LnCrIIIðÞONOhiþ
!hv
LnCrIV¼¼Oh
þþNO ð 24 ÞFor example, trans-[Cr(cyclam)(ONO) 2 ]þ (cyclam¼1,4,8,11-
tetraazacyclotetradecane) was suggested as photochemical pre-
cursor for NO delivery to physiological targets( 33 ). This water-
soluble and thermally stable system, which demonstrates a large
FNOfor NO release with UV light, seems very promising. How-
ever, its absorption cross section in the visible is very small, since
these transitions are ascribed to the Laporte forbidden ligand
field (d–d) bands. To improve the photosensitivity range, several
new macrocyclic ligands L with pendant aromatic chromophores
were prepared. Photoexcitation of aqueous solutions containing
these complexes at wavelengths corresponding to the chromo-
phore absorption bands led to the generation of NO (46,48).
Nanocrystal quantum dots were found to offer important
advantages as photosensitizing chromophores, including high
optical cross-sections for both single and two photon excitations
and the ability to tune the optical properties by varying the QD
diameter (120,121). Also visible light excitation of trans-[Cr
(cyclam)(ONO) 2 ]þin solutions containing thiol reducers such as
the biological antioxidant glutathione leads to a permanent reac-
tion even under anaerobic conditions, resulting in a high quan-
tum yield for the release of NO ( 122 ).
Density functional theory (DFT) studies of the series of
complexes [Cr(L)(H 2 O) 5 ]nþ (where L¼N 3 , NO, NS, or NSe)
show that the unpaired electron resides in a metal-based dxy
orbital and that the electronic structure in the equatorial plane
is similar in all four complexes and resembles Cr^3 þ. Thesdonat-
ing ability was found to follow the order N 3 NO<NSNSe,
whereas the p accepting ability followed the order
NO>NSNSe. Time dependent DFT calculations gave in all
four complexes a dx (^2) y 2 dxy transition energy around
17,500 cm^1 ( 123 ).
The Mn(III) complexes with pyridine derivatives, [Mn(PaPy 3 )
(NO)]ClO 4 and [Mn(PaPy 2 Q)(OH)]ClO 4 (where PaPy 3 H¼N,
N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide
and PaPy 2 QH¼N,N-bis(2-pyridylmethyl)-amine-N-ethyl-2-quin-
oline-2-carboxamide), though insensitive to dioxygen, react with
NO to afford the nitrosyl complexes [Mn(PaPy 3 )(NO)]ClO 4 and
[Mn(PaPy 2 Q)(NO)]ClO 4 via reductive nitrosylation. These dia-
magnetic nitrosyl [Mn NO]^6 complexes are soluble in water,
and release NO after exposure to visible radiation with high
quantum yields (0.385 and 0.694 atlirr¼550 nm, respectively).
The quantum yield of the quinoline derivative atlirr¼500 nm
314 ZOFIA STASICKA