3.2. Frontier molecular orbital analysis
The frontier orbital theoretical calculation leads to improvements of the capable electron delocalization and electron transition [10]. The formation patterns of HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital), and their calculated energies (EHOMO, ELUMO and Egap = EL-EH) for So have been presented in Figs. 4, Fig. S2, and Table 1. Taking notes on three forms (neutral, anion, cation) of parent moleculeDTS , both the HOMOs and LUMOs are definitely distributed on the entire molecule. In the symmetrical compounds, systems 1-4which encompassed HOMOs, are delocalizing the DTS core of neutral and cation, and DTS + phenyl groups of anion form, while LUMOs located at only phenyl rings of anion and DTS + phenyl of three remaining states. HOMO and LUMO contour plots of symmetrical compound 5 are also expandable to two pyridyl groups. However, this could not be found in terminal SiMe3 groups.
In asymmetric groups, frontier molecular orbitals of SMe groups in compounds 6 and 8 , especially HOMOs, differ from the none-distributions of SiMe in molecules 7-8 . Similar to compound 5 , except in HOMO neutral form, the evidence from the HOMO and LUMO images suggest that stabilization of compound 9is due to π-electron delocalization extension by introducing electron-withdrawing pyridyl groups. Considering the bulky systems of10-11 and in contrast to anion, cation, HOMO neutral forms are only located at SMe (Fig. S2). In addition, in all studied forms, it can be easily observed that the LUMO plots spread over the DTS backbone, phenyl, and SMe groups in dimeric compounds, but hardly penetrated two n-butyl groups in 11 . For additional information on frontier molecular orbitals, the percentages in HOMO and LUMO distribution can be found in Table 2. In the ground state, the DTS fragment is predominant to contribute to HOMO and LUMO. As such, compound 10 , where HOMO and LUMO have the lowest bandgap, are stabilized by two DTS units by at least 88.11%. However, the bridge containing a DTS in compound11 contributes to HOMO and LUMO only up to 32.88% and 21.94%, respectively. In other assessment, phenyl units in each compound get larger percent contribution toward LUMO than HOMO. This finding also agrees with the results in the geometric discussion. A divergence has been observed, which illustrates that substitution groups, including Br, SMe, SiMe3 and C5H3SiMe3 contribute to the percent distribution to HOMO more so than to LUMO, except for compounds 5 and 8 . Markedly, apart from the role of the DTS units, the key to LUMO energy adjustment depends on the substitution of the silyl atom while HOMO tuning is greatly affected by modification at 1,1’-DTS ring.
As calculated in Table 1, at ground state So, the band gap values (Eg) of DTS and its derivatives1-11 set out to amount to 2.768-4.044 eV when compared to that of a well-known OLED host material 4, 4’-bis(9-carbazolyl)-1,1’-biphenyl (CBP ). HOMO and LUMO energies for studied compounds range from –5.726 to –5.100 eV (–5.586 eV for CBP ) and from –2.163 to –1.605 eV (–1.502 eV for CBP ), respectively. These outcomes deal with prospective applications of silole derivatives in OLED [20]. Furthermore, the results of this situation reveal that the EH of 1-11 is higher than that of the parent molecule. Similar behavior arises from EL, at which, the remaining compounds are found to possess this parameter lower thanDTS, except for compounds 1 and 4 . Particularly, in terms of three groups 2-3 , 5-6 and9-11 , LUMO levels approach around –2.0 eV in comparison withDTS (–1.682 eV) and compound 4 (–1.670 eV). Herein, there are two optimal ways for lowering the LUMO layer, including the attachments of moieties Br, SMe, C5H3SiMe3 into each side of DTS skeleton and the increase of DTS units. The higher EH and the lower EL values correspond to the increase of the hole and electron injection capacities, establishing lowered Eg values accordingly [1,21]. Among the symmetrical groups, the band gap Eg is arranged in the order of DTS > 1 >4 > 3 > 2> 5 . When comparing DTS to 1 , the substitution of two phenyl rings on silyl atoms did not show a remarkable change in the Eg value, only a decrease by 0.069 eV was noticeable. However, for systems 2-5 , the Eg value significantly reduces up to 0.274, 0.238, 0.148 and 0.848 eV, respectively, in comparison with that one of compound1 . A similar action is found in the asymmetrical groups. The embedding of electron donors Br, SMe, or acceptor SiMe3in compounds 6-8 , can lead to a lower bandgap, but the π-delocalization extension through the addition of an electronegative pyridyl group in compound 9 seems to be the optimal choice. In innovative materials, OSCs-based siloles have become fashionable, wherein pyridyl units are injected to stimulate electron-transporting capacities of oligomer and polymers [22].