Fluorescence spectroscopy of all-trans-lycopene: comparison of the energy and the potential displacements of its 2Ag− state with those of neurosporene and spheroidene

The fluorescence spectra of all-trans-lycopene were recorded, and the 2Ag− energy was determined to be 13 300±300 cm−1 at both 295 and 170 K. The 2Ag− energies of open-chain carotenoids having the number of conjugated double bonds, n=7–11, so far determined by fluorescence spectroscopy, showed a linear relation, E (2Ag−)=220 550/(2n+1)+3700 cm−1. The displacements of the 2Ag− potential minima for lycopene, spheroidene and neurosporene were determined by spectral simulation of the 2Ag− fluorescence to be Δ1=1.2 and Δ2=1.9 along the ν1 and ν2 dimensionless normal coordinates. Based on the assumptions, i.e., (1) the 2Ag− energy follows the above linear relation, (2) only the ν1 mode plays a major role as a promoting and an accepting mode, and (3) the above potential displacements are the same among carotenoids with n=9–13, the Englman–Jortner equation predicted the relative rate constants of the 2Ag−-to-1Ag− internal conversion (the 2Ag− lifetime) to be 4.1 : 1.9 : 1.0 : 0.54 : 0.35, which is in agreement with those determined by time-resolved absorption spectroscopy, i.e., 4.5 : 2.0 : 1.0 : 0.47 : 0.28. An empirical equation predicting the 2Ag− lifetimes of those carotenoids is also proposed.