Multiphoton-gated cycloreversion reaction of a photochromic 1,2-bis(thiazolyl) perfluorocyclopentene diarylethene derivative

Stepwise Two-Photon-Induced Electron Transfer from Higher Excited States of Noncovalently Bound Porphyrin-CdS/ZnS Core/Shell Nanocrystals

There has been an increasing amount of interest in stepwise two-photon-absorption (2PA)-induced photochemical reactions because of their extremely lower power thresholds compared to that of the simultaneous process and drastic reaction enhancements in some cases. However, stepwise 2PA-induced photochemical reactions were reported only in single chromophores and covalently bound bichromophores and there are few reports on these reactions in noncovalently bound systems because of weak electronic interactions among chromophores. This study demonstrated the stepwise 2PA-induced electron transfer from higher excited states in noncovalently bound protoporphyrin IX·CdS/ZnS core/shell nanocrystals (NCs). The electron transfer from higher excited states of porphyrin to CdS NCs successfully overcomes the activation barrier associated with the wide bandgap ZnS shell, indicating that a high reduction potential can be obtained with the stepwise 2PA process. The concept presented in this study can be applied to various noncovalently bound multichromophore systems to explore nonlinear photoresponses.

Multiphoton-gated cycloreversion reaction of a fluorescent diarylethene derivative as revealed by transient absorption spectroscopy

The one- and two-photon cycloreversion reactions of a fluorescent diarylethene derivative with oxidized benzothiophene moieties were investigated by means of ultrafast laser spectroscopy. Femtosecond transient absorption spectroscopy under the one-photon excitation condition revealed that the excited closed-ring isomer is simply deactivated into the initial ground state with a time constant of 2.6 ns without remarkable cycloreversion, the results of which are consistent with the very low cycloreversion reaction yield (<10-5) under steady-state light irradiation. On the other hand, an efficient cycloreversion reaction was observed under irradiation with a picosecond laser pulse at 532 nm. The excitation intensity dependence of the cycloreversion reaction indicates that a highly excited state attained by the stepwise two-photon absorption is responsible for the marked increase of the cycloreversion reaction, and the quantum yield at the highly excited state was estimated to be 0.018 from quantitative analysis, indicating that the reaction is enhanced by a factor of >1800.

Photoionization and trans-to-cis isomerization of β-cyclodextrin-encapsulated azobenzene induced by two-color two-laser-pulse excitation

Azobenzene (1) and the complex resulting from the incorporation of 1 with cyclodextrin (1/CD) are attractive for light-driven applications such as micromachining and chemical biology tools. The highly sensitive photoresponse of 1 is crucial for light-driven applications containing both 1 and 1/CD to reach their full potential. In this study, we investigated the photoionization and trans-to-cis isomerization of 1/CD induced by one- and two-color two-laser pulse excitation. Photoionization of 1/CD, which was induced by stepwise two-photon absorption, was observed using laser pulse excitation at 266nm. Additionally, simultaneous irradiation with 266 and 532nm laser pulses increased the trans-to-cis isomerization yield (Υ_t→c) by 27%. It was concluded that the increase in Υ_t→c was caused by the occurrence of trans-to-cis isomerization in the higher-energy singlet state (S_n), which was reached by S₁→S_n transition induced by laser pulse excitation at 532nm. The results of this study are potentially applicable in light-driven applications such as micromachining and chemical biology tools.