158
K. Kamada et al.
was also obtained to be shorter than τ = 2 μs from a precise analysis of the UC
emission. These estimations of lifetimes are much faster than the natural lifetime
of
3 S
* , τ 0 = 16.2 μs determined by nanosecond transient absorption spectroscopy.
Thus, ISC = 1−τ/τ 0 = 0.88 or larger.
Since there is no direct method to characterize TTA , it is estimated from the other
values. The remaining efficiency is FL of the emitter. FL of rubrene is very high
and close to unity; however, it is very low in solid. The obtained value for the binary
solid of the cast sample was FL = 0.015. This value is almost close to the cast
sample without sensitizer ( FL = 0.015), suggesting PdTPTAP sensitizer did not
quench the emission of rubrene. With these values and Eq. (9.1), it was estimated
TTA = 0.36. Thus, the most determining process was found to be FL of rubrene. The
value of TTA was comparable to the reported highest value for rubrene in solution
[22]; nevertheless, further studies are needed to explorer the room to improve in
condensed solid.
9.2.4 Fixing Sensitizer Molecule on the Surface of Nanopore
The rapid-drying casting technique has been applied to prevent the segregation
of sensitizer molecules on the fabrication of solid. An alternative approach to
prevent segregation was explored. The method is fixing sensitizer molecules on
the solid surface. It was reported that, for example, porphyrin derivatives could be
fixed on the surface of clay nanosheets without aggregation [23]. Thus, a similar
approach can be adopted. As the platform to fix sensitizer, here, nanoporous glass
was employed. Nanoporous glass (NPG) has 3D-network of nanopores, enabling
extremely large pore surface area per volume. Thus, thick interaction volume like
bulk-hetero-junction can be possible.
To fix the sensitizer, we choose Pt(II) coproporphyrin I (PtCP), a tetracarboxylicacid derivative of PtOEP, as a sensitizer. The carboxylic acids of PtCP reacted with
the amino groups at the surface of the nanopores to fix it. NPG was treated with
the silane-coupling agent with the amino group to cover the surface with the amino
groups prior to the fixation (Fig. 9.7a–b). To fix the sensitizer, a piece of the aminotreated NPG was dipped or refluxed in PtCP solution of THF. The piece of NPG was
colored in pink, and its absorption spectrum well agreed with PtCP in THF solution
after subtraction of the scattering due to the nanoporous structure. This showed that
PtCP was fixed in its monomeric form on the surface of the nanopore.
This PtCP-fixed NPG was showed strong red emission assigned to the phosphorescence of PtCP under deaerated condition filled with Ar gas. Interestingly, it
was found that the phosphorescence QY (~0.25) was enhanced by compared to that
in THF solution (~0.16) for the same loading concentration (75 μM) as shown in
Fig. 9.7c. The high phosphorescence QY of PtCP fixed in NPG was unchanged
by increasing the loading concentration by a factor of ten and then decreased for
further increase. The phosphorescence decay of the samples fixed in NPG was single
exponential with the lifetime of 95 μs, which is nearly twice that in THF solution
K. Kamada et al.
was also obtained to be shorter than τ = 2 μs from a precise analysis of the UC
emission. These estimations of lifetimes are much faster than the natural lifetime
of
3 S
* , τ 0 = 16.2 μs determined by nanosecond transient absorption spectroscopy.
Thus, ISC = 1−τ/τ 0 = 0.88 or larger.
Since there is no direct method to characterize TTA , it is estimated from the other
values. The remaining efficiency is FL of the emitter. FL of rubrene is very high
and close to unity; however, it is very low in solid. The obtained value for the binary
solid of the cast sample was FL = 0.015. This value is almost close to the cast
sample without sensitizer ( FL = 0.015), suggesting PdTPTAP sensitizer did not
quench the emission of rubrene. With these values and Eq. (9.1), it was estimated
TTA = 0.36. Thus, the most determining process was found to be FL of rubrene. The
value of TTA was comparable to the reported highest value for rubrene in solution
[22]; nevertheless, further studies are needed to explorer the room to improve in
condensed solid.
9.2.4 Fixing Sensitizer Molecule on the Surface of Nanopore
The rapid-drying casting technique has been applied to prevent the segregation
of sensitizer molecules on the fabrication of solid. An alternative approach to
prevent segregation was explored. The method is fixing sensitizer molecules on
the solid surface. It was reported that, for example, porphyrin derivatives could be
fixed on the surface of clay nanosheets without aggregation [23]. Thus, a similar
approach can be adopted. As the platform to fix sensitizer, here, nanoporous glass
was employed. Nanoporous glass (NPG) has 3D-network of nanopores, enabling
extremely large pore surface area per volume. Thus, thick interaction volume like
bulk-hetero-junction can be possible.
To fix the sensitizer, we choose Pt(II) coproporphyrin I (PtCP), a tetracarboxylicacid derivative of PtOEP, as a sensitizer. The carboxylic acids of PtCP reacted with
the amino groups at the surface of the nanopores to fix it. NPG was treated with
the silane-coupling agent with the amino group to cover the surface with the amino
groups prior to the fixation (Fig. 9.7a–b). To fix the sensitizer, a piece of the aminotreated NPG was dipped or refluxed in PtCP solution of THF. The piece of NPG was
colored in pink, and its absorption spectrum well agreed with PtCP in THF solution
after subtraction of the scattering due to the nanoporous structure. This showed that
PtCP was fixed in its monomeric form on the surface of the nanopore.
This PtCP-fixed NPG was showed strong red emission assigned to the phosphorescence of PtCP under deaerated condition filled with Ar gas. Interestingly, it
was found that the phosphorescence QY (~0.25) was enhanced by compared to that
in THF solution (~0.16) for the same loading concentration (75 μM) as shown in
Fig. 9.7c. The high phosphorescence QY of PtCP fixed in NPG was unchanged
by increasing the loading concentration by a factor of ten and then decreased for
further increase. The phosphorescence decay of the samples fixed in NPG was single
exponential with the lifetime of 95 μs, which is nearly twice that in THF solution