Chemistry Reference
In-Depth Information
O
-
A
-
O
P O
O
3'
N
N
T
H
O
N
O
N
O
N
O
-
O
P
H
major groove
N
O
5'
N
O
O
HO
O
O
H
N
OH
HN
N
5'
H
O
P
O
N
N
minor groove
-
O
O
NH
C
H
N
O
O
N
3'
O
P
G
O
-
O
-
O
Figure 8.2
(Plate 10)
Complimentary pairs of nucleotides that make up double helix DNA
(Inset, A = adenine, T = thymine, G = guanine, C = cytosine, DNA = deoxyribonucleic acid).
Atoms are carbon (grey), hydrogen (white), nitrogen (blue), oxygen (red) and phosphorous
(pink) (See colour plate section)
8.2 Study of Photophysics: Toward PDT
The discussion of transition metal polyazine photophysics requires explanation of
general photophysical processes. Absorption of a photon by a chromophore causes
an electronic transition and populates the chromophore's electronic excited state.
There are several internal pathways by which the excited chromophore might relax.
The decay processes of electronically excited transition metal polyazine complexes
have been the subject of study for decades, and continue to be a strong focus in
research. Thorough studies of transition metal polyazine chromophores have led to
many applications of the complexes in photochemistry.
8.2.1 Electronic Excitation
Absorption of a photon of light by a chromophore in its electronic ground state
(GS) is coincident with an electronic transition, populating an electronic excited
state (ES) of the chromophore. The electronic transition promotes an electron from
a low energy occupied orbital to an unoccupied orbital of higher energy (Figure
8.3). The lowest energy transition promotes an electron from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO).
The orbitals involved in the transition must be electronically coupled to allow elec-
tronic transition. The probability of an electronic transition is governed by selection
rules. The symmetry selection rule for intensity of an electronic transition is expressed
in Equation (8.1):
