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

Two fluorescent pyrene derivatives, I and II, are reported in which the aromatic 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 the pyrene nucleus while II bears two bpy moieties linked to the 1 and 6 vertexes. The solvent-polarity dependence of the fluorescence emission properties 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 rate constants, k(f), and the short fluorescence lifetimes, tau (f), for both compounds are characteristic of strongly allowed transitions. The solvent and temperature dependence of the photophysical properties of I, such as the 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 for 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 interannular dihedral angle. In contrast, the experimental and calculated properties for compound II point to the occurrence of a highly stabilized L-1(a)-type excited state, thus experiencing very little influence of the forbidden L-1(b)-type state. Consistently, the fluorescence emission rate constant for II is found to be independent of solvent polarity and temperature.