ARTICLE pubs.acs.org/JPCC
Restricted Molecular Rotation and Enhanced Emission in Polymer Blends of Poly(fluorene-alt-naphthol) and Poly(vinyl pyrrolidone) with Mutual Hydrogen-Bond Interactions Rong-Hong Chien, Chung-Tin Lai, and Jin-Long Hong* Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, Republic of China
bS Supporting Information ABSTRACT: In this study, fluorescent alternative copolymer poly(fluorenealt-1,6-naphthol) (PFN) with inherent hydroxyl (OH) groups was blended with poly(vinyl pyrrolidone) (PVR) through facile hydrogen-bond (H-bond) interactions. The emission intensities of the resultant PFN/PVR mixtures of different compositions were shown to increase with increasing PVR content in both the chloroform solution and in the solid film states. The H-bond interactions between PFN and PVR resulted in the restricted molecular rotations of the fluorophores, which lead to the blockage of nonradiative energy decay channels and the accompanied emission enhancement. With the efficient H-bond interactions, the PFN/PVR blend with the low content (2.33 wt %) of fluorescent PFN component actually has a high quantum efficiency of 0.93, comparatively higher than other blends containing higher fluorescent PFN. The role of the H-bonding on the restricted molecular rotations of the PFN/PVR blends was therefore evaluated by the infrared, the 1H NMR spectroscopy and the dynamic light scattering.
’ INTRODUCTION Since the first silole compound with the aggregation-induced emission (AIE)1,2 feature was reported in 2001, lots of AIE3 42 or AIE enhancement43 54 (AIEE)-active organic and polymeric materials have been prepared and studied with the purpose to improve the fluorescent emission efficiency and to understand the operative mechanism responsible for the AIE3 42 or AIEE effect.43 54 In contrast with the aggregation-caused quench (ACQ), AIE-active materials possess enhanced emission efficiency in the solution aggregated and the solid film states. The enhanced emission in the solid state is especially advantageous for the optoelectronic device applications. Several mechanisms including the restricted intramolecular rotations (IMRs),3 35,42 54 formation of the J-aggregates,38 41 and excimer36,37 were previously suggested to account for the AIE or AIEE behavior. In the aggregated states of silole compounds, the restricted IMR of the phenyl rotors against the central silole stator reduces the nonradiative decay pathways and leads to the enhanced emission. The internal IMR of diisopropylattached hexaphenylsiloe (HP)24 compound was structurally hindered by the built-in, bulky isopropyl chains and with the inherent restriction on IMR, dilute solution of HP already possesses strong fluorescent emission. The same phenomenon was observed in the tetraphenylthiophene-derived quinoline (TP-Qu),33 in which the bulky C-2 quinoline rotor in TP-Qu efficiently hindered the molecular rotations and resulted in the indiscriminate, strong emissions between the solution and the aggregated states. r 2011 American Chemical Society
In addition to organic compounds, several polymeric fluorophores3,25 35,54 have been reported to exhibit AIE or AIEE characteristics. Among them, disubstituted polyacetylenes (PDAs)26 were thoroughly studied and characterized. Simulated results suggested the ease of the PDAs chains to form intramolecular excimers in the dilute solution state. However, the volume shrinkage of the polymer chain in the solvent mixture of poorer solvating power puts the fluorophoric units in closer vicinities, which adds restrictions on IMR and result in the enhanced emissions. The same result was also observed in vinyl polymer with pendant TP-Qu33 and TP groups.35 Fluorophores with inherent hydrogen bonds (H-bonds) tend to associate through preferable H-bond interactions to form aggregated structures with enhanced emission due to the operative AIE or AIEE effect. Previously, several organic compounds36 41 with inherent H-bonds were discovered to exhibit the AIE or AIEE properties. Among them, fluorenone-derivative36 of DSFO was reported to have enhanced excimer emissions due to intermolecular H-bonds. In this case, the DSFO dimeric structure was locked by intermolecular H-bonds in both the ground and the excited states. The emission proceeded without the structural rearrangement, and the nonradiative decay pathways that exist in common excimers are greatly reduced, leading to strong enhanced luminescence in the solid state. The salicylideneaniline-based organogelators38,39 are examples illustrating the enhanced emission promoted by gelling. Received: July 6, 2011 Revised: August 20, 2011 Published: September 08, 2011 20732
dx.doi.org/10.1021/jp206388y | J. Phys. Chem. C 2011, 115, 20732–20739
The Journal of Physical Chemistry C
ARTICLE
Figure 1. (A) Chemical structure of PFN and (B) the hydrogen-bond interactions between PFN and PVR.
The reversible sol gel conversion due to the tautomerism between the NH and the OH forms can be used to manipulate the emission behavior by heating and cooling. The AIE (or AIEE) property is ascribed to the formation of J-aggregate and the inhibition of IMR in the gel state. The above examples36 41 illustrated the fact that molecular rotations and fluorescence can be controlled by H-bonding. Poly(fluorene-alt-1,6-naphthol) (PFN, Figure 1) currently prepared54 in our lab was characterized to have AIEE property due to the hindered rotations imposed by the H-bonded hydroxyl (OH) functions. The intermolecular H-bonding gives restraint for the inherent fluorphores to rotate freely, which leads to the emission intensification. Intuitively, we thought of the possibility of further controlling on the molecular rotations and the emission behavior by facile H-bonding in a two component polymer blend system; in this case, the AIE-active PFN polymer was further used to blend with poly(vinyl pyrrolidone) (PVR) to construct the potential system to study (Figure 1). Here the OH groups in PFN can be preferably H-bonded to the amide carbonyl (CdO) groups in PVR. With the mutual H-bond interactions, homogeneous solutions and solid PFN/PVR samples of different compositions can be prepared and were characterized to elucidate the inter-relationships among the H-bonding, the involved molecular rotation, and the corresponding emission behavior.
’ EXPERIMENTAL SECTION PVR (Mn = 13 000, Mw = 29 000, and Mw/Mn = 2.2) was purchased from Aldrich Chem. PFN (Mn = 20 000, Mw = 26 000, and Mw/Mn = 1.3) was prepared from Susuki coupling reaction between 1,6-dibromonaphthol and 2,2-dibromo-9,9-dioctylfluorene.54 Instrumentation and Sample Preparation. 1H NMR spectra were recorded with a Varian VXR-500 MHz FT-NMR spectrometer. Tetramethylsilane was used as internal standard. FT-IR spectra were obtained from a Nicolet IR-200 spectrometer. Solution of organic compound or polymer in chloroform was dropped on a KBr pellet and dried at 100 °C under vacuum to prepare solid film for FT-IR analysis. Particle sizes were measured by a dynamic light scattering (DLS) instrument from a ZetaSizer Nano-S spectrometer at room temperature. A He Ne ion laser operating at 633 nm was used as light source. The conformation of PFN-PVR was formulated from the Materials Studio (MS) commercial software of Accelrys.
PL was obtained from a LabGuide X350 fluorescence spectrophotometer using a 450W Xe lamp as the continuous light source. UV vis absorption spectra were recorded with an Ocean Optics DT 1000 CE 376 spectrophotometer. Small quartz cell with dimension 0.2 1.0 4.5 cm3 was used to accommodate the solution sample. Stock solutions (10 2 M) of organic and polymeric fluorophores in chloroform were primarily prepared. Aliquot of the stock solution was then transferred to a 10 mL volumetric flask, into which appropriate volume of chloroform and ether was added dropwise under vigorous stirring to furnish 10 3 M solutions with different ether contents (0 90 vol %). Solid specimen was prepared by drop-casting sample solution on a quartz plate. Quantum yield (Φf) of solid samples was measured in an integrating sphere made by Ocean Optics. UV vis and PL emission spectra were immediately undertaken once the solutions were prepared.
’ RESULTS AND DISCUSSION As indicated in Figure 1A, the fluorescent PFN polymer54 possesses irregular chain sequence due to the random distribution of the 1- and the 6-naphthol links with the neighboring fluorene ring. Besides the irregularity on the chain links, the kinked segments introduced by the 1,6-disubstituted naphthol unit and the flexible 9,9-dioctyl side chains also contribute to the low Tg (with the detected midpoint at 104 °C) and the easy dissolution of PFN in common organic solvents such as chloroform, methylene chloride, tetrahydrofuran, toluene, and so on. By contrast, high content of hydrophobic aromatic moieties in PFN renders its insolubility in polar solvents such as methanol, ethanol, N,N-dimethylforamide, and dimethylsulfoxide. The preferable H-bond interactions between PFN and PVR facilitated the preparations of homogeneous PFN/PVR blends of various compositions from the corresponding solution state. Altogether, eight compositions with the code name of PFN/ PVR (x/y) were prepared and were summarized in Table 1. The x/y ratio in the code name refers to the molar ratio of the repeat units in PFN and PVR; therefore, it also represents the molar ratio between the inherent OH and CdO groups. To have sufficient restriction on the IMR, all OH functions in PFN are required to be H-bonded to the carbonyl groups of PVR; in other words, the x/y ratio must be
