Intramolecular Charge Transfer aromatic amines and their application towards molecular logic gate

Inorganica Chimica Acta 363 (2010) 2881–2885

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Intramolecular Charge Transfer aromatic amines and their application towards molecular logic gate Moorthy Suresh, Prasenjit Kar, Amitava Das * Central Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar 364002, Gujarat, India

a r t i c l e

i n f o

Article history: Received 21 January 2010 Received in revised form 12 March 2010 Accepted 15 March 2010 Available online 27 March 2010 Dedicated to Prof. Animesh Chakravorty

a b s t r a c t The emission response of 1-aminopyrene and 2-aminoanthracene was found to be switched ON or OFF by interrupting the Intramolecular Charge Transfer processes of these integrated systems using two binary ionic inputs like H+ or/and OH . These fluorescence responses in the presence of added H+ or/and OH could be correlated with Half-subtractor logic operation, revealing the possibility of using simple molecules for demonstrating complex logic operations. Ó 2010 Elsevier B.V. All rights reserved.

Keywords: Logic gate Half-subtractor Intramolecular Charge Transfer 1-Aminopyrene 2-Aminoanthracene INHIBIT gate

1. Introduction Implementation of computing at the molecular level is believed to be the alternative approach to silicon based processor technology as the nanoscaled dimension of the discrete molecules could lead to the realization of miniaturization of computers, which in turn requires low power consumption. To realize such a goal it was almost essential to search for molecules that were capable of performing Boolean operations [1,2] and this has emerged as an important area of present day research since the inception of the first molecular AND [3] logic gate, followed by other logic functions, such as XOR and INHIBIT [5,6]. This has helped in establishing different molecular devices that perform various basic Boolean logic operations such as AND [3], NOR [4], XOR [5], INH [6], XNOR [7], OR [8] and the integrated combinatorial circuits (Half-adder [9], Half-subtractor [10], Full-adder and Full-subtractor [11]). Integration of two simple Boolean logic functions into combinatorial circuits is an important step towards the complex logic information processing. We have recently reported combinatorial molecular Half-subtractor which is the combination of XOR and INHIBIT logic functions [12]. It would be highly desirable to have arithmetic operators which produce the same kind of output with sharp changes in the studied parameter. Fluorescence, being an essen* Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2567562. E-mail address: [email protected] (A. Das). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.03.037

tially zero-background technique, has clear advantages as output. The changes in the emission spectrum can also be followed at more than one wavelength, resulting in more than one logic function in a single molecular or supramolecular system. The photoinduced electron transfer (PET) and Intermolecular Charge Transfer (ICT) processes are particularly appropriate for this purpose, because the changes observed by the modulation of the frontier molecular orbitals with different inputs (analytes) usually yields clear and distinguishable emission signals. Among various operators, Halfsubtractor is a combinational circuit that subtracts two bits and produces their difference. This device requires two outputs: one generates the difference (D), while the other generates the borrow (B). These outputs are generated through XOR and INHIBIT gates, respectively. The development of molecular Half-subtractor is still at its early stage and the examples of a single molecule, which is capable of exhibiting operations for a Half-subtractor, are limited [13]. Herein we report the molecular Half-subtractor operation using two simple and commercially available aromatic amines, 1-aminopyrene (1-AP) and 2-aminoanthracene (2-AA) (Scheme 1), based on their fluorescence response with different ionic inputs (H+/ OH ). In this study, we could demonstrate the molecular logic gate operations like molecular Half-subtractor based on the changes in ICT or/and locally excited states (LE) using binary ionic inputs like H+ and OH . These ionic chemical inputs recognize their suitable sites in 1-AP and 2-AA and forms different emitting state which

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N H

N

H

H

1-AP

H

2-AA

Scheme 1. Chemical structures for compounds 1-AP and 2-AA.

emits their characteristic fluorescent signals. The emission signals at different wavelength can be used as outputs to their corresponding chemical inputs. Aromatic amines (1-AP and 2-AA) are typical examples of ICT character whose emission spectra can be altered by protonation and deprotonation by varying the pH. Both compounds exhibit an absorption band in the range 350–450 nm predominantly due to the Intramolecular Charge Transfer (ICT) transition with the homocyclic ring acts as an acceptor moiety and amino functionality with a lone-pair of electrons acts as the donor fragment. 2. Experimental section 2.1. Materials and methods 1-Aminopyrene (1-AP), 2-aminoanthracene (2-AA), trifluoroacetic acid and tetra butyl ammonium hydroxide (TBAH) were obtained from Sigma–Aldrich and were used as received. All other reagents used were procured from S.D. Fine chemicals India. All solvents were dried using standard procedure prior to use. Electronic spectra were recorded using a Shimadzu UV-3101 PC or a Cary 500 Scan UV–Vis–NIR spectrometer; while steady-state luminescence spectra and time resolved fluorescence decay experiments were carried out with a HORIBA JOBINYVON spectrophotometer. Time resolved fluorescence measurements were carried out using a diode laser based spectrofluorimeter from HORIBA JOBINYVON (USA). The instrument works on the principle of time-correlated single-photon counting (TCSPC). 340 nm LED was used as an excitation source for all the studies. 3. Results and discussion Spectral response of two amino derivatives (1-AP and 2-AA) studied, were found to be very sensitive to [H+] and [OH ] present in the medium [15]. The electronic absorption spectra of 1-AP 2-AA, 1-APH+, 2-AAH+ (respective protonated forms), 1-AP H+ and 2-AA H+ (respective deprotonated forms) are shown in Fig. 1. The amino group has a lone electron pair residing in an orbital on the nitrogen atom, with sp3 hybrid characteristics and thus, this

orbital is not expected to be perpendicular to the anthracene ring. However, it has a substantial component in the perpendicular direction and one would expect some interaction between the lone-pair and the p-system even in the ground electronic state (conjugation or resonance interaction). Upon excitation, however, an electron may be transferred completely from the nitrogen lone-pair into a vacant p*-orbital of the aromatic ring (Intramolecular Charge Transfer transition). This process results in greater conjugation of the amino group with the aromatic ring and produces rehybridization of the amino nitrogen atom to an sp2-type configuration (coplanar with the ring) during the electronic transition [14,15]. Fig. 1 reveals that the absorption spectra of 1-AP and 2-AA with a broad absorption band maximum at 401 nm (log e = 4.02) and 406 nm (log e = 3.58), respectively due the ICT transition. Apart from this ICT band, other intraligand p–p* transition bands with some anticipated red shift are present. The fluorescence spectra of 1-AP and 2-AA are unstructured with the emission maximum at 434 nm (kext = 362 nm) and 482 nm (kext = 335 nm), respectively. Intensity of these emission bands are much larger and broader than that for the solution of pyrene and anthracene of comparable absorbance due to the mixed n–p* and p–p* transitions (Fig. 2). Upon excitation of 1-AP and 2-AA, complete electron transfer takes place from the nitrogen lone-pair into a vacant p*-orbital of the aromatic rings with associated change in the rehybridization of the amino nitrogen into sp2 configuration and thus, the coplanarization of orbitals leads to the greater conjugation during the electronic transition [14,15]. When 1-AP and 2-AA fluoresces from its lowest singlet excited state, charge is transferred back from anthracene to amino group which leads to the rehybridisation of sp2 configuration into sp3 and thus diffusive long wavelength emission is observed at 434 and 482 nm, respectively (Fig. 2). 1-AP and 2-AA exist predominantly (>99%) in their respective protonated form 1-APH+ and 2-AAH+ on maintaining the certain acid concentration of the acetonitrile solution to 1.0  10 4 M (for 1-AP and 2-AA). Electronic spectra for 1-APH+ and 2-AAH+ are shown in Fig. 1. Protonation interrupts the charge transfer process between the amino group and the aromatic rings and this caused an effective bleaching of the lower energy ICT band and new higher energy structured absorption band with kmax at 309 (log e = 4.06), 322 (log e = 4.38) and 338 nm (log e = 4.52) for 1APH+ (pKa = 4.8 ± 0.1) [16a] and kmax at 339 (log e = 3.89), 357 (log e = 4.05), and 376 nm (log e = 3.96) for 2-AAH+ (pKa = 4 ± 0.04), [14] which resemble closely with the absorption spectra of pyrene and anthracene, respectively (Fig. 1). Absorption of spectra for 1-AP H+ and 2-AA H+ at [TBAH] = 0.01 M are similar to that for 1-AP and 2-AA in acetonitrile solution (Fig. 1) with associated ICT band at lower energy region. Fluorescence spectra for 1-AP and 2-AA, their respective protonated and deprotonated forms are shown in Fig. 2. Emission spectra for deprotonated 1-AP forms a new charge transfer

0.15 (c)

0.4 (b)

0.2

Absorbance

Absorbance

(c)

(b)

0.10 0.05

(a)

(a)

0.0 300

0.00 350 400 Wavelenght (nm)

Fig. 1. Electronic absorption spectra of (i) (a) 1-AP (2.0  10

5

M) and (ii) (a) 2-AA (2.0  10

350 400 450 Wavelength (nm) 5

M) in (b) TBAH, in (c) trifluoroacetic acid in acetonitrile solution at 27 °C.

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XOR (a, d)

Emission Intensity

Emission Intensity

High (0)

600 400 Low (1) (c)

200

(b)

INHIBIT High (1)

INHBIT High (1)

400 (a)

XOR High (0) (c, d)

200

Low (0)

(b) Low (1)

Low (0)

0

375

450 525 600 Wavelength (nm)

450 525 Wavelength (nm)

Fig. 2. Fluorescence spectra of (i) (a) 1-AP (1.0  10 7 M) and (ii) (a) 2-AA (4.0  10 7 M) in (b) 0.01 M TBAH in (c) 1.0  10 of TBAH and trifluoroacetic acid following excitation at 362 nm (i) and 335 nm (ii) at 27 °C.

Table 1 Life time data for 1-AP and 2-AA. 2-AA s (ns)

4.05 ± 0.05 (v2 = 1.065) kmon = 434 nm 4.32 ± 0.06 (67.39%) 12.80 ± 0.20 (32.61%) (v2 = 1.20), kmon = 434 nm 5.33 ± 0.04 (v2 = 1.01) kmon = 526 nm

9.34 ± 0.11 (v2 = 1.10) kmon = 434 nm 3.44 ± 0.02 (v2 = 0.85) kmon = 407 nm 8.35 ± 0.07 (v2 = 1.01) kmon = 480 nm

Input

Output (emission) +

OH

H

2-Aminoanthracene Borrow

0 0 1 1

1000

0 1 0 1

kemi = 407 nm

Difference (positive logic) kemi = 482 nm

Difference (negative logic) kemi = 482 nm

0 1 0 0

1 0 0 1

0 1 1 0

(low, 140.98) (high, 443.87) (low, 80.06) (low, 140.06)

(high, 329.23) (low, 88.95) (low, 82.18) (high, 329.23)

1000 +

1AP + H

100 1-AP

10 40

60 80 Time (ns)

-

1-AP + OH

100

Log (Counts)

TBAH

Table 2 Truth table for 2-AA.

1-AP s (ns)

Log (Counts)

HCl

M trifluoroacetic acid (d) in equimolar amount

of the protonated amine form (1-APH+) in acetonitrile medium saturated with dissolved oxygen could be responsible for a much lower lifetime component. No such excited state proton transfer process is known for 2AAH+. Protonation of the amino functionality in 2-AA is expected to inhibit the ICT process; while for 2-AA H+ favored ICT process quenches the emission of 2-AA due to increased charge density. A critical assessment of the fluorescence spectra of 2-AA, 2-AAH+ and 2-AA H+ suggests that the fluorescent output signals can be matched to XOR and INHIBIT Boolean logic operation to construct an integrated logic operator, Half-subtractor. In this electronic circuit, input signals are processed simultaneously by two parallel operating logic gates, XOR and INHIBIT, producing the difference (D) and borrow (B) bits, respectively. The truth table for unimolecular Half-subtractor has been matched (Table 2) to 2-AA by analyzing the fluorescence spectra of 2-AA at the wavelengths 407 and 482 nm (Figs. 2 and 5). The emission output at 407 nm has been monitored to correlate with INHIBIT function and emission at 482 nm correlated to XOR logic function. The disappearance ICT emission band at 482 and the appearance of emission at 407 nm from locally excited state (LE state)

emission band at longer wavelength with [TBAH] = 0.01 M due to the enhanced ICT process that may exist for the deprotonated form. Emission spectra for 1-AP and and 1-APH+ are similar. It was observed from previous report [15,16] that the excitation spectra of 1-AP resembled to that of pyrene spectra only in the pH range of 2.0–3.5 and this confirms that the 1-APH+ can only be observed from the excitation spectra at pH < 3.5. But, when pH > 3.5, 1APH+ dissociates rapidly into 1-AP and H+. Rate constant for a rapid excited state proton transfer process for [1-APH+]* is reported earlier as 1.8 ns in CH3CN–H2O (1:1 (v/v) mixture and generates 1-AP* in the medium. Thus, this rapid excited state proton transfer process for [1-APH+]* generates 1-AP* and could actually compete with the radiative decay process of the 1-APH+*. Emission decay profile for [1-APH+]* that could be best fitted to a biexponential time constants of 4.32 and 12.80 ns (Table 1, Fig. 3). Pyrene is known to have a high singlet excited state life time (400 ns in thoroughly degassed organic solvent), which decrease appreciably on derivatization by electron withdrawing substituents like fatty acids. Absorption spectra (Fig. 1) reveals that spectra for 1-APH+ resembles closely with that of pure pyrene and effective solvation

Neutral

4

2-AA

100 10

-

2AA + OH +

2AA + H

40

60

Time (ns)

Fig. 3. Single photon counting studies of (a) 1-AP and (b) 2-AA using excitation source of 340 nm LED in acetonitrile solution.

(low, 329.23) (high, 88.95) (high, 82.18) (low, 329.23)

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XOR (0)

INH (0)

600

600 XOR (1)

400

400

Input (0,0)

(a)

Emission Intensity

200 0

400

450

500

550

INH (0) Input (0,1)

(b)

200

60

0

0

400

450

500

550

600

XOR (0)

600

600 400

INH (0)

INH (1)

XOR (1)

400

Input (1,0)

(c)

(d)

200

200 0

Input (1,1)

400

450

500

550

0

600

400

450

500

550

600

Wavelength (nm) Fig. 4. Emission spectra of 1-AP (1.0  10

7

M), with different input sequences. kext = 362 nm.

Table 3 Truth table for 1-AP. Input H+

Output (emission) OH

1-Aminopyrene Borrow

0 1 0 1

kemi = 526 nm

Difference (negative logic) kemi = 434 nm

0 1 0 0

1 0 0 1

0 1 1 0

(low, 8.12) (high, 160.37) (low, 8.12) (low, 8.12)

400

(high, 668.98) (low, 213.81 (low, 265.23) (high, 668.98)

(high, 668.98) (low, 213.81) (low, 265.23) (high, 668.98)

XOR (0)

Input (0,0)

400

INH (1) Input (0,1)

XOR (1)

INH (0)

(b)

(a) 200 Emission Intensity

0 0 1 1

Difference (positive logic) kemi = 434 nm

correlated to INHIBIT logic function. Fluorescence response of 2-AA in the absence of both inputs (In 1 (OH )TBAH = 0 and In 2 (H+) = 0), the presence of (OH )TBAH (In 2 (H+) = 0 and In 1 (OH )T+ BAH = 1) or the simultaneous presence of both inputs (In 2 (H ) = 1 and In 1 (OH )TBAH = 1) showed emission mainly from ICT state at 482 nm (Table 2, Figs. 2 and 5). However, in the presence of H+ ([H+] = 1.0  10 4 M) as only ionic input (In 1 (OH )TBAH = 0, In 2 (H+) = 1) and appreciably enhanced emission intensity due to the locally excited state was observed at 407 nm. Thus, the truth table (Table 2) constructed based on these responses could be correlated to INHIBIT logic function.

0

200

400

450

500

Input (1,0)

400 INH (0)

450

500

450 Input (1,1)

XOR (1)

(c)

400

400

400

200

0

0

550

500

XOR (0)

INH (0)

(d)

200

0

550

400

450

500

Wavelength (nm) Fig. 5. Emission spectra of 2-AA (4.0  10

7

550

M) with different input sequences.

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The emission output signal of 2-AA at 482 nm resulted to an XNOR function as it showed off state (0), in the presence of either OH TBAH (In 1) or H+ (In 2). But, on state was observed when either, none (In 1 = 0 and In 2 = 0) or both chemical inputs (In 1 = 1 and In 2 = 1) were present (Table 2). Due to the complementary nature of XOR and XNOR logic operation [17], the XNOR logic operation at 482 nm can be converted into XOR logic gate by applying negative logic function at 482 nm. Thus, one could demonstrate that by using a single molecule (2-AA) it is possible to integrate two Boolean logic functions, XOR and INHIBIT, and thereby the Half-subtractor operation. Similar assessment of the fluorescence response for 1-AP, at 434 and 526 nm with different combinations of two ionic inputs like H+ (In = 1) and OH TBAH (In = 2) could also be correlated to that of Halfsubtractor operator (Table 3). On adding In 2 ([TBAH] = 0.01 M), a new long wavelength emission for 1-AP H+ at 526 nm was observed (Figs. 2 and 4); while for In 1 a partially quenched emission band was observed (Figs. 2 and 4). XOR and INHIBIT logic operations were examined at 434 and 526 nm, respectively. Fluorescence monitored at 526 nm matched with the truth table for INHIBIT function as it generates on output signal only when In 1 (H+) = 0 and In 2 (OH TBAH) = 1 and the remaining inputs sequences, In 1 = 0 and In 2 = 0, In 1 = 1 and In 2 = 1, In 1 = 1 and In 2 = 0, generate off output signals (Table 3). The truth table for XNOR function was derived when the fluorescence changes were monitored at 434 nm with two ionic inputs H+ (In 1) and OH (In 2). XNOR and XOR logic operators are complementary to each other and thus an output signal at 434 nm was transformed into XOR logic function by applying negative logic function at 434 nm. Thus, the ability of the molecule 2-AA to integrate the INHIBIT and XNOR operations could be used for demonstrating the Half-subtractor logic operation. 4. Conclusion It is established that the simple amino derivatives, e.g., 1-aminopyrene and 2-aminoanthracene, could be used for demonstrating the complex Boolean operation like arithmetic Halfsubtractor. The exact choice of chemical inputs and an appropriate wavelength selection make a molecule to perform complex arithmetic Half-subtractor (0–0, 1–0, 0–1 and 1–1) calculation. Acknowledgments CSIR and DST, Government of India, supported this work. M.S. and P.K. thank CSIR for Senior Research Fellowship and AD thanks Dr. P.K. Ghosh (CSMCRI, Bhavnagar) for his keen interest in this work.

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