Optical Spectroscopy of Single Pentacene Molecules in Disordered p-Terphenyl Crystals

Abstract

Fluorescence excitation spectroscopy of single dye molecules in crystals offers the possibility to determine molecular paramaters of single absorbers and their distribution instead of ensemble averages [1,2]. Informations about single molecules can be derived from the optical absorption spectra and from the time distribution of the fluorescence photons. The inhomogeneous broadening of an electronic transition reflects the variety of local environments accessible to the guest molecules and is therefore a measure for the degree of disorder or the defect concentration in the host crystal [3,4]. For pentacene molecules in p-terphenyl crystals (c ≈ 10−8mole/mole) we observed different types of inhomogeneous broadening: Some crystals show a narrow (FWHM ≈ 1 GHz) Lorentzian line with a satellite structure at short wavelengths due to 13C substitutional isomers of pentacene. The inhomogeneous line of other crystals is remarkable broadened and neither Lorentzian nor Gaussian. The intersystem crossing (ISC) rates k23 and k31 can be determined by measuring the intensity autocorrelation function of the fluorescence light [5]. In a crystal with low degree of disorder (narrow Lorentzian inhomogeneous broadening) the ISC rates of the molecules show a smaller distribution than in a crystal with higher degree of disorder (broadened line). The fully saturated fluorescence emission rates as calculated using the ISC rates are consistent with the experimentally measured photocount rates.