observation at 510 nm led expectedly to very similar average lifetimes for the ICT fluorophore in 1 and 1H+: τav ) 3.92 ns [τ1 ) 2.43 ns (70%), τ2 ) 7.45 ns (30%)] and τav ) 3.72 ns [τ1 ) 0.44 ns (3%), τ2 ) 2.20 ns (63%), τ3 ) 6.75 ns (34%)], respectively. These lifetimes are significantly shorter than measured for model compound 3 [τf ) 10.52 ns (100%)], which is in agreement with the above-mentioned involvement of PET (path b) from singlet-excited 4-amino-1,8-naphthalimide to 1,8-naphthalimide. It is noteworthy that the excited-state processes involving 1,8naphthalimide in the dyad seem to happen on a slower time scale as compared to the fluorescence lifetime of plain 4 [τf ) 128 ps (100%)]. This could be discussed in terms of structural differences between 1 and 4 and is not without precedent. For instance fluorescence lifetimes of bis-1,8-naphthalimides linked by oligomethylene spacers have been measured to be longer than observed for N-alkyl-1,8-naphthalimides.61 However, using the lifetime of model 4 for the calculation of rate constants for PET (path a) and EET would result in highly overestimated values and, thus, an erroneous picture for the competition of both processes. This problem can be overcome by using quantum yields of EET for 1 (ΦEET ) 0.27) and 1H+ (ΦEET ) 0.59) in eq 7, to estimate the quantum yield of PET in 1: ΦPET ) 0.54. Hence, in dyad 1 PET (path a) is twice as efficient than EET, which is experimentally verified by the increase of EET-sensitized 4-amino-1,8-naphthalimide fluorescence upon blocking of PET in 1H+ (see Figure 2). A similar value for

τf(1) τf(1H+)

(8)

Fluorescence Switching upon Addition of Fluoride Anions. One of the objectives of this work was the development of a tristable fluorescent switch. In order to achieve this aim, the monoprotonated form 1H+ was chosen as starting point. As described before, the experiments were performed by exciting selectively the 1,8-naphthalimide part at λexc ) 332 nm. Addition of basic anions, like the chosen fluoride (F-), to 1H+ is expected to reactivate PET (path a) due to deprotonation of the ammonium, yielding the formation of 1 (see Scheme 2).38,46,53 In agreement with the thus lowered efficiency of EET-sensitization, titration of 1H+ with F- (concentration range 1-20 µM) led to fluorescence quenching of the 4-amino-1,8-naphthalimide part by 41% (Figure 3). Noteworthy, this signal decrease relates exactly to the observed fluorescence increase in the reverse process, the protonation of the tertiary amine (FE ) 1.7, see above). Presence of F- in the range between 20 µM and 580 µM caused no further changes of the fluorescence output. However, based on earlier reports, F- in comparably high concentrations (>1 mM) can initiate deprotonation of the 4-amino group,54 which explains fluorescence quenching of 1H+ in presence of F- in the mM range (ca. 93% total quenching at 25 mM F-; Figure 3). Related observations were made when starting with neutral dyad 1 (see the Supporting Information). Furthermore, as shown in Figure 4, absorption spectra showed the parallel formation of a broad absorption band at ca. 525 nm. In

5810 J. Phys. Chem. C, Vol. 113, No. 14, 2009 SCHEME 4: Schematic Representation of the Action of Two Physically Separated F- Sources As Degenerate Inputs of a Ternary Logic Gate

Ferreira et al. TABLE 1: Truth Table for a Ternary NOR Gatea entry

pFtotal (sum of both inputs)

input Ab (F-)

input Bb (F-)

outputc (hνf ) 520 nm)

1 2 3 4 5 6 7 8 9

6.4 4.0 1.6 4.0 3.7 1.6 1.6 1.6 1.3

0 0 0 1 1 1 2 2 2

0 1 2 0 1 2 0 1 2

2 (528) 1 (312) 0 (35) 1 (312) 1 (312) 0 (35) 0 (35) 0 (35) 0 (26)

a Initial conditions: 9.7 µM 1H+ in acetonitrile, λexc ) 332 nm. 0: pF ) 6.7, 1: pF ) 4.0, 2: pF ) 1.6. c Fluorescence intensity of 4-amino-1,8-naphthalimide part at 520 nm. 0: If e 35, 1: If ) 312; 2: If ) 528. b

accordance with recent suggestions54 this new band was assigned to the deprotonated 4-amino-1,8-naphthalimide part, i.e., formation of (1-H)-, see Scheme 2. The resulting perturbation of the ICT state through interaction with F- is the likely reason for the observed fluorescence reduction. In agreement with this notion, direct excitation of the 4-amino-1,8-naphthalimide part at the quasi-isosbestic point of the absorption spectra at λexc ) 452 nm resulted in the same quenching effects like observed for excitation at 332 nm. This excludes that an alteration of EET efficiency caused the fluorescence reduction. For the sake of simplicity the EET pathway is not shown for (1-H)- in Scheme 2 (right structure), although it might actually happen. Implementation of Ternary Logic with 1H+ using Degenerate Chemical Input Signals. The obtained results demonstrate clearly tristable fluorescence switching, which is characterized by low, medium, and high output signals in three welldifferentiated concentration ranges of F-. The assignment of input and output levels to 0, 1, and 2, respectively, enables the realization of ternary logic functions. Like for most cases of binary logic, at least two inputs are necessary for a fully assigned logic operation. Akin to the situation encountered in conventional computers, where inputs of the same kind (voltage signals of logic level 0 or 1) are applied, degenerate chemical inputs (identical by type and nature, but distinguishable to the operator) have been used in recent works for the implementation of binary molecular logic operations.13,34,62 For the present case degenerate chemical inputs can be defined as two physically separated sources of F- anions (source A and source B in Scheme 4). The logic value of each input can be low (0; pF ) -log[F-] ) 6.7), medium (1; pF ) 4.0) or high (2; pF ) 1.6). The fluoride concentrations [F-]A and [F-]B, each corresponding to one of the possible three logic values (0, 1, 2), are additive and result in a total anion concentration (pFtotal, see Table 1), which gives rise to one of the three states of the switch and the related fluorescence output (Scheme 2). The fluorescence intensities If corresponding to the output levels are assigned in accordance with the titration curve shown in Figure 3b: low (0, If e 35), medium (1, If ) 312), high (2, If ) 528). These definitions lead to the truth table shown below (Table 1). The logic function, which is connected to the truth table, is the ternary pendant of the well-known binary NOR gate.12 The NOR gate can be imagined as an OR gate with reversed output. As for the binary version, the highest output (output ) 2) is only obtained, when both inputs are low (input A ) input B ) 0). For all other input situations, which involve one or two high logic values (input A and/or B ) 2), a low output (output ) 0) results. In ternary logic, reversing medium signal outputs (signal value 1) does not change their logic value. Thus, for entries 2, 4, and 5 in Table 1 output ) 1 applies in accordance with NOR being equal to a concatenation of OR and NOT gates.

Conclusions It was demonstrated that the well-known and often for sensing purposes applied general fluorophore-spacer-amine architecture can be used beneficially for the molecular exploration of alternative approaches to information processing. Herein, special attention was devoted to the possibility of ternary logic, which in conventional computing has recently started to regain the interest of computer engineers. In detail, a bifluorophoric dyad was designed to yield a one-channel fluorescence output with three stable signal levels, as the result of an interplay between photoinduced electron transfer and electronic energy transfer processes, as well as the manipulation of internal charge transfer. The competition between these processes was fine-tuned by acid-base reactions using fluoride anions as inputs. The definition of degenerate inputs with three logic values (low, medium, and high) and the reading out of corresponding fluorescence signal levels allowed the interpretation of the underlying logic operation as ternary NOR gate. The presented results show that molecular events can be related to alternative concepts of logic, which go beyond common binary operations. Acknowledgment. Financial support by the Spanish Ministry of Science and Innovation, Madrid (Ramo´n y Cajal Grant RYC2005-000175 and Project CTQ2008-06777-C02-02 for U.P.) and the Portuguese Foundation for Science and Technology, Lisbon (postdoctoral fellowship SFRH/BPD/34384/2006 for R.F.) is acknowledged. The authors thank Marek Kluciar for assistance in the synthesis of 1 and Dr. Alexander A. Fedorov (Instituto Superior Te´cnico Lisbon, Portugal) for help with the lifetime measurements. Note Added after ASAP Publication. This article was published ASAP on March 11, 2009. Equations 3 and 4 have been modified. The correct version was published on March 13, 2009.

Figure 4. Spectral changes of the UV/vis absorption of 1H+ (9.7 µM in acetonitrile) upon addition of TBAF (up to 25.4 mM).

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