MOLECULAR LUMINESCENCE SPECTRA OF COUMARINS AND FUROCOUMARINS AT $77^{\circ} K$

Abstract

The lowest excited singlet and triplet states of coumarins and furocoumarins (psoralens) have been assigned to the $(\pi,\pi^{*})$ type on the basis of luminescence and photoselection measurements in ethanol at $77^{\circ} K$, Surprisingly, coumarins and furoumarins without substituent at positions 3 and 4 of the pyrone moiety showed their 0-0 phosphorescence bands at essentially the same location regardless of size and substituent of these molecules. For example, the 0-0 bands are 457 (coumarin), 456, 450 (isopsoralen), 457 (8-methoxy-psoralen), and ca. 452 nm (4,5,8-trimethly psoralen). Polarizations of the 0-0 bands are negative or minimum, and out-of-plane vibration (possibly C-H) with maximum polarization is also present. 5-Methoxypsoralen shows its 0-0 band at 473 nm. Phosphorescence lifetimes of these compounds are found to be 0.5-1 sec. The above results are interpreted in terms of highly localized triplet states in the region of the pyrone double bond, and SCF molecular orbital data will be presented in support of the localized nature of the triplet states. Relative directions of the two transition moments $(\pi, \pi^{*})$ have also been determined from the lowest polarized fluorescence excitation spectra.