Using Buckyballs To Cut Off Light! Novel Fullerene ... - ACS Publications

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Chem. Mater. 2004, 16, 4790-4798

Using Buckyballs To Cut Off Light! Novel Fullerene Materials with Unique Optical Transmission Characteristics Han Peng,† Fennie S. M. Leung,† Andrew X. Wu,† Yuping Dong,† Yongqiang Dong,†,‡ Nai-Teng Yu,† Xinde Feng,‡ and Ben Zhong Tang*,†,‡ Department of Chemistry, Institute of Nano Materials and Technology, and Center for Display Research, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China, and Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received February 28, 2004. Revised Manuscript Received June 27, 2004

A simple rule governing the light transmission through a fullerene solution or solid is revealed, and a group of fullerene derivatives, glasses, and polymers are found to be able to continuously cut off lights of any wavelength in almost the entire UV and visible spectral region in a predictable and reliable manner. The solutions of aminated fullerenes 1-4 give structureless light transmission spectra well-resembling those of cutoff optical filters. The spectrum bathochromically moves with an increase in concentration (c), whose cutoff wavelength (λc) increases logarithmically with bc, i.e., λc ) R log bc + k, where b is the path length and R and k are constants. The spectra are reproducible and stable, changing little over a long period of time. In the solutions, the fullerene molecules aggregate into nanoclusters and their average sizes increase with increasing concentration, suggesting that the formation and growth of the fullerene nanoaggregates are responsible for the concentratochromism. The chromic effects are also observed in the solid state: the transmission spectra of the fullerenated glasses and polymers 5-7 all red shift with increasing C60 content. The spectrum of the parent polymer virtually does not vary with concentration, proving that the buckyball is the origin of the novel concentratochromism.

Introduction The Sun is a crucial existence to the Earth. The beneficial effects of the Sun to the Earth can never be overestimated: without the Sun, there would be no photosynthesis, no trees, no vegetables, ... On the other hand, the Sun also causes trouble on the Earth: under strong sunlight, milk spoils, beer rots, oil goes rancid, ...1 Radiation from the Sun is one of the major causes of ophthalmic damage, including cataract formation and tissue injury. Overexposure of the body to sunlight can result in erythema, skin cancer, and premature skin aging. The damaging effect of sunlight is wavelength (λ) dependent.1,2 The UV portion of sunlight (λ < 400 nm) is most harmful, which injures eyes, deteriorates the epidermis, and degrades foods. Intense blue and green lights (λ ≈ 400-550 nm) are detrimental to visual performance. Radiation with λ > 600 nm is generally harmless, but can, under certain circumstances, be * To whom correspondence should be addressed at the Department of Chemistry, The Hong Kong University of Science & Technology. Phone: +852-2358-7375. Fax: +852-2358-1594. E-mail: tangbenz@ ust.hk. † The Hong Kong University of Science & Technology. ‡ Peking University. (1) Consequences of Exposure to Sunlight; Giacomoni, P. U., Ed.; Elsevier: Amsterdam, 2001. (2) For reviews, see: (a) Albert, M. R.; Ostheimer, K. G. J. Am. Acad. Dermatol. 2003, 49, 1096-1106. (b) Luca, R. M.; Ponsonby, A. L. Med. J. Austr. 2002, 177, 594-598. (c) Mackerness, S. A. H. Plant Growth Regul. 2000, 32, 27-39. (d) Beissert, S.; Granstein, R. D. Crit. Rev. Biochem. Mol. 1996, 31, 381-404.

destructive due to its heating effect on the receptor segments in the retinal cells. To be protected from the injurious radiations of the Sun, humankind has made unremitting efforts in developing light-shielding materials.3 With the UV hole in the stratospheric ozone layer widening, more damaging light from the Sun will reach the Earth.4,5 The exploration of new optical materials that can cut off light of harmful wavelengths at will is thus of obvious contemporary public interest. Scientific and technological advancements also call for development of new optical materials that can tune and define the wavelength characteristics of transmitted light. Many photophysical processes and photochemical reactions are wavelength-sensitive,6 and optical filters with different transmission features are basic components of modern photonic toolboxes.7 Judicious use of (3) For reviews, see: (a) Ting, W. W.; Vest, C. D.; Sontheimer, R. Int. J. Dermatol. 2003, 42, 505-513. (b) Weinstock, M. A. Curr. Opin. Oncol. 2000, 12, 159-162. (c) Gasparro, F. P.; Mitchnick, M.; Nash, J. F. Photochem. Photobiol. 1998, 68, 243-256. (4) Atmospheric Photochemistry; Burrows, J. P., Moortgat, G. K., Eds.; Elsevier: Amsterdam, 2003. (5) For reviews, see: (a) Hollosy, F. Micron 2002, 33, 179-197. (b) Madronich, S.; McKenzie, R. L.; Bjorn, L. O.; Caldwell, M. M. J. Photochem. Photobiol., B 1998, 46, 5-19. (6) (a) Photoreaction Control and Photofunctional Materials; Tachiya, M., Arakawa, H., Sugihara, H., Eds.; Elsevier: Amsterdam, 2003. (b) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; M. Dekker: New York, 1993. (c) Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990. (d) CRC Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989.

10.1021/cm049680l CCC: $27.50 © 2004 American Chemical Society Published on Web 08/07/2004

Light Transmission through Fullerene Materials

the optical filters can prevent undesired events from happening but prompt desired ones to proceed. Photoresponsive materials are essential to a number of hightech processes, examples of which include photoresists in chip fabrication, photocurable polymer-dispersed liquid crystals in display devices, and photoreceptors in photovoltaic cells. Advanced pharmaceutical products, such as phototherapy reagents, photocurable dental resins, and phototriggering drug-delivery implants, need to be shielded from, and exposed to, light of specific wavelengths, to keep them dormant (during storage) and activated (when needed), respectively. Patients of cataract surgery are uncomfortable with blue light and need to wear medicaid lenses that can filter out the hurting rays. Many people (e.g., semiconductor cleanroom workers, UV sterilizers, laser operators, electric welders, and steel smelters) need to use light-filtering materials to protect themselves from prolonged exposure to constant radiation of injurious rays. Clearly, the needs and uses of optical filters are widespread. The exploration of new optical materials that can be easily prepared and whose transmission properties can be readily tuned will therefore undoubtedly contribute to scientific pursuit, technological innovation, and humankind healthcare. The manufacture of conventional optical filters is, however, arduous, and the manipulations of their cutoff wavelengths (λc) are difficult. The mainstream of the optical filters is colored glasses, which are made by incorporating metal ions into glass matrixes during the glass fabrication.7,8 The processes are complex and energy-consuming and involve many steps, including batching, melting, refining, and striking, in which large amounts (often 30-50 wt %) of many (normally 5-10) different components have to be admixed with silica at high temperatures (commonly 1000 deg), plus subsequent post-treatments such as annealing and finishing. Changing the λc of such a filter is a nontrivial job: the recipes (colorant components, feed ratios, etc.) need to be reformulated, and the processes (melting, annealing, etc.) must be reestablished accordingly. The glass making and its λc modulation are in some sense more an art than a science. Because plastics offer advantages of low cost, low density, high flexibility, high impact strength, etc., they are replacing glasses in many applications.9 The commonly used technology for making plastic-based optical filters is the dip dyeing process, in which “organic glasses” such as poly(methyl methacrylate) (PMMA), poly(bisphenol A carbonate) (PC), and CR-39 resins are immersed in the hot or cold dye solutions for some time (7) (a) Kochergin, V. Omnidirectional Optical Filters; Kluwer: Boston, 2003. (b) Macleod, H. A. Thin-film Optical Filters, 3rd ed.; Institute of Physics: Philadelphia, PA, 2001. (c) Madsen, C. K.; Zhao, J. H. Optical Filter Design and Analysis; Wiley: New York, 1999. (8) (a) Bray, C. Dictionary of Glass: Materials and Techniques; A&C Black: London, 1995. (b) International Commission on Glass. Dictionary of Glass-Making; Elsevier: Amsterdam, 1992. (c) Bansal, N. P.; Doremus, R. H. Handbook of Glass Properties; Academic Press: Orlando, FL, 1986. (d) Handbook of Glass Data; Elsevier: Amsterdam, 1983. (9) (a) Design, Fabrication, and Applications of Precision Plastic Optics; Ning, X., Hebert, R. T., Eds.; SPIE: Bellingham, WA, 1995. (b) Photonic Polymer Systems; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M., Gresser, J. D., Eds.; M. Dekker: New York, 1998. (c) Electrical and Optical Polymer Systems; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M., Gresser, J. D., Eds.; M. Dekker: New York, 1998.

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to allow the dyes to penetrate into the plastic matrixes.10 Because of the physical blending nature of the dyeing process, the distributions of the dyes in the plastics change with time when the filters are put into use, causing serious problems in their performance stability. The λc values achievable with the stained plastics are narrow in range and difficult to change, because the combined uses of many different dyes may darken the plastic filters to an unacceptable level. The working principle of an optical filter is basically the modulation of light transmission through a medium. A cutoff filter, for example, selectively blocks the path of light of certain wavelengths while allowing light of other wavelengths to pass with little or ideally no change.7 Light transmission through a dilute solution of isolated or nonaggregative species obeys the classic Beer-Lambert law:

A ) -(log T) ) abc

(1)

where A is the absorbance, T is the transmittance, a is the absorptivity, b is the path length, and c is the concentration of the absorbing solute.11 Spectroscopy textbooks and laboratory guides often remind us that the linear A-c correlation or the semilogarithmic T-c relationship defined by eq 1 only holds for dilute solutions. At high concentrations, deviations from Beer’s law occur, due to, for example, associative aggregations of the solutes.11,12 Almost all optical materials including optical filters are, however, practically used as “solid solutions” in the solid state, in which the colorant or dye “solutes” are admixed with the glass or plastic “solvents”. As mentioned above, such a filter commonly contains large amounts of different colorants or dyes, with 50% concentration not being unusual. Is there any “law” governing the light transmission through such a concentrated solution or solid mixture? How will λc change with c and/or b? These questions are of fundamental importance but have yet to be answered with satisfaction. C60 is a good optical limiter but has low solubility and poor transparency.13-15 During our research on the development of C60-based optical limiters with better processability, higher linear transmittance, and im(10) (a) Rosato, D. V. Rosato’s Plastics Encyclopedia and Dictionary; Hanser: Munich, 1993. (b) Cheremisinoff, N. P. Condensed Encyclopedia of Polymer Engineering Terms; Butterworth-Heinemann: Boston, 2001. (11) (a) Denny, R. C.; Sinclair, R. Visible and Ultraviolet Spectroscopy; Wiley: New York, 1987. (b) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; Wiley: New York, 1998. (12) (a) Mueller-Harvey, I.; Baker, R. M. Chemical Analysis in the Laboratory: A Basic Guide; Royal Society of Chemistry: Cambridge, U.K., 2002. (b) Laboratory Experiments; Phywe Systeme Gmbh: Go¨ttingen, Germany, 2003. (c) Christian, G. D. Analytical Chemistry, 6th ed.; Wiley: New York, 2004. (13) (a) Optical and Electronic Properties of Fullerenes and Fullerenebased Materials; Shinar, J., Vardeny, Z. V., Kafafi, Z. H., Eds.; M. Dekker: New York, 2000. (b) Fullerenes: from Synthesis to Optoelectronic Properties; Guldi, D. M., Martin, N., Eds.; Kluwer: Boston, 2002. (c) Fullerenes: Chemistry, Physics, and Technology; Kadish, K. M., Ruoff, R. S., Eds.; Wiley: New York, 2000. (14) For reviews, see: (a) Nierengarten, J. F. Top. Curr. Chem. 2003, 228, 87-110. (b) Kreher, D.; Cariou, M.; Liu, S. G.; Levillain, E.; Veciana, J.; Rovira, C.; Gorgues, A.; Hudhomme, P. J. Mater. Chem. 2002, 12, 2137-2159. (c) Innocenzi, P.; Brusatin, G. Chem. Mater. 2001, 13, 3126-3139. (d) Hollins, R. C. Curr. Opin. Solid State Mater. Sci. 1999, 4, 189-196. (e) Sun, Y. P.; Riggs, J. E. Int. Rev. Phys. Chem. 1999, 18, 43-90. (f) Tutt, L. W.; Boggess, T. F. Prog. Quantum Electron. 1993, 17, 299-338.

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proved nonlinear power-limiting performance,16 we needed to check the light transmissions through the fullerene derivatives, glasses, and polymers at different solution concentrations or fullerene contents. We found that the transmission spectrum of the fullerene solution continuously red shifts with fullerene concentration, with its λc varied with c in a semilogarithmic fashion.17,18 All of the fullerene materials were prepared by simple chemical reactions at normal temperatures, and the fullerene colorants or dyes were covalently attached to, or supramolecularly complexed through hydrogen bonds with, the glasses or plastics, hence suffering no compositional drift.18,19 These fullerene materials are thus a group of new optical filters with such remarkable attributes as easy fabrication, predictable wavelength tuning, and excellent performance stability. Experimental Section Materials. 2,2′-Azobisisobutyronitrile (AIBN) and C60 (>99.9%) were purchased from Acros and MER, respectively, and stored in a dark, cold place before use. MMA, CR-39, and tetraethyl orthosilicate (TEOS) were all purchased from Aldrich and purified by distillation under reduced pressure in our laboratories. PC was purchased from Waters and purified by repeated precipitation. All other reagents and solvents were purchased from Aldrich, some of which were further purified by distillation prior to use. The aminated fullerenes (AFs) 1-4, fullerene glasses AF/SiO2 and 4-SiO2, fullerenated PC (5), poly(C60-co-MMA) (6), and poly[C60-co-(CR-39)] (7) were prepared, respectively, by amination (eq 2), sol-gel reaction (eqs 3 and 4), macromolecular reaction (eq 5), and radical copolymerization (eq 6), according to our previously published procedures (Scheme 1).17a,18 The fullerene materials were thoroughly purified by repeated filtration, precipitation, and/or column chromatography, following the procedures established in our previous studies.16-18 The purified samples all gave spectroscopic data in agreement with their expected molecular structures, as demonstrated by the examples of NMR and MS spectra given in Figures 1 and S1 (Supporting Information), which are all well-resolved and readily assignable. The average degrees of amination (x) for 1-4 were estimated by spectral analyses and (15) (a) Tutt, L. W.; Kost, A. Nature 1992, 356, 225-226. (b) An, Y. Z.; Ellis, G. A.; Viado, A. L.; Rubin, Y. J. Org. Chem. 1995, 60, 63536361. (c) Beck, M. T.; Mandi, G. Fullerene Sci. Technol. 1997, 5, 291310. (d) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903-1907. (e) Goh, H. W.; Goh, S. H.; Xu, G. Q.; Lee, K. Y.; Yang, G. Y.; Lee, Y. W.; Zhang, W. D. J. Phys. Chem. B 2003, 107, 6056-6062. (f) Kuang, L.; Chen, Q. Y.; Sargent, E. H.; Wang, Z. Y. J. Am. Chem. Soc. 2003, 125, 13648-13649. (g) Li, H. M.; Li, Y. L.; Zhai, J.; Cui, G. L.; Liu, H. B.; Xiao, S. Q.; Liu, Y.; Lu, F. S.; Jiang, L.; Zhu, D. B. Chem.sEur. J. 2003, 9, 6031-6038. (16) (a) Tang, B. Z.; Xu, H.; Lam, J. W. Y.; Lee, P. P. S.; Xu, K.; Sun, Q.; Cheuk, K. K. L. Chem. Mater. 2000, 12, 1446-1455. (b) Xu, H.; Sun, Q.; Lee, P. P. S.; Kwok, H. S.; Tang, B. Z. Thin Solid Films 2000, 363, 143-145. (c) Xu, H.; Tang, B. Z. J. Macromol. Sci., Pure Appl. Chem. 1999, A36, 1197-1207. (d) Tang, B. Z.; Peng, H.; Leung, S. M.; Yu, N.-T.; Hiraoka, H.; Fok, M. W. In Materials for Optical Limiting II; Sutherland, R., Pachter, R., Hood, P., Hagan, D., Lewis, K., Perry, J. W., Eds.; Materials Research Society: Pittsburgh, PA, 1997; pp 69-74. (17) (a) Tang, B. Z.; Yu, N.-T.; Peng, H.; Leung, S. M.; Wu, X. U.S. Patent 6,066,272, 2000. (b) Peng, H.; Leung, S. M.; Tang, B. Z. Chin. J. Polym. Sci. 1997, 15, 193-198. (c) Tang, B. Z. Adv. Mater. 1996, 8, 939. (d) Peng, H.; Leung, S. M.; Au, C. F.; Wu, X.; Yu, N.-T.; Tang, B. Z. Polym. Mater. Sci. Eng. 1996, 75, 247-248. (18) (a) Peng, H.; Lam, J. W. Y.; Leung, F. S. M.; Poon, T. W. H.; Wu, A. X.; Yu, N.-T.; Tang, B. Z. J. Sol.-Gel Sci. Technol. 2001, 22, 205-218. (b) Tang, B. Z.; Peng, H.; Leung, S. M.; Au, C. F.; Poon, W. H.; Chen, H.; Wu, X.; Fok, M. W.; Yu, N.-T.; Hiraoka, H.; Song, C.; Fu, J.; Ge, W.; Wong, K. L. G.; Monde, T.; Nemoto, F.; Su, K. C. Macromolecules 1998, 31, 103-108. (c) Tang, B. Z.; Leung, S. M.; Peng, H.; Yu, N.-T.; Su, K. C. Macromolecules 1997, 30, 2848-2852. (19) Rousseau, F.; Poinsignon, C.; Garcia, J.; Popall, M. Chem. Mater. 1995, 7, 828-839.

Figure 1. 1H NMR spectrum of HxC60[NH(CH2)6OH]x (1) in chloroform-d/DMSO-d6 (upper panel) and 13C NMR spectrum of HxC60[NH(CH2)3Si(OCH2CH3)3]x (4) in CS2/chloroform-d (lower panel).

Scheme 1. Preparation of the Fullerene Materials Used in This Study

thermogravimetric measurements to be 7 (1), 11 (2), 11 (3), and 4 (4). While the fullerene glasses [C60/SiO2 (8)] prepared by blending small amounts of C60 (e.g., ∼0.01%) into TEOS solutions were morphologically inhomogeneous, those fabricated from the sol-gel processes of aminated fullerenes with TEOS (AF/SiO2 and 4-SiO2) were macroscopically homogeneous and visually clear, as shown by the examples given in

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Figure 2. Photographs of the sol-gel glasses HxC60[NH(CH2)3SiO3/2]x-/SiO2 (4-SiO2), in which the fullerene moieties are covalently bound to the glass substrates (A, B). A glass of C60/ SiO2 (8) made from physically blending the two components in the sol-gel mixture is shown in (C) for comparison. C60 content (wt %): (A) 0.10, (B) 0.52, (C) 0.0097. Figure 2, thanks to the homogeneous reactions as well as covalent bonding and/or supramolecular complexation between the components. The fullerene polymers 5-7 of high molecular weights (Mw up to ∼150 × 103) were obtained from the radical copolymerizations17 or polymer reactions.18 The C60 contents of the fullerene glasses and polymers were determined by spectroscopic and thermogravimetric analyses, as detailed in our previous papers.17,18 Instrumentation. The technical details of the instruments employed for the structural elucidations of the fullerene molecules including NMR, IR, MS, and TGA were given in our previous papers.16-18 Light transmission spectra of the fullerene materials were recorded on a Milton Roy Spectronic 3000 diode array spectrometer. The average power of its lamps was measured to be ∼0.14 mW/cm2 using a Newport silicon photodetector. For a solution sample, the transmission spectrum was measured at room temperature (∼23 °C) using a 1 cm2 quartz cell (b ) 1.0 cm). The corresponding pure solvent was used as the reference. Care was taken to make sure that the spectrum was taken in the transparent region of the solvent.11 For a glass or film sample, the path length was obviously the thickness of the glass or film and the spectrum was directly measured using a blank reference. For size analyses of fullerene nanoclusters in solutions, concentrated solutions of 1 in methanol were initially prepared, which were gradually diluted with the same solvent to desired concentrations. The sizes of the nanoparticles of 1 were measured on a PD2000DLS dynamic light scattering detector (Precision Detectors). For morphological analyses in the solid state, tiny drops (∼5-10 µL) of methanol solutions of 1 were deposited onto carbon-coated copper grids and the solvents were allowed to evaporate under ambient conditions. Morphological structures of the deposits were imaged, and electron diffraction (ED) patterns of the fullerene crystallites were collected on a JEOL 2010 transmission electron microscope operating at an accelerating voltage of 200 kV.

Results and Discussion Light Transmission through Aminated Fullerene Solutions. Amination is a versatile means for fullerene functionalization.20,21 Simply heating a C60/amine mixture at 100 °C under nitrogen for 1 day allowed us to attach multiple amine moieties to the buckyball (cf. Scheme 1, eq 1), rendering the fullerene soluble in many (20) For a review, see: Da Ros, T.; Prato, M. Chem. Commun. 1999, 663-669. (21) (a) Hirsch, A.; Li, Q. Y.; Wudl, F. Angew. Chem., Int. Ed. Engl. 1991, 30, 1309-1310. (b) Wudl, F.; Hirsch, A.; Khemani, K. C.; Suzuki, T.; Allemand, P. M.; Koch, A.; Eckert, H.; Srdanov, G.; Webb, H. M. ACS Symp. Ser. 1992, 481, 161-175. (c) Wilson, S. R.; Wu, Y. H. Org. Mass Spectrom. 1994, 29, 186-191. (d) Diekers, M.; Hirsch, A.; Pyo, S.; Rivera, J.; Echegoyen, L. Eur. J. Org. Chem. 1998, 1111-1121. (e) Miller, G. P.; Tetreau, M. C.; Olmstead, M. M.; Lord, P. A.; Balch, A. L. Chem. Commun. 2001, 1758-1759. (f) Ohno, T.; Moriwaki, K.; Miyata, T. J. Org. Chem. 2001, 66, 3397-3401.

Figure 3. Light transmission spectra of ethanol solutions of 1. Path length: 1.0 cm. Concentration (mg/mL): (A) 0.090, (B) 0.179, (C) 0.358, (D) 0.716, (E) 1.432, (F) 2.864, (G) 5.728, (H) 11.456, (I) 22.913, (J) 45.825, (K) 91.650.

common solvents including water.17a,22,23 The excellent solubility of the aminated fullerenes made it possible to check their optical power-limiting performance at concentrations as high as ∼100 mg/mL in ethanol,22 which could never be achieved by the C60 parent, whose solubility in ethanol is as low as 0.001 mg/mL.24 The concentrated solutions of the aminated fullerenes exhibited high optical nonlinearity: for example, the power suppression ratio Ft,s/Fi,m25-27 of an ethanol solution of 2 with a concentration of 2.18 mg/mL was as high as ∼25.16d The nonlinear optical property of an optical limiter is known to vary with its linear transmittance. We measured optical limiting effects of the aminated fullerene solutions at different linear transmittances by changing their concentrations,16d during which their light transmission spectra were taken on a UV-vis spectrometer. As can be seen from Figure 3, the transmission spectrum of the dilute ethanol solution of 1 with a concentration of 0.09 mg/mL is structureless, much like that of a cutoff optical filter. The λc7,28 of the solution is 239.7 nm, locating in the beginning part of the UV region. Interestingly, the transmission spectrum of 1 bathochromically moves almost horizontally with little change in shape when its solution concentration is increased. When the concentration is doubled, the λc is red-shifted to 272.1 nm. The solution with a concen(22) Leung, F. S. M. M.Phil. Thesis, Hong Kong University of Science & Technology, August 1996. (23) The solutions with low to medium concentrations are macroscopically homogeneous and visually transparent, whereas those with high concentrations are difficult to observe because they are deeply colored. (24) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379-3383. (25) Defined as a ratio of saturated transmitted fluence (Ft,s) to maximum incident fluence (Fi,m).26 (26) Peng, H.; Cheng, L.; Luo, J.; Xu, K.; Sun, Q.; Dong, Y.; Salhi, F.; Lee, P. P. S.; Chen, J.; Tang, B. Z. Macromolecules 2002, 35, 53495351. (27) For reviews, see: (a) O’Flaherty, S. M.; Hold, S. V.; Cook, M. J.; Torres, T.; Chen, Y.; Hanack, M.; Blau, W. J. Adv. Mater. 2003, 15, 19-32. (b) Kreher, D.; Cariou, M.; Liu, S. G.; Levillain, E.; Veciana, J.; Rovira, C.; Gorgues, A.; Hudhomme, P. J. Mater. Chem. 2002, 12, 2137-2159. (28) Defined here as the wavelength at which the light transmittance is 0.1% (or 1‰).

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Figure 4. Concentration dependence of cutoff wavelengths of HxC60(NHR)x solutions: (A) 1 in ethanol, (B) HxC60(NH-cycloC6H11)x (2) in ethanol, (C) HxC60[NH(CH2CH2O)2H]x (3) in DMSO, and (D) 4 in ethanol. Path length: 1.0 cm.

Figure 5. Time dependence of cutoff wavelengths of the ethanol solutions of 2 with different concentrations (mg/mL): (A) 12.984, (B) 6.492, (C) 3.246, (D) 1.623, (E) 0.811, (F) 0.406, and (G) 0.203. Path length: 1.0 cm.

tration of 91.7 mg/mL gives a λc of 713.3 nm, which locates in the ending part of the visible region. When λc is plotted against c, the former is found to increase with the latter in a semilogarithmic manner over a wide spectral range (Figure 4, line A); that is, the λc-c plot fits the following equation:

λc ) R log c + k

(7)

where R and k are the slope and the intercept of the semilogarithmic plot, respectively.18b Clearly, the λc of 1 can be changed over almost the entire UV and visible regions by simply varying one parameter, namely, its solution concentration. Further investigations on the transmission spectra of other aminated fullerenes reveal that this intriguing property is not an isolated phenomenon for 1 alone but a general characteristic for all the fullerene derivatives. Like 1, its congeners 2-4 all undergo similar spectral shifts with their solution concentrations, with their λc-c plots all following the same semilogarithmic relationship, as shown by lines B-D in Figure 4. The transmission spectra of the aminated fullerene solutions are stable, as demonstrated by the examples given in Figure 5. Little drifts in λc are observed when the spectra of the ethanol solutions of 2 are retaken after the solutions have been stored under ambient conditions for 3-5 days. An identical λc value is obtained, no matter how the solution with the same concentration is prepared: by thinning down (adding the solvent to a concentrated solution) or by thickening up (evaporating the solvent from a dilute solution). This proves the excellent reproducibility and reversibility of the spectral manipulation. Solvent Effect on Light Transmission (Solvatochromism). The change of the transmission spectra with concentration may be considered as a special kind of chromic effect, that is, concentratochromism, which may be caused by the change in the sizes of the fullerene nanoclusters with concentration (vide infra). If that is the case, the spectra should also be susceptible to solvent (solvatochromism), because the interactions of solvents with solutes should affect the formation and growth of the nanoclusters. Indeed, when the solvent

Figure 6. Solvent effect on the cutoff wavelengths of alcohol solutions of 1. Path length: 1.0 cm.

is changed from ethanol to methanol, the transmission spectrum of 1 is red-shifted, and the λc-c plot for the methanol solutions is located right above that for the ethanol solutions (Figure 6). Again the solvatochromism is a general phenomenon: many solvents can alter the light transmissions through the fullerene solutions. Their semilogarithmic λc-c plots all give straight lines, and examples of the R and k values obtained from the plots for the solutions of 1 in different solvents are given in Table 1. Evidently, the R and k values are dramatically affected by the solvents, but it is difficult to use a single parameter to explain the experimental results. The solubility parameters (δ) of DMSO (12.0) and 1-propanol (11.9)29 are, for example, almost identical, but the R values of 1 in these solvents are quite different (Table 1, nos. 3 and 5). The dielectric constants () of m-cresol (12.44) and o-cresol (6.76) are both lower than that of ethanol (25.3),30 but the R values of 1 in m- and o-cresols are lower (134.4) and higher (197.5), respectively, than that (29) Grulke, E. A. In Polymer Handbook, 4th ed.; Brandrup J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999.

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Table 1. Solvatochromism in the 1 Solutionsa no.

solute

solvent

Rb

kb

1 2 3 4 5 6 7 8c 9c 10c

1 1 1 1 1 1 1 PS PMMA PC

m-cresol EtOH DMSO n-BuOH n-PrOH MeOH o-cresol THF THF THF

134.4 154.8 166.6 167.1 170.5 182.3 197.5 0.6 4.3 12.3

459.3 399.7 432.9 391.5 438.9 438.6 420.8 246.9 255.3 264.4

a At room temperature, path length b ) 1 cm. b Obtained from eq 7 (λc ) R log c + k). c Comparison data. Abbreviations: PS ) polystyrene, PMMA ) poly(methyl methacrylate), and PC ) poly(bisphenol A carbonate).

in ethanol (154.8). The solvatochromism may thus be the result of many interactive forces such as complexing powers and steric effects, which is under further investigation in our laboratories. Noticing that R is an indication of how sensitively λc changes with c, the very high R value of 1 in o-cresol (∼200) means that its transmission spectrum moves very fast with an increase in concentration or that a certain λc value can be achieved in this solvent at a relatively low concentration. For example, an o-cresol solution of 1 with a concentration of ∼26 mg/mL gives a λc of ∼700 nm; to do the same job, an m-cresol solution with a concentration as high as ∼62 mg/mL is required. Self-Aggregation of Aminated Fullerenes into Nanoclusters. Will other molecules also exhibit the concentratochromism? To answer this question, light transmission spectra of THF solutions of some aromatic compounds (benzene, naphthalene, anthracene, phenylacetylene, etc.) are measured at varying concentrations. When the concentration of the phenylacetylene solution, for example, is increased 32-fold, its λc is shifted by merely 12.2 nm and accordingly its R value is small (8.5). The spectra of the PS solutions virtually do not change over a very wide concentration range (6.4751.78 mg/mL; Figure S2, Supporting Information) and its R value is again very small (0.6; Table 1, no. 8). Similarly, the PMMA and PC solutions give small R values. The concentratochromism is thus a unique optical transmission characteristic of the fullerene materials. What is the cause of this novel concentratochromism? C60 and many of its derivatives are known to aggregate into crystalline clusters in the solid state as well as in solutions.31,32 Will the aminated fullerenes also aggregate in their solutions? The answer to this question is a firm yes: examination of the methanol solutions of 1 by a particle size analyzer proves that the fullerene molecules aggregate into nanoclusters. The average size of the nanoclusters is found to increase with an increase in concentration (Figure 7). The aggregation of the fullerene molecules into nanoclusters is further confirmed by transmission electron microscopy (TEM) (30) Wohlfarth, C. In CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. R., Frederikse, H. P. R., Eds.; CRC: Boca Raton, FL, 1994; pp 6-155-6-188. (31) For reviews, see: (a) Braun, M.; Hirsch, A. Carbon 2000, 38, 1565-1572. (b) Bezmel’nitsyn, V. N.; Eletskii, A. V.; Okun’, M. V. Usp. Fiz. Nauk 1998, 168, 1195-1220. (c) Argentine, S. M.; Kotz, K. T.; Rudalevige, T.; Zaziski, D.; Francis, A. H.; Zand, R. Schleuter, J. A. Res. Chem. Intermed. 1997, 23, 601-620.

Figure 7. Concentration dependence of the average particle sizes of the 1 clusters formed in its methanol solutions.

Figure 8. TEM image of the 1 nanoclusters formed by depositing a methanol solution of 1 (0.025 mg/mL) onto a copper grid. Inset: electron diffraction pattern of the nanocrystallites of 1.

analyses, and ED patterns reveal that the nanoclusters formed by a dilute methanol solution of 1 on a carbon grid are crystalline (Figure 8). The aggregates of 1 obtained from its concentrated solutions are, however, noncrystalline, failing to show any defined ED patterns. (32) (a) Zhang, P.; Li, J.; Liu, D.; Qin, Y.; Guo, Z.-X.; Zhu, D. Langmuir 2004, 20, 1466-1472. (b) Matsumoto, M.; Inukai, J.; Tsutsumi, E.; Yoshimoto, S.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, M.; Murata, Y.; Komatsu, K. Langmuir 2004, 20, 1245-1250. (c) Manil, B.; Maunoury, L.; Huber, B. A.; Jensen, J.; Schmidt, H. T.; Zettergren, H.; Cederquist, H.; Tomita, S.; Hvelplund, P. Phys. Rev. Lett. 2003, 91, 215504. (d) Hasobe, T.; Imahori, H.; Kamat, P. V.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 14962-14963. (e) Song, T.; Dai, S.; Tam, K. C.; Lee, S. Y.; Goh, S. H. Polymer 2003, 44, 2529-2536. (f) Guo, Z. X.; Sun, N.; Li, J. X.; Dai, L. M.; Zhu, D. B. Langmuir 2002, 18, 90179021. (g) Zhou, S. Q.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944-1947. (h) Alargova, R. G.; Deguchi, S.; Tsujii, K. J. Am. Chem. Soc. 2001, 123, 10460-10467. (i) Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Adv. Mater. 2001, 13, 1614-1617. (j) Chen, X. L.; Jenekhe, S. A. Langmuir 1999, 15, 8007-8017. (k) Barrow, M. P.; Tower, N. J.; Taylor, R.; Drewello, T. Chem. Phys. Lett. 1998, 293, 302308. (l) Wang, Y. M.; Kamat, P. V.; Patterson, L. K. J. Phys. Chem. 1993, 97, 8793-8797. (m) Sun, Y. P.; Bunker, C. E. Chem. Mater. 1994, 6, 578-580.

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Scheme 2. Schematic Illustration of Nanocluster Growth with Concentration

The aminated fullerene is amphiphilic, whose hydrophobic core is adorned with hydrophilic pendants. When such molecules are mixed with methanol, the hydrophobic fullerene cores tend to cluster together to minimize their exposures to the incompatible environment, whereas the hydrophilic amine pendants tend to molecularly dissolve into the polar solvent. The net outcome of these two antagonistic forces is the “dissolution” of the fullerene molecules into the solvent as nanoclusters, whose fullerene kernels are surrounded by the well-solvated amine coronas. This process shares some similarity with one of our previously studied amphiphilic systems: the dissolution of the amino acidcontaining polyacetylenes into the polar solvents as nanostructured aggregates.33,34 When the concentration of the aminated fullerene is increased, more buckyballs will be aggregated and larger nanoclusters will be formed (Scheme 2). This increase in nanocluster size with concentration is probably the origin of the observed phenomenon of concentratochromism, because it is wellknown that the sizes of nanoclusters can affect their electronic and photonic properties to great extents.35-37 Both electronic absorption and light scattering are believed to be involved in the concentratochromic process, detailed mechanisms of which will be investigated in collaboration with our physicist colleagues. (33) For reviews, see: (a) Tang, B. Z. Polym. News 2001, 26, 262272. (b) Cheuk, K. K. L.; Li, B.; Tang, B. Z. Curr. Trends Polym. Sci. 2002, 7, 41-55. (c) Cheuk, K. K. L.; Li, B. S.; Tang, B. Z. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol. 8, pp 703-713. (34) (a) Li, B.; Cheuk, K. K. L.; Salhi, F.; Lam, J. W. Y.; Cha, J. A. K.; Xiao, X.; Bai, C.; Tang, B. Z. Nano Lett. 2001, 1, 323-328. (b) Li, B.; Cheuk, K. K. L.; Ling, L.; Chen, J.; Xiao, X.; Bai, C.; Tang, B. Z. Macromolecules 2003, 36, 77-85. (c) Li, B.; Cheuk, K. K. L.; Yang, D.; Lam, J. W. Y.; Wan, L.; Bai, C.; Tang, B. Z. Macromolecules 2003, 36, 5447-5450. (d) Cheuk, K. K. L.; Lam, J. W. Y.; Chen, J.; Lai, L. M.; Tang, B. Z. Macromolecules 2003, 36, 5947-5959. (e) Cheuk, K. K. L.; Lam, J. W. Y.; Lai, L. M.; Dong, Y.; Tang, B. Z. Macromolecules 2003, 36, 9752-9762. (f) Li, B.; Chen, J.; Zhu, C. Leung, K. K. L.; Wan, L.; Bai, C.; Tang, B. Z. Langmuir, in press. (35) For reviews, see: (a) Watanabe, A. J. Organomet. Chem. 2003, 685, 122-133. (b) Dalton, L. R. J. Phys.: Condens. Matter. 2003, 15, 897-934. (c) Leclere, P.; Hennebicq, E.; Calderone, A.; Brocorens, P.; Grimsdale, A. C.; Mullen, K.; Bredas, J. L.; Lazzaroni, R. Prog. Polym. Sci. 2003, 28, 55-81. (d) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18-52. (e) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545-610. (f) Herron, N.; Thorn, D. L. Adv. Mater. 1998, 10, 1173-1184. (g) Fendler, J. H. Adv. Chem. Ser. 1994, 240, 413-438. (h) Schmidt, H.; Krug, H. ACS Symp. Ser. 1994, 572, 183-194. (i) Ozin, G. A.; Ozkar, S. Chem. Mater. 1992, 4, 511-521. (36) (a) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (b) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (c) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609-611. (d) Meldrum, F. C.; Wade, V. J.; Nimmo, D. L.; Heywood, B. R.; Mann, S. Nature 1991, 349, 684-687.

Figure 9. Dependence of the cutoff wavelength (λc) on the C60 content (c) of the sol-gel glasses 4-SiO2 and HxC60[NH(CH2)6OH]x/SiO2 (1/SiO2) with varying glass thickness or path length (b ) 140-220 µm). C60 content (mg/g): for 4-SiO2, 0.52, 1.60, 2.50, 4.10; for 1/SiO2, 1.80, 3.60, 5.50.

Light Transmission through Fullerene Glasses and Polymers. The concentratochromism is, by its very nature, a solution property. The concentratochromic effect is of academic value, but solutions are difficult to use for practical applications. The aminated fullerenes were thus incorporated into silica glasses through a solgel process.17a,18a All the aminated fullerenes give macroscopically homogeneous glasses, as exemplified by the photographs of 4-SiO2 glasses A and B shown in Figure 2, due to the excellent solubility of the fullerene derivatives in the sol solutions and the good miscibility between the functionalized fullerenes and the silica gels aided by the hydrogen bond interaction.18a,19 In contrast, the buckyball distributions in the C60/SiO2 glasses prepared by physical blending are visually nonuniform, even when their C60 contents are very low (e.g., ∼0.01%; cf. Figure 2C). The color of sol-gel glass 4-SiO2 gradually deepens with increasing C60 content, visually manifesting the “concentratochromism”, if the glass is considered as a solid solution of the fullerene solute in the silica solvent. The light transmission spectra of the glasses are measured, but the plot of the λc obtained from the spectra versus the fullerene content (c) is scattered without showing a defined trend. Noticing that the glasses differ in thickness (b), replotting the λc data against bc (instead of c alone) offers a straight semilogarithmic line, as shown in Figure 9. The light transmission through the fullerene glasses thus observes the following λc-bc relationship:

λc ) R log(bc) + k

(8)

Similarly, the semilogarithmic λc-bc plot of 1/SiO2 (37) (a) 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. (b) Chen, H.; Lam, J. W. Y.; Luo, J.; Ho, Y.; Tang, B. Z.; Zhu, D.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574576. (c) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 15351546. (d) Chen, J.; Xie, Z. Lam, J. W. Y.; Law, C. C. W.; Tang, B. Z. Macromolecules 2003, 36, 1108-1117. (e) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319-4327.

Light Transmission through Fullerene Materials

Figure 10. Light transmission spectra of THF solutions of C60-containing poly(methyl methacrylate) [C60-PMMA (6)]. C60 content: 0.97 wt %. Path length: 1.0 cm. Polymer concentration (mg/mL): (A) 2.95, (B) 5.90, (C) 11.80, (D) 23.59, (E) 47.18, (F) 82.57.

glasses gives a straight line, verifying the generality of eq 8. It is noted that eq 7 is a special case of eq 8, because the latter is converted to the former when b ) 1. In other words, eq 8 is a general formula that regulates light transmissions through the fullerene materials in both the solid and solution states. The fullerene molecules were further integrated with organic polymers. Among commodity plastics, PC, PMMA, and CR-39 resin are the best-known organic glasses because of their outstanding optical transparency.10 In the preparations of the fullerene-containing sol-gel glasses discussed above, C60 has been prefunctionalized by the amination reaction (cf. Scheme 1, eq 2). In the syntheses of the fullerene polymers, however, C60 is used without prefunctionalization and is directly melded with the polymer chains at the molecular level via the polymer reactions (eq 5) or radical copolymerizations (eq 6) developed by other and our groups.17,18,38,39 The fullerenated PCs (5) and poly(C60-co-MMA)s (6) are soluble and can be cast into thin films, whereas the poly[C60-co-(CR-39)]s (7) are cross-linked gels and are hence used as prepared. Light transmission spectra of the polymer solutions again continuously red shift with concentration, as can be seen from the example shown in Figure 10 for the THF solutions of polymer 6. The λc-c plots of 5 and 6 give straight lines (Figure 11), following the semilogarithmic relationship defined by eq 8 (or eq 7 at b ) 1). The R values of 5 and 6 (∼168-176) are much higher than those of their parents PMMA and PC (∼4-12), verifying that the concentratochromism of the fullerenated polymers is endowed by their C60 moieties. It is (38) (a) Camp, A. G.; Lary, A.; Ford, W. T. Macromolecules 1995, 28, 7959-7961. (b) Cao, T.; Webber, S. E. Macromolecules 1995, 28, 3741-3743. (c) Bunker, C. E.; Lawson, G. E.; Sun, Y. P. Macromolecules 1995, 28, 3744-3746. (d) Cao, T.; Webber, S. E. Macromolecules 1996, 29, 3826-3830. (39) (a) Tang, B. Z.; Peng, H.; Leung, S. M. In Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; Electrochemical Society: Pennington, NJ, 1997; Vol. 4, pp 655-667. (b) Tang, B. Z.; Peng, H. In Recent Advances in Overseas Polymer Research; He, T. B., Hu, H. J., Eds.; Chemical Industry Press: Beijing, 1997; Chapter 10, pp 165-173. (c) Huang, Y.; Peng, H.; Lam, J. W. Y.; Xu, Z.; Leung, F. S. M.; Mays, J. W.; Tang, B. Z. Polymer 2004, 45, 4811-4817.

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Figure 11. Concentration dependence of the cutoff wavelengths of the THF solutions of C60-containing polycarbonate [C60-PC (5)] and poly(methyl methacrylate) [C60-PMMA (6)]. C60 content (wt %): 2.61 (5), 0.97 (6). Path length: 1.0 cm. Data for THF solutions of PC and PMMA are shown for comparison.

Figure 12. Dependence of the cutoff wavelength (λc) on the C60 content (c) of the C60-containing CR-39 resins (7) with different film thicknesses or path lengths (b). C60 content (mg/ g): 0.28 (solid circle), 0.12 (open circle), 0.50 (open triangle), 2.18 (solid triangle). Path length (µm): (A) 11.0, (B) 17.7, (C) 45.5, (D) 32.3, (E) 38.0.

noteworthy that 6 contains as little as ∼1% C60 but can be used to cut off light over a spectral span as wide as ∼250 nm by simply varying its concentration. The C60 content of 5 (2.61%) is higher than that of 6 (0.97%), and the λc-c plot of the former is located above that of the latter, further proving that the C60 moieties are responsible for the concentratochromism of the fullerenated polymers. The thin films of the fullerenated CR-39 resins (7) with different C60 contents and path lengths exhibit a clear concentratochromic effect in the solid state, whose semilogarithmic λc-bc plot well obeys eq 8 (Figure 12). Film C has a lower C60 content (0.12 g/mg) than film A (0.28 mg/g), but the λc of the former (332.8 nm) is higher than that of the latter (317.8 nm), because the former is thicker (45.5 µm) than the latter (11.0 µm). This evidently demonstrates the path length effect on the light transmissions through the polymer films. The fullerenated polymer data shown in Figures 10-12 prove that the concentratochromism is not a special

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feature of the aminated fullerenes but a general attribute of many fullerene materials. Indeed, other fullerene polymers, including C60-PVC, C60-PVA, and C60-PHEMA,40,41 all exhibit the interesting concentratochromic effect.17,18,22 Concluding Remarks In this work, the unique light transmission characteristics of a group of fullerene derivatives, glasses, and polymers are revealed. The fullerene materials show light transmission spectra resembling those of cutoff filters and enjoy the following remarkable advantages over their counterparts of traditional optical filters: (1) the fullerene materials can be prepared by uncomplicated reactions or processes at normal temperatures, (2) their λc values are tunable by changing just one single parameter, that is, the solution concentration or fullerene content, and (3) the spectral modulation is continuous, reversible, predictable, and reliable. The fullerene materials are thus a group of excellent optical filters that can be used to cut off lights at will over a very wide spectral region (∼240-710 nm). The fullerene nanoclusters are found to increase in size with solution concentration, suggesting a correlation between the nanocluster size and the concentratochromic effect. Beer’s law governs the light transmissions through dilute solutions of isolated solutes. The fullerene solutions studied in this work are, however, concentrated, and the fullerene solutes are aggregative. The light transmissions through the concentrated fullerene solutions are found to obey the following semilogarithmic relationship:17,18,22

λT ) R log(bc) + k

(9)

with the special case of λT ) λc (T ) 0.1%) reported in this paper in detail. The validity of this equation extends (40) Abbreviations: PVC ) poly(vinyl chloride), PVA ) poly(vinyl alcohol), and PHEMA ) poly(2-hydroxyethyl methacrylate). Note that PHEMA is a optical polymer for making contact lenses.41 (41) Hodd, N. B. Pocket Guide to Contact Lenses; Arlington: London, 1988.

to the solid, a special form of concentrated “solution” in which the solute normally aggregates. The λT-bc relationship is of academic interest, because it satisfactorily formulates the key terms for light transmissions through a solution and a solid into a united simple equation. The λT-bc relationship is also of practical value. Many electronic, optical, and photonic systems, for example, light-emitting diodes and photovoltaic cells, are working in the solid state, in which the fullerene molecules may serve as active components.42,43 The light transmission relationship defined by eq 9 may help in understanding how light emanates from, or penetrates into, the devices, which will in turn help in advancing the system designs and spawn technology innovations. Acknowledgment. We acknowledge the financial support by the Research Grants Council of Hong Kong (Grants N_HKUST606_03, 604903, HKUST6085/02P, 6121/01P, and 6187/99P) and by the University Grants Committee of Hong Kong through an Area of Excellence (AoE) scheme (AoE/P-10/01-1-A). Supporting Information Available: MS spectrum of 4 and light transmission spectra of PS and PC solutions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM049680L (42) For reviews, see: (1) Lam, J. W. Y.; Tang, B. Z. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2607-2629. (2) Armaroli, N. Photochem. Photobiol. Sci. 2003, 2, 73-87. (c) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807-815. (d) Brabec, C. J.; Sariciftci, S. N. Monatsh. Chem. 2001, 132, 421-431. (e) Godovsky, D. Y. Adv. Polym. Sci. 2000, 153, 163-205. (f) Prato, M. Top. Curr. Chem. 1999, 199, 173-187. (g) Diederich, F.; Gomez-Lopez, M. Chem. Soc. Rev. 1999, 28, 263-277. (h) Hummelen, J. C.; Bellavia-Lund, C.; Wudl, F. Top. Curr. Chem. 1999, 199, 93-134. (43) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789-1791. (b) Chen, J.; Peng, H.; Law, C. C. W.; Dong, Y.; Lam, J. W. Y.; Williams, I. D.; Tang, B. Z. Macromolecules 2003, 36, 4319-4327. (c) Lam, J. W. Y.; Luo, J.; Dong, D.; Cheuk, K. K. L.; Tang, B. Z. Macromolecules 2002, 35, 8288-8299. (d) Lam, J. W. Y.; Dong, Y.; Cheuk, K. K. L.; Luo, J.; Xie, Z.; Kwok, H. S.; Mo, Z.; Tang, B. Z. Macromolecules 2002, 35, 1229-1240. (e) Tang, B. Z.; Chen, H.; Xu, R.; Lam, J. W. Y.; Cheuk, K. K. L.; Wong, H. N. C.; Wang, M. Chem. Mater. 2000, 12, 213-221. (f) Chen, H.; Xu, R.; Sun, Q.; Lam, J. W. Y.; Wang, M.; Tang, B. Z. Polym. Adv. Technol. 2000, 11, 442449. (g) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129-9139.