Structural Control of the Photoluminescence of Silole Regioisomers

J. Phys. Chem. B 2005, 109, 10061-10066

10061

Structural Control of the Photoluminescence of Silole Regioisomers and Their Utility as Sensitive Regiodiscriminating Chemosensors and Efficient Electroluminescent Materials Zhen Li,† Yongqiang Dong,† Baoxiu Mi,† Youhong Tang,# Matthias Ha1 ussler,† Hui Tong,† Yuping Dong,† Jacky W. Y. Lam,† Yan Ren,‡ Herman H. Y. Sung,† Kam S. Wong,‡ Ping Gao,# Ian D. Williams,† Hoi Sing Kwok,§ and Ben Zhong Tang*,†,§,⊥ Departments of Chemistry, Physics, and Chemical Engineering, 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, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: January 19, 2005; In Final Form: March 30, 2005

We synthesized a group of silole regioisomers 1x,y, whose photoluminescence varied dramatically with its regiostructure. By internally hindering the intramolecular rotation, we succeeded in creating a novel silole (13,4) that is strongly luminescent in solutions and whose fluorescence quantum yield in acetone is as high as 83%. We revealed that 13,4 was a sensitive chemosensor capable of optically discriminating nitroaromatic regioisomers of p-, o-, and m-nitroanilines. Against general belief, crystal formation of 12,4 blue-shifted its emission color and boosted its emission efficiency. The light-emitting diode based on the crystal of 12,4 emitted a strong blue light (464 nm) in a high current efficiency (5.86 cd/A).

Introduction

SCHEME 1

Chromophoric aggregation often quenches light emission, which constitutes a formidable obstacle to the development of efficient light-emitting diodes (LEDs) because aggregation is inherently involved in the film-forming processes of luminophoric molecules.1 We have recently observed an exactly opposite phenomenon, aggregation-induced emission (AIE): silole molecules such as hexaphenylsilole (HPS) that are nonemissive in solutions are induced to emit efficiently when they aggregate in poor solvents or in solid films.2 A possible cause for this novel AIE effect is that the intramolecular rotations (IMRs) of the phenyl peripheries against the silacyclopentadiene (silole) core is hindered in the aggregates.2,3 In support of this hypothesis, photoluminescence (PL) of an HPS solution was found to be boosted by such external processes as increasing solvent viscosity and decreasing solution temperature.4 These observations prompted us to envision that the silole PL might also be tuned by internal control of its IMR process at the molecular level. In this work, we attached isopropyl (iPr) groups to the phenyl rings of HPS to internally hinder its IMR process. The steric effect of the bulky iPr groups was found to greatly affect the dynamics of the singlet excited states of the HPS derivatives (1x,y; Scheme 1). The silole with a high IMR barrier (13,4) became highly luminescent in solution. Its PL was quenched by nitroaromatic regioisomers to varying extents; in other words, it acted as a chemical sensor with a capability of optically discriminating isomeric structures. Intriguingly, the AIE effect blue-shifted the PL color of 12,4 and an LED based * To whom correspondence should be addressed. Phone: +852-23587375. Fax: +852-2358-1594. E-mail: [email protected]. † Department of Chemistry, The Hong Kong University of Science & Technology. ‡ Department of Physics, The Hong Kong University of Science & Technology. # Department of Chemical Engineering, The Hong Kong University of Science & Technology. § Center for Display Research, The Hong Kong University of Science & Technology. ⊥ Department of Polymer Science and Engineering, Zhejiang University.

on its crystal emitted a blue light in a high electroluminescence (EL) efficiency. Experimental Section Materials. Tetrahydrofuran (THF; Labscan) was distilled from sodium benzophenone ketyl under dry nitrogen immediately prior to use. Triethylamine (RdH) was distilled under normal pressure and kept over potassium hydroxide under nitrogen. Dichlorodiphenylsilane, lithium wire, copper(I) iodide, dichlorobis(triphenylphosphine)palladium(II), 2,6-diisopropyl-

10.1021/jp0503462 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/22/2005

10062 J. Phys. Chem. B, Vol. 109, No. 20, 2005 aniline, sodium nitrite, potassium iodide, dimethylformamide (DMF), phenylacetylene, trimethylsilylacetylene, triphenylphosphine, potassium hydroxide, concentrated hydrochloric acid, and o-, m-, and p-nitroanilines were all purchased from Aldrich and used as received without further purification. Instrumentation. 1H and 13C NMR spectra were measured on a Bruker ARX 300 spectrometer with tetramethylsilane (TMS; δ ) 0 ppm) as the internal standard. Mass spectra were recorded on a Finnigan TSQ 7000 triple quadrupole spectrometer operating in a chemical ionization (CI) mode with methane as the carrier gas. UV-vis spectra were measured on a Milton Roy Spectronic 3000 Array spectrophotometer. X-ray diffraction intensity data were collected at 298 or 100 K on a Bruker-Nonius Smart Apex CCD diffractometer with graphite-monochromated Mo KR radiation. Single crystals of silole 12,4 were grown from hexane. Processing of the intensity data was carried out with the SAINT and SADABS routines, and the structure solution and refinement were carried out by the SHELXTL suite of X-ray programs (Version 6.10). The Materials Studio (MS) software of Accelrys Inc. (http:// www.accelrys.com/mstudio/) was used to compute the rotation barrier (ER) of 1x,y. The MS modeling system contains a Discover simulation engine, which incorporates a broad spectrum of molecular mechanics and dynamics methodologies with demonstrated applicability to molecular design. On the basis of a carefully derived force field, Discover enables confident computations of minimum energy conformations as well as families of structures and dynamics trajectories of molecular systems. The rotation barrier of 1x,y is the average energy needed for an aryl ring at position 2, 3, 4, or 5 to rotate from its equilibrium position at the energy minimum (E0) to the position coplanar to the plane of the silacyclopentadiene or silole core (E), viz., ER ) E - E0. The E value is computed by freezing the aryl ring in question at a coplanar position with the silole core while allowing other aryl rings to take their equilibrium positions at their energy minimums. Releasing the frozen aryl ring to its energy-minimum equilibrium position then gives the E0 value for the ring. The reliability of the computations was examined by calculating the ER values for some “classic” molecules such as ethane and butane, which all coincided well with those given in standard organic chemistry textbooks. PL spectra of siloles 1x,y were recorded on a SLM 8000C spectrofluorometer. The concentrations of the silole solutions for the PL measurements and quenching experiments were very dilute, typically in the range of ∼4-10 µM. The PL quantum yields (ΦPL) of the silole solutions were measured with 9,10diphenylanthracene (DPA) in cyclohexane (ΦPL ) 90%) as the standard. The PL lifetime measurements were conducted with laser pulses of 267 nm as excitation wavelength, which was produced by the frequency summation of 800 nm with its 400 nm frequency-doubling from a Ti:sapphire regenerative amplifier (200 fs pulse width and 1 kHz repetition rate). For timeresolved PL measurements, the PL signals near the emission peaks were collimated and focused onto the entrance slit of a monochromator with the output plane connected to a synchroscan streak camera (Hamamatsu C4334, 20 ps resolution). In the optical setup, a flip mirror was inserted before the monochromator-streak camera system in order to reflect the PL to a fiber spectrometer (Ocean Optics Inc.) to record the timeintegrated PL spectra before each run of lifetime measurement. The laser energy level for excitation was ∼400 µW. Fabrication of EL Devices. The indium-tin oxide (ITO; 20 Ω/0) glasses were cleaned in ultrasonic baths of detergent solutions, followed by sequential rinsing with acetone, methanol,

Li et al. CHART 1

and deionized water before being dried in an oven at 100 °C. After 25 min of UV-ozone treatment, the glass substrates were transferred into a vacuum chamber with a base pressure of 2 × 10-4 Pa for device preparation. The EL devices based on silole 12,4 were fabricated through sequential vacuum depositions of multiple layers onto the glass substrates. The typical device configuration used in this work was Al (100 nm)/LiF (0.7 nm)/ AlQ3 (10 nm)/12,4 (40 nm)/NPB (70 nm)/ITO, where Al/LiF and ITO were cathode and anode, and AlQ3 [tris(8-hydroxyquinoline)aluminum] and NPB [N,N′-bis(1-naphthyl)-N,N′-diphenylbenzidine] were used as electron- and hole-transport layers, respectively (Chart 1). Silole Syntheses. The regioisomers of 1x,y were prepared according to the synthetic routes given in Scheme 1. The experimental procedures for their syntheses, isolations, and purifications as well as their characterization data are given below. Preparation of 2-Iodo-1,3-diisopropylbenzene (3). 2,6-Diisopropylaniline (2; 7.1 g, 40 mmol) was dissolved in 10 mL of water acidified with 10 mL of 37% hydrochloric acid. The mixture was cooled by an ice bath to ∼2 °C and a solution of sodium nitrite (2.8 g, 40 mmol) in 10 mL of water was added slowly under stirring. The resultant mixture was allowed to stir in the ice bath for 30 min and a solution of potassium iodide (8 g, 48 mmol) in 6 mL of water was added dropwise. The suspension was stirred vigorously at 0 °C for another 2 h, and was then extracted with diethyl ether several times. The ether solution was washed with water three times and dried over magnesium sulfate. Removal of the solvent under reduced pressure gave the crude product, which was purified on a silica gel column with hexane as the eluent. Colorless oil of 3 was obtained in 35% yield (4.0 g). 1H NMR (300 MHz, CDCl3) δ (TMS, ppm) 7.37 (m, 1H), 7.27 (d, 2H), 3.65 (m, br, 2H), 1.42 (d, 12H). 13C NMR (75 MHz, CDCl3) δ (TMS, ppm) 151.6, 129.0, 124.9, 40.0, 24.1. Preparation of 1-(2,6-Diisopropyl)phenyl-2-phenylacetylene (4). To a round-bottomed 250-mL flask were added 3.8 g of 3 (13.1 mmol), 70 mg of copper(I) iodide, 70 mg of triphenylphosphine, and 150 mg of dichlorobis(triphenylphosphine)palladium(II) in a glovebox. A solution of 1.4 g of tolan or diphenylacetylene (13.7 mmol) in 200 mL of triethylamine was injected. The resultant mixture was stirred at 70 °C for 3 days. The precipitate was removed by filtration. After removal of the solvent, the crude product was purified on a silica gel column with hexane as the eluent. Colorless oil of 4 was obtained in 65% yield (2.2 g). 1H NMR (300 MHz, CDCl3) δ (TMS, ppm) 7.85 (m, 2H), 7.67 (m, br, 4H), 7.48 (m, 2H), 3.96 (m, 2H), 1.66 (d, 12H). 13C NMR (75 MHz, CDCl3) δ (TMS, ppm): 151.43, 131.9, 129.2, 129.0, 128.8, 122.9, 98.1, 87.3, 32.6, 23.9. Syntheses of Regioisomers of 1x,y: 2,4-Bis(2,6-diisopropyl)phenyl-1,1,3,5-tetraphenylsilole (12,4), 2,5-Bis(2,6-diisopropyl)-

Photo- and Eletroluminescent and Chemosensory Siloles

J. Phys. Chem. B, Vol. 109, No. 20, 2005 10063 TABLE 1: Optical Properties of Siloles 1x,y (HPS 12,4 12,5 13,4

ΦPLa (%)

ER,Ph′/ER,Phb (kcal/mol)

fluorescence decaya A1/A2c τ1/τ2c

0.27 0.59 4.4 69

-/5.8 70.1/5.6 50.0/6.4 100.3/10.6

100/0 79/21 55/45 0/100

λmax (nm) solutiona solid 497 500 480 495

462 461d 478 487

∼0.02/-) 0.03/0.32 0.09/0.87 -/6.18

a Measured in THF. b Average rotation barriers for o-diisopropylphenyl (ER,Ph′) and phenyl rings (ER,Ph). c Fraction (A, %) and lifetime (τ, ns) of shorter (1) or longer lived species (2); data for HPS taken from ref 10a. d For amorphous film, λmax ) 481 nm.

Figure 1. Molecular structure of 12,4 with all hydrogen atoms omitted for clarity and with the atom-labeling schemes for Tables S1-S6 given in the Supporting Information.

phenyl-1,1,3,4-tetraphenylsilole (12,5), and 3,4-Bis(2,6-diisopropyl)phenyl-1,1,2,5-tetraphenylsilole (13,4). Under dry nitrogen, 141.7 mg of freshly cut lithium shavings (20.42 mmol) was added to a solution of 4 (5.35 g, 20.42 mmol) in 18 mL of THF at room temperature. The mixture was cooled to, and stirred at, 0 °C for 5 h and was then warmed to, and stirred at, room temperature for 16 h. The lithiation mixture was added dropwise to a solution of dichlorodiphenylsilane (1.5 mL, 7.08 mmol) in 120 mL of THF over 2 h. The resultant mixture was stirred at room temperature for 5 h and then refluxed for 18 h. The solvent was evaporated and the crude product was purified on an alumina column with hexane as the eluent. The silole regioisomers of 12,4, 12,5, and 13,4 were isolated and purified by repeated chromatographic separations on alumina columns with hexane as the eluent. After ∼20-times column purifications, 400 mg of 12,4, 23 mg of 12,5, and 10 mg of 13,4 were finally obtained as yellow-green solids. A single crystal of 12,4 was grown from hexane but attempted crystallizations of 12,5 and 13,4 from their solutions were unsuccessful. Characterization data for 12,4: 1H NMR (300 MHz, CDCl3) δ (TMS, ppm) 7.69 (d, 4H, Ar-H), 7.29 (m, 6H, Ar-H), 7.10 (d, 2H, Ar-H), 6.86 (m, br, 14H, Ar-H), 3.34 (m, 2H, CH of iPr at Ar-4), 2.89 (m, 2H, CH of iPr at Ar-2), 0.98 (m, 12H, CH3 of iPr at Ar-4), 0.69 (d, 6H, CH3 of iPr at Ar-2 toward Ar-3 side), 0.18 (d, 6H, CH3 of iPr at Ar-2 toward Ar-1 side). 13C NMR (75 MHz, CDCl ) δ (TMS, ppm) 147.1, 146.2, 136.7, 3

132.6, 130.9, 130.7, 130.5, 128.9, 128.4, 127.2, 126.9, 124.2, 123.5, 32.1, 31.7, 25.3, 24.9, 24.6, 24.2. MS (CI) m/e calcd for C52H54Si 706.4, found 706.3 (M+). UV (acetone, 1 × 10-5 M) λmax 367 nm. 12,5: 1H NMR (300 MHz, CDCl3) δ (TMS, ppm) 7.57 (d, 4H, Ar-H), 7.35 (m, 4H, Ar-H), 7.29 (d, 4H, Ar-H), 7.06 (m, 4H, Ar-H), 6.84 (m, br, 10H, Ar-H), 3.26 (m, 2H, CH of iPr at Ar-2 and -5 on Ar-3 and -4 sides), 2.89 (m, 2H, CH of iPr at Ar-2 and -5 on Ar-1 side), 0.98 (m, 12H, CH3 of iPr at Ar-2 and -5 on Ar-3 and -4 sides), 0.73 (m, 12H, CH3 of iPr at Ar-2 and -5 on Ar-1 side). 13C NMR (75 MHz, CDCl3) δ (TMS, ppm) 147.1, 146.7, 146.0, 136.6, 132.7, 131.0, 130.4, 130.2, 129.1, 128.9, 127.2, 126.9, 124.1, 123.4, 123.0, 32.1, 31.7, 30.7, 25.4, 25.0, 24.6, 24.2, 23.0. MS (CI) m/e calcd for C52H54Si 706.4, found 706.5 (M+). UV (acetone, 1 × 10-5 M) λmax 378 nm. 13,4: 1H NMR (300 MHz, CDCl3) δ (TMS, ppm) 7.90 (d, 4H, Ar-H), 7.43 (m, 6H, Ar-H), 7.10 (d, 2H, Ar-H), 7.02 (d, 2H, Ar-H), 6.89 (m, br, 12H, Ar-H), 3.00 (m, 4H, CH of iPr), 0.75 (d, 12H, CH3 of iPr at Ar-3 and -4 positioned outward), 0.46 (d, 12H, CH3 of iPr at Ar-3 and -4 positioned inward). 13C NMR (75 MHz, CDCl3) δ (TMS, ppm) 147.2, 137.2, 136.6, 130.7, 130.5, 128.9, 128.8, 128.3, 126.9, 124.2, 123.5, 32.1, 31.6, 25.8, 24.9, 24.6, 24.3. MS (CI) m/e calcd for C52H54Si 706.4, found 706.4 (M+). UV (acetone, 1 × 10-5 M) λmax 388 nm. Results and Discussion Synthesis. Siloles 1x,y were prepared by lithiation of 1-(2,6diisopropyl)phenyl-2-phenylacetylene (4) followed by coupling of the lithiated intermediates 5x,y with dichlorodiphenylsilane [Cl2Si(C6H5)2], as shown in Scheme 1. The isolation and purification of the products were achieved by subjecting the reaction mixture to column separation for ∼20 times. The vigorously purified silole regioisomers 12,4, 12,5, and 13,4 were obtained in a molar ratio of 40:2.3:1, which well reflects the

Figure 2. (A) PL spectra of acetone solutions of 1x,y (10 µM) and (B-D) photos of the 1x,y solutions taken under illumination of a UV light (365 nm).

10064 J. Phys. Chem. B, Vol. 109, No. 20, 2005

Figure 3. Fluorescence decay curves of THF solutions of siloles 1x,y. Concentration (mM) 7.50 (12,4), 0.35 (12,5), and 3.11 (13,4); excitation wavelength 367 nm.

Li et al.

Figure 5. AIE effect of 12,4: its quantum yield in water/acetone mixture versus the solvent composition. Silole concentration 10 µM; excitation wavelength 367 nm.

Figure 4. (A) Quenching of PL of 13,4 in THF by o-nitroaniline (oNA) and (B) Stern-Volmer plots for the PL quenching of a THF solution of 13,4 by o-, m- and p-NAs. Concentration of 13,4 3.96 µM; excited wavelength 389 nm.

steric effect involved in the synthesis of the regioisomers: 12,4 and 13,4 are the least and most spaciously crowded, respectively. All the silole regioisomers were fully characterized by spectroscopic methods, from which satisfactory analysis data were obtained (see the Experimental Section for details). Structure. While our efforts in growing the crystals of 12,5 and 13,4 failed, single crystals of 12,4 suitable for X-ray diffraction data collection were obtained from the recrystallization from its hexane solution. In the crystal structure of 12,4, the aryl peripheries are all out of the plane of the silole core, with the iPr-substituted phenyl rings twisted to larger angles (up to ∼81°) than the unsubstituted ones (up to ∼45°; Figure 1). This difference in torsion angle is clearly due to the steric effect of the bulky iPr groups at the ortho positions. Photoluminescence. Different from its nonemissive HPS parent,4a 1x,y is luminescent in solutions, although its PL efficiency varies dramatically with its regiostructure (Figure 2). The PL spectra do not look like single Gaussians, suggesting the existence of different conformers in the solutions. In acetone, the regioisomers of 1x,y emit blue-green lights with PL peaks (λmax values) in the region of 480-500 nm, whose ΦPL values increased in the order of 13,4 > 12,5 > 12,4. Similar results are obtained in such solvents as THF (Table 1), confirming that the ΦPL order observed in acetone is of generality. The ΦPL values of 1x,y are higher than those of “normal” siloles, which fall in the range of 0.031% to 0.51%,2,4,5-7 with ΦPL of ∼0.1% being most typical. The rotation barriers (ER) experienced by the aryl rings in 1x,y are generally higher than that in HPS. The ER values of the iPr-substituted phenyl rings are especially high (ER,Ph′ g 50 kcal/mol), practically inhibiting their IMR processes.8 It is known that a more rigid chromophore is often a

Figure 6. PL spectra of solid films of 1x,y.

more efficient emitter.9 The structural rigidification caused by the high rotation barriers has clearly played a role in making 1x,y more emissive than its HPS parent. Dynamics. While the singlet excited states of HPS rapidly decay single-exponentially (A1 ) 100%) with a very short lifetime (τ1 ∼ 20 ps),10 those of 1x,y relax double-exponentially with participation of a longer lived species (Figure 3 and Table 1). This species becomes more populated (or A2 is increased) from 12,4 to 12,5, accompanying an increase in the lifetime. It becomes the only transient species with a long lifetime (τ2 ) 6.18 ns) in the PL decay of 13,4. The changes in the orders of A and τ are in nice agreement with that in ΦPL. The variation in the ΦPL of 1x,y is thus a reflection of the extent to which the rapid-decaying nonradiative process is hindered and the longer lived species is populated. Silole 13,4 shows a very high ΦPL in solution, due to the structural stiffening caused by its high IMR barriers and the total blockage of the fast-decaying nonradiative channel of its singlet excited state. Chemosensor. The efficient emission of 13,4 in solution spurred us to explore its potential application as a chemosensor. Nitroaromatics such as 2,4-dinitrotoluene (DNT) and 2,4,6trinitrotoluene (TNT) are warfare explosives, detection of which has homeland-security and antiterrorism implications. Due to the commercial unavailability of these explosives, we used nitroanilines (NAs), a group of high-volume chemicals notorious for their “nitroaniline poisoning” and ecological hazard,11 as model compounds. The PL of 13,4 weakens upon addition of o-NA into its solution (Figure 4A) and its Stern-Volmer plot

Photo- and Eletroluminescent and Chemosensory Siloles

J. Phys. Chem. B, Vol. 109, No. 20, 2005 10065

Figure 7. (A) I-V-L characteristic and (B) current efficiency of an LED device of silole 12,4. Shown in the inset of panel B are the EL and PL spectra of its amorphous (a) and crystalline (k) films.

gives a quenching constant (KSV) as high as 16 750 M-1. The KSV varies when the regiostructure of the quencher changes, with KSV for the para isomer being ∼4-fold higher than that for its meta counterpart, revealing that 13,4 is capable of optically distinguishing the nitroaromatic regioisomers (Figures 4B). The KSV values for quenching the PL of silole 13,4 by the NAs are up to ∼61-fold higher than those of the polysiloles by nitrobenzene,5c which is, like NA, also a mononitroaromatic. This high sensitivity is apparently associated with the high PL efficiency of 13,4, while the regioisomeric discrimination is probably due to its selective interaction or preferred complexation with the analyte with a favorable electronic structure, noting that its PL quenching by p- or o-NA is more efficient than that by m-NA. The discriminatory interactions between an emitter and a set of regioisomers with different electronic structures suggest that other highly luminescent species can also be used as sensitive optical probes to distinguish regioisomeric quenchers. We exploited this possibility. DPA, for example, is an efficient bluelight emitter.12 Its KSV values for p-, o-, and m-NAs are very high with distinct difference, being 93 900, 53 500, and 10 200 M-1, respectively.13 This clearly proves the generality of the regioisomeric detection by luminophoric molecules; in other words, PL quenching can be used as a general, versatile tool to sensitively and selectively recognize regioisomeric structures. Aggregation. Silole 12,4 is weakly luminescent in solution but the AIE effect turns it into a strong fluorophore. When aggregated in, for example, a 90:10 water/acetone mixture, its ΦPL is increased by ∼80-fold to 43.8% (Figure 5). The λmax of the nanoaggregate (473 nm) is blue-shifted from that of the solution (500 nm), which is abnormal because aggregation commonly red-shits λmax. To double check this, we prepared a single crystal and amorphous film of 12,4 by crystallization from its solution and by freezing its melt with liquid nitrogen,14 respectively. The aggregates in the crystal and film both show blue-shifted spectra, with their respective λmax values at 461 and 481 nm. The solid films of 12,5 and 13,4 (crystals unavailable; vide supra) also give blue-shifted spectra (Figure 6 and Table 1) in comparison to those of their dilute solutions, confirming that the aggregation-induced blue shift is a general property of all the siloles. In the solid aggregates, the IMR processes of the siloles are more restricted and the aryl peripheries may be more twisted from the central silole cores, in comparison to those in the dilute solutions. The former effect (enhanced rigidity) increases the luminescence efficiency, while the latter effect (reduced conjugation) may have blue-shifted the emission spectra. Electroluminescence. Efficient blue LED is in great demand as a component for the construction of full-color, flat-panel

displays. The blue-light emission of the crystal of 12,4 makes it promising for LED applications, as the carrier mobility is higher in crystal than in the amorphous solid. We constructed an LED of Al/LiF/AlQ3/12,4/NPB/ITO using 12,4 as the active layer. The I-V-L characteristic of the device is shown in Figure 7A. The EL spectrum of 12,4 is similar to that of its crystal but different from that of its amorphous film (Figure 7B, inset), indicating that in the EL device the molecules of 12,4 are mainly in the crystalline state. Although the configuration of the LED device is yet to be optimized, it already exhibits an efficiency as high as 5.86 cd/A. This value is higher than those of some of the best nondoped blue LEDs developed recently based on, e.g., fluorene oligomers (1.53 cd/A) and anthracene derivatives (5.2 cd/A), in which the authors, like almost all the researchers in the area, tried hard to avoid close packing of the chromophoric dyes in order to get amorphous films.15 Silole-based LEDs with high efficiencies have been reported but their colors have been blue-green (495-510 nm).2,6,16 The device here is thus the first efficient blue LED based on a silole crystal. In summary, in this work, we demonstrated that internal control of the IMR process of a silole could tune its emission to a great extent. We succeeded in creating 13,4, a silole with its IMR process hindered and its fast-decaying nonradiative path blocked. It represents a new, rare example of a highly emissive silole in solution,7 whose ΦPL is up to ∼830-fold higher than that of a “normal” silole such as its parent form HPS. We revealed its high sensitivity and excellent selectivity (or regiodiscriminating capability) as an NA chemosensor. Against the general belief that crystal is “bad” for emission, we proved that crystal formation could boost ΦPL and blue-shift λmax. The phenomena observed and insights gained in this study are of value in guiding our future efforts in the structural designs of efficient luminophores that may find applications as chemosensors, biolabels, LEDs, photovoltaics, and vapochromics.17,18 Acknowledgment. This work was financially supported in part by the Research Grants Council of Hong Kong (603304, 604903, 6085/02P, and 6121/01P) and the National Science Foundation of China (N_HKUST606_03). B.Z.T. thanks the Cao Guangbiao Foundation of Zhejiang University for support. Supporting Information Available: Crystal structures with atom labels, tables of crystal data, structure solution and refinement, atomic coordinates, bond lengths and angles, and anisotropic displacement parameters for silole 12,4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Liu, D.; Hug, G. L.; Kamat, P. V. J. Phys. Chem. 1995, 99, 16768. (b) Blatchford, J. W.; Gustafson, T. L.; Epstein, A. J.; VandenBout,

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