The absorption and luminescence spectra of a series of 4,7-diaminocoumarins have been investigated in ethanol and acetonitrile solution. The pkJ and pkJ 1 values for several of the compounds have been measured. It has been found that the site of primary protonation is the nitrogen atom in the 7-position, and that the second protonation reaction occurs at the lactone oxygen atom. The effects of steric and electronic factors on the spectral-luminescence and acid-base characteristics of these compounds are discussed.
We have previously reported [1] the synthesis of 4,7-diaminocoumarins I-XIX from the corresponding 4-chloro derivatives. Interest in these compounds has been stimulated by several factors. First of all, many of these 4,7diaminocoumarins I-XIX are efficient luminophores, and several of these compounds (for example compounds I, III-V) also fluoresce in a short-wavelength region (400-440 nm) which is relatively rare for 7-aminocoumarins. In addition, compounds with similar structures (such as coumarins VI-IX) may have sharply different luminescence quantum yields. Second, according to [2], the excited singlet state (S~) for 7-aminocoumarins is a charge transfer state (CT state, formula A). For this reason it was not clear a priori what influence a strong electron-donating substituent in the 4-position would have on the nelectron density distribution in the ground and excited states. In the case of 4,7-diaminocoumarins we cannot exclude the possibility of a different CT state in which charge transfer occurs from the 4-amino group (formula B). NR1R ~ NR1R ~ a B Since many 4-aminocoumarins exhibit luminescence as well [3], the question arose which amino group would interact more efficiently with the lactone carbonyl group and, in general, on what factors do the spectral-luminescence properties of compounds I-XIX depend.
The absorption spectra of coumarins I-XIX in ethanol or acetonitrile solution contain, as expected, from three to five intense bands in the regions 245-260, 280-300, and 345-380 nm. The high oscillator strength values (f ~ 0.4-0.8), calculated according to [4] for long-wavelength absorption maxima in compounds I-XIX (Table I), indicate that the corresponding electronic transitions are not forbidden. We assume that the absorption maximum in the 345-380 nm range in the spectra ofcoumarins I-XIX corresponds to a type ACT state; the same conclusion has been reached previously for close structural analogs of the coumarins in question, namely coumarin-1 (XX) and coumarin-102 (XXI) [2]. In our further discusisons we shall limit ourselves primarily to consideration of the long-wavelength absorption maximum since, as has been explained, this band provides the most information and also correlates with the luminescence properties of the compounds. It is apparent from the data in Table 1 that replacement of a 4-methyl group by an amino group in the transition from compounds XX and XXI to 4,7-dia_minocoumarin derivatives with similar structures (I-IX, XVIII, XIX), is accompanied by a hypsochromic shift of the absorption bands by 10-15 run, and by a hypsofluoric shift of the emission bands by approximately 20-30 rim. The K. A. Timiryazev Agricultural Academy, Moscow..