2-methyltetrahydrofuran (MTHF) at 77 K ( 60 ). Although this
value is consistent with the earlier spectroscopic value reported
by Harriman, the phosphorescence spectra of the two studies
are appreciably different. It should be noted that Brauer
employed H 2 TPP available from Aldrich without further purifi-
cation and that the signal-to-noise ratio of H 2 TPP phosphores-
cence is poor. We used time-resolved PAC to study the triplet
state energy of H 2 TPP and obtainedFTET¼ 24 1 kcal/mol ( 32 ),
which indicates that the value reported by Moore is probably
too low. More recently, Röder et al. reported the spectral
positions of the phosphorescence spectrum of TPP in MTHF at
80 K ( 61 ) that are consistent with the spectrum presented by
Brauer. At present, the best estimate for the triplet energy of
H 2 TPP isET¼ 33 1 kcal/mol, which leads toFT¼0.730.05.
The phosphorescence of porphin and its deuterated derivatives
was studied in Xe matrices at 10 K ( 62 ). This significantly
enhances intersystem crossing rates and increases the phospho-
rescence yield. It was possible to identify the 0–0 band of the
phosphorescence at 790–794 nm (36 kcal/mol¼1.57 eV) in Xe
matrix and the corresponding band of the fluorescence at
613 nm (46.6 kcal/mol¼2.02 eV) in Ne matrix. The same S 1 – T 1
energy splitting for porphin and H 2 TPP would imply that the lat-
ter would have a triplet state energy of 33.5 kcal/mol.
The extremely weak phosphorescence of the free bases is due
to the presence of radiationless decay pathways from the triplet
to the ground state competing favorably with phosphorescence.
The deactivation of the triplet state was proposed to be mediated
by acisNH triplet intermediate, populated in a rate-determining
step by proton tunneling, followed by fast intersystem crossing to
the ground state of thecisform, and finally by return to the
ground statetrans species ( 62 ). Thecis triplet has not been
observed, but it is now established that the mechanism of
porphin tautomerization involves stepwise migration of two
hydrogen atoms, with the formation of a metastablecisNH inter-
mediate. The energy of this cis singlet intermediate is ca.
5.5 kcal/mol above the ground state ( 63 ), and the energy of the
cistriplet is expected to lie between this intermediate and the
transtriplet, which is the emissive^3 (p,p*) state.
The phosphorescence of diamagnetic metalloporphyrins can be
easily observed at 77 K. This is consistent with slower radiation-
less rates, and hence with the absence of metastablecis NH
intermediates, but the heavy-atom effect exerted by the central
metal ion can also contribute to enhance the phosphorescence
rate. For example, the phosphorescence lifetimes in the series
MgTPP, ZnTPP, and PdTPP decrease in the order 45, 26, and
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 209