**2. Theoretical methodology**

108 Organic Light Emitting Devices

compared with the available experimental data [87].

**S**

**C6H13 N**

**(a)**

useful properties which can further widen the absorption spectrum.

**n**

**Figure 2.** Chemical structure of compounds under study: (a): P3HTBT, (b): PCzBT.

devices [87], then the conversion efficiency may be increased.

Recently, the conjugated P3HT2BTCz compound, built as carbazole-thiophenebenzothiadiazole, has been copolymerized onto the backbone of the copolymer as shown in Fig. 3. This compound has been synthesized and experimentally characterized, using only photoluminescence and optical absorption spectroscopy. Their related intense and broad absorption bands as well as favorable excited-state energy levels make them good candidates for fabricating PSCs. Thus, if P3HT2BTCz compound is blended with [6,6] phenyl-C61-bytric acid methyl ester (PCBM) fullerene derivative into BHJ photovoltaic

Here further investigations of geometrical parameters, electronic structures, photo-physical and vibrational properties of these compounds are carried out, on the basis of quantumchemical calculations, providing a reasonable interpretation of the experimental results and

**S N**

and charge transporting properties [85]. A fundamental understanding of the ultimate relations between structure and properties of these materials is necessary for using them in photovoltaic cells. A number of studies demonstrate that the interplay between theory and experiment is very important in providing useful insights in understanding the molecular electronic structure of the ground and excited states as well as the nature of absorption and photoluminescence [86]. To rationalize our theoretical results, the simulated data are

In what follows, we elucidate the photophysical properties of the benzothiadiazole derivative compounds with structures as shown in Fig. 2 (a,b). These two D-A polymers provide a basis for a more comprehensive study of the backbone ring, heteroatom and fused ring effects on polymer properties. Therefore, it is of practical significance to extend our previous work to a comprehensive theoretical investigation on these two types of BTDbased derivatives. Moreover, poly(3-hexyl-thiophene) (P3HT) units have relative higher charge mobility in comparison with other conjugated polymers and have been widely used as π-conjugating spacers [88,89]. Its insertion in the polymer backbone serves the dual purpose of transporting carriers and providing sites for exciton dissociation [90]. Moreover, the incorporation of electron-withdrawing moieties (3HT) as side chains leads to some

> **N S N**

> > **n**

**N**

**(b)**

All molecular calculations are performed in the gas phase using Density Functional Theory (DFT) implemented in the GAUSSIAN (03) program [91]. We have used the B3LYP (Becke three-parameter Lee-Yang-Parr) exchange correlation functional [92,93] with 3-21G\* and 6- 31G\* as basis sets. In the first part, the calculation of conformational characteristics has been done by varying the torsion angle in steps of 20° from = 0° to = 180°. For each increment, the dihedral angle is held fixed while the remainder of the molecule is optimized. The energy differences in electronic states are always calculated relative to the corresponding absolute minimum conformation and then the relative potential energy surfaces are drawn.

In the optimization procedure of these compounds, the alkyl chains at the N-9 positions of carbazole (Cz) motifs and dioctyloxy groups in TBT-BIPY copolymer are replaced by methyl and methoxy groups, respectively. This has been proven that the presence of alkyl/alkoxy groups does not significantly affect the equilibrium geometry and hence the electronic and the optical properties [94]. Hexyl groups in 3HT motifs are then replaced by methyl groups. The optimization of the composite (P3HT2BTCz: PCBM) is done in two steps. First optimization with PM3 semi-empirical method was carried out, then the resulting structure was re-optimized by DFT/B3LYP/3-21G\* to find the equilibrium geometrical structure.

Optical absorption spectra are calculated using the Time-Dependant Density Functional Theory (TDDFT) [95] based on optimized ground state geometries [96]. Theoretically the transition energies and their respective intensities in a given configuration interaction (CI)

expansion of singly excited determinants are determined [97]. The electronic configurations for the lowest 50 singlet-singlet transitions are obtained using the same basis set. Then, the obtained data are transformed using the SWizard program [98] into simulated spectra as described in the literature [99]. Finally, the nature and the energy of vertical electron transitions (the main singlet-singlet electron transitions with highest oscillator strengths) of molecular orbital wave functions are presented. The photoluminescence (PL) spectrum has been derived from CIS/TDDFT calculation [100]. Similar procedures are applied on TBT-BIPY model compound on the basis of ground and lowest singlet excited-states, but with two additional methods (CIS/3-21G\* and the semi-empirical quantum-chemical ZINDO levels) for absorption and emission properties [101]. The vibrational properties as well as force constants are also examined through results derived from the Molecular Orbital Package (MOPAC 2000) [102].

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:

The optical properties of the copolymer were studied, in chloroform solution and recorded at ambient temperature, by using UV-Vis and fluorescence emission spectroscopies (Fig. 5). The TBT-BIPY solution showed a sharp peak absorption maximum at 436 nm corresponding

the polymer film. Obviously, the red shift of about 79 nm in the film state is due to the π-π\* stacking effect [106]. The optical band gap, defined by the onset absorption of the polymer in the chloroform solution state is 2.43 eV. The polymer showed low band gap when compared to that of TBT-BIPH (2.48 eV). This may be due to the strong interaction between electron acceptor (TBT) and strong electron acceptor segments (BIPY) in the polymer backbone. Then, this optical band gap of the copolymer could be attributed to the D-A

**0,354 eV**

1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4

Energy (eV)

**Figure 5.** Normalized optical absorption and photoluminescence spectra of TBT-BIPY copolymer. The

Fig. 5 includes also the fluorescence spectrum of copolymer that gives a bright blue-greenish fluorescence with the maximum emission wavelength of 498 nm with the excitation wavelength at 450 nm in chloroform solution state. This emission is corresponding to the

(2.43 eV) estimated from the onset position of the absorption (510 nm) essentially agrees

fluorescence takes places by migration of electrons in the conducting band to the valence band. It is worthy to note that the PL spectrum of the compound shows well-resolved structural features with maxima at 498, 527 and shoulder at about 580 nm assigned to the 0– 0, 0–1, and 0–2 intra-chain singlet transition, respectively (the 0–0 transition, the most

\* transition of the electronic absorption spectra. The band gap of the polymer

max value (498 nm, 2.48 eV) of the main fluorescence peak, indicating that the

0,0

0,2

0,4

0,6

Normalized Absorbance

0,8

1,0

Abs

\* electronic transition in the polymer backbone. This band appears at 517 nm for

**3.2. Optical absorption and emission properties** 

Theoretical Fit Components

R^2 = 0,997 Model : Lorentz

**(3)**

PL deconvolution spectrum was given in the same figure.

**(2)**

**(1)**

PL

to the 

structure of polymer matrix.

0,0

onset of

with the

0,2

0,4

Normalized PL Intensity

0,6

0,8

1,0

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 111

### **3. Part I: TBT-BIPY copolymer for Light Emitting Diodes (PLEDs)**

#### **3.1. Raman scattering spectroscopy**

The Raman spectrum recorded for the excitation line of 1064 nm is presented in Fig. 4a. We have found that the Raman spectrum is dominated by bands originating from the thiophene, the di-alkoxy-substituted phenylene and pyridine rings vibration. According to the literature [103,104], the major band in the spectrum can be attributed to C=C stretching vibration of the thienyl ring at roughly 1444 cm-1 and the relatively weaker band at about 1604 cm-1 can be assigned to the C=C stretching vibration of the phenylene ring. The 1302 cm-1 can be attributed to the interring Cthienyl-Cphenyl vibration. In addition, we notice a strong asymmetry in intensity of the dominant triplet, occurring at 1444, 1543 and 1604 cm-1, resulting from the short conjugation length of the material [105].

**Figure 4.** (a) Experimental and theoretical normalized Raman spectra and (b) Selected Raman vibrational modes of the calculated frequencies of TBT-BIPY copolymer.

#### **3.2. Optical absorption and emission properties**

110 Organic Light Emitting Devices

Package (MOPAC 2000) [102].

**3.1. Raman scattering spectroscopy** 

0,0

0,5

Normalized Raman Intensity

1120

1055

Experimental Theoretical

1180

1,0

expansion of singly excited determinants are determined [97]. The electronic configurations for the lowest 50 singlet-singlet transitions are obtained using the same basis set. Then, the obtained data are transformed using the SWizard program [98] into simulated spectra as described in the literature [99]. Finally, the nature and the energy of vertical electron transitions (the main singlet-singlet electron transitions with highest oscillator strengths) of molecular orbital wave functions are presented. The photoluminescence (PL) spectrum has been derived from CIS/TDDFT calculation [100]. Similar procedures are applied on TBT-BIPY model compound on the basis of ground and lowest singlet excited-states, but with two additional methods (CIS/3-21G\* and the semi-empirical quantum-chemical ZINDO levels) for absorption and emission properties [101]. The vibrational properties as well as force constants are also examined through results derived from the Molecular Orbital

**3. Part I: TBT-BIPY copolymer for Light Emitting Diodes (PLEDs)** 

resulting from the short conjugation length of the material [105].

1000 1100 1200 1300 1400 1500 1600

Wavennuber (cm-1)

**Figure 4.** (a) Experimental and theoretical normalized Raman spectra and (b) Selected Raman

1320

1302

1360

1356

1250

vibrational modes of the calculated frequencies of TBT-BIPY copolymer.

1234

The Raman spectrum recorded for the excitation line of 1064 nm is presented in Fig. 4a. We have found that the Raman spectrum is dominated by bands originating from the thiophene, the di-alkoxy-substituted phenylene and pyridine rings vibration. According to the literature [103,104], the major band in the spectrum can be attributed to C=C stretching vibration of the thienyl ring at roughly 1444 cm-1 and the relatively weaker band at about 1604 cm-1 can be assigned to the C=C stretching vibration of the phenylene ring. The 1302 cm-1 can be attributed to the interring Cthienyl-Cphenyl vibration. In addition, we notice a strong asymmetry in intensity of the dominant triplet, occurring at 1444, 1543 and 1604 cm-1,

1457

1444

1500

1550

1600 1604 1543 1487

<sup>1</sup> 1320 *cm*

<sup>1</sup> 1457 *cm*

<sup>1</sup> 1550 *cm*

<sup>1</sup> 1600 *cm*

**(a) (b)**

The optical properties of the copolymer were studied, in chloroform solution and recorded at ambient temperature, by using UV-Vis and fluorescence emission spectroscopies (Fig. 5). The TBT-BIPY solution showed a sharp peak absorption maximum at 436 nm corresponding to the \* electronic transition in the polymer backbone. This band appears at 517 nm for the polymer film. Obviously, the red shift of about 79 nm in the film state is due to the π-π\* stacking effect [106]. The optical band gap, defined by the onset absorption of the polymer in the chloroform solution state is 2.43 eV. The polymer showed low band gap when compared to that of TBT-BIPH (2.48 eV). This may be due to the strong interaction between electron acceptor (TBT) and strong electron acceptor segments (BIPY) in the polymer backbone. Then, this optical band gap of the copolymer could be attributed to the D-A structure of polymer matrix.

**Figure 5.** Normalized optical absorption and photoluminescence spectra of TBT-BIPY copolymer. The PL deconvolution spectrum was given in the same figure.

Fig. 5 includes also the fluorescence spectrum of copolymer that gives a bright blue-greenish fluorescence with the maximum emission wavelength of 498 nm with the excitation wavelength at 450 nm in chloroform solution state. This emission is corresponding to the onset of \* transition of the electronic absorption spectra. The band gap of the polymer (2.43 eV) estimated from the onset position of the absorption (510 nm) essentially agrees with the max value (498 nm, 2.48 eV) of the main fluorescence peak, indicating that the fluorescence takes places by migration of electrons in the conducting band to the valence band. It is worthy to note that the PL spectrum of the compound shows well-resolved structural features with maxima at 498, 527 and shoulder at about 580 nm assigned to the 0– 0, 0–1, and 0–2 intra-chain singlet transition, respectively (the 0–0 transition, the most

intense) [107]. The stocks shift was found to be 62 nm (0.35 eV). This shift points to large structural differences between the ground and excited states in the material. In addition, from PL deconvolution spectrum, it should be noted that the energy difference (~ 0.18 eV) agrees well with that of the most intense Raman vibration modes at around 1450 cm−1.

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:

copolymer, φ1 and φ4 are held fixed and the torsional angle φ3 (dihedral angle between the thiophene and pyridine rings) is calculated in the same way by varying the torsional angles (φ1 and φ4) as described above. From the conformational analysis of TDMP and BIPY, it is found that both show a minimum at the torsional angle around 0°, and they adopt co-planar conformations. However, when BIPY is connected to TBT unit, molecules get twisted out of the planarity with an angle φ3 = 40°. Accordingly, all the inter-ring dihedral angles are kept

The optimized structure of TBT-BIPY optimized using DFT//B3LYP/3-21G\* is shown Fig. 7.

**Figure 7.** Ground state B3LYP/3-21G\* optimized structure of 2-TBT-BIPY copolymer. The values written in red (blue) color represent S--O (N--H) distances in Angstrom. n (n=1-9) represents the

As shown in Table 1, the two TBT and bipyrdine units of 2-TBT-BIPY adopt planar conformations with dihedral angles inferior to 1°. Whereas, the dihedral angles 3, 5 and <sup>8</sup> for 2-TBT-BIPY are twisted out of plane of ~24°. In addition, the C-O-C angles are not

Dihedral Angle (°) Ground State Excited State

**Table 1.** Calculated dihedral angles in their ground- and excited-states of 2-TBT-BIPY copolymer.

1 -0.28 -0.21 2 -0.39 -0.31 3 23.61 20.31 4 0.007 -0.72 5 -24.52 -2.42 6 -0.20 -0.34 7 -0.55 -0.41 8 23.86 1.85 9 -0.088 -0.091

dihedral angle between rings and the values expressed in degree are the C-O-C angles.

affected along the polymer chains and are evaluated to be 119.5°.

constant at φ1 = φ2 = φ4 = 0° and φ3 = 40° during the geometry optimizations.

The selected bond lengths and twist angles are collected in Table 1.

*3.3.2. Ground- and excited-state structures* 

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 113

#### **3.3. Theoretical part**

#### *3.3.1. Conformational analysis*

In the absence of structural information, we have assumed that the oligomer tends to be planar because of two reasons: (i) interchain interactions (packing force) tend to significantly reduce the torsion angles between adjacent units in the solid state and (ii) electronic and optical properties are weakly affected by small changes in torsional angels. To determine the minimum energy configuration, we perform fully geometrical optimizations on TBT-BIPY with B3LYP/3-21G\*. Since there is only one type of substitution on the phenyl ring (substitution 2 is equivalent to the site 5), three different conformation types can occur in TBT-BIPY copolymer structure. The potential energy surface (PES) of copolymer is obtained by partial optimization as shown in Fig. 6.

**Figure 6.** Potential energy curves of thienylene-2,5-di-methoxy-phenylene, BIPY and TBT-BIPY obtained from DFT/B3LYP/3-21G\* level of theory.

As these structures show flexibility in the molecule, first of all, individual torsion potentials for the two structures of thiophene-di-methoxy-phenylene (TDMP) and bipyridine (BIPY) are obtained for each molecule as a function of the inter-ring C-C dihedral angle φ<sup>1</sup> (torsional angle between the thiophene and di-methoxy-phenylene rings) and φ4 (torsional angle between the two pyridine rings) by varying them from 0° (syn-planar) to 180° (antiplanar) in steps of 20°. Therefore, to construct the potential energy curve for TBT-BIPY copolymer, φ1 and φ4 are held fixed and the torsional angle φ3 (dihedral angle between the thiophene and pyridine rings) is calculated in the same way by varying the torsional angles (φ1 and φ4) as described above. From the conformational analysis of TDMP and BIPY, it is found that both show a minimum at the torsional angle around 0°, and they adopt co-planar conformations. However, when BIPY is connected to TBT unit, molecules get twisted out of the planarity with an angle φ3 = 40°. Accordingly, all the inter-ring dihedral angles are kept constant at φ1 = φ2 = φ4 = 0° and φ3 = 40° during the geometry optimizations.

## *3.3.2. Ground- and excited-state structures*

112 Organic Light Emitting Devices

**3.3. Theoretical part** 

*3.3.1. Conformational analysis* 

by partial optimization as shown in Fig. 6.

obtained from DFT/B3LYP/3-21G\* level of theory.

intense) [107]. The stocks shift was found to be 62 nm (0.35 eV). This shift points to large structural differences between the ground and excited states in the material. In addition, from PL deconvolution spectrum, it should be noted that the energy difference (~ 0.18 eV) agrees well with that of the most intense Raman vibration modes at around 1450 cm−1.

In the absence of structural information, we have assumed that the oligomer tends to be planar because of two reasons: (i) interchain interactions (packing force) tend to significantly reduce the torsion angles between adjacent units in the solid state and (ii) electronic and optical properties are weakly affected by small changes in torsional angels. To determine the minimum energy configuration, we perform fully geometrical optimizations on TBT-BIPY with B3LYP/3-21G\*. Since there is only one type of substitution on the phenyl ring (substitution 2 is equivalent to the site 5), three different conformation types can occur in TBT-BIPY copolymer structure. The potential energy surface (PES) of copolymer is obtained

**Figure 6.** Potential energy curves of thienylene-2,5-di-methoxy-phenylene, BIPY and TBT-BIPY

As these structures show flexibility in the molecule, first of all, individual torsion potentials for the two structures of thiophene-di-methoxy-phenylene (TDMP) and bipyridine (BIPY) are obtained for each molecule as a function of the inter-ring C-C dihedral angle φ<sup>1</sup> (torsional angle between the thiophene and di-methoxy-phenylene rings) and φ4 (torsional angle between the two pyridine rings) by varying them from 0° (syn-planar) to 180° (antiplanar) in steps of 20°. Therefore, to construct the potential energy curve for TBT-BIPY The optimized structure of TBT-BIPY optimized using DFT//B3LYP/3-21G\* is shown Fig. 7. The selected bond lengths and twist angles are collected in Table 1.

**Figure 7.** Ground state B3LYP/3-21G\* optimized structure of 2-TBT-BIPY copolymer. The values written in red (blue) color represent S--O (N--H) distances in Angstrom. n (n=1-9) represents the dihedral angle between rings and the values expressed in degree are the C-O-C angles.

As shown in Table 1, the two TBT and bipyrdine units of 2-TBT-BIPY adopt planar conformations with dihedral angles inferior to 1°. Whereas, the dihedral angles 3, 5 and <sup>8</sup> for 2-TBT-BIPY are twisted out of plane of ~24°. In addition, the C-O-C angles are not affected along the polymer chains and are evaluated to be 119.5°.


**Table 1.** Calculated dihedral angles in their ground- and excited-states of 2-TBT-BIPY copolymer.

It is worth noting that the interaction forces between the oxygen atom (negatively charged) and the sulfur atom (positively charged) in the TBT unit are attractive [108-110]. Similar results are found for the Bipyridine unit; in which intra-molecular interaction occurs between non-bonded nitrogen and hydrogen atoms (the atomic charges are listed in Table 2 referred to the individual atoms in the numbering sequence shown Fig. 8). In fact, the calculated bond lengths of S--O (N--H) bonds are found to be ~2.62 Å (~2.44 Å), which correspond to ~79% (~92%) of the sum of their Van der Waals radii, fall inside the Van der Waals contact distance of the S--O (3.32 Å) and N--H (2.64 Å) and outside of their covalent contacts of 1.70 Å for S-O and 0.91 Å for N-H. In this case, the planar conformations are stabilized by the non-bonded S--O and N--H interactions [111].

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:


vibrational modes are shown in Fig.4b.

**Figure 9.** The DFT//B3LYP/3-21G\* calculated energy levels for 2-TBT-BIPY copolymer.

The vibrational Raman frequencies are calculated using the same method on geometryoptimized structure and are directly compared to those obtained from the Raman spectroscopy measurements. In Fig. 4a, we have plotted the normalized theoretical and experimental Raman spectra of the TBT-BIPY copolymer compound. It is relevant to note here that the vibrational spectrum calculated by DFT methodology agree satisfactorily with the experimental spectrum both in relative intensities and peak positions. The deviation between the measured Raman scattering and theoretically vibrational frequencies are less than 30 cm-1. Moreover, it was found that there were no negative vibrational frequencies, which indicate that optimized structure was at the energy minimum. This implies that the theoretically determined structure of copolymer is the most accurate description of the electronic structure. Accordingly, the experimental and calculated Raman bands at 1444 and 1457 cm-1, respectively, assigned to the thiophene ring vibrations [103-104], are strongly resonant with the \* electronic transition of compound. The most important Raman



Energy Level (eV)


Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 115

L+2

L+1

L

Eg = 2,77 eV

H-2

H-1

H

**Figure 8.** 2-TBT-BIPY copolymer structure with individual atoms in the numbering sequence.


**Table 2.** Atomic charges of sulfur, oxygen, nitrogen and hydrogen atoms in S--O and N--H intramolecular interactions.

Further, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) as well as the HOMO-LUMO energy gap (LUMO-HOMO) are studied. Accordingly, for 2-TBT-BIPY, the HOMO is at -4.922 eV, LUMO at -2,152 eV and the energy difference between these levels is thus 2.77 eV. To further understand the optical property changes, Fig.9 illustrates the three highest occupied and three lowest unoccupied orbital levels for the 2-TBT-BIPY copolymer.

1

2 3 4

6 7 8 9

**O**

molecular interactions.

levels for the 2-TBT-BIPY copolymer.

14

11 10

15

**CH3**

13

**H3C**

12

**O**

16

17 18

**S**

19 20

21 22 23 24

26

**N**

25

27

28 29

**N**

32 31

**Figure 8.** 2-TBT-BIPY copolymer structure with individual atoms in the numbering sequence.

30

Atoms Ground State Excited State S5/O14 0,459/-0,565 0,513/-0,757 S20/O12 0,492/-0,566 0,560/-0,759 N25/H -0,619/0,220 -0,764/0,287 N28/H -0,619/0,219 -0,767/0,287 S37/O46 0,494/-0,565 0,568/-0,760 S52/O44 0,494/-0,565 0,569/-0,760 N57/H -0,618/0,220 -0,767/0,286 N60/ H -0,605/0,221 -0,753/0,288

**Table 2.** Atomic charges of sulfur, oxygen, nitrogen and hydrogen atoms in S--O and N--H intra-

Further, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) as well as the HOMO-LUMO energy gap (LUMO-HOMO) are studied. Accordingly, for 2-TBT-BIPY, the HOMO is at -4.922 eV, LUMO at -2,152 eV and the energy difference between these levels is thus 2.77 eV. To further understand the optical property changes, Fig.9 illustrates the three highest occupied and three lowest unoccupied orbital

33 34 35 36

37

**S**

38 39 40

46

**O**

47

Atomic charges (e)

CH3

43 42

41

48

49 50

**S**

51 52

53 54 55

56

59

60 61

**N**

64 63

62

57 58

**N**

44

**O**

45

**H3C**

5

S

It is worth noting that the interaction forces between the oxygen atom (negatively charged) and the sulfur atom (positively charged) in the TBT unit are attractive [108-110]. Similar results are found for the Bipyridine unit; in which intra-molecular interaction occurs between non-bonded nitrogen and hydrogen atoms (the atomic charges are listed in Table 2 referred to the individual atoms in the numbering sequence shown Fig. 8). In fact, the calculated bond lengths of S--O (N--H) bonds are found to be ~2.62 Å (~2.44 Å), which correspond to ~79% (~92%) of the sum of their Van der Waals radii, fall inside the Van der Waals contact distance of the S--O (3.32 Å) and N--H (2.64 Å) and outside of their covalent contacts of 1.70 Å for S-O and 0.91 Å for N-H. In this case, the planar conformations are

stabilized by the non-bonded S--O and N--H interactions [111].

**Figure 9.** The DFT//B3LYP/3-21G\* calculated energy levels for 2-TBT-BIPY copolymer.

The vibrational Raman frequencies are calculated using the same method on geometryoptimized structure and are directly compared to those obtained from the Raman spectroscopy measurements. In Fig. 4a, we have plotted the normalized theoretical and experimental Raman spectra of the TBT-BIPY copolymer compound. It is relevant to note here that the vibrational spectrum calculated by DFT methodology agree satisfactorily with the experimental spectrum both in relative intensities and peak positions. The deviation between the measured Raman scattering and theoretically vibrational frequencies are less than 30 cm-1. Moreover, it was found that there were no negative vibrational frequencies, which indicate that optimized structure was at the energy minimum. This implies that the theoretically determined structure of copolymer is the most accurate description of the electronic structure. Accordingly, the experimental and calculated Raman bands at 1444 and 1457 cm-1, respectively, assigned to the thiophene ring vibrations [103-104], are strongly resonant with the \* electronic transition of compound. The most important Raman vibrational modes are shown in Fig.4b.

By combining the experimental data (optical band gap and Raman frequencies) with DFT calculations, two units of TBT-BIPY copolymer were considered as model structure for predicting the optical and emission properties.

Photophysical Properties of Two New Donor-Acceptor Conjugated Copolymers and Their Model Compounds:

On the other hand and whatever the state is, the non-bonded S--O and N--H contacts were found to be considerably shorter than the sum of their Van der Waals radii. These distances vary from ~2.62 Å to ~2.64 Å (S--O) and from ~2.43 Å to ~2.46 Å (N--H), when excited from the ground to excited states, which confirm the occurrence of non-covalent intra-molecular interactions. We believe that attractive interaction forces can modify the C-O-C angles in the excited state. This indicates that the singlet excited state should be much more planar than

We have applied a variety of theoretical approaches, including CIS/3-21G\*, TD-B3LYP/3- 21G\* and ZINDO methods to study the optical and emission properties of TBT-BIPY copolymers. The theoretical results thus obtained are compared with the experimental ones. All the energy levels calculated using the Time Dependent Density Functional Theory (TD-DFT), the CIS/3-21G\* and the semi-empirical quantum-chemical ZINDO are used to predict the optical absorption and emission spectra of the ground (S0) and first excited (S1) optimized structures. The assignment of electronic transitions and their oscillator strengths

From theoretical calculations, the wavelength of transitions from the ground to the first excited state (S0S1) and from the first excited state to ground state (S1S0) having the largest oscillator strength as well as their corresponding molecular orbital character for 2- TBT-BIPY are listed in Table 3. The corresponding experimental optical absorption and emission wavelengths measured in TBT-BIPY copolymer in chloroform solution are also listed in the same table. Clarke et al [112] suggest that the importance of the HOMO-LUMO transition may be easily understood from the spectral distribution of molecular orbitals. Accordingly, to a first approximation, a significant overlap found between HOMO and LUMO implies an intense transition between HOMO to LUMO and vice-versa. Here, the

> Emission properties of excited state (S1S0)

> > L+2H-2

f MO/Character Coefficient

519.2 4.352 L H 89 79.8

(%)

80 5

Stokes shift (nm/eV)

> 41.5 (0.36)

> (0.13)

(0.43)

their ground state.

Method of calculation

max (nm)

CIS 357.2 4.364 HL

ZINDO 439.4 3.781 HL

*3.3.3. Electronic transitions* 

are also calculated using these three methods.

vertical S0S1 transition dominates the HL excitation by 60-81%.

f MO/Character Coefficient

(%)

60 22 9

81 5

TBT-BIPY copolymers calculated by CIS/3-21G\*, TD//B3LYP/3-21G\* and ZINDO methods.

max (nm)

TD-DFT 481.6 2.943 HL 78 508.8 3.224 L H 82 27.2

Exp 436 nm 498-527 nm 62 (0.35)

**Table 3.** The vertical transition energies (nm) and their oscillator strengths of absorption from the ground to the first excited state (S0S1) and emission from the first excited to ground (S1S0) states of

398.7 3.055 L H

Optical absorption properties of ground state (S0S1)

> H-1L+1 H-2L+2

> H-1L+1

(H=HOMO, L=LUMO, L+1=LUMO+1, etc.), f: Oscillator strength

Applications in Polymer Light Emitting Diodes (PLEDs) and Polymer Photovoltaic Cells (PPCs) 117

For better understanding of the optical and emission processes, we have firstly computed the bond lengths of the ground and excited states, where the changes of bond lengths can be compared. The values of bond lengths for the 2-TBT-BIPY copolymer, in their ground- and excited-states are shown in Fig. 10. It can be seen that some bond lengths increase and some decrease in the excited state. Furthermore, we find that all the bond lengths of two bipyridine as well as those of C-O-C are shortened. Whereas, in the left TBT unit, the C-C single bond of thiophene rings as well as that connecting the thiophene ring to phenylene and bipyridine rings increase. In addition, in the second TBT unit, double bonds of thiophene rings and single/double bonds of substituted phenylene rings also increase.

**Figure 10.** Bond length variation of ground (a) and excited (b) states of 2-TBT-BIPY copolymer as well as the difference in bond length between the excited and ground states (in Å) (c). The horizontal axis labels represent the bonds between adjacent atoms in the numbering sequence shown in Figure from the bottom.

On the other hand and whatever the state is, the non-bonded S--O and N--H contacts were found to be considerably shorter than the sum of their Van der Waals radii. These distances vary from ~2.62 Å to ~2.64 Å (S--O) and from ~2.43 Å to ~2.46 Å (N--H), when excited from the ground to excited states, which confirm the occurrence of non-covalent intra-molecular interactions. We believe that attractive interaction forces can modify the C-O-C angles in the excited state. This indicates that the singlet excited state should be much more planar than their ground state.
