A new novel series of vanillin-based hexaimine-Schiff bases: Synthesis, characterization and investigation of chromic and fluorescence properties

Abstract

Here, new symmetrical diimines D1 and D2 were prepared by the condensation reaction of o-vanillin with 2-nitro-p-phenylenediamine or 4-nitro-o-phenylenediamine, respectively. Diimines were reduced selectively to their amino derivatives, and reacted with terephthaldehyde to obtain of a novel symmetrical hexaimines (H1 and H2). Their structures were identified on the basis of microanalyses, IR, 1H/13C NMR, HRMS and UV–vis spectroscopy. The enol–keto tautomeric equilibrium in these Schiff bases were elucidated by chromically and fluorescence spectra with respect to the influence of solvent, acid–base and light. They acted non-emissive in the solid state (λexc = 365 nm). Diimines displayed a weak green, bluish-green/blue emission in basic DMSO and DMF solution (v/v, 1:1), but they were non-fluorescent in other media. In contrast to this, hexaimines emitted a strong greenish-blue fluorescence in pure, bluish-green/greenish-blue fluorescence in acidic, and purplish-blue/blue fluorescence in basic aqueous DMSO and DMF media.Mass spectrum of Schiff bases exhibited the ion peaks as following positions: (D1): m/z (RI%) = 447.1329 (100 %), 424.1517 (60 %), 322.1454 (20 %), 288.0988 (30 %), 102.1277 (55 %) (Fig. S7). (D2): m/z (RI%) = 422.1361 (20 %), 288.0996 (100 %) (Fig. 6). (H1): m/z (RI%) = 883.4259 (10 %), 528.2244 (100 %), 510.2038 (20 %), 392.1605 (10 %), 102.1276 (10 %) (Fig. S8). (H2): m/z (RI%) = 882.3320 (10 %) [M + 2H]+, 434.1278 (40 %), 392.1710 (50 %), 374.1588 (100 %), 322.1523 (10 %), 256.1142 (15 %), 102.1297 (15 %) (Fig. S9). Mass spectral pattern of D1 displayed [M + 3H]+ ion peak at m/z 424.1517. The base peak was observed at m/z 447.1329, which may be due to [M + 3H+Na]+ ion. And the following fragment ion appeared at m/z 288.0988, which may be due to [C14H14N3O4]+ ion (4-((2‑hydroxy-3-methoxybenzylidene)amino)-3-nitrobenzenaminium)). D2 showed [M + H]+ molecular ion at m/z 422.1361. The similar fragmentation was observed at m/z 288.0996. This base peak may be due to fragment [C14H14N3O4]+ ion (2-((2‑hydroxy-3-methoxybenzylidene)amino)-4-nitrobenzenaminium)). H1 exhibited [M + 3H]+ molecular ion at m/z 883.4259. The base peak was observed at m/z 528.2244, which may be due to [C35H24N6]+ ion (3,3′-(((2-((4-(((2-amino-5-(methyleneamino)phenyl)imino)methyl)benzylidene)amino)-1,4-phenylene) bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diylium)). In the case of H2, [M + 2H]+ ion peak was seen at m/z 882.3320. The following fragmentation was observed at m/z 434.1278, which may be belonged to fragment [C30H18N4]+ ion (3,3′-(((4-((4-((vinylimino)methyl)benzylidene)amino)1,2phenylene)bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diylium)). The base peak was recorded at m/z 374.1588, which may be due to fragment [C26H208N3]+ ion (3,3′-(((4-((2-vinylbut-2-en-1-ylidene)amino)-1,2-phenylene)bis(azanylylidene))bis(methanylylidene))bis(benzene-1-ylium)).