Energy-Transfer Processes in Donor–Acceptor Poly(fluorenevinylene- alt -4,7-dithienyl-2,1,3-benzothiadiazole)

June 23, 2017 | Autor: Eralci Therézio | Categoría: Engineering, Technology, CHEMICAL SCIENCES
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Energy-Transfer Processes in Donor–Acceptor Poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3benzothiadiazole) ARTICLE in THE JOURNAL OF PHYSICAL CHEMISTRY C · JUNE 2013 Impact Factor: 4.77 · DOI: 10.1021/jp400823d

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Article pubs.acs.org/JPCC

Energy-Transfer Processes in Donor−Acceptor Poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) E. M. Therézio,† Paula C. Rodrigues,‡,§ José R. Tozoni,† Alexandre Marletta,† and Leni Akcelrud*,‡ †

Physics Institute, Federal University of Uberlandia, CP 593, CEP 38400-902, Uberlandia - MG, Brazil Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Parana, CP 19081, CEP 81531-990, Curitiba - PR, Brazil § Institute of Physics, São Paulo University, CP 369, CEP 13084-971, São Carlos - SP, Brazil ‡

ABSTRACT: The emission ellipsometry technique was applied to the donor−acceptor structure, poly[9,9′-dioctyl-2,7-fluorenevinylene-alt-4,7-(di-2,5-thienyl)-2,1,3-benzothiadiazole], to quantify the energy transferred from the donor (fluorene) to the acceptor (thiophene−benzothiadiazole−thiophene). The relative contributions to the total emission from both donor and acceptor were determined and also the amount of the emitting species that lost coherence with the polarization of the excitation light. The results were discussed in terms of energy transfer via multiphonon processes.



INTRODUCTION In the framework of conjugated polymers photophysics, the electronic energy transfer is a well-established concept. In a general way it can be described as a “funneling” migration of higher energy excitons (usually isolated chromophores) to lower energy segments or exciton traps. This idea encompasses the view of the polymer system as a whole ensemble that works as an antenna for excitation light, but only very small parts of it are responsible for the light emission due to the downhill energy migration. Several kinds of anisotropy measurements have been used aiming to quantify or correlate the energy migration with emission intensity, provided a correlation between organization of the various molecular states and the corresponding anisotropy could be established. MEH-PPV has been the most explored structure in this regard, using singlemolecule excitation and emission simultaneous measurements1 and time-resolved stimulated emission anisotropy.2 Incorporation of the polymer in porous media3 was used to examine the dynamics of energy transfer, and the main conclusion was that the ordered conformational states and morphology are the driving forces for the anisotropy and electronic energy transfer.4,5 One important case of electronic energy transfer in conjugated polymer systems arises in the analysis and interpretation of the photophysics of donor−acceptor configurations. These are characterized by the presence of groups with different electron affinities forming a type of donor− acceptor (DA) system that may favor charge-separation processes.6 The interaction between an electron acceptor with a donor7 facilitates photoinduced charge separation in photovoltaic devices. Moreover, the DA structure allows the desired tuning of the energy levels, with systematic variation in © 2013 American Chemical Society

the polymer electronic structure. The origin of the wellseparated dual-absorption band sometimes encountered in semiconducting polymers using the “donor−acceptor” concept has been reviewed.6 Published data from experimental and theoretical studies confirm that the first absorption band in DA structures is related to the π → π* + 1 transition, characterizing the DA complex, and the second one at lower frequencies corresponds to the π → π* transition of the chromophoric group of the chain. This schematic picture of DA systems is widely accepted8−11 in qualitative terms, but the extent to which it occurs in a quantitative basis has not yet been addressed, to our knowledge. In the present contribution we aim to add further insight into this important subject using the emission ellipsometry (EE) technique to analyze in a quantitative manner the energy-transfer processes in these systems. The study was carried out using the DA structure poly[9,9′-octyl-2,7-fluorenevinylene-alt-4′,7′-(di-2,5-thienyl)2′,1′,3′-benzothiadiazole] (LaPPS37), depicted in Figure 1.



EXPERIMENTAL SECTION (1). Chemicals. The chemicals used were all purchased from Aldrich and used as received, without further purification, unless described in the specific chemical procedure. Solvents were from Aldrich (HPLC grade) and used as received. (2). Chemical Procedures. LaPPS37 was prepared using a combination of Suzuki and Wittig reactions (Figure 2). In brief, the compound (3) was prepared following the procedures Received: January 24, 2013 Revised: June 3, 2013 Published: June 4, 2013 13173

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hyde (4) were added under an argon atmosphere. To the mixture was added a solution of t-BuOK in anhydrous ethanol (8.5 mmol), and the reaction was allowed to proceed for 2 days at room temperature. After that, 5 mL of HCl 5% was added and the polymer was precipitated by pouring the mixture into an excess of methanol. The polymer was reprecipitated with CHCl3/methanol. Impurities and oligomers were eliminated by Soxhlet extraction, using methanol, acetone, hexane, and chloroform. Mn = 5200 g mol−1, PDI = 2.07. These values represent the fraction soluble in THF, which was the eluant and not a good solvent. The spectra were run with chloroform solutions, a good solvent. 1H NMR (CDCl3, δ): 7.66 (broad, 6H); 7.44 (broad, 4H); 7.00 (broad, 6H); 2.01 (broad, 4H); 1.07−0.66 (broad, 30H). (3). Sample. The cast film was obtained by dripping 100 μL of LaPPS37 (0.5 mg in 5 mL toluene) onto a quartz plate (1 × 2 cm2), thoroughly washing with detergent and ultrapure water, rinsing with ultrapure water and chromatographic grade toluene, and drying with argon flux.

Figure 1. Chemical structure of LaPPS37.

described elsewhere12 and the dialdehyde monomer was also synthesized according to the published procedure.13 The polymerization was run as follows: to a one-necked round-bottomed flask, 1.7 mmol of 2,7-bis[(p-triphenylphosphonium) methyl]-9,9′-dioctylfluorene (3) and 1.7 mmol of 5,5′-(2,1,3-benzothiadiazole-4,7-diyl)dithiophene-2-carbalde-

Figure 2. Synthetic route for LaPPS37 preparation. 13174

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(4). Equipment. The molar mass measurements were performed with a Waters 2690 gel permeation chromatograph, equipped with a Waters 996 refractive index photodiode detector. The calibration curves were built with polystyrene standards and tetrahydrofuran (THF) as eluant at a flow rate of 1.0 mL/min at 30 °C. 1H NMR spectra were recorded in chloroform on a Bruker system operating at 400 MHz at room temperature. (5). Spectroscopic Characterization. UV−vis absorbance measurements were performed using the spectrophotometer FEMTO 800 XI. Photoluminescence (PL) spectra were carried out by exciting the samples with the radiation at 514 nm (cutoff filter 550 nm, Newport OG 550) of an Ar+ laser (Coherent Innova 70C) and at 405 nm (cutoff filter 455 nm, Newport GG 455) of a diode laser (LaserLine-IZI) mantled in a He-closed cycle cryostat under vacuum (1.3 × 10−2 mbar). Linear and circular polarization of the excited light were carried out by introducing an achromatic linear polarizer (Newport 10LP-VISB) and an achromatic quarter-wave-plate (Newport 10RP54− 1) in the laser output path. The sample temperature ranged between 30 K and room temperature (290 K). The emission signal was analyzed with a spectrophotometer USB4000 Ocean Optics. For EE measurements, an achromatic quarter-waveplate (Newport 10RP54-1), compensator, and an achromatic polarizer (Newport 10LP-VIS-B) analyzer were introduced in the optical path before the spectrophotometer (USB4000 Ocean Optics). The EE curves were collected manually by rotating the compensator from 0 rad (0°) to 6.28 rad (360°) with steps of ∼0.17 rad (10°). Details of the experimental setup were published elsewhere.14

(or higher forms) in the electronic ground- or excitedstate15−17 H or J molecular aggregates or still the presence of the so-called β-phase has been reported.18 The state of aggregation and the macromolecular conformations associated with it can vary with temperature and excitation wavelength, altering the emission profile and making the assignment of the emitting center(s) under each condition very complex. This issue has been approached in the case of regioregular P3HT, whose spectrum presented well-defined progression peaks.19 However, the poorly resolved absorption or emission line shapes in the present case precluded a better description of the aggregated species. The emission also displays three main emissions at approximately 525, 670, and 760 nm regions attributed to fluorene (donor), thiophene−benzothiadiazole− thiophene (acceptor), and molecular aggregates (ππ), respectively, with excitation at either 405 or 514 nm. The overlap between the donor emission (π* + 1 → π) and the acceptor absorption (π → π*) spectra (Figure 3) fulfills the necessary condition for energy transfer from D to A chromophores. The absorption of fluorene peaks at 405 nm and is very small at 514 nm. Therefore its contribution to the global emission is significantly lower when the excitation is performed at 514 nm. Considering that the emission spectrum is composed of the contribution of donor and acceptor bands, switching the excitation from 405 to 514 nm will bring about a shift in its spectral mass center, characterizing not a “true” red shifting but the overall result of the sum of the two (uneven) contributions. This reasoning explains the shift of 665 nm (measured at the maximum of the fluorene emission, with excitation at 405 nm) to 680 nm (measured at the maximum of the acceptor emission, with excitation at 514 nm). The peak positions were assigned graphically considering the maximum of the intensity. They are in agreement with the literature.8,18 The assignment of the 405 nm band to the absorption of the fluorene unit deserves a rapid remark here because there is not a consolidated agreement in the literature about the assignment of the transition in the absorption spectrum of DA copolymers. Previous works attributed the band to the fluorene absorption,20,21 but the more recent publications prefer to see the band as a π−π* transition of the chain and not solely due to the fluorene unit, based on the weakness of fluorene as a donor (the real donor would be the thiophene unit) and thus behaving more as an “conjugation extender”. Particularly relevant in this respect is the work of Heeger et al.,11 in which the authors also refer to the partial charge transfer that is the main issue of the present contribution. Figure 4 shows the temperature dependency (30−290 K) of the PL, analyzed in parallel (Par.) and perpendicular (Per.) directions (according to an arbitrary laboratory referential). The sample was excited with linear polarized light at 405 (Figure 4a) and at 514 nm (Figure 4b). At lower temperature, Figure 4a displays well-resolved emission spectra, with a redshifted band at 684 nm, for both polarization directions. However, because of the line-shape enlargement and intensity decrease for 518, 574, and 684 nm bands, when the sample temperature increases, it was not possible to confirm the band shift for the donor group. The line broadening brought about by temperature increases was attributed to thermal disorder, altering the effective conjugation length and distribution and consequently the electron−vibrational modes coupling. In the perpendicular direction the intensity of the bands at 518 and 574 nm is significantly reduced in relation to the one at 684 nm, showing the temperature effect on the DA energy-transfer



RESULTS AND DISCUSSION Figure 3 displays the absorption and emission spectra of LaPPS37 in thin film form using nonpolarized light. The

Figure 3. Absorbance and emission spectra for LaPPS37 cast film at room temperature (290 K) in the UV−vis range. λex = 405 nm (black) and λex = 514 nm (gray).

absorption line is characterized by three bands at 405, 555, and 670 nm. The first two were attributed to the nonlocalized transitions π → π * + 1, π → π* corresponding to the chargetransfer complex and to the main chain chromophores, respectively. With the designation π* + 1 we refer in a symbolic way to any level of higher energy. Its precise location at the LUMO level is beyond the scope of this communication. The third band (ππ) → (ππ)* was assigned to the ground state of molecular aggregation. The assignment of red-shifted transitions to macromolecular association is widely spread in the literature on conjugated polymers. The formation of dimers 13175

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Figure 5. Photoluminescence intensity as a function of the sample temperature 30−290 K analyzed in parallel and perpendicular directions and photoexcited at 405 and 514 nm. Figure 4. Normalized photoluminescence spectra as a function of the sample temperature (30−290 K range) analyzed in parallel (Par.) and perpendicular (Per.) directions. Photoexcitation at (a) 405 and (b) 514 nm; the directions parallel and perpendicular of the linearly polarized excitations were arbitrarily taken using a laboratory referential.

temperature with a different dependence for the parallel and perpendicular directions at lower temperatures. This effect is smoothed out at higher temperatures. The emission coming from excitation at 514 nm in either the parallel or the perpendicular directions shows little dependence with temperature, and both have about the same intensity. In a previous publication23 we have shown that the fluorescence emitted by MEHPPV in solution has a significant amount of polarized light and that this result could be extended to other kinds of polymers, which is an unexpected result due to the isotropic character of the solution. The use of EE technique in association with Stokes’ parameters analyses allowed for the complete description of the polarization state of emitted light by the polymer. The method is based on exploiting the transformation that occurs in emission light polarization when the sample is excited using polarized light. It is possible to characterize any state of light polarization in terms of four measurable quantities, known as Stokes’ parameters. The first one describes the total intensity, and the remaining three describe the polarization state of the light.24,25 The rationale used to explain the retention of a certain amount of the linear polarization of the incident light in the emission of an isotropic polymer system is based on a model that considers the existence of a steady-state regime where light is absorbed by some chromophores aligned with the excitation, which will absorb and emit polarized light in the same direction before they lose the emission coherence due to molecular diffusion. In the perpendicular direction no emission is expected to occur under these conditions unless other photophysical events take place in the excited state, such as energy-migration or -transfer processes. In the latter case, some of the new emitting centers (acceptors) would assume conformations that will emit in the perpendicular direction. The assumption that the emission in the perpendicular direction of the linearly polarized light is originated only by transfer process is the basis for the present study. The use of the EE technique in the PL in association with a Stokes’ parameter analysis was applied to quantify the energy transfer in the DA LaPPS37. The theory underlying the results presented here, together with the set up used in the measurements, was already published elsewhere.23 In brief, in the EE experiment a fixed polarizer is used in front of the spectrophotometer, and the state of light polarization is analyzed in the scope of Stokes’ theory for electromagnetic field. The Stokes parameters used in

process. The behavior is enhanced with increasing the sample temperature, and was assigned to the increase in the density of accessible states via vibrational modes, thus favoring the π* + 1 → π* energy-transfer processes. A noteworthy feature is that the 762 nm band is detected only in the perpendicular direction, which agrees with its assignment to ππ stacking because this is a lower energy site (∼5 to 30 meV),22 and that the laser used had enough power (25 meV) to destabilize the molecular aggregate when excited in the parallel direction. For the perpendicular direction, the energy reaching the aggregate comes through transfer processes with much less energy. In this case, the ππ stacking structure is preserved and its emission can be detected. This assumption is strengthened by the fact that with direct excitation (630 nm) no emission signal was detected, showing that this chromophore emits only via transfer processes. Exciting the sample at 514 nm (Figure 4b), the emission in a general way follows the same trends observed for the excitation at 405 nm and 30 K (Figure 4a), with bands peaking at 577, 624, 694, and 760 nm. The peak associated with acceptor emission (694 nm) is also red-shifted by 15 nm, as occurred with excitation at 405 nm, discussed above. With increases in temperature, the relative intensity of the bands at 577 and 624 nm compared with that of the band at 694 nm (acceptor) decreases more slowly than the ratio observed in Figure 4πa. This is due to the fact that with excitation at 514 nm the contribution of the transition π* + 1 → π is smaller than that with excitation at 405 nm because with excitation at 514 nm the contribution of the donor is very small. The 760 nm band remained unchanged and well-resolved as before in perpendicular direction. For better visualization, the results previously described were put into graphical form (Figure 5). The following effects are discernible: the intensity of the emissions originated by excitation at 405 nm is higher than that brought about by the excitation at 514 nm, which was expected because with the former all of the chromophores are excited; the emission in the parallel direction for both excitations, 405 and 514 nm, was always higher than that in the perpendicular direction; finally, the emission with excitation at 405 nm is more sensitive to the 13176

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Figure 6. Emission ellipsometry curves of LaPPS 37 at room temperature (a) excited at λex = 405 nm and detected at λD = 525 nm and (b) excited at λex = 514 nm and detected at λD = 670 nm for polarization of the excitation in the parallel direction.

Table 1. Stokes Parameters S0, S1, S2, and S3 and Polarization Degree P of the Emission Ellipsometry Curves for LaPPS 37 Sample Excited with Linear Polarizationa

a

λex (nm)

λD (nm)

S1/S0 ± 0.01

S2/S0 ± 0.01

S3/S0 ± 0.01

P (%) ± 0.5

405 405 405 514 514

525 665 760 670 760

−0.40 −0.11 −0.07 −0.30 −0.08

−0.05 −0.01 0.01 −0.14 0.05

−0.08 −0.03 0.01 0.06 0.01

41.1 11.5 7.1 33.7 9.5

λex (nm) and λD (nm) stand for the wavelengths of the excitation and of the detection, respectively.

the method, S0, S1, S2, and S3, are obtained considering eq 1.14,25 I (θ ) =

or intermolecular energy-transfer processes between adjacent chromophores. The π* → π (∼665 nm) transition shows emission at the perpendicular direction relative to that of the excitation, with a contribution of 11% with S1 > 0. The emission in the perpendicular direction is a signature of the migration energy from the donor because direct excitation can only produce emission in the same direction as the incident light. The (ππ)* → (ππ) (∼760 nm) transition presents a totally random emission, with all of the parameters around zero. The excitation at 514 nm shows that for the transition π* → π (∼670 nm) the emission occurs in the parallel direction to the excitation, with S1 < 0 corresponding to 30% of the total emitted light, and with S2 < 0 corresponding to 14%. This result indicates that in the energy-transfer process a lower proportion of the photoexcited carriers decay radiatively, preferentially at the −45° direction in relation to the excitation direction. S3 is once more approximately zero. The results from excitations at 405 and 514 nm for the transition (ππ)* → (ππ) (∼760 nm) show that the emission is essentially isotropic and that the Stokes parameters are practically zero, agreeing with the assumption that this band is related to molecular aggregation. Additionally, to test the isotropy of the cast film, we excited the sample with circular polarization at 405 and 514 nm and detected it at 665 nm (donor) and 670 nm (acceptor), respectively. We obtained low values of the P parameter of ∼3% for both excitation wavelengths, as was anticipated for the isotropic cast film. Figure 7 displays the deconvolution of the emission spectrum carried out using circularly polarized light with λex = 405 nm, at room temperature. Five peaks are present in emission spectra at 30 K. At room temperature (300 K), however, it was difficult to identify all peaks due to the increase in the thermal disorder and line width. As a result, four Gaussian fittings were used to

1 [A + B sin(2θ ) + C cos(4θ ) + D sin(4θ )] 2 (1)

where I stands for the electric field intensity, θ is the angle between the quarter wave plate and the fixed linear polarizer (vertical), A = S0 − (S1)/2, B = S3, C = −(S1)/2, and D = −(S2)/2. S0 is associated with the total intensity of emitted light, S1 describes the amount of light linearly polarized in vertical or horizontal directions, S2 describes the amount of linear polarization rotated by +45° or −45°, and S3 describes the existence of circularly polarized light to the right or left. The Stokes parameters can be easily associated with the polarization degree parameter (P) of the light by:14,25 P=

S12 + S22 + S32 S0

(2)

Figure 6 shows the data from EE measurements using excitation in the parallel direction and linear light polarization for excitations at 405 and 514 nm. Table 1 summarizes the results obtained in simulations based in eq 1 for the Stokes parameters corresponding to the emissions of LaPPS 37, using eq 2 with linear polarization for the excitation light. The data were analyzed for each situation in the corresponding emission wavelengths, λD. The polarization degree P was also calculated. Exciting the donor chemical group (fluorene) at 405 nm, it was observed that the emission relative to π* + 1 → π + 1 (∼525 nm) occurs in the same direction as the excitation with S1 < 0 corresponding to 40% of the total emission. Considering the lower values of others Stokes parameters, we can assume that 60% of the donor emission is in random direction due to intra13177

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The proposed photophysical model was used to build Table 3. As mentioned, the absorption species are only those aligned in parallel direction (laser polarization), and those that maintain that direction will emit accordingly, whereas those that lose coherence by transfer process, via electron phonon coupling, for instance, will emit in random directions. The latter are designed as “any other” donor, acceptors, or ππ stacking in the Tables. Considering the results of EE (Table 1) and the spectral deconvolution (Table 2), the percentages of energy migration of luminescent excitons due to the DA character and polymer/polymer interaction were estimated. For donor luminescence band D + hνex → D*, the donor emission contribution about 16.1% was obtained (Table 2), D* → D + hνDo. The EE (Table 1) determines that 60% of the donor emission is in random directions, as D* → D′* and D′* → D′ + hνDo processes, and 40% of donor emission proceeds in the same direction as the excitation (parallel, see Table 1 S1 parameter). Therefore, the photoexcited carrier migration was calculated as 0.161 × 0.60 × 100% = 9.7%. It was not possible to distinguish the percentage of donor energy migration D* → D′* due to inter- or to intrachain processes in parallel direction (same as excitation direction). The subsequent calculations for acceptor and molecular aggregates followed the same procedure described for the donor. These results imply that ∼81% of the total emission from LaPPS37 corresponds to the migration of the photoexcited carriers from donor to acceptor when the donor is excited (405 nm).The EE results showed that from that amount, 89% represent the emission in different directions from that of the excitation. Thus 72% of the acceptor emission proceeds in a random direction. In addition, only 2.8% of the emission was due to energy migration from donor and acceptor to molecular aggregates (ππ stacking). The results demonstrated in a quantitative way the strong energy migration processes in this DA system, minimum of 84.6% (9.7 + 72.1 + 2.8%), for luminescent photoexcited excitons from the donor to other sites. The analyses performed with excitation at 514 nm are displayed in Figure 8 and Tables 4 and 5. Considering the emission bands at 300 K (i) 583 nm (π* + 1 → π + 1), (ii) 684 nm (π* → π), and (iii) 771 nm ((ππ)* → (ππ)), the relative proportion of each radiative transition to the total emission was determined to be 6.5, 86.6, and 6.9%, respectively, as listed in Table 4. Considering only the photophysical assignment for

Figure 7. Photoluminescence spectrum of LaPPS37 at room temperature, photoexcited at 405 nm with circularly polarized light.

estimate the contribution of each species on the emission spectra. At 300 K it was difficult to identify the five peaks shown in Figure 4 at 30 K, and thus only the four main ones were taken into account. Considering the emission bands (i) 529 and 572 nm (π* + 1 → π + 1), (ii) 669 nm (π* → π), and (iii) 769 nm ((ππ)* → (ππ)), the relative proportion of each radiative transition to the total emission was determined to be 16.1, 80.9, and 3.0%, respectively; the relative values of the contribution of each transition are listed in Table 2. Table 2. Multi Gaussian Adjustment of LaPPS37 Unpolarized PL Spectrum (shown in Figure 6) Excited at 405 nma transition

XC (nm)

fwhm (nm)

A (a.u.)

π*+1 → π +1 + hνDo

529 572 669 769

49.0 31.9 105.8 25.1

19.6 5.9 128.0 4.7

π* → π + hνAc (ππ)* → (ππ) + hνpi

E (%) 16.1 80.9 3.0

a XC stands for the band center, fwhm for full width at half-maximum, A for the relative area per transition, and E(%) for the percentage for (i) π*+1→π+1 + hνDo: 529 and 572 nm, (ii) π*→π + hνAc: 669 nm, and (iii) (ππ)*→(ππ) + hνpi 769 nm emission bands. Do, Ac, and pi as subscripts stand for donor, acceptor, and ππ stacking species, respectively.

Table 3. Photophysical Assignment of the Transitions Probed with λex= 405 nm (Donor) process

photophysical description

D + hνex → D* D* → D + hνDo D* → D′* D′* → D′ + hνDo D* → A* A* → A + hνAc D* → A′*

absorption vibrational relaxation + fluorescence energy transfer vibrational relaxation + fluorescence donor−acceptor transfer vibrational relaxation + fluorescence energy transfer

A′* → A′ + hνAc D* → (ππ)* and D* → A* + A* → (ππ)* (ππ)* → (ππ) + hνpi D* → (ππ)′* and D* → A′* + A′* → (ππ)′*

vibrational relaxation + fluorescence transfer from donor and acceptor to ππ stacking vibrational relaxation + fluorescence energy transfer

(ππ)′* → (ππ)′ + hνpi

ππ stacking

13178

rate and assignment 16.1% total donor emission (Table 2) 60% energy transferred to any other donor species (Table 1) 0.6*0.161*100% = 9.7% emission from any other donor 80.9% total acceptor emission (Table 2) 89.0% transferred energy to any other acceptor species (Table 1) 0.89*0.81*100% = 72.1% emission from any other acceptor

3.0% total ππ stacking emission (Table 2) 93% transferred energy to any other π−π stacked sites (Table 1) 0.93*0.03*100% = 2.8% emission from any other ππ stacked sites dx.doi.org/10.1021/jp400823d | J. Phys. Chem. C 2013, 117, 13173−13180

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the acceptor, indicating that all of its emission comes from the energy transferred from the donor. This finding is in agreement with widely reported literature data for DA copolymers including experimental and theoretical work. Analyzing the emission polarization, we were able to propose a photophysical model and demonstrated for the first time how it is possible to quantify the energy migration processes to donor, acceptor, and molecular aggregated sites.



AUTHOR INFORMATION

Corresponding Author

*Tel: +55-41-3027-0650. Fax:+55-41-3361-3186. E-mail: leni@ leniak.net. Notes

The authors declare no competing financial interest.

Figure 8. Photoluminescence spectrum of LaPPS37 at room temperature, photoexcited at 514 nm with circularly polarized light.



ACKNOWLEDGMENTS Thanks to FAPESP, CNPq, and National Institute of Organic Electronics (INEO) for financial support and fellowships.

Table 4. Multi Gaussian Adjustment of LaPPS37 Unpolarized PL Spectrum (Figure 7) Excited at 514 nma transition

XC (nm)

fwhm (nm)

A (a.u.)

E (%)

π* + 1 → π + 1 + hνDo π* → π + hνAc (ππ)* → (ππ) + hνpi

583 684 771

44.1 93.8 28.1

8.6 116.0 9.3

6.5 86.6 6.9



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a

XC stands for the band center, fwhm for full width at half-maximum, A for the relative area per transition, and E(%) for the percentage for (i) π* + 1 → π + 1 + hνDo: 583 nm, (ii) π* → π + hνAc: 684 nm, and (iii) (ππ)* → (ππ) + hνpi: 771 nm emission bands. Do, Ac, and pi as subscripts stand for donor, acceptor, and ππ stacking species, respectively.

acceptor, Table 5 was built using the obtained data. As a result, exciting the acceptor, 67.0% of the luminescent photoexcited carriers transfer energy to different chromophores. It is interesting to note the decrease in the percentage of carriers’ migration in the plane of the film to acceptor thiophene− benzothiadiazole−thiophene chemical group, ∼20%. This is most probably due to the low absorption of fluorene at that excitation wavelength.



CONCLUSIONS The results demonstrate that the great majority of the formed excitons are donating species able to transfer their energy to acceptors in random directions. These processes depend on the excitation wavelength and can occur via inter- or intramacromolecular pathways. In the present case, the transfer from the donor D (fluorene) to the acceptor A (thiophene− benzothiadiazole−thiophene) represents 81% of the total emission (Table 2) when the donor is photoexcited. Moreover, exciting the sample at 405 nm, there is not direct absorption of

Table 5. Photophysical Assignment of the Transitions Probed with λexc = 514 nm (Acceptor) process A + hνex → A* A* → A + hνAc A* → A′* A′* → A′ + hνAc A* → (ππ)* (ππ)* → (ππ) + hνpi A* → (ππ)′* (ππ)′* → (ππ)′ + hνpi

photophysical description absorption vibrational relaxation + energy transfer vibrational relaxation + transfer to ππ stacking vibrational relaxation + energy transfer vibrational relaxation +

rate and assignment

fluorescence fluorescence fluorescence fluorescence 13179

86.6% total acceptor emission (Table 3) 70.0% transferred energy to any other acceptor (Table 2) 0.70*0.866*100% = 60.6% emission from any other acceptor 6.9% total ππ stacking emission (Table 3) 92.0% transferred energy to any other ππ stacking (Table 1) 0.92*0.069*100% = 6.4% emission from any other ππ stacking dx.doi.org/10.1021/jp400823d | J. Phys. Chem. C 2013, 117, 13173−13180

The Journal of Physical Chemistry C

Article

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