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(P-C)(O)OH], where P-C = 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, epsilon (in M-1 cm(-1)), and fluorescence quantum yield, Phi(F)) in bio-imaging window (650-900 nm). For example, spectral characteristics of the tbpc derivatives have been determined as follows: e = 1.65 x 10(5) (absorption maximum 676 nm) and Phi(F) = 0.80 (emission maximum 686 nm) for [P(tbpc)(0)(OH)] while e = 1.45 x 10(5) (697 nm) and (Phi(F) = 0.27 (714 nm) for [P(tbpc)(OH)(2)](+), and e = 2.25 x 10(5) (662 nm) and Phi(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 Phi(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 bioimaging.