Intrachain Aggregation of Charge-Transfer Complexes in Conjugated

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Intrachain Aggregation of Charge-Transfer Complexes in Conjugated Polymer:Acceptor Blends from Photoluminescence Quenching Andrew Yurievich Sosorev, Dmitry Yurievich Paraschuk, Sergei Aleksandrovich Zapunidy, Grigoriy Sergeevich Kashtanov, and Olga Dmitrievna Parashchuk J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4000158 • Publication Date (Web): 08 Mar 2013 Downloaded from http://pubs.acs.org on March 9, 2013

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The Journal of Physical Chemistry

Intrachain Aggregation of Charge-Transfer Complexes in Conjugated Polymer:Acceptor Blends from Photoluminescence Quenching Andrew Yu. Sosorev, Olga D.

Parashchuk, Sergei A. Zapunidi, Grigoriy S.

Kashtanov, and Dmitry Yu. Paraschuk*. Faculty of Physics and International Laser Center, M.V. Lomonosov Moscow State University, Moscow 119991 Russia. *E-mail [email protected]; tel. +7(495)9392228

Abstract Recent studies of conjugated polymer donor-acceptor blends show that the donor and acceptor can form a weak charge-transfer complex (CTC) in the electronic ground state, and these CTCs can significantly change the photophysics in the blend. In this work, we study photoluminescence quenching in model polymer-acceptor blends of MEH-PPV with TNF (2,4,7trinitrofluorenone) in solution. Our experimental data show that the observed strong increase in the CTC concentration with acceptor content results only in a moderate quenching enhancement. We propose an extended Stern-Volmer relation to model photoluminescence quenching in conjugated polymers with statistically homogeneous distribution of CTCs over polymer chains. We compare the experimental data with the model and conclude that the CTCs are not randomly distributed within a chain but form intrachain CTC clusters. These findings imply that the CTCs can influence the morphology of donor-acceptor blends, which is of paramount importance for the performance of organic solar cells. Keywords: polymer solar cells, donor-acceptor blends, Stern-Volmer relation, selforganization. 1

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Introduction Blends of conjugated polymers with organic electron acceptors are actively studied as they are the heart of state-of-the-art polymer solar cells. In some type of blends, a weak donoracceptor charge-transfer complex (CTC) can be observed in the electronic ground state of the blend1-4. Some polymer CTCs can considerably absorb in the red and near-IR regions1,5-6 and generate mobile charges7-8 highlighting the CTC potential as low-bandgap materials for organic solar cells. The pathway from excitons to free charges generally includes the CTC states as key intermediates9-11. Furthermore, the ground-state interaction between the donor and acceptor in CTCs could influence the conformation of the polymer chains5,12-13 and the blend morphology14 and also enhance the photoxidation stability15. The formation of CTCs in donor-acceptor blends of conjugated polymers with organic acceptors appears to be a much more complex process compared with the CTC formation in lowmolecular-weight blends. Specifically, the CTC concentration in blend solution of poly(methoxy,5-(2’-ethyl-hexyloxy-1,4-phenylene-vinylene))

(MEH-PPV)

and

2,4,7-

trinitrofluorenone (TNF) shows a threshold increase with the acceptor content5. While the acceptor concentration is less than the threshold concentration, the CTC concentration is low, but when the threshold is exceeded, intense CTC formation starts. This threshold behavior of the CTC concentration is not usually observed in low-molecular-weight donor-acceptor blends, where a gradual increase of the CTC concentration is common (see, e.g. Ref. 16-18). To explain the threshold character of polymer complexation, the mechanism of positive feedback was proposed in Ref. 5. Briefly, the Raman data indicate that the conjugated chains become more planar upon CTC formation12. The locally planarized conjugated chain serves as a nucleation center for other CTCs as discussed in detail in Ref. 5. Therefore, the probability of subsequent CTC formation near the existing one is higher than the probability of CTC formation at a free (uncomplexed) conjugated segment. As a result, the CTCs are expected not to be homogeneously 2


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distributed over the polymer chains but rather form clusters. The CTC clusters were observed in MEH-PPV:TNF films by atomic force microscopy and x-ray diffraction19. These studies imply that CTC formation in MEH-PPV:TNF blends is a collective phenomenon. Unraveling the mechanisms of collective phenomena in polymers is a long-standing challenge in the polymer science. Moreover, the collective effects in conjugated polymers involving π-conjugated system are practically not studied. Photoluminescence (PL) quenching can highlight the process of CTC formation as the CTCs are expected to be efficient quenchers of the polymer PL. PL quenching in conjugated polymers was found to be very strong20-24 as compared to low-molecular-weight compounds. The enhanced PL quenching in conjugated polymers is attributed to exciton migration20-21,25-27. Excitons reach quenchers, dissociate, and hence cannot contribute to PL. Therefore, PL is very sensitive to the number and location of quenchers20, e.g., CTCs28. Correlation between the CTC concentration and the PL quenching data is expected to provide information about intrachain CTC distribution. In fact, PL quenching will be high if the CTCs are distributed homogeneously and will be low if the CTCs are aggregated. In this work, we observe that PL quenching in polymer:acceptor blends with CTCs is much less efficient than expected from the measured CTC concentration. We propose an extended Stern-Volmer relation to model PL quenching in conjugated polymers with statistically homogeneous distribution of CTCs over polymer chains. Then we compare the experimental data with the model and conclude that the CTCs are not randomly distributed over the chains but aggregate.

Experimental MEH-PPV (Mw=86000, Mn=420000, Sigma-Aldrich) and TNF were separately dissolved in chlorobenzene at initial concentrations 18.1 mM for MEH-PPV and 15.7 mM for TNF. Blends 3



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were prepared by mixing the solutions so that the MEH-PPV concentration in the blends was constant and equal to 7.25 mM, and the acceptor concentration was varied in the range 0–4.8 mM. Absorption spectra were recorded using a fiber-coupled spectrophotometer (Avantes). A quartz cuvette with a 100-µm-thick solution layer was used. PL spectra were recorded using a 1cm-thick quartz cuvette in the backscattering geometry. The samples were excited at a wavelength of 532 nm using a Nd:YAG laser with a maximum intensity of 200 mW/cm2. The PL spectra were recorded using a monochromator with 1-nm spectral resolution. The PL intensity was measured at 635 nm.

Absorption and Photoluminescence Data

Fig. 1 CTC concentration calculated from Fig. 2. PL quenching data (points) and their absorption spectra of MEH-PPV:TNF blend. approximation by Eq. 4 (line). The data were Inset in panel (a) shows CCTC(Ca) at low corrected to the blend absorption (see SI, Sec. 1). acceptor content.

The CTC concentration in MEH-PPV:TNF blends was calculated from the CTC absorption at 635 nm as described in Ref. 5 (see Supporting Information (SI), Sec. 1). Fig. 1 shows the CTC concentration as a function of the total (free and complexed) acceptor concentration. The inset in Fig. 1 presents the similar dependence at low acceptor concentration.

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The CCTC(Сa) dependence shows a clear threshold as was observed in the previous study5. When Ca is lower than the threshold concentration, Cat ≈ 2.25 mM, CCTC is low, while above Cat the CTC concentration increases rapidly with Ca. Fig. 2 presents PL quenching data in the SternVolmer coordinates. The PL quenching curve, which is linear below the threshold acceptor concentration, shows an upward curvature above the threshold. Therefore, the CTCs take part in PL quenching as expected. The linear approximation of the quenching curve below the threshold yields the effective bimolecular quenching constant kq’=(1.59±0.08)×1012 M-1s-1.This value is two orders higher than typical values for low-molecular-weight blends and well corresponds to those for conjugated polymers22-24. Therefore, very efficient PL quenching is observed below the threshold. Above the threshold, the CTC concentration increases in more than 50 times at the maximum acceptor concentration as compared to the threshold concentration (Fig. 1) so that about 20% of the repeat units are involved in the CTCs. One could expect that a drastic increase in the CTC concentration above the threshold would lead to strong PL quenching enhancement. However, the PL intensity decreases in only about five times as compared to the threshold concentration. This striking discrepancy between the quencher concentration and the PL quenching efficiency disagrees with high sensitivity of conjugated polymer PL to quenchers.

Model: Extended Stern-Volmer Relation To describe the PL quenching data qualitatively and address the origin of the observed discrepancy, we extend the Stern-Volmer relation (formulated for PL quenching of small molecular donors) to conjugated polymers. In solutions of low-molecular-weight donor and acceptor, two mechanisms of PL quenching, dynamic and static, are usually distinguished29. In dynamic quenching, an excited fluorophore (donor) is quenched by a collision with a free diffusing quencher (acceptor) molecule. Static quenching is observed if donor and acceptor 5



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molecules can form a non-emitting ground-state complex, e.g., CTC. The inverse PL intensity for combined dynamic and static quenching can be described by the Stern-Volmer relation29:

(

)(

)

I0 = 1 + K a ( Ca 0 − CCTC ) 1 + γ kqτ 0 ⋅ ( Ca 0 − CCTC ) I ,

(1)

where I0 is the PL intensity of pure fluorophore solution, and I is the PL intensity of the blend with the same fluorophore concentration, Ca0 is the initial acceptor (quencher) concentration, and CCTC is the CTC concentration. The first factor in the right hand side of Eq. (1) describes static quenching, and the second one describes dynamic quenching. The association constant Ka describes the equilibrium between concentrations of CTC, free donor and free acceptor in the blend. The bimolecular quenching constant kq characterizes the collisional frequency, τ0 is the PL lifetime in the absence of acceptor, γ is the quenching probability of single collision (usually it is close to unity, and we assume γ=1 below). The above model is formulated for low-molecular-weight blends and, therefore, does not include exciton migration over donor species and peculiarities of CTC formation in polymer blends. Now, we turn to the derivation of the PL quenching model for a conjugated polymer donor. Consider isolated infinite linear polymer chains and assume that an exciton is localized at one repeat unit that is a fluorophore in the model. The neglect of exciton delocalization does not change our conclusions as discussed in SI (sec.3) Assume that both the dynamic and static mechanisms contribute to PL quenching. Indeed, on the one hand, Wang et al. reported that quenching is dynamic in MEH-PPV:PCBM blends22. On the other hand, Zheng et al. concluded that quenching is static in MEH-PPV:C60 blends23. Static PL quenching was also reported for conjugated polymers with special functional groups, which form host-guest complexes with acceptor molecules20,24. In low-molecular-weight blends, time-resolved PL quenching is commonly used to distinguish static and dynamic quenching as the former does not affect PL lifetime, while the latter does. However, when an exciton migrates within the macromolecule, we 6


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suggest that the difference between static and dynamic quenching is not well defined as the both are associated with exciton migration and hence are dynamic in nature. Therefore, both the static and dynamic quenching should influence the PL lifetime, and it will be not easy to distinguish them. Because of this, we concentrate here on steady-state PL quenching. As the CTC formation in the polymer blends differs from that in low-molecular-weight blends and generally cannot be described by an association constant5, one should rewrite Eq. (1) in a form that does not contain the latter:

(

0

(

))

I0 Cd 0 = 0 1 + k qτ 0 ⋅ C a − C CTC I C d − n d ⋅ CCTC ,

(2)

where Cd0 and Ca0 are the initial concentrations of donor species (i.e., repeat units) and acceptor molecules, correspondingly; nd is the number of repeat units involved into one CTC.

Fig. 3. Illustration of quenching zone. The bell-shaped curve depicts the probability distribution for the exciton generated at particular unit to be quenched at the quencher ( ). In the model, we replace this distribution by a rectangular one with quenching zone length n. As a result, the chain is divided into segments from which the excitons are quenched and segments at which the excitons luminesce. Repeat units belonging to the quenching zone are shown by dark circles ( ), those out of the quenching zone are shown by light circles ( ).

Exciton motion facilitates both the dynamic and static quenching. A migrating exciton can visit repeat units, which are involved into CTC or collision with an acceptor molecule, and thus be quenched. In other words, these repeat units can quench excitons generated at other 7



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repeat units as a result of their migration. This quenching can be characterized by a probability distribution along the chain. The larger the distance between the exciton “birthplace” and the quencher, the lower this probability, as shown by a bell-shaped curve in Fig. 3. However, this probability distribution is unknown. We approximate it by a rectangular one with a length n units (“quenching zone”) centered at the quenching repeat unit. Fig. 3 illustrates the notion of quenching zone. All the excitons generated within the quenching zone are quenched with unit probability, while all the excitons generated out of this zone avoid quenching by this quencher. The quenching zone length of a CTC (nCTC units) should be larger than that of a repeat unit suffering a collision (nc units), as the CTC lifetime is longer than the collision duration. Indeed, a collision is a necessary stage of CTC formation, and the CTC can be treated as a prolonged collision. With the use of the quenching zones and assuming the statistically homogeneous CTC distribution over the chains, one can write down a Stern-Volmer-type relationship for the PL quenching at polymer chains:

I0 Cd 0 = 0 1 + nc ⋅ kqτ 0 ⋅ ( Ca 0 − CCTC ) I Cd − nCTC ⋅ CCTC

(

) (3)

However, Eq. (3) is incorrect if the quenching zones are overlapping. To calculate the PL intensity in this case, we should modify both the static and dynamic quenching described by the first and second factors in Eq. (3), respectively . In the static quenching factor, we should allow for overlapping between the quenching zones for different CTCs. In the dynamic quenching factor, we should take into account that the collisions may quench fewer units than nc because of overlapping between the collision and CTC quenching zones (Fig. 4). At the same time, we neglect the overlapping between the quenching zones of two collisions as we assume that the collision duration is very short.

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nCTC nc

CTC

CTC

Fig. 4. Possible location of the quenching zones. The quenching zone of the unit suffering a collision ( ) with length nc overlaps with the CTC quenching zone with length nCTC. Repeat units belonging to the quenching zones are shown by dark circles ( ), those out of the quenching zones are shown by light circles ( ).Quenching zones for different quenchers are shown by shading.

The CTCs divide polymer chains into segments of various lengths. To take into account the overlapping between CTC quenching zones, we calculate the distribution of these segments in length assuming that the CTCs are placed randomly at an infinite polymer chain. According to the Poisson statistics, the probability for a segment to have the length between l and l+dl is

w ( l ) = µ exp ( − µ l ) dl ,

(4)

where µ = CCTC / Cd 0 . A part of the segment is outside the quenching zones of two adjacent CTCs, and its average length is ∞

 n C  f = ∫ w ( l ) ⋅ φ ( l ) dl = exp  − CTC 0CTC  , Cd   0

(5)

where

l < nCTC 0, l − nCTC , l > nCTC

ϕ (l ) = 

(6)

is the quenching-free length of the segment with length l. Indeed, nCTC/2 units are quenched by the CTC on the left end of the segment, and the same number of units is quenched by the CTC

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on the other end. As a result, quenching by CTCs (first factor of Eq. (3)) in the case of the zone overlapping is described by

n C  I0 1 = = exp  CTC 0CTC  . I f  Cd 

(7)

To take into account the overlapping between the collision and CTC quenching zones (Fig. 4), we should correct nc in the second factor of Eq. (3) to the average fraction of repeat units outside the CTC quenching zones: nc ' = nc ⋅ f . Indeed, the quenching zone for this collision spreads over nc repeat units. A part of this zone (1 – on average) is already quenched as it is within the CTC quenching zone. Therefore, collision will quench the rest part ( on average) of the zone. As a result, we obtain the final expression for PL quenching which allows for zones overlapping:

n C I0 = exp  CTC 0CTC I  Cd

   nCTC CCTC  0 ⋅ τ ⋅ − k C C ( )  =  1 + ncoll ⋅ exp  −  0 a q CTC Cd 0    

 nCTC CCTC  0  + ncoll kqτ 0 ( Ca − CCTC ) 0  Cd 

= exp 

(8)

We will refer to Eq. (8) as to “extended Stern-Volmer relation”. Importantly, this relation implies the statistically homogeneous CTC distribution. For inhomogeneous quencher distribution (e.g., CTC aggregation), Eq. (8) is incorrect since the Poisson statistics (Eq. (4)) is not applicable in this case. Fig. 5 presents model PL quenching curves calculated for various parameters of the

model, nc and nCTC. The nCTC values are taken larger than nc as the CTC lifetime must be longer than the collision duration. For the simulation, the acceptor concentration range and CCTC(Ca) dependence are taken those for MEH-PPV:TNF blends (see Fig. 1). The threshold acceptor concentration in this blend is 2.25 mM, which results in a kink of the quenching curve in Fig. 5 10


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because of intensive CTC formation above the threshold. Below the threshold, the curve is linear, while above it, the curve becomes convex. (a)

(b)

Fig. 5. Simulated PL quenching curves. (a) nCTC=50 and various nc (b) nc=10 and various nCTC. Other parameters are those of MEH-PPV:TNF blends.

Naturally, an increase of either nc or nCTC enhances PL quenching. An increase of nc leads mainly to a larger curve slope below the threshold (Fig. 5a). At the same time, an increase of nCTC enlarges the slope above the threshold (Fig. 5b). As follows from Fig. 5, increase in nCTC has larger impact on PL quenching than that of nс.

Intrachain Aggregation of CTCs The extended Stern-Volmer relationship (Eq. (8)) was applied to fit our PL quenching data. The best fit shown in Fig. 2 perfectly corresponds to the experimental data. From this fit, one can estimate the lengths of quenching zones corresponding to a CTC (nCTC) and to a repeat unit colliding with an acceptor molecule (nc). We used three fixed parameters, Cd0=7.25 mM, τ0=0.4 ns, and kq=1.4×1010 M-1s-1. The latter was estimated using the Smoluchowski formula (SI, Sec. 2). The fitting parameters were nc and nCTC. Their best-fit values were found to be nc = kq '/ kq = 95±20 and nCTC =15±1 repeat units. The obtained nc value corresponds to the effective 11



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quenching constant kq’= kqnc = (1.5±0.2)×1012 M-1s-1 indicating efficient exciton migration. This value is close to the value from linear approximation of the PL quenching curve. The nc value is significantly larger than those usually reported for the exciton diffusion length in films of MEH-PPV 30 and other conjugated polymers31. Our PL data were recorded in a good solvent where the chain conformation is much more extended. As a result, long-range exciton migration is possible in the well-solvated chains27,32-33. The obtained nc value corresponds to the exciton migration length ~60 nm that is in good agreement with the recent data for well-solvated MEH-PPV single-molecules 34 and with earlier reports for poly(phenylene ethynylene)20,24. Surprisingly, the CTC quenching zone length, nCTC, was found to be much shorter than that of collision, nc. If our suggestion of the statistically homogeneous CTC distribution is right, these values of nCTC and nc mean that a CTC between a repeat unit and an acceptor molecule is about six times less efficient quencher than a repeat unit suffering a collision without bonding the acceptor molecule in a CTC. On the contrary, as mentioned above, the CTCs must be more efficient quenchers as the characteristic СTС lifetime is longer than the collision duration. Therefore, we conclude that the assumed statistically homogeneous CTC distribution cannot explain our PL quenching data. To reconcile the model and the experimental data, we suggest that the CTCs are aggregated within the chains so that effective nCTC becomes relatively small. (a)

(b)

Fig. 6. Schematic of CTC quenching zones for statistically homogeneous (a) and inhomogeneous (b) distributions of CTCs. For details, see the caption to Fig. 3.

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Fig. 6 illustrates statistically homogeneous (a) and inhomogeneous (b) CTC distributions. As follows from Fig. 6b, the aggregated CTCs, because of strong overlapping of the quenching zones, can result in much less efficient PL quenching compared with the statistically homogeneous CTC distribution. Accordingly, PL will be less sensitive to the CTC number in the aggregates, and a significant increase in the CTC concentration will result in a moderate increase in PL quenching. This explains our experimental data presented in Fig. 1 and Fig. 2. While the obtained nc~100 evaluates the exciton migration length, the obtained nCTC~15 only indicates that the CTC distribution over polymer chains is inhomogeneous. Indeed, Eq. (8) is inapplicable for the statistically inhomogeneous CTC distribution since nCTC is not a constant but depends on the CTC concentration. Two different explanations were earlier proposed to describe the absorption data in MEH-PPV:TNF blend: the model of acceptor penetration into the polymer coil35 and the hypothesis of positive feedback5. The statistically inhomogeneous intrachain CTC distribution reported in the current work is in accordance with the latter but contradicts the former. According to the latter, subsequent CTCs are preferentially formed near the existing ones that results in the formation of CTC aggregates5. By contrast, according to the former, CTCs are formed mainly on the coil surface below the threshold. Above the threshold, the acceptor molecules penetrate inside the coil resulting in a more homogenous CTC distribution. Therefore, PL quenching would be low below the threshold and high above it, but this is opposite to our PL quenching data. Note that the CTCs can link neighboring polymer chains so that interchain aggregates are formed13. The aggregates can modify one-dimensional exciton migration considered above. However, as discussed in SI (Sec. 4), including three-dimensional exciton migration only enhances our conclusion about inhomogeneous intrachain distribution of CTCs.

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Our model also assumes that the CTC formation does not change the polymer conformation. Nevertheless, the CTC formation results in planarization of the polymer chains according to the Raman spectroscopy data12. This should increase the exciton diffusion length32 enhancing the expected contribution of CTCs to PL quenching, i.e. should result in larger nCTC. As we observed the opposite (unexpectedly low nCTC), taking into account the CTC-induced polymer conformation supports our reasoning and conclusions. The above data suggest strong aggregation of CTCs. It is naturally to expect that the CTC aggregates will be inherited in the solid phase during film drying, as it was observed in pristine polymers36-37

and polymer:fullerene blends38. Therefore, the ground-stated charge-transfer

interaction could be used as a tool to influence and maybe to control the morphology of polymer bulk heterojunctions used as active layers in organic solar cells.

Conclusion In summary, the correlation between the CTC concentration and PL quenching in MEHPPV:TNF blends has been investigated. By using the extended Stern-Volmer relation, we conclude that the intrachain CTC distribution is not statistically homogeneous, but the CTCs aggregate resulting in a low sensitivity of PL quenching to the CTC concentration. These findings give evidence for the collective nature of the CTC formation at a single conjugated chain.

Supporting Information Supporting Information. Experimental details, estimation of bimolecular constant, and influence of interchain aggregation on PL quenching. This material is available free of charge via the Internet at http://pubs.acs.org.

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Acknowledgement This work is supported by Russian Ministry of Science and Education (contracts #16.740.11.0064 and 16.740.11.0249 ).

References (1) Bakulin, A. A.; Elizarov, S. G.; Khodarev, A. N.; Martyanov, D. S.; Golovnin, I. V.; Paraschuk, D. Yu.; Triebel, M. M.; Tolstov, I. V.; Frankevich, E. L.; Arnautov, S. A.; et al. Weak Charge-Transfer Complexes Based on Conjugated Polymers for Plastic Solar Cells. Synth. Met. 2004, 147, 221-225. (2) Goris, L.; Haenen, K.; Nesladek, M.; Wagner, P.; Vanderzande, D.; De Schepper, L.; D'Haen, J.; Lutsen, L.; Manca, J. V. Absorption Phenomena in Organic Thin Films for Solar Cell Applications Investigated by Photothermal Deflection Spectroscopy. Journal of Materials Science 2005, 40, 1413-1418. (3) Benson-Smith, J. J.; Goris, L.; Vandewal, K.; Haenen, K.; Manca, J. V.; Vanderzande, D.; Bradley, D. D. C.; Nelson, J. Formation of a Ground-State Charge-Transfer Complex in Polyfluorene/[6,6]-Phenyl-C-61 Butyric Acid Methyl Ester (PCBM) Blend Films and Its Role in the Function of Polymer/PCBM Solar Cells. Advanced Functional Materials 2007, 17, 451-457. (4) Bakulin, A. A.; Pshenichnikov, M. S.; van Loosdrecht, P. H. M.; Golovnin, I. V.; Paraschuk, D. Yu. In Physics of Nanostructured Solar Cells; Badescu, V., Paulescu, M., Eds.; Nova Science Publishers: 2010, p 463-504. (5) Parashchuk, O. D.; Bruevich, V. V.; Paraschuk, D. Yu. Association Function of Conjugated Polymer Charge-Transfer Complex. Phys. Chem. Chem. Phys. 2010, 12, 6021 6026. (6) Bakulin, A. A.; Khodarev, A. N.; Martyanov, D. S.; Elizarov, S. G.; Golovnin, I. V.; Paraschuk, D. Yu.; Arnautov, S. A.; Nechvolodova, E. M. Charge Transfer Complexes of a Conjugated Polymer. Doklady Chemistry 2004, 398, 204-206. (7) Bakulin, A. A.; Martyanov, D. S.; Paraschuk, D. Yu.; Pshenichnikov, M. S.; van Loosdrecht, P. H. M. Ultrafast Charge Photogeneration Dynamics in Ground-State ChargeTransfer Complexes Based on Conjugated Polymers. J. Phys. Chem. B 2008, 112, 13730-13737. (8) Ozimova, A. E.; Bruevich, V. V.; Dittrich, T.; Paraschuk, D. Yu. Enhanced Photostability and Red-Nir Photosensitivity of Conjugated Polymer Charge-Transfer Complexes. Macromolecular Symposia 2010, 296, 138-143. (9) Bakulin, A. A.; Martyanov, D.; Paraschuk, D. Yu.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S. Charge-Transfer Complexes of Conjugated Polymers as Intermediates in Charge Photogeneration for Organic Photovoltaics. Chem. Phys. Lett. 2009, 482, 99-104. (10) Drori, T.; Sheng, C. X.; Ndobe, A.; Singh, S.; Holt, J.; Vardeny, Z. V. Below-Gap Excitation of Pi -Conjugated Polymer-Fullerene Blends: Implications for Bulk Organic Heterojunction Solar Cells. Physical Review Letters 2008, 101, 037401. (11) Carsten, B.; Szarko, J. M.; Lu, L.; Son, H. J.; He, F.; Botros, Y. Y.; Chen, L. X.; Yu, L. Mediating Solar Cell Performance by Controlling the Internal Dipole Change in Organic Photovoltaic Polymers. Macromolecules 2012, 45, 6390-6395.

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Page 16 of 18

(12) Bruevich, V. V.; Makhmutov, T. S.; Elizarov, S. G.; Nechvolodova, E. M.; Paraschuk, D. Yu. Raman Spectroscopy of Intermolecular Charge Transfer Complex between a Conjugated Polymer and an Organic Acceptor Molecule. J. Chem. Phys. 2007, 127, 104905. (13) Parashchuk, O. D.; Laptinskaya, T. V.; Paraschuk, D. Yu. Macromolecular Dynamics of Conjugated Polymer in Donor-Acceptor Blends with Charge Transfer Complex. Physical Chemistry Chemical Physics 2011, 13, 3775-3781. (14) Elizarov, S. G.; Ozimova, A. E.; Paraschuk, D. Yu.; Arnautov, S. A.; Nechvolodova, E. M. Laser Light Scattering as a Probe of Phase Separation in Donor-Acceptor Conjugated Polymer Films. Proc. SPIE 2006, 6257, 293-302. (15) Golovnin, I. V.; Bakulin, A. A.; Zapunidy, S. A.; Nechvolodova, E. M.; Paraschuk, D. Yu. Dramatic Enhancement of Photo-Oxidation Stability of a Conjugated Polymer in Blends with Organic Acceptor. Appl. Phys. Lett. 2008, 92, 243311. (16) Grebenyuk, S. A.; Perepichka, I. F.; Popov, A. F. Evaluation of the Parameters of 1:1 Charge Transfer Complexes from Spectrophotometric Data by Non-Linear Numerical Method. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2002, 58, 2913-2923. (17) Matos, M. S.; Gehlen, M. H. Charge Transfer Complexes of 9-Vinyl-Carbazole with Π Acceptors in Homogeneous and in Micellar Solutions. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2004, 60, 1421-1426. (18) Emslie, P. H.; Foster, R.; Horman, I.; Morris, J. W.; Twiselton, D. R. Association Constants of Some Charge-Transfer Complexes of Aromatic Hydrocarbons with Electron Acceptors. Journal of the Chemical Society B: Physical Organic 1969, 1161-1165. (19) Parashchuk, O.D.; Grigorian, S.; Levin, E.E.; Bruevich, V.V.; Bukunov, K.; Golovnin, I.V.; Dittrich, T.; Dembo, K.A.; Volkov, V.V.; Paraschuk, D. Yu. Additive-Enhanced Local Order in Conjugated Polymer Films. Submitted to J. Phys. Chem. Lett. (20) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chemical Sensors Based on Amplifying Fluorescent Conjugated Polymers. Chemical Reviews 2007, 107, 1339-1386. (21) Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.; Sundstrom, V. Excited State and Charge Photogeneration Dynamics in Conjugated Polymers. Journal of Physical Chemistry B 2007, 111, 6303-6321. (22) Wang, J.; Wang, D. L.; Moses, D.; Heeger, A. J. Dynamic Quenching of 5-(2 'Ethyl-Hexyloxy)-P-Phenylene Vinylene (MEH-PPV) by Charge Transfer to a C-60 Derivative in Solution. Journal of Applied Polymer Science 2001, 82, 2553-2557. (23) Zheng, M.; Bai, F.; Li, F.; Li, Y.; Zhu, D. Э. The Interaction between Conjugated Polymer and Fullerenes. J. Appl. Polym. Sci. 1998, 70, 599-603. (24) Zhou, Q.; Swager, T. M. Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity. Journal of the American Chemical Society 1995, 117, 12593-12602. (25) Grage, M. M. L.; Wood, P. W.; Ruseckas, A.; Pullerits, T.; Mitchell, W.; Burn, P. L.; Samuel, I. D. W.; Sundstrom, V. Conformational Disorder and Energy Migration in MEHPPV with Partially Broken Conjugation. Journal of Chemical Physics 2003, 118, 7644-7650. (26) Becker, K.; Lupton, J. M.; Feldmann, J.; Setayesh, S.; Grimsdale, A. C.; Mullen, K. Efficient Intramolecular Energy Transfer in Single Endcapped Conjugated Polymer Molecules in the Absence of Appreciable Spectral Overlap. Journal of the American Chemical Society 2006, 128, 680-681. (27) Shibano, Y.; Imahori, H.; Sreearunothai, P.; Cook, A. R.; Miller, J. R. Conjugated “Molecular Wire” for Excitons. The Journal of Physical Chemistry Letters 2010, 1, 1492-1496. (28) Zapunidi, S. A.; Krylova, Yu. V.; Paraschuk, D. Yu. Analytical Model for Photoluminescence Quenching Via Forster Resonant Energy Transfer in a Conjugated Polymer Doped by Energy Acceptors. Physical Review B 2009, 79, 205208. 16


Page 17 of 18

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(29) Lakowicz, J. Principles of Fluorescence Spectroscopy, 3d Ed. 2006. (30) Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.; Miller, T. M. Photophysics of Phenylenevinylene Polymers. Synthetic Metals 1996, 80, 41-58. (31) Hwang, I.; Scholes, G. D. Electronic Energy Transfer and Quantum-Coherence in pi-Conjugated Polymers. Chemistry of Materials 2011, 23, 610-620. (32) Nesterov, E. E.; Zhu, Z.; Swager, T. M. Conjugation Enhancement of Intramolecular Exciton Migration in Poly(p-Phenylene Ethynylene)s. Journal of the American Chemical Society 2005, 127, 10083-10088. (33) Da Como, E.; Borys, N. J.; Strohriegl, P.; Walter, M. J.; Lupton, J. M. Formation of a Defect-Free pi-Electron System in Single Β-Phase Polyfluorene Chains. Journal of the American Chemical Society 2011, 133, 3690-3692. (34) Onda, S.; Kobayashi, H.; Hatano, T.; Furumaki, S.; Habuchi, S.; Vacha, M. Complete Suppression of Blinking and Reduced Photobleaching in Single Meh-Ppv Chains in Solution. The Journal of Physical Chemistry Letters 2011, 2, 2827-2831. (35) Parashchuk, O.; Sosorev, A.; Bruevich, V.; Paraschuk, D. Threshold Formation of an Intermolecular Charge Transfer Complex of a Semiconducting Polymer. Jetp Letters 2010, 91, 351-356. (36) Nguyen, T. Q.; Doan, V.; Schwartz, B. J. Conjugated Polymer Aggregates in Solution: Control of Interchain Interactions. Journal of Chemical Physics 1999, 110, 4068-4078. (37) Huang, W. Y.; Huang, P. T.; Han, Y. K.; Lee, C. C.; Hsieh, T. L.; Chang, M. Y. Aggregation and Gelation Effects on the Performance of Poly(3-Hexylthiophene)/Fullerene Solar Cells. Macromolecules 2008, 41, 7485-7489. (38) Peet, J.; Cho, N. S.; Lee, S. K.; Bazan, G. C. Transition from Solution to the Solid State in Polymer Solar Cells Cast from Mixed Solvents. Macromolecules 2008, 41, 8655-8659.

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For Table of Contents use only

Intrachain Aggregation of Charge-Transfer Complexes in Conjugated Polymer:Acceptor Blends from Photoluminescence Quenching Andrew Yu. Sosorev, Olga D.

Parashchuk, Sergei A. Zapunidi, Grigoriy S.

Kashtanov, and Dmitry Yu. Paraschuk*. Faculty of Physics and International Laser Center, M.V. Lomonosov Moscow State University, Moscow 119991 Russia. *[email protected]

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Intrachain Aggregation of Charge-Transfer Complexes in Conjugated

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