TDDFT modelling of bithiophene azo chromophores for optoelectronic applications

Dyes and Pigments 100 (2014) 261e268

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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig

A DFT/TDDFT modelling of bithiophene azo chromophores for optoelectronic applications Rajadurai Vijay Solomon, Rajangam Jagadeesan, Swaminathan Angeline Vedha, Ponnambalam Venuvanalingam* Theoretical & Computational Chemistry Laboratory, School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 April 2013 Received in revised form 10 September 2013 Accepted 11 September 2013 Available online 24 September 2013

Sixteen Bithiophene azo derivatives (BTAs) containing thiazole moiety have been reported recently, and seven of them along with five new candidates have been tested for optoelectronic properties using DFT/ TDDFT computations. FMO analysis clearly reveals that the substitution at bithiophene moiety largely stabilizes the HOMO while the LUMO is mainly localized on azo and thiazole moieties. Five new candidates with eNO2, eCN, eCOOH, COCH3 and eCOOCH3 as acceptors have been designed. Among the designed candidates, eNO2 substituted BTA (BTA8) is promising with an absorption maximum at 647 nm. Pushepull character has been quantified using NMR chemical shifts. Further, NBO analysis accounts for the n / p* and p / p* ground state stabilizing interactions arising from the heteroatoms in these BTAs. Four of the five designed candidates, are found to have reduced HOMOeLUMO energy gap, greater pushepull character along with higher wavelength of absorption and this render them very suitable for optoelectronic applications. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Azo dyes Thiophene Density functional theory TDDFT Optoelectronics NBO and NMR

1. Introduction Significant efforts have been focused on studying the electronic and structural properties of donoreacceptor substituted p-conjugated organic molecules with an aim to use them in light harvesting, data processing application and modern communication and photonic technologies [1e7]. Batches of dyes have been screened for their optoelectronic applications in recent times [8e 13]. Of these, Azo dyes are popular due to their wide applicability in day-to-day life [14e18], for instance in laser printing, molecular switches, non linear optical devices, xerography, textiles, foodstuffs, additives, cosmetics, leather, papers, organic solar cells and chemosensors [18e21]. Structurally, they are versatile for optoelectronic applications due to easily amenable absorption profile, intramolecular charge transfer characteristics and excitation energies corresponding to laser wavelengths. Over the years, various groups such as thiophenes, pyrroles, oxazoles, thiazoles and furans have been tried as substituents in this frame work in various combinations for better optoelectronic properties [22e24]. Raposo and her coworkers demonstrated that the incorporation of a heteroatom into the electron deficient five-membered ring of azo * Corresponding author. Tel.: þ91 431 2407053; fax: þ91 431 2407045. E-mail addresses: [email protected], [email protected] (P. Venuvanalingam). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.09.016

based organic pushepull molecules alters their optoelectronic properties [23e26]. This has stimulated a great deal of interest in such molecules and led to the present study where bithiophene azo derivatives (BTAs) have been examined using DFT/TDDFT methods [27]. The objective of the present work is to systematically investigate and understand the structureeproperty relationship of these BTAs and thus to design newer efficient BTA candidates for optoelectronic applications. In the present study twelve BTAs including five new ones have been investigated (Fig. 1). Different electron withdrawing groups (eNO2, eCN, eCOOH, eCOOCH3 and eCOCH3) have been screened as acceptors and eN(CH3)2 used as donor in all the new candidates and their frontier molecular orbitals, HOMOe LUMO energy gaps, absorption spectra have been studied. Effects of solvation on the spectra have been studied. Further AIM and NBO analysis have been carried out to understand the nature of delocalization and ground state stabilizing interactions respectively while the pushepull character present in these BTAs have been addressed from NMR chemical shift calculations.

2. Computational details The structures of the BTAs, considered here are shown in Fig. 1. B3LYP [28e31] functional is found to perform well for most of the pushepull organic molecules [32,33] and therefore all the

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S R1

S

N N N

S

R2

Fig. 2. A comparison of computed absorption maxima at various levels with experimentally reported values.

Fig. 1. The structures of BTAs chosen for the present study.

molecules have been optimized at B3LYP/6-311g(d,p) level using G09W [34]. Vibrational frequency analysis confirms that all the optimized geometries correspond to minima on the potential energy surface. Solvent influence on the electronic transition energies of BTAs has been investigated using polarizable continuum model (PCM) [35]. All these molecules have been optimized in solvent phase (1,4-Dioxane & DMSO) at B3LYP/6-311g(d,p) using PCM calculations. Further time dependent density functional theory (TDDFT) calculations have been performed to obtain the absorption spectra, singlet vertical excitation energies and oscillator strengths on the ground state optimized geometries [36]. The performance of different functionals such as B3LYP, CAMB3LYP [37], PBE [38,39], PBE0 [38,40] and M06 [41] have been examined in order to find out the suitable functional that estimates the absorption behaviour of these BTAs. Hence absorption maxima is computed in DMSO solvent and compared with the experimental values (Fig. 2). It is shown that the absorption maxima calculated at PBE0 level agrees well with experimentally observed absorption maxima and hence PBE0 is used for all the TDDFT calculations. Natural Bond Orbital (NBO) analysis has been carried out to find out various stabilizing interactions in the ground state [42,43]. Bader’s topological analysis has been done using AIM2000 package [44] to quantify the delocalization in these BTAs and the necessary wave functions have been generated at B3LYP/6-311g(d,p) level. To quantify the pushepull character, NMR chemical shifts (d) have been calculated using well known gauge-independent atomic orbital (GIAO) method using G09W [45].

substitution on the bithiophene moiety whereas in the case of BTA2 & BTA5 and BTA3 & BTA6, an electron releasing methoxy and ethoxy group is attached respectively. In BTA7, the electron withdrawing aldehyde group is attached. Selected bond parameters of B3LYP optimized geometries are given in Supporting information Table 1 (SIT1) and the data show that all these BTAs have extended p-delocalization. Fig. 3 indicates the nature of delocalization where the entire CeC, CeN, CeS and NeN bond lengths fall in between their respective single and double bond limits. Except the SeCeCeS dihedral angle which is 172 , other dihedral angles such as the CeNeNeC, NeCeSeC and NeCeNeN are closer to 0 or 180 . This is an excellent evidence for the enhanced delocalization of p-electrons throughout the molecule. Further the wiberg bond indices of the above mentioned bonds (SIT2) lie in the range 1.2e1.7  A and this confirms the p electron delocalization in nature of BTAs. In addition to this, the extent of p-delocalization has been quantified using Bader’s AIM analysis which is based on the electron density computed at the bond critical point [46e48]. The bond ellipticity (3 ) at the bond critical point (bcp) is used to understand the p-character of a

3. Results and discussion 3.1. Molecular structures Geometries of 12 BTAs including 5 new candidates are shown in Fig. 1. Diverse structural features prompted us to study how various substitutions alter the photophysical properties of these BTAs. Among the reported BTAs, those with R2 ¼ H, CH3 and R1 ¼ H, OMe & OEt are selected here (Fig. 1). In BTA 1 & BTA 4, there is no

Fig. 3. Illustration of p-delocalization from calculated CeC, CeN, CeS and NeN bond lengths at B3LYP/6-311g(d,p) level.

R.V. Solomon et al. / Dyes and Pigments 100 (2014) 261e268

Fig. 4. Frontier molecular orbital energy level diagram of BTAs.

chemical bond. The bond ellipticity (3 ), which is defined as (l1/l2) e 1, where l, the eigen value at the bcp, also provides quantitative evidence for the p character of a molecule. When 3 / 0, the bond belongs to a typical s bond, and larger it deviates from 0, stronger will be the p character. The calculated ellipticity values are in the range of 0.13e0.32 (SIT3) which indicate the existence of delocalization of p bond throughout the molecule. It is interesting to note that the bond ellipticity of CeC in BTAs is in the order of w101 and this clearly suggests that the bond ellipticities lie in between a pure CeC and C]C bonds. 3.2. Frontier molecular orbital analysis The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are not only used to understand the charge transfer within the molecule but also used to estimate the chemical reactivity and kinetic stability of the molecules [7,49]. The combined frontier molecular orbital energy level diagram is given in Fig. 4. The HOMOeLUMO gap lie over a range of 2.54 eVe

263

2.71 eV (Fig. 4). The energy gap of BTA1 is found to be maximum and is minimum for BTA 6. The HOMOs are observed in the range of 5.39 eV to 6.09 eV whereas the LUMOs in the range of 2.85 eV to 3.11 eV. The increasing order of HOMOeLUMO energy gap is as follows BTA6 < BTA5 < BTA3 < BTA7 < BTA2 < BTA4 < BTA1. It is interesting to note that the absence of pushepull groups on either ends leads BTA1 to be less reactive with wide HOMOeLUMO gap (5.71 eV) among the molecules studied here. When comparing the HOMOeLUMO energy gaps of BTA1 and BTA4, it is important to note that the substitution of electron releasing group (eCH3) at thiazole ring does not have any effect on the reduction of HOMOeLUMO energy gap. But electron releasing groups at bithiophene plays a significant role in the HOMOeLUMO energy gap reduction of these BTAs. For instance, ethoxy containing BTAs were found to have lower HOMOeLUMO energy gap than the corresponding methoxy substituted BTA. These findings clearly suggest that electron releasing groups at thiazole ring have little effect on the HOMOe LUMO energy gap reduction while the electron withdrawing groups at the thiazole ring further reduces the HOMOeLUMO energy gap. To account for the influence of solvent on the HOMOeLUMO energy gap geometry optimizations have been carried out in the solvent phase using PCM calculations and the results are summarized in SIT4. From the table, it is clear that as the polarity of the solvent increases, HOMOeLUMO gap decreases. The reduction in the band gap is up to w0.18 eV upon solvation and yet the trend in the HOMOeLUMO gap is the same as predicted in the gas phase. For instance, BTA6 has HOMOeLUMO gap of 2.53 eV in gas phase but it is reduced to 2.46 eV in 1,4-Dioxane and further it is reduced to 2.35 eV in DMSO. The frontier molecular orbitals such as HOMO  1, HOMO, LUMO and LUMO þ 1 are given in Fig. 5. It can be seen that there are some common characteristics observed in HOMOs and LUMOs of these BTAs. Particularly the HOMOs, LUMOs and LUMO þ 1s are evenly localized on the entire molecule. This indicates the overall delocalization and the pep* transitions arising out of these HOMOs in this class of molecules. But HOMO  1 is mainly localized on the azo and thiazole ring compared to the other end of the molecules. The HOMOs and LUMOs are often used to relate the spectral properties of molecules and provide decisive clues in designing newer molecules. Therefore it is essential to identify and

Fig. 5. Frontier molecular orbitals of BTAs.

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understand the nature of various segments of the molecule and their individual contributions towards HOMOs and LUMOs. Hence the contributions of various fragments of the molecules towards their HOMOs and LUMOs has been computed using QMForge program [50,51]. The whole molecule has been segmented into three fragments, namely azo, bithiophene and thiazole units as indicated in Fig. SIF1 (see Supporting information) and their corresponding percentage contributions are summarized in Table 1. From the table, it is clear that the bithiophene unit contributes significantly (63%e74%) to the HOMO and its contribution is diminished to 40e45% towards LUMO. In general the thiazole unit contributes largely towards LUMO than HOMO, yet in the wide HOMOeLUMO energy gap molecules (BTA1 & BTA4), the thiazole units were found to have lesser contributions (23 and 25%) to LUMO than to HOMO (26% and 30%) respectively. Invariably in all the molecules, the azo group predominantly contributes to the LUMO at least thrice higher than corresponding HOMO. Hence it is inferred that the substitutions on the bithiophene unit stabilize the HOMO whereas LUMO is mainly stabilized by azo and thiazole units. This study sheds light on the different segments and their contributions that are essential in designing newer candidates with desired HOMO and LUMO levels. 3.3. Electronic absorption spectra in gas phase from TDDFT calculations The ability to show high wavelength of absorption and emission makes the molecule an efficient candidate for optoelectronic applications and TDDFT is a useful tool in predicting excited state properties of a molecule. Therefore TDDFT calculations have been performed at PBE0/6-311g(2d,p) level to examine the vertical singlet excitations. The assessment and choice of this functional has been discussed in the computational details (Section 2). Experiments and earlier theoretical works report that azo dyes with two heterocyclic five-membered rings are efficient candidates for optoelectronic applications since they show higher wavelengths of absorption and emission than one five-membered ring [27]. Therefore it is expected to have higher wavelength absorptions in these BTAs. Hence the 10 low lying excited states of BTAs and their vertical transition energies, oscillator strength (f0) and percentage of contributions of various configurations to the excitations have been calculated and collected in Table 2. From the table, it is clear that BTAs (1, 4 & 7) with wide HOMOeLUMO gaps were found to have S0 / S1 excitations that correspond to their respective lmax values whereas the other BTAs show S0 / S1 transitions. As expected BTA6 (lowest HOMOeLUMO energy gap) absorbs at highest wavelength (487 nm) with the oscillator strength of 1.947. This is due to 98% contributions from HOMO / LUMO and are mainly from pep* transitions. BTA6 (wide HOMOeLUMO energy gap) absorbs at 458 nm with highest excitation energy (2.7 eV). In the BTA5, 98% contributions from HOMO / LUMO gives an intense absorption peak at 485 nm with second least excitation energy. The ethoxy substituted BTAs (3 & 6) were found to show w2 nm higher wavelength of absorption compared to corresponding methoxy substituted BTAs (2 & 4). HOMO  2 / LUMO and HOMO / LUMO

Table 2 Calculated absorption maxima of BTA derivatives from TDDFT//PBE0/6-311g(d,p) calculations in gas phase. Molecules Excitation lmax (nm) Oscillator E (eV) strength BTA1 BTA2 BTA3 BTA4 BTA5 BTA6 BTA7

S0 S0 S0 S0 S0 S0 S0

/ / / / / / /

S1 S1 S1 S1 S1 S1 S1

457.7 480.3 482.4 460.6 485.3 487.3 478.4

1.0075 1.0794 1.1132 0.9522 1.1632 1.1947 1.1347

2.7088 2.5817 2.5704 2.6920 2.5546 2.5445 2.5915

Major contribution HOMO HOMO HOMO HOMO HOMO HOMO HOMO

/ / / / / / /

LUMO(98%) LUMO(98%) LUMO(98%) LUMO(87%) LUMO(98%) LUMO(98%) LUMO(99%)

contributes 11% and 88% respectively towards 460 nm absorption of BTA6 (2.56 eV). Over all the excitations are mainly due to pep* transitions and the computed absorption maxima are well correlated with the calculated HOMOeLUMO energy gaps. 3.4. Effect of solvent on absorption spectra It is essential to understand the influence of solvents on the spectral properties of the molecules as a first hand note on the applications of these molecules. Since PCM is known as the most successful model to account for the solvent effects in literature [52,53], TDDFT calculations have been carried out in four different solvents (diethyl ether, dioxane, ethanol and DMSO) and the results are depicted in Fig. SIF2 (see Supporting information). The computed absorption maxima are in good agreement with the experiments and there is a shift in the absorption maxima upon solvation (up to w60 nm). For instance, BTA2 shows lmax of 471 nm in gas phase and the same is shifted to 528 nm in DMSO solvent. The absorption of all the molecules in solution phase corresponds to S0 / S1 transitions and is responsible for the most intense bands with higher oscillator strengths and mainly HOMO / LUMO transitions contribute to it. These are pep* transitions. 3.5. Natural Bond Orbital analysis NBO analysis offers useful insights on the intramolecular delocalization and donoreacceptor interactions based on the second order perturbation interactions between filled and vacant orbitals. It is interesting to understand the important ground state stabilization interactions that make the molecules to be stable in ground state. Hence NBO analysis has been carried out and the results are summarized in Table 3. The table lists the major second order perturbation interactions along with the corresponding donor and acceptor NBOs. It is interesting to note that in all the molecules, the lone pair on S atom participates in the stabilization of these BTAs through n / p* interactions contributing nearly 20e25 kcal/mol towards stabilization. Moreover the azo group also take part in the stabilization through pep* interactions (pCeCep*N]N). Yet the predominant stabilizing interactions in oxygen containing BTAs (2, 3, 5, 6 & 7) show n / p* interactions arising from lone pair of oxygen to the p* of adjacent CeC bond which is dominant than the pep*

Table 1 Molecular orbital composition (%) of various fragments towards FMOs of BTAs. Fragment

Bithiophene Azo Thiazole

BTA1

BTA2

BTA3

BTA4

BTA5

BTA6

BTA7

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

63.13 10.98 25.89

44.69 31.94 23.37

74.54 9.09 16.37

43.41 32.59 24.01

70.92 9.69 19.40

45.50 31.33 23.17

59.43 10.41 30.16

43.54 31.77 24.69

68.08 9.27 22.65

45.01 30.82 24.16

68.49 9.23 22.28

45.08 30.77 24.15

63.35 11.41 25.24

40.16 29.03 30.81

R.V. Solomon et al. / Dyes and Pigments 100 (2014) 261e268 Table 3 Second order perturbation interactions obtained for BTAs at B3LYP/6-311g(d,p) from NBO calculations. BTA

Donor(i)

Acceptor(j)

E(2) (kcal/mol)

E(j)  E(i) (a.u)

F(i,j) (a.u)

BTA1

p C12eC13

p*N16eN17 p*C18eN24 p*N15eN16 p*C17eN22 p*C1eC2 p*C1eC2 p*C17eN23 p*N15eN16 p*C17eN22 p*C8eC9 p*N15eN16 p*C1eC2 p*C1eC2 p*N15eN16 p*C1eC2 p*C17eN22 p*N15eN16 p*C8eC9 p*C17eN22 p*N15eN16

27.06 27.12 27.74 26.94 34.06 34.25 26.92 27.82 26.35 24.67 26.49 33.92 24.40 27.22 34.09 26.15 27.29 25.37 30.36 29.42

0.24 0.25 0.24 0.25 0.34 0.34 0.25 0.24 0.25 0.26 0.24 0.34 0.26 0.24 0.34 0.25 0.24 0.25 0.24 0.24

0.073 0.073 0.074 0.073 0.101 0.101 0.073 0.074 0.072 0.071 0.072 0.101 0.071 0.073 0.101 0.072 0.073 0.072 0.076 0.075

BTA2

BTA3

BTA4

BTA5

BTA6

BTA7

LP(2)S19 p C11eC13 LP(2)S18 LP(2)O23 LP(2)O24 LP(2)S18 p C11eC13 LP(2)S18 LP(2)S10 p C11eC13 LP(2)O27 LP(2)S5 p C11eC13 LP(2)O27 LP(2)S18 p C11eC13 LP(2)S10 LP(2)S18 p C11eC13

interactions. The most predominant interactions such as nS / p*Ce pCeC  p*NeN and nO / p*CeC are depicted in the Fig. 6 and the interacting orbitals are in proper position and orientation, facilitating the ground state stabilization of the BTA6. In Fig. 6a, the observed nep* interactions between lone pair of oxygen (LP2) and p* of CeC bond is shown. Similarly the predominant pep* interactions of CeC and N]N represented in Fig. 6b. Fig. 6c represents the intense interactions observed in BTA1 where the lone pair C,

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(n) of S atom and p* of CeC bond are involved. Overall the results highlight the importance of the incorporation of heteroatom towards the ground state stabilization of BTAs. 3.6. NMR chemical shift calculations The purpose of calculating the NMR chemical shielding is twofold; to check the reliability of the optimized structures and to quantify the pushepull nature of these BTAs. Recently Raposo and his coworkers have demonstrated the pushepull character of these BTAs experimentally through NMR chemical shielding analysis [27]. It is important to recall that the chemical shift of protons usually shifted to higher d values due to the substitution of electron withdrawing groups and shifted to lower d values in the case of electron releasing group. The Table 4 lists the experimental NMR chemical shielding values along with the computed data. From the table it is clear that the computed chemical shifts are in line with the experiments which further support the reliability of methods that have been adopted for the present study. For clarity, the numbering of the protons have been adopted as reported by Raposo et al. and indicated in Fig. SIF3 (see Supporting information). Due to perfect delocalization along with pushepull in the molecules, proton in positions (4-H) are expected to be slightly deshielded due to electron withdrawing groups induced paramagnetic anisotropy. The presence of aldehyde group in BTA7 makes the proton (4-H) to be highly deshielded with a larger chemical shift value than the rest of the BTAs. The protons of the bithiophene unit (30 -H, 40 -H, 300 -H and 400 -H) are shifted to lower chemical shifts as compared to the unsubstituted BTA1. For instance, the proton chemical shift of BTA5 & 6 is 7.46 ppm which is 0.37 ppm lesser than that of BTA1. When comparing methoxy and ethoxy substituents, the computed chemical shifts imply that both behave similarly towards shielding

Fig. 6. Illustration of n / p* and p / p* interactions observed in BTAs from NBO calculations.

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Table 4 Computed NMR chemical shifts in comparison with the experimentally reported values (EXP e Experimentally reported values & TH e Theoretically calculated values). BTA

BTA1 BTA2 BTA3 BTA4 BTA5 BTA6 BTA7

R1

H MeO EtO H MeO EtO H

R2

H H H CH3 CH3 CH3 CHO

5H

40 H

4H

30 H

300 H

400 H

500 H

EXP

TH

EXP

TH

EXP

TH

EXP

TH

EXP

TH

EXP

TH

EXP

TH

7.32 7.34 7.75 e e e e

7.08 7.02 7.01 e e e e

7.98 7.95 7.95 7.66 7.64 7.63 8.52

7.83 7.77 7.77 7.52 7.46 7.46 8.07

7.81 7.75 7.33 7.75 7.70 7.70 7.94

7.43 7.35 7.36 7.41 7.31 7.31 7.59

7.31 7.14 7.13 7.29 7.10 7.10 7.40

7.19 6.82 6.83 7.12 6.81 6.81 7.23

7.42 7.12 7.11 7.39 7.11 7.09 7.50

7.48 7.09 7.10 7.39 7.06 7.06 7.53

7.10 6.23 6.21 7.09 6.21 6.20 7.13

6.89 5.69 5.69 6.90 5.67 5.68 6.96

7.39 e e 7.37 e e 7.47

7.19 e e 7.18 e e 7.31

the bithiophene protons. Overall, the computed NMR chemical shifts accounts for the shielding and deshielding nature of protons due to the pushepull electronic environment in these BTAs. In short, a suitable donor on bithiophene ring plays a decisive role in tuning the HOMO whereas thiazole ring requires suitable acceptor in order to get a more promising candidate. Hence further computations to design newer BTAs with different acceptors have been attempted and discussed below. 3.7. Designing new BTA molecules Based on the computational results on BTA candidates (1e7), five new BTAs have been designed (Fig. 1) and the parent BTA skeleton has been carefully crafted with different acceptors (eNO2, eCN, eCOOH, eCOCH3 and eCOOCH3) with a view to get better optoelectronic properties like reduced HOMOeLUMO gap, higher wavelength of absorption and greater pushepull character. The calculated bond parameters of B3LYP/6-311g(d,p) optimized geometries show that these molecules have excellent p-conjugation. The frontier molecular energy level graph (Fig. 7) indicates that except BTA12, all the other new BTAs were found to have lower HOMOeLUMO energy gap than the best synthesized BTA (BTA6). Among the new candidates, the nitro substituted BTA8 have the lowest HOMOeLUMO energy gap (2.12 eV) where the competitive pushepull of electrons make it dynamic, eventually leading to high reactivity. BTA12 has a larger HOMOeLUMO energy gap with 2.54 eV and the increasing order of HOMOeLUMO energy gap is as follows BTA8
Fig. 7. Frontier molecular orbital energy level graph of designed BTAs.

The computed percentage contribution from various fragments is represented in Fig. 8 and SIT5. Contribution of azo moiety to HOMO is nearly 30% while bithiophene unit significantly contributes to HOMO than LUMO. Thus the calculated percentage contributions allow us to come to the conclusion that the tuning of acceptors greatly affects the LUMO while HOMO can be altered upon suitable donor substitution. From the HOMOs and LUMOs it is expected to show pep* transitions and this has been confirmed in computed absorption spectra (Supporting information SIF4). The designed candidates show wavelength of absorption in the range of 530e556 nm which is almost 50 nm higher than the best synthesized candidate BTA6. All excitations are assigned to HOMO / LUMO transitions with maximum oscillator strength (SIT6). Further, PCM calculations (Fig. 9) show that there is a shift in the absorption maxima (w80 nm) from gas phase to solvent phase, but polarity of the solvent has a very little influence (only upto 15 nm only) on the absorption spectra. Second order perturbation analysis clearly suggests that the participation of n / p* interactions of dimethyl amine moiety dominates other interactions in the stabilization of these designed BTAs (SIT7). For instance in BTA12, the p / p* interactions of C11eC13 and N15eN16 stabilizes 15 kcal/mol lesser than n / p* interactions from N23 and C1eC2. But the most prolific BTA8 was found to show strong n / p* interactions arising from nitro group than rest of the interactions. 3.8. Pushepull nature of designed candidates from NMR Chemical shifts of the designed candidates computed at B3LYP/ 6-311g(d,p) level show notable difference from the reported BTAs (1e7). Pushepull causes the p-delocalization throughout. Proton closer to the acceptors is deshielded due to this and those closer to the donor are slightly shielded. A comparison of the five designed candidates has been made with BTA1 (Fig. 10) where the delocalization and anisotropy due to pushepull is null. In BTA8, the nitro group deshielded the 40 -H proton downfield than that of in BTA1 and shields 40 -H to 1.19 ppm. Though all the designed candidates shows the same effect on substitution of various acceptors, against the same donor, the nitro group (BTA8) contributes significantly to the pushepull nature. In all the designed molecules, 30 -H protons which are placed closer to donor moiety experience lesser anisotropy due to substituents and are slightly shielded (w6.78 ppm), in comparison with BTA1 (7.19 ppm). It is interesting to note that 40 -H protons

Fig. 8. Molecular orbital contributions (%) of designed candidate (BTA8).

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pushing group and a suitable pulling group are essential to obtain improved absorption maxima and PCM calculations show that all the molecules were found to have positive solvatochromism. Further NBO analysis shed light on the necessary ground state stabilization interactions where n / p* and p / p* interactions complementing each other in the stabilization. The computed NMR chemical shields confirms the pushepull behaviour of these BTAs. Overall our study provides useful insights on the structureeproperty relationship that would be certainly handy to the experimentalist to carry the quest for better molecules with enhanced properties. Acknowledgement

Fig. 9. Effect of solvent on computed absorption spectra.

RVS thanks University Grants Commission (UGC), India for the financial support in the form of Junior and Senior Research Fellowships in Maulana Azad National Fellowship (Ref. No. F.40-17(C/ M)/2009(SA-III/MANF)). RJ thanks University Grants Commission (UGC), India for Rajiv Gandhi National Fellowship (Ref. No. F.17.1/ 2011-12/RGNF-SC-TAM-8488/(SA-III/website)). SAV thanks the University Grants Commission (UGC) and India Bishop Heber College, Tiruchirappalli for FDP program (Ref. No. F.ETFTNBD030/FIPXI PLAN). PV thanks UGC, India for the Award of Emeritus Fellowship for the Year 2012-13 (Ref. No. F.6-6/2012-13/EMERITUS-201213-GEN-1171/(SA-II)). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2013.09.016. References

Fig. 10. Computed NMR chemical shifts of designed BTAs in comparison with BTA1.

which are midway between the donor and acceptor in the pconjugation length are the ones which experience the pushepull lesser and do not show much difference in d values on comparison with BTA1. Hence BTA8, displays greater pushepull character resulting in higher reactivity. Thus the designed BTAs have reduced HOMOeLUMO energy gap, higher wavelengths of absorption, excellent push pull effects, show positive solvatochromism and are more susceptible to structural tuning for better optoelectronic properties. 4. Conclusions Totally 12 BTAs, 7 of them reported by Raposo et al. and five more new BTAs, have been newly designed and tested using DFT/ TDDFT methods. Our results show that among the experimentally synthesized molecules, BTA6 was found to be efficient with lowest HOMOeLUMO energy gap and high wavelength of absorption. While in the designed BTAs, except BTA12, all the candidates show promising optoelectronic properties. Solvation reduces the HOMOeLUMO energy gap but the order of reactivity remains the same both in the gas and solvent phases. All the molecules show excellent p-conjugation and planarity. Molecular orbital composition calculations reveal that the HOMO is mainly stabilized by the donor moiety (upto 70%) whereas azo group plays a significant role in the stabilization of LUMO. TDDFT results reveal that the better

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