Crystallization and Aggregation-Induced Emission in a Series of

Subscriber access provided by UNIV OF DURHAM

C: Physical Processes in Nanomaterials and Nanostructures

Crystallization and Aggregation-Induced Emission in a Series of Pyrrolidinylvinylquinoxaline Derivatives Nithiya Nirmalananthan, Thomas Behnke, Katrin Hoffmann, Daniel Kage, Charlotte F. Gers-Panther, Walter Frank, Thomas J. J. Mueller, and Ute Resch-Genger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01425 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Crystallization and Aggregation-Induced Emission in a Series of Pyrrolidinylvinylquinoxaline Derivatives Nithiya Nirmalananthan,†, ‡ Thomas Behnke,† Katrin Hoffmann,† Daniel Kage,† Charlotte F. Gers-Panther,§ Walter Frank,∥ Thomas J. J. Müller,*§ and Ute Resch-Genger*† †

Bundesanstalt für Materialforschung und –prüfung (BAM), Department 1, Richard Willstätter

Straße 11, D-12489 Berlin, Germany, Division Biophotonics. ‡

Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin,

Germany §

Institut für Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universität

Düsseldorf, Universitätstrasse 1, D-40225 Düsseldorf, Germany ∥

Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf,

Universitätstrasse 1, D-40225 Düsseldorf, Germany

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

ABSTRACT

Aggregation-induced emission (AIE) has been meanwhile observed for many dye classes and particularly for fluorophores containing propeller-like groups. Herein, we report on the AIE characteristics of a series of four hydrophobic pyrrolidinylvinylquinoxaline (PVQ) derivatives with phenyl, pyrrolyl, indolyl, and methoxythienyl substituents used to systematically vary the torsion angle between this substituent at the quinoxaline C2 position and the planar pyrrolidinylvinylquinoxaline moiety. These molecules, which are accessible via four- or five component one-pot syntheses, were spectroscopically studied in organic solvents and solventwater mixtures, as dye aggregates, solids, and entrapped in polystyrene particles (PSP). Steady state and time-resolved fluorescence measurements revealed a strong fluorescence enhancement for all dyes in ethanol−water mixtures of high water content, accompanying the formation of dye aggregates with sizes of a few hundred nm, overcoming polarity and H-bonding-induced fluorescence quenching of the charge-transfer (CT)-type emission of these PVQ dyes. The size and shape of these dye aggregates and the size of the AIE effect are controlled by the water content and the substituent-dependent torsion angle, that influences the nucleation process and the packing of the molecules during aggregation. Staining of 1 µm-sized carboxy-functionalized polystyrene particles with the PVQ dyes resulted also in a considerable increase in fluorescence quantum yield and lifetime, reflecting the combined influence of the restricted molecular motion and the reduced polarity of the dye microenvironment.


2

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

Bright and biocompatible fluorescent nanomaterials consisting of pure dyes or dyes entrapped in particles made from organic polymers, inorganic materials like silica or calcium phosphate and carbonate, and organic−inorganic hybrid particles are of increasing importance for signal amplification strategies, e.g., for optical assays, targeted probes in bioimaging studies and as sensor materials.1-6 This typically requires water solubility as well as functionalization with target-specific recognition elements, utilizing often molecules like polyethylene glycol (PEG) as spacers or surface coating to minimize unspecific interactions.7 A rapidly emerging and particularly promising technology is the use of dyes showing aggregation-induced emission (AIE).8-10 Except for dyes forming J-aggregates,11-13 the majority of hydrophobic organic fluorophores are highly fluorescent only as monomeric species in dilute solutions of apolar organic solvents. In all cases, which favor dye-dye interactions like concentrated solutions or hydrophilic environments,14-17 biomolecules labeled with a large number of fluorophores,18 or all types of particles surface functionalized with dyes,19-20 they are barely or even non-emissive. This has been exploited for the design of enzyme substrates or so-called smart or activatable probes.21-22 In all these systems, π − π stacking interactions of the chromophores with extended π-conjugated systems23 result in a strongly decreased emission due to the formation of non or barely emissive dye dimers or even higher aggregates, which then act as an energy sink for radiationless fluorescence resonance energy transfer (FRET) between chemically identical, yet spectroscopically distinguishable molecules (homo-FRET). These effects as well as potentially intermolecular fluorescence quenching by hydrogen-bonding interactions are summarized as aggregation-caused quenching (ACQ)24-26 and have been observed for many bioanalytically relevant dye classes such as xanthenes, cyanines, porphyrins, and BODIPYs.


3


Page 4 of 30

In 2001, B. Z. Tang and his coworkers observed the opposite behavior for 1-methyl-1,2,3,4,5pentaphenylsilole, i.e., an increase in emission intensity upon dye aggregation. This effect was analogously termed aggregation-induced emission (AIE).27 Since then, AIE has been reported for many different organic dyes, particularly for molecules with a twisted skeleton conformation and propeller-like moieties.26,

28

As a consequence, the occurrence of AIE and the size of the

resulting fluorescence enhancement were attributed to structural properties like conformational flexibility, intramolecular mobility (intramolecular rotations of certain substituents), and the packing structure of the molecules in the aggregates, favoring planarization and rigidization.26, 2930

This hampers non-radiative relaxation processes related to or involving molecular motions.31-32

In the last years, AIE dyes have obtained great attention, not only in fundamental studies of solid-state photophysics and photochemistry24 but also for applications in organic light-emitting diodes (OLED), advanced optical and electronic devices,14,

26, 29, 33-34

and lasers32 as well as

optical memory materials24 and solid-state emitters.35-36 In the life sciences, they have been used as optical reporters in assays,37 bioimaging probes,38 and as new types of turn-on chemo-/fluorescent sensors.30, 39-41 Moreover, multicomponent syntheses of AIE chromophores, that are highly convergent, have recently been summarized in an overview.42 Promising candidates for AIE dyes are synthetically easily accessible quinoxalines and their derivatives. This dye class is already used in a broad range of biological applications due to their antibacterial, antiviral, or anticancer properties.43 Here, we report on the structural and luminescence properties of a series of hydrophobic pyrrolidinylvinylquinoxaline (PVQ) derivatives with different aromatic substituents (see Figure 1), namely phenyl (1) and the heterocycles pyrrole (2), indole (3) and methoxythienyl (4). While the phenyl ring of dye 1


4

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


essentially represents an electroneutral substituent, the three heterocyclic moieties of dyes 2-4 are electron rich substituents with variable π-donor strength and polarizability.

Figure 1. PVQ derivatives with phenyl (1), pyrrolyl (2), indolyl (3), and methoxythienyl (4) substituent (R) at the quinoxaline C2 position of the planar PVQ moiety. These derivatives can be synthesized in a four- or five component one-pot approach utilizing glyoxylation and alkynylation of (hetero)arenes, Hinsberg cyclocondensation of 1,2diaminoarenes and the intermediate 1,2-dicarbonyl compounds, followed by a Michael addition with pyrrolidine.44 Based upon a study of the AIE behavior of these four PVQ dyes in mixtures of ethanol and water, the analytical characterization of the dye particles formed, and a comparison with their solid-state luminescence, we derived information on the influence of the substitution pattern on the optical properties of these PVQ derivatives as well as a correlation between the chemical and crystal structure and the size of the AIE effect. Furthermore, the effect of surfactants on the stability and size of the AIE particles was assessed. In addition, the PVQ dyes were entrapped in premanufactured 1 µm-sized carboxy-functionalized polystyrene nanoparticles3 to examine the impact of restricted molecular motion and reduced polarity of the dye microenvironment.


5


Page 6 of 30

EXPERIMENTAL SECTION

Materials. Triethylamine and the cationic and anionic surfactants cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. Carboxylated 1 µm-sized polystyrene particles (PSP) were obtained from Kisker Biotech GmbH (Germany). The solvents ethanol and THF (spectroscopic grade) were purchased from Merck and Sigma Aldrich (Germany), respectively. Milli-Q-water was obtained from a Millipore water purification system. Material preparation. PVQ dyes. The four PVQ derivatives bearing phenyl (1), pyrrolyl (2), indolyl (3), and methoxythienyl (4) substituents were synthesized following a previously described procedure.44 The chemical structures were confirmed by nuclear magnetic resonance (NMR) and mass spectrometry (MS).44 Solid crystals. 20 mg of each PVQ compound were dissolved in a minimum amount of a moderately polar solvent like dichloromethane, ethyl acetate or acetonitrile and placed in a screw-cap scintillation vial (20 mL). The cap was loosely screwed onto the vial, which was then placed in a larger screw-cap vessel (> 200 mL) filled with approximately 10 mL of an apolar solvent like n-hexane or n-pentane. The cap was screwed tightly onto the vial. Diffusion of the solvents induced crystallization and the crystals were collected after 2-5 days. Crystals for confocal laser scanning microscopy (CLSM) measurements. Crystals were obtained from suspensions of ethanol−water mixtures by using SDS and CTAB as additives and then transferred onto cover slips. AIE aggregates. For all spectroscopic AIE studies (absorption, emission, fluorescence lifetime, and quantum yield measurements), saturated ethanolic solutions of dyes 1 (14 mm), 2 (57 mm), 3 (14 mm), and 4 (2.7 mm) were prepared (V = 5 mL), respectively. 10 µL of these dye stock


6

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solutions were then added to ten ethanol−water mixtures of increasing water content from 0% to 99% in the presence of 10 µL triethylamine to avoid dye hydrolysis in water. The solutions were measured immediately after preparation. For size measurements of the aggregates with Dynamic Light Scattering (DLS), the solutions were stirred for 10 min before the measurement. Dye stained PSP. 1 µm-sized carboxy-functionalized PSP were stained representatively with the hydrophobic PVQ dye 1 following a previously described staining procedure from T. Behnke et al..3 For this purpose, 400 µL of a dye solution in THF (dye concentration of 6 mM) were added to 2400 µL of an aqueous suspension of 12 mg PSP (0.5 wt%). After shaking for 1 h, the dyeloaded particles were washed three times with Milli-Q-water to remove non-encapsulated dye and subsequently always resuspended in Milli-Q-water in an ultrasonic bath. Then, the particles were washed with an ethanol−water mixture 50/50 (V/V) to remove dye molecules adsorbed on the particle surface and larger aggregates. Instrumentation. DLS measurements of the AIE particles to determine their size and size distribution were done with the Zetasizer Nano Series from Malvern Instruments Ltd. using the program Zetasizer Software 7.02 for data analysis. All aggregates were studied in PS-semi-micro cuvettes from ratiolab® at T = 25 °C. Transmission Electron Microscopy (TEM) images of the AIE particles were recorded with the TEM-Tecnai G2 20 S-Twin from FEI. Crystal structure determinations. Suitable single crystals of 1-4 were selected with a polarization microscope and investigated with STOE IPDS type diffractometers at room temperature (RT), using Mo-Kα radiation (λ = 0.71073 Å). Corrections for Lorentz and polarization effects were applied. The structures were solved by direct methods (SHELXS)45 and subsequent ∆Fsyntheses. Approximate positions of all hydrogen atoms were found in different stages of the


7


Page 8 of 30

converging structure refinements by means of full-matrix least-squares calculations on F2 (SHELXL).45-46 In the refinements, the riding model was applied using idealized C-H bond lengths as well as H-C-H, C-C-H, and N-C-H angles. The Uiso(H) values were set to 1.5Ueq(Cmethyl) and 1.2Ueq (Cmethylene, Carene, Cvinyl). To account for residual electron density in the region of the pyrrolidinyl group in 1 and 4, as well as for elongated anisotropic displacement ellipsoids for some Cmethylene atoms that did not appear to be physically meaningful, a twoposition disorder was introduced with partial occupation sites. Appropriate distance and anisotropic displacement restraints had to be applied to stabilize the geometry of the minor occupied parts of the partial occupation site models. The images were generated with SHELXTL.47 The determination of the crystal structure of 3 suffers from low data completeness due to the fact, that only a ‘one circle experiment’ could be performed with the STOE IPDS at that time. The crystal data are given in Table 1 and selected details of the crystal structures in Table 2. Thee crystallographic data (excluding structure factors in case of 2-4) of the structures of 1-4 have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-1454594 (1), CCDC-1454592 (2), CCDC-1454071 (3), and CCDC1454593

(4)

(Fax:

+44-1223-336-033;

E-Mail:

[email protected],

http://www.ccdc.cam.ac.uk). Absorption spectra. Absorption spectra were recorded on a double beam spectrometer from Analytik Jena (Specord 210 Plus) in a 0°/180° geometry. The scan range was 300 nm to 800 nm, the scan speed 5 nm/s, and the integration time 0.2 s; the slit width and step width were set to 1 nm.


8

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fluorescence spectra. Fluorescence spectra were measured with a calibrated fluorometer FSP 920 (Edinburgh Instruments) equipped with a xenon lamp, Czerny–Turner double monochromators, a reference channel, and Glan-Thompson polarizers placed in the excitation and emission channels. The polarizers were set to 0° and 54.7°, respectively (magic angle conditions) to render detected emission intensities independent of possible polarization effects. The fluorescence spectra were recorded with spectral bandwidths of 2 nm and 4 nm in excitation and emission, respectively, an integration time of 0.2 s, and a step width of 1 nm. The scan range was 470 nm to 750 nm, 800 nm or 850 nm, with three repetitive scans performed for each sample. All spectra were subsequently corrected for the wavelength-dependent spectral responsivity of the fluorometer’s detection channel.48 Photoluminescence quantum yields (Φ). Φ values, that present the ratio of the number of emitted photons and absorbed photons, were determined absolutely with an integrating sphere setup from Hamamatsu (Quantaurus-QY C11347-11) previously evaluated by us.49 All Φ measurements were performed at 25 °C using special 10 mm x 10 mm long neck quartz cuvettes from Hamamatsu. With this setup, Φ values ≥ 0.01 can be reliably measured. Fluorescence decay kinetics. Fluorescence decay kinetics providing the fluorescence lifetimes (τ) of the PVQ dyes, aggregates, and particles, were recorded with the fluorometer FLS 920 (Edinburgh Instruments) equipped with a supercontinuum laser (Fianium SC400-2-PP), pulsed with a frequency of 10 MHz, and a fast multichannel plate photomultiplier (MCP-PMT) as detector. All samples were excited at the absorption maximum, while the emission was detected at the emission maximum employing a spectral bandwidth of the excitation and emission monochromator of 8 nm, a 4096-channel setting, and time ranges of 20 ns, 50 ns, and 100 ns, respectively. With this setup, τ values ≥ 0.2 ns can be reliably measured. The measured


9


Page 10 of 30

fluorescence decay kinetics were evaluated using the reconvolution procedure of the FAST program (Edinburgh Instruments). This procedure considers the measured instrument response function (IRF) which influences the fluorescence decays. All luminescence decay profiles could be analyzed with mono, bi- or tri-exponential fits with reduced χ² values between 0.8 and 1.2. From the multi-exponential decays, subsequently, the intensity-weighted average lifetimes were calculated. All spectroscopic measurements were performed with air saturated dye solutions at T = 25 °C by using 10 mm x 10 mm quartz cuvettes from Hellma GmbH filled with 3 mL of solvent or dye solution. Confocal laser scanning microscopy (CLSM). CLSM measurements were done with freshly prepared surfactant-stabilized dye crystals and dye-loaded PSP, all transferred onto a coverslip. Microscopic images were recorded with an Olympus FluoView™ FV1000 (Olympus GmbH, Hamburg, Germany). A multi-line argon ion laser (30 mW) was used as excitation source (excitation wavelength of 488 nm), which was reflected by a dichroic mirror (DM405/488) and focused onto the sample through an Olympus objective UPLSAPO 20x (numerical aperture N.A. 0.75). The emitted photons were recollected with the same objective and focused onto a PMT. The detection channel was set to “FITC channel 1” (variable band pass filter position 500 nm, detector range 100 nm). Fluorescence lifetime imaging (FLIM). FLIM measurements were performed with CLSM FV1000 equipped with a Picoquant FLIM-FCS upgrade kit employing an excitation wavelength of 440 nm. A beam splitter (20/80) was used to reflect the excitation light onto the sample. The repetition rate of the laser was 20 MHz. The emission light was collected with a 460-nm long pass filter and detected with an avalanche photodiode (APD). Data acquisition and analysis were


10

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


done with the TimeHarp 200 TCSPC PC board using SymPhoTime software (PicoQuant GmbH).

RESULTS AND DISCUSSION

Crystal structures and optical properties of solid PVQs. PVQ derivatives 1−4 were crystallized from a polar solvent using solvent diffusion (for further details, see experimental section) and then characterized by X-ray diffractometry. The crystal data obtained for the yellow (dyes 1 and 3), orange (dye 2), and red (dye 4) crystals are summarized in Table 1. Table 1. Space groups and unit cell parameters of dyes 1-4. dye

1

2

3

4

Crystal system,

orthorhombic,

monoclinic,

triclinic,

monoclinic,

space group

Pbca

P21/n

P-1

P21

a [Å]

10.316(1)

10.899(1)

7.755(1)

11.359(1)

b [Å]

18.301(1)

14.337(1)

9.966(1)

7.398(1)

c [Å]

17.355(1)

11.098(1)

12.744(1)

11.910(1)

α [°]

90

90

96.96(1)

90

β [°]

90

103.66(1)

91.77(1)

118.48(1)

γ [°]

90

90

107.21(1)

90

The quinoxaline ring and the vinylic double bond are almost coplanar, independent of the substituent at position 2, whereas the torsion angle between the quinoxaline plane and the different substituents vary significantly (see Table 2). While the methoxythienyl-substituted derivative 4 shows a nearly coplanar orientation with a torsion angle of around 20°, the phenyl-


11


Page 12 of 30

substituted quinoxaline 1 has an orthogonal arrangement. The pyrrolyl- and indolyl-substituted dyes 2 and 3 show moderate torsion angles of 48° and 36°. The different torsion angles reflect the substituent size and hence, its steric demand. Table 2. Crystal structures and torsion angles derived from XRD measurements of PVQ dyes 1−4. Derivative

1

2

3

4

87°

48°

36°

20°

pairs

pairs

columns, centrosym.

columns, acentric

3.461 Å

3.378 Å

3.649 Å

3.524 Å

3.471 Å

3.616 Å

Molecular structure in the crystal (30%displacement ellipsoids)

Torsion angle (XRD)

Packing motif

Type of Packing Dist Mpln R(C4N2)a to COR

not relevant; out of directiond

(C4N2‘)b Dist Mpln R(C4N2) to

3.639 Å

(COD)‘(or ‘‘)c


12

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

Mpln R(C4N2): Best least-squares plane of C4N2 ring of quinoxaline moiety; bCOR: Center of

ring; cCOD: Center of double bond of pyrrolidinylvinyl group; dDue to strong bending of molecules. As a result of the different torsion angles, the packing of the PVQ derivatives 1−4 differs significantly (see Table 2). While dyes 1 and 2, which show the largest torsion angles in this series, form only pair packing, PVQs 3 and 4 with their smaller torsion angles reveal a centrosymmetric or acentric column packing. Pair stacking yields a slightly denser packing of the molecules in the crystalline state compared to column stacking. The molecular packing in the solid state is expected to influence the fluorescence properties. On the one hand, a higher packing distance between the dye molecules reduces π − π -stacking interactions and should thus lead to an enhanced emission compared to π-stacked dyes. On the other hand, a smaller distance of the dye molecules in the crystal restricts intramolecular rotations, which can reduce non-radiative decay rates, thereby favoring radiative deactivation of electronically excited molecules. The competition between these two opposing effects determines the size of the observed AIE. We did not perform polarization-dependent absorption and emission studies to derive information on the orientation of the dipole moments of the dyes in the ground and excited state, because these measurements are affected by the order of the dye molecules in the particles and possible energy transfer processes between neighboring dyes. In the solid state, phenyl-substituted dye 1 shows the shortest wavelength emission band of this dye series centered at 569 nm, as follows from Figure 2. The pyrrolyl-, indolyl-, and methoxythienyl-substituted derivatives 2-4 display emission maxima at 576 nm, 588 nm, and 643 nm, respectively. Moreover, their emission spectra show a weak vibronic fine structure with an additional shoulder in the long-wavelength region. The red shift in emission resulting for 3


13


Page 14 of 30

compared to 1 and 2 with monocyclic aromatic substituents is ascribed to its more extended ߨsystem. The methylindole substituent in 3 is a stronger donor than the phenyl substituent in 1, which accounts for the red-shift. Although the pyrrole group in 2 has principally stronger electron-donating properties, the resulting red-shift is smaller because the planarization is hindered due to the methyl substituent. Dye 4 shows the longest wavelength emission within this series, which corresponds to a decreased HOMO-LUMO gap. This is attributed to the overall enhanced positive mesomeric effect of the 5-methoxythiophene moiety.50 The results of the Φ and fluorescence lifetime measurements with 1−4 are summarized in Figure 2b. In the solid state, pyrrolyl-substituted dye 2 shows the lowest Φ of 0.07 and also the shortest τ of 2.50 ns of this PVQ series and indolyl-substituted dye 3 reveals the most intense fluorescence with a Φ of 0.20 and the longest τ of 6.03 ns. This suggests that in the solid state, rotational motions are particularly restricted for 3. The almost constant ratio of both parameters indicates that similar processes determine the photophysics of all four PVQ derivatives.

Figure 2. a) Normalized emission spectra of 1−4 in the solid state (λex = 445 nm, 450 nm, 446 nm, and 454 nm, respectively); inset: photographs of the crystalline solids under natural light (upper panel) and under UV light (lower panel); b) absolutely determined Φ and τ derived from intensity-weighted fluorescence decay kinetics of 1−4 in the solid state.


14

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spectroscopic study of the PVQ derivatives in ethanol−water mixtures. A previous study of the spectroscopic properties of water-insoluble 1−4 in organic solvents of varying polarity had revealed a charge-transfer (CT)-type emission, strong positive solvatochromism, and particularly H-bonding-induced fluorescence quenching by protic solvents like ethanol and methanol or by protonation.44 However, the observation of solid-state fluorescence encouraged us to explore possible AIE effects in this differently substituted PVQ series and we hoped for AIE to overcome polarity- and H-bonding related fluorescence quenching and provide “turn-on” fluorescence in water. Thus, the emission of these dyes was examined in ethanol−water mixtures with increasing amounts of water. As surfactants can promote the initiation of crystallization and can even control the formation of various crystal phases,51-54 the cationic and anionic surfactants CTAB and SDS (concentration always below the critical micelle concentration (CMC)) were added to these solutions to initiate dye aggregation and crystal/particle formation. This led immediately to the formation of highly fluorescent micrometer-sized crystals (heterogenous sizes) with surfactant shape control. In the presence of SDS, we observed needle-like microstructures with lengths of 10-30 µm, while needle-like and hexagonal platelets were formed with CTAB. This is exemplarily shown for crystals of dyes 1 and 3 in Figure 3.


15


Page 16 of 30

Figure 3. CLSM images (λex = 488 nm, λem = 500 nm) of crystals of 1 and 3 formed after addition of SDS or CTAB. In order to gain a better insight into the formation of these highly fluorescent crystals, we studied the changes in the fluorescence properties during aggregation of the PVQ derivatives 1−4 in more detail using dilution series in ethanol−water mixtures of increasing water content from 0% to maximum 99%. Here, water was used to initiate the aggregation of these hydrophobic dyes. The onset and strength of dye aggregation was derived from changes in the absorption and emission spectra, Φ, and τ. The absorption spectra of dyes 1−4 in ethanol−water mixtures of varying water content shown in Figure 4 (left panel) revealed dye aggregation for water fractions exceeding 80% to 90%, depending on the solubility of the respective PVQ derivative in ethanol. The formation of dye aggregates is indicated by an increase in the baseline/background as typical for scattering, a red shift in absorption reflecting the increased polarity of the dye microenvironment (see Table 3), and a broadening of the dye´s absorption band. The latter is a clear hint of an increasing heterogeneity of the dye microenvironment. The smallest red shift was observed for dye 2, which


16

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


does not aggregate up to a water content of 90%. Dye 3 with the most extended conjugated πsystem exhibits the largest aggregation-induced red shift in absorption.


17


Page 18 of 30

Figure 4. Absorption and area-normalized, absorption-weighted emission spectra of 1-4 in ethanol−water mixtures of different water content excited in the corresponding maxima; a+b) phenyl, c+d) pyrrolyl, e+f) indolyl, and g+h) methoxythienyl; insets: change of the emission maxima of the PQV dyes 1-4 in ethanol−water mixtures of varying water content given as water fraction. Table 3. Spectroscopic properties of the PQV derivatives 1−4 in ethanol−water mixtures; the fluorescence was always excited in the absorption maximum.

λex, max [nm]

Red shift of λex, max [cm-1]

λem, max [nm] 0% water

98% water

1195

575

576

467

809

567

446

479

1545

454

488

1534

0% water

98% water

1

445

470

2

450

3 4

Red shift of λem, max [cm-1]

Stokes shift [cm-1] 0% water

98% water

30

5081

3915

569

62

4586

3839

561

567

189

4596

3240

576

613

1048

4665

4179

As shown in Figure 4 (right column), an increasing water fraction leads to a red shift in emission, reflecting the enhanced polarity of the dye´s microenvironment. The onset of dye aggregation at water fractions exceeding 80% to 90% results in a blue shift in emission for compounds 1-3 and an increase in emission intensity for all dyes. The former is attributed to a reduced polarity in dye microenvironment particularly for dye molecules in the dye particle core. For dye 1, this aggregation-induced hypsochromic shift of the emission maximum completely compensates even the initial polarity-induced red shift. Only dye 4 undergoes a continuous red shift in emission yielding a maximum Stokes shift of 1048 cm-1 for the highest water fraction. This indicates that with a decreasing torsion angle and an increasing dye−dye stacking distance


18

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(see Table 2), the shielding of the dye molecules in the inner core of the dye aggregates is diminished. The corresponding emission studies revealed a polarity-induced decrease in Φ and τ in the ethanol−water mixtures with increasing water content as to be expected for these PVQ fluorophores,44 caused by hydrogen bonds between solvent and dye molecules.26 With the beginning of dye aggregation, however, Φ and τ start to strongly increase for all dyes until aggregate formation is completed as shown in Table 4 and Figure 5. This indicates aggregationinduced restriction of the intramolecular flexibility and rotations, that act otherwise as nonradiative deactivation pathways of the electronically excited molecules.

c)

Figure 5. a) Φ and b) τ of 1−4 in ethanol−water mixtures with varying water content; c) images of 1−4 in ethanol−water solutions of 20/80 (V/V) and in ethanol−water solutions of 2/98 (V/V)


19


Page 20 of 30

under UV light revealing the very weak emission of the non-aggregated dyes and a strong aggregation-induced emission enhancement.

Table 4. Minimum and maximum τ and Φ of 1−4 in ethanol−water mixtures. dyes*)

before aggregation

*)

τ**) [ns]

Φ aggregates

before aggregation

aggregates

1

< 0.01

0.068 ± 0.002

< 0.2

4.60 ± 0.15

2

< 0.01

0.048 ± 0.001

< 0.2

2.69 ± 0.10

3

< 0.01

0.045 ± 0.003

< 0.2

1.61 ± 0.19

4

< 0.01

0.055 ± 0.002

< 0.2

1.32 ± 0.19

in ethanol−water mixtures, **) intensity-weighted

Moreover, an increasing aggregate size automatically increases the number of dye molecules in a less polar environment in the core of the dye aggregate, which are shielded from polarityand H-bonding-induced fluorescence quenching. Aggregate formation leads to a Φ enhancement by factors of 4.5 to 6.8 and an increase in τ by factors between 6.6 and 23 for dyes 1-4 (see Table 4). Apparently, the size of the AIE effect increases with an increasing torsion angle. This underline structural control of the aggregate emission by the torsion angle and the stacking of the PVQ dyes imposed by substitution pattern. The different enhancement factors observed in the Φ and fluorescence decay studies are ascribed to the formation of a mixture of fluorescent and nonfluorescent aggregates. Absorbing, yet non-emissive aggregates also contribute to Φ, defined as the number of emitted per number of absorbed photons, which results in an apparent decrease in Φ, whereas only emissive species are detected in the fluorescence decay measurements. The fact that the dye aggregates of 1-4 exhibit lower Φ and shorter τ than the corresponding crystalline


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solids (see Figure 2b) is attributed mainly to dye molecules at the aggregate surface, the emission of which is most likely partly quenched by the high polarity of the surrounding water molecules and H-bonding. Characterization of the dye aggregates. Application of dye aggregates as fluorescent reporters requires knowledge and eventual control of their size, shape, surface charge, and colloidal stability. To assess these features, the dye aggregates obtained after aggregation in ethanol−water mixtures were studied in the dried state with TEM. A representative TEM image of almost spherical aggregates of the pyrrolyl-substituted PVQ derivative 2 derived from an ethanol−water mixture with a water content of 99% is shown in Figure 6 (left). DLS was subsequently used to obtain the hydrodynamic diameters of the different dye aggregates and to monitor the reproducibility of the aggregation processes (see Figure 6). The resulting hydrodynamic diameter of similarly prepared dye aggregates (see section Material preparation; AIE aggregates) are between 190 nm and 430 nm and the polydispersity index (PDI) is between 0.1 and 0.2 except for dye 2, that reveals a PDI of 0.3. In solvent−water mixtures with a water content of 90% to 99%, dye 1 with its larger torsion angle forms always the largest and dye 4 with the smaller torsion angle the smallest aggregates. Obviously, the torsion angle clearly influences the nucleation process and the packing of the molecules during aggregation. Moreover, the hydrodynamic diameter of all dye aggregates decreased with increasing water content. This suggests faster aggregate nucleation for the hydrophobic dye molecules with increasing polarity, yielding a larger number of aggregate nuclei. Subsequently, less dye molecules are available during the growth process, yielding smaller aggregates. At longer growth times, however, all dye aggregates started to form larger macroscopic structures.


21


Page 22 of 30

All attempts to stabilize the nm-sized dye aggregates with surfactants like SDS and CTAB always resulted in the formation of larger µm-sized structures as discussed in a previous section.

Figure 6. Left: Exemplary TEM images of dye aggregates of 2 obtained in ethanol−water solutions of 1/99 (V/V); right: size of the aggregates of dyes 1−4 formed after ten minutes in ethanol−water mixtures of varying water content determined by DLS.

Encapsulation of the PVQ dyes in polymer particles. PVQ dye 1, showing the most pronounced AIE effect, was exemplarily incorporated into 1 µm-sized carboxy-functionalized polystyrene particles (PSP) using a simple swelling procedure.4 The spectroscopic properties of the dye-stained particles were studied with integrating sphere spectroscopy and fluorescence microscopy in dispersion and on the single-particle level. These experiments enable a straightforward assessment of the influence of steric restrictions of intramolecular rotations of the PSP-entrapped AIE dye molecules in a relatively apolar environment, although the PSP matrix is not expected to completely rigidize incorporated dye molecules. Moreover, we chose high concentrations of the encapsulated dyes to still encourage dye−dye interactions within the PSP matrix and hence AIE. Both effects should lead to an increase in the fluorescence efficiency of the PVQ dyes and a prolongation of their fluorescence lifetimes.


22

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As follows from Figure 7a and Table 5, in the less polar polystyrene matrix, the excitation and emission spectra of dye 1 are blue shifted compared to the spectra of the dye in ethanol and the dye aggregates in ethanol−water mixtures. b)

Figure 7. a) Normalized excitation and emission spectra (λex = 339 nm) of dispersed 1µm-sized PSP loaded with PVQ dye 1 and b) CLSM image of the PSP loaded with 6 mM dye 1; the inset displays fluorescence decay curves averaged over ten particles. Moreover, PSP encapsulation leads to a higher Φ and longer lifetimes than observed for the dye in ethanol, ethanol−water mixtures, and in the solid state (see Table 5). This reflects the desired combined influence of the reduced polarity of the dye microenvironment and at least partial restriction of molecular motions. Microscopic studies of single particles confirm homogeneous dye staining of the PSP (see Figure 7b). The fluorescence decay kinetics of individual dye loaded particles obtained with FLIM reveal a fluorescence lifetime τ FLIM = 6.10 ± 0.10 ns (averaged over 10 single particles), which closely matches with the data derived from ensemble studies (see Table 5). This encapsulation strategy can be hence utilized to fine-tune the emission properties of these AIE dyes and paves the road to colloidally stable nanoscale reporters which could be easily surface functionalized, e.g., with PEG linkers and/or target-specific biomolecules.1, 55


23


Page 24 of 30

Table 5. Φ and τ of phenyl substituted PVQ dye 1 in different matrices. λex[nm]

λem [nm]

τ**) [ns]

Φ

τFLIM [ns]

Solid state

445

569

0.187 ± 0.025

4.36 ± 0.19

-

EtOH

445

575

0.029 ± 0.001

0.49 ± 0.15

-

EtOH−water mixture (20/80 v/v)

454

613

< 0.01

< 0.2

-

470

576

0.068 ± 0.002

4.60 ± 0.15

-

440

526

0.194 ± 0.008

6.5 ± 0.57

6.10 ± 0.10

Dye aggregates in EtOH-water mixture Encapsulated 1 µm PSP **)

intensity-weighted

CONCLUSION AND OUTLOOK

In summary, a series of hydrophobic PVQ dyes with phenyl, pyrrolyl, indolyl, and methoxythienyl substituents at the quinoxaline C2 position of the planar dye moiety was synthesized by a four- or five compound one-pot reaction, thereby systematically varying the torsion angle between the planar plane and the substituent. Spectroscopic studies of these dyes revealed an AIE-typical high fluorescence in the solid state and upon dye aggregation in ethanol−water mixtures containing more than 80% water. The good correlation between the size of the torsion angle and the aggregation-induced fluorescence enhancement underlines structural control of AIE by substitution pattern in this dye class, compensating for otherwise observed polarity and H-bonding induced fluorescence quenching. While dye aggregation in ethanol−water mixtures leads to aggregates with hydrodynamic diameters < 450 nm, surfactantinduced dye crystallization provides µm-sized emissive structures. Aggregate size is controlled by the parameters torsion angle and the water content of the ethanol−water mixture, from which


24

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the dye particles are obtained. A decrease in torsion angle and an increase in water content lead to smaller aggregates. Entrapping of the PVQ dyes in carboxy-functionalized polymer particles at high dye loading concentrations presents an elegant way to overcome the low colloidal stability of the aggregates and even optimize their advantageous fluorescence properties. This underline the advantage of encapsulating AIE dyes compared to conventional fluorophores, which often show aggregationinduced fluorescence quenching, resulting in concentration-dependent fluorescence diminution at higher particle staining concentrations.3 Furthermore, this encapsulation strategy enables straightforward surface functionalization, e.g., with biomolecules for subsequent applications as nanoscale reporters in bioanalysis and the life sciences,1, 55 which is currently exploited by us. Moreover, the rational design of PVQ AIE chromophores will be studied in the future. One approach to enhance AIE effects in PVQ dyes could be the synthesis of derivatives with increased torsion angles and stronger electron donors in the 3-position of the planar PVQ plane and an enhancement of the emission solvatochromism in p-phenyl-substituted PVQ dyes with stronger donors. In addition, we will attempt to model the optical properties of such PVQ dye aggregates using quantum mechanical calculations to pave the road for the eventual prediction of molecular structures with optimum optical properties in the aggregated state. AUTHOR INFORMATION Corresponding Authors * U.R.-G.: e-mail, [email protected]; phone, ++49(0)30-8104-1134; fax, ++49(0)30-810471134.


25


Page 26 of 30

* T.J.J.M.: e-mail, [email protected]; phone, ++49(0)211-81-12298; fax, ++49(0)21181-14324. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was funded by Bundesanstalt für Materialforschung und -prüfung (BAM) within the funding programme “Menschen, Ideen” and the Fonds der Chemischen Industrie.

REFERENCES

(1) Quevedo, P. D.; Behnke, T.; Resch-Genger, U. Streptavidin conjugation and quantification— a method evaluation for nanoparticles. Anal. Bioanal. Chem. 2016, 408, 4133-4149. (2) Napp, J.; Behnke, T.; Fischer, L.; Würth, C.; Wottawa, M.; Katschinski, D. M.; Alves, F.; Resch-Genger, U.; Schäferling, M. Targeted Luminescent Near-Infrared Polymer-Nanoprobes for In Vivo Imaging of Tumor Hypoxia. Anal. Chem. 2011, 83, 9039-9046. (3) Behnke, T.; Würth, C.; Laux, E.-M.; Hoffmann, K.; Resch-Genger, U. Simple strategies towards bright polymer particles via one-step staining procedures. Dyes Pigm. 2012, 94, 247257. (4) Behnke, T.; Würth, C.; Hoffmann, K.; Hübner, M.; Panne, U.; Resch-Genger, U. Encapsulation of Hydrophobic Dyes in Polystyrene Micro- and Nanoparticles via Swelling Procedures. J. Fluoresc. 2011, 21, 937-944. (5) Wong, P. T.; Choi, S. K. Mechanisms of drug release in nanotherapeutic delivery systems. Chem. Rev. 2015, 115, 3388-432. (6) Sapsford, K. E.; Algar, W. R.; Berti, L.; Gemmill, K. B.; Casey, B. J.; Oh, E.; Stewart, M. H.; Medintz, I. L. Functionalizing Nanoparticles with Biological Molecules: Developing Chemistries that Facilitate Nanotechnology. Chem. Rev. 2013, 113, 1904-2074. (7) Yaghini, E.; Turner, H. D.; Le Marois, A. M.; Suhling, K.; Naasani, I.; MacRobert, A. J. In vivo biodistribution studies and ex vivo lymph node imaging using heavy metal-free quantum dots. Biomaterials 2016, 104, 182-191. (8) Reisch, A.; Klymchenko, A. S. Fluorescent Polymer Nanoparticles Based on Dyes: Seeking Brighter Tools for Bioimaging. Small 2016, 12, 1968-1992. (9) Zhang, X.; Zhang, X.; Tao, L.; Chi, Z.; Xu, J.; Wei, Y. Aggregation induced emission-based fluorescent nanoparticles: fabrication methodologies and biomedical applications. J. Mater. Chem. B 2014, 2, 4398-4414.


26

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Cheng, W.; Wang, G.; Pan, X.; Zhang, Y.; Tang, B. Z.; Liu, Y. Redox-Responsive Nanoparticles with Aggregation-Induced Emission (AIE) Characteristic for Fluorescence Imaging. Macromol. Biosci. 2014, 14, 1059-1066. (11) Nowak-Krol, A.; Rohr, M. I. S.; Schmidt, D.; Wurthner, F. A Crystalline pi-Stack Containing Five Stereoisomers: Insights into Conformational Isomorphism, Chirality Inversion, and Disorder. Angew. Chem. Int. Ed. Engl. 2017, 56, 11774-11778. (12) Würthner, F.; Kaiser, T. E.; Saha-Möller, C. R. J-Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials. Angew. Chem. Int. Ed. 2011, 50, 3376-3410. (13) Eisfeld, A.; Briggs, J. S. The J- and H-bands of organic dye aggregates. Chem. Phys. 2006, 324, 376-384. (14) Chen, J.; Xu, B.; Ouyang, X.; Tang, B. Z.; Cao, Y. Aggregation-Induced Emission of cis,cis-1,2,3,4-Tetraphenylbutadiene from Restricted Intramolecular Rotation. J. Phys. Chem. A 2004, 108, 7522-7526. (15) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced emission. Chem. Soc. Rev. 2011, 40, 5361-5388. (16) Shustova, N. B.; Ong, T.-C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dincă, M. Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal–Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission. J. Am. Chem. Soc. 2012, 134, 15061-15070. (17) Hu, R.; Leung, N. L. C.; Tang, B. Z. AIE macromolecules: syntheses, structures and functionalities. Chem. Soc. Rev. 2014, 43, 4494-4562. (18) Pauli, J.; Grabolle, M.; Brehm, R.; Spieles, M.; Hamann, F. M.; Wenzel, M.; Hilger, I.; Resch-Genger, U. Suitable Labels for Molecular Imaging - Influence of Dye Structure and Hydrophilicity on the Spectroscopic Properties of IgG Conjugates. Bioconjugate Chem. 2011, 22, 1298-1308. (19) Hennig, A.; Borcherding, H.; Jaeger, C.; Hatami, S.; Würth, C.; Hoffmann, A.; Hoffmann, K.; Thiele, T.; Schedler, U.; Resch-Genger, U. Scope and Limitations of Surface Functional Group Quantification Methods: Exploratory Study with Poly(acrylic acid)-Grafted Micro- and Nanoparticles. J. Am. Chem. Soc. 2012, 134, 8268-8276. (20) Hennig, A.; Hatami, S.; Spieles, M.; Resch-Genger, U. Excitation energy migration and trapping on the surface of fluorescent poly(acrylic acid)-grafted polymer particles. Photochem. Photobiol. Sci 2013, 12, 729-737. (21) Weissleder, R.; Tung, C.-H.; Mahmood, U.; Bogdanov Jr, A. In vivo imaging of tumors with protease-activated near-infrared fluorescent probes. Nature Biotechn. 1999, 17, 375. (22) Kobayashi, H.; Choyke, P. L. Target-Cancer-Cell-Specific Activatable Fluorescence Imaging Probes: Rational Design and in Vivo Applications. Acc. Chem. Res. 2011, 44, 83-90. (23) Qin, W.; Li, K.; Feng, G.; Li, M.; Yang, Z.; Liu, B.; Tang, B. Z. Bright and Photostable Organic Fluorescent Dots with Aggregation-Induced Emission Characteristics for Noninvasive Long-Term Cell Imaging. Adv. Funct. Mater. 2014, 24, 635-643. (24) Ning, Z.; Chen, Z.; Zhang, Q.; Yan, Y.; Qian, S.; Cao, Y.; Tian, H. Aggregation-induced Emission (AIE)-active Starburst Triarylamine Fluorophores as Potential Non-doped Red Emitters for Organic Light-emitting Diodes and Cl2 Gas Chemodosimeter. Adv. Funct. Mater. 2007, 17, 3799-3807. (25) Qin, W.; Ding, D.; Liu, J.; Yuan, W. Z.; Hu, Y.; Liu, B.; Tang, B. Z. Biocompatible Nanoparticles with Aggregation-Induced Emission Characteristics as Far-Red/Near-Infrared


27


Page 28 of 30

Fluorescent Bioprobes for In Vitro and In Vivo Imaging Applications. Adv. Funct. Mater. 2012, 22, 771-779. (26) Zheng, Z.; Yu, Z.; Yang, M.; Jin, F.; Zhang, Q.; Zhou, H.; Wu, J.; Tian, Y. Substituent Group Variations Directing the Molecular Packing, Electronic Structure, and AggregationInduced Emission Property of Isophorone Derivatives. J. Org. Chem. 2013, 78, 3222-3234. (27) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D., et al. Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem. Commun. 2001, 1740-1741. (28) Huang, J.; Sun, N.; Yang, J.; Tang, R.; Li, Q.; Ma, D.; Li, Z. 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, 7645-7654. (29) Tong, H.; Hong, Y.; Dong, Y.; Ren, Y.; Häussler, M.; Lam, J. W. Y.; Wong, K. S.; Tang, B. Z. Color-Tunable, Aggregation-Induced Emission of a Butterfly-Shaped Molecule Comprising a Pyran Skeleton and Two Cholesteryl Wings. J. Phys. Chem. A 2007, 111, 2000-2007. (30) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-induced emission: phenomenon, mechanism and applications. Chem. Commun. 2009, 4332-4353. (31) Zhao, Z.; Chen, S.; Shen, X.; Mahtab, F.; Yu, Y.; Lu, P.; Lam, J. W. Y.; Kwok, H. S.; Tang, B. Z. Aggregation-induced emission, self-assembly, and electroluminescence of 4,4'-bis(1,2,2triphenylvinyl)biphenyl. Chem. Commun. 2010, 46, 686-688. (32) Kokado, K.; Chujo, Y. Multicolor Tuning of Aggregation-Induced Emission through Substituent Variation of Diphenyl-o-carborane. J. Org. Chem. 2011, 76, 316-319. (33) Wu, W.-C.; Chen, C.-Y.; Tian, Y.; Jang, S.-H.; Hong, Y.; Liu, Y.; Hu, R.; Tang, B. Z.; Lee, Y.-T.; Chen, C.-T., et al. Enhancement of Aggregation-Induced Emission in Dye-Encapsulating Polymeric Micelles for Bioimaging. Adv. Funct. Mater. 2010, 20, 1413-1423. (34) Huang, J.; Sun, N.; Dong, Y.; Tang, R.; Lu, P.; Cai, P.; Li, Q.; Ma, D.; Qin, J.; Li, Z. Similar or Totally Different: The Control of Conjugation Degree through Minor Structural Modifications, and Deep-Blue Aggregation-Induced Emission Luminogens for Non-Doped OLEDs. Adv. Funct. Mater. 2013, 23, 2329-2337. (35) Li, Z.; Dong, Y. Q.; Lam, J. W. Y.; Sun, J.; Qin, A.; Häußler, M.; Dong, Y. P.; Sung, H. H. Y.; Williams, I. D.; Kwok, H. S., et al. Functionalized Siloles: Versatile Synthesis, AggregationInduced Emission, and Sensory and Device Applications. Adv. Funct. Mater. 2009, 19, 905-917. (36) Lu, H.; Zhao, X.; Tian, W.; Wang, Q.; Shi, J. Pluronic F127-folic acid encapsulated nanoparticles with aggregation-induced emission characteristics for targeted cellular imaging. RSC Adv. 2014, 4, 18460-18466. (37) Chan, C. P.-y.; Cheung, Y.-c.; Renneberg, R.; Seydack, M., New Trends in Immunoassays. In Biosensing for the 21st Century, Renneberg, R.; Lisdat, F., Eds. Springer Berlin Heidelberg: Berlin, Heidelberg, 2008; pp 123-154. (38) Wang, Y.; Chen, Y.; Wang, H.; Cheng, Y.; Zhao, X. Specific Turn-On Fluorescent Probe with Aggregation-Induced Emission Characteristics for SIRT1 Modulator Screening and LivingCell Imaging. Anal. Chem. 2015, 87, 5046-5049. (39) Chan, C. Y. K.; Zhao, Z.; Lam, J. W. Y.; Liu, J.; Chen, S.; Lu, P.; Mahtab, F.; Chen, X.; Sung, H. H. Y.; Kwok, H. S., et al. Efficient Light Emitters in the Solid State: Synthesis, Aggregation-Induced Emission, Electroluminescence, and Sensory Properties of Luminogens with Benzene Cores and Multiple Triarylvinyl Peripherals. Adv. Funct. Mater. 2012, 22, 378389.


28

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Galer, P.; Korošec, R. C.; Vidmar, M.; Šket, B. Crystal Structures and Emission Properties of the BF2 Complex 1-Phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione: Multiple Chromisms, Aggregation- or Crystallization-Induced Emission, and the Self-Assembly Effect. J. Am. Chem. Soc. 2014, 136, 7383-7394. (41) Wang, M.; Zhang, G.; Zhang, D.; Zhu, D.; Tang, B. Z. Fluorescent bio/chemosensors based on silole and tetraphenylethene luminogens with aggregation-induced emission feature. J. Mater. Chem. 2010, 20, 1858-1867. (42) Müller, T. J. J. Multicomponent and domino syntheses of AIE chromophores; Fujiki, M.; Tang, B. Z.; Liu, B. Aggregation Induced Emission: Materials and Applications; ACS Symposium Series e-book, 2016, pp 85-112. (43) Wiśniewski, Ł.; Deperasińska, I.; Staszewska, A.; Stefanowicza, P.; Berski, S.; Lipkowski, P.; Szewczuka, Z.; Szemik-Hojniak, A. Photophysical Properties of Dipeptides Containing Substituted 3-(Quinoxalin-6-yl) Alanine. Spectroscopic Studies and Theoretical Calculations. J. Phys. Chem. A 2010, 114, 9405-9412. (44) Gers-Panther, C. F.; Fischer, H.; Nordmann, J.; Seiler, T.; Behnke, T.; Würth, C.; Frank, W.; Resch-Genger, U.; Müller, T. J. J. Four- and Five-Component Syntheses and Photophysical Properties of Emission Solvatochromic 3-Aminovinylquinoxalines. J. Org. Chem. 2017, 82, 567578. (45) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr. 2008, A64, 112-122. (46) Sheldrick, G. M. SHELXT - integrated space-group and crystal-structure determination. Acta Crystallogr. 2015, C71, 3-8. (47) SHELXTL NT vers. 5.10,. BRUKER ANALYTCAL X-RAY SYSTEMS, 1997. (48) Resch-Genger, U.; DeRose, P. C. Characterization of photoluminescence measuring systems (IUPAC Technical Report). Pure Appl. Chem. 2012, 84, 1815-1835. (49) Würth, C.; Lochmann, C.; Spieles, M.; Pauli, J.; Hoffmann, K.; Schuttrigkeit, T.; Franzl, T.; Resch-Genger, U. Evaluation of a Commercial Integrating Sphere Setup for the Determination of Absolute Photoluminescence Quantum Yields of Dilute Dye Solutions. Appl. Spectrosc. 2010, 64, 733-741. (50) Saino, S.; Saikawa, M.; Nakamura, T.; Yamamura, M.; Nabeshima, T. Remarkable red-shift in absorption and emission of linear BODIPY oligomers containing thiophene linkers. Tetrahedron Lett. 2016, 57, 1629-1634. (51) Kumar, S.; Chawla, G.; Bansal, A. K. Role of Additives like Polymers and Surfactants in the Crystallization of Mebendazole. YAKUGAKU ZASSHI 2008, 128, 281-289. (52) Canselier, J. P. The effects of surfactants on crystallization phenomena. J. Disp. Sci. Techn. 1993, 14, 625-644. (53) Rodríguez-Hornedo, N.; Murphy, D. Surfactant-facilitated crystallization of dihydrate carbamazepine during dissolution of anhydrous polymorph. J. Pharm. Sci. 2004, 93, 449-460. (54) Wei, H.; Shen, Q.; Zhao, Y.; Zhou, Y.; Wang, D.; Xu, D. Effect of anionic surfactant– polymer complexes on the crystallization of calcium carbonate. J. Cryst. Growth 2004, 264, 424429. (55) Behnke, T.; Mathejczyk, J. E.; Brehm, R.; Würth, C.; Gomes, F. R.; Dullin, C.; Napp, J.; Alves, F.; Resch-Genger, U. Target-specific nanoparticles containing a broad band emissive NIR dye for the sensitive detection and characterization of tumor development. Biomaterials 2013, 34, 160-170.


29


Page 30 of 30

TOC GRAPHIC


30

Crystallization and Aggregation-Induced Emission in a Series of

Recommend Documents