7018 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Shediac et al.
[(porphinato)zinc(II)] chromophores via metal-mediated cross-
coupling chemistry in which the individual (porphinato)zinc(II)
units are linked via cylindrically π-symmetric ethyne or
butadiyne moieties at the meso- or â-positions of their respective
macrocycles,1,2,29,30 and demonstrated that controlling the nature
of the porphyrin-to-porphyrin linkage topology provides an
exquisite method to modulate the degree of both ground- and
excited-state electronic coupling between pigments in multi-
porphyrin systems.1,2,31,32
Mechanistic studies of excitation transfer processes in mul-
tiporphyrin systems over nanosecond-to-picosecond time do-
mains have typically required the syntheses of assemblies that
feature spectroscopically identifiable chromophoric entities that
function as energy donors and acceptors. The lowest excited
singlet and triplet states of (porphinato)metal complexes are
generally higher in energy than the analogous excited states of
their respective free base porphyrin macrocycles; hence, in
weakly coupled (porphinato)zinc(II)-spacer-porphyrin (PZn-
Sp-PH2) systems, energy transfer from an electronically excited
(porphinato)zinc(II) complex to the free base chromophore is
typically observed. The spacer moieties in these PZn-Sp-PH2
energy transfer systems have utilized hydrogen-bonded cy-
tosines,33 flexible diphenylcarboxy,34,35 and diphenylalkoxy
chains,36-40 as well as more rigid phenyl,7,8,41 diphenyla-
mide,12,13,42 diphenylpolyyne and -polyene,43 diphenylphenan-
throline,44,45 diphenylbipyridine,45 diphenylacetylene,3,4,24,41,46-50
and bis(phenylethynyl)arene51,52 units, to connect the porphyrin-
based pigments; energy delocalization between the porphyrinic
components is negligible due to the weak electronic coupling
afforded by these classes of pigment-to-pigment bridging
moieties.
In biological light harvesting assemblies that facilitate ultrafast
energy migration events, substantial interchromophore electronic
interactions cause individual pigment molecules to cease to be
electronically distinct,53 despite the anisotropic nature of the
protein environment and the variability in the chromophore
conformational landscape.54-57 With these facts in mind, we
have sought to probe the dynamical processes relevant to stron-
gly coupled multipigment systems that feature a small degree
of electronic asymmetry between their constituent chromo-
phoric building blocks. We have chosen meso-to-meso ethyne-
bridged bis(porphyrin) complexes1,2 as an archetypal bis(pig-
ment) system that features substantial ground- and excited-state
chromophore-chromophore coupling, and utilize PH2 and PZn
units to establish electronic asymmetry. While the nature of the
initially prepared excited state and ultrafast dynamical processes
inherent to complexes such as 5-[(10,20-bis[3′′,5′′-(di-tert-butyl)-
phenyl]porphinato)zinc(II)]-5′-[10′,20′-bis[3′′,5′′-(di-tert-butyl)-
phenyl]porphyryl]ethyne will be reported elsewhere, this study
contrasts the photophysics and spectroscopy of the singlet and
triplet excited states of this compound with its electronically
symmetric analogues bis[(5,5′,-10,20-bis[3′′,5′′-(di-tert butyl)-
phenyl]porphinato)zinc(II)]ethyne and bis[5,5′,-10,20-bis[3′′,5′′-
(di-tert-butyl)phenyl]porphyryl]ethyne, and probes the relative
roles played by the ethyne moiety and pyrrolic nitrogen
protonation in determining the nature of the S1 and T1 states of
these species over the nanosecond-to-miscrsecond time domain.
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