204
10 Importance of Molecular Crystals
temperature dependence is independent of others, whereas other modes fulfill the
assumption = z 0 (z = 1, 2, 3) with the common f 0 . The fits are successful. The
fact that the smallest activation energy (49 meV) is about half of the one with z = 1
(119 meV) implies that the former is of the hopping of individual protons. The latter
is, in contrast, the simultaneous hopping of two protons at a single side of the domain
wall. Note that the unit repeat distance of the H-bond chain in Phz-H 2 ca contains two
protons because of the presence of two molecular species. The two-proton transfer
causes a simple shift of a domain wall. For the simultaneous hopping of 2z protons,
the activation energy larger by a factor z is necessary. Considering these facts in
mind, we attribute the relaxation modes with z = 2 and 3 to simultaneous hopping
of 2z protons, which again cause a simple shift of a domain.
Similar situations, i.e., the existence of many relaxation modes governed by “quantized” activation energies recognized for related compounds [28], indicate the observability of correlated dynamics in quasi-one-dimensional systems. The possibility of
structural design, intrinsic to molecular systems, should play a vital role in a related
research field.
10.2 Molecular Crystals as Tunable Model System
10.2.1 Basis of Tunability
The structure of crystals dominates properties of materials irrespective of molecular
or non-molecular ones. If the effective Hamiltonian that describes the property under
one’s interest does not contain a coupling term(s) to lattice degrees of freedom, the
Hamiltonian parameters primarily depend on molecular properties and the crystal
structure. Such situations apply electronic and magnetic properties. In these cases, the
crystal structure is the condition given a priori. We can understand physical properties
in the light of standard theories of condensed matter physics based on molecular
properties, which are mostly calculable for a specified molecular structure based on
quantum chemical knowledge. The fundamental basis of the tunability for properties
of molecular crystals, thus, lies in the ability of molecular syntheses, which organic
and inorganic chemistries achieve. As long as the crystal structure remains unaltered,
the crystal properties vary according to the variation of molecular properties. We can
find an enormous amount of examples of this kind in the research fields of organic
conductors [29, 30] and molecular magnetism [31].
In contrast to the great success of standard theories of condensed matter, the
current achievement of crystal engineering is still insufficient. That is, we have no
reliable method to not only design but also predict crystal structures. Advancing
crystal engineering should be the primary task for utilizing molecular crystals in this
line.
10 Importance of Molecular Crystals
temperature dependence is independent of others, whereas other modes fulfill the
assumption = z 0 (z = 1, 2, 3) with the common f 0 . The fits are successful. The
fact that the smallest activation energy (49 meV) is about half of the one with z = 1
(119 meV) implies that the former is of the hopping of individual protons. The latter
is, in contrast, the simultaneous hopping of two protons at a single side of the domain
wall. Note that the unit repeat distance of the H-bond chain in Phz-H 2 ca contains two
protons because of the presence of two molecular species. The two-proton transfer
causes a simple shift of a domain wall. For the simultaneous hopping of 2z protons,
the activation energy larger by a factor z is necessary. Considering these facts in
mind, we attribute the relaxation modes with z = 2 and 3 to simultaneous hopping
of 2z protons, which again cause a simple shift of a domain.
Similar situations, i.e., the existence of many relaxation modes governed by “quantized” activation energies recognized for related compounds [28], indicate the observability of correlated dynamics in quasi-one-dimensional systems. The possibility of
structural design, intrinsic to molecular systems, should play a vital role in a related
research field.
10.2 Molecular Crystals as Tunable Model System
10.2.1 Basis of Tunability
The structure of crystals dominates properties of materials irrespective of molecular
or non-molecular ones. If the effective Hamiltonian that describes the property under
one’s interest does not contain a coupling term(s) to lattice degrees of freedom, the
Hamiltonian parameters primarily depend on molecular properties and the crystal
structure. Such situations apply electronic and magnetic properties. In these cases, the
crystal structure is the condition given a priori. We can understand physical properties
in the light of standard theories of condensed matter physics based on molecular
properties, which are mostly calculable for a specified molecular structure based on
quantum chemical knowledge. The fundamental basis of the tunability for properties
of molecular crystals, thus, lies in the ability of molecular syntheses, which organic
and inorganic chemistries achieve. As long as the crystal structure remains unaltered,
the crystal properties vary according to the variation of molecular properties. We can
find an enormous amount of examples of this kind in the research fields of organic
conductors [29, 30] and molecular magnetism [31].
In contrast to the great success of standard theories of condensed matter, the
current achievement of crystal engineering is still insufficient. That is, we have no
reliable method to not only design but also predict crystal structures. Advancing
crystal engineering should be the primary task for utilizing molecular crystals in this
line.