Photochromic transformations of spiropyrans in the crystalline state has been of great interest from the viewpoints of development of new polyfunctional materials combining different properties in the same crystalline lattice [1-3]. Here we report synthesis of (1',3',3'-trimethyl8nitrospiro[chroman-2,2'-indoline]6-yl)methanol 3 using Fischer’s base 1 [4] and 2-hydroxy-5-(hydroxymethyl)3-nitrobenzaldehyde 2. Raman studies of spiropyran 3 (closed and more stable) and open forms 4 are also reported. The reaction of aldehyde 2 with Fischer’s base 1 in benzene under reflux gave the corresponding chromene-2,2'-indoline 3 in high yield. It is readily seen that the Raman spectral features change dramatically with the transition from one state to another. Thus significant changes of the intensity of the bands at 1267 cm-1 and 1302 cm-1 corresponding to vibrational frequencies of -C-O- and -C-N- groups respectively are observed. Also the band at 1534 cm-1 (-NO2 – group) noticeably decreases in intensity relative to the band at 1592 cm-1. These spectral features of the transient species indicate that the C=C, C=O and C=N bonds are involved in conjugation of the merocyanine-like structure 4 (red-violet) and their stretching frequencies are shifted to lower frequencies.

The authors gratefully acknowledge funding through the grant CZ.1.07/2.3.00/30.0058 from
the Czech Ministry of Education, Youth and Sports as well as the bilateral BMBF project
01DK13029.
References:
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