Influence of Side-Chain
on Structural Order and Photophysical
Properties in Thiophene Based Diketopyrrolopyrroles: A Systematic
Study
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Abstract
In this work, we have synthesized a series of TDPP derivatives
with different alkyl groups such as <i>n</i>-hexyl (−C<sub>6</sub>H<sub>13</sub>) <b>3a</b>, 2-ethylhexyl (-(2-C<sub>2</sub>H<sub>5</sub>)C<sub>6</sub>H<sub>12</sub>) <b>3b</b>, triethylene
glycol mono methyl ether (-(CH<sub>2</sub>CH<sub>2</sub>O)<sub><b>3c</b></sub>H<sub>3</sub>, TEG) <b>3c</b>, and octadodecyl
(-(8-C<sub>8</sub>H<sub>17</sub>)C<sub>12</sub>H<sub>22</sub>) <b>3d</b>. <i>N</i>,<i>N</i> dialkylation of
thiophene-diketopyrrolopyrrole (TDPP, <b>1</b>) strongly influences
its solubility, solid state packing, and structural order. These materials
allow us to explicitly study the influence of alkyl chain on solid
state packing and photophysical properties. TDPP moiety containing
two different alkyl groups <b>3e</b> (TEG and 2-ethylhexyl)
and <b>3f</b> (TEG and <i>n</i>-hexyl) were synthesized
for the first time. The absorption spectra of all derivatives exhibited
a red shift in solid state when compared to their solution spectra.
The type of alkyl chains leads to change in the optical band gaps
in solid state. The fluorescence study reveals that TDPP derivatives
have strong π–π interaction in the solid state
and the extent of bathochromic shift is due to combination of intramolecular
interaction and formation of aggregates in solid state. This behavior
strongly depends on the nature of alkyl chain. The presence of strong
C–H···O inter chain interactions and CH−π
interactions in solid state exhibits strong influence on the photophysical
properties of TDPP chromophore