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Strategical Designing of Donor-Acceptor-Donor Based Organic Molecules for Tuning Their Linear Optical Properties Raja Sen, Samarendra P. Singh, and Priya Johari J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07381 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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The Journal of Physical Chemistry

Strategical Designing of Donor-Acceptor-Donor Based Organic Molecules for Tuning Their Linear Optical Properties Raja Sen, Samarendra P. Singh, and Priya Johari∗ Department of Physics, School of Natural Sciences, Shiv Nadar University, Greater Noida, Gautam Budhha Nagar, UP 201 314, India. E-mail: [email protected],[email protected]

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Abstract Low-energy linear absorption spectrum of a series of 48 donor-acceptor-donor (DA-D) scheme based thiophone-benzo-(bis-)X-diazole molecules with X = O, S, Se, or Te are calculated using time dependent density functional theory in order to propose strategical design of molecules that can efficiently absorb light in the infrared and visible region of the solar spectrum. Our study establishes that optical properties of the D-AD based organic molecules significantly depend on the donor-to-acceptor (D/A) ratio and the strength of the acceptor moiety. Thus, by choosing a suitable D/A ratio and type of the acceptor moiety, the linear absorption spectrum can be largely shifted, in general, while the optical gap can be engineered over a wide energy range of ∼ 0.2 − 2.3 eV, in particular. It is also noticed that the increase in acceptor units (i.e., when D/A 6 1) leads to increase in steric hindrance in between them. This, in turn, disrupts the effective conjugation length and increases the optical gap. However, this effect is found to dominate strongly in the bis-configurations of the molecules as compared to the non-bis compositions. In order to reduce this effect for rational designing of effective D-A-D type chromophores with less steric hindrance, the role of π-conjugated ethylene (-CH=CH-) linkage/spacer between the A-A units is explored further. Here, it is found that introduction of such linkage substantially decrease the steric hindrance and thereby, the optical gap as well. Besides, our study also highlights and explains the impact of the acceptor moiety in improving the absorption capabilities of these molecules in the low-energy region.

Introduction Over few decades, the concept of “spectral engineering” that relates the simple energy gap of a material to its more complex electromagnetic absorption or emission spectrum, has become interestingly popular in the field of organic semiconductor. 1 Within the “spectral engineering” toolbox, low band gap conjugated organic materials are of great interest. These materials play a crucial role and have drawn researcher’s attention as they possess interesting 2


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optical, electrochemical, morphological, and electronic properties. 2–5 This, together with the benefits of abundance availability, light weight and low cost, makes them a leading candidate for the next-generation opto-electronic devices. Moreover, overall diverse synthetic possibilities and manipulative structure-dependent electronic properties of organic molecules have also encouraged researchers to explore the horizons of designing and synthesizing new πconjugated organic materials, that can prove promising in the area of opto-electronic devices. Generally, in π-conjugated semiconductors, the semiconducting behaviour is being achieved due to delocalization of π-molecular orbitals along the conjugation length, mostly in the direction of molecular chain. But, due to relatively large band gap and moderate charge carrier density as compared to inorganic system like commonly used Si, their performance in opto-electronic devices is restricted. Although such a bottleneck performance has been greatly overcome in the recent years by adopting a donor (D)-acceptor (A) scheme based configuration. The low-lying H (Highest Occupied Molecular Orbital) of donor moiety and high-lying L (Lowest Unoccupied Molecular Orbital) of acceptor moiety results in the much smaller band gap than the individual parent moieties. Further, the H − L gap of the D-A based molecules can also be tuned by considering a proper choice of donor and acceptor moieties as the new H − L gap depends on the ionization potential of donor and the electron affinity of acceptor moieties. 6–10 This popular and generally applicable strategy enables us to rationally design and study organic molecules that can absorb light with wavelengths in the infrared and visible regions of the electromagnetic spectrum, and could be useful for the opto-electronic applications, in general, and organic solar cell, in particular. Although in recent years, a lot of emphasis has been paid on understanding the dependence of structural and electronic properties of D-A based organic molecules on the choice of donor and acceptor moieties and their configuration in the molecules. 6–16 But, in order to predict energy-efficiency of opto-electronic devices, optical absorption spectrum of these materials, especially in low energy region close to band edge, is highly desired. Amongst several investigated D-A molecules, 8,10,14–21 thiophene benzothiadiazole based D-A molecules

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are found to be a popular choice. The benzothiadiazole group acts both as an electron transporting and highly fluorescent chromophore. Interestingly, the coupling of benzothiadiazole with thiophene units represent a useful material combination, which provides redshifted UVvis and emission spectra compared to equivalent oligothiophenes, 6,22–29 essentially required in order to harvest low energy (viz. near infrared and visible) photons. Few years back, Sonar et al. 6 have shown through synthesis and characterization of solution processable thiophene (D) and benzothiadiazole (A) based D-A-D organic molecules that the opto-electronic properties of such molecules can be manipulated by a systematic control on the number of donor and acceptor moieties within the molecule. They examined a series of four molecules: 2T-1(NSN-B), 2T-2(NSN-B), 3T-1(NSN-B), and 4T-1(NSN-B), to study the role of D/A ratio on morphology, electronic and optical properties, as well as on the charge mobility, and also provided the absorption spectrum for all the four molecules. However, to establish a robust relation, and to have a deeper understanding about such system, these many molecules are not sufficient. There requires a detailed investigation on a series of molecules having different units and strength of donor or/and acceptor moiety. In this paper, we therefore, demonstrate and establish a relation between the donor-to-acceptor ratio and the strength of the acceptor moieties with the linear absorption spectrum of the D-A-D molecules. For this, together with different configurations of thiophene and benzothiadiazole, we also calculated optical absorption spectra of molecules having different chalcogenides in their acceptor moiety (benzo-X-diazole; X = O, S, Se, and Te). Moreover, other than considering different chalcogenides, the strength of the acceptor moiety is also varied by considering the bis-configuration of it, i.e., benzo-bis-X-diazole (Figure 1b). Thus, along with the effect on the absorption spectrum with respect to D/A ratio, we also emphasized on the dependence of the optical spectrum of these molecules on the strength of acceptor moiety. Sonar et al. 6 have also shown that the increase in acceptor units (i.e., in case of molecule 2T-2(NSN-B)) within the molecules enhances the steric hindrance between them, which then affect the absorption spectra in low energy region and increases the optical

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gap. In this work, we have, therefore, also made an effort to decrease the steric hindrance between the acceptor units by introducing ethylene (-CH=CH-) linkage between the A-A units via various possible combinations. Thereby, we examined and reported how all these three factors (D/A ratio, strength of the moiety, and role of π-conjugated linker between A-A moieties) together, as well as individually, influence the overall absorption spectrum of D-A-D based organic molecules. To obtain the absorption spectrum, transitions corresponding to each feature in the spectrum, as well as optical gap of a series of thiophene-benzo-X-diazole and thiophene-benzobis-X-diazole molecules, we performed time-dependent density functional theory (TDDFT) based calculations in conjugation with CAM-B3LYP xc- functional. It has been noticed that the absorption spectrum of all the examined D-A-D molecules show a dual-band spectrum, which is a typical hallmark of D-A and D-A-D based organic molecules. 1,9,30 The dual-band spectrum reveals the broad absorption characteristics of these molecules where the high energy band attributes to the π → π ∗ transition and the low band indicates an intra-molecular charge transfer transition. Here in our study, we consecutively studied the effect of donor and acceptor unit on the absorption spectrum and found that low, as well as high energy bands, can be shifted over a wide energy range through a proper controlling of D/A ratio, the strength of the acceptor moiety, and attaching a π-conjugated linker between A-A units within the molecules. We also demonstrate that how the intensity of peaks in the spectrum depend on the above mentioned parameters. Our systematic study elucidates the strong correlation between the configuration of the D-A-D molecules and it’s absorption spectrum, specifically in low-energy region, as that plays crucial role in predicting energy-efficiency of the organic opto-electronics devices.

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Computational Details Examined Molecules In the present work, we examined a series of 48 D-A-D scheme based organic molecules, in which thiophene was taken as the donor moiety and benzo-chalcogen-diazole was considered as the acceptor moiety, in non-bis as well as in bis-configuration. A typical structure of the investigated thiophene-benzo-X-diazole and thiophene-benzo-bis-X-diazole molecules, represented by general formula: mT-n(NXN-B)-pT and mT-n(NXN-B-NXN)-pT, respectively, is shown in Figure 1. In the general formula, m and p denote number of donor units present on the left and right side of the acceptor moiety, respectively, n signifies number of acceptor units present within the molecule, while X represents the chalcogenide atom, i.e., O, S, Se, or Te, present in the acceptor units. In order to investigate the effect of D/A ratio on the optical gap, in particular and the absorption spectra, in general, we varied number of donor (acceptor) units within the molecule by keeping the number of acceptor (donor) units same. This was done by having molecules with 1 to 4 acceptor units (n=1, 2, 3, 4) enclosed between 2 donor units (k=m+p=2) and molecules with total of 2 to 4 donor units (k=m+p=2, 3, 4) and single acceptor unit (n=1). This gave us a series of six molecules (having a particular type of acceptor moiety) with D/A ratio ∼ 0.5(n=4, m=1, p=1), 0.667 (n=3, m=1, p=1), 1.0 (n=2, m=1, p=1), 2.0 (n=1, m=1, p=1), 3.0 (n=1, m=2, p=1), and 4.0 (n=1, m=2, p=2) (Figure S1 in SI). The effect of strength of acceptor moiety on the optical properties was examined in two ways: (i) by considering the acceptor moieties in non-bis (Figure 1a) and bis (Figure 1b) configurations, and (ii) by changing the chemical composition of acceptor moieties by inducting different chalcogenides (X) in the acceptor units. Finally, to examine the role of π-conjugated linkage on the absorption spectrum and the optical gap for the molecules with D/A61, we introduced (-CH=CH-) linkage between the A-A units through various possible configurations. This gave us six new molecules with (-CH=CH-) linker, for both bis and non-bis configurations of the examined D-A-D molecules, as depicted in Figure

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Figure 1: Donor-Acceptor-Donor scheme based (a) thiophene-benzo-X-diazole and (b) thiophene-benzo-bis-X-diazole molecules, represented by general formula: mT-n(NXN-B)pT and mT-n(NXN-B-NXN)-pT, respectively, where m=1,2, n=1,2,3,4, and p=1,2. Here, k=m+p represents the total number of donor units and n denotes the number of acceptor units present in a molecule. Benzo-X-diazole and benzo-bis-X-diazole are the acceptor moieties, where X denotes the atoms from the chalcogen family, i.e., O, S, Se, and Te. S2 in SI. In general, to increase the solubility of these materials in organic solvents, they are synthesized with hexyl/octyl chains at the end of the terminal thiophene rings. However, for simplicity, instead of long alkyl chains we terminated the thiophene rings by methyl groups. We believe that replacement of hexyl/octyl chains by methyl groups will not affect our results in any significant way as the alkyl chains do not participate in photo-exciton. 31

Structure Relaxation and Optical Properties To demonstrate the absorption capability of examined molecules in the visible region of the solar spectrum, we studied the linear absorption spectrum of respective molecules using time-dependent density functional theory (TDDFT) as implemented in Gaussian09 software suite. 32 Prior to TDDFT calculations, all molecules were relaxed to their optimized ground state using density functional theory (DFT) in conjunction with hybrid B3LYP functional 33–35 and 6-311G(d,p) basis set. 36–39 However, specifically in case of molecules having tellurium (Te), Stuttgart-Dresden (SDD) valence basis set and effective core potential (ECP) were additionally used to describe the heavy Te atom. 40,41 The relaxed geometry of each molecule was obtained by considering atomic forces to be less than 0.0005 atomic 7



unit (au), while the convergence criteria for energy and density was set to 10−6 and 10−5 au, respectively. In TDDFT calculations, we computed lowest 50 singlet-singlet transitions for all molecules by adopting the range separated Coulomb attenuated xc-functional, CAMB3LYP 42 along with the same basis set used for DFT calculations. The CAM-B3LYP functional is already been demonstrated to provide reliable results for excitation energy in small D-A based molecules, where substantial intra-molecular charge transfer is involved. 43–45 For a better description of charge-transfer excitations, it splits the exchange term into shortand long-ranged components by considering 81% of B88 and 19% of HF exchange at shortrange, and 35% of B88 and 65% of HF exchange at long-range, with a range-separation parameter, γ =0.33. However, it is noteworthy to say that the choice of xc-functional is not always unique and more often it depends on the material under investigation. 31 Thus, in order to inspect the accuracy of an approximation for an appropriate explanation of experimental results, a comparative study with various xc-functional is essentially needed to provide a good quantitative guideline for subsequent research in this field. We therefore, along with CAM-B3LYP, considered various xc-functionals, such as generalized gradient approximation (GGA) based Perdew-Burke-Ernzerhof (PBE) functional 46 and hybrid functionals like Becke-3-parameters-Lee-Yang-Parr (B3LYP) 33–35 and Heyd-Scuseria-Ernzerhof (HSE06) 47–50 for describing the excitations. A Gaussian broadening of 0.3 eV was applied to plot the absorption spectra, while the components of transition dipole moment related to a pair of molecular orbitals were analyzed using Multiwfn software. 51

Results and Discussions Effect of Exchange-correlation Functional In order to study the effect of different xc-functionals on the optical properties, the absorption spectrum of one of the smallest considered molecule, 2T-1(NSN-B), was calculated using four different xc-functionals (Figure 2a). As expected, all spectra in the low energy 8


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region feature a typical dual band (I and II peaks), which is the hallmark of D-A based molecules. 1,9,30 The excitation energies, oscillator strengths, involved transitions, and the components of transition dipole moments corresponding to these I and II peaks of each spectrum are listed in Table S1 in SI. On analysing the data, we found that in all cases I and II peaks mainly originate due to |Hi → |Li and |Hi → |L + 1i transition, respectively, with prominent x−polarized dipole moment (Table S1 in SI). These common features clearly reflect a qualitative agreement between the results obtained using different xc-functionals. However, quantitatively, a substantial difference is observed in the peak positions and their intensities. In comparison to CAM-B3LYP, all xc-functionals predict a red-shifted spectrum. While PBE shifts the spectrum by ∼0.8 eV, HSE06 and B3LYP show a shift of ∼0.4 eV, as compared to the spectrum calculated using CAM-B3LYP. Also, in general, the I peak computed by all xc-functionals is found to be less intense as compared to the II peak, but, the difference between the intensities of both peaks (I and II) decreases on going from PBE to HSE06 or B3LYP to CAM-B3LYP. While, PBE estimates I peak to be ∼85% less intense than II, HSE06 and B3LYP predict the intensity of I peak ∼70% smaller than II peak. The CAM-B3LYP, however, calculates the intensity of I peak to be only ∼35% less intense than II peak, showing a good absorption capabilities in the low-energy region (visible region of the solar spectrum), as well, for a small molecule like 2T-1(NSN-B). Nevertheless, the accuracy of these predictions can only be validated on having a comparison with the experimental data. We, therefore, compare our results with the absorption spectrum measured by Sonar et al. 6 for a series of four molecules: 4T-1(NSN-B), 3T-1(NSNB), 2T-1(NSN-B), and 2T-2(NSN-B). A direct comparison between the calculated (using different xc-functionals) and measured H-L gap and peak positions for these four molecules are given in Tables S2 in SI & 1, respectively. It is clearly evident from Table 1 and Figure 2b that the peak positions and the relative intensities of I and II peaks calculated using CAMB3LYP with 6-311g(d,p) basis set are in very good agreement (∼ 95%) with the experimental results 6 for all the four molecules, while all other xc-functionals largely underestimate them.

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2T-1(NSN-B) -> PBE 2T-1(NSN-B) -> HSE06 2T-1(NSN) -> B3LYP 2T-1(NSN-B) -> CAM-B3LYP

40

II

4T-1(NSN-B) 3T-1(NSN-B) 2T-1(NSN-B) 2T-2(NSN-B)

II 40

II

II

I I

II

II

30

II

20

Intensity (a.u.)

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

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I I

10

I

II

30

I

20

I 10

I 0

0

1

2

3

4

0

5

0

1

Energy (eV)

2

3

4

5

Energy (eV)

(a)

(b)

Figure 2: (a) The low-energy linear absorption spectrum of 2T-1(NSN-B) computed using PBE, HSE06, B3LYP and CAM-B3LYP xc-functionals with 6-311g(d,p) basis set. (b) Calculated TDDFT spectrum of four experimentally investigated mT-n(NXN-B)-pT molecules obtained using CAM-B3LYP/6-311g(d,p) level of theory, where m=1,2, n=1,2, and p=1,2. Here, k=m+p represents the total number of donor units and n denotes the number of acceptor units present in a molecule. A Gaussian broadening of 0.3 eV is applied to plot the TDDFT spectrum. This is not surprising owing to the fact that except CAM-B3LYP, none of the examined xc-functionals incorporate the long-range interactions, which are crucial in describing the excited state properties of the systems where substantial intra-molecular charge transfer is involved, such as in D-A or D-A-D based molecules. 31,42–45 Also, noteworthy to mention here that though CAM-B3LYP accurately defines the optical properties, it highly overestimates the H − L gap (∆E). While, on the other hand, HSE06 calculated H − L gap (Table S2 in SI) agrees well with the measured electronic and optical gap. 6 This is not so surprising as it is a well known fact that CAM-B3LYP underestimates the H energy level (EH ) and overestimates the L energy level (EL ), 52–54 resulting a larger H − L gap. However, hybrid functionals (mostly HSE06, in case of organic molecules) that include 20% percentage of exact exchange, yield the H − L gap closer to the optical gap (if the transition corresponds to the first excited state, i.e., from H to L) at the DFT level only, due to systematic cancellation of errors. 55,56 Thus, in case where the first excited state 10


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corresponds to |Hi → |Li transition and one is just interested in knowing the optical gap and not the spectrum, one may safely rely on the Kohn-Sham gap computed using HSE06 xcfunctional within the framework of DFT. This is particularly useful in case of large molecules for which TDDFT calculations become extremely expensive. However, if the molecules are relatively small and exhibit intra-molecular charge transfer, then one should definitely follow the TDDFT route with CAM-B3LYP xc-functional that appropriately describes the effect of excitations, by including the correct 1/R-asymptotic behavior, where R is the charge separation distance. 45 In short, our comparative study on various xc-functional reveals that TDDFT within GGA approximation is unable to accurately predict excitation energies. At the same time, inclusion of certain percentage of exact exchange (Hartree-Fock) to the xc-functional (in case of HSE06 and B3LYP) can improve the results but only to some extent, as these functionals are known to be not suitable for the description of intra-molecular charge transfer excitations. On the other hand, CAM-B3LYP xc-functional appropriately describes the optical properties of system under consideration. Therefore, for rest of the study we will only present and discuss the results obtained using CAM-B3LYP xc-functional. Table 1: Position of low- and high-energy peaks in the dual band spectra of kT-n(NSNB) molecules calculated using TDDFT along with various xc-functionals with 6-311g(d,p) basis set. Here, k = 2 − 4 and n = 1 − 2 denote total number donor and acceptor units present in the molecules. Experimental values obtained by Sonar et al. 6 are also presented for comparison. Exp. 6

xc - functional PBE

HSE06

B3LYP

CAM-B3LYP

Name of the molecule I peak

II peak

I peak

II peak

I peak

II peak

I peak

II peak

I peak

II peak

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

4T-1(NSN-B)

1.23

2.34

1.65

2.76

1.61

2.71

2.04

3.37

1.95

3.39

3T-1(NSN-B)

1.31

2.64

1.75

2.98

1.69

2.92

2.13

3.48

2.03

3.55

2T-1(NSN-B)

1.43

3.03

1.87

3.50

1.80

3.46

2.23

3.92

2.25

3.90

2T-2(NSN-B)

1.57

2.96

1.89

3.50

1.84

3.45

2.20

4.17

2.21

4.00

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I

2T-4(NSN-B) 2T-3(NSN-B) 2T-2(NSN-B) 2T-1(NSN-B) 3T-1(NSN-B) 4T-1(NSN-B)

II

L+2

L+1

L

H

H-1

-1

3.11 eV

1.89 eV

3.28 eV

2.04 eV

3.81 eV

-4

I

2.26 eV

20

3.83 eV

II

I

-3 3.84 eV

II II

3.85 eV

II

I I

-2

1.93 eV

I

II 40

L+3

0

Energy (eV)

60

L+4

2.10 eV

1

80

1.99 eV

Effect of D/A ratio

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

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-5 -6 0

0

1

2

3

4

5

Energy (eV)

-7

(a)

2T4B

2T3B

2T2B

2T1B 3T1B

4T1B

(b)

Figure 3: (a) Low-lying linear absorption spectra of kT-n(NSN-B) molecules with varying D/A ratio. Molecules 2T-4(NSN-B), 2T-3(NSN-B), 2T-2(NSN-B), 2T-1(NSN-B), 3T1(NSN-B), and 4T-1(NSN-B) have donor to acceptor ratio as 0.5, 0.67, 1, 2, 3, and 4, respectively. A Gaussian broadening of 0.3 eV is applied to plot all TDDFT spectra. (b) Position of different molecular orbital energy levels of kT-n(NSN-B) molecules, obtained from the framework of DFT with HSE06 xc-functional and 6-311g(d,p) basis set. Here, red and blue arrows illustrate the dominant transitions, responsible for the origin of I and II peaks of the dual band spectrum of different kT-n(NSN-B) molecules, respectively.

Figure 4: Molecular orbitals corresponding to (a) H , (b) L, and (c) L + 1 molecular orbitals of 2T-1(NSN-B), drawn using an isosurface value of 0.02 |e|/au3

12


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Table 2: Excitation energies viz. peak positions, oscillator strength, major transitions involved, and the x− and y− polarized transition dipole moments corresponding to I and II peaks of the absorption spectrum, calculated using CAM-B3LYP xc-functional for kTn(NSN-B) molecules, where, k = 2 − 4 and n = 1 − 4. Name of

Peak

Peak

Oscillator

Transitions

Transition dipole moment Weights (a.u)

the molecule

2T-4(NSN-B)

position

energy (eV)

strength (a.u.)

involved

I

2.1166

1.9065

|Hi → |Li

II

4.3447

0.8242

2T-1(NSN-B)

3T-1(NSN-B)

4T-1(NSN-B)

y

0.8879

-6.2442

-0.0566

|Hi → |L + 4i

0.5616

-1.8601

-0.0492

|H − 1i → |L + 5i

0.1850

-0.8383

-0.0287

I

2.1424

1.4592

|Hi → |Li

0.9234

-5.9928

0.0000

II

4.1610

0.5828

|Hi → |L + 3i

0.4284

-1.6791

0.0000

|H − 1i → |L + 1i

0.1600

0.7921

0.0000

|H − 2i → |Li

0.1727

-0.0218

0.0000

2T-3(NSN-B)

2T-2(NSN-B)

x

I

2.1958

0.9649

|Hi → |Li

0.9550

5.3943

-0.0324

II

4.0962

0.6997

|Hi → |L + 2i

0.8098

2.3932

0.0764

I

2.2294

0.4256

|Hi → |Li

0.9806

4.1830

0.0000

II

3.9215

0.6256

|Hi → |L + 1i

0.9191

2.7078

0.0000

I

2.1347

0.6884

|Hi → |Li

0.9605

4.7734

0.2339

II

3.3980

0.4381

|Hi → |L + 1i

0.4629

1.8837

-0.2896

|H − 1i → |Li

0.4627

-0.2275

-0.4636

I

2.0396

0.9597

|Hi → |Li

0.9583

-5.3362

0.0000

II

3.3712

0.8772

|Hi → |L + 1i

0.8591

-2.7712

0.0000

To demonstrate the effect of number of donor and acceptor units present within the molecule (viz. D/A ratio), on the optical gap and linear absorption spectrum, we present the TDDFT calculated low-lying absorption spectra of thiophene-benzo-S-diazole (kT-n(NSNB)) molecules with different D/A ratio (D/A ∼ 2/4 (0.5), 2/3 (0.67), 2/2 (1), 2/1 (2), 3/1 (3), and 4/1 (4)), in Figure 3a. In particular, when the D/A ratio is 2, 3, or 4, donor units are increasing, while going from 2 to 0.5, acceptor units are increasing. On analysing the spectrum (Figure 3a), a typical dual-band spectrum is commonly noticed for all molecules, which reveals the broad absorption characteristics of D-A-D based organic molecules. 1,9,30 The low energy band in the spectrum indicates an intra-molecular charge transfer (ICT) ∗ from the donor to the acceptor unit (i.e., πdonor → πacceptor transition), while the high energy

band is attributed to the π → π ∗ transition along the conjugation length. The low energy band or the I peak in all spectra originates mainly due to |Hi → |Li transition where H and L are dominantly localized on the donor and the acceptor units, respectively (Figure 4 and Table 2). On the other hand, the II peak in the spectrum of all examined molecules is

13



found to originate primarily due to single particle |Hi → |L + ni transition, where n denotes the number of acceptor units present within the molecule (Table 2). This indicates that the origin of high energy peak in the spectrum strongly depends on the the number of acceptor units present in the molecule. However, when the orbitals related to these transitions are examined, we found that, irrespective to the value of n (i.e., number of acceptor units present in the molecule), all |L + ni orbitals possess similar character (Figure 4 and Figure S3 in SI). The electron density distribution of these |L + ni orbitals are mainly found to be localized on the donor units (i.e., thiophene rings) along with the thiadiazole side of the acceptor units. On analysing the energy of molecular orbitals in Figure 3b, it is noticed that |L + 4i, |L + 3i, |L + 2i, and |L + 1i orbitals of 2T-4(NSN-B), 2T-3(NSN-B), 2T-2(NSN-B), and 2T-1(NSNB) molecules, respectively, lie at almost same energy level, elucidating the stabilization of |L + ni orbital. On the other hand, when the number of acceptor units remains same (i.e., n=1) and donor units increase (k = 2, 3, 4), |L + 1i orbital shifts towards lower energy, showing destabilization of |L + 1i energy level in kT-1(NSN-B) molecules. For all examined molecules, the resultant spectrum is obtained by the absorption of strong x−polarized light where, the large value of transition dipole moment related to first peak over the second peak further establishes the phenomenon of strong intra-molecular charge transfer (ICT) from the donor to the acceptor unit (Table 2). 57,58 Furthermore, it is evident from the Figure 3a that the dual band characteristic does not depend on the D/A ratio but, the spectrum on the whole, does depend on that. The spectrum steadily gets redshifted with the increase in donor units of the molecule, which is in agreement with the measured UV-vis absorption spectra by Sonar et al. 6 Increase in donor units lead to the increase in conjugation length, in sync with the molecular length. This leads to stabilization of L and destabilization of H and L+1 (Figure 3b), which results into redshifting of the spectrum, and thereby, reducing the optical gap, when the D/A ratio increases from 2 to 4. 12,13 . Increase in acceptor units (i.e., when D/A ratio decreases from 2 to 0.5) however show a different trend in comparison with the spectrum obtained due to increase in donor

14


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units. Figure 3a shows that with the increase in number of acceptor units in the molecule, the first peak of the TDDFT spectrum gets slightly redshifted while, the second peak of the dual band spectrum shifts slightly towards higher energy (i.e., blue shifted) due to destabilization of L and stabilization of H and L + n energy levels (Figure 3b). However, this shift in both the peaks is relatively much less when D/A ratio decreases from 2 to 0.5, as compared to the case when D/A ratio increases from 2 to 4 (Figure 3a). This reveals that increase in D/A ratio (i.e., increase in donor units) can significantly help in shifting the whole spectrum towards lower energy, as a consequence of increase in conjugation length with the increase in molecular length. Although, increase in acceptor units (i.e., when D/A ratio decreases from 2 to 0.5) also increases the molecular length and helps to get redshifted spectrum as stated above, but, at the same moment, the spectrum show different behaviour in comparison with the redshifted spectra due to increase in donor moiety. For example, the molecular length of 2T-2(NSN-B) is higher than 3T-1(NSN-B) but, the absorption spectrum of 2T-2(NSNB) is found to be blue shifted and the optical gap to be at higher energy, as compared to 3T-1(NSN-B), in agreement with the experimentally measured absorption spectra by Sonar et al. 6 The reason behind the blue shift in the spectrum is the reduced conjugation length in 2T-2(NSN-B) due to increase of steric hindrance in between two acceptor units (dihedral angle between two acceptor unit is ∼28◦ ). Interestingly, the same phenomena is repeatedly found in the spectrum of 2T-3(NSN-B) and 2T-4(NSN-B) as compared to the spectrum of 3T-1(NSN-B) and 4T-1(NSN-B), where former gets blue shifted with respect to the later. Our calculations estimated the optical gap for 2T-4(NSN-B), 2T-3(NSN-B), 2T-2(NSN-B), 2T-1(NSN-B), 3T-1(NSN-B), and 4T-1(NSN-B) to be 2.12, 2.14, 2.20, 2.23, 2.13, and 2.04 eV, respectively, which is in good agreement with the available experimental results 6 along with the very good estimation of experimental H − L energy gap, calculated using DFT with HSE06 xc-functional and 6-311g(d,p) basis set (Figure 3b). It should also be noted here that the efficiency of organic photovoltaic (OPV) devices directly depends on the ability of effective absorption of photons by the device, which, in turn

15



depends on the the intensity of the absorption peaks. Therefore, together with the study of absorption energy or optical gap, the knowledge about intensity of the linear absorption spectra, in general, and oscillator strength (which signifies the intensity of the absorption peaks), in particular, is equally important. Our study elucidate that the intensity of the absorption peaks as well as their relative oscillator strength are also sensitive to the D/A ratio. It is found that with the increase in acceptor units as compared to increase in donor units, the oscillator strength increases drastically. In principle, oscillator strength depends on several factors, such as number of valence electrons, excitation gap, and transition dipole moments. On analysing Figure 3, it is evident that the intensity of the absorption peaks of 2T-1(NSN-B), 3T-1(NSN-B), 2T-2(NSN-B), 4T-1(NSN-B), 2T-3(NSN-B), and 2T-4(NSN-B) go along with the increase in number of electrons in these molecules. Thus, our study reveals that increase in acceptor units introduces the steric hindrance between the acceptor units of the molecule and reduces the conjugation length. This eventually results in a relatively higher optical gap, but such molecules are still promising as they absorb more light as compared to molecules with larger value of k (i.e., number of donor units) in kT-1(NSN-B).

Effect of Strength of the Acceptor Moiety As mentioned earlier, to study the effect of strength of the acceptor moiety on the absorption spectrum of the D-A-D molecules, we varied the strength of the acceptor moiety in two different ways: (i) by considering the bis-configuration of the acceptor moiety (benzo-Sdiazole), i.e., benzo-bis-S-diazole (Figure 1b), and (ii) by replacing “S” in the benzo-S-diazole by other chalcogenides like O, Se, or Te. Benzo-bis-S-diazole is known to be amongst the strongest acceptor because of the presence of hypervalent S atom, 59 while the presence of different chalcogenide atom (X) in the acceptor units (Benzo-X-Diazole; X=O, S, Se, and Te) has already been established to largely affect the strength of heterocyclic acceptor moieties. 9 Therefore, a comparison of absorption spectrum of molecules having these two different configurations of acceptor moieties (kT-n(NXN-B) and kT-n(NSN-B-NSN)) with spectrum 16


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of thiophene-benzo-S-diazole (kT-n(NSN-B)) molecules will be helpful in identifying the effect of strength of the acceptor moiety on the absorption spectrum of D-A-D molecules. We discussed the results of our study in subsequent subsections. Non-bis – bis-configurations The linear absorption spectrum of 2T-1(NSN-B) and 2T-1(NSN-B-NSN) are presented as a prototype in Figure 5a, to demonstrate the effect of non-bis and bis-configurations of the acceptor moiety on the optical properties of D-A-D molecules. Interestingly, the absorption spectrum of the molecule having bis-configuration of the acceptor moiety gets redshifted as compared to the non-bis configuration, but it is only the I peak (low energy band), which shifts substantially toward the lower energy, while no noteworthy difference is observed in the position of the II peak. This clearly elucidates the effect of strength of the acceptor moiety as the I peak originates due to intra-molecular charge transfer from donor to acceptor moiety and both the molecules differ only due to presence of different acceptor moiety. The relatively lower bond length alternation and thus, more delocalized electron density in Benzo-bis-SDiazole, destabilizes the L (which is localized over the acceptor moiety, Figure 4 and Figure S3 in SI) energy level of 2T-1(NSN-B-NSN) as compared to 2T-1(NSN-B). 12,13 Moreover, this low bond length alteration also affects the conjugation between the donor and acceptor units which results in destabilization of the H energy level (inset of Figure 5a). Thus, both of these effects, i.e., destabilization of H and L energy levels lead to ∼ 0.98 eV smaller optical gap and redshift of the absorption spectrum of 2T-1(NSN-B-NSN) molecule, as compared to 2T-1(NSN-B).

Similar to 2T-1(NSN-B), in 2T-1(NSN-B-NSN) also the I peak of the spectrum originates from |Hi → |Li transition, while, in contrast, instead of |Hi → |L + 1i, the II peak occurs majorly due to |Hi → |L + 2i, which also corresponds to π → π ∗ transition along the conjugation length and has almost the same characteristics as |L + 1i of 2T-1(NSN-B) (Figure S4 17



2T-1(NSN-B) 2T-1(NSN-B-NSN) L+1

L

H

70

II

-6 2T-1(NSN-B) 2T-1(NSN-B-NSN)

20

2T-4(NSN-B-NSN) 2T-3(NSN-B-NSN) 2T-2(NSN-B-NSN) 2T-1(NSN-B-NSN) 3T-1(NSN-B-NSN) 4T-1(NSN-B-NSN)

80

H+1

3.59 eV

1.12 eV

-4

3.81 eV

-2 2.26 eV

Energy (eV)

30

L+2

90

II

Intensity (a.u.)

0

40

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

I I

II

I

60

II

II

II II

II

50

I 40

I

30

I

20

10

Page 18 of 36

I I

10 0

0

1

2

3

4

5

0

0

1

Energy (eV)

2

3

4

5

Energy (eV)

(a)

(b)

Figure 5: (a) Comparison of absorption spectrum of 2T-1(NSN-B) and 2T-1(NSN-B-NSN) molecules to demonstrate the effect of strength of the acceptor moiety in case of non-bis and bis-configuration of it. Inset shows the different molecular energy levels for both the molecules. Here, red and blue arrows represent the dominant transitions, responsible for the origin of I and II peaks in the dual band spectrum of 2T-1(NSN-B) and 2T-1(NSN-BNSN) molecules, respectively. (b) Absorption spectra of kT-n(NSN-B-NSN) molecules with varying D/A ratio. Molecules 2T-4(NSN-B-NSN), 2T-3(NSN-B-NSN), 2T-2(NSN-B-NSN), 2T-1(NSN-B-NSN), 3T-1(NSN-B-NSN), and 4T-1(NSN-B-NSN) have D/A ratio as 0.5, 0.67, 1, 2, 3, and 4, respectively. A Gaussian broadening of 0.3 eV is used to plot all TDDFT spectrum. in SI). On analysing the intensity (viz. oscillator strength) of I peak, we found that it is correlated with the shift in the peak position and thus, in case of bis-configuration it decreases with the decrease in excitation energy, as compared to respective molecule with non-bis configuration. This happens as the relatively delocalized charge density in bis-configuration of molecule reduces the ability to separate intra-molecular charge and that leads to a decrease in the oscillator strength. The linear absorption spectrum of other thiophene-benzo-bis-Sdiazoles such as 4T-1(NSN-B-NSN), 3T-1(NSN-B-NSN), 2T-2(NSN-B-NSN), 2T-3(NSN-BNSN), and 2T-4(NSN-B-NSN) also show similar trends when compared with their respective non-bis counterparts, i.e., 4T-1(NSN-B), 3T-1(NSN-B), 2T-2(NSN-B), 2T-3(NSN-B), and 2T-4(NSN-B). Moreover, on analysing Figure 3a and 5b, we noticed that the spectrum of

18


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Table 3: Excitation energies viz. peak positions, oscillator strength, major transitions involved, and the x− and y− polarized transition dipole moments corresponding to I and II peaks of the absorption spectrum, calculated using CAM-B3LYP xc-functionals for kTn(NSN-B-NSN) molecules, where, k = 2 − 4 and n = 1 − 4. I and II peaks of the absorption spectrum, calculated using CAM-B3LYP xc-functionals for kT-n(NSN-B-NSN) molecules, where, k = 2 − 4 and n = 1 − 4. Name of

Peak

Peak

Oscillator

Transitions


the molecule

2T-4(NSN-B-NSN)

2T-3(NSN-B-NSN)

2T-2(NSN-B-NSN)

2T-1(NSN-B-NSN)

3T-1(NSN-B-NSN)

4T-1(NSN-B-NSN)

position

energy (eV)

strength (a.u.)

involved

I

1.2499

1.4296

|Hi → |Li

II

4.3022

1.0585

x

y

0.8967

7.5612

-0.2150

|Hi → |L + 8i

0.6085

1.8267

0.0361

|H − 1i → |L + 9i

0.2766

1.0133

0.0703

I

1.2935

1.0725

|Hi → |Li

0.9412

-7.2392

0.0000

II

4.2199

0.9373

|Hi → |L + 6i

0.7503

-2.2118

0.0000

|H − 1i → |L + 7i

0.1810

-0.7362

0.0000

I

1.3490

0.6890

|Hi → |Li

0.9853

6.5022

0.4852

II

4.1001

0.8063

|Hi → |L + 4i

0.8657

2.57520

0.0341

I

1.2495

0.2710

|Hi → |Li

0.9826

5.2579

0.0000

II

3.8375

0.7000

|Hi → |L + 2i

0.9420

3.0748

0.0000

I

1.1840

0.3907

|Hi → |Li

0.9726

-5.9503

-0.2989

II

3.3636

0.8690

|Hi → |L + 1i

0.5703

-2.2851

0.2752

|Hi → |L + 2i

0.2054

-0.6137

0.1836

I

1.1157

0.5221

|Hi → |Li

0.9714

6.6472

0.0000

II

3.0722

0.7248

|Hi → |L + 1i

0.2128

-1.7135

0.0000

|H − 2i → |Li

0.6861

-0.6430

0.0000

molecules having bis-configuration of acceptor moiety (kT-n(NSN-B-NSN)) also vary with respect to D/A ratio in a similar manner as the spectra of molecules with non-bis configuration (kT-n(NSN-B)), i.e., the spectrum of thiophene-benzo-bis-S-diazole molecules shifts toward lower energy when D/A ratio increases from 2 to 4, and relatively less effected when D/A ratio decreases from 2 to 0.5 In particular, our calculations predict the low (high) energy features of the dual band spectrum of 2T-4(NSN-B-NSN), 2T-3(NSN-B-NSN), 2T-2(NSN-B-NSN), 2T-1(NSN-B-NSN), 3T-1(NSN-B-NSN), and 4T-1(NSN-B-NSN) molecules to be at 1.25 (4.30), 1.29 (4.22), 1.35 (4.10), 1.25 (3.84), 1.18 (3.36), and 1.12 (3.07) eV, respectively. This clearly show that the spectrum shifts towards lower energy with the increase in donor units, i.e., in the increase of D/A ratio from 2 to 4, while with the increase in acceptor units, the enhanced steric hindrance between the acceptor units (dihedral angle > 50◦ ) disrupt the conjugation length, 12 which results into blue shift of the spectrum (both I and II peak) in case of 2T-n(NSN-B19



NSN), where n = 2, 3, and 4. Also, on comparing the peak positions of respective molecule in non-bis configuration, we found an approximate shift of 0.8 − 1.0 eV in the location of low energy band (peak I), while the high-energy band (peak II) remains almost at constant energy. This clearly reflects the dependency of optical properties of D-A-D based molecules on both, D/A ratio and strength of the acceptor moiety, and thereby, give us an opportunity to tune the optical gap in a wide energy range of ∼ 2 eV by appropriate variation in D/A ratio and selection of the non-bis or bis-configuration of acceptor unit. Similar to molecules with non-bis configuration (kT-n(NSN-B)), the low energy band or the I peak in the spectra of kT-n(NSN-B-NSN) molecules originates mainly due to |Hi → |Li transition where H and L are dominantly localized on the donor and the acceptor units, respectively (Figure S4 in SI and Table 3). On the other hand, in contrast, the II peak in the spectrum of all examined bis-molecules is found to be mainly due to single particle |Hi → |L + 2ni transition (instead of |Hi → |L + ni transition). Basically, the presence of two thiadiazole units in the bis-configuration of an acceptor unit leads to transition to the L + 2n energy level, where n denotes the number of acceptor units present within the molecule (Table 3). This again indicates that similar to kT-n(NSN-B), in kT-n(NSN-B-NSN) molecules also, the origin of high energy peak is sensitive to the number of acceptor units present in the molecule. Alternatively, when the number of acceptor units remains same (i.e., n=1) and donor units increase (i.e., k = 2, 3, 4), the II peak originates mainly due to |Hi → |L + 2i transition, irrespective of the number of donor units present in the molecule. For all examined molecules, the resultant spectrum is obtained by the absorption of strong x−polarized light where, the large value of transition dipole moment related to first peak over the second peak further establishes the phenomenon of strong intra-molecular charge transfer (ICT) from the donor to the acceptor unit (Table 3). 57,58

20


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Different Chalcogenides Variation of a single atom in the heterocyclic donor or acceptor unit can also change the strength of it, and thereby, the opto-electronic properties of the whole molecule. 12,20 We, therefore, substitute the “S” atom in the Benzo-S-Diazole and Benzo-bis-S-Diazole units of all investigated molecules by other chalcogenides (X) like “O”, “Se”, or “Te”, and studied the effect of change in the strength of the acceptor moiety due to presence of these distinct chalcogenides on the absorption spectrum. Figures 6a and b present the absorption spectrum of 2T-1(NXN-B) and 2T-1(NXN-B-NXN) molecules with X = O, S, Se, or Te, respectively. On examining the spectra we found that substitution of different chalcogenides do not effect the typical dual bands structure and regardless of the choice of chalcogenide atom, each spectrum exhibits a low and a high intensity band at low and high energy (within the visible range of solar spectrum), respectively. Interestingly, both the bands (I & II peak) consistently shift towards lower energy on going from O to S to Se to Te, in both non-bis and bis-configurations of the molecule. Moreover, the intensity of the I peak also reduces on going from O to Te, while the intensity of II peak increases in case of kT-n(NXN-B) and remains almost intact in kT-n(NXN-B-NXN). These results are in agreement with the calculations of Gibson et al., 20 who demonstrated similar variation in a series of cyclopentadithiophenebenzochalcogenodiazole copolymers. Moreover, such variations in the spectra are also very much similar to the trends observed in the previous subsection where we discussed the change in the acceptor strength due to non-bis and bis-configuration. Thus, it is clear that intramolecular charge transfer gets affected due to change in the strength of the acceptor moiety, even if the strength varies due to replacement of a single atom in the acceptor moiety. This eventually affects the low energy band of the spectrum and thereby, the optical gap.

21



50

II II

2T-1(NON-B) 2T-1(NSN-B) 2T-1(NSeN-B) 2T-1(NTeN-B)

2T-1(NON-B-NON) 2T-1(NSN-B-NSN) 2T-1(NSeN-B-NSeN) 2T-1(NTeN-B-NTeN)

II 40

II

I

Intensity (a.u.)

30

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

Page 22 of 36

20

I I I

II II II II

30

II’ 20

I I

10

I

10

I 0

0

1

2

3

4

0

5

0

1

Energy (eV)

2

3

4

5

Energy (eV)

(a)

(b)

Figure 6: Absorption spectrum of molecules containing different chalcogenide atom (X = O, S, Se, or Te) in the (a) non-bis configuration of acceptor moiety, i.e., 2T-1(NXN-B), and (b) bis-configuration of acceptor moiety, i.e., 2T-1(NXN-B-NXN). Individually both graphs demonstrate the effect of strength of the acceptor unit on the absorption spectrum due to presence of different chalcogenide atom in the acceptor unit, while comparison of both graphs elucidates the effect of strength of the acceptor moiety due to it’s non-bis and bis-configuration on the absorption spectrum.

Our calculations predict the location of I peak (viz. optical gap) to be at 2.35, 2.23, 2.04, and 1.97 eV for 2T-1(NON-B), 2T-1(NSN-B), 2T-1(NSeN-B), and 2T-1(NTeN-B), respectively, and 1.23, 1.25, 0.90, and 0.65 eV in case of 2T-1(NON-B-NON), 2T-1(NSN-B-NSN), 2T-1(NSeN-B-NSeN), and 2T-1(NTeN-B-NTeN), respectively. This suggests that the optical gap can be tuned and the spectrum can be shifted by ∼ 0.2 − 2.3 eV, just by changing the chalcogenide atom in the acceptor units of the D-A-D based molecules(Tables S3-S8 in SI). Similar behaviour is noticed in other investigated D-A-D molecules also, when D/A ratio was 0.5, 0.67, 1, 2, 3, and 4 (Figures S5 – S7 and Tables S3-S8 in SI). Thus, by considering a molecule with suitable D/A ratio, acceptor unit containing appropriate chalcogen atom, and choosing non-bis or bis-configuration of it, the absorption spectrum can be widely tuned and optical gap can efficiently be modulated between the infrared to visible region of the solar spectrum. 22


Page 23 of 36 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


Table 4: Excitation energies viz. peak positions, oscillator strength, major transitions involved, and the x− and y− polarized transition dipole moments corresponding to I and II peaks of the absorption spectrum calculated using CAM-B3LYP xc-functional, of 2T1(NXN-B) molecules with X = O, S, Se, or Te, presented in Figure 6a. Name of

Peak

Peak

Oscillator

Transitions


the molecule

position

energy (eV)

strength (a.u.)

involved

x

y

I

2.3484

0.5855

|Hi → |Li

0.9822

-4.5687

0.0000

II

4.1260

0.4785

I

2.2294

0.4256

|Hi → |L + 1i

0.8884

-2.2195

0.0000

|Hi → |Li

0.9806

4.1830

0.0000

II

3.9215

0.6256

|Hi → |L + 1i

0.9191

2.7078

0.0000

2T-1(NON-B)

2T-1(NSN-B)

I

2.0425

0.3306

|Hi → |Li

0.9801

4.0194

0.0000

II

3.8365

0.6812

|Hi → |L + 1i

0.9134

2.8885

0.0000

I

1.9726

0.2725

|Hi → |Li

0.9808

-3.9377

0.0000

II

3.9056

0.7218

|Hi → |L + 2i

0.8951

2.9850

0.0000

2T-1(NSeN-B)

2T-1(NTeN-B)

The redshift in the spectrum and decrease in the optical gap on having chalcogenide from O to S to Se to Te in the acceptor moiety arises due to decrease in the electronegativity of group 16 elements upon going down in the group (O→Te). Low electronegativity lowers the ionization potential of the heavier atom which leads to destabilization of L orbital (localized on acceptor units), and reduction in the optical gap or redshift of the spectrum. Moreover, on substituting heavier chalcogenide atom instead of a lighter one (O (lightest)→Te (heaviest)), the X-N bond length increases, which in turn changes the aromaticity of the benzene ring, 12 and destabilizes the H energy level, which also contributes to the lowering of optical gap. 20 As mentioned above, the intensity of low energy band also decreases on going from O to S to Se to Te. The reason of decrease in intensity is again the low electronegativity of heavier chalcogenides, which reduces the ability to separate intra-molecular charge, and thus, leads to lower excitation energy and corresponding decrease in the intensity of I peak of the absorption spectrum when moving from O to Te. 20 On analysing transitions from Table 4 and 5, and tables provided in Supporting Information (Tables S3 – S8), we found that irrespective to the type of chalcogenides, the low energy band corresponds to the |Hi → |Li transition, while high energy band in most of the cases, originates mainly due to |Hi → |L + 1i and |Hi → |L + 2i transition in molecules having non-bis and bis-configuration of acceptor 23



Page 24 of 36

Table 5: Excitation energies viz. peak positions, oscillator strength, major transitions involved, and the x− and y− polarized transition dipole moments corresponding to I and II peaks of the absorption spectrum calculated using CAM-B3LYP xc-functional, of 2T1(NXN-B-NXN) molecules with X = O, S, Se, or Te, presented in Figure 6b. Name of

Peak

Peak

Oscillator

Transitions


the molecule

2T-1(NON-B-NON)

position

energy (eV)

strength (a.u.)

involved

x

y

I

1.2302

0.3316

|Hi → |Li

0.9809

6.0249

0.0000

II

3.8822

0.6892

|Hi → |L + 1i

0.1125

0.4699

0.0000

|Hi → |L + 2i

0.8382

2.2754

0.0000

5.2579

0.0000

I

1.2495

0.2710

|Hi → |Li

0.9826

II

3.8375

0.7000

|Hi → |L + 2i

0.9420

3.0748

0.0000

I

0.8995

0.1605

|Hi → |Li

0.9825

-5.4060

0.0000

II

3.7160

0.7888

|Hi → |L + 3i

0.9505

-3.2623

0.0000

2T-1(NSN-B-NSN)

2T-1(NSeN-B-NSeN)

2T-1(NTeN-B-NTeN)

I

0.6491

0.0918

|Hi → |Li

0.9838

-2.0779

0.0007

II’

3.1035

0.3430

|H − 4i → |Li

0.8871

0.0016

4.1781

II

3.7385

0.8011

|Hi → |L + 4i

0.9437

-3.2516

0.0012

moiety, respectively, especially when there is increase in the donor units and when X = O, S, and Se. While, when the acceptor unit increases or when X=Te, the high energy band originates mainly due to transition from H to relatively higher unoccupied molecular orbital. Also, in all the examined molecules, the transitions are majorly dominated by x−polarized dipole moments.

Effect of the π–conjugated linker between A-A units As shown and discussed above, the steric hindrance between the acceptor units substantially influence the optical properties of the examined D-A-D based molecules by increasing the optical gap and varying the absorption capabilities. To reduce this steric hindrance and increase the conjugation length as well as the effective absorption capabilities of D-A-D molecules in low energy region, planner molecules are realized through introduction of π- conjugated ethylene (-CH=CH-) linkage in rational designing of D-A-D molecules. Interestingly, our results demonstrate that attaching a (-CH=CH-) linkage in between two acceptor units remarkably 24


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Figure 7: Comparison of absorption spectra of rationally designed D-A-D molecules with and without (-CH=CH-) linkage attached between the A-A units for demonstrating the role of π-conjugated linker on optical properties of the molecules having D/A 6 1. Figures (a)-(c) and (d)-(f) show comparison of linear absorption spectrum of 2T-n(NSN-B)-Lq and 2T-n(NSN-B-NSN)-Lq molecules, respectively, with the corresponding molecule having no linkage. Here, n varies from 2–4 and q=1–3. The optimized structures of these molecules are shown in Figure S1 & S2 of SI. reduce the dihedral angle between them ( ∼ 5◦ ). For example, in case of 2T-2(NSN-B) and 2T-2(NSN-B-NSN), the dihedral angle between the acceptor units was found to be ~ 28◦ and 50◦ , respectively. But, with the introduction of the π- conjugated (-CH=CH-) linkage, these angles are found to be reduced to less than 2◦ . The change of dihedral angle between A-A units is, therefore, found more effective in bis configuration of the molecules as compared to its non-bis counterpart. Moreover, as the active conjugation length also increases with the decrease of dihedral angle, the low energy spectrum of these newly designed molecules is found to shift towards the red end of the solar spectra as compared to similar molecules having no linkage (Figure 7). However, the amount of redshift of the I peak of the spectrum is inherently more distinguishable in each bis-configuration of the molecules than their respective non-bis configuration (Figure 7). For example, in case of molecule 2T-2(NSN-B-NSN) with (-CH=CH-) linkage (2T-2(NSN-B-NSN)-L1), the I peak of the spectrum is found to be redshifted by ∼ 0.27 eV as compared to the same molecule having no linkage (Figure 7d),

25



while just a minute shift of ∼0.08 eV is seen in case of 2T-2(NSN-B)-L1 (Figure 7a). The redshift of the low energy spectra, and hence, the decrease in the optical gap, are also noticed for other molecules with D/A ≤ 1, i.e., 2T-3(NSN-B) and 2T-4(NSN-B), and their respective bis-configurations. However, with the increase in acceptor units, there comes the possibility to include linkage at different locations to model various configurations. For example, in case of molecule 2T-3(NSN-B), we made two possible configurations: (i) A-L-A-L-A and (ii) A-L-A-A, where L denotes the linker between the acceptor (A) units. Here, an optimal efficiency is observed for the configuration where we put a (-CH=CH-) linker in between each A-A unit (i.e., configuration (i)). Thus, the absorption capability of the I peak and the optical gap are mostly found to enhance and reduce, respectively, from D-A-A-A-D to D-A-L-A-A-D to D-A-L-A-L-A-D (Figure 7b). The same effect or trend is noticed in case of 2T-3(NSN-B-NSN), 2T-4(NSN-B), and 2T-4(NSN-B-NSN) molecules (Figure 7e, 7c, and 7d). Thus, our calculations show that the introduction of a linkage or a spacer between the acceptor units makes the molecule planar by substantially reducing the steric hindrance between the acceptor units. This helps in lowering the optical gap, which was otherwise found to increase with the decrease in D-A ratio (in case of bis-configuration of the molecules). The graph, shown in Figure S8 of SI, clearly depicts the variation/reduction in the optical gap of the kT-n(NSN-B), and kT-n(NSN-B-NSN) molecules, when D/A ≤1. It is also noticed that the origin of I & II peaks does not depend on the (-CH=CH-) linker. In case of each molecule (Figure S2 in SI), I peak originates due to |Hi → |Li transition, while the origin of the II peak depends on the number of acceptor units present within the molecule (Table 9 and 10 in SI).

Conclusions In this paper, using the time dependent density functional theory based calculations in conjugation with CAM-B3LYP xc−functional, we studied the dependence of the linear optical

26


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absorption spectrum of donor-acceptor-donor based molecules on three factors: the D/A ratio, strength of the moiety, and role of π−conjugated linker between A-A moieties. Our calculated spectrum for all investigated molecules are found to exhibit a dual band in the low energy region. Both of these bands of the spectrum are found to be redshifted with the increase in the donor units (i.e., when D/A ≥ 2), while increase in the acceptor units (i.e., when D/A ≤ 2) leads to relative blue shift of the spectrum due to increased steric hindrance between the acceptor units, which is in excellent agreement with the measured spectra by Sonar et al. 6 The spectrum is also found sensitive to the strength of the acceptor moiety, but this does not influence the qualitative nature of the shift of the spectrum due to D/A ratio. When the strength of the acceptor moiety is varied by considering non-bis and bis-configuration of it, interestingly, it is the low-energy band of the spectrum that gets mainly affected. While, when “X” is changed from O to S to Se to Te in each acceptor unit, the whole spectrum gets redshifted. In both the cases, optical gap is found to decrease due to destabilization of both H and L energy levels. The optical properties of these strategically designed D-A-D molecules in low energy region are also found to vary with the introduction of a linkage between A-A units. Addition of π–conjugate-linker between A-A units reduces the steric hindrance between them and thereby, enhances the π–π interaction, which eventually helps in reducing the optical gap of these molecules. Overall, our calculations demonstrated that the linear absorption spectrum of thiophene-benzo-(bis-)X-diazole based D-A-D molecules can be significantly shifted and the optical gap can be widely tuned over a range of ∼ 0.2 − 2.3 eV, by strategically choosing D/A ratio and acceptor moiety of appropriate strength, as well as by introducing a π−conjugated linker between the A-A moieties. The intensity of the absorption peaks is also found to be sensitive to all three of these factors, suggesting the possibilities of tuning of absorption capabilities of these D-A-D molecules. In brief, the results obtained by our calculations establish a direct correlation between the linear optical properties and configuration of the D-A-D molecules, which will be helpful in further rational designing of promising molecules for opto-electronic applications,

27



in general, and solar cells, in particular.

Acknowledgement P.J. acknowledges the support provided by Grant No. SR/FTP/PS-052/2012 from Department of Science and Technology (DST), Government of India. The high performance computing facility available at the School of Natural Sciences, Shiv Nadar University, was used to perform all calculations.

Supporting Information Available The following files are available free of charge. Structure of all examined molecules, different energy orbitals, optical absorption spectra, HOMO-LUMO gap, excitation energies viz. peak positions, oscillator strength, major transitions involved, and the x− and y− polarized transition dipole moments corresponding to I and II peaks of the all examined absorption spectrum.

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