inorganic chemistry

atom ( 47 ). The rapid oxidation of MnIIto MnIIImakes this pro-
cess reversible. Laser photolysis of ClFeIIITPP at 355 nm in
degassed methanol gave a quantum yield of FeIITPP formation
of 0.02 ( 43 ). The system reverts to ClFeIIITPP in the presence
of oxygen. Nevertheless, it is in the closed-shell (ZnIITPP,
CdIITPP) or diamagnetic low-energy d orbital (PdIITPP,
ClInIIITPP) metalloporphyrins, and free-base porphyrins with
meso-fluorophenyl or chlorophenyl substituents, and their
derivatives with reduced macrocycles, that we can expect to find
a good balance between stability, long-lived triplet states with
high triplet quantum yields and intense red/infrared absorption.

B. RADIATIONLESSTRANSITIONS

The rates of radiationless transitions between electronic states
of porphyrins and their derivatives play a dominant role in their
photochemistry because they are the major decay channels of the
electronically excited states. Radiative channels, such as fluores-
cence, rarely exceed 10% of the overall decay rate constant at
room temperature. The lifetimes of the lowest electronic states
of free-base porphyrins and closed-shell metalloporphyrins vary
by more than 10 orders of magnitude with the nature of the sub-
stituents. The understanding of such variations is essential to
design and control the photochemistry of porphyrins and justifies
an incursion on the fundamentals of radiationless transitions.
Internal conversion between electronic states of the same spin
multiplicity and intersystems crossing between singlet and
triplet states take place because of weak interactions between
the initial and final states. The perturbing interaction is usually
limited in time and space, and the total Hamiltonian can be
considered as the sum of two terms

H¼H 0 þV (1)

where the time-independent operatorH 0 describes the unper-
turbed system andVis the perturbation. The probability of elec-
tronic transition per unit time is given by the solution of the
time-dependent Schrödinger equation

ðÞH 0 þVCð Þ¼x;y;z;t iℏ

dCðÞx;y;z;t

dt

(2)

which has the form (omitting the coordinates)

C¼

X

j

cjcjexp 

i

ℏ

Ejt



(3)

DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 203