Solvent and substituent effects on the fluorescent properties of coelenteramide analogues

Abstract

Coelenteramide 1 is the light emitter in aequorin bioluminescence. To establish the fluorescent character of 1, the fluorescence properties of 1 and a series of its analogues, 3a–f, possessing a substituent R [= CF₃, F, H, OCH₃, OH, N(CH₃)₂] at the para-position on the 5-phenyl group have been investigated in solvents of various polarity. The fluorescence emission maxima of 1 and 3d–f, possessing an electron-donating group R [= OCH₃, OH, N(CH₃)₂] shift to lower energy with increasing solvent polarity, while those of the analogues 3a–c (R = CF₃, F, H) are independent of the solvent polarity. The linear correlation between the fluorescence maxima of 1 and 3d–f and the solvent polarity scales can be explained by formation of the singlet excited state with a charge-transfer (CT) character. The quantum yields of CT fluorescence of 1 and 3d–f have been found to be higher than those of 3a–c. These results indicate that the solvatochromic fluorescence of 1 originates from the CT excited state and the existence of an electron donating hydroxy group on the 5-phenyl group is essential for determining a wavelength and a high fluorescence quantum yield of aequorin bioluminescence.