Supramolecular Aggregates of Tetraphenylethene Cored AIEgen

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Surfaces, Interfaces, and Applications

Supramolecular Aggregates of Tetraphenylethene Cored AIEgen towards Mechanoluminescent and Electroluminescent Devices Sandip Biswas, Debabrata Jana, Gundam Sandeep Kumar, Subrata Maji, Pronab Kundu, Uttam Kumar Ghorai, Rajendra P Giri, Bidisa Das, Nitin Chattopadhyay, Binay K Ghorai, and Somobrata Acharya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00165 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

Supramolecular Aggregates of Tetraphenylethene Cored AIEgen towards Mechanoluminescent and Electroluminescent Devices Sandip Biswas,† Debabrata Jana,‡ Gundam Sandeep Kumar,† Subrata Maji,† Pronab Kundu,┴ Uttam K. Ghorai, ╧ Rajendra P. Giri,§ Bidisa Das,╩ Nitin Chattopadhyay,┴ Binay K. Ghorai,‡ and Somobrata Acharya*,† †

Centre for Advanced Materials (CAM), Indian Association for the Cultivation of Science,

Jadavpur, Kolkata-700032, India. ╩

Technical Research Center (TRC), Indian Association for the Cultivation of Science, Jadavpur,

Kolkata-700032, India. ‡

Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,

Howrah 711103, India §

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, HBNI, 1/AF,

Bidhannagar, Kolkata 700064, India. ┴

Department of Chemistry, Jadavpur University, Kolkata-700032, India.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

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Department of Industrial Chemistry and Applied Chemistry, Swami Vivekananda Research

Center, Ramakrishna Mission Vidyamandira, Belurmath, Howrah 711202, India. KEYWORDS: Aggregation induced emission, Air-water interface, Spherical Aggregate, Mechanoluminescence, Light emitting diodes.

ABSTRACT:

Luminescent materials containing both the mechanoluminescence and

electroluminescence properties are the quest for sensing and optoelectronic applications. We report on the synthesis of a new tailor-made luminogen, 1,2-bis(4-(1-([1,1ʹ-biphenyl]-4-yl)-2,2diphenylvinyl)phenyl)-1,2-diphenylethene (TPE 5) using Suzuki coupling reaction with high yield. Aggregation induced emission (AIE) active complex TPE 5 forms supramolecular spherical aggregates at the air-water interface of a Langmuir trough. As a consequence, a large enhancement of luminescence is obtained from the mono and multilayer Langmuir Blodgett films of TPE 5 owing to the AIE effect. The luminogen TPE 5 exhibits reversible mechanoluminescence response displaying photoluminescence switching due to change in the crystalline states under external stimuli. The unique feature of luminescence enhancement upon aggregate formation is utilized for the fabrication of light emitting diodes with low threshold voltage using supramolecular aggregates as active layer. This work demonstrates an efficient strategy for obtaining controlled supramolecular aggregates of AIE-gen with a potential in the dual applications of mechanoluminescence and electroluminescence.

INTRODUCTION Organic solids with mechanoluminescence (MCL) and high photoluminescence (PL) efficiency have drawn tremendous attention for possible application in mechanosensors, optical recording, security inks, flat-panel displays and lighting sources.1-7 The mechanochromic luminescence is


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mainly caused by morphological transition from crystalline to amorphous state or between two different crystalline states under external stimulus such as mechanical stress, organic vapor, temperature etc.8-12 Development of organic luminescent material with high PL efficiency in the solid state is another important parameter for light emitting diodes (LEDs) and displays. However, a major problem associated with most of the organic luminogens is the quenching of luminescence in the aggregated state.13-14 On the contrary, luminescence increases by orders of magnitude in aggregated state owing to the aggregation induced emission (AIE) effect for the AIE luminogens.15-21 Since intense luminescence can be realized in solid state, the AIE effect adds advantages in fabricating solid state devices. Broadly, AIE phenomenon arises mostly due to restriction of intramolecular rotations (RIR) and intramolecular vibrations (RIV), which makes non-radiative relaxation channels of the excited state to decay via radiative pathways. Tetraphenylethene (TPE) and its derivatives are AIE luminogens with extended π-electron conjugation, which are of interest in developing mechanochromic materials with intense PL.22-25 The non-planar propeller shaped conformation of TPE is detrimental for π-π stacking, which usually results in quenching of the PL.26 Loosely packed aggregated structure of TPE often contains metastable morphology, which induces mechano-responsive PL behavior by changing the molecular conformation under external stimuli.19,26 TPE is a luminogenic material, however, attachment of other chromophores to TPE leads to outstanding improvement in the AIE properties. Recently, AIE luminogens with TPE moiety possessing mechanochromic behavior have been reported.22-25,27-33 These compounds are also called piezofluorochromic aggregationinduced emission (PAIE) materials since both piezofluorochromic and AIE properties can be realized.34-36 Hence, PAIE materials are advantageous in practical applications since PL can be tuned by AIE effect under external mechanical stimuli. However, reports on the compounds


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possessing both MCL and AIE properties are indeed limited. Additionally, AIE effect have been observed in most of the cases by using anti-solvent to induce aggregation in solution phase or using closely packed proximal effect in solid state.18,37,38 However, attempt to fabricate highly ordered aggregates of AIE active complexes using a supramolecular approach have been rare till date.39 Advantageously, the Langmuir-Blodgett (LB) technique offers control over the packing, orientation and ordering of molecules.40-44 Fabrication of highly ordered supramolecular aggregates using LB technique can restrict conformation of AIE active complexes in an efficient way to facilitate radiative pathways for enhanced luminescence, comparable to solid state.39 Herein, we report on the synthesis of a new AIE luminogen 1,2-bis(4-(1-([1,1ʹ-biphenyl]4-yl)-2,2-diphenylvinyl)phenyl)-1,2-diphenylethene (TPE 5, Compound 5 in Scheme 1) using a facile synthesis route. Fabrication of supramolecular spherical aggregates in a controllable and efficient way at the air-water interface using the LB technique is described. Insight investigation reveals prevention of π-π stacking of TPE 5 at the air-water interface. As a result, a large enhancement of PL in comparison to the solution is obtained owing to the RIR and RIV processes within the ordered spherical aggregates. The LB films of TPE 5 show mechanoresponse under external stimuli displaying reversibility of PL color tuning over repetitive cycles. The rapid enhancement of PL is utilized for fabricating LEDs with a low threshold voltage using TPE 5 as active layer. The devices show maximum luminance of ~2808 Cd m-2 and external quantum efficiency of ~2.5% respectively. RESULTS AND DISCUSSION Synthesis: A Pd-catalyzed Suzuki coupling reaction was used to synthesize TPE derivative TPE 5, which is outlined in Scheme 1. The synthetic steps and characterization techniques are described in supporting information in details. 4-(2,2-Diphenylvinyl)benzene (1) was prepared


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according to previously reported method.45 Compound 4 was synthesized from 4bromobenzophenone utilizing the conventional McMurry coupling procedure.46 Treatment of 1 with bromine in chloroform resulted in 4-(1-bromo-2,2-diphenylvinyl)biphenyl (2) with 62% yields.

O B

Br

ii

i

O

65%

62% 2

1

3

Br

3, iii 56% Br

4 5

Scheme 1. Steps for the synthesis of TPE 5. Reagents and conditions (i) Br2, CHCl3, Room temperature. (ii) (PPh3)2PdCl2, PPh3, bis(pinacolato)diboron, KOPh, toluene, 70 °C, 12 h. (iii) Pd(PPh3)4, K2CO3, toluene, H2O, Aliquot® 336, 90 °C, 16 h. The boronic ester (3) was prepared utilizing the procedure developed by Miyaura et al.47 Palladium catalyzed borylation of bromo derivative (2) with bis(pinacolato)diboron in presence of potassium phenoxide (KOPh) as a base yielded the boronic ester (3) in 65% yield. Finally, Suzuki

coupling

of

compound

4

with

boronic

ester

(3)

in

presence

of

Pd(PPh3)4/K2CO3/toluene/H2O provided TPE 5 with 56% yields. The structure of the final compound TPE 5 was confirmed by IR, 1H,

13

C NMR spectroscopic techniques and MALDI-

TOF MS measurements (Figure S1-S3) (supporting information, synthesis details).


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Surface activity at the air-water interface: TPE 5 is insoluble in water because of the presence of hydrophobic groups, hence it form a monolayer at the air-water interface at room temperature. The surface pressure (π)-area per molecule (A) isotherm shows a liquid-expanded phase at low pressure followed by a liquid-condensed region, which collapses slightly above 30 mN m-1 (Figure S4a). A limiting molecular area of ~9.2 Å2 per molecule is extracted from the π–A isotherm curve which is lower than the geometric area of a single TPE 5 molecule (~380 Å2) calculated using Density Functional Theory (DFT) in minimum energy conformation (Figure S5). A lowering of molecular area suggests possible formation of aggregates of TPE 5 molecules at the air-water interface. Lower surface activity at the air-water interface is expected from the hydrophobic nature of the TPE 5, which causes lesser interaction of TPE 5 with the water subphase. In order to investigate the monolayer behavior of TPE 5, we performed successive compression-expansion isotherm cycles below the collapse phase (Figure S4b). A large hysteresis is observed between the first compression and expansion isotherms suggesting formation of aggregated structures. The hysteresis reduces for the successive compression and expansion cycles suggesting that the major structural change of the monolayer occurs during the first compression. The irreversibility of the compression and expansion cycles indicates the existence of strong intermolecular interaction in the monolayer. The surface potential (∆V) - area per molecule (A) isotherm during the uniaxial compression process reveals possible dipolar orientation at the air-water interface (Figure S4a).48 An optimal dipole moment is expected for parallel arrangement of TPE 5 molecules where the direction of the effective molecular dipole moment is perpendicular to the air-water interface. The ∆V–A isotherm does not show a change at the initial stage of compression. The ∆V starts rising from a mean molecular area of ~12 Å2, which is higher than the limiting area per molecule of TPE 5 observed in the π–A isotherm


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curve. The difference originates from the fact that the π–A isotherm reflects short range interactions, while the ∆V–A isotherm is dominated by the long range dipole-dipole interactions.48 A rapid compression-induced increase of the effective molecular dipole moment is observed at higher surface pressure suggesting a preferred orientation of TPE 5 dipole moment perpendicular to the air-water interface. Morphological analysis: Various nano-structured morphology of different TPE derivatives in the aggregated state are reported in the literature recently.25,26,28,49,50 These aggregated structures are generally achieved by the addition of anti-solvent (mostly water) into the solutions of TPE

a

b

1.0µm

1.0µm

c

d

1.0µm

1.0µm

Figure 1. AFM topography images of TPE 5 at the air-water interface showing spherical aggregates at surface pressures (a) 5 mN m-1 (b) 10 mN m-1 (c) 15 mN m-1 (d) 20 mN m-1.


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derivatives. The fundamental difference of our work relies in studying the aggregation behavior of TPE 5 at the air-water interface with the aid of the surface pressure. LB monolayer films are lifted at different surface pressures following the π–A isotherm curve (Figure S4a). Atomic force microscopy (AFM) topographic image reveals formation of spherical shaped aggregates at a low surface pressure of π = 5 mN m-1 (Figure 1a). The spheres are found to be interconnected even at the low surface pressure. An increase in the packing density of the spherical aggregates is observed at higher surface pressures (Figure 1b and c). Finally, densely packed monolayer of spherical aggregates is obtained at π = 20 mN m-1 (Figure 1d). Cross-sectional analysis of AFM image suggests uniform height of ~25 ± 5 nm and diameter of ~175 ± 25 nm of the spherical aggregates for surface pressure of π = 5 mN m-1 (Figure S6). A control experiment by drop casting TPE 5 on top of mica surface from chloroform solution results in the irregular aggregates (Figure S7) suggesting that air-water interface determines the conformation of TPE 5 leading to the spherical aggregates. Interestingly, the height and the diameter of the spherical aggregates remain the same with increasing the surface pressure (Figure S8-S10). These observations imply that spherical aggregates of TPE 5 are formed upon spreading at the air-water interface irrespective of the compression stages. The increase of surface pressure effectively fills voids thus rigidifying Langmuir monolayer via close packing of the spherical aggregates. Energy optimized geometry of TPE 5 shows a twisted nature (Figure S5). A lower area per molecule value obtained from π–A isotherm curve (Figure S4a) in comparison to the total geometric area of TPE 5 (Figure S5) suggests a highly twisted configuration at the air-water interface. Benzene ring can serve both as a hydrogen bond donor and acceptor with water.51 Negatively charged benzene π-cloud donates electron density to the hydrogen atom of water acting as a hydrogen bond acceptor. On the other hand, the lone pairs on the oxygen atoms of


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water molecule interact with two hydrogens of the benzene ring making benzene a hydrogen bond donor. Hence, phenyl rings facilitate strong benzene-H2O interaction at the air-water interface forming a stable monolayer of TPE 5 at the air-water interface.51-54 Based on this concept, DFT calculation55 and AFM observations, we propose a tentative model for the aggregate formation at the air-water interface (Figure 2). The conformation of an isolated TPE 5 and the dimers were determined by full geometry optimization using M0656 functional with the 6-31G** basis-set.

Figure 2. Schematic showing formation of spherical aggregates of TPE 5 at the air-water interface. (i) Energy minimized dimeric structure of TPE 5 using DFT. C-H···π interactions are shown by red circles. The perpendicular distance for H atom interacting with the phenyl ring is marked by red lines. (ii) Equivalent cartoon structure of TPE 5 showing propeller shaped TPE units of adjacent molecules. (iii) Aggregation of the TPE 5 molecules into larger unit. (iv) Cross-


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sectional view of the spherical aggregate at the air-water interface composed by the stacking of TPE 5 molecules. (v) Close packed spherical aggregates with the aid of surface pressure. The phenyl rings in TPE 5 remain in highly twisted configuration (Figure S5). We observe that the dimeric assembly of adjacent TPE 5 molecules is feasible in anti TPE 5 conformer owing to the shorter width, which enables several prominent C-H···π interactions (Figure 2). It is evident that AIE luminogen TPE 5 contains propeller shaped TPE sections. In the aggregated state, the propeller shape of the TPE molecule prevents π-π stacking. Adjacent TPE 5 molecules stack via C-H···π interaction between hydrogen atoms in the phenyl rings of one TPE 5 molecule and the π electrons of the phenyl rings of another adjacent TPE 5 molecule (Figure 2). The perpendicular distances in the C-H···π interactions are found within the range of 2.55 Å to 2.8 Å. These CH···π interactions stiffen the conformation of the TPE 5 molecules and finally spherical aggregates are formed with higher units of TPE 5 molecules. Surface pressure brings these spherical aggregates into closer proximity as evidenced from the AFM observation (Figure 1). Higher surface pressure only causes the fulfilling of the voids of the monolayer film by close packing of these spherical aggregates (Figure 2). Photophysical properties and Aggregation induced emission (AIE) phenomenon: Figure 3a shows the comparison of UV-vis absorption spectra of monolayer LB films deposited at various surface pressures along with the solution absorption spectrum of TPE 5. The solution spectrum shows two distinct peaks at ~296 nm and ~320 nm respectively. The monolayer LB films show a red shifted board peak at ~330 nm in comparison to the solution spectrum suggesting aggregate formation in the LB films (Figure 3a). The absorption spectra of monolayer LB films show an increase in the absorbance with increasing surface pressure. However, the peak position remains unchanged suggesting the formation of a more compact monolayer with increasing surface pressure. Hence, the absorption spectra suggest that the aggregates of TPE 5 form immediately


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after spreading at the air-water interface and surface pressure brings the aggregates into closure proximity resulting in an increase in the absorbance. Photoluminescence (PL) spectra for monolayer LB films deposited at different surface pressures show a single broad peak centered at 498 nm (Figure 3b). The peak intensity increases rapidly with the increase in the surface pressure, although the peak position remains the same (Figure S11a). We have measured the PL quantum yield (QY) of TPE 5 solution and the monolayer films using a calibrated integrated

a

300

400

500

b

600

400

Wavelength (nm) 3 Layers 5 Layers 7 Layers 9 Layers 11 Layers 10-4 (M) 10-5 (M) 10-6 (M)

1125

10-7 (M)

400

500

600

500

600

Wavelength (nm)

d

AIE factor

c

5 mN/m 10 mN/m 15 mN/m 20 mN/m 25mN/m

Intensity (a.u.)

Absorbance (a.u.)

5 mN/m 10 mN/m 15 mN/m 20 mN/m 25 mN/m Solution

Intensity (a.u)

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


700

1050

5

3

7

9 11

975 2

4

Wavelength (nm)

6

8

10

12

14

Number of layers

Figure 3. (a) UV-vis absorption spectra of TPE 5 in chloroform and monolayer LB films lifted at different surface pressure. (b) PL spectra of LB monolayer at different surface pressure. PL is obtained with an excitation wavelength of 380 nm. (c) PL spectra of solution with different concentrations and different layer numbered LB films deposited at fixed surface pressure 20 mN m-1. The color codes are shown in the insets of the figure a, b and c. (d) Plot of AIE factor versus


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number of layers. Inset: Photographs of LB films with different number of layers deposited at fixed surface pressure 20 mN m-1 under 365 nm UV illumination. sphere. The solution (10-4 M) phase shows a low QY of 0.05% revealing TPE 5 as a weak luminescent molecule in chloroform. In comparison, QYs of the monolayer LB films are found to be in the range of ~45% to 47% within the surface pressure range of 5 mN m-1 to 25 mN m-1. We have investigated the three-dimensional aggregation property of TPE 5 by depositing multilayer LB films at a fixed surface pressure of 20 mN m-1. The extended AIE behavior of the multilayered LB films is compared with the TPE 5 solution of varying concentrations ranging from 10-7 M to 10-4 M. UV-vis absorption spectra of multilayer films show a peak at 330 nm which is red shifted with respect to solution spectra (Figure S11b). The absorbance of the LB films increases gradually with the increase in the layer number suggesting a uniform transfer of the monolayer during LB deposition process (Figure S11b). The PL spectra of multilayer LB films show a single broad peak at 498 nm (Figure 3c). The intensity of the PL from the LB films is immensely intense in comparison to the solution spectrum. For example, the PL from 3monolayer film shows significantly intense PL in comparison to the solution of high concentration (10-4 M). Additionally, the retention of the peak positions in the absorption and PL spectra of the LB films points out to the fact that the morphology of the TPE 5 aggregates remains identical in the Langmuir monolayer and upon transfer as LB films. The QY of 3monolayer LB film is ~48%, which is 950 fold higher than solution QY (0.05%). Moreover, the PL intensity of LB films increases sharply with the number of layers suggesting the three dimensional extended aggregation of TPE 5 (Figure 3c and Figure S11c). The PL QYs are found to be ~ 51% for 5 layers, 55% for 7 layers, 56% for 9 layers and 57% for 11 layers respectively. In comparison, a low QY of ~18% is recorded for dropcast film of TPE 5. Photograph of multilayer LB films deposited on quartz substrate show green color under the UV illumination of


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365 nm (inset, Figure 3d), which is in-line with the single peak PL observed for different number of layers. The morphology of multilayer LB films is characterized by AFM and scanning electron microscopy (SEM). Both the AFM and SEM observations reveal that TPE 5 retains the spherical shaped aggregated structure in multilayer LB film, however, the packing of multilayer film increases upon increasing the number of layers (Figure S12). The AIE effect is further quantified from the AIE factor (αAIE) which is the ratio of the QYs of aggregated state and solution. Figure 3d depicts the αAIE with the number of LB film layers deposited at π = 20 mN m-1. A low QY of 0.05% in the solution phase of TPE 5 is attributed to the active intermolecular rotation through which energy is dissipated.18 Drastic increase in the αAIE is observed with increase in the number of monolayer and ultimately it reaches to an optimum level for higher number of layers (Figure 3d). The multilayer LB films show intense PL owing to the RIR and RIV in the aggregated state.13,18 Increasing number of monolayers of the LB films rigidify the packing density thereby restricting the intramolecular vibration of phenyl rotors. Additionally van der Waals forces and interlayer C-H···π interaction33,57 in multilayer can further restrict and stiffen molecular conformation that facilitates strong AIE behavior. We have also measured the time resolved PL decays of the solution and LB films of TPE 5 and compared with the dropcast film of TPE 5 (Figure S13). A fast lifetime ~60 ps is measured for the solution in comparison to the longer lifetime of 2.20 µs for the 15 layer LB film and 1.9 µs for drop cast film (Figure S13a and S13b). Long lived excited state lifetime of LB film points out phosphorescent nature in ambient temperature.19,39 We anticipate that the intersystem crossing leading to phosphorescence is efficient mostly in the ordered aggregated LB films by preventing active molecular vibrations and rotations. Such an origin of longer PL lifetime corresponding to phosphorescence is often displayed by AIE complexes.57,58 Figure S14


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displays the comparison of UV-vis absorption and PL spectra of dropcast film with LB film as well as solution of TPE 5. A comparison of the photophysical properties (Table S2, supporting information) suggests that strong AIE activity of the TPE 5 can be realized at air-water interface. Mechanochromic

behavior:

Since

the

TPE

derivatives

are

susceptible

towards

vapochromism,23 we have tested the MCL activity of multilayer LB film of TPE 5 utilizing the vapochromic property. We have deposited multilayer LB film on a quartz substrate at π = 20 mN m-1 and fumed it in acetone vapor for 30 mins. The color of the substrate changes from green to cyan after acetone vapor treatment (Figure 4a). Interestingly, original green color can be reverted by thermal annealing of the fumed LB film. This observation suggests color reversibility of TPE 5 under external stimuli (Figure 4a). The nature of PL spectra of multilayer LB film of TPE 5 with repeated acetone fumigation and thermal annealing are shown in Figure 4b. Acetone fumigation causes a blue shift of the PL peak from 498 nm to 472 nm, which reverts back to 498 nm upon thermal annealing. Excellent color switching for several cycles has been obtained for the LB film under fumigation and annealing (inset, Figure 4b). We have examined the AIE properties of TPE 5 in liquid nitrogen at 77 K. Weakly emissive chloroform solution of TPE 5 turns into highly emissive in the glass form (Figure 4c). The PL spectrum in glass form shows a blue shift of ~43 nm in comparison to the solution phase at ambient condition owing to the rigid configuration of TPE 5 at low temperature, which changes the molecular conformation in glass form (Figure 4d). The original PL peak position at 498 nm of the solution state is reverted at ambient temperature (Figure 4d). The process is repeatable over several cycles (inset, Figure 4d) suggesting low temperature can act as an external stimulus for the MCL activity of TPE 5. While the solution of TPE 5 shows reversibility in PL with temperature, the LB film retains the PL peak at 498 nm at 77 K. Owing to the presence of flexible propeller geometry, TPE 5 can


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Figure 4. Photographs of 15 layer LB film on a quartz substrate deposited at π = 20 mN m-1 (a) Under the 365 nm UV illumination (left). Acetone fumigated LB film under the 365 nm UV illumination (right). After annealing treatment for 30 minutes at 150 °C the acetone fumigated LB film shows the same color under the 365 nm UV illumination as shown in left panel. (b) PL spectra of corresponding LB film upon acetone fumigation and annealing for different cycles. Switching of PL maxima of LB film upon acetone fumigation and annealing for different cycles is shown in the inset. (c) Photograph of TPE 5 in chloroform solution at ambient condition under UV illumination (left) and immediately after removal from liquid nitrogen under UV illumination (right). (d) Comparison of PL spectra of solution, corresponding low temperature glass form and LB film at 77 K. Inset: Switching of PL maxima of solution and glass form is shown in the inset.


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rigidify during the solution phase to glass formation process at low temperature making it susceptible for MCL response.27-36 However, the existence of close packed aggregates within the LB films restrict the RIR and RIV processes of TPE 5, thus preventing the PL to be tuned with temperature for the LB film. Notably, the example of AIE molecules with higher number of TPE moieties showing MCL behavior is rare.59 TPE 5 in the aggregated form shows MCL behavior and PAIE response in spite of containing higher TPE moieties.

Figure 5. XRD pattern of 15 layer LB film of TPE 5 at different conditions lifted on top of glass substrate at surface pressure of 20 mN m-1 (a) Pristine LB film (b) Upon acetone exposure of the pristine LB film for 30 mins (c) After thermal annealing of the acetone fumigated LB film for 30 minutes at 150° C. (d) Multilayer LB film lifted on filter paper, after fumigation under acetone exposure, the letters “AIE” are written on the paper with a metal spatula and subsequently the green colored path is erased by the acetone fume. Photographs are taken under UV light. In order to elucidate the origin of the PL spectra during the mechanochromic process, we have measured the photoluminescence excitation (PLE) spectra of TPE 5 under different conditions. The PLE peaks remain identical for the solution, glass form, and LB film before acetone exposure and for fumigated LB film (Figure S15). These observations confirm that the PL is not associated with the excimer formation, rather depends on the molecular conformation of TPE 5. Additionally, we have carried out PL measurements of TPE 5 in different solvents


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(toluene, acetone and ethanol) at 77 K in addition to chloroform (Figure S16). A similar blue shift in comparison to the LB films was observed in the glass form at 77 K irrespective of the nature of used solvent. This observation suggests that packing of TPE 5 at 77 K merely independent of the solvent molecules. The blue shifted PL in glass form in comparison to the solution or LB films suggests that TPE 5 rigidify at low temperature, which restricts the intramolecular rotation in glass form. It appears that the propeller shaped TPE 5 undergoes further rigid conformation at low temperature imparts strain which shortens the molecular conjugation length resulting in a blue shifted PL at low temperature. The strain relaxes at ambient temperature releasing the rigid configuration to revert the original PL in solution, which originates switchable PL with change in temperature. To understand the insight mechanism of MCL behavior of the multilayer LB film, we have carried out X-ray diffraction (XRD) measurements of LB film under different external stimuli (Figure 5a). Pristine LB film shows peaks at 3.3°, 6.58° and 9.18° and a broad peak at 22° corresponding to multiple aromatic interactions.39,60 Interestingly, acetone fumigation of the same LB film displays additional sharp reflections indicating a change in the molecular packing within the LB film (Figure 5b). It appears that spherical shaped aggregates absorb acetone vapor in the void spaces of the LB film. This reinforces the aggregates to reorient in presence of acetone vapor. Prolonged annealing of the LB film regains the original crystalline state by the evaporation acetone vapor thus resulting in the original green PL (Figure 5c). This observation is also supported by PL lifetime and QY measurements. The PL lifetime of the pristine LB film is 2.20 µs in comparison to 60 ps for the solution (Figure S13). The lifetime of the fumigated LB film is measured to be 1.85 ns (Figure S17) suggesting lower degree of packing in comparison to the pristine LB film. Additionally, the QY drops down from 56.14% to 34.43% for the fumigated


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multilayer LB film, which is attributed to the change in the molecular packing upon exposure to the acetone fume. The PL emission switching upon fumigation suggests that TPE 5 can be used as a security ink in optical recording, pressure deformation, volatile organic compound sensor etc. Such capability is demonstrated with a multilayer LB film deposited on filter paper. The pristine LB film shows green emission upon UV exposure (Figure 5d). The LB film is then exposed to acetone vapor for 30 mins, which turned into cyan color upon UV exposure (Figure 5d). We have written letters “AIE” on the acetone fumed filter paper LB film by streaking with a metal spatula (Figure 5d). The letters clearly emit a green color path under UV light, which can be distinguished easily from intense cyan background. Alternation of the crystalline structure during writing leads to the planarization of the molecular conformation in those streaked areas and causes increase in the conjugation length thus showing original green color emission of pristine LB film of TPE 5. Since it is difficult to obtain the crystallinity of TPE 5 from the local streaked letters using X-ray diffraction (XRD) technique, we have streaked the entire acetone fumigated LB film of TPE 5 for XRD measurements. The acetone fumigated LB film of TPE 5 on filter paper shows cyan color while the streaked LB film shows green color under UV illumination (Figure S18a). Following this, we have carried out XRD measurements of the acetone fumigated LB film and streaked LB film and compared with the pristine LB film (Figure S18b-S18d). Pristine LB film shows major peaks at 3.3°, 6.58° and a broad peak at 22° (Figure S18b). Acetone fumigation of the LB film displays additional XRD peaks indicating a change in the molecular packing (Figure S18c). Interestingly, the streaked LB film shows a similar XRD pattern like the pristine LB film. Notably, the 3.3° reflection becomes sharper in intensity (Figure S18d). The peaks at 3.3° and 6.58° are multiple reflections indicating improved crystallinity of the pristine and streaked LB films in comparison to the fumigated LB film. The appearance of


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higher order peaks points out towards the planarization of the TPE 5 molecules with increased conjugation length. Moreover, PL peaks of pristine and streaked LB films appear in the same position suggesting that the molecular packing of fumigated LB film reverts back to the pristine LB film upon streaking (Figure S18e). The letters “AIE” can be erased upon further acetone fume indicating the reorientation of the molecules in the streaked paths (Figure 5d). The process shows a facile way to write-read-erase processes on a flexible substrate using TPE 5. Hence TPE 5 affords the potential application of hiding information and sending messages secretly. Electrochemical behavior: The electrochemical property of the TPE 5 is investigated by cyclic voltammetry (CV) measurements and theoretically supported by the DFT calculations. The CV shows an onset oxidation potential (Eonset) of 0.73 V of TPE 5 (Figure 6a). Therefore, the highest occupied molecular orbital (HOMO) energy is estimated to be -5.13 eV following the relation, HOMO = - (4.4+Eonset) eV.61 Subsequently the lowest unoccupied molecular orbital (LUMO) energy is obtained from the relation, LUMO = HOMO + Eg, where Eg = 1240/λonset and λonset is the onset wavelength of the UV-vis absorption spectrum. We have estimated the optical bandgap energy of ~3.1 eV from the onset wavelength of solution UV-vis spectrum shown in figure 3a. Taking together, we have calculated the LUMO energy level to be -2.03 eV. Additionally, we have calculated the electronic structure of TPE 5 using DFT. Geometries are optimized using M06/6-31G** level as implemented in G09 program package.55 The plot of HOMO and LUMO and the corresponding energies are depicted in Figure 6b. The HOMO/LUMO energies computed from DFT are -5.27 eV and -1.40 eV respectively suggesting a HOMO-LUMO gap of 3.87 eV. The HOMO-LUMO gap is in line with the experimentally observed values from CV.


19


a

20

b

10

Current (µA)

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

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LUMO

-1.40 eV

0 3.87 eV -5.27 eV

-10

HOMO

-20

-30

1.0 0.5 0.0 -0.5 -1.0 -1.5

+ Potential vs. Ag / Ag (V)

Figure 6. (a) Cyclic voltammograms (CV) of TPE 5 in dichloromethane containing 0.1 (M) tetra-n-butylammonium hexafluorophosphate under N2 atmosphere. Scan rate = 250 mV S-1, concentration = 1 × 10-3 (M). (b) Molecular orbital plots of HOMO and LUMO of TPE 5 using DFT calculations. However, the HOMO-LUMO gap is expected to decrease in solution phase and upon aggregate formation. Moreover, it is evident that the frontier molecular orbitals (HOMO and LUMO) are delocalized all over the TPE 5 in spite of the non-planarity of the optimized geometry. Electroluminescence behavior: Since the TPE 5 shows high PL QY in the solid state, we have fabricated LED using TPE 5 as active element. We have used N,N´-Di(1-napthayl)-N,N´diphenyl-(1,1´-biphenyl)-4,4´-diamine (NPD) as the hole transport layer (HTL) and Bathophenanthroline (Bphen) as the electron transporting layer (ETL) (Figure 7a). The device structure consists of ITO/NPD/TPE 5 /Bphen/Al layers. The layers used within the device have been chosen following the energy level matching of the components (Figure 7b). The devices show stable electroluminescence (EL) over a wide range of operating voltages (Figure 7c). The


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EL retraces the PL suggesting that the EL is occurring from the active TPE 5 layer (Figure 7c). Corresponding Commission Internationale de l'´eclairage (CIE) chromaticity coordinates diagram indicates a nearly bias independent cyan color of the LED at different voltages (Figure 7d). The CIE chromaticity coordinates of the LED is (0.18, 0.33) at different voltages in comparison to CIE coordinates (0.19, 0.38) of the PL spectrum.

Figure 7. (a) Schematic diagram of LED device structure. (b) Energy level diagram of TPE 5 based multilayer OLED. (c) EL spectra of the device at different bias voltage along with the solid state PL. (d) CIE chromaticity co-ordinates of the EL and PL spectra. The current-voltage characteristic of working device is presented in Figure 8a. Subsequently, the recombination of charge carrier takes place within the active TPE 5 layers by external bias. A low turn-on voltage of ~3 V is obtained from the device. Luminance is detectable above the threshold voltage showing ~2808 Cd m-2 at 18 V (Figure 8b). Photograph of the working device is presented in the inset of figure 8b. LED device is uniform over the entire active device area


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without any inhomogeneity, demonstrating that the emission from TPE 5 is uniformly distributed in the working device. The device attains maximum current efficiency and power efficiency of ~10 Cd A-1 and ~8 lm W-1 respectively (Figure 8c). External quantum efficiency (EQE) of 2.5% is achieved by the TPE 5 LED (Figure 8d).

Figure 8. (a) Current density versus voltage curve of the LED. (b) Luminance versus voltage characteristics. Inset: Photograph of the working device at 18 V. (c) Current efficiency and power efficiency versus current density characteristics of TPE 5 LED. (d) External quantum efficiency versus voltage curve. A comparison of LED performance of TPE 5 with the reported LED devices fabricated using TPE derivatives with similar chemical structures to TPE 5 (Table S3) suggests robust LED performance manifesting TPE 5 as potent electroluminescent material.


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CONCLUSION In summary, we report on the supramolecular strategy to achieve spherical aggregates of AIE active TPE 5 at the air-water interface over a large area. Aggregate formation facilitates AIE phenomenon by inhibiting the rotation and vibration of TPE 5. As a result, a large enhancement of PL compared to solution phase is obtained from the mono and multilayer LB films. We demonstrate high contrast reversible mechanochromic behavior of TPE 5 where temperature acts as mechanical stimulus to change molecular packing for PL switching. The PL switching upon acetone fumigation also suggests that TPE 5 can be used as a security ink in optical recording, pressure deformation and volatile organic compound sensor on flexible substrates. TPE 5 LEDs exhibit low turn-on voltage and robust EL performances. These findings emphasize the importance of controlled organization of AIE complexes to explore optimal luminescence properties for efficient lighting and sensing applications. ASSOCIATED CONTENT Supporting Information: Experimental details for synthesis and characterization of TPE 5, acetone fumigation and thermal annealing procedure of LB film, OLED device fabrication, theoretical calculations, surface pressure-area isotherm, surface potential-area isotherm, hysteresis cycle of monolayer, energy optimized structure, high resolution AFM images, AFM and SEM images of multilayer LB film, additional spectroscopic data of LB films and glass form and XRD of LB films under different conditions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Tel: +91 33 2473 4971 (Ext.1104). ORCID Somobrata Acharya: 0000-0001-5100-5184 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank SERB Grant No EMR/2014/000664, DST, India for financial support. S.B. acknowledges CSIR India, for financial support. G.S.K. acknowledges DST-INSPIRE for fellowship. U.K.G. acknowledges West Bengal DST FIST and Central DST FIST programs for financial assistance. B.K.G. thanks CSIR India [No. 02(0150)/13/EMR-II.] for financial support.

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and

Aggregation-Induced-Emission

Compounds

Containing

Triphenylethylene and Tetraphenylethylene Moieties. Chem. Asian J. 2011, 6, 1470 – 1478. (60) Kar, H.; Ghosh, S. J-aggregation of a Sulfur-Substituted Naphthalenediimide (NDI) with Remarkably Bright Fluorescence. Chem. Commun. 2016, 52, 8818-8821.


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TOC

1.0µm

Fuming

MCL Annealing


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Supramolecular Aggregates of Tetraphenylethene Cored AIEgen

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