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Energy Transfer Processes in Donor-Acceptor Poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) Eralci Moreira Therézio, Paula C. Rodrigues, José Roberto Tozoni, Alexandre Marletta, and Leni Akcelrud J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400823d • Publication Date (Web): 04 Jun 2013 Downloaded from http://pubs.acs.org on June 4, 2013

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The Journal of Physical Chemistry

1

Energy Transfer Processes in Donor−Acceptor Poly(fluorenevinylenealt-4,7-dithienyl-2,1,3-benzothiadiazole) E. M. Therézio1, Paula C. Rodrigues2,3, José R. Tozoni1, Alexandre Marletta1, Leni Akcelrud2* 1

Physics Institute, Federal University of Uberlandia, CP 593, CEP 38400-902,

Uberlandia - MG, Brazil 2

Paulo Scarpa Polymer Laboratory (LaPPS) Federal University of Parana, CP 19081,

CEP 81531-990, Curitiba- PR, Brazil 3

Institute of Physics, São Paulo University, CP 369, CEP 13084-971, São Carlos-SP,

Brazil *

Corresponding author. Tel:+55-41-3027-0650; fax:+55-41-3361-3186;

e-mail:[email protected]

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],

in

order 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.

Keywords: energy transfer, donor-acceptor copolymer, emission ellipsometry

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2 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 single molecule 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 which 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 the polymer electronic structure. The origin of the well-separated 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 to this important ACS Paragon Plus Environment

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3 subject using the emission ellipsometry 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 – fluoreneylenevinylene – alt - 4’,7’ - (di - 2,5 - thienyl) 2’,1’,3’ benzothiadiazole] (LaPPS37), depicted in Figure 1. S S

n N

N S

Figure 1. Chemical structure of LaPPS37.



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4 EXPERIMENTAL SECTION

(i)

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.

(ii)

Chemical Procedures. LaPPS37 was .prepared using a combination of Suzuki and Wittig reactions

(Figure 2). Briefly, the compound (3) was prepared following the procedures described elsewhere12 and the dialdehyde monomer was also synthesized according to published procedure.13 The polymerization was run as follows: to a one neck round bottom flask (1.7 mmol) of 2,7-bis[(p-triphenylphosphonium) methyl]-9,9´-dioctylfluorene (3) and (1.7 mmol) and 5,5'-(2,1,3-benzothiadiazole-4,7-diyl)dithiophene-2-carbaldehyde (4) were added, under an argon atmosphere. To the mixture, a solution of t-BuOK in anhydrous ethanol (8.5 mmol) was then added, and the reaction allowed proceeding for two days at room temperature. After that 5 mL HCl 5% was added and the polymer was precipitated by pouring the mixture into an excess of methanol. The polymer was re-precipitated 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. 1

H 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).


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5

BuLi, C8H 17Br Et2O,

33% HBr in CH 3COOH

-80oC

BrH 2C

CH 2Br

paraformaldehyde

(1)

PPh 3,

(2)

(2)

P H 2C

CH 2P

DMF, 110oC

(3)

O O Br 2, HBr, reflux

Br

Br

OH S

OH N

N

N

Na 2CO3, PdCl2(PPh3) 2

S

B EtOH, Benzene, Aliquat

H

N

N

O

S

S

S N S

(4)

(3) + (4)

S

CHCl3, t-BuO

S

n N

N S

Figure 2 – Synthetic route for LaPPS37 preparation

(iii)

Sample The cast film was obtained by dripping 100 µL of LaPPS37 (0.5 mg in 5 mL

toluene) onto a quartz plate (1x2 cm2), thoroughly washed with detergent and ultrapure ACS Paragon Plus Environment

H


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6 water, rinsed with ultrapure water and chromatographic grade toluene and dried with argon flux.

(iv)

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

tetrahydrofurane (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.

(v)

Spectroscopic characterization UV-Vis absorbance measurements were performed using the spectrophotometer

FEMTO 800 XI. Photoluminescence (PL) spectra were carried out exciting the samples with the radiation at 514 nm (cut-off filter 550nm - Newport OG 550) of an Ar+ laser (Coherent Innova 70C) and at 405 nm (cut-off filter 455nm -Newport GG 455) of a diode laser (LaserLine-IZI) mantled in a He closed cycle cryostat under vacuum (1.3 x 10-2 mbar). Linear and circular polarization of the excited light were carried out introducing an achromatic linear polarizer (Newport 10LP-VIS-B) and an achromatic quarter-wave-plate (Newport 10RP54-1) in the laser output path. The sample temperature ranged between 30K and room temperature (290K). The emission signal was analyzed with a spectrophotometer

USB4000

Ocean

Optics.

For

emission

ellipsometry

(EE)

measurements, an achromatic quarter-wave-plate (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 (0º) to 6.28 rad (360º) with steps of ~0.17 rad (10º). Details of the experimental set up were published elsewhere.14


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7 RESULTS AND DISCUSSION

Figure 3 displays the absorption and emission spectra of LaPPS37 in thin film form using non-polarized light. The absorption line is characterized by three bands at 405, 555, and 670nm.The first two were attributed to the non localized transitions π→π*+1, π→π* corresponding to the charge transfer 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 ground state of molecular aggregation. The assignment of redshifted transitions to macromolecular association is widely spread in the literature on conjugated polymers. The formation of dimers (or higher forms) in the electronic ground or excited states15-17 H or J molecular aggregates or still the presence of the so called β-phase have been reported.18 The state of aggregation and the macromolecular conformations associated to it can vary with temperature and excitation wavelength altering the emission profile, turning the assignment of the emitting center(s) in each condition very complex. This issue has been approached in the case of regioregular P3HT which spectrum presented well defined progression peaks.19 However, the poor 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 approximately at 525, 670 and 760 nm regions attributed to fluorene (donor), thiophene-benzodiathiazole-thiophene (acceptor), and molecular aggregates (ππ), respectively; either with excitation at 405nm or 514nm. 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 405nm 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 by the contribution of donor and acceptor bands, switching the excitation from 405 nm to 514 nm will bring about a

shift in its spectral mass center , characterizing not a 'true"

redshifting, 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 ACS Paragon Plus Environment


8 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, since 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 which is the main issue of the present contribution.

0.04 T=290K

0.01

0.00 300

400

PL (a.u.)

0.02

λ ex=514nm

λ ex=405nm

0.03

Absorbance

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500

600

700

800

900

Wavelength (nm) t

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).

Figure

4

shows

the

temperature

dependence

(30–290

K)

of

the

photoluminescence, analyzed in parallel (Par.) and perpendicular (Per.) directions (according to an arbitrary laboratory referential). The sample was excited with linear


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9 polarized light at 405 nm (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, due to 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 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, since this is a lower energy site (~5 to 30 meV) 22 and that the laser used had power enough (25meV) to destabilize the molecular aggregate when excited in the parallel direction. On the other hand 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 4 b), 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 redshifted by 15nm, 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 to the band at 694 nm (acceptor) decreases slower than the ratio observed in Figure 4a. This is due to the fact that with excitation at 514 nm the contribution of the transition π*+1→π is smaller than with excitation at 405 nm, since 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.



10

Par. Per.

(a) λex=405nm

Par. Per.

(b) λex=514nm

290 K

290 K

270 K

270 K

240 K

240 K

210 K

210 K

PL (a. u.)

PL (a. u.)

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180 K 150 K

180 K 150 K

120 K

120 K

90 K

90 K 60 K

60 K

30 K

30 K

400

500

600

700

800

Wavelength (nm)

900

600

700

800

900

Wavelength (nm)

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

For better visualization, the results described above 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 those brought about by the excitation at 514 nm, which was expected, since with the former all the chromophores are excited; the emission in the parallel direction for both excitations, 405 and 514 nm was always higher than in the perpendicular direction; and finally, the emission with excitation at 405 nm is more sensitive to the temperature with a different dependence for the parallel and perpendicular directions at lower temperatures. This effect is smoothed out at higher temperatures; and the emission coming from excitation at 514 nm either in the parallel or perpendicular directions shows little dependence with temperature, and both have about the same intensity.


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11

1.2 λex = 405nm

1.0 Parallel

0.8

PL (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.4

Perpendicular λex = 515nm

Parallel

0.2 Perpendicular

0.0 50

100

150

200

250

300

Temperature (K) Figure 5. Photoluminescence intensity as a function of the sample temperature 30 – 290 K analyzed in parallel and perpendicular directions and photo-excited at 405 nm and 514 nm.

In a previous publication

23

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 emission ellipsometry 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, that 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 in these conditions, unless other photophysical events take place in the excited state, such as energy migration or transfer processes. In the later case some of the new emitting ACS Paragon Plus Environment


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12 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 emission ellipsometry technique in the photoluminescence (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 Briefly, in the emission ellipsometry 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 the method S0, S1, S2, and S3 are obtained considering equation (1). 14,25

2 4 4

(1)

where I stands for the electric field intensity, θ 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 +45º or -45º, and S3 describes the existence of circularly polarized light to the right or left. The Stokes parameters can be easily associated to polarization degree parameter (P) of the light by: 14,25

(2)

Figure 6 shows the data from emission ellipsometry measurements using excitation in the parallel direction and linear light polarization, for excitations at 405 and 514nm. Table 1 summarizes the results obtained in simulations based in equation (1), for the Stokes parameters corresponding to the emissions of LaPPS 37, using equation (2) with linear polarization for the excitation light. The data were analyzed for each


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13 situation in the corresponding emission wavelengths, λD. The polarization degree P of 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 of the excitation with S10. The emission in the perpendicular direction is a signature of the migration energy from the donor, since direct excitation can only produce emission in the same direction of the incident light. The (ππ)*→(ππ) (~760 nm) transition presents a totally random emission, with all the parameters around zero. On the other hand, 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

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