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K. Kamada et al.
9.1 Introduction
9.1.1 Background of the Multiple Spin Conversion Processes
in Excited States
In recent years, there is a growing need for efficient use of excited states of relatively
large molecules for obtaining unique optical functions and energy/material conversions. Photoexcitation and the use of its excitation energy have been governed by a
“one-photon absorption and single-molecule response” basis. This causes the limitation of the usage because of rapid annihilation processes between the molecules, even
if many numbers of excited molecules are generated in condensed systems. Thus,
the annihilation processes have been considered to be harmful to efficient photoluminescence and photo-induced chemical reactions for many years. However, the
annihilation processes and the intermolecular interaction in the excited state has the
potential to break the limitation beyond the conventional viewpoint and can employ
as a photosynergetic process. For example, it is known that there are the complex
excitation processes in which multiple molecules including excited states cooperatively change its spin state such as triplet–triplet annihilation (TTA) and singlet
fission (SF), which are the reverse processes each other as
3 M
∗
+
3 M
∗ TTA
SF
1 M
∗
+
1 M
where
1 M,
1 M
* , and
3 M
* are the ground, singlet and triplet excited states of the
molecule, M. TTA is involved in upconversion (UC) for converting from low energy
photons to a high-energy photon, and SF is involved in the generation of long-lived
triplet excitons by photoexcitation. These processes are now of particular interest
from the viewpoint of improving the conversion efficiency of solar cells and can be
a good example of the photosynergetic effect.
In Photosynergetic Project, the authors and coworkers aim at investigating TTAUC and SF to understand the basic mechanism of them and obtain the guideline for
a molecular design suitable for these processes by both theoretical and experimental
approaches. For SF process, theoretical works clarified that rational quantum design
of efficient materials based on diradical character [1], we have proposed (i) a novel
class of highly efficient SF systems called “pancake-bonded systems” [2], where
electronic couplings for both SF and charge mobility are found to be maximized
simultaneously in the face-to-face configuration, (ii) rational design guidelines for
singly-/doubly-bridged SF/TTA chromophores [3], and (iii) optimal packing forms of
oligorylenes [4]. Furthermore, we have clarified the size and structure dependences of
SF dynamics for molecular aggregates and have discussed dynamics design principles
using the quantum master equation method, including vibronic couplings [5–7]. As
experimental work, we combined SF and two-photon absorption (TPA), multiplephoton excitation, as a demonstration of the usage of higher excited states [8]. On
the other hand, for TTA-UC, we have experimentally and theoretically studied the
K. Kamada et al.
9.1 Introduction
9.1.1 Background of the Multiple Spin Conversion Processes
in Excited States
In recent years, there is a growing need for efficient use of excited states of relatively
large molecules for obtaining unique optical functions and energy/material conversions. Photoexcitation and the use of its excitation energy have been governed by a
“one-photon absorption and single-molecule response” basis. This causes the limitation of the usage because of rapid annihilation processes between the molecules, even
if many numbers of excited molecules are generated in condensed systems. Thus,
the annihilation processes have been considered to be harmful to efficient photoluminescence and photo-induced chemical reactions for many years. However, the
annihilation processes and the intermolecular interaction in the excited state has the
potential to break the limitation beyond the conventional viewpoint and can employ
as a photosynergetic process. For example, it is known that there are the complex
excitation processes in which multiple molecules including excited states cooperatively change its spin state such as triplet–triplet annihilation (TTA) and singlet
fission (SF), which are the reverse processes each other as
3 M
∗
+
3 M
∗ TTA
SF
1 M
∗
+
1 M
where
1 M,
1 M
* , and
3 M
* are the ground, singlet and triplet excited states of the
molecule, M. TTA is involved in upconversion (UC) for converting from low energy
photons to a high-energy photon, and SF is involved in the generation of long-lived
triplet excitons by photoexcitation. These processes are now of particular interest
from the viewpoint of improving the conversion efficiency of solar cells and can be
a good example of the photosynergetic effect.
In Photosynergetic Project, the authors and coworkers aim at investigating TTAUC and SF to understand the basic mechanism of them and obtain the guideline for
a molecular design suitable for these processes by both theoretical and experimental
approaches. For SF process, theoretical works clarified that rational quantum design
of efficient materials based on diradical character [1], we have proposed (i) a novel
class of highly efficient SF systems called “pancake-bonded systems” [2], where
electronic couplings for both SF and charge mobility are found to be maximized
simultaneously in the face-to-face configuration, (ii) rational design guidelines for
singly-/doubly-bridged SF/TTA chromophores [3], and (iii) optimal packing forms of
oligorylenes [4]. Furthermore, we have clarified the size and structure dependences of
SF dynamics for molecular aggregates and have discussed dynamics design principles
using the quantum master equation method, including vibronic couplings [5–7]. As
experimental work, we combined SF and two-photon absorption (TPA), multiplephoton excitation, as a demonstration of the usage of higher excited states [8]. On
the other hand, for TTA-UC, we have experimentally and theoretically studied the