A Theoretical Study on the Optical Spectroscopic Properties of Indigoids@B36

Optoelectronics have a strong effect on the different areas of technology. The electricity is transmitted by means of optoelectronic devices that can provide wireless electricity transmission with the help of light. An optoelectronic device includes various semiconductor alloys that lay on substrates. Optoelectronics technology after studies concerning cost reduction, performance improvement and large volume manufacturing, will shape the future.


Introduction
Optoelectronics have a strong effect on the different areas of technology. The electricity is transmitted by means of optoelectronic devices that can provide wireless electricity transmission with the help of light. An optoelectronic device includes various semiconductor alloys that lay on substrates.
Optoelectronics technology after studies concerning cost reduction, performance improvement and large volume manufacturing, will shape the future.
Recently, transistor properties of several halogensubstituted indigoids have been reported [1][2][3][4]. These molecules are of interest because of the minimal molecular structure consisting of electron-donating nitrogen atoms, working as a donor and electron withdrawing carbonyl groups, as an acceptor.
Due to their small energy gap, long wavelength absorption is related to the ambipolar transport. Glowacki and co-workers reviewed the history of indigo and its derivatives: Tyrian purple chemistry, physical properties, and their semiconducting characteristics in the solid state [1]. Pitayanatakul et.al.
developed 5,5'-dibromoindigo and 5,5'-diphenylindigo which showed excellent ambipolar transistor properties [2]. In another study, they investigated diiodoindigo with the iodine at the 5 position of indigo and this molecule showed excellent ambipolar transistor properties. They also found that iodineiodine interaction also affects the ambipolar performance [3]. Klimovich and co-workers synthesized and investigated nine different indigo derivatives for sustainable organic electronics. They solved the degradation problem of Organic Field-Effect Transistors (OFETs) in air, partially by introducing strong electron withdrawing substituents such as two CF 3 groups or four fl uorine atoms. They found that, it is necessary to design novel indigoids with further decreased LUMO energy levels down to at least -4.3eV for air stable device operation [4]. Many theoretical researchers have studied the electronic structure and spectroscopic properties of indigo and indigoids. Perpète and Jacquemin performed benchmark calculations for a set of 31 indigoid dyes with 24 functional. They calculated transition energies corresponding to the maximum experimental absorption wavelengths of indigoids at PCM-TD-X/6-311+G(2d,p)//PCM-PBE0/6-311G (d,p) level. They predicted that global hybrids with exact exchange between 20% and 25%, yield the smallest absolute deviation [5]. Amat and co-workers studied the theoretical and experimental investigation on the spectroscopic properties of indigo dye. They evaluated the effects of the intermolecular hydrogen bond in solid state by computing the vibrational spectra of the dimer [6]. Shityakov and co-workers developed a silicon model of indigoid based single electron transistor nanodevices, consisting of indigo and Nanostructures as semiconductors are often at the heart of modern optoelectronic devices. Recently, the successful synthesis of single-layer boron (referred to as borophene) opens the era of boron nanostructures. Due to the strong bonding of boron atoms, borophene has high resistance to mechanical impact. One of the unit of borophene sheet (B 36 ) consists of 36 boron atoms. B 36 is reported as a highly stable cluster with central hexagonal hole in its structure [8,9].
Several adsorption studies of the small gas adsorption of the B 36 cluster have been described by theoretical calculation methods [10,11]. Omidvar investigated the sensitivity of the B 36 toward two to six hydrogen cyanide (HCN) gas molecule. They found that the B 36 is sensitive to the concentrated HCN molecule [10].
Valadbeigi and co-workers studied adsorption of CO, N 2 , H 2 O, O 2 , H 2 and NO molecules on B 36 cluster using DFT adopting the B3LYP functional and 6-311+G(d,p) basis set. They found CO, O 2 and NO molecules are adsorbed on the B 36 more effectively through formation of chemical bonds. They also indicated that the outer boron atoms of the B 36 cluster adsorb the gas molecule better than the inner boron atoms [11]. Kootenaei and co-workers investigated the potential applicability of B 36 as electronic sensor for formaldehyde (HCOH) detection by using B97D/6-31G(d) level of theory calculation [12]. fi rst principles study on the adsorption of gas molecules on borophene [14]. We also assume that borophene may have good adsorption properties for indigoid molecules. The two atoms in the unit cell of borophene have six dispersion branches.
The branches of longitudinal acoustic (LA) and Transverse Acoustic (TA) correspond to vibration within the plane, and the other one (ZA) corresponds to out of plane vibration. When compares with graphene, the lifetime of borophene is four orders of magnitude smaller than that dominant ZA modes of graphene. However the lifetimes of LA and TA branches are generally longer than that of ZA, so the LA and TA branches are the main contributors for conductivity of borophene [15].
In this study, we performed first-principles calculations to investigate the adsorption of indigo and indigoid molecules on borophene. The results are expected to provide theoretical support concerning the application of optoelectronic properties in organic semiconducting devices.

Computational details
We performed all DFT calculations with Gaussian 09 program package [16]. Because of the fact that our indigoid molecules are hypothetical, we used a three steps approach for calculation. In the fi rst step we selected the B3LYP [17] and PBE0 [18] functional and 6-311G (d,p) basis set which are proven for indigoids optimization. TD-DFT calculations [19] were performed with the 6-311+G(2d,p) basis set which uses transition wavelengths for indigoids. The solvation effects were calculated for chloroform solution Polarizable Continuum Model (PCM) [20] and Conductor-like Polarizable Continuum Model (CPCM) [21]. In the second part, for fi nding the closest result to the experimental l max of indigo in chloroform, B3LYP functional and different basis sets were selected for optimization and TD-DFT calculations. In the third part, B 36 borophene and its complexes with indigo and indigoids were carried out using B3LYP functional with a 6-31G(d,p) basis set, a method used earlier for adsorption of molecules on borophene. To check that all structures were in the true local minimum, vibrational frequency analysis was performed with the same level of theory. The adsorption energy is calculated with the following equation: where E indigo@B36 corresponds to the electronic energy of the indigo and indigoids@B 36 complex.
Finally TD-DFT calculations for stable structures of indigo and indigoids adsorbed borophene calculated by B3LYP functional and 6-31+(2d,p) basis set.

Absorption spectra of indigo and indigoids
The optimized structures of indigo and indigoids with B3LYP functional and 6-31G (d,p) basis set are shown in Figure 1. To check which functional gives values closer to the experimental absorption spectra of indigo, on the optimized structures in chloroform, the lowest singlet-singlet transitions were computed with B3LYP and PBE0 functional. Table S1 shows the calculated absorption data of indigo with both functional (B3LYP and PBE0) and two different solvation models (PCM and CPCM) in chloroform. The computed transitions were compared with the experimental 285 and 604 nm absorption wavelengths [22]. Both B3LYP and PBE0 functionals gave almost the same values with PCM and CPCM calculations, except from extremely long wavelength. For the 285 nm wavelength, we obtained two closer results, 281, 277 nm and 272, 266 nm from B3LYP and PBE0 functional, respectively. 281 and 272 nm wavelengths have smaller oscillator strength, while 277 and 266 nm wavelengths have stronger oscillator strength. The longest wavelength which originated from the lowest singlet-singlet transition (S0S1) was compared with the l max at 694 nm of the experimental absorption band. B3LYP showed a deviation of 15 and 10 nm, while PBE0 showed 31 and 27 nm deviation from experimental results with PCM and CPCM solvation models, respectively. Therefore we selected the B3LYP functional for computing the closest result to the experimental l max by changing basis set. Table S2 shows the computed lowest excitations with different basis sets together with the experimental absorption maxima of indigo. The best fi t with the experimental data (3nm differences) is found at both B3LYP/6-311+G(2d,p)//B3LYP/6-31G (d,p) and B3LYP/6-311++G(2d,p)//B3LYP/6-31G(d,p) levels in CPCM solvation model. On the other hand, the values from B3LYP/6-31+G(2d,p)//B3LYP/6-31G (d,p) and B3LYP/6-31++G(2d,p)//B3LYP/6-31G (d,p) level showed also closer value with the experimental ones at 4 nm difference. Due to the large basis set and addition of diffuse functions, there was no infl uence on the absorption maxima accuracy. Therefore, we selected B3LYP/6-31+G(2d,p)//B3LYP/6-31G(d,p) level of theory for the absorption maxima of our hypothetic indigoid molecules and adsorbed indigo and indigoids on B 36. The selected TD-DFT absorption results of indigo and indigoids with B3LYP functional and 6-31G+(2d,p) basis set are given in Table 1 and spectra are illustrated in Figure 2. Bathochromic shift observed at the l max of halogen, nitro and carboxylic acid substituted indigoid molecules.    [23]. The electrical conductance which is calculated from theoretical computations has been usually simulated by the HOMO-LUMO gap change of semiconductor [24,25] Based on these studies, the large reduction of HOMO-LUMO gap causes signifi cant increase in the electrical conductance of borophene.

Adsorption of indigo and indigoids on B36
As can be seen in Figure 3 the indigoid-d@B 36 has the most narrow frontier orbital energy gap and it can be predicted

Absorption spectra of indigo and indigoids on B 36
The impact of indigo and indigoids@B 36 interactions on the adsorbed structures, optical properties are then discussed by TD-DFT. The absorption spectra of indigo and indigoids@B 36 were obtained by using B3LYP/6-31+G(2d,p) level and results are given in Table 3. It can be seen from the Table S1 and Table   S2

Conclusion
Borophene brings a new member to the magnifi cent 2D materials family and opens the way to exploring the boronbased microelectronic devices. However, the research of borophene is just in its beginning, and a lot of properties remain to be explored before borophene can be established as a valuable alternative for the next generation of electronic applications. To improve the performance of semiconductor behavior of borophene, we conducted the fi rst principle study of the adsorption of indigo and indigoids on a B 36 borophene surface. After the adsorption of indigo molecule on the B 36 , considering six different confi gurations, the strongest adsorption confi guration of indigo was found on the edge of B 36 . Five different indigoids molecule conducted this edge adsorption confi guration of B 36 . Due to indigo and indigoids@ B 36 adsorption process, we found that the HOMO-LUMO gap of B 36 is considerably decreased. Using TD-DFT method the absorption spectra of indigo and indigoids@B 36 were obtained from visible to near infrared region. From the absorption wavelengths of the indigo and indigoids@B 36 complexes, the charge transfer transitions were observed in the near infrared and visible regions. The fi ndings of this study suggests that the B 36 borophene structures can be used as organic semiconducting devices whose electronic conductions vary because of the interaction of indigo and indigoid molecules.

Acknowledgment
The presented numerical results were obtained at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA Resources).