Effect of Alkyl Chain Length on Charge Transport Property of Anthracene-Based Organic Semiconductors

Anthracene,
a simple planar building block for organic semiconductors, shows strong
intermolecular interactions and exhibits strong blue fluorescence.
Thus, its derivatives have a great potential to integrate considerable
charge carrier mobility and strong emission within a molecule. Here,
we systematically studied the influence of alkyl chain length on the
crystal structures, thermal properties, photophysical characteristics,
electrochemical behaviors, and mobilities for a series of 2,6-di­(4-alkyl-phenyl)­anthracenes
(Cn-Ph-Ants, where n represents
the alkyl chain length). Among them, Cn-Ph-Ants (n = 0, 1, 2, and 3) display similar layered
herringbone (LHB) packing motifs, which facilitate two-dimensional
charge transport and thereby enables high-performance organic field-effect
transistors (OFETs). All Cn-Ph-Ants exhibit
similar work functions and show strong blue fluorescence with photoluminescence
quantum yields (PLQY) of approximately 40% in toluene. In addition,
the absolute powder PLQYs of C0-, C2-, C3-, C4-, and C6-Ph-Ants are 24.6, 8.2,
5.7, 10.9, and 8.6%, respectively. Note that the alkyl chain length
shows a significant effect on the charge mobilities of Cn-Ph-Ants. Our newly synthesized C1-, C3-, and C4-Ph-Ants show hole mobilities of up to
2.40, 1.34, and 1.00 cm2 V–1 s–1, respectively, with mobilities of 3.40, 1.57, and 0.82 cm2 V–1 s–1 for C0-,
C2-, and C6-Ph-Ants, indicating an increasing
tendency of mobility with shorter alkyl chain length. This feature
is related to the microstructures of the thin films, which reveal
the enhanced film order, crystallinity, and grain size with a decrease
in the alkyl chain length. Moreover, we theoretically analyze the
intermolecular transfer integrals of HOMOs, which increase at T-shaped
contacts as the alkyl chain length decreases, which improves the intermolecular
charge transport properties, leading to the increases in mobility.
Interestingly, the anisotropy of the transfer integral tends to decrease
upon substitution with longer alkyl chains, suggesting that alkyl
chain adjustments may facilitate isotropic charge transport property
in 2,6-alkylated anthracenes.