Donor-π-acceptor based push-pull organic chromophores: chemosensing applications through modulation in intramolecular charge transfer

Abstract

Sensing of various targeted analytes including cations, anions and neutral molecules both qualitatively and quantitatively has been receiving growing attention amongst scientist working in diverse areas viz. environmental science, chemical and biochemical science, agricultural science, food processing, medicinal chemistry, pharmacy and health sciences. Several small cations and anions are functionally connected to a wide range of bio-chemical processes within organisms and in the external environment. Further, some small ions are also highly toxic to organisms bringing biological malfunction when present above their permissible limit. Consequently, early detection of these analytes is advantageous and of great concern to the scientific community.
Compared to classical methods for ion detection, chemosensors and especially optical chemosensors (including fluorescent and colourimetric chemosensors) demonstrate many advantages, such as easy and visual detection, high sensitivity, low background interference, and convenient applications in bio-imaging. Investigation of naked-eye colour change and change in UV-Vis absorption and fluorescence spectral behavior of chromogenic and fluorogenic receptors are the most versatile and simple means of analyzing molecular recognition. Among several chromogenic and fluorogenic chemosensors, compounds with Donor--Acceptor (D-π-A) push-pull molecular systems with inherent intramolecular charge transfer (ICT) character are gaining wide interest. The combination of a dipolar D--A signaling moiety with a polar ion recognition unit tends to increase the aqueous solubility of an organic chromophore, which is an essential property of a chemosensors for environmentally and biologically important ions. Presence of ICT may also possibly make them chromogenic and fluorogenic dual responsive chemosensors and provide diverse optical signals favorable for simultaneous detection of multiple analytes.
Although a large number of studies on optical chemosensors are found in the literature, the efficacy of most of these sensors is inadequate because of factors such as complex synthetic procedures, strong interferences, feeble sensitivity and low aqueous solubility. Further challenges for chemosensors are to make the transition from research tools into practically useful portable systems for on-site real time qualitative and quantitative detection of required analyte. It is difficult to make predictions regarding selective interaction between the species of interest (analyte) and receptors in a particular medium when several interfering species are present. Multifaceted behaviour is often observed as a function of receptor structures, mode of binding, mechanism of optical response, medium composition, presence of interfering ions (if any). Therefore, comprehensive investigations are usually required to fully understand the sensing mechanism, to test the feasibility of a particular chemosensor for a specific analyte, and to utilize their potentials to fabricate efficient dual-responsive chromo-fluorogenic chemosensors for specific applications.
Owing to the importance of D-π-A push-pull molecular systems to act both as chromogenic and fluorogenic dual-responsive chemosensors, and the fact that structurally different probes can unveil different aspects of receptor-analyte interactions in aqueous and semi-aqueous solvent systems, the present dissertation entitled “Donor--Acceptor Based Push-Pull Organic Chromophores: Chemosensing Applications through Modulation in Intramolecular Charge Transfer” aims at employing three different simple D-π-A push-pull dipolar chromophores, namely: (E)-4-(4-N,N-bis-(2-hydroxyethylthioethylamino)styryl)-1-methylpyridinium iodide (L1), 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde (L2), and N-((4-N,N-diethylamino)-2-hydroxybenzylidene) isonicotinohydrazide (L3) containing cyanine dye, azo dye and hydrazide Schiff base respectively as chromophores. These chromophores are proposed to transduce the event of molecular recognition of metal ions and anions to an readable optical signal. In this thesis attempt has been made to propose possible binding mode and mechanism of signal transduction based on experimental studies as well as theoretical calculations.
The designed D-π-A chromophores are synthesized in good yields and purified by simple procedures. These chromophores are found to be either soluble in pure aqueous or water-rich aqueous-organic binary solvent mixtures and are appropriate for sensing within environmental and biological samples. These chromophores exhibit very intense ICT absorption bands in the visible region. Further, these molecules are found to be weakly fluorescent, assigned to photo electron transfer (PET) due to the presence of non-bonding electrons. However this property became an advantage, as it facilitated a turn-on fluorescence response upon binding to the metal centre; the mechanism of which is believed to be chelation enhanced fluorescence (CHEF) through inhibition of PET. Selective ion recognition by these chemosensors is monitored through investigation of naked-eye colour change and a change in UV-Vis absorption and fluorescence spectral behaviour. Styrylpyridinium (cyanine) dye based D-π-A chromophore (L1), which is attached to a NS2O2 binding unit in its donor site is found to exhibit Hg2+ selective turn-off colourimetric response from orange to colourless. This is accompanied by a turn-on fluorescence response along with a blue shift of emission peak ascribed to the arrest of ICT and PET, through the formation of a conformationally rigid stable 1:1 L1+Hg2+ adduct. Similar Hg2+ selective turn-off colourimetric sensing behaviour is also observed for azo dye based chemosensor (L2) ascribed to the arrest of ICT through the formation of a 1:1 L2-Hg2+ adduct. As expected from the structure of the hydrazide-Schiff base based chemosensor (L3), it demonstrated both cation and anion selective chemosensing behaviour. Out of several metal ions and anions tested, L3 showed a selective response to Al3+ and AsO2-. The obvious colour change of L3 in the presence of these analytes is primarily due to the modulation of ICT upon ion complexation. Conversely, a combined effect of PET, ICT, and excited state intramolecular proton transfer (ESIPT) modulation is proposed to be responsible for prominent fluorescence enhancement.
Reversible selective molecular recognition events with turn-on or turn-off optical signal are used to construct different combinational molecular logic gates. The turn-on fluorescence responses of L1 and L3 in association with their appreciable cell membrane permeability are utilized for the intracellular detection of ions in living cells i.e. Hg2+ by using L1 and Al3+ and AsO2- by using L3. All the three chemosensors are successfully utilized to prepare disposable test paper strips, which can track the presence of targeted metal ions in aqueous media by simple naked-eye detection. This feature makes these test strips practical for the real time on-site detection of targeted analytes.
Therefore, the D--A based organic chromophores with simple chemical architectures can be promising candidates for chemosensing applications. It is worth mentioning that chromogenic and fluorogenic dual responsive chemosensing in combination with multiple interplaying sensing mechanisms can provide diverse optical signals, favourable for the simultaneous detection of multiple analytes. A good understanding of the factors affecting the sensing mechanism, mode of binding in different type of receptors, and the mechanism of signal transduction can lead to the design of novel probe molecules. A proper choice of donor, acceptor and -bridge are expected to deliver targeted properties i.e. better selectivity, sensitivity, aqueous solubility, NIR emission properties for practical applications in areas of molecular chemosensors.