Amphoteric phosphorous(V)-phthalocyanines as proton-driven switchable fluorescers toward deep-tissue bio-imaging

Spectral (optical absorption and emission) properties of three amphoteric phosphorous(V)-phthalocyanine derivatives, [P(Pc)(O)OH], where Pc = tetra(tert-butyl)phthalocyaninate (tbpc), tetrakis(2′,6′-dimethylphenoxy)phthalocyaninate (tppc), and octakis(4′-tert-butylphenoxy)phthalocyaninate (obppc), have been investigated in ethanolic solutions. Spectral changes upon protonation/deprotonation (the reaction sites have been determined to be their axial ligands by magnetic circular dichroism study) are drastic and rapid. All the initial ([P(Pc)(O)OH]), protonated ([P(Pc)(OH)₂]⁺), and deprotonated ([P(Pc)(O)₂]⁻) species are possessed with sufficient brightness (defined as the product of their molar extinction coefficient, ε (in M^{− 1} cm^{− 1}), and fluorescence quantum yield, Φ_F) in bio-imaging window (650–900 nm). For example, spectral characteristics of the tbpc derivatives have been determined as follows: ε = 1.65 × 10⁵ (absorption maximum 676 nm) and Φ_F = 0.80 (emission maximum 686 nm) for [P(tbpc)(O)(OH)] while ε = 1.45 × 10⁵ (697 nm) and Φ_F = 0.27 (714 nm) for [P(tbpc)(OH)₂]⁺, and ε = 2.25 × 10⁵ (662 nm) and Φ_F = 0.90 (667 nm) for [P(tbpc)(O)₂]⁻. Emission of tppc and obppc derivatives behave in essentially the same manner irrespective of nature of the peripheral substituents and hence Φ_F values are greater with increasing emission peak wavenumbers in line with the “energy gap law”. These characteristics make these compounds promising candidates as chemical probes for deep-tissue bio-imaging.