## 2 3D Optical Memory Systems by Photochromic Molecules Using Two-Photon Absorption
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Entry**} & \multicolumn{2}{c}{**Quantum yield**} & \multirow{2}{*}{**Conversion**} \\ \cline{2-3}  & **Cyclization** & & **Cycloreversion** \\ \hline
**3** & 0.17 & 0.63 & 47\% \\
**4** & 0.32 & 0.026 & 93\% \\
**5** & 0.12 & 0.017 & 76\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Quantum yields of diarylethene derivatives **3–5** in THF (tetrahydrofuran) [42]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Entry**} & \multicolumn{2}{c}{**Wavelength/nm**} & \multicolumn{2}{c}{**2 PA cross-section/GM**} \\ \hline
**3a** & 770 & 44 \\
**5a** & 770 & 43 \\
**5a** & 820 & 23 \\
**6** & 770 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Two-photon absorption (2 PA) cross-section values of **3a**, **5a**, and **6**[42]. In 2006 [45,46] and 2007 [47], the same group has reported two-photon resonance energy transfer (RET) from two-photon absorbing fluorene derivatives to the photochromic diarylethene molecule. In the latter report [47], the storage medium used was a polymer matrix of poly(methylmethacrylate-_co_-diethylvinylbenzylphosphonate) PMMA-co-VBP containing diarylethene **7**, which was a commercially available photochromic molecule, and two-photon absorbing fluorene derivatives **8** or **9**, as shown in Scheme 12. Data recording was achieved by two-photon excitation of the closed form of 7 at 800 nm. 2 PA cross-section values of **8** and **9** were 1200 GM at 800 nm [48] and 2000 GM at 720 nm [49], respectively. Data readout was also performed by two-photon irradiation of the fluorene derivatives at 800 nm using much lower laser intensities than those used for recording. Differences in the 2 PA efficiency of the two components made it possible for both the recording and readout using the same excitation wavelength at different laser intensities. This system provided nondestructive readout without loss of the initial fluorescence intensity greater than 20% after 10\({}^{4}\) readout cycles. **Scheme 12. ** Structures of diarylethene **7** and fluorene derivatives **8** and **9**. The multilayer bit-by-bit two-photon recording and two-photon readout was demonstrated by recording two layers on thick polymer films (\(\sim\)120 \(\upmu\)m), which consisted of PMMA-co-VBP with the closed form of diarylethene **7** and fluorene **8**. The two layers were separated by 50 \(\upmu\)m. The excitation beam was generated by a femtosecond laser at 800 nm and was focused onto the sample using an objective lens. For two-photon recording, the average power was 20 mW. For two-photon readout, the same laser was used with a lower average power of 6 mW. The same group reported two-photon 3D WORM (write-once read-many) optical data storage systems using photoacid generator (PAG) [50]. This is considered to be one type of irreversible photochromic system. The original study using PAG had already been reported in 2002 [51], where 3D fluorescence images of multilayer polymer films containing a fluorene derivative and PAG were presented. The writing was performed by acidification of upon 2 PA of a PAG at 730 nm. Theyemployed photosensitive polymeric systems composed of 5% of two-photon absorbing PAG **10**, 1% of two-photon upconverted fluorescence dye **11**, and 94% of polymer matrix **12** (phosphorylated poly(VBC-co-MMA)) (Scheme 13). 2 PA cross-section values of **10** and **11** were determined as 80 GM at 730 nm and 132 GM at 860 nm, respectively. Two-photon writing was performed using 200 fs pulses at 730 nm to induce the photoacid generation by 2 PA of PAG **10**, resulting in change in emission properties of **11**. Two-photon readout was performed at 860 nm in order to excite protonated dye **11**. From the two-photon writing and reading experiments, the maximum storage capacity was estimated to be approximately \(1.8\times 10^{13}\) bits/cm\({}^{3}\). **Scheme 13. ** Structures of two-photon absorbing PAG **10**, 2 PA fluorescence dye **11**, and polymer matrix **12**.