Synthesis and Photochemistry of Photochromic Compounds having Independent Functions

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Fulgide is one of the most promising candidate to be applied to the erasable-and-rewritable photomemory. We have synthesized several fulgides and studied the relation between the molecular structure including the reaction media and the photochromic properties and obtained highly efficient photochromic fulgides with the large coloring as well as bleaching quantum yields. Recently we have tried to lengthen the absorption maximum of the colored form of fulgides and to develop the nondestructive read-out method, which is the most difficult problem to be solved in order to materialize the photochromic optical memory.Synthesis of fulgide E ; (E) -3- [2- (5-dimethylamino-1, 2-dimethylindol-3-yl) ethylidene] -4-isopropylidene-3, 4-dihydrofuran-2, 5-dione ; was performed by Stobbe condensation of the corresponding substituted indole with diethyl isopropylidenesuccinate followed by hydrolysis and intramolecular acid anhydride formation processes. It has two distinctive characteristics : (i) the colored form (C) has the longest absorption maximum wavelength ever known as that of fulgid : (ii) the bleaching quantum yield of C is negligibly small. By adding strong acids such as trichloroacetic acid to a toluene solution, the protonation occured on the nitroge atom of the dimethylamino group to give EH or CH form. The latter recovered the photochromic nature to return to the bleached form EH. Because an equilibrium of rapid proton exchange exists between C, CH, E, and EH, the C and CH mixture entirely bleached to produce an E ad EH mixture. Similar reaction was observed in polymer films containing the fulgide and the acid. We suggest that the fulgide and trichloroacetic acid combination provides a model of a simple and efficient re-writable optical memory possessing non-destructive read-out ability. Namely, 554 nm light could be used for writing (or erasing), 403 nm light for erasing, and 780 nm light for non-destructive read-out.