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Langmuir 2009, 25, 2402-2406
Solvent Dependant Optical Switching in Carbazole-Based Fluorescent Nanoparticles Ravi M. Adhikari,† Bipin K. Shah,*,‡ Sujeewa S. Palayangoda,† and Douglas C. Neckers*,† Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403, and Department of Chemistry, Pittsburgh State UniVersity, Pittsburgh, Kansas 66762 ReceiVed August 19, 2008. ReVised Manuscript ReceiVed December 12, 2008 Suitably susbtituted ethynylphenyl carbazoles (PBM and PPM) form stable fluorescent organic nanoparticles. The emission of the nanoparticles can be reversibly switched on/off in the blue-green and orange-red regions by a change in the ratio of the tetrahydrofuran/water system used in their preparation. The size of the nanoparticles was found to be dependent on the solvent ratio, and the emissions were significantly red-shifted compared to those of dilute solutions of PBM and PPM in tetrahydrofuran. This is attributed to the formation of intermolecular charge transfer complexes in the nanoparticle state. The application of the nanoparticles as a chemical vapor sensor has been suggested.
Introduction
Scheme 1a
Fluorescent inorganic nanoparticles1 have found applications as biological labels,2 in photovoltaic cells,3 as light emitting diodes,4 and as optical sensors.5 Fluorescent organic nanoparticles (FONs), on the other hand, have received less attention, even though they allow wider variability and flexibility as materials and in synthesis. Since Nakanishi et al. reported use of FONs in third order nonlinear optics, composite and hybrid organic/ inorganic nanoparticles have been developed for applications as sensors and biological detectors.6-9 The electronic and optical properties of nanoparticles differ from those of bulk materials because they are structurally distinct and exhibit confinement effects caused by their finite size.10,11 Thus, given the diversity of organic compounds, the development of FONs should stimulate new applications in many fields. In this Article, the formation and photophysical characteristics of highly fluorescent, stable FONs based on 2-(4-(2-(4-(3,6-ditert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)benzylidene)malononitrile (PBM) and 2-((4-(2-(4-(2-(4-(3,6-di-tert-butyl-9Hcarbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)methylene)malononitrile (PPM) (Scheme 1) are reported. We demonstrate for the first time a direct correlation of the particle size with the solvent/nonsolvent (THF/water) ratio used to form the nanoparticles. We also report that the emission of PBM and PPM in the blue, blue-green, and orange-red regions can be reversibly * To whom correspondence should be addressed. E-mail: neckers@ photo.bgsu.edu (D.C.N.);
[email protected] (B.K.S.). † Bowling Green State University. ‡ Pittsburgh State University. (1) Contribution no. 671 from the Center for Photochemical Sciences. (2) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 2013. (3) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (4) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (5) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (6) Fernandez-Arguelles, M. T.; Yakovlev, A.; Sperling, R. A.; Luccardini, C.; Gaillard, S.; Medel, A. S.; Mallet, J.-M.; Brochon, J.-C.; Feltz, A.; Oheim, M.; Parak, W. J. Nano Lett. 2007, 7, 2613. (7) Su, X.; Zhang, J.; Sun, L.; Koo, T.-W.; Chan, S.; Sundararajan, N.; Yamakawa, M.; Berlin, A. A. Nano Lett. 2005, 5, 49. (8) Fu, H.-B.; Yao, J.-N. J. Am. Chem. Soc. 2001, 123, 1434. (9) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Itoh, O.; Nakanishi, H. Chem. Lett. 1997, 9, 1181. (10) Benzamin, G.; Huang, F.; Zhang, H.; Glenn, A. W.; Banfield, J. F. Science 2004, 305, 651. (11) Gesquiere, A. J.; Uwada, T.; Asahi, T.; Masuhara, H.; Barbara, P. F. Nano Lett. 2005, 5, 1321.
a Reagents and conditions: (a) tert-butyl chloride, zinc chloride, nitromethane, 40-50 °C, 5 h; (b) 1,4-diiodobenzene, Cu, potassium carbonate, 18-crown-6, o-dichlorobenzene, reflux, 12 h; (c) 4-ethynylbenzaldehyde, CuI, Pd(PPh3)2Cl2, triethylamine, room temperature, 12 h; (d) malononitrile, basic Al2O3, toluene, reflux 6 h; (e) 1,4-diethynylbenzene, CuI, Pd(PPh3)2Cl2, triethylamine, room temperature, 12 h.
switched on and off by changing the tetrahydrofuran (THF)/ water ratio.
Results and Discussion PBM and PPM were synthesized following our previously developed method.12 Carbazole (1) was converted to 3,6-ditert-butyl-9H-carbazole (2), which was further converted into 3,6-di-tert-butyl-9-(4-iodophenyl)-9H-carbazole (3) (Scheme 1). (12) Adhikari, R. M.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727.
10.1021/la802716w CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
Optical Switching in Carbazole-Based FONs
Figure 1. UV-visible spectra of PBM nanoparticle solutions (3.7 × 10-6 M) recorded at different THF/water ratios by volume.
Compound 3 was subjected to Sonogashira couplings with suitable substrates to obtain 4 and 6. PBM and PPM resulted from refluxing 4 and 6, respectively, with malononitrile and basic aluminum oxide in toluene. PBM and PPM nanoparticles were prepared by a precipitation technique.13 The nanoparticle suspensions formed were visibly transparent and stable under ambient conditions. UV-visible spectra of PBM in THF and as nanoparticles suspended in solvents differing in the THF/water ratio are displayed in Figure 1. PBM shows three distinct peaks at 295, 345, and 400 nm in THF. With the addition of water (THF/water ratio: 1/1), the absorption transition associated with the carbazole moiety (295 nm) gradually blue-shifts (275 nm) with a broadening of the peak. This may be due to the electronic coupling between the neighboring molecules as they approach each other in THF because of the hydrophobic nature of the PBM molecules. 1,3-Diphenyl-5-(2anthryl)-2-pyrazoline-based nanoparticles exhibit a similar behavior.14 The peak at 345 nm, associated with the absorption transition of the N-substituted ethynylphenyl moiety, remains unchanged. The 400 nm peak can be assigned to the transition from S0 to twisted intramolecular charge transfer (TICT) state.15,16 This band disappears on the addition of water due to the destabilization of the TICT caused by an increase in the solvent polarity. This is further supported by the fact that PBM shows negative solvatochromism (Supporting Information). As the ratio of water is further increased, a new absorption band (420 nm) appears. This is assigned to a transition of S0 to an intermolecular charge transfer (ICT) state17 that forms as the PBM molecules are cramped even more closely and begin nucleating into nanoparticles. It is likely that intermolecular interaction originates from overlapping of the carbazole moiety and the nitrile group of the neighboring molecules (Vide infra) and further increases as the nucleation progresses and the size of the particles becomes larger.18-21 The molecular overlap is also strengthened by an increase in the molecular dipole, and the (13) Palayangoda, S. S.; Xichen, C.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281. (14) Xiao, D.; Xi, L.; Yang, W.; Fu, H.; Shuai, Z.; Yan, F.; Yao, J. J. Am. Chem. Soc. 2003, 125, 6740. (15) Yu, M.-X.; Chang, L.-C.; Lin, C.-H.; Duan, J.-P.; Wu, F.-I.; Chen, I.-C.; Cheng, C.-H. AdV. Funct. Mater. 2007, 17, 369. (16) Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel, J. F.; Pratt, D. W. J. Chem. Phys. 2005, 122, 84309. (17) Fu, H. B.; Loo, B. H.; Xiao, D. B.; Xie, R. M.; Ji, X. H.; Yao, J. N.; Zhang, B. W.; Zhang, L. Q. Angew. Chem., Int. Ed. 2002, 41, 962. (18) Alivisatos, A. P. Science 1996, 271, 933. (19) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (20) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59.
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Figure 2. Fluorescence spectra of PBM nanoparticle solutions (3.7 × 10-6 M) recorded at different THF/water ratios by volume; excitation at 350 nm.
ICT state becomes more prominent. This results in the red-shifted absorption and emission (Vide infra).2,14,17 Additionally, Mie scattering may also be responsible for the red-shift in the absorption transition.4 There are significant differences between the fluorescence spectra of PBM in THF and the PBM nanoparticles formed in the THF/water medium (Figure 2). The change in the emission characteristics of the PBM samples at different water/THF ratios, in fact, follows the same pattern observed for absorption. The emission changes from blue-green in THF (410 and 485 nm) to blue (415 nm) and ultimately to orange-red as the water fraction is gradually increased, if one maintains the same concentration (3.7 × 10-6 M). The initial two emission maxima (410 and 485 nm) likely result from the locally excited state and the TICT, respectively. The latter emission disappears upon addition of water (THF/water ratio: 1/1), and the locally excited state becomes the only emitting state (∼415 nm). The ICT state of the PBM nanoparticles is responsible for the broad emission (525-700 nm) that appears at the higher water ratio. The particles are stable for hours as indicated by the retention of the color purity while in the dispersion medium. It is likely that an n-electron from nitrogen of the carbazole moiety of one molecule transfers to the nitrile moiety of the adjacent molecule, resulting in the ICT state. This leads to an increase in the dipole moment of PBM in the nanoparticle state, which in turn enhances the intermolecular interaction. A similar charge transfer phenomenon was observed in 1-phenyl-3-((dimethylamino)styryl)5-((dimethylamino)phenyl)-2-pyrazoline-based nanoparticles.8 The size of nanoparticles increases with an increase in the fraction of water (Vide infra). This allows the intermolecular electronic interactions to extend over a large number of molecules and to increase in magnitude. This is likely the reason for the significantly red-shifted emission of the PBM nanoparticles compared to that from the PBM individual molecules. A red-shifted emission is expected for an aggregate state14-17 arising from the extended orbital overlap of closely stacked molecules in nanoparticles.17 According to the molecular exciton model, head-to-tail alignment of the transition dipole (J-aggregation) shifts emission to the red region and enhances fluorescence.21 We observed the red-shifted emission from the PBM nanoparticles, but the fluorescence intensity was diminished. The main reason for an enhanced emission in J-aggregation is the planarization of the molecule in nanoparticles.21 It is likely that the PBM molecules undergo (21) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.-J.; Hanack, M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902–1907.
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Figure 4. (A) SEM image of PBM nanoparticles. (B) PBM nanoparticles formed at the THF/water ratio of (i) 1/5, (ii) 1/11, and (iii) 1/90, showing the size dependence on the THF/water ratio; bar of SEMs ) 0.5 µm. Figure 3. (A) Intermolecular charge transfer (ICT) state of the PBM nanoparticles and (B) absorption and emission transitions in PBM in THF and in THF/water solutions.
J-aggregation facilitating intermolecular charge transfer but retain the nonplanar or twisted structure in the ICT state. A schematic of the ICT state responsible for the red-shifted broad emission and energy associated with various excitations and emissions is depicted in Figure 3. Many organic systems exhibit emission from an excimer state that is significantly more red-shifted than that of individual molecules. To rule out the possibility of excimer of PBM being responsible for the red-shifted emission in solution, we further studied the PBM nanoparticles using a confocal microscope. A clear orange-red light emission was observed from the precipitated nanoparticles (the confocal image is provided in the Supporting Information). This also assured us that the orange-red emission is indeed associated with the PBM nanoparticles and is not a solution emission of PBM induced by the solvent gradient. These FONs were stable to a blue laser for hours. The formation of the PBM nanoparticle is clearly associated with the addition of water and follows the general Ostwald type ripening process. Water and THF are miscible, so the solubility of hydrophobic PBM decreases with the increasing fraction of water, eventually reaching a critical nucleation condition at which nuclei form throughout the solution and begin to grow as particles.22 Once the concentration of solute in solution falls due to the growing nuclei, particle growth stops so that the equilibrium mixture contains both particles and solution. This is the point that determines particle size. In a system containing a greater amount of water, the concentration at which particle growth stops is reached when most of the substance has fallen out of the solution. As mentioned earlier, the color of the emission of PBM changes instantaneously from blue-green to blue and ultimately to orangered as the water fraction increases. In fact, fluorescence switching to the orange-red region starts with a water ratio of ∼75% by volume. On increasing the water ratio to 92% by volume, a complete transition to orange-red fluorescence occurs. Similar results were obtained with samples of different concentrations in different THF/water ratios (see the Supporting Information). Thus, the change in emission is due to a change in nanoparticle size and is irrespective of the concentration of PBM. Figure 4 shows scanning electron microscope (SEM) images of the PBM nanoparticles formed at different THF/water ratios. Nanoparticles (22) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330.
Table 1. Fluorescence Lifetimes (τF) of PBM and PPM Recorded at Different THF/Water Ratios (Decay Monitored at the Corresponding λmax; Excitation at 360 nm) compound
THF/water ratio
λmax (nm)
τF1 (ns)
τF2 (ns)
PBM
1/0 1/3 1/3 1/5 1/11
450 450 590 590 590
3.0 0.5 1.2 2.1 4.4
3.0 12.7 14.5 32.3
1/0 1/40 1/45
460 590 590
3.5 10.5 13.0
45.0 49.0
PPM
of ∼100 nm were observed from a THF/water ratio of 1/5 (83% water by volume) with a polydispersity factor of 9%. Nanoparticles of larger sizes were formed at higher water ratios. For example, the size of the particles increased to ∼200 and ∼400 nm, respectively, when the water ratio was increased to 92% and 99.5% by volume. Though control of nanoparticle size by aging has been reported,14,17 our observations clearly suggest that the size of nanoparticles can be controlled by altering the solvent/ nonsolvent ratio. PPM, which contains one more phenylethynyl group than PBM, forms similar nanoparticles, the sizes of which depend on the THF/water ratio from which they are particulated. The water content needed to induce nanoparticle formation is higher for PPM (THF/water ratio of 1/5) than for PBM (THF/water ratio of 1/3). In the case of PBM, a structured higher wavelength absorption corresponding to the nanoparticles distinctly appears at a THF/water ratio of 1/11. However, in the case of PPM, the nanoparticle absorption appears distinctly only at a THF/water ratio of 1/50. To obtain further information on the nature of the excited state, we measured the fluorescence life times (τF) of PBM and PPM in dilute THF solutions and as nanoparticles dispersed in different fractions of THF/water (Table 1). Decays were monitored at the corresponding emission maxima (λmax). Decays monitored in THF solution could be fit monoexponentially, indicating the singlet excited state is exclusively formed. PBM and PPM showed similar lifetimes (τF) in THF (3.0 and 3.5 ns, respectively). Samples that contained different amounts of water and THF showed biexponential decay, suggesting involvement of multiple excited states. The fluorescence decay of a PBM sample containing a 1/3 ratio of THF/water was monitored at two different emission
Optical Switching in Carbazole-Based FONs Chart 1
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Information. The new FONs showed no solvent specificity, and the same fluorescence switching behavior was observed from exposure of the particles to THF, ethyl acetate, and acetonitrile vapors. Nonetheless, this preliminary observation implies that fluorescence switching of the PBM and PPM nanoparticles may find application in inexpensive sensors for organic vapors.
Conclusions maxima. The τF values (1.2 and 12.7 ns) observed at higher wavelength (590 nm) were correspondingly longer than those (0.5 and 3.0 ns) monitored at shorter wavelength (450 nm). The τF2 values were 12.7, 14.5, and 32.3 ns when the THF/water ratios were 1/3, 1/5, and 1/11, respectively. This suggests that nanoparticles are responsible for the longer lifetime components. In fact, the biexponential decay of the PBM nanoparticles monitored at 450 and 590 nm is due to overlap between the S1 and the ICT state, with the longer τF originating from the ICT state. The charge transfer state of organic molecules generally exhibits a longer decay time compared to the locally excited state.23 The τF values of the PBM nanoparticles increase with increasing fraction of water. This indicates that the larger the nanoparticle, the longer is the fluorescence lifetime. The PPM solutions exhibited similar fluorescence decay characteristics. Interestingly, the PPM nanoparticles showed higher lifetimes than the PBM nanoparticles. For example, the shorter (τF1 10.5 ns) and longer (τF2 45.0 ns) lifetimes of the PPM nanoparticles formed at a 1/40 THF/water ratio, respectively, were much higher than those of the PBM nanoparticles formed at a 1/11 THF/water ratio (τF1 4.4 ns and τF2 32.3 ns). This may be related to the differences in the ratios of THF/water used in these two cases, which would result in the PPM and PBM nanoparticles of different sizes. It is noted that the shorter component of lifetimes (τF1) of PBM and PPM also gradually increase as the water content increases. Surprisingly, 5 and 3,6-di-tert-butyl-9-(4-ethynylphenyl)phenyl-9H-carbazole (7, Chart 1), which are similar in chemical structure to PBM, form no nanoparticles under similar experimental conditions. Similarly, 6 (a compound similar in chemical structure to PPM) forms no nanoparticles. The presence of the malononitrile group on the side chain phenyl seems crucial for the formation of nanoparticles. We could not obtain nanoparticles from either 8 or 9 (Chart 1) under similar conditions, although these compounds contain mono- and dimalononitrile groups attached to the carbozole moiety. Thus, it is clear that when the malononitrile group is at the side chain phenyl group, it is structurally in a suitable position to induce the nucleation process. The emission of the PBM and PPM nanoparticles could be reversibly shifted, red or blue, through an adjustment in the THF/ water ratio. The fluorescence maximum of the PPM nanoparticles (Figure 5), for example, shifted from 410 to 590 nm with an increase in water content. Reversibly, increasing the THF ratio of such samples caused the fluorescence emission to attain its initial value (410 nm). To further investigate the on/off fluorescence switching of the PBM and PPM nanoparticles, we spotted a suspension of these FONs on a thin layer chromatography (TLC) plate coated with silica gel. The color of the emission from the plate was orange under illumination at 350 nm. Upon insertion of the plate into THF vapors, this color gradually fades eventually to be replaced by a blue color indicating that THF vapors were sensed by the PPM and PBM nanoparticles. A picture of TLC plates clearly showing the color change is provided in the Supporting (23) Schulman, S. G. Fluorescence and Phosphorescence Spectroscopy: Physicochemical Principles and Practice; Oxford: New York, 1977.
We have prepared novel PBM- and PPM-based FONs that show significantly red-shifted emissions compared to those of the free PPM and PBM molecules. An ICT state originating from the interaction of the carbazole moiety and the nitrile group of the adjacent molecules is responsible for such behavior of the nanoparticles. This is further supported by the fluorescence lifetime data. While the PBM and PPM molecules in THF showed a single lifetime associated with the locally excited state, the PBM and PPM nanoparticles exhibited biexponential fluorescence decay, with the longer lifetime arising from the ICT state. The sizes of FONs formed were found dependent on the THF/water ratio. A change in the THF/water ratio can also efficiently switch the emission of these FONs from orange-red to blue-green and vice versa. The interesting fluorescence switching behavior was used to sense organic vapors. Detailed studies of the application of these and similar other FONs to selectively sense organic vapors are underway.
Experimental Section Instrumentation. Mass spectra were recorded on a Shimadzu GCMS-QP5050A instrument equipped with a direct probe (ionization 70 eV). A Bruker spectrometer (working frequency 300.0 MHz for 1 H) was used to record the NMR spectra. CDCl3 was the solvent for NMR, and chemical shifts relative to tetramethylsilane at 0.00 ppm are reported in parts per million (ppm) on the δ-scale. Absorption and fluorescence spectra were recorded on a Shimadzu UV-2401 spectrophotometer and a Fluorolog-3 spectrometer, respectively. All measurements were carried out at room temperature unless otherwise specified. Synthesis. Compound 3 was synthesized according to our previous method12 and converted into PBM and PPM. 4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)benzaldehyde (4). A dry round-bottom flask was charged with 3 (1 mmol), 4-ethynylbenzaldehyde (1 mmol), triphenylphosphine (0.01 mmol), dry and distilled triethylamine (20 mL), CuI (0.01 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II) (0.01 mmol). The reaction mixture was stirred under argon for 5 h at room temperature, and the solvent was evaporated. The solid obtained was purified by chromatography (silica gel, 80% hexane in dichloromethane) to obtain pure 4 (60%) as a yellowish solid. 1H NMR (300 MHz, CDCl3): δ 1.5 (s, 18H), 7.42 (d, 2H), 7.5 (d, 2H), 7.62 (d, 2H), 7.72-7.78 (m, 4H), 7.92 (d, 2H), 8.15 (s, 2H), 10.1 (s,1H). 13C NMR (300 MHz, CDCl3): 31, 35, 84, 93, 108, 116, 121, 123, 125, 130, 132, 134, 135, 139, 142, 192. MS (ES) calculated, 484.2640; measured, 484.2642. 2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)benzylidene)malononitrile (PBM). Compound 4 (1 mmol), malononitrile (1.1 mmol), basic aluminum oxide (10 mmol), and toluene (30 mL) were added in a dry round-bottom flask. The mixture was refluxed under argon for 5 h. The mixture was filtered hot, and the residue was washed several times successively with hot ethyl acetate and dichloromethane. The filtrate was then dried, and the solid obtained was purified by chromatography (silica gel, 80% hexane in ethylacetate) to obtain pure PBM (60%) as a bright yellow solid. 1 H NMR (300 MHz, CDCl3): δ 1.5 (s, 18H), 7.44 (d, 2H), 7.5 (d, 2H), 7.64 (d, 2H), 7.72 (d, 2H), 7.79 (m, 3H), 7.95 (d, 2H), 8.15 (s, 2H). 13C NMR (300 MHz, CDCl3): 31, 35, 89, 95, 109, 112, 113, 116, 124, 127, 129, 129.5, 130, 132, 133, 139, 140, 143, 159. MS (EI) calculated, 531.2674; measured, 531.26742.
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Figure 5. Fluorescence spectra of PPM nanoparticle solutions (5.2 × 10-6 M) (A) recorded at different THF/water ratios by volume, excitation at 380 nm and (B) recorded during the reverse process of adding THF to a THF/water solution of PPM nanoparticles, excitation at 380 nm.
3,6-Di-tert-butyl-9-(4-(2-(4-ethynylphenyl)ethynyl)phenyl)-9Hcarbazole (5). Compound 3(3 mmol), 1,4-diethynylbenzene (4 mmol), triphenylphosphine (0.03 mmol), dry and distilled triethylamine (40 mL), CuI (0.03 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II) (0.03 mmol) were mixed in a dry round-bottom flask and stirred under argon at 0°C for 3 h. This mixture was allowed to warm to room temperature and stirred for 4 more hours. The solvent was evaporated, and the solid obtained was purified by chromatography (silica gel, 80% hexane in dichloromethane) to obtain pure 5 (81%) as a white solid. 1H NMR (300 MHz, CDCl3) δ 1.50 (s, 18 H), 3.2 (s, 1H), 7.3-7.6 (m, 10H), 7.7 (d, 2H), 8.18 (s, 2H); 13 C NMR (300 MHz, CDCl3) δ 31.5, 34.5, 78, 83, 89, 91, 109, 116, 120, 121, 123, 125, 130.5, 131, 132, 128, 139, 143; mass spectrum(DIP-MS) m/z M+ 479 (100%); HRMS (ES+) m/z found, 480.2691; calcd, 480.2691. 4-(2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)benzaldehyde (6). A dry round-bottom flask was charged with 5 (1 mmol), 4-iodobenzaldehyde (1 mmol), triphenylphosphine (0.01 mmol), dry triethylamine (20 mL), CuI (0.01 mmol), and trans-dichlorobis(triphenylphosphine)palladium(II) (0.01 mmol). The mixture was stirred for 5 h at room temperature under argon and the solvent was evaporated. The solid obtained was purified by chromatography (silica gel, 80% hexane in dichloromethane) to obtain pure 6 (60%) as a yellowish solid. 1H NMR (300 MHz, CDCl3) δ 1.50 (s, 18 H), 7.4 (d, 2H), 7.46 (d, 2H), 7.58 (m, 6H), 7.7 (m, 4H), 7.88 (d, 2H), 8.15 (s, 2H), 10 (s,1H). 13C NMR (300 MHz, CDCl3) δ 31.5, 34.5, 88.5, 89, 90, 92, 109, 116.5, 121, 122, 123.6, 123.8, 126.4, 129.3, 129.7, 131.5, 131.6, 132.3, 133, 135.4, 138.4, 139, 144, 191. Mass spectrum (DIP-MS) m/z M+ 583 (100%). HRMS (EI+) m/z found, 583.2878; calcd, 583.2875. 2-((4-(2-(4-(2-(4-(3,6-Di-tert-butyl-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)phenyl)-methylene)malononitrile (PPM). In a dry round-bottom flask was added 6 (1 mmol), malononitrile (1.1 mmol), basic aluminum oxide (10 mmol), and dry toluene (30 mL). The mixture was refluxed under argon for 5 h. The mixture was filtered hot, and the residue was washed several times successively with hot ethyl acetate and dichloromethane. The filtrate was then dried, and the solid obtained was purified by chromatography (silica gel, 80% hexane in ethylacetate) to obtain pure PPM (60%) as a bright yellow solid. 1H NMR (300 MHz, CDCl3) δ 1.50 (s, 18 H), 7.4 (d, 2H), 7.46 (d, 2H), 7.7 (m, 6H), 7.64 (d, 2H), 7.72 (m, 3H), 7.89 (d, 2H), 8.15 (s, 2H). 13C NMR (300 MHz, CDCl3) δ 31.5, 34.5, 83, 89, 90, 91, 95, 109, 113, 114, 117, 120, 121, 123, 123.2, 125, 129, 129.3, 130, 131, 132, 133, 138.4, 139, 143, 158. Mass spectrum (DIP-MS) m/z M+ 631 (100%). HRMS (EI+) m/z found, 631.29858; calcd, 631.29875. Fluorescence Lifetime (τF) Measurement. The samples containing PBM and PPM in THF and nanoparticles dispersed in THF/ water were placed in quartz cuvettes. Fluorescence decay profiles
of the argon-degassed (∼15 min) samples were recorded using a single photon counting spectrofluorimeter. Decays were monitored at the corresponding emission maximum of the compounds. In-built software allowed the fitting of the decay spectra (χ2 ) 1-1.5) and yielded the fluorescence lifetimes. Preparation of Nanoparticles. Samples of predetermined concentrations of PBM and PPM in THF were made. Appropriate volumes of these solutions were taken in different vials, and different amounts of distilled water were rapidly injected into those vials so as to maintain the concentrations of the final solutions the same. The formation of nanoparticles at the appropriate THF/water ratios could be clearly observed under a 366 nm UV lamp. In some experiments, different volumes of water were injected into the same quantity of THF solution to prepare the nanoparticle solutions of different concentrations. SEM Images of Nanoparticles. SEM images were recorded on a Hitachi S-2700 electron microscope at 15 kV. Samples for SEM were prepared by placing few drops of nanoparticle suspension onto a glass coverslip placed on an aluminum stub. The samples were allowed to dry in an oven (45 °C) before viewing under the electron microscope. To enhance the contrast and quality of the SEM images, the samples were sputter-coated with gold/palladium. Quartz PCI 7 imaging software was utilized to process the images and determine the size of the particles. Confocal Microscope Image of the PBM Particles. Confocal microscopy was performed using an Olympus FluoView 1000 microscope. The same method employed to prepare the samples for SEM images was used to prepare the samples for confocal microscopy (objective lens 100×). The excitation source was a diode blue laser (405 nm), and fluorescence was detected using standard three confocal channels (three photomultiplier detectors). The emission filter (low-pass,
