Aggregation-Induced Emission in PhenothiazineâTPE and âTPAN

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Aggregation-Induced Emission in Phenothiazine−TPE and −TPAN Polymers Ana Clara B. Rodrigues,† João Pina,† Wenyue Dong,‡ Michael Forster,‡ Ullrich Scherf,‡ and J. Seŕ gio Seixas de Melo*,† †

CQC, Department of Chemistry, University of Coimbra, P3004-535 Coimbra, Portugal Macromolecular Chemistry Group (buwmakro) and Institute for Polymer Technology, Bergische Universitat Wuppertal, Gauss-Str. 20, D-42097 Wuppertal, Germany

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‡

S Supporting Information *

ABSTRACT: Two phenothiazine-based polymerstetraphenylethylene- (PTzTPE) and triphenylacrylonitrile-substituted (PTzTPAN) polyphenothiazineswere studied in organic solvents, solid state, and THF:water and dioxane:water mixtures to investigate the occurrence of aggregation-induced emission (AIE). It is shown that AIE is present for the PTzTPE and PTzTPAN polymers in THF:water mixtures, although to a lesser extent in the latter case. The emission of PTzTPE was found to display the simultaneous emission of locally excited (LE) and charge transfer (CT) states with emission maxima at ∼480 and ∼640 nm, respectively. Dynamic light scattering measurements in THF:water mixtures point out to the formation of small nanoaggregates in which the polymers likely adopt a collapsed structure. The overall effect of the restriction of molecular movements of TPE or TPAN units and the poly(phenothiazine) backbone (thus leading to the enhancing of the excited state radiative channel over the internal conversion deactivation channel through the reduction of the “loose bolt” or free rotor effect), together with the decrease of the CT contribution with the increase of the water fraction was associated with the AIE effect observed in THF:water mixtures. For PTzTPE in dioxane:water mixtures an opposite effect, i.e., an aggregation caused quenching (ACQ), is observed with the increment of water fraction, f w, in the mixture (ϕF decreasing from 0.14 in pure dioxane to 0.042 for f w = 90%). This selective AIE behavior in specific solvents was attributed to the conversion of emissive LE states into dark CT states. DFT calculations of the phenothiazine-TPE and -TPA trimer units confirm the bent butterfly shape generally adopted by the phenothiazine moiety and the excited state charge transfer from the phenothiazine donor to the sidechain acceptor units.

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INTRODUCTION Aggregation-induced emission (AIE) has developed, in the past decade, as a major emerging research field in organic photophysics (over 3000 ISI publications), displaying a wide range of applications resulting from excellent luminous properties of solid state AIE organic materials. The great majority of luminogens are emissive in dilute solution but weakly fluorescent or even nonemissive in the solid state or in concentrated solutions. Under some circumstances this is a phenomenon known as aggregationcaused quenching (ACQ).1 Contrary to the ACQ effect, some luminogens are nonemissive or weakly emissive in dilute solutions but can become highly emissive upon aggregation in poor solvents or in the solid state. These observations led Tang and co-workers to coin this phenomenon as aggregationinduced emission (AIE).2 The mechanisms proposed for this phenomenon involve restriction of molecular motions (RIM), where M (motion) includes rotation and vibration, formation of J-aggregates, suppression of the charge-transfer (CT) state, and conformational planarization.3,4 Since the discovery of the AIE effect, a series of propeller-shaped luminescent molecules, such as 1,1,2,2-tetraphenylethene (TPE), were reported to be AIE-active compounds.5−8 AIE offers a new platform for © XXXX American Chemical Society

researchers to look into the light-emitting processes from luminogen aggregates, from which useful information about structure−property relationships may be collected and mechanistic insights may be gained. The discovery of the AIE effect paves the way for the development of new luminogen materials in the aggregate or solid state. By enabling light emission in the solid state, AIE has already demonstrated relevant significance in the technological applications of luminescent materials (e.g., as thin films in organic light-emitting diodes, OLEDs) and also as chemo- and biosensors.9−15 Understanding light emission enhancement in the aggregate state is therefore of great importance for their practical application. Our group have successfully characterized the AIE effect in polymers (2,3,3-triphenylacrylonitrile, TPAN), which exhibits so-called crystallization-induced emission (CIE)16,17 and was used for constructing AIE-active luminophores, that were attached to the backbone of electron-rich polycarbazoles and polytriphenylamines, thus designing donor−acceptor-type Received: August 15, 2018 Revised: October 2, 2018

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DOI: 10.1021/acs.macromol.8b01758 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthesis Scheme toward Monomers 1/2 and Copolymers PTzTPE and PTzTPAN, Respectively

polymers.18 In this work, two phenothiazine-based polymers tetraphenylethylene- (PTzTPE) and triphenylacrylonitrilesubstituted (PTzTPAN) polyphenothiazineshave been investigated in solid state, organic solvents, and THF:water mixtures to promote aggregation.

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from the monomers 1 and 2, respectively, in Yamamoto-type aryl− aryl couplings, using Ni(COD)2 as coupling reagent in a mixture of THF, COD, and Bpy under microwave (MW) heating for 12 min.20 The chemical structure of the obtained polymers was confirmed by GPC analysis as well as by 1H and 13C NMR spectroscopy. 10-[4-(1,2,2-Triphenylvinyl)phenyl]-10H-phenothiazine. Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (87 mg, 0.095 mmol) and tri-tert-butylphosphane (P(t-Bu)3) (0.02 mL, 0.095 mmol) were dissolved in dry toluene (20 mL) under argon and stirred for 10 min at room temperature (preformation of the catalyst). The catalyst was then added to a mixture of 10H-phenothiazine (1.53 g, 7.66 mmol), bromo-TPE (3.0 g, 7.29 mmol), and sodium tert-butylate (t-BuONa) (1.05 g, 10.94 mmol) in dry toluene (90 mL). The reaction mixture was stirred at 90 °C overnight. The reaction mixture was cooled to room temperature and treated with water. The mixture was extracted with chloroform three times. The organic phases were collected, dried over MgSO4, and concentrated under vacuum. The residue was purified by silica gel chromatography (eluent: dichloromethane/hexane = 3/7) to give desired compound as a light green solid in 91% yield (3.5 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.28 (d, J = 8.4 Hz, 2H), 7.23−7.05 (m, 17H), 7.02 (dd, J = 7.4, 1.7 Hz, 2H), 6.89 (td, J = 7.8, 1.6 Hz, 2H), 6.83 (t, J = 7.0 Hz, 2H), 6.17 (dd, J = 8.1, 1.0 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 144.12, 143.94, 143.58, 143.13 142.92, 142.05, 140.23, 138,89, 133.57, 131.39, 131.24, 131.22, 129.95, 127.87, 127.74, 127.61, 126.72, 126.70, 126.63, 122.37, 120.07, 117.72, 115.96.

EXPERIMENTAL SECTION

Materials. 1,1,2,2-Tetraphenylethylene (TPE) purchased from Sigma-Aldrich and N-methylphenothiazine (N-MePTz) from ACBR GmbH were used without further purification. All the solvents were of spectroscopic grade and were used as received. All reagents for the polymer synthesis were obtained from commercial suppliers and were used without further purification. All reactions were performed under an argon atmosphere by standard and Schlenk techniques. The solvents were used as commercial p.a. quality. Synthesis. The synthesis route toward the monomers and polymers is depicted in Scheme 1. Monobromo-TPE and monobromo-TPAN were synthesized according to the literature18,19 The N−C coupling of monobromo-TPE or monobromo-TPAN with phenothiazine was accomplished in a Buchwald−Hartwig protocol and afforded TPE/TPAN-substituted phenothiazine monomers. The corresponding dibromophenothiazine derivatives 1 and 2 were synthesized by bromination with N-bromosuccinimide (NBS) at 0 °C. The polymers PTzTPE and PTzTPAN were generated starting B


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Macromolecules 3,7-Dibromo-10-[4-(1,2,2-Triphenylvinyl)phenyl]-10H-phenothiazine (1). 10-[4-(1,2,2-Triphenylvinyl)phenyl]-10H-phenothiazine (2.50 g, 4.72 mmol) was dissolved in dichloromethane (30 mL) under argon and cooled by an ice bath. The solution was stirred under protection of light, and NBS (1.68 g, 9.44 mmol) was slowly added in three portions. The reaction mixture was slowly warmed to room temperature and stirred overnight. The reaction mixture was treated with water and extracted with chloroform three times. The organic layers were collected, washed with brine solution, and dried over MgSO4. The solution was concentrated under vacuum, and the product was purified by silica gel chromatography (eluent: dichloromethane/hexane = 1/4) to give desired compound as a light green solid in 77% yield (2.5 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.30 (d, J = 8.5 Hz, 2H), 7.25−7.14 (m, 11H), 7.14−7.02 (m, 8H), 6.96 (dd, J = 8.8, 2.3 Hz, 2H), 5.96 (d, J = 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 144.70, 143.49, 142.90, 142.69, 142.41, 140.00, 139.95, 137.97, 133.90, 131.37, 131.19, 131.16, 129.81, 129.64, 128.72, 127.93, 127.77, 127.61, 126.84, 126.81, 121.20, 116.99, 114.67. MS (APLI): m/z calcd 687.01; found 687.00. Elemental anal. calcd: C, 66.39%; H, 3.67%; N, 2.04%; S, 4.66. Found: C, 66.64%; H, 3.63%; N, 1.94%; S, 4.84%. 2-[4-(10H-Phenothiazin-10-yl)phenyl]-3,3-diphenylacrylonitrile. The synthesis procedure was similar to that described for the preparation of 10-[4-(1,2,2-triphenylvinyl)phenyl]-10H-phenothiazine, with Pd2(dba)3 (50 mg, 0.054 mmol), P(t-Bu)3 (11 mg, 0.054 mmol), 10H-phenothiazine (871 mg, 4.37 mmol), 2-(4-bromophenyl)-3,3-diphenylacrylonitrile (1.5 g, 4.16 mmol), t-BuONa (0.6 g, 6.25 mmol), and toluene (55 mL) as components. The product was purified by silica gel chromatography (eluent: dichloromethane/ hexane = 3/7) to give yellow powder in 85% yield (1.7 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.57−7.41 (m, 7H), 7.35−7.29 (m, 1H), 7.28−7.18 (m, 4H), 7.14−7.06 (m, 4H), 6.96 (td, J = 7.8, 1.7 Hz, 2H), 6.90 (td, J = 7.4, 1.2 Hz, 2H), 6.35 (dd, J = 8.1, 1.2 Hz, 2H). 13 C NMR (100 MHz, CDCl3): δ (ppm) 158.39, 140.05, 131.94, 130.80, 130.09, 129.93, 129.25, 128.60, 128.52, 128.26, 127.10, 126.87, 123.14, 122.48, 117.58, 110.68. 2-[4-(3,7-Dibromo-10H-phenothiazin-10-yl)phenyl]-3,3-diphenylacrylonitrile (2). The synthesis procedure was similar to that described for preparation of monomer 1 with 2-(4-(10H-phenothiazin-10-yl)phenyl)-3,3-diphenylacrylonitrile (1.41 g, 2.95 mmol), NBS (1.05 g, 5.90 mmol), and dichloromethane (20 mL) as components. The product was purified by silica gel chromatography (eluent: dichloromethane/hexane = 2/3) to give the desired compound as a yellow solid in 85% yield (1.6 g). 1H NMR (400 MHz, CDCl3): δ (ppm) 7.56−7.44 (m, 7H), 7.33 (t, J = 7.4 Hz, 1H), 7.25 (t, J = 7.5 Hz, 2H), 7.19 (d, J = 8.6 Hz, 2H), 7.15 (d, J = 2.3 Hz, 2H), 7.11− 7.03 (m, 2H), 7.00 (dd, J = 8.8, 2.3 Hz, 2H), 6.05 (d, J = 8.8 Hz, 2H). 13 C NMR (100 MHz, CDCl3): δ (ppm) 159.11, 142.57, 140.25, 139.79, 138.78, 135.24, 132.41, 130.79, 130.27, 129.92, 129.78, 129.41, 129.09, 128.56, 128.27, 122.51, 119.74, 117.70, 115.26, 110.28. MS (APLI): m/z calcd 635.97; found 636.98. Elemental anal. calcd: C, 62.28; H, 3.17; N, 4.40; S, 5.04. Found: C, 62.51%; H, 3.09%; N, 4.30%; S, 5.19%. Polymer PTzTPE. A mixture of monomer 1 (600 mg, 0.873 mmol), Ni(COD)2 (624 mg, 2.269 mmol), BPy (354 mg, 2.269 mmol), and COD (245 mg, 2.269 mmol) in THF (6 mL) was treated under microwave heating at 120 °C for 12 min. The reaction mixture was quenched with water and extracted with chloroform. The collected organic phases were washed with aqueous 2 M HCl, aqueous NaHCO3 solution, saturated, aqueous EDTA solution, and brine and were finally dried over MgSO4. Afterward, the solvents were removed under vacuum. The resulting solid was dissolved in a small amount of chloroform and precipitated into methanol (500 mL) to afford the polymer as a green solid. Subsequent Soxhlet extractions were performed with methanol, acetone, ethyl acetate, and chloroform. After reprecipitation of the chloroform-soluble fraction into methanol, a green polymer was obtained with 52% yield (251 mg). 1H NMR (600 MHz, C2D2Cl4): δ (ppm) 7.38 (s, 2H), 7.15−7.10 (m, 19H), 6.93 (s, 2H), 6.15 (s, 2H). 13C NMR (100 MHz, C2D2Cl4): δ (ppm) 144.49, 143.77, 143.25, 143.05, 142.58, 140.52, 133.90, 131.56,

131.38, 128.14, 127.98, 127.89, 126.92, 125.68, 124.96, 120.63, 116.36. Mn 17800 g/mol, Mw 110000 g/mol, Mw/Mn 6.15 (GPC, PS calibration). Polymer PTzTPAN. A mixture of monomer 2 (600 mg, 0.943 mmol), Ni(COD)2 (674 mg, 2.451 mmol), BPy (383 mg, 2.451 mmol), and COD (265 mg, 2.451 mmol) in THF (6 mL) was reacted under microwave heating at 120 °C for 12 min. The work-up procedure was similar to that described for the preparation of PTzTPE. The dark yellow polymer from the chloroform fraction was obtained in only moderate yield of 12% (55 mg). 1H NMR (600 MHz, C2D2Cl4): δ (ppm) 7.54−7.44 (m, 7H), 7.35−7.16 (m, 7H), 7.12−6.96 (m, 4H), 6.28−6.20 (m, 2H). 13C NMR (100 MHz, C2D2Cl4): δ (ppm) 159.24, 142.75, 140.21, 138.99, 132.44, 130.90, 130.27, 130.06, 129.61, 128.83, 128.54, 125.13, 124.82, 121.63, 120.62, 120.02, 110.94. Mn 5300 g/mol, Mw 7100 g/mol, Mw/Mn 1.34 (GPC, PS calibration). Instruments. NMR spectra were recorded on a Bruker AVANCE 400 or an AVANCE III 600. 1H and 13C NMR spectra were measured with tetramethylsilane (TMS) as internal standard. Gel permeation chromatography (GPC) measurements were performed on a PSS/AgilentSECurity GPC system equipped with polystyrene gel columns using chloroform as eluent. APLI (atmospheric pressure laser ionization) measurements were performed on a Bruker Daltronik Bremen with a micrOTOF detector. Elemental analyses were performed on a Vario EL (CHN) instrument. Absorption and fluorescence spectra were recorded on Cary 5000 UV−vis−NIR and Horiba-Jobin-Yvon Fluoromax spectrometers, respectively. Fluorescence spectra were corrected for the wavelength response of the system. The fluorescence quantum yields in solution and in the solid state (powder) were measured using the absolute method with a Hamamatsu Quantaurus QY absolute photoluminescence quantum yield spectrometer model C11347 (integration sphere). The absorption of the solutions was kept under 0.1 at the excitation wavelength to avoid the inner filter effect.21 Fluorescence decays were measured using a home-built picosecond time-correlated single photon counting (ps-TCSPC) apparatus described elsewhere.22 Excitation was performed with the second harmonic, 372 or 402 nm (generated with a Spectra-Physics GWU23PS module), from a picosecond Spectra-Physics mode-lock Tsunami laser (Ti:sapphire) model 3950 (80 MHz repetition rate, tuning range 700−1000 nm), pumped by a Millennia Pro-10s, continuous wave, solid state laser (532 nm). Alternatively, a picoQuant picoled model LDH-P-C-450B with λexc = 451 nm was also used as excitation source. The fluorescence decays and the instrumental response function (IRF) were collected using 1024 channels in a time scale up to 24.4 ps/channel (using a SpectraPhysics frequency divider, Pulse picker model 3980-2s, to reduce the fundamental laser repetition rate). Deconvolution of the fluorescence decay curves was performed using the modulation function method, as implemented by Striker in the SAND program and previously reported in the literature.23 The ground state molecular geometry was optimized on isolated entities in a vacuum without conformation restrictions using the density functional theory (DFT) by means of the Gaussian 09 program24 under the B3LYP/3-21G level.25,26 For the resulting optimized geometries time-dependent DFT calculations (using the same functional and basis set as those in the previously calculations) were performed to predict the vertical electronic excitation energies. Molecular orbital contours were plotted using the ChemCraft 1.7 program. Frequency analysis for each compound was also computed and did not yield any imaginary frequencies, indicating that the structure of each molecule corresponds to at least a local minimum on the potential energy surface. The time-resolved ultrafast transient absorption measurements were collected in a broadband HELIOS spectrometer (350−1600 nm) from Ultrafast Systems. Excitation at 430 nm was performed using an amplified femtosecond Spectra-Physics Solstice-100F laser (displaying a pulse width of 128 fs and 1 kHz repetition rate), coupled C


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Macromolecules with a Spectra-Physics TOPAS Prime F optical parametric amplifier (195−22000 nm). The probe light in the UV range was generated by passing a small portion of the 800 nm light from the Solstice-100F laser through a computerized optical delay (with a time window of up to 7.6 ns) and then focusing on to a sapphire crystal to generate a white-light continuum (450−800 nm). All the measurements were obtained in a 2 mm quartz cuvette, with absorptions in the range 0.3− 0.4 at the pump excitation wavelength. To avoid photodegradation, the solutions were stirred during the experiments or kept in movement using a motorized translating sample holder. The spectral chirp of the data was corrected using Surface Xplorer PRO program from Ultrafast Systems. Global analysis of the data (using a sequential model) was performed using Glotaran software.27 Dynamic light scattering studies were performed using a Zetasizer Nano ZS (Malvern Panalytical). The size distribution of the aggregates was measured in 10 mm quartz cuvettes with final volume of 1 mL, at 20 °C, in three consecutive runs of the same sample. The refractive index and viscosity of the THF:water mixtures were determined in advance at the experiment temperature and were seen to be in agreement with those found in the literature for different reported temperatures.28 Sample Preparation. A 3 mL stock solution of each polymer in THF or dioxane with absorption of 0.5−0.6 (at 400 nm) in 10 mm quartz cuvette was prepared. An aliquot (200 μL) of the stock solution was transferred to a 2 mL volumetric flask. After appropriate amount of THF or dioxane was added, water was added to furnish mixtures with different water fractions (f w = 0−90 vol %) with the same polymer concentration. The photophysical and DLS studies of the resultant mixtures were performed immediately after the sample preparation.

Figure 1. Normalized absorption and fluorescence emission spectra for PTzTPE, N-MePTz, and TPE in powder, dioxane, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). The vertical dashed lines are just meant to be guidelines to the eye. Color legend for lines: gray: absorption spectra of N-MePTz; blue: emission spectra of NMePTz; dark yellow: absorption spectra of TPE; purple: emission of TPE; black: absorption of PTzTPE; red: emission spectra of PTzTPE.

depicts two bands: (i) one at shorter wavelengths (∼300 nm), which is consistent with the absorption of the two chromophoric, phenothiazine and TPE, units, and (ii) a longer wavelength absorption band at ∼400 nm attributed to a charge transfer band. Similar behavior was previously reported for donor−acceptor phenothiazine−formylphenyl derivatives.29 In agreement with the absorption the fluorescence emission spectra also display two characteristic bands (Figure 1 and Table 1). The shorter wavelength (∼480 nm) emission band is assigned to a locally excited state (LE) and the longer wavelength, ∼640 nm, associated with a charge transfer (CT) emissive band. It is worth noting that although not clearly visible in dioxane, the CT band is present in all solvents and in the solid state, as can be seen from Figure 1 and from the timeresolved data (see below). Indeed, in THF and dioxane a good linear correlation is obtained when the LE/CT emission intensity ratio is plotted as a function of the PTzTPE absorption at 400 nm (CT band) collected at different concentrations (Figure S1). Table 1 shows that the fluorescence quantum yields for PTzTPE decrease with the increase of solvent polarity (from dioxane and THF, ε = 2.21 and 7.58, respectively, to DMF, ε = 36.7).30 This effect is characteristic of chromophores with CTtype transitions which interact strongly with polar solvents leading to the enhancement of the excited state radiationless decay channels.31 Moreover, in the solid state (powder) a broader absorption and an ∼14 nm red-shift in the absorption maxima were found when compared with the solution spectra (Figure 1). In addition, the fluorescence emission spectra, although redshifted, maintain the spectroscopic features observed in solution, and similar fluorescence quantum yields were found in polar solvents (ϕF in the 0.022−0.063 range depending of the solvent) and in the solid state (0.038) (Table 1). The broadening of the band and the red-shifted absorption and emission maxima together with an enhancement or constancy of the fluorescence quantum yield on going from solution to the solid state may suggest the presence of J-aggregates.32 Although at the supramolecular level some types of specific

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RESULTS AND DISCUSSION The structures and acronyms of the tetraphenylethylene- and triphenylacrylonitrile-substituted polyphenothiazines (PTzTPE and PTzTPAN, respectively) are depicted in Scheme 2. For comparison, the study of the model compound N-methylphenothiazine (N-MePTz) was also carried out. Figure 1 shows the absorption and emission spectra for PTzTPE and N-MeTPz in solution (tetrahydrofuran (THF) dioxane, and dimethylformamide (DMF)) and as a powder (solid state). In solution the absorption spectra of PTzTPE Scheme 2. Structures and Acronyms of the Investigated Tetraphenylethylene- and TriphenylacrylonitrileSubstituted Polyphenothiazines (PTzTPE and PTzTPAN, Respectively) Together with the N-Methylphenothiazine (N-MePTz) Model Compound

D


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Table 1. Spectral Data (Including Absorption, λabs, and Emission Maxima, λem, for the Locally Excited, LE, and Charge Transfer Bands, CT) and Fluorescence Quantum Yields (ϕF) for PTzTPE, PTzTPAN, N-MePTz, and TPE in Powder and Different Solvents at T = 293 Ka λAbs (nm) PTzTPE powder toluene dioxane THF DCM DMF PTzTPAN powder toluene dioxane THF DMF N-MePTz powder dioxane THF DMF TPE powder dioxane THF DMF

420 293/408 293/407 293/407 293/410 294/406

(sh) (sh) (sh) (sh) (sh)

410 293/395 290/394 292/400 293/403

(sh) (sh) (sh) (sh)

λem (nm) 520 474/503 474/506 473/505 476/509 479

ϕF

ΔSS (cm−1)

0.038 0.18 0.14 0.063 0.054 0.022

4579 3413 3473 3428 3382 3754

670 640 695 570

0.007 0.005 0.006 0.002

9465 9691 10992 7456

355 311 311 310

424/574 448 447 448

0.022 0.015 0.009 0.013

4584/10747 9833 9783 9937

381 238/308 239/308 306

454

0.23

(sh) (sh) (sh)/605 (CT) (sh)/627 (CT)

4220

a

sh = shoulder; CT = charge transfer.

Noteworthy, is that going to the solid state (powder) the fluorescence quantum yield of N-MePTz increases (ϕF= 0.022) when compared to solution. Moreover, a significant red-shift in the absorption maxima is observed (from 310 nm in dioxane to 355 nm in powder) together with a dual emission band (arising of a new emission band centered at 574 nm in addition to the short wavelength band centered at ∼424 nm), see Figure 1. In general, in the solid state strong π−π staking interactions occurs, which could prompt formation of excimers that can give rise to a red-shifted emission band. However, this would lead to aggregation caused quenching (ACQ). Moreover, it is expected that excimer formation should induce a strong quenching of the shorter wavelength emission band, which for N-MePTz is not observed. Indeed, it has been shown that the structure of the phenothiazine fused ring adopts a butterfly shape, bended along the S−N axis.34,37 This phenothiazine geometry promotes long intermolecular distances in the solid state, which hinder excimer and/or exciplex formation thus leading to higher fluorescence quantum yields. The characteristic dual emission observed in the solid state is thus better attributed to the existence of different conformations of the N-MePTz chromophore in the excited state. Significant conformational heterogeneity has been shown to occur for phenothiazine, because the nitrogen substituent (bridging element) can adopt a quasi-axial or quasi-equatorial position in the ground state and the hybridization of the nitrogen changes upon electronic excitation changes its ionization potential and concomitantly its donor properties.35 In the case of the TPE chromophore, as described in the literature, no emission was observed in diluted solutions, although, in the solid state it is strongly emissive.40 Indeed, as shown in this work, the TPE powder presents a strong blue emission centered at 454 nm with ϕF = 0.22.

molecular packing have been associated with the AIE mechanism, such as J-aggregation, dimer/excimer stacking, herringbone stacking, or even the weakly coupled H aggregation can also help to maintain the emission in solid states, probably caused by some specific emissive-favorable exciton coupling.33 However, herein we expect that the constancy of the emission efficiency going from polar solvents to the solid state (Table 1) results from the combination of two mechanisms. These include reduction of the nonradiative CT state and formation of amorphous aggregates with specific molecular packing where the restriction of intramolecular rotation (RIR) can efficiently prevent nonradiative relaxation pathways, which leads to AIE. As a result, the excited state radiative relaxation pathway is still active and competitive in the solid state. Indeed, previously the phenothiazine donor unit has shown to be is an excellent counterpart for azomethine acceptor units promoting an increase in the luminescence properties.34 This effect was related to the good conjugation of the donor phenothiazine with the acceptor units, increasing the rigidity, which hinders the nonradiative decay of the excited state via restriction of intramolecular rotations. The photophysical properties of the two model compounds, N-MePTz and TPE, were also studied in the solid state and in solution (dioxane, THF, and DMF) (Table 1). As previously reported,35−39 similar spectroscopic (displaying a single absorption and emission band centered at ∼311 and ∼448 nm, respectively, Figure 1 and Table 1) and photophysical properties were found for N-MePTz in polar and nonpolar solvents (ϕF ∼ 0.01, Stokes shift, ΔSS ∼ 9800 cm−1, and = 2.03 ns and τTHF = 1.76, which fluorescence lifetimes τdioxane F F were found to be the major component in the decays analysis with a contribution ≥98.3%). E


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Figure 2. (A) Emission spectra for PTzTPE polymer in THF:water mixtures (λexc = 400 nm) and (B) correlation of emission area, maximum emission wavelength and ratio of intensity of LE vs CT emission band with increasing water fraction for PTzTPE in THF:water mixtures. (C) Photos under UV irradiation (with λexc = 365 nm) green fluorescence emission of PTzTPE in THF:water mixtures (from left 0% to right 90% with increments of water fraction (v/v) of 10%).

69 nm and decreases upon the addition of water, until it reaches a diameter of 88 ± 35 nm in f w = 90% in THF (Figure S2). A colapsed structure of the polymer with the increase of the water fraction is then expected for the PTzTPE polymer. The fluorescence behavior in dioxane:water mixtures was also investigated for the PTzTPE polymer (see Figure S3). Noteworthy is the fact that in dioxane water mixtures an opposite effect to the AIE; i.e., aggregation-caused quenching (ACQ) is found for the PTzTPE polymer. In addition to the fluorescence quenching a bathochromic shift of the emission maximum by ∼8 nm (350 cm−1) is also observed (Figure S3). Moreover, the ϕF of the PTzTPE polymer decreases from 0.14 in pure dioxane solution to 0.042 in the mixture with the higher water fraction (f w = 90%). Indeed, selective AIE behavior in specific solvents was previously reported for D−A systems; the main cause of this phenomenon was described as the transition from the emissive LE state to the dark CT state.41,43 The strong decrease in the ratio of emission intensity of LE vs CT state (ILE/ICT) on going from pure dioxane to the mixture with f w = 10% (and higher f w) gives support for the formation of a nonemissive CT at the expenses of the LE state (Figure S3B). PTzTPAN Polymer. Figure 3 presents the absorption and fluorescence emission spectra for the PTzTPAN polymer in solution and in the solid state. Comparison with the PTzTPE polymer shows that similar absorption bands and maxima were found for PTzTPAN in THF and in the solid state (see Table 1). However, contrary to what was observed for PTzTPE, where a clear emission of the LE and CT states was observed, for PTzTPAN in THF a very weak fluorescence emission band centered at ∼570 nm was found (Figure 3). Moreover, a significant 100 nm red-shift was observed in the emission spectra on going from THF solution to the solid state. In addition, PTzTPAN was found nonemissive in DMF (Figure S4) and is weakly emissive with the decrease of solvent polarity (ϕF = 0.006 in dioxane) and even in the solid state (powder), ϕF = 0.007 (Table 1).

To evaluate the occurrence of AIE on the polymer PTzTPE, the fluorescence behavior in THF:water mixtures was investigated with different water fractions (f w, the volume percentage of water in THF:water mixtures). Increasing the water fraction in the mixed solvents should change the existing forms of the polymer from an extended conformation in pure THF solution to aggregated fluorophore particles in water mixtures. Figure 2 shows the emission spectra of the PTzTPE polymer in THF:water mixtures. The increment on the water amount in the mixture leads to a gradual increase in the total emission efficiency (ϕF) up to a value of 40% water fraction which is accompanied by an identical increase of the wavelength maxima with the water content in the mixture (Figure 2B). It is also worth noting the decrease of the CT band intensity (and therefore to the total fluorescence emission efficiency) with the gradual increment of water (see Figure 2B). This shows that for PTzTPE, concomitantly with the gradual increase in the AIE effect, a decrease of the CT contribution is observed, showing that the latter is in competition with the AIE process. Indeed, successive addition of water to THF increases the solvent polarity while decreases the solvating power and changes the medium viscosity. The former decreases the emission intensity of the CT state, with a promotion of the aggregate formation; the increase of the medium viscosity also correlates with the AIE effect by the restriction of torsional and vibrational motion of the polymer.41,42 Consequently, the total emission of PTzTPE is determined by the competition of different effects with the ϕF increasing from 0.063 in pure THF to 0.12 in the 10:90 THF:water (v/v) mixture. Dynamic light scattering (DLS) studies were also performed to determine the size of the aggregates of PTzTPE in THF:water mixtures (see Figure S2). These confirm the formation of nanoaggregates in the THF:water mixtures and shown a decrease in the aggregates average size with the increase of the water fraction, f w (Figure S2). It was found that for f w = 10% the PTzTPE aggregates have a diameter of 226 ± F


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Figure 3. (A) Normalized absorption and fluorescence emission spectra for PTzTPAN in THF solution and in the solid state (powder). (B) Emission spectra of PTzTPAN (λexc = 400 nm). (C) Photos under UV irradiation (with λexc = 365 nm) of PTzTPAN in THF:water mixtures (from left 0% to right 90% with increments of water fraction (v/v) of 10%). (D) Correlation of emission area with increasing water fraction in THF:water mixtures.

Figure 4. DFT//B3LYP/3-21G optimized ground state geometry together with the frontier molecular orbital energy levels and the relevant electronic density contours for the trimer model compounds of the phenothiazine-TPE and -TPA polymers.

On going to the THF:water mixtures, a clear enhancement of the fluorescence emission was observed for the mixtures with a water fraction ≥40% (see Figure 3B,D), and a significant red-shift was also found when compared with the emission bands in the mixtures up to 0−30% water content (see Figure 3B inset). This shift can be attributed to the typical effect observed for CT-type bands with the increase of solvent polarity (through increase of the water fraction).44 The ϕF increases 5 times with the maximum amount of water fraction: from 0.002 in pure THF to 0.010 for the mixture with f w = 90%. TD-DFT Calculations. Calculations using a trimer model compound of the investigated phenothiazine-TPE and -TPAN were performed to predict the geometry between the TPE and TPAN side substituents and the phenothiazine chromophores and more importantly to corroborate the attribution of the

low-energy absorption band to a charge transfer transition. Time-dependent density functional theory (TD-DFT) was used to obtain the vertical excitation energies, oscillator strengths (f), and excited state compositions in terms of excitations between the occupied and virtual orbitals for the investigated compounds (see Table S1). Although the predicted transitions are overestimated by 28 nm (0.02 eV) in the case of the trimer model compound of PTzTPE and 80 nm (0.06 eV) for PTzTPAN, when compared with the absorption maxima of the polymers in dioxane solution, these are well within the error range of the TD-DFT method for the prediction of CT-type transitions (with errors of up to 1 eV)39,45 in aromatic donor−acceptor systems. With these phenothiazine trimers the observed lowest energy absorption bands are associated with the predicted S0 → S1 transitions (with CT character as shown below), displaying predominantly G


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Macromolecules contribution from the HOMO → LUMO+2 orbitals (93%) in the case of the PTzTPE trimer, while for the PTzTPAN similar contributions from the H-1 → LUMO (43%) and HOMO → LUMO (46%) were found (see Table S1). In general, the electron density in the HOMO levels are spread over the phenothiazine moieties (acting as donor units), while in the case of the LUMO, the densities are localized in the TPE or TPA units (acceptor units) (see Figure 4). This gives further support for the occurrence of CT in the singlet excited state of these derivatives. Furthermore, the geometry optimization for these phenothiazine trimer units revealed that in agreement with the literature the phenothiazine moieties are slightly bent along the S−N axis (butterfly shape),34,37,45 with the thiazine central unit displaying angles between the C−N−C and C−S−C atoms of ∼123° and ∼99°, respectively (Figure S5). Moreover, between the backbone phenothiazine moieties dihedral angles of ∼140° were found, while for the attaching phenyl units of the TPE and TPA groups, dihedral angles in the ∼82°−97° range were obtained with respect to the central benzothiazine chromophore. Thus, the TPE and TPA units can be considered to be perpendicular to the phenothiazine units (see Figure S5). Time-Resolved Fluorescence Measurements. Timeresolved fluorescence emission was obtained for PTzTPE polymer in solution (THF, dioxane), in the solid state, and in THF:water and dioxane:water mixtures (see Figure 5 and Table 2). From the global analysis of the decays, in general, the fluorescence decays are fitted with sums of three exponentials (eq 1). The exception was PTzTPE in THF solution where the decays follow a biexponential decay law n

IPTzTPE(t ) =

∑ aie−t /τ

i

i=1

Figure 5. Room temperature fluorescence decays for PTzTPE collected with excitation at 404 and 451 nm in tetrahydrofuran (THF) solution and in the solid state (powder), respectively. For a better judgment of the quality of the fit, weighted residuals (W.R.), autocorrelation function (A.C.), and χ2 values are also presented. The dashed lines in the decays are the instrumental response function.

(1)

where τi are the decay times and ai the pre-exponential factors which represents the contribution of each exponential term at t = 0. In general, the shortest and the longest decay times are associated with the emission of the LE and CT species (τ1 and τ3) with decay times varying from 51 to 103 ps (LE) and 4.7 to 5.6 ns (CT) which can be seen by the dominance of the preexponential factor, ai1, when the decays are collected at 500 nm (in the LE state emission band) and ai3 for λem > 600 nm (CT band). It should also be emphasized that the CT species is already present in the ground state (in particular at the 451 nm excitation wavelength used to collect the decays), and this partially explains the absence of a rising component all over the CT emission band (and in particular at λem > 600 nm). Nevertheless, the most interesting feature of these decays is the presence of the intermediate component (τ2) in the solid state (powder), dioxane, and in the THF:water and dioxane:water mixtures (Table 2), although in THF this intermediate lifetime, τ2, is absent. In the solid state, in agreement to what was obtained in solution, the intramolecular charge transfer is also active as shown in Figure 5 from the presence of the characteristic decay components of the LE (36 ps) and CT (5.6 ns) species. This is further corroborated from the similar spectroscopic features found in solution and in solid state (Figure 1). Additionally, the intermediary decay time, τ2 = 1.64 ns, appears in the solid state associated with a higher pre-exponential factor and thus with higher emission contribution. Indeed, the decays collected in diluted THF:water and dioxane:water mixtures (Table 2)

show that the pre-exponential values associated with this intermediate component increases concomitantly with the increase of the water fraction (thus with the higher number of fluorescent aggregates in the mixture). Therefore, this intermediary decay component is assigned to the aggregated fluorophores (AIE effect). Moreover, even in diluted dioxane solution this intermediary component is found (τ2 = 2.27 ns), as shown by the significant pre-exponential values associated with this decay component in Table 2, mirroring the fact that aggregates are already present in pure dioxane. In contrast, for THF, only the decay times associated with the LE (51 ps) and CT (4.77 ns) species are observed. This behavior can be explained by the conformation adopted by the PTzTPE polymer in different polarity environments. In this case a strong interaction is expected between the polymer backbone and the more polar solvent THF molecules (in comparison to dioxane) due to the charge transfer nature of the PTzTPE polymer chromophoric units (with their higher excited state dipole moment). Indeed, in polar solvents the higher number of solvent molecules around the chromophoric units in the polymer can hinder the folding of the polymer (leading this to adopt a more extended conformation), thus minimizing the formation of intramolecular aggregates. This is further supported by the observed decrease in the fluorescence H


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Table 2. Fluorescence Decay Times (τi) and Pre-Exponential Factors (ai) Obtained from Global Analysis of the Decays for PTzTPE in THF:Water Mixtures Obtained with λexc= 404 and 451 nm and T = 293 K; χ2 Values Are Also Given as Criteria for the Judgment of the Quality of the Fitting THF:H2O (% v/v) 100:0 (THF) 90:10

80:20

50:50 10:90

dioxane:H2O (% v/v) 100:0 (dioxane)

80:20

50:50

10:90

λ (nm)

τ1 (ns)

500 640 500 600 680 500 600 680 500 640 500 600 680

0.051

500 600 680 500 600 680 500 600 680 500 600 680

τ2 (ns)

τ3 (ns) 4.77

0.12

1.30

4.92

0.22

1.67

5.10

0.16

1.12

4.50

0.26

1.34

4.91

0.103

2.27

4.73

0.38

1.67

4.99

0.34

1.44

4.83

0.24

1.12

4.14

ai2

ai3

χ2

0.970 0.353 0.824 0.441 0.178 0.772 0.335 0.021 0.736 0.250 0.727 0.471 0.209

0.118 0.224 0.233 0.176 0.291 0.288 0.217 0.270 0.248 0.350 0.467

0.030 0.647 0.058 0.335 0.589 0.052 0.374 0.691 0.047 0.706 0.025 0.179 0.324

1.13 1.28 1.09 1.06 1.02 1.26 0.96 1.21 1.15 1.06 1.21 1.21 0.94

0.751 0.419 0.022 0.699 0.349 −0.001 0.740 0.383 0.042 0.700 0.579 0.449

0.160 0.208 0.317 0.276 0.403 0.599 0.240 0.401 0.552 0.271 0.317 0.385

0.089 0.373 0.661 0.025 0.248 0.401 0.02 0.216 0.405 0.029 0.104 0.166

1.21 0.95 1.01 1.03 1.02 1.16 1.05 1.02 1.10 1.14 1.04 1.13

ai1

Figure 6. (A) Room temperature femtosecond time-resolved transient absorption data for PTzTPE in tetrahydrofuran solution collected with excitation at 430 nm. (B) The representative kinetic traces. (C) The evolution-associated difference spectra (EADS) obtained from the global analysis of the TA data. Also shown as insets in (B) are the decays at shorter times, and for a better judgment of the quality of the fits the residuals are also presented.

I


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− β), α(1 − β), and β are the fractions of light exciting these species in the ground state; and τ1 is the fluorescence lifetime associated with the decay of LE species, τ2 to the aggregate, and τ3 to CT species. It is worth mentioning that based on the time-resolved data in diluted THF solutions, no aggregation was observed for the PTzTPE polymer.

emission yields on going from nonpolar to polar solvents due to the enhancement of the nonradiative energy dissipation to the surrounding solvent molecules. In contrast, in the nonpolar dioxane, the polyphenothiazine backbone can adopt a folded conformation, which potentially promotes the formation of intramolecular aggregates, thus restricting the free rotation of the TPE units and decreasing the RIR effect. Femtosecond Transient Difference Absorption (fsTA) Spectra. To further investigate the absence of aggregation in the PTzTPE polymer in pure THF solutions, the timeresolved fs-TA spectra were recorded in the 450−800 nm range (Figure 6). The TA spectra for PTzTPE in THF present a positive excited state absorption band (ESA) overlapping with a negative TA band at shorter wavelengths, which in agreement with the fluorescence emission spectra is attributed to stimulated emission (SE) (see Figure 6). The positive ESA band is composed of two distinct TA bands with maxima centered at ∼670 and ∼545 nm. The former band appears right after laser excitation and is attributed to the locally excited state band (LE) and as it disappears gives rise to the appearance of the band centered around 545 nm that is associated with the intramolecular charge transfer band (CT). Indeed, the global analysis using a sequential model (Figure 6B) of the fs-TA data in the 455−780 nm range shows that the decays are well fitted with a sum of three exponentials shown in Figure 6B,C. The fast component of 1.4 ps is in good agreement with solvation time of THF (0.94 ps)46 and thus is assigned to solvent relaxation. It is worth mentioning that since excitation was near the 0−0 absorption transition, intramolecular vibrational relaxation is not expected to occur, and thus decay components associated with this fast processes were discarded. The second component (29 ps) is attributed to the locally excited state generated after solvent relaxation that according to the evolution-associated difference spectra (EADS) in Figure 6C is responsible for the positive redshifted transient absorption band centered at 670 nm. Finally, the presence of a negative pre-exponential value in the representative kinetic trace collected in the characteristic band of the CT state (545 nm, see Figure 6B) shows that this state is formed at the expenses of the LE state and decays with a transient lifetime of 4.9 ns. The fs-TA kinetic data (29 ps and 4.9 ns decay components) are in good agreement with the fluorescence lifetimes obtained by TCSPC (51 and 4.77 ps, respectively), thus showing that aggregation of the PTzTPE polymer does not occur in pure THF solution. On the basis of the overall data, we proposed the following kinetic Scheme 3, where LE(PTzTPE), CT(PTzTPE), and Agg (PTzTPE) correspond to the locally excited and charge transfer states and aggregated species, respectively; (1 − α)(1

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CONCLUSIONS The aggregation-induced emission effect in two phenothiazine drivative polymers, PTzTPE and PTzTPAN, was characterized in THF/water mixtures. Both polymers also present emission in solid state (powder), although PTzTPAN is weakly emissive (ϕF ≈ 0.01). DLS studies of PTzTPE in THF:water mixtures further support that the polymer adopts a collapsed conformation and, thus, decrease in molecular volume upon the addition of water. Although PTzTPE polymer presents AIE character in THF:water mixtures, in dioxane:water, the polymer showed the opposite effect, aggregation-caused quenching (ACQ). From the time-resolved data of PTzTPE it is shown that aggregates (with a characteristic lifetime) are present even in pure dioxane. From time-resolved emission and fs-transient absorption, it is shown that the LE state displays a characteristic decay time in the picosecond time range whereas the CT state decays with ∼4−5 ns (this state is formed both from direct excitation or at the expenses of the LE excited state). In bad solvents or mixtures of THF and dioxane with water, an intermediate decay time (with 1−2 ns) is found. The overall study points out the presence of the AIE effect in polymers containing phenothiazine and TPE units whose enhancement can be modulated by simple structural modification in the TPE unit (PTzTPE vs PTzTPAN).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01758. Photoluminescence spectra of PTzTPE in THF and dioxane in different concentrations and in dioxane:water mixtures; the particle size distribution studies of PTzTPE in THF:water mixtures, and absorption and emission spectra of PTzTPAN in pure solvents; original data of 1H NMR and 13C NMR spectra along with DFT//B3LYP/3-21G optimized ground state geometry of each polymer (PDF)

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AUTHOR INFORMATION

Corresponding Author

Scheme 3. Kinetic Scheme Summarizing the Different Species Present in the Experimental Conditions Undertaken in This Work

*E-mail: [email protected] (J.S.S.d.M.). ORCID

Ana Clara B. Rodrigues: 0000-0002-5128-0204 J. Sérgio Seixas de Melo: 0000-0001-9708-5079 Present Address

W.D.: School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, P. R. China. Notes

The authors declare no competing financial interest. J


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ACKNOWLEDGMENTS This work was supported by Project “Hylight” (no. 031625) 02/SAICT/2017 which is funded by the Portuguese Science Foundation and Compete Centro 2020. We acknowledge funding by Fundo Europeu de Desenvolvimento Regional (FEDER) through Programa Operacional Factores de Competitividade (COMPETE). The Coimbra Chemistry Centre is supported by the Fundaçaõ para a Ciência e a Tecnologia (FCT), Portuguese Agency for Scientific Research, through the Project PEst-OE/QUI/UI0313/2014. A.C.B.R. acknowledges project no. 22124-LLPT-Laserlab-Portugal (ref: CENTRO-01-0145-FEDER-000014) for a post-doctoral grant. J. Pina acknowledges the project “SunStorage - Harvesting and storage of solar energy” for financial support, reference POCI01-0145-FEDER-016387, funded by European Regional Development Fund (ERDF), through COMPETE 2020Operational Programme for Competitiveness and Internationalization (OPCI), and by national funds, through FCT. We also thank Dr. M. J. Moreno and Msc. J. A. Samelo for the assistance with DLS measurements and MSc. A. Alves and Msc. E. Melro for the assistance with rheological studies.

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