New regioisomeric naphthol–thiazole based ‘turn-on’ fluorescent chemosensor for Al³⁺

Abstract

Two new reactive and highly selective turn-on fluorescent chemosensors based on the position of ring annulation of the naphthalene–thiazole moiety for aluminum ions in ethanol, were synthesized and investigated. It was found that sensors 2 and 4 exhibited a remarkable enhancement of emission upon complexation with Al³⁺. A TD-B3LYP/6-31G(d,p) calculation was performed to characterize the nature of the fluorescence behavior of sensors 2 and 4 upon Al³⁺ complexation. The mechanism of fluorescence was based on the cation promoted hydrolysis of ester and subsequent complexation. The combination of experimental and computational analyses provides a more complete understanding of the molecular level origin of these types of unique photophysical properties.

Introduction

Aluminum is the third most prevalent element and the most abundant metal in the earth's crust, representing approximately 8.3% of total mineral components.¹ Due to its reactivity, it is widely dispersed in the environment around us in modern society: in water treatment, in food additives, aluminum-based pharmaceuticals, occupational dusts, and of course as the production of light alloy used in container and cooking utensils.2, 3 Dietary aluminum is ubiquitous but aluminum accumulation in tissues and organs results in their dysfunction and toxicity.4, 5 Al³⁺ toxicological effects are different from those of other heavy metals it usually interfere with the functioning of iron sulfur proteins that causes bone and joint diseases by defective mineralization and osteomalacia,⁶ neuronal disorder leading to dialysis encephalopathy, dementia, myopathy, idiopathic Parkinson's disease, impairment of memory, and Alzheimer's disease.7, 8, 9, 10 Its accumulation even causes cancer of lung, breast, and bladder11, 12 Moreover aluminum may cause anemia through decreased heme synthesis, decreased globulin synthesis, and increased hemolysis. Aluminum may also have a direct effect on iron metabolism: it influences absorption of iron via the intestine, it hinders iron's transport in the serum, and it displaces iron's binding to transferrin.¹³ Almost 40% of the world's acid soils is due to the effects of aluminum toxicity, which is the key factor hampering plant (i.e., crop) performance on the acid soils.1, 14 Therefore, the development of fluorescent sensors for aluminum is critical in biological and environmental monitoring.

However, the detection of Al³⁺ has always been problematic due to the lack of spectroscopic characteristics, poor coordination ability comparing to transition metals,¹⁵ and the pH-dependence in aqueous solution.¹⁶ Due to the high sensitivity, selectivity, rapid response time, and versatility fluorescence method is likely to be the most effective way to detect metal Al³⁺.¹⁷ Several fluorescent probes such as Schiff bases,18, 19, 20, 21, 22 hydrazones,²³ pyrollidine,24, 25 coumarin,²⁶ pyrimidine,²⁷ calixarene,28, 29 hydroxyflavone,30, 31 8-hydroxyquinoline,³² oxazoline and imidazoline,³³ boron dipyrromethene,³⁴ rhodamine,35, 36 and gold nano-particle derivatives37, 38 have been synthesized and used for this purpose. Most of the fluorescent probes reported for Al³⁺ till now have a few serious lacunae like interferences by Fe³⁺ and Cu²⁺ most commonly.

Chelation-enhanced fluorescence (CHEF) is strongly influenced by the nature of the substituents and their attachment positions on the fluorophore π-system, resulting in an absorption and emission peak shift.39, 40, 41, 42, 43 In principle, the more p-orbitals participating in a π-conjugated system, the more discrete π-orbitals are involved in bonding.⁴⁴ As a consequence, the HOMO–LUMO energy gap generally decreases with increasing size of a conjugated π-system, an effect that has been utilized on structurally related 2-(2′-hydroxyaryl)benzazole systems.45, 46, 47

Previously, turn-on Al³⁺ chemosensor based on photoinduced electron transfer using both sulfur and nitrogen as electron donors and the use of thiazolothiazole as an Al³⁺ and Cr³⁺sensor has been reported.48, 48(a), 48(b) We already have reported the synthesis of thiazole derivatives with different substituents at the 4-position as Zn²⁺, Cu²⁺, and Al³⁺ sensors.49, 50, 51, 52, 53 Naphthol group can act as an ideal signaling moiety because of its short fluorescence lifetime, low fluorescence quantum yield, good binding sites, and cheap cost. It in combination with an imine or a heterocyclic ring can as a very good Al³⁺ sensor.54, 55, 56, 57 We have also investigated the effect of the position of ring annulations in naphthol–thiazole based chemosensors, which produce an emission enhancement upon complexation with Zn²⁺, with an ‘on–off’ type fluoroionophoric switching property.⁵⁸ Schiff base fluorescent sensors are versatile organic blockers possessing comparable accessibilities and structure variations. Thus taking into account the Schiff base, electron donating property of the nitrogen in the thiazole ring, functional carboxylic group³⁷ affinity to Al³⁺, and ring annulation effects on the HOMO–LUMO energy difference we prepared three naphthol–thiazole based regioisomers 2, 3, and 4 (Fig. 1) having an extended π-conjugated nitrogen–oxygen-rich coordination environment, which provide a hard–base environment for the hard–acid Al³⁺.24, 25