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].