The impact of the π-electron conjugation length on the three-photon absorption cross section of fluorene derivatives

The three-photon absorption cross sections of three different fluorene derivatives, with extended π -electron conjugation lengths was experimentally measured and compared with shorter π -electron conjugation length analogs. The effect of the conjugation length on the three-photon absorption cross section σ3′ of this family of molecules has been elucidated. It is demonstrated that σ3′ of the asymmetric compound D-π -π -π -A is 6.6 times larger than its shorter configuration D-π -A, while for the symmetric compounds D-π -π -π -D and A-π -π -π -A a two-fold enhancement was found relative to their shorter conjugation length analogs. Measurements of the three-photon excitation of these compounds in THF solution (10-3 M) were accomplished with a tunable optical pulse generation pumped by a 25 ps Nd-YAG laser.