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Cyanine dyes and the related merocyanines and oxonols are commercially important dyes for high technology uses, including spectral sensitization of photographic films, optical filtering/light modulation, dye lasers, and biological staining agents. The primary chromophores and heterocycles in these dyes are outlined with specific reference to chemical reactions that allow the synthesis of almost every type of cyanine and related dye. Known structure‐property relationships for the cyanines are extensive, and examples are included for properties such as color, substituent effects, solubility, self‐association (aggregation), electrochemical behavior, photochemical reactions, and biological uses.

3,3-Dimethyl-2-bicyclo [3.3.l]nonanone (3) and 3,3-dimethyl-7-bicyclo [3.3.l]nonen-Z-one (5) have been obtained by methylation of 2-bicyclo [3.3.l]nonanone (1) and 7-bicyclo [3.3.1] nonen-%one (4), respectively. The ease of methylation of 1 and 4 is strikingly different and is attributed to the difficulty of generating the enolata ion from 4. The nmr spectrum of 3 is temperature dependent and analysis of the spectral data assuming boat e chair equilibrium in the ketone ring gives AH" = 1 kcal/mol. The chair-boat form for 3 ww assumed to be the more stable conformer, and some modified Wiberg-type calculations support this assumption. Rate of the bridgehead proton exchange for 3 in deuteriomethanol at 100.8" is 0.67 X 10-4 l./mol sec, and that of 5 is ca. 30 times slower.The chair conformation assigned the ketone ring of 5 is supported by spectral evidence.The difference is attributed partly to conformational influences.(1) The authors are pleased to make acknowledgment to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.(2) P. Rabe, R. Ehrenatein, and M. Jahn, Ann., S60, 265 (1908); P. Rabe, K.