Synthesis and photophysical properties of metal complexes of curcumin dyes: Solvatochromism, acidochromism, and photoactivity

Curcumin and its dyes have attracted attention due to their environment-sensitivity and optical properties. However, the free molecule has low photoactivity, which is a limitation for use in photodynamic therapy. To overcome this limitation, we proposed the chelation of a D-pi-A-pi-D curcumin dye (1) with the metals Cu (II) and Pd (II). The photophysical properties of the curcumin dyes were investigated in different solvents, using UV-vis spectroscopy and time-resolved/steady-state fluorescence techniques. In our results, all curcumin dyes exhibited a positive solvatochromism from non-polar to polar solvents, with the Stokes' shift in the range of 1895-4970 cm-1. The acidochromism studies were performed in chloroform solution and TLC plates using trifluoroacetic acid (TFA). The results exhibited a negative acidochromism and fluorescence quenching upon gradual addition of TFA, demonstrating reversibility upon addition of triethylamine (TEA). The main mechanism which influences the solvatochromism/acidochromism is the ICT system and fluorescence lifetime decays exhibited monoexponential fit for aprotic solvents and bi-exponential fit for protic solvents (EtOH and MeOH). Compared to the curcumin ligand (1), the metal complexes (1a-b) exhibited higher singlet oxygen quantum yields (phi Delta = 0.36 and 0.54, respectively). The in vitro antimicrobial photoactivity of the compounds was evaluated against six different microorganisms. The results showed that the metal complexes (1a-b) exhibited both antileishmanial (MIC = 2.29 mu M against Leishmania amazonensis promastigotes) and antifungal (IC50 = 10 mu M against Sporothrix brasiliensis yeasts) activities, while the ligand showed no activity, suggesting the chelation with the metals Cu (II) and Pd (II) may improve its photoactivity.

Automated summary

The chelation of D-π-A-π-D curcumin dye with metals Cu (II) and Pd (II) was found to improve its photoactivity, showing antifungal and antileishmanial activities. The solvatochromism and acidochromism mechanisms were mainly influenced by the ICT system.