Aggregation induced emissive carbazole-based push pull NLOphores: Synthesis, photophysical properties and DFT studies

Dyes and Pigments 124 (2016) 82e92

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Aggregation induced emissive carbazole-based push pull NLOphores: Synthesis, photophysical properties and DFT studies Sandip K. Lanke, Nagaiyan Sekar* Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT), Nathalal Parekh Marg, Matunga, Mumbai 400 019, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2015 Received in revised form 6 September 2015 Accepted 10 September 2015 Available online xxx

2-Alkoxy-9-methyl-9H-carbazole-3-carbaldehyde based push pull chromophore were efficiently synthesized by a multi-step reaction starting from 9H-carbazole-2-ol. The fluorescence properties of these DepeA chromophores were investigated in various solvents of varying polarities and in mixed solvent solutions of DMF and H2O. They exhibited low fluorescent intensity in solution but a high fluorescent intensity in aggregate forms and in their solid state due to the promising aggregation-induced emission enhancement characteristics. These dyes were fully characterized by FT-IR, 1H NMR and HRMS spectra. The ratio of ground to excited state dipole moment of the novel push pull chromophore were calculated by Bakhshiev and BiloteKawski correlations. All the four dyes show more or less twisted intramolecular charge transfer (TICT) characteristics. They show a large first hyperpolarizability value ranging from 189e721
1030 esu by theoretical method and 73e330
1030 esu by experimental method. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Aggregation induced emission Photophysical properties Twisted intramolecular charge transfer (TICT) Solvatochromism Polarizability Density functional theory (DFT)

1. Introduction Designing and synthesizing organic fluorophore with excellent optical and electronic properties have long been topics of interest of many researchers [1]. However when organic fluorophores are to be used in their solid states in which the molecules form aggregates, their emissions suffer from the “aggregation-caused quenching” (ACQ) effect due to strong pp stacking interactions in extended p-conjugated systems and dipoleedipole interactions in photoinduced DA charge transfer systems as exemplified by fluorescein [2]. The photophysical behaviour of the molecule is mainly affected by the structure property relationships such as planarity and rotatability, intramolecular restriction, intermolecular interactions, weak excimer formation and ACQ-to-AIE (Aggregation Induced Emission) transformation [3]. ACQ is theoretically well understood phenomenon but it is a harmful photophysical effect in terms of practical applications. There has been huge efforts directed to solve the ACQ problem by impeding chromophore aggregation, but it ended with limited success. The first report on Aggregation Induced Emission (AIE) which is totally

* Corresponding author. Tel.: þ91 22 3361 1111, þ91 22 3361 2222, þ91 22 3361 2707; fax: þ91 22 3361 1020. E-mail addresses: [email protected], [email protected] (N. Sekar). http://dx.doi.org/10.1016/j.dyepig.2015.09.013 0143-7208/© 2015 Elsevier Ltd. All rights reserved.

contrary to the ACQ phenomenon by Tang's group appeared in 2001 [4]. The important observation for the compounds showing aggregation induced emission enhancement (AIEE) characteristics that they exhibit weak luminescence or have almost no emissions in molecularly dissolved states, but their emission can be switched on when these molecules aggregate in concentrated solutions/solid states [4]. AIE based dyes are successfully utilized for a number of practical applications such as chemical sensors [5,6], fluorescent probes [7,8], live cell bioimaging [9e11] and OLEDs [12,13]. Number of fluorescent organic dyes showing AIE properties are well known, and they include arylethene derivatives [14], siloles [15e18], thienylazulene [19], pentacenequinone [20], salicylaldehyde azine derivatives [21], BODIPY dyes [22e24], ESIPT dyes [25,26] and isophorone based dyes [27]. However, in spite of the remarkable progress made in the synthesis of AIE molecules over the last decades, the DepeA based AIEE dyes are still less developed. Therefore exploration of new AIEE fluorophores is an exciting field to a synthetic chemist. Heterocycles can be used to enhance the stability of conjugated molecules. So after screening of several known fluorescent dyes, in this work we have investigated 2methoxy/2-ethoxy carbazole based pushepull AIE dyes. Carbazole is common heterocyclic compound containing electron donor nitrogen atom with interesting photo- and electro-chemistries and large number of derivatives can be synthesized by simple modification at 3, 6 and 9 positions on the carbazole core.

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

Carbazole derivatives have attracted much attention because of their interesting photochemical properties such as high fluorescence quantum yield, excellent photostability and sharp absorption and emission spectra. Carbazole derivatives have high triplet energy, excellent hole transporting ability, high luminescence efficiency and due to the rigid framework are oftenly used in organic light emitting diodes [28e30], dye sensitized solar cells [31e33], liquid crystals [34,35] and laser dyes [36]. Due to the promising photophysical properties of carbazole, several carbazole derivatives containing AIE luminogens with high thermal stability have been developed [37e39]. Studies on AIE emissive carbazole derivative with large third order nonlinear optical properties are reported by Yuliang Li and coworkers [40]. AIE carbazole derivatives have garnered much attention from many research groups because of their fundamental importance and practical applications in optoelectronic devices such as organic light-emitting diodes (OLEDs) [41], field effect transistors, live-cell imaging and fluorescent sensors [1,42,43]. In this paper we have synthesized N-alkyl carbazole derivatives with 2 methoxy/2-ethoxy substituents. This 2 alkoxy substituted carbazole moiety were used as a donor and cyano group at terminal position as an acceptor which can exhibit high values of dipole moment and hyperpolarizability. They are desired attributes for nonlinear optics and luminescent chromophores. Due to introduction of methoxy/ethoxy group in ortho-position towards the pbridge can more effectively conjugate with the p-system with the donation of lone pair electrons. The molecular design strategy is based on the structural changes like surrounding electron-donor and acceptor group position manipulation and its capability of effective intermolecular charge transfer (ICT) characteristics [44,45]. In addition, electron transfer or electron separation between the electron donor carbazole group and the accepting group like 2-(1phenylethylidene)malononitrile and 2-(3,5,5-trimethylcyclohex-2enylidene)malononitrile provides this class of compounds highly anisotropic structures and results in interesting photophysical properties [46]. 2. Experimental 2.1. Computational strategy All the computations were performed with Gaussian 09 package [47]. DFT method was used for the ground state optimization, while for the excited state optimization, time-dependent density functional theory (TD-DFT) was employed. The hybrid functional namely B3LYP (Becke3-Lee-Yang-Parr hybrid functional) [48e51] was used. The 6-31G(d) basis set was used for all atoms and later was ascertained in the literature [52]. The Polarizable Continuum Model (PCM) [53] was used to optimize the ground and excited state geometries in solvent environments. The excitation energies, oscillator strengths and orbital contribution for the lowest 10 singletesinglet transitions at the optimized geometry in the ground state were obtained by TD-DFT calculations using the same basis set as for the geometry minimization. The solvents used were toluene, tetrahydrofuran (THF), chloroform (CHCl3), dichloromethane (DCM), ethyl acetate (EtOAc), ethanol (EtOH), acetonitrile (ACN), N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO). 2.2. Materials and equipments All the reagents were obtained from S. D. Fine Chemicals (India), Sigma Aldrich and used as supplied without any further purification. All the solvents were of spectroscopic grade. The active methylene intermediates 2-(1-phenylethylidene)malononitrile (4a) and 2-(3,5,5-trimethylcyclohex-2-enylidene)- malononitrile

83

(4b) was prepared by the reported methods [54,55]. The reaction was monitored by thin layered chromatography (TLC) using 0.25 mm E-Merck silica gel 60 F254 precoated plates, which were visualized under UV light. Melting points were measured on standard melting point apparatus from Sunder industrial product, Mumbai and are uncorrected. The FT-IR spectra were recorded on PerkinseElmer 257 spectrometer using KBr discs. 1H NMR and 13C NMR spectra were recorded on VARIAN 500-MHz instrument (USA) using CDCl3 as solvent. The chemical shifts were reported in parts per million (ppm) relative to internal standard tetramethyl silane (TMS) (0 ppm) and coupling constants in Hz. Splitting patterns are described as singlet (s), doublet (d), triplet (t), quartet (q), or multiplet (m). Mass spectrometry was performed on a quadrupole/ time-of-flight tandem mass spectrometer (ESI) and recorded on Waters mass spectrometer. The visible absorption spectra of the dyes were recorded on a PerkineElmer Lambda 25 UVeVisible spectrophotometer. The quantum yields of dyes 5 and 6 in different solvents of varying polarities were calculated using Rhodamine 6G as reference standard (f ¼ 0.94 in ethanol, lexc ¼ 488 nm) [56]. 2.3. Synthesis and characterization 2.3.1. Synthesis of intermediates 2a, 2b and 3a The detailed experimental procedures for the synthesis of reported intermediates 2a, 2b and 3a are provided in the supporting information. 2.3.2. 2-ethoxy-9-ethyl-9H-carbazole-3-carbaldehyde (3b) POCl3 (4.9 mL, 52.00 mmol) was added drop wise to DMF (17 mL, 219.53 mmol) at 0e5  C, and stirred for 30 min maintaining the temperature 0e5  C. 2-ethoxy-9-ethyl-9H-carbazole (11.24 g, 47.00 mmol) was dissolved in 20 mL DMF was added dropwise within 30 min maintaining temperature between 0 and 5  C. Stirring was continued for next 20e30 min, reaction mixture was then brought to room temperature and heated at 70e75  C for 1 h. Completion of the reaction was monitored by TLC. The obtained reaction mass was poured into crushed ice stirred well and neutralized with sodium bicarbonate. The precipitate obtained was filtered off and dried. The crude product was purified by column chromatography on silica 100e200 mesh and using toluene as eluent. Yield 8.78 g (70%) m. p. 138e140  C (recrystallized in ethanol). 1 H NMR (CDCl3, 500 MHz) ¼ d 9.66 (s, 1H), 8.53 (s, 1H), 8.14 (d, 1H, J ¼ 7.5 Hz), 7.83e7.85 (t, 2H, J ¼ 7.5 & 8 Hz), 7.39 (d, J ¼ 8.0 Hz, 1H), 7.00 (s, 1H), 4.30 (q, J ¼ 7.2 Hz, 2H), 4.11 (q, J ¼ 6.9 Hz, 2H), 1.38 (t, J ¼ 6.9 Hz, 3H), 1.32 (t, J ¼ 7.2 Hz, 3H). HRMS m/z [M þ H]þ Calcd for C17H18NO2: 268.1338. Found: 268.1372. 2.3.3. (E)-2-(3-(2-Methoxy-9-methyl-9H-carbazol-3-yl)-1phenylallylide)malononitrile (5a) Under nitrogen atmosphere, 2-methoxy-9-methyl-9H-carbazole-3-carbaldehyde (2.39 g, 10 mmol) and 2-(1-phenylethylidene) malononitrile (1.68 g, 10 mmol) were dissolved in absolute ethanol (100 mL). 0.1 mL of piperidine was added, and the solution was stirred at reflux temperature for 24 h. After cooling the reaction mixture, the red solid was filtered, washed with methanol and dried. The crude product was further purified by column chromatography (Petroleum Ether: Ethyl Acetate (80:20): yield (2.91 g, 75%), 260e262  C. 1 H NMR (500 MHz, CDCl3) d 3.81 (s, 3H, NCH3), 3.97 (s, 3H, OCH3), 6.74 (s, 1H, ArH), 7.29 (d, J ¼ 7.8 Hz, 1H, ArH), 7.35 (d, J ¼ 6.2 Hz, 1H, ArH), 7.38e7.37 (m, 1H, ArH), 7.48e7.41 (m, 3H, ArH), 7.61e7.53 (m, 3H, ArH), 7.88 (d, J ¼ 15.5 Hz, 1H, ¼CH), 8.02 (d, J ¼ 7.7 Hz, 1H), 8.18 (s, 1H, ArH). 13C NMR (126 MHz, CDCl3) 29.4,

84

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

55.9, 90.8, 108.8, 113.9, 114.5, 116.5, 117.3, 120.0, 120.5, 122.3, 122.6, 125.6, 128.8, 129.0, 130.7, 133.7, 141.6, 144.7, 146.9, 159.1, 172.6. HRMS m/z [M þ H]þ Calcd for C26H20N3O: 390.1606. Found: 390.1571. 2.3.4. (E)-2-(3-(2-Ethoxy-9-ethyl-9H-carbazol-3-yl)-1phenylallylide)malononitrile (5b) Under nitrogen atmosphere, 2-ethoxy-9-ethyl-9H-carbazole-3carbaldehyde (2.67 g, 10 mmol) and 2-(1-phenylethylidene)malononitrile (1.68 g, 10 mmol) were dissolved in absolute ethanol (100 mL). 0.1 mL of piperidine were added, and the solution was stirred at reflux temperature for 24 h. After cooling the reaction mixture, the red solid was filtered, washed with methanol and dried. The crude product was further purified by column chromatography (Petroleum Ether: Ethyl Acetate (80:20): yield (3.25 g, 78%), 210e212  C. 1 H NMR (500 MHz, CDCl3) d 8.09 (s, 1H), 7.98 (t, 2H), 7.61e7.53 (m, 3H), 7.47e7.40 (m, 3H), 7.37 (d, J ¼ 8.1 Hz, 1H), 7.32 (d, J ¼ 15.4 Hz, 1H), 7.27 (dd, 1H), 6.75 (s, 1H), 4.30 (q, J ¼ 7.2 Hz, 2H), 4.19 (q, J ¼ 6.9 Hz, 2H), 1.53 (t, J ¼ 6.9 Hz, 3H), 1.44 (t, J ¼ 7.2 Hz, 3H). 13 C NMR (126 MHz, CDCl3) 13.6, 14.7, 37.8, 64.7, 78.1, 91.5, 108.8, 113.9, 114.6, 116.4, 117.4, 120.1, 120.3, 122.5, 123.0, 123.8, 125.5, 128.7, 129.1, 130.6, 133.9, 140.5, 143.7, 147.8, 158.8, 173.0. HRMS m/z [M þ H]þ Calcd for C27H26N3O: 408.2076. Found:.408.2046.

2.3.6. (E)-2-(3-(2-(2-Ethoxy-9-ethyl-9H-carbazol-3-yl)vinyl)5,5dimethylcylcohex-2-en-1-ylidene)malononitrile (6b) Under nitrogen atmosphere, 2-ethoxy-9-ethyl-9H-carbazole-3carbaldehyde (2.67 g, 10 mmol) and (E)-ethyl 2-cyano-2-(3,5,5trimethylcyclohex-2-en-1-ylidene)acetate (2.33 g, 10 mmol) were dissolved in absolute ethanol (100 mL). 0.1 mL of piperidine was added, and the solution was stirred at reflux temperature for 24 h. After cooling the reaction mixture, the red solid was filtered, washed with methanol and dried. The crude product was further purified by column chromatography (Petroleum Ether: Ethyl Acetate (80:20): yield (3.18 g, 73%), 232e234  C. 1 H NMR (500 MHz, CDCl3) d 8.27 (s, 1H), 8.04 (d, J ¼ 7.6 Hz, 1H), 7.65 (d, J ¼ 16.1 Hz, 1H), 7.46e7.42 (m, 1H), 7.37 (d, J ¼ 8.1 Hz, 1H), 7.29e7.25 (m, 1H), 7.15 (d, J ¼ 16.1 Hz, 1H), 6.79 (s, 2H), 4.32 (q, J ¼ 7.2 Hz, 2H), 4.26 (q, J ¼ 7.0 Hz, 2H), 2.54 (s, 2H), 2.51 (s, 2H), 1.60 (t, J ¼ 7.0 Hz, 3H), 1.45 (t, J ¼ 7.2 Hz, 3H), 1.08 (s, 6H). 13 C NMR (126 MHz, CDCl3) d 13.7, 14.8, 28.1, 32.0, 37.7, 39.12, 43.02, 64.5, 91.6, 108.6, 113.4, 114.2, 117.2, 117.6, 119.6, 119.9, 121.9, 123.1, 125.2, 126.7, 133.8, 140.4, 142.5, 155.8, 157.2, 169.4. HRMS m/z [M þ H]þ Calcd for C29H30N3O: 436.2389. Found:436.2314. 3. Result and discussion 3.1. Design and synthesis

2.3.5. (E)-2-(3-(2-(2-Methoxy-9-methyl-9H-carbazol-3-yl)vinyl) 5,5-dimethylcylcohex-2-en-1-ylidene)malononitrile (6a) Under nitrogen atmosphere, 2-methoxy-9-methyl-9H-carbazole-3-carbaldehyde (2.39 g, 10 mmol) and (E)-ethyl 2-cyano-2(3,5,5-trimethylcyclohex-2-en-1-ylidene)acetate (2.33 g, 10 mmol) were dissolved in absolute ethanol (100 mL). 0.1 mL of piperidine was added, and the solution was stirred at reflux temperature for 24 h. After cooling the reaction mixture, the red solid was filtered, washed with methanol and dried. The crude product was further purified by column chromatography (Petroleum Ether: Ethyl Acetate (80:20): yield (2.93 g, 72%), 236e238  C. 1 H NMR (500 MHz, CDCl3) d 8.26 (s, 1H), 8.03 (d, J ¼ 7.7 Hz, 1H), 7.63 (d, J ¼ 16.1 Hz, 1H), 7.44 (m, 1H), 7.36 (d, J ¼ 8.1 Hz, 1H), 7.28 (d, J ¼ 7.9 Hz, 2H), 7.13 (d, J ¼ 16.1 Hz, 1H), 6.80 (s, 2H), 4.05 (s, 3H), 3.83 (s, 3H), 2.53 (s, 2H), 2.51 (s, 2H), 1.08 (s, 6H). 13 C NMR (126 MHz, CDCl3) d 28.0, 29.3, 32.0, 39.2, 43.0, 55.9, 90.8, 108.6, 113.3, 114.1, 119.4, 119.7, 119.9, 122.0, 122.9, 125.2, 126.8, 133.6, 141.4, 143.5, 155.7, 157.8, 169.4. HRMS m/z [M þ H]þ Calcd for C28H24N3O: 418.1919. Found:.418.1862.

To enrich the aggregation induced emission research and to elaborate its practical applications to real word, we have synthesized four new AIE luminogens and devised a multistep reaction route for their synthesis (Scheme 1). 2-Methoxy-9-methyl-9H-carbazole (2a) and 2-ethoxy-9-ethyl-9H-carbazole (2b) were obtained from commercially available 9H-carbazol-2-ol (1) by simple methylation and ethylation using methyl iodide and ethyl iodide respectively. Further 2-methoxy-9-methyl-9H-carbazole-3-carbaldehyde (3a) and 2-ethoxy-9-ethyl-9H-carbazole-3-carbaldehyde (3b) were synthesized from the intermediate 2a and 2b via a VilsmeiereHaack reaction with phosphorus oxychloride and dry DMF. Isophorone, malononitrile, and acetophenone were available commercially. The compounds 2-(1-phenylethylidene)malononitrile (4a) and 2-(3,5,5trimethylcyclohex-2- enylidene)malononitrile (4b) were synthesized by reported procedures [54,55]. The dyes 5a, 5b, 6a and 6b were synthesized by using absolute ethanol and only piperidine as a catalyst for the Knoevenagel condensation. All the intermediates and final products were carefully purified and fully characterized by IR, 1H NMR, 13C NMR and HRMS spectroscopies, which confirmed

Scheme 1. Synthesis of carbazole based fluorophores 5 and 6.

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

ε (M cm )

40000 35000 30000 25000 20000 15000 10000 5000

Toluene THF EA CHCl3 DCM Acetonitrile EtOH DMF DMSO

300 250 Emission Intensity ( a. u.)

Toluene THF EA CHCl3 DCM Acetonitrile EtOH DMF DMSO

45000

85

200 150 100 50 0

0

450

300 340 380 420 460 500 540 580 Wavelength (nm)

500

550

600

650

700

750

Wavelength (nm)

Fig. 1. Absorption and emission spectra of dye 5b in solvents of varying polarities, lexc ¼ 488 nm (slit width: 5 nm/5 nm).

dyes 5a, 5b and 6b are similar to that of dye 6a. This indicates that the dyes show positive solvotochromism. The representative absorption and photoluminescence (PL) spectra of dye 5b in 9 solvents at a concentration c ¼ 5
106 mol L1 are shown in Fig. 1 and data for dye 5b are summarized in Tables 1 and 2. In general, the extension of the p-systems and the strong electron donating ability exerts an important influence on the absorption spectra. For example, the absorption maximum of dye 6a at 468 nm was red-shifted by 7 nm in toluene and 13 nm in DMSO relative to that of dye 5a by changing the electron withdrawing substituent from 2-(1-phenylethylidene)malononitrile (4a) to 2(3,5,5-trimethylcyclohex-2- enylidene)malononitrile (4b). The use of 4b leads to an elongation of conjugate chain as compared to that of 4a. They shift the emission bands very strongly to the red spectral region and showed very high Stokes shifts of around 104e150 nm. By changing N-methyl to N-ethyl group of carbazole core lead to change in absorption spectra by 1e10 nm. The absorption as well as emission spectra of dyes 5a, 5b, 6a and 6b are shown in Fig. S1 and the photophysical data are summarized in Tables S1 and S2. All the carbazole “pushepull” derivatives showed broad and structureless fluorescence spectra, which were completely different from those of the relevant model compounds. Dye 6b showed an emission maxima in the range of 578e641 nm. Dye 6b showed emission minimum 578 nm in toluene and maximum in DMSO at 641 nm. All the synthesized dyes 5a, 5b, 6a, and 6b showed solid state emission maxima under long UV light (365 nm). All extended pushepull derivative of carbazole were excited at 488 nm keeping all the spectroscopic parameters constant like source of light, slit

their expected molecular structures. Luminogens were soluble in common organic solvents, such as toluene, tetrahydrofuran (THF), ethyl acetate (EA), chloroform, dichloromethane (DCM), acetonitrile, N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) but were insoluble in water. 3.2. Photophysical properties The important characteristics of fluorescent molecules are absorption wavelength, emission wavelength, quantum yield, oscillator strength, fluorescence lifetime and dipole moment. To evaluate the effect of solvent polarity on absorptioneemission properties of synthesized dyes, the dyes 5ae5b and 6ae6b were tested in different solvents of varying polarity. Nine solvents were tested for the effect of solvents on their absorption-emission characteristics. All the spectroscopic studies were carried out at room temperature and the concentration of the solutions were 5
106 mol L1. All the four dyes showed the shortest absorption wavelength in ethyl acetate solution and the longest in DMSO. All these dyes showed absorption spectra in the range of 458e491 nm. These dyes showed their absorption maximum with the following order: 5a (461 nm) > 5b (462 nm) > 6a (468 nm) > 6b (474 nm) in toluene. From toluene to DMSO the dye 5a provides 15 nm red shift in the absorption band. Upon increasing the solvent polarity from toluene to DMSO, as shown in Tables S1 and S2 and Fig. S1, the absorption maxima and the Stokes shift of dyes 5a, 5b, 6a and 6b showed monotonically increasing tendency, especially for dyes 6a and 6b. The Stokes shift of dye 6a increases from 108 nm in toluene up to 150 nm in polar solvent like DMSO, and the behaviours of

Table 1 Effect of solvent polarity on photophysical properties of dye 5b.a

Dῡ (cm1)

Solvent

labsb (nm)

lemc (nm)

Dῡd (nm)

e

Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO

462 462 458 468 470 462 468 471 476

522 541 537 558 550 569 575 573 575

60 79 79 90 80 107 107 102 99

2488 3161 3212 3446 3095 4070 3976 3779 3617

a b c d e f g

Analysis were carried out at room temperature (25  C). labs ¼ absorption maximum. lem ¼ emission maximum. Dῡ ¼ Stokes shift (nm). Dῡ ¼ Stokes shift (cm1). s ¼ absorption cross section. FF ¼ quantum yield estimated using Rhodamine 6G as the standard.

ῡa þ ῡf (cm1)

s (cm2)
1014 f

FF g

40,802 40,129 40,456 39,289 39,458 39,220 38,759 38,683 38,400

1.1459 1.3128 1.4742 1.5592 1.0149 1.3125 1.4519 1.4006 1.0808

0.006 0.010 0.011 0.051 0.048 0.045 0.057 0.088 0.139

± ± ± ± ± ± ± ± ±

0.002 0.001 0.003 0.002 0.001 0.003 0.003 0.003 0.005

86

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

Table 2 Photophysical data of the dye 5b in solvents of varying polarities.a Solvent

ε (
104)

Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO

2.9957 3.4323 3.8541 4.0763 2.6534 3.4313 3.7959 3.6617 2.8255

a b c d e f g h i j

b

(M1 cm1)

Dῡ1/2 c (cm1)

Dῡ1/2 d (cm1)

t0 e (ns)

tf f (ns)

g

h

3688 3843 3672 3665 3744 3803 3855 3922 3964

3155 2707 2640 2491 2536 2466 2424 2406 2365

6.76 5.66 5.19 5.13 7.78 5.72 5.24 5.41 7.08

0.04 0.06 0.06 0.26 0.37 0.26 0.30 0.48 0.98

1.50 1.67 1.83 1.96 1.30 1.73 1.90 1.83 1.42

249 165 165 36.5 25.7 36.7 31.4 19.0 8.79

knr
108 (s1)

i

f

0.5307 0.6156 0.6588 0.4751 0.6943 0.6152 0.6785 0.6784 0.5323

jm

ge

(Debye)

7.22 7.78 8.01 6.88 8.33 7.78 8.22 8.24 7.34

Analysis were carried out at room temperature (25  C). ε ¼ molar extinction coefficient. Dῡ1/2 ¼ full width half maxima of the absorption band. Dῡi1/2 ¼ full width half maxima of the emission band. t0 ¼ theoretical radiative lifetimes. tf ¼ theoretical fluorescence lifetimes. kf ¼ fluorescence rate constant (kf ¼ FF/tf). knr ¼ Rate constant for nonradiative decay [knr ¼ (1FF)/tf)]. f ¼ oscillator strength. m ge ¼ transition dipole moment.

width, sample holder. Dyes 5a, 5b, 6a and 6b showed solid state emissions at 593, 629, 656 and 665 nm (Fig. 2). All these dyes showed more red shifted emission to their corresponding emission in solutions. The dyes 5a, 5b, 6a and 6b showed red shift of 20, 57, 24 and 30 nm in solid state as compared to their corresponding emission in DMF. This is because of dyes aggregate in the solid state and resulted into bathochromic shift. 3.3. Determination of dipole moment and ICT characteristics We studied solvent dependences of the spectroscopic and photophysical properties of the pushepull carbazole derivatives to evaluate the roles of the strong electron-donating and electron accepting group in the ground and excited-state characteristics. These pushepull carbazole analogues showed remarkable effect of solvent environment on their photophysical properties. All the four dyes 5a, 5b, 6a and 6b showed bathochromic shifts of DepeA charge transfer absorption band with an increase in the solvent polarity from toluene to DMSO suggesting larger stabilization of the excited-state energy relative to the stabilization energy in the ground state. To investigate the effect of solvent environment on the molecular and electronic structures of 5a-5b and 6ae6b, we evaluated the ground- (mg) and excited state dipole moments (me) of the derivatives. The ratio of the ground to excited state dipole moment of the synthesized novel push pull carbazole dyes 5a-5b and 6a-6b calculated by using BiloteKawski correlations [57e60]. The value for dielectric constant, refractive index, slope for the

140

Emission Intensity (a.u.)

kf
108 (s1)

5a ( 593 nm )

120

5b ( 656 nm )

100

6a ( 629 nm ) 6b ( 665 nm )

80

graph, regression coefficient are listed in Tables S3 and S4 and Fig. S2. Table 3 reveals that the ratio me =mg is more than unity, which implies that the excited state is more polar than the ground state. We have calculated dipole moment ratio me =mg computationally for better understanding of the excited state behaviour. We found that the dye 6a and 6b showed me =mg ratio more than unity. In case of dye 5a for polar solvent it was more than unity and for non polar solvent it was closed to unity. Moreover dye 5b showed ratio close to unity (Table S5). Which also supports that the excited state is more polar than the ground state. More quantitative analysis of the fluorescence solvatochromism can be carried out using Weller's equation. Weller equation is modified version of the LippertMataga equation, which allows the excited state dipole moment (me) to be estimated [61,62]. Here, the molecules show more charge transfer characteristics in the excited state hence the above described theories cannot be applied because the absorption spectrum of the charge transfer state is not known. Hence, in this case Weller's theory for exciplexes is more appropriate and only deals with the shift of the fluorescence spectrum [63]. In the Weller plot we have observed the regression coefficient is 0.3024, 0.8206, 0.8298 and 0.8273 for the dyes 5a, 5b, 6a and 6b respectively (Fig. 3b) which reveals that the molecules are showing more charge transfer characteristics in the S1 state. But to know more insight into the excited state that whether this molecules are showing intramolecular charge transfer characteristics or twisted intramolecular charge transfer we have used Rettig equation, which is the plot of emission wavenumber of emission versus Rettig function [63,64]. To understand TICT behaviour we have plotted Rettig's plot (Fig. 3c) and it is observed that the dyes show linear relationship with most of the nonpolar to polar solvents and the correlation coefficients are 0.4031, 0.8705, 0.8824 and 0.8778 for dyes 5a, 5b, 6a, and 6b respectively and from these observation it is confirmed that dye 6a has highest regression coefficient than other dyes and it shows more TICT character. So

60 Table 3 Excited state (me) and ground state (mg) dipole moment (in Debye) ratio value for dyes 5a, 5b, 6a and 6b.

40 20 0 550

600

650

700

750

Wavelength (nm) Fig. 2. Solid state emission spectra of dyes 5a-5b and 6a-6b.

800

Dye

jm1 þ m2j

jm2 e m1j

me mg

5a 5b 6a 6b

1210 3669 2886 2975

154 241 576 215

8 15 5 14

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87

Fig. 3. a) Correlation of Stokes shift versus EN T scale b) Weller's plot and c) Rettig's plot for the dyes 5a, 5b, 6a and 6b in selected solvents.

from the above observations we concluded that all the dyes show more or less twisted intramolecular charge transfer (TICT) characteristics. 3.4. Aggregation-induced enhanced emission (AIEE) properties The AIEE characteristics of the dyes 5a, 5b, 6a, and 6b were investigated in a mixture of DMF and water (Figs. 4 and 5). In DMF dyes 5a and 5b, 6a and 6b were well-dispersed and displayed weak fluorescence emissions in their solution state. To determine whether dyes 5a-5b and 6a-6b have AIEE characteristics, the

fluorescence spectra of dyes were measured in a series of DMF/ water mixtures with different volume fractions of water, since the dyes 5a, 5b, 6a and 6b were soluble in DMF but not in water (Fig. 4). We added different amounts of water, a poor solvent for the luminogens, to the pure DMF solutions by defining the water fractions (fw) of 0e90% and then monitored the change in the emission wavelength with the excitation wavelengths of 488 nm respectively. The concentration was maintained at 5
106 mol L1 Fig. 4 reveals that all dyes show higher emission intensity when water fractions (fw) was 90%. However it is noteworthy that with a gradual addition of water into the solution of dye 5a in DMF, the

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5a

180

PL Intensity (a.u.)

140 120 100 80 60 40 20

5b

250

0% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90%

160

PL Intensity (a.u)

88

0% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90%

200 150 100 50 0

0 500

600

430

700

480

Wavelength (nm)

PL Intensity (a.u.)

100 80 60 40 20 650

700

750

730

780

0% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90%

60 50 40 30 20 10

0 600

680

70

PL Intensity (a.u.)

120

550

630

6b

80 0% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90%

500

580

Wavelength (nm)

6a

140

530

0

800

500

Wavelength (nm)

550

600

650

700

750

800

Wavelength (nm)

Fig. 4. Emission spectra of dyes 5a, 5b, 6a and 6b in DMF/H2O mixtures with different water fractions (5
106 M), lexc ¼ 488 nm (slit width: 5 nm/5 nm).

595 590

1.00

585

0.80 580

0.60 0.40

575

0.20

570

0.00

0.00

I/I0

665

1.20

660

1.00

650

0.80

645

0.60

640

0.40 0.20

635

0.00

630 0 10 20 30 40 50 60 70 80 90 100

fw (%)

576

0.20

572

fw (%)

655

1.00

580

0.40

6b

670 665 660

0.80

I/I0

1.20

584

0.60

0 10 20 30 40 50 60 70 80 90 100

Emission Wavelength (nm)

6a

588

0.80

fw (%)

1.40

596 592

1.00

0 10 20 30 40 50 60 70 80 90 100

1.60

5b

1.20

655 650

0.60

645

0.40

640 0.20

635 630

0.00

Emission Wavelength (nm)

I/I0

1.20

1.40

I/I0

5a

1.40

Emission Wavelength (nm)

1.60

other dyes. The emission intensity of dye 6a was higher when water fractions (fw) was 90%. It is notable that the emission wavelength decrease slowly with when water fraction increases form 0e40 vol %, but increases sharply when fw was 50% again from 60% to 90% the

Emission Wavelength (nm)

emission intensity of 5a was dramatically weakened and the emission wavelength was red shifted to 583 nm when fw was 0e60 vol% but when fw >60%, emission intensity increase suddenly with red shifted emission. Similar observation are seen for the

0 10 20 30 40 50 60 70 80 90 100

fw (%)

Fig. 5. Plot of water fraction versus ratiometric fluorescence intensity (blue line) and water fraction versus emission wavelength (red line) of dyes 5a, 5b, 6a and 6b in DMF/H2O mixtures with different water fractions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

89

C32eC33 (0.044 Å), C36eN37 (0.006 Å), and bond shortening was observed for bond N19eC4 (0.034 Å), C3eC2 (0.012 Å), C1eC6 (0.018 Å), C5eC16 (0.038 Å), C15eC14 (0.028 Å), C21eC32 (0.007 Å), C33eC36(0.012 Å). This indicated that the dye shows effective charge transfer from donor nitrogen atom of carbazole core to acceptor cyano vinyl group. 3.6. Electronic vertical excitation and emission spectra (TDDFT)

Fig. 6. (a) Photograph of dyes 5a, 5b, 6a and 6b in daylight containing DMF:H2O (Vol %). (b) Photograph of coumarin dyes 5a, 5b, 6a and 6b in UV-light containing DMF:H2O (Vol%).

emission intensity increase slowly with red shift in emission. Similar enhancement was observed in the behaviours of dye 5b and dye 6b. The emission intensity of dye 5b was significantly quenched at the initial stage from fw ¼ 0 to 40 vol%. Meanwhile, the emission wavelength of dye 5b in the solvent mixture with fw ¼ 40 vol% was red-shifted to 580 nm, compared with that in pure DMF solution. It is noteworthy that fluorescence intensity of dye 6b dramatically recovers and intensifies with increasing fw from 50 to 90%, revealing the AIE phenomenon of fluorophores. The day light and UV light photographs of dyes 5a, 5b, 6a and 6b are shown in Fig. 6. The quantum yields of the dyes 5a, 5b, 6a and 6b are calculated in different water fraction (Fig. S4). All the synthesized dyes show highest quantum yields in 90% DMF:H2O mixture. 3.5. Optimized geometries of dyes 5a, 5b, 5c and 5d The optimized structure of dye 5a in chloroform solvent are shown in Fig. 7. The excited state optimized geometry reveals that the major bond lengthening was observed between the bonds C3eC4 (0.005 Å), C1eC2 (0.024 Å), C6eC5 (0.017 Å), C16eC15 (0.036 Å), C14eC20 (0.03 Å), N19eC11 (0.027 Å), C20eC21 (0.002 Å),

The optimized structures of dyes at the ground state in solvents of various polarities were subjected to TD. At least 10 excited states were calculated for each molecule. The computed energies of the vertical excitations, oscillator strength and their orbital contributions are listed in Table S6. The results of DFT and TD-DFT suggest that more influence of the change in solvent polarity was observed on the absorption spectra of the carbazole push pull dyes. The computed absorption values for these dyes are in better agreement with the experimental values. The dyes 5a and 6a show 0.65e2.34% deviation in all the solvent. However the dyes 6a and 6b show more deviation of about 2.81e7.04% as compared to the dyes 5a and 6a. In case of emission spectra dye 5a and 5b show two verticle emission closer to the experimental emission. The longer wavelength show 1.1e11% deviation with lower oscillator strength (f ¼ 0.008 to 0.5690) and shorter wavelength show 11.6e18.8% with higher oscillator strength (f ¼ 0.5610 to 0.9111) relative to experimental emission. While dye 6a and 6b shows single wavelength which is closer to the experimental result. The computed emission show 9.7e17.2% deviation relative to experimentally calculated wavelength. The computed energies of the vertical emission, oscillator strength and their orbital contributions are listed in Table S7. 3.7. Frontier molecular orbitals (FMO) The electronic distribution in the frontier molecular orbitals HOMO-1, HOMO, LUMO and LUMOþ1 for the dye 5a, 5b, 6a and 6b are shown in Fig. 8 and their respective energy values are summarized in Table S8. The HOMO and LUMO energy level diagram gives quantitative idea of electronic structure and excitation properties. Due to the increase in p conjugation there is a lowering of both HOMO and LUMO energies. The results indicate that calculated band gaps of the dyes 5a, 5b, 6a and 6b in the gas phase are 3.10, 3.08, 2.83 and 2.82 eV respectively and in toluene 3.01, 3.00, 2.75 and 2.74 eV while in DMF it significantly lowers upto

Fig. 7. Optimized geometry parameters of dye 5a in CHCl3 solvent in the ground state and excited state (bond length are in Å, dihedral angles are in degree).

90

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Fig. 8. Frontier molecular orbitals of dyes 5a, 5b, 6a and 6b in the ground state in CHCl3 solvent.

2.91, 2.90, 2.67 and 2.66 eV respectively. A similar trend was observed for non polar to polar solvent. The results reveal that increasing extension of the p-systems and electron-donating ability of the donors both can lower the band gaps obviously. As shown in Fig. 8 the occupied HOMO of these dyes is delocalized over the carbazole moiety, with maximum components arising from the

nitrogen lone pair and oxygen lone pair of methoxy/ethoxy group and is extended along the bridge to the central region of the pushepull dyes. In the LUMO orbital, which also has p-character, there is no significant contribution from the carbazole segment and the electron density has shifted toward the acceptor dicyanovinyl moiety.

Table 4 Measured linear and non-linear optical properties of dyes 5a, 5b, 6a and 6b in various solvents. Dye

Solvent

aa (Å)

fb

yge c (cm1)

mge d (D)

axx e (1024 cm3)

bxxx f (1030 cm5 esu1)

mbxxx (1048 esu)

gxxxx g (1036 esu)

5a

Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO Toluene THF EtOAc DCM CHCl3 Acetonitrile EtOH DMF DMSO

5.83 5.95 6.02 5.78 6.04 5.86 5.91 5.85 5.84 5.95 6.03 6.05 5.98 6.09 6.01 5.90 5.89 5.95 6.05 6.06 6.02 5.98 5.92 6.08 6.14 6.01 6.01 6.13 6.10 6.05 6.11 6.17 6.15 6.17 6.16 6.22

0.4342 0.4851 0.4958 0.5462 0.5628 0.4573 0.5620 0.4993 0.4682 0.5307 0.6156 0.6588 0.4751 0.6943 0.6152 0.6785 0.6784 0.5323 0.4277 0.5618 0.5395 0.3918 0.5635 0.3337 0.4841 0.4269 0.4778 0.9720 0.8398 0.4885 0.8781 0.6010 0.9778 0.8937 0.9874 0.8262

21,692 21,598 21,834 21,459 21,277 21,739 21,459 21,277 21,008 21,645 21,645 21,834 21,368 21,277 21,645 21,368 21,231 21,008 21,368 21,053 21,645 20,921 20,619 20,964 20,877 20,661 20,450 21,097 21,008 21,322 20,877 20,833 21,097 20,833 20,534 20,367

6.52 6.91 6.95 7.36 7.50 6.69 7.46 7.06 6.88 7.22 7.78 8.01 6.88 8.33 7.78 8.22 8.24 7.34 6.52 7.53 7.28 6.31 7.62 5.82 7.02 6.63 7.05 9.90 9.22 6.98 9.46 7.83 9.93 9.55 10.1 9.29

19.7 22.3 22.3 25.4 26.6 20.7 26.1 23.6 22.7 24.2 28.1 29.6 22.3 32.8 28.1 31.8 32.2 25.8 20.0 27.1 24.6 19.2 28.4 16.2 23.8 21.4 24.4 46.7 40.7 23.0 43.1 29.6 47.0 44.1 50.1 42.6

73 85 85 93 105 77 99 89 86 160 190 199 150 228 189 210 214 176 113 155 136 108 160 116 140 123 142 298 258 142 276 293 301 287 330 287

475.96 587.35 590.75 684.48 787.50 515.13 738.54 628.34 591.68 1155.20 1478.20 1593.99 1032.00 1899.24 1470.42 1726.20 1763.36 1291.84 736.76 1167.15 990.08 681.48 1219.20 675.12 982.80 815.49 1001.10 2950.20 2378.76 991.16 2610.96 2294.19 2988.93 2740.85 3333.00 2666.23

7.7 10.9 10.4 18.8 18.7 19.1 19.1 14.3 12.6 24.7 25.1 23.7 27.1 24.7 24.4 17.7 17.3 26.6 11.6 15.4 16.3 11.8 10.8 14.3 11.7 18.5 19.2 25.0 28.2 27.5 23.8 26.8 24.9 23.5 32.9 46.1

5b

6a

6b

a b c d e f g

Onsagar radii (Å). Oscillator strength (f). Wavenumbers of the absorption maxima in various solvents (yge). Transition dipole moment for absorption(mge). Linear polarizability (axx). First hyperpolarizability (bxxx). Second hyperpolarizability (gxxxx).

S.K. Lanke, N. Sekar / Dyes and Pigments 124 (2016) 82e92

3.8. Non-linear optical (NLO) properties by theoretical and solvatochromic method DepeA dyes are well known to have high first hyperpolarizability (b) value and such dyes are very useful for non linear optical application. In this section we have derived both experimental and theoretical values for non linear optical properties. The experimentally derived values for oscillator strength (f), transition dipole moment (mge), linear polarizability (axx) and first hyperpolarizabilility (bxxx) can be obtained using reported method [65] and which are listed in Table 4. The dipole moment (m), mean polarizability (a0), polarizability anisotropy (Da), static first hyperpolarizability (b0) and second hyperpolarizability (g) for the dyes 5a, 5b, 6a and 6b were calculated using B3LYP/6-31G(d) on the basis of the finite field approach [66,67] and which are gathered in Table S9. Here, we have correlated the theoretically derived values with experimentally derived values of the linear polarizability (axx), first hyperpolarizability (bxxx) and second hyperpolarizability (g) for the carbazole based push pull dyes 5a, 5b, 6a and 6b. According to donor and acceptor ability of chromophore the hyperpolarizability value changes. The computed first hyperpolarizability b0, of dyes was found to be ranging from 361.74, 716.90, 361.83 and 721.50
1030 esu for the dyes 5a, 5b, 6a, and 6b respectively in DMF while the experimentally calculated value for bxxx of dyes was found to be ranging from 86, 190, 214, 330
1030 esu1 cm5 respectively in DMF. Dye 6b has strong donor group like N-ethyl and O-ethyl carbazole and strong acceptor group like vinylisophorone hence it shows higher a, b, and g values as compared to other derivative. A similar trend was observed for linear polarizability (a) and second polarizability (g). The computed g value of dyes was found to be ranging from 1004.16, 1024.00, 2548.47 and 2570.77
1036 esu. for dyes 5a, 5b, 6a, and 6b respectively in DMSO. While the experimentally calculated value for gxxxx of dyes was found to be ranging from 12.6, 26.6, 19.2 and 46.1
1030 esu respectively in DMSO. In general, the hyperpolarizability mainly depends on conjugation path length and donor and acceptor capability. From the above observation it clarify that these dyes have shown a large hyperpolarizability as expected and which is quite higher as compared to urea and known dyes suggesting considerable charge transfer characteristics of the first excited state which is further supported by the large difference in the dipole moments between the ground and excited states and from the solvatochromism studies. The detailed theory used for calculating theoretical and experimental values for non linear optical properties are provided in the supporting information. 4. Conclusion In conclusion, the pushepull chromophore 5 and 6 from 2methoxy-9-methyl-9H-carbazole-3-carbaldehyde and 2-ethoxy-9ethyl-9H-carbazole-3-carbaldehyde were synthesized All the synthesized dyes were confirmed by spectral technique such as 1H NMR, 13C NMR and HRMS. The photophysical study revealed that there is not only a red shift imparted by the introduction of methoxy group at the 2-position on carbazole core but also there is increase in the quantum yields of these molecules. Also these molecules are showing very good aggregation induced emissive properties make them valuable candidates for number of practical application. The study of charge transfer characteristics of these push pull dyes by various solvent polarity parameter plots shows the evidence of charge transfer at the excited state. The dipole moment ratios calculated with different solvent polarity parameter plots suggests more polar excited states for the dyes 5a, 5b, 6a, and

91

6b. All the dyes show large hyperpolarizability and could be better candidates for non linear optical materials. Acknowledgements Sandip K. Lanke is thankful to University Grants Commission and Council of Scientific and Industrial Research, India for award of junior and senior research fellowships. Authors are also thankful for SAIF, Punjab University for recording HRMS spectra. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.09.013. References [1] Mei J, Hong Y, Lam JWY, Qin A, Tang Y, Tang BZ. Aggregation-induced emission: the whole is more brilliant than the parts. Adv Mater 2014:5429e79. [2] Thompson RB. Fluorescence sensors and biosensors. Boca Raton: CRC; 2006. [3] Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev 2011;40:5361e88. [4] Luo J, Xie Z, Lam JWY, Cheng L, Tang BZ, Chen H, et al. Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem Commun 2001;381: 1740e1. [5] Lu H, Xu B, Dong Y, Chen F, Li Y, Li Z, et al. Novel fluorescent pH sensors and a biological probe based on anthracene derivatives with aggregation-induced emission characteristics. Langmuir 2010;26:6838e44. [6] Alam P, Kaur G, Climent C, Pasha S, Casanova D, Alemany P, et al. New “aggregation induced emission (AIE)” active cyclometalated iridium (iii) based phosphorescent sensors: high sensitivity for mercury (ii) ions. Dalt Trans 2014;43:16431e40. [7] Dhara K, Hori Y, Baba R, Kikuchi K. A fluorescent probe for detection of histone deacetylase activity based on aggregation-induced emission. Chem Commun 2012;48:11534e6. [8] Song Z, Kwok RTK, Zhao E, He Z, Hong Y, Lam JWY, et al. A ratiometric fluorescent probe based on ESIPT and AIE processes for alkaline phosphatase activity assay and visualization in living cells. ACS Appl Mater Interfaces 2014;6:17245e54. [9] Wang D, Qian J, Qin W, Qin A, Tang BZ, He S. Biocompatible and photostable AIE dots with red emission for in vivo two-photon bioimaging. Sci Rep 2014;4:4279. [10] Liu G, Chen D, Kong L, Shi J, Tong B, Zhi J, et al. Red fluorescent luminogen from pyrrole derivatives with aggregation-enhanced emission for cell membrane imaging. Chem Commun 2015;51:8555e8. [11] Hu F, Huang Y, Zhang G, Zhao R, Yang H, Zhang D. Targeted bioimaging and photodynamic therapy of cancer cells with an activatable red fluorescent bioprobe. Anal Chem 2014;86:7987e95. [12] Huang J, Tang R, Zhang T, Li Q, Yu G, Xie S, et al. A new approach to prepare efficient blue AIE emitters for undoped OLEDs. Chem e A Eur J 2014;20: 5317e26. [13] Huang J, Sun N, Yang J, Tang R, Li Q, Ma D, et al. Blue aggregation-induced emission luminogens: high external quantum efficiencies up to 3.99% in LED device, and restriction of the conjugation length through rational molecular design. Adv Funct Mater 2014;24:7645e54. [14] Xu B, He J, Mu Y, Zhu Q, Wu S, Wang Y, et al. Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission. Chem Sci 2015;6:3236e41. [15] Xu L, Li Y, Li S, Hu R, Qin A, Tang BZ, et al. Enhancing the visualization of latent fingerprints by aggregation induced emission of siloles. Analyst 2014;139: 2332e5. €ußler M, et al. Functionalized siloles: [16] Li Z, Dong YQ, Lam JWY, Sun J, Qin A, Ha versatile synthesis, aggregation-induced emission, and sensory and device applications. Adv Funct Mater 2009;19:905e17. [17] Corey JY. Synthesis of siloles (and Germoles) that exhibit the AIE effect. Aggregation-induced emiss fundam appl, vol. 12. John Wiley and Sons Ltd; 2013. p. 1e37. [18] Yang J, Sun N, Huang J, Li Q, Peng Q, Tang X, et al. New AIEgens containing tetraphenylethene and silole moieties: tunable intramolecular conjugation, aggregation-induced emission characteristics and good device performance. J Mater Chem C 2015;3:2624e31. [19] Wang F, Han MY, Mya KY, Wang Y, Lai YH. Aggregation-driven growth of sizetunable organic nanoparticles using electronically altered conjugated polymers. J Am Chem Soc 2005;127:10350e5. [20] Kaur S, Gupta A, Bhalla V, Kumar M. Pentacenequinone derivatives: aggregation-induced emission enhancement, mechanism and fluorescent aggregates for superamplified detection of nitroaromatic explosives. J Mater Chem C 2014;2:7356e63.

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[44]

[45]

[46]

[47]

[48]

[49] [50] [51] [52]

[53]

[54]

[55] [56]

[57]

[58] [59] [60]

[61] [62]

[63] [64]

[65]

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