CONTENTS
Scheme 3.1: Emission changes through photo-induced electron transfer (PET) mechanism
Generally, π-stacked compounds cause fluorescence quenching but in AIE active fluorophores the fluorescence enhances under appropriate conditions with or without interactions with a substrate participating in aggregation. Molecules that show AIE emission are useful in various fields such as optoelectronics, sensors, probes.11
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Interplay of weak interactions and conformations of a fluorophore play important roles in aggregation induced emission.12 The mechanistic aspect of AIE are related to weak interactions and factors like conformational changes, geometrical changes, self-assembling, weak interactions, orbital symmetry.13 Fluorescence properties of naphthalene derivatives are utilized to recognize cis- or trans-isomers14 and biologically important metal ions.15 Fluorescence emissions of certain hydroxy-aromatic imines are influenced by supramolecular environments due to keto-imine inter-conversion.16 Quaternary ammonium salts influence fluorescence properties of hydroxy-aromatic compounds.17 These literatures suggest possibilities to suitably arrange molecules through hydrogen bonds to achieve aggregation induced emission from compounds having naphthalene unit tethered or directly connected to a unit for weak interactions such as hydrogen bonds.18 In this regard, hydroxy-aromatic oximes are suitable to form inter or intra molecular hydrogen bonds involving hydroxy- groups as well as to act as templates for π-interactions. Interplay of the weak interactions involving hydroxy-groups on hydroxy-aromatic aldoximes as seen in chapter 2 alters the hydrogen bonding patters of oxime counterparts. These occur upon change of position on a ring or increase in the number of hydroxy-groups attached to an aromatic ring. It is also an established fact that 2-hydroxynaphthalene derivatives19 are fluorescent due to excited state intra-molecular proton transfer process. In the case of 2-hydroxynaphthalene oxime this is due to the possibility of oxime-quinoid form shown in Fig. 3.1a.
(a) (b)
Figure 3.1: (a) Oxime-quinoid forms and (b) Two different arrangements of aromatic rings of 2-hydroxynaphthalene oxime (2.1.1), one is suitable for quenching of fluorescence and other is for AIE.
Due to these points, there are scopes to study the fluorescence of 2-hydroxynaphthalene derivatives through modulation of such non-covalent weak interactions. Reason to choose such molecules for structural and fluorescence emission study is explained with the two possible stacking arrangements among 2-hydroxynaphthalene oxime derivative (2.1.1) shown in Fig. 3.1b.
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The molecule 2-hydroxynaphthaldoxime shown in Fig. 3.1b has set of disc-like naphthalene molecules that may aggregate in parallel arrangements which are favourable arrangement to cause quenching of fluorescence as observed in aromatic fluorophores with π-stacking. This kind of quenching is known as Foster quenching process.20 But an arrangement involving C- H···π interactions as shown in Fig. 3.1b may be suitable for aggregation induced emission.21 It is also well known fact that non-covalently linked self-assemblies of fluorescent oxime molecules show interesting optical properties.22
In this chapter we have studied structural and emission properties of different hydroxy- aromatic oximes to understand their self-assemblies, molecular recognition and signal transduction properties.
3.2: Aggregation induced emission (AIE) of 2-hydroxynaphthaldoxime (2.1.1)
Oxime 2.1.1 shows solvatochromism behaviour in different solvent such as DMSO, DMF, DMA, THF, MeOH, acetonitrile and EtOH. But the fluorescence emission intensity of oxime 2.1.1 is different in each solvent. A strong emission was observed at 392 nm (Fig. 3.2a) upon excitation at 315 nm in DMSO solvent whereas in presence of other solvent it showed very weak emission at 392 nm. The emission at 392 nm in different solvents is due to excited state proton transfer. The exceptional high intensity of fluorescence emission observed from a solution in DMSO was due to higher basicity of DMSO helping proton transfer in excited state.23
(a) (b) (c)
Figure 3.2: (a) Emission spectra of oxime 2.1.1 (10-4 M) in solvents (inset is expanded spectra other than the one recorded in DMSO), (b) Emission spectra of oxime 2.1.1 (10-4 M) in solvents comprising of different proportions of DMSO and water (λex = 315 nm) and (c) fluorescence emission (under UV light) observed by naked eyes.
Oxime 2.1.1 in DMSO solvent shows a drastic change in fluorescence emission upon addition of water was due to its higher basicity induce proton transfer. A new emission peak
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at 446 nm was observed on addition of water to a solution of the oxime 2.1.1 in DMSO (Fig.
3.2b). The intensity of the new emission peak at 446 nm was increased with amounts of added water to the solution and it continuously increased till the volume of water was 80%
with respect to DMSO. The new emission peak appearing at higher wavelength is attributed to aggregation induced emission based on the literature on similar effect on emission spectra to show emission at higher wavelength on addition of water in related compounds.24 It may be noted that certain 2-naphthol tethered Schiff bases show fluorescence emissions due to excited state intramolecular proton transfer (ESIPT).25 In present case while growing the new AIE peak at 446 nm, it is also observed at intensity of emission peak at 392 nm due to ESIPT was enhanced upon adding water. Thus, the present example provided scope to study effect of water on ESIPT as well as on the generation of aggregation induced emission by adding water to a solution of it. Furthermore the crystal structure of the compound is known26 hence there is a scope to correlate packing pattern of the compound with fluorescence in solid state.
To a solution of oxime 2.1.1 in DMSO upon addition of water the AIE peak at 446 nm and ESIPT at 392 nm increase simultaneously. Upon excitation by UV-light the changes in emission intensities are noticeable naked eyes as shown in Fig. 3.2c. The oxime 2.1.1 is insoluble in pure water; hence it was not possible to record spectra of this compound in pure water. Similar trend in fluorescence spectral changes were observed in solutions of 2- hydroxynaphthaldoxime in other solvents such as methanol and tetrahydrofuran (THF), methanol upon addition of water. The fluorescence emissions in these two solvents at 392 nm were significant. There was also drastic increase in intensity of the new peak at 446 nm on addition of water in each case. Clear differences in enhancement of peak at 392 nm by water in different solvents were observed. This is attributed to the fact that, added water to other solvents enhanced ESIPT. Due to inherent basicity of DMSO, ESIPT occurred without water.
Upon addition of water ESIPT (ON state) is not significantly affected relative to the solutions of oxime 2.1.1 in other solvents. But in each case, aggregation induced emission occurred at 446 nm irrespective of the solvents under consideration.
Dynamic light scattering study of a solution of oxime 2.1.1 in DMSO and mixture of solvents DMSO-H2O (1:9, v/v) showed average particle size in each case to be different. In pure DMSO the average particle size was 561.2 nm with polydispersity 0.643; whereas average particle size was 623.5 nm with polydispersity 0.265 in 90 % water with 10 % DMSO solvent (Fig. 3.3a).
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(b)
Figure 3.3: (a) Dynamic light scattering (DLS) based particle size and (b) FESEM images (by drop caste method) of oxime 2.1.1 from (i) DMSO and (ii) DMSO-H2O (1:9, v/v) mixed solvent. Average particles size in Fig. b is (i) 26.2 nm and (ii) 163.2 nm.
These result showed that aggregate formed by oxime 2.1.1 in pure DMSO was different from mixed solvents of DMSO and water. In DMSO oxime 2.1.1 aggregated as relatively smaller average size particles than that in DMSO-H2O (1:9, v/v) mixed solvent. Similar result was found from Field Emission Scanning Electron Microscope (FESEM) studies, which are shown in Fig. 3.3b. The images were indicative of the changes in the sizes of the particles in DMSO and in mixed solvent of water and DMSO. 1H-NMR titration was performed by adding different amounts of D2O to a solution of oxime 2.1.1 in DMSO-d6 (Fig. 3.4). As the D2O concentration was increased in solution of the compound in DMSO-d6, the exchangeable OH proton appearing at chemical shift 11.53 ppm disappeared. There was no significant shift of the chemical shifts of the aromatic protons other than the proton labelled as b in Fig. 3.4.
Figure 3.4: 1H-NMR (600 MHz) spectra of oxime 2.1.1 in (i) DMSO-d6, (ii) 20 % D2O, (iii) 40% D2O and (iv) 50% D2O (aromatic regions are given for clarity).
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1H-NMR signal for hydrogen of N=CH of 2.1.1 appeared at 9.04 ppm, it was slightly shifted but the shift was not significant. Only drastic change in chemical shift position of the proton b was observed. The proton b originally appeared as a doublet of doublet 8.48 ppm which was shifted to 8.08 ppm (marked by arrows in Fig. 3.4).This shift is correlated to weak interactions involving concerned C-H bond between the oxime molecules caused by the changes in concentrations of D2O in the medium. The contribution of hydrogen bond with water to the weak interactions helped the molecules to organise such that self-assembly had C-H···π interactions (Fig. 3.1b). The 1H NMR spectra recorded in different solvents pointed out that the solvents not only influenced the chemical shifts of exchangeable OH protons, but also effected the CH=N proton. The trend in chemical shift (δCH=N) of this proton was acetonitrile-d3 > methanol-d4 > dimethylsulphoxide-d6. This has been attributed to the interactions of the solvents with labile OH changing the C=N character slightly. This could be a reason that the ESIPT is dependent on solvent as propensity to stabilise imine structure was guided by proton exchange process at the excited state. On the other hand, the aromatic C-H proton b appeared at 8.48, 8.26, 8.10 ppm in dimethylsulphoxide-d6, acetonitrile-d3,
methanol-d4 respectively. The rest of the peaks were not affected to show significant differences.
We have extended the study to examine the solid state self-assemblies of the compounds to correlate fluorescence properties of oximes 2.1.1 and 2.1.2 with anticipation that solid and solution property are cause difference in recognising of various guest molecules. We attempted crystallisation of these oxime derivatives in presence of different nitrogen containing molecules or nitrogen containing API molecules (Active Pharmaceutical Ingredient) such as 4,4'-bipyridine (44'-bipy), hexamethylenetetramine (HMTA), caffeine and theophylline.