Estudos espectroscopicos das propriedades de luminescencia da cumarina a 77K

AUTOR(ES)
DATA DE PUBLICAÇÃO

1984

RESUMO

The luminescence spectra and polarization spectra of coumarin (1,2-benzopyrone), obtained from Tonka Bean (Cumarú) which exist widely in Amazonia, have been investigated in polar and non-polar solvents at 77 K. The emission of coumarin in ethanol (~10M) gives the intensity ratio of phosphorescence and fluorescence as (Irel P/F = 1,29). The emission spectra of coumarin have been recorded also in n-hexane at (~10M) and at ~10M (Irel P/F = 0,1). It was observed that the fluoresrecence spectrum of coumarin in n-hexane at ~10M is more intense and red-shifted (~45 nm) in comparison with that at 10M. We assigned the emission spectrum at ~10M as originated from the excimer species whereas the less intense spectrum at 10M as monomer emission. Comparative studies of the photophisical behavior of coumarin in non-polar solvent (~10M) and polar (~10M), provided the evidences that the fluorescence and triplet formation in coumarin is strongly altered by the temperature, solvent and external heavy-atom. This is probably due to the efficient S1S0 internal conversion resulted from the proximity of the lowest energy S1(pp*) and S2(hp*) singlet states. The internal conversion rate increase with increasing temperature and decreasing solvent polarity. The intersystems crossing rate increases with the external heavy atom, which is in agreement the predictions from model calculations of the proximity effect and the photophysical behavior of some simple aromatic ketones with close-lying hp* and pp* singlet states. The enhancement in the intersystems crossing rate is probably vía the spin-orbit-vibronic coupling route between (hp*) and (pp*) states. The derivatives of coumarin can be divided into two groups: the group (A), constituded of coumarins with electron-donors substituents in the position seven, and the group (B) are electrons-withdraw substituents in the three position and the (-CH3) group in the six and seven positions. The group (A) of coumarin derivatives have different photophisical behavior than coumarin molecule in non-polar solvents, the internal conversion and the fluorescence are both more important for the desativation of excited species. However, we observed only fluorescence for group (A) in polar solvents. Because of photophisical caracteristics of these coumarins, they could be employed as the active media for dye-lasers, such as well-known active media 7-hydroxy-coumarin/ethanol. As shown in the literature, the coumarin 5,7-dímetoxycoumarin also show some interesting reactivities with biological materials, e.g. DNA. The group (B) has photophysical behavior similar to the coumarín molecules in both solvents. Theoretical calculations on the excitation energies of (pp*) and (hp*), singlet and triplet states have been calculated using HAM/3 SCF MO CI semiempirical method for all coumarins in non-polar solvents and for some coumarins in hydrogen-bonded solvents the results were used to assist the experimental interpretations. From electron distributions of HOMO and LUMO resulted from the HAM/3 calculation, we attempt also to correlate them with the chemical and photobiological activities.

ASSUNTO(S)

espectroscopia de luminescencia

ACESSO AO ARTIGO

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