Fluorescence Excitation Spectra and Quantum Efficiencies of Organic Crystals

Abstract

Fluorescence excitation spectra are measured for some organic crystals using excitation wavelengths down to 2200 A. The fluorescence quantum efficiency of crystalline p-terphenyl is found to be constant with wavelength of exciting radiation but apparent variations are found for trans-stilbene and particularly for anthracene crystals. These variations are due to surface escape of fluorescence the extent of which is sensitive to the depth of penetration of the exciting light in crystals which re-absorb their molecular fluorescence. The fluorescence excitation spectrum thus provides information about the absorption spectrum of the crystal. For excitation of the 001 crystal face of anthracene absorption maxima are found at 3960 A, 3740 A, 3540 A, 3370 A, 3220 A; these wavelengths are rather longer than hitherto reported. The first fluorescence maximum lies at 3980 A hence the 0-0 absorption and fluorescence transitions effectively coincide. Measurements made with an anthracene crystal having a very thin surface quenching layer of low fluorescence efficiency show that migration of electronic excitation energy through the crystal lattice occurs initially by a short range and non-radiative process. Possible transfer mechanisms are discussed and it is concluded that the most probable process is a quantum-mechanical resonance exchange between adjacent molecules resulting in a random diffusion of the excitation energy through the lattice. This is followed by the secondary and trivial process of fluorescence emission and re-absorption. The absolute photofluorescence quantum efficiencies of p-terphenyl and trans-stilbene crystals are measured by comparison with anthracene for which the quantum efficiency is 0.80 ± 0.05. At a temperature of 290°K the efficiencies of p-terphenyl and trans-stilbene crystals are respectively 0.52 and 0.65.