Synthesis, optical properties and ultrafast dynamics of aza-boron-dipyrromethene compounds containing methoxy and hydroxy groups and two-photon absorption cross-section

In this work, aza-boron-dipyrromethene (aza-BODIPY) compounds containing methoxy and hydroxy groups were synthesized. The effects of substitution and intramolecular charge transfer on linear and nonlinear optical absorptions (especially two photon absorption) of these BODIPY derivatives were investigated. Drastic spectral changes of aza-BODIPY compounds containing hydroxy group were observed in linear absorption spectra depending on basicity of solution. New bands in linear absorption spectra of phenolate form of these compounds appeared bellow 500 nm and above 700 nm in THF solution. These findings were caused by occurrence of the intramolecular charge transfer state in the phenolate form. Ultrafast pump probe spectroscopy experiments revealed that phenolic form shows only bleaching signal around 600 nm. On the other hand, the phenolate form showed fast growing nonlinear absorption signals bellow 500 nm and above 700 nm regions in addition to the bleaching signal around 600 nm. Intramolecular charge transfer state occurs above 700 nm and the lifetime of this state is found on the order of ultrafast time scales. Since the excitation of the fluorescence experiments is at 600 nm there is no fluorescence signal due to the relaxation of the energy through intramolecular charge transfer state. Aza-BODIPY compounds containing methoxy group did not show any two photon absorption properties due to weak electron-donating strength of methoxy group while aza-BODIPY compounds containing hydroxyl showed two photon absorption properties in wide spectral range (1200–1450 nm). Our results also indicated that intramolecular charge transfer enhances two photon absorption properties.