Plastic Transition to Switch Nonlinear Optical Properties Showing the Record High Contrast in a Single-Component Molecular Crystal

To
switch bulk nonlinear optical (NLO) effects represents an exciting
new branch of NLO material science, whereas it remains a great challenge
to achieve high contrast for “on/off” of quadratic NLO
effects in crystalline materials. Here, we report the supereminent
NLO-switching behaviors of a single-component plastic crystal, 2-(hydroxymethyl)-2-nitro-1,3-propanediol
(1), which shows a record high contrast of at least ∼150,
exceeding all the known crystalline switches. Such a breakthrough
is clearly elucidated from the slowing down of highly isotropic molecular
motions during plastic-to-rigid transition. The deep understanding
of its intrinsic plasticity and superior NLO property allows the construction
of a feasible switching mechanism. As a unique class of substances
with short-range disorder embedded in long-range ordered crystalline
lattice, plastic crystals enable response to external stimuli and
fulfill specific photoelectric functions, which open a newly conceptual
avenue for the designing of new functional materials.