PHOTOCHEMISTRY AND PHOTOINITIATOR PROPERTIES OF 2-SUBSTITUTED ANTHRAQUINONES .1. ABSORPTION AND LUMINESCENCE CHARACTERISTICS

The photophysical and photochemical properties of 15 2-substituted anthraquinones were determined in various solvents. The absorption spectra revealed the presence of two types of structure: (1) a structure with a long-wavelength band around 360 nm associated with the electron-donating effects of the substituent; (2) a structure with a long- wavelength band around 320 nm associated with the electron-withdrawing effects of the substituent. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state, with the relative positions of the lowest excited singlet pi pi* and second excited triplet n pi* states playing an important role in determining the luminescence behaviour and subsequent photoactivity. The electron-withdrawing and electron-donating abilities of the 2-substituent are important in this regard. Two of the anthraquinones, 2-(4-phenyl)thiazoleanthraquinone and 2-chloroamidoanthraquinone, give anomalous dual phosphorescence emissions from both the triplet pi pi* and n pi* states depending on the wavelength of excitation. The data are discussed in relation to the expected photoinitiation activities of the compounds.