Novel phosphorus(v) tetrabenzotriazacorroles: synthesis, characterization, optical, electrochemical, and femtosecond nonlinear optical studies

Abstract

A series of three novel tetrabenzotriazacorroles (TBCs) designed with an alkyl substituent tert-butyl group (TBC-tert), an electron donor phenothiazine group (TBC-PTZ) and an energy donor carbazole group (TBC-CBZ) on the peripheral position with phosphorus metal in the cavity have been synthesized. All three compounds were characterized using various spectroscopic techniques and we assessed their femtosecond third-order nonlinear optical (NLO) properties. TBCs exhibit the properties of both phthalocyanines and corroles as they are derived from parent phthalocyanines. The optical studies revealed a new band at ∼450 nm, which was absent in the parent phthalocyanine molecules, and all three compounds obeyed Beer–Lambert's law. Singlet-state quantum yields were measured in different solvents and were found to be in the range of 0.3 to 0.6 for TBC-tert, 0.21 to 0.25 in the case of TBC-PTZ and 0.31 to 0.41 for TBC-CBZ. Time-resolved fluorescence studies revealed lifetimes in the ns regime (typically few ns). The redox properties of the TBCs suggest that they are easier to oxidize and harder to reduce and exhibit multiple oxidation and reduction potentials. Using the Z-scan technique, the third-order NLO properties were investigated with kilohertz and megahertz repetition rate femtosecond pulses at 800 nm. We report the first observation of strong three-photon absorption in these molecules with coefficients of ∼10⁻²² cm³ W⁻² (∼10⁻¹³ cm³ W⁻²) with kHz (MHz) repetition rate fs pulse excitation.