The energies of low-lying singlet excited states of oligothiophenes, xT's (x = 3 to 8), and polythiophene are determined using femtosecond time-resolved near-infrared spectroscopy. The energies are linearly correlated with the reciprocal number of the thiophene rings for the oligomers 3T to 7T. The linear relation is not observed for 8T, indicating that the effective conjugation is formed over no more than seven rings in the excited states. Transient absorption spectra of polythiophene show that its thiophene chains are composed of planar segments with five or six thiophene rings.Organic polymers with large conjugate systems are a group of fundamental compounds for investigating the properties of onedimensional π-conjugate structure, such as large conductivity 1 and strong photoluminescence associated with photoexcitation.2 The molecular structure of the polymer significantly influences the electronic structure and the dynamics of excitations photogenerated in the polymer. In solution, the effective conjugation length can be shorter than the entire length of the polymer because solutesolvent interactions will create a number of kinks in the conjugate structure.Polythiophene is widely used as a model compound of the one-dimensional π-conjugate polymer, for evaluating the effective conjugation length in solid films and in solutions as well as for understanding the excitation dynamics. The excitation dynamics of oligothiophenes has been observed by time-resolved absorption 36 and emission spectroscopy. 7,8 The electronic or vibrational transition energies have been evaluated by steady-state absorption 911 and Raman spectroscopy 12,13 for neutral or charged oligothiophenes in the ground state, which are regarded as a partial structure of a polymer segmented with kinks. In the steadystate absorption studies, however, only one-photon allowed transitions from the ground state are observed while the other excited states are not monitored. Photodetachment photoelectron spectroscopy (PD-PES) 14,15 and high-resolution electron energy loss spectroscopy (HREELS) 16,17 have been performed for estimating the energies of the low-lying triplet and singlet states in solid films, respectively. The effects of solvents on the effective conjugation length in the low-lying excited states, however, are not well understood, because these methods are not easily applicable to solutions.New photophysics and photochemistry are expected when a polymer is photoexcited to an excited singlet state located higher than the lowest one. It is unclear if the effective conjugation length is preserved in the highly excited singlet states compared with the ground or lowest excited singlet state. In this study, we observed the highly excited singlet states of α-oligothiophenes and α-polythiophene by femtosecond time-resolved near-infrared absorption spectroscopy. The results for α-oligothiophenes composed of 3 to 7 thiophene rings show a systematic downshift of the absorption peak wavelength in the transient near-infrared absorption spectra at 11 ps, w...