Raman phonon spectra of pentacene polymorphs A. Brillante a, * , R.G. Della Valle a , L. Farina a , A. Girlando b , M. Masino b , E. Venuti a a Dipartimento di Chimica Fisica e Inorganica and INSTM-UdR Bologna, University of Bologna, Viale del Risorgimento, 4, I-40136 Bologna, Italy b Dipartimento Chimica GIAF and INSTM-UdR Parma, University of Parma, I-43100 Parma, Italy Received 13 March 2002 Abstract We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs. Ó 2002 Elsevier Science B.V. All rights reserved. 1.Introduction Pentacene is a well-known organic semicon- ductor, which is currently subject of renewed in- terest after the recent discovery of its high carrier mobilities, yielding a wealth of new and extraor- dinary phenomena [1]. The hints that its inter- esting transport properties can somehow be related to preparation, purity and crystal struc- ture of the material [2,3] have prompted several studies to revisit growing techniques and struc- tural investigations. On the basis of the five structural analyses so far reported on single crystals of pentacene [4–7] and of our recent theoretical work [8,9], it is clear that pentacene shows at least two polymorphic structures in the bulk phase. These two structures are found to correspond unambiguously to the two ‘inherent structures’ of minimum energy [8,9]. We have named the two polymorphs as phase C, after the structure of Campbell et al. [4] and as phase H, after the structure which results from the more recent crystallographic investigations [5–7]. In the present Letter we tackle the problem from a different point of view, that is, we focus our attention on the dynamical, rather than on the structural, properties of the lattice, as a means to discriminate between different crystal phases. This also provides a further check of theoretical pre- dictions advanced in [8,9]. Among the possible spectroscopic techniques to help the phase as- signment, we have chosen Raman scattering in the lattice phonon region (10–150 cm 1 ) which repre- sents the fingerprint of the individual crystal 3 May 2002 Chemical Physics Letters 357 (2002) 32–36 www.elsevier.com/locate/cplett * Corresponding author. Fax: +39-051-2093690. E-mail address: brill@ms.fci.unibo.it (A. Brillante). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2614(02)00441-4