Subscriber access provided by University of Otago Library
Article
Advantages of Mobile Liquid-Crystal Phase of AIE Luminogens for Effective Solid-State Emission Hoa Thi Bui, Jinhee Kim, Ho-Joong Kim, Byoung-Ki Cho, and Sung Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10026 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 9
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Advantages of Mobile Liquid-Crystal Phase of AIE Luminogens for Effective Solid-State Emission Hoa Thi Bui,[a] Jinhee Kim,[b] Ho-Joong Kim,[c] Byoung-Ki Cho,*,[b] and Sung Cho*,[a] [a]
Department of Chemistry, Chonnam National University, Gwangju 500-757, Korea; [b] Department of Chemistry and Institute of Nanosensor and Biotechnology, Dankook University, Gyeonggi-Do 448-701, Korea; [c] Department of Chemistry, Chosun University, Gwangju 501-759, Korea. Aggregation-induced emission, Tetraphenylethene, Liquid-crystal, Polycrystalline, Self-assembled
ABSTRACT: We investigate aggregation-induced emission (AIE) characters of bulk tetraphenylethene (TPE) derivatives with nonpolar and polar exterior chains. The bulk TPE derivatives with nonpolar dodecyl and polar di(ethylene oxide) chains are in liquid crystal (LC) and crystalline phases, respectively, at room temperature. The mobile LC character of the TPE derivatives in bulk is crucial for high AIE efficiency because the mobile LC character is effective in minimizing the number of undesirable non-emissive local sites in aggregates and inducing a homogeneous zigzag-stacked columnar structure with j-type coupled transition dipoles of the TPE derivatives. The mobile LC characteristics of AIE luminogens can be advantageous for highly efficient solid-state emission.
INTRODUCTION Aggregation-induced emission (AIE) luminogens have been widely investigated for the fabrication of efficient optoelectronic devices, solid-state dyes, chemical sensors, and bioprobes.1-10 Since typical AIE luminogens include several small aromatic rings and the constituent small aromatic rings act as excitation-energy dissipaters, the majority of AIE luminogens in the solution phase is non-emissive or weakly emissive because of strong interactions between flexible AIE luminogens and neighboring solvent molecules.11-13 On the other hand, after the aggregate formation of AIE luminogens, the restricted intramolecular motions of the constituent small aromatic rings in aggregates induce structural rigidity and consequently, deceleration of nonradiative decay processes as well as increased portion of radiative decay during excitation energy relaxation. Furthermore, the nonplanar geometry of constituent aromatic rings in AIE luminogens is also helpful in preventing aggregation-caused quenching (ACQ) due to strong intermolecular π−π interactions between AIE luminogens, e.g. inhibition of optically forbidden states caused by H-type couplings.14-16 Since the AIE character is strongly affected by the extent of restriction of intramolecular motions, we were interested in the structural parameters of AIE luminogens for the purpose of controlling agglomerated structures and AIE characteristics. We could selectively decorate flexible chains at the exterior of four individual branching sections of the tetraphenylethene (TPE) moiety.17-18 The TPE derivatives consist of an identical aromatic TPE core (central TPE unit and phenyl groups linked by 1,2,3-triazolyl groups) and peripheral flexible chains, i.e., nonpolar dodecyl chains for 1 and 2 and polar di(ethylene oxide) chains for 3 (Scheme 1).13, 19 In a previous report,13 1 and 2, with nonpolar exterior dodecyl chains, and 3, with polar exterior di(ethylene oxide) chains, form unidirectional ordered and emulsion-driven amorphous aggregates in THF/water solvent mixtures, respectively. This is mainly due to the different po-
larities of the peripheral flexible chains and subsequently, different solubilities of the TPE derivatives in THF/water solvent mixtures. Here, we focused on the solid-state AIE characters of bulk TPE derivatives, depending on the self-assembled structure and structural mobility. Since the TPE derivatives with dodecyl and di(ethylene oxide) chains have mobile liquid-crystal (LC) and solid phases at room temperature, respectively, they are good test beds to evaluate the effect of structural mobility on the AIE efficiencies of solid-state aggregates of AIE luminogens. If the restriction of intramolecular motion is the only overwhelming factor of AIE, the mobile LC phases of solid-state aggregates of AIE luminogens will be a disadvantageous condition. At first, we are interested in aggregates after fast drying, which is a common situation when a uniform thin layer is prepared by using coating or printing technique.20-21 Therefore, the bulk TPE derivatives were prepared by fast solvent evaporation with an aspirator vacuum pump. On the basis of spectroscopic results, we found that bulks 1 and 2, with columnar LC phases, exhibit high AIE efficiencies compared to their aggregate-counterparts in THF/water solvent mixtures and that the propeller-like TPE cores in bulks 1 and 2 form zigzagstacked structures and subsequently J-type dipole-dipole coupling between neighboring TPE derivatives. On the other hand, bulk 3, with polar exterior chains, is less emissive than its aggregate-counterpart in THF/water solvent mixtures and polycrystalline. Hence, the nonradiative decay processes are more efficient, probably due to numerous crystallographic defects and subsequently, large amounts of undesirable non-emissive electronic states. Since the structural mobility of the TPE derivatives can be tuned by the number of peripheral flexible chains and their intrinsic polarity, we could achieve highly efficient AIE of bulk AIE luminogens without further sample preparation. Consequently, we propose that the mobile LC characteristics of AIE luminogens can be advantageous for highly efficient AIE in solid-state light-emitting devices.
ACS Paragon Plus Environment
The Journal of Physical Chemistry Scheme 1. Molecular Structure of the TPE derivatives
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
R1
Page 2 of 9
RESULTS
R1
R2
R2 N
R3
N
N N
N
N
R3
3 R1
N
N N
N N
R1
N
R2
2
R2 R3
R3
1: R1=R2=R3=O(CH2)11CH3 2: R1=R2=O(CH2)11CH3, R3=H 3: R1=R2=O(CH2)2O(CH2)2OCH3, R3=H
1
EXPERIMENTAL Sample Preparation and Steady-State Spectroscopic Measurements. Tetraphenylethene derivatives (1, 2, 3) were synthesized according to previously reported methods.13, 18 Bulk samples were prepared by fast evaporation with an aspirator vacuum pump. Diffuse reflectance spectra were recorded with a PerkinElmer Lambda 900 UV-vis-NIR spectrometer, and steady-state emission spectra were obtained using a Hamamatsu Quantaurus-QY absolute PL quantum yield spectrometer. Differential Scanning Calorimetry and X-ray Diffraction Measurements. Differential scanning calorimetry (DCS) measurements were performed by using a Perkin Elmer DSC-7 with the 1020 thermal analysis equipment at a rate of 10 ℃/min. X-ray diffraction measurements were performed in transmission mode with synchrotron radiation at the 9A beamline of the Pohang Accelerator Laboratory (PAL), Korea. The samples were held in an aluminum sample holder with polyimide films on both sides. Time-Resolved Emission Measurements. A time-correlated single photon counting (TCSPC) system was used for the spontaneous emission measurements.22 The system consisted of a picosecond diode laser (PicoQuant). The excitation wavelength was fixed at 375 nm for all experiments. The excitation beam was focused onto a temperature-controlled cuvette containing the sample using a 5-cm-focal-length lens with s polarization. The emission from the sample was collected with a magic angle (54.7°) in order to prevent polarization-dependent signals, then was focused onto a monochromator (Dongwoo Optron) by a 2′′ plano-convex lens pair and detected using an APD detector (ID Quantique). The full width at half-maximum (fwhm) of the instrument response function obtained by using pump scattering was typically ∼180 ps in our TCSPC system. The number of emitted photons per unit time was always maintained at
