Synthesis and two photon absorption of diphenylamino endcapped oligo(9,9-diphenyl)fluorenes with enhanced functional properties

Synthetic Metals 220 (2016) 48–58

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Synthesis and two photon absorption of diphenylamino endcapped oligo(9,9-diphenyl)fluorenes with enhanced functional properties Qiang Fenga,b , Qi Zhanga,b , Tao Hana,b , Kang Sunb , Xiao Ling Zhanga,b , Ping Fang Xiaa,b , Zhong Hui Lib,* a College of Chemistry and Life Science, Chengdu Normal University, No.99 Haike Road Eastern Section, Wenjiang District, Chengdu 611130, Sichuan Province, PR China b Institute of Functional Molecules, Chengdu Normal University, No.99 Haike Road Eastern Section, Wenjiang District, Chengdu 611130, Sichuan Province, PR China

A R T I C L E I N F O

Article history: Received 8 November 2015 Received in revised form 3 May 2016 Accepted 20 May 2016 Available online xxx Keywords: Oligofluorene 9, 9-diphenyl derivatives 9, 9-diphenyl derivatives Synthesis Two photon absorption

A B S T R A C T

A homologous series of 9,9-diphenyl substituted oligofluorenes end-capped by diphenylamino groups, (Ph)-OF(n)-NPhs, n = 1–5, has been synthesized using Suzuki cross coupling reaction as a key step and were fully characterized by 1H NMR, 13C NMR, MS, and elemental analyses. The functional properties, including thermal stabilities, linear optical properties, fluorescence and electro-chemical properties, were investigated. The TPA cross sections of these newly synthesized oligofluorenes were measured by the TPF method. Compounds (Ph)-OF(4)-NPh and (Ph)-OF(5)-NPh exhibited larger two-photon absorption cross-sections, about 550 GM and 1390 GM at 800 nm, respectively. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction Two-photon absorption (TPA) has been known for about four decades [1]. It is one of the very important third-order nonlinear optical (NLO) effects and can be defined as a simultaneous absorption of two photons via virtual states in a medium. The process requires high peak power that is available from pulsed lasers. Even though two-photon processes have been known for a long time, materials that exhibit two-photon absorption have not yet found widespread applications. The reason for this is that most materials have relatively low two-photon absorption cross sections, d. That is, applications of two-photon absorption, such as two-photon laser scanning microscopy [2], 3D optical data storage [3], optical power limiting [4], 3D micro- and nanofabrication [5], and photo-dynamic therapy [6], depend critically on the availability of materials with high TPA cross sections and on the ability of these chromophores to perform specialized photophysical, photochemical, or photobiological functions. So, development of novel molecular materials with large two-photon absorption cross-sections still remains attractive prospectus.

* Corresponding author. E-mail address: [email protected] (Z.H. Li). http://dx.doi.org/10.1016/j.synthmet.2016.05.024 0379-6779/ã 2016 Elsevier B.V. All rights reserved.

The research for molecules endowed with large two-photon absorption cross-sections has mainly focused on push-pull (D-p-A) molecules [4b,7,8], pull–pull (A-p-A), push–push (Dp-D) or quadrupolar molecules [4b,7–10], where D is an electrondonating group, A is an electron-accepting group and p is a conjugated moiety. Otherwise, it has been observed that symmetrical conjugated molecules with two electron-donating (D) (or electron-withdrawing (A)) end groups exhibit highly nonlinear absorption properties [3], and large TPA cross-section values, d [11]. This enhancement in d was correlated to an intramolecular charge redistribution that occurs between the ends and the center of the molecule. Increasing the conjugation length of the molecule or increasing the extent of symmetrical charge from the ends to the middle, or vice versa, will result in a large increase of d [8,12]. The fluorenyl p-system was chosen as TPA material candidates due to its inherently high thermal and photochemical stabilities [13–15]. More importantly, fluorene can be readily functionalized onto its 2-,7-, and/or 9-positions. Recently, the synthesis and linear and nonlinear optical characterization of a number of fluorene derivatives with high two-photon absorptivities have been reported. [16–23] In our previous research [24], we have shown that the diphenylamino end-cap(s) can lower the first ionization potential which greatly reduces the energy barrier for the hole injection from ITO to the emissive oligofluorenes and does not perturb the planarity and alter the highly fluorescent nature of

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oligofluorene backbone. Furthermore, diphenylamino end-capped oligofluorenes showed enhanced thermal stability and exhibited superior amorphous morphological stability. However, the large difference (
20 nm) between the solution PL and thin film PL of (Bu)-OF(4)-NPh marked the presence of the intermolecular interaction among the quaterfluorene backbone [24a]. To prevent aggregation/excimer formation and induce amorphous state of fluorene-based molecular materials, a widely adopted approach is to introduce either spiro-linkage [25–27] or bulky substituent(s) or dendron(s) [28,29] at the C9-position of fluorene. In order to continuing our studies on the oligofluorene derivatives for optoelectronic applications [24], we herein describe the synthesis and two-photon absorption properties of a homologues series of

electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar salt bridge connecting to the oligomer solution dissolved in CH2Cl2 using 0.1 M of Bu4NPF6 as a supporting electrolyte with a scan rate of 100 mV/s and all the potentials were calibrated with ferrocene, (E1/2 (Fc/Fc+) = 0.45 V vs SCE) as an external. For two-photon-induced fluorescence method, fluorescein was used as a standard [38].

chromophores with symmetrical molecular structure of type D-p-D, based on 9,9-diphenyl substituted oligofluorene endcapped with diphenylamino groups acting as an electron donor group (D), (Ph)-OF(n)–NPhs, n = 1–5.

A single necked 100-mL round bottom flask was charged with diphenylamine (3.38 g, 20 mmol), sodium tert-butoxide (2.88 g, 30 mmol), palladium (II) acetate (0.11 g, 0.50 mmol), dry toluene (30 mL), 2,7-dibromo-9,9-diphenyl-9H-fluorene (1, 9.52 g, 20 mmol), [32] and tri-o-tolylphosphine (0.30 g, 1 mmol), and heated at 110  C for 24 h under a nitrogen atmosphere. After cooling, the reaction mixture was quenched with water and the solution was extracted with dichloromethane (3  50 mL). The combined organic extract was washed with water and dried over anhydrous Na2SO4. Evaporation of volatiles left a dark solid, which was separated by column chromatography using petroleum ether: dichloromethane (v/v = 8:1) as eluent affording compound 2 in 45% yield as a light yellow solid and compound (Ph)-OF(1)-NPh in 52% yield as a yellow solid. 2: 1H NMR (400 MHz, CDCl3, d) 7.47–7.60 (m, 4 H), 7.14–7.30 (m, 16 H), 7.00–7.10 (m, 6 H). 13C NMR (100 MHz, CDCl3, d) 152.6, 152.2, 147.8, 147.3, 145.0, 139.0, 133.1, 130.6, 129.3, 129.0, 128.2, 128.0, 126.7, 126.4, 124.2, 124.1, 122.9, 121.1, 120.7, 120.2, 65.6. MS (FAB) m/z 564.6 (M+). (Ph)-OF(1)-NPh: 1H NMR (270 MHz, CDCl3, d) 7.50 (d, J = 8.1 Hz, 2 H), 7.16–7.22 (m, 20 H), 7.04–7.12 (m, 10 H), 6.94–6.99 (m, 4 H). 13C NMR (60 MHz, CDCl3, d) 152.0, 147.5, 146.6, 145.5, 134.6, 129.0, 128.1, 127.9, 126.3, 123.9, 123.3, 122.5, 121.9, 120.0, 65.3. MS (FAB) m/z 652.4 (M+). [7-(Diphenylamino)-9,9-diphenyl-9H-fluoren-2-yl] boronic acid (3). To a 100 mL two-necked flask containing the solution of 2 (1.13 g, 2.0 mmol) in 30 mL of dried THF equipped with a magnetic stirrer, a N2 purge and a 78  C acetone-dry ice bath were added 1.5 M of n-butyl lithium/n-hexane (4.0 mL, 6.0 mmol) while maintaining a good stirring. After stirring for 0.5 h, the temperature was gradually lifted to room temperature for another 0.5 h, and then the reaction mixture was cooled to 78  C again, trimethyl borate (0.70 mL, 6.0 mmol) was added. After stirring further 2 h, water was first added to the reaction mixture and then HCl (6 M) was added in a dropwise fashion until an acidic mixture was obtained. The reaction mixture was poured into water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by flash column

2. Experimental 2.1. General procedures and requirements All the solvents were dried by the standard methods wherever needed. 1H NMR spectra were recorded using either a JEOL JHMEX270 FT NMR spectrometer or a Varian INOVA-400 FT NMR spectrometer and are referenced to the residual CHCl3 7.24 ppm or DMSO 2.5 ppm. 13C NMR spectra were recorded using a Varian INOVA-400 FT NMR spectrometer and are referenced to the CDCl3 77 ppm or DMSO-D6 39.5 ppm. Mass spectroscopy (MS) measurements were carried using fast atom bombardment (FAB) on the API ASTAR Pulsar I Hybrid Mass Spectrometer. Elemental Analysis was carried on the CARLO ERBA 1106 Elemental Analyzer. Thermal stabilities were determined by thermal gravimetric analyzer (PETGA6) with a heating rate of 10  C/min under N2. The glass transitions and melting transitions were extracted from the second run DSC traces which were determined by differential scanning calorimeter (PE Pyris Diamond DSC) with a heating rate of 10  C/ min under N2. All the physical measurements were performed in CHCl3 including electronic absorption (UV–vis) and fluorescence spectra. Electronic absorption (UV–vis) and fluorescence spectra were recorded using a Varian Cary 100 Scan Spectrophotometer and a PTI Luminescence Spectrophotometer, respectively. The fluorescence quantum yields in chloroform were determined by dilution method [31c] using quinine sulfate monohydrate (lexc = 313 nm, F = 48%) as a standard. The fluorescence decay curves were recorded at room temperature using nitrogen laser as excitation. The lifetimes were estimated from the measured fluorescence decay using iterative fitting procedure. E1/2 vs Fc+/Fc was estimated by cyclic voltammetric method (Voltammetric Analyzer CV-50W) using platinum disc electrode as a working

2.2. Synthesis (7-Bromo-9, 9-diphenyl-9H-fluoren-2-yl) diphenylamine (2) and N, N, N0 , N0 , 9,9-hexaphenyl-9H-fluorene-2, 7-diamine [(Ph)OF(1)-NPh].

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Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

chromatography using dichloromethane: ethyl acetate as a gradient eluent. The product boronic acid 3 was obtained as a yellow solid with an isolated yield of 86%. 1H NMR (270 MHz, DMSO-d6, d) 8.05 (s, 2 H), 7.78–7.89 (m, 4 H), 7.19–7.22 (m, 10 H), 6.69–7.05 (m, 12 H). 13C NMR (66 MHz, DMSO-d6, d) 152.3, 149.3, 147.1, 146.8, 145.3, 141.1, 133.9, 133.7, 131.4, 129.3, 128.2, 127.6, 126.5, 123.8, 123.1, 122.4, 121.6, 120.6, 118.7, 64.7. N, N, N0 , N0 , 9, 9, 90 , 90 -Octaphenyl-9H, 9H0 -2, 20 -bifluorene-7, 70 -diamine [(Ph)-OF(2)-NPh]. A mixture of (7-bromo-9,9diphenyl-9H-fluoren-2-yl)diphenylamine 2(452 mg, 0.8 mmol), [7-(diphenylamino)-9,9-diphenyl-9H-fluoren-2-yl] boronic acid 3 (517.0 mg, 1.0 mmol), 1:2 palladium (II) acetate: tri-o-tolylphosphine (5 mole%), toluene (20 mL), methanol (10 mL) and 2 M K2CO3 (2 mL) was heated at 75  C for overnight. After the reaction mixture was cooled to room temperature, it was poured into water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was further purified by silica gel column chromatography using 2:1 petroleum ether: dichloromethane as eluent. The product was a light yellow solid with an isolated yield of 71%. 1H NMR (400 MHz, CDCl3, d) 7.64 (d, J = 8.4 Hz, 2 H), 7.55 (d, J = 8.0 Hz, 2 H), 7.48–7.50 (m, 4 H), 7.13–7.20 (m, 30 H), 7.02–7.04 (m, 8 H), 6.94–6.99 (m, 6 H). 13C NMR (100 MHz, CDCl3, d) 152.7, 151.3, 147.6, 147.5, 145.8, 139.9, 139.1, 134.2, 129.1, 128.2, 126.6, 126.5, 124.7, 124.2, 123.0, 122.8, 121.5, 120.7, 119.7, 65.4. MS (FAB) m/z 969.8 (M+). Anal. Calcd for C74H52N2: C, 91.70; H, 5.41; N, 2.89. Found: C, 91.77; H, 5.50; N, 3.06. N, N, N0 , N0 , 9, 9, 90 , 90 , 900 , 900 -Decaphenyl-9H, 9H0 , 9H00 -2, 20 , 200 0 trifluorene-7, 7 -diamine [(Ph)-OF(3)-NPh]. A mixture of 2, 7dibromo-9,9-diphenyl-9H-fluorene 1 (381 mg, 0.8 mmol), [7(diphenylamino)-9,9-diphenyl-9H-fluoren-2-yl] boronic acid 3 (1035 mg, 2.0 mmol), 1:2 palladium (II) acetate: tri-o-tolylphosphine (5 mole%), toluene (30 mL), methanol (15 mL) and 2 M K2CO3 (4 mL) was heated at 75  C for overnight. After the reaction mixture was cooled to room temperature, it was poured into water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was further purified by silica gel column chromatography using 3:1 petroleum ether: dichloromethane as eluent, affording compound (Ph)-OF(3)-NPh as a yellow solid with an isolated yield of 65% and compound (Ph)-OF (2)-NPhH as a light yellow solid with an isolated yield of 21%. (Ph)OF(3)-NPh: 1H NMR (400 MHz, CDCl3, d) 7.74 (d, J = 8.0 Hz, 2 H), 7.64 (d, J = 8.0 Hz, 2 H), 7.49–7.57 (m, 10 H), 7.16–7.21 (m, 40 H), 7.03 (d, J = 7.6 Hz, 8 H), 6.95–6.99 (m, 6 H). 13C NMR (100 MHz, CDCl3, d) 152.8, 152.0, 151.3, 147.6, 145.9, 145.8, 140.8, 139.8, 139.3, 138.9, 134.1, 129.1, 128.3, 128.2, 126.7, 126.6, 124.7, 124.2, 123.0, 122.8, 121.5, 120.7, 120.3, 119.7, 65.7, 65.5. MS (FAB) m/z 1285.8 (M+). Anal. Calcd for C99H68N2: C, 92.49; H, 5.33; N, 2.18. Found: C, 92.19; H, 5.40; N, 2.34. (Ph)-OF(2)-NPhH: 1H NMR (270 MHz, CDCl3, d) 7.75 (d, J = 8.24 Hz, 2 H), 7.66(d, J = 8.24 Hz, 1 H), 7.51-7.58 (m, 6 H), 7.38 (d, J = 7.59 Hz, 1 H), 7.32 (d, J = 7.43 Hz, 1 H), 7.18-7.21 (m, 16 H), 6.957.05 (m, 10 H). 13C NMR (66 MHz, CDCl3, d) 152.6, 151.6, 151.2, 151.1, 147.4, 147.3, 145.8, 145.7, 140.8, 139.7, 139.6, 139.2, 139.1, 134.0, 129.0, 128.1, 128.0, 127.6, 127.4, 126.6, 126.5, 126.4, 126.1, 124.7, 124.6, 124.1, 122.9, 122.7, 121.4, 120.6, 120.2, 120.1, 119.6, 65.6, 65.5. MS (FAB) m/z 801.7 (M+). (7-Bromo-9,9-diphenyl-9H-fluoren-2-yl)-trimethyl-silane (4). To a 100 mL two-necked flask containing the solution of 1 (3.10 g, 6.5 mmol) in 30 mL dry THF equipped with a magnetic stirrer, a N2 purge and a 78  C acetone-dry ice bath were dropwise added 1.5 M of n-butyl lithium/n-hexane (4.8 mL, 7.16 mmol) while maintaining a good stirring. After stirring for 1 h, trimethylsilyl chloride (0.96 mL, 7.5 mmol) was carefully added. After stirring at room temperature for another 2 h, water was added to the reaction mixture. The reaction mixture was poured into water and extracted

with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by column chromatography using petroleum ether as eluent. The product 4 was obtained as a colorless liquid with an isolated yield of 81%. 1H NMR (400 MHz, CDCl3, d) 7.66–7.72 (m, 4 H), 7.49-7.52 (m, 2 H), 7.13–7.22 (m, 10 H). 13 C NMR (100 MHz, CDCl3, d) 153.4, 151.2, 149.9, 146.4, 139.8, 137.9, 135.3, 132.6, 130.9, 128.5, 128.2, 128.0, 127.6, 126.8, 121.9, 119.5, 65.5. MS (FAB) m/z 457.6 (M+). [7-(Trimethylsilyl)-9,9-diphenyl-9H-fluoren-2-yl] boronic acid (5). To a 100 mL two-necked flask containing the solution of 4 (2.13 g, 4.66 mmol) in 20 mL dry THF equipped with a magnetic stirrer, a N2 purge and a 78  C acetone-dry ice bath were dropwise added 1.5 M of n-butyl lithium/n-hexane (4.7 mL, 7.0 mmol) while maintaining a good stirring. After stirring for 1 h, trimethyl borate (0.80 mL, 7.0 mmol) was carefully added. After stirring at room temperature for another 2 h, water was first added to the reaction mixture and then HCl (6 M) was added in a dropwise fashion until an acidic mixture was obtained. The reaction mixture was poured into water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by column chromatography using dichloromethane: ethyl acetate (v/v = 1:1) as eluent. The boronic acid 5 was obtained as a light yellow solid with an isolated yield of 72%. 1H NMR (400 MHz, CDCl3, d) 8.11 (s, 2 H), 7.86–7.93 (m, 4 H), 7.54 (s, 2 H), 7.20–7.25 (m, 6 H), 7.12 (d, J = 7.20 Hz, 4 H), 0.19 (s, 9 H). 13C NMR (100 MHz, CDCl3, d) 150.0, 149.5, 145.5, 141.0, 140.2, 139.7, 133.5, 132.3, 131.5, 130.3, 128.1, 127.5, 126.4, 119.9, 119.4. 9, 9, 90 , 90 -Tetraphenyl-9H, 9H0 -2, 20 -bifluorene-7, 70 -bistrimethylsilane (6). A mixture of bromide 4 (1.46 g, 3.19 mmol), boronic acid 5 (1.94 g, 4.59 mmol), 1:2 palladium (II) acetate: tri-otolylphosphine (5 mol%), toluene (30 mL), methanol (15 mL) and 2 M K2CO3 (10 mL) was heated at 75  C for overnight under nitrogen atmosphere. After the reaction mixture was cooled to room temperature, it was poured into water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was further purified by a silica gel column chromatography using petroleum ether: dichloromethane (v/v = 4:1) as eluent. The product was obtained as a white solid with an isolated yield of 74%. 1H NMR (400 MHz, CDCl3, d) 7.84 (d, J = 8.0 Hz, 2 H), 7.80 (d, J = 7.6 Hz, 2 H), 7.65 (s, 4 H), 7.59 (t, J = 7.0 Hz, 4 H), 7.22–7.30 (m, 20 H). 13C NMR (100 MHz, CDCl3, d) 151.8, 150.5, 146.1, 140.9, 140.4, 140.0, 139.3, 132.5, 130.9, 128.2, 128.1, 126.7, 126.6, 124.8, 120.4, 119.5, 65.6. MS (FAB) m/z 779.6 (M+). 7, 70 -Diiodo-9, 9, 90 , 90 -tetraphenyl-9H, 9H0 -2, 20 -bifluorene (7). A mixture of compound 6 (1.18 g, 1.52 mmol), silver trifluoroacetate (0.81 g, 3.65 mmol) and chloroform (20 mL) was refluxed at 80  C for 0.5 h. After that a solution of iodine (0.93 g, 3.65 mmol) in THF (20 mL) was slowly added. The mixture was refluxed for another 2 h and cooled to room temperature. The reaction mixture was poured into a large quantity of water and extracted with dichloromethane (3  50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by a silica gel column chromatography using petroleum ether: dichloromethane (v/v = 4:1) as eluent. The product 7 was obtained as a white solid with an isolated yield of 83%. 1H NMR (400 MHz, CDCl3, d) 7.66-7.77 (m, 4 H), 7.49-7.56 (m, 4 H), 7.33-7.41 (m, 2 H), 7.17-7.28 (m, 22 H). 13C NMR (100 MHz, CDCl3, d) 153.5, 151.8, 151.4, 145.9, 141.5, 140.6, 139.7, 139.6, 139.4, 138.3, 136.7, 135.3, 128.4, 128.3, 128.1, 127.8, 127.5, 126.9, 126.7, 126.2, 124.8, 124.7, 121.9, 120.5, 120.4, 120.2, 92.8, 65.6. MS (FAB) m/ z 886.5 (M+). 9, 9, 90 , 90 , 900 , 900 -Hexaphenyl-9H, 9H0 , 9H00 -2, 20 , 200 terfluorene-7, 70 -bis-trimethylsilane (8). The synthetic procedure

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for preparation of compound 6 was followed using boronic acid 5 (1.27 g, 3 mmol) and 2,7-dibromo-9,9-diphenyl-9H-fluorene 1 (0.47 g, 1 mmol). The pure product 8 was separated by silica gel column chromatography using 4:1 dichloromethane: petroleum ether as eluent affording 0.95 g (86%) of a white solid. 1H NMR (400 MHz, CDCl3, d) 7.76 (s, 6 H), 7.54-7.58 (m, 12 H), 7.24 (s, 30 H). 13 C NMR (100 MHz, CDCl3, d) 152.0, 151.8, 150.5, 146.1, 145.8, 140.9, 140.8, 140.4, 140.1, 139.4, 138.9, 132.5, 130.9, 128.3, 128.2, 128.1, 126.8, 126.7, 126.6, 124.8, 124.7, 124.4, 120.4, 120.3, 119.5, 65.6, 1.0. MS (FAB) m/z 1095.7 (M+). N, N, N0 , N0 , 9, 9, 90 , 90 , 900 , 900 , 9000 , 9000 -Duodecphenyl-9H, 9H0 , 0 00 9H , 9H000 -2, 20 , 200 , 2000 -tetrafluorene-7, 7 -diamine [(Ph)-OF(4)NPh]. The Suzuki cross-coupling procedure for the preparation of (Ph)-OF(3)-NPh was followed using [7-(diphenylamino)-9, 9diphenyl-9H-fluoren-2-yl] boronic acid 3 (776 mg, 1.50 mmol) and 7,70 -diiodo-9, 9,90 ,90 -tetraphenyl-9H,9H0 -2,20 -bifluorene 7

51

(443 mg, 0.50 mmol). The pure product was separated by silica gel column chromatography using 2:1 dichloromethane: petroleum ether as eluent affording 649 mg (81%) of a light-yellow solid. 1 H NMR (400 MHz, CDCl3, d) 7.74 (d, J = 8.0 Hz, 4 H), 7.65 (d, J = 8.4 Hz, 2 H), 7.49–7.57 (m, 12 H), 7.13-7.24 (m, 50 H), 7.02–7.05 (m, 10 H), 6.94-6.99 (m, 6 H). MS (FAB) m/z 1601.9 (M+). Anal. Calcd for C124H84N2: C, 92.97; H, 5.29; N, 1.75. Found: C, 92.73; H, 5.27; N, 1.67. 7, 70 -Diiodo-9, 9, 90 , 90 , 900 , 900 -hexaphenyl-9H, 9H0 , 9H00 -2, 20 , 00 2 -terfluorene (9). The iodo-desilylation procedure above was followed using 9, 9, 90 , 90 , 900 , 900 -hexaphenyl-9H, 9H0 , 9H00 -2, 20 ,200 terfluorene-7,70 -bis-trimethylsilane 8 (1.28 g, 1.17 mmol), iodine (0.89 g, 3.51 mmol) and silver trifluoroacetate (0.72 g, 3.28 mmol). The pure product 9 was separated by silica gel column chromatography using 2:1 dichloromethane: petroleum ether as eluent affording 1.38 g (98%) of a white solid. 1H NMR (400 MHz,

Ph2NH Br

N

a

Br

+

Br

N

N

2, 45%

1

(Ph)-OF(1)-NPh, 25%

b 86% N

2, c

B(OH)2

N

N 2

71% (Ph)-OF(2)-NPh

3 1 c

N

N 3

N

+

(Ph)-OF(3)-NPh, 65%

d

1

81%

(Ph)-OF(2)-NPhH, 21%

b Si

4, c

72%

Br

Si

B(OH)2

4

Si 2

3, c

83%

I

I

e Si

Si 3 8

96%

3, c I 84%

I 9

N 4 (Ph)-OF(4)-NPh

7

7, c 86%

N

62%

2

6

5

74%

5

e Si

H 2

3

N

N 5 (Ph)-OF(5)-NPh

Scheme 1. Synthetic routes of (Ph)-OF(n)-NPh series. Reagents and Conditions: a, 1.0 equiv. Ph2NH, 5 mol% Pd(OAc)2: 2P(o-tolyl)3, NaOtBu, toluene, 110  C, N2, overnight. b, (i) 1.5 equiv. n-BuLi, THF, 78  C, 1 h; (ii) B(OCH3)3, 78  C
r.t., 2 h; (iii) 6 M HCl, 15 min c, 5 mol% Pd(OAc)2:2P(o-tolyl)3, K2CO3, toluene-methanol, N2, 75  C, overnight. d, (i) 1.0 equiv. n-BuLi, THF, 78  C, 1 h; (ii) 1.0 equiv. Me3SiCl, 78  C
r.t., 2 h e, I2, CF3COOAg, CHCl3, reflux, 4 h.

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CDCl3, d) 7.66–7.76 (m, 8 H), 7.48–7.52 (m, 10 H), 7.18–7.21 (m, 30 H). 13C NMR (100 MHz, CDCl3, d) 152.0, 151.8, 150.5, 146.1, 145.8, 140.9, 140.8, 140.4, 140.1, 139.4, 138.9, 132.5, 130.9, 128.3, 128.2, 128.1, 126.8, 126.7, 126.6, 124.8, 124.7, 124.4, 120.4, 120.3, 92.9, 65.7. MS (FAB) m/z 1203.4 (M+). N, N, N0 , N0 , 9, 9, 90 , 90 , 900 , 900 , 9000 , 9000 , 90000 , 90000 -Quatuodecphenyl0 9H, 9H0 , 9H00 , 9H0000 , 9H0000 -2, 20 , 200 , 2000 , 20000 -pentafluorene-7, 7 diamine [(Ph)-OF(5)-NPh]. The Suzuki cross-coupling procedure for the preparation of (Ph)-OF(3)-NPh was followed using [7(diphenylamino)-9,9-diphenyl-9H-fluoren-2-yl] boronic acid 3 (150 mg, 0.30 mmol) and 7,70 -diiodo-9,9,90 ,90 ,900 ,900 -hexaphenyl9H,9H0 ,9H00 -2,20 ,200 -terfluorene 9 (120 mg, 0.10 mmol). The pure product was separated by silica gel column chromatography using 2:1 dichloromethane: petroleum ether as eluent affording 163 mg (84%) of a light-yellow solid. 1H NMR (400 MHz, CDCl3, d) 7.74 (d, J = 7.6 Hz, 6 H), 7.65 (d, J = 8.8 Hz, 2 H), 7.49–7.57 (m, 16 H), 7.17–7.21 (m, 64 H), 7.03 (d, J = 8.0 Hz, 8 H), 6.94-6.96 (m, 6H). MS (FAB) m/z 1918.1 (M+). Anal. Calcd for C149H100N2: C, 93.29; H, 5.25; N, 1.46. Found: C, 93.12; H, 5.43; N, 1.62. 3. Results and discussion 3.1. Synthesis Synthetic routes for monodisperse diphenylamino end-capped oligo (9, 9-diphenylfluorene)s, (Ph)-OF(n)-NPh, n = 1-5 are outlined in Scheme 1. Amination of 1 [30] with diphenylamine (1 equivalent) catalyzed by Pd(OAc)2: 2P(o-tolyl)3 afforded monoamination product 2 in 45% yield and double-amination product (Ph)-OF(1)-NPh in a yield of 25% [31]. Transformation of the bromide compound 2 into the corresponding boronic acid 3 was achieved in 86% yield by lithium-bromine exchange at low temperature, followed by reaction with trimethyl borate at room temperature and subsequently acid hydrolysis [32]. Suzuki crosscoupling of the boronic acid 3 and the bromide 2 using Pd(OAc)2: 2P(o-tolyl)3 as a catalyst afforded the desired bifluorene derivative, (Ph)-OF(2)-NPh in a good yield (71%) [33]. Double palladium-catalyzed Suzuki cross-coupling of 1 with the boronic acid 3 yielded terfluorene derivative (Ph)-OF(3)-NPh in 65% yield, as well as mono-coupling product (Ph)-OF(2)-NPhH in 21% yield. In addition, transformation of the dibromide compound 1 into the corresponding the monobromide 4 was carried out in 70% yield by lithium-bromine exchange at low temperature, followed by reaction with trimethyl-silyl chloride at room temperature. Then, transformation of the monobromide 4 into the corresponding boronic acid 5 was easily carried out in a yield of 81% by lithium-bromine exchange at low temperature, followed by reaction with trimethyl borate at room temperature and subsequently acid hydrolysis. Suzuki cross-coupling between the boronic acid 5 and monobromide 4 using Pd (OAc)2: 2P(o-tolyl)3 as a catalyst afforded the desired trimethyl-silyl end-capped bifluorene, 6 in an excellent yield. Transformation of 6 into diiodide 7 was achieved in a good yield by iodine-desilylation at 80  C in the presence of silver trifluoroacetate (CF3COOAg) [34]. Then, double palladium-catalyzed Suzuki cross-coupling between diiodide 7 and boronic acid 3 yielded quaterfluorene derivative (Ph)-OF(4)NPh in a moderate yield. (Ph)-OF(5)-NPh was obtained in an excellent yield by using a similar reaction sequence as that of (Ph)OF(4)-NPh. 3.2. Thermal properties The thermal properties of the newly synthesized molecules were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). All oligo(9, 9-diphenylfluorene) s ((Ph)-OF(n)-NPh) show distinct high glass transition

Table 1 Thermal and electrochemical properties of (Ph)-OF(n)-NPh series. compound

Tga /  C

Tdecb /  C

OxidE1/2c / V

HOMOd / eV

LUMOe / eV

Eg f /eV

(Ph)-OF(1)NPh (Ph)-OF(2)NPh (Ph)-OF(3)NPh (Ph)-OF(4)NPh (Ph)-OF(5)NPh (Bu)-OF(3)-H (Bu)-OF(3)NPh

112

459

0.19

4.99

1.87

3.12

164

563

0.41

5.21

2.31

2.90

195

585

0.40

5.20

2.30

2.90

217

606

0.39

5.19

2.29

2.90

226

587

0.38

5.18

2.19

2.99

no 99

432 461

0.77 0.35

5.57 5.15

2.27 2.11

3.30 3.04

a Determined by differential scanning calorimeter from re-melt after cooling with a heating rate of 10  C/min under N2. b Determined by thermal gravimetric analyser with a heating rate of 10  C/min under N2. c E1/2 vs Fc+/Fc estimated by CV method using platinum disc electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar salt bridge connecting to the oligomer solution and all the potentials were calibrated with ferrocene, E1/2 (Fc/Fc+) = 0.45 V vs SCE. d Estimated from first ionization potential and E1/2(Fc/Fc+). e The difference between HOMO level and energy gap (Eg). f Estimated from onset absorption spectra of the sample in chloroform.

temperatures (Tg) ranging from 112  C to 226  C, which sequentially increase with an increase in fluorenyl units. (Table 1) It is noteworthy that the glass transition temperatures of the 9,9diphenyl substituted oligofluorenes, (Ph)-OF(n)-NPh, are higher than those of 9,9-dibutyl substituted oligofluorene analogues, (Bu)-OF(n)-NPhs [24a].The Tg is greatly enhanced by 73–100  C with a 9, 9-diphenylfluorenyl unit increase. These results suggest that incorporation of two phenyl groups at C9-position of oligofluorene can also be used as a tool to further improve morphologically stability. More importantly, the lower homologues of oligofluorenes, e.g., (Ph)-OF(1)-NPh and (Ph)-OF(2)-NPh, not only exhibit glass transition temperature, crystallization temperature, but also exhibit melting point temperature; however, the higher homologues of oligofluorenes, e.g., (Ph)-OF(3)-NPh to (Ph)-OF(5)-NPh, only exhibit glass transition temperature which means that they are amorphous. Furthermore, by TGA analysis, all (Ph)-OF(n)-NPhs were found to be thermally stable with decomposition temperatures (Tdecs) corresponding to a 5% weight loss upon heating (20  C/min) under nitrogen ranging from 459 to 606  C, and increased sequentially with a 9, 9-diphenyl-fluorenyl unit increase. However, decomposition temperatures of this series of oligofluorenes show a maximum at (Ph)-OF(4)-NPh, n = 4. (Table 1 and Fig. 1) In addition, decomposition temperatures of the three analogues with the same conjugation length, (Bu)-OF(3)-H, (Bu)-OF(3)-NPh and (Ph)-OF(3)-NPh, are 432  C, 461  C, and 585  C,[24a] respectively, indicating that the introduction of endcapped diphenylamino group onto the oligofluorene skeleton leads to an increase of about 30  C in decomposition temperature, while replacement of 9, 9-dibutyl groups with 9, 9-diphenyl groups leads to an enhancement of 124  C in decomposition temperature. Glass transition temperatures of these three analogues are not present, 99  C, and 195  C, respectively, indicating that the introduction of end-capped diphenylamino group onto the oligofluorene skeleton induces the formation of amorphous state, while the 9, 9-diphenyl substitution replacing of the 9,9-dibutyl substitution leads to a large increase of about 100  C in morphological stability.

Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

Weight % (%)

(a)

120

(b) 120

100

100

60 40

Weight % / %

80

(Ph)-OF(1)-NPh (Ph)-OF(2)-NPh (Ph)-OF(3)-NPh (Ph)-OF(4)-NPh (Ph)-OF(5)-NPh

20

53

(Ph)-OF(3)-NPh (Bu)-OF(3)-NPh (Bu)-OF(3)-H

80 60 40 20

0 200

400

600

800

0

1000

200

400

600

o

Temperature / C

800

1000

o

Temperature / C

Fig. 1. TGA analyses of (Ph)-OF(n)-NPh, n = 1-5 (a) and terfluorene derivatives (b), heated (10  C/min) from 50  C to 900  C.

Table 2 Optical properties of (Ph)-OF(n)-NPhs and (Bu)-OF(n)-NPhs. compound

labsa/nm

lema,b/nm

Kc/%

TPAlemmaxd /nm

d800e (GM)

d800f/MW

(Ph)-OF(2)-NPh (Ph)-OF(3)-NPh (Ph)-OF(4)-NPh (Ph)-OF(5)-NPh (Bu)-OF(3)-NPh (Bu)-OF(4)-NPh (Bu)-OF(5)-NPh

389(9.20) 389(14.7) 390(18.9) 390(21.8) 386(8.53) 387(14.2) 387(18.9)

428 435 439 439 432 432 433

85 86 86 80 93 94 93

438 443 446 446 446 440 446

129 230 550 1390 61 117 118

0.13 0.29 0.34 0.72 – – –

c d e f

Measured in CHCl3. Excited at the absorption maxima. Using quinine sulfate monohydrate(l313 = 48%) as a standard. Using nitrogen laser as excitation source. Determined by two-photon-induced fluorescence method using 800 nm femtosecond laser pulses with fluorescein as a standard. Ratio of d800 to molecular weight.

(a)

1.6

(b)

1.4

(Ph)-OF(1)-NPh (Ph)-OF(2)-NPh (Ph)-OF(3)-NPh (Ph)-OF(4)-NPh (Ph)-OF(5)-NPh

1.2 1.0 0.8 0.6 0.4 0.2 0.0 300

350

400

450

Wavelength / nm

500

550

1.2 Normalized PL Intensity / a.u.

b

Molar Absorptivity / 105 M -1 cm-1

a

(Ph)-OF(1)-NPh (Ph)-OF(2)-NPh (Ph)-OF(3)-NPh (Ph)-OF(4)-NPh (Ph)-OF(5)-NPh

1.0 0.8 0.6 0.4 0.2 0.0 300

350

400

450

500

Wavelength / nm

Fig. 2. (a)Absorption spectra and (b) emission spectra of (Ph)-OF(n)-NPhs, n = 1-5 in CHCl3.

550

600

54

Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

(b)

1.2

(Bu)-OF(3)-H (Bu)-OF(3)-NPh (Ph)-OF(3)-NPh

1.0 0.8 0.6 0.4 0.2 0.0 300

350

400

450

500

550

Wavelength / nm

Normalized PL Intensity / a.u.

Molar Absorptivity / 105 M-1 cm-1

(a)

1.2

(Bu)-OF(3)-H (Bu)-OF(3)-NPh (Ph)-OF(3)-NPh

1.0 0.8 0.6 0.4 0.2 0.0 300

350

400 450 500 Wavelength / nm

550

600

Fig. 3. (a) Absorption spectra and (b) emission spectra of terfluorene derivatives in chloroform.

3.3. Photophysical properties The photophysical characteristics of the new oligofluorenes were examined by UV–vis and fluorescence spectroscopy in chloroform solution. The absorption and emission spectral data are summarized in Table 2. The UV–vis absorption spectra of these oligofluorenes are depicted in Fig. 2a. The electronic spectra of these oligofluorenes exhibit two distinct bands arising from n!p* transition and p!p* transition, respectively. The n!p* transition originates from the transition of terminal diphenylamine moiety; therefore, all these oligofluorenes are expected to have a peak at
300 nm. The absorption maxima (labsmax) of the 9,9-diphenyl substituted oligofluorenes, (Ph)-OF(n)-NPhs, are slightly redshifted (D 4–5 nm) as compared to those of 9,9-dibutyl substituted oligofluorenes, (Bu)-OF(n)-NPhs [24a]; however, labsmax of these 9,9-diphenyl substituted oligofluorenes show sign of saturation at 390 nm even though emax increases sequentially with chain length. (Fig. 2a) This implies that diphenylamino end-caps play an important role in reducing the HOMO-LUMO energy gap and an effective conjugated length for the energy gap is reached when n = 2 in (Ph)-OF(n)-NPh series. Similar to the absorption, the emission maxima (lemmax) of 9, 9-diphenyl substituted oligofluorenes slightly shift to longer wavelengths (D 3–6 nm) as compared to those of 9,9-dibutyl substituted analogues [24a]. Upon excitation either at 308 nm attributed to the n!p* transition of triarylamine moiety or at 390 nm (lmax) corresponding to the p!p* transition of oligofluorene core, the emission spectra obtained are identical suggesting that energy or exciton can efficiently transfer from the triarylamine moiety to the emissive fluorene core. Consistent to the absorption behavior, the lemmax of (Ph)-OF(n)-NPh series also show a tendency of convergence at 439 nm when n = 4 further supporting an existence of the effective conjugated length for the energy gap. (Fig. 2b) The fluorescence quantum yields of (Ph)-OF(n)-NPhs which measured in chloroform using quinine sulfate monohydrate as a standard increase greatly from monofluorene to bifluorene and become saturated at ter- and quaterfluorenes (KFL
86%) and then decrease to 80% for (Ph)-OF (5)-NPh. On the other hand, the fluorescence quantum yields of (Ph)-OF(n)-NPh are quite smaller than those of corresponding 9, 9dibutyl substituted (Bu)-OF(n)-NPh series, suggesting that the 9, 9diphenyl substitution at C9-position of the oligofluorene skeleton may perturb the planarity and weaken highly fluorescent nature of

oligofluorene core. (Table 2) The fluorescence life-times of all 9,9diphenyl substituted oligofluorenes are in the nanosecond time scale suggesting that the emission originates from the singlet excited state to ground state. In all compounds, a small to moderate Stokes shift (D l = 17–49 nm) was observed, which can be explained by a small change in the molecular dipole moment upon electronic excitation. In addition, absorption maxima of the three analogues with the same backbone conjugation length, (Bu)OF(3)-H, (Bu)-OF(3)-NPh and (Ph)-OF(3)-NPh, are 353 nm, 386 nm, and 390 nm, respectively (Fig. 3a), and their fluorescence maxima are 394 nm, 432 nm, and 435 nm, respectively (Fig. 3b), suggesting that the introduction of end-capped diphenylamino group onto the oligofluorene skeleton produces a larger shift on absorption and fluorescence spectra of terfluorene derivatives than that of the replacement of 9,9-diphenyl group with 9,9-dibutyl group. 3.4. Electrochemical properties The electrochemical properties of these 9,9-diphenyl substituted oligofluorenes were investigated by using cyclic voltammetric method. Cyclic voltammetry was carried out in a three-electrode cell set-up with 0.1 M of Bu4NPF6 as a supporting electrolyte in CH2Cl2 to examine the electrochemical properties of the oligofluorenes. The results are tabulated in Table 1 and shown in Fig. 4. The 9,9-diphenyl substituted oligofluorenes, (Ph)-OF(n)-NPhs, n = 1–5, exhibit a reversible two-electron anodic redox couple, corresponding to two arylamine oxidations as well as a (ir) reversible one-electron anodic redox couple corresponding to the oxidation of oligofluorene skeleton. (Fig. 4) Except shorter homologue (Ph)-OF(1)-NPh, the arylamine oxidation and oligofluorene skeleton oxidation of other longer homologues proceed more easily with an increase in conjugated length as this electrochemically formed radical cation can be efficiently delocalized and hence stabilized. In general, with an incorporation of diphenylamino end-caps, the HOMO energy level of oligofluorenes moves up (relative to the vacuum level) to
5.20 eV as estimated by the electrochemical method. (Table 1) Such a high HOMO energy level greatly reduces the energy barrier for the hole injection from ITO (EHOMO = 5.0 eV) to the emissive oligofluorenes. As a result, (Ph)-OF(n)-NPhs can also be used as hole transport/ injection materials. Otherwise, in comparison with the longer homologues, (Ph)-OF(1)-NPh exhibits very different

Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

55

electrochemical behaviour, which has lower first/second oxidation potential indicating that its oxidation of triphenylamino center would proceed easier. 3.5. Two photon absorption properties Two-photon absorption cross-sections, d, can be determined by two different methods, a nonlinear transmission method employing a femtosecond white light continuum (WLC) and a two-photon induced fluorescence method (TPF). The TPA cross-sections for (Ph)-OF(n)-NPhs, n = 2-5, in CHCl3 as determined by the TPF method using 800 fs laser pulses as excitation wavelength exhibited TPA cross-section values of ca. 129, 230, 550, and 1390 GM, respectively. (Table 2) The strong TPA for (Ph)-OF(n)-NPhs (at lTPA = 800 nm) can be attributed to a transition from the ground state to the lowest-lying two-photon allowed state. This transition is expected and allowed for highly centrosymmetric molecules. The value of d800/MW for (Ph)-OF(n)-NPhs, n = 2-5 increases from 0.13 to 0.29 to 0.34–0.72 in chloroform with an increase in the number of fluorenyl units, indicating that the magnitude of the TPA cross-section exhibits a nearly linear increase with the number of the fluorenyl units. (Table 2) An enhancement in d has been reported in a triphenylamine-based multibranched structure, attributed, partly, to the electronic coupling between the branches [35], though a theoretical study revealed that the vibronic effects are the primary cause for such an enhancement [36]. Furthermore, increasing the number of chromophores of the system will increase the density of states, providing more effective coupling channels, which would in turn increase the TPA cross-section [35,37]. On the other hand, the large enhancement in d observed for this series of chromophores ((Ph)-OF(n)-NPhs, n = 2–5) can be correlated with intramolecular charge transfer from the terminal groups to the p-bridge. Hence, the TPA cross-section increases monotonically with the extent of charge transfer because the charge is transferred over a longer distance [38]. Remarkably, 9,9diphenyl-substituted oligofluorene (Ph)-OF(n)-NPh exhibit much larger TPA cross-sections than their corresponding 9,9-dibutylsubstituted counterparts (Bu)-OF(n)-NPh (Table 2). Fig. 5a shows TPA spectra of (Ph)-OF(n)-NPh and (Bu)-OF(n)-NPh, where n = 4 and 5, in which the maximum of the TPA cross-sections falls at around 710 nm, with dmax reaching 2559 GM for (Ph)-OF(5)-NPh and 221 GM for (Bu)-OF(5)-NPh, respectively. Consistently, the TPA cross-section enhancement for (Ph)-OF(n)-NPh series occurs in the full TPA spectra measured as compared to those of (Bu)-OF(n)NPhs. These results underline the importance of 9,9-diphenyl substitution onto a fluorenyl unit in enhancing the nonlinear optical properties. Fig. 6 shows that the allowed two-photon excited states of (Bu)-OF(4)-NPh and (Ph)-OF(4)-NPh are higher than their one-photon excited states, which is consistent with the TPA selection rule for the quadrupolar molecules [39]. The powersquared dependence of two-photon excited fluorescence for (Ph)OF(5)-NPh and (Bu)-OF(5)-NPh were also investigated. In both cases, the power-squared dependence of TPEF as shown in Fig. 5b was followed with the slope in the range of 1.91–2.01, which directly gives the experimental evidence of a two-photon excitation process. 4. Conclusion

Fig. 4. Cyclic voltammograms of the (Ph)-OF(n)-NPh series, n = 1-5 (versus Fc/Fc+).

A homologous series of 9,9-diphenyl substituted oligofluorenes end-capped by diphenylamino groups, (Ph)-OF(n)-NPhs, n = 1–5, has been synthesized using Suzuki cross coupling reaction as a key step and were fully characterized by 1H NMR, 13C NMR, MS, and elemental analyses. The functional properties, including thermal stabilities, linear optical properties, fluorescence and electrochemical properties, were investigated. The TPA cross sections of

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Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

Fig. 5. (a) TPA spectra of (Bu)-OF(n)-NPh and (Ph)-OF(n)-NPh, n = 4 and 5 in CHCl3 from 600 to 960 nm. (b) Logarithmic plots of the power dependence of relative two-photon induced fluorescence on pulse intensity using an 800 nm femtosecond laser as an excitation source for (Ph)-OF(5)-NPh and (Bu)-OF(5)-NPh.

Fig. 6. Normalized one- and two-photon excitation spectra of (a) (Bu)-OF(4)-NPh and (b) (Ph)-OF(4)-NPh. The two-photon excited wavelengths are divided by 2 for easy comparison.

these newly synthesized oligofluorenes were measured by the TPF method. Compounds (Ph)-OF(4)-NPh and (Ph)-OF(5)-NPh exhibited larger two-photon absorption cross-sections, about 550 GM and 1390 GM at 800 nm, respectively. Acknowledgment

[4]

We are grateful to the Scientific & Technological Support Project of Sichuan Province (No. 2009GZ0167 and 2013FZ0129) for financial support of this work. References [1] M. Göppert-Mayer, On elementary processes with two quantum steps, Ann. Phys. 9 (1931) 273–294. [2] (a) W. Denk, J.H. Strickler, W.W. Webb, Two-photon laser scanning fluorescence microscopy, Science 248 (1990) 73–76; (b) D.A. Parthenopoulos, P.M. Rentzepis, Three-dimensional optical storage memory, Science 245 (1989) 843–845; (c) A.R. Morales, K.D. Belfield, J.M. Hales, E.W. Van Stryland, D.J. Hagan, Synthesis of two-photon absorbing unsymmetrical fluorenyl-based chromophores, Chem. Mater. 18 (2006) 4972–4980. [3] (a) G.S. He, G.C. Xu, P.N. Prasad, B.A. Reinhardt, J.C. Bhatt, A.G. Dillard, Twophoton absorption and optical limiting properties of novel organic

[5] [6]

[7]

compounds, Opt. Lett. 20 (1995) 435–437; (b) J.E. Ehrlich, X.L. Wu, I.-Y.S. Lee, Z.-Y. Hu, H. Röckel, S.R. Marder, J.W. Perry, Two-photon absorption and broadband optical limiting with bis-donor stilbenes, Opt. Lett. 22 (1997) 1843–1845; (c) C.C. Corredor, Z.-L. Huang, K.D. Belfield, Two-photon 3D optical data storage via fluorescence modulation of an efficient fluorene dye by a photochromic diarylethene, Adv. Mater. 18 (2006) 2910–2914. (a) S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Finer features for functional microdevices, Nature 412 (2001) 697–698; (b) B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCord-Maughon, J. Qin, H. Röckel, M. Rumi, X.L. Wu, S.R. Marder, J.W. Perry, Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication, Nature 398 (1999) 51–54; (c) K.D. Belfield, M.V. Bondar, F.E. Hernandez, O.V. Przhonska, Photophysical characterization, two-photon absorption and optical power limiting of two fluorenylperylene diimides, J. Phys. Chem. C 112 (2008) 5618–5622. J.D. Bhawalkar, N.D. Kumar, C.F. Zhao, P.N. Prasad, Two photon photodynamic therapy, J. Clin. Laser Med. Surg. 15 (1997) 201–204. A. Abbotto, L. Beverina, R. Bozio, S. Bradamante, C. Ferrante, G.A. Pagani, R. Signorini, Push–pull organic chromophores for frequency-upconverted lasing, Adv. Mater. 12 (2000) 1963–1967. (a) B.A. Reinhardt, L.L. Brott, S.J. Clarson, A.G. Dillard, J.C. Bhatt, R. Kannan, L. Yuan, G.S. He, P.N. Prasad, Highly active two-photon dyes: design, synthesis, and characterization toward application, Chem. Mater. 10 (1998) 1863–1874; (b) Z. Liu, T. Chen, B. Liu, Z.-L. Huang, T. Huang, S. Li, Y. Xu, J. Qin, Two-photon absorption of a series of V-shape molecules: the influence of acceptor's strength on two-photon absorption in a noncentrosymmetric D–p–A–p–D

Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

system, J. Mater. Chem. 17 (2007) 4685–4689; (c) L. Mazur, R. Kolkowski, K. Matczyszyn, F. Mathevet, P. Rannou, A.-J. Attias, M. Samoc, Spectral dependence of nonlinear absorption and refraction in terthiophene-based organic semiconductors, Opt. Mater. 34 (2012) 1682– 1685; (d) E.Y. Choi, L. Mazur, L. Mager, M. Gwon, D. Pitrat, J.C. Mulatier, C. Monnereau, A. Fort, A.J. Attias, K. Dorkenoo, J.E. Kwon, Y. Xiao, K. Matczyszyn, M. Samoc, D.W. Kim, A. Nakao, B. Heinrich, D. Hashizume, M. Uchiyama, S.Y. Park, F. Mathevet, T. Aoyama, C. Andraud, J.W. Wu, A. Barsella, J.C. Ribierre, Photophysical, amplified spontaneous emission and charge transport properties of oligofluorene derivatives in thin films, Phys Chem. Chem. Phys. 16 (2014) 16941–16956; (e) M. Zhao, M. Samoc, P.N. Prasad, B.A. Reinhardt, M.R. Unroe, M. Prazak, R.C. Evers, J.J. Kane, C. Jariwala, M. Sinsky, Studies of third-order optical nonlinearities of model compounds containing benzothiazole, benzimidazole and benzoxazole units, Chem. Mater. 2 (1990) 670–678. M. Albota, D. Beljonne, J.-L. Brédas, 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. Röckel, M. Rumi, G. Subramaniam, W.W. Webb, X.L. Wu, C. Xu, Design of organic molecules with large two-photon absorption cross sections, Science 281 (1998) 1653–1656. (a) P.K. Frederiksen, M. JØrgensen, P.R. Ogilby, Two-photon photosensitized production of singlet oxygen, J. Am. Chem. Soc. 123 (2001) 1215–1221; (b) L. Ventelon, S. Charier, L. Moreaux, J. Mertz, M. Blanchard-Desce, Nanoscalep push–push dihydrophenanthrene derivatives as novel fluorophores for two-photon-excited fluorescence, Angew. Chem. Int. Ed. 40 (2001) 2098–2101; (c) B. Strehmel, A.M. Sarker, H. Detert, The influence of s and p acceptors on two-photon absorption and solvatochromism of dipolar and quadrupolar unsaturated organic compounds, ChemPhysChem 4 (2003) 249–259. (a) O. Mongin, L. Porrès, L. Moreaux, J. Mertz, M. Blanchard-Desce, Synthesis and photophysical properties of new conjugated fluorophores designed for two-photon excited fluorescence, Org. Lett. 4 (2002) 719–722; (b) M.K. Hwan, O.L. Yeon, S.L. Chang, S.K. Jong, R.C. Bong, Two-photon absorption properties of alkynyl-conjugated pyrene derivatives, J. Org. Chem. 73 (2008) 5127–5130. (a) L. Ventelon, L. Moreaux, J. Mertz, M. Blanchard-Desce, New quadrupolar fluorophores with high two-photon excited fluorescence, Chem. Commun. (1999) 2055–2056; (b) G.L.C. Moura, A.M. Simas, Two-photon absorption cross-sections from electronic structure methods: mesoionic compounds, Chem. Mater. 20 (2008) 4142–4155; (c) Z. Liu, P. Shao, Z. Huang, B. Liu, T. Chen, J. Qin, Two-photon absorption enhancement induced by aggregation due to intermolecular hydrogen bonding in V-shaped 2-hydroxypyrimidine derivatives, Chem. Commun. (2008) 2260–2262; (d) R. Bernard, C. Barsu, P.L. Baldeck, C. Andraud, D. Cornu, J.-P. Scharff, P. Miele, Synthesis and two photon absorption properties of 7,70 -(iminundecahydrocloso-dodecaborate)-9,90 -(dihexyl)-2,20 -bifluorene, Chem. Commun. (2008) 3765–3767. (a) M. Rumi, J.E. Ehrlich, A.A. Heikal, J.W. Perry, S. Barlow, Z. Hu, D. McCordMaughon, T.C. Parker, H. Roeckel, S. Thayumanavan, S.R. Marder, D. Beljonne, J.-L. Bredas, Structure?property relationships for two-photon absorbing chromophores: bis-donor diphenylpolyene and bis(styryl)benzene derivatives, J. Am. Chem Soc. 122 (2000) 9500–9510; (b) M. Williams-Harry, A. Bhaskar, G. Ramakrishna, T. Goodson III, M. Imamura, A. Mawatari, K. Nakao, H. Enozawa, T. Nishinaga, M. Iyoda, Giant thienyleneacetylene-ethylene macrocycles with large two-photon absorption cross section and semishape-persistence, J. Am. Chem. Soc. 130 (2008) 3252–3253. K.D. Belfield, M.V. Bondar, O.V. Przhonska, K.J. Schafer, Photostability of a series of two-photon absorbing fluorene derivatives, J. Photochem. Photobiol. A 162 (2004) 489–496. K.D. Belfield, M.V. Bondar, O.V. Przhonska, K.J. Schafer, One- and two-photon photostability of 9,9-didecyl-2,7-bis(,-diphenylamino)fluorene, Photochem. Photobiol. Sci. 3 (2004) 138–141. (a) K.D. Belfield, M.V. Bondar, O.V. Przhonska, K.J. Schafer, Photochemical properties of (7-benzothiazol-2-yl-9,9-didecylfluoren-2-yl) diphenylamine under one- and two-photon excitation, Photochem. Photobiol. A 162 (2004) 569–574; (b) S. Yao, K.D. Belfield, Synthesis of two-photon absorbing unsymmetrical branched chromophores through direct tris(bromomethylation) of fluorene, J. Org. Chem. 70 (2005) 5126–5132; (c) C.C. Corredor, Z.-L. Huang, K.D. Belfield, A.R. Morales, M.V. Bondar, Photochromic polymer composites for two-photon 3D optical data storage, Chem. Mater. 19 (2007) 5165–5173. K.D. Belfield, K.J. Schafer, W. Mourad, B.A. Reinhardt, Synthesis of new twophoton absorbing fluorene derivatives via Cu-mediated Ullmann condensations, J. Org. Chem. 65 (2000) 4475–4481. R. Kannan, G.S. He, L. Yuan, F.M. Xu, P.N. Prasad, A.G. Dombroskie, B.A. Reinhardt, J.W. Bauer, R.A. Vaia, L.-S. Tan, Diphenylaminofluorene-based twophoton-absorbing chromophores with various p-electron acceptors, Chem. Mater. 13 (2001) 1896–1904. O. Mongin, L. Porrès, C. Katan, T. Pons, J. Mertz, M. Blanchard-Desce, Synthesis and two-photon absorption of highly soluble three-branched fluorenylenevinylene derivatives, Tetrahedron Lett. 44 (2003) 8121–8125.

57

[19] K.D. Belfield, M.V. Bondar, A.R. Morales, O. Yavuz, O.V. Przhonska, A new blue light-emitting oligofluorene glass: synthesis, characterization and photophysical properties, J.Phys. Org. Chem. 16 (2003) 194–201. [20] (a) R. Kannan, G.S. He, T.-C. Lin, P.N. Prasad, R.A. Vaia, L.-S. Tan, 2004. Toward highly active two-photon absorbing liquids. Synthesis and characterization of 1,3,5-triazine-based octupolar molecules, Chem. Mater. 16 (2004) 185–194; (b) H. Ma, J. Leng, M. Liu, L. Zhao, Y. Jiao, Two-photon absorption and optical limiting of a fluorenyl-based chromophore with femtosecond laser pulse, Opt. Commun. 350 (2015) 144–147. [21] K.D. Belfield, A.R. Morales, J.M. Hales, D.J. Hagan, E.W. Van Stryland, V.M. Chapela, J. Percino, Linear and two-photon photophysical properties of a series of symmetrical diphenylaminofluorenes, Chem. Mater. 16 (2004) 2267–2273. [22] (a) K.D. Belfield, A.R. Morales, B.-S. Kang, J.M. Hales, D.J. Hagan, E.W. Van Stryland, V.M. Chapela, J. Percino, Synthesis, characterization, and optical properties of new two-photon-absorbing fluorene derivatives, Chem. Mater. 16 (2004) 4634–4641; (b) X. Feng, J.-Y. Hu, X.-F. Wei, C. Redshaw, T. Yamato, Influence of substituent position on thermal properties, photoluminescence and morphology of pyrene–fluorene derivatives, J. Mol. Struct. 1086 (2015) 216–222. [23] (a) M.H.V. Werts, S. Gmouh, O. Mongin, T. Pons, M. Blanchard-Desce, Strong modulation of two-photon excited fluorescence of quadripolar dyes by (de) protonation, J. Am. Chem. Soc. 126 (2004) 16294–16295; (b) C.L. Devi, K. Yesudas, N.S. Makarov, V.J. Rao, K. Bhanuprakash, J.W. Perry, Combined experimental and theoretical study of one- and two- photon absorption properties of D–p–A–p–D type bis(carbazolylfluorenylethynyl) arene derivatives: Influence of aromatic acceptor bridge, Dyes Pigm. 113 (2015) 682–691. [24] (a) Z.H. Li, M.S. Wong, Y. Tao, J.P. Lu, Diphenylamino end-capped oligofluorenes with enhanced functional properties for blue light emission: synthesis and structure-property relationships, Chem. Eur. J. 11 (2005) 3285–3293; (b) Z.H. Li, M.S. Wong, H. Fukutani, Y. Tao, Full emission color tuning in bisdipolar diphenylamino-endcapped oligoarylfluorenes, Chem. Mater. 17 (2005) 5032–5040; (c) Z.H. Li, M.S. Wong, Synthesis and functional properties of end-dendronized oligo(9, 9-diphenyl)fluorenes, Org. Lett. 8 (2006) 1499–1502; (d) Z.H. Li, M.S. Wong, H. Fukutani, Y. Tao, Synthesis and light-emitting properties of bipolar oligofluorenes containing triarylamine and 1 2,4-triazole moieties, Org. Lett. 8 (2006) 4271–4274; (e) Z.H. Li, M.S. Wong, Y. Tao, H. Fukutani, Ambipolar diphenylamino endcapped oligofluorenylthiophenes as excellent electron-transporting emitters, Org. Lett. 9 (2007) 3659–3662; (f) Z.Q. Gao, Z.H. Li, P.F. Xia, M.S. Wong, K.W. Cheah, Chin H. Chen, Efficient deep-blue organic light-emitting diodes: arylamine-substituted oligofluorenes, Adv. Funct. Mater. 17 (2007) 3194–3199. [25] W.-L. Yu, J. Pei, W. Huang, A.J. Heeger, Spiro-functionalized polyfluorene derivatives as blue light-emitting materials, Adv. Mater. 12 (2000) 828–831. [26] Y. Geng, D. Katsis, S.W. Culligan, J.J. Ou, S.H. Chen, L.J. Rothberg, Fully spiroconfigured terfluorenes as novel amorphous materials emitting blue light, Chem. Mater. 14 (2002) 463–470. [27] D. Katsis, Y. Geng, J.J. Ou, S.W. Culligan, A. Trajkovska, S.H. Chen, L.J. Rothberg, Spiro-linked ter-, penta-, and heptafluorenes as novel amorphous materials for blue light emission, Chem. Mater. 14 (2002) 1332–1339. [28] S. Setayesh, A.C. Grimsdale, T. Weil, V. Enkelmann, K. Müllen, F. Meghdadi, E.J. W. List, G. Leising, Polyfluorenes with polyphenylene dendron side chains: toward non-aggregating, light-emitting polymers, J. Am. Chem. Soc. 123 (2001) 946–953. [29] D. Marsitzky, R. Vestberg, P. Blainey, B.T. Tang, C.J. Hawker, K.R. Carter, Selfencapsulation of poly-2 7-fluorenes in a dendrimer matrix, J. Am. Chem. Soc. 123 (2001) 6965–6972. [30] (a) K.-T. Wong, Z.-J. Wang, Y.-Y. Chien, C.-L. Wang, Synthesis and properties of 9, 9-diarylfluorene-based triaryldiamines, Org. Lett. (2001) 2285–2288; (b) K.-T. Wong, Y.-Y. Chien, R.-T. Chen, C.-F. Wang, Y.-T. Lin, H.-H. Chiang, P.-Y. Hsieh, C.-C. Wu, C.-H. Chou, Y.-O. Su, G.-H. Lee, S.-M. Peng, Ter(9,9diarylfluorene)s: highly efficient blue Emitter with promising electrochemical and thermal stability, J. Am. Chem. Soc. 124 (2002) 11576–11577. [31] (a) T. Yamamoto, M. Nishiyama, Y. Koie, Tetrahedron Lett. 39 (1998) 2367; (b) J. Louie, J.F. Hartwig, A.J. Fry, Discrete high molecular weight triarylamine dendrimers prepared by palladium-catalyzed amination, J. Am. Chem. Soc. 119 (1997) 11695–11696; (c) B.H. Yang, S.L. Buchwald, Palladium-catalyzed amination of aryl halides and sulfonates, J. Organomet. Chem. 576 (1999) 125–146. [32] (a) A.R. Martin, Y. Yang, Palladium-catalyzed cross-coupling reactions of organoboronic acids with organic electrophiles, Acta Chem. Scand. 47 (1993) 221–230; (b) Z.H. Li, M.S. Wong, Y. Tao, M. D’Iorio, Synthesis and functional properties of strongly luminescent diphenylamino end-capped oligophenylenes, J. Org. Chem. 69 (2004) 921–927. [33] (a) N. Miyaura, T. Yanagi, A. Suzuki, The Palladium-catalyzed cross-coupling reaction of phenylboronic acid with haloarenes in the presence of bases, Synth. Commun. 11 (1981) 513–519; (b) N. Miyaura, A. Suzuki, Palladium-catalyzed cross-coupling reactions of organoboron compounds, Chem. Rev. 95 (1995) 2457–2483; (c) J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Arylaryl bond formation one century after the discovery of the Ullmann reaction, Chem. Rev. 102 (2002) 1359–1470.

58

Q. Feng et al. / Synthetic Metals 220 (2016) 48–58

[34] S.R. Wilson, L.A. Jacob, Iodination of aryltrimethylsilanes. A mild approach to (iodophenyl)alanine, J. Org. Chem. 51 (1986) 4833–4836. [35] S.-J. Chung, K.-S. Kim, T.-C. Lin, G.S. He, J. Swiatkiewicz, P.N. Prasad, Cooperative enhancement of two-photon absorption in multi-branched structures, J. Phys. Chem. B. 103 (1999) 10741–10745. [36] P. Macak, Y. Luo, H. Norman, H. Agren, Electronic and vibronic contributions to two-photon absorption of molecules with multi-branched structures, J. Chem. Phys. 113 (2000) 7055–7061. [37] J. Yoo, S.K. Yang, M.-Y. Jeong, H.C. Ahn, S.-J. Jeon, R.C. Bong, Bis-1 4- (pdiarylaminostryl)-2, 5-dicyanobenzene derivatives with large two-photon absorption cross-sections, Org. Lett. 5 (2003) 645–648.

[38] (a) M.A. Albota, C. Xu, W.W. Webb, Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm, Appl. Opt. 37 (1998) 7352–7356; (b) K.D. Belfield, M.V. Bondar, F.E. Hernandez, O.V. Przhonska, S. Yao, Twophoton absorption cross section determination for fluorene derivatives: analysis of the methodology and elucidation of the origin of the absorption processes, J. Phys. Chem. B 111 (2007) 12723–12729. [39] W.M. McClain, Two-photon molecular spectroscopy, Acc. Chem. Res. 7 (1974) 129–135.