Specific site binding of metal ions in presence of anionic micelle and construction of molecular logic gates

Azole Derivatives

6.1. Specific site binding of metal ions in presence of anionic micelle and construction of molecular logic gates

DMAPOP possesses three different metal ion binding sites. It was reported that Cu²⁺and Cd²⁺ form two different types of complexes with DMAPOP. Cu²⁺ binding sites are

Chart 6.0. Molecules used to construct logic gates (A) DMAPOP, (B) BHPBI and (C) DMAPIP-b.

pyridyl nitrogen and dimethylamino nitrogen, and Cd²⁺ binding sites are azole nitrogen and dimethylamino nitrogen (Chart 6.1.1).¹⁴⁶ In the present section, a surfactant is effectively used to control the metal ion binding site on DMAPOP. The interactions of Cu²⁺, Cd²⁺ in presence and in absence of micelle are utilized to develop different logic functions.

6.1.1. Metal ion binding in micellar medium

In presence of SDS (surfactant) the absorption spectrum of DMAPOP is red shifted and the fluorescence spectrum is blue shifted compared to those in aqueous medium (Figure 6.1.1). The results are consistent with the encapsulation of DMAPOP inside the micelle.

Figure 6.1.2 depicts the absorption spectra of DMAPOP in 8.2 mM of SDS at different concentrations of Cu²⁺ and Cd²⁺ metal ions. The pH of the solutions were kept at 6.1 ± 0.1. The addition of metal ion produce a little red shift in the absorption spectrum of DMAPOP. The fluorescence spectra of DMAPOP with increasing metal concentration is shown in Figure 6.1.3. In presence of Cu²⁺ the emission intensity gradually decreases.

It indicates that the excited Cu²⁺ complex of DMAPOP follows a non-radiative decay pathway. The binding constant of the molecule with the metal ion was obtained from the quenching equation (eq. 6.1.1.).¹⁴⁷

I0/I = 1 + K[Q] (eq. 6.1.1.)

where I and I0 are the fluorescence intensities in presence and absence of quencher, respectively. K is the binding constant and [Q] is the concentration of quencher. On the other hand, with gradual increase of Cd²⁺ a new red shifted band emerges at 463 nm (Figure 6.1.3.B).The binding constant of Cd²⁺ with DMAPOP was measured using Benesi-Hildebrand equation (eq. 6.1.2).¹⁴⁸

1/ (I - I⁰) = 1/(I^∞ - I⁰) + 1/((K[M])(I^∞ - I⁰)) (eq. 6.1.2.)

where I⁰, I^∞, I are the intensities of DMAPOP in micelle at zero, maximum and any

Chart 6.1.1. Different metal complexes of DMAPOP in presence of A) Cu²⁺, B) Cd²⁺ in acetonitrile.

Molecular Logic Gate

0.0E+0 2.0E+2 4.0E+2 6.0E+2

390 440 490 540

Intensity (a.u.)

Wavelength (nm) [SDS]

(mM) 0 32 (B)

0 0.04 0.08 0.12 0.16 0.2

310 360 410

Absorbance

Wavelength (nm)

[SDS]

(mM) 0 (A) 12

0 0.06 0.12 0.18 0.24 0.3

320 370 420

Absorbance

Wavelength (nm) 0 2.5

[Cu²⁺] (mM) (A)

0 0.06 0.12 0.18

300 350 400 450

Absorbance

Wavelength (nm) 0 1.15

[Cd²⁺] (mM) (B)

0 200 400 600 800

390 490 590

Intensity (a.u.)

Wavelength (nm) 0

2.52 Cd²⁺

(mM)

0.0 2.0 4.0 6.0

2.5 12.5 22.5 32.5 Binding constant x 10-3 (M-1)

[SDS] mM

(B)

0 100 200 300 400 500

380 430 480 530

Intensity (a.u.)

Wavelength (nm) 0

2.51 [Cu²⁺] (mM) (A)

Figure 6.1.1. (A) Absorption spectra, (B) emission spectra of DMAPOP with increasing SDS concentration in aqueous medium.

Figure 6.1.2. Absorption spectra of DMAPOP in 8.2 mM of SDS solution with gradual increase of (A) [Cu²⁺] and (B) [Cd²⁺].

Figure 6.1.3. Emission of DMAPOP in 8.2 mM of SDS solution with gradual increase of (A) [Cu²⁺], (B) [Cd²⁺], _𝒆𝒙 = 378 nm. In both cases the inset shows the binding constant of DMAPOP with the metal ion at different SDS concentration.

intermediate concentration of Cd²⁺, respectively. [M] and K are metal ion concentration and the binding constant, respectively. The binding constants for both the metal ion are maximum in 8.2 mM of SDS concentration, which is close to the critical micellar concentration of SDS (Figure 6.1.3. insets). In 8.2 mM of SDS solution the K obtained for Cu²⁺and Cd²⁺ were 9x10³ and 6.5x10³ M^-1, respectively.

The molecule has three basic centers pyridyl nitrogen, oxazole nitrogen and dimethylamino nitrogen. The binding of cations through the dimethylamino group results in a blue shift, and those through the ring nitrogen produces a red shift (Table 6.1.1). It is also expected that higher conjugation would provide more bathochromic shift in the absorption spectrum upon binding at the pyridyl nitrogen than at the azole nitrogen.¹⁴⁹ In acetonitrile, the metal complex through the pyridyl nitrogen produces 66 nm bathochromic shift.^{146, 150} Whereas the metal complex using the oxazole nitrogen exhibits less (only 16 nm) bathochromic shift in the absorption spectrum (Table 6.1.1.).¹⁴⁶ In micellar medium, the bathochromic shift is just about 12 nm for both Cd²⁺ and Cu²⁺

complexes. Again, neither Cd²⁺ nor Cu²⁺ generates any hypsochromic shift in the absorption spectra of DMAPOP in presence of SDS. The results not only indicate that both the metal ions bind only at one basic center but also suggests that not only Cd²⁺ but also Cu²⁺ bind through the oxazole nitrogen to form the complex with DMAPOP in micellar medium.

Table 6.1.1.Absorption band maxima (_𝑚𝑎𝑥^𝑎𝑏𝑠 , nm) and fluorescence band maxima (_𝑚𝑎𝑥^𝑒𝑚 , nm) in different media.

Medium In absence of metal ion In Presence of Cu²⁺ In Presence of Cd²⁺

_𝑚𝑎𝑥^𝑎𝑏𝑠 𝑚𝑎𝑥𝑒𝑚 _𝑚𝑎𝑥^𝑎𝑏𝑠 𝑚𝑎𝑥𝑒𝑚 _𝑚𝑎𝑥^𝑎𝑏𝑠 𝑚𝑎𝑥𝑒𝑚

Acetonitrile^a 359 418 310, 425 380 300, 375 370, 470

Water 364 447 364 447 364 447

SDS 372 438 384 385 463

a reference 146.

6.1.2. Mode of encapsulation and metal ion binding at selective site

Scheme 6.1.1. Encapsulation of DMAPOP in micelle and controlled metal ion binding.

Molecular Logic Gate

The molecule can go inside the micelle either partially or completely. The fluorescence decay experiment suggests the partial encapsulation of the fluorophore. In acetonitrile medium the fluorescence lifetime of the molecule is ∼1.56 ns.⁸¹ DMAPOP has shorter lifetime (1.12 ns) in micellar medium compared to acetonitrile, this may be due to the quenching of the fluorophore by water. To substantiate further, that the decrease in the lifetime is due to the quenching by water, the lifetime of DMAPOP in water was measured. The lifetime thus obtained, 0.43 ns, is shorter than that in micelle. Thus, the results suggest that DMAPOP is not completely buried inside the micelle but partially exposed to water. The other important concern is the orientation of the molecule inside the micelle and site of binding. The molecule can enter inside the micelle either with its dimethylamino group or its pyridyl group. The encapsulation of the dimethylamino group leads to the removal of the solvation shell around the dimethylamino group.

Therefore, the lone pair of the dimethylamino nitrogen does not take part in hydrogen bonding anymore. Instead, it delocalizes over the entire molecule. This leads to the red shift in the absorption spectra. The bathochromic shift in the absorption spectrum of DMAPOP in micelle indicates that the molecule enters inside the micelles through the dimethylamino group (Scheme 6.1.1.). Similar red shifts were also observed for analogues of DMAPOP when dimethylamino group enters inside the micelle.^{151, 152} Since, the dimethylamino group is present inside the micelle, despite the fact both Cu²⁺

and Cd²⁺ bind at dimethylamino nitrogen of DMAPOP in acetonitrile, the micelle effectively prevents the binding at dimethylamino nitrogen. The controlled binding of Cu²⁺ and Cd²⁺ suggests that the oxazole nitrogen is present near the anionic head of the surfactant. Consequently, not only Cd²⁺ but also Cu²⁺ binds with DMAPOP through the oxazole nitrogen. Since, the strong electrostatic interaction between the anionic head of the micelle and the metal ions increases the metal ion concentration near the azole nitrogen, both the metal ions bind at azole nitrogen (Scheme 6.1.1). At higher micellar concentration, due to the deformation of spherical micellar shape, the fluorophore goes inside the micellar core.¹⁵³ But the metal ions reside at the stern layer of the micelle. This restricts the metal ions to interact with the probe molecule, and the binding efficiency gradually decreases at higher micellar concentration.¹⁵⁴

6.1.3. Construction of molecular logic gates

In presence of micelle, the sensitivity of the molecule towards the metal ion in aqueous medium enhances. Even in presence of micelle, the non-radiative decay rate of DMAPOP decreases significantly, which results in enhanced emission of DMAPOP in

Cu²⁺ Micelle Output

0 0 0

0 1 1

1 0 0

1 1 0

Output = 438 nm emission

Molecular INH gate

Water SDS

Cu²⁺

SDS+Cu²⁺

0 0.2 0.4 0.6 0.8 1 1.2

Relative Intensity

Cd²⁺ Micelle Output

0 0 0

0 1 0

1 0 0

1 1 1

Output = 465 nm emission

Molecular AND gate

Water SDS

Cd²⁺

SDS+Cd²⁺

0 0.2 0.4 0.6 0.8 1 1.2

Relative Intensity

Cd²⁺ Micelle Output

0 0 0

0 1 1

1 0 0

1 1 1

Output = 438 nm emission