Chirped modulation of molecular vibration in quinoidal thiophene after sub-5 fs excitation Zhuan Wang a , Tetsuo Otsubo b , Takayoshi Kobayashi a,c,d,e, * a Department of Physics, University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan b Department of Applied Chemistry, Hiroshima University, Higashi-Hiroshima 739-8527, Japan c Department of Applied Physics and Chemistry, Institute of Laser Science, University of Electro-Communications, Chofugaoka 1-5-1, Chofu, Tokyo 182-8585, Japan d Institute of Laser Engineering, Osaka University, Yamadakami 2-6, Suita 565-0871, Ibaraki 567-0047, Japan e Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan Received 1 June 2006; in final form 23 August 2006 Available online 30 August 2006 Abstract Fourier transformation of the real-time trace of sub-5 fs pump-induced molecular vibration in quinoidal thiophene in solution revealed coupled modes to the electronic excitation at 206, 447, and 1466 cm 1 . Spectrogram analysis of 1466 cm 1 (C@C stretching) mode showed that the amplitude and frequency of the mode are modulated with another low-frequency mode at about 220 cm 1 , which is down chirped being induced by the coupling with modes of even lower frequency, resulting in the irreversible relaxation due to high mode density in the low-frequency range. This Letter thus reports on observation of the modulation of vibrational frequency with a chirped frequency. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction Thiophene-based oligomers can be readily synthesized [1–3] with different sizes and provide series of photophysi- cal conjugated systems with different p-electron delocaliza- tion lengths. They have been extensively studied theoretically and experimentally for better understanding of the basic physical properties of polythiophene, which has been one of the best-known conjugated polymers since 1990s [4–14]. They have also been studied from the view- point of application as promising materials for devices [10,15–18] such as light-emitting diodes [19–21]. Vibrational spectroscopy of polythiophenes has shown [11] that they contain all-s-trans sequences of thiophene rings linked at a- and a 0 -positions with a distribution of conjugation lengths depending on both sequential length and torsion angles of thiophene rings. This is because the distribution of conjugation lengths has been ascribed to the existence of disordered structures in the polymer chain. Polythiophene has a nondegenerate ground state like cis- polyacetylene, which is different from trans-polyacetylene with a degenerate ground state. The optical properties of doped polythiophene have thus been discussed in terms of polarons and bipolarons [13–15], which are self-localized excitation in conjugated polymers. A study of the processes following the creation of electron and hole by photoexcita- tion is important to understand the mechanics of the gen- eration of such nonlinear excitations. In the present study, we have performed sub-5 fs spec- troscopy of a derivative of quinoidal bithiophene [22] (Fig. 1) abbreviated as Bx-2, which is a good model molecule as a repeat unit of polythiophene with all-s-trans 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.08.106 * Corresponding author. Address: Department of Applied Physics and Chemistry and Institute of Laser Science, University of Electro-commu- nications, Chofugaoka 1-5-1, Chofu, Tokyo 182-8585, Japan. Fax: +81 42 443 5825. E-mail addresses: wang@ils.uec.ac.jp (Z. Wang), kobayashi@ils.ue- c.ac.jp (T. Kobayashi). www.elsevier.com/locate/cplett Chemical Physics Letters 430 (2006) 45–50