2,2'-(2,2'-Oxybis(ethane-2,1-diyl)bis(oxy))bis(N-(2,2'-bithiophen-5-ylmethylene)aniline)

Abstract

A new flexible fluorescent compound L derived from 1,5-bis(2-aminophenoxy)-3-oxapentane (A) has been synthesized by classical Schiff-base reaction between (A) and 2,2´-bithiophene carbaldehyde (B). The same synthesis was reproduced by a green methodology using an ultrasonication reaction in a classical sonication bath. The structure of the new compound was confirmed by elemental analysis, IR, ¹H-NMR, MALDI-TOF-MS and EI-MS-spectra, UV-vis and fluorescence emission spectroscopy.

1. Introduction

In the field of fluorescence chemosensors [1] with potential applications as emissive sensing materials, different approaches have been followed for their synthesis [2,3,4]. A Schiff-base condensation is a classical reaction starting from a primary amine and an active carbonyl group as an easy way to obtain emissive compounds provided with the C=N donor-group [5,6].

Due to the interest of the above suggestion, we planned to synthesize a new emissive compound that combines together two sensing sites, which are a polyoxa chain provided with two phenolic units and two bithiophene chromophores. In the present note, we are reporting a simple method for synthesizing a bithiophene derivative, using classical Schiff-base condensation in ethanolic solution [7,8,9,10] and a green method using an ultrasound-assisted synthesis [11], which does not need any catalyst. The work-up procedure was very simple and convenient.

2. Experimental

A mixture of 0.5 mmol of 1,5-bis(2-aminophenoxy)-3-oxapentane (A) (0.144 g) and 1 mmol of 2,2´-bithiophene carbaldehyde (B) (0.194 g) was added to 50 mL of absolute ethanol, and kept under reflux for 4 h. After that time, the mixture was allowed to cool to room temperature, and a yellow precipitate was formed, which was filtered off and washed with diethyl ether (3 portions of 10 mL) and with cold absolute ethanol (3 portions of 10 mL). The yellow powder was purified by silica gel column chromatography (eluent: petroleum ether/ethyl acetate, 3/7). The reaction was reproduced in the same solvent using a green methodology, employing a 1510 Branson ultrasonication cleaner bath during 2 h at room temperature with the same results. In order to reduce the quantity of residues, the reaction can be reproduced reducing the solvent employed (absolute ethanol) to 25 or 5 mL. In both cases, the reactants were reduced to 0.25 mmol and 0.5 mmol, and to 0.05 mmol and 0.1 mmol of the amine and the carbaldehyde precursors respectively.

Yield: 300 mg (94%).

EI-MS: m/z (rel. int%): 640.04 (100) ([L^•]⁺)

MALDI-TOF-MS: m/z (rel. int%): 641.08 (95) [L+H]⁺.

¹H NMR (CDCl₃): δ = 8.6 (s, 2H, N=C-H); 7.3–6.9 (m, C-H, Ar); 4.2 (t, 4H, CH₂); 4.0 (t, 4H, CH₂) ppm.

IR (cm^-1): 3068 (C-H, Ar), 2926 and 2872 (C-H, Alq), 1659 (C=N), 1609, 1583, 1547 and 1505 (C=C, Ar), 1127 (C-S), 1047 (C-O, Alq).

Elemental analysis: Calcd for C₃₄H₂₈N₂O₃S₄·0.5H₂O: C, 62.85; H, 4.50; N, 4.30; S, 19.75. Found: C, 62.95; H, 4.60; N, 4.05; S, 19.80%.

Uv-vis (Absolute ethanol, [L] = 9.3 × 10^-6 M, λ_max 360 nm; ε (M^-1cm^-1): 29760.

Fluorescence Emission (Absolute ethanol, [L] = 9.3E^-6 M): λ_max 452 nm.