Photonic Microresonators from Charge Transfer in Polymer Particles

Subscriber access provided by Stockholm University Library

Functional Nanostructured Materials (including low-D carbon)

Photonic Microresonators from Charge-Transfer in Polymer Particles - toward Enhanced and Tunable Two-Photon Emission Radhika Vattikunta, Dasari Venkatakrishnarao, Chakradhar Sahoo, Sri Ram Gopal Naraharisetty, Desai Narayana Rao, Klaus Müllen, and Rajadurai Chandrasekar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01600 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 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 27 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

ACS Applied Materials & Interfaces

Photonic Microresonators from Charge-Transfer in Polymer Particles - toward Enhanced and Tunable Two-Photon Emission Radhika Vattikunta,† Dasari Venkatakrishnarao,† Chakradhar Sahoo,£ Sri Ram Gopal Naraharisetty,£ Desai Narayana Rao,£ Klaus Müllen*‡ and Rajadurai Chandrasekar*† †

School of Chemistry, University of Hyderabad, Gachibowli, Hyderabad – 500046, India

£

School of Physics, University of Hyderabad, Gachibowli, Hyderabad – 500046, India

‡

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55218 Mainz, Germany

KEYWORDS: polymer particles, charge-transfer, optical microresonators, two-photon emission, non-linear optical properties

ABSTRACT: Novel photonic microresonators with enhanced non-linear optical (NLO) intensity are fabricated from polymer particles. As an additional advantage, they offer bandgap tunability from the visible (Vis) to the near-infrared (NIR) region. A special protocol including i) copolymerization of 4-(1-pyrenyl)-styrene, styrene, and 1,4-divinyl-benzene, ii) extraction of a dispersible and partly dissolvable, lightly cross-linked polymer network (PN) and iii) treatment of the blue emitting PN with electron acceptors (A)-molecules such as 1,2,4,5-tetracyanobenzene (TCNB)

ACS Paragon Plus Environment

1

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

Page 2 of 27

and 7,7,8,8-tetracyanoquinodimethane (TCNQ) furnishes orange and red emitting D-A chargetransfer (CT) complexes with the pendant pyrene units. These complexes, here named PNTCNB, and PN-TCNQ, respectively, precipitate as microparticles upon addition of water and subsequent ultrasonication. Upon electronic excitation, these spherical microparticles act as whispering-gallery-mode resonators by displaying optical resonances in the photoluminescence (PL) spectra due to light-confinement. Further, the trapped incident light increases the lightmatter interaction and thereby enhances the PL intensity, including the two-photon luminescence. The described protocol for polymer-based CT microresonators with tunable NLO emissions holds promise for a myriad of photonic applications. 1. INTRODUCTION Organic materials allow the tuning of bandgaps based upon concepts such as crystal polymorphism,1 crystalline-amorphous phase-changes,2 host-guest interactions,3 particle-size effects,4 π-conjugation length5,6 and CT complexes formation.7,8 This feature is imperative for applications such as lighting,9 electronics,10 microlasers,7,11-15 on-chip optical communications,16 microresonators,17-19 and sensors.20 Broad-band tuning from Vis to NIR, however, remains challenging.21-23 Moreover, most of the potential applications of organic materials are limited to one-photon luminescence and little work has been dedicated to their non-linear optical (NLO) properties. For instance, two-photon luminescence (TPL) is an up-conversion NLO process, which arises as a result of simultaneous absorption of two photons, is valuable for emerging technologies such as 3D-microfabrication,24 optical data storage,25 two-photon fluorescence imaging,24,26 and optical limiting27 and photodynamic therapy.28 On the other hand, most of the organic and polymeric NLO materials show much weaker TPL than the linear one-photon luminescence. To enhance optical emission intensity, polymers in the


2

Page 3 of 27 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


form of spherical microparticles are advantageous since they can act as high quality (Q) whispering-gallery-mode (WGM) resonators and increase the light-matter interaction. Basically, these resonators, upon optical pumping, tightly trap the light via repeated total internal reflection (TIR) which occurs at the air-microsphere surface boundary, and increase the photon lifetime, τ (as Q ∝ τ). The higher the Q factor, the better is the enhancement of the optical signal. For example, polystyrene (PS) microspheres doped with weakly luminescent molecules are known to act as WGM resonators displaying enhanced or amplified optical emissions.29,30 Here the superiority of PS comes from its ability to self-assemble into defect-free microspheres which provides resonators with high Q value. Surprisingly, no reports are available which address both issues: (i) bandgap tunability, both in the linear and NLO regimes, and (ii) enhancement of the optical emission using self-assembled optical microresonators. Therefore, we have envisioned novel microresonators made from cross-linked polymer particles with supramolecular CT type pendant groups exhibiting both tunable and enhanced oneand two-photon emissions. For this, we have selected pyrene as an electron donor (D) unit since it forms D-A complexes with many electron acceptor (A) molecules such as 1,2,4,5tetracyanobenzene (TCNB) and tetracyanoquinodimethane (TCNQ) and exhibit CT emissions at different wavelengths depending upon the strength of interaction within the D-A pair.31 To attach pyrene as a pendant group to the macromolecule, precipitation copolymerization of styrene and 4-(1-pyrenyl)-styrene (PyPS) with 1,4-divinyl-benzene (DVB) as a cross-linker is expected to be an ideal choice, since it furnishes spherical microparticles with smooth surface in case of poly(styrene-co-divinylbenzene).32 On the other hand, direct incorporation of A-units into the pendant D-groups during polymerization had proven challenging due to their reactions with the radical initiator, which adversely affected the particle formation. Alternatively, post-


3


Page 4 of 27

polymerization formation of D-A complexes had posed an additional challenge, in the form of poor particle dispersability in solvents. To overcome these setbacks, we have used the sol part of the polymer (PN) having a low-degree of cross-linking.39 The latter was used for the preparation of CT complexes, which were subsequently transformed into microparticles via precipitation followed by ultrasonication in a quantitate manner. The obtained particles are superior to PS particles due to their excellent mechanical stability and surface smoothness, features which are imperative to achieve high-Q resonators. This approach offered easy synthetic accessibility, widely tunable optical emissions, optical nonlinearity, and excellent solution processability for bottom-up and top-down technologies.7,33-35

Scheme 1. Synthesis of cross-linked polymer (PN) and its CT complexes, PN-TCNQ and PNTCNB emitting tunable PL and TPL.


4

Page 5 of 27 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


Here we present blue emitting lightly cross-linked polymer microparticles namely, PN wherein the pyrene serving as an electron donor is directly connected to PS (Scheme 1). PN forms orange and red emitting CT complexes viz. PN-TCNB and PN-TCNQ with TCNB and TCNQ, respectively in DMF and precipitates as microparticles upon adding water and subsequent ultrasonication. These microparticles display high-Q WGM resonance and as a result of increased light-matter interaction time, they exhibit enhanced one- and two-photon pumped optical emissions as compared to their corresponding homogenous thin films. 2. EXPERIMENTAL SECTION 2.1. Materials: 4-Vinylphenylboronicacid, DVB, TCNQ, TCNB and Pd(PPh3)4 were purchased from Sigma-Aldrich. Pyrene (98%) and azobisisobutyronitrile (AIBN) were obtained from Alfa aesar, and Avra chemicals, respectively. 2.2. Synthesis of 1-(4-Vinylphenyl)pyrene: 4-(1-Pyrenyl)-styrene was synthesized by a Suzuki coupling reaction according to a reported procedure.36 2.3. Synthesis of PN: Styrene (0.2 mL, 1.64 mmol, 1 equivalent), DVB (0.18 mL, 1.28 mmol, 0.78

equivalent),

4-(1-pyrenyl)-styrene

(500

mg,

1.64

mmol,

1

equivalent),

azobisisobutyronitrile (30 mg, 0.16 mmol, 0.1 equivalent), and THF (45 mL) were placed in a two-necked round-bottomed flask. The reaction mixture was degassed by freeze–pump–thaw cycles, stirred under nitrogen atmosphere for 30 min and subsequently subjected to reflux overnight. After cooling to room temperature, the formed sticky green semi-solid (gel + sol) was transferred into a beaker containing a large excess (250 mL) of methanol. The green colored precipitate was filtered through a Büchner funnel and washed with methanol for three times to remove any unreacted monomers and sol at room temperature. The gel product (PN) was dried under vacuum overnight. Yield (0.92 g) 74%. Further, the product was subjected to Soxhlet


5


Page 6 of 27

extraction (acetonitrile, CHCl3, and finally with THF) and the THF extracted soluble PN was used for further characterization. 1H-NMR (500 MHz, CDCl3, 298 K) δ (ppm) : 7.68.3(CH2CHC6H4 C16H9, where C16H9 = phenylpyrene), 6.3-7.2 (CH2CHC6H5, CH2CHC6H4 CH CH2, CH2CHC6H4 C16H9), 1.2–2.2 (CH2CHC6H5, CH2CHC6H4 CH CH2, CH2CHC6H4 C16H9). 13

C-NMR (500 MHz, CDCl3, 298 K) δ (ppm): 145, 131.50, 131.02, 130.24, 128.04, 127.67,

127.44, 125.63, 124.97, 124.71, 40.57, 37.11, 32.77, 31.93, 30.04, 29.71, 29.36, 27.10, 22.70, 19.73, 14.11. FTIR (KBr disk; ν in cm−1): 3024, 2920, 1600, 1494, 1452, 1260, 804, 698. 2.4. Synthesis of PN-TCNB and PN-TCNQ: To a dispersion of PN in DMF (1 mg/2.5 mL), TCNQ or TCNB (0.5 mg) was added and stirred at r.t. for two days to get black and brown colored solutions, respectively. These solutions upon solvent evaporation in vacuo provided 47% and 78% yields of PN-TCNB and PN-TCNQ, respectively (see Figure 2C). This reaction was further scaled up to 63 mg of PN with a proportional increase of acceptors and DMF. 2.5. Preparation of Microparticles: To a dispersion of PN (1 mg/1 mL) in DMF milli-pore water (15 µL) was added, the mixture ultrasonicated for 30 s and left undisturbed for 10 min at r.t. to precipitate microparticles. Later, two drops of this solution were dispersed on a clean glass cover slip and the mixture of DMF/water was slowly evaporated. In case of PN-TCNB and PN-TCNQ, milli-pore water (15 µL) was added directly into the reaction solutions and the above procedure was followed to prepare microparticles. 2.2. Instrumental Methods: 1H and

13

C NMR spectroscopy of the samples were performed on a

Bruker DPX 500 spectrometer; chemical shifts (δ) are expressed in parts per million (ppm) relative to the solvent proton as internal standard (CDCl3 = 7.26 ppm). Infra-red (IR) spectra were recorded on JASCO FT/IR-5300 or Nicolet 5700 FT-IR. Spin coating was performed on a


6

Page 7 of 27 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


glass substrate using a spin coater (Laurell Technologies Corporation; model WS-400B6NPP/LITE/8K). For thin-layer chromatography, silica gel plates Merck 60 F254 were used. UV-Vis absorption and fluorescence spectra were recorded on a Shimadzu-UV-3600 UV-VISNIR spectrophotometer and a Fluoromax spectrofluorimeter (Horiba, Jobin Yvon), respectively. Size and morphology of the microspheres were examined by using a Zeiss field-emission scanning electron microscope (FESEM) operating at 3 kV. Transmission electron microscopy (TEM) measurements were performed with a Tecnai G2 FEI F12 instrument operating at an accelerating voltage of 200 kV. Carbon-coated TEM grids (200 Mesh Type-B) were purchased from Ted Pella Inc., USA. 2.6. Atomic Force Microscope (AFM) Studies: AFM studies were carried out on a NT-MDT Model Solver Pro M microscope using a class 2R laser of 650 nm wavelength having maximum output of 1 mW. All calculations and image processing were carried out by using NOVA 1.0.26.1443 software provided by the manufacturer. The images were recorded in a semi-contact mode using a super sharp silicon cantilever (NSG 10_DLC) with a diamond-like carbon tip (NTMDT, Moscow). The dimension of the tip is as follows: cantilever length 5100 (65) mm, cantilever width 35 (65) mm, and cantilever thickness = 1.7–2.3 mm, resonance frequency = 190–325 kHz, force constant = 5.5–22.5 N/m, reflective side = Au, tip height = 10–20 nm, and DLC tip curvature radius 1–3 nm. 2.7. Laser Confocal Optical Microscope Set-up: The optical spectra of single microspheres were recorded on a WI-Tec alpha 300 AR confocal spectrometer equipped with a Peltier-cooled CCD detector. Using a 300 grooves/mm grating BLZ = 700 nm, the accumulation and integration time were typically 10 s and 1.0 s, respectively. Ten accumulations were performed for acquiring a single spectrum. A 405 nm (Diode laser) and 488 nm (Ar+) CW lasers were used as an excitation


7


Page 8 of 27

source for collecting the modes of PN, PN-TCNB, and PN-TCNQ microspheres, respectively. All measurements were performed at ambient conditions. A 150× objective (0.95 NA) was used for all the measurements. Laser power was estimated using THOR Labs power meter. 2.8. Experimental Set-up for TPL: For TPL measurements, a home built experimental set-up was developed by using a commercial Ti: Sapphire laser (Spectra Physics) having central wavelength 800 nm, pulse width 100 fs, and repetition rate 1 kHz (Scheme 2). The laser output was directed by a half-wave plate and polarizer combination to control the laser power. A lens having a focal length of 50 cm was used to focus the laser beam on the sample. The transmitted beam was filtered by a notch filter to cut down the fundamental excitation wavelength (800 nm),

Scheme 2. Experimental Set-up for TPL the filtered beam was collected by a fiber-coupled spectrometer (Ocean optics-2000 SD), and the spectrometer output was displayed by a computer. The experiment was performed by keeping the sample at focal volume having a spot size of 101.8 µm, and was exposed to linearly polarized light with different input powers.

2.9. Nonlinear Optical Absorption Co-efficient (β) Measurements: The β values of PN, PNTCNB, and PN-TCNQ in solution state were determined by carrying out open aperture z-scan measurements. Briefly, in a typical z-scan experimental technique, a single laser beam with a Gaussian profile was focused by using a lens. The sample in a 1-mm-thick cell was then


8

Page 9 of 27 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


translated along the direction of the focused beam. At the focal point, the sample experiences maximum intensity, which gradually decreases in both directions from the focus. The thickness of the sample was chosen such that it was smaller than the Rayleigh range of the focused beam, which was nearly 1mm, well calibrated neutral density filters were used to control the laser intensity. The data was then recorded by scanning the cell across the focus and the value of β was obtained by fitting the nonlinear transmission expression (given below) for an open aperture z-scan curve.

T 1

β L I

2 1

where z is the sample position, zo = πω02/λ is the Rayleigh range, ωo is the beam waist at the focal point (z = 0), λ is the laser excitation wavelength, Io is the intensity on the sample at focus, effective optical path length in the sample of length L is given as Leff = 1‐e‐

αoL

/αo, and αo is the

linear absorption coefficient. Figure 1 shows the open aperture Z-scan curve recorded for PN, PN-TCNB, and PN-TCNQ (black dotted lines), the red dotted lines represent the theoretical fit obtained by using above Equation. The β values were estimated from the theoretical fitting for PN, PN-TCNB, and PN-TCNQ, respectively having intensity at the focus 1.9 10

.

Figure 1. Open aperture z-scan curves of PN, PN-TCNB and PN-TCNQ.


9


Page 10 of 27

3. RESULTS AND DISCUSSION Donor monomer, namely 4-(1-pyrenyl)-styrene was synthesized via Suzuki coupling by reacting 1-bromopyrene and 4-vinylphenylboronic acid in 86% yield.36 To prepare CT polymer particles we have considered three routes: (i) our initial attempts to directly incorporate A-unit into D-unit during the polymerization of styrene, 4-(1-pyrenyl)-styrene together with DVB as a cross-linker was not fruitful, because of the decomposition of AIBN initiator in the presence of TCNQ43 leading to no particle formation.37 (ii) in our second effort, we have carried out the polymerization without the A-units by using equivalent amounts of styrene and 4-(1-pyrenyl)styrene together with DVB (0.78 equivalent) to obtain PN powder (yield 74%) (Scheme 1 and 3). FESEM examination of the precipitated PN powder after washing with methanol showed the

Scheme 2. Schematic representation of methods used for the preparation of PN, PN-TCNB and PN-TCNQ microparticles.


10

Page 11 of 27 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


formation of a mixture of coalesced microparticles and isolated microparticles. Although crosslinking density was not measured in this study, the aforementioned coalesced particles hint towards their low degree of polymer cross-linking.38,39 Additionally, post-polymerization integration of A-units into pyrene units turned out to be challenging, due to the poor dispersability of the PN microparticles in THF and DMF. The above mentioned two approaches clearly revealed a complex

Figure 2. PL spectra of PN and PN-TCNB and PN-TCNQ complexes (A) in dispersed solution state, B) in the solid state. C) Photographs of compounds under ambient conditions and UV light.

overlap between reality (difficulty in loading the acceptor into the particle, particle coagulation, poor particle dispersability) and our necessity (CT particles, spherical particles, cross-linked


11


Page 12 of 27

particles with smooth surface). (iii) Finally, the best tradeoff is using the sol part containing lightly cross-linked PN.39 Therefore we have Soxhlet extracted the sol at high temperature from the PN powder. The obtained fraction was dispersed and partly dissolved in DMF so that the acceptors (DMF soluble) units could be easily incorporated into the D-units (Scheme 3 and Figure S1-S3 for NMR spectra). Further, we have opted for this last route since this sol fraction in DMF readily precipitates as spherical microparticles upon addition of water followed by ultrasonication.29,30 Besides, these particles are still superior to linear chain PS particles in terms of mechanical stability and surface smoothness, which are crucial for tight light confinement. Both PN-TCNB and PN-TCNQ and their microparticles were prepared by the procedure given in the experimental section (see 2.4 and 2.5) (Scheme 3 and Figure 2C). The formation of PNTCNB and PN-TCNQ microparticles was confirmed by confocal microscopy. The mechanism of formation of these microparticles is possibly rather similar to that of polystyrene microspherical particles in THF/water under ultrasonication.29,30 The

13

C NMR spectra of PN-TCNB and PN-

TCNQ indicated D and A interactions by displaying up-field and down-field chemical shifts for the former and latter, respectively, compared to bare A-units (Figures S2 and S3). Infrared spectroscopy of PN-TCNB and PN-TCNQ indicated CT complex formation, as obvious from the high energy shift of the CN stretching frequency in comparison to the analogous frequency in the acceptors (Figure S4). The solution and solid-state absorption spectra of sol are presented in the Supporting Information (Figure S5). In DMF, PN-TCNQ and PN-TCNB showed slightly red-shifted high energy π-π* transitions compared to PN. Endorsing the intermolecular D-A interactions, the former compounds exhibited CT bands across visible (about 400-600 nm) regions as well (Figure S5A and S11). The PL spectrum of PN appeared in the blue region (381 and 410 nm)


12

Page 13 of 27 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


while PN-TCNB and PN-TCNQ displayed two distinct orange (λmax ~ 542 nm) and red (λmax ~ 587 nm) color CT emissions, respectively (Figure 2A). This finding corroborates D-A type CT interactions of varying strengths. Recently, Bardeen et. al. have detected no PL from pure pyrene/TCNQ in solution.36 In our case, the PS acts as a matrix and facilitates slow charge recombination, thus suppressing non-radiative relaxation and PL quenching. The estimated solution-state quantum yields of PN, PN-TCNB, and PN-TCNQ are about 46%, 3% and 8.4%, respectively.

Figure 3. A) and B) FESEM images of PN microspheres. C) AFM image of a single PN microsphere. The right inset demonstrates the surface smoothness. D) TEM image of PN microspheres. E) SAED pattern of microspheres. F) EDAX elemental (C and N) map of PNTCNB or PN-TCNQ microsphere.

In comparison to the solution, the absorption spectra of solid-state samples demonstrated a redshifted absorption (Figure S5B) and PL bands (Figure 2B). Notably, in contrast to the blue


13


Page 14 of 27

emission (λmax ~ 440-540 nm) of PN, the CT bands of PN-TCNB, PN-TCNQ showed larger redshifts of about 140 nm to 190 nm, respectively and appeared in the orange (λmax ~ 587 nm) and red (λmax ~ 632 nm) part of the electromagnetic spectrum. These shifts indicate that the CT interaction from pyrene to TCNQ is stronger than to TCNB, thus reducing the optical bandgap of the former complex. Examination of PN and PN-TCNB and PN-TCNQ microparticles under FESEM revealed their spherical shape with nearly uniform sizes (Figures 3A and 3B). The surface smoothness of particles is a very important parameter which determines the resonator quality. Particles which lack smooth surfaces normally reduce the Q factor of a resonator due to light scattering and in extreme cases40 (surface irregularity) even do not support the WGMs. To probe this, AFM analysis was performed. As can be seen in the inset of Figure 3C, the roughness of a representative microsphere is below the detection limit indicating the potential of the particles to be used as a resonator. TEM revealed dark contrast pointing out the dense packing of the lightly cross-linked polymer network within the microspheres (Figure 3D). The selected area electron diffraction (SAED) exhibited no diffraction spots pointing toward the amorphous nature of the particles (Figure 3E). Elemental distribution maps at the microsphere level using an energy dispersive X-ray (EDS) analysis displayed nitrogen distribution and established the incorporation of nitrogen containing A-units within PN (Figure 3F). To investigate the light-trapping potential of PN, PN-TCNB and PN-TCNQ microspheres, single-particle PL spectroscopy studies were performed on a confocal microscope set-up. A reflection mode geometry was employed for sample excitation and PL signal collection. For excitation continuous wave (Ar+ 488 nm and diode 405 nm) lasers were used. Point-focus illumination (spot size ~ 600 nm) of a single PN microsphere center with a 405 nm laser


14

Page 15 of 27 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


(objective 150×; NA 0.95) furnished a brilliant blue emission with a normal PL spectrum. Excitation of the microsphere at the edge displayed a blue ring-like PL and the corresponding PL spectrum showed a series of multiple pairs of sharp lines (or optical modes) indicating WGM resonance. These

Figure 4. Single-particle PL spectra of PN and PN-TCNB and PN-TCNQ exhibiting WGMs when at the edge of the particle and no modes when excited at the centre of the particle. The right insets reveal the corresponding WGMs emissions shown by the microspheres. The Q-factor versus D plot is shown at the right-bottom. + - sign denotes laser excitation and signal collection position.


15


Page 16 of 27

intense optical modes arise as a result of constructive wave interference of the circulating PL and their number increases with the resonator size. These resonance modes can be seen only if the PL frequencies match the frequencies of the resonator allowed-modes as described by the so-called Purcell effect.41 Similarly, PN-TCNB and PN-TCNQ microspheres upon excitation with 488 nm laser displayed orange and red emissions, respectively, together with WGMs. A pair of periodic sharp peaks identified in the PL spectra points to transverse electric (TE) and transverse magnetic (TM) modes (Figure 4) which are characteristics of a WGM resonator. Further, the spacing between two similar types of optical modes is known as the free spectral range (FSR) which varies inversely with microsphere diameter (D), FSR = λ2/ neff πD, where neff is the effective refractive index and λ peak wave length. In line with the above relation, the FSR values of the microspheres decreased sharply upon increasing D (estimated from the confocal microscope images) (Figure S7). A slight variation of the slopes indicated the experimental error in the estimation of D and disparity in the n values. Moreover, the Q value (Q = λ/∆ λ; where ∆ λ is the line width of a peak at full wave half maximum (FWHM) and λ is the wave length of the guided light) is an important parameter for a resonator. In our case, the Q factor of the microresonators is in the range of 600-1000, which is an improved value for polymer-based optical resonators19 (Figure 4). The PL spectra of PN, PN-TCNB, and PN-TCNQ microspheres showed a nearly 10 fold enhancement relative to the corresponding spin-coated homogeneous thin films under identical experimental conditions (Figure S8). The above singleparticle experiments obviously demonstrate the potential use of polymer network CT complexes to achieve bandgap tunable micro resonators producing enhanced optical emissions.


16

Page 17 of 27 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


Figure 5. A) One- and two-photon pumped PL from solution and microspheres of PN, PNTCNB, and PN-TCNQ. B) TPL spectra (solid line) of PN, PN-TCNB, and PN-TCNQ microspheres together with their OPL (solid fill) and C) the corresponding linear relationship between TPL intensity and excitation power in (mW)2 unit. The blue, orange, and red dotted lines correspond to PN, PN-TCNB, and PN-TCNQ, respectively.

In addition to one-photon emissions, CT complexes are known to show NLO properties upon excitation with high energy lasers because the applied electric field intensities are comparable to internuclear atomic fields. Therefore, initially, to determine the solution-state overall NLO


17


Page 18 of 27

response of the compounds, the z-scan technique was employed (see section 2.9 for more details). This method allowed us to estimate the non-linear absorption coefficient (β) values of PN, PN-TCNB and PN-TCNQ, which are 4.4×10-10, 3.18×10-10, and 3.28×10-10 cm/W, respectively (Figure 1). Further to investigate the NLO emissions of

thin films of

microresonators, each sample was excited with Ti: Sapphire 800 nm fs pulse laser (pulse width 100 fs, repetition rate 1 KHz by keeping the sample at focal volume having a spot size of 101.8 µm covering a dozen of particles. As per our expectation, a thin film of PN microresonators revealed an intense blue TPL at λmax ~488 nm nearly matching with its one-photon excited PL (Figure 5B). This result confirmed the third-order (χ(3)) NLO properties of PN microresonators. Likewise, CT type microresonators of PN-TCNB and PN-TCNQ also displayed TPL spectra at ~580 nm and ~602 nm, respectively, corresponding to their one-photon emission bands. The observed narrower FWHM of the TPL spectra in comparison to the one-photon PL spectra is probably due to reabsorbance effects of the samples.39 Upon increasing the pump power, the TPL intensity revealed a square dependence upon the pump energy, confirming that the up-converted emission indeed originates from two-photon absorption (Figures 5C and S9). It is important to note that the averaged TPL intensities of the microsphere samples are several orders of magnitude higher than those of the respective homogeneous thin films due to optical-resonator effect (Figure S10).

4. CONCLUSIONS In summary, we report a facile preparation of CT type polymer particle microresonators emitting enhanced TPL in the vis-NIR range. For this, initially, we have prepared a novel crosslinked polymer particle with pyrene as the pendant donor group. But poor solution dispersability


18

Page 19 of 27 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


of the polymer particles became one of the bottle-necks, impeding solution state integration of A-units into D-units of PN. On the other hand, to fabricate mechanically stable particles with good surface smoothness, polymer cross-linking was necessary. Therefore, as a quid pro quo, the sol part of the PN (containing lightly cross-linked polymer)36 was separated and dispersed in DMF to incorporate TCNB and TCNQ acceptors successfully. Subsequently, the resulting crosslinked polymer and its CT complexes (PN-TCNB and PN-TCNQ) were precipitated as spherical microparticles by adding water and subsequent ultrasonication. The formation mechanism of polymer particles from lightly cross-linked polymer networks is perhaps quite similar to dye incorporated PS microparticle formation.26,27 Microparticles composed of PN, PN-TCNB and PN-TCNQ emit blue, orange and red colors upon one-photon excitation due to CT interactions. Single-particle micro-PL spectra of these CT microparticles displayed multimodal WGMs in the vis-NIR range with a Q factor as high as 1000. Two-photon pumped (800 nm) NLO spectroscopy studies of microresonators exhibited TPL matching with their PL bands. The high Q microresonators also displayed enhanced emissions in both linear and non-linear optical regimes in comparison to their corresponding homogenous thin films, confirming improved light-matter interaction. Our approach demonstrates the efficacy of cross-linked polymer microparticle resonators to realize diverse visible-NIR range up-conversion CT emissions suitable for various NLO applications. ASSOCIATED CONTENT Supporting Information Materials, instrumental methods, sample preparation, and spectroscopy data. “This material is available free of charge via the Internet at http://pubs.acs.org.”


19


Page 20 of 27

AUTHOR INFORMATION Corresponding Authors [email protected] (RC) and [email protected] (KM) Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS RV and DV thank the UGC/CSIR for senior research fellowships. The authors acknowledge the SERB-New Delhi (EMR/2015/000186) and MPGA POLY (0006) for research funds. SRGN thanks SERB-New Delhi (EMR/2014/00516) for research fund. KM thanks the Gutenberg Research College for support.

REFERENCES 1) Siegrist, H.; Kloc, C.; Laudise, R. A.; Katz, H. E.; Haddon, R. C. Crystal Growth, Structure, and Electronic Band Structure of α-4T Polymorphs. Adv. Mater. 1998, 10, 379-382. 2) Srujana, P.; Radhakrishnan, T. P. Extensively Reversible Thermal Transformations of a Bistable, Fluorescence-Switchable Molecular Solid: Entry into Functional Molecular PhaseChange Materials. Angew. Chem. Int. Ed. 2015, 54, 7270-7274. 3) Dong, H.; Zhang, C.; Zhao, Y. S. Host–Guest Composite Organic Microlasers. J. Mater. Chem. C. 2017, 5, 5600-5609. 4) Fu, H. –B.; Yao, Y. Size Effects on the Optical Properties of Organic Nanoparticles. J. Am. Chem. Soc. 2001, 123, 1434-1439.


20

Page 21 of 27 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


5) Meier, H. Conjugated Oligomers with Terminal Donor–Acceptor Substitution. Angew. Chem. Int. Ed. 2005, 44, 2482-2506. 6) Narayana, Y. S. L. V.; Baumgarten, M.; Müllen, K.; Chandrasekar, R. Tuning the Solid State Emission of Thin Films/Microspheres Obtained from Alternating Oligo(3-octylthiophenes) and

2,6-Bis(pyrazole)pyridine

Copolymers

by

Varying

Conjugation

Length

and

Eu3+/Tb3+ Metal Coordination. Macromolecules. 2015, 48, 4801-4812. 7) Guo, X.; Baumgarten, M.; Müllen, K. Designing pi-Conjugated Polymers for Organic Electronics. Prog. Polym. Sci. 2013, 38, 1832-1908. 8) Ajayaghosh, A.; Praveen, V.; Vijayakumar, C.; George, S. J. Molecular Wire Encapsulated into Organogels: Efficient Supramolecular Light-Harvesting Antennae with Color-Tunable Emission. Angew. Chem. 2007, 119, 6260-6265. 9) Dong, H.; Wei, Y.; Zhang, W.; Wei, C.; Zhang, C.; Yao, J.; Zhao, Y. S. Broadband Tunable Microlasers Based on Controlled Intramolecular Charge-Transfer Process in Organic Supramolecular Microcrystals. J. Am. Chem. Soc. 2016, 138, 1118-1121. 10) Glowatzki, H.; Sonar, P.; Singh, S.P.; Mak, A. M.; Sullivan, M. B.; Chen, W.; Wee, A.T.S.; Dodabalapur, A. Band Gap Tunable N-Type Molecules for Organic Field Effect Transistors. J. Phys. Chem. C. 2013, 117, 11530-11539 11) Kozlov, V. G.; Bulovic, V.; Burrows, P. E.; Forrest, S. R. Laser Action in Organic Semiconductor waveguide and Double-Heterostructure Devices. Nature. 1997, 389, 362-364. 12) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Single-Nanowire Electrically Driven Lasers. Nature. 2003, 421, 241-245. 13) Venkatakrishnarao, D.; Narayana, Y. S. L. V.; Mohiddon, M.A.; Mamonov, E. A.; Mitetelo, N.; Kolmychek, I. A.; Maydykovskiy, A. I.; Novikov, V. B.; Murzina, T. V.; Chandrasekar,


21


Page 22 of 27

R. Two-Photon Luminescence and Second-Harmonic Generation in Organic Nonlinear Surface Comprised of Self-Assembled Frustum Shaped Organic Microlasers. Adv. Mater. 2017, 29, 16029560. 14) Lv, Y.; Li, Y. J.; Li, J.; Yan, Y.; Yao, J.; Zhao, Y. S. All-Color Subwavelength Output of Organic Flexible Microlasers. J. Am. Chem. Soc. 2017, 139, 11329-11332. 15) Zhang, W.; Yao, J.; Zhao, Y. S. Organic Micro/Nanoscale Lasers. Acc. Chem. Res. 2016, 49, 1691-1700. 16) Yan, R.; Gargas, D.; Yang, P. Nanowire Photonics. Nat. Photonics. 2009, 3, 569-576. 17) Venkatakrishnarao, D.; Chandrasekar, R. Engineering the Self-Assembly of DCM Dyes into Whispering-Gallery-Mode µ-Hemispheres and Fabry–Pèrot-Type µ-Rods for Visible–NIR (600–875 nm) Range Optical Microcavities. Adv. Optic. Mater. 2016, 4, 112-119. 18) Takazawa, K.; Inoue, J.; Mitsuishi, K.; Kuroda, T. Ultracompact Asymmetric Mach–Zehnder Interferometers with High Visibility Constructed from Exciton Polariton Waveguides of Organic Dye Nanofibers. Adv. Funct. Mater. 2013, 23, 839-845. 19) Tabata, K.; Braam, D.; Kushida, S.; Tong, L.; Kuwabara, J.; Kanbara, T.; Beckel, A.; Lorke. A.; Yamamoto, Y. Self-Assembled Conjugated Polymer Spheres as Fluorescent Microresonators. Sci. Rep. 2014, 4, 5902. 20) Zhao, Y. S.; Zhan, P.; Kim, J.; Sun, C.; Huang, J. Patterned Growth of Vertically Aligned Organic Nanowire Waveguide Arrays. ACS Nano. 2010, 4, 1630-1636.


22

Page 23 of 27 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


21) Pan, A,; Zhou, W.; Leong, E. S. P.; Liu, R.; Chin, A. H.; Zou, B.; Ning, C. Z. Continuous Alloy-Composition Spatial Grading And Superbroad Wavelength-Tunable Nanowire Lasers on a Single Chip. Nano Lett. 2009, 9, 784-788. 22) Pan, A.; Yang, H.; Liu, R.; Yu, R.; Zou, B.; Wang, Z. Color-Tunable Photoluminescence of Alloyed CdSxSe1-x Nanobelts. J. Am. Chem. Soc. 2005, 127, 15692-15693. 23) Liu, X.; Zhang, Q.; Yip, J. N.; Xiong, Q.; Sum, T. C. Wavelength Tunable Single Nanowire Lasers Based on Surface Plasmon Polariton Enhanced Burstein–Moss Effect. Nano Lett. 2013, 13, 5336-5343. 24) Strickler, J. H.; Webb, W.W. Two-Photon Excitation in Laser Scanning Fluorescence Microscopy. Proc. SPIE. 1991, 1398, 107. 25) Parthenopoulos, D. A. Rentzepis, P. M. Three-Dimensional Optical Storage Memory Science. 1989, 245, 843-845. 26) Denk, W.; Strickler, J.H.; Webb, W.W. Two-Photon Laser Scanning Fluorescence Microscopy. Science. 1990, 248, 73-76. 27) Sperber, P.; Penzkofer, A. S0-Sn Two-Photon Absorption Dynamics of Rhodamine Dyes. Opt. Quantum Electron. 1986, 18, 381-401. 28) Hu, W.; Xie, M.; Zhao, H.; Tang, Y.; Yao, S.; He, T.; Ye, C.; Wang, Q.; Lu, X.; Huang, W.; Fan, Q. Nitric Oxide Activatable Photosensitizer Accompanying Extremely Elevated TwoPhoton Absorption for Efficient Fluorescence Imaging and Photodynamic Therapy, Chem. Sci. 2018, 9, 999-1005. 29) Narayana, Y. S. L. V.; Venkatakrishnarao, D.; Mohiddon, M. A.; Biswas, A.; Viswanathan, N.; Chandrasekar, R. Visible−Near-Infrared Range Whispering Gallery Resonance from


23


Page 24 of 27

Photonic µ-Sphere Cavities Self-Assembled from a Blend of Polystyrene and Poly[4,7-bis(3octylthiophene-2-yl)benzothiadiazole-co-2,6-bis(pyrazolyl)pyridine]

Coordinated

to

Tb(acac)3. ACS. Appl. Mater & Interfaces. 2016, 8, 952-958. 30) Venkatakrishnarao, D.; Sahoo, C.; Radhika, V.; Annadhasan, M.; Naraharisetty, S. R. G.; Chandrasekar, R. 2D Arrangement of Polymer Microsphere Photonic Cavities Doped with Novel N-Rich Carbon Quantum Dots Display Enhanced One- and Two-Photon Luminescence Driven by Optical Resonances. Adv. Opt. Mater. 2017, 5, 1700695. 31) Dobrowolski, M. A.; Garbarino, G.; Mezouar, M.; A. Ciesielskiac.; Cyrańskia, M. A. Structural Diversities of Charge Transfer Organic Complexes. Focus on Benzenoid Hydrocarbons and 7,7,8,8-Tetracyanoquinodimethane. CrystEngComm. 2014, 16, 415-429. 32) Shim, S. E.; Yang, S.; Choe, S. Mechanism of the Formation of Stable Microspheres by Precipitation Copolymerization of Styrene and Divinylbenzene. J. Polym. Sci. A. Polym., Chem. 2004, 42, 3967-3974. 33) Kivala, M.; Boudon, C.; Gisselbrecht, J.-P.; Enko, B.; Seiler, P.; Müller, I. B.; Langer, N.; Jarowski, P. D.; Gescheidt, G.; Diederich, F. Organic Super-Acceptors with Efficient Intramolecular Charge-Transfer Interactions by [2+2] Cycloadditions of TCNE, TCNQ, and F4-TCNQ to Donor-Substituted Cyanoalkynes. Chem. Eur. J. 2009, 15, 4111-4123. 34) Edvinsson, T.; Li, C.; Pschirer, N.; Schöneboom, J.; Eickemeyer, F.; Sens, R.; Boschloo, G.; Herrmann, A.; Müllen, K.; Hagfeldt, A. Intramolecular Charge-Transfer Tuning of Perylenes: Spectroscopic Features and Performance in Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2007, 111, 15137-15140.


24

Page 25 of 27 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


35) Zhou,

G.; Baumgarten,

M.; Müllen,

K.

Mesitylboron-Substituted

Ladder-Type

Pentaphenylenes: Charge-Transfer, Electronic Communication, and Sensing Properties. J. Am. Chem. Soc. 2008, 130, 12477-12484. 36) Chung, F. J.; Liu, H. Y.; Jiang, B. Y.; He, G. Y.; Wang, S. H.; Wu, W. C.; Liu, C.L. Random Styrenic Copolymers with Pendant Pyrene Moieties: Synthesis and Applications in Organic Field-Effect Transistor Memory. J. Poly. Sci, Part A: Poly. Chem. 2016, 54, 910-917. 37) Dillon, R. J.; Bardeen, C. J. Time-Resolved Studies of Charge Recombination in the Pyrene/TCNQ Charge-Transfer Crystal: Evidence for Tunneling. J. Phys. Chem. A. 2012, 116, 5145-5150. 38) Downey, J. S.; McIsaac, G.; Frank, R. S.; Stover H. D. H. Poly(Divinylbenzene) Microspheres as an Intermediate Morphology Between Microgel, Macrogel, and Coagulum in Cross-Linking Precipitation Polymerization. Macromolecules. 2001, 34, 4534-4541. 39) Shim, S. E.; Yang, S.; Choi, H. H.; Choe, S. Fully Crosslinked Poly(Styrene-co‐ Divinylbenzene) Microspheres by Precipitation Polymerization and their Superior Thermal Properties J. Polym. Sci. Part A Polym Chem. 2004, 42, 835-845. 40) Narayana, Y. S. L. V.; Basak, S.; Baumgarten, M.; Müllen, M.; Chandrasekar, R. WhiteEmitting Conjugated Polymer/Inorganic Hybrid Spheres: Phenylethynyl and 2,6-Bis (pyrazolyl)pyridine Copolymer Coordinated to Eu(tta)3. Adv. Funct. Mater. 2013, 23, 58755880. 41) Purcell, E. M. Resonance Absorption by Nuclear Magnetic Moments in a Solid. Phys. Rev. 1946, 69, 674. 42) He, G. S.; Tan, L. S.; Zheng, Q.; Prasad, P. N. Multiphoton Absorbing Materials: Molecular Designs, Characterizations, and Applications. Chem. Rev. 2008, 108, 1245-1330.


25


Page 26 of 27

43) Acker, D. S.; Hertler, W. R. Substituted Quinodimethans. I. Preparation and Chemistry of 7,7,8,8-Tetracyanoquinodimethan, J. Am. Chem. Soc., 1962, 84, 3370-3374.


26

Page 27 of 27 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


Table of Contents Graphic


27

Photonic Microresonators from Charge Transfer in Polymer Particles

Recommend Documents