3.1. Geometries
Fig. 2 depicts the chemical structure of parent molecular DTSand its synthetic derivatives 1-9 that belong to the class of monomer; whereas two compounds 10-11 have fallen into the categorical dimer with/without the bridge. All were selected from several previous works of literature and have been identified as synthetic compounds [4,8,11,15-17]. The optimized geometrical structures of studied systems are shown in Figs. 3 and Fig. S1, including four states of neutral, cation, anion and S1. Meanwhile, the selective bond lengths/bond angles are tabulated in Table S1. The most striking feature is that not only previous calculations, but synthetic outcomes also confirmed that siloles presented in both planar, twist intramolecular transfer, and J-aggregate formations [4,8,18]. In all four states, the computational DFT method points out that the optimized planarity or co-planarity are observed for the compounds 5 and 10 . It is well known that the more co-planar the structures of all compounds are in the S1 state, the faster the transfer of the photo-induced electron from S0 to S1.(19) However, the dihedral angle θS-C4”-C4”-C3” in flexible thiophene bridge of compound 11 shows that the values of 74.7o/neutral form, 31.4o/anion, 27.8o/cation, and 13.3o/S1. This suggests that molecule11 has lost the co-planarity in the S1 state (Table S2).
We found that parent DTS and its analogs 1-5 have an equation in bond distances between C1-C2 and C1’-C2’ in each compound. These compounds also contain a C2 axis going through bond C4-C4 and the silicon atom, while compound 11possesses a C2 axis between C4”-C4”. ACi center has been observed via C1’-C1’ in DTS-DTS typical compound 10 [8]. The symmetrical property is further identified by means of electronic indices. For instance, compound 10 is comprised of the definite dipole moment of 0.00 Debye and is much lower than that of the bulky asymmetrical one of compound 11 (3.876 Debye) (Table 1).
Having a detailed analysis, from all studied compounds, C1-C2 and/or C1’-C2’ bond lengths of a typical neutral and anion appear to be consistently shorter than those of S1 and cation in each compound. Similarly, the values of C4-C4 and/or C4-C4’ bond distances in neutral mostly generate longer outcomes than those of the anion, cation and S1 forms. For dimeric compounds 10 and 11 , it can be seen from Table S3 that the structure of compound 10 is more planar than compound 11 in both states. It is interesting to note that for all compound which have substutient SMe (compounds3 , 6 , 8 , 9 , 10 ), the methyl groups of terminal methylthio moieties are almost perpendicular with the DTS units in the neutral, anion, and S1 states, and mostly planarization/co-planarization is found in cation ones. Because the more relaxation in structure between the neutral and cation form of these compounds can lead to the lager hole reorganization energy so these molecules will show the lager the λh than other compounds (as will be discussed later). The distortion in bond/dihedral of the compound 11 is lager than other compounds, which increase the reorganization energy of this compound.