We investigated the transitions of conformations and their effects on emission properties of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) single molecules in PMMA matrix during thermal annealing process. Total internal reflection fluorescence microscopy measurements reveal the transformation from collapsed conformations to extended, highly ordered rod-like structures of MEH-PPV single molecules during thermal annealing. The blue shifts in the ensemble single molecule PL spectra support our hypnosis. The transition occurs as the annealing temperature exceeds 100 °C, implying that an annealing temperature near the glass transition temperature Tg of matrix is ideal for the control and optimization of blend polymer films.
Conjugated polymers (CPs) have wide applications in organic light emitting diode (OLED), solar cells, organic lasers, organic thin-film transistors, and sensors.1,2 PPV and its derivatives are particular useful types of CPs for OLEDs and sensors due to their splendid optical and electrical properties.3,4 In device researches, CPs are usually mixed with cheap matrix polymers to reduce the cost and increase the durability of the composite materials. The properties of CPs are greatly influenced by intra- and intermolecular interactions.5 All these interactions are dependent on material morphology on a nanometer scale, leading to a huge diversity of the properties of individual CP molecules.6 The morphology of the composite polymer films can be manipulated by using different organic solvents, concentrations, annealing, and spin coating speeds, thereby tailor the optical and electrical properties of the polymer films in photoelectric devices on demand.
Annealing is widely used to achieve optimal film morphology and can be used to improve material properties such as electrical mobility in device applications.7,8 As post annealing techniques, thermal annealing, and solvent vapor annealing (SVA) are the most commonly used approaches to improve the chain packing and crystallinity.9,10 Although there are several studies on the impact of annealing process in bulk films, systematic comparison about their impacts on the morphology and electrical properties are rare. Recently, Noh's group has observed a remarkable difference between thermal annealing and SVA that the thiophene-based conjugated polymer poly(4,4′-bis-decyloxymethylquaterthiophene) (POQT) exhibits two polymorphic crystallite phases in thermal annealing process but single-phase crystallites in SVA process.11 In single molecule level, studies of poly[2-methoxy-5–(2′-ethyl-hexyloxy)–1,4-phenylene vinylene] (MEH-PPV), one of the most common of PPV derivatives, have been explored extensively by Barbara's group.12 They have observed the SVA-induced transition of single chain conformation of MEH-PPV in PMMA matrix and found that the MEH-PPV chains were in highly ordered conformations after SVA. Besides, they have also demonstrated the formation of aggregates for MEH-PPV/PMMA films during SVA.13 In their experiments, the formations of aggregates were estimated based on the increasing intensities and the decreasing number of particles per image area. Those aggregates containing 25–40 chains show 6–40 folds higher fluorescence intensities than isolated MEH-PPV chains and still display pronounced fluorescence blinking behaviors, indicating long-range energy transport.
The conformations and emission properties of MEH-PPV single molecules during thermal annealing process have not been studied yet in detail. The mechanism of annealing induced morphology change of MEH-PPV in composite polymer films remains unknown. Therefore, a systematic study of the thermal annealing and its impact on the emission properties of composite polymer films are of great importance. In this letter, we took on this mission and investigated the effect of thermal annealing on the conformations and the emission properties of single MEH-PPV molecules in PMMA matrix towards the improvement of OLED devices.
The chemical structure of MEH-PPV and PMMA is shown in Fig. 1(a). The samples were prepared by mixing a dilute toluene (for HPLC, ≥99.9%) solution of MEH-PPV with a 20 mg/ml toluene solution of PMMA, adjusting the final concentration of MEH-PPV to be 1 × 10−7 mg/ml (about 10−13 mol/l). The mixed solution was spin-coated on cleaned quartz substrates, yielding films with a thickness of 100 nm. The single molecule fluorescence of MEH-PPV was studied using a total internal reflection fluorescence microscopy (TIRFM) equipped with an Ar+ laser (514 nm). Images were acquired using a Leica AM TIRF microscope DMI6000 (Leica Microsystems) with an attached Hamamatsu Electron Multiplying Charge Coupled Device (EMCCD) camera at a depth of 110 nm, using HCX PL APO 100× oil objective with numerical aperture of 1.47. Ensemble single molecule fluorescence spectra were taken on a FLS-920 spectrometer (Edinburgh Instruments, Ltd.), with an excitation at 514 nm.
Fig. 1(b) shows the TIRFM image of MEH-PPV single molecules, in which each single molecule appears as a well separated bright spot. Fluorescence intermittency (blinking) is an important phenomenon observed in single quantum emitters. For conjugated polymers containing tens to hundreds of conjugated segments, the observation of fluorescence blinking implies fast localization of the excitation energy on one or a small number of chromophores. Previous studies have revealed that there are two main fluorescence time transients in MEH-PPV single molecules, corresponding to collapsed and extended chain conformations.14,15 A collapsed chain displays discrete fluorescence blinking due to a disruption of the energy transfer pathway to the emitting chromophore by the formation of a long-lived absorbing state or by transient bleaching of the emitting chromophore through said long-lived state.10 In contrast, an extended chain exhibits continuous photobleaching as one chromophore after another photo-oxidises. In our work, MEH-PPV single molecules in PMMA matrix show blinking on a few intensity levels and continuous photobleaching, shown in Fig. 1(c), implying that multiple chain conformations coexist. This result is in good agreement with the polarization excited fluorescence results of Barbara et al.12,16 that there is some heterogeneity in chain conformations while MEH-PPV spun from toluene is mostly folded into a rod-like structure.
Thermal treatment can play a very important role in modifying the morphology of amorphous polymers. To get insight into the interconnection of the morphology with single molecule fluorescence, MEH-PPV/PMMA blend samples were annealed at different temperatures ranging from 60 °C to 140 °C for 2 h in a vacuum oven and then characterized by TIRFM. Results show that all samples display both blinking and photobleaching. The fluorescence intensity histogram in Fig. 1(d) shows the proportions of molecules exhibiting specific time transients under different annealing temperatures. As illustrated, the proportion of MEH-PPV molecules exhibiting blinking behaviors decreases while that of molecules exhibiting continuous photobleaching increases. This implies that thermal annealing prompts MEH-PPV molecules to change their collapsed chain conformations to more extended ones. However, previous studies of spectra and morphology have demonstrated that single nanoparticle of CPs emitting like bulk films without any fluorescence blinking behaviors.17,18 Consequently, additional evidences are needed.
Accordingly, we compared the fluorescence intensity of each emitting spot per image area before and after thermal annealing at different temperatures. Fig. 2 shows the histograms of all samples based on a statistical analysis over 120 molecules. Each distribution is fitted to a single peak Gaussian function, showing that the center positions are similar for annealed samples and the widths are narrowing to a certain extent with increasing annealing temperatures. The intensities of annealed samples maintain mean values at about 3.5 ± 0.3 thousand counts (see Fig. S1),19 without any significant difference, suggesting no formations of aggregates. Consequently, the increase of MEH-PPV molecules exhibiting photobleaching is not due to the formation of intermolecular aggregates during the thermal annealing process. Besides, the narrower widths of distributions imply a trend of monospecific conformations for annealed samples.
For highly ordered, rod-like chains, nearly all of the chromophores have the same in-plane orientation, and the energy transfer in such a chain is a multistep funneling process, which has been simulated using an incoherent Förster-type mechanism.20 Those results have demonstrated that even in the limit of Förster resonance energy transfer (FRET), the high organization of the conjugated polymer leads to energy across the entire polymer chain. Besides, such highly ordered, rod-like structure has been attributed to an extended chain conformation.20 Accordingly, we can explain the evolutions of molecules exhibiting specific fluorescence transients. Thermal annealing changes the morphology of polymer thin films similar to SVA. During the annealing process, PMMA acts like a sieve and restrains the diffusion of MEH-PPV molecules. However, external heat induces the motion of PMMA segments, allowing the MEH-PPV molecules to twist or orientate orderly. According to the foregoing, we assume that thermal annealing improves the motions of MEH-PPV molecules and translates the collapsed chain conformation into a more extended one. This is the cause for the increase of MEH-PPV molecules exhibiting continuous photobleaching. Notably, this transformation is most distinct when the temperature rises from 100 °C to 120 °C. The glass transition temperature (Tg) of PMMA, which is about 106 °C,21 is a probable reason for this. When the annealing temperature is higher than Tg, the PMMA chains can motion freely, leading to the conformational transition of MEH-PPV molecules embedded there. As the temperature rises higher further, the conformational transition becomes slighter than before.
Anisotropy parameters acquired by polarized fluorescence measurements can characterize the chain conformations of single polymer chains sufficiently. However, the ensemble single molecule fluorescence measurement provides useful information as well. Fig. 3(a) shows the absorption and PL spectra of pure MEH-PPV film. The absorption spectrum shows a maximum absorbance at about 500 nm and the PL spectrum exhibits a primary peak at about 575 nm, a small peak at ∼625 nm, and a shoulder peak with a long tail at ∼700 nm, as illustrated in Fig. 3(a). These peaks are assigned to the purely electronic transition (0–0) and the first (0–1), second (0–2) vibronic transitions of MEH-PPV, respectively. The absorption spectra of all annealed samples are not discussed here, as the concentrations of MEH-PPV in the blend films are extremely low that the absorption spectra are thus very challenge to be acquired by a commonly used instrument like Perkin-Elmer Lambda 750. Fig. 3(b) shows the ensemble single molecule fluorescence spectra of blend films annealed at different temperatures. The PL spectrum of the sample annealed at 60 °C displays a primary peak at about 575 nm, a shoulder peak at ∼540 nm, and another shoulder peak with long tail at ∼620 nm. When annealed at 80 °C and 100 °C, the samples show similar PL spectra with lower intensities. Obvious transformation of the PL spectra shape was observed when samples annealed at a temperature higher than 100 °C. The strength of the peaks at 540 nm and 575 nm is nearly the same when annealed at 120 and 140 °C.
The decrease in the intensity of the PL spectra upon thermal annealing has been reported by the previous studies.7,22 In bulk films, the structural relaxation that occurs upon thermal annealing of polymer films allows the formation of aggregates or interchain polaron pairs that can reduce or quench PL or electroluminescence (EL) yield. Liu et al.23 have observed that MEH-PPV annealed at 140 °C exhibits a lower EL efficiency than one annealed at 70 °C. Obviously, such an interpretation is not applicable to our experiments, as the concentration of MEH-PPV in PMMA is so dilute that rarely intermolecular aggregates exist.
The PL spectra of MEH-PPV diluted to PMMA matrix exhibit a blue component compared with the pure film, which might originate from isolated chains.24 Furthermore, obvious blue shifts were observed for the PL spectra with increasing annealing temperatures, as illustrated in Fig. 3(b). The blue shift of emission spectra upon dilution of MEH-PPV in PMMA has been observed by the previous works,5,25,26 which can be explained by the cutting-off of the energy transfer to low energy traps and suppressing of generation of interchain excitons, and conformational changes of the polymer backbones.27 Assuming that only intrachain excitons are responsible for the PL, the blue shift can be attributed to the thermally induced torsional motions shortening the conjugation lengths and differences in the Huang-Rhys coupling factors (S) for chains with distinct conjugation lengths, leading to differences in the relative intensities of the zero- and higher-order phonons, producing changes in the spectral profile.28,29 The temperature dependent PL spectra have demonstrated that the thermally induced torsion and libration modes increase the conformation disorders to shorten the conjugation segments in the MEH-PPV with increasing temperature and thus reduce the extent of π - electron delocalization and increase the energy of the π - π* transition, leading to a blue shift in the PL spectra of MEH-PPV in solid matrix at high temperature.30 To our experiments, the increasing motion ability of PMMA molecules promotes the torsion and libration of MEH-PPV single molecules and thus increases the torsion defects at the backbone and shortens the conjugation length. This should be the cause of the blue shifts in the PL spectra with higher annealing temperatures. From another point of view, the blue shifts in the PL spectra verify the foregoing supposition about the change of conformations. The intramolecular energy transfer is efficient in a collapsed chain, leading to an emission at lower energy site, while an extended chain emits at a higher energy site due to the less of chain-chain interactions.15,31
In fact, previous studies about perylene based chromophore have demonstrated that spontaneous spectral jumps, originating from conformational changes, are indeed directly observed in sequences of single-molecule spectra.32,33 The change of the amino group conformation results in in-resonance and off-resonance conformers, leading to disparity spectra. The conformations are also responsible for the emissions of MEH-PPV.34 The single chain of MEH-PPV always folds at the defect sites (saturated vinylene linkages) into a rod-like structure due to its specificity of the molecular structure. In this case, a slight helicity can be seen owing to the steric interactions of the alkyl side groups.35 The helix conformation decreases the degree of molecular planarity of the polymer π-conjugated backbone, leading to a blue shifted spectrum.
To further clarify the effect of annealing temperature on the emission of MEH-PPV, we analyzed the PL spectra using Gaussian functions to adjust the electronic and vibronic bands. All Gaussian fitting curves results in four peaks, with centers at about 540, 575, 620, and 670 nm, respectively. Fig. 4(a) shows the Gaussian fitting result of the PL spectrum of the sample annealed at 60 °C. Other fitting results are displayed in supplementary material (Fig. S2).19 The positions of the four peaks change with increasing annealing temperature differently. The blue emission at 540 nm and the 0–2 peak remain in the same positions, while the 0–0 peak shifts to a higher energy frequency since the annealing temperature higher than 100 °C, and the 0–1 peak exhibits blue shifts gradually with increasing annealing temperatures. The evolutions of the 0–0 and the 0–1 peaks with increasing annealing temperature are shown in Fig. 4(b).
Based on the descriptions above, we attribute the changes in the 0–0 and the 0–1 emission peaks to the conformational transition. Little conformational changes of MEH-PPV are caused by the secondary motions of side groups since the PMMA segments cannot motion freely under a temperature lower than its Tg. In this condition, rarely changes in intrachain interaction but more in interchain interactions occur. As a result, the 0–0 peak shows no changes, while the 0–1 peak exhibits obvious blue shifts at low temperature. When the annealing temperature is higher than the Tg of PMMA, the motions of PMMA segments allow the MEH-PPV molecules to twist or orient, resulting to more extended rod-like structures. On the other hand, the thermally induced defects reduce the effective conjugation lengths, leading to blue shifts of the 0–0 and 0–1 peaks.
From the four-peak Gaussian fitting results, we can obtain information about electron-phonon coupling in MEH-PPV molecules. The electron-phonon coupling strength in a molecule can be described by the Huang-Rhys factor, S. The S value can be obtained by calculating the related strength of the 0–1 peak intensity compared to the 0–0 peak intensity: S = I0–1/I0–0 and could be related to the effective conjugation length of the polymer system. According to the empirical function proposed by Yu et al.36 for the PPV, the relationship between the Huang-Rhys factor and the conjugation length can be described by S = a exp(−n2/b), where a and b are empirically chosen to be 3.2 and 38, and n is the conjugation length. Fig. 4(c) displays the calculated Huang-Rhys values and conjugation lengths at different annealing temperatures. As illustrated, the S value increases while the n value decreases with increasing annealing temperature. This indicates that the molecular torsions and the low frequency vibrational modes that change the chain planarity are increased, which also means that the effective conjugation length and the electron delocalization reduce. These calculated results are in good agreement with our explanations about the blue shifts of the PL spectra that the torsion defects of MEH-PPV single molecules increase and the conjugation lengths decrease with increasing annealing temperature. Therefore, the reduced conjugation length promotes the single chain to be more extended state.
In summary, thermal annealing promotes the transformation from collapsed conformations to extended, highly ordered rod-like structures of MEH-PPV single molecules. Since the Tg of PMMA is about 106 °C, the conformational transition is observed most obviously when the annealing temperature rises from 100 °C to 120 °C. The blue shifts in the ensemble single molecule PL spectra, the calculated Huang-Rhys factors, and conjugation lengths support our hypnosis. The transition is slight relatively as the annealing temperature higher than 100 °C, implying that an annealing temperature near the Tg of matrix is ideal for manipulating the morphology of blend polymer films in devices. This result is believed to be very meaningful for other blend polymer films.
This work was supported by the Natural Science Foundations of China (Grant Nos. 51233008, 51403244, and 51173215). Y. Chen would also like to thank the support from the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Project No. 20143000041050352).