  We report the first investigation of relativistic effects on the UV-vis
spectra of two prototype complexes for so-called photo-activated chemotherapy
(PACT), trans-trans-trans-[Pt(N3)2(OH)2(NH3)2] and
cis-trans-cis-[Pt(N3)2(OH)2(NH3)2].In PACT, design of new drugs requires
in-depth understanding of the photo-activation mechanisms. A first step is
usually to rationalize their UV-vis spectra for which time-dependent density
functional theory (TD-DFT) is an indispensable tool. We carried out TD-DFT
calculations with a systematic series of non-relativistic(NR),
scalar-relativistic (SR), and four-component (4c) Hamiltonians. Large
differences are found between spectra calculated within 4c and NR frameworks,
while the most intense features (found at higher energies below 300 nm) can be
reasonably well reproduced within a SR framework. Yet the underlying
transitions can be strongly influenced by spin-orbit coupling introduced in the
4c framework: while this can affect both intense and less intense transitions
in the spectra, the effect is most pronounced for weaker transitions at lower
energies, above 300 nm. Since the investigated complexes are activated with
light of wavelengths above 300 nm, employing a method with explicit inclusion
of spin-orbit coupling may be crucial to rationalize the activation mechanism.
All calculations were carried out with both the CAM-B3LYP and B3LYP
functionals; we generally find the former to perform best in comparison with
experimental spectra.
