274
T. Mutai
in preparing multiple polymorphic phases and obtaining appropriate crystal data,
because some phases often have amorphous or microcrystalline states.
14.2 Molecular Design
14.2.1 Designing Polymorph-Dependent Luminescent
Compound
The ability to (i) form polymorphic phases and (ii) transduce conformational changes
and molecular interactions into altered luminescence is required for a molecule
to display PDL. It would be interesting if the polymorphic phases could be interconverted by external stimuli, because it may lead to luminescence-switchable
materials.
Our strategy to realize the above points is to introduce adequate flexibility in the
molecular conformation, especially around the aromatic luminescent core. As shown
in Scheme 14.2, two π-electronic planes connected by a covalent single bond are
“moderately” fixed with a weaker hydrogen bond, so that various conformations and
packing modes could be tolerated during crystallization.
The second strategy is to take advantage of a characteristic luminescence that
stands on the intramolecular hydrogen bond: in this case, N···H–O. It has been demonstrated that the phenolic proton transfers to the nitrogen atom promptly after photoexcitation, which is termed excited-state intramolecular proton transfer (ESIPT), and
the generated excited species is deactivated upon emitting low-energy light (see next
section for details). ESIPT luminescence, especially in our system, is supposed to be
susceptible to the molecular conformation and the effects of surrounding molecules;
therefore, different ESIPT luminescence might be expected in different polymorphic
phases.
14.2.2 Mechanism of ESIPT Luminescence
A photoinduced ESIPT [36] is reported to be a remarkably fast process (rate constant
k ≈ 1 × 10
13 s
−1 ) [37, 38]. In the ground state, the acidities (pK a ) of phenolic–OH
and hydrogen-bonded nitrogen are about 9 and 6, respectively; therefore, they may
exist as an enol (N···H–O). Upon photoexcitation, the phenolic –OH becomes quite
acidic (pK a ~ 2) and the proton transfer to the nitrogen (ESIPT) and subsequent
tautomerization to the keto form are promoted (Scheme 14.3) [39]. Emission from
the keto form is characterized by a large Stokes shift (≈10,000 cm
−1 ), which enables
long-wavelength fluorescence upon excitation with UV light. However, molecules
which exhibit efficient ESIPT fluorescence are rather limited. The most studied
T. Mutai
in preparing multiple polymorphic phases and obtaining appropriate crystal data,
because some phases often have amorphous or microcrystalline states.
14.2 Molecular Design
14.2.1 Designing Polymorph-Dependent Luminescent
Compound
The ability to (i) form polymorphic phases and (ii) transduce conformational changes
and molecular interactions into altered luminescence is required for a molecule
to display PDL. It would be interesting if the polymorphic phases could be interconverted by external stimuli, because it may lead to luminescence-switchable
materials.
Our strategy to realize the above points is to introduce adequate flexibility in the
molecular conformation, especially around the aromatic luminescent core. As shown
in Scheme 14.2, two π-electronic planes connected by a covalent single bond are
“moderately” fixed with a weaker hydrogen bond, so that various conformations and
packing modes could be tolerated during crystallization.
The second strategy is to take advantage of a characteristic luminescence that
stands on the intramolecular hydrogen bond: in this case, N···H–O. It has been demonstrated that the phenolic proton transfers to the nitrogen atom promptly after photoexcitation, which is termed excited-state intramolecular proton transfer (ESIPT), and
the generated excited species is deactivated upon emitting low-energy light (see next
section for details). ESIPT luminescence, especially in our system, is supposed to be
susceptible to the molecular conformation and the effects of surrounding molecules;
therefore, different ESIPT luminescence might be expected in different polymorphic
phases.
14.2.2 Mechanism of ESIPT Luminescence
A photoinduced ESIPT [36] is reported to be a remarkably fast process (rate constant
k ≈ 1 × 10
13 s
−1 ) [37, 38]. In the ground state, the acidities (pK a ) of phenolic–OH
and hydrogen-bonded nitrogen are about 9 and 6, respectively; therefore, they may
exist as an enol (N···H–O). Upon photoexcitation, the phenolic –OH becomes quite
acidic (pK a ~ 2) and the proton transfer to the nitrogen (ESIPT) and subsequent
tautomerization to the keto form are promoted (Scheme 14.3) [39]. Emission from
the keto form is characterized by a large Stokes shift (≈10,000 cm
−1 ), which enables
long-wavelength fluorescence upon excitation with UV light. However, molecules
which exhibit efficient ESIPT fluorescence are rather limited. The most studied