Conjugated Polyelectrolyte Dendrimers: Aggregation, Photophysics

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Conjugated Polyelectrolyte Dendrimers: Aggregation, Photophysics, and Amplified Quenching Fude Feng,† Seoung Ho Lee,† Sung Won Cho,† Sevnur Kömürlü,† Tracy D. McCarley,‡ Adrian Roitberg,† Valeria D. Kleiman,† and Kirk S. Schanze*,† †

Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-2804, United States

‡

S Supporting Information *

ABSTRACT: Conjugated polyelectrolyte dendrimers (CPDs) are monodisperse macromolecules that feature a fully πconjugated dendrimer core surrounded on the periphery by ionic solubilizing groups. CPDs are soluble in water and polar organic solvents, and they exhibit photophysics characteristic of the π-conjugated chromophores comprising the dendrimer core. Here we describe the synthesis and photophysical characterization of series of three generations of CPDs based on a phenylene ethynylene repeat unit structure that is surrounded by an array of anionic sodium carboxylate groups. Molecular dynamics simulations indicate that the first-generation CPD is flat while the second- and third-generation CPDs adopt oblate structures. Photophysical studies, including absorption, fluorescence spectroscopy, and lifetimes, show that the ester protected precursor dendrimers exhibit highly efficient blue fluorescence in THF solution emanating from the phenylene ethynylene chromophore that is in the dendrimer core. By contrast, the water-soluble CPDs have much lower fluorescence quantum yields and the absorption and fluorescence spectra exhibit features of strong chromophore−chromophore interactions. The results are interpreted as suggesting that the CPDs exist as dimer or multimer aggregates, even in very dilute solution. Fluorescence quenching of the anionic CPDs with the dication electron acceptor N,N′-dimethylviologen (MV2+) is very efficient, with Stern− Volmer quenching constants (KSV) increasing with generation number. The third-generation CPD exhibits highly efficient amplified quenching, with KSV ∼ 5 × 106 M−1.

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INTRODUCTION Dendrimers are well-defined macromolecules that have a branched architecture containing a central core and a large number of terminal substituents.1−4 Fully π-conjugated dendrimers which function as “molecular photonic antenna” to harvest and transport excitation energy are receiving significant attention due to their unique photochemical, photophysical, and electrochemical properties.1,5−9 Moore and co-workers carried out pioneering work on fully πconjugated dendrimers based on the phenylene ethynylene skeleton.10−13 They designed and synthesized dendritic structures optimized for vectorial energy transfer and studied exciton migration from the periphery to the core by incorporating a perylene unit as a fluorescent trap.13,14 This work shows that the fully conjugated phenylene ethynylene systems feature localized excitonic states that undergo ultrafast © 2012 American Chemical Society

energy transfer driven by an energy gradient to a core acceptor unit.15−17 Here we report an investigation of a series of conjugated polyelectrolyte dendrimers (CPDs). These monodisperse dendritic structures contain a fully π-conjugated phenylene ethynylene skeleton functionalized on the periphery with a large number of anionic groups which render the macromolecules soluble in polar solvents such as water and methanol. This work is part of a larger program that has examined the photophysics of conjugated polyelectrolytes (CPEs), which feature a π-conjugated backbone substituted with ionic solubilizing groups.18−21 CPEs exhibit “amplified quenching” wherein an ionic fluorescence quencher induces highly efficient Received: September 10, 2012 Published: September 12, 2012 16679

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solid (32 g, 90%) 1H NMR (300 MHz, CDCl3), δ (ppm): 6.45 (s, 1H), 3.87 (s, 2H), 2.15 (m, 6H), 1.93 (m, 6H), 1.36 (s, 27H). 13C NMR (75 MHz, CDCl3), δ (ppm): 172.5, 165.2, 80.8, 60.5, 58.1, 42.5, 34.5, 29.8, 28.2. LC-MS m/z (M+ + Na), 514.1. Compound 2. To a solution of compound 1 (24.6 g, 50 mmol) in acetonitrile (300 mL) was added 4-iodophenol (11.0 g, 50 mmol). The mixture was stirred under argon for 30 min, followed by addition of anhydrous potassium carbonate (6.9 g, 50 mmol) and potassium iodide (1.7 g, 10 mmol). After reacting overnight at 80 °C, the mixture was cooled to room temperature. After removal of solvents under vacuum, the residue was extracted with ethyl acetate/water. The organic layer was washed with water and brine and then dried over anhydrous sodium sulfate. The solvent was removed to give compound 2 as white solid (30.0 g, 90%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.61 (d, 2H, J = 9.0 Hz), 6.75 (d, 2H, J = 9.0 Hz), 6.56 (s, 1H), 4.36 (s, 2H), 2.20 (t, 6H, J = 7.4 Hz), 2.03 (t, 6H, J = 7.4 Hz), 1.43 (s, 27H). 13C NMR (75 MHz, CDCl3), δ (ppm): 172.68, 167.10, 157.18, 138.72, 117.22, 84.64, 80.89, 67.57, 57.86, 30.10, 29.81, 28.28. LC-MS m/z (M+ + Na), 698.1. Compound 6. Compound 3 (7.0 g, 20 mmol) was dissolved in diisopropylamine (100 mL) and combined with Pd(PPh3)2Cl2 (28 mg, 0.4 mmol, 2 mol %) and CuI (15 mg, 0.8 mmol, 4 mol %). The mixture was degassed and warmed to 60 °C, and then a degassed solution of 5 (3.1 g, 10 mmol) in THF (20 mL) was added dropwise. The reaction mixture was cooled to room temperature after reaction overnight at 60 °C. The solvent was removed, and residue was extracted with dichloromethane/water. The organic layer was washed with saturated ammonium chloride, water, and brine and then dried over anhydrous sodium sulfate. The solvent was removed, and the crude product was purified by silica chromatography (hexane) to give compound 6 as light yellow solid (5.1 g, 62%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.56−7.54 (m, 9H), 1.14 (s, 21H), 0.25 (s, 9H). 13 C NMR (75 MHz, CDCl3), δ (ppm): 135.31, 135.02, 134.82, 134.24, 124.58, 124.06, 123.68, 123.43, 103.27, 96.07, 89.07, 88.87, 18.91, 11.52, 0.09. MS (ESI) m/z (M+), 842.3999, calcd 842.4011. Compound 7. A mixture of compound 6 (4.2 g, 5 mmol) and ground anhydrous potassium carbonate (5.52 g, 40 mmol) in dichloromethane/methanol (100 mL/100 mL) was stirred at room temperature overnight. After removal of potassium carbonate and solvent, the residue dissolved in dichloromethane was washed with water and brine to give compound 7 as light yellow solid (2.0 g, 71%). 1 H NMR (300 MHz, CDCl3), δ (ppm): 7.60−7.57 (t, 9H), 3.12 (s, 4H), 1.14(s, 21H). 13C NMR (75 MHz, CDCl3), δ (ppm): 136.03, 135.80, 135.59, 134.75, 125.08, 124.15, 123.98, 123.64, 105.55, 93.41, 89.61, 89.20, 82.32, 79.33, 19.30, 11.91. MS (ESI) m/z (M+ − 4H), 549.2249, calcd 554.2430. Compound DW1. Compound 2 (13.5 g, 20 mmol) was dissolved in a mixture of THF (50 mL) and isopropylamine (150 mL) and combined with Pd(PPh3)2Cl2 (28 mg, 0.4 mmol, 2 mol %) and CuI (15 mg, 0.8 mmol, 4 mol %). After thoroughly degassing the solution by bubbling argon, trimethylsilylacetylene (2.9 g, 30 mmol) was added, and the reaction mixture was stirred overnight at room temperature. The solvent was removed, and the residue was extracted with dichloromethane/water. The organic layer was washed with saturated ammonium chloride, water, and brine and then dried over anhydrous sodium sulfate. The solvent was removed, and the crude product was purified by silica chromatography (hexane/ethyl acetate, 8/1) to give compound DW1 as white solid (12.5 g, 97%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.45 (d, 2H, J = 8.9 Hz), 6.89 (d, 2H, J = 8.9 Hz), 6.51 (s, 1H), 2.20 (t, 6H, J = 7.4 Hz), 1.97 (t, 6H, J = 7.4 Hz), 1.43 (s, 27H), 0.24 (s, 9H). 13C NMR (75 MHz, CDCl3), δ (ppm): 172.69, 167.18, 157.22, 133.89, 114.74, 80.89, 67.46, 57.84, 30.09, 29.79, 28.28, 0.21. LC-MS m/z (M+ + Na), 668.2. Compound DW2. Compound 2 (0.54 g, 0.8 mmol) was dissolved in a mixture of THF (5 mL) and isopropylamine (20 mL) and combined with Pd(PPh3)2Cl2 (2.7 mg, 0.004 mmol, 5 mol %) and CuI (1.5 mg, 0.008 mmol, 10 mol %). After thoroughly degassing the solution by bubbling argon, a degassed solution of compound 5 (0.1 g, 0.33 mmol) in THF (2 mL) was added, and the reaction mixture was stirred overnight at room temperature. The solvent was removed, and

quenching with Stern−Volmer constants (K SV ) >10 6 M−1.19,22,23 Amplified quenching is believed to arise due to ion-pairing between the CPE and the quencher ion, combined with exciton delocalization and transport in the conjugated backbone.19,24 While amplified quenching in CPE/quencher ion systems is well-documented, open questions remain regarding the underlying mechanism and its relationship to CPE structure and state of aggregation.25 In an effort to provide more insight into the structure−property relationship for amplified quenching, we have designed conjugated polyelectrolyte dendrimers, with an aim to explore the quenching effect in structurally well-defined (monodisperse) polyelectrolyte systems. In particular, at the outset we expected that CPDs would be less prone to aggregate than CPEs, especially in very dilute solution, making it possible to study amplified quenching when it occurs in well-defined, molecularly dissolved conjugated polyelectrolyte systems. As outlined in this report, photophysical study of the set of three CPDs reveals that despite their monodisperse nature, they display complex photophysics, indicative of considerable interchromophore interactions, especially in aqueous solution. Molecular structure simulations, coupled with detailed photophysical experiments including absorption, fluorescence emission and excitation, and lifetime spectroscopy, provide insight into the origin of the interchromophore interactions. In particular, the first-generation CPD is relatively flat in structure, and it displays the greatest tendency to form multimer (aggregate) structures in aqueous solution. By contrast, secondand third-generation CPDs feature oblate structures, and consequently they are less prone to aggregate. Nonetheless, in the higher generation dendrimers photophysical results reveal considerable interchromophore interaction, and the results are interpreted as arising from the existence of dimer or multimer structures as well as due to interactions of discrete π-conjugated chromophore units within individual dendrimer molecules. Despite these complications, the dendrimers exhibit amplified quenching when treated with the cationic electron acceptor quencher ion methyl viologen (MV2+). The KSV values increase with generation number, with the third-generation CPD exhibiting quenching that is as efficient as seen in πconjugated polyelectrolyte systems.19,20

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EXPERIMENTAL SECTION

General Information and Materials. UV absorption spectra were measured on a Shimadzu UV-1800 spectrophotometer. Luminescence spectra were measured on a PTI (Photon Technology International) fluorescence spectrometer. Fluorescence lifetimes were determined by time-correlated single photon counting on a FluoTime 100 spectometer (PicoQuant) equipped with 370 nm diode laser as excitation source. Dynamic light scattering (DLS) spectroscopy (Malvern Nanosizer ZS, equipped with a 633 nm laser.) was used to determine the size of the CPDs in water. All chemicals were from commercial sources unless specially mentioned. Compounds 1, 3, 4, and 5 were prepared according to the reported procedures.26−28 For all experiments with the CPDs in aqueous solution, the pH was adjusted to 8.0 by addition of a dilute solution of sodium hydroxide. Compound 1. The solution of di-tert-butyl 4-amino-4-(3-(tertbutoxy)-3-oxopropyl)heptanedioate29,30 (30.0 g, 72.2 mmol) in ethyl acetate (200 mL) was mixed with aqueous K2CO3 (40.1 g, in 200 mL of water). To the mixture was added dropwise a solution of chloroacetyl chloride (10.4 g, 91.7 mmol) in ethyl acetate (50 mL) and allowed to react overnight at room temperature. The organic layer was washed by water and brine and dried over anhydrous sodium sulfate. The solvent was removed under vacuum to give 1 as a white 16680

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Scheme 1. Synthetic Routes to DW1, DW2, and DW3a

a Reagents and conditions: (i) 2-chloroacetyl chloride, K2CO3, ethyl acetate/H2O; (ii) 4-iodophenol, K2CO3, KI, MeCN, Δ, overnight; (iii) trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, overnight; (iv) trimethylsilylacetylene, Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, Δ, overnight; (v) triisopropylsilylacetylene, Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, Δ, overnight; (vi) K2CO3, dichloromethane/ methanol; (vii) 2, Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, overnight; (viii) Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, Δ, overnight; (ix) 2, Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, Δ, overnight.

the residue was extracted with dichloromethane/water. The organic layer was washed with saturated ammonium chloride, water, and brine and then dried over anhydrous sodium sulfate. The solvent was removed, and the crude product was purified by silica chromatography (hexane/ethyl acetate, 5/1) to give compound DW2 as light yellow solid (0.3 g, 65%). 1H NMR (300 MHz, CDCl3), δ (ppm): 7.58 (s, 1H), 7.55 (s, 2H), 7.51 (d, 4H, J = 8.4 Hz), 6.96 (d, 4H, J = 8.4 Hz), 6.57 (s, 2H), 4.42 (s, 4H), 2.22 (t, 12H, J = 7.4 Hz), 2.04 (t, 12H, J = 7.4 Hz), 1.44 (s, 54H), 1.14 (s, 21H). 13C NMR (75 MHz, CDCl3), δ (ppm): 172.72, 167.18, 157.42, 134.42, 133.98, 133.61, 124.39, 124.20, 116.69, 115.04, 90.22, 87.39, 80.93, 67.56, 57.90, 30.14, 29.83, 28.29, 18.88, 11.48. MS (ESI) m/z (M+ + Na), 1424.7969, calcd 1423.7992. Compound DW3. Compound 2 (3.2 g, 4.8 mmol) was dissolved in a mixture of THF (30 mL) and isopropylamine (50 mL) and combined with Pd(PPh3)2Cl2 (16.2 mg, 0.024 mmol, 5 mol %) and CuI (9.0 mg, 0.048 mmol, 10 mol %). After thoroughly degassing the solution by bubbling argon, a degassed solution of compound 7 (0.55 g, 1.0 mmol) in THF (5 mL) was added, and the reaction mixture was stirred overnight at 50 °C. The solvent was removed, and the residue

was extracted with dichloromethane/water. The organic layer was washed with saturated ammonium chloride, water, and brine and then dried over anhydrous sodium sulfate. The solvent was removed, and the crude product was purified by silica chromatography (hexane/ethyl acetate, 3/1) to give compound DW3 as a light yellow solid (2.3 g, 86%). 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64−7.60 (m, 9H), 7.51 (d, 8H, J = 8.4 Hz), 6.96 (d, 8H, J = 8.4 Hz), 6.58 (s, 4H), 4.42 (s,8H), 2.21 (t, 24H, J = 7.4 Hz), 2.03 (t, 24H, J = 7.4 Hz), 1.43 (s, 108H), 1.15 (s, 21H). 13C NMR (125 MHz, CDCl3), δ (ppm): 172.73, 167.17, 157.49, 134.00, 133.66, 124.49, 123.79, 123.70, 116.66, 115.11, 90.47, 89.25, 88.92, 87.33, 80.96, 67.60, 57.94, 30.19, 29.88, 28.32, 18.89, 11.52. MS (MALDI-TOF) m/z (M+.), 2741.7, calcd 2745.5. General Synthesis of Ph-PG-n. DWn (3.0 equiv, n = 1, 2, 3) was deprotected by reaction with TBAF (1.2 equiv) in THF. When the deprotection was complete, the mixture was passed through a short silica column, concentrated, and dissolved in a small amount of THF. The solution was degassed before adding into a degassed mixture of 1,3,5-tris((4-iodophenyl)ethynyl)benzene31 (1.0 equiv), Pd(dba)2 16681

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Scheme 2. Synthesis of Ph-PG-ns (n = 1, 2, and 3) and Ph-G-ns (n = 1, 2, and 3)a

Reagents and conditions: (i) TBAF; (ii) 1,3,5-tris((4-iodophenyl)ethynyl)benzene, Pd(dba)2, CuI, diisopropylamine, toluene, 60 °C, 20 h; (iii) (a) TFA; (b) 1.0 M Na2CO3.

a

(ppm): 7.70 (d, 9H), 7.67 (s, 3H), 7.65 (d, 18H), 7.56 (t, 36H), 6.98 (d, 24H, J = 8.9 Hz), 6.59 (s, 12H), 4.43 (s, 24H), 2.22 (t, 72H, J = 7.4 Hz), 2.04 (t, 72H, J = 7.4 Hz), 1.44 (s, 308H). 13C NMR (125 MHz, CDCl3), δ (ppm): 172.74, 167.16, 157.53, 134.50, 134.00, 133.66, 131.97, 124.53, 124.03, 123.68, 116.65, 115.11, 90.52, 89.43, 88.81, 87.31, 80.95, 67.60, 57.93, 30.18, 29.87, 28.32. MS (MALDI-TOF) m/ z (M•+), 8139.9, calcd 8139.9. General Synthesis of Ph-G-n. To a solution of Ph-PG-n (1.0 equiv) in dichloromethane was added trifluoroacetic acid (100 equiv). After reaction at room temperature for 4 h, the solvent was removed under vacuum. Dichloromethane was added, decanted, and another potion of dichloromethane was added and removed under vacuum. The residue was dissolved by addition of 1 M sodium carbonate until a clear solution was obtained. The solution was then dialyzed against water (pH 8.5) with a membrane with a 500 Da cutoff over 3 days, followed by filtration and evaporation under vacuum to give watersoluble dendrimers. Characterization of Ph-G-1. Compound Ph-G-1 was obtained as light-yellow solid (300 mg, 90%). %). 1H NMR (300 MHz, D2O), δ (ppm): 8.15−7.18 (br, 27H), 2.70 (br, 6H), 2.10 (br, 18H), 1.95 (br, 18H). Characterization of Ph-G-2. Compound Ph-G-2 was obtained as glassy light-yellow solid (200 mg, 80%). 1H NMR (300 MHz, D2O), δ (ppm): 8.26−7.22 (br, 48H), 2.77 (br, 12H), 2.19 (br, 36H), 2.03 (br, 36H).

(0.02 equiv), and CuI (0.04 equiv) in toluene/diisopropyamine (1:1). After reaction under an argon atmosphere at 60 °C for 20 h, the reaction mixture was cooled to room temperature and the solvent was removed by evaporation under vacuum. The residue was subjected to silica chromatography using dichloromethane (containing 2−12% acetone) as the eluent to afford pure Ph-PG-n. Characterization of Ph-PG-1. Compound Ph-PG-1 was obtained as light yellow solid (50 mg, 40%). 1H NMR (500 MHz, CDCl3), δ (ppm): 7.66 (s, 3H), 7.52 (d, 18H), 6.96 (d, 6H, J = 8.9 Hz), 6.57 (s, 3H), 4.42 (s, 6H), 2.21 (t, 18H, J = 7.4 Hz), 2.03 (t, 18H, J = 7.4 Hz), 1.44 (s, 81H). 13C NMR (125 MHz, CDCl3), δ (ppm): 172.49, 166.95, 157.15, 133.34, 131.62, 131.43, 123.94, 123.69, 122.30, 116.66, 114.82, 91.10, 90.40, 89.45, 88.37, 80.70, 67.33, 57.67, 29.92, 29.61, 28.06. MS (MALDI-TOF) m/z (M•+), 2092.8, calcd 2092.1. Characterization of Ph-PG-2. Compound Ph-PG-2 was obtained as light yellow solid (30 mg, 28%). 1H NMR (500 MHz, CDCl3), δ (ppm): 7.69 (s, 3H), 7.63 (d, 9H), 7.54 (t, 24H), 6.97 (d, 12H, J = 8.9 Hz), 6.58 (s, 6H), 4.43 (s, 12H), 2.22 (t, 36H, J = 7.4 Hz), 2.04 (t, 36H, J = 7.4 Hz), 1.44 (s, 154H). 13C NMR (125 MHz, CDCl3), δ (ppm): 172.51, 166.93, 157.28, 133.75, 133.42, 131.70, 124.20, 123.70, 122.89, 116.45, 114.87, 90.18, 89.96, 89.60, 87.10, 80.71, 67.36, 57.71, 29.95, 29.64, 28.03. MS (MALDI-TOF) m/z (M•+), 4105.4, calcd 4109.0. Characterization of Ph-PG-3. Compound Ph-PG-3 was obtained as light yellow solid (50 mg, 19%). 1H NMR (500 MHz, CDCl3), δ 16682

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Characterization of Ph-G-3. Compound Ph-G-3 was obtained as glassy light-yellow solid (30 mg, 80%). 1H NMR (300 MHz, D2O), δ (ppm): 8.2−7.0 (br, 90H), 2.72 (br, 24H), 2.12 (br, 72H), 1.99 (br, 72H). Molecular Simulations. The AMBER 11 package32 was used for molecular dynamics simulations of the dendrimers. The general AMBER force field (GAFF)33 was used throughout. Each dendrimer was solvated in a periodic box with water for Ph-G-n and tetrahydrofuran for Ph-PG-n. A 10.0 Å buffer distance between the solute and the edge of the solvent box was used. The cutoff distance for direct sum of interparticle interactions was 8.0 Å. Sodium counterions were added prior to solvation for Ph-G-n. Each system was first minimized in constant volume with a 500 kcal mol−1 Å−1 restraint on the solute in 500−1000 steps with the steepest descent method and then 500−1000 steps with the conjugate gradient method. It was then minimized with no restraint in 1000−2000 steps using the steepest descent method and 1500−2000 more steps using the conjugate gradient method. The system was then heated to 300 K in 300 ps with a 20 kcal mol−1 Å−1 restraint on the solute. A 1 fs time step and the Langevin34 thermostat were used for heating and equilibration with a 2 ps−1 collision frequency. Constant pressure and temperature equilibration followed at 1 bar with 1 ps relaxation time and 300 K for 1 ns. Finally constant volume and temperature equilibration at 300 K was performed using the Andersen35 thermostat. Here the SHAKE36 algorithm was used, with time step of 2 fs. Visualization and analysis were done using the VMD37 package and the ptraj module within AMBER, based on the final equilibration. Dynamic Light Scattering. The dynamic light scattering was determined at fixed back scattering angle of 173°, and the hydrodynamic radius (RH) of the scattering particles was calculated through the Stokes−Einstein equation: RH = κBT/(6πηD0), where κB is Boltzmann constant, T is the absolute temperature, η is the absolute zero-shear viscosity of the medium, and D0 is translational diffusion coefficient. The data were obtained from cumulant analysis and plotted by intensity distributions and volume distributions. Samples were prepared at 1 or 10 μM in water (pH 8.0), filtered through a cellulose membrane (0.2 μm), transferred to a disposable cuvette, and measured at 25 °C.

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purified by passing through a short silica column and then used immediately in a Sonogashira coupling reaction with 1,3,5tris((4-iodophenyl)ethynyl)benzene in a 3:1 stoichiometric ratio to afford the target dendrimers Ph-PG-n. The Sonogashira coupling reaction for the third-generation dendrimer, Ph-PG-3, was much less efficient than for the first and second generations, likely due to the steric hindrance by the bulky peripheral groups. It was found that the reaction yields could be optimized (especially for Ph-PG-3) by thoroughly degassing the reaction solution with argon, using very pure dendrons DWn and highly active palladium catalyst. Additional optimization of reaction conditions was made by elevating the reaction temperature to 60 °C, increasing the volume of the reaction solution, and reducing the reaction time. Optimization resulted in a modest isolated yield for Ph-PG-3 of 19%. Characterization of Ph-PG-n was performed by gel permeation chromatography (GPC), 1H NMR, and MALDITOF mass spectroscopy utilizing electron-transfer matrices.38−40 By using GPC analysis, impurities can be detected based on the difference in the molecular weight. The molecular weights are roughly doubled stepwise with increasing generation number. As indicated in the GPC chromatograms shown in Figure 1 for the Ph-PG-n series, the retention time

RESULTS

Synthesis of Dendrons. The synthetic route to the dendrons (DWn, n = 1, 2, 3) is shown in Scheme 1. Synthesis of the first-generation dendron DW1 began with dendritic compound 1, which was prepared from nitromethane in three steps.30 Reaction of 1 with an equivalent of 4-iodophenol in the presence of anhydrous potassium carbonate catalyzed by potassium iodide afforded 2 quantitatively. The iodo group of 2 was converted to trimethylsilylethynyl through a Sonogashira coupling reaction, providing DW1 in good yield. Secondgeneration dendron DW2 was produced by Sonogashira coupling of 5 with 2 equiv of 2. The tetra(trimethylsilylethynyl) derivative 6 was obtained by reaction of 5 with 2 equiv of 3. Compound 6 was subsequently treated with potassium carbonate to result in selective removal of the four TMS groups, followed by Sonogashira coupling with 2 to give the third-generation dendron DW3. Synthesis and Characterization of Dendrimers Ph-PGn. As shown in Scheme 2, synthesis of the precursor dendrimers Ph-PG-n (n = 1, 2, 3) follows a convergent strategy. (The peripheral carboxyl groups are protected as tertbutyl esters in these structures, and this is indicated by the “P” in the acronym “Ph-PG-n”.) The core unit 1,3,5-tris((4iodophenyl)ethynyl)benzene was obtained by a one-step Sonogashira coupling reaction of 1,3,5-triethynylbenzene with excess 1,4-diiodobenzene. Dendrons DWn (n = 1, 2, 3) were treated with TBAF; the resulting terminal acetylenes were

Figure 1. Gel permeation chromatography of dendrimers Ph-PG-ns (n = 1, 2, and 3).

decreases smoothly as the generation increases. The GPC trace for each Ph-PG-n sample exhibits a narrow symmetrically shaped profile with a polydispersity index (PDI) ∼ 1.02 using polystyrene standards for calibration, which clearly suggests the isolated pure form of dendrimer molecules. However, the GPC determined Mn values are higher than theoretical molecular weights by a factor of 74%, 66%, and 24% for Ph-PG-1, Ph-PG2, and Ph-PG-3, respectively (Table 1). This deviation arises because the GPC is calibrated using polystyrene standards; in Table 1. Characterization of Dendrimers by MALDI-TOF and GPC dendrimer

Mn(MALDI-TOF),a g/mol

Mn(GPC),b g/mol

PDI (Mw/Mn)b

Ph-PG-1 Ph-PG-2 Ph-PG-3

2093 4105 8139

3630 6810 10100

1.02 1.02 1.02

a

Molecular weight measured by MALDI-TOF. bMolecular weight and polydispersity index measured by GPC. 16683

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contrast to flexible coil-shaped polystyrene molecules, the conjugated dendrimers are rigid and likely have a proportionally increased hydrodynamic volume, especially at low molecular weight.41 The 1H NMR spectra, especially the aromatic region which contains a wealth of information, allow us to confirm the dendrimer structures. In each case as shown in Figure 2 (see

Figure 3. MALDI-TOF mass spectrum of Ph-PG-3. The ∗ label shows the molecular ion peak, and the ∗∗ label indicates the structure arising from loss of all of the tert-butyl ester groups.

Preparation of Water-Soluble Conjugated Polyelectrolyte Dendrimers: Ph-G-n (n = 1, 2, 3). The water-soluble CPDs (Ph-G-n, Scheme 2) were produced by elimination of the tert-butyl groups under acid catalysis using trifluoroacetic acid. (The acronym for the CPDs, Ph-G-n, reflects the absence of the ester protecting groups by dropping the “P” from the precursor acronym, Ph-PG-n.) The solubility of the CPDs in aqueous media was significantly improved after treatment of the elimination products with aqueous sodium carbonate which converted the acid form to a sodium salt form. The Ph-G-n CPDs were purified by dialysis in aqueous solution;44 to avoid complications associated with formation of the carboxylic acids by protonation, the dialysis was performed against water with pH 8.5. The peaks associated with the tert-butyl ester groups were not visible in the 1H NMR spectra of the resulting Ph-G-n series measured in deuterium oxide, which clearly indicates that the elimination reactions went to completion. Dendrimer Structures: Molecular Simulations and Dynamic Light Scattering. In order to gain insight into the 3D structure and conformation of the CPDs, as well as their interactions with solvent, the structures were examined by using the AMBER 11 package for molecular dynamics simulation. The simulations were done in order to facilitate interpretation of photophysical results, which as described below suggest that the structure of the dendrimers is solvent dependent. The methods used for the simulations are provided in the Experimental Section. The simulations were carried out using the ester protected precursors, Ph-PG-n, in tetrahydrofuran (THF) as well as the ionic dendrimers, Ph-G-n (as Na+ salts), in water. Several different approaches were used to visualize the results of the simulations, and the most useful are shown in the images and plots in the paper and the Supporting Information. One method we found to be particularly useful for visualizing the conformational space sampled by the structures is the “oxygen occupancy contour” image which illustrates the region of space sampled by the ester (or carboxylate) oxygen atoms in the dendrimer periphery (Figure 4 and Figure S2). As discussed below, the dendrimer core is planar, but the peripheral units in the second and third generation can take on conformations in which the periphery is out of plane. In order to quantify the

Figure 2. Stacked plot of the aromatic regions of the 1H NMR spectra of Ph-PG-1 (top), Ph-PG-2 (middle), and Ph-PG-3 (bottom) in CDCl3 (298 K, 500 MHz). The arrows show the positions of the core singlet resonances; the ∗ label indicates the amide singlet resonances; the ∗∗ label indicates the doublet resonances of the protons ortho to the alkoxy substituents.

Figure S1 for full 1H NMR spectra), the doublets at δ = 6.98 ppm are assigned to the protons ortho to the alkoxy groups in the peripheral phenylene units, the signal at δ ∼ 7.6 ppm is assigned to the protons on the trisubstituted benzene units, and the signals at δ = 7.50−7.58 ppm are assigned to the protons on the disubstituted phenylene units. A singlet δ ∼ 7.69 ppm arising from the three protons on the core phenylene unit is detected for all of the three generations, confirming that these molecules contain a C3 rotational axis. The relative integrated area of the singlet resonance due to the core at δ ∼ 7.69 ppm and the proton resonances in the region δ 7.6−7.7 ppm, assigned to the protons on the trisubstituted benzene units in the dendrimer periphery, follows the predicted ratio of 1:3 for Ph-PG-2 and 1:3:6 for Ph-PG-3. Final confirmation of the dendrimer structures comes from mass spectrometry. In particular, peaks corresponding to the mass of the molecular ions (M•+) are observed for each of the Ph-PG-n dendrimers by using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry.42,43 Although the intensity of the M+ peak decreases with increasing generation number, the molecular ion peak of PhPG-3 (m/z = 8139) is easily distinguished, accompanied by the presence of peaks corresponding to dendrimer fragments (Figure 3).43 Impurities with larger molecular weight are not observed. Interestingly, a strong peak corresponding to the dendrimer structure in which all of the tert-butyl groups are lost was observed at m/z = 6119, consistent with the predicted value of 6120. These results strongly support the proposed structures of dendrimer molecules. 16684

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which appear at times to come into close proximity, even when they are attached to adjacent dendrimer branches. Visualization of the dynamics (see movie file in Supporting Information) reveals that the disubstituted phenylene rings rapidly rotate around their long axis, whereas the trisubstituted rings undergo comparatively slower torsions. Because of the rotation of the phenylene units, the dendrimer branches are not always parallel to the plane of the core, resulting in a lowered overall planarity of the dendrimer structure (increasing with generation number). As seen by the plots in Figure 5 and Figure S3, Ph-PG-1 is relatively “flat” (95% of the conformations are less than 1 nm thick), while Ph-PG-2 and Ph-PG-3 are relatively less so, existing in conformations that are overall oblate in shape rather spherical. Simulations were also performed on the ionic dendrimers, Ph-G-n, in water solution. Overall, the results are generally similar to those seen for the ester forms, but with a few important distinctions (Figures 4 and 5 and Figure S2). First, Ph-G-1 in water adopts a relatively flat conformation in analogy to Ph-PG-1 in THF. The second- and third-generation CPDs also have relatively flat cores; however, electrostatic repulsion between the anionic carboxylate units in the dendrimer perhiphery leads to significant population of out-of-plane conformations arising from rotation of the trisubstituted phenylene rings. The significant result is that the conformations of Ph-G-2 and Ph-G-3 are more three-dimensional than those of corresponding ester-type dendrimers in THF. (This difference is especially evident by comparing the thickness histograms in Figure 5 for Ph-PG-3 and Ph-G-3.) Overall, we anticipate that due to their relatively more oblate structures, and the surrounding negative charge density, Ph-G-2 and PhG-3 will be more stable in solution as molecularly dissolved CPDs. By contrast, as noted above, due to its relatively flat overall structure, Ph-G-1 is more likely to undergo selfassembly (due to hydrophobic interactions) to form supramolecular “pancake stack” assemblies. In order to provide experimental results concerning the size of the CPDs, dynamic light scattering (DLS) was carried out on the Ph-G-n series in aqueous solution (c = 1.0 and 10 μM), and the results are shown in Figure 6 and Figure S5. Interestingly, the DLS results for the 1 μM solutions suggest a similar average diameter (∼3.5 nm) for each generation of the CPDs, which is

Figure 4. Oxygen occupancy contour images of (a, b) Ph-PG-3 in THF and (c, d) Ph-G-3 in water.

extent of out-of-plane geometry, we constructed plots illustrating the effective thickness of the dendrimers by plotting the length of a vector which runs perpendicular from the plane defined by the central core phenylene ring to the last (para) aromatic carbon in the dendrimer periphery (Figure 5).

Figure 5. Thickness from the core plane to last aromatic carbon.

Another method for quantifying the out-of-plane conformations sampled is shown in Figure S3, where we show plots of the angle subtended between the plane defined by the central core phenylene ring and a vector that runs from the center of the core phenylene to the last (para) aromatic carbon in the dendrimer periphery. Finally, the plots in Figure S4 provide insight regarding the overall radius of the dendrimers. As shown in Figure 4 and Figure S2, the ester dendrimers Ph-PG-n have a flat core, and the main structural difference between the different generations lies in the orientations of the peripheral branches. The peripheral ester substituents of PhPG-1 are positioned far away from each other, whereas Ph-PG2 and Ph-PG-3 possess crowded peripheral ester substituents

Figure 6. Dynamic light scattering analysis of (a) Ph-G-1, (b) Ph-G-2, and (c) Ph-G-3 in water at 1 mM concentration.. 16685

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length band, which is prominent in Ph-PG-2 and Ph-PG-3 and increases in intensity with generation number, is due to the chromophores in the dendrimer periphery, which consist mainly of P-E-P units. Compared to the Ph-PG-ns in THF, significant differences in the absorption spectra of the ionic Ph-G-n CPDs are observed in methanol and water solutions (Figure 7b,c). The most noticeable difference is seen in the absorption of Ph-G-1, which is significantly blue-shifted in both solvents, compared to that of the ester dendrimer in THF. The overall band shape of the absorption of Ph-G-1 is strongly suggestive of the presence of an H-aggregate (or dimer/multimer), which could be produced by face-to-face (pancake stack) self-assembly of Ph-G-1 molecules.45,46 The spectra of Ph-G-2 and Ph-G-3 also exhibit differences compared to the corresponding esters, with the appearance of new and/or shifted bands in the short wavelength region. These band shifts are indicative of interchromophore interactions. Interestingly, the long wavelength shoulder band (λ ∼ 350 nm) arising from the core chromophore is relatively unchanged in Ph-G-2 and Ph-G-3, suggesting that this unit is sheltered from the environment by the dendrimer periphery. Overall, there is a pronounced hypochromism in the absorption spectra of the Ph-G-n series compared to the Ph-PG-n, which also is suggestive of the existence of interchromophore (π−π) interactions for the CPDs in methanol and water. Importantly, there is little or no concentration dependence in the absorption spectra for the PhG-n series in aqueous solution (0.1−4 μM range, Figure S6). This result suggests that the effects seen arise largely due to intradendrimer interactions, with the exception of Ph-G-1, where we believe that there exists a strong tendency for the structure to self-assemble into H-multimers. The fluorescence spectra, lifetimes, and quantum yields were measured for the family of dendrimers, and the results are provided in Figure 8 and Table 2 as well as in the Supporting Information (Figures S7 and S9). First, the fluorescence spectra of the ester dendrimers Ph-PG-n in THF are shown in Figure 8a. Here it is seen that the fluorescence of Ph-PG-2 and PhPG-3 is essentially identical, characterized by a structured band with a well-defined vibronic progression. The emission exhibits only a very small Stokes shift from the low-energy absorption (shoulder), suggesting that it emanates from the core chromophore. Interestingly, Ph-PG-1 exhibits structureless fluorescence, with a broad band centered at 380 nm. The unique appearance of the Ph-PG-1 fluorescence spectrum is also attributed to the effect of the electron-donating alkoxy substituent that is in direct conjugation with the P-E-P-E-P core chromophore. This substituent likely introduces a degree of charge transfer character into the singlet excited state.47 The fluorescence spectra of the Ph-PG-n series are independent of excitation wavelength, indicating that energy transfer from the periphery to the core is efficient. (This subject is the topic of a study that will be reported separately.) The fluorescence quantum yields for the Ph-PG-n series in THF were all large, with that of Ph-PG-1 ∼1.0 (Table 2). Fluorescence lifetimes were determined at three wavelengths across the emission band with excitation at 370 nm (the tail of the core absorption), and the results are shown graphically in Figure S9. Ph-PG-1 features a single-exponential decay (700 ps) which is invariant across the emission band, while Ph-PG-2 and Ph-PG-3 feature biexponential decays, with median lifetimes of ∼1.2 ns. The fluorescence quantum yields and lifetimes of the Ph-PG-n series are typical of phenylene ethynylene oligomers and

comparable to the calculated size for Ph-G-1 (∼3.0 nm) and Ph-G-2 (∼4.0 nm) and slightly smaller than calculated for PhG-3 (∼4.6 nm). When the experiments are carried out at 10fold higher concentration, similar results are obtained for Ph-G2 and Ph-G-3; however, a much larger scattering size is observed for Ph-G-1, consistent with aggregation of this CPD (Figure S5). The most significant result of the DLS analysis is that it shows that large supramolecular aggregate structures are not present in the dilute solutions used for the photophysical studies. However, it is important to note that it may not be possible to discern the presence of dimeric or multimeric structures in these solutions from the DLS. Absorption and Fluorescence Spectroscopy. A key objective of this research is to understand the optical and excited state properties of the CPD systems and how they respond to the solution environment. First, we look at the properties of the ester forms of the dendrimers (Ph-PG-n) in THF solution. These structures are very soluble in this solvent, and they are likely to exist as molecularly dissolved species. As shown in Figure 7a, the absorption spectrum for each PhPG-n exhibits a major band at short wavelength combined with

Figure 7. Absorption spectra of (a) Ph-PG-ns (n = 1, 2, and 3) in THF, (b) Ph-G-ns (n = 1, 2, and 3) in methanol, and (c) Ph-G-ns (n = 1, 2, and 3) in water.

an additional band (shoulder) at longer wavelength, λ ∼ 350 nm. The long wavelength band is due to the dendrimer core chromophore consisting of a P-E-P-E-P moiety (P = 1,4phenylene and E = ethynylene). Note that the long wavelength band is slightly red-shifted for Ph-PG-1. This is due to the effect of the electron-donating alkoxy group that is located on the terminal phenylene unit which is conjugated with the P-EP-E-P core chromophore. This structural feature is unique to Ph-PG-1; in the higher generation dendrimers the alkoxy groups are in the dendrimer periphery and are not strongly conjugated with the core chromophore. The shorter wave16686

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the fact that the fluorescence intensity on the red side of the peak fluorescence is enhanced considerably for Ph-G-2, and to a lesser extent for Ph-G-3, especially in water. As outlined below in more detail, the broadened and red-shifted fluorescence is believed to arise from chromophore− chromophore interactions. In the case of Ph-G-1 this likely is caused by the existence of H-aggregates produced by face-toface interaction of two or more Ph-G-1 molecules. For Ph-G-2 and Ph-G-3, we believe that the chromophore−chromophore interactions arise mainly from interdendrimer interaction within dimer or small multimer aggregates.54 Interestingly, the fact that the fluorescence broadening is more pronounced for Ph-G-2 compared to Ph-G-3 suggests that the former may have a stronger propensity to form multimer structures. This is consistent with the structure simulations which show that the latter is more oblate and would be expected to have a lower propensity to form multimer structures. Fluorescence lifetime studies of the Ph-G-n series reinforce the notion that there is enhanced interchromophore interaction for the CPDs. In particular, as shown in Figure S9d−i, their fluorescence decays are all biexponential, with significant contributions from components with τ ∼ 1 and 5 ns. Importantly, the amplitude of the long lifetime component increases with the emission wavelength, coinciding with the region where the broad long wavelength emission is observed in the steady-state spectra. The long lifetime of the broad, redshifted fluorescence is consistent with it arising from an excimer-like state due to interchromophore interaction.53 Careful, concentration-dependent fluorescence experiments were carried out on the Ph-G-n series in aqueous solution, in each case starting at 100 nM concentration (Figure S7). Interestingly, in every case, at very low concentration (100 nM) a well-defined 0−0 band at ∼370 nm can be seen in the spectra. With increasing concentration the fluorescence broadens with enhancement on the red-side, with the effect decreasing in the sequence Ph-G-1 > Ph-G-2 > Ph-G-3. This finding supports the idea that dendrimer−dendrimer interactions are mainly responsible for the spectral shifts and broadening seen for the Ph-G-n CPDs and further that the extent of these interdendrimer interaction decreases with increasing generation number. Finally, we examined the effect of ionic strength on the fluorescence of Ph-G-3. As shown in Figure S8, addition of NaCl to a solution of the CPD has a pronounced effect on the fluorescence band shape. In particular, with increasing ionic strength, the intensity of the broad, red-shifted emission increases substantially to the point where it is the only emission observed in 100 mM salt. At high ionic strength, the salt screens the electrostatic repulsion between individual CPD macromolecules, resulting in the formation of aggregates from which the broad excimer like emission is dominant.

Figure 8. Fluorescence spectra of (a) Ph-PG-ns (n = 1, 2, and 3) in THF, (b) Ph-G-ns (n = 1, 2, and 3) in methanol, and (c) Ph-G-ns (n = 1, 2, and 3) in water. λex = 320 nm.

polymers in solvents of low to moderate polarity and when they are not aggregated.48−51 The biexponential decays observed for the higher generation dendrimers likely arise due to the existence of slowly equilibrating conformers which give rise to heterogeneity in the decay kinetics. The fluorescence properties of the ionic CPDs Ph-G-n in the polar solvents methanol and water differ substantially from those of the ester precursors. First, as seen in Table 2, the fluorescence quantum yields for the Ph-G-n series in methanol and water are much lower compared to the ester precursors, with yields less than 0.1 in all cases. Low fluorescence quantum yields have been observed in previous studies of phenylene ethynylene oligomers and polymers in aqueous solution.18,19,21,50−53 The effect has been attributed to aggregation as well as the introduction of new nonradiative pathways in the polar solvent environment.53 Turning to the fluorescence spectra shown in Figure 8b,c it is seen that in all cases the spectra for Ph-G-n are broadened and red-shifted compared to those of the corresponding Ph-PG-n dendrimers. The spectrum of Ph-G-1 in water is especially broad; however, noticeable is

Table 2. Photophysical Properties of Dendrimers in Various Solvents Ph-PG-n/THFa

Ph-G-n/CH3OHb

dendrimer generation

λabs max (nm)

λem max (nm)

ΦFc

1st 2nd 3rd

333 319 315

380 360 360

1.00 0.84 0.80

λabs max (nm) 292 327 292

Ph-G-n/H2Ob

λem max (nm)

ΦFc

λabs max (nm)

λem max (nm)

ΦFc

394 356 356

0.06 0.01 0.03

292 327 292

421 360 356

0.05 0.01 0.03

a

THF is used as the solvent for Ph-PG-ns (n = 1, 2, and 3). bCH3OH and H2O are used as the solvent for Ph-G-ns (n = 1, 2, and 3). cQuinine sulfate in 0.1 M H2SO4 is used as actinometer, ΦF = 0.55. 16687

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experiment finds that the lifetime is nearly unchanged over a range of added quencher where the emission intensity is reduced 5-fold (Figure S11). Thus, the quenching process is very fast on the time scale of the TCSPC system (resolution ∼100 ps), indicating that “static quenching” dominates. Overall, this quenching behavior of the Ph-G-3/MV2+ system is very similar to that observed when anionic conjugated polyelectrolytes are quenched by MV2+.23,24,57 This point will be discussed in more detail below.

Fluorescence Quenching of by MV2+. A number of investigations have explored amplified fluorescence quenching of conjugated polyelectrolytes by oppositely charged quencher ions. For example, anionic poly(phenylenevinylene)s and poly(phenylene ethynylene)s are quenched with very high efficiency by positively charged quencher ions, in particular p yr i d i niu m s a l t s su c h a s N ,N ′- d i m e t h yl v i o l o g e n (MV2+).23−25,55,56 An objective of this study was to explore whether conjugated polyelectrolyte dendrimers exhibit the amplified quenching effect. In particular, we were especially interested in exploring quenching under conditions where the CPDs are not aggregated. The quenching studies were carried out in water with the concentration of each CPD set at 1 μM. The Stern−Volmer (SV) plots (I0/I − 1 vs [MV2+]) are shown in Figure 9, and

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DISCUSSION This study has focused on the synthesis and photophysical characterization of a series of fully π-conjugated polyelectrolyte dendrimers (CPDs) that are surrounded on the periphery by anionic carboxylate units. The dendrimers were prepared by an iterative procedure resulting in their formation in a form where the ionic groups are protected as tert-butyl esters (Ph-PG-n series). This set of “precursor” dendrimers is fully soluble in organic solvents such as THF and CHCl3. The ester protecting groups were removed by acid treatment, resulting in the formation of the ionic CPDs (Ph-G-n) which are very soluble in alcohol and water. While there have been a number of previous studies that explored the properties of fully πconjugated dendrimers,10−13,15−17 as well as examples of ionic, water-soluble systems,44,58−60 this is the first study to fully explore the photophysical properties of conjugated dendrimers that feature polyelectrolyte character, making them soluble in aqueous solution. Molecular dynamics simulations provide insight regarding the CPD structures. The planar structure of the core units has a strong influence on the overall 3-D structure of the dendrimers. Specifically, the first generation Ph-G-1 is essentially flat, and this is believed to result in a structure that has a strong propensity to aggregate in solution with the monomer units in a pancake stack arrangement. The second- and third-generation CPDs Ph-G-2 and Ph-G-3 take on more globular structures, with the extent of out of plane conformations populated increasing with generation number. (The video file in the Supporting Information showing the simulation dynamics for Ph-G-2 and Ph-G-3 affords considerable insight concerning the available conformations for these structures.) We suggest the increased globular structure of Ph-G-3 compared to Ph-G-2 makes the former somewhat less prone to form multimer aggregate structures. DLS experiments were carried out to specifically probe for the existence of large aggregate structures present in the CPD solutions. Interestingly, these results reveal that the overall size is nearly the same for each of the three polyelectrolyte dendrimer generations; while the result does not preclude the existence of dimers or multimers, it strongly suggests that the CPDs do not exist as large aggregate structures under the conditions of the photophysical investigations. Given that there is no evidence for the formation of large aggregates in the CPD solutions, it is important to consider the origin of the significant differences in the photophysical properties of the (ester) Ph-PG-n and (ionic) Ph-G-n series; namely, for the Ph-G-n series the absorption is blue-shifted and the fluorescence is broadened and (much) less efficient compared to that of the Ph-PG-n dendrimers in THF. For the first-generation CPD (Ph-G-1) is it likely that the dendrimers exist as multimeric structures in which the individual molecules are stacked together in a face-to-face manner. This type of packing architecture can explain the

Figure 9. Stern−Volmer plot of Ph-G-ns (n = 1, 2, and 3) in water as a function of MV2+ concentration.

complete fluorescence spectra as a function of [MV2+] are shown in Figure S10. First, it is immediately evident that the overall quenching efficiency increases dramatically with the generation number of the dendrimer. Stern−Volmer constants (KSV) determined from the initial slopes of the quenching plots are 1.8 × 105 M−1 (Ph-G-1), 1.3 × 106 M−1 (Ph-G-2), and 4.7 × 106 M−1 (Ph-G-3). Aside from the difference in overall efficiency, the SV plots for the Ph-G-n series differ with generation number. Specifically, for Ph-G-1 and Ph-G-2 the plots exhibit downward curvature (e.g., less efficient quenching at higher [MV2+]), while that of Ph-G-3 features slightly upward curvature (which is more typical of a system exhibiting amplified quenching). The downward curvature in the SV plots for the Ph-G-1 and Ph-G-2 is believed to arise due to the presence of multimer aggregates in the CPD solutions. In essence, there are likely regions with the CPD assemblies where the MV2+ quencher is not able to come within close proximity to the fluorescent core chromophores. The quenching efficiency is most remarkable in Ph-G-3, with >90% quenching observed at [MV2+] = 1.0 μM and 98% quenching at 2.5 μM. Importantly, the 90% quenching efficiency is seen at 1:1 Ph-G-3:MV2+ stoichiometry, indicating that on average a single viologen is able to quench the emission of an entire Ph-G-3 macromolecule. In order to examine the dynamics of the MV2+ quenching process, the fluorescence lifetime of Ph-G-3 was monitored as a function of [MV2+]. This 16688

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important to comment on the distinct downward curvature seen in the Stern−Volmer quenching plot for Ph-G-2. This effect is believed to arise due to the existence of multimers in which a fraction of the core chromophores are sterically inaccessible to the MV2+ quencher ion.61 An important finding of this work is that the highest generation dendrimer, Ph-G-3, displays the most pronounced amplified quenching effect. This is because of the interplay of the two features that have been previously identified as being key to the effect: (1) a large ion-pairing association constant between the π-conjugated polyelectrolyte and the oppositely charged quencher ion and (2) efficient energy transport (and delocalization) within the π-conjugated polyelectrolyte chromophore.19 Clearly these two factors are optimized in the high molecular weight dendrimer, giving rise to a Stern−Volmer quenching efficiency that rivals that seen in the most efficient CPE−quencher systems.

significant blue-shift in the absorption, which is characteristic of an H-aggregate structure. The situation is clearly more complicated for the higher generation dendrimers, Ph-G-2 and Ph-G-3. In these systems we posit that the aggregate optical signatures arise from a combination of factors. First, given the oblate structures of Ph-G-2 and Ph-G-3, it is likely that it is still possible for the CPDs to form dimer or multimers, driven by solvophobic forces. The interactions that occur between the large π-conjugated chromophores in the core result in the effects seen in the optical spectra within these multimer structures. The fact that the red-shifted aggregate fluorescence band is less significant in Ph-G-3 suggests that the extent of aggregation is less in this system. The second factor that may contribute to the difference in the optical spectra of Ph-G-2 and Ph-G-3 may be the existence of interchromophore interactions that can occur within a single dendrimer molecule which are caused by direct, through-space contacts between the conjugated branches. The Stern−Volmer quenching experiments carried out using the MV2+ quencher ion provide unequivocal evidence for the amplified quenching effect in the Ph-G-n series. While the overall dependence of the SV quenching on generation number is complex, in each case the KSV values obtained at low quencher concentration are 100−1000-fold greater than typical for quenching in small molecule systems where the fluorophore and quencher ion are oppositely charged.18,52 However, we believe that the underlying mechanism for the amplified quenching is different for the different generation dendrimers. First, it is clear that ion-pairing between the negatively charged CPDs and the cationic MV2+ is important in each case. Moreover, it is probable that one reason for the trend that KSV increases in the sequence Ph-G-1 < Ph-G-2 < Ph-G-3 is due to the increased electrostatic attraction (and consequent increase in ion-pair association constant) between the higher generation CPDs and MV2+. Similar trends in increased quenching efficiency with generation number have been observed in quenching studies of ionic (nonconjugated) chromophore loaded dendrimer systems.61,62 Second, it is important to consider the contribution of energy transport within the CPDs as a contributor to the amplified quenching effect. We believe that the contribution of intradendrimer energy transfer to the amplified quenching likely increases with generation number. Specifically, in Ph-G-1, the overall quenching efficiency is only ∼50% at a quencher:dendrimer ratio of 10:1. In this system the quenching enhancement compared to a small molecule chromophore/ quencher system is likely only due to the large association constant for ion-pairing between Ph-G-1 and MV2+. There is probably little or no contribution to quenching from exciton transport either within an individual CPD molecule or within Ph-G-1 aggregates. At the opposite extreme is Ph-G-3, where the 90% quenching is seen at a 1:1 quencher:dendrimer ratio; as noted above, this indicates that a single MV2+ is able to completely quench an entire Ph-G-3. In addition, time-resolved fluorescence (with 100 ps time resolution) indicates that the quenching is “static”; i.e., it occurs in less than the 100 ps time resolution. This effect must arise due to very rapid transport of singlet excitons within the CPD structure, bringing the exciton into close proximity of the MV2+ ion for quenching to occur. Indeed, we recently reported ultrafast fluorescence upconversion experiments on structurally similar CPDs where it was observed that energy transfer from the branched periphery to the core occurs within 5 ps of excitation.63 Finally, it is

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SUMMARY AND CONCLUSIONS In summary, we have synthesized and carried out detailed photophysical studies of a series of conjugated polyelectrolyte dendrimers. The dendrimer consists of a rigid, planar phenylene ethynylene core, with branched phenylene ethynylene units in the periphery of the second- and third-generation structures. The π-conjugated core is surrounded by a number of anionic carboxylate units which render the structures soluble in polar solvents, including methanol and water. Molecular dynamics studies reveal that the dendrimer structures range from flat to oblate with increasing generation number. On the basis of the available photophysical data, we propose that Ph-G1 exists as aggregates in methanol and aqueous solution. It is also likely that Ph-G-2 and Ph-G-3 form dimer or multimer structures in these solvents as well, but the degree of interdendrimer interactions likely decreases with increasing generation number. Fluorescence quenching studies carried out with MV2+ show that all of the CPDs are quenched very efficiently, with KSV ranging from 105 to 107 M−1. The increase in quenching efficiency with generation number arises due to an increase in the ion-pair association constant coupled with an increasing contribution of exciton transport within the CPD structure. Despite their complex structure and photophysical properties, the unique structures of the CPDs with their ionic periphery and π-conjugated hydrophobic core provide a significant opportunity to study light-harvesting and energyand charge-transfer with guest molecules bound by electrostatic and/or hydrophobic forces. Future studies will take advantage of these properties to study the dynamics of intercomponent energy and charge transfer.

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ASSOCIATED CONTENT

S Supporting Information *

Proton NMR spectra of Ph-PG-n series, molecular simulation images for Ph-PG-1, Ph-PG-2, Ph-G-1, and Ph-G-2, histograms showing structural data from molecular simulations, concentration-dependent absorption and fluorescence spectra, effect of NaCl on fluorescence of Ph-G-3, plots of emission lifetime data, fluorescence spectra of Ph-G-n series with added MV2+ and fluorescence lifetime of Ph-G-3 as a function of MV2+ concentration (10 figures); movie (.mpg) files of molecular simulations for Ph-PG-3 and Ph-G-3. This material is available free of charge via the Internet at http://pubs.acs.org. 16689

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the United States Department of Energy (Grant DEFG02-03ER15484) for support of this work. REFERENCES

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