thus raising their energy. Hyper-type spectra exhibit additional
bands coming either from charge transfer (CT) from the metal
p orbitals into the empty porphyrinsp orbitals (d^1 – d^5 transition
metals, such as FeIII, MnIII, CrIII) or from the porphyrinsporbitals
into vacancies in the transition metal's d orbital (cation with an
open shell outer electronic configuration, such as SnIIor PbII)
( 36 ). Figure 5 compares the UV–Vis absorption spectra of free-base
and metalloporphyrins.
Paramagnetic complexes such as CuIITPP or CoIITPP have one
odd electron that can couple to the spin of the porphyrin triplet
yielding a“tripdoublet”and a“tripquartet”state. Similarly, that
odd electron can couple its spin with that of the porphyrin first
excited singlet state, leading to singmultiplet states. Moreover,
the singmultiplet states couple efficiently with the tripmultiplet
states resulting in rapid intersystems crossing from the excited
singlet state to the triplet manifold. This coupling mechanism
deactivates the singlet states rapidly and quenches almost
completely the fluorescence of paramagnetic complexes of
porphyrins (FF< 10 ^4 ). However, this coupling is responsible
for the observation of direct absorption from the ground state
doublet of CuIITPP to the “spin-forbidden” tripdoublet, at
670 – 710 nm( 37 ). The phosphorescence from lowest energy (p,p)
state of CuIITPP, which is the tripquartet, to the ground state dou-
blet becomes relatively intense and long-lived (FP¼ 6 10 ^2 and
tT¼ 600 ms) at 77 K in methylcyclohexane ( 38 ). The luminescence
lifetime is reduced to 29 ns at room temperature, and this is
ascribed to phosphorescence from the tripdoublet state which is
thermally repopulated from the tripquartet state ( 39 ).
The lowest energy (p,p) state of CoIITPP is also the
tripquartet, but this and other CoII-porphyrins are not lumines-
cent. This peculiar behavior is due to the presence of a porphyrin
a2u(p)!metal dz 2 CT state near the energy of the tripquartet
state. In fact, very weak absorption peaks in the 850–905 nm
region have been assigned to this CT transition( 40 ). The decay
of the lowest energy (p,p) state of CoIITPP to the CT state occurs
with a lifetime of 220 fs in benzene, and the decay of the CT state
takes place in 16 ps ( 41 ).
In addition to the hypsochromic shift of the Soret and Q bands
of diamagnetic NiIITPP, there are various optically invisible
lower-energy electronic states in which an electron is promoted
from the metal to the porphyrin ring or vice versa and those that
correspond to d–d excitation within the metal orbitals.
Calculations have shown that there is a multitude of CT states
below the first optically allowed state and explain its lack of fluo-
rescence ( 42 ). The radiationless depopulation of the Q state
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 201