2.1 Introduction
19
of luminescent mechanochromism [10]. Structurally related tetrafluoropyridyl gold
isocyanide complex 2 (Fig. 2.1a) shows more prominent phase transitions than 1
in response to mechanical stimulation and solvent addition [11]. As-prepared solid
2Y displaying yellow emission undergoes phase transitions to form 2B (blue emission) and 2G (green emission) by immersion in acetone and subsequent evaporation, respectively (Fig. 2.1c). The resulting 2G shows mechanochromism, affording
an orange-emitting amorphous 2O phase that does not contain solvent molecules
(Fig. 2.1c). In this mechanochromism, the transformation from 2G to 2O upon
grinding is caused by amorphization during solvent release.
On the basis of our previous studies, we envisioned that aryl gold isocyanide complexes are a promising candidate for solid-state emitting materials that can response
to volatile molecules. Our intensive studies [9, 11, 12] have revealed that aryl gold
isocyanide complexes tend to form multiple crystalline structures with different emission colors. Under appropriate external stimuli, these crystal structures change to
various other structures in an interconvertible manner [11, 12d, e, 13]. The structural
changes include the modification of intermolecular interactions (Au···Au, Au/π, and
CH/π interactions) and their conformation (dihedral angle between the aromatic ring
on Au and that in the isocyanide ligand θ Isocyanide ). Addition of solvent often causes
changes in the solid-state structures of aryl gold isocyanide complexes. In the study of
complex 2, simple grinding releases acetone molecules [11], suggesting that grinding
to induce solvent release can be used as a reactivation process [14]. We anticipated
that the further structure design of aryl gold isocyanide complexes will enable the
development of solvent-responsive emitting compounds that show grinding-induced
reactivation of solvent detection properties even after solvent release.
As a new design of aryl gold isocyanide complex, we focus here on the 2,2
-
substituted biphenyl structure. In 2,2
-disubstituted biphenyl compounds (Fig. 2.2a),
the rotation around the single bond between the phenyl groups is restricted to some
extent by the steric repulsion between the two substituents. The reported dihedral
angles between the phenyl rings of the 2,2
-disubstituted biphenyl structure (θ Biphenyl ,
Fig. 2.2a) range from 30° to 60° [15]. As preliminary attempt, we calculated the
potential energy surface of a model compound M as a simple gold isocyanide complex
possessing biphenyl structure to investigate the rotation of the single bond between
two phenyl rings (PBEPBE/SDD; Fig. 2.2b). We obtained a shallow potential energy
curve (ΔE < 2 kJ mol
−1 within the range of θ Biphenyl = 40°–80°) with an energy
minimum at 63.5°. This indicates that a large structure change with various θ Biphenyl
is possible with very small free energy loss. Such a structure with restricted freedom
may enable structural variation of the crystalline phases of biphenyl gold complexes
[16]. In addition, 2,2
-disubstituted biphenyl compounds can adopt (R)- and (S)conformations based on their axially chiral biphenyl structure (Fig. 2.2c). In solution,
these conformers rapidly interconvert between each other, but in the solid state,
the conformer should be fixed in (R)- or (S)-conformation by the intermolecular
interactions between neighboring molecules. These chiral structures in the solid state
19
of luminescent mechanochromism [10]. Structurally related tetrafluoropyridyl gold
isocyanide complex 2 (Fig. 2.1a) shows more prominent phase transitions than 1
in response to mechanical stimulation and solvent addition [11]. As-prepared solid
2Y displaying yellow emission undergoes phase transitions to form 2B (blue emission) and 2G (green emission) by immersion in acetone and subsequent evaporation, respectively (Fig. 2.1c). The resulting 2G shows mechanochromism, affording
an orange-emitting amorphous 2O phase that does not contain solvent molecules
(Fig. 2.1c). In this mechanochromism, the transformation from 2G to 2O upon
grinding is caused by amorphization during solvent release.
On the basis of our previous studies, we envisioned that aryl gold isocyanide complexes are a promising candidate for solid-state emitting materials that can response
to volatile molecules. Our intensive studies [9, 11, 12] have revealed that aryl gold
isocyanide complexes tend to form multiple crystalline structures with different emission colors. Under appropriate external stimuli, these crystal structures change to
various other structures in an interconvertible manner [11, 12d, e, 13]. The structural
changes include the modification of intermolecular interactions (Au···Au, Au/π, and
CH/π interactions) and their conformation (dihedral angle between the aromatic ring
on Au and that in the isocyanide ligand θ Isocyanide ). Addition of solvent often causes
changes in the solid-state structures of aryl gold isocyanide complexes. In the study of
complex 2, simple grinding releases acetone molecules [11], suggesting that grinding
to induce solvent release can be used as a reactivation process [14]. We anticipated
that the further structure design of aryl gold isocyanide complexes will enable the
development of solvent-responsive emitting compounds that show grinding-induced
reactivation of solvent detection properties even after solvent release.
As a new design of aryl gold isocyanide complex, we focus here on the 2,2
-
substituted biphenyl structure. In 2,2
-disubstituted biphenyl compounds (Fig. 2.2a),
the rotation around the single bond between the phenyl groups is restricted to some
extent by the steric repulsion between the two substituents. The reported dihedral
angles between the phenyl rings of the 2,2
-disubstituted biphenyl structure (θ Biphenyl ,
Fig. 2.2a) range from 30° to 60° [15]. As preliminary attempt, we calculated the
potential energy surface of a model compound M as a simple gold isocyanide complex
possessing biphenyl structure to investigate the rotation of the single bond between
two phenyl rings (PBEPBE/SDD; Fig. 2.2b). We obtained a shallow potential energy
curve (ΔE < 2 kJ mol
−1 within the range of θ Biphenyl = 40°–80°) with an energy
minimum at 63.5°. This indicates that a large structure change with various θ Biphenyl
is possible with very small free energy loss. Such a structure with restricted freedom
may enable structural variation of the crystalline phases of biphenyl gold complexes
[16]. In addition, 2,2
-disubstituted biphenyl compounds can adopt (R)- and (S)conformations based on their axially chiral biphenyl structure (Fig. 2.2c). In solution,
these conformers rapidly interconvert between each other, but in the solid state,
the conformer should be fixed in (R)- or (S)-conformation by the intermolecular
interactions between neighboring molecules. These chiral structures in the solid state