3.3. Optical properties
To understand the electronic transitions, the TD-DFT with various methods at 6-311G(d,p) level of theory has been carried out on the basis of the optimized structures DTS and compounds 1-11 in THF . From the computed results in Table 3, it is clear that λ­em estimated at WB97XD method agrees well with the experimental values but the λ­abs are so different. The λ­em, λ­abs calculated at PBE0 are more agreement with the experimental values than those of B3LYP so we choose PBE0 method in our research. Together with several experimental data, available data from Table 4 and Fig. 5 reveal the excitation state, wavelength absorption λabs and emission λem (in nm), oscillator strength f , and main transition contribution.
Derivatives 1-11 have red shifts with reference to the parent compound DTS . In comparison with DTS , the symmetric group 1-5 shows red shifts of the respective absorption maximums by 10, 34, 31, 24 and 103 nm while the asymmetric compounds6-9 have bathochromic  shifted with the deviations being 32, 29, 27, 66 nm respectively. These phenomena associated with the increase of π-system (two phenyl groups) and the influence of functional groups at 1,1’-positions. So far, in short, pyridyl chromophore in compounds 5 and push-pull type DTS derivative 9 mainly causes for a better red shift than auxochromes, such as SMe in the same class of compounds [8].
Analogously, the dime compounds (10 and 11) , on the one hand, tend to have longer wavelengths. For instance, the absorption bands λabs peak at 496 and 445 nm, and the emission bands λem located at 648 and 573 nm in the visible region. On the other hand, it is relative to DTS and monomers1-9 , compounds 10-11 still show red shifts. In addition, the theoretical substantial stoke shifts calculation with λem > λabs in each studied compound would lay a good foundation to design high performance pure optical material, using DTS compounds especially dimeric types in OLED emission layer [20]. The high ranges of oscillator strengthsf abs (0.23-1.62) and f em(0.30-2.05) of studied compounds have been strongly linked to π → π* electronic transitions [23]. This also confirms that the electronic effect of the π-conjugative system is responsible for reducing the optical band gap and also increasing the fluorescence intensity [8,10].