Synthesis of branched-chromophores with enhanced two-photon absorption via core effect

Optical Materials 31 (2009) 805–811

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Synthesis of branched-chromophores with enhanced two-photon absorption via core effect Deqiang Wang a, Chuanxiang Qin a, Xiaomei Wang a,*, Wanli Jiang b, Xiangyun Fang c, Junfang Zhao c, Guoqiang Chen a a

Institute of Material Science and Engineering, Suzhou University, Suzhou, China State Key Laboratory of Crystal Materials, Shandong University, Jinan, China c Laboratory of Ultra-fast Laser, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing, China b

a r t i c l e

i n f o

Article history: Received 25 April 2008 Received in revised form 7 September 2008 Accepted 8 September 2008 Available online 26 October 2008 PACS: 78.66.Q 82.50.P 42.70.M 42.70.N

a b s t r a c t The synthesis of new triphenylamine-branching chromophores with dibenzofuran-core and dibenzothiophene-core was reported and the molecular two-photon absorption (TPA) enhancement via core effect and generation effect was investigated. Comparatively, the core effect shows more significant contribution to TPA enhancement than the generation effect. This is of greatly valuable since optimizing dendritic core is laborsaving in comparison with synthesizing dendrimers with high generation. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Two-photon absorption enhancement Core effect Generation effect Dibenzofuran Dibenzothiophene

1. Introduction Multibranched chromophores can provide an effective architectural strategy for the purpose of enhancing two-photon absorption (TPA) that possesses lots of potential applications such as 3D microfabrication, 3D fluorescence microscopy, photodynamic therapy (PDT), optical-power limiting and frequency up-converted lasing [1–8]. These exciting applications accelerate molecule– engineering to develop new chromophores with strong TPA. Up to now, new multibranched and dentritic molecules via either increasing molecular generation number or placing appropriate molecular core have been reported, wherein triphenylamine unit [9–11,3] is frequently used for branching unit. Usually, triphenylamine-branching chromophores exhibit three fold TPA enhancements in comparison with one-branching counterpart [12–16]. And triphenylamine-chromophores with generation 2 show 5–10 folds increase in TPA cross-section relative to those with genera-

* Corresponding author. Tel.: +86 512 62092786; fax: +86 512 67246786. E-mail address: [email protected] (X. Wang). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.09.002

tion 1 [17–19,31]. Meanwhile, most investigations suggest that the electron coupling [20,21] and vibration coupling [22,23] have contribution to the enhancement of TPA, however, generation effect [18] and core effect upon TPA enhancement are seldom investigated. Herein, we reported the synthesis of two series of novel triphenylamine-branching chromophores with dibenzothiophene and dibenzofuran as ‘‘core”, respectively. The influences of the generation- and core-effect upon TPA enhancement were comparatively studied. The experimental results show that the core effect possesses more significant contribution to TPA enhancement than the generation effect. This is of valuable since optimizing of appropriate molecular ‘‘core” is laborsaving in comparison with increasing of molecular generation. 2. Experimental 2.1. Chemicals Synthetic routes of chromophores are shown in Scheme 1. According to the convergent approach, the multibranched chro-

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D. Wang et al. / Optical Materials 31 (2009) 805–811

R2

CH 2PPh3Br

R2

R1

N

N

X

+

R1

Wittig reaction

N X

dry THF

R3

CH 2PPh3Br

R1 R2 OT-G1

compd 4 X = O compd 1 R 1 = -CHO; R2 = R 3 = H;

X= O; R 1 =Br; R 2 =

N

CH=CH2

compd 5 X = S

compd 2 R 1 = -CHO; R 2 = Br; R 3 =

compd 3 R 1 = -CHO; R2 = R 3 =

OT-G1.5

X = O; R1 = R2 =H

N

N

CH=CH 2

OT-G2

X= O; R1 = R2 =

ST-G1

X = S; R 1 = R2 =H

N

CH=CH2

CH=CH2

ST-G1.5

ST-G2

X= S; R 1 = Br; R2 = X= S; R1 = R2 =

N

N

CH=CH2

CH=CH2

Scheme 1. The synthetic routes of triphenylamine-branching chromophores.

mophores were begun at preparation of outside branching segments and then coupling them to the core units. Synthesis of the requisite branching and core units were reported previously [16,24,25]. For dibenzofuran-cored chromophores, OT-G1 (denoted to generation 1) is the smallest chromophore with dibenzofuran ‘‘core” flanked on both sides by one triphenylamine unit. OT-G1.5 and OT-G2 (denoted to generation 1.5 and generation 2, respectively) are the multibranched chromophores with dibenzofuran ‘‘core” flanked on both sides by two triphenylamine and three triphenylamine units, respectively. By analogy, the dibenzothiophene-cored chromophores (ST-G1, ST-G1.5, ST-G2) were named.

form = 1:3 as eluent) to obtain pale yellow solid (67%). Melting point of 125 °C. MS (EI, m/z): 706.5 (100%, M+). Elem. Anal. Calcd. (%) for C52H38N2O: C, 88.36; H, 5.42; N, 3.96. Found. C, 88.91; H, 5.25; N, 4.02. 1H NMR (CDCl3, ppm): d 8.059 (s, 2H, dibenzofuran), 7.986, 7.967 (d, 2H, J = 7.6 Hz, dibenzofuran), 7.621, 7.601 (d, 2H, J = 8.0 Hz, dibenzofuran), 7.575, 7.554, 7.542, 7.520 (q, 4H, J = 7.3 Hz), 7.482–7.414 (m, 8H), 7.372, 7.353, 7.334 (t, 4H, J = 7.6 Hz), 7.133, 7.114 (d, 12H, J = 7.6 Hz), 7.034, 7.017 (m, 4H, J = 6.8 Hz).

2.3. Synthesis

2.3.2. 2,8-Bis{E-40 -(E-4-(N,N-diphenylamino)styryl)-400 -(bromo)-E-4(N,N-diphenylamino)styryl}dibenzofuran (OT-G1.5) By similar procedure as described for OT-G1, chromophore OTG1.5 was synthesized on replacing compd. 1 with 4-((4-bromophenyl)-{4-[2-(4-diphenylaminophenyl)-vinyl]-phenyl}amino) benzaldehyde (comp. 2) [16]. The crude products were purified by chromatography on a silica gel column (petroleum/chloroform, 5/ 1 V/V) to produce the greenish-yellow powder (50%). Melting point of 142 °C. Elem. Anal. Calcd. (%) for C92H66Br2N4O: 78.74; H, 4.74; N, 3.99. Found. C, 79.02; H, 4.65; N, 3.86. MS (MALDI–TOF–TOF): 1403.6 (100%, M+), 705. 1H NMR (CDCl3, ppm): d 8.064 (s, 2H, dibenzofuran), 7.988, 7.969 (d, 2H, J = 7.6 Hz, dibenzofuran), 7.627, 7.605 (d, 2H, J = 8.8 Hz, dibenzofuran), 7.575, 7.555, 7.545, 7.523 (q, 8H, J = 7.0 Hz), 7.481, 7.462, 7.448, 7.428 (m, 8H), 7.395, 7.375 (d, 8H, J = 8.0 Hz), 7.376, 7.356, 7.337 (t, 24H, J = 7.8 Hz), 7.116, 7.094 (d, 4H, J = 8.8 Hz), 7.054, 7.034 (d, 8H, J = 8.0 Hz).

2.3.1. 2, 8-Bis [-E-4-(N,N-diphenylamino)styryl]dibenzofuran (OT-G1) Under the conditions of stirring and nitrogen atmosphere at room temperature, 2.9 g (11 mmol) of 4-(N,N-diphenylamino)benzaldehyde (compd. 1) was added to mixture of 4.47 g (5.1 mmol) of [2,8-dibenzofurannediylbis(methylene)]bis(triphenylphosphonium)dibromide (compd. 4) [25] and 0.78 g (7.0 mmol) of potassium tert-butoxide in 80 mL dry THF. After refluxing for 24 h, THF was removed at reduced pressure. The mixture was poured into water, neutralized to pH = 7 with 1 N HCl and extracted with dichloromethane. After removing solvent, the residue was chromatographed on a silica gel column (petroleum:chloro-

2.3.3. 2,8-Bis{E-40 ,400 -di-(E-4-(N,N-diphenylamino)styryl)-E-4-(N,Ndiphenylamino)styryl}dibenzofuran (OT-G2) By similar procedure as described for OT-G1, chromophore OTG2 was synthesized on replacing compd. 1 with 4-(bis(4-((E)-4(diphenylamino)styryl)phenyl)amino)benzaldehyde (compd. 3) [16]. The crude products were purified by chromatography on a silica gel column (petroleum/chloroform, 5/1 V/V) to produce the greenish-yellow powder. Yield 37%. Melting point 178 °C. Elem. Anal. Calcd. (%) for C132H98N6O: C, 88.86; H, 5.54; N, 4.71. Found. C, 89.01; H, 5.25; N, 4.56. MS (MALDI–TOF–TOF): 1784.5 (100%, M+), 974. 1H NMR (CDCl3, ppm): d 8.069 (s, 2H, dibenzofuran),

2.2. Measurements Electron impact (Mode laser) mass spectra and EI mass spectra were obtained on a 4700 Proteome Analyzer (MALDI–TOF–TOF) produced by ABI Company and on a HP 5989 mass spectra instrument, respectively. 1H NMR of the chromophores was performed on an INOVA-400 spectrometer. Element analyses were performed on Perkin 2400 (II) autoanalyzer. Linear optical spectra (including absorption and fluorescence) and two-photon-induced fluorescence as well as two-photon absorption cross-section were obtained as the same methods reported previously [24].

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2.3.6. 2,8-Bis{E-40 ,400 -di-(E-4-(N,N-diphenylamino)styryl)-E-4-(N,Ndiphenylamino)styryl}dibenzothiophene (ST-G2) A solution of 2.2:1 equiv of compd. 3 and compd. 5 was added in dry THF in the presence of 2 equiv of potassium tert-butoxide as catalyst. The crude products were purified through column chromatography to give bright yellow powders (20%). Melting point 168 °C. MS (MALDI–TOF–TOF): 1800 (M+), 990. Elem. Anal. Calcd. (%) for C132H98N6S: C, 88.06; H, 5.49; N, 4.67. Found. C, 88.15; H, 5.58; N, 4.61. 1H NMR (CDCl3, ppm): d 8.285 (s, 2H, dibenzothiophene), 7.825, 7.803 (d, 1H, J = 8.8 Hz, dibenzothiophene), 7.663, 7.642 (d, 1H, J = 8.4 Hz, dibenzothiophene), 7.489, 7.467 (d, 2H, J = 8.8 Hz, dibenzothiophene), 7.415, 7.393 (d, 4H, J = 8.8 Hz), 7.383, 7.361 (d, 6H, J = 8.8 Hz), 7.276, 7.257, 7.237, 7.217, (q, 28H, J = 7.9 Hz), 7.121, 7.102 (d, 22H, J = 7.6 Hz), 7.057–7.007 (q, 20H, J = 6.7 Hz), 6.972–6.869 (m, 12H).

Linear optical properties of dibenzofuran-cored and dibenzothiophene-cored chromophores are presented in Table 1 and Fig. 1. We noticed that the absorption bands of intramolecular charge transfer (ICT) beyond 350 nm showed obvious red-shift from generation 1 to 1.5 and further to 2. These can be explained the extension of p-electron conjugation with the generation number increasing. For the given generation, dibenzothiophene-cored chromophores exhibit red-shifted spectra in comparison with dibenzofuran-cored chromophores. For example, the absorption peaks are at 372 nm for ST-G1 and at 364 nm for OT-G1. And the absorption peaks are at 405 nm for ST-G2 and at 399 nm for OTG2. These suggest that strong electron coupling between core

1.0

0.8

OT-G1 (364 nm) OT-G1.5 (383 nm) OT-G2 (399 nm)

0.8

0.6

0.6 435 nm

0.4

0.4

0.2

0.2

0.0 300

350

400

450

500

550

Fluorescence intensity (a.u)

1.0 443 nm 456 nm

0.0 600

Wavelength (nm) 1.0

1.0

446 nm

405 nm

0.8

460 nm ST-G1 ST-G1.5 ST-G2

0.6

0.8

480 nm

392 nm

0.6

0.4

0.4 372 nm

0.2

0.2

Fluorescence intensity (a.u)

2.3.5. 2,8-Bis{E-40 -(E-4-(N,N-diphenylamino)styryl)-400 -(bromo)-E-4(N,N-diphenylamino)styryl}dibenzo thiophene (ST-G1.5) A solution of 2.2: 1 equiv of compd. 2 and compd. 5 was added in dry THF in the presence of 2.2 equiv of potassium tert-butoxide as catalyst. The crude products were purified through column chromatography to give bright yellow powders (24%). Melting point of 140 °C. Elem. Anal. Calcd. (%) for C92H66Br2N4S: C, 77.85; H, 4.69; N, 3.95. Found. C, 77.5; H, 4.68; N, 4.02. MS (MALDI– TOF–TOF): 1418.8 (M+), 801.5. 1H NMR (CDCl3, ppm): d 7.826, 7.806, (d, 2H, J = 8.0 Hz, dibenzothiophene), 7.464, 7.445 (d, 4H, J = 7.6 Hz, dibenzothiophene), 7.381, 7.359 (d, 10H, J = 8.8 Hz), 7.281–7.242 (t, 24H, J = 7.8 Hz), 7.124, 7.104, 7.086 (t, 16H, J = 7.6 Hz), 7.049–7.011 (t, 10H, J = 7.6 Hz).

3.1. Linear optical properties

Absorbance

2.3.4. 2,8-Bis[-E-4-(N,N-diphenylamino)styryl]dibenzothiophene (ST-G1) By similar procedure as described for OT-G1, chromophore ST-G1 was obtained. A solution of 2.2:1 equiv of compd. 1 and [2,8-dibenzothiophenediylbis(methylene)]bis(triphenylphosphonium)dibromide (compd. 5) [25] was added in dry THF in the presence of 2.2 equiv of potassium tert-butoxide as catalyst. The crude products were purified through column chromatography to give bright yellow brittle microcrystals in yield 84%. Melting point 120–122 °C. Td: 245 °C. Elem. Anal. Calcd. (%) for C52H38N2S: C, 86.39; H, 5.30; N, 3.87. Found. C, 86.42; H, 5.26; N, 3.83. MS (m/ z, EI): 722.25 (100%) [M+]. 1HNMR (CDCl3, ppm): d 8.262 (s, 2H, dibenzothiophene), 7.806, 7.786 (d, 2H, J = 8.00 Hz, dibenzothiophene), 7.642, 7.623 (d, 2H, J = 7.6 Hz), 7.457, 7.436 (d, 4H, J = 8.4 Hz), 7.294, 7.274 (d, 6H, J = 8.00 Hz), 7.142, 7.123 (d, 10H, J = 7.6 Hz), 7.095–7.025 (m, 12H).

3. Results and discussion

Absorbance

7.992, 7.970 (d, 2H, J = 8.8 Hz, dibenzofuran), 7.630, 7.610 (d, 2H, J = 8.0 Hz, dibenzofuran), 7.578–7.520 (q, 20H, J = 7.7 Hz), 7.483, 7.442 (m, 16H), 7.398, 7.379, 7.358 (t, 12H, J = 8.0 Hz), 7.117, 7.099 (d, 8H, J = 7.2 Hz), 7.054, 7.033 (d, 36H, J = 8.4 Hz).

0.0

0.0 300

350

400

450

500

550

600

Wavelength (nm) Fig. 1. Linear absorption (left) and one-photon fluorescence (right) for dibenzofuran-cored and dibenzothiophene-cored chromophores (THF, 1  105 mol dm3).

Table 1 One-photon and two-photon absorption properties of dibenzofuran-cored and dibenzothiophene-cored chromophores in THF Compounds

3 kOPA cm1) max (nm)/e(mol dm

kOPF max (nm)

Uf

kTPF max (nm)

dTPA max (GM)

a dTPA max =WM (GM)

b dTPA max =N (GM)

OT-G1 OT-G1.5 OT-G2 ST-G1 ST-G1.5 ST-G2

364/14832 383/35510 399/36530 372/39383 392/51856 405/99882

435 443 456 446 460 480

0.74 0.79 0.80 0.34 0.29 0.38

438 471 489 460 483 491

19 800 1097 180 1582 3766

0.024 0.57 0.62 0.24 1.11 2.09

19 533 546 180 1055 1883

a b

Molecular weight. Generation number.

D. Wang et al. / Optical Materials 31 (2009) 805–811

and branching occurs within dibenzothiophene-cored chromophores and results in their red-shifted absorption, in comparison with dibenzofuran-cored chromophores. 3.2. Two-photon fluorescence (TPF) and two-photon absorption (TPA) cross-section Two-photon fluorescence (TPF) spectra of dibenzofuran-cored and dibenzothiophene-cored chromophores at different pumped powers are presented in Figs. 2 and 3. It is noticed that the relationship between TPF intensity and pumped power follow almost the square law below the excitation powers of 50 mW, as shown in Fig. 2 (d–f). Meanwhile, one can see that the TPF intensities are increased with the generation from 1 to 1.5 and to 2, with the concomitance of spectral red-shift. For example, the TPF integral for OT-G1, OT-G1.5 and OT-G2 is increased in the ratio 1:7:9 at the

a

350

TPF intensity (a.u)

300

438 nm

OT-G1

250 200 150 100 50 0 300

same pumped powers, when the peaks are red-shifted from 438 nm (OT-G1) to 474 nm (OT-G1.5) and to 487 nm (OT-G2). And the TPF integral for ST-G1, ST-G1.5 and ST-G2 is increased in the ratio 1:10:19 at the same pumped power as shown in Fig. 3. Meanwhile, one can see that dibenzothiophene-cored chromophores present stronger TPF than dibenzofuran-cored chromophores for the given generation. For instance, the TPF integrals of OT-G1 and ST-G1 are in the ratio 1:3.3 and those of OT-G2 and ST-G2 are in the ratio 1:3.7 at pumped power about 40 mW. These suggest that the TPF enhancement is associated with not only molecular generation but also molecular core. In comparison Fig. 1 (right) with Fig. 2 (a–c), one can see that the maximum TPF position for higher generation (such as 1.5 and 2) chromophores shows the obvious red-shifted relative to the one-photon fluorescence (OPF). These can be explained that the red-shift of TPF peak is due to the reabsorption of relatively

d

P = mW 103.5 98.8 92.7 86.1 80.1 72.3 66.9 59.1 54.7 49.6 43.6 35.3

4.6 4.4

4.0

500

600

Slope = 1.7 Slope = 2

3.8 3.6 1.5

400

OT-G1

4.2

Lg TPF

808

1.6

1.7

700

Wavelength (nm)

474 nm

2500

OT-G1.5 TPF intensity (a.u.)

2000 1500 1000 500 0 300

400

500

600

300

2.1

5.3 5.2

OT-G1.5

5.1 5.0 4.9 Slope = 1.7 Slope = 1.8

4.8 4.7 4.6 4.5 1.5

700

1.6

1.7

1.8

1.9

2.0

Lg I0

f 487 nm

OT-G2

400

500 Wavelength (nm)

P = mW 93.5 87.5 83.6 76.3 69.2 62.1 56.5 51.9 44.8 39.1 36.6

600

5.4 5.3 5.2

OT-G2

5.1

Lg TPF

TPF intensity (a.u.)

3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 250 0

2.0

5.4

Wavelength (nm)

c

1.9

e

p = mW 101.5 97.0 92.2 86.6 80.2 74.5 67.8 60.6 52.4 49.4 42.5 36.5

Lg TPF

b

1.8

Lg I0

5.0 4.9

Slope = 1.7 Slope = 1.9

4.8 4.7 4.6

700

4.5 1.5

1.6

1.7

1.8

1.9

2.0

2.1

Lg I0

Fig. 2. Two-photon excited fluorescence spectra of OT-G1, OT-G1.5 and OT-G2 under different pumped powers at 800 nm (THF, c = 1  104 mol dm3).

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D. Wang et al. / Optical Materials 31 (2009) 805–811

a

2000 1500 1000

0 400

1000 800 600 400

ST-G1 (444 nm)

b 450

500

550

600

Wavelength (nm) Fig. 3. Two-photon excited fluorescence spectra of dibendithiophene-cored chromophores under same pumped powder at 800 nm (THF, c = 1  104 mol dm3).

concentrated solution (1  104 mol dm3) used in TPF measurement, compared to dilute solution (1  105 mol dm3) used in OPF measurement. Similar results for dibenzothiphene-cored system can also be observed in Fig. 3. However, for OT-G1 and STG1, their TPF peaks are almost the same as OPF peaks, suggesting that the influence of concentration effect of higher generation (such as 1.5 and 2) chromophores upon TPF behaviors are much more obvious. Additionally, the structured difference in OPF and TPF spectra suggest that OPF and TPF emissions are from the different vibrational states at the first excited electron state (S1). According to two-photon fluorescence method, TPA cross-section of chromophores in the range of 700–880 nm is displayed in Fig. 4, wherein two obvious TPA regions beyond 700 nm and about 800 nm (insert) are shown. Due to the limitation of our detecting equipment, we cannot observe the TPA peak beyond 700 nm. Supposed that heterofluorene-cored chromophoress with noncentrosymmetric V-shaped configuration [24], the lowest energy band (corresponding to the most intense OPA) is two-photon allowed [26]; the maximum TPA peak for chromophoress locating by the side of 700 nm is reasonable. Moreover, for dibenzothiophenecored chromophores, the measured TPA cross-section at 800 nm almost accord with those obtained by Z-scan femtosecond technique at the same wavelength [17,30], indicating that two-photon fluorescence methods in this work are reliable.

a

80 60 40 20 0 760 780 800 820 840 860 880

700 720 740 760 780 800 820 840 860 880

4000 3000 2000

500

ST-Gn OT-Gn

300 200 100 0

760 780

Fig. 4. Two-photon fluorescence excitation spectra of dibenzofuran-cored (a) and dibenzothiophene-cored (b) chromophores, obtained by Ti: sapphire femtosecond laser pulse in 700–880 nm regions.

Secondly, one can see that the maximum TPA cross-sections (dTPA max ) display the significant enhancement with the generation. For instance, the dTPA max values of dibenzofuran-cored chromophores show the dramatic increase from 19 GM (OT-G1) to 800 GM (OTG1.5) and to 1097 GM (OT-G2), while those of dibenzothiophenecored chromophores exhibit obvious enhancement from ST-G1 (180 GM) to ST-G1.5 (1582 GM) and to ST-G2 (3766 GM), as shown in Table 1. The relationship of dTPA max vs. generation number (N) was plotted in Fig. 5 a, which displays unambiguously the linear dependence of dTPA max and N values. It is known that generation effect that improves molecular TPA is owing to the increase of chromophore density [27,28] and molecular dipole moment change (4lge) between ground- and excited-states [20]. Based on Lippert–Mataga equation, molecular dipole moment change (4lge) can be deduced from the relationship between Stokes shift (4m) and orientation polarizability (4f), as shown in Eqs. (1), (2) [29].

line 1 ST-Gn OT-Gn

/ N, GM

/ WM, GM

δTPA

1.0

line 2

0.0 0 1.0

1.5

2.0

Generation number (N)

860

Wavelength (nm)

0.5

500

840

700 720 740 760 780 800 820 840 860 880

1.5

line 2

800 820

0

line 1

1000

ST-G2 ST-G1.5 ST-G1

400

1000

2.0

1500

OT-G2 OT-G1.5 OT-G1

100

b

2000

δTPA

120

200 0

500

Cross-section (GM)

ST-G1.5 (482 nm)

TPA cross-section (GM)

TPF intensity

3000 2500

1200

TPA cross-section (GM)

ST-G2 (491 nm) 3500

Cross-section (GM)

4000

1.0

1.5

2.0

2.5

Generation number (N)

Fig. 5. The maximum dTPA divided by generation number (N) and molecular weight (WM) with the generation number.

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D. Wang et al. / Optical Materials 31 (2009) 805–811

Dl2ge Df þ C cha3 2 e1 n 1 where Df ¼  2e þ 1 2n2 þ 1 if k ¼ Dl2ge =a3 Dm ¼ mabs  mem ¼

ð2Þ

Thus, the relationship between 4m and 4f of chromophores was presented in Fig. 6, wherein the solid lines represent the fitting results. As presented in Eq. (2) and Fig. 6, the slope (k) of line is in proportion to 4lge and k values of chromophores with generation 1 are smaller than chromophores with generation 2. Therefore, 4lge values of chromophores with generation 1 are smaller than chromophores with generation 2. Association with the fact that dTPA and 4lge are all increased with the generation number, it was concluded that the generation effect that enhances TPA cross-section is correlative to large dipole moment change (4lge). 3.3. TPA cross-section enhancement by the core effect Also in Fig. 5 (a–b), one can see that line 1 is located above line 2, which suggests that the maximum dTPA values divided either by generation number (N) or by molecular weight (WM) for dibenzothiophene-cored chromophores are always larger than those for dibenzofuran-cored chromophores. Evidently, the TPA enhancement is strongly associated with not only the generation number but also the molecular core. And unambiguously, the core effect shows more significant contribution to TPA enhancement than the generation effect.

Intramolecular cherge trnasfer

Dm1kDf 1Dlge

0.02 0.00 -0.02 -0.04 -0.06 -0.08

(a) OT-G1

-0.10

S0 S1

-0.12 -0.14 N(Ph)3C=C

Dibenzofuran

N(Ph)3C=C

0.00

Intramolecular charge transfer

then

0.04

ð1Þ

-0.02 -0.04 -0 .06

(b) ST-G1

-0.08 -0.10 S0 S1

-0.12 -0.14 N(Ph)3C=C

Dibenzothiophene

N(Ph)3C=C

Fig. 7. Charge density distribution in the ground- (S0) and the excited- (S1) states for OT-G1 (a) and ST-G1 (b).

a -1

Stokes shift (cm )

5500 5000 OT-G1

4500 4000 OT-G2

3500 3000 2500 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Δf

b

5600

Using the HF ab initio method in Gaussian 98 program, we calculated the charge density within OT-G1 and ST-G1. As shown in Fig. 7, there shows smaller charge transfer on the core within OT-G1 and ST-G1 in the ground state. However, in the excited state, larger excited charge redistribution on the core can be seen. For instance, the charge on dibenzothiophene-core (|0.13|e) shows about four folds larger than that on dibenzofuran-core (|0.03|e). Moreover, the excited charge variation (4e) on dibenzothiophene-core (0.11e) is larger than dibenzofuran-core (0.04e). Association with the fact that dibenzothiophene-cored chromophores show much larger TPA cross-section relative to dibenzofurancored chromophores, one can suggest that large excited charge transfer (4e) on the core plays an important role in TPA cross-section enhancement. 4. Conclusions

-1

Stokes shift (cm )

5200 4800

ST-G1

4400 4000 3600

ST-G2

3200 2800 0.00

0.05

0.10

0.15

0.20

0.25

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Δf Fig. 6. The relationship between 4m and 4f for dibenzofuran-cored (a) and dibenzothiophene-cored chromophores (b).

We reported firstly the synthesis of triphenylamine-branching chromophores based on dibenzofuran and dibenzothiophene ‘‘core” and investigated the TPA enhancement via generation effect and core effect. dTPA max values of dibenzofuran-cored chromophores show the dramatic increase from 19 GM (OT-G1) to 800 GM (OTG1.5) and to 1097 GM (OT-G2), while those of dibenzothiophenecored chromophores exhibit obvious enhancement from ST-G1 (180 GM) to ST-G1.5 (1582 GM) and to ST-G2 (3766 GM). Generation effect is originated from larger dipole moment change (4lge) while the core effect is associated with larger charge transfer on the core. Comparatively, core effect is much larger than the generation effect. This is of greatly valuable since optimizing dendritic core is laborsaving in comparison with synthesizing dendrimers with high generation.

D. Wang et al. / Optical Materials 31 (2009) 805–811

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 50273024 and 50673070), the foundation for the author of National Excellent Doctoral Dissertation of PR China (FANEDD, Grant No. 200333), and International Cooperation Project of Suzhou (SWH 0616) for financial support. References [1] A. Bar-Haim, J. Klafter, J. Phys. Chem. B. 102 (1998) 1662. [2] C.C. Kwok, M.S. Wong, Chem. Mater. 14 (2002) 3158. [3] Y. Wang, G.S. He, P.N. Prasad, T. Goodson III, J. Am. Chem. Soc. 127 (2005) 10128. [4] W. Zhou, S.M. Kuebler, K.L. Braun, T. Yu, J.K. Cammack, C.K. Ober, J.W. Perry, S.R. Marder, Science 296 (2002) 1106. [5] K.D. Belfield, K.J. Schafer, W. Mourad, B.A. Reinhardt, J. Org. Chem. 65 (2000) 4475. [6] P.K. Frederiksen, M. Jorgensen, P.R. Ogilby, J. Am. Chem. Soc. 123 (2001) 1215. [7] G.Y. Zhou, X.M. Wang, D. Wang, Z.S. Shao, M.H. Jiang, Appl. Opt. 41 (2002) 1120. [8] C. Wang, X.M. Wang, Z.S. Shao, X. Zhao, G.Y. Zhou, D. Wang, Q. Fang, Q.M.H. Jiang, Opt. Commun. 192 (2001) 315. [9] G.S. He, L.-S. Tan, Q.D. Zheng, P.N. Prasad, Chem. Rev. 108 (2008) 1245. [10] J. Yoo, S.K. Yang, M.-Y. Jeong, H.C. Ahn, S.-J. Jeon, B.R. Cho, Org. Lett. 5 (2003) 645. [11] Q.D. Zheng, G.S. He, P.N. Prasad, Chem. Mater. 17 (2005) 6004. [12] L. Porrès, O. Mongin, C. Katan, M. Charlot, T. Pons, J. Mertz, M. Blanchard-Desce, Org. Lett. 6 (2004) 47. [13] H.J. Lee, J. Sohn, J. Hwang, S.Y. Park, H. Choi, M. Cha, Chem. Mater. 16 (2004) 456.

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