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Self-Assembly Tuning of #-Cyanostilbene Fluorogens: Aggregates to Nanostructures Anuji K Vasu, Mithun Radhakrishna, and Sriram Kanvah J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06225 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017
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Self-Assembly Tuning of α-Cyanostilbene Fluorogens: Aggregates to Nanostructures Anuji K Vasu†, Mithun Radhakrishna‡ and Sriram Kanvah†*
*Department of Chemistry, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar 382 355, India. ‡
Department of Chemical Engineering, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar 382 355
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ABSTRACT
Two AIE active α-cyanostilbene fluorophores substituted with octyl and hexadecyl chains were synthesized and examined for their absorption and fluorescence properties. The alkoxy substitution tunes the self-assembly in water and exhibits intensity enhanced bathochromic shift resulting in well-defined ṇanospheres with the formation of nice dendrimeric structures. MonteCarlo simulations validate our experimental observations and show the greater propensity of selfassembly formation for cyanostilbene containing hexadecyl chain compared to derivative containing octyl chain. The results are substantiated by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) studies. Such tunable self-assembled nanostructures seem promising for material and biological applications.
INTRODUCTION Self-organization of small organic molecules into nanostructures yields novel materials that have biological and organic electronic applications1-5. In the recent years, a diversity of fluorescent assemblies based on π-conjugated organic molecules has attracted tremendous attention for use in organic optoelectronics. The desirable optical and electrical properties of the π-conjugated molecules along with its synthetic feasibility and ability to incorporate donor or acceptor moieties or other functional groups have led to their prevalent use for electronic and biological applications6-7. Weak intermolecular interactions such as van der Waals forces, hydrogen bonding, dipole−dipole attraction, hydrophobic effect, electrostatic interaction, π−π stacking and metal−ligand coordination inbuilt in the molecular structure play a major role in the formation of such self-assemblies8-10. Based on the optimum non-covalent interactions, in
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general, systems possessing coplanar π-conjugated systems, or those with long alkyl chains and hydrogen bonding interactions preferentially form self-assembled functional nano or micro structures11. Any chemical substitution that deviates from the coplanar system can result in loss of the propensity to form self-assembled structures, and it is well established that the nonplanar chromophores do self-assemble in solution and solid state depending on the substituents or the molecular conformation12-13. It is also known that isolated α-cyanostilbenes bearing a cyano group on the double bond exhibit internal steric repulsions due to the ‘twist elasticity’ of the system14. But specific molecular interactions due to restricted rotation results in the formation of H or J- aggregates with unique optical properties15-18. Further suitable substitution such as trifluoromethyl (CF3) group, PEGylation or cholesterol yields hierarchical self-assemblies19-21. This enhanced fluorescence is commonly known as aggregation-induced enhanced emission (AIEE) or aggregation-induced emission (AIE)22-25 and attributed to intramolecular planarization, restriction to the intramolecular rotation and obstruction of non-radiative relaxation pathways of the excited species26. We have recently developed such fluorophores with charge transfer characteristics and exhibiting AIE behavior27-30. In an attempt to tune the morphology of the aggregates/ self-assemblies, we designed alkyl substituted α-cyanostilbene fluorophores. Alkyl chains are commonly attached to π-conjugated systems to improve solubility and enhance molecular packing modes in the aggregated state31. This combination of the cyanostilbene motif and introducing a strongly hydrophobic alkyl chain on the terminal rings could enhance the balance of the non-covalent interactions playing a key role in the formation of self-assembled micro or nanostructures exhibiting interesting fluorescence properties. In quest of this, we designed and synthesized two α-cyanostilbene units substituted with Octyl and Hexadecyl alkyl units [Figure-1]. Examination of their UV-Vis absorption, emission in water
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indeed reveals they readily form self-assemblies in water and these observations are substantiated with Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Dynamic Light Scattering (DLS) studies. Further, coarse grained molecular Monte Carlo simulations validate our experimental observations and provide us a fundamental understanding of the driving forces for structural rearrangements. The details are given in the following sections.
EXPERIMENTAL SECTION Materials and instruments All the reagents required for the synthesis are obtained from Acros, Aldrich, Alfa Aesar and SD Fine chemicals. The synthesized compounds were characterized using 1H and
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C NMR
(Bruker Avance III-500 MHz) and Mass spectrometry (Waters Synapt G2S). Solvents used for absorption and fluorescence investigations were dried and distilled before use. UV-visible absorption spectra were recorded using UV-Visible spectrophotometer (Analytik Jena, Specord plus 210) and steady-state fluorescence emission and excitation studies were conducted using Fluorolog-3, spectrofluorimeter with a slit-width of 2 nm /3 nm. Fluorescence quantum yields of these compounds were determined using quinine sulphate (Φ = 0.546 in 0.5 M H2SO4) as a reference standard. Fluorescence lifetime measurements were performed using a picosecond time-correlated single photon counting (TCSPC) setup (Edinburgh Instruments Ltd, Lifespec II model) employing a picosecond light emitting diode lasers (Nano LED, λex = 405 nm with pulse width 57.3 ps) as the excitation source. The detection system consists of a high speed red photomultiplier. The lamp profile was recorded by placing a scattered (diluted Ludox solution in water) in place of the sample. The fluorescence lifetime values were determined by
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deconvoluting the instrument response function with decay using FAST decay analysis software. The quality of the fit has been evaluated by the fitting parameters such as χ2 (
