Ultrafast Singlet Excited-State Polarization in Electronically Asymmetric Ethyne-Bridged Bis(porphinato)zinc(II) Complexes

The excited-state dynamics of two conjugated bis(porphinato)zinc(II) (bisPZn) species, bis(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenylporphinato)zinc(II)]ethyne (DD) and (5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenylporphinato)zinc(II)]-(5',-15'-ethynyl-10',20'-bis(heptafluoropropyl)porphinato)zinc(II)ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bisPZn compounds display intense near-infrared (NIR) transient S(1)-->S(n) absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S(1)-->S(0) oscillator strength on a 2-20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin-porphyrin conjugation, which varies as a function of the pigment-pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (tau approximately 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S(1)-->S(0) transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S(1)-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation.