Effects of Polymer Packing Structure on Photoinduced Triplet

Apr 26, 2012 - Annamaria Petrozza*†‡, Daniele Fazzi‡, Igor Avilov§, David Beljonne§, Richard H. Friend†, and Ji-Seon Kim⊥. † Cavendish L...
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Effects of Polymer Packing Structure on Photoinduced Triplet Generation and Dynamics Annamaria Petrozza,*,†,‡ Daniele Fazzi,‡ Igor Avilov,§ David Beljonne,§ Richard H. Friend,† and Ji-Seon Kim⊥ †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge. CB3 0HE United Kingdom Center for Nano Science and Technology @PoliMI, Istituto Italiano di Tecnologia, via Pascoli 70/3 20133 Milano, Italy § Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium ⊥ Department of Physics & Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom ‡

ABSTRACT: The study of photoexcitations dynamics as a function of thin film microstructures for solution-processed organic semiconductors is essential to provide physical insights needed to further developments in the field of organic electronics as a whole. Here, the effects of polymer packing structure on triplet generation and dynamics in poly(9,9-di-noctylfluorene-alt-benzothiadiazole) (F8BT) thin films of different molecular weights (Mn = 9−255 kg/mol), both in the pristine and annealed states, were studied by photoinduced absorption spectroscopy and quantum chemical calculations. For pristine films, the lowest molecular weight gives rise to the strongest triplet absorption signal resulting from the enhanced generation through an efficient intersystem crossing process. Upon annealing, an increase in the triplet lifetime is measured. These changes are associated with a restructuring of F8BT molecules packing dictated by the strong dipole on the BT unit, which subsequently affects the interchain exciton migration.

1. INTRODUCTION The polyfluorenes comprise a class of conjugated polymer which has shown important characteristics such as the ability to tune a wide range of optical and electronic properties by copolymerizing basic fluorene monomers with other appropriately chosen conjugated units.1−3 One of these, poly(9,9-di-noctylfluorene-alt-benzothiadiazole) (F8BT), has emerged as an attractive candidate for many optoelectronic applications.4−7 It emits yellowish-green light with a high luminescence quantum yield (∼60−80%),8 and it has both high electron affinity (∼2.95 eV)9 and field-effect electron mobility (∼0.005 cm2 V−1 s−1).10 On the basis of these properties, F8BT plays an important role as an efficient electron transport agent in optoelectronic devices. In previous studies on F8BT thin films, a restructuring of the polymer packing structure was observed as a function of F8BT molecular weight and upon annealing, which affects both optoelectronic and charge-transport properties of the films.8 In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the fluorene (F8) and the benzothiadiazole (BT) units; moreover, BT units in neighboring chains are close to one another (e.g., face to face (FTF) structure).8 Annealing films to sufficiently high temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain (e.g., alternating (A) structure), © 2012 American Chemical Society

and the torsion angle between F8 and BT units is reduced. This affects the efficiencies of interchain charge transfer and exciton migration making these processes more difficult for the alternating structure. The effects of these distinctive polymer packing structures on the photophysical processes involving the triplet excitations, however, have not been fully investigated yet. Because of a stronger localization of the triplet exciton wave function with respect to the singlet wave function, the direct effect on triplet dynamics of structural changes in conjugated polymer chain microarrangements is definitely more difficult to identify with respect to emissive singlet excitons. However, when working on the device optimization, the introduction of different local chain packing structures can be fundamental for improving the optoelectronic properties of the system; therefore, a complete understanding and control of excitons dynamics as a function of polymer chain microstructures gain great importance. Here, we study the generation and the lifetime of triplet excitons in F8BT thin films by using photoinduced absorption spectroscopy and quantum chemical calculations. We show that the triplet exciton generation efficiency, on the basis of the intersystem crossing process, and the triplet lifetime are Received: October 30, 2011 Revised: April 23, 2012 Published: April 26, 2012 11298

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strongly dependent on the F8BT molecular weight and on the annealing process used. Then, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations help us to predict the excited-state energetics for both singlet and triplet states of F8BT and to identify the most efficient pathway for the singlet → triplet (Sn → Tm) transition.

In eqs 2a and 2b, ω is the pump modulation frequency and N is a prefactor proportional to the pump intensity and the triplet generation efficiency. The bimolecular triplet recombination leads to sublinear dependence of the triplet population on Nabs. Thus, a further term has to be added to eq 1

2. EXPERIMENTAL METHODS F8BT materials ranging in molecular weight (Mn) from 9 to 255 kg/mol were used. Thin F8BT films were spin-coated from o-xylene solutions on spectrosil substrates to obtain an optical density of ∼0.8 at 463 nm. Spectrosil substrates were precleaned by two successive 10 min ultrasonic baths in acetone and isopropanol. For annealing, the films were heated above the Tg at 180 °C8 for 1 h in a nitrogen atmosphere and then were slowly cooled. Photoinduced absorption (PIA) spectroscopy is a form of pump/probe modulation spectroscopy used to study photogenerated species characterized by a long lifetime (≈μs−ms). Every sample is excited by a continuous wave (CW) pump source at 2.54 eV mechanically modulated at 224 Hz. The laser intensity has been constantly monitored to avoid long-term fluctuations. The transmission (T) of the sample is monitored using a probe beam (white light source 150 W halogen bulb) and is collected by an amplified Si photodiode; its output signal is measured by a lock-in amplifier to detect small changes in the transmission (ΔT). The lock-in amplifier is set in-phase with the pump source by adjusting the phase to obtain all the luminescence signal (the short lifetime signal, ∼ns) as a positive signal in the X-channel (in-phase channel). In this way, the induced absorption due to long-lived excited states (i.e., triplet states) will be read as a negative signal in the X-channel and a positive signal in the Y-channel (out-of-phase channel). PIA spectra are corrected for any luminescence signal. In fact, if both photoluminescence and induced absorption features occur at the same wavelength, they would cancel each other out. To compensate for this, two readings are taken at each wavelength: one with the probe beam incident through the sample and one without under photoexcitation. These are then subtracted to calculate the true induced absorption signal. All measurements were performed at 20 K. The data have been reproduced several times, and the samples have been measured in a different order. The data analysis has been performed describing the triplet monomolecular recombination process by the simple rate equation

(3)

dn n = g (t ) − dt τ

dn n = g (t ) − − β n 2 dt τ

where β is the bimolecular recombination rate. Electronic structures and geometries of F8BT oligomers have been optimized by using DFT method with a hybrid exchangecorrelation functional (B3LYP)11 and double split valence basis sets 6-31G and 6-31G(d). This approach is known to provide reliable trends when ground- and excited-state properties of conjugated compounds have to be described.12 Since the electronic and optical properties of conjugated oligomers saturate rapidly with increasing the chain length (n), we considered F8BTn oligomers with n = 3, 4 (repeat units) as good representatives for the polymer.9 Both the singlet ground state (S0) and the first triplet state (T1) geometries have been optimized at the restricted and unrestricted level ((U)B3LYP/ 6-31G(d,p)) for the case of F8BT4. For both molecular structures, we have carried out a vibrational frequencies analysis to check that the optimized geometries correspond to minima on the potential energy surfaces (no imaginary frequencies). Vertical electronic transition energies have been evaluated at the TD-DFT level (TD(U)-B3LYP/6-31G(d,p)) for both singlet, S0 → Sn, and triplet, T1 → Tn, transitions on the basis of the optimized S0 and T1 geometries.13 All simulations have been carried out with the Gaussian09 suite of programs.14

3. RESULTS Figure 1a shows the PIA spectra ranging from 1.12 to 2.48 eV of pristine F8BT films of different molecular weights (Mn = 9, 90, 255 kg/mol). For all spectra, the ground-state bleaching is observed at ∼2.46 eV. Below 2.3 eV, broad photoinduced absorption bands appear showing the most intense transition at ∼1.44 eV and less bands at ∼1.2, ∼1.32, and ∼2.2 eV. T1 → Tn transitions are typically responsible for absorption in this spectral range in conjugated polymers.15−19 A significant increase in PIA signal intensities is observed in Figure 1a as the F8BT molecular weight decreases from 90 to 9 kg/mol, but no further changes occur between 90k and 255k. No measurable shift in the absorption peak positions is noticed. Figure 1b and c shows the comparison of PIA spectra between pristine and annealed F8BT films for 9 and 255 kg/ mol molecular weight, respectively. Different effects of the annealing process are observed on the spectra for F8BT with different molecular weights. For the case of low molecular weight film (9 kg/mol), a reduction in the PIA signal is observed after annealing; a red-shift of about 70 meV in the ground-state bleaching peak is observed as well as a red-shift of the T1 → Tn transitions band of about 25 meV. In contrast, for higher molecular weight films (90k and 255k), a strong intensity enhancement for both in-phase and out-of-phase triplet main absorption transitions is observed after annealing. No shifts of the triplet absorption peaks or of the ground-state bleaching peak are observed. To help rationalize these data, a detailed DFT and TDDFT study has been carried out. The analysis of the triplet excited state wave functions in F8BT15 indicates that the experimental absorption band appearing at 1.4−1.7 eV arises from several electronic transitions involving occupied and unoccupied

(1)

where g(t) is the excitation generation rate which is proportional to the intersystem crossing (ISC) efficiency, γ, and to the absorbed photons on a unit cross section area per second, Nabs, and τ is the monomolecular lifetime. Assuming a sinusoidal generation rate, the in-phase and quadrature components can be described by the following equations in order to extract the monomolecular lifetime: ⎛ ΔT ⎞ Nτ ⎜ ⎟ = ⎝ T ⎠in (ωτ )2 + 1

(2a)

⎛ ΔT ⎞ Nωτ 2 ⎜ ⎟ = ⎝ T ⎠out (ωτ )2 + 1

(2b) 11299

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Figure 2. (a) Simulated theoretical T1 → Tn absorption spectra (in absorption cross sections) of FBT trimer optimized without geometry constraints. Black vertical lines show the position of the calculated transitions; their height corresponds to the oscillator strength of transition. The first height transitions are marked with letters from a to h. (b) Comparison of T−T absorption spectra of the trimers, optimized imposing constraints on the value of the dihedral angle between fluorene and BT subunits (from 0° for fully planar structure to ∼32° for the structure optimized without geometrical restrictions).

Figure 1. (a) Photoinduced absorption spectra of F8BT pristine films of different molecular weights (M n = 9, 90, 255 kg/mol). Photoinduced absorption spectra of (b) F8BT/9k and (c) F8BT/ 255k annealed films compared to the respective pristine films. The inphase signal is shown for each sample; the correction phase with respect to the photoluminescence signal is ∼0.36 for the pristine samples and ∼0.53 for the annealed samples. Excitation is 113 mW cm−2 at 2.54 eV.

these transitions involve molecular orbitals that are much localized over either BT or fluorene units. At last, the peak at 2.17 eV (T1 → T12 transition, peak h in Figure 2) involves electronic excitations from delocalized occupied molecular orbitals to unoccupied molecular orbitals mostly confined on BT units thus having a considerable charge transfer character (see Table 1). In previous studies, it has been demonstrated that reducing the molecular weight of the copolymer and annealing films above the Tg at 180 °C8 induces a reduction of the torsion angle between F8 and BT units, which thus is a planarization of the polymer chain.8 Therefore, on the basis of the detailed analysis of the triplet absorption spectra and excited states shown in Figure 1, the effect of a reduced dihedral angle between adjacent fluorene and benzothiadiazole units on triplet−triplet excitations has been theoretically investigated. The calculated T1 → Tn absorption spectrum is shown in Figure 2b, where the torsion angle between F8 and BT units varies from 0 (virtually fully planar structure) to ∼32° (structure optimized without geometrical constraints). According to the calculations, planarization causes the red-shift of the absorption bands (by up to ∼0.08−0.09 eV for fully planar

molecular orbitals which have different spatial delocalization and symmetry. The calculated TD-UB3LYP/6-31G T1 → Tn transitions for F8BT3, as computed upon the UB3LYP T1 optimized equilibrium geometry, are presented in Figure 2a. The character of different T1 → Tn transitions has been determined on the basis of the so-called natural transition orbitals (NTOs) approach as described in detail in ref 20. The natural transition orbitals for the most intense transition band (named c) are presented in Table 1. This transition, that contributes to the most intense experimental band, is predicted as the T1 → T4 excitation, and the molecular orbitals involved are delocalized over both BT and fluorene units. The energy of this transition is predicted at 1.26 eV to be compared to the measured 1.44 eV value with an oscillator strength f = 0.622. Other triplet−triplet transitions are predicted (see bars a, b, d, e, f, and g in Figure 2) with lower oscillator strength excitation; 11300

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Table 1. Contour Plots of the Pairs of Natural Transition Orbitals Contributing the Most to the Description of the Main T1 → Tn Transitions of FBT Trimer (n = 3)a

a

Only dominant α-NTOs are presented. The energies and oscillator strengths of transitions are given in the first column.

Figure 3. Dependence of the photoinduced absorption at 1.44 eV on the laser intensity for the (a) 9k and (b) 255k F8BT thin films in the pristine (open circles) and annealed (filled circles) state. Modulation frequency dependence of the photoinduced absorption at 1.44 eV for pristine (open markers) and annealed (filled markers) (c) 9k and (d) 255k F8BT thin films. Experimental values of in-phase (circles) and out-of-phase (triangles) PIA signals are shown together with their global fits (solid line) to eq 2aand 2b. Excitation was 90 mW cm−2 at 2.54 eV.

and d shows the PIA signals of the triplet absorption peak at the same transition energy as for the intensity dependence study as a function of the modulation frequency taken from 9k and 255k pristine and annealed F8BT films. The frequency dependence data, taken at low excitation intensity, have been fitted using eq 2a and 2b. The results of the fitting are reported in Table 2. The ideal behavior expected in the presence of monomolecular recombination process shows the in-phase signal independent of modulation frequency at lower modulation frequency and a decrease as a power law of ω−2 at higher frequencies; the out-of phase signal should reach a maximum at the frequency where the in-phase signal reduces to half of its zero frequency and then decreases as a power law of ω−1. However, not a completely accurate fit is shown at higher frequency from all F8BT samples, whose in-phase and out-of-phase signals decrease with a law power ω−a with a < 1.

F8BT3) and induces an enhancement of the absorption crosssection of the main T1 → Tn transition. These data, as further explained in the next section, are in good accordance with the changes observed in the PIA spectra of 9 kg/mol molecular weight F8BT films (Figure 2). To gain insight into the characteristic lifetime of triplet excitons, the modulation frequency dependence of the main triplet absorption transition has been measured. Figure 3a and b shows the excitation intensity dependence of the PIA signal at 1.44 eV that has been first taken to identify the presence of a monomolecular decay regime. The intensity dependence curves show a linear trend over about 2 decades and become sublinear at higher intensities with a higher turning point for annealed samples. This is the signature of a bimolecular recombination regime at high excitation intensities and a monomolecular decay regime in the low intensity excitation region. Figure 3c 11301

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Table 2. Monomolecular Triplet Lifetime τ, ISC Efficiency γ, and Bimolecular Recombination Rate β of Pristine and Annealed F8BT Thin Films of Different Molecular Weights sample 9k pristine 255k pristine 9k annealed 255k annealed

τ [ms]

β (10−15 cm3 s−1)

± ± ± ±

1.61 ± 0.01 1.5 ± 0.01 0.7 ± 0.01 0.04 ± 0.01

0.28 0.26 0.43 0.44

0.02 0.03 0.03 0.04

strongly on both the generation efficiency of triplet exciton via ISC process (γ) and the triplet exciton lifetime (τ). On the basis of the previous observations, we consider that more efficient triplet generation (γ) via ISC process is responsible for a significant increase in the triplet absorption for the low molecular weight F8BT film. On the other hand, the longer monomolecular triplet lifetime (τ) induced by annealing leads to enhanced triplet absorption, in particular, for high molecular weight films. For the annealed low molecular weight film, the reduction in the γ value and, hence, in the density of triplets is so large that no significant enhancement in triplet absorption is observed although annealing increases its τ value.

γ 0.028 0.017 0.015 0.016

± ± ± ±

0.002 0.001 0.001 0.001

As it has been shown by Epshtein et al., this is indicative of the presence of a distribution of lifetimes and has be modeled as a dispersive process21 including a parameter α which is the width of the distribution of the lifetimes with τ0 being the center of the distribution. We have fitted the experimental data shown in Figure 3 also by using this model; the quality of the fit is improved and the mean lifetime τ0 is found to correspond to the monomolecular lifetime obtained from eq 2a and 2b. The parameter α is about 0.85, which indicates modest dispersion.21 The bimolecular recombination parameter, β (see eq 3), has been calculated too. First, the equivalent bimolecular lifetime, τbi, has been measured by frequency dependence measurements at high excitation densities (1 W/cm2 excitation intensity). Then, β has been calculated according to the relationship β = 1/(τbinbi)[s−1 cm−3] obtaining nbi directly from the intensity dependence curves22 (Figure 3c and d). The values obtained for pristine and annealed samples of different molecular weights are shown in Table 2. In summary, it is found that (a) the monomolecular triplet lifetime τ shows a similar value of about 0.27 ms for all pristine F8BT films; (b) annealing the films increases the monomolecular lifetime by a factor of 2 to about 0.44 ms; (c) the bimolecular recombination parameter results are mainly affected when annealing high molecular weight films exhibiting a reduction of about 1 order of magnitude. Finally, the effects of change in molecular weight and annealing on triplet generation efficiency have been investigated. The intersystem crossing efficiency (γ) has been calculated solving the rate equation for the monomolecular decay kinetics (eq 1) under the steady-state condition using the monomolecular triplet lifetime previously calculated.23 We observe that the ISC efficiency (γ) is almost doubled from 1.7% to 2.8% as the molecular weight of pristine F8BT films decreases. After annealing, γ decreases to about 1.5% for the lowest molecular weight (9k) film, but no significant reduction is found for high molecular weight films (all the values are reported in Table 2). As chain planarization can induce an enhancement of the absorption cross section of the main T1 → Tn transition (see Figure 2 b), the drop of the ISC efficiency of the 9k annealed F8BT film may be even underestimated. The relative change in transmission (ΔT/T) measured by CW PIA spectroscopy is governed mainly by three parameters: the density of triplet excitons (n), the triplet absorption cross section (σ), and the film thickness (d). The film thickness has been kept constant for all samples (d ∼ 100 nm). Then, according to theoretical studies on the dependence of chain length on the triplet absorption cross section of the F8BT trimer, presented by Ford et al., a small triplet absorption cross section (σ ∼ 2 × 10−16 cm2) and weak chain-length dependence has been found.23 Therefore, the change in the intensity of triplet absorption signals is expected because of the changes in the density of triplet excitons (n) created under photoexcitation. In fact, the density of triplet excitons depends

4. DISCUSSION In general, the origin of the ISC process in F8BT can be attributed to the heavy-atom effect associated with the sulfur atoms as in the case of thiophene-based conjugated materials.24 However, the results presented above show that the efficiency of this ISC process depends strongly on the local packing structure of F8BT polymer chains, which varies as a function of molecular weight and upon annealing. First, we will discuss the effects of molecular weight and annealing on the ISC efficiency. To get insights into the physics governing the ISC phenomenon, it can be useful to qualitatively recall here the dependence of the ISC transfer rate (kISC) from the molecular parameters affecting the phenomenon. The transfer from a singlet excited state Sn to a triplet excited state Tm can be described by the golden-rule expression25−27 kISC =

⎛ (λ + ΔE)2 ⎞ 2π 1 0 1 0 ⟨ ΨS |HSO|3Ψ T ⟩2 exp⎜ − ⎟ ℏ 4λRT ⎠ ⎝ 4πλRT (4)

where ΨS and are the wave functions of singlet and triplet excited states, respectively, and HSO is the spin−orbit Hamiltonian. ΔE is the energy difference between the initial singlet (S) and the final triplet states (T) (i.e., singlet−triplet energy splitting), whereas λ corresponds, to the first approximation, to the energy variation in the initial excited state when switching from the singlet to the triplet equilibrium geometry. Reorganization energy, λ, associated to the Sn → Tm process in organic compounds is usually assumed to be about ∼0.1−0.3 eV; for F8BT, thanks to its relatively rigid polymer backbone conformation, λ can be considered, as a good approximation, lower than 0.3 eV.26,27 Equation 4 highlights two independent factors affecting the ISC rate (kISC) which can be discussed to analyze the experimental data: the spin orbit coupling matrix element (⟨ 1 Ψ s 0 |H SO | 3 Ψ T 0 ⟩ 2 ) and the so-called Franck−Condon weighted density of states (FCWD) term [1/(4πλRT)1/2]exp[−(λ + ΔE)2/(4λRT)]. To our knowledge, there is no evident explanation which relates the enhancement of the ISC efficiency to a stronger spin−orbit coupling matrix element as the polymer molecular weight is reduced. The following analysis will be focused on the FCWD factor and, more specifically, on the ΔE term that enters in the exponential function. This approach is certainly not generally valid and depends on the relative magnitude between the driving force versus the reorganization energy (see eq 4).26,27 Experimental and theoretical findings show that polymers are characterized by a rather large splitting between the lowest singlet (S1) and triplet (T1) excited states Δ(ST) ∼ 0.7−0.9 1

11302

0

3

ΨT0

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eV28 in organic compounds. In Figure 4a, the calculated energy level diagram of excited singlet and triplet states is reported.

= 1.9) and S3 (2.55 eV, f = 0.09), TD-DFT calculations predict the presence of several triplet states; in particular, S3 is in quasiresonance with T2 and T3 states with Δ(ST) ≈ 0.05−0.08 eV, while for S1, Δ(ST) is higher with respect to T2/3, namely, Δ(ST) ≈ 0.3−0.5 eV. The calculated energy difference can be considered sufficiently low to induce significant overlap between the vibrational wave functions of the initial singlet state (Snvib) and the final triplet (Tmvib) state (e.g., qualitatively high FC factors) thus satisfying one of the requirements for efficient ISC, that is, triplet and singlet states within an energy range commensurate with the reorganization energy. At this stage, information on the singlet excitons dynamics is critical to understand the fate of the primary excitation as a function of different F8BT molecular packings. The F8BT films under investigation exhibit at least two main relaxed emissive states: one at ∼2.13 eV (low energy state, LES) and the other at ∼2.28 eV (high energy state, HES), which can be assigned to the dipole allowed singlet states S1 and S3 calculated above. The exact nature of these emissive states is not well-known yet, however, they were previously determined to be distinct emissive states through grazing angle photoluminescence measurements29 and selective quenching of the LES by a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) quenching interface.30 A distinctive bimodal emission has also been shown by Grey et al. through single molecule spectroscopy when going from short to long F8BT chains.31 In Figure 4b, we show that there is a gradual variation in the relative intensity ratio between these states with the HES becoming more intense as the molecular weight decreases. Upon annealing, the HES is dominant for all molecular weight films. The relative intensities of the LES and HES are thought to be strongly correlated with the two different local chain arrangements which strongly affect the exciton migration.8 From these findings, we propose the following mechanism for the population of T1 state in different molecular packing structures. Excitation can be transferred trough ISC from S1 or S3 states (dipole allowed) to high energy T2 or T3 states. From TDDFT calculations, a better energy resonance has been evaluated for the case of S3 → T2/T3, showing the lower Δ(ST) energy gap, which thus is a more efficient triplet generation process. From PL and PIA measurements (see Figures 1 and 3), it is plausible to suggest that a higher triplet density of population for the low molecular weight chain arrangement is a consequence of a higher population of the HES favored by the alternating molecular packing. In fact, upon annealing, the population of the HES increases for all molecular weight films. However, this enhanced HES population is not followed by an efficient ISC process especially for the low molecular weight sample whose ISC efficiency (e.g., γ) is nearly twice smaller. As a consequence of annealing, a fundamental effect is the reduction of the torsion angle between the F8 and BT unit along the polymer chain. From TDDFT calculations reported in Figure 2, we have shown that the effect of planarization consists of causing a red-shift of the main absorption bands (∼0.08−0.09 eV for fully planar FBT3). Interestingly, this effect is experimentally observable (see Figure 1b) only in the low molecular weight sample, which shows a shift of the main triplet transition of ∼0.025 eV (see Figure 1b). The same sample also shows the most appreciable drop in ISC efficiency (see data reported in Table 2). The decreasing ISC efficiency upon annealing is thus ascribed to a planarization of the polymer chains. Planarization decreases the expectation value of

Figure 4. (a) TD-(U)B3LYP/6-31G* excited state vertical transition energies evaluated for FBT oligomer (n = 4) starting from the optimized ground state structure (S0 → Sn transitions) and the triplet structure (T1 → Tn transitions). (b) Normalized luminescence spectra of pristine and annealed films for F8BT/255k and F8BT/9k. The proposed local chain packing structures are also shown. Inset FTF: the initial packing structure of the high molecular weight pristine films. The BT units exhibit a relatively high torsion angle with respect to the F8 units, and in neighboring polymer chains, the BT units are adjacent to each other. Inset A: the packing structure for the low molecular weight pristine films or annealed films. Adjacent polymer chains have been translated with respect to one another so that the BT units in one chain are adjacent to the F8 units in the adjacent chain (alternating structure). This structure forces the BT units into a geometry that is more planar with the F8 units.

According to our quantum chemical calculations, the adiabatic transition from S0 to T1 is located at ∼1.6 eV; the calculated energy difference between S1 and T1 is Δ(ST) = 0.79 eV. Such a large exchange energy makes the ISC process from the singlet S1 to the lowest triplet T1 state difficult; for this reason, it is very likely that ISC proceeds through high energy triplet states.26 Close to the dipole allowed singlet states S1 (2.38 eV, f 11303

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the spin−orbit coupling and, consequently, the kISC value. It has been shown for the case of oligothiophenes that the expectation value of spin−orbit coupling required for the ISC process decreases when the torsion angle between adjacent rings decreases.26 This is explained by taking into account changes in the energetic separation between π and σ electronic structures. The σ−π mixing decreases as the torsion angle gets smaller leading to a reduced spin−orbit interaction between singlet and triplet states.28,32,33 Also in heterocyclic compounds, a strong spin−orbit coupling has been observed for transitions which involve molecular orbitals with some σ character because of efficient σ−π mixing.24,28,34,35 The presence of a more planar structure of F8BT chains with reduced torsion angle between the F8 and the BT units induced by annealing can give rise to a decrease in the σ−π mixing and thus a reduction in the ISC efficiency. Finally, in this frame, an increase in triplet monomolecular lifetime upon annealing can be understood on the basis of the local packing structure of F8BT molecules induced by thermal treatment. The polymer chains are not able to fully reach the lowest energy configuration during the relatively fast spincoating process. When they obtain sufficient time and energy during thermal annealing, they can rearrange themselves to adopt the lowest energy configuration. In this configuration, the BT units would occupy positions adjacent to F8 units in neighboring polymer chains (alternating structure, Figure 4b). This alternating packing structure allows less overlap between BT units in neighboring chains and reduces the rate of triplet exciton transfer between polymer chains and finally to the exciton quenching sites.36

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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5. CONCLUSIONS The effects of polymer packing structure on triplet generation and dynamics were studied in F8BT thin films. A significant increase in the triplet absorption for pristine low molecular weight F8BT film was observed and was attributed to enhanced triplet generation efficiency via an efficient intersystem crossing process. In addition, annealing results in an increase in the monomolecular triplet lifetime leading to enhanced triplet absorption for all molecular weight films. These changes are strongly correlated with a solid-state restructuring of F8BT molecules as a function of molecular weight and upon annealing, which subsequently affects the efficiency of interchain exciton migration and thus the efficiency of intersystem crossing and the triplet lifetime.

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ACKNOWLEDGMENTS

The authors thank CDT for F8BT materials supply and the EPSRC for funding. J.S.K. thanks the World Class University (WCU) Project through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R32-10051). D.B. is a Research Director of Belgian National Science Foundation (FNRS). 11304

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