The introduction of trifluoromethyl (H 2 TCF 3 P) or phenyl
groups (H 2 TPP) in the fourmeso-positions of porphin leads to rel-
atively small and nearly identical blue shifts of the Q and B
bands. However, the fluorescence lifetime is reduced by nearly
one order of magnitude from H 2 TPP to H 2 TCF 3 P, and the
fluorescence quantum yields follow the same trend( 34 ). This is
consistent with the internal heavy-atom effect to be discussed
in the next section.
The presence of strongly electron-withdrawing groups, such as
fluorines, in the phenyl rings of H 2 TPP or MTPP lower the
energy of a2urelative to that of a1u, as the latter has nodes at
the methine carbon atoms ( 35 ). This is confirmed by the blue
shifts of the Q bands observed when fluorine atoms are
introduced in the phenyl rings of free-base or zinc TPPs ( 32 ).
The atomic orbitals of metal ions with a closed shell of valence
electrons interact only weakly with thep-molecular orbitals of
the porphyrin. The absorption and emission spectra of the
corresponding metalloporphyrins are largely determined by the
porphyrin'sp-electrons and are classified as“regular.”An exam-
ple of this class is ZnIITPP. Regular metalloporphyrins givenor-
malspectra, with the Soret band in the near-UV (390–425 nm)
and two Q bands. The longest wavelength band occurs within
the range 570–610 nm for complexes with the macrocycle
substituted in the b-positions and between 590 and 630 when
the macrocycle is substituted in themeso-positions ( 36 ). It should
be remembered that the number of Q bands decreases from four to
two upon the coordination of the metal ions to free-base
porphyrins because of the change from D2htoD4hsymmetry.
Metalloproteins with closed metal shells, such as MgII(TPP) and
ZnII(TPP), are less fluorescent than the parent free-base porphy-
rin (FF¼0.033 for ZnTPP in deaerated toluene) ( 32 ), have shorter
fluorescence lifetimes (ts¼1.9 ns for ZnTPP) ( 33 ), and higher
intersystems crossing to triplet states (FT¼0.86 for ZnTPP in
deaerated toluene) ( 32 ) promoted by the spin–orbit coupling
mechanism described in the next section. In PdTPP, this becomes
so pronounced thatFF¼ 2 10 ^4 andFT1( 18 ).
“Irregular”metalloporphyrins show mainly three types of spec-
tra, called normal,hypso, and hyper. The normal spectra are
observed when the metal d or f electrons are of such low energy
that they do not interact appreciably with the porphyrin p-
electrons. Hypso-type spectra resemble the normal spectra,
except for a hypsochromic (blue) shift. The origin of this shift is
the back-bonding between dxyand dyzorbitals of metals from
groups VIII to IB (d^6 – d^9 transition metals, such as low-spin FeII,
CoII, NiII, PdII, or CuII) and the porphyrins's emptyp* orbitals
200 LUIS G. ARNAUT