inorganic chemistry

C.4. Release of bioactive molecules

The selective activation of compounds with potential therapeu-
tic effects and the controlled delivery of bioactive molecules trig-
gered by light are topics of intensely growing interest
(6,197– 200 ). Besides the search for light-sensitive prodrugs
activated by photochemical cleavage, isomerization or
photoredox processes ( 201 – 203 ), especially the release of small
molecules such as NO, CO, CS 2 , and H 2 S, have attracted a lot
of interest in the past years ( 204 – 208 ).
Inorganic photochemistry offers all the necessary tools
required for the design and optimization of such systems. The
typical reactivity patterns observed for photoexcited transition
metal complexes can be readily exploited for the controlled deliv-
ery of therapeutically relevant compounds from physiologically
inert precursor species. Such a process was for the first time
described in the nineteenth century, when Haldane discovered
by serendipity that the dioxygen binding activity of inactivated
carboxy-hemoglobin (COHb) was regenerated by the rapid disso-
ciation of CO upon exposure to sunlight( 209 ). Today, the basic
features of inducing this kind of reactivity are very well settled
(210,211), and current research efforts can focus on the optimiza-
tion of the required molecular properties such as bioavailability,
solubility, drug-targeting strategies, and photoreactivity in the
therapeutically reasonable spectral regions including the
improvement of two-photon absorption cross sections or NIR-
photosensitivity. The structures of two recent examples of a pho-
toactivated NO-delivering transition metal complex sensitive to
NIR light ( 26 ) and a water soluble organometallic compound
( 27 ) which was tested for the controlled photorelease of CO are
shown in Fig. 23. Efforts in our own group currently focus on

OC

N OC

NO

Mn

N

26 27

N

N

N CO

CO

PR 3

CO

W

R =

SO–

3

FIG. 23. Examples of photolabile nitrosyl ( 26 ) and carbonyl
complexes ( 27 ) that have been suggested for the light-triggered release
of nitric oxide ( 212 ) and carbon monoxide ( 210 ).

PHOTOSENSITIZATION AND PHOTOCATALYSIS 275