Tunable photophysical properties of two 2,2 '-bipyridine-substituted pyrene derivatives

Two fluorescent pyrene derivatives, I and II, are reported in which the aro matic ring is connected to the 2,2'-bipyridine (bpy) complexing unit via a single C-C bond. Compound I contains one bpy moiety at the 1 position of th e pyrene nucleus while II bears two bpy moieties linked to the 1 and 6 vert exes. The solvent-polarity dependence of the fluorescence emission properti es of both compounds at room temperature points to the existence of a polar excited-state possessing a small extent of intramolecular charge transfer (ICT). The large fluorescence quantum yields, Phi (f), the high radiative r ate constants, k(f), and the short fluorescence lifetimes, tau (f), for bot h compounds are characteristic of strongly allowed transitions. The solvent and temperature dependence of the photophysical properties of I, such as t he increase of k(f) with increasing polarity and temperature, appears to be due to intermixing between closely lying polar S-1(L-1(a)) and forbidden S -2(L-1(b)) states. Semiempirical theoretical calculations were performed fo r I and allowed to confirm the assignment of the electronic and structural nature of the two lowest-energy excited states as a function of the interan nular dihedral angle. In contrast, the experimental and calculated properti es for compound II point to the occurrence of a highly stabilized L-1(a)-ty pe excited state, thus experiencing very little influence of the forbidden L-1(b)-type state. Consistently, the fluorescence emission rate constant fo r II is found to be independent of solvent polarity and temperature.