Aggregation-Induced Emission Enhancement of 2-(2'-Hydroxyphenyl

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J. Phys. Chem. B 2007, 111, 5861-5868

5861

Aggregation-Induced Emission Enhancement of 2-(2′-Hydroxyphenyl)benzothiazole-Based Excited-State Intramolecular Proton-Transfer Compounds Yan Qian,† Shayu Li,† Guoqi Zhang,† Qian Wang,† Shuangqing Wang,† Huijun Xu,† Chengzhang Li,‡ Yi Li,*,‡ and Guoqiang Yang*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China, and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: January 4, 2007; In Final Form: March 9, 2007

A novel class of 2-(2′-hydroxyphenyl)benzothiazole-based (HBT-based) excited-state intramolecular protontransfer (ESIPT) compounds, N,N′-di[3-Hydroxy-4-(2′-benzothiazole)phenyl]isophthalic amide (DHIA) and N,N′-di[3-Hydroxy-4-(2′-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) has been feasibly synthesized and the properties of their nanoparticles in THF/H2O mixed solvent were investigated. Both compounds were found to exhibit aggregation-induced emission enhancement (AIEE) due to restricted intramolecular motion and easier intramolecular proton transfer in solid state. On identical experimental conditions, the emission of DHBIA aggregates increased more remarkably than that of DHIA. Different aggregation forms of these two organic compounds, due to the steric hindrance of a single tert-butyl group, could be responsible for the notably different degrees of the fluorescence enhancement. Their aggregation modes were investigated on the basis of time-dependent absorption, scanning electron microscope (SEM) images, and molecular modeling with theoretical calculation. The photophysical dynamics were also depicted based on the extremely fast ESIPT four-level cycle.

1. Introduction Highly fluorescent organic nanoparticles (FONs) have attracted numerous research interests due to more variability and flexibility in molecular design as well as their great potentials in light emitting diodes (LEDs), photovoltaic cells, optical sensors, photoemitters, and etc.1-4 However, the fluorescence intensity of solid state or nanoparticles generally descends greatly in comparison with their highly fluorescent dilute solutions. That is a result of concentration quenching caused by enhanced nonradiative deactivation of the excited state, due to the strong intermolecular vibronic interactions such as exciton coupling and excimer formation in the aggregate. Excited-state intramolecular proton-transfer (ESIPT) fluorescent dyes, attributed to their intrinsic peculiar four-level cyclic proton-transfer reactions,5 are a family of intensively investigated functional materials for their wide applications in chemosensors,6 electroluminescent materials,7 laser dyes,8 biochemistry,9 UV-photostabilizers10 as well as the peculiar photophysical process.11 It is well-known that ESIPT molecules are normally more stable as enol forms in the ground state and more stable as keto forms in the excited state. In the ESIPT process, an extremely fast four-level photophysical cycle (E-E*-K*-KE), mediated by intramolecular H-bonds, occurs immediately after photoexcitation of the intramolecular H-bonded molecules. Therefore, an abnormally large Stokes shift without selfabsorption is detected, providing an ideal scheme for UVphotostabilizers or proton-transfer lasers. * Corresponding authors. Tel.: 86-10-62569563. Fax: 86-10-62569564. E-mail: [email protected] (G.Y.). Tel.: 86-10-82543518. Fax: 86-1082543518. E-mail: [email protected] (Y.L.). † Institute of Chemistry, Chinese Academy of Sciences. ‡ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.

To achieve a favorable device with high performance, it is essential to develop new ESIPT systems available in highly dense solid-state systems. In the solid state, however, the problem of the concentration quenching, generally resulting in a remarkable decrease of the fluorescence efficiency, still remains a prominent challenge in the practical applications of ESIPT compounds. Recently, it has been investigated that specific structures such as J-aggregation with a head-to-tail arrangement12 and dendritic architecture13 with site-isolation or restriction of rotation and torsion of the intermolecular bond, by greatly blocking the nonradiative channel, could effectively suppress the selfquenching and thus will notably enhance the luminescence intensity in their aggregate state or solid state. This kind of peculiar enhanced emission in the solid state was the so-called aggregation-induced emission enhancement (AIEE).12-17 However, discoveries of the organic AIEE systems are quite limited, for instance, 1-cyano-trans-1,2-bis(4′-methylbiphenyl)ethylene (CN-MBE),12 siloles-based compounds,13 thienylazulene,14 arylethene derivatives,15 and 1,4-di[(E)-2-phenyl-1propenyl]benzene (PPB).16 Recently enhanced emission from a dendrimer with a core of 2-(2′-hydroxyphenyl)benzoxazole, one of the most popular ESIPT compounds, was also reported.17a To enlarge the family of the specific AIEE-active compounds, it is necessary to carry out more extensive investigations in this field, especially to study the structure-property relationships. Aiming at obtaining highly fluorescent compounds in condensed solid state of more desirable performance, we try to develop new ESIPT materials associated with AIEE properties. Our group has previously reported that N,N′-bis(salicylidene)p-phenylenediamine (p-BSP),17b based on another intramolecular proton-transfer compound N-salicylideneaniline (SA), exhibited obvious AIEE phenomenon. Herein, a novel class of ESIPT

10.1021/jp070076i CCC: $37.00 © 2007 American Chemical Society Published on Web 05/04/2007

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SCHEME 1: Synthesis of DHBA and DHBIA

materials with significant AIEE propertiessN,N′-di[3-hydroxy4-(2′-benzothiazole)phenyl]isophthalic amide (DHIA) and its tert-butyl substituted compound, N,N′-di[3-hydroxy-4-(2′-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA)s have been designed and synthesized. Both the 2-(2′-hydroxyphenyl)benzothiazole-based (HBT-based) compounds exhibit obvious phenomenon of AIEE, but in considerably different degrees. Such different extents of the emission enhancement could be ascribed to their distinct molecular stacking models, and their different aggregation forms were believed to be induced by the single group of tert-butyl, which is confirmed by the time-dependent absorption profiles of the nanoparticles/ aggregates as well as molecular modeling by molecular mechanic calculation. 2. Experimental Section Materials. 5-tert-Buylisophthalic acid (98%) was obtained from Aldrich, isophthalic acid (98%) and polyphosphoric acid (PPA) from Beijing Chemical Reagents, 2-aminothiophenol (98%) from Acros and 4-aminosalicylic acid (99%) from Alfa Aesar. All the chemicals were used directly without further purification. THF was HPLC grade from Aldrich and was used as received. Synthesis of DHIA and DHBIA. DHIA (N,N′-di[3-hydroxy4-(2′-benzothiazole)phenyl]isophthalic amide). 2-(4-amino-2hydroxyphenyl)benzothiazole (AHBA) was synthesized using previously published methods.18a A 0.415 g sample of isophthalic acid (2.5 mmol) and 0.972 g 1,1′-carbonyldiimidazole (CDI, 6 mmol) were, in a nitrogen atmosphere and protected from moisture, stirred in 30 mL toluene and refluxed for 5h,18b,c The bath was cooled down, adding 1.21 g AHBA (5 mmol), and refluxed again. After AHBA was completely dissolved on heating, the reaction mixture was stirred overnight with plenty of pale yellow precipitate emerging. The solid was filtered to give 1.265 g of crude product (yield 82.4%), which was further purified by repeatedly washing with CH2Cl2/CH3OH (v/v ) 10: 1) to give a light yellow solid (yield 76%). Mp 388 °C. 1H NMR (400 Hz, d-DMSO) δ ) 11.71 (2H, s), δ ) 10.66 (2H, s), δ ) 8.57 (1H, s), δ ) 8.19 (4H, dd, J ) 8.40, 3.45), δ ) 8.13 (2H, d, J ) 7.84), δ ) 8.03 (2H, d, J ) 8.06), δ ) 7.83 (2H, d, J ) 1.63), δ ) 7.74 (1H, t, J ) 7.74), δ ) 7.54 (2H, t, J ) 7.59), δ ) 7.44 (4H, d, J ) 8.04). MALDI-TOF m/z: 615.4. Anal. calcd. for C34H22N4O4S2 (614.11): C 0.6643; H 0.0361; N 0.0911. Found: C 0.6636; H 0.0359; N 0.0921. DHBIA (N,N′-di[3-hydroxy-4-(2′-benzothiazole)phenyl]5-tertbutyl-isophthalic amide). DHBIA was synthesized as same as

DHBA except using 5-tert-butylisophthalic acid instead of isophathlic acid. The crude product was washed repeatedly by CH2Cl2 to give a pale yellow solid (70%), pure enough for element analysis. Mp 322 °C. 1H NMR (400 Hz, d-DMSO) δ ) 11.74 (2H, s), δ ) 10.63 (2H, s),δ ) 8.42 (1H, s), δ ) 8.21∼8.12 (6H, m), δ ) 8.04 (2H, d, J ) 7.99), δ ) 7.83 (2H, s, J ) 8.06), δ ) 7.83 (2H, d, J ) 1.63), δ ) 7.74 (1H, t, J ) 7.74), δ ) 7.54 (2H, t, J ) 7.50), δ ) 7.44 (4H, d, J ) 7.23) , δ ) 1.43 (9H, s). MALDI-TOF m/z: 671.5. Anal. calcd for C38H30N4O4S2 (670.17): C 0.6804; H 0.0451; N 0.0835. Found: C 0.6784; H 0.0469; N 0.0820. Preparation of Nanoparticles. The nanoparticles of DHIA and DHBIA were prepared by rapidly injecting 50 µL DMSO solution of DHIA (2 × 10-3 mol/L) and 50 µL THF solution of DHBIA (2 × 10-3 mol/L), respectively, into a 20 mL THF/ H2O mixture (THF/H2O ) 38/62 (v/v)) with vigorous stirring for 30 s at room temperature. The samples, with concentrations both of 5 × 10-6 mol/L, were immediately taken for UV-vis absorption and fluorescence detection at different time intervals. The scanning electron microscope (SEM) samples were prepared by dropping the mixture solutions at variable evolving time onto a silicon plate and immediately evaporating the solvent in vacuum. Characterization. Differential scanning calorimetry (DSC) and thermal gravimetry (TG) curves were obtained by a NETZSCH STA 409 PC/PG instrument. UV-vis absorption spectra were recorded with a Hitachi U-3010 spectrophotometer. Photoluminescence spectra were carried out on a Hitachi F-4500 spectrophotometer. Time-resolved fluorescence measurements were performed using a LifeSpec-ps fluorescence lifetime analytical spectrometer (Edinburgh Instruments). A pulsed LED from 370 nm was used to excite the samples. Fluorescence quantum yields were determined with excitation at 350 nm, using quinine sulfate as a standard. The size and morphology of DHIA and DHBIA nanoparticles were examined with a Hitachi S-4300 field-emission scanning electron microscope (SEM). Geometry optimization of molecules was carried out by molecular mechanic calculation using the Hyperchem 7.0 package.24 3. Results and Discussion Synthesis and Thermal Stability Tests. N,N′-di[3-hydroxy4-(2′-benzothiazole)phenyl]isophthalic amide (DHIA) was successfully synthesized (Scheme 1), without protection of the hydroxyl (phenol) group, by reacting isophthalic acid with

2-(2′-Hydroxyphenyl)benzothiazole-Based Compounds

Figure 1. DSC (a) and TGA (b) curves of DHIA (solid line) and DHBIA (dashed line).

AHBA at the presence of CDI.18b,c The product was found to be dissolved well in DMF, DMSO, dioxane, and pyridine but to possess poor solubility in other common organic solvents. To enhance the solubility, we chose the 5-tert-butylisophthalic acid to act as the reactant instead. The desired compound N,N′di[3-hydroxy-4-(2′-benzothiazole)phenyl]5-tert-butyl-isophthalic amide (DHBIA) was obtained, which could, as expected, be well dissolved in THF and acetone. The tert-butyl substituents not only introduced more alkyl parts to the molecules but also reduced the tight stacking and strong intermolecular interaction. It could be preliminarily presumed that the poor solubility of DHIA was attributed to the extremely tight molecular stacking, which was consistent with its higher melting point than DHBIA (388 °C for DHIA and 322 °C for DHBIA; see the DSC curves in Figure 1a). The compounds of DHIA and DHBIA, containing two HBT units, were both thermally stable, and their melting points were above 320 °C. When the materials were heated up to ∼410 °C, as revealed in thermal gravimetric analysis (TGA) curves (Figure 1b), almost no losses of thermograms weight could be detected. Since good thermal stability was essential and necessary for perfect operation of an optical device, this new series of materials could be potential better candidates for desirable devices with high performance. Absorption and Fluorescence Spectroscopy. As shown in Figure 2, the profiles of either the absorption spectra or the fluorescence spectra of DHIA and DHBIA resembled each other in both shape and position. This suggested that the substitution of the tert-butyl did not bring great changes to the conjugation of the molecular structure. UV-vis absorption spectra (Figure 2a) revealed that both compounds exhibited two absorption peaks; one short-wavelength band at ∼312 nm was due to coupling between thiazole and the fused phenyl ring while another band at a longer wavelength of ∼353 nm corresponded to the coupling between bezothiazole and the hydroxyphenyl ring.19 In the emission spectra (Figure 2b), two emission peaks were observed; one small peak at 393 nm was attributed to the enol form and another emission at 509 nm was due to the keto tautomer form after the ESIPT process. Furthermore, as

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Figure 2. Absorption (solid line) and emission (dashed line) spectra of DHIA (a) and DHBIA (b) in THF solutions (5 × 10-6 mol/L).

Figure 3. Photographs of DHIA (a) and DHBIA (b) with concentrations both of 5 × 10-6 mol/L in THF dilute solution (left) and in THF/ H2O mixture (right, THF:H2O ) 38:62 (v/v)) under irradiation at 365 nm.

expected, large stokes shifts of ∼155 nm for both ESIPT compounds were detected, due to proton-transfer energy losses during the transition into the keto excited state. This indicated that these ESIPT compounds possessed great potentials in application such as proton-transfer lasers and optical devices. Enhanced Emission Phenomenon. The molecularly dissolved dilute THF solutions of DHIA and DHIBA (5 × 10-6 mol/L)were transparent and gave very weak green luminescence peaking at ∼510 nm when excited at ∼350 nm at room temperature (ΦF,THF for DHIA ) 0.0021 and ΦF,THF for DHBIA ) 0.0017). The little difference in their fluorescence efficiency also suggested that, compared to DHIA, the incorporation of a tert-butyl substitutent hardly changed the electronic conjugation of the molecular structure of DHBIA. After the addition of water, a poor solvent for both dyes, into their molecularly dispersed dilute solutions, impressive AIEE phenomena (Figure 3) were observed for both compounds, which was due to the rapid formation of a super-saturated solution and the subsequent consecutive growth of particles. SEM Observation. Scanning electron microscope (SEM) images were obtained to check whether the observed spectroscopic change proceeded with aggregate formation. For DHIA (Figure 4), the particle size was about 100 nm at a time of 2 h.

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Figure 4. FE-SEM of 5 × 10-6 mol/L DHIA particles obtained from suspensions of mixed THF/H2O solvent (THF:H2O ) 38:62 (v/v)) in the time course of 2 (a), 4 (b), and 21 h (c).

Figure 5. FE-SEM of 5 × 10-6 mol/L DHBIA particles obtained from suspensions of mixed THF/H2O solvent (THF:H2O ) 38:62 (v/v)) in the time course of 15 min (a), 30 min (b), and 1 h (c).

Larger particles with a size range of 150-250 nm were obtained at 4 h. After a long ripening time of 21 h, cubic-shaped particles ranging from 200 to 400 nm were clearly observed, indicating some kind of specific ordered arrangement of DHIA molecules. For DHBIA (Figure 5), the growth of aggregates was much faster. The size of particles reached 20-150 nm at 15 min and 100-400 nm at 30 min, respectively, both with a wider distribution in size. At a time of 1 h, the molecules grew into larger rods in a regularly long rectangular shape, with widths of ∼80 nm and various lengths ranging from 280 to 580 nm. The favored one-dimensional growth of DHBIA molecules was consistent with the computationally simulated head-to-tail alignment as well as the J-aggregation band in its time-resolved absorption spectrum, which would be discussed later. The SEM images from smaller nanoparticles to much bigger cubes or rods gave us the direct evidence of molecular aggregation during the emission enhancement. That is to say, both dyes were AIEEactive. Restricted intramolecular/intermolecular motion13 has been suggested as one of the most significant mechanisms of AIEE phenomenon. In dilute solutions of molecular dispersed DHIA and DHBIA, two end-substituted HBT units of the molecules could rotate freely around the single bonds and the radiant decay would be effectively restricted by this kind of intramolecular torsion. While in the aggregate state, the intramolecular rotation and torsion were greatly impeded and therefore the nonradiative decay channel was effectively restricted, which in turn populated the irradiative state of the excited molecules and resulted in a great increase of fluorescence. Hence, it was easy to understand why the molecularly dispersed dilute solutions of DHIA and DHBIA were so weakly luminescent while their nanoparticles and aggregates were highly emissive. Time-Dependent Fluorescence and Absorption Spectra. To detect the kinetic formation process of the aggregation of DHIA and DHBIA particles, the time-dependent evolution of the absorption and fluorescence spectroscopic investigations were carried out in the mixed THF/H2O solvents (v/v ) 38/ 62). Time-dependent fluorescence spectra further verified the AIEE phenomenon of both DHIA and DHBIA (Figure 6), in which a prominent emission enhancement was observed for either fluorophore. A dramatic increase of the fluorescence in DHBIA particles was detected as shown in Figure 6b. The THF solution was

Figure 6. Time-dependent evolution of the fluorescence spectra of 5 × 10-6 mol/L DHIA (a) and DHBIA (b) in the mixed THF/H2O solvent (THF:H2O ) 38:62 (v/v); excited at 350 nm, corrected by absorption at 350 nm).

weakly fluorescent, whereas its aggregates suspensions in THF/ H2O mixed solvent were highly emissive. With increasing the aging time, the intensity of luminescence was gradually intensified. The maximum of fluorescence wavelength (λf) assigned to excited keto emission was almost unchanged as compared the molecularly dispersed THF dilute solution, remaining at ∼510 nm in the whole evolving time. Simultaneously, the fluorescence quantum yields strikingly increased to 0.19, ∼112 times higher than that in pure THF solvent (ΦF,THF ) 0.0017) and ∼21 times higher than that of the initial sample in a THF/ H2O mixture (0 min, ΦAF,0min ) 0.009). At the same time, the fluorescence intensity around 410 nm due to the enol fluores-

2-(2′-Hydroxyphenyl)benzothiazole-Based Compounds TABLE 1: Fluorescence Quantum Yieldsa of DHIA and DHBIA in Dilute THF Solvent and THF/H2O Mixed Solventb ΦF,THFc A d ΦF,0min A e ΦF,max

DHIA

DHBIA

0.0021 0.012 0.029f

0.0017 0.009 0.19g

Keto emission. b THF/H2O ) 38:62 (v/v). c In dilute THF solvent. Aggregates in mixed solvents at the first minute. e Aggregates in mixed solvents when ΦF arrived at the maximum. f At 21 h. g At 6 h 20 min. a

d

cence was also enhanced, but less drastically with respect to that at ∼510 nm, by 9 times or so compared to the preliminary sample at time zero. With the aging time being more than 1 h and 30 min, the intensity of the luminescence began to exhibit a much slow increase. For DHIA, the obvious red-shift of the fluorescence wavelength maximum (λf) was observed. Compared with λf of 510 nm in THF dilute solution, the λf values of 0 min and 21 h in the mixed solvent of THF:H2O ) 38:62 (v/v) were red-shifted to 516 and 519 nm, respectively. The larger bathochromic shift of luminescence could indicate that the molecules of DHIA had stronger intermolecular interactions than those of DHBIA. The fluorescence efficiency of the keto form that peaked at 510 nm was enhanced by 14 times as compared to that in THF solvent and was 2.4 times higher than that at the beginning (0 min). The increase in the fluorescence of the particles or aggregates was less dramatic than that of DHBIA. And, the efficiency of enol fluorescence of DHIA at shorter wavelength regions from 370 to 450 nm also increased (about 1.9 times higher than at the beginning). Meanwhile, the evolving process of the DHIA was much slower than that of DHBIA, suggesting that the time needed for the full growth of DHIA particles was much lengthened. The faster aggregation of DHBIA was probably owing to stronger hydrophobic interaction caused by the bulky tert-butyl group. Similar investigation of AIEE phenomenon of another intramolecular proton-transfer compound, p-BSP, was previously reported by our group17 except that p-BSP presented a decrease in luminescence at the beginning, possibly for intermolecular charge-transfer induced self-quenching.20 In contrast, however, DHIA and DHBIA exhibited a significant increase of fluorescence over the entire evolving time. Moreover, it should be pointed out that the absolute fluorescence efficiency could be higher than its relative values summarized in Table 1, owing to the underestimation caused by Mie scattering of the particle suspensions. After a certain period of aging time, the intensity of fluorescence of both compounds was no longer intensified but gradually weakened instead. It was speculated to be assigned to the precipitation of larger aggregates in mixed solvents, and actually, lots of green fluorescent suspensions were discernible when the solutions were moved under UV irradiation. For DHBIA in the time-dependent evolution process, as revealed in Figure 7b, with the maximum absorption wavelength (λabs) remaining almost unchanged at 353 nm, the maximum absorption intensity gradually descended due to the continuous consumption of the free DHBIA molecules. In the first 55 min, the intensity of maximum absorption decreased very rapidly, at the time range of 55-90 min descending at a lower rate and afterward even more slowly. Besides the typical tails in the visible region caused by Mie scattering,21 a newly formed peak ranging from 380 to 400 nm appeared after 55 min and became more obvious at ripening times of 6 h 20 min (See Figure 7b inset). This new absorption was attributed to J-aggregation12 or

J. Phys. Chem. B, Vol. 111, No. 21, 2007 5865 assigned to the intermolecular interaction/aggregation14 of DHBIA particles. The newly appeared J-aggregation band, compared to the typical intense and narrow J-aggregation band,22 was relatively weak and broad, suggesting a less optimal way of J-aggregation, probably due to a lesser lattice order.23 Moreover, the nearly unchanged position with time of the new band, due to a relatively weak delocalization of the π-electron conjugation, again indicated a less optimal aggregation form of DHBIA molecules. The profile of time-dependent absorption of DHIA (Figure 7a) was generally similar to that of DHBIA. The slower decline of absorption intensity for DHIA also indicated a lower aggregation rate than that of DHBIA. Besides, the timedependent evolving absorption spectra showed us two striking differences: (1) The absorption intensity of DHIA that ranged in a shorter wavelength region from 280 to 320 nm increased gradually in the evolution process, accompanied by the decrease of the main absorption at ∼350 nm. For DHBIA, both the absorptions, whether at 280-320 nm or ranging from 320 to 380 nm, presented a gradual decline over the entire aging time. (2) The absorption of DHIA ranging from 280-320 nm was slightly blue-shifted with time, whereas for DHBIA a new peak at a longer wavelength of ∼390 nm was detected. Similar results were observed in other solvents (Figure 7c and d). All the absorption results revealed that the DHIA could form an H-aggregate but the J-aggregate was formed for DHIBA. It was well-known that H-aggregation (face-to-face) usually led to a blue-shift in the absorption and a significant decrease in luminescence but J-aggregation (head-to-tail) often caused a redshift in absorption together with an enhanced emission. However, in our case for DHIA, although the absorption was blueshifted, the luminescence increased greatly. A similar observation was also made for the compound of PPB,16 which exhibited a blue-shifted absorption but strongly enhanced emission due to combined effects of a herringbone-type aggregation with an edge-to-face arrangement. Here arose a question about whether DHIA possessed an H-aggregation or another specific aggregation fashion. If H-aggregation was present, there must be some other factors contributing to the luminescence enhancement and overcoming its fluorescence quenching. Molecular Modeling. It was well documented that the spatial configuration of molecules played an important role in determining their stacking modes and thus influenced the properties of the resulting aggregate. For DHIA and DHBIA, the noteworthy discrimination in both the absorption/fluorescence spectra and their aggregates shapes allowed us to hypothesize that DHIA and DHBIA might present different aggregation modes. And such differences could possibly facilitate the discriminated degrees in their emission enhancement, about 8 times between these two compounds. Thus, it was necessary for us to carry on deeper investigations into the discrepancy in the aggregation modes. Since it was hard to obtain the single crystals in a large size for crystal analysis, molecular modeling on the basis of molecular mechanic calculation using the Hyperchem 7.0 package was applied.24 To enlarge conjugation for minimizing thermal energy in the system, two possible arrangements (face-to-face/head-to-tail) were possessed by aggregates of DHIA and DHBIA molecules. In a parallel face-to-face aggregation, full π-π stacking was the primary alignment for molecules of DHIA (shown in Figure 8a). The distances between molecules were 3.5 Å, which was a common interval in planar aromatic system in crystal stacking.25 Molecules of DHIA had, somewhat, the tendency to aggregate head-to-tail as well, but with enhanced thermal

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Figure 7. Time-dependent evolution of the absorption spectra of 5 × 10-6 mol/L (a) DHIA in the mixed solvent of THF:H2O ) 38:62 (v/v), (b) DHBIA in the mixed solvent of THF:H2O ) 38:62 (v/v) (inset: absorption at 6 h 20 min), (c) DHIA in the mixed solvent of THF:hexane ) 15:85 (v/v), and (d) DHBIA in the mixed solvent of THF:hexane ) 5:95 (v/v).

Figure 8. Simulated stacking modes of DHIA (a) and DHBIA (b) molecules utilizing the π-π stacking interaction of HBT subunits.

energy. For the coexistence of two ways of aggregation, the face-to-face packing was dominant. However for DHBIA, due to the bulky tBu groups, the molecules were not able to possess an energy-favored full parallel π-π stacking. Forcing two molecules parallel to each other, the intermolecular distances will arrive at more than 5 Å, which indicated a much weaker intermolecular interaction and the corresponding crystal could not be formed. Thus the face-to-face aggregation, in energy optimization, is not the favored packing fashion for DHBIA. When adopting a head-to-tail aggregation (Figure 8b), the distance between two HBT subunits of two adjacent molecules would be restrained at 3.4 Å, which indicated an intensified π-π stacking between the tail HBT arm of one molecule and the head HBT arm of another molecule. Meanwhile, the whole molecular structure became more planar in this stacking mode.

With such a head-to-tail aggregation, microscopically onedimensional molecular stacking could be observed for DHBIA aggregates. Their simulated aggregation fashions correlated well with distinguish growth tendency of DHIA and DHBIA particles in SEM observations respectively, as we described above. Analysis of AIEE. Generally, H-aggregation would induce a decreased luminescence with a blue shift in absorption attributed to faster internal conversion to the lowest excitonic state than emission, while for J-aggregation, it would result in an enhanced emission with a J-aggregate absorption band usually in the longer wavelength range, due to the allowed transition to the lowest exciton state.26 In our research, J-aggregates of DHBIA indeed possessed an enhanced emission. But for DHIA, the enhanced emission seemed to contradict the common perception of nonfluorescence or decreased fluorescence of the H-aggregates.26 Similar observation of the abnormal enhanced emission in H-aggregates of merocyanine dyes was reported by Wu¨rthner et al.27 They ascribed such a phenomenon to a nonvanishing transition probability between the ground and the lowest exciton state from which a weak fluorescence arises, as a result of vibronic coupling or a small rotational twist between two chromophores. In our case, the enhanced emission for H-aggregates of DHIA could be ascribed to the extremely fast proton transfer after the excitation of the molecules. The discussion was carried out primarily on the keto emission, which was the dominant emission whether in THF solutions or in aggregates. On the basis of the four-level photophysical cycle (E-E*-K*-K-E), the quantum efficiency Φ of the keto form could be expressed as,28

Φ ) ΦESIPTΦF(K) ) kF(K) kESIPT (1) kF(E) + kIC(E) + kISC(E) + kESIPT kF(K) + kIC(K) + kISC(K)

2-(2′-Hydroxyphenyl)benzothiazole-Based Compounds

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TABLE 2: Photophysical Parameters of DHIA and DHBIA in Dilute THF Solvent and in Aggregates DHIA DHBIA a

aggregate in THF aggregate in THF

kF(E) (s-1)

kIC(E) (s-1)

τK (ns)

Aggre THF (ΦESIPT )/(ΦESIPT )

THF (ΦAggre F(K) )/(ΦF(K))

1012 s-1). Meanwhile, the calculated increase in ΦESIPT also indicated the negligible influence of the enhanced kF(E). Moreover, kIC(E) would decrease26 after J-aggregation, contributing to the enhanced ΦESIPT. Thus, the offset of two opposite effectssH-aggregation induced decrease in ΦESIPT and restricted rotation induced increase in ΦF(K)sled to a moderate enhancement in DHIA emission. As for DHBIA, both positive effectssJ-aggregation induced increase in ΦESIPT and restricted rotation induced increase in ΦF(K)scontributed to the significant emission enhancement. That could be a rational understanding for smaller emission enhancement of DHIA aggregates compared to that of DHBIA. 4. Conclusion In summary, DHIA and DHBIA were both AIEE-active molecules which were weakly luminescent in molecularly dispersed THF solutions and highly luminescent in their aggregates, mainly caused by the restricted intramolecular motion as well as larger population of intramolecular H-bonds. Further investigation revealed that the different stacking modes were possibly responsible for different extents in the emission enhancement. From molecular modeling, such a difference in aggregation modes was solely induced by incorporation of a single tert-butyl group in the molecule. Prospectively, the HBTbased AIEE molecules may find high-tech applications such as functioning as luminescent and sensory materials in optical displays, biomedical imaging, and environmental protection. Acknowledgment. We sincerely express our thanks to the National Natural Science Foundation of China (Grant Nos. 20333080, 50303019, and 50221201) and the National Basic Research Program (2007CB808004) for the financial support. References and Notes (1) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49.

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