arene: A DFT study

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Accepted Manuscript Understanding the structure, reactivity and absorption spectra of borazine doped pillar[5]arene: A DFT study Himakshi Sharma, Bhabesh Ch. Deka, Bapan Saha, Pradip Kr. Bhattacharyya PII: DOI: Reference:

S2210-271X(18)30294-9 https://doi.org/10.1016/j.comptc.2018.07.011 COMPTC 2836

To appear in:

Computational & Theoretical Chemistry

Received Date: Revised Date: Accepted Date:

29 April 2018 18 July 2018 18 July 2018

Please cite this article as: H. Sharma, B. Ch. Deka, B. Saha, P. Kr. Bhattacharyya, Understanding the structure, reactivity and absorption spectra of borazine doped pillar[5]arene: A DFT study, Computational & Theoretical Chemistry (2018), doi: https://doi.org/10.1016/j.comptc.2018.07.011

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Understanding the structure, reactivity and absorption spectra of borazine doped pillar[5]arene: A DFT study

Himakshi Sharma, Bhabesh Ch. Deka, Bapan Saha and Pradip Kr. Bhattacharyya * Department of Chemistry, Arya Vidyapeeth College, Guwahati-781 016, India E-mail: [email protected] (HS) [email protected] (BCD) [email protected] (BS) [email protected] (PKB)

*corresponding author

Abstract Pillar[5]arene and its derivatives act as useful complexing agents and find utility in various fields of chemistry. Effects of doping borazine unit in pillar[5]arene with respect to its geometry, reactivity and spectroscopic properties are discussed in the light of density functional theory and density functional reactivity theory. Reactivity parameters namely energy of the HOMO, global hardness and chemical potential are used to explain the reactivity of the undoped and doped pillar[5]arene. Aromaticity of the chosen molecule is determined by Nucleus Independent Chemical Shift (NICS) calculations. Absorption spectra of the chosen molecules are analyzed using time dependent density functional theory. Results suggest enhanced stability of pillar[5]arene system upon doping with of borazine unit. Blue shifts in absorption spectra of pillar[5]arene systems upon such doping are also evident which is remarkable from the viewpoint of its opto-electronic utility.

Keywords: Pillar[5]arene, borazine, DFT, reactivity, absorption spectra 1

1. Introduction Pillar[5]arenes are pillar shaped cyclophanes made up of hydroquinone units bridged via methylene groups at para-positions [1]. Presence of bridging methylene groups grants pillar[5]arene with the amenability to chemical modifications and its cylindrical framework enables it to accommodate different guest molecules. These two features provide a valuable platform for synthesizing numerous supramolecular systems based on pillar[5]arene [2]. Pillar[5]arenes have found utility as polyrotaxanes and pseudorotaxanes, [3-6] artificial transmembrane channels, [7-11] supramolecular polymers, [12] chiral sensors, organic nanotubes, [13] drug delivery and controlled release systems [14] as well as data storage in optical devices [15]. Incorporation of hetero atoms like B, N or P has been used as a tool in manipulating the electronic structure and intrinsic properties of various polyaromatics such as graphene, coronene etc [16, 17]. Enhancement in magnetic, optical, electrical and gas sensing property, capacity to adsorb biomolecules like amino acids by two dimensional polyaromatics doped with B, N or BN in comparison to undoped system is reported in various studies on graphene and carbon nanotubes. It has been suggested that random distribution of B and/or N centres imparts semiconducting behavior in them [18]. As achieved in case of graphene or coronene systems, physicochemical properties of pillar[5]arene systems also might be tuned for specific purposes by imparting a change in symmetrical electron density around the neighbouring carbon atoms and replacing arene ring with borazine unit can be envisaged as a useful protocol in this regard. Borazine, the inorganic benzene and benzene possess similar physical properties with different chemical properties [19]. Lower aromaticity of borazine and involvement of electropositive (B) and electronegative (N) atoms make this molecule an attractive hub for non-covalent interaction by other species. Moreover as reported by earlier 2

studies, distortion of an otherwise symmetrical electron density leads to a change in HOMOLUMO gap of a molecule which influences the vibrational spectra and thereby its optoelectronic properties [20-22]. A number of theoretical studies delving into aspects like reactivity, absorption spectra of pillararenes

and

functionalized

pillararenes

are

available

in

literature

[23].

Notwithstanding the potential applicability of hetero-atom doped pillar[5]arenes systems in different field of chemistry, studies on doped pillar[5]arene systems at the molecular level are sparse. Density functional theory (DFT) has become one of the most effective tools in providing accurate ground state electronic structure of macrocyclic systems. On the other hand density functional reactivity theory (DFRT) which utilizes a number of reactivity descriptors in rationalizing the reactivity patterns of a molecular system has been consistently impressive in evaluating the reactivity parameters of a wide range of molecular systems [24]. Some of the reactivity descriptors extensively used by computational chemists include energy of HOMO (EHOMO), global hardness () (also called chemical hardness) and chemical potential (µ). A clear understanding on the stability/reactivity of a doped system as envisaged in the present study requires evaluating the above mentioned descriptors for exploring potential synthetic applications in future. Optoelectronic application of molecular systems as considered in the present study requires an in-depth understanding as well as controlled manipulation of their spectroscopic properties [25, 26]. Time-Dependent Density Functional Theory (TDDFT) has found increasing popularity among chemists for interpreting the optical properties of macro molecular systems [27, 28]. TDDFT treats the electronically excited-states of a molecular system resulting from light-matter interactions and its vitality as a quantum chemical 3

methodology for studying absorption spectra of macromolecules have been well reviewed [29, 30]. In view of the forgoing facts, an attempt has been made to analyze the geometry, reactivity, and absorption spectra of pillar[5]arene systems substituted with borazine units using DFT and TDDFT. Pillar[5]arene exists as two chiral planar stereoisomers -the Rp and Sp, and as

identified by Ogoshi, four conformational isomers are available for Rp

pillar[5]arene; cone, partial cone, 1,2-alternate and 1,3-alternate conformers [31]. Out of these four isomers, the cone isomer possesses the most symmetrical structure [32] and is considered for the present study. Moreover it is reported that stabilization energy, reactivity and absorption spectra of doped coronene molecules significantly depend upon the number of dopant atoms [33]. Therefore in the present study, pillar[5]arene systems substituted by one to five borazine units are considered. 2. Theoretical and computational details In DFRT, using the finite difference approximation, global hardness and chemical potential can be approximated as [34] η = (IP-EA)/2

(1)

μ =  (IP+EA)/2

(2)

where, IP and EA are the first vertical ionization potential and electron affinity respectively, of the chemical system. Gas phase ground state geometrical minima of the considered pillar[5]arene systems are computed by using 6-31+G(d,p) basis set with Becke three parameter exchange and Lee, Yang and Parr correlation functional (B3LYP) [35] and are confirmed by the absence of any imaginary frequency on subsequent frequency calculations. Electronic transitions in the absorption spectra of the chosen molecular systems are assigned by performing TDDFT calculations on the optimized structures using TD method (N=20) [36] using B3LYP 4

functional. Tozer and collaborators concluded that reliability of B3LYP functional in calculating variety of transitions of medium size chromogens are at par with other present day functional such as CAM-B3LYP, PBE, M06-2X etc [37]. Further, the effect of solvent polarity on the chosen reactivity parameters and optical spectra of the pillar[5]arene systems is analyzed by using PCM (Polarizable Continuum Model) [38] considering different solvents with a range of dielectrics, viz. cyclohexane, ( = 2.02), ethanol ( = 24.85), DMSO ( = 46.83), and water ( = 78.35). All calculations are carried out using Gaussian09 programme [39]. 3. Results and discussion 3.1. Geometry of the Chosen systems Geometry optimization is carried out for pillar[5]arene system as well as for systems in which arene ring(s) is substituted with 1 to 5 number of borazine units. In case of replacement of 2 and 3 arene rings by borazine units, both adjacent and non-adjacent positions are explored. The optimized geometries of the title systems are shown in Fig. 1. Cartesian coordinates of the complexes are provided in Supplement Table S1. The considered pillar[5]arene system, i.e. its cone conformation possesses a regular structure with dihedral angle of ~89.0° [32]. Doping leads to a significant change in the tilting of the plane of borazene and benzene unit. The BCCC dihedral angle becomes ~76° while NCCC dihedral angle is ~113°. This can be traced to the accumulation of electron density around the N atom of the borazine unit and electron deficiency in the B atom of the borazine unit linked to a benzene unit. Thus it is evident that doping of borazine unit in pillar[5]arene system leads to structural distortion which might impact upon its reactivity and spectroscopic properties.

5

(a) pillar[5]arene

(b) pillar-1-bora

(e)

(f) pillar-3-bora-adj

pillar-3-bora

(c) pillar-2-bora

(g) pillar-4-bora

(d) pillar-2-bora-adj

(h) pillar-5-bora

Fig. 1: Optimized geometry of title systems obtained at B3LYP/6-31+G(d,p) level of theory. 3.2 Stability in terms of Reactivity descriptors Chemical stability of a species are estimated in gas phase in terms of various reactivity descriptors such as EHOMO, chemical hardness () and chemical potential (μ). Solvent dielectric imparts significant perturbation in the electron density and changes the different types of energies of a chemical species including its HOMO, ionization potential or electron affinity. Therefore to gain a better understanding of the reactivity pattern of doped pillar[5]arene systems the effect of solvent dielectric on the chosen reactivity descriptors are also studied.

6

35

Difference in EHOMO

30 25 20 15 10 5 0 1

2

3

4

5

Number of borazine unit in the pillar[5]arene

Fig. 2: Difference in EHOMO as the number of borazine increases. Table 1: EHOMO, HOMO-LUMO gap (HLG), chemical hardness () and chemical potential (μ ) at gas phase obtained at B3LYP/6-31+G(d,p) level of theory (values are in kcal mol-1). System pillar[5]arene

EHOMO 124.54

HLG 93.89

η 77.72

μ 71.42

pillar-1-bora

125.51

96.92

77.90

73.15

pillar-2-bora

126.51

95.28

77.77

75.22

pillar-2-bora-adj

126.90

91.81

78.13

76.58

pillar-3-bora

128.88

90.54

78.30

78.89

pillar-3-bora-adj

127.91

88.52

79.30

79.89

pillar-4-bora

132.21

84.84

81.50

86.35

pillar-5-bora

156.95

101.3

86.37

95.20

7

solvent phases.

8

3-b

pill ar5-b

4-b ora

ora

j

ora

j

ora

ora -ad

pill ar-

3-b

2-b

ora

ora -ad

pill arar-

pill

2-b

ar-

pill

pill ar-

ren e

1-b

5] a

pill ar-

ar[

pill

-1

chemical potential (in kcal mol )

pill ar2b ora -ad j pill ar3-b o pill ra ar3-b ora -ad j pill ar4-b ora pill ar5-b ora

pill

en e

r[5 ]ar

ar1-b ora ar2-b ora

pill

pill a

-1

chemical hardness (in kcal mol )

Pil la

bo ra r-2 -bo raad j pill ar3-b ora pill ar3-b ora -ad j pill ar4-b ora pill ar5-b ora

ar2-

pill

bo ra

are ne ar1-

pill

ar[ 5]

pill

-1

EHOMO (in kcal mol ) -110

-120

Gas Cyclohexane Ethanol DMSO Water

-130

-140

-150

3a. EHOMO Gas Cyclohexane Ethanol DMSO Water

80

60

40

20

0

3b. Chemical Hardness

-60

-65

Gas Cyclohexane Ethanol DMSO Water

-70

-75

-80

-85

-90

-95

3c. Chemical Potential

Fig. 3: Variation of (a) EHOMO, (b) chemical hardness and (c) chemical potential in gas and

3.2.1 Energy of HOMO (EHOMO) Energy of HOMO (EHOMO) is a measure of the ability of a species to donate electrons. More negative the value of EHOMO, higher is the stability of HOMO. Moreover, along with the energy of LUMO (ELUMO), it governs the absorption spectra of a species. Calculated EHOMO values of undoped pillar[5]arene and the doped systems considered in the present study are presented in Table 1. It is observed that EHOMO is dropped upon incorporation of borazine unit. Incorporation of one borazine unit in pillar[5]arene leads to a drop of 0.72 kcal mol-1 in EHOMO. As the number of doped borazine unit increases, the difference in EHOMO between undoped and doped pillar[5]arene drops exponentially as depicted in Fig. 2. Results thus suggest that doping borazine unit lowers the electron donating ability of pillar[5]arene molecules. Previously in case of graphene system, substitutional doping of 3BN units is reported to raise the EHOMO by ~22 kcal mol-1[18]. Similar trend is observed in coronene systems also (EHOMO is raised by 7-15 kcal mol-1). This implies that substitutional doping of B and N atoms in isoelectronic manner into carbon clusters like graphene or coronene leads to a destabilised HOMO. However substituting arene rings in pillar[5]arene systems with isoelectronic borazine units stabilizes the HOMO. Thus it is pertinent to mention that substitution of arene rings with borazine unit(s) is feasible as EHOMO drops exponentially with increase in the number of borazine units. From the shape of the HOMO, the donor behaviour of the complexes can be rationalised. It is evident in Fig. 4 that in case of undoped pillar[5]arene the HOMO is localised over all the aromatic benzene rings. In doped pillar[5]arene systems namely, pillar-1-bora, pillar-2-bora, pillar-3-bora and pillar-4-bora also, the HOMO is seen to be localised only over the aromatic benzene part. However, in case of pillar-5-bora, where all the benzene rings are doped with borazine units HOMO is localised over all the borazine rings and this eventually leads to a spiky fall in EHOMO (From 9

125.5 kcal mol-1 in pillar-1-bora to 156.95 kcal mol-1 in pillar-5-bora). Thus, we can conclude that electron donating ability is associated with benzene part of the pillar[5]arene and doped pillar[5]arene with an exception in pillar-5-bora. In solvent phase also there is lowering of EHOMO with doping of each single borazine unit in almost the same range as observed in gas phase. However as regard to a particular system, insertion of solvent phase raises the EHOMO of each of the considered systems by ~7 kcal mol-1 as we move from gas to cyclohexane phase.

(a) pillar[5]arene

(b) pillar-1-bora

(c) pillar-2-bora

(d) pillar-2- bora-adj

(e) pillar-3-bora

(f) pillar-3-bora-adj

(g) pillar-4-bora

(h) pillar-5-bora

Fig. 4: Shapes of HOMO of the undoped and doped pillar[5]arene (obtained at isosurface=0.02).

10

In cyclohexane and ethanol media, EHOMO values are almost identical and in high dielectric media like DMSO and water, a significant drop in EHOMO is noticed upto pillar-3-bora; in pillar-4-bora the drop is marginal while in case of pillar-5-bora EHOMO is identical in all the chosen solvents. (Fig. 3a and Supplement Table S2). Thus insertion of solvent phase may be useful in tuning the reactivity and subsequently the solution phase chemistry of doped pillar[5]arene systems. 3.2.2 Vertical Ionization Potential(VIPs) and Vertical Electron Affinity(VEAs): Many experimental and theoretical studies have been made to study the ionization potential (IPs) and electron affinity (EAs) of atoms and molecules [40-42]. These parameters are very important properties for atoms and molecules, as they are fundamental in assessing the electron donating and accepting ability. For the following chemical processes, M  e- → M+ M + e- → M The calculation formulas defined according to the energy differences are as follows: VIP = E+(Geo=0) E0(opt) VEA= E0(opt)  E(Geo=0)

(1) (2)

Where VIP is the vertical ionization potential, VEA is the vertical electron affinity, E0(opt) is the energy of optimized neutral (0) species, E +(Geo=0) is the energy of cation in the optimized neutral geometry, and E(Geo=0) is the energy of anion in the optimized neutral geometry. The values of VIP and VEA are presented in Table 2. As the number of borazine unit increases, the VIP increases. This indicates higher ionization potential of pillar-5-bora which possesses the maximum VIP value (181.58 kcal mol-1). Similar ionization potential values can also be expected from the observed EHOMO values. In case of pillar-5-bora, having 11

more negative EHOMO value (156.95 kcal mol-1) and more stable and possess the highest VIP(181.58 kcal mol-1). Therefore, we can expect linear relationship between EHOMO and VIPs, to examine the relationship these two parameters are plotted (EHOMO vs VIP). The EHOMO and VIPs plots along with R2 value for undoped and doped pillar[5]arene are provided

185

-1

Vertical Ionization potential (in kcal mol )

in Fig.5.

2

R =0.878

180 175

pillar[5]arene

170 pillar-1-bora

165 160

pillar-2-bora pillar-2-bora-adj

155

pillar-3-bora pillar-3-bora-adj pillar-4-bora

150 145 -160

pillar-5-bora

-155

-150

-145

-140

-135

-130

-125

-120

-1

EHOMO (in kcal mol )

Fig.5: Plot of EHOMO vs. VIP (in kcal mol-1) From Table 2, VEAs value indicates that pillar[5]arene and doped borazine upto two units are unstable with respect to vertical electron affinity the gas phase. Upon further doping with three, four and five borazine units in the arene rings of the pillar[5]arene increase in VEAs are obtained.

12

Table 2: Vertical Ionization Potential (VIP) and Vertical Electron Affinity (VEA) calculated at B3LYP/6-31+G(d,p) level of theory (values are in kcal mol-1). System

Vertical Ionization Potential (VIP)

Vertical Electron Affinity ( VEA)

pillar[5]arene

149.15

6.30

pillar-1-bora

151.06

4.74

pillar-2-bora

152.99

2.55

pillar-2-bora-adj

154.72

1.55

pillar-3-bora

157.19

0.58

pillar-3-bora-adj

159.20

0.59

pillar-4-bora

167.85

4.86

pillar-5-bora

181.58

8.83

3.2.3 Chemical hardness (η) Chemical hardness is a measure of stability of a system in changing environment and bears significance from chemical viewpoint. A popular method to calculate chemical hardness (and chemical potential also) is by combining Finite difference approximation [34] and Koopman’s theorem [42, 43]. However questions been raised regarding the reliability of calculation of HOMO and LUMO by DFT in certain cases, the reactivity descriptors have been extensively applied owing to their computational efficiency [44]. Although physical significance of HOMO and LUMO obtained by using DFT have also received support from a number of studies [45]. In the present study, the vertical ionization potential and electron affinities values are employed for calculation of chemical hardness and chemical potential. Chemical hardness of undoped pillar[5]arene and the doped systems calculated by equation 1 are presented in Table 1. It is observed that substituting with the first borazine unit leads to an increase in η value by ~0.18 kcal mol-1 from that of undoped pillar[5]arene (77.72 kcal mol-1). However each successive substitution further leads to increase in η value upto 5 13

borazine units and in case of pillar-5-bora it becomes 86.37 kcal mol-1. Substituting all the five arene rings results in increase in η value of 8.65 kcal mol-1 from that of undoped pillar[5]arene. Moreover it is important to note that substituting of borazine units at adjacent position leads to increase value of η (and hence higher chemical stability) than they are placed at alternative positions and the difference in η value between such pair in case of pillar-2-bora and pillar-3-bora are 0.36 and 1.00 kcal mol-1 respectively. The higher chemical stability of adjacently substituted systems as well as extra stability of pillar[5]arene can be justified from relatively greater symmetry in their structures. Furthermore, a linear relationship between EHOMO and η is observed which substantiates an increase in chemical stability of the pillar[5]arene system upon doping with borazine units. Introduction of borazine unit in pillar[5]arene leads to increase in η values in solvent phase also. The increase is marginal upto doping by 4 borazine units. However in case of pillar-5-bora the increase is quite high (Values obtained at B3LYP/6-31+G(d,p) level of theory are depicted in Fig. 3b). For instance, gas phase η values for pillar[5]arene, pillar-1bora, pillar-2-bora, pillar-3-bora, pillar-4-bora and pillar-5-bora are 77.72, 77.90, 77.77, 78.30, 81.50 and 86.37 kcal mol-1 respectively. With respect to a particular doped pillar[5]arene system, η values decreases in solvent phases irrespective of the dielectric constant. 3.2.4 Chemical potential (μ) Chemical potential of a species represents the drop in energy when an infinitesimal amount of electronic charge enters into it. It is a measure of how hospitable the species is to the ingress of electronic charge which fits in. Variation of chemical potential accounts for the additive effect on the energy of HOMO and LUMO. More negative the μ value of a system, greater is the stability of the system. The gas phase μ value of the title systems are presented 14

in Table 1. The μ value of pillar[5]arene is 71.42 kcal mol-1, upon doping with one borazine unit i.e, in pillar-1-bora, μ increases to 73.15 kcal mol-1. Insertion of further borazine unit leads to more negative μ value and becomes highest negative in case of pillar-5-bora (95.20 kcal mol-1) which predicts the highest stability of pillar-5-bora among the chosen systems. Similar to gas phase, variation in chemical potential with doping as well as changing dielectric constant of solvent medium exhibit similar trend in most of the cases and are represented in Fig. 3c and in Supplement Table S2. Thus, analysis of reactivity parameters namely energy of HOMO, chemical hardness and chemical potential in both gas and solvent phases unequivocally advocates an increased chemical stability of pillar[5]arene systems upon doping with increase in number of borazine unit(s). 3.2.5 Aromaticity of undoped and borazine doped Pillar[5]arene: There has been great interest among the researchers to estimate aromaticity in terms of NICS values [46, 47]. NICS considers the magnetic shielding produced at a particular point in a ring. NICS values can be both positive and negative depending on the ring currents. Positive NICS indicates antiaromaticity whereas negative NICS value indicates aromaticity and NICS value close to zero is termed as non aromatic. NICS values are computed at the ring centre (NICS(0)) as well as at a distances of 1 Ǻ ((NICS (1)) above and below the plane of the ring centre. Herein, the arene rings in pillar[5]arene are numbered as ring1, ring2,….ring5 and in borazine doped pillar[5]arene rings are numbered with respect to the position of borazine. The observed variation in NICSiso values are presented in Table 3. NICSiso value is a distance dependent parameter and it changes as the distance from the plane increases. In pillar[5]arene, NICSiso values are all negative (9 to 11 ppm). However, NICSiso values at the plane of the ring are more negative. This indicates that all the arene rings in pillar[5]arene are highly aromatic. In case of pillar-1-bora, it is seen that NICSiso obtained for borazine rings 15

are less negative compared to arene rings. Similar results are also obtained for pillar-2-bora, pillar-2-bora-adj, pillar-3-bora, pillar-3-bora-adj and pillar-4-bora. The NICSiso values for doped pillar[8]arene, where NICSiso for borazine rings are in the range of 3.2 to 4.9 ppm at the plane and ~3-5 ppm above and below the ring indicating their lower aromatic character than the undoped arene rings of the pillar[8]arene (9 to 11 ppm ) which is highly aromatic. Thus, on doping with borazine leads to significant drop in aromatic character which is believed to influence the EHOMO of the considered pillar[8]arene system. Lower aromaticity of borazine doped systems can be attributed to its unsymmetrical distribution of electron density. Although incorporation of borazine unit lowers the aromaticity, drop in EHOMO and rise in η values are observed with increase in number of borazine units. This questions the suitability of aromaticity to be a criterion for determining the stability of the considered systems. 3.3 Absorption spectra of the species: The absorption spectra of an aromatic system depend upon the extent delocalization of π-electrons. Due to the presence of borazine unit in place of benzene at different position, the π-electron distribution is perturbed and hence their corresponding absorption spectra are anticipated to get changed. Therefore, the absorption spectra of the systems are calculated at B3LYP/6-31+G(d,p) level of theory. Although, other functional like CAM-B3LYP, M06-2X can also be used for the purpose, we have used B3LYP functional because of its popularity and ability to produce results that are in close agreement with the experimental result [33]. To validate this we have calculated the UV-visible spectra of benzene and borazine molecule using these functionals and on comparing it is seen that the result produced by B3LYP functional (λmax(benzene)=168.5, λmax(borazine)=128.8 and 157.1 nm) is quite close to the 16

Table 3: Calculation of aromaticity (NICSiso in ppm) at the centre, above and below the rings of pillar[5]arene and borazine doped pillar[5]arene.

System pillar[5]arene

pillar-1-bora

pillar-2-bora

pillar-2-bora-adj

pillar-3-bora

pillar-3-bora-adj

pillar-4-bora

pllar-5-bora

Ring Ring 1 Ring 2 Ring 3 Ring 4 Ring 5 Ring 1(Bora) Ring 2 Ring 3 Ring 4 Ring 5 Ring 1(Bora) Ring 2 Ring 3 Ring 4(Bora) Ring 5 Ring 1(Bora) Ring 2(Bora) Ring 3 Ring 4 Ring 5 Ring 1(Bora) Ring 2(Bora) Ring 3 Ring 4 (Bora) Ring 5 Ring 1(Bora) Ring 2(Bora) Ring 3 (Bora) Ring 4 Ring 5 Ring 1(Bora) Ring 2(Bora) Ring 3 (Bora) Ring 4 (Bora) Ring 5 Ring 1(Bora) Ring 2(Bora) Ring 3 (Bora) Ring 4 (Bora) Ring 5 (Bora)

isoabove 9.95 10.49 9.68 10.93 10.09 3.61 10.07 11.32 9.94 9.68 3.95 9.72 11.76 3.95 10.37 3.51 3.86 10.14 9.69 11.64 4.35 2.87 9.29 3.68 10.35 3.74 5.16 3.53 10.21 9.28 3.52 4.87 6.48 4.06 10.25 3.88 4.08 3.90 3.60 4.68

17

isocentre 10.73 11.49 10.32 11.33 10.60 4.91 10.26 11.37 10.83 10.10 4.81 10.46 10.87 4.68 11.41 4.86 4.92 10.75 10.37 10.85 4.79 3.21 10.02 4.81 9.94 4.98 4.91 4.72 10.7 10.06 4.81 4.96 4.97 4.85 10.78 4.81 4.87 4.85 4.85 4.81

isobelow 9.78 11.03 9.37 10.81 9.87 4.02 10.24 10.89 9.78 9.83 3.57 9.46 12.10 4.34 10.72 3.78 3.93 10.11 9.56 11.69 4.35 2.95 9.47 3.88 10.16 3.60 6.2 3.58 10.19 9.29 3.36 4.76 6.86 3.95 10.30 3.71 4.23 4.02 3.39 4.51

experimental result for the same [48]. The peak for borazine at 157.1 nm is attributed to HOMO-1→LUMO (π→ π*) transition while the peak at 128.8 nm can be ascribed to transition from HOMO-4→LUMO (σ→σ*) transition. The UV-visible spectra of the concerned system in gas phase are shown in Fig. 6a and λmax values with their corresponding energy, molecular orbitals involved and oscillator strength is shown in Table 4. Referring to the Fig. 6a and Table 4 we can say that substituting benzene by isoelectronic borazine unit leads to significant variation in the absorption spectra of the systems under consideration irrespective of its number and position. For example, in case of unsubstituted pillar[5]arene, only one peak with absorption maxima at 280.6 nm (ultraviolet range) is observed whereas in presence of borazine moiety two blue shifted distinct peaks are observed. In contrast to this, pillar-5-bora in which all the benzene rings are substituted by borazine rings, exhibits one blue shifted absorption maxima (at 208.1 nm, quite significant). Interestingly, the absorption maxima for both pillar[5]arene and pillar-5-bora can be attributed to HOMO-2→LUMO transition. The observed maxima corroborates well with the reported values for pillar[5]arene isomers [32] Moreover, the absorption maxima are also red shifted from the individual aromatic units present in the considered systems. For instance, in case of benzene, C 6H4(OH)2 and borazine, B3N3H4(OH)2 derivatives, one strong and two weak absorption maxima are observed. For the former, the absorption maxima occur at 175.8 (strong), 207.8 (weak) and 259.5(weak) nm whereas for the borazine derivative the maxima are at 160.4 (strong), 186.4(weak) and 217.9 (weak) nm. The shapes of HOMO-2 and LUMO orbitals are depicted in Fig.7 which are π-type and thus validates the favorable π-π* transition. However, in case of mixed benzene-borazine system, both the peaks are blue shifted as compared to pillar[5]arene which can be attributed the transition due to arising due to the presence of borazine derivative in the systems. For instance, in case of pillar-1-bora, the absorption 18

maxima are observed at 272.7 and 225.5 nm respectively. Similar result is also observed for other systems, Fig.6, Table 4. Furthermore, position of borazine unit also has crucial impact on the absorption spectra. As an example, for pillar-2-bora the observed maxima are at 265.6 (HOMO-1→LUMO+1) and 219.2 nm (HOMO-5→LUMO) whereas for pillar-2-bora-adj the maxima are obtained at 273.6 (HOMO-1→LUMO) and 222.3 nm (HOMO→LUMO+6). Similar red shifts are also observed for pillar-3-bora-adj as compared to pillar-3-Bora with similar involved transition. However, for pillar-4-bora maximum blue shift is observed for higher energy transition (maxima at 266.2 and 203.5 nm). Moreover, presence of solvent dielectric exerts some effect of the absorption spectra of benzenoid compounds and thereby we calculated the absorption spectra of the said compounds in cyclohexane, ethanol, DMSO and water, represented in Figs. 6(b-e) and Supplement Table S3. Referring to Figs. 6(b-e) and Supplement Table S3, it is seen that the absorption maxima show insignificant red shift (