Donor−Acceptor-Substituted Anthracene-Centered Cruciforms: Synthesis, Enhanced Two-Photon Absorptions, and Spatially Separated Frontier Molecular Orbitals

pubs.acs.org/cmPublished on Web 10/15/2009r 2009 American Chemical Society

Chem. Mater. 2009, 21, 5125–5135 5125DOI:10.1021/cm9020707

Donor-Acceptor-Substituted Anthracene-Centered Cruciforms:

Synthesis, Enhanced Two-Photon Absorptions, and Spatially Separated

Frontier Molecular Orbitals

Hai Chang Zhang, Er Qian Guo, Yan Li Zhang, Pei Hua Ren, andWen Jun Yang*

Key Laboratory of Rubber-plastics (QUST), Ministry of Education, School of Polymer Science andEngineering, Qingdao University of Science and Technology, 53 Zhengzhou Road,

Qingdao, 266042, China

Received July 8, 2009. Revised Manuscript Received October 1, 2009

This paper reports the design, synthesis, two-photon absorption (TPA) properties, and frontiermolecular orbital (FMO) features of a new class of cruciforms. These new cruciforms utilize theanthracene ring as a π-center and exhibit very large and significantly enhanced two-photonabsorption cross sections (δ) that are two to four times as large as the sum of the δ values of thetwo corresponding linear analogues over a relatively wide range of wavelengths (740-900 nm).Quantum chemical calculations and solvatochromic behaviors reveal that donor-acceptor-substituted anthracene-centered cruciforms show spatially separatedHOMOandLUMO.Althoughthe enhancement of δ in anthracene-centered cruciforms seems to be not relative to their FMOfeatures, this is thoroughly different from the known characteristics of benzene-centered analoguesshowing no enhancement of δ. On considering that the only difference in these cruciforms is theπ-center, it is primarily concluded that the anthracene core plays a crucial role in the enhancementof δ.

1. Introduction

Conjugated molecules exhibiting large two-photon ab-sorption (TPA) cross sections (δ) have been receiving in-creasing attention owing to their potential applications inphotosciences, such as two-photon fluorescence imaging,1

optical power limiting,2 two-photon up-conversion lasing,3

three-dimensional (3D) optical data storage,4 3D microfab-rication,5 and photodynamic therapy.6 Themost extensivelyinvestigated structural motifs are donor-bridge-acceptor(D-π-A) dipoles, donor-bridge-donor (D-π-D)quad-rupoles, multibranched compounds, dendrimers, octupoles,and porphyrin derivatives. The most extensively utilizedπ-centers are benzene, biphenyl, fluorene, dithienothiophene,

dihydrophenathrene, and pyrazine. The results of thesestudies reveal that δ increases with the donor/acceptorstrength, conjugation length, molecular dimensionality, andplanarity of the π-center.7

For decades, linear ππ-conjugated oligomers have beenthe mainstay of advanced molecular materials for newelectronic and optoelectronic devices. Higher order, multi-dimensional π-conjugated oligomeric systems, such as 2Darylene-based π-conjugated oligomers, started to draw ser-ious researchattentiononlywithin thepast fewyears.Recentrepresentative contributions include cross-shaped oligo-(phenylene ethynylene) (OPEs),8 oligo(phenylene vinylene)(OPVs),9 oligo(thiophenes) (OTs),10 swivel cruciformOPVs9b and OTs,11 cruciform OPE/OPV co-oligomers,12

*Corresponding author. E-mail: [email protected].(1) (a) Cahalan, M. D.; Parker, I.; Wei, S. H.; Miller, M. J. Nature

2002, 2, 872. (b) Zipfel, W. R.; Williams, R. M.; Webb, W. W. Nature2003, 21, 1369. (c) Kim, H. M.; Cho, B. R.Acc. Chem. Res. 2009, 42,863.

(2) (a) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl.Phys. Lett. 1995, 67, 2433. (b) Oliveira, S. L.; Correa, D. S.; Misoguti,L.; Constantino, C. J. L.; Aroca, R. F.; Zilio, S. C.; Mendonca, C. R.Adv. Mater. 2005, 17, 1890.

(3) He, G. S.; Zhao, C. F.; Bhawalkar, J. D.; Prasad, P. N.Appl. Phys.Lett. 1995, 67, 3703.

(4) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.;Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S.M.; Lee, Y.S.; McCord-Maughon, D.; Qin, J.; Rockel, H.; Rumi, M.; Wu, X.-L.; Marder, S. R.; Perry, J. W. Nature 1999, 398, 51.

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(8) (a) Spitler, E. L.; Shirtcliff, L. D.; Halley, M. M. J. Org. Chem.2007, 72, 86. (b)Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M.M. J. Am. Chem. Soc. 2005, 127, 2464.

(9) (a) Kang, H.; Evmenenko, G.; Dutta, P.; Clays, K.; Song, K.;Marks, T. J. J. Am. Chem. Soc. 2006, 128, 6194. (b) He, F.; Tian, L.;Tian, X.; Xu, H.; Wang, Y.; Xie, W.; Hanif, M.; Xia, J.; Shen, F.; Yang,B.; Li, F.;Ma,Y.; Yang,Y.; Shen, J.Adv. Funct.Mater. 2007, 17, 1551.

(10) Sun, X.; Liu, Y.; Chen, S.; Qiu, W.; Yu, G.; Ma, Y.; Qi, T.; Zhang,H.; Xu, X.; Zhu, D. Adv. Funct. Mater. 2006, 16, 917.

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(12) (a)Wilson, J. N.; Bunz,U.H. F. J. Am.Chem. Soc. 2005, 127, 4124.(b) Wilson, J. N.; Smith, M. D.; Enkelmann, V.; Bunz, U. H. F. Chem.Commun. 2004, 1700. (c) Zhou, N.;Wang, L.; Thompson,D.W.; Zhao,Y.Tetrahedron Lett. 2007, 48, 3563. (d) Zucchero, A. J.;Wilson, J. N.;Bunz, U. H. F. J. Am. Chem. Soc. 2006, 128, 11872.

5126 Chem. Mater., Vol. 21, No. 21, 2009 Zhang et al.

and others.13-15 The emergence of these new conjugatedmaterials has greatly widened the scope of molecular can-didates applicable in molecular sensors,8a,12 switches,16 non-linearopticalmaterials,17-21organic field effect transistors,10

photoluminescence,8a andelectroluminescence.14b,15Amongthem, donor/acceptor-functionalized cross-shaped chromo-phores, namely, cruciforms, have been the subject of con-siderable study due to their unique and interestingoptoelectronic properties. Bunz et al. have demonstratedthat donor-acceptor-substituted benzene-centered cruci-forms show independent electronic shifts in HOMO andLUMO into opposite directions.12 As a sequence, the in-tramolecular charge-transfer emission and absorption ofsuch cruciforms is dependent upon the solvent polarity,protons, and coordination of metal cations. These cruci-forms with spatially separated HOMO and LUMO havebeen considered as valuable functional scaffolds for differ-ential metal ion sensor arrays.8a,12

We considered that cruciforms exhibiting the highdegree of conjugation and multiple pathways for intra-molecular electronic and photonic transfer have potentialfor the enhancement of δ. If this type of cruciforms withenhanced δ and spatially separated frontier molecularorbitals is synthesized, it is possible that such cruciformsfind useful applications including functional two-photonsensor arrays for protons and metal ions. To date,there have been a few studies on TPA properties ofcruciforms. Feng et al. theoretically predicted thattetrakis(phenylethynyl)benzenes (TPEBs) end-cappedby donor/acceptors would show large δ only in theUV-visible regions (λ< 600 nm).18 Soon afterward,Haley et al. experimentally confirmed that some TPEBshad 240-520GMof δ at 700-900 nm. These δ values arenotably smaller than the sum of the cross section of twocorresponding linear analogues.19 Marder et al. synthe-sized donor/acceptor capped tetrastyryl chromophores

with benzene or pyrazine core to investigate their TPAproperties. The results still showed no enhancement ofδ compared with the corresponding linear analogues.20 Jenet al. synthesized a bis(di(n-butyl)aminostyryl)benzene/fluorene cross-conjugated polymer for metal ion sensing.This cross-conjugated polymer showed no enhancement ofδ and fluorescence quenching effect upon addition of zinc.21

In a word, the enhanced δ in cruciform configuration is notobtained at present. Moreover, almost all the cruciformsreported before utilize benzene as the π-center.Recently, the utility of the anthracene as a special and

efficient π-center for the two-photon chromophores hasbeen demonstrated by several groups.22 The anthraceneunit could be in principle linked in two different ways,9,10- or 2,6-linkages, to form two different conjugationpathways with two different quinoid characters. As asequence, the electronic and photonic properties of an-thracene-centered conjugated molecules are highly de-pendent on both the nature of the active building blocksand the way in which they are linked. Very recently, Yanget al. have reported the effect of 9,10-substitutents on theTPA and fluorescence properties of 2,6-bis(p-dihexyl-aminostyryl)anthracene derivatives.23 However, the at-tempt to synthesize 2,6-bis(p-dicyanostyryl)anthracenederivatives was unsuccessful due to their poor solubilityand new synthetic method inaccessibility. In the currentwork, we employ new synthetic routes to prepare two newdonor-acceptor substituted anthracene-centered cruci-forms (CDAE and CDAV, Scheme 1) and two new linearanalogues (B26A and B910V, Scheme 2). Furthermore,their one- and two-photon properties are investigated.For the sake of comparison, the corresponding tetrado-nor cruciforms,C4DE andC4DV, and another two linearanalogues, 9,10-bis(p-didecylaminophenylethynyl)anth-racene (B910E) and 2,6-bis(p-dihexylaminostyryl)anth-racene (B26D), are also included. It should be noticedthat the one- and two-photon properties of C4DE,C4DV, B910E, and B26D have been reported in previousworks,22a,b,23 but their FMO features are not clear andtheir enhanced δ compared with the corresponding linearanalogues is not recognized.

2. Experimental Section

Materials. Toluene, benzene, and tetrahydrofuran were

distillated over metallic sodium before use. Other solvents and

reagents (analytical grade) were used as received, unless other-

wise claimed. p-Didecylamino-phenylacetylene, p-dihexylamino-

benzaldehyde, p-didecylaminostyrene, p-cyanobenzaldehyde,

p-cyano-styrene, 2,6-dimethyl-9,10-dibromoanthracene, 2,6-bis-

(p-dihexylaminostyryl)anthracene (B26D), 9,10-bis-(p-(didecyl-

amino)phenylethynyl)anthracene (B910E), 2,6-bis(p-(dihexyl-

amino)styryl)-9,10-bis(p-(didecylamino)styryl)anthracene

(13) (a) Miao, Q.; Chi, X.; Xiao, S.; Zeis, R.; Lefenfeld,M.; Siegrist, T.;Steigerwald,M.L.;Nuckolls, C. J.Am.Chem. Soc. 2006, 128, 1340.(b) Tolosa, J.; Diez-Barra, E.; Sancheez-Verdu, P.; Rodriguez-Lopez, J.Tetrahedron Lett. 2006, 47, 4647. (c) Zhou, N. Z.; Wang, L.;Thompson, D. W.; Zhao, Y. M. Org. Lett. 2008, 10, 3001. (d) Klare,J. E.; Tulevski, G. S.; Sugo, K.; Picciotto, A. D.;White, K. A.; Nuckolls,C. J. Am. Chem. Soc. 2003, 125, 6030.

(14) (a) Wang, H. Y.; Wan, J. H.; Jiang, H. J.; Wen, G. A.; Feng, J. C.;Zhang, Z. J.; Peng, B.; Huang, W.; Wei, W. J. Polym. Sci., Part A:Polym.Chem. 2007, 45, 1066. (b)Wang, H.Y.; Feng, J. C.;Wen,G. A.;Jiang, H. J.;Wan, J. H.; Zhu, R.;Wang, C.M.;Wei,W.; Huang,W.NewJ. Chem. 2006, 30, 667.

(15) (a) Cheng, G.; He, F.; Zhao, Y.; Duan, Y.; Zhang,H. Q.; Yang, B.;Ma, Y. G.; Liu, S. Y. Semicond. Sci. Technol. 2004, 19, L78. (b) He,F.; Xu,H.; Yang, B.; Duan,Y.; Tian, L.; Huang,K.;Ma,Y. G.; Liu, S. Y.;Feng, S.; Shen, J. C. Adv. Mater. 2005, 17, 2710. (c) Duan, Y.; Zhao,Y.; Chen, P.; Li, J.; Liu, S. Y.; He, F.; Ma, Y. G.Appl. Phys. Lett. 2006,88, 263503.

(16) Grunder, S.; Huber, R.; Horhoiu, V.; Gonzalez, M. T.; Schonen-berger, C.; Calame, M.; Mayor, M. J. Org. Chem. 2007, 72, 8337.

(17) Yi, Y.; Zhu, L.; Suai, Z. Macromol. Theory Simul. 2008, 17, 12.(18) Zhang, X.-B.; Feng, J.-K.; Ren, A.-M.; Sun, C.-C. Opt. Mater.

2007, 29, 955.(19) Slepkov, A. D.; Hegmann, F. A.; Tykwinshi, R. R.; Kamada, K.;

Ohta, K.; Marsden, J. A.; Spitler, E. L.; Miller, J. J.; Haley, M. M.Opt. Lett. 2006, 31, 3315.

(20) Rumi, M.; Pond, S. J. K.; Meyer-Friendrichsen, T.; Zhang, Q.;Bishop, M.; Zhang, Y.; Barlow, S.; Marder, S. R.; Perry, J. W.J. Phys. Chem. C 2008, 112, 8061.

(21) Huang, F.; Tian, Y.; Chen, C.-Y.; Cheng, Y.-J.; Young, A. C.; Jen,A. K.-Y. J. Phys. Chem. C 2007, 111, 10673.

(22) (a) Yang,W. J.; Kim,D. Y.; Jeong,M.-Y.; Kim,H.M.; Lee, Y. K.;Fang, X.; Jeon, S.-J.; Cho, B. R.Chem.;Eur. J. 2005, 11, 4191. (b)Yang, W. J.; Kim, C. H.; Jeong, M.-Y.; Lee, S. K.; Piao, M. J.; Jeon, S.-J.; Cho, B. R.Chem.Mater. 2004, 16, 2783. (c) Kim, S.; Zheng,Q.; He,G. S.; Bharali, D. J.; Pudavar, H. E.; Baev, A.; Prasad, P. N.Adv. Funct.Mater. 2006, 16, 2317. (d) Strehmel, B.; Amthor, S.; Schelter, J.;Lambert, C. ChemPhysChem 2005, 6, 893.

(23) Yang, W. J.; Seo, M. S.; Wang, X. Q.; Jeon, S. J.; Cho, B. R.J. Fluoresc. 2008, 18, 403.

Article Chem. Mater., Vol. 21, No. 21, 2009 5127

(C4DV), and 2,6-bis(p-(dihexylamino)styryl)-9,10-bis(p-(didecyl-

amino)phenylethynyl)-anthracene (C4DE) are from previous

works.22a,b,23,24

2,6-Bis(diethylphosphorylmethyl)-9,10-dibromoanthracene (A).

A mixture of 2,6-dimethyl-9,10-dibromoanthracene (2.6 g, 7.3

mmol), NBS (2.9 g, 16 mmol), and benzoyl peroxide (71 mg,

0.29mmol) in benzene (100mL)was refluxed for 4 h.Themixture

was poured into methanol, and the precipitate that collected was

dried in vacuum. This intermediate was added to triethyl phos-

phite (50mL), and the resultingmixture was refluxed for 6 h. The

solvent was removed in vacuum, and the residuewas purified by a

column chromatography on silica gel using ethyl acetate/CH2Cl2(2:1) as the eluent.Yield: 2.4 g (54%).Mp: 251-253 �C. 1HNMR

(300 MHz, CDCl3): δ 8.52 (d, J = 9.0 Hz, 2H), 8.45 (d, J =

3.0Hz, 2H), 7.60 (dd, J=9.0Hz, 3.0 Hz, 2H), 4.07 (m, 8H), 3.42

(d, J= 21 Hz, 4H), 1.27 ppm (t, J= 7.5 Hz, 12H). Anal. Calcd

for C24H30Br2O6P2: C, 45.31;H, 4.75; Br, 25.12; O, 15.09; P, 9.74.

Found: C, 45.33; H, 4.71.

2,6-Bis(diethylphosphorylmethyl)-9,10-di(p-didecylaminophe-

nylethynyl)anthracene (B). A solution of p-(didecylamino)-

phenylacetylene (1.3 g, 3.9 mmol) in toluene (10 mL) was added

to a degassed mixture containingA (1.1 g, 1.7 mmol), Pd(PPh3)4(0.24 g, 0.20 mmol), CuI (38 mg, 0.20 mmol), and (i-Pr)2NH

(25 mL) in toluene (60 mL) by a syringe. The suspension was

stirred overnight at 80 �C. The solvent was removed in vacuum,

and the product was separated by a column chromatography on

silica gel using ethyl acetate/hexane (1:1) as the eluent. Yield: 1.7

g (81%). Mp 174-176 �C. 1H NMR (300 MHz, CDCl3): δ 8.62

(d, J=9.0 Hz, 2H), 8.52 (d, J=3.0 Hz, 2H), 7.58 (m, 6H), 6.68

Scheme 2. Synthesis and Molecular Structures of the Corresponding Linear Analogues Studied in This Work

Scheme 1. Synthesis and Molecular Structures of the Cruciforms Studied in This Work

(24) Lee, S. K.; Yang, W. J.; Choi, J. J.; Kim, C. H.; Jeon, S. J.; Cho, B.R. Org. Lett. 2005, 7, 323.


(d, J=9.0 Hz, 4H), 4.06 (m, 8H), 3.44 (d, J=21 Hz, 4H), 3.33

(t, J=7.5Hz, 8H), 1.63 (m, 8H), 1.29 (m, 68H), 0.89 ppm (t, J=

6.0Hz, 12H).Anal. Calcd forC80H122N2O6P2: C, 75.67;H, 9.68;

N, 2.21; O, 7.56; P, 4.88. Found: C, 75.83; H, 9.71; N, 2.20.

2,6-Bis(diethylphosphorylmethyl)-9,10-di((p-didecylamino)-

styryl)anthracene (C). A pressure tube containing a mixture

of p-(didecylamino)styrene (1.88 g, 4.7 mmol), A (0.14 g,

0.22 mmol), Pd(OAc)2 (4.9 mg, 0.022 mmol), tris-(o-tolyl)-

phosphine (0.045 g, 0.15 mmol), N(C2H5)3 (1 mL, 10 mL), and

THF (1 mL) was sealed under nitrogen and refluxed for 24 h.

The mixture was separated by a column chromatography on

silica gel using ethyl acetate/hexane (1/1) as the eluent. Yield:

0.19 g (69%). Mp 125-127 �C. 1H NMR (300 MHz, CDCl3):

δ 7.67 (d, J = 9.0 Hz, 2H), 7.61 (d, J = 10.0 Hz, 2H), 7.56 (s,

2H), 7.42 (d, J= 9.0 Hz, 4H), 7.14 (d, J= 15.0 Hz, 2H), 7.08

(d, J = 18.0 Hz, 2H), 6.62 (d, J = 9.0 Hz, 4H), 4.12 (m, 8H),

3.42 (d, J=21Hz, 4H), 3.28 (t, J=7.5 Hz, 8H), 1.59 (m, 8H),

1.32 (m, 68H), 0.91 ppm (t, J= 6.0 Hz, 12H). Anal. Calcd for

C80H126N2O6P2: C, 75.43; H, 9.97; N, 2.20; O, 7.54; P, 4.86.

Found: C, 75.51; H, 9.92; N, 2.24.

2,6-Bis(p-cyanostyryl)-9,10-bis(p-didecylaminophenylethynyl)-

anthracene (CDAE). LDA (1.5 M in cyclohexane; 1.1 mL, 1.6

mmol) was added dropwise to a stirred solution of B (0.79 g,

0.71mmol) in anhydrous THF (40mL) at-78 �C underN2. The

mixture was stirred for 1 h, and then p-cyanobenzaldehyde

(0.24 g, 1.8 mmol) in THF (10 mL) was added dropwise over a

period of 3 min. After the mixture was stirred for 1 h at -78 �Cand for 6 h at room temperature, 1 mL of water was added, and

the solvent was evaporated. The residue was dissolved into

CH2Cl2 and washed several times with water. The solvent was

evaporated, and the crude product was separated by a column

chromatography on silica gel using hexane/CH2Cl2 (2:1) as

the eluent. Yield: 0.39 g (57%). Mp 156-158 �C. 1H NMR

(300MHz, CDCl3): δ 8.51 (d, J=9.0Hz, 2H), 8.44 (s, 2H), 7.74

(d, J = 9.0 Hz, 2H), 7.59 (m, 12H), 7.34 (d, J = 16.5 Hz, 2H),

7.15 (d, J= 16.5 Hz, 2H), 6.68 (d, J= 9.0 Hz, 4H), 3.33 (t, J=

6.0Hz, 8H), 1.64 (m, 8H), 1.33 (m, 56H), 0.91 ppm(t, J=7.5Hz,

12H). 13C NMR (75 MHz, CDCl3): δ 148.52, 142.05, 134.34,

133.33, 132.81, 132.62, 132.15, 128.31, 128.21, 127.35, 127.17,

123.45, 119.37, 119.00, 111.57, 110.61, 108.95, 104.81, 102.52,

84.75, 51.27, 32.16, 29.97, 29.86, 29.83, 29.60, 27.53, 27.43, 22.96,

14.40 ppm.Anal. Calcd (%) forC88H110N4: C, 86.36;H, 9.06;N,

4.58. Found (%): C, 86.50; H, 9.02; N, 4.62.

2,6-Bis(p-cyanostyryl)-9,10-bis(p-didecylaminophenylethyny)-

anthracene (CDAV). CDAV was synthesized by the same pro-

cedure as described forCDAEexcept thatCwas used. The crude

product was separated by a column chromatography on silica

gel using hexane/CH2Cl2 (2:1) as the eluent. Yield: 0.46 g (68%).

Mp 101-103 �C. 1H NMR (300 MHz, CDCl3): δ 7.88 (m, 4H),

7.60 (m, 12H), 7.29 (m, 8H), 7.08 (d, J= 18.0 Hz, 2H), 6.88 (d,

J=9.0 Hz, 4H), 3.42 (t, J=7.5Hz, 8H), 1.76 (m, 8H), 1.36 (m,

56H), 0.90 ppm (t, J = 7.5 Hz, 12H). 13C NMR (75 MHz,

CDCl3):δ 148.39, 133.24, 132.54, 132.49, 132.31, 132.19, 131.78,131.52, 128.49, 127.68, 127.53, 125.23, 121.92, 119.98, 118.78,

117.19, 111.96, 111.41, 108.39, 103.48, 51.21, 32.13, 29.89, 29.81,

29.75, 29.56, 27.45, 27.37, 22.92, 14.36 ppm.Anal. Calcd (%) for

C88H114N4: C, 86.08; H, 9.36; N, 4.56. Found (%): C, 86.15; H,

9.30; N, 4.52.

2,6-Dibromo-9,10-anthraquinone. 2,6-Diaminoanthraquinone

(4.8 g, 20 mmol), t-Bu ONO (5.2 g, 50 mmol), CuBr2 (11.1 g,

50 mmol), and CH3CN (85 mL) were added to a one-neck flask,

and the mixture was heated at 65 �C for 2 h. The reaction was

quenched by adding 20% HCl (aq) solution to the product

mixture. The solution was filtered, washed with CH3CN, and

the product was recrystallized with 1,4-dioxane. Yield: 4.2 g

(86%). Mp: >300 �C. 1H NMR (300 MHz, CDCl3): δ 8.44

(d, J = 3.0 Hz, 2H), 8.18 (d, J = 9.0 Hz, 2H), 7.95 ppm (dd,

J = 9.0 Hz, 3.0 Hz, 2H).

2,6-Dibromo-9,10-dihexyloxyanthracene.To a two neck flask,

2,6-dibromo-9,10-anthraquinone (2.0 g, 5.4 mmol), Bu4NþBr-

(1.6 g, 4.9 mmol), Na2S2O4 (1.9 g, 11 mmol), and water (50 mL)

were added under nitrogen. The mixture was stirred for 10 min,

and CH2Cl2 (60 mL) was added. When the solution turned to a

green color, 20% NaOH (aq) was added and stirred for 2 h. To

this solution was added n-hexyl bromide (8.8 g, 54 mmol), and

the mixture was stirred for 8 h. The product was purified on a

silica column using hexane/CH2Cl2 (5:1) as the eluent. Yield:

2.0 g (70%). Mp: 127-129 �C. 1H NMR (300 MHz, CDCl3): δ8.36 (d, J=9.0 Hz, 2H), 8.21 (s, 2H), 7.64 (dd, 2H, J=9.0 Hz,

3.0 Hz), 4.11 (t, J = 7.0 Hz, 4H), 1.69 (m, 4H), 1.28 (m, 12H),

0.91 (t, J = 7.5 Hz, 6H). Anal. Calcd (%) for C26H32Br2O2: C,

58.22; H, 6.01; Br, 29.80; O, 5.97. Found (%): C, 58.15; H, 6.04.

9,10-Dihexyloxy-2,6-bis(p-cyanostyryl)anthracene (B26A).

To a pressure tube, 2,6-dibromo-9,10-dihexyloxyanthracene

(127 mg, 0.24 mmol), p-cyanostyrene (80 mg, 0.58 mmol),

Pd(OAC)2 (8.7 mg, 39 μmol), tri-o-tolylphospine (5.9 mg,

20 μmol), and THF (5 mL), Et3N (10 mL) were added under

nitrogen. The mixture was heated for 6 h at 70 �C and poured

into CH2Cl2. The product was purified on a silica column using

hexane/ethyl acetate (5:1) as the eluent. Yield: 0.11 g (74%).Mp:

211-213 �C. 1H NMR (300 MHz, CDCl3): δ 7.67 (d, J =

9.0 Hz, 2H), 7.64 (d, J= 9.0 Hz, 2H), 7.56 (s, 2H), 7.36 (d, J=

9.0Hz, 4H), 7.02 (d, J=16.5Hz, 2H), 6.93 (d, J=16.5Hz, 2H),

6.60 (d, J=9.0 Hz, 4H), 4.12 (t, J=7.5 Hz, 4H), 1.96 (m, 4H),

1.32 (m, 12H), 0.91 ppm (t, J=6.2Hz, 6H). 13CNMR(75MHz,

CDCl3): δ 145.21, 137.56, 133.32, 132.28, 131.93, 131.75, 131.32,129.98, 128.02, 126.95, 125.01, 124.87, 120.10, 111.93, 66.41,

32.02, 30.29, 27.12, 22.97, 14.34 ppm. Anal. Calcd (%) for

C44H44N2O2: C, 83.51; H, 7.01; N, 4.43; O, 5.06. Found (%):

C, 83.46; H, 7.04; N, 4.46.

9,10-Bis-bromomethylanthracene. A mixture of anthracene

(1.9 g, 10.5 mmol), (CH2O)n (1.2 g), 33% HBr in acetic acid

(15 mL), and a catalytic amount of AlCl3 was stirred for 3 h at

50 �C. The resulting solid was washed with water and dried in

vacuum. The residue product was recrystallized in toluene to

afford an off-white solid. Yield 2.2 g (62%). 1H NMR (300

MHz, CDCl3): δ 8.36 (m, 4H), 7.65 (m, 4H), 5.58 ppm (s, 4H).

9,10-Bis(diethylphosphorylmethyl)anthracene. 9,10-Bis(bromo-

methyl)anthracene (1.2 g, 3.3 mmol) was added to triethyl phos-

phate (10mL), and the resulting solution was refluxed for 6 h. The

solvent was removed in vacuum and the residue was purified by a

column chromatography on silica gel using ethyl acetate/CH2Cl2(2:1) as the eluent. Yield: 1.1 g (71%). 1H NMR (300 MHz,

CDCl3): δ 8.38 (m, 4H), 7.57 (m, 4H), 4.24 (d, J= 18.0 Hz, 4H),

3.86 (m, 8H), 1.06 ppm (t, J= 7.5 Hz, 12H).

9,10-Bis(p-dihexylaminostyryl)anthracene (B910V). Synthe-

sized by the same procedure as described for CDAE except that

9,10-bis(diethylphosphorylmethyl)anthracene (0.24 g, 0.50

mmol) and p-dihexylaminobenzaldehyde (0.41 g, 1.4 mmol)

were used. The crude product was separated by column chro-

matography on silica gel using hexane/CH2Cl2 (5:1) as the

eluent. Yield: 0.25 g (66%). Mp: 141-143 �C. 1H NMR

(300 MHz, CDCl3): δ 8.46 (m, 4H), 7.70 (d, J = 15.0 Hz, 2H),

7.56 (d, J=9.0Hz, 4H), 7.44 (m, 4H), 6.84 (d, J=18.0Hz, 2H),

6.74 (d, J=9.0 Hz, 4H), 3.35 (t, J=7.5 Hz, 8H), 1.65 (m, 8H),

1.37 (m, 24H), 0.94 ppm (t, J = 6.5 Hz, 12H). 13C NMR (75

MHz, CDCl3): δ 148.21, 137.56, 133.32, 129.98, 128.02, 126.95,

125.01, 124.87, 120.10, 111.93, 51.41, 32.02, 27.55, 27.12, 22.97,


14.34 ppm. Anal. Calcd (%) for C54H72N2: C, 86.57; H, 9.69; N,

3.74; O, 5.06. Found (%): C, 86.48; H, 9.74; N, 3.81.

Measurements. All spectroscopic measurements were per-

formed in toluene or dichloromethane solutions (spectroscopic

grade). One-photon absorption spectra were recorded on a

Hewlett-Packard 8453 diode array spectrophotometer, and the

fluorescence spectrawereobtainedwith anAmicoBowman series

2 luminescence spectrometer. The fluorescence quantum yield

was determined in toluene by the literature method using fluore-

scein in water (pH = 11) or rhodamine B in methanol as the

reference.25 The certainty formeasuredUVorPL is<2%.NMR

spectra were recorded on a JEOL JNM-LA300 (300 MHz)

spectrometer in chloroform solutions with tetramethylsilane

(TMS) as an internal standard. The elemental analysis was

performed on a Perkin-Elmer 2400.

The two-photonabsorption cross-sectionof the compoundshas

beenmeasuredwith the two-photon-induced fluorescencemethod

by using the femto-second laser pulses as described.23,24,26

Samples were dissolved in toluene at concentrations of 1.0 �10-5 M, and the two-photon induced fluorescence intensity was

measured at 740-960 nm by using fluorescein (8 � 10-5 M in

water, pH = 11) as the reference. The two-photon properties

of this reference have been well documented in the literature.26

The intensities of the two-photon induced fluorescence

spectra of the reference and sample emitted at the same

excitation wavelength were determined. The TPA cross sec-

tion, measured by using the two-photon-induced fluorescence

measurement technique, can bemeasured by using the follow-

ing equation,

δ ¼ SsΦrφrcr

SrΦsφscsδr

where the subscripts s and r stand for the sample and reference

molecules. The intensity of the signal collected by a PMT

detector was denoted as S. Φ is the fluorescence quantum

yield. φ is the overall fluorescence collection efficiency of the

experimental apparatus. The number density of the molecules

in solution was denoted as c. δr is the TPA cross section of the

reference molecule. The whole experimental uncertainty in δis about 15%.

3. Results and Discussion

Synthesis and structures of cruciforms CDAE, CDAV,C4DE, and C4DV are shown in Scheme 1, and those of thecorresponding linear analogues B910V, B26A, B26D, andB910E are illustrated in Scheme 2. 2,6-Bis(p-diethylphos-phorylmethyl)-9,10-dibromoanthracene (A), 2,6-di(p-dihexyl-aminostyryl)-9,10-bis(p-dide-cylaminophenylethynyl)-anthracene (C4DE), 2,6-di(p-dihexylaminostyryl)-9,10-bis(p-didecylamino-styryl)anthracene (C4DV), 9,10-bis-(p-didecylaminophenylethynyl)anthracene (B910E), 2,6-di(p-dihexyl-aminostyryl)anthracene (B26D), and 2,6-di-bromo-9,10-anthaquinone were available from previousstudies.22a,b,23,24 Compounds B and C were prepared bySonogashira coupling betweenA and p-didecyl-aminophe-nylacetylene and by Heck coupling between A andp-didecylaminostyrene, respectively.CDAE andCDAVwereobtained from the Wittig reaction of p-cyanobenzaldehyde

with B and C, respectively. The Sandmeyer reaction of2,6-diaminoanthraquinone followed by the reductivealkylation afforded 2,6-dibrmo-9,10-dihexyoxylanthra-cene. B26A was derived from Heck coupling of 2,6-dibromo-9,10-dihexyl-oxyanthracene and p-cyanostyr-ene. To prepare B910V, first, the bromomethylation ofanthracene with HBr/CH2O followed by treating withtriethylphosphite afforded 9,10-bis(diethylphosphoryl-methyl)-anthracene, and second, the Wittig reactionof 9,10-bis(diethylphosphorylmethyl)anthracene andp-dihexylaminobenzaldehyde was employed. All com-pounds were unambiguously characterized by 1H and13C NMR and elemental analysis.The normalized one-photon absorption and emission

spectra for cruciforms CDAE, CDAV, C4DE, and C4DV

and the corresponding linear analogues B26A, B910E,B26D, and B910V in toluene are displayed in Figures 1and 2. The peak positions of emission spectra (λem) andthe longest wavelength absorption brand (λmax) are sum-marized in Table 1. Overall, the λmax and λem of 9,10-disubstitution linear analogues (B910E and B910V) arelocated in the longer wavelength region compared withthat of 2,6-disubstitution ones (B26D and B26A)(Figure 1, Table 1), which will decrease their fluorescence

Figure 1. Normalized absorption spectra of the cruciforms C4DV,CDAV, C4DE, and CDAE and the linear analogues B26D, B26A,B910E, and B910V in toluene (at concentrations of 1 � 10-5 M).

(25) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.(26) Xu, C.; Webb, W. W. J. Opt. Soc. Am. B 1996, 13, 481.


quantum yields (Φ) due to the lower energy of theemitting states that facilitate the nonoradiative pathways.B910V shows the lowestΦ because of its strongly twistedstructure (vide post). In comparison with the two corre-sponding linear analogues (Figure 1), every cruciformshows a featureless and obviously red-shifted long wave-length absorption band (Figure 1) and much lowerextinction coefficient (εmax) at λmax (Table 1). These arevery different from that reported in benzene-centeredcruciforms,19-21 indicating that cross-conjugation in

anthracene-centered cruciforms has resulted in a signifi-cant ICT effect between 9,10- and 2,6-branches. The peakabsorption positions (λmax) of all cruciforms are red-shifted by approximately 30-80 nm from those of thecorresponding linear analogues (Table 1). Moreover,as shown in Figure 1 and Table 1, the λmax of thedonor-acceptor substituted cruciforms are longer thanthat of the corresponding tetradonor-substituted cruci-forms, that is, CDAE > C4DE, CDAV > C4DV, andthe λmax of 9,10-diaminophenylethynyl-substituted cru-ciforms are longer than that of the corresponding9,10-diaminostyryl-substituted cruciforms, that is,C4DE > C4DV, CDAE > CDAV. These are in propor-tion to the λmax of the two corresponding linear analo-gues. Furthermore, as seen in Table 1, the εmax of thedonor-acceptor substituted cruciforms are weakerthan that of the corresponding tetradonor-substitutedcruciforms, that is, CDAE < C4DE, CDAV < C4DV,and the εmax of 9,10-diaminophenylethynyl-substitutedcruciforms are stronger than that of the correspon-ding 9,10-diaminostyryl-substituted cruciforms, thatis, C4DE> C4DV, CDAE> CDAV. There is a qualita-tive correlation between the εmax of cruciforms and that ofthe corresponding linear analogues, that is, the higherthe εmax of the two corresponding linear analogues, thehigher the εmax of the corresponding cruciform.The stronger ICT effect in anthracene-centered cruci-

forms decreases the energy gap between the ground andFranck-Condon states and then significantly red-shiftsthe fluorescence emission of cruciforms compared withthat of the corresponding linear analogues (Figure 2).Moreover, the λem of donor-acceptor substituted cruci-forms show larger red-shift than that of the correspond-ing tetradonor-substituted cruciforms, that is, CDAE >C4DE, CDAV > C4DV, implying that acceptor-substi-tution may have stabilized the emitting states more thanthe donor-substitution. On the other hand, the emissionsof 9,10-diaminostyryl-substituted compounds are weak-er, broader, and more red-shifted than that of the corre-sponding 9,10-diaminophenylethynyl-substituted com-pounds. λem are in the order C4DV > C4DE, CDAV >CDAE, although λmax are in the order, C4DE > C4DV,CDAE > CDAV, indicating that 9,10-diaminostyrylsubstitution has stabilized the emitting states more thanthe 9,10-diaminophenylethynyl substitution. As a se-quence, 9,10-diaminostyryl-substituted compounds show

Table 1. One- and Two-Photon Properties of Cruciforms and the Corresponding Linear Analogues in Toluene or in Dichloromethanea

compound λmaxb 10-4εmax

c λemd Δνe Φf λTPA

g δmaxh

B26Di 456 6.50 487 1400 0.78 800 1100

B26Al 463 4.42 498 1520 0.57 800 470B910Ej 489 5.06 541 1970 0.53 780 480 iB910Vl 466 2.82 561 3600 0.055 780 160C4DVk 489 (495) 2.17 576 (596) 3090 (3420) 0.036 (0.031) 800 3940CDAV

l 503 (512) 0.83 608 (680) 3430 (4825) 0.051 (0.034) 860 2570C4DE

k 532 (536) 2.38 563 (585) 1035 (1530) 0.43 (0.40) 800 3010CDAE

l 541 (549) 1.32 595 (656) 1680 (2970) 0.38 (0.25) 880 2670

aThe data in the parentheses are measured in dichloromethane. bThe peak wavelength of the longest wavelength one-photon absorption band innanometers. cThe molar absorption coefficient in each λmax.

dThe peak one-photon fluorescence spectra in nanometers. e Stokes shift in cm-1.fFluorescence quantum yield. gThe peak two-photon excitation spectra in nanometers. hThe peak two-photon absorption cross section in 10-50 cm4 sphoton-1 (GM). iThe data are taken from ref 22a. jThe data are taken from ref 22b. kThe data are taken from ref 23. lThis work.

Figure 2. Normalized fluorescence spectra of the cruciforms and theirlinear building blocks in toluene excited at each peak λmax (at concentra-tions of 1 � 10-5 M).


larger fluorescence Stokes shifts (Δν) than do the others(Table 1). Also, Stokes shifts of the donor-acceptorsubstituted cruciforms are larger than that of theall-donor substituted cruciforms. To linear analogues,9,10-conjugation branches show redder emission andlarger Stokes shift than do 2,6-conjugation branches. Asseen in Table 1, the fluorescence quantum yields (Φ) ofcruciforms are lower than those of the correspondinglinear analogues. This is due to the lower energy of theemitting states in cruciforms that facilitate the nono-radiative pathways. Nevertheless, 9,10-diaminophenyl-ethynyl-substituted compounds are still stronglyfluorescent. The much smaller Φ for 9,10-diaminostyr-yl-substituted compounds C4DV, CDAV, and B910V

may be due to the severely distorted 9,10-distyrylanthra-cene framework because of a large internal steric hin-drance between the anthrylene ring and vinylenemoiety.22c In the monomeric form, this will limit mole-cular effective conjugation and enhance fluorescencequenching by free intramolecular torsional motion.22c

Moreover, the weak, broad, and large red-shifted emis-sion of these compounds implies the existence of large-amplitude relaxation in the excited state, such as twistedintramolecular charge transfer, which often acts as anintramolecular fluorescence quencher because of the nπ*characteristics.27

The TPA cross sections (δ) were determined with thetwo-photon-induced fluorescence measurement techni-que by using femto-second laser pulses (160 fs, 1 kHz),28

which can avoid possible complications due to the ex-cited-state excitation. These chromophores have no linearabsorption beyond 600 nm in the solution spectra. If thesolutions are irradiated with an 800 nm of laser, thereshould not be any one-photon absorption induced fluore-scence. We have observed that the fluorescence intensityof CDAE solution is gradually increased with the inputlaser power. Moreover, a power-law dependence of ex-ponent 2.08 in the plot of logarithmic output fluorescenceintensity (up-conversion signal) versus logarithmic inputlaser power is found (Figure 3), which is indicative of atwo-photon excitation process. On the other hand, thegood overlap between one- and two-photon excitationfluorescence forCDAE indicates that the emission occursfrom the same excited states, regardless of the mode ofexcitation (inset in Figure 3).As shown in Figure 4, all compounds exhibit large δ at

800 nm. This is a profitable factor for some practicalapplications because the most common Ti/xapphire laseremits an intense beam around 800 nm even thoughmodern laser techniques can easily vary the excitation

wavelengths. Among these linear analogues, B26D andB910V show the largest and smallest δ, respectively; theformer can be ascribed to both theD-π-Dmotif and themore effective utility of π electrons of anthracene ring,7c

Figure 3. Depndence of up-conversion fluorescence of CDAE on theinput laser power (800 nm). The inset is the one-photon (OPF) and two-photon (TPEF) fluorescence spectra of CDAE.

Figure 4. Two-photon excitation spectra of the cruciforms (a, top)and the building blocks (b, bottom) in toluene (at concentrations of1 � 10-5 M).

(27) (a) Valeur, B. Molecular Fluorescence: Principles and applications;Wiley-VCH,Weinheim,Germany: 2001. (b) Rettig,W.Angew.Chem.,Int. Ed. Engl. 1986, 25, 971.

(28) (a) Rumi,M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J.W.; Barlow, S.;Hu, Z.; McCord-Maughon, D.; Parker, T. C.; R€ockel, H.;Thayumanavan, S.; Marder, S. R.; Beljonne, D.; Br�edas, J.-L.J. Am. Chem. Soc. 2000, 122, 9500. (b) Cho, B. R.; Son, K. H.; Lee, S.H.; Song, Y.-S.; Lee, Y.-K.; Jeon, S.-J.; Choi, J.-H.; Lee, H.; Cho, M.J. Am. Chem. Soc. 2001, 123, 10039. (c) Lee, W. H.; Lee, H.; Kim, J.A.; Choi, J. H.; Cho, M.; Jeon, S. J.; Cho, B. R. J. Am. Chem. Soc.2001, 123, 10658.


and the latter is mainly due to the distorted conforma-tion.22c An interesting result is that all cruciforms showlarge δ values over a relatively wide range of wavelengths(Figure 4a), which will greatly enhance the choice ofexcitation wavelength for two-photon events and willalso be useful for a variety of applications, includingoptical limiting. The results may be ascribed to themolecular size.7c As the molecular size increases throughthe extended π-conjugation, the density of states willincrease, providing more effective coupling channelsbetween the ground and two-photon allowed states,which would in turn increase the δ over a wide range ofwavelengths. Themost interesting result from this study isthat, to any a given excitation wavelength in the range ofexperimental wavelengths (740-960 nm), the δ value ofevery cruciform is over the sum of the TPA cross sectionof the two corresponding linear analogues (Figure 4). Inthis context, according to themaximal TPA cross sections(δmax) (Table 1), δC4DE (3010 GM) = 1.9[δB26D (1100GM)þ δB910E (480 GM)], δCDAE (2670 GM)= 2.8[δB26A(470 GM) þ δB910E (480 GM)], δC4DV (3940 GM) =3.1[δB26D (1100 GM) þ δB910V (160 GM)], and δCDAV

(2570 GM) = 4.1[δB26A (470 GM) þ δB910V (160 GM)].These results clearly indicate that a cooperatively en-hanced TPA effect is obtained in anthracene-centeredcruciforms. After further comparison among these cruci-forms, the δmax of the tetradonor-substituted cruciform is

more than that of donor-acceptor-substituted cruci-form, that is, δC4DE > δCDAE, δC4DV > δCDAV. The en-hancement factor of δmax (vide supra) is CDAV (4.1) >CDAE (2.8), C4DV (3.1) > C4DE (1.9) and CDAV

(4.1) > C4DV (3.1), CDAE (2.8) > C4DE (1.9); theformer implies that 9,10-diaminostyryl substitution ismore effective than 9,10-diaminophenylethynyl substi-tution in enhancing δ, and the latter suggests thatdonor-acceptor-substitution pattern can effectively en-hance δ more than the tetradonor-substitution pattern.It is also noted that the maximum TPA cross sections(δmax) of CDAE and CDAV appear at 880 and 860 nm,respectively (Figure 4 and Table 1), which suggests aninteresting possibility that the wavelength of the max-imal TPA (λTPA) could be tuned by attaching appro-priate donor and/or acceptor substituents. Figure 5depicts the one- and two-photon excitation spectra ofthese anthracene-centered cruciforms. Themost notablecharacteristic of the two-photon spectra of these cruciformsstudied here shows two or three TPA peaks within ex-perimental wavelengths, and the densest two-photonallowed states (according to half the wavelength of themaximal δ, λTPA/2) are located at shorter wavelengths(higher energy) than the Franck-Condon state (one-photon allowed states, ICT absorption λICT). The appear-ance of the red-shifted two-photon bands (low energypeaks) might be ascribed to the conjugation-extending

Figure 5. Normalized one-photon absorption (OPA) and two-photon excitation (TPE) spectra of cruciforms studied here. The two-photon spectrum isplotted against λ/2 (twice the photon energy).


effect or the enhanced degree of ICT.28,30 However, it isnot clear whether these cuciforms have another TPAstate at low energy (>480 nm) since the investigation ofthe TPA properties at λ > 960 nm is limited by ourinstrument, and further investigations are needed.Inmany TPA studies,7 the enhancement of δ is ascribed

to a variety of features, including the good overlapbetween one- and two-photon absorption spectra, thehigh degree of conjugation, the multiple pathways forintramolecular electronic and photonic transfer, the ef-fective electronic coupling between branches, the ex-tended conjugation molecular size and planarity, and soforth. Indeed, the anthracene-centered cruciforms haveall the above features for the enhancement of δ andexhibit large and enhanced TPA cross sections. However,the benzene- or pyrazine-centered cruciforms also showthe main characteristics of enhancing δ; why the greatlyenhanced TPA effect occurred in the anthracene-centeredand not in the benzene-centered cruciforms and the realorigin of this dichotomy are not clear at present. Onconsidering that the main difference in these cruciformstructures is the π-center, it is concluded that the anthra-cene core plays a crucial role in enhancing the TPA crosssection and that the different types of π-centers andmolecular symmetry might involve different typesof states upon two-photon excitation. Nevertheless, incomparison with benzene- or pyrazine-centered cruci-forms,20 there are two apparent differences to be ob-served, and one is a largely red-shifted ICT absorptionband in anthracene-centered cruciforms (Figure 1). Thiswould predict a larger δ, because the smaller the energygap, the higher the probability of the two-photon excita-tion.28b,c,29 Another is that 2,6,9,10-tetrasubstituted

anthracene derivatives are not really a centrosymmetricmolecule due to non-straight-line conjugation along the2,6-direction. Thus, whether the molecular exciton modelfor two-photon absorption in octupoles31 or in benzene-centered cruciforms20 can be used to describe anthracene-centered cruciforms still requires investigation.Organic chromophores with disjoint FMO structures

are still quite rare and mostly consist of benzene-centeredcruciforms at present. Anthracene-centered chromo-phores that extend both in the 2,6-direction and in the9,10-direction may be a new class of cross-conjugatedchromophores with a disjoint FMO structure. Althoughthe effect of some 9,10-substitutents on the TPA proper-ties of 2,6-bis(p-dihexylaminostyryl)-anthracene deriva-tives have been reported, the FMO features ofanthracene-centered cruciforms exhibiting significantlyenhanced δ compared with the corresponding linearanalogues are not known.23 It would be very interestingto know their FMO features because this may provide anew class of parent chromophores for one-photon andeven two-photon sensors. As illustrated in Figure 6,HOMO and LUMO of anthracene-centered cruciforms(calculated with B3LYP/6-31G*) are very intriguing.Their FMO features vary with both the nature of theend groups and the way in which they are linked. If thefour end groups of anthracene-centered cruciforms are allmethylene (CRRV, CRRE), the HOMO is similar toLUMO, implying that FMOs are spatially superimposa-ble upon one another. The cruciforms with such an FMOarrangement have been called congruent. However, theelectron density ofCRRV concentratesmostly toward theanthracene ring and that of CRRE is spread out over thewhole π-system. Interestingly, when two ends of one

Figure 6. Frontier molecular orbital plots of CRR, C4DE, C4DV, CDAV, and CDAE calculated with B3LYP/6-31G* using Spartan, HOMO (left) andLUMO (right).

(30) Chung, S.-J.; Rumi,M.; Alain, V.; Barlow, S.; Perry, P. J.;Marder,S. R. J. Am. Chem. Soc. 2005, 127, 10844.

(29) Lee,W.-H.; Cho,M.; Jeon, S.-J.; Cho, B.R. J. Phys. Chem.A 2000,104, 11033.

(31) Beljonne,D.;Wenseleers,W.; Zojer, E.; Shuai, Z.; Vogel,H.; Pond,S. J. K.; Perry, J. W.; Marder, S. R.; Bredas, J.-L. Adv. Funct.Mater. 2002, 12, 631.


branch are capped with donors and that of another withacceptors (i.e., CDAE and CDAV), most of the HOMOdensity is located predominantly on the donor-π-donorbranch, while much of the LUMO density lies on theacceptor-π-acceptor branch. That is, the donor-accep-tor substitution disjoints the FMOs on their respectivebranches, with the central anthracene ring acting as anintegral part of both orbitals. For tetradonor cruciformC4DV andC4DE, this is not the case. Instead, theHOMOof C4DE is almost spread out over the whole π-systemand the HOMO of C4DV is mainly on the 2,6-branch,while the LUMO density of C4DE is convergent towardthe anthracene ring and the LUMO density of C4DV

almost concentrates on the anthracene ring. Since theFMOs of tetradonor cruciforms are different from that ofboth donor-acceptor-substituted cruciforms (CDAE

and CDAV) and methyl-subsituted cruciforms (CRREand CRRV), we propose to call cuciforms with such anFMO feature semidisjoint. It should be noted that thespatial separation of FMOs of cruciforms CDAV andC4DV are more distinct. This implies a stronger ICTeffect along the 9,10-distyryl direction although there is alarge internal steric hindrance between the anthrylenecenter and vinylene moiety, which is consistent thatCDAV and C4DV show large red-shift emissions(Figures 2 and 7). Although the degree of enhancementof δ was higher in the donor-acceptor cruciform CDAE

and CDAV than in the tetradonor cruciform C4DE andC4DV, the absolute value of δ of C4DE and C4DV werelarger than that of CDAE and CDAV. The relationshipbetween the enhancement of δ and the FMO features inanthracene-centered cruciforms are unclear, based on thelimited studies. Therefore, in the current study, in com-parison with benzene-centered cruciforms,19-21 we haveonly concluded that the π-center characteristics in thecruciform configuration are responsible for the enhance-ment of δ. It should be also noted that cruciforms CDAV

and C4DV contain a 9,10-distyrylanthracene unit thatcan exhibit aggregation-enhanced emission (AEE).22c

This is a special class of organic fluorophores that are

currently at the very initial state of investigation. To datefewAEE fluorophores with cruciform configuration havebeen synthesized, to the best our knowledge. The inves-tigation of their AEE effect is another interesting subjectand is underway in our laboratory.As expected for an excited state effect, the solvent

polarity only slightly affects the cruciform absorptions(Table 1). However, increasing solvent polarity (fromtoluene to dichloromethane) leads to a significantly red-shifted emission by 60-70 nm and a decreased Φ byapproximately 34% for donor-acceptor substitutedcruciforms CDAE and CDAV (Figure 6 and Table 1).This is because an electron from the HOMO, uponphotonic excitation, will be advanced into the LUMO,to give a highly polarized excited state that should besignificantly stabilized by polar solvents. This shoulddecrease the band gap between LUMO and HOMOand promote the accessibility of nonradiative deexcita-tion pathways via vibronic coupling.32 For semidisjointcruciforms C4DE and C4DV, a moderately red-shiftedemission by approximately 24 nm and slightly decreasedΦ under 10% were observed with the increase of solventpolarity (Figure 6 and Table 1), which illustrates thatsemidisjoint cruciforms are weak polar chromophores.The solvatochromic behaviors are qualitatively consis-tent with the FMO features of anthracene-centered cruci-forms, indicating that solvent stabilization of the charge-transfer excited-state has a dramatic effect on the opticalproperties.

4. Conclusions

We have synthesized two new donor-acceptor substi-tuted anthracene-centered cruciforms having p-cyanosty-ryl groups at 2,6-positions and having p-aminostyryl(CDAV) or p-aminophenylethynyl (CDAE) groups at9,10-positions using a new synthetic route. Their one-photon, two-photon absorption properties and frontiermolecular orbital features have been investigated andcompared with the corresponding tetradonor-substitutedcruciforms (C4DE, C4DV) and the corresponding linearanalogues. The results show that all cruciforms exhibitlargely red-shifted emission and significantly enhancedTPA cross section compared with the corresponding ana-logues. This is thoroughly different from the knowncharacteristics of benzene-centered cruciforms, whichshow no enhancement of TPA cross section. Quantumchemical calculationand solvatochromic behavior indicatethat donor-acceptor substitution have spatially separatedthe HOMO and LUMO of anthracene-centered cruci-forms (CDAV and CDAE). Although the enhanced TPAcross section seems to be independent of the frontiermolecular orbital (FMO) features, these cruciformswith new π-center exhibiting enhanced δ and/or spatiallyseparated FMO may ultimately find useful applicationsas two-photon sensor and optical limiting materials.

Figure 7. Normalized fluorescence spectra of the cruciforms in dichloro-methane excited at each peak λICT (at concentrations of 1 � 10-5 M).

(32) (a) Tolbert, L. M.; Nesselroth, S. M.; Netzel, T. L.; Raya, N.;Stapleton,M. J. Phys. Chem. 1992, 96, 4492. (b) Caspar, J. V.;Meyer,T. J. J. Phys. Chem. 1983, 87, 952.


Further synthesis of new anthracene-centered cruciformsand investigation of their photonic and optoelectronicproperties are presently underway in our laboratory.

Acknowledgment. Financial support for this researchwas provided by NSFC of China (No. 50573036) andOpen Project of State Key Laboratory of Supramolecular

Structure and Materials (SKLSSM200707). Dr. Yangthanks Professor B. R. Cho (Korea University, Korea) forthe instruction of knowledge and technique.

Supporting Information Available: NMR spectra of new

compounds (PDF). This material is available free of charge

via the Internet at http://pubs.acs.org.

Documents

Donor−Acceptor-Substituted Anthracene-Centered Cruciforms: Synthesis, Enhanced Two-Photon Absorptions, and Spatially Separated Frontier Molecular Orbitals