276
T. Mutai
As Douhal et al. reported [71, 72], HPIP exhibits ESIPT fluorescence (1 IPT * →
1 IPT ) [37, 38, 73] in nonpolar organic solutions, albeit with a low quantum yield (Φ =
0.01–0.08). The fluorescence appears at a significantly lower energy (~600 nm) than
that of HBI (~480 nm), probably because of the decreased energy level of 1 IPT * and
increased energy level of 1 IPT owing to solvent rearrangement and/or conformational
changes. This explanation is supported by the ESIPT fluorescence observed in rigid
media. In a polymer matrix [74, 75] and frozen organic solution [61], where effects
based on molecular motion are suppressed, the fluorescence is largely blue-shifted
to ~520 nm.
In polar environments, where intramolecular hydrogen bonding is prevented,
HPIP exists predominantly as the “open-enol” species and shows normal fluorescence
(Scheme 14.4b).
14.3 Polymorph-Dependent Luminescence of HPIP
14.3.1 Overview
Two crystal polymorphs of HPIP (1) exhibit bright photoluminescence of different
colors, blue-green and yellow, both of which are ESIPT luminescence. X-ray crystallographic analysis revealed different molecular conformations and packing in these
two crystal polymorphs, and interconversion between the polymorphs can be realized
via a heat-mode dry process.
Ab initio quantum chemical calculations were performed to investigate the
enhanced ESIPT luminescence in the solid state.
14.3.2 Crystalline Polymorphism of HPIP
Slow and fast cooling of a heated aqueous ethanol solution of 1 gives polymorphic crystals 1-BG and 1-Y, respectively. In the infrared spectra of 1-BG and 1-Y,
the O–H stretching band appears at around 3135 cm
−1 in both crystals, indicating
the intramolecular hydrogen bonding between the O–H and nitrogen (N1) in the
imidazopyridine ring.
X-ray crystallographic analysis of 1-BG (Pbca, Z = 8, Fig. 14.1) and 1-Y (P2 1 /c,
Z = 8, Fig. 14.2) was performed at room temperature [76]. The asymmetric unit
of 1-BG and 1-Y is composed of one and two molecules, respectively, and each
molecule clearly participates in an intramolecular hydrogen bond (O–H···N1). The
most notable conformational difference was the torsion angle θ (N1–C2–C1
–C2
)
composed of the phenyl ring and the imidazopyridine ring. While the two aromatic
rings are nearly coplanar (θ = –1.0° and 1.3°) in 1-Y, a larger torsion angle was
T. Mutai
As Douhal et al. reported [71, 72], HPIP exhibits ESIPT fluorescence (1 IPT * →
1 IPT ) [37, 38, 73] in nonpolar organic solutions, albeit with a low quantum yield (Φ =
0.01–0.08). The fluorescence appears at a significantly lower energy (~600 nm) than
that of HBI (~480 nm), probably because of the decreased energy level of 1 IPT * and
increased energy level of 1 IPT owing to solvent rearrangement and/or conformational
changes. This explanation is supported by the ESIPT fluorescence observed in rigid
media. In a polymer matrix [74, 75] and frozen organic solution [61], where effects
based on molecular motion are suppressed, the fluorescence is largely blue-shifted
to ~520 nm.
In polar environments, where intramolecular hydrogen bonding is prevented,
HPIP exists predominantly as the “open-enol” species and shows normal fluorescence
(Scheme 14.4b).
14.3 Polymorph-Dependent Luminescence of HPIP
14.3.1 Overview
Two crystal polymorphs of HPIP (1) exhibit bright photoluminescence of different
colors, blue-green and yellow, both of which are ESIPT luminescence. X-ray crystallographic analysis revealed different molecular conformations and packing in these
two crystal polymorphs, and interconversion between the polymorphs can be realized
via a heat-mode dry process.
Ab initio quantum chemical calculations were performed to investigate the
enhanced ESIPT luminescence in the solid state.
14.3.2 Crystalline Polymorphism of HPIP
Slow and fast cooling of a heated aqueous ethanol solution of 1 gives polymorphic crystals 1-BG and 1-Y, respectively. In the infrared spectra of 1-BG and 1-Y,
the O–H stretching band appears at around 3135 cm
−1 in both crystals, indicating
the intramolecular hydrogen bonding between the O–H and nitrogen (N1) in the
imidazopyridine ring.
X-ray crystallographic analysis of 1-BG (Pbca, Z = 8, Fig. 14.1) and 1-Y (P2 1 /c,
Z = 8, Fig. 14.2) was performed at room temperature [76]. The asymmetric unit
of 1-BG and 1-Y is composed of one and two molecules, respectively, and each
molecule clearly participates in an intramolecular hydrogen bond (O–H···N1). The
most notable conformational difference was the torsion angle θ (N1–C2–C1
–C2
)
composed of the phenyl ring and the imidazopyridine ring. While the two aromatic
rings are nearly coplanar (θ = –1.0° and 1.3°) in 1-Y, a larger torsion angle was