Diphenylalanine-Derivative Peptide Assemblies with Increased Aromaticity Exhibit Metal-like Rigidity and High Piezoelectricity

Diphenylalanine (FF) represents the
simplest peptide building block that self-assembles into ordered nanostructures
with interesting physical properties. Among self-assembled peptide
structures, FF nanotubes display notable stiffness and piezoelectric
parameters (Young’s modulus = 19–27 GPa, strain coefficient d33 = 18 pC/N). Yet, inorganic alternatives remain
the major materials of choice for many applications due to higher
stiffness and piezoelectricity. Here, aiming to broaden the applications
of the FF motif in materials chemistry, we designed three phenyl-rich
dipeptides based on the β,β-diphenyl-Ala-OH (Dip) unit:
Dip-Dip, cyclo-Dip-Dip, and tert-butyloxycarbonyl
(Boc)-Dip-Dip. The doubled number of aromatic groups per unit, compared
to FF, produced a dense aromatic zipper network with a dramatically
improved Young’s modulus of ∼70 GPa, which is comparable
to aluminum. The piezoelectric strain coefficient d33 of ∼73 pC/N of such assembly exceeds that of
poled polyvinylidene-fluoride (PVDF) polymers and compares well to
that of lead zirconium titanate (PZT) thin films and ribbons. The
rationally designed π–π assemblies show a voltage
coefficient of 2–3 Vm/N, an order of magnitude higher than
PVDF, improved thermal stability up to 360 °C (∼60 °C
higher than FF), and useful photoluminescence with wide-range excitation-dependent
emission in the visible region. Our data demonstrate that aromatic
groups improve the rigidity and piezoelectricity of organic self-assembled
materials for numerous applications.