## 2 3D Optical Memory Systems by Photochromic Molecules Using Two-Photon Absorption
In the same year, [PERSON] _et al._ reported diarylethene derivatives (Scheme 9) that exhibited two-photon photochromism with a maximum 2 PA cross-section value of \(\sim\)44 GM at 770 nm [42]. To enhance 2 PA, they introduced the D-\(\pi\)-D structure into diarylethenes by using indole as the donor unit. The 1,4-bis(ethynyl)benzene or 1,4-bis(ethenyl)benzene unit was employed as the \(\pi\)-conjugation chain. The aryl groups can control the absorption property and reactivity of the diarylethene units. When thiophene is introduced into perfluorocyclopentene at the 3-position, the absorption tail extends to 800 nm, which overlaps with laser diodes. Therefore, the thiophene ring was attached to perfluorocyclopentene at the 2-position (**1a**, **2a**, and **3a**). As 4-thiazol and 4-oxazolly substituents are known to shift the absorption maximum of the closed-ring isomer to shorter wavelength, **4a** and **5a**, having 5-methyl-2-phenyl-4-thiazol and 5-methyl-2-phenyl-4-oxazolly groups, were synthesized. In addition, **6** was the reference compound for 2 PA. The one-photon photochromic property of **1** by irradiation at 365 nm was examined. Only one of the diarylethene units underwent photocyclization, and a compound having two closed-rings was not obtained. The energy transfer from the open-ring to the closed-ring may prevent the cyclization of the second diarylethene unit. As compound **2**, which had ethenyl groups instead of ethynyl, exhibited low solubility, its colored form could not be isolated. Compound **3a** also gave only one closed-ring isomer with the low conversion of 47%, which was attributed to the larger cycloreversion quantum yield than that of cyclization, as shown in Table 1. Although **4** exhibited good photochromic reactivity with the cycloreversion quantum yield of 0.026, which decreased to 1/25 of that of **3b** and the cyclization quantum yield almost twice, the absorption tail of **4b** extended to 800 nm, which was overlapped with 800 nm laser light. In order to shift the absorption band of the closed-ring form to shorter wavelength, the 5-methyl-2-phenyl-4-thiazolyI group was replaced with a 5-methyl-2-phenyl-4-oxazolyI group (**5**). Although the conversion of 76% is moderate, almost no absorption existed at 800 nm, allowing two-photon photochromic investigation with femtosecond 820 nm laser light. They measured 2 PA cross-sections by femtosecond Z-scan method, as shown in Table 2. They also examined two-photon photochromic reaction of **5** in solution with femtosecond pulses at 820 nm. In the same year, [PERSON] also reported the two-photon photochromism in an azo dye (Scheme 10) [43]. The _cis-trans_ photoisomerization of the commercially available azo dye in DMSO was induced by two-photon excitation employing femtosecond pulses. The 2 PA cross-section of the dye was not described. The two-photon photoreaction was monitored by \({}^{1}\)H NMR, and was kinetically analyzed. **Scheme 10. ** Structure of the _trans_ form of the azo dye. In 2006 [44], [PERSON] _et al._ reported two-photon photochromism of the diarylethene derivative 3,4-bis-(2,4,5-trimethyl-thiophen-3-yl)furan-2,5-dione as shown in Scheme 11. They examined the steady-state excitation anisotropy and MO (molecular orbital) calculations to explain its linear absorption property. The 2 PA cross-section of the open form was determined to be 80 GM at 674 nm by a femtosecond fluorescence method. The two-photon photochromic reaction was investigated using picosecond pulses. The 2 PA photocyclization quantum yield was estimated to be 0.22 \(\pm\) 0.05, which corresponded to the value obtained under one-photon excitation. **Scheme 11. ** The photochromic reactions of the diarylethene derivative 3,4-bis-(2,4,5-trimethyl-thiophen-3-yl)furan-2,5-dione.