1 research outputs found
Synthesis and Photophysical Properties of Novel Meta-Conjugated Organic Molecules with 1,3,5-Benzene Branching Units
The synthesis and photophysical investigation of three
novel meta-conjugated
molecules based on 3,1,2-benzothiadiazole and thiophene-2,5-diyl derivatives
linked through 1,3,5-benzene branching units are described. Each of
them is a symmetrical molecule with two branching units, four identical
lateral thiophene-containing fragments, and one central benzothiadiazole-containing
fragment. To study the effect of the chemical structure on their photophysical
properties, the molecules with different linearly conjugated lateral
and central fragments due to incorporation of additional thiophene
rings were synthesized and compared. It was shown that absorption
spectra of the meta-conjugated molecules can be represented as a sum
of absorption bands of model compounds for their peripheral and central
fragments containing a common benzene ring being branched at the 1,3,5-benzene
unit in the meta-conjugated molecules. Therefore, they cannot be considered
simply as isolated π-conjugated systems of their peripheral
and central fragments. Instead, DFT calculations showed that several
transitions between the orbitals located in different regions of the
meta-conjugated molecule are responsible for the formation of their
absorption spectra, and they strongly depend on the degree of their
overlapping. Theoretical absorption spectra reconstructed from the
DFT data demonstrated a good agreement with the experimental results:
the transitions with larger oscillator strength correspond to the
bands with higher molar extinction coefficients and vice versa. It
was shown that luminescence spectral maxima of the meta-conjugated
molecules monotonically shift to the lower energy from 489 to 540
and 613 nm with increasing the number of thiophene rings in the peripheral
and central fragments, respectively. However, luminescence quantum
yield of the meta-conjugated molecules critically depends on the length
of linearly conjugated fragments in its structure decreasing from
24% to 1.3% with increasing the number of thiophene rings in the lateral
fragments but increasing to 90% in the molecule with more thiophene
rings in both types of the fragments. The results obtained are well
correlated to the ratio of radiative and nonradiative deactivation
rate constants of the meta-conjugated molecules that indicates a high
rate of internal conversion between the excited states corresponding
to different fragments of the molecule. The CV measurements allowed
estimating the HOMO, LUMO, and bandgap values of the target and model
compounds, which confirm the presence of meta-conjugation within the
molecules investigated. Thus, connection of linearly conjugated fragments
through meta-positions (meta-conjugation) of a benzene ring leads
to an intermediate option between fully conjugated and nonconjugated
molecules due to partial delocalization of electron density through
the 1,3,5-substituted benzene branching center