Optoelectronic properties, ground- and excited state dynamics, and synthesis of strongly coupled porphyrin-based donor-acceptor systems

This thesis focuses on the synthesis, spectroscopy, time-dependent dynamics, and photophysics of strongly coupled donor-acceptor (D-A) systems based on the porphyrin chromophore. Initial work reexamined ultrafast photoinduced electron transfer (PET) on electronically excited (5-($1\sp\prime,4\sp\prime$-benzoquinonyl)-10,15,20-triphenylporphinato) magnesium(II) $\{$MgPQ$\}$ in toluene, tetrahydrofuran (THF), and pyridine. These results show that the PET process involving the locally-excited MgP*Q state is solvent independent ($k\sb{ET}=2.86\times 10\sp{12}$ s$\sp{-1}$ in THF), while the thermal charge recombination reaction is solvent dependent. Moreover, these studies indicate that the photoinduced charge separation reaction may be rate-limited by electronic population equilibration between near degenerate porphyrin Q$\sb{\rm x}$ and Q$\sb{\rm y}$ levels. In an effort to enhance the electronic coupling in the porphyrin ground- and electronically excited-states, a new class of porphyrins in which an intervening ethynyl moiety connects aryl groups to the porphyrin meso 5- and 15-positions has been synthesized. Electronic spectroscopy, electrochemistry, and electronic structure calculations were carried out on various (5,15-bis ((aryl) ethynyl) -10,20-diphenylporphinato) zinc(II) compounds as well as an X-ray structural determination of (5,15-bis ((4$\sp\prime$-fluorophenyl)ethynyl) -10,20-diphenylporphinato) zinc(II) and 5,15-bis ((4$\sp\prime$-methoxyphenyl)ethynyl) -10,20-diphenylporphyrin. The data presented are indicative of: (i) removal of the steric barrier between the porphyrin and arylethynyl-phenyl aromatic systems, (ii) enhanced $\pi$-conjugative interactions, and (iii) ability of this new structural motif to readily modify energetics of porphyrin frontier orbitals. FT-Raman, resonance Raman, and transient resonance Raman spectroscopic studies carried out in THF for (5,15-bis ((4$\sp\prime$-nitrophenyl)ethynyl) -10,20-diphenylporphinato) copper(II) (I) and electronically asymmetric (5- ( (4$\sp\prime$-(dimethylamino)phenyl) ethynyl) -15- ((4$\sp\prime\sp\prime$-nitrophenyl)ethynyl) -10,20-diphenylporphinato) copper(II) (II) are consistent with unusually long-lived triplet excited states that possess significant porphyrin-to-nitro charge-separated character and substantial nuclear reorganization with respect to the ground state. The long lived nature of the triplet excited states, in fact, are borne out by triplet-triplet transient absorption measurements as 16.4 ns and 8.5 ns for I and II in THF, respectively. Hyper-Rayleigh scattering (HRS) experiments show that the molecular first-order hyperpolarizability ($\beta$) values of compound II and its (porphinato)zinc(II) analog are exceptionally large with the latter possessing $\beta$ values near $5000\times 10\sp{-30}$ esu at incident irradiation wavelengths of both 830 and 1064 nm; suggesting that this chromophoric structural motif may find utility in the development of electrooptic devices as well as materials for efficient frequency doubling of incident irradiation (second harmonic generation)