posted on 2021-10-19, 19:47authored byAndrea Pinto, Carla Cunha, Gabriel Aullón, João Carlos Lima, Laura Rodríguez, J. Sérgio Seixas de Melo
Six
gold(I) complexes (R3P-Au-Coum) containing three
different alkynylcoumarin chromophores (Coum) with different electron-donating
and electron-withdrawing characteristics and two different water-soluble
phosphanes (PR3 = PTA (a) and DAPTA (b)) have been synthesized
(1a,b, unsubstituted coumarin; 2a,b, 4-methyl substituted coumarin; 3a,b, 3-chloro and 4-methyl substituted coumarin). A comprehensive
study of the photophysical properties of the R3P-Au-Coum,
together with their propynyloxycoumarin precursors 1–3, was performed in solution at room and low temperatures.
Spectral and photophysical characteristics of the R3P-Au-Coum
essentially depend on the electronic characteristics of the propynyloxycoumarin
ligand. The presence of the Au(I) atom was found to be responsible
for an increase of the intersystem crossing, with triplet state quantum
yield values, ϕT, ranging from ∼0.05 to 0.35
and high coumarin phosphorescence quantum yield values for derivatives 1 and 2; fluorescence dominates the deactivation
in derivatives 3. Efficient singlet oxygen photosensitization
was observed for the new compounds 3a,b.
From TDDFT calculations, the relevant HOMO and LUMO of the compounds,
i.e., those involved in the transitions, are dominated by the frontier
orbitals associated with the coumarin core. The Au(I)-phosphane structure
introduces a new transition assigned to an intraligand transition
involving the phosphane ligand, and π(CC) system, to
the p orbitals of phosphorus and gold atoms.