2,7-Di-t-butyldibenzofulvene (tBu(2)DBF), a bulky dibenzofulvene derivative, was polymerized using n-butyllithium as initiator in tetrahydrofuran at -78 degrees C and in toluene at 0 degrees C. tBu(2)DBF afforded mainly oligomers up to trimer even at [monomer](0)/[initiator](0) = 20 ([monomer](0) = 0.2 M) at -78 degrees C and 0 degrees C, indicating that this monomer is much less reactive than dibenzofulvene (DBF), its parent compound. The reaction at the same [monomer](0)/[initiator](0) ratio at an elevated [monomerlo gave a small amount of insoluble polymer. The oligomers indicated a hypochromic effect in the absorption spectra and only monomer emission in the fluorescent spectra. The conformation of the trimer and the dimer was examined by means of NOESY NMR spectra and semiempirical calculations. In the trimer conformation, the fluorene moieties of the central and the initiation-side monomeric units were found to be closely stacked on top of each other, while the termination-side monomeric unit appeared to be in a faster conformational dynamics compared with the other monomeric units. Although the dimer seemed to have a relatively flexible conformation, a pi-stacked structure appeared to be involved in the conformational dynamics to show hypochromicity. The results of this study suggest that the reported intramolecular excited dimer (excimer) formation of the poly- and oligo(DBF)s [J Am Chem Soc 2003, 125, 15474] is based on a slight, local conformational change upon photo absorption, leading to a closer pi-stacked alignment of two neighboring fluorene units than that in the ground state. Such a local conformational transition may be difficult for the tBu(2)DBF trimer because of steric repulsion of the t-butyl groups. (c) 2005 Wiley Periodicals, Inc.