2 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
Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface
Single
crystals of thiophene–phenelyne co-oligomers (TPCOs)
have previously shown their potential for organic optoelectronics.
Here we report on solution growth of large-area thin single-crystalline
films of TPCOs at the gas–liquid interface by using solvent–antisolvent
crystallization, isothermal slow solvent evaporation, and isochoric
cooling. The studied co-oligomers contain identical conjugated core
(5,5′-diphyenyl-2,2′-bithiophene) and different terminal
substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated
films are molecularly smooth over areas larger than 10 × 10 μm<sup>2</sup>, which is of high importance for organic field-effect devices.
The low-defect structure of the TPCO crystals is suggested from the
monoexponential kinetics of the PL decay measured in a wide dynamic
range (up to four decades) and from low crystal mosaicity assessed
by microfocus X-ray diffraction. The TPCO crystal structure is solved
using a combination of X-ray and electron diffraction. The terminal
substituents affect the crystal structure of TPCOs, bringing about
the formation of a noncentrosymmetric crystal lattice with a crystal
symmetry <i>Cc</i> for the bulkiest trimethylsilyl terminal
groups, which is unusual for linear conjugated oligomers. Comparing
the different crystal growth techniques, it is concluded that the
solvent–antisolvent crystallization is the most robust for
fabrication of single-crystalline TPCOs films. The possible nucleation
and crystallization mechanisms operating at the gas–solution
interface are discussed