Intrachain Interactions Behind the Formation of Charge Transfer

Mar 3, 2015 - In this work, we demonstrate the complex excited-state nature of the ... quality and temperature dependent CT state formation arising fr...
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Inter/Intrachain Interactions Behind the Formation of Charge Transfer States in Polyspirobifluorene: A Case Study for Complex Excited-State Dynamics in Different Polarity Index Solvents Murat Aydemir, Vygintas Jankus, Fernando B. Dias, and Andrew P. Monkman* Physics Department, OEM Research Group, Durham University, Rochester Building, South Road, DH1 3LE County Durham, United Kingdom S Supporting Information *

ABSTRACT: In this work, we demonstrate the complex excited-state nature of the conjugated polymer, polyspirobifluorene (PSBF), using steady-state and time-resolved spectroscopy techniques to understand the origin of excited charge transfer state (CT) formation and their contribution to the total photoluminescence (PL). The measurements were compared in two solvents with different polarity, for example, methyl cyclohexane (MCH) and 2methyltetrahydrofuran (2-MeTHF), which allow us to reveal solvent quality and temperature dependent CT state formation arising from “inter/intrachain” interaction phenomena. The inter/intrachain interactions are explained by means of spatial conformational changes of the polymer chain configuration, such as coiling and collapse of the backbone with concomitant side chain reorganization. It has been found that the PL emission at room temperature (RT) demonstrates a mixed state configuration containing contributions from 1 (π, π*) excited states along with the CT states contribution, with the spectra arising from a mixture of the two emissive species. However, with decreasing temperatures to ca. 145 K (prior to the freezing point) in 2-MeTHF, the two emissive species become separated, with the emission from the CT state showing a red-shift with decreasing temperature. At 145 K, we observe the formation of an unstructured, wholly new emission band, which is strongly red-shifted relatively to the 1(π, π*) excited-state and shows classic Gaussian line shape. This emission is attributed to the formation of “inter/intrachain” CT states. In the case of frozen solutions (∼90 K), the spectra dramatically blue-shifts and loses all contribution from the “inter/intrachain” species, and emission then arises completely from the pure “intrachain” CT excitonic state. The behavior of the polymer is strongly dependent on both solvent quality and temperature effects on the excited state geometry relaxation by means of the local solvent−solute interactions that stabilize the CT states, due to solvation of the new charge distribution, and also changes on the transition states via manipulating energy barriers.

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INTRODUCTION

Generally conjugated polymers adopt random coil conformation, and this configuration changes depending on their environment. Thermal processing, solution viscosity, and polarity all affect the final conformational relaxation of the polymer chains resulting in new photophysical properties,6,11 which show dramatical differences when comparing the solution and solid-state phases due to the closer proximity of the chains in solid-state.12,13 In general, rigid-rod type polymers demonstrate more extended conformations comparing with the polymers which have more flexible backbones.14 Conformational disorder also affects the charge transport properties of polymers, and usually this is attributed to the presence of traps due to disorder15,16 which also affect the exciton dynamics and quantum yield of the polymers.17,18 Aggregation of polymer chains either in solution or in solid-state, may result in exciton

Organic light-emitting diodes (OLEDs) have attracted a great deal of interest due to their potential applications in display and lighting technologies.1,2 Over the past two decades, demanding commercial applications have paved the way for improving the efficiency, stability, and charge trasport features of OLEDs to make them competitive with inorganic LEDs.3,4 Conjugated polymers still hold the promise of cheap solution processing for large area applications such as lighting, and much work has focused on highly emissive blue-emitting polyfluorene derivatives, in order to understand the nature and complex excitedstate dynamics of conjugated polymer systems.5−7 Inherently, conjugated polymers show conformational and energetic disorder which give rise to wave function localization on conjugated segments of different lengths and results in formation of a broad density of states (DOS).8,9 This strongly disordered intramolecular conformation and intermolecular packing nature dominates electrical transport in these polymers.10 © 2015 American Chemical Society

Received: December 15, 2014 Revised: February 23, 2015 Published: March 3, 2015 5855

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backbone and is designed to prevent aggregation and crystallization of the PSBF. Conjugation is enhanced by means of spatial overlap of π orbitals between orthogonally positioned fluorene units, that is, spiroconjugation. This gives rise to enhanced charge carrier mobility, photochemical stabilization, and nonaggreagation.6,7 Recently, Monkman and co-workers have reported on the investigation of PSBF to understand the excited-state photophysics6,7 and how it affects the high-efficiency of devices.22 These reports demonstrated the complex excited-state behavior, emphasizing the formation of CT states by means of the spiroconjugation between the backbone and spiro groups in solid-state films. The CT states energetically lie slightly below the singlet-excited state, and lead to the conclusion that the observed long-lived fluorescence resulted from the back and forth energy exchange between these energetically close states at room temperature (RT) (∼14 meV). In solution, solvent−solute interactions shorten intermolecular distances and tend to favor rotational and translational relaxations.23 Hintschich et al. highlighted the important role of geometrical orientation of the side groups of PSBF to increase the orbital coupling and enhance spiroconjugation, which facilitates the relaxation of the initially excited-state and stabilizes the long-lived CT state.6 Our new systematic observations highlight the solvent polarity index and temperature dependent properties of these inter/intrachain interactions and emissive CT states, and their dominating role on total fluorescence with decreasing temperatures. Inter/intrachain energy migration in polymers, through various conformational relaxed states to low energy emissive

quenching by means of forming nonemissive states, such as interchain excitons and excimers.19 A detailed energy migration process due to the inter/intrachain interactions was reported by Chen et al.20 governed by formation of weak complexes between polymer chains. The relative efficiencies of inter/ intrachain interaction mechanisms was used by Nguyen et al. to explain the exciton diffusion process in an alkoxy-substituted polymer.21 In this report, we describe the effects that chain−chain interactions have in polyspirobifluorene (PSBF). Structurally, the PSBF consists of a polyfluorene backbone with additionally substituted fluorene unit with electron-donating alkoxy side chains by a spiro-linkage at the 9-position (Figure 1). This

Figure 1. Chemical structure of polyspirobifluorene.

spiro-linkage means that the fluorene group containing the four branched alkoxy groups is rigidly perpendicular to the

Figure 2. Temperature dependence of steady-state emission of PSBF (a) in air-saturated MCH solution, (b) in air-saturated 2-MeTHF solution, (c) in degassed MCH solution, and (d) in degassed 2-MeTHF solution. 5856

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Figure 3. Lifetime decays of PSBF (a) in degassed MCH solution and (b) in degassed 2-MeTHF solution.

MeTHF), and the results were compared in air-saturated and degassed solutions. Figure 2a and c shows the photoluminescence (PL) behavior in nonpolar air-saturated (Figure 2a) and degassed (Figure 2c) MCH solution as a function of temperature. The feature at 426 nm is assigned to a 1(π, π*) transition with the 0−0 transition having the highest intensity with vibronic structure at high temperatures. However, on cooling down to 145 K, the spectra gradually lose the structured shape as well as shift to the red by ca. 30 nm. Also, a triplet-state has a moderate contribution to the total PL, which can be seen from integrating the ratios of intensity values measured with and without oxygen: 1.44 (295 K O2 out/in) and 1.13 (145 K O2 out/in). More details about the intensity and the structural shapes of spectra are given in the Supporting Information (SI) for both air-saturated and degassed solutions. The steady-state PL emission behavior of PSBF in polar, airsaturated (Figure 2b) and degassed (Figure 2d) 2-MeTHF solutions are presented as a function of temperature. The feature at 424 nm is assigned to a 1(π, π*) transition with the 0−0 transition having the highest intensity with vibronic structure at high temperatures. However, on cooling down to 145 K, all the vibronic structure on the PL is lost, and a new emission band, with no vibronic structure, is formed. This new emission is strongly red-shifted to 500 nm and shows classic Gaussian line shape. The contribution of a triplet state in the total PL emission can again be seen integrating the ratio of intensity values measure with and without oxygen: 1.25 (295 K O2 out/in) and 1.49 (145 K O2 out/In). As a result of cooling we observed moderate separation of the two emitting species in the sample and a clear isoemissive point is observed, showing that the two emitting species are kinetically linked to each other. In the case of frozen solutions (∼90 K), the spectra dramatically shift (∼70 nm) to the blue and lose all contribution from the unstructured band seen at 145 K; in this case, phosphorescence emission can be observed in the steady-state emission spectra. More details about the intensity and the structural shapes of spectra are given in the SI for both air-saturated and degassed solutions. In chloroform (see the SI), similar behavior is seen as in 2MeTHF. Upon freezing of the solvent, the strongly red-shifted 500 nm feature is lost and the spectra return to the mixed 1(π, π*) and 450 nm band observed at room temperature.

states, will be greatly enhanced through chain coiling or collapsing of the single chains.24 Once the chain folding process happens, multiple funnels are formed along the polymer chains which play important roles in exciton migration to quenching sites.

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EXPERIMENTAL SECTION Absorption spectra were collected using a UV-3600 double beam spectrometer (Shimadzu), and fluorescence spectra were collected using Fluoromax and Fluorolog fluorescence spectrometers (Jobin Yvon). The solutions were degassed in a long necked quartz cuvvette using three freeze−thaw cycles and then mounted in a liquid nitrogen cryostat (Janis Research). Nanosecond gated luminescence and lifetime measurements (from 400 ps to 1 s) were made using a high energy pulsed Nd:YAG laser emitting at 355 nm (SL312, EKSPLA), and the pulse duration was approximately 150 ps. And the energy of per pulse was chosen around 100 μJ. Emission was focused onto a spectrograph and detected on a sensitive gated iCCD camera (Stanford Computer Optics) having sub-nanosecond resolution. Details on decay measurements by exponentially increasing gate and delay times can be found elsewhere.25 Also, a nitrogen laser (LTB Lasertechnik Berlin) was used as an excitation source of temperature dependent delayed fluorescence measurements, which has a 3 ns pulse width and 60−100 μJ per pulse energy and emits at 3.68 eV (337 nm). Picosecond time-resolved fluorescence decays were collected using the time-correlated single photon counting technique (impulse response function, IRF: 19.5 ps). A vertically polarized picosecond Ti:sapphire laser (Coherent) was used as an excitation source, the excitation wavelength was 367 nm, and the power of the laser was 54 MW. Emission was collected using a polarizer at its magic angle (54°), which is crucially important to remove polarization effects. For detection of the emission, a double monochromator (Acton Research Corporation) coupled to a microchannel plate photomultiplier tube (Hamamatsu R3809U-50) was used.

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RESULTS

Steady-State Measurements. The steady-state behavior of PSBF as a function of temperature was observed in two solvents with different polarity index, namely, in methyl cyclohexane (MCH) and 2-methyltetrahydrofuran (25857

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Figure 4. Laser fluency dependence of DF (a) in degassed MCH solution and (b) in degassed 2-MeTHF solution. The excitation wavelength of 337 nm has been used at 0.2 μs delayed and 1 μs integration times.

Time-Resolved Measurements. In order to clarify our observations in the steady-state measurements, nanosecond gated time-resolved spectroscopy25 was used to measure the decay dynamics in solutions. The key element of the measurement was that more than 12 orders of magnitude in intensity and more than 10 decades of time can be recorded in a single experiment, therefore, it is possible to capture both prompt (PF) and delayed fluorescence (DF) simultaneously (in one curve) as shown in Figure 3 using log−log scales. The initial fast part of the decay is assigned to PF, and the long slower decay component is assigned to DF. Figure 3a shows the lifetime decay of PSBF in degassed MCH solution at 295 and 145 K. The PF follows a biexponential decay with the lifetimes of 3 ± 1 ns and 11 ± 1 ns and at longer delay times a power law regime (IDF ∼ t−m) occurs with slopes of −1.29 at 295 K and −1.31 at 145 K. The delayed emission has the same spectrum as the PF and is ascribed to delayed emission from triplet− triplet annihilation. The power law behavior can be understood in terms of triplet diffusion mechanism, which consists of nonequilibrium and equilibrium dispersive regimes.26 In the nonequilibrium dispersive regime, at early times, “hot” triplet exciton migration is time dependent (decelerates) toward lowenergy sites. In this regime the DF intensity is proportional to the variation of the triplet population, d[T]/dt, and the DF decay follows a power law regime (IDF ∼ t−m) with m values approaching −1. Once the triplets have thermalized, the nondispersive migration dominates and the DF decays with m approaching −2 at certain time. The DF lifetimes were determined as 252 ± 7 ns at 295 K and 5.5 ± 0.2 μs at 145 K. The ratio between DF and PF intensities, DF/PF, was calculated as 0.0092 for 295 K and 0.0059 for 145 K. More details about the shape of spectra are given for both temperatures for various delay times in the SI. Figure 3b shows the lifetime decay of PSBF in degassed 2MeTHF solution at 295 and 145 K. The PF follows a biexponential decay with decay components of 3 ± 1 and 12 ± 1 ns, and at longer delay times a power law regime IDF ∼ t−m, decaying with slope −2.34 at 295 K. However, at 145 K, the initial power regime slope is −1.29 and turns to slope of −2.10 only at longer times. This is consistent with the triplet diffusion mechanism described above. The DF lifetimes were calculated as 284 ± 5 ns at 295 K and 6.2 ± 0.3 μs at 145 K. And the ratio of DF/PF intensities were calculated as 0.014 for 295 K and 0.058 for 145 K. More details about shape of the spectra of PF

and DF spectra are given for both temperatures for various delay times in the SI. The delayed fluorescence quantum yield (ΦDF) can be determined from directly from the DF/PF ratio. Addressing this point, the integration of the initial exponential decay give the PF value and the integration of the following power law parts give the DF contributions of the total sample emission, hence, the intensity ratio of DF to PF intensity is defined as in eq 2 Total fluorescence, that is, PF + DF, of the sample is given by eq 1 as Φf + Φf (ΦTΦDF) + Φf (ΦTΦDF)2 + ... +∞

= Φf

∑ (ΦTΦDF)m

(1)

m=0

From eq 1 the DF/PF ratio is determined: DF PF FinstNsΦf (ΦTΦDF + ΦT 2ΦDF 2 + ...) = FinstNsΦf ΦTΦDF = 1 − ΦTΦDF

R=

(2)

where Finst is an instrumental function, Ns is the number of excited states, Φf is fluorescence quantum yield, ΦT is triplet yield, and ΦDF is DF yield from triplet fusion (TF). Finst is the same for PF and DF since they were recorded during the same measurements, likewise the initial number of excited states after pulsed excitation. All quenching mechanisms including intersystem crossing (ISC), nonradiative decay rate, and quenching due to exciton migration are accounted for in Φf and are identical for both types of emission, once again as they come from the same CT state, so eq 2 is simplified and gives eq 3 below, from which ΦDF is determined. ΦDF =

R (1 + R )ΦT

(3)

In order to determine ΦDF, R is calculated from the data in Figure 3 at each temperature. The starting point for the DF calculation was taken as the inverse time of the intersystem crossing rate of PSBF.7 We assumed DF starts when 67% of triplets have formed, such that the concentrations were high 5858

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Figure 5. Temperature dependence of DF (a) in degassed MCH solution and (b) in degassed 2-MeTHF solution. The excitation wavelength of 337 nm has been used at 0.2 μs delayed and 1 μs integration times.

Figure 6. Log−log scaled fluorescence lifetime measurements using time-correlated single photon counting method (TCSPC) at four different wavelengths 425, 445, 460, and 500 nm (a) in air-saturated 2-MeTHF solution, (b) in degassed 2-MeTHF solution, (c) in air-saturated MCH solution, and (d) in degassed MCH solution.

enough to achieve TTA. The value of ΦT = 0.12 ± 0.02 was taken from the previous femtosecond ground state recovery measurements made by King et al.7 So using this information we arrive at various values of ΦDF in different solutions at distinct temperatures, such as 0.076 ± 0.002 at 295 K and 0.049 ± 0.003 at 145 K in MCH solution and 0.12 ± 0.01 at 295 K and 0.46 ± 0.02 at 145 K in 2-MeTHF solution. For, PSBF it is not possible to achieve ΦDF > 0.2 from TF alone. Clearly, in

PSBF, 2ET > ETn (ET = 2.22 eV, ETn = 3.77 eV)22 so that the maximum ΦDF can only be 0.2. As our measurements require that ΦDF >0.2, we conclude that there must be a further contribution to DF. The DF dependence on excitation intensity, measured in degassed solutions, is shown in Figure 4. Here a delay time of 0.2 μs and the integration time of 1 μs were chosen. The results for MCH solution in Figure 4a shows a slope of ∼1.5 at low 5859

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Table 1. Fluorescence Lifetime Decays in Two Different Polarity Solutions at Four Various Wavelength Positions Have Been Calculateda λx = 376 nm air-saturated solution material PSBF in MCH

in 2-MeTHF

a

degassed solution

425 nm

445 nm

460 nm

500 nm

425 nm

445 nm

460 nm

500 nm

τ1 = 1.53 ns τ2 = 0.37 ns τ3 = 25 ps χ2 = 1.05 τ1 = 3.80 ns τ2 = 2 ps τ3 = 9.3 ps χ2 = 1.06

τ1 = 1.88 ns τ2 = 0.38 ns τ3 = 39 ps χ2 = 1.04 τ1 = 3.89 ns τ2 = 7.1 ps τ3 = 11 ps χ2 = 1.06

τ1 = 1.96 ns τ2 = 0.43 ns τ3 = 43 ps χ2 = 1.08 τ1= 3.94 ns τ2 = 8.5 ps τ3 = 14.7 ps χ2 = 1.08

τ1 = 2.01 ns τ2 = 0.44 ns τ3 = 49 ps χ2 = 1.03 τ1 = 4.64 ns τ2 = 9.3 ps τ3 = 17.9 ps χ2 = 1. 05

τ1 = 1.74 ns τ2 = 0.36 ns τ3 = 24 ps χ2 = 1.05 τ1 = 4.87 ns τ2 = 1.1 ps τ3 = 9.6 ps χ2 = 1.06

τ1 = 2.15 ns τ2 = 0.42 ns τ3 = 44 ps χ2 = 1.03 τ1 = 5.24 ns τ2 = 5.9 ps τ3 = 12 ps χ2 = 1.06

τ1 = 2.27 ns τ2 = 0.49 ns τ3 = 48 ps χ2 = 1.05 τ1 = 5.32 ns τ2 = 6 ps τ3 = 20.1 ps χ2 = 1.07

τ1 = 2.37 ns τ2 = 0.51 ns τ3 = 56 ps χ2 =1.08 τ1 = 5.87 ns τ2 = 9.1 ps τ3 = 30.6 ps χ2 = 1.04

As an excitation wavelength of 376 nm was chosen, and the effect of oxygen on the fluorescence lifetime has been revealed.

laser dose (≤10 μJ), and slope ∼1.2 at high excitation doses (≤100 μJ). These dependencies indicate that monomolecular (long-lived CT emission) and bimolecular (TTA) processes are both contributing to the observed delayed fluorescence. Approaching the value of slope ∼1.5 at low intensity is indicative of nearly equal contributions of the TTA and CT emission on delayed fluorescence. In the case of high laser intensity conditions, we have previously shown that the TTA intensity dependence turns over slope 126 and this is what gives rise to the observed reduction in slope at high intensity, once again both monomolecular and bimolecular processes contribute to the total DF signal. Figure 4b shows the DF intensity dependence with excitation dose in 2-MeTHF. At low laser fluence conditions (≤10 μJ), in particular at 145 K, the TTA is the dominant process giving DF showing nearly perfect quadratic dependence with slope approaching the value of ∼2. The 3(π, π*) triplet state of PSBF with an energy of ET = 2.22 eV and lifetime of seconds at low temperature22 decays predominantly via TTA with little competition from nonradiative decay. At 145 K, the CT state is stabilized in the polar solvent27 and leads to increased triplet state 3(π, π*) generation.28 This higher triplet density and also higher triplet mobility favor TTA, whereas, at 295 K, the slope was found to be ∼1.68 showing relatively less contribution of TTA. Here the nonradiative decay is competing to depopulate the 3(π,π*) triplet state, and as there are fewer CT states on the polymer chain the triplet yield is also much lower. At high laser fluence conditions, the observed slopes ∼1.52 at 295 K and ∼1.3 at 145 K indicate more complex mixed behavior. At high intensities, the dependence once again turns over to linear behavior, indicative of high triplet density and good triplet mobility giving efficient TTA. We also studied the behavior of the DF as a function of temperature in the same solutions, the delay and integration times were chosen as 0.2 and 1 μs, respectively. In particular, in MCH, Figure 5a, the DF emission becomes more intense with decreasing temperatures and also red-shifts (ca.19 nm) from a mixed emission to pure 450 nm emission. The DF behavior in 2-MeTHF solution is seen (Figure 5b), the DF is dominated by the strongly red-shifted 500 nm Gaussian band and its intensity increases rapidly below 220 K. Figure 6 shows the fluorescence decays of PSBF at RT, and the solid lines are triexponential fits to the decays at four different emission wavelengths (425 nm, 445, 460, and 500 nm) using an excitation wavelength at 376 nm. In the polar 2MeTHF (Figure 6a,b), there is clear evidence that the long

decay component becomes more important at longer wavelengths, giving indication for the quenching of the 1(π, π*) state into a long-lived species. In nonpolar MCH, the fluorescence decays were relatively long-lived, comparing with the nonpolar environment (Figure 6c,d), which again is consistent with lower formation of the CT state, which quenches the 1(π, π*) singlet state. The decay components are determined by fitting the fluorescence decay with three discrete exponential functions with simultaneous deconvolution of the apparatus response function (see Table 1). The effect of oxygen is especially noticed in the polar solvent where the DF component at longer wavelengths is quenched.

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DISCUSSION Our observations revealed the complex excited state dynamics, and how this is affected by solvent polarity in a supposed “simple” luminescence polymer, PSBF in solution phases. In particular, the majority of initially excited exciton states do not decay directly to the ground state. Instead, they relax into longlived states with charge transfer character. The measurements were taken in dilute solutions, and so interchain interactions would be thought to be rather unlikely, especially with the spiro configuration of the alkoxy branched side chains. However, from time-resolved luminescence measurements, we see that initially photocreated 1(π, π*) state gives rise to new lower energy and longer lived states. This process is strongly affected by solvent polarity and temperature, and in 2-MeTHF occurs within a nanosecond and is more effective at low temperatures. The relaxed states show clear CT state character, especially in polar solvent (2-MeTHF), but even in MCH and CHL solutions the CT character of the relaxed state is clearly observed, as typified by the observation of unstructured Gaussian emission bands and solvent dependent energies. The red-shift CT emissions are further stabilized as the temperature is decreased. Comparing emission in MCH to 2MeTHF, we see in MCH the stabilization of a single CT feature at low temperature, whereas in the more polar solvent at RT mixed 1(π, π*) and CT emission is observed. And as the temperature is decreased, a new even further red-shift CT band forms and dominates emission at low temperatures. The striking formation of this CT state in 2-MeTHF as a function of temperature is accompanied by a clear isoemissive point observed in steady-state measurements with decreasing temperatures. The observation of the isoemissive point in polar environment indicates the presence of two different 5860

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Understanding that the natural intersystem crossing rate in PSBF is enhanced by the presence of CT excited states enables us to explain the oxygen dependencies observed in the emission. From laser fluence measurements, it is clear that the delayed fluorescence from PSBF is a mixture of long-lived geminate CT state recombination and triplet−triplet annihilation giving rise to triplet fusion. Even though we are working in dilute solution, through the enhanced intrachain triplet generation it is perfectly possible to have multiple triplet excitations on a single chain. The work of Burrows et al.30 on luminescence polymers with pulse radiolysis measurements clearly showed that the creation of more than 30 triplet excitations per chain is possible. Thus, intrachain TTA clearly occurs and contributes to the total luminescence of the sample. However, oxygen will quench these triplet states and removes the TF contribution from the total luminescence. The mixed contribution of long-lived CT emission and TF in the total delayed fluorescence at low temperature is substantiated by the large overall contribution of DF. We calculate the value of ΦDF in different solutions at distinct temperatures: 0.076 ± 0.002 at 295 K and 0.049 ± 0.003 at 145 K in MCH solution and 0.12 ± 0.01 at 295 K and 0.46 ± 0.02 at 145 K in 2-MeTHF solution. In 2-MeTHF, the stabilization of the charge transfer state is greatly increased by the polar solvent and so at room temperature the DF contribution is much higher than in the MCH case. Moreover, as the temperature is decreased, the chain collapse increases more rapidly in the poor solvent (2-MeTHF), causing “inter/ intrachain” CT states to form, which leads to higher triplet production and enhanced triplet migration such that they find each other more easily and annihilate, resulting in enhanced DF from TTA. Whereas in MCH, the CT state is not stabilized so strongly and chain collapse does not occur, and hence, triplets can only encounter each other when there is more than one triplet per chain. As the temperature is reduced, the triplet mobility decreases and the rate of TTA also decreases. The DF/PF ratios are thus controlled by both the temperature dependence of the triplet population and the triplet annihilation rate. So, we observe greatly enhanced values of ΦDF at low temperatures where the CT states are strongly stabilized. As we know that for PSBF the maximum DF yield cannot surpass 20% as in this polymer ETN < 2ET1 (ET = 2.22 eV, ETN = 3.77 eV) and given the oxygen effect, DF must include TTA and long-lived CT state contributions together. It is therefore clear why all laser fluence dependence of DF measurements showed mixed contributions of monomolecular and bimolecular. Also at low temperatures in 2-MeTHF, the chains have coiled and collapse which will increase the probability of TTA events and contribute to enhance TTA and give the DF intensity dependent behavior that is observed. In addition, the photoinduced absorption features of PSBF have shown previously;7 that two excited state absorption bands are observed in PSBF: PA1 is due to the singlet excited state absorption, and PA2 is assigned to the formation of CT states. The large lifetime decay differences observed between PA1 (rapid process) and the ground state recovery (slow process) showed the dominating role of the PA2 band in the recovery of the ground state. Again, previous observations are in agreement that the CT state is the dominant state in total fluorescence observed at low temperature. Moreover, laser fluence dependence measurements confirmed that both PA1 and PA2 absorption bands show a linear dependence upon the excitation

emissive species kinetically linked to give the total sample fluorescence. Referring first to the emission in MCH, Hintschich et al.6 reported previously that in nonpolar solvents the energetic relaxation of the 1(π, π*) to CT state is accompanied by the conformational changes of the polymer chain. This explains the important role of the solvent viscosity and the temperature on the observed spectra. TD-DFT calculations also indicate that charge transfer occurs from the polymer backbone into the spiro side groups and form a self-trapped excitonic state having charge transfer character. This intrachain species give rise to emission at 460 nm, as we see in MCH and 2-MeTHF as well. Moreover, it is the emission of this intrachain CT exciton that dominates the delayed emission in this polymer, which is indicative of the far longer lifetime of this self-trapped CT exciton species. As discussed by previously,6 the mechanism for the formation of this new CT species is driven by spiroconjugation between backbone and spiro-side groups. As temperature decreases the quality of the solvent decreases and this effects the geometry of the polymer chain and also the side chains, such that formation of the intrachain CT state is stabilized. Therefore, even in MCH at low temperatures the emission from this CT state dominates, and very little 1(π, π*) emission is observed. In the far more polar 2-MeTHF, we observe a stronger stabilization of this intrachain CT species, such that at room temperature the emission spectra is a mixture of 1(π,π*) and CT exciton. This is strongly reflected in the time-resolved emission decay as well where very rapid quenching of the 1(π, π*) to the CT exciton is observed. As the temperature is decreased, we observe the formation of a wholly new emission band, strongly red-shifted to 500 nm and with classic Gaussian line shape. The observation of an isoemissive point confirms this to be a new emitting species that is kinetically linked with the intrachain states that are quenched to form this new band. However, once the solvent is frozen, we observe the complete loss of this 500 nm feature and a return to pure intrachain CT exciton emission. This behavior is also observed in CHL (see the SI). This clearly indicates that chain motion is required to stabilize the formation of the state emitting at 500 nm, and implies that this state has interchain character and can be understood to form as the dilute chains coil or collapse as the 2MeTHF becomes a progressively worse solvent as the temperature decreases. Clearly, this feature is strongly stabilized by the solvent polarity, and requires the interaction between excited CT state and, more probably a neighboring ground state molecule, (the probability that in dilute solution that two excited interact is too small to be considered). This situation is analogous to the excimer or exciplex formation. Returning to the intrachain CT exciton state. As we previously demonstrated, such intrachain CT states lead to increased formation of triplet states.28 This can clearly be seen in our new measurements as well. For the case of frozen 2MeTHF, the spectra have lost all contribution from intra/ interchain species and emission arises completely from the intrachain CT exciton. In this case, phosphorescence at 567 nm can be observed in the steady-state emission spectra. This matches perfectly with the phosphorescence we have previously reported using gated detection measurements,29 and strongly reinforces our premise that the relaxed intrachain species have charge transfer character and lead to enhanced 3(π, π*) triplet exciton formation. 5861

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density, ruling out bimolecular processes giving rise to the decay of these states, that is, singlet−singlet annihilation.

CONCLUSION Measurements of PSBF in two different polarity solvents as a function of temperature revealed the complex excited state behavior of PSBF. It has been shown that inter/intrachain interactions play a crucial role in forming the additional CT states and they greatly make contribution into the total PL spectra. The results demonstrate that environmental variations, such as quality of solvent, solvent polarity, and temperature, have great effects on spatial conformations of the polymer structure and excited state stabilization. The strength of spiroconjugation is very sensitive to orbital coupling and spatial conformations of backbone and side chains. Once the relative conformations are changed by polarity and the temperature, two emissive species are resolved, a fast decaying 1(π, π*) state and long-lived intrachain CT state. As temperature is decreased, we observe the formation of a wholly new emission band, strongly red-shifted to 500 nm (at 145 K) with classic Gaussian line shape in 2-MeTHF, which occurs as a result of chain collapse giving an “inter/intrachain” interaction, a result of the polymer chain folding on itself due to decreasing the quality of the solvent. The emission has CT character, and is only observed once the polar solvent is able to stabilize it. However, once the solvent is frozen we observed the complete loss of inter/intrachain emission and a return to pure intrachain CT exciton emission. This clearly indicates that chain motion is required to stabilize the formation of the inter/intrachain state and the observed structureless PL spectra can be attributed to an increase of vibronic coupling due to changes in temperature and polarity of solvent. Our work sheds light on understanding the complex excitedstate nature of PSBF, and the mixed contribution of long-lived CT emission and TF into the total delayed fluorescence is presented by calculating of ΦDF values in 2-MeTHF as 0.46 ± 0.02 (145 K) and 0.12 ± 0.01 (295 K). Thus, we observe greatly enhanced values of ΦDF at low temperatures where the CT states are strongly stabilized. As we know that for PSBF the maximum DF yield from TTA alone cannot surpass 20%,22 however, the calculated ΦDF value (0.46 ± 0.02) shows that the DF must include contributions from bimolecular (TTA) and monomolecular (CT) processes which has been confirmed by laser fluence dependence measurements. ASSOCIATED CONTENT

S Supporting Information *

Additional details about the steady-state and time-resolved spectroscopy of polyspirobifluorene. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 We are grateful to Ministry of National Education of Turkey for supplying Ph.D. scholarship. 5862

DOI: 10.1021/jp512467g J. Phys. Chem. C 2015, 119, 5855−5863

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DOI: 10.1021/jp512467g J. Phys. Chem. C 2015, 119, 5855−5863