Synthesis and characterization of the photophysical properties of novel heterocyclic imines

As part of an on-going research to develop efficient heterocyclic systems for optical applications (NLO, photochromic, chemosensor, etc) we report in this work the synthesis and the photophysical characterization of novel heterocyclic imines 1a-b and 2, functionalized with thiadiazole or benzothiazole as acceptor groups linked to an aryl ring.


Introduction
The imine-bridged organics compounds have been target of high interest for the application of photosensitive materials.These compounds are an important group due to the diversity of applications in medicinal, materials and supramolecular chemistry due to their biological properties as well as their ability as chemosensors, photochromic, two-photon absorption and nonlinear optical materials.[3] Additionally, imines having intramolecular hydrogen-bonding are a versatile class of compounds for the design of optical materials since proton transfer in these systems origin the change of their optical properties.Recently, we have showed that the kinetic rate of the thermal cis-trans re-isomerization of pyrrolidene Schiff bases, at room temperature, can be controlled through proper substitution at the imine-aryl moiety of the molecule. 4erefore, as part of an on-going research to develop efficient heterocyclic systems for optical applications (NLO, photochromic, chemosensor, etc) [5][6][7][8] we report in this communication the synthesis and the photophysical characterization of novel heterocyclic imines 1 and 2 functionalized with thiadiazole or benzothiazole as acceptor groups linked to an aryl ring, having in mind their potential optical (linear and nonlinear) applications.

Instruments
NMR spectra were obtained on a Varian Unity Plus Spectrometer at an operating frequency of 300 MHz for

Synthesis
General procedure for the synthesis of imines 1 and 2: a solution of 2-hydroxy-3methoxybenzaldehyde (2 mmol), amino derivative (2 mmol), acetic acid (2 drops) in ethanol (15 mL) was heated at reflux and monitored by TLC, which determined the reaction time (9-24 h).After cooling and solvent evaporation, the crude product was recrystallized from light petroleum/dichloromethane or ethanol to afford the pure imines 1 and 2.

Optical properties
The UV-visible spectra of imines 1-2 were recorded.in ethanol solutions (10 -4 M).The position of the absorption bands is influenced by the structure of the compounds, for example, by the different heterocyclic group linked to the aryl-imine moiety (Table 1).
Chromophore 2, which have the benzothiazole heterocycle linked to the aryl ring show a marked bathochromic shift in their charge transfer (CT) band compared with (aryl)thiadiazole imines (1a, 84 nm) or (1b, 44 nm) due to the stronger electronacceptor ability of NO 2 -benzothiazole heterocycle as well as due to more extensive electron delocalization (Figure 1).0

Electronic structure analysis
The most characteristic signals in the 1 H NMR spectra of this family of Schiff bases is that corresponding to the CH=N protons.Compounds 1-2 have the CH=N protons resonate at 9.27-9.50ppm, whereas aryl OH protons were found in the 10.80-11.0ppm interval.All protons in compound 2 exhibits signals that are shifted upfield, compared to the corresponding (aryl)thiadiazole derivatives 1a-b, due to the strongest electron acceptor ability of the nitro-functionalized benzothiazole electron-deficient heterocycle compared to the (aryl)thiadiazole groups (Table 2). 6 The 1 H NMR chemical shifts reflect a charge separation in the ground state, consequently the analysis of these data in push-pull organic systems such as imines 1-2 bearing thiadiazole and benzothiazole electron-deficiente heterocycles as acceptor groups confirms their push-pull character with a significant intramolecular charge transfer (ICT) from the donor aryl-imine moiety to the acceptor groups and a high polarizability of the whole donor-acceptor derivatives.

Conclusions
In summary, we have synthetized in good to excellent yields new heterocyclic imines functionalized with (aryl)thiadiazole or benzothiazole acceptors groups linked to the imne-aryl moiery through position 2 of the thiadiazole or benzothiazole heterocycles, using simple experimental procedures.
Modulation of the electronic and optical properties of compounds 1-2, was achieved through the variation of the electronic acceptor ability of the heterocycle as well as increase of the π-conjugated bridge.
Due to their push-pull character and marked acidity of OH protons the new heterocyclic systems could have potential applications for NLO chromophores and/or as colorimetric chemosensors.

Table 1 )
. All compounds were characterized by 1 H and 13 C NMR, UV-Vis and IR, the data obtained are in full agreement with the proposed formulation.

Table 1 .
Yields and UV-visible data for compounds 1-2 in ethanol.

Table 2 .
Chemical shifts of protons of compounds 1-2 in deuterated DMSO at 300 and 400 MHz, with chemical shift values in ppm.