DOI: 10.1002/chem.200700130 Chiral Supramolecular Assemblies of a Squaraine Dye in Solution and Thin Films: Concentration-, Temperature-, and Solvent-Induced Chirality Inversion Kuthanapillil Jyothish, Mahesh Hariharan, and Danaboyina Ramaiah* [a] Introduction Squaraine dyes have been the subject of many recent inves- tigations. [1] The current interest in these dyes may be attrib- uted to their interesting photophysical and photochemical properties [2] which make them suitable for a variety of appli- cations. These include photoconductors in organic solar cells, [3] photoreceptors in copiers and laser printers, [4] IR ab- sorbers in organic optical disks, [5] sensors for metal ions [1g] and proteins, [6] and sensitizers in photodynamic therapeutic (PDT) applications. [7] These dyes are known to form aggre- gates in solution, organized media, Langmuir–Blodgett films, and microcrystals. [8] Depending on the nature of the substituents and the medium, these dyes form both H- (pack-of-cards) and J-type (end-to-end) aggregates. [8,9] Recently, dye aggregates have received considerable at- tention in the fields of highly sensitive biosensors, [10] field- effect transistors, [11] and supramolecular electronics. [1a] Dye aggregates that can form chiral assemblies are particularly important, since switching between the different supra- molecular chiral states could, in principle, have potential ap- plications in data storage, optical devices, LCDs, and chro- matographic chiral separation. [12] Moreover, stable chiral ag- gregates with favorable photophysical properties that exhibit reversible switching between P (plus, right-handed) and M (minus, left-handed) supramolecular assemblies could have applications as chiroptical materials. [13,14] Abstract: We prepared novel cholester- ol-appended squaraine dye 1 and model squaraine dye 2 and investigated their aggregation behavior in solution and thin films using photophysical, chi- roptical, and microscopic techniques. Investigations on the dependence of aggregation on solvent composition (good/poor, CHCl 3 /CH 3 CN) demon- strated that squaraine dye 1 forms two novel H-type chiral supramolecular as- semblies with opposite chirality at dif- ferent good/poor solvent compositions. Model compound 2 formed J-type achiral assemblies under similar condi- tions. 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-de- pendent experiments. With increasing temperature, the chirality of the kineti- cally controlled aggregate was lost and, on cooling, the aggregate with the op- posite 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 thermody- namically 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 kineti- cally 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 condi- tions, 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-ag- gregated solutions. Keywords: aggregation · chirality · dyes/pigments · helical structures · supramolecular chemistry [a] K. Jyothish, M. Hariharan, Dr. D. Ramaiah Photosciences and Photonics Chemical Sciences and Technology Division Regional Research Laboratory (CSIR) Trivandrum - 695019 (India) Fax: (+ 91)471-249-0186 E-mail: rama@csrrltrd.ren.nic.in d_ramaiah@rediffmail.com Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 5944–5951 5944