Chapter 20
Functional Photoactive Materials Based
on Flexible π Molecules
Shohei Saito
Abstract Flapping molecules (FLAP) have been developed as a new series of photofunctional systems. The molecular design is based on the concept of the rigid/flexible
hybridization of π conjugated units. FLAP bearing bulky substituents works as a
ratiometric viscosity probe that shows no polarity dependence, while FLAP with
long alkyl chains provides a “light-melt adhesive,” a highly cohesive columnar liquid
crystals that can be melt by ultraviolet light irradiation.
Keywords Flapping molecule · Viscosity probe · Liquid crystal · Adhesive
20.1 Introduction
Applications of π-conjugated molecules cover dyes/pigments and aromatic polymers, and in recent years, they have been used in a wide range of fields as organic
electronics materials, fluorescent probes, and building blocks for self-organized and
supramolecular structures. However, most of these uses relies on the merit of “rigidity
of the skeleton” inherent to the π-conjugated molecular system mainly composed
of aromatic rings. For example, porphyrins and fullerenes tend to undergo electron
transfer because their molecular shape does not change and they have low reorientation energy [1]. In addition, perylene and fluorescein having a condensed polycyclic
structure show slow nonradiative decay when photoexcited because molecular vibration is suppressed, and thereby exhibit high fluorescence quantum yield [2]. Similarly,
polycyclic triphenylene has a flat structure and is easy to stack, and it has been put
to practical use as a discotic liquid crystal material [3]. However, in a meaning, the
fact that the basic molecular skeleton is rigid could be considered that it is difficult to
convert the physical properties derived from the flexibility of the structure, and that
static physical properties are still expressed. Based on this molecular scientific point
of view, we tried to create new molecular technologies by actively giving flexibility to
rigid π-conjugated molecules and utilizing the movement of the π-electron skeleton.
S. Saito (B)
Graduate School of Science, Department of Chemistry, Kyoto University, Sakyo, Kyoto
606-8502, Japan
e-mail: s_saito@kuchem.kyoto-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
H. Miyasaka et al. (eds.), Photosynergetic Responses in Molecules
and Molecular Aggregates, https://doi.org/10.1007/978-981-15-5451-3_20
349
Functional Photoactive Materials Based
on Flexible π Molecules
Shohei Saito
Abstract Flapping molecules (FLAP) have been developed as a new series of photofunctional systems. The molecular design is based on the concept of the rigid/flexible
hybridization of π conjugated units. FLAP bearing bulky substituents works as a
ratiometric viscosity probe that shows no polarity dependence, while FLAP with
long alkyl chains provides a “light-melt adhesive,” a highly cohesive columnar liquid
crystals that can be melt by ultraviolet light irradiation.
Keywords Flapping molecule · Viscosity probe · Liquid crystal · Adhesive
20.1 Introduction
Applications of π-conjugated molecules cover dyes/pigments and aromatic polymers, and in recent years, they have been used in a wide range of fields as organic
electronics materials, fluorescent probes, and building blocks for self-organized and
supramolecular structures. However, most of these uses relies on the merit of “rigidity
of the skeleton” inherent to the π-conjugated molecular system mainly composed
of aromatic rings. For example, porphyrins and fullerenes tend to undergo electron
transfer because their molecular shape does not change and they have low reorientation energy [1]. In addition, perylene and fluorescein having a condensed polycyclic
structure show slow nonradiative decay when photoexcited because molecular vibration is suppressed, and thereby exhibit high fluorescence quantum yield [2]. Similarly,
polycyclic triphenylene has a flat structure and is easy to stack, and it has been put
to practical use as a discotic liquid crystal material [3]. However, in a meaning, the
fact that the basic molecular skeleton is rigid could be considered that it is difficult to
convert the physical properties derived from the flexibility of the structure, and that
static physical properties are still expressed. Based on this molecular scientific point
of view, we tried to create new molecular technologies by actively giving flexibility to
rigid π-conjugated molecules and utilizing the movement of the π-electron skeleton.
S. Saito (B)
Graduate School of Science, Department of Chemistry, Kyoto University, Sakyo, Kyoto
606-8502, Japan
e-mail: s_saito@kuchem.kyoto-u.ac.jp
© Springer Nature Singapore Pte Ltd. 2020
H. Miyasaka et al. (eds.), Photosynergetic Responses in Molecules
and Molecular Aggregates, https://doi.org/10.1007/978-981-15-5451-3_20
349