Single-Molecule Piezoelectric Deformation: Rational Design from First-Principles Calculations

Conventional piezoelectric materials
change shape in response to
an applied external electric field, frequently deforming at grain
boundaries in addition to intrinsic unit cell changes. We detail a
computational investigation, using density functional theory (DFT)
calculations of single-molecule piezoelectrics. Rather than deforming
along covalent bond lengths or angles, these molecular springs, derivatives
of [6]­helicene and phenanthrene, change conformation in response to
the applied field, up to 15% of the molecular length. A substituted
[6]­helicene has a predicted piezoelectric coefficient of 48.8 pm/V,
and one of the phenanthrenes yields a piezoelectric coefficient of
up to 54.3 pm/V, which is significantly higher than polymers such
as polyvinylidine difluoride (PVDF) and comparable to conventional
inorganic materials such as zinc oxide (ZnO). We discuss structural
properties that are found to yield large piezoresponse and hypothetical
target molecules with up to 64% length change and a predicted piezoelectric
coefficient of 272 pm/V. Based on these findings, we believe a new
class of highly responsive piezoelectric materials may be created
from the “bottom up”, yielding immense electromechanical
response.