## 2 3D Optical Memory Systems by Photochromic Molecules Using Two-Photon Absorption
In 2007, we reported a photochromic porphyrin-perinaphthotiohidigo conjugate having high 2 PA efficiency [52, 53, 54]. As mentioned in the Introduction, we previously found that the ethynylene linking between porphyrins enhanced significantly the 2 PA efficiency [20, 21]. Following the similar line, a new photochromic molecule **13** (Scheme 14) by linking porphyrin and photochromic \(\pi\)-systems using the ethynylene bridge was designed. In the molecular design, the energy level of the photochromic moiety should be lower than the S\({}_{1}\) state of the zinc porphyrin part because this excitation energy that is generated by rapid relaxation after the two-photon excitation to the porphyrin S\({}_{2}\) state must transfer to the photochromic part for switching. Therefore, usual photochromic compounds, such as azobenzenes and diarylethenes, cannot be used for porphyrin. However, the S\({}_{1}\) state of the _trans_-isomer of perinaphthotiohidigo PNT (2.0 eV) [55] is lower than that of zincporphyrin. The _trans_-isomer of **13** exhibited a strong Soret band at 435 nm and a broad absorption at 655 nm originated from porphyrin and _trans_-PNT, respectively. The pure _cis_-isomer of **13** can be generated from the _trans_-isomer by one-photon irradiation at \(>\)700 nm with a quantum yield of 5% and the _trans_-isomer can be transferred from the _cis_-isomer by 500 nm irradiation with a 15% quantum yield. When the solution of the _cis_-isomer is kept in the dark at room temperature for a period of seven days, no change was observed in the UV-vis spectrum, showing that it is relatively thermally stable. On the other hand, the _trans_-isomer exhibited no change in the UV-vis spectrum when kept in the dark for several months. The 2 PA cross-section values were determined by a femtosecond open-aperture Z-scan method. The 2 PA maxima for both isomers appeared around at 850 nm with a value of 2000 GM for the _trans_-isomer and 700 GM for the _cis_-isomer. The value for the _trans_-isomer is two orders of magnitude larger than that of the ethynylporphyrin monomer (\(>\)20 GM). The strong electronic communication between porphyrin and PNT probably leads to a significant enhancement of 2 PA in **13**. Further, photoisomerization of the _trans_-isomer into the _cis_-isomer by the two-photon irradiation was examined using 200 fs pulses with a peak intensity of 0.53 GW/cm\({}^{2}\) and a beam waist around 40 \(\upmu\)m at 890 nm, where no one-photon absorption exists. After irradiating \(4.6\times 10^{10}\) shots for a period of 10 min, around \(2\times 10^{-11}\) moles of the _trans_-isomer were converted to the _cis_-isomer (7% conversion). This result shows that it might take around 0.2 ms to isomerize the _trans_-isomer molecules within a spherical volume with a diameter of 40 \(\upmu\)m using the same experimental conditions. A new diarylethene-fluorenyl derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9\(H\)-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (**14**) (Scheme 15) exhibiting efficient reversible phototransformation and large 2 PA cross-sections was reported by [PERSON] _et al_. [56]. Cyclization and cycloreversion processes of **14** were investigated in hexane, cyclohexane, and dichloromethane. A high quantum yield of close to 1.0 was observed for the direct photochromic transformation of **14** in nonpolar solvents. The lifetimes of the excited states of the open and closed forms of **14** were determined by a femtosecond transient absorption measurement to be \(\sim\)0.7 and \(\sim\)0.9 ps in dichloromethane, respectively. The rate constants of the photochromic reactions of the open and closed forms of **14** were determined as \(k_{\rm OF}\approx 5.7\times 10^{10}\) s\({}^{-1}\) and \(k_{\rm CF}\approx 2.6\times 10^{8}\) s\({}^{-1}\), respectively. The time for photochromic transformation of **14** in nonpolar solvents estimated to be less than 1 ps. The nature of cyclization and cycloreversion processes was investigated by TD-DFT calculation.