Synthesis and spectroscopic properties of symmetrically substituted two-photon absorbing molecules with rigid elongated π-conjugation

Journal of Molecular Structure 833 (2007) 82–87 www.elsevier.com/locate/molstruc

Synthesis and spectroscopic properties of symmetrically substituted two-photon absorbing molecules with rigid elongated p-conjugation Bo Liu, Jun Liu, Hai-Qiao Wang, Yuan-Di Zhao, Zhen-Li Huang

*

Key Laboratory of Biomedical Photonics of Ministry of Education and Hubei Bioinformatics and Molecular Imaging Key Laboratory, Huazhong University of Science and Technology, Wuhan 430074, China Received 30 April 2006; received in revised form 31 August 2006; accepted 6 September 2006 Available online 22 December 2006

Abstract A series of symmetrical molecules with substituted acetylene as central rigid elongated conjugation have been designed and synthesized. These molecules consist of a typical D-p-D structure, where N,N-dihexylamino and substituted acetylene are employed as donor (D) and p-conjugated center (p), respectively. Single and two-photon spectroscopic properties of these molecules were investigated systematically. One derivative with great enhancement on the two-photon absorption cross-section in 820 nm, which is among the best output wavelength range of a typical Ti:Sapphire femtosecond laser, could be obtained by inserting an anthracene ring into the rigid elongated p-conjugation. Such kind of structure modification is expected to be helpful in designing better organic nonlinear optical materials for biological imaging.  2006 Elsevier B.V. All rights reserved. Keywords: Optical materials; Chemical synthesis; Two-photon absorption; Structure–property relationships; Biological imaging

1. Introduction Organic molecules that can simultaneously absorb two photons have attracted a lot of interest over recent years, due to their potential applications on several areas including two-photon fluorescence imaging, optical limiting and stabilization, two-photon pumped lasing, two-photon photodynamic therapy, and 3-D data storage [1–3]. As molecules with large two-photon absorption (2PA) crosssections and sound solubility are required in practical applications, many novel organic molecules have been designed and synthesized to fulfill these requirements in the past [4–8]. Inside the several classes of 2PA materials, one of the most frequent adopted strategies is using double bonds (C@C) as the conjugation bridge and aromatic rings such as phenyl, biphenyl, fluorene, dithieniothiophene, and dihydrophenanthrene as the p-centers, respectively [9,10].

*

Corresponding author. Tel./fax: +86 27 87792202. E-mail address: [email protected] (Z.-L. Huang).

0022-2860/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.09.007

However, it is worthy of noticing that, although C@C bond is an excellent conjugation bridge for the intramolecular charge transfer from donor to acceptor, it readily undergoes trans to cis photoisomerization, which may hamper the efficiency and the lifetime of the materials [10,11]. In particular, Mertz et al. pointed out that this kind of photoisomerization could lead to localized photoinduced flip-flop when marked to biological membranes; however, no flip-flop could be observed when they replaced vinyl by the corresponding ethynyl molecules with a triple-bonded linker arm, which cannot be isomerized [11]. This finding implies that acetylene derivatives would be a new kind of promising candidates with potential applications in two-photon fluorescence imaging. Therefore, detailed information on the relationship between the chemical structure of acetylene derivatives and their two-photon spectroscopic properties will be helpful for designing and developing efficient organic probes that possess high 2PA cross-sections without photoinduced flip-flop for biological imaging. Unfortunately, limited number of reports was focusing on 2PA molecules containing triple bonds (C„C) as p-bridges [9,10].

B. Liu et al. / Journal of Molecular Structure 833 (2007) 82–87

(C6H13)2N

I 1

(a)

2

(b)

(C6H13)2N Br

Br 3

(a)

Br

Br

(a)

Rigid conjugated linker

Perkin-Elmer 240 C instrument. Schlenck technique was used for reactions performed under inert gas. 2.2. Synthesis

(C6H13)2N

(C6H13)2N

83

4

N(C6H13)2

1 2 3

2.2.1. General procedure for the synthesis of molecules 1–4 [13] Corresponding aryl halides (1.5 mmol), PdCl2(PPh3)2 (2 mol%), 4-N,N-dihexylaminophenyl-acetylene (0.4 g, 1.4 mmol) and a trace amounts of CuI were added into a 50 ml Schlenck reaction tube contained triethylamine (1 ml) and CH3CN (25 ml) (previously purged with pure Ar gas). The reaction mixture was stirred under argon/ hydrogen (7:3) mixture gas atmosphere at 60 C for 10 h. The solvent was removed under vacuum. Water (40 ml) was then added and the mixture was extracted with ether and dried over MgSO4. The solvent was evaporated and the residue was purified by column chromatography (silica gel, light petroleum ether/ethyl acetate, 1:1) to give products. 2.2.2. 1,4-Bis[p-(N,N-dihexylamino)phenyl]-1,2-ethyne (1) Green oil. Yield: 80%. 1H NMR (300 MHz, CDCl3, ppm): 7.31 (d, 4H), 6.54 (d, 4H), 3.24 (t, 8H), 1.55 (m, 8H), 1.26 (m, 24H), 0.89 (t, 12H). Anal. Calcd for C38H60N2: C, 83.76%; H, 11.10%; N, 5.14%. Found: C, 83.81%; H, 11.16%; N, 5.20%.

4

Fig. 1. Reagents and conditions: (a) PdCl2(PPh3)2, CuI, triethylamine, CH3CN, under argon/hydrogen (7:3) mixture gas atmosphere, 60 C, 10 h. (b) PdCl2(PPh3)2, CuI, triethylamine, CH3CN, under oxygen atmosphere, 60 C, 10 h.

In this paper, four symmetrical organic molecules with substituted acetylene as central rigid elongated conjugation (Fig. 1) have been designed and synthesized. Their 2PA cross-sections were measured by two-photon induced fluorescence measurement method. Basing on these experimental single-photon and two-photon spectroscopic results, structure–property relationships of these molecules were discussed, which would provide a better insight into the design of efficient 2PA molecules containing triple bonds as p-bridges for biological imaging. 2. Experimental 2.1. Chemicals and instrument All solvents were purified by standard procedures and distilled prior to use. Dichlorobis(triphenylphosphine) palladium(II) (PdCl2(PPh3)2) (Fluka) was used without further purification. 4-N,N-Dihexylaminophenyl-acetylene was prepared according to the literature [12]. 1H NMR spectra were recorded on a Mercury VX-300 (Varian, 300 MHz) Spectrometer. Elemental analyses were performed on a

2.2.3. 1,4-Bis[p-(N,N-dihexylamino)phenyl]-1,3-butadiyne (2) Reaction under oxygen atmosphere, yellow oil. Yield: 92%. 1H NMR (300 MHz, CDCl3, ppm): 7.33 (d, 4H), 6.53 (d, 4H), 3.25 (t, 8H), 1.57 (m, 8H), 1.31 (m, 24H), 0.90 (t, 12H). Anal. Calcd for C40H60N2: C, 84.45%; H, 10.63%; N, 4.92%. Found: C, 84.48%; H, 10.70%; N, 4.96%. 2.2.4. 1,4-Bis[p-(N,N-dihexylamino)phenylethynyl] benzene (3) Yellow solid, mp: 50–53 C. Yield: 63%. 1H NMR (300 MHz, CDCl3, ppm): 7.41 (d, 4H), 7.35 (d, 4H), 6.54 (d, 4H), 3.24 (t, 8H), 1.56 (m, 8H), 1.31 (m, 24H), 0.89 (t, 12H). Anal. Calcd for C46H64N2: C, 85.66%; H, 10.00%; N, 4.34%. Found: C, 85.70%; H, 10.08%; N, 4.50%. 2.2.5. 9,10-Bis[p-(N,N-dihexylamino)phenylethynyl] anthracene (4) Red solid, mp: 58–60 C. Yield: 78%. 1H NMR (300 MHz, CDCl3, ppm): 8.69 (d, 4H), 7.59 (m, 4H), 7.30 (m, 4H), 6.65 (d, 4H), 3.26 (t, 8H), 1.58 (m, 8H), 1.32 (m, 24H), 0.91 (t, 12H). Anal. Calcd for C54H68N2: C, 87.04%; H, 9.20%; N, 3.76%. Found: C, 87.09%; H, 9.25%; N, 3.85%. 2.3. Single and two-photon spectroscopic measurement The linear (single-photon) absorption spectra were recorded on a UV-2550 UV–visible Spectrophotometer

84

B. Liu et al. / Journal of Molecular Structure 833 (2007) 82–87

(SHIMADZU, Japan). The single-photon fluorescence (SPF) spectra and the quantum yields were measured on a LS-55 Luminescence Spectrometers (Perkin-Elmer, USA). The quantum yields g were measured by the optically dilute measurement method using fluorescein in 0.1 M sodium hydroxide as the reference standard (g = 0.90) [14]. The experimental set-up for the 2PA cross-section measurements was similar to the reference [15]. In brief, a mode-locked Ti:Sapphire laser (Mai Tai, Spectra-Physics Inc., USA) with the spectral range from 720 to 960 nm was used as the excitation source. The average output power, pulse width and repetition rate were 1.5 W, 100 fs, and 82 MHz, respectively. After passing through a Pocket cell (Model 302, Conoptics Inc., USA), which was used to control the laser power, the laser was focused into the sample cell (polished on all sides) by a focusing lens (f = 6 cm). The excitation light was adjusted to close to the wall as near as possible in order to reduce the re-absorption effect, The fluorescence emission was collected by an objective lens (10·/NA 0.30, Olympus, Japan) and then was focused by another objective lens (10·/NA 0.25, DHC Inc., China) into a fiber optics spectrometer (HR2000, Ocean Optics Inc., USA), which was used to record fluorescence spectra. In addition, a liquid barrier filter (1 cm pathlength, 1 mol l 1 CuSO4 solution) was placed in front of the fiber optics spectrometer to exclude excitation illumination.

3. Results and discussion 3.1. Single-photon spectroscopic properties The normalized linear absorption spectra of molecules 1–4 at a concentration of 1 · 10 5 mol l 1 are presented in Fig. 2. Several absorption peaks could be observed in the linear absorption spectra of all four molecules in the wavelength range from 270 to 600 nm, while almost no lin-

1 2 3 4

1 2 3 4

1.0 Normalized intensity (a.u.)

Normalized linear absorption (a.u.)

1.0

ear absorption was observed beyond 600 nm. As shown in Figs. 1 and 2, when replacing the central triple bond (acetylenyl) with two triple bonds (butadiynyl), the dominant absorption band has a small red-shift, while the shoulder peak around 386 nm has negligible change in the position and notable increase in the intensity. Furthermore, when inserting a phenyl group into the center of butadiynyl group (from molecules 2 to 3), the absorption band slightly shifts to longer wavelength. However, changing from molecule 3 into 4 has significant influence on the absorption spectra, where a broad absorption band locating between 410 and 550 nm can be observed. This is probably due to the reason that the anthracenyl group has smaller aromatic resonance energy than the phenyl group, which may facilitate the p-orbital delocalization and cause a large bathochromic shift [10]. The SPF spectra of the four molecules at a concentration of 1 · 10 6 mol l 1 are shown in Fig. 3. When increasing the p-conjugation length, the emission peaks red-shift steadily and slowly (molecules 1–3). Significant red-shift can be noticed by changing from molecules 3 to 4, which is similar to that in the linear absorption. However, none of these fluorescence spectra have multiple peaks, which indicates that the emission occurs from the lowest excited state with the largest oscillator strength [16]. Table 1 compares the linear absorption and emission peaks of all investigated molecules in solvents of different polarity. From Table 1, it can also be seen that, with the increase of the solvent polarity for each molecule (polarity: DMF > CHCl3 > toluene) [17], the maximum peaks of the linear absorption spectra have slight red-shifts, while the SPF spectra have notable red-shifts. This kind of behavior is typical for organic chromophores with larger dipole moment in the excited state than in the ground state [17,18]. Moreover, all four molecules have moderate quantum yield ranging from 0.20 to 0.35 in toluene, 0.14–0.36 in CHCl3 and 0.12–0.40 in DMF. Unfortunately, no simple structure–quantum yield relations can be seen.

0.5

0.5

0.0

0.0

400 300

350

400 450 500 Wavelength (nm)

550

600

Fig. 2. Normalized linear absorption spectra of molecules 1–4 in CHCl3.

500 600 Wavelength (nm)

700

Fig. 3. Normalized single-photon fluorescence spectra of molecules 1–4 in CHCl3.

B. Liu et al. / Journal of Molecular Structure 833 (2007) 82–87

85

Table 1 Linear spectroscopic properties and quantum yields of molecules 1–4 in different solventsa Molecule

In Toluene

1 2 3 4 a b

In CHCl3

In DMF

k1PA max

kspf max

b

g

k1PA max

kspf max

g

k1PA max

kspf max

g

356 357 364 465

416 433 419 541

0.35 0.28 0.32 0.20

357 358 365 467

418 435 438 551

0.18 0.14 0.36 0.18

360 361 367 469

442 457 462 607

0.28 0.20 0.40 0.12

spf k1PA max , kmax : peak wavelengths(unit: nm) of single-photon absorption (the first absorption band) and single-photon fluorescence. single-photon fluorescence quantum yield, where fluorescein in 0.1 M sodium hydroxide was used as the reference standard (g = 0.90) [14].

3.2. Two-photon spectroscopic properties Two-photon induced fluorescence (2PF) of molecules 2– 4 were measured in solvents of varying polarity at a concentration of 5 · 10 4 mol l 1 using femtosecond laser as excitation source. The data were recorded in the excitation wavelength range from 720 to 960 nm with a step size of 20 nm. The two-photon induced fluorescence of molecule 1 is too low to be detected. The profiles of the 2PF spectra are found to be identical to the corresponding SPF spectra, while the differences in emission peaks between SPF and 2PF are typically less than 5 nm, which indicates that re-absorption is quite small in these studies. Fig. 4 shows

In CHCl3 In DMF

Normalized intensity (a.u.)

1.0

0.5

0.0 450

500

550

600 650 Wavelength (nm)

700

750

800

Fig. 4. Normalized two-photon fluorescence spectra of molecules 4 in CHCl3 and DMF.

the representative 2PF spectra of molecule 4 in CHCl3 and DMF, under 820 nm excitation. Table 2 summarizes the two-photon spectroscopic performances of molecules 2–4 in solvents of different polarity. The solvatochromism of the two-photon process is similar to that of the singlephoton process for these molecules. The 2PA cross-sections (r2PA) of molecules 2–4 were obtained by comparison the 2PF with a fluorescein (both at a concentration of 5 · 10 4 mol l 1) calibration standard. The detailed procedure was similar to the reference [15] and had been described in our previous work [19]. Using similar procedures, the 2PA cross-sections for molecules 2–4 were calculated and shown in Fig. 6 and Table 2. From Table 2 and Fig. 6, one notices that the 2PA crosssections of these molecules vary significantly. Molecule 4 is found to have the largest 2PA cross-section (r2PA = 344 GM) in toluene (see Table 2). To explain the origin of these differences, one has to take a close look at the parity selection rules for symmetric molecules: two-photon transitions from the ground state to an excited state with the same parity (gerade fi gerade) are allowed whereas single-photon transitions from the ground state to an excited state with opposite parity (gerade fi ungerade) are allowed [20]. So for organic molecules with symmetry structures, the lowest-lying two-photon allowed transition is from the ground state to the second excited state (S0 fi S2) [20], which explains why the twice frequency of TPA peak should be on the blue side of the corresponding peak of the linear absorption [21]. Specifically, as seen from Fig. 2 and Table 1, the first absorption bands (S0 fi S1) in the single-photon absorption spectra of molecules 2, 3 and 4 in toluene are located around 357, 364 and 465 nm, respectively. From Fig. 6 and Table 2, the strong 2PA peak for molecule 4 is located at 820 nm, which is (1) corresponding to

Table 2 Two-photon spectroscopic properties of molecules 2–4 in different solventsa Molecule

2 3 4

In toluene

In CHCl3

In DMF

k2PF max

r2PE max

r2PA max

k2PF max

r2PE max

r2PA max

k2PF max

r2PE max

r2PA max

437 422 543

8.4 20.8 68.8

40 65 344

439 442 556

5.3 12.6 32.1

31 35 178

460 465 609

5.4 23.2 38.28

36 58 319

a 2PF 2PA kmax : peak wavelength (unit: nm) of two-photon fluorescence; r2PE max , rmax : two-photon action cross-section and two-photon absorption cross-section values given in GM at 720 nm for molecules 2 and 3, and 820 nm for molecule 4. 1 GM (Go¨ppert–Mayer) = 10 50 cm4 s photon 1.

86

B. Liu et al. / Journal of Molecular Structure 833 (2007) 82–87

410 nm in the single-photon absorption spectrum, and (2) located on the blue side of the corresponding single-photon absorption peak (465 nm, S0 fi S1) (see Figs. 2 and 5). From the parity selection rules for symmetric molecules, this 2PA peak at 820 nm may be attributed to a transition from the ground state to the second excited state (S0 fi S2), which is the lowest-lying two-photon allowed state [20]. However, this S0 fi S2 transition is nearly forbidden by single-photon absorption (ca. k1PA = 410 nm), as a valley in the single-photon absorption at 410 nm appear (see Fig. 2). Basing on this selection rule, for molecules 2 and 3, the maximum r2PA values should be located at wavelengths shorter than 720 nm, which could be readily predicted from the 2PA spectra of molecules 2 and 3 (see Fig. 6). That is to say, a significant enhancement of the r2PA value could be expected when excited at shorter wavelengths.

4. Conclusion Four novel substituted acetylene with typical D-p-D structures have been designed and synthesized. The linear and two-photon spectroscopic properties of these molecules were studied. The parity selection rules for symmetric molecules were used to explain the two-photon spectroscopic behaviors of these molecules. One derivative with great enhancement on the two-photon absorption crosssection in 820 nm, which is among the best output wavelength range of commercial Ti:Sapphire femtosecond laser, has been obtained by inserting an anthracene ring into the rigid elongated p-conjugation. Such kind of structure modification provides a better insight into the design of efficient 2PA molecules containing triple bonds as p-bridges for biological imaging. Acknowledgements

400 Toluene CHCl3 DMF

σ2PA (GM)

300

200

We gratefully acknowledge the support of the National Nature Science Foundation of China (Grant No. 30400117). Dr. Yuan-Di Zhao appreciates the partial supports from the National Natural Science Foundation of China (Grant Nos. 30200058 and 30370387). The authors thank Prof. Kevin D. Belfield and Dr. Sheng Yao at the University of Central Florida (USA) for their helpful suggestions. References

100

720

760

800 840 880 Wavelength (nm)

920

960

Fig. 5. Two-photon absorption spectra of molecule 4 in different solvents.

400 2 3 4

σ2PA (GM)

300

200

100

0 720

760

800

840

880

920

960

Wavelength (nm) Fig. 6. Two-photon absorption spectra of molecules 2, 3 and 4 in toluene.

[1] J.D. Bhawalkar, G.S. He, P.N. Prasad, Rep. Prog. Phys. 59 (1996) 1041. [2] S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777. [3] R. Zipfel, R.M. Williams, W.W. Webb, Nat. Biotechnol. 21 (2003) 1369. [4] M. Albota, D. Beljonne, J.L. Bredas, J.E. Ehrlich, J.Y. Fu, A.A. Heikal, S.E. Hess, T. Kogej, M.D. Levin, S.R. Marder, D. McCord-Maughon, J.W. Perry, H. Rokeel, M. Rumi, G. Subramaniam, W.W. Webb, X.L. Wu, C. Xu, Science 281 (1998) 1653. [5] L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-Desce, Angew. Chem. Int. Ed. 40 (2001) 2098. [6] A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D. Pedron, R. Signorini, Org. Lett. 4 (2002) 1495. [7] Z.L. Huang, N. Li, H. Lei, Z.R. Qiu, H.Z. Wang, Z.P. Zhong, Z.H. Zhou, Chem. Commun. 20 (2002) 2400. [8] K.D. Belfield, K.J. Schafer, Chem. Mater. 14 (2002) 3656. [9] Y. Iwase, K. Kamada, K. Ohta, K. Kondo, J. Mater. Chem. 13 (2003) 1575. [10] W.J. Yang, C.H. Kim, M.Y. Jeong, S.K. Lee, M.J. Piao, S.J. Jeon, B.R. Cho, Chem. Mater. 16 (2004) 2783. [11] T. Pons, L. Moreaux, J. Mertz, Phys. Rev. Lett. 89 (2002) 288101. [12] B. Traber, J.J. Wolff, F. Rominger, T. Oeser, R. Gleiter, M. Goebel, R. Wortmann, Chem. Eur. J. 10 (2004) 1227. [13] A. Elangovan, Y.H. Wang, T.I. Ho, Org. Lett. 5 (2003) 1841. [14] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991. [15] C. Xu, W.W. Webb, J. Opt. Soc. Am. B. 13 (1996) 481. [16] W.J. Yang, D.Y. Kim, M.Y. Jeong, H.M. Kim, Y.K. Lee, X.Z. Fang, S.J. Jeon, B.R. Cho, Chem. Eur. J. 11 (2005) 4191. [17] C. Reichardt, Chem. Rev. 94 (1994) 2319.

B. Liu et al. / Journal of Molecular Structure 833 (2007) 82–87 [18] C.M. Whitaker, E.V. Patterson, K.L. Knott, R.J. McMahon, J. Am. Chem. Soc. 118 (1996) 9966. [19] Z.L. Huang, H. Lei, N. Li, Z.R. Qiu, H.Z. Wang, J.D. Gou, Y. Luo, Z.P. Zhong, X.F. Liu, Z.H. Zhou, J. Mater. Chem. 13 (2003) 708.

87

[20] K.D. Belfield, A.R. Morales, J.M. Hales, D.J. Hagan, E.W. Van Stryland, V.M. Chapela, J. Percino, Chem. Mater. 16 (2004) 2267. [21] H. Lei, Z.L. Huang, H.Z. Wang, X.J. Tang, L.Z. Wu, G.Y. Zhou, D. Wang, Y.B. Tian, Chem. Phys. Lett. 352 (2002) 240.