104
4 Mechano-Responsive Luminescence via Crystal-to-Crystal Phase …
solids, Eisenberg et al. have reported seminal mechano-responsive luminescent crystals that exhibit increasing emission intensity upon grinding, due to changes of the
molecular arrangement, which release acid vapors [2a]. Generally, the luminescence
changes of crystalline MRL solids occur due to changes of the crystal structure
following mechanical stimulation.
The difference in lattice energy between the polymorphic solid structures that are
interconverted upon mechanical stimulation are considered crucial for the design for
mechano-responsive solid materials [1]. However, predicting the crystal structures
of polymorphs and the associated energy differences is usually difficult. Nevertheless, Wallach’s rule can serve as an empirical guide for the understanding of polymorph stability: the density of crystal phases with chiral space groups (hereafter
denoted “chiral crystals”) is lower than that of crystals with centrosymmetric space
groups (henceforth denoted “achiral crystals”) [3]. In general, the thermodynamic
stability of polymorphs increases with their density [4]. Transforming chiral crystals by mechanical stimulation into achiral crystals under concomitant changes of
the luminescent or optical properties, represents an unprecedented design concept
for mechano-responsive functional materials. Previously, thermally induced phase
transitions from chiral crystals to achiral crystals have been reported [5]. However,
reports of mechano-responsive “chiral-crystal-to-achiral-crystal phase transitions”
accompanied by emission changes remain, to the best of our knowledge, elusive, and
could be used for a new, rational design of MRL materials.
We have previously revealed that many aryl gold(I)-isocyanide compounds exhibit
MRL properties in the solid state under simultaneous prominent changes of the
molecular arrangement [2e−f, 6e−g, 7]. Very recently, we have designed an aryl
gold(I)-isocyanide complex with a binaphthyl moiety, which exhibits a stable axially chiral structure and distinct mechano-responsive behavior between racemic
and homochiral crystals [2f]. However, it was not designed for the interconversion
between chiral and achiral crystals. To implement the aforementioned concept into
MRL materials, a molecule that can form chiral and achiral crystals is required.
Therefore, we have designed a new aryl gold(I)-isocyanide complex with a biphenyl
unit (1) that is able to adopt M- (M-1) or P-conformations (P-1) (Fig. 4.1a). 1 is
achiral in solution, as the C ipso –C ipso bond in the biphenyl moiety can rotate freely.
In the solid state, however, the free rotation is inhibited by intermolecular interactions with other molecules, which results in the formation of either M-1 or P1. We anticipated that 1 should be able to form both chiral and achiral crystals,
and that mechanical stress should be able to interconvert these crystals through the
change of molecular chirality (Fig. 4.1b). In fact, all the reported crystal-to-crystal
mechanochromic compounds exhibited phase transitions from one achiral crystal to
another achiral crystal [6]. Herein, we report the first MRL material that exhibits a
chiral-crystal-to-achiral-crystal phase transition (Fig. 4.1c).
4 Mechano-Responsive Luminescence via Crystal-to-Crystal Phase …
solids, Eisenberg et al. have reported seminal mechano-responsive luminescent crystals that exhibit increasing emission intensity upon grinding, due to changes of the
molecular arrangement, which release acid vapors [2a]. Generally, the luminescence
changes of crystalline MRL solids occur due to changes of the crystal structure
following mechanical stimulation.
The difference in lattice energy between the polymorphic solid structures that are
interconverted upon mechanical stimulation are considered crucial for the design for
mechano-responsive solid materials [1]. However, predicting the crystal structures
of polymorphs and the associated energy differences is usually difficult. Nevertheless, Wallach’s rule can serve as an empirical guide for the understanding of polymorph stability: the density of crystal phases with chiral space groups (hereafter
denoted “chiral crystals”) is lower than that of crystals with centrosymmetric space
groups (henceforth denoted “achiral crystals”) [3]. In general, the thermodynamic
stability of polymorphs increases with their density [4]. Transforming chiral crystals by mechanical stimulation into achiral crystals under concomitant changes of
the luminescent or optical properties, represents an unprecedented design concept
for mechano-responsive functional materials. Previously, thermally induced phase
transitions from chiral crystals to achiral crystals have been reported [5]. However,
reports of mechano-responsive “chiral-crystal-to-achiral-crystal phase transitions”
accompanied by emission changes remain, to the best of our knowledge, elusive, and
could be used for a new, rational design of MRL materials.
We have previously revealed that many aryl gold(I)-isocyanide compounds exhibit
MRL properties in the solid state under simultaneous prominent changes of the
molecular arrangement [2e−f, 6e−g, 7]. Very recently, we have designed an aryl
gold(I)-isocyanide complex with a binaphthyl moiety, which exhibits a stable axially chiral structure and distinct mechano-responsive behavior between racemic
and homochiral crystals [2f]. However, it was not designed for the interconversion
between chiral and achiral crystals. To implement the aforementioned concept into
MRL materials, a molecule that can form chiral and achiral crystals is required.
Therefore, we have designed a new aryl gold(I)-isocyanide complex with a biphenyl
unit (1) that is able to adopt M- (M-1) or P-conformations (P-1) (Fig. 4.1a). 1 is
achiral in solution, as the C ipso –C ipso bond in the biphenyl moiety can rotate freely.
In the solid state, however, the free rotation is inhibited by intermolecular interactions with other molecules, which results in the formation of either M-1 or P1. We anticipated that 1 should be able to form both chiral and achiral crystals,
and that mechanical stress should be able to interconvert these crystals through the
change of molecular chirality (Fig. 4.1b). In fact, all the reported crystal-to-crystal
mechanochromic compounds exhibited phase transitions from one achiral crystal to
another achiral crystal [6]. Herein, we report the first MRL material that exhibits a
chiral-crystal-to-achiral-crystal phase transition (Fig. 4.1c).