TD-DFT Study of Dipole Moment

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C: Energy Conversion and Storage; Energy and Charge Transport

From Molecules to Devices: A DFT/TD-DFT Study of Dipole Moment and Internal Reorganization Energies in Optoelectronically Active Aryl Azo Chromophores Ujla Daswani, Usha Singh, Pratibha Sharma, and Ashok Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04070 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

From Molecules to Devices: A DFT/TD-DFT Study of Dipole Moment and Internal Reorganization Energies in Optoelectronically Active Aryl Azo Chromophores Ujla Daswani,1 Usha Singh,2 Pratibha Sharma,1 Ashok Kumar*,1 1

School of Chemical Sciences, Devi Ahilya Vishwavidyalaya, Takshashila Campus, Khandwa Road, Indore (M.P.) 452001, India 2 Department of Physics, IPS Academy, Rajendra Nagar, Indore (M.P.) - 452012, India. *Corresponding author’s E-mail: [email protected]

Abstract Present paper elicits a very useful, computational exploration of molecular architectonics of benzothiazole scaffold. The investigation elucidates hopping transport phenomenon in donor-acceptor ensembles lying within a same molecule, comprising an azo group as the core entity. Author reply: In depth electronic and charge transfer behavior of certain substituted azo benzothiazole (organic π conjugated) considering notions of Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) has been investigated. Moreover, the effect of structural variation in aryl azo moiety (-CF3, -SCH3) in presence/absence of solvent has been examined. Also, the impact of disparate solvents viz., polar protic, polar aprotic, and nonpolar solvents has been deduced. Interestingly, results

indicate

that

(E)-2-((4-(trifluoromethyl)phenyl)diazenyl)benzo[d]thiazole

(BAF)

and

(E)-2-

(phenyldiazenyl)benzo[d]thiazole (BAB) have affirmed to be the promising candidates for the organic charge transfer material in Organic Light Emitting Diodes (OLEDs). It was observed that the substituent (-SCH3)

deeply

perk

up

the

properties

of

resulting

compound

i.e.,

(E)-2-((4-

(methylthio)phenyl)diazenyl)benzo[d]thiazole (BAS) which demonstrated to be an efficient entrant for photovoltaic devices (Dye Sensitized Solar Cells (DSSCs)) as dictated by the internal reorganization energies. Furthermore, in order to substantiate these results vis-à-vis to gain a deep insight to consider these molecules as powerful hole/electron carrier mobilizer, their electron density has also been computed. Results obtained by Natural Bond Orbital (NBO) analysis, provide a strong support to the intramolecular charge transfer properties (ICT). An unprecedented explanation of change in the dipole moment substantiates the ICT properties. Besides, HOMO-LUMO gaps, ionization potentials (IPs), electron affinities (EAs), chemical hardness, and light harvesting efficiency (LHE) have been computed to comprehend the nature of the moiety in a more ameliorate way. Author reply: Also, vibrational findings of BAS placed it as a propitious candidate for in vivo biosensing applications.

1

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1. Introduction Conjugated heterocyclic compounds have emerged as a major contributor of photovoltaic devices,1 organic light-emitting diodes,2 electro generated chemiluminescence, biological imaging probes, biosensors3 LASERS,4 field-effect transistors5,6 and sensors owing to the presence of hole/electron hopping transport propensity in them. Investigatory studies on such a unique structure-property relationship lend a platform to design new architecture of organic electronic devices.7-10 Moreover, conjugated11 heterocyclic compounds offer excellent superiority over existing inorganic materials in terms of their low cost,12 easy assembly,13 light weight,14 possible recyclability and flexible substitution15 in their skeletal architecture. Moreover, substitution in the ring profoundly alters the physical properties of organic compounds viz., solubility enhancement upon attaching long chain of alkyl substituents. However, alteration in electronic energy levels can be accustomed by introducing dopants. Additionally, use of metal-free organic compounds16 have created bonus rewards viz., no noble metal resource restriction, high molar absorption coefficient17 tunable spectral absorption response from the Vis to NIR regions.18 These striking features have provided a great opportunity and flexibility towards advancement in this sphere. Moreover, in designing and development of such compounds, computational techniques provide a molecular rationale to experimentally substantiate the findings.19-24 In order to unwrap the assessment and to design the new materials with optoelectronics applications, special focus is mandatorily required to understand the hopping properties of holes/electrons, HOMOLUMO gaps, ionization potentials, electron affinities and also the effect of appending donor-acceptor groups in the structure. Though the holes/electrons mechanism operating in organic materials has been studied, however, theoretical understanding in terms of molecular design is still very scarce. In the extension of our work on computational tactics,25-31 our group reports for the first time the justification of change in the dipole moment which substantiates the ICT properties. The major objective of this treatise is to elucidate properties of unexplored molecules utilizing computational implements thereby enabling options for future optoelectronic contrivances with amended performances. 2. Computational All molecular calculations were performed by notions of DFT with B3LYP hybrid functional32-39 implanted in the Gaussian 09 program without symmetry constraints and the 6-311++G(d,p) basis set, to assess complete structure. Absorption and Emission spectra of all the compounds were simulated using 2


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the TD-DFT with above mentioned functional and basis sets. The TD-DFT has been considered as a reliable means for theoretically predicting optoelectronic properties of organic systems. In order to visualize the molecules, a graphical user interface Gauss View 05 was utilized in the current analysis. In a view to develop a computational design of the transport phenomenon taking place in the compounds, the reorganization energies of holes and electrons of the moiety were simulated. Further, to gain additive comprehension in elucidating the reorganization energy, electron density has also been investigated. In order to understand the polarizability property the dipole moments of all the seven states ( ,

,

and

,

,

,

) are computed. Similarly, the Ionization Potential, the Electron Affinity, and the

chemical hardness are computed for the fully optimized compounds without any imaginary harmonic frequencies. Using TD-DFT 6-311++G(d,p),

/

is obtained from

and

from

and

from

to compute the value of reorganization energies. To harvest the light efficiency of the hole transport properties of the organic materials as in Dye Sensitized Solar Cells (DSSCs), Light Harvesting Efficiency (LHE) of the moiety is figured out. The gas phase reorganization energies were evaluated while considering the concept of solvatochromism. We also looked in to the effect of various solvents including polar protic (methanol), polar aprotic (acetone), and nonpolar (cyclohexane) on reorganization energies as a default model. Electron/charge hopping transport has been appraised on the basis of Marcus theory40 with the help of the

following equation

(1): (1)

Where kB represents Boltzmann constant, T is the temperature, ħ is the Planck’s constant, V informs about the coupling matrix element (transfer integral) between the two adjacent moieties dictated largely by orbital overlap, and λ is the reorganization energy due to geometric relaxation accompanying charge transfer. The charge transfer rate is greatly influenced by V and λ. However, λ exerts this predominantly and comprises of external reorganization energy (λext) and internal reorganization energy (λint). The λext represents the impact of enraptured medium on charge transfer; then again λint is the measure of auxiliary change amongst ionic and neutral states. λext governs the energy change of the electronic and nuclear polarization in biological environments and have small values for condensed-state systems41,42 and therefore, λext can be ignored. 3



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Therefore, the prime focus is to envisage the λint of the targeted organic moieties. Using B3LYP/6311++G(d,p) basis set, the reorganization energy43 for electron (λe) and hole (λh) of the molecules have been computed for the optimized neutral, anionic and cationic geometries. λe and λh are defined by equation (2) and (3) as

(2)

(3)

Where,

represent the energy of the cation (anion) of the optimized structure of the neutral

molecule. Likewise,

represents the energy of the cation (anion) calculated with the optimized

cation (anion) structure,

which represent the energy of the neutral molecule calculated at the

optimized cationic (anionic) state. Lastly,

is the energy of the neutral molecule at the optimized

ground state. The absolute hardness44 of the targeted moieties has been determined from a perspective of Molecular Orbital Theory using the following equation (4) (4)

Where, IP stands for Adiabatic Ionization Potential45 and EA stands for Adiabatic Electron Affinity.45 These were calculated using the following formula (equation 5 and 6) (5) (6) Where,

,

, and

are the total energies of neutral, cationic and anionic forms of the molecules

computed at the B3LYP/6-311++G(d,p) level, respectively.

3. Results and discussion For the sake of convenience and to gain deep insight about hopping transport in Donor-Acceptor moieties, all calculations were carried out in two segments (i) in the absence of a solvent (gas phase) and (ii) in the presence of a solvent (liquid phase). The influence of solvent systems was examined using default model. The three different categories of solvents viz., Methanol (polar-protic), Acetone (polar4


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aprotic) and Cyclohexane (non-polar) were used to compute the energies of optimized neutral ( cationic (

) and anionic (

),

) moieties using TD-DFT theory with all positive harmonics.

3.1 Structural Parameters The optimized structural parameters of BAS, BAB, and BAF in neutral, cationic and anionic states are computed at the B3LYP/6-311++G(d,p) level in all the solvents and gas phase and are mentioned in this report. The optimized geometries of BAS, BAB and BAF for neutral, cationic and anionic states are shown in Fig. 1 (a-l). In the Fig. 1 (a-l) all the pink, yellow, blue, orange and cream balls represents carbon, sulfur, hydrogen, nitrogen and fluorine, respectively. It is revealed from the angles of the optimized anion structure of BAS depicted in Fig. 1 (b) and (d) that substituent -SCH3 flips away from the observer in comparison to the neutral state. However, no structural change was observed in cationic state of BAS.

(a) BAS optimized neutral angles

(c) BAS optimized neutral dihedral angles

(b) BAS optimized anion angles

(d) BAS optimized anion dihedral angles

Further, it is important to notice from the structures of BAB (Fig. 1 (e-h)) and BAF (Fig. 1 (i-l)) that a twisted geometry is resulted for their cationic optimized structures owing to the co-planarity in benzothiazole and arly azo chromophoric fraction along with the tilt in the benzene ring. It is inferred that deviation from the planarity due to the flip in -SCH3 group of BAS as well as twist in BAB and BAF structures causes significant changes in the dihedral angles.

5



(e) BAB optimized neutral angles

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(f) BAB optimized cation angles

(g) BAB optimized neutral dihedral angles

(h) BAB optimized cation dihedral angles

(i) BAF optimized neutral angles

(j) BAF optimized cation angles

(k) BAF optimized neutral dihedral angles

(l) BAF optimized cation dihedral angles

Fig. 1. Structural parameters of BAS, BAB and BAF in neutral ( (

), cationic (

) and anionic states

)

Such changes in dihedral angle are correlated with the total reorganization energy. Dihedral angle decreases with increasing strength of the donor moiety (-SCH3), which in turn makes the BAS more 6


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rigid and planar in comparison to BAB and BAF. This has resulted in the increase of the hopping of holes and, thus enhancing the activity of BAS as a Hole Transport Material (HTM), which is a prerequisite feature for molecules to behave as DSSCs. 3.2 Reorganization energies Reorganization energy (λ) is one of the key parameters governing the hopping rate. It is equivalent to activation energy barrier of a hole/electron transfer process which needs to be minimized to augment the performance of optoelectronic devices. It is, therefore, a prerequisite to have a low value of λ for high hopping charge mobility. Theoretical investigations to calculate charge mobility in the case of π conjugated heterocyclic compounds is determined by hopping transport process at room temperature. In order to compute reorganization energies of electron/hole, total energies of the neutral, cationic, anionic and other excited states are determined (provided in supplementary data) for the polar protic solvent ca., methanol. If λe < λh , material is an electron transporter and if λh < λe material behaves as a hole transporter. According to the concept of reorganization energies Table 1 reveals that BAB and BAF are electron transporter while BAS is a hole transporter. It is worth citing herein that with the augmentation of the substituent (-SCH3) at the para position an electron transporter molecule (BAB) can act as a hole transporter (BAS). Furthermore, different substitution viz, -CF3 (BAF) induces partial delocalization in – CF3 which obstruct back electron delocalization. Moreover, a high value of λh (0.9364) for BAF validates its behavior as a hole blocking agent. Thus, the results obtained from analysis of solvatochromic behavior (Table 1) demonstrate that the increase in dielectric constant of the solvents brings about the best internal reorganization energies of compounds. Table 1: Internal reorganization energies in the absence/presence of solvents having different dielectric constants:

S. No.

1

Solvent free Compound

BAB

Methanol

Acetone

Cyclohexane

(dielectric

(dielectric

(dielectric

constant = 32.6)

constant = 20.7)

constant = 2.02)

λe

λh

λe

λh

λe

λh

λe

λh

0.4830

0.6770

0.4538

0.7156

0.4547

0.7148

0.4710

0.6905

Transporter

Electron (λe< λh) 7



2

BAF

0.5272

0.8398

0.4631

0.9364

0.4644

Page 8 of 30

0.9422

0.4986

0.8245

Electron (λe< λh)

3

BAS

0.6493

0.4349

0.5610

0.2077

0.5637

0.2113

0.6164

0.3994

Hole (λh< λe)

It is evident from the data depicted in Table 1, that methanol having highest dielectric constant, behaves as the best media for transporting/blocking materials. It is observed that with no substituent ca., BAB is acting as an electron transporter. Further, upon replacing H by -CF3 at para position (BAF) of the benzene ring, an obstruction in the migration of holes is observed. The value of λe is approximately equal for BAB and BAF in all the solvent/gas phases and the difference of λh and λe in BAF is large in comparison to BAB (almost double, Table 1). Also, the dipole moments of BAB are almost equal for all the seven states whereas for BAF there is significant change in dipole moments (Table 7). Therefore, BAF is considered to be the better hole blocking material than the BAB. However, on the contrary to our surprise if H is replaced by -SCH3 in para position (BAS) of the benzene ring, the targeted moiety behaves as the Hole transporter. Conclusively, for the first time we are observing that with a pertinent substitution in the electron transporter moiety it can behaves as a hole transporter. Author reply: Upon comparative analysis of Table 1 results with others reported in literature, BAS was found to have even much better value of λh (0.207 eV) than the standard hole transport material (0.290 eV) N,N'-diphenyl- N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD).46 Although λe value for BAF was found to be much greater than the standard AlQ347 (λe = 0.276 eV) but it was found to be much better (λe = 0.657 eV) as compared to the sample (HEGRUG) reported in paper.48 Moreover, no one has reported the difference between λh and λe values affecting their behavior as better hole blocking agent viz., BAF as in the present case. In order to understand their activity, attention has been paid to its chemistry. In general, benzothiazole ring acts as an electron acceptor and substituted benzene ring acts as an electron donor in push pull chromophoric systems. However, with optimized results, it was observed that addition of electron donating group like -SCH3 to the para position of (E)-2-(phenyldiazenyl)benzo[d]thiazole reverses the nature of the moiety. It behaves as appropriate HTM for construction of functional organic materials (Fig. 2 (a) and (b)). The rationale behind this umpolung activity could be ascribed to the presence of 8


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electron rich sulfur atom which minimizes nuclear separations. Because of electron accepting nature of benzothiazole it withdraws the electrons from the donor ring. This donation is supported by the lone pair of electrons resided on sulfur atom in -SCH3 group. This donation has resulted in the substantial increase in the hole density around -SCH3 group and instigated non planarity in -SCH3 group (only -CH3 group flips away) as compared to rest of the rings in the system which boosts the efficiency of BAS as HTM.

(a)

(b)

Fig. 2 (a) Umpolung activity after addition of –SCH3 (b) Activity of BAS While, the addition of an electron withdrawing group like -CF3 at the para position of (E)-2(phenyldiazenyl)benzo[d]thiazole supports the nature of the targeted building block and remains as an electron transport material (ETM) (Fig. 3) only. It is noteworthy to mention here that –CF3 group being strong acceptor attracts the electrons more strongly towards itself as compared to the benzothiazole ring and thereby creating high electron density (Fig. 3a and 3b) around it, thus, behaving as a potent hole blocking material. This hole quenching property of -CF3 derivative is also supported by its nonflexibility and hence, this rigidity constrains increases the lifetime of the species under investigation. As a result, such a structural framework of BAF as hole blocking material can be a supportive advantage for this to serve as electron transporter as well.

(a)

(b)

9



Page 10 of 30

Fig. 3 (a) Umpolung activity after addition of –CF3 (b) Activity of BAF 3.3 Natural Bond Orbital (NBO) Analysis and Electron Density The NBOs correspond to the accurate picture of Lewis structure, in which two-center bonds and lone pairs are localized. It gives highest possible percentage of the total electron density of all the orbital’s. The Lewis-type NBOs establish the localized Natural Lewis Structure (NLS) representation of the wavefunction, while the residual “non-Lewis”-type NBOs complete the span of the basis and illustrate the residual “delocalization effects”. Second order perturbation analysis of Fock matrix provides an insight about the overlap of orbitals and change of charge between the atoms involved viz., donor (Lewis-type (bonding or lone pair)) and acceptor (non-Lewis-type (antibonding or Rydgberg)) NBO orbitals with the loss of occupancy. This delocalization of electrons in a conjugated pi system can be anticipated with the help of stabilization energy49 E(2). (7) Where qi is the donor orbital occupancy, εi and εj are the diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. Furthermore, it provides details of charge transfer in a conjugative molecular architechtonics.50 Specifically, in this case it has been performed using methanol solvent to validate intramolecular charge transfer interactions. A number of interactions with large E(2) value have been reported which illustrate intensive interaction between electron donors and electron acceptors. Observed orbital overlaping (Table 2) shows migration of electron from right to left Fig. 4(a) due to the presence of –SCH3 group.

(a)

(b)

Fig. 4 (a) Electron mobility in neutral BAS state (b) Electron mobility in cationic BAS state based on NBO analysis in methanol

Table 2 Donor acceptor interaction in BAS in neutral state with stabilization energy S. No.

Donor NBO orbitals

Acceptor NBO orbitals

E(2)(kJ/mol) 10


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1.

LP(2) S26

BD*(2) C21-C23

20.00

2.

BD(2) C21-C23

BD*(2) C16-C18

22.86

3.

BD(2) C16-C18

BD*(2) N14-N15

27.39

4.

BD(2) N14-N15

BD*(2) C7-N13

15.21

5.

BD(2) C7-N13

BD*(2) C1-C2

16.85

6.

BD(2) C1-C2

BD*(2) C3-C4

18.30

7.

BD(2) C3-C4

BD*(2) C5-C6

18.32

Cationic state Fig. 4(b) reveals an opposite movement of electrons i.e., from left to right (movement of holes from right to left) which substantiates the occurrence of intramolecular charge transfer (ICT) in accordance with the results reported in Table 1.

Table 3 Donor acceptor interaction in BAS in cationic state with stabilization energy S. No.

Donor NBO orbitals


E(2)(kJ/mol)

1.

LP(2) S12

LP*(2) C2

37.27

2.

LP*(2) C2

BD*(2) C3-C4

19.08

3.

BD(2) C3-C4

BD*(2) C5-C6

6.16

4.

BD(2) C5-C6

BD*(2) C1-N13

11.46

5.

BD(2) C1-N13

BD*(2) C7-N14

17.54

6.

BD(2) C7-N14

BD*(2) N15-C16

8.10

7.

BD(2) N15-C16

BD*(2) C18-C21

4.55

8.

BD(2) C18-C21

BD*(2) C23-S26

13.40

In addition to values mentioned in above Table 3 another interaction is also observed between π N15C16 to π* C7-N14 with stabilization energy of 20.35 kJ/mol. Thus, N15 was found to be the focal centre for charge transfer i.e., the region of density enrichment (red) is principally localized on the acceptor moiety (Fig. 5). While the density depletion zone (green) is mostly located on the N14 of azo chromophore of BAS indicating one of the best intramolecular charge transfer.

11



Page 12 of 30

Fig. 5: Electron density in BAS Likewise NBO results of BAF depicts hole blocking property i.e., a completely opposite behavior to that of BAS. Upon analyzing its neutral state in methanol (Table 4) an orbital overlap has been observed from right to left thus, validating potential of –CF3 as a strong electron acceptor (Fig. 6(a)). However, analysis of anionic state demonstrates no transfer of electron in either directions therefore, it acts as hole blocking material (Fig. 6(b)).

Table 4 Donor acceptor interaction in BAF in neutral state with stabilization energy S. No.

Donor NBO orbitals


E(2)(kJ/mol)

1.

LP(2) S25

BD*(2) C1-C2

19.95

2.

BD(2) C1-C2

BD*(2) C3-C4

14.90

3.

BD(2) C3-C4

BD*(2) C5-C6

19.34

4.

BD(2) C5-C6

BD*(2) C1-C2

19.12

5.

BD(2) C1-C2

BD*(2) C7-N24

13.41

6.

BD(2) C7-N24

BD*(2) N12-N13

10.37

7.

BD(2) N12-N13

BD*(2) C14-C16

10.10

8.

BD(2) C14-C16

BD*(2) C19-C21

20.65

12


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(a)

(b)

Fig. 6 (a) Electron mobility in neutral BAF state (b) Electron mobility in anionic BAF state based on NBO analysis in methanol

3.4 HOMO-LUMO and Band Gaps Optimization of optoelectronic devices prior to their synthesis helps in addressing the challenge which limits the further improvisation of such devices. The simulated band gaps and HOMO-LUMO levels of all the seven states ( HOMO LUMO gaps of

,

, ,

, ,

)

are computed at the B3LYP/6-311++G(d,p) level.

for BAS and

,

,

for BAF are shown in the

Fig. 7.

(a)

13



Page 14 of 30

(b) Fig. 7. (a) HOMO-LUMO gaps of BAS (b) HOMO-LUMO gaps of BAF HOMO-LUMO gap ranges approximately from 2.7 eV to 3.6 eV for BAS and BAF for different states which shows that both organic compounds are behaving like a semiconductors hence can be used as DSSCs or OLED. The values of frontier molecular orbitals that are obtained for all the seven states in neutral, cationic and anionic moieties are shown in the Table 5.

Table 5: Frontier molecular orbitals for neutral, cationic and anionic states of BAS, BAB and BAF ENERGY (eV) BAS

BAB

BAF

HOMO

LUMO

HOMO

LUMO

HOMO

LUMO

-6.1383

-3.2321

-6.7005

-3.3225

-6.8510

-3.4966

-7.2151

-3.9459

-7.7534

-4.2229

-7.7076

-4.1541

-3.5592

-0.4862

-3.6145

-0.4606

-3.7674

-0.6277

-7.0086

-4.0496

-7.3773

-4.6564

-7.5395

-4.8145

-5.9443

-3.2958

-6.0279

-3.4090

-6.0831

-3.5290

-4.1187

-0.5167

-4.0795

-0.4367

-4.2403

-0.6299 14


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-6.5599

-3.7911

-6.5384

-3.7579

-6.7261

-3.9443

A close examination of frontier molecular orbitals (Table 5) revealed that the increment in HOMO energy upon introduction of the electron-donating group viz., -SCH3 as in BAS due to which the hole reorganization energy decreases (hopping of holes λh< λe) was observed. Contrarily, decrement in the HOMO and LUMO energy upon introduction of an electron-withdrawing group (-CF3) in BAF increases the hole reorganization energy (electron hopping λe< λh). Thus, based upon the requirement of HOMO-LUMO gap, such theoretical computation provides a very fruitful means to guide the synthesis of future organic electronics. Theoretically the optical energy band gap between frontier molecular orbitals is simulated as E = ELUMO - EHOMO. This potential energy band gap is considered to play a vital role in power conversion efficiency of electronic devices associated with charge transfer (CT) processes. Conjugation can be considered as other important parameter to stimulate the light-harvesting efficiency in optoelectronic devices. We observed that there was a variation in energy of all three targeted moieties after evaluation of BAB (no substitution), BAS (-SCH3 substituted), and BAF (-CF3 substituted). It was found that presence of electron-donating substituent such as -SCH3, lowers the HOMO-LUMO gap by increasing the EHOMO. On the contrary, presence of electron-withdrawing substituent such as, CF3 lowers the HOMO-LUMO gap by lowering the ELUMO in addition to decreased EHOMO. Therefore, this outcome implies that these substituted azo benzothiazoles have the potential to increase the shortcircuit density of photovoltaic by enhancing short-circuit current. Accordingly, it will boost Power Conversion Efficiency (PCE) and work efficiently as organic photovoltaics due to broader absorption range thereby, results in increased number of generated charge carriers. Thus, by modulating the HOMO-LUMO energy band gap, BAS has been proven to be a promising candidate with potential application in DSSCs. Also, current literature reveals that energy level of the LUMO of the organic compound in DSSCs must be higher than the conduction band of the TiO2 (-4.1 eV). Furthermore, the energy level of HOMO of the organic compound must be lower than -4.80 eV.51 From our results of BAS as shown in Table 5 it is very much evident that LUMO energy is greater than -4.1eV while HOMO energy is less than -4.8 eV, which could be potentially a promising Hole transporter.

15



Page 16 of 30

Additionally, -CF3 based derivatives have been reported to be used as Hole blocking layers in OLEDs.52 Thus, the optimization of BAF affirms substitution of -CF3 as a vital factor to boost efficiency of OLEDs. 3.5 Hardness Essentially, chemical hardness is considered as the reference to judge the resistance to intramolecular charge transfer process. Small value of chemical hardness (less than 2 eV) corresponds to a significant charge transfer and consequently brings about a framework with higher competence.53 For all the targeted moieties, chemical hardness is found to be less than 2 eV (Table 6) which reveals them as pertinent charge transport materials. Table 6: Calculated values of Chemical hardness (eV) of BAS, BAB and BAF:

η

BAS

BAB

BAF

1.157596838133

1.3371512611

1.277669511705

3.6 Absorption and Emission spectra Addition of groups like –SCH3 and -CF3 increases the co-planarity and conjugation of the system at ground level. Author reply: The absorption and emission spectra at neutral, cationic and anionic states were calculated to substantiate different activity of BAS and BAF and to demonstrate their opposite transport behavior. In conjunction with conjugation at excited level, a twist in the structures of BAB and BAF are observed while there is tilting of CH3 in –SCH3 by 270.435° of dihedral angle. This property was interpreted as an indication for a large dipole moment of the emitting/absoption state and hence for its charge-transfer character. As BAB and BAF have same nature, only BAF is discussed in this section. In the present findings, only those transitions are discussed in which either emission or absorption phenomenon takes place, i.e.

to

/

(absorption) and

to

/

to

(emission). Fig. 8 (a)

and (b) for BAS and BAF illustrates that when the neutral moiety accepts or donates an electron, resulting in red shift in the spectrum is induced. At the same time if cationic molecule (anionic) accepts (donates) an electron (Fig. 8 (c) and (d)) a blue shift in the spectrum is observed. Generally a wide band gap in Near Infra Red (NIR) is considered suitable for biosensing in vivo conditions.54,55 Longer-wavelength light has a larger penetration depth into biological samples offering a 16


Page 17 of 30

longer working distance for in vivo applications. For the BAS cationic observed in the NIR (1078.04 nm) region. Also, the wavelength of

moiety the red shift is

is observed in the NIR (986 nm)

region (Fig. 8 (a) and (c)). Thus, a wide band gap (Fig. 7(a)) obtained for BAS in NIR makes it suitable optogenetic candidate for bioimaging applications addressing the challenge of poor tissue penetration (Fig. 9). The sensitivity of -N=N- double bond will also encourage the use of azo derivatives as cleavable linkers in chemical biology. These findings facilitate to focus on exploring specific applications of biosensors, making the materials more valuable for practical applications.

1.2 1.4

Peak value = 406.38 nm

+


E0

1.2

_

E0

1.0

0

E0

0

Intensity (a.u)

Intensity (a.u)

1.0 0.8 0.6


E0

0.8

0.6

0.4


0.4

0.2

0.2

0.0

0.0 0

200

400

600

800

1000

1200

0

1400

200

400

(a)

800

1000

(b)


1.4 1.4

600

Wavelength (nm) BAF

Wavelength (nm) BAS


1.2 1.1

1.2 1.2

1.0

0

E+

0.9

E-

+

0.8

-

1.0 1.0

E+

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


0.8 0.8 0.6 0.6

Peak value = 986 and 1326 nm

0.4 0.4

0

E-

0.7 0.6 0.5 0.4

Peak value = 447.55 and 528.14 nm

0.3 0.2

0.2 0.2

0.1 0.0 0.0

0.0 00

200 200

400 400

600 600

800 800

1000 1000

1200 1200

1400 1400

0

100

200

300

400

500

600

700

800

900

Wavelength (nm) BAF

Wavelength (nm) BAS

(c)

(d)

Fig. 8. Absorption spectra of (a) BAS for

to

states (b) BAF for

(d) BAF for

to

(c) BAS for

to

to

Tuning benzene ring with electronegative fluorine atom viz., -CF3 group in BAF elongates π-conjugation which results in broadening and a red shift of the absorption bands and subsequently decreased the band 17



gap and thus can have an improved photo-current response. For all the four states of BAF (

,

,

,

) wavelength in visible region is observed (Fig. 8) which reveals that BAF is a potent, promising material for Organic Light Emitting Diodes (OLEDs) as a hole blocking material (high λh). Sample with wide band gap in NIR region

BIOIMAGING NIR

Fig. 9. BAS as a promising optogenetic candidate for bioimaging applications For

the

in

,

and

, new

and

depth

study

of

emission

and

absorption

of

the

targeted

are optimized using TD-DFT 6-311++G(d,p) basis set and named as

new.

Then

new

from

new/

new

,

new

from

new

and

new

moieties, new,

from

new, new

are

obtained with all positive harmonics. To portray the structural understanding of the targeted moiety graphs are plotted between wavelength and oscillator strength at which maximum intensity is obtained as shown in the Fig. 10.

1.4

501.13 nm

+

468.23 nm

E0

Absorption

0

1.2

E0

1.0

E0 new

+

Intensity (au)

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 18 of 30

Emission

0

E0 new

0.8 0.6

1076.84 nm 984.99 nm

0.4 0.2 0.0 400

500

600

700

800

900

Wavelength (nm) BAS

1000

1100

1200

(a) BAS absorption

to

and emission

new to

new

18


Page 19 of 30

501.13 nm

1.4

+

E+

Emission

468.23nm

0

E+

1.2

+

Absorption

Intensity (au)

1.0

E+ new 0

E+ new 0.8 0.6

1076.79 nm 984.96 nm

0.4 0.2 0.0 400

500

600

700

800

900

1000

1100

1200

Wavelength (nm) BAS

1.2

(b) BAS emission

470.96 nm

1.0

to

and

absorption

new to

new

-

E0

Absorption

0

406.38 nm

E0 -

Intensity (au)

E0 new

Emission

0.8

0

E0 new

0.6

0.4

511.37 nm 528.12 nm

0.2

0.0 375

400

425

450

475

500

525

550

575

600

Wavelength (nm) BAF

(c) BAF absorption

to

and emission

new to

new

1.2

471.01 nm 406.39 nm

1.0

-

E-

Emission

0

E0

0.8

Intensity (au)

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


Absorption

E- new -

E- new 0.6

0.4

511.34 nm 528.14 nm

0.2

0.0 350

375

400

425

450

475

500

525

550

575

600

Wavelength (nm) BAF

(d) BAF emission

to

and absorption

new to

new.

Fig. 10. Comparative analysis of shifts obtained during absorption and emission (a) BAS absorption to emission new (b) BAS emission to absorption new (c) BAF absorption to emission new (d) BAF emission to absorption new

19



These figures illustrates that ground state (

,

,

,

Page 20 of 30

and

) has more intensity (oscillator

strength) as compared to any of the cationic or anionic state. Higher oscillator strength implies higher transition which demonstrates that absorption transition is better than emission transition for the targeted moieties. The wavelength of the optimized BAS molecule is found to be 468.23 nm for . When

new is optimized and

984.99 nm for

and 501.1 nm for

and 1078.84 nm for

new is obtained using TD-DFT wavelengths are found to be . A red shift as observed in

whereas

shows blue shift. That

is emission of radiations is followed by absorption. This observation strongly suggests the decrement in the difference of the wavelengths between the two transitions as depicted in Fig. 10 (a). During transition from

to

state, the wavelengths are found to be 948.96 nm and 501.1 nm,

respectively. When the optimized molecule

new is converted into

new state the wavelengths are

468.23 nm and 1076.79 nm, respectively using TD-DFT level as illustrated in Fig. 10 (b). In case of molecule there is red shift while for

blue shift is observed. The behavior of this conversion is entirely

opposite to the conversion shown in Fig. 10 (a). The plausible rationale for the reason could be the absorption followed by emission of the radiations. On comparing the Fig. 10 (a) and 10 (b) it depicts that both the phenomena are entirely opposite. The intensity versus λ graph of BAF (Fig. 10 (c)) depicts that wavelength conversion from

new it is 470.96 nm. However, the wavelength of

is 511.37 nm and for optimized

is 406.38 nm and after state after conversion from

new it is 528.12 nm. In this particular scenario both the emission

and absorption cases of BAF molecule shows red shift. Again when BAF molecule is converted from respectively. Using TD-DFT level when

to

wavelengths are 528.14 nm and 471.01 nm,

new is converted to

new wavelengths are 511.34 nm

406.39 nm respectively (Fig. 10 (d)). For both the emission and absorption cases the blue shift is observed which is opposite to the results depicted in Fig. 10 (c) where red shift is observed. Different behavior obtained for BAS and BAF moieties also substantiate different transport (hole transport/hole blocking) properties of both the compounds under theoretical investigation. 3.7 Dipole Moment 20


Page 21 of 30 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


The computed dipole moment (in Debye) of the BAB molecule for any of the seven cases is approximately equal as shown in the Table 7. Whole of the BAB moiety is acting along the X axis as a result, polarization should take place along the same axis only. The arrow of the dipole moment acts along the Y axis (Fig. 11 (a)) therefore, the flipping of the dipole moment along the Y axis (Fig. 11 (b)) does not change it significantly. This shows that the molecule does not polarized to a significant extent hence, possesses approximately equal dipole moment for all the seven cases. Our observation also shows that just flipping of Y axis does not allow any changes to the dipole moment. The computation of the dipole moments implies that there is need of some other groups to attach to the moiety for better polarization of the molecule which allows better hole/electron transportation. The calculated dipole moment (Table 7) of and the dipole moment of

state for the BAS moiety is found to be 4.1742 Debye

is found to be 12.8416 which is more than 207% the dipole moment of

state. The reason being –SCH3 (at the beginning two separate lone pair of electrons has high electron density) when gets attached to other to create BAS has high hole density while benzothiazole present in the moiety which is a strong acceptor suggesting most of the electrons are localized near the benzothiazole moiety. Fig. 12 depicts that dipole moment flip towards the more electron density area. Between the Fig. 12 (a) and (b) the arrow of the dipole moment acting along the positive X axis shifts significantly towards the benzothiazole group which increases the dipole moment by three folds. Again the calculated dipole moment of

is 3.0161 Debye which is slightly less than around 28%. Similarly,

while comparing the Fig. 12 (a) and (c) it is clear that the arrow acting along the positive X axis shifts away from the benzothiazole moiety which clearly depicts the decrement in the dipole moment. In case of BAF the calculated dipole moment of

state is 4.7965 Debye while for

moment is 23.2402 Debye which is more than 384% of

state dipole

state. In the BAF moiety -CF3 is strong

acceptor as well as a hole blocking agent therefore the electron density is very high near the -CF3 group. Fig. 13 illustrates that dipole moment acts towards the -CF3 group that is towards the high electron density area. When we compare the Fig. 13 (a) and (b), the arrow of the dipole moment acting along the negative X axis shift towards the -CF3 group thereby, increasing the dipole moment from 4.7965 to 23.2402 Debye. For the

state dipole moment is 1.9849 which is 58% less than the

state. The

difference between Fig. 13 (a) and (c) suggests that the arrow acting along the negative X axis shifts away from the –CF3 group causing the significant decrement in the dipole moment.

21



Page 22 of 30

Table 7: Dipole Moment (Debye) of BAS, BAB and BAF in methanol solvent: Dipole moment (Debye)

(a)

BAS

BAB

BAF

4.1742

2.3553

4.7965

12.8416

2.4203

23.2402

4.6522

3.3036

1.9908

10.7128

3.3825

11.2398

5.2062

3.0115

4.2237

3.0161

2.6907

1.9849

2.4963

2.5796

5.1537

(b)

(c)

Fig. 11. Variation in dipole moment of BAB in states (a)

(a)

(b)

Fig. 12. Variation in dipole moment of BAS in states (a)

, (b)

and (c)

(c) , (b)

and (c) 22


Page 23 of 30 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


(a)

(b)

(c)

Fig. 13. Variation in dipole moment of BAF in states (a)

, (b)

and (c)

Comparison of dipole moment and energy of the BAS and BAF molecules as shown in Fig. 14 (a) and (b) revealed that both the physical quantity go hand in hand. If energy increases dipole moment also increases and vice versa. When an electron is donated (accepted) the value of both the physical quantities increases (decreases). If we convert the molecule to

to

(

) state i.e. cationic (anionic)

state external energy is supplied (removed) to overcome the binding energy of the molecule result in (

) state energy increases (decreases) and potentially the dipole moment also changes accordingly.

3.8 Light Harvesting Efficiency (LHE) The ability of the materials and the molecules to capture photons of solar light is called as Light Harvesting Efficiency, given by the formula: LHE = 1-10-f where f is the oscillator strength at the λmax. Higher the value of f, higher is the light capturing ability LHE of the Dye Sensitized Solar Cells (DSSCs).56 Significant value of LHE (Table 8) is observed for BAS at optimized ground state 0.93973 as compared to BAB (LHE = 0.82522) and BAF (LHE = 0.89341) ensuring its hole transporter property.

23



-1500.70

Dipole moment Energy

-1500.75

12

-1500.80 -1500.85

10

-1500.90

8

-1500.95

6

-1501.00

4

Dipole moment (Debye)

Energy (atomic units)

14

-1501.05 2 -1501.10 0 +

E+0

0

E00

+

E++

−

E0

0

E+

−

0

E−

E−

BAS

(a)

-1400.2

Dipole moment Energy

Energy (Atomic Units)

-1400.3

24 22 20 18

-1400.4

16 14

-1400.5

12 10

-1400.6

8 6

-1400.7

Dipole moment (Debye)

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 24 of 30

4 2

-1400.8

0

E0

+

E0

-

E0

+

E+

0

E+

-

E-

(b)

0 0

E-

BAF

Fig. 14. Correlation diagram between calculated total energy and dipole moment of (a) BAS (b) BAF Moreover, it has been corroborated that BAS is good hole transporter by observing cationic ( anionic (

) and

) species having values of LHE as 0.63926 and 0.15141, respectively. Similarly, BAB is a

good electron transporter supported by LHE values of cationic (

) and anionic (

) species (Table 8). 24


Page 25 of 30 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


The value of LHE of anionic species (= 0.46284) >> cationic species (= 0.00069) for BAF clearly validates its behavior as hole blocking agent. Table 8: λmax (nm), Oscillator Strength (f) and LHE of BAS, BAB, BAF in methanol solvent: BAS λmax

f

BAB LHE

BAF

λmax (nm)

f

LHE

λmax (nm)

f

LHE

(nm) 468.23

1.2199 0.93973

398.69

0.7575

0.82522

406.38

0.9723

0.89341

1076.84

0.4428 0.63926

1404.74

0.0012

0.00276

1413.74

0.0003

0.00069

545.85

0.713

519.25

0.0849

0.17757

511.37

0.2699

0.46284

0.15141

4. Conclusions In summary, the crucial prerequisite for the heterocyclic compounds to be the main constituent of electrically conductive materials is their ability to freightage charge intra/inter molecularly, either in the form of holes or as electrons. Upon introducing substituents viz., -SCH3/-CF3 in the para position of phenyl ring in (E)-2-(phenyldiazenyl)benzo[d]thiazole, the same compound behaves as hole transporter/hole blocking material. Among the three tested solvents, polar protic solvent methanol yielded best results. A close inspection of factors such as solvent polarity and substitution modulates the activity of the targeted moiety to behave as optoelectronics (DSSCs or OLEDs) as well as optogenetic materials, which is a noteworthy culmination of the present study. We also report the significant changes in dipole moment values of BAS and BAF to substantiate the result to the greater extent. The LHE values strongly suggests BAS to be a promising material for hole transport device. Undoubtedly, all these findings reported herein can be considered as the valuable assets to design, predict and functionalize several conjugated heterocyclic compounds as compelling organic optoelectronics devices. Associated Content The additional evaluated results have been provided in the supplementary data. Acknowledgements 25



Page 26 of 30

We are thankful to the University Grants Commission for providing support through SAP status (No. F.540/13/DRS-I/2016 (SAP-I)) to the Department of Chemistry.

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Hole Transporting agent

Intramolecular Charge Transfer at different transitions

Hole Blocking agent

Table of Contents Graphic

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TD-DFT Study of Dipole Moment

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