Facile synthesis of highly stable BF3-induced meso-tetrakis (4-sulfonato phenyl) porphyrin (TPPS4)-J-aggregates: structure, photophysical and electrochemical properties

Abstract

We report herein, a novel method for the formation of highly stable BF₃-induced J-aggregates by interaction between meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS₄) and BF₃O(C₂H₅)₂. The aggregates were characterized by NMR (¹H, ¹¹B and ¹⁹F), optical absorption, cyclic voltammetry, FT-IR and fluorescence spectroscopic techniques. TPPS₄ readily forms a 1 : 2 adduct with BF₃ which further converts into BF₃-induced TPPS₄ aggregates whose spectroscopic properties strongly depend on the concentration of BF₃. The optical absorption spectrum shows the formation of J-type aggregates with an apparent association constant (logK_app) of 4.2 ± 0.1. The steady state emission shows formation of 1 : 2 (TPPS₄–BF₃) adducts at ∼685 nm and J-aggregates exhibit emission at 732 nm with a red shift of ∼17 nm in comparison to J-aggregates of TPPS₄ with TFA. The fluorescence lifetime of TPPS₄ : BF₃ (1 : 2) adducts exhibits ∼3.68 ns and aggregates show a lifetime of 4.32 ± 0.2 ns with major abundance. NMR study reveals that proton transfer occurs from pyrrole N–H to the SO₃⁻ groups and J-aggregates were stabilized by strong intermolecular hydrogen bonding interaction between N₂–BF₂ and SO₃H. Cyclic voltammetry shows a decrease in the reduction peak current along with a change in the peak potentials for the aggregates. The PXRD pattern of the aggregates exhibits orthorhombic structure with interplanar distance of 4.87 Å. The variations in unusual stability, photophysical, electrochemical properties and nature of the aggregates were rationalized with aggregates of TPPS₄ dication.