Random Structural Modification of a Low-Band-Gap BODIPY-Based

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Random Structural Modification of a Low Band Gap BODIPY-Based Polymer Léo Bucher, Shawkat M. Aly, Nicolas Desbois, PaulLudovic Karsenti, Claude P. Gros, and Pierre D Harvey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00117 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Physical Chemistry

Random Structural Modification of a Low Band Gap BODIPY-Based Polymer Léo Bucher,a,b Shawkat M. Aly,a Nicolas Desbois,b Paul-Ludovic Karsenti,a Claude P. Gros,b* and Pierre D. Harveya* a

Département de Chimie, Université de Sherbrooke, 2500, Bd de l’Université J1K 2R1

Sherbrooke, QC, Canada b

Université de Bourgogne Franche-Comté, ICMUB (UMR CNRS 6302), 9, Avenue Alain

Savary, BP 47870, 21078 Dijon Cedex, France Submitted to J. Phys. Chem.

ABSTRACT A BODIPY-thiophene polymer modified by extending conjugation of the BODIPY chromophore is reported. This modification induces tunability of energy levels and therefore, absorption wavelengths, in order to target lower energies.

ACS Paragon Plus Environment

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INTRODUCTION Over the past decade, organic conjugated polymeric materials have received considerable attention due to their excellent electrical and optical properties.1, 2 Interestingly, conjugated polymeric materials can provide optimized devices with flexibility, costeffectiveness and easy processability and therefore combine their excellent processing advantages and mechanical properties with the desired electrical, optical, electronic and magnetic properties of metals and semiconductors. For these reasons they are interesting candidates for alternative or specific applications in solar cells.3-6 Nowadays, among polymer solar cells (PSCs), bulk heterojunction (BHJ) devices exhibit the most efficient structure.7, 8 These devices, consisting of a blend between electron donor and electron acceptor materials, have been used to achieve high Power Conversion Efficiency (PCE) bringing them closer and closer to the efficiency goal that would allow them to be industrially interesting.9, 10 Conjugated polymers need to present suitable HOMO and LUMO energy levels leading to low band gaps approaching those of the materials used for the cell design, broad and strong absorption, good film forming property, and high hole mobility in polymer blend. Since about 54% of the sunlight energy is distributed in the visible region from 380 to 800 nm, an ideal active layer for a polymer solar cell should display a broad and strong absorption spectrum in this range and especially in the near-infrared (NIR) window (> 750 nm) where the photon flux is at maximum.9 Several methods have been used to shift absorption toward lowest energies such as the use of push-pull type copolymer, involving alternated electron rich (donor) and electron deficient (acceptor) units.11-18 BOronDIPYrromethene (BODIPY) is an interesting chromophore with very impressive optical properties, such as very high molar extinction coefficients and fluorescence quantum yield.19,

20

Moreover it shows a good photo- and thermal stability that make it

suitable candidate for photovoltaic applications. A well-known pathway to extend BODIPY conjugation is a Knoevenagel-like reaction that has already been used to go further in NIR.2123

For instance, it has been used many times to reach the therapeutic window generally

targeted in medical imaging.24-26 Thiophene has also been exploited in many very efficient semi-conducting conjugated polymers for applications in OLED or showing high PCEs in photovoltaics, making it as an ideal unit to use in these organic polymers.1,

27-32

Several

studies were reported using BODIPY-thiophene conjugates in small molecules or in polymer solar cells.33-44 Promising results were found there, and BODIPY-based polymers have recently shown quite high charge transport properties, but this chromophore hasn’t prove its whole efficiency in OPV yet.45


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In this investigation, post-functionalized conjugated BODIPY-thiophene-containing polymers in pseudo-doping manner are prepared as well as a series of model compounds (Chart 1). Both M1 and M2 represent small molecule analogs for P1 and P2 respectively, and will help, as well-defined structures, to better understand the polymers behavior. Indeed, the BODIPY moiety in a BODIPY-thiophene copolymer is randomly extended in order to generate a wide absorption shifted to the NIR region. The combination of unfunctionalized, semi- and bi-functionalized BODIPY units in the polymer backbone results in an overlay of absorption spectra designed to harvest as much photons as possible. In this report, the synthesis and characterization along with a full photophysical study of these conjugated copolymers in solution and as thin films are presented, in order to find out mechanisms and kinetics which are involved within donor material from the photon absorption until the loss of an electron to the benefit of the fullerene acceptor. Polymers

S

S

n

N

N B F F

Z

R1

P1

N B F F

n

N

R1

Z

P2

Models S

S

R2

N B F F

R2

S

R2

N

R2

R2

S N

R2

M1

R2

N B F F

R2

M2

R1 = Z=

CH3 or

R2 =

Chart 1: Structures of polymers P1 and P2 and models M1 and M2.

RESULTS AND DISCUSSION Synthesis and characterization. The syntheses of BODIPYs 1-3 have been previously reported and were used with a slight modification.46-48 After deprotection of the silyl groups of compound 3 with TBAF at 0 °C for 15 min, monomer 4 was obtained in 82% yield (Scheme 1). Noteworthy, longer reaction time or higher temperatures (as already noticed at rt) leads to slow decomposition of the BODIPY product. Thiophene 5 (scheme 2) was synthesized by halogenation with N-iodosuccinimide under acidic conditions as described in the litterature.49 Polymer P1 was obtained by palladium-catalyzed Sonogashira coupling


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between

equivalent

amount

of

4

and

5

(Scheme

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3)

in

the

presence

of

tetrakis(triphenylphosphine)palladium(0), copper(I) iodide and triethylamine in dry THF.

i.

+

2

ii. N

N H O

I

I

N

N

N

B

B

F F

F F

1

2 iii.

H

H N

iv.

Si

Si

N

N

B

N B

F F

F F

4

3

Scheme 1: Synthesis of the diethynyl-BODIPY 4. i. 1) TFA, DCM, rt, 7 h 2) p-chloranil, rt, 4 h 3) Et3N, BF3/Et2O, rt, 3 h; ii. N-iodosuccinimide, DCM, rt, 2 h; iii. Pd(PPh3)4, CuI, dry THF, dry Et3N, ethynyltrimethylsilane, 50 °C, 5 h; iv. TBAF, DCM, 0 °C, 15 min.

The thermal gravimetric analysis (TGA) showed a decomposition temperature of 390 °C for P1 and 230 °C for P2 (Figure S20), implying a quite high thermal stability. P1 undergoes a Knoevenagel condensation between its a-methyl and mesitaldehyde in order to obtain P2. This reaction takes place in hot toluene (85 °C) under reduced pressure (300 mbar) in order to remove any water (as byproduct). The reaction was monitored by MS and TLC. It appears that the condensation reaction did not occur at all 3- and 5-methyl positions of each BODIPY unit. This is probably due to some steric hindrance in the polymeric chain, leading to a mixture of mono-, di- and un-substituted BODIPYs. Interestingly, this feature is highly desirable to broaden the absorption spectrum of the resulting polymeric material while concurrently shifting it towards the NIR region. This synthetic feature can be seen as a pseudo “doping” effect on P1 where the starting polymer (P1) has been randomly and partially functionalized. S

i.

R1

R1

R1 =

i.

R1

S

S

ii. Br

i. R2

R2

6

I

5 S

I

R2 =

Br

S

I

8 S

I

R2

R2

7

Scheme 2: Syntheses of thiophene derivatives 5-8. i. N-iodosuccinimide, CHCl3/AcOH, rt, 8 h (for 5) or 12 h (for 7 and 8); ii. 2-(ethylhexyl)magnesium bromide 1 M in Et2O, Ni(dppp)Cl2, dry THF, reflux, 12 h.


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4

+

5

i.

S N

N

n

B F F

R1

P1 Z=

CH3 or

ii.

R1 = S N

N Z

B F F

n Z

R1

P2

Scheme 3: Synthesis of polymers P1 and P2. i. Pd(PPh3)4, CuI, dry Et3N, dry THF, 60 °C, overnight; ii. mesitaldehyde, piperidine, p-toluenesulfonic acid, dry toluene, 85 °C, 300 mbar, 2 h.

The amount of functionalized BODIPY units has been evaluated by 1H NMR spectroscopy by comparing the integrations of the aromatic protons (as exemplified in Figure 1) using the two H-resonances at the meta positions of the meso-mesityl group of BODIPY (7.00 ppm) and the four H’s on the double bonds formed during the reaction (8.40 and 7.41 ppm). By comparing these integrations with the total integration for the peaks at 6.93 and 6.91 ppm corresponding to the proton at the 4-position of the thiophene unit and the four newly added H’s for the mesityl groups, respectively, the number of added styryl arms per α-methyl group is found to be equal to 0.8 (i.e. showing that 80% of these sites have been functionalized).

Figure 1: Comparison between aromatic regions in 1H NMR (300 MHz, CDCl3) spectra of P1 and P2.


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Models M1 and M2 were prepared for interpretation purposes of the optical and photophysical properties of the new polymers. In these cases, the synthesis pathway is also starting from BODIPY 4 but involves the use of mono-halogenated thiophene unit (Scheme 4). The symmetric thiophene 6 (bearing alkyl chains on both 3 and 4 positions) was conveniently used instead of 3-octylthiophene and one of its a-position was substituted with iodine in presence of 1 equivalent of N-iodosuccinimide to obtain 7. The bis(3octylthiophene) compound 9 have also been synthesized from compound 8 in order to verify if alkyl chains have any influence on the optical and photophysical properties. Even if compound 8 was previously described in literature, its synthesis was found very tedious, especially the purification step.50 Indeed, the complete separation of the mono-iodo product 8 from starting material turned out impossible. Furthermore, the formation of the second possible mono-iodo isomer (i.e. with iodo group at opposite side from alkyl chain) was not observed. Fortunately, starting material did not react with BODIPY 4 in the synthesis of 9. By assuming that the presence of two alkyl chains on the thiophene moiety does not affect the photophysical data in comparison with those for only one chain (as illustrated in compound 9 and polymers P1 and P2), then the M1 and M2 derivatives appear as appropriate models. Several attempts were made in order to obtain the mono-styryl derivative as another potential model compound but the isolation of such a species remained elusive.

8

S

i.

S N

N B F F

R1

R1

9

4

+

7

S

i. R2

S N

R2

N B F F

R2

R2

M1

R1 =

ii.

R2 =

S

S

R2

N

R2

R2

N B F F

R2

M2

Scheme 4: Synthesis of 9 and model compounds M1 and M2. i. Pd(PPh3)4, CuI, dry Et3N, dry THF, 60 °C, overnight (for 9) and 4 h (for M1); ii. mesitaldehyde, piperidine, p-toluenesulfonic acid, dry toluene, 85 °C, 300 mbar, 1.5 h.


6

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The mono- and bis-adducts for this kind of condensation are usually easily performed and separable. Unexpectedly, it appears that the mono-styryl intermediate reacts faster than the starting BODIPY-containing materials leading preferentially to the isolated di-styryl derivative. An explanation would be that the acidity of the remaining protons at the 3- (or 5-) position is exacerbated due to involvement of the first styryl arm in electron delocalization pathway of the molecule, making the carbon-hydrogen bond even more electron deficient. At this point, the combination of model compounds M1 and M2 appears appropriate and sufficient as model compounds for P1 and P2.

Figure 2: Normalized absorption spectra of BODIPY-containing materials 1 (orange), 4 (purple), P1 (blue) and P2 (green) at 298 K in 2-MeTHF.

Optical and electrochemical properties. The UV-vis absorption spectra of 1, 4, P1, and P2 in 2-MeTHF exhibit a strong low-energy signal corresponding to the characteristic S0 → S1 transition of the BODIPY chromophore (Figure 2). The expected red-shifts upon extending the conjugation of the core, i.e. 1 → 4 → P1 → P2, are unambiguously observed. Specifically, the addition of two ethynyl groups at 2 and 6 positions moves the maximum absorption from 501 to 541 nm. The co-polymerization with thiophene generates P1, which induces a red-shift of the maximum by another 87 nm. Finally, the addition of styryl substituents shifts the band to 688 nm. So, these modifications led to an overall 189 nm shift. Figure 3 compares experimental absorptions between P1 and P2 with corresponding model compounds M1 and M2 at room temperature both in solution and in thin film. In each case, maxima between polymers and corresponding models are shifted about 50 nm (M1 → P1) and 30 nm (M2 → P2).


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These observations are expected since conjugated character of polymers allows large electronic delocalization all along the backbone that leads to a decrease of the band gap and this bathochromic displacement. This is therefore a clue for an efficient communication between units through the polymer.

Figure 3: Absorption spectra of M1 (dashed blue), M2 (dashed green), P1 (solid blue) and P2 (solid green in 2-MeTHF (top) and in thin film (bottom) at 298 K.

The absorption spectra of P1 and P2 exhibit weaker bands at higher energy at ~ 400 nm, and are readily attributed to S0 → S2 transitions. It is noteworthy that the absorption in thin films presents band broadening and slight red-shift compared to solution. This highlights interchain interactions between polymer backbones at the solid state. Interestinlgy, this phenomenon is highly desirable since it will contribute to organization and good morphology inside the BHJ, as well as better charge mobilities, that would lead to better performances of the photovoltaic device.51, 52


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Figure 4: Representations of HOMOs and LUMOs for M1 and M2 with corresponding calculated energy levels in THF.

DFT calculations were carried out at the B3LYP/6-31g*, and the geometry optimization (Figure 4) shows that LUMO orbitals of both M1 and M2 are mainly localized at the BODIPY core while HOMO populates more largely ethynyl bridge and thienyl rings, demonstrating a charge transfer behavior and the role of adding the thiophene units in order to shift absorption toward NIR. The addition of styryl arms reduces theoretical band gap (from 2.43 to 2.08 eV) which is in good agreement with the shift to low energies noticed from optical properties. Table 1 presents main transitions extracted from TD-DFT calculations and indicates that the main absorption bands for M1 and M2 (600 - 700 nm) are due to HOMO → LUMO transitions. Transitions of higher energies (400 - 500 nm) come almost fully from BODIPY core as well.

Table 1: Calculated positions of the electronic transitions in visible window and major contributions for M1 and M2

Wavelength (nm) 595 506 409

M1 Major contributions HOMO→LUMO (98%) H-1→LUMO (99%) H-2→LUMO (91%)

Wavelength (nm) 676 519 446 409


M2 Major contributions HOMO→LUMO (98%) H-2→LUMO (97%) H-3→LUMO (92%) HOMO→L+1 (91%)

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Figure 5: Experimental absorption spectra of M1 (dashed blue) and M2 (dashed green) at 298 K in 2-MeTHF superimposed with their corresponding calculated spectra (black).

It has to be noted that in measured absorption spectrum, M2 presents a band around 480 nm, which is not there for M1, that could be attributed to styryl arms. Indeed, if we look at the comparison with computed absorption spectra presented in Figure 5, this new band at 446 nm can be observed. This H-3 → LUMO transition clearly corresponds to the movement of electronic population from styryl moiety to the rest of BODIPY core (Figure S46). More data about frontier molecular orbitals and computed oscillator strength are given in the SI (Figures S37-S39 and S43-S45, Tables S2-S5).

Figure 6: Reduction waves (top) and oxidation waves (down) from cyclic voltammograms of P1 (blue) and P2 (green) polymers in thin films at a scan rate of 50 mV.s-1 in 0.1 M TBAPF6/MeCN solution.


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Electrochemical properties were also investigated. Cyclic voltammograms in thin films are presented in Figure 6 where both P1 and P2 show irreversible oxidation and reduction behavior at potentials around -1.0 V and +0.5 V vs Fc+/Fc which correspond to BODIPY unit. In comparison with BODIPY small molecules, characterized by sharp and reversible oxidation and reduction, each wave is very broad, which is typical with long polymeric material. The oxidation and reduction onsets, along with absorption wavelength onset are summarized in Table 2. The electrochemical gaps thus obtained are in good agreement with optical gaps estimated from the onset of the absorption spectra in thin films with values of 1.77 eV for P1 and 1.63 eV for P2. As band gaps are under 2 eV, these polymers are considered as being low band gap polymers and suitable to reach high PCEs. Actually, this structural modification of P1 leads to several improvements for the potential of the polymer that follows what have been previously described as guidelines in the literature.9, 10

In one hand the band gap is reduced, allowing to absorb more in the NIR window, until

800 nm where number of photons is at maximum.53 In the other hand, we can notice in Figure 7 that if the HOMO energy is almost unchanged, the LUMO of the donor P2 (-3.73 eV) is brought closer to the LUMO of the PC61BM acceptor which is favorable for efficient electron transfer since the theoretical optimum energy between them should be 0.3 eV.6 We would so expect an improvement in exciton dissociation inside the active layer of the BHJ from P1 to P2.

Table 2: Optical and electrochemical properties of the P1 and P2 polymers in thin films 𝒇𝒊𝒍𝒎

𝝀𝒐𝒏𝒔𝒆𝒕 (nm)a

b b 𝑬𝑶𝒙 𝑬𝑹𝒆𝒅 𝒐𝒏𝒔𝒆𝒕 (V) 𝒐𝒏𝒔𝒆𝒕 (V)

𝒐𝒑𝒕

𝑬𝒈 (eV)

𝑬𝒆𝒍𝒆𝒄 𝒈 (eV)

HOMO(eV)

LUMO(eV)

P1

712

0.59

-1.18

1.74

1.77

-5.39

-3.62

P2

770

0.56

-1.07

1.61

1.63

-5.36

-3.73

a

b

Determined from drawn tangent lines to the absorption lowest energy and baseline. Determined from the onset potentials of oxidation and reduction peaks.


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Figure 7: Diagram of energy levels HOMO and LUMO of synthesized polymers P1 and P2, and LUMO of PC61BM acceptor. Calculated with data extracted from cyclic voltammograms and absorption spectra for P1 and P2 and from literature for PC61BM.54

Fluorescence properties were then investigated. Emission spectra with corresponding absorption are presented in Figure 8. Emission are slightly red-shifted for both P1 and P2 about 50 nm, 20 nm and 80 nm, at 298 K, 77 K and in thin film respectively. These Stoke shift values are comparable with BODIPY small molecules. Both absorption and emission bands notoriously shift to the red side of the spectra upon cooling to 77 K and especially at solid state. The fluorescence lifetimes measured by TCSPC (Time-correlated Single Photon Counting) method at 298 K and 77 K in 2-MeTHF are summarized in Table 3.

Figure 8: Absorption (solid line) and emission (dashed line) of P1 (blue) and P2 (green) at 298 K and 77 K in 2-MeTHF and in thin film.


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Table 3: Fluorescence lifetimes of compounds in 2-MeTHF determined by TCSPC 𝝺ex (nm)

298 K

𝝺em (nm)

4

378

77 K 378

M1

443

443

650

630

M2

633

633

710

710

P1

378

378

660

665

P2

378

378

720

720

a

298 K 560

77 K 560

𝝉 ± ∆ (ns) [cont.]a

𝜒2

298 K

77 K

298 K

77 K

5.53 ± 0.30 [100 %] 1.62 ± 0.20 [100 %] 2.84 ± 0.22 [100 %] 0.22 ± 0.03 [15 %] 0.71 ± 0.14 [85 %] 0.18 ± 0.02 [14 %] 0.56 ± 0.04 [15 %] 1.31 ± 0.16 [71 %]

6.41 ± 0.46 [100 %] 3.07 ± 0.37 [100 %] 3.56 ± 0.23 [100 %] 0.72 ± 0.10 [10 %] 1.23 ± 0.12 [90 %] 0.16 ± 0.02 [6 %] 0.76 ± 0.10 [20 %] 1.72 ± 0.34 [74 %]

1.081

1.032

1.051

1.034

1.063

1.010

1.088

1.017

1.028

1.010

contribution of the component

All lifetimes increase when at low temperature because structures are immobilized and radiative relaxation becomes predominant. Fluorescence lifetime for M1 is lower than those of other 2,6-diethynyl BODIPY described in literature, this could be attributed to the presence of the thienyl units and a possible heavy atom effect coming from sulfur that would lead to increase intersystem crossing (ISC) and lower the lifetime.55 It is noticed that lifetimes are shorter for polymers (P1, P2) compared to models (M1, M2), and it is easily understandable since even though these polymers are quite rigid, the backbone still enables some vibrations in the molecule, and further non-radiative deactivation processes. We can notice that lifetime is longer when styryl are added compared to initially, from M1 to M2, and the same behavior is found again from P1 to P2. P2 presents a nanosecond component and several hundreds of picoseconds components. As expected, behaviors of M1 and 9 were found very similar (cf. comparison between M1 and 9, Figure S49 and Table S6 in SI) and changing alkyl chains on the thienyl part doesn’t seem to influence optical properties. The use of long solubilizing chains on the BODIPY has been minimized in order to prevent or diminish any excited state


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non-radiative deactivation processes. Photophysical properties have not been often welldescribed for BODIPY-thiophene copolymers. Few papers have presented good characterization but there is still a lack for data at 77 K or even in solid state, particularly concerning lifetimes and kinetics.39, 40 This information should not be neglected since long lifetime excited state could be related to longer exciton migration and lead preferentially to exciton dissociation then creation of an electric current. In our case, it was difficult to find out time-resolved properties on thin films with TCSPC method. Because solid state reflects reality of what is happening inside the solar cell, we carried out femtosecond transient absorption experiment, which is more suitable for this configuration in solid state. Then we investigated changes in lifetimes as function of the ratio of polymer/fullerene in the BHJ.

Transient absorption (TA). The TA spectra of thin films for P1 and P2 collected in absence and presence of PC61BM acceptor after photoexcitation with ~100 fs laser at lex of 640 nm have been measured. Figure 9 shows the collected TA spectra for neat P1 and P2 films at different delay times.

Figure 9: Transient absorption (TA) of P1 and P2 thin films after photoexcitation with ~100 fs laser at lex of 640 nm; delay times are indicated on curves.


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TA spectra collected for thins films of P1 and P2 shows broad signal mostly covering the entire spectrum 380 – 1100 nm range. Both TA spectra have one main positive differential transmittance (DT/T) band with maximum at 650 and 750 nm for P1 and P2, respectively. These two bands can be attributed to the ground state bleach (GSB) as indicated from the corresponding ground state absorption given in Figure 8. Spectra exhibited also the S1-Sn signals, i.e. excited state (ES) photoinduced absorption, where those for P1 came as negative DT/T band at 515 nm whereas that for P2 came as doublet at ~ 480 and 550 nm. Furthermore, the ES showed some weak features in the near IR range of the spectra where a long tail with maxima at 900 nm for P1 while weak shoulder recorded for P2 over the same spectral range. Although no significant changes recorded in TA spectra collected for thin films of the polymers blends with PC61BM yet one clear band detected as negative DT/T band at about 380 nm which has no record in the free polymer film’s TA. In order to investigate the origin of this band TA spectra of PC61BM collected under the same experimental conditions and laser excitation where an observable TA signal detected at 380 nm. In line of the experimental results we assign the 380 nm TA signal for photoinduced absorption of the PC61BM acceptor based on resemblance with that collected for alone, see Figure 10.

Figure 10: TA of thin films: a) free PC61BM and blends of polymer:PC61BM (1:3): b) P1 and c) P2. TA collected with photoexcitation at lex of 640 nm with ~100 fs laser; delay times are given on curves.


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Kinetic profiles associated with the TA signals fitted by global analysis using Glotaran software where three major components were estimated from GSB for P1 and P2; see Table 4. The very fast lifetime for these films can be assigned to exciton-exciton annihilation which is common for solid films.56 By comparison with the fluorescence lifetimes given in Table 3, we can assign the longer lifetimes for the S1 lifetime. It is worth mentioning here that all fitting traces exhibited a minor component (~10%) of lifetime turned out to be beyond the maximum detection of the time delay for our experimental set up, i.e. 7.5 ns, which can be assigned for triplet state lifetime. In order to address the interaction with PC61BM, further measurements were carried out for P1 and P2 in their blends using this acceptor; see Figure 11.

Figure 11: Kinetic traces from TA of thin films: a) Excited state decay at lmon = 380 nm of PC61BM free (black) and in blend of 1:3 with P1 (blue) and P2 (green); b) ground state bleach at lmon = 640 nm of P1 free (black) and blend of 1:3 with PC61BM (green); and c) ground state bleach at lmon = 740 nm of P2 free (black) and blend of 1:3 with PC61BM (green). TA collected with photoexcitation at lex of 640 nm with ~100 fs laser; red lines showing fitted curves.

From the kinetic trends collected for the polymers in their PC61BM blends it is evident that GSB gets slower in presence of the acceptor as compared to traces collected for the free polymer films. On the other hand, comparison of the ES decay kinetics at the 380 nm band, assigned for PC61BM, we clearly notice a faster decay profile for the polymer/PC61BM blends. Accordingly, we can assume that in these blends a photoinduced electron transfer takes place from the polymer to the PC61BM acceptor which in turn yield the radical ions, i.e. polymer+•/PC61BM‒•. This radical formation can account for the significant change detected in the ES lifetime of PC61BM as well as the change in kinetics of GSB the polymers in the


16

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blends. Indeed, this electron transfer action is thermodynamically feasible taken in account the energy level diagram given in Figure 7. Furthermore, confirmation of charge separated state formation came from analysis of the TA signal for the ES of the polymer in PC61BM blends evolution with time delay as shown in Figure 12. While signal collected at early time after excitation match in position that of the free polymer, increasing delay times shift for TA signal to higher wavelengths. Further shift detected as delay time extended for even longer delay time for the case of both P1/ and P2/PC61BM blends.

Figure 12: Comparison of TA evolution for polymer/PC61BM (1:3) blend: a) P1, and b) P2; after photoexcitation at lex of 640 nm with ~100 fs laser; grey curves show TA for free polymer thin film. Delay times at which TA signal collected are given on graph.

Such spectral changes in the TA spectra can be correlated to the induced interaction with the PC61BM, i.e. photoinduced electron transfer.57-59 Accordingly, we can anticipate the similarity detected at early delay time due to the development of the photoinduced absorption of the polymer (S1-Sn) which in turn, with the PC61BM acceptor in the vicinity, yield the tight or electrostatic-bonded radical pair leading to the first observed spectral shift. Finally, a complete charge separation occurs as indicated by the final spectral shift manifested at the long delay times. Upon examine the TA signal of P2/PC61BM blend an extra shift detected at even very long delay time (278 ps), this can be assumed to formation of the triplet state as indicated long lifetime component in the kinetic trace of GSB in Figure 11. The rates for the charge separation (kCS) can be estimated from the change recorded in PC61BM lifetime for the polymers in their blends, see Table 4. Lifetimes reported in this table


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extracted from kinetic profile of TA signal fitted by global analysis and it is worth to mention here that some minor component of very short lifetime (less than 1 ps) was used to achieve the best fitted curve and it was neglected in the given table. Since the GSB recovery reflects the charge recombination to repopulate the neutral species we can use the extracted lifetimes associated with this signal to compare the rates of charge recombination in P1 and P2 blends with PC61BM.60 The extracted lifetimes as well as the kinetic traces given in Figure 11 clearly demonstrates much slower GSB kinetic for P2/PC61BM blend. Moreover, the contribution of a long lifetime component, >> 7.5 ns, becomes more predominant for this blend as indicated by the kinetic trace (~60%). Thus, we can anticipate that charge recombination in P2/PC61BM blend is significantly slower than the corresponding rates in P1/PC61BM blend. Moreover, recombination through populating triplet state is most likely to occur for the case of P2/PC61BM blend as suggested by the major contribution of the long-lived component in the kinetic trace. Table 4: Lifetimes and rates for charge separation (kCS) of polymer/PC61BM blends. 𝝺mon (nm)a

𝝉 ± ∆ (ps)

kCS (s-1)

P1

640

P2

740

PC61BM

380

P1/PC61BM (1:3)

380

1.39 ± 0.01 (59%) 7.76 ± 0.06 (24%) 115.46 ± 0.87 (12%) Long component (5%) 0.49 ± 0.01 (62%) 2.41 ± 0.01 (23%) 61.29 ± 0.87 (7%) Long component (8%) 114.75 ± 0.68 (35%) 1163.20 ± 12.15 (65%) 21.74 ± 0.09 (50%) 492.39 ± 3.62 (30%)

3.73 x1010 1.17 x109

3.41 ± 0.03 (28%) 56.42 ± 0.57 (28%) 516.99 ± 5.24 (28%) Long component (14%) 19.40 ± 0.128 (55%) 359.08 ± 6.13 (25%)

4.28 x1010 1.92 x109

640

P2/PC61BM (1:3)

380 740

10.61 ± 0.19 (13%) 76.62 ± 1.51(12%) 1172 ± 32 (12%) Long component (60%) a Monitoring wavelength associated to the lifetime


18

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In conclusion, in this work, we have demonstrated a strategy to improve the potential of a donor polymer based on BODIPY core for photovoltaic applications. We used a quite simple starting polymer comprising alternated units of BODIPY and thiophene to postfunctionnalize it by classic Knoevenagel condensation. This results with extended BODIPY backbones, shifting absorption of the polymer toward lower energies. It allows the polymer to potentially absorb more photons coming from the sun and brings its LUMO closer to PC61BM one in order to increase the electron transfer efficiency. This was confirmed by using ultrafast transient absorption spectroscopy, where charge recombination rate was found significantly slower after adding styryl arms. This supports our idea that one-step post-polymerization functionalization is a synthetic strategy could really improve the PCE of a device, for instance, by increasing the electron transfer efficiency and the lifetime of the charge-separated state.


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MATERIALS AND METHODS Instrumentation. 1H NMR was performed on a Bruker 300 Avance III NanoBay or a Bruker DRX 400 spectrometer using deuterated solvent. Mass spectra were obtained by MALDITOF with a Bruker Ultraflex II LRF 200 spectrometer using dithranol as matrix. High resolution mass measurements were carried out using a Bruker Ultraflex II LRF 2000 spectrometer (HR-MS MALDI-TOF) or Waters Synapt MALDI HRMS TOF mass spectrometer (Waters, Ontario, Canada) with dithranol as matrix. Molecular weights of the polymers were determined by gel permeation chromatography (GPC) by using a Viscotek GPCmax/VE 2001 GPC equipped with a PLgel MIXED-B columns set from Agilent Technologies. The experiments were run with THF as eluent at 45 °C and calibrated with polystyrene standards. Electrochemistry. The redox properties of polymers were studied using cyclic voltammetry (CV). Polymers P1 and P2 were drop-casted on a 2 mm diameter platinum disk electrode from chloroform solution (10 mg.mL-1) to form thin films. The cyclic voltammograms were recorded in a three-electrode cell against Ag/Ag+ reference in a 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile as supporting electrolyte at a scan rate of 50 mV.s-1 and using platinum mesh as counter electrode. All voltammograms were calibrated afterwards against the Fc+/Fc redox couple. The HOMO and LUMO levels of polymers were estimated from the obtained potentials by assuming the absolute HOMO energy level of ferrocene to be -4.8 eV, according to following equations:61

Steady-state fluorescence spectroscopy. Absorption spectra were measured on a Varian Cary 300 Bio UV-Vis spectrometer at 298 K and on a Hewlett-Packard 8452A diode array spectrometer with a 0.1 second integration time at 77 K. Steady-state emission spectra, both UV−vis (up to 830 nm) and NIR (from 800 nm) emissions, were measured by QuantaMaster 400 phosphorimeter from Photon Technology International, upon excitation by a xenon lamp and recording with a NIR PMT-7-B detector. All fluorescence spectra were corrected for instrument response. The absorption and fluorescence spectra on films were obtained by spin coating solutions of compounds in chlorobenzene (10 mg.mL-1) on quartz discs at 2000 rpm for 30 s. Remaining solvent is dried by thermal annealing for 10 min at 120 °C in the air.


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Fluorescence spectra were acquired when samples (on quartz discs) were at 25° from incident excitation beam in order to inhibit scattering. Time-resolved fluorescence spectroscopy. Fluorescence lifetime measurements were made on an Edinburgh Instruments FLS980 phosphorimeter equipped with single monochromators, using a 378 nm (fwhm = 90 ps), 477 nm or 633 nm picosecond pulsed diode lasers as excitation sources. Data collection on the FLS980 system is done by time correlated single photon counting (TCSPC). Quantum yield measurements. Measurements were performed in distilled and degassed 2-methyltetrahydrofuran (2-MeTHF). Quartz cuvettes of 3 mL with path length of 1 cm equipped with a septum were used. A minimum of three replicate measurements (i.e., different solutions) were performed for each quantum yield. The sample concentrations were chosen to obtain an absorbance of about 0.05. The ﬂuorescence quantum yield (ΦF) measurements were performed with a slit width of 1.0-3.0 nm for both excitation and emission monochromators. Relative quantum efficiencies were obtained by comparing the areas under the corrected emission spectra of the sample relative to a known standard, and the following equation was used to calculate quantum yield: ΦF(sample) = ΦF(standard) (Isample/Istandard) (Fstandard/Fsample) (ηsample2/ηstandard2), where ΦF(standard) is the reported quantum yield of the standard, I is the integrated emission spectrum, F is the absorptance (F= 1-10-A, where A is the absorbance) at the excitation wavelength, and η is the refractive index of the solvents used. Nile blue was used as a standard for all quantum yield measurements with a reported quantum yield of 0.27 in MeOH.62 Computations. Calculations were performed with Gaussian 0963 at the Université de Sherbrooke with Mammouth supercomputer supported by “Le Réseau Québécois de Calculs de Haute Performances”. The DFT64-67 and TD-DFT68-70 were calculated with the B3LYP71-73 method. 6-31g* basis sets were used for every atoms constituting BODIPY core, ethynyl and thienyl core, and 3-21g* was used to optimize alkyl side chains on thienyl and mesityl at meso position of the BODIPY.74-79 All calculations were done in presence of THF as solvent. The calculated absorption spectra and related MO contributions were obtained from the TDDFT/singlets output file and gaussSum 2.2.80 The model compounds were optimized before the TD-DFT calculations. Only the relevant (stronger oscillator strength and wave function coefficients) molecular orbitals are shown.


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Femtosecond transient absorption experiments. The samples were prepared as follow for TA experiments. Quartz discs (1” diameter) were cleaned in an ultrasonic bath sequentially with deionized water, acetone and isopropanol. After drying substrates in an oven, solutions of polymers P1 or P2 (15 mg.mL-1 in o-DCB) with gradient of PC61BM (solution of 15 mg.mL-1 in o-DCB) concentration, from 1:0 to 1:3 ratios (w:w), were spin coated at 2000 rpm (~100 nm thick). Films were annealed in the air for 15 min at 100 °C. EXPERIMENTAL SECTION Materials. The analysis was performed both at the « Plateforme d’Analyses Chimiques et de Synthèse Moléculaire de l’Université de Bourgogne » and at the « Université de Sherbrooke ». Syntheses of compounds 1, 2, 3, 5, 6 and 8 were inspired from ones as previously reported.47,46,48,49 Unless otherwise stated, all chemicals were of analytical reagent quality and were used as received. 2,4-Dimethylpyrrole, mesitaldehyde, 3-octylthiophene, 3,4-dibromothiophene were all purchased from Sigma-Aldrich. Dry THF was obtained by distillation over Na/benzophenone, dry toluene was distilled over Na while dry triethylamine was obtained by distillation over CaH2. Dry 2-MeTHF was obtained by filtration over alumina followed by distillation under inert atmosphere using CaH2 as a drying agent then was degassed by repeated cycles of vacuum and purging with an inert gas (Ar) while submerged in a sonic bath. Once degassed the 2-MeTHF solution was stored inside a glove box under an argon atmosphere (O2 levels less than 10 ppm). Synthesis. 8-Mesityl-1,3,5,7-tetramethyl-BODIPY (1).47 1,3-Dimethylpyrrole (3.50 mL, 33.84 mmol), mesitaldehyde (2.50 mL, 16.95 mmol) and degassed DCM (100 mL) were added to a twonecked round-bottom flask under argon. Then TFA (194 µL, 2.54 mmol) was added dropwise to the solution and the mixture was stirred for 7 h at rt protected from direct light. p-Chloranil (4.98 g, 20.28 mmol) was added and the reaction was further stirred for 4 h. Triethylamine (22.76 mL, 169.50 mmol) was added in one portion and 30 min later boron trifluoride diethyl etherate 46.5 % (31.08 mL, 253.50 mmol) was added dropwise. The reaction mixture was stirred for 3 h at rt then solvents were removed under vacuum and the residue was dissolved in DCM, washed with water, dried with MgSO4 then evaporated under vacuum. The crude product was purified by SiO2 column chromatography (hexanes/DCM 7:3) to yield 36% (2.24 g) of pure compound 1 as shiny orange powder. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.94 (s, 2H), 5.96 (s, 2H), 2.56 (s, 6H), 2.33 (s, 3H), 2.09 (s, 6H), 1.38 (s, 6H). 13C NMR (75 MHz,


22

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CDCl3): (ppm) δ 155.2 (Cq), 142.4 (Cq), 141.8 (Cq), 138.7 (Cq), 135.1 (CH), 131.3 (Cq), 130.8 (Cq), 129.1 (CH), 120.9 (Cq), 21.4 (CH3), 19.7 (CH3), 14.8 (CH3), 13.5 (CH3). UV-vis (THF) λmax (x 103 L.mol-1.cm-1): 501 (82.3), 475 (20.9). ESI+ Infusion MS (m/z) for C22H25BF2N2: found [M+H]+ 367.2158, calcd. 367.2156. 2,6-Diiodo-8-mesityl-1,3,5,7-tetramethyl-BODIPY

(2).46

8-Mesityl-1,3,5,7-tetramethyl-

BODIPY 1 (900 mg, 2.46 mmol) and N-iodosuccinimide (1.84 g, 8.20 mmol) and DCM (35 mL) were added to a round-bottom flask under nitrogen. The reactional mixture was stirred at room temperature for 2 h then washed with water, dried over MgSO4 then evaporated under vacuum.

The

crude

compound

was

purified

by

SiO2 column

chromatography

(hexanes/chloroform 7:3) to yield 87% (1.32 g) of pure compound 2 as dark pink powder. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.98 (s, 2H), 2.65 (s, 6H), 2.36 (s, 3H), 2.06 (s, 6H), 1.40 (s, 6H). 13C NMR (75 MHz, CDCl3): (ppm) δ 157.8 (Cq), 156.5 (Cq), 153.5 (Cq), 144.7 (Cq), 141.8 (Cq), 139.4 (Cq), 134.9 (Cq), 131.0 (Cq), 129.4 (CH), 21.4 (CH3), 19.7 (CH3), 16.2 (CH3), 15.9 (CH3). UV-vis (THF) λmax (x 103 L.mol-1.cm-1): 533 (71.9), 499 (26.7). ESI+ Infusion MS (m/z) for C22H23BF2I2N2: found [M+H]+ 619.0089, calcd. 619.0090. 8-Mesityl-1,3,5,7-tetramethyl-2,6-di[(trimethylsilyl)ethynyl]-BODIPY (3).48 2,6-Diiodo-8mesityl-1,3,5,7-tetramethyl-BODIPY 2 (250 mg, 0.41 mmol) and copper(I) iodide (46.2 mg, 0.24 mmol) were introduced in a round-bottom flask. After 3 cycles of vacuum/argon, tetrakis(triphenylphosphine)palladium(0) (140.3 mg, 0.12 mmol), dry THF (35 mL) and distilled triethylamine (35 mL) were added. The mixture was heated to 50 °C then ethynyltrimethylsilane (345 µL, 2.43 mmol) was added in one portion. The reaction was kept stirring for 5 h then the solvent was removed under vacuum. The residue was taken up in DCM, washed with water and dried over MgSO4. The crude product was purified by SiO2 column chromatography (DCM/petroleum ether 6:4) to yield 94% (213 mg) of pure compound 3 as green/pink powder. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.96 (s, 2H), 2.64 (s, 6H), 2.34 (s, 3H), 2.04 (s, 6H), 1.47 (s, 6H), 0.21 (s, 18H). 13C NMR (75 MHz, CDCl3): δ 158.7 (Cq), 144.3 (Cq), 143.1 (Cq), 139.2 (Cq), 135.1 (Cq), 134.9 (Cq), 130.7 (Cq), 130.3 (Cq), 129.3 (CH), 101.8 (Cq), 97.4 (Cq), 21.4 (CH3), 19.6 (CH3), 13.7 (CH3), 12.5 (CH3), 0.2 (CH3). UV-vis (THF) λmax (x 103 L.mol-1.cm-1): 554 (96.5), 515 (35.4). MALDI/TOF MS (m/z) for C32H41BF2N2Si2: found [M]+• 558.2898, calcd. 558.2870. 2,6-Diethynyl-8-mesityl-1,3,5,7-tetramethyl-BODIPY (4). 8-Mesityl-1,3,5,7-tetramethyl2,6-di[(trimethylsilyl)ethynyl]-BODIPY 3 (300 mg, 0.54 mmol) and DCM (300 mL) were added to a round-bottom. The mixture was stirred for 15 min then 1 M tetrabutylammonium fluoride in THF (1.13 mL, 1.13 mmol) was added dropwise at 0 °C. The reaction was left


23


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Page 24 of 54

stirring for 15 min at this temperature and a 10% hydrochloric acid solution (50 mL) was added. This mixture was washed with a saturated NaHCO3 solution then water, dried over MgSO4 then evaporated under vacuum. The crude compound was purified by SiO2 column chromatography (hexane/DCM 7:3) to yield 82% (184 mg) of pure compound 4 as orange/pink powder. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.97 (s, 2H), 3.32 (s, 2H), 2.65 (s, 6H), 2.35 (s, 3H), 2.06 (s, 6H), 1.48 (s, 6H). 13C NMR (75 MHz, CDCl3): (ppm) δ 158.8 (Cq), 144.9 (Cq), 143.6 (Cq), 139.3 (Cq), 134.8 (Cq), 130.5 (Cq), 130.3 (Cq), 129.4 (CH), 114.9 (Cq), 84.2 (CHalcyne), 76.2 (Cq), 21.4 (CH3), 19.6 (CH3), 13.7 (CH3), 12.4 (CH3). UV-vis (THF) λmax (x 103 L.mol-1.cm-1): 541 (81.2), 509 (26.7). MALDI/TOF MS (m/z) for C26H25BF2N2: found [M]+• 414.2097, calcd. 414.2078. 2,5-Diiodo-3-octylthiophene

(5).49

3-Octylthiophene

(108.7

µL,

0.51

mmol),

N-

iodosuccinimide (285 mg, 1.27 mmol) and 6.0 mL of a 1:1 mixture CHCl3/glacial acetic acid were introduced in a round-bottom flask under nitrogen. The reaction was stirred at rt for 8 h then quenched with NaHCO3. The organic phase was washed with water, dried with MgSO4 then evaporated under vacuum. Crude product was purified by SiO2 column chromatography (heptane) and 203 mg of pure compound 5 was obtained in 89% yield as yellow oil. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.89 (s, 1H), 2.57 (t, 3J = 7.6 Hz, 2H), 1.53 (m, 2H), 1.30 (m, 10H), 0.89 (t, 3J = 6.8 Hz, 3H). 13C NMR (75 MHz, CDCl3): (ppm) δ 149.6 (Cq), 137.9 (CH), 77.2 (Cq), 76.0 (CH2), 32.1 (CH2), 32.0 (CH2), 30.1 (CH2), 29.5 (CH2), 29.4 (CH2), 29.3 (CH2), 22.8 (CH2), 14.3 (CH3). APCI+ Infusion MS (m/z) for C12H18I2S: found [M+H]+ 448.9284, calcd. 448.9291. P(BdP-OT) (P1). 2,5-Diiodo-3-octylthiophene 5 (53.6 mg, 0.119 mmol) and copper(I) iodide (13.6 mg, 0.071 mmol) and tetrakis(triphenylphosphine)palladium(0) (41.2 mg, 0.036 mmol) were introduced in a round-bottom flask under argon atmosphere. After 3 cycles of vacuum/argon, distilled THF (10 mL) and dry triethylamine (329.0 µL, 2.380 mmol) were added and the mixture was heated to 60 °C. Then a solution of 2,6-diethynyl-8-mesityl1,3,5,7-tetramethyl-BODIPY 4 (49.5 mg, 0.119 mmol) in THF (10 mL) was added dropwise. The reaction was stirred overnight then the solvent was removed under vacuum. The residue was taken up in CHCl3, washed with water and dried over MgSO4. The crude product was passed through a Celiteâ plug with CHCl3 as solvent to remove insoluble material, then purified by gel permeation chromatography (pure CHCl3) to finally yield 85% (61 mg) of polymer P1 as dark blue solid film. 1H NMR (300 MHz, CDCl3): (ppm) δ. 6.98 (s, 2H), 6.95 (s, 1H), 2.69 (s, 6H), 2.63 (m, 2H), 2.36 (s, 3H), 2.08 (s, 6H), 1.50 (s, 6H), 1.23 (m, 12H), 0.86 (m, 3H). UV-vis (THF) λmax (x 103 L.mol-1.cm-1): 628 (27.8), 421 (6.3).


24

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FF (THF) = 20.2%

(Nile

Blue

as

reference).

GPC

(THF,

polystyrene

standard):

Mn = 31.8 kDa; Mw = 70.0 kDa; PDI: 2.2. P(sBdP-OT) (P2). P(BdP-OT) P1 (23.4 mg, 0.04 mmol of monomer), mesitaldehyde (24.1 µL, 0.16 mmol), dry toluene (8 mL), piperidine (15.8 µL, 0.16 mmol) and recristallized ptoluenesulfonic acid (1 crystal) were introduced in an evaporating flask. The mixture was heated up to 85 °C at 300 mbar on a rotary evaporator for 30 min until solvent was almost completely removed. Then, dry toluene (8 mL) and piperidine (15.8 µL, 0.16 mmol) were added to the flask again and the reaction was heated under the same conditions than before: this last step was repeated 3 more times until no more change was visible by TLC monitoring. The residue was taken up in CHCl3, the crude product was passed through a Celiteâ plug with CHCl3 as solvent to remove insoluble material, then purified by gel permeation chromatography (pure CHCl3) to finally yield polymer P2 (23 mg) as dark green solid film. 1

H NMR (300 MHz, CDCl3): (ppm) δ. 8.39 (d, 3J = 16.5 Hz, 1.6H), 7.42 (d, 3J = 16.5 Hz,

1.6H), 7.00 (s, 2H), 6.92 (m, 4.3H), 2.69 (m, 1.2H), 2.58 (m, 2H), 2.42 (m, 9.6H), 2.37 (s, 3H), 2.30 (s, 4.8H), 2.12 (m, 6H), 1.57 (m, 6H), 1.22 (m, 12H), 0.85 (m, 3H). UV-vis (THF) λmax: 688, 377. FF (THF) = 14.5% (Nile Blue as reference). GPC (THF, polystyrene standard): Mn = 23.2 kDa; Mw = 55.1 kDa; PDI: 2.4. 3,4-Di(2’-ethylhexyl)thiophene (6).32 3,4-Dibromothiophene (1.686 mg, 6.97 mmol), [1,3bis(diphenylphosphino)propane]dichloronickel(II) (1.13 g, 2.09 mmol) were introduced in a round-bottom flash. After 3 cycles of vacuum/argon, dry THF (35 mL) was added. A solution of (2-ethylhexyl)magnesium bromide 1.0 M in Et2O (17.43 mL, 17.42 mmol) was added dropwise to the mixture at 40 °C. The reaction was left refluxing overnight then quenched with water. The solvent was removed under vacuum and the residue was taken up in Et2O. The organic phase was washed with water and dried over MgSO4 then evaporated under reduced pressure. The crude product was purified by SiO2 column chromatography (petroleum ether) to yield 56% (1.207 g) of pure compound as colorless oil. The product 6 was used in the next step without any further purification. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.90 (s, 2H), 2.51 (d, 3J = 7.2 Hz, 4H), 1.58 (m, 2H), 1.27 (m, 16H), 0.89 (m, 12H). 13

C NMR (75 MHz, CDCl3): (ppm) δ 141.3 (Cq), 120.9 (CH), 39.7 (CH), 33.5 (CH2), 32.9

(CH2), 29.2 (CH2), 26.1 (CH2), 23.2 (CH2), 14.2 (CH3), 11.0 (CH3). APCI+ Infusion MS (m/z) for C20H36S: found [M+H]+ 309.2611, calcd. 309.2610. 3,4-Di(2’-ethylhexyl)-2-iodothiophene (7). 3,4-Di(2’-ethylhexyl)thiophene (300 mg, 0.97 mmol) 6, N-iodosuccinimide (217.8 mg, 0.97 mmol) and 10 mL of a 1:1 mixture


25


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Page 26 of 54

CHCl3/glacial acetic acid were introduced in a round-bottom flask under nitrogen. The reaction was stirred at rt overnight then quenched with NaHCO3. The organic phase was washed with water, dried with MgSO4 then evaporated under vacuum. Crude product was purified by SiO2 column chromatography (hexanes) and we obtained 162 mg of pure compound 7 in 38% yield as yellow oil. 1H NMR (300 MHz, CDCl3): (ppm) δ 7.02 (s, 1H), 2.46 (m, 4H), 1.58 (m, 2H), 1.27 (m, 16H), 0.89 (t, 3J = 7.3 Hz, 12H).

13

C NMR (75 MHz,

CDCl3): (ppm) δ 145.4 (Cq), 141.4 (Cq), 120.9 (CH), 75.5 (Cq), 40.0 (CH), 39.5 (CH), 35.2 (CH2), 34.1 (CH2), 32.8 (CH2), 32.8 (CH2), 29.1 (CH2), 29.1 (CH2), 29.07 (CH2), 26.0 (CH2), 25.9 (CH2), 23.2 (CH2), 14.4 (CH3), 14.3 (CH3), 11.4 (CH3), 11.0 (CH3). APCI+ Infusion MS (m/z) for C20H35IS: found [M+H]+ 435.1576, calcd. 435.1577. 2,6-Di(ethynyl-2’-[3’,4’-di(2’’-ethylhexyl)thiophene])-8-mesityl-1,3,5,7-tetramethylBODIPY (M1). 3,4-Di(2’-ethylhexyl)-2-iodothiophene 7 (105.0 mg, 0.242 mmol) and copper(I) iodide (13.7 mg, 0.072 mmol) and tetrakis(triphenylphosphine)palladium(0) (41.9 mg, 0.036 mmol) were introduced in a round-bottom flask under argon atmosphere. After 3 cycles of vacuum/argon, distilled THF (10 mL) and triethylamine (334.8 µL, 2.420 mmol) were added and the mixture was heated to 60 °C. Then a solution of 2,6-diethynyl-8-mesityl1,3,5,7-tetramethyl-BODIPY 4 (50.1 mg, 0.121 mmol) in THF (5 mL) was added dropwise. The reaction was refluxed for 4 h then the solvent was removed under vacuum. The residue was taken up in CHCl3, washed with water and dried over MgSO4. The product was purified by SiO2 column chromatography (CHCl3/hexanes 7:3) to yield 57% (71 mg) of M1 as dark blue sticky solid. 1H NMR (300 MHz, CDCl3): (ppm) δ 6.99 (s, 2H), 6.80 (s, 2H), 2.70 (s, 6H), 2.58 (d, 3J = 7.3 Hz, 4H), 2.42 (d, 3J = 7.0 Hz, 4H), 2.36 (s, 3H), 2.10 (s, 6H), 1.64 (m, 4H), 1.51 (s, 6H), 1.26 (m, 32H) 0.85 (m, 24H). FF (THF) = 27.6% (Nile Blue as reference). MALDI/TOF MS (m/z) for C66H93BF2N2S2: found [M]+• 1026.6865, calcd. 1026.6847. 2,6-Di(ethynyl-2’-[3’,4’-di(2’’-ethylhexyl)thiophene])-8-mesityl-1,7-dimethyl-3,5-di(mesitylstyryl)BODIPY

(M2).

2,6-Di(ethynyl-2’-[3’,4’-di(2’’-ethylhexyl)thiophene])-8-

mesityl-1,3,5,7-tetramethyl-BODIPY M1 (36.0 mg, 0.04 mmol), mesitaldehyde (20.6 µL, 0.14 mmol), dry toluene (5 mL), piperidine (27.7 µL, 0.28 mmol) and recristallized ptoluenesulfonic acid (1 crystal) were introduced in an evaporating flask. The mixture was heated up to 85 °C at 300 mbar on a rotary evaporator for 30 min until solvent was almost completely removed. Then, dry toluene (5 mL) and piperidine (27.7 µL, 0.28 mmol) were added to the flask again and the reaction was heated under the same conditions than before: this last step was repeated one more time. The reaction was monitored by TLC and stopped when BODIPY starting material M1 was totally consumed. The crude product was taken up


26

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in CHCl3 then washed with water, dried over MgSO4 and evaporated. The compound M2 was purified by SiO2 column chromatography (7:3 hexanes/CHCl3) and isolated as shiny green sticky solid with a yield of 20%. 1H NMR (300 MHz, CDCl3): (ppm) δ 8.46 (d, 3J = 16.7 Hz, 2H), 7.41 (d, 3J = 16.7 Hz, 2H), 7.02 (s, 2H), 6.90 (s, 4H), 6.81 (s, 2H), 2.56 (d, 3J = 7.3 Hz, 4H), 2.42 (s, 12H), 2.40 (m, 4H), 2.39 (s, 3H), 2.29 (s, 6H), 2.14 (s, 6H), 1.59 (s, 6H) 1.56 (m, 4H), 1.22 (m, 32H) 0.86 (m, 12H), 0.77 (m, 12H). FF (THF) = 63.4% (Nile Blue as reference). MALDI/TOF MS (m/z) for C86H113BF2N2S2: found [M]+• 1286.8405, calcd. 1286.8414. 2-Iodo-3-octylthiophene (8).50 3-Octylthiophene (200 µL, 0.94 mmol), N-iodosuccinimide (209.9 mg, 0.94 mmol) and 8 mL of a 1:1 mixture CHCl3/glacial acetic acid were introduced in a round-bottom flask under nitrogen. The reaction was stirred at rt overnight then quenched with NaHCO3. The organic phase was washed with water, dried with MgSO4 then evaporated under vacuum. Crude product was purified by SiO2 column chromatography (hexanes) and 200 mg of a yellow oil was obtained with 66% yield. Product 8 was used in the next step with no further purification. NMR data are reported in literature.50 APCI+ Infusion MS (m/z) for C12H19IS: found [M+H]+ 323.0324, calcd. 323.0325. 2,6-Di(ethynyl-2’-[3’-octylthiophene])-8-mesityl-1,3,5,7-tetramethyl-BODIPY

(BdP-

DEHT) (9). 2-Iodo-3-octylthiophene 8 (59.9 mg, 0.19 mmol) and copper(I) iodide (9.5 mg, 0.05 mmol) and tetrakis(triphenylphosphine)palladium(0) (29.2 mg, 0.03 mmol) were introduced in a round-bottom flask under argon atmosphere. After 3 cycles of vacuum/argon, distilled THF (6 mL) and triethylamine (233.5 µL, 1.69 mmol) were added and the mixture was heated to 60 °C. Then a solution of 2,6-diethynyl-8-mesityl-1,3,5,7-tetramethyl-BODIPY 4 (35.0 mg, 0.08 mmol) in THF (6 mL) was added dropwise. The reaction was refluxed overnight then the solvent was removed under vacuum. The residue was taken up in CHCl3, washed with water and dried over MgSO4. The product 9 was purified by SiO2 column chromatography (CHCl3/hexanes 7:3) to yield 12% (8 mg) of dark blue sticky solid. 1H NMR (300 MHz, CDCl3): (ppm) δ 7.15 (d, 3J = 5.2 Hz, 2H), 6.99 (s, 2H), 6.86 (d, 3J = 5.2 Hz, 2H), 2.71 (s, 6H), 2.70 (m, 4H), 2.36 (s, 3H), 2.10 (s, 6H), 1.61 (m, 4H), 1.52 (s, 6H), 1.25 (m, 20H) 0.86 (m, 6H). MALDI/TOF MS (m/z) for C50H61BF2N2S2: found [M]+• 802.4369, calcd. 802.4340; found [M-F]+ 783.4412, calcd. 783.4353.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 1

H NMR and

13

C NMR spectra, ESI, MALDI/TOF or APCI MS spectra, absorption,

emission, and excitation spectra at 298 K and 77 K, TGA thermograms, Optimized geometry (DFT, B3LYP), Representations of the frontier MOs for M1 with corresponding energies, Percent distribution of MOs, Computed oscillator strength (F) as a function of the calculated positions of the first 75 electronic transitions, Lifetime data. AUTHOR INFORMATION Corresponding Authors *Tel : +001-819-8212005. Email: [email protected] (P.D.H.) *Tel : + 33-(0)3-80396112. Email: [email protected] (C.P.G.) Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), le “Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT)”, the “Centre d’Etudes des Matériaux Optiques et Photoniques de l’Université de Sherbrooke (CEMOPUS)”. We are thankful to Prof. Gessie Brisard from the Université de Sherbrooke for e-chem measurements. The “Centre National de la Recherche Scientifique” (ICMUB, UMR CNRS 6302) is gratefully thanked for financial support. L.B. also gratefully acknowledges the French Research Ministry for a PhD fellowship. Support was provided by the CNRS, the “Université de Bourgogne Franche-Comté” and the “Conseil Régional de Bourgogne” through the PARI CDEA project. We are also thankful to the “Consulat Général de France à Québec” for a “Samuel de Champlain” travel grant.


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(70) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. Molecular Excitation Energies

to

High-Lying

Bound

Functional Response Theory:

States

from

Characterization and

Time-Dependent DensityCorrection of the Time-

Dependent Local Density Approximation Ionization Threshold. J. Chem. Phys. 1998, 108, 4439-4449. (71) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange J. Chem. Phys. 1993, 98, 5648−5652. (72) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (73) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results Obtained with the Correlation Energy Density Functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200-206. (74) Binkley, J. S.; Pople, J. A.; Hehre, W. J. Self-consistent Molecular Orbital Methods. 21. Small Split-valence Basis Sets for First-row Elements. J. Am. Chem. Soc. 1980, 102, 939-947. (75) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. Self-Consistent Molecular-Orbital Methods. 22. Small Split-Valence Basis Sets for Second-Row Elements. J. Am. Chem. Soc. 1982, 104, 2797−2803. (76) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; Defrees, D. J.; Pople, J. A.; Binkley, J. S. Self-Consistent Molecular Orbital Methods. 24. Supplemented Small Split-Valence Basis Sets for Second-Row Elements. J. Am. Chem. Soc. 1982, 104, 5039−5048. (77) Dobbs, K. D.; Hehre, W. J. Molecular Orbital Theory of the Properties of Inorganic and Organometallic Compounds 4. Extended Basis Sets for Third-and Fourth-Row, MainGroup Elements. J. Comput. Chem. 1986, 7, 359−378. (78) Dobbs, K. D.; Hehre, W. J. Molecular Orbital Theory of the Properties of Inorganic and Organometallic Compounds 5. Extended Basis Sets for First-Row Transition Metals. J. Comput. Chem. 1987, 8, 861−879. (79) Dobbs, K. D.; Hehre, W. J. Molecular Orbital Theory of the Properties of Inorganic and Organometallic Compounds. 6. Extended Basis Sets for Second-Row Transition Metals. J. Comput. Chem. 1987, 8, 880-893. (80) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. Software News and Updates cclib: A Library for Package-Independent Computational Chemistry Algorithms. J. Comput. Chem. 2008, 29, 839−845.


35


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TOC Graphic


36

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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


S N

S

n

N

B F F

N

R1

Z

B F F

S

R2

N

S

R2

N B

R2

R1

Z

P2

P1

S

n

N

S

R2

R2

N

R2

F F

F F

M1

M2

R1 = Z=

CH3 or

R2 = Chart 1


R2

N B

R2


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

i.

+

2

ii. N

N H O

N

Page 38 of 54

I

I N

N

B

B

F F

F F

1

2 iii.

H

H N

iv.

Si

Si

N

N

B F F

N B F F

3

4 Scheme 1


Page 39 of 54

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S

i.

R1

R1

R1 =

i.

R1

S

S

ii. Br

i. R2

R2

6 Scheme 2


I

5 S

I

R2 =

Br

S

I

8 S

I

R2

R2

7


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

4

+

5

Page 40 of 54

i.

S N

N

n

B

R1

F F

P1 Z=

CH3 or

ii.

R1 = S N

N Z

B F F

P2 Scheme 3


n Z

R1

Page 41 of 54

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


8

S

i.

S N

N B

R1

R1

F F

9

4

+

7

S

i.

S

R2

N

N B

R2

R2

R2

F F

M1

R1 =

ii.

R2 =

S

S

R2

N

R2

R2

N B F F

M2 Scheme 4


R2


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

Figures

Figure 1


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Figure 2



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Figure 3


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Figure 4



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

Figure 5


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Figure 6



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Figure 7


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Figure 8



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

Figure 9


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Figure 10



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

Figure 11


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Figure 12



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TOC Graphic


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