Microscale polarity in polymer films probed by fluorescence of a benzanthrone derivative on ensemble and single-molecule level Akifumi Hattori a,b , Hisaya Sato a , Martin Vacha b, * a Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16 Naka-machi, Koganei, Tokyo 184-8588, Japan b Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama 2-12-1-S8, Meguro-ku, Tokyo 152-8552, Japan Received 27 November 2006; in final form 13 December 2006 Available online 19 January 2007 Abstract We report fluorescence lifetime studies of a charge-transfer dye, dimethyl-amino-N-acetyl-3-aminobenzanthrone, embedded in a series of polymer matrices. In all polymers studied, the fluorescence lifetime has two components, indicating the existence of two classes of molecular environments. Microscopic studies on sub-ensembles of the dye molecules carried out in PMMA and PVAc matrices reveal that the polymer films are spatially heterogeneous with respect to their physical properties on scales of more than few hundreds of nm. The heterogeneity further increases on molecular scale, as observed by fluorescence lifetime distribution of the dye in the PMMA matrix. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Amorphous polymer solids are complex systems with large structural heterogeneities on microscopic level. The heterogeneities arise from distributions in molecular weights of the polymers and from complex chain confor- mations and entanglements. The structural heterogeneities give rise to heterogeneities in dynamical processes which may be further enhanced in spin-coated polymer films by the highly non-equilibrium thermodynamic states. With the potential of applications of polymer solids for nano- scale devices there appears a need for characterization of polymer physical properties and their dynamics on sub- micrometer scales. Optical absorption, scattering and luminescence methods provide many convenient tools for polymer characterization. Among them, the use of fluo- rescent probes and labels has proven very efficient in the study of polymer dynamics. Fluorescent probes have been used to observe orientation, rotational motion, conforma- tional changes and diffusion of polymers, to monitor intra- and intermolecular reaction rates, to probe hydro- phobicity, microviscosity, polarity, membrane, surface potential, etc. [1]. However, the bulk of these studies have been done using large ensembles of fluorescence molecules. The results of such studies are physical parameters which are averaged over large numbers of polymer molecules and such data have only limited relevance in the context of nanotechnology applications. One of the most important physical parameters is local polarity of the polymer. It is important in itself for the structural and dynamical characterization of the local envi- ronment of the material, and it is also important in charac- terizing the local polarity changes in optoelectronic devices in which external electric field or current is applied. The devices which are being most intensely developed include organic light emitting diodes and organic field effect tran- sistors. Fluorescence probes and labels have been widely used in the characterization of polymer polarities. Charge-transfer (CT) dyes utilize the high sensitivity of the CT fluorescence band wavelength position on the sol- vent polarity [2–5]. Pyrene is highly suitable as a polarity probe of polymers due to the strong dependence of the ratio of its I and III vibrational bands [6–8]. For other dyes, such as coumarin [9] or dansyl [10,11], fluorescence lifetime and/or fluorescence wavelength position are used 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.093 * Corresponding author. Fax: +81 3 5734 2425. E-mail address: vacha@op.titech.ac.jp (M. Vacha). www.elsevier.com/locate/cplett Chemical Physics Letters 435 (2007) 311–316