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].