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Chiral Supramolecular Assemblies of a Squaraine Dye in Solution and Thin Films: Concentration‐, Temperature‐, and Solvent‐Induced Chirality Inversion
Author(s) -
Jyothish Kuthanapillil,
Hariharan Mahesh,
Ramaiah Danaboyina
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700130
Subject(s) - chirality (physics) , solvent , supramolecular chirality , chemistry , supramolecular chemistry , moiety , solvent effects , photochemistry , j aggregate , chemical engineering , organic chemistry , molecule , physics , chiral symmetry breaking , quantum mechanics , nambu–jona lasinio model , engineering , quark
We prepared novel cholesterol‐appended squaraine dye 1 and model squaraine dye 2 and investigated their aggregation behavior in solution and thin films using photophysical, chiroptical, and microscopic techniques. Investigations on the dependence of aggregation on solvent composition (good/poor, CHCl 3 /CH 3 CN) demonstrated that squaraine dye 1 forms two novel H‐type chiral supramolecular assemblies with opposite chirality at different good/poor solvent compositions. Model compound 2 formed J‐type achiral assemblies under similar conditions. The supramolecular assembly of 1 observed at lower fractions of the poor solvent could be assigned to the thermodynamically stable form, while a kinetically controlled assembly is formed at higher fractions of the poor solvent. This assignment is evidenced by temperature‐ and concentration‐dependent experiments. With increasing temperature, the chirality of the kinetically controlled aggregate was lost and, on cooling, the aggregate with the opposite chirality was formed. On further heating and cooling the aggregates thus formed resulted in no significant changes in chirality, that is they are thermodynamically stable. Similarly, at lower concentrations, the thermodynamically stable form exists, but at higher concentration aggregation was found to proceed with kinetic control. Based on these observations it can be assumed that formation of the kinetically controlled assembly might be largely dependent on the presence of the nonpolar cholesterol moiety as well as the amount of poor solvent present. However, under solvent‐free conditions, structurally different aggregates were observed when drop cast from solutions containing monomer, whereas a left‐handed CD signal corresponding to the thermodynamically controlled assemblies was observed from pre‐aggregated solutions.