Induction of Mesomorphism through Supramolecular Assembly in Metal Coordination Compounds of "salphen"-Type Schiff Bases: Photoluminescence and Solvatochromism

• Published in: European journal of inorganic chemistry 2016-10, Vol.2016 (28), p.4604-4614
• Main Authors: Chakraborty, Sutapa, Mondal, Paritosh, Prasad, S. Krishna, Rao, D. S. Shankar, Bhattacharjee, Chira R.
• Format: Article
• Language: English
• Subjects: Agglomeration; Aggregation; Condensing; Coordination compounds; Copper; Density functional calculations; Differential scanning calorimetry; Diffraction; Ligands; Maxima; Metallomesogens; Photoluminescence; Schiff bases; Solvent effects; X-rays
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201600513

Mesomorphism induced by metal (VO2+, Ni2+, Cu2+ and Zn2+) coordination and its influence on the photophysical properties of a new series of symmetric “salphen”‐type N,N‐bis(4‐n‐alkoxysalicylidene)‐4,5‐dichloro‐1,2‐phenylenediamine ligands (n = 12, 14, 16) bearing highly electronegative spacer substituents and alkoxy arms of varied length have been investigated. The mesomorphism has been probed by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and variable‐temperature powder X‐ray diffraction (XRD). The Schiff base ligands are non‐mesomorphic; however, coordination to NiII and ZnII ions induces columnar mesomorphism, this being stable down to room temperature in the latter case. Interestingly, the complexes of CuII and VOIV did not show any mesomorphism. The free ligands and the ZnII complexes are luminescent both in the condensed state and in solution, with emission maxima in the condensed state being significantly red‐shifted with respect to those in solution. The energy‐optimized structure of a representative ZnII complex was ascertained to be distorted square planar in a DFT study employing the GAUSSIAN 09 program. The ZnII and VOIV complexes exhibited solvatochromic effects, showing interesting aggregation behavior in the former case. The effects of metal coordination (M = VO2+, Ni2+, Cu2+ and Zn2+) on mesomorphism and photophysical properties of a new series of salphen ligands have been investigated. The NiII and ZnII complexes show columnar oblique and columnar rectangular phases, respectively, the latter being stable down to room temperature thus affording room temperature mesomorphism. Further, the axial vacant coordination sites in the square‐planar ZnII and the square‐pyramidal VO(IV) centers facilitate solvatochromism, with the former exhibiting formation of aggregates or monomers in solvents of different coordinating ability.