New Re(I) carbonyl complexes are
proposed as candidates for photodynamic
therapy after investigating the effects of the pyridocarbazole-type
ligand conjugation, addition of substituents to this ligand, and replacement
of one CO by phosphines in [Re(pyridocarbazole)(CO)
3
(pyridine)]
complexes by means of the density functional theory (DFT) and time-dependent
DFT. We have found, first, that increasing the conjugation in the
bidentate ligand reduces the highest occupied molecular orbital (HOMO)–lowest
unoccupied molecular orbital (LUMO) energy gap of the complex, so
its absorption wavelength red-shifts. When the enlargement of this
ligand is carried out by merging the electron-withdrawing 1
H
-pyrrole-2,5-dione heterocycle, it enhances even more the
stabilization of the LUMO due to its electron-acceptor character.
Second, the analysis of the shape and composition of the orbitals
involved in the band of interest indicates which substituents of the
bidentate ligand and which positions are optimal for reducing the
HOMO–LUMO energy gap. The introduction of electron-withdrawing
substituents into the pyridine ring of the pyridocarbazole ligand
mainly stabilizes the LUMO, whereas the HOMO energy increases primarily
when electron-donating substituents are introduced into its indole
moiety. Each type of substituents results in a bathochromic shift
of the lowest-lying absorption band, which is even larger if they
are combined in the same complex. Finally, the removal of the π-backbonding
interaction between Re and the CO trans to the monodentate pyridine
when it is replaced by phosphines PMe
3
, 1,4-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane
(DAPTA), and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP)
causes another extra bathochromic shift due to the destabilization
of the HOMO, which is low with DAPTA, moderate with PMe
3
, but especially large with CAP. Through the combination of the PMe
3
or CAP ligands with adequate electron-withdrawing and/or
electron-donating substituents at the pyridocarbazole ligand, we have
found several complexes with significant absorption at the therapeutic
window. In addition, according to our results on the singlet–triplet
energy gap, all of them should be able to produce cytotoxic singlet
oxygen.