Raman Spectra and Quantum Chemistry Calculations of Fluorene-Dibenzothiophene-S,S- dioxide Oligomers José L. Zafra a , Irene I. Perepichka b , Igor F. Perepichka c , Martin R. Bryce d , Francisco J. Ramírez a , Juan Casado a , Juan T. López Navarrete a a Departamento de Química Física, Universidad de Málaga, Málaga 29071, Spain b Département de Chimie, Université de Montréal, Montréal, Quebec H3C 3J7, Canada c Centre for Materials Science, University of Central Lancashire, Preston PR1 2HE, United Kingdom d Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom An electroactive material is electroluminescent if its response to an electric current is a visible light emission without thermal energy generation. Since the first organic- based electroluminescent device (OLED) was available in the 1990s, remarkable progress has been made in synthesizing, developing and optimizing new organic molecules to be used in electronics and optics. Fluorene-based (F) polymers are one of the most promising materials for electroluminiscent applications, due to their efficient emission, high physico-chemical stability and easy tuning of the emission wavelength by side-chain modifications or copolymerization [1]. Moreover, the charge-transport can be improved by introducing electron-acceptor moieties into the polymer backbone to form alternating copolymers, Thus, co-polymers and co-oligomers of F with dibenzothiophene-S,S-dioxide (S) have shown high luminescence efficiency, improved electron affinity, stability towards p- and n-doping, and excellent performance in OLEDs [2,3]. In a previous work, steady- state and time-resolved fluorescence was used to investigate an interplay between the local and charge transfer excited states in some FS co-oligomers [4]. Here we report Raman spectroscopy studies of those co-oligomers, Fig. 1, as models to investigate the conjugational effect of modulating structural variables such as chain length and donor- acceptor separation. Raman spectra of the co-oligomers are shown in Fig. 1. Taking FSF as the model molecule, the most outstanding changes are observed in the spectrum of FASAF, where the intense band at 1596 cm -1 in FSF downshifts by 4 cm -1 . This feature has been successfully predicted by DFT calculations and the associated eigenvector describes an aromatic-quinoid transition, accordingly with the Effective Conjugation Coordinate (ECC) theory [4], in which the benzene rings vibrate as in the 8a Wilson’s mode [5]. This is in agreement with a greater quinoid character for FASAF originating from extension of π-conjugation along the major molecular axis. In addition, the C≡C stretching vibration was measured about 70 cm -1 lower than for aliphatic C≡C groups, thus evidencing bond relaxation. 683 Downloaded 05 Nov 2010 to 147.143.2.5. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/about/rights_permissions CP1267, XXII International Conference on Raman Spectroscopy edited by P. M. Champion and L. D. Ziegler © 2010 American Institute of Physics 978-0-7354-0818-0/10/$30.00