The photophysical properties and their temperature dependence are
reported for the sterically encumbered
nonplanar zinc and free base
2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrins
(ZnDPP and H2DPP),
and
2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrins
(ZnOETPP and H2OETPP), and the zinc
complex of
5,10,15,20-tetra-tert-butylporphyrin
(ZnT(t-Bu)P). Compared to planar
5,10,15,20-tetraphenylporphyrins (ZnTPP and H2TPP), the above compounds
exhibit reduced lifetimes of the lowest excited singlet
state, reduced fluorescence yields, and large shifts between their
absorption and emission maxima at room
temperature. ZnT(t-Bu)P, which is known to adopt a
ruffled conformation, displays dramatically altered
photophysical properties including a 7 ps 1(π,π*)
lifetime compared to one of ∼2 ns for ZnTPP at 296 K.
Equally noteworthy is the return of the ZnT(t-Bu)P
singlet lifetime to a “normal” value of 2.5 ns at 78
K.
An analogous temperature dependence has been observed previously
for the free base analog
H2T(t-Bu)P.
The other porphyrins investigated, with different modes of
nonplanarity, display smaller temperature variations
but also tend toward more normal properties at low temperatures. A
more extreme case of perturbation to
the tetrapyrrole electronic structure is found in zinc
2,3,5,5‘,7,8,12,18-octamethyl-13,17-bis(3-methoxy-3-oxopropyl)isoporphyrin perchlorate, a porphyrin tautomer with an
interrupted π system. This zinc
isoporphyrin
also exhibits a short excited state lifetime of 130 ps at 296 K, which
again increases to 0.8 ns at 78 K. The
results for the various nonplanar porphyrins and for the isoporphyrin
in several solvents indicate that the
principal cause of the altered excited state lifetimes is the ability
of the molecules to traverse multiple
conformational surfaces in the excited state. These surfaces
appear to be separated by only small energy
barriers that vary with the types of conformational distortions and
their concomitant perturbations of the
electronic structures of the chromophores.