Twisted Intramolecular Charge Transfer and Aggregation-Induced

Aug 17, 2009 - Rongrong Hu,† Erik Lager,§ Angélica Aguilar-Aguilar,§ Jianzhao Liu,† Jacky W. Y. Lam,†. Herman H. Y. Sung,† Ian D. Williams,...
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J. Phys. Chem. C 2009, 113, 15845–15853

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Twisted Intramolecular Charge Transfer and Aggregation-Induced Emission of BODIPY Derivatives Rongrong Hu,† Erik Lager,§ Ange´lica Aguilar-Aguilar,§ Jianzhao Liu,† Jacky W. Y. Lam,† Herman H. Y. Sung,† Ian D. Williams,† Yongchun Zhong,‡ Kam Sing Wong,‡ Eduardo Pen˜a-Cabrera,*,§ and Ben Zhong Tang*,†,⊥ Departments of Chemistry and Physics, The Hong Kong UniVersity of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China, Departamento de Quı´mica, UniVersidad de Guanajuato, Col. Noria Alta S/N, Guanajuato, Gto. 36050, Mexico, and Department of Polymer Science and Engineering, Institute of Biomedical Macromolecules, Key Laboratory of Macromolecular Synthesis and Functionalization of the Ministry of Education, Zhejiang UniVersity, Hangzhou, China ReceiVed: April 1, 2009; ReVised Manuscript ReceiVed: July 30, 2009

Boron dipyrromethene (BODIPY) derivatives 1 and 2 consisting of donor and acceptor units with dual photoresponses to solvent polarity and luminogen aggregation are developed through taking advantage of twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE) processes. In nonpolar solvents, the locally excited (LE) states of the BODIPY luminogens emit intense green lights. Increasing solvent polarity brings the luminogens from the LE state to the TICT state, causing a large bathochromic shift in the emission color but a dramatic decrease in the emission efficiency. The red emission is greatly boosted by aggregate formation or AIE effect: addition of large amounts of water into the solutions of 1 and 2 in the polar solvents causes the luminogens to aggregate supramolecularly and to emit efficiently. The emission can be enhanced by increasing solvent viscosity and decreasing solution temperature, indicating that the AIE effect is caused by the restriction of the intramolecular rotations in the aggregates of the luminogens. Introduction 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene or boron dipyrromethene (BODIPY) derivatives are a group of luminogenic molecules that usually emit in the long wavelength region (λem up to near-IR) in high fluorescence quantum yields (ΦF up to unity).1,2 Recent studies, however, have found that the emission behaviors of the BODIPY luminogens are sensitive to the intramolecular rotations of their chromophoric units and to the steric interactions between their components. The BODIPY parent (I; Chart 1), whose synthesis has remained elusive for decades1 and has just been realized by us recently,3 is highly fluorescent. When photoexcited in ethanol, it emits a green light in a high efficiency (ΦF ) 93%).3 Intuitively, one would think that adding an aromatic ring to the BODIPY base would make it more emissive. However, the BODIPY derivative with a phenyl ring at the 8 position (II) is much less luminescent: its ΦF is ∼4.9-fold lower than that of BODIPY in the same solvent, because the rotational movement of the phenyl ring around the single bond in II nonradiatively deactivates its excited state to a large extent.1 Putting two methyl groups at the 1 and 7 positions exerts steric effect on the rotational motion of the phenyl ring. As a result of the restriction on the intramolecular rotations, the resulting BODIPY derivative (III) becomes more emissive.1 Similarly, the BODIPY derivative carrying a bulkier naphthyl group (V) is a more efficient emitter (ΦF ) 99%) than its

CHART 1: Examples Illustrating the Effects of Rotational Freedom and Steric Hindrance on the Light-Emitting Behaviors of BODIPY Luminogens

* To whom correspondence should be addressed. E-mail: [email protected] (B.Z.T) and [email protected] (E.P.-C.). † Department of Chemistry, HKUST. § Universidad de Guanajuato. ‡ Department of Physics, HKUST. ⊥ Zhejiang University.

10.1021/jp902962h CCC: $40.75  2009 American Chemical Society Published on Web 08/17/2009

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Figure 1. Aggregation-induced emission (AIE). Intramolecular rotation makes tetraphenylethene (TPE) nonluminescent in the solution state, whereas restriction of the intramolecular rotation makes it highly emissive in the aggregate state. Photographs taken for a dilute solution of TPE in acetonitrile and for its aggregates suspended in an acetonitrile/ water mixture with 99% water under UV illumination.

counterpart bearing a smaller phenyl group (IV; ΦF ) 78%) because the former experiences severer steric effect than the latter (Chart 1).4 On the other hand, attaching a group with multiple rotatable junctions, e.g., 4-(N,N-dimethylamino)styryl, is detrimental to the emission efficiency, as can be readily understood from the comparison between the ΦF values of V (99%) and VI (64%).4 The more the rotatable junctions, the lower the emission efficiency. Thus, attaching another 4-(N,Ndimethylamino)styryl group to VI further weakens its emission. The active intramolecular rotations of the multiple rotating units collectively deactivate the excited state of VII nonradiatively, bringing its ΦF to a value as low as 40%. The above examples clearly manifest the detrimental and beneficial effects of intramolecular rotation and steric hindrance, respectively, on the luminescence efficiencies of the BODIPYbased luminogens. Employing the chemical reactions to limit the rotational freedom of, and to increase the steric hindrance between, the chromophoric components is often a nontrivial mission and sometimes requires elaborate structural design and painstaking synthetic effort. The multiple chemical modifications can also make molecular structure of a BODIPY derivative rather complex. Furthermore, the intramolecular rotations in the 4-(N,N-dimethylamino)styryl group in BODIPY derivatives VI and VII, for example, are difficult, if not impossible, to limit by structural modifications through chemical reactions. It will be really nice if the intramolecular rotations can be restricted by simple physical processes, rather than by complicated chemical procedures. We have recently discovered a novel phenomenon of aggregation-induced emission (AIE).6 We have found that many luminogenic molecules with great freedom in intramolecular rotation are nonemissive when dissolved in their good solvents but become highly luminescent when aggregated in their poor solvents or fabricated into solid thin films, due to the physical restriction on the intramolecular rotations in the aggregates.7,8 A typical example of the AIE system is shown in Figure 1: a dilute solution of tetraphenylethene (TPE) in acetonitrile is nonluminescent, whereas its nanoaggregates suspended in an acetonitrile/water mixture with 99% water are highly emissive. A natural question we asked ourselves is if the AIE effect can be utilized to make highly luminescent nanoparticles of BODIPY derivatives with “simple” molecular structures. We are interested in the BODIPY-based luminogens because many of them emit red light, an attribute especially useful to the development of efficient bioimaging systems. We have recently developed a synthetic protocol for BODIPY derivatization and functionalization.9 Utilizing Liebeskind-Srogel as well as Suzuki coupling, we have succeeded in the preparation of a large number of new BODIPY derivatives consisting of electron donor (D) and acceptor (A) units. Many of these

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Figure 2. Solvatochromism of BODIPY derivatives comprised of donor (D) and acceptor (A) units (1). Fluorescence of 1 shifts to red in color and decreases in intensity with an increase in solvent polarity (CH ) cyclohexane, THF ) tetrahydrofuran).

CHART 2: Chemical Structures of BODIPY Luminogens Studied in This Work

molecules emit green light in reasonable efficiencies in nonpolar solvents, such as cyclohexane (CH) and hexane. Intriguingly, their emission colors are bathochromically shifted (to as far as near-IR) but their emission intensities are significantly decreased (to as weak as invisible) when the polarity of solvent is increased. As can be seen from the example shown in Figure 2, the emission color of BODIPY derivative 1a is shifted from green to red but its ΦF value is decreased from 23% to ∼6% when the solvent is changed from CH to tetrahydrofuran (THF). A more dramatic change in emission efficiency is observed in the case of 1b: although spectroscopic analysis indicates that it emits a red light (684 nm) in THF, the emission is invisible to the naked eyes because its ΦF value drops to as low as ∼1%.9 What is the cause for the shift in the emission color and the decrease in the emission intensity with the solvent polarity? Is there any simple way to revitalize the red emission in the polar medium? We are particularly interested to see whether the red emission can be boosted by the AIE process. In this work, we systematically investigated the emission behaviors of BODIPY derivatives 1 and 2 (Chart 2) in a variety of solvents with very different polarities. We also studied the photophysical properties of their aggregates in aqueous media, which is of implication to the development of biosensor systems. In this paper, we offer our answers to the questions posed above: the color shift and emission weakening with solvent polarity is caused by a twisted intramolecular charge transfer (TICT) process,10,11 and the red emission in the TICT state in the aqueous media can be rejuvenated by AIE, a simple physical process. The physical confinement in the luminogen aggregates works in the same way as the steric hindrance induced by the structural modifications through chemical reactions discussed in Chart 1. It restricts the intramolecular rotation and rigidifies the molecular conformation, leading to a great enhancement in the emission efficiency.

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TABLE 1: Optical Transitions of BODIPY Luminogens 1a and 2a in Different Solventsa 1a solvent hexane cyclohexane toluene chloroform ethyl acetate THF ethanol acetonitrile methanol

2a

λab (nm) λem (nm) ΦF (%) λab (nm) λem (nm) ΦF (%) 514 515 517 516 512 513 512 511 511

534 529 572 632 644 663 653 n.d. n.d.

17.3 23.0 38.2 12.7 5.7 6.1 0.3 n.d. n.d.

515 516 518 515 513 515 514 512 512

532 533 540 672 684 688 534 n.d. n.d.

18.4 17.7 21.3 6.2 1.2 1.1 0.1 n.d. n.d.

Abbreviations: λab ) absorption maximum, λem ) emission maximum, ΦF ) fluorescence quantum yield [estimated by using Rhodamine B as standard (ΦF ) 70% in ethanol)], and n.d. ) not detectable (signal too weak to be accurately determined). Emission spectra were measured by exciting the solutions (10 µM) at their absorption maxima. a

Figure 3. Photographs of 1a and 2a taken under UV illumination in different solvents: (A) hexane, (B) cyclohexane, (C) toluene, (D) chloroform, (E) ethyl acetate, (F) THF, (G) ethanol, (H) acetonitrile, and (I) methanol.

Experimental Section General Information. THF (Labscan) was distilled from sodium benzophenone ketyl under nitrogen immediately prior to use. All the BODIPY derivatives studied in this work were prepared according to our previously published procedures.9 Absorption spectra were measured on a Milton Roy Spectronic 3000 Array spectrophotometer in dilute solutions. Emission spectra were recorded on a Perkin-Elmer LS 55 spectrofluorometer at room temperature (∼25 °C) as well as other specific temperatures (-75 to 65 °C). Particle sizes of BODIPY aggregates in THF/water mixtures were measured on a Bookhaven Instruments Corporation 90 Plus/B1-MAS Zetaplus Zeta Potential Analyzer. Images of transmission (TEM) and scanning electron microscopy (SEM) were taken on JEOL-100CX and JSM-6700F electron microscopes, respectively. Preparation of Aggregates. Stock THF solutions of the BODIPY derivatives with a concentration of 0.1 mM were prepared. An aliquot (1 mL) of this stock solution was transferred to a 10 mL volumetric flask. After adding an appropriate amount of THF, water was added dropwise under vigorous stirring to furnish a 10 µM solution in a THF/water mixture with a specific water fraction (fw). The water content was varied in the range of 0-99 vol %. BODIPY solutions or aggregates in other solvent mixtures (e.g., THF/hexane and ethanol/glycerol) were prepared by similar experimental procedures. Absorption and emission spectra and/or nanoparticle sizes of the resulting BODIPY solutions and aggregates were measured immediately after the sample preparation. Results and Discussion Solvent Effect. We first studied the optical properties of 1a and 2a in different solvents. Both of the BODIPY derivatives show absorption spectra in a similar wavelength region that changes little with the variation in solvent (Supporting Information, Figure S1). Their absorption peaks (λab) appear at 511-518 nm, due to the π-π* transition of the BODIPY unit. The λab is generally blue-shifted with an increase in the solvent polarity, but the extent of spectral shift is small, being merely a few nanometers (Table 1). In sharp contrast, the luminescence behaviors of the BODIPY solutions vary dramatically with solvent. Upon excitation by UV light, the solutions of 1a in nonpolar solvents such as hexane and CH emit a strong green light, while its toluene solution emits a yellow light (Figure 3). The color of the emission of its solutions in relatively polar solvents such as chloroform, ethyl acetate, and THF is red,

Figure 4. Emission spectra of solutions of (A) 1a and (B) 2a in solvents with different polarities (∆f). Solution concentration: 10 µM. Solutions were excited at their absorption maxima (cf., Table 1).

whereas in highly polar media such as ethanol, acetonitrile, and methanol, the emission becomes nearly invisible. Evidently, the light emission of 1a is bathochromically shifted in color and weakened in intensity with increasing solvent polarity. A similar phenomenon is observed in the solutions of 2a (Figure 3). In this case, the emission intensity drops even faster: the light emission from a solution in a relatively polar solvent (e.g., ethyl acetate) is already so weak that it can hardly be seen with the naked eyes. In addition to the visual observations, we examined the solvent effects on the emission behaviors of the BODIPY derivatives using spectroscopic techniques. Their emission spectra are shown in Figure 4 and the numerical data are summarized in Table 1. To see whether there is any correlation between the optical properties of the luminogens and the solvent polarity, the solvent polarity parameters (∆f) are also given in the figure, which are expressed by the following equation:

∆f ) f(ε) - f(n2) =

ε-1 n2 - 1 - 2 2ε + 1 2n + 1

(1)

where ε and n are the permittivity (or dielectric constant) and the refractivity (or refractive index) of the solvent, respectively.12,13 In a nonpolar solvent of hexane (∆f ≈ 0), 1a shows a single, sharp emission peak at 534 nm (Figure 4A). Its cyclohexane solution gives a similar emission spectrum. The emission spectrum is sensitively affected by solvent polarity. Thus, although the ∆f value (0.014) of toluene is only slightly higher than that of hexane, the λem value of 1a is already shifted to the yellow spectral region (λem ) 572 nm) with a shoulder at ∼548

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ε ) ε0 - R(T - T0) - β(T - T0)2 - γ(T - T0)3

(2) n ) n0 - a(T - T0) - b(T - T0)2

Figure 5. Transition of the locally excited (LE) state of 4-(N,Ndimethylamino)benzonitrile (DMABN) to the twisted intramolecular charge transfer (TICT) state through intramolecular rotation of its donor (D) and acceptor (A) units at the excited state.

nm. When the solvent polarity is further increased, the shoulder peak disappears and the whole spectrum is shifted to the red region, with a concomitant decrease in the emission intensity. In the very polar solvents of acetonitrile and lower alcohols (∆f ≈ 0.3), the emission spectra are so weak that they become basically flat lines parallel to the abscissa. Similarly, the emission of 2a is red-shifted in color and weakened in intensity with an increase in the solvent polarity (Figure 4B). TICT Process. What is the cause of the solvent effect observed above? The BODIPY derivatives are comprised of D and A units, as depicted in Figure 2. Such D-A molecules often show solvatochromic effects.12,13 For example, the emission of 4-(N,N-dimethylamino)benzonitrile (DMABN) is red-shifted with an increase in solvent polarity.10 The solvatochromism of DMABN has been well explained by a TICT mechanism (Figure 5).11 In the locally excited (LE) or Franck-Condon state, it takes a planar conformation. In a nonpolar solvent, the excited luminogen is in equilibrium with solvent molecules and its planar conformation stabilized by electronic conjugation gives a sharp emission spectrum. In a polar solvent, however, the luminogen is not in equilibrium with the surrounding solvent molecules. Intramolecular rotation brings the luminogen from the LE state to the TICT state, at which there is a total charge separation between the D and A units. In the new equilibrium state, the twisted molecular conformation is stabilized by the solvating effect of the polar solvent. Each luminogen molecule has a different twisting angle and hence emission characteristic, the collection of which thus gives rise to a broad emission spectrum. Elevation of the HOMO level narrows the band gap, thereby red-shifting the emission spectrum. Although the TICT process enables a luminogen to red-shift in emission color, its emission intensity is weakened because of the susceptibility of the TICT state to various nonradiative quenching processes. The BODIPY derivatives may have undergone the TICT transitions in the polar solvents. To examine whether the TICT process is really involved, we checked the temperature effect because it is known that the emission of a TICT luminogen is sensitive to temperature variations. As can be seen from Figure 6, increasing the temperature of a THF solution of 1a from -75 to 60 °C leads to a continuous increase in the emission intensity accompanied by a gradual blue-shift in the emission maximum. Analogous effects are observed in a THF solution of 2a: its emission is intensified and blue-shifted with increasing temperature (Figure 6B). Permittivity and refractivity of a solvent are known to decrease with an increase in temperature (T) in a nonlinear fashion, as expressed by the following empirical equations:14

(3)

For example, Kawski et al. determined the differences in ε and n values of 1,2-dichloroethane at -20 and 100 °C to be ∆ε ) -4.92 and ∆n ) -0.049, respectively,14 corresponding to a ∆(∆f) value of -0.0112 or a big drop in the solvent polarity. Equations 1-3 are general rules applicable to all solvents. Thus in the solutions of the BODIPY derivatives, the polarity of THF must have decreased, or the solvent must have become more “hydrophobic”, along with the temperature raise from -75 to 65 °C. This increase in the solvent hydrophobicity makes the BODIPY luminogens less twisted in conformation, thereby enhancing their emission intensities and blue-shifting their emission colors. To collect more information about the hydrophobic effect, we systematically changed the hydrophobicity of the solvent by admixing polar THF and nonpolar hexane together in different ratios and measured the emission spectra of the BODIPY derivatives in the solvent mixtures with different compositions. When the hexane fraction (fh) in the THF/hexane mixture is increased from 0 to 99%, the emission color of 1a is blue-shifted in the order of red f orange f yellow f green with increasing solvent hydrophobicity (Figure 7A). Spectroscopic analysis reveals that in pure THF, the emission spectrum of 1a is dominated by its TICT emission (Figure 7B). When fh in the solvent mixture is progressively increased from 0 to 50%, the spectrum is continuously blue-shifted and the emission is gradually intensified. In the solvent mixture with 60% hexane, the TICT emission is further hypsochromically shifted, with the LE emission becoming apparent as a shoulder in the green region. A dual fluorescence is observed in the solvent mixture with fh ) 70%. In the mixture with 80% hexane, the shapes of the dual fluorescence peaks are reversed: the LE emission becomes the prevailing peak, while the TICT emission changes to a shoulder peak. In the solvent mixtures with fh g 90%, the TICT peak in the longer wavelength region disappears, and the LE emission becomes the only peak observed in the shorter wavelength region. The spectral data of 1a are summarized and plotted in Figure 7C. Its LE emission is negligibly weak in the THF/hexane mixtures with low hexane contents (fh e 50%). Afterward, the LE emission starts to swiftly increase and becomes very strong

Figure 6. (A) Temperature dependence of emission spectra of THF solution (10 µM) of 1a. (B) Plots of maximum emission intensities (I) and wavelengths (λ) of 1a (red solid lines) and 2a (blue dashed lines) versus temperature. Excitation wavelength (nm): 512 (for 1a), 513 nm (for 2a).

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Figure 7. (A) Photographs of 1a in THF/hexane mixtures with different fractions of hexane (fh) taken under UV illumination. (B) Emission spectra of 1a in the THF/hexane mixtures. (C) Plots of maximum emission intensity (I) and wavelength (λem) of 1a versus hexane fraction (fh) in the THF/hexane mixture. Solution concentration: 10 µM. Excitation wavelength: 512 nm.

at fh of 99%. The TICT emission in the longer wavelength region is intensified with increasing fh, similar to what was observed in the heating experiment described above (Figure 6) and proving that the temperature effect is indeed due to the increase in the solvent hydrophobicity. The TICT peak disappears at fh ≈ 70%, as evidenced by the sudden plummet in the λem - fh plot. Similar changes are observed in the case of 2a (Supporting Information, Figure S2). Its TICT-LE transition, however, occurs at a lower fh, probably because it is more hydrophobic than 1a. The changes in its LE emission intensities (I/I0 ) 80) and peak positions (∆λem ) λTICT - λLE ) 157 nm) are also larger than those (I/I0 ≈ 43, ∆λem ) 113 nm) observed in the system of its less hydrophobic congener 1a. AIE Phenomenon. After studying the emission behaviors of 1a and 2a in the THF/hexane mixtures, we investigated their optical properties in THF/water mixtures. Since the BODIPY derivatives are not soluble in water, they must aggregate in the aqueous mixtures with high water fractions (fw). We are interested to learn how the aggregation affects the emissions of the TICT luminogens. The photographs of the solutions of 1a in the aqueous mixtures taken under UV illumination are shown in Figure 8A. The THF solution of 1a emits a red light, which is quenched when a “small” amount of water is added into THF, as evidenced by the invisibility of its emission in the aqueous mixtures with fw e 70%. The emission of 1a is revitalized when a “large” amount of water (fw > 70%) is added into THF. The visual observation is verified by the spectroscopic analysis. Thus, upon photoexcitation, 1a emits a red light of 663 nm in the THF solution (Figure 8B). When water is added, the light emission is dramatically weakened and the emission color is bathochromically shifted, due to the increase in solvent polarity and the shift to TICT state. The light emission is invigorated from fw ≈ 70% and is further intensified with a further increase in fw (Figure 8C). Meanwhile, the emission color is gradually blue-shifted. In the aqueous mixture with fw of 99%, the emission is hypsochromically shifted to 643 nm (Figure 8B). BODIPY derivative 2a shows similar behavior (Figure 9). In THF, its TICT emission appears at 688 nm. Addition of “small”

amounts of water weakens and red-shifts its emission, whereas in the aqueous mixtures with “large” amounts of water, the emission is intensified and blue-shifted. All the BODIPY solutions in the aqueous mixtures are macroscopically homogeneous and visually transparent without precipitate, even at fw as high as 99%, suggesting that the luminogen aggregates are nanosized. The formation of nanoaggregates is supported by the absorption spectra shown in Figure S3 (Supporting Information): level-off tails are seen in the visible spectral region in the mixtures with high fw values. Such tails are commonly observed in nanoparticle suspensions and are attributed to the light scattering effect of the aggregates. ζ-Potential particle size measurements provide further supporting evidence. Whereas no signal is detected in the solvent mixtures with low fw values, particles with sizes of several hundreds of nanometers are found in the mixtures with high fw values (Table 2). TEM and SEM images also show the formation of nanoparticles in these aqueous mixtures (Supporting Information, Figure S4). The different emission behaviors of the BODIPY derivatives in the aqueous mixtures with varying fw values bring about different decay dynamics, as shown in Figure S5 (Supporting Information). In THF, the excited state of 1a decays in a singleexponential fashion. In other words, all of its excited species decay through the same pathway. The lifetime for the excited state of 1a is 3.52 ns (Table 3). In the aqueous mixture containing 50% water, the excited state of 1a still decays single exponentially but in a much faster rate, with the lifetime shortened by 9.5 times. The decay curve for the excited state of 1a in the aqueous mixture with 95% water, however, is better fitted by a double-exponential function. One component is shorter lived (τ1 ) 0.53 ns), while another has a very long lifetime (τ2 ) 9.04 ns). Even for the shorter lived species, its lifetime is longer than that in the mixture with fw of 50%. The excited state of 2a decays in a similar way. The longer lifetimes for the excited states of the aggregates of the BODIPY derivatives account for the higher emission intensities in the aqueous mixtures with higher fw values.16

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Figure 8. (A) Photographs of 1a in THF/water mixtures with different fractions of water (fw) taken under UV illumination. (B) Emission spectra of 1a in the THF/water mixtures. (C) Plot of maximum emission intensity of 1a versus water fraction in the aqueous mixture. Solution concentration: 10 µM. Excitation wavelength: 512 nm.

TABLE 2: Optical Transitions and Particle Sizes of 1a and 2a in THF/Water Mixturesa 1a

Figure 9. (A) Emission spectra of 2a in THF/water mixtures with different fractions of water (fw). (B) Plots of maximum emission intensity of 2a versus water fraction in the THF/water mixture. Solution concentration: 10 µM. Excitation wavelength: 513 nm.

We also studied the emission behaviors of the BODIPY derivatives in the solid state and compared them with those in pure THF and in 90% THF/water mixture (Supporting Information, Figure S6). The emission peaks of the luminogen aggregates in the aqueous mixture are blue-shifted from those in pure THF. The environment inside the aggregates is less polar than the medium outside, which explains why the aggregate emission is blue-shifted from that of the dissolved molecules in the polar solvent. The emission peaks of their thin films are further blue-shifted from those in the aqueous mixture, due to the complete elimination of the solvent polarity effect on the photophysical properties. As briefly mentioned in the Introduction, aggregate formation imposes physical restraints on the intramolecular rotations.8 The rotations of the multiple aromatic rings in the BODIPY luminogens are restricted in the nanoaggregates suspended in the aqueous mixtures with high water contents. This blocks the nonradiative channels and populates the radiative excitons, thereby making the luminogens more emissive in the aggregate state. If this mechanism is indeed at work, the BODIPY

2a

fw (vol %)

λab (nm)

λem (nm)

d

λab (nm)

λem (nm)

d

0 10 30 50 65 70 75 80 85 90 95 99

513 514 514 514 515 515 518 520 523 521 520 526

663 684 693 n.d. n.d. n.d. 654 654 648 645 645 643

0 0 0 0 300 380 800 550 280 160 110 130

515 515 515 516 516 518 524 522 521 521 522 526

688 705 n.d. n.d. n.d. 677 670 670 664 662 658 658

0 0 0 210 260 470 400 490 340 150 100 150

a Solution concentration: 10 µM. Excitation wavelength (nm): 512 (1a) and 513 (2a). Abbreviations: fw ) water fraction, λab ) absorption maximum, λem ) emission maximum, d ) particle size measured by ζ-potential analyzer, and n.d. ) not detectable (signal too weak to be accurately determined).

derivatives should become more luminescent even in their solutions when viscosity is increased and temperature is decreased, because both of the thickening and cooling processes are known to hamper intramolecular rotations.16 Glycerol is a highly viscous liquid that is fully miscible with many polar solvents. We evaluated the viscosity effect on the emission behaviors of 1a in the ethanol/glycerol mixtures with different fractions of glycerol (fg). As shown in Figure 10, the light emission of 1a is enhanced when more than 10% of glycerol is added into ethanol. The emission intensity of 1a in the ethanol/glycerol mixture with 90% glycerol becomes ∼3fold higher than that in the ethanol solution. We then studied the temperature effect on the light emission of 1a in the THF/water mixture with 90% of water. The emission spectrum is progressively intensified with almost no shift in the emission peak when the temperature is decreased from 65 to

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TABLE 3: Fluorescence Decays of Excited States of BODIPY Luminogens 1a and 2a in THF/Water Mixturesa 1 fw (vol %)

A1/A2

τ1 (ns)

0 50 95

1/0 1/0 0.98/0.02

3.52 0.37 0.53

2 τ2 (ns)

A1/A2

τ1 (ns)

τ2 (ns)

9.04

1/0 1/0 0.98/0.02

1.30 0.10 0.19

1.54

Dynamic parameters determined from I ) A1 exp(-t/τ1) + A2 exp(-t/τ2), where A1/A2 and τ1/τ2 are the fractions (A) and lifetimes (τ) of shorter (1) and longer (2) lived species. Solution concentration: 10 µM. Excitation wavelength (nm): 512 (for 1a) and 513 (for 2a). a

Figure 12. (A) Emission spectra of 1b in THF/water mixtures with different fractions of water (fw). (B) Plots of maximum emission intensities of 1b and 2b versus water fraction in the THF/water mixture. Solution concentration: 10 µM. Excitation wavelength (nm): 499 (for 1b) and 501 (for 2b).

Figure 10. (A) Emission spectra of 1a in ethanol/glycerol mixtures with different fractions of glycerol (fg). (B) Plot of maximum emission intensity of 1a versus glycerol fraction in the ethanol/glycerol mixture. Solution concentration: 10 µM. Excitation wavelength: 512 nm.

Figure 11. (A) Temperature dependence of the emission spectrum of 1a in a THF/water mixture containing 90% water. (B) Plots of maximum emission intensities of 1a and 2a versus temperature. Concentration: 10 µM. Excitation wavelength (nm): 512 (for 1a) and 513 (for 2a).

-10 °C (Figure 11A). A similar temperature effect is observed in the case of 2a (Figure 11B). Such a cooling effect has commonly been observed in typical AIE systems.17,18 This implies that the intramolecular rotations of the luminogens in the aggregates suspended in the aqueous mixture are restricted but not fully prohibited at ambient temperature, which is understandable, as we have previously observed that even in the solid film, an AIE luminogen can still undergo intramolecular rotation to a limited extent.19 Cooling does not change the polarity inside the aggregates but suppresses the intramolecular rotations of the BODIPY molecules residing there to a greater extent, thereby boosting their light emissions. It is of interest to note that 2a is less emissive than 1a at all the temperatures. Although 2a contains more aromatic rings, it also has more rotatable junctions. The more active intramolecular rotation in

2a is probably the reason why it is less emissive than 1a, its counterpart with a smaller number of rotatable junctions. To check whether the AIE effect is a general phenomenon for the BODIPY luminogens of similar structures, we further investigated the fluorescence behaviors of BODIPY derivatives 1b and 2b under the experimental conditions analogous to those used in the photophysical studies of their counterparts 1a and 2a. As shown in Figure 12A, the emission of 1b is weak in the aqueous mixtures with low fw values but is intensified when large amounts of water are added into THF. The aggregates of 1b in the aqueous mixture with fw of 95% fluoresce in an intensity that is 2.3- and 215-fold higher than those of its solutions in pure THF and the 50% THF/water mixture, respectively (Figure 12B). Similarly, the aggregates of 2b in the 95% THF/water mixture are 1.5- and 50-fold more emissive than its solutions in pure THF and the 50% THF/water mixture, respectively. Here again, 2b is found to be less emissive than 1b at all the solvent compositions, due to the same reason that the former has more rotatable joints than the latter. The results obtained in the systems of 1b and 2b are remarkable, considering that in the systems of 1a and 2a, their light emissions in pure THF are recovered but not enhanced by the luminogen aggregation in the aqueous mixtures with high water contents (cf., Figures 8 and 9). In the systems of 1b and 2b, their emissions are not just regained but greatly enhanced by the AIE effect. The emission enhancement is realized by making a subtle change to the structures of the BODIPY derivatives, viz., replacing the ethyl groups by the hydrogen atoms (cf., Chart 2). This demonstrates the ready tunability of the optical properties of the BODIPY luminogens and offers attractive possibility for creating new BODIPY-based luminogens with desired emission colors and high emission efficiencies through structural design or molecular engineering. Mechanistic Discussion. To collect more information on, and to gain more insight into, the AIE processes of the BODIPY derivatives, we studied their molecular geometries and packing arrangements in the solid state. The single crystal structures of 1a and 1b are plotted in Chart 3 and their numerical data are given in Tables S1 and S2 (Supporting Information). The dihedral angle between the central benzene ring and the BODIPY base in 1a is 49°, while those between the terminal benzene rings and the central benzene plane are both 45°. The molecule of 1b takes a conformation similar to that of 1a but the two benzene rings are twisted out of the plane of the central benzene ring to larger extents (55° and 57°). Such a nonplanar molecular conformation weakens the intermolecular interaction,

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CHART 3: Single Crystal Structures of 1a and 1b

Hu et al. case of 2b, for example, the emission becomes practically invisible at an fw value as low as ∼10%. When a large amount of water is added into THF, the resulting aqueous mixture becomes so poor in solvating power that it cannot dissolve the luminogen molecules any more. The molecules thus cluster together to form nanoaggregates. The luminogen molecules residing there are in a less polar environment but their intramolecular rotations are physically restricted. This thus leads to the observed blue shift in emission color and enhancement in emission intensity. Concluding Remarks

CHART 4: Proposed Mechanism for TICT and AIE Processes in BODIPY Luminogens

reduces the likelihood of excimer formation, and hence increases the emission efficiency of the luminogen in the solid state. The interplane distances for the molecules of 1a and 1b are 5 and 7 Å, respectively, suggestive of a lack of strong intermolecular interactions that tend to reduce the possibility of radiative recombination as commonly seen in most “conventional” luminophores with strong face-to-face π-π interactions.20 Putting all the experimental data together enables us to draw a clear overall picture for the emission behaviors of the BODIPY luminogens (Chart 4). In a nonpolar solvent with a high hydrophobicity (e.g., hexane), the BODIPY molecule takes a planar conformation and shows only the LE emission. In a polar solvent with a relatively high hydrophilicity (e.g., THF), the conformation of the BODIPY molecule is twisted and the TICT state emerges, which red-shifts and weakens the emission of the luminogen. Addition of water into THF results in an increase in the solvent polarity, which further twists the luminogen conformation and moves the equilibrium to the TICT state, leading to a further bathochromic shift in the emission color but dramatic drop in the emission intensity. Because of the very high hydrophilicity of water, not much water is needed to completely quench the light emission of the luminogen. In the

In this work, we have studied the emission behaviors of a group of BODIPY derivatives consisting of D and A units in the solution and aggregate states. The luminogenic molecules show solvatochromism and their emissions are tunable from visible to near-IR by changing solvent polarity, due to the involved LE-TICT transition. In nonpolar solvents, the fluorescence spectra of the luminogens are associated with the emissions from their LE states, whereas in polar solvents, weaker and redder emissions from the TICT states dominate their fluorescence spectra. Addition of large amounts of poor solvents such as water into the solutions of the luminogens induces the BODIPY molecules to aggregate, which boosts their TICT emissions through the AIE process. In the TICT process in the solution, the intramolecular rotation decreases the emission efficiency and red-shifts the emission color, whereas in the AIE process in the aggregates, the intramolecular rotation is restricted and the local environment becomes less polar, thereby causing an increase in the emission efficiency and a blue shift in the emission color. We have so far developed a great variety of AIE systems with emission colors covering the whole visible spectral region, which have mainly been realized by changing or modifying chemical structures of the luminogens through molecular engineering endeavors.8 For example, to synthesize a red lightemitting AIE luminogen, we have taken approaches such as incorporating large chromophores into the luminogen structure and lengthening its π-electronic conjugation. In this work, we have succeeded in the generation of a group of new AIE luminogens, whose emissions well extend into near-IR, by taking advantage of their TICT processes. In other words, we have found a simple physical means to boost the TICT-induced red emissions. Since many TICT systems have been reported in the past decades,10,11 our work may stimulate research efforts directed to the development of AIE luminogen systems with ready color tunability, especially those with efficient red light emissions in the aggregate or solid state, which are of great value to the development of bioimaging systems and lightemitting devices. Acknowledgment. The work reported in this paper was partially supported by the Research Grants Council of Hong Kong (603509, 603008, 602706, and CUHK2/CRF/08), the National Natural Science Foundation of China (20634020), the Ministry of Science and Technology of China (2009CB623605), and the FOMIX-CONCYTEG program of Mexico (GTO-2007C02-69094). Supporting Information Available: Absorption spectra of 1a and 2a in solvents with different polarities and THF/water mixtures of different compositions, emission spectra of 2a in THF/hexane mixtures and 1b and 2b in THF/water mixtures, time-resolved emission spectra or fluorescence decay curves of

TICT and AIE of BODIPY Luminogens 1a and 2a in THF/water mixtures, TEM and SEM photomicrographs of the nanoparticles of 1a and 2a formed in THF/ water mixtures, and single crystal data and structure refinements for 1a and 1b. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891. (2) (a) Aurore, L.; Kevin, B. Chem. ReV. 2007, 107, 4891. (b) Qin, W. W.; Baruah, M.; Auweraer, M. V.; Schryver, F. C. D.; Boens, N. J. Phys. Chem. A 2005, 109, 7371. (c) Kollmannsberger, M.; Rurack, K.; Genger, U. R.; Daub, J. J. Phys. Chem. A 1998, 102, 10211. (d) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184. (e) Kuimova, M. K.; Yahioglu, G.; Levitt, J. A.; Suhling, K. J. Am. Chem. Soc. 2008, 130, 6672. (f) Eduardo, P. C.; Ange´lica, A. A.; Martha, G. D.; Erik, L.; Rubi, Z. V.; Jazmin, G. V.; Fabian, V. G. Org. Lett. 2007, 9, 3985. (g) Sameiro, M.; Gonc¸alves, T. Chem. ReV. 2009, 109, 190. (h) Lopez, A. F.; Banuelos, J.; Martinez, V.; Arbeloa, T.; Lopez, A. I. Int. ReV. Phys. Chem. 2005, 24, 339. (3) Arroyo, I. J.; Hu, R.; Merino, G.; Tang, B. Z.; Pen˜a-Cabrera, E. J. Org. Chem. 2009, 74, 5719. (4) Zheng, Q.; Xu, G.; Prasad, P. N. Chem.sEur. J. 2008, 14, 5812. (5) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U. Org. Lett. 2009, 11, 2105. (6) Luo, J.; 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. (7) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y. P.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535. (8) (a) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009, 4332. (b) Liu, J.; Lam, J. W. Y.; Tang, B. Z. J. Inorg. Organomet. Polym. Mater. 2009, 19, 249. (c) Qian, L.; Zhi, J.; Tong, B.; Yang, F.; Zhao, W.; Dong, Y. P. Prog. Chem. 2008, 20, 673. (d) Yang, B.; Ma, Y. G.; Shen, J. C. Chem. J. Chin. UniV. 2008, 29, 2643. (9) Lager, E.; Liu, J. Z.; Aguilar-Aguilar, A.; Tang, B. Z.; Pen˜a-Cabrera, E. J. Org. Chem. 2009, 74, 2053. (10) (a) Valeur, B. Molecular Fluorescence: Principles and Applications; Wiley-VCH: Weinheim, Germany, 2002; Chapter 3. (b) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel, F.; Miehe, J. A. AdV. Chem. Phys. 1987, 68, 1. (c) Grabowski, Z. R.; Rotkiewicz, K. Chem. ReV. 2003, 103, 3899. (11) (a) Yang, J. S.; Liau, K. L.; Li, C. Y.; Chen, M. Y. J. Am. Chem. Soc. 2007, 129, 13183. (b) Palayangoda, S. S.; Cai, X. C.; Adhikari, R. M.; Neckers, D. C. Org. Lett. 2008, 10, 281. (c) Yuan, M. S.; Liu, Z. Q.; Fang, Q. J. Org. Chem. 2007, 72, 7915. (d) Cogan, S.; Zilberg, S.; Haas, Y. J. Am. Chem. Soc. 2006, 128, 3335. (e) Singh, T. S.; Mitra, S. J. Lumin. 2007, 127, 508. (f) Lippert, E.; Luder, W.; Moll, F.; Magele, W.; Boos, H.; Prigge, H.; Seibold-Blankenstein, I. Angew. Chem. 1961, 73, 695. (g) Luis, S.; Manuela, M.; Bjorn, O. R.; Roland, L. J. Am. Chem. Soc. 1995, 117, 3189. (12) (a) Suppan, P.; Ghoneim, N. SolVatochromism; Royal Society of Chemistry: Cambridge, UK, 1997. (b) Baumann, W.; Bischof, H.; Frohling, J. C.; Brittinger, C.; Tettig, W.; Rotkiewicz, K. J. Photochem. Photobiol. A: Chem. 1992, 64, 49.

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