J. Phys. Chem. B 2000, 104, 3429-3437
3429
Picosecond Near-Field Microspectroscopic Study of a Single Anthracene Microcrystal in Evaporated Anthracene-Tetracene Film: Inhomogeneous Inner Structure and Growth Mechanism Hiroyuki Yoshikawa, Keiji Sasaki, and Hiroshi Masuhara* Department of Applied Physics, Osaka UniVersity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ReceiVed: July 13, 1999
Topographies, fluorescence images, fluorescence spectra, and fluorescence decay curves of anthracenetetracene two-component evaporated films were observed by ps near-field fluorescence microspectroscopy. Individual anthracene microcrystals were formed on tetracene microcrystalline films, although layer-on-layer film was expected. It is interesting to note that the central part of individual anthracene microcrystals shows intense tetracene monomer fluorescence, while edge parts give relatively weak fluorescence. This means that energy transfer from anthracene to tetracene took place more preferably in the central part of the anthracene crystal than in the surrounding area. This inhomogeneity cannot be simply explained as an energy transfer at interface between anthracene and tetracene crystals. The characteristic position dependence of energy transfer dynamics implied special crystal growth and molecular diffusion mechanisms under the present sample preparation.
1. Introduction Near-field scanning optical microscopy (NSOM) is a powerful technique for investigating photophysical and photochemical phenomena occurring in nanometer-sub-micrometer dimension. NSOM has been applied to some organic solids such as a single dye molecule on solid surface, microcrystal, J-aggregate, and so on.1-7 Particularly, organic solid systems are attracting much attention as materials for the NSOM studies, since their spatial inhomogeneity is extremely important for understanding their physical and chemical properties and developing their applications. The fundamental studies on mechanisms of exciton diffusion and excitation energy relaxation processes of organic solid systems are necessary and important also for developing organic optical and electronic devices. Energy transfer, proton transfer, electron transfer, and so on, have been studied for a long time in bulk, while in the solid state, some microdomains, defect sites, surfaces, and interfaces which act as energy traps or barriers, exist, so that excitation energy relaxation processes should indicate inhomogeneous properties. It is obvious in photophysical and photochemical studies of solid state that space- and time-resolved spectroscopic measurements are indispensable. However, many works in this field are based only on measurements of topography and fluorescence images. The characteristic advantage of NSOM, which cannot be obtained by other scanning probe microscopes (SPM), is the capability of measuring local spectroscopic properties and dynamic relaxation processes. In the past few years we have developed an NSOM system, where fluorescence spectra and their picosecond fluorescence decay curves can be measured, and applied it to organic crystals; particularly, to two-component crystals.8-10 For utilizing energy transfer, electron transfer, and proton transfer, most organic solid devices consist of at least two kinds of molecules, of which one is the donor and the other is the acceptor. It is important to know how component molecules are condensed and form * Corresponding author. Tel: +81-6-6879-7837. Fax: +81-6-6876-8580. E-mail:
[email protected].
characteristic structures, because they determine performance as a device. Further, two-component crystals will be less homogeneous than one-component conventional molecular crystals. Hence NSOM is expected to be an important and indispensable tool for studying photophysical and photochemical properties of two-component crystals. Along this idea, we have studied charge-transfer crystals by picosecond near-field fluorescence microspectroscopy and reported some interesting new features of the crystal.8-10 Another interesting photophysical property is energy transfer whose rate is a function of distance and relative orientation between donor and acceptor molecules. Therefore, energy transfer analysis by using the near-field system gives information on donor-acceptor conformation at each ∼100 nm local spot. Also NSOM studies on single donor and acceptor pairs are very interesting, and actually Ha et al. measured fluorescence spectra of the pairs linked by DNA and estimated the distance and orientation between donor and acceptor.11 In the present work another approach is applied; energy transfer efficiency in the crystal phase is used to estimate the local distribution of donor and acceptor molecules. This is the first demonstration of energy transfer dynamics of organic crystals with a hundred nanometer resolution by using the nearfield technique. We have adopted anthracene as an energy donor and tetracene as an acceptor, and applied the picosecond near-field microspectroscopy system for analyzing an anthracene-tetracene evaporated film. These molecules are well-known as an energy transfer pair in the solid state, and spectroscopic properties have been studied by many researchers in bulk crystals.12-17 A sample film is prepared by two-step evaporation of tetracene and anthracene, and it is expected that anthracene and tetracene layers are formed as layer-on-layer. However, in this sample many single anthracene microcrystals are formed on the tetracene microcrystalline film and highly efficient energy transfer is surprisingly observed. We consider that local mixing of two-component molecules is the origin of the efficient energy transfer, and measure systematically topography, fluorescence image, fluo-
10.1021/jp992361a CCC: $19.00 © 2000 American Chemical Society Published on Web 03/29/2000
3430 J. Phys. Chem. B, Vol. 104, No. 15, 2000
Yoshikawa et al.
Figure 1. Far-field fluorescence images of (a) tetracene, (b) anthracene-tetracene, and (c) anthracene evaporated films observed by a conventional fluorescence microscope. (d) and (e) are molecular structures of anthracene and tetracene, respectively. Excitation wavelength is 365/366 nm.
rescence spectra, and decay curves with a resolution in the submicrometer range. An inhomogeneity in the energy transfer dynamics in single anthracene microcrystals is also examined and considered. The relation between the energy transfer dynamics and the tetracene distribution in single anthracene crystals is discussed, and a novel mechanism for the crystal growth in the two-component evaporated film is proposed. 2. Experimental Section A picosecond near-field fluorescence microspectroscopy system was reported elsewhere.8-10 An NSOM (Seiko Instruments, SPA 300) was used by modifying the probe-sample distance regulation method from tapping to shear-force mode feedback. The distance could be controlled to be ∼10 nm. A tapered optical-fiber probe was prepared by pulling an optical fiber (Newport: F-SA) by using a micro pipet puller with a CO2 laser (Sutter, P2000), and coated with aluminum in a highvacuum evaporation chamber having a fiber-rotating mechanism. The second harmonic pulse of a mode-locked Ti:sapphire laser (Coherent, Mira 900) was introduced to the other cleaved end of the optical fiber as an excitation light source. Its wavelength and pulse width were 390 nm and 200 fs, respectively, the repetition rate was 76 MHz, and the power at the input end
was less than 100 µW. To measure the tetracene microcrystalline film, the 488 nm line from an argon ion laser (OmNichrome, 543-AP,
