Effect of Conjugated Backbone Protection on Intrinsic and Light

Article pubs.acs.org/cm

Effect of Conjugated Backbone Protection on Intrinsic and LightInduced Fluorescence Quenching in Polythiophenes Dibakar Sahoo,† Kazunori Sugiyasu,‡ Yuxi Tian,† Masayuki Takeuchi,‡ and Ivan G. Scheblykin*,† †

Chemical Physics, Lund University, P.O. Box 124, 22100 Lund, Sweden Macromolecules Group, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

‡

S Supporting Information *

ABSTRACT: Polythiophenes (PTs), particularly regioregular poly(3hexylthiophene-2,5-diyl) (rr-P3HT), are important materials in photovoltaics. The photophysical properties of PTs are still poorly understood, because of their aggregation tendency and formation of interchain species which can be avoided by insulating the conjugated backbone via selfthreading. We investigated two polymers, rr-P3HT and its insulated analog, imbedded in PMMA at low concentrations. The exciton decay dynamics and fluorescence quantum yield were analyzed as a function of excitation power densities over the range from 1 × 10−4 to 100 W/cm2. For both polymers, substantial (up to 5 times) photoinduced fluorescence quenching was observed owing to singlet−triplet annihilation and quenching by other long-living charged photoproducts. We found that chain insulation eliminates static (or ultrafast) fluorescence quenching, but has no effect on slow dynamic quenching at time scales longer than 10 ps. We propose that static quenching is solely due to chain aggregation, whereas the dynamic quenching is a consequence of intrachain processes.

1. INTRODUCTION Conjugated polymers (CPs) are charge-conducting, lightabsorbing, and light-emitting organic semiconductors. CPs can bear neutral (excitons) and charged (polarons) excited states localized on a portion of the chain that can migrate over the chain or between the neighboring chains. CPs are primary materials for organic electronics and are used as active materials in light-emitting diodes, field-effect transistors, and solar cells.1−3 Because of their luminescence, CPs and their oligomers are often used in biological applications as imaging labels with great environmental sensitivity. 4 In these applications, interpolymer interactions and the internal organization of CP chains at the molecular and supramolecular levels determine the functions and properties.5 Therefore, the diversity and complexity of hierarchical supramolecular assemblies, characteristic of organic materials, are advantages in material design, because one can expect unprecedented “collective” functions through controlling aggregation morphologies. However, this complexity, in turn, gives rise to ambiguity in many important fundamental issues related to CPs. For example, photophysical processes such as light absorption and fluorescing states, mechanisms of exciton quenching, energy and charge transfer, and the role of the polymer conformation in optical and electronic properties remains controversial.5 Revealing the excited state deactivation mechanisms is very important to the fundamental understanding of CPs and their applications. Exciton quenching can be dynamic (fluorescence © 2014 American Chemical Society

lifetime is affected) or static (fluorescence lifetime does not change upon quenching). Ultrafast dynamic quenching, which is much faster than the time resolution of the experimental setup, also appears as static quenching. Although much literature has been devoted to exciton quenching, little attention has focused on static quenching, and its nature is not yet understood.6−8 One of the proposed mechanisms is direct photogeneration of polaron pairs instead of neutral excitons.8 Recently, single molecule spectroscopy studies revealed that static quenching is not homogeneous, meaning that a portion of a single chain can be quenched more than others.9 Diverse deactivation processes responsible for the quenching of the singlet excited S1 state can be divided into excitation power independent and depended (bimolecular) processes. At low excitation intensity limits, the averaged intensity and peak intensity (in the case of pulsed excitations) are very low, and no accumulation of photoproducts occurs. Excitation intensity-independent quenching includes S1 → S0 + heat S1 → T1

internal conversion to the ground state

intersystem crossing (ICS)

S1 → S0 + P+ + P−

exciton dissociation to a charge pair

Received: June 17, 2014 Revised: July 30, 2014 Published: July 30, 2014 4867

dx.doi.org/10.1021/cm5021959 | Chem. Mater. 2014, 26, 4867−4875

Chemistry of Materials S1 + Q → S0 + Q*

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intermolecular interactions that can lead to formation of interchain species such as interchain electronic states. Insulation greatly increases the fluorescence of such polymers7,36,37 and improves the performance of organic electronics devices.38,39 Recently, the insulation strategy was applied to PTs.40 The covalently linked cyclic side chains not only insulated the backbone, but also enhanced the effective conjugation length by making the polymer more planar, resulting in an excellent intrinsic hole mobility.40 We thus envisioned that this unique structure would enable us to examine the complex photophysical processes of PTs. In the present contribution, we will shed light on the dynamic and static/ultrafast exciton quenching processes in PTs by comparing the optical properties of “insulated” and “naked” PTs dispersed in a PMMA matrix at different conditions. For the first time, exciton decay dynamics in these materials will be carefully analyzed as a function of excitation power over the range from 1 × 10−4 to 100 W/cm2. We will attempt to elucidate the effect of the insulation on dynamic and static exciton quenching and their dependence on excitation power. In addition to understanding fundamental excited state processes in PTs, our results will also have direct applications in single molecule spectroscopy and organic electronics.

energy transfer to a permanent

quencher (e.g., chemical or structural defect)12

At high power, bimolecular process starts to play a role. Accumulation of photoproducts such as singlet and triplet excitons, polarons, photogenerated defects, and charge transfer states10−12 can occur leading to the following excitation intensity-dependent (bimolecular) quenching exciton annihilation processes5,13 S1 + S1 → S0 + S* singlet−singlet annihilation (S−SA) S1 + T1 → S0 + T* singlet−triplet annihilation (S−TA)

S1 + PhotoP → S0 + PhotoP* singlet−photo−product annihilation (S−PhotoPA)

Here by photoproducts we consider long-living (with lifetimes larger than microseconds) states other than triplets. Such states are the main reason for fluorescence blinking of single conjugated polymer molecules and aggregates at time scale of seconds as has been shown by experiments with electric field.14−16 S-TA appears at the single molecule level as fast blinking at microsecond time scale,17,18 whereas S-SA reveals as fluorescence photon antibunching.19 Notably, excitation conditions (CW or pulsed light) determine which of the three annihilation processes is dominant.20 S-SA is dominant when the peak excitation intensity is high, whereas the average excitation intensity is low. S-TA and S-PhotoPA require high average excitation intensity, easily reached under CW excitation, to accumulate a sufficient concentration of long-living photoproducts.17 Exciton annihilation is more pronounced in pristine films as compared to solutions, due to the three-dimensional (3D) characteristics of exciton migration in the films versus quasi one-dimensional (1D) intrachain exciton migration in solution. Thus, in order to reach annihilation conditions in an isolated polymer chain, an exciton has to find another exciton in the same chain, which requires much higher excitation power than that for densely packed 3D pristine polymer films. Polythiophenes (PTs) such as poly(3-hexylthiophene-2,5diyl) (P3HT) are very important materials for solar cell applications,21,22 yet suffer from the same aforementioned complexity issues. In the pristine films and blends (i.e., the common form in devices), interpolymer interactions and structural disorder drastically affect the optical and electronic properties.23−26 For example, there is an order of magnitude difference in the fluorescence quantum yield (QY) and large difference in optical spectra between solution and pristine films.27 Low QYs are usually attributed to ultrafast or direct formation of polaron states28−31 or the presence of a lowenergy charge-transfer state32 related to H-aggregate geometry.26 Understanding the details of exciton deactivation mechanisms in PTs is necessary to develop highly efficient, organic electronic devices. Within the past decade, a new class of conjugated polymers with “insulated” conjugated backbones has emerged, namely insulated molecular wires.33−35 The insulation reduces

2. MATERIALS AND METHODS 2.1. Samples. Regioregular poly(3-hexylthiophene-2,5-diyl) (rrP3HT) (Figure 1) and N,N′-bis(2,6-dimethylphenyl) perylene3,4:9,10-tetracarboxylic diimide (DXP) were purchased from SigmaAldrich. Insulated PT (Figure 1) was synthesized according to the published procedure40 and possessed the following parameters: Mn = 15.7 kDa relative to polystyrene, polydispersity index (PDI)=1.4, length ca. 32 thiophene units, diameter = 1.2 nm. The same backbone polymer rr-P3HT (Aldrich) possessed Mn = 39kD with PDI = 2.6,

Figure 1. Absorption and fluorescence spectra of (a) insulated PT (excitation at 514 nm) and (b) “naked” P3HT (excitation at 458 nm) in solutions (dashed lines) and in solid PMMA films (solid lines). Chemical structures of the polymers are also shown. Insulated PT in PMMA (a) was spin-cast from solution with ODsol = 0.1. No indications of aggregation were observed. The band around 600 nm (b) in absorption of “naked” P3HT in toluene solution with addition of PMMA indicates aggregation (OD of this solution was 0.3 in the maximum). Aggregation in solid PMMA films is observed as concentration dependence of the fluorescence spectra when the films were prepared from solutions of different optical densities ODsol= 0.001, 0.01, and 0.1 (from thin to thick line, respectively). 4868


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Where Nall is the number of emitted photons and ηdet is the detection efficiency of the setup. The detection efficiency is the product of the fluorescence photon collection efficiency for the particular sample geometry and objective lens, transmission of the optics, and quantum efficiency of the CCD. The excitation power density P(x,y) (W/cm2) can be not uniform over the sample usually having a Gaussian shape. However, we assume that the sample (a thin film) is uniform at the scale of the P(x,y) variations. Because Φ can be dependent on the excitation power density, it is also a function of coordinates in the sample. For further convenience we use absorption cross section density of the film expressed as

corresponding to 235 thiophene units on average. Samples were prepared by spin-casting at 3000 rpm of the solutions in toluene (QY measurements) or chloroform (relative QY measurements and fluorescence kinetics) containing 9 mg mL−1 poly(methyl methacrylate) as the matrix polymer (PMMA, Aldrich, Mw ≈ 996 000, PMMA was added in the last dilution stage) on cleaned glass substrates to yield a uniform film with a thickness of ∼40 nm (measured by a profilometer). We chose PMMA because it is a standard matrix for immobilization and isolation of molecules in single-molecule spectroscopy measurements. The concentration can be followed by the corresponding optical densities of the solutions (ODsol) indicated in the text. The P3HT/PMMA weight/weight ratio is equal to 0.0023ODsol at our experimental conditions. For the fluorescence decay measurements we used concentrations about 1000 times higher (about 1 × 10−4 w/w conjugated polymer/PMMA ratio) than those typically utilized in single-molecule experiments. 2.2. Optical Setup. A home-built, epi-fluorescence microscope based on an Olympus X71 with an Ar-ion laser as an excitation source was used.41 The fluorescence of the sample was collected by an objective lens (Olympus LUCPlanFl 40 × , NA 0.6) and imaged by EMG CCD Camera ProEM-512, Princeton Instrument (see the Supporting Information). The polymers and DXP dye were excited by linearly polarized light from the Ar-ion laser at 458 and 514 nm, respectively. The excited area of the sample had a diameter of ca. 30 μm. The exposure time of the camera varied depending on the excitation power. In BOD measurements (see below) the background signal measured from a glass slide containing pure PMMA film was subtracted. All experiments were carried out under a nitrogen atmosphere at room temperature. The fluorescence lifetime was measured on the same microscope using a 485 nm pulsed diode laser (PicoQuant GmbH), a timecorrelated single-photon counting unit (PicoHarp 300, PicoQuant GmbH), and an avalanche photodiode (Micro Photo Devices). The time-correlated single photon counting regime system had an instrument response function (IRF) of about 50 ps, measured using the Raman scattering of the excitation laser from water. Additional details are given in the Supporting Information. The time-tagged time-resolved (TTTR) mode was used to detect the absolute arrival times of all photons together with synchronization pulses from the laser in order to obtain the fluorescence decay kinetics and absolute fluorescence intensity. 2.3. Brightness per Optical Density (BOD) to Measure Fluorescence QY of the Thin Films. Fluorescence QY is one of the most important benchmarks of a fluorophore and characterizes its ability to emit light via photoexcitation

Φ=

number of emitted photons number of absorbed photons

β=

(3)

Where σ is the absorption cross-section (cm ) of molecules, N is number of molecules in the sample, S is the sample area (cm2), c is the concentration of the molecules in the film (cm−3), h is the thickness of the film, and ODfilm is the optical density of the film. Because ODfilm is too small to be measured, another parameter to control the sample concentration is needed. For this purpose, we chose the optical density of the solution in the 1 cm thick cell (ODsol) from which the sample was spin-cast. Because ODfilm is proportional to ODsol 2

β = ODfilm ln 10 = ODsol K

(4)

Where coefficient K is dependent on the concentration of the matrix polymer in solution and the matrix film thickness. The number of photons with energy hν emitted during the exposure time τ is then:

Nall =

⎛ τ ⎞ ⎜ ⎟β ⎝ hν ⎠

∫

P(x , y)Φ(P(x , y))dx dy

(5)

Assuming weak dependence of Φ on P(x,y), the equation can be simplified

Nall ≈

⎛ τ ⎞ ⎜ ⎟β Φ(⟨P ⟩) ⎝ hν ⎠

⎛

⎞

∫ P(x , y)dxdy = ⎝ hτν ⎠β Φ(⟨P⟩)I ⎜

⎟

(6)

Where I is the total excitation power (W), and ⟨P⟩ is the averaged excitation power density. Using eqs 2, 4, and 6, we can express the observed emission intensity Ndet [photons] detected by the CCD camera as

⎛ τ ⎞ Ndet ≈ Kηdet⎜ ⎟ODsol Φ(⟨P⟩)I ⎝ hν ⎠

(7)

BOD is the number of detected photons normalized to the sample concentration in solution (ODsol), the excitation power density (P) and exposure time (τ)

(1) BOD =

Here we present the theoretical background for fluorescence QY measurement technique proposed by us. The method is specially designed for measuring the fluorescence QY of thin matrix films doped with a low concentration of fluorophores using fluorescence microscopy. Such films cannot be measured by absolute methods like integrating spheres because of their negligible optical density. A fluorescence microscope is an excellent platform for relative and absolute measurements of fluorescence QY. Visual observation via a microscope allows for precise control of the excitation beam and the sample and reference sample positions, and provides good repeatability of the measurements. A detailed account of the technique will be published elsewhere. We introduce here a parameter termed brightness per optical density (BOD).42 BOD can be seen as fluorescence brightness, which is used to measure the fluorescence QY of individual molecules,6,7,43 but measured for an ensemble. The number of photons detected by the camera (Ndet) from the entire excitation spot is

Ndet = Nallηdet

σN σcV = = σch = ODfilm ln 10 S S

Ndet 1 ≈ Kη Φ(⟨P⟩) IτODsol hν det

(8)

−1 −1

Therefore, BOD (W s ) is directly proportional to the fluorescence QY of the sample. The proportional constant reflects the sample preparation (K) and the setup (ηdet is fluorescence photon detection efficiency) properties. Because BOD is proportional to fluorescence QY, we can use BOD of the DXP dye with a known QY (ΦDXP = 0.93)44 to calculate QY of PT under different conditions. However, BOD is sensitive to the orientation distribution of chromophores because ηdet of the setup is dependent on this distribution. In general, we can write Φsample =

BODsample ξ BODreference

Φreference

(9)

Where ξ accounts for different distributions of the dipole moment orientations of the sample and the reference. We assume here that small molecules of DXP possess an orientation distribution that is nearly random. CP chains are elongated and probably oriented in the sample plane, because of the shear force during spin coating and the small thickness (40 nm) of the PMMA film.45 For such situation, a

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simple estimation gives ξ = 1.5. See the Supporting Information for more details. 2.4. Accuracy of Fluorescence Quantum Yield Measurements Using BOD Methodology. Any QY measurement technique suffers from substantial errors. The BOD method as we used it here gives relative QY. We obtain absolute QY by measuring a standard dye DXP of known QY at the same conditions as the samples of the polymers. Using a microscope for excitation and fluorescence detection makes uncertainties due to fluctuations of the laser beam or the sample position negligible. High collection efficiency of the objective lens and CCD detector with 95% quantum efficiency allows measuring QY of samples containing just several hundreds of fluorophores in the excited volume. The main source of random errors is variation of the PMMA film thickness that leads to proportional variation of the luminescent material amount in the excited spot. When concentration gets too low, fluorescent background starts also to be an important source of errors. Overview of the random errors can be seen in Figure 2, where several

When absolute QY is calculated, the main source of systematic error up to 40% is the factor ξ, see eq 9. It reflects difference in orientation distribution of small reference dye molecules and long CP chains. Since it is a systematic error, it influences absolute values of QY only. In other words, it does not influence the ratio Φnaked PT/Φinsulated PT. The error in the estimation of the factor ξ can just shift values of Φ and Φsk0 (see Table 1) up and down, but it cannot change the ratio of these values calculated for different polymers. Therefore, such uncertainty does not influence the conclusions of the paper because most of them are based on relative values.

3. RESULTS AND DISCUSSION 3.1. Absolute Fluorescence Yield of the Polymers, P3HT Aggregation in PMMA Matrix. BOD for the insulated and naked PTs (ODsol ≈ 0.1) imbedded in the PMMA layer and BOD for the reference dye DXP in PMMA were measured under low excitation power (see Table 1). Fluorescence QY was calculated using eq 9. More than 10 times higher QY of the insulated polymer (Φ = 0.23) in comparison with that of the naked polymer (Φ = 0.019) was observed. To understand the origin of such strong fluorescence quenching, we measured the concentration dependence of BOD for both polymers dispersed in PMMA (Figure 2). The fluorescence QY of “naked” P3HT increased nearly 8 times upon decreasing its concentration in PMMA, although it did not change for insulated PT. Therefore, the quenching must be due to the aggregation of P3HT in PMMA matrix at higher concentrations. The amplitude of the QY drop due to aggregation is in agreement with the literature data.31 The BOD concentration dependence shows that only at ODsol < 1 × 10−3 (ca. 10−6 P3HT/PMMA w/w), chains of ”naked” P3HT become isolated in the PMMA matrix and their QY reaches the value close to that for the insulated counterpart. Thus, backbone insulation prevents the molecules from aggregation, or at least prohibits electronic interactions between the conjugated backbones of the two chains. Fluorescence spectra of “naked” P3HT in PMMA also showed concentration dependence in the studied concentration range (Figure 1). The fluorescence spectra were red-shifted to 675 nm, a typical indication of P3HT aggregation. 26,31,46 At the lowest concentration, however, the 0−0 fluorescence peak was at 630 nm (Figure 1b) in agreement with spectra of single P3HT chains of the same molecular weight imbedded in PMMA.18 We also checked the absorption spectra of P3HT in toluene with 1% PMMA (the composition that was used for spincasting). When ODsol was larger than 0.1 a clear P3HT aggregates absorption band was observed at 600 nm (Figure 2b), whereas no indication of aggregation was observed in pure

Figure 2. BOD and fluorescence quantum yield of the polymers as functions of their concentration in the PMMA films given in the units of the optical density of the corresponding solutions used for spincasting and P3HT/PMMA w/w ratio. “Naked” P3HT was excited at 458 nm with power density of 3.8 W/cm2, insulated PT was excited at 514 nm with power density of 2.4 W/cm2. Note that because of S-T annihilation, QY of the polymers appeared smaller than that measured at low excitation power limit (compare with Figure 3). The error bars for QY are shown. Lines are guides to the eye.

measurements were done for each concentration and error bars estimated. In practice we had error of relative QY better than 15% if the concentration was not too low.

Table 1. Photophysical Parameters of the Insulated PT and “Naked” P3HT Polymers and the Reference Dye DXP at Different Conditionsa matrix/solution

in PMMA

fluorophore BOD of doped PMMA film Φ ⟨τ⟩amp = ∫ f(t)dt [ns] Φsk0= Φ/⟨τ⟩ampc (ns−1)

b

insulated PT ODsol ≈ 0.1 5.3 × 1013 (± 15%) 0.23c (± 20%) 0.085 2.5 (± 20%)

”naked” P3HT (aggregated) ODsol ≈ 0.1 4.4 × 1012 (± 15%) 0.02−0.04c,d 0.076 0.25−0.5d

in toluene solution DXP 1.45 × 1014 (± 15%) 0.9344

insulated PT

”naked” P3HT

DXP

0.6140 0.26 2.3

0.3331 0.2 1.65

0.9344

The error of Φ and Φsk0 is indicated without considering the uncertainty in orientation factor ξ. It can bring a systematic error (a factor) which, however, is the same for both polymers. Therefore, the ratio of the parameters for the insulated PT and “naked” P3HT is unaffected by the uncertainty in ξ. bMeasured using 40× NA = 0.6 dry objective lens at excitation power density 7.2 × 10−4 W/cm2. cΦ is calculated with eq 9 with ξ = 1.5. dThe range comes from possible difference of the absorption spectrum of P3HT in PMMA films in comparison to that in solution due to aggregation, see the text for details. a

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generation of polaron pairs and ultrafast relaxation of the exciton to a change transfer (CT) state.28−32,49 Since generation of such states is an interchain process, it can be eliminated by insulation of the PT backbone, as we observed experimentally. This hypothesis will be further supported below by the analysis of the fluorescence decay kinetics. 3.2. Fluorescence QY of Polymers As a Function of Excitation Power Density. BOD and fluorescence QY for both polymers dispersed in PMMA films at different concentrations (indicated by the optical density ODsol) are plotted in Figure 3 as a function of excitation power density. For both polymers, the fluorescence QY remained constant up to a certain excitation power, and then decreased because of photoinduced fluorescence quenching. Note that the decrease in the QY was not due to photobleaching, as this process was fully reversible at our experimental conditions; see Figure S2 in the Supporting Information. Excitation power dependence of the fluorescence QY normalized to the low power limit is presented in Figure 4.

toluene of the same or even higher concentrations. This shows that PMMA induces P3HT aggregation. It is especially pronounced in solid PMMA films when P3HT molecules experience decrease of the solvent quality due to evaporation of toluene. Unfortunately, we were not able to measure absorption spectra of the resulted films because their optical density was too low. Aggregation shifts absorption maximum to lower energy and decreases the absorption cross section at 458 nm up to 2 times in the case of fully aggregated polymer.47 Because OD at this wavelength was used to calculate BOD and QY, this can lead to up to 2 times underestimation of QY of aggregated P3HT in PMMA as shown by error bars and other means in Figures 2 and 3 and Table 1.

Figure 3. BOD of “naked” P3HT, insulated PT, and reference dye DXP dispersed in PMMA matrices as functions of excitation power density. The fluorescence quantum yield scale (right axis) is applied to the polymers only because of the difference in the orientation distributions between polymers and the DXP dye. The vertical arrow shows where the data for DXP should be if the orientation distributions of DXP molecules and PTs were the same, see the text for details. The excitation wavelength and the corresponding optical densities of the solutions the samples were spin-cast from are indicated in the legend. To calculate BOD, we used OD at the excitation wavelength. The solid line shows the upper limit for QY of aggregated P3HT (see text for details).

Figure 4. Normalized fluorescence quantum yield of “naked” P3HT, insulated PT and DXP dispersed in PMMA matrices as a function of excitation power density. The excitation wavelength and the corresponding OD values for each solution the samples were spincast from are indicated. Lines are guides for the eye.

For different polymers QY start to decrease at different excitation power density (P). There was also a clear difference in the slope of QY(P) for insulated and ”naked” PTs. The polymer that was most susceptible to the photoinduced quenching was aggregated “naked” P3HT, while insulated PT showed the least quenching. As discussed in the introduction, photogenerated triplet and other long-living photoproducts can quench singlet excitons by different annihilation processes5 when accumulated at sufficient concentrations. Because the triplet and charge transfer state concentration are proportional to the excitation power density, the QY should decrease with increasing of P, as observed experimentally. Since the ISC yield in PTs can be as large as 75%,50,51 the dominant reason for QY excitation intensity dependence must be the S-TA.17,18,20 Note that even the highest excitation power used was not enough to induce S-SA20 (see the Supporting Information). Moreover, long living photo products like CT or polaron states can form. Thus, intensity-dependent quenching must partially reflect also S-PhotoPA. Isolated chains of “naked” P3HT showed stronger excitation power dependence than insulated PT. One obvious reason

According to the literature, long chains of P3HT dispersed in PMMA are prone to self-aggregation, or in other words, they possessed collapsed conformation.18,32 Probably the same happens in our case with long “naked” P3HT chains at low concentration. However, in spite of self-aggregation we did not observe substantial fluorescence quenching in these isolated chains. Indeed QY of the short insulated PT and long isolated “naked” P3HT in PMMA are very close at the low excitation power limit (Figure 3). It means that it is not enough just to collapse the chain for fluorescence quenching. To decrease QY of a chain by an order of magnitude the intersegment interaction should be highly specific (probably an ordered structure with close contacts between parallel segments like in P3HT films), which is difficult to reach for a single chain because its conformation obeys principle of self-avoidance.48 Low QY of P3HT aggregates and pristine films is usually related to formation of H-aggregates−highly ordered structures with chains arranged in a parallel fashion.26 However, the particular mechanism of the quenching is unknown. The mechanisms discussed in literature include direct (or ultrafast) 4871



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3.3. Static vs Dynamic Quenching. Fluorescence decay kinetics were measured for similar concentrations (ODsol = 0.1) of “naked” and insulated PTs. Under such conditions, because of the aforementioned aggregation effect, the QY of the “naked” P3HT was about 10 times smaller than that of the insulated PT (Table 1). However, the fluorescence decays of both polymers were very similar (see Figure S3 in the Supporting Information). This is an illustrative example of the incompatibility of the fluorescence lifetime and QY due to the presence of static quenching that has been referred to as the formation of “dark matter” in single-molecule CP studies.6,7 We can write the following general equation of the fluorescence quantum yield Φ

must be the difference in the chain length. P3HT chains were in average 7 times longer than those of the insulated PT. If the ST annihilation radius was larger than the insulated polymer chain length, more efficient quenching should be observed for the given excitation power for the longer polymer. The same mechanism can be even more important for aggregated P3HT, because quenching in a 3D aggregate is more efficient than in the quasi 1D case of a single chain. In this regard it is interesting to compare our study with a recent paper by Steiner et al.18 where relative fluorescence QY of individual P3HT molecules imbedded in PMMA was measured as a function of the chain length while the excitation power was constant and equal to 50 W/cm2. Figure 4 shows that at such excitation power ensemble averaged QY of P3HT of 39 kDa is about 2 times lower due to annihilation processes, which is in good quantitative agreement with the single molecule study by Steiner et al. In the cited paper, the dependence of the S-TA was satisfactory explained by assuming that one triplet state is enough to substantially quench chains as long as several hundreds of thiophene units possessing self-collapsed conformation. “Naked” P3HT studied by us had similar length. If one decreases the length of the chain by 7 times keeping the conditions for energy transfer the same as for the long chain, 7 times larger excitation power is required for the same decrease of QY. So, one would expect to see QY(P) for insulated PT shifted toward higher P by a factor of 7 in comparison with the same curve for “naked” P3HT. Experimentally observed shift is by a factor of 5, which agrees with the prediction taking into account quite large distribution of the molecular weight of the polymers. However, isolated “naked” P3HT and insulated PT must have different conformation. P3HT in PMMA looks as a random coil with segmented conjugation,18,32 whereas the insulated short PT has a rodlike shape with planar conjugation. This can potentially influence QY(P) via differences in, for example, energy transfer efficiency, ISC yields, and photoproduct lifetimes. However, in spite of these differences, from the previous paragraph, we conclude that “per unit length” the conditions of S-TA are similar for these polymers. Aggregated P3HT possess the highest excitation power dependent quenching. The same quenching effect is reached for aggregated chains at ca. 4 times lower excitation power density than for isolated “naked” P3HT. This means that excitation can migrate efficiently over densely packed P3HT aggregates consisted of several (about 4) chains of 39 kDa molecular weight in agreement with the literature data on other polymers.14 Note that such efficient migration is observed in spite of 10 times quenching due to static quenching mechanisms that will be discussed below. S-T annihilation in photosynthetic aggregates has been studied in details both experimentally and theoretically since more than 40 years ago.52−54 It is interesting that the dependences of QY on the excitation power reported here are much weaker than predicted by the theory. In our experiments, in order to decrease the relative QY from 0.9 to 0.1, the excitation power needed to be changed by approximately 5 orders of magnitude, whereas only 3 orders of magnitude were reported and theoretically predicted for S-T annihilation in photosynthetic aggregates.53 A possible explanation is that in CPs, there is a very broad distribution of parameters determining the annihilation efficiency, as discussed elsewhere.42

Φ = ΦDΦS

(10)

Where ΦD and ΦS are the components of the fluorescence QY determined solely by dynamic processes that are visible in the fluorescence decay, and static (or ultrafast) processes that are not reflected in the fluorescence decay. Below, we will separate the contributions of the static and dynamic exciton quenching mechanisms by simultaneous measuring of fluorescence decays and QY as a function of excitation power density. The fluorescence decays were fitted using the multiexponential decay model f (t ) =

∑ A i e −t / τ / ∑ A i

⟨τ ⟩amp =

i

∑ Aiτi/∑ Ai = ∫ f (t )dt ∝ ΦD

(11) (12)

Where Ai and τi are the amplitude and lifetime of the decay component, i, and f(t) are the normalized fluorescence decay profile ( f(0) = 1), and ⟨τ⟩amp is the amplitude weighted average lifetime that is proportional to the dynamic quenching ΦD. The following equation derived in ref 7 connects parameters of fluorescence QY, fluorescence decay, and radiative decay rate k0 Φ = ΦDΦS = k 0 ΦS

∫ f (t )dt = k 0ΦS⟨τ⟩amp

(13)

Therefore, static and dynamic quenching can be addressed independently if one knows the fluorescence QY and amplitude-averaged fluorescence lifetime. Figure 5 shows the normalized total QY (Φ/Φ0) and normalized QY due to dynamic quenching (ΦD/ΦD0), obtained from the same experiment where the fluorescence kinetics was measured with full control of the excitation and emission intensities. Normalization was carried out at the low excitation power limit. Parameters of the decay components as functions of excitation power are given in the Supporting Information. We can summarize the observations as follows: (i) Higher excitation power opens up extra nonradiative deactivation channels leading to decreased ΦD for both polymers. Remarkably, the excitation power density dependence of normalized ΦD is exactly the same for both polymers (Figure 5e). Because ”naked” P3HT was in the aggregated form for lifetime measurements, while chains were separated for the insulated PT, the powerdependent dynamic quenching must originate from intrachain processes that were similar for both types of PTs. (ii) There was no buildup of static quenching in the insulated PT with increasing excitation intensity (Figure 5c). Combining this with the absolute QY measurements 4872



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Figure 6. Quenching processes in aggregated “naked” P3HT at different excitation power density regimes. “Fast” means that the processes cannot be time-resolved in our experimental setup (≪10 ps). Such processes appear as static quenching. “Slow” means that the kinetics can be resolved, the processes appear as dynamic quenching. A, absorption; F, fluorescence; nr, nonradiative transition. Figure 5. Dynamic vs static quenching. Excitation power dependence of the fluorescence quantum yield normalized at the low excitation power limit for the “naked” aggregated P3HT (right column) and insulated PT (left column) dispersed in PMMA films. Φ, total quantum yield; ΦD, quantum yield due to dynamic quenching; ΦS, quantum yield due to static quenching (Φ = ΦDΦS). The samples were prepared from solutions with OD ≈ 0.1. Graph (e) shows the data for both polymers for comparison.

discussed above, we can conclude that excitons in insulated PT are deactivated via dynamic quenching only, meaning that the entire decrease of QY is reflected in the corresponding decrease of the fluorescence lifetime at time scales from 10 ps and longer. (iii) There is a buildup of static quenching with increasing excitation power density in “naked” P3HT (Figure 5f). Therefore, at high powers, together with increased dynamic quenching, new channels of static (or ultrafast) quenching arise. Power-dependent static quenching must be related to the chain aggregation. Thus, the insulation of the conjugated backbone prevents photoinduced static quenching. Moreover, the values of ΦSk0 for insulated PT in solution and in PMMA films are the same within the limits of experimental errors (2.3 and 2.5 ns−1 respectively, Table 1) and close to the typical radiative rate constant in CPs.7 This means that if there is no static quenching in liquid solutions, there must be no static quenching in PMMA for insulated PT as well. 3.4. Summary of the Quenching Processes. We observed that the excitation power density affects “naked” P3HT and insulated PT embedded in a PMMA matrix differently, as summarized in Figures 6 and 7. At the low excitation power limit, we observed a ∼3−10 times difference in the extent of static quenching between the two polymers, related to the aggregation of “naked” P3HT. Our methods cannot reveal the origin of the quenching. However, we can tell that this deactivation process is either static or ultrafast (≪10 ps) leaving fluorescence lifetime unchanged. There is also no comprehensive explanation of this effect in the literature. According to some reports29−31,49 light absorption in PT films can generate singlet excitons or polaron pairs similar as was suggested for poly(p-phenylenevinylene).8 There are also indications in the literature that low QYs can be due to population of low-energy interchain CT state.32 We refer to such processes as static quenching leading to a branching ratio η between the formation of emissive excitons and “dark” states

Figure 7. Quenching processes in insulated PT at different excitation power density regimes. “Slow” means that the kinetics can be resolved, so the processes appear as dynamic quenching.

(Figure 6): η = 0.1−0.3 for aggregated “naked” P3HT and η = 1 for insulated PT. Increased excitation power density opens new channels of dynamic quenching in “naked” P3HT. Moreover, the dynamic quenching power dependence remains identical to that for insulated PT. That means that the power-dependent dynamic quenching (slow S-T, S-PhotoP annihilation) must be an intrachain process that cannot be prevented by conjugated backbone insulation. However, at higher excitation power, increasing of the static quenching was observed in “naked” P3HT, whereas it was not the case in insulated PT. It must be related to ultrafast S-PhotoP or/and S-T annihilation processes at the interchain level. Having several P3HT chains or chain segments densely packed in 3D not only facilitates electronic interactions between the excited species, but also reduces the distance between the excitations in comparison to the situation in separated rodlike chains of insulated PT. The situation is different when chains of P3HT are protected by threading through its own cyclic side chains and cannot aggregate. Independently on excitation power insulation prevents formation (or direct excitation) of “dark” states. Basically at all studied conditions, the insulated PT possessed only dynamic fluorescence quenching and this quenching was fully visible via the fluorescence decay profile. Dynamic quenching increases with increasing excitation power density. This dynamic quenching is related to slow (slower than 10 ps) S-T and S-PhotoP intrachain annihilation processes (Figure 7).

4. CONCLUSIONS Aggregation of P3HT induces static fluorescence quenching that decreases fluorescence QY up to one order of magnitude in 4873



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(6) Lin, H.; Tian, Y.; Zapadka, K.; Persson, G.; Thomsson, D.; Mirzov, O.; Larsson, P. O.; Widengren, J.; Scheblykin, I. G. Nano Lett. 2009, 9, 4456−4461. (7) Thomsson, D.; Camacho, R.; Tian, Y.; Yadav, D.; Sforazzini, G.; Anderson, H. L.; Scheblykin, I. G. Small 2013, 9, 2619−2627. (8) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.; Miller, T. M. Synth. Met. 1996, 80, 41−58. (9) Camacho, R.; Thomsson, D.; Sforazzini, G.; Anderson, H. L.; Scheblykin, I. G. J. Phys. Chem. Lett. 2013, 4, 1053−1058. (10) Xu, B.; Holdcroft, S. J. Am. Chem. Soc. 1993, 115, 8447−8448. (11) Monkman, A. P.; Burrows, H. D.; Hartwell, L. J.; Horsburgh, L. E.; Hamblett, I.; Navaratnam, S. Phys. Rev. Lett. 2001, 86, 1358−1361. (12) Schwartz, B. J. Annu. Rev. Phys. Chem. 2003, 54, 141−172. (13) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; 2nd ed.; Oxford University Press: New York, 1999. (14) Scheblykin, I.; Zoriniants, G.; Hofkens, J.; De Feyter, S.; Van der Auweraer, M.; De Schryver, F. C. ChemPhysChem 2003, 4, 260−267. (15) Gesquiere, A. J.; Park, S. J.; Barbara, P. F. J. Am. Chem. Soc. 2005, 127, 9556−9560. (16) Hania, P. R.; Scheblykin, I. G. Chem. Phys. Lett. 2005, 414, 127− 131. (17) Yu, J.; Lammi, R.; Gesquiere, A. J.; Barbara, P. F. J. Phys. Chem. B 2005, 109, 10025−10034. (18) Steiner, F.; Vogelsang, J.; Lupton, J. M. Phys. Rev. Lett. 2014, 112, 137402. (19) Tinnefeld, P.; Weston, K. D.; Vosch, T.; Cotlet, M.; Weil, T.; Hofkens, J.; Mullen, K.; De Schryver, F. C.; Sauer, M. J. Am. Chem. Soc. 2002, 124, 14310−14311. (20) Zaushitsyn, Y.; Jespersen, K. G.; Valkunas, L.; Sundstrom, V.; Yartsev, A. Phys. Rev. B 2007, 75, 195201. (21) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; Mcculloch, I.; Ha, C. S.; Ree, M. Nat. Mater. 2006, 5, 197−203. (22) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15, 1617−1622. (23) Thiessen, A.; Vogelsang, J.; Adachi, T.; Steiner, F.; Vanden Bout, D.; Lupton, J. M. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, E3550− E3556. (24) Yamamoto, T. NPG Asia Mater. 2010, 2, 54−60. (25) Barnes, M. D.; Baghar, M. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 1121−1129. (26) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Phys. Rev. Lett. 2007, 98, 206406. (27) Li, Y. N.; Vamvounis, G.; Holdcroft, S. Macromolecules 2002, 35, 6900−6906. (28) Yu, G.; Phillips, S. D.; Tomozawa, H.; Heeger, A. J. Phys. Rev. B 1990, 42, 3004−3010. (29) Jiang, X. M.; Osterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.; Janssen, R. A. J.; Vardeny, Z. V. Adv. Funct. Mater. 2002, 12, 587−597. (30) Ruseckas, A.; Theander, M.; Andersson, M. R.; Svensson, M.; Prato, M.; Inganas, O.; Sundstrom, V. Chem. Phys. Lett. 2000, 322, 136−142. (31) Cook, S.; Furube, A.; Katoh, R. Energy Environ. Sci. 2008, 1, 294−299. (32) Brazard, J.; Ono, R. J.; Bielawski, C. W.; Barbara, P. F.; Vanden Bout, D. A. J. Phys. Chem. B 2013, 117, 4170−4176. (33) Frampton, M. J.; Anderson, H. L. Angew. Chem., Int. Ed. 2007, 46, 1028−1064. (34) Terao, J. Polym. Chem. 2011, 2, 2444−2452. (35) Terao, J.; Kimura, K.; Seki, S.; Fujihara, T.; Tsuji, Y. Chem. Commun. 2012, 48, 1577−1579. (36) Frampton, M.; Sforazzini, G.; Brovelli, S.; Latini, G.; Townsend, E.; Williams, C. C.; Charas, A.; Zalewski, L.; Kaka, N. S.; Sirish, M.; Parrott, L. J.; Wilson, J. S.; Cacialli, F.; Anderson, H. L. Adv. Funct. Mater. 2008, 18, 3367−3376. (37) Terao, J.; Tsuda, S.; Tanaka, Y.; Okoshi, K.; Fujihara, T.; Tsuji, Y.; Kambe, N. J. Am. Chem. Soc. 2009, 131, 16004−16005.

comparison with isolated chains in PMMA matrix. Such static quenching can be fully prevented by insulating the πconjugated backbone with self-threaded side chains. Fluorescence QY of polythiophenes decreases dramatically with increasing of the excitation power due to singlet−triplet annihilation and annihilation with long-living photoproducts (most probably charged states). Even at excitation power densities as small as 0.1W/cm2 (equal to the Sun power density on the Earth), the QY of P3HT is already about 20% lower because of photoinduced quenching processes, highlighting that excitation power density effects must be taken into account in single-molecule and single-nanoparticle P3HT studies when orders of magnitude higher excitation powers are used. The annihilation process are very fast (observed as static quenching) in aggregated P3HT (interchain processes), and are much slower (observed as dynamic quenching) within a single chain (intrachain processes). Finally, we conclude that the backbone protection dramatically reduces exciton quenching via suppressing the formation of charged pairs and charge transfer states. It totally eliminates excitation power-dependent and -independent static quenching, while having no influence on dynamic quenching associated with intrachain processes.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental setup, simple estimation of the factors of 2D and 3D absorbing dipoles, conversion between BOD and QY, photostability of polymers during the experiment time, estimation of excitation power for S−S annihilation, fluorescence decay kinetics of polymers in different excitation powers, parameters of decay components of polymers at different excitation power, comparison of QY measured from two different experiments, and comparison of absolute ⟨τ⟩amp in both polymers. 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 These studies were financially supported by The Royal Physiographic Society in Lund, Knut & Alice Wallenberg Foundation, Crafoord Foundation, Carl Trygger Foundation, and The Swedish Research Council. D.S acknowledges the Wenner Gren Postdoctoral Fellowship.

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REFERENCES

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Effect of Conjugated Backbone Protection on Intrinsic and Light

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