Relaxation Dynamics of Photoexcited Excitons in Rubrene Single Crystals Using Femtosecond Absorption Spectroscopy S. Tao, 1 N. Ohtani, 1 R. Uchida, 1 T. Miyamoto, 1 Y. Matsui, 1 H. Yada, 1 H. Uemura, 1 H. Matsuzaki, 1, * T. Uemura, 2 J. Takeya, 2 and H. Okamoto 1,† 1 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan 2 Institute of Scientific and Industrial Research, Osaka University, Ibaraki 567-0047, Japan (Received 24 October 2011; revised manuscript received 6 April 2012; published 30 August 2012) The relaxation dynamics of an exciton in rubrene was investigated by femtosecond absorption spectroscopy. Exciton relaxation to a self-trapped state occurs via the coherent oscillation with 78 cm 1 due to a coupled mode of molecular deformations with phenyl-side-group motions and molecular displacements. From the temperature dependence of the decay time of excitons, the energy necessary for an exciton to escape from a self-trapped state is evaluated to be 35 meV ( 400 K). As a result, a self-trapped exciton is stable at low temperatures. At room temperature, excitons can escape from a self-trapped state and, subsequently, they are dissociated to charged species. The exciton dissociation mechanism is discussed on the basis of the results. DOI: 10.1103/PhysRevLett.109.097403 PACS numbers: 78.47.jb, 72.20.Jv, 78.40.q, 78.47.D Rubrene is an important material for organic field effect transistors (FETs) because of its highest mobility among organic molecular semiconductors [1–3]. The Hall-effect measurements suggest a band transport mechanism [4,5]. In addition, a potential of rubrene as a photoconductive and light-emitting material has also been attracting much at- tention [6–8]. When we consider the applications of mo- lecular semiconductors to optical devices, it is important to clarify the nature of a molecular exciton. In the present study, we focus on the relaxation dynamics of an exciton in rubrene. Rubrene is a derivative of tetracene with four phenyl side groups [Fig. 1(a)]. The characters of molecular orbitals in isolated rubrene and tetracene molecules are similar to each other [9]. However, their crystal structures are sig- nificantly different. The crystal structure of rubrene pro- jected on the ab plane is shown in the inset of Fig. 1(b). Tetracene backbones are aligned in a slipped-cofacial configuration, forming a -stacking structure along the b axis. This results in the large mobility ð>20 cm 2 =VsÞ along b [4,5]. In tetracene, such a -stacking structure is not formed and ð2 cm 2 =VsÞ is much lower [10]. Photoluminescence (PL) properties of tetracene and ru- brene are also considerably different from each other; in tetracene, both free excitons (FEs) and self-trapped exci- tons (STEs) were identified [11,12], while in rubrene only the PL peak (2.18 eV) with a large Stokes shift ( 140 meV) was observed [6], suggesting the formation of STEs [13]. More recently, we reported photoinduced absorption (PA) spectra due to excitons and photocarriers in rubrene by femtosecond (fs) pump-probe (PP) spectros- copy and suggested that an exciton is dissociated to charged species [13]. This is a special property which is not seen in other molecular semiconductors [14,15]. However, it has not been fully understood yet. Here, we investigated temperature dependences of PA signals due to excitons and charged species. At low tem- peratures, we observed a coherent oscillation on the PA signals of excitons. This oscillation was assigned to a 0 0 4 8 0.2 0.4 1 2 1 2 3 0 200 400 0 0.5 1 0.2 0.4 1 2 0 1 1.5 2 0 1 2 0 4 8 (a) Photon energy (eV) ∆OD (10 -2 ) A B B C Absorption E//b 0.1 ps 200 ps 200 ps (c) Delay time (ps) 0.1 ps 0.37 eV 1.15 eV 294 K 0.35 eV 1.20 eV 5 K Normalized ∆OD A 5 K (d) 294 K ∆OD (10 -2 ) ∆OD (10 -2 ) Photon energy (eV) (b) C Photon energy (eV) b a 200 ps FIG. 1 (color online). (a) Molecular structure of rubrene. (b) ÁOD spectra of rubrene on the ab plane with the pump and probe pulses polarized ==b. The pump energy was 2.23 eV at 294 K and 2.27 eV at 5 K. In the upper panel, the thin solid line shows the absorption spectrum (E==b). (c) Time evolutions of PA bands A and B. (d) Expanded PA band C at 294 K. The thick solid line shows the FIA [18]. PRL 109, 097403 (2012) PHYSICAL REVIEW LETTERS week ending 31 AUGUST 2012 0031-9007= 12=109(9)=097403(5) 097403-1 Ó 2012 American Physical Society