Field-Induced Stimulated Emission in a Polymer–Liquid Crystal

Article pubs.acs.org/JPCC

Field-Induced Stimulated Emission in a Polymer−Liquid Crystal Mixture Luca Moretti,† Luigino Criante,‡ Guglielmo Lanzani,‡ Sandro De Silvestri,† Giulio Cerullo,† and Francesco Scotognella*,† †

Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Center for Nano Science and Technology CNST-IIT@PoliMi, Istituto Italiano di Tecnologia, via Pascoli 70/3, 20133 Milano, Italy

‡

S Supporting Information *

ABSTRACT: We present a spectroscopic study on a novel blend of a light-emitting polymer (F8BT) and a liquid crystal (5CB). We investigate the possibility to control the optical behavior of such blend with an external stimulus. By means of ultrafast pump−probe spectroscopy we observe a modulation of the stimulated emission of the polymer driven by an external applied voltage. We attribute the rise of the stimulated emission to a rearrangement of the polymer that modifies its packing as a consequence of the alignment of the liquid crystal. Such field-induced stimulated emission modulation can find applications in information and communication technology, lasing and optical sensing.

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INTRODUCTION The combination of materials with different electrical or optical properties may lead to new functionalities, beyond the performances of the isolated components, especially when the materials are intermixed at the nanometre and micrometre scales. Such mixtures are strikingly important for the fabrication of, for example, polymer-oxide hybrid solar cells1 and polymeric bulk heterojunction solar cells.2 Both these devices employ, as light-absorbing materials, conjugated polymers (CPs). CPs are organic semiconductors that have gained wide popularity in photonics due to their appealing properties:3 they display a combination of broad band emission, high optical gain, versatility in processing, they are ductile and can be electrically pumped (so far in LEDs).4,5 Indeed many optoelectronic devices based on CPs have been demonstrated such as solar cells,6,7 phototransistors,8,9 photodetectors,10,11 optically pumped lasers 12−14 and organic light emitting diodes (OLEDs).15,16 Another class of materials that is employed in blends are liquid crystals (LCs).17,18 LCs can self-assemble into various phases that exhibit properties and responses not possible in crystals or liquids, while possessing all the fluid and solid state crystalline properties.19−21 LCs are used for their capability to be ordered in a certain configuration by different types of external stimuli, such as electric field, magnetic field or optical radiation. Their large anisotropy allows the control of their optical properties, which is exploited in a variety of devices comprising displays for televisions, computers and phones, optical shutters and modulators, and 3D glasses for cinema or television.22−25 Driven by this general concept of mixing materials with different properties, in this work we devised a system in which a © XXXX American Chemical Society

LC, acting as electro-optic material (i.e., a material in which an electric field modulates the optical properties) is mixed with a CP showing optical gain, thus, obtaining a new functional optical material. LCs and CPs have already been mixed together in the literature but the latter component has been primarily used as an inert matrix.18 To the best of our knowledge no studies in which the CP has been used as an active optical material have been performed previously. Indeed here an external trigger acting primarily onto the LC in the blend offers a handle for controlling the photophysics of the CP and thus its photonic performances. We investigate the interaction of the mixed LCs and CPs by using the ultrafast pump−probe technique. We choose poly(9,9-di-n-octylfluorene)-co-(benzothiadiazole), named F8BT, as CP component, because it is commercially available and it shows excellent performances as an active layer in light emitting devices.26−29 As LC we choose to use 5CB (4-cyano-4′-pentylbiphenyl), a nematic LC that is available in the market. Upon applying an external voltage to the blend, we affect the photoexcitation dynamics in F8BT, demonstrating a new combined electro-optical effect. In the blends, the CP is used as optically active part of the device while the LCs act as an external electro-actuator inducing supramolecular order. Because CPs are highly anisotropic systems, the orientation induced by the LC is transduced into a change of the overall optical properties of the blend. In addition, LC orientation deeply affects the interchain interactions in the polymer and, thus, the excited state deactivation pathways. In this way we obtain a new cooperative Received: June 4, 2015 Revised: September 12, 2015

A

DOI: 10.1021/acs.jpcc.5b05347 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C effect that is characteristic of the blend and represents a new functionality that can be exploited in photonic applications.

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EXPERIMENTAL METHODS The CP we use is poly(9,9-di-n-octylfluorene)-co-(benzothiadiazole) (F8BT, Sigma-Aldrich, average molecular weight 255 kg/mol, i.e., around 10000 monomers in one chain). As LC we employed 4-cyano-4′-pentylbiphenyl (K15, or 5CB) by SigmaAldrich (molecular formula CH3(CH2)4C6H4C6H4CN and molecular weight 249.35 g/mol). We mixed the CP with the LC, without solvent, with a concentration of 1:400 in weight. This ratio was chosen after different attempts in order to have a balance between two factors: the mixability of the two materials and a sufficient absorption of the polymer. To promote the mixing, initially the blend was heated at about 50 °C (above its nematic-to-isotropic phase transition temperature that is around 37 °C, at which the mixture starts to become more transparent) for 30 min with the stirring on at 3000 rpm. Then it was also sonicated 10 min at about 40 °C. This entire procedure was repeated five times. In order to apply an electric field to the sample we adopted a homemade cell built by putting face to face two pieces of glass covered by indium tin oxide (ITO) as conductive electrode. Two thin (23 μm) stripes of Mylar are used as spacers to create a cavity in which the mixture is infiltrated by capillarity. To avoid a nonuniform infiltration, the composite was heated above the nematic-to-isotropic phase transition temperature of the mixture before injecting it into the cell. The laser system used for the pump−probe measurements is based on an amplified Ti:sapphire laser, with 1 mJ output energy, 1 kHz repetition rate, and central wavelength of 800 nm (1.59 eV). Excitation pulses at 400 nm (3.18 eV), with ≈150 fs duration, are obtained by second-harmonic generation in a βbarium borate (BBO) crystal. Pump pulses are focused onto a 200 μm spot, and the pump fluences were kept below 1 mJ/ cm2. Probing is performed in the visible region by using white light generated in a thin sapphire plate. Chirp free differential transmission (ΔT/T) spectra are collected using a fast optical multichannel analyzer (OMA) with dechirping algorithm.30 We applied an external sinusoidal AC (1001 Hz, to have a frequency different from the one of the laser pulses) voltage to the cell containing the polymer/liquid crystal blend and we performed steady-state absorption and pump−probe measurements as a function of the electric field strength. We thus compared the differential transmission signal with the one acquired without electric field. We applied an AC voltage and not a DC one, as the latter tends to trigger undesirable electrochemical reactions among the LC molecules, generating impurities and degrading the sample, moreover triggering the formation of charges in the LC that screen the external field. The pump beam is impinging on the device faces with an angle of around 10°. The field acts on the liquid crystal part of the sample in order to align the mesogen such that the LCs will be aligned perpendicularly with respect to the device faces and in this way with the same angle of 10° with respect to the incoming pump and probe beams (see Figure 1).

Figure 1. Schematization of the device and the laser beams impinging on the materials during the pump−probe measurements. The pump− probe measurements have been performed with or without the electric field applied and compared in order to understand the behavior of the polymer in the two different configurations. Pump beam is chopped and on it are shown the two different polarizations used, vertical V and horizontal O with respect to the plane of the two beams. Probe beam, after focusing on the sample, is sent on a grating, and the dispersed light is detected by a CCD.

one can see, this effect decreases with the application of the field (green dotted curve), due to the alignment of the rods in a unique nematic domain all over the cell. The absorption spectra of the polymer in solution (blue dotted curve) and in film (light blue curve) are different from those in the blend. This is due to a different packing of the polymer chains in the presence of the solvent, the LCs or no other materials. Figure 2b shows normalized spectra for comparison. A similar red shift occurring from solution to film, due to an increase in the effective conjugation length of the polymer, is also visible moving from the blend without field to the one with the applied field. Taking into account the physical characteristics of both mixed materials, it is important to emphasize the idea that the interplay between CP conformation and LC orientation depends strongly on the relative proportions of the two components and plays an important role in the system’s thermodynamics.31 The diluted mixture of polymer adopts an anisotropic conformation because of its coupling to the solvent’s director field. Figure 3a shows as a dashed line the ΔT/T spectrum of the F8BT film at 0.5 ps pump−probe delay, following excitation at 400 nm with a fluence of ≈0.4 mJ/cm2. At probe wavelengths shorter than ∼520 nm, corresponding to the ground state absorption of the polymer, we observe a positive signal, assigned to photobleaching (PB) of the S0 → S1 transition. For probe wavelengths between 520 and 600 nm we observe a positive signal which is attributed to stimulated emission (SE) from the photoexcited S1 state. At wavelengths longer than 600 nm we observe a negative signal, which is assigned to photoinduced absorption (PA) from S1 to a higher lying singlet state. At 200 ps (line in Figure 3a), the shape of the ΔT/ T spectrum changes dramatically, and the SE band is overwhelmed by a new broad PA band, which is assigned to polaron pairs. The weight of the polaron pair band increases with the pump fluence and correspondingly the lifetime of the SE band decreases from 100 ps to less than 10 ps (comparison between dashed light blue line and blue dots in Figure 3b). These results show, in agreement with previous literature, that singlet−singlet annihilation (SSA), leading to charge generation, dominates the excited state dynamics in F8BT films32 within the first few picoseconds. The high SSA yield is easily

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RESULTS AND DISCUSSION Figure 2a shows steady state absorption spectra of F8BT and of the F8BT:5CB blend. The strong background in the absorption of the blend (red dotted line) is due to high light scattering by the disordered LCs domains in the nematic phase. Indeed, as B

DOI: 10.1021/acs.jpcc.5b05347 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Absorption spectra: (a) pure F8BT in a toluene solution (1 mg in 4 mL) (blue dots); pure F8BT in spin-coated film (light blue dots); mixture of F8BT and 5CB (1 mg in 400 mg) infiltrated into a device without (red dots) and with (green dots) electric field applied; (b) normalized and background subtracted spectra around the peak of absorption.

Figure 3. Pump−probe measurements performed on the F8BT film (blue) and on the device without (light red) and with (green) applied field, at comparable fluences of ≈0.6 mJ/cm2. (a) The ΔT/T spectra at 0.5 ps (dashed line) and 200 ps (full line) time delay of the polymer film; the spectra have been normalized to the peak of the PB. (b) The ΔT/T dynamics at 550 nm, corresponding to the peak of the SE band of the polymer: the dashed light blue one is related to a lower fluence measurement (≈0.04 mJ/cm2). (c) The ΔT/T spectra at 200 ps for the blend without (red dashdot line) and with (green line) applied field: a different behavior of the polymer is clearly visible in the region from 500 to 600 nm. In panel d) the dynamics and fits at 550 nm wavelength are shown (red dots for no field, green dots for field): here the difference in SE behavior is clear.

delayed photoluminescence.34 PA and PB both have an initial fast decay (due to excitons) followed by a very long-lived tail (due to polarons) and have very similar dynamics over the whole time range.35 To better understand the SE band one could refer to the photoluminescence of this polymer: it shows two bands related to different emitting states (around 530 and 570 nm, related to the molecular weight of the used polymer). As reported in the literature the one at higher energy is the

explained by the different localizations of HOMO and LUMO levels,33 that results in singlet excitons with a strong chargetransfer character. In SSA the reaction S1 + S1 → S0 + Sn leads to the formation of higher energy excited singlet states Sn, increasing the probability of autoionization forming geminate charge pairs with large electron−hole separation and long lifetimes. Geminate recombination by through-space tunneling repopulates the low lying emissive singlet state resulting in C

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Table 1. Parameters of a Multiexponential Fit at the Three Wavelengths Representing the Three Different Bands of Signala no field

a

field

ps (frac)

τ1

τ2

τ3

R2

480 nm 550 nm 640 nm

2.2 (0.35) 6.6 (0.26) 1.7 (−0.3)

21 (0.45) 44.8 (0.34) 21.9 (−0.3)

811 (0.18) 412 (−0.39) 688.5 (−0.38)

0.99855 0.98916 0.99810

τ1 2.12 (0.2) 1.2 (−0.28)

τ2

τ3

R2

23.7 (0.33) 22.2 (0.42) 11.3 (−0.21)

711.2 (0.45) 855.8 (0.56) 790.5 (−0.47)

0.99390 0.98881 0.99098

Decay lifetimes are given in ps; in parentheses the corresponding fractions of the decay. Also shown is the R2 parameter of the fit.

dominant one when the chains are aligned,36−38 while the one at lower energy corresponds to disordered and aggregated chains. In the blend, the ΔT/T spectra and dynamics (Figure 3c,d) at 0.6 mJ/cm2 are different with respect to those of the neat polymer at comparable excitation fluence. Without electric field (Figure 3c, red dash-dot line) the PA band related to the polaron pairs at 200 ps delay is broader and more intense than the one seen in the film (or in the toluene solution at the same fluence), overwhelming the SE band (between ≈520 and 600 nm). By applying a bias (140 Vrms at 1 kHz on a sample thickness of 23 μm, corresponding to a field of ≈6 V/μm) we see however (Figure 3c green line) a long-living SE band, peaking at around 530 nm. As one can see in Figure 3d also the dynamics at the center of the polymer SE band is radically changing with the applied field: from a SE quickly decaying into a PA we pass to a long-lived SE. Such a long living SE is more similar to the behavior of the polymer in toluene solution (data not shown), which for different excitation fluences always showed very low PA signal overlapped to the SE, resulting in no change of sign in the ΔT/T dynamics at 550 nm. We attribute the broad polaron pairs PA band observed in the blend under zero bias to a strong interchain interaction in the aggregates (that, as shown before in Figure 2, is different from the one in the neat polymer film), resulting in a more efficient formation of interchain polaron pairs from higher singlet states ionization. Furthermore, the back transfer to singlet states has low probability, and consequently the polaron pair states are long-lived and the SE band is heavily quenched.32,35 We conjecture that, by applying the electric field, the conformation and the orientation of the polymer aggregates changes, reducing interchain interaction and, thus, yielding a reduction of the polaron pair population that, in turn, implies a long-lived SE band. Indeed the smaller polaron population could be related to the disentanglement of the chains caused by LC alignment and to the enhanced chain separation provided by the aligned LC molecules within the blend. The fact that the SE band in the biased device has much longer lifetime than in the neat polymer film has important implications for optoelectronics and lasing applications. Fitting the dynamics with a three-exponential function, using time constants τ1, τ2, τ3 (see Table 1) we could quantify the different behavior with or without field applied. Note that SSA is a bimolecular phenomenon and usually dynamics should be fitted with a different, more complex model;32 here we use exponential decay as a simple approach to quantitatively compare these two measurements. The dynamics at 480 nm (related to PB) show similar short time decays but a preponderance of long-lived component when the field is applied (see Figure S1 in the Supporting Information). The decrease of the overall ΔT/T signal is a proof of the orientation of the polymer perpendicular to the sample plane: the polymer electric dipole is perpendicular to the optical electric field of the

pump beam, reducing its absorption. An initial faster decay of the PA band related to polaron pair states (at 640 nm) with the field could be ascribed to intrachain recombination (see Figure S1 in the Supporting Information). Instead, a longer τ3 is due to a lower number of remaining polaron pairs, which do not easily recombine since they are created in less connected chains (with respect to the more intertwined chains without the electric field). We choose to analyze the 640 nm wavelength in order to be sure to be out of the SE band region and to still have a signal from the polaron pairs states; of course there is an underlying signal from singlet PA. Indeed, we would like to remark that the most significant change with the applied field is that the dynamics of the positive signal at 550 nm, mostly due to SE, is characterized by a long lifetime component (τ3, which is the 56% of the total decay, is around 850 ps). By gradually increasing the amplitude of the electric field, the SE of the F8BT achieves a sort of asymptotic value (Figure 4

Figure 4. Amplitude of ΔT/T signals for the blend under increasing applied voltage. We have performed measurements in different areas of the sample and here we show the average values at time delay of 200 ps of the ΔT/T at 530, 550, and 570 nm (standard deviation shown and explained in Table S1 of SI). The dashed lines are related to the polarization “VV” of the pump and probe beams, the straight lines instead are related to the “OO” polarizations.

shows the trend at 200 ps delay): after an increase of the signal the last two values have similar amplitudes. A threshold for this phenomenon is thus present between 60 Vrms and 120 Vrms. The value is higher in the mixture, with respect to the usual reported values for pure liquid crystal phases, due to the polymer network - liquid crystal interaction. It is indeed related to the complete “homeotropic” electric field induced alignment of the liquid crystal and thus of the polymer network chains, not to be confused with the so-called Freedericksz transition threshold in the pure LC39 or doped mixture.40 In this case, the reorientation process involves competition between fieldinduced torque and the elastic distortion of the complex liquid crystal mixture. Even though a theory that completely explains our specific system could be complicated and beyond the scope D

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of this manuscript, the results suggest to consider an intermediate regime between strong coupling (e.g., mainchain polymers, as considered by Bosch et al.41) and complete decoupling (Brochard’s theory for side-chain polymers31) in order to study the interplay between the polymer’s conformational entropy and liquid crystalline order. Anyway, the relative proportions of the two components play an important role in the system’s thermodynamics and in the LC−CP interaction. Moreover, unlike rigid rod-like molecules of equal length, a polymer network chains seems to offer a greater elastic resistance to the reorientation assisted by liquid crystal. For this reason (as shown in the crossed polarizers images shown in Figure S2 of SI), although the reorientation begins already at low voltages, to obtain a complete “homeotropic” electric field induced alignment of the liquid crystal and thus of the polymer network chains, voltages over 100 Vrms are necessary. The polarization-dependent measurements allow us to observe the differences in the excitation ratio of the polymer chains. With pump and probe beams impinging on the sample with a vertical polarization (“VV”) (perpendicular with respect to the plane of the two laser beams, see Figure 1 for the configuration), the rise of the SE is observed only at the higher electric field intensities (dashed lines in Figure 4). Instead in the horizontal polarization measurement (“OO”, in which the pump beam excites more efficiently the polymer chains oriented in the direction of the electric field), owing to a greater dipole moment projection of the aligned chains in that direction, we have observed a higher SE signal also at lower electric field amplitudes (straight lines in Figure 4). This tends to confirm the alignment of the polymer chains. The threshold behavior and the differences between the two beam polarizations is visible also at longer time delays (see Figure S3). We propose that the mechanism explaining the voltage induced change in photophysics of blend is the change of the CP chain packing induced by the alignment of the LCs. This is consistent with the red shift of the absorption spectrum of the mixture (as previously shown in Figure 2). The CP morphological configuration so obtained displays a new photophysics, with a long-lived SE band not competing with PA. It is also visible that the blend behaves differently with respect to the neat polymer. This could be explained with different interchain or intrachain interaction mechanisms with respect to the pure polymer due to low concentration or to an interaction within the liquid crystal matrix.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05347. Graphics of 480 and 640 nm dynamics and related fits; cross polarized optical microscopy images of the blend under different voltages; table of standard deviation for the increasing voltage behavior averages at 200 ps; graphic of the behavior of the blend under increasing voltage at 400 ps of time delay (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Italian Ministry of University and Research (Project PRIN 2010-2011 “‘DSSCX’”, Contract 20104XET32).

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REFERENCES

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CONCLUSIONS

In this work we demonstrated a new electro-optical effect: the electric field induced switching of SE in a blend of a lightemitting polymer (F8BT) with a liquid crystal (5CB). The phenomenon has been characterized by ultrafast transient absorption measurements. We show that the alignment of the LCs under a bias changes the interchain interactions in the polymer, in turn affecting the deactivation pathways of the photoexcited state. In particular, the quenching of charge generation leads to a long-lived SE. We find that this effect occurs above a threshold in the electric field amplitude related to the LC dynamics. This new electro-optical effect could be exploited in a optoelectronic device in which optical gain is activated by the electric field. E

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DOI: 10.1021/acs.jpcc.5b05347 J. Phys. Chem. C XXXX, XXX, XXX−XXX