The impact of charge defects and resonance enhancement on the two-photon absorption activity of spirofluorene and ladder-type pentaphenylene derivatives

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Abstract
Novel two-photon absorption (TPA) chromophores that contain spirofluorene and ladder-type pentaphenylenes (LPPs) as π-centres with diarylamine groups as electron donors (D) in the D–π–D arrangement are prepared. The spirofluorenes with different donors at the both terminals exhibit TPA activity increase as the donor strength increases (from N-ethylcarbazoyl 1, triphenylamino 2, to N,N-dibutylaminophenyl 3). Compared to the spirofluorene derivatives, the LPP derivatives show the larger TPA efficiency due to better coplanarity throughout the molecule. As the LPP unit added from single (4), double (5), to triple (6), the increment of the TPA cross-section exceeds the one-fold of a LPP unit, which can be attributed to the cooperative enhancement of the TPA cross-section (σ2) between the LPP units. We have also explored the effect of introducing charge defects and examined the resonance enhancement by chemical oxidation on the TPA properties of LPP derivatives 4 and 5. Nearly an order of magnitude enhancement was found in the TPA cross-section of dye 4, which was interpreted as the role of charge transfer from LPP π-centre to the cationic diarylamine end groups.

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