EPR spectroscopy and photophysics of the lowest photoactivated triplet state of a series of highly conjugated (porphinato)Zn arrays

The lowest photoexcited metastable triplet state of a family of highly conjugated (porphinato)zinc(II) arrays in which ethyne or butadiyne units bridge the macrocycle carbon frameworks, along with their ethyne- and butadiyne-elaborated monomeric porphyrinic building blocks, and parent (porphinato)zinc(II) precursor molecules were studied by electron paramagnetic resonance (EPR), transient triplet-triplet absorption, and phosphorescence emission. All EPR spectra of the excited monomeric precursors and dimeric and trimeric porphyrin arrays reflect symmetries of rhombic distortion (\D\ greater than or equal to 3\E\) at 4 K and exhibit electron spin polarization patterns verifying that intersystem crossing from the first excited singlet state occurs predominantly through the \T-z> spin state sublevel. The zero field splitting (ZFS) parameters of the (porphinato)zinc monomers that beat ethynyl and butadiynyl groups as well as the conjugated porphyrin arrays are essentially temperature independent over a 4-100 K temperature range. This behavior is in contrast to the Line shape changes typically seen in most monomeric closed-shell metalloporphyrin triplet states, signifying that the lowest excited triplet state possesses a symmetry lower than D-4h and hence not susceptible to Jahn-Teller instabilities. The magnitudes of the D values for all compounds except the meso-to-meso butadiyne-linked dimer lie in the range expected for monomeric (porphinato)zinc complexes, indicating that (1) there is no global delocalization of the triplet excitation and (2) the excitation can be considered to be localized on one of the monomeric subunits in the conjugated supramolecular chromophoric systems on the time scale probed by EPR. The type and mode of chromophore-to-chromophore connectivity-does effect both spin distribution and spin alignment in the low temperature photoactivated triplet states of conjugated (porphinato)zinc arrays. For example, in the meso-to-meso butadiyne linked dimer, the \D\ value actually increases approximately 30% with respect to its monomeric precursor; this anomaly is interpreted as resulting from spin alignment in the molecular plane. A most striking feature is the persistence of electron spin polarization in most of the dimeric and trimeric porphyrin arrays under conditions of steady state illumination up to temperatures approaching the glass transition temperature (T approximate to 120 K). Progressive power saturation experiments carried out on one of the dimeric porphyrin systems, in which an ethyne moiety bridges two (porphinato)zinc chromophores at their respective mesocarbon positions, reveals an exceptionally weak temperature dependence on the saturation parameter, P-1/2, along with no significant changes in either the ZFS parameters or line widths with increasing microwave power or temperature, indicating that spin lattice relaxation is long relative to de-excitation to the ground state over a broad temperature range. This anomalous effect is discussed from the standpoint of relaxation occurring primarily through matrix disorder modes.