Chapter 4
Stepwise Two-Photon Photochromism
Jiro Abe, Yoichi Kobayashi, and Katsuya Mutoh
Abstract Stepwise two-photon absorption (2PA) is a well-known nonlinear optical
phenomenon where two photons are sequentially absorbed to proceed a photophysical or photochemical reaction. The stepwise 2PA can produce a higher excited state
with lower energy photons than the energy level of the excited state. Therefore, it can
be an efficient tool to explore and realize nonlinear photoresponsive materials beyond
Kasha’s rule. Moreover, if a photogenerated chemical species is used as an intermediate state of the stepwise 2PA, the power threshold to induce the stepwise 2PA can
be greatly reduced compared with that of a simultaneous 2PA. We focus on the stepwise 2PA processes related to photochromic reactions because the combination of
photochromic reactions and stepwise 2PA processes is beneficial to develop unique
nonlinear photoresponsive materials. We introduce several anomalous phenomena
beyond one-photon photophysical and photochemical reactions.
Keywords Two-photon absorption · Nonlinear optical response ·
Photochromism · Imidazole dimer · Biradical · Naphthopyran
4.1 Introduction
Stark–Einstein law states that the primary act of light absorption by a molecule is a
one-photon process [1]. This law can be applied in various photophysical and photochemical reactions using continuous wave (CW) light sources. However, this law
would be violated when intense light sources such as laser pulses are used [2–6].
Under these conditions, nonlinear optical phenomena such as simultaneous or stepwise two- or multiple-photon absorptions may be observed. Nonlinear optical properties have several features beyond one-photon reactions, and therefore, they have
J. Abe (B) · K. Mutoh
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1
Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
e-mail: jiro_abe@chem.aoyama.ac.jp
Y. Kobayashi
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1
Nojihigashi, Kusatsu, Shiga 525-8577, Japan
© 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_4
57
Stepwise Two-Photon Photochromism
Jiro Abe, Yoichi Kobayashi, and Katsuya Mutoh
Abstract Stepwise two-photon absorption (2PA) is a well-known nonlinear optical
phenomenon where two photons are sequentially absorbed to proceed a photophysical or photochemical reaction. The stepwise 2PA can produce a higher excited state
with lower energy photons than the energy level of the excited state. Therefore, it can
be an efficient tool to explore and realize nonlinear photoresponsive materials beyond
Kasha’s rule. Moreover, if a photogenerated chemical species is used as an intermediate state of the stepwise 2PA, the power threshold to induce the stepwise 2PA can
be greatly reduced compared with that of a simultaneous 2PA. We focus on the stepwise 2PA processes related to photochromic reactions because the combination of
photochromic reactions and stepwise 2PA processes is beneficial to develop unique
nonlinear photoresponsive materials. We introduce several anomalous phenomena
beyond one-photon photophysical and photochemical reactions.
Keywords Two-photon absorption · Nonlinear optical response ·
Photochromism · Imidazole dimer · Biradical · Naphthopyran
4.1 Introduction
Stark–Einstein law states that the primary act of light absorption by a molecule is a
one-photon process [1]. This law can be applied in various photophysical and photochemical reactions using continuous wave (CW) light sources. However, this law
would be violated when intense light sources such as laser pulses are used [2–6].
Under these conditions, nonlinear optical phenomena such as simultaneous or stepwise two- or multiple-photon absorptions may be observed. Nonlinear optical properties have several features beyond one-photon reactions, and therefore, they have
J. Abe (B) · K. Mutoh
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1
Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
e-mail: jiro_abe@chem.aoyama.ac.jp
Y. Kobayashi
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1
Nojihigashi, Kusatsu, Shiga 525-8577, Japan
© 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_4
57