Tunable Photophysical Properties of Two 2,2‘-Bipyridine-Substituted Pyrene Derivatives

Abstract

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, Φ f, the high radiative rate constants, k f, and the short fluorescence lifetimes, τ 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 ( 1 L a ) and forbidden S 2 ( 1 L 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 1 L a -type excited state, thus experiencing very little influence of the forbidden 1 L b -type state. Consistently, the fluorescence emission rate constant for II is found to be independent of solvent polarity and temperature