Conjugated Fluorenes Prepared From Azomethines Connections-II: The Effect of Alternating Fluorenones and Fluorenes on the Spectroscopic and Electrochemical Properties

The photophysics and electrochemistry of fluorene and fluorenone azomethine derivatives were examined in order to understand the deactivation pathways responsible for the quenched fluorescence of these compounds, which should otherwise be fluorescent. Steady-state fluorescence showed that the fluorene singlet excited state is quenched both by fluorenone (1) and a model aliphatic azomethine compound (14) with k(q) approximate to 10(10) M-1 s(-1). The quencher concentration required to deactivate 95% of the excited singlets formed was 8.4 mM for fluorenone and 34 mM for 14. Intramolecular photoinduced electron transfer (PET) from fluorene to both I and 14 was found as the principle deactivation mode of the fluorene's excited state. The high degree of conjugation of the azomethines promotes intersystem crossing to the triplet manifold by narrowing the singlet-triplet energy gap, which is also in part responsible for the reduced fluorescence observed for the fluorenone azomethine derivatives 5-11. Fluorescence quenching by PET was corroborated from the electrochemical and spectroscopic data by applying the Rehm-Weller equation. Meanwhile, the inherent fluorene fluorescence can be restored by protonating the azomethine, resulting from suppressed PET. Although PET is exergonically favorable (-242 kJ/mol for 1 and -96 kJ/mol for 14), intersystem crossing still occurs. The resulting fluorene triplet state is efficiently quenched by both 1 (k(q) = 7 x 10(9) M-1 s(-1)) and 14 (k(q) = 5 x 10(9) M-1 s(-1)) confirming that the absence of triplet signal by laser flash photolysis is a result of rapid intramolecular energy transfer to the two quencher sites. A concentration-dependent second emission was observed for the fluorenone containing azomethines assigned to the formation of excimers.