A Picosecond Hard X-ray Study of the Fluorescence Dynamics of Anthracene Derivatives and 8-Hydroxyquinoline Complex Microcrystals † Hideho Odaka, Toshifumi Miura, Koji Hatanaka, ‡ Sabine Wiebel, § and Hiroshi Fukumura* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed: April 5, 2009; ReVised Manuscript ReceiVed: May 10, 2009 A 20 ps pulsed hard X-ray source (4 -15 keV) was utilized for exciting microcrystals of anthracene, 9,10- dichloroanthracene, 9,10-diphenylanthracene tris (8-hydroxyquinoline) aluminum, and bis(8-hydroxyquinoline) zinc, and the generation process of these fluorescence states was studied. It was found that the fluorescence rise time by hard X-ray excitation was much slower than the rise time by UV excitation for all samples. In addition, there was a tendency that molecules including heavier atoms show faster fluorescence rise time and lower fluorescence intensity. It is considered that because the X-ray absorption occurs at the heaviest atom in a molecule to yield a photoelectron which further generates ionized tracks consisting of electron-hole pairs, the amount of the electron-hole pairs leading to fluorescence states would depend on the initial photoelectron energy. The observed fast rise for the molecules including heavy atoms is attributed to the rapid recombination of electron-hole pairs, which may be affected by the initial distance distribution of electron-hole pairs, the density of electron-hole pairs, and the crystal structures. Introduction The development of ultrafast spectroscopy has extensively advanced molecular photochemistry, and now ultrafast X-ray and electron pulses are expected to open up new research fields in chemistry and physics of excited states. 1 Ultrafast X-ray and electron pulses can be used for pump pulses to generate excited states as well as for probe pulses to detect photoinduced changes. The interaction of molecules with X-ray, γ ray, and high-energy electrons is known to be quite different from that with UV and visible light. 2 Thus, it is meaningful and interesting to study ultrafast initial processes in radiation chemistry by the use of ultrafast X-ray and electron pulses. X-ray induced luminescence from various materials has long been studied since the finding of X-ray, being called scintilla- tion. 3 The mechanism to convert a single X-ray photon to multiple visible photons is roughly understood. When the X-ray photon energy is below 100 keV, the major process in a conventional scintillation material is the photoelectric effect, yielding a high-energy electron from one of the heaviest atoms in the material. The high-energy electron travels within 1 ps in the scintillation material, generating multiple electron-hole pairs. Electrons and holes then migrate in the material and recombine to produce excited states emitting visible photons. The kinetics of the recombination process has been extensively studied, and the mechanism involving electron thermalization, electron Brownian motion in a Coulomb field for high temper- ature, and electron tunneling for low temperature is widely accepted. 4-6 Recently, fluorescence dynamics and quantum efficiency of anthracene single crystals have been measured with 1 ns pulses having a variety of photon energies, ranging from 3 to 700 eV. 7 The fluorescence intensity was found to be proportional to the excitation photon energy for above 75 eV, whereas the photochemical nature showing fine spectroscopic structures becomes prominent in the action spectrum below 40 eV. However, no difference was found in the fluorescence dynamics irrespective of the excitation photon energy, which is ascribed to the excitation pulse duration being longer than ultrafast decay processes specific to radiation chemistry. The fluorescence quantum efficiency when excited with X-rays is generally known to be lower than a few percentages. 3 It is considered that the low efficiency is due to high density excitation in ionized tracks called spurs, where S 1 -S 1 annihilation, ultrafast internal conver- sion from highly excited states, and other quenching processes involving chemical reaction and color center formation may occur. 7,8 Accordingly, the ultrafast initial process to yield fluorescent states by X-ray excitation should be clarified for an in-depth understanding of the scintillation mechanism. Photoexcitable X-ray tubes are now available for the study of scintillation rise and decay dynamics of various materials. 9-12 When picosecond pulse lasers are used as the excitation source of this kind of apparatus, the X-ray pulse durations remain longer in the range of 35-100 ps. Recently we have utilized a femtosecond pulse laser to drive an X-ray tube and generated 20 ps X-ray pulses for measuring the fluorescence dynamics of a Re compound in solution and solid states. 13 In this report, we use the same excitation source and study the ultrafast scintil- lation dynamics of anthracene derivatives and 8-hydroxyquino- line complexes. Anthracene is well-known as a representative of organic scintillators for its high scintillation efficiency. 14 The excited state dynamics of anthracene derivatives have been extensively studied in solid states in relation to their crystal structures. 15 8-Hydroxyquinoline complexes such as tris(8- hydroxyquinoline) aluminum (Alq 3 ) have also been studied from an application viewpoint as an organic light-emitting device material. 16,17 Here, in order to study the effect of heavy atoms in molecules, we have selected bis(8-hydroxyquinoline) zinc (Znq 2 ) as a sample. † Part of the “Hiroshi Masuhara Festschrift”. ‡ Present address: Center for Ultrafast Intense Laser Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan. § Present address: Schneider Electric Industries, 37 Quai Paul Louis Merlin, 38000 Grenoble, France. J. Phys. Chem. C 2009, 113, 11969–11974 11969 10.1021/jp9031332 CCC: $40.75  2009 American Chemical Society Published on Web 05/29/2009