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Colorless endoperoxide (1a) of benzo [1,2,3-kl:4,5,6-k'l']dixanthene (2a) changes into colored 2a in the presence of acid at room temperature. Interaction between 1a and acid was estimated using the AM1 molecular orbital calculation. The calculation shows that 1a reacts with a proton more easily at an endo oxygen atom than at a xanthene oxygen atom, and that the protonated species goes through a carbocation (4a) to give colored 2a. In order to propose novel functional dyes, which change their color more sensitively by the addition of acid, we studied the interaction between protons and the endoperoxides containing nitrogen (1b) or sulfur (1c) instead of oxygen. Nitrogen analog 1b is a hypothetical molecule. The calculated proton affinity of 1b or 1c at peripheral heteroatoms was about 34 kcal mol -1 greater than that of 1a. The deoxygenation of endoperoxide (1) to give 2 is expected to be accelerated by replacement of the peripheral heteroatoms. Replacement of the peripheral heteroatom from oxygen to nitrogen also showed larger proton affinity in case the protonation would take place at the endo oxygen. Such replacement of the peripheral atoms would be effective modification for the acceleration of color forming reaction because of their greater proton affinity.

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