Design and Synthesis of Two New Two-Photon Absorbing Pyridine

Jan 22, 2009 - Department of Chemistry, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province, Anhui University, 230039, Hefei,...
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Design and Synthesis of Two New Two-Photon Absorbing Pyridine Salts as Ligands and Their Rare Earth Complexes Yu-Peng Tian,*,†,‡,§ Lin Li,† Yong-Hong Zhou,† Peng Wang,† Hong-Ping Zhou,*,† Jie-Ying Wu,† Zhang-Jun Hu,† Jia-Xiang Yang,† Lin Kong,† Gui-Bao Xu,‡ Xu-Tang Tao,‡ and Min-Hua Jiang‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1499–1504

Department of Chemistry, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui ProVince, Anhui UniVersity, 230039, Hefei, P. R. China, State Key Laboratory of Crystal Materials, Shandong UniVersity, 250100, Jinan, P. R. China, and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, 210093, Nanjing P. R. China ReceiVed September 12, 2008; ReVised Manuscript ReceiVed December 15, 2008

ABSTRACT: Two new donor-bridge-acceptor (D-π-A) type ligands (L1: trans-4-[(p-N,N-dimethylamino)styryl]-N-acetic-acid pyridinium and L2: trans-4-[(p-N-2-hydroxyethyl-N-methylamino)styryl]-N-acetic-acid pyridinium) were designed and synthesized, which have double functions, two-photon absorption and coordination ability. Fourteen novel rare earth complexes have been prepared by the reaction of rare earth salts with the two ligands in ethanol solution, respectively. The two novel ligands and their complexes were characterized by single crystal X-ray diffraction determination. Linear and nonlinear optical properties of the ligands and three representative complexes are described. All these compounds exhibit relatively large two-photon absorption (TPA) cross sections. When pumped by a 1064 nm laser beam, two-photon-pump cavity lasing was observed in these compounds. Analysis of the lasing efficiencies and TPA cross sections show that rare earth ions have a significant influence on two-photon absorption properties of the complexes by adjusting the density of the chromophores. Introduction The design and synthesis of materials with large two-photon absorption (TPA) cross sections are attracting intense research activity because of their potential applications in optical data storage,1 three-dimensional fluorescence imaging,2 photodynamic therapy,3 two-photon-pumped (TPP) lasing, and so on.4 To the best of our knowledge, most of the reported materials with strong TPA properties are organic chromophores.5 It would not be surprising that the growth of the field around 10 years ago was to be eclipsed by a completely new set, rapid development and application of TPA materials, enabled by our improved understanding of the detailed relationships between structure and nonlinear optical properties.6 More recently, based on the concept that the increase of chromophore density would improve the TPA cross section, multibranched compounds, dendrimers, and octupoles have also been widely investigated.7 However, synthesis of multipolar molecules with strong TPA properties requires a lot of synthetic skills and generally afford a very low yield of the desired products. Surprisingly, limited study has been devoted to metal complexes,8 especially the rare earth complexes, for this purpose. Rare earth ions are rather hard cations and possess an extremely high coordination number, which can easily assemble simple ligands containing oxygen atoms in a variety of multipolar arrangements and serve as either as a noninteracting center or a multidimensional template for increasing the molecular number density of two-photon active components (ligands), or as an important part of the structure to control the intramolecular charge-transfer process that drives the TPA process.9 For nonlinear optics, the correlation between the structure and TPA properties for the metal complexes is currently of significant importance for materials design, because metal complexes can * To whom correspondence should be addressed. Phone: +86-5515108151. Fax: +86-5515107342. E-mail: [email protected]. † Anhui University. ‡ Shandong University. § Nanjing University.

combine the merits of organic and inorganic materials in themselves, which is broad and exciting in materials science.10 One practical issue is to design appropriate organic ligands with high TPA properties for rare earth ions; however, up to now, no crystal structure has been reported for rare earth complexes with large TPA activity. According to the literature reports, molecules with high TPA activities typically contain electron donor and acceptor groups connected via polarizable π-systems.11 Pyridinium rings act as powerful electron acceptors groups in their corresponding salts.12 These increase in molecular TPA responses which arise from the greater electronwithdrawing abilities of other N-alkyl- with respect to N-ethylpyrimidium groups.13 Based on the considerations above, two new π-conjugated N-acetic-acid pyridinium salts as ligands were designed. We herein report the synthesis, linear optical absorption, fluorescence, and TPA properties (TPP lasing) of two novel N-acetic-acid pyridinium-based ligands and their 14 novel rare earth complexes. X-ray structure determinations for both ligands and their complexes have been carried out. Optical properties including linear absorption, single-photon and two-photon excited fluorescence, and two-photon pumped lasing behavior for them in solution are presented in this article. Mindful of the length of the article, we chose three complexes as representatives to be studied in detail. Experimental Procedures Synthesis of π-Conjugated N-Acetic-acid Pyridinium. The synthetic procedure for the ligands (L1 and L2) is described below, which are in Scheme 1. Their structures are given in Figure 1. trans-4-[(p-N,N-Dimethylamino)styryl]-N-acetic-acid Pyridinium. BrCH2COOH (27.78 g, 0.2 mol) was dissolved in 50 mL of EtOH, and then NaOH (8 g, 0.2 mol) in 5 mL of water was added dropwise during 1 h at room temperature. The resulting solution was stirred for 2 h, and then 4-methylpyridine (20 mL, 0.2 mol) was added. The mixture was heated to 40 °C and stirred for 12 h. The final pH of the solution was 7. To the solution above, N,N-dimethylaminobenzaldehyde (29.8 g, 0.2 mol) and 10 drops of piperidine were added. The solution

10.1021/cg801022h CCC: $40.75  2009 American Chemical Society Published on Web 01/22/2009

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Scheme 1. Synthetic Route to L1 and L2

[TbL13(phen)(OH)2(H2O)] · NO3 · L1 · 8H2O (1) was isolated by treating rare earth metal(III) nitrate with L1 and 1,10-phenanthroline (phen) in a molar ratio of 1:3:2 in 95% ethanol solution. For the second kind of rare metal complexes, [Tb2L16(OH)4(H2O)2] · 2NO3 · 16H2O (2) and [Tb2L26(H2O)2] · 6NO3 · 7H2O (3) were obtained by the reactions of corresponding rare earth metal(III) nitrate and L1 or L2 in a molar ratio of 1:3 in EtOH · H2O solution. It is interesting that phen does play a crucial role in the formation of different structures, in which there are two kinds of ligands, L1 and phen, both of them can coordinate to rare earth metal ions. For the second, treating Tb(NO3)3 · 6H2O with only L1 or L2 leads to the formation of 2 or 3. It is obviously that the density of the ligands increases from 4 to 6 in one complex molecule when we omit phen in our design strategy. Structural Studies. From Figure 1, it can be seen that the ligands are perfectly planar. Actually, the dihedral angles between the benzene and the pyridinium ring are nearly zero in the two ligands. Least-square plane calculations show that the maximum deviations to their corresponding planes are no more than 0.1 Å in L1. The bond lengths of the benzene ring and the pyridinium ring are all of an aromatic character. The bridge bond lengths of C6-C9-C10-C11 are also highly conjugated. The planar and the conjugated geometric configuration indicate that the ligand has a highly delocalized π-electron system.15 L2 has the same structural features, which are necessary for TPA activity. A summary of the crystal data for complexes 1-3 is given in Table 1 and 4-14 is shown in Table S1, Supporting Information. Selected bond lengths and angles of 1-3 are listed in Table 2. In the crystal structure of 1, TbIII is eight-coordinated by three oxygen atoms from three L1, two nitrogen atoms of 1,10-phenanthroline ligand, the other coordinated sites are occupied by three oxygen atoms from two hydroxyl groups and one water molecule. Complexes 2 and 3 have similar structural features. The cation of 2 and 3 (Figure 2) contains two TbIII centers bridged by two carboxylates (2) or four carboxylates (3), providing a dimeric core of [Tb2L2] with Tb · · · Tb distance of 4.735 Å in 2 and 4.347 Å in 3. It can be seen that the six ligands can be divided into three models (A, B, C) in complexes 2 and 3. For model A, the ligands act as a bridging mode. For model B, the two oxygen atoms from each ligand coordinate to Tb1 (or the Tb1A) atom forming four-member rings. For model C, the other ligands bond to the TbIII through the monodentate fashion. Each TbIII is eight-coordinate bonded to eight oxygen atoms. The average Tb-O distance is 2.410(7) Å in 2 and 2.373(4) Å in 3, which are longer than those previously reported.16 Comparing the bond lengths, angles and the distortion of the plane in L1, L2, and their corresponding complexes, there are no significant differences after coordinating with rare earth metal ions, which suggests that rare earth metal ions do not influence the electron distribution in the ligands. This is very important in our design strategy. Linear Absorption and Single-Photon Excited Fluorescence (SPEF). Detailed data of the linear optical properties is summarized in Table 3. Linear absorption spectra of L1, L2, and complexes 1-3 in DMF with a solution concentration of C ) 1.0 × 10-5 mol/L are shown in Figure 3. One can see that the linear absorption maximum of L2 was slightly red-shifted when comparing with L1 in Figure 3. This shift between the two ligands is indicates that increasing electron-donation stabilizes the excited-state more than the ground-state from L1 to L2, which suggests that significant charge redistribution takes place upon excitation.8c,17 One can also observe that the

was heated to reflux for 20 h. After being cooled to room temperature, the solution was poured into 1000 mL of acetone and filtered under a vacuum. The filtration was washed with a small amount of water and dried in air. The product was recrystallized from methanol and microcrystals of L1 were obtained. Yield: 29.5 g (64%). Anal. Calc. (found) for C17H20N2O3: C, 68.11 (68.47), H, 8.06 (8.12), N, 9.31 (9.60)%. IR (KBr, pellet, cm-1): 3384s; 1639vs; 1585vs; 1361vs; 1171vs; 803w; 530w. ES-MS, m/z (rel. intensity): 283.2 (M + H+, 86%), 305.1 (M + Na+, 98%), 564.9 (2M + H+, 28%), 587.0 (2M + Na+, 100%). 1H NMR (DMSO-d6, 400 MHz, ppm) δ 3.02 (s, 6H), 4.82 (s, 2H), 6.78 (d, 2H, J ) 8.8 Hz), 7.16 (d, 1H, J ) 16.0 Hz), 7.59 (d, 2H, J ) 8.8 Hz), 7.86 (d, 1H, J ) 16.0 Hz), 7.99 (d, 2H, J ) 6.8 Hz), 8.57 (d, 2H, J ) 6.8 Hz). trans-4-[(p-N-2-Hydroxyethyl-N-methylamino)styryl]-N-aceticacid Pyridinium. L2 was synthesized by method similar to that used in synthesizing L1, except N-2-hydroxyethyl-N-methylaminobenzaldehyde was used instead. Yield: 34.7 g. (52%). Anal. Calc. (found) for C17H20N2O3: C, 65.44 (65.65), H, 6.71 (6.52), N, 8.48 (8.65)%. IR (KBr, pellet, cm-1): 3415s; 1649vs; 1587vs; 1372vs; 1186vs; 815w. ES-MS, m/z (rel. intensity): 331.2 (M + H+, 82%), 683.5(2M+Na+, 100%). (DMSO-d6, 400 MHz, ppm) δ 3.04 (s, 3H), 3.50 (t, 2H, J ) 5.8 Hz), 3.58 (t, 2H, J ) 5.6 Hz), 4.95 (s, 2H), 6.79 (d, 2H, J ) 8.8 Hz), 7.14 (d, 1H, J ) 16.0 Hz), 7.58 (d, 2H, J ) 9.2 Hz), 7.88 (d, 1H, J ) 16.0 Hz), 8.00 (d, 2H, J ) 6.8 Hz), 8.59 (d, 2H, J ) 7.2 Hz). Synthesis of the Rare Earth Complexes. The procedure for synthesizing 1-3 is described below (the others are included in the Supporting Information). [TbL13(phen)(OH)2(H2O)] · NO3 · L1 · 8H2O (1). Reaction of Tb(NO3)3 · 6H2O (0.453 g, 0.001 mol), L1 (0.846 g, 0.003 mol), phen · H2O (0.793 g, 0.002 mol) in a mol ratio of 1:3:2 in 95% EtOH solution (50 mL) at 80 °C for 8 h yielded red solutions. After cooling of the sample, dark red microcrystals of 1 were obtained. Yield: 51%; elemental analysis calcd (%) for C80H100N11O22Tb: C 55.65, H 5.84, N 8.92%; found: C 55.95, H 5.50, N 8.72%. [Tb2L16(OH)4(H2O)2] · 2NO3 · 16H2O (2). Tb(NO3)3 · 6H2O (0.453 g, 0.001 mol) was dissolved in 5 mL water, and L1 (0.846 g, 0.003 mol) dissolving in 30 mL of ethanol was added dropwise. The mixture was stirred for 10 h at 80 °C. Microcrystals of 2 were obtained by filtration after cooling. Yield: 60%; elemental analysis calcd (%) for C102H148N14O40Tb2: C 48.46, H 5.90, N 7.76%; found: C 48.29, H 5.62, N 7.90%. [Tb2L26(H2O)2] · 6NO3 · 7H2O (3). This compound was prepared as that for 2, except that L2 was used instead. Yield: 65%; elemental analysis calcd (%) for C108H146N18O49Tb2: C 46.36, H 5.26, N 9.01%; found: C 46.06, H 5.50, N 9.36%. X-ray Structural Determinations. Single crystals of the ligands used in X-ray determination were obtained by slow evaporation of EtOH solution and those of 1-14 were obtained by slow evaporation of their mother liquors. X-ray diffraction data of the ligands was collected on a Bruker Smart P4 X-ray diffratometer and those for 1-14 were collected on a Bruker Smart 100 CCD area detector diffractometer. Both of the radiation sources were Mo KR (λ ) 0.71073 Å). Empirical absorption correction was applied to the data. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2. All the nonhydrogen atoms were located form the trial structure and then refined anisotropically with SHELXTL using the full matrix least-squares procedure.14 The hydrogen atom positions were geometrically idealized and generated in idealized positions and fixed displacement parameters (Table 1 and Table S1, Supporting Information).

Results and Discussion Syntheses. Our design strategy has three steps. For the first kind of rare metal complexes, mononuclear complex

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Crystal Growth & Design, Vol. 9, No. 3, 2009 1501

Figure 1. ORTEP drawings of L1 and L2, with H atoms and solvent molecule omitted for clarity. Table 1. Crystallographic Data for L1, L2, and Complexes 1-3

formula Mr T [K] crystal system space group a [Å] b [Å] c [Å] R [°] β [°] γ [°] V [Å3] F(000) crystal size [mm3] range [°] index range measured data data with I > 2σ(I) goodness-of-fit R1 [I > 2σ(I)] wR2 [I > 2σ(I)]

L1

L2

1

2

3

C17H20N2O3 300.35 293(2) monoclinic P21 5.8780(6) 8.4735(12) 15.5832(16) 90.00 91.907(7) 90.00 775.73(16) 320 0.38 × 0.33 × 0.25 2.62-28.00 -7 e h e 1 -11 e k e 1 -20 e l e 20 2874 2250 0.955 0.0522 0.1285

C18H22N2O4 330.38 293(2) monoclinic P21/c 15.646(16) 10.158(11) 10.456(11) 90.00 104.279(18) 90.00 1610(3) 704 0.38 × 0.31 × 0.13 2.41-25.03 -18 e h e 18 -10 e k e 12 -9 e l e 12 7566 2727 0.873 0.0772 0.1851

C80H100N11 O22Tb 1726.63 298(2) triclinic P1j 14.830(5) 17.692(6) 17.974(6) 107.338(5) 99.936(5) 102.501(6) 4250(2) 1796 0.46 × 0.39 × 0.31 2.13-23.37 -15 e h e 16 -19 e k e 13 -19 e l e 20 18424 11739 0.897 0.0546 0.1007

C102H148N14 O40Tb2 2528.18 298(2) triclinic P1j 13.421(7) 14.936(8) 18.031(10) 74.280(9) 72.802(8) 75.939(9) 3271(3) 1308 0.48 × 0.34 × 0.18 1.61-25.03 -15 e h e 15 -17 e k e 17 -21 e l e 19 16650 11097 0.923 0.0717 0.1582

C108H146N18 O49Tb2 2798.30 293(2) triclinic P1j 12.532(11) 15.748(14) 17.757(16) 100.188(13) 107.017(14) 102.701(14) 3158(5) 1410 0.31 × 0.23 × 0.17 1.59-25.03 -12 e h e 14 -18 e k e 18 -21 e l e 15 16843 11000 0.890 0.055 0.1073

absorption maximum shows no significant difference between the corresponding the ligand and complexes in Figure 3 and Table 3. This result suggests that the linear absorption is due to the ligand and the rare metal earth ions have little influence on the energy gap between the ground and the first excited state, which corresponds to the structural studies above. But the absorbance values increase in the order 3-2-1, which may different from the investigation of the increase of chromophore number density in the references. This may be caused by rare earth ions perturbing the chromophore and the systematic study of the reason is in progress. The one-photon fluorescence spectra of L1, L2, and complexes 1-3 are shown in Figure 4 in DMF. The concentration of the solution was 1.0 × 10-6 mol/L. Crystal structural studies have revealed that there are four pyridine salt ligands in 1 and six pyridine salt ligands in 2 and 3. So it is not surprising to find that these complexes have higher relative fluorescence intensities than those of their ligands. In fact, the fluorescence intensities show the sequence: 1 > 3 > 2 > L2 > L1. This fact suggests that the rare metal earth ions may significantly increase the molecular density without causing aggregation due to π-π packing. When the number of the ligand reaches six in 2 with the structure of two bridged LnIII centers, the head influence of the rare earth metal ions may be the enhancement of the intersystem crossing or other nonradiation processes to quench fluorescence.18 Comparing 2 and 3, there is obvious enhancement in one-photon fluorescence intensity, indicating that increasing electron-donating ability reduces the nonradiation processes to quench fluorescence in the complexes, which may be caused by the center rare earth ions.

Two-Photon Excited Fluorescence (TPEF). From Table 3, one can see that the λmax of the two-photon excited fluorescence (TPEF) is at 616 and 615 nm for L1 and L2 and those for 1 and 2 are red-shifted ∼10 nm, compared with that of L1, but not red-shifted for 3. Comparing the peak position of TPEF with those of SPEF for these compounds, one can find the peak positions of TPEF are red-shifted 15-38 nm. These red-shifts are attributed to the reabsorption effect of the fluorescence within the solutions. The reabsorption effect expands rapidly with the increase of the concentration in the samples. For SPEF, a dilute solution of 1 × 10-6 M was used and the reabsorption effect can be ignored, when in the case of TPEF, we used a concentrated solution of 0.001 M and the reabsorption can no longer be ignored. TPA cross sections for the ligands and their rare earth complexes were measured by the open aperture Z-scan technique with a ps-laser source. Ps-laser was used to avoid the excitedstate absorption, and relatively accurate TPA cross sections may be obtained. Table 3 presents the TPA cross sections (σ) of the two ligands and their three rare earth complexes. It is obvious that the σ values for the metal complexes are larger than those of their corresponding ligands, which may result from their different chemical nature. As mentioned above, there are four or six ligands (chromophores) per molecule for these complexes. The density of chromophore in the complexes is much larger than their corresponding ligands. The results are consistent with the theoretical explanation that the TPA cross section would be enhanced with the increase of chromophore density. On the other hand, the differences in the TPA cross sections for the three rare earth complexes are not obvious. Though 2 and 3

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Table 2. Selected Bond Lengths [Å] and Angles [°] for L1, L2, and Complexes 1-3

Table 3. Optical Properties of L1, L2, and Complexes 1-3 in DMF Solutiona

Compound L1 C17-O1 C17-O2 C3-N1 C9-C10 O1-C17-O2

1.240 1.255 1.366 1.328 127.1

C10-C11 C16-N2 C16-C17 C6-C9 N2-C16-C17

1.449 1.470 1.521 1.444 113.5

Complex 1 Tb(1)-O(1) Tb(1)-O(3) Tb(1)-O(5) Tb(1)-O(7) O(5)-Tb(1)-O(7) O(5)-Tb(1)-O(1) O(7)-Tb(1)-O(9)

2.412(4) 2.377(4) 2.279(4) 2.325(5) 145.62(15) 142.62(15) 101.92(16)

Tb(1)-O(8) Tb(1)-O(9) Tb(1)-N(7) Tb(1)-N(8) O(7)-Tb(1)-O(8) O(3)-Tb(1)-O(8) O(3)-Tb(1)-O(1)

2.421(4) 2.361(4) 2.635(5) 2.592(5) 142.08(15) 134.37(15) 131.43(14)

compd λ(1a)max[a] λ(1f)max[b] λ(2f)max[c] λ(2f)max[d] L1 1 2 L2 3

459 460.5 463 467 468.5

591 590 595 602 596

616 627 624 615 616

617 628 632 616 628

η [%]

ET σ [mJ] [GM]

3.10 ( 0.28 3.88 ( 0.30 1.21 ( 0.13 4.26 ( 0.38 2.95 ( 0.28

0.90 65.66 0.81 224.6 0.55 237.5 0.93 76.65 1.81 266.4

a [a], [b], [c], and [d] are one-photon absorption, one-photon fluorescence, two-photon fluorescence, and two-photon pumped up-conversion lasing maxima peaks in nm, respectively. η is the two-photon pumped up-conversion lasing efficiency. ET is the threshold of the sample. σ is the two-photon absorption cross section. 1 GM ) 1 × 10-50 cm4 s photon-1.

Complex 2 Tb(1)-O(1) Tb(1)-O(2) Tb(1)-O(3) Tb(1)-O(4)A O(7)-Tb(1)-O(2) O(8)-Tb(1)-O(2)

2.411(7) 2.631(6) 2.416(5) 2.289(5) 140.2(2) 138.7(3)

Tb(1)-O(5) Tb(1)-O(7) Tb(1)-O(8) Tb(1)-O(9) O(4)A-Tb(1)-O(2) O(4)A-Tb(1)-O(3)

2.295(6) 2.421(7) 2.434(7) 2.387(10) 126.4(2) 108.7(2)

Compound L2 C1-O1 C2-O2 C5-C8 C9-C10 O1-C1-O2

1.211(6) 1.237(6) 1.501(11) 1.425(10) 127.7(5)

C1-C2 C2-N1 C8-C9 C13-N2 N2-C2-C1

1.518(7) 1.448(6) 1.258(11) 1.345(6) 111.9(4)

Figure 3. Linear absorption spectra of L1, L2, and complexes 1-3 in DMF.

Complex 3 Tb(1)-O(1) Tb(1)-O(2) Tb(1)-O(4) Tb(1)-O(11) O(7)-Tb(1)-O(2) O(5)-Tb(1)-O(4)

2.377(4) 2.299(4) 2.350(4) 2.468(4) 142.02(2) 123.64(2)

Tb(1)-O(5) Tb(1)-O(7) Tb(1)-O(12) Tb(1)-O(10) O(7)-Tb(1)-O(5) O(2)-Tb(1)-O(1)

2.320(4) 2.281(4) 2.416(5) 2.474(4) 141.03(2) 121.30(2)

have a higher chromophore density than that of 1, their cross sections do not increase proportionally. This structure-property

Figure 4. Single-photon fluorescence spectra of L1, L2, and complexes 1-3 in DMF.

Figure 2. Perspective view of the cations of 1, 2, and 3. H atoms, solvent molecules, and anions are omitted for clarity.

relationship is very similar to that of “nonconjugated dendrimers” in which no strong cooperative enhancements are found. The result indicates the rare earth complexes in the present report here can be referred to as a new type of dendrimer. In order to obtain efficient TPP lasing, a large TPA cross section value is necessary. From Table 3, we can see the compounds studied here have relatively large TPA cross section values which makes it possible to get a TPP lasing when pumped by a 1064 nm IR laser beam.12c,19 TPP Lasing. To date, the number of reported novel TPP lasing dyes is below one 10th of the total number of reported novel two-photon absorbing chromophores. However, it should be noted that the major requirements for TPP lasing dyes are different from those for chromophores designed for other applications, where a larger TPA cross section is the most important consideration. Here, for the TPP lasing purpose, the most essential requirements for the lasing dyes are the ease of establishing population inversion and a higher lasing efficiency. Thus, it is important to realize that the best two-photon absorbing materials may not necessarily produce two-photon pumped lasing. This may be the reason why only a small number of the

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ligands are in monoclinic. The two novel ligands and three representative complexes also have been systematically investigated for their optical properties. Their efficiencies of twophoton pumped (1064 nm) lasing are in the range of 1.21-4.26%. It is worth noting that the rare earth metal ions can offer the advantages of adjusting the chromophore density without causing aggregation with the ease of chemical modification toward different functionalities. The definite function of the new ligand can be tunable, and further investigation is currently underway.

Figure 5. SPEF, TPEF, and lasing spectra for a typical complex 1 in DMF.

reported novel two-photon active chromophores can be employed for TPP lasing. Two-photon-pumped lasing offers distinct merits; some of these are20 (i) frequency up-conversion without the restriction of phase matching, (ii) the use of a long interaction length, and (iii) the minimization of local thermal damage because of weaker absorption. The two main requirements for TPP lasing are to maintain high concentration of the dye and high optical quality of the bulk. Figure 5 shows the lasing spectrum for 1 comparing with SPEF and TPEF, representatively. The central wavelengths of these compounds are listed in Table 3. From Figure 4 and Table 3, one can find that the lasing peaks are slightly redshifted from the peaks of the TPEF because of reabsorption in the solution of the samples. The efficiencies of TPP lasing, calculated by η ) (Eup/Epump) × 100%, where Eup is lasing energy and Epump is the pump laser energy, are also measured and listed in Table 3. It is easy to find that the complexes possess larger TPA cross sections, but the lasing efficiency of 1 is higher than that of ligand, while that of 3 is lower than its ligand. This suggests that a significant nonradiative transition might occur in complexes 2 and 3. Experimental results have confirmed that a large TPA cross section is essential for TPP lasing, but does not always guarantee TPP lasing. One reason may be that nonradiative decay exists in the excited state. Jones et al.21 have proposed that intramolecular charge-transfer (ICT) excited-state with high emission is able to transform into a no-fluorescent twisted intramolecular charge-transfer state (TICT). As mentioned above, the ligand molecules interact with each other in a head-to-tail manner. This manner not only provides an entropically favorable state in both the ground-state and the excited-state but also a rigid structure. The barrier between the ICT and TICT might be raised due to the restriction of configuration. Consequently, TPP lasing would be enhanced. In the case of complexes, the head-to-tail manner of the ligand no longer exists and the barrier between the ICT and TICT decreased.13 Thus, the lasing efficiencies of these complexes decreased rapidly. Of course, the moderate high up-conversion efficiencies may be the result of the strong reabsorption of the samples. The threshold values of these samples are listed in Table 3. The threshold pump energy for 1 and 2 is lower than that of L1 (0.9 mJ), but the value of 3 is higher than that of L2 (0.93 mJ). The systematic study of the reason is still in progress. Conclusions Fourteen new rare earth complexes with chromophores L1 and L2 as ligands have been synthesized and characterized by single crystal X-ray diffraction. The results of X-ray analysis show that all the complexes crystallize in triclinic while both

Acknowledgment. The work was supported by grants for the National Natural Science Foundation of China (50532030, 20771001 and 50703001), the Foundation of Scientific Innovation team of Anhui Province (2006KJ007TD). Supporting Information Available: Chemicals, measurements information, and crystal data of 4-14. This material is available free of charge via the Internet at http://pubs.acs.org.

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