Molecules in electronically excited states play important roles in many photofunctional processes. Dr. Hiroshi Miyasaka has developed various ultrafast time-resolved detection systems and applied them to the elucidation of the dynamics and mechanism of elementary photochemical reactions and photofunctional processes in the condensed phase. On the basis of these results, he and his colleagues have realized and revealed new photochemical responses in highly excited states of molecular systems by utilizing multiphoton absorption and multiple excitation. These studies have contributed to the development of photochemical science beyond the restriction of one-photon excitation and one-molecule response in the lowest excited state. His major achievements are summarized below.

1. Direct elucidation of photochemical reactions and photofunctional processes in condensed phase
As an example of his research results using time-resolved measurements, the study on the initial process of photoconductivity in organic solids is introduced. Some solid systems containing aromatic compounds show photoconductive properties via the hopping process of the cationic state. Initial event in this photoconduction is the photoinduced charge separation between the aromatic molecule (or group), D, and the guest electron acceptor molecule, A, followed by the charge shift reaction of the cationic state, such as A^- D⁺ D → A^- D D⁺, in competition with the charge recombination in the initial ion pair. Although this charge shift reaction is an endothermic process against the Coulomb attraction in the initial ion pair, it proceeds with high efficiency in actual systems. To elucidate important factors beyond the conventional energy gap dependence of electron transfer reactions, he has devised and applied the detection of transient dichroism signals to the selective observation of the charge shift process and revealed that the delocalization process of the cationic state over 2 to 3 aromatic molecules in the time range of 100 fs to several ps after the photoinduced charge separation. This delocalization reduces the effective Columbic attraction and reorganization energy for the reaction, allowing the effective charge shift process taking place from several hundreds of ps to several ns. Furthermore, based on this finding, he prepared a heterogeneous adsorption system with aromatic polymers and electron acceptor, in which the inter-ionic distance in the charge separated state can increase by the sequential charge shift processes and this distance is stochastically fixed at trapping sites of the cationic state. This system realized the long-lived charge-separated state surviving for more than 8 hours even by the steady light irradiation at room temperature. From these results, he and his colleagues have demonstrated the molecular system beyond the conventional limit of the energy gap dependence in electron transfer reactions. He and his colleagues also advanced their research on the basic photochemical reactions such as electron, proton, and excitation energy transfer, ionization, and isomerization in the ultrafast time region and elucidated detailed mechanisms of these reactions, intramolecular vibration regulating the reaction, and the role of environments in the reaction.

2. Ionization by multiple photons and multiple excitations and its applications
Using two-photon excitation with ultraviolet laser pulses, Dr. Miyasaka has precisely elucidated reaction behaviors of highly excited states as well as ionized states in the condensed phase, such as solvation process of the ejected electron, excited state formation by the recombination of the ionized state and so forth. Based on these results, he and his colleagues have realized two-photon induced ultrafast (≤ 100 fs) formation of the intermolecular charge-separated state via the rapid capture of the electron from the highly excited state. This charge-separated state is at the energy level higher than the S₁ state by > 0.5 eV and shows no remarkable geminate recombination owing to the long inter-ionic distance (ca. 3 nm) resulting in the formation of free ions via the ionic dissociation process in polar solutions, which can survive more than several microseconds. By demonstrating this long-lived charge-separated state at the high energy level, he and his colleagues have provided a principle for the formation of two-photon-induced charge-separated state with preferable properties, which are difficult to attain in the conventional framework of one-photon electron transfer reactions.

3. Isomerization reaction by multiple photons and multiple excitations and development to new photo-functions
Photoisomerization of 6π-electron systems, such as fulgide and diarylethene derivatives, is one of the typical photochemical reactions where the character of electronic states well correlates with reaction profiles. During the investigation of these photoisomerization reactions, he and his colleagues found that the ring-opening isomerization reaction could be significantly enhanced by the stepwise visible two-photon excitation; a derivative with the low quantum yield (ca. 1%) of the ring-opening reaction by the visible one-photon excitation shows the reaction yield > 50 % at higher excited state attained by the visible stepwise two-photon excitation, while such reaction enhancement does not occur by the UV one-photon absorption with similar energy of the visible two-photon. He and his colleagues have elucidated the mechanism of this two-photon gated reaction by developing femtosecond two-wavelength two-pulse excitation system and applying it to many derivatives. From these results, it was shown that the orbital symmetry could mainly regulate the reaction behaviors even in highly excited states. Not only the enhancement of the reaction, he and his colleagues have also realized the suppression of the reaction using stimulated emission and advanced the reaction control method. They have expanded their research to the isomerization reaction by off-resonant multiphoton absorption and found a new reaction pathway from the forbidden excited state. On the basis of these results, they realized one-color reaction control of both directions of the isomerization reactions by inducing three-photon ring-closing and two-photon ring-opening reactions.
These studies have provided basic principles for the realization of new photo-responses beyond the Kasha's law, as well as the development of photoresponsive functional molecular systems with the light intensity threshold and an important viewpoint to the development of new functions such as one-color mesoscopic space patterning of materials.

As described above, Dr. Miyasaka has elucidated mechanisms of various photochemical reactions in the condensed phase by developing and using time-resolved detection systems with ultrashort pulsed lasers. On the basis of these results, he has extended his research to realize photoresponses beyond the restrictions in conventional photochemistry by utilizing the multiple-photon excitation processes and elucidated their detailed mechanisms. These studies have greatly contributed to the development of new photoresponsive molecules and molecular aggregates. Accordingly, these achievements were recognized as worthy of the Chemical Society of Japan Award.