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In this paper we demonstrate 3D two-photon recording and two-photon readout in photochromic polymer composites containing a mixture of 1,2-bis(2-methylbenzobthiophen-3-yl)hexafluorocyclopentene (diarylethene 1) and fluorene derivatives 2,2‘-(9,9-didecyl-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl)bis(4,1-phenylene)) dibenzodthiazole (2) or poly(9,9-didecyl-2,7-dipheylaminofluorene) (3). The recording mechanism in this system is based on two-photon excitation of the closed form of diarylethene 1at 800 nm. The readout mechanism is based on the modulation of the emission intensity of fluorene derivatives 2or 3by the closed form of diarylethene 1through Resonance Energy Transfer (RET). Föster distances (R0) and critical concentrations (A0) were calculated from the spectral overlap of the donor's emission (fluorene derivatives) and the acceptor's absorption (closed form of diarylethene 1) in solution and in polymer films of PMMA-co-VBP. This system was demonstrated to be suitable for recording data by two-photon excitation in thick storage media. The RET-based readout method proved to be essentially nondestructive (exhibiting a loss of the initial fluorescence emission less than 20% of the initial emission after 10 000 readout cycles), providing a solution to a long-standing challenge in photochromic optical data storage. [ABSTRACT FROM AUTHOR]