Synthesis and Aggregation-Enhanced One - American Chemical Society

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Dibutylaminophenyl- and/or Pyridinyl-Capped 2,6,9,10Tetravinylanthracene Cruciforms: Synthesis and AggregationEnhanced One- and Two-Photon Excited Fluorescence Jinfeng Li, Tongliang Liu, Meng Zheng, Mingxiao Sun, Deteng Zhang, Haichang Zhang, Pingping Sun, Shanfeng Xue, and Wenjun Yang* Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, 53-Zhengzhou Road, Qingdao, 266042, China S Supporting Information *

ABSTRACT: This article reports the synthesis, one- and two-photon absorption (2PA), and emission properties of 2,6,9,10-tetraarylvinylanthracene derivatives in both THF solution and aqueous media. The results show that the center-crossed integration of aggregation-induced emissionactive 9,10-diarylvinylanthracenes and aggregation-quenched emissionactive 2,6-diarylvinylanthracenes does not surely constitute aggregationenhanced emission (AEE) dyes, and only the dibutylamino-containing cruciforms exhibit AEE effect. The investigation of two-photon properties reveals that the dibutylamino-containing cruciforms show large 2PA cross sections (δmax up to 1500 GM) in both THF solution and THF/water (1/ 9) mixture, and the two-photon fluorescence action cross sections in aqueous media are about 10 times higher than those in THF solution because of their much higher fluorescence quantum yields in aqueous media. This work has demonstrated that the nature of the building blocks could significantly affect the aggregation behaviors and fluorescence properties of anthracene-centered cruciforms. Cruciforms with excellent 2PA and AEE properties could be constructed by using anthracene as the π-center and suitable aromatic units as building blocks.

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INTRODUCTION In recent years, high order, multidimensionall π-conjugated oligomeric systems, such as 2D arylene-based oligomers, have started to draw serious research attention, and the typical systems include cross-shaped oligo(phenylene-ethynylene) (OPE),1 oligo(phenylenevinylene) (OPV),2 oligo(thiophenes) (OT),3 swivel cruciform OPVs2b and OTs,4 cruciform OPE/ OPV co-oligomers,5 and others.6−8 The emergence of these new π-conjugated materials has greatly widened the scope of molecular candidates applicable in molecular sensors,1a,5 switches,9 nonlinear optical materials,10,11 organic field effect transistors,3 photoluminescence,1a and electroluminescence.7b,8 Among them, donor- and/or acceptor-functionalized crossshaped chromophores with an aromatic ring as the common πcenter, namely, cruciforms, have shown unique and interesting optical properties. For instance, Bunz et al. have demonstrated that donor−acceptor-substituted benzene-centered cruciforms exhibit not only interesting aggregation behaviors and solidstate properties5b but also spatially separated HOMO and LUMO that could be used to construct valuable functional scaffolds for differential metal ion one-photon sensor arrays.1a,5 We have been interested in anthracene-centered cruciforms. Anthracene unit could be, in principle, linked in two ways, 9,10or 2,6- linkages, to form two different conjugation pathways with different quinoid characters. As a sequence, their electronic © 2013 American Chemical Society

and photonic properties are highly dependent on both the nature of the active building blocks and the linking ways (bond and position). For example, linear and branched 9,10divinylanthracene (9,10-DVA) derivatives show weak solution fluorescence but aggregation-enhanced emission (AEE),12a,14 and linear 2,6-divinyl-anthracene (2,6-DVA) derivatives exhibit strong solution fluorescence but aggregation-quenched emission (AQE).15 Moreover, various non-centro-symmetric cruciforms could be formed when the same and/or different building blocks are linked at the 2,6- and/or 9,10-positions of anthracene, which are different from benzene-centered cruciforms. Also, the enhanced two-photon absorption (2PA) effect has occurred only in the anthracene-centered11 and not in the benzene- or pyrazine-centered10 cruciforms, although they are all characteristic of spatially separated HOMO and LUMO. This implies that different types of π-centers and molecular symmetry could endow cruciforms with different optical properties. Conjugated organic molecules exhibiting large 2PA cross section (δ) and strong two-photon excited fluorescence (2PEF) have many unique applications and advantages over the Received: December 28, 2012 Revised: March 28, 2013 Published: April 2, 2013 8404

dx.doi.org/10.1021/jp312794t | J. Phys. Chem. C 2013, 117, 8404−8410

The Journal of Physical Chemistry C

Article

Chart 1. Structure of the Anthracene-Centered Cruciforms Studied in This Work

Scheme 1. Synthetic Route to the Cruciforms Studied in This Work: (i) Pd(AcO)2/tri-o-tolylphosphine/N(C2H5)3/THF; (ii) tBuOK/THF

common one-photon-active dyes.9 For some application purposes, it is necessary that 2PA dyes not only have large δ

but also are water-soluble or dispersible and remain highly fluorescent in aqueous media.12 Up to now, a variety of organic 8405



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Figure 1. Absorption (a) and emission (b) spectra of the cruciforms in THF at a concentration of 1 × 10−5 M.

Table 1. Spectroscopic Data of the Cruciforms in THF solution (s) and in Aqueous Media (a)a Cpd

λsmax

λsem

Φs

λamax

λaem

Φa

δs/GM

δa/GM

BzBz BzPy PyBz AmBz AmPy PyAm

435 442 439 461 475 481

540 547 561 574 596 570

0.64 0.44 0.52 0.016 0.019 0.018

442 450 454 465 498 490

546 562 556 569 585 567

0.14 0.02 0.01 0.18 0.15 0.16

336 682 1011

839 1191

λ (in nm) is the peak wavelength of absorption (max) and emission (em) in THF solution (s) and THF/water (1/9) mixture (a). Φ is the fluorescence quantum yield. δ is the two-photon absorption cross section in 10−50•cm4•s•photon−1 (GM).

a

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molecules exhibiting large δ (>1000 GM) and strong 2PEF have been extensively developed, but the 2PA dyes reported to date are mostly hydrophobic and exhibit reduced fluorescence emissions in aqueous media by self-aggregation. There have been a variety of organic dyes exhibiting aggregation-enhanced one-photon emission; however, conjugated organic molecules with aggregation-enhanced 2PEF are rarely reported.12 Moreover, the AEE dyes reported to date are almost linear, propellerlike, and branched organic molecules, and there are few reports on center-crossed cruciforms with AEE effect to the best of our knowledge. While we have previously reported the enhanced δ and spatially separated frontier molecular orbitals of some donor−acceptor-substituted anthracene-centered cruciforms in organic solvent,11b it is not clear whether the center-crossed integration of AEE-active 9,10-diarylvinylanthracene and AQEfeatured 2,6-diaryl-vinylanthracene branches could endow these donor- and/or acceptor-substituted cruciforms with aggregation-enhanced one- and two-photon excited fluorescence in aqueous media. In the current work, we design and synthesize a series of 2,6,9,10-tetravinylanthracenes end-capped with dibutyl-aminophenyl and/or pyridine groups (Chart 1) to investigate the one- and two-photon excited fluorescence and emission properties in both THF solution and aqueous media. If this type of cruciforms exhibiting aggregation-enhanced oneand two-photon excited fluorescence is constructed, it would lay a foundation on the development of one- and two-photon dyes applicable in biophotonics, such as photodynamic therapy, fluorescence imaging, and valuable functional scaffolds for differential metal ion sensor arrays.5,16 We now report that dibutylamino-containing 2,6,9,10-tetraarylvinyl-anthracene cruciforms exhibit not only aggregation-enhanced one-photon excited fluorescence but also large δ and aggregation-enhanced 2PEF.

RESULTS AND DISCUSSION The structures of donor (dibutylamino)- and/or acceptor (pyridine)-substituted anthracene-centered cruciforms studied in this work are shown in Chart 1, and the synthetic routes are depicted in Scheme 1. 2,6-Bis(diethoxyphosphoryl-methyl)9,10-dibromoanthracene (PA) was from a previous work.11 The Heck coupling between PA and styrene, p-dibutylaminostyrene, and 4-vinylpyridine produced corresponding 2,6-bis(diethylphoshorylmethyl)-9,10-diarylvinyl-anthracenes (BPA, PPA, and APA). The target cruciforms were obtained by the Wittig reaction of BPA with 4-t-butyl-benzaldehyde or 4pyridylaldehyde, PPA with 4-t-butyl-benzaldehyde or 4dibutylaminobenzaldehyde, and APA with 4-t-butylbenzaldehyde or 4-pyridylaldehyde, respectively. Thanks to the cruciform configuration, these compounds are all soluble in common organic solvents, such as chloroform, tetrahydrofuran, dichloromethane, and toluene. Their structures and compositions were unambiguously characterized by 1H and 13C NMR and elemental analysis. (See the Supporting Information.) The normalized one-photon absorption and emission spectra of these cruciforms in THF solution are displayed in Figure 1, and the corresponding spectroscopic data are summarized in Table 1. All cruciforms show two main absorption bands, and the peak wavelength of the lowest energy band (λsmax) of the cruciform without donor/acceptor substituents (BzBz) is at 435 nm. When the benzene ring is replaced by pyridine at 2,6or 9,10-positions (BzPy, PyBz), only slightly red-shifted λsmax are observed, implying that pyridine has played a weak acceptor role. However, when dibutylaminostyryl moiety (donor) is incorporated into the 9,10-positions of cruciforms (AmBz), especially when the donor and acceptor units are introduced together into two different branches (AmPy, PyAm), significantly red-shifted λsmax are observed (Figure 1a and Table 1), indicating that the dibutylamino unit is a strong electron-donating moiety and can greatly enhance intra8406



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The Φ values of AmBz, AmPy, and PyAm in aqueous media (Φa = 15−18%) are obviously higher than those in THF solution (Φs = 1.6 to 1.9%). On the contrary, BzBz, BzPy, and PyBz with high Φs (44−64%) exhibit obviously decreased Φa values (14% for BzBz, and 1 to 2% for PyBz and BzPy). This implies that BzBz, BzPy, and PyBz, like 2,6-DVA molecules, are AQE-active dyes, and PyAm, AmPy, and AmBz exhibit AEE effect as 9,10-DVA molecules.12a,14 That is, the nature of building blocks could significantly affect the aggregation and fluorescence properties of anthracene-centered cruciforms, although they are all 2,6,9,10-tetravinylanthracene derivatives, and the center-crossed integration of an AIE-active 9,10-DVA and an AQE-active 2,6-DVA does not surely constitute a AEEactive molecule. It is observed that the peak absorption wavelengths of the ICT band in aqueous media (λamax) are all bathochromic shift relative to those in THF solution (λsmax) (Table 1); however, some cruciforms (BzBz, BzPy and PyBz) exhibit red-shifted emission and decreased Φa and others (PyAm, AmPy and AmBz) show blue-shifted emission and increased Φa by self-aggregation. This implies that the aggregation and fluorescence behaviors observed for these cruciforms could not be well-explained by H- or J-type aggregation,19and the real origin of this dichotomous behavior is not clear at present. On considering that the main difference for these cruciforms is the presence and absence of dibutylamino groups (Chart 1), it could be concluded that the dibutylamino groups have played an important role in determining the cruciform’s aggregation and fluorescence properties, and a possible way is to suppress the intermolecular π−π interactions caused mainly by planar 2,6-diarylvinylanthracene moieties. PyAm, AmPy, and AmBz have not only multiple pathways for intramolecular electronic and photonic transfer but also donor and donor−acceptor end groups. Moreover, they have exhibited AEE effect. Therefore, it is possible to obtain a class of cruciforms with large 2PA cross sections (δ) and strong twophoton excitation fluorescence (2PEF) in aqueous media. Figure 4 shows the 2PEF spectra of cruciforms PyAm, AmPy, and AmBz in THF and THF/water (1/9) mixture. It is shown that these cruciforms are all 2PA-active dyes in both THF solution and aqueous media. Importantly, the 2PEF intensities in aqueous media are about three to four times higher than those in THF solution, indicating that these cruciforms exhibit not only aggregation-enhanced one-photon emission but also aggregation-enhanced 2PEF. Because these chromophores have no linear absorption beyond 650 nm in both THF solution and

molecular charge transfer (ICT)13 between 9,10- and 2,6branches. Figure 1b shows that the emission spectra of the cruciforms are red-shifted by the substitution of donor and acceptor moieties due to the ICT effect that could decrease the energy gap between the ground and Franck−Condon states. The peak emission wavelengths in THF solution (λsem) are in the order: AmPy > AmBz > PyAm > PyBz > BzPy > BzBz, which are almost in the same order as λsmax except of BzPy and PyAm (Table 1). Like other anthracene-based conjugated molecules,12a,14a,15 there is a certain overlap between the emission spectra and weak ICT absorption bands for these cruciforms even if in dilute solution. Notably, the measurement of fluorescence quantum yields (Φ) by dilute solution method using rhodamine B in methanol as ref 18 indicates that some cruciforms (PyAm, AmPy, and AmBz) are weakly fluorescent (Φs = 1.6 to 1.9%) and others are highly fluorescent (PyBz, BzPy, and BzBz) (Φs = 44−64%). Thus, the low Φ for some cruciforms could be not simply ascribed to the existence of reabsorption. Cruciforms studied here could be regarded as the integration of AEE-active 9,10-DVA and AQE-active 2,6-DVA by crosssharing one anthracene ring (Chart 1). To understand the aggregate fluorescence properties, we added a large amount of water to the THF solution of the compound under vigorous stirring, which is a simple and usual method.17 The fluorescence images (Figure 2) and absorption and emission spectra in

Figure 2. Fluorescence images of cruciforms in THF solution and THF/water (1/9) under UV lamp. The concentration of dye is kept at 1 × 10−5 M.

THF/water mixture (1/9) (Figure 3) are conducted. The corresponding spectral data are summarized in Table 1, and the emission spectra under different THF/water ratios are depicted in Figure S1 (see the Supporting Information). Fluorescence images indicate the enhanced fluorescence intensity for AmBz, AmPy, and PyAm and the decreased fluorescence intensity for BzBz, BzPy, and PyBz in THF/water (1/9) mixture relative to those in THF solution (Figure 2). This has been evidenced by the measurement of the fluorescence quantum yields (Table 1).

Figure 3. Absorption (a) and emission (b) spectra of the cruciforms in THF/water (1/9) at 1 × 10−5 M. 8407



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Figure 4. TPEF spectra of cruciforms PyAm, AmPy, and AmBz in THF and THF/water (1/9) at a concentration of 1 × 10−5 M excited at 800 nm under the input laser power of 150 mW.

aqueous media (Figures 1a and 3a), the fluorescence emission under the irradiation of 800 nm laser should result from twophoton excitation. Like one-photon excited fluorescence, 2PEF spectra in aqueous media are also hypsochromic shift relative to those in THF solution (Figures 2 and 4). Moreover, there is a good overlap between one- and two-photon excitation fluorescence spectra in both THF solution and aqueous media for each cruciform. These findings indicate that the one- and two-photon excitation fluorescence occurs from the same excited states, regardless of the mode of excitation. 2PA spectra of the three cruciforms both in THF solution and aqueous media were measured with two-photon-induced fluorescence measurement technique by using femtosecond (fs) laser pulses (160 fs, 1 kHz),20 which could avoid possible complications due to the excited-state absorption. As shown in Figure 5, the three compounds all exhibit considerably large δ

soluble or dispersible two-photon dyes are still scarce at present, these anthracene-centered molecules with large δ and enhanced 2PEF in aqueous media are potential materials for some practical applications. Moreover, the large δ and high Φδ appears at 800 nm, which is a profitable factor because the most common Ti/sapphire laser emits an intense beam around 800 nm even though modern laser techniques can easily vary the excitation wavelengths.

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CONCLUSIONS A new series of tetraarylvinylanthracene derivatives have been synthesized, and their one- and 2PA and emission properties are investigated in both THF solution and aqueous media. It is found that only dibutylamino-containing cruciforms show AEE effect, implying that the center-crossed integration of the AEEactive 9,10-diaryl-vinylanthracenes and the AQE-active 2,6diarylvinylanthracenes does not surely constitute a AEE-active cruciform, and the nature of the building blocks could significantly affect the aggregation and fluorescence properties of anthracene-centered cruciforms. Interestingly, the donor− acceptor-substituted anthracene-centered cruciforms are highly 2PA-active in both THF solution and aqueous media, and the two-photon fluorescence action cross sections in aqueous media are ca. 10 times higher than that in THF solution. Overall, we have obtained water-dispersible anthracenecentered cruciforms with large δ and aggregation-enhanced one- and two-photon excited fluorescence, and these twophoton dyes may ultimately find useful applications, such as two-photon bioimaging and photodynamic therapy as well as functional scaffolds for differential metal ion one- and twophoton sensor arrays.

Figure 5. Two-photon absorption spectra of cruciforms PyAm, AmPy, and AmBz in THF and THF/water mixture (1/9) at a concentration of 1 × 10−5 M under the input laser power of 150 mW.

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis, characterization and measurement. This material is available free of charge via the Internet at http:// pubs.acs.org.

within experimentally available excitation wavelengths. To a given excitation wavelength, the δ values are in the order PyAm > AmPy > AmBz. This is ascribed to the fact that the 2PA process is a third-order nonlinear optical property with a strong dependence on the ICT process, and donor−acceptorsubstituted cruciforms (PyAm and AmPy) exhibit a stronger ICT effect than only donor-substituted cruciform (AmBz). Interestingly, they still exhibit large δ in aqueous media, and the δ values are similar to those in THF solution. As a sequence, the two-photon fluorescence action cross sections (Φδ) are greatly enhanced by self-aggregation in aqueous media because of the much higher Φ in aqueous media than in THF solution. This enhanced Φδ (about 10-fold, Figure S2 in the Supporting Information) benefits from the AEE effect of dibutylaminocontaining anthracene-centered cruciforms. Because water-

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-532-84023977. Tel:+86-532-84023670. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC of China (No. 51073083, 51173092), NSF of Shandong Province (No. ZR2010EM023, ZR2012EMQ003), and Open Project of State Key Laboratory 8408



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