Hydrogen Bonds and Enhanced Aggregation Emission of Organic and

ARTICLE pubs.acs.org/JPCC

Hydrogen Bonds and Enhanced Aggregation Emission of Organic and Polymeric Fluorophores with Alternative Fluorene and Naphthol Units 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 organic compound FN and polymer PFN with alternative fluorene and naphthol units were prepared to assess the influence of the hydrogen bonds (H-bonds) on the emission behavior. Both FN and PFN exhibit the enhanced aggregation emission behavior with enhanced emissions on solution mixtures of higher degree of aggregation. For solid FN and PFN films, the fractions of the H-bonded hydroxyl groups decrease upon being heated to high temperature, which results in aggregate dissociation and reductions in the aggregation emissions. Meanwhile, procedures leading to the increase of the H-bonds (such as shearing or enhanced aggregation by addition of nonsolvent in the solution preparative step) resulted in solid films with higher emission intensity. Variations on the H-bonds can be detected by infrared spectroscopy and are in line with the emission behavior.

’ INTRODUCTION Polyfluorenes1
6 (PFs) are one of the most widely studied and promising luminescent polymers because of their superior properties, and a number of PFs and their derivatives have been prepared and characterized as promising blue-light emitting polymers. Nevertheless, undesired low-energy green emission usually appears upon thermal annealing7 and leads to great reduction in the emission intensity. This low-energy green emission has been attributed to the interchain aggregates6 in the solid state. To suppress the detrimental interchain aggregation, copolymerizations of fluorene with rigid anthracene,9 kinked carbazole,10 bulky spirofluorene,11 and dendrimer-like12,13 monomers had been attempted to prepare copolyfluoroenes with steric hindrance for interchain approaches and aggregations. The aggregation-caused quench (ACQ) on emission of PFs can be therefore blocked by prohibiting the interchain aggregation process. In contrast to ACQ observed in PFs and their derivatives, Tang’s group discovered that silole molecule (1-methyl-1,2,3,4,5pentaphenylsilole) emits more efficiently in the aggregated or solid state than in the dilute solution.14,15 This interesting phenomenon was designated as aggregation-induced emission (AIE)16
55 or aggregation-induced emission enhancement (AIEE)56
61 to emphasize the unusual phenomenon that the originally nonluminescent or weakly luminescent solution can be tuned to emit strongly when nanoaggregates formed upon nonsolvent inclusion. In view of the superior properties of the AIE- or AIEE-active materials in the application film state, lots of organic and polymeric materials have been discovered and characterized to exhibit AIE or AIEE properties. Fluorophores with inherent hydrogen bonds (H-bonds) tend to associate through preferable H-bond interactions to form r 2011 American Chemical Society

aggregated structures and therefore to generate enhanced emission with the potential AIE or AIEE feature. Previously, several organic compounds49
54 with inherent H-bonds were discovered to exhibit enhanced aggregation emission properties. Among them, fluorenone derivatives49,50 were reported to have enhanced emissions due to the formation of dimer (excimer) species with intermolecular H-bond interactions. The salicylideneaniline-based organogelators51,52 are examples illustrating the enhanced emission in the aggregated gel states. The reversible sol
gel conversion due to the tautomerism between the NH and OH forms can be used to manipulate the emission behavior by heating and cooling. The AIE or AIEE properties are ascribed to the formation of J-aggregate, and the inhibition of intramolecular rotation in the gel state blocks the nonradiative decay channels and leads to the emission enhancement. In this study, organic FN and polymeric PFN fluorophores (Scheme 1) with alternative fluorene and naphthol units were synthesized and used to evaluate the influence of the inherent H-bonds on the emission behavior. Distinct from the aggregation emission reduction observed in the polyfluorene copolymers, the present fluorene-based FN and PFN were actually AIE- and AIEEactive materials, respectively. The inter- and/or intrachain H-bond interactions among the inherent hydroxyl
OH groups of FN and PFN are supposed to promote interactions among fluorophores and generate aggregated structures responsible for the AIE or AIEE properties. In contrast to normal PFs, the interchain H-bond associations of polymer PFN are therefore beneficial for the emission Received: April 8, 2011 Revised: May 14, 2011 Published: May 25, 2011 12358

dx.doi.org/10.1021/jp203261s | J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C

ARTICLE

Scheme 1. Synthetic Procedures of Organic Compound FN and Copolymer PFN

efficiency. Infrared spectral investigation on the hydroxyl
OH groups was used to characterize the influence of H-bonding on the resolved emission. The relationship between H-bond and emission will be evaluated in this study.

’ EXPERIMENTAL SECTION Materials. 2,7-Dibromofluorene, n-butyllithium (1.6 M in hexane), 1-bromooctane, fluorene, and Aliquat 336 were purchased from Aldrich Chemical Co. 1,6-Dibromo-2-naphthol (DBN) was purchased from ACROS Chemical Co. These starting materials were used directly without further purification. Tetrahydrofuran (THF) and toluene were distilled from sodium/benzophenone under nitrogen. Tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] was synthesized according to the literature.62 Syntheses of FN Organic Compound and PFN Copolymer. Organic compounds 3 and 5 were prepared according to literature procedures.63 Other intermediates, FN, and polymer PFN were prepared according to Scheme 1 and the detailed procedures are given below. Preparation of 9,9-Dioctylfluorene (1). An aliquot of 83 mL (132.8 mmol) of n-butyllithium (1.6 M in hexane) was added dropwise to a solution of 10 g (60.2 mmol) of fluorene in THF (140 mL) at
78 °C. The mixture was stirred at
78 °C for 1 h, and 26.7 g (138.5 mmol) of 1-bromooctane in THF (40 mL) was added dropwise to the mixture. The resultant mixture was stirred for 30 min before it was warmed up to room temperature to react for another 3 h. The resultant mixture was poured into water and extracted with diethyl ether. The organic layer was washed with brine, dried over magnesium sulfate, and concentrated by rotary evaporator. Excess 1-bromooctane was removed by further distillation (100 °C oil bath) to give 20 g of 9,9-dioctylfluorene (yield 85%) 1H NMR (300 MHz, CDCl3, Figure S1, Supporting Information): δ 7.70 (d, 2H, aromatic He), 7.2
7.4 (m, 6H, aromatic Hf-Hh), 1.99 (t, 4H, Ha), 1.05
1.23 (m, 20H, Hb),

0.86 (t, 6H, Hd), 0.64 (m, 4H, Hc). MS m/e: calcd for C29H42, 390.33; found, 390.4 (Mþ); Anal. Calcd for C29H42: C, 89.16; H, 10.84. Found: C, 89.10; H, 10.70. Preparation of 2-Bromo-9,9-dioctylfluorene (2). To a solution of 9,9-dioctylfluorene (5 g, 12.8 mmol) in CHCl3 (40 mL) at 0 °C were added 32 mg (0.2 mmol) of ferric chloride and 2 g (12.8 mmol) of bromine. This procedure was operated in the dark to avoid the undesired bromination on the aliphatic substituent. The solution was then warmed to room temperature before continuous reaction for 4 h. The resultant slurry was poured into water and washed with sodium thiosulfate to the extent that the red color disappeared. The aqueous layer was extracted with CHCl3 twice, and the combined organic layers were dried over magnesium sulfate to afford 5 g (yield 83%) of the product. 1H NMR (300 MHz, CDCl3, Figure S2, Supporting Information): δ 7.26
7.70 (m, 7H, aromatic He-Hj), 1.99 (t, 4H, Ha), 1.18
1.30 (m, 20H, Hb), 0.88 (t, 6H, Hd), 0.64 (m, 4H, Hc). MS m/e: calcd for C29H41Br, 468.24; found, 468.5 (Mþ); Anal. Calcd for C29H41Br: C, 74.18; H, 8.80. Found: C, 74.00; H, 8.40. Preparation of 2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2yl)-9,9-dioctylfluorene (4). A hexane solution of n-BuLi (1.6 M, 3.2 mL, 5.1 mmol) was added to compound 2 (2.0 g, 4.3 mmol) in THF (40 mL) at
78 °C. The reaction mixture was stirred for 1 h before the addition of 2-isopropoxy-4,4,5,5-tetramethyl1,3,2-dioxaborolane (1.2 mL, 6.0 mmol). The mixture was then warmed to room temperature and stirred overnight. The resultant mixture was neutralized with dilute HCl (2 M) solution. The whole mixture was then extracted with ethyl ether and the organic layer was washed with brine (150 mL) and sequentially dried over sodium sulfate and magnesium sulfate. The solvent was then removed under reduced pressure to afford 1.2 g (yield 55%) of the product. 1H NMR (300 MHz, CDCl3, Figure S3, Supporting Information): δ 7.6
7.85 (m, 6H, He
Hg), 7.30 (m, 1H, Hh), 2.17 (t, 4H, Ha), 1.40 (s, 12H, Hi), 1.03
1.22 (m, 20H, Hb), 0.84 (t, 6H, Hd), 0.59 (m, 4H, Hc). MS m/e: calcd for C35H53BO2, 12359

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C 516.41; found, 516.2 (Mþ); Anal. Calcd for C35H53BO2: C, 81.37; H, 10.34; O, 6.19. Found: C, 81.05; H, 10.29; O, 6.35. Preparation of FN Organic Compound. To the mixture of compound 4 (0.2 g, 0.39 mmol), DBN (0.25 g, 0.82 mmol), Aliquat 336 (40 mg, 0.08 mmol), and [Pd(PPh3)4] (93 mg, 0.0078 mmol) was added a degassed mixtures of aqueous K2CO3 (2 M, 12 mL) and toluene (18 mL). The whole reaction mixture was vigorously stirred at 80 °C for 12 h before it was cooled and quenched in 150 mL of methanol. The resultant precipitates were filtered and dried to obtain the crude products. The solvent was then removed under reduced pressure. The crude products were recrystallized from ethyl acetate to afford the title product as white powder. 1H NMR (300 MHz, CDCl3, Figure S4, Supporting Information): δ 7.26
7.79 (m, 19H, aromatic Hd-Hn), 3.48 (s, 1H, Ho) 0.4
2.1 (m, 68H, Ha-Hc). Tm = 53 °C (DSC). MS m/e: calcd for C66H88O, 920.68; found, 920.4 (Mþ); Anal. Calcd for C66H88O: C, 88.64; H, 9.63; O, 1.74. Found: C, 88.44; H, 9.53; O, 1.99. Preparation of Poly(9,9-dioctylfluorene-alt-naphthol) (PFN). To the mixture of 2,7-bis(4,4,5,5-tetramethyl-1,3,2dioxaborolan2-yl)-9,9-dioctylfluorene (2.34 g, 4 mmol), DBN (1.32 g, 4.4 mmol), Aliquat 336 (40 mg, 0.08 mmol), and [Pd(PPh3)4] (22 mg, 0.02 mmol) was added a degassed mixture of toluene (24 mL) and aqueous K2CO3 (2 M, 16 mL) solution. The whole reaction mixture was vigorously stirred at 85 °C for 48 h before it was cooled and quenched by 400 mL of methanol. The resultant precipitate was filtered and dried to obtain the solid residue. This crude product was then purified by Soxhlet extraction with methanol to give yellow powder as the final product. 1H NMR (300 MHz, CDCl3, Figure S5, Supporting Information): δ 7.3
7.81 (b, 11H, aromatic He-Hp), 5.25
5.35 (b, 1H, Hq), 1.8
2.2 (br, 4H, Ha), 0.81
1.58 (b, 30H, Hb-Hd). Mn = 20 000, Mw = 26 000 and Mw/Mn = 1.3 (GPC). Tg = 104 °C (DSC). Instrumentation and Sample Preparation. 1H NMR spectra were recorded with a Varian VXR-300 MHz FT-NMR instrument. Tetramethylsilane was used as internal standard. A VG Quattro GC/MS/MS/DS instrument was used to determine the molar mass of the organic molecules. The sample was charged into the rapidly moving gas and converted into the corresponding ions, which were further separated based on their mass-to-charge ratios (m/z). Molecular weight and molecular weight distribution were determined at 40 °C from GPC, using a Waters 510 HPLC organic equipped with three Ultrastyragel columns (100, 500, and 1000 Å) connected in series and a 410 differential refractometer and an UV detector. Polymer solution was eluted by THF solvent with a flow rate of 0.6 mL/min. The melting point (Tm) of the organic molecules and the glass transition temperatures (Tg) of polymers were obtained from a TA Q-20 DSC calorimeter with a scan rate of 20 °C/min. PL emission spectra were obtained from a LabGuide X350 fluorescence spectrophotometer using a 450 W Xe lamp as the continuous light source. UV
vis absorption spectra were recorded with an Ocean Optics DT 1000 CE 376 spectrophotometer. Quartz cell with dimensions of 0.2  1.0  4.5 cm3 was used for the UV
vis absorption and PL emission spectra measurements. Stock solutions of the organic and polymeric fluorophores with a concentration of 10
4 M in chloroform were primarily prepared. Aliquots of these stock solutions were transferred to 10 mL volumetric flasks, into which appropriate volumes of chloroform and methanol were added dropwise under vigorous stirring to furnish 10
5 M solutions with different methanol contents (0
90 vol %). UV
vis and PL emission

ARTICLE

spectroscopy were immediately performed once the solutions were prepared. Solid samples were prepared by drop-casting sample solutions (10
3 M in chloroform) over quartz plates. Fluorescence quantum yields (ΦF) of the FN and PFN in the solution mixtures (10
5 M) with varied compositions were determined by comparison with a quinine sulfate standard (10
5 M in 0.1 N H2SO4). Integrating sphere was used for film sample. FT-IR spectra were obtained on a Nicolet IR-200 spectrometer. Sample solution was dropped on a KBr pellet and dried at 100 °C under vacuum to prepare the solid film for FT-IR analysis. Computer simulation was formulated from the Materials Studio (MS) commercial software of Accelrys Inc.

’ RESULTS AND DISCUSSION Synthesis. As illustrated in Scheme 1, the organic molecule FN and the copolymer PFN were synthesized from the Suzuki coupling reaction between fluorene-based mono- (or di-) boronic esters of compound 4 (or 5) and 1,6-dibromonaphthol (DN). To have compound 4 (or 5), the starting fluorene was subjected to a three-step synthetic process including dioctylation of fluorene to result in compound 1, mono- (or di-) bromination of compound 1 to obtain compound 2 (or 3) and the following boronization to yield the desired boronic esters of 4 (or 5). The intermediates and the organic FN and polymeric PFN products were carefully identified by 1H NMR (Figure S1
S5, Supporting Information), mass spectroscopy, and elemental analysis (Experimental Section). The resulting PFN polymer has Mn = 20 000, Mw = 26 000, and Mw /Mn = 1.3 from the GPC analysis (Figure S6, Supporting Information). As indicated in Scheme 1, PFN chain contains both the fluorene-1- and fluorene-6napnthol dimeric units, which constitutes the enormous possibilities of the chain sequences and the irregular chain structures. Besides the irregular chain structure, the kinked 1,6-naphthol unit and the long 9,9-dioctyl side chain introduce large interchain space allowed polymer chain to flow at relatively low temperature (a low Tg at 104 °C was detected from the DSC scan; Figure S7, Supporting Information). The factors contributing to the low Tg also render the PFN polymer for easy dissolution in common organic solvents such as methylene chloride, chloroform, tetrahydrofuran, toluene, etc. Nevertheless, the large portions of hydrophobic aromatic rings also make PFN insoluble in polar solvents such as dimethyl sulfoxide, N,N-dimethylforamide, and methanol. Chloroform and methanol were therefore chosen as the respective good and poor solvents to study the AIEE effect. Characterization of AIE or AIEE Effects by Solution Emission Spectra. Characterization of AIE (or AIEE) effect was primarily conducted on the small-molecule compound of FN. For all dilute (10
5 M) solutions of FN in chloroform/methanol mixtures, the UV
vis absorption spectra (Figure S8A, Supporting Information) contain one broad band at 339 nm. A level-off longwavelength (>350 nm) tail due to Mie scattering was observed in solutions with high (>60 vol %) methanol content. Aggregate formations are responsible for the Mie scattering. The solution emission spectra of FN (Figure 1A) showed complex response toward methanol inclusion. Solution of FN in pure chloroform showed no fluorescent emission, indicating the absence of aggregated form to exert the AIE-operative emission. With the inclusion of methanol (70 vol %) in the mixtures, a broad band at ∼408 nm was observed instead. Meanwhile, the long-wavelength shoulder became visible 12360

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C

ARTICLE

Figure 1. (A) Solution (10
5 M) fluorescence emission spectra of FN in chloroform/methanol solutions with different volume fractions of methanol (excited at 350 nm) and (B) solution quantum yield (ΦF) in relationship to the volume fraction of methanol.

Figure 2. (A) Solution (10
5 M) fluorescence emission spectra of PFN in chloroform/methanol solutions with different volume fractions of methanol (excited at 350 nm) and (B) solution quantum yield (ΦF) in relationship to the volume fraction of methanol.

and progressively developed its contribution with increasing methanol content. This complex emission behavior is in line with the sol
gel H-bonded systems,51,52 and the dual emissions at 397 and 411 nm are due to the respective monomer and J-aggregate emissions whereas the shoulderlike emission is attributed, considering the bathochromic shift, to the formation of large aggregates. In accord with the AIE behavior, the increasing aggregation with methanol inclusion resulted in the observed emission enhancement. The enhanced emission can be verified by the increasing solution quantum yields (ΦF, Figure 1B) from 0.02 (in pure chloroform) to 0.249 (in 90 vol % methanol solution). The UV
vis absorption spectra (Figure S8B, Supporting Information) of PFN contain one broad band centered at the similar position (∼338 nm) as that of the FN solution. Unlike the small-mass FN molecules, solution of PFN in pure chloroform (Figure 2A) already emitted moderately, which suggests the existence of aggregated structures for the PFN dilute solution. The emission spectra of PFN in chloroform/methanol mixtures contain a broad emission band besides the shoulderlike

aggregation emission in the long-wavelength region. The broad emission band may consist of several overlapped emission peaks in considering the large number of the structural possibilities due to the irregular chain sequences of PFN. Similar to FN solution, the long-wavelength aggregation band red-shifted with increasing methanol content in the solution mixtures. In accord to the AIEE effect, the increase of aggregate emission was accompanied by the emission intensification. This emission enhancement is correlated with the increase of ΦFs from 0.23 (in pure chloroform) to 0.45 (in 90 vol % methanol solution) as shown in Figure 2B. Comparatively, the polymer solutions have higher emission efficiency compared to FN solutions under the same experimental condition. The dimensions of the nanoaggregates formed were determined from dynamic light scattering. The result summarized in Figure 3 indicates that the detected average hydrodynamic diameters (Dhs) decrease from 300 to 220 nm from the 0 to 90 vol % methanol solution mixtures. The suspended nanoparticles tended are therefore shrunk in mixtures with increasing 12361

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C methanol content, which is reasonable in considering that shrinkage of the hydrophobic polymer aggregates reduces the unfavorable contacts with the poor solvent of methanol. The volume shrinkage also forces the involved aromatic fluorophores to pack in more coplanar arrangement, which enhances the π-stacking interactions among the aromatic rings and results in the enhanced conjugation and the red shift of the long-wavelength aggregation emissions with increasing methanol inclusion. The steric crowdedness imposed by the volume shrinkage also imposes restraint for the aromatic rings of the fluorophores to rotate freely; in other words, the restrictions on the intramolecular rotation in the shrunk nanoaggregates are largely promoted to the extent that efficiently block the nonradiative decay channel and lead to the observed emission enhancements observed in Figure 2A. Film Emission in Relationship to H-Bond. Solid FN and PFN films cast from chloroform solution were subjected to further emission investigations. The resultant FN and PFN films have the respective ΦF values of 0.43 and 0.61, which are comparatively higher than their solution analogues. Higher

Figure 3. Hydrodynamic diameter of PFN (10
5 M) solution in chloroform/methanol mixtures with different volume fractions of methanol.

ARTICLE

extent of aggregation in the solid samples therefore promotes the solid emission compared to solutions with lower degree of aggregation. Emission spectra of solid FN and PFN samples at different temperatures are summarized in Figure 4, A and B, respectively. Comparatively, emissions of PFN film are in the longer wavelength regions compared to emissions of FN film. Both the FN and PFN films at room temperature contain the long-wavelength shoulder emissions in association with the aggregations in the solid samples. With increasing temperatures, the decreasing contributions of the aggregation emissions are in line with the corresponding reductions on the total emission intensity. For organic FN compound, the observed flat band (Figure 4A) is due to the overlapped emission bands, which will be discussed later. Molecular associations through the H-bond interactions should promote the tendency to aggregate and therefore enhance the emission from the AIE or AIEE effect. The hydroxyl
OH stretching bands in the infrared spectra can be used to probe for the extent of the H-bond interactions and to correlate with the emission variations under different experimental conditions. Infrared hydroxyl stretching bands of FN and PFN in the temperature ranges of 25
200 °C are summarized in Figure 5, A and B, respectively. The results in Figure 5 clearly show that the broad H-bonded OH stretching bands between 3500 and 3100 cm
1 decreased progressively with increasing temperature and, in contrast, the sharp free OH stretching peaks at ∼3540 cm
1 persisted for both samples after heating at high temperatures. The dissociations of the H-bonds are in line with the decrease of AIE or AIEE-derived fluorescence observed in Figure 4. The chain arrangements in the solid specimen are supposed to be reminiscent of the chain conformations in the precursor solution and the solution aggregated states. With this aspect, films from different preparative solution states may have varied chain arrangements to exhibit different fluorescent emission behavior. Therefore, solid films prepared from pure CHCl3 and from the 80 vol % methanol solutions (10
3 M) were primarily prepared and compared with the sheared film, which was prepared by applying unidirectional shearing forces during solvent evaporation steps on the dilute (10
3 M) solutions of FN and PFN in chloroform. Under external shearing forces, the molecular arrangements in the mobile solution state can be

Figure 4. Fluorescent emission spectra of (A) FN and (B) PFN films at different temperatures (excited at 350 nm). 12362

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C

ARTICLE

Figure 5. Hydroxyl
OH stretching bands of (A) FN and (B) PFN films at different temperatures.

Figure 6. Fluorescent emission spectra and hydroxyl stretching bands (insets) of (A) the FN and (B) the PFN films prepared from different preparative solution states (excited at 350 nm).

altered and so are the involved H-bonds and the related AIEEoperative emission. The PL emission and the FTIR spectra of the FN and PFN films prepared from different solution states are compared in Figure 6, A and B, respectively. In contrast to the flat emission band pattern observed for the FN film (as P-film) (Figure 6A) from the pure chloroform solution, films from the 80 vol % methanol solution (as M-film) and from the sheared solution state (as S-film) showed the distinct monomer and J-aggregate emissions at 394 and 409 nm, respectively. The fraction of the aggregation emission shoulder is in line with the resolved emission intensity, which is in the order of M-film > S-film > P-film. The relative order of the emission intensity can be further verified from the corresponding ΦF values of the M-film (=0.67), the sheared S-film (=0.54), and the P-film (=0.43). Intermolecular H-bonds should promote molecular aggregation and are considered to be beneficial for the AIE- or AIEEoriented emission. To support this proposition, the hydroxyl
OH stretching bands (inset in Figure 6A) of the solid films

were subjected to the deconvolution procedures to separate the free OH (indicated by the dashed line) from the broad H-bonded OH bands. The result suggests that a high fraction (19%) of free OH for the P-film when compared to the S-film (7%) and to the M-film (3%). High fractions of H-bonded OH groups in the Mand the S-films thus promote the molecular aggregations and lead to emission enhancement due to AIEE effect. The corresponding UV
vis spectra (Figure S9A, Supporting Information) indicate the bathochromic shifts of the absorption peaks from 348 nm for the P-film to 356 nm for the S-film and then to 358 nm for the M-film. Unidirectional shearing on the solution mobile state forces FN molecules to pack in more compact and coplanar arrangements. After solvent evaporation, the more coplanar geometries of the involved fluorophores are locked in the solid state and result in the observed red shift of the absorption band. As judged from the relative fraction of the H-bonded OH groups and from the absorption maxima, the nanoaggregates formed in the 90 vol % methanol solution seem to secure more compact molecular packing in the M-film as 12363

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C compared to the sheared S-film. The compact arrangement in the M-film resulted in the higher extent of aggregation and emission efficiency as compared to the S-film. Nanoaggregate formation and shearing caused the same effect on the PFN sample as the corresponding emission spectra; the hydroxyl stretching (Figure 6B) and the UV
vis absorption

Figure 7. Simulated molecular arrangements of FN dimer associated with intermolecular H-bonded
OH functions.

ARTICLE

(Figure S9B, Supporting Information) spectra of the P-, the sheared S-, and the M-PFN films were inspected. Effect of preparation procedures on the emission efficiency can be emphasized from the corresponding ΦF values (0.61 for the P-, 0.77 for the S-, and 0.85 for the M-PFN films). It is noted at this point that the emission spectra of the sheared S- and the M-PFN films are composed of overlapped bands centered at the respective positions of 433 and 445 nm. It is difficult to assign origins of the bands due to the complicated chain structures originating from the random 1,6-links; further discussion will be given later in this study. The nanoparticles formed in the 80 vol % methanol solution are supposed to be small in size (as the DLS result in Figure 3 suggested). The PFN chain segments in the small nanoparticles are supposed to pack intimately, which in turn ensures the close contacts of the involved fluorophores after solvent removal and results in the enhanced emission due to the extensive aggregations. Analogous to the FN system, nanoaggregates formed from the 90 vol % methanol solution have higher degree of H-bond association (hence, extent of aggregation) than that in the sheared film. The M-PFN film therefore possesses a high ΦF value of 0.85. H-Bond and Hindered Rotation. It is envisaged that the H-bonds in the aggregates add difficulty for molecular rotations and enhance emission intensity by blocking the nonradiative decay channels. The concept can be best illustrated by the FN dimer associated with the interreacting H-bonded hydroxyl

Figure 8. Simulated molecular conformers of two heptamers of the same chain sequence with the preferred H-bonded interactions (1 and 6 refer to the position of naphthol attached to the neighboring fluorene ring). 12364

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C groups. The energy minimization on the FN dimer was conducted using the Molecular Studio (MS) software, and the resultant molecular conformation of the FN dimer in Figure 7 shows an oblique head-to-tail arrangement, which is supposed to be responsible for the observed J-aggregate emission of FN molecules. Here, the two FN molecules of the dimer are associated to each other by the H-bonded hydroxyl groups with an
O
H---O
distance of 3.19 Å. The four fluorene rings in the dimer are actually tilted away from the individually linked naphthol rings due to the inherent 1,6linkage. With the bended chain and the dioctyl substituents, the fluorene rings of the dimer are supposed to be locked firmly by the surrounding entities when the dimers are situated within the closely packed aggregated domains. The restricted molecular rotations of the fluorene rings in the aggregates enhance the emission due to the blockage of the nonradiative decay channels. Situations in polymeric PFN are complicated due to the irregular 1,6-linkage in the polymer chain. To simplify the situation, only heptamer sets with identical chain sequence are discussed here (possible combined sets for heptamers with different chain sequences are too enormous) and were subjected to energy minimization by MS. Altogether, nine possible heptamer sets with their constituent chain sequence symbolized by either 1F or 6F (as 1-naphtholfluorene or 6-naphtholfluorene) are possible, and the resultant interchain arrangements after energy minimizations were summarized in Figure 8. All nine interchain arrangements are held by four H-bonding pairs with the formulated
O
H---O
distances included in Figure 8. Here, the H-bonds with intermolecular distances larger than 4 Å are considered to be less effective and are prevalent in certain arrangements (e.g., 1F6F6F6F). In any case, all heptamer chains in Figure 8 are bent with all kinds of conformations responsible for the overlapped emission bands observed in the PFN polymer. The bent conformers are supposed to block the interchain approaches from closer contacts and result in large interchain distances among all arrangements in Figure 8. Upon applying the external shearing force, the large interchain distances allow spaces for the conformational changes from the bent to the more aligned chains, which promote aggregations through interchain H-bonds and result in emission enhancements. This conformational change may transform the originally ineffective H-bonds into more intimately bonded ones.

’ CONCLUSIONS Small-molecule FN and polymer PFN with alternative fluorene
naphthol units were successfully synthesized by the facile Suzuki coupling reaction. The resultant FN and PFN all show the typical AIE and AIEE behavior in chloroform/methanol solution mixtures, respectively. The solid FN and PFN films have their AIEEoperative emission behavior related to the fractions of the H-bonded hydroxyl groups. The decrease of the H-bonded hydroxyl groups at higher temperature reduced the corresponding emission intensity. In contrast, procedures leading to enhanced H-bonding (such as shearing or extensive aggregation by adding methanol solvent in the solution preparative step) led to the enhanced emission for the resultant FN and PFN films. ’ ASSOCIATED CONTENT

bS

Supporting Information. All supporting data including the sample characterizations (1H NMR, DSC, and GPC) and the absorption spectra of FN and PFN solution mixtures and solid

ARTICLE

films from different preparative methods. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: þ886-7-5252000, ext 4065.

’ ACKNOWLEDGMENT We appreciate the financial support from the National Science Council, Taiwan, Republic of China, under Contract No. NSC 98-2221-E-110-005-MY2. ’ REFERENCES (1) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. Adv. Mater. 2000, 12, 1737–1750. (2) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867–2873. (3) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 188, 7416–7417. (4) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv. Mater. 1997, 9, 798–802. (5) Kreyenschmidt, M.; Klaerner, G.; Fuhrer, T.; Ashenhurst, J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller, R. D. Macromolecules 1998, 31, 1099–1103. (6) Chen, S. A.; Lu, H. H.; Huang, C. W. Adv. Polym. Sci. 2008, 212, 49–84. (7) Gong, X.; Iyer, P. K.; Moses, D.; Bazan, G. C.; Heeger, A. J.; Xiao, S. S. Adv. Funct. Mater. 2003, 13, 325–330. (8) Teetsov, J.; Fox, M. A. J. Mater. Chem. 1999, 9, 2117–2122. (9) Kl€arner, G.; Davey, M. H.; Chen, W.-D.; Scott, J. C.; Miller, R. D. Adv. Mater. 1998, 10, 993–997. (10) Xia, C.; Advincula, R. C. Macromolecules 2001, 34, 5854–5859. (11) Yu, W. L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater. 2000, 12, 828–831. (12) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; M€ullen, K.; Meghdadi, F.; List, E. J.; Leising, G. J. Am. Chem. Soc. 2001, 123, 946–953. (13) Marsitzky, D.; Vestberg, R.; Blainey, P.; Tang, B. T.; Hawker, C. J.; Carter, K. R. J. Am. Chem. Soc. 2001, 123, 6965–6972. (14) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740–1741. (15) Tang, B. Z.; Zhan, X.; Yu, G.; Lee, P. P. S.; Liu, Y.; Zhu, D. J. Mater. Chem. 2001, 11, 2974–2978. (16) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332–4353. (17) Bhongale, C. J.; Hsu, C. S. Angew. Chem., Int. Ed. 2006, 45, 1404–1408. (18) Tong, H.; Hong, Y.; Dong, Y.; H€aussler, M.; Lam, J. W. Y.; Guo, Z.; Li, Z.; Guo, Z.; Tang, B. Z. Chem. Commun. 2006, 3705–3707. (19) Tong, H.; Dong, Y.; Hong, Y.; H€aussler, M.; Lam, J. W. Y.; Sung, H. H.-Y.; Yu, X.; Sun, J.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. C 2007, 111, 2287–2294. (20) Dong, S.; Li, Z.; Qin, J. J. Phys. Chem. B 2009, 113, 434–441. (21) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410–14415. (22) Wang, Z.; Shao, H.; Ye, J.; Tang, L.; Lu, P. J. Phys. Chem. B 2005, 109, 19627–19633. (23) Tong, H.; Hong, Y.; Dong, Y.; Ren, Y.; H€aussler, M.; Lam, J. W. Y.; Wong, K. S.; Tang, B. Z. J. Phys. Chem. B 2007, 111, 2000–2007. (24) Chen, J.; Xu, B.; Quyang, X.; Tang, B. Z.; Cao, Y. J. Phys, Chem. A 2004, 108, 7522–7526. (25) Yuan, C. X.; Tao, X. T.; Wang, L.; Yang, J. X.; Jiang, M. H. J. Phys. Chem. C 2009, 113, 6809–6814. (26) Liu, Y.; Tao, X.; Wang, F.; Dang, X.; Zou, D.; Ren, Y.; Jiang, M. J. Phys. Chem. C 2008, 112, 3975–3981. 12365

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366

The Journal of Physical Chemistry C (27) Zhao, Z.; Chen, S.; Shen, X.; Mahtab, F.; Yu, Y.; Lu, P.; Lam, J. W. Y.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2010, 46, 686–688. (28) Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.; Hong, Y.; Ji, L.; Zhu, Z.; Jim, C. K. W.; Yu, G.; Li, Q.; Li, Z.; Liu, Y.; Qin, J.; Tang, B. Z. Chem. Commun. 2007, 70–72. (29) Li, Y.; Li, F.; Zhang, H.; Xie, Z.; Xie, W.; Xu, H.; Li, B.; Shen, F.; Ye, L.; Hanif, M.; Ma, D.; Ma, Y. Chem. Commun. 2007, 231–233. (30) Qian, L.; Tong, B.; Shen, J.; Shi, J.; Zhi, J.; Dong, Y.; Yang, F.; Dong, Y.; Lam, J. W. Y.; Liu, Y.; Tang, B. Z. J. Phys. Chem. B 2009, 113, 9098–9103. (31) Yang, Z.; Chi, Z.; Yu, T.; Zhang, X.; Chen, M.; Xu, B.; Liu, S.; Zhang, Y.; Xu, J. J. Mater. Chem. 2009, 19, 5541–5546. (32) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Adv. Mater. 2010, 22, 1–5. (33) Xu, B.; Chi, Z.; Yang, Z.; Chen, J.; Deng, S.; Li, H.; Li, X.; Zhang, Y.; Xu, N.; Xu, J. J. Mater. Chem. 2010, 20, 4135–4141. (34) Zhao, Z.; Chen, S.; Lam, J. W. Y.; Jim, C. K. W.; Chan, C. Y. K.; Wang, Z.; Lu, P.; Deng, C.; Kwok, H. S.; Ma, Y.; Tang, B. Z. J. Phys. Chem. C 2010, 114, 7963–7972. (35) Zhao, Z.; Chen, S.; Lam, J. W. Y.; Lu, P.; Zhong, Y.; Wong, K. S.; Kwok, H. S.; Tang, B. Z. Chem. Commun. 2010, 46, 2221–2223. (36) Wang, W.; Lin, T.; Wang, M.; Liu, T. X.; Ren, L.; Chen, D.; Huang, S. J. Phys. Chem. B 2010, 114, 5983–5988. (37) Li, Z.; Dong, Y.; Mi, B.; Tang, Y.; H€aussler, M.; Tong, H.; Dong, Y.; Lam, J. W. Y.; Ren, Y.; Sung, H. H. Y.; Wong, K. S.; Gao, P.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. B 2005, 109, 10061–10066. (38) Kokado, K.; Chujo, Y. Macromolecules 2009, 42, 1418–1420. (39) Qin, A.; Jim, C. K. W.; Tang, Y.; Lam, J. W. Y.; Liu, J.; Mahtab, F.; Gao, P.; Tang, B. Z. J. Phys. Chem. B 2008, 112, 9281–9288. (40) Pucci, A.; Rausa, R.; Ciardelli, F. Macromol. Chem. Phys. 2008, 209, 900–906. (41) Li, Y.; Vamvounis, G.; Holdcroft, S. Macromolecules 2002, 35, 6900–6906. (42) Liu, J.; Lam, J. W. Y.; Tang, B. Z. J. Inorg. Orangomet. Polym. 2009, 19, 249–285. (43) Lai, C. T.; Hong, J. L. J. Phys. Chem. C 2009, 113, 18578–18583. (44) Liu, J.; Zhong, Y.; Lam, J. W. Y.; Lu, P.; Hong, Y.; Yu, Y.; Yue, Y.; Faisal, M.; Sung, H. H. Y.; Williams, I. D.; Wong, K. S.; Tang, B. Z. Macromolecules 2010, 43, 4921–4936. (45) Qin, A.; Lam, J. W. Y.; Tang, L.; Jim, C. K. W.; Zhao, H.; Sun, J.; Tang, B. Z. Macromolecules 2009, 42, 1421–1424. (46) Lai, C. T.; Hong, J. L. J. Phys. Chem. B 2010, 114, 10302–10310. (47) Chou, C. A.; Chien, R. H.; Lai, C. T.; Hong, J. L. Chem. Phys. Lett. 2010, 501, 80–86. (48) Chien, R. H.; Lai, C. T.; Hong, J. L. J. Phys. Chem. C 2011, 115, 5958–5965. (49) Liu, Y.; Tao, X.; Wang, F.; Shi, J.; Sun, J.; Yu, W.; Ren, Y.; Zou, D.; Jiang, M. J. Phys. Chem. C 2007, 111, 6544–6549. (50) Liu, T.; Tao, X.; Wang, F.; Dang, X.; Zou, D.; Ren, Y.; Jiang, M. J. Phys. Chem. C 2008, 112, 3975–3981. (51) Xue, P.; Lu, R.; Chen, G.; Zhang, Y.; Nomoto, H.; Takafuji, M.; Ihara, H. Chem.—Eur. J. 2007, 13, 8231–8239. (52) Chen, P.; Lu, R.; Xue, P.; Xu, T.; Chen, G.; Zhao, Y. Langmuir 2009, 25, 8395–8399. (53) Camerel, F.; Bonardi, L.; Schmutz, M.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 4548–4549. (54) Zhang, P.; Wang, H.; Liu, H.; Li, M. Langmuir 2010, 26, 10183– 10190. (55) Zhou, T.; Li, F.; Fan, Y.; Song, W.; Mu, X.; Zhang, H.; Wang, Y. Chem. Commun. 2009, 3199–3201. (56) Fang, H. H.; Chen, Q. D.; Yang, J.; Xia, H.; Gao, B. R.; Feng, J.; Ma, Y. G.; Sun, H. B. J. Phys. Chem. C 2010, 114, 11958–11961. (57) Yang, Z.; Chi, Z.; Xu, B.; Li, H.; Zhang, X.; Li, X.; Liu, S.; Zhang, Y.; Xu, J. J. Mater. Chem. 2010, 20, 7352–7359. (58) Lamere, J. F.; Saffon, N.; Santos, I. D.; Fery-Forgues, S. Langmuir 2010, 26, 10210–10217. (59) Li, H.; Chi, Z.; Xu, B.; Zhang, X.; Yang, Z.; Li, X.; Liu, S.; Zhang, Y.; Xu, J. J. Mater. Chem. 2010, 20, 6103–6110.

ARTICLE

(60) Dong, S.; Li, Z.; Qin, J. J. Phys. Chem. B 2009, 113, 434–441. (61) Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.; Hong, Y.; Ji, L.; Zhu, Z.; Jim, C.; Yu, G. Chem. Commun. 2007, 70–72. (62) Brandsma, L.; Vasilevsky, S. F.; Verkruijsse, H. D. Application of Transition Metal Catalysts in Organic Synthesis, corrected ed.; Springer: Berlin, 1999; p 5. (63) Yang, R.; Tian, R.; Hou, Q.; Yang, W.; Cao, Y. Macromolecules 2003, 36, 7453–7460.

12366

dx.doi.org/10.1021/jp203261s |J. Phys. Chem. C 2011, 115, 12358–12366