122
Y. Kubota
coefficient (ε) (promotion of radiative processes: increase in k f ), and (4) change to
favourable perturbation of the electronic structure.
For advantage (1), a dye having a flexible π-conjugated skeleton is prone to induce
molecular rotation and vibration. Thus, a flexible dye easily promotes non-radiative
processes, which increases the non-radiative rate constant (k nr ). Boron complexation of a flexible dye causes rigidization (restriction of vibration and rotation),
which is reflected in the decrease in k nr . For instance, the k nr values of bispyrrole
squarylium dye and the corresponding boron complex are 0.24 × 10
9 s
−1 and 0.03
× 10
9 s
−1 , respectively, which suggest the restriction of non-radiative processes by
boron complexation (Fig. 5.2b). Roughly, Φ f is represented by the following formula:
Φ f = k f /(k f + k nr ). Therefore, a decrease in k nr results in an increase in Φ f . In many
cases, the main reason for the increase in Φ f by boron complexation is considered
to be due to the restriction of the flexible π-conjugated structure.
For advantage (2), in the pyridomethene skeleton as an example, hydrogen shifts
such as 1,3-H shift (imine-enamine tautomerization) and 1,5-H shift are possible
(Fig. 5.2a). In the excited state, a hydrogen shift results in a decrease in Φ f . Since
boron complexation of pyridomethene prevents the hydrogen shift, it contributes to
the increase in Φ f .
For advantage (3), boron complexation often leads to an increase in ε due to
the increased rigidity. According to the Strickler–Berg equation (Strickler and Berg
1962), the radiative rate constant (k f ) is proportional to the integral of the molar
extinction coefficient curve. Thus, boron complexation is advantageous in that it
promotes radiative processes, which leads to an increase in Φ f .
For advantage (4), in some cases, boron complexation leads to a favourable perturbation of the electronic structure, such as the inversion of the energy level between
the
1 (n, π*) and
1 (π, π*) states (Yoshino et al. 2007). The n-π* transition is known
to be an orbital overlap forbidden. Thus, when the S 0 → S 1 transition of a dye is
the n-π* transition, the ε value is very small, and consequently, Φ f becomes small.
Since in many cases, the π-π* transition is allowed, the change from S 0 → S 1 (n,
π*) transition to S 0 → S 1 (π*, π*) transition is considered to enhance the Φ f value.
5.2.2 Representation Method for Four-Coordinate
Organoboron Complexes
There are three alternative representation methods to express the structure of fourcoordinate organoboron complexes. Take pyridomethene-BF 2 for example. Chemical
structural formulas 1a, 1b, and 1c represent exactly the same molecule (Fig. 5.3).
Structure 1a, which has a negative formal charge on the boron atom and a positive
formal charge on the nitrogen atom, is commonly used in organic chemistry. As
shown in structure 1b, when pyridomethene-BF 2 is represented as a coordination
compound, or complex, of a metal, the bond of a neutral ligand and an anionic ligand
to a metal atom can be represented as an arrowed line and a solid line, respectively.
Y. Kubota
coefficient (ε) (promotion of radiative processes: increase in k f ), and (4) change to
favourable perturbation of the electronic structure.
For advantage (1), a dye having a flexible π-conjugated skeleton is prone to induce
molecular rotation and vibration. Thus, a flexible dye easily promotes non-radiative
processes, which increases the non-radiative rate constant (k nr ). Boron complexation of a flexible dye causes rigidization (restriction of vibration and rotation),
which is reflected in the decrease in k nr . For instance, the k nr values of bispyrrole
squarylium dye and the corresponding boron complex are 0.24 × 10
9 s
−1 and 0.03
× 10
9 s
−1 , respectively, which suggest the restriction of non-radiative processes by
boron complexation (Fig. 5.2b). Roughly, Φ f is represented by the following formula:
Φ f = k f /(k f + k nr ). Therefore, a decrease in k nr results in an increase in Φ f . In many
cases, the main reason for the increase in Φ f by boron complexation is considered
to be due to the restriction of the flexible π-conjugated structure.
For advantage (2), in the pyridomethene skeleton as an example, hydrogen shifts
such as 1,3-H shift (imine-enamine tautomerization) and 1,5-H shift are possible
(Fig. 5.2a). In the excited state, a hydrogen shift results in a decrease in Φ f . Since
boron complexation of pyridomethene prevents the hydrogen shift, it contributes to
the increase in Φ f .
For advantage (3), boron complexation often leads to an increase in ε due to
the increased rigidity. According to the Strickler–Berg equation (Strickler and Berg
1962), the radiative rate constant (k f ) is proportional to the integral of the molar
extinction coefficient curve. Thus, boron complexation is advantageous in that it
promotes radiative processes, which leads to an increase in Φ f .
For advantage (4), in some cases, boron complexation leads to a favourable perturbation of the electronic structure, such as the inversion of the energy level between
the
1 (n, π*) and
1 (π, π*) states (Yoshino et al. 2007). The n-π* transition is known
to be an orbital overlap forbidden. Thus, when the S 0 → S 1 transition of a dye is
the n-π* transition, the ε value is very small, and consequently, Φ f becomes small.
Since in many cases, the π-π* transition is allowed, the change from S 0 → S 1 (n,
π*) transition to S 0 → S 1 (π*, π*) transition is considered to enhance the Φ f value.
5.2.2 Representation Method for Four-Coordinate
Organoboron Complexes
There are three alternative representation methods to express the structure of fourcoordinate organoboron complexes. Take pyridomethene-BF 2 for example. Chemical
structural formulas 1a, 1b, and 1c represent exactly the same molecule (Fig. 5.3).
Structure 1a, which has a negative formal charge on the boron atom and a positive
formal charge on the nitrogen atom, is commonly used in organic chemistry. As
shown in structure 1b, when pyridomethene-BF 2 is represented as a coordination
compound, or complex, of a metal, the bond of a neutral ligand and an anionic ligand
to a metal atom can be represented as an arrowed line and a solid line, respectively.