SPECTRAL SHIFTS AND BROADENING OF FLUORESCENCE OF ANTHRACENE AND TETRACENE IN SEVERAL HOST CRYSTALS AT HIGH PRESSURES

The fluorescence spectra of anthracene in biphenyl, -4-terphenyl, naphthalene, and phenanthrene and those of tetracene in -4>-terphenyl, naphthalene, phenanthrene, and anthracene were measured at pressures as great as 50 kbar. The pressure effects on the spectra of the guests are generally analogous to the pressure effects in solution or the gas phase. Due to the increased dispersion interaction between these nonpolar molecules upon compression, the energies of the electronic transitions are shifted toward the red (lower energy), and the spectral bands are broadened, while the integrated intensity remains approximately constant. As the pressure is released, the fluorescence spectra return to those obtained before compression. The spectral shifts with pressure are satisfactorily interpreted in terms of a second-order perturbation theory using a dipole-dipole approximation. For three of the eight guest-host combinations, the fluorescence spectra above 35 kbar were considerably broader than expected. The similar broadening observed for the pure host crystals suggests that mixed excimers are produced at high pressures. On the basis of the present data, however, it is not possible to give an unambiguous explanation of the extensive broadening.