Volume 10 1. number 3 CHEMICAL PHYSICS LETTERS 21 October 1983 INTENSE NON-PHOTOCHEMICAL HOLE-BURNING SPECTRA WITH DISCRETE PHONON STRUCTURE IN AN AROMATIC ORGANIC GLASS (DIPHENYLANTHRACENE) R. JANKOWIAK and H. BASSLER Faclrbereich Ph_vsikalische Chemie. Philipps-UniversitS Marburg, Ifans-Meerwein-Strasse, D-3550 Alarburg, West Germany Reccned 6 June 1983:in find form 2 August 1983 Hole burning in 3 tetracemxioped 9,lO-diphenylanthracene glass deposited at 4 -2 K occurs at an initial efficiency of 0.2 k O.l_ The hole depth in the saturation limit is 0.6. The spectra show sharp phonon structures. The role of the phenyl rins for nonphotochernicJ hole burning (NPHB) is emphasized. Hole-burning spectroscopy * is a static method for investigating the dynamics of a molecular excitation_ avoiding the complications of modern picosecond tech- niques [ 21. The price to be paid as a compensation for its simplicity is confinement to a narrow class of ma- terials_ It requires matrices that are transparent at 4 K and ,rllow for structural relaxation after optical excita- tron. These conditions are usually met with amorphous solids. If sample preparation is intended to be done at .rmbient temperature, alcoholic glasses [3,4] are the ob- vious candidates for matrix materials. Their disadvan- t.rge is then low solubility for large molecules and low efficiency for non-photochemical hole burning (NPHB). The range of potential solvent materials widened con- siderably with the discovery of hole burning in certain polymer matrices [5,6] and in vapor-deposited amor- phous linthrscenr [7,S] _ Since most non-polymeric organic compounds are to some extent volatile and dopable in the vapor phase, investigation of low-temperature-deposited organic layers offers a simple, yet powerful tool for exploring the structural conditions to be met by a solvent/guest pair in order that NPHB be observable_ In this letter, we report zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA on NPHB studies employing a 9,l O-diphenyl- anthracene (DPA) glass doped with tetracene (TC). Among the NPHB systems studied so far, this material appears to be unique with regard to efficiency (only * For .r review on NPHB, see ref. [ 1 ]_ =:5 photons are needed to remove one TC molecule from its absorbing site), signal amplitude (in the satura- tion limit the relative hole depth is ~0.6) and spectral resolution of the phonon structure. By comparison with an unsubstituted anthracene as a host material, we comment on the major role played by the phenyl rings in the mechanism of the site interconversion pro- cess responsible for NPHB. The experimental procedure was similar to that described previously [7] _ Because of the comparable vapor pressures of DPA and TC, both compounds were mixed in the desired concentratton ratio and co- sublimed. The substrate temperature during layer depo- sition was 4.2 K, the deposition rate was of the order of 50 A/s and the layer thickness was typically 2000 A. To burn holes into the origin of the So + S absorption profile of TC and to record fluorescence excitation spectra as a measure for hole profiles, we used a line- narrowed N,-laser-pumped dye laser (bandwidth 0.1 cm -I)_ Hole burn‘ g m was done at a time-averaged in- tensity of 2.6 i 0.9 mW/cm”, otherwise the intensity was reduced by a factor of 220. Fig. 1 presents NPHB spectra recorded with two different samples in the short- and the long-burning- time limit, respectively. The depth of the zero phonon hole as a function of burning time is shown in fig_ 2_ The basic observations are: (i) In addition to the zero phonon hole, the spectra display a rich phonon structur which is almost mirror-symmetric to the origin. In con- 374 0 OOY-2614/83/OXlO-0000/S 03.00 0 1983 North-Holland