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In order to make a comparison between the properties of low molecular weight poly(phenyleneethynylene) type compounds and the corresponding high molecular weight polymers, we have synthesized oligomers and model molecules of poly(phenyleneethynylene) derivatives by two different routes. This paper describes the synthesis and the X-ray and optical characterization of conjugated soluble oligomers, as well as model trimers and pentamers of the same structure in which the solubility was improved by fixing flexible alkyl chains of 10 or 12 carbons to the backbone. The electron density on the phenyl ring was also enhanced or reduced by introducing either electron-donor or electron-acceptor groups. The oligomers were synthesized by polycondensation using a palladium catalyzed coupling reaction, between a bromoaryl and an ethynylaryl unit, with HBr elimination. This method allows the insertion of a triple bond between two phenyl groups. It was initially used in our laboratory in order to obtain high molecular weight polymers. It has been adapted to favor the production of lower molecular weight compounds, by shortening the reaction time or by an important increase of the amount of catalyst. The trimers and pentamers were synthesized step by step by using a route which involves a selective protection−deprotection method, followed by the palladium coupling reaction. Model compounds and oligomers were characterized by UV−visible and Raman spectroscopies, as well as nonlinear optical measurements of the third order susceptibilities (χ(3)). These measurements allowed us to show the influence of the donor/acceptor groups and the chain lengths on the absorption wavelength. In addition, the nonlinear optical measurements show that the χ(3) value of the pentamer (1.6 × 10-10 esu) is close to the value of the polymer (4.6 × 10-10 esu).

Two-pulse photon echoes were observed in organic dye molecules in solution using 6-fs optical pulses. The results show an initial rapid dephasing of the echo due to level multiplicity followed by quantum beats. The temporal decay of the quantum beats is characterized by a T2 of about 60 fs.PACS numbers: 42.50.Md, 72.20.My, Recent hole-burning experiments performed with large molecules in solution 1 suggest that it might be possible to observe photon echoes in these systems. The photonecho technique has proven useful in a variety 2 " 4 of systems to study processes which influence the dephasing of an induced polarization. The use of coherent optical transients to study such processes in molecules has been frustrated by the very fast dephasing rates in such systems. Recent advances in short-pulse techniques have led to the generation of optical pulses as short as 6 fs. 5 With this increased time resolution we have recently observed two-pulse photon echoes from bulk GaAs and directly measured the rate of polarization dephasing. 6 We report here the observation of femtosecond photon echoes and quantum beats from organic dye molecules in solution.Several attempts have been made to observe coherent transient processes in organic dyes, using techniques in both the time domain 7 " 10 and the frequency domain. 1,11,12 In the experiments reported here we observe photon echoes using a two-pulse sequence. Two pulses, one having wave vector ki and the other wave vector k2, generate an echo in the momentum-matched direction 2k2 -kj. 2 The echo is then separated spatially from the exciting pulses. The energy of the generated echo is measured as a function of the relative time delay between the exciting pulses.The primary utility of a photon echo is to measure the polarization dephasing time, T2. For a two-level system which is purely inhomogeneously broadened the echo energy will decay exponentially with the relative time delay r between the two pulses aswhere T ec ho s=: T2/4. For the case of a homogeneously broadened transition the radiated signal at 2k2~~ki, corresponding to the polarization free decay, will relax as exp( -It/Ti)^ These relations show that an optical pulse shorter than 7V4 must be used to time resolve the echo decay.The experiment was performed using compressed pulses phase corrected to third order in a manner described previously. 5 The duration of the pulses was measured to be in the range 6 to 10 fs using the secondharmonic up-conversion technique. The pulse repetition rate was 8 kHz and the pulse energy was about 1 nJ. The energy of the pulse was much less than that needed for a n pulse so the echoes observed in the experiments described here are in the small-signal perturbation limit. The pulses were split in two in a modified Michelson interferometer configuration to form the two excitation pulses. The two pulses were focused with a 5-cm focallength lens into a flowing stream of ethylene glycol containing the molecule under study. One pulse was delayed with respect to the other using a steppermotor-con...

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