Copolymers Comprising Monomers with Various Dipoles and

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Copolymers Comprising Monomers with Various Dipole and Quadrupole as Active Material in Organic Field Effect Transistors Saumya Singh, Sundaresan Chithiravel, and Kothandam Krishnamoorthy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08225 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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The Journal of Physical Chemistry

Copolymers Comprising Monomers with Various Dipole and Quadrupole as Active Material in Organic Field Effect Transistors Saumya Singh, Sundaresan Chithiravel and Kothandam Krishnamoorthy* Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, CSIRNetworks of Institute for Solar Energy, Pashan Road, Pune 411008.

ABSTRACT: Copolymers of 4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) and diketopyrrolopyrrole (DPP) were synthesized. The BODIPY has a permanent dipole and the DPP has quadrupole. The dipole and quadrupole in the monomers are expected to bring the polymers closer and improve the charge transport properties. By judicious choice of these monomers, the electron wave function is evenly distributed through the molecules. However, we notice that the torsional angle at the connecting point of BODIPY and DPP is a function of the methyl moieties at the β, β' position of the BODIPY. We found that the polymer comprising DPP and BODIPY without methyl moiety at β, β' position showed a torsional angle of 27°. The lowest among the three polymers studied in this work. The absorption spectrum of the polymer showed transitions due to vibronic coupling indicating linearity along the polymer backbone. The bandgap of the polymer was found to be 1.2 eV. The thermally stable polymer showed an ambipolar charge transport of 0.01 cm2/Vs.

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1. INTRODUCTION 4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene, popularly known as BODIPY has been an attractive fluorophore due to excellent absorption and emission properties.1,2 Variety of BODIPY derivatives could be easily synthesized due to the modularity of the backbone and structural robustness.3 BODIPY has been explored as co-monomer to synthesize polymer with low bandgap.4-6 The initial attempts of to synthesize low bandgap BODIPY based donor acceptor polymer were unsuccessful.7-10 Later, a copolymer of BODIPY and perylenediimide exhibited a band gap of 1.5 eV.11 The polymer forms a good film and the authors could fabricate field effect transistors. Although the bandgap was low, the charge carrier mobility was found to be low (1 x 10-5 cm2/Vs). In another approach, a copolymer of BODIPY was synthesized, wherein the substitution on the BODIPY core was varied. The BODIPY core has methyl moieties in α, and β' positions. By removing the methyl moieties at β' positions and copolymerizing the monomer with thiophene, a low band gap polymer (1.5 eV) with a hole mobility of 0.17 cm2/Vs was achieved.12 So far, BODIPY based polymers are unipolar charge transporters.6,11-14 Recently, single layer organic solar cell with impressive short circuit current density (Jsc) has been reported in an inverted configuration.15 The increase in Jsc has been attributed to better charge extraction, which originates from better contacts and good charge carrier mobility in the organic semiconductors. Thus, it is essential to synthesize polymers with low band gap and ambipolar charge transport. In one of the previous attempts, a BODIPY copolymer was reported to exhibit ambipolar charge transport (10-5 cm2/Vs) by time of flight (TOF) measurement.5 We hypothesized that a BODIPY based ambipolar charge transport polymer can be synthesized by modulating the bandgap as well as packing of the polymer. The first objective is to modulate the bandgap of the polymer by variations in BODIPY core. The BODIPY 1 has a phenyl moiety in


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the meso position of the BODIPY. BODIPY 2 has an alkyl chain in the meso position, which is expected to decrease strain in the ring. In BODIPY 3, the alkyl chain in the meso position was retained and the methyl moieties in the β' positions are removed to further decrease the strain on the ring. The variation in substitution on the BODIPY backbone modulates the dipole (vide infra). Thus, by changing the substitution on BODIPY, the effect of dipole on charge transport can also be studied. We have chosen diketopyrrolopyrrole (DPP) as a co-monomer due to the flat nature of the core and the presence of quadrupole.16-21 The quadrupole facilitate better packing of the molecules.22,23 Thus, the combination of these monomers is expected to impart better packing.24 The alkyl chain in the DPP was kept constant. Three co-polymers comprising DPP and BODIPY have been synthesized. Highest hole mobility of 0.03 cm2/Vs and electron mobility of 0.01 cm2/Vs have been measured for one of the copolymers. The bandgap of the polymer with high hole and electron mobility is 1.2 eV. 2. RESULTS AND DISCUSSION BODIPY 1 was synthesized by following the procedure reported by our group.25 BODIPY 2 was synthesized by condensing daconoyl chloride with 2,4-dimethyl pyrrole in presence of BF3OEt2. BODIPY 3 was synthesized by condensation of daconoyl chloride with 2methyl pyrrole in presence of BF3OEt2. The diiodo derivatives of BODIPYs were synthesized by reacting the corresponding BODIPY with N-iodosuccinimide. The DPP derivative (M1) that is essential for the copolymer synthesis was prepared by following the reported procedure.26 The suzuki polymerization (Scheme 1) was carried out by reacting the M1 with corresponding BODIPY in presence of Pd(dppf)Cl2.27 The polymers were purified by soxhlet extraction. The polymer comprising BODIPY 1, BODIPY 2 and BODIPY 3 will be mentioned as P1, P2 and P3, respectively. P1, P2, and P3 are soluble in chloroform. To determine the molecular weight of the


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Scheme 1. Synthetic route to diiodoBODIPY monomers (1a – 3a) and polymers (P1 – P3). polymers, gel permeation chromatograms were obtained by using polystyrene as standard and chloroform as eluent. The Mw of P1, P2 and P3 was found to be 3.24 x104, 4.86 x 104 and 8.05 x 104, respectively. The PDI of P1 and P2 were found to be 2.7 and 2.4, respectively. However, the PDI of P3 was found to be 4.2. This is likely due to the reactivity difference of β' unsubstituted BODIPY that was used as monomer in the synthesis of P3. The thermal stability of the polymers were studied by thermogravimetric analysis (Figure 1a). All the polymers were stable upto 300 °C. We observed 5% weight loss after 350 °C for all the polymers (Table S1). To study the


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(b) P1 P2 P3

100

Heat Flow (mW)

Weight % (%)

(a)

80 60 40 20 0

125

250

375

P1 P2 P3

1.8 0.9 0.0 -0.9 -1.8 -100

500

Temperature (°C)

0

100

200

300

Temperature (°C)

(d) 0.9 0.6

Solution P1 P2 P3

0.3 0.0 400

600

800

1000

Normalized Absorbance

(c) Normalized Absorbance

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


0.9 0.6

Thin film P1 P2 P3

0.3 0.0 400

Wavelength (nm)

600

800

1000

Wavelength (nm)

Figure 1. TGA curves (a) and DSC plots (b) of polymers P1, P2, and P3. UV-vis absorption spectra of polymers P1, P2, and P3 in dilute chloroform solution (c) and as thin-film on quartz substrates (d). melting and crystallization behaviour of the polymers, differential scanning calorimetry curves were recorded between -50 and 300 °C (Figure 1b). The polymer P1 showed Tc and Tm at 230 °C and 275 °C, respectively. On the other hand, P2 and P3 didn't show thermal transition in the thermal window chosen for the DSC studies. Considering the possible degradation of the polymers above 300 °C, the DSC curves were not recorded above this temperature. The band gap of the polymers were determined by UV-vis absorption spectroscopy. A dilute solution of the polymer was prepared in chloroform and the spectra were recorded between 300 and 1100 nm


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(Figure 1c). Polymers P1 and P2 showed single peak with λ max at 647 and 630 nm, respectively. On the contrary, polymer P3 showed a peak at 878 nm and strong shoulder at 800 nm. This splitting is likely due to vibronic coupling indicating a high degree of linearity in the polymer back bone.28-30 The increased linearity in P3 is due to the absence of methyl moiety at the β' positions of BODIPY. Thin films of the polymers were spun on quartz substrates and the UV-vis absorption spectra were recorded (Figure 1d). The λmax of P1, P2 and P3 were shifted to 696, 678 and 905. The bathochromic shift in the λmax of the films is due to improved packing of the polymer chains in the solid state. The λmax further bathochromic shifted to 730, 700 and 927 upon thermal annealing of the polymer films at 150 °C for P1, P2, and P3, respectively. On thermal annealing, polymers P1 and P2 display emergence of low energy shoulder peaks, indicating improved interchain π-π stacking interactions (Figure S1).31-33 The impact of thermal annealing on the absorption spectra of the P3 was not very pronounced, only a slight red shift was observed. This implies that P3 with planar backbone forms an ordered structure even in the pristine film form. From the higher wavelength onset of the absorption spectra, the band gap was calculated to be 1.4, 1.4 and 1.2 eV for P1, P2, and P3, respectively. The lowest band gap of 1.2 eV observed for P3 is due to better conjugation between the monomers as a result of decreased torsional angle between a BODIPY without β' methyl group and DPP. A discussion on the torsional angle is provided in DFT calculation section (vide infra). The difference is dipole moment between BODIPY 1 and BODIPY 2 is 1.28, but the band gap is found to be same (1.4 eV) for polymers comprising these monomers. On the other hand, the dipole moment difference between BODIPY 2 and BODPY 3 is 0.78. It is interesting to note that the band gap of P3 (BODIPY 3 comprising polymer) is 0.2 eV less than that of P2 (BODIPY 2 comprising polymer). Thus, it is not possible to arrive at a conclusion based on the available data. Cyclic


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Figure 2. Front and side view of model compounds representing polymers (P1 – P3) obtained from DFT calculations. voltammetry was used to calculate the band edges of thin films of the polymers.34 The polymers were drop casted on Pt working electrodes from a chloroform solution. The cyclic voltammograms (CVs) were recorded in 0.1 M tertrabutylammonium chloride dissolved in acetonitrile. The polymers are insoluble in acetonitrile, hence this solvent has been chosen to record CVs. All the CVs showed large capacitance in the anodic and cathodic sweeps (Figure S2). However, the faradaic peaks corresponding to oxidation and reduction of the polymers were clearly discernible. The onset of oxidation of P1, P2, and P3 were found to be 0.65, 0.68, and 0.57 V respectively. The lower oxidation onset of P3 is due to shallow HOMO energy level, which was found to be -5.3 eV. The HOMO of P1 and P2 are -5.4 and -5.4 eV, respectively. The lowest band gap was calculated to be 1.7 eV for P3, which is 0.2 eV less than that of the band gap measured for P1 (1.9 eV) (Table S1). The trend in band gap variation is same irrespective the method used for the calculation. Density functional theory (DFT) analysis was carried out to investigate the optimal geometric structure and the electronic properties of polymers (P1 – P3).


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DFT calculations were done at B3LYP/6-31G∗∗ level using Gaussian 09 on model compounds representing the DPP and BODIPY monomers and the repeat units in the corresponding polymers.35,36 The theoretical HOMO and LUMO levels are given in the table S2. The HOMO and LUMO levels for DPP monomer are -4.95 and -2.50 eV, respectively. For BODIPY (1 – 3) the HOMO/LUMO energy levels are -5.28/-2.28 eV, -5.36/-2.31 eV, and -5.53/-2.51 eV. The replacement of phenyl ring by alkyl chain (BODIPY 1 to BODIPY 2) didn't impact the energy levels significantly. On the other hand, the removal of methyl moieties fromβ,β' positions impacted the HOMO and LUMO energy level (BODIPY 3). These BODIPY monomers exhibit strong molecular dipoles, oriented toward the 4,4 ′-fluorine substituents, and the values are 5.42 D, 4.14 D, and 3.36 D for BODIPY 1, 2, and 3, respectively. In polymers, DPP and BODIPY (1 – 3) monomers are connected in an alternate fashion. Since substituent groups and orbital energy levels are different for BODIPY (1 – 3), corresponding polymers (P1 - P3) will exhibit variation in orbital energy levels and degree of planarity in the conjugated backbones. The geometrically optimized structures of two repeating units representing polymers are given in figure 2. The inter-ring torsion angles are ~ 47°, ~51°, and ~27° for P1, P2, and P3, respectively. Side views of conjugated backbones of model compounds demonstrate that P1 has a twisted coil structure while P2 and P3 have an alternate twist in the backbone. With the lowest inter-ring torsion angle, P3, containing dimethyl BODIPY core, has the most coplanar conjugated backbone compared to other two polymers. Please recall the vibronic coupling observed in the UV-vis absorption spectrum of P3, which indicated planarity in the polymer backbone. This matches with DFT analysis of the polymer. In each of the model compounds, both LUMO and HOMO wave functions are delocalized along the backbone (Figure S4), and this is a characteristic feature of ambipolar molecules reported in the literature.37-40 Thin film morphology of the polymers were


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(a)

(b) P2_as cast

(c) P3_as cast

P2_annealed

(e)

(d)

(f) P2

P3_annealed Intensity (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


10

As cast Annealed_100 oC Annealed_150 oC

15 20 25 2θ (degree)

30

P3

Intensity (a.u.)

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10

As cast Annealed_100 oC Annealed_150 oC

15 20 25 2θ (degree)

30

Figure 3. AFM height images of as-spun polymer thin film of P2 (a) and P3 (b) and thermally annealed film of P2 (c) and P3 (d). X-ray diffraction (XRD) patterns of as cast and thermally annealed polymer thin films: (e) of P2 and (f) of P3.

studied using atomic force microscopy. The as prepared film of P1 (Figure S5a) and P2 (Figure 3a) on silane modified silicon substrates showed film with isolated domains. On the contrary, P3 showed fibrillar morphology although the fibres are short (Figure 3b). Upon thermal annealing, the morphology of P1 didn't vary significantly (Figure S5b). In case of P2, the domain boundaries became unclear (Figure 3c). The fibres of P3 are connected upon thermal annealing (Figure 3d). This is likely to facilitate better charge transport. The thin film XRD of three polymers were recorded as a function of temperature. The as prepared film for P3 showed a


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sharp peak at a 2θ of 22.05 corresponding to d spacing of 4.03 Å. The intensity of the peak was very weak for P2. The intensity of the peak for P1 was between P2 and P3 (Figure S5c). Upon thermal annealing, the peak for P3 became intense and sharp, indicating improved crystallinity (Figure 3f). However, thermal annealing didn't increase the intensity of the peak for P2 (Figure 3e). The peak continued to be weak and broad. Thus, P3 is more crystalline than other polymers. Next, we fabricated organic thin film transistors using polymers P1, P2, and P3. Thin films of the polymers were spun on pre-fabricated substrates comprising gold source and drain electrodes on top of the SiO2 gate dielectric. The gate potentials are applied on n-doped silicon beneath the SiO2 gate dielectric. The drain voltage (VD) was swept between 0 and -100 V while keeping the gate voltage (VG) at constant negative voltage. For P1, at a VG of 0 V, the drain current (ID) was immeasurably small and it started increasing after -65 V (VD) (Figure S6a). At an applied VG of 20 V, the ID increased after -85 V (VD). Contrary to this, the ID increased from - 8V for a VG of 60 V. The current continued to increase until -25 V resembling typical linear regime in output characteristic curve. The ID for all other VG showed typical linear and saturation regimes. We also noticed leakage current in the output characteristic curves. The transfer characteristic curves were recorded by holding VD at a constant potential of -80 V and sweeping the VG between 10 and -100 V. A U shaped transfer characteristic curve was observed indicating a possible ambipolar charge transport (Figure S6b). However, we didn't observe proper output characteristic curves while sweeping the VD between 0 and 80 V (Figure S6c). Thus, we are not able to calculate electron mobility for P1. The hole mobility was calculated to be 2.5 x 10-5 cm2/Vs (Table 1). The device was annealed at 100 and 150 °C and the device metrics were measured. The first striking difference was observed in the output characteristic curves. For the negative sweep, a typical ambipolar output characteristics curve was observed (Figure S7a).


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Table 1. Summary of OFET properties of polymers P1, P2 and P3 Polymer

P1

P2

T a (°C)

VT (V)

Ion/Ioff

µe (cm2V-1s-1) (µmax )b

as-spun 100 150

2.53 × 10-5 8.44 × 10-4 5.25 × 10-3 (6.08 × 10-3)

-30 -37 -17

6.5 × 103 3.6 × 103 3.1 ×105

-

as-spun 100

6.0 × 10-5 1.62 × 10-4 (3.1 × 10-4) 7.4 × 10-5

-32 -25

1.11 × 103 6.91 × 103

-

-28

-

-3

-24 -30

8.08×103 102

as-spun 150 175

a

Electron

µh (cm2V-1s-1) (µmax )b

150 P3

Hole

6.74 × 10 2.44 × 10-2 (3.08 × 10-2) 1.76 × 10-2

-33

1.3 × 102 8.08 × 103

VT (V)

Ion/Ioff

52 58

3.2 × 103 7.5 × 104

5.24 × 10-3 4.80 × 10-3

40 50

0.2 × 102 0.3 × 102

1.12 × 10-2 (1.53 × 10-2)

40

0.3 × 102

5.43 × 10-4 1.05 × 10-3 (3.37 × 10-3)

Annealing temperature b maximum mobility Similarly, ambipolar output characteristic curves were observed for positive sweep as well (Figure S7c). From these curves, we were able to calculate hole and electron mobilities, which were found to be 6 x 10-3 cm2/Vs (µh) and 3.3 x 10-3 cm2/Vs. Thus, the hole mobility increased by two orders upon thermal annealing. The electron mobility increased by several orders and comparable to that of hole mobility.41 In case of P2, the output characteristic curves were noisy, although linear and saturation regimes are discernible (Figure S8a). The hole mobility was measured to be 6 x 10-5 cm2/Vs. Upon thermal annealing at 100 °C, the µh increased to 3 x 10-4 cm2/Vs (Figure S9a). We attribute this lower mobility to increased torsional angle. Please note that the torsional angle was the highest (51°) among the three polymers under investigation. Due to this torsional angle, thermal annealing didn't improve the packing of the polymer films. In


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(a)

(b) 0V -20 V -40 V -60 V -80 V -100 V

-300

-9x10

-4

-6x10

ID (A)

ID (µ A)

-450

-2

-4

-150

-3x10

ID (A) 1/2 1/2 ID (A)

-2

-2x10

-2

-2x10 -4

-3x10

-3

-8x10

ID 1/2(A)1/2

0

0 0

-20

-40

-60

0

-80 -100

0 -20 -40 -60 -80 -100

VD (V)

VG(V)

(c)

(d) 0V 20 V 40 V 60 V 80 V 100 V

80

ID (A) 1/2 1/2 ID (A)

-4

6x10

ID (A)

ID (µ A)

120

40

-2

2x10

-4

4x10

-4

-3

2x10

9x10

0

0 0

20

40

60

80

100

0

-2

3x10

20

40

60

ID 1/2(A)1/2

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

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0 80 100

VG(V)

VD (V)

Figure 4. OFET characteristics of thermally annealed film of P3: Output (a) and transfer (b) characteristic curves while measuring hole transport (annealed at 150 °C), and output (c) and transfer (d) characteristic curves while measuring electron transport (annealed at 175 °C). (Channel width (W) = 10 mm, Channel length (L) = 5µm)

case of P3, typical linear and saturation regimes were observed for both the as prepared (Figure S10) and thermally annealed films (Figure 4). From the output curves, presence of contact resistance could be clearly visualised. Increase in ID as a function of thermal annealing indicates the increase in charge carrier mobility. The µh and µe were found to be 6.7 x 10-3 cm2/Vs and 5.2 x 10-3 cm2/Vs, respectively. The hole and electron mobilities are calculated to be 0.01 cm2/Vs upon thermally annealing the device at 175 °C. This is the highest ambipolar charge transport of


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a BODIPY comprising polymer. It is essential to recall that the dipole moment decreased linearly from BODIPY 1- BODIPY 3. However, the charge carrier mobility varied non linearly for P1P3. It is essential to note that the dipole moment was calculated for the monomers (BODIPY). The charge carrier mobilities are calculated for the polymers, which also comprise a monomer with quadrupole. Thus, the comparison becomes difficult. In addition, we found that polymer P3 is more crystalline than other polymers. Therefore, it is difficult to separate out the parameter that impacted the charge transport. 3. SUMMARY Three copolymers of BODIPY have been designed and synthesized. The molecular dipole of the BODIPY varied between 5.42 D and 3.36 D depending on the substitution on the backbone. A copolymer of BODIPY with dipole of 3.36 D and DPP showed a band gap of 1.2 eV. This is 0.2 eV less than that of a polymer comprising BODIPY with a dipole of 5.42 D. The decrease in bandgap is attributed to better conjugation between the BODIPY and the comonomer DPP. Indeed, the DFT calculation showed that the dihedral angle between the BODIPY and DPP is 27°, which is significantly lower than that of other two monomer combinations. The polymer with a low bandgap also showed a fibrillar morphology in thin film. The low bandgap polymer showed a hole and electron carrier mobility of 6 x 10-3 and 5 x 10-3 cm2/Vs. Upon thermal annealing, the hole and electron mobility were found to reach 0.01 cm2/Vs. By judicious choice of monomers, we have synthesized a BODIPY based polymer with ambipolar charge transport. ASSOCIATED CONTENT


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Supporting Information. Experimental procedure, figures and schemes. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Funding Sources Council of Scientific and Industrial Research (TAPSUN, NWP 54) ACKNOWLEDGMENT KK thank CSIR for funding (TAPSUN, NWP 54). SS thank UGC for senior research fellowship.

REFERENCES (1) Ulrich, G.; Ziessel, R.; Harriman, A. The Chemistry of Fluorescent Bodipy Dyes: Versatility Unsurpassed. Angew. Chem., Int. Ed. 2008, 47, 1184−1201. (2) Boens, N.; Leen, V.; Dehaen, W. Fluorescent Indicators Based on BODIPY. Chem. Soc. Rev. 2012, 41, 1130−1172. (3) Loudet, A.; Burgess, K. BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties. Chem. Rev. 2007, 107, 4891−4932.


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Copolymers Comprising Monomers with Various Dipoles and

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